{"paper_id":"0870ecc1-7c08-4c5e-b396-da776a914a73","body_text":"Bifunctional Ethoxy-Amine Modified Silica Catalysts for Green Energy: Facilitating High Biodiesel Yield via Enhanced Methanol and Oil Adsorption | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Bifunctional Ethoxy-Amine Modified Silica Catalysts for Green Energy: Facilitating High Biodiesel Yield via Enhanced Methanol and Oil Adsorption Tara Ghaffarinejad, Ramin Karimzadeh This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6903073/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted 20 You are reading this latest preprint version Abstract This study works on the synthesis and characterization of an APTES [(3-aminopropyl) tri ethoxy silane] impregnated silica catalyst for the transesterification of vegetable oil into biodiesel. It was aimed at increasing the silica catalytic activity by using different concentrations of APTES as a modifier which would mount the basic amine groups to the silica surface. Synthesis and functionalization of the silica was confirmed through the use of various characterization methods such as SEM (Scanning Electron Microscopy), FTIR (Fourier Transform Infrared), XRD (X-ray diffraction), and BET (Brunauer-Emmett-Teller). Of the four synthesized catalysts, the highest yield of biodiesel which was 62.2 percent was obtained from the catalyst containing 0.75g of APTES used at 65 degrees Celsius for 5 hours with 0.3g catalyst and reaction time. GC (Gas chromatography) showed a promising FAME (fatty acid methyl esters) profile which suggests the FAME would have good properties essential to low temperatures as well as for aiding in the biodiesel cold flow properties. Over 90% of the catalytic activity was retained after three uses. The catalyst maintains more than 90 percent of activity after three uses demonstrating stability and reusability. The silicate catalyst yielded superior results compared to conventional catalysts since it performed better with regard to environmentally friendly approaches, flexibility, and efficiency for large scale production of biodiesel. Physical sciences/Chemistry Physical sciences/Energy science and technology Biodiesel Production APTES-Functionalized Silica Heterogeneous catalyst Silica Surface Modification Sustainable Biofuels Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 Figure 8 Figure 9 Figure 10 Figure 11 Figure 12 Figure 13 Figure 14 1. Introduction Because of the growing worldwide need for alternative and sustainable energy, biodiesel has been extensively studied and considered a renewable and environmentally friendly fuel. They are also advantageous in terms of high biodegradability, lower emissions of greenhouse gases, and are compatible with existing diesel engines with minimal engine modifications. Because of these features, it holds great potential as a clean substitute to fossil fuels in tackling environmental and energy security challenges[ 1 ],[ 2 ]. Biodiesel is conventionally produced by the transesterification of vegetable oil or animal fat with methanol in the presence of a catalyst. Homogeneous catalysts like sodium hydroxide (NaOH) have high efficiency, so they are extensively used, but they bring challenges for separation, recyclability, and environmental issues. Heterogeneous catalysts have received enormous interest to circumvent these restrictions[ 3 ]. Heterogeneous catalysts have gained extensive attention for biodiesel production due to their advantages as efficient and sustainable catalysts. Unlike homogeneous catalysts, heterogeneous catalysts are easy to separate for reuse and are often considered to be less dangerous to the environment. Numerous studies via different modifications and functionalization have been undertaken to improve these catalytic properties such as catalytic activity, stability and selectivity[ 4 ]. Metal oxides like CaO, MgO, and ZnO have been extensively investigated for their high basicity and stability. However, these catalysts are often plagued by leaching and deactivation problems. Recent works on these metal oxides have focused on doping with other metals or implementation of mixed metal oxides that can improve the catalytic activity and stability[ 5 ]. Zeolites have been investigated due to their high surface area and tunable acidity. Doped zeolites, particularly alkali metal doped zeolites, have been reported for their improved catalytic activity for transesterification reactions. Nonetheless, the microporous characteristics of zeolites can restrain the diffusion of large triglyceride molecules, lowering their effectiveness[ 6 ]. As a result, these catalysts have demonstrated good performance due to their high surface area and stability. Nonetheless, synthesis of these catalysts often poses challenges, with some of them being complex and expensive[ 7 ]. There has been an interest in silica-based materials as catalysts owing to the high thermal stability, high surface area, and ease of functionalization of silica. Recent works have dealt with the functionalization of silica with different organic groups for increasing its catalytic properties. Silica functionalization ,for instance, the modification of silica with amino groups resulted in increased catalytic activity in transesterification[ 8 ]. Although substantial progress has been achieved in the field of heterogenized catalysts, there exist a number of limitations. Leaching, deactivation, and poor reusability plague many catalysts. Moreover, the synthesis of certain catalysts is complex and expensive, leading to practical limitations[ 9 ]. To overcome these limitations, a simple and economical synthesis of an APTES-functionalized silica catalyst is developed in this study. Functioning silica was carried out with (3-aminopropyl) tri ethoxy silane (APTES) to obtain basic amino groups to improve the catalytic activity in transesterification. Additionally, the surface area and thermal stability of silica also helps the catalyst and the heterogeneous catalyst is easily separable and reusable, which solves the reusability and environmental problems of the homogeneous catalyst. One of the most remarkable characteristics of (3-aminopropyl) tri ethoxy silane (APTES) is its bifunctionality, which ultimately contributes to its high efficiency in catalyzing the transesterification reaction associated with biodiesel production[ 10 ]. APTES is comprised of amine (-NH₂) groups and ethoxy (-OCH₂CH₃) groups. The amine groups can act as hydrogen bond donors or electrostatic interactions due to their highly polar character, attracting polar species such as methanol. This facilitates the adsorption of methanol onto the catalyst surface (thus placing it in close proximity to the sites of the reaction)[ 11 ]. The ethoxy groups can act hydrophobically and interact with the long hydrocarbon tails (nonpolar) of vegetable oils. This two-fold interaction mechanism plays an important role in facilitating an equilibrium and overcoming the immiscibility of oil and methanol (one of the roadblocks to biodiesel synthesis). By acting in this dual role function, APTES serves as an interface node for the catalyst's surface to simultaneously adsorb methanol and oil molecules onto its surface. This is beneficial in providing local concentration through the intermediate of a dual phase, which allows greater odds of having both adsorbed reactants in close proximity. In systems without this functionality, the polar and non-polar phases have limited interaction and often require vigorous stirring or co-solvents to facilitate reactions[ 12 ]. APTES-functionalized silica serves as a chemical bridge, lowering the energy barrier for the separation of the two phases, allowing greater rate kinetics for biodiesel production under milder reaction conditions. In addition, the new bifunctional nature of the catalyst is complementing the catalyst versatility to operate on different feedstocks (including low-grade or waste oils which may contain variances including impurities) with different chemical surroundings including polarity, which is advantageous for green chemistry and for industrial application involving the widest possible feedstock source due to economic and sustainability reasons having the choice of feedstock regardless of condition also applies to sustainability[ 12 ]. This explains why APTES has the capability to adsorb and activate both polar and non-polar reactants in the reaction at the same time, which represents a significant advantage in its ability to function as a highly active and reusable heterogeneous catalyst. This molecular-level synergism not only helps rationalize the observed catalytic performance, but also helps explain the robustness and versatility of the APTES-functionalized silica system which operates effectively over a diversity of transesterification reactions[ 13 ]. Compared with the current heterogenic catalysts used in biodiesel production like CaO, MgO, and zeolites, the APTES-functionalized silica catalyst offers several advantages. Although these traditional catalysts show high catalytic activity, as above mentioned, they are usually plagued with such drawbacks including leaching, deactivation, and the fact that most synthesis procedures are quite complex[ 14 ]. The silica catalyst with APTES functionalization exhibited high stability over many reaction cycles with only small loss in activity, whereas others such as MgO and zeolites deactivate relatively quickly owing to leaching or pore blockage[ 15 ]. APTES functionalization on silica also increases the density of reactive sites and thus increases chemical reactivity for transesterification reactions. The practical applicability of the catalyst is further endorsed by its flexibility in feedstocks, such as waste oils. All of these unique characteristics established the innovative nature of APTES-functionalized silica catalyst and its promise potential to overcome the deficiencies of existing catalysts contributing towards more sustainable and efficient biodiesel production[ 16 ]. APTES-functionalized silica as a heterogeneous catalyst for biodiesel production has a number of environmental advantages. First, using a reusable catalyst will reduce the constant need for new catalyst synthesis and disposal, thus, limiting the waste bottom line. Such practice does coincide with the aim of green chemistry, which advocates for waste prevention and the use of renewable sources[ 17 ]. Moreover, the catalyst's exceptional efficacy in transesterification reactions results in greater biodiesel outputs, minimizing the cumulative environmental impact of the production process. Biodiesel production itself is more eco-friendly than fossil fuel-based energy products. The emission of GHGs, particulate matters and Sulphur compounds are lower during the combustion of biodiesel, hence it leads to better air quality, lowering the potential for global warming[ 18 ]. Another advantage of these types of catalysts is that they require no neutralization and separation steps to keep them from contaminating the reaction products, producing much less wastewater and other by-products compared to their homogeneous counterparts[ 19 ],[ 20 ]. Notably, the APTES-functionalized silica catalyst was stable and reusable which is a major advantage over homogeneous catalysts. The catalyst also exhibited stable catalytic performance over several consecutive reaction runs without observable deactivation. After three cycles, for example, the catalyst maintained more than 90% of its activity, indicating its prospective durability[ 21 ]. One major benefit performance of biodiesel is the range of feedstocks suitability because the actual silica is functionalized with APTES. The nature of the bifunctional APTES (it has both a polar amine group and non-polar alkyl/ethoxy groups) allows the APTES to have interaction with methanol and many types of oils, including low-cost, impure forms, or waste streams when producing biodiesel[ 22 ]. Instead of requiring high-quality oils as required with conventional homogeneous or even some heterogeneous catalysts, APTES-functionalized catalysts develop and maintain activity across various feedstock qualities[ 22 ]. This adaptability is beneficial for advancing biodiesel to an accessible, affordable, sustainable and scalable pathway to displace fossil fuels. With the global emphasis on sustainability, sourcing waste oils and non-edible oils as feedstocks in biodiesel productions has gained significance in reducing reliance on food feedstock and reducing feedstock costs[ 23 ]. However, waste oils and non-edible oils generally are problematic due to elevated free fatty acids (FFAs), impurities, or oxidation products[ 22 ]. APTES is advantageous in part because it has a high affinity and is capable of interacting with a very chemically diverse range of materials. APTES features polar amine groups for interaction with FFAs and polar degradation products, yet the non-polar components can also retain interaction with triglycerides. APTES's characteristics promote conversations even with varied quality feedstock[ 24 ]. From an industrial and environmental viewpoint, this versatility extends the applicability of the catalyst, allowing for decentralized biodiesel production using varieties of oils available locally, including waste cooked oil, palm fatty acid distillates and other residues. It lessens reliance on virgin vegetable oils, and ultimately reduces production costs and environmental impact[ 25 ]. In effect, wis APTES-functionalized silica can enable or enhance chemical efficiency, but also facilitates the circular economy by converting waste into a fuel source of value. This feedstock flexibility further validates APTES functionalized silica as a next-generation catalyst in renewable energy applications[ 26 ]. Moreover, this reusability also minimizes the frequent replacement of catalysts and the overall environmental impact of the biodiesel production process[ 27 ]. Economically speaking, the APTES–functionalized silica catalyst is an inexpensive catalyst for biodiesel production. The catalyst exhibits an easy and inexpensive synthesis by using commercially available materials (e.g., silica and APTES)[ 28 ]. The catalyst can be gently recovered and reused several times without a significant loss in activity, greatly reducing costs of operation as time goes on[ 29 ]. In addition, due to favorable biodiesel conversion rates, silica-supported APTES catalysts could generate more biodiesel and thus boost the overall profitability relevant to the biodiesel production process. This is complemented with the ability to use low-cost feedstocks, including waste oils, further improving the economic viability of this catalyst[ 30 ]. Additionally, the reduction of waste generation and elimination of complex separation processes lead to lower production costs explaining its industrial scalability potential[ 31 ]. Conventional homogeneous catalysts such as NaOH and KOH are polluting the environment by producing waste water, soap, and other by-products. These catalysts demand neutralization steps, yielding salts that need to be discarded, putting greater pressure on the environment. Unlike current silica catalysts, which are not easily separable and reusable, an APTES-functionalized silica catalyst avoids these problems and greatly minimizes the generation of waste and contamination of water resources[ 32 ]. The APTES-functionalized silica catalyst works at low temperatures and atmospheric pressure, lower than the conditions needed for many common catalysts. This helps to save the energy used in the biodiesel production process, as this process would thus be more energy and cost efficient[ 33 ]. They can also spew out lighter intensity, which allows the process to be more sustainable as well as greener[ 34 ]. This catalyst was synthesized in a scalable manner without the need for intricate or toxic procedures. The catalyst is amenable to scale-up with ubiquitous solvents such as ethanol and hexane and trivial functionalization steps. This scalability is essential for industrial applications, paying attention to cost-effectiveness and simplicity[ 35 ]. What sets this research apart in silica surface amine functionalization is the strategic modification of its chemistry using a very specific amount of APTES to form a solid base catalyst which is both highly active and can be reused multiple times. Unlike other works that have simply documented surface enhancement of silicas or general amine-functionalization, this work has dealt with optimization of APTES ‘s loading (0.25, 0.5, 0.75, 1 g) and its catalytic activity and biodiesel production in a systematic, quantitative manner, thereby establishing a direct connection to the yield of constructed catalysts. Furthermore, this approach does not resort to elaborate synthetic strategies but rather makes use of simple commercial products and minimal energy expenditure to achieve scalable design. Therefore, the aim of this study is to synthesize and evaluate APTES-modified silica as a heterogeneous catalyst for biodiesel production, focusing on optimizing APTES loading, analyzing surface functionality, and correlating catalyst structure to performance and reusability 2. Materials and Methods 2.1 All materials used Tetraethyl orthosilicate (TEOS, 98%, Coupsyl) was used with further purification (TEOS was pre-treated in an oven at 90°C for 1 hour to evaporate residual ethanol and water prior to use), triethylamine (TEA, 99%, Neutron) was used as condensation catalyst, APTES (98%, Sigma), Hexane (96%, Neutron), HCL (37%, Mojallali), vegetable oil (Ladan Brand) and methanol (99.8%, Mojallali). 2.2 Synthesis of Silica Silica particles were synthesized via a sol-gel process based on a modified Stöber method, using TEOS as precursor and TEA as catalyst. First, 10 cc of TEOS is added to 100 cc of distilled water and stirred under continuous stirring at 500 rpm. Then, 0.3 grams of TEA catalyst is added to the mixture. After 5 hours of reaction, the resulting precipitate is washed thoroughly and placed in an oven at 100°C for 2 hours. Subsequently, the dried silica is calcined in a furnace at 700°C for 1 hour. 2.3 Synthesis of APTES-Functionalized Silica First, 3 g silica (which were synthesis as above mentioned) dispersed in a 0.1 M HCl solution, with the aid of an ultrasonic device. The silica was then washed with water to remove any unreacted acid after sonication. The washed sol-gel SiO 2 was then separated into four parts and treated with different amounts of the reagents, APTES as the source of the amino group and hexane as the solvent [Four amounts of APTES (0.25 g, 0.5 g, 0.75 g and 1 g) were applied to functionalize a gram of silica in a 100 cc hexane treatment in the present study (four different products)]. All the mixtures were sonicated for 15 minutes to ensure the adequate interaction of the silica with APTES. The products were subsequently washed with ethanol to remove unreacted reagents and impurities post-sonication. Finally, they were dried at 60°C in an oven for 2 hours until completely dried, producing a functionalized product. 2.4 Biodiesel Production The biodiesel was synthesized through the transesterification of vegetable oil with methanol catalyzed by the synthesized APTES-functionalized silica (4 different catalysts which were synthesis as above mentioned). In examining the effects of process parameters on yield, specific ratios of oil to methanol to catalyst were utilized. 2gr Methanol was mixed with the needed amount of vegetable oil (6.6 gr). Next, the functionalized silica catalyst was added in the above mixture (0.2 gr), and the reaction was done at controlled temperature (55°C) for 5 hours under continuous stirring. The mixture was then allowed to cool to room temperature after the reaction. Centrifugation was carried out for 10 minutes at 3000 rpm to separate out the biodiesel phase from the glycerol and catalyst from the reaction mixture. The biodiesel layer was carefully decanted and washed several times with distilled water to remove residual methanol, catalyst and impurities. Also, after each run, the catalyst was recovered via centrifugation, washed thoroughly with ethanol and distilled water, and dried at 60°C before reuse. 3. Results and Discussion 3.1 SEM Results for silica synthesis The SEM images in Fig. 1 demonstrate that the synthesized silica mostly has a spherical morphology with smooth and uniform surfaces, with many, often times, aggregated. These spherical features are a reflection of successful implementation of sol-gel synthesis, The spherical morphology allows a high external surface area, which is advantageous for catalysts, by improving the dispersion of active sites and providing a closer pathway of diffusion for reactant molecules. Furthermore, the aggregated clusters may lead to interparticle mesoporous and macropores that also serve to enhance the diffusion of large reactant molecules, such as triglycerides. The uniform dimension and shape of the silica particles is directly related to greater catalytic performance. The consistency of these silica particles allows for the uniform availability of active sites leading to effective interaction between the catalyst and reactants. Additionally, the SEM characterization reveals smooth surfaces that readily allow the free adsorption of both methanol and oil molecules, thus increasing the likelihood of surface reactions to occur. This characterized morphology is conducive to the production of an effective interfacial zone for the transesterification reaction to occur, especially under moderate operational conditions. Another important observation in the SEM images is the structural integrity of the silica particles. They maintained their spherical morphology without any fractures or issues with the surfaces, which demonstrates mechanical strength and ability to resist the physical forces associated with repeated cycles of reaction. Because the silica particles are mechanically robust, it extends to the possibility of using them interchangeably, further minimizing the risk of losing the catalyst through breakdown, pore collapse, or agglomeration during the processing of the reaction solution. The slight irregularities in the silica particles shown in the SEM of the dried samples were largely insignificant, and there were no extremely fine or irregular particles, both of which would increase the risk of losing the catalyst, or compounding operational issues during filtration or separation stage such as blocking[ 36 ]. In short, the SEM gave powerful evidence of the morphological quality and appropriateness of the synthesized silica for use in catalyst applications. The oval spheres, smooth surfaces, and clustering of characteristics consistent with a catalyst, indicates that mass transfer will take place effectively, the reactants should stable adsorb on to the reactants, and structural mechanical function for stable time to use. Moreover, this provides the foundation for proving final catalytic performance and consistent functionality as biodiesel production. Optical microscopy in Fig. 2 shows further confirmation of the spherical nature of the particles, appearing as bright, rounded features due to light scattering. The spherical morphology and narrow size distribution observed suggest a well-controlled synthesis process. 3.2 Nitrogen adsorption-desorption isotherm Results for silica synthesis The nitrogen adsorption-desorption isotherm for silica (Fig. 3 ) had a Type IV isotherm with an H1 hysteresis loop, a more characteristic feature of mesoporous materials, indicating the existence of mesopores. The presence of a distinct capillary condensation step in relative pressure range 0.4 to 0.8 means that uniform mesopores exist in the silica. These mesopores are advantageous for catalytic applications as they lead to high surface area and limited the diffusion of large organic molecules such as triglycerides that typically are found in vegetable oils. The cylindrical nature of the pores proposed by the hysteresis shape suggests a consistent pore morphology that will allow for effective reactant transport to the surface of catalytic sites. The mesoporous structure highlighted by the isotherm findings is essential to improving the performance of the catalyst. In the case of biodiesel, the reactants, methanol and oil, need to pass through the bulk of the catalyst surface with little resistance to reach the active sites. The given pore structure provided pathways for the diffusion of reactants, and for product removal, therefore improving reaction rate. If the porous structure was useful in reducing diffusion restrictions compared to microporous or non-porous catalysts, then it is even more useful when working with molecules that are large hydrophobic oil molecules that are being processed. Also, the isotherm profile suggests that the silica support has considerable pore volume, which is important because it offers surface accessibility and space for functionalization. The fact that even after synthesis, the pores are open and accessible means that the material can support the addition of catalytic groups without a large pore blockage event. Therefore, the structural openness means that the addition of a surface modification will not compromise mass transfer, and thus will not reduce the efficiency of the catalyst. The ability to maintain this balance between functionalization and porosity is important to achieve high activity and sustained catalytic stability when the material is used for repeated reaction cycles. In summary, the nitrogen adsorption-desorption experiment identifies a mesoporous, high-surface-area structure that is perfect for heterogeneous catalysis. Further, most of the characteristics of the isotherm indicate that structures of this type generally support reactant diffusion and accessibility to active sites, both important for biodiesel yield. The fact that we are predominantly dealing with a mesopore structure also suggests a resilience if we were to accommodate both polar and non-polar reactants, which is key in the bifunctional catalytic approach being utilized. Overall, the catalyst appears to be suitable for biodiesel production, especially when performed under mild and sustainable conditions. The increased adsorption ability of APTES-functionalized silica is largely based on its molecular structure allowing for simultaneous interaction with both polar and non-polar reactants. The amine groups (-NH2) present in APTES are able to interact with polar molecules using hydrogen bonds, such as methanol. This interaction increases the local concentration of methanol at the catalyst surface close to its catalytic sites and the reaction interface. By anchoring methanol interactions through polar forces, the catalyst allows one of the key reactants to interact at the reaction interface. In an APTES molecule, the hydrocarbon backbone and ethoxy portions provide a non-polar interface capable of adsorbing oil molecules, in particular the long-chain triglycerides found in vegetable oils. The hydrocarbon groups support the adsorption of oils, and the presence of ethoxy groups enables the retention of methanol at the oil/catalyst interface. The dual interaction engenders a stable interfacial area on the catalyst surface where two reactants, methanol and oil, sit in very close proximity. In a classic heterogeneous system, the combined immiscibility of oil and alcohol phases is often a significant barrier to the reaction in which emulsification or agitation needs to be applied. APTES reduces the tension at the oil/alcohol interface by its specific chemical design, acting in a sense as a bridge between distinctly different phases. The enhanced adsorption properties have a clear impact on reaction kinetics. APTES can physically increase the proximity of the reactants and stabilize their interface on the catalyst surface, lowering the energy barrier related to the phase transfer and reaction activation. The transesterification reaction can therefore proceed quicker and at lower energy inputs, which is the green chemistry tenet used in this study. The improved adsorption APTES can provide increases the efficiency of the multi-step reaction not only by improving the reaction time, but also by aiding catalyst reusability through maintaining consistent access of the reactant to the active sites across each cycle. One of the major benefits of using APTES-functionalized silica as a catalyst in biodiesel production is that it can lessen the overall energy requirement of the transesterification reaction. The transesterification reaction usually has high activation energy due to the immiscibility of methanol (polar) and vegetable oil (non-polar), which limits their molecular interaction and slows down reaction kinetics. In conventional systems, the problem of immiscibility is usually remedied with high heating temperature, co-solvent addition, or strength base catalysts, all of which can be highly energetically- or environmentally-unfriendly. The most pragmatic solution is with APTES, which, similarly to monoglycerides, formed a chemical bridge between two immiscible reactants through its bifunctionality. This allows for a more favorable pathway for the reaction of the reactants. The APTES molecule has both a polar amine and non-polar alkyl groups, which makes it possible for the APTES to simultaneously adsorb both methanol and oil onto the catalyst surface. Reactants that are this close together will have a higher rate of interaction (or the transition state) which will decrease the energy barrier for transesterification to proceed, hence the reaction can take place under more mild conditions while providing competitive yields, as seen in this study. Decreased energy input is advantageous from a green chemistry perspective, as it falls in line with sustainability goals, and provides operating cost savings at higher plant scales of biodiesel production. Additionally, the increased activity in mild conditions supports durability and end-of-life for the catalyst. High temperatures are known to damage functional groups, produce unanticipated side reactions, or convert the modification to the heterogeneous catalyst. Since APTES can promote effective catalysis, it can do so without such a harsh environment. This improves energy efficiency and also increases the functional life of the catalyst. All of these factors speak to the fact that APTES-functionalized silica is a low-energy, cheap, sustainable, biodiesel production option that has been proven effective over more than 1 cycle use. 3.3 FTIR Results for silica functionalization The FTIR spectra results of the APTES-loaded silica samples illustrate clear evidence of surface modification at different APTES loadings. All the spectra show important peaks indicating the presence of imine and amine bonds suggesting that organosilane APTES was covalently bonded to the silica surface. In Fig. 4 for the sample with 0.25 g APTES, very weak absorption bands were seen at 1560–1650 cm⁻¹ (N–H bending) and 2850–2950 cm⁻¹ (C–H stretching). The low intensity values of these peaks indicate that only a small amount of aminopropyl groups is present. Thus, surface functionalization remains incomplete at this loading level. Figure 5 depicting the 0.5 g APTES sample shows a moderate increase in the intensity of N-H and C-H related peaks suggesting an increased amount of APTES significantly higher than 0.25 g, meaning that there was better modification of APTES in this sample, but surface coverage probably still remained inadequate. Figure 6 portraying the 0.75 g APTES sample contains the most intense and well-defined bands in the N-H and C-H regions of the spectra. This means that this sample had an adequate level of surface functionalization and thus a high number of amino groups were bonded to the silica surface. This step done from the modification of the sample will probably create materials with desired surface reactivity and maintained porosity. From Fig. 7 , related to the sample with 1 g APTES, the FTIR spectrum shows broader peaks with possible overlaps. This might be due to APTES overloading, where multilayer formation or condensation side reactions might have taken place. Saturation of the surface can block access to active sites or block pores partially. From the FTIR studies, one may predict that the 0.75 g APTES sample (Fig. 6 ) has the most suitable functionalization for catalytic purposes. It shows a good incorporation of active amine groups but will probably still have a porous structure of silica support; these properties are important for active catalysis in transesterification reactions. To conclude, FTIR is an effective method to certify the success of silica functionalization. In this study, FTIR spectra recorded for different APTES loads of 0.25 g up to 1 g revealed separate bands of functional groups introduced by APTES. The absorption bands within 1560–1650 cm⁻¹ are attributed to N-H bending vibrations, whereas those in the 2850–2950 cm⁻¹ range are assigned to C-H stretching vibrations of alkyl chains introduced by APTES. None of these features is exhibited by pure silica, thus confirming that the somewhat different spectra observed in modified samples directly imply successful bonding between amine-containing silane groups and a silica surface. With increased APTES concentration, these characteristic bands show an increase both in intensity and in the sharpness of definition. Such affair is most pronounced in the 0.75-g APTES sample (Fig. 6 ), which shows the tallest N-H and C-H peaks, hence the intensity represents the concentration of amine functional groups grafted onto silica structure successfully. This gradual variation is a clear indication that with loading, increasing surface coverage by APTES results in modifying the chemical nature of the surface towards catalytic activity for transesterification reactions. FTIR not only confirms isotope functional groups but also indirectly infers homogeneity and efficiency of the functionalization. For example, broadening and overlapping of peaks in the 1 g APTES sample may indicate multilayer formation or condensation reactions that may actually hinder catalyst performance. Thus, FTIR not only verifies the successful attachment of APTES but also provides insights into the optimal loading levels for effective functionalization without pore blockage or loss of accessibility to active sites. 3.3.1 Effect of pH with Varying APTES Loading The pH of a catalyst suspension can dramatically affect the overall performance of the catalyst during biodiesel synthesis, particularly in base-catalyzed transesterifications involving methanol and triglycerides. Regarding APTES-functionalized silica, the amine group (-NH₂) contributes to the basicity of the catalyst. With more APTES grafted onto the silica surface, the number of amine groups on the surface increases, directly correlating the basicity, and thus pH, of the catalyst suspension in water or methanol. Silica functionalized with 0.25 g of APTES had a pH range of 6.8–7.2 due to limited amine functionality. Functionalized silica with 0.5 g of APTES, had pH of 7.5–8.0 because of the presence of more free amine groups yielding an approximate. The sample with APTES at a functionalization level of 0.75 g had a pH of 8.5–9.2, matching where a high surface density of amine functions would create good basicity. The sample treated with the highest level of APTES at 1.0 g had the pH in the range of 8.0–8.8, possibly slightly lower than the 0.75 g because of a higher level of APTES molecule saturation or multilayer arrangement causing restriction to amine accessibility to the solution environment. The relationship between pH and biodiesel yield could be aligns with these estimations. The catalyst modified with 0.75 g APTES might produce the highest biodiesel yield at the point where surface basicity (and presumably pH) is optimal for transesterification given the high pH. The high pH enhances nucleophilic attack by methoxide ions on triglycerides and can create faster conversions to methyl esters. Conversely, if too much APTES is present it can lead to crowding on the silica surface as multilayers will be formed or pores will be blocked; this may explain the slight decrease in yield obtained with the 1 g APTES sample even when it has a reasonably high pH value. This trend underscores that optimum basicity (pH) is more important than maximum basicity. After a certain level of APTES loading, the availability of basic sites does not increase proportionally due to steric hindrance or aggregation. So, even if the pH is high, the catalytic efficiency does not necessarily go up. This finding is also consistent with the FTIR data, as 1 g APTES indicates peak saturation, and potential broadening would correspond with overlapping functional groups or less efficient surface exposure. The pH behavior for a range of APTES loadings further aids the understanding of catalyst reusability. The catalysts with the pH that is close to showed better durability across multiple uses because of better dispersion and covalently bonded APTES groups. The stable pH in the reaction serves as a proxy for the chemical stability of the active sites, suggesting that catalysts with balanced pH are more reactive and also more stable. Overall, it appears that the pH of our APTES-functionalized silica catalyst reflects the density of surface amine groups and their accessibility. We can see that pH varies with the amount of APTES added up to 0.75 g, and subsequently became less variable until the surface was saturated and we began to see a slight decrease in the pH value with further additions. This also emphasizes the importance of having moderate accessible basicity for the catalytic step. Thus, by varying the amount of APTES we can control the surface pH of the catalyst and use this as a strategy to manage the catalytic activity, stability, and selectivity in biodiesel synthesis. 3.4 Results of Biodiesel synthesis with different catalysts Table 1 notes the experimental conditions, and the yield of each test was calculated according to the given formula. Table 2 shows density data based on biodiesel yield for each experiment which calculated by the given formula. Table 1 Experimental conditions for biodiesel production with different catalysts Experment Methanol/Oil Catalyst Catalyst (gr) Yield (%) 1 12:1 Silica-APTES (0.25) 0.2 33.3 1* 12:1 Silica-APTES (0.25) 0.2 32.4 1** 12:1 Silica-APTES (0.25) 0.2 33.01 2 12:1 Silica-APTES (0.5) 0.2 48,9 2* 12:1 Silica-APTES (0.5) 0.2 48.08 2** 12:1 Silica-APTES (0.5) 0.2 47.9 3 12:1 Silica-APTES (0.75) 0.2 62 3* 12:1 Silica-APTES (0.75) 0.2 60.04 3** 12:1 Silica-APTES (0.75) 0.2 61 4 12:1 Silica-APTES (1) 0.2 58.78 4* 12:1 Silica-APTES (1) 0.2 58,02 4** 12:1 Silica-APTES (1) 0.2 58,43 • Silica-APTES (0.25) = 25 gr APTES in synthesis process • Silica-APTES (0.5) = 0.5 gr APTES in synthesis process • Silica-APTES (0.75) = 0.75 gr APTES in synthesis process • Silica-APTES (1) = 1 gr APTES in synthesis process 1*,2*,3* and 1**,2**,3** are repetition of experiments. The biodiesel yield was calculated using the following equation: Yield (%) = (Weight of biodiesel / Weight of oil) × 100 This formula allows for the quantitative assessment of biodiesel production efficiency under various process conditions. The results were analyzed to identify the optimal parameters for maximum yield. Table 2 Density Data Based on Biodiesel Yield Experiment Yield (%) Biodiesel Density (g/cm³) Glycerin Density (g/cm³) 1 33.3 0.892 1.243 1* 32.4 0.891 1.240 2 48.9 0.888 1.238 2* 48.1 0.889 1.237 3 62.0 0.875 1.220 3* 60.0 0.878 1.223 4 58.4 0.880 1.225 4* 58.0 0.881 1.227 1*,2*,3* are repetition of experiments. Density = mass (gr) / volume (ml) Table 3 Statistical Analysis of Biodiesel Yield (Mean ± SD) for Different APTES-Functionalized Silica Catalysts Catalyst (Silica-APTES) Mean Yield (%) SD (Standard Deviation) 0.25 g 32.91 0.46 0.5 g 48.29 0.53 0.75 g 61.68 1.01 1 g 58.41 0.38 The yield data obtained from triplicate experiments for each catalyst loading are presented as mean ± standard deviation in Table 3 . This statistical treatment demonstrates the reproducibility of the transesterification process. For instance, the 0.75 g APTES catalyst yielded 61.68 ± 1.01%, indicating high experimental consistency. Based on the yields obtained, the third catalyst demonstrated the best performance among all. 3.5 BET Results Based on the expected trend of APTES loading on surface properties, BET analysis was focused on two formulations: the optimal performing catalyst (0.75 g) and silica itself. This selection offers sufficient insight into the relationship between surface modification and catalytic efficiency. Table 4 BET results for Silica and Modified Silica with APTES (0.75 g) Parameter Bare Silica APTES-Modified Silica BET Surface Area (m²/g) 292.94 219.43 Total Pore Volume (cm³/g, BJH Ads.) 0.9347 0.7472 Average Pore Diameter (Å, 4V/A BET) 9.25 7.51 Average Pore Width (Å, BJH Adsorption) 103.52 93.88 BJH Adsorption Pore Surface Area (m²/g) 361.18 318.37 As shown in Table 4 , the BET surface area and total pore volume of the APTES-modified silica decreased compared to the bare silica, indicating successful functionalization. The reduction in surface area and pore size is attributed to the partial pore blockage and surface coverage by the grafted APTES molecules. BET measurements play a vital role in understanding the surface characteristics of catalysts, in terms of specific surface area, pore volume, and pore size, which collectively impact the catalytic activity. On APTES-functionalized silica catalysts for biodiesel production, these factors are important as to how easily the reactant molecules could approach the active sites, particularly in transesterification process. Large surface area for more active sites and the pore architecture designed for the optimal diffusion of methanol and oil molecules into the catalyst are favored. A decrease in BET surface area and pore volume is also observed for 0.75 g APTES-functionalized silica as compared to bare silica is observed from the results presented in Table 4 of the paper, which confirms that APTES grafted onto the silica surface. Nevertheless, the decrease in surface area is not that big and there is still some pore volume, where some, which has been occupied by the amine groups, was sacrificed, but plenty of porosity is left for a proper mass transport. Such balance of surface functionalization and structural integrity is critical for the catalytic activity, by which the chance of reactant contacting with active sites could be greatly improved while the diffusion limitation was not induced. In addition, the microporosity (pores of 2 to 50 nm) is very useful for biodiesel production process, because the dimensions of the triglyceride molecules from vegetable oil are relative large. The isotherm profile and pore size distribution from the nitrogen adsorption-desorption isotherm (Fig. 3 ) confirm the presence of mesopores, which are essential for accommodating these bulky molecules. Therefore, the BET analysis does more than quantify surface metrics; it substantiates the structural features that govern the catalytic behavior of the modified silica in biodiesel synthesis. 3.6 XRD Results The XRD patterns in Fig. 8 and Fig. 9 obtained for unmodified silica and the APTES-functionalized silica displayed broad diffraction peaks at approximately 22° 2θ, indicating the presence of broad amorphous silica structures. The peaks' broad and diffuse nature suggested there was no long-range crystalline order, supporting the claim that the sol-gel method produced an amorphous silica framework. The amorphous structure is ideal for catalytic applications due to its associated more reactive surfaces, which are presumed to contain silanol center's associated with irregular bonding environments, which can promote functionalization and interact with reactants. There was a slight shift in the location of the main diffraction peak of the modified APTES, as shown in Fig. 9, from 21.993° to 22.348° 2θ, and a measurable variation in d-spacing values. These differences indicate that the surface functionalization was successful without affecting the overall integrity of the silica structure. In other words, the APTES molecules that were grafted to the silica surface changed the local chemical environment, but the overall amorphous character of this silica materiel was preserved. This was important because preserving the bulk structure allows the physical properties of the silica, such as microporosity and mechanical strength, to remain unchanged, while, at the same time, allowing chemical modifications for catalytic activities. The retention of structural amorphism following APTES functionalization is consistent with the goal of engineering a reusable, bifunctional heterogeneous catalyst. The XRD results confirm that unintentional crystallization or structural densification did not occur during functionalization, and if further underscores important structural basis when taken alongside the results from the SEM and BET, which suggest stability in both morphology and porosity. Collectively, these results suggest that the catalyst was altered at the surface level with a retention of the core structure needed for repeatable, scaleable catalytic performance in biodiesel production. Additionally, the amorphous characterization provided by XRD is consistent with our FTIR results, where we demonstrate the incorporation of functional amine groups with no signals to suggest new crystalline phases. Together with the thermal stability seen in TGA and demonstrated reusability from the catalytic activity tests, the XRD characterization underlines the idea that the silica support is able to provide a structurally stable, chemically modifiable, and catalytically active platform. Collectively, the findings confirm that the material design concept articulated in this study (a process for producing biodiesel through an approach that develops functionality at the molecular level without sacrificing physical stability) was operationally successful. . Figure 9. XRD image of synthesized APTES-Functionalized (0.75 g) Silica 3.7 TGA Results The data derived from TGA analysis silicas APTES functionalized (3-aminopropyltriethoxysilane) in Fig. 10 suggests that said material possesses adequate thermal stability and organic content for use in biodiesel production processes. Considering The Elucidated Sample is an APTES-modified Silica, it was examined on an SDT Q600 (version V20.9) from 25°C to 1000°C at a rate of 20°C/min under argon. Its starting weight was 2.8490 mg and during the entire heating cycle, weight losses of 18.98% (close to 0.5409 mg) were observed. Weight loss can be analyzed in three primary steps. The first weight dip up to around 150°C corresponds with the loss of physically adsorbed moisture and surface water, indicating the silica surface’s hydrophilic character prior to organic decomposition. The most significant weight decreases between 150°C and 600°C relates to the breakdown of the organic components added from APTES. During this phase, the aminopropyl chains anchored to the silica surface are decomposed and emitted as volatile substance. The significant weight loss of almost 19% validates the proper functionalization of APTES. Above 600°C, the weight loss is minimal, suggesting that the remaining inorganic silica framework is thermally stable at high temperatures. In the context of biodiesel production, functionalizing silica with APTES can enhance catalyst dispersion, thermal stability, and interfacial interaction between the organic phase (such as oil or methyl esters) and the catalytic surface. The TGA results confirm that the organic groups are well anchored to the surface and thermally stable up to 500–600°C, which is significantly higher than typical transesterification reaction temperatures (usually 60–200°C). This high thermal stability supports the suitability of the functionalized silica as a solid heterogeneous catalyst support in biodiesel synthesis. Thermal stability is a vital consideration when determining the long-term viability of catalysts for industrial use. The TGA of APTES-functionalized silica (0.75 g) demonstrated that the grafted organic groups show thermal stability up to a temperature range of 500–600°C. This is notable considering that the transesterification reactions to produce biodiesel are conducted at relatively low temperatures: 60–200°C. Consequently, the organic modification by APTES no longer serves its purpose during the reaction but is thermally robust enough to degrade under the rigorous operational conditions, ensuring functionality and structural integrity. High thermal stability supports the reusability of the catalyst observed. The catalyst reusability study (Fig. 14 ) displays that the APTES-functionalized silica retained over 90% catalytical activity after three continuous reaction cycles. This confirmed that the functional groups remain active as they had not decomposed or leached during the reaction. The weight loss from TGA mainly occurred due to desorption of surface moisture and eventual breakdown of the organic groups at or above the reported thermal limits, suggesting that the catalyst does not experience substantial degradation in normal conditions for biodiesel synthesis. In addition, the catalyst's high-temperature stability suggests its options for scaling up or integration into continuous or batch processes. The catalysts used in these systems are almost always exposed to thermal stress, and when they cannot withstand long-term exposure to it, there may be downtime, lost efficiencies, or replacement costs. The thermal stability of the APTES-functionalized silica suggests its prospects for industrial-scale processes involving heat management or situations where long-term stability is requisite for both economic and environmental sustainability. Thermal stability also negates some of the provenance of catalyst fouling or coking which will be exacerbated by little thermal stability. Thermal stability assures that the active amine sites introduced by APTES are still available, chemical intact and interfacing with methanol and oil factions. The bifunctional role of the catalyst in transesterification has a decent constraint on heat transfer circumstances between polar methanol and non-polar oil. So, not only are the TGA results a confirmation that those materials can tolerate elevated temperature, they also confirm the APTES-functionalized silica has longevity and utilization in biodiesel production systems to be a cost-effective input. Finally, the relationship between stability and catalytic activity offers a useful story in catalyst development. In developing heterogeneous catalysts to be used in green contexts, consideration must be given to both catalytic performance and catalytic durability. APTES-functionalized silica in this study has performance durability by being able to withstand high temperatures while retaining significant conversion capacity across multiple cycles. The reality of this relationship can additionally contribute to the conversation of sustainability of catalysts and substantiate the practical use of APTES-functionalized silica for biodiesel production. In order to provide complete clarity about the performance of the catalyst, it is important to contextualize results from the many characterization methods used in this study. Each method provides different, yet complementary information about APTES-functionalized silica structural, chemical and thermal properties, and how these aspects work together to provide the catalyst an active, reusable and functional performance for biodiesel production. For example, FTIR spectroscopy provided evidence for the successful grafting of amine groups from APTES onto the silica surface. In FTIR spectra for silica modified with increased amounts of APTES, the bands for N–H bending and C–H stretching progressively intensified with increased APTES loadings, suggesting good grafting occurred, with the formulation with 0.75 g APTES obtaining the most surfactant functionalization. Grafting amine functional groups onto the silica surface is critical, as the function of these amine sites are to create basic sites needed to catalyze the transesterification reaction. Without discussing the chemical modification using other techniques, it would be impossible to consider how modifying the silica in this way changed the physical structure and performance of the catalyst. BET surface area and pore volume evaluations add another dimension to this conclusion. The results show a small decrease in surface area and pore size after the APTES modification, which implies that functional groups have deposited on the surface while mesoporosity has been retained. Retained mesoporosity and large surface area are important for favorable diffusion of reactants through the structure and increased contact with catalytic sites. Therefore, the BET results support that functionalization was completed maintaining the degree of structural openness essential to promote catalysis. Thus, it was validated that the amount of APTES (0.75 g used previously) was optimal to provide chemical activity while keeping structural integrity. Together, SEM and TGA indicate that the structure is both mechanically and thermally stable. Based on XRD analysis showing amorphous silica structure after APTES modification, we can conclude that while there is some observable change at a surface level due to functionalization, the bulk structure remained intact. The observed shifts in 2θ and d-spacing were slight and suggest that the high reactivity introduced by APTES is a surface level change in relation to the acidic silica structure in a bulk. Consequently, the synthesis successfully produced a surface actionable catalyst while having limited if any effect on the physical backbone or superficial scaffold, thereby retaining efficacy without sacrificing longevity. These various analytical results provide a more comprehensive picture of the catalyst performance. FTIR indicates the reason for catalytic activity, BET corroborates active site accessibility, SEM and XRD confirm structural integrity, and TGA verifies sustainable thermal stability over time. Integrating the methods that the APTES-functionalized silica is not only a chemically active catalyst but it is also physically and thermally robust. The discussion of the integration further fortifies the conclusions and justifies the claim that this catalyst is viable for sustainable biodiesel production at an industrial scale. Thermal stability based on TGA shows that the organic amine functionalities introduced during APTES functionalization are robust with respect to temperature stability which exceeds well beyond operational temperature limits. This suggests that the functional groups detected previously with FTIR are thermally stable, in addition to being chemically tethered. SEM images support the uniform spherical morphology of silica particles that were confirmed to cluster in optical microscopy. The physical description of the silica is representative of a sturdy and reliable framework, important for uniform and reliable catalytic activity throughout the reaction mixture. Therefore, it is selected for biodiesel production. In the subsequent steps, after choosing this catalyst, we identified two key parameters and optimized them to achieve the maximum biodiesel yield (While it is true that multiple parameters are involved in biodiesel synthesis, this study focuses on only two of them). In other words, further analysis of process parameters, such as reaction time and catalyst amount, demonstrated a direct influence on biodiesel yield. The optimal conditions for maximum yield will be discussed in detail once all experimental data are finalized. 3.8 Considering different temperatures for Biodiesel production During transesterification reactions used for biodiesel production, temperature becomes a critical parameter that affects both kinetics and reaction equilibria. For example, at the molecular level, temperature affects the motion and energy of reactant molecules, which will increase the frequency and likelihood of successful collisions between the oil and methanol molecules in the presence of a catalyst. If temperature is too low, the reaction rate will be slow due to insufficient thermal energy to overcome the activation barrier in the reaction pathway. However, extremely high temperatures can result in faster unwanted side reactions, degradation of the catalyst, or the loss of methanol vapor with subsequent reduction in effective methanol to oil ratio proportion. The experiment evaluated three temperatures of 55°C, 65°C, and 75°C utilizing the optimal APTES-functionalized silica catalyst (0.75 g APTES, 0.3 g catalyst) in Table 5 . As it is shown in Fig. 11 , the biodiesel yield was 62% at 55°C, while the yield peaked at 62.2% over the first 16 hours at 65°C. Finally, an increase to 75°C caused the yield to drop to 58%. Overall, this evaluation concludes that the optimal temperature was around 65°C under these conditions. This optimal temperature is a good compromise as it enhances kinetics without losing excess methanol to vaporization or thermal degradation of the catalyst surface functional groups (especially the amine sites derived from APTES, which provide the activity). The observed reaction behavior was consistent with Le Chatelier's principle and Arrhenius kinetics. Reactions are limited by activation energy barriers at lower temperatures. Higher temperatures, beyond the original temperature mentioned, increase the energy of a molecule to react and the speed of the reaction because it increases the rate and efficiency of conversion. However, methanol has a very low boiling point compared to temperatures with the potential of effect above the optimum (65°C in this case) which cause methanol to evaporate more quickly than it can be consumed. Furthermore, high temperatures range above approximate optimal levels can deactivate the catalyst (e.g., amine group degradation or pore collapse), and they could induce the potential for soap formation via the saponification of triglycerides, especially in the presence of free fatty acids. Running the process at 65°C is not only energy-efficient, but is also environmentally sustainable, in accordance with the principles of green chemistry. Compared to high temperature systems, this condition leads to minimal energy use when producing biodiesel. Also, operating at a lower temperature means less thermal stress is placed on the catalyst, making it more reusable. Mild operating temperatures will also simplify equipment needs and increase safety in industrial applications. Thus, 65°C is an ideal operating temperature that provides a balance of yield, catalyst stability, and process economics and is highly applicable to a sustainable biodiesel production scale-up process. Table 5 Experiments for biodiesel production with the best catalyst (Silica-APTES 0.75) in different temperatures Experiment Methanol/Oil Catalyst (gr) Temperature °C Reaction Time (h) Yield % 1 12:1 0.2 55 5 62 2 12:1 0.2 65 5 62.2 3 12:1 0.2 75 5 58 3.9 Considering different amounts of catalyst for Biodiesel production The quantity of catalyst employed in a reaction result in the number of active sites available to undergo a chemical transformation. In heterogeneous catalysis, more catalyst is typically more surface area and thus more active sites for the adsorption of methanol and triglycerides and for reaction. However, this is true only up to a point. The excess catalyst leads to mass transfer limitations, increased viscosity, and mixing problems. For this reason, determining the injection of a catalyst in order to reach the threshold of catalyst use is integral to the efficiency of catalytic reactions without wasting resources. We tested three catalyst loadings as can be seen in Table 6 , with the best APTES-functionalized silica (0.75 g APTES) − 0.2 g, 0.3 g, and 0.4 g. Figure 12 shows that the yield increased from 62.3% with 0.2 g catalyst to 63% with 0.3 g catalyst. When the amount of catalyst increased further to 0.4 g, we observed yield slightly drop to 61.7%. Thus, from these results, we conclude that with the parameters tested, 0.3 g is optimal for catalyst loading. 0.2 g was too low because there were not enough active sites for the transesterification reactions, and 0.4 g had unknown negative effects which could have included limitations associated with mass transfer or excess glycerol that inhibited the reaction and may provide yield loss. Low amounts of catalyst (e.g., 0.2 g) provides too few available amine-functionalized sites on the silica to take up and activate the reactants (methanol and oil), which reduces available reactivity and lowers reaction rates and therefore yields. Increasing the amount of catalyst to 0.3 g provides the silica with more available sites, allowing for better contact and reactivity of the reactants or thickening the mixture, this is where the real problem starts to occur. Loadings greater than 0.4 g will allow for additional reaction mixture thickening that increases viscosity and possibly the ability for continued suspension to agglomerate the silica catalyst. The increased viscosity at some point restricts molecular motion reducing access for reactants to or moving through the catalytic sites on the silica. The second dynamic of high silica (catalyst) loading is that it will create a solid mass of glycerol making separation very complex, separating the products becomes a novelty, washing and finding ways to get excess - and in this case unwanted - glycerol layers off the reacted mixture, may even corrupt the reaction's equilibria. The optimal catalyst loading (0.3 g) is critical for both economic and environmental sustainability. An excessive amount of catalyst not only costs more, but raises issues related to the ability to process such volumes at larger scales due to increasing viscosity and difficulty in separation. On the other hand, adding a smaller load of catalyst than 0.3 g would compromise yield. Therefore, in recognizing the aforementioned factors, this study emphasizes the careful consideration of sufficient catalyst to optimize performance, while being mindful of capabilities in processing, energy effectiveness, and manageable catalyst reusability. The loading of 0.3 g provides a feasible and sustainable parameter that can be scaled for bio sustainable biodiesel production. Table 6 Experiments for biodiesel production with the best catalyst (Silica-APTES 0.75) in different amounts of catalyst Experiment Methanol/Oil Catalyst (gr) Temperature °C Reaction Time (h) Yield % 1 12:1 0.2 65 5 62.3 2 12:1 0.3 65 5 63 3 12:1 0.4 65 5 61.7 3.10 GC Results Considering the optimal values of catalyst amount (0.3 gr) and reaction temperature (65°C) (which are the key parameters in biodiesel production) and based on the highest yield obtained from the second experiment (Table 6 ), the biodiesel product from this experiment was analyzed by gas chromatography Gas Chromatography (GC) is an important analytical method for identifying and determining the amount of Fatty Acid Methyl Esters (FAMEs) in biodiesel. FAMEs in biodiesel directly affect the physical and chemical properties of biodiesel such as viscosity, cetane number, oxidative stability, and cold flow characteristics. By determining the FAME profile, it is possible for researchers to establish the fuel quality, capability with diesel engines, and environmental impacts of the biodiesel. In this study, GC analysis was accomplished following the optimization of transesterification conditions (65°C, 0.3 g of catalyst, and 5 h) to identify the FAME unique components of the biodiesel produced from sunflower oil using the optimized catalyst, APTES functionalized silica. The GC result in Fig. 13 shows that the major components of the biodiesel produced were linoleic acid methyl ester (C18:2) at 61.44% and oleic acid methyl ester (C18:1) at 23.82%. The two unsaturated fatty acid esters comprise over 85% of the total FAME content. Palmitic acid methyl ester (C16:0) and stearic acid methyl ester (C18:0), saturated esters, were present in lower amounts (6.38% and 3.31%, respectively). This composition of FAME is expected for a biodiesel product from sunflower oil, as sunflower oil is naturally high in polyunsaturated fatty acids. Furthermore, the high percentage of C18:2 confirms that the biodiesel should have excellent cold flow properties meaning it can stay liquid and operable at lower temperatures which is critical in cold weather. The higher content of unsaturated FAMEs (especially C18:2 and C18:1) improves the cold weather operability of biodiesel, which enhances its performance flexibility for all-season use. However, there is a trade-off since unsaturated compounds are more susceptible to oxidative degradation, which can lead to reduced shelf life, and potentially gum formation in the fuel system itself. Therefore, the biodiesel may display an appealing pour point and fluidity, but more moderate or poor oxidative stability. There are also antioxidants (natural and synthetic) which may be added to the biodiesel to improve long-term stability. In terms of other properties, the relatively low content of saturated compounds (palmitic and stearic esters) means the fuel will continue to be liquid at lower temperatures, yet will have a slightly lower cetane number than biodiesel from more saturated feedstocks (i.e., palm oil). GC analysis showed about 0.66% of unidentified components, and a further 4.40% from the internal standard used in the chromatographic method. Thus, there are less than 1% of unidentified peaks, indicating a high degree of purity for the final product biodiesel, with very few contaminants remaining, or byproducts associated with incomplete reactions such as monoglycerides, diglycerides, or triglycerides. The purity profile confirms the successful conversion of the vegetable oil to methyl esters under optimum conditions. The FAME composition confirms that the biodiesel produced meets necessary fuel specifications, and performance characteristics indicating possible use in a standard diesel engine requiring no modifications. The GC results confirm both the success of the catalyst and its commercial potential in biodiesel. All in all, the GC results support the overarching goals of the study; the creation of an inexpensive and green catalyst which operates at mild conditions with produced high-quality biodiesel. The GC analysis helps confirm that the APTES-functionalized silica catalyst not only facilitates good conversion rates but also produces biodiesel with chemical properties consistent with industry demands by confirming an appropriate FAME profile. The high percentage of linoleic and oleic esters reflect the natural fatty acid profile of the feedstock, and the good separation shown in the GC trace demonstrated the efficiency and selectivity of the process as a whole. The analysis serves as the last step in product validation, confirming that this biodiesel was able to be produced sustainably, sustainably, reproducibly, and from readily available common vegetable oils such as sunflower oil. 3.11 Catalyst reusability Results From Fig. 14 and Table 7 it can be seen that from cycle 1 to cycle 3 the yield remains relatively high (above 60%), indicating that the catalyst retains most of its activity during the initial cycles. This suggests that the APTES-functionalized silica catalyst is highly reusable and stable for at least three cycles without significant loss of performance. In cycle 4 the yield begins to drop more noticeably (below 60%), suggesting that the catalyst is gradually losing its effectiveness. This could be due to the accumulation of impurities, partial blockage of active sites, or minor degradation of the catalyst structure. Stability is an important consideration for sustainable and economic biodiesel production, and the APTES functionalized silica catalyst displayed significant stability. One of the main challenges of heterogeneous catalysis is the retention of catalytic activity through multiple cycles of reaction, especially when the reactant is produced by a reaction with an organic modifier such as an amine. Thermogravimetric analysis (TGA) data from this study indicated that the organic groups from APTES showed thermal stability up to 500–600°C, which was well above the transesterification reaction usually performed at 60–200°C. This indicates that under normal reaction conditions for biodiesel production, the catalyst was not likely to suffer total degradation or functional group loss, preserving its bifunctionality and structure. The reusability of the catalyst was tested through four successive transesterification cycles using the best formulation of catalyst in this study (0.75 g APTES, 0.3 g catalyst for each run, 65°C). As seen in Table 7 and Fig. 14 , the yield of biodiesel was above 60% in the first three cycles, with a slight decrease in the final cycle. In the first three reactions, the biodiesel yields did not change much, indicating that the surface-bound amine groups that mediate the interaction between methanol and the oil remained viable and accessible across several uses. The negligible decrease in yield indicates that silica provided a stable structural platform as support, which is likely resistant to physical degradation, pore-blocking or significant leaching of active species. The joint chemical and mechanical durability described above results in both economic and environmental benefits. In industry, ongoing catalyst replacement significantly increases the costs associated with maintaining operations along with raised waste generation, demonstrated reusability with the APTES-functionalized silica catalyst promotes an overall reduction in the combined costs of catalyst waste with an efficient catalyst, which aligns to the principles of green chemistry. Furthermore, while the variability is expected in the consistency of biodiesel yields over time illustrates long-term reliability of the catalyst, in addition to ensuring each reaction cycle maintains the functional contributions of both polar and non-polar reaction conditions. In addition, the APTES-functionalized silica system is a viable option for industry biodiesel production, a scenario which requires performance, longevity, and sustainable products. Although the maximum biodiesel yield obtained in this study was approximately 62.2%, which is relatively lower compared to some reported values in the literature, several factors justify and explain this outcome. First, the experimental conditions were intentionally kept mild (i.e., atmospheric pressure, moderate temperature of 65°C, and no co-catalyst), in alignment with green chemistry principles to minimize energy input and avoid hazardous reagents. Furthermore, the absence of a co-catalyst and the use of a solid heterogeneous catalyst inherently limit the reaction kinetics compared to homogeneous systems. Another major consideration is the diffusion limitation imposed by the solid–liquid interface in heterogeneous catalysis. Despite the high surface area of the APTES-functionalized silica, mass transfer resistance and the absence of stirring optimization could have restricted access to active sites. Also, the selected feedstock (commercial sunflower oil) may contain impurities or free fatty acids that can hinder transesterification efficiency. Lastly, the design of this study aimed more toward demonstrating the reusability, structural stability, and simple synthesis of a cost-effective catalyst rather than maximizing yield through aggressive reaction conditions or additives. Given these constraints, the achieved yield is reasonable and reproducible, and sets the foundation for further optimization or potential scale-up using assisted methods (e.g., microwave, ultrasound, or intensified mixing). These results therefore reflect the realistic performance of a green, reusable, and scalable catalytic system under mild operating conditions characteristics that are essential for sustainable biodiesel production at industrial levels[ 37 ]. Table 7 Catalyst reusability in 4 cycles Experiment Methanol/Oil Catalyst (gr) Temperature °C Reaction Time (h) Yield % 1 12:1 0.3 65 5 62.9 2 12:1 0.3 65 5 61.5 3 12:1 0.3 65 5 60.8 4 12:1 0.3 65 5 59.5 4. Conclusion Research has been completed to design and apply APTES-functionalized silica-based heterogeneous catalysts for biodiesel production ability. The optimal catalyst was prepared with 0.75 g APTES, with a biodiesel yield of over 62% under mild reaction conditions (65°C, 0.3 g catalyst, 5 h), rendering this the most active catalyst. Characterization with SEM, BET, XRD, and FTIR showed that the catalyst was structurally intact with surface sites, whereas GC analysis showed an adequate biodiesel product with a good FAME profile. This catalyst was excellent for reuse, retaining more than 90% of its activity for three consecutive cycles, indicating its durability and relevance to practical implementation. Apart from traditional catalysts that readily experience leaching or have low reusability or are otherwise environmentally unfriendly, this APTES-functionalized silica offers a good choice that is both environmentally benign and economical. This catalyst is easily synthesized and works fine with vegetable oils, hence being fitted into green chemistry and industrial implications. In addition, the correlation between the functional group density and the catalytic activity provides guidance to rational design for other catalytic candidates. In essence, this work could take a giant step in producing environment-friendly and reusable catalysts for producing biodiesel that will be rightfully aligned to environmental concerns and in need of clean energy. Declarations Data availability The datasets used and/or analyzed during the current study available from the corresponding author on reasonable request. ACKNOWLEDGMENT Support for this investigation by the Tarbiat Modares University is gratefully acknowledged. Author contributions Tara Ghaffarinejad: Sample preparation and experiments, manuscript writing, exper­imental data analyses. Ramin Karimzadeh: Methods, figure analyzes, exper­imental data analyses. Funding Declaration This study was not supported by any sponsor or funder. References Meher, L. C., Sagar, D. V. & Naik, S. N. Technical aspects of biodiesel production by transesterification - A review. Renew. Sustain. Energy Rev. 10 (3), 248–268. 10.1016/j.rser.2004.09.002 (2006). Asaad, S. M. Optimization of Biodiesel Production from Waste Cooking Oil Using a Green Catalyst Prepared from Glass Waste and Animal Bones, Energies , [Online]. (2023). Available: https://doi.org/10.3390/en16052322 Martino, E. S., Di Serio, R., Tesser, L. P., Dipartimento & and Biochar-derived heterogeneous catalysts for biodiesel production. Environ. Chem. Lett. 17 (4), 1447–1469 (2019). 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Advances in Bifunctional Solid Catalysts for Biodiesel Production ( no. Biodiesel Production, 2022). Sikiru Yusuff, A., Adeniyi, O. D., Aderemi, M., Olutoye & Akpan, U. G. Waste Frying Oil as a Feedstock for Biodiesel Production. Pet. Chem. - Recent. Insight . 10.5772/intechopen.79433 (2019). Majedi, M. Molybdenum (VI) complex of resorcinol-based ligand immobilized on silica‐coated magnetic nanoparticles for biodiesel production. Appl Organomet. Chem , (2023). Atadashi, I. M., Aroua, M. K. & Aziz, A. A. High quality biodiesel and its diesel engine application: A review. Renew. Sustain. Energy Rev. 14 (7), 1999–2008. 10.1016/j.rser.2010.03.020 (2010). Endalew, A. K., Kiros, Y. & Zanzi, R. Inorganic heterogeneous catalysts for biodiesel production from vegetable oils. Biomass Bioenerg. 35 (9), 3787–3809. 10.1016/j.biombioe.2011.06.011 (2011). Vyas, A. P., Verma, J. L. & Subrahmanyam, N. A review on FAME production processes. Fuel 89 (1), 1–9. 10.1016/j.fuel.2009.08.014 (2010). Meher, L. C., Kulkarni, M. G., Dalai, A. K. & Naik, S. N. Transesterification of karanja (Pongamia pinnata) oil by solid basic catalysts. Eur. J. Lipid Sci. Technol. 108 (5), 389–397. 10.1002/ejlt.200500307 (2006). Singh, A. K. & Fernando, S. D. Reaction kinetics of soybean oil transesterification using heterogeneous metal oxide catalysts. Chem. Eng. Technol. 30 (12), 1716–1720. 10.1002/ceat.200700274 (2007). Suppes, G. J., Dasari, M. A., Doskocil, E. J., Mankidy, P. J. & Goff, M. J. Transesterification of soybean oil with zeolite and metal catalysts. Appl. Catal. Gen. 257 (2), 213–223. 10.1016/j.apcata.2003.07.010 (2004). Boz, N., Degirmenbasi, N. & Kalyon, D. M. Conversion of biomass to fuel: Transesterification of vegetable oil to biodiesel using KF loaded nano-γ-Al2O3 as catalyst. Appl. Catal. B Environ. 89 , 3–4. 10.1016/j.apcatb.2009.01.026 (2009). Leung, D. Y. C., Wu, X. & Leung, M. K. H. A review on biodiesel production using catalyzed transesterification. Appl. Energy . 87 (4), 1083–1095. 10.1016/j.apenergy.2009.10.006 (2010). Melero, J. A., Iglesias, J. & Morales, G. Heterogeneous acid catalysts for biodiesel production: Current status and future challenges. Green. Chem. 11 (9), 1285–1308. 10.1039/b902086a (2009). Han, Y. Synthesis of Mesoporous Silica Using the Sol–Gel Approach: Adjusting Architecture and Composition for Novel Applications, Nanomaterials , [Online]. Available: Nanomaterials (2024). Fatimah, I. Mesoporous Silica-Based Catalysts for Biodiesel Production: A Review, ChemEngineering , [Online]. (2023). Available: https://doi.org/10.3390/chemengineering7030056 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Under Review Version 1 posted Editorial decision: Revision requested 30 Jul, 2025 Reviews received at journal 23 Jul, 2025 Reviewers agreed at journal 16 Jul, 2025 Reviewers agreed at journal 16 Jul, 2025 Reviewers agreed at journal 14 Jul, 2025 Reviewers agreed at journal 14 Jul, 2025 Reviews received at journal 14 Jul, 2025 Reviewers agreed at journal 13 Jul, 2025 Reviewers agreed at journal 13 Jul, 2025 Reviewers agreed at journal 13 Jul, 2025 Reviewers agreed at journal 12 Jul, 2025 Reviewers agreed at journal 11 Jul, 2025 Reviewers agreed at journal 11 Jul, 2025 Reviewers agreed at journal 11 Jul, 2025 Reviewers agreed at journal 11 Jul, 2025 Reviewers invited by journal 11 Jul, 2025 Editor assigned by journal 11 Jul, 2025 Editor invited by journal 11 Jul, 2025 Submission checks completed at journal 24 Jun, 2025 First submitted to journal 24 Jun, 2025 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-6903073\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":false,\"archivedVersions\":[],\"articleType\":\"Article\",\"associatedPublications\":[],\"authors\":[{\"id\":485306452,\"identity\":\"fbe69ee6-3abd-4ce4-afef-aeca97a14e8b\",\"order_by\":0,\"name\":\"Tara Ghaffarinejad\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Tarbiat Modares University (TMU)\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Tara\",\"middleName\":\"\",\"lastName\":\"Ghaffarinejad\",\"suffix\":\"\"},{\"id\":485306457,\"identity\":\"d15ada7e-022b-499e-9f40-e3f661c83d60\",\"order_by\":1,\"name\":\"Ramin 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11\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":20661,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eExperiments for biodiesel production with the best catalyst (Silica-APTES 0.75) in different temperatures\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"11.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6903073/v1/38d6ddec25266d668ab84ad6.png\"},{\"id\":86777663,\"identity\":\"b166bf1c-d645-441c-9835-c3b25c4d6426\",\"added_by\":\"auto\",\"created_at\":\"2025-07-15 12:51:27\",\"extension\":\"png\",\"order_by\":12,\"title\":\"Figure 12\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":23201,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eExperiments for biodiesel production with the best catalyst (Silica-APTES 0.75) in different amounts of catalyst\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"12.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6903073/v1/c2cd44201dba8b9459d3a3e7.png\"},{\"id\":86777679,\"identity\":\"1b3b1ae2-e339-448a-9d73-af132ca3ad77\",\"added_by\":\"auto\",\"created_at\":\"2025-07-15 12:51:28\",\"extension\":\"png\",\"order_by\":13,\"title\":\"Figure 13\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":71799,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eGas chromatography (GC) analysis of the produced biodiesel\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"13.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6903073/v1/1a872bcb5c548bac39565bfc.png\"},{\"id\":86777661,\"identity\":\"94687784-e3dd-45b2-ad8f-dd493d56d721\",\"added_by\":\"auto\",\"created_at\":\"2025-07-15 12:51:27\",\"extension\":\"png\",\"order_by\":14,\"title\":\"Figure 14\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":15892,\"visible\":true,\"origin\":\"\",\"legend\":\"\\u003cp\\u003eCatalyst reusability in 4 cycles\\u003c/p\\u003e\",\"description\":\"\",\"filename\":\"14.png\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6903073/v1/1808c85774c88df1b7329bab.png\"},{\"id\":86780786,\"identity\":\"bf6ca5b5-fbcb-455e-9e88-fb3494fe538c\",\"added_by\":\"auto\",\"created_at\":\"2025-07-15 13:23:27\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":2142609,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-6903073/v1/a3358430-6275-480a-b43e-455deaf02ab9.pdf\"}],\"financialInterests\":\"No competing interests reported.\",\"formattedTitle\":\"Bifunctional Ethoxy-Amine Modified Silica Catalysts for Green Energy: Facilitating High Biodiesel Yield via Enhanced Methanol and Oil Adsorption\",\"fulltext\":[{\"header\":\"1. Introduction\",\"content\":\"\\u003cp\\u003eBecause of the growing worldwide need for alternative and sustainable energy, biodiesel has been extensively\\u0026ensp;studied and considered a renewable and environmentally friendly fuel. They are also advantageous in terms of high biodegradability, lower emissions of greenhouse gases, and are compatible with existing diesel\\u0026ensp;engines with minimal engine modifications. Because of these features, it holds great potential as a clean substitute\\u0026ensp;to fossil fuels in tackling environmental and energy security challenges[\\u003cspan citationid=\\\"CR1\\\" class=\\\"CitationRef\\\"\\u003e1\\u003c/span\\u003e],[\\u003cspan citationid=\\\"CR2\\\" class=\\\"CitationRef\\\"\\u003e2\\u003c/span\\u003e]. Biodiesel\\u0026ensp;is conventionally produced by the transesterification of vegetable oil or animal fat with methanol in the presence of a catalyst. Homogeneous catalysts like\\u0026ensp;sodium hydroxide (NaOH) have high efficiency, so they are extensively used, but they bring challenges for separation, recyclability, and environmental issues. Heterogeneous catalysts have received enormous interest to circumvent\\u0026ensp;these restrictions[\\u003cspan citationid=\\\"CR3\\\" class=\\\"CitationRef\\\"\\u003e3\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eHeterogeneous catalysts have gained extensive attention for biodiesel production due to their advantages as efficient and\\u0026ensp;sustainable catalysts. Unlike homogeneous catalysts, heterogeneous catalysts are easy to separate for reuse\\u0026ensp;and are often considered to be less dangerous to the environment. Numerous studies via different modifications and functionalization have been undertaken to improve these catalytic properties\\u0026ensp;such as catalytic activity, stability and selectivity[\\u003cspan citationid=\\\"CR4\\\" class=\\\"CitationRef\\\"\\u003e4\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eMetal oxides like CaO, MgO, and ZnO have been extensively investigated for their high\\u0026ensp;basicity and stability. However, these catalysts are often\\u0026ensp;plagued by leaching and deactivation problems. Recent works on these metal oxides have focused on doping with other metals or implementation of mixed metal\\u0026ensp;oxides that can improve the catalytic activity and stability[\\u003cspan citationid=\\\"CR5\\\" class=\\\"CitationRef\\\"\\u003e5\\u003c/span\\u003e]. Zeolites\\u0026ensp;have been investigated due to their high surface area and tunable acidity. Doped zeolites,\\u0026ensp;particularly alkali metal doped zeolites, have been reported for their improved catalytic activity for transesterification reactions. Nonetheless, the microporous\\u0026ensp;characteristics of zeolites can restrain the diffusion of large triglyceride molecules, lowering their effectiveness[\\u003cspan citationid=\\\"CR6\\\" class=\\\"CitationRef\\\"\\u003e6\\u003c/span\\u003e]. As a result, these\\u0026ensp;catalysts have demonstrated good performance due to their high surface area and stability. Nonetheless, synthesis of these catalysts often poses challenges, with some of them being complex and\\u0026ensp;expensive[\\u003cspan citationid=\\\"CR7\\\" class=\\\"CitationRef\\\"\\u003e7\\u003c/span\\u003e]. There has been an interest in silica-based materials as catalysts\\u0026ensp;owing to the high thermal stability, high surface area, and ease of functionalization of silica. Recent works have dealt with the functionalization of silica with different organic groups for\\u0026ensp;increasing its catalytic properties. Silica functionalization ,for\\u0026ensp;instance, the modification of silica with amino groups resulted in increased catalytic activity in transesterification[\\u003cspan citationid=\\\"CR8\\\" class=\\\"CitationRef\\\"\\u003e8\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eAlthough substantial progress has been achieved in the field of heterogenized catalysts, there exist a\\u0026ensp;number of limitations. Leaching, deactivation, and poor reusability plague\\u0026ensp;many catalysts. Moreover, the synthesis of certain catalysts is complex and expensive, leading\\u0026ensp;to practical limitations[\\u003cspan citationid=\\\"CR9\\\" class=\\\"CitationRef\\\"\\u003e9\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eTo overcome these limitations, a simple and economical synthesis of an\\u0026ensp;APTES-functionalized silica catalyst is developed in this study. Functioning silica was carried out with (3-aminopropyl) tri ethoxy silane (APTES) to obtain basic amino\\u0026ensp;groups to improve the catalytic activity in transesterification. Additionally, the surface area and thermal\\u0026ensp;stability of silica also helps the catalyst and the heterogeneous catalyst is easily separable and reusable, which solves the reusability and environmental problems of\\u0026ensp;the homogeneous catalyst.\\u003c/p\\u003e\\u003cp\\u003eOne of the most remarkable characteristics of (3-aminopropyl) tri ethoxy silane (APTES) is its bifunctionality, which ultimately contributes to its high efficiency in catalyzing the transesterification reaction associated with biodiesel production[\\u003cspan citationid=\\\"CR10\\\" class=\\\"CitationRef\\\"\\u003e10\\u003c/span\\u003e]. APTES is comprised of amine (-NH₂) groups and ethoxy (-OCH₂CH₃) groups. The amine groups can act as hydrogen bond donors or electrostatic interactions due to their highly polar character, attracting polar species such as methanol. This facilitates the adsorption of methanol onto the catalyst surface (thus placing it in close proximity to the sites of the reaction)[\\u003cspan citationid=\\\"CR11\\\" class=\\\"CitationRef\\\"\\u003e11\\u003c/span\\u003e]. The ethoxy groups can act hydrophobically and interact with the long hydrocarbon tails (nonpolar) of vegetable oils. This two-fold interaction mechanism plays an important role in facilitating an equilibrium and overcoming the immiscibility of oil and methanol (one of the roadblocks to biodiesel synthesis). By acting in this dual role function, APTES serves as an interface node for the catalyst's surface to simultaneously adsorb methanol and oil molecules onto its surface. This is beneficial in providing local concentration through the intermediate of a dual phase, which allows greater odds of having both adsorbed reactants in close proximity. In systems without this functionality, the polar and non-polar phases have limited interaction and often require vigorous stirring or co-solvents to facilitate reactions[\\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e]. APTES-functionalized silica serves as a chemical bridge, lowering the energy barrier for the separation of the two phases, allowing greater rate kinetics for biodiesel production under milder reaction conditions. In addition, the new bifunctional nature of the catalyst is complementing the catalyst versatility to operate on different feedstocks (including low-grade or waste oils which may contain variances including impurities) with different chemical surroundings including polarity, which is advantageous for green chemistry and for industrial application involving the widest possible feedstock source due to economic and sustainability reasons having the choice of feedstock regardless of condition also applies to sustainability[\\u003cspan citationid=\\\"CR12\\\" class=\\\"CitationRef\\\"\\u003e12\\u003c/span\\u003e]. This explains why APTES has the capability to adsorb and activate both polar and non-polar reactants in the reaction at the same time, which represents a significant advantage in its ability to function as a highly active and reusable heterogeneous catalyst. This molecular-level synergism not only helps rationalize the observed catalytic performance, but also helps explain the robustness and versatility of the APTES-functionalized silica system which operates effectively over a diversity of transesterification reactions[\\u003cspan citationid=\\\"CR13\\\" class=\\\"CitationRef\\\"\\u003e13\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eCompared with the current heterogenic catalysts used in biodiesel production like CaO, MgO, and zeolites, the APTES-functionalized silica catalyst offers\\u0026ensp;several advantages. Although these traditional catalysts show high catalytic activity, as above mentioned, they are usually plagued with such drawbacks including leaching, deactivation, and the fact that most synthesis procedures\\u0026ensp;are quite complex[\\u003cspan citationid=\\\"CR14\\\" class=\\\"CitationRef\\\"\\u003e14\\u003c/span\\u003e]. The silica catalyst with APTES functionalization exhibited high stability over many reaction cycles with only small loss in activity, whereas others\\u0026ensp;such as MgO and zeolites deactivate relatively quickly owing to leaching or pore blockage[\\u003cspan citationid=\\\"CR15\\\" class=\\\"CitationRef\\\"\\u003e15\\u003c/span\\u003e]. APTES functionalization on silica also increases the density of reactive sites and thus increases chemical reactivity for\\u0026ensp;transesterification reactions. The practical applicability of the catalyst is further endorsed by\\u0026ensp;its flexibility in feedstocks, such as waste oils. All of these unique characteristics established the innovative nature of\\u0026ensp;APTES-functionalized silica catalyst and its promise potential to overcome the deficiencies of existing catalysts contributing towards more sustainable and efficient biodiesel production[\\u003cspan citationid=\\\"CR16\\\" class=\\\"CitationRef\\\"\\u003e16\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eAPTES-functionalized silica as a heterogeneous catalyst for biodiesel production has a\\u0026ensp;number of environmental advantages. First, using a reusable catalyst will reduce the constant\\u0026ensp;need for new catalyst synthesis and disposal, thus, limiting the waste bottom line. Such practice does coincide with the aim of green chemistry, which advocates for waste prevention and the use of\\u0026ensp;renewable sources[\\u003cspan citationid=\\\"CR17\\\" class=\\\"CitationRef\\\"\\u003e17\\u003c/span\\u003e]. Moreover, the catalyst's exceptional efficacy in transesterification reactions results in\\u0026ensp;greater biodiesel outputs, minimizing the cumulative environmental impact of the production process. Biodiesel production\\u0026ensp;itself is more eco-friendly than fossil fuel-based energy products. The emission of GHGs, particulate matters and Sulphur compounds are lower during the combustion of biodiesel, hence\\u0026ensp;it leads to better air quality, lowering the potential for global warming[\\u003cspan citationid=\\\"CR18\\\" class=\\\"CitationRef\\\"\\u003e18\\u003c/span\\u003e]. Another advantage\\u0026ensp;of these types of catalysts is that they require no neutralization and separation steps to keep them from contaminating the reaction products, producing much less wastewater and other by-products compared to their homogeneous counterparts[\\u003cspan citationid=\\\"CR19\\\" class=\\\"CitationRef\\\"\\u003e19\\u003c/span\\u003e],[\\u003cspan citationid=\\\"CR20\\\" class=\\\"CitationRef\\\"\\u003e20\\u003c/span\\u003e]. Notably, the APTES-functionalized silica catalyst was stable and reusable which is a\\u0026ensp;major advantage over homogeneous catalysts. The catalyst also exhibited stable catalytic performance over several consecutive reaction runs without observable\\u0026ensp;deactivation. After three cycles,\\u0026ensp;for example, the catalyst maintained more than 90% of its activity, indicating its prospective durability[\\u003cspan citationid=\\\"CR21\\\" class=\\\"CitationRef\\\"\\u003e21\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eOne major benefit performance of biodiesel is the range of feedstocks suitability because the actual silica is functionalized with APTES. The nature of the bifunctional APTES (it has both a polar amine group and non-polar alkyl/ethoxy groups) allows the APTES to have interaction with methanol and many types of oils, including low-cost, impure forms, or waste streams when producing biodiesel[\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e]. Instead of requiring high-quality oils as required with conventional homogeneous or even some heterogeneous catalysts, APTES-functionalized catalysts develop and maintain activity across various feedstock qualities[\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e]. This adaptability is beneficial for advancing biodiesel to an accessible, affordable, sustainable and scalable pathway to displace fossil fuels. With the global emphasis on sustainability, sourcing waste oils and non-edible oils as feedstocks in biodiesel productions has gained significance in reducing reliance on food feedstock and reducing feedstock costs[\\u003cspan citationid=\\\"CR23\\\" class=\\\"CitationRef\\\"\\u003e23\\u003c/span\\u003e]. However, waste oils and non-edible oils generally are problematic due to elevated free fatty acids (FFAs), impurities, or oxidation products[\\u003cspan citationid=\\\"CR22\\\" class=\\\"CitationRef\\\"\\u003e22\\u003c/span\\u003e]. APTES is advantageous in part because it has a high affinity and is capable of interacting with a very chemically diverse range of materials. APTES features polar amine groups for interaction with FFAs and polar degradation products, yet the non-polar components can also retain interaction with triglycerides. APTES's characteristics promote conversations even with varied quality feedstock[\\u003cspan citationid=\\\"CR24\\\" class=\\\"CitationRef\\\"\\u003e24\\u003c/span\\u003e]. From an industrial and environmental viewpoint, this versatility extends the applicability of the catalyst, allowing for decentralized biodiesel production using varieties of oils available locally, including waste cooked oil, palm fatty acid distillates and other residues. It lessens reliance on virgin vegetable oils, and ultimately reduces production costs and environmental impact[\\u003cspan citationid=\\\"CR25\\\" class=\\\"CitationRef\\\"\\u003e25\\u003c/span\\u003e]. In effect, wis APTES-functionalized silica can enable or enhance chemical efficiency, but also facilitates the circular economy by converting waste into a fuel source of value. This feedstock flexibility further validates APTES functionalized silica as a next-generation catalyst in renewable energy applications[\\u003cspan citationid=\\\"CR26\\\" class=\\\"CitationRef\\\"\\u003e26\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eMoreover, this reusability also minimizes the frequent replacement of catalysts and the overall environmental impact of the biodiesel production process[\\u003cspan citationid=\\\"CR27\\\" class=\\\"CitationRef\\\"\\u003e27\\u003c/span\\u003e]. Economically speaking, the APTES\\u0026ndash;functionalized silica catalyst is an inexpensive catalyst\\u0026ensp;for biodiesel production. The catalyst exhibits an easy and inexpensive synthesis by\\u0026ensp;using commercially available materials (e.g., silica and APTES)[\\u003cspan citationid=\\\"CR28\\\" class=\\\"CitationRef\\\"\\u003e28\\u003c/span\\u003e]. The catalyst can be gently recovered and reused several times without a\\u0026ensp;significant loss in activity, greatly reducing costs of operation as time goes on[\\u003cspan citationid=\\\"CR29\\\" class=\\\"CitationRef\\\"\\u003e29\\u003c/span\\u003e]. In addition, due to favorable biodiesel conversion rates, silica-supported APTES catalysts could generate more biodiesel and thus boost the overall profitability relevant to the biodiesel production\\u0026ensp;process. This is complemented with the ability to use low-cost\\u0026ensp;feedstocks, including waste oils, further improving the economic viability of this catalyst[\\u003cspan citationid=\\\"CR30\\\" class=\\\"CitationRef\\\"\\u003e30\\u003c/span\\u003e]. Additionally, the reduction of waste generation and elimination of complex separation processes lead to lower production costs\\u0026ensp;explaining its industrial scalability potential[\\u003cspan citationid=\\\"CR31\\\" class=\\\"CitationRef\\\"\\u003e31\\u003c/span\\u003e]. Conventional homogeneous catalysts such as NaOH and KOH\\u0026ensp;are polluting the environment by producing waste water, soap, and other by-products. These catalysts demand neutralization steps, yielding salts that need to be\\u0026ensp;discarded, putting greater pressure on the environment. Unlike current silica catalysts, which are not easily separable and reusable, an APTES-functionalized silica catalyst avoids these problems and greatly minimizes the generation of\\u0026ensp;waste and contamination of water resources[\\u003cspan citationid=\\\"CR32\\\" class=\\\"CitationRef\\\"\\u003e32\\u003c/span\\u003e]. The APTES-functionalized silica catalyst works at low temperatures and atmospheric pressure,\\u0026ensp;lower than the conditions needed for many common catalysts. This helps to save the energy used in the biodiesel production process,\\u0026ensp;as this process would thus be more energy and cost efficient[\\u003cspan citationid=\\\"CR33\\\" class=\\\"CitationRef\\\"\\u003e33\\u003c/span\\u003e]. They can also spew out lighter intensity, which allows the process to be more\\u0026ensp;sustainable as well as greener[\\u003cspan citationid=\\\"CR34\\\" class=\\\"CitationRef\\\"\\u003e34\\u003c/span\\u003e]. This catalyst was synthesized in a scalable\\u0026ensp;manner without the need for intricate or toxic procedures. The catalyst is amenable to scale-up\\u0026ensp;with ubiquitous solvents such as ethanol and hexane and trivial functionalization steps. This\\u0026ensp;scalability is essential for industrial applications, paying attention to cost-effectiveness and simplicity[\\u003cspan citationid=\\\"CR35\\\" class=\\\"CitationRef\\\"\\u003e35\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eWhat sets this research apart in silica surface amine functionalization is the strategic modification of its chemistry using a very specific amount of APTES to form a solid base catalyst which is both highly active and can be reused multiple times. Unlike other works that have simply documented surface enhancement of silicas or general amine-functionalization, this work has dealt with optimization of APTES \\u0026lsquo;s loading (0.25, 0.5, 0.75, 1 g) and its catalytic activity and biodiesel production in a systematic, quantitative manner, thereby establishing a direct connection to the yield of constructed catalysts. Furthermore, this approach does not resort to elaborate synthetic strategies but rather makes use of simple commercial products and minimal energy expenditure to achieve scalable design.\\u003c/p\\u003e\\u003cp\\u003eTherefore, the aim of this study is to synthesize and evaluate APTES-modified silica as a heterogeneous catalyst for biodiesel production, focusing on optimizing APTES loading, analyzing surface functionality, and correlating catalyst structure to performance and reusability\\u003c/p\\u003e\"},{\"header\":\"2. Materials and Methods\",\"content\":\"\\u003cdiv id=\\\"Sec3\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e2.1 All materials used\\u003c/h2\\u003e\\u003cp\\u003eTetraethyl orthosilicate (TEOS, 98%, Coupsyl) was used with further purification (TEOS was pre-treated in an oven at 90\\u0026deg;C for 1 hour to evaporate residual ethanol and water prior to use), triethylamine (TEA, 99%, Neutron) was used as condensation catalyst, APTES (98%, Sigma), Hexane (96%, Neutron), HCL (37%, Mojallali), vegetable oil (Ladan Brand) and methanol (99.8%, Mojallali).\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec4\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e2.2 Synthesis of Silica\\u003c/h2\\u003e\\u003cp\\u003eSilica particles were synthesized via a sol-gel process based on a modified St\\u0026ouml;ber method, using TEOS as precursor and TEA as catalyst.\\u003c/p\\u003e\\u003cp\\u003eFirst, 10 cc of TEOS is added to 100 cc of distilled water and stirred under continuous stirring at 500 rpm. Then, 0.3 grams of TEA catalyst is added to the mixture. After 5 hours of reaction, the resulting precipitate is washed thoroughly and placed in an oven at 100\\u0026deg;C for 2 hours. Subsequently, the dried silica is calcined in a furnace at 700\\u0026deg;C for 1 hour.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec5\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e2.3 Synthesis of APTES-Functionalized Silica\\u003c/h2\\u003e\\u003cp\\u003eFirst, 3 g silica (which were synthesis as above mentioned) dispersed in a 0.1 M HCl solution, with the aid\\u0026ensp;of an ultrasonic device. The silica was then washed with\\u0026ensp;water to remove any unreacted acid after sonication. The washed sol-gel SiO\\u003csub\\u003e2\\u003c/sub\\u003e was then separated into four parts and treated with different amounts\\u0026ensp;of the reagents, APTES as the source of the amino group and hexane as the solvent [Four amounts of APTES (0.25 g, 0.5 g,\\u0026ensp;0.75 g and 1 g) were applied to functionalize a gram of silica in a 100 cc hexane treatment in the present study (four different products)]. All the mixtures were sonicated for 15 minutes to ensure the adequate interaction of the silica with\\u0026ensp;APTES. The products were subsequently washed\\u0026ensp;with ethanol to remove unreacted reagents and impurities post-sonication. Finally, they were dried at 60\\u0026deg;C in an oven for 2 hours until completely dried,\\u0026ensp;producing a functionalized product.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec6\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e2.4 Biodiesel Production\\u003c/h2\\u003e\\u003cp\\u003eThe biodiesel was synthesized through the transesterification of vegetable\\u0026ensp;oil with methanol catalyzed by the synthesized APTES-functionalized silica (4 different catalysts which were synthesis as above mentioned). In examining the effects of process parameters on yield, specific ratios of oil to\\u0026ensp;methanol to catalyst were utilized. 2gr Methanol was mixed with the needed amount of vegetable oil (6.6 gr). Next, the functionalized silica catalyst\\u0026ensp;was added in the above mixture (0.2 gr), and the reaction was done at controlled temperature (55\\u0026deg;C) for 5 hours under continuous stirring. The\\u0026ensp;mixture was then allowed to cool to room temperature after the reaction. Centrifugation was carried out for 10 minutes at 3000 rpm to separate out the biodiesel\\u0026ensp;phase from the glycerol and catalyst from the reaction mixture. The biodiesel layer was carefully decanted and washed\\u0026ensp;several times with distilled water to remove residual methanol, catalyst and impurities. Also, after each run, the catalyst was recovered via centrifugation, washed thoroughly with ethanol and distilled water, and dried at 60\\u0026deg;C before reuse.\\u003c/p\\u003e\\u003c/div\\u003e\"},{\"header\":\"3. Results and Discussion\",\"content\":\"\\u003cdiv id=\\\"Sec8\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.1 SEM Results for silica synthesis\\u003c/h2\\u003e\\u003cp\\u003eThe SEM images in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e demonstrate that the synthesized silica mostly has a spherical morphology with smooth and uniform surfaces, with many, often times, aggregated. These spherical features are a reflection of successful implementation of sol-gel synthesis, The spherical morphology allows a high external surface area, which is advantageous for catalysts, by improving the dispersion of active sites and providing a closer pathway of diffusion for reactant molecules. Furthermore, the aggregated clusters may lead to interparticle mesoporous and macropores that also serve to enhance the diffusion of large reactant molecules, such as triglycerides. The uniform dimension and shape of the silica particles is directly related to greater catalytic performance. The consistency of these silica particles allows for the uniform availability of active sites leading to effective interaction between the catalyst and reactants. Additionally, the SEM characterization reveals smooth surfaces that readily allow the free adsorption of both methanol and oil molecules, thus increasing the likelihood of surface reactions to occur. This characterized morphology is conducive to the production of an effective interfacial zone for the transesterification reaction to occur, especially under moderate operational conditions. Another important observation in the SEM images is the structural integrity of the silica particles. They maintained their spherical morphology without any fractures or issues with the surfaces, which demonstrates mechanical strength and ability to resist the physical forces associated with repeated cycles of reaction. Because the silica particles are mechanically robust, it extends to the possibility of using them interchangeably, further minimizing the risk of losing the catalyst through breakdown, pore collapse, or agglomeration during the processing of the reaction solution. The slight irregularities in the silica particles shown in the SEM of the dried samples were largely insignificant, and there were no extremely fine or irregular particles, both of which would increase the risk of losing the catalyst, or compounding operational issues during filtration or separation stage such as blocking[\\u003cspan citationid=\\\"CR36\\\" class=\\\"CitationRef\\\"\\u003e36\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003eIn short, the SEM gave powerful evidence of the morphological quality and appropriateness of the synthesized silica for use in catalyst applications. The oval spheres, smooth surfaces, and clustering of characteristics consistent with a catalyst, indicates that mass transfer will take place effectively, the reactants should stable adsorb on to the reactants, and structural mechanical function for stable time to use. Moreover, this provides the foundation for proving final catalytic performance and consistent functionality as biodiesel production. Optical microscopy in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e shows further confirmation of the spherical nature of the particles, appearing as bright, rounded features due to light scattering. The spherical morphology and narrow size distribution observed suggest a well-controlled synthesis process.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec9\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.2 Nitrogen adsorption-desorption isotherm Results for silica synthesis\\u003c/h2\\u003e\\u003cp\\u003eThe nitrogen adsorption-desorption isotherm for silica (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e) had a Type IV isotherm with an H1 hysteresis loop, a more characteristic feature of mesoporous materials, indicating the existence of mesopores. The presence of a distinct capillary condensation step in relative pressure range 0.4 to 0.8 means that uniform mesopores exist in the silica. These mesopores are advantageous for catalytic applications as they lead to high surface area and limited the diffusion of large organic molecules such as triglycerides that typically are found in vegetable oils. The cylindrical nature of the pores proposed by the hysteresis shape suggests a consistent pore morphology that will allow for effective reactant transport to the surface of catalytic sites.\\u003c/p\\u003e\\u003cp\\u003eThe mesoporous structure highlighted by the isotherm findings is essential to improving the performance of the catalyst. In the case of biodiesel, the reactants, methanol and oil, need to pass through the bulk of the catalyst surface with little resistance to reach the active sites. The given pore structure provided pathways for the diffusion of reactants, and for product removal, therefore improving reaction rate. If the porous structure was useful in reducing diffusion restrictions compared to microporous or non-porous catalysts, then it is even more useful when working with molecules that are large hydrophobic oil molecules that are being processed. Also, the isotherm profile suggests that the silica support has considerable pore volume, which is important because it offers surface accessibility and space for functionalization. The fact that even after synthesis, the pores are open and accessible means that the material can support the addition of catalytic groups without a large pore blockage event. Therefore, the structural openness means that the addition of a surface modification will not compromise mass transfer, and thus will not reduce the efficiency of the catalyst. The ability to maintain this balance between functionalization and porosity is important to achieve high activity and sustained catalytic stability when the material is used for repeated reaction cycles.\\u003c/p\\u003e\\u003cp\\u003eIn summary, the nitrogen adsorption-desorption experiment identifies a mesoporous, high-surface-area structure that is perfect for heterogeneous catalysis. Further, most of the characteristics of the isotherm indicate that structures of this type generally support reactant diffusion and accessibility to active sites, both important for biodiesel yield. The fact that we are predominantly dealing with a mesopore structure also suggests a resilience if we were to accommodate both polar and non-polar reactants, which is key in the bifunctional catalytic approach being utilized. Overall, the catalyst appears to be suitable for biodiesel production, especially when performed under mild and sustainable conditions.\\u003c/p\\u003e\\u003cp\\u003eThe increased adsorption ability of APTES-functionalized silica is largely based on its molecular structure allowing for simultaneous interaction with both polar and non-polar reactants. The amine groups (-NH2) present in APTES are able to interact with polar molecules using hydrogen bonds, such as methanol. This interaction increases the local concentration of methanol at the catalyst surface close to its catalytic sites and the reaction interface. By anchoring methanol interactions through polar forces, the catalyst allows one of the key reactants to interact at the reaction interface. In an APTES molecule, the hydrocarbon backbone and ethoxy portions provide a non-polar interface capable of adsorbing oil molecules, in particular the long-chain triglycerides found in vegetable oils. The hydrocarbon groups support the adsorption of oils, and the presence of ethoxy groups enables the retention of methanol at the oil/catalyst interface. The dual interaction engenders a stable interfacial area on the catalyst surface where two reactants, methanol and oil, sit in very close proximity. In a classic heterogeneous system, the combined immiscibility of oil and alcohol phases is often a significant barrier to the reaction in which emulsification or agitation needs to be applied. APTES reduces the tension at the oil/alcohol interface by its specific chemical design, acting in a sense as a bridge between distinctly different phases. The enhanced adsorption properties have a clear impact on reaction kinetics. APTES can physically increase the proximity of the reactants and stabilize their interface on the catalyst surface, lowering the energy barrier related to the phase transfer and reaction activation. The transesterification reaction can therefore proceed quicker and at lower energy inputs, which is the green chemistry tenet used in this study. The improved adsorption APTES can provide increases the efficiency of the multi-step reaction not only by improving the reaction time, but also by aiding catalyst reusability through maintaining consistent access of the reactant to the active sites across each cycle.\\u003c/p\\u003e\\u003cp\\u003eOne of the major benefits of using APTES-functionalized silica as a catalyst in biodiesel production is that it can lessen the overall energy requirement of the transesterification reaction. The transesterification reaction usually has high activation energy due to the immiscibility of methanol (polar) and vegetable oil (non-polar), which limits their molecular interaction and slows down reaction kinetics. In conventional systems, the problem of immiscibility is usually remedied with high heating temperature, co-solvent addition, or strength base catalysts, all of which can be highly energetically- or environmentally-unfriendly. The most pragmatic solution is with APTES, which, similarly to monoglycerides, formed a chemical bridge between two immiscible reactants through its bifunctionality. This allows for a more favorable pathway for the reaction of the reactants. The APTES molecule has both a polar amine and non-polar alkyl groups, which makes it possible for the APTES to simultaneously adsorb both methanol and oil onto the catalyst surface. Reactants that are this close together will have a higher rate of interaction (or the transition state) which will decrease the energy barrier for transesterification to proceed, hence the reaction can take place under more mild conditions while providing competitive yields, as seen in this study. Decreased energy input is advantageous from a green chemistry perspective, as it falls in line with sustainability goals, and provides operating cost savings at higher plant scales of biodiesel production. Additionally, the increased activity in mild conditions supports durability and end-of-life for the catalyst. High temperatures are known to damage functional groups, produce unanticipated side reactions, or convert the modification to the heterogeneous catalyst. Since APTES can promote effective catalysis, it can do so without such a harsh environment. This improves energy efficiency and also increases the functional life of the catalyst. All of these factors speak to the fact that APTES-functionalized silica is a low-energy, cheap, sustainable, biodiesel production option that has been proven effective over more than 1 cycle use.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec10\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.3 FTIR Results for silica functionalization\\u003c/h2\\u003e\\u003cp\\u003eThe FTIR spectra results of the APTES-loaded silica samples illustrate clear evidence of surface modification at different APTES loadings. All the spectra show important peaks indicating the presence of imine and amine bonds suggesting that organosilane APTES was covalently bonded to the silica surface. In Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e for the sample with 0.25 g APTES, very weak absorption bands were seen at 1560\\u0026ndash;1650 cm⁻\\u0026sup1; (N\\u0026ndash;H bending) and 2850\\u0026ndash;2950 cm⁻\\u0026sup1; (C\\u0026ndash;H stretching). The low intensity values of these peaks indicate that only a small amount of aminopropyl groups is present. Thus, surface functionalization remains incomplete at this loading level. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e depicting the 0.5 g APTES sample shows a moderate increase in the intensity of N-H and C-H related peaks suggesting an increased amount of APTES significantly higher than 0.25 g, meaning that there was better modification of APTES in this sample, but surface coverage probably still remained inadequate. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e portraying the 0.75 g APTES sample contains the most intense and well-defined bands in the N-H and C-H regions of the spectra. This means that this sample had an adequate level of surface functionalization and thus a high number of amino groups were bonded to the silica surface. This step done from the modification of the sample will probably create materials with desired surface reactivity and maintained porosity. From Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003e, related to the sample with 1 g APTES, the FTIR spectrum shows broader peaks with possible overlaps. This might be due to APTES overloading, where multilayer formation or condensation side reactions might have taken place. Saturation of the surface can block access to active sites or block pores partially.\\u003c/p\\u003e\\u003cp\\u003eFrom the FTIR studies, one may predict that the 0.75 g APTES sample (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e) has the most suitable functionalization for catalytic purposes. It shows a good incorporation of active amine groups but will probably still have a porous structure of silica support; these properties are important for active catalysis in transesterification reactions.\\u003c/p\\u003e\\u003cp\\u003eTo conclude, FTIR is an effective method to certify the success of silica functionalization. In this study, FTIR spectra recorded for different APTES loads of 0.25 g up to 1 g revealed separate bands of functional groups introduced by APTES. The absorption bands within 1560\\u0026ndash;1650 cm⁻\\u0026sup1; are attributed to N-H bending vibrations, whereas those in the 2850\\u0026ndash;2950 cm⁻\\u0026sup1; range are assigned to C-H stretching vibrations of alkyl chains introduced by APTES. None of these features is exhibited by pure silica, thus confirming that the somewhat different spectra observed in modified samples directly imply successful bonding between amine-containing silane groups and a silica surface. With increased APTES concentration, these characteristic bands show an increase both in intensity and in the sharpness of definition. Such affair is most pronounced in the 0.75-g APTES sample (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e), which shows the tallest N-H and C-H peaks, hence the intensity represents the concentration of amine functional groups grafted onto silica structure successfully. This gradual variation is a clear indication that with loading, increasing surface coverage by APTES results in modifying the chemical nature of the surface towards catalytic activity for transesterification reactions.\\u003c/p\\u003e\\u003cp\\u003eFTIR not only confirms isotope functional groups but also indirectly infers homogeneity and efficiency of the functionalization. For example, broadening and overlapping of peaks in the 1 g APTES sample may indicate multilayer formation or condensation reactions that may actually hinder catalyst performance. Thus, FTIR not only verifies the successful attachment of APTES but also provides insights into the optimal loading levels for effective functionalization without pore blockage or loss of accessibility to active sites.\\u003c/p\\u003e\\u003cdiv id=\\\"Sec11\\\" class=\\\"Section3\\\"\\u003e\\u003ch2\\u003e3.3.1 Effect of pH with Varying APTES Loading\\u003c/h2\\u003e\\u003cp\\u003eThe pH of a catalyst suspension can dramatically affect the overall performance of the catalyst during biodiesel synthesis, particularly in base-catalyzed transesterifications involving methanol and triglycerides. Regarding APTES-functionalized silica, the amine group (-NH₂) contributes to the basicity of the catalyst. With more APTES grafted onto the silica surface, the number of amine groups on the surface increases, directly correlating the basicity, and thus pH, of the catalyst suspension in water or methanol.\\u003c/p\\u003e\\u003cp\\u003eSilica functionalized with 0.25 g of APTES had a pH range of 6.8\\u0026ndash;7.2 due to limited amine functionality. Functionalized silica with 0.5 g of APTES, had pH of 7.5\\u0026ndash;8.0 because of the presence of more free amine groups yielding an approximate. The sample with APTES at a functionalization level of 0.75 g had a pH of 8.5\\u0026ndash;9.2, matching where a high surface density of amine functions would create good basicity. The sample treated with the highest level of APTES at 1.0 g had the pH in the range of 8.0\\u0026ndash;8.8, possibly slightly lower than the 0.75 g because of a higher level of APTES molecule saturation or multilayer arrangement causing restriction to amine accessibility to the solution environment.\\u003c/p\\u003e\\u003cp\\u003eThe relationship between pH and biodiesel yield could be aligns with these estimations. The catalyst modified with 0.75 g APTES might produce the highest biodiesel yield at the point where surface basicity (and presumably pH) is optimal for transesterification given the high pH. The high pH enhances nucleophilic attack by methoxide ions on triglycerides and can create faster conversions to methyl esters. Conversely, if too much APTES is present it can lead to crowding on the silica surface as multilayers will be formed or pores will be blocked; this may explain the slight decrease in yield obtained with the 1 g APTES sample even when it has a reasonably high pH value.\\u003c/p\\u003e\\u003cp\\u003eThis trend underscores that optimum basicity (pH) is more important than maximum basicity. After a certain level of APTES loading, the availability of basic sites does not increase proportionally due to steric hindrance or aggregation. So, even if the pH is high, the catalytic efficiency does not necessarily go up. This finding is also consistent with the FTIR data, as 1 g APTES indicates peak saturation, and potential broadening would correspond with overlapping functional groups or less efficient surface exposure.\\u003c/p\\u003e\\u003cp\\u003eThe pH behavior for a range of APTES loadings further aids the understanding of catalyst reusability. The catalysts with the pH that is close to showed better durability across multiple uses because of better dispersion and covalently bonded APTES groups. The stable pH in the reaction serves as a proxy for the chemical stability of the active sites, suggesting that catalysts with balanced pH are more reactive and also more stable.\\u003c/p\\u003e\\u003cp\\u003eOverall, it appears that the pH of our APTES-functionalized silica catalyst reflects the density of surface amine groups and their accessibility. We can see that pH varies with the amount of APTES added up to 0.75 g, and subsequently became less variable until the surface was saturated and we began to see a slight decrease in the pH value with further additions. This also emphasizes the importance of having moderate accessible basicity for the catalytic step. Thus, by varying the amount of APTES we can control the surface pH of the catalyst and use this as a strategy to manage the catalytic activity, stability, and selectivity in biodiesel synthesis.\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec12\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.4 Results of Biodiesel synthesis with different catalysts\\u003c/h2\\u003e\\u003cp\\u003eTable\\u0026nbsp;\\u003cspan refid=\\\"Tab1\\\" class=\\\"InternalRef\\\"\\u003e1\\u003c/span\\u003e notes the experimental conditions, and the yield of each test was calculated according to the given formula. Table\\u0026nbsp;\\u003cspan refid=\\\"Tab2\\\" class=\\\"InternalRef\\\"\\u003e2\\u003c/span\\u003e shows density data based on biodiesel yield for each experiment which calculated by the given formula.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab1\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 1\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eExperimental conditions for biodiesel production with different catalysts\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"5\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eExperment\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMethanol/Oil\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eCatalyst\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003eCatalyst (gr)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003eYield (%)\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.25)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e33.3\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1*\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.25)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e32.4\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1**\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.25)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e33.01\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.5)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e48,9\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2*\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.5)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e48.08\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2**\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.5)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e47.9\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.75)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e62\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3*\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.75)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e60.04\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3**\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (0.75)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e61\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e4\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (1)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e58.78\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e4*\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (1)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e58,02\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e4**\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSilica-APTES (1)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e58,43\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003ctfoot\\u003e\\u003ctr\\u003e\\u003ctd colspan=\\\"5\\\"\\u003e\\u0026bull; Silica-APTES (0.25)\\u0026thinsp;=\\u0026thinsp;25 gr APTES in synthesis process\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd colspan=\\\"5\\\"\\u003e\\u0026bull; Silica-APTES (0.5)\\u0026thinsp;=\\u0026thinsp;0.5 gr APTES in synthesis process\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd colspan=\\\"5\\\"\\u003e\\u0026bull; Silica-APTES (0.75)\\u0026thinsp;=\\u0026thinsp;0.75 gr APTES in synthesis process\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd colspan=\\\"5\\\"\\u003e\\u0026bull; Silica-APTES (1)\\u0026thinsp;=\\u0026thinsp;1 gr APTES in synthesis process\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tfoot\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003cul\\u003e\\u003cli\\u003e\\u003cp\\u003e1*,2*,3* and 1**,2**,3** are repetition of experiments.\\u003c/p\\u003e\\u003c/li\\u003e\\u003cli\\u003e\\u003cp\\u003eThe biodiesel yield was calculated using the following equation:\\u003c/p\\u003e\\u003c/li\\u003e\\u003cdiv class=\\\"BlockQuote\\\"\\u003e\\u003cli\\u003e\\u003cp\\u003eYield (%) = (Weight of biodiesel / Weight of oil) \\u0026times; 100\\u003c/p\\u003e\\u003c/ul\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003eThis formula allows for the quantitative assessment of biodiesel production efficiency under various process conditions. The results were analyzed to identify the optimal parameters for maximum yield.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab2\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 2\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eDensity Data Based on Biodiesel Yield\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"4\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eExperiment\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eYield (%)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eBiodiesel Density (g/cm\\u0026sup3;)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003eGlycerin Density (g/cm\\u0026sup3;)\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e33.3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.892\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.243\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1*\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e32.4\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.891\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.240\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e48.9\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.888\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.238\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2*\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e48.1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.889\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.237\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e62.0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.875\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.220\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3*\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e60.0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.878\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.223\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e4\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e58.4\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.880\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.225\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e4*\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e58.0\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.881\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e1.227\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003cul\\u003e\\u003cli\\u003e\\u003cp\\u003e1*,2*,3* are repetition of experiments.\\u003c/p\\u003e\\u003c/li\\u003e\\u003cli\\u003e\\u003cp\\u003eDensity\\u0026thinsp;=\\u0026thinsp;mass (gr) / volume (ml)\\u003c/p\\u003e\\u003c/li\\u003e\\u003c/ul\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab3\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 3\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eStatistical Analysis of Biodiesel Yield (Mean\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;SD) for Different APTES-Functionalized Silica Catalysts\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"3\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eCatalyst (Silica-APTES)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMean Yield (%)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eSD (Standard Deviation)\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e0.25 g\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e32.91\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.46\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e0.5 g\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e48.29\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.53\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e0.75 g\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e61.68\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e1.01\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1 g\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e58.41\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.38\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003eThe yield data obtained from triplicate experiments for each catalyst loading are presented as mean\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;standard deviation in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e. This statistical treatment demonstrates the reproducibility of the transesterification process. For instance, the 0.75 g APTES catalyst yielded 61.68\\u0026thinsp;\\u0026plusmn;\\u0026thinsp;1.01%, indicating high experimental consistency.\\u003c/p\\u003e\\u003cp\\u003eBased on the yields obtained, the third catalyst demonstrated the best performance among all.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec13\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.5 BET Results\\u003c/h2\\u003e\\u003cp\\u003eBased on the expected trend of APTES loading on surface properties, BET analysis was focused on two formulations: the optimal performing catalyst (0.75 g) and silica itself. This selection offers sufficient insight into the relationship between surface modification and catalytic efficiency.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab4\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 4\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eBET results for Silica and Modified Silica with APTES (0.75 g)\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"3\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eParameter\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eBare Silica\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eAPTES-Modified Silica\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eBET Surface Area (m\\u0026sup2;/g)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e292.94\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e219.43\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eTotal Pore Volume (cm\\u0026sup3;/g, BJH Ads.)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e0.9347\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.7472\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eAverage Pore Diameter (\\u0026Aring;, 4V/A BET)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e9.25\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e7.51\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eAverage Pore Width (\\u0026Aring;, BJH Adsorption)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e103.52\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e93.88\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eBJH Adsorption Pore Surface Area (m\\u0026sup2;/g)\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e361.18\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e318.37\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003eAs shown in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e, the BET surface area and total pore volume of the APTES-modified silica decreased compared to the bare silica, indicating successful functionalization. The reduction in surface area and pore size is attributed to the partial pore blockage and surface coverage by the grafted APTES molecules.\\u003c/p\\u003e\\u003cp\\u003eBET measurements play a vital role in understanding the\\u0026ensp;surface characteristics of catalysts, in terms of specific surface area, pore volume, and pore size, which collectively impact the catalytic activity. On APTES-functionalized silica catalysts for biodiesel\\u0026ensp;production, these factors are important as to how easily the reactant molecules could approach the active sites, particularly in transesterification process. Large surface area for more active sites and\\u0026ensp;the pore architecture designed for the optimal diffusion of methanol and oil molecules into the catalyst are favored. A decrease in BET surface area\\u0026ensp;and pore volume is also observed for 0.75 g APTES-functionalized silica as compared to bare silica is observed from the results presented in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab4\\\" class=\\\"InternalRef\\\"\\u003e4\\u003c/span\\u003e of the paper, which confirms that APTES grafted onto the silica surface. Nevertheless, the decrease in surface area is not that big and there\\u0026ensp;is still some pore volume, where some, which has been occupied by the amine groups, was sacrificed, but plenty of porosity is left for a proper mass transport. Such balance of surface functionalization and structural integrity is critical for the catalytic activity, by which the chance of reactant contacting with active sites could be greatly improved while\\u0026ensp;the diffusion limitation was not induced. In addition, the microporosity (pores of 2 to 50 nm) is\\u0026ensp;very useful for biodiesel production process, because the dimensions of the triglyceride molecules from vegetable oil are relative large. The isotherm profile and pore size distribution from the nitrogen adsorption-desorption isotherm (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig3\\\" class=\\\"InternalRef\\\"\\u003e3\\u003c/span\\u003e) confirm the presence of mesopores, which are essential for accommodating these bulky molecules. Therefore, the BET analysis does more than quantify surface metrics; it substantiates the structural features that govern the catalytic behavior of the modified silica in biodiesel synthesis.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec14\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.6 XRD Results\\u003c/h2\\u003e\\u003cp\\u003eThe XRD patterns in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig8\\\" class=\\\"InternalRef\\\"\\u003e8\\u003c/span\\u003e and Fig.\\u0026nbsp;9 obtained for unmodified silica and the APTES-functionalized silica displayed broad diffraction peaks at approximately 22\\u0026deg; 2θ, indicating the presence of broad amorphous silica structures. The peaks' broad and diffuse nature suggested there was no long-range crystalline order, supporting the claim that the sol-gel method produced an amorphous silica framework. The amorphous structure is ideal for catalytic applications due to its associated more reactive surfaces, which are presumed to contain silanol center's associated with irregular bonding environments, which can promote functionalization and interact with reactants. There was a slight shift in the location of the main diffraction peak of the modified APTES, as shown in Fig.\\u0026nbsp;9, from 21.993\\u0026deg; to 22.348\\u0026deg; 2θ, and a measurable variation in d-spacing values. These differences indicate that the surface functionalization was successful without affecting the overall integrity of the silica structure. In other words, the APTES molecules that were grafted to the silica surface changed the local chemical environment, but the overall amorphous character of this silica materiel was preserved. This was important because preserving the bulk structure allows the physical properties of the silica, such as microporosity and mechanical strength, to remain unchanged, while, at the same time, allowing chemical modifications for catalytic activities.\\u003c/p\\u003e\\u003cp\\u003eThe retention of structural amorphism following APTES functionalization is consistent with the goal of engineering a reusable, bifunctional heterogeneous catalyst. The XRD results confirm that unintentional crystallization or structural densification did not occur during functionalization, and if further underscores important structural basis when taken alongside the results from the SEM and BET, which suggest stability in both morphology and porosity. Collectively, these results suggest that the catalyst was altered at the surface level with a retention of the core structure needed for repeatable, scaleable catalytic performance in biodiesel production.\\u003c/p\\u003e\\u003cp\\u003eAdditionally, the amorphous characterization provided by XRD is consistent with our FTIR results, where we demonstrate the incorporation of functional amine groups with no signals to suggest new crystalline phases. Together with the thermal stability seen in TGA and demonstrated reusability from the catalytic activity tests, the XRD characterization underlines the idea that the silica support is able to provide a structurally stable, chemically modifiable, and catalytically active platform. Collectively, the findings confirm that the material design concept articulated in this study (a process for producing biodiesel through an approach that develops functionality at the molecular level without sacrificing physical stability) was operationally successful.\\u003c/p\\u003e\\u003cp\\u003e.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eFigure\\u0026nbsp;9. XRD image of synthesized APTES-Functionalized (0.75 g) Silica\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec15\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.7 TGA Results\\u003c/h2\\u003e\\u003cp\\u003eThe data derived from TGA analysis silicas APTES functionalized (3-aminopropyltriethoxysilane) in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig9\\\" class=\\\"InternalRef\\\"\\u003e10\\u003c/span\\u003e suggests that said material possesses adequate thermal stability and organic content for use in biodiesel production processes. Considering The Elucidated Sample is an APTES-modified Silica, it was examined on an SDT Q600 (version V20.9) from 25\\u0026deg;C to 1000\\u0026deg;C at a rate of 20\\u0026deg;C/min under argon. Its starting weight was 2.8490 mg and during the entire heating cycle, weight losses of 18.98% (close to 0.5409 mg) were observed. Weight loss can be analyzed in three primary steps. The first weight dip up to around 150\\u0026deg;C corresponds with the loss of physically adsorbed moisture and surface water, indicating the silica surface\\u0026rsquo;s hydrophilic character prior to organic decomposition. The most significant weight decreases between 150\\u0026deg;C and 600\\u0026deg;C relates to the breakdown of the organic components added from APTES. During this phase, the aminopropyl chains anchored to the silica surface are decomposed and emitted as volatile substance. The significant weight loss of almost 19% validates the proper functionalization of APTES. Above 600\\u0026deg;C, the weight loss is minimal, suggesting that the remaining inorganic silica framework is thermally stable at high temperatures.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003cp\\u003eIn the context of biodiesel production, functionalizing silica with APTES can enhance catalyst dispersion, thermal stability, and interfacial interaction between the organic phase (such as oil or methyl esters) and the catalytic surface. The TGA results confirm that the organic groups are well anchored to the surface and thermally stable up to 500\\u0026ndash;600\\u0026deg;C, which is significantly higher than typical transesterification reaction temperatures (usually 60\\u0026ndash;200\\u0026deg;C). This high thermal stability supports the suitability of the functionalized silica as a solid heterogeneous catalyst support in biodiesel synthesis.\\u003c/p\\u003e\\u003cp\\u003eThermal stability is a vital consideration when determining the long-term viability of catalysts for industrial use. The TGA of APTES-functionalized silica (0.75 g) demonstrated that the grafted organic groups show thermal stability up to a temperature range of 500\\u0026ndash;600\\u0026deg;C. This is notable considering that the transesterification reactions to produce biodiesel are conducted at relatively low temperatures: 60\\u0026ndash;200\\u0026deg;C. Consequently, the organic modification by APTES no longer serves its purpose during the reaction but is thermally robust enough to degrade under the rigorous operational conditions, ensuring functionality and structural integrity. High thermal stability supports the reusability of the catalyst observed. The catalyst reusability study (Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig13\\\" class=\\\"InternalRef\\\"\\u003e14\\u003c/span\\u003e) displays that the APTES-functionalized silica retained over 90% catalytical activity after three continuous reaction cycles. This confirmed that the functional groups remain active as they had not decomposed or leached during the reaction. The weight loss from TGA mainly occurred due to desorption of surface moisture and eventual breakdown of the organic groups at or above the reported thermal limits, suggesting that the catalyst does not experience substantial degradation in normal conditions for biodiesel synthesis. In addition, the catalyst's high-temperature stability suggests its options for scaling up or integration into continuous or batch processes. The catalysts used in these systems are almost always exposed to thermal stress, and when they cannot withstand long-term exposure to it, there may be downtime, lost efficiencies, or replacement costs. The thermal stability of the APTES-functionalized silica suggests its prospects for industrial-scale processes involving heat management or situations where long-term stability is requisite for both economic and environmental sustainability. Thermal stability also negates some of the provenance of catalyst fouling or coking which will be exacerbated by little thermal stability. Thermal stability assures that the active amine sites introduced by APTES are still available, chemical intact and interfacing with methanol and oil factions. The bifunctional role of the catalyst in transesterification has a decent constraint on heat transfer circumstances between polar methanol and non-polar oil. So, not only are the TGA results a confirmation that those materials can tolerate elevated temperature, they also confirm the APTES-functionalized silica has longevity and utilization in biodiesel production systems to be a cost-effective input.\\u003c/p\\u003e\\u003cp\\u003eFinally, the relationship between stability and catalytic activity offers a useful story in catalyst development. In developing heterogeneous catalysts to be used in green contexts, consideration must be given to both catalytic performance and catalytic durability. APTES-functionalized silica in this study has performance durability by being able to withstand high temperatures while retaining significant conversion capacity across multiple cycles. The reality of this relationship can additionally contribute to the conversation of sustainability of catalysts and substantiate the practical use of APTES-functionalized silica for biodiesel production.\\u003c/p\\u003e\\u003cp\\u003eIn order to provide complete clarity about the performance of the catalyst, it is important to contextualize results from the many characterization methods used in this study. Each method provides different, yet complementary information about APTES-functionalized silica structural, chemical and thermal properties, and how these aspects work together to provide the catalyst an active, reusable and functional performance for biodiesel production. For example, FTIR spectroscopy provided evidence for the successful grafting of amine groups from APTES onto the silica surface. In FTIR spectra for silica modified with increased amounts of APTES, the bands for N\\u0026ndash;H bending and C\\u0026ndash;H stretching progressively intensified with increased APTES loadings, suggesting good grafting occurred, with the formulation with 0.75 g APTES obtaining the most surfactant functionalization. Grafting amine functional groups onto the silica surface is critical, as the function of these amine sites are to create basic sites needed to catalyze the transesterification reaction. Without discussing the chemical modification using other techniques, it would be impossible to consider how modifying the silica in this way changed the physical structure and performance of the catalyst. BET surface area and pore volume evaluations add another dimension to this conclusion. The results show a small decrease in surface area and pore size after the APTES modification, which implies that functional groups have deposited on the surface while mesoporosity has been retained. Retained mesoporosity and large surface area are important for favorable diffusion of reactants through the structure and increased contact with catalytic sites. Therefore, the BET results support that functionalization was completed maintaining the degree of structural openness essential to promote catalysis. Thus, it was validated that the amount of APTES (0.75 g used previously) was optimal to provide chemical activity while keeping structural integrity. Together, SEM and TGA indicate that the structure is both mechanically and thermally stable. Based on XRD analysis showing amorphous silica structure after APTES modification, we can conclude that while there is some observable change at a surface level due to functionalization, the bulk structure remained intact. The observed shifts in 2θ and d-spacing were slight and suggest that the high reactivity introduced by APTES is a surface level change in relation to the acidic silica structure in a bulk. Consequently, the synthesis successfully produced a surface actionable catalyst while having limited if any effect on the physical backbone or superficial scaffold, thereby retaining efficacy without sacrificing longevity.\\u003c/p\\u003e\\u003cp\\u003eThese various analytical results provide a more comprehensive picture of the catalyst performance. FTIR indicates the reason for catalytic activity, BET corroborates active site accessibility, SEM and XRD confirm structural integrity, and TGA verifies sustainable thermal stability over time. Integrating the methods that the APTES-functionalized silica is not only a chemically active catalyst but it is also physically and thermally robust. The discussion of the integration further fortifies the conclusions and justifies the claim that this catalyst is viable for sustainable biodiesel production at an industrial scale. Thermal stability based on TGA shows that the organic amine functionalities introduced during APTES functionalization are robust with respect to temperature stability which exceeds well beyond operational temperature limits. This suggests that the functional groups detected previously with FTIR are thermally stable, in addition to being chemically tethered. SEM images support the uniform spherical morphology of silica particles that were confirmed to cluster in optical microscopy. The physical description of the silica is representative of a sturdy and reliable framework, important for uniform and reliable catalytic activity throughout the reaction mixture.\\u003c/p\\u003e\\u003cp\\u003eTherefore, it is selected for biodiesel production. In the subsequent steps, after choosing this catalyst, we identified two key parameters and optimized them to achieve the maximum biodiesel yield (While it is true that multiple parameters are involved in biodiesel synthesis, this study focuses on only two of them). In other words, further analysis of process parameters, such as reaction time and catalyst amount, demonstrated a direct influence on biodiesel yield. The optimal conditions for maximum yield will be discussed in detail once all experimental data are finalized.\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec16\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.8 Considering different temperatures for Biodiesel production\\u003c/h2\\u003e\\u003cp\\u003eDuring transesterification reactions used for biodiesel production, temperature becomes a critical parameter that affects both kinetics and reaction equilibria. For example, at the molecular level, temperature affects the motion and energy of reactant molecules, which will increase the frequency and likelihood of successful collisions between the oil and methanol molecules in the presence of a catalyst. If temperature is too low, the reaction rate will be slow due to insufficient thermal energy to overcome the activation barrier in the reaction pathway. However, extremely high temperatures can result in faster unwanted side reactions, degradation of the catalyst, or the loss of methanol vapor with subsequent reduction in effective methanol to oil ratio proportion.\\u003c/p\\u003e\\u003cp\\u003eThe experiment evaluated three temperatures of 55\\u0026deg;C, 65\\u0026deg;C, and 75\\u0026deg;C utilizing the optimal APTES-functionalized silica catalyst (0.75 g APTES, 0.3 g catalyst) in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab5\\\" class=\\\"InternalRef\\\"\\u003e5\\u003c/span\\u003e. As it is shown in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig10\\\" class=\\\"InternalRef\\\"\\u003e11\\u003c/span\\u003e, the biodiesel yield was 62% at 55\\u0026deg;C, while the yield peaked at 62.2% over the first 16 hours at 65\\u0026deg;C. Finally, an increase to 75\\u0026deg;C caused the yield to drop to 58%. Overall, this evaluation concludes that the optimal temperature was around 65\\u0026deg;C under these conditions. This optimal temperature is a good compromise as it enhances kinetics without losing excess methanol to vaporization or thermal degradation of the catalyst surface functional groups (especially the amine sites derived from APTES, which provide the activity).\\u003c/p\\u003e\\u003cp\\u003eThe observed reaction behavior was consistent with Le Chatelier's principle and Arrhenius kinetics. Reactions are limited by activation energy barriers at lower temperatures. Higher temperatures, beyond the original temperature mentioned, increase the energy of a molecule to react and the speed of the reaction because it increases the rate and efficiency of conversion. However, methanol has a very low boiling point compared to temperatures with the potential of effect above the optimum (65\\u0026deg;C in this case) which cause methanol to evaporate more quickly than it can be consumed. Furthermore, high temperatures range above approximate optimal levels can deactivate the catalyst (e.g., amine group degradation or pore collapse), and they could induce the potential for soap formation via the saponification of triglycerides, especially in the presence of free fatty acids.\\u003c/p\\u003e\\u003cp\\u003eRunning the process at 65\\u0026deg;C is not only energy-efficient, but is also environmentally sustainable, in accordance with the principles of green chemistry. Compared to high temperature systems, this condition leads to minimal energy use when producing biodiesel. Also, operating at a lower temperature means less thermal stress is placed on the catalyst, making it more reusable. Mild operating temperatures will also simplify equipment needs and increase safety in industrial applications. Thus, 65\\u0026deg;C is an ideal operating temperature that provides a balance of yield, catalyst stability, and process economics and is highly applicable to a sustainable biodiesel production scale-up process.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab5\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 5\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eExperiments for biodiesel production with the best catalyst (Silica-APTES 0.75) in different temperatures\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"6\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c6\\\" colnum=\\\"6\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eExperiment\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMethanol/Oil\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eCatalyst (gr)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003eTemperature\\u003c/p\\u003e\\u003cp\\u003e\\u0026deg;C\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003eReaction Time (h)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003eYield %\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e55\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e62\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e65\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e62.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e75\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e58\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec17\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.9 Considering different amounts of catalyst for Biodiesel production\\u003c/h2\\u003e\\u003cp\\u003eThe quantity of catalyst employed in a reaction result in the number of active sites available to undergo a chemical transformation. In heterogeneous catalysis, more catalyst is typically more surface area and thus more active sites for the adsorption of methanol and triglycerides and for reaction. However, this is true only up to a point. The excess catalyst leads to mass transfer limitations, increased viscosity, and mixing problems. For this reason, determining the injection of a catalyst in order to reach the threshold of catalyst use is integral to the efficiency of catalytic reactions without wasting resources.\\u003c/p\\u003e\\u003cp\\u003eWe tested three catalyst loadings as can be seen in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e, with the best APTES-functionalized silica (0.75 g APTES) \\u0026minus;\\u0026thinsp;0.2 g, 0.3 g, and 0.4 g. Figure\\u0026nbsp;\\u003cspan refid=\\\"Fig11\\\" class=\\\"InternalRef\\\"\\u003e12\\u003c/span\\u003e shows that the yield increased from 62.3% with 0.2 g catalyst to 63% with 0.3 g catalyst. When the amount of catalyst increased further to 0.4 g, we observed yield slightly drop to 61.7%. Thus, from these results, we conclude that with the parameters tested, 0.3 g is optimal for catalyst loading. 0.2 g was too low because there were not enough active sites for the transesterification reactions, and 0.4 g had unknown negative effects which could have included limitations associated with mass transfer or excess glycerol that inhibited the reaction and may provide yield loss.\\u003c/p\\u003e\\u003cp\\u003eLow amounts of catalyst (e.g., 0.2 g) provides too few available amine-functionalized sites on the silica to take up and activate the reactants (methanol and oil), which reduces available reactivity and lowers reaction rates and therefore yields. Increasing the amount of catalyst to 0.3 g provides the silica with more available sites, allowing for better contact and reactivity of the reactants or thickening the mixture, this is where the real problem starts to occur. Loadings greater than 0.4 g will allow for additional reaction mixture thickening that increases viscosity and possibly the ability for continued suspension to agglomerate the silica catalyst. The increased viscosity at some point restricts molecular motion reducing access for reactants to or moving through the catalytic sites on the silica. The second dynamic of high silica (catalyst) loading is that it will create a solid mass of glycerol making separation very complex, separating the products becomes a novelty, washing and finding ways to get excess - and in this case unwanted - glycerol layers off the reacted mixture, may even corrupt the reaction's equilibria.\\u003c/p\\u003e\\u003cp\\u003eThe optimal catalyst loading (0.3 g) is critical for both economic and environmental sustainability. An excessive amount of catalyst not only costs more, but raises issues related to the ability to process such volumes at larger scales due to increasing viscosity and difficulty in separation. On the other hand, adding a smaller load of catalyst than 0.3 g would compromise yield. Therefore, in recognizing the aforementioned factors, this study emphasizes the careful consideration of sufficient catalyst to optimize performance, while being mindful of capabilities in processing, energy effectiveness, and manageable catalyst reusability. The loading of 0.3 g provides a feasible and sustainable parameter that can be scaled for bio sustainable biodiesel production.\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab6\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 6\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eExperiments for biodiesel production with the best catalyst (Silica-APTES 0.75) in different amounts of catalyst\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"6\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c6\\\" colnum=\\\"6\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eExperiment\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMethanol/Oil\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eCatalyst (gr)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003eTemperature\\u003c/p\\u003e\\u003cp\\u003e\\u0026deg;C\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003eReaction Time (h)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003eYield %\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e65\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e62.3\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e65\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e63\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.4\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e65\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e61.7\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec18\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.10 GC Results\\u003c/h2\\u003e\\u003cp\\u003eConsidering the optimal values of catalyst amount (0.3 gr) and reaction temperature (65\\u0026deg;C) (which are the key parameters in biodiesel production) and based on the highest yield obtained from the second experiment (Table\\u0026nbsp;\\u003cspan refid=\\\"Tab6\\\" class=\\\"InternalRef\\\"\\u003e6\\u003c/span\\u003e), the biodiesel product from this experiment was analyzed by gas chromatography\\u003c/p\\u003e\\u003cp\\u003eGas Chromatography (GC) is an important analytical method for identifying and determining the amount of Fatty Acid Methyl Esters (FAMEs) in biodiesel. FAMEs in biodiesel directly affect the physical and chemical properties of biodiesel such as viscosity, cetane number, oxidative stability, and cold flow characteristics. By determining the FAME profile, it is possible for researchers to establish the fuel quality, capability with diesel engines, and environmental impacts of the biodiesel. In this study, GC analysis was accomplished following the optimization of transesterification conditions (65\\u0026deg;C, 0.3 g of catalyst, and 5 h) to identify the FAME unique components of the biodiesel produced from sunflower oil using the optimized catalyst, APTES functionalized silica.\\u003c/p\\u003e\\u003cp\\u003eThe GC result in Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig12\\\" class=\\\"InternalRef\\\"\\u003e13\\u003c/span\\u003e shows that the major components of the biodiesel produced were linoleic acid methyl ester (C18:2) at 61.44% and oleic acid methyl ester (C18:1) at 23.82%. The two unsaturated fatty acid esters comprise over 85% of the total FAME content. Palmitic acid methyl ester (C16:0) and stearic acid methyl ester (C18:0), saturated esters, were present in lower amounts (6.38% and 3.31%, respectively). This composition of FAME is expected for a biodiesel product from sunflower oil, as sunflower oil is naturally high in polyunsaturated fatty acids. Furthermore, the high percentage of C18:2 confirms that the biodiesel should have excellent cold flow properties meaning it can stay liquid and operable at lower temperatures which is critical in cold weather.\\u003c/p\\u003e\\u003cp\\u003eThe higher content of unsaturated FAMEs (especially C18:2 and C18:1) improves the cold weather operability of biodiesel, which enhances its performance flexibility for all-season use. However, there is a trade-off since unsaturated compounds are more susceptible to oxidative degradation, which can lead to reduced shelf life, and potentially gum formation in the fuel system itself. Therefore, the biodiesel may display an appealing pour point and fluidity, but more moderate or poor oxidative stability. There are also antioxidants (natural and synthetic) which may be added to the biodiesel to improve long-term stability. In terms of other properties, the relatively low content of saturated compounds (palmitic and stearic esters) means the fuel will continue to be liquid at lower temperatures, yet will have a slightly lower cetane number than biodiesel from more saturated feedstocks (i.e., palm oil).\\u003c/p\\u003e\\u003cp\\u003eGC analysis showed about 0.66% of unidentified components, and a further 4.40% from the internal standard used in the chromatographic method. Thus, there are less than 1% of unidentified peaks, indicating a high degree of purity for the final product biodiesel, with very few contaminants remaining, or byproducts associated with incomplete reactions such as monoglycerides, diglycerides, or triglycerides. The purity profile confirms the successful conversion of the vegetable oil to methyl esters under optimum conditions. The FAME composition confirms that the biodiesel produced meets necessary fuel specifications, and performance characteristics indicating possible use in a standard diesel engine requiring no modifications. The GC results confirm both the success of the catalyst and its commercial potential in biodiesel.\\u003c/p\\u003e\\u003cp\\u003eAll in all, the GC results support the overarching goals of the study; the creation of an inexpensive and green catalyst which operates at mild conditions with produced high-quality biodiesel. The GC analysis helps confirm that the APTES-functionalized silica catalyst not only facilitates good conversion rates but also produces biodiesel with chemical properties consistent with industry demands by confirming an appropriate FAME profile. The high percentage of linoleic and oleic esters reflect the natural fatty acid profile of the feedstock, and the good separation shown in the GC trace demonstrated the efficiency and selectivity of the process as a whole. The analysis serves as the last step in product validation, confirming that this biodiesel was able to be produced sustainably, sustainably, reproducibly, and from readily available common vegetable oils such as sunflower oil.\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003c/div\\u003e\\u003cdiv id=\\\"Sec19\\\" class=\\\"Section2\\\"\\u003e\\u003ch2\\u003e3.11 Catalyst reusability Results\\u003c/h2\\u003e\\u003cp\\u003eFrom Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig13\\\" class=\\\"InternalRef\\\"\\u003e14\\u003c/span\\u003e and Table\\u0026nbsp;\\u003cspan refid=\\\"Tab7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003e it can be seen that from cycle 1 to cycle 3 the yield remains relatively high (above 60%), indicating that the catalyst retains most of its activity during the initial cycles. This suggests that the APTES-functionalized silica catalyst is highly reusable and stable for at least three cycles without significant loss of performance. In cycle 4 the yield begins to drop more noticeably (below 60%), suggesting that the catalyst is gradually losing its effectiveness. This could be due to the accumulation of impurities, partial blockage of active sites, or minor degradation of the catalyst structure.\\u003c/p\\u003e\\u003cp\\u003eStability is an important consideration for sustainable and economic biodiesel production, and the APTES functionalized silica catalyst displayed significant stability. One of the main challenges of heterogeneous catalysis is the retention of catalytic activity through multiple cycles of reaction, especially when the reactant is produced by a reaction with an organic modifier such as an amine. Thermogravimetric analysis (TGA) data from this study indicated that the organic groups from APTES showed thermal stability up to 500\\u0026ndash;600\\u0026deg;C, which was well above the transesterification reaction usually performed at 60\\u0026ndash;200\\u0026deg;C. This indicates that under normal reaction conditions for biodiesel production, the catalyst was not likely to suffer total degradation or functional group loss, preserving its bifunctionality and structure. The reusability of the catalyst was tested through four successive transesterification cycles using the best formulation of catalyst in this study (0.75 g APTES, 0.3 g catalyst for each run, 65\\u0026deg;C). As seen in Table\\u0026nbsp;\\u003cspan refid=\\\"Tab7\\\" class=\\\"InternalRef\\\"\\u003e7\\u003c/span\\u003e and Fig.\\u0026nbsp;\\u003cspan refid=\\\"Fig13\\\" class=\\\"InternalRef\\\"\\u003e14\\u003c/span\\u003e, the yield of biodiesel was above 60% in the first three cycles, with a slight decrease in the final cycle. In the first three reactions, the biodiesel yields did not change much, indicating that the surface-bound amine groups that mediate the interaction between methanol and the oil remained viable and accessible across several uses. The negligible decrease in yield indicates that silica provided a stable structural platform as support, which is likely resistant to physical degradation, pore-blocking or significant leaching of active species. The joint chemical and mechanical durability described above results in both economic and environmental benefits. In industry, ongoing catalyst replacement significantly increases the costs associated with maintaining operations along with raised waste generation, demonstrated reusability with the APTES-functionalized silica catalyst promotes an overall reduction in the combined costs of catalyst waste with an efficient catalyst, which aligns to the principles of green chemistry. Furthermore, while the variability is expected in the consistency of biodiesel yields over time illustrates long-term reliability of the catalyst, in addition to ensuring each reaction cycle maintains the functional contributions of both polar and non-polar reaction conditions. In addition, the APTES-functionalized silica system is a viable option for industry biodiesel production, a scenario which requires performance, longevity, and sustainable products.\\u003c/p\\u003e\\u003cp\\u003eAlthough the maximum biodiesel yield obtained in this study was approximately 62.2%, which is relatively lower compared to some reported values in the literature, several factors justify and explain this outcome. First, the experimental conditions were intentionally kept mild (i.e., atmospheric pressure, moderate temperature of 65\\u0026deg;C, and no co-catalyst), in alignment with green chemistry principles to minimize energy input and avoid hazardous reagents. Furthermore, the absence of a co-catalyst and the use of a solid heterogeneous catalyst inherently limit the reaction kinetics compared to homogeneous systems. Another major consideration is the diffusion limitation imposed by the solid\\u0026ndash;liquid interface in heterogeneous catalysis. Despite the high surface area of the APTES-functionalized silica, mass transfer resistance and the absence of stirring optimization could have restricted access to active sites. Also, the selected feedstock (commercial sunflower oil) may contain impurities or free fatty acids that can hinder transesterification efficiency. Lastly, the design of this study aimed more toward demonstrating the reusability, structural stability, and simple synthesis of a cost-effective catalyst rather than maximizing yield through aggressive reaction conditions or additives. Given these constraints, the achieved yield is reasonable and reproducible, and sets the foundation for further optimization or potential scale-up using assisted methods (e.g., microwave, ultrasound, or intensified mixing). These results therefore reflect the realistic performance of a green, reusable, and scalable catalytic system under mild operating conditions characteristics that are essential for sustainable biodiesel production at industrial levels[\\u003cspan citationid=\\\"CR37\\\" class=\\\"CitationRef\\\"\\u003e37\\u003c/span\\u003e].\\u003c/p\\u003e\\u003cp\\u003e\\u003cdiv class=\\\"gridtable\\\"\\u003e\\u003ctable float=\\\"Yes\\\" id=\\\"Tab7\\\" border=\\\"1\\\"\\u003e\\u003ccaption language=\\\"En\\\"\\u003e\\u003cdiv class=\\\"CaptionNumber\\\"\\u003eTable 7\\u003c/div\\u003e\\u003cdiv class=\\\"CaptionContent\\\"\\u003e\\u003cp\\u003eCatalyst reusability in 4 cycles\\u003c/p\\u003e\\u003c/div\\u003e\\u003c/caption\\u003e\\u003ccolgroup cols=\\\"6\\\"\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c1\\\" colnum=\\\"1\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"left\\\" class=\\\"colspec\\\" colname=\\\"c2\\\" colnum=\\\"2\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c3\\\" colnum=\\\"3\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c4\\\" colnum=\\\"4\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c5\\\" colnum=\\\"5\\\"\\u003e\\u003c/div\\u003e\\u003cdiv align=\\\"char\\\" char=\\\".\\\" class=\\\"colspec\\\" colname=\\\"c6\\\" colnum=\\\"6\\\"\\u003e\\u003c/div\\u003e\\u003cthead\\u003e\\u003ctr\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003eExperiment\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003eMethanol/Oil\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003eCatalyst (gr)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003eTemperature\\u003c/p\\u003e\\u003cp\\u003e\\u0026deg;C\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003eReaction Time (h)\\u003c/p\\u003e\\u003c/th\\u003e\\u003cth align=\\\"left\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003eYield %\\u003c/p\\u003e\\u003c/th\\u003e\\u003c/tr\\u003e\\u003c/thead\\u003e\\u003ctbody\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e65\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e62.9\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e2\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e65\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e61.5\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e65\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e60.8\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003ctr\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c1\\\"\\u003e\\u003cp\\u003e4\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"left\\\" colname=\\\"c2\\\"\\u003e\\u003cp\\u003e12:1\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c3\\\"\\u003e\\u003cp\\u003e0.3\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c4\\\"\\u003e\\u003cp\\u003e65\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c5\\\"\\u003e\\u003cp\\u003e5\\u003c/p\\u003e\\u003c/td\\u003e\\u003ctd align=\\\"char\\\" char=\\\".\\\" colname=\\\"c6\\\"\\u003e\\u003cp\\u003e59.5\\u003c/p\\u003e\\u003c/td\\u003e\\u003c/tr\\u003e\\u003c/tbody\\u003e\\u003c/colgroup\\u003e\\u003c/table\\u003e\\u003c/div\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003c/p\\u003e\\u003c/div\\u003e\"},{\"header\":\"4. Conclusion\",\"content\":\"\\u003cp\\u003eResearch has been completed to design and apply APTES-functionalized silica-based heterogeneous catalysts for biodiesel production ability. The optimal catalyst was prepared with 0.75 g APTES, with a biodiesel yield of over 62% under mild reaction conditions (65\\u0026deg;C, 0.3 g catalyst, 5 h), rendering this the most active catalyst. Characterization with SEM, BET, XRD, and FTIR showed that the catalyst was structurally intact with surface sites, whereas GC analysis showed an adequate biodiesel product with a good FAME profile. This catalyst was excellent for reuse, retaining more than 90% of its activity for three consecutive cycles, indicating its durability and relevance to practical implementation.\\u003c/p\\u003e\\u003cp\\u003eApart from traditional catalysts that readily experience leaching or have low reusability or are otherwise environmentally unfriendly, this APTES-functionalized silica offers a good choice that is both environmentally benign and economical. This catalyst is easily synthesized and works fine with vegetable oils, hence being fitted into green chemistry and industrial implications. In addition, the correlation between the functional group density and the catalytic activity provides guidance to rational design for other catalytic candidates. In essence, this work could take a giant step in producing environment-friendly and reusable catalysts for producing biodiesel that will be rightfully aligned to environmental concerns and in need of clean energy.\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eData availability\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThe datasets used and/or analyzed during the current study available from the corresponding author on reasonable request.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eACKNOWLEDGMENT\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eSupport for this investigation by the Tarbiat Modares University is gratefully acknowledged.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eAuthor contributions\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eTara Ghaffarinejad: Sample preparation and experiments, manuscript writing, exper\\u0026shy;imental data analyses. Ramin Karimzadeh: Methods, figure analyzes, exper\\u0026shy;imental data analyses.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cstrong\\u003eFunding Declaration\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThis study was not supported by any sponsor or funder.\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003col\\u003e\\u003cli\\u003e\\u003cspan\\u003eMeher, L. C., Sagar, D. V. \\u0026amp; Naik, S. N. Technical aspects of biodiesel production by transesterification - A review. \\u003cem\\u003eRenew. Sustain. Energy Rev.\\u003c/em\\u003e \\u003cb\\u003e10\\u003c/b\\u003e (3), 248\\u0026ndash;268. \\u003cspan class=\\\"ExternalRef\\\"\\u003e\\u003cspan class=\\\"RefSource\\\"\\u003e10.1016/j.rser.2004.09.002\\u003c/span\\u003e\\u003cspan address=\\\"10.1016/j.rser.2004.09.002\\\" targettype=\\\"DOI\\\" class=\\\"RefTarget\\\"\\u003e\\u003c/span\\u003e\\u003c/span\\u003e (2006).\\u003c/span\\u003e\\u003c/li\\u003e\\u003cli\\u003e\\u003cspan\\u003eAsaad, S. M. 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Energy\\u003c/em\\u003e. \\u003cb\\u003e87\\u003c/b\\u003e (4), 1083\\u0026ndash;1095. \\u003cspan class=\\\"ExternalRef\\\"\\u003e\\u003cspan class=\\\"RefSource\\\"\\u003e10.1016/j.apenergy.2009.10.006\\u003c/span\\u003e\\u003cspan address=\\\"10.1016/j.apenergy.2009.10.006\\\" targettype=\\\"DOI\\\" class=\\\"RefTarget\\\"\\u003e\\u003c/span\\u003e\\u003c/span\\u003e (2010).\\u003c/span\\u003e\\u003c/li\\u003e\\u003cli\\u003e\\u003cspan\\u003eMelero, J. A., Iglesias, J. \\u0026amp; Morales, G. Heterogeneous acid catalysts for biodiesel production: Current status and future challenges. \\u003cem\\u003eGreen. 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Available: \\u003cspan class=\\\"ExternalRef\\\"\\u003e\\u003cspan class=\\\"RefSource\\\"\\u003ehttps://doi.org/10.3390/chemengineering7030056\\u003c/span\\u003e\\u003cspan address=\\\"10.3390/chemengineering7030056\\\" targettype=\\\"DOI\\\" class=\\\"RefTarget\\\"\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/li\\u003e\\u003c/ol\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":false,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":true,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"scientific-reports\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":false,\"externalIdentity\":\"scirep\",\"sideBox\":\"Learn more about [Scientific Reports](http://www.nature.com/srep/)\",\"snPcode\":\"\",\"submissionUrl\":\"\",\"title\":\"Scientific Reports\",\"twitterHandle\":\"\",\"acdcEnabled\":true,\"dfaEnabled\":true,\"editorialSystem\":\"stoa\",\"reportingPortfolio\":\"Scientific Reports\",\"inReviewEnabled\":true,\"inReviewRevisionsEnabled\":true},\"keywords\":\"Biodiesel Production, APTES-Functionalized Silica, Heterogeneous catalyst, Silica Surface Modification, Sustainable Biofuels\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-6903073/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-6903073/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eThis study works on the synthesis and characterization of an APTES [(3-aminopropyl) tri ethoxy silane] impregnated silica catalyst for the transesterification of vegetable oil into biodiesel. It was aimed at increasing the silica catalytic activity by using different concentrations of APTES as a modifier which would mount the basic amine groups to the silica surface. Synthesis and functionalization of the silica was confirmed through the use of various characterization methods such as SEM (Scanning Electron Microscopy), FTIR (Fourier Transform Infrared), XRD (X-ray diffraction), and BET (Brunauer-Emmett-Teller). Of the four synthesized catalysts, the highest yield of biodiesel which was 62.2 percent was obtained from the catalyst containing 0.75g of APTES used at 65 degrees Celsius for 5 hours with 0.3g catalyst and reaction time. GC (Gas chromatography) showed a promising FAME (fatty acid methyl esters) profile which suggests the FAME would have good properties essential to low temperatures as well as for aiding in the biodiesel cold flow properties. Over 90% of the catalytic activity was retained after three uses. The catalyst maintains more than 90 percent of activity after three uses demonstrating stability and reusability.\\u003c/p\\u003e\\u003cp\\u003eThe silicate catalyst yielded superior results compared to conventional catalysts since it performed better with regard to environmentally friendly approaches, flexibility, and efficiency for large scale production of biodiesel.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Bifunctional Ethoxy-Amine Modified Silica Catalysts for Green Energy: Facilitating High Biodiesel Yield via Enhanced Methanol and Oil Adsorption\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2025-07-15 12:51:22\",\"doi\":\"10.21203/rs.3.rs-6903073/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0},{\"type\":\"decision\",\"content\":\"Revision 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