Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash

Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash | 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 Research Article Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash Xianglian Wu, Aisha Nulahong, Changmin Tuo, Jian Li, Fei Xu, Tiezhen Ren, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4846209/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 04 Mar, 2025 Read the published version in Silicon → Version 1 posted 7 You are reading this latest preprint version Abstract Solid waste fly ash has a great impact on the environment and pollutes water, air and soil to different degrees. Therefore, it is of great significance to realize the high-value utilization of fly ash. In this paper, high purity SiO 2 was successfully extracted from fly ash by high temperature calcination, alkali fusion activation and pickling, and then different morphologies of Silicalite-1 zeolites were synthesized by using SiO 2 extracted from fly ash as silicon source, TPAOH as templating agent and NaOH as alkali source. The influencing factors such as crystallization time, crystallization temperature, NaOH dosage, TPAOH dosage and different hydrosilica ratios were investigated separately. The results show that under the conditions of crystallization time of 12 h, crystallization temperature of 180 ℃ and molar ratio of water-silicon of 42, the prepared P-S (plate-like Silicalite-1) has good morphology, high relative crystallinity and adsorption capacity of CO 2 of 1.89 mmol/g. The CO 2 adsorption capacity of the S-S (spherical-like Silicalite-1) prepared by adjusting the amount of TPAOH is 1.34 mmol/g. The CO 2 adsorption capacity of the prepared C-S (cross-type Na-Silicalite-1) is only 1.06 mmol/g. The CO 2 adsorption capacities follow P-S > S-S > C-S. The results may provide a valuable reference for zeolite-based adsorbents in the adsorption removal or recovery of CO 2 . Fly ash SiO2 Silicalite-1 Morphology CO2 adsorption 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 Figure 15 1. Introduction Fly ash, as the main solid waste of thermal power plants [ 1 ] , can produce about 250 ~ 350 kg of fly ash for every 1 t of raw coal burned [ 2 ] . The massive accumulation of fly ash not only encroaches on land resources, but also pollutes the environment [ 3 ] and poses a threat to human health [ 4 ] , so it is of great significance for the high-value utilization of fly ash [ 5 ] . The main oxide compositions of fly ash are SiO 2 and A1 2 O 3 , and the structure of both exists mainly in crystalline phase [ 6 ] . Fly ash rich in Si and AI has the same chemical composition as zeolite [ 7 ] . Zeolite molecular sieves are silicate inorganic minerals with skeletal elements consisting of silicon, aluminum and their coordinating atoms [ 8 ] . It is characterized by the [SiO 4 ] −4 tetrahedron and [AlO 4 ] −5 tetrahedron constituting the primary structural unit. Various skeletal structures are formed by interconnecting shared oxygen atoms [ 9 ] . This unique internal structure and physicochemical properties make zeolite molecular sieves have broad application prospects in the fields of wastewater treatment, exhaust gas adsorption and industrial catalysis [ 10 ] . With the increasing demand for zeolite molecular sieve, more and more attention has been paid to the preparation of low-cost molecular sieve using fly ash as raw material [ 11 ] . As one of the global environmental problems, the “greenhouse effect” caused by the excessive use of fossil fuels has attracted the attention of various countries [ 12 ] . As the source of the “greenhouse effect”, CO 2 capture and recovery has become the most promising method to solve the greenhouse effect at this stage [ 13 ] . Fly ash has a high silica and aluminum content, and can be processed under certain conditions to obtain zeolite molecular sieves, which can be used to adsorb pollutants in water and capture CO 2 in the air [ 14 ] . Silicalite-1 molecular sieve is an all-silicon molecular sieve without aluminum element, with good hydrophobicity and CO 2 adsorption and separation properties [ 15 ] . Wang [ 16 ] prepared microporous and mesoporous Silicalite-1 molecular sieves and investigated their adsorption and separation properties for CO 2 /N 2 /CH 4 /C 2 H 6, and the results demonstrated that microporous Silicalite − 1 has high adsorption capacity for CH 4 , N 2 , CO 2 and C 2 H 6 , while the selectivity of mesoporous Silicalite-1 is relatively high.Wang [ 17 ] synthesized ZSM-5 and Silicalite-1 with rice husk ash as silicon source and studied their adsorption properties for CO 2 under humid conditions, the adsorption capacities are 81.69cm 3 /g and 69.96cm 3 /g. Razavian [ 18 ] prepared Silicalite-1 by ultrasonic method and hydrothermal method respectively, and compared their CO 2 /CH 4 adsorption properties. The results showed that the ultrasonic treated Silicalite-1 had high selectivity, while the hydrothermal method Silicalite-1 had high adsorption capacity. Therefore, fly ash can be used to prepare Silicalite-1 with high specific surface area and good adsorption performance for CO 2 [ 19 ] . The morphology of MFI zeolite is closely related to their structure, micropore size and crystal size, which have a significant role in adsorption performance [ 20 ] . The difference in the preferred orientation of pore channels will affect the morphology and diffusion efficiency [ 21 , 22 ] . Chen et al. [ 23 ] investigated the adsorption properties of toluene on Silicalite-1 zeolites with different morphologies and found that morphologies have significant and different effects on toluene adsorption under dry or wet conditions. Under dry conditions (303 K), the dynamic adsorption capacity followed S-S (spherical Silicalite-1) > P-S (plate-like Silicalite-1) > B-S (brick-like Silicalite-1). Under humid conditions (RH = 50%, 303 K), the dynamic adsorption capacity was in the order of P-S > S-S > B-S.Liu et al [ 24 ] found that the particle size of zeolite molecular sieves could be varied by the length of the b-axis to adjust the distribution of aromatic hydrocarbons. Wang et al. [ 25 ] compared the catalytic performance of nano-scale and micron-scale ZSM-5 in different reactions (disproportionation of toluene, alkylation of toluene with methanol and trimethylbenzene cracking) and found that nano-scale ZSM-5 exhibited excellent reactivity due to increased pore openings and high accessibility of acid sites. Wang et al. [ 26 ] synthesized straight channel-covered hemispherical bicrystalline zeolites and obtained p-xylene with greater than 99% selectivity by optimizing shape-selective catalysis of toluene in sinusoidal channels. Based on the advantages of molecular sieve morphology, grain size and adsorbability, the effect of MFI zeolite morphology on CO 2 adsorption removal was proposed, which has been rarely reported so far. In order to promote the high-value utilization of fly ash and reduce the production cost of molecular sieves, in this work, high purity SiO 2 was successfully extracted from fly ash by high-temperature calcination, alkali fusion activation and acid leaching, and then Silicalite-1 zeolites with three different morphologies (plate-like, spherical-like and cross-type), were synthesized using SiO 2 as the silica source, and then used for the adsorption of CO 2 . Finally, the purity, morphology, pore size and CO 2 adsorption performance of the Silicalite-1 zeolites with different morphologies were determined by characterization means such as XRD, SEM, FT-IR and BET. 2. Experiment Materials and Methods 2.1 Materials and reagents Fly ash, purchased from Xinjiang Changji thermal power plant. Hydrochloric acid (HCl, 38%), sodium carbonate (Na 2 CO 3 ), tetrapropyl ammonium hydroxide (TPAOH, 25%) and sodium hydroxide (NaOH) were all analytically pure, and were acquired from Sinopharm Group Chemical reagent Co, LTD. Deionized water for homemade. 2.2 Sample preparation 2.2.1 Extraction of SiO 2 from fly ash The fly ash was placed in a box-type resistance furnace and roasted at 600 ℃ for 2 h to remove the unburned carbon. After cooling to room temperature, the fly ash was activated with Na 2 CO 3 as the activator. Fly ash and Na 2 CO 3 were mixed according to the mass ratio of 1:1 and calcined at 850 ℃ for 100 min in the box resistance furnace, and the calcined clinker was obtained by thorough grinding after cooling. The calcined clinker was stirred with 5 mol/L HCl according to the solid-liquid ratio of 1 g :10 mL on a magnetic stirrer for 4 h, and then filtered, washed with distilled water until neutral and dry, and SiO 2 is obtained. 2.2.2 Preparation of P-S (plate-like Silicalite-1) P-S were prepared by using SiO 2 extracted from fly ash as the silica source, TPAOH as the template agent and distilled water as the solvent. Weighing a certain mass of deionized water, TPAOH and the prepared SiO 2 at room temperature according to a certain molar ratio on a magnetic stirrer after stirring vigorously for 4 h, and then the mixture was transferred to a 100 mL stainless steel reactor PTFE liner, and then placed into the oven has been preheated, crystallization at a certain temperature for a certain period of time. After crystallization is completed, take out of the reactor, cool the product, filter, wash to neutral, dry and grind. Finally, the product was roasted in a muffle furnace at 550 ℃ for 6 h, so as to remove the template agent and obtain the P-S (plate-like Silicalite-1). 2.2.3 Preparation of S-S (spherical-like Silicalite-1) S-S were prepared by adjusting the dosage of TPAOH with SiO 2 extracted from fly ash as the silica source, TPAOH as the templating agent and distilled water as the solvent, keeping the crystallization time, crystallization temperature and water-to-silica ratio certain. The specific steps were the same as 2.2.2. 2.2.4 Preparation of C-S (cross-type Na-Silicalite-1) C-S was prepared with SiO 2 extracted from fly ash as silicon source, TPAOH as template agent, NaOH as base source and distilled water as solvent. The specific steps were the same as 2.2.2. 2.3 Characterization X-ray powder diffraction test (XRD): The D/max-2400 X-ray diffractometer of Rigaku Company in Japan, the radiation source is Cu target Kα radiation, the scanning speed is 10°/min, the scanning range is 5°-80°. X-ray fluorescence analyzer (XRF): SRF3400 X-ray fluorescence spectrometer from Bruker, Germany; Scanning electron microscope (SEM): Beijing KYKY-2800B. FT-IR: IFS88 from Bruker, Germany, with a range of 700–4000 cm − 1 . Specific surface area test (BET): Micrometrics Tristar II instrument was used to analyze the specific surface area, pore structure, and nitrogen adsorption and desorption curves of the catalysts. The samples were degassed under vacuum at 200°C for 8 h prior to the test, and the N 2 adsorption and desorption tests were performed on the samples under 77 K liquid nitrogen. The samples were purged at 300 ℃ for 12 h and then tested for CO 2 adsorption at 25 ℃. 3. Results and discussion 3.1 Investigation of the synthesis conditions of P-S 3.1.1 Effect of different template agents The type of template agent also affects the synthesized Silicalite-1 molecular sieve, and TPABr and TPAOH are the most commonly used template agents. Under the same experimental conditions, TPABr and TPAOH were used as templates to investigate the effects of different TPA + on the preparation of Silicalite-1 molecular sieve. As can be seen from Fig. 1, the sample with TPAOH as the template agent has an obvious characteristic peak of Silicalite-1 molecular sieve. However, when TPABr is used as the template agent, only amorphous SiO 2 diffraction peaks appear, indicating that Silicalite-1 molecular sieve cannot be successfully synthesized when only TPABr is used as the template agent. At this time, Br - in the system can only balance charge and cannot promote silicate rearrangement to form Silicalite-1 molecular sieve crystals. When TPAOH is the template agent, the reaction process contains the appearance of OH - , which provides the base source for the synthesis environment of the sample and balances the negative charge at the same time, playing a structure-oriented role, so the Silicalite-1 molecular sieve can be successfully synthesized. Figure 2.is the SEM image of Silicalite-1 zeolite synthesized under different template agents. It can also be seen from the figure that when the template agent is TPAOH, the crystalline phase structure of Silicalite-1 zeolite with smooth surface, regular morphology and uniform size appears, and the grain size is between 6–8 µm. When TPABr was used as template, only amorphous phase structure appeared. Therefore, TPAOH was selected as the template agent to synthesize Silicalite-1 zeolite. 3.1.2 Effect of crystallization time Figure 3.shows the XRD (a) and crystallinity (b) diagrams of P-S synthesized with different crystallization times, as shown in the figure: when the crystallization time is 2 h, it can be clearly seen that the characteristic peaks belonging to MFI-type molecular sieves appeared in the 2θ of 7.94 °, 8.87 °, 23.08 °, 23.31 ° and 23.96 °, but with lower peak heights and weaker peak strengths. The initial induction period of ZSM-5 zeolite molecular sieve, which is also of MFI type, is about 5 h without the addition of crystal seed, and Silicalite-1 has entered the crystal growth stage at 2 h, which is a shorter induction period than that of ZSM-5. This may be due to the fact that there is no involvement of aluminate in the crystallization process of all-silica molecular sieves, thus shortening the crystallization time. With the prolongation of the crystallization time, the intensity of the characteristic diffraction peaks was gradually enhanced, indicating that more and more amorphous silicate sols have begun to transform to Silicalite-1 zeolite molecular sieve crystals, and the intensity of the characteristic diffraction peaks of Silicalite-1 zeolite gradually showed a downward trend when the crystallization time was greater than 12 h, which may be attributed to the fact that the crystallization time is too long, the crystals reacted with solution, making the crystals grow excessively. This may be due to the long crystallization time, the reaction between the crystals and the solution, which makes the crystals grow excessively and thus transcrystallization phenomenon occurs, which reduces its crystallinity [ 27 ] . As can be seen from Fig. (b), the crystallinity of the samples reached the highest when the crystallization time was 12 h, and the crystallinity of the samples gradually decreased when the time was extended. Figure 4.is the SEM diagram of P-S synthesized under different crystallization times. As can be seen from the diagram, when the crystallization time is 2 h, there is no obvious regular crystal phase structure, which is due to the short crystallization time so that the basic molecular sieve skeleton cannot be formed. When the crystallization time is 7 h, a very small amount of P-S crystal phase structure appears, and the surface is smooth and flat, the grain size is between 4–6 µm, but still dominated by large amorphous particles.When the crystallization time was extended to 12 h, regular and dispersed plate crystals appeared with good morphology and uniform size, and the grain size was between 6–8 µm. However, with the continuous extension of crystallization time, the molecular sieve appeared obvious agglomeration and fracture phenomenon. When the crystallization time is 24 h, almost no single crystal structure can be found, and all are large agglomerated crystals. When the crystallization time is 36 h, P-S crystals not only agglomerate, but also fracture, with a large number of broken crystals attached to the surface of the molecular sieve. The results indicated that the long crystallization time was not conducive to the synthesis of P-S, but would promote the target product to continue to react with the solution, resulting in crystallization transformation.Therefore, based on the above analysis, 12 h is selected as the best crystallization time. 3.1.3 Effect of crystallization temperature A higher crystallization temperature may shorten the induction period and promote the growth of the molecular sieve crystals, because the higher temperature helps rearrangement between atoms of the substance and promotes the formation of nuclei and the growth of crystals. However, when the crystallization temperature is too high, it may lead to other side reactions of the formed crystals, such as crystal transformation or crystal dissolution. When the crystallization temperature is low, it is not conducive to the atomic activity and rearrangement between the reactive substances, resulting in a decrease in the reaction rate [ 28 ] . Figure 5.shows the XRD (a) and crystallinity (b) plots of P-S synthesized at different temperatures. From the figure, it can be seen that the products at five crystallization temperatures all showed the characteristic peaks of MFI molecular sieves between 2θ of 7.94 °, 8.87 °, 23.08 °, 23.31 ° and 23.96 °. When the crystallization temperature was 150 ℃, the characteristic diffraction peak strength of Silicalite-1 zeolite was weaker, which indicated that only the basic skeleton structure could be formed at 150 ℃. With the continuous increase of the crystallization temperature, the peak area of the characteristic peaks of Silicalite-1 gradually becomes higher, and when the temperature reaches 180 ℃, the characteristic diffraction peak intensity of Silicalite-1 reaches the highest. As the crystallization temperature continues to increase, the characteristic diffraction peak intensity decreases, which may be due to the fact that the crystallization temperature is too high, which makes the crystal transcrystallization or rupture phenomenon, thus leading to the decrease in the intensity of the characteristic peaks and the degree of crystallinity. Figure 6.shows the SEM images of P-S synthesis at different crystallization temperatures. It can be seen from the figure that when the crystallization temperature is 150 ℃, although a small amount of plate crystalline phase structure appears, it still contains a large amount of bulk amorphous material. When the crystallization temperature is 160 ℃, the crystallization is relatively complete and more uniform plate structure appears, but a large number of small particles are still gathered on the crystal surface, which may be due to the low temperature and the adhesion of SiO 2 which is not involved in crystallization to the crystal surface. When the crystallization temperature is 170 ℃, the small particles gathered on the surface of Silicalite-1 crystals decrease, indicating that the unreacted amorphous SiO 2 has begun to transform into Silicalite-1 crystals. When the crystallization temperature is 180 ℃, the P-S crystal structure with uniform size and smooth surface appears. When the crystallization temperature was further extended to 190 ℃, cracks appeared on the surface of some samples and a large number of samples were agglomerated. According to the above analysis, either too high or too low temperature is not conducive to the synthesis of P-S, so 180℃ is chosen as the best synthesis temperature. 3.1.4 Effect of water amount Water is not only a solvent in the synthesis of molecular sieve, but also has an important impact on the composition and structure of silicate gels as well as the formation and growth of crystal nuclei by changing the concentration of silicate ions in the solution and adjusting the alkalinity [ 29 ] . Figure 7.shows the XRD pattern and crystallinity of P-S synthesized under different water amounts. As can be seen from the figure, pure phase Silicalite-1 zeolite can be synthesized within the molar ratio of H 2 O/SiO 2 of 21 ~ 49, and no other crystalline heterogeneous peaks appeared. When H 2 O/SiO 2 = 21, the characteristic peak-to-peak strength of Silicalite-1 molecular sieve is low, which may be due to the large silicate concentration, incomplete crystallization and low crystallinity caused by too low water content. The crystallinity gradually increased with the increase of water volume, and the relative crystallinity reached the optimum when the molar ratio of H 2 O/SiO 2 was 42. While the water-silicon ratio continues to increase to 49, the relative crystallinity begins to decrease, which may be due to too much water, low concentration of silicate, and the existence of a large number of incomplete crystallized silica, resulting in a decrease in crystallinity. Figure 8.is the SEM photo of P-S synthesized with different water-silicon ratios. It can be seen from the SEM image that when the molar ratio of H 2 O/SiO 2 is 21, although there is a small amount of Silicalite-1 crystals, there are still a large number of amorphous substances attached to the crystal surface. With the increase of the water-silicon ratio, the amorphous material gradually decreases. When the molar ratio of H 2 O/SiO 2 is 42, the crystal with smooth surface and uniform particle size is between 6–8 µm. Continue to increase the water-silicon ratio to 49, at this time, a large number of incomplete crystallization substances appear on the crystal surface again, indicating that too much water will lead to a relatively low concentration of silicate, making a large amount of SiO 2 is not fully crystallized. Based on the above analysis, the water-silicon ratio 42 is selected as the best water-silicon ratio. 3.2 Investigation of the synthesis conditions of S-S 3.2.1 Influence of TPAOH dosage During the crystallization process of molecular sieves, the template agent will affect the interaction of silicaluminate, which have an impact on the gelation and nucleation process. Organic template agents are expensive, and their dosage also determines the cost of molecular sieve production. Figure 9 shows the XRD (a) and relative crystallization (b) of Silicalite-1 zeolites synthesized under different TPA + /SiO 2 molar ratio. It can be seen from the figure: When the molar ratio is 0.01, there is no characteristic peak belonging to MFI zeolite molecular sieve, indicating that the guiding effect is weak when the dosage of template agent is small, and it cannot provide structural guiding effect for amorphous SiO2 in the system [ 30 ] . With the increase of the amount of template agent, within a certain range, the characteristic diffraction peaks and peaks of MFI zeolites are strong, and the relative crystallinity of Silicalite-1 is also high, indicating that TPAOH as the template agent can be synthesized within a certain range of Silicalite-1 with high crystallinity and single crystalline phase. When the molar ratio of TPAOH/SiO 2 is 0.28, the relative crystallinity reaches the maximum. The relative crystallinity decreased slightly with increasing the amount of TPAOH. It may be because the concentration of the template agent is too high, which will lead to excessive growth or aggregation of the crystal, thus affecting the relative crystallinity. Figure 10.is the SEM diagram of Silicalite-1 molecular sieve with different dosage of template agent. According to the diagram, when the dosage of template agent is too low, it cannot fully polymerize with silicate to form gel, resulting in the failure to form crystal nucleus. With the increase of the concentration of template agent, TPA + can bind to silicate ions and promote the formation of silicate gel. Silica tetrahedrons tend to form pores or cages on the gel surface, increasing the number of crystal nuclei [ 31 ] . When the molar ratio of TPAOH/SiO 2 increases from 0.07 to 0.28, the particle size becomes smaller and smaller. When the molar ratio is 0.28, spherical-like crystals with smooth surface and uniform size appear.When the molar ratio of TPAOH/SiO 2 is greater than 0.28, the dosage of template agent exceeds a certain threshold, and the contribution to molecular sieve synthesis is reduced. This is due to the large size of TPA + , which may prevent silicon species from entering the molecular sieve skeleton [ 32 ] , and affect the formation of molecular sieve skeleton to a certain extent, resulting in larger molecular sieve particle size. Based on the above analysis, a molar ratio of TPAOH/SiO 2 of 0.28 is the best choice for the synthesis of S-S. 3.3 Investigation of the synthesis conditions of C-S 3.3.1 Influence of NaOH dosage The amount of NaOH has a great influence on the synthesis of Silicalite-1 zeolite, which can adjust the pH value of the reaction medium, thus affecting the reaction rate and the crystal structure of the product. Alkaline environment can promote the reaction to a certain extent, but too high alkalinity may cause side reactions or affect the crystal morphology of the product [ 33 ] . Figure 11.shows the XRD (a) and relative crystallinity (b) of Silicalite-1 molecular sieve with different amounts of NaOH. It can be seen from the figure that the synthesized zeolites at different proportions all show the standard MFI structure characteristic diffraction peaks at 2θ of 7.94 °, 8.87 °, 23.08 °, 23.31 ° and 23.96 °, and the crystallinity of the five products does not change significantly, indicating that Na-Silicalite-1 zeolite with high crystallinity can be synthesized in a certain range when SiO 2 extracted from fly ash is used as the silicon source. Figure 12.shows the SEM images of Silicalite-1 zeolites at different NaOH dosages, from which it can be seen that the grain size decreases gradually with the increase of NaOH dosage. When the molar ratio of NaOH/SiO 2 is 0.011, there are plate-like structures of different sizes in the system, when the molar ratio of NaOH/SiO 2 is 0.055, at this time, there are not only plate-like structures in the system, but also smaller spherical-like structures, and when the molar ratio of NaOH/SiO 2 is 0.11, the crystalline structure of the system at this time becomes cross-type. When the molar ratio of NaOH/SiO 2 is 0.165, at this time it is a spherical-like crystal structure with uniform size. Continue to increase the dosage of NaOH to the molar ratio of NaOH/SiO 2 is 0.22, at this time, the solution is more alkaline, the surface of the molecular sieve is etched, and it becomes similar to the amorphous material morphology. 3.4 Characterization of different morphologies of Silicalite-1 zeolites and their CO 2 adsorption properties 3.4.1 Nitrogen adsorption-desorption Figure 13.shows the nitrogen adsorption-desorption curves and pore size distribution of Silicalite-1 zeolites with different morphologies. As can be seen from the figure, both the P-S and S-S have no obvious hysteresis loops and are type I isotherms. The main performance is: the adsorption capacity rises rapidly under low relative pressure, and the adsorption saturation occurs after reaching a certain relative pressure. This is because in the narrow micropores, the adsorbent-adsorbent interaction is enhanced, which leads to the micropores being filled rapidly at very low relative pressure, but when the saturation pressure is reached, the adsorbent condenses, resulting in the curve beginning to flatline.However, C-S has an insignificant hysteresis ring, and the adsorption curve is a composite isotherm with type I as the main and type IV as the auxiliary. According to the analysis of pore size distribution by BJH theory, it can be seen that the pore size distribution of Silicalite-1 zeolites with three kinds of morphology are mainly concentrated in the range of 1 nm ~ 4 nm, which mainly exists in the form of micropores and has relatively narrow mesoporous pores. Table 1 shows the pore structure parameters of Silicalite-1 zeolites with different morphologies, from the table, it can be seen that the specific surface area of S-S is 460.24 m 2 /g, of which the specific surface area of the micropores is 275.25 cm 2 /g, and the total pore volume is 0.365 cm 3 /g, of which the microporous pore volume is 0.141 cm 3 /g, which indicates that the prepared P-S have high specific surface area and pore volume. Not only that, the P-S have larger specific surface area, total pore volume and microporous pore volume than the other two morphologies of Silicalite-1. The specific surface area of S-S is much lower than that of P-S and C-S, except for the specific surface area, the rest of the parameters of S-S and C-S are not much different. Table 1 Pore structure parameters of Silicalite-1 zeolites with different morphologies: (a) P-S; (b) S-S; and (c) C-S. S BET (m 2 /g) S micro (m 2 /g) V total (cm 3 /g) V micro (cm 3 /g) Average pore size/nm a 460.24 275.25 0.365 0.141 3.16 b 413.83 304.77 0.183 0.120 1.76 c 454.58 298.95 0.201 0.119 1.771 3.4.2 FT-IR analysis Figure 14.shows the FT-IR spectra of Silicalite-1 zeolites with different morphologies. It can be seen from the figure that all the three morphologies of Silicalite-1 have skeleton vibration peaks belonging to MFI-type molecular sieves at 1230, 1080 and 800 cm − 1 , among which the peak at 1230 cm − 1 is attributed to the tensile vibration of the five-membered ring unique to MFI, the peak at 1080 cm − 1 is attributed to the asymmetric telescopic vibration of the Si-O-Si external connection, and the peak at 800 cm − 1 is attributed to the telescopic vibration of Si-OH. and the 800 cm − 1 peak is attributed to the stretching vibration of Si-OH. However, the FT-IR spectra of C-S did not show any obvious Na-O peaks, probably due to less Na in the skeleton, and thus the absorption vibration peaks were not obvious. 3.4.3 CO 2 adsorption isotherm Figure 15.shows the CO 2 adsorption isotherms of Silicalite-1 zeolites with different morphologies. It can be seen from the figure that both P-S and S-S overlapped the adsorption curve and desorption curve of CO 2 , indicating that their adsorption properties for CO 2 are reversible and mainly dominated by physical adsorption. In the low pressure region, the adsorption capacity rises rapidly, indicating that there is strong adsorption in the low pressure region, and this adsorption process is related to the large number of micropores in the sample.In the medium pressure section, the adsorption curve rose slowly, indicating that the mesoporous content of the sample was low. When the adsorption pressure is increased to 101.325KPa, the CO 2 adsorption capacity of the prepared P-S is 1.89mmol /g, indicating that it can be used as an excellent CO 2 adsorption material. The adsorption capacity of S-S for CO 2 is 1.34mmol /g, which is much lower than that of P-S.This may be due to the fact that P-S has a larger specific surface area, total pore volume and microporous pore volume than the remaining two morphologies of Silicalite-1. In addition, it can be seen from the figure that the adsorption curve and desorption curve of C-S for CO 2 do not completely coincide, indicating that the adsorption performance of C-S for CO 2 is partially irreversible. Although it is mainly physical adsorption, there is still a certain amount of chemical adsorption due to the introduction of alkaline metals. Under the test condition of 25 ℃, when the adsorption pressure was increased to 101.325 KPa, the adsorption capacity of C-S for CO 2 was 1.06 mmol / g. Although a small amount of chemical adsorption phenomenon appeared in C-S, its adsorption performance was much lower than that of P-S and S-S,which could be attributed to the decrease of CO 2 adsorption capacity due to the reduction of the specific surface area and the micropore of molecular sieves by adding inorganic alkali. Among the three morphologies of Silicalite-1 zeolites, the adsorption effect of P-S was better due to the slightly larger specific surface area compared with the others, so it can be hypothesized that the CO 2 adsorption performance is mainly related to the specific surface area of the molecular sieves. 4. conclusion Silicalite-1 zeolite molecular sieves with different morphologies were prepared using SiO 2 extracted from fly ash as silicon source. The results show that under the conditions of crystallization time of 12 h, crystallization temperature of 180 ℃ and molar ratio of H 2 O/SiO 2 of 42, the P-S prepared has good morphology, uniform size and high relative crystallinity. The specific surface area is 460.24 m 2 /g, the specific surface area of the micropores is 275.25 cm 2 /g, and the total pore volume is 0.365 cm 3 /g, among which the micropore volume is 0.141 cm 3 /g, indicating that the prepared P-S has a high specific surface area and pore volume. S-S was prepared by adjusting the amount of TPAOH. When TPAOH/SiO 2 = 0.28, the prepared S-S had better morphology and higher relative crystallinity. The specific surface area is 413.83 m 2 /g, the specific surface area of the micropores is 304.77 cm 2 /g, the total pore volume is 0.183 cm 3 /g, and the micropore volume is 0.120 cm 3 /g. When preparing Na-Silicalite-1 molecular sieve, it is found that when NaOH/SiO 2 = 0.11, C-S can be synthesized. The specific surface area is 454.58 m 2 /g, the specific surface area of the micropores is 298.95 cm 2 /g, the total pore volume is 0.201 cm 3 /g, and the micropore volume is 0.119 cm 3 /g. Under the test conditions of 298 K and 1bar, the adsorption capacity of P-S for CO 2 is 1.89 mmol/g, and that of S-S for CO 2 is 1.34 mmol/g, while the adsorption capacity of C-S for CO 2 is only 1.06 mmol/g, and the adsorption capacity of CO 2 is P-S > S-S > C-S. Declarations Author Contribution Wu Xianglian wrote the main manuscript text Aisha Nulahong was responsible for overseeing the experimentChangmin Tuo,Tiezhen Ren,Abulikemu Abulizi ,Jian Li and Fei Xu Data Availability Data is provided within the manuscript . Fund Project: Supported by the 2022 Xinjiang Uygur Autonomous Region Natural Science Foundation Project (Joint Fund) (2002D01C378) Declaration of Interest Statement ： There are no conflicts of interest to declare.All authors disclosed no relevant relationships. 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Supplementary Files GraphicalAbstract.docx Highlights.docx Cite Share Download PDF Status: Published Journal Publication published 04 Mar, 2025 Read the published version in Silicon → Version 1 posted Editorial decision: Revision requested 08 Sep, 2024 Reviews received at journal 08 Sep, 2024 Reviewers agreed at journal 29 Aug, 2024 Reviewers invited by journal 26 Aug, 2024 Editor assigned by journal 16 Aug, 2024 Submission checks completed at journal 16 Aug, 2024 First submitted to journal 02 Aug, 2024 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-4846209","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":351190557,"identity":"8a1a6ad0-1140-4f94-b828-add1c10c57eb","order_by":0,"name":"Xianglian Wu","email":"","orcid":"","institution":"Xinjiang University","correspondingAuthor":false,"prefix":"","firstName":"Xianglian","middleName":"","lastName":"Wu","suffix":""},{"id":351190558,"identity":"505cdb5c-ae3d-4d53-af04-0385c00d2e6f","order_by":1,"name":"Aisha 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crystallinity pattern (b) of Silicalite-1 molecular sieve with different dosage of template agent\u003c/p\u003e","description":"","filename":"9.png","url":"https://assets-eu.researchsquare.com/files/rs-4846209/v1/f315dc14b6fdbd37ba97a572.png"},{"id":64382988,"identity":"d262d4e7-0911-4c9b-96ca-471f08e94d7a","added_by":"auto","created_at":"2024-09-12 12:03:59","extension":"png","order_by":10,"title":"Figure 10","display":"","copyAsset":false,"role":"figure","size":1721823,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of Silicalite-1 molecular sieve with different amounts of template agent\u003c/p\u003e","description":"","filename":"10.png","url":"https://assets-eu.researchsquare.com/files/rs-4846209/v1/5716e71d849ca2320572b122.png"},{"id":64382987,"identity":"8565d7a7-4976-44fb-bbc3-7a596dd15668","added_by":"auto","created_at":"2024-09-12 12:03:59","extension":"png","order_by":11,"title":"Figure 11","display":"","copyAsset":false,"role":"figure","size":119378,"visible":true,"origin":"","legend":"\u003cp\u003eXRD plots (a) and relative crystallinity (b) of Silicalite-1 zeolites synthesized with different NaOH dosages\u003c/p\u003e","description":"","filename":"11.png","url":"https://assets-eu.researchsquare.com/files/rs-4846209/v1/6f87200206ed881451f54c3e.png"},{"id":64383594,"identity":"42abf0c3-2468-4b44-9dd5-02b851ebff72","added_by":"auto","created_at":"2024-09-12 12:11:58","extension":"png","order_by":12,"title":"Figure 12","display":"","copyAsset":false,"role":"figure","size":1235793,"visible":true,"origin":"","legend":"\u003cp\u003eSEM image of Na-Silicalite-1 zeolites with different amounts of NaOH\u003c/p\u003e","description":"","filename":"12.png","url":"https://assets-eu.researchsquare.com/files/rs-4846209/v1/eb0ca3d10e574ab93f7bf338.png"},{"id":64382990,"identity":"d8b5eda7-8e77-47bb-91cc-48cdd251f6f2","added_by":"auto","created_at":"2024-09-12 12:03:59","extension":"png","order_by":13,"title":"Figure 13","display":"","copyAsset":false,"role":"figure","size":151903,"visible":true,"origin":"","legend":"\u003cp\u003eNitrogen adsorption-desorption diagrams and pore size distributions of Silicalite-1 zeolites with different morphologies: (a) P-S; (b) S-S; and (c) C-S.\u003c/p\u003e","description":"","filename":"13.png","url":"https://assets-eu.researchsquare.com/files/rs-4846209/v1/f3685b4236f063cf87b4defc.png"},{"id":64384467,"identity":"68b124db-05eb-4aca-9b42-dee222c6343c","added_by":"auto","created_at":"2024-09-12 12:19:58","extension":"png","order_by":14,"title":"Figure 14","display":"","copyAsset":false,"role":"figure","size":39818,"visible":true,"origin":"","legend":"\u003cp\u003eFT-IR spectra of Silicalite-1 zeolites with different morphologies: (a) P-S; (b) S-S; and (c) C-S.\u003c/p\u003e","description":"","filename":"14.png","url":"https://assets-eu.researchsquare.com/files/rs-4846209/v1/2cf01aec779630743a34b87d.png"},{"id":64382986,"identity":"24999175-8e54-4fee-b2f5-fa5199882fd3","added_by":"auto","created_at":"2024-09-12 12:03:58","extension":"png","order_by":15,"title":"Figure 15","display":"","copyAsset":false,"role":"figure","size":126570,"visible":true,"origin":"","legend":"\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e adsorption isotherms of Silicalite-1 zeolites with different morphologies： (a) P-S; (b) S-S; and (c) C-S.\u003c/p\u003e","description":"","filename":"15.png","url":"https://assets-eu.researchsquare.com/files/rs-4846209/v1/055afa521f57a8135cc85a97.png"},{"id":78181494,"identity":"0b8508d8-2f79-447e-a90d-6f7c5f98b6a7","added_by":"auto","created_at":"2025-03-10 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12:11:58","extension":"docx","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":12631,"visible":true,"origin":"","legend":"","description":"","filename":"Highlights.docx","url":"https://assets-eu.researchsquare.com/files/rs-4846209/v1/876fa482dbbb25c25e691be9.docx"}],"financialInterests":"No competing interests reported.","formattedTitle":"Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash","fulltext":[{"header":"1. Introduction","content":"\u003cp\u003eFly ash, as the main solid waste of thermal power plants\u003csup\u003e[\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]\u003c/sup\u003e, can produce about 250\u0026thinsp;~\u0026thinsp;350 kg of fly ash for every 1 t of raw coal burned\u003csup\u003e[\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]\u003c/sup\u003e. The massive accumulation of fly ash not only encroaches on land resources, but also pollutes the environment\u003csup\u003e[\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]\u003c/sup\u003e and poses a threat to human health\u003csup\u003e[\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]\u003c/sup\u003e, so it is of great significance for the high-value utilization of fly ash\u003csup\u003e[\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]\u003c/sup\u003e. The main oxide compositions of fly ash are SiO\u003csub\u003e2\u003c/sub\u003e and A1\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e, and the structure of both exists mainly in crystalline phase\u003csup\u003e[\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]\u003c/sup\u003e. Fly ash rich in Si and AI has the same chemical composition as zeolite\u003csup\u003e[\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]\u003c/sup\u003e. Zeolite molecular sieves are silicate inorganic minerals with skeletal elements consisting of silicon, aluminum and their coordinating atoms\u003csup\u003e[\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]\u003c/sup\u003e. It is characterized by the [SiO\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026minus;4\u003c/sup\u003e tetrahedron and [AlO\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026minus;5\u003c/sup\u003e tetrahedron constituting the primary structural unit. Various skeletal structures are formed by interconnecting shared oxygen atoms\u003csup\u003e[\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e]\u003c/sup\u003e. This unique internal structure and physicochemical properties make zeolite molecular sieves have broad application prospects in the fields of wastewater treatment, exhaust gas adsorption and industrial catalysis \u003csup\u003e[\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]\u003c/sup\u003e. With the increasing demand for zeolite molecular sieve, more and more attention has been paid to the preparation of low-cost molecular sieve using fly ash as raw material\u003csup\u003e[\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eAs one of the global environmental problems, the \u0026ldquo;greenhouse effect\u0026rdquo; caused by the excessive use of fossil fuels has attracted the attention of various countries\u003csup\u003e[\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e]\u003c/sup\u003e. As the source of the \u0026ldquo;greenhouse effect\u0026rdquo;, CO\u003csub\u003e2\u003c/sub\u003e capture and recovery has become the most promising method to solve the greenhouse effect at this stage\u003csup\u003e[\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]\u003c/sup\u003e. Fly ash has a high silica and aluminum content, and can be processed under certain conditions to obtain zeolite molecular sieves, which can be used to adsorb pollutants in water and capture CO\u003csub\u003e2\u003c/sub\u003e in the air\u003csup\u003e[\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e]\u003c/sup\u003e. Silicalite-1 molecular sieve is an all-silicon molecular sieve without aluminum element, with good hydrophobicity and CO\u003csub\u003e2\u003c/sub\u003e adsorption and separation properties\u003csup\u003e[\u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]\u003c/sup\u003e. Wang\u003csup\u003e[\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e]\u003c/sup\u003e prepared microporous and mesoporous Silicalite-1 molecular sieves and investigated their adsorption and separation properties for CO\u003csub\u003e2\u003c/sub\u003e/N\u003csub\u003e2\u003c/sub\u003e/CH\u003csub\u003e4\u003c/sub\u003e/C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6,\u003c/sub\u003e and the results demonstrated that microporous Silicalite \u0026minus;\u0026thinsp;1 has high adsorption capacity for CH\u003csub\u003e4\u003c/sub\u003e, N\u003csub\u003e2\u003c/sub\u003e, CO\u003csub\u003e2\u003c/sub\u003e and C\u003csub\u003e2\u003c/sub\u003eH\u003csub\u003e6\u003c/sub\u003e, while the selectivity of mesoporous Silicalite-1 is relatively high.Wang\u003csup\u003e[\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]\u003c/sup\u003e synthesized ZSM-5 and Silicalite-1 with rice husk ash as silicon source and studied their adsorption properties for CO\u003csub\u003e2\u003c/sub\u003e under humid conditions, the adsorption capacities are 81.69cm\u003csup\u003e3\u003c/sup\u003e/g and 69.96cm\u003csup\u003e3\u003c/sup\u003e/g. Razavian\u003csup\u003e[\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]\u003c/sup\u003e prepared Silicalite-1 by ultrasonic method and hydrothermal method respectively, and compared their CO\u003csub\u003e2\u003c/sub\u003e/CH\u003csub\u003e4\u003c/sub\u003e adsorption properties. The results showed that the ultrasonic treated Silicalite-1 had high selectivity, while the hydrothermal method Silicalite-1 had high adsorption capacity. Therefore, fly ash can be used to prepare Silicalite-1 with high specific surface area and good adsorption performance for CO\u003csub\u003e2\u003c/sub\u003e\u003csup\u003e[\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e]\u003c/sup\u003e.\u003c/p\u003e \u003cp\u003eThe morphology of MFI zeolite is closely related to their structure, micropore size and crystal size, which have a significant role in adsorption performance\u003csup\u003e[\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]\u003c/sup\u003e. The difference in the preferred orientation of pore channels will affect the morphology and diffusion efficiency\u003csup\u003e[\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]\u003c/sup\u003e. Chen et al.\u003csup\u003e[\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]\u003c/sup\u003e investigated the adsorption properties of toluene on Silicalite-1 zeolites with different morphologies and found that morphologies have significant and different effects on toluene adsorption under dry or wet conditions. Under dry conditions (303 K), the dynamic adsorption capacity followed S-S (spherical Silicalite-1)\u0026thinsp;\u0026gt;\u0026thinsp;P-S (plate-like Silicalite-1)\u0026thinsp;\u0026gt;\u0026thinsp;B-S (brick-like Silicalite-1). Under humid conditions (RH\u0026thinsp;=\u0026thinsp;50%, 303 K), the dynamic adsorption capacity was in the order of P-S\u0026thinsp;\u0026gt;\u0026thinsp;S-S\u0026thinsp;\u0026gt;\u0026thinsp;B-S.Liu et al\u003csup\u003e[\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]\u003c/sup\u003e found that the particle size of zeolite molecular sieves could be varied by the length of the b-axis to adjust the distribution of aromatic hydrocarbons. Wang et al.\u003csup\u003e[\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]\u003c/sup\u003e compared the catalytic performance of nano-scale and micron-scale ZSM-5 in different reactions (disproportionation of toluene, alkylation of toluene with methanol and trimethylbenzene cracking) and found that nano-scale ZSM-5 exhibited excellent reactivity due to increased pore openings and high accessibility of acid sites. Wang et al.\u003csup\u003e[\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e]\u003c/sup\u003e synthesized straight channel-covered hemispherical bicrystalline zeolites and obtained p-xylene with greater than 99% selectivity by optimizing shape-selective catalysis of toluene in sinusoidal channels. Based on the advantages of molecular sieve morphology, grain size and adsorbability, the effect of MFI zeolite morphology on CO\u003csub\u003e2\u003c/sub\u003e adsorption removal was proposed, which has been rarely reported so far.\u003c/p\u003e \u003cp\u003eIn order to promote the high-value utilization of fly ash and reduce the production cost of molecular sieves, in this work, high purity SiO\u003csub\u003e2\u003c/sub\u003e was successfully extracted from fly ash by high-temperature calcination, alkali fusion activation and acid leaching, and then Silicalite-1 zeolites with three different morphologies (plate-like, spherical-like and cross-type), were synthesized using SiO\u003csub\u003e2\u003c/sub\u003e as the silica source, and then used for the adsorption of CO\u003csub\u003e2\u003c/sub\u003e. Finally, the purity, morphology, pore size and CO\u003csub\u003e2\u003c/sub\u003e adsorption performance of the Silicalite-1 zeolites with different morphologies were determined by characterization means such as XRD, SEM, FT-IR and BET.\u003c/p\u003e"},{"header":"2. Experiment Materials and Methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.1 Materials and reagents\u003c/h2\u003e \u003cp\u003eFly ash, purchased from Xinjiang Changji thermal power plant. Hydrochloric acid (HCl, 38%), sodium carbonate (Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e), tetrapropyl ammonium hydroxide (TPAOH, 25%) and sodium hydroxide (NaOH) were all analytically pure, and were acquired from Sinopharm Group Chemical reagent Co, LTD. Deionized water for homemade.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.2 Sample preparation\u003c/h2\u003e \u003cdiv id=\"Sec5\" class=\"Section3\"\u003e \u003ch2\u003e2.2.1 Extraction of SiO\u003csub\u003e2\u003c/sub\u003e from fly ash\u003c/h2\u003e \u003cp\u003eThe fly ash was placed in a box-type resistance furnace and roasted at 600 ℃ for 2 h to remove the unburned carbon. After cooling to room temperature, the fly ash was activated with Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e as the activator. Fly ash and Na\u003csub\u003e2\u003c/sub\u003eCO\u003csub\u003e3\u003c/sub\u003e were mixed according to the mass ratio of 1:1 and calcined at 850 ℃ for 100 min in the box resistance furnace, and the calcined clinker was obtained by thorough grinding after cooling. The calcined clinker was stirred with 5 mol/L HCl according to the solid-liquid ratio of 1 g :10 mL on a magnetic stirrer for 4 h, and then filtered, washed with distilled water until neutral and dry, and SiO\u003csub\u003e2\u003c/sub\u003e is obtained.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec6\" class=\"Section3\"\u003e \u003ch2\u003e2.2.2 Preparation of P-S (plate-like Silicalite-1)\u003c/h2\u003e \u003cp\u003eP-S were prepared by using SiO\u003csub\u003e2\u003c/sub\u003e extracted from fly ash as the silica source, TPAOH as the template agent and distilled water as the solvent. Weighing a certain mass of deionized water, TPAOH and the prepared SiO\u003csub\u003e2\u003c/sub\u003e at room temperature according to a certain molar ratio on a magnetic stirrer after stirring vigorously for 4 h, and then the mixture was transferred to a 100 mL stainless steel reactor PTFE liner, and then placed into the oven has been preheated, crystallization at a certain temperature for a certain period of time. After crystallization is completed, take out of the reactor, cool the product, filter, wash to neutral, dry and grind. Finally, the product was roasted in a muffle furnace at 550 ℃ for 6 h, so as to remove the template agent and obtain the P-S (plate-like Silicalite-1).\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec7\" class=\"Section3\"\u003e \u003ch2\u003e2.2.3 Preparation of S-S (spherical-like Silicalite-1)\u003c/h2\u003e \u003cp\u003eS-S were prepared by adjusting the dosage of TPAOH with SiO\u003csub\u003e2\u003c/sub\u003e extracted from fly ash as the silica source, TPAOH as the templating agent and distilled water as the solvent, keeping the crystallization time, crystallization temperature and water-to-silica ratio certain. The specific steps were the same as 2.2.2.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section3\"\u003e \u003ch2\u003e2.2.4 Preparation of C-S (cross-type Na-Silicalite-1)\u003c/h2\u003e \u003cp\u003eC-S was prepared with SiO\u003csub\u003e2\u003c/sub\u003e extracted from fly ash as silicon source, TPAOH as template agent, NaOH as base source and distilled water as solvent. The specific steps were the same as 2.2.2.\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Characterization\u003c/h2\u003e \u003cp\u003eX-ray powder diffraction test (XRD): The D/max-2400 X-ray diffractometer of Rigaku Company in Japan, the radiation source is Cu target Kα radiation, the scanning speed is 10\u0026deg;/min, the scanning range is 5\u0026deg;-80\u0026deg;. X-ray fluorescence analyzer (XRF): SRF3400 X-ray fluorescence spectrometer from Bruker, Germany; Scanning electron microscope (SEM): Beijing KYKY-2800B. FT-IR: IFS88 from Bruker, Germany, with a range of 700\u0026ndash;4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. Specific surface area test (BET): Micrometrics Tristar II instrument was used to analyze the specific surface area, pore structure, and nitrogen adsorption and desorption curves of the catalysts. The samples were degassed under vacuum at 200\u0026deg;C for 8 h prior to the test, and the N\u003csub\u003e2\u003c/sub\u003e adsorption and desorption tests were performed on the samples under 77 K liquid nitrogen. The samples were purged at 300 ℃ for 12 h and then tested for CO\u003csub\u003e2\u003c/sub\u003e adsorption at 25 ℃.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Investigation of the synthesis conditions of P-S\u003c/h2\u003e \u003cdiv id=\"Sec12\" class=\"Section3\"\u003e \u003ch2\u003e3.1.1 Effect of different template agents\u003c/h2\u003e \u003cp\u003eThe type of template agent also affects the synthesized Silicalite-1 molecular sieve, and TPABr and TPAOH are the most commonly used template agents. Under the same experimental conditions, TPABr and TPAOH were used as templates to investigate the effects of different TPA\u003csup\u003e+\u003c/sup\u003e on the preparation of Silicalite-1 molecular sieve. As can be seen from Fig.\u0026nbsp;1, the sample with TPAOH as the template agent has an obvious characteristic peak of Silicalite-1 molecular sieve. However, when TPABr is used as the template agent, only amorphous SiO\u003csub\u003e2\u003c/sub\u003e diffraction peaks appear, indicating that Silicalite-1 molecular sieve cannot be successfully synthesized when only TPABr is used as the template agent. At this time, Br\u003csup\u003e-\u003c/sup\u003e in the system can only balance charge and cannot promote silicate rearrangement to form Silicalite-1 molecular sieve crystals. When TPAOH is the template agent, the reaction process contains the appearance of OH\u003csup\u003e-\u003c/sup\u003e, which provides the base source for the synthesis environment of the sample and balances the negative charge at the same time, playing a structure-oriented role, so the Silicalite-1 molecular sieve can be successfully synthesized.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;2.is the SEM image of Silicalite-1 zeolite synthesized under different template agents. It can also be seen from the figure that when the template agent is TPAOH, the crystalline phase structure of Silicalite-1 zeolite with smooth surface, regular morphology and uniform size appears, and the grain size is between 6\u0026ndash;8 \u0026micro;m. When TPABr was used as template, only amorphous phase structure appeared. Therefore, TPAOH was selected as the template agent to synthesize Silicalite-1 zeolite.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec13\" class=\"Section3\"\u003e \u003ch2\u003e3.1.2 Effect of crystallization time\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;3.shows the XRD (a) and crystallinity (b) diagrams of P-S synthesized with different crystallization times, as shown in the figure: when the crystallization time is 2 h, it can be clearly seen that the characteristic peaks belonging to MFI-type molecular sieves appeared in the 2θ of 7.94 \u0026deg;, 8.87 \u0026deg;, 23.08 \u0026deg;, 23.31 \u0026deg; and 23.96 \u0026deg;, but with lower peak heights and weaker peak strengths. The initial induction period of ZSM-5 zeolite molecular sieve, which is also of MFI type, is about 5 h without the addition of crystal seed, and Silicalite-1 has entered the crystal growth stage at 2 h, which is a shorter induction period than that of ZSM-5. This may be due to the fact that there is no involvement of aluminate in the crystallization process of all-silica molecular sieves, thus shortening the crystallization time. With the prolongation of the crystallization time, the intensity of the characteristic diffraction peaks was gradually enhanced, indicating that more and more amorphous silicate sols have begun to transform to Silicalite-1 zeolite molecular sieve crystals, and the intensity of the characteristic diffraction peaks of Silicalite-1 zeolite gradually showed a downward trend when the crystallization time was greater than 12 h, which may be attributed to the fact that the crystallization time is too long, the crystals reacted with solution, making the crystals grow excessively. This may be due to the long crystallization time, the reaction between the crystals and the solution, which makes the crystals grow excessively and thus transcrystallization phenomenon occurs, which reduces its crystallinity \u003csup\u003e[\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e]\u003c/sup\u003e. As can be seen from Fig. (b), the crystallinity of the samples reached the highest when the crystallization time was 12 h, and the crystallinity of the samples gradually decreased when the time was extended.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;4.is the SEM diagram of P-S synthesized under different crystallization times. As can be seen from the diagram, when the crystallization time is 2 h, there is no obvious regular crystal phase structure, which is due to the short crystallization time so that the basic molecular sieve skeleton cannot be formed. When the crystallization time is 7 h, a very small amount of P-S crystal phase structure appears, and the surface is smooth and flat, the grain size is between 4\u0026ndash;6 \u0026micro;m, but still dominated by large amorphous particles.When the crystallization time was extended to 12 h, regular and dispersed plate crystals appeared with good morphology and uniform size, and the grain size was between 6\u0026ndash;8 \u0026micro;m. However, with the continuous extension of crystallization time, the molecular sieve appeared obvious agglomeration and fracture phenomenon. When the crystallization time is 24 h, almost no single crystal structure can be found, and all are large agglomerated crystals. When the crystallization time is 36 h, P-S crystals not only agglomerate, but also fracture, with a large number of broken crystals attached to the surface of the molecular sieve. The results indicated that the long crystallization time was not conducive to the synthesis of P-S, but would promote the target product to continue to react with the solution, resulting in crystallization transformation.Therefore, based on the above analysis, 12 h is selected as the best crystallization time.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec14\" class=\"Section3\"\u003e \u003ch2\u003e3.1.3 Effect of crystallization temperature\u003c/h2\u003e \u003cp\u003eA higher crystallization temperature may shorten the induction period and promote the growth of the molecular sieve crystals, because the higher temperature helps rearrangement between atoms of the substance and promotes the formation of nuclei and the growth of crystals. However, when the crystallization temperature is too high, it may lead to other side reactions of the formed crystals, such as crystal transformation or crystal dissolution. When the crystallization temperature is low, it is not conducive to the atomic activity and rearrangement between the reactive substances, resulting in a decrease in the reaction rate\u003csup\u003e[\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]\u003c/sup\u003e. Figure\u0026nbsp;5.shows the XRD (a) and crystallinity (b) plots of P-S synthesized at different temperatures. From the figure, it can be seen that the products at five crystallization temperatures all showed the characteristic peaks of MFI molecular sieves between 2θ of 7.94 \u0026deg;, 8.87 \u0026deg;, 23.08 \u0026deg;, 23.31 \u0026deg; and 23.96 \u0026deg;. When the crystallization temperature was 150 ℃, the characteristic diffraction peak strength of Silicalite-1 zeolite was weaker, which indicated that only the basic skeleton structure could be formed at 150 ℃. With the continuous increase of the crystallization temperature, the peak area of the characteristic peaks of Silicalite-1 gradually becomes higher, and when the temperature reaches 180 ℃, the characteristic diffraction peak intensity of Silicalite-1 reaches the highest. As the crystallization temperature continues to increase, the characteristic diffraction peak intensity decreases, which may be due to the fact that the crystallization temperature is too high, which makes the crystal transcrystallization or rupture phenomenon, thus leading to the decrease in the intensity of the characteristic peaks and the degree of crystallinity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;6.shows the SEM images of P-S synthesis at different crystallization temperatures. It can be seen from the figure that when the crystallization temperature is 150 ℃, although a small amount of plate crystalline phase structure appears, it still contains a large amount of bulk amorphous material. When the crystallization temperature is 160 ℃, the crystallization is relatively complete and more uniform plate structure appears, but a large number of small particles are still gathered on the crystal surface, which may be due to the low temperature and the adhesion of SiO\u003csub\u003e2\u003c/sub\u003e which is not involved in crystallization to the crystal surface. When the crystallization temperature is 170 ℃, the small particles gathered on the surface of Silicalite-1 crystals decrease, indicating that the unreacted amorphous SiO\u003csub\u003e2\u003c/sub\u003e has begun to transform into Silicalite-1 crystals. When the crystallization temperature is 180 ℃, the P-S crystal structure with uniform size and smooth surface appears. When the crystallization temperature was further extended to 190 ℃, cracks appeared on the surface of some samples and a large number of samples were agglomerated. According to the above analysis, either too high or too low temperature is not conducive to the synthesis of P-S, so 180℃ is chosen as the best synthesis temperature.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec15\" class=\"Section3\"\u003e \u003ch2\u003e3.1.4 Effect of water amount\u003c/h2\u003e \u003cp\u003eWater is not only a solvent in the synthesis of molecular sieve, but also has an important impact on the composition and structure of silicate gels as well as the formation and growth of crystal nuclei by changing the concentration of silicate ions in the solution and adjusting the alkalinity\u003csup\u003e[\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e]\u003c/sup\u003e. Figure\u0026nbsp;7.shows the XRD pattern and crystallinity of P-S synthesized under different water amounts. As can be seen from the figure, pure phase Silicalite-1 zeolite can be synthesized within the molar ratio of H\u003csub\u003e2\u003c/sub\u003eO/SiO\u003csub\u003e2\u003c/sub\u003e of 21\u0026thinsp;~\u0026thinsp;49, and no other crystalline heterogeneous peaks appeared. When H\u003csub\u003e2\u003c/sub\u003eO/SiO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;21, the characteristic peak-to-peak strength of Silicalite-1 molecular sieve is low, which may be due to the large silicate concentration, incomplete crystallization and low crystallinity caused by too low water content. The crystallinity gradually increased with the increase of water volume, and the relative crystallinity reached the optimum when the molar ratio of H\u003csub\u003e2\u003c/sub\u003eO/SiO\u003csub\u003e2\u003c/sub\u003e was 42. While the water-silicon ratio continues to increase to 49, the relative crystallinity begins to decrease, which may be due to too much water, low concentration of silicate, and the existence of a large number of incomplete crystallized silica, resulting in a decrease in crystallinity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;8.is the SEM photo of P-S synthesized with different water-silicon ratios. It can be seen from the SEM image that when the molar ratio of H\u003csub\u003e2\u003c/sub\u003eO/SiO\u003csub\u003e2\u003c/sub\u003e is 21, although there is a small amount of Silicalite-1 crystals, there are still a large number of amorphous substances attached to the crystal surface. With the increase of the water-silicon ratio, the amorphous material gradually decreases. When the molar ratio of H\u003csub\u003e2\u003c/sub\u003eO/SiO\u003csub\u003e2\u003c/sub\u003e is 42, the crystal with smooth surface and uniform particle size is between 6\u0026ndash;8 \u0026micro;m. Continue to increase the water-silicon ratio to 49, at this time, a large number of incomplete crystallization substances appear on the crystal surface again, indicating that too much water will lead to a relatively low concentration of silicate, making a large amount of SiO\u003csub\u003e2\u003c/sub\u003e is not fully crystallized. Based on the above analysis, the water-silicon ratio 42 is selected as the best water-silicon ratio.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec16\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Investigation of the synthesis conditions of S-S\u003c/h2\u003e \u003cdiv id=\"Sec17\" class=\"Section3\"\u003e \u003ch2\u003e3.2.1 Influence of TPAOH dosage\u003c/h2\u003e \u003cp\u003eDuring the crystallization process of molecular sieves, the template agent will affect the interaction of silicaluminate, which have an impact on the gelation and nucleation process. Organic template agents are expensive, and their dosage also determines the cost of molecular sieve production. Figure\u0026nbsp;9 shows the XRD (a) and relative crystallization (b) of Silicalite-1 zeolites synthesized under different TPA\u003csup\u003e+\u003c/sup\u003e/SiO\u003csub\u003e2\u003c/sub\u003e molar ratio. It can be seen from the figure: When the molar ratio is 0.01, there is no characteristic peak belonging to MFI zeolite molecular sieve, indicating that the guiding effect is weak when the dosage of template agent is small, and it cannot provide structural guiding effect for amorphous SiO2 in the system\u003csup\u003e[\u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e]\u003c/sup\u003e. With the increase of the amount of template agent, within a certain range, the characteristic diffraction peaks and peaks of MFI zeolites are strong, and the relative crystallinity of Silicalite-1 is also high, indicating that TPAOH as the template agent can be synthesized within a certain range of Silicalite-1 with high crystallinity and single crystalline phase. When the molar ratio of TPAOH/SiO\u003csub\u003e2\u003c/sub\u003e is 0.28, the relative crystallinity reaches the maximum. The relative crystallinity decreased slightly with increasing the amount of TPAOH. It may be because the concentration of the template agent is too high, which will lead to excessive growth or aggregation of the crystal, thus affecting the relative crystallinity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;10.is the SEM diagram of Silicalite-1 molecular sieve with different dosage of template agent. According to the diagram, when the dosage of template agent is too low, it cannot fully polymerize with silicate to form gel, resulting in the failure to form crystal nucleus. With the increase of the concentration of template agent, TPA\u003csup\u003e+\u003c/sup\u003e can bind to silicate ions and promote the formation of silicate gel. Silica tetrahedrons tend to form pores or cages on the gel surface, increasing the number of crystal nuclei\u003csup\u003e[\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e]\u003c/sup\u003e. When the molar ratio of TPAOH/SiO\u003csub\u003e2\u003c/sub\u003e increases from 0.07 to 0.28, the particle size becomes smaller and smaller. When the molar ratio is 0.28, spherical-like crystals with smooth surface and uniform size appear.When the molar ratio of TPAOH/SiO\u003csub\u003e2\u003c/sub\u003e is greater than 0.28, the dosage of template agent exceeds a certain threshold, and the contribution to molecular sieve synthesis is reduced. This is due to the large size of TPA\u003csup\u003e+\u003c/sup\u003e, which may prevent silicon species from entering the molecular sieve skeleton\u003csup\u003e[\u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e]\u003c/sup\u003e, and affect the formation of molecular sieve skeleton to a certain extent, resulting in larger molecular sieve particle size. Based on the above analysis, a molar ratio of TPAOH/SiO\u003csub\u003e2\u003c/sub\u003e of 0.28 is the best choice for the synthesis of S-S.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec18\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Investigation of the synthesis conditions of C-S\u003c/h2\u003e \u003cdiv id=\"Sec19\" class=\"Section3\"\u003e \u003ch2\u003e3.3.1 Influence of NaOH dosage\u003c/h2\u003e \u003cp\u003eThe amount of NaOH has a great influence on the synthesis of Silicalite-1 zeolite, which can adjust the pH value of the reaction medium, thus affecting the reaction rate and the crystal structure of the product. Alkaline environment can promote the reaction to a certain extent, but too high alkalinity may cause side reactions or affect the crystal morphology of the product\u003csup\u003e[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]\u003c/sup\u003e. Figure\u0026nbsp;11.shows the XRD (a) and relative crystallinity (b) of Silicalite-1 molecular sieve with different amounts of NaOH. It can be seen from the figure that the synthesized zeolites at different proportions all show the standard MFI structure characteristic diffraction peaks at 2θ of 7.94 \u0026deg;, 8.87 \u0026deg;, 23.08 \u0026deg;, 23.31 \u0026deg; and 23.96 \u0026deg;, and the crystallinity of the five products does not change significantly, indicating that Na-Silicalite-1 zeolite with high crystallinity can be synthesized in a certain range when SiO\u003csub\u003e2\u003c/sub\u003e extracted from fly ash is used as the silicon source.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFigure\u0026nbsp;12.shows the SEM images of Silicalite-1 zeolites at different NaOH dosages, from which it can be seen that the grain size decreases gradually with the increase of NaOH dosage. When the molar ratio of NaOH/SiO\u003csub\u003e2\u003c/sub\u003e is 0.011, there are plate-like structures of different sizes in the system, when the molar ratio of NaOH/SiO\u003csub\u003e2\u003c/sub\u003e is 0.055, at this time, there are not only plate-like structures in the system, but also smaller spherical-like structures, and when the molar ratio of NaOH/SiO\u003csub\u003e2\u003c/sub\u003e is 0.11, the crystalline structure of the system at this time becomes cross-type. When the molar ratio of NaOH/SiO\u003csub\u003e2\u003c/sub\u003e is 0.165, at this time it is a spherical-like crystal structure with uniform size. Continue to increase the dosage of NaOH to the molar ratio of NaOH/SiO\u003csub\u003e2\u003c/sub\u003e is 0.22, at this time, the solution is more alkaline, the surface of the molecular sieve is etched, and it becomes similar to the amorphous material morphology.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv id=\"Sec20\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Characterization of different morphologies of Silicalite-1 zeolites and their CO\u003csub\u003e2\u003c/sub\u003e adsorption properties\u003c/h2\u003e \u003cdiv id=\"Sec21\" class=\"Section3\"\u003e \u003ch2\u003e3.4.1 Nitrogen adsorption-desorption\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;13.shows the nitrogen adsorption-desorption curves and pore size distribution of Silicalite-1 zeolites with different morphologies. As can be seen from the figure, both the P-S and S-S have no obvious hysteresis loops and are type I isotherms. The main performance is: the adsorption capacity rises rapidly under low relative pressure, and the adsorption saturation occurs after reaching a certain relative pressure. This is because in the narrow micropores, the adsorbent-adsorbent interaction is enhanced, which leads to the micropores being filled rapidly at very low relative pressure, but when the saturation pressure is reached, the adsorbent condenses, resulting in the curve beginning to flatline.However, C-S has an insignificant hysteresis ring, and the adsorption curve is a composite isotherm with type I as the main and type IV as the auxiliary. According to the analysis of pore size distribution by BJH theory, it can be seen that the pore size distribution of Silicalite-1 zeolites with three kinds of morphology are mainly concentrated in the range of 1 nm\u0026thinsp;~\u0026thinsp;4 nm, which mainly exists in the form of micropores and has relatively narrow mesoporous pores.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eTable\u0026nbsp;1 shows the pore structure parameters of Silicalite-1 zeolites with different morphologies, from the table, it can be seen that the specific surface area of S-S is 460.24 m\u003csup\u003e2\u003c/sup\u003e/g, of which the specific surface area of the micropores is 275.25 cm\u003csup\u003e2\u003c/sup\u003e/g, and the total pore volume is 0.365 cm\u003csup\u003e3\u003c/sup\u003e/g, of which the microporous pore volume is 0.141 cm\u003csup\u003e3\u003c/sup\u003e/g, which indicates that the prepared P-S have high specific surface area and pore volume. Not only that, the P-S have larger specific surface area, total pore volume and microporous pore volume than the other two morphologies of Silicalite-1. The specific surface area of S-S is much lower than that of P-S and C-S, except for the specific surface area, the rest of the parameters of S-S and C-S are not much different.\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\u003ePore structure parameters of Silicalite-1 zeolites with different morphologies: (a) P-S; (b) S-S; and (c) C-S.\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=\"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 \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\u0026nbsp;\u003c/th\u003e \u003cth align=\"left\" colname=\"c2\"\u003e \u003cp\u003eS\u003csub\u003eBET\u003c/sub\u003e(m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c3\"\u003e \u003cp\u003eS\u003csub\u003emicro\u003c/sub\u003e(m\u003csup\u003e2\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c4\"\u003e \u003cp\u003eV\u003csub\u003etotal\u003c/sub\u003e(cm\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c5\"\u003e \u003cp\u003eV\u003csub\u003emicro\u003c/sub\u003e(cm\u003csup\u003e3\u003c/sup\u003e/g)\u003c/p\u003e \u003c/th\u003e \u003cth align=\"left\" colname=\"c6\"\u003e \u003cp\u003eAverage pore size/nm\u003c/p\u003e \u003c/th\u003e \u003c/tr\u003e \u003c/thead\u003e \u003ctbody\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ea\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e460.24\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e275.25\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.365\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.141\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e3.16\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003eb\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e413.83\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e304.77\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.183\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.120\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.76\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003ctr\u003e \u003ctd align=\"left\" colname=\"c1\"\u003e \u003cp\u003ec\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e \u003cp\u003e454.58\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e \u003cp\u003e298.95\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e \u003cp\u003e0.201\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c5\"\u003e \u003cp\u003e0.119\u003c/p\u003e \u003c/td\u003e \u003ctd align=\"char\" char=\".\" colname=\"c6\"\u003e \u003cp\u003e1.771\u003c/p\u003e \u003c/td\u003e \u003c/tr\u003e \u003c/tbody\u003e \u003c/colgroup\u003e \u003c/table\u003e\u003c/div\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec22\" class=\"Section3\"\u003e \u003ch2\u003e3.4.2 FT-IR analysis\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;14.shows the FT-IR spectra of Silicalite-1 zeolites with different morphologies. It can be seen from the figure that all the three morphologies of Silicalite-1 have skeleton vibration peaks belonging to MFI-type molecular sieves at 1230, 1080 and 800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e, among which the peak at 1230 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to the tensile vibration of the five-membered ring unique to MFI, the peak at 1080 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to the asymmetric telescopic vibration of the Si-O-Si external connection, and the peak at 800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e is attributed to the telescopic vibration of Si-OH. and the 800 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e peak is attributed to the stretching vibration of Si-OH. However, the FT-IR spectra of C-S did not show any obvious Na-O peaks, probably due to less Na in the skeleton, and thus the absorption vibration peaks were not obvious.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec23\" class=\"Section3\"\u003e \u003ch2\u003e3.4.3 CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherm\u003c/h2\u003e \u003cp\u003eFigure\u0026nbsp;15.shows the CO\u003csub\u003e2\u003c/sub\u003e adsorption isotherms of Silicalite-1 zeolites with different morphologies. It can be seen from the figure that both P-S and S-S overlapped the adsorption curve and desorption curve of CO\u003csub\u003e2\u003c/sub\u003e, indicating that their adsorption properties for CO\u003csub\u003e2\u003c/sub\u003e are reversible and mainly dominated by physical adsorption. In the low pressure region, the adsorption capacity rises rapidly, indicating that there is strong adsorption in the low pressure region, and this adsorption process is related to the large number of micropores in the sample.In the medium pressure section, the adsorption curve rose slowly, indicating that the mesoporous content of the sample was low. When the adsorption pressure is increased to 101.325KPa, the CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of the prepared P-S is 1.89mmol /g, indicating that it can be used as an excellent CO\u003csub\u003e2\u003c/sub\u003e adsorption material. The adsorption capacity of S-S for CO\u003csub\u003e2\u003c/sub\u003e is 1.34mmol /g, which is much lower than that of P-S.This may be due to the fact that P-S has a larger specific surface area, total pore volume and microporous pore volume than the remaining two morphologies of Silicalite-1.\u003c/p\u003e \u003cp\u003eIn addition, it can be seen from the figure that the adsorption curve and desorption curve of C-S for CO\u003csub\u003e2\u003c/sub\u003e do not completely coincide, indicating that the adsorption performance of C-S for CO\u003csub\u003e2\u003c/sub\u003e is partially irreversible. Although it is mainly physical adsorption, there is still a certain amount of chemical adsorption due to the introduction of alkaline metals. Under the test condition of 25 ℃, when the adsorption pressure was increased to 101.325 KPa, the adsorption capacity of C-S for CO\u003csub\u003e2\u003c/sub\u003e was 1.06 mmol / g. Although a small amount of chemical adsorption phenomenon appeared in C-S, its adsorption performance was much lower than that of P-S and S-S,which could be attributed to the decrease of CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity due to the reduction of the specific surface area and the micropore of molecular sieves by adding inorganic alkali. Among the three morphologies of Silicalite-1 zeolites, the adsorption effect of P-S was better due to the slightly larger specific surface area compared with the others, so it can be hypothesized that the CO\u003csub\u003e2\u003c/sub\u003e adsorption performance is mainly related to the specific surface area of the molecular sieves.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"4. conclusion","content":"\u003cp\u003eSilicalite-1 zeolite molecular sieves with different morphologies were prepared using SiO\u003csub\u003e2\u003c/sub\u003e extracted from fly ash as silicon source. The results show that under the conditions of crystallization time of 12 h, crystallization temperature of 180 ℃ and molar ratio of H\u003csub\u003e2\u003c/sub\u003eO/SiO\u003csub\u003e2\u003c/sub\u003e of 42, the P-S prepared has good morphology, uniform size and high relative crystallinity. The specific surface area is 460.24 m\u003csup\u003e2\u003c/sup\u003e/g, the specific surface area of the micropores is 275.25 cm\u003csup\u003e2\u003c/sup\u003e/g, and the total pore volume is 0.365 cm\u003csup\u003e3\u003c/sup\u003e/g, among which the micropore volume is 0.141 cm\u003csup\u003e3\u003c/sup\u003e/g, indicating that the prepared P-S has a high specific surface area and pore volume. S-S was prepared by adjusting the amount of TPAOH. When TPAOH/SiO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.28, the prepared S-S had better morphology and higher relative crystallinity. The specific surface area is 413.83 m\u003csup\u003e2\u003c/sup\u003e/g, the specific surface area of the micropores is 304.77 cm\u003csup\u003e2\u003c/sup\u003e/g, the total pore volume is 0.183 cm\u003csup\u003e3\u003c/sup\u003e/g, and the micropore volume is 0.120 cm\u003csup\u003e3\u003c/sup\u003e/g. When preparing Na-Silicalite-1 molecular sieve, it is found that when NaOH/SiO\u003csub\u003e2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;0.11, C-S can be synthesized. The specific surface area is 454.58 m\u003csup\u003e2\u003c/sup\u003e/g, the specific surface area of the micropores is 298.95 cm\u003csup\u003e2\u003c/sup\u003e/g, the total pore volume is 0.201 cm\u003csup\u003e3\u003c/sup\u003e/g, and the micropore volume is 0.119 cm\u003csup\u003e3\u003c/sup\u003e/g.\u003c/p\u003e\n\u003cp\u003eUnder the test conditions of 298 K and 1bar, the adsorption capacity of P-S for CO\u003csub\u003e2\u003c/sub\u003e is 1.89 mmol/g, and that of S-S for CO\u003csub\u003e2\u003c/sub\u003e is 1.34 mmol/g, while the adsorption capacity of C-S for CO\u003csub\u003e2\u003c/sub\u003e is only 1.06 mmol/g, and the adsorption capacity of CO\u003csub\u003e2\u003c/sub\u003e is P-S\u0026thinsp;\u0026gt;\u0026thinsp;S-S\u0026thinsp;\u0026gt;\u0026thinsp;C-S.\u003c/p\u003e\n"},{"header":"Declarations","content":"\u003ch2\u003eAuthor Contribution\u003c/h2\u003e\u003cp\u003eWu Xianglian wrote the main manuscript text Aisha Nulahong was responsible for overseeing the experimentChangmin Tuo,Tiezhen Ren,Abulikemu Abulizi ,Jian Li and Fei Xu\u003c/p\u003e\u003ch2\u003eData Availability\u003c/h2\u003e\u003cp\u003eData is provided within the manuscript .\u003c/p\u003e\u003cp\u003e\u003cstrong\u003eFund Project: Supported by the 2022 Xinjiang Uygur Autonomous Region Natural Science Foundation Project (Joint Fund) (2002D01C378)\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDeclaration of Interest Statement\u003c/strong\u003e\u003cstrong\u003e：\u003c/strong\u003e\u003cstrong\u003eThere are no conflicts of interest to declare.All authors disclosed no relevant relationships.\u003c/strong\u003e\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eGOLLAKOTA A,VOLLI V,SHU C M.Progressive utilisation prospects of coal fly ash:A review[J]. 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Journal of Porous Materials, 2011, 18:509-515.\u003c/li\u003e\n\n\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":"[email protected]","identity":"silicon","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scon","sideBox":"Learn more about [Silicon](https://www.springer.com/journal/12633)","snPcode":"12633","submissionUrl":"https://submission.nature.com/new-submission/12633/3","title":"Silicon","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"Fly ash, SiO2, Silicalite-1, Morphology, CO2 adsorption","lastPublishedDoi":"10.21203/rs.3.rs-4846209/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4846209/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eSolid waste fly ash has a great impact on the environment and pollutes water, air and soil to different degrees. Therefore, it is of great significance to realize the high-value utilization of fly ash. In this paper, high purity SiO\u003csub\u003e2\u003c/sub\u003e was successfully extracted from fly ash by high temperature calcination, alkali fusion activation and pickling, and then different morphologies of Silicalite-1 zeolites were synthesized by using SiO\u003csub\u003e2\u003c/sub\u003e extracted from fly ash as silicon source, TPAOH as templating agent and NaOH as alkali source. The influencing factors such as crystallization time, crystallization temperature, NaOH dosage, TPAOH dosage and different hydrosilica ratios were investigated separately. The results show that under the conditions of crystallization time of 12 h, crystallization temperature of 180 ℃ and molar ratio of water-silicon of 42, the prepared P-S (plate-like Silicalite-1) has good morphology, high relative crystallinity and adsorption capacity of CO\u003csub\u003e2\u003c/sub\u003e of 1.89 mmol/g. The CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of the S-S (spherical-like Silicalite-1) prepared by adjusting the amount of TPAOH is 1.34 mmol/g. The CO\u003csub\u003e2\u003c/sub\u003e adsorption capacity of the prepared C-S (cross-type Na-Silicalite-1) is only 1.06 mmol/g. The CO\u003csub\u003e2\u003c/sub\u003e adsorption capacities follow P-S\u0026thinsp;\u0026gt;\u0026thinsp;S-S\u0026thinsp;\u0026gt;\u0026thinsp;C-S. The results may provide a valuable reference for zeolite-based adsorbents in the adsorption removal or recovery of CO\u003csub\u003e2\u003c/sub\u003e.\u003c/p\u003e","manuscriptTitle":"Comparative studies on CO2 adsorption performance of different morphologies of Silicalite-1 zeolites synthesized from fly ash","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-09-12 12:03:53","doi":"10.21203/rs.3.rs-4846209/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-09-09T00:38:08+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-09-08T08:02:15+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"24338537361161419179826165433993585820","date":"2024-08-29T09:58:16+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-08-27T01:38:27+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-08-16T12:23:24+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-08-16T12:22:07+00:00","index":"","fulltext":""},{"type":"submitted","content":"Silicon","date":"2024-08-02T06:49:42+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"silicon","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"scon","sideBox":"Learn more about [Silicon](https://www.springer.com/journal/12633)","snPcode":"12633","submissionUrl":"https://submission.nature.com/new-submission/12633/3","title":"Silicon","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"649d59dc-a484-4701-8e9e-bcb77b97f0eb","owner":[],"postedDate":"September 12th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2025-03-10T17:10:18+00:00","versionOfRecord":{"articleIdentity":"rs-4846209","link":"https://doi.org/10.1007/s12633-025-03269-9","journal":{"identity":"silicon","isVorOnly":false,"title":"Silicon"},"publishedOn":"2025-03-04 15:57:39","publishedOnDateReadable":"March 4th, 2025"},"versionCreatedAt":"2024-09-12 12:03:53","video":"","vorDoi":"10.1007/s12633-025-03269-9","vorDoiUrl":"https://doi.org/10.1007/s12633-025-03269-9","workflowStages":[]},"version":"v1","identity":"rs-4846209","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4846209","identity":"rs-4846209","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}