Catalytic Pyrolysis of Cotton Stalk for Bio-Oil Production: Effectiveness of CaO and Fe2O3 Catalysts in Ex-Situ Mode

Catalytic Pyrolysis of Cotton Stalk for Bio-Oil Production: Effectiveness of CaO and Fe2O3 Catalysts in Ex-Situ Mode | 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 Catalytic Pyrolysis of Cotton Stalk for Bio-Oil Production: Effectiveness of CaO and Fe 2 O 3 Catalysts in Ex-Situ Mode Barnali Bhui, Bhavin Dineshbhai Soni, Rohit Kumar Singh, Urvish Patel, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-7893658/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract The present study explores non-contact catalytic pyrolysis of cotton stalk, a representative waste agro-residue, using calcium oxide (CaO) and ferric oxide (Fe₂O₃) to enhance bio-oil quality and carbon recovery. Pyrolysis was performed at 450–550°C, with optimum bio-oil yield observed at 500°C. CaO incorporation (40 wt.%) reduced total bio-oil yield by 22.7 wt.% but increased the organic fraction by 139 wt.%, achieving a maximum organic phase yield of 15.3 wt.% and a high calorific value of 30.5 MJ kg⁻¹, indicating enhanced deoxygenation and acid neutralization via carbonate formation and base-catalyzed cracking. In contrast, Fe₂O₃ (10 wt.% loading) elevated the organic fraction by 25.8 wt.% while reducing total liquid yield to 26.4 wt.%, functioning through Fe³⁺/Fe²⁺ redox cycles that promote vapor-phase reforming and water–gas shift reactions, enriching the pyro-gas with H₂, CO, and CH₄. The organic phase exhibited calorific values of 28.1–30.5 MJ kg⁻¹, while the char retained a stable heating value of 18.3–19.1 MJ kg⁻¹, highlighting energy densification without compromising char quality. Post-reaction XRD and FTIR analyses revealed structural stability of the spent catalysts, with CaO transforming into CaCO₃ and Ca(OH)₂ and Fe₂O₃ yielding Fe(OH)₃, FeO, and Fe₃O₄, consistent with carbonation, hydration, and redox transformations under reducing pyrolysis conditions. Overall, the study demonstrates that ex-situ catalytic pyrolysis enhances bio-oil selectivity, energy content, and stability, while simultaneously producing a cleaner, value-added char. The findings underscore the potential of agro-residue valorization for sustainable waste-to-energy conversion, offering a carbon-negative route for high-calorific bio-oil production. Catalytic Pyrolysis Ex-situ Agro Residue Organic Phase HCV Sustainable Carbon Recovery Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Highlights • Non-contact catalytic pyrolysis enhances bio-oil selectivity and energy density • CaO acts primarily as a basic sorbent and deoxygenation agent • CaO boosts organic frac. by 139% & achieves 30.5 MJ kg⁻¹ calorific value • Fe₂O₃ results in redox-active hydrogenation & reforming catalyst • Fe₂O₃ promotes H₂-rich pyro-gas via Fe³⁺/Fe²⁺ redox and reforming reactions • Structural stability of spent catalysts confirmed through XRD and FTIR analyses Novelty statement The novelty of this work lies in its non-contact catalytic configuration, which enables the interaction of catalytic vapors without direct catalyst–biomass contact, thereby minimizing catalyst fouling and secondary char formation. This configuration effectively enhances organic-phase selectivity, deoxygenation efficiency, and bio-oil stability, while maintaining catalyst reusability. Furthermore, the mechanistic elucidation of carbonate formation (CaO) and Fe³⁺/Fe²⁺ redox cycles (Fe₂O₃) provides new insights into vapor-phase catalytic transformations during pyrolysis—an area that remains underexplored in current biomass conversion literature. This study contributes to the journal’s objectives by advancing sustainable thermochemical conversion technologies and offering a carbon-negative route for waste-to-energy valorization. 1. Introduction India approximately generates 657 million tonnes of agro-residues every year (surplus amount accounting for 230 million tonnes). This surely can be considered as a potential source to meet the present challenges faced by conventional energy sources. Further, the energy consumption rate is escalating at an alarming rate; thus, an alternative pathway needs to be implemented on an immediate basis to address the energy crisis. The primary energy sources (coal and petroleum) could only satisfy 80% of the total energy demand [ 1 ]. The substantial amount of secondary agro-residues can be beneficial in many aspects such as (a) energy production with no environmental degradation (since it is carbon neutral), (b) waste utilization promoting no landfill disposal, (c) lower NOx and SOx generation than conventional fuels, (d) low ash content; and (e) large scale employment, etc. [ 2 , 3 ]. Agro-residues such as wheat straw, cotton stalk, rice husk, bagasse, groundnut shell, etc., are abundant in the agricultural fields in India, accounting for about 228.52 MMT [ 4 ]. These residues are generally discarded and thus are often considered a serious issue for disposal. The farmers usually opt for open burning, which causes serious damage to the soil and the environment. Thus, a feasible and environmentally friendly technique is needed to utilize the residue such that the maximum extractable energy can be recovered. Agro-based residues can produce renewable energy by various thermochemical processes (pyrolysis, combustion, and gasification). India presently produces 6 million tonnes of cotton, thereby generating 15 million tonnes of cotton stalk (CS) annually [ 5 ]. It is not suitable as animal fodder due to its high lignin content. Thus, recent literature has focused on thermally degrading it to produce value-added products. The thermochemical methods are preferred over biochemical methods since it is cheaper, less complicated, require less time to process, and additional pre-processing of lignin is not required. Pyrolysis process has been proven to be more efficient and practical process for the production of bio-oil and bio-char than other thermal treatment processes [ 6 ]. It is seen that special attention is given to converting renewable feedstock into bio-oil since it has 5–10 times higher heating value than its raw sample [ 7 ]. Further, the bio-oil served multiple purposes, such as fuel in boilers and furnaces, or producing various chemical feedstocks [ 8 ]. The catalytic pyrolysis process is preferred over the non-catalytic process as the obtained bio-oil possesses improved quality compared to the non-catalytic process [ 9 , 10 ]. The catalytic process can be conducted in two modes, i.e., in-situ and ex-situ modes. For in-situ-based pyrolysis, the catalyst is mixed with the solid feed and is introduced into a single reactor. When both the feed and the catalysts are fed into two separate reactors, it is referred to as ex-situ pyrolysis. Thus, the pyrolyzed products (mainly in the vapor and liquid phases) only interact with the catalyst in the secondary reactor. Lin et al. [ 11 ] performed cotton stalk pyrolysis in a fluidized bed reactor in the presence of CaO in in-situ mode. They observed that the bio-oil quality improved during catalytic pyrolysis due to the reduction in oxygenated compounds. They concluded that the presence of CaO is beneficial since it promotes dehydration reactions. Chen et al. [ 12 ] tested the performance of CaO with CS in a fixed-bed reactor for bio-oil production in an in-situ manner. The optimum temperature was selected at 600°C for high CaO functioning since further high temperatures resulted in secondary cracking of oil. The organic fraction reduced when operated beyond 20 wt.% CaO loading. They observed that CaO plays multiple roles in pyrolysis, i.e., as an absorbent, reactant, and as catalyst. The bio-oil quality improved in the presence of CaO due to the increase in ketone, furans, and hydrocarbon content, while a decrease in acidic and anhydro-sugar contents. The major challenge in the upscaling of biomass pyrolysis is the bio-oil quality. It should be of superior quality either at the production point or by upgrading it. The untreated bio-oils usually contain water, acids, aldehydes, etc., which reduces the potential of the bio-oil [ 13 ]. It can be upgraded either by hydro-processing, esterification, or by catalytic cracking [ 14 , 15 ]. Catalytic pyrolysis is preferred over non-catalytic due to the improvement in bio-oil quality since it promotes deoxygenation, dehydrogenation, cracking, and aromatization [ 16 , 17 ]. It cracks the heavy molecules into a lesser viscous oil, thereby providing better stability and higher heating value. Iisa et al. [ 18 ] observed that bio-oil generated from pine in a fluidized bed reactor had 45% lower oxygen content during catalytic pyrolysis when compared to the non-catalytic process. The ex-situ process is preferred to the in-situ process since catalyst regeneration is easy due to the non-mixing of catalysts with the char (unlike the in-situ process). Further, catalyst deactivation and coking occur at a faster rate in the in-situ configuration [ 18 ]. Shafaghat et al. [ 19 ] conducted pyrolysis in ex-situ and in-situ mode using empty fruit bunches at 400–600°C. They observed that the ex-situ mode produced more aromatics and fewer oxygenated components than the in-situ mode. Similarly, Leng et al. [ 20 ] compared the in-situ pyrolysis and ex-situ pyrolysis by pyrolyzing poplar at 700°C. They observed that the carbonaceous residue during ex-situ and in-situ was found to be 18.6% and 31.3%. The higher residues resulted from the mixing of the catalyst with the char particles. The present study aims to provide a comparative analysis of the effect of catalysts on CS pyrolysis in a batch reactor in an extensive manner. It is mainly focused on the bio-oil yield and quality including the effect on catalyst of volatiles during ex-situ pyrolysis. The process optimization (temperature, catalyst, and catalyst loading) was implemented to obtain the maximum organic phase in the bio-oil. No reported literature has compared the performance of CaO and Fe 2 O 3 based pyrolysis with any biomass in ex-situ mode. Thus, exhaustive data is presented. The detailed characterization of both catalysts was also conducted to determine their composition after pyrolysis. Further, the basic characterization of the bio-oil obtained during non-catalytic and catalytic trials was also examined. 2. Materials and methods 2.1 Feed material The feed material (Cotton stalk) was acquired from the nearby village of Jambusar, Bharuch, Gujarat. Before proceeding to experiments, the feed material was crushed via a hammer mill up to a maximum size of 2 mm and was later used in experiments. The feed material was characterized for its physical and chemical characteristics according to the ASTM methods and is reported in Table 1 . Table 1 Physical and compositional characterization of feedstock (Cotton Stalk) Components Value Component Value Ultimate Analysis Proximate Analysis Carbon (wt.%) 36.1 Moisture (wt.%) 12.0 Hydrogen (wt.%) 7.4 Volatile matter (wt.%) # 75.9 Nitrogen (wt.%) 3.0 Ash (wt.%) # 6.7 Sulphur (wt.%) Fixed carbon (wt.%)* # 17.4 Oxygen (wt.%)* 47.6 Lignocellulosic Analysis Hemicellulose (wt.%) 12.5 Bulk Density (kg/m 3 ) 190.0 Cellulose (wt.%) 37.6 Calorific Value (MJ/kg) 15.5 Lignin (wt.%) 26.0 *by balance; # dry basis 2.2 Catalysts During this catalytic study, the cost of the catalyst and its availability are the major concerns. In this regard, the catalysts used in this study are of industrial grade and are readily available at a lower cost in the market. CaO was procured from local vendors, while Fe 2 O 3 was purchased from Tata Pigments Ltd. The particle sizes of CaO and Fe 2 O 3 were 0.4–25.0 mm and 0.4-4.0 mm, respectively. 2.3 Experimental procedure The experimental system consisted of a 25 L stainless steel (SS316) pyrolysis reactor, electrically heated and equipped with an internal mechanical stirrer to ensure uniform mixing and effective heat transfer within the feedstock, as shown in Fig. 1 . A separate vertical catalytic reactor (or secondary chamber) was integrated downstream of the primary reactor to facilitate ex-situ catalytic upgrading. This chamber, equipped with an independent heating system, was designed to prevent condensation of volatiles and ensure that only gaseous products interacted with the catalyst bed. The feedstock was introduced through the reactor lid, which was subsequently sealed to maintain system integrity. Nitrogen was continuously purged at a rate of 100 mL/min throughout the experiment to sustain an inert environment. Heating was initiated at a controlled rate until the target temperature was achieved, followed by an isothermal holding period of 30–60 min, depending on the experimental conditions. As the temperature increased, the biomass underwent thermal decomposition, generating volatile compounds that were carried by the nitrogen stream from the primary reactor into the catalytic chamber, where secondary catalytic reactions occurred at varying loadings. The resulting gas stream was then directed through a condenser for recovery of condensable liquids (bio-oil), while the non-condensable gases were subsequently flared. Gas samples were collected at specified intervals for compositional analysis. Each experimental condition was repeated twice to ensure reproducibility, and the mean values were reported. 2.4 Characterization methods The proximate analysis (moisture content, volatile matter, ash, and fixed carbon) was estimated as per the ASTM standards i.e., moisture content (D-871-82), volatile matter (ASTM E-872), and ash content (ASTM E-1755-01). The ultimate analysis was conducted by CHNS Analyzer (2400 Series II, Perkin Elmer, USA). The calorific value was estimated using an automated bomb calorimeter (C5000, IKA, Germany). The FTIR of bio-oils and catalysts were analyzed using Bruker Alpha II. The range was maintained at 400 cm − 1 to 4000 cm − 1 with a resolution of 4 cm − 1 . The 1 H NMR spectra of the liquid fraction were obtained from 400 MHz FT-NMR (Bruker). The phase identification of the catalysts during the pre- and post-pyrolysis was done by X-ray diffraction (Xpert MPD). The non-condensable gases after passing through the condensers are collected and subsequently characterized using a gas chromatograph (Make: Sigma IPL, Vadodara). The gas composition values were determined on a dry basis vol. %. 3. Results and discussion 3.1 Batch Pyrolysis : To investigate the maximum bio-oil recovery during non-catalytic pyrolysis, cotton stalk feedstock was pyrolyzed at temperatures ranging from 450°C to 550°C, and the product yield distribution is presented in Table 2 . As the temperature increased from 450°C to 550°C, the bio-char yield decreased significantly from 48.5 ± 0.8 wt.% to 36.0 ± 1.2 wt.%, indicating enhanced devolatilization and secondary cracking reactions that convert solid carbonaceous material into volatile compounds [ 5 , 15 ]. Conversely, the pyro-gas yield exhibited a steady increase from 31.7 ± 1.7 wt.% to 40.6 ± 1.9 wt.% with rising temperature, which can be attributed to the thermal decomposition of heavier organic molecules into lighter gaseous species at higher temperatures [ 21 ]. The bio-oil fraction initially increased from 19.8 ± 1.5 wt.% at 450°C to a maximum of 27.1 ± 1.1 wt.% at 500°C, suggesting that this temperature favored the condensation of primary pyrolysis vapors. However, further increase in temperature to 525°C and 550°C led to a decrease in liquid yield to 23.4 ± 0.8 wt.% and 21.4 ± 0.08 wt.%, respectively. This decline is associated with secondary cracking and reforming of condensable vapors into non-condensable gases, thereby reducing the overall liquid recovery [ 22 ]. These observations confirm that 500°C represents the optimum temperature for maximizing bio-oil yield during non-catalytic pyrolysis of cotton stalks. Table 2 Effect of operating temperature on CS pyrolysis products Temperature (°C) Bio-char (wt.%) Bio-oil (wt.%) Organic phase (wt. %) Pyro-gas (wt.%) 450 48.5 ± 0.8 19.8 ± 1.5 4.9 ± 0.8 31.7 ± 1.7 475 45.2 ± 0.9 22.1 ± 1.2 5.3 ± 0.9 32.7 ± 1.5 500 41.4 ± 0.8 27.1 ± 1.1 9.1 ± 01.1 31.5 ± 1.4 525 39.3 ± 1.1 23.4 ± 0.8 5.8 ± 0.9 37.3 ± 1.9 550 36.0 ± 1.2 21.4 ± 0.8 6.3 ± 0.9 42.6 ± 1.9 3.2 Catalyst Characterization: The FTIR analysis of both catalysts is compared in Fig. 2 . The peaks of CaO after pyrolysis contain two additional peaks when compared to CaO before pyrolysis (raw CaO). A peak at 3200 cm − 1 indicates the existence of the OH group [ 5 ], which is attributed to the presence of Ca(OH) 2 . This confirms the fact that CaO absorbs the aqueous phase of the bio-oil. Further, the C = O peak is due to the absorption of CO 2 to form CaCO 3 . Thus, CaO gets converted to CaCO 3 and Ca(OH) 2 after interacting with the bio-oil and pyro gases. However, significant peaks could not be observed in both cases of Fe 2 O 3 (before and after pyrolysis). The crystalline phase composition of CaO and Fe₂O₃ catalysts after pyrolysis was analyzed using X-ray diffraction (XRD) to examine their structural transformations during the reaction as reported in Fig. 3 . In the case of CaO, distinct phase transformations were observed, confirming its active participation during the reaction. The emergence of new diffraction peaks at 18°, 34.1°, and 50.8° corresponds to Ca(OH)₂, while additional peaks at 29.4°, 39.4°, 43.2°, 47.4°, and 48.5° are characteristic of CaCO₃. These results indicate that CaO underwent carbonation and hydration reactions during pyrolysis, forming CaCO₃ and Ca(OH)₂, respectively. The carbonation reaction occurs via CO₂ absorption from the pyrolysis vapors (CaO + CO₂ → CaCO₃), whereas the hydration reaction results from CaO’s interaction with moisture or reactive hydrogen-containing species (CaO + H₂O → Ca(OH)₂). The presence of these phases was further corroborated by FTIR analysis, thereby confirming the chemical conversion of CaO into its carbonate and hydroxide forms. Such transformations validate the CO₂ absorption and water-capture roles of CaO, consistent with its CO₂-sequestration and mild cracking behavior observed in the gas-phase composition. The XRD profile of Fe₂O₃ before and after pyrolysis revealed partial reduction and phase restructuring. The post-pyrolysis catalyst exhibited additional peaks corresponding to Fe(OH)₃, FeO, and Fe₃O₄, signifying that Fe₂O₃ underwent a redox transformation under the reducing pyrolysis environment. These transformations can be attributed to interactions between Fe₂O₃ and the volatile organic compounds and reducing gases (CO and H₂), resulting in stepwise reduction of Fe³⁺ to Fe²⁺ and formation of mixed oxide phases. The appearance of Fe(OH)₃ further indicates the participation of water–gas shift and reforming reactions, leading to the formation of surface hydroxide groups. The coexistence of hydroxide and oxide phases in both catalysts after pyrolysis suggests that deoxygenation of bio-oil intermediates occurred via catalytic redox and adsorption mechanisms. Thus, the XRD findings confirm that CaO primarily functioned as a CO₂ absorbent and basic cracking medium, while Fe₂O₃ acted as a redox catalyst, promoting extensive deoxygenation and hydrogen production during pyrolysis. 3.3 Catalytic effect on product distribution: The elemental nature of Ca and Fe dictates their catalytic roles, where CaO acts primarily as a basic sorbent and deoxygenation agent, optimizing the liquid-phase quality but reducing overall oil yield, whereas Fe₂O₃ serves as a redox-active hydrogenation and reforming catalyst, enhancing syngas production and gaseous fuel value (Fig. 4 ). In CaO-loaded systems, an inverse relationship between liquid and gas yields was observed, wherein bio-oil decreased from 32.2 wt.% under non-catalytic conditions to 24.4–24.6 wt.% at higher CaO loadings (1:0.4–1:0.6), while the pyro-gas fraction increased from 27.4 wt.% to 35.6 wt.%. The bio-char yield remained largely unchanged (~ 40–41 wt.%), indicating that CaO predominantly facilitates a redistribution between condensable and gaseous fractions rather than affecting solid residue. This behavior arises from the strong basicity and oxygen affinity of CaO, which catalyzes decarboxylation, decarbonylation, and dehydration of oxygenated intermediates [ 23 , 24 ]. Mechanistically, the adsorption and decomposition of carboxylic acids and carbonyl compounds on CaO surfaces generate CO₂, CO, and H₂ while producing light hydrocarbons and aromatics. The dual functionality of CaO as a CO₂ sorbent further enhances the conversion of heavy volatiles by shifting equilibrium toward gas production and reducing oxygen content in the bio-oil, thereby improving its thermal stability and calorific value [ 10 , 23 ]. At higher loadings, the increased availability of basic sites intensifies secondary cracking, progressively reducing condensable oil while promoting pyro-gas and light organics. Additionally, CaO can influence char formation by providing active surfaces for polymeric carbon deposition and modifying reaction equilibria via CO₂ sequestration, although these effects are generally marginal or non-monotonic [ 3 ]. Fe₂O₃ exhibits a fundamentally different catalytic profile, enhancing hydrogen and carbon monoxide concentrations in the pyro-gas while moderately increasing methane formation and slightly reducing CO₂. For example, H₂ increased from 16.6 to 21.5 vol.%, CO from 20.6 to 24.8 vol.%, and CH₄ from 6.8 to 14.6 vol.% under Fe₂O₃ catalysis. Fe₂O₃ functions through transition-metal-mediated redox reactions, where Fe³⁺ is reduced to Fe²⁺ and Fe⁰ under the reducing environment of pyrolysis. These reduced species provide active metallic sites for C–C and C–O bond scission, promoting vapor-phase cracking, reforming, and methanation reactions. The redox cycling (Fe₂O₃ ⇌ Fe₃O₄ ⇌ Fe⁰) enables oxygen transfer between organic intermediates and the oxide lattice, facilitating water–gas shift (CO + H₂O ⇌ CO₂ + H₂) and methanation (CO + 3H₂ → CH₄ + H₂O) reactions that substantially increase the H₂ and CO content of the syngas [ 12 , 25 – 27 ]. Char yield remains relatively stable under Fe₂O₃ catalysis, reflecting a balance between solid carbon gasification via redox reactions and secondary char formation from volatile condensation. Comparatively, the two catalysts emphasize different reaction pathways: CaO predominantly acts through basicity-driven cracking, deoxygenation, and CO₂ capture, shifting condensable fractions toward permanent gases and improving bio-oil quality, whereas Fe₂O₃ leverages redox activity to enhance hydrogen-rich syngas production, facilitate methanation, and reform oxygenated vapors into light gases [ 23 ]. While both catalysts reduce heavy bio-oil content, CaO favors light organics and CO₂ evolution, whereas Fe₂O₃ promotes H₂ and CO enrichment, highlighting its utility in syngas-oriented applications. The selection of catalyst type and loading thus enables targeted control over product distribution and energy density, with CaO being more effective for reducing oxygenated liquids and Fe₂O₃ for hydrogen-enriched gaseous outputs [ 22 , 25 ]. 3.4 Bio-oil Characterization The physical and chemical properties of bio-oils obtained under non-catalytic and catalytic conditions were reported in Table 3 . The higher organic phase fraction observed in the CaO-assisted system corresponds to enhanced cracking and reforming of condensable vapors into light oxygenates and hydrocarbons, driven by the strong basicity and CO₂ sorption capacity of CaO. This effect promotes deoxygenation through decarboxylation and decarbonylation pathways, which is reflected in the significantly increased heating value (30.5 MJ/kg) compared to the non-catalytic oil (22.3 MJ/kg). Similar deoxygenation-induced improvements in calorific value have been reported by Wang et al. [ 7 ] and Reza et al. [ 3 ], indicating that CaO facilitates the conversion of oxygen-rich compounds into energy-dense hydrocarbons. Fe₂O₃ also enhanced the heating value to 28.1 MJ/kg, primarily due to its redox-mediated reforming reactions and partial hydrogenation of unsaturated volatiles [ 25 ]. However, the slightly lower organic fraction compared to CaO suggests that Fe₂O₃ catalysis favors gas-phase reforming and syngas formation more strongly than condensable liquid stabilization [ 17 ]. This density increment is associated with the higher proportion of aromatic and polymeric compounds in the catalytic oils, resulting from secondary vapor-phase reactions and partial condensation [ 9 , 17 ]. The higher viscosity may limit direct utilization in conventional burners, such levels remain comparable to those of typical lignocellulosic bio-oils, suggesting feasibility after minor upgradation steps such as mild hydrotreatment or blending [ 21 ]. The solid and ash contents were generally low across all samples, increasing slightly from 2.1 wt.% to 5.3 wt.% and 0.1 wt.% to 0.8 wt.%, respectively, with catalytic addition. Despite this, the values remain within acceptable limits reported for catalytic pyrolysis oils [ 28 ]. The pH values of all bio-oils were in the range of 4.6–5.1, indicating mildly acidic character and confirming substantial neutralization of carboxylic acids by CaO’s strong basicity [ 7 ]. The catalytic systems not only improved the organic phase yield and energy content but also mitigated the acidity and instability inherent to raw bio-oils [ 8 , 12 ]. Table 3 Comparative analysis of bio-oil physical properties obtained during non-catalytic and catalytic pyrolysis at 500°C. Bio-oil properties Non-catalytic Optimum catalyst loading 40 wt.% CaO 10 wt.% Fe 2 O 3 Organic phase (wt.%) 6.6 15.8 8.3 Heating value (MJ/kg) 22.3 30.5 28.1 Density (kg/m 3 ) 1.03 1.04 1.10 Viscosity at 50°C (cP) 81.9 89.4 96.5 Solid content (wt. %) 2.1 3.2 5.3 Ash content (wt. %) 0.1 0.5 0.8 pH value 4.6 5.1 5.0 During the functional group identification via FTIR, the peak at 3200–3600 cm − 1 (Fig. 5 ) represents stretching vibrations of the -OH group indicating the presence of alcohol or water content in the organic phase [ 20 ]. C = C is associated with lignin at ~ 1629 cm − 1 . The C-H bonds signify the presence of aliphatic hydrocarbons. Peaks of C-O manifest the presence of esters and amides. The FTIR spectra for all three cases are found to be similar. This suggests that bio-oil derived from cotton stalk pyrolysis contains water, alcohols, acids, aromatics, and ethers as possible functional groups. Similar peaks are also obtained by Ma et al. [ 21 ] and Lazzari et al. [ 29 ]. The organic fraction of the liquid product has been characterized using 1 H NMR to identify the chemical composition. It is seen that the 1 H NMR spectrum is found to be similar in the case of non-catalytic and catalytic processes. Peaks at 0.5 to 1 ppm mainly represent the aliphatic protons (-CH 2 and -CH 3 ). The weak intensity during this range indicates a lower content of aliphatic in the non-catalytic case. The aliphatic content was 11% of the total protons in the case of non-catalytic, while CaO and Fe 2 O 3 -based pyrolysis yielded 20% and 16%. The second intensive peaks at 1.5-3 ppm constitute CH 3 -C = C (aromatic or olefin), CH 3 -N. This is found to be stronger in the presence of CaO (40%). This demonstrates that the water from the bio-oil was absorbed in the case of CaO only. The aromatics, olefins, and phenols of bio-oil are represented in the region between 6–9 ppm. Non-catalytic case produced 57% aromatics, while 40% and 48% in the case of CaO and Fe 2 O 3 . Thus, non-catalytic pyrolysis contains more aromatic protons when compared to catalytic. This demonstrates that oxygenated components are retained in the non-catalytic bio-oil. 3.5 Gas Analysis The composition of non-condensable gases produced during the pyrolysis of CS under non-catalytic and catalytic conditions at optimum operating ratios demonstrates significant variations depending on the type of catalyst employed, as presented in Table 4 . The introduction of catalysts markedly influenced these proportions, with both CaO and Fe₂O₃ enhancing the overall content of combustible gases, albeit via different mechanisms. Specifically, the H₂ concentration increased to 19.9 vol.% with 40 wt.% CaO and further to 21.5 vol.% with 10 wt.% Fe₂O₃, indicating that the catalysts effectively promoted dehydrogenation and water–gas shift reactions, which are critical for hydrogen production [ 25 , 28 ]. The CO content showed a moderate increase from 20.6 vol.% in non-catalytic pyrolysis to 21.6 vol.% with CaO and 24.8 vol.% with Fe₂O₃, reflecting the catalysts’ influence on decarbonylation and reforming pathways [ 10 ]. Similarly, CH₄ levels rose substantially, reaching 11.5 vol.% with CaO and 14.6 vol.% with Fe₂O₃, which can be attributed to the secondary cracking of bio-oil intermediates into lighter hydrocarbons [ 22 ]. Table 4 Composition of evolved gas during catalytic and non-catalytic pyrolysis at 500 o C Composition Non-catalytic process Catalytic process 40 wt.% CaO 10 wt.% Fe 2 O 3 H 2 16.6 ± 2.4 19.9 ± 2.1 21.5 ± 2.8 CO 20.6 ± 1.8 21.6 ± 1.2 24.8 ± 1.1 CH 4 6.8 ± 0.7 11.5 ± 1.1 14.6 ± 0.8 CO 2 32.4 ± 1.4 26.7 ± 0.4 29.4 ± 0.9 The reduction in CO₂ content from 32.4 vol.% to 26.7 vol.% in the presence of CaO underscores the dual functionality of this catalyst. CaO acts as an efficient CO₂ absorbent, forming CaCO₃ via the carbonation reaction (CaO + CO₂ → CaCO₃), which effectively removes CO₂ from the gas phase. In addition, the basic sites on CaO promote mild cracking of bio-oil oxygenates, leading to partial decomposition into light gases while preserving a significant portion of bio-oil [ 3 ]. In contrast, Fe₂O₃ functions primarily as a redox-active catalyst, facilitating extensive cracking of high-molecular-weight bio-oil molecules into H₂, CO, and CH₄ via oxidative and reforming pathways, while only slightly reducing CO₂ content through decarbonylation reactions. The enhanced methane formation under Fe₂O₃ catalysis further indicates conversion of larger oxygenated intermediates into light hydrocarbons, highlighting its strong reforming capability [ 7 , 10 , 28 ]. Mechanistically, the effects of the two catalysts can be represented schematically: in CaO-catalyzed pyrolysis, bio-oil intermediates undergo mild cracking, CO₂ is sequestered via carbonation, and combustible gas formation is moderately enhanced, resulting in a balanced distribution of bio-oil and syngas. In Fe₂O₃-catalyzed pyrolysis, bio-oil molecules are extensively cracked and reformed into H₂, CO, and CH₄, with water–gas shift and decarbonylation reactions predominating, leading to a hydrogen-rich syngas with lower CO₂ content [ 25 ]. This schematic interpretation clarifies how catalyst selection can direct pyrolysis toward either bio-oil preservation or maximum syngas yield. The obtained results are consistent with literature reports indicating that CaO primarily functions as a CO₂ sorbent with mild cracking activity, whereas Fe₂O₃ enhances bio-oil decomposition and syngas formation via redox and reforming mechanisms [ 7 , 10 , 28 ]. 4. Conclusions Pyrolysis of the cotton stalk (CS) is performed in a batch-wise pyrolysis reactor to generate bio-oil and bio-char. Based on the above results; it can be concluded that both calcium oxide (CaO) and ferric oxide (Fe₂O₃) are effective in directing the thermochemical conversion pathways toward enhanced product quality and energy recovery. The maximum organic phase yield of 15.3 wt.% and the highest bio-oil calorific value of 30.5 MJ/kg at 40 wt.% CaO loading signify its superior capability for deoxygenation and acid neutralization through carbonation and base-catalyzed cracking mechanisms. Fe₂O₃ catalysis, optimized at 10 wt.% loading, operates through Fe³⁺/Fe²⁺ redox cycles, promoting vapor-phase reforming and water–gas shift reactions, thereby enriching the pyro-gas with H₂, CO, and CH₄. Although the char composition remained largely unaltered under ex-situ operation, its calorific value (18.3–19.1 MJ/kg) underscores potential reuse in thermal applications. Overall, it was concluded that CaO favors decarboxylation and CO₂ capture, whereas Fe₂O₃ enhances hydrogen evolution through oxygen transfer and methanation pathways. The catalytic data indicate that 500°C is optimal for bio-oil maximization in non-catalytic runs, yet selective catalyst choice enables precise control over product profiles. CaO is thus suited for high-quality, low-oxygen bio-oil production, while Fe₂O₃ is advantageous for hydrogen-rich syngas generation. These findings confirm that catalyst type and loading critically determine the reaction pathways, enabling tailored optimization of bio-oil, syngas, or char yields in sustainable biomass-to-energy systems. Declarations Acknowledgment The research work was financially supported by the Indian Council of Agricultural Research (ICAR), New Delhi, Government of India under the Coordinated Research Project (CRP) under the Energy from Agriculture (EA) program. 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10:10:44","extension":"xml","order_by":25,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":120901,"visible":true,"origin":"","legend":"","description":"","filename":"WAVED25023090structuring.xml","url":"https://assets-eu.researchsquare.com/files/rs-7893658/v1/2e9f91551bfc5d8ace4f6f7e.xml"},{"id":96365614,"identity":"e2e2aeab-1c75-4005-9596-674da8bb4332","added_by":"auto","created_at":"2025-11-20 10:10:37","extension":"html","order_by":26,"title":"","display":"","copyAsset":false,"role":"acdc-reference","size":127599,"visible":true,"origin":"","legend":"","description":"","filename":"earlyproof.html","url":"https://assets-eu.researchsquare.com/files/rs-7893658/v1/1eb4d29ba1cba0aa610a31a1.html"},{"id":96365854,"identity":"8e5fb070-2a39-4830-8f02-48c834d5629c","added_by":"auto","created_at":"2025-11-20 10:10:52","extension":"jpg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":92339,"visible":true,"origin":"","legend":"\u003cp\u003eExperimental setup for ex-situ catalytic pyrolysis\u003c/p\u003e","description":"","filename":"1.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7893658/v1/ab5fb63a3d5237b731d2a86e.jpg"},{"id":96322733,"identity":"14885c9b-7b33-4646-a2c5-da558692da91","added_by":"auto","created_at":"2025-11-19 19:42:04","extension":"jpg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":74197,"visible":true,"origin":"","legend":"\u003cp\u003eFTIR spectra of (a) CaO \u0026amp; (b) Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e before and after pyrolysis\u003c/p\u003e","description":"","filename":"2.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7893658/v1/85018ce74c2fe3e3afb323ba.jpg"},{"id":96322735,"identity":"3b7ffea0-d2d0-4d7f-be58-02cd23a651a6","added_by":"auto","created_at":"2025-11-19 19:42:05","extension":"jpg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":183292,"visible":true,"origin":"","legend":"\u003cp\u003eXRD patterns of the raw and used catalysts of (a) fresh CaO, (b) used CaO, (c) fresh Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3,\u003c/sub\u003e and (d) used Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e","description":"","filename":"3.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7893658/v1/d4b31c1dda240cb3f6310fc6.jpg"},{"id":96366164,"identity":"a1e7c3c2-8942-406e-905c-b9cf5ab9c46c","added_by":"auto","created_at":"2025-11-20 10:11:15","extension":"jpg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":250769,"visible":true,"origin":"","legend":"\u003cp\u003eYield of pyrolyzed products at different catalyst loading at 500°C.\u003c/p\u003e","description":"","filename":"4.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7893658/v1/277cf1fb29641305f3760a3f.jpg"},{"id":96322737,"identity":"f54116d9-b0b8-4d33-b001-4c1553f4fb7d","added_by":"auto","created_at":"2025-11-19 19:42:05","extension":"jpg","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":134157,"visible":true,"origin":"","legend":"\u003cp\u003eComparative analysis of FTIR peaks of derived bio-oil at optimum conditions.\u003c/p\u003e","description":"","filename":"5.jpg","url":"https://assets-eu.researchsquare.com/files/rs-7893658/v1/02ced360de2bed0af295995b.jpg"},{"id":98775059,"identity":"3fbc66d1-194f-40e5-ac73-09404c4fc37f","added_by":"auto","created_at":"2025-12-22 12:18:12","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1514021,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-7893658/v1/117578a6-f566-4cfb-be33-72937b6d248c.pdf"}],"financialInterests":"","formattedTitle":"\u003cp\u003eCatalytic Pyrolysis of Cotton Stalk for Bio-Oil Production: Effectiveness of CaO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e Catalysts in Ex-Situ Mode\u003c/p\u003e","fulltext":[{"header":"Highlights","content":"\u003cp\u003e\u0026bull; Non-contact catalytic pyrolysis enhances bio-oil selectivity and energy density\u003c/p\u003e\u003cp\u003e\u0026bull; CaO acts primarily as a basic sorbent and deoxygenation agent\u003c/p\u003e\u003cp\u003e\u0026bull; CaO boosts organic frac. by 139% \u0026amp; achieves 30.5 MJ kg⁻\u0026sup1; calorific value\u003c/p\u003e\u003cp\u003e\u0026bull; Fe₂O₃ results in redox-active hydrogenation \u0026amp; reforming catalyst\u003c/p\u003e\u003cp\u003e\u0026bull; Fe₂O₃ promotes H₂-rich pyro-gas via Fe\u0026sup3;⁺/Fe\u0026sup2;⁺ redox and reforming reactions\u003c/p\u003e\u003cp\u003e\u0026bull; Structural stability of spent catalysts confirmed through XRD and FTIR analyses\u003c/p\u003e\u003cp\u003eNovelty statement\u003c/p\u003e\u003cp\u003eThe novelty of this work lies in its non-contact catalytic configuration, which enables the interaction of catalytic vapors without direct catalyst\u0026ndash;biomass contact, thereby minimizing catalyst fouling and secondary char formation. This configuration effectively enhances organic-phase selectivity, deoxygenation efficiency, and bio-oil stability, while maintaining catalyst reusability. Furthermore, the mechanistic elucidation of carbonate formation (CaO) and Fe\u0026sup3;⁺/Fe\u0026sup2;⁺ redox cycles (Fe₂O₃) provides new insights into vapor-phase catalytic transformations during pyrolysis\u0026mdash;an area that remains underexplored in current biomass conversion literature. This study contributes to the journal\u0026rsquo;s objectives by advancing sustainable thermochemical conversion technologies and offering a carbon-negative route for waste-to-energy valorization.\u003c/p\u003e"},{"header":"1. Introduction","content":"\u003cp\u003eIndia approximately generates 657\u0026nbsp;million tonnes of agro-residues every year (surplus amount accounting for 230\u0026nbsp;million tonnes). This surely can be considered as a potential source to meet the present challenges faced by conventional energy sources. Further, the energy consumption rate is escalating at an alarming rate; thus, an alternative pathway needs to be implemented on an immediate basis to address the energy crisis. The primary energy sources (coal and petroleum) could only satisfy 80% of the total energy demand [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e]. The substantial amount of secondary agro-residues can be beneficial in many aspects such as (a) energy production with no environmental degradation (since it is carbon neutral), (b) waste utilization promoting no landfill disposal, (c) lower NOx and SOx generation than conventional fuels, (d) low ash content; and (e) large scale employment, etc. [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. Agro-residues such as wheat straw, cotton stalk, rice husk, bagasse, groundnut shell, etc., are abundant in the agricultural fields in India, accounting for about 228.52 MMT [\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e]. These residues are generally discarded and thus are often considered a serious issue for disposal. The farmers usually opt for open burning, which causes serious damage to the soil and the environment. Thus, a feasible and environmentally friendly technique is needed to utilize the residue such that the maximum extractable energy can be recovered.\u003c/p\u003e\u003cp\u003eAgro-based residues can produce renewable energy by various thermochemical processes (pyrolysis, combustion, and gasification). India presently produces 6\u0026nbsp;million tonnes of cotton, thereby generating 15\u0026nbsp;million tonnes of cotton stalk (CS) annually [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e]. It is not suitable as animal fodder due to its high lignin content. Thus, recent literature has focused on thermally degrading it to produce value-added products. The thermochemical methods are preferred over biochemical methods since it is cheaper, less complicated, require less time to process, and additional pre-processing of lignin is not required. Pyrolysis process has been proven to be more efficient and practical process for the production of bio-oil and bio-char than other thermal treatment processes [\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e]. It is seen that special attention is given to converting renewable feedstock into bio-oil since it has 5\u0026ndash;10 times higher heating value than its raw sample [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. Further, the bio-oil served multiple purposes, such as fuel in boilers and furnaces, or producing various chemical feedstocks [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eThe catalytic pyrolysis process is preferred over the non-catalytic process as the obtained bio-oil possesses improved quality compared to the non-catalytic process [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. The catalytic process can be conducted in two modes, i.e., in-situ and ex-situ modes. For in-situ-based pyrolysis, the catalyst is mixed with the solid feed and is introduced into a single reactor. When both the feed and the catalysts are fed into two separate reactors, it is referred to as ex-situ pyrolysis. Thus, the pyrolyzed products (mainly in the vapor and liquid phases) only interact with the catalyst in the secondary reactor. Lin et al. [\u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e] performed cotton stalk pyrolysis in a fluidized bed reactor in the presence of CaO in in-situ mode. They observed that the bio-oil quality improved during catalytic pyrolysis due to the reduction in oxygenated compounds. They concluded that the presence of CaO is beneficial since it promotes dehydration reactions. Chen et al. [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e] tested the performance of CaO with CS in a fixed-bed reactor for bio-oil production in an in-situ manner. The optimum temperature was selected at 600\u0026deg;C for high CaO functioning since further high temperatures resulted in secondary cracking of oil. The organic fraction reduced when operated beyond 20 wt.% CaO loading. They observed that CaO plays multiple roles in pyrolysis, i.e., as an absorbent, reactant, and as catalyst. The bio-oil quality improved in the presence of CaO due to the increase in ketone, furans, and hydrocarbon content, while a decrease in acidic and anhydro-sugar contents. The major challenge in the upscaling of biomass pyrolysis is the bio-oil quality. It should be of superior quality either at the production point or by upgrading it. The untreated bio-oils usually contain water, acids, aldehydes, etc., which reduces the potential of the bio-oil [\u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e]. It can be upgraded either by hydro-processing, esterification, or by catalytic cracking [\u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Catalytic pyrolysis is preferred over non-catalytic due to the improvement in bio-oil quality since it promotes deoxygenation, dehydrogenation, cracking, and aromatization [\u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. It cracks the heavy molecules into a lesser viscous oil, thereby providing better stability and higher heating value. Iisa et al. [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e] observed that bio-oil generated from pine in a fluidized bed reactor had 45% lower oxygen content during catalytic pyrolysis when compared to the non-catalytic process.\u003c/p\u003e\u003cp\u003eThe ex-situ process is preferred to the in-situ process since catalyst regeneration is easy due to the non-mixing of catalysts with the char (unlike the in-situ process). Further, catalyst deactivation and coking occur at a faster rate in the in-situ configuration [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e]. Shafaghat et al. [\u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e] conducted pyrolysis in ex-situ and in-situ mode using empty fruit bunches at 400\u0026ndash;600\u0026deg;C. They observed that the ex-situ mode produced more aromatics and fewer oxygenated components than the in-situ mode. Similarly, Leng et al. [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e] compared the in-situ pyrolysis and ex-situ pyrolysis by pyrolyzing poplar at 700\u0026deg;C. They observed that the carbonaceous residue during ex-situ and in-situ was found to be 18.6% and 31.3%. The higher residues resulted from the mixing of the catalyst with the char particles.\u003c/p\u003e\u003cp\u003eThe present study aims to provide a comparative analysis of the effect of catalysts on CS pyrolysis in a batch reactor in an extensive manner. It is mainly focused on the bio-oil yield and quality including the effect on catalyst of volatiles during ex-situ pyrolysis. The process optimization (temperature, catalyst, and catalyst loading) was implemented to obtain the maximum organic phase in the bio-oil. No reported literature has compared the performance of CaO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e based pyrolysis with any biomass in ex-situ mode. Thus, exhaustive data is presented. The detailed characterization of both catalysts was also conducted to determine their composition after pyrolysis. Further, the basic characterization of the bio-oil obtained during non-catalytic and catalytic trials was also examined.\u003c/p\u003e"},{"header":"2. Materials and methods","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e\u003ch2\u003e2.1 Feed material\u003c/h2\u003e\u003cp\u003eThe feed material (Cotton stalk) was acquired from the nearby village of Jambusar, Bharuch, Gujarat. Before proceeding to experiments, the feed material was crushed via a hammer mill up to a maximum size of 2 mm and was later used in experiments. The feed material was characterized for its physical and chemical characteristics according to the ASTM methods and is reported in Table \u003cspan refid=\"Tab1\" class=\"InternalRef\"\u003e1\u003c/span\u003e.\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\u003ePhysical and compositional characterization of feedstock (Cotton Stalk)\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eComponents\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eValue\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eComponent\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eValue\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003e\u003cem\u003eUltimate Analysis\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003e\u003cem\u003eProximate Analysis\u003c/em\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCarbon (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e36.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eMoisture (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e12.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHydrogen (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e7.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eVolatile matter (wt.%)\u003csup\u003e#\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e75.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eNitrogen (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e3.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eAsh (wt.%)\u003csup\u003e#\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e6.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSulphur (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eFixed carbon (wt.%)*\u003csup\u003e#\u003c/sup\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e17.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOxygen (wt.%)*\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e47.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colspan=\"2\" nameend=\"c2\" namest=\"c1\"\u003e\u003cp\u003e\u003cb\u003eLignocellulosic Analysis\u003c/b\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHemicellulose (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e12.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eBulk Density (kg/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e190.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCellulose (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e37.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u003cp\u003eCalorific Value (MJ/kg)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u003cp\u003e15.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eLignin (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e26.0\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e\u003cem\u003e*by balance;\u003c/em\u003e \u003csup\u003e\u003cem\u003e#\u003c/em\u003e\u003c/sup\u003e \u003cem\u003edry basis\u003c/em\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c3\"\u003e\u0026nbsp;\u003c/td\u003e\u003ctd align=\"left\" colname=\"c4\"\u003e\u0026nbsp;\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=\"Sec4\" class=\"Section2\"\u003e\u003ch2\u003e2.2 Catalysts\u003c/h2\u003e\u003cp\u003eDuring this catalytic study, the cost of the catalyst and its availability are the major concerns. In this regard, the catalysts used in this study are of industrial grade and are readily available at a lower cost in the market. CaO was procured from local vendors, while Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e was purchased from Tata Pigments Ltd. The particle sizes of CaO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e were 0.4\u0026ndash;25.0 mm and 0.4-4.0 mm, respectively.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec5\" class=\"Section2\"\u003e\u003ch2\u003e2.3 Experimental procedure\u003c/h2\u003e\u003cp\u003eThe experimental system consisted of a 25 L stainless steel (SS316) pyrolysis reactor, electrically heated and equipped with an internal mechanical stirrer to ensure uniform mixing and effective heat transfer within the feedstock, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003e. A separate vertical catalytic reactor (or secondary chamber) was integrated downstream of the primary reactor to facilitate ex-situ catalytic upgrading. This chamber, equipped with an independent heating system, was designed to prevent condensation of volatiles and ensure that only gaseous products interacted with the catalyst bed. The feedstock was introduced through the reactor lid, which was subsequently sealed to maintain system integrity. Nitrogen was continuously purged at a rate of 100 mL/min throughout the experiment to sustain an inert environment. Heating was initiated at a controlled rate until the target temperature was achieved, followed by an isothermal holding period of 30\u0026ndash;60 min, depending on the experimental conditions.\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAs the temperature increased, the biomass underwent thermal decomposition, generating volatile compounds that were carried by the nitrogen stream from the primary reactor into the catalytic chamber, where secondary catalytic reactions occurred at varying loadings. The resulting gas stream was then directed through a condenser for recovery of condensable liquids (bio-oil), while the non-condensable gases were subsequently flared. Gas samples were collected at specified intervals for compositional analysis. Each experimental condition was repeated twice to ensure reproducibility, and the mean values were reported.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec6\" class=\"Section2\"\u003e\u003ch2\u003e2.4 Characterization methods\u003c/h2\u003e\u003cp\u003eThe proximate analysis (moisture content, volatile matter, ash, and fixed carbon) was estimated as per the ASTM standards i.e., moisture content (D-871-82), volatile matter (ASTM E-872), and ash content (ASTM E-1755-01). The ultimate analysis was conducted by CHNS Analyzer (2400 Series II, Perkin Elmer, USA). The calorific value was estimated using an automated bomb calorimeter (C5000, IKA, Germany). The FTIR of bio-oils and catalysts were analyzed using Bruker Alpha II. The range was maintained at 400 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e to 4000 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e with a resolution of 4 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The \u003csup\u003e1\u003c/sup\u003eH NMR spectra of the liquid fraction were obtained from 400 MHz FT-NMR (Bruker). The phase identification of the catalysts during the pre- and post-pyrolysis was done by X-ray diffraction (Xpert MPD). The non-condensable gases after passing through the condensers are collected and subsequently characterized using a gas chromatograph (Make: Sigma IPL, Vadodara). The gas composition values were determined on a dry basis vol. %.\u003c/p\u003e\u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cp\u003e\u003cb\u003e3.1 Batch Pyrolysis\u003c/b\u003e: To investigate the maximum bio-oil recovery during non-catalytic pyrolysis, cotton stalk feedstock was pyrolyzed at temperatures ranging from 450\u0026deg;C to 550\u0026deg;C, and the product yield distribution is presented in Table \u003cspan refid=\"Tab2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. As the temperature increased from 450\u0026deg;C to 550\u0026deg;C, the bio-char yield decreased significantly from 48.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 wt.% to 36.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2 wt.%, indicating enhanced devolatilization and secondary cracking reactions that convert solid carbonaceous material into volatile compounds [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e]. Conversely, the pyro-gas yield exhibited a steady increase from 31.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7 wt.% to 40.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.9 wt.% with rising temperature, which can be attributed to the thermal decomposition of heavier organic molecules into lighter gaseous species at higher temperatures [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The bio-oil fraction initially increased from 19.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5 wt.% at 450\u0026deg;C to a maximum of 27.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1 wt.% at 500\u0026deg;C, suggesting that this temperature favored the condensation of primary pyrolysis vapors. However, further increase in temperature to 525\u0026deg;C and 550\u0026deg;C led to a decrease in liquid yield to 23.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8 wt.% and 21.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.08 wt.%, respectively. This decline is associated with secondary cracking and reforming of condensable vapors into non-condensable gases, thereby reducing the overall liquid recovery [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e]. These observations confirm that 500\u0026deg;C represents the optimum temperature for maximizing bio-oil yield during non-catalytic pyrolysis of cotton stalks.\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab2\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 2\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eEffect of operating temperature on CS pyrolysis products\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"5\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c5\" colnum=\"5\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eTemperature\u003c/p\u003e\u003cp\u003e(\u0026deg;C)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eBio-char\u003c/p\u003e\u003cp\u003e(wt.%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003eBio-oil\u003c/p\u003e\u003cp\u003e(wt.%)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003eOrganic phase\u003c/p\u003e\u003cp\u003e(wt. %)\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c5\"\u003e\u003cp\u003ePyro-gas\u003c/p\u003e\u003cp\u003e(wt.%)\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e450\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e48.5\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e19.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e4.9\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e31.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.7\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e475\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e45.2\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e22.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e5.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e32.7\u0026thinsp;\u0026plusmn;\u0026thinsp;1.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e500\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e41.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e27.1\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e9.1\u0026thinsp;\u0026plusmn;\u0026thinsp;01.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e31.5\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e525\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e39.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e23.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e5.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e37.3\u0026thinsp;\u0026plusmn;\u0026thinsp;1.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003e550\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e36.0\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e21.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e6.3\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c5\"\u003e\u003cp\u003e42.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cdiv id=\"Sec8\" class=\"Section2\"\u003e\u003ch2\u003e3.2 Catalyst Characterization:\u003c/h2\u003e\u003cp\u003eThe FTIR analysis of both catalysts is compared in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e. The peaks of CaO after pyrolysis contain two additional peaks when compared to CaO before pyrolysis (raw CaO). A peak at 3200 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e indicates the existence of the OH group [\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e], which is attributed to the presence of Ca(OH)\u003csub\u003e2\u003c/sub\u003e. This confirms the fact that CaO absorbs the aqueous phase of the bio-oil. Further, the C\u0026thinsp;=\u0026thinsp;O peak is due to the absorption of CO\u003csub\u003e2\u003c/sub\u003e to form CaCO\u003csub\u003e3\u003c/sub\u003e. Thus, CaO gets converted to CaCO\u003csub\u003e3\u003c/sub\u003e and Ca(OH)\u003csub\u003e2\u003c/sub\u003e after interacting with the bio-oil and pyro gases. However, significant peaks could not be observed in both cases of Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e (before and after pyrolysis).\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe crystalline phase composition of CaO and Fe₂O₃ catalysts after pyrolysis was analyzed using X-ray diffraction (XRD) to examine their structural transformations during the reaction as reported in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. In the case of CaO, distinct phase transformations were observed, confirming its active participation during the reaction. The emergence of new diffraction peaks at 18\u0026deg;, 34.1\u0026deg;, and 50.8\u0026deg; corresponds to Ca(OH)₂, while additional peaks at 29.4\u0026deg;, 39.4\u0026deg;, 43.2\u0026deg;, 47.4\u0026deg;, and 48.5\u0026deg; are characteristic of CaCO₃. These results indicate that CaO underwent carbonation and hydration reactions during pyrolysis, forming CaCO₃ and Ca(OH)₂, respectively. The carbonation reaction occurs via CO₂ absorption from the pyrolysis vapors (CaO\u0026thinsp;+\u0026thinsp;CO₂ \u0026rarr; CaCO₃), whereas the hydration reaction results from CaO\u0026rsquo;s interaction with moisture or reactive hydrogen-containing species (CaO\u0026thinsp;+\u0026thinsp;H₂O \u0026rarr; Ca(OH)₂). The presence of these phases was further corroborated by FTIR analysis, thereby confirming the chemical conversion of CaO into its carbonate and hydroxide forms. Such transformations validate the CO₂ absorption and water-capture roles of CaO, consistent with its CO₂-sequestration and mild cracking behavior observed in the gas-phase composition.\u003c/p\u003e\u003cp\u003eThe XRD profile of Fe₂O₃ before and after pyrolysis revealed partial reduction and phase restructuring. The post-pyrolysis catalyst exhibited additional peaks corresponding to Fe(OH)₃, FeO, and Fe₃O₄, signifying that Fe₂O₃ underwent a redox transformation under the reducing pyrolysis environment. These transformations can be attributed to interactions between Fe₂O₃ and the volatile organic compounds and reducing gases (CO and H₂), resulting in stepwise reduction of Fe\u0026sup3;⁺ to Fe\u0026sup2;⁺ and formation of mixed oxide phases. The appearance of Fe(OH)₃ further indicates the participation of water\u0026ndash;gas shift and reforming reactions, leading to the formation of surface hydroxide groups. The coexistence of hydroxide and oxide phases in both catalysts after pyrolysis suggests that deoxygenation of bio-oil intermediates occurred via catalytic redox and adsorption mechanisms. Thus, the XRD findings confirm that CaO primarily functioned as a CO₂ absorbent and basic cracking medium, while Fe₂O₃ acted as a redox catalyst, promoting extensive deoxygenation and hydrogen production during pyrolysis.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec9\" class=\"Section2\"\u003e\u003ch2\u003e3.3 Catalytic effect on product distribution:\u003c/h2\u003e\u003cp\u003eThe elemental nature of \u003cem\u003eCa\u003c/em\u003e and \u003cem\u003eFe\u003c/em\u003e dictates their catalytic roles, where CaO acts primarily as a basic sorbent and deoxygenation agent, optimizing the liquid-phase quality but reducing overall oil yield, whereas Fe₂O₃ serves as a redox-active hydrogenation and reforming catalyst, enhancing syngas production and gaseous fuel value (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003e). In CaO-loaded systems, an inverse relationship between liquid and gas yields was observed, wherein bio-oil decreased from 32.2 wt.% under non-catalytic conditions to 24.4\u0026ndash;24.6 wt.% at higher CaO loadings (1:0.4\u0026ndash;1:0.6), while the pyro-gas fraction increased from 27.4 wt.% to 35.6 wt.%. The bio-char yield remained largely unchanged (~\u0026thinsp;40\u0026ndash;41 wt.%), indicating that CaO predominantly facilitates a redistribution between condensable and gaseous fractions rather than affecting solid residue. This behavior arises from the strong basicity and oxygen affinity of CaO, which catalyzes decarboxylation, decarbonylation, and dehydration of oxygenated intermediates [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e, \u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e]. Mechanistically, the adsorption and decomposition of carboxylic acids and carbonyl compounds on CaO surfaces generate CO₂, CO, and H₂ while producing light hydrocarbons and aromatics. The dual functionality of CaO as a CO₂ sorbent further enhances the conversion of heavy volatiles by shifting equilibrium toward gas production and reducing oxygen content in the bio-oil, thereby improving its thermal stability and calorific value [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eAt higher loadings, the increased availability of basic sites intensifies secondary cracking, progressively reducing condensable oil while promoting pyro-gas and light organics. Additionally, CaO can influence char formation by providing active surfaces for polymeric carbon deposition and modifying reaction equilibria via CO₂ sequestration, although these effects are generally marginal or non-monotonic [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eFe₂O₃ exhibits a fundamentally different catalytic profile, enhancing hydrogen and carbon monoxide concentrations in the pyro-gas while moderately increasing methane formation and slightly reducing CO₂. For example, H₂ increased from 16.6 to 21.5 vol.%, CO from 20.6 to 24.8 vol.%, and CH₄ from 6.8 to 14.6 vol.% under Fe₂O₃ catalysis. Fe₂O₃ functions through transition-metal-mediated redox reactions, where Fe\u0026sup3;⁺ is reduced to Fe\u0026sup2;⁺ and Fe⁰ under the reducing environment of pyrolysis. These reduced species provide active metallic sites for C\u0026ndash;C and C\u0026ndash;O bond scission, promoting vapor-phase cracking, reforming, and methanation reactions. The redox cycling (Fe₂O₃ ⇌ Fe₃O₄ ⇌ Fe⁰) enables oxygen transfer between organic intermediates and the oxide lattice, facilitating water\u0026ndash;gas shift (CO\u0026thinsp;+\u0026thinsp;H₂O ⇌ CO₂ + H₂) and methanation (CO\u0026thinsp;+\u0026thinsp;3H₂ \u0026rarr; CH₄ + H₂O) reactions that substantially increase the H₂ and CO content of the syngas [\u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan additionalcitationids=\"CR26\" citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eChar yield remains relatively stable under Fe₂O₃ catalysis, reflecting a balance between solid carbon gasification via redox reactions and secondary char formation from volatile condensation. Comparatively, the two catalysts emphasize different reaction pathways: CaO predominantly acts through basicity-driven cracking, deoxygenation, and CO₂ capture, shifting condensable fractions toward permanent gases and improving bio-oil quality, whereas Fe₂O₃ leverages redox activity to enhance hydrogen-rich syngas production, facilitate methanation, and reform oxygenated vapors into light gases [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e]. While both catalysts reduce heavy bio-oil content, CaO favors light organics and CO₂ evolution, whereas Fe₂O₃ promotes H₂ and CO enrichment, highlighting its utility in syngas-oriented applications. The selection of catalyst type and loading thus enables targeted control over product distribution and energy density, with CaO being more effective for reducing oxygenated liquids and Fe₂O₃ for hydrogen-enriched gaseous outputs [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e, \u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec10\" class=\"Section2\"\u003e\u003ch2\u003e3.4 Bio-oil Characterization\u003c/h2\u003e\u003cp\u003eThe physical and chemical properties of bio-oils obtained under non-catalytic and catalytic conditions were reported in Table \u003cspan refid=\"Tab3\" class=\"InternalRef\"\u003e3\u003c/span\u003e. The higher organic phase fraction observed in the CaO-assisted system corresponds to enhanced cracking and reforming of condensable vapors into light oxygenates and hydrocarbons, driven by the strong basicity and CO₂ sorption capacity of CaO. This effect promotes deoxygenation through decarboxylation and decarbonylation pathways, which is reflected in the significantly increased heating value (30.5 MJ/kg) compared to the non-catalytic oil (22.3 MJ/kg). Similar deoxygenation-induced improvements in calorific value have been reported by Wang et al. [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e] and Reza et al. [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e], indicating that CaO facilitates the conversion of oxygen-rich compounds into energy-dense hydrocarbons. Fe₂O₃ also enhanced the heating value to 28.1 MJ/kg, primarily due to its redox-mediated reforming reactions and partial hydrogenation of unsaturated volatiles [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. However, the slightly lower organic fraction compared to CaO suggests that Fe₂O₃ catalysis favors gas-phase reforming and syngas formation more strongly than condensable liquid stabilization [\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. This density increment is associated with the higher proportion of aromatic and polymeric compounds in the catalytic oils, resulting from secondary vapor-phase reactions and partial condensation [\u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. The higher viscosity may limit direct utilization in conventional burners, such levels remain comparable to those of typical lignocellulosic bio-oils, suggesting feasibility after minor upgradation steps such as mild hydrotreatment or blending [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e]. The solid and ash contents were generally low across all samples, increasing slightly from 2.1 wt.% to 5.3 wt.% and 0.1 wt.% to 0.8 wt.%, respectively, with catalytic addition. Despite this, the values remain within acceptable limits reported for catalytic pyrolysis oils [\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The pH values of all bio-oils were in the range of 4.6\u0026ndash;5.1, indicating mildly acidic character and confirming substantial neutralization of carboxylic acids by CaO\u0026rsquo;s strong basicity [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e]. The catalytic systems not only improved the organic phase yield and energy content but also mitigated the acidity and instability inherent to raw bio-oils [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab3\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 3\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eComparative analysis of bio-oil physical properties obtained during non-catalytic and catalytic pyrolysis at 500\u0026deg;C.\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\".\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eBio-oil properties\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\" morerows=\"1\" rowspan=\"2\"\u003e\u003cp\u003eNon-catalytic\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003eOptimum catalyst loading\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e40 wt.% CaO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10 wt.% Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eOrganic phase (wt.%)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e6.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e15.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e8.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eHeating value (MJ/kg)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e22.3\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e30.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e28.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eDensity (kg/m\u003csup\u003e3\u003c/sup\u003e)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e1.03\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e1.04\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e1.10\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eViscosity at 50\u0026deg;C (cP)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e81.9\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e89.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e96.5\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eSolid content (wt. %)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e2.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e3.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.3\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eAsh content (wt. %)\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e0.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e0.5\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e0.8\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003epH value\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c2\"\u003e\u003cp\u003e4.6\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c3\"\u003e\u003cp\u003e5.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\".\" colname=\"c4\"\u003e\u003cp\u003e5.0\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eDuring the functional group identification via FTIR, the peak at 3200\u0026ndash;3600 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e) represents stretching vibrations of the -OH group indicating the presence of alcohol or water content in the organic phase [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e]. C\u0026thinsp;=\u0026thinsp;C is associated with lignin at ~\u0026thinsp;1629 cm\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e. The C-H bonds signify the presence of aliphatic hydrocarbons. Peaks of C-O manifest the presence of esters and amides. The FTIR spectra for all three cases are found to be similar. This suggests that bio-oil derived from cotton stalk pyrolysis contains water, alcohols, acids, aromatics, and ethers as possible functional groups. Similar peaks are also obtained by Ma et al. [\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e] and Lazzari et al. [\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003c/p\u003e\u003cp\u003eThe organic fraction of the liquid product has been characterized using \u003csup\u003e1\u003c/sup\u003eH NMR to identify the chemical composition. It is seen that the \u003csup\u003e1\u003c/sup\u003eH NMR spectrum is found to be similar in the case of non-catalytic and catalytic processes. Peaks at 0.5 to 1 ppm mainly represent the aliphatic protons (-CH\u003csub\u003e2\u003c/sub\u003e and -CH\u003csub\u003e3\u003c/sub\u003e). The weak intensity during this range indicates a lower content of aliphatic in the non-catalytic case. The aliphatic content was 11% of the total protons in the case of non-catalytic, while CaO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e-based pyrolysis yielded 20% and 16%. The second intensive peaks at 1.5-3 ppm constitute CH\u003csub\u003e3\u003c/sub\u003e-C\u0026thinsp;=\u0026thinsp;C (aromatic or olefin), CH\u003csub\u003e3\u003c/sub\u003e-N. This is found to be stronger in the presence of CaO (40%). This demonstrates that the water from the bio-oil was absorbed in the case of CaO only. The aromatics, olefins, and phenols of bio-oil are represented in the region between 6\u0026ndash;9 ppm. Non-catalytic case produced 57% aromatics, while 40% and 48% in the case of CaO and Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e. Thus, non-catalytic pyrolysis contains more aromatic protons when compared to catalytic. This demonstrates that oxygenated components are retained in the non-catalytic bio-oil.\u003c/p\u003e\u003c/div\u003e\u003cdiv id=\"Sec11\" class=\"Section2\"\u003e\u003ch2\u003e3.5 Gas Analysis\u003c/h2\u003e\u003cp\u003eThe composition of non-condensable gases produced during the pyrolysis of CS under non-catalytic and catalytic conditions at optimum operating ratios demonstrates significant variations depending on the type of catalyst employed, as presented in Table\u0026nbsp;\u003cspan refid=\"Tab4\" class=\"InternalRef\"\u003e4\u003c/span\u003e. The introduction of catalysts markedly influenced these proportions, with both CaO and Fe₂O₃ enhancing the overall content of combustible gases, albeit via different mechanisms. Specifically, the H₂ concentration increased to 19.9 vol.% with 40 wt.% CaO and further to 21.5 vol.% with 10 wt.% Fe₂O₃, indicating that the catalysts effectively promoted dehydrogenation and water\u0026ndash;gas shift reactions, which are critical for hydrogen production [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e]. The CO content showed a moderate increase from 20.6 vol.% in non-catalytic pyrolysis to 21.6 vol.% with CaO and 24.8 vol.% with Fe₂O₃, reflecting the catalysts\u0026rsquo; influence on decarbonylation and reforming pathways [\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e]. Similarly, CH₄ levels rose substantially, reaching 11.5 vol.% with CaO and 14.6 vol.% with Fe₂O₃, which can be attributed to the secondary cracking of bio-oil intermediates into lighter hydrocarbons [\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e].\u003c/p\u003e\u003cp\u003e\u003cdiv class=\"gridtable\"\u003e\u003ctable float=\"Yes\" id=\"Tab4\" border=\"1\"\u003e\u003ccaption language=\"En\"\u003e\u003cdiv class=\"CaptionNumber\"\u003eTable 4\u003c/div\u003e\u003cdiv class=\"CaptionContent\"\u003e\u003cp\u003eComposition of evolved gas during catalytic and non-catalytic pyrolysis at 500 \u003csup\u003eo\u003c/sup\u003eC\u003c/p\u003e\u003c/div\u003e\u003c/caption\u003e\u003ccolgroup cols=\"4\"\u003e\u003cdiv align=\"left\" class=\"colspec\" colname=\"c1\" colnum=\"1\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c2\" colnum=\"2\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c3\" colnum=\"3\"\u003e\u003c/div\u003e\u003cdiv align=\"char\" char=\"\u0026plusmn;\" class=\"colspec\" colname=\"c4\" colnum=\"4\"\u003e\u003c/div\u003e\u003cthead\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u003cp\u003eComposition\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u003cp\u003eNon-catalytic process\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colspan=\"2\" nameend=\"c4\" namest=\"c3\"\u003e\u003cp\u003eCatalytic process\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003ctr\u003e\u003cth align=\"left\" colname=\"c1\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c2\"\u003e\u0026nbsp;\u003c/th\u003e\u003cth align=\"left\" colname=\"c3\"\u003e\u003cp\u003e40 wt.% CaO\u003c/p\u003e\u003c/th\u003e\u003cth align=\"left\" colname=\"c4\"\u003e\u003cp\u003e10 wt.% Fe\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e3\u003c/sub\u003e\u003c/p\u003e\u003c/th\u003e\u003c/tr\u003e\u003c/thead\u003e\u003ctbody\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eH\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e16.6\u0026thinsp;\u0026plusmn;\u0026thinsp;2.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e19.9\u0026thinsp;\u0026plusmn;\u0026thinsp;2.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e21.5\u0026thinsp;\u0026plusmn;\u0026thinsp;2.8\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCO\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e20.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.8\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e21.6\u0026thinsp;\u0026plusmn;\u0026thinsp;1.2\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e24.8\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCH\u003csub\u003e4\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e6.8\u0026thinsp;\u0026plusmn;\u0026thinsp;0.7\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e11.5\u0026thinsp;\u0026plusmn;\u0026thinsp;1.1\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e14.6\u0026thinsp;\u0026plusmn;\u0026thinsp;0.8\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003ctr\u003e\u003ctd align=\"left\" colname=\"c1\"\u003e\u003cp\u003eCO\u003csub\u003e2\u003c/sub\u003e\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c2\"\u003e\u003cp\u003e32.4\u0026thinsp;\u0026plusmn;\u0026thinsp;1.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c3\"\u003e\u003cp\u003e26.7\u0026thinsp;\u0026plusmn;\u0026thinsp;0.4\u003c/p\u003e\u003c/td\u003e\u003ctd align=\"char\" char=\"\u0026plusmn;\" colname=\"c4\"\u003e\u003cp\u003e29.4\u0026thinsp;\u0026plusmn;\u0026thinsp;0.9\u003c/p\u003e\u003c/td\u003e\u003c/tr\u003e\u003c/tbody\u003e\u003c/colgroup\u003e\u003c/table\u003e\u003c/div\u003e\u003c/p\u003e\u003cp\u003eThe reduction in CO₂ content from 32.4 vol.% to 26.7 vol.% in the presence of CaO underscores the dual functionality of this catalyst. CaO acts as an efficient CO₂ absorbent, forming CaCO₃ via the carbonation reaction (CaO\u0026thinsp;+\u0026thinsp;CO₂ \u0026rarr; CaCO₃), which effectively removes CO₂ from the gas phase. In addition, the basic sites on CaO promote mild cracking of bio-oil oxygenates, leading to partial decomposition into light gases while preserving a significant portion of bio-oil [\u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e]. In contrast, Fe₂O₃ functions primarily as a redox-active catalyst, facilitating extensive cracking of high-molecular-weight bio-oil molecules into H₂, CO, and CH₄ via oxidative and reforming pathways, while only slightly reducing CO₂ content through decarbonylation reactions. The enhanced methane formation under Fe₂O₃ catalysis further indicates conversion of larger oxygenated intermediates into light hydrocarbons, highlighting its strong reforming capability [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\u003cp\u003eMechanistically, the effects of the two catalysts can be represented schematically: in CaO-catalyzed pyrolysis, bio-oil intermediates undergo mild cracking, CO₂ is sequestered via carbonation, and combustible gas formation is moderately enhanced, resulting in a balanced distribution of bio-oil and syngas. In Fe₂O₃-catalyzed pyrolysis, bio-oil molecules are extensively cracked and reformed into H₂, CO, and CH₄, with water\u0026ndash;gas shift and decarbonylation reactions predominating, leading to a hydrogen-rich syngas with lower CO₂ content [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e]. This schematic interpretation clarifies how catalyst selection can direct pyrolysis toward either bio-oil preservation or maximum syngas yield. The obtained results are consistent with literature reports indicating that CaO primarily functions as a CO₂ sorbent with mild cracking activity, whereas Fe₂O₃ enhances bio-oil decomposition and syngas formation via redox and reforming mechanisms [\u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e, \u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e].\u003c/p\u003e\u003c/div\u003e"},{"header":"4. Conclusions","content":"\u003cp\u003e\u003cdiv class=\"BlockQuote\"\u003e\u003cp\u003ePyrolysis of the cotton stalk (CS) is performed in a batch-wise pyrolysis reactor to generate bio-oil and bio-char. Based on the above results; it can be concluded that both calcium oxide (CaO) and ferric oxide (Fe₂O₃) are effective in directing the thermochemical conversion pathways toward enhanced product quality and energy recovery. The maximum organic phase yield of 15.3 wt.% and the highest bio-oil calorific value of 30.5 MJ/kg at 40 wt.% CaO loading signify its superior capability for deoxygenation and acid neutralization through carbonation and base-catalyzed cracking mechanisms. Fe₂O₃ catalysis, optimized at 10 wt.% loading, operates through Fe\u0026sup3;⁺/Fe\u0026sup2;⁺ redox cycles, promoting vapor-phase reforming and water\u0026ndash;gas shift reactions, thereby enriching the pyro-gas with H₂, CO, and CH₄. Although the char composition remained largely unaltered under ex-situ operation, its calorific value (18.3\u0026ndash;19.1 MJ/kg) underscores potential reuse in thermal applications. Overall, it was concluded that CaO favors decarboxylation and CO₂ capture, whereas Fe₂O₃ enhances hydrogen evolution through oxygen transfer and methanation pathways. The catalytic data indicate that 500\u0026deg;C is optimal for bio-oil maximization in non-catalytic runs, yet selective catalyst choice enables precise control over product profiles. CaO is thus suited for high-quality, low-oxygen bio-oil production, while Fe₂O₃ is advantageous for hydrogen-rich syngas generation. These findings confirm that catalyst type and loading critically determine the reaction pathways, enabling tailored optimization of bio-oil, syngas, or char yields in sustainable biomass-to-energy systems.\u003c/p\u003e\u003c/div\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003ch2\u003eAcknowledgment\u003c/h2\u003e\u003cp\u003eThe research work was financially supported by the Indian Council of Agricultural Research (ICAR), New Delhi, Government of India under the Coordinated Research Project (CRP) under the Energy from Agriculture (EA) program. The authors wish to acknowledge the support of the ICAR and the collaborative organizations under the ICAR-CRP for their cooperation and in providing the requisite research facilities. The authors would also like to thank Director, SPRERI for allowing us to conduct the research work at their institute.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\u003cli\u003e\u003cspan\u003eTagade, A., Kirti, N., Sawarkar, A.N.: Pyrolysis of agricultural crop residues: an overview of researches by Indian scientific community. Bioresource Technol. Rep. \u003cb\u003e15\u003c/b\u003e, 100761 (2021). \u003cspan class=\"ExternalRef\"\u003e\u003cspan class=\"RefSource\"\u003ehttps://doi.org/10.1016/j.biteb.2021.100761\u003c/span\u003e\u003cspan address=\"10.1016/j.biteb.2021.100761\" targettype=\"DOI\" class=\"RefTarget\"\u003e\u003c/span\u003e\u003c/span\u003e\u003c/span\u003e\u003c/li\u003e\u003cli\u003e\u003cspan\u003eRobinson, A.M., Hensley, J.E., Medlin, J.W.: Bifunctional catalysts for upgrading of biomass-derived oxygenates: a review. 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A., Primaz, C. T., Silva, A. N., Ferrao, M.F., \u0026hellip; Caramao, E. B. (2018). Classification of biomass through their pyrolytic bio-oil composition using FTIR and PCA analysis. Industrial Crops and Products, 111, 856\u0026ndash;864. https://doi.org/10.1016/j.indcrop.2017.11.005\u003c/span\u003e\u003c/li\u003e\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":true,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true},"keywords":"Catalytic Pyrolysis, Ex-situ, Agro Residue, Organic Phase, HCV, Sustainable Carbon Recovery","lastPublishedDoi":"10.21203/rs.3.rs-7893658/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-7893658/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eThe present study explores non-contact catalytic pyrolysis of cotton stalk, a representative waste agro-residue, using calcium oxide (CaO) and ferric oxide (Fe₂O₃) to enhance bio-oil quality and carbon recovery. Pyrolysis was performed at 450\u0026ndash;550\u0026deg;C, with optimum bio-oil yield observed at 500\u0026deg;C. CaO incorporation (40 wt.%) reduced total bio-oil yield by 22.7 wt.% but increased the organic fraction by 139 wt.%, achieving a maximum organic phase yield of 15.3 wt.% and a high calorific value of 30.5 MJ kg⁻\u0026sup1;, indicating enhanced deoxygenation and acid neutralization via carbonate formation and base-catalyzed cracking. In contrast, Fe₂O₃ (10 wt.% loading) elevated the organic fraction by 25.8 wt.% while reducing total liquid yield to 26.4 wt.%, functioning through Fe\u0026sup3;⁺/Fe\u0026sup2;⁺ redox cycles that promote vapor-phase reforming and water\u0026ndash;gas shift reactions, enriching the pyro-gas with H₂, CO, and CH₄. The organic phase exhibited calorific values of 28.1\u0026ndash;30.5 MJ kg⁻\u0026sup1;, while the char retained a stable heating value of 18.3\u0026ndash;19.1 MJ kg⁻\u0026sup1;, highlighting energy densification without compromising char quality. Post-reaction XRD and FTIR analyses revealed structural stability of the spent catalysts, with CaO transforming into CaCO₃ and Ca(OH)₂ and Fe₂O₃ yielding Fe(OH)₃, FeO, and Fe₃O₄, consistent with carbonation, hydration, and redox transformations under reducing pyrolysis conditions. Overall, the study demonstrates that ex-situ catalytic pyrolysis enhances bio-oil selectivity, energy content, and stability, while simultaneously producing a cleaner, value-added char. The findings underscore the potential of agro-residue valorization for sustainable waste-to-energy conversion, offering a carbon-negative route for high-calorific bio-oil production.\u003c/p\u003e","manuscriptTitle":"Catalytic Pyrolysis of Cotton Stalk for Bio-Oil Production: Effectiveness of CaO and Fe2O3 Catalysts in Ex-Situ Mode","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-11-19 19:42:00","doi":"10.21203/rs.3.rs-7893658/v1","editorialEvents":[{"type":"communityComments","content":0}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"researchsquare","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":true,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"/submission","title":"Research Square","twitterHandle":"researchsquare","acdcEnabled":true,"dfaEnabled":false,"editorialSystem":"","reportingPortfolio":"","inReviewEnabled":false,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"8f166410-c1ed-4ab9-b77f-41f8758bcb76","owner":[],"postedDate":"November 19th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"posted","subjectAreas":[],"tags":[],"updatedAt":"2025-12-14T16:01:23+00:00","versionOfRecord":[],"versionCreatedAt":"2025-11-19 19:42:00","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-7893658","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-7893658","identity":"rs-7893658","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}