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

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.