Microalgae for biofuel: Comparing the performance of solvent-free sulfated zirconia catalysts and zeolite Y for the catalytic pyrolysis of freshwater microalgae (Chlorella vulgaris)

Abstract

Microalgae's high photosynthetic efficiency and CO₂ sequestration capability makes it an attractive feedstock for biofuels and value-added compounds. Zeolite Y is widely used in biomass conversion due to its affordability and high catalytic activity, but its limited acidity and susceptibility to coking present challenges. This study investigates whether sulfated zirconia, with its superacidic properties, can serve as an alternative catalyst to zeolite Y for improving bio-oil quality in Chlorella vulgaris pyrolysis. Sulfated zirconia catalysts with zirconium-to-sulfate ratios of 1:1 and 1:6 (wt/wt) were synthesized using a solvent-free method and evaluated alongside zeolite Y using Pyrolysis Gas Chromatography/Mass Spectrometry (Py-GCMS) and isothermal pyrolysis at 400, 500, and 600°C. Py-GC/MS analysis revealed that zeolite Y, sulfated zirconia (1:1), and sulfated zirconia (1:6) increased aromatic content by 227 %, 69 %, and 50 %, respectively, than the non-catalytic process. Additionally, sulfated zirconia at 400 °C produced bio-oil with a 29.9 % higher heating value (HHV) than the non-catalytic process. This is comparable to the highest HHV of 37.8 % achieved with zeolite Y at 500 °C. These results suggest that sulfated zirconia catalysts effectively enhance aromatic production while suppressing nitrogen-containing and acidic compounds, making them a viable alternative to zeolite Y in Chlorella vulgaris pyrolysis. Furthermore, a comparison of catalytic and non-catalytic reaction mechanisms highlights the significant influence of acid site distribution on product selectivity, demonstrating the potential of sulfated zirconia in optimizing biofuel composition.