In-depth into the mechanism of aromatic production from catalytic pyrolysis of wet-torrefied microalgae with HZSM-5 coated biochar

Abstract Bio-oil from pyrolysis of microalgal biomass tends to exhibit high oxygen and nitrogen content, which is challenging for the commercial application of algae-based fuels. This study started with wet torrefaction of Chlorella to improve its fuel properties, and then Zeolite Socony Mobile-5 (HZSM-5) was used to synthesize HZSM-5 coated biochar (HZSM-5@biochar) catalyst for catalytic pyrolysis. During the process, up to 96.06% of aromatics with benzene, toluene and xylene (BTX) selectivity of 83.24% and a yield of 94.64 mg g −1 was obtained under specific conditions (wet torrefaction at 200 °C, pyrolysis at 500 °C, catalyst-to-feedstock ratio of 20:1). Meanwhile, the content of oxygenates and nitrogenates was reduced from 82.14% under non-catalytic condition to 3.26%. The catalytic conversion pathway was hypothesized by catalytic experiments with model compounds. The transformation process of oxygen- and nitrogen-containing functional groups was probed by XPS and in situ DRIFTS to help elucidate the deoxygenation and denitrogenation mechanism during the catalytic pyrolysis of Chlorella . Besides, the excellent stability of HZSM-5@biochar was demonstrated by six-cycle experiments. The composite catalyst also showed lower yield of coke (0.33%) with less aromaticity after a single use compared to HZSM-5, whose coke yield was 1.87%. Scanning electron microscopy analysis revealed that compared to HZSM-5@biochar, the used HZSM-5 formed more amorphous components suspected to be coke. Graphical Abstract