Catalytic pyrolysis of microalgal lipids to liquid biofuels: Metal oxide doped catalysts with hierarchically porous structure and their performance

Catalytic pyrolysis of lipids is an alternative pathway for alternative aviation fuel production with advantages of H2-free and mild reaction conditions. However, it is still hindered by coking of catalysts and low selectivity of kerosene. In this research, composite catalysts were synthesized via postsynthesis method (alkali treatment) followed by doping of metal oxides (ZnO, La2O3, and CeO2). Catalytic performance of as-synthesized catalysts was evaluated during catalytic pyrolysis of microalgal lipids. The characterization of BET, XRD, SEM, NH3-TPD for catalysts indicated that alkaline treatment increased the mesoporous structure (5.28–5.91 nm) of microporous zeolite HZSM-5 to form a hierarchically porous structure without apparent disruption of crystal structure of zeolite, while doping of metal oxides (about 10 wt%) enhanced the performance of hetero-atoms (oxygen and nitrogen) removal. The acidity of as-synthesized catalysts significantly decreased from 1175.61 μmol/g (HZSM-5) to 406.77 μmol/g (Ce-MHZ). After catalytic pyrolysis carbon number distribution and compound type of produced liquid fuels tended to high kerosene components (above 40%) and increased hydrocarbon content with reduced content of oxygen-containing compounds. The hierarchically porous zeolite also presented aromatization for aromatic hydrocarbons in alternative aviation fuels. Catalytic mechanism was also elucidated including complex reactions of decarboxylation, decarbonylation, carbon chain cracking, and aromatization.