Acid-base bifunctional catalyst with coordinatively unsaturated cobalt-nitrogen sites for the simultaneous conversion of microalgal triglycerides and free fatty acids into biodiesel.

An acid-base bifunctional catacknalyst with coordinatively unsaturated cobalt-nitrogen active sites Co-N _x (x < 4) was synthesized to convert microalgal lipids with high acid value into biodiesel. Pyrolysis destroyed Co-N ₄ coordination structure in ZIF-67 and released coordinatively unsaturated Co-N _x and uncoordinated N sites, which resulted in the Lewis/Brønsted acid ratio increasing from 0.1 to 11.45 and the basicity increasing from 0.96 to 6.05 mmol/g. According to DFT calculations, the adsorption energy of free fatty acid (FFA) on Co-N ₂ site (-1.003 eV) exceeded that on Co-N ₄ site (-0.271 eV). The strong interaction between Co-N ₂ site and FFA increased electropositivity of carbonyl carbon atom in FFA from 1.379 to 1.529 eV and promoted esterification. The pyrolysis-induced defects generated more mesopores to promote the transportation of lipid molecules inside the catalyst. Therefore, the conversion efficiency of microalgal lipids into biodiesel over the ZC-450 catalyst (96.7%) was higher than that over the ZIF-67 catalyst (69.5%).
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