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 co-ordination structure in ZIF-67 and released coordinatively unsaturated Co-N-x and uncoordinated N sites, which resulted in the Lewis/Bronsted 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%).

Automated summary

In this study, an acid-base bifunctional catalyst was successfully synthesized and its catalytic performance was improved through pyrolysis process. The pyrolysis process destroyed the coordination structure of the catalyst, releasing more active sites and increasing the Lewis acid/Bronsted acid ratio and basicity. Additionally, more mesopores were generated inside the catalyst, facilitating the transportation and conversion of lipid molecules. The use of this catalyst achieved higher conversion efficiency of microalgal lipids compared to traditional catalysts.