Phosphotungstic acid-modified zeolite imidazolate framework (ZIF-67) as an acid-base bifunctional heterogeneous catalyst for biodiesel production from microalgal lipids

To provide more acid-base active sites for catalyzing esterification and transesterification, a novel bifunctional heterogeneous catalyst was synthesized by modifying ZIF-67 with phosphotungstic acid (HPW) for biodiesel production. The proportion of coordinated Co-N bonds in ZIF-67 decreased from 32.4% to 21.7%, because they were destroyed by HPW to form coordinatively unsaturated Co cations and N- extremities of imidazolate ligands. Then terminal W = O groups in HPW interacted with Nextremities in ZIF-67 via covalent W-O-N bonds, which strengthened catalysts recyclability. The released coordinatively unsaturated Co cations and N- extremities enhanced Lewis acid-base properties of catalysts, facilitating conversion of microalgal lipids to fatty acid methyl ester. The ratio of Lewis acid sites to Bronsted acid sites increased from 0.1 to 0.66 and Lewis base sites increased from 0.45 to 4.53 mmol/g. The conversion efficiency of 98.5% over optimum HPW-modified ZIF-67 (weight ratio = 0.25) was higher than that of 72.7% over pure ZIF67 catalyst.