Ultrafine particles of zeolitic imidazolate frameworks induced by efficient nucleation facilitated conversion of microalgal lipids

Abstract

In order to improve property and utilization of active sites on catalyst surfaces, novel ultrafine particles of zeolitic imidazolate framework (ZIF-67) were synthesized for microalgal biodiesel production. The synergy of N,N-dimethylformamide (DMF) and ethanol promoted deprotonation of imidazole and efficient nucleation of ZIF-67, which reduced crystal size from ∼300 nm (using methanol as solvent) to ∼20 nm. Meanwhile, the ratio of unsaturated coordinated Co-Nx (x = 1,2,3) to saturated CoN4 increased from 0.40 to 0.57, and pyridinic N content on catalyst surface increased from 16.59% to 32.25%, which enhanced the acidity and basicity of catalyst. The total acidity of optimal ZIF-67 increased from 0.125 to 0.691 mmol/g, while total basicity increased from 0.078 to 0.137 mmol/g. Density functional theory (DFT) calculations clarified that unsaturated Co-Nx sites and pyridinic N exhibited excellent catalytic activity. Therefore, ZIF-67-DE catalyst (using DMF and ethanol as solvents) effectively converted microalgal lipids into biodiesel with a catalytic efficiency of 97.8%.