Synergetic metal-Nx and metal nanoparticles derived from Zn/Co-zeolitic imidazolate frameworks boosted conversion of microalgal lipids

Abstract

In order to enhance acid-base active sites and their utilization efficiency to efficiently convert microalgal lipids into biodiesel, an acid-base catalyst was synthesized through pyrolyzing ZnCo bimetallic zeolitic imidazolate frameworks (ZIFs). Pyrolysis destroyed saturated coordinated metal-N4 sites in ZIFs and produced more unsaturated metal-Nx and metal nanoparticles, which enhanced Lewis acidity. The increased pyridinic N and unsaturated metal-Nx promoted Lewis basicity. Meanwhile, Zn doping reduced sizes of catalyst crystals and metal nanoparticles, which improved utilization efficiency of active sites. Hence, compared with pyrolyzed ZIF-67, the acidity of optimal catalyst increased from 0.205 to 1.535 mmol/g, whereas basicity increased from 0.069 to 0.405 mmol/g. DFT calculations revealed that catalytic performances of metal nanoparticles and unsaturated metal-Nx were better than that of saturated metal-N4. Therefore, the catalytic efficiency of C-Zn/Co-ZIF = 4 (molar ratio of Zn to Co was 4) catalyst was highest at 96.2%, because of the minimal catalyst crystals and metal nanoparticles.