Zeolitic imidazolate framework (ZIF-8)-derived acid-base bifunctional single-atom catalysts with Zn-Nx coordination for microalgal lipids conversion

To strengthen active sites in zeolitic imidazolate frameworks (ZIFs) to effectively convert microalgal lipids into biodiesel, acid-base bifunctional single-atom catalysts with Zn-Nx coordination were synthesized through pyrolyzing ZIF-8. The saturated coordinated Zn-N4 sites were destroyed and converted to unsaturated coordinated Zn sites and surrounding N atoms at 700 °C, which enhanced Lewis acidity and basicity, respectively. The Zn-N content in the catalyst decreased from 77.68 % to 43.26 %, while that of pyridinic N increased from 8.14 % to 36.73 %. The acidity of the catalyst increased from 0.498 to 3.979 mmol/g, whereas basicity increased from 0.219 to 0.650 mmol/g. According to density functional theory calculations, unsaturated coordinated Zn-Nx (x = 1, 2, 3) sites exhibited lower free energies and reaction energy barriers in free fatty acid esterification. The energy barriers for rate-determining steps over Zn-N4, Zn-N3, Zn-N2, and Zn-N1 sites were + 1.01 eV, +0.82 eV, +0.63 eV, and + 0.54 eV, respectively. Therefore, the optimum catalyst exhibited a high conversion efficiency of 97.9 %, which was better than that of ZIF-8 (85.4 %).