Synthesis of core-shell structured cobalt-based zeolitic imidazolate framework to expose active metal sites for catalytic conversion of acidic microalgal lipids to biodiesel

To improve metal atom utilization of Co-based zeolitic imidazolate framework (ZIF) in conversion of microalgal lipids to biodiesel, core-shell structured ZIF-67 with two-dimensional ultrathin layered cobalt hydroxide was synthesized to allow more Co atoms to be distributed on the crystal surface. The competition between deprotonated 2-methylimidazole and hydroxyl anions for coordination to Co2+ led to the formation of more crystal defects and thus a higher number of coordinatively unsaturated Co-Nx (x < 4) and pyridinic-N sites, which enhanced acid-base properties of catalyst. By density functional theory calculations, the adsorption energies of acetic acid on CoN1, CoN2, CoN3, and Co(OH)2 sites were − 1.370, −0.785, −0.585, and − 0.631 eV, respectively, while that on CoN4 site was −0.186 eV. Stronger interaction between acetic acid and active sites enhanced electron transfer to increase the electropositivity of carbonyl carbons, which promoted nucleophilic attack of methanol on carbonyl carbons in esterification reaction. The pyridinic-N sites and cobalt hydroxyl groups promoted the deprotonation of methanol to form methoxy anions to attack triglycerides in transesterification step. Thus, the optimum catalyst achieved efficiency of 96.1% in catalytic conversion of microalgal lipids to biodiesel, which was higher than that of conventional ZIF-67 (75.4%).