Ultrafine metal-organic framework exposed coordinatively unsaturated cobalt-nitrogen active sites for catalytic conversion of microalgal lipids to biodiesel

To improve atomic utilization of metal-organic framework for biodiesel production from microalgal lipids with high acid value, ultrafine ZIF-67 was proposed to release metal atoms normally encapsulated in the framework to form coordinatively unsaturated cobalt-nitrogen active sites Co-N-x (x < 4) on crystal surfaces. With reducing ZIF67 crystal size from 2.3 mu m to 30 nm, coordinatively saturated Co-N-4 content decreased from 81.3% to 56.2%, while unsaturated Co-N-x content increased from 18.7% to 43.8% and pyridinic-N content increased from 11.6% to 32.6%. The exposed Co-N-x and pyridinic-N enhanced Lewis acid-base properties, resulting in an increased Lewis/Bronsted acidity ratio from 0.10 to 15.89 and total basicity from 0.69 to 1.89 mmol/g. Density functional theory calculation clarified adsorption energy of acetic acid on Co-N-3, Co-N-2, and Co-N-1 sites were -0.585, -0.785, and -1.370 eV, respectively, while that on Co-N-4 site was -0.186 eV. The stronger interaction between acetic acid and Co-N-1 site led to more electrons transfer to increase electropositivity of carbonyl carbon from 1.379 to 1.537 eV, which facilitated the nucleophilic attack of methanol for esterification. The optimum catalyst achieved conversion efficiencies of 99.3% and 97.7% for free fatty acid and triglyceride in acidic lipids to biodiesel, respectively.