Acid-base bifunctional Fe-NC catalyst with Fe-N4 and Fe nanoparticles active sites derived from Fe-doped ZIF-8 boosted microalgal lipid conversion

Abstract

To enhance catalytic acid-base sites in ZIFs and efficiently promote conversion of microalgal lipids, a bifunctional catalyst with single-atom Fe-N4 sites and Fe nanoparticles (NPs) was synthesized by pyrolyzing Fe-doped ZIF-8. Compared with ZIF precursors, Fe nanoparticles and coordinated Fe ions in Fe-N4 sites enhanced Lewis acidity, while increased uncoordinated N atoms enhanced Lewis basicity. Therefore, the acidity of Fe-NC increased from 0.473 to 1.845 mmol/g, while basicity increased from 0.172 to 0.439 mmol/g. DFT calculations showed that the adsorption energies of acetic acid on the Fe–N4 and Fe NP were −0.536 eV and −0.397 eV, respectively, exceeding that (-0.190 eV) on the Zn–N4 site. The energy barriers for rate-determining steps over the Fe-N4 and Fe NP were + 0.32 eV and + 0.21 eV, respectively, which were lower than that (+1.01 eV) over the Zn-N4 site. The maximum biodiesel yield reached 98.3% over Fe-NC catalyst, higher than that (84.5%) over ZIF-8 catalyst.