Experimental and computational studies of Zn (II) complexes structured with Schiff base ligands as the efficient catalysts for chemical fixation of CO2 into cyclic carbonates

Abstract

The present work aimed at comparably studying the catalytic performance of Zn (II) complexes applied as the efficient catalysts in accelerating the transformation of CO2 into cyclic carbonates. The physicochemical properties of the synthesized complexes were determined by XRD, NMR, FT-IR, TGA, NH3-TPD, CO2-adsorption. The synthesized samples were found to possess the Lewis acid/basic sites derived from the zinc ions and nitrogen-containing ligands, which can synergistically catalyze the coupling reaction. In addition, the sample containing iodide ion ligands showed the highest catalytic performance compared with other Zn (II) complexes, suggesting that the amount and distribution of the Lewis acid sites were not the primary factors for determining the catalytic activity, and the difference in activity of halogen ion ligands (I−, Br−, Cl−) has a larger influence on the coupling process, which made the CO2 insertion and ring closure of the formed intermediate more easily occur when iodide ion ligands participated. It can be drawn that the catalytic activity of the Zn (II) complexes was simultaneously associated with halide ion ligands and substitution group of the ligand. Moreover, 98.62% conversion of epichlorohydrin (ECH) and 96.97% selectivity to chloropropene carbonate (TOF=56.65 h − 1) was achieved at the optimal operation parameters (110 °C, 2.5 MPa, 8 h, 1.25 wt.% catalyst of ECH). Furthermore, the proposed pathway of CO2 coupling with epoxide was confirmed by DFT calculation, with the result confirming that the process of epoxide ring opening was the key step for the coupling reaction.