New findings in the catalytic activity of zinc glutarate and its application in the chemical fixation of CO2 into polycarbonates and their derivatives

A heterogeneous zinc glutarate (ZnGA) catalyst and its derivatives were prepared from various zinc and glutarate sources. The hydrothermal reaction between zinc perchlorate hexahydrate and glutaronitrile afforded ZnGA single crystals (sc-ZnGA), with a monoclinic lattice unit cell and a P2/c space group, as determined by X-ray single-crystal structural analysis. The structural details of the ZnGA catalyst are crucial in helping to elucidate its activity in the copolymerization reactions between carbon dioxide (CO2) and alkylene oxides. X-ray absorption studies provided direct evidence that CO2 and propylene oxide (PO) are reversibly adsorbed onto the Zn ion centers on the ZnGA surface. Compared to CO2, PO was found to insert more easily into the Zn–O bond of the ZnGA catalyst, suggesting that the ZnGA-catalyzed copolymerization is initiated by PO rather than CO2. The activity of the ZnGA catalyst in the copolymerization of CO2 and PO was found to depend on the zinc source used, and its ability to produce a catalyst of large surface area and high crystallinity (≥77%). Modification of the glutarate ligand with electron-donating or withdrawing substituents failed to enhance the ZnGA catalyst activity further, indicating that glutarate is the best ligand for the Zn metal ion to achieve a high catalytic activity in the CO2 copolymerization with PO. The ZnGA-catalyzed copolymerization was further optimized to maximize the yield of alternating poly(propylene carbonate), and also extended to the terpolymerization of CO2 and PO with δ-valerolactone (VL). Terpolymers with high molecular weights and yields could be obtained by adjusting the PO/VL feed ratios. In addition, the terpolymers were found to exhibit excellent enzymatic and biological degradability.