NEXAFS spectroscopy study of the surface properties of zinc glutarate and its reactivity with carbon dioxide and propylene oxide

The surface state of polycrystalline zinc glutarate (ZnGA) catalyst and its catalytic adsorption of carbon dioxide (CO2) and propylene oxide (PO) were investigated by using near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The outermost layer of ZnGA catalyst was found to contain more hydrocarbon units (i.e., glutarate ligand component) than the inner layers. The ZnGA catalyst was found to reversibly react with CO2 and to readily react with PO via adsorption onto the catalyst surface and insertion into the ZnO bond. Experiments in which the catalyst was treated with CO2 followed by PO and vice versa showed that each of these components can replace the other component on the catalyst surface. This reversible adsorption and insertion of CO2 and PO on the ZnGA surface provides a clue to the mechanism underlying the production of alternating poly(propylene carbonate) in the ZnGA-catalyzed copolymerization of CO2 and PO. However, in comparison to CO2, PO was more easily adsorbed onto the ZnGA catalyst and inserted into the ZnO bond. As a consequence, PO significantly modified the catalyst surface. This suggests that the ZnGA-catalyzed copolymerization is initiated by PO rather than CO2.