X-ray absorption and NMR spectroscopic investigations of zinc glutarates prepared from various zinc sources and their catalytic activities in the copolymerization of carbon dioxide and propylene oxide

Abstract

The local and microstructures of zinc glutarates synthesized from various zinc sources were investigated by X-ray absorption and solid-state carbon-13 nuclear magnetic resonance spectroscopy, and related to their catalytic activities in the copolymerization of carbon dioxide and propylene oxide. It was found that the local structure around the Zn atoms of the zinc glutarate catalysts consists basically of tetrahedrally coordinated carboxyl oxygen atoms with a ZnO bond distance in the range 1.95–1.96 Å, and that the nearest neighbor Zn atom distance is 3.19–3.23 Å. These results suggest that the catalysts have a network structure composed of layers interconnected by glutarate ligands. However, the first-shell structures of the catalysts tested are somewhat different, which might originate from differences in the catalysts' overall crystallinity and crystal quality (crystal size and perfection) produced by their different synthetic routes. The surface areas of the catalysts also varied with synthetic route. In the copolymerization, one catalyst with low surface area but the highest crystallinity and best crystal quality shows the highest catalytic activity, which is contrary to the usual expectation of increased catalytic activity with increased catalyst surface area. Therefore, the catalytic activities of zinc glutarates in the copolymerization seem to depend primarily on their morphological structures rather than on their surface areas. The surface areas of zinc glutarates may play a crucial role in improving the catalytic activity in the copolymerization when they first meet the morphological requirements (i.e., high crystallinity and crystal quality).