Behavior of homogeneous and immobilized zinc-based catalysts in cycloaddition of CO2 to propylene oxide

Homogeneous and heterogeneous zinc-based catalysts were tested in the reaction of propylene oxide and CO2 to propylene carbonate without the use of additional solvents. For the preparation of the heterogeneous catalysts, the homogeneous zinc pyridine bromide and acetate complexes were modified for chemical anchoring on a silica surface. All intermediate stages of the series, starting with the zinc pyridine-based complex and ending with the anchored complex, were catalytically tested and characterized. Rates in terms of turnover frequencies (TOFs) > 1100 h−1 were obtained with the homogeneous zinc pyridine bromide-based catalysts, and TOFs of 330 h−1 were obtained with the corresponding heterogeneous catalyst. Using acetate instead of bromide as a ligand significantly decreased the performance. The differences in activity could be traced to structural differences unraveled by X-ray absorption spectroscopy (XAS). For the heterogeneous catalysts, special attention was paid to the deactivation behavior and the reusability. The catalyst structures were followed in situ by XAS using a specially designed batch reactor cell. This provided important information on the fate of zinc and bromide under reaction conditions and also gave new insight into the reaction mechanism.