Strategies for enhancing the catalytic performance of Metal-Organic Frameworks in the fixation of CO2 into cyclic carbonates

Metal-organic frameworks with accessible Lewis acid sites are finding increasing application in the field of heterogeneous catalysis. However, the structural instability of MOFs when they are exposed to high temperature or high pressure often limits their applicability. In this study, two strategies were applied to achieve a MOF catalyst with high stability, activity and selectivity in the reaction of CO2 with styrene oxide to produce styrene carbonate. In the first approach, a MOF with linkers with high connectivity as MIL-100(Cr) was studied, leading to promising activity and recyclability in consecutive catalytic runs without loss of activity. In the second strategy, a MOF with linkers with lower connectivity but with encapsulated Keggin phosphotungstic acid was prepared. However, the activity of this catalyst decreased upon reuse as a consequence of deterioration of the metal-organic framework. Further investigations were dedicated to the enhancement of the catalytic performance of MIL-100 and included the variation of the metal centre as well as the type and loading of organic salts acting as nucleophile source. This allowed tuning the nature of the organic halide to the specific porous structure of MIL-100(Cr) in order to prevent diffusion limitations. The best catalytic performance was obtained with MIL-100(Cr) in combination with EMIMBr, which achieved very high styrene carbonate yield (94%) with complete selectivity after 18h of reaction at mild temperature (60°C)