Organic-inorganic multifunctional hybrid catalyst giving catalytic synergies in cooperative coupling between CO2 and propylene oxide to propylene carbonate

An organic-inorganic hybrid catalyst was synthesized by silylation of silanol (Si-OH) groups with ammonium bromide group on the mesopore wall of mesoporous zeolite having a hierarchically nanoporous architecture, conferring both electrophilic and nucleophilic functions due to the acid sites of the zeolite and bromide in the functional group, respectively. In the coupling of carbon dioxide and propylene oxide for production of propylene carbonate, the hybrid catalyst exhibited a synergistic enhancement in activity leading to a two-fold higher product yield than the catalysts having a single catalytic function. Comprehensive reaction studies revealed that the catalytic synergy could only be obtained by achieving close proximity between the ammonium bromide group and acid sites on the zeolite surface. The acidity was also crucial, where no synergy was observed for a similarly designed organic-inorganic hybrid catalyst constructed on weakly acidic ordered mesoporous silica SBA-15. Comparison of the activation energy of the catalysts supported the observation that the zeolitebased hybrid catalyst with stronger acidity gave rise to a lower activation energy (37.3 kJ mol(-1)) than the SBA-15-based hybrid catalyst with weaker acidity (51.3 kJ mol(-1)). The mesoporous zeolite-based hybrid catalyst was very active under neat conditions without additional organic solvent, and showed good recyclability without significant loss of the initial activity for up to five runs.