Substrate dependence on the fixation of CO2 to cyclic carbonates over reusable porous hybrid solids

N,N,N-tributyl-N-propylammonium iodide-functionalized mesoporous silicas (SBA-15 and a silica gel) proved to be efficient, robust and reusable catalysts for the solvent-free synthesis of cyclic carbonates (CCs) from epoxides and CO2. The reusability of the catalysts was found to be dependent on the nature of the support and the substrate. In the presence of 1,2-epoxybutane (1,2-EB) as substrate, the SBA-15-based catalyst (PHS-1) exhibited a constant catalytic activity for five reaction cycles (yield > 96.0%) under mild conditions (1.0 MPa CO2, 100 degrees C and 4 h). Conversely, in the presence of styrene oxide, a gradual decrease in yield was observed. Based on textural properties and C-13 CP-MAS NMR measurements, such decline was attributed to the adsorption of solid reaction product (styrene carbonate - SC) on the catalyst surface. Using a chromatographic silica gel as support and an improved catalyst recovery step gave rise to a highly efficient, robust and fully reusable catalyst (PHS-2) for the synthesis of CCs. Notably, PHS-2 led to a 33-50% decrease in reaction time, and maintained an excellent and steady catalytic activity over 5 cycles in the synthesis of SC (yield > 96% and selectivity > 97%). The amount of trapped solid SC product on PHS-2 surface was also significantly reduced.