POSS-Al-porphyrin-imidazolium cross-linked network as catalytic bifunctional platform for the conversion of CO₂ with epoxides

Two heterogeneous catalysts were prepared with the aim of following the promising path of CO₂ fixation into epoxides. The synthetic procedure involves a radical copolymerization of an octavinylsilsesquioxane as inorganic core building block and tetrastyrylporphyrin aluminum chloride monomer (TSP-AlCl) in presence (POSS-TSP-AlCl-imiBr) or in absence (POSS-TSP-AlCl) of a bis-vinylimidazolium bromide salt (bis-imiBr), in order to investigate if the bifunctional heterogeneous material can display better catalytic performance than the separate species. All the solids were fully characterized and tested in the synthesis of cyclic carbonates starting from CO₂ and several epoxides. The synergic cooperation of the two co-catalytic species (Lewis acid/imiBr) was observed. POSS-TSP-AlCl-imiBr showed to be a recyclable material with an excellent activity and TON and TOF values up to 16,000 and 5000, respectively. These excellent results, even under mild reaction and solvent-free conditions, were attributed to a twofold effect: the proximity between the two active sites due to the direct covalent bond between the porphyrin and the imidazolium component and the increasing local concentration of active sites effect that is obtained by functionalizing the silica cage at all its vertices.