Polyoxometalate-nano Gold Hybrid Mesostructured Catalyst for Green Cyclohexene Epoxidation

Background: The design of a highly active catalyst is a key point in olefins epoxidation. Recently, polyoxometalates have received an increasing interest in selective oxidation of organic compounds. Their scope as nanoparticle carriers is of immense importance as the high anionic charge associated with them prevents sintering of the nanoparticles which is a major issue related to nanoparticle based catalysts thus increasing the stability of the catalyst. Objective: We have immobilized Na9PMo11O39 (PMo11) and gold nanoparticles on chloropropyltriethoxysilane functionalized SBA-15 mesoporous material. We investigated the molecular oxygen activation ability and stability of this heterogeneous hybrid catalytic system for green epoxidation of cyclohexene under mild reaction conditions. Method: PMo11 was prepared according to the literature, while PMo11/CPTES-SBA-15, Au/CPTES-SBA-15 and Au/PMo11/CPTES-SBA-15 were prepared by wet deposition method. The catalytic performance was evaluated in a PTFE lined autoclave and analysis was done by gas chromatography. Results: Cyclohexene epoxidation was carried out on different heterogeneous catalytic systems which included polyoxometalate and CPTES-SBA-15 support, nano gold and CPTES-SBA-15 support and the hybrid material Au/PMo11/CPTES-SBA-15 with varying amounts of doped nanogold. All the samples showed good conversion of cyclohexene and significant selectivity for oxidized products with molecular oxygen as an oxidant under mild conditions. 1 % Au/PMo11/CPTES-SBA-15 delivered 48.1 % conversion and 35.9 % selectivity to epoxide. Conclusion: The CPTES-SBA-15 supported Au/PMo11 hybrid catalytic system demonstrated relatively high catalytic activity and selectivity for the green epoxidation of cyclohexene with molecular oxygen as an oxidant under mild reaction conditions. The hybrid catalyst was regenerated and reused for three cycles without compromising the catalytic performance.