A comparison of methods for the heterogenization of the chiral Jacobsen catalyst on mesostructured SBA-15 supports

Mesostructured SBA-15 materials, with both pure silica and aluminosilicate compositions, have been investigated as supports for the heterogenization of the chiral Mn(salen) catalyst using different methods: impregnation, direct ion exchange, indirect ion exchange, and coordinative anchoring on organosilica-functionalized supports. In the last case, two different types of SBA-15 functionalization agents were tested: aminopropyltrimethoxysilane and aminophenyltrimethoxysilane. In all the methods studied, the incorporation of the Mn(salen) catalyst does not modify the SBA-15 structure although a significant decrease in the surface area, pore volume and pore size is observed. Enantioselective styrene epoxidation using m-CPBA as oxidant and NMO as co-oxidant has been used as test reaction for the different heterogeneous catalysts synthesized. The stability of the heterogenized systems has been studied by carrying out reusing tests. The main problem encountered is the extensive leaching of the Mn(salen) complex that occurs with most of the catalytic systems. However, a significant stability has been found for the catalyst prepared by anchoring the Mn(salen) complex on a SBA-15 support previously functionalized with aminophenyltrimethoxysilane. In this case, the extension of the Mn(salen) leaching has been reduced below 10% by increasing both the styrene/oxidant and catalyst/styrene ratios.

Mesostructured SBA-15 materials, with both pure silica and aluminosilicate compositions, have been investigated as supports for the heterogenization of the chiral Mn(salen) catalyst using different methods: impregnation, direct ion exchange, indirect ion exchange, and coordinative anchoring on organosilica-functionalized supports. In the last case, two different types of SBA-15 functionalization agents were tested: aminopropyltrimethoxysilane and p-aminophenyltrimethoxysilane. In all the methods studied, the incorporation of the Mn(salen) catalyst does not modify the SBA-15 structure although a significant decrease in the surface area, pore volume and pore size is observed. Enantioselective styrene epoxidation using m-CPBA as oxidant and NMO as co-oxidant has been used as test reaction for the different heterogeneous catalysts synthesized. The stability of the heterogenized systems has been studied by using the same catalyst in repetitive reaction runs. The main problem is the extensive leaching of the Mn(salen) complex. After three reaction cycles, the best results were obtained with the catalyst prepared by anchoring the Mn(salen) complex on a SBA-15 support previously functionalized with aminophenyltrimethoxysilane. Moreover, the extension of the Mn(salen) leaching can be reduced in this catalyst system below 10% by increasing both styrene/oxidant and catalyst/styrene ratios. Figure optionsDownload as PowerPoint slide