Mn porphyrins immobilized on non-modified and chloropropyl-functionalized mesoporous silica SBA-15 as catalysts for cyclohexane oxidation

Abstract

In this work, solid catalysts were prepared by the heterogenization of Mn porphyrins (MnP) on mesoporous silica SBA-15 by two simple strategies: (1) immobilization of the three cationic isomers of Mn(III) N-methylpyridiniumporphyrins (MnTM-X-PyPCl5, X=2, 3,4) on mesoporous silica SBA-15 by electrostatic interaction; and (2) anchoring of the three neutral isomers of Mn(III) N-pyridylporphyrins (MnT-X-PyPCl, X=2, 3, 4) on chloropropyl-functionalized mesoporous silica SBA-15Cl via covalent bonding. SBA-15 and SBA-15Cl solids were characterized by FTIR, TGA-DTA, elemental analysis, SAXS, textural analysis, SEM, TEM, and 13C CPMAS NMR (for SBA-15Cl only). The resulting MnP-supported materials with MnP loadings of 0.3% w/w were characterized by diffuse reflectance UV/VIS spectroscopy and explored as catalysts for hydroxylation of cyclohexane using iodosylbenzene as oxygen donor. Changes on SBA-15Cl silanol and CCl bands on FTIR spectra, and UV/VIS analyses confirm the porphyrin immobilization. The heterogenized catalysts were more efficient and selective toward alcohol than the corresponding homogeneous systems. The covalently immobilized catalysts SBA-15Cl/MnT-X-PyPCl (X=2, 3, 4) showed total oxidation yields and selectivities slightly higher than those of SBA-15/MnTM-X-PyPCl5. In both solid systems, the efficiency and selectivity remained essentially unaffected by the immobilized MnP isomer. The strong interaction between MnP and the mesoporous silica, in both SBA-15 and SBA-15Cl systems, was verified by the low-leaching of MnP from the materials even after exhaustive washings. The solid catalysts were recovered and reused in recycling studies showing high resistance against oxidative destruction with no significant changes in catalytic efficiency upon recycling.