Manganese(III) porphyrin-encapsulated Ti,Si-mesoporous molecular sieves as heterogeneous catalysts for the epoxidation of alkenes

摘要

The mesoporous molecular sieves containing different amounts of titanium were synthesized using cetyltrimethylammonium bromide (CTAB) as the structure-directing agent and tetraethyl orthotitanate (TEOT) as the source of titanium. The heterogeneous catalysts were obtained by encapsulating cationic manganese-porphyrin, [meso-tetrakis(4-trimethylammoniophenyl)porphyrinato]manganese(III) pentachloride (MnTAPP) into the titanium-substituted mesoporous molecular sieves. Both the type of catalysts, i.e., supported and unsupported were characterized by FTIR, TGA, low-angle X-ray diffraction, nitrogen adsorption isotherm, scanning electron microscopy and DRUV-vis spectroscopy. A combined study on the characterization and catalytic performance of MnTAPP supported systems suggests the successful encapsulation of MnTAPP inside the mesoporous molecular sieves. The catalytic activity of the manganese(III) porphyrin-encapsulated materials was tested in the epoxidation of cyclohexene and styrene using iodosylbenzene (PhIO) as oxygen source. When compared to the homogeneous system, the manganese(III) porphyrin-encapsulated materials show higher activity for the epoxidation reactions. The activity and selectivity of these heterogeneous catalysts were also compared with a heterogeneous catalyst containing no titanium and with unsupported molecular sieves. The former shows lower activity than the heterogeneous catalyst prepared by encapsulating MnTAPP onto titanium-substituted molecular sieves, whereas the latter was inactive in the oxidation of both cyclohexene and styrene. The selectivity of these heterogeneous catalysts does not change appreciably after three times of reusing, but their catalytic activity decreases marginally. The decrease in the activity may be due to some leaching and/or decomposition of MnTAPP complex under the reaction conditions. (c) 2006 Elsevier B.V. All rights reserved.