Dioxomolybdenum(VI)-modified mesoporous MCM-41 and MCM-48 materials for the catalytic epoxidation of olefins

The dioxomolybdenum(vi) complex [MoO2Cl2(L-L)], containing the bidentate 1,4-diazabutadiene ligand RN= C(Ph)-C(Ph)=NR [R = (CH2)(3)Si(OEt)(3)], was immobilised in the ordered mesoporous silica MCM-41 by carrying out a grafting reaction in dichloromethane. The grafted material (3.3 wt.-% Mo) was tested as a catalyst for the epoxidation of cyclooctene with tert-butyl hydroperoxide (TBHP) at 55 degreesC. Selectivity to the epoxide was very high and the observed kinetic profile was similar to that of the complex [MoO2Cl2(L-L)] in the homogeneous phase. On recycling several times, some activity was lost from the first to second runs, but thereafter stabilised. Tethered complexes of this type were also prepared by a stepwise approach. The mesoporous silicas MCM-41 and MCM-48 were first treated with a toluene solution of the ligand L-L under reflux. Before isolating the grafted materials, the silylating agent Me3SiCl was added to remove the residual Si-OH groups on the surface of the mesoporous materials. Oxomolybdenum species were subsequently introduced into the ligand-silicas by pore volume impregnation of a solution of the complex [MoO2Cl2(THF)(2)] in CH2Cl2 at room temperature. The modified materials have been characterised by powder X-ray diffraction, N-2 adsorption, and solid-state MAS NMR (C-13, Si-21). Despite having lower metal loadings (1 wt.-% Mo), the materials exhibited similar catalytic activities for the epoxidation of cyclooctene to that of the modified MCM-41 with 3.3 wt.-% of Mo. However, selectivities to the epoxide were lower. The MCM supported catalysts have also been tested for the epoxidation of cyclododecene and alpha-pinene with TBHP. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003).