Dioxomolybdenum(VI) modified mesoporous materials for the catalytic epoxidation of olefins

Organic–inorganic hybrid heterogeneous catalyst systems were synthesized by the reaction of the dioxomolybdenum(VI) complex MoO2Cl2(THF)2 with the mesoporous silica MCM-41 functionalized with a pyrazolylpyridine ligand (MCM-41-PP). Two catalysts were prepared, one of which involved the postsynthesis trimethylsilylation of MCM-41-PP to remove the residual surface silanol groups. The model complex MoO2Cl2L {L = ethyl[3-(2-pyridyl)-1-pyrazolyl]acetate} was also synthesized. Elemental analysis of the supported mesoporous materials indicated molybdenum loadings of 8.0 wt.% (0.83 mmol g−1) for MCM-41-PP·MoO2Cl2 and 7.0 wt.% (0.73 mmol g−1) for silylated MCM-41-PP·MoO2Cl2. The supported materials were further characterized by N2 adsorption, 13C/29Si (CP) MAS NMR, IR and Raman spectroscopy. The spectroscopic data are consistent with the successful formation of tethered complexes of the type MoO2Cl2(PP), although the materials also contained an excess of dioxomolybdenum(VI) species that were probably not coordinated directly with the surface-bound ligands. The modified materials are active and selective in the epoxidation of cyclooctene at 328 K using t-BuOOH (in decane) as the oxidant and no additional solvent. The initial specific reaction rates were about 350 mol molMo−1 h−1 for the modified materials and also the model complex MoO2Cl2L. Stability was checked by recycling the solid catalysts several times. Some activity is lost from the first to second runs, but thereafter stabilizes. The catalytic performance of the hybrid materials was further investigated in the oxidation of α-pinene, (R)-(+)-limonene, trans-2-octene and 1-octene.