Comparison of liquid-phase olefin epoxidation catalysed by dichlorobis-(dimethylformamide)dioxomolybdenum(VI) in homogeneous phase and grafted onto MCM-41

A mesoporous silica-supported molybdenum oxide catalyst with a loading of 0.17 Mo/nm(2) was prepared by liquid phase deposition of MoO2Cl2(dMf)(2) (1) onto MCM-41 (dmf = dimethylformamide). Powder X-ray diffraction and N-2 adsorption studies of Mo-MCM-41 indicate that the texture properties of the support were preserved during the grafting experiment. On the basis of evidence from FTIR spectroscopy, C-13 and Si-29 MAS NMR, and Mo K-edge EXAFS, the Mo atoms in this catalyst are mainly present as isolated {MoO2[(-O)(3)SiO](2)(dmf)(2)} species, with a small contribution from oxo-bridged dimers. Catalysis tests show that complex 1 and Mo-MCM-41 are highly active oxidation catalysts for liquid-phase epoxidation of unfunctionalised olefins using tert-butylhydroperoxide as oxidant, from ambient to 55 degrees C. The high stability and recyclability of the heterogeneous Mo-MCM-41 catalyst is attributed to the strong metal oxide-support interaction. With cyclooctene, 1-octene, trans-2-octene and norbornene substrates, the corresponding epoxides were the only observed reaction products. The reactions of the other substrates gave by-products such as 8,9-p-menthen-1,2-diol from (R)-(+)-limonene, campholenic aldehyde from alpha-pinene, and benzaldehyde from styrene. The addition of dichloromethane as a co-solvent had a beneficial effect on catalytic performance, and in the case of (R)-(+)-limonene allowed the epoxide to be formed as the only reaction product. (C) 2008 Elsevier B.V. All rights reserved.