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/nm2 was prepared by liquid phase deposition of MoO2Cl2(dmf)2 (1) onto MCM-41 (dmf = dimethylformamide). Powder X-ray diffraction and N2 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, 13C and 29Si MAS NMR, and Mo K-edge EXAFS, the Mo atoms in this catalyst are mainly present as isolated {MoO2[(–O)3SiO]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 °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 α-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.

Graphical abstractIsolated MoVI active sites have been grafted onto the inner surfaces of MCM-41 mesoporous silica via a MoO2Cl2(dmf)2 precursor, to generate a stable and recyclable catalyst for the liquid-phase epoxidation of unfunctionalised olefins using tert-butylhydroperoxide as oxidant. The derivatised material was characterised by powder X-ray diffraction, N2 adsorption, FTIR spectroscopy, 13C and 29Si MAS NMR, and Mo K-edge EXAFS.Figure optionsDownload full-size imageDownload as PowerPoint slide