Surface Modification of Mesoporous, Macroporous, and Amorphous Silica with Catalytically Active Polyoxometalate Clusters

Transition metal-substituted polyoxometalates (TMSP) of the type [M^II(H₂O)PW₁₁O₃₉]^5- (M=Co, Zn) and [SiW₉O₃₇{Co^II(H₂O)}₃]^10- have been chemically anchored to modified macroporous (400 nm pores), mesoporous (2.8 nm pores), and amorphous silica surfaces. The materials were characterized by solid-state ³¹P MAS-NMR, UV-vis, FT-IR spectroscopy, and N₂ adsorption experiments to verify cluster attachment and the structure of the TMSP on the support. Based on the spectroscopic evidence, clusters were attached datively to the surface, and the integrity of the [CoPW₁₁] cluster was maintained for nonaqueous impregnation with TBA₅[CoPW₁₁]; partial degradation of the cluster occurred when it was impregnated from aqueous solution using the K₅[CoPW₁₁] salt. Both the amine surface groups and the cobalt centers in the clusters were found to be necessary to prevent cluster loss during washing or reaction processes. The catalytic activities of the supported TMSP clusters were tested by the epoxidation of cyclohexene to cyclohexene oxide in the presence of isobutyraldehyde. The percent conversion of the substrate and the amount of product formed per unit time were similar for [CoPW₁₁] clusters on each of the three silica supports, but slightly lower than for purely homogeneous reactions. [SiW₉Co₃] clusters with three available cobalt centers exhibited higher catalytic activity with nearly identical performance on a silica support or in homogeneous solution.