Highly Selective Oxidation of Alkylphenols to Benzoquinones with Hydrogen Peroxide over Silica-Supported Titanium Catalysts: Titanium Cluster Site versus Titanium Single Site

Titanium-silica catalysts have been prepared by supporting titanium(IV) precursors with different nuclearity {mononuclear titanocene dichloride Ti(Cp)2Cl2, dinuclear titanium diethyl tartrate and the tetranuclear titanium peroxo complex (NH4)8[Ti4(C6H4O7)4(O2)4]·8H2O onto the surface of silica materials with different textural characteristics. The supported catalysts have been explored as highly active and reusable catalysts for the oxidation of 2,3,6-trimethylphenol (TMP) and 2,6-dimethylphenol (DMP) to 2,3,5-trimethyl-1,4-benzoquinone (TMBQ, vitamin E key intermediate) and 2,6-dimethyl-1,4-benzoquinone (DMBQ), respectively, using aqueous hydrogen peroxide as green oxidant. Catalysts prepared by grafting mononuclear Ti(Cp)2Cl2 revealed a strong dependence of the product selectivity on the surface concentration of titanium active centers. Mesoporous materials with titanium surface concentration in the range of 0.6-1.0 Ti/nm2 were identified as optimal catalysts for the transformation of alkylphenols to benzoquinones. Catalysts having <0.6 Ti/nm2 produced a mixture of benzoquinones and dimeric by-products. Conversely, when di-/tetranuclear titanium precursors were employed for the catalyst preparation, a diminution of the titanium surface concentration had no impact on the benzoquinone selectivity, which was typically as high as 96-99%. DR-UV spectroscopic studies revealed that the catalysts capable of producing alkylbenzoquinones with nearly quantitative yields possess titanium dimers and/or subnanometer-size clusters homogeneously distributed on a silica surface. On the contrary, catalysts with isolated titanium sites give a considerable amount of dimeric by-products. This is the first example which clearly demonstrates the advantages of titanium cluster-site catalysts over titanium single-site catalysts in hydrogen peroxide-based selective oxidation reaction.