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Highly Selective Oxidation of Alkylphenols to Benzoquinones with Hydrogen Peroxide over Silica‐Supported Titanium Catalysts: Titanium Cluster Site versus Titanium Single Site
Author(s) -
Kholdeeva Oxana A.,
Ivanchikova Irina D.,
Guidotti Matteo,
Pirovano Claudio,
Ravasio Nicoletta,
Barmatova Marina V.,
Chesalov Yurii A.
Publication year - 2009
Publication title -
advanced synthesis and catalysis
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.541
H-Index - 155
eISSN - 1615-4169
pISSN - 1615-4150
DOI - 10.1002/adsc.200900109
Subject(s) - catalysis , titanium , chemistry , hydrogen peroxide , inorganic chemistry , selectivity , benzoquinone , mesoporous material , nuclear chemistry , organic chemistry
Titanium‐silica catalysts have been prepared by supporting titanium(IV) precursors with different nuclearity {mononuclear titanocene dichloride Ti(Cp) 2 Cl 2 , dinuclear titanium diethyl tartrate and the tetranuclear titanium peroxo complex (NH 4 ) 8 [Ti 4 (C 6 H 4 O 7 ) 4 (O 2 ) 4 ]⋅8 H 2 O} 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) 2 Cl 2 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/nm 2 were identified as optimal catalysts for the transformation of alkylphenols to benzoquinones. Catalysts having <0.6 Ti/nm 2 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.