The selective catalytic oxidation of organic compounds with an environmental attractive oxidant, aqueous H2O2, is a challenging goal in fine chemistry. Over the past two decades, heterogeneous Titanium(IV)-based catalysts have received much attention for their application in this field. Highly active and selective catalysts can be produced by dispersing titanium atomically in a silica matrix, isomorphously substituting titanium for silicon in molecular sieve frameworks, or grafting isolated titanium species to the surface of silica, mesoporous molecular sieves, layered aluminosilicates, polyoxometallates. It has been established that the titanium active sites in these catalysts are atomically isolated from one another and tetrahedrally coordinated under vacuum conditions and readily expand their coordination sphere to include one or two ligands. Addition of hydrogen peroxide yields the active oxygen-donating intermediate that is active for oxidation reactions. Despite the numerous mechanistic studies on these materials, the mechanism of their catalytic action and the nature of the oxidizing species still remains a matter of discussion. All isolated and well characterized Ti-2-peroxo complexes appeared to be inactive towards organic substrate in a stoichiometric reaction, in contrast to the 2-peroxo or bis-peroxo d0 complexes of Mo, W, and Re, which are structurally similar to Ti(2-O2) species and epoxidize alkenes. Hydroperoxo titanium species were proposed to be the active intermediates: however, no hydroperoxo complex has yet been isolated and structurally characterized. The hydroperoxo oxygens may coordinate the titanium center in a (n1) or bidentate (n2) manner, and many theoretical studies have been devoted to describe the relative stabilities of these two species. Few experimental evidences about alkylperoxo species formed in heterogeneous systems such as Ti-MCM-41 have been obtained and characterized by X-ray absorption fine structure (XAFS). Some data of alkylperoxo structures are obtained from molecular compounds such as silicon-titanium complexes obtained from silsesquioxane-type species, or from homogeneous species obtained by chiral trialkanolamines and Ti(IV)alkoxides. NMR data suggest that Ti-2-alkylperoxo complexes are the active oxidants in the catalytic cycle.
Equilibria between peroxo and hydroperoxo species in the titanosilicates: An in situ high-resolution XANES investigation
PRESTIPINO, Carmelo;BONINO, Francesca Carla;DAMIN, Alessandro Ali;BORDIGA, Silvia;ZECCHINA, Adriano;LAMBERTI, Carlo
2004-01-01
Abstract
The selective catalytic oxidation of organic compounds with an environmental attractive oxidant, aqueous H2O2, is a challenging goal in fine chemistry. Over the past two decades, heterogeneous Titanium(IV)-based catalysts have received much attention for their application in this field. Highly active and selective catalysts can be produced by dispersing titanium atomically in a silica matrix, isomorphously substituting titanium for silicon in molecular sieve frameworks, or grafting isolated titanium species to the surface of silica, mesoporous molecular sieves, layered aluminosilicates, polyoxometallates. It has been established that the titanium active sites in these catalysts are atomically isolated from one another and tetrahedrally coordinated under vacuum conditions and readily expand their coordination sphere to include one or two ligands. Addition of hydrogen peroxide yields the active oxygen-donating intermediate that is active for oxidation reactions. Despite the numerous mechanistic studies on these materials, the mechanism of their catalytic action and the nature of the oxidizing species still remains a matter of discussion. All isolated and well characterized Ti-2-peroxo complexes appeared to be inactive towards organic substrate in a stoichiometric reaction, in contrast to the 2-peroxo or bis-peroxo d0 complexes of Mo, W, and Re, which are structurally similar to Ti(2-O2) species and epoxidize alkenes. Hydroperoxo titanium species were proposed to be the active intermediates: however, no hydroperoxo complex has yet been isolated and structurally characterized. The hydroperoxo oxygens may coordinate the titanium center in a (n1) or bidentate (n2) manner, and many theoretical studies have been devoted to describe the relative stabilities of these two species. Few experimental evidences about alkylperoxo species formed in heterogeneous systems such as Ti-MCM-41 have been obtained and characterized by X-ray absorption fine structure (XAFS). Some data of alkylperoxo structures are obtained from molecular compounds such as silicon-titanium complexes obtained from silsesquioxane-type species, or from homogeneous species obtained by chiral trialkanolamines and Ti(IV)alkoxides. NMR data suggest that Ti-2-alkylperoxo complexes are the active oxidants in the catalytic cycle.
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