Enantioselective epoxidation of beta-methylstyrene catalyzed by immobilized Mn(salen) catalysts in different mesoporous silica supports

Mesoporous silica-supported chiral Mn(salen) catalysts were prepared and evaluated in the heterogeneous asymmetric epoxidation of 15-methylstyrene with NaClO as an oxidant. Homogeneous and immobilized Mn(salen) catalysts exhibit similar cis/trans ratios and ee values when trans-substrate is used but different cis/trans ratios and ee values when cis-substrate is used. The production of trans-epoxides is favored in the heterogeneous asymmetric epoxidation of cis-beta-methylstyrene, whereas the production of cis-epoxides is favored in homogeneous reaction. In addition, the cis/trans ratio and ee value of transepoxides produced from cis-beta-methylstyrene change sequentially with changes in support materials, but the ee value of cis-epoxides does not. The textural properties of immobilized Mn(salen) catalysts can be sequentially adjusted by changing pore dimension and channel length, after which the motion restriction and confinement effect in nanochannels of immobilized Mn(salen) catalysts can be adjusted sequentially. Our results reflect that the collapse step of trans-intermediates is considerably affected by the confinement effect. whereas the rotation step of cis-intermediates is greatly influenced by motion restriction. The motion restriction in nanochannels increases the likelihood of rotation for the C-C single bond of cis-intermediates and favors the production of trans-epoxides. (c) 2008 Published by Elsevier Inc

Iron(III) species dispersed in porous silica through sol-gel chemistry

Fe/SiO2 catalysts have been prepared by two different sol-gel methods, cogelation and dissolution. The cogelation and dissolution preparative methods lead to xerogels with fundamentally different pore width distributions. The nature of the iron species obtained has been examined in detail by UN-visible and Mossbauer spectroscopy, and magnetic, transmission electron microscopy, and X-ray diffraction studies. There is no evidence for the presence of any ordered iron(III) oxides in the samples but all three contain two types of iron species, specifically paramagnetic high-spin iron(III) ions isolated in silica and iron(III) containing nanoparticles with a broad width distribution centered on 1.5 nm, nanoparticles that contain antiferromagnetically coupled clusters of a few ligated bridged iron(III) ions. (C) 2007 Elsevier B.V. All rights reserved

An efficient hybrid, nanostructured, epoxidation catalyst: Titanium silsesquioxane-polystyrene copolymer supported on SBA-15

A novel interfacial hybrid epoxidation catalyst was designed with a new immobilization method for homogeneous catalysts by coating an inorganic support with an organic polymer film containing active sites. ne titanium silsesquioxane (TiPOSS) complex, which contains a single-site titanium active center, was immobilized successfully by in-situ copolymerization on a mesoporous SBA-15-supported polystyrene polymer. The resulting hybrid materials exhibit attractive textural properties (highly ordered mesostructure, large specific surface area (> 380 m(2)g(-1)) and pore volume (-0.46 cm(3) g(-1))), and high activity in the epoxidation of alkenes. In the epoxidation of cyclooctene with tert-butyl hydrogen peroxide (TBHP), the hybrid catalysts have rate constants comparable with that of their homogeneous counterpart, and can be recycled at least seven times. They can also catalyze the epoxidation of cyclooctene with aqueous H2O2 as the oxidant. In two-phase reaction media, the catalysts show much higher activity than their homogeneous counterpart due to the hydrophobic environment around the active centers. They behave as interfacial catalysts due to their multifunctionality, that is, the hydrophobicity of polystyrene and the polyhedral oligomeric silsesquioxanes (POSS), and the hydrophilicity of the silica and the mesoporous structure. Combination of the immobilization of homogeneous catalysts on two conventional supports, inorganic solid and organic polymer, is demonstrated to achieve novel heterogeneous catalytic ensembles with the merits of attractive textural properties, tunable surface properties, and optimized environments around the active sites