Redox molecular sieve catalysts for partial oxidation with hydrogen peroxide.

摘要

Density functional theory has been used to study model epoxidation and Baeyer-Villiger reaction mechanisms for Ti(IV)-H₂O₂ and Sn(IV)-H₂O₂ catalytic oxidation systems. The titanium and tin catalysts have been modeled with unconstrained single coordination sphere clusters using a B3LYP/ECP methodology. Activation of hydrogen peroxide via formation of a metal hydroperoxo intermediate proceeds with similar energetics over titanium and tin. Oxygen transfer from the metal hydroperoxo intermediate to the olefin substrate is the rate-determining step for the epoxidation mechanism. The overall reaction kinetics for epoxidation of ethylene or 2,3-dimethyl-1-butene are similar for Ti(IV)-H₂O₂ and Sn(IV)-H₂O ₂ in the absence of solvent coordination. The effects of localized solvent coordination on the formation, structure, and epoxidation reactivity of titanium hydroperoxo intermediates have been elucidated. The Baeyer-Villiger reaction mechanism proceeds through a Criegee intermediate that contains a five-membered chelate ring with the metal center; rearrangement of the chelated Criegee intermediate to the product ester is the rate-determining step. The intrinsic reaction rate for Baeyer-Villiger oxidation of acetone or 2-methyl-3-pentanone is approximately five orders of magnitude slower with Ti(IV)-H₂O ₂ than with Sn(IV)-H₂O₂. Natural bond orbital analysis provides an enriched understanding of the computational results. These quantum chemical calculations offer valuable insights into experimentally observed reactivity trends for titanium- and tin-containing redox molecular sieve catalysts.; Trimethylchlorosilane and trimethylethoxysilane have been shown to be effective silylation agents for MCM-41 mesoporous molecular sieves under anhydrous, high temperature, vapor-phase reaction conditions. Fundamental understanding of the silylation process has been obtained using in-situ infrared spectroscopy to monitor how the concentration and distribution of surface silanols change as a result of thermal treatment or silylation. Reaction temperatures above 200°C are required to achieve high organic loadings with both silylation agents. At 350°C, trimethylchlorosilane reacts with most lone and hydrogen-bonded silanols in non-dehydroxylated MCM-41. Silylation protocols incorporating a dehydroxylation pre-treatment step at 700°C yield lower organic loadings because the siloxane bridges produced during dehydroxylation do not completely react with the silylation agents or their co-products. Silylation of titanium-containing MCM-41 redox molecular sieve catalysts enhances their hydrophobicity and thereby improves their activity and selectivity for the epoxidation of cyclohexene with aqueous hydrogen peroxide.