The Polyoxometalate / NOx / Brx Catalystsfor Aerobic Sulfoxidations Open Access

Luo, Zhen (2009)

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Abstract In attempts to make a nitrate and bromide containing transition-metal- substituted-polyoxometalate (POM) catalyst developed in our group practically applicable, an in situ layer-by-layer self-assembly procedure has been developed to immobilize this POM onto solid supports. The solid-supported POMs efficiently catalyze aerobic sulfoxidation under ambient conditions. The addition of protic acids to the system significantly enhances the catalytic activities while water elongates the induction period and inhibits the reaction. To obtain disorder-free X-ray crystal structures which provide insightful albeit indirect evidence regarding the mechanism of the POM-catalyzed sulfoxidation, divacant [γ-SiW10O36]8- and trivacant [α-P2W15O56]12- POMs have been used as synthetic precursors of the catalysts. Two multi-copper-substituted polytungstosilicates, K9Na2Cu0.5[γ-Cu2(H2O)SiW8O31]2 ·38H2O and K3H4Cu0.5 {Cu[Cu7.5Si2W16O60(H2O)4(OH)4]2} ·9H2O, and a series TBA salts of Well-Dawson sandwich POMs have been prepared and characterized by X-ray crystallography, IR and elemental analysis. The complex [( n-C4H9)4N]11H5[Cu4(P2W15O56)2], proved to be inactive in the absence of NO3- and Br-; however, a system containing NO3 -, Br- and copper-substituted POM is the most effective (rapid and selective) catalytic system for aerobic sulfoxidation to date. In order to probe the mechanism of the catalytic reaction, the kinetics of sulfoxidation catalyzed by simplified systems that contain only nitrate and bromide from different sources were investigated. The yellow active specie of the NOx/Br system previously documented in our research group has been unequivocally identified to be TBABr3. A mechanism based on the bromine sulfoxidation and aerobic bromine regeneration catalyzed by NOx is proposed and the kinetic data fit the equations associated with the mechanism.

Table of Contents

Chapter One: Introduction of Transition-metal-substituted Polyoxometalates (TMSPs) and TMSPs Catalyzed Sulfoxidation...1

Chapter Two: Synthesis and Characterization of Solid- Support Catalytic TMSP Materials for Aerobic Sulfoxidation...20

Chapter Three: Kinetics and Mechanistic Studies of the NOx/Br Systems for Catalytic Aerobic Sulfoxidation...60

Chapter Four: Multi-Copper Polyoxoanions. 1. Synthesis, Characterization, X-ray Crystal Structures, and magnetism of a new dimeric silicotungstate...92

Chapter Five: Multi-Copper Polyoxoanions. 2. Synthesis, Characterization, X-ray Crystal Structure and Magnetism of a One-dimensional Silicotungstate Array...114

Chapter Six: Multi-Copper Polyoxoanions. 3. Synthesis, structure, magnetism and catalytic properties of a polyoxometalate with coordinatively unsaturated d-electron-transition metal centers...139

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