The Polyoxometalate / NOx / Brx Catalysts for Aerobic Sulfoxidations Open Access

Luo, Zhen (2009)

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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

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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