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Kinetic and mechanistic studies of polyoxometalate (POM) reaction with lignin and model compounds Kim, Yong Sik

Abstract

Polyoxometalates (POMs) are a rapidly growing class of metal-oxygen-cluster anions. The properties of POMs can be modified by altering the POMs chemical composition and structure. Due to low cost, commercial availability, and synthetic tractability POMs have found application in various fields of chemistry and technology. POMs are reusable and thermally stable to oxidative conditions, making them an attractive alternative to chlorine for the delignification of wood pulp. The research addressed in this dissertation deals with detailed kinetic and mechanistic studies of K₅[SiVW₁₁O₄₀]·12H₂O, a POM used in the delignification of wood pulp, oxidation of phenolic lignin model compounds and milled wood lignin (MWL). Results from lignin model studies suggest an overall second-order reaction rate; first order with respect to both POM and phenolic substrate. It was observed that electron-transfer from neutral phenols was slower than that from the corresponding phenoxide anions. Hammett studies revealed the reaction involved the formation of an electron-deficient radical intermediate where the rate-determining step is electron transfer from a neutral substrate. The structure of the substituted phenol, in terms of its electron donating/withdrawing character, along with the position of the substituent on the aromatic ring heavily influences the reaction rates. Increasing the number of ortho methoxyl groups dramatically increased the reaction rate, e.g. phenol < guaiacyl < syringyl model structures. The ortho methoxyl group(s) resonance stabilizes and delocalizes the forming phenoxyl radical intermediate. Similarly, the reaction rate of para-substituted guaiacyl and syringyl model compounds showed a dependence on the nature of the para-substituent; inductive or resonance conjugated electron withdrawing effects and inductive donating effects. The effect of POM oxidation on the chemical structure of a Lodgepole pine MWL is investigated. ¹³C nuclear magnetic resonance (NMR) spectroscopic data revealed an approximate 28% decrease in β-O-4 inter-unit linkages after POM treatment, the decrease in β-O-4 inter-unit linkages being accompanied by an increase in carbonyl content. These results suggest that POM oxidation involves side-chain (such as α-OH/β-O-4) oxidation. ¹³C NMR spectroscopy along with gel permeation chromatography revealed an increase in the degree of condensation which supports the idea that radical coupling is a major reaction pathway in this process.

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