Gold-Catalysed Oxidation of Lignin-Derived Building Blocks

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Abstract

The use of heterogeneous catalysts containing Au nanoparticles supported on TiO2 has been explored for oxidative aqueous phase transformations of sustainable phenolic and benzoic acid derivatives that can be obtained from lignin. Au/TiO2 catalysts were chosen because of their high activity for ambient pressure oxidations of gas phase species, and because their synthesis is facile and reproducible through a modified deposition-precipitation method. The aerobic oxidation of syringic, vanillic, and ferulic acid as well as of guaiacol, eugenol and anisole was investigated at temperatures up to 70°C under (i) atmospheric air sparging in an open reactor and (ii) at 10 atm air pressure in a closed reactor system. The catalysts were characterised by Transmission Electron Microscopy (TEM), Inductively Coupled Plasma (ICP) Optical Emission Spectroscopy and the reproducibility of their catalytic activity independently monitored by determining their activity for carbon monoxide oxidation in a gas flow reactor.The oxidation of syringic acid, vanillic acid, ferulic acid over Au/TiO2 resulted in the formation of 2,6-dimethoxy benzoquinone, guaiacol, and vanillin, respectively, indicating high selectivity for decarboxylation followed by selective oxidation at the position releasing the leaving group. Guaiacol was found to form tetraguaiacol, while eugenol produced quinone methide. Generally, higher air pressure strongly accelerated the transformations, indicating that availability of oxidants formed from O2 is the rate limiting step in the observed transformations. No transformations took place when O2 was excluded from the systems. Overall, guaiacol was found to react fastest, followed by syringic acid, ferulic acid, then vanillic acid. Anisole was found to be unreactive, even at elevated air pressure. The overall reaction pattern emerging from these studies is that the aerobic oxidation in the presence of Au/TiO2 mimics known biotransformations, for example peroxidase-catalysed oxidations involving H2O2.To assess how the functional groups on the aromatic ring influence reactivity the oxidation of p-hydroxybenzoic acid and of 2,6-dimethoxybenzoic acid was also assessed. It was found that decarboxylation of p-hydroxybenzoic acid proceeds, albeit rather slowly, forming phenol, with no further oxidation to hydroquinone or benzoquinone. Taken together these results indicate that the methoxy moieties influence reactivity through both their inductive and resonance effects: leaving of the carboxylic acid group appears to be enhanced through the inductive effect, while further oxidation at the phenolic site seems to be activated through the resonance effect in ortho-position. In line with this hypothesis, it was recently found that dimethoxybenzoic acid converts fast.

Keyword(s)

Gold catalyst; Lignin

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