Gold catalysts for sustainable chemical processing

Abstract

The focus of this thesis is the development of supported gold catalysts with application in selective hydrogenation directed at sustainable chemical processing. The gas phase hydrogenation of nitrobenzene (to aniline), p- and o-chloronitrobenzene (to pand o-chloroaniline), p-nitrobenzonitrile (to p-aminobenzonitrile), nitrocyclohexane (to cyclohexanone oxime) and phenylacetylene (to styrene) have been examined. Utilisation of an array of complementary characterisation techniques has facilitated an explicit correlation of catalyst performance with structure/performance that is underpinned by detailed thermodynamic and mechanistic kinetic analysis. Taking the conversion of p-chloronitrobenzene as a model system, it is established that reaction temperature has little effect on equilibrium composition whereas an increase in H2/p-chloronitrobenzene ratio results in p-chloroaniline hydrodechlorination/hydrogenation to aniline, benzene and cyclohexane. Gold on non-reducible oxides (Al2O3 and ZrO2) is 100% selective in catalysing the hydrogenation of nitrobenzene, p-chloronitrobenzene and p-nitrobenzonitrile to the target amine product. Use of reducible oxides, notably Ce0.62Zr0.38O2, resulted in hydrodechlorination (of p-chloronitrobenzene) through the catalytic action of oxygen vacancies generated during catalyst activation. The use of Mo2C as Au carrier serves to enhance activity (relative to Al2O3) as a result of increased surface reactive hydrogen associated with the carbide. Incorporation of Pd with Au on both Al2O3 and Mo2C further increases rate while retaining product exclusivity Selectivity is particularly challenging in the conversion of nitrocyclohexane as an aliphatic nitro-compound and sensitive to the support where preferential oxime formation (selectivity as high as 95%) over Au/Al2O3 while Au/TiO2, Au/ZrO2 and Au/CeO2 are non-selective, promoting cyclohexylamine and cyclohexanone formation. Moreover, application of Au/Al2O3 in phenylacetylene hydrogenation delivers full selectivity to the target styrene. Process sustainability has been examined in terms of hydrogen utilisation and productivity in moving from batch to continuous processing. Hydrogenation rate also shows a dependence on reactant carrier where a switch from an alcoholic to an aqueous feed serves to increase turnover. The results in this thesis can be applied in the development of cleaner alternative routes to a range of fine chemicals