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