New catalyst formulations based on gold and molybdenum nitrides and carbides : application in selective hydrogenation
Abstract
The development of selective heterogeneous catalytic systems is of fundamental
importance for the realisation of sustainable green chemical processes. In this thesis, the
hydrogenation of nitroarene, aldehyde and carboxylic acid compounds is examined in
continuous gas phase operation, for the synthesis of a target functionalised aromatic
amine and alcohol. Novel catalysts based on Mo nitride, carbide and supported Au have
been synthesised and subjected to an array of complementary characterisation
measurements that provide catalyst structure/performance correlations with detailed
kinetic and mechanistic analysis.
The use of Mo2N and Mo2C as catalysts served to promote the selective
hydrogenation of nitrobenzene to aniline and p-chloronitrobenzene to p-chloroaniline
where the incorporation of nano-scale Au increased reaction rate. Activity has been
correlated to hydrogen adsorption/release capacity, which shows a dependence on the
degree of nitridation and crystallographic structure. In contrast, hydrogenation of
benzaldehyde is limited by C=O activation, which is facilitated by the Mo component.
Alumina supported Au has exhibited 100% selectivity in the hydrogenation of
benzaldehyde and 4-nitrobenzaldehyde to the target alcohol, where Au particle size and
surface Lewis acidity play crucial roles. The hydrogenation of benzoic acid over Au
supported on a CeO2-ZrO2 mixed oxide has shown promising results with the possibility
of a one step transformation to the alcohol.
The results presented in this thesis establish feasible catalytic routes to high
value amines and alcohols where critical process optimisation is demonstrated in terms
of catalyst composition/surface structure and reaction conditions.