New catalyst formulations based on gold and molybdenum nitrides and carbides : application in selective hydrogenation
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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.