Development of gold and copper catalysts for sustainable chemical processing
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Supported Au and Cu catalysts have been developed for clean/sustainable production of value fine chemicals (including alcohols, ketone, amines and imines) from selective reduction (of benzaldehyde, nitrobenzene and furfural) and coupled (dehydrogenation-hydrogenation) reaction in the continuous gas phase operation. Critical catalyst physicochemical properties are characterised by applying a range of techniques and correlated to the catalytic response. The role of support, Au particle size and electronic character in determining catalytic activity and selectivity in the hydrogenation of benzaldehyde and nitrobenzene over oxide supported nano-scale (2-8 nm) Au has been established. Hydrogenation (turnover frequency) TOF increases with decreasing Au size (from 8 to 4 nm) with measurably lower TOF over Au < 3 nm. Repulsion of –C=O and –NO2 functionalities with respect to Auδ+ and strong binding to surface oxygen vacancies have been found to lower hydrogenation rates. Promotional effect of water via catalytic dissociation has been found to enhance the selective benzaldehyde hydrogenation rate. Two catalytic routes for imine (N-benzylideneimine) synthesis in continuous gas phase operation have been established. Reductive coupling of benzaldehyde with nitrobenzene (using external hydrogen) over supported Au generated up to 99% selectivity to the target imine. Coupling of benzyl alcohol dehydrogenation with nitrobenzene hydrogenation (in the absence of external hydrogen supply) over Au/TiO2 + Cu/SiO2 mixture produced imine with full hydrogen utilisation. Incorporation of Au/TiO2 to Cu/SiO2 created a synergy between Cu and Au and enhanced catalyst stability. A tandem dehydrogenation/amination/reduction process has been developed for high throughout production of benzylamine in continuous gas phase operation over Cu/SiO2 and Au/TiO2. A synergy between Cu/SiO2 and Au/TiO2 serves to promote benzylamine formation with 81% yield achieved through an optimization of process parameters. Coupling of 2-butanol dehydrogenation with nitrobenzene hydrogenation over Cu/SiO2 in the absence of an external H2 supply delivered exclusive production of both 2-butanone and aniline at full conversion. Hydrogen utilisation efficiency was appreciably greater (by a factor of up to 50) in the coupled system relative to conventional stand-alone hydrogenation using pressurised hydrogen. Selective conversion of biomass-derived furfural to furfuryl alcohol over supported Au catalysts has been established. A series of approaches (e.g. promotional effect of water via catalytic dissociation, increased spillover hydrogen with addition of oxide support and coupling strategy) directed at increasing the surface availability of reactive hydrogen were adopted to enhance furfuryl alcohol production and hydrogen utilisation. Continuous production of γ-butyrolactone has been established in both stand-alone hydrogenation of succinic acid (using external H2) and reaction coupling with formic acid decomposition (as a source of H2) over Cu/SiO2. Pd/SiO2 and Ni/SiO2 promoted propanoic acid formation at higher reaction rates. The results presented in this thesis establish feasible catalytic routes to high value alcohols, imines and amines where critical process optimisation is demonstrated in terms of catalyst composition/surface structure and reaction conditions.