Palladium and gold catalysts for sustainable chemical processing
The main focus of this thesis is the investigation of sustainable routes for the production of commercially important higher and functionalised aliphatic and aromatic amines through the application of (oxide and carbon) supported palladium and gold catalysts. In the hydrogenation of butyronitrile as a model aliphatic nitrile, unsupported Pd promoted the formation of primary and secondary amines. The acid-base character of the support and available surface reactive hydrogen are critical catalyst variables. The greater acidity of Pd/C (relative to Pd/Al2O3) resulted in the predominant formation of the tertiary (tributyl-) amine where spillover hydrogen serves to elevate hydrogenation rate. The combination of Ba with Pd (supported bimetallic) proved effective in promoting hydrogenation activity with 100% selectivity to the secondary amine, which is attributed to a decrease in acidity and modification to Pd dispersion that enhances surface hydrogen. The feasibility of an alternative route for the synthesis of higher aliphatic secondary and tertiary amines from primary and secondary amine feedstock has been demonstrated. Control of contact time is key where the use of a multiple catalyst beds in series facilitates higher yields. This is accounted for in terms of surface reaction mechanism. This configuration was also efficient for the synthesis of benzylamine from benzonitrile. Hydrogenation selectivity was further assessed by considering the reduction of functionalized nitroarenes (p-chloronitrobenzene (p-CNB) and p-nitrobenzonitrile (p-NBN)). It is shown that the redox nature of the support has a direct impact on the activity and selectivity response. The formation of Pdδ+ (on carbon) activates the nitro group with subsequent C-Cl bond scission with the formation of p-chloroaniline (p-CAN) and aniline (AN). The occurrence of Pdδ- (on SiO2 and Al2O3) favours interaction via the aromatic ring that activates both –NO2 and –Cl for attack generating AN and nitrobenzene. The formation of a PdZn alloy (established by XPS analysis) in addition to Pd0 selectively activates the –NO2 group and promotes the sole formation of p-CAN at all levels of conversion (and close to 100%). Exclusive conversion of p-NBN to p-aminobenzonitrile was achieved over a series of oxide (CeO2, Fe2O3, Fe3O4, TiO2, ZrO2 and Al2O3) supported (1 mol %) Au catalysts. Hydrogen uptake is structure sensitive and favoured by smaller nano-scale metal particles with a consequent increase in activity. Reaction over Au/TiO2 delivered the highest specific hydrogenation rate, which is explained on the basis of –NO2 activation at the metal-support interface that is facilitated by Nδ+ interaction with electron rich gold (Auδ-, demonstrated by XPS). This effect is shown to extend to TiO2 supported Ag and Pd. Supported Au is also effective in the selective hydrogenation of benzaldehye in liquid phase operation using water as a green solvent. 100% yield of the target benzyl alcohol was attained over Au/Al2O3 whereas Pt/Al2O3 generated toluene and benzene as significant (hydrogenolysis) by-products. Solvent effects were evaluated where a direct correlation between selective hydrogenation rate and dielectric constant is demonstrated and ascribed to competitive adsorption, which was more severe for less polar alcohol solvents. Solvation by polar water facilitated benzaldehyde activation. The same activity and selectivity trends were found to also apply to continuous gas phase reaction. The results presented in this thesis demonstrate, for the first time, direct participation of the support in the catalytic hydrogenation of aliphatic nitriles over Pd-based catalysts. This can be harnessed to enhance amine production in a sustainable continuous flow gas operation process. Moreover, secondary and tertiary aliphatic amines can be selectively produced from the correspondent primary and secondary amines over Pd in continuous mode. The use of reducible supports can result in the formation of an alloy phase and surface defects with beneficial selectivity and activity effects in the production of functionalized amines. The selective catalytic action of supported Au catalysts has been established in achieving 100% yield of benzyl alcohol (from benzaldehyde) using water as a benign solvent.