The Liquid Phase Hydrogenation of Nitrobenzene Over Supported Copper Catalysts

A series of silica-supported copper catalysts have been prepared, characterised and used for the hydrogenation of nitrobenzene. The catalysts were prepared using a wet impregnation technique, with three different metal salts (copper nitrate, copper acetate and copper carbonate) deposited onto two different forms of silica (Cab-O-Sil and C-10), with nominal metal loadings of approximately 10 wt%. Although different pretreatments were applied to produce derivatives of the catalyst precursors, the four principal substrates used are referred to as Cu(N)/Cab, Cu(A)/Cab, Cu(C)/Cab and Cu(N)/C-10. The catalysts have been the subject of numerous characterisation techniques including atomic absorption spectroscopy, BET surface area determination, thermal gravimetric analysis, temperature programmed reduction and nitrous oxide decomposition; with the latter technique used to measure the metal surface areas of the active catalysts. Selected samples were also analysed using differential scanning calorimetry, variable temperature x-ray diffraction and electron microscopy. The characterisation experiments provided insight into the nature of the catalysts. The choice of metal salt used in the preparation, rather than the choice of support material, was found to be the major factor affecting catalyst performance. The activation procedure applied to the catalysts was optimised with respect to metal surface area. The apparatus and experimental protocol for liquid phase nitrobenzene hydrogenation over copper/silica catalysts was developed. A batch slurry reactor was found to be preferable over a bubble-phase vessel. Catalyst activation was conducted in situ, whilst the design allowed control over a range of variables, e.g. mass of catalyst, concentration of reactant, reaction temperature, etc. The choice of solvent for the reaction was not trivial, and the suitability of various solvents was assessed. The use of an alcohol solvent led to the formation of unwanted by-products, arising via an alkylation reaction which occurred between the hydrogenation product, aniline, and the solvent itself, hexan-l-ol. A preliminary investigation of this side reaction was conducted, revealing the sequential formation of N-hexylenebenzenamine and N-hexylaniline. It is believed that the alkylation reaction occurs on the silica support, whilst the hydrogenation step occurs on the metal crystallites; in the presence of nitrobenzene the hydrogenation of the nitro group is favoured over that of N-hexylenebenzenamine. During the assessment of t-butyl benzene as a solvent it was found that its history bore marked effect on catalyst activity. It was established that the presence of sulphur within different batches of t-butyl benzene was responsible for an observed enhancement of catalyst performance. The overall concentration of sulphur within the solvent samples was determined, and it was found that five different sulphur species were present. A conclusive assignment of their identities, however, did not prove possible. Dodecane, ethyl benzene and mesitylene were also found to be unsuitable as solvents for this reaction. t-Butyl benzene was ultimately adopted as the reaction medium. Comprehensive testing with n-butyl benzene did reveal the existence of two minor reactions involving this solvent, but these were not found to affect the major reaction. The optimised technique, 'benchmarked' against a platinum/silica catalyst, allowed the behaviour of the different copper catalysts to be evaluated with reproducible results. The Cu(N)/Cab catalyst had the greatest activity and the highest metal surface area, which was attributed to the combination of the copper nitrate salt and powdered Cab-O-Sil support used in its preparation. No by-products were observed from the reaction of nitrobenzene; all of the copper catalysts showed complete selectivity to aniline. Following complete consumption of the nitrobenzene, however, it was apparent that the concentration of aniline in solution decreased through time. This was attributed to hydrogenolysis of the aniline product, with the consequent formation of ammonia and a phenyl species. (Abstract shortened by ProQuest.)