| Summary: | Selective hydrogenation of a multi-functional compound to achieve high yield of a particular product is often involved in the production of fine chemicals and pharmaceuticals. The control of selectivity can be difficult and can be affected by a number of variables such as the interaction of the reactants and intermediates with the catalyst, the particle size, promoters, steric factors and adsorption geometries. In this study, two selective hydrogenation reactions were studied namely the gas-phase hydrogenation of furfural to furfuryl alcohol and liquid-phase hydrogenation of 4-nitroacetophenone. The hydrogenation of furfural is the sole production route for furfuryl alcohol which is used widely in the chemical industry. However, a variety of products can be formed through the hydrogenation of furfural depending on the catalyst used. The industrial process is conducted at high temperature and pressure using a copper chromite catalyst. However, the main drawback of this method is the toxicity of the catalyst. In this study, silica supported copper catalysts proved to be active and selective alternative catalysts for the hydrogenation of furfural to furfuryl alcohol. The higher the copper loading the greater the furfural conversion as more Cu sites were present with both catalysts achieving 98% selectivity for furfuryl alcohol. Ceria was investigated as a promoter for copper catalysts and the incorporation of 1% CeOx was found to enhance the selectivity towards furfuryl alcohol. The presence of Ce3+ sites was thought to polarise the carbonyl bond facilitating nucleophillic attack by dissociated hydrogen present on Cu. However, 5% CeOx promoter was found to reduce the selectivity of the catalyst possibly by the blocking Cu active sites. Pd was also investigated as a promoter was found to enhance the activity and selectivity of the catalyst as it activated hydrogen allowing for more facile hydrogenation of the carbonyl group. Cu1132, a BASF copper-chromite catalyst used for the production of furfuryl alcohol by hydrogenation of furfural exhibited moderate activity but excellent selectivity towards furfuryl alcohol (almost 100%). For all catalysts, deactivation was observed over time on stream mainly due to carbon laydown. Polyaromatic coke formation on the surface of copper catalysts blocked pores and significantly reduced the activity of the catalyst. 5% Cu + 5% CeOx/SiO2 catalyst showed a slow deactivation compared to all of the other catalysts and post-reaction XRD suggested sintering was the cause of deactivation. TPD of 5% Cu + 1% CeOx/SiO2 showed desorption of furfural, 2-methylfuran and furan suggesting that the catalyst was poisoned causing deactivation. An important reaction in the pharmaceutical industry is the hydrogenation of 4-nitroacetophenone (4-NAP) which yields 4-aminoacetophenone (4-AAP); a chemical intermediate used in the production of the hypoglycemic drug acetohexamide. Further hydrogenation of the carbonyl group yields 1-(4-aminophenyl)ethanol (4-APE) which can be dehydrated to give a substituted styrene that can be polymerised. As the consecutive hydrogenation of 4-NAP has not been the subject of significant study, this reaction was systematically investigated using a series of Rh/SiO2 catalysts. Functional group hydrogenation followed the order NO2 >> C=O > Ph > OH with the nitro group being hydrogenated approximately an order of magnitude faster than the carbonyl group, while hydrogenation of either the phenyl ring or the alcohol function is a factor of two slower than carbonyl hydrogenation. This combination of kinetic controls allows high selectivity to 4-AAP (99%) and 4-APE (94%) to be achieved at different times in the reaction. The presence of 4-NAP inhibits 4-AAP hydrogenation due to strong adsorption of the 4-NAP while deuterium studies revealed the presence of a kinetic isotope effect for both 4-NAP and 4-AAP. Full kinetic analysis of the reaction system gave activation energies of ~48 kJ mol-1 for 4-NAP and 4-AAP hydrogenation, with orders of reaction of ~1 for hydrogen and a zero order dependence for 4-NAP. Although 4-NAP inhibits 4-AAP hydrogenation when present, 4-AAP hydrogenation is faster after 4-NAP hydrogenation than over a fresh catalyst. The reason for this may be that 4-NAP adsorption causes a surface reconstruction which allows easier hydrogen transfer or sub-surface hydrogen. The hydrogenation of both 4-NAP and 4-AAP showed an antipathetic particle size effect with an increase in TOF with increasing metal crystallite particle size. This suggests that the hydrogenation reaction takes place on the plane face surface as opposed to edge and corner sites. However the electronic changes in small metal particles of this size are also significant and it is likely that the antipathetic particle size effect is a combination of both an electronic and geometric effect. Addition of 4-methylcyclohexylamine (4-MCHA) to 4-NAP and 4-AAP hydrogenation systems results in an enhancement of rate for both reactants. For 4-NAP hydrogenation, this is due to electron donation from the acyclic amine causing a reduction of the reactant-surface bond strength since it is clear that 4-NAP forms a strong bond to the surface as shown by the zero order kinetics and the inhibition of 4-AAP hydrogenation, a reduction in the strength of 4-NAP adsorption would enhance the rate of 4-NAP hydrogenation. However, 4-AAP is not strongly bound to the surface so a weakening the carbonyl interaction is unlikely to lead to an enhanced rate. Just as the rate enhancement observed for 4-AAP hydrogenation after 4-NAP hydrogenation has been attributed to changes in hydrogen concentration in the rhodium by strong adsorption of 4-NAP, 4-MCHA is also strongly adsorbed which means it may promote 4-AAP hydrogenation by a similar process.
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