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The catalytic hydrogenation of substituted benzenes to saturated cyclic products is an industrially relevant reaction. It is important in the production of fine chemicals, petroleum and in the fuel industry. It is used in the process of lowering the aromatic content in diesel fuels to follow up the up-to-date environmental legalisation. It has been widely reported that aromatic ring hydrogenation is a structure insensitive reaction, however more recent studies have suggested that structure sensitivity may indeed exist. Therefore, the demand to perform more research on substituted benzenes to understand their behaviour during the hydrogenation has increased. Unlike most of what was found in literature, this study involved substituents which include methyl, ethyl, propyl, hydroxyl and methoxy groups. These reactions were performed at different parameters of temperatures, H2 pressure and concentrations and over Rh/SiO2 catalyst. Different mechanisms were suggested for the hydrogenation of aromatic compounds. A stepwise mechanism is generally accepted to explain the reaction mechanism. This suggestion was built on the fact aromatic adsorption is zero order in aromatics which suggests a strong adsorption of the substrates. This mechanism was confirmed in this work by the observation of alkyl cyclohexenes as intermediates during the hydrogenation of alkyl benzenes. Interesting points were observed during the hydrogenation of phenols. Firstly, cyclohexane was formed independently and directly from the original phenol. This observation was not found in most of previous studies. The other point was that phenol and anisole reacted in different ways from each other. Phenol was found to react in three independent routes, the formation of cyclohexanone, the formation of cyclohexanol and the formation of cyclohexane. Whereas, cyclohexanol was not formed directly from anisole, it was formed from cyclohexanone and after the total conversion of anisole. Competitive hydrogenations were also executed in order to investigate the behaviour of different groups in the same reaction. The findings of these tests were different from what was observed during the solo tests. As for the hydrogenation of alkylbenzenes, a steric effect might explain the differences between these substrates. It was observed that the reaction rate decreased as the alkyl group attached to the ring increased. These findings were not the same during the competitive hydrogenation. n-Propylbenzene, which has a larger group attached to the aromatic ring, showed higher reactivity in the presence of toluene and ethylbenzene, which suggests that the steric effect was not the only factor affecting the hydrogenation of substituted benzenes. These findings were explained by an electronic effect applied by the alkyl groups attached to the ring. The effect of these groups increases as the group size increases. In addition to steric and electronic effects, the mode of adsorption was also suggested to affect the competitive hydrogenations of phenols. Different modes of adsorption and different medium species formed leads to different behaviour during the competitive hydrogenation. In addition, NMR analysis was performed on selected samples from toluene and deuterated toluene reactions with deuterium and hydrogen. Toluene reaction with deuterium showed that all hydrogen atoms were replaced by deuterium at the beginning of the reaction. It was also shown from NMR results that -CD3 group was contacted to the surface which confirms the ability of -CH3 group to be adsorbed to the surface as well as the aromatic ring. |
