title: Hydrogenation and low temperature hydrodeoxygenation of oxygen substituted aromatics over a Rh/SiO2 catalyst
Creators: name: Kirkwood, Kathleen
Publisher: name: University of Glasgow
Year: 2020

description: One of the main areas of energy consumption is the transportation sector, where at present the vast majority of fuel comes from crude oil. Well documented environmental concerns over the use of fossil fuels has driven a search for alternative sources of energy and one focus is on second generation biofuels from agricultural waste. The most abundant source for candidate biofuels is lignocellulosic biomass which contains hemicellulose (25-35 %), cellulose (40-50 %) and lignin (15-20 %), with the exact composition dependent on the plant species present. The focus of this study is on the components of bio-oil derived from lignin and in particular the p-hydroxyphenyl and guaiacol based monomers. Lignocellulosic biomass can be converted into liquid bio-oil via fast pyrolysis at 673–873 K in the absence of air with multiple reactions taking place. The result is a bio-oil that contains over 300 individual compounds, many of them oxygenates, limiting their value as increased oxygen content is the cause of many of the negative properties of bio-oil such as low heating value, corrosiveness, high viscosity and instability. The challenge to try and selectively remove the oxygen has gathered considerable momentum, with efforts focused on catalytic hydrodeoxygenation (HDO) and hydrogenation. Currently, there is limited understanding of the mechanisms involved in this process and our study aims to increase knowledge in this area. Hydrogenation of dihydroxybenzene, cresol and methoxyphenol isomers was studied over a Rh/SiO2 catalyst at mild operating conditions (303-343 K, 3 barg H2). Reaction temperature and substrate concentration were varied to assess their effect on reactivity and HDO activity. Isomers were reacted together to uncover the effect of the competitive environment on reaction rate and behaviour. To gain further insight into the reaction mechanism, substrates were reacted with deuterium to ascertain overall and product KIE values. TPO analysis was carried out on spent catalysts under standard conditions to investigate the extent of carbon laydown. Dihydroxybenzene isomers were studied initially, and deoxygenated products (cyclohexanone, cyclohexanol and cyclohexane) were formed in significant amounts; the meta and para isomers (resorcinol and hydroquinone) favoured HDO product formation over conventional hydrogenated products (hydroxycyclohexanone and cyclohexanediol). The lack of any deoxygenation or isomerisation activity when the aromatic substrate was replaced by cis-cyclohexanediol led to the proposal of an independent and direct route for both HDO and hydrogenated products. The HDO products are believed to form via highly reactive surface intermediates whereby the position of the double-bond β-γ to hydroxyl group facilitates the C-OH bond cleavage more readily. The trans isomer was shown not to occur via an isomerization mechanism, and instead was proposed to form exclusively from hydrogenation of the ketone intermediate product. To understand the effect changing a substituent had on the hydrogenation reactivity and HDO favourability, the cresol isomers were tested under identical reaction conditions. In this instance, it was the ortho isomer which showed the greatest favourability towards the deoxygenated product (methylcyclohexane), however, all three isomers favoured the standard hydrogenated products (methylcyclohexanone and methylcyclohexanol) overall. A clear substituent effect was shown to exist, with a shift in the HDO favourability of each isomer and a change in overall order in reactivity apparent when a hydroxyl group was replaced with a methyl group. The methyl group on the ortho isomer was shown to interact directly with the catalyst surface through NMR analysis, this exchange was significantly reduced with both the meta and para isomers. This suggests the existence of a different mode of adsorption for the ortho isomer and may explain its increased favourability towards HDO. The effect of substituent was investigated further by studying the hydrogenation of the methoxyphenol isomers under identical reaction conditions. Similar to the dihydroxybenzenes, the meta and para isomers favoured HDO products (cyclohexanone, cyclohexanol, cyclohexane and methoxycyclohexane) over conventional hydrogenated products (methoxycyclohexanone, methoxycyclohexanol). Order in reactivity was also in accordance with that observed for the dihydroxybenzenes indicating the presence of two oxygen-containing substituents resulted in similar substrate behaviour. The hydrogenation of 4-methoxyphenol was particularly interesting as secondary hydrogenation was completely absent: the only instance that this was observed. This is postulated to be a result of the adsorption/desorption kinetics when the bulky methoxy group and hydroxyl group are in the para position to one another. A significant reduction in reactivity was observed for all substrates in the competitive environment. When combined, the three isomers of each substrate and the corresponding isomers from each set of substrates, gave a uniform rate constant, independent of substituent nature and position. This shows competition for limited surface hydrogen and active sites on the catalyst surface is the major factor influencing reactivity in these competitive situations. Calculated overall and product KIE values from competitive deuterated reactions showed a change in reaction mechanism from that observed during individual hydrogenation, and again highlights the complex nature of these reactions. This substantial change in behaviour and reactivity observed in the competitive environment substantially limits the value of studying a single model compound as a route to understanding a true bio-oil feed. It is notable that during the deuterated reaction of ortho-cresol, no cis-2-methylcyclohexanol was detected and formation of the HDO product, methylcyclohexane, was delayed until 60 minutes into the reaction; significantly different behaviour to that observed during the hydrogenation reaction. Extended run reactions of the dihydroxybenzene isomers showed a substantial drop in conversion associated with deactivation of the catalyst; confirmed by spent catalyst TPO analysis to be a result of carbonaceous deposits. TPO analysis of post reaction catalysts from standard reactions for each substrate showed the existence of a direct relationship between percentage of carbon found and substituent nature. The post-reaction catalysts of those substrates that exhibited the highest levels of HDO showed the presence of additional carbon, phenolic in nature, arising from the formation of these products.