Low temperature hydrogenation and hydrodeoxygenation of oxygen-substituted aromatics over Rh/silica: part 1: phenol, anisole and 4-methoxyphenol

Abstract

The hydrogenation and competitive hydrogenation of anisole, phenol and 4-methoxyphenol was studied in the liquid phase over a Rh/silica catalyst at 323 K and 3 barg hydrogen pressure. The rate of conversion of the reactants to products gave an order of anisole ≫ phenol > 4-methoxyphenol with hydrogenation and hydrodeoxygenation products being produced. Anisole, the most reactive substrate, was rapidly converted to methoxycyclohexane, cyclohexane, cyclohexanone and cyclohexanol, while phenol was hydrogenated to cyclohexanone, cyclohexanol and cyclohexane. In both cases cyclohexanol was produced as a secondary product from cyclohexanone hydrogenation. The yield of cyclohexane, the hydrodeoxygenation (HDO) product was > 20% from both reactants and was formed as a primary product from the aromatic species. Hydrogenation of 4-methoxyphenol was selective to 4-methoxycyclohexanone with no alcohol formation, while the hydrogenolysis products revealed that the catalyst was more active for demethoxylation than dehydroxylation. A comparative strength of adsorption was determined from competitive hydrogenation and gave an order of anisole > phenol > 4-methoxyphenol. Competitive, pair hydrogenation inhibited HDO and stopped cyclohexane from being produced from phenol and 4-methoxyphenol, although it was still produced from anisole. An increased rate of hydrogenation for 4-methoxyphenol was observed for competitive reactions with phenol and anisole but not when all three reactants were present. In contrast to the pair reactions, when all three reactants were present HDO occurred with all aromatics producing cyclohexane. Replacing hydrogen with deuterium revealed an inverse kinetic isotope effect for ring hydrogenation of 4-methoxyphenol but not phenol or anisole, which both had a positive KIE.

Hydrogenation and Hydrodeoxygenation of Oxygen-Substituted Aromatics over Rh/silica: Catechol, Resorcinol and Hydroquinone

“…As may be expected, an increase in temperature resulted in higher selectivity to hydrogenolysis products as shown in Figure 4. Similar behaviour was reported with anisole hydrogenation over rhodium [14].…”

“…It has been suggested that cyclohexane is formed from cyclohexanol [17], however other authors [18] have suggested direct conversion of phenol to cyclohexane. We have shown that cyclohexanol is stable under these conditions [14] and does not form cyclohexane, therefore cyclohexane is a direct product from resorcinol. As may be expected, an increase in temperature resulted in higher selectivity to hydrogenolysis products as shown in Figure 4.…”

“…The bulk of the literature up until now has reported HDO as occurring at high temperature and pressure, however, a recent study examining phenol hydrogenation over rhodium at low temperature and pressure (≤343 K, 2-5 barg hydrogen) found a 20% yield of cyclohexane [14]. In light of this, the occurrence of dihydroxybenzene hydrogenolysis under similar conditions was a possibility and worthy of investigation.…”

Competitive Hydrogenation and Hydrodeoxygenation of Oxygen-Substituted Aromatics over Rh/Silica: Catechol, Resorcinol and Hydroquinone

“…Under these conditions we have shown that cyclohexane, cyclohexanone and cyclohexanol are primary products. Cyclohexanone can be hydrogenated to cyclohexanol but only once more strongly bound aromatic species have been hydrogenated [11]. At the end of the reaction conversion of hydroquinone and resorcinol is ~ 26% respectively and the overall yield of HDO products is ~ 32%, when this is compared with the HDO product yield at 26% conversion for the individual DHB isomer reactions the yield for each of them was ~ 15% giving a combined yield of 30% [6].…”

“…In this study competitive hydrogenation and low temperature HDO was examined. Studies of competitive hydrogenation are still not common [7][8][9][10] and studies of competitive HDO even less so [11][12][13]. Nevertheless, it is important that competitive reactions are considered given that when lignocellulosic biomass is converted into liquid bio-oil it can contain over 300 individual compounds.…”