Abstract
The hydrogenation and hydrodeoxygenation (HDO) of dihydroxybenzene isomers, catechol (1,2-dihydroxybenzene), resorcinol (1,3-dihydroxybenzene) and hydroquinone (1,4-dihydroxybenzene) was studied in the liquid phase over a Rh/silica catalyst at 303–343 K and 3 barg hydrogen pressure. The following order of reactivity, resorcinol > catechol > hydroquinone (meta > ortho > para) was obtained. Kinetic analysis revealed that catechol had a negative order of reaction whereas both hydroquinone and resorcinol gave positive half-order suggesting that catechol is more strongly adsorbed. Activation energies of ~30 kJ·mol−1 were determined for catechol and hydroquinone, while resorcinol gave a value of 41 kJ·mol−1. Resorcinol, and similarly hydroquinone, gave higher yields of the hydrogenolysis products (cyclohexanol, cyclohexanone and cyclohexane) with a cumulative yield of ~40%. In contrast catechol favoured hydrogenation, specifically to cis-1,2-dihydroxycyclohexane. It is proposed that cis-isomers are formed from hydrogenation of dihydroxycyclohexenes and high selectivity to cis-1,2-dihydroxycyclohexane can be explained by the enhanced stability of 1,2-dihydroxycyclohex-1-ene relative to other cyclohexene intermediates of catechol, resorcinol or hydroquinone. Trans-isomers are not formed by isomerisation of the equivalent cis-dihydroxycyclohexane but by direct hydrogenation of 2/3/4-hydroxycyclohexanone. The higher selectivity to HDO for resorcinol and hydroquinone may relate to the reactive surface cyclohexenes that have a C=C double bond β-γ to a hydroxyl group aiding hydrogenolysis. Using deuterium instead of hydrogen revealed that each isomer had a unique kinetic isotope effect and that HDO to cyclohexane was dramatically affected. The delay in the production of cyclohexane suggest that deuterium acted as an inhibitor and may have blocked the specific HDO site that results in cyclohexane formation. Carbon deposition was detected by temperature programmed oxidation (TPO) and revealed three surface species.
Highlights
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, which results in a bio-oil that contains over 300 individual compounds
Xylene hydrogenation over a similar catalyst gave an order of reactivity of para > ortho > meta, which is the reverse of that found with dihydroxybenzenes [12], while the hydrogenation of dimethoxybenzenes over a rhodium catalyst gave an order of activity of para > meta > ortho [15], indicating that changing the substituents results in a change in the ordering of the reactivity of the isomers
A comparison of hydrogenation and deuteration of toluene, ethylbenzene and propyl benzene revealed that all three displayed an inverse kinetic isotope effect, which was concluded to be due to the change in hybridisation of the carbon atom from sp2 to sp3 which takes place during the hydrogenation of the aromatic ring—an explanation which may explain the inverse KIE values calculated for catechol and resorcinol
Summary
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, which results in a bio-oil that contains over 300 individual compounds. Through the course of their study, benzene, the complete HDO product, was undetected [7] They proposed a mechanism whereby aryl C–O bond hydrogenolysis and aromatic ring hydrogenation occurred in parallel, where the latter reaction dominated. Studies of dihydroxybenzenes on the effect of symmetry, number of substituents, relative rates and reaction order have recorded similar but by no means uniform trends to that observed with xylenes. The bulk of the literature up until now has reported HDO as occurring at high temperature and pressure, 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]. The occurrence of dihydroxybenzene hydrogenolysis under similar conditions was a possibility and worthy of investigation
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