Hydrogenation and hydrodeoxygenation of oxygen-substituted aromatics over Rh/silica: catechol, resorcinol and hydroquinone

Kirkwood, K. and Jackson, S. D. (2020) Hydrogenation and hydrodeoxygenation of oxygen-substituted aromatics over Rh/silica: catechol, resorcinol and hydroquinone. Catalysts, 10(5), 584. (doi: 10.3390/catal10050584)

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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.

Item Type:Articles
Additional Information:This work was funded by Innospec Ltd. and University of Glasgow.
Glasgow Author(s) Enlighten ID:Kirkwood, Miss Kathleen and Jackson, Professor David
Creator Roles:
Jackson, D.Conceptualization, Methodology, Resources, Supervision, Writing – review and editing
Kirkwood, K.Investigation, Methodology, Writing – original draft
Authors: Kirkwood, K., and Jackson, S. D.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Catalysts
ISSN (Online):2073-4344
Published Online:22 May 2020
Copyright Holders:Copyright © 2020 by the authors
First Published:First published in Catalysts 10(5):584
Publisher Policy:Reproduced under a Creative Commons license

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