Abstract
The competitive 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 323 K and 3 barg hydrogen pressure. Under competitive hydrogenation conditions an order of reactivity of ortho > para > meta was observed. Catechol initially inhibited resorcinol and hydroquinone hydrogenation but not HDO suggesting separate sites for hydrogenation and HDO. When resorcinol and hydroquinone were reacted competitively, HDO became the favoured reaction. The data suggested that cyclohexane and cyclohexanone were primary products. At low dihydroxybenzene (DHB) conversion the ratio of HDO products was dependent upon DHB isomer. When all three DHB isomers were reacted together, initially 86% of the HDO yield came from catechol with the rest from hydroquinone. When resorcinol finally reacted, HDO products were produced first. Reaction of DHB isomers in pairs using deuterium instead of hydrogen revealed changes in kinetic isotope effect (KIE). The presence of competing reactants had a dramatic effect on the energetics of hydrogenation and HDO reactions of individual components, reinforcing the view that hydrogenation and HDO are mechanistically separate. This effect on reaction energetics observed when more than one substrate was present, highlights the limitations of studying one single model compound as a route to understanding the processes required for the upgrading of a true bio-oil feed.
Highlights
The focus of this study is on the hydrogenation and hydrodeoxygenation of components of bio-oil derived from lignin, dihydroxybenzenes
The data suggested that cyclohexane and cyclohexanone were primary products
3-hydroxycyclohexanone and cis-1,3-cyclohexanediol delayed until 10 and 25 min respectively, whereas catechol reacted immediately. This is in marked contrast to individual hydrogenations where both dihydroxybenzene isomers formed hydrogenated products from the outset
Summary
The focus of this study is on the hydrogenation and hydrodeoxygenation of components of bio-oil derived from lignin, dihydroxybenzenes. Substituted phenols are used as model compounds for bio-oil HDO [1,2,3,4] as this class of Centre for Catalysis Research, School of Chemistry, University of Glasgow, Glasgow G12 8QQ, Scotland, UK molecules are commonly produced in lignin decomposition [5]. In a previous study [6] the hydrogenation and hydrodeoxygenation (HDO) of catechol, resorcinol and hydroquinone was reported. That study highlighted that significant HDO was possible at low temperatures (< 343 K). In this study competitive hydrogenation and low temperature
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