Hydrogenation of alkylaromatics over Rh/silica

Alshehri, Feras; Weinert, Hanns Micha; Jackson, S. David

doi:10.1007/s11144-017-1251-6

Cited by 11 publications

(26 citation statements)

References 23 publications

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“…This behavior was similar to that observed with the ring hydrogenation of alkylaromatics, where an inverse KIE was observed [39]. It was suggested that this was a secondary inverse KIE, which could be related to the change in hybridization of the carbon (C-H) from sp 2 to sp 3 as would be the case in hydrogenation of the aromatic ring [39]. There are few other examples of this in the literature but an inverse KIE was also observed with graphene hydrogenation [40].…”

Section: Deuterium Reactionssupporting

confidence: 88%

“…Determination of individual KIEs for each reaction occurring in the 4-methoxyphenol hydrogenation reaction revealed that the hydrogenation of 4-methoxyphenol to 4-methoxycyclohexanone had an inverse KIE, while surprisingly production of cyclohexane revealed no KIE. This behavior was similar to that observed with the ring hydrogenation of alkylaromatics, where an inverse KIE was observed [39]. It was suggested that this was a secondary inverse KIE, which could be related to the change in hybridization of the carbon (C-H) from sp 2 to sp 3 as would be the case in hydrogenation of the aromatic ring [39].…”

Section: Deuterium Reactionssupporting

confidence: 83%

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

Alshehri

Feral

Kirkwood

et al. 2019

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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.

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Section: Deuterium Reactionssupporting

confidence: 88%

Section: Deuterium Reactionssupporting

confidence: 83%

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

Alshehri

Feral

Kirkwood

et al. 2019

Reac Kinet Mech Cat

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“…In single reactant tests, resorcinol was the most reactive of the three isomers [6]. Nevertheless, a similar result was observed with xylene isomers [16]. Meta-xylene was more reactive than ortho-xylene in single reagent tests but in a competitive environment ortho-xylene was much more active.…”

Section: Discussionsupporting

confidence: 63%

“…The change to a normal KIE for resorcinol implies a significant change in surface energetics for resorcinol ring hydrogenation, which is consistent with hydrogenation being affected by the presence of a second reactant [14,15]. The iKIE was explained by the change from sp 2 hybridized carbon to sp 3 hydridized carbon being the rate determining step [16], however the normal KIE suggests that hydrogen bond breaking or making is involved in the rate determining step. Similar behaviour was observed with competitive deuteration of catechol and hydroquinone in that a positive KIE of 1.3 for catechol and 1.2 for hydroquinone was calculated.…”

Section: Discussionmentioning

confidence: 55%

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

Kirkwood

Jackson

2021

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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.

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Hydrogenation of alkynyl substituted aromatics over rhodium/silica

Gregory

Jackson

2021

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The cascade reactions of phenylacetylene to ethylcyclohexane and 1-phenyl-1-propyne to propylcyclohexane were studied individually, under deuterium and competitively at 343 K and 3 barg pressure over a Rh/silica catalyst. Both systems gave similar activation energies for alkyne hydrogenation (56 ± 4 kJ mol−1 for phenylacetylene and 50 ± 4 kJ mol−1 for 1-phenyl-1-propyne). Over fresh catalyst the order of reactivity was styrene > phenylacetylene ≫ ethylbenzene. Whereas with the cascade hydrogenation starting with phenylacetylene, styrene hydrogenated much slower phenylacetylene even once all the phenylacetylene was hydrogenated. The activity of ethylbenzene was also reduced in the cascade reaction and after styrene hydrogenation. These reductions in rate were likely due to carbon laydown from phenylacetylene and styrene. Similar behavior was observed with the 1-phenyl-1-propyne cascade. Deuterium experiments revealed similar positive KIEs for phenylacetylene (2.6) and 1-phenyl-1-propyne (2.1). Ethylbenzene hydrogenation/deuteration gave a KIE of 1.6 obtained after styrene hydrogenation in contrast to the inverse KIE of 0.4 found with ethylbenzene hydrogenation/deuteration over a fresh catalyst, indicating a change in rate determining step. Competitive hydrogenation between phenylacetylene and styrene reduced the rate of phenylacetylene hydrogenation but increased selectivity to ethylbenzene suggesting a change in the flux of sub-surface hydrogen. In the competitive reaction between 1-phenyl-1-propyne and propylbenzene, the rate of hydrogenation of 1-phenyl-1-propyne was increased and the rate of alkene isomerization was decreased, likely due to an increase in the hydrogen flux for hydrogenation and a decrease in the hydrogen species active in methylstyrene isomerization.

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Hydrogenation of alkylaromatics over Rh/silica

Cited by 11 publications

References 23 publications

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

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

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

Hydrogenation of alkynyl substituted aromatics over rhodium/silica

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