Hydrogenation of alkynyl substituted aromatics over rhodium/silica

Gregory, J. W. and Jackson, S. D. (2021) Hydrogenation of alkynyl substituted aromatics over rhodium/silica. Reaction Kinetics, Mechanisms and Catalysis, 133(2), pp. 669-686. (doi: 10.1007/s11144-021-02039-z)

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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.

Item Type:Articles
Glasgow Author(s) Enlighten ID:Jackson, Professor David and Gregory, Mr Joseph
Creator Roles:
Gregory, J. W.Funding acquisition, Validation, Formal analysis, Investigation
Jackson, S. D.Supervision, Project administration, Formal analysis, Writing – original draft, Writing – review and editing
Authors: Gregory, J. W., and Jackson, S. D.
College/School:College of Science and Engineering > School of Chemistry
Journal Name:Reaction Kinetics, Mechanisms and Catalysis
ISSN (Online):1878-5204
Published Online:04 August 2021
Copyright Holders:Copyright © 2021 The Authors
First Published:First published in Reaction Kinetics, Mechanisms and Catalysis 133(2): 669-6786
Publisher Policy:Reproduced under a Creative Commons License

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