The selective hydrogenation of alkynyl-substituted aromatic molecules over rhodium/silica

Gregory, Joseph William (2019) The selective hydrogenation of alkynyl-substituted aromatic molecules over rhodium/silica. MSc(R) thesis, University of Glasgow.

Abstract

Kinetic studies involving phenylacetylene and 1-phenyl-1-propyne hydrogenation cascades over 2.5% w/w rhodium/silica were carried out to assess aspects of selectivity and catalytic behaviour. The reactions were performed over the temperature range 313 K – 343 K in the liquid phase using 2-propanol (IPA) as a solvent. Typically 0.1g of catalyst was used with 3 barg hydrogen pressure and 10 mmol of aromatic in a stirred tank reactor. Rate constants and activation energies for phenylacetylene, ethylbenzene, 1-phenyl-1-propyne and propylbenzene hydrogenation were calculated and, for molecules with analogous functionality, were found to increase with a decrease in the mass of the substituent.

For the hydrogenation of 1-phenyl-1-propyne, the cis/trans ratio of β-methylstyrene was calculated and a kinetic isotope effect observed. The cis/trans ratio was typically at its greatest in the initial stages of the reaction and decreased with time. Increasing the temperature increased the cis/trans ratio, however this trend was reversed at higher temperatures, suggesting that there was a change in reaction pathway and meaning that the nature of the reaction was temperature-dependent. Competitive hydrogenation was carried out which showed that, in some instances, aromatic molecules with larger substituents can completely outcompete molecules with smaller substituents. This effect was reflected in an increased selectivity for alkylarenes with shorter-chain substituents when related alkylarenes with longer chain substituents were present, for example the presence of propylbenzene in a reaction mixture improved ethylbenzene selectivity.

Styrene selectivity during the hydrogenation cascade of phenylacetylene was monitored and found to be largely unaffected by altering the temperature and by changing the reactive hydrogen isotope, however an enhancement was made possible by competition with ethylbenzene and 1-phenyl-1-propyne. The majority of unsaturated functional groups assessed in this study appeared to undergo hydrogenation at the same active site of the catalyst, however kinetic results suggested the terminal alkyne group of phenylacetylene reacted at a different site.

These kinetic results also provided evidence that subsurface hydrogen was the active hydrogen species in some of the hydrogenation processes observed. The findings of this study have further developed our understanding of the hydrogenation of alkynylaromatics.

• Item Type: Thesis (MSc(R))
• Qualification Level: Masters
• Keywords: alkynylaromatic, unsaturated, rhodium, rhodium/silica, hydrogenation, phenylacetylene, 1-phenyl-1-propyne, styrene, ethylbenzene, ethylcyclohexane, methylstyrene, propylbenzene, propylcyclohexane, competitive.
• Subjects: Q Science > QD Chemistry
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Jackson, Prof. Samuel David
• Date of Award: 2019
• Depositing User: Mr Joseph William Gregory
• Unique ID: glathesis:2019-75124
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 25 Oct 2019 15:53
• Last Modified: 25 Oct 2019 15:54
• Thesis DOI: 10.5525/gla.thesis.75124
• URI: https://theses.gla.ac.uk/id/eprint/75124