Selective hydrogenation of terminal alkynes using supported palladium-silver nanoparticles

Williams, Jake 2023. Selective hydrogenation of terminal alkynes using supported palladium-silver nanoparticles. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

Hydrogenation of terminal alkynes is an important process in the refining of chemical feedstocks used as monomers in polymerisation reactions. The presence of alkynes in their respective alkene streams can poison the Ziegler-Natta catalysts used in the polymerisation of these alkenes. Removal of these alkynes by hydrogenation, without hydrogenating the alkene stream is imperative. This thesis investigates the use of supported Pd-Ag nanoparticles to selectively hydrogenate terminal alkynes to the respective alkenes, whilst mitigating the formation of the alkane and any oligomers. The catalysts were produced using sol-immobilisation which is novel for the production of Pd-Ag catalysts. The sol-immobilisation method added polyvinyl alcohol (PVA) to dissolved metal nitrate salts to form a colloid, which was reduced in situ to form the sol. The sol was supported to form the catalyst. The nanoparticles produced using this method were 3 - 7 nm in diameter and were supported on titania, θ,α-alumina and γ-alumina. Whilst most of the deposited metal on the catalysts consisted of nanoparticles, some agglomerated structures, denoted as “coral-like”, were observed with TEM and AC-STEM/EDX. These corals were only seen on bimetallic catalysts and were more prevalent with higher proportions of Pd. The corals were formed from chains of distinct nanoparticles, suggesting that this was not a product of sintering, which would typically merge nanoparticles together. The EDX element mapping showed that the Pd and Ag which formed the bimetallic nanoparticles, were alloyed. The catalysts were tested for liquid-phase hydrogenation with phenylacetylene and 1-octyne. The hydrogenation of phenylacetylene showed that the reaction with these catalysts and conditions underwent a sequential hydrogenation, as opposed to a parallel hydrogenation, meaning that whilst phenylacetylene was present, no ethylbenzene was formed. This was explored further by introducing styrene into the reactor and evaluating the phenylacetylene hydrogenation in a competitive environment. When the reaction was started with styrene and a measure of phenylacetylene was added during the reaction, the formation of ethylbenzene was stopped until the phenylacetylene had been depleted. The hydrogenation of 1-octyne showed the same sequential hydrogenation pathway in that no octane was formed while 1-octyne was present. Also, no isomerisation of 1-octene occurred while 1- Preface ii octyne was present, but once 1-octyne was depleted the concentration of 2-octene (cis/trans) increased. 3- and 4-octene were not observed. The most active and selective catalyst for liquid phase hydrogenation was a high Pd-low Ag composition. The final chapter in this thesis concerns gas-phase hydrogenation of acetylene, undertaken with equal partial pressures of acetylene and ethylene to emulate the conditions seen from the Hüls process, as opposed to a small amount of acetylene in an excess of ethylene which would emulate conditions seen in steam-cracking. The most active catalyst with the best selectivity to ethylene was found to be one that contained a low Pd-high Ag composition as a high Pd composition over-hydrogenated the ethylene to ethane. The addition of the high proportion of silver diluted the active sites on the palladium so that the hydrogenation was controlled and hence selectivity to ethylene increased. The reactor conditions were varied to optimise conversion, selectivity to ethylene, and run-time and the knowledge this yielded will be used to optimise catalysts and conditions in the future of this project.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Chemistry
Date of First Compliant Deposit:	3 May 2024
Last Modified:	03 May 2024 08:20
URI:	https://orca.cardiff.ac.uk/id/eprint/168686

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