Heterogeneously catalysed isomerisation of allylbenzene.
MSc(R) thesis, University of Glasgow.
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The current method for the isomerisation of aromatic alkenes involves super stoichiometric quantities of liquid base (KOH) in higher alcohols [1-7]. Acid catalysis of such transformations produces a cis:trans ratio near to one . It is well documented that solid bases catalyse double bond migration through the abstraction of an allylic proton forming an anionic intermediate [9-11]. Solid bases hold a significant advantage over acid catalysis as they promote double bond migration without carbon-carbon bond interruption . In addition, solid base catalysts provide an environmentally benign method of producing a mixture of cis/trans isomers, which could potentially find application in the pharmaceutical  and perfumery industries .
Activated potassium carbonate supported on alumina was found to be active at 50°C for the isomerisation of allylbenzene. Where isomerisation of aliphatic alkenes has reportedly given high cis:trans ratios, the isomerisation of aromatic alkenes appears to favour the formation of the trans-isomer. The formation of the trans-isomer suggests that the thermodynamic stability of this isomer controls the outcome of the reaction.
The optimum K2CO3 loading was found to be 15% with higher loadings leading to pore blockage and reduced surface areas. Spray impregnation was found to produce an active catalyst that produced high conversions, which however rapidly became deactivated. The alumina supplier had a significant effect on the product alkene selectivity. Engelhard AL-3992E was found to produce a catalyst that allowed for high product selectivity.
Catalyst deactivation occurred via two processes. BET analysis showed a 25% reduction in surface area post reaction which, served as evidence of coke formation. Additionally, TGA-DSC analysis showed that under thermal treatment benzene was desorbed from the catalyst post reaction. It is thought that aromatic reactants/products act as poisons. Chemisorption through multiple bonds and back-bonding  causes aromatic species to block active sites required for further reaction. It was also noted that with increasing LHSV, the deactivation of the catalyst increased. This led to the determination of a negative first order of reaction, with respect to the concentration of allylbenzene.
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