Mechanism of alkene epoxidation

• Main Author: Husain, A. H.
• Published: University of Cambridge 2003
• Subjects: 547.4
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.604836

Heterogeneous silver catalysed ethene epoxidation is a strategically important selective oxidation giving rise to ethene epoxide a versatile monomer that is largely converted to glycols and used as antifreeze. Several aspects of this commercially important reaction were investigated using electrochemical promotion, a technique which involves electropumping of ions from a solid ion-conducting electrolyte to the surface of a porous catalytically active metal film. The most notable result in this work from ethene epoxidation was the elucidation of the NO_xpromotion effect. It was shown that adsorbed alkali and parts per million levels of nitric oxide added in the gas feed were minimum and sufficient conditions for the appearance of this phenomenon. Another commercially daunting and scientifically challenging reaction that was studied in this work was the heterogeneous propene epoxidation on silver catalysts. Using electrochemical promotion this work has shown that ethene and propene epoxidation react in a very different and contrasting manner to the addition of cholorine (CI), alkali and nitric oxide (NO) promoters. Basically potassium (K) alone is bad for ethene epoxide (EO), good for propene epoxide (PO), CI is good for EO, indifferent for PO, K+CI is good for EO, bad for PO and K+NO is good for EO, bad for PO. X-ray photoelectron spectroscopic investigation of the silver catalyst revealed that the substantially different state of oxygenation of the silver subsurface region could be the origin of the very different selectivity observed with these two alkenes. The underlying reason for the observed promotion in selectivity with adsorbed chlorine in ethene epoxidation was investigated by studying the effect of all four halogen promoters’ viz., fluorine, chlorine, bromine and iodine on supported silver on alumina catalysts. It was found that chlorine is the best promoter amongst them all. The behaviour of the halogens had a direct bearing to their electron affinity. And it seems that electronic affects are at work and geometric effects due to site blocking can be ruled out.