Isomerization and exchange reactions of dimethylbutenes over copper-exchanged 13X type zeolites

Abstract

The isomerization reaction of 3,3-dimethylbut-1-ene (I), to produce 2,3-dimethylbut-1-ene (II) and 2,3-dimethylbut-2-ene (III), has been investigated over a series of copper-exchanged X-type zeolites. The reaction is believed to proceed via carbonium ion intermediates and the ability of the copper zeolites to exhibit acidic-type behaviour was demonstrated. The results emphasized the important role that the copper cations play in the reaction, and the intermediate formation of a hydrocarbon–copper complex was postulated. The observed poisoning effect of water was explained in terms of the preferential formation of a copper–aquo complex. The isomerization of II was shown to be a more facile process.The exchange reactions of I and II, with deuterium oxide and deuterium, were examined over one of the copper-containing zeolites. Substantial exchange was only observed under conditions where isomerization also took place. With deuterium oxide, the exchange of I was multiple in character whereas that of II was stepwise. These differences were rationalized in terms of the relative rates of desorption of II and III from the surface and the rate of formation of a tertiary carbonium ion from the initial olefin. The exchange reactions of the olefins with deuterium all occurred via a stepwise process.