Active sites on CaO and BaO for isomerization of butenes and exchange of butenes with deuterium: Selective poisoning of the exchange sites with oxygen

Abstract

The reactions of cis-2-butene, ethylene, and propylene with D 2, and coisomerization of cis-2-butene d 0 d 8 were carried out over CaO and BaO evacuated at different temperatures, and the effects of poisoning with oxygen were examined. Over CaO evacuated at 500, 600, or 1038 °C, the exchange of butene with D 2 did not substantially occur but the isomerization occurred. Over CaO evacuated at 700 or 800 °C, and BaO evacuated at 600 or 800 °C, both the exchange and isomerization reactions occurred. The exchange rate of ethylene with D 2 was much slower than that of propylene or butene. Oxygen poisoned the exchange reaction of butene more selectively than it did the isomerization. Results of the coisomerization of cis-2-butene d 0 d 8 over BaO evacuated at 600 °C showed that rapid hydrogen scrambling among butene molecules, which accompanied the isomerization, occurred in addition to the isomerization which involved an intramolecular hydrogen transfer. Poisoning with oxygen resulted in an elimination only of the hydrogen scrambling. Oxygen did not poison the isomerization reaction which involved an intramolecular hydrogen transfer. Two types of active sites are proposed on CaO and BaO, whose appearances are quite dependent on the evacuation temperature. One of them (Site I) is active only for the isomerization that involves intramolecular hydrogen transfer, and the other (Site II) is active both for the isomerization and exchange reactions. The nature of these sites and the reaction mechanisms over these sites are discussed.