The mechanisms of dimethylbutene isomerization over alumina

Abstract

Reactions of dimethylbutenes over pure and chlorided γ-alumina have been studied in the presence of selective poisons and/or with various host molecules for deuterium label. The extent of H 2S poisoning has been used to determine the relative roles of π-allylic species and alkyi carbonium ions in the isomerization of the 2,3-dimethylbutenes on alumina at 273 K. The absence of poisoning by H 2S of skeletal isomerization of 3,3-dimethylbut-l-ene at 352 K is evidence that H2S does not inhibit reaction through carbonium ions whereas it is known to poison activity involving π-allylic species. The isomerization of 3,3-dimethylbut-l-ene in the presence of suitably labeled alkenes showed that vinylic and allylic hydrogen atoms can equilibrate with Brønsted-acid-type hydrogen on alumina at about 273 K. But adsorbed vinyl species have been found to retard skeletal isomerization, suggesting identity, or at least close proximity, of the sites for Brønsted acid reactions and vinylic CH exchange. Catalyst chloriding enhanced isomerization but retarded exchange of CH bonds with D. The influence of activation temperature and of impurities is discussed for reactions of dimethylbutenes over alumina.