Monomolecular mechanisms of isobutanol conversion to butenes catalyzed by acidic zeolites: Alcohol isomerization as a key to the production of linear butenes

Abstract

Density functional theory calculations are performed to explore the mechanisms of poorly understood monomolecular conversions of isobutanol to butenes, belonging to an important class of transformations of biobased molecules into chemicals. Except for the lowest reaction temperatures (<405 K), at which a conventional mechanism via alkoxide intermediates dominates, the alkenes formation proceeds via newly explored routes involving alcohol isomerization followed by dehydration according to a E2 mechanism. The carbenium ion deprotonation in the latter step is facilitated by the water molecule created during the reaction. In contrast, the isomerization of alkenes is found to be unfavorable at all reaction conditions. A favourable unconventional mechanism of formation of linear alkenes from isobutanol is discovered, in which the isomerization and dehydration are combined into a single reaction step. Our work represents a key in unraveling why linear butenes are formed in some zeolites when isobutanol is used as the dehydration reagent.