Carboranes: the strongest Brønsted acids in alcohol dehydration

Abstract

Alcohol dehydration is an important reaction for the production of olefins (polymer feedstock) and known to evolve on solid acids through the formation of carbenium ions. In this work, we employed density functional theory calculations to study the dehydration mechanisms of biomass-derived alcohols catalyzed by fully halogenated carborane superacids, the strongest known Brønsted acids to date. We considered three carborane acids with shells consisting of fluorine, chlorine and bromine as well as alcohols of different substitution (primary, secondary and tertiary). Our results demonstrate that the fluorinated carborane is the most active alcohol dehydration catalyst, with the reaction following an intramolecular ^βH elimination mechanism. We found that water, a dehydration product, competes with alcohols for the catalytic (Brønsted acid) sites, increasing at the same time the dehydration barriers. Due to the increased stability of the conjugate base of the carboranes, the acid can dissociate under dehydration conditions and H₃O⁺ can act as the acid catalyst, in addition to the non-dissociated carborane. Solvation by polar protic solvents decreases the reaction barriers due to stabilization of the carbenium ions formed at the transition states. We show strong linear correlations between calculated reaction barriers and carbenium ion stability, a known alcohol reactivity descriptor. Interestingly, by calculating the alcohol dehydration slopes on various Lewis and Brønsted catalysts, we reveal a slope dependence on the degree of carbenium ion character at the transition state, which in turn, depends on the reaction mechanism. This slope dependence can be attributed to the degree of charge separation at the transition states. As alcohol dehydration mechanisms can be very difficult to elucidate experimentally, this observation opens new avenues for identifying potential dehydration mechanisms.