Understanding the Importance of Carbenium Ions in the Conversion of Biomass-Derived Alcohols with First-Principles Calculations

Dehydration reactions play a key role in the conversion of biomass derivatives to valuable chemicals, such as alcohols to alkenes. Both Lewis and Brønsted acid-catalyzed dehydration reactions of biomass-derived alcohols involve transition states with carbenium ion characteristics. In this work, we employed high-level ab initio theoretical methods to investigate the effect of molecular structure on the physicochemical properties of a set of alcohols that appear to control dehydration chemistry. Specifically, we calculated the carbenium ion stability (CIS, alkene-binding H⁺) and proton affinity (PA, alcohol-binding H⁺) of various C2–C8 alcohols to show the effect of alcohol size and degree of primary heteroatom substitution on the properties of the reactive species. Our results show a strong linear correlation between CIS and PA, following the substitution order of the reacting alcohols (i.e., primary < secondary < tertiary). Additionally, the calculated binding free energy (BE) of water on the formed carbenium ions was found to be exothermic and to decrease in magnitude with increasing alcohol substitution level. We demonstrate that the CIS and/or the PA are excellent structural descriptors for the alcohols and, most importantly, they can serve as reactivity descriptors to screen a large number of alcohols in the conversion of biomass-based alcohols involving the formation of carbenium ions. We demonstrate this concept in both Lewis and Brønsted acid-catalyzed dehydration reactions.