Understanding Water Effects in Alcohol Dehydration Activity on γ-Al2O3 Using Microkinetic Modeling

Increased consumption of declining petroleum resources in the production of plastics and chemicals necessitates the utilization of alternative feedstocks. Specifically, solid acid-catalyzed dehydration of biomass-derived alcohols has attracted significant attention as an excellent alternative to forming olefins, the building blocks for polymers production. γ-Al2O3 is an active dehydration catalyst and surface reactions evolve via Lewis acid-catalyzed (concerted-E2) mechanism. Specifically, the tri-coordinated surface Al sites of γ-Al2O3 are the strongest Lewis acid sites. However, the formation of water (dehydration product) can compete with alcohol adsorption and change the surface acidity of γ-Al2O3. Using DFT (Density Functional Theory) calculated reaction energy profiles we constructed a single-site microkinetic model of alcohol dehydration on γ-Al2O3. Our model addresses the activity of different-substituted alcohols, ethanol, propanol, isopropanol, and t-butanol and rationalizes the higher activity of tertiary over primary alcohols. Importantly, our model demonstrated that (competitive) water adsorption dominates on the active sites. Reaction path analysis suggests that shifting the equilibrium of surface bound species to alcohols would have a significant kinetic effect in increasing the olefin formation rates.