(330e) Developing Relationships for the Lewis-Catalyzed Alcohol Dehydration On Alumina

A fundamental understanding on the dehydration of simple alcohols can help us elucidate and eventually control the selective dehydration of polyols and complicated biomass molecules (i.e., glycerol). Herein, we use Density Functional Theory (DFT) calculations to study the alcohol dehydration on γ-Al₂O₃. The dehydration mechanism for primary (ethanol, 1-propanol), secondary (2-propanol), and tertiary (2-methyl-2-propanol) alcohols was investigated. We found a remarkable agreement on the alcohol dehydration barriers between theory and TPD-TGA (Thermogravimetric Analysis) reactivity measurements in high vacuum. The dehydration barriers of the alcohols correlate with the corresponding carbenium ion stabilities. This is because the transition state of alcohol dehydration on Lewis acid sites has carbenium characteristics. A key-feature of the suggested mechanism is the water removal after the alkene formation. The water desorption value is larger than the activation energies of the alkenes formation, explaining how water poisons the alumina surface. Finally, expanding this study by modifying the Lewis acid site with various metals, we demonstrate a series of linear trends between dehydration barriers of the alcohols and their carbenium ion stabilities.