Elucidating the Catalytic Valorization of Ethanol over Hydroxyapatite for Sustainable Butanol Production: A First-Principles Mechanistic Study

The widespread concern about the burning of fossil fuels and its effects on the environment has driven our research efforts toward sustainable energy solutions. As a result, there is a strong drive to improve the eco-friendly production of chemical compounds sourced from renewable resources. Here, we report a first-principle mechanistic study of the conversion of ethanol to butanol using a hydroxyapatite (HAP) catalyst. Basically, we combined density functional theory calculations, the unity bond index-quadratic exponential potential approximation to analyze the reaction mechanism, and microkinetic simulations to address the influence of the kinetic parameters on the chemical distribution of formed species on the HAP catalyst. From our calculations and analyses, the sequence of elementary reaction steps follows the Guerbet reaction pathway, which involves ethanol dehydrogenation, aldol condensation, and subsequent hydrogenation steps. The results indicate that the bifunctional nature of the HAP surface is key to facilitate the initial dehydrogenation of ethanol and subsequent C–C coupling via aldol condensation, determining reactions to the formation of C4 species. Furthermore, microkinetic analysis shows that butanol is the main product, with minimal formation of other C4 byproducts. However, higher initial ethanol coverages decrease the rate of conversion because of limited active sites. The conversion of the aldol intermediate is crucial for efficient butanol production. These findings provide valuable information for the future development of HAP-based catalysts for sustainable biofuel production.