Elucidating the Catalytic Valorization of Ethanol\nover Hydroxyapatite for Sustainable Butanol Production: A First-Principles\nMechanistic Study

The widespread concern\nabout the burning of fossil fuels and its\neffects on the environment has driven our research efforts toward\nsustainable energy solutions. As a result, there is a strong drive\nto improve the eco-friendly production of chemical compounds sourced\nfrom renewable resources. Here, we report a first-principle mechanistic\nstudy of the conversion of ethanol to butanol using a hydroxyapatite\n(HAP) catalyst. Basically, we combined density functional theory calculations,\nthe unity bond index-quadratic exponential potential approximation\nto analyze the reaction mechanism, and microkinetic simulations to\naddress the influence of the kinetic parameters on the chemical distribution\nof formed species on the HAP catalyst. From our calculations and analyses,\nthe sequence of elementary reaction steps follows the Guerbet reaction\npathway, which involves ethanol dehydrogenation, aldol condensation,\nand subsequent hydrogenation steps. The results indicate that the\nbifunctional nature of the HAP surface is key to facilitate the initial\ndehydrogenation of ethanol and subsequent C–C coupling <i>via</i> aldol condensation, determining reactions to the formation\nof C<sub>4</sub> species. Furthermore, microkinetic analysis shows\nthat butanol is the main product, with minimal formation of other\nC<sub>4</sub> byproducts. However, higher initial ethanol coverages\ndecrease the rate of conversion because of limited active sites. The\nconversion of the aldol intermediate is crucial for efficient butanol\nproduction. These findings provide valuable information for the future\ndevelopment of HAP-based catalysts for sustainable biofuel production.