Elucidating the Crystal Face- and Hydration-Dependent Catalytic Activity of Hydrotalcites in Biodiesel Production

Abstract

Hydrotalcites (HTs) are an important class of heterogeneous catalysts for a wide variety of traditionally base-catalyzed organic transformations and have been considered as an alternative to homogeneous catalysts in the transesterification of fats/oils in biodiesel production. Previous experimental observations show that HT-like materials possess a crystal face-dependent activity that varies between related organic transformations, as well as a profound influence for HT hydration on its catalytic activity. We use a combination of molecular dynamics simulations and periodic plane-wave density functional theory to elucidate both the crystal face- and hydration-dependent activity of Mg−Al HT. We develop a new force field for HT, based on the existing ClayFF force field, and utilize this model to examine the interaction of a model ester reactant with HT in the presence of differing solvents, representative of conditions for common transesterification and hydrolysis reactions. Our results suggest that the variations in the activity of the HT crystal faces can be explained in terms of dramatically different adsorption free energies of the ester onto the various faces, changing with the nature of the solvent. We further conclude that the observed hydration dependence of the HT activity is due to the deprotonation of interfacial acidic Mg(Al)−O−H hydroxyl hydrogen by interlayer hydroxide counterions in the absence of interlayer hydration, leading to a loss of Brønsted basicity.