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 faceand 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.

Atomic-resolution three-dimensional hydration structures on a heterogeneously charged surface

“…All parameters for this model were optimised to reproduce experimental thermodynamic and structural properties of liquid water as well as vibrational spectra. We also checked all simulations using the nonbonded three-body harmonic potential energy term for Mg–O–H interactions 55 and modified charges 56 . These made no significant difference to our results on the standard step edge.…”

Vibrational Analysis of Brucite Surfaces and the Development of an Improved Force Field for Molecular Simulation of Interfaces

“…The small deviations from perfect symmetry are a consequence of numerical factors during the DFT optimization, rather than being due to any physically significant distortion mechanism. Yu and Schmidt 23 derived an Mg–O–H term for a specific hydrotalcite edge structure that uses a θ 0 value of 132°. This value was determined solely for (11̅0) edge surfaces.…”

“…Distributions are shown for simulations applying (1) only previous Clayff parameters, including a Morse potential for O–H bond stretch, and (2) the new three-body term in addition to previous Clayff parameters (noted as NB3B). To implement the three-body term, a value of 120° was assumed for θ 0 (from the DFT-derived distribution) and a value of 6.35 kcal mol –1 rad –2 was taken for k (from the Mg–O–H term used by Yu and Schmidt 23 ). The choice of the force constant value will be discussed below.…”

“… 13 However, it was noted that the implementation of the three-body term was problematic, and it was suggested as an optional feature for surface simulations. 13 On the basis of the Mg–O–H parameters developed by Yu and Schmidt 23 for the (11̅0) hydrotalcite edge, we initially implemented a k -parameter of 6.35 kcal mol rad –2 . This value was determined by comparing the classical and DFT phonon spectra of the brucite surface.…”

“…A nonbonded three-body term was recently developed for Mg–O–H interactions on the (11̅0) hydrotalcite (Mg 6 Al 2 (OH) 16 CO 3 ·4H 2 O) surface; however, this implementation required parametrization of several other Clayff terms for application to the specific hydrotalcite structure. 23 In this study, we also replace the O–H bonded interaction with a recently developed Morse potential. 24 The Morse potential provides a description of the O–H bond stretch interaction that is more accurate than a harmonic function that describes an equivalent potential energy response with either bond compression or bond extension.…”