Quasielastic Neutron Scattering and Molecular Dynamics Simulation Study on the Molecular Behaviour of Catechol in Zeolite Beta

Abstract

The dynamics of catechol in zeolite Beta was studied using quesielastic neutron scattering (QENS) experiments and molecular dynamics simulations at 393 K, to understand the behaviour of phenolic monomers relevant in the catalytic conversion of lignin via metal nanoparticles supported on zeolites. Compared to previous work studying phenol, both methods observe that the presence of the second OH group in catechol can hinder mobility significantly, as explained by stronger hydrogen-bonding interactions between catechol and the Brønsted sites of the zeolite. The instrumental timescale of the QENS experiment allows us to probe rotational motion, and the catechol motions are best fit to an isotropic rotation model with a $$D^{rot}$$ D rot of 2.9 × 10$$^{10}$$ 10 s$$^{-1}$$ - 1 . While this $$D^{rot}$$ D rot is within error of that measured for phenol, the fraction of molecules immobile on the instrumental timescale is found to be significantly higher for catechol. The MD simulations also exhibit this increased in ‘immobility’, showing that the long-range translational diffusion coefficients of catechol are lower than phenol by a factor of 7 in acidic zeolite Beta, and a factor of $$\sim$$ ∼ 3 in the siliceous material, further illustrating the significance of Brønsted site H-bonding. Upon reproducing QENS observables from our simulations to probe rotational motions, a combination of two isotropic rotations was found to fit the MD-calculated EISF; one corresponds to the free rotation of catechol in the pore system of the zeolite, while the second rotation is used to approximate a restricted and rapid “rattling”, consistent with molecules anchored to the acid sites through their OH groups, the motion of which is too rapid to be observed by experiment.

The effect of molecular shape and pore structure on local and nanoscale cresol behaviour in commercial zeolite catalysts

“…5. The same dynamics were observed in simulations of catechol and phenol in H-Beta investigated by Hernandez-Tamargo et al 48 . The isotropic model has a radius corresponding to that of the cresol molecules (∼2.9 Å), while the model of rapid rattling is actually a model of isotropic rotation with radii shorter than the molecular dimensions, from 0.5-0.9 Å.…”

“…While the distribution of acid sites in each system is important to catalysis, it is highly dependent on the synthesis conditions 12 . The Al distributions applied in this study were determined based on the literature 48,57,58 . Dempsey's rule 59 was applied for determining distances between acid sites, consistent with previous studies for the frameworks used [60][61][62] .…”

“…where N is the number of hydrogen atoms in the cresols, and d i is the coordinate vector of the hydrogen atom i with respect to the cresol centre of mass, thus sampling rotational motions 48 . The angular brackets represent an NVE ensemble averaged over the set of initial times, t 0 .…”

Neutron spectroscopy as a method for classical force-field parameterization: Past methods, present successes and future challenges

“…The limited examples of surface studies, typically employ comparisons of MD or MC to that of neutron reflectometry [9][10][11]. Although MD models have been routinely compared to Neutron spectroscopy techniques for surface and confined/pore diffusion [12,13] there are a clear lack of examples of accurate parametrization to produce reliable models of these phenomena. Returning to the accuracy of the self diffusion coefficient for water models, it is worth pointing out that it isn't completely surprising that models which correctly predict structural and thermodynamic properties also provide a good proxy for the selfdiffusion coefficient.…”

“…The reproduction of the self-diffusion coefficient can therefore be loosely thought of as a proxy for dynamical properties of the system in general, and thus seems like a good choice as a figure of merit for fitting classical force-fields. Notwithstanding the modest successes of MD models in reproducing dynamical properties for small molecules, many of the cases of scientific interest in the modern era are based on much more complex multi component systems, within porous materials [13,15], or interacting with a substrate [16]. Neutron spectroscopy has been a very successful probe of the interaction of small organic molecules in porous materials or on surfaces.…”