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

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 Drot of 2.9 × 1010 s−1. While this Drot 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 ∼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

Influence of Topology and Brønsted Acid Site Presence on Methanol Diffusion in Zeolites Beta and MFI

Detailed insight into molecular diffusion in zeolite frameworks is crucial for the analysis of the factors governing their catalytic performance in methanol-to-hydrocarbons (MTH) reactions. In this work, we present a molecular dynamics study of the diffusion of methanol in all-silica and acidic zeolite MFI and Beta frameworks over the range of temperatures 373–473 K. Owing to the difference in pore dimensions, methanol diffusion is more hindered in H-MFI, with diffusion coefficients that do not exceed 10×10−10 m2s−1. In comparison, H-Beta shows diffusivities that are one to two orders of magnitude larger. Consequently, the activation energy of translational diffusion can reach 16 kJ·mol−1 in H-MFI, depending on the molecular loading, against a value for H-Beta that remains between 6 and 8 kJ·mol−1. The analysis of the radial distribution functions and the residence time at the Brønsted acid sites shows a greater probability for methylation of the framework in the MFI structure compared to zeolite Beta, with the latter displaying a higher prevalence for methanol clustering. These results contribute to the understanding of the differences in catalytic performance of zeolites with varying micropore dimensions in MTH reactions

Tautomerization of Phenol at the External Lewis Acid Sites of Scandium-, Iron- and Gallium-Substituted Zeolite MFI

We have employed density functional theory calculations to analyze the possible tautomerization of phenol mediated by three different Lewis acid sites at the external (010) surface of zeolite MFI. A silicon atom of the silanol group was substituted by Sc, Fe, and Ga metal atoms, which adopted a formal charge of 3+. This substituted silanol was dehydrated in order to form three-coordinated Lewis acid sites. The tautomerization of phenol involves the adsorption of the molecule on the Lewis site, the dissociation of the phenolic O–H bond and the transfer of the proton to the zeolite framework. This proton is transferred from the zeolite to the C atom at ortho positions to the phenolic O atom, thus generating the tautomer. The acidity of the substituted Lewis sites follows a strength order of Ga < Fe < Sc, where the Sc substitution provides the lowest energy barriers: 33 kJ/mol for the dissociation of the O–H bond, and 32 kJ/mol for the formation of the C–H bond, calculated with the GGA functional PBE including Grimme’s dispersion corrections. We observed that the GGA functional PBE produces binding energies and energy barriers with a difference of less than 13 kJ/mol compared to the meta-GGA TPSS and rev-TPSS and the hybrid-GGA HSE06

Mercury exchange in zeolites Na-A and Na-Y studied by classical molecular dynamics simulations and ion exchange experiments

Classical molecular dynamics simulations have been employed to study the exchange of Na+ for Hg2+ in zeolite Na-A, with a Si/Al ratio of 1, and zeolite Na-Y, with Si/Al ratios of 2 and 5, in dry and hydrated conditions within the temperature range 330 – 360 K, to understand factors underpinning the performance of zeolites for water decontamination. A classical forcefield based on DFT energies has been developed for the interaction between the Hg2+ ions and the zeolite O atoms. In terms of water diffusion, zeolite Na-A shows the lowest calculated diffusivity, followed by zeolite Na-Y (Si/Al=2) and Na-Y (Si/Al=5), as a consequence of differing pore dimensions and extra-framework ion loadings. In the absence of speciation anions, the Hg2+ ions are consistently adsorbed at the pore windows in both the LTA and FAU framework types. The reduced pore size of zeolite A leads to an average hydration number per Hg2+ ion of <1.0, whilst the wider pore aperture of zeolite Y exerts less steric hindrance, and thus the Hg2+ hydration number reaches values between 1.0 and 2.0 in zeolite Y. These observations might indicate that Hg2+ ions are more strongly immobilized in zeolite A than in zeolite Y. Preliminary measurements of mercury removal using these zeolites, as synthesised from bauxite and kaolin, seem to support these findings

Local and Nanoscale Methanol Mobility in Different H-FER Catalysts

The dynamical behaviour of methanol confined in zeolite H-FER has been studied using quasielastic neutron scattering (QENS) and classical molecular dynamics (MD) simulations to investigate the effects of the Si/Al ratio on methanol dynamics in different Brønsted acidic FER catalysts. Quasielastic neutron scattering probed methanol mobility at 273 - 333 K in a commercial FER sample (Si/Al = 10) at methanol saturation, and in a FER sample synthesised from naturally sourced Ghanaian kaolin (FER-GHA, Si/Al = 35-48), also at saturation. Limited mobility was observed in both samples and an isotropic rotation model could be fitted to the observed methanol motions, with average mobile fractions of ~20% in the commercial sample and ~15% in the FER-GHA, with rotational diffusion coefficients measured in the range of 0.82 – 2.01 × 1011 s-1. Complementary molecular dynamics simulations were employed to investigate methanol mobility in H-FER over the same temperature range, at a loading of ~6 wt% (close to experimental saturation) in both a fully siliceous H-FER system and one with a Si/Al = 35 ratio to understand the effect of the presence of Brønsted acid sites on local and nanoscale mobility. The simulations showed that methanol diffusivity was significantly reduced upon introduction of Brønsted acid sites into the system by up to a factor of ~3 at 300 K, due to strong interactions with these sites, with residence times of the order of 2-3 ps. The MD-calculated translational diffusivities took place over a timescale outside the observable range of the employed QENS spectrometer, varying from 0.34 – 3.06 × 10-11 m2s-1. QENS observables were reproduced from the simulations to give the same isotropic rotational motions with rotational diffusion coefficients falling in a similar range to those observed via experiment, ranging from 2.92 – 6.62 × 1011 s-1 between 300 to 400 K