Methanol diffusion in zeolite HY: a combined quasielastic neutron scattering and molecular dynamics simulation study

Abstract

The diffusion of methanol in zeolite HY is studied using tandem quasielastic neutron scattering (QENS) experiments and molecular dynamics (MD) simulations at 300-400 K. The experimental diffusion coefficients were measured in the range 2-5 × 10(-10) m(2) s(-1) and simulated diffusion coefficients calculated in the range of 1.6-3.2 × 10(-9) m(2) s(-1). Activation energies were measured as 8.8 and 6.9 kJ mol(-1) using QENS and MD respectively. Differences may be attributed predominantly to the experimental use of a dealuminated HY sample, containing significant defects such as strongly adsorbing silanol nests, compared to a perfect simulated crystal containing only evenly distributed Brønsted acid sites. Experimental and simulated diffusivities measured in this study are lower than those obtained from those previously calculated in siliceous faujasite, due to methanol H-bonding to Brønsted acid sites as observed in the MD simulations. However, both experimental and simulated diffusivities were significantly higher than those obtained in NaX, due to the higher concentration of extraframework cations present in the previously studied structures.

Neutron spectroscopy as a tool in catalytic science

“…The significant broadening in HY is completely consistent with diffusing methanol. MD simulations 29 supported this interpretation and showed the presence of hydrogen-bonded methanol in the pores. In contrast, the close fit to the resolution function in H-ZSM-5 suggests that methanol is immobile on the instrumental time scale.…”

“…This combination of techniques has proved to give unique insight into molecular behaviour in catalytic systems 15,[25][26][27][28][29] and will feature prominently in this article.…”

Molecular behaviour of phenol in zeolite Beta catalysts as a function of acid site presence: a quasielastic neutron scattering and molecular dynamics simulation study

“…We observed that 1 ns was enough to obtain mean-squared displacement (MSD) plots with acceptable linearity to calculate the self-diffusivity of phenol throughout the pore system of zeolite Beta. 38,39 The MSD was calculated from the variation in the coordinates of the centre of mass of the molecules. The selfdiffusion coefficients were derived from the Einstein relationship:…”

“…The dynamical characteristics derived from the MD analysis are an essential complement to the QENS experiments, as shown in previous reports on a range of zeolite-sorbate systems. [35][36][37][38][39] QENS observables such as the intermediate scattering function and its temporal Fourier transform, the dynamic structure factor, may be calculated directly to compare simulation results with experiment. 40,41 In the present work, we have studied the mobility of phenol in zeolite Beta using QENS experiments complemented by density functional theory (DFT) calculations and classical MD simulations.…”

Transport Phenomena in Zeolites in View of Graph Theory and Pseudo‐Phase Transition

“…Due to the interactions between CO 2 molecules, it does not behave like ideal gas. Catlow et al studied methanol diffusion in zeolite HY with quasi‐elastic neutron scattering, which can hardly be done by other methods due to the reactivity of methanol and crystal size …”