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

Abstract

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.

Local and nanoscale methanol mobility in different H-FER catalysts

“…The residence times at 300 K, 350 K and 400 K were calculated at 3.00, 2.50 and 1.96 ps respectively. These values fall in a very similar range to that observed in zeolite beta and slightly below that of ZSM-5, 39 which suggests that regardless of the different pore dimensions, the length of time spent at a BAS for methanol is relatively consistent.…”

“…These self-diffusivities are an order of magnitude lower than those reported for methanol simulated at loadings of ∼1.1 and ∼2.2 wt% in MFI, which reach up to 10 × 10 −10 m 2 s −1 , and ∼3 orders of magnitude smaller than zeolite beta at loadings of 1.7 and 3.4 wt%. 39 They are also ∼2 orders of magnitude lower than simulated values in zeolite HY at loadings of ∼1.7 wt%. 29 The observed differences in diffusivity can be rationalised by considering both the loading, which is significantly higher in our FER systems – increasing the number of adsorbate–adsorbate interactions – as well as the topology of the zeolite, where FER has the smallest pore size across these systems.…”

“…37,38 This complementarity is particularly clear when MD is used to reproduce QENS observables, such as the intermediate scattering function (ISF) or elastic incoherent structure factor (EISF), allowing for direct comparison between the two techniques. Classical MD was recently employed by Botchway et al 39 to probe the dynamics of methanol in zeolites beta and MFI as a function of Si/Al ratio. The study showed that the zeolite topology had a significant effect on the methanol dynamics, with diffusion coefficients in ZSM-5 not exceeding 10 × 10 −10 m 2 s −1 , compared to diffusion rates two orders of magnitude higher in zeolite beta, owing to the larger pore dimensions.…”

A quantitative multiscale perspective on primary olefin formation from methanol

“…Ab initio static simulations offer a clear description of the potential energy surface, which can be used to determine molecular structures, charge distribution, and vibrational modes (with corresponding frequencies), together with kinetic and thermodynamic observables that can indicate reaction intermediates and pathways used to decipher experimental data. [38][39][40] Molecular dynamics techniques have the ability to simulate real-time dynamics of atoms in zeolite pores, and has been used to investigate surface adsorption, proton transfers and diffusion effects, 41,42 whilst positive bias metadynamic simulations can uncover rare-events and map out the free energy surface for pathways leading to meta-or un-stable intermediates. 43,44 All of these techniques have been successfully applied to complement experimental analysis and improve the development of heterogeneous catalysts.…”

“…Quasielastic neutron scattering can yield values for self-diffusion coefficients, which are useful for isotopic and jump models. 200 Using a combination of quasielastic neutron scattering and molecular dynamic simulations, the mobility of initial species was studied in H-ZSM-5 catalysts (Si/Al = 30) and H-Y (Si/Al = 30), 41,42,172,201 with the latter considered because it is inactive for the MTH process and thus provides a reference material. O'Malley et al 41,172 showed from quasielastic neutron scattering measurements (Fig.…”