Methanol diffusion and dynamics in zeolite H-ZSM-5 probed by quasi-elastic neutron scattering and classical molecular dynamics simulations

Santhosh K. Matam, Ian P. Silverwood, Lotfi Boudjema, Alexander J. O'Malley, C. Richard A. Catlow

Research output: Contribution to journalComment/debatepeer-review

1 Citation (SciVal)

Abstract

Zeolite ZSM-5 is a key catalyst in commercially relevant processes including the widely studied methanol to hydrocarbon reaction, and molecular diffusion in zeolite pores is known to be a crucial factor in controlling catalytic reactions. Here, we present critical analyses of recent quasi-elastic neutron scattering (QENS) data and complementary molecular dynamics (MD) simulations. The QENS experiments show that the nature of methanol diffusion dynamics in ZSM-5 pores is dependent both on the Si/Al ratio (11, 25, 36, 40 and 140), which determines the Brønsted acid site density of the zeolite, and that the nature of the type of motion observed may vary qualitatively over a relatively small temperature range. At 373 K, on increasing the ratio from 11 to 140, the observed mobile methanol fraction increases and the nature of methanol dynamics changes from rotational (in ZSM-5 with Si/Al of 11) to translational diffusion. The latter is either confined localized diffusion within a pore in zeolites with ratios up to 40 or non-localized, longer-range diffusion in zeolite samples with the ratio of 140. The complementary MD simulations conducted over long time scales (1 ns), which are longer than those measured in the present study by QENS (≈1-440 ps), at 373 K predict the occurrence of long-range translational diffusion of methanol in ZSM-5, independent of the Si/Al ratios (15, 47, 95, 191 and siliceous MFI). The rate of diffusion increases slightly by increasing the ratio from 15 to 95 and thereafter does not depend on zeolite composition. Discrepancies in the observed mobile methanol fraction between the MD simulations (100% methanol mobility in ZSM-5 pores across all Si/Al ratios) and QENS experiments (for example, ≈80% immobile methanol in ZSM-5 with Si/Al of 11) are attributed to the differences in time resolutions of the techniques. This perspective provides comprehensive information on the effect of acid site density on methanol dynamics in ZSM-5 pores and highlights the complementarity of QENS and MD, and their advantages and limitations. This article is part of the theme issue 'Exploring the length scales, timescales and chemistry of challenging materials (Part 2)'.

Original languageEnglish
Article number20220335
Number of pages20
JournalPhilosophical transactions. Series A, Mathematical, physical, and engineering sciences
Volume381
Early online date11 Sept 2023
DOIs
Publication statusPublished - 30 Oct 2023

Bibliographical note

unding.The UK Catalysis Hub is kindly thanked for resources and support provided via our membershipof the UK Catalysis Hub Consortium and funded by EPSRC grant nos. EP/R026939/1, EP/R026815/1,EP/R026645/1, EP/R027129/1

Data accessibility: The raw experimental data resulting from our neutron beamtime RB1920441 can be found atdoi:10.5286/ISIS.E.RB1920441

Keywords

  • Brønsted acidity
  • diffusion and dynamics
  • H-ZSM-5
  • methanol
  • QENS
  • Si/Al ratio

ASJC Scopus subject areas

  • General Mathematics
  • General Engineering
  • General Physics and Astronomy

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