Methanol dynamics in H-ZSM-5 with Si/Al ratio of 25: a quasi-elastic neutron scattering (QENS) study

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

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11 Citations (SciVal)

Abstract

Methanol dynamics in zeolite H-ZSM-5 (Si/Al of 25) with a methanol loading of ~ 30 molecules per unit cell has been studied at 298, 323, 348 and 373 K by incoherent quasi-elastic neutron scattering (QENS). The elastic incoherent structure factor (EISF) reveals that the majority of methanol is immobile, in the range between 70 and 80%, depending on the measurement temperature. At 298 K, ≈ 20% methanol is mobile on the instrumental timescale, exhibiting isotropic rotational dynamics with a rotational diffusion coefficient (DR) of 4.75 × 1010 s−1. Upon increasing the measurement temperature from 298 to 323 K, the nature of the methanol dynamics changes from rotational to translational diffusion dynamics. Similar translational diffusion rates are measured at 348 and 373 K, though with a larger mobile fraction as temperature increases. The translational diffusion is characterised as jump diffusion confined to a sphere with a radius close to that of a ZSM-5 channel. The diffusion coefficients may be calculated using either the Volino–Dianoux (VD) model of diffusion confined to a sphere, or the Chudley–Elliot (CE) jump diffusion model. The VD model gives rise to a self-diffusion co-efficient (Ds) of methanol in the range of 7.8–8.4 × 10–10 m2 s−1. The CE model gives a Ds of around 1.2 (± 0.1) × 10–9 m2 s−1 with a jump distance of 2.8 (either + 0.15 or − 0.1) Å and a residence time (τ) of ~ 10.8 (either + 0.1 or − 0.2) ps. A correlation between the present and earlier studies that report methanol dynamics in H-ZSM-5 with Si/Al of 36 is made, suggesting that with increasing Si/Al ratio, the mobile fraction of methanol increases while DR decreases.

Original languageEnglish
Pages (from-to)699-706
Number of pages8
JournalTopics in Catalysis
Volume64
DOIs
Publication statusPublished - 29 Jun 2021

Bibliographical note

Funding Information:
The UK Catalysis Hub is kindly thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC Grant: EP/R026939/1, EP/R026815/1, EP/R026645/1, EP/R027129/1. AJOM acknowledges Roger and Sue Whorrod for the funding of the Whorrod Fellowship. The authors acknowledge the ISIS Neutron and Muon Source at the STFC Rutherford Appleton Laboratory for granting access to OSIRIS instrument and the beamline facilities. The raw experimental data resulted from our beamtime RB1920441 can be found at https://doi.org/10.5286/ISIS.E.RB1920441 .

Funding

The UK Catalysis Hub is kindly thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC Grant: EP/R026939/1, EP/R026815/1, EP/R026645/1, EP/R027129/1. AJOM acknowledges Roger and Sue Whorrod for the funding of the Whorrod Fellowship. The authors acknowledge the ISIS Neutron and Muon Source at the STFC Rutherford Appleton Laboratory for granting access to OSIRIS instrument and the beamline facilities. The raw experimental data resulted from our beamtime RB1920441 can be found at https://doi.org/10.5286/ISIS.E.RB1920441 . The UK Catalysis Hub is kindly thanked for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC Grant: EP/R026939/1, EP/R026815/1, EP/R026645/1, EP/R027129/1. AJOM acknowledges Roger and Sue Whorrod for the funding of the Whorrod Fellowship. The authors acknowledge the ISIS Neutron and Muon Source at the STFC Rutherford Appleton Laboratory for granting access to OSIRIS instrument and the beamline facilities. The raw experimental data resulted from our beamtime RB1920441 can be found at https://doi.org/10.5286/ISIS.E.RB1920441.

Keywords

  • Diffusion
  • H-ZSM-5
  • Methanol
  • QENS
  • Rotational
  • Translational
  • Zeolite

ASJC Scopus subject areas

  • Catalysis
  • General Chemistry

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