Local and Nanoscale Methanol Mobility in Different H-FER Catalysts

Alex Porter, Cecil Botchway, Bright Kwakye-Awuah, Carlos Hernandez-Tamargo, Santhosh K. Matam, Sandra Mchugh, Ian P. Silverwood, Nora H. de Leeuw, Alexander O'Malley

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Abstract

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.
Original languageEnglish
Pages (from-to)1663-1677
Number of pages15
JournalCatalysis Science & Technology
Volume12
Issue number5
Early online date20 Jan 2022
DOIs
Publication statusPublished - 7 Mar 2022

Bibliographical note

This work was supported by the UK Engineering and Physical Sciences Research Council (EPSRC), grant no. EP/R513155/1 at the University of Bath and EP/K009567 at Cardiff University. NHdL, CHB and BKA also acknowledge the Royal Society and the UK Department for International Development for funding under the Africa Capacity Building Initiative (ACBI), which has supported this research. BKA would also like to acknowledge the Natural Environment Research Council for their funding through the grant NE/R009376/1. This work has made use of the Balena High Performance Computing (HPC) Service at the University of Bath and HPC Wales at Cardiff University. We would also like the thank Dr Gabriele Kociok-Kohn for running and maintenance of the PXRD instrument as well as the whole of MC2 at the University of Bath. AJOM acknowledges Roger and Sue Whorrod for the funding of a Whorrod Fellowship. SKM would like to thank the UK Catalysis Hub for resources and support provided via our membership of the UK Catalysis Hub Consortium and funded by EPSRC grants: EP/R026939/1, EP/R026815/1, EP/R026645/1, EP/R027129/1. The ISIS neutron and muon source at the STFC Rutherford Appleton Laboratory are thanked for access to neutron beam facilities; the data from our experiment RB2010237 can be found at DOI: 10.5286/ISIS.E.RB2010237.

Keywords

  • Zeolite
  • QENS
  • Molecular dynamics (MD)
  • Molecular dynamics

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