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Issue 13, 2020
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Molecular behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts as a function of Si/Al ratio: a quasielastic neutron scattering study

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Abstract

The dynamical behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts of differing Si/Al ratios (36 and 135) was probed using quasielastic neutron scattering to understand the effect of catalyst composition (Brønsted acid site concentration) on the behaviour of species present during the initial stages of the H-ZSM-5 catalysed methanol-to-hydrocarbons process. At room temperature in H-ZSM-5(36) isotropic methanol rotation was observed (rotational diffusional coefficient, DR = 2.6 × 1010 s−1), which contrasted qualitatively with H-ZSM-5(135) in which diffusion confined to a sphere matching the 5.5 Å channel width was observed, suggesting motion is more constrained in the lower Si/Al catalyst. At higher temperatures, confined methanol diffusion is exhibited in both catalysts with self-diffusion coefficients (Ds) measured in the range of 8–9 × 10−10 m2 s−1. However, the population of molecules immobile over the timescale probed by the instrument is significantly larger in H-ZSM-5(36), consistent with the far higher number of Brønsted acid adsorption sites. For dimethyl ether, diffusion confined to a sphere at all temperatures is observed in both catalysts with Ds measured in the range of 9–11 × 10−10 m2 s−1 and a slightly smaller fraction of immobile molecules in H-ZSM-5(135). The larger Ds values obtained for dimethyl ether arise from the sphere of confinement being larger in H-ZSM-5(36) (6.2 Å in diameter) than the 5.5 Å width of the pore channels. This larger width suggests that mobile DME is sited in the channel intersections, in contrast to the mobile methanol which is sited in the channels. An even larger confining sphere of diffusion was derived in H-ZSM-5(135) (∼8 Å in diameter), which we attribute to a lack of Brønsted sites, allowing for a larger free volume for DME diffusion in the channel intersections.

Graphical abstract: Molecular behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts as a function of Si/Al ratio: a quasielastic neutron scattering study

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Supplementary files

Article information


Submitted
03 Apr 2020
Accepted
23 May 2020
First published
26 May 2020

This article is Open Access

Catal. Sci. Technol., 2020,10, 4305-4320
Article type
Paper

Molecular behaviour of methanol and dimethyl ether in H-ZSM-5 catalysts as a function of Si/Al ratio: a quasielastic neutron scattering study

T. Omojola, I. P. Silverwood and A. J. O'Malley, Catal. Sci. Technol., 2020, 10, 4305
DOI: 10.1039/D0CY00670J

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