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.
Highlights
Zeolite ZSM-5 plays a crucial role in many commercial petrochemical and environmental processes due to its unique porous architecture and chemical/catalytic properties [1]
The experimental quasi-elastic neutron scattering (QENS) data collected at all studied temperatures were fitted to the combination of a delta function, a single Lorentzian function that accounts for the quasi-elastic broadening of the scattering function and a linear background function convoluted with the resolution data measured at 10 K
This intensity decreases with increasing Q, and the intensity of the quasielastic component fit by the Lorentzian function increases which is typically observed for methanol loaded H-ZSM-5 [17]
Summary
Zeolite ZSM-5 plays a crucial role in many commercial petrochemical and environmental processes due to its unique porous architecture and chemical/catalytic properties [1]. One study concluded that methanol diffuses along the ZSM-5 channels with a diffusion coefficient in the order of 10–11 m2s–1 using the high resolution IN10 instrument at ILL, Grenoble [14], while other studies employing lower resolution spectrometers (with wider energy windows) were not able to observe long range methanol diffusion [15–17]. We note that these measurements were conducted on different instruments with varied instrument resolutions [14–17]
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