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

Methanol dynamics in zeolite H-ZSM-5 (Si/Al of 25) with a methanol loading of similar to 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, approximate to 20% methanol is mobile on the instrumental timescale, exhibiting isotropic rotational dynamics with a rotational diffusion coefficient (D-R) of 4.75 x 10(10) 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 (D-s) of methanol in the range of 7.8-8.4 x 10(-10) m(2) s(-1). The CE model gives a Ds of around 1.2 ( 0.1) x 10(-9) m(2) s(-1) with a jump distance of 2.8 (either + 0.15 or - 0.1)angstrom and a residence time (tau) of similar to 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 D-R decreases.