The effect of Si/Al ratio on local and nanoscale water diffusion in H-ZSM-5: A quasielastic neutron scattering and molecular dynamics simulation study

Abstract

The dynamics of water confined in H-ZSM-5 (protonated form of the Zeolite Socony Mobil – 5) has been studied using quasielastic neutron scattering (QENS) and classical molecular dynamics simulations (MD). QENS measurements probed water confined in ZSM-5 samples with Si/Al ratios of 15, 40 and 140 at 2.8 wt% loadings. In the lower silica samples, fitting of the elastic incoherent structure factor (EISF) showed that water diffusion was confined to a sphere (with radii ranging from 3.4 to 4.3 Å), suggesting the mobile water was located within the MFI (framework type of H-ZSM-5) channel intersections, giving localised diffusion coefficients in the range of ∼0.9–1.8 × 10−9 m2s−1. In the high silica zeolite, the diffusion was observed to be far less confined and more long range in nature, with diffusion coefficients significantly higher than in the lower silica systems (∼1.8–4.8 × 10−9 m2s−1). MD simulations further investigated the effect of the Si/Al ratio on water diffusivity at 2.8 wt% loading (9 molecules/unit cell (UC)) in H-ZSM-5 with Si/Al ratio = 15, 47, 95 and fully siliceous. The Si/Al ratio had a significant effect on the MD calculated nanoscale diffusivity of water, reducing the self-diffusion coefficient by a factor of 2 from a fully siliceous system to that with Si/Al = 15, due to the strong coordination and increased residence time of water molecules at the Brønsted acid sites which range from ∼5 ps to ∼2 ps in the Si/Al = 15 and Si/Al = 95 systems respectively. QENS observables, both the EISF and quasielastic line broadenings, were reproduced from the MD trajectories upon sampling the experimental timescale giving both qualitative and quantitative agreement with the QENS experiments. Fitting of the MD calculated EISF showed that the experimentally observed diffusion confined to a sphere of radii ranging from 3.5 to 6.8 Å was also present in our simulations, with diffusion coefficients calculated to within a factor of 0.5 of experiment.