Multiscale studies of framework structure, composition, and defect influences on small molecule behaviour in zeolites

The dynamics of water and methanol confined within different zeolites has been studied using classical molecular dynamics and quasielastic neutron scattering to understand the effects of zeolite structure and composition (Si/Al ratio, framework topology, and presence of defects) on the behaviour of small molecules confined in zeolite systems. This is with the aim to expand the current understanding of structure-property relationships regarding guest-host interactions and the dynamical behaviour of confined species to enable further development of these materials for use as molecular sieves, in water purification and as catalysts. The effect of the Si/Al ratio and inclusion of silanol nest defects on the diffusion of water at various loadings confined within H-FAU zeolites was investigated using classical molecular dynamics. Water loading has a significant effect on its diffusion, showing increased diffusivity as the loading was increased – plateauing as saturation is reached. Brønsted acid sites (BASs) show strong interactions with the water molecules and thus hinder molecular movements. As the Si/Al ratio is decreased, the higher concentration of BASs results in decreased diffusivity of the water. Silanol nests had no significant effect on the movement of water within the zeolite in our simulations. Complementary quasielastic neutron scattering (QENS) experiments were conducted on H-FAU samples. Again, a positive correlation between water diffusivity and Si/Al ratio was observed. Water was observed to be mostly confined to the FAU supercages on the timescale of experiment (1 – 100 ps). A good qualitative agreement was seen between the MD and QENS via the generation of QENS observables. Water diffusion within H-MFI was also studied. Similarly, a positive correlation between water diffusivity and Si/Al ratio was observed. Water was characterised as undergoing confined diffusion on the experimental timescale, within the MFI intersections, however, in the high silica sample the confinement was significantly reduced. QENS observables reproduced from MD trajectories gave good qualitative and quantitative agreement to the experiment. Finally, the effect of Si/Al ratio and defect presence on the diffusion of methanol within zeolite H-FER was probed using QENS and classical molecular dynamics. Classical molecular dynamics showed that decreasing the Si/Al ratio slowed the diffusion of methanol by a factor of 2 or more due to hydrogen bonding between the BAS and methanol molecules. The QENS measurements investigated two FER samples; a commercial sample (Si/Al = 20) and a sample synthesized from Ghanaian Bauxite and Kaolin (Si/Al = 35). The methanol was found to be mostly immobilised in both samples on the timescale observed by the OSIRIS QENS spectrometer (1- 100 ps), with at least 80% of methanol molecules appearing immobile. The EISF was best characterised by isotropic rotation of the methanol molecules. Similar observations were made via the QENS observables reproduced from the MD trajectories, the major difference being that almost 100% of the molecules were mobile in the MD simulations. No translational diffusion was observed over the experimental timescale in any sample – a potential indicator for methoxylation within the sample or due to oversaturation of the zeolite.