PFG NMR diffusion measurements of CH4 and CO2 through large ZSM-58-crystals

• Large crystals above 100 μm of ZSM-58 (DDR) have been synthesised and characterised.
• Self-diffusion of CH4 and CO2 in ZSM-58 has been determined by PFG NMR.
• The self-diffusivity of CO2 is about two orders of magnitude higher than CH4.
• Self-diffusion of CH4 increases with increased loading of binary mixtures.

Large crystals of zeolite ZSM-58 having the DDR-topology have been synthesised. The crystals are diamond shaped and 100–160 μm long. Basic characterisation involving X-ray diffraction and N2-porosimetry proved good structure quality and microporosity of the sample. 1H and 13C PFG NMR self-diffusion studies of single gas and binary 1:1 mixtures with CO2 and CH4 show that – in agreement with the topology of the ZSM-58 – the intracrystalline molecular diffusion occurs in a two dimensionally interconnected micropore space. The self-diffusion coefficients of the smaller carbon dioxide exceed those of the larger methane molecules by about two orders of magnitude, which is clearly the effect of increased diffusional resistance and shape selectivity within the narrow 8-ring window system of the ZSM-58 micropores. Carbon dioxide diffusion is rather constant and methane diffusion increases with increasing total loading. For methane this indicates that the narrow windows act as barriers which are easier passed if additional interactions with adsorbed molecules are present. These trends for the loading dependences in both the single-component and the binary mixture diffusion studies confirm earlier predictions by kinetic MC and MD computer simulations. However, at low loadings the predicted values for methane self-diffusion are significantly smaller than the experimental NMR data.

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