Dependence of the diffusion coefficients of methane in silicalite on diffusion distance as investigated by ¹H PFG NMR

¹H PFG NMR was performed to measure the diffusion coefficient of methane in three MFI-type siliceous zeolite silicalite samples, which were synthesized under different hydrothermal conditions resulting in different crystal sizes or intracrystalline defects. Measured diffusivities of methane decreased at the diffusion distance around 7 μm for all samples. The distance almost corresponds with the distance between cracks in the intracrystalline. However, the well-calcined sample, which has no detectable cracks as observed by scanning electron microscope, also showed a drastic decrease in the diffusion coefficient. This result suggests the existence of an intracrystalline transport barrier in the single crystal, the size of which is below the SEM resolution.