Diffusion of benzene in HZSM-5 zeolite in powder or pellet form and of 2,3-dimethylpentane in HY zeolite pellets, more or less deactivated by formation of coke during n-heptane cracking, was studied by 1H NMR imaging and 129Xe NMR of adsorbed xenon used as a probe. The shape and the evolution of the signal intensity of the 1D profile, representing the proton distribution along the axis of the cylindrical tube containing the catalyst, made it possible to determine the intracrystalline molecular diffusion coefficient of benzene when this was adsorbed (≈10-14m2/s). Comparison of the 129Xe NMR spectra, recorded during benzene adsorption, with those simulated assuming intracrystalline diffusion gave a similar value of the diffusion coefficient (≈ 10-15m2/s). For dimethylpentane in HY coked pellets, the NMR profile shows a maximum, which moved during dimethylpentane adsorption. This was interpreted as the superposition of two competing processes: intercrystalline diffusion and intracrystalline diffusion of dimethylpentane from uncoked to highly coked zones of the crystallites. 1D and 2D images at equilibrium adsorption of the probe clearly confirmed the heterogeneous distribution of the coke shown by 129Xe NMR spectroscopy.

Hydrocarbon Diffusion Measurements and Coke Distribution in Zeolite Pellets: a Study by 1H NMR Imaging and 129Xe NMR Spectroscopy. J.L.Bonardet, T.Domeniconi, P.N'Gokoli-Kékélé, M.A.Springuel-Huet, J.Fraissard: Langmuir, 1999, 15[18], 5836-40

Table 22

Parameters for Cation Exchange

Diffusion in Heulandite