Hydrocarbon Diffusion Measurements and Coke Distribution in Zeolite Pellets: A Study by 1H NMR Imaging and 129Xe NMR Spectroscopy

Abstract

Diffusion of benzene in HZSM-5 zeolite in powder or pellet form and of 2,3-dimethylpentane (DMP) 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, make it possible to determine the intracrystalline molecular diffusion coefficient of benzene when this is adsorbed (≈10-14 m2 s-1). Comparison of the 129Xe NMR spectra, recorded during benzene adsorption, with those simulated assuming intracrystalline diffusion gives a similar value of the diffusion coefficient (≈10-15 m2 s-1). For DMP in HY coked pellets, the NMR profile shows a maximum, which moves during DMP adsorption. This was interpreted as the superposition of two competing processes: intercrystalline diffusion and intracrystalline diffusion of DMP from uncoked to highly coked zones of the crystallites. 1D and 2D images at equilibrium adsorption of the probe clearly confirm the heterogeneous distribution of the coke shown by 129Xe NMR spectroscopy.