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.

In Situ NMR Spectroscopy of Combustion

“…[12][13][14][15][16] The value of 129 Xe NMR using thermally polarized (i.e., nonoptically pumped) xenon for intracrystalline hydrocarbon diffusion studies at low temperature in heterogeneous catalysts has been demonstrated extensively in the recent past. [17][18][19][20][21] The combustion process within the coil region of the NMR probe head (Figure 1) takes place on and above the surfaces of molecular sieve pellets (i.e., only partially dehydrated Na-X zeolite). Although catalytic activity of this material is not expected, the reaction may still be influenced by the zeolite surface in contact with the flame.…”

Magnetic Resonance Imaging

“…This was achieved by imbibing cyclohexane within the catalyst. Microimaging has also been used at the single-pellet level [34] and in small beds of pellets to map coke distribution [35,36]. Therefore, by spatially resolving the T 1 of cyclohexane within the catalyst, the distribution of the hexachloroplatinate dianion is determined.…”

“…More recently, changes in liquid distribution within a catalyst pellet during reaction have been studied [33]. For example, Bonardet et al [35] imaged the 1 H spin density associated with 2,3-dimethylpentane probe molecules adsorbed on pellets of HY zeolites coked to levels of 7.5 and 10% w/w. Figure 3 shows the spatial maps of the liquid phase within the catalyst pellet under catalytic reaction conditions.…”

“…H,14 C, 32 P and35 S as tracers may provide quantitative information about reaction mechanisms and rate-limiting steps in reaction kinetics, but no in situ information from the catalyst surface can be obtained. However, due to the specific radiation characteristics, radionuclides such as 11 C, 13 N,15 O and 18 F can be applied for quantitative in situ heterogeneous studies.…”

In Situ MRI Study of 1-octene Isomerisation and Hydrogenation within a Trickle-bed Reactor

“…To a lesser extent measurements of molecular diffusion [4][5][6][7], liquid distribution during reaction [8] and coke distribution [9][10][11] within both single catalyst pellets and arrays of such pellets have been addressed. These studies have used both spatially unresolved and microimaging MR methods.…”