Diffusion Measurements of Hydrocarbons in Zeolites with Pulse-Field Gradient Nuclear Magnetic Resonance Spectroscopy (original) (raw)
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Intracrystalline Diffusion in Mesoporous Zeolites
ChemPhysChem, 2012
Specially synthesized extra-large crystalhtes of zeolite LTA with two (micro-and meso-) pore spaces. Gas-kmetic order-of-magintentionally added mesoporosity are used for an in-depth study of guest diffusion in hierarchical nanoporous materials by the pulsed field gradient NMR technique. Using propane äs a guest molecule, intracrystalline mass transfer is demonstrated to be adequately described by a single effective diffusivity resulting from the weighted average of the diffusivities in the nitude estimates of the diffusivities are in satisfactory agreement with the experimental data and are thus shown to provide a straightforward means for predicting and quantifying the benefit of hierarchically structured nanoporous materials ir comparison with their purely microporous äquivalent.
NMR Imaging as a Tool for Studying the Diffusion and Co-Diffusion of Gases in Zeolite Catalysts
H NMR imaging is a powerful technique for studying the diffusion of pure hydrocarbons (for example benzene and n-hexane) during their adsorption in or desorption from a fixed bed of zeolite crystallites. This technique is used to visualize the progression of the diffusing molecules in the zeolite bed and to determine their intracrystallite diffusion coefficients. More importantly, NMR imaging is today the only technique able to give the time dependence of the distribution of two gases during their competitive diffusion.
The Journal of Chemical Physics, 2002
Pulsed-field gradient nuclear magnetic resonance (PFG NMR) has been applied to study molecular diffusion in industrial fluid catalytic cracking (FCC) catalysts and in USY zeolite for a broad range of molecular displacements and temperatures. The results of this study have been used to elucidate the relevance of molecular transport on various displacements for the rate of molecular exchange between catalyst particles and their surroundings. It turned out that this rate, which may determine the overall rate and selectivity of FCC process, is primarily related to the diffusion mode associated with displacements larger than the size of zeolite crystals located in the particles but smaller than the size of the particles. This conclusion has been confirmed by comparative studies of the catalytic performance of different FCC catalysts. D
Exploring Mass Transfer in Mesoporous Zeolites by
2012
With the advent of mesoporous zeolites, the exploration of their transport properties has become a task of primary importance for the auspicious application of such materials in separation technology and heterogeneous catalysis. After reviewing the potential of the pulsed field gradient method of NMR (PFG NMR) for this purpose in general, in a case study using a specially prepared mesoporous zeolite NaCaA as a host system and propane as a guest molecule, examples of the attainable information are provided.
Microporous and Mesoporous Materials, 2008
Self-diffusion coefficients for intracrystalline diffusion of hydrocarbon molecules adsorbed in large crystals of NaX zeolite have been measured by the pulsed field gradient (PFG) NMR technique, at ambient temperature and at different diffusion times (from 6 to 12 ms). Two NMR pulse sequences, stimulated and 13-interval bipolar spin echo, were used to examine the influence of internal field gradients on diffusion data. For both sequences the effective self-diffusion coefficient of the guest molecules was found to decrease with increasing observation times. The extrapolated intracrystalline diffusion coefficient is independent of the NMR sequence. In contrast, the estimated extent of molecular diffusion depends strongly on the pulse program. For the small molecules (butane to hexane), the domain size, R, of restricted diffusion obtained with the stimulated spin-echo sequence is smaller than the crystal dimension whereas R is always comparable to it when the 13-interval pulse sequence is used. This shows the effect of internal field gradients on the diffusion data leading to wrong values of R if the stimulated pulse sequence is used. The light hydrocarbons diffuse freely inside the zeolite particles whereas the crystal boundaries act as reflecting surfaces, as previously observed. On the other hand, even with the 13-interval pulse sequence, the smaller values of R obtained for large molecules as n-heptane and octane shows that their displacement is hindered by restrictions in the NaX macro-crystals.
Exploring Mass Transfer in Mesoporous Zeolites by NMR Diffusometry
Materials, 2012
With the advent of mesoporous zeolites, the exploration of their transport properties has become a task of primary importance for the auspicious application of such materials in separation technology and heterogeneous catalysis. After reviewing the potential of the pulsed field gradient method of NMR (PFG NMR) for this purpose in general, in a case study using a specially prepared mesoporous zeolite NaCaA as a host system and propane as a guest molecule, examples of the attainable information are provided.
Diffusion of Benzene and Propylene in MCM-22 Zeolite. A Molecular Dynamics Study
Journal of Physical Chemistry B, 1999
Molecular dynamics simulations have been performed to study the diffusion of a mixture of benzene and propylene, for the cumene synthesis process, in purely siliceous MWW (MCM-22), a zeolite containing two separate channel systems: the 10-member ring (MR) sinusoidal and the 12-MR supercages interconnected by 10-MR windows system. The diffusion processes in each channel system of MWW at 650 K have been studied independently. We have found that in order to obtain quantitative or semiquantitative diffusion coefficients, the framework should be optimized. A large diffusivity for propylene in both channel systems, and especially in the supercage system, is observed, whereas benzene is not seen to diffuse in either of the two channel systems, and only intracage mobility is seen in the supercage voids. The positions of minimum energy, where the molecules are expected to react, have been located in both channels. The diffusion of benzene in the supercage system seems to be temperature-activated, and when the temperature is increased, intercage diffusion will probably occur. Radial distribution functions show that condensation reactions between benzene-propylene and propylene-propylene are possible, which indicate the necessity of working in an excess of benzene. The results of the simulations of diffusion suggest that the formation of cumene probably occurs at the external surface or close to the external surface of the MCM-22 zeolite crystals.
Concentration dependence of intracrystalline self-diffusion in zeolites
Adsorption Science & Technology, 1985
The nmr pulsed field gradient technique is applied to study molecular self-diffusion in zeolitic adsorbate—adsorbent systems. Depending on the zeolite type and on the nature of the adsorbate—adsorbent interaction, the concentration dependence of the self-diffusion coefficients reveals considerable differences. Furthermore, intracrystalline molecular transport may be significantly influenced by the co-adsorption of a second molecular species. Supported by additional nuclear magnetic relaxation and sorption rate measurements microdynamic models for the observed diffusion behaviour are established.
Chemistry of Materials, 2005
Diffusivities of n-octane in particles of industrial fluid catalytic cracking (FCC) catalysts and in zeolite USY, which is the main zeolitic component of the particles, are reported. Diffusion measurements have been performed by using pulsed field gradient (PFG) NMR for a broad range of molecular displacements and temperatures. The recorded diffusivities are used to evaluate the relevance of various transport modes in the particles of FCC catalysts, such as diffusion in the micropores of the zeolite crystals located in the particles, diffusion through the surface layer of these crystals, and diffusion in the meso-and macropores of the particles, for the rate of molecular exchange between catalyst particles and the surrounding atmosphere. This rate is shown to be primarily related to the diffusion in the meso-and macropores of the particles under the condition of fast molecular exchange between these pores and the zeolite crystals located in the particles. The diffusivity associated with this type of diffusion (i.e., the intraparticle diffusivity) is found to correlate well with the catalytic performance of FCC catalysts having the same fractions of the same zeolite USY but different systems of meso-and macropores.