Transport diffusion and self-diffusion of benzene in NaX and CaX zeolite crystals studied by ZLC and tracer ZLC methods (original) (raw)
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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.
NMR Study of the Influence of a Carrier Gas on Zeolitic Diffusion
Zeitschrift für Physikalische Chemie, 1984
Self-diffusion of cyclohexane in NaX zeolites in contact with an argon atmosphere is measured by means of the nmr pulsed field gradient technique. Using two zeolite samples with mean crystallite diameters of 25 ¡Jtm and 2.92 [im, respectively, in a temperature region of 293 K gj T 5= 500 K both inter-and intracrystalline transport phenomena are studied. It is found that the additional argon molecules do not affect the intracrystalline diffusivity of cyclohexane, while intercrystalline diffusivities are drastically retarded. For the interpretation of these "long-range" diffusivities a straightforward model is proposed.
1984
Under constant-volume-variable-pressure conditions the sorption uptake of benzene by NaX zeolite crystals has been investigated with respect to the influence of external thermal conditions on the uptake rate. The information obtained permits quantitative conclusions concerning the influence of the sorption heat generated on the uptake rate, i.e. the decrease in apparent diffusivities calculated under the erroneous presumption of the isothermicity of the sorption process considered. A corresponding tentative mechanism which takes into account the generation of thermal interface barriers is proposed. Furthermore, there are experimental conditions for which the uptake of benzene by NaX zeolite crystals should be considered as an isothermic process. Within that parameter region agreement was obtained between the sorption diffusion data (corrected by the Darken equation) and the n.m.r. self-diffusivities, as reported earlier.
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.
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.
The Journal of Physical Chemistry B, 2001
The PFG NMR technique is applied to investigate the intracrystalline diffusion of methane and n-butane molecules in MFI-type zeolites at several temperatures (from 123 up to 383 K) and over a wide range of diffusion times (from 2 to 35 ms). The intracrystalline self-diffusion coefficients of the guest molecules recorded at low temperatures were observed to decrease with increasing root-mean-square displacements. The comparison of the experimental results with the results of the Monte Carlo simulations of diffusion allowed us to rule out the restriction of diffusion by crystal boundaries as a possible explanation of the observed dependencies of the diffusivities on the root-mean-square displacement. These dependencies are tentatively attributed to the existence of intracrystalline transport barriers in MFI-type crystals. The intersections between the elementary building blocks of the crystals and/or intergrowth sections of MFI crystals are suggested as the possible candidates for the transport barriers.
Langmuir
Removing carbon dioxide is important for the upgrading of biogas or natural gas into compressed or liquefied methane, and adsorption-driven separation of CO2 could be further advanced by developing for example new adsorbents. Zeolite adsorbents can select CO2 over CH4, and we here confirmed that the adsorption of CH4 on zeolite |Na12-xKx|-A was significantly lower for samples with a high K + content, i.e. x > 2. Nevertheless, these samples adsorb CH4 after long equilibration times as determined with 1 H NMR experiments. To assess further the intracrystalline diffusion of CH4 in these zeolites, pulsed-field gradient NMR experiments were performed. In large crystals of zeolites |Na12-xKx|-A, the long-time diffusion coefficients of CH4 did not vary with x, and the mean square displacement was about 1.5 µm irrespective of the diffusion time. Also for zeolite |Na12|-A samples of three different particle sizes (~0.44, ~2.9, ~10.6 µm), the mean-square displacement of CH4 was 1.5 µm and largely independent of the diffusion time. This similarity provided further evidence for an intracrystalline diffusion restriction for CH4 within the medium-and large-sized zeolite A crystals, and possibly of clustering and close contact among the small zeolite A crystals. The long-time diffusion coefficient of adsorbed CH4 was (at 1 atm and 298 K) about 1 × 10-10 m 2 /s irrespective of the size of the zeolite particle or the studied content of K + in zeolites |Na12-xKx|-A and |Na12|-A. The T1 relaxation time for adsorbed CH4 on zeolites |Na12-xKx|-A with x > 2 was smaller than for those with x < 2, indicating that the short-time diffusion of CH4 was hindered.
Diffusion of methanol in NaX crystals: Comparison of i.r., ZLC, and PFG-n.m.r. measurements
Zeolites, 1994
A new method of following sorption/desorption rates in zeolite crystals by monitoring the transient response of the surface temperature by i.r. has been developed. Unlike conventional sorption rate measurements, this method is not limited to isothermal or quasi-isothermal conditions, and the rapid response of the i.r. detector means that the method can be applied to study relatively fast systems. The shape of the response curves can provide useful information concerning the nature of the masstransfer resistance, thus allowing the intrusion of a surface barrier to be detected. The validity of this technique has been confirmed by a detailed study of sorption/desorption rates for methanol in NaX crystals. At room temperature, the results obtained by the new method are consistent with the data obtained, for the same system, by both zero-length column (ZLC) and pulsed field gradient (PFG) n.m.r. self-diffusion measurements. The PFG n.m.r, data show an unusual variation of self-diffusivity and activation energy with loading; the limiting activation energy, extrapolated to zero loading, agrees well with the ZLC value (11 k J/tool). Sorption rates in successive experiments, after "regeneration" at 300°C, show a regular decline accompanied by an increasing surface resistance to mass transfer. This was attributed to a slow buildup of "coke" at the crystal surface. Butterworth-Heinemann provides, in principle, an alternative way of measuring the sorption rate and, hence, the time constant for intracrystalline diffusion. This approach offers two major advantages: It does not require a near isothermal system and the response time of an infrared detector is short, so the method should be applicable to the study of fairly rapid sorption processes. We report here a new experimental technique, developed at the CNRS laboratory, that depends on monitoring, by an infrared detector, the surface temperature of a sample of zeolite Crystals following a small change in sorbate pressure. This technique must be distinguished from the method recently introduced by Karge and Niessen 3 in which FTi.r. measurements are used to monitor the adsorbed phase concentration and diffusivities are then determined from the (isothermal) transient sorption curve. To confirm the validity of the new method, the same system (methanol-NaX zeolite) has also been studied by the pulsed field gradient (PFG) n.m.r, method and by the zero-length column (ZLC) method. This particular
Intracrystalline Diffusion of Benzene in ZSM-5 Type Zeolites
Zeitschrift für Physikalische Chemie, 1989
Zusammenfassung Sorptionskinetische Daten, die die Beladungs-und Temperaturabhangigkeiten des intrakristallinen Transports von Benzen in Pentasilstrukturen (ZSM-5 und Silikalith) beschreiben, lassen sich in hinreichender Weise im Rahmen der Vorstellungen iiber konstante mittlere molekulare Sprunglangen interpretieren.