Wilfried Heink

The propagator representation of molecular transport in microporous crystallites

Abstract NMR diffusion data may easily be represented by the propagator, that is, by the probability that after a time interval Δ a molecule, initially at position z = 0, will be found at position z. Such a propagator representation is used for visualizing the mean features of molecular diffusion in biporous systems as, e.g., a powder sample of zeolite crystallites. As an example, propagator representations for ethane in NaX and NaCaA zeolites in dependence on the temperature and on the crystallite radii are given. The observed diffusion characteristics are in satisfactory agreement with theoretical estimates of the influence of inter- and intracrystalline transport on the overall diffusion behavior.

Microdynamics of methane, ethane and propane in ZSM-5 type zeolites

The n.m.r. pulsed field-gradient technique has been used to study systematically the intracrystalline self-diffusion of methane, ethane and propane in ZSM-5. In conjunction with the information obtained from nuclear magnetic relaxation studies the elementary steps of diffusion are found to be activated jumps between the channel intersections. Only for sorbate concentrations > 2.5 molecules per intersection is a decrease in the jump lengths observed. The results are compared with alkane self-diffusion measurements in zeolites A and X, as well as with the self-diffusion of water in ZSM-5 and zeolites A and X.

High-temperature pulsed field gradient nuclear magnetic resonance self-diffusion measurements of n-alkanes in MFl-type zeolites

The coefficients of intracrystalline self-diffusion of the n-alkanes from propane to n-alkanes from propane to n-hexane adsorbed in zeolite ZSM-5 are studied by means of the pulsed field gradient (PFG) NMR method with field gradient amplitudes up to 25 T m–1 over a temperature range from –20 to +380 °C. The diffusivities are found to decrease monotonically with increasing chain lengths. They are in satisfactory agreement with the results of quasielastic neutron scattering and MD simulations as well as with non-equilibrium measurements by the frequency response and single-step adsorption technique. However, in comparison with zero-length-column (ZLC) desorption and permeation studies, these data, though exhibiting the same trends of chain length dependence, are ca. two orders of magnitude larger. As expected, the diffusivities are intermediate between those for zeolite NaX and NaCaA, exhibiting, however, the smallest activation energies. This indicates that the differences in the potential energies of the adsorbate molecules on their intracrystalline trajectories are in general smaller than in zeolites NaCaA and NaX.

Self-diffusion measurements of n-alkanes in zeolite NaCaA by pulsed-field gradient nuclear magnetic resonance

Pulsed-field gradient (PFG) NMR is applied to the study of self-diffusion of the n-alkanes from methane to n-heptane, adsorbed in zeolite NaCaA. Using magnetic field gradients up to 10 T m–1 over a temperature range from – 100 to 350 °C, diffusivities between 10–13 and 10–8 m2 s–1 can be covered. For methane and ethane in NaCaA the diffusivity is found to increase by up to two orders of magnitude with increasing concentration. In contrast to the case for zeolites NaX and HZSM-5, there is a dramatic jump in the chain-length dependence of the diffusivities between ethane and propane, corresponding to the fact that the critical molecular diameter of propane and the longer n-alkanes is close to the diameter of the free apertures between adjacent supercages. With further increasing chain length there is a monotonic decrease in the diffusivities. The values obtained are in satisfactory agreement with the results of uptake measurements and neutron scattering experiments.

N.m.r. evidence of the existence of surface barriers on zeolite crystallites

Traditional n.m.r. pulsed field gradient technique and n.m.r. tracer desorption technique are applied in search of surface barriers on zeolite crystallites. Investigating the system ethane/NaCa-A it is found that the existence of surface barriers is no general peculiarity of zeolite crystallites. It is shown, however, that different ways of pretreatment as well as the preadsorption of other molecular species may lead to the formation of surface resistances. These changes in the structure of the crystallite surface may take place without any change in the intracrystalline mobility of the ethane molecules.

P.f.g. n.m.r. study of the influence of the exchangeable cations on the self-diffusion of hydrocarbons in zeolites

Abstract The molecular mobility of methane and n -hexane adsorbed in X- and A-type zeolites with different calcium contents is studied by pulsed-field gradient (p.f.g.) n.m.r. In contrast to sodium, the calcium ions are found to be of substantial influence on the intracrystalline diffusivities. As a consequence of this interaction, the diffusivities of methane in NaCaA and NaCaX are found to be quite close to each other in both the absolute values and the dependence on temperature and concentration. For n -hexane, the diffusivities in NaCaA are significantly below the diffusivities in NaCaX. This can be attributed to the larger critical diameter of the n -hexane molecules, which is of the order of the diameters of the windows between adjacent supercages in zeolite A. The proposed model of molecular interaction is confirmed by comparison with the results of MD simulations and by n.m.r. tracer desorption measurements with zeolite alpha.

Application of zeugmatography to study kinetics of physical adsorption

Abstract Spatial resolution of nuclear magnetic resonance signals may be achieved by use of magnetic field gradients. This new technique, called “zeugmatography”, has been applied in the present work to study dynamic processes, especially the kinetics of physical adsorption in microporous systems. The method provides information similar to the results of Dubinins X-ray technique, but is not limited to X-ray contrast adsorbates. After a general discussion, examples of application of “dynamic zeugmatography” are given. Mass transfer for sorption of butane in NaCaA zeolites of different shape and of water in NaX zeolites could be observed directly.

On the Use of Pulsed Field Gradients in a High-Field NMR Spectrometer to Study Restricted Diffusion in Zeolites

Abstract A probe system is described which allows the application of magnetic field gradient pulses with amplitudes up to 24 T/m for self-diffusion measurements by means of a PFG NMR spectrometer operating at a proton resonance frequency of 400 MHz. It is demonstrated that PFG NMR measurements in the high field of a superconducting magnet necessitate precautions which are of minor relevance for measurements with iron magnets. Taking advantage of the large gradient intensity and the high sensitivity in signal detection of the described system, attenuation patterns of the NMR spin echo are observed for the first time that represent the PFG NMR analogue of the diffraction pattern of a sphere.

On the limits of the application of the n.m.r. pulsed-field gradient technique for self-diffusion measurements in zeolites

Abstract The main limitations for an application of the n.m.r. pulsed-field gradient technique to self-diffusion studies in zeolites are reviewed. It is shown that small amounts of highly mobile molecules adsorbed on the outer surface or within intracrystalline cracks will, in general, lead to an enhanced damping of the n.m.r. signal that may be interpreted erroneously as a high overall mobility of the adsorbed molecules if the two-phase character of the signal is not taken into consideration. As an example, for benzene adsorbed on ZSM-5-type zeolites, the n.m.r. pulsed-field gradient technique is shown to provide only an upper limit of the intracrystalline diffusivity of the order of 10 −10 m 2 s −1 . This value does not conflict with the previously reported uptake diffusivities of the order of 10 −13 m 2 s −1 .

19F NMR diffusion studies of molecules adsorbed on zeolites

Abstract The 19 F nmr pulsed field gradient technique is applied to study molecular self-diffusion of fluorine containing molecules adsorbed on zeolites. Most favourable conditions for the nmr self-diffusion measurements are provided by the X type zeolites. In this case, self-diffusion coefficients at both single-component (SF 6 , CF 4 , CHF 2 Cl and C 6 F 6 ) and two-component (C 6 F 6 /C 6 H 6 adsorption are determined. In A type zeolites, molecular diffusion is dramatically reduced due to the smaller diameter of the windows between the micropores of the zeolite crystallites. The diffusion coefficients of the fluorocarbons investigated are compared with those previously obtained for the corresponding hydrogen-containing compounds.