C. Krause

Comparative QENS and PFG NMR diffusion studies of water in zeolite NaCaA

Quasi-elastic neutron scattering and pulsed field gradient NMR, in combination with temperature-programmed desorption, were applied to study water diffusion in NaCaA samples obtained by exchange from a batch of NaA zeolite of commercial origin. Both techniques exhibit similar trends, viz. increasing water diffusivities with increasing loading (at least up to medium pore filling factors) and with decreasing calcium content. There are, however, notable differences in both the absolute values of the diffusivities and their temperature dependencies. These differences are explained by the real structure of the zeolite samples and the different sensitivity of the measuring techniques to the deviations from ideal crystallinity.

Intracrystalline Transport Resistances in Nanoporous Zeolite X

By applying pulsed-field gradient nuclear magnetic resonance (PFG NMR) in comparison to quasi-elastic neutron scattering (QENS), we sense by measurement of the diffusion of n-octane on different length scales, transport resistances in faujasite type X (which is isostructural with type Y and differs by the lower Si/Al ratio only) with mutual distances of less than 300 nm. Direct observation of the real structure of zeolite X by transmission electron microscopy identifies them as stacking faults of mirror-twin type on (111)-type planes of the cubic framework. Thus, direct experimental proof is given that, in general, nanoporous host systems such as zeolite crystals cannot be considered as a mere arrangement of cavities. It is rather the presence of structural defects that dominates their properties as soon as transport phenomena with practical relevance, including chemical conversion by heterogeneous catalysis and chemical separation by molecular sieving and selective adsorption, become relevant.

Pulsed field gradient NMR studies of diffusion in MCM-41 mesoporous solids

Abstract Pulsed field gradient NMR is able to monitor the displacement of guest molecules in porous materials without interfering with the microstructure and microdynamics of the system under study. Using this technique, the propagation pattern of benzene, n -hexadecane, propylenecarbonate, ethylbenzene and diethyl ether in ordered mesoporous materials of type MCM-41 was studied. The evaluation of the self-diffusion of benzene allows a quick and easy assessment of the transport resistance in the nanoporous material acting over diffusion length scales of 1–10 μm. The fast diffusivities observed with benzene, ethylbenzene and diethyl ether in MCM-41 suggest that the transport processes for these sorbate molecules are governed by gas or vapour phase diffusion, which is reduced by interactions with the silica walls. Measurements over shorter diffusion lengths (⩽1 μm), which were realised using n -hexadecane, showed that the self-diffusion in the hexagonal channels of MCM-41 is anisotropic. The finite value of the self-diffusion component perpendicular to the channel axis indicates the existence of permeable defects like “windows” in the silica matrix.

Nanoporous Glass as a Model System for a Consistency Check of the Different Techniques of Diffusion Measurement

The remarkable differences in the guest diffusivities in nanoporous materials commonly found with the application of different measuring techniques are usually ascribed to the existence of a hierarchy of transport resistances in addition to the diffusional resistance of the pore system and their differing influence due to the differing diffusion path lengths covered by the different measuring techniques. We report diffusion measurements with nanoporous glasses where the existence of such resistances could be avoided. Molecular propagation over diffusion path lengths from hundreds of nanometers up to millimeters was thus found to be controlled by a uniform mechanism, appearing in coinciding results of microscopic and macroscopic diffusion measurement.

Diffusion of n-hexane in 5A zeolite studied by the neutron spin-echo and pulsed-field gradient NMR techniques

The neutron spin-echo (NSE) and pulsed-field gradient (PFG) NMR techniques are applied to investigate the microdynamics of n-hexane diffusion in a commercial sample of zeolite NaCaA. In the PFG NMR measurements molecular root mean square displacements in the range from 100 nm to 2 μm are covered, while the NSE experiments applied to deuterated guest molecules provide information about the rate of molecular fluxes over distances of a few nm. Taking into account these differences, the obtained diffusivity data are found to be in reasonable agreement.

Diffusion of guest molecules in MCM-41 agglomerates

The pulsed field gradient nuclear magnetic resonance method has been used to study self-diffusion of cyclohexane in a commercial MCM-41 material at different external gas pressures from zero to saturated vapor pressure. It is found that the effective diffusivities exhibit three different regions with increasing pressure: decrease at low pressures, a sudden drop at intermediate pressures, and increase at higher pressures. In addition, in the region of irreversible adsorption (hysteresis loop) the diffusivities are also found to differ on the adsorption and the desorption branches. A simple analytical model taking account of different molecular ensembles with different transport properties due to the complex architecture of the porous structure is developed which provides a quantitative prediction of the experimental data. The analysis reveals that the effective diffusivity is predominantly controlled by the adsorption properties of the individual mesoporous MCM-41 crystallites which, in combination with high transport rates, provide a simple instrument for fine tuning of the transport properties by a subtle variation of the external conditions.

Tracing pore-space heterogeneities in X-type zeolites by diffusion studies.

Pore-space homogeneity of zeolite NaX was probed by pulsed field gradient (PFG) NMR diffusion studies with n-butane as a guest molecule. At a loading of 0.75 molecules per supercage, a wide spectrum of diffusivities was observed. Guest molecules in the (well-shaped) zeolite crystallites were thus found to experience pore spaces of quite different properties. After loading enhancement to 3 molecules per supercage, however, molecular propagation ideally followed the laws of normal diffusion in homogeneous media. At sufficiently high guest concentrations, sample heterogeneity was thus found to be of no perceptible influence on the guest mobilities anymore.

A Surprising Drop of the Diffusivities of Benzene in a Mesoporous Material of Type MCM-41 at Medium Pore Filling Factors

Pulsed Field Gradient (PFG) NMR is applied to study molecular diffusion of benzene in a commercial sample of MCM-41. Over a very small concentration range at medium pore filling factors the diffusivity is found to sharply drop by up to one order of magnitude, while it remains essentially constant over the total remaining range from vanishing concentrations up to over-saturation. Similarly extreme deviations from monotonous concentration patterns have so far not been described in the literature. The observed effect may be rationalised as a consequence of the onset of capillary condensation in the transport pores in the hyper-structure of the MCM-41 particles under study.

Diffusion of n-alkanes in zeolites : the benefit of observation over different length scales

Abstract First results of an international research collaboration to study zeolitic diffusion are presented. By using a variety of experimental techniques, molecular diffusion paths are probed over different time and length scales. As host-guest systems the homologous series of n-alkanes in zeolites NaCaA and silicalite-1 have been selected. Since the diffusivities are found to be the smaller the longer are the diffusion paths covered during the experiments, the observed differences may be explained by the existence of transport resistances acting in addition to the intracrystalline zeolite bulk phase. In selective experiments, indications of such resistances are traced from the range of some 100 nm up to crystal dimensions.

Water relaxation behavior inside the pores and cages of some molecular sieves

Abstract Molecular dynamics of water confined to some molecular sieves is studied by broadband dielectric spectroscopy (10 −2 to 10 9 Hz). Faujasite as well as molecular sieves with hexagonal cylindrical pores or cellular structure were used as confining matrix. It was found that the mean relaxation time of confined water has unusual non-monotonic saddle-like temperature dependence. Such temperature dependence was interpreted using a model recently developed by Ryabov et al. [J. Phys. Chem. B 2001, 105, 1845] to analyze the dynamics of water in nanoporous glasses. The unusual behavior is the result of two competing processes: orientational fluctuations the water molecules following an Arrhenius-like temperature dependence and the formation of the defects necessary for this reorientation. The number of defects decreases with increasing the temperature. The temperature dependence of the relaxation time has been proven to be a quite fundamental phenomenon characteristic for water in confining geometries. The defects involved in the dynamics of confined water might be related to the presence of OH groups as defects of the confining framework.