Frank Stallmach

Spin Echo NMR Diffusion Studies

The basic principles of NMR diffusion studies and a generalized approach to calculate NMR spin echo attenuation due to diffusion in the presence of gradients of the polarizing magnetic field are presented. By means of this generalized approach, PFG NMR sequences for diffusion studies, including those using alternating pulsed field gradients (APFG) and modern techniques for advanced cross-term suppression by magic pulsed field gradient (MPFG) ratios, are reviewed. Gradient systems and experimental procedures for the generation of high-intensity pulsed field gradients are discussed and recent examples of their successful application in diffusion studies with porous materials are given.

The Potentials of Pulsed Field Gradient NMR for Investigation of Porous Media

PFG NMR self-diffusion studies provide information on the translational mobility of fluid molecules. Since in porous media the diffusion path of fluid molecules in the pore space is affected by interaction with the pore wall, PFG NMR measurements are sensitive to structural peculiarities of the confining porous medium. The pore space properties which can be investigated depend on length scales set by the PFG NMR experiment in respect to the typical size of the structural feature studied. Based upon these length scales, an interpretation pattern for PFG NMR self-diffusion studies in porous media is given. PFG NMR self-diffusion studies in macro- and microporous systems such as sedimentary rocks and zeolite crystallites, respectively, are reviewed.

129Xe and 13C PFG n.m.r. study of the intracrystalline self-diffusion of Xe, CO2, and CO

Using 129Xe and 13C PFG n.m.r., the temperature dependence of the coefficients of self-diffusion of xenon, carbon monoxide, and carbon dioxide in zeolites X, NaCaA, and ZSM-5 (silicalite) are studied. In all cases, the measured diffusivities are found to follow an Arrhenius dependence. The determined activation energies and preexponential factors may be rationalized by relating specific properties of the sorbate gases (kinetic diameter, permanent electrostatic moments) to those of the considered zeolites (free diameter of the pores, cation content).

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.

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.

Self-Diffusion of Chain Molecules in the Metal–Organic Framework IRMOF-1: Simulation and Experiment

Metal-organic frameworks (MOFs) possess characteristics, such as tunable pore size and chemical functionality, that make them attractive candidates for separations, catalysis, gas storage, and sensing applications. The rate of diffusion of guest molecules in the pores is an important property for all of these potential applications. In this work, the self-diffusion of hydrocarbons in IRMOF-1 was studied as a function of chain length with a combination of molecular dynamics simulations and pulsed field gradient NMR experiments. Excellent agreement is seen between the experiments and simulations, and the self-diffusion coefficients in IRMOF-1 are on the same order as those in the bulk liquid. Additionally, the effect of concentration on diffusivity was found to be very small for low to moderate loadings. Molecular dynamics simulations also provided insights about the preferential diffusion pathways of these guests in IRMOF-1.

NMR studies of carbon dioxide and methane self-diffusion in ZIF-8 at elevated gas pressures

Self-diffusion measurements with methane and carbon dioxide adsorbed in the Zeolitic Imidazolate Framework-8 (ZIF-8) were performed by 1H and 13C pulsed field gradient nuclear magnetic resonance (PFG NMR). The experiments were conducted at 298 K and variable pressures of 7 to 15 bar in the gas phase above the ZIF-8 bed. Via known adsorption isotherms these pressures were converted to loadings of the adsorbed molecules. The self-diffusion coefficients of carbon dioxide measured by PFG NMR are found to be independent of loading. They are in good agreement with results from molecular dynamic (MD) simulations and resume the trend previously found by IR microscopy at lower loadings. Methane diffuses in ZIF-8 only slightly slower than carbon dioxide. Its experimentally obtained self-diffusion coefficients are about a factor of two smaller than the corresponding values determined by MD simulations using flexible frameworks.

Self-diffusion studies of pore fluids in unconsolidated sediments by PFG NMR

Abstract The self-diffusion of water and hexadecane in medium and coarse sands from glacial sand deposits in central Germany were investigated by pulsed field gradient nuclear magnetic resonance (PFG NMR). Due to the restriction of the diffusion path at the pore/grain interface, the measured apparent self-diffusion coefficients ( D ( Δ )) in the pore space depend on the observation time ( Δ ) in the PFG NMR experiment. Although the bulk self-diffusion coefficients of water and hexadecane differ by about one order of magnitude, the apparent self-diffusion coefficients in the pore space obey the same characteristic time-behaviour, which depends only on geometrical properties of the pore system. Using the “short-time diffusion” model, surface-to-volume ( S / V ) ratios and inherent self-diffusion coefficients ( D 0 ) of the pore fluids were extracted from these diffusion measurements. The S / V ratios obtained are independent of the pore fluid used and agree with known geometrical properties of the sand grains. Moreover, the D 0 values are consistent with the corresponding bulk self-diffusion coefficients measured separately. In contrast to these results of PFG NMR, simultaneous investigations of longitudinal ( T 1 ) nuclear magnetic relaxation reveal that the relaxation time of the pore fluid is a less suitable parameter for a quantitative estimation of geometrical properties of the pore/grain interface in these unconsolidated sediments since it depends on chemical properties of the fluid/grain interface.

129Xe n.m.r. self-diffusion measurements — a novel method to probe diffusional barriers on the external surface of zeolite crystallites

129 Xe n.m.r. pulsed-field gradient measurements have been carried out to probe the surface permeability of zeolites NaX, NaCaA, and ZSM-5. Although for zeolite NaX the passage through the external surface of the crystallite is found to be of minor importance for the observed exchange rates, in zeolites NaCaA and ZSM-5, this process is found to be significantly retarded. Similar experiments with methane do not provide any stringent indication of the existence of diffusional barriers on the external surface (“surface barriers”). One has to conclude, therefore, that owing to its somewhat larger diameter in comparison to methane, xenon represents a sensitive tool for probing structural distortion in the surface layer of zeolites.

Boundary effects of molecular diffusion in nanoporous materials: A pulsed field gradient nuclear magnetic resonance study

The boundary conditions of intraparticle diffusion in nanoporous materials may be chosen to approach the limiting cases of either absorbing or reflecting boundaries, depending on the host-guest system under study and the temperature of measurement. Pulsed field gradient nuclear magnetic resonance is applied to monitor molecular diffusion of n-hexane and of an n-hexane-tetrafluoromethane mixture adsorbed in zeolite crystallites of type NaX under either of these limiting conditions. Taking advantage of the thus-established peculiarities of mass transfer at the interface between the zeolite bulk phase and the surrounding atmosphere, three independent routes for probing the crystal size are compared. These techniques are based on (i) the measurement of the effective diffusivity under complete confinement, (ii) the application of the so-called NMR tracer desorption technique, and (iii) an analysis of the time dependence of the effective diffusivity in the short-time limit where, by an appropriate variation of the adsorbate and the measuring conditions, the limiting cases of reflecting and adsorbing boundaries could be considered. All these techniques are found to yield coinciding results, which are in excellent agreement with the crystal sizes determined by microscopy.