S. Vasenkov

Evidence for the existence of intracrystalline transport barriers in MFI-type zeolites: a model consistency check using MC simulations

The results of Monte Carlo simulations of diffusion on cubic lattices with diffusion anisotropy and regularly spaced permeable transport barriers are reported. By varying the simulation parameters in a broad range it was attempted to reproduce the experimental dependencies of diffusivities on the root mean square displacement reported earlier for MFI-type zeolites. The experimental dependencies were originally attributed to the existence of permeable transport barriers in MFI-type zeolites. Using this assumption and applying a simple model based on the transition state theory a satisfactory agreement between the experimental and the simulation results for selected sets of simulation parameters is obtained. This supports the assumption about the existence of permeable internal transport barriers in MFI-type zeolites.

Pulsed field gradient nuclear magnetic resonance study of long–range diffusion in beds of NaX zeolite: Evidence for different apparent tortuosity factors in the Knudsen and bulk regimes

The pulsed field gradient nuclear magnetic resonance method is applied to study self-diffusion of ethane in beds of zeolite NaX for displacements which are orders of magnitude larger than the size of individual crystals. Comparison of the measured diffusivities with those calculated using simple gas kinetic theory indicates that for the same bed of NaX crystals the apparent tortuosity factor in the Knudsen regime is significantly larger than that in the bulk regime. This finding is tentatively attributed to the more pronounced geometrical trapping by surface imperfections in the Knudsen than in the bulk regime. Tortuosity factors, which are much larger in the Knudsen regime than in the bulk regime, were also recently obtained by dynamic Monte Carlo simulation of gas diffusion in various porous systems.

On the diffusion of water in silicalite-1: MD simulations using ab initio fitted potential and PFG NMR measurements

Abstract Molecular dynamics simulations of water diffusion in silicalite-1 are reported. The simulations are carried out using an ab initio fitted silicalite-1–water potential based on quantum chemical calculations. In addition, preliminary results of pulsed field gradient (PFG) NMR diffusion measurements of water and small alkane molecules in silicalite-1 samples are presented. Pre-adsorption of water in silicalite-1 samples was found to change the intra-crystalline diffusivities of small alkane molecules in silicalite-1. This is interpreted as an indirect evidence that under our experimental conditions water molecules occupy a significant part of the silicalite-1 channel system. The preliminary results of the PFG NMR diffusion measurements of water in silicalite-1 samples are discussed in terms of the contributions of extra- and intra-crystalline water to the measured signals. An-order-of magnitude agreement between the measured and the simulated intra-crystalline diffusivities of water in silicalite-1 is obtained.

Molecular dynamics under confinement to one dimension: options of measurement and accessible information

Two types of host systems for one-dimensional molecular arrangements are considered, namely zeolites containing one- and two-dimensional arrays of channels of sub-nanometre dimension and porous silicon with channel diameters in the range of a few nanometres. After a discussion of the potential of zeolites as host systems, in particular for molecular arrangements under the conditions of single-file diffusion and of molecular traffic control, actual diffusion measurements by means of pulsed-field gradient NMR and interference/IR microscopy are shown to reveal substantial differences between the real and ideal zeolite structure. In contrast, porous silicon with one-dimensional channel arrays is successfully exploited as a host system allowing the experimental observation of such most important features of molecular confinement like hysteresis in mesoscalic systems and surface diffusion. Thus, the attainable experimental insight offers promising conditions for a comparison of the results with those of the theoretical treatment of the observed phenomena.

Structure-mobility relations of molecular diffusion in nanoporous materials

Depending on the measuring conditions, pulsed field gradient (PFG) NMR measurements of molecular diffusion in beds of nanoporous particles may provide information about the propagation rate of guest molecules in both the intra- and interparticle spaces, as well as through the interface between them. Recent progress in both PFG NMR instrumentation and computational techniques have initiated studies of novel aspects in each of these areas, which are reviewed in this communication. They concern the possibility of multicomponent diffusion measurements with ultra-high pulsed field gradients, the peculiarities of molecular diffusion in channel networks, the determination of the surface-to-volume ratio of nanoporous particles and the dependence of the tortuosity factor of long-range diffusion on the diffusion mode in the intercrystalline space.

PFG NMR observation of an extremely strong dependence of the ammonia self-diffusivity on its loading in H-ZSM-5

Ammonia diffusion in H-ZSM-5 (Si/Al = 20) has been studied by the PFG NMR method for different ammonia loadings at 298 K. The results obtained indicate that the self-diffusion coefficient of ammonia changes by up to two orders of magnitude if ammonia loading is varied by a factor of only 2 between 1.6 and 0.8 mmol/g. This unusually strong loading dependence is attributed to the strong interaction of ammonia molecules with the limited number of acid sites of the H-ZSM-5 framework.

Modeling molecular diffusion in channel networks via displacements between the channel segments

Molecular diffusion in channel networks (zeolite silicalite-1) is studied by molecular trajectories as a sequence of displacements between the individual channel segments. Alternatively to the method introduced by Karger (J. Karger, J. Phys. Chem., 1991, 95, 5558) for predicting correlated diffusion anisotropy in channel networks, in this concept the diffusants are assumed “to lose their memory” on moving through a channel segment rather than a channel intersection. The pros and cons of this novel approach are illustrated by analysing own simulations with 1-butene as a diffusant.

19-O-04-Interference microscopy as a tool of choice for investigating the role of crystal morphology in diffusion studies

Publisher Summary This chapter discusses interference microscopy as a tool of choice for exploring the role of crystal morphology in diffusion studies. Interference microscopy is applied to carry out investigations of the influence of the regular intergrowth effects commonly observed in large silicalite crystals on adsorption or desorption of adsorbate molecules. The intracrystaline concentration profiles measured by the interference microscopy during the adsorption of isobutane are compared with those simulated using the Monte Carlo method. The comparison of the simulated and the measured profiles helps to rule out the uptake of isobutane from gas phase into silicalite crystals directly through the internal interfaces separating the intergrowth sections of the crystals.

Percolation diffusion of guest molecules in NaCaA zeolites: field gradient NMR studies and Monte Carlo simulations

Percolation diffusion of guest molecules in NaCaA zeolite is simulated by the Monte Carlo method and monitored by the pulsed field gradient (PFG) nuclear magnetic resonance (NMR) technique. The simulations are performed to evaluate possible manifestations of anomalous percolation diffusion in NaCaA zeolite observable by PFG NMR. The observation of normal diffusion of ethane in NaCaA by PFG NMR was found to be in agreement with the results of Monte Carlo simulations. The conditions of the observation of anomalous percolation diffusion by PFG NMR are discussed.

Monitoring the intracrystalline distribution of guest molecules in zeolites

Abstract Interference and FTIR microscopy are applied to study intracrystalline concentration profiles of guest molecules in large SAPO-5, CrAPO-5 and ferrierite zeolite crystals. By using both techniques, the high spatial resolution of interference microscopy is complemented by the ability of FTIR spectroscopy to pinpoint adsorbates by their characteristic IR bands. In this way the experimental results are shown to unveil a number of remarkable deviations in the real structure of zeolite crystals from their textbook patterns. Combining Interference and FTIR microscopy with dynamic Monte-Carlo simulations the influence of the internal structure and surface barriers on intracrystalline diffusion was investigated quantitatively.