Siegfried Fritzsche

About the influence of lattice vibrations on the diffusion of methane in a cation-free LTA zeolite

Abstract The influence of lattice vibrations on the diffusion of methane in a cation-free zeolite of structure Type LTA is examined. It is shown that contrary to earlier published results the self-diffusion coefficients obtained with flexible and with rigid lattices are practically the same. This finding is true over a wide range of temperatures and for different interaction parameters. The reason why earlier papers did not state this independence of D on the lattice vibrations is explained.

On entropic barriers for diffusion in zeolites: A molecular dynamics study

The self-diffusion of ethane in cation-free Linde type A zeolite has been studied by molecular dynamics simulations for various temperatures. These simulations predict that the diffusivity decreases with increasing temperature between 150 K and 300 K for a low loading of one molecule per cage. The rate of cage-to-cage crossings shows the same temperature dependence. We explain this phenomenon based on an analysis of the activation entropy that controls motion through eight-ring windows separating adjacent cages. The diffusivity and the cage-to-cage rate constant both decrease with temperature because heating the system moves ethane away from eight-ring windows, on average, which increases the entropic barrier for cage-to-cage motion.

An MD simulation on the applicability of the diffusion equation for molecules adsorbed in a zeolite

Abstract Diffusion of methane in a cation-free analog of zeolite A is studied by molecular dynamics (MD). The first four moments of the particle displacement profiles are evaluated by MD and compared with expressions derived from the solution of the diffusion equation. Diffusion coefficients from these expressions are found to coincide with each other for times larger than 30–60 ps depending on the occupation number.

Molecular dynamics consideration of the mutual thermalization of guest molecules in zeolites

Abstract Molecular dynamics (MD) is applied to simulate methane self-diffusion in a model zeolite ZK4. It is found that the mutual interaction of the adsorbed molecules is sufficient to guarantee their thermalization, so that in first-order considerations, the energy exchange with the zeolite framework may be neglected.

On the diffusion mechanism of methane in a cation-free zeolite of type ZK4

Abstract Molecular dynamics (MD) simulations were performed in order to examine the mechanism of diffusion of guest molecules in a cation-free zeolite of type ZK4. The peculiarities of molecular motion are illustrated by the probability distribution of the residence times in the large cavities and by the velocity autocorrelation functions in combination with trajectory studies. Depending on the potential parameters used the diffusivities may increase or decrease with increasing concentration. The diffusion coefficients are found to follow an Arrhenius law which is examined for different loadings.

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.

Influence of exchangeable cations on the diffusion of neutral diffusants in zeolites of type LTA. An MD study

Abstract Diffusion of methane in LTA type zeolites has been calculated from molecular dynamics simulations. Taking into account the polarization interaction of the exchangeable cations in the zeolite NaCaA with the methane molecules, the self-diffusion coefficients in NaCaA decrease by one order of magnitude in comparison with its cation-free analogue ZK4. The order of magnitude of the calculated values is in agreement with NMR data.

An MD Study on the Correlation between Transport Diffusion and Self-Diffusion in Zeolites

Non-equilibrium Molecular Dynamics (MD) simulations in a model zeolite of structure type LTA for permeation studies following the Wicke-Kallenbach method are performed. The transport diffusivities are found to increase with increasing concentration. The concentration dependence is satisfactorily represented by the thermodynamic factor dmpl dine. For small concentrations, the transport diffusivities approach the self-diffusivities determined by MD simulations under equilibrium conditions. Molecular transportation in the direction perpendicular to the concentration gradient is unaffected by the non-equilibrium conditions.

Grundlagen der Statistischen Physik

Die zur Berechnung der gewunschten strukturellen, thermodynamischen und kinetischen Daten mittels Computersimulationen benotigten Ausgangsgrosen wie radiale Verteilungsfunktionen, Zustandssummen und Korrelationsfunktionen sowie die dabei benotigten Relationen werden durch die Statistische Physik — Statistische Thermodynamik im Gleichgewicht und Statistische Theorie irreversibler Prozesse im Nicht-Gleichgewicht — bereitgestellt. Die Grundlagen werden im folgenden Kapitel kurz zusammengestellt. Ausfuhrlichere Darstellungen entnimmt man der Literatur [14–18].

The importance of various degrees of freedom in the theoretical study of the diffusion of methane in silicalite-1

The self diffusion coefficient of methane in silicalite-1 is influenced by the flexibility of the lattice unlike the self diffusion coefficient of methane in the cation-free zeolite of type A. In the present paper, besides the influence of lattice vibrations on this process, the influence of internal vibrations of the methane molecule and the applicability of several spherical models of this molecule are examined. The method of moments [Chem. Phys. Lett. 198 (1992) 283] is generalized to anisotropic diffusion.