Andreas Schüring

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.

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.

Modelling Diffusion in Zeolites by Molecular Dynamics Simulations

Abstract The diffusion of molecules sorbed in zeolites is of growing interest for understanding the mechanisms of chemical processes with regard to selectivity and reactivity [1]. MD simulations give insight into physical systems on the molecular level allowing to study and visualize the motion of molecules even beyond the possibilities of experiments [2,3]. Single system parameters can easily be varied to study their influence, also those parameters that are fixed in reality (e.g., the size of particles). We present a cross section of our recent work to illustrate the capabilities of MD: The self diffusion coefficients (D) of a mixture of methane and xenon in silicalite show remarkable deviations from those of the pure species. This is shown and confirmed by PFG NMR experiments [4]. Simulating ethane in zeolite A the mechanism of diffusion has been studied. The effects of rotation on the diffusion lead to cases where D decreases with growing temperature [5]. The independence of self diffusion on lattice vibrations is proven even for zeolites with windows of guest particle size comparing simulations with rigid and vibrating zeolite lattice [6].

Random walk treatment of dumb-bell molecules in an LTA zeolite and in chabazite

Abstract We analyse the mechanisms of self diffusion in zeolite-guest systems which show a non-monotonic temperature dependence of the self-diffusion coefficient D: ethane in a cation-free LTA zeolite [1] and chlorine in silicious chabazite. In these systems D is influenced by jump rates for crossing energetic barriers as well as for crossing entropic barries [1]. The entropy-controlled jump rates were found to decrease with increasing temperature. Employing random walk descriptions, simple analytical formulas are derived which relate the self-diffusion coefficient of the guest molecules with the jump rates for crossing the different barriers. The simple ansatz we use can be transfered to other zeolite-guest systems.

A new type of diffusional boundary effect at the edges of single-file channels

The tracer exchange of guest molecules between one-dimensional channels of zeolites and the surrounding gas phase is studied by molecular dynamics and Monte Carlo simulations under the conditions of single-file diffusion. The shape of the resulting concentration-distance curves reveals a novel diffusional boundary effect. For the same degree of tracer exchange the molecular concentration near the channel boundaries is always closer to the equilibrium concentration in the case of single-file diffusion than in the case of normal diffusion. Using analytical considerations we predict the length of the boundary region of channels showing this deviation and find good coincidence with the corresponding results of simulations.

Influence of the Methane–Zeolite a Interaction Potential on the Concentration Dependence of Self-Diffusivity

Studies on the diffusion of methane in a zeolite structure type LTA (as per IZA nomenclature) have indicated that different types of methane–zeolite potentials exist in the literature in which methane is treated within the united-atom model. One set of potentials, referred to as model A, has a methane oxygen diameter of 3.14 Å, while another set of potential parameters, model B, employs a larger value of 3.46 Å. Fritzsche and co-workers (1993) have shown that these two potentials lead to two distinctly different energetic barriers for the passage of methane through the eight-ring window in the cation-free form of zeolite A. Here, we compute the variation of the self-diffusivity (D) with loading (c) for these two types of potentials and show that this slight variation in the diameter changes the concentration dependence qualitatively: thus, D decreases monotonically with c for model A, while D increases and goes through a maximum before finally decreasing for model B. This effect and the surprising congruence of the diffusion coefficients for both models at high loadings is examined in detail at the molecular level. Simulations for different temperatures reveal the Arrhenius behaviour of the self-diffusion coefficient. The apparent activation energy is found to vary with the loading. We conclude that beside the cage-to-cage jumps, which are essential for the migration of the guest molecules, at high concentrations migration within the cage and guest–guest interactions with other molecules become increasingly dominant influences on the diffusion coefficient and make the guest–zeolite interaction less important for both model A and model B.

16-P-18 - The mutual influence of dynamic processes acting in different time scales

Publisher Summary This chapter discusses the diffusion and relaxation processes of different nonspherical guest molecules in zeolites by using molecular dynamics (MD) simulations with rigid and flexible lattice and molecules. Interrelations among such processes are studied by different tools, such as correlation functions. The results show that there exist cases where a strong mutual influence can be found and others in which there is only small influence.