Zeolite diffusion molecular dynamics

Sastre, G., Catlow, C.R.A., and Corma, A. (1999) Diffusion of benzene and propylene in MCM-22 zeolite a molecular dynamics study. J. Phys. 

Diffusion in Zeolites by Molecular Dynamics Simulations , Mol. SimuL, 2000, 25, 27... 

L. Leherte, J.-M. Andre, E. G. Derouane, and D. P. Vercauteren, /. Chem. Soc., Faraday Trans., 87,1959 (1991). Self-Diffusion of Water into a Ferrierite-Type Zeolite by Molecular Dynamics Simulations. 

Leroy, F., and Rousseau, B. 2004. Self-diffusion of n-alkanes in MFl type zeolite using molecular dynamics simulations with an anisotropic united atom (AUA) forcefield Mol. Simul. Vol. 30 pp 617-620. 

Nuclear magnetic resonance provides means to study molecular dynamics in every state of matter. When going from solid state over liquids to gases, besides mole- cular reorientations, translational diffusion occurs as well. CD4 molecule inserted into a zeolite supercage provides a new specific model system for studies of rotational and translational dynamics by deuteron NMR. 

MO LCAO methods, 34 136 Molecular-beam surface scattering, 26 26, 27 Molecular Cage, 34 226 Molecular design in cyclodextrin, 32 427 Molecular dynamics diffusion in zeolites, 42 2, 4-6 argon, 42 20... 

Skoulidas AI, Sholl DS (2001) Direct tests of the darken approximation for molecular diffusion in zeolites using equilibrium molecular dynamics. J. Phys. Chem. B. 105 3151-3154... 

Molecular-dynamic simulations are characterized by a solution of Newton s laws of motion for the molecules travelling through the zeolite pore system under control of the force field given by the properties of the host lattice, by interactions between the host and the molecules, and by interactions between the molecules. To date this has been possible only for the diffusion of simple molecules (e.g. methane or benzene) inside a zeolite lattice of limited dimensions [29, 37, 54], To take into account the effects of a chemical reaction as well would require quantum-mechanical considerations however, such simulations are in their infancy. 

Abstract The chemical activation of light alkanes by acidic zeolites was studied by a combined Classical Mechanics/Quantum Mechanics approach. The diffusion and adsorption steps were investigated by Molecular Mechanics, Molecular Dynamics and Monte Carlo simulations. The chemical reactions step was studied at the DPT (B3LYP) level with 6-31IG basis sets and 3T and 5T clusters to represent the acid site ofthe zeolite. 

In principle, the diffusion steps (a) and (e) could be studied through molecular dynamics simulations as long as rehable forces fields are available to describe the zeolite structure and its interaction with the substrates. Also, if the adsorption takes place without charge transfer between the reagents/products and the zeolite, steps (b) and (d) could also be investigated either by molecular dynamics or Monte Carlo simulations. Step (c) however can only be followed by quantum mechanical techniques because the available force fields cannot yet describe the breaking and formation of chemical bonds. 

The diffusion step was investigated by molecular dynamics (MD) simulations. For the adsorption step, molecular mechanics (MM), molecular dynamics (MD) and Monte Carlo (MC) techniques were used, under the same simulation conditions, in order to compare their performance. The influence of the force-field, the loading of alkane molecules and the relaxation of the zeolite framework on the adsorption energies were also investigated. For the chemical reaction... 

It has been demonstrated that the combined application of various NMR techniques for observing molecular rotations and migrations on different time scales can contribute to a deeper understanding of the elementary steps of molecular diffusion in zeolite catalysts. The NMR results (self-diffusion coefficients, anisotropic diffiisivities, jump lengths, and residence times) can be correlated with corresponding neutron scattering data and sorption kinetics as well as molecular dynamics calculations, thus giving a comprehensive picture of molecular motions in porous solids. 

Of all the porous solids, diffusion in zeolites has certainly been studied most extensively, in part because there seemed to be an enormous difference between macroscopic and microscopic diffusion constants (from MD and from NMR). It is not practical to discuss all this work here, but references to other such molecular dynamics simulations are given in the papers of [69]. 

P. Santikary and S. Yashonath, Molecular Dynamics Investigation of Sorption of Argon in NaCaA Zeolite, J. Chem. Soc. Faraday Trans. 88 (1992) 1063-1066 C. Fritzsche, R. Haberlandt, J. Kaerger, H. Pfeifer and K. Heinsinger, A MD Simulation on the Applicability of the Diffusion Equation for Molecules Adsorbed in a Zeolite, Chem. Phys. Lett. 198 (1992) 283-287. 

Paschek D and Krishna R. Diffusion of binary mixtures in zeolites Kinetic Monte Carlo versus molecular dynamics simulations. 

R. C. Runnebaum and E. J. Maginn, /. Phys. Chem. B, 101, 6394 (1997). Molecular Dynamics Simulations of Alkanes in the Zeolite Silicalite Evidence for Resonant Diffusion Effects. 

P. Demontis and G. B. Suffritti, in Zeolites and Related Microporous Materials State of the Art 1994, J. Weitkamp, H. G. Karge, H. Pfeifer, and W. Holderich, Eds., Elsevier Science Publishers, Amsterdam, 1994, pp. 2107-2113. Molecular Dynamics Simulations of Diffusion in a Cubic Symmetry Zeolite. 

Force-field calculations could be simple energy minimization or advanced monte-carlo and molecular dynamics calculations. The major assumption here is the transferability of force-field parameters among the related materials. These calculations can provide wealth of information such as the relative ordering of adsorption sites on surface, diffusion mechanism of molecules particularly inside zeolites, energy barrier for difihision, diffusion coefficients, heats of adsorption and more importantly, the effect of temperature on all these properties. 

Molecular dynamics (MD) simulations have been used to simulate non-equilibrium binary diffusion in zeolites. Highly anisotropic diffusion in boggsite provides evidence in support of molecular traffic control. For mixtures in faujasite, Fickian, or transport, diffusivities have been obtained from equilibrium MD through appropriate correlation functions and used in macroscopic models to predict fluxes through zeolite membranes under co- and counterdiffusion conditions. For some systems, MD cannot access the relevant time scales for diffusion, and more appropriate simulation techniques are being developed. 

Abstract As a non-invasive technique, NMR spectroscopy allows the observation of molecular transport in porous media without any disturbance of their intrinsic molecular dynamics. The space scale of the diffusion phenomena accessible by NMR ranges from the elementary steps (as studied, e.g., by line-shape analysis or relaxometry) up to macroscopic dimensions. Being able to follow molecular diffusion paths from ca. 100 nm up to ca. 100 xm, PPG NMR has proven to be a particularly versatile tool for diffusion studies in heterogeneous systems. With respect to zeolites, PFG NMR is able to provide direct information about the rate of molecular migration in the intracrystalline space and through assemblages of zeolite crystallites as well as about possible transport resistances on the outer surface of the crystallites (surface barriers). 

The derivation of Dt from coherent QENS is similar to a computation of Dt from the fluctuations in an equilibrium density distribution. This was accomplished by Tepper and co-workers for Ar in AIPO4-5 [6]. Using the Green-Kubo formahsm, they were able to extract this non-equilibrium quantity from just one equihbrium simulation. Moreover, the calculations being performed in reciprocal space, the variation of the diffusivity upon the wave vector was used to check when the system was in the linear regime [6]. The first application of non-equihbrium molecular dynamics (NEMD) to zeolites was performed by Maginn et al. on methane in sihcalite [7]. Standard equi-libriiun MD techniques were later used by Sholl and co-workers to determine the concentration dependence of diffusivities [8]. 

In recent molecular dynamics (MD) studies of propane in Na-Y zeolite, the HWHM obtained from the simulations have been compared with jump diffusion models and with the experiment [19]. Figure 4 shows fits of different models to the MD data, at three temperatures. The error bars on the MD points is too large to select the best model. However, the oscillatory behavior expected for the CE model does not seem to be present either in the MD data or in the experimental QENS broadenings (Fig. 5). 

Molecular dynamics calculations have also been carried out on a zeolite A system using the above force field. Simulations were carried out over a range of temperatures using a 12 ion model, and a full cell which contains 96 ions. It was shown that even at room temperature there is not much motion of the ions they tend to oscillate about their original positions. When the temperature was raised to 600K a marked difference was noted, and it was found that over a time scale of ca. 2ps there is a bulk movement of ions which occupy the sites in the 8-rings, in a concerted transportation process, This is found to repeat every Bps and shows evidence that bulk diffusion of ions through the structure may take place. 

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