Note: Chevron, Energy Technology Company, 100 Chevron Way, Richmond, California 94802-0627 Reference EPFL-ARTICLE-200588doi:10.1021/cr8002642 URL: http://pubs3.acs.org/acs/journals/doilookup?in_doi=10.1021/cr8002642 URL: http://pubs.acs.org/doi/pdfplus/10.1021/cr8002642 Record created on 2014-08-14, modified on 2017-05-12

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

Molecular dynamics simulations are used to study the flow of methane, ethane, and ethylene through carbon nanotubes at room temperature. The interatomic forces in the simulations are calculated using a classical, reactive, empirical bond-order hydrocarbon potential coupled to Lennard−Jones potentials. The simulations show that the intermolecular and molecule−nanotube interactions strongly affect both dynamic molecular flow and molecular diffusion. For example, molecules with initial hyperthermal velocities slowed to thermal velocities in nanotubes with diameters less than 36 A. In addition, molecules moving at thermal velocities are predicted to diffuse from areas of high density to areas of low density through the nanotubes. Normal-mode molecular thermal diffusion is predicted for methane for nearly all the nanotube diameters considered. In contrast, ethane and ethylene are predicted to diffuse by normal mode, single-file mode, or at a rate that is transitional between normal-mode and single-file diffusi...

/pdf/a-computational-study-of-molecular-diffusion-and-dynamic-3gzpg7v1b3.pdf

A Computational Study of Molecular Diffusion and Dynamic Flow through Carbon Nanotubes

Molecular modeling has become a useful and widely used tool to predict diffusion coefficients of molecules adsorbed in the pores of zeolites and other nanoporous materials. These simulations also provide detailed, molecular-level information about sorbate structure, dynamics, and diffusion mechanisms. We review recent advances in this field, including prediction of various transport coefficients (Fickian, Onsager, Maxwell–Stefan) for single-component and multicomponent systems from equilibrium and non-equilibrium molecular dynamics (MD) simulations, elucidation of anomalous diffusion effects induced by the confining pore structure, and prediction of slow diffusion processes beyond the reach of MD simulations.

Recent developments in the molecular modeling of diffusion in nanoporous materials

Accurate and efficient predictions of hydrocarbon diffusivities in zeolitic imidazolate frameworks (ZIFs) are challenging, due to the small pore size of materials such as ZIF-8 and the wide range of diffusion time scales of hydrocarbon molecules in ZIFs. Here we have computationally measured the hopping rates of 15 different molecules (kinetic diameters of 2.66–5.10 A) in ZIF-8 via dynamically corrected transition state theory (dcTST). Umbrella sampling combined with the one-dimensional weighted histogram analysis method (WHAM) was used to calculate the diffusion free energy barriers. Both the umbrella sampling and dynamical correction calculations included ZIF-8 flexibility, which is found to be critical in accurately describing molecular diffusion in this material. Comparison of the computed diffusivities to extant experimental results shows remarkable agreement within an order of magnitude for all the molecules. The dcTST method was also applied to study the effect of hydrocarbon loadings. Self and tra...

Temperature and Loading-Dependent Diffusion of Light Hydrocarbons in ZIF-8 as Predicted Through Fully Flexible Molecular Simulations.

A dynamically corrected transition state theory method is presented that is capable of computing quantitatively the self-diffusivity of adsorbed molecules in confined systems at nonzero loading. This extention to traditional transition state theory is free of additional assumptions and yields a diffusivity identical to that obtained by conventional molecular-dynamics simulations. While molecular-dynamics calculations are limited to relatively fast diffusing molecules, our approach extends the range of accessible time scales significantly beyond currently available methods. We show results for methane, ethane, and propane in LTL- and LTA-type zeolites over a wide range of temperatures and loadings, and demonstrate the extensibility of the method to mixtures.

/pdf/molecular-simulation-of-loading-dependent-diffusion-in-3agm3iugbg.pdf

Molecular simulation of loading-dependent diffusion in nanoporous materials using extended dynamically corrected transition state theory

The diffusion of benzene and p-xylene in zeolite NaY at infinite dilution is studied by means of constrained reaction coordinate dynamics. Molecular dynamics (MD) simulations are used to determine the potential of mean force along the coordinate perpendicular to the window connecting two supercages of the zeolite. A comparison of the results of simulations with a fixed and with a flexible lattice shows that the consideration of lattice flexibility does not significantly influence the potential of mean force for cage-to-cage jumps. The changes in the orientation of the aromatic ring during the cage-to-cage jump are explicitly examined. The transition states for cage-to-cage migration are defined by a plane perpendicular to the reference coordinate. The components of the force along the reaction coordinate are studied as functions of the position, and the orientation is studied for the transition states. Differences between the results for benzene and p-xylene are discussed.

Diffusion of aromatic molecules in zeolite nay. 1. constrained reaction coordinate dynamics

Molecular dynamics simulations of individual trajectories that are started in the transition state for cage-to-cage diffusion are presented for benzene and p-xylene in zeolite NaY. Furthermore, diffusion coefficients and the activation energies for the diffusion are determined from a hopping model that considers dynamical corrections. The transmission coefficients for both molecules are small, particularly at low temperatures. The influence of the window adsorption site of benzene on the transmission coefficient is investigated. The hopping model fails for the treatment of the diffusion of benzene at high temperatures T > 700 K, because the diffusion cannot be divided into different jump events at such temperatures. The results for the diffusion coefficient of p-xylene are in very good agreement with experimental results, whereas for benzene an activation energy results that is slightly too high. This fact is related to the possibility of a dependence of the activation energy on the exact positions of the...

Thomas Mosell

Papers

Diffusion of aromatic molecules in zeolite nay. 1. constrained reaction coordinate dynamics

Diffusion of Aromatic Molecules in Zeolite NaY. 2. Dynamical Corrections