Propane/propylene diffusion in zeolites: framework dynamics (original) (raw)
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Molecular Dynamics Simulations of the Diffusion of Small Chain Hydrocarbons in 8-Ring Zeolites†
The Journal of Physical …, 2010
Molecular dynamics (MD) simulations were performed to study the microscopic motion of methane, ethane, propene, and propane adsorbed in three pure silica zeolites with windows made of 8 SiO 4 tetrahedral units: Si-LTA, Si-IHW, and Si-ITE. The zeolite framework and guest structures have been simulated allowing full flexibility, using the well-known BKS model and the potential of Oie et al., respectively. The MD approach followed allows us to calculate the intra-and intercage dynamics of the smaller adsorbates, that is, methane and ethane, in statistically meaningful time and length scales in the temperature range studied, whereas for the larger size guest molecules the analysis of intercage motion is limited to higher temperatures. Calculated self-diffusion coefficients for methane, ethane, and propene show a decreasing trend correlated with increasing guest kinetic diameter sizes and decreasing critical window size, confirming experimental measurements on the molecular sieving properties of 8-ring zeolite frameworks. The microscopic motion of propane diffusing in Si-ITE suggests a somewhat anomalous diffusion process, which can be related to the levitation effect. Thus, guest diffusion translational motion is shown to be highly influenced by the topological features of the framework, with the dimensionality of the diffusion path exerting the most noticeable influence. The microscopic understanding of the host-guest dynamics can be used to highlight the trade-off between propane/propylene selectivity and diffusional differentiation in these materials. † Part of the "Alfons Baiker Festschrift".
Diffusion of propane, propylene and isobutane in 13X zeolite by molecular dynamics
Chemical Engineering …, 2010
This paper presents single component diffusion data of propane, propylene and isobutane in zeolite 13X obtained by molecular dynamics (MD) simulations, especially its dependence on temperature and concentration. Our results are critically compared to experiments and previous simulation data, when available. One novelty of this work is that the diffusion coefficients are computed taking the framework cations of zeolite 13X into consideration. Furthermore, to our knowledge, we present the first simulation results for propylene diffusion in faujasite frameworks. From the mean squared displacements, self-diffusion coefficients of 7.5×10−9, 9.1×10−9, and 9.6×10−10 m2 s−1 for 2 molecules/unit cell were calculated for propane, propylene, and isobutane at 373 K, respectively. The simulations show that the diffusivity decreases with increasing loadings for all adsorbates studied. Finally, transport diffusivities were estimated from the self-diffusion coefficient and the equilibrium adsorption isotherms by using the Darken equation.
The Journal of Physical Chemistry C, 2013
Molecular dynamics (MD) and transition state theory (TST) methods are becoming efficient tools for predicting diffusion of molecules in nanoporous materials. The accuracy of predictions, however, often depends upon a major assumption that the framework of the material is rigid. This saves a considerable amount of computational time and is often the only method applicable to materials for which accurate force fields to model framework flexibility are not available. In this study, we systematically characterize the effect of framework flexibility on diffusion in four model zeolites (LTA, CHA, ERI, and BIK) that exhibit different patterns of window flexibility. We show that for molecules with kinetic diameters comparable to (or larger than) the size of the window the rigid framework approximation can produce order(s) of magnitude difference in diffusivities as compared to the simulations performed with a fully flexible framework. We also show that simple recipes to include the effect of framework flexibility are not generally accurate. To account for framework flexibility effects efficiently and reliably, we introduce two new methods in which the flexible structure is approximated as a set of discrete rigid snapshots obtained from simulations of dynamics of an empty framework, using either classical or, in principle, ab initio methods. In the first method, we perform MD simulations of diffusion in a usual manner but replace the rigid structure with a new random snapshot at a certain characteristic frequency corresponding to the breathing motion of the window, while keeping positions of adsorbate molecules constant. In the second method, we directly compute cage to cage hopping rates in each rigid snapshot using TST and average over a distribution of snapshots. Excellent agreement is obtained between diffusivities predicted with these two new methods and direct MD simulations using fully flexible structures. Both methods are orders of magnitude more efficient than the simulations with the fully flexible structure. The new methods are broadly applicable for fast and accurate predictions of both infinite dilution and finite loading diffusivities of simple molecules in zeolites and other nanoporous materials, generally without the need for an accurate flexible force field.
Selective Diffusion of C8 Aromatics in a 10 and 12 MR Zeolite. A Molecular Dynamics Study
The Journal of Physical Chemistry B, 1998
Molecular dynamics simulations employing a flexible framework are used to simulate the diffusion of o-and p-xylene in purely siliceous zeolite CIT-1. The simulations are performed at 500 K, investigating two loadings (corresponding to 1 molecule/unit cell and 0.25 molecule/unit cell) for the ortho isomer and one loading (0.25 molecule/unit cell) for the para isomer. For the former system the diffusion coefficient decreases from 7.79 × 10-6 cm 2 /s for the lower to 3.56 × 10-6 cm 2 /s for the higher loading. The diffusion coefficient for the para isomer (25.18 × 10-6 cm 2 /s) is substantially greater. Graphical analyses reveal a jump diffusion mechanism, which in the case of the ortho isomer takes place in the 12 MR channels of the structure, while for the para isomer, incursions into the 10 MR channel are observed. The results of the MD simulations are complemented and reinforced by calculations of the activation energies for the diffusion of the two isomers in the two channels. Diffusivity measurements of both isomers in B-CIT-1 (Si/Al) 35) by FTIR have also been carried out in order to compare the values obtained theoretically and experimentally.
Diffusion of Benzene and Propylene in MCM-22 Zeolite. A Molecular Dynamics Study
Journal of Physical Chemistry B, 1999
Molecular dynamics simulations have been performed to study the diffusion of a mixture of benzene and propylene, for the cumene synthesis process, in purely siliceous MWW (MCM-22), a zeolite containing two separate channel systems: the 10-member ring (MR) sinusoidal and the 12-MR supercages interconnected by 10-MR windows system. The diffusion processes in each channel system of MWW at 650 K have been studied independently. We have found that in order to obtain quantitative or semiquantitative diffusion coefficients, the framework should be optimized. A large diffusivity for propylene in both channel systems, and especially in the supercage system, is observed, whereas benzene is not seen to diffuse in either of the two channel systems, and only intracage mobility is seen in the supercage voids. The positions of minimum energy, where the molecules are expected to react, have been located in both channels. The diffusion of benzene in the supercage system seems to be temperature-activated, and when the temperature is increased, intercage diffusion will probably occur. Radial distribution functions show that condensation reactions between benzene-propylene and propylene-propylene are possible, which indicate the necessity of working in an excess of benzene. The results of the simulations of diffusion suggest that the formation of cumene probably occurs at the external surface or close to the external surface of the MCM-22 zeolite crystals.
Molecular Dynamics Studies of Light Hydrocarbons Diffusion in Zeolites
1997
Molecular dynamics simulation techniques have been used to study the diffusion of methane, ethane, propane and i-butane into the zeolite ZSM-5. From the trajectories, the meansquare displacements were obtained and the diffusion coefficients determined using Einstein's diffusion equation. The results, when compared to the available experimental data, indicate that the simulations can provide a realistic representation of the microscopic process of diffusion into the zeolite pores.
Simulating the properties of small pore silica zeolites using interatomic potentials
Chemical Society Reviews, 2012
Despite the sustained use of forcefield methodologies to study SiO 2 polymorphs few reviews on the subject are available in the literature. The present study is an attempt to help fill this gap, focusing on classical forcefields used to reproduce and predict properties of pure silica zeolites (or zeosils) such as cell parameters, SiO distance and especially pore size. Instead of an exhaustive study we have focused on an application where diffusion of hydrocarbons makes important the use of pure silica zeolites. A particular area of interest is small pore zeosils containing 8-rings as the largest window, which are industrially interesting for their ability to perform kinetic separations of mixtures of C3 hydrocarbon molecules whose dimensions are of similar characteristics. A set of forcefields have been selected from the literature to analyze their accuracy and transferability when predicting structural, mechanical and dynamical properties of small pore pure silica zeolites and their performance at selective diffusion of C3 hydrocarbons.
The Journal of Physical Chemistry C, 2010
Molecular-dynamics simulations are performed to understand the role of host-framework flexibility on the diffusion of methane molecules in the one-dimensional pores of AFI-, LTL-, and MTW-type zeolites. In particular, the impact of the choice of the host model is studied. Dynamically corrected Transition State Theory is used to provide insights into the diffusion mechanism on a molecular level. Free-energy barriers and dynamical correction factors can change significantly by introducing lattice flexibility. In order to understand the phenomenon of free-energy barriers reduction, we investigate the motion of the window atoms. The influence that host-framework flexibility exerts on gas diffusion in zeolites is, generally, a complex function of material, host model, and loading such that transferability of conclusions from one zeolite to the other is not guaranteed.
Journal of the American Chemical Society, 1996
A recently-developed Monte Carlo method is used to simulate the energetics of n-alkanes from butane to decane in a variety of different all-silica zeolite structures (MFI, MOR, FAU, RHO, LTA, and FER). Where possible, the predicted values of the heat of adsorption are compared to experimental data and are generally found to be in good agreement. On the basis of the energetic data, the graphs of heat of adsorption as a function of mean pore diameter appear to show a maximum between 4 and 5 Å. However, close inspection of the location and conformation of the alkanes in small pore zeolites reveals that the molecules adopt highly coiled conformations localized exclusively in regions of maximum void volume. In the case of the small pore zeolites studied heresRHO and LTAsthese maximum void volumes are the R-cages and the alkanes "feel" a larger pore diameter than that generally used to characterize the zeolite (that of the channels). It is necessary to obtain information on the location and conformations of sorbed molecules to fully understand the trends in the heat of adsorption as a function of pore diameter.