Transport Diffusivities of CH4, CF4, He, Ne, Ar, Xe, and SF6 in Silicalite from Atomistic Simulations

Skoulidas, Anastasios I.; Sholl, David S.

doi:10.1021/jp014279x

Cited by 215 publications

(263 citation statements)

References 44 publications

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“…͑11͔͒, unlike the case of mesoporous materials in which the possibility of a dual transport mechanism involving gas and surface diffusions is well documented. The steric effects that cause a rapid decrease in self-diffusivities as a function of loading in micropores, 61,62 also exist in rough amorphous mesoporous materials, as we have just demonstrated. It is suggested that whenever trapping occurs in microporous or mesoporous materials, selfdiffusivities will rapidly decrease with pore loading, but transport ͑collective͒ diffusivities will be a much weaker function of pore loading.…”

Section: ͑11͒supporting

confidence: 57%

Knudsen self- and Fickian diffusion in rough nanoporous media

Malek

Coppens

2003

The Journal of Chemical Physics

247

147

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The effect of pore surface roughness on Knudsen diffusion in nanoporous media is investigated by dynamic Monte Carlo simulations and analytical calculations. A conceptual difference is found between the roughness dependence of the macroscopic, transport diffusivity and the microscopic, self-diffusivity, which is reminiscent of diffusion in zeolites, where a similar difference arises due to adsorption effects and intermolecular interactions. Because of the dependence of the self-diffusivity on molecular residence times, self-diffusion may be roughness dependent, while transport diffusion is not. Detailed proofs are given. The differences become significant when the pore surface is rough down to molecular scales, as is the case, e.g., for many common sol-gel materials. Simulations are in good agreement with analytical calculations for several tested rough, fractal pore structures. These results are important for the interpretation of experimental diffusion measurements and for the study of diffusion-reaction processes in nanoporous catalysts with a rough internal surface.

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Section: ͑11͒supporting

confidence: 57%

Knudsen self- and Fickian diffusion in rough nanoporous media

Malek

Coppens

2003

The Journal of Chemical Physics

247

147

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“…The Lennard-Jones parameters for N 2 -zeolite interactions are due to Makrodimitris et al [13]. The force fields for Ne and Ar are those listed in Skoulidas and Sholl [14]. The force field for Kr was taken from Talu and Myers [15].…”

Section: Molecular Simulation Methodologymentioning

confidence: 99%

Loading Dependence of Self‐Diffusivities of Gases in Zeolites

Krishna

Baten

2007

Chem Eng & Technol

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Experimental data on the self-diffusivities, D i,self , of a variety of gases (CH 4 , N 2 , Kr, C 2 H 6 , and C 3 H 8 ) in three different zeolites, LTA, FAU, and MFI, show different dependences on the molar loading, q i . In LTA, D i,self appears to increase with q i for all molecules except N 2 . In FAU and in MFI the D i,self shows a sharp decrease with increasing q i . In order to gain insights into the causes behind the loading dependences, molecular dynamics (MD) simulations were carried out to determine the self-diffusivities of seven gases (CH 4 , N 2 , Kr, C 2 H 6 , C 3 H 8 , Ar, and Ne) in six different all-silica zeolite structures (MFI, AFI, FAU, CHA, DDR, and LTA). The simulation results show that the variation of D i,self with q i is determined by a variety of factors that include molecular size and shape, and degree of confinement within the zeolite. For one-dimensional channels (AFI) and intersecting channel structures (MFI), the D i,self invariably decreases with increasing q i . For zeolite structures that consist of cages separated by windows (FAU, CHA, DDR, LTA), the size of the windows is an important determinant. When the windows are wide (FAU), the D i,self decreases with q i for all molecules. If the windows are narrow (CHA, DDR and LTA), the D i,self often exhibits a sharp increase with q i , reaches a maximum and reduces to near-zero values at saturation. The sharpness with which D i,self increases with q i , is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of D i,self with q i .

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“…In most practical cases, close to saturation the increase in chemical potential needed to add an additional molecule is so large that one obtains a unique saturation loading. However, as pointed out by Skoulidas and Sholl, 139 this procedure depends on the range of data that are fitted. As an alternative, they proposed to define the maximum loading as the loading where the isosteric heat of adsorption is zero.…”

Section: Adsorption Isothermsmentioning

confidence: 99%