Diffusion of hydrocarbon mixtures in MFI zeolite: Influence of intersection blocking

Branched and cyclic hydrocarbons such as iso-butane, 2-methylpentane, 3-methylpentane, 2,2-dimethyl-butane and benzene are preferentially adsorbed at the intersections of the channels of MFI zeolite; they serve as bottlenecks for molecular traffic in mixtures with linear C1–C6 alkanes. Molecular dynamics simulations show that as the loadings of tardier iC4, 2MP, 3MP, 22DMB, and Bz is progressively increased to four molecules per unit cell the diffusivity of the more mobile linear alkane reduces nearly to zero. The reduction in the n-alkane diffusivity is quantitatively similar irrespective of the branched/cyclic hydrocarbon.     

A Molecular Dynamics investigation of the influence of framework flexibility on self-diffusivity of ethane in Zn(tbip) frameworks

Configurational-Bias Monte Carlo simulations of the adsorption isotherm of ethane in Zn(tbip) (H₂tbip = 5-tert-butyl isophthalic acid), a representative of metal-organic frameworks, show an inflection at a loading, q_i = 6 molecules per unit cell. This inflection causes the inverse thermodynamic factor, 1/Γ_i, to display a minimum at q_i = 6, along with a maximum at q_i ≈ 10. Molecular Dynamics (MD) simulations of the self-diffusivity D_i,self, taking account of the framework flexibility of Zn(tbip) show that the D_i,self − q_i dependence follows that of 1/Γ_i − q_i, with a minimum at q_i = 6. Remarkably, MD simulations assuming a rigid framework yield significantly lower values of the self-diffusivities, and show a monotonic decrease of D_i,self with q_i.     

The composition dependence of self and transport diffusivities from molecular dynamics simulations

Using molecular dynamics (MD) simulations, we determine the composition dependence of the self-diffusivity and transport diffusivity of a methane/ethane mixture at high pressure. The transport diffusivity is generated from the self-diffusivities using the Darken equation. We perform a careful analysis of the molecular dynamics simulations and show that it is possible to reproduce the results in the microcanonical, canonical, and isobaric–isothermal ensembles. We demonstrate that in order to capture the sensitive dependence of the diffusivities on composition, it is necessary to run simulations with larger systems and for longer durations than is typical. We report the trends in the diffusivities as a function of composition, temperature, pressure, and density. We modify an existing empirical correlation, which when combined with a corresponding states chart, is capable of quantitatively reproducing the simulated diffusivity dependence on composition, temperature, pressure, and density. Finally, we quantify the effect that the choice of equation of state (EOS) used to evaluate the thermodynamic factor in the Darken equation has on the transport diffusivity.     

An investigation of the characteristics of Maxwell-Stefan diffusivities of binary mixtures in silica nanopores

Diffusion of pure components (hydrogen (H₂), argon (Ar), krypton (Kr), methane (C1), ethane (C2), propane (C3), n-butane (nC4), and n-hexane (nC6)) in silica nanopores with diameters of 1, 1.5, 2, 3, 4, 5.8, 7.6, and 10 nm were investigated using molecular dynamics (MD). The Maxwell-Stefan (M-S) diffusivity (?_i,s) and self-diffusivities (D_i,self,s) were determined for pore loadings ranging to 10 molecules nm⁻³. The MD simulations show that zero-loading diffusivity ?_i,s(0) is consistently lower, by up to a factor of 10, than the values anticipated by the classical Knudsen formula; the differences increase with increasing adsorption strength. Only when the adsorption is negligible does the ?_i(0) approach the Knudsen diffusivity value.MD simulations of diffusion in binary mixtures C1-H₂, C1-Ar, C1-C2, C1-C3, C1-nC4, C1-nC6, C2-nC4, C2-nC6, and nC4-nC6 in the different pores were also performed to determine the three parameters ?_1,s, ?_2,s, and ?₁₂, arising in the M-S formulation for binary mixture diffusion. The ?_i,s in the mixture were found to be practically the same as the values obtained for unary diffusion, when compared at the same total pore loading. Also, the ?_i,s of any component was practically the same, irrespective of the partner molecules in the mixture. Furthermore the intermolecular species interaction parameter ?₁₂, could be identified with the binary M-S diffusivity in a fluid mixture at the same concentration as within the silica nanopore. The obtained results underline the overwhelming advantages of the M-S theory for mixture diffusion in nanopores.Our study underlines the limitations of the commonly used dusty-gas approach to pore diffusion in which Knudsen and surface diffusion mechanisms are considered to be additive.