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Diffusion of methane, ethane, propane and n-butane was studied within the micropores of several metal-organic frameworks (MOFs) of varying topologies, including the MOFs PCN-14, NU-125, NU-1100 and DUT-49. Diffusion coefficients of the pure components, as well as methane/ethane, methane/propane and methane/butane binary mixtures, were calculated using molecular dynamics simulations to understand the effect of the longer alkanes on uptake of natural gas in MOFs. The calculated self-diffusion coefficients of all four components are on the order of 10 −8 m 2 /s. The diffusion coefficients of the pure components decrease as a function of chain length in all of the MOFs studied and show different behaviour as a function of loading in different MOFs. The self-diffusivities follow the trend D PCN-14 NU-125≈NU-1100>DUT-49. By comparing the diffusion of pure methane and methane mixtures with the higher alkanes, it is observed that the diffusivity of methane is unaffected by the presence of the higher alkanes in the MOFs considered, indicating that the diffusion path of methane is not blocked by the higher alkanes present in natural gas. [ABSTRACT FROM AUTHOR]

Highlights: [•] Calculated hydrogen Maxwell–Stefan diffusivity in Mg-functionalized MOFs. [•] Highlighted relationship between hydrogen heat of adsorption and M–S diffusivity. [•] Showed M–S diffusivity decreases exponentially with increasing heat of adsorption. [•] Lessons from this study can be applied to diffusion in other host/guest systems. [ABSTRACT FROM AUTHOR]

Knudsen dynamics simulations were performed in cylindrical pores with multiple sections of different diameters. The number and location of hits between particles and the pore walls can be related to the known distributions in the uniform diameter cases. Both sweep-gas and pass-through modes of operation were examined. The contact of the particles traveling inside the pores with the pore walls can be regulated by adjusting the aspect ratio (section length/section diameter) of the pore sections and the ratio of their cross-sectional areas. The results were found to be independent of the transition angle between sections except for very smooth transitions. Analytical expressions were developed to relate the transmission probability in pores of multiple sections to the transmission probabilities of the constituent sections. [Copyright &y& Elsevier]

Thin-film membranes have been made from cavity-containing supramolecular building blocks known as molecular squares and evaluated for simple test separations. Permeation properties of single components and binary gas mixtures were measured through membranes made of several different molecular squares. For a small square with pyrazine edges, permeation increased in the order cyclohexane, consistent with the selectivities predicted from the sorption coefficients. Diffusion rates also play an important role in the permeation selectivity. For a larger square with porphyrin edges, mixture selectivities for 4-picoline over toluene were observed to be around 7–9, depending on the partial pressure. Functionalization of the interior of the porpyrin square allowed a mixture selectivity of 7 for benzene over cyclohexane. Independently measured adsorption isotherms and effective diffusion coefficients proved very helpful in understanding the effects of sorption and diffusion on the observed permeabilities. [Copyright &y& Elsevier]