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Zeolites of type ferrierite are exploited as a host system for monitoring the evolution of guest concentration (methanol) in nanoporous host materials upon adsorption. Additional transport resistances at the crystal surface have been removed so that uptake is exclusively controlled by the diffusion resistance of the pore space. Since the crystal shape deviates from a simple parallelepiped, the primary imaging data do not immediately reflect true local concentrations. A simple algorithm is developed which overcomes this complication. The determined transient concentration profiles ideally comply with the requirements for the application of the Boltzmann-Matano integration method for determining diffusivities. The resulting diffusivities (along the direction of the '10-ring channels') are found to exceed those along the 8-ring channels by three orders of magnitude. [ABSTRACT FROM AUTHOR]

Abstract: The self-diffusion of three equimolar mixtures in the metal-organic framework Zn(tbip) has been investigated by molecular dynamics simulations. The simulations take due account of lattice flexibility. The diffusional characteristics are discussed in relation to molecule properties and lattice geometry. The results show that Zn(tbip) may be a useful material for separating methane/ethane and /ethane mixtures at low concentrations, and /methanol mixtures at high concentrations. [Copyright &y& Elsevier]

Abstract: Adsorption and diffusion of n-butane/iso-butane mixtures in individual silicalite-1 crystals has been investigated using infrared (IR) microscopy. The equilibrium sorption isotherm for an equimolar gas phase mixture is calculated using Configurational Bias Monte-Carlo simulations. The comparison between simulation results and the experimental data enabled the determination of absolute values of concentration in the IR experiments. n-Butane uptake under presence of iso-butane and, vice versa, iso-butane uptake under presence of n-butane have been studied experimentally. It is shown that the n-butane counter-uptake is not limited by the n-butane diffusivity, but by the availability of free sites, which is, in turn, determined by the mobility of iso-butane. A site-percolation threshold, given by the number of iso-butane molecules blocking the ‘traffic junctions’ of the channel network, has been employed for explaining the occurrence of network regions which are initially inaccessible for n-butane in the counter-uptake process. [Copyright &y& Elsevier]

Abstract: The adsorption and intra-crystalline diffusion of n-butane (nC4), iso-butane (iC4), 2-methylbutane (2MB), and 2,2-dimethylpropane (neoP) in CuBTC (Cu3(BTC)2 where BTC=benzene-1,3,5-tricarboxylate) has been investigated using infrared microscopy (IRM), combined with molecular simulations. Both experiments and simulations show strong inflection characteristics in the adsorption isotherms. The primary cause of the inflection is due to the strong preference for adsorption within, and in the regions close to the mouths of tetrahedral pockets. The isotherm inflection has a significant influence on the dependence of the Maxwell–Stefan diffusivity, Ði , on the fractional occupancy, Ði . Both IRM experiments and simulations show that the Ði –θi behavior appears to be influenced by the loading dependence of the inverse thermodynamic factor . For nC4 and iC4, the Ði increase sharply by about one order of magnitude as the occupancy θi increases from 0 to about 0.2. This increase is caused by the fact that the diffusion characteristics undergo a transition from being dominated by hops across narrow 4.6Å windows at low loadings to hops across large 9Å windows for loadings higher than 8 molecules per unit cell. For loadings higher than 8 molecules per unit cell the Ði –θi dependence of nC4 and iC4 is dictated by 1/Γi , characteristic of more open channel structures such as FAU zeolite. For 2MB and neoP, the IRM experiments show evidence of phase transition and except for the region in which this occurs the Ði follows the occupancy dependence of 1/Γi for the entire range of loadings. [Copyright &y& Elsevier]

There is experimental evidence from adsorption and permeation studies that methane can enter the Zeolitic Imidazolate Framework (ZIF)-90 framework despite the fact that the critical diameter of methane (3.8 Å) is larger than the window size of ZIF-90 (3.5 Å) assuming a rigid framework. Therefore, adsorption and diffusion of methane in the ZIF-90 were investigated by Molecular Dynamics (MD) and Gibbs Ensemble Monte Carlo. Various interaction force fields have been tested and a suitable one has been proposed. Results of structural and dynamical properties of methane in ZIF-90 are presented. Like for methane in ZIF-8, no gate opening was found in this paper for methane in ZIF-90 up to a pressure of 260 bar. Therefore, the adsorption and diffusion of methane in ZIF-90 can be explained by a flexible framework. [ABSTRACT FROM AUTHOR]

Adsorption and diffusion of hydrogen in the metal organic framework ZIF-11 will be discussed in this paper using molecular dynamics (MD) and Gibbs Monte Carlo (GMC) computer simulations. Reliable force fields, needed for these simulations, are only partially available. Adsorption simulations, in comparison with experiments, have been used to fit some missing interaction parameters. The lattice flexibility turns out to have relatively small influence on adsorption. The obtained parameter set has been used to investigate the structure of ZIF-11 and the self-diffusion of hydrogen within the flexible framework. [ABSTRACT FROM AUTHOR]

Highlights: [•] Guest induced gate opening is examined by MD simulations of N2 in flexible ZIF-8. [•] The parameter set used reproduces XRD measurements of the size of the windows. [•] Density distributions and radial correlation functions give insight and explanations. [ABSTRACT FROM AUTHOR]

Abstract: Recent adsorption, diffusion and permeation studies give experimental evidence that ethane can easily adsorb and diffuse in the ZIF-8 framework. This finding is surprising since the pore window of ZIF-8 – derived from XRD data – is virtually too small for ethane migration. Therefore, the diffusion mechanism of ethane in ZIF-8 and the influence of the diffusing guest molecules on the flexible lattice are investigated in detail in this paper with focus on the diffusion limiting step, the passage of the ZIF-8 window between adjacent cavities. Contrary to zeolite research, well established interaction parameter sets are still not available for MOF simulations. Hence, a large part of this paper is devoted the comparison of several parameter sets for the molecular interactions. Also, the conservation not only of the unit box size in NPT simulations but also the conservation of the geometrical structure of the lattice (most important the “windows” between adjacent pores) has been examined. Employing a suitable parameter set, two interesting effects can be observed: (i) The window size of ZIF-8 is smaller at higher ethane loadings, while simultaneously (ii) at higher loadings the guest molecules within the cavity exert forces that press a given probe molecule toward the window. These two effects nearly compensate each other leading to a weak dependence of the self-diffusivity upon the concentration of guest molecules. However, no gate-opening mechanism by a torsional motion of the linker molecule methylimidazolate could be found in our MD simulations because of a too large activation energy for this motion. [Copyright &y& Elsevier]