Adsorption and diffusion of alkanes in CuBTC crystals investigated using infra-red microscopy and molecular simulations

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, Ð_i, on the fractional occupancy, Ð_i. Both IRM experiments and simulations show that the Ð_i-θ_i behavior appears to be influenced by the loading dependence of the inverse thermodynamic factor 1/Γ _i=dlnθ_i/dln ρ_i. For nC4 and iC4, the Ð_i increase sharply by about one order of magnitude as the occupancy θ_i 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 4.6 Å windows at low loadings to hops across large 9 Å windows for loadings higher than 8 molecules per unit cell. For loadings higher than 8 molecules per unit cell the Ð_i-θ_i dependence of nC4 and iC4 is dictated by 1/Γ_i, 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 Ð_i follows the occupancy dependence of 1/Γ_i for the entire range of loadings.