Influence of adsorption thermodynamics on guest diffusivities in nanoporous crystalline materials

Abstract

Published experimental data, underpinned by molecular simulations, are used to highlight the strong influence of adsorption thermodynamics on diffusivities of guest molecules inside ordered nanoporous crystalline materials such as zeolites, metal–organic frameworks (MOFs) and Zeolitic Imidazolate Frameworks (ZIFs). For cage-type structures (e.g. LTA, CHA, DDR and ZIF-8), the variation of the free energy barrier for inter-cage hopping across the narrow windows, −δF_i, provides a rationalization of the observed strong influence of pore concentrations, c_i, on diffusivities. In open structures with large pore volumes (e.g. FAU, IRMOF-1, CuBTC) and within channels (MFI, BEA, MgMOF-74, MIL-47, MIL-53), the pore concentration (c_i) dependence of the self- (D_i,self), Maxwell–Stefan (D_i) and Fick (D_i) diffusivities are often strongly dictated by the inverse thermodynamic correction factor, 1/Γ_i ≡ ∂ln c_i/∂ln p_i; the magnitudes of the diffusivities are dictated by the binding energies for adsorption. For many guest–host combinations D_i−c_i dependence is directly related to the 1/Γ_i vs. c_i variation. When molecular clustering occurs, we get 1/Γ_i > 1, causing unusual D_i vs. c_i dependencies. The match or mis-match, between the periodicity of the pore landscape and the conformations of adsorbed chain molecules often leads to non-monotonic variation of diffusivities with chain lengths.