Adsorption of Methane–Nitrogen mixtures with flexible Zeolitic Imidazolate Framework-7 (ZIF-7)

Abstract

ZIF–7 is a pressure–regulated flexible metal–organic framework with a notably lower admission pressure for CH₄ relative to N₂, making it highly-prospective for curbing fugitive emissions of methane. Here, the separation of CH₄ from N₂ by an improved variant of ZIF–7 was studied over a range of pressures, temperatures, and compositions using multiple complementary techniques. Pure gas adsorption isotherms measured from (283 to 323) K at up to 16 MPa reveal this variant of ZIF-7 exhibits 50–60 % higher adsorption capacities for both CH₄ and N₂ in the large–pore phase than previous ZIF–7 samples. Binary mixture adsorption isotherms from dynamic column breakthrough experiments were consistent with those predicted from pure–gas isotherms using Ideal Adsorbed Solution Theory. Methane uptake increased significantly when the pore phase transition pressure for the mixture was reached, while the uptake of nitrogen remained small. Consequently, the equilibrium CH₄-over–N₂ selectivity of ZIF-7 is always larger than 5.5 and can increase to around 9, significantly above that of conventional adsorbents. Further characterisation showed the rate of the adsorption on ZIF–7 initially decreases as the pore phase transition begins before it increases significantly. Raman spectra acquired at various pressures along the adsorption isotherms revealed changes in certain vibrational modes associated with the structural change. Collectively, these observations indicate that the conceptually simple gating mechanism often used to describe the pore phase transitions in ZIF-7 in fact depends on interactions between the guest molecules and the adsorbent framework that continue to operate once the material is in the large–pore state.