(669c) Adsorption Equilibria of CO2/CH4/N2 Binary Mixtures on Flexible Zeolitic Imidazolate Frameworks (ZIF-7)

In conjunction with a high surface area and pore volume, structural transformation in flexible Metal-Organic Framework (MOFs), which generally attributed to the alterations between narrow-pore or non-porous (np) and large-pore or porous (lp) states of the adsorbent framework, makes these materials one of the most promising in the field of gas separation. Although the structural transition of these materials in the presence of pure gases and their adsorption capacity has been studied widely in the literature, limited research investigated the adsorption behaviour and gas separation performance of these hybrid adsorbents in the presence of gas mixtures experimentally. Experiments with binary mixtures provide more accurate and direct measurements of an adsorbent's equilibrium selectivity for the component gases than obtained from estimates based only on pure fluid capacity measurements.

Here, a dynamic column breakthrough (DCB) apparatus was first used to measure the adsorption capacities of pure CO₂, CH₄ and N₂ on flexible ZIF-7 at temperatures in the range 263.15–303.15 K and pressures up to 900 kPa. Structural changes in ZIF-7 were observed for CO2, and CH4. Equilibrium adsorption capacities of the component gases in the binary mixtures of CO₂ + N₂, CO₂ + CH₄ and CH₄ + N₂ were then determined by these dynamic experiments. In addition, we have investigated the application of a new adsorption isotherm model, which was successfully regressed to the pure capacity data over the ranges of temperature and pressure measured; this model avoids the need for piecewise definitions or discontinuities in the definition of the adsorption isotherm. The IAST selectivity estimated from the pure fluid adsorption data were compared with the measured selectivities in the gas mixtures; reasons for the discrepancies observed will be discussed.