Adsorption of CO₂, CH₄, N₂O, and N ₂ on MOF-5, MOF-177, and zeolite 5A

Adsorption equilibrium and kinetics of CO₂, CH₄, N₂O, and N₂ on two newly discovered adsorbents, metal-organic frameworks MOF-5 and MOF-177 and one traditional adsorbent, zeolite 5A were determined to assess their efficacy for CO₂, CH ₄, and N₂O removal from air and separation of CO ₂ from CH₄ in pressure swing adsorption processes. Adsorption equilibrium and kinetics data for CO₂, CH₄, N₂O, and N₂ on all three adsorbents were measured volumetrically at 298K and gas pressures up to 800 Torr. Adsorption equilibrium capacities of CO₂ and CH₄ on all three adsorbents were determined gravimetrically at 298 K and elevated pressures (14 bar for CO ₂ and 100 bar for CH₄). The Henry's law and Langmuir adsorption equilibrium models were applied to correlate the adsorption isotherms, and a classical micropore diffusion model was used to analyze the adsorption kinetic data. The adsorption equilibrium selectivity was calculated from the ratio of Henry's constants, and the adsorbent selection parameter for pressure swing adsorption processes were determined by combining the equilibrium selectivity and working capacity ratio. Based on the selectivity and adsorbent selection parameter results, zeolite 5A is a better adsorbent for removing CO₂ and N₂O from air and separation of CO₂ from CH₄, whereas MOF-177 is the adsorbent of choice for removing CH ₄ from air. However, both MOF adsorbents have larger adsorption capacities for CO₂ and CH₄ than zeolite 5A at elevated pressures, suggesting MOF-5 and MOF-177 are better adsorbents for CO₂ and CH₄ storage. The CH₄ adsorption capacity of 22 wt.% on MOF-177 at 298K and 100 bar is probably the largest adsorption uptake of CH₄ on any dry adsorbents. The average diffusivity of CO₂, CH₄ and N₂O in MOF-5 and MOF-177 is in the order of 10^-9 m²/s, as compared to 10^-11 m²/s for CO₂, CH₄ and N₂O in zeolite 5A. The effects of gas pressure on diffusivity for different adsorabte-adsorbent systems were also investigated.