(608a) Determining CO2/H2s and CO2/CH4 Binary Adsorption Selectivities in Metal-Organic Frameworks for Biogas Purification

Biogas is a promising alternative to natural gas as the global energy sources shift from fossil fuels to renewables. Biogas contains mainly CH₄ (35-70%), CO₂ (15-60%), and N₂ (15-50%), with trace impurities such as NH₃, H₂O, and H₂S. To reduce Greenhouse Gas release from biogas and prevent pipeline corrosion and catalyst poisoning in the transport and downstream process of biogas, removal of H₂S and CO₂ is critical. Besides the traditional acid gas removal methods involving aqueous amine scrubbing, less energy-intensive adsorption methods using microporous materials such as zeolites, metal-organic frameworks (MOFs), and porous organic cages (POCs) are being developed. In particular, MOFs have received much attention as they exhibit high CO₂ uptake and have tunable selectivity. Much research has focused on measuring single-component adsorption isotherms and predicting the mixture selectivity using the Ideal Adsorbed Solution Theory (IAST). As the functional groups and unsaturated metal sites often interact with quadrupolar CO₂ molecules more strongly than CH₄ or N₂, IAST predictions may not capture the nonideal adsorption behavior.¹ Mixture containing H₂S and CO₂ poses more challenges in binary adsorption selectivity prediction due to the competitive sorption of H₂S and CO₂ to active sorption sites.² Thus, understanding the mixture adsorption performance is valuable in adsorption process design and adsorbent selection. This work aims to measure the binary adsorption uptakes of CO₂/H₂S and CO₂/CH₄ in MIL-125-NH₂(Ti), amine-appended Mg₂(dobpdc), MIL-101(Cr), and MOF-74(Mg, Ni). A zero-length column methodis used, where a small amount of MOF is first saturated with the gas mixture of interest. Then the mixture gas is desorbed using an inert gas under equilibrium conditions.³ A mass spectrometer is used to measure the concentration of the desorption effluent gas. The experimental mixture gas results are compared to the IAST predictions, and the pre and post-adsorption MOFs are characterized by SEM, N₂ physisorption, PXRD, and XPS to understand the competitive adsorption behavior and H₂S stability in these MOFs.

1. Gharagheizi, F.; Sholl, D. S., Comprehensive Assessment of the Accuracy of the Ideal Adsorbed Solution Theory for Predicting Binary Adsorption of Gas Mixtures in Porous Materials. Industrial & Engineering Chemistry Research 2021, 61 (1), 727-739.
2. Joshi, J. N.; Zhu, G.; Lee, J. J.; Carter, E. A.; Jones, C. W.; Lively, R. P.; Walton, K. S., Probing Metal-Organic Framework Design for Adsorptive Natural Gas Purification. Langmuir 2018, 34 (29), 8443-8450.
3. Brandani, F.; Ruthven, D., Measurement of adsorption equilibria by the zero length column (ZLC) technique part 2: Binary systems. Industrial & Engineering Chemistry Research 2003, 42 (7), 1462-1469.