The separation of CO2 from H2 is highly significant in the context of two distinct applications: (i) the capture of CO2 emissions like those produced from coal-fired power plants, and (ii) the purification of hydrogen gas, which is synthesized on megaton scales annually. Pressure-swing adsorption (PSA) is advantageous over other separations techniques such as liquid absorbents, membrane or cryogenic separation due to the high purity and yield of hydrogen that can be produced. In a PSA system, CO2 adsorbs onto a surface at high pressure in the presence of other gases, and the porous material can be regenerated for another purification cycle by simply dropping the pressure to ambient conditions. Porous materials such as zeolites and activated carbons are used in PSA systems for CO2/H2 separations, however due to the maturity of these technologies only modest improvements in CO2/H2 separation performance can be expected in the future. For PSA to be the most economical separation technique in all scenarios, much greater efficiencies must be achieved than what can be realized with these adsorbents. To address this need, investigators at University of California at Berkeley have investigated a novel group of adsorbents, metal-organic frameworks, for PSA separation of CO2 from H2 and other gases such as CH4. Metal-organic frameworks are a group of porous crystalline materials composed of metal cations or clusters joined by multifunctional organic linkers. The high surface area and low bulk densities of these materials result in both large gravimetric and volumetric capacities for CO2. Five metal-organic frameworks have been studied by the investigators, where single-component CO2, H2 and CH4 adsorption isotherms were measured at 313 °K at pressures up to 40 bar. Mixtures of CO2, H2 and CH4 were simulated using these single-component data as a starting point. The best-performing materials exhibited much higher selectivities and working capacities for CO2 than activated carbons and zeolites.
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