Adsorption of CO2, CO, H2, and N2 on Zeolites, Activated Carbons, and Metal-Organic Frameworks with Different Surface Nonuniformities

Abstract

The single-component adsorption of CO2, CO, N2, and H2 at 25 and 35 °C was studied using microporous faujasite-framework zeolites (NaY and NaX), activated carbons (GCN and MSP), and metal–organic frameworks (A100 and Z1200) as starting points for the separation of CO2 from syngases produced by gasifying biomass-based solid wastes. The indicated adsorption isotherms and uptake of the adsorbates strongly depended on the adsorbates themselves as well as on the adsorbents because of significant differences in the surface features, such as surface nonuniformity, and in the molecular properties. The selectivity of CO2 to the other gases also varied with the adsorbents due to the distinctive energetic characteristics. The surfaces of the zeolites were the most energetically heterogeneous ones, yielding higher CO2 uptake at low pressures, while the two activated carbons and A100 had moderate surface heterogeneities, and MSP showed the highest CO2 uptake at high pressures, such as 6 bar, at which the micropore volume and surface area are important. Z1200, which has highly homogeneous surfaces and no high-affinity-binding sites, exhibited the lowest CO2 adsorption capacity regardless of equilibrated pressure. The surface nonuniformities of the six sorbents were consistent with the calculated isosteric heats of CO2 adsorption. CO2 could be reversibly adsorbed on NaY and MSP but not on GCN, with some metal impurities, although all these adsorbents showed a fully reversible process for CO adsorption. The estimated working capacity for CO2 adsorption at 25 °C was 0.78–6.50 mmol/g, depending on the sorbents used. The highest value was disclosed for MSP, the surface energetic heterogeneity of which was between that of zeolites and Z1200. Such a high working capacity bodes well for use in our later applications.