Adsorption kinetics of CO2, O2, N2, and CH4 in cation-exchanged clinoptilolite

The rate of adsorption of pure CO2, O-2, N-2, and CH4 on natural untreated clinoptilolite-rich volcanic tuff (Cp) from Tehuacan (Puebla, Mexico), and on cation-exchanged clinoptilolite samples (Ca-Cp, K-Cp, and Na-Cp) has been measured at 20 degreesC using a glass high-vacuum volumetric apparatus. The X-ray diffraction pattern of Cp showed that the main crystalline phases correspond to clinoptilolite-heulandite and minor amounts of mordenite, cristobalite, feldspar (albite), quartz, smectite, and opal. The adsorption rates of gases in the initial period (t < 180 s) were measured with a custom acquisition data card capable of registering pressure and time data five times per second, simultaneously. The influence of cation exchange on adsorption kinetics of the gases depended on the gas-adsorbent contact time (t). In the initial period, the adsorption rate of all gases on all samples decreased in the order Ca-Cp > K-Cp > Cp > Na-Cp, and the affinity decreased in the order CO2 much greater than N-2 > O-2 > CH4, whereas at equilibrium (t --> infinity s) the adsorption uptake decreased in the sequence CO2 much greater than CH4 > N-2 > O-2. The slow adsorption of methane in Na-Cp was probably due to diffusional difficulties as a result of channel blockage by Na+ cations. By cation exchange of Cp an adsorbent can be tailored for the separation of N-2/O-2, N-2/CH4, and CO2/CH4 mixtures.