Adsorption equilibria of CO2 and small hydrocarbons in AEI-, CHA-, STT-, and RRO-type siliceous zeolites

AEI, CHA, and STT siliceous zeolites and RUB-41 (RRO) were prepared with low concentration of internal silanols and were characterized by X-ray diffraction, N2 adsorption isotherms, electron microscopy, and 29Si MAS NMR spectroscopy. Adsorption isotherms of methane, ethylene, ethane, propylene, propane, and CO2 were measured volumetrically on siliceous zeolites AEI, CHA, RRO, and STT at various temperatures ranging from 273 K to 323 K up to a pressure of 101.3 kPa. The Clausius-Clapeyron equation was used to calculate the isosteric heats of adsorption. The amount adsorbed depends strongly on adsorbate polarizability in the low-pressure region, and on the adsorbent micropore volume in the high-pressure region. Adsorption entropy losses are independent of temperature and increase with increasing adsorption enthalpy. RRO showed the highest pure-component selectivity of CO2 over CH4, and potential for the separation of the natural gas mixtures; AEI, CHA, and RRO-type materials showed potential for the kinetic separation of propylene/propane.