Expanding Pore Size of Al-BDC Metal–Organic Frameworks as a Way to Achieve High Adsorption Selectivity for CO2/CH4 Separation

Abstract

The mesostructured Al-BDC metal–organic frameworks (MOFs) with an average pore size of 2.58 nm were prepared via a simplified washing and drying process and applied to the separation of CO2/CH4 mixtures. The adsorption equilibrium and thermodynamics of CH4 and CO2 were studied in the dynamic processes by the volumetric–chromatographic and inverse gas chromatographic (IGC) methods. The experiments represent that the Al-BDC MOF with large pore size has a much higher CO2/CH4 selectivity of ca. 24 at 303 K in the pressure range 0–1.0 MPa and therefore appears to be a good candidate for the separation of CH4 from CO2. The initial heats of adsorption of CH4 and CO2 on the mesostructured Al-BDC MOFs were determined to be 11.5 and 25.2 kJ mol–1 by the IGC method, respectively, which are significantly reduced by ca. 25% compared with that on the microporous Al-BDC MOFs. The results indicate that the expanded pore size not only greatly increases the selectivity of CO2 adsorption over CH4 but also reduces the adsorption heat, revealing that it should be the desired method to obtain a satisfactory absorbent for CO2/CH4 separation.