Co-adsorption and Separation of CO2-CH4 Mixtures in the Highly Flexible MIL-53(Cr) MOF

The present study attempts to understand the use of the flexible porous chromium terephthalate Cr(OH)(O2C-C6H4-CO2) denoted MIL-53(Cr) (MIL = Material from Institut Lavoisier) for the separation of mixtures of CO2 and CH4 at ambient temperature. The coadsorption of CO2 and CH4 was studied by a variety of different techniques. In situ synchrotron X-ray Powder Diffraction allowed study of the breathing of the solid upon adsorption of the gas mixtures and simultaneously measured Raman spectra yielded an estimation of the adsorbed quantities of CO2 and CH4, as well as a quantification of the fraction of the narrow pore (NP) and the large pore (LP) form of MIL-53. Quantitative coadsorption data were then measured by gravimetry and by breakthrough curves. In addition, computer simulation was performed to calculate the composition of the adsorbed phase in comparison with experimental equilibrium isotherms and breakthrough results. The body of results shows that the coadsorption of CO2 and CH4 leads to a similar breathing of MIL-53(Cr) as with pure CO2. The breathing is mainly controlled by the partial pressure of CO2, but increasing the CH4 content progressively decreases the transformation of LP to NP. CH4 seems to be excluded from the NP form, which is filled exclusively by CO2 molecules. The consequences in terms of CO2/CH4 selectivity and the possible use of MIL-53(Cr) in a PSA process are discussed.