CO Adsorption on Variably Hydrated Cation-Exchanged Montmorillonite-Rich Clays.

Layered swelling clay minerals like montmorillonite (MMT) can competitively and synergistically adsorb CO and HO in their interlayer galleries. This work examines how different interlayer cations, relative humidity levels (and amount of cosorbed HO), and (de)hydration history affect CO adsorption on MMT and MMT-rich bentonite at near-ambient pressure and temperature. For CO to be adsorbed, the MMT requires either large (e.g., Cs) or hydrated interlayer cations to provide a sufficiently wide interlayer gallery, and must not have too much HO adsorbed competitively with CO. Na-MMT and initially anhydrous Mg- and Ca-MMT studied under increasing relative humidity conditions adsorb little CO. However, Mg- and Ca-MMT can effectively adsorb CO if first hydrated and then mildly dried such that the cations remain hydrated while the competitively adsorbed excess HO is removed. Because of their high stability and the favorable shape of their CO adsorption isotherms, low-cost (near-)natural Mg- and Ca-bentonite can be used for (cyclic) CO adsorption and separation purposes, similar to the more expensive Cs-bentonite.