Azeotropic Adsorption and Selectivity Reversals in Zeolites

The applicability of the Ideal Adsorbed Solution Theory (IAST) for mixture adsorption in zeolites is contingent on three tenets being satisfied: (i) all of the adsorption sites within the microporous material are equally accessible to each of the guest molecules, implying a homogeneous distribution of guest adsorbates within the pore landscape, (ii) there are no preferential locations of any guest species in the pore landscape, and (iii) there is no molecular clustering, implying that the surface area of each guest molecule remains unchanged on mixture adsorption.For water(1)/ethanol(2) mixture adsorption in CHA and DDR zeolites, the hydrogen bonding between the three different molecular pairs (water-water, water-ethanol, and ethanol-ethanol) causes molecular clustering and violation of mandate (iii) of the IAST.For CO2(1)/alkane(2) and CO2(1)/alkene(2) mixture adsorption in cation-exchanged zeolites NaX, LTA-4A, and LTA-5A, the CO2 molecules congregate around the extra-framework cations.Consequently, the partner alkane, or alkene, molecules experience less competition than anticipated in the IAST prescription that demands a homogeneous distribution of guest adsorbates.Segregation effects in CO2(1)/C3H8(2) mixture adsorption in MOR zeolite result from the fact the C3H8 cannot locate within the side pockets, that are occupied exclusively by CO2.For all the guest/host combinations examined here, the failure of the IAST leads to the phenomenon of azeotropic adsorption, in which the mole fraction of component 1 in the bulk fluid phase equals that in the adsorbed phase mixture, 1 1 y x = ; at this composition, the adsorption selectivity 1 ads S = .For water/ethanol mixtures, the adsorption selectivity strongly favors water for ethanol-rich feed mixtures, and this is beneficial in the pervaporation technology for selective water removal.Also, at the azeotropic mixture composition, 1 1 y x = , the ratio of activity coefficients 2 1 γ γ equals the ratio of sorption pressures 0 0 1 2 P P .This equality is the analog of the corresponding relation for azeotropy in vapor/liquid equilibrium for which 2 1 γ γ equals the ratio of vapor pressures 1 2 vap vap P P .This communication shows how activity coefficient models such as NRTL, Wilson, and Margules need to be properly modified to include the influence of pore occupancy.Also addressed is the extension of the Herrington thermodynamic consistency to mixture adsorption equilibrium.