Chalcogenide Aerogels as Sorbents for Noble Gases (Xe, Kr)

High-surface-area molybdenum sulfide (MoS_x) and antimony sulfide (SbS_x) chalcogels were studied for Xe/Kr gas separation. The intrinsic soft Lewis basic character of the chalcogel framework is a unique property among the large family of porous materials and lends itself to a potential new approach toward the selective separation of Xe over Kr. Among these chalcogels, MoS_x shows the highest Xe and Kr uptake, reaching 0.69 mmol g^-1 (1.05 mmol cm^-3) and 0.28 mmol g^-1 (0.42 mmol cm^-3) respectively, at 273 K and 1 bar. The corresponding isosteric heat of adsorption at zero coverage (Q_st⁰) is 22.8 and 18.6 kJ mol^-1 and both are the highest among the selected chalcogels. The IAST (10:90) Xe/Kr selectivity at 273 K for MoS_x is 6.0, whereas for SbS_x chalcogels, it varies in the range 2.0-2.8. The higher formal charge of molybdenum, Mo⁴⁺, in MoS_x versus that of antimony, Sb³⁺, in SbS_x coupled with its larger atomic size could induce higher polarizability in the MoS_x framework and therefore higher Xe/Kr selectivity.