Temperature-Dependent Nitrous Oxide/Carbon Dioxide Preferential Adsorption in a Thiazolium-Functionalized NU-1000...

Solvent-assisted ligand incorporation (SALI) of the ditopic linker 5-carboxy-3-(4-carboxybenzyl)thiazolium bromide [(HPhTz)Br] into the zirconium metal-organic framework NU-1000 [Zr₆O₄(OH)₈(H₂O)₄(TBAPy)₂, where NU = Northwestern University and H₄TBAPy = 1,3,6,8-tetrakis(p-benzoic-acid)pyrene], led to the SALIed NU-1000-PhTz material of minimal formula [Zr₆O₄(OH)₆(H₂O)₂(TBAPy)₂(PhTz)]Br. NU-1000-PhTz has been thoroughly characterized in the solid state. As confirmed by powder X-ray diffraction, this material keeps the same three-dimensional architecture of NU-1000 and the dicarboxylic extra linker bridges adjacent [Zr₆] nodes ca. 8 Å far apart along the crystallographic c-axis. The functionalized MOF has a BET specific surface area of 1560 m²/g, and it is featured by a slightly higher thermal stability than its parent material (T_dec = 820 vs. 800 K, respectively). NU-1000-PhTz has been exploited for the capture and separation of two pollutant gases: carbon dioxide (CO₂) and nitrous oxide (N₂O). The high thermodynamic affinity for both gases [isosteric heat of adsorption (Q_st) = 25 and 27 kJ mol^-1 for CO₂ and N₂O, respectively] reasonably stems from the strong interactions between these (polar) "stick-like" molecules and the ionic framework. Intriguingly, NU-1000-PhTz shows an unprecedented temperature-dependent adsorption capacity, loading more N₂O in the 298 K ≤ T ≤ 313 K range but more CO₂ at temperatures falling out of this range. Grand canonical Monte Carlo simulations of the adsorption isotherms confirmed that the preferential adsorption sites of both gases are the triangular channels (micropores) in close proximity to the polar pillar. While CO₂ interacts with the thiazolium ring in an "end-on" fashion through its O atoms, N₂O adopts a "side-on" configuration through its three atoms simultaneously. These findings open new horizons in the discovery of functional materials that may discriminate between polluting gases through selective adsorption at different temperatures.