Temperature-Dependent Nitrous Oxide/Carbon Dioxide Preferential Adsorption in a Thiazolium-Functionalized NU-1000 Metal–Organic Framework

Solvent-assisted
ligand incorporation (SALI) of the ditopic linker
5-carboxy-3-(4-carboxybenzyl)­thiazolium bromide [(H2PhTz)­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 (Tdec = 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 (Qst) = 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.