Comparative Study on Temperature-Dependent CO2 Sorption Behaviors of Two Isostructural N-Oxide-Functionalized 3D Dynamic Microporous MOFs.

By functionalization of the achiral carboxylate-based pyridine-N ligand 2,2'-bipyridine-3,3'-dicarboxylate (H2bpda) with N-oxide groups, the axially chiral ligand 2,2'-bipyridine-3,3'-dicarboxylate 1,1'-dioxide (H2bpdado) has been obtained. On the basis of H2bpdado and auxiliary N-donor ligands, two isostructural 3D dynamic porous Cu(II) metal-organic frameworks (MOFs), {[Cu0.5(bpdado)0.5(L)0.5]·3H2O}n (L = 1,2-bis(4-pyridyl)ethane (bpa), trans-1,2-bis(4-pyridyl)ethene (bpe) for 1 and 2, respectively), have been synthesized, which contain N-oxide "open donor sites" (ODSs) and carboxyl sites on the pore surfaces. The modification of pyridine-N into the N-oxide group not only transforms the nonporous structure into a porous framework but also endows the N-oxide group with unique charge-separated plus electron-rich character, which may provide an enhanced affinity toward CO2 molecules. Interestingly, both 1 and 2 present reversible structural transformation upon dehydration and rehydration. The adsorption properties of 1 and 2 have been investigated by N2, H2, CH4, and CO2 gases, and they reveal evident adsorption for CO2 and CH4. Both MOFs have high CO2 uptake, CO2 sorption affinity, and sorption selectivities of CO2 over CH4 and N2. Remarkably, 1' and 2' exhibit intriguingly comparable temperature-dependent CO2 sorption behaviors that can probably be attributed to the difference in bpa and bpe. First, at 195 K, 1' and 2' exhibit stepwise adsorption and hysteretic desorption behavior for CO2, but in the second step, the isotherms of 2' display a starting pressure greater than that of 1'. Then, at 298 K, their CO2 isotherms all show nonclassical type I adsorption, while peculiarly, at 273 K, the CO2 isotherm of 1' still exhibits uncommon stepwise adsorption but that of 2' does not. Thus, these temperature-dependent CO2 sorption behaviors indicate that there exist different threshold temperatures and pressures of channel expansion for 1' and 2'.