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

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), {[Cu_0.5(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'.