NO2-functionalized bis(pyrazolate) MOFs as CO2 capture materials at ambient conditions

The MOFs M(BPZNO2) (M = Co, Cu, Zn) (H2BPZNO2 = 3-nitro-4,4'-bipyrazole) were prepared through solvothermal routes and fully investigated in the solid state. They show a good thermal stability both under nitrogen and in air, with decomposition temperatures peaking up to 663 K for Zn(BPZNO2). Their crystal structure is characterized by 3-D networks with square (M = Co, Zn) or rhombic (M = Cu) channels decorated by the polar NO2 groups. As revealed by N2 adsorption at 77 K, they are micro-mesoporous materials with BET specific surface areas ranging between 400 and 900 m2/g. Remarkably, at the mild conditions of 298 K and 1.2 bar, Zn(BPZNO2) adsorbs 21.8 wt. CO2 % (4.95 mmol/g). It shows a Henry CO2/N2 selectivity of 15 and an IAST selectivity of 12 at p = 1 bar. As a CO2 adsorbent, this compound is the best performing MOF to date among those bearing a nitro group as a unique chemical tag. High-resolution PXRD at 298 K and different CO2 loadings revealed, for the first time in a NO2-functionalized MOF, the insurgence of primary host-guest interactions involving the C(3)-NO2 moiety of the framework and the oxygen atoms of carbon dioxide, as confirmed by Grand Canonical Monte Carlo simulations. This interaction mode is markedly different from that observed in NH2-functionalized MOFs, where the carbon atom of CO2 is involved.