Tuning Carbon Dioxide Adsorption Affinity of Zinc(II) MOFs by Mixing Bis(pyrazolate) Ligands with N-Containing Tags

The four zinc(II) mixed-ligand metal-organic frameworks (MIXMOFs) Zn(BPZ)(BPZNO₂)_1-, Zn(BPZ)(BPZNH₂)_1-, Zn(BPZNO₂)(BPZNH₂)_1-, and Zn(BPZ)(BPZNO₂)(BPZNH₂)_1- (H₂BPZ = 4,4'-bipyrazole; H₂BPZNO₂ = 3-nitro-4,4'-bipyrazole; H₂BPZNH₂ = 3-amino-4,4'-bipyrazole) were prepared through solvothermal routes and fully investigated in the solid state. Isoreticular to the end members Zn(BPZ) and Zn(BPZX) (X = NO₂, NH₂), they are the first examples ever reported of (pyr)azolate MIXMOFs. Their crystal structure is characterized by a three-dimensional open framework with one-dimensional square or rhombic channels decorated by the functional groups. Accurate information about ligand stoichiometric ratio was determined (for the first time on MIXMOFs) through integration of selected ligands skeleton resonances from ¹³C cross polarized magic angle spinning solid-state NMR spectra collected on the as-synthesized materials. Like other poly(pyrazolate) MOFs, the four MIXMOFs are thermally stable, with decomposition temperatures between 708 and 726 K. As disclosed by N₂ adsorption at 77 K, they are micro-mesoporous materials with Brunauer-Emmett-Teller specific surface areas in the range 400-600 m²/g. A comparative study (involving also the single-ligand analogues) of CO₂ adsorption capacity, CO₂ isosteric heat of adsorption (Q_st), and CO₂/N₂ selectivity in equimolar mixtures at p = 1 bar and T = 298 K cast light on interesting trends, depending on ligand tag nature or ligand stoichiometric ratio. In particular, the amino-decorated compounds show higher Q_st values and CO₂/N₂ selectivity vs the nitro-functionalized analogues; in addition, tag "dilution" [upon passing from Zn(BPZX) to Zn(BPZ)(BPZX)_1-] increases CO₂ adsorption selectivity over N₂. The simultaneous presence of amino and nitro groups is not beneficial for CO₂ uptake. Among the compounds studied, the best compromise among uptake capacity, Q_st, and CO₂/N₂ selectivity is represented by Zn(BPZ)(BPZNH₂)_1-.