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)x(BPZNO2)1–x, Zn­(BPZ)x(BPZNH2)1–x, Zn­(BPZNO2)x(BPZNH2)1–x, and Zn­(BPZ)x(BPZNO2)y(BPZNH2)1–x−y (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 (Qst), 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 Qst values and CO2/N2 selectivity
vs the nitro-functionalized analogues; in addition, tag “dilution”
[upon passing from Zn­(BPZX) to Zn­(BPZ)x(BPZX)1–x] 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, Qst, and CO2/N2 selectivity
is represented by Zn­(BPZ)x(BPZNH2)1–x.