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.

四种二价锌混合配体金属有机框架（mixed-ligand metal–organic frameworks, MIXMOFs），分别为Zn(BPZ)ₓ(BPZNO₂)₁₋ₓ、Zn(BPZ)ₓ(BPZNH₂)₁₋ₓ、Zn(BPZNO₂)ₓ(BPZNH₂)₁₋ₓ以及Zn(BPZ)ₓ(BPZNO₂)ᵧ(BPZNH₂)₁₋ₓ₋ᵧ（其中H₂BPZ=4,4'-联吡唑；H₂BPZNO₂=3-硝基-4,4'-联吡唑；H₂BPZNH₂=3-氨基-4,4'-联吡唑），通过溶剂热法制备，并在固态下进行了全面表征。这些框架与单配体端基成员Zn(BPZ)和Zn(BPZX)（X=NO₂、NH₂）具有等网状结构，是迄今为止首次报道的（吡）唑啉盐混合配体金属有机框架。其晶体结构呈现三维开放骨架，带有一维方形或菱形孔道，孔道表面修饰有功能性取代基。研究人员通过对合成原样品的¹³C交叉极化魔角旋转固态核磁共振光谱中选定配体骨架共振峰的积分，首次在混合配体金属有机框架体系中确定了配体化学计量比。与其他聚(吡唑啉盐)金属有机框架类似，这四种混合配体金属有机框架具有优异的热稳定性，热分解温度介于708~726 K之间。77 K下的氮气吸附测试结果表明，它们属于微介孔材料，布鲁诺尔-埃米特-特勒（Brunauer–Emmett–Teller, BET）比表面积介于400~600 m²/g范围内。研究团队同时结合单配体类似物开展对比研究，考察了1 bar、298 K下等摩尔混合气体中的二氧化碳吸附容量、二氧化碳吸附等量热（Qst）以及CO₂/N₂吸附选择性，揭示了随配体取代基性质或配体化学计量比变化的有趣趋势。具体而言，氨基修饰的框架相较于硝基功能化的同类框架，展现出更高的吸附等量热与CO₂/N₂选择性；此外，通过从Zn(BPZX)向Zn(BPZ)ₓ(BPZX)₁₋ₓ的转变实现的取代基“稀释”效应，可提升CO₂相对于N₂的吸附选择性。同时存在氨基与硝基基团对二氧化碳吸附量并无增益。在所研究的所有框架中，Zn(BPZ)ₓ(BPZNH₂)₁₋ₓ实现了吸附容量、吸附等量热与CO₂/N₂选择性之间的最优平衡。