3. Structure Diversity of TIF-Ax
TIF-A1 is a 4-connected metal–organic framework with Zn2+ as the metal center, adenine (ad) as the main ligand, and isonicotinic acid (Hint) as the auxiliary ligand. Both ligands bind to Zn2+ via N and carboxyl O on heterocycles. It was observed that TIF-A1~A3 has a different topology, which was regulated by changing the type of imidazole salts in the framework. 2-NH2-TIF-A1 and 3-NH2-TIF-A1 were prepared by functional modification, and their functional groups could effectively adjust the pore environment and pore size. Cd-ad-int (int= isonicotinic acid) changed the metal center to have different building units, thus adjusting the structural network. Zn-thp-nit (thp = theophylline, nit = nicotinic acid) changed the isonicotinic acid ligand into nicotinic acid, which changed the angle of connection between the ligand and the metal. In TIF-A4 ~ A8, monocarboxylic acid with a coordination angle of ca. 120° was selected to replace the Hint, and a dia topological structure and layered structure were obtained.
4. Special Properties of TIF-Ax
4.1. Solvent Stability
To realize the wide application of MOFs, they should first have good stability in harsh conditions, such as high temperature, high pressure, and acid and alkali environments. Interestingly, TIF-A1 can be stable in various organic solvents, such as water,
N,
N′-dimethylformamide (DMF),
N,
N′-diethylformamide (DEF), acetonitrile (CH
3CN), ethyl acetate, toluene, n-hexane, dichloromethane (DCM), chloroform, methanol (MeOH), ethanol (EtOH), isopropanol (IPA), and n-butanol (n-BuOH). Furthermore, TIF-A1 can also maintain the framework after being soaked in a solution of pH= 2–1
[63]. The exceptional stability of TIF-A1 makes it a promising candidate for further industrial applications.
4.2. Guest Selectivity
It is well known that solvent plays an important role in the structural variety of MOFs. Different solvent systems tend to produce different MOFs. However, it was found that the construction of TIF-A1 was independent of the solvents. TIF-A1 can accommodate amide solvents, including
N,
N′
-dimethylformamide (DMF), ethyl urea (e-urea),
N-methyl-1,2-pyrrolidone (NMP), and
N,
N′-dimethylacetamide (DMA). Interestingly, the crystallization process of TIF-A1 exhibited a strict selective trapping ability for these four solvent guests, and its selective order was DMF > e-urea > NMP > DMA (Scheme 2). This order of selection may be related to the size of the guest and the interaction between the host and guest. When the size of the guest is relatively small, the size of the guest is the main factor affecting the selection of guest molecules for TIF-A1. The four solvent guests are arranged in that order of molecular size: DMF < DMA < e-urea < NMP. DMF is very suitable to exist in the pores of TIF-A1. When the size of the guest is relatively large, TIF-A1 needs more energy to expand the pore to accommodate them. However, under this condition, the interaction between the host and the guest is dominant, and the energy needed to expand the pore will be partially offset. There is a strong N-H∙∙∙N hydrogen bond between e-urea and TIF-A1. There are non-classic C-H∙∙∙O and C-H∙∙∙N hydrogen bonds between NMP and TIF-A1. There is only a non-classic C-H∙∙∙O hydrogen bond between DMA and TIF-A1. Therefore, the selection order for TIF-A1 is e-urea > NMP > DMA. However, if the molecule is too large, it still depends mainly on the size of the guest. For example, TIF-A1 cannot trap 1,3-dimethyl-2-imidazolidinone (DMI) or
N,
N′-dimethylpropyleneurea (DMPU)
[64].
Scheme 2. The solvents used for the synthesis of TIF-A1.
4.3. Flexibility
As mentioned above, TIF-A1 showed a certain degree of contraction and expansion during crystallization to adapt to different guest molecules. Considering the porosity of TIF-A1, the flexibility of it was further explored by different gases
[65]. The de-solvated form denoted as TIF-A1′ was obtained through methanol exchange. Unexpectedly, TIF-A1′ showed normal adsorption behavior for most common gases, such as N
2, H
2, and CO
2, while it only showed multistep adsorption for C
2H
2. With the combination of in situ single-crystal XRD and calculation simulation, the flexibility of TIF-A1 was derived from the strong interaction between acetylene and the uncoordinated carboxylate O atoms of the int ligand. The C
2H
2-induced flexibility of TIF-A1 is intrinsic and was not affected by the sample size or defects. However, 2-NH
2-TIF-A1 exhibited normal acetylene adsorption because the presence of a strong H-bond between the -NH
2 of ad and the uncoordinated carboxylate O atoms of 2-NH
2-int prevented C
2H
2 from approaching the active site and made the rotation of 2-NH
2-int ligand more complex.
5. Application
5.1. CO2 Separation
Considering the presence of abundant active sites on TIF-A1, its CO
2 absorption was first evaluated. After becoming fully activated, TIF-A1 exhibited high CO
2 uptake at 273 K and 1 bar (107 cm
3/g)
[64]. In contrast, the highest CO
2 adsorption in ZIFs at that time was ZIF-69, with a value of 70 cm
3/g
[22]. The high CO
2 adsorption capacity of TIF-A1 can be attributed to the synergistic effect of int and ad ligands. Because the -NH
2 group amino and pyrimidine N atoms in ad form an electron-rich system, they are potential CO
2 bond sites. As mentioned above, the uncoordinated carboxylate O atoms of int ligand can also be active sites. In addition, the specific pore size and environment of TIF-A1 also play important roles in CO
2 adsorption. Furthermore, the special interaction between TIF-A1 and CO
2 indicates that TIF-A1 has the potential ability to selectively adsorb CO
2, which makes it possible to apply TIF-A1 in the capture and separation of CO
2 in flue gas. The flue gas mainly contains CO
2 and N
2, which are similar in molecular size, and it is difficult to achieve the separation effect by the pure physical adsorption of porous materials. The N
2 adsorption of TIF-A1 was 4.7 cm
3/g (273 K) and 1.4 cm
3/g (298 K), which shows that TIF-A1 barely adsorbed N
2. Henry’s constant indicates that the adsorption selectivity of TIF-A1 for CO
2 over N
2 was 90 (273 K,1 bar) and 60 (298 K,1 bar), respectively
[58]. The excellent separation selectivity of TIF-A1 was further demonstrated by a breakthrough experiment
[63].
5.2. NH3 Adsorption
Due to the strong electronegativity of the N sites in TIF-A1, it can not only form an electron-rich system to attract CO
2 but also form a strong hydrogen bond with NH
3. Therefore, the NH
3 adsorption of 3-NH
2-TIF-A1 was studied. At 298 K and 1 bar, the maximum NH
3 adsorption capacities of 3-NH
2-TIF-A1 (obtained in DMA) was 9.8 mmol/g (obtained in DMA) and 7.1 mmol/g (obtained in DMF), which were higher than those of MOF-5 and ultrahigh porous MOF-177
[59]. 3-NH
2-TIF-A1 has abundant active sites, including an uncoordinated oxygen atom and -NH
2 group of 3-NH
2-int, uncoordinated N atoms and -NH
2 group of the ad ligand. Among them, the -NH
2 group on ad and the N of the pyrimidine ring at its para-position had lower
Eads values compared to those of the other four sites, which can provide more stable adsorption sites.
5.3. C2 Separation
In addition to CO
2, TIF-A1 also exhibited a good C
2 separation ability. In 2022, Ding and co-workers applied TIF-A1 to trap C
2H
2 and C
2H
6 simultaneously from the ternary mixture gas of C
2H
2/C
2H
4/C
2H
6 and performed the purification of C
2H
4 [66]. In the ternary mixture of C
2H
2/C
2H
4/C
2H
6, a strong electrostatic interaction occurred between C
2H
2 and the uncoordinated carboxyl O with high polarity in TIF-A1, and van der Waals (vdW) interaction occurred between C
2H
6 and the aromatic heterocycles with low polarity. TIF-A1 not only provides a strong binding site for the adsorption of C
2H
2 and C
2H
6, but also oblate C
2H
6 and linear C
2H
2 are very suitable for the spindle-shaped cage of TIF-A1, which can store the target molecule well. The results show that TIF-A1 separated 99.9% ethylene from C
2H
2/C
2H
4/C
2H
6 (1/10/89) at 298 K and 1 bar, and the yield was 1.43 mmol/g, representing the best purification capacity at that time.
5.4. CO2 Cycloaddition
In 2021, Wang and co-workers selected ZnX
2 (X = Cl, Br, I) instead of ZnNO
3 and selected a suitable solvent environment to synthesize a series of X-TIF-A1 with many halogen ions in the framework as catalysts for the CO
2 activation reaction
[67]. Due to their potent nucleophilic nature, halogen ions induced the ring-opening of propylene oxide, facilitating the reaction between CO
2 and propylene oxide to generate propylene carbonate—a pivotal step in catalyzing CO
2 conversion. The outcomes demonstrated that the yield of the CO
2 activation reaction catalyzed by ZnI
2-ad-int-DMF exhibited an upward trend with increasing reaction temperature. Notably, the yield achieved an impressive 98.5% at 140 °C. Subsequent to three cycles of reuse, ZnI
2-ad-int-DMF experienced a moderate decline in catalytic activity. However, its framework remained stable despite exposure to high-temperature, high-pressure, and solvent conditions. Importantly, no discernible blocking phenomenon was observed as the iodine content decreased from 69 μmol/g to 51 μmol/g
[67].
5.5. Heavy Metal Adsorption
In 2022, Ma and co-workers used chitosan as the TIF-A1 growth template and prepared TIF-A1/chitosan composite beads using a secondary growth method for Pb(II) adsorption in water
[68]. Several small TIF-A1 crystals formed rod-shaped clusters and aggregated in the pores of chitosan. The average particle size of TIF-A1/chitosan was 0.2 cm. Such a large composite ball can effectively avoid the harm of pipeline blockage caused by the difficulty of separation and recovery of powdery MOF materials in practical applications. The polar groups -COO
−, -OH, C=N, and -NH
2 in the TIF-A1/chitosan structure coordinate with Pb(II).
The maximum adsorption of TIF-A1/chitosan for Pb(II) was 397.3 mg/g at 25 °C and pH = 6. Furthermore, in the mixed solution of multiple metal ions, TIF-A1/chitosan hardly adsorbed other ions, and the adsorption removal efficiency of Pb(II) was 99.17%. Especially when the concentration of Pb(II) was 100 ppb, the removal efficiency of trace Pb(II) was 99.95%, and the residual amount of Pb(II) met the international drinking water standards
[68]. After five adsorption/desorption cycles, TIF-A1/chitosan could still maintain high adsorption performance, and the removal efficiency was more than 99%. Furthermore, the crystal structure of TIF-A1 was not destroyed, and TIF-A1 was still attached to the chitosan matrix. In conclusion, TIF-A1/chitosan is a promising adsorbent for the removal of trace Pb(II) in drinking water treatment because of its excellent performance and reusability.