Tetrahedral Imidazolate Frameworks with Auxiliary Ligands: Comparison
Please note this is a comparison between Version 1 by Hao Tong and Version 2 by Peter Tang.

Zeolitic imidazolate frameworks (ZIFs) are an important subclass of metal–organic frameworks (MOFs).  A new kind of MOF, namely tetrahedral imidazolate frameworks with auxiliary ligands (TIF-Ax) was reported, by adding linear ligands (Hint) into the zinc–imidazolate system. Introducing linear ligands into the M2+-imidazolate system overcomes the limitation of imidazole derivatives. Thanks to the synergistic effect of two different types of ligands, a series of new TIF-Ax with interesting topologies and a special pore environment has been reported, and they have attracted extensive attention in gas adsorption, separation, catalysis, heavy metal ion capture, and so on.

  • zeolite
  • metal–organic frameworks
  • zeolitic imidazolate frameworks
  • tetrahedral imidazolate frameworks
  • TIF-A1
  • adsorption and separation

1. Introduction

Zeolite and metal–organic frameworks (MOFs) are two kinds of crystalline porous materials [1][2][3][1,2,3]. Both of them have received much attention not only because of their wide applications but also for their fascinating structures that can be rationally designed and predicted [4][5][6][7][4,5,6,7]. Recently, zeolitic imidazolate frameworks (ZIFs) have been developed by employing tetrahedral metal (M2+ = Zn2+, Co2+, etc.) and imidazolate (im-) derivatives to mimic the tetrahedral center and bridge O in zeolite [8][9][10][11][12][13][8,9,10,11,12,13]. At the same time, the M-im-M (M = metal ion) angle is close to the Si-O-Si angle in zeolite. The network diversity of ZIFs is mainly achieved by changing the substitute groups of imidazole or mixing different imidazolate derivatives (link–link interactions) [3][14][15][16][17][18][19][20][21][22][3,14,15,16,17,18,19,20,21,22]. ZIFs have been widely used in gas adsorption/separation [23][24][25][23,24,25], catalysis [26][27][26,27], electrocatalysis [28][29][30][31][32][33][34][35][36][37][28,29,30,31,32,33,34,35,36,37], biomedicine [38], and more [39][40][41][42][43][44][45][46][39,40,41,42,43,44,45,46]. Considering the wide applications of ZIFs, researchers have never stopped exploring and synthesizing new ZIFs with interesting topologies [47][48][47,48].
Theoretically, beyond the limitation of imidazole, any bridge ligand with −1 charge can be used to replace imidazole in ZIFs to construct 4-connected frameworks [49]. In fact, the ZIF analogs based on triazole and tetrazole derivatives and the combination of them have also been reported [11][16][50][51][52][53][54][55][56][57][11,16,50,51,52,53,54,55,56,57]. Additionally, the attempt of some linear ligands generally leads to the formation of the non-zeolite dia (cubic diamond) topology, suggesting the importance of bent ligands.
In 2011, the researchers reported a new kind of MOF, namely tetrahedral imidazolate frameworks with auxiliary ligands (TIF-Ax), by adding linear ligands into the zinc–imidazolate system (Scheme 1) [22]. The most obvious difference between TIF-Ax and ZIFs is that the introduction of Hint makes the angle of M-int-M larger than that of M-im-M. Therefore, introducing linear ligands into the M2+-imidazolate system overcomes the limitation of imidazole derivatives. Thanks to the synergistic effect of two different types of ligands, a series of new TIF-Ax with interesting topologies and a special pore environment has been reported, and they have attracted extensive attention in gas adsorption, separation, catalysis, heavy metal ion capture, framework flexibility, and so on. More importantly, a variety of facile and rapid synthesis methods have been developed to realize the near-kilogram synthesis of them, paving the way for the large-scale application of TIF-Ax in the future (Table 1).
Scheme 1.
Strategy for the synthesis of TIF-Ax.
Table 1.
Summary of reported TIF-Ax.

Name

Formula

Space Group

Topology

Ref.

TIF-A1

[Zn(ad)(int)](DMF) 1

Pna21

dmp

[22]

TIF-A2

Zn2(im)3(int) 2

Pca21

dia

[22]

TIF-A3

Zn2(im)(int)2(OH)

C2/c

neb

[22]

2-NH2-TIF-A1

[Zn(ad)(2-NH2-int)](DMF)

Pna21

dmp

[58][62]

3-NH2-TIF-A1

[Zn(3-NH2-int)(ad)](DMF)

[Zn(3-NH2-int)(ad)](DMA) 3

Pna21

dmp

[59][63]

Zn-thp-nit

[Zn(thp)(nit)] 4

Pbca

[60][64]

Cd-ad-int

[Cd2(ad)2(int)2(DMF)

(H2O)](DMF)

P21/n

mog

[61][65]

TIF-A4

Zn(im)(Ac) 5

Ima2

dia

[62][66]

TIF-A5

Zn(2-mim)(Ac) 6

P21/c

sql

[62][66]

TIF-A6

Zn(2-eim)(Ac) 7

P21/c

sql

[62][66]

TIF-A7

Zn(2-pim)(Ac) 8

Pna21

sql

[62][66]

TIF-A8

[Zn2(OH-)(Ac)(2-cim)2](DMSO) 9

Cmc21

sql

[62][66]

1 ad = adeninate; int = isonicotinate; DMF = N,N′-dimethylformamide; 2 im = imidazolate; 3 DMA = N,N′-dimethylacetamide; 4 thp = theophylline; nit = nicotinic acid; 5 Ac = acetic acid; 6 mim = 2-methylimidazolate; 7 eim = 2-ethylimidazolate; 8 pim = 2-propylimidazolate; 9 cim = imidazolate-2-carboxaldehyde; DMSO = dimethyl sulfoxide.

2. Synthesis Method

All of the TIF-Ax compounds were initially synthesized using the solvothermal method (for detailed information, refer to the original papers). Based on the solvothermal method, diverse improved methods have been developed to serve various purposes.

2.1. Facile Synthesis

TIF-A1 was originally synthesized by the solvothermal method using Zn(NO3)2∙6H2O, adenine, and isonictinic acid (Hint) in N,N′-dimethylformamide (DMF) at 120 °C for 3 days with a high yield (88%) [22].

2.2. Metal Sources

Inspired by the aforementioned outcomes achieved through stirring and heating at 120 °C in a brief timeframe, and mindful of the potential for the reaction between nitrate and carboxylic acid to generate corrosive nitric acid, which might compromise the equipment’s integrity and influence the stability of the final MOFs, alternative metal sources were employed to investigate the feasibility of substituting Zn(NO3)2∙6H2O. Similarly, the yields of TIF-A1 synthesized from ZnO and Zn(CH3COO)2∙2H2O were 75.5% and 80.5%, respectively [63][67]. This shows that the metal source has good substitutability.

2.3. Upscale Synthesis

The potential for synthesizing a kilogram of TIF-A1 was further investigated using the aforementioned methods. Interestingly, through the scaling up of reactant quantities, a yield of over 800 g of TIF-A1 was achieved in a single heating process at 120 °C [63][67]. The resulting samples exhibited a similar crystallinity, morphology, and BET surface area to those obtained through the original solvothermal method.

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 (CH3CN), 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][67]. 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][68].
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][69]. 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 N2, H2, and CO2, while it only showed multistep adsorption for C2H2. 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 C2H2-induced flexibility of TIF-A1 is intrinsic and was not affected by the sample size or defects. However, 2-NH2-TIF-A1 exhibited normal acetylene adsorption because the presence of a strong H-bond between the -NH2 of ad and the uncoordinated carboxylate O atoms of 2-NH2-int prevented C2H2 from approaching the active site and made the rotation of 2-NH2-int ligand more complex.

5. Application

5.1. CO

2

Separation

Considering the presence of abundant active sites on TIF-A1, its CO2 absorption was first evaluated. After becoming fully activated, TIF-A1 exhibited high CO2 uptake at 273 K and 1 bar (107 cm3/g) [64][68]. In contrast, the highest CO2 adsorption in ZIFs at that time was ZIF-69, with a value of 70 cm3/g [22]. The high CO2 adsorption capacity of TIF-A1 can be attributed to the synergistic effect of int and ad ligands. Because the -NH2 group amino and pyrimidine N atoms in ad form an electron-rich system, they are potential CO2 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 CO2 adsorption. Furthermore, the special interaction between TIF-A1 and CO2 indicates that TIF-A1 has the potential ability to selectively adsorb CO2, which makes it possible to apply TIF-A1 in the capture and separation of CO2 in flue gas. The flue gas mainly contains CO2 and N2, which are similar in molecular size, and it is difficult to achieve the separation effect by the pure physical adsorption of porous materials. The N2 adsorption of TIF-A1 was 4.7 cm3/g (273 K) and 1.4 cm3/g (298 K), which shows that TIF-A1 barely adsorbed N2. Henry’s constant indicates that the adsorption selectivity of TIF-A1 for CO2 over N2 was 90 (273 K,1 bar) and 60 (298 K,1 bar), respectively [58][62]. The excellent separation selectivity of TIF-A1 was further demonstrated by a breakthrough experiment [63][67].

5.2. NH

3

Adsorption

Due to the strong electronegativity of the N sites in TIF-A1, it can not only form an electron-rich system to attract CO2 but also form a strong hydrogen bond with NH3. Therefore, the NH3 adsorption of 3-NH2-TIF-A1 was studied. At 298 K and 1 bar, the maximum NH3 adsorption capacities of 3-NH2-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][63]. 3-NH2-TIF-A1 has abundant active sites, including an uncoordinated oxygen atom and -NH2 group of 3-NH2-int, uncoordinated N atoms and -NH2 group of the ad ligand. Among them, the -NH2 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. C

2

Separation

In addition to CO2, TIF-A1 also exhibited a good C2 separation ability. In 2022, Ding and co-workers applied TIF-A1 to trap C2H2 and C2H6 simultaneously from the ternary mixture gas of C2H2/C2H4/C2H6 and performed the purification of C2H4 [66][70]. In the ternary mixture of C2H2/C2H4/C2H6, a strong electrostatic interaction occurred between C2H2 and the uncoordinated carboxyl O with high polarity in TIF-A1, and van der Waals (vdW) interaction occurred between C2H6 and the aromatic heterocycles with low polarity. TIF-A1 not only provides a strong binding site for the adsorption of C2H2 and C2H6, but also oblate C2H6 and linear C2H2 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 C2H2/C2H4/C2H6 (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. CO

2

Cycloaddition

In 2021, Wang and co-workers selected ZnX2 (X = Cl, Br, I) instead of ZnNO3 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 CO2 activation reaction [67][71]. Due to their potent nucleophilic nature, halogen ions induced the ring-opening of propylene oxide, facilitating the reaction between CO2 and propylene oxide to generate propylene carbonate—a pivotal step in catalyzing CO2 conversion. The outcomes demonstrated that the yield of the CO2 activation reaction catalyzed by ZnI2-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, ZnI2-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][71].

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][72]. 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 -NH2 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][72]. 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.
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