1.2 Design principles of water stable MOFs 3
1.2.4 Hydrophobicity 11
Water-based hydrolysis of MOFs occurs mainly in two primary steps. At first, the water molecule comes close to the metal cluster, which allows the interaction between the electrophilic metal ion and nucleophilic water molecule. In the second step, the energetics of this interaction must be greater to overcome the activation energy barrier of the hydrolysis reaction.39 The framework hydrophobicity can play a key role to control the first step of the reaction. It has been observed that the stability of MOFs under humid conditions can be improved by incorporating hydrophobic functional groups in the ligands.51 The direct installation of ligands during MOF synthesis or post-synthetic modification (PSM) of ligands was widely adopted for the synthesis of hydrophobic frameworks.
1.2.4.1 Hydrophobic ligands
The use of hydrophobic ligands during the synthesis is a simple and single-step protocol for the synthesis of hydrophobic MOFs. The presence of hydrophobic groups in the framework structure increases the pore hydrophobicity and repeal the water molecules, which prevents further interactions of water molecules with metal ions. Methyl, ethyl, phenyl or long chain alkyl groups including fluorinated functional groups are commonly used for the design of hydrophobic ligands.
The pore hydrophobicity decreases the water adsorption, which can be directly verified by performing the water adsorption isotherms on the sample. The fluorous metal-organic frameworks
(FMOFs) synthesized by Omary et al. with a fluorous ligand, 3,5-bis(trifluoromethyl)-1,2,4- triazolate showed no detectable water adsorption even at near 100% relative humidity, confirming the hydrophobicity of the framework.52 Few water-stable MOFs constructed with hydrophobic ligands are summarized in Table 1.2.
Table 1.2 Summary of some representative stable MOFs constructed with hydrophobic ligands.
MOFs Surface
Modification Type
Synthesis Method
Structure of Organic Ligand
Water Contact Angle [°]
Ref.
PESD‐1 aromatic ring direct synthesis
>150 53
UPC-21 multi- aromatic rings
direct synthesis
145 ± 1 54
FMOF-1/
FMOF-2
fluorinated aromatic ring
direct synthesis
- 52
MOFF-1 fluorinated aromatic ring
direct synthesis
108 ± 2 55
MOFF-2 fluorinated aromatic ring
direct synthesis
151 ± 1 55
MOFF-3 fluorinated aromatic ring
direct synthesis
135±2 55
UHMOF‐
100
fluorinated aromatic ring
direct synthesis
176 56
Al-MIL‐
53‐AM4
alkyl chain (C4)
post- synthetic modification
>150 57
Al-MIL- 53‐AM6
alkyl chain (C6)
post- synthetic modification
>150 57
F-ZIF-90 fluorinated aromatic ring
post- synthetic modification
159.1 58, 59
1.2.4.2 Post-synthetic modification
Post-synthetic modification (PSM) is another well accepted method to tune the surface functionality of MOF materials. The transformation of hydrophilic to hydrophobic MOFs via PSM method was accomplished by several research groups. This transformation can be accomplished by mainly two methods. First, the ligand present in MOFs can be functionalized by PSM method.
Second, the surface of the MOF material can be coated by various techniques.
1.2.4.2.1 Post-synthetic functionalization of ligands
Cohen et al. showed post-synthetic covalent modification of metal-organic frameworks with long alkyl substituents to protect these materials against moisture.57 The MOF constructed with polar 2-amino-1,4-benzenedicarboxylate (NH2-BDC) displayed hydrophilic properties. The amino group was modified with different alkyl anhydrides to form amide-functionalized MOFs.
The introduction of hydrophobic alkyl chains via PSM improves the water resistance and change the physical properties (i.e., hydrophobicity) of the MOF. Contact angle measurements are commonly used to examine the hydrophobic properties of materials. The materials with water contact angles (WCA) >150° are considered as superhydrophobic. After modification, Al-MIL- 53-NH2 MOF possessed superhydrophobic properties with contact angles greater than 150°.
Superhydrophobic zeolitic imidazole framework (F-ZIF-90) was reported by Huang et al.
by using a fluorine-functionalized imidazolate as an organic linker.58,59 PSM of ZIF-90 with
pentafluorobenzylamine via amine condensation reaction resulted in superhydrophobis surface of the MOF material, which showed high steam stability.
1.2.4.2.2 Post-synthetic surface modification
The hydrophobic surface coating of MOF material is another method to stabilize MOFs under humid conditions. A well-known MOF called UiO-66-NH2 was coated with microporous organic network (MON). The MON-coated UiO-66-NH2 was synthesized via Sonogashira coupling of tetra(4-ethynylphenyl)methane with 1,4-diiodobenzene or 4,4′-diiodobiphenyl (Figure 1.8).60 The MON thickness on UiO-66-NH2 was controlled by changing the amount of tetra(4- ethynylphenyl)methane. Water contact angle measurement showed the chemical changes of the surface properties of MOF@MONs as compared to the original UiO-66-NH2 MOF. The water contact angle increased up to 145° for the MON-coated MOF material and it showed an adsorption of a model organic compound, toluene, in water.
Figure 1.8 Schematic presentation for the synthesis of MOF@MON hybrid materials. Reproduced with permission from ref. 60. Copyright 2014 American Chemical Society.
Vapor deposition technique was also adopted to modify the MOF surface with hydrophobic polydimethysiloxane (PDMS) materials to enhance their moisture or water resistance. Yu et al.
developed a general strategy for PDMS coating on MOF-5, HKUST-1 and ZnBT as representative vulnerable MOFs (Figure 1.9).61 The PDMS-coated MOFs showed nearly 100% retention of
porosity as confirmed by surface area analysis. Compared to the pristine MOFs, all PDMS-coated samples displayed water contact angles of 130 ± 2°, which revealed hydrophobic character of the surface.
A plasma-enhanced chemical vapor deposition (PECVD) of perfluorohexane on Cu-BTC MOF was used for the pore surface modification with hydrophobic -CF3 group.62 The plasma- treated MOF showed enhanced stability against degradation by water. The Monte Carlo simulations suggested that the perfluorohexane sites prevent the formation of water clusters within the Cu-BTC MOF, thereby improving the water stability.
Figure 1.9 Schematic presentation of PDMS-coating on the surface of MOFs and the enhancement of water resistance of MOFs. Reproduced with permission from ref. 61. Copyright 2014 American Chemical Society.
Park et al. described the formation of amorphous carbon coatings on the surface of MOFs that prevented hydrolysis.63 The controlled heat treatment of IRMOF‐1 under nitrogen atmosphere led to the formation of an amorphous carbon coating on its surface, which shielded the framework from decomposition under humid conditions. The overheated MOF produced ZnO@carbon material that did not display microporosity or chemical characteristics of MOFs. The carbon- coated MOF prepared at 510° C showed undamaged crystalline structure after 2 h of soaking in water (Figure 1.10).
Figure 1.10 Schematic presentations and XRPD patterns of IRMOF-1 (a) and carbon-coated IRMOF-1 after thermal treatment at increasing temperatures (b and c). At a temperature of 550
°C, the structure of IRMOF-1 transforms into ZnO nanoparticles@amorphous carbon (d).
Reproduced with permission from ref 63. Copyright 2012 Wiley Online Library.