Abstract
Tens of thousands of metal–organic frameworks (MOFs) have been developed in the past two decades, and only ≈100 of them have been demonstrated as porous and hydrophobic. These hydrophobic MOFs feature not only a rich structural variety, highly crystalline frameworks, and uniform micropores, but also a low affinity toward water and superior hydrolytic stability, which make them promising adsorbents for diverse applications, including humid CO2 capture, alcohol/water separation, pollutant removal from air or water, substrate‐selective catalysis, energy storage, anticorrosion, and self‐cleaning. Herein, the recent research advancements in hydrophobic MOFs are presented. The existing techniques for qualitatively or quantitatively assessing the hydrophobicity of MOFs are first introduced. The reported experimental methods for the preparation of hydrophobic MOFs are then categorized. The concept that hydrophobic MOFs normally synthesized from predesigned organic ligands can also be prepared by the postsynthetic modification of the internal pore surface and/or external crystal surface of hydrophilic or less hydrophobic MOFs is highlighted. Finally, an overview of the recent studies on hydrophobic MOFs for various applications is provided and suggests the high versatility of this unique class of materials for practical use as either adsorbents or nanomaterials.
Highlights
BUT-155 showed a high porosity (BET surface area: 2070 m2 g−1 and pore volume: 0.82 cm3 g−1), as well as exceptional stability against water, which was verified by powder X-ray diffraction (PXRD) patterns, N2 adsorption studies, scanning electron microscopy (SEM) images and single-crystal X-ray diffraction (SXRD) studies for the BUT-155 crystals after treatment in water for 10 days at room temperature or in boiling water for 24 h (Figure 7c,d)
The results revealed that ZIF-8 and ZIF-71 exhibit promising separation performances for 1-butanol–water, 1-propanol–water, and 2-propanol–water pairs
The authors later investigated the high-pressure intrusion−extrusion of aqueous KCl, LiCl, and NaCl solutions with ZIF-8.[283]. It was found that the intrusion of the aqueous electrolyte solutions into the pores of ZIF-8 occurred at higher pressures (29–52 MPa) than that for pure water intrusion (27 MPa) (Figure 36c), and the stored energy for a ZIF-8–NaCl solution (4 m) system was increased to 26.0 J g−1
Summary
Among the over 20 000 MOFs reported so far,[6] ≈100 MOFs have been described as hydrophobic Many of these MOFs were reported without extra experimental evidence except crystal structures supporting their hydrophobicity, some experimental analysis methods are often used to assess the hydrophobicity of MOFs qualitatively or quantitatively (Figure 1), namely, the water adsorption isotherm measurement, competitive adsorption of a vapor mixture of water and hydrocarbon vapor (mostly toluene) in a breakthrough experiment, and contact angle of liquid water. These methods are discussed in detail . It should be noted that the crystal structures of MOFs cannot be used to predict the hydrophobicity/hydrophilicity of their external surfaces, because the external surface is a kind of crystal defect, and the bonding environment of the atoms on the surface can be very different from those inside the bulk
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