Abstract
Metal-organic frameworks (MOFs) have diverse potential applications in catalysis, gas storage, separation, and drug delivery because of their nanoscale periodicity, permanent porosity, channel functionalization, and structural diversity. Despite these promising properties, the inherent structural features of even some of the best-performing MOFs make them moisture-sensitive and unstable in aqueous media, limiting their practical usefulness. This problem could be overcome by developing stable hydrophobic MOFs whose chemical composition is tuned to ensure that their metal-ligand bonds persist even in the presence of moisture and water. However, the design and fabrication of such hydrophobic MOFs pose a significant challenge. Reported syntheses of hydrophobic MOFs are critically summarized, highlighting issues relating to their design, characterization, and practical use. First, wetting of hydrophobic materials is introduced and the four main strategies for synthesizing hydrophobic MOFs are discussed. Afterward, critical challenges in quantifying the wettability of these hydrophobic porous surfaces and solutions to these challenges are discussed. Finally, the reported uses of hydrophobic MOFs in practical applications such as hydrocarbon storage/separation and their use in separating oil spills from water are summarized. Finally, the state of the art is summarized and promising future developments of hydrophobic MOFs are highlighted.
Highlights
Hydrophobic surfaces are defined as substrates with an apparent contact greater than 90° with respect to water, while surfaces of superhydrophobic materials have contact angles above 150° and very low adhesion to water droplets because drops partially rest on an air cushion
This review provides a comprehensive summary of the characterization, preparation and applications of hydrophobic metal−organic frameworks (MOFs) and their composites, highlighting state-of-the-art strategies
We discussed the basics of wetting of hydrophobic materials, followed by four strategies for preparing hydrophobic MOFs, namely (a) the use of hydrophobic ligands, (b) postsynthetic grafting of hydrophobic side chains onto reactive sites, (c) the targeted exploitation of surface corrugation to induce hydrophobicity, and (d) the preparation of hydrophobic hierarchical porous composite structures
Summary
Over the past two decades, novel hybrid nanoporous metal−organic frameworks (MOFs) have been developed as a new class of tunable hybrid materials comprised of ordered networks formed from organic ligands and metal cations.[1,2,3,4,5,6,7,8,9,10] They are typically synthesized under mild conditions via coordination-directed self-assembly processes and are known as metalorganic coordination networks (MCNs) and porous coordination polymers (PCPs).[11,12,13,14] Due to their high surface areas, large porosity, tunable pore sizes, and functionalities, MOFs have prospective applications in fields such as gas storage/separation, sensing or recognition, proton conduction, and magnetism.[14,15,16,17,18,19,20,21,22,23,24,25,26,27,28] the advantageous unique structural features of even some of the best-performing eminent MOFs are readily degraded because of their high moisture sensitivity, which may limit their practical applications.[29,30,31,32] there is an ongoing search for highly hydrophobic, porous, sorbent materials to be employed in various large-scale applications in industry such as oil spill cleanup, hydrocarbon storage/separation or water purification.[33,34,35,36,37] Many academics, industrial scientists, and engineers have conducted research on the fabrication of superhydrophobic surfaces, which involves hydrophobic surface modification and creating surface roughness on the micrometer or nano- scale. We offer some perspectives on future directions for this promising field
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