Abstract
Metal-organic frameworks, popularly known as MOFs, have come of age—thanks to custom-built design principles enabling them to serve a myriad of task-specific applications. In an era where >6000 new MOFs are annually published, driven by academic research, a new generation of MOFs that can find use in niche markets is on the rise. One of the foremost challenges that plague the translation of MOFs into materials suited for commercialization is their water-influenced degradation, largely accountable to hydrolytic instability and the ubiquity of water. Hydrophobic MOFs offer one of the most promising solutions which can enable these porous materials to leverage their functionalization guided properties even under extreme humid conditions, a criterion that application-ready materials should necessarily serve. In this contribution, our discussion focuses on the state-of-the-art hydrophobic MOFs and MOF-derived composites. Outlining a brief overview of the structure-property correlation aspects in each of these promising hydrophobic MOFs with little or no influence exhibited to water and/or humidity, a future outlook is put forward to enable potentially better design strategies leading to futuristic MOFs tolerant to water/moisture. Concise premise of this review allows us to limit our discussion to the design principles in action behind the most hydrophobic MOFs/derived composites reported thus far and to discuss their prime applications viz., oil/water separation inclusive of self-cleaning, hydrocarbon separation, and tackling marine oil spillage. When synergized, such improved porous material design approaches lean toward a greener environment of tomorrow.
Highlights
Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs),[1] according to the International Union of Pure and Applied Chemistry (IUPAC) recommendations 2013, are defined by metal and organic ligand derived coordination networks containing potential voids.[2]
A comparable scitation.org/journal/apm approach of grafting octadecylamine onto the coordinatively unsaturated metal centers (UMCs) of MIL-101(Cr), UiO-66, ZIF-8-melamine sponge (ZIF)-67, and HKUST-1 led to superhydrophobic surface traits for all the postPSM MOFs, whereas the S-MIL-101(Cr)/commercially available filter paper composite exhibited high organic solvent sorption capacities coupled with high oil/water separation performance (>99.5%), with no external pressure.[90]
As it transpires from the discussion, that hydrophobicity in MOFs can be engineered has been realized in the recent times
Summary
Metal-organic frameworks (MOFs) or porous coordination polymers (PCPs),[1] according to the International Union of Pure and Applied Chemistry (IUPAC) recommendations 2013, are defined by metal and organic ligand derived coordination networks containing potential voids.[2] Key features such as high surface area, targeted pore functionalization, thermal/mechanical stability, and network modularity have spurred enormous research interest in the last two decades to establish MOFs as the most sought-after class of new-generation materials,[3] promising for a range of applications These largely include gas storage,[4] chemical separation,[5] sensing,[6] heterogeneous catalysis,[7] drug delivery,[8] energy storage,[9] and environmental remediation.[10] Regardless of their high promises, there has been a shortfall in their practical deliverables, primarily owing to the high water affinity of MOFs.[11] Most of the MOFs being labile or hemilabile,[12] humidity influenced coordination bond (M-L) cleavage renders hydrolytic instability to them, overshadowing their performances. It highlights a range of surface property-derived applications hydrophobic MOFs can potentially afford (Fig. 1)
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