Hydrophobicity: a key factor en route to applications of metal–organic frameworks

Compositional modularity can be leveraged in metal–organic frameworks (MOFs) like no other, flying high in terms of structural diversity and task-specific adaptability. Hydrophobic MOFs and MOF-derived hydrophobic composites are a relatively small subclass (<100) of MOFs that exhibit surface hydrophobicity. Induced by high surface roughness, superhydrophobic and ultrahydrophobic MOFs (ca. 25) constitute an even smaller, yet conspicuous, subset of MOFs that strongly repel water, usually translating into chemical and hydrolytic robustness. Whereas the superhydrophobic and ultrahydrophobic MOFs/MOF-derived composites reveal exciting properties for oil/water separation, self-cleaning, carbon capture, etc., only a perceptive and systematic evaluation of pore and surface hydrophobicity will lead material chemists to forge ahead with these bespoke physisorbents to higher levels of technological readiness. Water is ubiquitous on planet Earth. Whereas freshwater is proverbially mentioned as life, water in its vapor and liquid forms is infamous as a nemesis to solid adsorbents/catalysts in terms of negatively impacting their relevant performance parameters. Despite early promises, modularly composed metal–organic frameworks (MOFs) are no exception to this oft-encountered handicap of water instability. Thanks to the recent emergence of high surface hydrophobicity, some water-stable MOFs have been realized. We herein put forward a critical analysis of the hydrophobic MOFs and MOF composites known thus far. Recent developments in this area are contextualized from the perspective of future growth as regards several applications of immediate relevance to energy and environmental sustainability for a greener tomorrow. Water is ubiquitous on planet Earth. Whereas freshwater is proverbially mentioned as life, water in its vapor and liquid forms is infamous as a nemesis to solid adsorbents/catalysts in terms of negatively impacting their relevant performance parameters. Despite early promises, modularly composed metal–organic frameworks (MOFs) are no exception to this oft-encountered handicap of water instability. Thanks to the recent emergence of high surface hydrophobicity, some water-stable MOFs have been realized. We herein put forward a critical analysis of the hydrophobic MOFs and MOF composites known thus far. Recent developments in this area are contextualized from the perspective of future growth as regards several applications of immediate relevance to energy and environmental sustainability for a greener tomorrow. difference between the two dynamic WCAs, advancing and receding WCAs. studies the influence of molecular solid-state structures upon crystal packing, crystal structure, and physicochemical properties. hydro = water; philicity = affinity. Surfaces that register static WCA < 90° are generally adjudged hydrophilic. hydro = water; phobicity = lack of affinity, stemming from the Greek word phobos, meaning fear or horror. Surfaces that register static WCA > 90° are generally adjudged hydrophobic. a combination of seven performance parameters that must be exhibited by an adsorbent solid with respect to commodity chemical purifications. These are (i) stability; (ii) recyclability (including regenerability); (iii) kinetics; (iv) selectivity; (v) adsorption capacity; (vi) scale-up; and (vii) cost [5.Mukherjee S. et al.Crystal engineering of porous coordination networks to enable separation of C2 hydrocarbons.Chem. Commun. 2020; 56: 10419-10441Crossref PubMed Google Scholar]. surfaces on which water droplets elicit static WCA > 150° and CAH is < 20°. For simplicity (and lack of experimental data), only the first criterion is typically considered. a metric for assessing the maturity of technologies that enables consistent and uniform understanding of technical maturity across different technological sectors, addressing the entire innovation chain [57.European Commission Horizon 2020 – Work Programme 2016-2017 General Annexes; Annex G. EC, 2017Google Scholar]. angle formed by a water droplet at the three-phase boundary where a water, gas, and solid intersect; can be static or dynamic (based on nature of the three-phase contact lines). a simplified scale for surface wetting assessment, where static WCAs are the only metric of defining surface wetting. surfaces on which water droplets elicit static WCA > 170° and negligible CAH. For simplicity (and lack of experimental data), only the first criterion is typically considered.