Recent Hydrophobic Metal-Organic Frameworks and Their Applications

The focus of discussion of this review is the application of the most recent synthesized hydrophobic metal-organic frameworks (MOFs). The most promising hydrophobic MOFs are mentioned with their applications and discussed. The various MOFs considered are sub-sectioned into the main application areas, namely alcohol adsorption and oil/water-alcohol/water separation, gas separation and storage, and other applications such as self-cleaning and liquid marbles. Again, the methods of synthesis are briefly described, showing how the features of the end product aid in their applications. The efficiency of the MOF materials and synthesis methods are highlighted and briefly discussed. Lastly, the summary and outlook section concludes the write-up giving suggestions that would be useful to present-day researchers.


Introduction
Metal-organic frameworks (MOFs) are an attractive crystalline class of porous materials that entail metal ions and organic bridging ligands coordinated together in a controlled manner to enhance several requisite features over their conventional porous inorganic and organic counterparts such as activated carbons and zeolites [1][2][3][4][5]. These materials have gained enormous attention from various scientists around the world.
Over the years a vast variety of MOFs have been produced and have served diverse purposes in a wide range of industries. MOFs have attracted the attention of many scientists due to their large surface area, designable pore structure and functionality, high thermal stability and huge porosity [1,6,7]. These special properties have made them promising in diverse applications including gas storage [8,9] and separation [10][11][12][13], chemical sensing [14,15], catalysis [16][17][18][19][20], wastewater treatment [21], proton conductors [22], and drug delivery [23]. Down the line, a disturbing problem was realized with these impactful materials upon their application in moisture-bound areas; most MOFs degrade under moisture conditions, limiting their functions in some industries [6,24].
Instability of MOFs is ascertained when the original structure of a MOF is distorted, and this is mainly realized by performing powder X-ray diffraction (XRD) and nitrogen sorption isotherms. With the sorption isotherms, the surface area of the MOF reduces and, with the XRD, the pattern of the peaks metamorphose or change [24].
It was reported that MOFs get degraded via two main mechanisms namely: (1) ligand displacement and (2) hydrolysis [25]. These mechanisms were arrived at through computational chemistry and confirmed experimentally. The ligand displacement occurs when a water molecule gets

Alcohol Adsorption and Oil/Water-Alcohol/Water Separation
The development of renewable and clean energy sources such as bioalcohols has attracted attention in recent years because of emerging scarcity of fossil resources and the accompanying critical environmental concerns [44]. Adsorptive separation by using porous materials is considered to be one of the most cost-effective and environmentally friendly methods of recovering bioalcohols from fermentation broth produced from biomass. The use of hydrophobic MOFs for adsorptive separation of alcohol/water mixture is a probable solution to this problem as they have a less strong affinity to water compared to other porous materials such as activated carbons, zeolites and polymeric resins which have as well been tested for this application [45][46][47][48][49]. Adjustable internal surface properties and tunable pore size [50,51], and hydrophobicity [45,52] are important criteria for the adsorption performances of such materials.
Moreover, hydrophobic MOFs for this same common reason of moisture stability hold promise for oil/water separation compared to other porous adsorbent materials such as organoclays, zeolites, activated carbons, sand and cotton fibers which have been used to tackle oil spillage [53][54][55]. Furthermore, fabricated functional materials are significant for preparing efficient oil-absorbing materials such as microporous polymers [45], macroporous gels [56], cross-linked polymer gels [57], meshes/membranes [58], sponges [59] and graphene [60]. This points out the necessity of the use of hydrophobic MOFs to solve such problems. Wettability of surface area, water resistance, and aqueous stability are the most important criteria for this application.
The exterior of hydrophobic surfaces and in many cases superhydrophobic surfaces have low surface energy [61,62] which is a phenomenon of superhydrophobic (SH) MOFs. Alcohol adsorption from aqueous solution is one essential application area for such materials. Liu et al. fluorinated Zeolitic Imidazolate Framework, ZIF-90 through an amine condensation reaction with pentafluorobenzylamine [63]. This type of post-functionalization was possible due to the presence of aldehyde groups in the ZIF-90 framework (Figure 2), proved by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectra. XRD proved that the morphology of the asprepared ZIF-90 was maintained after fluorination. The reported CA as shown in Table 1 was 152.4° which proved its hydrophobicity. It achieved a 98% recovery of ethanol from an ethanol/water

Alcohol Adsorption and Oil/Water-Alcohol/Water Separation
The development of renewable and clean energy sources such as bioalcohols has attracted attention in recent years because of emerging scarcity of fossil resources and the accompanying critical environmental concerns [44]. Adsorptive separation by using porous materials is considered to be one of the most cost-effective and environmentally friendly methods of recovering bioalcohols from fermentation broth produced from biomass. The use of hydrophobic MOFs for adsorptive separation of alcohol/water mixture is a probable solution to this problem as they have a less strong affinity to water compared to other porous materials such as activated carbons, zeolites and polymeric resins which have as well been tested for this application [45][46][47][48][49]. Adjustable internal surface properties and tunable pore size [50,51], and hydrophobicity [45,52] are important criteria for the adsorption performances of such materials.
Moreover, hydrophobic MOFs for this same common reason of moisture stability hold promise for oil/water separation compared to other porous adsorbent materials such as organoclays, zeolites, activated carbons, sand and cotton fibers which have been used to tackle oil spillage [53][54][55]. Furthermore, fabricated functional materials are significant for preparing efficient oil-absorbing materials such as microporous polymers [45], macroporous gels [56], cross-linked polymer gels [57], meshes/membranes [58], sponges [59] and graphene [60]. This points out the necessity of the use of hydrophobic MOFs to solve such problems. Wettability of surface area, water resistance, and aqueous stability are the most important criteria for this application.
The exterior of hydrophobic surfaces and in many cases superhydrophobic surfaces have low surface energy [61,62] which is a phenomenon of superhydrophobic (SH) MOFs. Alcohol adsorption from aqueous solution is one essential application area for such materials. Liu et al. fluorinated Zeolitic Imidazolate Framework, ZIF-90 through an amine condensation reaction with pentafluorobenzylamine [63]. This type of post-functionalization was possible due to the presence of aldehyde groups in the ZIF-90 framework (Figure 2), proved by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FT-IR) spectra. XRD proved that the morphology of the as-prepared ZIF-90 was maintained after fluorination. The reported CA as shown in Table 1  proved its hydrophobicity. It achieved a 98% recovery of ethanol from an ethanol/water mixture within 20 h of contact whereas the unfluorinated or unmodified ZIF-90 adsorbed only 7% of ethanol from a similar mixture. The superhydrophobic MOF exhibits high adsorptive separation qualities for the removal of other bioalcohols such as methanol, isopropanol and butanol as well as its mixtures ( Figure 3a). Interestingly, the material can be regenerated by simple thermal regeneration under vacuum and is highly adsorptive even after five successive cycles. These qualities make this material suitable for recovery of bioalcohols from aqueous solution and can be employed commercially to produce biofuels. Table 1.
Properties of hydrophobic MOFs highlighting contact angle (CA), postsynthetic modification (PSM), SC (self-cleaning), pH stability range (SR), moisture stability (MS), thermal stability, ionic solution stability range (ISSR), Brunauer-Emmett-Teller (BET) surface area (SA) and n-octadecylphosphonic acid (OPA).  Figure 3a). Interestingly, the material can be regenerated by simple thermal regeneration under vacuum and is highly adsorptive even after five successive cycles. These qualities make this material suitable for recovery of bioalcohols from aqueous solution and can be employed commercially to produce biofuels.    Recently, the Burg group presented a new method called nanomechanical mass correlation spectroscopy (MCS) [67] to measure the effective mass density of MOF nanoparticles in different solvent systems [68]. Three different MIL-101(Cr) MOF species were employed: MIL-101(Cr) nanoparticles and its derivatives functionalized at the coordinatively unsaturated metal sites with pyridine or pyrazine using postsynthetic modification [69]. MIL-101(Cr) with different inner pore functionalizations were suspended in binary mixtures of ethanol and water, and methoxyperfluorobutane (HFE-7100) and ethanol. Mass fluctuations resulting from the flow of this suspension through a suspended microchannel resonator were measured. The pyridine-modified MOF, which is more hydrophobic than the other two kinds, records a lower effective density in the polar mixture, which is probably due to increased ethanol content within the particles. It was said that this occurred because the particles aggregated with ethanol in the interstices and/or ethanol solvation layer was formed around the nanoparticles. The tuning of the inner functionalization of the MOF is also a contributing factor to this effect. The differences observed contribute new information about the specific interaction between the different solvent components and the internal surface. Again, the findings in this work show new opportunities for the use of MOF nanoparticles in the separation of solvent mixtures founded on the selective enhancement of a solvent component in the pores.

MOF
Oil/water and alcohol/water separation is another prime use of hydrophobic MOFs. Zhang et al. conducted a diauxic growth strategy on their previous MOF UPC-21 making it highly hydrophobic, with CA 145 ± 1° and exhibiting high oleophilicity [65]. The filtrate obtained from the initial MOF synthesis with the ligand H4L and metal compound Cu(NO3)2 was further placed again in the same condition as the previous MOF to obtain highly hydrophobic UPC-21 with high yield. The main contributing factor to the hydrophobicity of this MOF was the intelligent choice of organic ligand H4L. It carries multi-aromatic hydrocarbon units in its pentiptycene core, which are hydrophobic and arranged in a 2D layer, and is the key reason for the hydrophobic behavior of UPC-21. It was used to separate water from several organic solvents and oils by simple filtration method and achieved approximately 99% separation efficiency. Figure 4 depicts how the separation was conducted. It is a promising material for oil recovery during the occurrence of oil spills. Recently, the Burg group presented a new method called nanomechanical mass correlation spectroscopy (MCS) [67] to measure the effective mass density of MOF nanoparticles in different solvent systems [68]. Three different MIL-101(Cr) MOF species were employed: MIL-101(Cr) nanoparticles and its derivatives functionalized at the coordinatively unsaturated metal sites with pyridine or pyrazine using postsynthetic modification [69]. MIL-101(Cr) with different inner pore functionalizations were suspended in binary mixtures of ethanol and water, and methoxyperfluorobutane (HFE-7100) and ethanol. Mass fluctuations resulting from the flow of this suspension through a suspended microchannel resonator were measured. The pyridine-modified MOF, which is more hydrophobic than the other two kinds, records a lower effective density in the polar mixture, which is probably due to increased ethanol content within the particles. It was said that this occurred because the particles aggregated with ethanol in the interstices and/or ethanol solvation layer was formed around the nanoparticles. The tuning of the inner functionalization of the MOF is also a contributing factor to this effect. The differences observed contribute new information about the specific interaction between the different solvent components and the internal surface. Again, the findings in this work show new opportunities for the use of MOF nanoparticles in the separation of solvent mixtures founded on the selective enhancement of a solvent component in the pores.
Oil/water and alcohol/water separation is another prime use of hydrophobic MOFs. Zhang et al. conducted a diauxic growth strategy on their previous MOF UPC-21 making it highly hydrophobic, with CA 145 ± 1 • and exhibiting high oleophilicity [65]. The filtrate obtained from the initial MOF synthesis with the ligand H 4 L and metal compound Cu(NO 3 ) 2 was further placed again in the same condition as the previous MOF to obtain highly hydrophobic UPC-21 with high yield. The main contributing factor to the hydrophobicity of this MOF was the intelligent choice of organic ligand H 4 L. It carries multi-aromatic hydrocarbon units in its pentiptycene core, which are hydrophobic and arranged in a 2D layer, and is the key reason for the hydrophobic behavior of UPC-21. It was used to separate water from several organic solvents and oils by simple filtration method and achieved approximately 99% separation efficiency. Figure 4 depicts how the separation was conducted. It is a promising material for oil recovery during the occurrence of oil spills.  The organic linker component has an impactful influence over controlling the chemical and structural properties of the resultant material [3,70]. Hence, the systematic synthesis of new functionalized MOFs requires a meticulous choice of the organic linker component. In view of this, Mukherjee et al synthesized an ultrahydrophobic MOF named UHMOF-100 with a careful choice of the organic ligand [32]. The new carboxylic low symmetry organic linker was cleverly synthesized, having in mind that such have a remarkable impact on the contact surface traits of the resulting material with liquid water [71]. The initial product formed, named UHMOF-100a contained dimethylformamide (DMF) guest molecules which were washed off to obtain UHMOF-100. Powder X-ray diffraction (PXRD) patterns show that the porous crystallinity and water stability of the latter are far higher than that of the former (Figure 5d). UHMOF-100 has by far recorded the highest water CA of approximately 176° and this is evident from Table 1. UHMOF-100-spray-coated polymeric hydrophobic membrane (UHMOF-100/PDMS/PP) was exploited in oil-water separation (Scheme 1), with substantial oil hexadecane, crude oil, toluene, biodiesel, and CCl4. The absorption capacities ranged from 40 wt.% to 70 wt.% with negligible difference in absorption capacities even after 10 cycles. Again, when water-in-oil emulsions prepared from dichloromethane, toluene and hexadecane were each poured on the upper side of UHMOF-100/PDMS/PP, emulsion droplets instantly demulsified. Oil components permeated right away through the membrane retaining water wholly above the membrane. It also proved efficient in alcohol-water separation by showing a sharp contrast between alcohol sorption isotherms and that of water. The performance of this MOF is partially attributed to unsaturated copper open metal sites and guest-accessible voids (Figure 5c). The organic linker component has an impactful influence over controlling the chemical and structural properties of the resultant material [3,70]. Hence, the systematic synthesis of new functionalized MOFs requires a meticulous choice of the organic linker component. In view of this, Mukherjee et al. synthesized an ultrahydrophobic MOF named UHMOF-100 with a careful choice of the organic ligand [32]. The new carboxylic low symmetry organic linker was cleverly synthesized, having in mind that such have a remarkable impact on the contact surface traits of the resulting material with liquid water [71]. The initial product formed, named UHMOF-100a contained dimethylformamide (DMF) guest molecules which were washed off to obtain UHMOF-100. Powder X-ray diffraction (PXRD) patterns show that the porous crystallinity and water stability of the latter are far higher than that of the former (Figure 5d). UHMOF-100 has by far recorded the highest water CA of approximately 176 • and this is evident from Table 1. UHMOF-100-spray-coated polymeric hydrophobic membrane (UHMOF-100/PDMS/PP) was exploited in oil-water separation (Scheme 1), with substantial oil hexadecane, crude oil, toluene, biodiesel, and CCl 4 . The absorption capacities ranged from 40 wt.% to 70 wt.% with negligible difference in absorption capacities even after 10 cycles. Again, when water-in-oil emulsions prepared from dichloromethane, toluene and hexadecane were each poured on the upper side of UHMOF-100/PDMS/PP, emulsion droplets instantly demulsified. Oil components permeated right away through the membrane retaining water wholly above the membrane. It also proved efficient in alcohol-water separation by showing a sharp contrast between alcohol sorption isotherms and that of water. The performance of this MOF is partially attributed to unsaturated copper open metal sites and guest-accessible voids (Figure 5c). Modification of the external surface of zirconium-based MOFs via a facile method as shown in Scheme 2 through the incorporation of n-octadecylphosphonic acid (long alkyl chains) was Modification of the external surface of zirconium-based MOFs via a facile method as shown in Scheme 2 through the incorporation of n-octadecylphosphonic acid (long alkyl chains) was performed by Ma et al. [62]. One important objective of this work was to impart hydrophobicity on the MOFs without the intact structure and internal channel environment being adjusted. It resulted in a superhydrophobic MOF of CA above 150 • , was also used for oil-water and alcohol-water separation and absorbed up to about 99% of oil from oil-water mixture (Figure 3b). performed by Ma et al. [62]. One important objective of this work was to impart hydrophobicity on the MOFs without the intact structure and internal channel environment being adjusted. It resulted in a superhydrophobic MOF of CA above 150°, was also used for oil-water and alcohol-water separation and absorbed up to about 99% of oil from oil-water mixture (Figure 3b). Again, obtaining hydrophobic MOFs by coating the MOF surface with hydrophobic polymer crystals is inappropriate for this application. The reason is that these layers interact weakly with the water and do not promote water droplets from spreading on its surface [42]. The use of hydrophobic organic linkers in synthesis usually results in low surface area MOFs and is very evident in UHMOF-100 with BET of 469.2 m 2 ·g −1 . UHMOF-100 showed the highest efficiency for crude oil but the reverse was true for UPC-21 MOF. OPA-UiO-66 performed a better methanol adsorption than the fluorinated ZIF-90 MOF. Toluene performed much better with OPA-UiO-66 than UHMOF-100. The alcohol removal percentage with fluorinated ZIF-90 increased with a decrease in polarity of adsorbate. These interesting observations reiterate that the methods used for the synthesis of the hydrophobic MOFs were effective for their respective applications; varying materials and methods yield varying specific results upon application.

Gas Separation and Storage
Gas separation and storage has been one strong application area for MOFs as a whole. The metal clusters linked by organic ligands in MOFs yields porous three-dimensional networks with large pore volumes, high surface areas, low densities, good thermal stability, controlled porosities and the possibility of adjusting the pore size and chemical compositions [72][73][74][75]. These properties make this class of materials highly suitable for gas separation and storage [76][77][78][79]. Adsorption selectivity and uptake capacity are the most important for gas separation purposes [80]. The separation of various gases is an essential industrial process [81][82][83][84][85] because of the high quality and purity requirements for their practical use [86][87][88][89][90]. Again, carbon dioxide is a greenhouse gas that needs to be reduced in the atmosphere to avoid the dramatic consequences of global warming. Increased surface area, pore size, and moisture stability of MOFs are the most efficient approaches to maximize CO2 uptake [91,92]. Hydrophobic MOFs promise to be of highly valuable use in this rather broad area because of their structural stability in moist environments.
The UPC-21 hydrophobic MOF synthesized by the Sun group was also tested for separation of gases (C1-C3). It was found to compete with other MOFs for acetylene uptake especially [93] and other light hydrocarbons. It takes up to 196.5 cm 3 g −1 at 273 K and 139.5 cm 3 g −1 at 295 K of acetylene [65]. Moreover, it has a good uptake performance for other light hydrocarbons such as C2H4, C2H6, C3H6 and C3H8 at 273 K and 1 bar, values being 123.1, 137.6, 124.1 and 116.2 cm 3 g −1 respectively ( Figure 6). In comparison with other promising MOFs such as UTSA-67a [94] and Cu(etz) [95], UPC-21 has a higher uptake for light hydrocarbons (C2-C3). Again, it is of great importance to note that UPC-21 displays much higher selectivity for C2H6/CH4 at 295 K (Figure 7) than the previously Again, obtaining hydrophobic MOFs by coating the MOF surface with hydrophobic polymer crystals is inappropriate for this application. The reason is that these layers interact weakly with the water and do not promote water droplets from spreading on its surface [42]. The use of hydrophobic organic linkers in synthesis usually results in low surface area MOFs and is very evident in UHMOF-100 with BET of 469.2 m 2 ·g −1 . UHMOF-100 showed the highest efficiency for crude oil but the reverse was true for UPC-21 MOF. OPA-UiO-66 performed a better methanol adsorption than the fluorinated ZIF-90 MOF. Toluene performed much better with OPA-UiO-66 than UHMOF-100. The alcohol removal percentage with fluorinated ZIF-90 increased with a decrease in polarity of adsorbate. These interesting observations reiterate that the methods used for the synthesis of the hydrophobic MOFs were effective for their respective applications; varying materials and methods yield varying specific results upon application.

Gas Separation and Storage
Gas separation and storage has been one strong application area for MOFs as a whole. The metal clusters linked by organic ligands in MOFs yields porous three-dimensional networks with large pore volumes, high surface areas, low densities, good thermal stability, controlled porosities and the possibility of adjusting the pore size and chemical compositions [72][73][74][75]. These properties make this class of materials highly suitable for gas separation and storage [76][77][78][79]. Adsorption selectivity and uptake capacity are the most important for gas separation purposes [80]. The separation of various gases is an essential industrial process [81][82][83][84][85] because of the high quality and purity requirements for their practical use [86][87][88][89][90]. Again, carbon dioxide is a greenhouse gas that needs to be reduced in the atmosphere to avoid the dramatic consequences of global warming. Increased surface area, pore size, and moisture stability of MOFs are the most efficient approaches to maximize CO 2 uptake [91,92]. Hydrophobic MOFs promise to be of highly valuable use in this rather broad area because of their structural stability in moist environments.
The UPC-21 hydrophobic MOF synthesized by the Sun group was also tested for separation of gases (C 1 -C 3 ). It was found to compete with other MOFs for acetylene uptake especially [93] and other light hydrocarbons. It takes up to 196.5 cm 3 g −1 at 273 K and 139.5 cm 3 g −1 at 295 K of acetylene [65]. Moreover, it has a good uptake performance for other light hydrocarbons such as C 2 H 4 , C 2 H 6 , C 3 H 6 and C 3 H 8 at 273 K and 1 bar, values being 123.1, 137.6, 124.1 and 116.2 cm 3 g −1 respectively ( Figure 6). In comparison with other promising MOFs such as UTSA-67a [94] and Cu(etz) [95], UPC-21 has a Materials 2018, 11, 2250 9 of 21 higher uptake for light hydrocarbons (C 2 -C 3 ). Again, it is of great importance to note that UPC-21 displays much higher selectivity for C 2 H 6 /CH 4 at 295 K (Figure 7) than the previously reported MOFs. The high density of open Cu (II) sites, optimized pore size, and multi-aromatic rings are the main contributions to its advantage over other MOFs in this application area [65].       Considering the capture of CO 2 from flue gas, a major requirement is that the incoming gas stream is dehydrated since water can bring about a noticeable reduction in the CO 2 sorption capabilities [96][97][98]. Coating MOF surfaces with a hydrophobic polymer to render them water-resistant was recently illustrated by Qian et al. via a facile solution-immersion process [66]. The approach was attempted on three representative water-sensitive MOFs: NH 2 -MIL-125(Ti), ZIF-67 and HKUST-1 to prove its universality. The MOFs were coated with organosilicon, (DC 1-2577), depositing a layer of hydrophobic high molecular polymer on its external surface, not deeply penetrating the internal pores of the MOFs. The resulting MOFs retained their crystal structure, morphology and surface area when exposed to liquid water, and with a water CA of 146 • . The surface hydrophobic MOFs retained up to about 86% of their CO 2 sorption and desorption capabilities after being exposed to water, while that of the as-synthesized MOFs is about 21%. Scanning Electron Microscope (SEM) images shown in Figure 8 give further proof of the stability of the coated MOFs in water. Considering the capture of CO2 from flue gas, a major requirement is that the incoming gas stream is dehydrated since water can bring about a noticeable reduction in the CO2 sorption capabilities [96][97][98]. Coating MOF surfaces with a hydrophobic polymer to render them waterresistant was recently illustrated by Qian et al via a facile solution-immersion process [66]. The approach was attempted on three representative water-sensitive MOFs: NH2-MIL-125(Ti), ZIF-67 and HKUST-1 to prove its universality. The MOFs were coated with organosilicon, (DC 1-2577), depositing a layer of hydrophobic high molecular polymer on its external surface, not deeply penetrating the internal pores of the MOFs. The resulting MOFs retained their crystal structure, morphology and surface area when exposed to liquid water, and with a water CA of 146°. The surface hydrophobic MOFs retained up to about 86% of their CO2 sorption and desorption capabilities after being exposed to water, while that of the as-synthesized MOFs is about 21%. Scanning Electron Microscope (SEM) images shown in Figure 8 give further proof of the stability of the coated MOFs in water. Moreover, a group of other scientists with the approach of facile vapor deposition technique, by polydimethylsiloxane (PDMS)-coating treatment [99], made hydrophobic MOFs that were suitable for CO2 sorption [29]. PDMS forms a thin protective hydrophobic surface layer on the MOFs, aiding in the retention of the crystalline structures and high porosity under moisture conditions. XRD patterns, BET values and N2 sorption isotherms of the coated MOFs remain relatively same after exposure to moisture. However, the original representative water-sensitive MOFs reported otherwise in all cases.
Again, water-stable, zinc-doped Zeolitic Imidazolate Framework-67 (ZIF-67) was synthesized for CO2 capture applications [100]. The water stability of ZIF-67 was enhanced by facile Zn-doping during the crystallization process. The MOF exhibited a far better stability in terms of crystal structure, morphology, CO2 uptake capacity and surface area as compared to the undoped MOF. This Moreover, a group of other scientists with the approach of facile vapor deposition technique, by polydimethylsiloxane (PDMS)-coating treatment [99], made hydrophobic MOFs that were suitable for CO 2 sorption [29]. PDMS forms a thin protective hydrophobic surface layer on the MOFs, aiding in the retention of the crystalline structures and high porosity under moisture conditions. XRD patterns, BET values and N 2 sorption isotherms of the coated MOFs remain relatively same after exposure to moisture. However, the original representative water-sensitive MOFs reported otherwise in all cases.
Again, water-stable, zinc-doped Zeolitic Imidazolate Framework-67 (ZIF-67) was synthesized for CO 2 capture applications [100]. The water stability of ZIF-67 was enhanced by facile Zn-doping during the crystallization process. The MOF exhibited a far better stability in terms of crystal structure, morphology, CO 2 uptake capacity and surface area as compared to the undoped MOF. This study realized a new approach to improve the water stability of MOFs and a more proper retention ability when exposed to moist environments.
A bridging ligand strategy was used to prepare polyMOFs that were found to be suitable for CO 2 /N 2 separations [28]. This is because the material reported relatively high CO 2 sorption but very low N 2 sorption. The exceptional water stability exhibited by these materials is attributed to the hydrophobicity of polymer ligands and cross-linking of the polymer chains within the MOFs. The following strategies form the bedrock upon which this method was derived. Some earlier reports prepared polymer MOF hybrid materials through PSM, whereby chemical cross-linking of the MOF is achieved through organic ligands to form polymeric monoliths [101][102][103]. Polymerization of polymer chains conducted inside the channels of MOFs has been another approach to synthesizing hybrids of MOFs and polymers [104][105][106].
In addition, a group of researchers has reported a new strategy to divide MOF open channels into confined and hydrophobic compartments by in situ polymerization [107][108][109] of aromatic acetylenes [110]. It was executed by adsorption and encapsulation of 1,2-diethynlybenzene (DEB) monomer into MOF-5 and further heating to a higher temperature to afford polynaphthylene (PN) into the inner channels via Bergman cyclization and subsequent radical polymerization. The resulting MOF, named PN@MOF-5 exhibited a significantly improved moisture stability, doubled CO 2 capacity (78 vs. 38 cm 3 /g at 273 K and 1 bar) and 23 times higher CO 2 /N 2 selectivity (212 vs. 9) compared with the pristine MOF-5.
With reference to recent literature, hydrophobic MOFs have been more extensively applied in CO 2 capture in the broad area of gas separation and storage. Such works have re-visited old ways or invented new strategies of synthesizing hydrophobic MOFs for such a purpose. Among them, creating a permeable hydrophobic layer on the surface of MOFs is the commonest strategy. This strategy has the advantage of avoiding setbacks such as reduced porosity, tedious procedure, complex instrumentation, and so on. Again, intrinsic properties such as surface area, pore texture, and crystalline structure are retained. Using a hydrophobic organic ligand realizes lower surface area compared to the other methods, including the in situ polymerization method. The CO 2 sorption retention capacities of the hydrophobic MOFs made by the vapor deposition method was much higher (at least 96%) than those of the solution-immersion method (at least 76%), after exposure to moisture. These results may not be solely based on the processes adopted but the ligands and MOFs experimented with as well.
NMOF-1 is a hydrophobic MOF that has been tested for the self-cleaning application [64]. It was synthesized via coordination directed self-assembly of dialkoxyoctadecyl-oligo-(p-phenyleneethynylene) dicarboxylate (OPE-C18) with Zinc II in DMF/H 2 0 mixture. This strategy was adapted from an earlier reported work [27]. OPE-C 18 , being a hydrophobic alkyl chain when exposed on the MOF surface, decreased surface energy hence increasing hydrophobicity. NMOF-1 is thermally stable in that the characteristic PXRD peaks are retained even at temperatures as high as 300 • C. A glass surface was coated with the MOF, dust particles placed on it and water droplets introduced on the surface. The water droplets in no time rolled off the surface laden with the dust particles ( Figure 9). This phenomenon depicts the self-cleaning nature of NMOF-1. Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) jointly confirmed the hierarchal surface roughness nature of NMOF-1, trapped with air pockets (Figure 10). This is the main property of this MOF that contributes to its superhydrophobicity and subsequently its self-cleaning ability. This work realized MOF with a very high CA and corrosion resistance with a pH stability range of 1-9. This makes it a promising material for industrial applications in stain-resistant textiles, anti-biofouling paints in ships and so on. introduced on the surface. The water droplets in no time rolled off the surface laden with the dust particles ( Figure 9). This phenomenon depicts the self-cleaning nature of NMOF-1. Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) jointly confirmed the hierarchal surface roughness nature of NMOF-1, trapped with air pockets (Figure 10). This is the main property of this MOF that contributes to its superhydrophobicity and subsequently its selfcleaning ability. This work realized MOF with a very high CA and corrosion resistance with a pH stability range of 1-9. This makes it a promising material for industrial applications in stain-resistant textiles, anti-biofouling paints in ships and so on.  introduced on the surface. The water droplets in no time rolled off the surface laden with the dust particles ( Figure 9). This phenomenon depicts the self-cleaning nature of NMOF-1. Field Emission Scanning Electron Microscopy (FESEM) and Atomic Force Microscopy (AFM) jointly confirmed the hierarchal surface roughness nature of NMOF-1, trapped with air pockets (Figure 10). This is the main property of this MOF that contributes to its superhydrophobicity and subsequently its selfcleaning ability. This work realized MOF with a very high CA and corrosion resistance with a pH stability range of 1-9. This makes it a promising material for industrial applications in stain-resistant textiles, anti-biofouling paints in ships and so on.

Liquid Marbles
A less common application area for hydrophobic MOFs is liquid marbles. Liquid marble is a ball-like object formed by virtue of a liquid being entirely encapsulated in a hydrophobic powder [145]. Liquid marbles have functioned as miniature reactors [146], micro-pumps [147], pH sensors [148], gas sensors [149] and water pollution sensors [150]. Several hydrophobic particles such as superhydrophobic silica aerogels and Fe 3 O 4 have been explored in this field but least can be said of hydrophobic MOFs. The perfluorooctyl-modified NH 2 -MIL-53(Al) developed by Chin et al. turned out highly hydrophobic and was used to make liquid marbles [151]. Interfacial polymerization of ethyl-2-cyanoacrylate was further carried out on the outer surface of the marbles to produce stable liquid capsules ( Figure 11). This work revealed that this approach to interfacial polymerization can be used industrially for the economical formation of biodegradable liquid capsules.

Liquid Marbles
A less common application area for hydrophobic MOFs is liquid marbles. Liquid marble is a ball-like object formed by virtue of a liquid being entirely encapsulated in a hydrophobic powder [145]. Liquid marbles have functioned as miniature reactors [146], micro-pumps [147], pH sensors [148], gas sensors [149] and water pollution sensors [150]. Several hydrophobic particles such as superhydrophobic silica aerogels and Fe3O4 have been explored in this field but least can be said of hydrophobic MOFs. The perfluorooctyl-modified NH2-MIL-53(Al) developed by Chin et al turned out highly hydrophobic and was used to make liquid marbles [151]. Interfacial polymerization of ethyl-2-cyanoacrylate was further carried out on the outer surface of the marbles to produce stable liquid capsules ( Figure 11). This work revealed that this approach to interfacial polymerization can be used industrially for the economical formation of biodegradable liquid capsules. Liquid marbles have broadly served as micro-reactors for polymerization [152] and other reactions [146]. Their property of low adhesion to surfaces makes them act well as non-wetting liquid droplets that display various dynamic characteristics; liquid droplets can be manipulated. MOFs being materials with several advantages can make a significant contribution to the liquid marbles story. Liquid marbles have broadly served as micro-reactors for polymerization [152] and other reactions [146]. Their property of low adhesion to surfaces makes them act well as non-wetting liquid droplets that display various dynamic characteristics; liquid droplets can be manipulated. MOFs being materials with several advantages can make a significant contribution to the liquid marbles story.

Summary and Outlook
This review has given a concise update on the recent applications of hydrophobic MOFs in general. The research on hydrophobic MOFs began about a decade ago and just like any other material explored in science, they come with their imperfections. The evolution of this special kind of MOFs came about as scientists attempted to solve the chemical and thermal instability nature of most MOFs, as they were known to be mostly moisture-instable. Scientists managed to solve this particular problem and obviously produced new application areas for MOFs, which is a very recommendable achievement. However, scientists should produce water-stable MOFs that are still applicable to the removal of contaminants in wastewater other than oils as this is the main advantage activated carbon has over MOFs. One major application area of hydrophobic MOFs is in oil-water separation and most of these MOFs can do just that. Researchers should then move to the next step of developing hydrophobic MOFs that can further separate these oils into the various classes of hydrocarbons, which is a more successful recovery of oils. We think that designing MOFs with specific pore sizes would be a stepping stone to overcoming this problem. The advantage is that for their practical use, these hydrocarbons are required at high quality and purity. Furthermore, the application areas of hydrophobic MOFs as it stands now are limited; an area such as catalysis, if explored, will bring about an unrecoverable revolution in hydrophobic MOFs. In an area such as the Fischer-Tropsch Synthesis, it is thought that hydrophobicity can partially suppress water-gas shift reaction and ultimately decrease carbon dioxide production; hydrophobic MOFs could be of use here. Again, we recommend researchers to study the areas of liquid marbles and self-cleaning more. Moreover, it is recommended that researchers look at applying hydrophobic MOFs whose applications are yet to be explored. This is likely to reveal some major setbacks in designing and synthesizing hydrophobic MOFs, bringing about more novel methods of producing hydrophobic MOFs that have lesser and lesser disadvantages in the various application fields. MOFs find themselves in a vast area of application in physics, biology, chemistry, engineering and so on. The untiring collaboration of scientists from a plethora of fields will achieve endless solutions to problems in the society.