1.3 Applications of water stable MOFs 16
1.3.1 Adsorption 16
1.3.1.2 Adsorption of other molecules/ions 20
UiO-66 0.75/1.2 1160 0.52 0.40 88
UiO-66 0.75/1.2 1290 0.49 0.42 87
UiO-66-NO2 < 0.75/1.2 792 0.4 0.37 94
UiO-66-NH2 < 0.75/1.2 1123 0.52 0.34 94
UiO-66-2,5- (OMe)2
n.d. 868 0.38 0.42 94
MOF-841 0.9 1390 0.53 0.48 87
a Measured at lower relative humidity p/p0 = 0.5– 0.7.
Carbon dioxide (CO2) is a main greenhouse gas, which is released through human activities such as deforestation and burning fossil fuels including natural processes such as volcanic eruptions.101 The annual global emission of CO2 has been increased by approximately 80%
between 1970 and 2004. The porous and robust MOF materials have shown remarkable progress for CO2 adsorption in last decades.102 The capture of CO2 gas in presence of water have been accomplished by several water-stable MOFs. The selective capture of CO2 in 65% RH was reported by Yaghi et al. for IRMOF-74-III-CH2NH2 and IRMOF-74-III-(CH2NH2)2.103,104 The high affinity towards CO2 gas by amine functional groups was attributed to the strong dipole- quadrupole interactions between amine functional group and CO2. Am amide-CO2 hydrogen bonding interaction was also suggested for CO2 adsorption by a flexible metal‐organic framework, {[Mn2(2,6‐ndc)2(bpda)2]⋅5DMF}n (2,6‐ndc = 2,6‐naphthalene dicarboxylate; bpda = N,N′‐bis (4‐
pyridinyl)‐1,4‐benzene dicarboxamide).105
1.3.1.2.2 Adsorption of small molecules
Using isoreticular chemistry, a series of MOFs with tunable aperture size was reported with rare-earth metal ions (Eu3+, Tb3+ and Y3+) for the separation of controlled and selective solvent molecules and light hydrocarbons. The fumarate-based fcu-MOF displayed complete exclusion of branched paraffins from normal paraffins. The fcu-MOF constructed with bulky 1,4- naphthalenedicarboxylate (1,4-NDC) ligand exhibited an exceptionally high selectivity for n- C4H10 over CH4.106,107 The copper(II) paddle wheel based hydrolytically stable MOF [Cu4(tdhb)]
(BUT-155), constructed with 3,3′,5,5′-tetrakis(3,5-dicarboxyphenyl)-2,2′,4,4′,6,6′- hexamethylbiphenyl ligand, showed a high performance for selective adsorption of soft-base type aniline over water or phenol.76
The fluorous metal-organic framework (FMOF-1) constructed from silver(I) 3,5-bis (trifluoromethyl)-1,2,4-triazolate showed hydrophobic behavior with a high capacity and affinity to C6-C8 hydrocarbons of oil components.52 Based on its high water stability, the material could be used for oil-spill cleanup. Another fluorinated ultrahydrophibic MOF called UHMOF-100 was synthesized by Ghosh et al. This MOF exhibited excellent oil adsorption capacities and reusability features.56 Based on the excellent features of this MOF, a MOF-coated membrane was fabricated for the separation of oil from water (Figure 1.13). Post-synthetically modified MOF@MON hybrid
materials, reported by Son et al., demonstrated an excellent performance for the adsorption of toluene in water.60
Figure 1.13 Schematic presentation of fluorinated ultrahydrophobic pore surface, utilized as a potential method to obtain ultrahydrophobicity in MOFs. Reproduced with permission from ref.
63. Copyright 2016 Wiley Online Library.
Water-stable MOF materials were successfully utilized for the adsorptive removal of various organics from contaminated water.108-112 The well-studied Zr(IV)-based UiO-66 MOF was employed for the adsorptive removal of an anionic dye, methyl orange (MO).112 Experimental data suggested that the adsorption capacity of UiO-66 toward MO was higher than that of methylene blue (MB). The adsorption and removal of phthalic acid and diethyl phthalate from water by UiO- 66 and UiO-66-NH2 MOFs were also investigated.113 A comparative study supported that ZIF-8 MOF has higher adsorption capacity as compared to UiO-66 framework. The adsorptive removal of naproxen and clofibric acid, two typical PPCPs (PPCP = pharmaceuticals and personal care product) by MOF materials was reported by Jhung et al. The Cr-MIL-101 and Fe-MIL-101 compounds showed higher removal efficiency as compared to porous activated carbons.114
1.3.1.2.3 Adsorption of ions
Separation of cation mixtures via chromatography is another promising application for water-stable MOFs. Zhang et al. reported a 3D pillar-layer framework, [Zn(trz)(H2betc)0.5]·DMF,
with uncoordinated carboxyl groups which exhibited exceptional stability.115 The pillar-layer MOF can selectively adsorb Cu2+ ions and it was employed for the column chromatographic separation of Cu2+/Co2+ mixture. The selective adsorption of Cu2+ over Cd2+ and Ni2+ ions was studied by Biswas et al. with a water-stable thienothiophene based Zr(IV) MOF.116 Phosphorylurea-derived Zr(IV)-MOFs with UiO-68 topology were synthesized by Lin et al., which showed efficient sorption capacity for uranyl ions.117 Two isostructural mesoporous MOFs, PCN-100 and PCN-101, constructed with Zn4O(CO2)6 secondary building blocks and extended ligands containing amino functional groups, were employed to capture metal ions like Cd2+ and Hg2+.118
The removal of toxic ionic pollutant such as arsenic was accomplished by Wang et al. using Zr-UiO-66 MOF across a broad pH range of 1 to 10.119 The superior arsenic removal performance of UiO-66 MOF could be attributed to the crystalline structure containing zirconium oxide clusters.
These clusters provide active sites along with two binding sites within the adsorbent framework, i.e., hydroxyl group and benzenedicarboxylate ligand. Two MOFs namely [Zn2(Tipa)2(OH)]·3NO3·12H2O (FIR-53, Tipa = tris(4-(1H-imidazol-1-yl)phenyl)amine)) and [Zn(Tipa)]·2NO3·DMF·4H2O) (FIR-54) were synthesized by Zhang et al., which can efficiently trap the inorganic pollutant ion Cr2O72– via single-crystal-to-single-crystal (SC-SC) approach.120 A mesoporous cationic thorium-based MOF called SCU-8 containing channels with a large inner diameter of 2.2 nm showed a rapid removal of oxo anions like ReO4− and Cr2O72− by driving forces including electrostatic interactions, hydrogen bonds, hydrophobic interactions and Van der Waals interactions.121