Functionalized Metal-Organic Frameworks (MOFs)
1.4 HISTORY
It is well established that the reaction of Zn2+ ions with carboxylic acid produces a centered tetrazinc cluster, [Zn4O(RCOO)6] where the four Zn2+ ions are bridged together by the µ4-oxo group and six bridging carboxylates. For the first time, Yaghi and coworkers showed that it is possible to interconnect such tetrazinc clusters into an infinite polymeric cubic network by employing a rigid linear dicarboxylic acid instead of a monocarboxylic acid. Thus, the reaction of Zn(OAc)2·2H2O with terepthalic acid in presence of a non-coordinating base such as Et3N yields a highly robust cubic 3-dimensional MOF, Zn4O(BDC)3 (BDC = 1,4-benzenedicarboxylate). This coordination framework features an exceptionally high surface area of 3909 m2g-1 and it is popularly abbreviated as MOF-5.72 A series of Cr(III), Al(III) and Zr(IV) based MOFs have been
thoroughly investigated and among those UiO-66,42 MIL-10138, 39 and MIL-5338, 73 are very well known for their extraordinary physiochemical stability and potential applications. Since the preparation of MOFs having the ability to survive when exposed to moist air in order to avoid framework collapse is a challenging goal and requires research attention, herein we have aimed to synthesize new Al(III), Cr(III) and Zr(IV) based MOFs that can overcome these drawbacks with retention of their chemical and physical properties.
Figure 1.8 Ball-and-stick representation of the 3D cubic framework structure of UiO-66. (a,b) Spatial arrangement of the octahedral (cyan spheres) and tetrahedral (yellow spheres) cages in the framework. (c,d) Magnified representation of the octahedral and tetrahedral cages. Zr atoms are shown as octahedra (colour codes: Zr, blue; C, blue; O, dark grey). The hydrogen atoms and guest molecules have been removed from all structural plots for clarity. The figure was drawn by using atomic coordinates provided in ref.74.
The Zr(IV)-based MOF called UiO-66 (UiO = University of Oslo)42 has attracted notable attention in recent years owing to its high thermal and chemical stability as well as potential properties for CO2/CH4 gas separation.75, 76 The 3D cubic framework of this MOF is built up of hexanuclear [Zr O (OH) ]12+ building units in which the triangular faces of the Zr -octahedron are
alternatively capped by µ3-O and µ3-OH groups (Figure 1.8). All the [Zr6O4(OH)4]12+ building units are interconnected by the carboxylate groups of twelve 1,4-benzenedicarboxylate (BDC) dianions leading to the formation of [Zr6O4(OH)4(BDC)6] framework having octahedral (Figure 1.8 c) and tetrahedral (Figure 1.8 d) cages. Each Zr atom exists in a square-antiprismatic geometry and coordinates with eight O atoms. One square face of the square antiprism comprises of O atoms from the carboxylate groups, whereas the O atoms from the μ3-O and μ3-OH groups construct the other face of the square antiprism.
The synthesis of UiO-66 was performed by using commercially available ZrCl4 and 1,4‐
benzenedicarboxylic acid (H2BDC) applying N,N’‐dimethylformamide (DMF) as solvent medium at 120 °C in a pre‐heated oven for 24 h. This coordination framework exhibits an exceptionally high Langmuir surface area of 1187 m2 g-1. Extending the length of the linker to two and three benzene ring dicarboxylic acids resulted in two more very potential MOFs popularly abbreviated as UiO-67 and UiO-68 with increased surface area of 3000 and 4170 m2 g-1, respectively. The structural integrity of UiO-66 against different solvents like water, DMF, benzene and acetone, confirmed by X-ray powder diffraction (XRPD) measurements, also proved the efficacy of this material for industrial applications. Functionalization of UiO-66 framework through attachment of different functional groups to the BDC ligand (UiO-66-X, X = -CH3, -(CH3)2, -NO2, -NH2, - OH, -CO2H, -SO3H) enhanced the CO2 uptake compared to that of the un-functionalized UiO-66 material.21 Moreover, the modulated synthesis of the MOF materials having UiO-66 framework topology by the addition of different modulators/additives (e.g., benzoic acid, acetic acid, formic acid, conc. HCl and H2O)77, 78 have been carried out in order to investigate their effect on the crystallinity of the MOF materials.
Ferey and coworkers have developed a very interesting series of MOFs known as the MIL- n (MIL = Material of the Institute Lavoisier) which possess very high thermal and chemical stability in addition to their remarkable porosity.38, 39 Cr-MIL-101 is the most popular amongst these MOFs on account of its high moisture stability, high thermal stability (up to 400 °C) and extraordinarily high surface area of 4100 m2 g-1.38, 79 This chromium terephthalate MOF having formula of [Cr3F(H2O)2O(BDC)3]·nH2O (where n∼25) were synthesized by the reaction of Cr(NO3)3·9H2O with H2BDC ligand and HF in H2O under hydrothermal conditions (at 220 °C for 8 h).
Figure 1.9 (a) Framework structure of Cr-MIL-101 having MTN topology and consisting of smaller (green) and larger (red) mesoporous cages. The structure is derived from (b) supertetrahedra (ST), which consist of trimeric oxido-centered [Cr3(μ3-O)(F)(H2O)2]6+ building units at the vertices cross-linked by the BDC linkers. The smaller and larger cages possess only pentagonal (c) or a combination of pentagonal (c) and hexagonal (d) windows, respectively. Colour codes: Cr, green octahedra; C, gray; O, red. The MTN framework (a) and the portions of Cr-MIL-101 network (b-d) have been drawn by using the atomic coordinates provided in “Database of Zeolite Structures”80 and ref.38, respectively.
The framework structure of Cr-MIL-101 is constructed from the linkage of BDC dianions with inorganic trimeric [Cr3O(H2O)2F(CO2)6] building units. Within the trimeric secondary building units, the three Cr atoms are in an octahedral environment and the six coordination sites are occupied by four O atoms from the bridging bidentate dicarboxylates, one µ3-O atom and one O atom from the terminal water or fluorine group. The structure of Cr-MIL-101 features corner- sharing supertetrahedra which are made from the linkage of inorganic trimers and BDC dianions.
The framework has two types of quasi-spherical cages limited by 12 pentagonal faces for the smaller (29 Å) and by 16 faces (12 pentagonal and 4 hexagonal) for the larger (34 Å) (Figure 1.9).
Besides the parent and functionalized Cr-MIL-101-X (X = -H, -F, -Cl, -Br, -I, -CH3, -NO2, -NH2, -OH, -CO2H, -SO3H, etc.)81 MOFs, several other hydrolytically stable Fe-based MIL-101-X (X =
-H, -Cl, -NH2 and -(CH3)2)82, 83 materials were also reported. The synthesis of non- and amino- functionalized vanadium-based materials MIL-101 materials has been reported by the groups of Van Der Voort and Zou.84 Nevertheless, the rate of exchange of linkers by water molecules for both V(III) and Fe(III)-based MIL-101 materials made them highly sensitive to moisture. Apart from the parent and functionalized Cr(III), Fe(III) and V(III)-based MIL-101 materials, the synthesis, thermal and chemical properties of Al-MIL-101-NH2 compound have been also studied.40, 85, 86 However, the synthesis procedures of Al-MIL-101 derivatives having any other functional group than amino is so far unknown.