Metal Organic Frameworks (MOFs) have revolutionized the fields of solid state chemistry and materials science since the discovery of MOF-5 in 1999.1 These materials are intriguing for two distinct reasons: ultra high porosity and fascinating structures.
With regards to porosity, MOFs can reach BET surface areas of up to 10400 m²/g, which approaches the absolute limit for solid materials.2 Additionally, their pore sizes and functionalities can be tailored to match specific molecules for adsorption. As a result of their high surface areas and ability to be functionalized, MOFs are candidates for a wide range of applications, including gas storage (e.g. natural gas or hydrogen), gas phase separations, catalysis, and chromatography.
From a crystallographic standpoint, the plurality of MOF structures was the inspiration for comprehensive theories of crystallographic nets. For example, the reticular chemistry concept of O'Keeffe and Yaghi describes nets composed of linear links and vertex figures. The inorganic secondary building units (SBUs) of a MOF structure can be simplified to these vertices, and nearly every new MOF structure can be related to one of these net types.3 Therefore, most publications that address MOF structures use the terms "structure design" or "crystal engineering". Many possible structures, however, are not realized due to the limitations of current laboratory methods. Almost all MOF structures that have been published thus far are synthesized in a solvothermal black box using the concepts of zeolite chemistry.
In contrast to traditional methods, the "controlled SBU approach" (CSA), uses preformed SBU precursors in a simple substitution reaction, eliminating trial-and-error scanning. CSA is an easy method for making tailored MOF structures containing metal centers other than the commonly used ones. Similar structures can be altered to yield control over properties like chemical stability, catalytic activity, magnetism, and polarity. Additionally, the controlled SBU approach can be used as a low temperature route to prevent thermal decomposition of linker molecules.
Figure 1. Schematic representation of the CSA procedure for two MIL-88 homologues.
The first example of CSA, shown in Figure 1, was given by Férey in 2004 with the synthesis of MIL-88 and MIL-89, using a µ3-oxo-centered trimeric iron acetate, Fe3OAc6∙ClO4 (Aldrich Prod No. 749141) as a precursor.4 In 2009, the Férey group demonstrated the CSA for producing two absolute water stable UiO-66-type MOFs,5 using a six-centered Maltese cross-shaped zirconium methacrylate oxocluster, Zr6O4(OH)4(Mc)12 (Aldrich Prod No. 749168) as a preformed SBU.5
The four-centered octahedral SBU of the Isoreticular Metal Organic Framework (IRMOF) type (Figure 2) can be found in a large number of soluble complexes containing very different metal centers. This type of carboxylate complex is named after the basic beryllium acetate, which was first reported in 1901.6 However, IRMOFs containing metal centers other than zinc were not reported until 2010. The difficulty in synthesizing non-zinc analogs is presumably due to limitations of the solvothermal reaction pathway. The central oxide ion of the SBU must be formed through nitrate decomposition, which only takes place in the presence of zinc ions.7 In 2010, three homologues of the archetypical MOF-5 were shown to be producible by CSA: the common MOF-5(Zn) using basic zinc acetate, Zn4OAc6, (Aldrich Prod No. 741051); the largely moisture-stable MOF-5(Be) with basic beryllium acetate, Be4OAc6; and the paramagnetic MOF-5(Co) using cobalt oxo pivalate, Co8O2Piv12 (Aldrich Prod No. 749125), which can described as a SBU dimer.8 A schematic representation of the general reaction route is given in Figure 2.
The successful synthesis of these materials suggests that this method can be applied to the whole series of IRMOFs and MOF-177 like structures.
Figure 2. Schematic representation of the CSA for MOF-5 homologues. A basic beryllium acetate-type SBU precursor is represented as an octahedron.
The synthetic procedures for obtaining MOFs through the controlled SBU approach are described below.
Mil-88A is formed by mixing Fe3OAc6∙ClO4, fumaric acid, sodium hydroxide, deionized water and methanol in a 1:3:1.5:50:1000 ratio. The resulting orange gel is aged at 100 °C for 3 days in a closed vessel. The light orange solid is filtered, washed with methanol and acetone, and dried at room temperature. Fumaric acid can be substituted for trans,trans-muconic acid to form MIL-89, or 2,6-naphthalenedicarboxylic acid to form MIL-88C. MIL-88C is isostructural to MIL-88A, but with larger pores.
Zr6O4(OH)4(Mc)12, trans,trans-muconic acid and DMF (molar ratio 1:30:1485) are mixed at room temperature under continuous stirring. The reactants are placed in a Teflon-lined steel autoclave, and heated at 150 °C for 1 h. The reaction yields a crystalline white powder, with small traces of a yellow impurity, which is easily removable by suspension in DMF. The DMF is removed by filtration, and the product is dried at 100 °C. This procedure leads to an isomorph of a previously reported MOF, UiO-66, which contains terephthalate linkers and can be synthesized in the same manner. The muconate linker can be found in its cis conformation so that it occupies the same space as a terephthalate molecule. A similar crystalline product can be obtained at room temperature over 96 h. In contrast to CSA, the same solvothermal procedure with ZrCl4 yielded an unusable amorphous product. Figure 3 illustrates the Maltese cross-shaped SBU molecular detail of the UiO-66 structure—and the conformations of the linker molecules in both UiO-66 structures.
Figure 3. Maltese cross-shaped UiO-66 SBU Precursor without carboxylates. Tetrahedral arrangement of UiO-66. Configurations of cis-muconate and terephthalate build the same type of structure when Zr6O4(OH)4(Mc)12 is used as a CSA precursor.
When using Zn or Co precursors, it is necessary to use moisture-free reagents and solvents. The best CSA results, marked by high surface areas and pure phases, can be obtained by using an inert gas technique, solvents distilled over P2O5, and terephthalic acid dried by sublimation in vacuum. A beneficial general procedure is to drip a 0.03 M solution of the linker acid into the same volume of a 0.01 M precursor solution (0.005 M in the case of Co8O2Piv12) under rapid stirring at a rate of 1.5 ml/min. Every IRMOF has a typical optimal temperature for CSA at which nearly inclusion free, highly crystalline material accrues. These temperatures are 100 °C for MOF-5(Zn), 90 °C for MOF-5(Co), and 189 °C for MOF-5(Be) when using terephthalic acid. The optimal temperature for IRMOF-3 with 2-aminoterephthalic acid is 70 °C. Room temperature synthesis leads to materials of about 60-70 % of the theoretical specific pore volume calculated from the crystal structure. Exceeding the optimal temperature leads to the protonation of the central oxide ion of the precursor and a non-porous dimer terephthalate occurs. In the case of cobalt, a 10 % excess of the precursor and traces of a stabilizing base (e.g. diethylamine) are beneficial. In all cases, the use of the solvent N,N-diethylformamide yields products with higher surface areas than the use of N,N-dimethylformamide. Formamide solvents are currently the only solvents known to produce high quality IRMOFs.
Procedure for activation (removal of solvent):
CSA typically leads to powdery products with particle sizes of 150 nm-5µm. Known structures can be identified by Powder X-ray Diffraction (PXRD) and the use of a powder pattern simulation program like PowderCell.10 New structures can be solved by using Rietveld refinements using an initial structure model. The structure model can be selected from the possible crystallographic nets listed in the RCSR database in its SBU-linker combination and pre-refined by molecular modeling.11 The porosity characterization can be carried out by measuring the deep temperature nitrogen or argon adsorption isotherms and evaluating the surface areas by the BET model. Only the isothermal regions of positive BET c-parameters should be used for surface determination.12 The specific pore volume can also be determined from the adsorption isotherm by using the t-plot method.13