*2.2.2. Mechanochemical synthesis*

The mechanochemical synthesis is named due to the chemical reaction that occurs as a consequence of mechanical energy supplied to the system [63]. In the mechanical grinding process, several phenomena occur, such as


In the synthesis of MOFs, the metal salt and the organic linker are ground together in the absence of solvents. In 2002, Belcher et al. [64] reported the synthesis of a 1D copper coordi‐ nation polymer, using mechanochemical synthesis (Figure 3).

**Figure 3.** Basic unit of construction of coordination polymer [Cu(O2C-Me)2]2(μ-dpp) dpp = 2,3-bis(2-pyridyl)pyrazine. Gray, red, white, violet, and orange spheres correspond to atoms of C, H, N, O and Cu, respectively.

In other recent studies [65], MOFs were synthesized using 12 metal salts and 5 organic linkers to obtain 60 different solids. As a result, 38 microcrystalline MOFs were identified using X-ray diffraction techniques. Their structure patterns are found on the CSD database (Cambridge Structure Database), including microporous MOFs [Cu(INA)2]*n* (INA = isonicotinate) and Cu3(BTC)2.

## *2.2.3. Electrochemical synthesis*

Electrochemical synthesis of MOFs was reported by BASF in 2005 [66], in order to eliminate the use of anions such as nitrate, perchlorate, and chloride, which act as counterions or as impurities in the network. In this synthesis method, the metal salts are replaced by metal ions produced from the anodic dissolution in the reaction medium. The dissolution also contained organic linkers; in cathodes, metal deposition occurred. In particular, for the synthesis of Cu3(BTC)2, copper metal bars that function as electrodes (anode and cathode) were employed in the electrochemical cell with organic linker (BTC = benzene-1,3,5-tricaboxylic), dissolved in methanol [67], with an applied voltage between 12 and 19 V and a current of 1.3 A for 150 min. The result was the oxidation of the copper bar acting as the anode to form Cu2+, which reacts with the organic linker. Furthermore, in cathode, water reduction took place to produce hydrogen. At the end of the reaction, a greenish-blue precipitate was formed, which was filtered and dried to obtain Cu3(BTC)2. Using these synthesis pathways, materials can be produced with high purity and ease of being industrially scalable.

Other studies on the electrochemical synthesis of MOFs are presented in Table 1.

### *2.2.4. Microwave or ultrasound-assisted synthesis*

Synthesis assisted by ultrasound or microwave is an alternative to the solvothermal synthesis. In microwave-assisted synthesis, the reaction mixture is subjected to nonionizing radiation,


H3BTC = benzene-1,3,5-tricarboxilic acid; H2BCD = terephtalic acid; MeIm = 2-methyl-1H-imidazole; BIm = benzimidazole.

**Table 1.** MOFs synthesized electrochemically.

**Figure 3.** Basic unit of construction of coordination polymer [Cu(O2C-Me)2]2(μ-dpp) dpp = 2,3-bis(2-pyridyl)pyrazine.

In other recent studies [65], MOFs were synthesized using 12 metal salts and 5 organic linkers to obtain 60 different solids. As a result, 38 microcrystalline MOFs were identified using X-ray diffraction techniques. Their structure patterns are found on the CSD database (Cambridge Structure Database), including microporous MOFs [Cu(INA)2]*n* (INA = isonicotinate) and

Electrochemical synthesis of MOFs was reported by BASF in 2005 [66], in order to eliminate the use of anions such as nitrate, perchlorate, and chloride, which act as counterions or as impurities in the network. In this synthesis method, the metal salts are replaced by metal ions produced from the anodic dissolution in the reaction medium. The dissolution also contained organic linkers; in cathodes, metal deposition occurred. In particular, for the synthesis of Cu3(BTC)2, copper metal bars that function as electrodes (anode and cathode) were employed in the electrochemical cell with organic linker (BTC = benzene-1,3,5-tricaboxylic), dissolved in methanol [67], with an applied voltage between 12 and 19 V and a current of 1.3 A for 150 min. The result was the oxidation of the copper bar acting as the anode to form Cu2+, which reacts with the organic linker. Furthermore, in cathode, water reduction took place to produce hydrogen. At the end of the reaction, a greenish-blue precipitate was formed, which was filtered and dried to obtain Cu3(BTC)2. Using these synthesis pathways, materials can be

Gray, red, white, violet, and orange spheres correspond to atoms of C, H, N, O and Cu, respectively.

100 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

produced with high purity and ease of being industrially scalable.

*2.2.4. Microwave or ultrasound-assisted synthesis*

Other studies on the electrochemical synthesis of MOFs are presented in Table 1.

Synthesis assisted by ultrasound or microwave is an alternative to the solvothermal synthesis. In microwave-assisted synthesis, the reaction mixture is subjected to nonionizing radiation,

Cu3(BTC)2.

*2.2.3. Electrochemical synthesis*

which does not change the electronic structure of the material. The energy supplied to the material by electromagnetic waves through interactions of molecular type offers a number of advantages, such as a uniform controlled heating as well as a great speed with which energy is generated [69]. The characteristic frequency of this radiation is between 300 MHz and 300 GHz (wavelengths between 0.01 and 1 m). This synthetic method has been applied to organic molecules [70] and inorganic materials [71].

Generally, the microwave synthesis is carried out in minutes and offers a better method to control the morphology of the material and the selectivity of the phases. For example, MOF-5 (Figure 4) was synthesized using microwave at 368 K for 9 min, with a yield of 27% [72], while using the solvothermal synthesis, a yield of 60% was achieved after 7 days.


Table 2 shows the conditions of microwave-assisted synthesis of MOFs.

EMIm = 1-ethyl-3-methylimidazolium; TMA = trimesate; pyz = pyrazine; oba = 4,4′-oxydibenzoic acid; BTC = ben‐ zene-1,3,5-tricarboxilic; H3IDC = 4,5-imidazoledicarboxylic acid; bbi = 1,1′-(1,4-butanediyl)bis(imidazole); NDC = 2,7 naphthalene dicarboxylate; DPNI = *N,N* di(4-pyridyl)-1,4,8-naphthalenetetracarboxydiimide.

**Table 2.** Conditions of microwave-assisted synthesis of MOFs [73].

Ultrasound-assisted synthesis is another route for obtaining materials, where you can get MOFs with small crystal size in a short reaction time. In this synthesis, the reaction mixture is subjected to ultrasound (part of the spectrum of the sound whose frequency is approximately 19 kHz) to generate high temperatures (above 5000 K) and pressures at specific locations within the mixture. Such increases in temperature and pressure are due to the phenomenon of "cavitation", which involves the creation, expansion, and destruction of small bubbles that appear when the reaction mixture is treated with ultrasound [74]. In this case, acoustic radiation mechanical energy is converted into thermal energy. Among the MOFs synthesized by this method are MOF-5, MOF-177, Cu3BTC2, Zn-2,2′bipiridina-5,5′dicaboxilato, Zn3(BTC)2 12H2O [Zn (1,4-bencendicarboxilato) (H2O)]n [75].

## *2.2.5. Synthesis of MOFs using near supercritical water conditions*

Motivated by the resolution of the problem that exists with the use of solvents (1.3.1), Schröder and Poliakoff [76] developed a new methodology for the synthesis of MOFs, building for its acronym high-temperature water (HTW). Due to these properties, the HTW has been studied as a means of organic reactions [77, 78], destruction of contaminants [79] and formation of nanoparticles [80, 81]. Water properties change dramatically as it approaches its critical point (647 K, 220 bar) [82]. For example, the dielectric constant decreases to values of typical nonpolar solvents, and therefore, organic compounds, such as organic ligands of MOFs, can be dis‐ solved.

Water can potentially be reused after the reaction has been completed and, if necessary, ion exchange may be employed in order to remove any traces of unreacted organic ligand and metal ions. HTW presents some technical difficulties due to the high pressures and accelerated corrosion of the reactors. However, Schröder and Poliakoff [76], in 2012, first reported the possibility of using HTW (573 K) as solvent for the synthesis of a MOF with high performance. The new MOF, {[Zn2(L)] (H2O)3}∞; (L = 1,2,4,5-tetrakis(4-carboxyphenyl)-benzene) (Figure 4), is synthesized using only water as reaction medium at 573 K and 80 bar.

**Figure 4.** View of the crystal structure of the structure {[Zn2(L)] (H2O)3}∞. Green, red, black and gray represent Zn atoms, O, C, and H, respectively [76].

## **2.3. Characterization and evaluation methods of MOFs**

Ultrasound-assisted synthesis is another route for obtaining materials, where you can get MOFs with small crystal size in a short reaction time. In this synthesis, the reaction mixture is subjected to ultrasound (part of the spectrum of the sound whose frequency is approximately 19 kHz) to generate high temperatures (above 5000 K) and pressures at specific locations within the mixture. Such increases in temperature and pressure are due to the phenomenon of "cavitation", which involves the creation, expansion, and destruction of small bubbles that appear when the reaction mixture is treated with ultrasound [74]. In this case, acoustic radiation mechanical energy is converted into thermal energy. Among the MOFs synthesized by this method are MOF-5, MOF-177, Cu3BTC2, Zn-2,2′bipiridina-5,5′dicaboxilato, Zn3(BTC)2

Motivated by the resolution of the problem that exists with the use of solvents (1.3.1), Schröder and Poliakoff [76] developed a new methodology for the synthesis of MOFs, building for its acronym high-temperature water (HTW). Due to these properties, the HTW has been studied as a means of organic reactions [77, 78], destruction of contaminants [79] and formation of nanoparticles [80, 81]. Water properties change dramatically as it approaches its critical point (647 K, 220 bar) [82]. For example, the dielectric constant decreases to values of typical nonpolar solvents, and therefore, organic compounds, such as organic ligands of MOFs, can be dis‐

Water can potentially be reused after the reaction has been completed and, if necessary, ion exchange may be employed in order to remove any traces of unreacted organic ligand and metal ions. HTW presents some technical difficulties due to the high pressures and accelerated corrosion of the reactors. However, Schröder and Poliakoff [76], in 2012, first reported the possibility of using HTW (573 K) as solvent for the synthesis of a MOF with high performance. The new MOF, {[Zn2(L)] (H2O)3}∞; (L = 1,2,4,5-tetrakis(4-carboxyphenyl)-benzene) (Figure 4),

**Figure 4.** View of the crystal structure of the structure {[Zn2(L)] (H2O)3}∞. Green, red, black and gray represent Zn

is synthesized using only water as reaction medium at 573 K and 80 bar.

12H2O [Zn (1,4-bencendicarboxilato) (H2O)]n [75].

102 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

solved.

atoms, O, C, and H, respectively [76].

*2.2.5. Synthesis of MOFs using near supercritical water conditions*

The different methods of synthesis of MOFs can generate homogeneous solids that allow carrying out processes of heterogeneous catalysis. Once the reaction finished, it is desirable that the physicochemical characteristics of material prevail. There are different characteriza‐ tion techniques for determining the homogeneity of the material, structural characteristics, and stability of the MOF. Analytical methods that are useful and applicable are listed below. However, others characterizations may exist which are also useful in the evaluation of MOFs, such as heterogeneous catalysis.

*Powder X-ray diffraction (XRD)* is used in determining the crystallographic MOFs by comparing the diffractogram of MOF before and after the catalytic process. In certain processes, the stability of the structure is also determined. Additionally, it is possible to determine the purity of the catalyst and some crystallographic parameters as red parameter, size of lattice, and crystal size.

*Fourier transform infrared spectroscopy (FTIR)* provides information about functional groups present in the network of the MOF. It is possible to make a comparison to determine the changes once the network has carried out the catalytic reaction.

*Nuclear magnetic resonance (NMR)* is a widely used technique in the characterization of products, by-products, and intermediates of the catalysed reaction. It is possible to determine the chemical environment inside the catalyst using probe molecules.

*Nitrogen physisorption. The* texture parameters such as surface area, pore volume, and average pore size are determined by this technique. The shape of the isotherm provides information about the homogeneity of the solid.

*Ultraviolet-visible diffuse reflectance spectrum* provides information about the environment metal coordination before and after carrying out a catalysed reaction.

*Thermogravimetric analysis (TGA)* is useful to determine the thermal stability of the MOF. In some processes, it is necessary to conduct a heat treatment prior to the catalysed reaction and treatment parameters are determined by TGA. It is possible to obtain a model which is highly suitable for the process reaction desired.

*Scanning electron microscopy (SEM) and transmission electron microscopy (TEM)* are able to show the morphology, defects, grain boundaries, mixtures of crystalline phases, and grain size, among others. In some solids, even the crystallinity and porosity can be determined by these techniques.

*Energy-dispersive X-ray* (EDX) is used to determine the elemental materials analysis. The MOF is studied before and after the catalytic process for identifying the presence of new elements and their percentage.

*Gas absorption analyser* can be used to analyze the adsorption capacity for MOF for a particular gas or vapour.

*Gas/liquid chromatography-mass spectrometry (GC/LC-MS)* is a powerful technique to analyse the catalytic reaction and determine the amount and type of products.

*Raman spectroscopy is* widely used in the characterization of noncrystalline or low-crystalline catalysts. Comparing the spectra before and after the reaction provides information about the incorporation of new components into the MOF network.

*Temperature-programmed reduction (TPR)* is used in determining redox reaction parameters. The catalytic activity in redox conditions can be determined by this technique.
