Transition Metals-Based Metal-Organic Frameworks, Synthesis, and Environmental Applications

Abstract

This work illustrates examples of metal-organic frameworks (MOFs) derived from transition metals and their environmental applications in areas of catalysis, sorption, and hydrogen evolution. Explanation of some of the techniques employed for their synthesis has been discussed. On the other hand, the advantages of the use of hybrid materials such as the metal-organic frameworks are exposed in this book as well a detailed description of the different linkers and metals used for the synthesis of this kind of porous materials going through the methodologies and techniques utilized by different authors to obtain good-quality crystalline applicable materials. Adjustments of linker geometry, length, ratio, and the functional group can tune the size, shape, and internal surface property of an MOF for a targeted application. The uses of MOFs are exploring new different areas of chemistry such as catalysis, adsorption, carrier systems, hydrogen evolution, photocatalysis, and more. Different examples of MOFs from Scandium to Zinc are well described in this book, and finally, a brief description of some common environmental applications such as metals and azo dyes sorption, hydrogen evolution, and catalyst in the transesterification process of vegetable oils to produce biodiesel is explored and commented.

1. Introduction

Metal-organic frameworks (MOFs) have been widely reported in the literature during the last two decades, and the number of articles published is exponentially increasing due to the opportunity to obtain a great diversity of novel crystalline porous materials with different topologies and most important with a countless number of applications in different fields of chemistry such as catalysis, photocatalysis, gas storage, separation, sorption, hydrogen evolution, and more [1, 2, 3, 4, 5, 6, 7, 8, 9]. In general, crystalline porous materials can have several terminologies, for example, metal-organic material (MOM), coordination polymer (CP), coordination network (CN), porous coordination polymer (PCP), porous coordination network (PCN), microporous coordination polymer (MCP), and metal-organic coordination network (MOCN), which are habitually used by different scientists to designate, at least some, MOFs. All these subclasses of coordination compounds have very similar structural features with slight differences and therefore confer an inevitable overlap in their properties. For example, coordination polymer (CP) is a common term that has occasionally been used as an alternative word for MOF. The term “coordination polymer” implies that coordination compounds that constitute one-, two-, or three-dimensional (1D, 2D, or 3D) polymeric structures via linking of the metal ions by bridging ligands are CPs [1]. The synthesis of MOFs requires two principal components, the ligands commonly called linkers, and the metals in the form of different salts such as nitrates, chlorates, perchlorates, or sulfates. The ligands are usually any atom or any organic molecule, the latter possesses in its structure different functional groups as carboxylic acid, amine, pyridine group, among others, with the ability to donate at list a lone electron pair to the metal, commonly known as Lewis’s base. The pore size depends mostly on the length of the ligand. On the other hand, the second component corresponds to the metal, which may vary from the s-block, p-block, transition metals, or even rare earth metals. The obtention of MOFs in 1D, 2D, and 3D dimensions depends directly on the metal employed for the synthesis, which has to do with its oxidation state and the types of geometries that the metal can acquire. And finally, a very important issue to consider is the technique used to obtain the MOFs that involves several parameters such as solvent or solvent mixture, temperature, pressure, pH value among others, all these variables together may or may not give rise to the desired materials. Different synthesis methods to obtain MOFs have been applied in the last 20 years in addition to the room temperature or slow evaporation synthesis, which are conventional electric (CE) heating, microwave (MW) heating, electrochemistry (EC), mechanochemistry (MC), ultrasonic (US) methods, and a combination of the previous techniques before mentioned. Conventional step-by-step methods, as well as high-throughput methods, have been employed in some of the studies. There have been various studies regarding the morphology of the MOF products, in addition to the crystal size or shape, thin films, membranes, and various other shapes made of MOFs have been reported, which require the application of different synthesis methods [2].

2. Linkers

The geometry and connectivity of a linker dictate the structure of the resulting MOF, which are broadly classified into neutral (MOFs) and ionic (iMOFs), based on the charge of the framework backbone. Ionic MOFs (iMOFs) are subdivided into cationic (iMOF-C) and anionic (iMOF-A). Azolate based linkers may give neutral as well as ionic framework depending on the connectivity of the linker, metal coordination environment, and coordination geometry. Figure 1 shows the type of MOF obtained according to the N-donor linker base used [3].

Figure 1.

Classification of MOFs constructed from N-donor linkers [3].

Many of the systems reported from the use of azolate-based linkers are neutral in nature, wherein the charge balance is either via anionic donor groups of azolates or the presence of coordinating anion used in the synthesis. For instance, all the ZIF-series MOFs, which are constructed from imidazolates, are electrically neutral. Likewise, pyrazolate and triazolate-based systems have been found to self-assemble with metal ions/clusters into neutral MOFs. A good example of a nitrogen-containing linker is the 4,4′-bipyridine, which is an aromatic organic compound with nitrogen hetero atoms located in the para positions of the aromatic rings. Nitrogen atoms possess an electron lone pair located in the plane of the carbon atoms of the ring, such arrangement allows a very specific coordination binding to the metal. Figure 2 shows different N-donor linkers used in the synthesis of coordination polymeric materials [3].

Figure 2.

Typical donor groups in N-donor linkers.

Typical N-donor linkers can have 1, 2, 3, 4 or more nitrogen atoms. It’s very important to select the appropriate linker to obtain the desired pore size, i.e., the linker 4,4′-bipyridine, which was reported by Wang and coworkers, describes a MOF synthesized through hydrothermal conditions and using two different linkers [4]. As could be measured from the .cif file, the distance between the Cd atoms is in the range of 11.59–7.50 Å. Also, the rings of the N-ligands are almost planar showing π-π interactions. Due to the electron-rich cloud inside the pore, small cations, or even small molecules with a deficiency of electrons, could interact with each other to form stable systems. This analysis can be deduced from the packing material obtained by X-ray diffraction. Figure 3 shows a small portion of the material, which is composed of two different organic linkers where the cadmium atoms are hexacoordinated arranged in an octahedral fashion [4].

Figure 3.

Pore size and distances between Cd atoms in the MOF described by Wang.

The exchange of the 4,4′-bipyridine linker for a small one like the pyrazine linker leads to a decrease in the distance between the metal atoms and hence the pore size is reduced. Arenzano and coworkers reported the pillared MOF shown in Figure 4, which is composed of two different organic linkers, pyrazine and 2-amine isophthalic acid, which possess free amino groups in the final structure that can interact with other atoms or molecules by hydrogen bonding or even donating the electron pair to another metal atom to form a coordination bond. In the same way as the material reported by Wang, this keeps the π-π interactions between the pyrazine rings. The distances between cobalt atoms are in the range from 7.34 Å to 7.14 Å. The material that possesses free functional NH₂ groups able to form hydrogen bonding interactions can be used as a carrier system [5].

Figure 4.

Distances between cobalt atoms and free amino groups present in MOF.

Carboxylate linkers possess two oxygen atoms, which contain four-electron lone pairs ready to be donated to a metal to form a coordination bond; for this reason, the carboxylate ligand, [RCO₂]⁻, is one of the most versatile and hence also one of the most abundant ligands in coordination chemistry. A great difference between the N-ligands and carboxylate ligands is the coordination modes. Although simple coordination modes are exhibited by the carboxylate and multicarboxylate family of ligands, their utility has been immense, Figure 5 [6].

Figure 5.

Carboxylate coordination modes.

Designing the network topology in MOFs can be done within two main routes, changing the cluster connectivity or altering the linker’s topological geometry. In the same way as the N-linkers, the carboxylate length may vary the pore size in the final structure. Linkers can be functionalized, that is, functional groups are added in different sites of the benzene ring to confer certain desired properties in the final material [6, 7, 8, 9, 10]. Figure 6 shows different carboxylate linkers, which differ mainly in length, generally used in the synthesis of coordination polymers. Metal clusters adopt established geometries that cannot be easily modified to change the connectivity. The creation and modification of pore space with optimized size, functionality, and diversity can be precisely tuned at the molecular level by rationally designing building blocks and synthetic procedures. Therefore, logical designing of linker geometry is essential for discovering specified topologies as an ideal platform for designing MOFs.

Figure 6.

Linkers based on the carboxylate functional group to synthesize MOFs.

3. S-block metals-based MOFs

MOFs made up of s-block main group elements, alkali and alkaline earth metals, have been less considered in both the fundamental and applied chemistry because of their low stability and most importantly, preconceived chemical features Despite the limited structures and difficulties in the structural formation of s-block MOFs originating from the unpredicted coordination behavior of metals in group 1A and 2A, s-block MOFs adopt many distinctive, appealing, and intriguing features that are suited to many applications. S-block MOFs inherit the characteristics of s-block main group elements that are (1) naturally high abundance, making them inexpensive; (2) less toxicity, unlike the other MOFs based on transition metals, thus expanding their applicability into various fields, especially in biological processes; and (3) low density, one of the essential factors for gas sorption applications [11].

4. Transition metal-based MOFs

4.1 Scandium MOFs

Scandium-based MOFs are stable and widely reported in the literature, reports on highly selective CO₂ capture by small pore scandium-based MOF, hydrogen storage, catalysis, sorption, and more [12, 13, 14, 15]. Recently, Stock and coworkers described a Scandium MOF, which was synthesized under solvothermal reaction conditions using 4,4′-oxidibenzoic. The crystal structure of Sc-CAU-21 was determined from single crystal X-ray diffraction data and showed two different types and sizes of channels. Cadmium atoms present an octahedral geometry with bond distances in the range of 2.059–2.083 Å. Figure 7. The pore size can give us an idea of the molecules, which could be absorbed in the Sc-CAU-21 [16].

Figure 7.

Types of channels with electronic charge distribution observed in red color in MOF Sc-CAU-21.

The IBUs (Inorganic Building Units) are connected by 4,4′-oxidibenzoate linker molecules to form a 3Dframework, which is isostructural to the nonporous Al-CAU-21. Doping of Sc-CAU-21 was carried out to tune the luminescence properties where Sc-CAU-21 showed a linker-based blue emission, the (co)doping of Dy³⁺ and Eu³⁺ ions resulted in a single-phase white-light-emitting phosphor [16].

4.2 Titanium MOFs

There are many reports on the synthesis of titanium-based MOFs, i.e., in photocatalytic hydrogen evolution, sorption, drug delivery, photoactive materials, and more [17, 18, 19, 20]. Martí-Gastaldo and coworkers have described a hydroxamate titanium MOF, which was synthesized from the ligand Benzene-1,4-dihydroxamic acid following a solvothermal technique. The technique indicates that the ligand was suspended in a mixture of 7.2 mL of N,N-dimethylformamide and 2.1 mL of AcOH in a 25 mL Schott bottle. The bottle was sealed and heated in an oven at 120°C for 48 h. MUV-11 is a crystalline, porous material that combines photoactivity with outstanding chemical stability in acid conditions intrinsic to the introduction of siderophore metal binders, Figure 8 [21]. The structure presents a very distorted octahedral geometry. Sorption of different ions, small and medium-size molecules can result because of the high porosity. The aromatic rings and oxygen atoms present in the structure can interact with different guest molecules via Van der Walls, electrostatic hydrogen bonding, or π-π interactions.

Figure 8.

Pore size representation of a small part of the MOF, dimethyl ammonium molecules were omitted for clarity [21].

4.3 Vanadium MOFs

While MOFs based on divalent metals, such as Zn²⁺ and Cu²⁺, have received much attention over the past decade, less progress has been achieved on the synthesis of new MOFs containing tri- and tetravalent metals [22]. Among these, vanadium MOFs are particularly rare. Applications of vanadium-based-MOFs can be found in areas of chemistry such as catalysis, adsorption, separation of N₂, CO₂, CH₄, magnetism, and others [23, 24, 25]. Ferey and coworkers reported three-dimensional vanadium (III) dicarboxylate, derived from terephthalic acid in HF, MIL-71. MIL-71 was prepared from a mixture of metallic vanadium, HF, 1,4-benzenedicarboxylic acid, and deionized water heated 3 days at 473 K under hydrothermal conditions. MIL-71 exhibits two features according to the authors: (i), it is the first solid with a two-dimensional inorganic subnetwork among the series of hybrid vanadocarboxylates, and (ii) compared with other trivalent cations, it exemplifies once more the peculiar behavior of V(III), which easily oxidizes into V(IV) [26]. Finally, the Vanadium atoms show an octahedral geometry where four oxygen atoms coordinated to the metal occupy the equatorial positions and two fluorine atoms that occupy the apical positions as observed from Figure 9.

Figure 9.

Molecular representation, where the vanadium atom is hexacoordinated displaying an octahedral geometry.

More interesting to observe is the pore size obtained in MIL-71 in the different planes. Figure 9 represents the pores in the MOF, the left side denotes the 010 planes with distances of 3.57 Å × 10.72 Å and the right plane 100 with distances of 3.57 Å × 3.87 Å. The right side represents the front part, and the left side represents the lateral view, forming in this way a rectangle. Red color represents oxygen atoms, green color represents fluorine atoms, gray color represents carbon atoms, and purple color represents vanadium atoms. Metals and many small molecules could be adsorbed in this material due to the great number of electrons spported by the oxygen and fluorine atoms and the aromatic rings in the MOF, Figure 10.

Figure 10.

Lateral view plane 010 (left) and frontal view plane 100 (right) of MIL-71.

4.4 Chromium MOFs

Chromium-based MOFs have been widely used with different applications such as environmental remediation, gases sorption, catalysis, carrier systems, among others [27, 28, 29, 30, 31]. Feng and coworkers have recently reported an ultrastable High-Connected Chromium Metal-Organic Framework, which was obtained using a Teflon cup, weighing, and combining chromium nitrate nonahydrate, 1,4-benzenedicarboxylic acid, 2,4,6-tri(4-pyridyl)-1,3,5-triazine, and 50 μL hydrofluoric acid dissolved in water. The solvothermal technique indicates that after being stirred for an hour, the vessel was sealed and was subsequently placed in a 220°C preheated oven for 2 days. As mentioned before, the importance to follow the technique is imperative to obtain the correct product, because a minimal variation of temperature, reaction time, pH, and even stir velocity can conduce to a different product. The obtained MOF gives rise to a tridimensional structure, and the material presents high porosity. The chromium atoms in the structure are hexacoordinated showing an octahedral geometry with typical bond distances in the range from 1.94 Å to 2.17 Å, Figure 11 [32].

Figure 11.

Illustration showing graphic depicts of the porosity of Cr-based MOF.

4.5 Manganese MOFs

Manganese-based MOFs are very stables, and the reports in the literature are very large, which include catalysis, materials with magnetic properties, transport, energy, gas sorption, among others [33, 34, 35, 36, 37]. Manganese(II) ions/clusters have been used as secondary building units (SUBs) to build MOFs in fields of gas adsorption and magnetism attributed to their specific electronic configuration.

Wang and coworkers reported a manganese-based MOF applied in the decomposition of ozone, which means that the MOF works like a catalytic species to decompose the ozone in water. The technique employed for the synthesis was solvothermal, weighing the corresponding moles of Mn (NO₃)₂·6H₂O and the linker H₄TTPE, which were dissolved in a mixture of DMAc/H₂O. The yellow solution obtained was stirred at room temperature for 5 min, sealed in a 25 mL Teflon-lined bomb, and heated at 120°C for 72 h [38].

The material shows according to the X-ray structure, two different kinds of manganese atoms, one manganese being pentacoordinate with a very distorted trigonal bipyramidal geometry, and the second manganese being surrounded by six atoms accommodated in an octahedral fashion. The X-ray shows two different types of pores in the structure, and the water molecules are coordinated to the manganese metal atom. The two water molecules occupy part of the interior of the pores, and although both pores are similar in size, it could be a slight difference in sorption capacity when small molecules go inside the pore and interact either with the benzene electron-rich rings pores or the more exposed electron-rich pores that contain oxygen and nitrogen atoms from the tetrazole moiety, Figure 12.

Figure 12.

Manganese-based MOF shows the pores for possible sorption of ions and molecules.

4.6 Iron MOFs

Iron-based MOFs have been widely reported in biological applications due to their high stability, ease of synthesis, and low toxicity of the metal to the human being. Application in areas such as environmental remediation, adsorption of volatile compounds, catalysis, drug delivery, water remedying, glucose biosensing, among others [39, 40, 41, 42, 43].

Long and coworkers described an iron MOF that was synthesized from anhydrous ferrous, 1,4-dihydroxyterephthalic acid, DMF, and methanol. The reaction mixture was heated at 393 K and stirred for 18 h to afford a red-orange precipitate. The solid was collected by filtration and washed with 100 mL of DMF to yield 2.0 g (91%) [44].

Although the reaction implies a solvothermal method for 18 h, worth it for the high yielding obtained. In Figure 12, we observe a small representation of the MOF, orange, gray, and red spheres represent Fe, C, and O atoms, respectively, and the hydrogen atoms were omitted for clarity. The iron atom possesses two different coordination ways, square pyramidal and octahedral. The pores are well defined and the oxygen atoms possessing two-electron lone pairs can interact with different metals and molecules. Even more, all the channels are well defined, and Figure 13 shows a small portion of the crystal.

Figure 13.

Well-defined pores in the MOF Fe₂(dobdc), where solvent molecules were omitted for clarity.

4.7 Cobalt MOFs

Cobalt-based MOFs have been broadly studied due that cobalt salts being cheap and easy to obtain, even more, the cobalt atom can form Penta or Hexa coordinate geometries, increasing the possibilities of coordination modes. These materials found applications in oxygen and hydrogen evolution, magnetism and superconductivity, catalysis, electrocatalysis, synthesis of nanomaterials, and more [45, 46, 47, 48, 49].

Burgos and coworkers could develop nanosheets of cobalt MOF for enhanced electrocatalytic water oxidation. The X-ray studies show that there exists just one type of cobalt atom, which is hexacoordinate with an octahedral geometry, and the pyridine solvent is coordinated to the cobalt atom. The pyridine rings are close enough to interact via π-π stacking with an average distance of 9.25 Å. The cobalt atoms are hexacoordinated showing a distorted octahedral geometry with coordination distances in the range of 2055–2179 Å. Because this material contains two different types of cavities, both with an electron-rich environment, metals or small molecules can interact in the surroundings to produce stable states [50].

The use of a well-defined cobalt cluster as the starting compound for the synthesis directs the construction of a Co-MOF with an unusual topology. In this MOF, the layered double nanosheets are held together by π − π stacking interactions between labile pyridine ligands. It has been shown that this material delaminates in the presence of water and that the original 3D layered structure can be regenerated by solvothermal treatment with pyridine so that the individual nanosheets have associated memory Figure 14.

Figure 14.

A small part of the crystal shows the proximity of the pyridine groups that originate the π-π interactions.

4.8 Nickel MOFs

The combination of porosity and the presence of coordinatively unsaturated Ni²⁺ sites are also of special interest because of catalytic properties and the strong H₂ binding affinity. Added to this, the chemical and thermal stability and the presence of accessible Lewis acid sites are some of the reasons why Nickel-based MOFS find application in a great variety of fields of chemistry and reports can be found on catalysis for the ethylene oligomerization, CO₂/CH₄ separation, magnetic and conducting materials, methanol oxidation, among others [51, 52, 53, 54, 55].

Gong and coworkers reported the synthesis of Nickel-based MOFS using the microwave-assisted [MA] technique combined with the solvothermal reaction. In contrast to conventional methods, MA methods allow the rapid and systematic investigation of large synthesis fields. This enables the effective discovery of new compounds, the fast optimization of synthesis conditions, and because of the large amount of data, it allows the extraction of reaction trends. MA methods have been successfully applied in the investigation of porous MOFs.

The resulting material was obtained as a crystalline solid and the X-ray diffraction showed the structure that possesses a hexacoordinated cobalt atom arranged in an octahedral fashion with a Ni-Ni interaction of 2.713 Å. The coordination bond distances are in the range from 1.972 to 2.009 Å [56]. Part of the structure is shown in Figure 15.

Figure 15.

Part of the structure of Ni-based MOF obtained by microwave-assisted technique.

4.9 Copper MOFs

Reports on MOFs containing copper include novel application in coordination strategies to control the growth orientation of the crystals, modification, and adsorption of different volatile organic compounds, MOF nanoparticles applied for sensitive fluorescent detection of ferric ion, copper-based MOFs for sensitive colorimetric detection of staphylococcus aureus, separation of CO₂ over CH₄ or N₂, among others [57, 58, 59, 60, 61].

Park and coworkers adopted a facile green synthesis for the preparation of a copper-based MOF applied in the cycloaddition reaction of CO₂ and epoxide. The coordination geometry around each Cu in the bimetallic cluster is octahedral, where the square base is established by the four BDC units with their four carboxylate oxygen atoms. The PNU-25 structure contains 3D channels with rectangular windows of dimensions 15.21 × 10.80, which undergo interpenetration via various supramolecular interactions forming an overall triple interpenetrated network, as depicted in Figure 16 [62].

Figure 16.

The PNU-25 structure contains 3D channels left, with copper metal atoms being hexacoordinated within an octahedral geometry, right.

The heterogeneous PNU catalysts efficiently catalyzed the synthesis of cyclic carbonates by the coupling of epoxide and CO₂ under ambient pressure and lower reaction temperature. The PNU catalysts demonstrated remarkably good thermal stability for the cycloaddition reaction. The coordinatively unsaturated Cu(II) units and the basic N atoms resulted in a large number of acidic-basic sites, facilitating the conversion of epoxides.

4.10 Zinc MOFs

One of the important applications of zinc-based MOFs is the separation of materials, gases, and compounds. By using the modified MOFs, diverse gases, organic and inorganic compounds were separated, i.e., H₂/CO₂, Xe, and Kr, O₂/N₂/CH₄, other applications such as catalysis in organic synthesis, removal and detection of antibiotics in water, sensing, photocatalytic activity, semiconductive and magnetic properties, thin-film nanocomposite membrane incorporated with Porous Zn-Based Metal-Organic Frameworks, among others [63, 64, 65, 66, 67].

Yang and coworkers develop a low toxicity MOF for the detection of organic and inorganic contaminants from water. The material was obtained according to the mixture of the linker Httb and Zn (NO₃)₂·6H₂O in a mixture of solvents, H₂O, and CH₃CN, and the mixture was stirred for 20 min before adding in a steel vessel at 160°C for 5 days to obtain the Zn-based MOF, with yield 64%.

The MOF exhibits high water and chemical stability as well as excellent fluorescence properties. The remaining binding sites show higher sensitivity and better fluorescence response to the representative organic micropollutant TNP and inorganic pollutants (Fe³⁺ and Cr₂O₇²⁻ in wastewater. The Zn metal is hexacoordinated to four nitrogen atoms from the tetrazole ring and two oxygen atoms belonging to water molecules, Figure 17 [68]. The final structure presents π-π interactions between the aromatic rings.

Figure 17.

A small fraction of Zn-ttb-bdc MOF shows two coordination water molecules.

5. Synthesis OF MOFs

There are many reports in the literature about the ways and techniques commonly used, frequently called reaction conditions, which are in some cases very specific to synthesize the metal transition-based MOFs. The different synthesis methods that have been applied in the last 20 years in addition to room temperature synthesis are conventional electric (CE) heating, microwave (MW) heating, electrochemistry (EC), mechanochemistry (MC), and ultrasonic (US) methods that have been employed.

Conventional step-by-step methods, as well as high-throughput methods, have been employed in some of the studies. In addition to the crystal size or shape, thin films, membranes, and various other shapes made of MOFs have been reported, which require the application of different and specific synthesis methods. Nevertheless, many other synthetic methods and parameters, such as temperature, reaction time, pressure, pH, and solvent, must be considered as well. Numerous different synthetic approaches, including slow diffusion, hydrothermal, and solvothermal, can be applied to produce MOFs relying on the resulting structures and features [69]. Figure 18 presents a resume of the different techniques applied for the synthesis of MOFs, where we observe the conventional heating, sonochemistry, microwave-assisted, using different temperature programs that let the correct nucleation and growth of single crystals.

Figure 18.

Strategies and methods for the synthesis of crystalline materials [2].

6. Environmental applications of transition metal-based MOFs

As could be discussed in the previous part, there are many different techniques to achieve the synthesis of MOFs, and something very important to consider is the combination of procedures that can allow the formation of materials with specific pore sizes and shapes. The environmental applications of MOFs depend on the final structure obtained, which is very related to the technique employed and the metals and linkers used for the synthesis. Pore size and electron charge distribution into the pores and the surface let those metals or even neutral or charged molecules interact with each other. In this section, we’ll focus on three environmental applications, heavy metal atoms, and azo dyes removal from water, hydrogen evolution, and catalysis directed to the transesterification process of non-edible oils to obtain biofuels.

6.1 Metals and azo dyes removal from wastewater

There are many reports in the literature of heavy metal atoms removed from aqueous mediums using crystalline materials. The remotion of those metals from wasted water is due to the weak interactions generated between the metals and the electron-rich environment of the pores. In the same way, the remotion of azo dyes (medium-size molecules). i.e., Congo red, methylene blue, and methyl orange from wastewater, has been widely described, due to those azo dyes are considered toxic and carcinogenic pullulans in the water.

Heavy metals such as arsenic, cadmium, chromium, copper, lead, mercury, nickel, and zing with a density over 5 g cm⁻³polluting our water are a rapidly growing global concern. These elements can be found within the environment, be it in water reservoirs, the atmosphere, or soil [70]. The recovery and elimination of toxic metal ions from wastewater are of concern with increasing awareness toward the need for protecting nature. Mercury and lead are the most toxic species and can cause bioaccumulation in kidneys, brain, lung tissues, gastrointestinal tract, central nervous system, and reproductive system [71].

To date, several approaches have been applied to remove the heavy metals from water including nanofiltration, membrane separation, ion exchange, resin, photocatalytic degradation, chemical moisture, membrane filtration, freezing, chemical deposition, biological treatment, reverse osmosis, adsorption, etc. [72].

Studies to investigate the sorption mechanisms governing the sorption process and to determine whether the sorption mechanism are controlled by a chemical or physical mechanism, Morsali et al. applied conventional sorption kinetics models such as pseudo-first-order, pseudo-second-order, and intraparticle diffusion to the study. The correlation coefficient of the pseudo-second-order model for Cd(II) is close to 1 (R² = 0.9999) and exhibited that the pseudo-second-order model is more consistent with experimental data than the other model. Therefore, it is compatible with chemical sorption, which indicates that transfer, exchange, or sharing of electrons has taken place. Therefore, supramolecular interactions between cadmium ion and the free electrons of ligand nitrogens and the electrostatic interaction between the cation and the dipole of nitrogen on the dihydropyrazine ring of ligand are proposed as the dominant interactions between the metal ion and the structure [73].

The X-ray structure showed that the aromatic rings from the linker are available to interact through supramolecular interactions with different metals or even small molecules through week interactions Figure 19.

Figure 19.

A small portion in MOF TMU-60 and pore size obtained after the synthesis.

The synthesis was achieved in DMF at 60°C, and then the solution was transferred to a Teflon autoclave and heated to 120°C for 72.

Large amounts of dyes are commonly used in many manufacturing, such as textiles, clothing, and printing. Among these, azo dyes are one of the largest groups that are heavily used, but most difficult to be degraded, and even more, the azo dyes and their derivative products are toxic to the aquatic environment and are mutagenic and carcinogenic to humans. Therefore, the treatment of wastewaters containing these dyes is necessary [74].

Numerous studies have considered new processes to eliminate emerging organic contaminants (EOCs) from water, i.e., ozonization, chlorination, sonodegradation, biodegradation, inorganic heterogeneous catalysis, activated carbon treatment, and more. Among these technologies, metal-organic frameworks (MOFs) have been recently investigated for the removal of contaminants in water [75].

Separation or degradation requires strong research efforts to modify MOFs via controlling their pore diameter (adding functional groups, creating defects) or to construct MOF composites to engineer materials as improved adsorbents and/or catalysts for contaminated water treatment. MOFs and MOF composites have been proposed for the removal of a wide range of contaminants, including dyes, pharmaceuticals, plasticizers, herbicides and pesticides, industrial products, among others. While a few reviews have already documented the use of MOFs in the removal of novel contaminants [76].

Choe et al. reported the adsorptive removal of various dyes using a Zr porphyrinic MOF, PCN-224. The plausible mechanism for adsorptive removal revealed multiple interactions between the dye and porphyrin linker/Zr6 node via π − π interactions and hydrogen bonding, respectively. Such results demonstrate that PCN-224 is an excellent adsorbent, providing superior water stability, pore aperture of suitable size, and multiple interaction sites Figure 20 [77].

Figure 20.

Zr porphyrinic metal-organic framework (MOF) for removal of azo dyes [77].

The best performance of dye adsorption onto PCN-224 comes from structural properties such as appropriate pore aperture, volume as well as various types of interactions such as π−π interaction, hydrogen bonding, and electrostatic interaction. Significantly, an interaction between the sulfonate group of the MO molecule and the Zr6 node of PCN-224 was demonstrated through experimental and theoretical studies. Arrangement of tripod form between Zr node and sulfonate group of the MO molecule can form hydrogen bonding.

6.2 Hydrogen evolution

Highly efficient hydrogen evolution reactions (HERs) will determine the mass distributions of hydrogen-powered clean technologies in the future. That’s why hydrogen evolution is one of the topics more explored in the last years because molecular hydrogen is the best environmentally friendly fuel available, which reacts with oxygen to produce energy and water.

The procedures employed to produce hydrogen are still expensive because of the catalysts employed to obtain the gas and the conditions and hence the high-cost obtention. Current industrial hydrogen production methods include coal gasification (followed by water-gas shift reaction), steam reforming, cryogenic distillation, and water splitting.

The specific surface area of MOFs can range from 1000 to over 6000 m² g⁻¹, thanks to their tailorable porous structures, which play a significant role in enhancing the catalytic HER process. Large surface area and pore volume ensure sufficient contact between the electrolyte (or reactant solution) and the surface of the catalyst, which essentially improves catalytic performance by exposing more active sites for the catalytic reactions to take place. It is that research on MOFs for HER primarily focuses on the following three techniques: electrocatalytic, photocatalytic, and chemocatalytic HER [78].

MOFs, MOF supports, and MOF derivatives can be utilized as catalysts in the abovementioned hydrogen production methods. Most of the materials used for photocatalytic hydrogen evolution PHE applications include inorganic oxides such as TiO₂, ZnO, and SrZrO₃, due to their high stability. The application of metalorganic frameworks in these processes is limited due to the loss of efficiency attributed to the recombination of electrons and holes [79]. Even so, together with the unique porous structure of MOFs, a remarkable hydrogen evolution reaction HER performance can be achieved using different overpotential in phosphate buffer solution (PBS, pH = 7.0) [80].

In the same way, the significant effect of crystallinity in the photocatalytic activity of metal-organic frameworks was demonstrated through the evaluation of different samples with different crystallinity in the HER reactions. The samples with high crystallinity produce too a higher amount of hydrogen, which is attributed to the lower recombination supported by the experiments of photoluminescence and electrochemical impedance directly related to a high-ordered material.

Rivera et al. described the synthesis of a BDC-Zn MOF, which was firstly used for methyl orange and methyl blue sorption and followed by PHE under solar light. MO presented the best adsorption result, with a maximum adsorption capacity of 2100 mg/g, which is higher than all the MOFs reported in the literature. For HER, the activity was enhanced 24 times in photocatalyst with MO adsorbed, and 27 times for the MB adsorbed (from 47 to 1148 and 1259 μmol/gh, respectively). This result is attributed to better light adsorption and a decrease in charge recombination. It’s important to mention that even though the reflux method presented the disadvantage that it is not possible to obtain single crystals, the reaction conditions such as temperature, pressure, and time are more ecological in comparison with the traditional MOF 5 solvothermal synthesis and analogous [81].

Figure 21 shows the mechanism for HER. In the first step, MO/MB molecules linker to BDC-Zn MOF captures the light from the solar simulator. Light produces the transference of electrons from the conduction (LUMO orbital analog) to the valence band (HOMO orbital analog), this produces a hole formed in the conduction band, which oxidizes water molecules, finally, the electron in the valence band reduces H⁺ to H₂. At the same time the species O₂⁻ and OH⁻ are formed, and these could affect MOFs by redox reactions; however, possibly MO/MB acts as a sacrificial agent avoiding the BDC-Zn MOF structure collapse by the capture of these species. Therefore, growing hydrogen production is observed.

Figure 21.

Mechanism of HER by using the BDC-Zn-MOF obtained under solvothermal conditions [81].

6.3 Catalytic transesterification reaction

Biodiesel is green and renewable energy, which is a promising substitute to replace fossil combustibles. Normally, biodiesel is produced via transesterification/esterification with the assistance of a homogeneous or heterogeneous catalyst. Biodiesel is a series of fatty acid alkyl ester (FAAE), which is normally derived from transesterification of vegetable oil or animal fat (triglyceride) and alcohol with the assistance of catalyst as shown in Figure 22.

Figure 22.

Transesterification reaction of triglycerides in alcohol as a solvent.

In general, transesterification is the most used technique for biodiesel production, which is proceeded in three steps. In the first step, triglyceride reacts with alcohol generating monomolecular FAAE and diglyceride. Then, diglyceride reacts with alcohol resulting in monomolecular FAAE and monoglyceride. Finally, monoglyceride reacts with alcohols giving rise to monomolecular FAAE and glycerol [82].

Industrial biodiesel production is based on the transesterification of triglycerides with methanol or ethanol using stoichiometric amounts of strong Brönsted base (e.g., sodium or potassium hydroxide/methoxide) and homogeneous acid catalysis, such as H₂SO₄, HCl, H₃PO₄, but the problem with this homogeneous catalyst is their difficult recovery from the media. The ease of heterogeneous catalyst to be recovered at the end of the reaction just by a simple filtration reduces dramatically the cost in the biodiesel obtention process.

One of the properties of MOFs compared with other porous solids is the facile introduction of the desired active sites. Solvent/water molecules weakly bounded around metal nodes in MOFs could be removed upon activation; thus, these coordinatively unsaturated metal sites behave as Lewis acid sites [83].

MOFs can be used in combination with other materials to form composites or as catalytic support to solve two specific problems, stability, and recyclability issues. The porous materials, metal-organic frameworks (MOFs) are suitable for enzyme immobilization owing to their ideal features of tunable pore size, and topological and compositional versatility. Currently, multiple enzyme-MOF composites have been successfully created using physical adsorption or covalent attachment strategies. Also, because MOFS catalyze many reactions, they accelerate the velocity of the reaction and low the transition states of the molecules to react faster and slower reaction times and energy consumption [84].

To increase the catalytic activity in MOFs, three different strategies can be used. The first strategy is post modification of MOFs, which results in the linker modification to increase the active sites and hence the catalytic activity. The second strategy is the formation of composites, which results in mixing the MOFs with different basic or acid materials together to obtain better results and structural stability in MOFs. The third strategy is to develop new linkers that incorporate the specific active sites that remain free once the MOFs are obtained [85]. Finally, because the transesterification process requires very large reactions times, the energy consumption is high; therefore, different synthesis techniques can be used in combination. Rivera et al. could obtain through hydrothermal synthesis a 3D-Co-based MOF. The Ultrasonic-assisted synthesis is a powerful technique, the yields obtained are increased, and the reaction times are decreased using milder conditions.

Rivera et al. reported the green synthetic approach that involves the ultrasonic technique that allows in terms of energy conservation higher purity products. The high catalytic activity exhibited by the catalyst reported has to do in part with the free carboxylic acid groups that remain free in the structure, as revealed by the X-ray diffraction. The application of this catalyst resulted in 80% of total conversion after 12 h at just 60°C [86].

The use of composite MOFs is widely described in the literature, and the idea to produce biodiesel is to add new basicity or acidity sites to the material to increase the catalytic activity or simply to get better stability. Zhang et al. synthesized the reusable and highly active Fe-BTC and UiO-66 metal-organic framework by hydrothermal method, and the MOF was applied for the acid-catalyzed esterification of oleic acid with methanol. Typically, the esterification reactions of oleic acid with methanol were performed in a 50 mL stainless steel reactor equipped with a magnetic stirrer. The mechanism proposed for the esterification reaction is shown in Figure 23.

Figure 23.

Possible reaction mechanism for the esterification reaction process [87].

Characterization analyses indicated that the ZrSiW/UiO-66 possesses an appropriate structure and high acidity. The highest oleic acid conversion of 98.0% was obtained using the ZrSiW/UiO-66 nano-hybrids nanocatalyst under the optimal esterification reactions: 150°C, 0.24 g catalyst, 1:20 molar ratio of oleic acid to methanol, and a reaction time of 4 h [87]. The easy preparation of MOF composites lets these materials find better opportunities to be investigated [87].

7. Conclusions

Metal-organic materials are easy to synthesize and have a wide variety of applications. MOF materials are constructed from linkers and different metal atoms, which can interact via coordination bonds to produce 1D, 2D, and 3D porous structures. The final porous structure depends primarily on the linkers and metals used for the synthesis but equally important to obtain specific crystalline structures and hence high-ordered and porous materials is to apply the correct technique that involves the correct pressure, reaction time, temperature, pH value, solvent, or a mixture of solvents, stirring, or even microwave and ultrasonic-assisted synthesis, which can be applied and combined with other techniques to obtain desirable size and porous structures in the MOFs. The environmental applications of MOFs such as sorption of metals and azo dyes depend on the final structure obtained, the pore size, and the electron charge distribution into the pores, and the surface allows metals or even neutral or charged molecules to interact with the MOF to form different contacts such as hydrogen bonding, π-π interactions, or simply interact through Van der walls forces. On the other hand, the catalytic activity for the transesterification process to obtain biodiesel depends mainly on the availability of metal centers, which are acid sites that can be increased by adding more acid sites by a post-modification synthesis or simply by using a specific linker containing the acid sites in the original structure. Finally, the hydrogen evolution reaction depends mainly on the structure of MOFs (crystallinity) and the linkers, that is, delocalized systems that allow HOMO and LUMO orbitals to get less separated, and hence the electrons can get transferred more efficiently to oxidase water.

Acknowledgments

Thanks to committee pro-mejoras from Universidad Veracruzana, Facultad de Ciencias Químicas, Orizaba, Ver. México, who supported the funding of this project.

Conflict of interest

“The authors declare no conflict of interest.”