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Titanium is one of the most attractive elements, due to its unique advantages such as stability, recyclability, activity under light absorption, cheapness, and safety. The special characteristics of titanium include different oxidation states, high coordination number of Ti4+, and the ability to form strong bonds with oxygen and different ligands, making it a good candidate for the construction of the new composite named metal–organic framework or briefly MOF. MOFs are composites that have opened a new window toward the scientific world due to their special structure that makes them have some properties, including the highest surface activity, high porosity, tunable pore, and high flexibility in design that make them useful in different applications, such as gas storage and separation, liquid separation and purification, electrochemical energy storage, catalysis, and sensing. Titanium, due to the mentioned properties, has been used as a node in the structure of different MOFs and applied in different fields.
Faculty of Science, Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Islamic Republic of Iran
Faezeh Mokari
Faculty of Science, Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Islamic Republic of Iran
Mostafa Gholizadeh
Faculty of Science, Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Islamic Republic of Iran
Yahya Absalan*
Faculty of Science, Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Islamic Republic of Iran
*Address all correspondence to: y.absalan@ferdowsi.um.ac.ir;, yahyaabsalan2014@gmail.com
1. Introduction
Coordination polymers (CPs) are a class of compounds formed by metal ions (or clusters) coordinated to multidentate organic and/or inorganic ligands. Metal–organic frameworks (MOFs) are a special group of CPs with porous and crystalline one-, two- or three-dimensional structures. These compounds are composed of metal ions or well-defined secondary building units and organic linkers that are connected by coordination bonds and other weak interactions or noncovalent bonds, such as H-bonds, π-electron stacking, or van der Waals interactions [1]. In the 1960s, the MOF kind of structure was firstly introduced by Tomic and others [2, 3, 4]. In the 1990s, research on MOF was developed by Robson et al. [5, 6] and later by Yaghi et al. [7].
After the discovery, synthesis, and characterization of the first known MOF structure, MOF-5 ([Zn4 (BDC) 3] (DMF) x) by Yaghi and coworkers in 1999, a rapid advancement has been made in the chemistry of MOFs [8]. The increasing interest in MOF research can be attributed to their amazing properties, including high specific surface area, ultra-high porosity, tunable surface chemistry, functional diversity, and crystallinity [9].
The main goal of this chapter is the use of titanium as a node in the design of metal–organic frameworks. To achieve this goal, we will specifically investigate the characteristics and properties of titanium as a component in the design of metal–organic frameworks. In the following, considering the features and advantages mentioned in the text for titanium, we will introduce the types of titanium-based MOFs, synthetic methods, and applications. Eventually, the comparison of the properties and performance of titanium-based MOFs with other metal–organic frameworks is discussed.
MOFs are constructed by connecting inorganic and organic building units through coordination bonds. The inorganic units can be monovalent (Cu+, Ag+, etc.), divalent (Mg2+, Mn2+, Fe2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, etc.), trivalent (Al3+, Sc3+, V3+, Cr3+, Fe3+, Ga3+, In3+, lanthanides3+, etc.), or tetravalent (Ti4+, Zr4+, Hf4+, Ce4+, etc.) metal cations or clusters, namely secondary building units (SBUs) (e.g., Zn4O(COO)6, Cu2(COO)4, Cr3O(H2O)3(COO)6, and Zr6O4(OH)10(H2O)6(COO)6) [10]. These inorganic units are connected by coordination bonds to organic linkers, which commonly contain carboxylate, phosphonate, pyridyl, and imidazolate, or other azolate functional groups [11].
Different synthesis methods have been developed and applied to synthesize MOFs to achieve the required properties in terms of shape, surface morphology, and others for various applications. The major synthesis methods of MOFs include hydrothermal (solvothermal) synthesis, mechanochemical synthesis, microwave-assisted synthesis, sonochemical synthesis, and electrochemical synthesis [12, 13, 14, 15].
Solvothermal/hydrothermal synthesis is the most conventional method for the synthesis of these compounds. If water is used as a solvent, this method is termed as a hydrothermal method. This method generally consists of the self-assembly of soluble precursors, which contain the metal and the organic component. The key point of this method is the selection of the appropriate solvent because the solvent acts not only as a reaction medium but also as a structure-directing agent. Some examples of this synthesis include MOF-5 [16], MOF-74 [17], ZIF-8 [18], and Uio-66 [19]. The advantages of this technique are a large operating temperature range, relatively easy transposition, and high yield. Disadvantages include high energy consumption and a long reaction time [20].
Another commonly used method is mechanochemical synthesis, which is a green approach performed by grinding a mixture of the organic linker and metal salt in an agate mortar and pestle or in a ball mill and in the absence of solvent. Examples of this method include HKUST-1 [21], ZIF-4, and ZIF-8 [22].
In microwave-assisted synthesis, energy for the reaction is provided in the form of microwave radiation (MW). Advantages include a reduction in crystallization time and higher yields, on the other hand, disadvantages include difficulty in controlling the shape and size and the lack of a direct method for scale-up [20]. Examples include CoMOF-74 [23] and Cr-MIL-100 [24].
The ultrasonic/sonochemical method is another efficient synthesis of MOFs under ultrasonic irradiation. The impact of ultrasound on liquid and colloid systems is mainly caused by acoustic cavitation, which would be the production, growth, and implosion of bubbles within the solvent. The advantages of this method include homogeneous particle size and morphology in short periods of time and the formation of stable structures due to the high energy and high pressure created during this process [20]. Fe-MIL-53 [25] and MOF-177 [26] are some examples of MOF materials synthesized through this ultrasonic method.
In the electrochemical method, metal ions rather than metal salts are introduced in the anode, whereas an organic linker is placed at the cathode and the electrochemical cell is filled with conducting salt. This technique was reported by BASF in 2005. The advantages of this method are avoiding anions, such as nitrates, from metal salts, lower temperatures of reaction, and extremely quick synthesis [12]. Some examples of this synthesis are ZIF-8 and MIL-53 (Table 1) [27].
Synthetic method
Advantages
Disadvantages
Examples
Ref.
Hydro(solvo)thermal
Large operating temperature range (i.e., between 80 and 250°C)
Easy transposition
High yield
High energy consumption
Long reaction time
Expensive pressure-sealed metal vessels and heating ovens
MOFs have been attracting great attention in the past several decades mainly because of their outstanding properties, including high specific surface area, ultra-high porosity, tunable surface chemistry, functional diversity, and crystallinity [9]. These properties make them good candidates for use in gas storage and separation [28, 29], catalysis, drug delivery, and sensing [30].
Common methods of gas storage require high-pressure tanks and multistage compressors, which are difficult and expensive. MOFs provide a simpler and safer storage method, owing to their high surface area, high porosity, and tunable pore structure. To date, numerous MOFs have been applied for gas storage. PCN-12 [31], MOF 210 [32], and NU-100 [33] are some examples for H2 storage. MOFs also have great potential to perform gas separations as adsorbents. In 2014, Yang et al. reported a hydroxyl–functionalized Al-MOF (NOTT-300) with high selectivity for C2H4/C2H6 separation [34].
MOFs have shown great capability in heterogeneous catalysis due to their unique features. Numerous MOFs have been reported as catalysts in photocatalysis reactions, electrocatalysis reactions, and organic transformations, such as oxidation, epoxidation, and Knoevenagel condensation [35]. These compounds are commonly employed as heterogeneous catalysts in the form of pristine solids or post-synthetically modified ones. One of the earliest studies in the field of MOF catalysis was reported in 1994 by Fujita et al. They constructed a two-dimensional (2D) network of Cd(4,4′-bpy)2(NO3)2 by reaction between Cd(NO3)2 and (4,4′-bpy), and used it as a heterogeneous catalyst for the cyanosilylation of aldehydes [36]. The first experimental evidence of photocatalytic activity on a MOF (MOF-5) was published in 2007 [37]. To date, many researches have been made in the area of MOF-based photocatalysts. For example, Leng et al. reported an indium-based MOF, USTC-8(In), which exhibits an excellent photocatalytic activity for H2 production [38].
Researches on MOFs as drug carriers have been developed in biomedical fields due to their features, such as drug loading ability, biodegradability, and adaptable functionality. According to the enhanced stability, enormous porosity, and large pore volume of the MIL family, they are an attractive candidate for the storage and controlled release of biologically important molecules [39]. Serrey and Férey et al. demonstrated encapsulating drug molecules (Ibuprofen) in chromium carboxylate MOFs, MIL – 100 and MIL – 101, exhibiting drug storage capacities of 35 wt% and 140 wt%, respectively, and controlled drug release behavior of 5 to 6 days under physiological conditions [40].
In addition to the applications mentioned above, MOFs can be used as sensors in different fields. A sensor is a device that can receive, respond, and detect a signal. In MOF-based devices, the sensor responds to different external stimulations, such as stress, mass, optical, and environmental changes. MOFs, which can be excited by absorbing UV–visible light, are considered as photoluminescent. Both the metal and the linker increase luminescence. Lanthanides like Eu, Tb, Dy, Sm, Nd, Gd, Er, and Yb are employed as luminescent metal ions owing to their electronic transition from d- to f-orbitals. Naphthalene, anthracene, pyrene, perylene, and stilbene types of ligands are used for the synthesis of luminescent MOFs [15]. As an example, HKUST-1 has shown a color change from turquoise to green as a result of the exchange of Cu2+-coordinated water with pyridine (Table 2) [44].
MOFs can be divided into different types based on their metal nodes. Metal nodes may vary from the s-block, p-block, transition metals, or even rare earth metals. S-block main group elements, alkali and alkaline earth metals are rarely used because of their low stability and difficulties in the structural formation of MOFs. In spite of their limitations, they have some advantages, including naturally high abundance, less toxicity, and low density, which lead to different applications [48]. Most reported MOFs are based on transition metals that can divide into different groups.
4.1 Transition-metal-based MOFs
4.1.1 Zr-based MOFs
Zr-based MOFs are useful in different fields according to their extraordinary mechanical, thermal, chemical stability and high oxidation state of Zr(IV) compared with M(I), M(II), and M(III)-based MOFs (M stands for metal elements). Zr-MOFs are applied in catalysis, molecule adsorption and separation, drug delivery, and fluorescence sensing, and as porous carriers [49]. For instance, Feng et al. reported Zr-PCN-221(Fe) as the catalyst for the selective oxidation of cyclohexane with tert-butyl hydroperoxide (TBHP) as the oxidant [50]. Additionally, Zr-based MOFs can act as a semiconductor in photocatalytic reactions. As an example, Lillerud and coworkers reported the first Zr-based MOFs including UiO-66 and UiO-66(NH2) for photocatalytic water splitting [51].
4.1.2 Cu-based MOFs
Copper (Cu) as an abundant and inexpensive metal with divalent copper ions having tetra- or hexa-coordinated structure can be used as another metal node in the synthesis of MOFs. Also, it has different oxidation states, such as Cu0, CuI, CuII, and CuIII, leading to a wide variety of redox reactions [52].
Park and coworkers reported the green synthesis of PNU-25 applied in the cycloaddition reaction of CO2 and epoxide [53]. In another study, a semi-conductive copper organic MOF, that is, {[CuICuII2-(DCTP)2]NO3·1.5 DMF}n with 2.1 eV bandgap was involved in UV/visible photocatalytic H2 generation and oxidation of holes for photocatalytic degradation of methyl blue with use of visible light and without the aid of photosensitizer [54].
4.1.3 Fe-based MOFs
According to the Earth’s abundance of iron, development in various applications of Fe-based MOFs in CO2 reduction, photo-degradation, water splitting, and organic transformations occurred [55]. They are widely applied in photocatalytic reactions like H2 production. For example, the photocatalytic visible light H2 production of MIL-100(Fe) in presence of MEOH was reported by Wang et al. [56].
4.1.4 Co-based MOFs
Based on the inexpensiveness and availability of cobalt salts, cobalt-based MOFs have been widely used in different applications, such as oxygen and hydrogen evolution, catalysis, electrocatalysis, synthesis of nanomaterials, and more [57, 58, 59]. These structures can provide a transition porous framework and ultra-high surface area for photocatalytic H2 production. For instance, Yang et al. reported Co-based zeolitic imidazolate MOF (ZIF-67) with RuN3 as a photosensitizer for efficient photocatalytic H2 production from H2O [60]. In another study of Co-MOFs, Dirk Volkmer and coworkers reported the synthesis and the catalytic performance of MFU-1 MOF in the oxidation of cyclohexene using TBHP as an oxidant [61].
4.1.5 Ni-based MOFs
Ni-based MOFs are another useful group of metal–organic frameworks with properties such as high porosity, presence of coordinatively unsaturated Ni2+ sites and Lewis acid sites so they can be used in different fields. Miyake et al. first described the synthesis and application of MOFs with Ni-bipyridyl complex as the linker along with Ni as the nodes in ethylene oligomerization [62]. In another study, [Ni(phen) (oba)]n- 0.5nH2O 2D MOF has been studied as supporting material for metal sulfides as a visible light active photocatalyst for effective H2 production [63].
4.1.6 Ti-based MOFs
4.1.6.1 The overall properties of titanium (Ti)
4.1.6.1.1 The chemistry of Ti
What makes titanium an attractive candidate to use as a metal node in several MOFs? Indeed, divalent transition metal cations, such as Zn2+, Cu2+, Co2+, and Ni2+, lead to thousands of extraordinary porous MOFs. Nevertheless, they are often unstable in the presence of water. To enhance the strength of cation–ligand bond and gain stability in MOFs, increasing the charge of a metal cation and its polarizing power charge over ionic radius are considered [64].
Among the elements in the periodic table, titanium or Ti has drawn considerable attention due to its fascinating intrinsic features. To mention about its abundance, it is the ninth most abundant element in the Earth’s crust and is the second most abundant transition metal after iron. This element, located in the first row of the periodic table and above the zirconium, has a [Ar] 3d2 4s2 electronic configuration. In comparison with Zr4+, it has a stronger interaction with oxygen due to its smaller ionic radius. Its common oxidation states are +2, +3, and + 4. Ti+4 ion is a frequent oxidation state, which is the most stable in ambient conditions (air and water), while Ti3+ could be generated under reductive conditions but quickly oxidized back to Ti4+ in the presence of oxygen. The reasons for its tendency to hydrolyze into various titanium oxo clusters, or directly to generate TiO2 precipitation are low electronegativity and high polarizability [65, 66, 67].
Due to the all features mentioned above, titanium is a highly desirable alternative to constructing stable and diverse MOFs. There are several Ti-MOFs that were published, including MIL-91 [68], MIL-125 [69], NTU-9 [70], MIL-101 [71], COK-69 [72], Ti-CAT-5 [73], MIL-167 [74], MIP-177 [75], MIL-100 [76], Ti-TBP [77], and ZSTUs [78], applied in different fields. However, Ti-MOFs have seldom been reported due to some reasons, which will be summarized into three main parts [79]:
Strong interaction between reactive titanium sources and ligand as a result of the high charge to radius (Z/r) value on Ti4+ causes poor association/dissociation of metal–ligand bond and formation of crystalline products. To achieve a crystalline structure, choosing suitable synthetic conditions, such as the organic solvents, ratio of the reacted solution mixture, pH, and temperature, should be considered.
Another limitation is the extreme hydrolysis of reactive titanium sources
Some of the titanium carboxylates show low symmetry or inappropriate connectivity that leads to hinder the formation of periodic networks with organic linkers [80].
4.1.6.1.2 History
MIL-22 was the first Ti-MOF, synthesized by Serre, Fèrey, and coworkers, using phosphorus ligand and TiO2 under high temperature in 1999 (Figure 1a) [81]. This compound showed limited porosity. For a quite long time, no highly porous Ti-MOF was reported. Subsequently, in 2009, MIL-125 was discovered by the same group (Figure 1b) [82]. The discovery of this MOF created a scientific boom in the synthesis of new Ti-based MOFs material. It consists of 12-connected [Ti8O8(OH)4(COO)12] clusters and 1,4-benzenedicarboxylate (BDC) linker and demonstrates permanent porosity with a BET surface area of 1550 m2g−1 [69]. Meanwhile, titanium oxo clusters, which can be seen as TiO2 nanoclusters, affect the construction of novel Ti-based MOFs [83].
Figure 1.
(a) Structure of MIL-22, (b) structure of MIL-125.
Up to now, different types of Ti-MOFs have been introduced. The development of a titanium-based MOFs timeline is presented in Figure 2 [65].
Figure 2.
The timeline for the development of crystal structures of titanium-based MOFs.
4.1.6.1.3 Comparison of the Ti-MOF with other MOFs
In this regard, Ti-MOFs possess several advantages over other groups of MOFs, which are as follows:
Strong Ti–O bonding, which is the result of the small ionic radius of Ti4+, leads to rigid MOF structures.
The high coordination number of Ti4+ causes structural diversity in metal-oxo clusters.
Ti-based MOFs with large surface area and high porosity can be effective photocatalysts in which Ti–O clusters act as isolated titanium oxide quantum dots, making them like the Ti-grafted zeolites and the semiconducting titanium oxide.
Their ability to store charge effectively because of the reversible redox transition between Ti4+/Ti3+ under photo-excitation leads to using them in photo-driven catalytic redox reactions.
Their photocatalytic properties can be developed and improved by functionalization of ligands and composition with other highly active species like metal nanoparticles and graphene.
They form rigid frameworks, which are the result of strong metal–ligand bonding.
Their well-defined porosity enhances photocatalytic activities.
Their low cost and low toxicity make them good candidates as catalysts [64, 84].
To realize the importance of Ti-MOFs among all families of MOFs, their properties should be discussed, most notably about two important ones such as porosity and optical properties [85].
Porosity is one of the most effective features in photocatalytic reactions accomplished by MOFs. Owing to this property, the transfer of photoelectric charge happens rapidly, thus the electron–hole recombination reduces. Moreover, porosity lets the active sites to be in contact with the substrate more effectively by reducing the diffusion resistance [86]. In addition, this property prepares large spaces for encapsulating different types of catalysts [87]. As a result, Ti-MOFs can be used in different fields of chemistry.
There are various strategies used for the synthesis of porous Ti-MOFs, which are good to mention:
Partial post-synthetic metal exchange; for instance, porous TiIII-MOF-5 and TiIV-UiO-66 were synthesized by exchanging their metals with ZnII and ZrIV [88].
Assembly of clusters; MTM-1, a porous Ti-MOF, is synthesized by a combination of {[Ti6O6][Oipr]6}6+ cluster and cross-connected Cu2I2 dimer through isononanol (INA) [85].
Isoreticular expansion, an effective strategy, is used to increase the pore size, surface area, and porosity by self-assembly of metal centers and organic ligands. In this strategy, long and geometrically equivalent organic ligands influence the porosity of MOF. As an example, MOF-902 has a greater porosity in comparison with MOF-901 because of its longer organic building blocks [89]. In spite of the fact that the longer the organic ligands are, the higher porosity occurs, and the stability of Ti-MOFs diminishes.
The optical property of MOFs is an important factor for photocatalysis. In general, photocatalytic reactions consist of three main steps: light absorption, charge separation and transfer, and redox reaction [90]. To enhance the light absorption intensity, modification of organic ligands is utilized. In the same way, it can affect MOF’s bandgap. As an example, inserting the -NH2 functional group on the ligand of MIL-125 yields a small bandgap [91].
To conclude, compared to other MOFs, Ti-MOFs are fascinating structures with various applications in different fields due to their high thermal and chemical stability, novel structural diversity, photocatalytic properties, low cost, and low toxicity (Figure 3) [85]. The geometry and length of organic building blocks and metal nodes can affect the chemical stability and functional properties of MOFs. For instance, the carboxylate ligand-based MOFs with low-valent ions, such as Zn2+, Co2+, and Mn2+, show lower chemical stability. However, those with high-valent ions, such as Zr4+, Ti4+, and Cr3+, usually have high chemical stability [65].
Figure 3.
The number of photocatalytic publications on MOFs and Ti-MOFs reported annually from 2009 to 2019.
In comparison with other metal-based MOFs, for example, Zr-MOFs, Fe-MOFs, or Cu-MOFs, the titanium-based MOFs exhibit richer structural and topological diversity, due to the variable nuclearity of titanium cores.
Owing to the low-level control of titanium chemistry and highly unpredictable polycondensation of titanium cation, titanium-based MOF structures are less commonly reported compared with the Zr-MOFs, Fe-MOFs, or Cu-MOFs.
4.1.6.1.4 Stability
The stability of M4+-MOFs makes them an appealing choice to concentrate on. Many factors are involved in the stability of MOFs, such as metal nodes, organic ligands, metal–ligand coordination, operating environment, hydrophobicity of the pore surface, and defects [92, 93, 94]. Thermodynamic stability of MOFs is affected by metal–ligand coordination. Based on Pearson’s hard/soft acid/base (HSAB) theory, strong bonds are formed between hard Lewis’s acids and bases or soft Lewis’s acids and bases. Carboxylate-based ligands are hard bases that can generate stable MOFs with group 4 metal cations such as Ti4+ [66]. Likewise, kinetic factors have an important role in the stability of MOFs. Strong connection between metal-oxo clusters and rigid ligands leads to dense and rigid frameworks, which increase the stability of MOFs. Hence, good stability in M4+-based MOFs is expected because M4+ cations tend to have more ligands to balance their charges. Another factor that affects the stability of MOFs is the operating environment. For instance, the resistance of carboxylate-based ligands in acids and bases is completely different. They show excellent stability in acids, while their stability is quite weak in bases [95].
4.1.6.1.5 The coordination behavior of titanium ions
Generally, each metal ion prefers to coordinate with other components in a special way. For example, Cu2+ ions tend to assemble with four carboxylate groups to form Cu(COO)4 paddle-wheel geometry [44] and zirconium ions construct the clusters like Zr6O4(OH)4(CO2)12 as 6-C, 8-C, 10-C, or 12-C nodes [49]. With regard to titanium ions, the coordination with other components changes with reaction systems. The diversity in titanium-oxo clusters makes it hard to predict the topology of titanium-based MOFs. Ti4+ constructs octahedrally six-coordinated structures by O forming TiO6 units that are bridged by μ2-O or μ3-O atoms. They lead to the creation of titanium-oxo clusters that vary based on their synthetic conditions. For instance, the cluster in MIL-125 or NH2-MIL-125, Ti8O8(OH)4 is completely different from Ti3(μ3-O) cluster in its polymorph Ti-MIL-101 [69, 71].
4.1.6.1.6 Applications
Most of the investigations have demonstrated that Ti-MOFs have several applications due to their diversity of stable structures, including photocatalytic reactions and organic transformations such as H2 productions [96], CO2 conversion [97], H2O2production [98], pollutant degradation [99], and many other organic reactions. As an example, in 2018, an efficient photocatalytic H2 production, accomplished by a Ti-MOF known as MIL-125-(SCH3)2, was reported by Han et al. [100]. In 2018, Yamashita and coworkers reported the first example of a Ti-based MOF, MIL-125-NH2 with NiO nanoparticles (NPs), used in photocatalytic H2O2 production [101]. The photocatalytic reduction of CO2 with Co/NH2-MIL-125(Ti) was reported by Fu et al. under visible light irradiation [102].
4.1.6.2 Titanium precursor for MOF synthesis
Titanium precursors used in MOFs synthesis include hydrous TiO2, titanium chlorides, titanium alkoxides, titanium complexes, and titanium-oxo-clusters.
MIL-25 [103] and MIL-22 [81], two Ti natural phosphonates, were the first structures that were formed by hydrous TiO2 as a precursor. Hydrous forms of TiO2 are rarely used for the synthesis of MOFs due to their limitations, such as high reaction temperature (>200 C), long reaction time (3–4 d), and highly corrosive and toxic reaction media (40% HF aqueous solution) [104].
Titanium chlorides extremely hydrolyze and release hydrochloride fumes in the reaction medium [105]. This is the reason why they are seldom used in Ti-MOFs synthesis. TiCl3 is an exception in the construction of di or trivalent Ti-MOFs. Whereas Ti4+ is not a suitable selection because of the imbalanced charge and ionic range. According to the reductive nature of Ti3+, exposure to air inhibits the synthesis [106].
In terms of titanium alkoxides, Ti(OR)4, hydrophobic character and hindrance of alkoxy groups cause them less hydrolyzed than TiCl4. The construction of complexes of Ti cations with bidentate ligands, such as carboxylates, diketonates, catecholates, and phosphonates, leads to a stable structure [64].
An efficient way to control the reaction degree of MOF synthesis is the utilization of stable six-coordinated Ti compounds (Ti-oxo-clusters) [80]. Owing to the different sizes, geometry, and coordination numbers of Ti clusters, they have the potential to be chosen as building blocks of MOFs. The original cluster might be transformed during the reaction. For instance, in the synthesis of PCN-22 by Ti6O6(iOPr)6(abz)6 as Ti precursor, this cluster was completely transformed to Ti7O6-oxo SBU in the final MOFs [107]. In comparison with Ti alkoxides, Ti clusters are useful to adjust the reaction rates and obtain larger single crystals [106].
4.1.6.2.1 Synthesis methods
New synthetic methods of Ti-MOFs have been proposed to remove the restrictions. In this section, we have introduced some strategies to synthesize these structures, including direct synthesis, cooperative self-assembly strategy, and vapor-assisted crystallization method. Besides, the synthetic strategies of Ti-MOF composites and derived porous materials are discussed [85].
4.1.6.2.1.1 Direct synthesis
Most of the Ti-MOFs have been synthesized by direct methods, such as hydrothermal/solvothermal, microwave, and ultrasonic synthesis. The most conventional method for direct synthesis of Ti-MOFs is the hydrothermal/solvothermal technique, which includes the interaction between metal–salt and an organic linker in a suitable solvent(s) and autoclave conditions. Choosing the proper solvent and concentration of reactants are two important factors that affected the crystallinity and morphology of Ti-MOFs [106]. As an example, the morphology of NH2-MIL-125 microcrystals could be changed from circular plate to octahedron by changing the concentration of the solution [108]. In 2015, Yaghi and coworkers reported the first three-dimensional (3D) extended Ti catecholate MOF (Ti-CAT-5) by solvothermal reaction of the Ti ion and hexatopic catecholate linker, 2,3,6,7,9,11-hexahydroxytriphenylene (H6THO) at 180 C for 4 h [73].
Other direct methods for synthesis of Ti-MOFs are ultrasonic- and microwave-assisted techniques that have advantages including obtaining structures with different molecular sizes and shapes in short reaction time and low temperature [104]. Kim and coworkers have synthesized NH2-MIL-125 by the microwave-assisted method in a shorter reaction time in comparison with the solvothermal technique. As an example of sonochemical synthesis, Han and coworkers have reported NH2-MIL-125 in a uniform size distribution of around 300 nm [109].
In spite of the advantages of direct synthesis of Ti-MOFs, this methodology has some limitations like the formation of unpredicted structures and byproducts. The utilization of Ti-oxo clusters can approximately solve this problem. To obtain an ideal structure that does not disturb the Ti–O bonding in the clusters, combining the MOFs and COFs known as the coordination-covalent combination strategy was introduced [104]. The first example of this strategy was reported by Yaghi and coworkers. This reported synthesis involves in situ generation of an amine-functionalized titanium oxo cluster, Ti6O6(OCH3)6(AB)6 (AB = 4-aminobenzoate), which was linked with benzene-1,4-dialdehyde using imine condensation reactions, typical of COFs. The crystal structure of MOF-901 with known structure and topology was achieved [110].
4.1.6.2.1.3 Post-synthetic cation exchange method
Another strategy to overcome the conventional methods’ limitations is the cation exchange method. This technique can avoid the high reactivity and vigorous hydrolysis of Ti precursors and lead to desired pore and topography in Ti-MOFs [111]. The example for post Ti-exchange was first reported by Cohen et al. Ti ion exchange occurred in Zr-MOF, UiO-66, became a better option for Ti-MOF [112].
4.1.6.2.1.4 Vapor-assisted crystallization method
To overcome the limitations of microporous structures in catalytic applications of Ti-MOFs by creating mesopores in MOFs structures, the vapor-assisted crystallization method (VAC) was introduced by Hicks et al. [113].
4.1.6.2.1.5 Synthesis of Ti-MOF composites
According to the importance of MOFs supported metal nanoparticle compounds in catalysis, highly dispersed nanoparticle-doped Ti-MOFs were prepared. Not only noble metal nanoparticles, such as silver (Ag), gold(Au), platinum (Pt), palladium (Pd), and non-noble metal nanoparticles, such as nickel (Ni) have been successfully embedded into the pores of Ti-MOFs [114, 115] but also quantum dots, such as graphene-like MoS2 sheets, Ag2S, CdS, and CuS have been deposited onto MIL-125 and a series of Ti MOFs based composites [116].
4.1.6.2.2 Different types of Ti-MOFs
Ti-MOFs can be categorized based on their structures (one-dimensional (1D) chain structures, two-dimensional (2D) layer structures, and three-dimensional (3D) network structures) and ligands in their secondary building units (phosphonates, carboxylates, salicylates, and catecholates).
4.1.6.2.2.1 Types of Ti-MOFs based on their structures
1D and 2D Ti-MOF structures are rarely reported. For instance, MIL-168 has 1D zigzag chains running parallel to each other (Figure 4a) [74]. To form an ordered 1D zigzag, a single bidentate ligand occupies two cis positions to create an octahedral environment around the metal ion [117].
Figure 4.
(a)Crystal structure of MIL-168, a 1D coordination chain. (b) Crystal structure of MIL-169, 2D coordination network. (c) a view of the structure of poly[bis(μ3–3,5-di-ethyl-1,2,4-triazolato-κ3N1:N2:N4)trisilver nitrate] along the [110] direction, showing the three-dimensional framework.
Generally, 2D network structures can be divided into two types: “tilt penetration” and “parallel penetration” [118]. In two-dimensional MIL-169 (Ti2(DOBDC)4O(H2O)2 dimer, DOBDC4 = 2,5-dioxido-1,4-benzenedicarboxylate), the Ti ion is chelated by two DOBDC ligands through the six-membered ring (Figure 4b) [74].
In terms of 3D MOFs, they possess different complicated structures. For example, Jiang et al. reported the synthesis and structure of a 3D chiral structure, a 3-connected SrSi2 net with (10,3) topology, which exhibited a novel doubly interwoven SrSi2 topology (Figure 4c) [119].
4.1.6.2.2.2 Types of Ti-MOFs based on ligands in secondary building units
It is noteworthy to discuss about the structures of Ti-MOFs based on different kinds of ligands due to their obvious directing effects toward the finally formed Ti-oxo SBUs.
4.1.6.2.2.2.1 Carboxylate-based Ti-MOFs
In general, Ti-MOFs are mostly constructed from carboxylate ligands. Ti cations with polydentate carboxylate acids have a tendency to form large Ti-oxo-carboxylate clusters, leading to a wide variety of Ti-MOFs’ structures. Ti-oxo-carboxylate clusters with the general formula TinOm(OR)x(OOCR/)y can be synthesized by a solution of a metal cation precursor (very often Ti(OR)4) with carboxylate ligand and/or a controlled amount of water. It is important to control different factors such as nature and the amount of the metal cation precursor, the ligand (R/COOH), the solvent, and the reaction conditions (time, pressure, presence of water or not, inert conditions or not, etc.). In 2015, Bueken et al. reported a new Ti-MOF based namely COK-69 by reaction of trans-1,4-cyclo-hexanedicarboxylate (cdc) linkers and an unprecedented [TiIV3 -(μ3-O)(O)2(COO)6] cluster. In COK-69, three neighboring Ti atoms joined by a μ3-O2− ion and six carboxylates give the formation of the Ti3O3 cluster. Subsequently, the porous framework is formed by a connection between the cluster and six cdc2− linkers [120].
4.1.6.2.2.2.2 Phosphonate-based Ti-MOFs
A series of different phosphonate-based Ti-MOFs, typically MIL-22 [81], MIL-25n (n = 2, 3) [103], and MIL-91(Ti) [68], has been introduced by Férey et al. For example, the monoclinic MIL-22 is synthesized by corner-shared trimers of Ti-oxo octahedrals and PO4 from diphosphonic linkers, forming a 2D array of microporous channels.
4.1.6.2.2.2.3 Salicylate-based Ti-MOFs
As an example of using salicylate organic linkers in the structure of MOF, in 2013 Hong et al. introduced Ti-salicylate MOF, namely ZTOF-1. Under solvothermal conditions, the reaction of zinc and titanium precursors with asymmetrical dicarboxylate ligand, 2-hydroxyterephthalic acid (H3obdc) leads to orange crystals of ZTOF-1. In this structure, Ti coordinates with the “salicylate” end of the ligand, while Zn coordinates with the carboxylate end [121].
4.1.6.2.2.2.4 Catecholate-based Ti-MOFs
Up to now only one Ti-MOF based on polycatecholate ligand known as Ti-CAT-5 was reported, in spite of the richness of Ti-catecholate clusters and the potential of these structures to apply in different fields. Ti-CAT-5 was constructed by the connection of isolated Ti-centers and the tritopic catechol derivative ligand, H6THO [73].
Metal–organic frameworks, composed of metal ions or well-defined secondary building units and organic linkers, are utilized in different fields, such as gas storage and separation, liquid separation and purification, electrochemical energy storage, catalysis, and sensing, due to their unique properties. According to the features of titanium, such as different oxidation states, high coordination number of Ti4+, and the ability to form strong bonds with oxygen and different ligands, it is used as a metal node in MOF design. Titanium-based MOFs, as a branch of metal–organic frameworks, have recently attracted the researchers’ attention because of their special properties, including high thermal and chemical stability, novel structural diversity, photocatalytic properties, low cost, and low toxicity. In this regard, the characteristics, types of Ti-MOFs, methods of synthesis, applications, and comparison of Ti-MOFs with other MOFs are discussed in this chapter. In comparison with other metal-based MOFs, for example, Zr-MOFs, Fe-MOFs, or Cu-MOFs, the titanium-based MOFs exhibit richer structural and topological diversity, high thermal and chemical stability, and high photocatalytic activity.
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Written By
Behnaz Delaram, Faezeh Mokari, Mostafa Gholizadeh and Yahya Absalan
Submitted: 09 July 2022Reviewed: 04 August 2022Published: 03 November 2022
Open access peer-reviewed chapter
