**Abstract**

Ionothermal synthesis employs ionic liquids for synthesis of metal organic frameworks (MOFs) as solvent and template. The cations and anions of ionic liquids may be finely adjusted to produce a great variety of reaction environments and thus frameworks. Organisation of the structures synthesised from related ionic liquid combinations give rise to provocative chemical trends that may be used to predict future outcomes. Further analysis of their structures is possible by reducing the complex framework to its underlying topology, which by itself brings more precision to prediction. Through reduction, many seemingly different, but related classes of structures may be merged into larger groups and provide better understanding of the nanoscopic structures and synthesis conditions that gave rise to them. Ionothermal synthesis has promised to enable us to effectively plan the synthesis ahead for a given purpose. However, for its promise to be kept, several difficult limitations must be overcome, including the inseparable cations from the solvent that reside in the framework pore.

**Keywords:** ionothermal synthesis, metal organic frameworks, imidazolium-based ionic liquids, chemical trend analysis, structure simplification

### **1. Introduction**

Three things are necessary to consider in the preparation of metal organic frameworks (MOFs): the metal, organic ligand, and solvent. Often neglected is the influence exerted by the solvent on the eventual framework, unlike the metal and the ligand that the structure always constitutes of. Varying the key characteristics of the solvent, such as hydrophilicity, is often the deciding factor in the reaction yield and the nature of the final compound [1]. Until 2002, when Jin et al. first used ionic liquids to synthesise metal organic framework [2], the list of solvents in inorganic synthesis was limited to few organic solvents and water [3]. This new synthesis method received growing attention in the field of MOFs to open a new realm of novel structures and provocative findings regarding the very nature of nanoscale synthesis. One aspect of ionothermal synthesis that contributed to its attention must have been its simplicity; the overall process comprises no more than mixing the metal salt and the organic ligand with the ionic solvent and incubating at a high temperature for long enough time. Unfortunately, however, the growth seems to have ceded in the recent years as shown in **Figure 1**. Given its distinctive potentials, this chapter is dedicated to introduce the field and draw more efforts for the full realisation of what the methodology dare to have promised. Before we move to the discussion of ionothermal synthesis and its potentials, the chemistry of ionic liquids

**Figure 1.** *Number of papers published annually under the topic of the ionothermal synthesis of MOFs since its first report.*

must be first visited since it is this distinctive nature that lies behind all positive aspects, and limitations too, of ionothermal synthesis.

Ionic liquids are simply salts in the liquid state as opposed to the liquids typically used as solvents, [4] which are predominantly comprised of electrically neutral molecules. While most salts may be brought to their liquid states by heating, the term 'ionic liquids' is exclusively used for those that stay fluid around or below 100°C to distinguish them from the older phrase 'molten salts' [5]. One reason behind the attention that ionothermal methodology receives may be directly induced from the term 'ionic liquid' itself. The liquids are held by ionic interactions that far outcompetes the most intermolecular interactions in other solvents, including the renowned hydrogen bonds in water. Such strong interaction is responsible for their low vapour pressure [6], which could resolve the safety and environmental concerns associated with conventional organic solvents [5]. Such characteristics function as the exact same advantages in synthesis of MOFs. Nevertheless, the synthesis has greater potential in which the reaction environment can be finely tuned by modifying the solvent ions [7]. There are only several hundreds of molecular solvents, whereas a million binary combinations and a million of millions of ternary combinations possible for ionic liquids [5], hence their nickname 'designer solvent [8]'. Efforts in the field need to be focused not only on collecting outcomes from as many combinations as possible, but also, more importantly, on comprehending the laws of chemistry lying behind the trend observable in those data. This shall, as more than enough possible combinations await, ultimately enable designing the product for a given purpose, rather than vice versa.

This chapter will focus on showing the potentials of ionothermal synthesis by presenting a set of related syntheses in an organised manner. A series of such ionic liquids (RMI-X) may be prepared with 1-alkyl-3-methylimidazolium (RMI) and halide ions(X) [9]. This series of solvents exhibit finest tunability, in addition to their stability, with its variable length of alkyl side chain of the cation and the anion species variable along the halide column, which places them among the most extensively studied solvents for ionothermal synthesis of MOFs. To confirm their dominance in structure reports and the focus of our discussion on them, investigations have been made about the number of MOFs synthesised with several popular ionic liquids. The scope of our search—the list of cations and anions comprising the most common ionic liquids—has been illustrated in **Figure 2**. According to Cambridge Structural Database (CSD), it was shown that much of the reported MOFs is synthesised from

**85**

**in the product**

**Figure 2.**

*Ionothermal Synthesis of Metal-Organic Framework DOI: http://dx.doi.org/10.5772/intechopen.79156*

ionic liquids that contains imidazolium and halide ions. There are no MOF crystal containing pyridium cation, and only few crystals synthesised from tetramethyl ammonium is reported as MOF. Synthesis using pyrrolidinium cation shows about 100 crystals, which corresponds to co-crystal form, showing that no crystal exhibits MOFs including pyrrolidinium cation. Extensiveness of data is the foundation of all successful discussions. With the extensiveness of RMI-X now taken for granted, structures synthesised from conditions with piecemeal differences, namely the length of the side chain of the cation, halide ions, and core metal atom of the structure were analysed to explore the effect on the final product arising from such variations.

*Common cations and anions that composes an ionic liquid. Different combination of cations and anions results in various characteristics and gives rise to the diversity of topology and structure in synthesised MOFs. (a) Imidazolium (R1 = methyl, ethyl, butyl; R2 = hydrogen, methyl; R3 = methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, benzyl, allyl, vinyl); (b) Ammonium; (c) Pyridinium (R1 = hydrogen, methyl); (d) Pyrrolidinium (R1 = ethyl, propyl, butyl, hexyl); (e) Bis((trifluoromethyl)sulfonyl)amide; (f) Alkyl sulphate (R1 = methyl, ethyl); (g) Trifluoromethanesulfonate; (h) Hexafluorostibate(V); (i) Hexafluorophosphate(V); (j) Thiocyanate; (k) Acetate; (l) Bis(cyanide)amide; (m) Halide ion; (n) Tetrafluoroborate; (o) L-lactate.*

**2. Gradual difference in the solvent brings about gradual difference** 

An important characteristic of ionothermal synthesis is that the characteristics of the solvents may be gradually varied and investigate the difference induced in the final product. While the solvent can be substituted with a complete different class of cations or anions to provide a completely reshaped environment, more minor changes can be made to the ions so that the change is gradual and quantifiable. Changing the length of the alkyl side chains attached to imidazolium cations, or changing the anions within the halide column to gradually change the size of the solvent ions is one example that will be mainly discussed in the chapter. This way, we may grasp a better understanding of the relation between the beginning and the end of this nanoscopic synthesis. Actually, organic solvents hold the exact same advantage, seeing that even the size variation of imidazolium cations is actually an organic one. However, in ionic liquids, this variation is expanded to a twodimensional table for binary combinations, and possibly to even four-dimensional

*Ionothermal Synthesis of Metal-Organic Framework DOI: http://dx.doi.org/10.5772/intechopen.79156*

#### **Figure 2.**

*Recent Advancements in the Metallurgical Engineering and Electrodeposition*

must be first visited since it is this distinctive nature that lies behind all positive

*Number of papers published annually under the topic of the ionothermal synthesis of MOFs since its first* 

used as solvents, [4] which are predominantly comprised of electrically neutral molecules. While most salts may be brought to their liquid states by heating, the term 'ionic liquids' is exclusively used for those that stay fluid around or below 100°C to distinguish them from the older phrase 'molten salts' [5]. One reason behind the attention that ionothermal methodology receives may be directly induced from the term 'ionic liquid' itself. The liquids are held by ionic interactions that far outcompetes the most intermolecular interactions in other solvents, including the renowned hydrogen bonds in water. Such strong interaction is responsible for their low vapour pressure [6], which could resolve the safety and environmental concerns associated with conventional organic solvents [5]. Such characteristics function as the exact same advantages in synthesis of MOFs. Nevertheless, the synthesis has greater potential in which the reaction environment can be finely tuned by modifying the solvent ions [7]. There are only several hundreds of molecular solvents, whereas a million binary combinations and a million of millions of ternary combinations possible for ionic liquids [5], hence their nickname 'designer solvent [8]'. Efforts in the field need to be focused not only on collecting outcomes from as many combinations as possible, but also, more importantly, on comprehending the laws of chemistry lying behind the trend observable in those data. This shall, as more than enough possible combinations await, ultimately

enable designing the product for a given purpose, rather than vice versa.

This chapter will focus on showing the potentials of ionothermal synthesis by presenting a set of related syntheses in an organised manner. A series of such ionic liquids (RMI-X) may be prepared with 1-alkyl-3-methylimidazolium (RMI) and halide ions(X) [9]. This series of solvents exhibit finest tunability, in addition to their stability, with its variable length of alkyl side chain of the cation and the anion species variable along the halide column, which places them among the most extensively studied solvents for ionothermal synthesis of MOFs. To confirm their dominance in structure reports and the focus of our discussion on them, investigations have been made about the number of MOFs synthesised with several popular ionic liquids. The scope of our search—the list of cations and anions comprising the most common ionic liquids—has been illustrated in **Figure 2**. According to Cambridge Structural Database (CSD), it was shown that much of the reported MOFs is synthesised from

Ionic liquids are simply salts in the liquid state as opposed to the liquids typically

aspects, and limitations too, of ionothermal synthesis.

**84**

**Figure 1.**

*report.*

*Common cations and anions that composes an ionic liquid. Different combination of cations and anions results in various characteristics and gives rise to the diversity of topology and structure in synthesised MOFs. (a) Imidazolium (R1 = methyl, ethyl, butyl; R2 = hydrogen, methyl; R3 = methyl, ethyl, propyl, butyl, pentyl, hexyl, octyl, benzyl, allyl, vinyl); (b) Ammonium; (c) Pyridinium (R1 = hydrogen, methyl); (d) Pyrrolidinium (R1 = ethyl, propyl, butyl, hexyl); (e) Bis((trifluoromethyl)sulfonyl)amide; (f) Alkyl sulphate (R1 = methyl, ethyl); (g) Trifluoromethanesulfonate; (h) Hexafluorostibate(V); (i) Hexafluorophosphate(V); (j) Thiocyanate; (k) Acetate; (l) Bis(cyanide)amide; (m) Halide ion; (n) Tetrafluoroborate; (o) L-lactate.*

ionic liquids that contains imidazolium and halide ions. There are no MOF crystal containing pyridium cation, and only few crystals synthesised from tetramethyl ammonium is reported as MOF. Synthesis using pyrrolidinium cation shows about 100 crystals, which corresponds to co-crystal form, showing that no crystal exhibits MOFs including pyrrolidinium cation. Extensiveness of data is the foundation of all successful discussions. With the extensiveness of RMI-X now taken for granted, structures synthesised from conditions with piecemeal differences, namely the length of the side chain of the cation, halide ions, and core metal atom of the structure were analysed to explore the effect on the final product arising from such variations.
