**5. Outstanding properties of ionothermally prepared MOFs**

In previous sections, we have explored through the diverse structures prepared by ionothermal synthesis and several perspectives through which the groups of structures may be analysed to gain deeper insights. The last step is to find a practical use for those insights. The versatility of ionothermal synthesis, that its reaction environment may be easily altered and related to the change in products, directly

leads us towards the diversity of structures that may be prepared through the methodology. As such, ionothermal synthesis promises a variety of potential uses, although most of them have obstacles yet to be resolved along the way towards practical employment.

## **5.1 Ion exchange is the key to make use of the pores**

Many of the most popularly studied application of MOFs make use of the frameworks as molecular sieves. The nanoscale pores of MOFs can selectively filter out any chemicals that do not fit into them and this selectivity can be chosen by the industry among the diversity of reported structures. The first use of ionothermally prepared MOFs is probably also the same. In this case, ionothermal synthesis has one advantage that the solvent functions as a template and can be varied in size to modify the pore size. However, it is a double-edged sword that actually limits the practicality of ionothermal synthesis. To make any use of the pores, the templates occupying the pores must be removed. The problem is that they hardly ever do.

The void volume of the structures synthesised with the cation varied in size has been compared in **Tables 4**, **5** and **7**. Frameworks with the solvent resident in their channels, or cavities, tend to have compact structure with the void volume as low as 0% of the unit cell volume. For your reference, MOF-5, a representative framework, has a void cavity as large as 70% of unit cell volume. This absence of void volume arose because of the large solvent cations stuck in the cavity, rather than the framework itself. When calculated with the resident cations completely removed, void volume was increased to approximately 50% of unit cell volume. In theory, the large volume occupied by the cations may be decreased by subjecting the framework to ion exchange with smaller cations, so that the rest may be used purposefully. Unfortunately for now, this possibility seems to stay only in theory. Given its important position—the first step in bringing ionothermal synthesis to practicality, tremendous efforts have been put into making this exchange possible, but they rarely succeeded. In one case that we tested, evacuation of cations was observed in [BMI]2[Co2(BTC)2(H2O)2] crystals upon treatment with water, but only when accompanied with significant destruction of the framework [31]. Nevertheless, Li et al. reported partial but stable ion exchange with [EMI]2[In2Co(OH)2(BTC)2Br2] crystals [32], suggesting a new possibility for the ionothermal synthesis methodology.

#### **5.2 Placing metal atoms in proximity to yield novel characteristics**

The limitations posed by the irreplaceable templates have indeed disappointed the researchers and presumably many of you, too. However, even if the pores of ionothermally prepared MOFs are totally unusable, they still have some valuable characteristics. It is very common in the world of nanoscience that a substance acquires some characteristics completely different from those of its macroscopic bulk. One of the most frequently reported application is detection of chemicals via photoluminescence that changes upon encounter with specific chemicals. This includes the photoluminescence of europium ions in [HMI][Eu(DHBDC)2, where DHBDC indicates 2,5-dihydroxytelephtalic acid, capable of detect Ba2+ ions quantitatively [25], and [RMI][Eu2(BDC)3Cl] for detection of aniline [18]. In addition, ionothermally prepared [EMI][Dy3(BDC)5], a rod-shaped polymer, has been shown to exhibit slow magnetic relaxation behaviour like single-molecule magnets [22]. It seems like ionothermally prepared structures may be applied for any purposes that exist in the field of nanochemistry.

**101**

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

uses from the framework itself, rather than the voids.

RMI 1-alkyl-3-methylimidazolium EMI 1-ethyl-3-methylimidazolium PMI 1-propyl-3-methylimidazolium BMI 1-butyl-3-methylimidazolium PEMI 1-pentyl-3-methylimidazolium HMI 1-hexyl-3-methylimidazolium

BTC 1,3,5-benzenetricarboxylic acid BDC 1,4-benzene dicarboxylic acid NDC 1,4-naphthalene dicarboxylic acid

DHBDC 2,5-dihydroxytelephtalic acid

CCDC Cambridge crystallographic data centre

MOF metal organic framework CSD Cambridge structural database PXRD powder X-ray diffraction

4,4′-bpy 4,4′-bipyridine

OAc acetate

In a system of different chemical outcomes from related starting conditions, it is often difficult to track what has caused that difference. The ionothermal synthesis methodology is excellent in this aspect. Changes can be made in a gradual and continuous manner to observe how the result reacts to them. By organising the results based on their solvents, we can connect the dots to find useful correlations that can be both used for intra- and extrapolation. The relations are often very intuitive. For this simplicity to not lead to inaccuracy, there is a need to carefully examine the frameworks and how they react with the reaction environment—the ionic liquid that functions both as solvent and template. In the course of simplifying the frameworks for examination follows merging of structures into large topology groups, which can be used to better organise once separated correlations from various chemical systems. Despite of all the positive characters of ionothermal synthesis, however, there is a limit to their practical application—the irreplaceable reaction templates. For this rich and potentially richer field of material synthesis to contribute to the industry, efforts must be made to either resolve the issue or to find

**6. Conclusion**

**List of abbreviations**

**Cations**

**Others**

**Organic ligands**
