Abstract
Traditional synthesis of metal-organic frameworks (MOFs) involves the reaction of a metal-containing precursor with an organic linker in an organic solvent at an elevated temperature, in what is termed a 'solvothermal' reaction. More recently, many examples have been reported of MOF synthesis in ionic liquids (ILs), rather than an organic solvent, in 'ionothermal' reactions. The high concentration of both cations and anions in an ionic liquid allows for the formation of new MOF structures in which the IL cation or anion or both are incorporated into the MOF. Most commonly, the IL cation is included in the open cavities of the MOF, countering the anionic charge of the MOF framework itself and acting as a template around which the MOF structure forms. Ionic liquids can also serve other structure-directing roles, for example, when an IL containing a single enantiomer of a chiral anion leads to a homochiral MOF, even though the IL anion is not itself incorporated into the MOF. A comprehensive review of ionothermal syntheses of MOFs, and the structure-directing effects of the ILs, is given.
Highlights
Metal–organic frameworks (MOFs) are crystalline framework materials that consist of metal atoms or cluster ‘nodes’ joined by multifunctional organic linking ‘struts’
If a zeolite is synthesized with a tetraalkylammonium cation in place of the alkali-metal cation, the organic cation can act as a structural template, as it is incorporated into the zeolite galleries (Lok et al, 1983), and it is possible to computationally design appropriate tetraalkylammonium cations for the synthesis of specific zeolites (Davis et al, 2016)
A large number of MOF and MOF-like framework materials have been synthesized in ionic liquids (ILs) solvents
Summary
Long before the first MOFs were synthesized, zeolites were well known and industrially significant framework materials with porosity on the molecular scale (Sels & Kustov, 2016). At the largest cation size examined, [C5mim]+, a different MOF is obtained, [C5mim]2[Co3(BDC)4], which has a three-dimensional framework It contains Co3 units very similar to the other compounds, with six BDC2À linkers directed outward to form a 2D framework. Co3(BDC)3(imidazole), which contains Co3 units with a total of six bridging carboxylate groups, as in the MOFs of Fig. 4, but with the ends of the Co3 units capped by imidazole rather than bromide ligands, resulting in a neutrally charged framework The formation of [C2C2im][NaCu(BDC)2] rather than [C2mim][NaCu(BDC)2] or an alternative [C2mim]+containing MOF shows again the strong structure-directing effect of the IL cation, as the thermodynamic drive to form [C2C2im][NaCu(BDC)2] apparently overwhelms the fact that [C2mim]+ is present in much higher concentration, especially at the beginning of the reaction
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