Ionic liquids are ionic substances in liquid form, or ionic substances with melting point below a certain point. All liquids that are used every day, including water and petrol, consist mostly of electronegative molecules with no ionic mobility. But ionic liquids are mostly charged ions and short-lived ion-ion combinations. Ionic liquids are a very versatile thing; they make great solvents and can also be used as deionisers. Particularly liquid salts at room temperature are very important for batteries. All soluble salts that do not decompose or evaporate are normally considered ionic liquids. Ionic liquids synthesis is an important work in the case of ionic liquids. COFs and MOFs are also synthesised by some custom ionic liquids.
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In the wake of this network chemistry, we have now discovered a class of new crystallised porous materials — covalent organic frameworks (COFs) — that are made entirely of organic scaffolds, free of metal ions. As an alternative to conventional volatile organic solvents, ionic liquids are not only green, but have wide industrial applications. Unlike most organic solvents, ionic liquids are very thermally stable and have a wide liquid range. Importantly, ionic liquids have plenty of hydrogen bond acceptors, donors and conjugated molecules.
Ionic liquids can bind hydrogen bonds with conjugated small molecules in large numbers. By solventing with ionic liquids and as model substances using -ketoenamine-linked COFs, the role of cationic and anionic properties of ionic liquids in the generation of solution-processable COFs was investigated. Using the results from the experiments, it is evident that the length of alkyl chains of imidazolium ionic liquids can be used to raise the concentration of COFs colloidal solutions. In 1-methyl-3-butylimidazolium bromide ([C4mim][Br], for instance, the concentration of the COFs colloidal solution obtained after treatment was 0.2 mg/mL, and in 1-methyl-3-octylimidazolium bromide ([C8mim][Br]) the concentration of the COFs colloidal solution obtained after treatment was 0.8 mg/mL. The chemical and crystal structure of the treated COFs is retained, and particle size is decreased to around 100 nm. They are easily dissolved in common organic solvents like ethanol, dichloromethane, acetone and water to make a stable colloidal solution and perform solution processing. And, to test for generality, COFs bound with imine and azine were heated in the highly favourable [C8mim][Br]. The COFs colloidal solution left behind was 0.9 mg/mL with the particle size down to around 100 nm and the chemical structure and crystal structure were maintained.
MOF is a very useful porous material used for catalysis, separation, electrochemistry and more. MOF materials are typically prepared through solvothermal process which is not only energy intensive but also long reaction time. How to get MOF to crystallize and get MOF prepared very fast in room temperature is also an important and daunting research question. With the use of ionic liquids in place of organic solvents, a number of Zr-based MOFs could be prepared in rapid and room temperature. For instance, in the presence of ionic liquid 1-hexyl-3-methylimidazolium chloride as solvent, Zr-based MOF can be made at room temperature in only 0.5 hours, and only a fraction of product is produced after 120 hours of reaction at room temperature in conventional solvent dimethylformamide. The synthesis of Zr-based MOF in ionic liquid was observed on the spot and dynamically with small-angle X-ray scattering equipment at the 1W2A beamline of Beijing Synchrotron Radiation Facility, and with X-ray absorption equipment at the 1W1B beamline. Combining with in situ characterization using nuclear magnetic resonance, the growth and formation law of Zr-based MOF crystals and the ionic liquid-driven MOF crystallisation was discovered. MOF made with room temperature ionic liquid can benefit from low energy usage, speed, additive-free, environmental protection and recyclable ionic liquids. Moreover, MOF crystallisation rate and particle size can also be controlled by varying the composition and shape of the ionic liquid. The Zr-containing MOF produced this way has the advantage of being tiny in size, very specific in surface area, and with lots of active sites. It is a very good catalytic compound, it catalyses the Meerwein-Ponndorf-Verley reaction well and it is universal for a wide range of substrates.
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Ionic Liquids Synthesis
