Issue 27, 2012 - Advances in the Application of Ionic Liquids

Ionic liquids are liquids composed entirely of ions, with a series of unique properties that traditional organic solvents and electrolytes do not have: a wide temperature range in the liquid state; It has the dual function of solvent and catalyst; Greater polarity controllability; High electrochemical stability; designability; Low toxicity and recyclable
.
The application of ionic liquids as reaction solvents, template agents, electrolytes, and as solvents and template agents in material synthesis has developed rapidly
.
There are many types of ionic liquids, and different combinations of cations and anions can be designed and synthesized by changing different combinations of cations and anions
.

First, the synthesis method

1.
The direct synthesis method
synthesizes ionic liquids in one step such as acid-base neutralization reaction or quaternary amination reaction, which is economical and simple to operate, has no by-products, and the product is easy to purify
.
Hlrao et al.
acid-base neutralization method synthesized a series of tetrafluoroborate ionic liquids
with different cations.
In addition, a variety of ionic liquids can also be prepared in one step by quaternary amination reaction, such as haloated 1-alkyl 3-methylimidazole salt, halopyridine salt, etc
.

2.
The two-step synthesis method
is difficult to obtain the target ionic liquid, and the two-step synthesis method
must be used.
There are many
applications for the preparation of ionic liquids by the two-step method.
The preparation of commonly used tetrafluoroborate and hexafluorophosphate ionic liquids is usually two-step
.
Firstly, halogen salts containing target cations were prepared by quaternary amination.
The target ionic liquid
is then obtained by substituting the halogen ions with the target anion or adding Lewis acid.
It is important to note that during the exchange of X-(halogen) anions with the target anion Y-, the reaction must be carried out as completely as possible to ensure that no X anion remains in the ionic liquid of interest, since the purity of the ionic liquid is critical
for its application and characterization of its physicochemical properties.
The synthesis of high-purity binary ionic liquids is usually prepared
by anion exchange using ion exchange resins in ion exchangers.

Second, the application field

Ionic liquids have been used as reaction solvents
in many types of chemical reactions.
Not only as a green solvent in separation process, electrochemistry, organic synthesis, polymerization reaction and other aspects of the application prospects, but also due to its unique physical and chemical properties, it is expected to be used as a new functional material, which is a hot field
in fine chemical research and development at home and abroad in recent years.

1.
Chemical separation of ionic liquids has the characteristics of high solubility, low vapor pressure, immiscible with many solvents, can be recycled repeatedly, does not pollute water and air, etc.
, and is very suitable as a solvent
for separation
and purification.
For example, the use of ionic liquid to extract butanol from fermentation broth to recover butanol; Especially in liquid-liquid extraction and separation, ionic liquids can dissolve some organic compounds, inorganic compounds and organometallic compounds, and are immiscible with a large number of organic solvents and can be recycled
repeatedly.
For example, 1-butyl-3 methylimidazolium hexafluorophosphate [bmim]PF6 ionic liquid is used as solvent, naphthalene is non-volatile solute, and CO2 is used as supercritical fluid to combine
the two green extraction processes.
The extraction rate of oil shale treated by [bmim]PF6 ionic liquid is 10 times higher than that of hexane.
The research team at Alabama University in the United States used a dissolved imidazole [bmim] PF6 mixture to remove H2S and CO2 in natural gas, which worked well
.
Chinese chemist Deng Youquan and others applied ionic liquids to the field of solid-solid separation for the first time, and used [bmim]PF6 as an extractant for separating tauric acid and sodium sulfate solid mixture, effectively separating tauric acid with a recovery rate of more than 97%.


2.
Electrochemical due to the high electrochemical stability of ionic liquids, high conductivity and wide electrochemical
window, and inherent ion conductivity, non-volatile, non-combustion, can reduce self-discharge, as a battery electrolyte does not need to react
at high temperature.
Ionic liquids can be used to make new high-performance batteries, such as high-voltage conductivity batteries, fuel cells, electric double-layer capacitors, sensors, etc
.
The Swiss Federal Institute of Technology uses ionic liquid as the electrolyte of solar cells, and experiments show that the thermally stable temperature of ionic liquid 1,2-dimethyl-4-fluoroimidazole boron tetrafluoride (DMFPBF4) is 300 °C, which can coexist stably with lithium in a wide temperature range, and the electrochemical window of DMFPBF4/LiBF4 is greater than 4V, and the LiMnO4/Li battery with it as the electrolyte shows a high charge-discharge cycle efficiency (96%)
.

3.
Chemical reaction ionic liquids have good solubility to dissolve the catalyst and the reaction
matrix in the same phase, which is conducive to the reaction
.
In some reactions, ionic liquids can be used as both solvents and catalysts to facilitate the reaction
.

(1) Fu-Gram reaction The relatively mature catalysts of the Fu-Gram reaction
include zeolite, solid acid, etc
.
However, due to green synthesis and cost considerations, many chemists have changed the traditional solvent to ionic liquid for related research
.
In recent years, chemists have made more efforts
in this regard.
For example, Chinese chemist Deng Youquan and others have made relevant research
on the carbonylation of alkanes.
They reported for the first time the direct carbonylation of several alkanes with CO in a super-acidic room-temperature ionic liquid composed of halogenated 1-alkyl pyridine and 1-methyl-3-alkylimidazole salts and anhydrous AlCl3 as ketones
.

(2) The hydrogenation reaction has been widely reported on the application of ionic liquids to hydrogenation reactions, and the advantages of using ionic liquids instead of ordinary solvents in the reaction are: the reaction
rate is several times faster than in ordinary solvents; The mixture of ionic liquids and catalysts used can be reused
.
Studies have shown that ionic liquids act as solvents and catalysts during the reaction
.

(3) Heck reaction Heck reaction is the reaction

of alkenes and halogenated aromatic hydrocarbons or aromatic anhydride under the action of a catalyst (such as palladium metal) to generate aromatic olefins.
The application of ionic liquids in such reactions can better overcome the problems
of catalyst loss and organic solvent volatilization in traditional reactions.
In 2000, Vincenzo et al.
of the University of London reported that after the application of ionic liquids to the Heck reaction, the reaction rate was very fast, and the yield increased to more than
90%.
A research group conducted a Heck reaction in a three-phase system [bmim]PF6/water/hexane, the catalyst used remained in the ionic liquid and could be recycled, while the product was dissolved in the organic layer, and the by-products formed by the reaction were extracted to the aqueous phase for easy separation
.

Using ionic liquid as the reaction solvent provides an environment different from the traditional molecular solvent for the chemical reaction, which may change the reaction mechanism, so that the catalyst activity and stability are better, and the conversion rate and selectivity are higher
.
The catalyst is dissolved in an ionic liquid and can be recycled with ionic
liquids.
The catalyst has the characteristics of high homogeneous catalytic efficiency and easy separation of
heterogeneous catalysis.
In addition, the types of reactions that can use ionic liquids include Diels-Alder ring addition reaction, epoxidation of olefins, organic polymerization reaction, asymmetric catalysis, etc
.

(4) The application
of oxidative extraction desulfurization using ionic liquid extraction to achieve deep desulfurization has attracted extensive attention
from chemists at home and abroad 。 Pennsylvania State University Professor Zhang et al.
discussed the application of ionic liquids for oxidative desulfurization in fuel oil, and they used ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate [emim]BF4 and [bmim]PF6 instead of traditional solvents to extract sulfur-containing compounds in fuel oil, and at the same time chemical oxidation in ionic liquids to achieve the purpose of desulfurization, which improved the desulfurization efficiency and avoided the pollution and safety caused
by the use of organic solvents.

III.
Prospects and Prospects

As a new class of chemical substances, the use of ionic liquids is constantly being developed and has broad
prospects.
Ionic liquids replace traditional organic solvents in separation and purification, chemical reactions, material preparation, etc.
, realize "green chemical industry", and can improve product yield and selectivity
.
However, the large-scale application of ionic liquids in industry still needs a lot of work
.

The design and synthesis of new functionalized ionic liquids is another important direction of ionic liquid research, such as the development of ionic liquids with catalytic functions, chiral ionic liquids (used in asymmetric catalysis), temperature-controlled ionic liquids (applied to fuel desulfurization reaction), polar or non-polar ionic liquids (applied to extraction desulfurization or coupled with oxidative desulfurization to construct a more complete oxidation-extraction integrated desulfurization system
).

Intersecting the research of ionic liquids with other disciplines and developing functional materials based on ionic liquids, such as energy storage materials, optical materials, smart materials, electrical materials, etc.
, will be a new hot spot in the next few years and will show good application prospects
.