Emerging Trends in Ionic Liquid Formulations: A Critical Review

Only a small number of specialist research organizations were aware of the concept of "ionic liquids" (ILs) twenty years ago. Even those groups either tried ILs or thought they might be the most appropriate response solvents for a wide range of reactions, such as carbonization, esterification, depolymerization, Beckmann restructuring, acidic breakdown, polymerization, alkylation, and pre-extractions (Singh & Savoy, 2020). Chemicals termed ionic liquids have recently transformed chemical industries and research facilities. Reducing the usage of poisonous, hazardous, and environmentally detrimental substances requires certain elements, which are found in environmentally favourable compounds like solvents. Liquids with ions These are liquid-at-100 degrees organic materials made of ions. The fact that scientists are trying to find a practical substitute for volatile solvents in the industry is one of the factors driving the current surge in ionic fluid research(Ehsan Kianfar & Sajjad Mafi, 2020) While salts that melt below 373 K are connected with this description, higher melting point salts are not excluded by the phrase "ionic liquid." Actually, the terms "ionic liquid" and "molten salt," which are commonly used interchangeably to refer to high temperature liquids, are practically interchangeable.(Rooney et al., 2009)." There are two straightforward reasons why solvents rank highly among harmful chemicals: I They are widely utilised; (ii) These are typically flammable liquids that are difficult to confine; and the key to the ongoing success of IL research will be our capacity to revise definitions, reclassify, and develop new design strategies(Jordan & Gathergood, 2015).  ILs are considered superior molecular and/or environmentally friendly solvent replacement for the widely used volatile organic solvent. A small number of ILs, such as Olah's reagent, are protic ILs and are only considered ionic when described as deep eutectic solvents. Furthermore, at ambient conditions, ILs exhibit low vapor pressure, low viscosity, and non-volatility. In comparison to mineral acids and bases, they have a wide range of temperature stability, tuneable solubility, acidity, basicity, and extremely low corrosivity, among other characteristics. ILs have no adverse effects on air photochemistry and do not exhibit the explorer risk associated with volatile organic because of their incredibly low vapor pressure, solvents. Additionally, the non-volatility of the majority of ILs results in their non-flammability at ambient circumstances, as demonstrated by a number of low-boiling solvents, including acetone, dichloromethane, pet ether, and many more (Singh & Savoy, 2020)  Recently, it has been demonstrated that certain ILs can dissolve a wide range of substrates that are typically insoluble in organic solvents, including ascorbic acid, amino acids, fatty acids, triglycerides, and up to 10–20 percent (wt.) cellulose. ?-D-glucose, sucrose, lactose, and ?-cyclodextrin are among the other carbohydrates that can dissolve, as well as >100 g L-1. These natural products' ability to undergo chemical changes in homogenous systems has been made possible by the solubility of carbohydrates in ILs. These ionic liquids (ILs) have anions, usuallystrong hydrogen bonds with carbohydrates to dissolve them, such as chloride (Cl?), dicyanamide (dca?), formate (HCOO?), and acetate (OAc?). These ILs also have the potential to denature enzymes, preventing enzymatic reactions from transforming dissolved substrates. Complicated ligand clusters have the potential to enhance the catalytic activity of their reaction processes(Olivier-Bourbigou et al., 2010). Consequently, it is necessary to raise the stability as well activity of enzymatic processes in ILs, especially those ILs that inactivate enzymes. For these reasons, a number of common non-aqueous enzymology techniques for stabilising and activating enzymes have been adapted for use in ionic media, along with the development of certain novel techniques. This review's objective is to provide an overview of these techniques and offer guidance to researchers looking to advance them in order to expand the field of biocatalytic applications in ionic liquids(Zhao, 2010). Besides from being utilized as replacement solvents, ionic liquids have recently been used made progress in the development of functional ionic liquids or "task-specific ionic liquids"(TSIL). The phrase "task specific ionic liquids" or "functionalized ionic liquids" refers to an effort to turn ionic liquids from mere reaction media into actual working systems by utilising its potential "design" capacity(Ratti, 2014). Interaction of ILs with metal ions or other cocatalysts. As a result, compared to other catalysts, ILs have many advantages (like zeolites, enzymes, metallic salts, solid acid catalysts, etc.) For example, techno-economic and environmental factors are crucial components of IL catalytic system adoption and readily managed because they are applicable to an ongoing procedure with a small effluent output(Tiong et al., 2017). A good catalyst should be highly reactive and selective, with ease of separation and reusability being highly desirable from an industrial perspective. Using ILs increases the process's economic viability while lowering the quantity of reactions and purification steps(Ong et al., 2021). Since air- and moisture-stable ILs are easily handled and utilised in a wide range of conditions, we concentrate on their current analytical applications in this study. t. Nevertheless, only a small number of studies on other facets of IL research—such as synthesis/catalysis and the clarification of ILs' structures and properties—were chosen for this study(Qureshi et al., 2014). Compared to molten salts at high temperatures, at room temperature, ILs are known to be liquid salts. Their distinct range of physio-chemical characteristics renders them appropriate for a multitude of uses where traditional organic solvents prove to be insufficiently efficacious or inappropriate. Short noted that while there were few patent applications for intellectual labours (ILs) in 1980, there were 100 in 2000, and by 2004 there were over 800. This demonstrates unequivocally how highly academics and business associate themselves with the ILs(Keskin et al., 2007).  According to Wilkes and colleagues: Finding a new ionic liquid is not too difficult, but finding out its physical and chemical properties is a much more significant investment if you want to use it as a solvent. The best trick would be a technique to predict the composition of an ionic liquid with a given set of attributes. Ionic fluids obtained more focus in recent years because of their diverse range of applications. New generations and families of ionic liquids having wider specialized as well as desired attributes have been added to the range of ionic liquids now available. This growing interest has prompted several reviews of the physicochemical characteristics of ionic liquids, the creation of new groups of ionic liquids, chemical engineering, and a variety of configurations (liquid phase, multiphase, immobilized on supports, etc.) in which they have ILs. have been used, as well as pilot or industrial development. Because the early ILs, like organo-aluminate ILs, were unstable in the presence of water and air, their range of uses was restricted. Moreover, these ILs weren't inert to different chemical substances.  After preliminary research on the synthesis and application of air-stable ILs, such as 1-n-butyl-3-methylimidazolium tetrafluoroborate [BF4] and 1-n-butyl-3-methylimidazolium hexafluorophosphate [PF6], the number of air- and water-stable ILs started to rise quickly. More benefits have been discovered recently for ionic liquids (ILs) than for green solvents alone. Their numerous uses have been discovered, such as the substitution of volatile organic IL solvents, the development of new materials, effective thermal conductivity, the encouragement of enzyme-catalysed reactions, the hosting of different catalysts, the purification of gas, homogeneous and heterogeneous catalysis, biological reaction media, and the removal of metal ions. (Keskin et al., 2007)

According to Welton, ILs are not brand-new; in fact, several ILs, including [EtNH3][NO3], were originally documented in 1914. The first deliberate IL in the literature dates back to the 1970s and was developed for nuclear weapon batteries. Molten salts based on aluminium chloride were used for electroplating in the 1940s at temperatures as high as hundreds of degrees Celsius(Keskin et al., 2007).  However, as the activities of the latter time period unfold, unfavourable long-term environmental implications can and have become a significant legacy. The earlier years of the last century seem to contain entirely helpful achievements in science and technology toward humans(Park & Kazlauskas, 2003). Armstrong and colleagues synthesised 39 imidazolium- and pyrrolidinium-based di-cationic ILs in 2005 to investigate the relationship between structure and characteristics. The head groups reacted with four distinct conventional anions because they were connected by an alkyl chain that ranged in length from three to twelve carbons (Br, NTf2, BF4, and PF6)(Nusaibah Masri et al., 2016).  17 ammonium-based symmetrical di-cationic ILs by combining 3 equivalent moles of 1-alkylbromide with tetramethyl ethylenediamine, tetramethylpropylenediamine, or tetramethylhexylenediamine in a cold bath using acetone as the solvent for 24 hours (Nusaibah Masri et al., 2016)Although ILs have long been recognised, their widespread usage as solvents in catalysis and synthesis reactions has just lately gained attention(Keskin et al., 2007).  The chemists eventually found one salt that is liquid at normal temperature after searching for salts that stay liquid at lower temperatures. A small team of researchers started creating ILs and assessing their characteristics after Wilkes and his associates carried out more refinement of their ILs for use as battery electrolytes. ILs emerged as one of the most promising materials for use as solvents in the late 1990s. The early ILs' range of applications was limited because they were unstable in the presence of water and air, such as organo-aluminate ILs. Moreover, these ILs weren't sensitive to other chemical molecule. (Keskin et al., 2007)Ethyl ammonium nitrate [EtNH 3] [NO 3] (m.p. 12oC), which was originally documented in the literature in 1914, is the first IL(Singh & Savoy, 2020). Since IL research is still in its early stages, scientists are currently analysing some of the fundamental physical features of ILs, such as density and viscosity.[8]. The literature is seeing a sharp rise in the number of studies on ILs and their particular uses. As an illustration, the most extensively researched cation until 2001 was 1-n-ethyl-3-methylimidazolium. But as of right now, Imidazolium salts with 1-3 dialkyl are the most widely employed and studied group of ILs. The goal of research on ILs going forward is to commercialise them so that they can be used as materials, reagents, solvents, and catalysts in extensive chemical applications(Keskin et al., 2007). In the first of these, Paul Walden searched for liquid salts that were melted at temperatures that allowed him to utilize his equipment without needing to make any additional adjustments.(Welton, 2018). He found that the melting point of [EtNH3] [NO3] is 12 °C. Additionally, this was the first time a protic ionic liquid (PIL) was used. Hiroyuki Ohno later made a new discovery of PILs (Hirao et al. 2000), and they went on to become an important subclass of ionic liquids.(Welton, 2018). Walden was curious in the connection between the conductivity and these liquid salts' molecular size. Sadly, this discovery's potential was disregarded for a very long time, with the exception of a passing mention in a 1927 study on a number of fused salts (Sugden and Wilkins 1929).(Welton, 2018) (Hagiwara et al., n.d.). The report by Cooper and O'Sullivan is thorough and must have to be read by everyone, even though it is not surprising that the Wilkes group is most frequently credited with discovering this novel and practically significant class of solvent liquids—they were previously well-known for their study of molten salts at ambient temperatures. The possibilities that awaited In the domains of materials science and synthetic chemistry, these novel low vapour pressure solutions were quickly recognized by groups like Gratzel in Switzerland and MacFarlane in Australia. If not for the vision of Seddon and other synthetic organic chemists, it is unlikely that either group's innovations would have gotten any attention for the chloroaluminates.(Welton, 2018)If viscosity had a higher Arrhenius activation energy than conductivity performed, the fractional Walden rule likely explained the larger halide ions' quasilattice being broken by the smaller silver ion. When it came to Specifically, silver iodide was an excellent conductive even in its crystallized state condition at certain temperatures around, the moment at which melting occurs this picture was particularly clear.(Austen Angell et al., 2012)

These Walden rule aberrations are currently referred to as "superionic" behaviour within the domain of glass-forming solid electrolytes are found in ionic liquids, which are valued, particularly protonated Ionic liquids (which could act as electrolytes in fuel cells) (Cooper & Angell, 1983)(Poole et al., 1989) Later research revealed that the purported stability of these—both hydrolytic and thermal—was a little exaggerated.

Aim: Emerging Trends in Ionic Liquid Formulations: A Critical Review

Objective:

1. Tailored Properties: Customize the chemical structure and composition of ionic liquids to achieve desired physical and chemical properties, such as low volatility, high thermal stability, or specific solvation capabilities.

Thermophysical Properties: Understanding and optimizing the thermophysical properties of ionic liquids, such as viscosity, density, and thermal conductivity, to tailor them for specific applications.

Ionic Liquid Classes:

Anti-Aprotic Ionic Fluids: Most ionic liquids include organic molecule ions as their cation, which is perhaps why the number of papers in this field has increased dramatically since the mid-1990s. Resonant stabilised cations of dialklyimidazolium and alkyl pyridinium which date returning to Weir and Hurley in the midst of the 20th century, are two examples. They combined different metal chlorides and nitrates with N-substituted alkyl and aryl pyridinium halides to create low-liquids, which they then used to carry out electro-chemical extractions (though they seem to be best known for their efforts on aluminium deposition on which they took the first patents). The cyclic and non-cyclic tetraalkylammonium salts, such as those containing cations of alkylpyrrolidinium and especially those with ether oxygenated sidechains, have been developed much more recently(Cooper & Angell, 1983). Anions with an oxidic character, such as nitrate perchlorate, or a more common fluorinated-oxidic character, such as triflate, typically compensate for the charge of such cations Triflate (trifluoromethane sulfonate, CF3SO3) and bis-trifluoromethanesulfonyl-imide (CF3SO2)2N, or NTf2) ions have been identified in PF6, BF4, and NTf3 are some of the most prevalent of the latter. Due to van der Waals interactions ' reduction in viscosity (caused by the fluorine electrons' tightly bound, therefore unpolarizable, nature), the fluorinated anions are more noticeable. Henderson mentioned that the NTf2 anion is particularly successful because it has two isoenergetic forms, which allows it to provide an additional (and hence liquid-stabilizing) mode to the configurational entropy. This anion is rather massive.,.(Austen Angell et al., 2012)

1. Protic Ionic Liquids: A proton is simply transferred from pure Brønsted acid to pure Bronsted base to generate these. As previously mentioned, the Ohno laboratory in Japan was the driving force behind the invention of the first artificial intelligence (IL) in the contemporary age. This procedure creates a proton potential in the liquid result, which gives this class of ionic liquids a unique tunability due to its nature and reversibility. The NTf2 anion that was discussed in the preceding section works just as effectively to reduce these liquids' cohesiveness and, consequently, their fluidity. Protic ionic liquids, however, have the potential to become more conductive than aprotic ones through the manipulation of cohesion through adjustments to the proton transfer energy. In fact, some of the most conductive liquids ever discovered have been produced by utilising these properties. (Austen Angell et al., 2012)
2. Inorganic Ionic Liquids: These can be created by exploiting the same packing issues that result in low-melting ILs of the organic cation type, in both aprotic and protic forms. Aprotic examples include lithium chlorate (melting point 115 C) and its glass-forming eutectic with lithium perchlorate; protic examples include hydrazinium nitrate, Tm ¼ 80 C; low-melting mixtures of ammonium salts; and lastly, the largely unexplored cases of salts with inorganic molecular cations, like PBr3Cl+, SCl3 +, ClSO2NH3 +, etc., with the appropriate weak base anions. These examples may be few in number, but they do exist. (Austen Angell et al., 2012)
3. Solvate (Chilate) Ionic Liquids: Recognizing this as a mostly unexplored class of ILs is essential since the class contains cases of multivalent cation salts that would not normally be able to satisfy the condition of Tm < 100>

ADVANTAGES:

The following are the primary benefits of ionic liquids:

• They have extreme polarity.
• They have a low vapour pressure and are non-volatile.
• They are typically stable and heat resistant to 300 degrees Celsius.
• They are liquid at temperatures as high as 200 °C.(Keskin et al., 2007)
• These substances have extremely high electrical conductivity.
• Numerous typical organic solvents are incompatible with these substances.
• ILs are polar nonaqueous substitutes for phase transfer procedures.(Keskin et al., 2007)
• Numerous different organic, inorganic, and organometallic compounds can be dissolved by ILs.(Keskin et al., 2007)
• The ability to modify the cation or anion to affect qualities like viscosity, melting temperature, water miscibility, and density is one benefit of RTILs' chemical structures.(Park & Kazlauskas, 2003)
• Ionic liquids with synergistic benefits including enhanced yields, safer delivery of reactive species, or more selectivity. (Chowdhury et al., 2007)
• For certain base-catalysed procedures, the combination of an ionic liquid and an inorganic base has lower catalytic efficiency and is less convenient for recycling.(Ratti, 2014)
• Although this has a lot of benefits, it also prevents the employment of catalysts or reagents that are sensitive to water, which is frequently more significant. Furthermore, it might be extremely challenging to eliminate trace levels of organic compounds from water.(Welton, 1999)

DISADVANTAGES:

Nevertheless, there are certain drawbacks to PEG modification as well. For instance, there may be a lot of work involved in the preparation, and various immobilisation batches may have varied enzyme catalytic characteristics.(Park & Kazlauskas, 2003)

FIG: Different techniques for the recovery and purification of ILs, as well as their attributes, benefits, and drawbacks(Singh & Savoy, 2020)

Variations In Ionic Liquid Generations:

1. Initial generation: These Ionic liquids are compounds that are commonly used as solvents. By changing the cations or anions in these compounds, one can improve their unique physical properties. Ionic liquids are presented as the first generation in figure (Ehsan Kianfar & Sajjad Mafi, 2020)

Applications:

• Applications for ILs are being investigated in the fields of analytical chemistry, electrochemistry, separation chemistry, cellulose processing, polymers, energy, and organic synthesis and catalysis.(Qureshi et al., 2014)
• Exceptional task specificity with a broad range of uses in extraction and separation, solvent and catalysts, electrochemistry, and lowest environmental release
• Chemistry in engineering, analytical, physical, synthetic, biological, and more(Singh & Savoy, 2020)
• Among the applications of ILs are as liquid crystals, solvents for organic and organometallic synthesis and catalysis, electrolytes in electrochemistry, fuel and solar cells, lubricants, stationary phase for chromatography matrices for mass spectroscopy, supports for enzyme immobilizations, separation technologies, templates for synthesis nanomaterials and materials for tissue preservation, preparation of polymer-gel catalytic membranes and production of high conductivity materials.(Keskin et al., 2007)
• Anions and cations of ILs can independently modify the physicochemical characteristics of the IL (melting point, viscosity, density, conductivity, refractive index, and so forth) and present characteristics for a specific application (regulating the hydrophobicity and solute solubility as opposed to chirality, hydrophilicity, and the addition of functional groups for catalysis. objectives of responsiveness, etc.)(Ong et al., 2021)
• Key places where ionic liquid is used:

Replacement of solvent Gas purification Both heterogeneous and homogeneous catalysts Media for biological reactions

• These solvents are economically feasible for use in several applications, including polymer impregnation and dry cleaning.(Keskin et al., 2007)

Ionic liquids' basic characteristics include their environmental effects and their green elements.(Singh & Savoy, 2020) There are two main methods for making ionic liquids: the first is metathesis of a halide salt with an anion of choice (e.g., silver, group 1 metal, ammonium salt); the other technique is neutralization processes between acids and bases.(Welton, 1999) The first room temperature ionic liquids [EtNH3] [NO3] (m.p12 degree c) was discovered in 1914. But it wasn’t until the discovery of binary ionic liquids – comprising of mixtures of N-alkyl pyridinium or 1,3-dialkylimidazolium chloride and aluminium (III)chloride-that interest began to rise.(Ratti, 2014) These ionic liquids are always mixtures of organic salts or at least one organic component. N-alkyl pyridinium, N, N?-dialkylimidazolium, alkylammonium, and alkyl phosphonium cations are the most commonly utilized salts. (Welton, 1999) Maintaining the chirality of the original biological applications, over 100 ILs were synthesised directly from either natural AAs or AA ester salts. (Jadhav et al., 2021) Remarkably, the first polysaccharide-based RTIL was produced in 2007 by synthesizing a polysaccharide. The RTILs were made by combining 1-ethyl-3- 5 methylimidazolium hydroxide with carboxymethylated chitosan.(Jadhav et al., 2021) The one-step synthesis of 1-methylimidazolium halides produced a stochiometric amount of waste but also maximum atom economy or efficiency. (Singh & Savoy, 2020) In comparison to the conventional conductive heating process, the microwave irradiation approach uses less energy, accelerates reaction rates, increases the desired product's selectivity, and requires less reagent quantities. In some circumstances, the synthesis of ILs without the use of solvents is carried out, resulting in less hazardous waste that is simpler to dispose of. (Singh & Savoy, 2020)Halides and organic substrates containing water are the most significant ionic liquid pollutants. Usually, these substrates come from unreacted materials, which are the starting points of ionic liquid synthesis that don't alter throughout. The primary attributes of ionic liquids are their green components and their influence on the environment.

• Procedures for making ionic liquids:

1. Alkylation
2. Solvent-free synthesis
3. Creation of chiral
4. Ionic liquid synthesis with a unique performance
5. Ion exchange synthesis (Ehsan Kianfar & Sajjad Mafi, 2020)

1. Alkylation:

A nucleophile, or suitable precursor, and an alkylating agent such as d-alkyl sulk or haloalkane can be used to form an alkyl cation in a variety of ionic liquids based on ammonium, imidazole, pyridinium and phosphonium. For example, imidazole's alkalization easily yields alkyl imidazole, the main ingredient in ionic liquids based on imidazole. The same is true for halogens of alkanes, which are employed to create alkyl salts of pyrimidine. Halides Phosphorus tetrachloride asymmetric compounds, for example, are often created by raising the halo alkanes and the third type of phosphine's nucleus friendship

2. Solvent- Free Synthesis:

The production of non-alcoholic ionic liquids involves two fundamental steps:

The production of unwanted halide salts following the use of a haloalkane as an alkylating agent during the double anion exchange process. Keep in mind that haloalkanes can be difficult o work with, especially if they have high boiling temperatures that cause them to separate from the reaction product at the end. Furthermore, halide salts- which are produced by double substitution- have a major influence on the physicochemical properties of ionic liquids. Impurities in halide salts or the deactivation of catalysts using an intermediary to allow the ionic liquid, the reaction's solvent, to accelerate metals can both cause poisoning. A wide variety of ionic liquids can be used to make ionic liquids with specific uses or enhanced physical- chemicals properties  (adjusting physiochemical parameters). Ionic liquids consist of various anions and cations. Anions like BF4-, BF6, Br-, Cl-, and so on are frequently seen.(Ehsan Kianfar & Sajjad Mafi, 2020)

3. Creation of chiral:

For asymmetric synthesis, the use of crystal ionic fluids as a catalyst and solvent is essential material. These molecules come from a variety of chiral sources, including naturally occurring chiral compounds obtained from plants and animals, including amino acids, sugars and terpenes, as well as pre-crystal precursors such as synthetic enantiomers produced industrially. These ionic liquids are often created by an anionic exchange following the alkalization of a pre-crystal precursor.(Ehsan Kianfar & Sajjad Mafi, 2020)

4. Ionic liquid synthesis with a unique performance:

Ionic liquids can be used with large-scale cations or anions. Functional ionic liquids or ionic liquids with special properties like medications, lubricants, and rocket propulsion, are useful solvents in chemical synthesis, catalysts and material applications. Consequently, there is a growing trend in the analysis of the manufacturing process for these materials. There are numerous reported methods for producing ionic liquids with special qualities. Ionic fluids are composed of typically active halo alkanes, and the causative agents in them are imidazolium, phosphonium and pyridinium. One of the initial reports of the emergence of ionic immunosuppressive beverage intended to remove metal ions from water-based solutions. Because anionic exchange occurs between commercial acids and alkaline salts, it is possible to make various ionic liquids using an ionised agent. Other methods exist for sterilising anion, like interested fiction. This approach has been suggested for the creation of ionic liquids containing an anion functionalized from alkyl sulphates.(Ehsan Kianfar & Sajjad Mafi, 2020)

5. Ion exchange synthesis:

Imidazole diacetyl cations are utilized in two processes to prepare hexafluorophosphate and tetrafluoroborate ionic liquids, two common ionic liquids used in conventional research. The halide salt is initially produced by reacting the necessary cation with the alkyl. The necessary anion is then introduced to the anion halide twice. In this procedure, an anion of a mineral—such as a metal salt group or silver salts- replaces an anion of an elemental salt.(Ehsan Kianfar & Sajjad Mafi, 2020)

Table: To create ILs linked to 1, 3 di-alkyl imidazolium cations, anionic metathesis is used