 |
| |
 |
Frequently Asked Questions
|
|
What are Ionic Liquids?
Ionic liquids are purely ionic, salt-like materials, which are per definition liquid below 100°C. Commonly, they have melting points below room temperature, with some even below 0°C.
Comparing a typical Ionic Liquid such as 1-ethyl-3-methyl-imidazolium ethylsulfate (m.p. < - 20°C) with a typical inorganic salt, e.g. table salt (NaCl, m.p. 801°C), one can easily see that the Ionic Liquid has a significantly lower symmetry. Consequently, solidification of the Ionic Liquid will take place at lower temperatures. The strong ionic interaction within Ionic Liquids results in a negligible vapor pressure, unless decomposition occurs. It makes the material non-flammable and highly stable thermally, mechanically and electrochemically. Furthermore, it imparts very appealing solvent properties and immiscibility with water or organic solvents that results in biphasic systems.
The cation has a strong impact on the Ionic Liquid’s properties and will often define its stability. Furthermore, choice of the anion controls the chemistry and functionality of the Ionic Liquid in general. Although possible combinations of cations and anions can theoretically lead to as many as 1018 Ionic Liquids, a realistic number is magnitudes lower. About 1,000 Ionic Liquids are described in the literature today, and approximately 300 are commercially available. The following illustration shows typical structures that combine organic cations with inorganic or organic anions:
 |
 |
 |
cation
(organic) |
imidazolium-
|
pyridinium-
|
pyrrolidinium-
|
 |
 |
 |
phosphonium-
|
ammonium-
|
sulfonium-
|
 |
 |
 |
alkylsulfat-
|
tosylate-
|
methanesulfonate-
|
anion
(organic) |
 |
 |
anion
(inorganic) |
How pure are Ionic Liquids?
What exactly is purity? Most people would define purity by the actual content of the desired compound expressed in weight percent. This already is not an easy thing to do with Ionic Liquids. As salts they intrinsically consist of two compounds, an cation and an anion. For example a sample of EMIM CI might contain 5 w% of EMIM HSO4 as impurity. This means the Ionic Liquid is 100 w% pure in terms of the cation EMIM and 95 w% pure in terms of the anion chloride.
Weight % is only one of many possible definitions of purity. A catalysis chemist would define purity as being free of any coordination species, like halides which deactivate the metal by formation of stable complex compounds. An electrochemist would define purity by having no oxidisable impurities which narrow down the electrochemical window. An engineer might prefer not to have impurities that affect the viscosity and finally the end user will define purity as being free of residual potentially toxic alkylating agents. These examples show, that only the targeted application defines what purity in this case means. In the end Ionic Liquid manufactures do not sell purity, but performance.
Does "technical grade" mean "impure"?
BASF for example provides large scale quantities of Ionic Liquids at a "technical grade". These materials have a certain quality derived from the actual manufacturing process. As an example the parameters for BASIONIC™ LQ01 (EMIM EtOSO3) from a typical ton scale production are as follows:
|
Actual content
|
>99 w%
|
|
Acutal color
|
yellow
|
|
Actual content of 1-Methylimidazole
|
0,2 w%
|
|
Actual water content
|
400 ppm
|
|
Actual content of alkylating agent
|
8 ppm
|
|
Actual chloride content
|
2 ppm
|
|
This shows that the initial quality is already very high and “technical grade” does by far not mean “impure”. However, as mentioned above, the actual quality has to meet specific needs of the customer. It can turn out that actual content and color for a given application is not relevant, but that the water content must be below 50 ppm. In this case the quality requirements are less challenging regarding actual content, but very demanding regarding water. It not only makes no sense it is even impossible to provide a purity from the beginning that will meet all imaginable future quality issues.
|
|
Is the color of Ionic Liquids a problem?
Colored materials are quite often perceived as being impure. In fact, most Ionic Liquids are colorless liquids. However, they tend to become colored especially during prolonged thermal treatment. The good news is that the color persistently stays in the Ionic Liquid and cannot be extracted in any organic product or solvent. Currently no one has managed to isolate the colorant because the quantities are just too low. It is assumed that oligomers of the imidazole or even radical ions might cause the color. For commercial applications the color is usually not a problem, since products are not affected. BASF has been running a pilot plant for an extractive distillation process for 3 months continuously. The color of the Ionic Liquid changed to black, but the performance stayed constant without the need for any purge. During the entire run the product was colorless clear and fully inline with the required specification. For this application it would have been a waste of money to start with a colorless high price Ionic Liquid as the colorless Ionic Liquid would also have turned black under the required high temperatures.
|
|
How stable are Ionic Liquids?
Generally Ionic Liquids show remarkably high thermal stabilities of > 200°C. A decomposition pathway of the usually very stable imidazolium-based Ionic Liquid is the back alkylation of the anion.
The back reaction depends on the nucleophilicity of the anion at which temperature this reaction occurs. It has been determined that onset measurements from DSC alone are not suitable to determine the thermal stability. A more valuable indication is provided by TGA analysis which shows the loss of weight due to the distillation of the volatile alkylating agents. For EMIM Acetate the temperature at which 10% loss of weight is observed is for example 215°C. EMIM Methanesulfonate or EMIM Ethylsulfate show higher thermal stabilities with corresponding decomposition temperatures of 330°C.
Under basic conditions imidazolium-based Ionic Liquids tend to form carbenes, which can undergo further decomposition, like irreversible disproportionation.
|
|
Are Ionic Liquids toxic?
There is no general answer to this question. Since Ionic Liquids can consist of many chemically different types of cations and anions this question has to be answered case by case. The "magic Ionic Liquid" which meets all requirements in toxicity, ecotoxicity, stability and performance simply does not exist. Up to now there is still very limited information available regarding a full toxicological profile of Ionic Liquids. Only recently some valuable data has been published [14]. This data was collected for the notification process of the corresponding Ionic Liquids and represents examples from the most common classes of cations: EMIM, BMIM and ammonium.
|
|
BMIM CI1)
|
EMIM EtOSO32)
|
MTEOA MeOSO33)
|
|
Acute oral toxicity
|
toxic
|
not harmful
|
not harmful
|
|
Skin irritation
|
irritant
|
non-irritant
|
non-irritant
|
|
Eye irritation
|
irritant
|
non-irritant
|
non-irritant
|
|
Sensitization
|
non-sensitizing
|
non-sensitizing
|
non-sensitizing
|
|
Mutagenicity
|
non-mutagenic
|
non-mutagenic
|
non-mutagenic
|
|
Biological degradability
|
not readily degradable
|
not readily degradable
|
readily biodegradable
|
|
Toxicity to daphniae
|
acute toxic
|
acutely not harmful
|
acutely not harmful
|
|
Toxicity to fish
|
acutely not harmful
|
-
|
acutely not harmful
|
|
1) BMIM Cl = 1-Butyl-3-methylimidazolium chloride
2) EMIM EtOSO3 = 1-Ethyl-3-methylimidazolium ethylsulfate
3) MTEOA MeOSO3 = Tris-(2-hydroxyethyl)-methylammonium methylsulfate
These examples show that Ionic Liquids can be toxic as well as completely harmless and biodegradable. However, the harmless Ionic Liquids may not necessarily provide the best performance. In the end a balanced decision between toxicological and performance properties has to be made to gain the best fit to the targeted application. Some Ionic Liquids may only be suitable for being handled by skilled personnel in chemical plants, others for a use closer to the end user will have to be non-toxic and readily biodegradable.
|
|
Are Ionic Liquids green?
Besides their technical and economical potential Ionic Liquids are often claimed to be green solvents or green materials. The "greeniness" has often been justified with an important property being that Ionic Liquids have no vapour pressure, hence being not released to the environment by evaporation. Certainly, there might be applications where exactly this property can help to make a particular process greener than a corresponding process with organic solvents. However, this is not necessarily and not generally the case. It might be misleading to assume that non-volatility alone makes a material "green". A more precise approach is to evaluate the whole process from "cradle to grave". This obviously includes the manufacture of raw materials as well as the final product. For example energy consumption in every step has to be considered as well as emissions. BASF is using the eco-efficiency analysis tool in order to evaluate which process from a set of possible alternatives is the most sustainable one. This analysis has been done for the Ionic Liquid based BASIL™ process which was shown to be more sustainable with regard to both, economics and environment. BASIL™ has been awarded with the eco-efficiency label.
|
|
How can Ionic Liquids be recycled?
Recycling of Ionic Liquids is easy, when protonated cations are used. In this case the Ionic Liquids can be switched off by deprotonation. The resulting amine or imidazole is a conventional liquid which can be distilled for recycling or purification purpose.
It is more difficult with the alkylated cations. Apart from a purification or recycling by a liquid-liquid extraction two principal distillations methods haven been reported. The first is the formation of distillable carbenes [WO 01/77081] the second is the back-alkylation of the anion [WO 01/15175; DE 10002420].
Imidazolium cations can be deprotonated by bases to form neutral carbenes. These carbenes are surprisingly stable and can be distilled. The Ionic Liquid can be recycled by further reaction of the carbene with an acid.
This controlled decomposition reaction even allows for a recycling or purification process of the Ionic Liquid. In this case the Ionic Liquid is thermally cleaved. The neutral imidazole and alkylating agent are distilled off, collected and re-reacted.
|
|
How can Ionic Liquids be disposed?
Ionic Liquids are frequently reported as being non-flammable. This is only true up to the temperatures at which decomposition takes place. Some Ionic Liquids already start to decompose at 120°C others are stable up to nearly 400°C. Upon decomposition neutral and volatile molecules are formed which of course can burn. This explains why Ionic Liquids indeed have flash points even if they are usually much higher than 100°C. However, Ionic Liquids can easily be disposed by incineration which is usually done at temperatures of several hundred degrees celsius. At these very high temperatures even the toughest organic Ionic Liquids will give up.
How can I get the Ionic Liquid I really need?
- You can either visit web sites and try by yourself to find the right IL or you may ask BASF:
- BASF does not offer an extremely broad portfolio of Ionic Liquids - but BASF offers to define specific Ionic Liquids (including specific properties) in discussions with customers
| Inquiry |
|
lab trials |
|
pilot plant |
|
production |
|
- product selection
- basic specification
- can-do-evaluation
- secrecy agreement
|
|
- lab experiments
- kg's samples
- product definition
- spec. parameters
- basic tox data
- process definition
- price calculation
|
|
- pilot plant trials
- 100kg's sample
- def. specification
- plant decision
- notification
- price quote
- neg. contract
|
|
- contract signed
- production plant run
- product on ton's scale
> in spec
> on time
|
|
|
| step by step to the product you exactly need | |
Why should I think about using Ionic Liquids?
- The property profile of Ionic Liquids looks that it may fit to an already realized application where I feel this needs to be improved
- The property profile of Ionic Liquids looks that it might enable me to realize an idea I could not realize so far with other materials
- I have a problem with one of my applications or processes or I have an idea for a new product or a new process, which I cannot sufficiently realize with what I have so far and I’m questioning myself whether Ionic Liquids may help me
In all these cases, just talk to us, we will be able in a short period of time to give you advice whether IL’s may help you and if so we will guide you through the whole development process with our IL expertise.
What prices can be expected for Ionic Liquids?
- Prices will mainly depend on the quantity in which specific Ionic Liquids are produced - most of the Ionic Liquids today are manufactured only in kg quantities and therefore are offered at high prices. The price will be also influenced by specific requirements on specification parameters if these make additional manufacturing steps necessary.
- BASF expects to see prices standard quality Ionic Liquids with growing demands in tons quantities below 30€/kg (this expectation is based on first production runs carried our at BASF on a ton scale)
- The often mentioned correlation "imidazolium salts are expensive, pyridinium, pyrrolidinium, ammonium, phosphonium etc. salts are less expensive" does not correlate with our calculations. Based on the backward integration within BASF we have internally access to a very broad range of Ionic Liquid precursors enabling us to carry out these calculations with a high degree of accuracy. As it comes to larger quantities, always the performance of the Ionic Liquid will decide which compound is the best as the differentiation in prices will be only small.
Which Ionic Liquid should I use?
In many publications today - for several reasons - most often imidazolium salts are mentioned as the preferred Ionic Liquid materials. From our experience imidazolium salts indeed offer a lot of advantages and we therefore also set a focus on these. Nevertheless, with each application we evaluate, we also check other opportunities, targeting to identify the optimal Ionic Liquid showing the best fit between the properties and the requirements in the desired application.
The discussion on potential prices for Ionic Liquids are often limited to statements like imidazolium salts expensive and ammonium salts cheap. From our point of view there is not really a basis for these statements as we have not seen big differences in our cost calculations for Ionic Liquids with different cations. On the other side the anion should not be underestimated as in many cases the anion will define the performance of the Ionic Liquid as well as the price (for example in electrochemical applications, where often fluorinated anions are required).
|
|
|
|
|
|
