WO2011154890A1

WO2011154890A1 - Process for reusing ionic liquids

Info

Publication number: WO2011154890A1
Application number: PCT/IB2011/052467
Authority: WO
Inventors: Nikolaus Nestle; Markus Braun; Michael Siemer; Thomas Wisniewski
Original assignee: Basf Se; Basf (China) Company Limited
Priority date: 2010-06-10
Filing date: 2011-06-07
Publication date: 2011-12-15
Also published as: WO2011154370A1

Abstract

Process for reusing an ionic liquid, wherein a) after use of the ionic liquid, a composition comprising the used ionic liquid and other constituents is obtained, b) optionally, this composition is worked up to increase the content of the ionic liquid in the composition and to remove other constituents either entirely or partly, c) the T₂ value of the composition is determined by TD-NMR measurement (this composition will hereinafter be referred to as TD-NMR composition) and d) depending on the T₂ value determined, the TD-NMR composition is either worked up further or the TD-NMR composition is used as ionic liquid for a desired application.

Description

Process for reusing ionic liquids

The invention relates to a process for reusing an ionic liquid, wherein a) after use of the ionic liquid, a composition comprising the used ionic liquid and other constituents is obtained, b) optionally, this composition is worked up to increase the content of the ionic liquid in the composition and to remove other constituents either entirely or partly, c) the T2 value of the composition is determined by TD-NMR measurement (this composition will hereinafter be referred to as TD-NMR composition) and d) depending on the T2 value determined, the TD-NMR composition is either worked up further or the TD-NMR composition is used as ionic liquid for a desired application.

Ionic liquids are important for many industrial applications. They can be used, for example, as solvent, electrolyte or working liquid, including, for example, hydraulic fluids, lubricants, absorption media in circular processes, damping liquids or force transmission media.

They are suitable as solvents for synthetic or natural polymers which are otherwise sparingly soluble or insoluble. In this context, they are of particular industrial importance as solvents for cellulose, since cellulose films, cellulose beads or cellulose fibers can be produced from the solutions obtained, as is also described in

WO 2003/029329, WO 2009/062723 and WO 2007/076979. In such uses, ionic liquids are not consumed but merely contaminated. Since ionic liquids are generally complex chemical compounds which are significantly more expensive than standard solvents, there is interest in a process for working up and reusing ionic liquids. A distillation process for recovering and purifying ionic liquids is described in WO 2009/027250. Such processes for purifying ionic liquids are also complicated, with the outlay naturally increasing greatly with the purity required.

For reuse of an ionic liquid, it is sufficient for the ionic liquid to have the minimum purity for the respective use without disproportionate disadvantages having to be accepted; a higher purity is not necessary. An important feature of any work-up of ionic liquids is therefore a measurement method by means of which it can be determined whether and when the desired purity has been achieved. The measurement method should be very simple and quick to carry out and thus also be suitable for processes carried out continuously. In particular, it has to be a method which for compositions comprising predominantly or virtually exclusively ions can be carried out with the necessary accuracy. TD-NMR (time domain nuclear magnetic resonance) is a known analytical method. In TD-NMR, the sample to be examined is located in a magnetic field. This results in nuclear spin magnetization which can be excited by irradiation with resonant high- frequency pulses. The decay of the observed signals of the excited nuclear spin magnetization (transverse relaxation, T2) and the return of the nuclear spin system to the thermodynamic equilibrium state (longitudinal relaxation, Ti) can be characterized by relaxation times.

To utilize T2 information for analytical purposes, the decay of the detectable

magnetization is measured as a function of time and this measurement curve is evaluated by means of suitable models. It is possible to determine, depending on the nature of the sample, one or more relaxation times. As an example of the utilization of T2 values as analytical parameters, reference may be made to Nikolaus Nestle and Karl Haberle, Analytica Chimica Acta 654 (2009), pages 35 to 39. The T₂ values determined in this way are dependent on the chemical structure and composition of the sample and external conditions, in particular the temperature. If a plurality of relaxation times are determined for a sample, these can be converted into average values by suitable calculation methods (e.g. by calculation of the harmonic mean).

It was an object of the present invention to provide a simple and economical process for recovering ionic liquids. The purity of the ionic liquid should be determined quickly and with sufficient accuracy by a simple measurement method, so that it can be decided whether and to what degree further purification or work-up of the ionic liquid has to be carried out before the ionic liquid is reused. The measurement method should if possible also be suitable for continuous processes and preferably also be able to be carried out on-line.

We have accordingly found the process defined at the outset.

Process step a)

In process step a), an ionic liquid is used.

The term ionic liquid refers to salts (compounds composed of cations and anions) which have a melting point at atmospheric pressure (1 bar) of less than 200°C, preferably less than 150°C, particularly preferably less than 100°C. Possible ionic liquids also include mixtures of different ionic liquids.

Preferred ionic liquids comprise an organic compound as cation (organic cation).

Depending on the valence of the anion, the ionic liquid can comprise further cations, including metal cations, in addition to the organic cation.

The cations of particularly preferred ionic liquids are exclusively one organic cation or, in the case of polyvalent anions, a mixture of different organic cations. Suitable organic cations are, in particular, organic compounds having heteroatoms such as nitrogen, sulfur, oxygen or phosphorus; in particular, the organic cations are compounds having an ammonium group (ammonium cations), an oxonium group (oxonium cations), a sulfonium group - (sulfonium cations) or a phosphonium group (phosphonium cations).

In a particular embodiment, the organic cations of the ionic liquids are ammonium cations, which for the present purposes are nonaromatic compounds having a localized positive charge on the nitrogen atom, e.g. compounds having tetravalent nitrogen (quaternary ammonium compounds) or compounds having trivalent nitrogen, where one bond is a double bond, or - aromatic compounds having a delocalized positive charge and at least one nitrogen atom, preferably from one to three nitrogen atoms, in the aromatic ring system.

Preferred organic cations are quaternary ammonium cations, preferably those having three or four aliphatic substituents, particularly preferably C1 -C12 alkyl groups, on the nitrogen atom, which substituents may be substituted by hydroxyl groups.

Preference is likewise given to organic cations which comprise a heterocyclic ring system having from one to three, in particular one or two, nitrogen atoms as

constituents of the ring system.

Possibilities are monocyclic, bicyclic, aromatic or nonaromatic ring systems. Mention may be made by way of example of bicyclic systems as described in WO 2008/043837. The bicyclic systems of WO 2008/043837 are diazabicyclo derivatives, preferably comprising a 7-membered ring and a 6-membered ring, which comprise an amidinium group; mention may be made of, in particular, the 1 ,8-diazabicyclo[5.4.0]undec-7- enium cation. The solvent in process step a) is particularly preferably an ionic liquid having a cation selected from among quaternary ammonium cations or from among cations comprising a heterocyclic ring system having from one to three nitrogen atoms as constituents of the ring system.

Very particular preference is given to ionic liquids having cations comprising a heterocyclic ring system having one or two nitrogen atoms as constituents of the ring system.

Possible organic cations of this type are, for example, pyridinium cations, pyridazinium cations, pyrimidinium cations, pyrazinium cations, imidazolium cations, pyrazolium cations, pyrazolinium cations, imidazolinium cations, thiazolium cations, triazolium cations, pyrrolidinium cations and imidazolidinium cations. These cations are described, for example, in WO 2005/1 13702. If it is necessary for a positive charge on the nitrogen atom or in the aromatic ring system, the nitrogen atoms are in each case substituted by a hydrogen atom or an organic group having generally not more than 20 carbon atoms, preferably a hydrocarbon group, in particular a C1 -C16-alkyl group, in particular a C1 -C10-alkyl group, particularly preferably a C1 -C4-alkyl group.

The carbon atoms of the ring system can also be substituted by organic groups having generally not more than 20 carbon atoms, preferably a hydrocarbon group, in particular a C1 -C16-alkyl group, in particular a C1 -C10-alkyl group, particularly preferably a C1 - C4-alkyl group.

Particularly preferred ammonium cations are quaternary ammonium cations, imidazolium cations, pyrimidinium cations and pyrazolium cations.

Particular preference is given to imidazolium cations as are comprised in formula I (see below).

The ionic liquids can comprise inorganic or organic anions.

Such anions are described, for example, in the abovementioned WO 03/029329,

WO 2007/076979, WO 2006/000197 and WO 2007/128268.

Preference is given to anions from the group of alkylsulfates

RaOSOs^",

where R_a is a C1 -C12-alkyl group or a C5-C12-aryl group, preferably a C1 -C6-alkyl group or a C6-aryl group (tosylate), alkylsulfonates where R_a is a C1 -C12-alkyl group, preferably a C1-C6-alkyl group, methanesulfonate, halides, in particular chloride, bromide or iodide, pseudohalides such as thiocyanate, isothiocyanate, dicyanamide, halogen-comprising anions, e.g. tetrafluoroborate,

hexafluorophosphate, aluminates, e.g. tetrachloroaluminate, carboxylates R _aCOO^";

where R_a is a C1 -C20-alkyl group, preferably a C1-C8-alkyl group, in particular acetate, phosphates,

in particular dialkylphosphates of the formula RaRbP , where R_a and Rb are each, independently of one another, a C1 -C6-alkyl group; in particular, R_a and Rb are the same alkyl group, mention may be made of dimethylphosphate and diethylphosphate, and phosphonates, in particular monoalkylphosphonic esters

of the formula R_aRbP03^" , where R_a and Rb are each, independently of one another, a

C1 -C6-alkyl group.

Very particularly preferred anions are the carboxylates R_aCOO^";

where R_a is a C1 -C20-alkyl group, preferably a C1-C1 1 -alkyl group, in particular acetate.

The solvent is particularly preferably an imidazolium salt of the formula I below,

where

R1 is an organic radical having from 1 to 20 carbon atoms,

R2, R3, R4 and R5 are each an H atom or an organic radical having from 1 to 20 carbon atoms,

X is an anion and

n is 1 , 2 or 3. In formula I, preference is given to R1 and R3 each being, independently of one another, an organic radical having from 1 to 10 carbon atoms. In particular, R1 and R3 are each an aliphatic radical, in particular an aliphatic radical without further heteroatoms, e.g. an alkyl group. Particular preference is given to R1 and R3 each being, independently of one another, a C1 -C10-alkyl group or a C1 -C4-alkyl group.

In formula I, preference is given to R2, R4 and R5 each being, independently of one another, an H atom or an organic radical having from 1 to 10 carbon atoms; in particular, R2, R4 and R5 are each an H atom or an aliphatic radical. Particular preference is given to R2, R4 and R5 each being, independently of each other, an H atom or an alkyl group; in particular, R2, R4 and R5 are each, independently of one another, an H atom or a C1 -C4-alkyl group. Very particular preference is given to R2, R4 and R5 each being an H atom. n is preferably 1 .

X is preferably one of the abovementioned preferred anions; X is particularly preferably a carboxylate. The ionic liquid may optionally also be used in combination with nonionic solvents.

Possible nonionic solvents here are, in particular, those which mix homogeneously with the ionic liquid in the set mixing ratio. Mention may be made by way of example of water, acetone, dioxane, dimethyl sulfoxide, dimethylacetamide, formamide, N- methylmorpholine N-oxide and dichlormethane. Such mixtures of ionic liquids with nonionic solvents preferably comprise at least 50% by weight, particularly preferably at least 70% by weight, of ionic liquids, based on the total weight of the ionic liquid and nonionic solvent.

Furthermore, the ionic liquid or the mixture of ionic liquid and nonionic solvent can comprise additives which are desired or required for the respective use, e.g.

thickeners, stabilizers, corrosion inhibitors, antifoams, etc.

Industrial uses of the ionic liquid or the mixtures of ionic liquids with nonionic solvents are, for example, uses as solvent, as electrolyte, in particular as electrolyte for the production of aluminum or coating of any substrates with aluminum (aluminum plating), or working liquid, including, for example, hydraulic fluids, lubricants, absorption media in circular processes, damping liquids or force transmission media.

In process step a), the use, preferably one of the above uses, particularly preferably a use as solvent, is carried out. In particular, the use is a use as solvent for synthetic or natural polymers which are otherwise sparingly soluble or insoluble.

In this context, the use of ionic liquids or mixtures thereof with nonionic solvents as solvent for polysaccharides and in particular for cellulose is of particular industrial importance since cellulose films, cellulose beads or cellulose fibers can be produced from the resulting solutions, as is also described in WO 2003/029329, WO2009/062723 and WO2007/076979.

For the purposes of the present application, the term cellulose refers to cellulose, hemicellulose, modified cellulose (cellulose esters or cellulose ethers) and mixtures thereof with lignin, in particular with less than 40 parts by weight of lignin per 100 parts by weight of cellulose. Particular preference is given to using cellulose in the form of pulp in such applications.

To produce cellulose films, cellulose beads or cellulose fibers, the dissolved cellulose has to be precipitated from the solution by addition of a coagulant. Compounds suitable as coagulant are any compounds in which the cellulose does not dissolve, e.g. water or methanol, in particular water. The coagulant can naturally also be used in the form of a mixture with other solvents, e.g. the ionic liquid; however, such mixtures should comprise the coagulant in sufficient amounts; suitable mixtures are, for example, mixtures of water and ionic liquid in a weight ratio of water to ionic liquid of from 100:0 to 60:40. The coagulation is carried out in a known manner in such a way that the cellulose is obtained in the desired form as film, beads or fibers and is separated off in this form. In general, such processes, in particular the production of cellulose fibers, are carried out continuously. A use process which is preferred for the purposes of the present invention is therefore a continuous process in which

cellulose is dissolved in an ionic liquid or in a solvent mixture comprising ionic liquid and

cellulose is, after addition of a coagulant, obtained in the desired form, e.g. as fibers, beads or film, and separated off, with water being preferred as coagulant and the water being used, in particular, in the form of a mixture with ionic liquid.

The composition obtained after process step a) comprises

- the ionic liquid used or the mixture of ionic liquid and nonionic solvent used, insofar as these have not been consumed in the use,

where applicable additives which have been added before the use, additives or impurities which have got into the ionic liquid or the mixture as a result of the use. In the case of the production of cellulose films, cellulose beads or cellulose fibers, the latter additives or impurities are, in particular, the coagulant, preferably water, and any residual cellulose, including, as indicated above, hemicelluloses, which has not been separated off or other materials which may be comprised in cellulose, in particular low molecular weight sugars such as monosaccharides, disaccharides or oligosaccharides.

Process step b)

In process step b), a further work-up of the composition obtained after process step a) can be carried out in order to increase the content of the ionic liquid in the composition and/or to remove other constituents either entirely or partly.

Carrying out process step b) immediately is advisable when it is certain that the composition obtained is not suitable for a reuse of the ionic liquid without further work- up. If the composition obtained could possibly be suitable for a reuse without further work-up, process step b) can firstly be omitted and the TD-NMR measurement

(process step c) can be carried out directly.

In process step b), it is possible, for example, to separate off solid in a simple manner, e.g. by filtration. As separation methods, mention may also be made of ultrafiltration and reverse osmosis. Volatile compounds, e.g. solvents having a boiling point below 150°C at atmospheric pressure (1 bar), can be removed entirely or partly by distillation. If the composition obtained is inhomogeneous and forms two phases, the phase which comprises no ionic liquid or only small proportions of ionic liquid can be removed.

Optionally, the ionic liquid can also be distilled from the composition, e.g. by molecular distillation as described in WO 2009/027250.

Process step c)

In process step c), the T2 value of the composition obtained after process step a) and optionally b) is determined by TD-NMR measurement. The composition obtained after process steps a) and b) and used for the TD-NMR measurement will hereinafter be referred to as TD-NMR composition.

The TD-NMR composition comprises the constituents which have been introduced during the use and have not been separated off in process step b).

In particular, the TD-NMR composition comprises a nonionic solvent.

This can be a solvent such as water, methanol, ethanol, acetone, dioxane, dimethyl sulfoxide, dimethylacetamide, formamide, N-methylmorpholine N-oxide or

dichloromethane. In the case of the above-described use as solvent, e.g. for cellulose, the TD-NMR composition comprises, in particular, water or methanol, particularly preferably water, which has been used as coagulant.

The TD-NMR compositions usually comprise at least 0.1 part by weight, particularly preferably at least 1 part by weight, very particularly preferably at least 5 parts by weight, of nonionic solvents, in particular water, per 100 parts by weight of ionic liquid. The TD-NMR compositions generally comprise less than 50 parts by weight, in particular less than 40 parts by weight, particularly preferably less than 30 parts by weight, very particularly preferably less than 20 parts by weight and in a particular embodiment less than 10 parts by weight, of nonionic solvents, in particular water, per 100 parts by weight of ionic liquid.

Apart from nonionic solvents, inorganic cations in particular are possible constituents of the TD-NMR composition.

Such cations, e.g. alkali metals, alkaline earth metals, iron cations, copper cations, zinc cations, may be comprised in the TD-NMR composition in a total amount of at least 0.01 part by weight, in particular at least 0.1 part by weight, per 100 parts by weight of ionic liquid. In general, the TD-NMR composition will comprise less than 10 parts by weight, in particular less than 5 parts by weight, of inorganic cations per 100 parts by weight of ionic liquid.

The TD-NMR composition can naturally comprise further constituents, in particular those which have been mentioned above, However, preferred TD-NMR compositions comprise not more than 20 parts by weight, in particular not more than 10 parts by weight and particularly preferably not more than 5 parts by weight, of further constituents apart from nonionic solvents and the above cations.

Overall, preference is given to TD-NMR compositions which comprise at least 60% by weight, in particular at least 80% by weight, very particularly preferably at least 90% by weight, of ionic liquid.

The determination of the T2 value by TD-NMR measurement can be carried out in a known manner using commercially available TD-NMR instruments. TD-NMR instruments can be obtained, for example, from Oxford Instruments or from Bruker (Rheinstetten, Germany). The T2 value is temperature- and pressure-dependent. In general, the measurement is carried out on samples at atmospheric pressure (1 bar) and temperatures in the range from 10 to 50°C, in particular from 15 to 40°C. Optionally, a calibration curve or a corresponding evaluation could be recorded under the conditions selected for carrying out the measurement; the content of nonionic solvent and/or inorganic cations corresponding to the measured T2 can then be taken or determined directly from this calibration curve or evaluation.

It is important here that the measured T2 value corresponds reproducibly and with sufficient accuracy to the content of nonionic solvent and inorganic cations, as shown in the examples in the present patent application.

Apart from the TD-NMR measurement, other analytical methods can naturally also be employed in process step c) if further information about the composition of the samples is desired. Mention may be made by way of example of customary methods of analytical spectroscopy, in particular IR, Raman or UV-Vis spectroscopy, and also TXRF (total X-ray fluorescence), ICP-MS (ICP mass spectrometry) or LIFS (laser- induced fluorescence spectroscopy). Process step d)

In process step d), the TD-NMR composition is either worked up further (first case) or the TD-NMR composition is used for a desired use as ionic liquid (second case) as a function of the T2 value determined.

In the first case, a process step b) is carried out. This can be a first process step b) or a renewed process step b), i.e. a repetition of the process step b) previously carried out or, if necessary, a new work-up measure. Process step c) is subsequently carried out again.

In the second case, or after process step b) has been successfully carried out once more, the composition can be reused as ionic liquid.

In general, the composition is reused as ionic liquid for the same application. The process of the invention is, owing to its effectiveness and speed, particularly advantageous for continuous processes.

The use in process step a) is therefore preferably a continuous use process which is supplemented by the then likewise continuous process for work-up, measurement and reuse according to process steps a) to d). As indicated above, the process of the invention is particularly suitable for the use of ionic liquid as solvent for cellulose, i.e. for a process in which, accordingly

a) cellulose is dissolved in ionic liquid or in a solvent mixture comprising ionic liquid and cellulose is, after addition of a coagulant, isolated and separated off in the desired form, e.g. as fibers, beads or film, b) optionally, the resultant composition which comprises ionic liquid, the coagulant and further constituents is worked up to increase the content of ionic liquid in the composition and remove the coagulant or the further constituents either entirely or partly, c) the T2 value of the composition is then determined by TD-NMR measurement (hereinafter referred to as TD-NMR composition) and d) the TD-NMR composition is either worked up further or the TD-NMR composition is reused as ionic liquid as per process step a) as a function of the T2 value determined.

The process of the invention is a simple and economical process for the reuse of ionic liquids. The purity of the ionic liquid can be determined quickly and with sufficient accuracy by the TD-NMR measurement in process step c), so that a decision can be made as to whether and to what extent further purification or work-up of the ionic liquid has to be carried out before reuse. Contamination of the ionic liquid with nonionic solvents, e.g. water, in particular can be determined reproducibly with good accuracy. The same applies to contamination with metal cations, in particular paramagnetic cations such as iron(lll) cations.

The measurement method in process step c) is suitable for continuous processes and can also be carried out on-line.

Examples

Starting materials used

As ionic liquid, use was made of ethylmethylimidazolium acetate, an imidazolium salt of the formula I above with R1 =ethyl, R3=methyl, n=1 and X=acetate, referred to as EMIM acetate for short. Defined amounts of water and salts having inorganic cations (NaCI, FeS0₄ x 7H2O) were added to EMIM acetate and the T2 value was determined by TD-NMR

measurement.

Measurement method

The recording and evaluation of TD-NMR measurements is known and also described, for example, in Nikolaus Nestle and Karl Haberle, Analytica Chimica Acta 654 (2009), pages 35 to 39. The samples were examined at 30°C by means of the SE-CPMG method at 20 MHz in a Bruker minispec mq20. In the SE-CPMG method, the transverse relaxation of the nuclear spin magnetization is recorded by means of a solid-state echo and a subsequent CPMG curve (CPMG: Carr-Purcell-Meiboom-Gill sequence; this comprises a sequence of many, possibly several 1000, spin echoes and allows recording of the transverse magnetization decay curves of a liquid sample in one "shot"). A description of the method may also be found, for example, in the above-cited article by Nestle and Haberle.

To carry out the evaluation, the magnetization decay curves were normalized and then fitted by means of a Levenberg-Marquardt fit using a biexponential model.

To convert the evaluation results into simple calibration parameters, it is useful in each case to work with suitably calculated means for the relaxation time. Here, the harmonic mean of the relaxation times 1/<1 T2IL> is employed, since the physical information is comprised in the reciprocals of the relaxation times (relaxation rates).

Pe3 + PeA

Addition of water

Measurements were carried out on EMIM acetate with addition of increasing amounts of water (500 ppm, 2000 ppm, 1 % by weight, 5% by weight, 10 % by weight and 25% by weight).

The mean 1/<1/T2ii_> displays a clear relationship which is shown in figure 1 .

Up to a water content of about 10%, the relaxation time increases largely linearly with the amount of water. After that, saturation effects occur but the relaxation time continues to increase monotonically. In figure 1 , R2 is the correlation coefficient of the linear regression, which in the ideal case is 1 . The deviations between the water concentrations and the regression lines for added amounts of water up to 10% are less than 0.06 percentage points. In the region of nonlinear dependence, a greater number of measurement points at smaller intervals may be required for the desired accuracy of the calibration curve.

The water content of samples can then be taken from the calibration curve. The calibration curve as per figure 2 applies for a temperature of 30°C. If samples at different temperatures are taken in the recycling process, corresponding calibration curves have to be constructed for these temperatures.

Addition of metal salts

Measurements were carried out on EMIM acetate with addition of Na and iron(lll) salts of various anions:

EMIM acetate + 200 ppm NaS0₄

EMIM acetate + 200 ppm NaCI

EMIM acetate + 1000 ppm NaS0₄

EMIM acetate + 1 % by weight NaS0₄

EMIM acetate + 200 ppm FeS0₄ x 7 H₂0

EMIM acetate + 1000 ppm FeS0₄ x 7 H₂0

EMIM acetate + 1 % by weight FeS0₄ x 7 H₂0

EMIM acetate always comprised 0.15% by weight of water. For the dependence of 1/<1 T2IL>, a linear increase in the reciprocal <1/T2ii_> with the salt concentration is expected in the case of the salts. Corresponding plots for the trials using different amounts of sodium salts of various anions or of iron sulfate are shown in figures 2 and 3.

The correlation is excellent in the case of the sodium salts. At 200 ppm of NaCI and 200 ppm of NaSC , the results were identical within measurement accuracy; the cation content could therefore be determined accurately regardless of the type of anion.

In the case of iron sulfate, the correlation is likewise excellent for the first three values. The value for the highest concentration, on the other hand, surprisingly displays a significant deviation. The reason for this is presumably the simultaneous introduction of water by the iron sulfate heptahydrate, which at 1 % can no longer be disregarded.

In figures 2 and 3, R2 is once again the correlation coefficient of the linear regression, which in the ideal case is 1 .

The cation content of samples can be determined from the calibration curves of figures 2 and 3. This naturally applies both in the case of a linear calibration curve and for a nonlinear calibration curve; in the case of nonlinear behavior, it is merely necessary to use more measurement points for constructing the calibration curve. The calibration curves in figures 2 and 3 once again apply for a temperature of 30°C. If samples are taken at other temperatures in the recycling process, corresponding calibration curves have to be constructed for these temperatures.

Claims

1 . A process for reusing an ionic liquid, wherein a) after use of the ionic liquid, a composition comprising the used ionic liquid and other constituents is obtained, optionally, this composition is worked up to increase the content of the ionic liquid in the composition and to remove other constituents either entirely or partly, the T2 value of the composition is determined by TD-NMR measurement (this composition will hereinafter be referred to as TD-NMR composition) and depending on the T2 value determined, the TD-NMR composition is either worked up further or the TD-NMR composition is used as ionic liquid for a desired application.

The process according to claim 1 , wherein the ionic liquid is an ionic liquid having a cation selected from among quaternary ammonium cations and cations comprising a heterocyclic ring system having from one to three nitrogen atoms as constituents of the ring system.

The process according to either claim 1 or 2, wherein the ionic liquid comprises imidazolium salts of the formula I below,

where

R1 is an organic radical having from 1 to 20 carbon atoms,

X is an anion and

n is 1 ,

2 or

3.

4. Process according to any of claims 1 to 3, wherein the use of the ionic liquid in process step a) is a use as solvent for synthetic or natural polymers.

5. The process according to any of claims 1 to 4, wherein the use is a use as

solvent for polysaccharides.

6. The process according to any of claims 1 to 5, wherein the use is a use as

solvent for cellulose.

7. The process according to any of claims 1 to 6, wherein

- cellulose is dissolved in an ionic liquid or in a solvent mixture comprising ionic liquid and - cellulose is, after addition of a coagulant, obtained in the desired form, e.g. as fibers, beads or film, and separated off.

8. The process according to claim 7, wherein the coagulant is water.

9. The process according to any of claims 1 to 8, wherein the TD-NMR composition comprises a nonionic solvent.

10. The process according to any of claims 1 to 9, wherein the TD-NMR composition comprises water.

1 1 . The process according to any of claims 1 to 10, wherein the TD-NMR

composition comprises less than 40 parts by weight of water per 100 parts by weight of ionic liquid.

12. The process according to any of claims 1 to 1 1 , wherein the TD-NMR

composition comprises less than 5 parts by weight of inorganic cations per 100 parts by weight of ionic liquid.

13. The process according to any of claims 1 to 12, wherein the TD-NMR

composition comprises at least 80% by weight of ionic liquid.

14. The process according to any of claims 1 to 13, wherein the TD-NMR

composition is reused as ionic liquid for the same application.

15. The process according to any of claims 1 to 14, wherein the process is a

continuous process. The process according to any of claims 1 to 15, wherein the process is a continuous process in which a) cellulose is dissolved in an ionic liquid or in a solvent mixture comprising ionic liquid and cellulose is, after addition of a coagulant, obtained in the desired form, e.g. as fibers, beads or film, and separated off, b) optionally, the composition obtained, which comprises ionic liquid, the coagulant and further constituents, is worked up to increase the content of the ionic liquid in the composition and to remove the coagulant or the other constituents either entirely or partly, c) the T2 value of the composition is determined by TD-NMR measurement (hereinafter referred to as TD-NMR composition) and d) depending on the T2 value determined, the TD-NMR composition is either worked up further or the TD-NMR composition is reused as ionic liquid as per process step a).