Removal of impurities from ionic liquids

Abstract

According to an example aspect of the present invention, there is provided a distillation method for purifying ionic liquids from impurities such as hydroxycarboxylic acids, which accumulate to the ionic liquid for example during their use in cellulose dissolution.

Description

REMOVAL OF IMPURITIES FROM IONIC LIQUIDS FIELD

[0001] The present invention relates to a method of purification of ionic liquids usedfor example in the dissolution of cellulose. The dissolution of cellulose produces as sideproducts for example hydroxycarboxylic acids, which are detrimental for continued use ofthe ionic liquid. The produced hydroxycarboxylic acids accumulate into the ionic liquidduring recycling. BACKGROUND

[0002] The removal of impurities, such as recently found hydroxycarboxylic acidsand ash compounds, are important for the use of ionic liquids in the ionic liquid based fibermanufacturing processes. These hydroxycarboxylic acids are formed during dissolution ofcellulose and should be removed due to for example 1) economic viability of the processand 2) the product fiber quality.

[0003] The impurities thus consist of (1) degraded fractions of hemicelluloses,typically in the form of oligosaccharides, but also short polymers, (2) hydroxycarboxylic acids formed by alkali-catalyzed beta-alkoxyelimination reactions ("primary and secondary peeling") from the reducing ends of polysaccharides, especially from hemicelluloses, as well as (3) inorganic components present in the raw material, pulp and / or recycledcellulose-based waste textiles, and (4) hydrolysis products of ionic liquid, H-mTBD, andthe corresponding lactam of the superbase, which reacts with the anion to form A-mTBD. In addition, there are impurities from the input of textile waste, such as additives fortextiles, for example elastane, residues of synthetic polymers, colours, finishing chemicals,etc.

[0004] In closed process cycles, even the smallest amounts of these impuritiesaccumulate in the solvent, which leads to massive process disruptions and quality losses in the fibres, both in terms of mechanical and optical properties.

[0005] Some relating prior art exists. For example, Saad (2021) focuses on recoveryof [mTBDH][OAc] in the presence of different impurities and finding suitable operatingconditions for removing the impurities using a short path distillation unit. KCl, NaCl, CaCl2, lactic acid, xylan and hydrolysis products were the impurities used in understandingtheir interaction with the ionic liquid. Also, initial results for the removal of lactic acidwere obtained. However, quantification of the lactic acid was not possible at the time dueto non-availability of the accurate analytics.

[0006] Other relevant prior art publications comprise Earle et al. (2006), whereindistillation and the volatility or certain ionic liquids has been mentioned for the first time, King et al. (2011) reporting about the distillability of [TMGH][OAc], also a superbase-based ionic liquid to high purity, and Parviainen et al. (2013), where also the distillabilityof superbase-based ionic liquids, including [DBNH][CO2Et] and [mTBDH][Ac] has beenhighlighted. These publications, however, fail to disclose an effective solution forpurifying ionic liquids from for example hydroxycarboxylic acids.

[0007] Thus, there is a need for a novel solution for purification of ionic liquids byan effective removal or essential reduction of hydroxycarboxylic acids from such.SUMMARY OF THE INVENTION

[0008] The invention is defined by the features of the independent claims. Somespecific embodiments are defined in the dependent claims.

[0009] An object of the invention is to provide a method for the purification ofdistillable ionic liquids such as protic ionic liquids, in particular acid superbase conjugates(such as [mTBD][acetate], [DBNH][acetate], [DBUH][acetate] and other ionic liquids) byremoval of non-volatile impurities comprising 1) short chain polysaccharides, 2)hydroxycarboxylic acids and 2) inorganic components.

[0010] According to one aspect, distillation methods (such as batch distillation, shortpath distillation and continuous distillation) are used for removal or reduction ofhydroxycarboxylic acids.

[0011] According to another aspect, the hydroxycarboxylic acids are reacted furtherby heat treatment to a higher molecular weight substance and are thereafter separated by distillation.

[0012] These and other aspects, together with the advantages thereof over knownsolutions are achieved by the present invention, as hereinafter described and claimed.

[0013] The method of the present invention is mainly characterized by what is statedin the characterizing part of claim 1.

[0014] Next, the present technology will be described more closely with reference tocertain embodiments. EMBODIMENTS

[0015] The present purification / separation technology enables for examplecontrolling of the hydroxycarboxylic acid content during cellulose dissolution using ionicliquids and recycling of the valuable ionic liquid.

[0016] FIGURE 1 is a photo of a batch distillation column used in some of theexperiments.

[0017] FIGURE 2 is a drawing showing a schematic set-up of a short pathdistillation equipment usable in the present method. Feed (top of equipment) and theresidue (bottom part of equipment) flows on the wall as a thin film. The wiper spreads the feed evenly to the wall. Distillate is condensed in the center of the apparatus.

[0018] FIGURE 3 is a drawing showing a schematic set-up of the batch distillationcolumn used, having: 1. Heater, 2. Round bottom flask, Vigreaux column, 4 distillatereflux controller, 5. Condenser, 6. Additional distillate condenser, 7. Round bottom flasks for distillate fraction collection, 8. vacuum line, 9. Liquid nitrogen trap, 10 Vacuum control valve, T temperature probes, P pressure transducer.

[0019] FIGURE 4 is a chart showing measured vapor pressures of thehydroxycarboxylic acids and the [mTBDH][acetate] 3:2 complex.

[0020] FIGURE 5 shows a simplified scheme of the monofilament Ioncell® fiberspinning process integrated with ionic liquid recycling. Pulp and ionic liquid are fed into a kneader unit (10) to dissolve pulp and provide a dope. The dope is then directed to a spinning unit (13) in which filaments are spun or films are extruded into a spin bath (14). After spinning or extruding the filaments or films are washed with water and stretched (15). Washed fibres are recovered and the washing filtrate from the washing and stretchingunit (15) is disposed of. The spin bath (14) solution is filtered in a filter unit (21) anddirected to the recovery of the ionic liquid and to a first thin-film evaporation stage (17). The distillate from the first thin film evaporation stage (17) is directed to the spin bath (14) and the residue from the first thin film evaporation stage (17) is directed to a second thin film evaporation stage (18). Distillate from the second thin film evaporation stage (18) is directed to the spin bath (14) and recycled ionic liquid is recovered from the second thin film evaporation stage (18) and directed to the kneader (10) for dissolving further pulp.

[0021] One important aspect of the present invention is that the ionic liquid to bepurified is taken for purification after the water has been removed by distillation.Depending on the degree of closure of the circuits, different quantities of the ionic liquidmust be subjected to this purification. This proportion is between 2 and 25 % of the total ionic liquid. It is also important that the ionic liquid supplied for purification does notexceed a certain water content. It should therefore be less than 5 wt-%, preferably less than3 wt-%.

[0022] The present method is thus carried out by first feeding a partial flow of therecycled ionic liquid from the evaporation plates to remove the water to contents below 5%, preferably below 3 % by weight (by using batch, shorth path distillation equipment orcontinuous distillation equipment). The ionic liquid is then evaporated, and the impuritiesremain to a high degree in the heavy non-volatile fraction / residue. The pure ionic liquid iscondensed and an ionic liquid with a high purity is obtained.

[0023] According to one embodiment of the present invention, a method forpurification of ionic liquids comprises at least the following steps:˗ feeding an ionic liquid mixture comprising an ionic liquid complex and at least onehydroxycarboxylic acid into a batch, short path or continuous distillationequipment,˗ evaporating the ionic liquid mixture in the distillation equipment, wherein vaporpressure of the ionic liquid complex is preferably higher than vapor pressure of thehydroxycarboxylic acid(s) in the mixture,˗ removing the hydroxycarboxylic acid(s) from a heavy non-volatile fraction / residue,and ˗collecting the condensed pure ionic liquid from a distillate.

[0024] In one embodiment, the ionic liquid is first fed to a heated reactor or tube andthe hydroxycarboxylic acids react to heavy molecular weight substances. The reactor canbe continuous, semi batch or batch reactor. Subsequently the ionic liquid is purified bydistillation as defined above.

[0025] According to one embodiment, the method comprises feeding of the ionicliquid mixture having a water content lower than 5 wt-%, preferably lower than 3 wt-% to a heated batch, semi-batch or continuous reactor or tube prior to distillation.

[0026] According to one embodiment, the proportion of the ionic liquid complex fedinto the batch, short path or continuous distillation is between 2 to 25 wt-% of the ionicliquid mixture. Thus, only 2-25 % of the amount of concentrated ionic liquid complexneeds to be purified in order not to deteriorate the spinning behaviour and fibre quality.

[0027] According to one embodiment, it is preferable that the condensates fromwater evaporation and ionic liquid purification are distributed in the process in such a waythat a stoichiometric 1:1 acid to base ratio of the ionic liquid is maintained in thecontinuous process. During the purification distillation a small amount of acid is lost to thebottom product with the heavy impurities. To retain 1:1 acid to base ratio, the lost acid should be replaced by adding such acid to a suitable location.

[0028] According to one embodiment, the superbase of the ionic liquid complex ismTBD (7-Methyl-1,5,7-triazabicyclo[4.4.0]dec-5-ene), DBN (1,5-Diazabicyclo[4.3.0]non-5-ene), DBU (1,8-Diazabicyclo[5.4.0]undec-7-ene), mTBN (a mixture or pure componentof the following: 5-methyl-1,5,7-triazabicyclo[4.3.0]non-5-ene & 7-methyl-1,5,7-triazabicyclo[4.3.0]non-5-ene), TBN (1,5,7-triazabicyclo[4.3.0]non-5-ene) or TBU (1,5,7-Triazabicyclo[4.4.0]dec-5-ene).

[0029] According to one embodiment, the conjugate acid of the ionic liquid complexis mTBDH, DBNH or DBUH.

[0030] According to one embodiment, the vapor pressure of 3:2 complex of the ionicliquid is lower than the vapor pressure(s) of the pure acetic acid, lactic acid and glycolicacid.

[0031] According to one embodiment, the ionic liquid complex is[mTBDH][acetate], [DBNH][acetate] or [DBUH][acetate].

[0032] According to one embodiment, the hydroxycarboxylic acid is itaconic acid,succinic acid, lactic acid, adipic acid, citric acid, malic acid, glycolic acid, glutaric acid, tricarballylic acid, acetic acid or any other C2 to C6 hydroxycarboxylic acids, and / or any combination thereof.

[0033] According to one embodiment, the ionic liquid can be simultaneouslypurified from hydroxycarboxylic acids and from non-volatile or low-volatile impuritieslike salts (CaCl2, NaCl, CaCO3) using distillation methods. Separate process units are notneeded for hydroxycarboxylic acid and heavy impurity removal, and economic benefits arethereby achieved.

[0034] According to one embodiment, the distillation is carried out in conditions,wherein a temperature of the evaporator is around 140 °C and a temperature of thecondenser is around 65 °C, and by using pressure around 0.4 mbar and flowrate of 1.2 to1.8 ml / min.

[0035] According to one embodiment, the method disclosed is able to remove atleast 80%, preferably at least 85%, more preferably at least 90% and most suitably at least95% of the hydroxycarboxylic acids present in the ionic liquid feed.

[0036] One further embodiment is a condensed ionic liquid, which is produced bythe method disclosed herein, wherein the ionic liquid mixture purified in the method isobtained for example from used textiles.

[0037] Reference throughout this specification to one embodiment or anembodiment means that a particular feature, structure, or characteristic described inconnection with the embodiment is included in at least one embodiment of the present invention. Thus, appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the sameembodiment. Where reference is made to a numerical value using a term such as, forexample, about or substantially, the exact numerical value is also disclosed.

[0038] While the forgoing examples are illustrative of the principles of the presentinvention in one or more particular applications, it will be apparent to those of ordinary skill in the art that numerous modifications in form, usage and details of implementation can be made without the exercise of inventive faculty, and without departing from the principles and concepts of the invention. Accordingly, it is not intended that the invention be limited, except as by the claims set forth below.

[0039] The verbs “to comprise” and “to include” are used in this document as openlimitations that neither exclude nor require the existence of also un-recited features. The features recited in depending claims are mutually freely combinable unless otherwiseexplicitly stated. Furthermore, it is to be understood that the use of "a" or "an", that is, asingular form, throughout this document does not exclude a plurality. INDUSTRIAL APPLICABILITY

[0040] The present technology provides means to purify and thereafter recyclevaluable ionic liquids, by controlling of the non- or low-volatile non-cellulosic impuritiesof the ionic liquids that are used in for example methods of cellulose dissolution. Not onlyhydroxycarboxylic acids and ash, but also impurities from the recycling of used textilescan be removed by the method disclosed herein. In addition, the method disclosed herein issuitable for removing hydrolysis and lactam products of the ionic liquid, H-mTBD and A- mTBD, as formed in the presence of water. EXAMPLEShort path distillation was performed for the ionic liquid mixture at 0.4 mbar pressure andwiper speed of 360 rpm. The flow rate utilized for the distillation were varied slightly dueto manual adjustment and ranges from 1,25 - 1,41 ml / min. The contaminated ionic liquid(IL) was fed into the feed and purified IL was recovered as the distillate fraction. The massbalances and the state of the fractions for the respective distillation are presented in Table 4.HPLC analysis was performed for all distillation fractions to identify and quantify the massof hydroxycarboxylic acids present. The results from the HPLC analysis are presented inTable 5. The results indicated that all hydroxycarboxylic acids except citric acid wassuccessfully removed from the ionic liquid after distillation. The mass of citric acid that was present in the ionic liquid after distillation was 0,014 g, which is around 5% of the mass of citric acid that was added to the ionic liquid at the beginning of the distillation.Hydroxycarboxylic acids were not detected in any of the water-cooled trap and liquidnitrogen trap fractions. Loss in the mass balance is due to the fact that some of the IL remains in the devices and / or the hydrocarboxylic acids react to form unidentifiable compounds. Table 1 shows the distillation data and properties of distillation samples, Table 2 mass balances of batch distillation and Table 3 distillation experimental conditions.Table 1. Distillation data and properties of distillation samples. Initial Initial bottom Final bottom condensate Final condensate Pressure Mass SampleA:B Molar Ratio State of sample(g)91.3 93.8 150.4 151.3 1.3 36.2 BaseDistillate 22 145.1 146.7 170.5 192.9 0.9 114.9 1.5 light brown, solidlight brown, mostly Distillate 32 146.7 134.5 192.9 217.9 0.9 30.5 1.5crystals with some liquidTable 2. Mass balance of batch distillation in g. * - no peak was observed for water in NMR spectrum and quantification of water with KarlFischer titration had not been performed, thus assumed as 0. ** - Mass of water based upon NMR peak area of water in sample analysed. *** -Mass of water calculated based on Karl Fischer Titration data. **** - Loss identified as holdup volume of column.Balance, g m(final)- Feed, g Distillate 1, g Distillate 2, g Distillate 3, g Residue, g Water-cooled trap, gm(init)mTBD 148,4 36.2 71.9 19.3 0.0 0.0 -31.2Acetic acid 65,1 0.0 42.3 11.3 2.8 0.0 -13.3H-mTBD-1 13,3 0.0 0.0 0.0 7.4 0.0 -6.7H-mTBD-2 6,6 0.0 0.8 0.0 17.1 0.0 10.8A-mTBD-1 & A-mTBD-2 8,3 0.0 0.0 0.0 12.3 0.0 3.5Water 16,4 0.0* 0.0* 0.0* 0.0* 17.5** 17.5Loss*** -19.3Table 3. Distillation experimental conditions. Experiment Conditions Temperature (°C) Experiment NumberPre-heater Evaporator Condenser Pressure (mbar) Flowrate (ml / min)1 90,0 140,0 65,0 0,4 1,272 90,0 140,0 65,0 0,4 1,773 90,0 140,0 60,0 0,4 1,254 90,0 140,0 60,0 0,4 1,265 90,0 140,0 60,0 0,4 1,41Table 4. Mass balances for short-path distillation performed.Mass Experiment Distillation percentage of NumberFractions Mass, g feed, wt-% State of fractions1 Feed 99,50 - Brown liquidYellow liquid at end of the experiment. Crystalized at roomDistillate 76,90 77,29temperature. Dark brown, viscous fluid. Solid atResidue 3,81 3,83room temperature Water CooledTrap 8,30 8,34 Yellow, clear liquidLiquid N2 Trap 6,13 6,16 Colourless, clear liquidLoss 4,36 4,38 -Feed 95,40 - Brown liquidPale yellow liquid at end of 72,87 76,38experiment.Crystalised at room temperature. Distillate Dark brown, viscous fluid. HighlyResidue 4,71 4,94viscous paste at room temperature Water CooledTrap 9,03 9,46 Yellow, clear liquidLiquid N2 Trap 6,33 6,64 Colourless, clear liquidLoss 2,46 2,58 -Feed 93,44 - Brown liquidYellow liquid at end of experiment. Crystalized at roomDistillate 71,11 76,11temperature.Dark brown, viscous fluid. HighlyResidue 4,15 4,45viscous paste at room temperature Water CooledTrap 7,72 8,27 Yellow, clear liquidLiquid N2 Trap 5,86 6,27 Colourless, clear liquidLoss 2,65 2,84 -Feed 90,91 - Brown liquidYellow liquid with some crystals formation at end of experiment. Fully crystalised at roomDistillate 70,03 77,04temperature. Dark brown, viscous fluid. Highly 5,12 5,63viscous paste at room temperatureResidue Water CooledTrap 7,32 8,06 Yellow, clear liquidLiquid N2 Trap 5,67 6,24 Colourless, clear liquidLoss 2,76 3,04 -Feed 90,87 - Brown liquidYellow liquid at end of experiment. Crystalised at roomDistillate 71,49 78,67temperature. Dark brown, viscous fluid. HighlyResidue 4,32 4,76viscous paste at room temperature 6,66 7,33 Yellow, clear liquidWater CooledTrapLiquid N2 Trap 5,53 6,08 Colourless, clear liquidLoss 2,87 3,16 -Table 5. Quantity of hydroxy acids present in the distillate fractions of the [mTBDH][acetate] determined through HPLC. Lactic acid was foundin runs 1 and 2 even though none was added to feed. The [mTBDH][acetate] used was from the pilot plant and lactic acid was formed during thefiber spinning piloting. Experiment Experiment ExperimentExperiment Number 4 Experiment Number 5Number 1 Number 2 Number 3 Mass of Hydroxy Acids (g) Distillation Succinic Lactic Itaconic Lactic Adipic Citric Malic Succinic Lactic Tricarballyli Glycolic Glutaric Fractions Acid Acid Acid Acid Acid Acid Acid Acid Acid c Acid Acid AcidFeed 3,87 0,0 0,0057 0,23 0,0 0,49 0,76 0,55 0,30 0,24 0,15 0,0Residue 0,22 0,02 0,23 0,0055 0,0096 0,30 0,73 0,52 0,31 0,29 0,19 0,0Distillate 0,0 0,0 0,0 0,0 0,0 0,014 0,0 0,0 0,0 0,0 0,0 0,0Trap 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0LN2 Trap 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0 0,0REFERENCE SIGNS LISTfilter 2kneader unit 10spinning unit 13spin bath 14washing and stretching unit 15first thin film evaporation stage 17 second thin film evaporation stage 18distillation unit 19filter 21CITATION LIST Non-patent literature:1. Saad M., Recovery of protic liquid by short patch distillation, Kemian tekniikankorkeakoulu, 2021, https: / / aaltodoc.aalto.fi / browse / author?scope=2eef0435-e78a-4a1f- 9120-69416dcf524d&value=Saad,%20Mohammed&bbm.return=12. Earle, Martyn J. Esperança, José M. S. S. Gilea, Manuela A. Canongia Lopes, José N.Rebelo, Luís P. N. Magee, Joseph W. Seddon, Kenneth R. Widegren, Jason A. The distillation and volatility of ionic liquids. Nature, Vol 439, 7078, 831-834. Doi:10.1038 / nature04451. The distillation and volatility of ionic liquids | Nature3. King, Alistair W. T. Asikkala, Janne Mutikainen, Ilpo Jaervi, Paula Kilpelaeinen, Ilkka.Distillable Acid-Base Conjugate Ionic Liquids for Cellulose dissolution and processing. Angew. Chem., Int.Ed., Vol 50, 6301-6305, S6301 / 1-S6301 / 5. https: / / doi.org / 10.1002 / anie.2011002744. Parviainen, Arno, King, Alistair W. T., Mutikainen, Ilpo, Hummel, Michael, Selg,Christoph, Hauru, Lauri K. J., Sixta, Herbert, Kilpelainen, Ilkka. ChemSusChem, Vol 6, 2161-9. https: / / doi.org / 10.1002 / cssc.201300143

Claims

CLAIMS:

1. A method for purification of ionic liquids, characterized in that the method comprisesat least the following steps: ˗feeding an ionic liquid mixture comprising an ionic liquid complex and at least onehydroxycarboxylic acid into a batch, short path or continuous distillationequipment, ˗evaporating the ionic liquid mixture in the distillation equipment, wherein vaporpressure of the ionic liquid complex is higher than vapor pressure of thehydroxycarboxylic acid(s) in the mixture,˗ removing the hydroxycarboxylic acid(s) from a heavy non-volatile fraction / residue,and ˗collecting the condensed pure ionic liquid from a distillate.

2. The method according to claim 1, characterized in feeding the ionic liquid mixturehaving a water content lower than 5 wt-%, preferably lower than 3 wt-% to a heated batch,semi-batch or continuous reactor or tube prior to distillation.

3. The method according to claim 1 or 2, characterized in that the proportion of the ionicliquid complex fed into the batch, short path or continuous distillation is between 2 to 25 wt-% of the ionic liquid mixture.

4. The method according to any of the preceding claims, characterized in maintaining astoichiometric 1:1 acid to base ratio of the ionic liquid.

5. The method according to any of the preceding claims, characterized in that the ionicliquid complex comprises a superbase selected from mTBD, DBN, DBU, mTBN, TBN andTBU.

6. The method according to any of the preceding claims, characterized in that the ionicliquid complex comprises a conjugate acid selected from mTBDH, DBNH and DBUH.

7. The method according to any of the preceding claims, characterized in that the vaporpressure of 3:2 complex of the ionic liquid is lower than the vapor pressure(s) of the pureacetic acid, lactic acid and glycolic acid.

8. The method according to any of the preceding claims, characterized in that the ionicliquid complex is [mTBDH][acetate], [DBNH][acetate] or [DBUH][acetate].

9. The method according to any of the preceding claims, characterized in that thehydroxycarboxylic acid is itaconic acid, succinic acid, lactic acid, adipic acid, citric acid,malic acid, glycolic acid, glutaric acid, tricarballylic acid, acetic acid or any other C2 to C6 hydroxycarboxylic acids, and / or any combination thereof.

10. The method according to any of the preceding claims, characterized in furthercomprising removal of low- or non-volatile impurities, such as CaCl2, NaCl and CaCO3.

11. The method according to any of the preceding claims, characterized in that thedistillation is carried out in conditions, wherein a temperature of the evaporator is around140 °C and a temperature of the condenser around 65 °C, and by using a pressure around0.4 mbar and a flowrate of 1.2 to 1.8 ml / min.

12. The method according to any of the preceding claims, characterized in removing atleast 80%, preferably at least 85%, more preferably at least 90% and most suitably at least95% of the hydroxycarboxylic acids present in the ionic liquid feed.

13. A condensed ionic liquid produced by the method according to any of claims 1 to 12,wherein the ionic liquid mixture purified in the method is obtained from used textiles.

14. Use of the method according to any of claims 1 to 12 for removing hydroxycarboxylicacids, ash and / or impurities from the recycling of used textiles.

15. Use of the method according to any of claims 1 to 12 for removing hydrolysis and lactam products of the ionic liquid, H-mTBD and A-mTBD, as formed in the presence of water.

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