Process for the extraction of metal pollutants from treated cellulosic biomass

Abstract

The present invention relates to a process for extracting oxidised metal pollutants from treated cellulosic or lignocellulosic biomass to recover the metal. The treatment also generates a cellulosic or lignocellulosic biomass which can to be used as a feedstock for biofuel, for making cellulose containing materials, and provides a source of other renewable chemicals.

Claims

1. A process for the extraction of metal pollutants from treated lignocellulosic biomass, the process comprising (a) contacting the treated lignocellulosic biomass with an ionic liquid, said ionic liquid comprising an anion and an organic cation, wherein the contacting produces a cellulose-rich solid material and a liquid phase, wherein the liquid phase comprises a hemicellulose fraction, a lignin fraction, and metal pollutants, the anion is selected from the group consisting of chloride (Cl.sup.−), acetate ([OAc].sup.−), and hydrogen sulfate (HSO4].sup.−) and the cation is selected from (i) a cation of general formula: ##STR00004## wherein X is N; and A1 to A4 are each independently selected from H, an aliphatic, C3-6 carbocycle, C6-10 aryl, alkylaryl, and heteroaryl; or (ii) a cation containing a nitrogen-containing heterocyclic moiety.

2. The process of claim 1 wherein the cation is selected from (i) a cation of general formula: ##STR00005## wherein X is N; and A1 to A4 are each independently selected from H, an aliphatic, C3-6 carbocycle, Co-10 aryl, alkylaryl, and heteroaryl.

3. The process of claim 2 wherein at least one of A1 to A4 is H.

4. The process of claim 1 wherein the organic cation is a protic organic cation.

5. The process of claim 1 wherein the cation is an alkylammonium, an alcoholammonium, or a mixture thereof.

6. The process of claim 5 wherein the cation is N,N-dimethyl-N-butylammonium ([DMBA].sup.+).

7. The process of claim 1 wherein the cation containing a nitrogen-containing heterocyclic moiety is an imidazolium based cation or a mixture thereof.

8. The process of claim 7 wherein the imidazolium based cation is selected from 1-ethyl-3-methylimidazolium ([EMIM].sup.+), 1-butylimidazolium ([HBIM].sup.+) and 1-methylimidazolium ([HMIM].sup.+).

9. The process of claim 1 wherein the ionic liquid is selected from N,N-dimethyl-N-butylammonium hydrogen sulfate ([DMBA][HSO4]), 1-butylimidazolium hydrogen sulfate ([HBIM][HSO4]), 1-methylimidazolium chloride ([HMIM]C I), 1-methylimidazolium hydrogen sulfate ([HMIM][HSO4]), or triethylammonium hydrogen sulfate ([TEA][HSO4]), or mixtures thereof.

10. The process of claim 1 wherein the organic cation is derived from its conjugate base, and wherein the ionic liquid comprises at least 5% molar excess of the conjugate base.

11. The process of claim 1 wherein the ionic liquid is contacted with the biomass in the form of a composition comprising the ionic liquid and 10-40% v/v water.

12. The process of claim 1 further comprising (b) separating the cellulose-rich solid material from the liquid phase.

13. The process of claim 12 further comprising (c(i)) washing the cellulose-rich solid material at least once with a washing fluid that is the ionic liquid or an organic solvent miscible with the ionic liquid.

14. The process of claim 13 further comprising (c(ii)) separating the cellulose-rich solid material from the washing fluid.

15. The process of claim 12 further comprising (d) adding an anti-solvent to the ionic liquid obtained in (b) to precipitate the lignin.

16. The process of claim 12 further comprising removing water from the ionic liquid after step (b).

17. The process of claim 15 further comprising removing water from the ionic liquid after step (d).

18. The process of claim 1 further comprising electrodeposition of the metal pollutant from the ionic liquid.

19. The process of claim 18 wherein after the electrodeposition, the ionic liquid is re-used in step (a).

20. The process of claim 13 further comprising electrodeposition of the metal pollutant from the ionic liquid, wherein after the electrodeposition, the ionic liquid is re-used in step (c(i)).

21. The process of claim 1 further comprising saccharification of the cellulose containing solid residue obtained in step (a) to obtain glucose.

22. The process of claim 12 further comprising saccharification of the cellulose containing solid residue obtained in step (b) to obtain glucose.

23. The process of claim 14 further comprising saccharification of the cellulose containing solid residue obtained in step (c(11)) to obtain glucose.

24. A method of fractionating lignocellulosic biomass, the method comprising: (a) contacting the lignocellulosic biomass with an ionic liquid, said ionic liquid comprising an anion and a N,N-dimethyl-N-butylammonium ([DMBA].sup.+) cation.

25. The method of claim 24, wherein the ionic liquid is N,N-dimethyl-N-butylammonium hydrogen sulfate ([DMBA][HSO4]).

Description

(1) The invention will now be described in the examples below which refer to the following figures:

(2) FIG. 1 is a flow chart of the biomass pre-treatment and copper extraction process. CRM stands for Cellulose Rich Material.

(3) FIG. 2 displays the composition of the copper (II) treated wood as analysed by compositional analysis.

(4) FIG. 3 shows copper (II) concentrations in IL measured by inductively coupled plasma optical emission spectrophotometry ICP OES

(5) FIG. 4 shows currents measured at −1V in [HC.sub.1im][HSO.sub.4].

(6) FIG. 5 shows cyclic voltammograms of recycled and fresh [HC.sub.1im][HSO.sub.4] doped with CuO to saturation;

(7) The temperature and time period are not essential for the extraction of the copper (II), but they will have a major impact on the sugar yields achieved from enzymatic saccharification of the pulps.

EXAMPLE 1. DECONSTRUCTION OF BIOMASS AND EXTRACTION OF METALS IN VARIOUS IONIC LIQUIDS

(8) A flow chart of the deconstruction and extraction process is summarized in FIG. 1. An ionic liquid/water mixture was prepared by adding the required amount of water to the dried ionic liquid. The water content was confirmed by Karl-Fischer titration in triplicate. Pre-treatments were run in triplicate. 10±0.05 g of ionic liquid/water master-mix was weighed into a glass pressure tube and the exact weight recorded. Copper azole treated softwood (1.0 g oven-dried basis) with particle sizes of 180-850 μm was added and the tube tightly closed and the contents mixed with a Vortex shaker until all of the biomass had been in contact with the ionic liquid. The vial was placed in a preheated convection oven at 170° C. for 30 min. After the incubation, the mixture was transferred into a 50 mL centrifuge tube. This was facilitated by diluting with 40 mL of ethanol. The contents were mixed using a vortex shaker and left at room temperature for one hour. The tube was then centrifuged and the solids and liquids decanted carefully into a round bottom flask. The solid was further washed by repeating the washing step 2-3 more times. The remaining solid (pulp) was then transferred into a cellulose thimble and further washed by Soxhlet extraction with refluxing ethanol (150 mL) for 22 hours. The pulp was left to dry in the thimble on the bench overnight. The ethanol used for the Soxhlet extraction was combined with the previous washes and evaporated under reduced pressure at 40° C., leaving the dried ionic liquid/lignin mixture. To the dried ionic liquid/lignin mixture, 30 mL of water was added in order to precipitate the lignin. The suspension was transferred into a 50 mL falcon tube, shaken for one minute and then left at room temperature for at least 1 hour. The tube was centrifuged and the supernatant decanted and collected in a round bottom flask. This washing step was repeated three more times and the washings combined.

(9) The air-dried pulp yield was determined by weighing the recovered biomass from the cellulose thimbles. The oven-dried yield was determined as described below. The lid of the centrifuge tube containing the lignin was pierced and the tube put into a vacuum oven overnight for drying the lignin at 40° C. under vacuum. The dried lignin was weighed the next day to determine the lignin yield.

(10) Saccharification

(11) Enzymatic saccharification was performed in triplicate according to LAP “Enzymatic saccharification of lignocellulosic biomass” (NREL/TP-510-42629), issue date Mar. 21, 2008. The enzymes were Novozymes experimental enzyme mixture NS-22201. Glucose yields were calculated based on the glucose content of the untreated biomass. 50 μL of enzyme solution was used per 100 mg of sample.

(12) Compositional Analysis

(13) The glucan, hemicellulose and lignin content of copper treated timber was determined following the LAP procedures “Preparation of samples for compositional analysis” (NREL/TP-510-42620), issue date Aug. 6, 2008 and Determination of Structural Carbohydrates and Lignin in Biomass” (NREL/TP-510-42618 version Aug. 3, 2012). The extractives in untreated copper treated timber were removed and quantified according to the LAP “Determination of extractives in biomass” (NREL/TP-510-42619), issued Jul. 17, 2005. The oven-dry weight (ODW) of lignocellulosic biomass was determined according to the procedure described in the LAP “Determination of Total Solids in Biomass and Total Dissolved Solids in Liquid Process Samples” (NREL/TP-510-42621) issued Mar. 31, 2008.

(14) Metal Content

(15) Inductively coupled plasma optical emission spectrophotometry (ICP-OES) was used to analyse the ionic liquid liquor on its metal content and run in triplicate on a Perkin Elmer Optima 2000 DB instrument. A mixed metal standard for ICP analysis was obtained from Sigma Aldrich (TraceCERT grade) and diluted to the required concentrations with 5% nitric acid.

(16) The water content of the ionic liquid liquors was determined by Karl-Fischer titration. The liquors were diluted to below 10 ppm of copper and a maximum of 1 wt % ionic liquid concentration with 5% nitric acid and then analysed by ICP-OES.

(17) Approximately 100 mg of ground wood samples were weighed out and the exact weight recorded (±0.1 mg, Mettler Toledo NewClassic MS). The samples were digested in 1 mL 69% nitric acid in a closed PTFE vessel (MARSXpress vessels and microwave with power/time control by CEM with the following sequence: 300 W at 83% power for 5 min, 600 W at 66% power for 5 min and 1200 W at 58% power for 6 min). The obtained solution was cooled in a freezer for an hour before diluting to 10 mL with 5% nitric acid and filtration through a 0.4 μm PTFE syringe filter.

(18) The measured wood's copper content was used to calculate the percentage of copper extracted into the ionic liquid.

(19) Results and Discussion

(20) Ionic Liquid Screening Experiments for the Extraction of Copper(II) from CA Treated Softwood

(21) FIG. 2 displays the composition of the copper treated wood as analysed by compositional analysis.

(22) TABLE-US-00001 TABLE 1 Composition of copper treated wood used in pre-treatments. Glu stands for glucan, Xyl for xylan, Gal for galactan, Ara for arabinan, Man for mannan, AIL for acid insoluble lignin, ASL for acid soluble lignin, Extr. for extractives. Glu Xyl Gal Ara Man AIL ASL Ash Extr. 47.9 ± 0.8 6.5 ± 0.1 0.8 ± 0.0 0.4 ± 0.0 13.6 ± 0.3 25.4 ± 1.1 3.3 ± 0.1 BDL 2.1 ± 0.0

(23) The following ionic liquids were screened for their suitability to fractionate/pre-treat biomass and to extract copper from the biomass, where [TEA] stands for triethylammonium, [DMBA] for N,N-dimethyl-N-butylammonium, [DEA] for diethylammonium, [DEtOHA] for diethanolammonium, [HMIM] for 1-methylimidazolium and [EMIM] for 1-ethyl-3-methylimidazolium:

(24) TABLE-US-00002 TABLE 2 Ionic liquids screened. Ionic liquid Type of ionic liquid [TEA][HSO.sub.4] Protic, symmetric, tertiary amine based, weakly coordinating anion [DMBA][HSO.sub.4] Protic, asymmetric, tertiary amine based, weakly coordinating anion [HMIM][HSO.sub.4] Protic, imidazole based, weakly coordinating anion [DEA][HSO.sub.4] Protic, symmetric, secondary amine based, weakly coordinating anion [DEtOHA][Cl] Protic, symmetric, secondary amine based, alcohol side chain, strongly coordinating anion [HMIM][Cl] Protic, imidazole based, strongly coordinating anion [EMIM][Cl] Aprotic, imidazole based, strongly coordinating anion [EMIM][OAc] Aprotic, imidazole based, strongly coordinating anion [EMIM][OTf] Aprotic, imidazole based, mildly coordinating anion

(25) Table 3 displays the copper (II) extraction, saccharification yield as well as pulp and lignin yields after pre-treatment of the copper azole treated softwood with the tested ILs as a percentage of the total initial biomass.

(26) TABLE-US-00003 TABLE 3 Results from screening experiments. Sacchar- Copper ification Pulp Lignin Ionic liquid extraction/% yield/% yield/% yield/% [TEA][HSO.sub.4] 87 ± 1 55.2 ± 2.6 56.7 ± 0.3 8.7 ± 0.2 [DMBA][HSO.sub.4] 93 ± 0 72.3 ± 4.1 42.6 ± 0.3 19.4 ± 1.4  [HMIM][HSO.sub.4] 82 ± 1 15.7 ± 2.6 58.9 ± 0.5 7.0 ± 1.7 [HMIM][Cl] 98 ± 2 75.7 ± 2.5 43.1 ± 0.8 14.3 ± 0.7  [DEA][HSO.sub.4] 85 ± 2  0.1 ± 0.1 34.7 ± 1.0 7.3 ± 0.4 [DEtOHA][Cl] 81 ± 4 11.0 ± 0.3 94.5 ± 0.3 BDL [EMIM][OAc] 86 ± 2 43.0 ± 1.9 92.1 ± 1.2 BDL [EMIM][Cl] 92 ± 1 28.8 ± 2.9 75.9 ± 1.8 1.7 ± 0.2 [EMIM][OTf] 68 ± 1  9.7 ± 0.3 92.7 ± 0.4 BDL Untreated —  9.9 ± 0.2 100 —

(27) The data shown here suggests that [HSO.sub.4].sup.−, [Cl].sup.− and [OAc].sup.− ILs are capable of the extraction of 81-98% of the present copper (II) from treated softwood. The only IL studied here that extracted significantly lower amount of copper was [EMIM][OTf] which extracted 68% of the copper (II). A wider range of results was obtained for the saccharification of the recovered cellulose rich pulp; the highest glucose yields were obtained for enzymatic saccharification of [DMBA][HSO.sub.4] and [HMIM][Cl] pre-treated biomass (above 70% of theoretical). Lower yields but still significant improvements compared to untreated biomass were obtained with [TEA][HSO.sub.4], [EMIM][OAc] and [EMIM][Cl].

(28) CCA Treated Wood

(29) Pre-treatments of chromated copper arsenate (CCA) treated softwood with [HBIM][HSO.sub.4], where [HBIM] stands for 1-butylimidazolium, were conducted at 170° C. for 30 min. Saccharification yields obtained were 52.5% and the metal extraction is displayed in table 4. All three metals were extracted nearly quantitatively (≥98%).

(30) TABLE-US-00004 TABLE 4 Metal Contents as measured by ICP-OES and relative metal extraction in pulp after [HC.sub.4im][HSO.sub.4] pre-treatment of the CCA treated wood for 1 hour at 150° C. Standard error of measurements in brackets. CCA Treated Wood Arsenic(V) Chromium(VI) Copper(II) Metal Content/ppm 4268 (605)   4664 (745)   2784 (365)   Metal Extracted 99% (0.06%) 99% (0.14%) 98% (0.46%)

(31) Mixed Infeed and Processed Wood

(32) Mixed unprocessed and processed wood waste obtained (unprocessed wood is chipped waste wood of various origin, processed wood is the same type of wood that had part of the metals, mainly iron, removed mechanically) were pre-treated at 170° C. for 30 min with two different ILs, [HC.sub.1im][Cl] and [HC.sub.1im][HSO.sub.4]. The original metal content as well as the amounts extracted for unprocessed and processed wood are displayed in Tables 5 and 6 respectively. Higher saccharification yields were obtained with [HC.sub.1im][Cl] and measured to be 53 and 60% for unprocessed and processed wood respectively.

(33) TABLE-US-00005 TABLE 5 Metal Contents as measured by ICP-OES and relative metal extraction in pulp after pre- treatment of the unprocessed mixed wood. Standard error of measurements in brackets. Unprocessed Mixed Wood Zinc(II) Lead(II) Iron(II/III) Chromium(VI) Copper(II) Metal Content/ppm 138 (0.3)  173 (8.4)  567 (12.4)  9.1 (1.2).sup.  37 (6.9)  Metal Extracted with 82% (1.50%) 12% (1.85%) 55% (3.67%) 69% (1.71%) 54% (2.07%) [HC.sub.1im][HSO.sub.4] Metal Extracted with 86% (3.02%) 85% (1.29%) 65% (4.67%) 80% (3.46%) 90% (0.59%) [HC.sub.1im][Cl]

(34) TABLE-US-00006 TABLE 6 Metal Contents as measured by ICP-OES and relative metal extraction in pulp after pre- treatment of the processed mixed wood. Standard error of measurements in brackets. Processed Mixed Wood Zinc(II) Lead(II) Iron(II/III) Chromium(VI) Copper(II) Metal Content/ppm 114 (0.3)  367 (18.7)  331 (31.7)  55 (2.1)  82 (5.0)  Metal Extracted with 70% (6.20%) 39% (4.47%) 52% (14.86%) 77% (2.66%) 48% (2.74%) [HC.sub.1im][HSO.sub.4] Metal Extracted with 89% (0.41%) 96% (1.92%) 56% (9.28%)  93% (0.54%) 97% (0.29%) [HC.sub.1im][Cl]

(35) The presented results suggest very high extraction efficiencies in the range of 80-99%, for zinc (II), lead (II), chromium (VI) and copper (II) are possible with [HC.sub.1im][Cl] in the absence of a chelating agent.

EXAMPLE 2 COPPER SOLUBILITY AND ELECTRODEPOSITION OF COPPER FROM VARIOUS IONIC LIQUIDS COPPER SOLUBILITY

(36) Solubility measurements were conducted in order to establish solubility limits in one of the investigated protic ionic liquids 1-methylimidazolium hydrogen sulfate ([HC.sub.1im][HSO.sub.4], shown below, top). For comparison, solubility was also tested in the more inexpensive triethylammonium hydrogen sulfate [TEA][HSO.sub.4] (shown below, bottom).

(37) ##STR00003##

(38) The solubilities were tested by dissolving copper oxide in the ionic liquid until no further dissolution was observed overnight. The remaining solids were filtered off and the obtained solutions subjected to ICP-OES. In order to investigate if excess base improved solubilities, 5 wt % excess amine/imidazole was used in a further test. The results are displayed in FIG. 3. The maximum copper(II) solubilities at 20 wt % water are also summarised in table 7.

(39) TABLE-US-00007 TABLE 7 Copper(II) solubilities in ionic liquid systems at 20 wt % water. Ionic Liquid Copper(II) solubility/ppm [HC.sub.1im][HSO.sub.4] 4752 ± 715 [TEA][HSO.sub.4] 2043 ± 433 [HC.sub.1im][HSO.sub.4] 5% excess base 10845 ± 2050 [TEA][HSO.sub.4] 5% excess base 13712 ± 4068

(40) FIG. 3 shows that the water content plays a deciding role in the copper dissolution capability of the ionic liquid. Solubility peaks at around 60 wt % and 80 wt % water in the case of [HC.sub.1im][HSO.sub.4] and [TEA][HSO.sub.4] respectively. Peak solubility is around 72,000 ppm or 7.2 wt % in the case of [HC.sub.1im][HSO.sub.4] and 33,000 ppm or 3.3 wt % in the case of [TEA][HSO.sub.4]. At higher base contents the dissolution capacity of [HC.sub.1im][HSO.sub.4] is improved to around 93,000 ppm or 9.3 wt % at 60 wt % water. Pre-treatments however cannot be conducted at water contents above around 30% which means that solubilities at around 20 wt % water are more important for the here conducted study. From table 7 we can see that at 20 wt % water the solubility in [HC.sub.1im][HSO.sub.4] was improved from 4752 ppm to 10845 ppm by the addition of 5% excess base while in the case of [TEA][HSO.sub.4] dissolution capability was improved from 2043 ppm to 13712 ppm. [HC.sub.1im][HSO.sub.4] and [TEA][HSO.sub.4] with 5 wt % excess base therefore reach a similar dissolution capacity within the measurement error.

(41) While these tests were conducted by dissolving Cu(II) oxide, copper bound in the biomass is expected to be extracted in a similar manner.

(42) Electrochemical properties of [HC.sub.1im][HSO.sub.4]

(43) For a successful deposition of copper from the ionic liquid, electrochemical stability of the IL needs to be guaranteed in order to make the process viable. FIG. 4 shows maximum currents obtained at −1 V vs. Ag in [HC.sub.1im][HSO.sub.4] with different water contents. The almost dry IL (2 wt % water) shows very small currents at very reducing potentials, suggesting that the ionic liquid is electrochemically stable under these conditions. The higher water content ILs exhibit higher current densities due to the occurrence of water reduction leading to hydrogen evolution. The current density reaches a maximum at 25 wt % water and then decreases again. The hydrogen evolution reaction can be linked to acidity of the medium. The data presented here suggest that acidity of the ionic liquid maximises at around 25 wt % water.

(44) Copper Deposition

(45) The deposition of copper out of an ionic liquid liquor was shown for [HC.sub.1im][HSO.sub.4] as well as for the less expensive triethylammonium hydrogen sulfate ([TEA][HSO.sub.4]) by means of cyclic voltammetry and chronoamperometry. In both cases, liquor from biomass pre-treatment was saturated with copper (II) ions by dissolution of CuO and filtering off undissolved solid. The liquors tested contained 20 wt % water in order to mimic the conditions of pre-treatment.

(46) Preliminary results of solubility tests of CuO in the two protic ionic liquids show a higher solubility in the imidazolium salt than in the alkylammonium salt. Therefore the focus of further tests has been on the imidazolium salt. In order to establish the effect of biomass degradation products present in the liquor on the deposition behaviour of copper, a cyclic voltammogram (FIG. 5) of copper (II) saturated fresh ionic liquid was compared to that obtained from the copper (II) saturated recycled ionic liquid. The oxidation current peak due to Cu.fwdarw.Cu.sup.II+2e.sup.− at ca. 0.1 V in the positive-going voltammetric scan of the recycled liquor was shifted slightly towards higher potentials compared to the fresh ionic liquid and exhibited a small side peak before the main peak. However, the potentials (ca. −0.15 V) of current onsets for the oxidation (Cu.fwdarw.Cu.sup.II+2e.sup.−) and reduction (Cu.sup.II+2e.sup.−.fwdarw.Cu) were almost identical. Integration of current-time data further confirmed that the copper deposition was not measurably affected by biomass degradation products, as charge efficiencies in both cases were around 94% under the tested conditions.