Treatment of lignocellulosic biomass with aromatic chemicals as scavengers

Abstract

A method for using at least one compound as a scavenger, in particular as a scavenger in the hydrolytic treatment of lignocellulosic biomass is provided. The compound includes at least one aromatic ring substituted with at least two moieties each having a free electron pair, wherein the at least two moieties are arranged in meta-position to each other, and at least two alkyl or alkenyl moieties.

Claims

1. A process for the production of a cellulosic feedstock by hydrolytic treatment of lignocellulosic biomass wherein at least one compound is added to the lignocellulosic biomass during at least one of before, during or after the aqueous treatment autohydrolysis of the lignocellulosic biomass, wherein the compound is selected from at least one compound of ##STR00007## the general formula (II) ##STR00008## wherein X.sub.1, X.sub.2, X.sub.3=OH, R=unsubstituted or substituted alkyl substituent(s) or unsubstituted or substituted alkenyl substituent(s), and n=2. ##STR00009##

2. The process according to claim 1, wherein the aqueous treatment of the lignocellulosic biomass is a liquid hot water treatment or an aqueous steam treatment.

3. The process according to claim 1, wherein lignocellulosic biomass consists of lignocellulosic material containing fermentable carbohydrate.

4. The process according to claim 1, wherein the aqueous treatment autohydrolysis of the lignocellulosic biomass is carried out under inert gas, oxygen containing, ozone containing or hydrogen containing atmosphere.

5. The process according to claim 1, wherein the aqueous treatment autohydrolysis of the lignocellulosic biomass is carried out in a temperature range from 100 to 260 C., and at a pressure within the range from 0 to 500 bar.

6. The process according to claim 1, wherein the pH during the aqueous treatment of the lignocellulosic biomass is adjusted by the addition of at least one acid.

7. The process according to claim 1, wherein the pH during the aqueous treatment of the lignocellulosic biomass is adjusted by the addition of at least one base.

8. The process according to claim 1, wherein the water to dry biomass ratio is in the range between 1:5 and 100:1.

9. The process according to claim 1, wherein the at least one compound used as scavenger is added an amount between 0.01 to 20% w/w based on dry biomass loading.

10. The process according to claim 1, wherein any unreacted scavenger compound is recovered after autohydrolysis treatment and recycled to the autohydrolysis treatment stage.

11. The process according to claim 1, wherein the reacted scavenger compound is recovered after autohydrolysis with a laccase treatment and recycled to the autohydrolysis treatment stage.

12. The process according to claim 1, wherein the hydrolytically treated lignocellulosic biomass is washed before entering the enzymatic hydrolysis stage.

13. The process according to claim 1, wherein the lignocellulosic biomass comprises lignocellulosic material containing fermentable carbohydrate comprising at least one of softwood spruce, hardwood beech, herbs or agricultural residues.

14. The process according to claim 1, wherein the aqueous treatment autohydrolysis of the lignocellulosic biomass is carried out under inert gas or air.

15. The process according to claim 1, wherein the aqueous treatment autohydrolysis of the lignocellulosic biomass is carried out in a temperature range from 190 to 240 C.

16. The process according to claim 1, herein the aqueous treatment autohydrolysis of the lignocellulosic biomass is carried out at a pressure within the range from 1 to 40 bar.

17. The process according to claim 1, wherein the pH during the aqueous treatment of the lignocellulosic biomass is adjusted by the addition of at least one inorganic acid which comprises nitric acid, hydrochloric acid, phosphoric acid or mixtures thereof.

18. The process according to claim 1, wherein the pH during the aqueous treatment of the lignocellulosic biomass is adjusted by the addition of at least one organic acid which comprises acetic acid, formic acid or mixtures thereof.

19. The process according to claim 1, wherein the pH during the aqueous treatment of the lignocellulosic biomass is adjusted by the addition of at least one inorganic base which comprises at least one alkali salt.

20. The process according to claim 19, wherein the alkali salt comprises sodium hydroxide.

21. The process according to claim 1, wherein the pH during the aqueous treatment of the lignocellulosic biomass is adjusted by the addition of at least one organic base which comprises pyridine.

22. The process according to claim 1, wherein the water to dry biomass ratio is in the range between 1:1 and 5:1.

23. The process according to claim 1, wherein the compound is: ##STR00010## wherein R.sub.1 and R.sub.2 have the meaning of R and are the same or different.

24. The process according to claim 1, wherein each R is independently selected from a C.sub.1-C.sub.10 alkyl substituent or a C.sub.1-C.sub.10 alkenyl substituent.

25. The process according to claim 1, wherein each R is independently selected from a C.sub.1-C.sub.5 alkyl substituent or a C.sub.1-C.sub.5 alkenyl substituent.

26. The process according to claim 1, wherein each R is independently selected from a methyl, ethyl, n-propyl, i-propyl, n-butyl, i-butyl, t-butyl, propenyl, butenyl or pentenyl substituent.

27. The process according to claim 1, wherein the compound comprises dimethylphloroglucinol.

28. The process according to claim 1, wherein the compound is dimethylphloroglucinol.

29. A cellulosic feedstock or lignin fraction obtainable by a process according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention will now be explained in more detail by means of the examples with reference to the Figures.

(2) FIG. 1 a scheme for lignin reaction in acidic media;

(3) FIG. 2 a process scheme for biological conversion of lignocellulose biomass to sugars;

(4) FIG. 3 an embodiment of a preferred scavenging compound;

(5) FIG. 4 a scheme showing different transitions states of 2-naphthol as scavenger;

(6) FIG. 5 a scheme showing the reaction of an aliphatic alcohol and carbocation;

(7) FIG. 6 structures of different compounds tested for their scavenging activity;

(8) FIG. 7 a first diagram showing the scavenging effect of different tested compounds by means of the wood composition after hydrolytic treatment;

(9) FIG. 8 a second diagram showing the scavenging effect of different tested compounds by means of glucose yield after hydrolytic treatment;

(10) FIG. 9 structures of different compounds tested for their scavenging activity and their respective activation positions; and

(11) FIG. 10 structures of further scavenging compounds according to the invention.

DETAILED DESCRIPTION

I. Reagents

(12) The following compounds were evaluated as additives (see also FIGS. 5 and 9): 2-naphtol (Fluka, 99%), methanol (Fluka, 99.8%), ethanol (Fluka, 99.8%), 2-propanol (Scharlau, 99.8%), phenol (Fluka, 99%), resorcinol (Chemie Brunschwig, 98%), catechol (Sigma-Aldrich, 99%), hydroquinone (Sigma-Aldrich, 99%), phloroglucinol dihydrate (Aldrich, 97%), dimethylphloroglucinol (Green Pharma, 95%).

II. Autohydrolysis Treatment, Biomass Analysis and Enzymatic Hydrolysis (FIG. 2)

(13) Spruce wood, sourced from a forest in the canton of Solothurn, Switzerland and previously knife-milled through a 1 mm screen size in a Retsch SM200 cutting mill, was sieved with a Retsch AS200 vibratory sieve shaker to obtain a particle size fraction between 0.18 and 1 mm. The spruce dry matter (74.630.39% w/w) composition was determined by standard NREL methods (Sluiter & Hames et al., 2008; Sluiter & Ruiz et al., 2005): glucan 45.180.55%, mannan 17.740.28%, acid soluble lignin (ASL) 4.770.42%; acid insoluble lignin (AIL) 28.730.06%, extractives 4.89% and ash 0.22%0.01 (total 101.53%).

(14) The autohydrolysis treatment of the biomass was carried out in a Parr MRS 5000 Multi-Reaction system, which consists of six 75 mL stainless steel reactors, each equipped with a suspended magnetic stirrer. Heat is provided to each reactor through an electrically heated aluminum external jacket. The reactor was loaded with 2.5 g of spruce wood (including moisture) and 39.2 g of distilled water, resulting in a biomass loading of 6% w/w. The amount of the autohydrolysis additive depended on its molar mass. For comparison reasons, All additives were dosed at the same molar ratio of 0.2054 mol scavenger/mol lignin C9 unit. This molar ratio is the same as in the pretreatment of biomass with 2-naphthol at a loading of 4% w/w of the biomass load, which was used as a benchmark. The corresponding concentrations of each additive are shown in Table 1.

(15) TABLE-US-00001 TABLE 1 Autohydrolysis additive loadings for biomass autohydrolysis as weight percentage of the amount of biomass (including moisture) in the reactor. Substance Loading/% w/w Methanol 0.89 Ethanol 1.28 2-Propanol 1.67 2-Naphthol 4.00 Phenol 2.61 Resorcinol 3.05 Cathechol 3.05 Hydroquinone 3.05 Phloroglucinol 3.50 Dimethylphloroglucinol 4.24

(16) Before the autohydrolysis treatment the reactor was purged three times with nitrogen at 15 bar and the stirrer speed was set at 400 min.sup.1. The reaction temperature of 210 C. was reached in approximately 12 minutes by preheating the aluminum jacket to 350 C. before inserting the reaction vessel. After a reaction time of 120 min at 210 C., the reactor was cooled immediately to room temperature by submerging it in water.

(17) The contents of the reactor were vacuum filtered and the solids washed with 300 mL of boiling water to remove any unreacted scavenger as well as byproducts from the pretreatment. The volume and pH of the filtrate were recorded and the pretreated biomass was stored in an airtight bag at 8 C. for further use and analysis.

(18) The content of glucan, mannan, acid-soluble and acid-insoluble lignin in the pretreated biomass as well as sugar concentrations in the autohydrolysis liquor were determined according to NREL standard procedures (Sluiter & Hames et al., 2006; Sluiter et al., 2008). However, owing to the small amount of biomass used in the pretreatment, the procedures were downscaled by a factor of 56. All analyses were done in triplicate.

(19) Enzymatic hydrolysis was carried out according to a NREL standard procedure (Selig, Weiss et al., 2008). Deviating from this procedure, the sodium citrate buffer was added as to obtain a pH of 5.0 and a sodium azide concentration of 0.005 mol/L after sample preparation. 230 L of Accelerase 1500 enzyme complex, with an activity of 26 FPU/mL were added to each vial. This corresponds to an enzyme dosing of 60 FPU/g cellulose. The enzymatic hydrolysis of the samples was carried out continuously for 120 hours in an INFORS HT Minitron incubator at 50 C. and a shaking speed of 210 min.sup.1. The concentration of sugars in the supernatant was determined with HPLC.

III. Effect of Autohydrolysis Additives

(20) In the following, the present invention will be explained with the help of representative experiments. In particular, the effectiveness of the described carbocation scavengers in autohydrolysis for improving the digestibility of the resulting cellulose is demonstrated. Compared to a state of the art autohydrolysis treatment, the digestibility of the cellulose can be improved. The yields of the enzymatic hydrolysis for each compound are benchmarked against such a control sample (no scavenger) where a yield of 64.8% was obtained.

(21) It is important to recall that the concentration of all tested scavengers was equivalent, in molar terms, to 4% w/w 2-naphthol (0.2054 mol scavenger/mol lignin C9 unit). The yield obtained with this concentration of 2-naphthol, which is an already known effective scavenger, is shown for comparison, too. The newly described scavengers have a different mode of action and can be more effective than 2-naphtol.

(22) The pH of the filtrates recovered from the treatments with the different additives showed very similar pH (3.02-3.15), indicating that there were no differences in the autohydrolysis environment for the generation of carbocations.

(23) The composition of the various samples after autohydrolysis with the different additives is shown in FIG. 7. It can be observed that hemicellulose was removed completely, which is typical for an autohydrolysis treatment. Samples showed similar contents of cellulose and lignins except for biomass pretreated with phloroglucinol and dimethylphloroglucinol, which showed a noticeably higher content of acid insoluble lignin (AIL) than the others. This is due to the incorporation of these very reactive additives into the lignin structure.

(24) The glucose yields obtained in the enzymatic hydrolysis after 120 hours are shown in FIG. 8. It can be observed that aliphatic alcohols did not have an effect on the glucose yield of the enzymatic hydrolysis. The likely explanation for this is that they are simply too weakly nucleophilic and thus unable to avoid repolymerization reactions (i.e. the carbocations were mostly attacked by other lignin structures and not by the alcohol). The conclusion from these results is that alcohol scavengers have to be activated more towards electrophilic substitution to have an effect, aliphatic alcohols are not reactive enough.

(25) One option therefore is to make use of aromatic groups, since an aromatic ring with its high electron density can enhance the affinity to the positively charged carbocations. The results with aromatic alcohols show interesting relationships between the structure of the molecules and their reactivity. Phenol shows no effect, probably also because it is not reactive enough and cannot prevent lignin repolymerization to a large enough extent. This is interesting with regard to the fact, that in an autohydrolysis-delignification process phenol did indeed have a significant positive effect on lignin extraction (Lora & Wayman, 1980). This also demonstrates the fundamental difference in the working principle of an extraction process to the biochemical degradation process presented in this document.

(26) The addition of another OH group to the phenol molecule would be expected to increase the overall reactivity of the molecule, but not all positions are equally reactive because the hydroxyl groups direct substitutions towards ortho and para positions relative to them. This is shown in FIG. 9. It is likely that only the free positions of resorcinol, which receive the positive mesomeric effect from both OH groups and not just from one (as in catechol and hydroquinone), are reactive enough to compete with lignin structures in attacking the formed carbocations. This explains why practically no effect was observed from phenol, catechol and hydroquinone.

(27) However, because resorcinol has three reactive positions, it can undergo several substitutions and promotes the crossing of lignin molecules. This results in more condensed lignin structures and explains the significant worsening of the yield during the enzymatic hydrolysis. This explanation is further supported by results obtained with phloroglucinol, which showed an even larger decrease in the yield of the hydrolysis than resorcinol. Phloroglucinol has a similar structure to resorcinol (OH groups in meta position to each other) but its three free positions are even more reactive as they are activated by three OH groups. This results in more crossing reactions and in even more condensed structures, explaining the dramatic worsening of the glucose yield in the enzymatic hydrolysis.

(28) In summary, it is necessary that the added substances be highly nucleophilic in order to be able to participate in reactions with carbocations formed in the lignin structure. Aliphatic alcohols and monocyclic aromatic alcohols with ring positions that are activated just by a single OH group (e.g. phenol, catechol) are not reactive enough. Aromatic alcohols with ring positions activated by two or more OH groups (e.g. resorcinol, phloroglucinol) do indeed react but act as crossing agents, worsening the glucose yield of the enzymatic hydrolysis.

(29) In this line of thought, a resorcinol- or phloroglucinol-derived molecule with just a single reactive site should prove as a very effective scavenger. When methylating two of the three free ring positions in phloroglucinol, dimethylphloroglucinol is obtained which has just a single ring position available for the reaction with the lignin carbocation. Indeed, dimethylphloroglucinol proved as a very effective additive. The cellulose conversion was increased to 92.1%, meaning dimethylphloroglucinol is even a more effective additive than the known scavenger 2-naphtol (compare FIG. 8). No compound has been known so far which can suppress lignin condensation in autohydrolysis as effective or even more than 2-naphtol.

(30) This new class of scavengers is based on the activation of an aromatic ring position by several (at least two) OH groups and the simultaneous sterical blocking/deactivation of excessive reactive sites. In distinction to this class of scavengers, 2-naphtol is composed of two aromatic rings and just one OH group. I particular, its scavenging effect is based on the preservation/loss of aromaticity of its aromatic ring system.

(31) This new class of scavengers offers possible alternatives to 2-naphthol, which are molecules based on resorcinol or phloroglucinol with occupying/blocking excessive reactive sites by covalent bonds (see FIG. 10).

IV. References

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