Lignin processing

Abstract

A method of depolymerizing a lignin includes oxidizing the lignin to provide an oxidized lignin wherein benzylic OH of -O-4 linkages have been converted to carbonyl. The oxidized lignin is depolymerized with a metal selected from the group consisting of zinc, magnesium, aluminum and titanium or mixtures thereof, in the presence of an ammonium salt or carbon dioxide. Also described are methods for manufacturing phenolic products from lignin and a method for the cleavage of a -O-4 linkage in a substrate.

Claims

1. A method of depolymerizing a lignin, the method comprising: oxidizing benzylic OH of -O-4 linkages of a lignin with molecular oxygen in the presence of a quinone and a source of nitrogen dioxide to provide an oxidized lignin wherein the benzylic OH of -O-4 linkages have been converted to carbonyl; and depolymerizing the oxidized lignin with a metal selected from the group consisting of zinc, magnesium, aluminum and titanium or mixtures thereof, in the presence of an ammonium salt or carbon dioxide, and producing at least one phenol according to any one of formulas I, II, or III: ##STR00023##

2. The method of claim 1, wherein the process is carried out as a one pot process, with both the oxidation and depolymerization steps carried out one after the other.

3. The method of claim 1, wherein the quinone is selected from the group consisting of: 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ), p-chloroanil, o-chloroanil, benzoquinone, 2-chloroanthroquinone, and 1,4,5,8-tetrachloroanthroquinone.

4. The method of claim 1, wherein the source of nitrogen dioxide is selected from the group consisting of: alkyl nitrites, nitrogen dioxide (NO.sub.2), nitrite salts, nitric acid/hydrochloric acid mixtures and nitric oxide (NO).

5. The method of claim 1, wherein the quinone is employed at an amount of from 2 to 30% by weight of the lignin.

6. The method of claim 5, wherein the quinone is employed at an amount of from 5 to 20% by weight of the lignin.

7. The method of claim 1, wherein the oxidizing step is carried out in an alcoholic solvent or mixture comprising an alcoholic solvent.

8. The method of claim 7, wherein the oxidizing step is carried out in the presence of a co-solvent selected from the group consisting of: alkyl ethers, alkyl nitriles, carboxylic acids and aromatic solvents.

9. The method of claim 7, wherein the oxidizing step is carried out in a solvent system comprising 2-methoxyethanol, a mixture of 2-methoxyethanol and 1,2-dimethoxyethane, or a mixture of 2-methoxyethanol, 1,2-dimethoxyethane and water.

10. The method of claim 1, wherein the oxidizing step is carried out in a solvent selected from the group consisting of: water, methanol, ethanol, glycerol, glycol ethers and mixtures thereof.

11. The method of claim 1, wherein the oxidizing step is carried out at a temperature of from 20 C. to 130 C.

12. The method of claim 1, wherein an ammonium salt is employed in the depolymerization of the oxidized lignin.

13. The method of claim 12, wherein the ammonium salt is selected from the group consisting of ammonium chloride, ammonium formate, ammonium acetate, ammonium sulphate and ammonium bisulphate.

14. The method of claim 1, wherein zinc is employed as the metal in the depolymerization of the oxidized lignin.

15. The method of claim 1, wherein in the depolymerization of the oxidized lignin is carried out in a solvent comprising water and an alcohol.

16. The method of claim 15, wherein in the depolymerization of the oxidized lignin is carried out in a solvent comprising water and glycol ether.

17. The method of claim 16, wherein in the depolymerization of the oxidized lignin is carried out in a solvent comprising water and 2-methoxyethanol.

18. The method of claim 17, wherein the depolymerization of the oxidized lignin is carried out in a water and 2-methoxyethanol mixture in a ratio of from 9:1 to 7:3 of 2-methoxyethanol:water, by volume.

19. The method of claim 15, wherein the depolymerization of the oxidized lignin is carried out in the presence of a co-solvent selected from the group consisting of: alkyl ethers, alkyl nitriles, carboxylic acids and aromatic solvents.

20. The method of claim 1, wherein in the depolymerization of the oxidized lignin is carried out at from 20 C. to 120 C.

21. A method of depolymerizing an oxidized lignin, the method comprising: reacting an oxidized lignin, wherein benzylic OH of -O-4 linkages of the oxidized lignin have been converted to carbonyl, with a metal selected form the group consisting of zinc, magnesium, aluminum and titanium or mixtures thereof, in the presence of an ammonium salt or carbon dioxide; depolymerizing the oxidized lignin; and producing at least one phenol according to any one of formulas I, II, or III: ##STR00024##

22. A method of manufacture of a phenolic product of general formula Z: ##STR00025## wherein R.sup.1 and R.sup.2 are, independently for each occurrence, H or OMe and R.sup.3 is H or OH; wherein the method comprises oxidizing a lignin to an oxidized lignin wherein benzylic OH of the -O-4 linkages have been converted to carbonyl; and depolymerizing the oxidized lignin with a metal selected from the group consisting of zinc, magnesium, aluminum, titanium and mixtures thereof, in the presence of an ammonium salt or carbon dioxide.

23. The method of claim 22, wherein the phenolic product of formula Z is at least one of formulas I, II, or III: ##STR00026##

24. A method for the cleavage of a -O-4 linkage in a substrate, the method comprising: oxidizing a -O-4 linkage of the substrate to provide an oxidized product wherein benzylic OH of the -O-4 linkage is converted to carbonyl; and depolymerizing the oxidized product with a metal selected from the group consisting of zinc, magnesium, aluminum, titanium and mixtures thereof, in the presence of an ammonium salt or carbon dioxide to produce at least one phenol according to any one of formulas I, II, or III: ##STR00027##

25. A method of producing at least one phenol according to any one of formulas I, II, or III: ##STR00028## the method comprising: oxidatively converting benzylic OH of -O-4 linkages of a lignin to carbonyl with molecular oxygen in the presence of a quinone and a source of nitrogen dioxide to provide an oxidized lignin; and reacting the oxidized lignin with a metal selected from the group consisting of zinc, magnesium, aluminum and titanium or mixtures thereof, in the presence of an ammonium salt or carbon dioxide; and cleaving a phenolic ether linkage adjacent the carbonyl.

Description

DESCRIPTION OF SOME PREFERRED EMBODIMENTS AND EXPERIMENTAL RESULTS MODEL COMPOUND STUDIES

(1) Methoxy substituted -O-4 linked diaryl compounds (1a to 1d) and veratryl alcohol (1e) were subjected to an oxidation procedure using DDQ, .sup.tBuONO and O.sub.2 as the oxidising system and 2-methoxyethanol as solvent. Oxidised products 2a to 2e were obtained in high yields as indicated beside each entry in Table 1 below, which highlights the selectivity and reactivity of the catalytic oxidation conditions employed.

(2) TABLE-US-00001 TABLE 1 0
Model Polymer Studies

(3) A polymer incorporating equal proportions of both S and G units was prepared as a model for a -O-4 rich hardwood lignin and the results of the oxidation experiments assessed using semi-quantitative 2D-HSQC NMR experiments.

(4) The polymer was prepared according to the following reaction (Scheme 3)

(5) ##STR00017##

(6) The polymer was initially treated under identical conditions to those used for the monomer studies (Table 2 below, Entry 1). The outcome of the reaction indicated that whilst the catalytic oxidation still proceeded, the conversion was lower than seen in the monomer series where close to complete conversion was achieved. As a result further optimisation of the reaction conditions was sought. Increasing the DDQ loading to 10 mol % improved conversions significantly (Entry 2). It was additionally found that the addition of 1,2-dimethoxyethane as a non-protic co-solvent markedly improved conversions (Entries 3 and 4). At 10 mol % DDQ and 10 mol % .sup.tBuONO oxidation reached 91%. However, to achieve almost complete oxidation the loading of DDQ and .sup.tBuONO was increased to 20 mol % (Entry 6). The beneficial effect of the co-solvent in this reaction may in part, be explained by an increase in the effective lifetime of DDQ under these conditions. It is interesting to note that G units appear significantly easier to oxidise than the S units. This observation is consistent with the reaction proceeding through the formation of a benzyl cation intermediate which is resonance stabilised by the para-methoxy substituent but destabilised by the meta-methoxy substituents through inductive electron withdrawal. This means the G unit, which is substituted with only one meta-methoxy group, is easier to oxidise than the S unit bearing two meta-methoxy groups.

(7) TABLE-US-00002 TABLE 2 DDQ tBuONO Solvent Conversion (%)[b] Entry (mol %) (mol %) System[a] Total G S 1 5 10 A 50 64 35 2 10 10 A 74 83 64 3 5 10 B 69 81 57 4 10 10 B 91 100 82 5 15 15 B 97 100 94 6 20 20 B 99 100 98 [a]A = 2-methoxyethanol, B = 2-methoxyethanol/1,2-dimethoxyethane (2:3) [b]Conversions were determined by integration of the aromatic cross peaks characteristic of oxidised and unoxidised structures in the 2D HSQC NMR. Error analysis indicated the standard deviation for these measurements was less than <0.05.
Lignin Experiments

(8) In experiments using actual lignin as a substrate described below, a typical dioxasolv process was used to extract lignin from Birch sawdust (Betula pendula). Birch sawdust is heated to reflux for 1 hour in a 8:2 mixture of dioxane and 2M aq. HCl using 1:8 mass:volume ratio. The liquor containing the lignin is then separated by filtration, partially concentrated under reduced pressure and precipitated in water. The precipitated lignin is then collected by filtration, dissolved in dioxane/water 9:1 and purified by precipitation with diethyl ether. The isolated lignin was syringyl rich and contained a high proportion of -O-4 linkages, smaller amounts of - (resinol) and only just detectable amounts of the -5 linkage (Scheme 1). Oxidation under catalytic DDQ conditions proceeded smoothly to give an oxidised lignin in which the appearance of the desired alpha ketone -O-4 structure could be readily identified in the 2D HSQC NMR spectrum. Concurrent to the appearance of the cross peaks assigned to the alpha ketone structure, complete disappearance of the cross peaks assigned to the unoxidised -O-4 structure was observed. The Integration relative to the sum of the aromatic region suggested that the selectivity seen in model studies was maintained in lignin itself as the relative volume integrals of the -O-4 cross peaks before and after oxidation are almost identical (22.0 protons and 23.4 protons per 100 aromatic protons respectively).

(9) General Procedure for Lignin Oxidations:

(10) To a solution of lignin in 2-methoxyethanol (14 mL/g) or 2-methoxyethanol/1,2-dimethoxyethane (DME) (2:3, 14 mL/g) was added DDQ followed by .sup.tBuONO. The reaction mixture was placed under an O.sub.2 atmosphere (balloon) and stirred at 80 C. for 14 hrs. The oxidised lignin was isolated by precipitation in 10 volumes of Et.sub.2O and filtering, alternatively the lignin solution was used in the next step without any further processing.

(11) Results of experiments are given in the table below.

(12) TABLE-US-00003 Degree of DDQ .sup.tBuONO oxidation Entry (wt %) (mol %) Solvent (%).sup.[a] 1 5 10 2-methoxyethanol/DME 46 2 10 10 2-methoxyethanol/DME 77 3 10 20 2-methoxyethanol/DME 78 4 10 20 2-methoxyethanol 66 .sup.[a]as determined by the relative integration of S.sub.2,6 and S.sub.2,6 aromatic cross peaks in the NMR spectrum corresponding to the syringyl rings before and after oxidation. The regions corresponding to the syringyl units in lignin before and after oxidation are well resolved and so this comparison is used as a measure of oxidation.
Zinc Mediated Depolymerisation of Lignin:
From Isolated Oxidised Lignin:

(13) To a solution of lignin (600 mg) in 2-methoxyethanol (8.4 mL) was added water (2.1 mL) followed by NH.sub.4Cl (740 mg) and zinc dust (900 mg). The reaction mixture was heated at 80 C. for 1 hour, cooled and filtered to remove excess zinc. The reaction mixture was then added to water (30 mL), acidified to pH 1 by the addition of 1M HCl which causes the lignin to flocculate and then the mixture was filtered. The residual lignin was washed with EtOAc and the aqueous filtrate extracted with EtOAc (520 mL). The organic extracts and washings were combined, washed with sat. NaHCO.sub.3, brine, dried (MgSO.sub.4) and concentrated in vacuo. The crude extract was purified by column chromatography petroleum ether:ethyl acetate (0 to 55%) to yield 3 products. II (3.5 mg, 0.58 wt %), I (29 mg, 5 wt %) and III (3.4 mg, 5.7 wt %).

(14) One Pot Procedure:

(15) Lignin (2.40 g) was treated according to the general procedure for catalytic oxidation in 2-methoxyethanol/1,2-dimethoxyethane. After the reaction time water (7 mL) was added followed by NH.sub.4Cl (3.0 g) and zinc dust (3.60 g). The mixture was then heated at 80 C. for 1 hr, cooled and filtered to remove excess zinc. The reaction mixture was then added to water (100 mL), acidified to pH 1 by the addition of 1M HCl which causes the lignin to flocculate and then the mixture was filtered. The residual lignin was washed with EtOAc and the aqueous filtrate extracted with EtOAc (550 mL). The organic extracts and washings were combined, washed with sat. NaHCO.sub.3, brine, dried (over MgSO.sub.4) and concentrated in vacuo. The crude extract was purified by column chromatography on silica with petroleum ether:ethyl acetate (0 to 55%) to yield 3 products. 11 (11 mg, 0.46 wt %), 1 (110 mg, 4.6 wt %) and III (12 mg, 0.50 wt %).

(16) Isolated Products:

(17) ##STR00020##

3-Hydroxy-1-(4-hydroxy-3,5-dimethoxyphenyl)propan-1-one (I)

(18) White solid. M.p. 109-110 C. Spectral data are consistent with those reported in the literature.

(19) .sup.1H NMR (400 MHz, CDCl.sub.3) =7.25 (s, 2H), 6.08 (s, 1H), 4.03 (t, J=5.4, 2H), 3.95 (s, 6H), 3.19 (t, J=5.4, 2H), 2.74 (s, 1H).

(20) .sup.13C NMR (101 MHz, CDCl.sub.3) 199.00, 147.0, 140.3, 128.4, 105.6, 58.4, 56.6 (2C), 40.0.

(21) ##STR00021##

3-Hydroxy-1-(4-hydroxy-3-methoxyphenyl)propan-1-one (II)

(22) Pale yellow amorphous solid. Spectral data are consistent with those reported in the literature.

(23) .sup.1H NMR (500 MHz, CDCl.sub.3) =7.587.50 (m, 2H), 6.95 (d, J=8.1, 1H), 6.17 (s, 1H), 4.02 (t, J=5.3, 2H), 3.96 (s, 3H), 3.19 (t, J=5.3, 2H), 2.78 (s, 1H).

(24) .sup.13C NMR (126 MHz, CDCl.sub.3) =199.2, 150.9, 146.8, 129.8, 123.8, 114.1, 109.7, 58.5, 56.2, 39.9.

(25) ##STR00022##

1-(4-Hydroxy-3,5-dimethoxyphenyl)propan-1-one (III)

(26) Colourless solid. M.p. 102-104 C. (lit. 109-111 C.). Spectral data are consistent with those reported in the literature.

(27) .sup.1H NMR (400 MHz, CDCl.sub.3) =7.27 (s, 2H), 5.92 (s, 1H), 3.96 (s, 6H), 2.97 (q, J=7.3, 2H), 1.23 (t, J=7.3, 3H).

(28) .sup.13C NMR (101 MHz, CDCl.sub.3) =199.4, 146.9, 139.6, 128.7, 105.5 (2C), 56.6 (2C), 31.5, 8.7.

REFERENCES

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