Oxidative Degradation of Dimeric Lignin Model Compounds - ACS

Chapter 11

Oxidative Degradation of Dimeric Lignin Model Compounds 1

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Downloaded by STANFORD UNIV GREEN LIBR on September 17, 2012 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch011

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Carmen Canevali , Franca Morazzoni , Marco Orlandi , Bruno Rindone , Roberto Scotti , Jussi Sipila , and Gosta Brunow 1

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Dipartimento di Scienza dei Materiali, Universitàdi Milano-Bicocca, Via R.Cozzi 53, 20125 Milano, Italy Dipartimento di Scienze dell' Ambiente e del Territorio, Università di Milano-Bicocca, Piazza dalla Scienza 1, 20126 Milano, Italy Laboratory of Organic Chemistry, P.O. Box 55, FIN-00014, University of Helsinki, Helsinki, Finland 2

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The oxidation of dimeric lignin model compounds (arylglycerol - β - aryl ethers and phenylcoumarans) by molecular oxygen, catalyzed by [N, N' - bis(salicylidene) ethane - 1, 2 - diaminato] cobalt(II), [Co(salen)], was studied in order to find novel processes for the production of chemicals by degradation of polyphenols contained in waste watersfromthe pulp and paper industry. All the studied compounds reacted at room temperature in chloroform with high conversion already after 30 minutes; after 48 h the reaction was essentially complete and interesting low molecular weight compounds were isolated with good yields. An EPR investigation of the reaction mixtures indicates the presence of a phenoxy cobalt radical, [Co (salen)(RO )(RO )], as a reactive intermediate for ROH as phenolic substrate. The electron donor properties of the phenolic substrate and the stability of the phenoxy radical control the yields of the oxidation products. III

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© 2001 American Chemical Society In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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Introduction The use of renewable biomass as raw material for the production of chemicals is an important component of international energy policy. One of the most underused carbon source materials in the biosphere is lignin, present in the woody structure of higher plants, typically 30% in dry weight (/). It is a structurally highly intricate aromatic polymer of oxygenated phenylpropane units (2, 3, 4, 5, 6). Several interunit carbon-carbon and carbon-oxygen bonds are present in its structure, the relative abundance of these interunit linkages varying for the different types of wood. Many studies are focused on their oxidative degradation to give useful low molecular weight aromatic compounds such as vanillin. These processes are commercially important and may be performed by oxidation with nitroaromatics (7), with air in alkaline solution (8), with ozone (9) and by electrochemical means (10). Other degradation systems use dioxygen as the oxidant and cobalt-complexes as catalysts. Drago et al. showed that dioxygen was able to oxidize, in very good yield, isoeugenol to vanillin, using [l5is(salicylidene-y-iminopropyl)rnethylamine]cobalt(II) [Co(SMDPT)], as catalyst (//); recently Bozell et al. reported the oxidation of para-substituted phenolic compounds to benzoquinones using [N,N'-bis(salicylidene)-ethane-l, 2diaminato]cobalt(II), [Co(salen)], as catalyst (12,13). We have recently reported that simple model compounds of the polyphenols contained in the waste waters from the paper industry and agroindustrial activity can be degraded by oxidation with dioxygen, using [Co(salen)] as catalyst (14). The mechanism of the reaction was elucidated by studying the degradation of several phenolic propenoids with different electron donor properties and by monitoring the process by electron paramagnetic resonance (EPR) spectroscopy (15). Among the investigated compounds, Emethyl ferulate (compound 1 in Scheme 1) has higher electron donor capacity than methyl E-4-hydroxycinnamate (compound 2 in Scheme 1). Structures like I and 2 are very simple model compounds for lignin, containing only one aromatic ring with an extended double bond-ester conjugation (16). However, since only a very small amount of such structures are present in lignin waste material, we decided to test model compounds representing more abundant structural units (17) as substrates of oxidation. Arylglycerol-p-aryl ethers are the most abundant structural units of lignin (17), phenylcoumarans are also abundant and contain an intermonomeric carbon-carbon bond (C -C ) (18). The model compounds with two aromatic rings are indicated as "dimeric" lignin model compounds, to distinguish them from phenols containing one ring, indicated as "monomeric" model compounds. Thus, the oxidation of arylglycerol-P-aryi ethers (compounds 3, 4, 5 in Scheme 2) and phenylcoumarans (compound 6 in Scheme 3) by molecular oxygen, catalyzed by [Co(salen)], was studied with the aim of obtaining further insight into the oxidation mechanism of lignin model compounds. ?

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In Oxidative Delignification Chemistry; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

199 COOCH

3

0 (1 MPa) Co(salen) 2

c

o

C

H

yu on 2

C

H

0

3

25 °C


1 R - H; R' = OCH 2 R = R' = H

3

7 R = H; R' = OCH 9 R = R' = H

3

8 R = H; R' = OCH 10 R = R' = H

Scheme 1: Monomelic Compounds

3 R = OCH ; R' = OH; R" = H 3

4 R = R" = H; R' = OH

11 R = OCH ; R' *= H 12 13 R = R' = H 3

5 R = R" = H; R' = OCH3 Scheme 2: Dimeric Compounds OH

10 Scheme 3: Dimeric Compounds


3

200 The reactivity was measured as conversion (disappearance of starting material) and the product distribution was determined after methylation using GC-MS. The reactions were monitored by EPR spectroscopy.

Experimental


Reagents The complex [Co(salen)] (99%) and the 1, 4 - benzoquinone, used as standard, werefromAldrich. Chloroform (Fluka) was used as received. The dimeric substrates 3 - (4 - hydroxy - 3 - methoxyphenyl) - 2 - (2 methoxyphenoxy) - 1 , 3 - propanediol 3; 3 - (4 - hydroxyphenyl) - 2 - (2 methoxyphenoxy) - 1 , 3 -propanediol 4; 3 - (4 - methoxyphenyl) - 2 - (2 methoxyphenoxy) - 1, 3 - propanediol 5 were prepared according to the method of Sipila and Syrjanen (19). Ε - methyl - [(2RS - 3SR) - 2, 3 dihydro - 2 - (4 - hydroxy - 3 - methoxyphenyl) - 7 - methoxy - 3 methoxycarbonyl - 1 - benzofuran - 5 - yl] propenoate 6 was prepared according to the method of Bolzacchini et al. (20).

Apparatus NMR spectra were recorded on a Bruker A M 300 instrument; IR spectra were recorded on a FT-1R Jasco instrument; mass spectra were performed by the Direct injection System mode with positive Electron Impact using a VG 7070 EQ Instrument. EPR spectra were recorded on a Bruker EMX EPR spectrometer working at the X-band frequency, equipped with a variable temperature BVT 2000 unit (Bruker). HPLC analysis were performed on WATERS 600 E. The detector was a HP 1040 Diode Array Detector. The GCMSD analysis were performed using a HP 5890 gas chromatograph, interfaced with a quadrupol detector (HP 5970), operating in Electron Impact mode 70 eV.

Oxidations 3

A solution (40 cm ) of substrate (0.06 M) and [Co(salen)] (0.006 M) was placed in a glass vessel (100 cm ) and inserted into an autoclave (250 cm ). The autoclave was charged with dioxygen (1 MPa) and left at 25 °C for the required time. The solvent was then evaporated under reduced pressure at room temperature and the residue was resolved on a silica gel column with a gradient of ethyl acetate-hexane (3:7 to 7:3) as eluents. 3


3

201 The quinones 11, 13 (Scheme 2) were identified by comparison with authentical samples {21). The conjugated methylenic aldehyde 12 (Scheme 2), the benzofuran structure 15 and the open ring product 14 (Scheme 3) were identified by the following spectroscopic data, obtained by mass spectrometry, ^ - N M R , C NMR, IR and UV spectroscopies. Compound 12: m/z = 178(M ), 149,108,77; ^ - N M R (CDCi ): δ 3.79 (s, 3 H), 5.05 (d, 1 H, J = 2 Hz), 5.25 (d, 1 H, J = 2 Hz), 6.85-7.25 (m, 4 H, aromatics), 9.45 (s, 1 H); C-NMR (CDC1 ): δ 56.4(CH ), 110.1(CH ) 114.8(CH), 118.3(CH), 121.6(CH), 123.7(CH), 143.0(C), 155.0(C), 166.9 (C), 189.5(CH); IR (nujol) = 1703, 1617 cm ; m.p. = 160 °C. Compound 14 (isolated after methylation): m/z 472(M ), 457, 422, 428; *H-NMR (CDC1 ): δ 3.70 (s, 3H), 3.75 (s, 3H), 3.78 (s, 3H), 3.85 (s, 3H), 4.00 (s, 3H), 4.06 (s, 3H), 4.10 (s, 3H), 6.80-7.20 (m, 5H), 6.31 (d, 1H, J=16Hz), 7.75 (d, 1H, J=16Hz); C-NMR (CDC1 ) δ: 51.8(CH ), 51.3(CH ), 52.3(CH ), 56.8(CH ), 56.8(CH ), 56.8(CH ), 56.8(CH ), 93.0(CH), 112.8(CH), 112.8(CH), 115.8(CH), 117.1(CH), 119.3(CH), 119.4(CH), 121.4(C), 127.8(CH), 127.9 (C), 141.3(C), 144.6(CH), 146.8(C), 146.9(C), 148.3(C), 158.9.4(C), 166.3(C), 166.4(C); IR (nujol): 1720, 1460, 1149 cm" ; UV (CH C1 ): 228, 300,400 nm. Compound 15: m/z = 290(M ), 359, 277,199; *H-NMR (CDC1 ): δ 3.83 (s, 3 H), 3.83 (s, 3 H), 3.92 (s, 3 H), 6.45 (d, 1H, J=16Hz), 7,75 (d, 1H, J=16Hz), 7.05 (s, 1H), 7.85 (s, 1H), 8.29 (s, 1H); C-NMR (CDC1 ): δ 52.3(CH ), 52.3(CH ), 56.8(CH ), 115.8(CH), 116.1(C), 118.3(CH), 118.4(CH), 127.2 (C), 130.6(C), 141.3(C), 145.6(CH), 148.3(C), 154.4(CH), 166.3(C), 166.4(C); IR (nujol) 1724, 1644 cm" ; UV (CH C1 ): 230, 265,289 nm. The conversion (measured as disappearance of starting material) at 30 min, 1 h, 5 h and 48 h was obtained by high performance liquid chromatographydiode array detector (HPLC-DAD) analysis, using calibration curves with biphenyl as internal standard . The product distribution was determined 48 h from the onset of the reaction by dissolving the residue in a mixture of acetone (20 cm ) and dimethyl sulphate (0.3 cm ), and adding K 2 C O 3 (0.435 g). After refluxing for 2 h, the solid was filtered out, the solvent evaporated under reduced pressure; the residue was dissolved in C H 2 C I 2 and analyzed by gas liquid chromatography-mass spectrometry (GLC-MS) using biphenyl as internal standard. 13

+

3

13


3

3

2 5

-1

+

3

13

3

3

3

3

3

3

3

3

1

2

2

+

3

l3

3

3

3

3

1

2

2

3

3


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EPR measurements Deoxygenated solutions were prepared by dissolving [Co(salen)] (0.006 M) in the solvent previously outgassed with a nitrogen stream; the substrate (0.06 M) was then added. Oxygenation with 0.1 MPa dioxygen pressure was performed by bubbling dioxygen for 15 minutes into the deoxygenated solutions. In the reaction conditions (1 MPa dioxygen pressure) aliquots of the solution containing [Co(salen)] (0.006 M) and the substrate (0.06 M) were taken at the following reaction times (min): 5, 10, 15, 20, 30, 45, 60, 120, 180, and immediately cooled in liquid nitrogen in order to slow down the reaction. The EPR spectra were recorded at - 150 °C. The g values were measured by standardization with diphenylpicrylhydrazyl (DPPH) and the relative amounts of the paramagnetic species were obtained by double integration of the resonance line areas.

Results Oxidations All the reactions were carried out in chloroform because of the high conversion found in this solvent for the previously reported cobalt-catalyzed oxidation of monomeric compounds (/5). Reactions were followed for 48 h. After this time all the substrates had almost completely reacted. The results from previous oxidations of cinnamatea (15) are shown in Table I. Tables II and III show the data from the oxidations of the dimeric compounds described in the present paper.

Table I - Conversion % at different reaction times and product distribution % after 48 h for the oxidation in chloroform of the monomeric compounds shown in Scheme 1. Compound

1(15) 2(15)

Product distribution (%) after 48 h 30 min 1 h 5 h 48 h Benzoic acid Benzaldehyde derivative derivative 8 39 1 61 60±5 84 ±6 88±4 99±1 10 43 9 17 1±0.5 6±3 37±4 60±4 Conversion (%)


203 Table II - Conversion % at different reaction times and product distribution % after 48 h for the oxidation in chloroform of the β-Ο-4 type dimeric compounds shown in Scheme 2.


30 min 3 4 5

Product distribution (%) after 48 h Aldehyde Quinone 48 h derivative 12 derivative 81 11 16 99±1 20 13 2 98±1 Not detected 98±1 Not detected

Conversion (%)

Compound

90±6 70±5 50±10

Ih

5h

98±5 99±1 83±4 96±3 66±8 70±7

Table III - Conversion % at different reaction times and product distribution % after 48 h for the oxidation in chloroform of the β-5 type dimeric compound shown in Scheme 3. Conversion (%)

Compound

30 min 1 h 6

99±1

5 h 48 h

99±1 99±1

99±1

Product distribution (%) after 48 h Fragmentation Oxygenation products product 7 10 14 55 15 15 8 5

All the dimeric compounds showed high conversion within 30 minutes of reaction onset. The dimeric compounds 3, 4 showed higher conversion than the monomeric phenols 1, 2 which have the same substituents ortho to the hydroxy group. Compounds 3, 4 gave quinones 11, 13 and the conjugated aldehyde 12 (Scheme 1). Aldehyde 12 has been already isolated as the product of chemical degradation of the β-aryl ether type compound with oxygen in alkaline media and microbiological transformation by Mn Peroxidase (22). Compound 5, in which a phenolic hydroxyl group lacks, showed the lowest conversion values of the examined dimeric model compounds. Neither quinone nor conjugated aldehyde 12 derivatives were observed; nevertheless, there was the formation of not characterized polymerization products, together with a small amount of vanillin. From the oxidation of the phenylcoumaran 6, four main compounds were isolated and identified: 7, 8,14,15 (Scheme 3).


204 EPR investigation The spectra of [Co(salen)] in a chloroform deaerated solution and under dioxygen (0.1 MPa, 1 MPa) atmosphere were described in a previous paper


US). The reactions of the different substrates with dioxygen (1 MPa) in the presence of [Co(salen)] were monitored by EPR spectroscopy, recording the spectra at - 150 °C on aliquots of the reaction solutions taken at different reaction times and immediately cooled in liquid nitrogen (see experimental). The zero-time solutions were measured under nitrogen atmosphere. Tables IV and V summarize the EPR data of the compounds described in the present paper. For comparison purposes, the spectroscopic behavior of the previously studied Ε-methyl ferulate 1 and methyl E-4-hydroxycinnamate 2 (15) is also reported. [Co(salen)] - 3-(4-hydroxy-3-methoxyphenyl)-2~(2-methoxyphenoxy)-l,3' propanediol (3). The spectrum of the zero-time solution containing [Co(salen)] and the β-Ο4 type compound 3, under nitrogen atmosphere, is the same as that of [Co(salen)]; no interaction with the substrate was observed. The same result was obtained with the other dimeric substrates 4, 5, 6. After contact with O 2 at 0.1 MPa pressure for 15 minutes, the lines of the phenoxy cobalt radical [Co^ (salen)(RO")(RO )], where ROH is the substrate, I

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Table IV - EPR data for the oxidation in chloroform of the monomeric compounds reported in Scheme 1. Compound

Atmosphere

1(75)

g

A(G)

= 3.250 g = 1.912 As under N 2.0015

Aj =93.60 A3-30 As under N 18.70

g l

3

O 0.1 MPa 2

0

2(15)

0

2

2

1 MPa

1 MPa

g l

=2.087

2

Aj =20.70

2.0015

18.70

2.000

16.20

Paramagnetic Species Planar [Co(salen)] 2

Planar [Co(salen)] Phenoxy cobalt radical Superoxocobalt derivative Phenoxy cobalt radical Phenoxy cobalt radical



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Table V - EPR data for the oxidation in chloroform of the dimeric compounds reported in Schemes 2 and 3. g

A(G)

= 3.250 g = 1.912 As under N 2.002

Aj =93.60 A ~30 As under N 17.78

Compound Atmosphere 3

N

2

g l

3

O 0.1 MPa 2

4

Paramagnetic Species Planar [Co(salen)]

3

2

0

2

1 MPa

2.002

17.78

0

2

1 MPa

Not read

Not read

2.002

17.58

5

0

2

1 MPa

Not read

Not read

6

0

2

1 MPa

Not read

Not read

2.003

18.30

2

Planar [Co(salen)] Phenoxy cobalt radical Phenoxy cobalt radical Superoxocobalt derivative Phenoxy cobalt radical Superoxocobalt derivative Superoxocobalt derivative Phenoxy cobalt radical



206

ι—·—ι—ι—ι—ι—·—·—ι—ι—ι—ι—ι 3200

3250

3300

ι ι—ι—ι—ι—r—I—ι—r—i—ι—*—ι— [Co (salen)(ROH)(0 ")] [Co (salen)(ROH)(0 -)] + ROH -> [Co (salen)(ROH)(RO )] + H 0 " [Co (salen)(ROH)(RO )] + H 0 " ~> [Co (salen)(RO-)(RO )] + H 0 2

2

m

m

e

2

m

2

#

m

2

e

2

2

(A) (B) (C)

First, the aromatic hydroxyl of ROH and dioxygen co-ordinate with [Co(salen)] with the formation of the EPR active superoxocobalt derivative, [Co (salenXROHX02")l; this first step is favored by the electron-donor properties of the phenol (23). Then the resulting superoxocobalt derivative reacts with another phenol ligand, the 0 ~ abstracting a hydrogen atom from ROH; for this step the more basic the co-ordinated phenol, the greater the possibility that the superoxide abstracts a hydrogen atom from ROH (24). An EPR active phenoxy cobalt radical, [Co (salen)(RO")(RO*)], forms and its stability depends on the ability of the phenol to delocalize the unpaired electron. At the end the R O radical probably undergoes decomplexation from cobalt and is further attacked by 0 giving the final reaction products. The same type of paramagnetic species observed during the oxidation of monomeric cinnammates 1 and 2 were detected with the dimeric compounds 3, HI

2

iIi

e

2



209 4, 5 and 6: the superoxocobalt derivative was observed in the first reaction minutes of compounds 4 and 6 and was the only paramagnetic species at each reaction time of compound 5. Compound 3 showed only phenoxy cobalt radical at each time of reaction. For the formation of the phenoxy cobalt radical, it is necessary that the aromatic hydroxyl co-ordinates with [Co(salen)]; in fact the radical does not form in the case of compound 5. Step Β (the formation of the phenoxy radical) depends on the electron donor character of the ROH ligand (24). In fact in the first minutes of reaction the amount of the phenoxy cobalt radical increased with the electron donor character of the ligands (4 « 3 ~ 6), a trend expected from the substituents on the phenolic ring. The same trend is present in the substrate conversion after 30 minutes from the onset of reaction. The dimeric compounds react faster than the corresponding resonance stabilized monomeric esters 1 and 2 because of their stronger nucleophilic character, in particular phenylcoumaran 6 is completely degraded after 30 minutes. Non phenolic model compounds are not degraded, but nevertheless they react to give principally polymerization products. The oxidative degradation of phenolic dimeric model compounds by molecular oxygen catalyzed by [Co(salen)] has given good results, the reaction is fast and interesting low molecular weight compounds such as quinones 11, 13, vanillin 8 and a-(2-metoxyphenoxy)-acrolein 12 were isolated with good yields. This oxidative system can be developed to degrade real waste lignin because the work up is easy, the amount of cobalt used is low and the cobalt can be recovered at the end of reaction.

References 1. Prince,R.C.; Stiefel, E.I.Tibs 1987, 12, 334. 2. Dordick, J. S.; Marletta, Μ. Α.; Klibanov, A. M. Proc. Natl.Acad.Sci. USA 1986, 83, 6255. 3. Hammel, H.; Kalyanaraman, B.; Kent Kirk, T. Proc.Natl.Acad.Sci. USA 1986, 83, 3707. 4. Swan, G. A. Fortschr. Chem.Org.Naturstoffe 1974, 31, 521. 5. Higuchi, T. Biosynthesis and biodegradation ofwood components, Academic Press, New York, 1985. 6. Wariishi, H.; Valli, K.; Gold, M. Biochemistry 1989, 28, 6017. 7. Chum, H. L.; Baaizer, M. M. The Electrochemistry of Biomass and Derived Material, ACS Monograph Washington, 1985. 8. Janson, J.; Fullerton, T. Holzforschung 1987, 41(6), 359-362. 9. Quesada, J.; Rubio, M.; Gomez, D. HRC-Jowmal of High Resolution Chromatography 1997, 20(10), 565-568.



210 10. Pardini, V. L.; Vargas, R. R.; Viertler, H.; Utley, J. H. P. Tetrahedron 1992, 48, 7221-7228. 11. Drago, R. S.; Corden, B. B.; Bames, C. W. JACS 1986, 108, 2453-2454. 12. Bozell, J. J.; Hames, B. R.; Dimmel, D.R.J.Org.Chem. 1995, 60, 2398-2404. 13. Elder, T.; Bozell, J. J. Hokforschung 1996, 50, 24-30. 14. Bolzacchini, E.; Brambilla, A. M.; Orlandi, M.; Rindone, B. Life Chemistry Report 1995,13,71; Bassoli, Α.; Brambilla, Α.; Bolzacchini, E.; Chioccara, F.; Morazzoni, F.; Orlandi, M.; Rindone, Β. (II) ACSSymp.Ser. 1996, 626, 92. 15. Bolzacchini, E.; Bocchio Chiavetto, L.; Canevali, C.; Morazzoni, F.; Orlandi, M.; Rindone, B. J. Mol. Catal. A: Chemical 1996, 112, 347; Bolzacchini, E.; Canevali, C.; Morazzoni, F.; Orlandi, M.; Rindone, B.; Scotti,R.J. Chem. Soc. Dalton Trans. 1997, 4695. 16. Yan Ping Sun; Yates, B.; Abbot,J.;Chen, C. L. Holzforschung 1996, 50, 226-232. 17. Adler, E. in Lignin Chemistry - Past, present andfuture. Wood Sci. Technol. 1977, 11, 169; KilpelainenI.,AnnalesAcademiaeScientiarumFennicae1994, 255. 18. Cui, F.; Wijesekera, Α.; Dolphin, D.; Farrel, R.; Skerer, P. Journal ofBiotechnology 1993,30,15-26. 19. Sipila J.; Syrjanen, K.Holzforschung1995,49,325. 20. Bolzacchini, E.; Meinardi, S.; Orlandi, M.; Rindone, Β.; Brunow, G.; Pietikainen, P.; Rummakko, P. Green Oxidation: horseradish peroxidase(HRP)-catalyzed regio and diastereoselective preparation of dilignols, P. Tundo, P.Anastas eds Green Chemistry Challenging Perspectives, 2000, pp. 21-33, Oxford University Press New York. 21. Capdevielle, P.; Maumy, M. Tetrahedron Lett. 1983, 5611-5614. 22. Gierer, J.; Imsgard, F.; Noren, I. Acta Chem.Scand.Β 1977, 561. 23. Jovanovic, S. V.; Tosic, M.; Simic, M.G.J.Phys.Chem.1991, 95, 10824. 24. Drago, R.S.Coord.Chem. Rev. 1992, 117, 185.


Oxidative Degradation of Dimeric Lignin Model Compounds - ACS

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