Oxidation of Lignin Using Aqueous Polyoxometalates in the Presence of Alcohols

DOI: 10.1002/cssc.200800050

Oxidation of Lignin Using Aqueous Polyoxometalates inthe Presence of AlcoholsTobias Voitl and Philipp Rudolf von Rohr*[a]

Introduction

Lignin is the second most abundant natural raw material aftercellulose, if mass is considered. In the pulp and paper industry,vast amounts of lignin are separated from the cellulose, mainlyby chemical pulping. The most prominent pulping process isKraft pulping, which makes up 95% of the world production oflignin.[1] The lignin processed in Kraft pulp mills (~50)106 tonnes per year) is mainly used as a fuel to cover internalenergy needs. In contrast to the thermal utilization of lignin,only a small amount of lignin (~1)106 tonnes per year) is iso-lated from spent liquors and used chemically.[2] Kraft lignin,however, can be precipitated by neutralizing alkaline blackliquor. Partial precipitation of lignin represents a cost-effectivemethod to increase the capacity of Kraft pulp mills, as carriedout in the LignoBoost process and demonstrated in a mini-plant that produces approximately 4000 tonnes of lignin annu-ally.[3] In this way, Kraft lignin becomes available on the marketand can be used to replace fossil resources. Hence, we consid-er Kraft lignin as a renewable raw material that can be convert-ed into low-molecular-weight compounds of industrial interest.

Kraft lignin is chemically distinct from natural lignin as aresult of the reactions that take place during pulping withsodium hydroxide and sodium sulfide. Typically, the ligningrown in softwood consists of units stemming from the poly-merization of about 94% coniferyl alcohol, 1% syringyl alcohol,and 5% coumaryl alcohol which are linked mainly through b-O-4 aryl ether (48%) bonds as shown for spruce lignin.[4]

During Kraft pulping of pines, up to 85% of the phenolic b-aryl ether bonds are cleaved.[5] Kraft lignin dissolved in blackliquor can be viewed as a highly heterogeneous mixture oflignin fragments. About 50% of Kraft lignin has a molecularweight of less than 3000 Da.[6] Furthermore, radical couplingreactions between low-molecular-weight phenols and poly-meric lignin lead to the formation of stable biphenyl struc-tures.[7] Thus, Kraft lignin precipitated from black liquor is a dis-

perse biopolymer with a high percentage of relatively stablecarbon–carbon bonds.

Two main objectives must be met in order to degrade Kraftlignin and thereby generate a mixture of low-molecular-weightcompounds. First, a thorough depolymerization of Kraft ligninrequires the cleavage of most of the interunit linkages, eventhose of the carbon–carbon type. This cleavage can be ach-ieved by oxidation using polyoxometalates (POMs) as shown inreactions with lignin dimer models.[8] Second, the repolymeri-zation of lignin fragments has to be prevented. The degrada-tion of lignin using POMs involves one-electron transfer, whichgenerates intermediate lignin radicals. Radical coupling ofthese lignin fragments can lead to polymerization, thus inhibit-ing an efficient degradation of lignin.[9] Therefore, in this studywe have considered the addition of radical scavengers thatcouple with the lignin fragments before repolymerizationoccurs.

Polyoxometalates have previously been used in the delignifi-cation of pulp, where they were employed to remove residuallignin from unbleached chemical pulp.[10] In these studies, itwas shown that a series of POMs are capable of selectively ap-plying oxygen to the breakdown of residual lignin and that itis possible to fully convert residual lignin into CO2 and H2O bywet oxidation.[11] In this context, self-buffering aqueous sys-tems of POMs were designed to keep the pH constant duringdelignification.[12] In general, most of the key properties, suchas redox potentials and solubilities in aqueous and organic

A novel approach has been developed in order to use Kraft ligninas a renewable resource for the production of chemicals. Theconcept is based on the use of polyoxometalates as reversible ox-idants and on the use of radical scavengers, which prevent ligninfragments from repolymerizing. The oxidation of Kraft lignin,which is a potential source of functionalized phenols, byH3PMo12O40 in water yields a relatively small amount of mono-meric species detected by GC-MS. The addition of methanol tothe reaction resulted in an increase in the yield of monomeric

products by a factor of up to 15. Vanillin and methyl vanillateare the main products obtained, in a maximum yield of 5 wt%based on dry Kraft lignin. Methanol plays a decisive role in theprevention of repolymerization by reducing lignin–lignin conden-sation reactions. Furthermore, it is proposed that methanol gen-erates small amounts of CCH3 and CH3OC radicals through theacid-catalyzed formation of dimethyl ether which couple withlignin fragments.

[a] T. Voitl, Prof. P. Rudolf von RohrInstitute of Process EngineeringETH Z3rich, 8092 Z3rich (Switzerland)Fax: (+41)44-632-1325E-mail : [email protected]

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.200800050.

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medium, can be tuned significantly by choosing precursors toobtain the appropriate POM.[13] Furthermore, POMs are highlystable at the temperatures and pH values used within thisstudy. The POM used in the present study, H3PMo12O40, is com-mercially available, and its redox potential is high enough tooxidize typical lignin subunits and low enough to be reoxi-dized by molecular oxygen.[14]

The introduction of radical scavengers to a running depoly-merization represents a way to gain control over the degrada-tion process. In general, every substance that can generatesmall radicals such as CCH3 or CH3OC was considered for its useas a coupling agent in our investigations. If a balance betweenfreshly generated radicals from bond cleavage in the depoly-merization of lignin and the provision of small radicals by aradical scavenger can be established, it should be possible tosuppress repolymerization of lignin by competitive couplingwith a radical scavenger. In a sense, such a concept has beenstudied before in the catalytic hydrotreatment of lignin at ele-vated temperatures (400–500 8C). In those studies, hydrogenderived from the reaction atmosphere or from hydrogen-donor solvents such as tetralin were employed to stabilize freeradical lignin fragments.[15] However, hydrogen-donor solventsand/or H2 atmosphere has shown to be of limited use for im-proving overall yields of phenols.[16] Also, phenol was em-ployed as a coupling agent in the supercritical oxidation oflignin resulting in a 99% yield of products that were soluble inTHF.[17] In this reaction, however, the phenol must be presentin excess (phenol/lignin=10:1–15:1) and is integrated into theproducts, making such a depolymerization process uneconomi-cal for recovering phenols.

Results and Discussion

Lignin Oxidation

The oxidation of Kraft ligninfrom spruce wood by aqueousH3PMo12O40 was studied undernitrogen and under oxygen.When the reaction is carried outunder an inert atmosphere, thePOM acts as a stoichiometric ox-idant. Thus, the oxidation oflignin is accompanied by reduc-tion of the polyoxoanion[PMo12O40]

3� so that a secondstep, reoxidation of the POM, isnecessary for its recycling. In re-actions conducted in the pres-ence of oxygen, the POM canundergo reduction and reoxida-tion in the same process stepand therefore acts as a catalyst.The two steps involved in theoxidative degradation of ligninmay be summarized by the re-actions shown in Equations (1)

and (2), where the number of protons (m) released from lignindoes not necessarily match the number of electrons (n) trans-ferred to the polyoxoanion and where lignin(ox) represents theproducts (polymeric, oligomeric, and monomeric) obtainedfrom oxidative cleavage of lignin.

ligninþ ½PMo12O40�3� ! ligninðoxÞ þ ½PMo12O40�ð3þnÞ� þmHþ

ð1Þ

½PMo12O40�ð3þnÞ� þ ðn=4ÞO2 þ nHþ ! ½PMo12O40�3� þ ðn=2ÞH2O

ð2Þ

Table 1 summarizes the amount of monomeric species re-covered from the oxidation of lignin using H3PMo12O40 as wellas from conventional alkaline oxidations in the presence ofCu2+ and Fe3+ . The oxidation of Kraft lignin (1 g) usingH3PMo12O40 under nitrogen generally resulted in the produc-tion of small amounts of monomeric products. The yield ofmonomeric products was as low as 0.11 wt% when the reac-tion was carried out in pure water. It was possible to recoverabout 40% more products by adding chloroform (30 mL;Table 1, entry 2) to the reaction mixture. This in situ extractionmay provide a means to remove monomeric products fromthe reaction mixture as soon as they are formed in the degra-dation and thereby prevent them from repolymerization. How-ever, a more positive effect on the yield can be seen whenmixtures of water and methanol or ethanol are used as sol-vents for the oxidation of lignin.

Figure 1 illustrates the effect of adding simple alcohols tothe reaction. The amount of vanillin, as well as all other mono-meric products recovered from the degradation, increased sig-nificantly when 40 vol% ethanol/water or 40 vol% methanol/water mixtures were used as solvents. Furthermore, ethyl vanil-late and methyl vanillate were produced as additional productsin the reactions conducted in ethanol/water and methanol/

Table 1. Amount of products obtained from the oxidation of lignin in aqueous solution (100 mL) at 170 8C for20 min.[18]

Entry CatalystACHTUNGTRENNUNG(amount ACHTUNGTRENNUNG[mmol])

Solvent GasACHTUNGTRENNUNG(amount [g])

LigninACHTUNGTRENNUNG(amount [g])

Products[mg]

Yield[%][a]

1 POM (5) H2O N2 (4.60) spruce (1) 1.05 0.112 POM (5) 100 mL H2O+30 mL CHCl3 N2 (4.60) spruce (1) 1.45 0.163 POM (5) 40 vol% EtOH/H2O N2 (4.60) spruce (1) 2.12 0.234 POM (5) 40 vol% MeOH/H2O N2 (4.60) spruce (1) 3.86 0.425 POM (5) H2O O2 (2.68) spruce (1) 1.84[b] 0.206 POM (5) 40 vol% MeOH/H2O O2 (2.68) spruce (1) 8.10 0.887 POM (5) 60 vol% MeOH/H2O O2 (2.68) spruce (1) 22.61 2.478 POM (5) 80 vol% MeOH/H2O O2 (2.68) spruce (1) 26.78[c] 2.929 POM (5) 80 vol% MeOH/H2O O2 (2.68) poplar[d] (1) 29.80[e] 3.25

10 CuSO4/FeCl3 (3.1/0.3) H2O 12 g NaOH O2 (6.94) spruce (10) 145 1.5811 CuSO4/FeCl3 (3.1/0.3) H2O 12 g NaOH O2 (6.94) agricultural[f] (10) 131 1.3812 CuSO4/FeCl3 (3.1/0.3) 40 vol% MeOH/H2O 12 g NaOH O2 (6.94) agricultural[f] (10) 154 1.6213 CuSO4/FeCl3 (3.1/0.3) 40 vol% EtOH/H2O 12 g NaOH O2 (6.94) agricultural[f] (10) 136 1.44

[a] Weight of products identified by GC-MS divided by the weight of dry lignin employed. [b] Vanillin: 1.54 mg;acetovanillone: 0.09 mg; further products : 0.21 mg. [c] Vanillin : 11.41 mg; acetovanillone: 0.66 mg; methyl va-nillate: 12.49 mg; further products : 2.23 mg. [d] Organosolv lignin. [e] Vanillin : 4.30 mg; acetovanillone:0.30 mg; methyl vanillate: 5.55 mg; syringaldehyde: 7.23 mg; acetosyringone: 0.96 mg; methyl syringate:9.45 mg; further products : 2.01 mg. [f] Granit lignin.

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Ph. Rudolf von Rohr and T. Voitl

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water, respectively. The yield of monomeric products increasedby a factor of 3.7 in the case of the methanol/water system.The formation of methyl vanillate and ethyl vanillate suggeststhat simple alcohols are incorporated into the lignin-derivedproducts. This observation led us to study the reactions of al-cohols in acidic medium with respect to their interaction withlignin and lignin-derived products.

In all experiments conducted under nitrogen atmosphere,the resulting aqueous mixtures had a dark-blue color which isa clear indication that the polyoxoanion is reduced to a greatextent.[19] Thus, the amount of oxidant (5 mmol POM) mightnot be sufficient to depolymerize most of the sample of lignin(1 g) leading to monomeric products with high yields. Owingto the high molecular weight of POMs, it does not seem to beeconomical to increase the amount of oxidant used, as theamount of POM (5 mmol) employed for the oxidation of lignin(1 g) already corresponds to a weight ratio of 9:1. Hence, thetwo-step approach in which the POM is used as an oxidant inthe first step under nitrogen and is regenerated in a secondstep by oxygen would require large quantities of the POM inthe overall process. Therefore, it seems more promising to in-vestigate and develop systems in which the POM can act as acatalyst for lignin degradation under oxygen.

The POM used in this study appeared to become only slight-ly reduced in reactions with lignin in the presence of molecularoxygen as indicated by the yellow-green color of the liquidproduct. Hence, H3PMo12O40 undergoes reduction and reoxida-tion cycles and acts as a catalyst for lignin oxidation under themild conditions employed.

The yield of monomers generally increased in those reac-tions conducted in the presence of molecular oxygen as com-pared to those performed under nitrogen. For instance, almosttwice as much product was obtained in the oxidation of lignin

in 100% water. The use of methanol/water mixtures also re-sulted in the formation of significant amounts of vanillin andmethyl vanillate. This positive effect was even more pro-nounced in reactions under oxygen, as the amount of vanillinproduced increased by a factor of 7.4 when spruce lignin wasdegraded in 80 vol% methanol/water as compared to degrada-tion in pure water (Table 1, entry 8 vs entry 5). In this experi-ment, the yield of monomeric products increased by a factorof 14.6. In the oxidation of poplar lignin, syringaldehyde andmethyl syringate were produced along with vanillin andmethyl vanillate in amounts that are consistent with the struc-ture of poplar lignin (Table 1, entry 9).

The addition of methanol or ethanol to the conventional ox-idation of lignin under alkaline conditions did not result in animprovement of the yield of monomeric products (Table 1, en-tries 10–13). Only a slight change in the product distributionwas observed (Supporting Information). The degradation oflignin in 40 vol% methanol/water showed a relatively high se-lectivity towards typically lignin-derived products such as vanil-lin, syringaldehyde, acetovanillone, and acetosyringone. Notraces of methyl vanillate or ethyl vanillate were observedamongst the products in any experiment.

The oxidation of Kraft lignin from softwood usingH3PMo12O40 under oxygen and in the presence of methanolyielded up to 3% of monomeric products. This yield lies in thesame range as the values reported for the oxidation of Kraftlignin with oxygen in alkaline medium.[20] While a relativelyhigh yield of aldehydes (up to 15%) can be obtained from thealkaline oxidation of steam-exploded hardwood lignin or acid-hydrolyzed hardwood lignin,[21] only a small amount of mono-mers was obtained from alkaline oxidation of the Kraft ligninused in our investigations (Table 1). Thus, the method intro-duced herein appears to be as suitable as conventional alkalineoxidation when applied to Kraft lignin. However, on the basisof the results of the first series of experiments we are con-vinced that a substantial improvement of the yield is possible.The development of POM systems especially designed tobreak most of the interunit linkages in Kraft lignin, the utiliza-tion of more effective radical scavengers, and the optimizationof reaction time and temperature represent just a few of themany parameters that might lead to an improved process.

An interesting feature of the treatment of Kraft lignin withaqueous solutions of the POM is the complete dissolution ofthe treated lignin in acidic media (pH 1.2). No solids werefound after the reaction of Kraft lignin with the POM in mostof the experiments. As Kraft lignin is not soluble in acidic solu-tion, this observation indicates that full conversion of lignintakes place. In contrast, the conversion of Kraft lignin in con-ventional alkaline oxidation did not exceed a value of 80%.

To verify the effectiveness of H3PMo12O40 as an oxidant orcatalyst in lignin degradations under nitrogen or oxygen, weconducted the corresponding control experiments in the pres-ence and absence of the POM. The proton concentration in re-actions performed in the absence of POM was adjusted by theaddition of a few drops of concentrated sulfuric acid to thesame value (pH 1.1) that was obtained when the POM(1.825 g) was dissolved in 20 mL solvent. The yields of the

Figure 1. GC-MS analysis of products obtained from the oxidation of Kraftlignin (1 g) in aqueous solution (100 mL) under nitrogen at 170 8C for20 min: A) no POM, 40 vol% EtOH/H2O; B) 0.05m POM, 100 vol% H2O;C) 0.05m POM, 40 vol% EtOH/H2O; D) 0.05m POM, 40 vol% MeOH/H2O.

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Lignin Oxidation Using Polyoxometalates

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main products, vanillin and methyl vanillate, are summarizedin Table 2. In the control experiments, the internal standard forGC-MS analysis (4-ethylresorcinol) was added immediatelyprior to extraction. Assuming a similar extraction behavior of4-ethylresorcinol and lignin-derived compounds, the values re-ported in Table 2 represent the amount of vanillin and methylvanillate produced from Kraft lignin. The comparatively lowervalues reported in Table 1, on the other hand, correspond tothe amount of products recovered by extraction, as the inter-nal standard was added directly to the extract.

The degradation of lignin in the presence of the POM gener-ally resulted in a higher yield of the targeted products vanillinand methyl vanillate. For instance, degradation of Kraft ligninwith oxygen in 80 vol% methanol/water at acidic pH yielded2.76% monomers in the absence of [PMo12O40]

3� and 5.18%monomers in its presence. From the data presented in Table 2,it is clear that the POM catalyzes the production of monomersfrom lignin. It can also be seen that the positive effect of meth-anol on the yield of monomers is independent of the usage ofthe POM and depends on the pH value. Traces of monomericproducts were obtained at neutral pH in 40 vol% ethanol/water (see Figure 1), and the addition of methanol or ethanolhad virtually no effect on the alkaline oxidation (see Table 1,entries 12 and 13).

From a comparison of studies on the oxidation of dimericlignin model compounds using oxygen and using POMs, onecan conclude that a broader range of lignin structures are ac-cessible for oxidation in the presence of an appropriate POM.Gierer and Nilvebrandt studied the degradation of a series ofmodel compounds in the presence of oxygen in acidicmedia.[22] In their studies, all lignin model dimers (phenolic andnon-phenolic) that contain a conjugated double bond or thatcan be converted into such a structure (by elimination ofwater, alkyl, or aryl at the Ca atom) were degraded to givemonomers in varying amounts. The dimer 1 (see Figure 2), onthe other hand, was recovered unchanged after treatmentwith oxygen in acidic water (pH 2.3) at 140 8C for 2 h. The di-phenylmethane model 2, in contrast, was partially degradedinto monomers by oxidation with [SiVW11O40]

5� in a potassiumacetate buffer (pH 5) at room temperature during 1 h.[8] An effi-

cient degradation of Kraft lignin requires the cleavage of di-phenylmethane structures, which result from condensation re-actions during Kraft pulping.[23]

Reactions of Simple Alcohols in Lignin Degradations inAcidic Aqueous Medium

The degradation of lignin is accompanied by repolymerizationreactions of intermediate lignin fragments. The repolymeriza-tion occurs through condensation reactions and through radi-cal coupling reactions. The relative importance of these repoly-merization pathways can be estimated from a study on thetreatment of lignin model compounds with POMs. Evtuguinet al. studied the oxidation of monomeric lignin model com-pounds using oxygen in the presence of [PMo7V5O40]

8� inacidic media.[24] From the relative amounts of dimeric productsthat resulted from the condensation and radical coupling reac-tions, it was concluded that repolymerization proceeds in aratio of 2:1 through condensation and radical coupling, respec-tively. The influence of simple alcohols on both types of repo-lymerization reactions will be discussed later (see below).While the interaction of alcohols with lignin in condensationreactions has been reported before, we found experimentalevidence that also radical coupling reactions with radicals gen-erated from methanol (CCH3 and CH3OC) may occur.

The condensation reactions of lignin in acidic media havebeen well studied and are caused by structures that form reso-nance-stabilized carbonium ions.[25] As an example, protonationof hydroxy groups at the Ca atom leads to the elimination ofwater and the formation of a benzyl carbonium ion. Then, elec-trophilic addition of the thus-formed carbonium ions on the C6

or C5 position (weakly nucleophilic) of a further lignin unit re-sults in the formation of a diphenylmethane substructure andthe release of a proton. However, when other nucleophilic spe-cies are added to the reaction these may well react withlignin-derived carbonium ions and compete with lignin–lignincondensation. It has been reported that the addition of etha-nol or acetone decreases lignin condensation in the acidic de-lignification of wood.[26]

The formation of methyl vanillate and ethyl vanillate resultsfrom reactions of lignin fragments with methanol and ethanolat acidic conditions. The incorporation of methanol into theproduct methyl vanillate was confirmed experimentally. Themethyl vanillate produced in reactions with methanol has amolecular weight of 182 gmol�1 [MS (70 eV): m/z (%): 182 (47),

Table 2. Production of vanillin and methyl vanillate from Kraft lignin at170 8C for 20 min in the presence and absence of H3PMo12O40.

Acid Solvent [mL] pH Gas Products [mg] Yield[a]

H2O MeOH start end vanillin methylvanillate

[%]

H2SO4 20 0 1.13 0.98 N2 0.47 0 0.26POM 20 0 1.13 0.91 N2 3.35 0 1.84H2SO4 4 16 1.00 1.17 N2 1.06 0.80 1.01POM 4 16 1.15 1.27 N2 1.06 1.30 1.29H2SO4 20 0 1.11 0.95 O2 2.17 0 1.18POM 20 0 1.13 0.92 O2 4.49 0 2.45H2SO4 4 16 1.13 1.28 O2 2.94 2.12 2.76POM 4 16 1.13 1.24 O2 4.99 4.51 5.18

[a] Combined amount of vanillin and methyl vanillate divided by theweight of dry lignin employed.

Figure 2. Representative structures formed from lignin during Kraft pulping.



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151 (100), 123 (22)] as detected by GC-MS. Oxidation of ligninby H3PMo12O40 under nitrogen in 40 vol% CD3OD/H2O solutionresulted in the production of methyl vanillate with a molecularweight of 185 gmol�1 [MS (70 eV): m/z (%): 185 (55), 151 (100),123 (21)] . The formation of methyl vanillate must be exclusive-ly attributed to a reaction with CD3OD, as no signal was ob-tained at m/z 182.

The formation of methyl vanillate clearly results from esterifi-cation of vanillic acid. It is known that vanillin and vanillic acidare monomeric products in the acidic degradation of ligninunder reflux.[27] In the acidic degradation of lignin with oxygen,vanillic acid is also formed by the route shown in Scheme 1.The acyl radical 3 may be generated from vanillin by homolytic

cleavage of the aldehydic hydrogen[22] or by homolytic cleav-age of the Ca–Cb bond in the degradation of lignin.[28] In thepresence of oxygen, 3 reacts very quickly to form a peroxoacylintermediate which leads to the formation of vanillic acid.[29]

Then, acid-catalyzed esterification of vanillic acid may yieldmethyl vanillate. Indeed, 4-hydroxybenzoic acid was fully con-verted into its methyl ester in an experiment carried out undernitrogen in 40 vol% methanol/water and in the presence ofH3PMo12O40. Therefore, if vanillic acid is formed in lignin oxida-tions, esterification with methanol will result in the formationof methyl vanillate. The fact that we have detected only tracesof vanillic acid in our experiments may be explained by theloss of vanillic acid in condensation reactions or its further oxi-dation leading to CO2 and H2O. Hence, one can state that va-nillic acid is an intermediate in the studied systems that can berecovered by its conversion into the corresponding methylester.

A further possible route to methyl vanillate formation is de-picted in Scheme 2 and is based on the fact that alcohols reactwith aldehydes under acidic conditions to give acetals andhemiacetals. If the hemiacetal 7 is formed from vanillin andmethanol, it could be directly oxidized to methyl vanillate.However, heating a sample of vanillin (15 mg) to 170 8C for20 min in 40 vol% methanol/water (20 mL) under nitrogen andin the presence of 1 mmol H3PMo12O40 did not result in the for-mation of any products. Thus, vanillin appears to be highlystable and the production of methyl vanillate does not pro-ceed via hemiacetal formation.

Finally, we considered the possibility that methanol and eth-anol act as radical scavengers. Then, the formation of esters inlignin oxidations should result from radical coupling of ligninfragment 3 with radicals generated from methanol (8) or etha-nol (9 ; see Scheme 3). The results from lignin oxidations areconsistent with this scheme and with the concept of using rad-ical scavengers to hinder repolymerization of lignin-derived

products. However, the mechanism by which radicals 8 and 9are generated from methanol and ethanol is not clear.

It is very unlikely that methanol directly generates radicalsCH3OC and HC by homolytic cleavage of the OH bond becauseof the high bond dissociation energy (BDE) for this reaction(105 kcalmol�1).[30] If methanol would form radicals accordingto the reaction shown as Equation (3); the coupling of HC withlignin fragments should be detectable. This ; however; was notmeasurable in an experiment performed with CH3OD. The va-nillin produced in the oxidation lignin in the presence ofCH3OD and detected by GC-MS showed the same characteristicions (m/z 152, 123, 109, and 81) as a pure sample of vanillin(reference). Hence, the reaction [Eq. (3)] does not proceedunder the conditions investigated.

CH3OH! CH3OC þ HC BDE ¼ 105 kcalmol�1 ð3Þ

A reasonable way to form radicals from methanol is present-ed in Equations (4) and (5). In a first step, condensation ofmethanol catalyzed by the strong Brønsted acid H3PMo12O40

can lead to the production of dimethyl ether (DME).[31] Indeed,the production of DME has been confirmed by GC-TCD analy-sis of the gas phase from lignin degradations performed in thepresence of methanol and a POM. Then, homolytic cleavage ofthe CO bond in DME can generate radicals CH3OC and CCH3 as aresult of the relatively low BDE (84 kcalmol�1). Interestingly,note that the production of DME from methanol is quite sub-stantial as the conversion of methanol reached 40% in a reac-tion without lignin after 20 min at 170 8C under nitrogen.

2 CH3OH! CH3OCH3 þ H2O ð4Þ

CH3OCH3 ! CH3OC þ CCH3 BDE ¼ 84 kcalmol�1 ð5Þ

If the generation of radicals from methanol and ethanol pro-ceeds via ether formation, then DME and diethyl ether (DEE)may directly be employed as radical scavengers. The use ofDME and DEE instead of methanol and ethanol did indeed

Scheme 1. Formation of methyl vanillate via vanillic acid as intermediate.

Scheme 2. Methyl vanillate produced from vanillin via hemiacetal formation.

Scheme 3. Radical coupling during the degradation of lignin.



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result in the formation of methyl vanillate and ethyl vanillate,respectively. However, only traces of methyl vanillate (5) wereproduced in reactions performed in pure water with DMEadded, as the solubility of DME in pure water is low.[32] Addi-tion of diethyl ether to the aqueous reaction media might alsolead to the extraction of the homogeneous catalyst[33] so thatonly traces of products (including 10) are obtained in a degra-dation of lignin performed in water with added DEE. Note thatit is also conceivable that DME and DEE are first hydrolyzed tomethanol and ethanol, which may then react according toScheme 1.

More direct evidence of the occurrence of radical couplingreactions was obtained by a study with CD3OD. If DME gener-ates radicals CH3OC and CCH3, one will also detect couplingproducts of CCH3 with lignin fragments. From the products ob-tained from lignin oxidations by H3PMo12O40 in methanol/water, one can see that a certain amount of acetovanillone(11) may be produced from the coupling of radical 3 with amethyl radical (Scheme 4). This theory was confirmed in theoxidation of lignin using H3PMo12O40 under nitrogen in40 vol% CD3OD/H2O. The mass spectral data for the acetovanil-lone produced clearly showed that a part of the produced ace-tovanillone has a molecular weight of 169 gmol�1 [MS (70 eV):m/z (%): 169 (2), 166 (100)] .

Conclusions

In summary, we have developed a novel method to oxidativelydegrade lignin using aqueous POMs in the presence of alco-hols for the purpose of converting Kraft lignin into chemicalsof industrial interest. The oxidation of Kraft lignin in acidicmedia using oxygen yielded twice as many monomers in thepresence of the catalyst H3PMo12O40. Addition of methanol andethanol to the aqueous reaction mixture also showed a posi-tive effect on the yield of monomeric products. Relatively largeamounts of methyl vanillate and ethyl vanillate were foundamongst the products when the degradation was carried outin methanol/water and ethanol/water, respectively. The com-bined yield of monomers obtained from the depolymerizationof Kraft lignin was thereby increased by a factor of 14.6. Theproducts obtained from the oxidation of lignin are completelysoluble in acidic media.

The role of simple alcohols in the degradation of lignin inacidic media was studied in detail. Methanol and ethanol mayprevent lignin–lignin condensation by competitive reactionswith intermediate carbonium ions. Vanillic acid can be recov-ered from the reaction by its conversion into the methyl ester.

Furthermore experiments with deuteriated methanol suggestthat radical coupling of lignin fragments with CH3OC and CCH3

produced from methanol occurs through acid-catalyzed forma-tion of DME.

Future studies will focus on the improvement of the yield ofmonomeric products by developing a more efficient POM andradical scavenger system. This will also include the analysis ofoligomeric and polymeric products to get a better understand-ing of the many steps involved along the reaction pathway.

Experimental Section

Materials : All chemicals were used as received: Kraft lignin (Aldrich471003), H3PMo12O40·xH2O (Fluka 79563), CHCl3 (J.T. Baker 7386),CH3OH (Fluka 65543), C2H5OH (Merck 100983), CH3OCH3 (Aldrich295299), CH3CH2OCH2CH3 (Fluka 31700), CH3OD (Aldrich 151939),CD3OD (Aldrich 151947), C10H22 (Fluka 30560), C8H10O2 (Alfa AesarB21575), H2SO4 (Fluka 84720), NaOH (Schweizerhall 12626), anhy-drous CuSO4 (Fluka 61230), FeCl3·6H2O (Fluka 44944), HCl (Sigma–Aldrich 258148). He, N2, and O2 were supplied by PanGas.

Analysis: The GC-MS system (Fisons Instruments GC8000/MD800)was equipped with a Rtx-5MS (Restek) capillary column (30 m)0.25 mm)0.25 mm). Split injections (1 mL) were performed with anautosampler (GC Pal) at a split ratio of 25:1 using helium as thecarrier. The column was initially kept at 40 8C for 5 min, and thetemperature was then ramped at a rate of 20 Kmin�1 to 280 8C,where it was kept for 5 min. Gas-phase samples were analyzed ona gas chromatograph (Hewlett Packard 5890) equipped with athermal conductivity detector (TCD), a 20 foot (6 m) Supelcocolumn packed with 100/120-mesh Hayesep D, and using heliumas the carrier. The column was initially kept at 100 8C for 8 min,and the temperature was then ramped at a rate of 20 Kmin�1 to220 8C, where it was kept for 10 min.

Lignin Oxidation

In a typical experiment, the POM H3PMo12O40·xH2O (9.125 g) wasdissolved in either H2O or H2O/alcohol (100 mL) to give nominalPOM concentrations of 0.05m based on the formula weight ofH3PMo12O40. This solution was transferred to a 500 mL autoclave(PREMEX AG), and powdery lignin (1 g) was added. The reactorwas sealed, purged three times with N2 or O2, and then filled to apressure of typically 5 bar. The mixture was heated to 170 8C at arate of 8 Kmin�1. After keeping the mixture at 170 8C for 20 min,the liquid sample was taken by quenching it through a cooler. Theliquid sample was filtered once (Whatman filter paper, Grade 40)and then extracted four times with chloroform (4)10 mL). For theexperiments performed in H2O/alcohol mixtures, the alcohol wasremoved by using a rotary evaporator (BNCHI AG) prior to the ex-traction steps. n-Decane (30 mL) was added to the organic phase asan internal standard for GC-MS analysis, and the solution wasmade up to a fixed volume of 50 mL with additional chloroform.

Alkaline oxidation of Kraft lignin from spruce was carried out in theautoclave mentioned above according to a method described pre-viously.[21]

Mechanistic Studies

All mechanistic studies were carried out in a 50 mL autoclaveequipped with a glass liner (Parr Instrument Company). Typically,

Scheme 4. Acetovanillone formed by radical coupling of lignin fragment 3with methyl radicals.



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H3PMo12O40·xH2O (1.825 g) and Kraft lignin (200 mg) were added tothe glass liner. The liner was placed in the reactor, and a total of20 mL solvent was added. The solvent systems comprised one ormore of the following compounds: H2O, MeOH, CD3OD, CH3OD,CH3CH2OCH2CH3. The reactor was sealed, purged three times withN2, and then filled to a pressure of typically 10 bar. The mixturewas heated to 170 8C at a rate of 12 Kmin�1. After keeping the re-actants at 170 8C for 20 min, the heating jacket was removed andthe reactor was allowed to cool to 30 8C. A gaseous sample wascollected with a gas-tight syringe for GC-TCD analysis. The liquidsample was collected and extracted three times with chloroform(3)10 mL). n-Decane (2 mL) was added to the organic phase as aninternal standard for GC-MS analysis.

In the experiment performed with dimethyl ether, the reactor wasloaded with H2O (20 mL), H3PMo12O40·xH2O (1.825 g), and Kraftlignin ( 200 mg), then sealed, and purged three times with 10 barN2. Then, gaseous dimethyl ether was added to the reactor at 20 8Cand a pressure of 5.1 bar. Afterwards, the pressure was raised to10.4 bar with additional N2. The remaining procedure was conduct-ed as described above.

The production of dimethyl ether was studied in an experimentconducted under nitrogen. The reactor was loaded with MeOH(4 mL), H2O (6 mL), and H3PMo12O40·xH2O (912.5 mg). The reactorwas sealed, purged three times with N2, and then filled to a pres-sure of 10 bar at 20 8C. The mixture was heated to 170 8C, keptthere for 20 min, and then cooled to room temperature. A capillarywas filled with a small amount of the gas phase, and the pressureand temperature were recorded. The capillary was separated fromthe reactor, and its content was released into a gas-tight syringefor GC-TCD analysis. The total amount of dimethyl ether producedwas calculated iteratively using Aspen Properties 2004.1 (NRTL-RK)with the following input data (initial values): T=21 8C, p=9.37 bar,MeOH (98.875 mmol), H2O (342.66 mmol), N2 (12.39 mmol), andyDME=0.0267 molmol�1.

In the control experiments (Table 2), H3PMo12O40·xH2O (1.825 g)was dissolved in the solvent (20 mL)—corresponding to a nominalconcentration of 0.05m based on the formula weight ofH3PMo12O40—and the pH was measured. However, the POM is ahydrate containing several water molecules per Keggin structureso the pH typically reached a value of 1.1. In the corresponding ex-periment in the absence of a POM, the pH was adjusted to approx-imately the same value by the addition of a few drops of concen-trated sulfuric acid. Kraft lignin (200 mg) was added after prepara-tion of the reaction media in a glass liner, and the liner was trans-ferred to the 50 mL autoclave. The reactor was sealed and purgedthree times with N2 or O2 then filled to a pressure of typically10 bar. The mixture was heated to 170 8C at a rate of 12 Kmin�1.After keeping the reactants at 170 8C for 20 min, the heating jacketwas removed and the reactor was allowed to cool to 30 8C. A gas-eous sample was collected with a gas-tight syringe for GC-TCDanalysis. The pH of the liquid sample was measured. The aqueousphase was collected and extracted three times with chloroform(3)10 mL). The internal standard 4-ethylresorcinol (1 mL of a1 gL�1 solution in chloroform) for GC-MS analysis was added to theextraction funnel immediately prior to extraction.

Acknowledgements

We thank Prof. James A. Dumesic and Dr. Rodrigo Amandi forvaluable discussions.

Keywords: alcohols · lignin · oxidation · polyoxometalates ·reactive intermediates

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Received: March 7, 2008Revised: June 17, 2008Published online on August 7, 2008



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Oxidation of Lignin Using Aqueous Polyoxometalates in the Presence of Alcohols