Lignin Degradation Reactions in Aerobic Delignification Catalyzed by

Chapter 20

Lignin Degradation Reactions in Aerobic Delignification Catalyzed by Heteropolyanion [PMo V O ] 8-

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Downloaded by STANFORD UNIV on June 22, 2013 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch020

D. V. Evtuguin and C. Pascoal Neto Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal

The fundamental principles and the mechanisms of lignin oxidative degradation in the reaction system [PMo V O ] /O are reviewed. The delignification was shown to follow a complex mechanism including heterolytic and homolytic oxidative stages with the catalyst. In addition, minor contributions of acidolysis and autooxidation reactions have been noted. The phenolic lignin structural units are involved in oxidation with [PMo V O ] and VO + ions dissociated from the parent heteropolyanion. The oxidation rate of nonphenolic structures is 5-7 times lower than that of phenolic ones. The influence of heterogeneous character of the catalysis on the mechanisms of lignin oxidation is discussed. 8-

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Recent trends for highly selective lignin oxidation reactions with dioxygen include the application of metal-oxygen anion clusters (polyoxometalates) as regenerable redox reagents (l 2) or as catalysts (3,4). The principles of oxidative delignification with polyoxometalates (POMs) have been widely discussed previously (1-4) and can be formulated briefly as follows: the POMs, having an energetic barrier to lignin oxidation lower than that of dioxygen, oxidizes the lignin and then, the reduced form of the POM is re-oxidized by 0 (Fig. 1). The final products of the lignin oxidation and the oxygen reduction are f

2

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

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328

Figure 1. General scheme ofPOM catalyzed oxygen delignification (POMs are presented as Keggin compounds in polyhedral-filling representation). carbon dioxide and water, respectively. The oxidation of lignin and re-oxidation of the POMs can occur in one stage (aerobic oxidation of lignin in the presence of POM) or in separate stages (step I as shown in Fig. 1 is performed under anaerobic conditions). In the first case POMs may be considered as oxidative catalysts. POMs catalyzed oxygen delignification was successfully used both for the pulping and bleaching needs (J-7). Due to the complete oxidation of organic matter and because bleaching liquors can be continuously re-used, POM based oxidation processes are considered as a very promising approach towards Total Effluent Free (TEF) bleaching plants (1,5). Between a large variety of POMs, the heptamolybdopentavanadophosphate heteropolyanion (HPA-5) with general formula [PMo V O ] " is particularly adapted for the catalytic purposes mentioned above (5-7). The aim of this paper was to review the fundamentals of the oxidation catalysis with HPA-5 and to summarize the results of latest investigations of the lignin degradation mechanisms in the reaction system HPA-5/0 . 8

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Heteropolyanion [PM07V5O40] " as a Redox Catalyst 8

The nomenclature and structural varieties of POMs are well described elsewhere (8). POMs are composed primarily by early transition metal cations in their high oxidation states and oxoanions with a variety of structures.

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

329 Particularly, heteropolyanions (ΗΡΑ) are soluble salts of polyoxoanions having the general formula [X M O ] ", where X is a heteroatom (X=P, Si, B, etc.) and M is an addenda atom (M= W , V , Mo^, etc.). ΗΡΑ of T symmetry, quasispherical structures derived from assemblies of twelve M O octahedra around a tetrahedron containing a heteroatom, are known as Keggin compounds (Fig. 2). The heteropolyanion [PMo V O ] " (HPA-5) belongs to Keggin type HPA-n (where η is the number of vanadium atoms in ΗΡΑ) series [ P M 0 1 2 V O40] ". The main structural features and redox properties of HPA-5 are similar to those of others HPA-n (n=2,3,4,6) and were extensively studied in last q

x

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(3+n)

n

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decades (9,10).

HPA-n (n=2-6) are well known highly selective catalysts in oxidative organic syntheses due to their easily re-oxidation by oxygen even at room temperatures (9,10). The basic reversible redox reactions involved in organic electron donor's oxidation with HPA-n may be shown as follow (9): Substrate + [HPA] + mH Hm[HPA] +oxidized substrate (1) H [HPA] + m/4 0 α-carbonyl structures (16). According to the voltammetric studies in aqueous solutions of HPA-5 (pH 2) and monomeric model compounds it was proposed that both HPA-n (n < 5) and V 0 ions can oxidize phenolic lignin units (16). The mechanism of oxidative cleavage of β-Ο-4 linkages in phenolic structures has been studied using 1 -(3 -methoxy-4-hydroxypheny l)-2-(2methoxyphenoxy)ethanol (A) as a lignin model compound (17). The main reaction pathways suggested, based on the analysis of the products obtained (III-VII), are shown in Fig. 4. Two consecutive one-electron oxidations of the aromatic ring with the catalyst (both V 0 and HPA-n) are followed by hydrolytic degradation of the formed cyclohexadienyl cation Y. This reaction intermediate was confirmed using Electrospray Ionisation Mass Spectrometry (ESI/MS) applied in the positive mode (17). The heterolytic cleavage of alkylphenyl bonds was also suggested to occur in the oxygen delignification of wood catalyzed by HPA-5 (15). 2

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334 The occurrence of acid-catalyzed (acidolytic) cleavage of β-Ο-4 linkages was concluded based on appearance of acetoguaiacone (VI) as a typical acidolysis product (Fig. 4). The formation of benzyl cation Ζ (Fig. 4), a known intermediate of the acidolytic degradation, was detected by means of ESI/MS (/7). The occurence of acid-catalyzed degradation of lignin was additionally concluded from the study of eucalyptus wood oxygen delignification catalyzed by HPA-5 in acidic ethanol-water solutions (15). However, the contribution of acidolysis in the cleavage of β-aryl ether bonds of lignin is much lower than that inserted by oxidative degradation with HPA-5: the delignification degree increased five times with addition of HPA-5 at the same conditions (100 °C, 5h, oxygen pressure 0.5 MPa; pH 2) of the oxygen pulping as compared to the control experiment without HPA-5 (3). Since the catalytic oxidative delignification with HPA-5 is performed under acidic conditions (pH 2-3) and elevated temperatures (80-100 °C), the lignin condensation reactions take place slowing the delignification rates (3 15). The condensation reactions are especially important in the case of guaiacyl type lignin (softwood delignification). The addition of organic solvents to the pulping/bleaching liquor prevents substantially the undesirable condensation reactions thus promoting the delignification (3,15). t

Reactivity of non-phenolic lignin units in the reaction system HPA-5/0 . Non-phenolic lignin structural units are hard to oxidize in the reaction system HPA-5/0 . However, their contribution in the lignin oxidative degradation is not negligible (16). The comparison of the oxidation extent of vanillyl and veratryl alcohols, as phenolic and non-phenolic lignin model compounds, respectively, under the same reaction conditions (90 °C; 40 min; P°0 = 0.5 Mpa) showed a conversion of veratryl alcohol 5-6 times lower than that of vanillyl alcohol (16). V 0 ions were suggested to be the active species in the oxidation of non phenolic model compounds. The reaction mechanism of the oxidative cleavage of β-Ο-4 linkages in non phenolic lignin structures was elucidated using a dimeric model compound 1(3,4-dimethoxyphenyl)-2-(2-methoxyphenoxy)ethanol (B) (17). Based on the identified oxidation products (III, VII, Χ-ΧΠΙ), the homolytic cleavage of β-Ο4 and Ca-Οβ linkages in Β were suggested as the main reaction routes (Fig. 5). The rate-limiting reaction step is one-electron electron oxidation of the guaiacyl group with the catalyst followed by the formation of the corresponding cationradical (Fig.5). The formation of products such as vanillin (XII) and 3,4dihydroxybenzoic acid (XIII) indicated the occurrence of the demethoxylation reactions. In general, the reaction pathways discovered were very close to those 2

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HCOH

) CHCJ

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3

V S

^3L_ V

[Ο

OCH,

V(V)_* V(IV) CH _OHQ> Downloaded by STANFORD UNIV on June 22, 2013 | http://pubs.acs.org Publication Date: March 26, 2001 | doi: 10.1021/bk-2001-0785.ch020

2

HCOH

V CHOT 3

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Lignin Degradation Reactions in Aerobic Delignification Catalyzed by

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