Chemistry of delignification

chemical analysis in pulp

1. Chemistry of delignification Part 2: Reactions of lignins during bleaching By Audrey Zahra

2. Ozonation of aromatic and olefinic structures Ozone may be represented as a resonance hybrid comprising four mesomeric structures ( Scheme 33). The terminal positively charged oxygens constitute the electrophilic sites ofthe molecule being more reactive than the nucleophilic sites. Thus, as was shown for the other oxidation reactions, the initial step of ozonation involves clectrophilic attack of the activated positions by the oxidant (Schemes 34 and 35). This is true not only for the oxidative hydroxylation (Scheme 34, reaction 1 a) and demethoxylation (reaction 1 b) proceeding via elimination of oxygen, but also for the 1,3-dipolarcycloadditions (2 a and 2 b) which are followed by formation of hydrogen peroxide.

3. While the oxidative opening of phenolic and non-phenolic nuclei by hydrolysis of the cycloaddition product is straight-forward (2 a ), the cleavage of olefinic structures takes a more complicated course (2b).

4. As can be seen in Schemes 34 and 35, ozonation of aromatic and olefinic structures is accompanied by the generation of molecular oxygen (1, 3 and 4) or hydrogen peroxide (2). Alkaline decomposition of the latter (see below) and of the starting oxidant provides intermediary hydroxy and hydroperoxy radicals, known to be powerful oxidants.

5. Oxidation of aromatic and olefmic structures with peroxyacetic acid The reacting species in this oxidation is the hydroxonium ion , HO+, arising by heterolytic cleavage of the peroxidic bond ha the oxidant . HO+ ions attack the same activated sites as the previously treated bleaching reagents . Schemes 36 and 37 summarize the main reaction types of phenolic and non-phenolic lisnin-related structures , studied in model experiments . These reaction types are: ring hydroxylation oxidative demethylation oxidative ring cleavage displacement of side chains cleavage offl -aryl ether bonds epoxidation

6. Liwnin-degrading bleaching by the action of hydroxonium ions can also be achieved using acidic solutions of hydrogen peroxide . Although no detailed studies concerning the mechanism of liLgnin breakdown by this reagent have been reported so far , it may be assumed that the reactions accounting for this process are si milar to those observed when peracetic acid is used .

7. Nucleophilic addition and displacement reactions following initial electrophilic attack During the introductory electrophilic steps of lignin-degrading bleaching described in section A , quinonoid and other enone structures are generated ( Schemes 24, 26, 27, 29, 32, 34, 36, 37) which are susceptible to subsequent attack by nucleophlles . The nucleophilic reactions may take place during the same bleaching step as the initial electrophilic reaction and involve nucleophilic species originally present or generated either from the electrophilic bleaching reagent or from intermediary lignin structures ( cf . e.g . formation of hydroperoxide and peroxide anions during oxygen bleaching , p . 11). Nucleophilic reactions also participate in the delignification process during later steps of conventional bleaching sequences , where added nucleophiles are the reacting species .

8. Nucleophilic reactions during the introductory bleaching step Thus , in oxidative dealkylation by chlorine ( Scheme 24) and by chlorine dioxide ( Scheme 27), the initial electrophilic addition of the oxidant is followed by hydrolytic , i.e . nucleophilic , processes . In chlorination reactions of olefinic structures ( schemes 25 and 26), the addition of electrophilic chl o ronium ions is followed by that of nucleophilic chloride ions . The nucleophile and the appropriate enone substrate may also be formed in the same molecule. In such instances, the subsequent intramolecular attack gives rise to cyclic intermediates. This is illustrated by the formation of dioxetane structures during oxygen bleaching (Schemes 30 and 32 a).

9. Nucleophilic reactions during subsequent bleaching steps Illustrative examples are given in the following sections : Reactions involving hydroxide ions . In conventional bleaching , the introductory step , i.e . treatment with chlorine / chlorine dioxide in acidic media , is followed by extraction with alkali . C hlorine linked to side chains or to quinonoid moieties can be replaced by hydroxide ions via participation of a neighbouring hydroxyl group ( formation of oxiranes ) or via nucleophilic addition ( formation of cyclohexadienone intermediates ), respectively ( Scheme 38). H H H H H H H H

10. Similar nucleophilic addition of hydroxide ions , resulting in increased solubility , may also take place in non-chlorinated quinonoid structures to give hydroxy-substituted catechols or , via a benzylic acid type of rearrangement , yield a-hydroxy carboxylic acids of the cyclopentadiene type ( Scheme 39).

11. Oxidation of enone structures by hydroperoxide ions . In analogy to hydroxide ions , hydroperoxide ions add to quinonoid and other enone structures ( schemes 40 and 41) to give hydroperoxide -, and subsequently oxirane -, dioxetane - or hydroxy quinone intermediates . Further alkaline and/ or oxidative degradation affords end products mainly of the carboxylic acid type . Hydroperoxide ions may be used in different stages of bleaching sequences. In lignin-degrading bleaching they usually complete the effect of electrophilic reagents, operative in earlier steps. In lignin-retaining bleaching they remove chromophores from residual litmins in high yield pulps. Phenolic structures are virtually stable towards alkaline hydrogen peroxide, provided homolytic decomposition of the oxidant to give hydroxy and hydroperoxy (or superoxide anion) radicals can be effectively inhibited. These radicals, together with their reaction product, the biradical oxygen, are responsible for the degradation of aromatic substrates observed when non-stabilized hydrogen peroxide solutions are used. The same species are considered as being involved in the oxidation of hydroxyl groups in wood polysaecharides to give carbonyl groups. This reaction initiates alkaline carbohydrate degradation by "peeling" during hydrogen peroxide and oxygen bleaching (cf. also reaction with chlorine radicals, p. 6).

12. Oxidation of enone structures by hypochlorite ions . As is true for hydroperoxide ions , hypochlorite ions constitute strongly nucleophilic species adding readily to enone structures , in particular to quinonoid structures . The reaction proceeds via hypochlorite esters to intermediates of the oxirane type ( Scheme 42). Reactions involving hypochlorite ions (Scheme 42) bear strong resemblance to those of hydroperoxide ions (cf. Schemes 40 and 41). Both nucleophilic species can be used to render the alkaline extraction step more effective with regard to lignin removal and increase of brightness, and to continue lignin degradation in later bleaching steps.

13. The generation of these radical species from nucleophiles , like the opposite process involving the formation of nucleophiles such as HOO- from radicals such as ‘ O 2’ ( p . 11), makes the differentiation between nucleophilic and electrophilic bleaching steps less distinct . As shown above , nucleophiles and electrophiles present in a bleaching liquor may be thought to co-operate in the degradation of residual lignin . The efficiency of a particular bleaching sequence may thus depend , at least in part , on a well balanced alternate action of electrophilic and nucleophilic species .

14. Lignin-retai n ing bleaching This bleaching variant is used in the production of high-yield mechanical , chemi-mechanical and chemical pulps with the aim of removing chromophoric groups without degrading and dissolving lignocellulosic material . Lignin-retaining bleaching can be achieved by the action of nucleophilic reagents . Best results are obtained when the decomposition of the bleaching reagents , catalyzed by heavy metals, is minimized by the choice of appropriate reaction conditions and by the use of complexing and other stabilizing agents. Conversion of chromophoric enone structures , e.g . quinonoid groups , into colourless structures can also be achieved by addition of nucleophiles . An example of this "additive" bleaching mode is the treatment of mechanical and other high-yield pulps with sodium sul f ite which adds to quinones to give aromatic sul fonic acid structures ( Scheme 43)

15. Selectivity of delignification Pulping reactions Acidic cleavage of benzyl-aryl and benzyl-alkyl ether linkages parallels acidic cleavage of glycosidic bonds . Both processes proceed via the corresponding hydroxonium - and carbonium ions ( Scheme 44).

16. Alkaline cleavage of the same types of bond , requiring the participation of a free phenolic hydroxyl group in the para-position , may be regarded as a vinylogous β - elimination , a reaction type which constitutes the key step in alkaline peeling of carbohydrates ( Scheme 45)

17. Bleaching reactions The alkaline oxygenation of phenolic structures in li gnins and enolic structures in carbohydrates , here in their carbanion form , give the corresponding hydroperoxide intermediates (scheme48).

18. These intermediates then undergo intramolecular nucleophilic attack of the carbonyl carbon with formation of dioxetanes followed by rearrangement with cleavage of the carbon-carbon bond of the dioxetane ring system ( Scheme 49). In lignins , this results in rupture of the originally aromatic ring ; in carbohydrates , in shortening of the reducing end units by one carbon atom , released as formic acid , and in formation of a stable aldonic acid end group .

19. Another striking analogy exists in the alkaline autoxidation of enediol structures ( Scheme 50). Both catechol structures in li g nins and enediol structures in carbohydrates are readily autoxidized to give the respective dicarbonyl structures, i.e. ortho-quinones and 2,3-diketones.

20. Hydrogen peroxide is thereby formed ( cf . Scheme 32b) which adds in the known manner to one of the two neighbouring carbonyl groups affording the corresponding hydroperoxide intermediate ( Scheme 51). The subsequent transformation of these intermediates via formation of dioxetanes and cleavage of carbon-carbon bonds are also completely analogous .

21. Concluding remarks Delignification during pulping is due essentially to nucleophilic reactions . Both the addition of pulping chemicals and the intramolecular attack by ionized neighbouring groups are nucleophilic processes . This is also true for the competing condensation reactions . Delignification during lignin-degrading bleaching is initiated by electrophilic reactions which are followed by nucleophilic processes either during the same or during subsequent bleaching steps . Nucleophilic reactions generate new phenolic and enolic structures which maybe attacked by electrophiles, while electrophilic reactions result in the formation of enone structures which may be the substrate for subsequent nucleophilic attack. The elucidation of this interplay facilitates the understanding of the problems encountered in technical delignification . Thus, the incomplete removal of lignin during conventional pulping processes can be explained by the supposition that after a certain period of time all enone structures have been consumed by addition and/or elimination reactions and, thereby, all possibilities of nucleophilic attack have been exhausted.

22. Thus , the chemistry of delignification can now be described and summarized in terms of reaction mechanisms generally accepted in organic chemistry . However , there are still gaps in our knowledge which will be filled by future work . The following topics are suggested : 1. Investigation of the possibility of formation of limain -carbohydrate linkages during delignification processes and of the behaviour of such linkages under the conditions of pulping and bleaching. 2. Confirmation of the delignitication mechanisms by isolation of further lignin degradation products from pulping and bleaching spent liquors and by characterization of residual lignins . 3. Extension of the mainly qualitative studies hitherto carried out to include the kinetics of the lignin- and carbohydrate-degrading reactions and their dependency on various parameters.