Kraft pulping quinone methide intermediates

Fig. 7-25. Main reactions of the phenolic /8-aryl ether structures during alkali (soda) and kraft pulping (Gierer, 1970). R = H, alkyl, or aryl group. The first step involves formation of a quinone methide intermediate (2). In alkali pulping intermediate (2) undergoes proton or formaldehyde elimination and is converted to styryl aryl ether structure (3a). During kraft pulping intermediate (2) is instead attacked by the nucleophilic hydrosulfide ions with formation of a thiirane structure (4) and simultaneous cleavage of the /3-aryl ether bond. Intermediate (5) reacts further either via a 1,4-dithiane dimer or directly to compounds of styrene type (6) and to complicated polymeric products (P). During these reactions most of the organically bound sulfur is eliminated as elemental sulfur.

In soda pulping there are similar condensation reactions as those described above for kraft pulping. In this case, however, the black liquor does not have strong nucleophiles (such as sulphide or hydrosulphide) that might block condensation of the quinone methide intermediates with the carbanions. Hence, lignin condensation may be more prevalent for soda than for kraft pulping. 

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