Effect of anthraquinones on endwise degradation of hydrocellulose in relation to alkaline pulping

Abstract

Anthraquinones promote early delignification and stabilisation of cellulose during alkaline pulping. Alkali-catalysed endwise depolymerisation of hydrocellulose was studied at 97°C under nitrogen. In the presence of sodium anthraquinone-2-sulphonate (AQS), the rates of both the chain-propagated unzipping reaction and the competing termination reaction were lowered. Since the former reaction was inhibited to a greater extent than the latter, less dissolution of cellulose occurred at this temperature. These findings are inconsistent with a mechanism that hypothesises coupling of an oxidative stabilisation of cellulosic chain-ends with chemical reduction of an anthraquinone catalyst. The inhibitory action of AQS at 97°C is ascribed to its possible action of diminishing the degree of ionisation of cellulosic reducing chain-ends. As a possible mechanism for the accelerated dissolution of cellulose at pulping temperatures, it is suggested that anthraquinones promote intrachain scissions in cellulose, enhancing the dissolution of chain fragments before unzipping. An anomalously high rate of peeling near neutrality is attributed to an uncatalysed reaction occurring simultaneously with the base-catalysed degradation.