Abstract

2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ, 1) is the most prominent representative of cellulosic key chromophores, which occur almost ubiquitously in all types of aged cellulosics. The degradation of DHBQ by chlorine dioxide under conditions of industrial pulp bleaching (“D stage”) was studied, i.e. in moderately acidic medium (pH 3) at temperatures between 50 and 90 °C. The degradation in the presence of excess ClO₂ generates rhodizonic acid (RhA, 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone, 2) as a secondary chromophore which is even more stable and more potent as a chromophore than the starting DHBQ, especially in the form of its salts. At least a threefold ClO₂ excess is needed for complete DHBQ consumption. The reaction from DHBQ to RhA involves pentahydroxybenzene (PHB, I) as an intermediate which is either readily further oxidized to RhA by excess ClO₂ or slowly reconverted to DHBQ in the absence of ClO₂. The RhA yield after 30 min reaction time had a maximum of 83% at a DHBQ/ClO₂ molar ratio of 1:5, and decreased with increasing ClO₂ charge, reaching 38% at a DHBQ/ClO₂ ratio of 1:8 and above. Degradation of DHBQ by ClO₂ is 42 times faster than that of RhA (50 °C, pH 3). RhA is present in aqueous medium in the form of its dihydrate, 2,3,5,5,6,6-hexahydroxycyclohex-2-ene-1,4-dione, which contains two pairs of geminal diols at C-5 and C-6. At pH 5 and above it forms an aromatic C₆O₆²⁻ dianion, so that the RhA salts are very stable. These salts are intensively colored, not only the ones with transition metal cations, but also those with monovalent (Na⁺, K⁺) and especially divalent (Ca²⁺, Mg²⁺) main group metals, and usually have very low solubility so that they precipitate on the pulp fibers. It was demonstrated that the inferior ClO₂-bleachability of some pulps is due to the conversion of DHBQ into colored RhA and its respective salts.

Graphic abstract

中文翻译：

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二氧化氯纸浆漂白条件下纤维素关键发色团2,5-二羟基-[1,4]-苯醌（DHBQ）的降解：作为第二发色团的重氮根酸盐的形成-结合实验和理论研究

摘要

2,5-二羟基-[1,4]-苯醌（DHBQ，1）是纤维素关键发色团的最主要代表，其主要存在于所有类型的老化纤维素中。研究了在工业纸浆漂白（“ D阶段”）条件下，即在中等酸性介质（pH 3）中，温度在50至90°C之间，二氧化氯对DHBQ的降解。在过量ClO ₂存在下的降解会产生作为次生生色团的Rhodizonic酸（RhA，5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone，2），它作为次生发色团，甚至更稳定，更有效。比起始DHBQ的发色团，尤其是其盐的形式。至少三倍的ClO ₂完全DHBQ消耗需要过量。从DHBQ到RhA的反应涉及作为中间体的五羟基苯（PHB，I），该中间体容易被过量的ClO ₂进一步氧化成RhA，或者在没有ClO ₂的情况下缓慢地重新转化为DHBQ 。30分钟的反应时间后的RHA收率在DHBQ / CLO的最大的83％有₂摩尔比为1：5，并且随着CLO降低₂电荷，在一个DHBQ / CLO达到38％₂比为1：8和以上。ClO ₂降解DHBQ是RhA（50°C，pH 3）的42倍。RhA以其二水合物2,3,5,5,6,6-六羟基环己-2-烯-1,4-二酮的形式存在于水相介质中，在C-5和C处含有两对双生二醇。 -6。在pH 5和更高的温度下，它会形成芳族C ₆ O ₆ ^2-阴离子，因此RhA盐非常稳定。这些盐不仅带有过渡金属阳离子，而且带有一价（Na ⁺，K ⁺）尤其是二价（Ca 2 ⁺，Mg ²⁺）主族金属的盐都有很深的着色，通常溶解度很低，因此它们沉淀在纸浆纤维上。证明了劣质的ClO ₂一些纸浆的可漂白性是由于DHBQ转化为有色RhA及其相应的盐。

图形摘要