Degradation of the cellulosic key chromophore 2,5-dihydroxy-[1,4]-benzoquinone (DHBQ) under conditions of chlorine dioxide pulp bleaching: formation of rhodizonate as secondary chromophore—a combined experimental and theoretical study

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 ClO ₂ 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 ClO ₂ 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 ClO ₂ or slowly reconverted to DHBQ in the absence of ClO ₂. The RhA yield after 30 min reaction time had a maximum of 83% at a DHBQ/ClO ₂ molar ratio of 1:5, and decreased with increasing ClO ₂ charge, reaching 38% at a DHBQ/ClO ₂ ratio of 1:8 and above. Degradation of DHBQ by ClO ₂ 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 C ₆O ₆ ²⁻ 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 (Ca ²⁺, Mg ²⁺) main group metals, and usually have very low solubility so that they precipitate on the pulp fibers. It was demonstrated that the inferior ClO ₂-bleachability of some pulps is due to the conversion of DHBQ into colored RhA and its respective salts. Graphic abstract: [Figure not available: see fulltext.].