Degradation of the Cellulosic Key Chromophore 5,8-Dihydroxy-[1,4]-naphthoquinone by Hydrogen Peroxide under Alkaline Conditions

Abstract

5,8-Dihydroxy-[1,4]-naphthoquinone (DHNQ) is one of the key chromophores in cellulosic materials. Its almost ubiquitous presence in cellulosic materials makes it a target molecule of the pulp and paper industry's bleaching efforts. In the presented study, DHNQ was treated with hydrogen peroxide under alkaline conditions at pH 10, resembling the conditions of industrial hydrogen peroxide bleaching (P stage). The reaction mechanism, reaction intermediates, and final degradation products were analyzed by UV/vis, NMR, GC-MS, and EPR. The degradation reaction yielded C-C carboxylic acids as the final products. Highly relevant for pulp bleaching are the findings on intermediates of the reaction, as two of them, 2,5-dihydroxy-[1,4]-benzoquinone (DHBQ) and 1,4,5,8-naphthalenetetrone, are potent chromophores themselves. While DHBQ is one of the three key cellulosic chromophores and its degradation by HO is well-established, the second intermediate, 1,4,5,8-naphthalenetetrone, is reported for the first time in the context of cellulose discoloration.

Cyclic peroxides as key intermediates in the degradation of cellulosic key chromophores by alkaline hydrogen peroxide: first direct proof by 17O NMR

“…The pKa values of DHNQ (2), 7.8 and 9.2, indicate that formation of the corresponding dianion 2a requires alkaline media. At a reaction pH of 10, chosen to set the same conditions as in previous experiments (Hosoya and Rosenau 2013a;Zwirchmayr et al 2017), both 1 and 2 are fully deprotonated and present as their dianions 1a and 2a. This brings about drastic changes in the NMR spectra.…”

“…This is another nice support for the resonance structure in O NMR observation Scheme 2 DHNQ 2, its dianion 2a and its primary intermediate 2b, formed upon reaction with H 2 O 2 under alkaline conditions. As the final products, low molecular weight degradation products (mainly carbonate, acetate, and maleate) are generated (Zwirchmayr et al 2017 1a with its fully delocalized charges and double bonds, which is the main reason for the recalcitrance of the compound towards common bleaching agents that act on (localized) double bonds.…”

“…In the case of experiments on possible oxygenated intermediates, such long measurement times are evidently unfeasible because the intermediates have a lifetime of only several minutes at room temperature, as known from previous kinetic studies (Hosoya and Rosenau 2013a;Zwirchmayr et al 2017). Therefore, isotopic enrichment had to be employed and, in addition, lower temperatures (0°C) to decrease the reaction rate.…”

“…by treatment with alkaline hydrogen peroxide, has been studied in detail. The kinetics as well as the reaction mechanism, stable intermediates and final degradation products, mostly C 1 to C 4 acids and hydroxyacids and carbon dioxide, have been established (Hosoya and Rosenau 2013a;Zwirchmayr et al 2017). One question that remained open, however, was the intermediacy of cyclic peroxides 1b and 2b (Schemes 1, 2).…”

“…One question that remained open, however, was the intermediacy of cyclic peroxides 1b and 2b (Schemes 1, 2). The occurrence of these intermediates has been postulated from in-depth computational treatments of the reaction pathways (Hosoya and Rosenau 2013a;Zwirchmayr et al 2017), and NMR experiments provided some faint indication of their existence, but solid proof was still missing. In particular 1 H and 13 C NMR spectroscopy was inconclusive insofar as resonances of putative intermediates were in agreement with the cyclic peroxides' structure, but would fit also to other structurally similar compounds.…”