Cellulose (2020) 27:3623–3649 https://doi.org/10.1007/s10570-020-03014-y (0123456789().,-volV)( 0123456789().,-volV) ORIGINAL RESEARCH 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 Matthias Guggenberger . Hubert Hettegger . Nele Sophie Zwirchmayr . Takashi Hosoya . Markus Bacher . Sara Zaccaron . Stefan Bo¨hmdorfer . Heidemarie Reiter . Martin Spitzbart . Thomas Dietz . Klaus Eibinger . Arnulf Kai Mahler . Heribert Winter . Thomas Ro¨der . Antje Potthast . Thomas Rosenau Received: 24 November 2019 / Accepted: 20 January 2020 / Published online: 10 February 2020 Ó The Author(s) 2020 Abstract 2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ, stable and more potent as a chromophore than the 1) is the most prominent representative of cellulosic starting DHBQ, especially in the form of its salts. At key chromophores, which occur almost ubiquitously least a threefold ClO2 excess is needed for complete in all types of aged cellulosics. The degradation of DHBQ consumption. The reaction from DHBQ to DHBQ by chlorine dioxide under conditions of RhA involves pentahydroxybenzene (PHB, I)asan industrial pulp bleaching (‘‘D stage’’) was studied, intermediate which is either readily further oxidized to i.e. in moderately acidic medium (pH 3) at tempera- RhA by excess ClO2 or slowly reconverted to DHBQ tures between 50 and 90 °C. The degradation in the in the absence of ClO2. The RhA yield after 30 min presence of excess ClO2 generates rhodizonic acid reaction time had a maximum of 83% at a DHBQ/ (RhA, 5,6-dihydroxycyclohex-5-ene-1,2,3,4-tetrone, ClO2 molar ratio of 1:5, and decreased with increasing 2) as a secondary chromophore which is even more ClO2 charge, reaching 38% at a DHBQ/ClO2 ratio of M. Guggenberger Á H. Hettegger Á N. S. Zwirchmayr Á T. Dietz M. Bacher Á S. Zaccaron Á S. Bo¨hmdorfer Á A. Potthast Á Evonik-Degussa, Rodenbacher Chaussee 4, T. Rosenau (&) 63457 Hanau-Wolfgang, Germany Department of Chemistry, Institute for Chemistry of Renewable Resources, University of Natural Resources K. Eibinger and Life Sciences (BOKU), Muthgasse 18, 1190 Vienna, Zellstoff Po¨ls AG, Dr. Luigi-Angeli-Str. 9, 8761 Po¨ls, Austria Austria e-mail: [email protected] A. K. Mahler Á H. Winter T. Hosoya SAPPI Papier Holding GmbH, Brucker Str. 21, Graduate School of Life and Environmental Sciences, 8101 Gratkorn, Austria Kyoto Prefectural University, Shimogamo-hangi-cho 11-5, Sakyo-ku, Kyoto-shi, Kyoto, Japan T. Ro¨der Lenzing AG, Werkstraße 2, 4860 Lenzing, Austria H. Reiter Á M. Spitzbart Mondi Uncoated Fine & Kraft Paper GmbH, Marxergasse T. Rosenau 4A, 1030 Vienna, Austria Johan Gadolin Process Chemistry Centre, A˚ bo Akademi University, Porthansgatan 3, 20500 A˚ bo, Turku, Finland 123 3624 Cellulose (2020) 27:3623–3649 1:8 and above. Degradation of DHBQ by ClO2 is 42 relevant transition metals in pulp production, and times faster than that of RhA (50 °C, pH 3). RhA is therefore also those that are regularly monitored in present in aqueous medium in the form of its pulp. The content of (transition) metals varies depend- dihydrate, 2,3,5,5,6,6-hexahydroxycyclohex-2-ene- ing on the wood species and also shows strong 1,4-dione, which contains two pairs of geminal diols seasonal fluctuations for a particular wood species. at C-5 and C-6. At pH 5 and above it forms an aromatic Our model pulp studies in this regard were rather 2- C6O6 dianion, so that the RhA salts are very stable. inconclusive: in some cases, the bleachability corre- These salts are intensively colored, not only the ones lated clearly with the metal content, in other cases not with transition metal cations, but also those with at all. It seemed logical to attribute the brightness monovalent (Na?,K?) and especially divalent (Ca2?, reversion behavior to transition metal-induced radical Mg2?) main group metals, and usually have very low processes and oxidative pulp damage. But then it solubility so that they precipitate on the pulp fibers. It remained completely unclear why the supposed chro- was demonstrated that the inferior ClO2-bleachability mophores formed during these processes should not be of some pulps is due to the conversion of DHBQ into immediately destroyed—quasi ‘‘in situ’’—in the colored RhA and its respective salts. D-stage, and, in particular, why calcium and magne- sium, which as main group metal ions are innocent Graphic abstract with regard to inducing redox chemistry and radical Keywords Aging Á Bleaching Á Brightness reactions, showed in some cases just as negative reversion Á Cellulose Á Chlorine dioxide Á effects as the transition metals did. Chromophores Á Computational chemistry Á Density Because it was known that the three cellulosic key functional theory (DFT) Á 2,5-Dihydroxy-[1,4]- chromophores (Korntner et al. 2015) can develop benzoquinone Á Pulp bleaching Á Rhodizonate Á directly from oxidatively damaged cellulose (Rosenau Rhodizonic acid Á Yellowing et al. 2005, 2017), several pulp samples with poor bleachability were tested for the content of key chromophores according to the CRI method (Rosenau et al. 2004a, 2011). This technique has been developed Introduction for the analysis of aromatic and quinoid traces of chromophores in cellulosic matrices, which are other- In recent years, there have been several reports of wise inaccessible due to their extremely low concen- lowered bleachability of cellulosic pulps upon chlo- trations. CRI thus is the only approach available to rine dioxide bleaching (so-called ‘‘D stage’’) in the date that allows cellulosic chromophores to be isolated final bleaching stage, which was associated with an and their structures to be identified, covering up to increased content of transition metal ions. Manganese, 75% of the total mass of chromophores isolated. It copper and iron are the ‘‘best-known’’ and most should be recalled that the CRI method works with 123 Cellulose (2020) 27:3623–3649 3625 highly bleached pulps and addresses chromophores agents attack localized double bonds in their usual with ‘‘carbohydrate history’’, i.e. generated from mode of action, they have a hard time with DHBQ cellulosics and hemicelluloses, thus excluding pulps which lacks such ‘‘proper’’ double bonds due to its with higher lignin contents and lignin-based chro- resonance delocalization. Fast and reliable methods to mophores that would overrun this analytical method. detect and quantify DHBQ, even at the very low In all pulps with deficient final D-stage bleachabil- concentrations in cellulosic pulps, have become ity, coming from different mills in three continents, we available (Potthast et al. 2018), which is a prerequisite found high contents, between 3 and 6 ppm, of the key for monitoring both bleaching sequences and model chromophore 2,5-dihydroxy-[1,4]-benzoquinone compound studies. (DHBQ, 1) (Hosoya et al. 2013a), whereas the content Several previous accounts have addressed the of DHBQ in a highly bleached pulp is usually in the chemistry of DHBQ, which has recently been low ppb range. DHBQ is known to be formed from reviewed (Hosoya et al. 2013a). The strong resonance oxidative damage (carbonyl groups) along the cellu- stabilization of the corresponding dianion and promi- lose chains (Rosenau et al. 2005). Also, effects of nent keto-enol interconversions are typical features of metal ions in the chemistry of DHBQ have been this compound. Its preferred reaction paths are elec- observed (Hosoya et al. 2015). The content of the two trophilic substitutions, mainly at C-3 and C-6 (Bras- other key chromophores, 2,5-dihydroxy-[1,4]-naph- sard and L’Ecuyer, 1958; Jimenez-Alonso et al. 2007; thoquinone (DHNQ) and 2,6-dihydroxyacetophenone Misiolek et al. 2009), while the oxygens are rather (DHAP), in the difficult-to-bleach pulps was not resistant (Keegstra et al. 1996a). Such processes occur increased beyond normal levels (\ 10 ppb). for instance with S-ylides (Rosenau et al. 2004b)or DHBQ (1) is a nearly ubiquitous cellulosic chro- iodonium species (Papoutsis et al. 1994). Typical mophore because it is both a prime survivor of examples for nucleophilic reactions, which occur at bleaching treatments and a regeneration product from either C-1, C-2, C-4 and C-5 with little regioselectiv- condensation of low-molecular weight fragments ity, are reactions with amines (Manthey et al. 1989; upon cellulose aging (Rosenau et al. 2007, 2008). Zhang and Jin 2003), and the follow-up chemistry to Even a seemingly negligible oxidative modification, N-heterocycles (Placin et al. 2000; Seillan et al. 2008; such as a single hydroxyl group oxidized to a carbonyl Tang et al. 2009). As the compound is prone to both group, can lead to the formation of a DHBQ moiety nucleophilic and electrophilic attack, there is a wide along the cellulose chain (Rosenau et al. 2008, 2017). range of options for chemical modifications of the The high stability of DHBQ and its dianion towards structure. Reduction to the hydroquinone form pro- bleaching treatments is attributed to exceptional ceeds easily, but requires stabilization of the product resonance stabilization (Hosoya and Rosenau in the form of alkyl or acyl derivatives (Schlotter et al. 2013b, c). The double bonds in the peculiar system 1994; Keegstra et al. 1996b). These previous accounts of DHBQ have much greater stability towards bleach- focused on DHBQ in organic synthesis. Its pivotal role ing agents (under both acidic and alkaline conditions) in cellulose aging and in pulp bleaching chemistry has than conventional double bonds conjugated with each brought its degradation and ‘‘discoloration chemistry’’ other or with carbonyl groups. In acidic media and in to the fore in recent studies (Hosoya and Rosenau solid state, the two hydroxyl protons are equidistant to 2013b, c).