Cyclic peroxides as key intermediates in the degradation of cellulosic key chromophores by alkaline hydrogen peroxide: first direct proof by 17O NMR
Cyclic peroxides as key intermediates in the degradation of cellulosic key chromophores by alkaline hydrogen 17 peroxide: first direct proof by O NMR
0 S. Nomura T. Erata Graduate School of Engineering, Hokkaido University , Sapporo , Japan
1 T. Hosoya Graduate School of Life and Environmental Sciences, Kyoto Prefectural University , Shimogamo-hangi-cho 11-5, Sakyo-ku, Kyoto-shi, Kyoto , Japan
2 M. Bacher N. S. Zwirchmayr S. Nomura A. Potthast T. Rosenau (&) Division of Chemistry of Renewable Resources, Department of Chemistry, University of Natural Resources and Life Sciences , Muthgasse 18, 1190 Vienna , Austria
3 T. Rosenau Johan Gadolin Process Chemistry Centre, A ̊ bo Akademi University , Porthansgatan 3, 20500 A ̊ bo/Turku , Finland
4 T. Vuorinen Department of Bioproducts and Biosystems, Aalto University , Vuorimiehentie 1, 02150 Espoo , Finland
5 T. Dietz Evonik-Degussa , Rodenbacher Chaussee 4, 63457 Hanau-Wolfgang , Germany
6 L. Gille Department of Biomedical Sciences, University of Veterinary Medicine Vienna , Veterina ̈rplatz 1, 1210 Vienna , Austria
2,5-Dihydroxy-[1,4]-benzoquinone (DHBQ) and 5,8-dihydroxy-[1,4]-naphthoquinone (DHNQ) are two key chromophores which are almost ubiquitous in cellulosic materials. Their fate under conditions of alkaline peroxide bleaching (P stage) has been established previously, but the intermediacy of cyclic peroxides, which so far had only been postulated, remained an open issue. By means of 17O NMR spectroscopy, additionally supported by other NMR techniques, it was demonstrated that both DHBQ and DHNQ form cyclic peroxides as primary intermediates in the reaction with hydrogen peroxide under alkaline conditions. These intermediates are subsequently further degraded to products already known. The experimental confirmation of the cyclic peroxides is an important step in the understanding of reaction mechanisms in pulp bleaching chemistry.
Graphical Abstract
Introduction
Three compounds have been established as key
chromophores in cellulosic matrices:
2,5-dihydroxy1,4-benzoquinone (1), 5,8-dihydroxy-naphthoquinone
(2), and 2,5-dihydroxyacetophenone
(Korntner et al.
2015)
. Because of their exceptionally strong
resonance stabilization they are the main survivors of
bleaching attempts, and due to their exceptional
thermodynamic stability they are readily re-formed
from low-molecular weight degradation products
upon aging and brightness reversion. It is their
ubiquity in cellulosic materials that makes them prime
targets of mechanistic studies, which aim both at a
better understanding of their bleaching chemistry and
at high efficiency of their removal or destruction—in
terms of both reactivity and costs.
The degradation of the key chromophores under
conditions of industrial peroxide bleaching (‘‘P
stage’’), i.e. 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 C1 to C4 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). 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 1H and 13C 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.
The task how to prove the existence of such
transient cyclic peroxide intermediates along the
degradation pathway from the chromophores to the
final low-molecular weight products was not trivial.
Complicating was evidently the fact that the
intermediates—as per definition—were not stable, so that they
could not be isolated or determined by
chromatographic techniques. In addition, they were present in a
rather complex mixture, and their high symmetry
made conventional 1H and 13C NMR techniques less
meaningful due to low numbers of characteristic
resonances (note, for instance, that the dianion of
DHBQ (1a) produces a singlet in 1H NMR and two
singlets in 13C NMR).
We came up with 17O NMR
(Berger et al. 1997;
Boykin 1990; Klemperer 1978; Chandrasekaran et al.
1984; Gerothanassis 2010a, b)
as a means to tackle this
problem. This variant of non-metal NMR is still
relatively little used, which is mainly due to the fact
that 17O is rather rare (0.037%) and 17O-labelled
compounds are consequently rather uncommon and
very expensive. In addition, 17O is a spin 5/2 nucleus
with moderate quadrupole moment, which produces
notoriously broad resonances, and low receptivity of
0.06 relative to 13C. This, however, is to a certain
extent counterbalanced (...truncated)