Salient Reactions in Lignin During Pulping
and Oxygen Bleaching: An Overview
D.S. ARGYROPOULOS
INTRODUCTION
While the ether bonds of lignin cleave
efficiently under alkaline pulping conditions,
the formation of condensed structures decrease
its reactivity and diminish the solubility of re-
sidual lignin from within the pulp matrix. Dur-
ing the past several years, a number of
modifications have been proposed for the con-
ventional kraft pulping process, aimed at en-
hancing delignification. Rapid displacement
heating and modified continuous cooking are
two examples of these processes. Among the
reactions that are known to interfere with the
process of alkaline delignification are those in-
volving the formation of carbon–carbon bonds.
On another front, regulatory and eco-
nomic pressures have driven the pulp and paper
industry to implement new delignification and
bleaching practices. A major transition is the
partial replacement of chlorine or chlorine di-
oxide delignification stages with oxygen
delignification. However, the effectiveness of
an oxygen-delignification stage is limited to the
50% level. Beyond this level, severe cellulose
degradation takes place, resulting in the deteri-
oration of pulp viscosity and strength charac-
teristics. This selectivity issue is a major draw-
back of oxygen delignification.
Aromatic condensation reactions also
have been thought to decrease the reactivity of
residual lignin toward oxygen delignification
[1,2]. The reactivity of various condensed units
present in kraft pulp has been explored using
degradation techniques. Condensed structures
such as diphenylmethane (DPM) and biphenyls
were found to be resistant during oxygen
delignification and were held responsible for
the slow phase of oxygen delignification [1,3].
The reactivity of the condensed structures
within residual kraft lignin under oxygen-
delignification conditions was recently evalu-
ated in detail [4,5]. Even under homogeneous
oxidative conditions with no mass-transfer lim-
itations operating, the 5-5¢biphenyl condensed
structures were found to be fairly stable toward
the oxidation [5]. Despite the many research ef-
forts, the question of which structures are re-
sponsible for activation and which are
responsible for lignin deactivation during oxy-
gen delignification still remain unanswered. In
view of the above, we focussed our efforts on
defining the formation of DPM and 5-5'
biphenolic structures in lignin as they occur
during conventional kraft and soda pulping
conditions [6]. The accumulated kinetic data
for the formation of these species under soda
and kraft pulping conditions was then corre-
lated to known chemistry [6]. Furthermore, our
continuing efforts to understand the fundamen-
tals of oxygen delignification prompted us to
focus on systematically examining the residual
lignins within an oxygen-delignified fibre [7].
To do this, a newly developed method for iso-
lating residual lignin [8], which provides resid-
ual lignin in high yield, was coupled with a
systematic sampling of delignified pulp and
spent liquor at various stages of oxygen
delignification. The isolated residual and
solubilized lignins were then subjected to de-
tailed spectroscopic analyses [8,9]. This ap-
proach allowed a systematic assessment of the
structural changes occurring within the lignin
during the oxidation process. This paper will at-
tempt to summarize our findings in this impor-
tant area.
EXPERIMENTAL
Details of the experimental procedures
discussed in this overview have already ap-
peared in the literature [4–11]. More specifi-
cally, the details of treating milled-wood lignin
under homogeneous kraft or soda pulping con-
ditions are given in [6]; while the details of the
oxygen-delignification experiments dealing
with lignin samples isolated from fibres are
given in [7]. The new residual lignin isolation
protocol applied during the overall effort de-
scribed in this paper has been published in [8],
while the novel quantitative 13C nuclear mag-
netic resonance (NMR) spectroscopic acquisi-
tion protocol using internal standards is
described in detail in [9]. Traditional
308 JOURNAL OF PULP AND PAPER SCIENCE: VOL. 29 NO. 9 SEPTEMBER 2003
D.S. Argyropoulos
Dept. Wood Paper Sci.
North Carolina State Univ.
Raleigh, NC, USA
27695-8005
(dimitris.argyropoulos@ncsu.edu)
This overview attempts to summarize our efforts focussed on defining the formation of the various condensed structures in lignin as they emerge dur-
ing conventional kraft, EMCC, soda and oxygen- delignification conditions. The role and fate of the newly discovered structures of dibenzodioxocins
are also discussed and their significance toward pulping and bleaching operations emphasized. Finally, the fate and role of the condensed phenolic
and p-hydroxyphenyl structures under oxygen-delignification conditions are discussed.
Nous résumons ici notre travail visant à définir la formation de diverses structures condensées de lignines développées lors de la délignification à
l’oxygène, du procédé EMCC, et de la cuisson kraft classique et à la soude. Nous discutons aussi du rôle et du sort des oxanthrènes
(dibenzodioxocines) et mettons l’accent sur leur signification lors de la mise en pâte et du blanchiment. Nous discutons aussi du sort et du rôle des
structures p-hydroxyphényles et phénoliques condensées lors de la délignification à l’oxygène.
