Kinetic study and real-time monitoring strategy for tempo-mediated oxidation of bleached eucalyptus fibers

Fig. 4 Correlation between 
t
final
and k
1
for all the tested 
conditions
Fig. 5 Effect of the pH of the medium on the NaOH versus 
CC relation. Inset figure: influence on the apparent reaction 
rate

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In addition, we hypothesize that kinetic control is 
favored by the initial effect of surface oxidation, rap-
idly disrupting the supramolecular structure of the 
fiber. In cases of optimal conditions, this latency time 
is absent. Hence, the presence or lack of mass trans-
fer limitations depends on proper TEMPO activation 
and on the initial rates of reaction, affected by catalyst 
concentration.
Figure 
5
schematizes the different processes under-
gone by a cellulosic fiber through TEMPO-medi-
ated oxidation, from its supramolecular structure 
to a molecular scale. We claim that there is synergy 
involving regioselective oxidation, depolymerization, 
and subsequent processes of hydration, spacing, peel-
ing, and unbundling. First, carboxylate groups grant 
the presence of more water molecules while hinder-
ing cellulose–cellulose intermolecular interactions. 
Hence, a higher hydration degree results in more 
effective mass transfer phenomena, therefore expos-
ing the β-1,4 acetal bonds to oxidative cleavage by 
BrO
–
/ClO
–
. This effect is also highlighted by how 
mechanical refinement improves the reaction rate, as 
it increases the availability of groups susceptible to 
oxidation (Fig. 
2
D).
It should be noted that these oxidants are 
consumed by both reactions, OH(6) oxidation and 
oxidative cleavage of glycosidic bonds. For lower 
TEMPO concentrations, a lower carboxyl content is 
achieved, possible due to a higher consumption of 
oxidants into oxidative cleavage, since the presence of 
aldehyde groups formed by the first stage of oxidation 
remains low during the reaction time. In other words, 
a higher ratio of TEMPO to ClO
–
/BrO
–
grants higher 
selectivity to OH(6) oxidation, avoiding excessive 
depolymerization (Spier et al. 
2017
) (Fig. 
6
).
The degree of polymerization of polysaccharides 
and the carboxyl content attained by TEMPO-medi-
ated oxidation are unequivocally correlated, both 
increasing with the oxidative charge and pH (Shinoda 
et al. 
2012
; Serra et al. 
2017
). While it is known that 
a pH around 10 is ideal for the stability of TEMPO, it 
has been as well reported that the higher the pH, the 
higher the extent of the depolymerization (Spier et al. 
2017
; Lin et al. 
2018
). This is due to both the higher 
Fig. 6 Schematic representation of oxidation, depolymerization, hydration, and partial disruption of fibers and microfibers

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concentration of hydroxide ions and the availability 
of deprotonated hyprobromite and hypochlorite ions.
Whilst a high enough concentration of oxidants 
is required for TEMPO activation, we envisage fur-
ther research on plausible alternative methodologies 
that uphold this activated TEMPO without majorly 
compromising the supramolecular structure of fibrils. 
These conditions could involve controlled addition of 
oxidants or use of HBr, which would grant lower val-
ues of pH since the early stages of reaction while not 
affecting the Br
–
availability; therefore not hindering 
the catalysis provided by it during TEMPO activation 
followed by –CHO oxidation into –COO
–
. These two 
measures have the potential to achieve high carboxyl 
content in fibers with higher degree of polymerization 
than the ones reported elsewhere (Serra et al. 
2017
; 
Lin et al. 
2018
).