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

Conclusions
The present work provides a deep understanding of 
the reaction mechanism of cellulose with TEMPO/
NaBr/NaOH system, as well as the effect of process 
conditions on the reaction kinetics. In a first part of 
the study, it was found that regardless the process 
conditions, the NaOH consumption exhibited a lin-
ear relationship with the carboxyl content of the fib-
ers, which indicates that the evolution of the reaction 
could be directly monitored by means of considering 
the amount of NaOH per gram of fiber. This infers the 
possibility of using a calibration curve to estimate the 
oxidation degree based on NaOH addition, potentially 
skipping a characterization step that could be per-
formed only for validation purposes. In addition, this 
relationship was found to be the same at varying tem-
perature, NaBr content and surface area of the fibers, 
while different TEMPO contents were found to have 
different impact on this correlation. This was attrib-
uted to the capacity of TEMPO to protect the fibers 
and then, preventing their depolymerization. In light 
of the competing OH(6) oxidation and the oxidative 
cleavage of β-1,4 bonds, we advise against the com-
mon practice of quenching the reaction once the pH 
remains stable. This way, the last period of the pro-
cess still shows consumption of ClO
–
, but particularly 
directed to depolymerization. Instead, the process 
should be finished in a predetermined, optimized 
NaOH consumption.
The obtained results were consistent with those 
previously reported for cotton or regenerated cel-
lulose, particularly in terms of energy of activation, 
which validates the study. Modifying the process 
conditions only resulted in positive impact for tem-
perature, where the required time for complete con-
version was found to be significantly decreased at 
25 and 30 °C. However, this was not observed at 
increasing TEMPO and NaBr contents, and only a 
slight reduction was observed at increasing PFI revo-
lutions. Further, the increase on surface area resulted 
in lower yield reactions, which implies the generation 
of higher amount of waste in the form of dissolved 
substances. Finally, pH was found to have low effect 
on oxidation, at least between 10.5 and 11.5, where 
no significant differences were observed at relatively 
high CC.
The present work was conducted at a NaClO dos-
age of 5 mmol/g and results are consistent with a 
pseudo-first order kinetics. However, the need of con-
ducting further studies at different NaClO amounts, 
as well as further understanding the mechanism of 
TEMPO, is clear. Moreover, despite the proven use-
fulness of monitoring NaOH consumption and CC, 
other characteristics of the oxidized fibers should be 
taken into account: rheology, transmittance, conduc-
tivity. Besides these properties, which can be meas-
ured in-line, the effects of TEMPO-mediated oxida-
tion on remaining hemicelluloses can impact the 
quality and usability of the end product.
Acknowledgments Authors wish to acknowledge the 
financial support of the funding agencies listed in the 
“Funding” section for their support to develop the present 
work. Marc Delgado-Aguilar and Quim Tarrés are Serra 
Húnter Fellows.