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

Fig. 2 Evolution of CC of the oxidized pulp with time as function of temperature (A), TEMPO dosage (B), NaBr dosage (C) and 
CD (D)

---

 Cellulose
1 3
Vol:. (1234567890)
which may be attributed to their lower activation 
energy or frequency factor. This delay on the 
increase of the CC was not considered to determine 
the reaction rate, as it may interfere on the slope 
and increase the fitting error of the data. Table 
2
shows the obtained reaction rates (slope of Eq. 
3
), 
the logarithm of the initial CH
2
OH concentration 
(y-intercept) and the experimental data fitting (R
2
) 
for each temperature. In addition, the evolution of 
the reaction rate (k
1
) as function of temperature is 
provided in Fig. 
3
A for further clarification, as well 
as its fitting to the Arrhenius equation (Fig. 
3
B).
The obtained reaction rates are in agreement with 
some previously reported for other raw materials, 
such as cotton (Dai et al. 
2011
) or regenerated 
cellulose (Sun et al. 
2005
). In addition, the reaction 
rate evolved linearly with temperature from 5 to 
25 °C (R
2
= 0.9877), where an important change on 
the slope was observed to be stabilized at 35 °C. This 
indicates that temperature imparts a positive effect on 
reaction kinetics until certain point, where selectivity 
starts to decrease, affecting the conversion of the 
alcohol groups into carboxyl. Oxidative reactions, but 
also exothermic reactions in general, tend to decrease 
their selectivity with the increase of temperature, 
mainly because it becomes harder to maintain locally 
optimal concentrations of feed, oxidant, and product 
(Towler and Sinnott 
2021
). The average y-intercept 
accounted for 7.24 ± 0.06, which results in an average 
initial concentration of CH
2
O of 1400 µeq/g. The 
evolution of the kinetic constants as function of 
temperature allowed the determination of activation 
energy (Ea) using the Arrhenius equation (Eq. 
6
), as 
reflected in Fig. 
3
B, where a good linear relationship 
between the different temperatures and the reaction 
rate can be observed. The slope accounted for 
− 8864.37, which resulted in an Ea of 73.70 kJ/mol 
for the selected conditions of catalyst concentration 
and oxidizer amount. This value of Ea is of the same 
order of magnitude than those reported for cotton 
or regenerated cellulose, which validates the kinetic 
study with a commercial bleached kraft pulp (Sun 
et al. 
2005
; Dai et al. 
2011
).
In the case of the effect of TEMPO concentration, 
Fig. 
2
B shows the evolution of the CC, in µeq/g, as 
function of time for the tested catalyst concentrations. 
As expected, not only the kinetic constant decreased 
with lower TEMPO catalyst concentration, but the 
oxidation was considerably lower for the cases using 
2 and 4 mg/g of TEMPO, particularly in the case 
of the former. These results are in accordance with 
the ones reported by Sun et al (
2005
) and Lin et al 
(
2018
), where it was demonstrated the possibility of 
reaching between 800 and 850 µeq/g of CC, which is