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

Table 2 Reaction rate (k
1
), y-intercept and experimental data 
fitting (R
2
) as function of temperature
Temperature 
(°C)
k
1
(s
−1
) [10
–4
]
y-intercept
R
2
5
0.58
7.21
0.9700
10
1.40
7.32
0.9803
15
2.90
7.22
0.9775
20
3.63
7.35
0.9789
25
5.16
7.20
0.9476
30
5.50
7.20
0.9384
35
5.27
7.21
0.9545
Fig. 3 Evolution of k
1
as function of temperature (A) and Arrhenius Plot for the oxidation of BEKP from 5 to 25 °C, linear 
regression, and experimental data fitting (R
2
) (B)

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Cellulose 
1 3
Vol.: (0123456789)
the maximum value found in previous works for an 
NaClO amount of 5 mmol/g.
The lag time at the beginning of each kinetic 
curve is related to the availability of TEMPO and 
Br
–
. Although the rate equations considered here are 
empirical and not mechanistic, this lag is due to the 
activation of the catalyst (TEMPO+). This stage is 
faster than the regioselective oxidation of primary 
–OH groups (Pääkkönen et al. 
2015
), especially in the 
case of high Br
–
doses, but it is of utmost relevance. 
In fact, we have observed that both lag and total reac-
tion times decrease with TEMPO concentration for a 
given dose of NaBr (Figure S1), and that those times 
are roughly but significantly intercorrelated (Pear-
son’s r = 0.914).
In a previous work, the authors already observed 
that reaction time increased at decreasing TEMPO 
concentration, but no further discussion was pro-
vided except for a significant reduction on production 
costs (Serra et al. 
2017
). The catalyst concentration 
that can be most commonly found in the literature is 
16 mg/g, as it corresponds to 1 mol of TEMPO per 
mol of cellulose (Tarrés et al. 
2017
; Levanič et al. 
2020
). Indeed, the stoichiometric relationship corre-
sponds to 1:1 and, as revealed in Fig. 
2
B, doubling up 
the TEMPO:cellulose ratio (32 mg/g) had no effect 
on reaction time. Table 
3
shows the evolution of the 
obtained reaction rates as function of the catalyst con-
centration, as well as the value of the y-intercept and 
the experimental fitting (R
2
) of Eq. 
3
.
The effect of TEMPO concentration was signifi-
cant, as k
1
was decreased to 0.47 s
−1
, from the max-
imum of 3.63 s
−1
for 16 mg/g at 20 °C. In this case, 
the average y-intercept was found at 7.30 ± 0.07, 
corresponding to an initial CH
2
O concentration of 
1477 µeq/g.
Differently from the TEMPO concentration, 
reducing the amount of Br
−
available (Fig. 
2
C) 
was not found to affect the oxidation degree of 
the resulting fibers, as all the batches resulted in 
fibers with a CC near to 800 µeq/g. This might 
indicate that the concentration of TEMPO holds a 
higher interference than NaBr when considering 
side reactions than the Br
−
concentrations (Spier 
et al. 
2017
). However, due to the reaction scheme 
provided in the previous section, the availability 
of Br
−
influences the reaction rate, particularly to 
k
1
and, thus, the evolution of the primary alcohol 
conversion into carboxyl as function of time. 
Table 
4
shows the evolution of the reaction rate as 
function of the co-catalyst concentration, along with 
the value of the y-intercept and the experimental 
fitting (R
2
) of Eq. 
3
. Again, no significant changes 
were found between 100 and 200 mg/g, while the 
reduction of the NaBr dosage from 100 to 50 and 
25 mg/g dramatically affected the reaction kinetics, 
clearly indicating that operating below 100 mg/g 
would be detrimental in terms of production.
The decrease on k
1
with the reduction of NaBr 
concentration can be explained by the fact that the 
availability of Br
−
dictates the pace of radical for-
mation, which is a fundamental step of the oxida-
tive reaction. In addition, a decrease in the oxida-
tion degree with higher concentrations of NaBr has 
been previously reported, hypothesizing that higher 
presence of Br
−
would suppress the formation of the 
secondary oxidant (HBrO) and, therefore, hindering 
the consecution of the reactions (Lin et al. 
2018
). 
Thus, doubling the amount of NaBr would be detri-
mental for the reaction rate, clearly indicating that it 
should remain at 100 mg/g.
Finally, Fig. 
2
D reveals an increase on the reaction 
rate with the refining degree of the BEKP. The 
mechanical action of mechanical refining has been 
reported to increase the surface area of fibers, as well