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

Table 5 Reaction rate (k
1
), y-intercept and experimental data 
fitting (R
2
) as function of refining degree of fibers
PFI (rev) °SR CD (µeq/g) k
1
(s
−1
) 
[10
–4
]
y-intercept R
2
0
16
33.05
3.63
7.35
0.9798
2500
19
53.61
3.78
7.31
0.9955
5000
24
71.88
3.88
7.29
0.9945
7500
32
78.00
4.05
7.39
0.9985
10,000
41
102.69
4.12
7.37
0.9884

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Cellulose 
1 3
Vol.: (0123456789)
costs. In this context, TEMPO prices have been 
regarded as a key barrier for the industrial-scale 
production of nanoscale oxycellulose (Clauser et al. 
2022
). The improvement in terms of reaction time 
due to the increase of the initial surface area of the 
fibers, this is PFI refining, is not justified because 
of the lower yield (waste generation) and additional 
energy consumption (4.44 kWh/kg for 10,000 PFI 
revolutions).
According to the literature, pH of roughly 10–11 
should be maintained to ensure optimal conditions 
of reaction. As seen from Fig. 
5
, the difference 
in pH from 10.5 to 11.5 shows, at the beginning 
of the process, higher reaction rates (inset figure) 
and a higher conversion for the same consumption 
of NaOH (Lin et al. 
2018
). Differences beyond 
0.5–0.6 mmol COO
–
/g are non-significant, and thus 
the final oxidation degree does not depend on pH over 
this narrow range. Differing from previous results, 
reactions conducted with pH values above 11 did not 
result in fibers with lower oxidation degree.
Taking into account the pK
a
values of hypobro-
mous and hypoclorous acids, respectively 8.6 and 
7.5 (Kim et al. 
2021
), their ionization under the pH 
range studied is quantitative, with little difference at 
pH 10.5 or pH 11.5. Therefore, all possible effects 
of OH
–
concentration are related to cellulose itself. 
With a pK
a
value between 12 and 13 for the primary 
hydroxyl groups (Bialik et al. 
2016
), an increase in 
pH from 10.5 to 11.5 is enough to explain why the 
polymer is more accessible during the first stages of 
the reaction in the latter case.
Insights into the progressive accessibility of primary 
hydroxyl groups through OH(6) oxidation and 
depolymerization
Reservations to reality compel us to justify why 
homogeneous models satisfactorily describe the 
TEMPO-mediated oxidation of cellulosic fibers, 
while etherification of cellulose, for instance, tend 
to be better fitted by chemisorption models (Aguado 
et al. 
2018
). It is possible to observe from Fig. 
2
that 
for lower temperature and concentration of catalysts 
and co-catalysts, there is an increase in the latency 
time until the reaction assumes an apparent first-
order pattern. That could be explained by lower ener-
getic availability from a thermodynamic perspective.