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

Table 1 Reaction 
conditions for each kinetic 
study
NaClO: 5 mmol/g; pH: 10.5
Variable
Temperature 
(°C)
TEMPO 
(mg/g)
NaBr (mg/g)
PFI refining (rev)
Temperature
5
16
100
0
10
16
100
0
15
16
100
0
20
16
100
0
25
16
100
0
30
16
100
0
35
16
100
0
TEMPO catalyst
20
2
100
0
20
4
100
0
20
8
100
0
20
32
100
0
NaBr catalyst
20
16
25
0
20
16
50
0
20
16
200
0
Refining degree
20
16
100
2500
20
16
100
5000
20
16
100
7500
20
16
100
10,000

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Cellulose 
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of k
1
, which will be assumed as the kinetic constant 
of the whole reaction (Eq. 
3
).
where [R-CH
2
OH]
0
and [R-COOH]
t
are the initial 
concentration of primary alcohols and the carboxyl con-
tent at time t in the BEKP fibers, respectively, and both 
expressed in µeq/g. k
1
is the kinetic constant, expressed 
in reciprocal time units. A value of 1500 µeq/g was 
assumed for [R−CH
2
OH
]
0
, as it has been reported 
to be the maximum CC that can be achieved through 
TEMPO-mediated oxidation (Saito et al. 
2007
). Higher 
values might be found in the literature but can be attrib-
uted to differences on the quantification methods (Fuji-
sawa et al. 
2011
).
Conversion, expressed as the relationship between 
the reacted CH
2
O to COO
−
groups compared to the 
experimental oxidation limit, this is the maximum car-
boxyl content achieved, was calculated according to 
Eq. 
4
.
where CC
t
is the carboxyl content at time t, CC
0
is 
the initial carboxyl content of the neat fiber, and 
CC
max
is the maximum carboxyl content experimen-
tally achieved during oxidation. The time required 
for a complete conversion (t
final
) was calculated from 
the linear regression resulting from Eq. 
5
, where 
(1 −
X)
1∕2
was expressed as function of time (Sbiai 
et al. 
2011
).
The activation energy (Ea) was calculated according 
to the Arrhenius equation, which is given in Eq. 
6
.
where k is the kinetic constant, A is the pre-exponen-
tial factor, also known as Arrhenius factor, R is the 
universal gas constant, and T is the temperature.
(3)
ln
([
R−CH
2
OH
]
0
− [
R − COOH]
t
)
=
−
k
1
t + ln
([
R − CH
2
OH
]
0
)
(4)
X =
CC
t
−
CC
0
CC
max
−
CC
0
(5)
(1 −
X)
1∕2
= 1 −
t
t
final
(6)
k = A ⋅ e
−
Ea
RT
Results and discussion
Validation of NaOH consumption as real-time 
monitoring parameter
The relationship between NaOH consumption during 
TEMPO-mediated oxidation and the oxidation degree 
of fibers has been already reported (Sun et al. 
2005
). 
However, this correlation may depend on the avail-
ability of –CH
2
OH groups at the fiber surface and 
their concentration. An appropriate study correlating 
the NaOH consumption with the CC of the TEMPO-
oxidized fibers at different reaction conditions (i.e. 
temperature, TEMPO/NaBr concentration, surface 
area of the fibers) is required for suitably monitoring 
the evolution of the reaction in real time. This would 
underpin the hypothesis of using the NaOH consump-
tion for the real-time monitoring of the TEMPO-
mediated oxidation kinetics and, thus, minimizing the 
use of time-consuming techniques such as the deter-
mination of the CC during an industrial batch produc-
tion of TEMPO-oxidized fibers.
Figure 
1
 shows the correlation between the 
NaOH consumption, in mmol/g, and CC, in µeq/g, 
at different temperatures (Fig. 
1
A), different 
TEMPO concentrations (Fig. 
1
B), different NaBr 
concentrations (Fig. 
1
C), and different cationic 
demand of the fibers (Fig. 
1
D).
The CC of the samples evolved linearly with the 
NaOH consumption during TEMPO-mediated oxi-
dation in all cases. However, while the variation of 
the temperature (Fig. 
1
A), the NaBr concentration 
(Fig. 
1
C) and the initial cationic demand of the fibers 
(Fig. 
1
D) did not affect the tendency, leading to simi-
lar slopes and y-intercepts (Tables S1, S3 and S4 from 
the Supplementary Material), the case of TEMPO 
catalyst (Fig. 
1
B) differed from the rest (Table S2). In 
this case, as the amount of TEMPO increased from 2 
to 16 mg/g in the reaction media, the consumption of 
NaOH decreased for a certain CC. However, a similar 
slope was found between 16 and 32 mg/g indicating 
no effect of increasing the dosage of TEMPO catalyst 
from 16 mg/g. As revealed in Tables S1 to S4, the 
correlation factors (R
2
) were around 0.99 in all cases, 
indicating an excellent fitting of the linear regression 
for all the reaction conditions and, thus, the suitabil-
ity of NaOH consumption as an indicator of the CC 
during TEMPO-mediated oxidation. Albeit it is not 
shown in Fig. 
1
, additional NaOH was added to the 

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 Cellulose
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Vol:. (1234567890)
fibers, and no change was observed on the CC, find-
ing its maximum at 800 µeq/g. This is in accordance 
with previously published studies, where this maxi-
mum was already reported at 5 mmol/g of NaClO 
addition (Serra et al. 
2017
).
The linear regressions from Fig. 
1
A, C, D 
exhibited similar average slopes and y-intercepts 
(corresponding to the theoretical initial CC of the 
fibers), and low standard deviation. Except for the 
case represented in Fig. 
1
B, corresponding to variable 
amounts of TEMPO, the correlation between the CC 
and the NaOH consumption was the same regardless 
the reaction conditions, which is of interest for the 
industrialization of the reaction. This clearly confirms 
that NaOH consumption could be easily used as real-
time monitoring parameter of the TEMPO-mediated 
oxidation at large scale, and reveals a new opportunity 
for this reaction not only in batch processing, but also 
for continuous production. Focusing on the exception, 
the lowest CC at low TEMPO addition (2 to 4 mg/g) 
for a certain NaClO addition (i.e. 5 mmol/g) was 
previously observed by Serra et al. (
2017
). Lin et al. 
(
2018
) also found a strong influence of TEMPO 
during the formation of carboxyl groups at the fibers. 
However, the authors worked only in two conditions 
regarding TEMPO, in absence and containing 
16 mg/g. Considering the reaction mechanism, 
widely described in the literature, limiting the 
presence of TEMPO has a direct influence over the 
reaction from Eq. 
1
, corresponding to the formation 
of the aldehyde group, which is the most determinant 
in the process of TEMPO-mediated oxidation of 
cellulose. Further, the lower generation of aldehyde 
groups also limits the formation of carboxyl groups, 
which is the selected parameter to monitor the 
oxidative reaction (Saito and Isogai 
2004
; Sun et al. 
2005
; Dai et al. 
2011
; Isogai et al. 
2011
). The slope 
between the CC and NaOH consumption reveals that 
the reaction maximum conversion from –CH
2
OH to 
–COO
−
groups is satisfactorily achieved for dosages 
between 8 and 32 mg/g of TEMPO, to be significantly 
decreased below 8 mg/g. This can be clearly observed 
in Table S2, where the slopes at different TEMPO 
dosages are provided and similar values from 8 to