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

Experimental
Materials
Bleached eucalyptus kraft pulp (BEKP) was kindly 
supplied by Torraspapel, S.A. (Zaragoza, Spain). 
All the chemicals involved in this study, both for 
TEMPO-mediated oxidation and characterization 
of the materials, were obtained from Merck (Barce-
lona, Spain). All reagents were used without further 
purification.
BEKP fibers preparation and characterization for 
TEMPO-mediated oxidation
BEKP was supplied in the form of dried (10 wt% of 
water content) laminates. Prior to TEMPO-mediated 
oxidation, BEKP fibers were disintegrated in a labo-
ratory disintegrator, according to ISO 5263-1:2004. 
Briefly, 30 g (oven-dried weight) of BEKP were sus-
pended in 1.5 L of deionized water and disintegrated 
in a laboratory disintegrator for 20 min at 3000 rpm. 
In the case of mechanically refined pulps, the whole 
suspension was vacuum-filtered using a 200 mesh 
filter to be later diluted to 10 wt% consistency. The 
high-consistency pulps were then refined at 2500, 
5000, 7500 and 10,000 revolutions in a PFI mill 
(Metrotec, model NPFI 02), according to ISO 5264-
2:2011. The PFI refiner counts on an energy moni-
toring and data acquisition system that provides the 
required energy to mechanically refine the pulps.
Schopper–Riegler degree (°SR) was measured 
according to ISO 5267-1:1999 in a 95,587 PTI 
Schopper–Riegler tester, obtaining 16, 19, 24, 32, 
and 41°SR for 0–10,000 PFI revolutions, respec-
tively. Cationic demand (CD) was also measured 
in a Mütek Particle Charge Detector PCD-06 from 
BTG Instruments (Weßling, Germany). Briefly, 0.1 g 
of dry BEKP were mixed with a known excess of 
poly(diallyldimethylammonium chloride) (polyDAD-
MAC) in a deionized water medium. The suspen-
sion was then centrifuged for 30 min at 10,000 rpm 
and the supernatant, once removed, was titrated with 
sodium polyethylene sulfonate (PES-Na) until the iso-
electric point (0 mV) (Serra-Parareda et al. 
2021a
). 
Concretely, cationic demands of 33.04, 53.62, 71.88, 
78.00, and 102.69 µeq/g were obtained for the 
refined pulps between 0 and 10,000 PFI revolutions, 
respectively.
TEMPO-mediated oxidation: sampling for the kinetic 
study
TEMPO-mediated oxidation was conducted in a 
stirred batch tank reactor equipped with temperature 
control under different conditions, aiming at deter-
mining the effect of different process parameters 
to the kinetics of the reaction. In a typical experi-
ment, for the oxidation of 10 g (oven-dried weight) 
of BEKP, the required amount of TEMPO and NaBr 
catalysts were previously dispersed in 500 mL of 
deionized water. After complete dissolution of the 
catalysts, 10 g of BEKP were incorporated into the 
stirred batch reactor and additional deionized water 
was added, until reaching a consistency of 1 wt% 
with respect to BEKP. Once the fibers were fully dis-
persed, the reaction was started by means of adding 
the required amount of NaClO. The starting pH of the 
reaction was 11.5 dropping to 10.5 in the early stages 
of the reaction and maintained at 10.5 by means of 
gradually adding a 0.5 M NaOH solution. The con-
sumed NaOH was constantly monitored as function 
of time. In order to obtain the correlation between the 
consumed NaOH and the carboxyl content, one of the 
determining parameters to monitor the evolution of 
the oxidation, small batches were prepared in parallel, 
stopping the reaction at different times. The reactions 
were stopped after a certain amount of NaOH con-
sumption (0.33, 0.66, 1.00 and 1.17 mmol) per gram 
of fiber oxidized with 5 mmol/g of NaClO.
The kinetic study was conducted considering 
four different approaches: (i) effect of temperature 
(5–35 °C), (ii) effect of TEMPO catalyst concen-
tration (2–32 mg/g), (iii) effect of NaBr amount 
(25–200 mg/g), and (iv) effect of refining degree of 
BEKP (0–10,000 revolutions of PFI). The rest of the 
oxidation conditions are described in Table 
1
.
All samples were stored in hermetic plastic bags 
at 4 °C to prevent any degradation. The effect of pH 
was also assessed, increasing from 10.5 to 11.5 main-
taining the rest of the conditions at 20 °C, 16 mg/g 
of TEMPO, 100 mg/g of NaBr and using unrefined 
BEKP as substrate.
CC was determined by adapting a methodology 
introduced by Weber and Husemann (
1942
), as 
described in more detail elsewhere (Tarrés et al. 
2017
). Briefly, 3–5 mg of dry fiber are added in 
a solution containing 5 mL of methylene blue 
(300 mg/L) and 5 mL of a borate buffer solution at pH 

---

 Cellulose
1 3
Vol:. (1234567890)
8.5. After gentle stirring, followed by centrifugation 
at 10,000 rpm for 20 min, 2 mL of the supernatant 
were transferred into a 25 mL flask containing 
2.5 mL of HCl 0.1 M, the volume was completed 
with distilled water. The absorbance at 664 nm was 
then measured. This method allows for simultaneous 
measurements of several samples per run, speeding 
up the characterization process in comparison to 
conventional titrations.
TEMPO-mediated oxidation reaction mechanism and 
kinetics
The oxidation mechanism of cellulose by the system 
TEMPO/NaBr/NaClO has been extensively described 
in the literature since the last century (Farkas et al. 
1949
; de Nooy et al. 
1994
; Saito and Isogai 
2004
; Dai 
et al. 
2011
). The oxidation of the primary alcohols 
of cellulose chains, located in carbon 6 (C6), cata-
lyzed by TEMPO and with NaBr, has been described 
as a reaction with three main steps, consisting of (i) 
the formation of HBrO from NaClO, (ii) the genera-
tion of TEMPO
+
, and (iii) the formation of carboxyl 
groups from primary alcohols in C6. This mechanism 
previously described in detail in the literature (Saito 
and Isogai 
2004
; Sun et al. 
2005
).
The main reactions occur in the third step, 
consisting of the oxidation of primary alcohol to an 
intermediate aldehyde (Eq. 
1
) and the oxidation of 
the intermediate aldehyde to carboxyl (Eq. 
2
).
where R-CHO-H
2
O from Eq. 
2
 corresponds to the 
hydrated aldehyde, whose formation is neglected 
in terms of the rate-controlling stage, as it has been 
reported significantly more rapid than the oxidation 
of the alcohol (Friedlander et al. 
1966
; de Nooy et al. 
1995
).
The oxidation of alcohol to the intermediate 
aldehyde (Eq. 
1
) has been reported to be a rate-
determining step, as k
2
> k
1
, and the oxidation of 
cellulose can be assumed as a consecutive first-
order reaction system (Jiang et al. 
2000
; Sun et al. 
2005
; Dai et al. 
2011
). Thus, the kinetics of the 
TEMPO-catalyzed oxidation, in terms of primary 
alcohols consumption, can be expressed as function 
(1)
R−CH
2
OH + TEMPO
+
+ OH
−
k
1
�
�������
→ R − CHO + TEMPOH + H
2
O
(2)
R−CHO−H
2
O + (H)OBr + OH
−
k
2
�
�������
→ R − COOH + Br
−
+ 2H
2
O