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

Keywords Cellulose · Kinetics · Regioselective 
oxidation · TEMPO-mediated oxidation · Carboxyl 
content
Introduction
At the present time, the most popular pathway for 
the regioselective oxidation of the primary hydroxyl 
groups of cellulose involves a stable aminoxyl radical, 
(2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO). 
Its key advantages comprise the use of aqueous 
media and its ease of regeneration in proper systems, 
allowing for catalytic amounts to be employed 
(Balea et al. 
2019
; Turk et al. 
2020
; Fedorov et al. 
2020
; Serra-Parareda et al. 
2021b
). When activated 
Supplementary Information The online version 
contains supplementary material available at 
https:// doi. 
org/ 10. 1007/ s10570- 022- 05013-7
.
A. Mazega · R. Aguado · Q. Tarrés · M. À. Pèlach · 
M. Delgado-Aguilar (*) 
LEPAMAP-PRODIS Research Group, University 
of Girona, C/ Maria Aurèlia Capmany, 61, 17003 Girona, 
Spain
e-mail: [e-mail]
A. Mazega · R. Aguado · Q. Tarrés · N. Fiol · 
M. À. Pèlach · M. Delgado-Aguilar 
Chemical Engineering Department, University of Girona, 
C/ Maria Aurèlia Capmany, 61, 17003 Girona, Spain
A. F. Santos 
Department of Chemical Engineering, Federal University 
of Paraná, Curitiba, PR 81531-990, Brazil

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to its N-oxoammonium form, herein referred to as 
TEMPO+, it selectively oxidizes primary alcohols to 
aldehydes, remaining unreactive towards secondary 
or tertiary alcohols.
The activation of TEMPO can be attained electro-
chemically on an anode, supplying a certain current 
(Zeng et al. 
2022
), but it is more typically carried out 
in oxidative media at pH 9–11.5, comprising bromide 
and hypochlorite ions (Tarrés et al. 
2022
). It involves 
the loss of one electron from the radical form (ami-
noxyl), or two electrons from the hydroxylamine form 
(TEMPOH) (Nutting et al. 
2018
). It should be noted 
that the Br
–
|BrO
–
||ClO
–
|Cl
–
system causes both the 
generation of TEMPO
+
and the conversion of car-
bonyl groups to carboxylate groups (de Nooy et al. 
1995
). Furthermore, these BrO
–
and ClO
–
ions, not 
TEMPO, are responsible for the depolymerizing side 
reaction (Spier et al. 
2017
). Among these ions, ClO
–
, 
whose standard reduction potential (0.81 V) is higher 
than that of BrO
–
(0.76 V), is the spent oxidant (Kuc-
era 
2019
). The other oxidants, TEMPO
+
and BrO
–
, 
are regenerated along the process, and thus regarded 
as catalyst and co-catalyst, respectively (Saito and 
Isogai 
2004
; Filipova et al. 
2020
; Serra-Parareda et al. 
2021a
).
Back in 1996, one of the earliest reports (if not the 
earliest) of the TEMPO-mediated oxidation of cellu-
losic fibers concluded that, unlike for water-soluble 
polysaccharides, the conversion was not quantitative 
even in excess of hypochlorite (Besemer et al. 
1998
). 
Some years later and up to date, research groups 
focused on nanocellulose have taken huge advantage 
of this apparent limitation, since the purpose is gener-
ally isolating, hydrating and/or or unbundling fibrils, 
not dissolving them or completely disrupting their 
crystalline structure (Tarrés et al. 
2016
; Isogai et al. 
2018
; Beaumont et al. 
2021
). Nonetheless, it may be 
worth clarifying that the claim only holds true for cel-
lulose I, as a quantitative conversion of OH(6) has 
been reported for cellulose II, cellulose III, and amor-
phous cellulose (Isogai et al. 
2011
).
Many applications require a partial oxidation of 
cellulose, not even (or not necessarily) reaching the 
highest conversion. To attain a desirable content of 
carboxylate groups (CC), it is common practice to 
select a proper ratio of ClO
–
to cellulose, and then to 
perform the reaction until all hypochlorite has been 
spent (Tarrés et al. 
2017
). Along the reaction, NaOH 
is added to keep the pH within a certain interval, 
usually around 10 pH units. Then, the endpoint of 
the reaction is marked by the attainment of constant 
pH without further addition of alkali. This signals 
the complete consumption of hypochlorite, but 
carrying out the reaction until total conversion 
of the limiting reagent presents drawbacks. The 
most evident one is the time spent, which is one 
of the reasons why the upscalability of the process 
remains a challenge (Sanchez-Salvador et al. 
2021
). 
Moreover, it should be pointed out that ClO
–
is not 
only consumed in oxidizing hydroxyl groups, but 
also in the oxidative cleavage of β-1,4 glycosidic 
bonds. In this context, evaluating the effects of the 
so-called catalyst and co-catalyst on the reaction 
rate is key to ease optimization and monitoring on 
a large scale. Finally, fibers are complex structures 
that cannot be reduced to cellulose I crystallites, 
and the possibility of mechanical refining to display 
higher surface area, and thus higher surface charge 
(Serra-Parareda et al. 
2021b
), should be explored.
All considered, this works seeks to undertake a 
comprehensive kinetic study on the TEMPO-medi-
ated oxidation of cellulosic fibers from eucalyptus 
wood. While softwood-sourced nanocellulose usu-
ally displays better properties and usability, hard-
wood pulps are more effectively oxidized (Tarrés 
et al. 
2019
). This reaction has already been found to 
follow apparent first-order kinetics in different sys-
tems (Sun et al. 
2005
; Dai et al. 
2011
; Sang et al. 
2017
). Nonetheless, some knowledge gaps are still 
to be filled by broadening the interval of tempera-
ture values, by observing the effects of refining, and 
by assessing how the concentrations of TEMPO and 
Br
–
affect the process. For a wide range of condi-
tions, we plot the NaOH consumption against the 
CC, thus offering a plausible strategy for real-time 
monitoring, without the need of sampling for ex 
situ measurements, which are usually expensive 
and time-consuming (Balea et al. 
2021
). Then, the 
influence of TEMPO dosage, Br
–
dosage, tempera-
ture, and surface charge on the reaction rate was 
assessed. In light of the results, the progressive 
availability of primary hydroxyl groups in terms 
that apply to chemical pulp fibers was discussed. 
Further, the present study can serve as precursor 
of subsequent studies for continuous production of 
TEMPO-oxidized cellulose fibers, leaving behind 
uncertainty in batch processes.

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