In this short communication, the degradation of cellulose for delignified pulp by deep eutectic solvents was evaluated. The pulp was delignified using different DES systems based on choline chloride and lactic acid (1:9), oxalic acid dehydrate (1:1), malic acid (1:1), and system alanine : lactic acid (1:9). This paper shows that cellulose degradation can be characterized in terms of the percentage degree of polymerization loss of cellulose. Among the investigated DES (Choline chloride:oxalic acid; choline chloride:malic acid; alanine:lactic acid; choline chloride:lactic acid), the most suitable seemed to be a treatment using the alanine:lactic acid system that provided a relatively low degradation of cellulose and high delignification efficiency for the removed of lignin from pulp.

1. Behaviors of Pulp During Delignification in Solutions
of Deep Eutectic Solvents: Degradation of Cellulose
Michal Jablonský, Veronika Majova, Andrea Škulcová, Aleš Ház, Petra Strižincová, František Kreps, Zuzana
Burčová, Alexandra Sládková, Igor Šurina
Faculty of Chemical and Food Technology, Slovak University of Technology in Bratislava,
Radlinského 9, 812 37 Bratislava
michal.jablonsky@stuba.sk, veronika.majova@stuba.sk, andrea.skulcova@stuba.sk, ales.haz@stuba.sk,
petra.strizincova@stuba.sk, frantisek.kreps@stuba.sk, zuzana.burcova@stuba.sk, alexandra.sladkova@stuba.sk,
igor.surina@stuba.sk
Abstract—In this short communication, the degradation of
cellulose for delignified pulp by deep eutectic solvents was
evaluated. The pulp was delignified using different DES systems
based on choline chloride and lactic acid (1:9), oxalic acid
dehydrate (1:1), malic acid (1:1), and system alanine : lactic acid
(1:9). This paper shows that cellulose degradation can be
characterized in terms of the percentage degree of
polymerization loss of cellulose. Among the investigated DES
(Choline chloride:oxalic acid; choline chloride:malic acid;
alanine:lacticacid; choline chloride:lacticacid), the most suitable
seemedto be a treatment using the alanine:lacticacidsystem that
provided a relatively low degradation of cellulose and high
delignification efficiency for the removed of lignin from pulp.
Keywords—deep eutectic solvents; delignification; pulp;
degradation
I. INTRODUCTION
The goal of oxygen delignification is to reduce the lignin
content in unbleached pulp before using more expensive
bleaching chemicals [1-3]. Oxygen delignification can be done
at a medium or high consistency. Incorporation of the oxygen
delignification stage into pulp bleaching operations became
common practice. In general, the high kappa brownstock
exhibited improved pulp bleachability over the low kappa
brownstock. Analysis of the residual lignin structure of
brownstocks and bleach effluents plays an important role of
condensed phenolics in the controlling oxygen delignification
technology [3]. In the 1970s, oxygen delignification has
become a powerful delignification technology to produce
elemental chlorine-free and total chlorine-free pulps [4]. The
limitation of oxygen delignification is the low reactivity of
oxygen. Valorisation is a critical component of an economic
and environmental lignocellulosic biorefinery [5, 6]. In recent
years, increasing research attention has been focused on
breakthrough development processes. One of the promising
technologies is the use of green solvents such as deep eutectic
solvents (DESs), natural deep eutectic solvents (NADESs) or
low-transition-temperature mixtures (LTTMs). DESs or
NADESs are thermodynamically stable, liquid mixtures of two
or more components, and they have a lower melting point than
either of the individual components [7]. The benefits of using
green solvents as a reaction mediumare highlighted by the fact
that they are biodegradable, low-toxic, recyclable, and they can
be easily prepared using cheap raw materials. Up to now, they
have been widely used in extraction [8, 9], delignification of
different biomass [10-16] or in pre-treatment of cellulose [17].
A nowadays newpossibility for the use of green solvents in the
pulp and paper industry is explored. Great possibilities are in
the field of substitution of oxygen delignification. However, it
is necessary to find the effect of solvents on the removal of
lignin but also the effect on degradation of cellulose. Large
numbers of mathematical models, hypotheses, and theories for
cellulose degradation can be found in the following literature
[18-22]. The first kinetic model of cellulose deterioration has
been proposed by Ekenstam [22]. The decrease of the degree of
polymerization straightforwardly affects all the mechanical
properties. In this study, the results obtained from the
delignification of pulp by DESs composed of choline chloride
and three organic acids, as well as alanine with lactic acid, are
reported. Degradation of cellulose was evaluated according to
the work of Ding and Wang [20] and by the parameter of the
accumulated degree of polymerization loss of cellulose.
II. EXPERIMENTAL
All chemicals were obtained from Sigma Aldrich
(Bratislava, Slovakia). The mixture was stirred in a water bath
at 70 °C – 80 °C to form a homogeneous liquid. Four deep
eutectic solvents were used for the experiment. The DESs were
mixed from choline chloride, oxalic acid dehydrate (DES1,
molar ratio 1:1), malic acid (DES2, 1:1), lactic acid (90 %
solution) (DES3, 1:9), and alanine:lactic acid (DES4, 1:9). The
hardwood kraft pulp was obtained from Mondi SCP,
Ružomberok, Slovakia (Kraft pulp 1) and BUKÓZA Holding
a.s., Hencovce, Slovakia (Kraft pulp 2). Characteristic
chemical properties of the pulp before and after DES
delignification are listed in Table I. [17]. Pulp (50 g absolute
dry weight) and 115 mL water were added into individual
DESs at a ratio of 1:20 (wt/wt). The consistency of pulp was
4.8%. The DESs, choline chloride, and lactic acid (1:9), oxalic
acid (1:1), malic acid (1:1), and system alanine: lactic acid
(1:9) were used for delignification. Delignification was carried
out for 1 h in a drying oven at a temperature of 60 °C. The
delignified pulp was washed with deionized water. The Kappa
This work was supported by the Slovak Research and Development
Agency under contracts no.APVV-14-0393, APVV-16-0088 and APVV-15-
0052. Theauthors are grateful for support fromthe project "National Center
for Research and Application of Renewable Energy Sources," ITMS
26240120016,the project "Competence Center for NewMaterials, Advanced
Technologies andEnergy "ITMS 26240220073" Science and Technology
Park STU "ITMS26240220084",co-financed from the European Regional
Development Fund.

2. number and viscosity were determined by standard procedures
described elsewhere [13].
TABLE I. THE CHARACTERISTIC CHEMICAL PROPERTIES OF PULP
BEFORE AND AFTER DESDELIGNIFICATION
Sample
Properties
Kappa
no.
Viscosity
(mL/g)
Degree of
polymerization
Kraft pulp 1 21.7 789 1157
Oxygen delignified
pulp
(industrial) *
11.8 569 805
DES1 13.3 648 930
DES2 13.2 772 1130
DES3 13.5 775 1134
DES4 12.3 784 1149
Kraft pulp 2 14.3 851 1258
DES1 11.1 490 683
DES2 12.3 780 1113
DES3 11.8 745 1085
DES4 11.2 800 1160
III. RESULTS AND DISCUSSION
In recently published work, Majova et al. [13] described the
effect of DES on the delignification of pulp by parameters such
as degree of polymerization, cellulose chain scission number,
the selectivity of delignification and efficiency of
delignification. The results showed that pulp with a higher
initial Kappa number or lignin content would possess a greater
fraction of easily removable lignin fragments. In this work,
pulps with different initial Kappa numbers (21.7; 14.3) were
used. The efficiency of oxygen delignification expressed as
change of Kappa units was 45.6 %. The efficiency of
delignification ranged from 37.8 to 43.3%. The best system
was alanine:lactic acid (DES4). For kraft pulp 2 (initial Kappa
number 14.3), the efficiency of delignification was from 14.0%
to 22.4%. The decrease in DP is associated with scission of
cellulose chains. This phenomenon is clearly documented in
several papers [18-22]. Ding and Wang [20] introduced
continuous scalar variable δ, and named it ‘‘percentage
retention of DP”:
δ = DPt/DP0 (1)
where DP0 denotes the initial degree of polymerization and
DPt the real degree of polymerization, decreased as a result of
deterioration due to degradation of cellulose. At the beginning
of processing-related degradation (t = 0), δ =1, during
degradation, its value decreases, and δ = 0 which means total
damage of the sample. Then, the degradation variable of
cellulose can be defined in terms of the percentage of DP loss
as:
ωDP = 1 - δ = 1 - DPt/DP0 (2)
where ωDP is the accumulated DP loss of cellulose.
The extent of degradation can be, thus, also expressed using
the above relation. At the beginning ωDP = 0, i.e. an average DP
is preserved. ωDP = 1 represents a (theoretical) total damage of
the sample depending on specific experimental conditions. It is
generally accepted that, when DP has decreased to an average
DP of about 200, the paper will lose all its mechanical strength.
If DP0 = 1 000, then δ = 0.2, and the accumulated degradation
critical value ωDPcr = 0.8.
When we compare results from the degradation of
cellulose, it is obvious that the influence of DESs differs. When
using ChCl/oxalic acid, the degradation of cellulose expressed
as DP loss is 0.2, 0.02 for ChCl/malic acid, 0.02 for ChCl/lactic
acid, for alanine/lactic acid is 0.01. At the same pulp (initial
kappa 21.7) which undergoes oxygen delignification, DP loss
of cellulose was 0.3. Delignification proceeds at a reasonable
rate only at a high temperature and pH. However, these
conditions favour cellulose degradation [23]. And thus, the
cellulose fibre can be expected to be affected by deformation,
curl and degradation by oxidation and alkaline hydrolysis. For
DES delignified kraft pulp with initial Kappa 14.3, ωDP 0.46
(DES1), 0.12 (DES2) 0.14 for DES3 and 0.08 for DES4 was
respecting. The choice of green solvents greatly influences the
delignification and degradation of cellulose. The results show
that the behaviour of degradation is dependent on the initial
content of lignin in the pulp. If first kappa number is high,
DESs delignification is selective, and there is no significant
degradation of the cellulose. When kappa number is low, the
degradation of cellulose is intense, and thus the initial content
of lignin in pulp has an impact on the selectivity of
delignification.
TABLE II. PROPERTIES OF PULP BEFORE AND AFTER DES
DELIGNIFICATION
Sample δ * ωDP **
Kraft pulp 1 1.00 0.00
Oxygen delignified pulp
(industrial) *
0.70 0.30
DES1 0.80 0.20
DES2 0.98 0.02
DES3 0.98 0.02
DES4 0.99 0.01
Kraft pulp 2 1.00 0.00
DES1 0.54 0.46
DES2 0.88 0.12
DES3 0.86 0.14
DES4 0.92 0.08
* δ – percentage retention of the degree of polymerization, ** ωDP –accumulated degree of
polymerization loss of cellulose
Advantages of DES delignification against oxygen
delignification: delignification using DESs was carried out for
1 hour, i.e. a shorter reaction time than for oxygen
delignification. Other advantages of DES application are lower
temperature (60°C) and unpressurised system which reduces
equipment cost. The ostensible disadvantage is the necessity to

3. introduce a system for DES recycling. The recycled DES can
repeatedly be used for delignification. The benefit is that the
lignin obtained from DES delignification is relatively pure.
Moreover, the lower degree of cellulose degradation and thus
higher selectivity happen at DES delignification. Given the fact
that DES and oxygen delignification led to a similar degree of
delignification, reduced emissions from the bleach plant,
reduced consumption of bleaching chemicals, higherbrightness
value in a given bleaching sequence can be expected when
applying DES delignification. The mentioned benefits may
contribute to a broader research of applying DESs for
delignification after batch and later also its introduction into the
process of bleached pulp production.
IV. CONLUSIONS
DES delignification has potential to replace oxygen
delignification after kraft pulping. In this study, pulp was
delignified with different DES systems based on choline
chloride and lactic acid (1:9), oxalic acid dehydrate (1:1), malic
acid (1:1), and alanine:lactic acid (1:9). The impact of DES
delignification was evaluated as the accumulated degree of
polymerization loss of cellulose. Deep eutectic solvents are
breakthrough discovery and open the way to pulp production at
low temperatures and at atmospheric pressure. Our research is
entirely in line with the European strategy of this development
of science and industry with a view to 2050.
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