Delignification of pulp using deep eutectic solvents

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/323676633
Delignification of pulp using deep eutectic solvents
Article · March 2018
CITATIONS
0
READS
73
4 authors:
Some of the authors of this publication are also working on these related projects:
Printed composite electrodes based on conductive polymers and carbon nanomaterials for hybrid solar cells. (Slovak University of
Technology - Grant for the Excellent Teams of Young Scientists - 2015/2017) View project
Complex utilization of bark extractives View project
Michal Jablonsky
Slovak University of Technology in Bratislava
167 PUBLICATIONS   336 CITATIONS
SEE PROFILE
Veronika Majová
SEE PROFILE
Skulcova Andrea
SEE PROFILE
Ales Haz
SEE PROFILE
All content following this page was uploaded by Michal Jablonsky on 10 March 2018.
The user has requested enhancement of the downloaded file.

Journal of Hygienic Engineering and Design
76
Abstract
Lot of works during the last decades have been focused
on the new modes of pulp processing. One promising
technology is the use of deep eutectic solvents. Deep
eutectic solvents (DES) have opportunities to open
new paths in the field of delignification methods.
This study was conducted to investigate the effects
of deep eutectic solvent treatment on physical and
chemical properties of delignified pulp. In the follow-
ing experiment we used as an initial pulp the kraft pulp
(Kappa No. 21.7; Degree of polymerization 1157). The
pulp was treated with two different DES system based
on choline chloride with lactic acid (1 : 9), and system
alanine : lactic acid (1 : 9). The efficiency of delignifica-
tion expressed as a decrease in kappa number on the
unit change of the initial kappa number of pulp.
The order by the delignification efficiency growth is as
follows: choline chloride : lactic acid (37.8%) > alanine
: lactic acid (43.3%). During delignification by DESs, a
degradation of pulp chain occurs, however a decrease
in degree of polymerization was only 23 units versus
kraft unbleached pulp, which represents maximum de-
crease by 2%. Delignified pulp with DESs has a bright-
ness 34% and unbleached pulp achieved brightness
27%, therefore, it achieves the increase in brightness
by 26%.The physical strength properties of DES delig-
nified pulps were assessed in terms of tensile, tear and
burst index and stiffness. Application of deep eutec-
tic solvents were achieved to reduce tensile index by
13.2%, burst index by 14.3% and a tear index by 9.8%,
and the pulp stiffness was increased by 4% again the
unbleached pulp.
The results indicate that application of DESs might be
an interesting alternative to oxygen delignification of
pulp following kraft cooks.
Key words: Delignification, Deep eutectic solvents, Green
chemistry, Pulp.
1. Introduction
Recently, many studies concerning the environmen-
tal impact of deep eutectic solvents have shown that
despite their unique properties and clear advantages
in an ever wide range of applications and processes
(Florindo et al., [1]). Valorisation is a key component
of an economic and environmental lignocellulosic
biorefinery (Jablonsky et al., [2]; Surina et al., [3]). Lots
of works during the last decades has been focused on
the new pulp processing. Many kraft pulp mills are us-
ing oxygen delignification before bleaching to reduce
the amount of chlorinated organic compounds in the
bleach plant effluent (Springer and MsSweenyr, [4]).
Extending conventional oxygen treatments to remove
more than 50% of the residual pulp lignin would fur-
ther decrease bleach chemical demands and increase
environmental benefits (Allison and McGrouther, [5]).
The reasons for the limit of lignin removal during ox-
ygen delignification are still under debate (Gellerst-
edt and Heuts, [6]; Shin et al., [7]; Kontturi et al., [8]),
but the most explicit evidence suggests that covalent
bonds between lignin and carbohydrates hinder the
selective removal of lignin after a certain point (Chirat
and Lachenal, [9]; Axelsson et al., [10]). Independent of
developing a more efficient result from oxygen delig-
nification, exist of the field of research which focus on
improving the strength properties of fibres (Kontturi
et al., [8]).
DESs have been also applied for the pretreatment and
fractionation of lignocellulosic biomass. Interestingly,
Original scientific paper
UDC 676.03
DELIGNIFICATION OF PULP USING DEEP EUTECTIC SOLVENTS
Michal Jablonsky1*
, Veronika Majova1
, Andrea Skulcova1
, Ales Haz1
1
Department of Wood, Pulp, and Paper, Institute of Natural and Synthetic Polymers,
Slovak University of Technology, Radlinského 9, 81237 Bratislava, Slovak Republic
*
e-mail: michal.jablonsky@stuba.sk

77
Francisco and co-workers [11] found that lignin had
relatively high solubility in DES systems, especially in
acidic DESs, being the solubility of cellulose and starch
small or null in most of the cases. Kroon et al., [12],
tested the solubility of lignin, starch and cellulose for
selected low transition temperature mixtures. Kroon et
al., [12] observed very low solubility of cellulose in se-
lected mixtures. In addition, Jablonský et al., [2], tested
the delignification of wheat straw by choline chloride
based DESs at 60 0
C for 24 h, and showed that the DESs
cannot selectively remove lignin from biomass. Re-
cently, Kumar et al., [13], studied the pretreatment of
rice straw using natural DESs (lactic acid/betaine, lactic
acid/choline chloride). The best delignification effect
was achieved by lactic acid/choline chloride at molar
ratio of 5 : 1 at 60 0
C for 12 h. The results of the other
studies indicated that DESs could be used as the prom-
ising media for delignification.
2. Materials and Methods
Choline chloride (ChCl) (≥ 98% mass fraction purity)
was purchased from Sigma-Aldrich and was dried un-
der vacuum prior to use. Lactic acid (90% solution) and
alanine were purchased from Sigma-Aldrich and used
as supplied.
2.1 Pulp characterization
The hardwood kraft pulp was obtained from Mondi
SCP, Ružomberok, Slovakia. Characterization chemical
properties of pulp before and after DES delignification
are listed in Table 2 and Table 3. The Kappa number of
the pulp was used to estimate the lignin content, and
was determined according to Tappi standard method
T-236. The viscosity of the pulp was used to estimate
the intrinsic viscosity or degree of polymerization (DP)
of the cellulose within the pulp fibre. The measure-
ments were determined by dissolving the pulp in cu-
priethylene diamine solution, and then measuring the
elution times in a capillary viscometer at constant tem-
perature.
In order to estimate the amount of scissions, the de-
gree of polymerization (DP) of the carbohydrates was
calculated from intrinsic viscosity using Mark-Houwink
equation, where ([η]) is the intrinsic viscosity (ml/g) of
pulp (Rydholm [14]).
(1)
Freeness of beaten pulps was measured according to
TAPPI T227 om-99. The handsheet for testing of pa-
permaking properties was formed according to TAPPI
T205 sp-02. The handsheet of each beating condition
was measured for optical and strength properties such
as brightness (TAPPI T452 om-98), tensile strength
(TAPPI T494 om-96), tearing strength (TAPPI T414 om-
98), bursting strength (TAPPI T407 om-97).
2.2 Preparation and characterization of DESs
Deep eutectic solvents were prepared by heating
method. The heating method is based on mixing the
two components, which are then heated at 70 0
C un-
der constant stirring until a homogeneous liquid is
formed. Physicochemical properties of deep eutectic
solvents were measured in different temperature rang-
es. The viscosity of the deep eutectic solvents reagent
was measured using Ubbelohde viscometer in range
28 - 90 0
C. The density of DESs was determined by spe-
cific gravity bottles. Table 1 shows prepared DESs and
some of them properties.
Table 1. Deep eutectic solvents
Sample
HBA :
HBD
Molar
ratio
Density
[g/cm3
]
at 25 0
C
Viscosity
[mPa∙s]
at 28 0
C
DES1
ChCl :
lactic acid
1:9 1217 70.9
DES2
Alanine :
lactic acid
1:9 1230 168.0
Legend: ChCl - choline chloride, HBA - hydrogen bond acceptor, HBD
- hydrogen bond donor.
2.3 Evaluation of deep eutectic delignification of
pulp
Selectivity of bleaching expressed as an increase in
brightness on the unit change of the intrinsic viscosity:
(2)
Where: B0
- initial brightness of pulp; Bt
- brightness of
pulp after delignification; [η]0
- initial intrinsic viscosity
of pulp; [η]t
- intrinsic viscosity of pulp after delignifi-
cation.
The selectivity of delignification (SlcK
) expressed as a
decrease in kappa number on the unit change of the in-
trinsic viscosity was calculated from following equation:
(3)
Where: K0
- initial kappa number of pulp; Kt
- kappa
number of pulp after delignification; [η]0
- initial intrin-
sic viscosity of pulp; [η]t
- intrinsic viscosity of pulp after
delignification.

78
The efficiency of delignification (Efcκ
) expressed as a
decrease in kappa number on the unit change of the
initial kappa number of pulp was calculated from fol-
lowing equation:
(4)
Where: K0
- initial kappa number of pulp; Kt
- kappa
number of pulp after delignification.
3. Results and Discussion
3.1 Physicochemical properties of deep eutectic
solvents
In this report we presented the research work on
preparation and characterization of two deep eutectic
solvents.
3.1.1 Viscosity
Viscosity data can be used for selection of optimal ra-
tio of hydrogen bond donor and acceptor. In gener-
al, viscosity of DES is mainly affected by the chemical
structure of DES such as type of HBD and HBA, tem-
perature and water content. Viscosity is also important
parameter for industrial applications, and is a critical
property that must be accounted for equipment de-
sign and fluid flow calculation. These DESs systems are
liquid at room temperature but their viscosity can be
so high that there is a problem with flow and penetra-
tion. Preheating is a very simple technique that can be
used to reduce the viscosity. Viscosities are decreasing
with increasing temperature as show Figure 1 because
of the weakening of van der Waals and hydrogen bond
interactions. Depending on the molecular interactions
are viscosities of DESs much higher than some conven-
tional organic solvents. The obtained data shows that
the DESs composed of choline chloride and lactic acid
have lower viscosity values than the remaining DES (al-
anine : lactic acid), most likely due to the length of their
chains in HBA, because it molar ratio is the same (1 : 9).
3.1.2 Density
The experimental density results for the samples of
DESs as a function of temperature are plotted in Figure
2. The obtained data shows that the DESs composed
of choline chloride and lactic acid have lower density
values than the remaining DES (alanine : lactic acid). As
observed, the density decreases linearly with tempera-
ture for DESs in the whole temperature range studied,
and a linear equation was used to express the correla-
tion with the temperature:
ρ = a + bT (5)
0.0027 0.0028 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
lactic acid: alanine (9:1)
choline chloride: lactic acid (1:9)
lnη
1/T(K-1
)
Figure 1. Effect of temperature on the
viscosity of DESs in range 28 - 90 0
C
Figure 3. pH for lactic acid based DESs (c = 0.5 mol/L)
as a function of temperature from 23 - 60 0
C
20 30 40 50 60 70
1180
1190
1200
1210
1220
1230
lactic acid: alanine (9:1)
ρ(kgm-3
)
T (°C)
Figure 2. Densities of lactic acid based DESs
as a function of temperature from 25 - 65 0
C
20 25 30 35 40 45 50 55 60 65
0.8
1.0
1.2
1.4
1.6
1.8
2.0
2.2 lactic acid: alanine (9:1)
pH
T (°C)
Where: ρ corresponds to density in kg m−3
;T is the tem-
perature in 0
C and; a and b are the fitting parameters.
The a and b values derived from e.q. 5 for the studied
DESs are presented in Table 2.
3.1.3 pH properties
pH is physical property that has essential impact on
reaction. pH is depended on hydrogen bond donor
and it is decreasing with increased temperature. DESs
based on lactic acid and choline chloride or alanine
are non-toxic and eco-friendly mixtures and it is very
important to know their pH value. The pH values indi-
cate that this type of solvents can be used in industrial
application where acidic medium is needed (Figure 3).

79
3.1.4 DES delignification
The original pulp had a Kappa number equal to 21.7.
After DES delignification, the Kappa number decreased
to 13.5 for the ChCl: lactic acid system and to 12.3 for
alanine: lactic acid. The order by the delignification
efficiency (EfcK
) growth is as follows: choline chloride:
lactic acid (37.8%) alanine: lactic acid (43.3%). During
delignification using DESs, a degradation of pulp chain
occurs,howeveradecreaseindegreeofpolymerization
was only 23 units versus kraft unbleached pulp, which
represents maximum decrease by 2%. Delignified pulp
with DESs has brightness 34% and unbleached pulp
achieved brightness 27%, therefore, it achieves the
increase in brightness by 26%. The selectivity parame-
ter of bleaching for the alanine : lactic acid system was
127.7%, and for ChCl : lactic acid was 50.2%. The selec-
tivity of delignification (SlcK) expressed as a decrease
in the kappa number on the unit change of intrinsic
viscosity was highest for alanine : lactic acid (206.5%)
than for choline chloride : lactic acid was 58.3%. Based
on the parameters of selectivity, the better system was
alanine : lactic acid. This is consistent with the fact that
if the lignin is removed, also the brightness should be
increased. DES based on choline chloride: lactic acid
degrades the cellulosic chain very slightly compared
to alanine : lactic acid DES. The selectivity of delignifi-
cation has not the effect on direct enhancement of the
strength properties of paper (Table 3).
Indeed, there are several factors which dictate the
strength properties of a fiber network. The degree of
polymerization of the cellulose in the fibres, represent-
ed by viscosity, is only one of them (Kontturi et al., [8]).
Although the fibre strength is an important proper-
ty of quality, the strength of the fibre network is also
strongly influenced by the bonding between the fibres
(Rydholm, [14]). Therefore, it is very important to mon-
itor the impact of delignification on the mechanical
properties of the fibres. The physical strength prop-
erties of delignified pulps using DESs were assessed
in terms of tensile, tear and burst index and stiffness
(Table 4). Application of deep eutectic solvents were
achieved to reduce tensile index by 13.2%, burst index
by 14.3% and tear index by 9.8%.The pulp stiffness was
increased by 4% against the unbleached pulp.
Table 2. Values of parameters a and b for three equations which describe viscosity, density and pH of studied DES
DES
ln(η) = a*exp(b/T) ρ = a + bT *pH = a + bT
T € 28; 90 0
C T € 25; 65 0
C T € 23; 60 0
C
a b a b a b
lactic acid : alanine
(9 : 1)
45
.45*10-3 1425.11 1249.90 - 78.56*10-3
2.67 - 20.77*10-3
choline chloride : lactic acid
(1 : 9)
38.93*10-3
1415.48 1236.41 - 75.53*10-3
2.18 - 22.58*10-3
* pH determination was measured for DES concentration c = 0.5 mol/L
Table 4. Mechanical properties of kraft pulps after DES treatment
Sample
Beating
[°SR]
Tensile index
[Nm/g]
Burst index
[kPa∙m2
/g]
Tear index
[mN∙m2
/g]
Stiffness
[mN]
Pulp 30 72.02 4.2 7.1 126
DES1 30 62.49 3.6 6.4 131
DES2 30 63.00 3.6 6.6 130
Table 3. Characterization chemical properties of pulp before and after DES delignification
Parameters Kappa n.
Viscosity
(mL/g)
Degree of
polymerization
Brightness
(%)
SlcB
SlcK
EfcK
Kraft pulp 21.7 789 1157 27.02 - - -
DES1:
ChCl : lactic acid
13.5 775 1134 34.05 50.21 58.27 37.8
DES2:
alanine : lactic acid
12.3 784 1149 33.38 127.70 206.48 43.3

80
UV-Vis analysis of DES extracted liquor showed a
strong peak at 267 nm, suggesting that the extract-
ed solution contains components with aromatic
rings (Figures 4 and 5). These prominent peaks corre-
spond to the principle aromatic components of lignin
(Kumar et al., [13]).
- Results indicate that application of DESs might be
an interesting alternative to oxygen delignification of
pulp following kraft cooks.
Acknowledgement
This work was supported by the Slovak Research and
Development Agency under the contracts No. APVV-
15-0052 and APVV-16-0088. The authors would like
to thank for financial assistance from the STU Grant
scheme for financial assistance from the STU Grant
scheme for Support of Young Researchers under the
contract no.1625, 1688.
5. References
[1] Florindo C., Oliveira F. S., Rebelo L. P. N., Fernandes A.
M., Marrucho I. M. (2014). Insights into the synthesis and
properties of deep eutectic solvents based on cholinium
chlorideandcarboxylicacids. ACS Sustainable Chemistry
Engineering, 2, (10), pp. 2416-2425.
[2] Jablonsky M., Skulcova A., Kamenska L., Vrska M., Vrska
M. (2015). Deep eutectic solvents: Fractionation of wheat
straw. BioResources, 10, (4), pp. 8039-8047.
[3] Surina I., Jablonsky M., Haz A., Sladkova A., Briskarova
A., Kacik F., Sima, J. (2015). Characterization of non-
wood lignin precipitated with sulphuric acid of various
concentrations. BioResources, 10, (1), pp. 1408-1423.
[4] Springer E. L., MsSweenyr J. D. (1993). Treatment of
softwood kraft pulps with peroxymonosulfate before
oxygen delignification. Tappi J., 76, (8), pp. 194-199.
[5] Allison R. W., McGrouther K. G. (1995). Improved oxygen
delignification with interstage peroxymonosulfuric acid
treatment. Tappi J., 78, (10), pp. 134-142.
[6] Gellerstedt G., Heuts L. (1997). Changes in the lignin
structureduringatotallychlorinefreebleachingsequence,
Journal of Pulp and Paper Science, 23, (7), pp. 335-340.
[7] Shin S. J., Schroeder L. R., Lai Y. Z. (2006). Understanding
factors contributing to low oxygen delignification of
hardwood kraft pulps. Journal of Wood Chemistry and
Technology, 26, (1), pp. 5-20.
[8] Kontturi E., Mitikka-Eklund M., Vuorinen T. (2007).
Strength enhancement of fiber network by carboxymethyl
cellulose during oxygen delignification of kraft pulp.
BioResources, 3, (1), pp. 34-45.
[9] Chirat C., Lachenal D. (1998). Limits of oxygen
deligrufication. Tappi Pulping Conference Proceedings,
Tappi Press, Atlanta, USA, pp. 619-624.
[10] Axelsson P., Gellerstedt G., Lindström M. E. (2004).
Condensation reactions of lignin during birch kraft
pulping as studied by thioacidolysis. Journal of Pulp and
Paper Science, 30, (12), pp. 317-322.
[11] Francisco M., van den Bruinhorst A., Kroon M. C. (2012).
New natural and renewable low transition temperature
mixtures (LTTMs): screening as solvents for lignocellulosic
biomass processing. Green Chemistry, 14, pp. 2153-2157.
200 300 400 500 600 700 800
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
Absorbance
Wavelength (nm)
pure DES
extract of DES
200 300 400 500 600 700 800
-0,5
0,0
0,5
1,0
1,5
2,0
2,5
3,0
3,5
Absorbance
Wavelength (nm)
pure DES
extract of DES
Figure 4. Spectral analysis of extract for DES1
(choline chloride : lactic acid (1 : 9))
Figure 5. Spectral analysis of extract for DES2
(alanine : lactic acid (1 : 9))
4. Conclusions
- Physical properties (densities, viscosities and pH)
were studied for DESs prepared using choline chlo-
ride : lactic acid and alanine : lactic acid at the same
molar ratio (1 : 9). The obtained data shows that the
DESs composed of choline chloride and lactic acid had
lower density, viscosity and pH values than the second
DES (alanine : lactic acid).
- Actually, it has been documented that due to selec-
tivity and efficiency of delignification, the best DES
system is alanine : lactic acid. Obtained results showed
that the type of DES influenced on mechanical proper-
ties of delignified pulp.

81
[12] Kroon M. C., Casal M. F., van den Bruinhorst A. (2012).
Pretreatment of lignocellulosic biomass and recovery
of substituents using natural deep eutectic solvents/
compound mixtures with low transition temperatures.
International patent: WO2013/153203 A1.
[13] Kumar A. K., Parikh B. S., Pravakar M. (2016). Natural
deep eutectic solvent mediated pretreatment of rice
straw: bioanalytical characterization of lignin extract
and enzymatic hydrolysis of pretreated biomass residue.
Environmental Science and Pollution Research, 23, 10,
pp. 9265-9275.
[14] Rydholm S. A. (1965). Pulping processes. Interscience
Publishers, New York, USA.
View publication statsView publication stats

Delignification of pulp using deep eutectic solvents

More Related Content

What's hot (16)

Similar to Delignification of pulp using deep eutectic solvents (20)

More from Michal Jablonsky (19)

Recently uploaded (20)

Delignification of pulp using deep eutectic solvents