Alcoholysis of polyethylene terephthalate to produce dioctyl terephthalate using choline chloride-based deep eutectic solvents as efficient catalysts

Article abstract of DOI:10.1039/c8gc03791d

Dioctyl terephthalate (DOTP) is a new kind of green and non-toxic plasticizer. The traditional process to prepare DOTP is costly and complicated, thus it is very necessary to find an efficient and environmentally-friendly way to obtain DOTP. In this study, we prepared DOTP from the alcoholysis of polyethylene terephthalate (PET) by using 2-ethyl-1-hexanol (2-EH) as the solvent, and used choline chloride-based deep eutectic solvents (ChCl-based DESs) as the catalysts due to their cheapness, low toxicity and ease of preparation. Under the optimized conditions (ChCl/Zn(Ac)₂ 1:1, 5 wt%, 180 °C, 60 min, PET:2-EH 1:5 molar ratio), the conversion of PET and the yield of DOTP were 100% and 84.7%, respectively. Furthermore, the possible mechanism was proposed through density functional theory (DFT) calculations and experimental results. It revealed that the hydrogen bonds (H-bonds) formed between 2-EH and DESs played a key role in accelerating the degradation process. Furthermore, the kinetics of the reaction was also investigated, and the results indicated that the alcoholysis of PET is a first-order reaction with an activation energy of 95.05 kJ mol^-1. This work confirmed that ChCl-based DESs are favorable for the degradation of PET to prepare DOTP under mild conditions, which can also be used for other polymer degradation strategies.

Full text of DOI:10.1039/c8gc03791d

View Article Online
View Journal
Green
Chemistry
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: L. Zhou, Z. Ju, X.
Lu, B. Liu, H. Yao, J. Xu, Q. Zhou, Y. Hu and S. Zhang, Green Chem., 2019, DOI: 10.1039/C8GC03791D.
Volume 18 Number
7
7
April 2016 Pages 1821–2242
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Green
Chemistry
Cutting-edge research for a greener sustainable future
www.rsc.org/greenchem
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
author guidelines.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the ethical guidelines, outlined
in our author and reviewer resource centre, still apply. In no
event shall the Royal Society of Chemistry be held responsible
for any errors or omissions in this Accepted Manuscript or any
consequences arising from the use of any information it contains.
ISSN 1463-9262
CRITICAL REVIEW
G. Chatel et al.
Heterogeneous catalytic oxidation for lignin valorization into valuable
chemicals: what results? What limitations? What trends?
rsc.li/green-chem

Page 1 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
Alcoholysis of Polyethylene Terephthalate for Dioctyl Terephthalate Using
Choline Chloride-Based Deep Eutectic Solvents as Efficient Catalysts
Lei Zhou a,b, Xingmei Lu b,c, Zhaoyang Ju , Bo Liu , Haoyu Yao , Junli Xu , Qing Zhou, ,
b
b
b
b
b,c
Yufeng Hu, a* Suojiang Zhang b,c*
a
State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing, Beijing
1
02249 China
b
CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase
Complex Systems, Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190 China
c
College of Chemical and Engineering, University of Chinese Academy of Sciences, Beijing, 100049,
China
Abstract
Dioctyl terephthalate (DOTP) is a new kind of green and non-toxic plasticizer. The
traditional process to prepare DOTP is costly and complicated, thus, it is very necessary
to find an efficient and environmentally-friendly way to obtain DOTP. In this study, we
prepared DOTP from the alcoholysis of polyethylene terephthalate (PET) by 2-ethyl-1-
hexanol (2-EH) as solvent, and used choline chloride-based deep eutectic solvents
(ChCl-based DESs) as the catalysts due to their cheapness, low toxicity and easy
preparation. Under the optimized conditions (ChCl/Zn(Ac) 1:1, 5 wt%, 180 °C, 60 min,
2
PET:2-EH 1:5 molar ratio), the conversion of PET and yield of DOTP were 100% and
Corresponding author. Tel./Fax: +86-010-897338, +86-010-82544800.
E-mail addresses: huyf3581@sina.com (Y. F. Hu); sjzhang@ipe.ac.cn. (S. J. Zhang)

Green Chemistry
Page 2 of 26
View Article Online
DOI: 10.1039/C8GC03791D
8
4.7%, respectively. Furthermore, the possible mechanism was proposed through
density functional theory (DFT) calculations and experimental results. It revealed that
the hydrogen bonds (H-bonds) formed between 2-EH and DESs played a key role to
accelerate the degradation process. Furthermore, the kinetics of the reaction was also
investigated, and the results indicated that the alcoholysis of PET is a first-order
reaction with an activation energy of 95.05 kJ/mol. This work confirmed that ChCl-
based DESs are favorable for the degradation of PET to prepare DOTP under mild
conditions, which can also be used for other polymer degradation strategies.
Introduction
1
Dioctyl terephthalate (DOTP), a new kind of green plasticizer to replace dioctyl
phthalate, has received more and more attention due to its good insulation, cold
2
resistance, heat resistance, pullout resistance, softness and low volatility. DOTP can
be used in a lot of fields, not only as a non-toxic main plasticizer for medical polyvinyl
3
chloride products or plastic toys, but also as the lubricant or additive in the manufacture
of nitrile rubber, artificial leather products, polyethylene, etc. However, DOTP is
4
usually prepared from p-phthalic acid (PTA) or dimethyl terephthalate in industry,
both of which have some shortcomings such as long reaction process, high energy
consumption and low economic efficiency. While using polyethylene terephthalate
(PET) as raw material promises to overcome these problems, it can reduce production
costs greatly. Meanwhile PET is a widely used polyester,5, 6 and many studies showed
it can be degraded by hydrolysis,7, 8 aminolysis9, 10 and alcoholysis11, 12 to obtain high
value products like PTA,13,
14
p-phenylenediamine15 or bis (2-hydroxyethyl)

Page 3 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
1
6
terephthalate. Moreover, the global PET market is expected to grow rapidly from 6.5
million tons in 2000 to 23.4 million tons by 2020, with a compound annual growth rate
of approximately 7.0%.8, 17 Environmental problems are getting increasingly serious,
therefore, preparing DOTP from PET is a good choice for both economic effect and
green issues.
In recent years, researchers have developed a variety of catalysts such as acids,
bases and metal salts16, 18, 19 or some intensification methods, including microwave-
assisted technology20, 21 and supercritical method22-24 for PET degradation. Although
these methods have already been widely studied and applied in PET recycling, there
are still some drawbacks to resolve. For example, acid or base catalysts can cause
corrosion of equipment or environmental pollution, and the catalytic efficiency isn’t
very high. Although microwave-assisted technology can increase the reaction rate, it
usually requires high temperature and tremendous power. High yield of DOTP can be
obtained by the supercritical method, but the reaction conditions are very severe (300–
4
50 °C, 14–50 MPa). Thus, it is important to find an efficient way for the alcoholysis
of PET to prepare DOTP under mild reaction conditions.
Deep eutectic solvents (DESs) are fleetly emerging as a new kind of sustainable
2
5
and green solvent, since being introduced by Abbott et al. in 2003, and they have been
research hotspots for a long time.26-28 Furthermore, DESs are also considered to be a
new generation of ionic liquids (ILs), with similar excellent properties,29, 30 and a lot of
ILs have already been widely used for the degradation of PET,16, 31 DESs were usually
synthesized by simply mixing the hydrogen bond acceptors (HBAs) and hydrogen bond

Green Chemistry
Page 4 of 26
View Article Online
DOI: 10.1039/C8GC03791D
2
7
donors (HBDs) together. It is noteworthy that DESs have many advantages compared
with the conventional ILs, which rest with the simpler preparation, cheaper materials
and no need for further purification,32, 33 so DESs are deemed to have great application
prospects.34, 35 Wang et al. showed that urea/metal salt DESs have high catalytic
activity on PET alcoholysis with ethylene glycol (EG) by the action of both H-bond
36
3
7
and coordination bond. Cho et al. used the DESs for the modification of PET polyester
surfaces. These examples fully demonstrate that DESs have great potential in the field
of PET degradation. In addition, Multiple studies have shown that choline chloride
(ChCl) has a potential green profile owing to its low toxicity and high level of
3
8
biodegradability, and the ChCl-based DESs are generally considered to be non-toxic
3
9
salts, some of which have been approved as nutritional additives. Compared with
2
+
40
other transition metals, Zn tend to have higher catalytic activity for PET degradation
-
and COOH is also friendly to the environment, so zinc acetate (Zn(Ac) ) can be a good
2
choice as the HBA.36
In this work, in order to get an efficient and green catalyst for the PET degradation,
ChCl and Zn(Ac) were selected as HBD and HBA. Then a series of DESs
2
(
ChCl/Zn(Ac) from molar ratio 2:1 to 1:6) were synthesized to be investigated, and the
2
reaction conditions such as reaction temperature, reaction time, ratio of solvent and
content of catalyst were optimized. The prepared DESs were characterized by Fourier
transform infrared spectroscopy (FT-IR), proton nuclear magnetic resonance
spectroscopy (1H NMR), differential scanning calorimetry (DSC) and
thermogravimetric analysis (TGA). Through high performance liquid chromatograph

Page 5 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
(
HPLC), the yield of DOTP was obtained and the main product was identified by FT-
1
IR and H NMR. In addition, the DES-catalyzed mechanism of PET degradation was
proposed by density functional theory (DFT) calculations and scanning electron
microscopy (SEM). Finally, the kinetics of the reaction was studied to get the reaction
rate constants of different temperatures and the activation energy.
Experimental
Materials
PET pellets (2.0×2.5×2.7 mm) were purchased from Jingdong Commercial Co., Jiangsu
Province, China. The pellets were ground into 40-60 mesh by small grinding miller
(
(
(
ZN-02). Choline chloride (ChCl), anhydrous zinc acetate (Zn(Ac) ), cobalt acetate
2
Co(Ac) ), ferric chloride (FeCl ), tetrabutyl titanate (Ti(OC H ) ), 2-ethyl-1-hexanol
2
3
4
9 4
2-EH), dioctyl terephthalate (DOTP) standard sample and the other reagents were
purchased from Sinopharm Chemical Reagent Beijing Co., China. All reagents were
used as purchased without further purification.
Preparation of Choline Chloride-Based DESs
A series of ChCl-based DESs were synthesized by mixing ChCl and Zn(Ac) from 2:1
2
to 1:6 (molar ratio) at a moderate temperature for 2h-8h with strong agitation in a round
flask. Until a clear and homogeneous liquid formed, the reaction was stopped and the
product was collected.
Characterizations of catalysts and main products
Fourier transform-infrared (FT-IR) spectra of the synthesized DESs and the main
product were obtained with a Nicolet 380 (Thermo Fisher Scientific, U.S.A.)

Green Chemistry
Page 6 of 26
View Article Online
DOI: 10.1039/C8GC03791D
-1 1
spectrometer, using KBr as blank in the range 4000-400 cm . H Nuclear magnetic
1
resonance ( H NMR) spectra were recorded on an AVANCE-III 600 NMR
spectrometer operating at 600 MHz in DMSO-d6 solution. Thermogravimetric analysis
(TGA) of the synthesized DESs was done using DTG-60H in nitrogen atmosphere by
heating sample from 25 ºC to 600 ºC at the rate of 10 ºC/min. Differential scanning
calorimeter (DSC) scans of the synthesized DESs were obtained using DSC-1 STARe
system by heating from 0 ºC to 500 ºC at a heating rate of 10 ºC/min in an atmosphere
of nitrogen with a flow rate of 20 mL/min. The morphologies of fresh PET and residual
PET were observed with scanning electron microscope (SEM, Hitachi, SUB020, Japan).
The content of product was analyzed with a ACQUITY HPLC (Waters, 2695, U.S.A.)
equipped with a refractive index detector (Waters, 2414, U.S.A.), and BET C 18 column
(Waters, U.S.A.) under the condition of an oven temperature 30 °C, detector
temperature 50 °C, solvent chromatographic is grade ethanol and flow rate 0.2 mL/min.
Gel permeation chromatography (GPC) analysis were performed on a PL-GPC 50
system (Agilent, U.S.A.) under the condition of oven temperature of 30 °C, solvent
trichloromethane and flow rate 1.0 mL/min. Three columns (PLgel 5um Guard, length
5
0mm, 7.5mm i.d. PLgel 5um MIXED-C, length 300mm, 7.5mm i.d. PLgel 5um
MIXED-D, length 300mm, 7.5mm i.d.) were used. The detector used on the GPC is a
refractive index (RI) detector.
General procedure for the catalytic degradation of PET
In each experiment, 5 g of PET pellets were degraded, using 2-EH in the presence of
ChCl or one kind of previously synthesized DES catalysts. A 100 mL double-necked

Page 7 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
round-bottom glass flask equipped with a thermometer, a magnetic stirrer and a reflux
condenser was needed for all the degradation experiments. The degradation reactions
were carried out under atmospheric pressure at reaction temperatures ranging from 155
°
C to 185 °C for reaction times of 20-180 min. The flask was immersed in an oil bath
at aspecific temperature for the required time. When the reaction terminated, the
reaction mixture was cooled to room temperature. Then the reaction mixture was
filtered by a decompress filter, the flask washed with ethanol, to separate the liquid
product from the undegraded PET (because of the high viscosity, the catalyst adhered
to the flask wall). The filter residue was dried at 80 °C for 24 h, weighted to calculate
the conversion of PET, which is defined by the following Eq.(1):
푊0 ― 푊1
퐶표푛푣푒푟푠푖표푛 표푓 푃퐸푇 =
× 100%
(1)
푊0
Where W represents the initial weight of PET, W represents the weight of
0
1
undegraded PET. In addition, the filtrate was distilled at atmospheric pressure to
remove ethanol and then vacuum distillation was used to remove EG and 2-EH. A light
yellow transparent liquid was collected and all characterization techniques showed that
this light yellow transparent liquid was DOTP. Moreover, the product can be further
purified by silica gel column. The selectivity and yield of DOTP are defined by Eqs.
(2) and (3):
푛퐷푂푇푃
푆푒푙푒푐푡푖푣푖푡푦 표푓 퐷푂푇푃 = 푛푢푛푖푡푠 × 100%
(2)
(3)
푌푖푒푙푑 표푓 퐷푂푇푃 = 푆퐷푂푇푃 × 퐶푃퐸푇
where nDOTP represents the number of moles of DOTP, nunits represents the number
of moles of the degraded PET units, SDOTP represents the selectivity of DOTP and CPET

Green Chemistry
Page 8 of 26
View Article Online
DOI: 10.1039/C8GC03791D
represents the conversion of DOTP.
DFT calculations
All calculations were operated using the Gaussian 09 program. The M06-2X-D3/6-
11+G**, lanl2DZ method was used for structure optimizations, and subsequent
3
frequency calculations at the same level verify the optimized structures to be ground
states without imaginary frequencies (NImag = 0).
Results and Discussion
Screening of catalysts for PET degradation
Under the same reaction conditions, the catalytic performance of DESs (ChCl/Zn(Ac)2
from molar ratio 2:1 to 1:6) in PET alcoholysis was investigated, and the results were
shown in Figure 1. It showed that with the change of the ratio, the CPET and YDOTP
increased first and then decreased, and the best catalytic performance appeared when
the ratio of the two components was 1:1, the CPET and YDOTP were almost 100% and
8
4%, respectively. In addition, the catalytic performance of the DESs synthesized from
the other metal salts and some kinds of other catalysts together were shown in Table 1.
According to the experimental results, some metal salts (Zn(Ac) , Mn(Ac) , FeCl , etc.)
2
2
3
can catalyze the degradation of PET, and Zn(Ac) showed the best catalytic
2
performance. Moreover, it was found that when ChCl was used alone as catalyst, the
PET degradation rate was very low. But when Zn(Ac) was added, 5 g PET can be
2
completely degraded in 1 h, which indicated that metal ions may played the main
catalytic role and ChCl played the role of cocatalyst. Compared with references,1, 2, 41
the yield of DOTP in this work (84.2 %) was lower than that reported in the references

Page 9 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
(
93.1 %, 97 % and 97.6 % respectively). However, the time required for complete
degradation of PET using ChCl/Zn(Ac) as catalyst was 1 hour, which was much
2
shorter than that reported in the references (5h, 2h and 8h respectively). In addition, the
experimental conditions adopted in this work were milder, and the methods of the
references usually need higher temperature and pressure, or needs substantial cosolvent
such as [Bmim]Cl.
Figure 1. Performance of DESs with different proportion of ChCl and Zn(Ac) . Reaction
2
conditions: PET (5.0 g), 2-EH (17.0 g), DES (0.25 g), atmospheric pressure, 185 ºC, 60 min.
Table 1. The catalytic activities of PET degradation with different catalysts a
Temperature
ºC)
Conv.PET
(%)
YieldDOTP
(%)
Entry
Catalysts
time (h)
Ref.
(
1
2
3
4
5
6
7
8
–
185
185
185
185
185
185
185
185
1
1
1
1
1
1
1
1
1.5
3.4
–
ChCl
–
ChCl/Zn(Ac)
2
(1:1)
(1:1)
(1:1)
(1:1)
≈100.0
96.5
90.4
91.7
94.5
46.4
84.2
80.6
74.1
76.2
78.3
40.6
ChCl/Mn(Ac)
2
This work
ChCl/Co(Ac)
2
ChCl/Cu(Ac)
2
ChCl/FeCl
3
(1:1)
Zn(Ac)
2

Green Chemistry
Page 10 of 26
View Article Online
DOI: 10.1039/C8GC03791D
9
Mn(Ac)
2
185
1
44.2
36.6
1
0
1
2
3
4
5
6
7
Co(Ac)
2
185
185
185
185
185
185
200
300
1
1
1
1
1
1
5
2
32.9
33.8
43.8
31.6
41.4
78.5
100
27.1
27.6
35.7
26.5
34.5
63.8
93.1
97.0
1
1
1
1
1
1
1
Cu(Ac)
2
FeCl
CoCl
SO
Ti(OC
3
2
H
2
4
4
9 4
H )
[
b]
1
2
Zn(Ac)
2
–[c]
100
[HO
3
S-(CH
2 3 3
) -NEt ]Cl-
4
1
1
8
210
8
100
97.6
[d]
FeCl
3
a
Experiment conditions: Conditions: PET (5.0 g), 2-EH (17.0 g), catalysts (0.25 g),
atmospheric pressure.
b
2
-EH (10.0 g), [Bmim]Cl as cosolvent (10.0 g), catalyst (0.06 g).
c
2
-EH (20.4 g) and at high pressure.
d
[Bmim]Cl as cosolvent (10.0 g), catalyst (1.00 g).
Characterizations of DESs
FT-IR, NMR, DSC and TGA were carried out to identify the structure of the prepared
DES (ChCl/Zn(Ac) (1:1)). FT-IR spectra were shown in Figure 2. It showed that the
2
spectrum of DES basically contains all functional groups of two kinds of raw materials,
-1
such as peaks at 950.5, 950. 2 and 950.3 cm , but there are some obvious differences
-1
between them. Compared with the peaks at 1481.5 and 1562.3 cm on the FT-IR
-1
spectra of ChCl and Zn(Ac) , it can be seen that a new peak at 1587.1 cm appeared,
2
which indicated that a new amido bond (in-plane bending vibration) was formed by the
coordination between ChCl and Zn(Ac) . At the same time, methyl group and carbon-
2

Page 11 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
-1
chlorine bond in ChCl showed obvious red shifts from 1409.5 to 1391.4 cm and from
-1
6
21.2 to 618.3 cm , which suggested that the new bonds formed also have an effect on
the bond energies of other functional groups.
Figure 2. FT-IR spectra of DES, Zn(Ac)2 and ChCl
1
1
H NMR spectra were shown in Figure 3. By comparing the H NMR peaks of two
raw materials and DES, it was found that DES basically contains all the peaks of the
two raw materials. However, it is worth noting that an obvious signal at δ 5.660 ppm
on the spectrum of ChCl disappeared on the spectrum of DES, which indicated that the
hydroxyl on ChCl participated in the synthesis of DES and formed H-bonds with the
oxygen on carbonyl group of Zn(Ac)2.
1
Figure 3. H NMR patterns of DES, Zn(Ac) and ChCl
2
The DSC curves of DES was depicted in Figure. 4. There was a sharp endothermic

Green Chemistry
Page 12 of 26
View Article Online
DOI: 10.1039/C8GC03791D
peak, and the melting onset temperature of DES was 182.4 °C, which was lower than
that of ChCl (302 °C) and Zn(Ac) (237 °C). This accorded with the physical and
2
chemical properties of DESs, and proved that a homogeneous DES has been prepared.
The TGA curve of the DES was shown in Figure 5. It showed an obvious weight loss
at approximately 214 °C, and the weight loss was nearly 72% in the range 214–478 °C.
The decomposition temperature of the DES (214 °C) was higher than the reaction
temperature (180 °C), so DES can exist stably in the reaction system and can hardly be
decomposed by heat.40
Figure 4. DSC curves of DES (ChCl-Zn(Ac) (1:1))
2
Figure 5. TGA curves of DES (ChCl-Zn(Ac) (1:1))
2
Effect of experiment conditions on the PET degradation

Page 13 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
The effect of reaction temperature on PET degradation in a particular set of condition
(viz. 5 g PET, 60 min, 5 wt% catalyst/PET ratio, PET:2-EH 1:5 molar ratio, atmospheric
pressure) using ChCl/Zn(Ac) (1:1) as catalyst and the results were shown in Figure 6.
2
When the temperature as low as 155 ºC, The degradation rate of PET was very low,
only about 35%, and the yield of DOTP was 32%, but the selectivity was higher at low
temperature, which exceeded 90%. With the increasing of reaction temperature from
1
55 °C to 185 °C, the degradation rate of PET and yield of DOTP increased, but the
selectivity of DOTP decreased. The highest CPET and YDOTP were obtained at 180 °C,
which were 100% and 84.5%, respectively. When the temperature continued to rise, the
CPET remained unchanged, but the side reactions increased so that the YDOTP and SDOTP
decreased.
Figure 6. Effect of temperature on PET degradation. Reaction conditions: PET (5.0 g), 2-EH
(17.0 g), DES (0.25 g), atmospheric pressure, 60 min.
The effect of reaction time on PET degradation was investigated and presented in
Figure 7 by reacting 5 g of PET with 5 wt% catalyst/PET ratio and PET:2-EH 1:5 molar
ratio at 180 °C under atmospheric pressure. It was observed that as time went on, the

Green Chemistry
Page 14 of 26
View Article Online
DOI: 10.1039/C8GC03791D
CPET increased from 70% to 100%. Meanwhile, YDOTP and SDOTP reached the
maximums 84.4% and 84.6% when the reaction time was 60 min. After 60 min, there
was a slight decrease in the YDOTP and SDOTP as the extension of time, due to the existing
equilibrium between DOTP, dimer and oligomer.
Figure 7. Effect of time on PET degradation. Reaction conditions: PET (5.0 g), 2-EH (17.0 g),
DES (0.25 g), atmospheric pressure, 180 ºC.
To study the effect of the alcohol amount on the PET degradation, the experiments
were performed at the following reaction conditions: 5 g PET, 5 wt% catalyst, 60 min,
1
80 °C, under atmospheric pressure and the results were shown in Figure 8. When the
addition of 2-EH was small, although the CPET could reach more than 90%, the YDOTP
and SDOTP were less than 60%. When 2-EH reached five times the molar amount of
PET, the maximums of YDOTP and SDOTP appeared, which were 84.5% and 84.7%
respectively. The amount of 2-EH continued to increase and the YDOTP and SDOTP
decreased slightly, which may be owing to the lower concentration of reactants and the
side reactions caused by excessive 2-EH.

Page 15 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
Figure 8. Effect of alcohol amount on PET degradation. Reaction conditions: PET (5.0 g), DES
0.25 g), atmospheric pressure, 180 ºC, 60 min.
(
The effect of catalyst amount on PET degradation was investigated at the following
reaction conditions: 5 g PET, PET:2-EH 1:5 molar ratio, 60 min, 180 °C, atmospheric
pressure, and the results were shown in Figure 9. It showed that when the catalyst
amount reached 5 wt% of PET, the degradation rate reached almost 100%, and YDOTP
and SDOTP reached 83.6% and 83.2%, respectively. Continued to increase the amount
of catalyst, the degradation rate remained unchanged. By comparing the above
diagrams, it can be found that the reaction temperature and time have more influence
on PET degradation, but the amount of alcohol and catalyst has less influence on the
reaction.

Green Chemistry
Page 16 of 26
View Article Online
DOI: 10.1039/C8GC03791D
Figure 9. Effect of catalyst amount on PET degradation. Reaction conditions: PET (5.0 g), 2-EH
17.0 g), atmospheric pressure, 180 ºC, 60 min.
(
Degradation mechanism for PET alcoholysis
In this work, the mechanism was studied by DFT calculations and the M06-2X-D3/6-
3
11+G**, lanl2DZ method was used for structure optimizations. The interaction
between DES and 2-EH was investigated in Figure 10, and it can be found that there
are some possible stable interaction structures among the Zn(Ac) , ChCl and 2-EH,
2
4
2
which is mainly formed by H-bonds and coordination bonds. Among all these modes,
structure B has the lowest energy, meaning it is more stable than the others. The H-
bonds were formed by hydrogen of hydroxyl in ChCl and oxygen of carbonyl in
Zn(Ac) , and by hydrogen of hydroxyl in 2-EH and oxygen of carboxylate in Zn(Ac) .
2
2
The coordination bonds were formed between chloride ion in ChCl and zinc ion in
Zn(Ac) . In addition, Table 2 shows that the interaction between 2-EH and DES will
2
bring changes to the bond lengths, and the O–H bond length of the hydroxyl group in
2
-EH after interaction with DES in structures A, B, C and D, become longer than those
before interaction (original 0.958 to 0.971, 1.006, 0.973 and 0.973), which leads to the
hydrogen to be lost more quickly.18

Page 17 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
Figure 10. Optimized interaction structures among Zn(Ac) , ChCl and 2-EH by M06-2X-D3/6-
2
3
11+G**, lanl2DZ with relative energies. Red: O; green: Cl; blue: N; white: H; dark grey: C; steel
grey: Zn.
Table 2. Changes in bond length for 2-EH while it interacts with DES
Bond
O1-H1
O1-C1
C1-C2
C2-C3
C3-C4
2-EH
0.958
1.420
1.523
1.532
1.534
A
B
C
D
0.971
1.419
1.524
1.532
1.533
1.006
1.431
1.522
1.538
1.533
0.973
1.417
1.529
1.533
1.534
0.973
1.446
1.523
1.532
1.533
The interaction among DES, 2-EH and PET was studied in Figure 11, and it shows
that different energy required for four different reaction paths. Zn2+ attacked the
carbonyl group on PET to form carbon cation, which follows the acid-catalyzed
3
6
-
mechanism, COOH simultaneously attacked the hydroxyl group on 2-EH, and then

Green Chemistry
Page 18 of 26
View Article Online
DOI: 10.1039/C8GC03791D
activated the carbonyl group, and this process in line with the base-catalyzed
mechanism. Under this synergistic catalysis, the long chains of PET were rapidly
broken, which has greatly accelerated the degradation rate of PET and then the
transesterification reaction has completed. Furthermore, based on the DFT calculations
and the experiments, the possible degradation mechanism of PET catalyzed by DES
(
ChCl/Zn(Ac) 1:1) is illustrated in Scheme 1.
2
Figure 11. Optimized interaction structures among DES, 2-EH and PET with relative energies.
Red: O; green: Cl; blue: N; white: H; dark grey: C; steel grey: Zn.

Page 19 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
Scheme 1. Proposed mechanism for the PET degradation catalysed by DES
In studies of the PET degradation, the reaction is usually considered to occur on
the surfaces of PET pellets, and then with the aggrandizement of cracks on the surfaces
and increasing exposed areas, the reaction accelerated.43, 44 The images of fresh PET
and residual PET after different reaction time from 20 min to 60 min were observed by
SEM, and the results were illustrated in Figure 12. The image presented in Figure 12
(a) shows that the surface of fresh PET is relatively smooth. After 20 minutes of
reaction, the surface of PET was no longer smooth and there were a few cracks, as
displayed in Figure 12 (b). Then, after a longer reaction time, the surface cracks of
PET increased and a lot of holes appeared, the images was shown in Figure 12 (c) and
Figure 12 (d). These images showed that the degradation process of PET began on the
surface and slowly penetrated into the inner layer as the long chains were broken down.
With DES as catalyst, PET particles were gradually eroded and many pores and fissures

Green Chemistry
Page 20 of 26
View Article Online
DOI: 10.1039/C8GC03791D
were formed on the surface, which reduced the degree of polymerization and this was
more favorable for the further degradation.
Figure 12. SEM of fresh PET (a) and residual PET (20min (b), 40 min (c), 60min (d))
Kinetics of the PET alcoholysis
There has been some debates that the reaction order is usually considered to be first-
order40, 45 in the depolymerization kinetics of polymers. Consequently, the PET
alcoholysis with the DES (ChCl/Zn(Ac) 1:1) as catalyst was initially assumed to be
2
controlled by the first-order kinetic equations:
푑(퐶푃퐸푇)
=
―푘퐶푃퐸푇
where k is the rate constant of the reaction and CPET is the concentration of the PET
at time t
(4)
푑푡
퐶푃퐸푇 = 퐶푃퐸푇0(1 ― 푋)
(5)
where X is the conversion of PET. And then Eq. (4) can be written as Eq. (6):
푑푋
=
푘(1 ― 푋)
(6)
푑푡
Eq. (6) was integrated over time to provide Eq. (7):

Page 21 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
1
푙푛1
= 푘푡
(7)
― 푋
using the rate constant, the activation energy (E ) can be obtained by Arrhenius
a
equation:
―
퐸푎
푙푛푘 =
+푙푛퐴
(8)
푅푇
where A is the pre-exponential factor, R is the gas constant (8.31 J/k mol), and T
is the temperature in Kelvin.
The effect of the reaction temperature on the alcoholysis rate of PET in the presence
of DES (ChCl/Zn(Ac) 1:1) is shown in Figure 13. The activation energy for this
2
reaction, calculated from the slope of the Arrhenius plot shown in Figure 14, was 95.05
kJ/mol. All the linear correlation coefficients in Table 3 are higher than 0.98, which
suggests that ln[1/(1-X)] was proportional to the reaction time at different temperatures
and that this process followed a first-order kinetic reaction. In addition, with the rise of
temperature, the reaction constant increases rapidly, which indicates that the
temperature has great influence on the PET degradation and this is consistent with the
previous experiments.
Figure 13. Effect of the temperature on the rate of alcoholysis of PET (Reaction condition: PET
(5.0 g), 2-EH (17.0 g), DES (0.25 g), atmospheric pressure)

Green Chemistry
Page 22 of 26
View Article Online
DOI: 10.1039/C8GC03791D
Table 3. Linear regression results for the data in Figure 15 and Figure 16
Temp (K)
Rate constant
0.01236
R2
Ea (kJ/mol)
R2
4
4
4
4
4
23
33
38
48
58
0.9828
0.9905
0.9846
0.9941
0.9819
0.02285
0.03318
95.05
0.9852
0.05406
0.09825
Figure 14. Arrhenius plots of the rate constant of alcoholysis of PET.
Conclusion
In summary, ChCl-based DES (ChCl/Zn(Ac) ) were successfully synthesized and used
2
as the catalysts in PET alcoholysis, and after a different proportion of screening, it was
finally confirmed that the best molar ratio of ChCl and Zn(Ac) is 1:1. The effects of
2
reaction temperature, time, 2-EH amount and catalyst dosage on the results were
investigated, and it is found that temperature and time are the main influencing factors.
Under the optimized conditions (DES 5 wt%, PET:2-EH 1:5, 180 °C, 60 min), the CPET
and the YBHET were 100% and 84.7%. The possible mechanism proposed by DFT

Page 23 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
calculations revealed that the H-bonds formed between 2-EH and DESs played a key
role, and the synergistic effect can enormously improve catalytic activity of the DESs.
Moreover, the kinetics of the reaction was also investigated, and the results indicating
that the alcoholysis of PET is a first-order reaction with an activation energy of 95.05
kJ/mol. Eventually, this work is supposed to provide some valuable references for
polymer wastes degradation and other interesterification.
Acknowledgments
This work was supported by National Basic Research Program of China (973 Program),
(
2015CB251401), Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-
JSC011), National Natural Scientific Fund of China (No. 21878292, No.21476234, No.
1776289), the Strategic Priority Research Program of Chinese Academy of Science
No. XDA21060300), K. C. Wong Education Foundation.
2
(
Reference
1
.
J. Y. Chen, J. X. Lv, Y. M. Ji, J. Y. Ding, X. Y. Yang, M. H. Zou and L. Y. Xing, Polym.
Degrad. Stab., 2014, 107, 178-183.
2
3
.
.
F. Liu, J. Chen, Z. Li, P. Ni, Y. Ji and Q. Meng, J. Anal. Appl. Pyrolysis, 2013, 99, 16-22.
E. Langer, S. Waskiewicz, M. Lenartowicz-Klik and K. Bortel, Polym. Degrad. Stab., 2015,
1
19, 105-112.
4
5
.
.
J. Ding, J. Chen, Y. Ji, P. Ni, Z. Li and L. Xing, J. Anal. Appl. Pyrolysis, 2014, 106, 99-103.
M. Furukawa, N. Kawakami, K. Oda and K. Miyamoto, ChemSusChem, 2018, DOI:
1
0.1002/cssc.201802096.
6
7
8
.
.
.
K. E. Garcia, R. Navarro, A. Ramirez-Hernandez and A. Marcos-Fernandez, Polym. Degrad.
Stab., 2017, 144, 195-206.
S. Joo, I. J. Cho, H. Seo, H. F. Son, H. Y. Sagong, T. J. Shin, S. Y. Choi, S. Y. Lee and K. J.
Kim, Nature Communications, 2018, 9.
C. Jehanno, I. Flores, A. P. Dove, A. J. Muller, F. Ruiperez and H. Sardon, Green Chemistry,
2
018, 20, 1205-1212.

Green Chemistry
Page 24 of 26
View Article Online
DOI: 10.1039/C8GC03791D
9
1
.
R. K. Padhan and A. A. Gupta, Construction and Building Materials, 2018, 158, 337-345.
0.
H. Salmi-Mani, G. Terreros, N. Barroca-Aubry, C. Aymes-Chodur, C. Regeard and P. Roger,
Eur. Polym. J., 2018, 103, 51-58.
1
1.
2.
N. D. Pingale, V. S. Palekar and S. R. Shukla, J. Appl. Polym. Sci., 2010, 115, 249-254.
S. B. Wang, C. S. Wang, H. P. Wang, X. L. Chen and S. B. Wang, Polym. Degrad. Stab., 2015,
1
1
14, 105-114.
1
3.
H. P. Austin, M. D. Allen, B. S. Donohoe, N. A. Rorrer, F. L. Kearns, R. L. Silveira, B. C.
Pollard, G. Dominick, R. Duman, K. El Omari, V. Mykhaylyk, A. Wagner, W. E. Michener, A.
Amore, M. S. Skaf, M. F. Crowley, A. W. Thorne, C. W. Johnson, H. L. Woodcock, J. E.
McGeehan and G. T. Beckham, Proceedings of the National Academy of Sciences of the United
States of America, 2018, 115, E4350-E4357.
1
1
1
4.
5.
6.
F. Dubelley, E. Planes, C. Bas, E. Pons, B. Yrieix and L. Flandin, Polymer, 2018, 142, 285-292.
Z. Leng, R. K. Padhan and A. Sreeram, Journal of Cleaner Production, 2018, 180, 682-688.
Q. Wang, Y. R. Geng, X. M. Lu and S. J. Zhang, Acs Sustainable Chemistry & Engineering,
2
015, 3, 340-348.
1
1
7.
8.
J. Sun, D. J. Liu, R. P. Young, A. G. Cruz, N. G. Isern, T. Schuerg, J. R. Cort, B. A. Simmons
and S. Singh, Chemsuschem, 2018, 11, 781-792.
Q. Wang, X. Q. Yao, S. F. Tang, X. M. Lu, X. P. Zhang and S. J. Zhang, Green Chemistry,
2
012, 14, 2559-2566.
1
2
9.
0.
A. S. Goje and S. Mishra, Macromolecular Materials and Engineering, 2003, 288, 326-336.
P. Sharma, B. Lochab, D. Kumar and P. K. Roy, Acs Sustainable Chemistry & Engineering,
2
016, 4, 1085-1093.
J. Y. Cho, C. J. Hong and H. M. Choi, Industrial & Engineering Chemistry Research, 2013, 52,
309-2315.
2
2
1.
2.
2
A. M. Al-Sabagh, F. Z. Yehia, G. Eshaq and A. E. ElMetwally, Industrial & Engineering
Chemistry Research, 2015, 54, 12474-12481.
2
2
2
3.
4.
5.
B. Geyer, G. Lorenz and A. Kandelbauer, Express Polymer Letters, 2016, 10, 559-586.
Q. L. Liu, R. S. Li and T. Fang, Chem. Eng. J., 2015, 270, 535-541.
A. P. Abbott, G. Capper, D. L. Davies, R. K. Rasheed and V. Tambyrajah, Chem. Commun.,
2
003, DOI: 10.1039/b210714g, 70-71.
2
6.
7.
D. Z. Troter, Z. B. Todorovic, D. R. Dokic-Stojanovic, O. S. Stamenkovic and V. B. Veljkovic,
Renewable & Sustainable Energy Reviews, 2016, 61, 473-500.
2
L. Duan, L. L. Dou, L. Guo, P. Li and E. H. Liu, Acs Sustainable Chemistry & Engineering,
2
016, 4, 2405-2411.
2
2
3
8.
9.
0.
O. S. Hammond, D. T. Bowron and K. J. Edler, Green Chemistry, 2016, 18, 2736-2744.
Y. Xie, H. Dong, S. Zhang, X. Lu and X. Ji, Green Energy & Environment, 2016, 1, 195-200.
H. Cruz, N. Jordao, P. Amorim, M. Dionisio and L. C. Branco, Acs Sustainable Chemistry &
Engineering, 2018, 6, 2240-2249.
3
3
1.
2.
T. T. L. Bui, D. A. Nguyen, S. V. Ho and H. T. N. Uong, J. Appl. Polym. Sci., 2016, 133, 11.
T. N. Gu, M. L. Zhang, T. Tan, J. Chen, Z. Li, Q. H. Zhang and H. D. Qiu, Chem. Commun.,
2
014, 50, 11749-11752.
3
3
3.
4.
D. V. Wagle, H. Zhao and G. A. Baker, Acc. Chem. Res., 2014, 47, 2299-2308.
D. V. Wagle, H. Zhao, C. A. Deakyne and G. A. Baker, Acs Sustainable Chemistry &
Engineering, 2018, 6, 7525-7531.

Page 25 of 26
Green Chemistry
View Article Online
DOI: 10.1039/C8GC03791D
3
3
5.
6.
S. H. Gage, D. A. Ruddy, S. Pylypenko and R. M. Richards, Catal. Today, 2018, 306, 9-15.
Q. Wang, X. Q. Yao, Y. R. Geng, Q. Zhou, X. M. Lu and S. J. Zhang, Green Chemistry, 2015,
7, 2473-2479.
1
3
3
7.
8.
J. Y. Cho, H. M. Choi and K. W. Oh, Textile Research Journal, 2016, 86, 1318-1327.
J. Patil, S. Ghodke, R. Jain and P. Dandekar, Acs Sustainable Chemistry & Engineering, 2018,
6
, 10578-10586.
39.
40.
41.
42.
43.
K. Radosevic, M. C. Bubalo, V. G. Srcek, D. Grgas, T. L. Dragicevic and I. R. Redovnikovic,
Ecotoxicology and Environmental Safety, 2015, 112, 46-53.
A. M. Al-Sabagh, F. Z. Yehia, A. M. F. Eissa, M. E. Moustafa, G. Eshaq, A. M. Rabie and A.
E. ElMetwally, Polym. Degrad. Stab., 2014, 110, 364-377.
S. W. Liu, L. Zhou, L. Li, S. T. Yu, F. S. Liu, C. X. Xie and Z. Q. Song, Journal of Polymer
Research, 2013, 20.
Z. Y. Ju, W. H. Xiao, X. M. Lu, X. M. Liu, X. Q. Yao, X. C. Zhang and S. J. Zhang, Rsc
Advances, 2018, 8, 8209-8219.
D. P. R. Kint, A. M. De Ilarduya, J. J. Bou and S. Munoz-Guerra, Journal of Polymer Science
Part a-Polymer Chemistry, 2002, 40, 76-87.
4
4
4.
5.
X. Zhou, X. Lu, Q. Wang, M. Zhu and Z. Li, Pure Appl. Chem., 2012, 84, 789-801.
A. M. Al-Sabagh, F. Z. Yehia, A.-M. M. F. Eissa, M. E. Moustafa, G. Eshaq, A.-R. M. Rabie
and A. E. ElMetwally, Industrial & Engineering Chemistry Research, 2014, 53, 18443-18451.

Green Chemistry
Page 26 of 26
View Article Online
DOI: 10.1039/C8GC03791D
To prepare DOTP more cheaply and
efficiently, PET was used as raw material
and ChCl-based DES was synthesized as
catalyst.