Degradation of polycarbonate to produce bisphenol A catalyzed by imidazolium-based DESs under metal-and solvent-free conditions

Article abstract of DOI:10.1039/d0ra09215k

Bisphenol A (BPA) is an important chemical raw material, but the traditional preparation process of BPA is costly and complicated, so it is necessary to find an efficient and environmentally friendly method for the production of BPA. Deep eutectic solvents (DESs) have attracted widespread attention due to their low cost, low toxicity, low melting point, non-volatilization, easy preparation, recyclablility and biodegradability. In this work, a series of imidazolium-based DESs were synthesized and used for the degradation of polycarbonate (PC), and BPA was obtained from the methanolysis of PC catalyzed by DESs under metal- and solvent-free conditions. It was found that imidazolium-based DES [EmimOH]Cl-2Urea showed excellent catalytic activity and reusability. Under the optimized reaction conditions (the mass ratio of DES to PC is 0.1?:?1, the molar ratio of CH3OH to PC is 5?:?1, 120 °C, reaction time 2 h), the PC conversion and BPA yield were almost 100% and 98%, respectively. Moreover, the kinetics of methanolysis catalyzed by [EmimOH]Cl-2Urea was investigated in the temperature range 100-120 °C, and the results indicated that it is a pseudo-first order reaction with an activation energy of 133.59 kJ mol-1. In addition, a possible catalytic mechanism of PC methanolysis is proposed.

Full text of DOI:10.1039/d0ra09215k

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Degradation of polycarbonate to produce
bisphenol A catalyzed by imidazolium-based DESs
Cite this: RSC Adv., 2021, 11, 1595
under metal-and solvent-free conditions†
a
Wenwen Huang,a Hui Wang,b Weiyue Hu,a Daoshan Yang,
Shitao Yu,c
a
Fusheng Liuc and Xiuyan Song
*
Bisphenol A (BPA) is an important chemical raw material, but the traditional preparation process of BPA is
costly and complicated, so it is necessary to find an efficient and environmentally friendly method for the
production of BPA. Deep eutectic solvents (DESs) have attracted widespread attention due to their low
cost, low toxicity, low melting point, non-volatilization, easy preparation, recyclablility and
biodegradability. In this work, a series of imidazolium-based DESs were synthesized and used for the
degradation of polycarbonate (PC), and BPA was obtained from the methanolysis of PC catalyzed by
DESs under metal- and solvent-free conditions. It was found that imidazolium-based DES [EmimOH]Cl-
2Urea showed excellent catalytic activity and reusability. Under the optimized reaction conditions (the
mass ratio of DES to PC is 0.1 : 1, the molar ratio of CH3OH to PC is 5 : 1, 120 ꢀC, reaction time 2 h), the
PC conversion and BPA yield were almost 100% and 98%, respectively. Moreover, the kinetics of
methanolysis catalyzed by [EmimOH]Cl-2Urea was investigated in the temperature range 100–120 ꢀC,
Received 29th October 2020
Accepted 7th December 2020
DOI: 10.1039/d0ra09215k
and the results indicated that it is
a pseudo-first order reaction with an activation energy of
rsc.li/rsc-advances
133.59 kJ molꢁ1. In addition, a possible catalytic mechanism of PC methanolysis is proposed.
unrecyclable catalyst and harsh reaction conditions, resulting
in high cost and high energy consumption. Compared with
1. Introduction
these methods, alcoholysis, especially methanolysis, is a more
important approach for PC recycling due to its mild reaction
conditions and relatively easy recovery of raw material mono-
mers. Various catalysts, including concentrated base or super-
bases (NaOH,14,15 DBU16 and TBD17), ZnO-NPs/NBu4Cl
nanoparticles,18 mesoporous molecular sieves (CaO, SrO, BaO)/
SBA-15 (ref. 19) and CaO–CeO2/SBA-15 (ref. 20) and some ionic
liquids ([Bmim]Cl,21 [Bmim]OAc,22 [Bmim]Cl-2.0FeCl3,23
[HDBU][LAc]24 and [HDBU][Im]25) have been used to catalyze the
PC methanolysis (Table S1†) and have several advantages, such
as high PC conversion and yield of BPA,14,21–25 easy to separate
solid catalyst,18,20 low temperature (60, 70 ꢀC)14,22,25 and short
reaction time.24 However, there are also some disadvantages,
such as equipment corrosion,14,15 cumbersome operation, waste
water generation,14,15 large amount of solvent (toluene,14 or
THF18,20) or catalyst,21,22 and the synthesis process of ionic
liquids are very complicated, costly, metal ions contaminate the
product23 and cannot be biodegraded. As a result, nding a new
simple, environmentally benign and low-cost synthetic process
is still an unsolved issue in this eld as the methods mentioned
above are not green.
Polycarbonate (PC), as a high-performance engineering plastic,
has been widely used in compact disk manufacturing, glass
assembly industry, automotive industry, medical equipment,
food packaging, etc.1 With the rapid growth of PC production
and sales, more and more PC waste has been generated.
Although this PC waste is non-toxic and harmless, it is bulky
and difficult to degrade naturally, which may cause serious
environmental pollution and waste resources. From the
perspective of resource conservation and sustainable develop-
ment, chemical recycling of this discarded PC is still an
important issue and has attracted widespread attention.2,3
Several chemical methods, including thermal pyrolysis,4–6
glycolysis,7,8 hydrolysis9–11 and aminolysis12,13 have been studied
by many researchers. Some good results with high conversion
and high yield have been obtained, but there are still many
disadvantages, such as low purity of BPA, slow reaction rate,
aState Key Laboratory Base of Eco-Chemical Engineering, College of Chemistry and
Molecular Engineering, Qingdao University of Science and Technology, Qingdao
266042, China. E-mail: xiuyansong@126.com
bCollege of Chemistry and Pharmaceutical Sciences, Qingdao Agricultural University,
Qingdao 266109, China
To develop a cost-effective and environmentally friendly
method, we focus on exploring a new and affordable catalyst for
the methanolysis of PC under “green” conditions. Deep eutectic
solvents (DESs) were rst described by Abbott26 et al. in 2003,
cCollege of Chemical Engineering, Qingdao University of Science and Technology,
Qingdao 266042, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra09215k
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and are usually synthesized by simply mixing hydrogen bond measured on a BRUKER AV 500 Digital NMR spectrometer in d6-
acceptors (HBAs) and hydrogen bond donors (HBDs). As an DMSO solution. The TGA curves were recorded with a NETZSCH
alternative, DESs not only have similar properties to ionic STA 409 PC/PG thermogravimetric analyzer in a nitrogen
liquids, but also have many other advantages, including simple atmosphere at a ow rate of 100 mL minꢁ1 over a temperature
preparation, cheap raw materials, low toxicity, biodegradability range from room temperature to 600 ꢀC at a heating rate of
and no further purication.27 Because of these properties, DESs 20 ꢀC minꢁ1. The Mw and Mn of fresh PC and residues were
have gradually become a new type of green solvents and cata- checked by Gel permeation chromatography (GPC) (TOSOH-
lysts, and have been widely used in biocatalysis,28,29 extrac- 8320) under the condition of room temperature, solvent tetra-
tion,30,31 carbon dioxide capture,32–34 biomedical applications,35 hydrofuran (THF) as the mobile phase at a ow rate of 1.0
material synthesis36–38 and other elds. In recent decades, DESs mL minꢁ1. The GPC is equipped with a differential refraction
as a reaction medium have attracted widespread attention, detector and TSK gel Super Mutipore HZ-M*2 columns. Cali-
making them have broad application prospects in industrial bration of GPC was performed with standard polystyrene
production.39
samples.
Although their application as catalysts in PC degradation has
not been reported, DESs such as urea/metal salt, ChCl/Zn(Ac)2
and 1,3-dimethylurea/Zn(OAc)2 have been successfully used for
the alcoholysis of polyethylene terephthalate (PET),40–42 and
achieved some good results. At present, DESs are mainly made
of choline chloride (ChCl) and urea or certain metal salts, but
almost no other types of DESs have been reported. In our
previous work, ChCl-2Urea43 was successfully used to catalyze
the PC methanolysis, with 100% CPC and 99% YBPA at 130 ꢀC for
2.5 h. Compared with ChCl-2Urea, imidazolium-based DES has
lower viscosity, lower melting point, wider liquid phase range,
higher electrical conductivity and better thermal stability.
Therefore, this kind of DESs will have a broader application
elds in the feature.
In order to obtain an efficient and green catalyst for PC
methanolysis, a series of [EmimOH]Cl/urea DESs (molar ratio
from 4 : 1 to 1 : 5) was synthesized by using [EmimOH]Cl and
urea as HBA and HBD in this work. The effects of reaction
conditions, such as reaction temperature, reaction time, PC/
methanol molar ratio, and DESs/PC mass ratio were further
examined in detail. In addition, the recycling of [EmimOH]Cl/
urea, the mechanism of PC methanolysis and the kinetics of
this reaction were also investigated.
2.2 Synthesis of imidazolium-based DESs
2.2.1 Preparation of precursor [EmimOH]Cl. In a 250 mL
three-necked ask equipped with a reux condenser, a ther-
mometer and a magnetic stirrer, adding (32.843 g, 0.4 mol) N-
methylimidazole, (32.330 g, 0.4 mol) 2-chloride ethanol and
150 mL ethyl acetate. Under nitrogen atmosphere, the mixture
was heated to reux and stirred for 48 h, then cooled to room
temperature, and a large amount of white precipitate appeared
at the bottom of the ask. The precipitate was collected by
ltration, washed three times with ethyl acetate, and dried for
ꢀ
8 h under vacuum at 80 C to obtain a white solid powder, 1-
hydroxyethyl-3-methylimidazolium chloride [EmimOH]Cl. The
product weighed 50.54 g with the yield of 77.5%. 13C NMR (126
MHz, d6-DMSO), d ¼ 136.8 (s), 123.2 (s), 122.6 (s), 59.2 (s), 51.5
(s), 35.6 (s) (see Fig. S1†).
2.2.2 Preparation of [EmimOH]Cl-2Urea. Take (8.187 g,
0.05 mol) precursor [EmimOH]Cl and (6.004 g, 0.1 mol) urea,
and put into a 100 mL round bottom ask equipped with
a reux condenser and a thermometer. Under a nitrogen
atmosphere, the reaction was heated and stirred at 80 ꢀC for
2.0 h, until a colorless, transparent and homogeneous liquid
was completed, the reaction was over and the product
[EmimOH]Cl-2Urea (13.8 g) was collected with the yield of
97.5%. [EmimOH]Cl-2Urea 13C NMR (126 MHz, d6-DMSO), d ¼
159.7 (s), 136.8 (s), 123.3 (s) 122.6 (s), 59.3 (s), 51.5 (s), 35.6 (s)
(see Fig. S2†).
2. Experimental
2.1 Material and instruments
Pure PC pellets (3 mm length and 2.5 mm diameter) were used
as model plastics and purchased from Idemitsu Kosan Co., Ltd.
of Japan. The weight average molecular weight (Mw) and
number average molecular weight (Mn) were 23 563 g molꢁ1 and
8993 g molꢁ1, respectively. N-Methylimidazole, 2-chloroethanol
and urea were obtained from Shanghai McLean Biochemical
Technology Co., Ltd., China. Methanol, oxalic acid (OA), suc-
cinic acid (SA), benzoic acid (BA), p-toluenesulfonic acid (PTSA),
aniline (AN), acetamide (AA), glycerol (Gly), glucose (Glc),
anhydrous manganese chloride (MnCl2) and anhydrous cupric
According to the above method, DES of [EmimOH]Cl and urea
with a molar ratio of 4 : 1 to 1 : 5 were synthesized, and other DESs
were synthesized by mixing [EmimOH]Cl with different HBAs,
such as [EmimOH]Cl-OA, [EmimOH]Cl-SA, [EmimOH]Cl-BA,
[EmimOH]Cl-PTSA, [EmimOH]Cl-2CuCl2, [EmimOH]Cl-2MnCl2,
[EmimOH]Cl-2AN, [EmimOH]Cl-2AA, [EimmOH]Cl-2Gly and
[EMIMOH]Cl-2Glc.
2.3 General procedure for catalytic methanolysis of PC
chloride (CuCl2) were purchased from Sinopharm Chemical In each experiment, (4.0 g, 15.7 mmol) PC (w1) was degraded,
Reagent Beijing Co., Ltd., China. All chemicals (AR) were using methanol catalyzed by urea or one kind of synthesized
utilized directly without any further treatment.
DESs. The mass ratio of catalyst to PC ranges from 0.05 : 1 to
Infrared (IR) spectra of the main degradation product and 0.1 : 1, and the amount of methanol ranges from 0.047 mol to
DES were recorded on a BRUKER FT-IR VERTEX 70 spectrom- 0.11 mol. A 50 mL autoclave equipped with a thermometer and
eter in the range of 4000–400 cmꢁ1 by liquid lm or KBr discs. a stirrer was put into an oil bath for all the methanolysis
1H and 13C Nuclear magnetic resonance (NMR) spectra were experiments. The reaction was carried out under autogenous
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Scheme 1 Reaction formula of PC methanolysis catalyzed by [EmimOH]Cl-2Urea.
pressure at reaction temperatures ranging from 110–125 ꢀC for [EmimOH]Cl-OA, [EmimOH]Cl-SA, [EmimOH]Cl-BA, [EmimOH]
reaction time of 1.0–2.5 h. Aer the methanolysis reaction was Cl-PTSA (entries 2–5), and some neutral DESs like [EmimOH]Cl-
completed, the autoclave was cooled to room temperature. The 2Gly, [EmimOH]Cl-2Glc (entries 8 and 9) had no catalytic effect
unreacted PC residue (w2) was collected by ltration and dried on the PC methanolysis. The Lewis acidic DESs like [EmimOH]
at 100 ꢀC for 4 h, weighed (w2) to calculate the conversion of PC Cl-2MnCl2 and [EmimOH]Cl-2CuCl2 (entries 6 and 7), although
(CPC), which is determined by eqn (1):
they can catalyze this reaction, the CPC is very low and no BPA is
obtained. However, basic DESs like [EmimOH]Cl-2Urea,
[EmimOH]Cl-2AN and [EmimOH]Cl-2AA (entries 10–12) can
catalyze the methanolysis of PC. The reason may be that PC
methanolysis is a transesterication reaction, basic catalysts are
benecial to this reaction. It was also found that when [Emi-
mOH]Cl and urea were used as catalyst alone (entries 13 and
14), the methanolysis cannot take place. However, when adding
the DES formed by the same quality of [EmimOH]Cl and urea
(molar ratio is 1 : 2), [EmimOH]Cl-2Urea exhibited very good
catalytic activity with 100% CPC and 99% YBPA (entry 12). This
high catalytic performance may be caused by the synergistic
effect between [EmimOH]Cl and urea. Compare with ChCl-
2Urea,43 the catalyst not only decreased the reaction tempera-
ture but also shorted reaction time.
w1 ꢁ w2
CPC
¼
ꢂ 100%
(1)
w1
where w1 refers to the initial weight of PC, w2 refers to the
weight of unreacted PC. Additionally, a light yellow viscous
liquid was obtained by removing methanol and dimethyl
carbonate (DMC) from the distilled ltrate under atmospheric
pressure, which was then dissolved in an equal volumes of ethyl
acetate and distilled water till the mixture separated into two
phases. The upper liquid was removed ethyl acetate by rotary
evaporation to obtain a white solid, which was dried for 8 h
under vacuum to obtain the product BPA (w3). The lower liquid
was mainly DES and water, which was treated by vacuum
distillation to remove water. The residue was mainly DES, can
be reused for the next experiment without any further treat-
ment. Yield of BPA (YBPA) is dened by eqn (2):
3.2 Characterization of [EmimOH]Cl-2Urea
w3
MðPCÞ
YBPA
¼
ꢂ
ꢂ 100%
(2)
The structure of [EmimOH]Cl-2Urea was characterized by FT-IR
and 1H NMR, and the results are shown in Fig. 2. The [Emi-
mOH]Cl characteristic absorption peaks at 3285, 3093, 1642,
1572, 1167, 1072 cmꢁ1, and peaks of urea at 3443, 3346, 1642,
1572, 1167 and 1072 cmꢁ1. As can be seen that the IR spectrum
of DES contains almost all the functional groups of [EmimOH]
Cl and urea, such as the peaks of –NH2, –C]O and C–Cl. But the
w1 MðBPAÞ
where w3 represents the weight of the obtained BPA, M(PC) and
M(BPA) are the molar mass of the repeating unit of PC and
molecular weight of BPA, which are 254 g molꢁ1 and 228 g
molꢁ1, respectively.
The reaction formula is shown in Scheme 1.
3. Results and discussion
3.1 Selection of catalyst for PC methanolysis
The effect of different molar ratio of DESs on the methanolysis
of PC was investigated, and the results are shown in Fig. 1.
When n([EmimOH]Cl) : n(urea) was added from 4 : 1 to 1 : 2,
the CPC increased from 62% to 100%, and the YBPA increased
from 62% to 99%, respectively. In other words, when the molar
ratio of [EmimOH]Cl/urea is 1 : 2, PC is completely degraded
into products. The reason may be due to the formation of
hydrogen bonds between HBD and HBA, when n([EmimOH]
Cl) : n(urea) ¼ 1 : 2, the synergistic effect between them is
better, improving the catalytic performance of DES. Continue to
increase the molar ratio to 1 : 5, the CPC and YBPA decreased
slightly, the reason may be caused by the operation process.
In addition, the catalytic performance of synthesized DESs in
the PC methanolysis was evaluated and the results were pre-
sented in Table 1. It was indicated that this reaction cannot
Fig. 1 Catalytic performance of DESs with different molar ratio of
[EmimOH]Cl and urea (reaction conditions: PC (4.0 g), DES (0.4 g),
occur without any catalyst (entry 1). Some acidic DESs like n(PC) : n(CH3OH) ¼ 1 : 5, 120 ꢀC and 2.0 h).
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Table 1 Catalytic activities of different catalysts in the methanolysis of
3.3 Qualitative analysis of methanolysis product
PCa
When the methanolysis was over, an equal volume of ethyl
acetate and distilled water was put into the mixture. Aer
removing the ethyl acetate in the upper phase by rotary evapo-
ration, the product was mainly BPA. The FT-IR and 1H NMR of
the obtained product and standard BPA were shown in Fig. 3. In
the FT-IR of the obtained product, the peak at 3335 cmꢁ1
corresponds to the O–H stretching vibration of phenolic
hydroxy group, the peak at 2964 cmꢁ1 corresponds to the C–H
stretching vibration of methyl, the peaks at 1446–1612 cmꢁ1
corresponds to the C]C stretching vibration of the benzene
ring, and the peaks at 1177–1237 cmꢁ1 corresponds to the C–O
stretching vibration. The peak at 827 cmꢁ1 corresponds to C–H
out-of-plane bending vibration of benzene ring, indicating it
was para-substituted benzene.
Entry
Catalyst
YBPA/%
CPC/%
1
2
3
4
5
6
7
8
Blank
0
0
0
0
0
0
0
0
0
95
0
99
0
0
0
0
0
0
0
4
2
0
0
99
6
100
0
0
[EmimOH]Cl-OA
[EmimOH]Cl-SA
[EmimOH]Cl-BA
[EmimOH]Cl-PTSA
[EmimOH]Cl-2MnCl2
[EmimOH]Cl-2CuCl2
[EmimOH]Cl-2Gly
[EmimOH]Cl-2Glc
[EmimOH]Cl-2AN
[EmimOH]Cl-2AA
[EmimOH]Cl-2Urea
[EmimOH]Clb
9
10
11
12
13
14
Ureac
1
From the H NMR of the obtained product, it can be seen
a
Experimental conditions: PC (4.0 g), catalyst (0.4 g), n(PC) : n(CH3OH)
that the peak at d ¼ 1.50 ppm (s, 6H) belongs to the hydrogen of
–CH3, The symmetrical peaks appears at d ¼ 6.61 ppm (d, 4H),
6.95 ppm (d, 4H) attribute to the hydrogen of benzene ring,
indicating a para-substitution. The peak at d 9.14 ppm (s, 2H)
b
c
¼ 1 : 5, T ¼ 120 ꢀC, t ¼ 2 h. m([EmimOH]Cl) ¼ 0.234 g. m(Urea) ¼
0.166 g.
1
represents the hydrogen of –OH. Regardless of IR or H NMR,
peak at 3336 cmꢁ1 belongs to the N–H stretching vibration, is
wider than that of [EmimOH]Cl and urea, and moved to a low
wave number, showing a signicant red shi. In addition, the
peaks of C]O and N–H also have obvious red shis.
the spectrum of obtained product is almost consistent with that
of standard BPA, which proves that the obtained product is BPA.
In the 1H NMR of [EmimOH]Cl, the peaks at d ¼ 9.3–9.5, 7.84
and 7.81 ppm are assigned to the H of the imidazole ring. The
multiple peaks at d ¼ 5.5–5.6 ppm attributed to the H of –OH.
The triplet peak appeared at d ¼ 4.2–4.3 ppm belongs to 2H of
–CH2CH2OH, another triplet peak at d ¼ 3.6–3.7 ppm is ascribed
to 2H of –CH2CH2OH. The singlet peak at d ¼ 3.8–3.9 ppm
belongs to 3H of N–CH3 in the imidazole ring. Compared with
the 1H NMR peaks of [EmimOH]Cl, DES shows a single peak at
d ¼ 5.6–5.7 ppm, which is attributed to –NH2 in urea. From 1H
NMR, DES basically contains all the peaks of the raw material.
From the above results, it can be seen that due to the
interaction between HBD and HBA, a hydrogen bond is formed
between [EmimOH]Cl and urea, resulting in obvious red shis
shown at the peaks of N–H and C]O in the IR spectrum. In the
1H NMR of DES, some characteristic peaks, C–H for instance
move toward high eld, and the chemical shi values decrease.
3.4 Effect of experimental conditions on PC methanolysis
The inuences of reaction conditions on PC methanolysis were
investigated and the results are shown in Table 2. Obviously,
reaction temperature is the main inuencing factor on the PC
methanolysis. When the temperature increased from 110 to
120 ꢀC, the CPC and YBPA increased from 14%, 8% to 100%, 99%,
respectively (entries 1–3). However, when the reaction temper-
ꢀ
ature was raised to 125 C (entry 4), the CPC and YBPA did not
increase signicantly. The reason may be that the methanolysis
of PC is a transesterication reaction, and it reached equilib-
rium at 125 ꢀC. When the reaction time was extended from
1.25 h to 2.0 h (entries 3 and 5–6), the CPC increased from 24%
to 100%, meanwhile, the YBPA reached the maximum 99% at
2.0 h. When m(DES) : m(PC) was added from 0.05 : 1 to 0.1 : 1,
both CPC and YBPA increased clearly, the CPC from 56% to 100%
and the YBPA from 53% to 99%, respectively. Moreover, as the
Fig. 2 FT-IR and 1H NMR spectra of DES, [EmimOH]Cl and urea.
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Fig. 3 FT-IR and 1H NMR spectra of obtained product and standard BPA ((a) standard BPA; (b) obtained product) (reaction conditions: 4.0 g PC,
0.4 g DES, n(PC) : n(CH3OH) ¼ 1 : 5, 120 ꢀC and 2.0 h).
Table 2 Effects of reaction conditions on methanolysis of PCa
Entry
Temperature/ꢀC
Time/h
m(DES) : m(PC)
n(CH3OH) : n(PC)
CPC/%
YBPA/%
1
2
3
4
5
6
7
8
9
10
110
115
120
125
120
120
120
120
120
120
2.0
2.0
2.0
2.0
1.25
1.5
2.0
2.0
2.0
2.0
0.1 : 1
0.1 : 1
0.1 : 1
0.1 : 1
0.1 : 1
0.1 : 1
0.05 : 1
0.075 : 1
0.1 : 1
0.1 : 1
5 : 1
5 : 1
5 : 1
5 : 1
5 : 1
5 : 1
5 : 1
5 : 1
3 : 1
7 : 1
14
55
100
100
24
73
56
86
98
8
48
99
98
19
66
53
81
97
97
99
a
Experimental conditions: PC (4.0 g).
molar ratio of methanol to PC changed from 3 : 1 to 7 : 1 [EmimOH]Cl-2Urea aer reused for 4 times. In addition, the
(entries 3 and 9–10), the CPC and YBPA increased slightly. When TGA curve of fresh DES is almost the same as that of reused one,
the molar ratio of methanol to PC was 5 : 1, the CPC and YBPA demonstrating that the [EmimOH]Cl-2Urea has an excellent
reached the maximum values, which were 100% and 99%, thermal stability. Moreover, the TGA curves also shows that the
respectively. When n(CH3OH) : n(PC) increased from 5 : 1 to decomposition process of DES is mainly divided into two stages.
7 : 1 (entries 3 and 10), the CPC and YBPA had a slight decrease, The weight loss was nearly 38% in the range of 175–252 ꢀꢀC,
which may be due to excessive methanol addition, resulting in another weight loss is about 58% in the scope of 252–320 C.
a decrease in the reactant concentration.
3.5 Reusability of [EmimOH]Cl-2Urea
From the perspective of environmental protection and
sustainable development, it is necessary to study the reusability
of DES in PC methanolysis. Under the following conditions: PC
(4.0 g), DES (0.4 g), n(PC) : n(CH3OH) ¼ 1 : 5, 120 ꢀC and 2.0 h,
the reusability of DES was investigated and the results were
shown in Fig. 4. It can be clearly seen that even if the DES is
reused for 4 times, the CPC and YBPA are not apparently reduced,
indicating that [EmimOH]Cl-2Urea has good reusability
performance. Aer reused for 4 times, the CPC and YBPA will
decrease slightly due to the loss of catalyst during operation.
The fresh and reused [EmimOH]Cl-2Urea were characterized
by FT-IR and TGA, and results were shown in Fig. 5. Comparing
with the IR spectrum of fresh [EmimOH]Cl-2Urea, the reused
one basically contained all the characteristic absorption peaks,
indicating that there was no change in chemical structure of PC (4.0 g), DES (0.4 g), n(PC) : n(CH3OH) ¼ 1 : 5, 120 ꢀC and 2.0 h).
Fig. 4 Reusability results of [EmimOH]Cl-2Urea (reaction conditions:
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Fig. 5 FT-IR spectra and TGA curves of fresh and reused [EmimOH]Cl-2Urea.
Fig. 6 FT-IR spectra of PC residues (A) and BPA (B) obtained at different conversions.
ꢀ
When the temperature exceeds 320 C, the mass of the sample a yield of 90% is almost identical with that of the BPA standard
gradually approaches zero, meaning the sample is completely sample (Fig. 6(B)).
decomposed. However, the methanolysis temperature is only
The PC residues were also determined by GPC and the
120 ꢀC, lower than the decomposition temperature of results are shown in Table 3. When the CPC increased from 0 to
[EmimOH]Cl-2Urea, which shows that DES can stably exist and 46% and 78%, the Mn of PC residue decreased from 8993 to
hard to decompose under the above conditions.
5589, 5423 g molꢁ1, and the Mw of PC residue reduced from
23 563 to 14 130, 13 933 g molꢁ1. The results indicate that with
the increase of PC, the long-chain PC decomposed into some
short-chain oligomers, which reduced the degree of polymeri-
zation and this was more favorable for further degradation.
To further investigate this mechanism, 1H NMR spectros-
copy analysis was performed in d6-DMSO. As shown in Fig. 7, in
the absence of DES, the chemical shi of hydroxyl proton in
methanol is 4.08 ppm. As the amount of DES increases (from
curve 1 to 4), the signals of hydrogens in –OH move towards the
3.6 Mechanism of PC methanolysis catalyzed by [EmimOH]
Cl-2Urea
To study the mechanism of PC methanolysis, FT-IR was used to
characterize the PC residues and BPA products at different
conversions. As shown in Fig. 6(A), the PC residue at low
conversion of 46% showed characteristic peaks similar to that
of PC raw material. With increasing PC conversion, the –OH
peak at about 3500 cmꢁ1 increases slightly, and the –C]O peak
at about 1760 cmꢁ1 becomes stronger. This might be due to the
breakage of long chains into short chains during the reaction,
resulting in more –OH and –C]O being exposed. The BPA
product obtained at low yield of 29% showed an obvious
difference in the carbonyl group at about 1730 cmꢁ1, indicating
there are some oligomers in BPA product. As the YBPA increased
to 78% and 90%, the carbonyl peak gradually weakened until
completely disappeared. The structure of BPA obtained at
Table 3 Molecular weights of PC residues obtained at different
conversions (a 0%; b 46%; c 78%)
Entry
Mn
Mw
Mw/Mn
a
b
c
8993
5589
5423
23 563
14 130
13 933
2.620
2.528
2.569
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amino group of urea and the hydroxyl group of methanol,
improving the electronegativity of the oxygen atom in CH3OH,
which is conducive to –OCH3 attacking the carbon atom in the
ester group of PC, resulting in the breaking of the ester bond.
PC gradually decomposes into different molecular weight olig-
omers. These oligomers then reacted with methanol repeatedly
to form smaller oligomers and dimers until BPA and dimethyl
carbonate were produced.
3.7 Kinetics of PC methanolysis
The methanolysis of PC previously reported in our group20–25,43
and the glycolysis of PET40–42 reported by Zhang groups. Kinetic
studies of chemical degrading polymers were generally
considered to be a pseudo-rst order reaction. Therefore, the
methanolysis of PC catalyzed by the DES [EmimOH]Cl-2Urea is
also assumed to be a pseudo-rst order, and the kinetic equa-
tion is as follows:
dðCPC
dt
Þ
¼ ꢁkCPC
(3)
where k represents the rate constant of the reaction, and CPC is
the concentration of PC at time t.
Fig.
7
1H NMR spectra of [EmimOH]Cl-2Urea DES and CH3OH
mixture with different molar ratio; the symbols in the 1H NMR spectra
identify the N–H (triangle) of urea.
CPC ¼ CPC0(1 ꢁ X)
(4)
where X represents the conversion of PC, so eqn (3) can be
converted to eqn (5):
lower eld. When the amount of DES is 0.7 mmol mLꢁ1, the
chemical shi of hydrogen in –OH reached 4.10 ppm (Dd ¼
+0.02 ppm). In addition, as the concentration of methanol
increased (from curve 4 to 1), the N–H signal shied from 5.43
to 5.42 ppm. However, as the concentration of DES increased
from 0.1 to 0.4, 0.7 mmol mLꢁ1, the signals of –CH3 (d ¼ 3.14
ppm) do not change obviously. Therefore, it can be reasonably
speculated that the change of chemical shi is due to the strong
hydrogen bond interaction between the hydroxyl group in
methanol and the amino group in DES, which is similar to what
reported by Zhang's group.40–42
dX
¼ kð1 ꢁ XÞ
(5)
(6)
dt
Eqn (5) is integrated against time to get eqn (6).
1
ln
¼ kt
1 ꢁ X
Under the conditions of PC (4.0 g), m(DES) : m(PC) ¼ 0.1 : 1,
and n(CH3OH) : n(PC) ¼ 5 : 1, the effects of reaction tempera-
ture on the rate of PC methanolysis were shown in Fig. 8(A), and
the corresponding linear regression results were shown in Table
4. It can be seen from Table 4 that all of the linear correlative
coefficients were almost 0.98, indicating that ln 1/(1 ꢁ X) was
Based on the above experimental results, a probable mech-
anism of PC methanolysis catalyzed by [EmimOH]Cl-2Urea is
proposed in Scheme 2. First, the H-bond is formed between the
Scheme 2 Proposed mechanism of PC methanolysis catalyzed by [EmimOH]Cl-2Urea.
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Fig. 8 (A) Effect of reaction temperature on methanolysis rate of PC catalyzed by [EmimOH]Cl-2Urea; (B) Arrhenius plot of rate constant of PC
methanolysis (reaction conditions: PC (4.0 g), m(DES) : m(PC) ¼ 0.1 : 1, n(CH3OH) : n(PC) ¼ 5 : 1).
Table 4 Linear regression results of the data in Fig. 8A
Reaction temperature/ꢀC
Regressive equation
Rate constant
Linear correlation coefficient
120
115
105
100
y ¼ ꢁ2.857 + 2.859x
y ¼ ꢁ3.478 + 1.994x
y ¼ ꢁ1.725 + 0.676x
y ¼ ꢁ0.984 + 0.318x
2.859
1.994
0.676
0.318
0.976
0.993
0.993
0.992
proportional to time (t) at different reaction temperatures. In (0.4 g), PC (4.0 g) and n(CH3OH) : n(PC) ¼ 5 : 1 for 2.0 h at
ꢀ
other words, the rate of PC methanolysis was proportional to PC 120 C, the CPC and YBPA reached 100% and 99%, respectively.
concentration. So it can prove that the methanolysis of PC is Moreover, [EmimOH]Cl-2Urea had good thermal stability and
also a pseudo-rst order reaction. This reaction rate constants can be reused for 5 times without any apparent decrease in the
can be calculated from the slope values of straight line in CPC and YBPA. The kinetics results suggest that the methanolysis
Fig. 8(A), and they were 2.859 hꢁ1, 1.994 hꢁ1, 0.676 hꢁ1 and of PC catalyzed by [EmimOH]Cl-2Urea is a pseudo-rst order
0.318 hꢁ1, respectively, which represented the rate constants of reaction with Ea of 133.59 kJ molꢁ1. This study can provide
PC methanolysis reaction at 120 ꢀC, 115 ꢀC, 105 ꢀC and 100 ꢀC. important guidance for the design of effective DESs used to
Moreover, the rate constant increases rapidly with the reac- promote the degradation reaction of polymer wastes and other
tion temperature, showing that the PC methanolysis is signi- transesterication.
cantly inuenced by reaction temperature, which is consistent
with the previous experimental results (Table 2, entries 1–4).
Using the above rate constant, the reaction activation energy
(Ea) can be calculated by the Arrhenius eqn (7).
Conflicts of interest
There are no conicts to declare.
Ea
ln k ¼ ln A ꢁ
(7)
RT
Acknowledgements
where A is pre-exponential factor, R is molar gas constant (8.314
J kꢁ1 molꢁ1) and T is the Kelvin temperature. The Arrhenius plot
of the rate constant was shown in Fig. 8(B), and Ea of PC
methanolysis calculated from the slope of the Arrhenius plot is
133.59 kJ molꢁ1, which is lower than that of methanolysis of PC
catalyzed by ChCl-2Urea43 (141.47 kJ molꢁ1).
This research was supported nancially by the National Natural
Science Foundation of China (No. 51673106), the Science and
Technology Research Project of Shandong Province (No.
2019GSF109015), the Taishan Scholars Projects of Shandong
(ts201511033) and Shandong Key Laboratory of Reactions and
Separation of Muti-phase Liquid (No. 2019MFRSE-B03).
4. Conclusions
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