European Polymer Journal 45 (2009) 1535–1544
Contents lists available at ScienceDirect
European Polymer Journal
journal homepage: www.elsevier.com/locate/europolj
Glycolysis of poly(ethylene terephthalate) catalyzed by ionic liquids
Hui Wang a, Yanqing Liu a, Zengxi Li a,*, Xiangping Zhang b, Suojiang Zhang b,*, Yanqiang Zhang b
a
b
Graduate University of Chinese Academy of Sciences, No. 19A Yuquan Road, Shijingshan, Beijing 100049, China
Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China
a r t i c l e
i n f o
Article history:
Received 4 November 2008
Received in revised form 5 January 2009
Accepted 19 January 2009
Available online 30 January 2009
Keywords:
Poly(ethylene terephthalate)
Glycolysis
Ionic liquid
Catalysis
a b s t r a c t
Poly(ethylene terephthalate) (PET) from an industrial manufacturer was depolymerized by
ethylene glycol in the presence of a novel catalyst: ionic liquids. It was found that the purification process of the products in the glycolysis catalyzed by ionic liquids was simpler
than that catalyzed by traditional compounds, such as metal acetate. Qualitative analysis
showed that the main product in the glycolysis process was the bis(hydroxyethyl) terephthalate (BHET) monomer. Thermal analysis of the glycolysis products was carried out
by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The
influences of experimental parameters, such as the amount of catalyst, glycolysis time,
reaction temperature, and water content in the catalyst on the conversion of PET, selectivity of BHET, and distribution of the products were investigated. Results show that reaction
temperature is a critical factor in this process. In addition, a detailed reaction mechanism of
the glycolysis of PET was proposed.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Poly(ethylene terephthalate) (PET) is a thermoplastic
polyester with excellent tensile and impact strength,
chemical resistance, clarity, processability, transparency,
and appropriate thermal stability [1]. Tremendous quantities of this material are consumed in the manufacture of video and audio tapes, X-ray films, food packaging, and
especially of soft-drink bottles. With the widespread use
and increasing consumption of PET, the amount of waste
PET is growing rapidly. Although this kind of polyester
does not create a direct hazard to the environment, it does
not decompose readily in nature. Thus, the effective recycling of PET wastes for the preservation of resources and
protection of the environment has received considerable
attention. PET wastes can be converted by mechanical
methods into extruded or molded articles, whose properties are inferior to those of the original. PET decomposition
and its conversion into reusable chemical products be* Corresponding authors. Fax: +86 10 82627080 (Z. Li).
E-mail addresses: [email protected] (Z. Li), [email protected]
(S. Zhang).
0014-3057/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.eurpolymj.2009.01.025
comes one of the important recycling strategies for this
material [2].
Several processes for PET depolymerization have been
put forward, such as alcoholysis process with methanol
[3,4], glycolysis with ethylene glycol or other glycol
[2,5–7], and hydrolysis under the promotion of acidic or basic conditions [8,9]. Alcoholysis to dimethyl terephthalate
(DMT) with liquid or gaseous methanol has the obvious disadvantage of volatilization of methanol. The glycolysis process involves the insertion of a diol into PET chains to give
bis(hydroxyethyl) terephthalate (BHET), which is easily
integrated into conventional PET products. The main drawback of this process is that the reaction products are not discrete chemicals but BHET along with higher oligomers,
which are difficult to purify with conventional methods
[5]. Hydrolysis of PET under acidic or basic conditions gives
terephthalic acid (TPA), along with corrosion and pollution
problems [10].
Since glycolysis of PET can produce the BHET monomer,
which has been widely used in the production of unsaturated polyesters and rigid or flexible polyurethanes [11],
the glycolysis process was chosen for further investigation
in our study. This process is very sluggish without a catalyst.
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H. Wang et al. / European Polymer Journal 45 (2009) 1535–1544
Therefore, various kinds of catalysts, such as titanium–
phosphate [2], metal acetate [5], and solid superacids
[12] are required to facilitate the process. Baliga and Wong
[10] have found that the glycolysis products had one to
three repeating units depending on the catalyst used, and
zinc acetate had the best result among the tested metal
acetates (lead, zinc, cobalt, and manganese), yielding the
highest amount of BHET monomer. However, Troev et al.
[2] have pointed out that titanium–phosphate showed
markedly higher catalytic activity than zinc acetate in the
glycolysis of PET. Although these catalysts are effective in
the glycolysis of PET, it is difficult to separate the catalysts
from the products, as illustrated in the literature [5]. Thus,
the catalysts could not be recycled and reused, and the
purity as well as application fields of the depolymerization
products will be seriously threatened.
Room-temperature ionic liquids (RTILs) are a kind of organic molten salt with a melting point below 100 °C. Scientists have shown great interest in RTILs because of their
unique features, such as the strong solvent power for organic and inorganic compounds, thermal stability, nonvolatility, electrochemical stability, and low flammability
[13–15]. In the last decade, ionic liquids have been widely
used in extraction [16], catalysis [17], electrochemistry
[18], and organic synthesis [19,20]. Recently, it has been
reported that chloroaluminate(I) ionic liquids could be
used in the catalytic cracking of polyethylene to light alkanes [21]. Kamimura et al. [22] have employed quaternary ammonium ionic liquids to depolymerize polyamide
plastics, and the caprolactam monomer was obtained.
However, we have found no reports on the use of ionic liquids in the depolymerization of PET or in the catalysis of
traditional depolymerization processes. The special properties of ionic liquids make it easy to separate the catalyst
from the solid glycolysis products. Thus, ionic liquids seem
to be a promising catalyst for the depolymerization of PET.
A simple glycolysis system catalyzed by ionic liquids
was set up in our laboratory. The BHET monomer, dimer,
and oligomers were found to be the glycolysis products.
As composition of the glycolysis products is complex,
product distribution was illustrated in this study. Moreover, we investigated influences of the reaction parameters on the conversion of PET and selectivity of the
main product. Thermal properties of the glycolysis products were also studied. To further understand this recycling process, the mechanism of glycolysis of PET was
revealed.
2. Experimental
2.1. Materials
PET pellets (2.0 2.5 2.7 mm) were supplied by
Jindong Commercial Co. Ltd., Jiangsu Province, China. Reagent grade ethylene glycol, phenol, and 1,1,2,2-tetrachloroethane were obtained from Sinopharm Chemical Reagent
Beijing Co., Ltd., China. Standard sample of BHET (99.5%)
was obtained from Sigma–Aldrich China Inc. Materials for
synthesizing ionic liquids, and methanol with chromatogram grade for High Performance Liquid Chromatography
(HPLC) analysis were purchased from J&K Chemical Ltd.,
China. The materials were used without any further treatment. Ionic liquids were synthesized according to the procedures described in literatures [23–25].
2.2. Synthesis of ionic liquids
2.2.1. Synthesis of 1-butyl-3-methylimidazolium chloride
([bmim] Cl)
Equal molar amounts of chlorobutane and 1-methylimidazole were added to a round-bottom flask fitted with a
reflux condenser. The mixture was refluxed for 48–72 h
at 70 °C with stirring until two phases formed. The top
phase, containing unreacted starting material, was decanted and acetone was added with thorough mixing. Then
the flask was frozen in the refrigerator for 24 h. Crystals of
[bmim]Cl were formed. The liquid phase, containing unreacted material and acetone, was decanted followed by the
addition of fresh acetone and this step was repeated twice.
After the third decanting of acetone, any remaining acetone was evaporated with a rotary evaporator. The product, [bmim]Cl, was obtained and dried in the vacuum
oven at 70 °C for 48 h.
2.2.2. Synthesis of 1-butyl-3-methylimidazolium bromine
([bmim] Br)
This synthesis followed the same procedure as for
[bmim]Cl described above, although bromobutane was
used instead of chlorobutane.
2.2.3. Synthesis of 1-butyl-3-methylimidazolium dihydrogen
phosphate ([bmim] H2PO4)
[bmim]H2PO4 was obtained by dropwise addition of
one equivalent of concentrated phosphoric acid (85%) to
a cooled solution of [bmim]Cl (one equivalent) in anhydrous methylene chloride. The mixture was refluxed for
48 h with stirring, and the HCl by-product formed in the
reaction was distilled out of the condenser under a stream
of dry nitrogen and was dissolved in deionized water at
0 °C. (The acid aqueous solution was monitored by titration
with NaOH.) When the formed HCl had been completely
removed, CH2Cl2 was evaporated with a rotary evaporator.
The obtained ionic liquid was dried in a vacuum oven at
70 °C for 48 h.
2.2.4. Synthesis of 1-butyl-3-methylimidazolium hydrogen
sulphate ([bmim] HSO4)
This synthesis followed the same procedure as for
[bmim]H2PO4 described above, although concentrated sulfuric acid was used instead of phosphoric acid.
2.2.5. Synthesis of (3-amino-propyl)-tributyl-phosphonium
glycine ([3a-C3P(C4)3][Gly])
Equal molar amounts of tri-n-butylphosphine and 3-bromopropylamine hydrobromate were added to a round-bottom flask containing acetonitrile as solvent. The mixture
was vigorously stirred for 8 h at 60 °C, and then 48 h at
80 °C. The solvent acetonitrile was removed by vacuum
evaporation at 70 °C. Then, n-hexane was added to the flask
to dissolve the remaining material with thorough stirring.
After standing for 30 min, two phases were formed. The
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H. Wang et al. / European Polymer Journal 45 (2009) 1535–1544
top phase, containing unreacted starting material and
n-hexane, was decanted. The bottom phase was the product
fraction. This washing process was repeated until no starting
material exited in the top phase. The obtained product, [3aC3P(C4)3]Br, was dried in a vacuum oven at 70 °C for 24 h.
Then [3a-C3P(C4)3]Br was dissolved in deionic water, and
the solution was added to the pretreated ionic exchange resin column, resulting in the transformation of Br1 into
OH1. The obtained solution of [3a-C3P(C4)3][OH] was evaporated to remove most of the water. Equal molar amount of
glycine (determined by the amount of [3a-C3P(C4)3]Br) was
added to the solution of [3a-C3P(C4)3][OH] with stirring for
24 h at room temperature. Then, water was removed by vacuum evaporation. The final product [3a-C3P(C4)3][Gly] was
dried in a vacuum oven at 70 °C for 48 h.
2.2.6. Synthesis of (3-amino-propyl)-tributyl-phosphonium
alanine ([3a-C3P(C4)3][Ala])
This synthesis followed the same procedure as for
[3a-C3P(C4)3][Gly] described above, although alanine was
used instead of glycine.
2.3. Glycolysis of PET
A 50 mL round-bottom three-necked flask equipped
with a thermometer and a reflux condenser was loaded
with 5.0 g of PET, 20.0 g of ethylene glycol, and certain
amount of ionic liquids. The glycolysis reactions were carried out under atmospheric pressure at reaction temperatures ranging from 160 to 195 °C for glycolysis times of
5–10 h. The flask was immersed in an oil bath at a specific
temperature for the required time.
When each glycolysis reaction finished, the undepolymerized PET pellets were quickly separated from the liquid
phase before the products precipitated. Then an excess
amount of cold distilled water was used to wash the undepolymerized PET pellets, and the water was then mixed
with the product fraction. The undepolymerized PET was
collected, dried, and weighed. The conversion of PET is defined by Eq. (1):
Conversion percentage of PET ¼
W0 W1
100%
W0
ð1Þ
where W0 represents the initial weight of PET and W1 represents the weight of undepolymerized PET. Meanwhile,
the glycolysis product mixture was vigorously agitated
(cold distilled water would dissolve the remaining ethylene glycol, ionic liquids, and the monomer) and then filtered. The collected filtrate was concentrated to about
100 mL by heating at the boiling point. The concentrated
filtrate was stored in a refrigerator at 0 °C for 24 h. White
crystalline flakes were formed in the filtrate, then separated and dried. This was labeled as fraction A. The solid
insoluble in cold distilled water was boiled in 600 mL distilled water with vigorous stirring, then filtered. The collected filtrate in this step was concentrated to about
200 mL by heating at the boiling point. Upon immersing
this concentrated filtrate in an ice bath, floccules were
formed in the filtrate, then filtered and dried. This fraction
was labeled B. The fraction insoluble in boiling water was
labeled C. The selectivity is defined by Eq. (2):
Selectivity ðmol%Þ ¼
moles of specific products
100%
moles of depolymerized PET units
ð2Þ
2.4. Determination of molecular weight of PET
Viscosity-average molecular weight of PET was determined by viscosity method. Viscometry analysis was
performed with an Anton Paar AMVn-Automated Microviscometer at 25 °C. PET samples were dissolved in the
mixture of phenol and 1,1,2,2-tetrachloroethane with a
weight ratio of 60/40 at 110 °C. The procedure was carried
out according to ASTM D 4603.
2.5. Characterization
HPLC analysis of the main glycolysis product was performed using an Agilent-1001 high performance liquid
chromatography equipped with a reverse-phase ZorbaxC8 column (150 mm long and 4.6 mm diameter) and an
ultraviolet detector set at 254 nm. The HPLC experiments
were performed using methanol/water of ratio 70/30
(v/v) as the mobile phase at a flow rate of 1.0 mL/min.
The solution for HPLC analysis was prepared by dissolving
about 4 mg crystals in 50 mL of methanol/water (70/30
v/v) mixture. 1H NMR and 13C NMR of the main glycolysis
product were recorded with a Brucker ARX-400 Advance
Spectrometer operating at 400 MHz. The spectra were obtained in d6-acetone solution. The mass spectrum was performed using a Bruker Daltonics BiflexIII MALDI-TOF
instrument with electrospray ionization (ESI). DSC scans
of the products were obtained using DSC-2910 by heating
from room temperature to 300 °C at a rate of 10 °C/min in
an atmosphere of nitrogen. A thermogravimetric analyzer
(TGA-2050) was used to measure the weight loss of the
products in a nitrogen atmosphere during a temperature
range from room temperature to 500 °C at a heating rate
of 10 °C/min. The morphology of PET was examined by a
scanning electron microscope (JSM 6700F, Japan).
3. Results and discussion
3.1. Selection of the catalysts
The catalytic effect of the synthesized ionic liquids on
the glycolysis of PET was investigated, and the results are
summarized in Table 1.
Table 1
Catalytic effect of different ionic liquids on the glycolysis of PETa.
Entry
Ionic liquids
Temperature (°C)
Conversion of PET (%)
1
2
3
4
5
6
7
–
[bmim]H2PO4
[bmim]HSO4
[3a-C3P(C4)3][Gly]
[3a-C3P(C4)3][Ala]
[bmim]Cl
[bmim]Br
180
175
170
180
180
180
180
10.1
6.9
0.5
100
100
44.7
98.7
a
Reaction conditions: 1 atm, 8 h, amount of ionic liquid 1.0 g.
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H. Wang et al. / European Polymer Journal 45 (2009) 1535–1544
Table 1 shows that basic ionic liquids ([3a-C3P(C4) 3][Gly],
[3a-C3P(C4)3][Ala]) and neutral ionic liquids ([bmim]Cl,
[bmim]Br) do accelerate the glycolysis process (entries
4–7). But when acidic ionic liquids ([bmim]H2PO4, [bmim]
HSO4) are used (entries 2 and 3), the maximum temperature
of the system was below 180 °C and the conversion of PET is
diminished. The effect could probably be attributable to the
instability of the synthesized acidic ionic liquids at relatively
higher temperature. The catalytic effect of basic ionic liquids
is desirable and the conversion of PET could reach 100% (entries 4 and 5). However, this kind of ionic liquids is quite complex to synthesize (three steps are needed) and price of the
materials is very high. Thus, basic ionic liquids were not further studied. Neutral ionic liquids, which also have good catalytic effects and appropriate price, became the alternative.
The conversion of PET catalyzed by [bmim]Cl and [bmim]Br
could reach 44.7% and 98.7%, respectively (entries 6 and 7).
Although [bmim]Br has a better effect, [bmim]Cl, being much
more stable than [bmim]Br, was chosen as the depolymerization catalyst for further investigation.
3.2. Qualitative analysis of the products
Qualitative analysis of the main product fraction A in
the glycolysis of PET catalyzed by [bmim]Cl was performed. The HPLC chromatograms are shown in Fig. 1. It
indicates that the main product in the presence of
[bmim]Cl has the same retention time as the standard
BHET under the same chromatogram conditions. This
means that the main product in the glycolysis process catalyzed by [bmim]Cl is probably bis(hydroxyethyl) terephthalate (BHET) [5]. Fig. 1 also indicates that purity of
the main product obtained in our research is high. Fractions B and C are considered to be the dimer and oligomers,
as shown by the melting points of those fractions.
In order to confirm that fraction A is BHET, 1H NMR, 13C
NMR, and mass spectrum of fraction A were performed. 1H
NMR and 13C NMR spectra of this fraction are reproduced
in Fig. 2 for illustration. The signal at d 8.15 ppm indicates
the presence of the four aromatic protons of the benzene
ring. Signals at d 4.4 and 3.9 ppm are characteristic of the
methylene protons of COO–CH2 and CH2–OH. And the triplet
at d 4.2 ppm is characteristic of the protons of the hydroxyl.
The signals of 13C NMR are in accordance with those predicted in 1H NMR, also shown in Fig. 2. The NMR spectra also
accord very well with those reported in the literature [5].
It is clear from the mass spectrum of fraction A in Fig. 3 that
the peak up to m/e 277 with intensity almost 100% was obtained. This peak is related to fraction A ionized by Na+ (in electrospray ionization, the fraction could be ionized by H+, Na+, or
K+). Thus, the molecular weight of fraction A is 254 g/mol,
which is the same as the molecular weight of BHET.
3.3. Thermal analysis of the products
The DSC thermograms of the glycolysis products are
shown in Fig. 4. The DSC scan of fraction A is shown as
curve 1. The melting onset temperature and peak temperature of fraction A are 105 and 109 °C, respectively. Curve
2, representing the DSC scan of fraction B, shows a sharp
endothermic peak at 171 °C. The melting temperatures of
fractions A and B agree very well with the known melting
points of BHET and its dimer [26]. Curve 3 in Fig. 4 displays
the DSC thermal analysis curve of fraction C. It shows a
broader peak centered at about a maximum of 240 °C,
and this peak can be related to the presence of a small
amount of a mixture of higher oligomers. The successful
separation of the dimer from oligomers is confirmed by
the complete disappearance of the peak at 171 °C in the
DSC curve of fraction C.
Three TGA curves are shown in Fig. 5. These curves all
indicate a weight loss starting around 400–420 °C. Curve
a shows the TGA curve of the BHET monomer. The first
weight loss is about 60% and starts around 200–220 °C.
This phenomenon is attributed to the thermal decomposition of the BHET fraction. The second weight loss of about
30% starting around 400–420 °C is due to the thermal
decomposition of the PET produced by the BHET thermal
polymerization during the thermogravimetric analysis
process [6]. Curve b displays the TGA curve of dimer. The
first weight loss of about 25% starts around 250–270 °C.
This weight loss is attributed to the thermal decomposition
of the dimer. The second weight loss of about 65% begins
around 400–420 °C. Similarly, this weight loss is considered as the thermal decomposition of the PET produced
by the dimer thermal polymerization during the thermo-
Fig. 1. Comparison of HPLC chromatogram of the standard BHET (I) and that of the main product in the glycolysis process catalyzed by [bmim]Cl (II).
H. Wang et al. / European Polymer Journal 45 (2009) 1535–1544
Fig. 2. 1H NMR (a,b),
1539
13
C NMR (c) spectra of fraction A in d6-acetone.
gravimetric analysis process. Curve c shows the TGA curve
of oligomers, and it also shows two major weight loss
processes. The first weight loss of about 10% at around
320–330 °C is attributed to the thermal decomposition of
oligomers. The second weight loss of about 80% starting
around 400–420 °C, is also due to the thermal decomposition of the PET produced by the thermal polymerization of
oligomers during the thermogravimetric analysis process.
These analysis results also indicate that during the thermogravimetric analysis process, the weight of the PET produced by the thermal polymerization of oligomers is
higher than that produced by the thermal polymerization
of dimer, and the weight of the PET produced by the thermal polymerization of dimer is higher than that produced
by the thermal polymerization of BHET.
3.4. Influences of reaction conditions
products is shown in Table 2. It shows that PET can also
be partially depolymerized by ethylene glycol in the absence of catalyst, perhaps due to the solvent effect of ethylene glycol in the glycolysis process. The conversion of
PET increases with the amount of [bmim]Cl. The selectivity
of BHET first appears to increase with the amount of catalyst, and then varies insignificantly when the amount of
catalyst is more than 2.0 g. The composition of the products in Table 2 indicates that when no catalyst was used,
the weight percentage of BHET monomer in the glycolysis
products is only 1.8%. Thus, with no catalyst, most of the
glycolysis products are the oligomers. Subsequently, when
[bmim]Cl is added, the proportion of BHET in the products
dramatically increases from 1.8% to more than 70.0%.
Therefore, the presence of the catalyst significantly increases the selectivity of BHET monomer in the glycolysis
process.
3.4.1. Influence of the amount of catalyst
The influence of the amount of catalyst on the conversion of PET, selectivity of BHET, and composition of the
3.4.2. Influence of glycolysis time
The effects of glycolysis time on the glycolysis conversion and BHET selectivity are presented in Fig. 6, and the
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H. Wang et al. / European Polymer Journal 45 (2009) 1535–1544
Fig. 3. The mass spectrum of fraction A.
Table 2
Influence of the amount of catalyst on the glycolysis process.a
Endo
3
2
Amount
of IL (g)
Conversion
of PET (%)
Selectivity
of BHET (%)
0.0
1.0
2.0
3.0
4.0
10.1
44.7
54.1
66.9
70.1
1.8
53.7
63.0
59.9
59.4
a
Composition (wt%)
BHET
Dimer
Oligomers
1.8
72.4
76.6
75.1
78.0
25.4
5.7
4.9
4.4
1.9
72.8
21.9
18.5
20.5
20.1
Reaction conditions: 1 atm, 180 °C, 8 h.
1
40
80
120
160
200
240
280
Temperature (°C)
Fig. 4. DSC scans of fraction A (curve 1), fraction B (curve 2), and fraction
C (curve 3).
c
100
100
b
Conversion of PET (%)
Weight (%)
80
60
a
40
influence of glycolysis time on the product distribution is
given in Fig. 7. Fig. 6 indicates that with increasing glycolysis time, the conversion of PET increases distinctly. If the
reaction time is extended to 10 h, the glycolysis conversion
could be close to 100%. It also shows that the selectivity of
BHET reaches a maximum value when the reaction time is
20
0
69
Conversion of PET
Selectivity of BHET
90
66
80
63
70
60
60
57
Selectivity of BHET (%)
0
50
0
100
200
300
Temperature (
400
500
)
Fig. 5. TGA curves of BHET (curve a), dimer (curve b), and oligomers
(curve c).
5
6
7
8
9
10
54
Glycolysis time (h)
Fig. 6. Effects of glycolysis time on the conversion of PET and selectivity
of BHET (1 atm, 180 °C, with 4.0 g of [bmim]Cl).
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H. Wang et al. / European Polymer Journal 45 (2009) 1535–1544
80
60
Composition (wt%)
Composition (wt%)
80
40
20
0
5
6
7
8
9
BHET
Dimer
Oligomers
set at 6 h. However, the effect of reaction time on the product distribution illustrated in Fig. 7 is quite complex: with
the increasing of reaction time, the weight percentage of
BHET in the final products first increases then decreases;
while that of the dimer first decreases then reaches a constant value. Correspondingly, the weight percentage of the
oligomers first decreases then increases. Thus, it may be
concluded that during the depolymerization process, PET
was first depolymerized into oligomers; afterwards, oligomers were converted into dimer, then BHET monomer in
the presence of ethylene glycol; with increasing of glycolysis time, BHET would further polymerize into dimer, then
oligomers. This would account for the tendency for the
selectivity of BHET to first increase, then decrease with
an increase of reaction time.
3.4.3. Influence of reaction temperature
Fig. 8 indicates the effects of reaction temperature on
the conversion of PET and selectivity of BHET, and Fig. 9
presents the influence of glycolysis temperature on the
product distribution. Fig. 8 shows that the glycolysis conversion is very close to 0% when the reaction temperature
is below 160 °C. Subsequently, with increasing glycolysis
temperature, the conversion of PET increases apparently
100
80
80
60
60
40
40
20
20
0
0
160
170
180
190
Selectivity of BHET (%)
Conversion of PET (%)
Conversion of PET
Selectivity of BHET
40
20
170
175
180
185
Temperature (
Fig. 7. Effect of glycolysis time on distribution of the products (1 atm,
180 °C, with 4.0 g of [bmim]Cl).
100
60
0
165
10
Glycolysis time (h)
BHET
Dimer
Oligomers
200
Temperature (°C)
Fig. 8. Effects of reaction temperature on the conversion of PET and
selectivity of BHET (1 atm, 8 h, with 4.0 g of [bmim]Cl).
190
195
200
)
Fig. 9. Effect of reaction temperature on distribution of the products
(1 atm, 8 h, with 4.0 g of [bmim]Cl).
and rapidly reaches 100% when the temperature is set at
190 and 195 °C. Further, the selectivity of BHET increases
as the reaction temperature rises. When the glycolysis
temperature is 170 °C, the selectivity begins to reach
7.9%. Then it dramatically increases from 7.9% to 59.4%
when the glycolysis temperature rises to 180 °C. However,
the increasing tendency is retarded when the reaction temperature is above 180 °C. Fig. 9 indicates that the proportion of BHET monomer in the products increases with
increase of reaction temperature, and that of oligomers decreases with increase of glycolysis temperature. Thus, high
temperature is beneficial to the formation of BHET monomer. Therefore, the glycolysis temperature is a critical factor in the glycolysis of PET.
3.4.4. Influence of water content in ionic liquid
[bmim]Cl is a moisture-sensitive substance, and its
property will be seriously influenced by the existence of a
small amount of water. To study the effects of water content in [bmim]Cl on the conversion of PET and selectivity
of BHET, experiments were carried out under atmospheric
pressure at 180 °C with reaction time of 8 h, the amount
of [bmim]Cl 4.0 g, and the addition of certain amount of
water into the 4.0 g of [bmim]Cl. The results are shown in
Fig. 10. It shows that with the increase of water content
in [bmim]Cl, the conversion of PET decreases apparently
and the selectivity of BHET decreases slightly. The Cl in
[bmim]Cl would associate with the proton in water and
hydrogen bonds were formed, which resulted in the reduction of the amount of effective [bmim]Cl and further caused
decrease of the glycolysis conversion and selectivity of
BHET. Therefore, [bmim]Cl should be stored in vacuum.
3.5. Mechanism of the glycolysis of PET
To further understand the depolymerization of PET, the
mechanism should be revealed. Mechanism of the methanolysis process was detailedly discussed by Genta et al. [27],
and that of this glycolysis process was also mentioned by
previous studies [10,28,29]. However, adequate convictive
data on this mechanism were not supplied in these researches. In order to further investigate the mechanism,
PET conversion or BHET selectivity (%)
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H. Wang et al. / European Polymer Journal 45 (2009) 1535–1544
Conversion of PET
Selectivity of BHET
70
60
50
40
30
Table 3
Viscosity-average molecular weight of fresh PET and residual PET.
Reaction
temperature (°C)
Viscosity-average
molecular weight of PETb
Fresh PET
160
170
180
190
26,280
4330
2370
1720
No residual
b
Calculate from the Mark–Houwink equation: ½g ¼ KMv a , where
K = 75.5 103 mL/g and a = 0.685.
20
10
0
0
2
4
6
8
10
Water content in [bmim]Cl (wt%)
Fig. 10. Effects of water content in [bmim]Cl on the conversion of PET and
selectivity of BHET.
we determined the molecular weight of fresh and residual
PET, and examined the morphology by scanning electron
microscope.
The viscosity-average molecular weight determination
of fresh PET and residual PET is undertaken, and the results
are shown in Table 3. It shows that the molecular weight of
fresh PET is 26,280, and that of residual PET significantly
decreases to 4330 after PET was depolymerized at
160 °C. And molecular weight of residual PET continuously
decreases with increase of reaction temperature. This suggests that during the depolymerization process, ethylene
glycol first penetrated into the PET pellets, causing swelling of the pellets and destruction of the polymer chains,
which resulted in the depolymerization of PET into lower
polyester. This assumption could be further confirmed by
the scanning electron micrographs revealing the morphology of fresh PET and residual PET, as illustrated in Fig. 11.
The image presented in Fig. 11(a) shows that the surface of
fresh PET is relatively smooth. However, the surface of
residual PET, as displayed in Fig. 11(b)–(d), seems to exhibit porous structure, indicating the penetration of ethylene
glycol into PET pellets. Based on these results, the reaction
pathway was proposed, as shown in Fig. 12. First, PET with
polymerization degree of n was depolymerized into PET
with polymerization degree of m (m < n). Afterwards, PET
with lower molecular weight was converted into oligomers
in the presence of ethylene glycol. Oligomers were further
depolymerized into dimer, then BHET monomer; or oligomers were directly converted into monomer. With increas-
Fig. 11. Scanning electron micrographs of fresh PET (a) and residual PET (after reacting at 160 °C (b), 170 °C (c), 180 °C (d)).
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H. Wang et al. / European Polymer Journal 45 (2009) 1535–1544
HOCH2CH2OH
O
O
O
O
C
C
O
O
C
C
OCH2CH2
OCH2CH2
n
m
Oligomers
HOCH2CH2O
O
O
O
O
C
C
OCH2CH2O C
C
OCH2CH2OH
Dimer
HOCH2CH2O
O
O
C
C
OCH2CH2OH
BHET
Fig. 12. Reaction pathway of glycolysis of PET.
ing glycolysis time, BHET would further polymerize into
dimer, then oligomers. These assumptions could be confirmed by data on the influence of experimental parameters, such as reaction time and temperature, on the
glycolysis conversion, BHET selectivity, and product distribution, as illustrated above.
4. Conclusions
The glycolysis of PET was catalyzed by different kinds of
ionic liquids at atmospheric pressure. [bmim]Cl was chosen as the glycolysis catalyst for our investigation. The
main product from the depolymerization catalyzed by
ionic liquids is BHET monomer. DSC scans show that the
melting temperatures of the products agree very well with
the known melting points of BHET and dimer. TGA curves
of BHET monomer, dimer, and oligomers all show two major weight loss processes. The second weight loss starting
around 400–420 °C is attributed to the thermal decomposition of the PET produced by the thermal polymerization of
BHET, dimer, and oligomers, respectively, during the thermogravimetric analysis process. The effects of reaction
parameters, such as the amount of catalyst, glycolysis time,
reaction temperature, and water content in ionic liquid on
the conversion of PET, selectivity of BHET, and distribution
of the products have been illustrated. The results show that
the conversion of PET increases with increasing amount of
catalyst, glycolysis time, and reaction temperature, but decreases with the addition of water in [bmim]Cl. The selectivity of BHET is enhanced by increasing amount of catalyst
and reaction temperature; and it has maximum value for
an optimum value of glycolysis time, but decreases with
the increase of water content in the ionic liquid. However,
the influences of reaction conditions on the product distribution are quite complex. In addition, a reaction pathway
for the glycolysis process was suggested.
Acknowledgments
This research was supported financially by the National
High Technology Research and Development Program of
China (863 Program) (No. 2006AA06Z371) and by the
National Natural Science Funds for Distinguished Young
Scholar (No. 20625618).
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