Degradation of poly(ethylene terephthalate) catalyzed by metal-free choline-based ionic liquids

Article abstract of DOI:10.1039/d0gc00327a

Glycolysis of poly(ethylene terephthalate) (PET) is a prospective way for degradation of PET to its monomer bis(hydroxyethyl) terephthalate (BHET) which can be polymerized again to form new qualified PET materials, and hence provides possibilities for a permanent loop recycling. However, most of the reported glycolysis catalysts are metal-based, leading to high cost and negative environmental impact. In this study, we developed a series of choline-based ionic liquids (ILs) without metals and applied them in the glycolysis of PET as catalysts. Choline acetate ([Ch][OAc]), which is cheaper, more biologically compatible and environmentally friendly in comparison with conventional imidazolium metal-based ILs, can achieve a comparable or even better performance than them. Under optimum conditions (PET (5.0 g), ethylene glycol (EG) (20.0 g), [Ch][OAc] (5 wt%), 180 °C, 4 h, atmospheric pressure), the yield of BHET reached up to 85.2%. Additionally, the reaction kinetics was studied and proved to be the shrinking-core model. The apparent activation energy is 131.31 kJ mol^-1, and the pre-exponential factor is 1.21 × 10¹³ min^-1. Finally, based on the experimental results and density functional theory (DFT) calculations, a possible mechanism was proposed. The promotion of the glycolysis reaction is attributed to the activation of EG by the formation of hydrogen bonds between EG and the IL.

Full text of DOI:10.1039/d0gc00327a

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Degradation of Poly(ethylene terephthalate) Catalyzed by Metal-free
Choline-Based Ionic Liquids
Yachan Liu,a,b Xiaoqian Yao, Haoyu Yao, Qing Zhou, Jiayu Xin,
a,c
a,c
a,c
a,b,c
Xingmei Lu,a,b,c,
*
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and Suojiang Zhanga,b,*
aBeijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process
and Engineering, State Key Laboratory of Multiphase Complex Systems, Institute of Process
Engineering, Chinese Academy of Sciences, Beijing 100190, PR China
bSino Danish College, University of Chinese Academy of Sciences, Beijing 100049, PR China
cCollege of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,
Beijing 100049, PR China
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E-mail: xmlu@ipe.ac.cn, sjzhang@ipe.ac.cn
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Abstract
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Glycolysis of poly(ethylene terephthalate) (PET) is a prospective way for degradation of PET to its
monomer bis(hydroxyethyl) terephthalate (BHET) which can be polymerized again to form new
qualified PET materials, and hence provides possibilities for a permanent loop recycling. However,
most of the reported glycolysis catalysts are metal-based, leading to high cost and negative
environmental impact. In this study, we developed a series of choline-based ionic liquids (ILs)
without metals and applied them in the glycolysis of PET as catalysts. Choline acetate ([Ch][OAc]),
which is cheaper, more biologically compatible and environmentally friendly in comparison with
conventional imidazolium metal-based ILs, can achieve a comparable or even better performance
than them. Under the optimum conditions (PET (5.0 g), ethylene glycol (EG) (20 g), [Ch][OAc] (5
wt %), 180 °C, 4 h, atmospheric pressure), the yield of BHET reached up to 85.2 %. Additionally,
the kinetics of reaction was studied and proven to be the shrinking-core model. The apparent
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activation energy is 131.31 kJ/mol, and the pre-exponential factor is 1.21x10 min . Finally, based
on the experimental results and density functional theory (DFT) calculations, the possible
mechanism was proposed. The promotion of the glycolysis reaction is attributed to the activation of
EG by the formation of hydrogen bond between EG and the IL.
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Key words: Poly(ethylene terephthalate); glycolysis; ionic liquids; catalysts; metal-free
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1. Introduction
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Poly(ethylene terephthalate) (PET) is a thermoplastic polymer widely used in synthetic fibres,
bottles, food packaging and plastic film due to its chemical stability, high structural strength,
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transparency, and safety. However, these excellent properties also make PET difficult to be
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naturally degraded by microbes , resulting in great environmental pressure to treat gradually
accumulated huge amount of PET waste. Hence, it is a crucial challenge to develop environmentally
benign processes to degrade PET economically and effectively.
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Glycolysis is a kind of chemical recycling methods that converts waste PET into monomer
bis(hydroxyethyl) terephthalate (BHET) through various chemical reactions. Compared with other
recycling methods, the most prominent advantage of glycolysis is that the monomer can be re-
polymerized into new PET materials under relatively mild reaction conditions and low volatile
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solvents. During the past decade, a great number of catalysts for this process have been developed.
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The most commonly used catalysts include metal acetates , oxides , hydroxides , carbonates ,
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sulfates and halides , and those with excellent catalytic performance are usually composed of
transition metals such as Zn and Mn. These costly metal-based catalysts are inevitably not so
environmentally friendly, as they bring extra metal elements migrating and storing in soil, water and
atmosphere and ultimately harming environment and human beings. Furthermore, the disposal of
metals also faces similar problems, associating with great expenses and the emissions of acids/bases
and harmful gas.10 Therefore, the glycolysis catalyst composed of low cost and environmentally
benign materials is highly desirable.
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Ionic liquids (ILs), considered as green solvents, offer a promising approach for reducing costs
and protecting environment due to their unique features like thermal stability, non-volatility, and
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low flammability. They have been widely used in many fields including extraction , absorption
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and catalysis . ILs have also been applied to the degradation of PET. A growing number of
imidazolium-based ILs have been recently reported to catalyze the glycolysis of PET, since Wang
et al.15 first used 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) as solvents dissolving and
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degrading PET pellets. For example, 1-butyl-3-methylimidazolium hydroxide ([Bmim][OH]) , 1-
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allyl-3-methylimidazolium halometallate ([Amim][ZnCl ]) , and 1-butyl-3-methylimidazolium
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halometallate ([Bmim][CoCl ]) . Among these imidazolium-based ILs, ILs containing metals
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have relatively better catalytic activity with shorter reaction time (even several minutes) and higher
BHET yield (usually more than 80 %). Here, metal salts are discovered to interact with hydroxyl
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hydrogen in EG17-20 or oxygen of ester group in PET21, 22 to activate reactants. Nevertheless, metal-
free ILs do not have such good catalytic activity, showing a performance with longer reaction time
(usually more than 3 h) and lower BHET yield (usually less than 80 %). It is reasonable to speculate
that nonmetals cannot form such strong interaction with reactants as metals. However, most of these
imidazolium-based ILs still work with costly and valuable metals. Furthermore, imidazolium-based
ILs are toxic to microbes like some enzymes23, 24
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To conclude, the efficient PET degradation is expected to be achieved over the green media metal-
free ILs. Previous work16-20, 25 has proven that the key is the synergic interaction between anion and
cation of the IL. Specifically, the cation of IL interacts with oxygen of carbonyl in PET (C=O),
while the anion interacts with hydrogen of hydroxyl in EG at the same time. In this regard, anion of
IL is expected to form hydrogen bonds as strong as possible with hydroxyl hydrogen in EG, thus
enhancing the electronegativity of hydroxyl oxygen in EG as well as the nucleophilic attack ability
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to ester group of PET. Acidic groups including acetate , halide ion
and bicarbonate and metal
salts17-20 have been reported, and anions of carboxylate stand out. Meanwhile, cation of IL interacts
with carbonyl oxygen in PET, and the most common used was imidazolium16-20, 25, 26. Thus, to
achieve the green efficient process, it is key to design appropriate cation that is metal-free and
biologically compatible to coordinate with the anion of IL and promote the efficiency of PET
catalytic degradation. To overcome the concerns about toxicity, economy and environment of
conventional imidazolium cation, people have been looking for alternative green cations, which has
sparked interest in (2-hydroxyethyl) trimethylammonium (known as “choline”) since 2007, a kind
of cheaper, more biocompatible and biodegradable ILs. In specific, choline acetate ([Ch][OAc]) has
been thought to be biocompatible alternative to 1-ethyl-3-methylimidazolium acetate
([Emim][OAc]) for biomass pretreatment due to its natural origin and lower toxicity.28 Plus, the
potential deep eutectic solvent (DES) interaction between choline structure and EG is expected to
promote PET depolymerization rapidly.29 Therefore, choline cation is expected to replace
imidazolium cations to achieve efficient PET degradation combing with anions of metal-free acidic
groups here.
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Based on the above consideration, a series of metal-free choline-based ILs were prepared and
catalyzed the glycolysis of PET effectively. The structure and purity of the main product was
characterized by nuclear magnetic resonance (NMR), mass spectra with electrospray ionization
(ESI-MS), fourier transform infrared analysis (FT-IR), thermogravimetric analysis (TGA),
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differential scanning calorimetry (DSC), X-ray diffraction (XRD) and elemental analysis. The
reaction conditions in terms of temperature, time and catalyst dosage were optimized. The reaction
kinetics was investigated, and the apparent activation energy and the pre-exponential factor were
calculated. Finally, the degradation mechanism was proposed according to experimental results and
density functional theory (DFT) calculations.
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2. Experimental
2.1 Materials
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PET pellets were supplied by Jindong Commercial Co.Ltd., Jiangsu Province, China. Ethylene
glycol, choline hydroxide ([Ch][OH]) aqueous solution (46 %), choline chloride ([Ch]Cl), formic
acid, acetic acid, propionic acid, butyric acid, methane-sulfonic acid, sulfuric acid and diethyl ether
were purchased by Sinopharm Chemical Reagent Beijing Co.,Ltd. The PET particles of 40-60 mesh
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were obtained by grinding the PET pellets. ILs were synthesized according to reported procedures .
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2.2 Synthesis of ILs
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Six choline-based ILs (choline formate ([Ch][For]), choline acetate ([Ch][OAc]), choline propionate
([Ch][Pro]), choline butyrate ([Ch][But]), choline mesylate ([Ch][Mesy]), choline bisulfate
([Ch][HSO ]) were prepared by acid-base neutralization method. [Ch][OH] aqueous solution (0.1
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mol [Ch][OH]) was loaded in a 250 ml round-bottom flask under the conditions of ice bath and
stirring. A slight excess of acid (0.11 mol) was slowly added to the solution drop by drop, and the
reaction then continued for 7 hours. Water was removed by rotary evaporation at 60 °C. Unreacted
acid were removed by the addition of diethyl ether. Solvent diethyl ether remaining in the solution
was removed by rotary evaporation at room temperature. Finally, the resulting IL was dried in a
vacuum oven at 70 °C for 24 h.
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2.3 General procedure of PET glycolysis
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EG and IL in a 80:1 mass ratio were mixed firstly with stirring. Then 5.0 g PET, a certain amount of
mixture of EG and IL were added to a 50 mL three-necked round-bottom flask with a thermometer,
a magnetic stirrer and a reflux condenser. The glycolysis reactions were carried out at reaction
temperatures ranging from 145 ℃ to 195 ℃ for reaction times from 0.5 h to 5 h. The flask was
immersed in an oil bath at a specific temperature for the required time.
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After each reaction finished, the hot reaction solution was quickly filtered. The filter cake was un-
depolymerized PET, and the filtrate was a mixture of remaining EG, catalyst, monomer and
oligomers. Methanol was used to wash the filter cake to prevent precipitation of products. Un-
depolymerized PET was then collected, dried and weighed. The PET conversion is defined as below.
WPET ― W1
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PET conversion =
× 100%#(1)# #
WPET
Where WPET is the initial weight of PET, W is the weight of un-depolymerized PET.
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About 400 mL distilled water was added to the filtrate, and then the solution was heated at 70 ℃
with a magnetic stirrer. After the solution became clear with a few floccule, the hot solution was
quickly filtered. The filter cake was oligomers, and the filtrate was a mixture of remaining EG,
catalyst, monomer and distilled water. The filtrate was diluted with distilled water to 1000 mL in a
volumetric flask. The BHET yield is defined as below.
WBHET
MBHET
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BHET yield =
× 100%#(2)
WPET
MPET
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MBHET and MPET are the molecular mass of BHET and the PET repeat unit, respectively. WBHET is
the weight of BHET, which is calculated by HPLC according to the peak area of BHET.
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To obtain the product, the remaining solution was concentrated to about 70 mL, and then stored in a
refrigerator at 0 ℃ for 12 h. Finally, White needle-like crystals were precipitated, then separated
and dried. The product was then characterized by NMR, ESI-MS, FT-IR, TGA, DSC, XRD and
elemental analysis.
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2.4 Characterization
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NMR spectra of ILs and product were recorded using AVANCE III HD 600 (Bruker, Switzerland)
in d -DMSO solution. MS were obtained on a mircroTOF instrument (Bruker, Germany) with ESI.
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FTIR was carried out on a Nicolt 380 spectrometer (Thermo Fisher Scientific, U.S.A). Element
analysis was employed using a Vario EL cube (Elementar, Germany). DSC measurement of the
product was performed using a DSC1 (Mettler-Toledo, Switzerland) by heating samples from room
temperature to 180 ℃ at a rate of 10 ℃ /min under nitrogen atmosphere. TGA of both product and
PET particle were obtained using DTG-60H (SHIMADZU, Japan) by heating samples from room
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temperature to 600 ℃ at a rate of 10 ℃ /min under nitrogen atmosphere. XRD test was performed
by a Smartlab(9) (Rigaku, Japan) in the range of 2θ = 4~80°. High performance liquid
chromatography (HPLC) analysis of the product was conducted by a ACQUITY HPLC with
refractive index detector and BET C18 column (Waters, U.S.A). The mobile phase was methanol
aqueous solution (50v:50v) at a flow rate of 0.2 mL /min with injection volume of 10 μL and
running time of 30 min.
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2.5 DFT calculations
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All calculations were carried out using the Gaussian 03 program. The B3LYP/6-311+G(d,p)
method was used for structure optimizations, and subsequent frequency calculations at the same
level verified the optimized structures to be ground states without imaginary frequencies (NImag =
0). There are possible positions in which ion-pairs locates around dimer, and different initial
configurations were optimized at B3LYP/6-31++G**.
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3. Results and discussion
3.1 Screening of Catalysts
Table 1. The catalytic activities with different catalysts for PET glycolysis.
Temperature
℃)
Time
CPET
(%)
1.6
YPET
(%)
–
Ref
Catalyst
–
(
(h)
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this worka
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[
[
[
[
[
[
[
Ch][For]
Ch][OAc]
Ch][Pro]
Ch][But]
Ch]Cl
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97.1
94.3
65.1
33.3
3.2
84.5
82.9
40.5
9.9
–
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Ch][Mesy]
Ch][HSO4]
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4.2
–
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–
ref 16b
[Bmim][OH]
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100
82.8
10.7
100
71.2
14.1
–
[
Bmim][HCO3]
Bmim]Br
2
[
2
ref 31c
[Bmim][OAc]
3
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Reaction conditions: [a]: PET (5.0 g), EG (20 g), catalysts (0.25 g), atmospheric pressure; [b]: PET
2 g), EG (20 g), catalysts (0.1 g), atmospheric pressure; [c]: PET (3 g), EG (20 g), catalyst (1 g),
(
atmospheric pressure
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The catalytic effect of choline-based ILs was investigated under the same reaction conditions, and
the results are shown in Table 1. It was found that the PET degradation rate was very low without
catalysts. The order of catalytic activity was [Ch][For] > [Ch][OAc] > [Ch][Pro] > [Ch][But] >
[Ch][Mesy] > [Ch]Cl > [Ch][HSO ]. Among the synthesized ILs, [Ch]Cl, [Ch][Mesy], [Ch][HSO ]
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had little catalytic activity. The conversion of PET was less than 5 % at 180 ℃ for 3 h using these
three ILs. On the contrary, ILs containing carboxylate, especially [Ch][For] and [Ch][OAc], showed
excellent catalytic activity under the same reaction conditions. The almost complete degradation of
PET was achieved at 180 ℃ for 3 h, and the yield of BHET was above 80 % using [Ch][For] and
[Ch][OAc]. In particular, in ILs containing carboxylate, the catalytic activity order is that [Ch][For] >
[Ch][OAc] > [Ch][Pro] > [Ch][But]. The higher catalytic activity was believed to arise from the
smaller size ion and the electron-deficient structure caused by alkyl group which could enhance the
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interaction between cation of IL and oxygen of ester in PET. Similarly, Wang et al. confirmed that
the same tendency occurred when catalysts of PET glycolysis were [Amim][FeCl ] and
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[Bmim][FeCl ], [Amim]Cl and [Bmim]Cl. Besides, Ju et al. found that the increase of anion
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volume led by alkyl chain promoted charge distribution and reduced the binding energies
interaction between PET and ion pairs using DFT calculations. Therefore, it is speculated that the
smaller size of anion made the attack of IL to PET easier and enhanced the interaction between EG
and anion of IL here.
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To screen an ideal catalyst, the cost of ILs should be considered. The raw materials of [Ch][For]
and [Ch][OAc] are all easily available chemicals which can be manufactured on large scale. The
prices of industry grade choline hydroxide (50 %), formic acid (85 %) and acetic acid (99 %) were
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around $500~1500 per ton, $500~800 per ton, $500~650 per ton respectively. The costs of ILs
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were estimated by equation 3 reported .
M ∗ P + M ∗ P
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IL price =
#(3)
M + M1
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Where M and M are the molecular weights of the raw materials, P and P are their prices.
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The prices of [Ch][For] and [Ch][OAc] are approximately $1130 per ton and $1074 per ton,
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respectively. The price of [Ch][For] is slightly lower than [Ch][OAc]. These ILs are obviously
cheaper than most common used imidazolium-based ILs.
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Although the catalytic activity of [Ch][OAc] was slightly lower than [Ch][For], the raw material of
[Ch][OAc], acetic acid, is more ideal for its relatively less toxicity and slightly cheaper cost.
Furthermore, compared with other reported metal-free ILs16, 31 like [Bmim][OAc], [Bmim][OH] and
[Bmim][HCO ], [Ch][OAc] still performed higher catalytic activity. In summary, the catalysts
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studied in this research show relatively high catalytic activity and simplify the subsequent
refinement of the main product like elimination of metal residue. [Ch][OAc] was chosen as the best
catalyst in the following research.
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3.2 Analysis of the main product and PET raw material
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The structure of main product obtained by glycolysis was analysed by ESI-MS, NMR, FT-IR, and
element analysis while the purity was determined by HPLC. Furthermore, the physical properties
were checked by DSC, TGA and XRD.
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Figure 1. (a) ESI-MS patterns, (b) H NMR spectrum and (c) C NMR spectrum of the main
product.
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In Figure 1. (a), the ESI-MS patterns show that the peak with the highest intensity appears at m/z =
+
277.0885, which is consistent with the molecular weight of BHET after being ionized by Na . Thus,
it can be preliminarily inferred that the main product is BHET.
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The NMR spectra show the following signals. H NMR (d6-DMSO, 600MHz): δ=8.13 (s, 4H), 4.95
(t, 2H), 4.32 (t, 4H), 3.72 (q, 4H); 13C NMR (d6-DMSO, 500MHz): δ=165.13, 133.72, 129.48,
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67.00, 58.95. In the H NMR spectrum, the single peak at 8.13 ppm corresponds to four aromatic
protons of the benzene ring. The triplet at 4.95 ppm represents two protons of the hydroxyl groups.
The triplet at 4.32 ppm and quartet at 3.72 ppm indicate protons of methylene groups attached to the
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hydroxyl groups and carbonyl groups, respectively. The C NMR spectrum also shows consistency
with the structure of BHET. Besides, there are no other signals in the NMR spectra, which indicates
the high purity of BHET.
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The elemental analysis results are performed in Table S1 after preliminary determination of
structure of main product. The results are close to the theoretical results, demonstrating the high
purity of BHET. The purity of the main product was also checked by HPLC. As shown in Figure S3,
there is only one sharp single peak at 24.036 min, indicating the high purity of the main product.
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Figure 2. (a) FT-IR spectra, (b) XRD spectra, (c) TGA curves of PET and the main product and (d)
DSC curve of the main product.
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The FTIR spectra of PET and main product are depicted in Figure 2. (a). The similarity between
two curves illustrates the similarity in structures between PET raw material and the main product.
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The peaks of BHET curve are analysed as follows. The peaks at 3347 cm and 1130 cm ascribe to
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O-H bands. Absorbance at 2977 cm ascribes to the vibration of alkyl C-H bonds. The C=O
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stretching vibration and C-O vibration are at 1722 cm and 1274 cm , respectively. The absorption
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bands at 700–800 cm are assigned to the vibration of benzene ring. These peaks are all consistent
with reported BHET17, 18, 31. The XRD spectra of PET raw material show a typical diffraction peak
of a polyester crystal structure. These typical peaks are broad and appear at 2θ = 16.4°, 17.7° and
23.0°, respectively. The diffraction peaks of the monomer become very sharp with relatively high
intensity compared to those of PET, which indicates that the monomer obtained by this method has
high crystallinity. Figure 2. (c) compares the TGA curves of BHET monomer and PET material.
The PET raw material only shows a weight loss peak at 410 °C, which is caused by the thermal
decomposition of PET. The TGA curve of the product BHET shows two weight loss peaks, with the
first weight loss at about 240 °C caused by thermal decomposition of the product BHET and the
second weight loss at 409 °C caused by the re-polymerization of some BHET to PET. A sharp
endothermic peak appears in the DSC curve of the main product. The temperature corresponding to
this endothermic peak is the melting point of the BHET monomer, which is 109.81 °C. This value is
consistent with the melting point of the BHET standard17, 18, 31
.
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3.3 Optimization of Reaction Conditions
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Figure 3. (a) The effect of temperature on PET glycolysis reaction. Reaction conditions: PET (5.0 g),
EG (20.0 g), catalyst ([Ch][OAc], 0.25g), 3 h, atmospheric pressure; (b) The effect of reaction time
on PET glycolysis reaction. Reaction conditions: PET (5.0 g), EG (20.0 g), catalyst ([Ch][OAc],
0.25 g), 180 ℃, atmospheric pressure; (c) The effect of catalyst dosage on PET glycolysis reaction.
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Reaction conditions: PET (5.0 g), EG (20.0 g), 3 h, 180 ℃, atmospheric pressure.
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The effect of temperature on the conversion of PET and the yield of BHET were investigated, and
the results are shown in Figure 3. (a). It can be concluded that temperature plays a crucial role in the
PET glycolysis reaction, affecting the reaction rate of PET glycolysis greatly. When the temperature
was 145 ℃, the conversion of PET and the yield of BHET were only 19.0 % and 16.1 %,
respectively. With temperature raising from 145 ℃ to 180 ℃, the conversion of PET and the yield
of BHET increased greatly, from 19.0 % to 94.3 % and from 16.1 % to 82.9 %, respectively. When
the temperature was over 180 ℃, take 195℃ as an example, the conversion of PET and the yield of
BHET was 100.0 % and 85.8 %, the increase tendency was totally retarded. Thus, the temperature
of 180 ℃ was chosen to be applied in the other optimum reactions.
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The effect of reaction time on the conversion of PET and the yield of BHET were examined, and
the results are performed in Figure 3. (b). The conversion of PET and the yield of BHET firstly
increased over glycolysis time with a gradually decreasing rate. However, when the reaction time
was prolonged to 5 h, the conversion of PET remained as high as 100 % while the yield of BHET
slightly decreased. Therefore, the reaction time of 4 h was preferred, and the conversion of PET and
the yield of BHET reached maximum values, 98.2 % and 85.2 % respectively. This phenomenon
can be attributed to the equilibrium between BHET monomers and oligomers.
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The effect of catalyst dosage on the conversion of PET and the yield of BHET were studied, and the
results are shown in Figure 3. (c). The conversion of PET increased sharply when initially adding
0.125 g [Ch][OAc]. After that, the increase rate came to be gradually stable. It reached 94.3 % at
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0.25 g and finally 100 % at 1.0 g. As for the yield of BHET, it firstly increased and then decreased
slightly. It reached a peak of 82.8 % at 0.25 g. The catalyst dosage of 0.25 g was chosen to be
applied in the other reactions because the conversion of PET was high enough with the maximum
yield of BHET. The tendency is consistent with the effect of time on the reaction. Due to the
synergic interaction of anion and cation in the IL forming hydrogen bonds with reactants, more
catalyst means more possibility to interact with reactants. Thus, more catalyst can accelerate the
glycolysis reaction and shorten equilibrium time26, 31. The maximum value and the subsequent
decline period of BHET yield come earlier with the increase of catalyst dosage.
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3.4 Reaction Kinetics
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The kinetics of PET glycolysis is affected by many factors, including the molar ratio of EG and
PET, the phase of catalysts, the shape and size of PET, and the agitation rate. Based on these
reaction conditions, researchers have proposed many various kinetics models of PET glycolysis
reaction, like the shrinking-core model36, 37, random scission model38, 39 and other mathematic
models31, 40. In this study, the catalyst can dissolve in EG while the PET raw material cannot
dissolve in the EG at the same time. SEM spectra (Figure S4) indicates that the reaction occurs in
the surface of PET particles. PET particles of 40-60 mesh are so small that can be regarded as
spheres. Thus, the reaction can be considered as the solid-liquid two-phase reaction occurred in the
surface of PET. Based on the above hypotheses, the kinetics model for this reaction is assumed as
the shrinking-core model initially. The kinetic equation of this shrinking-core model is as follows3.
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′
퐾 푡 = 1 ― (1 ― 푋) #(4)
[
]
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As said above, temperature is a crucial factor in PET glycolysis. The effect of temperature is mainly
investigated, and the results are shown in Figure 4. (a). Take experiment results of 160 ℃, 170 ℃,
and 180 ℃ into the shrinking-core model, and the results show a good linear relationship with linear
correlation coefficients over 0.98. The Arrheniius curve obtained subsequently in Figure 4. (b)
performs good linear correlation with a correlation coefficient of 0.99 as well. The fitted Arrhenius
equation is y = -15.79346x+30.12538. This proves that experiment results are consistent with model
assumed before. The apparent activity energy is 131.31 KJ/mol and the pre-exponential factor is
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1.21x10 calculating from the slope of the Arrheniius curve.
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Figure 4. (a) Effect of the temperature on the rate of glycolysis of PET. Reaction condition: PET
(5.0 g), EG (20.0 g), catalyst ([Ch][OAc], 0.25 g), atmospheric pressure; (b) Arrhenius plot of the
rate constant of glycolysis of PET.
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The apparent activation energy for this reaction is still higher than those glycolysis reactions
catalyzed by catalysts containing metals (36.5-92.2 kJ/mol)41-45. Therefore, the seeking for more
effective metal-free ILs catalysts is still meaningful to reduce the activation energy as well as
facilitate high efficient glycolysis reaction.
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3.5 Reaction Mechanism
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Figure 5. H NMR spectra of [Ch][OAc] and EG mixture with different weight ratios. Signals for -
-
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OH (EG) in dashed; -CH COO in ▲.
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Based on the previous study, the catalytic activity is mainly attributed to the hydrogen bonds
forming between EG and IL catalyst. To verify this speculation, the interaction between EG and
1
[Ch][OAc] was researched by H NMR. The results are shown in Figure 5. In the spectrum of pure
EG, the chemical shift of the hydrogen of hydroxyl in EG (dashed) is 4.44 ppm, and this number
shifts to 4.71 ppm and 5.53 ppm respectively when the ratio of EG and [Ch][OAc] turns to 1 and
1/3. That is, with the increase of [Ch][OAc] dosage, the signals of hydrogen of hydroxyl in EG
move to downfield. Moreover, the chemical shifts of hydrogen of methyl group in acetate anion (▲)
move to upfield. Therefore, it is reasonable to speculate that the change in chemical shift is due to
strong hydrogen bonds forming between oxygen of acetate anion and hydrogen of hydroxyl in EG.
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(a) 449.21 kJ/mol
(c) 450.72 kJ/mol
(e) 423.75 kJ/mol
(b) 453.28 kJ/mol
(d) 448.80 kJ/mol
(f) 419.93 kJ/mol
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(g) 402.71 kJ/mol
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Figure 6. Optimized structures of dimer and ion-pairs at the level of B3LYP/6-31++G**. Dark grey:
C; red: O; white: H; blue: N; green: Cl; yellow: S. (a) [Ch][For]; (b) [Ch][OAc]; (c) [Ch][Pro]; (d)
[Ch][But]; (e) [Ch]Cl; (f) [Ch][Mesy]; (g) [Ch][HSO4].
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Apart from the interaction between EG and IL catalyst, the interaction between another reactant
PET and IL catalyst was further researched by DFT calculations. Due to the computational limits,
dimer with similar structure unit was used to represent long chain PET. The interaction of dimer
with ion-pairs led to the form of several kinds of structures. The hydrogen bonds were formed
between oxygen and hydrogen in most case, while hydrogen bond between chloride and hydrogen
was also found. The hydrogen bonds were formed between oxygen of ester in PET and hydrogen of
hydroxyl in choline cation, between hydrogen of benzene ring in PET and oxygen of anion in ILs,
between hydrogen of methylene in PET and oxygen of anion in ILs, and between hydrogen of
benzene or methylene in PET and chloride anion in ILs. The interactions brought changes in the
bond lengths, bond angles and charge density. The optimized conformers were finally obtained to
compare their interactions and shown in Figure 6. The ion pairs stayed with similar positions
around dimer to compare the strength of their interactions. The order of binding energy between
dimer and ILs is [Ch][OAc]> [Ch][Pro]> [Ch][For]> [Ch][But]> [Ch]Cl> [Ch][Mesy]> [Ch][HSO4].
In general, the dimer-[Ch][carboxylate] system has relatively higher binding energies compared
with other three choline-based ILs, which is consistent with the catalytic activity. It is a little
different from the order of catalytic activity, and the distinction may be ascribed to the steric effect
caused by ion size and the difference between dimer and PET. Furthermore, compared with dimer-
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[Amim][OAc] structure of 435.01 kJ/mol at the same level of B3LYP/6-31++G** , dimer-
[Ch][OAc] structure has a higher binding energy of 453.28 kJ/mol. It demonstrates that choline
cation is a greener alternative to imidazolium cations for PET catalytic degradation.
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Figure 7. Proposed mechanism of the PET glycolysis process catalyzed by [Ch][OAc].
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Based on both experiment results and DFT calculations, the possible mechanism for PET glycolysis
catalyzed by [Ch][OAc] is proposed in Figure 7. The synergic interaction between the cation and
anion of IL is the key. The hydrogen bond is formed between acetate anion and hydrogen of
hydroxyl in EG, enhancing the electronegativity of hydroxyl oxygen in EG as well as the
nucleophilic attack ability. Meanwhile, the hydrogen of hydroxyl in choline cation protonates the
carbon of carbonyl in PET and makes it more electrophilic. Then, the hydroxyl bond is broken as
well as the ester bond of PET. Subsequently, new C-O bond forms between the carbon of carbonyl
in PET and hydroxyl oxygen in EG and the hydrogen detached from the hydroxyl in EG is bonded
to the oxygen in another part of PET. The catalyst continues to repeat these above steps, and PET
gradually degrades into oligomers, eventually forming BHET monomers.
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4. Conclusion
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In this study, we developed a series of choline-based ILs catalysts without metals, which can
achieve a comparable or even better performance than reported metal-based catalysts. [Ch][For] and
[Ch][OAc] exhibit higher catalytic activity under the same reactions. The operation parameters
were investigated with results indicating temperature plays important role in the glycolysis. Under
the optimum conditions (5.0 g PET of 40-60 mesh, 20.0 g EG, 5wt % catalyst of [Ch][OAc], 180 ℃,
4 h, atmospheric pressure), the yield of BHET reached 85.2 %. The reaction kinetics was
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investigated, and the results indicated the model is shrinking-core model with the apparent
1
activation energy of 131.31 kJ/mol. H NMR spectra and DFT calculations confirmed the possible
reaction mechanism. The synergic effect of the cation and anion in IL makes the attack of oxygen in
EG on the carbon of carbonyl in PET much easier. The results of this study show that this structure
can realize PET degradation without costly and precious metal-based catalysts, and therefore
guiding the design of more efficient and economic IL catalysts applied in practical PET recycling
industry.
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Corresponding Authors
*E-mail xmlu@ipe.ac.cn (X.L.)
*E-mail sjzhang@ipe.ac.cn (S.Z.)
Acknowledgements
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This research was supported financially by the National Natural Scientific Fund of China
(No.21878292, No.21776289, No.21921005), K. C. Wong Education Foundation (No.GJTD-2018-
04) and the Strategic Priority Research Program of Chinese Academy of Science (No.
XDA21060300).
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