Ultrafast homogeneous glycolysis of waste polyethylene terephthalate

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Kinetics, Catalysis, and Reaction Engineering

Ultrafast homogeneous glycolysis of waste polyethylene terephthalate via a dissolution-degradation strategy Bo Liu, Xingmei Lu, Zhaoyang Ju, Peng Sun, Jiayu Xin, Xiaoqian Yao, Qing Zhou, and Suojiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b03854 • Publication Date (Web): 05 Nov 2018 Downloaded from http://pubs.acs.org on November 10, 2018

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Industrial & Engineering Chemistry Research

Ultrafast homogeneous glycolysis of waste polyethylene terephthalate via a dissolution-degradation strategy Bo Liu,a,b Xingmei Lu,*,a,b Zhaoyang Ju,a,c Peng Sun,a Jiayu Xin,a Xiaoqian Yao,a Qing Zhoua and Suojiang Zhang*,a,b a

Beijing Key Laboratory of Ionic Liquids Clean Process, CAS Key Laboratory of Green Process and

Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, P. R. China. b School

of Chemistry and Chemical Engineering, University of Chinese Academy of Sciences,

Beijing 100049, P. R. China. c College

of Engineering, China Agricultural University, Beijing 100083, P. R. China.

*[email protected] Tel: +86-010-82544800; Fax: +86-010-82544800

Abstract Recycling of discarded polyethylene terephthalate (PET) is an important issue for both environmental protection and resource conservation purposes. In this work, a dissolution-degradation strategy has been developed for recycling PET by adding solvents such as aniline, nitrobenzene, 1-methyl-2-pyrrolidinone (NMP) or dimethyl sulfoxide (DMSO) into the traditional PET glycolysis system. The results show that the 1

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conversion of PET reaches 100% and the yield of monomer bis(hydroxyalkyl) terephthalate (BHET) reaches 82% during 1 minute with zinc acetate as catalyst in DMSO at 463 K. Importantly, this strategy can be applied to a variety of catalysts. The simulation and in situ IR results indicate that the π-π interaction between PET and aromatic solvents plays a key role in PET dissolution, which leads to fast degradation. This promising dissolution-degradation strategy can improve the glycolysis efficiency of PET dramatically and may be applied to the degradation process of other polyesters. Keywords: Polyethylene terephthalate; glycolysis; dissolution; homogeneous reaction; π-π interaction

1. Introduction Polyethylene terephthalate (PET) is one of the thermoplastic polyesters, which is widely used in packaging, film and clothes field due to its excellent chemical immutability, safety, creep resistance and translucency. The annual global production of PET has exceeded 50 million tons1. However, the huge production and mostly disposable usage of PET is also followed by serious environment problems and waste of resources2. Therefore, recycling of PET waste has become a significant and urgent issue for both environmental protection and resource conservation purposes. The methods for recycling PET can be classified into two categories, physical method and chemical method. Physical method is the most common way for recycling PET in industry due to its low cost and easy to operation, but this method is a downcycling 2


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process because the products obtained from discarded PET show worse quality than raw PET3,4. In comparison to the physical method, chemical methods have a much brighter prospect for extensive application owing to the monomer product obtained at these processes, which could be used for reproducing PET or other polyester. There are numerous types of chemical methods for recycling PET, including glycolysis5,6, methanolysis7,8, hydrolysis9,10, ammonolysis11,12 and aminoglycolysis13,14. Due to the advantage of mild reaction conditions and low-volatility solvents, glycolysis method has become a more attractive approach. However, the slow reaction rate of chemical methods is still a problem with the need to be solved. In the view of an industrial implementation, high reaction efficiency will reduce the cost and consumption of energy. During the last decade, researchers have been devoted to developing various catalytic systems15,16, employing supercritical technology17,18 and microwave-assisted methods19 to improve reaction rate, but the higher reaction temperature and pressure are usually needed. For instance, the reaction time can be sharply shortened to 30 minutes with supercritical technology, but it was achieved under harsh conditions (15.3 MPa, 723 K)17. Meanwhile, catalysts have also been extensively investigated, which include metal salts20,21, ionic liquids22-26, polyoxometalate27, metal oxides28-30 and urea31. For example, when [bmim]2[ZnCl4] (bmim=1-butyl-3-methyl-imidazolium) was used as catalyst for PET glycolysis reaction, the reaction time was shortened to 1.5 h under mild reaction condition (atmosphere, 448 K)32. However, the chemical recycling of PET with ultrafast reaction rate and high selectivity under mild condition is still a significant and 3



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challenging work. It was well known that using homogenous catalysts can greatly improve the reaction rate of PET degradation32-34. And our previous research has demonstrated that the glycolysis reaction takes place on the surface of the PET particle, and the reaction rate is restricted by the surface area of PET25. Therefore, we anticipate designing a homogenous glycolysis system to improve the reaction rate significantly by concentrating on the solvents used in PET dissolution. As one of the engineering plastics, the solubility of PET in different solvents has already

been

investigated,

the

chloroform/hexafluoroisopropanol36,

commonly

used

solvents

1-methyl-2-pyrrolidinone

include

m-cresol35,

(NMP)37

and

nitrobenzene38. However, these solvents are reported to be used as mobile phase for gel permeation chromatography or to recycle PET waste by dissolution-reprecipitation method39. The utilization of these solvents in PET glycolysis has been seldom reported13, which encouraged us to explore the effect of solvents in PET glycolysis. For glycolysis system, an ideal solvent must have a good solubility to PET and should not react with either PET or ethylene glycol (EG). Hence, we developed a dissolution-degradation method for recycling PET by adding solvents in PET glycolysis reaction in this work. Solvents, including aniline, nitrobenzene, NMP and dimethyl sulfoxide (DMSO) are all suitable for PET glycolysis process. Under the conditions of EG (10 g), DMSO (20 g), PET (5 g), catalyst (zinc acetate, 0.25 g), and in atmosphere pressure at 463 K for 1 minute, the conversion of PET and yield of BHET were 100% and 82%, respectively. The structure and purity of the 4


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product were characterized by proton nuclear magnetic resonance (1H NMR), electrospray ionization mass spectroscopy (ESI-MS), high performance liquid chromatography (HPLC), Fourier Transform infrared spectroscopy (FT-IR), elemental analysis and inductive coupled plasma emission spectrometer (ICP). The universality of this method was tested by using different catalysts. Moreover, the kinetics difference of PET glycolysis between homogeneous system and heterogeneous system was investigated. Finally, the dissolution mechanism of PET was proposed via density functional theory (DFT) calculation and the experiment of in situ IR.

2. Materials and methods 2.1 Materials PET pellets were purchased from Jingdong Commercial Co., Jiangsu Province, China. The raw PET pellets were smashed to 40-60 mesh. Ethylene glycol, dimethyl sulfoxide, nitrobenzene, 1-methyl-2-pyrrolidinone, aniline, zinc acetate, zinc acetate dihydrate (Zn(OAc)2·2H2O), zinc nitrate (Zn(NO3)2·6H2O), zinc sulphate (ZnSO4·7H2O), cobalt acetate

(Co(OAc)2·4H2O),

nickel

acetate

(Ni(OAc)2·4H2O),

copper

acetate

(Cu(OAc)2·H2O), manganese acetate (Mn(OAc)2·4H2O) were obtained from Sinopharm Chemical Reagent Beijing Co.. The materials were used without any further purification. Urea/Zn(OAc)2 mixture, [Bmim]Zn(OAc)3 and K6SiW11ZnO39(H2O) were synthesized according to literatures27,32,40. 2.2 General procedure for glycolysis of PET in EG 5 g PET powder and 30 g EG were added to a 50 mL three-necked round-bottom flask, 5



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which was equipped with a reflux condenser, a thermometer and a magnetic stirrer. After the mixture was heated to target temperature, catalyst was added into the flask. The glycolysis reactions were carried out under atmospheric pressure at reaction temperatures ranging from 433 K to 463 K for reaction times of 5 min-300 min. When the reaction was finished, the reaction mixture was poured into 800 mL distilled water to separate the residual PET powder and oligomer from the liquid. The residual PET powder and oligomer were filtrated out, dried at 353 K to constant weight and weighed, respectively. The conversion of PET and the yield of BHET are calculated by the following equations: Conversion of PET initial weight of PET - weight of undepolymerized PET = × 100% (1) initial weight of PET weight of BHET Molar mass of BHET Yield of BHET = initial weight of PET Molar mass of PET

(2)

Meanwhile, the collected filtrate was poured into a 1000 mL volumetric flask to constant volume. The yield of BHET was calculated through quantitative analysis of HPLC. The residual solvent was concentrated to 60 mL. The concentrated filtrate was stored in a refrigerator at 277 K overnight. White needle-like crystals of BHET were formed and then filtered and dried. The structure and purity of the BHET were characterized by 1H NMR, ESI-MS, HPLC, FT-IR and elemental analysis. 2.3 General procedure for glycolysis of PET in solvents 5 g PET powder, 20 g solvent and 10 g EG were added to a 50 mL three-necked round-bottom flask, which was equipped with a reflux condenser, a thermometer and a magnetic stirrer. After the mixture was heated to target temperature, catalyst was added 6


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into flask. The glycolysis reactions were carried out under atmospheric pressure at reaction temperatures ranging from 428 K to 463 K for reaction times of 1 min-20 min. When the reaction finished, the reaction mixture was poured into 800 mL distilled water to separate the solid from the liquid. The solid was filtrated out, dried at 353 K to constant weight and weighed. The collected filtrate was poured into a 1000 mL volumetric flask to constant volume. The yield of BHET was calculated through quantitative analysis of HPLC. The residual solid was analyzed by GPC to calculate the conversion of PET. Conversion of PET = (N0 - N1)/N0 × 100%

(3)

Where N0 represents the initial number of ester bond in PET, and N1 represents the residual number of ester bond in products. The residual solvent was concentrated to 60 mL. The concentrated filtrate was stored in a refrigerator at 277 K overnight. White needle-like crystals of BHET were formed and then filtered and dried. The structure and purity of the BHET were characterized by 1H NMR, ESI-MS, HPLC, FT-IR, elemental analysis and ICP. 2.4 Characterization To characterize the structure and purity of the main product, several kinds of analysis methods were used. 1H NMR analysis was carried out using Bruker ECA-600 instrument while the solvent was d6-DMSO. FT-IR spectra were carried out using a Nicolet 380 spectrometer (Thermo Fisher Scientific, U.S.A.). The HPLC analysis were measured using an ACQUITY HPLC with TUV detector and BET C18 column (Waters, U.S.A.) 7



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under the conditions of an oven temperature of 303 K, detector temperature of 323 K, solvent methanol/water (70:30 v/v) and flow rate 0.1 mL/min. MS (Thermo Scientific, U.S.A.) instrument equipped with electron ionization (EI) was used to analysis the mass of product. GPC analysis were performed on a PL-GPC 50 system (Agilent, U.S.A.) under the conditions of oven temperature of 303 K, solvent trichloromethane and flow rate 1.0 mL/min. Three columns (PLgel 5um Guard, length 50 mm, 7.5 mm i.d. PLgel 5 um MIXED-C, length 300 mm, 7.5 mm i.d. PLgel 5 um MIXED-D, length 300 mm, 7.5 mm i.d.) were used. The detector used on the GPC is a refractive index (RI) detector. In situ IR spectra were carried out using ReactIR (Mettler-Toledo, U.S.A.) operating by the ReactIR 15 with a LN2 MCT detector, DiComp probe connected by K6 Conduit and Sampling 3000 to 650 at 8 wavenumber resolution. Element analysis were carried out using a Vario EL cube elemental analyzer (Elementar, Germany). The models used were CHNS model (Comb. Rube (1423 K), Reduct Rube (1123 K)) and O model (Comb. Rube (1423 K), Reduct Rube (273 K)). The content of metal in products was measured using an inductively coupled plasma atomic emission spectrometer ICPE-9000 (Shimadzu, Japan). Samples were pretreated as follows before element analysis measurement. Samples (0.02 g) were added to the 10 M HNO3 aqueous solution and heated at 358 K until all samples were dissolved. The solution was diluted by deionized water to adjust the pH to meet the demand of ICPE-9000. Four calibration standards of Zn were prepared including concentration at 0 mg/L, 0.1 mg/L, 1 mg/L and 10 mg/L. The amount of Zn in the samples was calculated according to the following equation. 8


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Zn% = (CZn × V × Dilution)/Wsample × 100%

(4)

Where CZn is the concentration of Zinc ion, V is the volume of sample, Wsample is the weight of sample.

3. Results and discussion 3.1 The effect of solvents on PET glycolysis in homogenous system Four solvents, including aniline, nitrobenzene, NMP and DMSO, were used in homogenous PET glycolysis process. The products obtained in these four solvents were confirmed to be BHET characterized by HPLC, NMR, ESI-MS, FT-IR, elemental analysis and ICP, respectively (Figure S1-S6, Table S1). In order to investigate the difference between heterogeneous and homogenous PET degradation process, PET glycolysis reactions were carried out under identical reaction conditions. In heterogeneous system, ethylene glycol was used as solvent, PET particles were dispersed in ethylene glycol in a solid form. The yield of BHET reached only 21.88% at 453 K for 5 minutes (Figure 1). However, PET particles were dissolved in reaction solutions rapidly in homogenous system. When the temperature increased to 453K, the PET particles were dissolved in 3 minutes. The BHET-yield reached about 50% at 1 minute and greater than 70% with the reaction time increasing to 5 minutes. Moreover, the results of the BHET-yield over time in different solvents showed high consistency which meant the glycolysis of PET in different solvents should follow the same kinetic model. It can be determined that the surface area of PET is a key factor limiting the reaction rate. Although the reaction efficiency has no obvious difference in these four solvents systems 9



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for the PET glycolysis reaction, different boiling points of these solvents would determine their operating temperature range which should be considered in industrial applications.

Figure 1 The change of BHET-yield with reaction time in different solvents. Reaction condition: 453K, 5min, atmospheric pressure, catalyst (Zn(OAc)2 0.25 g), PET (5.0 g), EG (10.0 g), solvent (20.0 g)

3.2 The universality of dissolution-degradation strategy with different catalysts The dissolution-degradation strategy can improve the efficiency of glycolysis of PET effectively in homogeneous systems. To find out whether this strategy could be universally applicable to various catalysts, several types of catalysts, including metal salts, polyoxometalate, ionic liquid and deep eutectic solvents were tested. The comparison of the BHET-yield between homogeneous system and heterogeneous system under identical reaction conditions (Table 1) depicted that the reaction rates in the homogeneous systems all increased significantly, regardless of which type of catalysts were used. When Zn(OAc)2·2H2O was utilized as catalyst in homogenous system, the 10


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BHET-yield reached 82 % with a reaction time of 1 min. Other metal salts such as Cu(OAc)2 and Ni(OAc)2, which are not generally used in PET glycolysis process, also showed good catalytic activity. Moreover, the yields of BHET catalyzed by polyoxometalate, ionic liquid and deep eutectic solvents are also higher than that in heterogeneous system. All these results indicate that the addition of a solvent increases the reaction rate of PET glycolysis. Furthermore, this effect is widely applicable and not affected by the type of catalysts, which is a great advantage for applying this method in industry. It is reasonable to infer that this strategy can also be useful for the degradation of some other polyester, thus providing a feasible method for recycling the disposable waste polyesters. Table 1 Comparison of the yield of BHET catalyzed by different catalysts between homogenous system and heterogeneous system Catalyst

Time (min)

Yield of BHET (%)

Yield of BHET (%)

Homogenous

Heterogeneous

Zn(OAc)2·2H2O

5

83.881.12

42.981.76

Zn(OAc)2·2H2O

1

82.972.44

20.112.01

Zn(NO3)2·6H2O

5

78.642.60

25.692.37

Zn(NO3)2·6H2O

1

53.482.46

7.322.33

ZnSO4·7H2O

5

57.212.14

1.270.29

Co(OAc)2·4H2O

5

78.691.87

24.731.88

Ni(OAc)2·4H2O

5

21.651.82

1.220.20

Cu(OAc)2·H2O

5

14.681.01

1.160.32

Mn(OAc)2·4H2O

5

80.771.70

42.152.37

Urea/Zn(OAc)2

5

77.902.28

48.171.51

K6SiW11ZnO39(H2O)

5

65.942.86

38.772.62

[Bmim]Zn(OAc)3

5

72.442.29

38.402.53

Reaction conditions: homogenous system: 463 K, atmospheric pressure, catalyst (0.25 g), PET (5.0 g) EG (10 g) DMSO (20 g); heterogeneous system: 463 K, atmospheric pressure, catalyst (0.25 g), PET 11



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(5.0 g) EG (30.0 g )

3.3 Kinetics difference between homogeneous system and heterogeneous system For the better understanding about this dissolution-degradation strategy, the difference of PET glycolysis kinetic between homogeneous system and heterogeneous system was investigated. For heterogeneous system, the reaction takes place at the surface of PET particle. So the kinetic model may be closer to the shrinking-core model41. If the PET particle is regarded as a sphere, then the mass change of PET at time ‘t’ can be represented as follows: dmp = ρpdV = 4πρpR2dR

(5)

where mp is the mass of PET particle, ρp is the density of PET particle, R is the radius of PET particle at time ‘t’. According to the material balance, the conversion rate of sphere powder is equal to the flow of mass transfer surface, then ― dmp/dt = ― bdmA/dt = bkCAsSR

(6)

Where b is the ratio between PET and EG in stoichiometric number, mA is the mass of EG, k is the reaction rate constant, CAs is the concentration of EG in reaction surface, SR is the reaction area. Equation (7) can be derived from equations (5) and (6) ― ρpdR/dt = bkCAs

(7)

For the mass transfer process of sphere particle, when the flow rate around sphere particle is slow, and the diameter of sphere particle is small enough, the Re will be very tiny, Sherwood number ShD = 2.0. 12


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k = D/R

(8)

where D is the diffusion coefficient of EG in reaction surface. Equation (9) can be derived from equations (7) and (8). ―

∫

R

RdR =

bCAsD

t

ρp

0

R0

∫ dt

(9)

where R0 is the initial radius of PET particle. t = K[1 ― (R/R0)2]

(10)

K = (ρpR20)/(2bCAsD)

(11)

And the conversion of PET, X is calculated by 4 πρ R3 3 p X=1― 4 πρ R3 3 p 0

(12)

From (11) and (12), equation (13) can be derived as follow 2

[

K′t = 1 ― (1 ― X)

]

3

(13)

And K’ =1/K. For homogeneous system, the reaction takes place in solution. When EG is much excessive, the reaction can be treated as the first-order reaction then Kt = ln(1 ― X)

(14)

Due to the wide temperature range of PET dissolution in aniline, aniline was chosen as a representative of these four solvents to investigate the kinetic of homogeneous PET glycolysis reaction. Figure 2 and Figure 3 show the kinetic data of PET glycolysis in heterogeneous and homogenous system, respectively. All the data maintain excellent 13



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graphical linearity (R2 >0.98), which indicates that the results are consistent with each statistical model, respectively. The activation energy of the heterogeneous system is 124.6 kJ/mol, whereas the activation energy of the homogenous system is 75.2 kJ/mol. This difference between the activation energies is attributed to the mass transfer process on the surface of the PET, which limits the reaction rate of the heterogeneous system.

Figure 2 Kinetic of glycolysis of PET in heterogeneous system (a) effect of the temperature on the rate of glycolysis of PET, (b) Arrhenius plots of the rate constant of glycolysis of PET. (Reaction condition: catalyst (Zn(OAc)2, 0.05 g), 1 atm, PET (5.0 g), EG (30.0 g))

Figure 3 Kinetic of glycolysis of PET in homogenous system (a) effect of the temperature on the rate 14


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of glycolysis of PET, (b) Arrhenius plots of the rate constant of glycolysis of PET. (Reaction condition: catalyst (Zn(OAc)2, 0.05 g), 1 atm, PET (5.0 g), EG (10.0 g), aniline (20.0 g))

3.4 Dissolution mechanism of PET in different solvents As a type of transesterification, the glycolysis mechanism of PET has been proposed by different researchers15,42,43. For example, with the Zinc acetate as the catalyst in PET glycolysis process, Zn2+ acts as Lewis acid to activate carbonyl groups in PET, which makes the carbonyl group of PET more vulnerable to nucleophilic attack44. However, the mechanism for PET dissolution still remains elusive, although many empirical formulas and data exist on this topic. In order to explore the dissolution mechanism of PET in various solvents, in situ IR and DFT calculations were employed in this work. Limited by the calculation quantity, long-chain PET is not suitable for DFT calculation. For the purpose of simplifying calculations, BHET monomer was chosen as a representation of PET. Although PET has more complex properties and dissolution behavior as a polymer, BHET has functional groups similar to those of PET, such as benzene ring, ester group and hydroxyl group. Therefore, BHET should be a partial replacement for PET during the simulation. The results of the calculations are shown in Figure 4. The order of relative energy between the solvents and BHET was aniline > nitrobenzene > NMP > DMSO. It was a little bit different from the order of heat of dissolution and solubility (Table S3, Figure S7). In general, the solubility of PET is increased with the increasing of relative energy between solvents and PET. The distinction may be attributed to the structural differences between PET and BHET and the impact caused by the solvation effects in the 15



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reaction. For aniline and nitrobenzene, the calculation results indicate that the location of benzene ring in solvents were approximately parallel to the benzene ring in BHET (Figure 4(a), Figure 4(b)). It can be assumed that π-π interaction existed between these two benzene rings. Furthermore, the distance between the hydrogen of amino group in aniline and oxygen of carbonyl in BHET was greater than 3 Å, which was hard to form hydrogen bonding. Hence, π-π interaction might play a key role in PET dissolution for aromatic solvents. However, the interaction between DMSO and BHET was mainly attributed to hydrogen bonding formed between oxygen of carbonyl in BHET and sulfur in DMSO.

Figure 4 Optimized interaction structures among solvents and BHET by m062x/6-311+ (d, p) with relative energies. white: H; grey: C; blue: N; yellow: S.(a)aniline, (b)nitrobenzene, (c) NMP, (d) DMSO

To verify the authenticity of the simulation, in situ IR was employed in this work. As 16


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shown in Figure 5, the red shifts of the flexural vibration of benzene ring at 878 cm-1 and 1020 cm-1 can be observed with the increase of PET concentration in aniline and nitrobenzene, respectively. However, the red shifts of benzene ring vibration were not observed in NMP and DMSO. Therefore, the red shifts were caused by some interaction between PET and solvents. It can be speculated that π-π interaction was the main driving force in PET dissolution for solvents containing benzene rings. Furthermore, naphthalene and tetrahydronaphthalene were chosen to prove the above assumption because they have similar structure but different aromaticity. It was found that the solubility of PET in naphthalene is much larger than solubility in tetrahydronaphthalene, which is in accord with our guess. The dissolution mechanism of non-aromatic solvents may be related to the hydrogen bond between solvents and PET, which needs further research.

17



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Figure 5 In situ IR spectra in different solvents with the increase of PET (a) Aniline (b) Nitrobenzene (c) NMP (d) DMSO.

4. Conclusions To summarize, a dissolution-degradation strategy for PET glycolysis was developed, which is universally applicable to various solvents and catalysts. Compared with general PET glycolysis processes, this strategy reduced the reaction time to 1 minute and decreased the activation energy by 49.40 kJ/mol. The proposed mechanism for PET dissolution depicted that the π-π interaction between the aromatic solvents and PET plays an important role in this process, which provides an important guidance for searching more solvents for PET dissolution. However, the dissolution mechanism for non-aromatic solvents needs further investigation. This strategy has considerable potential to be applied to other polymer degradation processes.

Author Information Corresponding Author Dr. Xingmei Lu. Tel: +86-010-82544800; E-mail: [email protected] ORCID Xingmei Lu 0000-0003-4712-656X Jiayu Xin 0000-0002-0728-294X

Conflicts of interest 18


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There are no conflicts to declare.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. (1) 1H NMR spectra of the main product obtained in solvents (2) ESI-MS spectra of the main product, (3) HPLC results of the main product, (4) FT-IR spectrum of the main product, (5)

Photographs of products obtained from different solvents, (6) ICP results of products obtained from different solvents (7) ICP results of products produced in four solvents, (8) GC-MS spectrum of reaction solution using aniline as solvent, (9) Solubility and solubility heat of PET in different solvents, (10) Detail method of DFT calculation, (11) Solubility of PET in naphthalene and tetrahydronaphthalene.

Acknowledgements This work was supported financially by National Basic Research Program of China (2015CB251401), National Natural Scientific Fund of China (No. 21878292, No. 21476234, No.21776289).

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Ultrafast homogeneous glycolysis of waste polyethylene terephthalate

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