Lewis AcidâBase Synergistic Catalysis for Polyethylene Terephthalate

Subscriber access provided by UNIV OF NEW ENGLAND

Article

Lewis acid-base synergistic catalysis for PET degradation by 1,3-dimethylurea/Zn(OAc)2 deep eutectic solvent Bo Liu, Wenzhao Fu, Xingmei Lu, Qing Zhou, and Suojiang Zhang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05324 • Publication Date (Web): 31 Dec 2018 Downloaded from http://pubs.acs.org on January 1, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Lewis acid-base synergistic catalysis for PET degradation by 1,3-dimethylurea/Zn(OAc)2 deep eutectic solvent Bo Liu†,‡, Wenzhao Fu§, Xingmei Lu†,‡,*, Qing Zhou†, Suojiang Zhang†,‡

†

Beijing 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, P. R. China.

‡ School

of Chemistry and Chemical Engineering, University of Chinese Academy of

Sciences, Beijing 100049, P. R. China.

§

State Key Laboratory of Chemical Engineering, East China University of Science

and Technology, 130 Meilong Road, Shanghai 200237, P. R. China. *[email protected] Tel: +86-010-82544800; Fax: +86-010-8254480 Bo Liu e-mail: [email protected] Wenzhao Fu

e-mail: [email protected]

Qing Zhou e-mail: [email protected] Suojiang Zhang e-mail: [email protected] 1 ACS Paragon Plus Environment

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

ABSTRACT: Deep eutectic solvents (DESs) become more attractive in catalytic field due to their biodegradation, low toxicity and designability. This study focused on the active sites and influencing factors of 1,3-dimethylurea (1,3-DMU) based DESs in PET glycolysis process. It is found that the active site of urea derivatives is the amino group, and the basicity and steric hindrance of the amino group affect its catalytic activity. Additionally, the mechanism of PET glycolysis reaction catalyzed by DES was investigated. The outstanding catalytic activity of DES can be attributed to the synergistic effect of acid and base formed between metal salts and 1,3DMU. Under the optimization conditions, PET (5.0 g), ethylene glycol (20.0 g), catalyst (n(1,3DMU)/n(Zn(OAc)2) 4/1, 0.25 g) at 190 °C for 20 minutes, the PET-conversion is up to 100% and the yield of bis(hydroxyalkyl) terephthalate (BHET) is 82%. Furthermore, the kinetic research shows that the glycolysis of PET follows the shrink-core model and the apparent activity energy is 148.89 kJ/mol.

KEYWORDS: Polyethylene terephthalate; glycolysis; deep eutectic solvents; synergistic catalysis; mechanism

INTRODUCTION As a kind of thermoplastic polyester with excellent physical and mechanical properties, creep resistance, electrical insulation, transparency and safety, polyethylene terephthalate (PET) has fantastically wide applications in electronic devices, films, fibers, and food packaging1. However, PET is extremely difficult to degrade in the natural environment. With the large consumption of PET, discarded PET has caused serious ecological disasters2. Therefore, chemical recycling of PET has become an important topic. Glycolysis method has become a

ACS Paragon Plus Environment

2

Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


more important approach for PET recycling compared with other methods including methanolysis3, hydrolysis4, aminolysis5, ammonolysis6 and enzymolysis7 because of the mild reaction condition and low-volatility of diols. Numerous catalysts, including metal oxide8, zeolites9, polyoxometalate10 and ionic liquids11-13, were developed to applied in PET glycolysis process with some advantages, such as high PET conversion and BHET selectivity. However, there are also some defects, for example harsh reaction conditions and low reaction efficiency. Interestingly, deep eutectic solvent (DES) has shown excellent application potential in PET glycolysis field because of its high efficiency, low toxicity and designability14. DES is a eutectic mixture of hydrogen bond acceptor (HBA) such as quaternary salt and metal salts with hydrogen bond donor (HBD) such as amide, acid and alcohol at room temperature15, which have been applied in various fields of chemistry including electrochemistry16-17, gas absorption18, organic synthesis19-20, extraction21, biomass processing22-23 and polymerizations24-25. Our previous work14 displayed that DES composed by urea and metal salts showed remarkable catalytic activity. The mechanism of PET degradation catalyzed by DES was ascribed to the synergetic effect of coordination bonds and hydrogen bonds formed between the DES and ethylene glycol. However, this mechanism was proposed based on the calculation results of density functional theory (DFT), which lacks further experimental verification. Moreover, the accurate catalytic sites of DESs, especially the active group on urea, are still unclear. In some other reactions catalyzed by DESs, hydrogen bonding between DES and reactants were claimed to play the key role for the activation of reactants during the DES catalytic reaction, but the specific form of interaction between reactant and DES are still uncertain26-28. Therefore, it has an important guiding role in developing more efficient DES catalysts if the research work could confirm that which functional group on DES plays a major catalytic role in the degradation reaction of PET.


3


Page 4 of 21

Exhilaratingly, Fukushima29 reported that the cyclic amidines can be used as catalyst in PET glycolysis. Moreover, the catalytic activity of cyclic amidines is correlated to their basicity. Inspired by this research, we speculate that the amino group on urea is a catalytic site and hydrogen bonding site of DES and the catalytic activity of urea is attributed to its basicity. Meanwhile, metal salts showed catalytic activity as Lewis acid in PET glycolysis reaction30. Therefore, the high catalytic activity of PET glycolysis catalyzed by DES may be attributed to acid-base synergistic catalysis. In this work, several urea derivatives, ketones and amines were used as catalysts in PET glycolysis process for the purpose of verifying the mechanism of PET glycolysis. The hydrogen bonding effect between EG and DES was verified by 1H NMR and

13C

NMR. A series 1,3-

dimethylurea/metal salts were synthesized and used in theglycolysis of PET, which has not been reported. The influences of metal salts in DESs were investigated. The reaction mechanism of DES-catalyzed PETglycolysis was proposed. Moreover, the effect of reaction conditions and the kinetic of PET glycolysis were investigated. The structure of product was analyzed by 1H NMR, ESI-MS, TGA and DSC.

EXPERIMENTAL SECTION Material. PET pellets (2.0 × 2.5 × 2.7 mm) were supplied by Jindong Commercial Co. Ltd., Jiangsu Province, China. In experiment, the size of PET particles was 40~60 mesh which was obtained by grinding from PET pellets. EG, urea, 1,3-dimethylurea (1,3-DMU), tetrahydro-2pyrimidinone, 1,1-dimethylurea (1,1-DMU), tetramethylurea (TMU), 2-imidazolidinone, thiourea, zinc acetate, manganese acetate, cobalt acetate, nickel acetate, copper acetate, ferric chloride were supplied by Sinopharm Chemical Reagent Beijing Co. Ltd. China.


4

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Synthesis of DESs. A series of deep eutectic solvents were synthesized as described elsewhere31. In a typical procedure, mixed 1,3-dimethylurea with metal salts at molar ratios from 2:1 to 10:1 at 80~100 °C for 4 h until the raw materials turn to homogeneous, clear liquids. Finally the resulting liquid was dried in a vacuum oven at 60 °C for one day. General glycolysis process of PET. 5.0 g of PET and a certain amount of EG were added to a 50 mL three-necked flask with a thermometer, a magnetic stirrer and a reflux condenser. Catalyst was added into the flask when the mixture heated to target temperature. The glycolysis of PET was performed at reaction temperature ranging from 150 °C to 197 °C for reaction time from 20~60 min under atmospheric pressure. After the reaction completed, the reaction solution was added to 700 mL distilled water. Undepolymerized PET powder and Oligomer were separated by filtration respectively. The PET and Oligomer were dried at 70 °C for 12 h and weighed. The PET-conversion and the BHET-yield are calculated by equation (1) and (2), respectively. Conversion of PET =

Yield of BHET =

W0 - W1 W0

WBHET M BHET W0 M PET

× 100%

(1)

× 100%

(2)

Where W0 is the initial weight of PET, W1 is the weight of undepolymerized PET, WBHET is the weight of BHET, MBHET is the molar mass of BHET, MPET is the molar mass of PET. The collected filtrate was diluted with distilled water to 1000 mL in volumetric flask. The BHET-yield was defined by HPLC. The remaining solution was concentrated to 70 mL. After storing at 0 °C overnight, white needle-like crystals were precipitated from concentrated solution. High purity BHET was obtained after filtration, washing by water and drying. The products were analyzed by 1H NMR, ESI-MS, TGA and DSC.


5


Page 6 of 21

Characterizations. HPLC analysis of the main product was performed using a ACQUITY HPLC which was equipped with refractive index detector and BET C18 column (Waters, U.S.A.) under the condition of an column temperature 30 °C, detector temperature 50 °C, solvent methanol/water (50:50) and flow rate 0.2 mL/min. NMR analysis of the PET glycolysis products was performed on Bruker ECA-600. The spectra were obtained in d6-DMSO solution. The mass spectra were carried out on a microTOF instrument (Bruker, Germany) equipped with electron ionization (ESI). The FT-IR analysis were run using a Nicolet 380 spectrometer (Thermo Fisher Scientific, U.S.A). DSC scans of the product was recorded with a DSC1 (Mettler-Toledo, Switzerland) by heating from room temperature to 200 °C at a rate of 10 °C/min under the nitrogen atmosphere. DSC scans of the DESs was recorded with a DSC1 (Mettler-Toledo, Switzerland) by heating from -80 °C to 120 °C at a rate of 5 °C/min under the nitrogen atmosphere. The viscosity of DES was measured by Discovery Hybrid Rheometer (DHR-2) (TA Instruments, USA) by heating from 90 °C to 130 °C at a rate of 5 °C/min. The TG analysis was obtained using DTG-60H (SHIMADZU, Japan) with temperature range from 25 °C to 600 °C at a heating rate of 10 °C/min under the nitrogen atmosphere. ICPE-9000 (Shimadzu, Japan) was employed to analysis the content of Zinc in product and EG. The pretreatment of samples before test were shown as follow. 20 mg samples were added to 10 mL 10 M HNO3 aqueous solution and heated at 80 °C for 2 h. After the solution cooling to room temperature, 1 mL solution was taken and diluted to 10 mL with deionized water. The content of Zinc in the samples was calculated by equation (3). Content of Zn% = (CZn × V × Dilution)/Wsample × 100%

(3)

Where CZn is the concentration of Zinc ion determined by ICPE-9000, mg/L; V is the volume of test solution, 0.01 L; Dilution = 10; Wsample is the weight of sample, mg.


6

Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The nitrogen content of the samples was measured by the Kieldahl method. N% = [1.401 × C × (Vt ― Vb)]/Wsample

(4)

Where, C= the concentration of titrated acid. 0.02 mol/L; Vt= titration volume of sample, ml; Vb= titration volume of blank sample, ml; Wsample= the weight of the sample, g.

RESULTS AND DISCUSSION Active site and influence factors of urea derivatives in DESs. To investigate the influence factors of urea derivatives on catalytic activity, a series of urea derivatives were used as catalysts under the same conditions (Scheme 1). As shown in Figure 1, 1,3-DMU showed the highest catalytic activity. It was found that the yield of BHET reached 67 % using 1,3-DMU (5.0 wt%), EG (10.0 g) and PET (2.0 g) for 90 minutes of glycolysis at 160 °C under the pressure of 3 MPa, while the yield of BHET catalyzed by urea under the same conditions only reached 49 %. The reason for this difference can be attributed to the electronegativity enhancement of the nitrogen atom in 1,3-DMU with the amino group substituted by two methyl groups. Other urea derivatives including TMU, 2-imidazolidinone and tetrahydro-2-pyrimidinone showed basically none catalytic activity in PET glycolysis process. The reason probably is that the steric hindrance effect limited the progress of the catalytic reaction. To determine the position of the active site in 1,3-DMU, a series of ketones and amines were subjected to PET glycolysis experiments as representative of carbonyl and amine groups, respectively. Table 1 shows the yield of BHET catalyzed by different ketones and amines. It can be seen that ketones, including acetone and 3-pentanone, exhibit very low catalytic activity under the conditions of PET (2.0 g), EG (10.0 g ), catalyst (20% molar ratio of PET) at 160 °C for 90 min under the pressure of 5MPa. However, the amine shows the opposite result with high catalytic activity under the same reaction conditions. Moreover, the yield of BHET catalyzed by


7


Page 8 of 21

tri-n-propylamine is lower than that catalyzed by n-propylamine, because the steric hindrance of nitrogen on tri-n-propylamine is larger than that on n-propylamine. The similar results can be observed among the BHET-yield catalyzed by di-n-butylamine, diisobutylamine and di-secbutylamine. Based the above results, it is reasonable to claim that the active site of urea derivatives in DESs is the amino group. Moreover, both the basicity and the steric hindrance of amino group affect the catalytic activity of urea derivatives.

Scheme 1. Molecular formula of urea derivatives

Figure 1. PET glycolysis using different urea derivatives. (160 °C, 90 min, ncatalyst :nPET =20%, PET (2.0 g), EG (10.0 g), 3MPa)A urea, B 1,3-DMU, C 1,1-DMU, D TMU, E 2imidazolidinone, F Tetrahydro-2-pyrimidinone, G thiourea.)


8

Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Yield of BHET catalyzed by different ketones and amines Catalysts Yield of BHET/% Aceton 0.10 3-pentanone 0.10 N-propylamine 72.88 Di-n-propylamine 73.75 Tri-n-propylamine 26.07 Diisopropylamine 70.13 Di-n-butylamine 76.85 Diisobutylamine 47.55 Di-sec-butylamine 63.84 Reaction conditions: PET (2.0 g), EG (10.0 g ), catalyst (20% molar ratio of PET), 160 °C, 90 min, 5MPa Effect of metal salts in DES. In order to find an effective DES catalyst, a series of transition metal salts including Zn(OAc)2, Mn(OAc)2, Co(OAc)2, Ni(OAc)2, Cu(OAc)2 and FeCl3 were employed to react with 1,3-DMU to synthesize various kinds of DESs, respectively. The characterizations of DESs and main product are shown in Figure S1-S10 and Table S1-S2. The results of PET glycolysis catalyzed by these DESs were showed in Figure 2. It was found that 1,3-DMU/Zn(OAc)2 showed the best catalytic activity among these DESs at the same reaction conditions. In DESs with different metal salts, the catalytic activity sequence is that Zn(OAc)2 > Mn(OAc)2 > Co(OAc)2 > Ni(OAc)2 > Cu(OAc)2 > FeCl3, which is accord with the catalytic activity order catalyzed by metal salts32 and first-row transition metal-containing ionic liquids33. It was reported that Zn(OAc)2 can react with polyol to form ZnO34. Moreover, ZnO was an effective catalyst in urea glycolysis process to synthesis ethylene carbonate35. However, 3Methyl-oxazolidin-2-one was detected in the mixture of DES and EG after heating, but not detected in the mixture of DES, EG and PET (Figure S11). The reason for the difference was speculated that the PET glycolysis reaction and the 1,3-DMU glycolysis reaction are mutually


9


Page 10 of 21

competing and the hydrolysis of Zn(OAc)2 is also inhibited in the presence of PET. The effect among EG, 1,3-DMU, Zn(OAc)2 and PET is complicated, which need our further research.

Figure 2. Effect of different DESs on BHET yield (170 °C, 60 minutes, PET (5.0 g), EG (20.0 g), catalyst (0.25 g)) Proposed mechanism of PET glycolysis. In the PET glycolysis reaction or other transesterification reaction, zinc salts are generally considered to be a Lewis acid catalyst to attack the carbonyl oxygen on the ester group30, 36-38. As mentioned above, amino group is the active site in 1,3-DMU. The hydrogen bonding interaction between amino groups in 1,3-DMU and EG may play the key role in PET glycolysis. To confirm that inference, we conducted 1H NMR spectroscopy analyses of the system in d6-DMSO. As shown in Figure 3, the chemical shift of hydroxyl in EG OH is 4.45 ppm without the addition of DES. With the increasing of DES, the signals of hydroxyl protons moved to downfield. When the weight ratio of EG and DES reached 1:3, the OH reached 4.51 ppm (= +0.06 ppm). The signals of CH2 (=3.39 ppm) did not show obvious change with the change of EG: DES weight ratio. From the other side, the amido resonances shifted from 5.77 ppm to 5.73 ppm (= -0.04 ppm) with the increasing of EG. Moreover, the other signals of DES including


10

Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N-CH3 (=2.52) and CH3COO (=1.82 ppm) were basically unchanged. In order to verify the possibility of a strong hydrogen bond interaction between the carbonyl group and the ethylene glycol,

13C

NMR spectroscopy was used to measure the chemical shift changing of carbonyl

carbon in 1,3-DMU and methylene carbon in EG. The results showed that their signals remained the same (Figure S12). Therefore, it is reasonable to speculate that the change in chemical shift is due to the strong hydrogen bonding interaction between the hydroxyl group in EG and the amino group in 1,3-DMU.

Figure 3. 1H NMR spectra of 1,3-DMU/Zn(OAc)2 DES and EG mixture with different weight ratio. The symbols in the 1H NMR spectra identify the N-H (triangle) of 1,3-DMU and O-H (diamond) of EG On the basis of experiment results, the possible glycolysis mechanism of PET wastes catalyzed by DESs should be attributed to Lewis acid-base synergistic catalysis effect which is illustrated in Scheme 2. It can be seen that hydrogen bond was formed between nitrogen of


11


Page 12 of 21

amino group in 1,3-DMU and hydrogen of hydroxyl in EG. The electronegativity of hydroxyl oxygen in EG is enhanced through hydrogen bonding which means EG is more active to attack the carbonyl group of ester. Meanwhile, Zn2+ acting as a Lewis acid protonates the carbonyl group of the ester, making the carbon of carbonyl group in PET more electrophilic. Thus, the carbonyl group of PET is more susceptible to nucleophilic attack by ethylene glycol. The synergistic effect of acid and base leads to the breaking of the ester bond, producing the oligomers and dimers, and to BHET at last. The results of PET glycolysis catalyzed by Zn(OAc)2, 1,3-DMU and 1,3-DMU/Zn(OAc)2 DES respectively have confirmed that the existence of synergetic catalytic effect between 1,3-DMU and Zn(OAc)2 (Table S3).

Scheme 2. Proposed mechanism for PET glycolysis catalyzed by 1,3-DMU/Zn(OAc)2. Influences of reaction conditions. 1,3-DMU/Zn(OAc)2 DES exhibits pretty good catalytic performance in PET glycolysis. Some factors affecting the glycolysis of PET were investigated, and the results were depicted at Figure 4. The influence of different molar ratio of 1,3-DMU and Zn(OAc)2 was depicted in Figure 4(a). It can been seen that when the molar ratio of 1,3-DMU and Zn(OAc)2 is 4, the PET-conversion andthe BHET-yield reach the maximum value, respectively. This may be related to the coordination number of the Zn2+. When the molar ratio is less than 4, the incomplete coordination of Zn2+ leads to a lower catalytic activity.


12

Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4(b) presents the influence of reaction temperature on the BHET-yield and the PETconversion. With the increasing of reaction temperature, the PET-conversion increases obviously and reaches 100% which the temperature is 190 °C. The BHET-yield reaches the peak value (82%) at 190 °C. When the temperature is further increased to 197 ° C, the yield of BHET decreases slightly (80%). This is because that the chemical reaction equilibrium between BHET monomer and the oligomer is changed with the increasing of temperature. Figure 4(c) displays the influence of reaction time on PET glycolysis. The DESs catalyst exhibits excellent catalytic activity. When the reaction time is 5 min, the PET-conversion reaches 64% and theBHET-yield reaches 54%. With the reaction time extending to 20 minutes and above, the BHET yield reaches a maximum of 82% and remains constant. To investigate the influence of the dosage of catalyst on the PET glycolysis, a range of 1.07.0 catalysts weight percentage were tested as shown in Figure 4(d). It is obviously seen that the yield of BHET increased rapidly with the augmentation of catalyst concentration from 0% to 3%. The PET glycolysis is catalyzed efficaciously by a very low concentration of catalyst (1.0 wt%) with a BHET yield of 58%. When the concentration of catalyst reaches to 5%, the PET is completely degraded, and the BHET yield reaches the maximum equilibrium value (82%). The PET glycolysis process is a heterogeneous reaction process with solid (PET) – liquid (EG) two phases. The glycolysis reaction proceeds on the surface of PET solid, and the product BHET is solved in EG39. Therefore, the amount of ethylene glycol needs to meet the requirements of completely wetting the surface of PET particles while dissolving BHET. Otherwise, the efficiency of PET glycolysis will be affected. The influence of ethylene glycol was shown in Figure 4(e). It is obvious that the yield of BHET increases with the increasing of


13


Page 14 of 21

ethylene glycol amount from 5.0 g to 20.0 g. When the amount of EG arrives to 20 g, the glycolysis of PET reaches equilibrium. After a comprehensive study of the factors affecting PET glycolysis, it can clearly be seen that reaction temperature is the crucial factor for PET glycolysis reaction. The optimized reaction conditions are obtained: EG (20.0 g), catalyst (n(1,3-DMU)/n(Zn(OAc)2) 4/1, 0.25 g), PET (5.0 g), atmospheric pressure at 190 °C for 20 min, the PET-conversion and BHET-yield are 100% and 82%, respectively.

Figure 4. Optimization of PET glycolysis reaction conditions. (a) Influence of molar ratio of 1,3DMU and Zn(OAc)2 Reaction conditions: PET (5.0 g), EG (20 g), catalyst (n(1,3DMU)/n(Zn(OAc)2), 0.25 g), atmospheric pressure, 170 °C, 60 minutes. (b) Influence of reaction temperature. Reaction conditions: PET (5.0 g), EG (20 g), catalyst (n(1,3-DMU)/n(Zn(OAc)2) 4/1, 0.25 g), atmospheric pressure, 20 minutes. (c) Influence of reaction time. Reaction conditions: PET (5.0 g), EG (20 g), catalyst (n(1,3-DMU)/n(Zn(OAc)2) 4/1, 0.25 g), atmospheric


14

Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


pressure, 190 °C. (d) Influence of catalyst ratio. Reaction conditions: PET (5.0 g), EG (20 g), catalyst (n(1,3-DMU)/n(Zn(OAc)2) 4/1), atmospheric pressure, 20 minutes, 190 °C. (e) Influence of EG dosage. Reaction conditions: PET (5.0 g), 190 °C, catalyst (n(1,3-DMU)/n(Zn(OAc)2) 4/1, 0.25 g), atmospheric pressure, 20 minutes. Kinetic of PET glycolysis catalyzed by 1,3-DMU/Zn(OAc)2. Several factors have effects on the kinetic of PET glycolysis which includes EG/PET molar ratio, phase state of catalyst, size and shape of PET and stirring rate. Based on these reaction conditions, many different kinetic models of PET glycolysis were proposed, such as modified shrink-core model40, random scission model41 and other mathematic models42-43. In our experiment, the catalyst 1,3-DMU/Zn(OAc) 2 was soluble in EG. Therefore, the PET glycolysis reaction is a two-phase reaction. PET used in experiment was ground into small particles of 40-60 mesh, which can be regarded as spherical. Therefore, the kinetic model suitable for our experiment conditions is the shrink-core model. The kinetic equation of shrinkcore model is shown as equation 5. The detail derivation process has been described in our previous work44.

[

2

Kt = 1 ― (1 ― X)

]

3

(5)

As mentioned above, temperature is the crucial factor for PET glycolysis. Therefore, temperature is main factor in investigating the influencing factors of kinetics. The effect of temperature on the reaction rate analyzed by equation 5 is shown in Figure 5(a). The results show that the experimental data present excellent linearity at different temperatures. An Arrhenius plot of the apparent rate constant obtained from the slope of the straight lines is depicted in Figure 5(b). As observed, data fell on a straight line with an excellent linear


15


Page 16 of 21

correlation factor (R2> 0.99). The apparent activity energy was calculated from the slope of Arrhenius plot. The apparent activity energy is 148.89kJ/mol.

Figure 5. (a) Effect of the temperature on the rate of PET glycolysis, (b) Arrhenius plot of the rate constant of PET glycolysis. (Reaction condition: PET (5.0 g), EG (20.0 g), catalyst (n(1,3DMU)/n(Zn(OAc)2) 4/1, 0.25 g), atmospheric pressure) Recycling of the catalyst. From the view of environmental protection and economy, recycling of catalyst is an important issue. The results of PET-conversion and BHET-yield were shown in Figure 6. It can be seen that the catalyst still work efficiently after 5 times recycle. The ICP result displays that the zinc content compared with initial addition decreased to 24.82% after 6 times recycle (Table S3). The content of nitrogen in residual EG accounts for 22.15 % of the initial addition after 6 times recycling. If the catalyst loss during the experiment can be strictly controlled, the cycle times of the catalyst can be more than six times.


16

Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Recycling of catalyst (Reaction condition: 190 °C, 20 min, PET (5.0 g), EG (20.0 g), catalyst (n(1,3-DMU)/n(Zn(OAc)2) 4/1, 0.25 g)).

CONCLUSION In conclusion, it was demonstrated that 1,3-DMU/Zn(OAc)2 DES can behave as an efficient catalyst for PET glycolysis process. It was found that the yield of BHET reached 82% under the conditions of 1,3-DMU/Zn(OAc)2 (5 wt%), EG (20.0 g) and PET (5.0 g) for 20 minutes of glycolysis at 190 °C. Reaction temperature is a critical factor in PET glycolysis reaction. The kinetic results showed that the glycolysis of PET catalyzed by 1,3-DMU/Zn(OAc)2 followed the shrink-core model with the apparent activation energy 148.89kJ/mol. The study of the glycolysis mechanism revealed that the extraordinary performance of catalyst can be attributed to acid-base synergetic effect between Zn(OAc)2 and 1,3-DMU. This work can provide guidance for developing more efficient DES catalysts in PET glycolysis process.

ASSOCIATED CONTENT AUTHOR INFORMATION Corresponding Author Dr. Xingmei Lu. Tel: +86-010-82544800; Fax: +86-010-82544800; E-mail: [email protected]


17


Page 18 of 21

ORCID Xingmei Lu 0000-0003-4712-656X

CONFLICTS OF INTEREST There are no conflicts to declare.

SUPPORTING INFORMATION The supporting information is available free of charge via the Internet at http://pubs.acs.org. (1) FT-IR spectra of 1,3-DMU/Zn(OAc)2 DES and the raw materials, (2) FT-IR spectra of 1,3-DMU based DES with different metal salts, (3) 1H NMR spectrum of the main product, (4) ESI-MS spectrum of the main product, (5) TGA curves of PET and the main product, (6) DSC curve of the main product, (7) HPLC result of the main product of PET glycolysis, (8) DSC curve of 1,3DMU/Zn(OAc)2 DESs, molar ratio from 1:1 to 10:1, (9) Effect of temperature on 1,3DMU/Zn(OAc)2 (molar ratio 4:1) viscosity, (10) TGA curves of DESs, (11) Recycling of the residual Zn, (12) Effect of catalysts on the yield of BHET, (13) GC-MS chromatogram of DES/EG and DES/EG/PET mixture, (14) 13C NMR spectra of 1,3-DMU/Zn(OAc)2 DES and EG mixture with different weight ratio.

ACKNOWLEGEMENTS This work was supported financially by National Basic Research Program of China (973 Program), (2015CB251401), National Natural Scientific Fund of China (No. 21878292, No. 91434203, No. 21776289), K. C. Wong Education Foundation, the Strategic Priority Research Program of Chinese Academy of Science (No. XDA21060300) and Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-JSC011).

REFERENCES (1)

Karayannidis, G. P.; Nikolaidis, A. K.; Sideridou, I. D.; Bikiaris, D. N.; Achilias, D. S., Chemical Recycling of


18

Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


PET by Glycolysis: Polymerization and Characterization of the Dimethacrylated Glycolysate. Macromol. Mater. Eng. 2006, 291 (11), 1338-1347, Doi 10.1002/mame.200600243. (2) Sanchez, A. C.; Collinson, S. R., The Selective Recycling of Mixed Plastic Waste of Polylactic Acid and Polyethylene Terephthalate by Control of Process Conditions. Eur. Polym. J. 2011, 47 (10), 1970-1976, Doi 10.1016/j.eurpolymj.2011.07.013. (3) Kurokawa, H.; Ohshima, M.; Sugiyama, K.; Miura, H., Methanolysis of Polyethylene Terephthalate (PET) in the Presence of Aluminium Tiisopropoxide Catalyst to Form Dimethyl Terephthalate and Ethylene Glycol. Polym. Degrad. Stab. 2003, 79 (3), 529-533, Doi 10.1016/S0141-3910(02)00370-1. (4) Zhang, L. R.; Gao, J.; Zou, J. Z.; Yi, F. P., Hydrolysis of Poly(Ethylene Terephthalate) Waste Bottles in the Presence of Dual Functional Phase Transfer Catalysts. J. Appl. Polym. Sci. 2013, 130 (4), 2790-2795, Doi 10.1002/app.39497. (5) Paszun, D.; Spychaj, T., Chemical Recycling of Poly(Ethylene Terephthalate). Ind. Eng. Chem. Res. 1997, 36 (4), 1373-1383, Doi 10.1021/Ie960563c. (6) Mittal, A.; Soni, R. K.; Dutt, K.; Singh, S., Scanning Electron Microscopic Study of Hazardous Waste Flakes of Polyethylene Terephthalate (PET) by Aminolysis and Ammonolysis. J. Hazard Mater. 2010, 178 (1-3), 390-396, Doi 10.1016/j.jhazmat.2010.01.092. (7) Yoshida, S.; Hiraga, K.; Takehana, T.; Taniguchi, I.; Yamaji, H.; Maeda, Y.; Toyohara, K.; Miyamoto, K.; Kimura, Y.; Oda, K., A Bacterium That Degrades and Assimilates Poly(Ethylene Terephthalate). Science 2016, 351 (6278), 1196-1199, Doi 10.1126/science.aad6359. (8) Wi, R.; Imran, M.; Lee, K. G.; Yoon, S. H.; Cho, B. G.; Kim, D. H., Effect of Support Size on the Catalytic Activity of Metal-Oxide-Doped Silica Particles in the Glycolysis of Polyethylene Terephthalate. J. Nanosci. Nanotechno. 2011, 11 (7), 6544-6549, Doi 10.1166/jnn.2011.4393. (9) Shukla, S. R.; Palekar, V.; Pingale, N., Zeolite Catalyzed Glycolysis of Poly(Ethylene Terephthalate) Bottle Waste. J. Appl. Polym. Sci. 2008, 110 (1), 501-506, Doi 10.1002/app.28656. (10) Geng, Y. R.; Dong, T.; Fang, P. T.; Zhou, Q.; Lu, X. M.; Zhang, S. J., Fast and Effective Glycolysis of Poly(Ethylene Terephthalate) Catalyzed by Polyoxometalate. Polym. Degrad. Stab. 2015, 117, 30-36, Doi 10.1016/j.polymdegradstab.2015.03.019. (11) Wang, H.; Li, Z. X.; Liu, Y. Q.; Zhang, X. P.; Zhang, S. J., Degradation of Poly(Ethylene Terephthalate) Using Ionic Liquids. Green Chem. 2009, 11 (10), 1568-1575, Doi 10.1039/b906831g. (12) Wang, H.; Liu, Y. Q.; Li, Z. X.; Zhang, X. P.; Zhang, S. J.; Zhang, Y. Q., Glycolysis of Poly(Ethylene Terephthalate) Catalyzed by Ionic Liquids. Eur. Polym. J. 2009, 45 (5), 1535-1544, Doi 10.1016/j.eurpolymj.2009.01.025. (13) Wang, H.; Yan, R. Y.; Li, Z. X.; Zhang, X. P.; Zhang, S. J., Fe-Containing Magnetic Ionic Liquid as an Effective Catalyst for the Glycolysis of Poly(Ethylene Terephthalate). Catal. Commun. 2010, 11 (8), 763-767, Doi 10.1016/j.catcom.2010.02.011. (14) Wang, Q.; Yao, X. Q.; Geng, Y. R.; Zhou, Q.; Lu, X. M.; Zhang, S. J., Deep Eutectic Solvents as Highly Active Catalysts for the Fast and Mild Glycolysis of Poly(Ethylene Terephthalate)(PET). Green Chem. 2015, 17 (4), 24732479, Doi 10.1039/c4gc02401j. (15) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V., Novel Solvent Properties of Choline Chloride/Urea Mixtures. Chem. Commun. 2003, (1), 70-71, Doi 10.1039/b210714g. (16) Nkuku, C. A.; LeSuer, R. J., Electrochemistry in Deep Eutectic Solvents. J. Phys. Chem. B 2007, 111 (46), 13271-13277, 10.1021/jp075794j. (17) Smith, E. L.; Abbott, A. P.; Ryder, K. S., Deep Eutectic Solvents (DESs) and Their Applications. Chem. Rev. 2014, 114 (21), 11060-11082, Doi 10.1021/cr300162p. (18) Xie, Y. J.; Dong, H. F.; Zhang, S. J.; Lu, X. H.; Ji, X. Y., Solubilities of CO2, CH4, H2, CO and N2 in Choline Chloride/Urea. Green Energy Environ.2016, 1 (3), 195-200, Doi org/10.1016/j.gee.2016.09.001. (19) Keshavarzipour, F.; Tavakol, H., Deep Eutectic Solvent as a Recyclable Catalyst for Three-Component Synthesis of Beta-Amino Carbonyls. Catal. Lett. 2015, 145 (4), 1062-1066, Doi 10.1007/s10562-014-1471-6. (20) Chen, Z. Z.; Zhou, B.; Cai, H. H.; Zhu, W.; Zou, X. Z., Simple and Efficient Methods for Selective Preparation of Alpha-Mono or Alpha,Alpha-Dichloro Ketones and Beta-Ketoesters by Using Dcdmh. Green Chem. 2009, 11 (2), 275-278, Doi 10.1039/b815169e. (21) Gu, T. N.; Zhang, M. L.; Tan, T.; Chen, J.; Li, Z.; Zhang, Q. H.; Qiu, H. D., Deep Eutectic Solvents as Novel Extraction Media for Phenolic Compounds from Model Oil. Chem. Commun. 2014, 50 (79), 11749-11752, Doi


19


Page 20 of 21

10.1039/c4cc04661g. (22) Loow, Y. L.; New, E. K.; Yang, G. H.; Ang, L. Y.; Foo, L. Y. W.; Wu, T. Y., Potential Use of Deep Eutectic Solvents to Facilitate Lignocellulosic Biomass Utilization and Conversion. Cellulose 2017, 24 (9), 3591-3618, Doi 10.1007/s10570-017-1358-y. (23) Kandanelli, R.; Thulluri, C.; Mangala, R.; Rao, P. V. C.; Gandham, S.; Velankar, H. R., A Novel Ternary Combination of Deep Eutectic Solvent-Alcohol (DES-Ol) System for Synergistic and Efficient Delignification of Biomass. Bioresource Technol. 2018, 265, 573-576, Doi 10.1016/j.biortech.2018.06.002. (24) Park, T. J.; Lee, S. H., Deep Eutectic Solvent Systems for FeCl3-Catalyzed Oxidative Polymerization of 3Octylthiophene. Green Chem. 2017, 19 (4), 910-913, Doi 10.1039/c6gc02789j. (25) Li, R. A.; Zhang, K. L.; Chen, G. X.; Su, B.; Tian, J. F.; He, M. H.; Lu, F. C., Green Polymerizable Deep Eutectic Solvent (PDES) Type Conductive Paper for Origami 3d Circuits. Chem. Commun. 2018, 54 (18), 2304-2307, Doi 10.1039/c7cc09209a. (26) Singh, B.; Lobo, H.; Shankarling, G., Selective N-Alkylation of Aromatic Primary Amines Catalyzed by BioCatalyst or Deep Eutectic Solvent. Catal. Lett. 2011, 141 (1), 178-182, Doi 10.1007/s10562-010-0479-9. (27) Azizi, N.; Yadollahy, Z.; Rahimzadeh-oskooee, A., Odrourless Strategy for Deep Eutectic Solvent-Mediated Ring Opening of Epoxides with in Situ Generated S-Alkylisothiouronium Salts. Synlett. 2014, 25 (8), 1085-1088, Doi 10.1055/s-0033-1341050. (28) Sanap, A. K.; Shankarling, G. S., Choline Chloride Based Eutectic Solvents: Direct C-3 Alkenylation/Alkylation of Indoles with 1,3-Dicarbonyl Compounds. Rsc Adv. 2014, 4 (66), 34938-34943, Doi 10.1039/c4ra05858e. (29) Fukushima, K.; Coady, D. J.; Jones, G. O.; Almegren, H. A.; Alabdulrahman, A. M.; Alsewailem, F. D.; Horn, H. W.; Rice, J. E.; Hedrick, J. L., Unexpected Efficiency of Cyclic Amidine Catalysts in Depolymerizing Poly(Ethylene Terephthalate). J. Polym. Sci. Pol. Chem. 2013, 51 (7), 1606-1611, Doi 10.1002/pola.26530. (30) Lopez-Fonseca, R.; Duque-Ingunza, I.; de Rivas, B.; Arnaiz, S.; Gutierrez-Ortiz, J. I., Chemical Recycling of Post-Consumer PET Wastes by Glycolysis in the Presence of Metal Salts. Polym. Degrad. Stab. 2010, 95 (6), 10221028, Doi 10.1016/j.polymdegradstab.2010.03.007. (31) Sirvio, J. A.; Visanko, M.; Liimatainen, H., Deep Eutectic Solvent System Based on Choline Chloride-Urea as a Pre-Treatment for Nanofibrillation of Wood Cellulose. Green Chem. 2015, 17 (6), 3401-3406, Doi 10.1039/c5gc00398a. (32) Pingale, N. D.; Palekar, V. S.; Shukla, S. R., Glycolysis of Postconsumer Polyethylene Terephthalate Waste. J. Appl. Polym. Sci. 2010, 115 (1), 249-254, Doi 10.1002/app.31092. (33) Wang, Q.; Geng, Y. R.; Lu, X. M.; Zhang, S. J., First-Row Transition Metal-Containing Ionic Liquids as Highly Active Catalysts for the Glycolysis of Poly(Ethylene Terephthalate) (PET). Acs Sustain. Chem. Eng. 2015, 3 (2), 340348, Doi 10.1021/sc5007522. (34) Li, X. Q.; Zhang, J.; Ju, Z. Y.; Li, Y.; Xu, J. L.; Xin, J. Y.; Lu, X. M.; Zhang, S. J., Facile Synthesis of Cellulose/ZnO Aerogel with Uniform and Tunable Nanoparticles Based on Ionic Liquid and Polyhydric Alcohol. Acs Sustain. Chem. Eng. 2018, 6 (12), 16248-16254, Doi 10.1021/acssuschemeng.8b03106. (35) Wang, J. P.; Chen, Y. M.; Shen, W. H.; Zhu, Z. Q.; Xu, Y. S.; Fang, Y. J., The Role of Zinc Oxide in Carbonylation of Ethylene Glycol to Ethylene Carbonate with Urea: A Precursor for Homogeneous Catalyst. Catal. Commun. 2017, 89, 52-55, Doi 10.1016/j.catcom.2016.10.016. (36) Chakraborty, P.; Adhikary, J.; Sanyal, R.; Khan, A.; Manna, K.; Dey, S.; Zangrando, E.; Bauza, A.; Frontera, A.; Das, D., Role of Ligand Backbone of Tridentate Schiff-Base on Complex Nuclearity and Bio-Relevant Catalytic Activities of Zinc(Ii) Complexes: Experimental and Theoretical Investigations. Inorg. Chim. Acta. 2014, 421, 364-373, Doi 10.1016/j.ica.2014.06.018. (37) Wang, Z. Q.; Yang, X. G.; Liu, S. Y.; Hu, J.; Zhang, H.; Wang, G. Y., One-Pot Synthesis of High-MolecularWeight Aliphatic Polycarbonates Via Melt Transesterification of Diphenyl Carbonate and Diols Using Zn(OAc)2 as a Catalyst. Rsc Adv. 2015, 5 (106), 87311-87319, Doi 10.1039/c5ra18275a. (38) Agura, K.; Hayashi, Y.; Wada, M.; Nakatake, D.; Mashima, K.; Ohshima, T., Studies of the Electronic Effects of Zinc Cluster Catalysts and Their Application to the Transesterification of Beta-Keto Esters. Chem-Asian J. 2016, 11 (10), 1548-1554, Doi 10.1002/asia.201600062. (39) Al-Sabagh, A. M.; Yehia, F. Z.; Harding, D. R. K.; Eshaq, G.; ElMetwally, A. E., Fe3O4-Boosted Mwcnt as an Efficient Sustainable Catalyst for PET Glycolysis. Green Chem. 2016, 18 (14), 3997-4003, Doi 10.1039/c6gc00534a. (40) Yoshioka, T.; Motoki, T.; Okuwaki, A., Kinetics of Hydrolysis of Poly(Ethylene Terephthalate) Powder in


20

Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sulfuric Acid by a Modified Shrinking-Core Model. Ind. Eng. Chem. Res. 2001, 40 (1), 75-79, Doi 10.1021/Ie000592u. (41) Sanchez-Jimenez, P. E.; Perez-Maqueda, L. A.; Perejon, A.; Criado, J. M., A New Model for the Kinetic Analysis of Thermal Degradation of Polymers Driven by Random Scission. Polym. Degrad. Stab. 2010, 95 (5), 733739, Doi 10.1016/j.polymdegradstab.2010.02.017. (42) Lopez-Fonseca, R.; Duque-Ingunza, I.; de Rivas, B.; Flores-Giraldo, L.; Gutierrez-Ortiz, J. I., Kinetics of Catalytic Glycolysis of PET Wastes with Sodium Carbonate. Chem. Eng. J. 2011, 168 (1), 312-320, Doi 10.1016/j.cej.2011.01.031. (43) Viana, M. E.; Riul, A.; Carvalho, G. M.; Rubira, A. F.; Muniz, E. C., Chemical Recycling of PET by Catalyzed Glycolysis: Kinetics of the Heterogeneous Reaction. Chem. Eng. J. 2011, 173 (1), 210-219, Doi 10.1016/j.cej.2011.07.031. (44) Liu, B.; Lu, X. M.; Ju, Z. Y.; Sun, P.; Xin, J. Y.; Yao, X. Q.; Zhou, Q.; Zhang, S. J., Ultrafast Homogeneous Glycolysis of Waste Polyethylene Terephthalate Via a Dissolution-Degradation Strategy. Ind. Eng. Chem. Res. 2018, 57 (48), 16239-16245, Doi 10.1021/acs.iecr.8b03854.

For Table of Contents Only

synopsis The process of PET glycolysis catalyzed by DES is an effective and green method to recycle PET waste.


21

Lewis AcidâBase Synergistic Catalysis for Polyethylene Terephthalate

Recommend Documents

Lewis AcidâBase Synergistic Catalysis for Polyethylene Terephthalate