Ultrasmall Cobalt Nanoparticles as a Catalyst for PET Glycolysis: A

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Ultrasmall cobalt nanoparticles as a catalyst for PET glycolysis: A green protocol for pure hydroxyethyl terephthalate precipitation without water Fernanda Reis Veregue, Cleiser Thiago Pereira da Silva, Murilo Pereira Moises, Joziane Gimenes Meneguin, Marcos Rogério Guilherme, Pedro Augusto Arroyo, Silvia Luciana Favaro, Eduardo Radovanovic, Emerson Marcelo Girotto, and Andrelson Wellington Rinaldi ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b02294 • Publication Date (Web): 26 Jul 2018 Downloaded from http://pubs.acs.org on July 26, 2018

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ACS Sustainable Chemistry & Engineering

Ultrasmall cobalt nanoparticles as a catalyst for PET glycolysis: A green protocol for pure hydroxyethyl terephthalate precipitation without water Fernanda Reis Veregue†, Cleiser Thiago Pereira da Silva†, Murilo Pereira Moisés†,ǁ, Joziane Gimenes Meneguin†,‡, Marcos Rogério Guilherme§, Pedro Augusto Arroyo‡, Silvia Luciana Favaro†, Eduardo Radovanovic†, Emerson Marcelo Girotto†, Andrelson Wellington Rinaldi†* †

Materials Chemistry and Sensors Laboratory - LMSen, State University of Maringá, Av. Colombo 5790, CEP 87020-900. Maringá, PR, Brazil. ‡

Department of Chemical Engineering, State University of Maringá, Av. Colombo 5790, CEP 87020-900. Maringá, PR, Brazil. ǁ

Federal University of Techonology Paraná, Rua Marcílio Dias 869/870, CEP 86812-460. Apucarana, PR, Brazil.

§

Department of Chemistry, State University of Maringá, Av. Colombo 5790, CEP 87020-900. Maringá, PR, Brazil.

*Corresponding author email address: [email protected] ABSTRACT Polyethylene terephthalate (PET) is a very stable polymer widely used in the modern world. Due to its stability, this polymer can remain in the environment for several years before its complete degradation. The glycolysis reaction of PET has emerged as a green approach to obtain the PET monomer, thus avoiding such environmental problems and adding value to this waste. In this work, PET waste was depolymerized by glycolysis using ultrasmall cobalt nanoparticles (1.5 wt%) as the catalyst for the production of bis-2-hydroxyethyl terephthalate (BHET). A capping agent (tannic acid, TA) and a borohydride reduction approach were used to obtain such ultrasmall cobalt nanoparticles (~3 nm). A PET depolymerization yield of 96% was achieved within 3 h at 180 °C. The precipitation of 77% of pure BHET was achieved without the need for water. The remaining ethylene glycol solution containing the ultrasmall cobalt nanoparticle catalyst was reused five times for this glycolysis process, demonstrating the feasibility of solvent reuse without the need for any treatment. A reaction mechanism is proposed in order to explain the high BHET yield obtained by this ultrasmall cobalt nanoparticle catalyst stabilized with TA. Keywords: Borohydride reduction, depolymerization, glycolysis reaction, PET recycling, reusability, ultrasmall nanoparticles. 1 ACS Paragon Plus Environment

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INTRODUCTION Polyethylene terephthalate, which is widely known as PET or PETE, is a thermoplastic polymer that has attractive properties, such as chemical stability and moderate hardness1. For this reason, PET has been used for the manufacture of plastic bottles to store liquids, such as water, soft drinks, juices, and beer. Owing to the worldwide use of PET, recycling of this polymer has attracted the attention of many research groups2-6 as it may remain on the planet for many years before complete decomposition, causing a serious environmental problem. PET recycling can be performed by two main approaches: mechanical recycling7 and chemical recycling8-12. The chemical recycling route, also called depolymerization, is considered both economical and environmentally friendly because the obtained product can be reused to manufacture PET7. Depolymerization may be performed via glycolysis13-15. Basically, PET shredded into very small pieces is boiled with 1,2-ethanediol, usually called ethylene glycol (EG). The product of this reaction is bis-2-hydroxyethyl terephthalate (BHET)16,17, although sometimes, incomplete depolymerization may afford oligomers or dimers18. To enhance the glycolysis reaction rate, metal salts19-21, such as the acetate, sulfate, and chloride derivatives of different ions, can be used as the catalyst19. However, researchers have recently undertaken vast efforts toward the use of oxide nanoparticles17,20,22 and composites23,24 for PET depolymerization. Some advantages of the use of nanoparticles instead of metal salts for such glycolysis processes are their higher selectivity toward the production of BHET and the reusability of the catalyst for further runs. A considerable number of papers17,20,22,25,26 have shown that nanoparticles are very efficient for the PET glycolysis reaction. However, in most studies, the nanoparticles were synthesized by sol–gel methods27,28 , affording average particle sizes typically larger than 50 nm. Furthermore, in some cases, the glycolysis reaction was carried out at temperatures and pressures higher than those normally used, e.g., 5 atm and 260 °C, respectively20. Nanoparticles with an average size below 10 nm can be obtained by chemical reduction29. This approach has been widely established in the literature for the synthesis of metallic (gold29-30 and silver31) nanoparticles. However, a capping agent such as citrate or tannic 2 ACS Paragon Plus Environment

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acid (TA) must be used in the synthesis to constrain the growth of nanoparticles and avoid their agglomeration. Cobalt nanoparticles have been applied in many areas of science32, for example, as a catalyst for environmental remediation in waste-water treatments33, in the degradation of herbicides34 and dyes35, or in PET depolymerization processes20. In this study, the synthesis of ultrasmall cobalt nanoparticles was performed by the reduction of Co+2 using TA as the capping agent. TA is a polyphenolic molecule widely used in the synthesis of nanoparticles. In addition, this compound can self-assemble into supramolecular structures via hydrogen molecular interactions36,37. The synthesized ultrasmall cobalt nanoparticles were characterized and applied for PET chemical recycling. New optimizations of the PET glycolysis process38 are always beneficial, and here, a new optimization is described as the successful precipitation of BHET without using water. In addition, the catalyst was reused at least five times, with no significant drop in the BHET production.

EXPERIMENTAL SECTION Materials. Cobalt (II) chloride CoCl2.6H2O (Sigma Aldrich 98%), sodium borohydride NaBH4 (Sigma Aldrich 96%), TA (Sigma Aldrich ACS reagent), ethanol (Sigma Aldrich > 99.5%), and ethylene glycol (Fmaia) were used as received without further purification. Plaspet Reciclagens Ltda Company, Maringá, Brazil, generously provided transparent PET pellets (dimensions ~1 mm) obtained from waste soft-drink bottles.

Synthesis of ultrasmall cobalt nanoparticles. The ultrasmall cobalt nanoparticles were synthesized via a chemical reduction approach with sodium borohydride, which is a strong reducing agent, in ethanol. This type of approach allows for the formation of ultrasmall nanoparticles of different metals.

First, CoCl2.6H2O (12.14 mg, 51 µmol) and TA (50 mg,

30 µmol) were solubilized in 20 mL of ethanol and left in an ice bath for 20 min. Then, an 3 ACS Paragon Plus Environment


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ethanol solution containing sodium borohydride (5 mL, 0.1 mol L-1) was rapidly added to the solution containing cobalt ions. The reaction was allowed to proceed under stirring for 2 h. The sequential steps in the nanoparticle synthesis are shown in the Supplementary Information (see Figure S1). Finally, ethanol was evaporated on a thermal plate, and the catalyst powder (~50 mg) was obtained, which was subsequently characterized.

PET depolymerization. The experimental design displayed in Scheme 1 describes the whole process for the depolymerization of PET. A detailed step-by-step guide with images of the PET glycolysis process and BHET precipitation without water are provided in the SI.

Scheme 1: Experimental set-up for PET glycolysis and BHET precipitation without water. Ultrasmall cobalt nanoparticle catalyst (60 mg) was dispersed in 100 mL of ethylene glycol using an ultrasonic bath. Then, this solution was heated to 180 °C and 4.0 g of pricked PET obtained from commercial bottles was added. The glycolysis reaction was allowed to proceed for 2, 3, or 4 h. After the specified reaction time, the temperature of the solution was decreased to 120 °C using a water bath for separation of PET not depolymerized. The thus obtained solid material was separated from the liquid phase by filtration and the PET conversion, i.e., the amount of depolymerized polymer, was calculated using Equation 1:

PET conversion %

initial final × 100 initial

Equation 1 4 ACS Paragon Plus Environment

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where ‘W initial’ refers to the initial weight of PET and ‘W final’ refers to the weight of PET that was not depolymerized. The remaining solution containing the solubilized BHET was placed in a refrigerator at 4 °C for 24 h; the obtained precipitate was filtered off, washed with cold water, and then dried in an oven at 80 °C for 24 h. Using an analytical balance, the weight of BHET was measured, and the BHET molar yield was calculated using Equation 2: filtered !! BHET yield % × 100 " initial !!" Equation 2 where, ‘WPET (initial)’ and ‘WBHET (filtered)’ refer to the initial weight of PET and the weight of BHET obtained, respectively. MMBHET and MMPET correspond to the molar mass of BHET (254 g mol-1) and the PET repeating unit (192 g mol-1), respectively. The catalyst was not recovered from the final solution, but left there for reuse in subsequent depolymerization processes.

Characterization Fourier Transform Infrared (FT-IR) Spectroscopy: The IR spectrum of the catalyst in the form of KBr pellets was obtained on a Bruker Vertex 70 v equipment in the region of 400–4000 cm−1 with a resolution of 4 cm−1, and 256 accumulated spectra. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS): The SEM image and EDS profile were obtained using a Shimadzu Superscan SS-550 microscope. The sample was coated with gold by sputtering. Transmission Electron Microscopy (TEM): TEM analysis was performed on a JEOL–JEM 1400 microscope. The sample was dispersed in isopropyl alcohol and dropped onto a lacey carbon copper grid.

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Thermogravimetric Analysis (TGA): TG analysis was performed using a TGA-50 thermogravimetric analyzer (Shimadzu). The sample (6.0 mg) was placed in an aluminum pan and heated from 25 to 1000 °C at the rate of 10 °C min-1 under flowing N2(g) (10 mL min−1). 13

C and 1H Nuclear Magnetic Resonance (13C-NMR and 1H-NMR) Spectroscopy: 13C-NMR and 1H-

NMR spectra were recorded on a Varian Mercury Plus BB 300 MHz spectrometer operating at 75.34 MHz for 13C with a contact time of 1 ms and recycle time of 1.36 s. DMSO-d6 was used as the solvent. Differential Scanning Calorimetry (DSC): DSC data were obtained using a DSC-50 calorimeter (Shimadzu) previously calibrated with an indium standard. The sample was placed on an aluminum pan and then heated from 25 to 300 °C at the rate of 5°C min-1 under flowing N2(g) (10 mL min−1). Inductively Coupled Plasma Mass Spectrometry (ICP-MS): ICP-MS was obtained with an Agilent 7900 (Agilent Technologies, Tokyo, Japan), equipped with an Octapole Reaction System (ORS4) and quadrupole mass analyzer. The sample (20 mg) was digested in piranha solution.

RESULTS AND DISCUSSION Synthesis of cobalt nanoparticles. The catalyst nanoparticles were synthesized by a common reduction approach using sodium borohydride as the reducing agent. TA is widely used to stabilize noble-metal nanoparticles and avoid their agglomeration, and here, it was used for the same purpose. The progress of the cobalt nanoparticle reduction was monitored by sequential imaging, as seen in Figure S1. The brown color solution at the start of the reaction (1 min) suggests the formation of ultrasmall cobalt nanoparticles after the addition of sodium borohydride. After 2 h of reaction, the solution displays a dark blue/tannin color, indicating the end of the reaction. By simple ethanol solvent evaporation at 80 °C, the material was collected as a dark powder for further characterization. The FT-IR spectrum of the powder shows the characteristic bands of TA (Figure S2), indicating that the TA structure is preserved. Indeed, the main bands 6 ACS Paragon Plus Environment

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for the carboxylic (νC=O at 1723 cm-1) and hydroxyl (νOH at 3444 cm-1) groups are observed, indicating that the TA structure is preserved after the nanoparticle synthesis. In order to visualize the morphology of this material, SEM images were obtained and an EDS analysis was carried out, as shown in Figure 1 (a) and Figure 1 (b), respectively. The material presents a spherical morphology resulting from the use of TA. In addition, the EDS data confirm the presence of cobalt in the material, as seen in the EDS spectrum, Figure 1 (b). The TEM microscopy images in Figure 1 (c) and Figure 1 (d) show extremely small nanoparticles well dispersed in an organic spherical structure. The nanoparticles have an average size of 3.07 ± 0.59 nm, see Figure S3 (a). The size of the organic spherical structure containing the nanoparticles was calculated to be 60.23 ± 10.14 nm, see Figure S3 (b).



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Figure 1: (a) SEM image and (b) EDS spectrum of the material obtained after ethanol evaporation. (c-d) Bright-field TEM images of the ultrasmall cobalt nanoparticles, taken at magnifications of x130,000 and x330,000, respectively. The origin of the organic spherical structure as a dendrimer may be related to hydrogen bonding and electrostatic interactions between TA molecules. TGA of the material (Figure S4) shows several weight loss steps above 200 °C. The first step at 46 °C is attributed to the evaporation of ethanol used in the synthesis, and the second step at 240 °C is attributed to the decomposition of TA, as reported in the literature39. The steps in the broad range of 250–450 °C are presumably related to polymorphic structures of cobalt oxide (CoyOx, where y and x are the number of cobalt and oxygen atoms, respectively), which are converted into Co3O4 at 450 °C.

PET glycolysis. Most papers published on PET glycolysis processes indicate the use water to separate and precipitate BHET after depolymerization. Herein, it is demonstrated that BHET precipitation can be achieved without water as the precipitating agent (see the SI step-by-step protocol guide). PET glycolysis using 1.5 wt% of the catalyst was performed first for 3 h at 180 °C. The powder obtained was analyzed by

13

C-NMR and 1H-NMR spectroscopy in DMSO-d6. The

13

C-

NMR spectrum after 3 h of glycolysis reaction is shown in Figure 2. The signals at 59.0, 67.0, 166.0, 134.0, and 130.0 ppm in Figure 2 are attributable to the BHET monomer reported in the literature20,22,40. Furthermore, the signal at ca. 63.0 ppm corresponding to the BHET dimer is not observed in this spectrum. The 1H-NMR spectrum (see Figure S5) confirms that the powder collected is BHET, as it displays the signals for all BHET hydrogens in the correct proportion. These results demonstrate that BHET precipitation from the EG glycolysis reaction solution can be achieved at 0 °C after 24 h.


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Figure 2:

13

C-NMR spectrum of BHET obtained from PET glycolysis using ultrasmall cobalt

nanoparticle catalyst. The DSC curves corroborated the

13

C-NMR and 1H-NMR spectral results, as only an

endothermic peak at 111 °C was observed after 2, 3, and 4 h of glycolysis reaction for the isolated BHET powder (Figure 3). Such a sharp endothermic peak at 111 °C is related to the melting point of BHET. The endothermic peak at 170 °C attributed to the BHET dimer16 was not observed, demonstrating again that the glycolysis process does not produce BHET dimers. It is noteworthy that 3 h is the minimum glycolysis time required to obtain a good BHET yield.



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Figure 3: DSC curves of BHET obtained after 2, 3 and 4 h of PET glycolysis. The effect of the reaction time on the PET glycolysis conversion is presented in Figure 4 (a), showing that the BHET yield depends on the reaction time. After 3 h of reaction, a yield of ~77% was achieved, while 2 h of reaction yielded only 49% of BHET. After 4 h, a drop in the BHET yield to 70% was observed. The performance of the catalyst contained in the EG solution previously employed in other glycolysis runs was evaluated, as shown in Figure 4 (b). In these experiments, the solution containing the catalyst was successfully reused in four cycles. It was not possible to recover the catalyst from the solvent as it was highly dispersed in it. In addition, water was not required to precipitate BHET, thus enabling the reuse of the solvent–catalyst system. In addition, we noted certain catalyst losses during the BHET filtration process, see Scheme 1. Nevertheless, the BHET production yield remained at around 80%, demonstrating the great efficiency of the PET glycolysis reaction and the feasibility of this process.


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Figure 4: (a) BHET yield as a function of time. (b) BHET yield obtained in each PET glycolysis cycle after the reuse of EG with cobalt nanoparticles. The BHET material isolated from the five cycles was subjected to

13

C-NMR and DSC

analyses, as shown in Figure 5 (a) and Figure 5 (b), respectively. Again, the 13C-NMR and DSC data confirmed that only BHET was produced in the five runs. All the signals for the BHET monomer were present in the

13

C-NMR spectrum. In addition, the DSC curves displayed only

the same sharp peak at 111 °C. The reusability of the EG solution containing the catalyst and the results of the 13C-NMR and DSC analyses demonstrate that the BHET production and yield of this catalytic reaction does not change for at least five cycles. However, the PET conversion started at 100% in the first cycle and decreased throughout the runs to proximally 80%, (Figure S6). These results may be explained by the catalyst losses during the filtration of BHET in each cycle, which may reduce the PET conversion while maintaining the BHET yield. The glycolysis of PET is usually accompanied by the production of dimer and oligomers. The conversion of PET is 96% and the yield of BHET is 77%. This means that more than ~20% of either dimer or oligomers are produced. In order to investigate this issue, 13C-NMR spectrum of the EG solution after the PET glycolysis reaction (see Figure S7) was obtained. No peak for higher macromolecules (dimers and trimers) was observed, which was attributed to high concentration of EG. In the case of some dimers remain in solution; these can be depolymerized in a further glycolysis cycle.



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Figure 5: (a) 13C-NMR spectra and (b) DSC curves of BHET obtained after five glycolysis cycles using EG/cobalt nanoparticles. 12 ACS Paragon Plus Environment

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The PET depolymerization reaction was also performed with cobalt chloride and TA as the catalyst under the same conditions used for PET glycolysis (3 h and 180 °C); the results for the PET conversion are presented in Figure S8. No significant catalytic activity toward PET conversion was observed when using these reagents. The glycolysis reactions carried out without a catalyst or with just EG afforded 15% conversion. When cobalt chloride was used as the catalyst, the conversion increased by ~35%; such a catalytic activity has already been described in detail in the literature19. Interestingly, using TA as the catalyst led to a reduction in the conversion to ~3%, Figure S8. A large number of PET depolymerization processes with various catalysts have been reported in the literature. In addition to the ultrasmall nanoparticles used in this work, the precipitation of BHET was here achieved directly from the EG solution. This means that only a simple filtration step is necessary to remove BHET from the solvent, as seen in Scheme 1. Furthermore, from an industrial point of view, if no water is added to EG, this solvent can be used again in the catalytic process without further treatment. Thus, these features make our PET depolymerization process more environmentally friendly and economical. This is feasible because BHET is the main product obtained from the PET depolymerization process using this catalyst. Table 1 compares the data collected from the literature on PET depolymerization and PET glycolysis. To the best of our knowledge, the size of the cobalt nanoparticles used in this PET glycolysis process is the smallest among those reported in the literature so far. In addition, even with a slightly lower BHET yield (77%) as compared to that in previous reports, we can say that this catalyst is very effective for BHET production at 180 °C and 1 atm pressure. This demonstrates that our catalyst is very promising for PET glycolysis processes.



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Table 1: Comparative data collected from the literature on PET glycolysis and ultrasmall cobalt nanoparticles. Catalyst Material

Size

BHET Yield (%)

Conditions

Ref.

Temperature/Pressure ZnO

70 – 100 nm

Co3O4

40 nm

Mn3O4 Microcomposite MnO4/SMPs Microcomposite ZnO/SMPs* Nanocomposite MnO4/SNPs Nanocomposite ZnO/SNPs* Ϫ-Fe2O3 ZnO/silica CeO/silica Ultrasmall cobalt nanoparticles

o

20

o

20

o

20

o

23

o

23

o

23

o

23

o

25

85.3

o

270 C/ -

41

78.6

o

270 C/ -

41

o

This work

67 63

260 C/5 atm 260 C/5 atm

31 nm

74

260 C/5 atm

1 – 20 µm

̴ 77

300 C/1.1 MPa

1 – 20 µm

̴ 68

300 C/1.1 MPa

150 ± 10 nm

≥ 90

300 C/1.1 MPa

150 ± 10 nm

̴ 88

300 C/1.1 MPa

8 – 14 nm

90

300 C/1.1 MPa

750 nm 750 nm 3.07 ± 0.59 nm

77

180 C/1 atm

*SNPs: silica nanoparticles and SMPs: silica microparticles. Based on previous reports22,25,26 where metal oxides were used for PET glycolysis, we propose here a reaction mechanism similar to those already reported (Figure 6). The pair of free electrons present in the carbonyl moieties of PET interacts with the cobalt ions, resulting in electronic charge delocalization; consequently, the carbonyl moieties are more susceptible to nucleophilic attack by EG. At this point, the main catalytic effect is from the cobalt ions, and although TA has a very low catalytic effect, synergy between the ultrasmall cobalt nanoparticles and TA may play an important role in this glycolysis reaction.


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(a)

(b)

Figure 6: Schematic illustration of (a) organic spherical structures containing ultrasmall cobalt nanoparticles and (b) reaction mechanism of PET glycolysis. The organic spherical structure formed by TA possibly prevents the aggregation of the ultrasmall nanoparticles, and consequently, avoids their precipitation, which would otherwise reduce the efficiency of PET glycolysis. In addition, since the TA molecules bear many hydroxyl groups, they may interact with EG, promoting it into an active state, as reported in the literature26,42. This activated EG may then enhance the BHET yield of the PET glycolysis process. In addition, the ultrasmall size of these cobalt nanoparticles results in high selectivity toward BHET production owing to their good dispersity in the organic spherical structures. Therefore, a higher availability of Co cations is achieved, meaning an increased catalytic effect on the glycolysis process to produce BHET. Another point to consider here is that the PET glycolysis kinetics21,43 depends on pricked PET size used for the glycolysis reaction. Very small pricked PET sizes (1 mm) would mean faster PET depolymerization into BHET. ICP-MS results indicated that the precipitated BHET contains 2.27±0.08 ppm of cobalt, which is considered to be very low, showing that BHET is a safe monomer for reuse. Furthermore, the catalyst is easily adsorbed on precipitated BHET. This has no great influence in the reuse of BHET. 15 ACS Paragon Plus Environment


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It must be noted that the use of cobalt nanoparticles in the PET depolymerization has been previously reported. However, for commercial utilization, we propose that the nanoparticles could be pelletized into appropriate shapes, for example, in a fixed bed reactor. The pelletization process employed here is the same as that used to prepare the KBr pellets for the FT-IR analysis. By applying 2 TON of pressure on 200 mg of the catalyst with a common hydraulic press, ultrasmall nanoparticle pellets can be obtained, see Figure S9. This is possible because during the ultrasmall cobalt nanoparticle synthesis, the capping agent TA remains on the final material, as seen in the FT-IR spectrum (Figure S2), enabling the formation of hydrogen bonds during the pelletization process. The glycolysis process discussed in this paper employs low-temperature and ambientpressure conditions, as opposed to other processes reported in the literature20,22,24,44. Moreover, the glycolysis reaction time is comparable to that in other reported works. Another attractive feature of our glycolysis process is the BHET precipitation, whereby no water is required to recover the monomer. The BHET molecules somehow precipitate under these conditions (with the catalyst still present in the solvent); thus, the catalyst also plays a key role in the isolation of the BHET monomer.

CONCLUSIONS Ultrasmall cobalt nanoparticles were synthesized by a simple and easy method. This material was applied as the catalyst for the glycolysis of PET.

13

C-NMR and DSC analyses

confirmed that the BHET monomer is the sole product precipitated in at least 77% yield. The solution with the catalyst was directly recycled for at least five runs without the need for any solvent treatment. The presence of dimers in the precipitated BHET was not detected throughout the five cycles. The BHET precipitation process was found to proceed without the use of water. We suggest that the formed spherical TA structures, where the cobalt nanoparticles are highly dispersed, play an important role in this precipitation process. Furthermore, as a novel advantage of our material, the ultrasmall cobalt nanoparticles can be easily pelletized. Here, it has been demonstrated that the catalyst need not be removed from 16 ACS Paragon Plus Environment

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the solvent, facilitating its reuse, and that the precipitation of BHET does not require the use of water, rendering this a clean, sustainable, economical, and environmentally friendly process for PET recycling.

Supporting Information FTIR results, particle size distribution curves, TEM image, TGA results, 1H-NMR and

13

C-NMR

spectra, PET conversion as a function of glycolysis cycles, PET conversion with cobalt chloride, EG, and TA. Pictures of BHET, PET not depolymerized, EG, PET, ultra-small cobalt nanoparticle powder, solution of catalyst ultra-small cobalt nanoparticles, and ethanol solutions containing CoCl2.6H2O and tannic acid.

Acknowledgements The authors thank the COMCAP–UEM for the SEM and TEM analyses, and the Brazilian Agencies for fellowship CNPQ (Process: 577527/2008-8, 310820/2011-1, and 118454/2017-0), Fundação Araucária/PR (Process: 830/2013), FUNDECT/MS (Process: 23/200.146/210) and CAPES for financial support. The technical support provided by B.N. Safabi in ICP-MS analysis is also acknowledged. Conflicts of interest The authors declare no competing financial interests. References 1. Awaja, F.; Pavel, D., Recycling of PET. European Polymer Journal 2005, 41 (7), 1453-1477; (b) Welle, F., Twenty years of PET bottle to bottle recycling—An overview. Resources, Conservation and Recycling 2011, 55 (11), 865-875, DOI 10.1016/j.resconrec.2011.04.009.



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Page 24 of 25

For Table of Contents Use Only

Ulstrasmall nanoparticle catalyst

No water required to precipitate BHET BHET-to-PET conversion

Synopsis PET glycolysis is catalyzed by ultrasmall cobalt nanoparticles (~3 nm) and the precipitation of hydroxyethyl terephthalate is achieved without water.


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PET glycolysis is catalyzed by ultrasmall nanoparticles and the precipitation of BHET is achieved without water 163x98mm (150 x 150 DPI)

ACS Paragon Plus Environment

Ultrasmall Cobalt Nanoparticles as a Catalyst for PET Glycolysis: A

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