Preparing cedrene from ethylene-vinyl acetate copolymer and polyethylene terephthalate of waste solar cells

Journal of Cleaner Production 254 (2020) 120065

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Preparing cedrene from ethylene-vinyl acetate copolymer and polyethylene terephthalate of waste solar cells Baojia Qin, Mi Lin, Zhe Huang, Ruijun Qiu, Jujun Ruan*, Yetao Tang, Rongliang Qiu School of Environmental Science and Engineering, Sun Yat-Sen University, 135 Xingang Xi Road, Guangzhou, 510275, People’s Republic of China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 November 2019 Received in revised form 6 January 2020 Accepted 7 January 2020 Available online 10 January 2020

Quantities of waste solar cells have been produced due to their widespread sustainable energy applications. Due to the complicated composition of waste solar cells, the disposal of waste solar cells has been the hot spot of sustainable development and environmental protection research. To our knowledge, little information has been reported about the disposal of waste solar cells. For the safe disposal of waste solar cells, and to produce economic benefits, the present study applied the density functional theory to design the transformation pathways of organics in waste solar cells. The ethylene-vinyl acetate copolymer and polyethylene terephthalate of waste solar cells were transformed to 3,6,8,8-tetramethyl2,3,4,7,8,8a-hexahydro-1H-3a,7-methanoazulene (cedrene). It is the precursor of precious woundhealing medicine. The transformation pathways exhibited a total energy barrier of 58.79 kcal/mol, and thermodynamic calculations indicated the temperature required for breaking this energy barrier was 777.50 K. 3,6,8,8-Tetramethyl-2,3,4,7,8,8a-hexahydro-1H-3a,7-methanoazulene was produced by vacuum heating at a temperature of 778.15 K, and the material had the most substantial proportion that accounted for 19.85 wt% in the products of vacuum heating. This paper provides insight into preparing the raw materials of medicine from the nonmetallic material of waste solar cells. © 2020 Elsevier Ltd. All rights reserved.

Handling editor: Giorgio Besagni Keywords: Waste solar cell Ethylene-vinyl acetate copolymer Polyethylene terephthalate Vacuum heating Quantum chemical calculation

1. Introduction Solar energy is a kind of renewable clean energy that has been widely used in industry, agriculture, transportation, and other fields (You et al., 2013; Celik et al., 2017; Cheng et al., 2009; Osborne, 2015). Solar cells are the primary devices that convert solar energy into electricity based on the photovoltaic effect, which has been massively produced. In 2012, waste solar cells were classified as electronic wastes by the European Union (Dempsey, 2001; Fu et al., 2015; Gustafsson et al., 2014). Organic components account for 15% of solar cells, mainly including the ethylenevinyl acetate (EVA) copolymer and polyethylene terephthalate (PET), which are used as adhesives and pose a threat to the environment. However, waste solar cells contain also abundant resources, such as organic components, tempered glass, copper, silver, and high-purity polysilicon (Mcdonald and Pearce, 2012). In other words, recovering waste solar cells can be valuable for sustainable development and the recycling of resources.

* Corresponding author. E-mail address: [email protected] (J. Ruan). https://doi.org/10.1016/j.jclepro.2020.120065 0959-6526/© 2020 Elsevier Ltd. All rights reserved.

The reported technologies of recovering waste solar cells are still in their infancy. Kang et al. (2012) reported a new procedure to recover high-purity silicon (99.99%) from waste photovoltaic modules. Doi et al. (2001) proposed an organic solvent method to recover silicon cells from crystalline silicon PV modules and suggested that dissolving EVA by trichloroethylene can recover silicon cells without any additional damage. Zhang and Xu (2016) employed nitrogen pyrolysis and vacuum-gasificationcondensation to recover organic components, glass, and gallium from solar cell modules. However, these technologies only acquired a single portion or a mixture of low-value components from waste solar cells, and they somehow produced pollutants during the recovery process. In this study, we pursued a more eco-friendly, efficient, and economical technique for high-value waste solar cell recovery. The density functional theory (DFT) is widely used in explaining the reaction mechanism in the complex system via forecasting the physical and chemical properties of the molecule, the chemical reaction pathway, and kinetic characteristics (Li et al., 2016; Ping et al., 2012). The reaction pathway of partial oxidation and steam reforming of dimethyl ether under the cold plasma condition were investigated by Pan and Liu (2007). Pan et al. (2007) also obtained

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the key steps and the optimum temperature of cold plasma electrons by using DFT calculations. Jarvis et al. (2011) used phenethyl phenyl ether as a model compound to study the pyrolysis pathways of polyphenylene ether at different temperatures by the DFT calculations and experiments. Wang et al. (2001) studied the thermodynamics and kinetics of the pyrolysis of methyl benzene by quantum chemical calculations and provided a theoretical basis for the formation of methane through the pyrolysis of methyl benzene. To realize the safe disposal and obtain high-value products, DFT can be a novel approach for designing the co-disposal products of EVA and PET in vacuum heating. Thermal decomposition was confirmed to be a green technology to dispose solid wastes, such as printed circuit board and toner (Ruan et al., 2017, 2018; Qiu et al., 2019; Huang et al., 2019). To our knowledge, the heating process generates free radicals due to macromolecular bond breakage, thereby allowing the combination of these free radicals to form a new compound (Dong et al., 2017; Chen et al., 2017). Therefore, the free radical reactions can be a pivotal issue in the decomposition and reformation of EVA and PET. On this basis, the present study applied the DFT calculation to analyze and design the decomposition process of EVA and PET from the waste solar cell. The highvalue product was simultaneously configured via the codecomposition of EVA and PET. In this study, we designed and verified a new decomposition pathway of EVA and PET to form a specific compound. The reaction energy, reaction transition state, and electron transfer between molecular orbitals of the decomposition process were obtained via DFT calculations. The vacuum heating experiments for decomposing EVA and PET were performed to confirm the parameters from the DFT calculations. This paper provides a new method to dispose of the organic components of waste solar cells with green and high value-added benefits. 2. Experimental setup 2.1. Density functional theory The present study employed Materials Studio software. DFT calculations were accomplished via the DMol3 module to obtain the information on the reaction, including the Gibbs free energy (DG), transition state, and molecular orbitals. During the transition state calculations, the molecular structures of the reactants and products were established and optimized, which then were paired with the atoms to establish the reaction pathway. At the same time, the calculation task was set up as TS Search. GGA was chosen as the calculation method, BP was used as the functional form, DND was used as the basis set, and medium was used as the convergence parameter to calculate the Frequency. The transition state was calculated and verified, and its frequency was queried. A transition state with only one virtual frequency was considered as reasonable. The transition state was then further optimized, and the calculation task was set up as TS Optimization. The optimized transition state was calculated with other settings unchanged. During the reaction DG calculation, the molecular structure was established and the calculation task was set up as Geometry Optimization. GGA was used as the calculation method, BLYP was used as the functional form, DND was used as the basis set, and medium was used as convergence parameter to calculate the frequency. The calculated structural file was analyzed by Thermodynamic Properties, the corrected data was obtained in the output file, and the DG of the reaction was calculated by Equation (1):

DGTreaction

¼

h

i

ETTcorr ðproductÞ  ETTcorr ðreactantÞ

 627:51

where DGTreaction is the free energy of the reaction at a certain temperature (kcal/mol); ETTcorr ðproductÞ is the energy of the product at a certain temperature (Ha); and ETTcorr ðreactantÞ is the energy of the reactant at a certain temperature (Ha). During the calculation process of molecular orbitals, the molecular structure was established and the calculation task was set up as Geometry Optimization. LDA was chosen as the calculation method, PWC was used as the functional form, DND was used as the basis set, and medium was used as convergence parameter to calculate Orbitals. In the Orbitals analysis on the calculated structure file, the Highest Occupied Molecular Orbital (HOMO) and Lowest Unoccupied Molecular Orbital (LUMO) of the molecule were set as the output, and the energy eigenvalue of the orbit were obtained. 2.2. EVA and PET of the waste solar cells The solar cells were provided by the Trina Solar Corporation in Changzhou, Jiangsu Province, China. The solar cell had an EVA-toPET ratio of approximately 2:1. The EVA and PET mixture was collected after the crushing and separation of the waste solar cell. The mixture and polymer molecular structure of EVA and PET are presented in Fig. 1. 2.3. Apparatus and TGA analysis The vacuum tube furnace was manufactured by the Shanghai Alarge Furnace Co., Ltd. in Shanghai, China (Fig. S1). It consisted of three temperature sections, a control panel, a vacuum ionization gauge, a diffusion pump, and a mechanical vacuum pump. The three temperature sections were equipped with independent silicon molybdenum rod heating elements, and the temperature of each zone in the system was controlled separately by the control panel (Qin et al., 2019). Section A was the heating area, and sections B and C were the condensation areas. The EVA and PET samples were decomposed in Section A, and the products were condensed in Section B and C at condensation temperatures of 453 K and 353 K, respectively. The decomposition process was investigated using a Thermal Gravimetric Analyzer (TGA; NETZSCH TG 209). EVA and PET were fed into the sample holder and then heated from 303.14 K to 1173.14 K at a constant heating rate of 5 K/min. Nitrogen (N2) gas was purged at a flow rate of 30 mL/min and balanced at a flow rate of 40 mL/min. The products were investigated by Gas Chromatography-Mass Spectrometry (GC-MS; Voyager). Both helium and nitrogen were set at a pressure of 0.5 MPa, and were vacuumed for 4 h. The sample volume was set at 0.5 mL with four plunger strokes through a TG-5SilMS column. 2.4. Reaction kinetics in the heating process The reaction kinetics mainly expressed the effects of the factors during the heating process (Sierra, 2012). The fundamental equation of the kinetics can be expressed as Equation (2):

da ¼ kðTÞf ðaÞ dt

(2)

where a is the decomposition rate and k is the reaction rate constant, which can be expressed as Equation (3):

  Ea k ¼ A exp  RT

(3)

(1) where Ea is the energy barrier (kJ/mol); A is the frequency factor (min1); T is the thermodynamic temperature (K); and R is the

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Fig. 1. (a) EVA and PET mixture from the waste solar cell; (b) Polymer molecular structure of EVA; (c) Polymer molecular structure of PET.

molar gas constant (8.314 J/(mol$K)). Equation (3) expresses the relationship between Ea and T. f ðaÞ is expressed as a function of the reaction rate and mass loss rate. The pyrolysis reaction commonly follows Equation (4):

f ðaÞ ¼ ð1  aÞn

(4)

where n is the reaction order, wherein the decomposition process of mixed solid organic matter contained many elementary reactions due to its complicated composition and following pyrolysis. Thus, the entire pyrolysis process was actually a total package reaction that contained several elementary reactions. The present study set the value of n as 1 given that the reaction order of elementary reactions is generally 1. The decomposition reaction employed constant heating rate b, which can be expressed as Equation (5):

b¼

dT dt

(5)

According to Equation (2), Equation (3), Equation (4), and Equation (5), the established reaction kinetic equation can be expressed as Equation (6):

0

1

  A Ea B C  ln@ A ¼ ln 1a b RT da dT

(6) 0

1

da B dT C The corresponding relationships between ln@1 aA and

1 T

were

fit to obtain k and A. According to Equation (3), the relationship between Ea and T can be determined. 3. Results and discussion 3.1. Decomposition of EVA and PET in vacuum heating In this study, the monomers of EVA and PET were selected as the model compounds. The bond lengths of the EVA and PET monomers are shown in Fig. 2. In general, longer bond lengths indicate a smaller bond energy that is more prone to breakage. The decomposition reaction started from the thin spot of molecules. The chemical bonds of the EVA and PET molecules were fractured due to the sufficient energy supply. According to Fig. 2, for the EVA monomer molecule, the CeC bond of CH3eCH2CH(OCOCH3)CH3 was the longest, and as such it

Fig. 2. (a) Bond length of the EVA monomer; (b) Bond length of the PET monomer.

may be preferentially fractured. The free radicals CH3 and CH3CH(OCOCH3)CH2 were generated by the breakage of the CeC bond of CH3eCH2CH(OCOCH3)CH3. The reaction pathway and the reaction enthalpy change were analyzed and calculated. The results are shown in Fig. 3. As shown in Scheme 1 in Fig. 3, the transient state (TS1) proceeded in the breakage of the CeC bond of CH3eCH2CH(OCOCH3)CH3, which exhibited an reaction enthalpy change of 0.178 kcal/mol and a reaction energy barrier of 8.655 kcal/mol. With the continuous supply of energy, CH3CH(OCOCH3)CH2 may be further decomposed. The CeC bond of CH3eCH(OCOCH3)CH2 also exhibited a thin spot on its molecule structure that would be fractured. The free radicals CH3 and CH(OCOCH3)CH2 were generated in the system following the breakage of the CeC bond. The reaction pathway and the reaction enthalpy change are shown in Scheme 2 of Fig. 3. The breakage of CeC bond of the CH3CH(OCOCH3)CH2 suggested a transient state (TS2), thereby exhibiting a reaction enthalpy change of 34.305 kcal/ mol and a reaction energy barrier of 35.464 kcal/mol. As such, the generated free radical CH(OCOCH3)CH2 was further decomposed. The chemical bonds of CH(OCOCH3)CH2 may further break, with the breakage of CeC in the previous two steps. The CeO bond of CH2CHeOCOCH3 became relatively weak and was further fractured. The reaction pathway and the reaction enthalpy change of breakage of the CeO bond were analyzed (Scheme 3 in Fig. 3). Breakage of the CeO bond of CH2CHeOCOCH3 indicated a transient state (TS3) that exhibited a reaction enthalpy change of 3.588 kcal/mol and a reaction energy barrier of 5.439 kcal/mol. The free radicals CHCH2 and CH3COO were subsequently generated in the system. The PET molecular monomers were simultaneously decomposed during the decomposition of the EVA molecular monomers.

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Fig. 3. Decomposition of EVA and radical formation.

According to Fig. 2, the HOOCC6H4COOCH2eCH2OH molecule exhibited the longest CeC bond, indicating its preferential fracturing. With the breakage of the CeC bond of HOOCC6H4COOCH2eCH2OH, the free radicals HOOCC6H4COOCH2 and CH2OH were produced. The reaction pathway and the reaction enthalpy change were analyzed and are shown in Scheme 5 of Fig. 4. CeC bond breakage of HOOCC6H4COOCH2eCH2OH presents the transient state (TS4), which exhibited a reaction enthalpy change of 0.29 kcal/mol and reaction energy barrier of 1.774 kcal/mol. The free radical CH2OH was then further decomposed. On the basis of the structure of CH2OH, the CeO bond of CH2eOH was the longest and relatively weak, suggesting its breakage to create new products. The reaction pathway and the enthalpy change following CeO bond breakage were analyzed. The results are shown in Scheme 6 of Fig. 4. The process of CeO bond breakage exhibited a reaction enthalpy change of 0.017 kcal/mol, indicating the transient state (TS5) and an energy barrier of 7.454 kcal/mol. The new free radical OH and free radical CH2 were produced in the system. Moreover, the EVA and PET molecules exhibited other fractured paths to produce other free radicals, which could be combined to generate other products. In this study, we only discussed the formation of free radicals required for the target product. The whole decomposing process required a total energy supply of up to 58.79 kcal/mol. During this process, the free radical CHCH2 and free radical CH2 were formed in this system. 3.2. Reaction thermodynamic According to the above DFT calculations, the total energy barrier was up to 58.786 kcal/mol during the decomposition of EVA and

PET. As such, it was necessary for the decomposition of EVA and PET to absorb 58.786 kcal/mol of heat from outside. According to the thermodynamic analysis, the temperature corresponding to the energy barrier could be obtained. TGA was performed on EVA and PET, and the TG and derivative thermogravimetric (DTG) curves of the mixture of EVA and PET are presented in Fig. 5. The temperature interval of the highest decomposition rate (659.14 Ke734.14 K) was selected to fit the reaction kinetics. According to Equation (5) and 0 1 da B dT C 1 TGA, the corresponding relationships between ln@1 aA and T was

obtained and is shown in Fig. 5. The fitting linear relationship can be expressed as (7):

y ¼ ax þ b

(7)

where a is the slope of the fitting linear and b is the intercept of the fitting linear. The energy barrier Ea and frequency factor A were obtained from the values of a and b, respectively. The values of a and b were 27923.15 and 34.24 from the fitting linear. According to Equation (6), Ea and A can be expressed as (8) and (9):

Ea ¼  R  a

(8)

A ¼ expðbÞ  b

(9)

Ea was calculated to be 232.15 kJ/mol, which meant when the temperature T reached 734.14 K, the reaction presented an energy barrier of 232.15 kJ/mol. Meanwhile, the frequency factor A was 3.70  1015 min1. Substituting these values into Equation (3), the

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Fig. 4. Decomposition process of PET and radical formation.

Fig. 5. (a) TG and DTG curves of EVA and PET; (b) Reaction kinetics curve and its fitting linear.

reaction rate constant k was calculated to as 0.11 min1. According to Equation (2) and the kinetic parameters, we concluded that the temperature required to break through the energy barrier of 58.79 kcal/mol was 777.50 K during the decomposition process. 3.3. Combination of radicals and the formation of the product According to the above DFT and thermodynamics analyses, the free radicals CHCH2 and CH2 were mainly formed in the system

when the temperature reached 777.50 K. Because CHCH2 and CH2 had multiple unpaired electrons in the system, different electrons combined with each other to form different products. Hence, the free electron of CH2 would bond with the free electron on the bilateral C of CHCH2, and the free radicals CH2CHCH2 and CH2CH2CH were produced, respectively. Fig. 6 presents the two combination methods between CHCH2 and CH2. The DG (778.15 K) of the two reactions were calculated to be 52.34 kcal/ mol and 43.52 kcal/mol, respectively. The two combination pathways were both exothermic and were able to occur

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Fig. 6. Combinations, formed products, and HOMO-LUMO orbits of the free radicals CH2 and CH2CH.

spontaneously in the system. Bigger absolute values of DG of an exothermic reaction indicates that the reactions proceeded more easily. The CH2CHCH2 was generated when the CH2 combined with the CH of CHCH2. Meanwhile, the free electron of the HOMO energy level was transferred to the LUMO energy level, forming a chemical bond and generating the corresponding products. According to molecular orbit theory (Fukui, 1970), the molecular orbitals of the reaction process should satisfy the following conditions: (1) symmetric matching of reactant molecular orbitals; (2) similar energy levels of reactant molecular orbitals (the low value of EHOMO-LUMO); and (3) maximum overlap of the reactant molecular orbital. The HOMO and LUMO of CHCH2 and CH2 are shown in Fig. 6, wherein the HOMO of CHCH2 and the LUMO of CH2 exhibited symmetry. EHOMO-LUMO of HOMO of CHCH2 and LUMO of CH2 was 0.0666 Ha, which was lower than the EHOMO-LUMO of HOMO of CH2 and LUMO of CHCH2 (0.0779 Ha). Lower EHOMO-LUMO values indicate lower electronic transfer barriers. Meanwhile, the orbitals exhibited more overlapping when the CH of HOMO of CHCH2 combined with the LUMO of CH2, indicating combined energy and the formation of a more stable product. Thus, when CH2 combined with CHCH2, the free electron transferred to the CH2 LUMO from the CH of HOMO of CHCH2, forming CH2CHCH2. The production of CH2CHCH2 allowed its bonding to other free radicals. The combination of CH2CHCH2 with CHCH2 allowed the free electrons of CH and CH2 from CH2CHCH2 to combine with the free electrons of CH and CH2 from CHCH2. The four combination pathways of CH2CHCH2 and CHCH2 are presented in Fig. 7. As such, CH2CH(CH2)CHCH2, CHCH2CH(CH2)CH2, CH2CHCH2CHCH2, and CH2CHCH2CH2CH were produced, which had DGs (778.15 K) of 32.46 kcal/mol, 11.12 kcal/ mol, 20.43 kcal/mol, and 27.22 kcal/mol, respectively. CH2CH(CH2)CHCH2 had the lowest DG (778.15 K) for its generation. The HOMO and LUMO of CH2CHCH2 and CHCH2 are shown in Fig. 7. The CH of HOMO of CH2CHCH2 had symmetry with the CH of LUMO of CHCH2. The EHOMO-LUMO between the HOMO of CH2CHCH2 and the LUMO of CHCH2 was calculated as 0.0493 Ha,

which is lower than the EHOMO-LUMO (0.0778 Ha) between the HOMO of CHCH2 and the LUMO of CH2CHCH2. Meanwhile, the combination of the CH of LUMO of CHCH2 with the CH of HOMO of CH2CHCH2 indicated more orbital overlapping. Therefore, when CH2CHCH2 bonded with CH2CH, the free electron then transferred from the CH of the HOMO of CH2CHCH2 to CH of the LUMO of CHCH2, thus forming CH2CH(CH2)CHCH2. The free radical CH2CH(CH2)CHCH2 had four unpaired electrons. The structure was very unstable because the unpaired electrons interacted with each other to form the more stable structure (Harman, 1956). According to Fig. 8, CH2CH(CH2)CHCH2 translated into CH2C(CH3) ¼ CHCH2, and the DG (778.15 K) of this reaction was 105.33 kcal/mol, indicating it was an exothermic reaction. Meanwhile, 3,6,8,8-tetramethyl-2,3,4,7,8,8a-hexahydro-1H-3a,7methanoazulene is a kind of terpenoid, and its form followed the experimental isoprene rule. Isoprene (C5H8) was considered to be a basic unit and could connect with each other to form terpenoids (Sun, 2004). Therefore, the monomer of isoprene polymers CH2C(CH3) ¼ CHCH2 was formed in the system. The selfcombination of the monomer of isoprene polymers then synthesized the terpenoids. In this study, three monomers of isoprene polymers combined with themselves, and the results are shown in Fig. 8. The DG (778.15 K) of this reaction was 154.48 kcal/mol; the combination pathway was exothermic, and the molecular orbitals were symmetric. The free electron of HOMO of CH2C(CH3) ¼ CHCH2 transferred to the LUMO of CH2C(CH3) ¼ CHCH2 to form a chemical bond. The reaction was spontaneous in the system, thus generating 3,6,8,8-tetramethyl2,3,4,7,8,8a-hexahydro-1H-3a,7-methanoazulene. 3.4. Vacuum heating experiment of EVA and PET According to the above calculation results, 3,6,8,8tetramethyl-2,3,4,7,8,8a-hexahydro-1H-3a,7-methanoazulene was generated in the co-decomposition of EVA and PET. Therefore, vacuum heating experiments were introduced to verify the calculation results. EVA and PET were decomposed in the vacuum tube furnace. According to the reaction kinetic

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Fig. 7. Combinations, formed products, and HOMO-LUMO orbits of the free radicals CHCH2 and CH2CHCH2.

calculations, the temperature of section A was set as 778.15 K, the heating rate was set as 5 K/min, and the retention time was set as 30 min. The samples were decomposed and gasified in section A. The gasified materials were then cooled down and collected in section B (453 K) and section C (353 K). An approximately 6.0 g mixture of EVA and PET was fed into the crucible placed in section A. After the vacuum heating process, an approximately 1.950 g residue was remained in crucible A. The gasification products were condensed in sections B and C. Approximately 2.592 g and 0.673 g of condensation oils were collected in sections B and C, respectively. No gas was collected in the whole vacuum heating process. Subsequently, 0.785 g oil was diffused on the inner surface of the alundum tube of the furnace. GC-MS was used to analyze the compositions of the condensed oils and the results were presented in Fig. 9. GC-MS revealed the molecular structures and contents of the condensation oils. The components of the condensed oils in sections B and C whose mass percentages were greater than 1.0 wt% are presented in Table 1. In section B, eight substances had the proportions greater than 5.0 wt %.3,6,8,8-Tetramethyl-2,3,4,7,8,8a-hexahydro-1H-3a,7methanoazulene had the largest proportion, accounting for 19.85 wt%. The second and third proportions were butyl acetate and 2,6,10,15-tetramethylheptadecane, accounting for 16.21 wt% and 11.68 wt%, respectively. In section C, six substances had the proportions greater than 5.0 wt%, to which 3,6,8,8-tetramethyl-

2,3,4,7,8,8a-hexahydro-1H-3a,7-methanoazulene had the second proportion, accounting for 12.15 wt%. The first and third proportions were 2,6,10-trimethyltetradecane and diisobutyl phthalate, which accounted for 35.96 wt% and 11.54 wt%, respectively. 3,6,8,8-Tetramethyl-2,3,4,7,8,8a-hexahydro-1H-3a,7methanoazulene was observed in the condensed oils following the vacuum heating process of EVA and PET. However, the yield of the main product was not sufficient. Further studies should be conducted to improve the yield of the main product.

4. Conclusions To obtain high-value products from the disposal of EVA and PET of waste solar cell, the present study applied the DFT method to calculate the optimum heating pathway and vacuum heating to verify the 3,6,8,8-tetramethyl-2,3,4,7,8,8a-hexahydro-1H-3a,7methanoazulene (cedrane) formation from the decomposition of EVA and PET. The transform route was designed according to the energy calculation of the EVA and PET decomposition via DFT. Projected vacuum heating was then implemented, and 3,6,8,8tetramethyl-2,3,4,7,8,8a-hexahydro-1H-3a,7-methanoazulene was successfully formed in the oil products, which accounted for the most substantial proportion (19.85 wt%). This final product is an expensive raw material that is employed in the manufacturing of wound-healing medicine. As such, this study realized the highly valued recycling of waste solar cells.

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Fig. 8. HOMO-LUMO orbits of the free radical CH2C(CH3) ¼ CHCH2 and the formation of 3,6,8,8-tetramethyl-2,3,4,7,8,8a-hexahydro-1H-3a,7-methanoazulene.

Fig. 9. (a) Components of the oil collected from the condensing temperature of 453 K; (b) Components of the oil collected from the condensing temperature of 353 K.

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Table 1 Components of the condensation oils in section B and section C. Condensation temperature 453 K (Section B)

Condensation temperature 353 K (Section C)

Components

Chemical formula

Proportion (>1.0 wt%)

Components

Chemical formula

Proportion (>1.0 wt%)

Butyl acetate N-(4-bromobut-1-yl) piperidin-2-oneO2-Methylhexadecan-1-ol 2,6,10-Trimethyltetradecane 3,6,8,8-Tetramethyl-2,3,4,7,8,8a-hexaHydro-1H3a,7-methanoazulene Dibutyl phthalate Diisobutyl phthalate 6-Methyloctadecane

C6H12O2 C9H16BrNO C17H36O C17H36 C15 H24

16.21 1.84 1.69 10.55 19.85

C6H12O2 C9H16BrNO C17H36O C17H36 C15H24

6.31 2.29 3.50 35.96 12.15

C16H22O4 C16H22O4 C19H4O

5.39 9.75 8.86

C16H22O4 C16H22O4 C23H30N2O5

6.70 11.54 1.30

All-trans-retinal 2,6,10,15-Tetramethylheptadecane Potassium,2,6-ditert-butylphenolate Hexadecane 1,1-Didodecoxyhexadecan e Octadecane, 6-methyl-

C20H28O C21H44 C16H34S C16H34 C40H82O2 C19H40

1.94 11.68 1.10 6.76 1.55 1.60

Butyl acetate N-(4-bromobut-1-yl) piperidin-2-oneO2-Methylhexadecan-1-ol 2,6,10-Trimethyltetradecane 3,6,8,8-Tetramethyl-2,3,4,7,8,8a-hexaHydro-1H3a,7-methanoazulene Dibutyl phthalate Diisobutyl phthalate Aspidospermidin-17-ol, 1-acetyl-19,21-epoxy15,16-dimethoxyGeranyl isovalerate 12-Methyl-E,E2,13-octadecadien-1-ol 2,5-Furandione, 3-dodecylPhthalic acid, isobutyl octadecyl ester 7-Methyl-Z-tetradecen-1-ol acetate 13-Heptadecyn-1-ol

C15H26O2 C19H36O C16H26O3 C30H50O4 C17H32O2 C17H32O

1.15 5.22 1.59 1.03 3.45 1.89

Authors contribution: Baojia Qin wrote the manuscript, Mi Lin prepared Figure 5, Zhe Huang and Ruijun Qiu helped Baojia Qin to prepare Figure 2, Figure 3, and Figure 6, Jujun Ruan designed and revised the manuscript, Yetao Tang and Rongliang Qiu revised and improved the manuscript. Declaration of competing interest The authors declare no competing interest. Acknowledgements This work was supported by the National Key R&D Project of China (Grant no. 2019YFC1604600), the 111 Project (Grant no. B18060), the Natural Science Foundation of Guangdong Province, China (Grant no. 2019A1515011448), the Science and Technology Planning Projects of Guangdong Province (Grant no. 2019B110209001), and the Pearl River Star of Science and Technology (Grant no. 201710010032). The authors are grateful to the reviewers who helped us improve the paper with many pertinent comments and suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2020.120065. References Celik, I., Mason, B.E., Phillips, A.B., Heben, M.J., Apul, D., 2017. Environmental impacts from photovoltaic solar cells made with single walled carbon nanotubes. Environ. Sci. Technol. 51, 4722e4732. Chen, Y., Zhang, L., Xu, Z., 2017. Vacuum pyrolysis characteristics and kinetic analysis of liquid crystal from scrap liquid crystal display panels. J. Hazard Mater. 327, 55e63. Cheng, Y., Yang, S.H., Hsu, C.S., 2009. Synthesis of conjugated polymers for organic solar cell applications. Chem. Rev. 109 (11), 5868e5923. Dempsey, P.E., 2001. Solar PV markets surge in the United States and Canada. Environ. Sci. Technol. 35 (3), 62A-62A. Doi, T., Tsuda, I., Unagida, H., Murata, A., Sakuta, K., Kurokawa, K., 2001. Experimental study on PV module recycling with organic solvent method. Sol. Energy Mater. Sol. Cells 67, 397e403. Dong, L., Huang, Z., Ruan, J., Zhu, J., Huang, J.X., Huang, M.Z., Kong, S.F., Zhang, T.,

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