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Thermal decomposition mechanisms of poly(vinyl chloride): A computational study
Waste Management xxx (2018) xxx–xxx
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Thermal decomposition mechanisms of poly(vinyl chloride): A computational study Jinbao Huang a, Xinsheng Li b,⇑, Guisheng Zeng c,⇑, Xiaocai Cheng a, Hong Tong a, Daiqiang Wang a a
School of Mechatronics Engineering, Guizhou Minzu University, Guiyang 550025, China School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China c Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China b
a r t i c l e
i n f o
Article history: Received 23 January 2018 Revised 9 March 2018 Accepted 19 March 2018 Available online xxxx Keywords: Poly(vinyl chloride) Thermal decomposition mechanisms Density functional theory
a b s t r a c t The studies on the pyrolysis mechanisms of waste PVC contribute to development and application of pyrolysis technology for mixed waste plastics. In the article, the thermal decomposition mechanisms of model compound of poly(vinyl chloride) (PVC) have been investigated by employing density functional theory methods at M06-2X/6–31++G(d,p) level in order to illuminate the elimination of HCl and the formation of hydrocarbons. Various possible pyrolysis paths for the formation of main products were proposed, and the thermodynamic and kinetic parameters in every path were calculated. The calculation results show that the HCl elimination can occur through the concerted reaction and the energy barrier of HCl elimination changes from 167.4 to 243.3 kJ/mol; allyl group can obviously reduce the activation energy of HCl elimination, and the branched-chain can lower the energy barrier of HCl elimination at the carbon sites near the branch chain; a free radical is more easily converted into aromatic compound through a series of isomerizations, cyclization and dehydrogenation; the conjugated polyene could be decomposed in parallel reaction channels: one is the evolution of aromatics, another is the formation of small molecule products. The above analysis is consistent with previous experimental results and analysis. Ó 2018 Published by Elsevier Ltd.
1. Introduction Plastic products are highly convenient for people’s use on account of their resistance to degradation, versatility, light weight and low price. The production of plastic increased on an average of 10% every year on a global basis since 1950 as continuous innovation and research for better product in the field occurred (Diaz Silvarrey and Phan, 2016). The short cycle life of plastics leads to the emergence of a large number of plastic wastes. The municipal plastic wastes (MPW) comprise a mixture of thermoplastics (such as polyethylene, PE; polypropylene, PP; polystyrene, PS; poly(vinyl chloride), PVC; and polyethylene terephthalate, PET). The rate of MPW generation has increased steadily at 5% per year while that of MPW recycling is only at 3% per year (Singh and Ruj, 2016). The remains are either incinerated or disposed in landfills. Pyrolysis, a thermochemical decomposition, provides an excellent alternative to transform MPW into energy fuels or valuable chemicals. ⇑ Corresponding authors. E-mail addresses: [email protected] (J. Huang), [email protected] (X. Li), [email protected] (G. Zeng).
There have been a number of researches on the pyrolysis of individual and mixed plastics with and without PVC (Aboulkas et al., 2010; Kim and Kim, 2004; Kumagai et al., 2015; Zheng et al., 2007). It is well known that the thermal decomposition of mixed plastics containing PVC generates HCl, which is corrosive, and some chlorinated hydrocarbons, which act as precursors of toxic emissions such as polychlorinated dibenzodioxins (PCDD), dibenzofurans (PCDF) and polychloro biphenyls (PCB) upon combustion afterwards (Bhaskar et al., 2005; Cao et al., 2016; Shen et al., 2016). Among all sorts of plastics, PVC is one of the most important but also potential environmentally harmful polymers for the content of chlorine. Therefore, studies on the thermal decomposition mechanism of PVC are an important consideration in developing pyrolysis technology for mixed waste plastics. Yuan et al. (2014) reported high efficiency chlorine removal from polyvinyl chloride (PVC) pyrolysis with a gas-liquid fluidized bed reactor, and the experimental results showed that dechlorination efficiency is mainly temperature dependent and 300 °C is a proper reaction temperature; under this temperature 99.5% of Cl removal efficiency can be obtained within reaction time around 1 min after melting is completed. Zhou et al. (2016) investigated the influence
https://doi.org/10.1016/j.wasman.2018.03.033 0956-053X/Ó 2018 Published by Elsevier Ltd.
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of process conditions on the formation of polycyclic aromatic hydrocarbons (PAH) during the pyrolysis of PVC; slow pyrolysis produced higher HCl yield, and lower gas and tar yield than fast pyrolysis; the PAH yield obtained from the slow pyrolysis was much lower compared to fast pyrolysis. The reaction kinetics of the thermal decomposition of PVC was studied by Castro et al. (2012), and a kinetic model was developed for the decomposition temperatures lower than 340 °C, in which almost all chlorine is removed from the pure PVC, with an activation energy of 133.8 kJ/ mol. Gui et al. (2013) investigated the effects of peak temperature, holding time, and heating rate on the formation of nascent tar during the polyvinylchloride pyrolysis, and proposed a new four-stage mechanism, including (1) dechlorination accompanied with inner cyclization, (2) aromatic chain scission, (3) release of quasi-3 rings or 3 rings group, and (4) release of 2 rings group. In general, PVC thermal degradation mainly consists of two stages: in the first stage, dehydrochlorination was the main reaction of PVC decomposition, leading to the release of HCl and the formation of conjugated polyene; in the second stage, aromatic hydrocarbons are formed from cyclization reactions of the conjugated polyene and also small molecule hydrocarbons are generated (Aracil et al., 2005; Jordan et al., 2001; Kim, 2001; Starnes, 2002; Yu et al., 2016). Quantum chemistry theoretical methods is increasingly being applied to predict possible reaction pathways and to investigate the detailed thermal decomposition mechanism (Huang et al., 2014, 2016, 2018; Younker et al., 2012). The pyrolysis mechanism of PVC is complex and experimental studies are hard to get insight into the detailed decomposition mechanism. In the present study, the thermal decomposition processes of model compound of PVC (shown in Fig. 1) are investigated by using density functional theory methods M062X with the 6-31++G(d,p) basis set in order to expound the elimination of HCl and the formation of aromatic hydrocarbons. Due to computational ‘‘cost”, we used a model compound of PVC (an oligomer containing 4 repeated units) instead of actual PVC polymers to investigate the pyrolysis mechanism of PVC. The model compound of PVC holds a chemical structure similar to the real PVC and is an intermediate appeared in the process of PVC pyrolysis, so the pyrolysis mechanism of model compound of PVC is similar to that of PVC and there is a similar product distribution during the pyrolysis of both PVC model and PVC. 2. Calculation methods The M06-2X hybrid exchange-correlation functional (Zhao and Truhlar, 2008), which is confirmed to be more accurate than the method B3LYP in previous studies (Parthasarathi et al., 2011; Zhang et al., 2015), has been employed in all calculations. All calculations have been performed with the version of Gaussian 09 (Frisch et al., 2013) program package. The calculation details are the same as those in the literature (Huang et al., 2018).
Fig. 1. Model compound of poly(vinyl chloride) (MPVC) and the BDE values marked in blue, calculated by using M06-2X/6-31++G(d,p) (kJ/mol).
3. Results and discussion 3.1. Bond dissociation energies The values of the bond dissociation energies (BDEs) in MPVC, calculated by using M06-2X/6-31++G(d,p), are shown in Fig. 1. On the basis of these calculations, the weakest bond in PVC is the single bond between the chlorine atom and carbon atom (352.9 kJ/mol), and the average BDE value of branched-chain CACl bond for MPVC is 355.6 kJ/mol. Compared with CACl bond, the BDE of main-chain CAC bond is obviously higher, and the average BDE value of main-chain CAC bond for MPVC is 373.8 kJ/mol. Therefore, the cleavage of the branched-chain CACl bond is more likely to occur in the thermal decomposition of PVC. Experimental results of the thermal decomposition of PVC (Castro et al., 2012; Gui et al., 2013; Yuan et al., 2014; Zhou et al., 2016) showed that the main products are HCl, aromatic hydrocarbons, small molecule hydrocarbons (for instance, methane, ethane, ethylene) and so on. Based on related experimental results, we propose various possible decomposition paths for the evolution of the main products, and the kinetic parameters in all paths were calculated.
3.2. Elimination mechanism of HCl The first decomposition step in the PVC pyrolysis is the elimination of HCl. Schemes 1 and 2 describe seven proposed thermal decomposition paths for elimination mechanism of HCl and the kinetic schematic diagram is shown in Fig. 2. The optimized molecular structures in these decomposition paths are given in Figs. 3 and 4. In path (1), the elimination of HCl starts from the left end of the model compound 1; in path (2), the elimination of HCl starts from the right end of the model compound 1; in paths (3) and (4), the elimination of HCl starts from the middle of the model compound 1; in path (5), the rupture of main chain CAC bond occurs, meanwhile there is a transfer of chlorine atom from one carbon atom to another carbon atom. Paths (6) and (7) describe the elimination of HCl for the model compound with branched-chain ACH2CH2Cl (or ACHClCH3). In reaction path (1), model compound 1 is decomposed into intermediate 2 and HCl via four-member ring transition state TS1 with an energy barrier of 214.9 kJ/mol through a concerted reaction, and the elimination reaction of HCl absorbs an energy of 92.4 kJ/mol; the intermediate 2 is decomposed into intermediate 3 and HCl via transition state TS2 with an energy barrier of 207.4 kJ/mol, and the elimination reaction of HCl absorbs an energy of 61.3 kJ/mol; the intermediate 3 is decomposed into intermediate 4 and HCl via transition state TS3 with an energy barrier of 203.5 kJ/mol, and the elimination reaction of HCl absorbs an energy of 61.7 kJ/mol; the intermediate 4 is decomposed into conjugated polyene 5 and HCl via transition state TS4 with an energy barrier of 225.4 kJ/mol, and the elimination reaction of HCl absorbs an energy of 54.5 kJ/mol. In reaction path (2), model compound 1 is decomposed into intermediate 6 and HCl via transition state TS5 with an energy barrier of 243.3 kJ/mol, and the elimination of HCl absorbs an energy of 81.6 kJ/mol; the intermediate 6 is decomposed into intermediate 7 and HCl via transition state TS6 with an energy barrier of 205.8 kJ/mol, and the elimination of HCl absorbs an energy of 57.2 kJ/mol; the intermediate 7 is decomposed into intermediate 8 and HCl via transition state TS7 with an energy barrier of 183.0 kJ/mol, and the elimination of HCl absorbs an energy of 60.9 kJ/mol; the intermediate 8 is decomposed into conjugated polyene 5 and HCl via transition state TS8 with an energy barrier of 177.8 kJ/mol, and the elimination of HCl absorbs an energy of 64.2 kJ/mol.
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Scheme 1. The proposed thermal decomposition paths for the elimination of HCl.
In reaction path (3), model compound 1 is decomposed into intermediate 9 and HCl via transition state TS9 with an energy barrier of 216.7 kJ/mol, and the elimination of HCl absorbs an energy
of 83.1 kJ/mol; the intermediate 9 is decomposed into intermediate 10 and HCl via transition state TS10 with an energy barrier of 187.8 kJ/mol, and the elimination of HCl absorbs an energy of 61.9 kJ/-
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Scheme 2. The proposed thermal decomposition paths for the elimination of HCl.
mol; the intermediate 10 is decomposed into intermediate 11 and HCl via transition state TS11 with an energy barrier of 176.3 kJ/mol, and the elimination of HCl absorbs an energy of 61.1 kJ/mol. In reaction path (4), model compound 1 is decomposed into intermediate 12 and HCl via transition state TS12 with an energy barrier of 213.2 kJ/mol, and the elimination of HCl absorbs an energy of 83.9 kJ/mol; the intermediate 12 is decomposed into intermediate 13 and HCl via transition state TS13 with an energy barrier of 194.2 kJ/mol, and the elimination of HCl absorbs an energy of 66.5 kJ/mol; the intermediate 13 is decomposed into intermediate 8 and HCl via transition state TS14 with an energy barrier of 226.3 kJ/mol, and the elimination of HCl absorbs an energy of 55.3 kJ/mol. In reaction path (5), model compound 2 is decomposed into chloroethylene 14 and intermediate 15 via transition state TS15 with a higher energy barrier of 361.3 kJ/mol through a rupture of main chain CAC bond, and the reaction absorbs an energy of 113.9 kJ/mol; the intermediate 15 is decomposed into intermediate 16 and HCl via transition state TS16 with an energy barrier of 203.3 kJ/mol, and the elimination of HCl absorbs an energy of 63.9 kJ/mol; the intermediate 16 is decomposed into intermediate 17 and HCl via transition state TS17 with an energy barrier of 227.3 kJ/mol, and the elimination of HCl absorbs an energy of 56.0 kJ/mol. Chloroethylene 14 can be decomposed into ethyne and HCl via transition state TS18 with an energy barrier of 302.2 kJ/mol, and the elimination of HCl absorbs an energy of 119.4 kJ/mol.
In reaction path (6), model compound 18 is decomposed into intermediate 19 and HCl via transition state TS19 with an energy barrier of 187.5 kJ/mol, and the elimination of HCl absorbs an energy of 76.7 kJ/mol; the intermediate 19 is decomposed into intermediate 20 and HCl via transition state TS20 with an energy barrier of 195.4 kJ/mol, and the elimination of HCl absorbs an energy of 60.4 kJ/mol; the intermediate 20 is decomposed into intermediate 21 and HCl via transition state TS21 with an energy barrier of 167.4 kJ/mol, and the elimination of HCl absorbs an energy of 64.9 kJ/mol; the intermediate 21 is decomposed into intermediate 22 and HCl via transition state TS22 with an energy barrier of 227.5 kJ/mol, and the elimination of HCl absorbs an energy of 55.0 kJ/mol. In reaction path (7), model compound 23 is decomposed into intermediate 24 and HCl via transition state TS23 with an energy barrier of 199.9 kJ/mol, and the elimination of HCl absorbs an energy of 65.8 kJ/mol; the intermediate 24 is decomposed into intermediate 25 and HCl via transition state TS24 with an energy barrier of 202.7 kJ/mol, and the elimination of HCl absorbs an energy of 70.4 kJ/mol; the intermediate 25 is decomposed into intermediate 26 and HCl via transition state TS25 with an energy barrier of 204.1 kJ/mol, and the elimination of HCl absorbs an energy of 56.9 kJ/mol. By comparing energy barriers of the HCl elimination in all reaction paths, we can find that the activation energy of concerted reaction of HCl elimination, which changes from 167.4 kJ/mol to 243.3 kJ/mol, is significantly lower than the average BDE value of
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Fig. 2. The kinetic schematic diagram in the elimination processes of HCl. Energy values are in kJ/mol.
CACl bond (355.6 kJ/mol), so the concerted reaction may be the major reaction channel and free radical reaction is less likely to occur with regard to the HCl elimination in the thermal decomposition of PVC. Allyl group (ACH@CHACHClA) generated after a HCl elimination can obviously reduce the activation energy of HCl elimination at allylic chlorine site, for example, in path (2), the energy barrier of first HCl elimination is 243.3 kJ/mol, but the energy barrier of second HCl elimination is 205.8 kJ/mol and the energy barrier of third HCl elimination is 183.0 kJ/mol and the energy barrier of fourth HCl elimination is 177.8 kJ/mol. However, butenyl group (ACH@CHACH2ACHClA) generated after a HCl elimination might have little influence over the activation energy of HCl elimination, for example, in path (1), the energy barrier of first HCl elimination is 214.9 kJ/mol, and the energy barrier of sec-
ond HCl elimination is 207.4 kJ/mol, and the energy barrier of third HCl elimination is 203.5 kJ/mol, and the energy barrier of fourth HCl elimination is 225.4 kJ/mol. In path (5), the energy barrier of the rupture of main chain CAC bond (361.3 kJ/mol) is significantly higher than that of HCl elimination, and the energy barrier of HCl elimination of chloroethylene is higher than that of HCl elimination of PVC, so the reaction path (5) is less likely to occur in the first stage of PVC thermal decomposition. In the process of synthesis of poly(vinyl chloride), some defect structures may occur, such as conformational isomer (Starnes, 2005). Paths (6) and (7) give the elimination reaction of HCl for the model compounds 18 and 23 of PVC isomers. It is inferred from kinetic analysis that the branched-chain (ACH2CH2Cl or ACHClCH3) has a significant effect on the activation energy of
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Fig. 3. Optimized molecular structures in the thermal decomposition paths for elimination of HCl. Some key interatomic distances are given in Å.
HCl elimination at the carbon sites near the branch chain, while the activation energy of HCl elimination at the carbon sites far away from the branch chain is rarely affected. The activation energy of HCl elimination at tertiary carbon site in path (6) is lower than that of HCl elimination in path (1), and the activation energy of HCl elimination at the carbon site connected with the branch chain in path (7) is also lower than that of HCl elimination in path (1).
Based on the above analyses, dehydrochlorination reactions, which result in the release of HCl and the formation of conjugated polyene, might occur through a concerted reaction with fourmember ring transition state in the first stage of PVC thermal decomposition; the initiation of PVC decomposition might be due to the random allylic chlorine atoms or branches connected with the carbon atom on the main-chain, which is consistent with previous reports (Starnes, 2002; Yu et al., 2016).
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Fig. 4. Optimized molecular structures in the thermal decomposition paths for elimination of HCl. Some key interatomic distances are given in Å.
3.3. Evolution mechanisms of aromatics In the second stage of PVC thermal decomposition, aromatic hydrocarbons can be evolved through cyclization reactions of the conjugated polyene. Scheme 3 describes the proposed evolution processes of aromatic hydrocarbons and the kinetic schematic diagram is shown in Fig. 5. The optimized molecular structures in evolution processes of aromatics are given in Fig. 6. In reaction path (8), model compound 5 can be transformed into isomer 27 via transition state TS26 with an energy barrier of 258.9 kJ/mol through the rotation of dihedral angle C(2)AC(3)AC(4)AC (5); the isomer 27 can be transformed into isomer 28 via transition state TS27 with a low energy barrier of 21.9 kJ/mol through the rotation of dihedral angle C(3)AC(4)AC(5)AC(6); the isomer 28 can be transformed into isomer 29 via transition state TS28 with a low energy barrier of 23.1 kJ/mol through the rotation of the dihedral angle C(1)AC(2)AC(3)AC(4). The isomer 29 can further be transformed into isomer 30 via transition state TS29 with an energy barrier of 97.4 kJ/mol through cyclization and the cyclization reaction releases an energy of 77.8 kJ/mol. The compound 30 can be transformed into compound 31 through a dehydrogenation reaction with a dissociation energy of 317.5 kJ/mol, or decomposed into radicals 32 and 34 through CAC homolytic cleavage with a dissociation energy of 315.7 kJ/mol. The compound 31 can be transformed into styrene 35 via transition state TS30 with an energy barrier of 103.2 kJ/mol through a dehydrogenation, or decomposed into benzene 33 and vinyl 34 via transition state TS31 with an energy barrier of 110.9 kJ/mol. Styrene 35 and ethylene can polymerize to form compound 36 via transition state TS32 with an
energy barrier of 120.3 kJ/mol through cycloaddition reaction. The compound 36 can be decomposed into compound 37 and H2 via transition state TS33 with an energy barrier of 165.2 kJ/mol. The compound 37 can be transformed into compound 38 through a dehydrogenation reaction with a dissociation energy of 330.4 kJ/mol, and the compound 38 can further be transformed into naphthalene 39 via transition state TS34 with an energy barrier of 139.5 kJ/mol; the compound 37 can also decomposed directly into naphthalene 39 and H2 via transition state TS35 with a high energy barrier of 425.7 kJ/mol. Styrene 35 and chloroethylene can polymerize to form bicyclic compound 40 via transition state TS36 with an energy barrier of 122.3 kJ/mol through cycloaddition reaction. The compound 40 can be decomposed into compound 41 and H2 via transition state TS37 with an energy barrier of 168.0 kJ/mol. The compound 41 can decomposed into naphthalene 39 and HCl via transition state TS38 with an energy barrier of 145.3 kJ/mol; the compound 41 can also transformed into free radical 38 through a dechlorination with a dissociation energy of 273.6 kJ/mol. In reaction path (9), model compound 5 can be decomposed into vinyl and free radical 42 through CAC homolysis with a high dissociation energy of 488.4 kJ/mol. Radical 42 can undergo isomerization to form isomer 43 via transition state TS39 with a low energy barrier of 25.6 kJ/mol; the isomer 43 can undergo isomerization to form isomer 44 via transition state TS40 with an energy barrier of 209.8 kJ/mol; the isomer 44 can undergo isomerization to form isomer 45 via transition state TS41 with a low energy barrier of 21.2 kJ/mol; the isomer 45 can undergo isomerization to form isomer 46 via transition state TS42 with a low energy barrier
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Scheme 3. The proposed evolution processes of aromatic hydrocarbons in thermal decomposition.
of 9.1 kJ/mol. The isomer 46 can further transformed into isomer 32 via transition state TS43 with a low energy barrier of 8.3 kJ/mol through cyclization and the cyclization reaction releases an energy of 236.7 kJ/mol. The radical 32 can be transformed into benzene via transition state TS44 with an energy barrier of 110.7 kJ/mol through a dehydrogenation. In reaction path (10), model compound 17 can undergo isomerization to form isomer 47 via transition state TS45 with a low
energy barrier of 25.3 kJ/mol; the isomer 47 can undergo isomerization to form isomer 48 via transition state TS46 with an energy barrier of 235.4 kJ/mol; the isomer 48 can undergo isomerization to form isomer 49 via transition state TS47 with a low energy barrier of 23.3 kJ/mol. The isomer 49 can further transformed into isomer 50 via transition state TS48 with an energy barrier of 88.2 kJ/mol through cyclization and the cyclization reaction releases an energy of 103.9 kJ/mol. The compound 50 can be transformed
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Fig. 5. The kinetic schematic diagram in evolution processes of aromatics. Energy values are in kJ/mol.
into compound 32 through a dehydrogenation with a dissociation energy of 314.6 kJ/mol. Based on a comparison of rate-determining steps in the evolution processes of aromatics, it can be found that a free radical
(such as 42) is more easily converted into aromatic compound through a series of isomerizations, cyclization and dehydrogenation, but it is difficult for formation of the radical. The energy barrier of HCl elimination of chlorinated aromatic hydrocarbon
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Fig. 6. Optimized molecular structures in evolution processes of aromatics.
(145.3 kJ/mol) is obviously lower than that of aliphatic hydrocarbon (167.4 243.3 kJ/mol), so there is the possibility of formation of chlorinated aromatics before all HCl eliminations in PVC thermal decomposition.
3.4. Formation processes of small molecule hydrocarbons Scheme 4 describes the proposed thermal decomposition paths for the formation of small molecule hydrocarbons and the
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Scheme 4. The proposed thermal decomposition paths for the formation of small molecule hydrocarbons.
kinetic schematic diagram is shown in Fig. 7. The optimized molecular structures in the formation processes of small molecule hydrocarbons are given in Fig. 8. In reaction path (11), model compound 5 undergoes homolytic thermal scission reaction to generate radicals 42 and 34 with a high dissociation energy of 488.4 kJ/mol. The vinyl 34 can be transformed into ethyne 54 via transition state TS49 with an energy barrier of 171.3 kJ/mol through a dehydrogenation, or into ethylene 51 through a hydrogenation reaction with a binding energy of 454.6 kJ/mol. Ethylene 51 can further be transformed into ethyl 52 via transition state TS50 with a low energy barrier of 15.3 kJ/mol through a hydrogenation and the hydrogenation reaction releases an energy of 154.8 kJ/mol; ethyl 52 can be transformed into ethane 53 through a hydrogenation reaction with a binding energy of 317.3 kJ/mol. In reaction path (12), the radical 42 can be decomposed into radical 55 and ethyne 54 via transition state TS51 with an energy barrier of 198.2 kJ/mol, and the radical 55 can further be transformed into butadiene 56 through a hydrogenation reaction with a binding energy of 462.1 kJ/mol. In reaction path (13), model compound 11 can be transformed into the radical 57 via transition state TS52 with a low energy barrier of 14.3 kJ/mol through a hydrogenation; the radical 57 can be decomposed into
compound 58 and methyl via transition state TS53 with an energy barrier of 184.8 kJ/mol, and the methyl can further be transformed into methane through a hydrogenation reaction with a binding energy of 433.0 kJ/mol. In reaction path (14), model compound 11 can be transformed into the radical 59 via transition state TS54 with a low energy barrier of 22.4 kJ/mol through a hydrogenation, and the radical 59 can be decomposed into radical 60 and propylene 61 via transition state TS55 with an energy barrier of 165.6 kJ/mol. Through comparison of energy barriers of rate-determining steps in all formation paths of small molecule hydrocarbons, it is easy to know that the dissociation energy of CAC bond of the conjugated polyene is very high, so it is difficult to form small molecule products CnHm; however, the presence of hydrogen can obviously reduce the energy barriers in formation paths of small molecule products. In the evolution processes of aromatics, some hydrogen radicals can be formed, which contribute to the formation of small molecule products. Therefore, in the second stage of PVC pyrolysis, the conjugated polyene formed in the first stage could be decomposed in parallel reaction channels: one is the evolution of aromatics, another is the formation of small molecule products.
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Fig. 7. The kinetic schematic diagram in formation of small molecule hydrocarbons. Energy values are in kJ/mol.
4. Conclusions Waste PVC is one of the potential environmentally harmful polymers for the content of chlorine, and then studies on the pyrolysis mechanisms of PVC are an important consideration in developing pyrolysis technology for mixed waste plastics. In the present study, the thermal decomposition mechanisms of model compound of PVC were investigated by using density functional theory methods M062X with the 6-31++G(d,p) basis set in order to clarify the elimination of HCl and the formation processes of aromatic hydrocarbons and small molecule hydrocarbons. The main conclusions are made as follows: (1) The HCl elimination can occur through the concerted reaction in the thermal decomposition of PVC, and the energy barrier of HCl elimination changes from 167.4 kJ/mol to 243.3 kJ/mol, which is significantly lower than the average BDE value of CACl bond (355.6 kJ/mol). (2) Allyl group (ACH@CHACHClA) generated after a HCl elimination can obviously reduce the activation energy of HCl
(3)
(4)
(5)
(6)
elimination, however, butenyl group (ACH@CHACH2ACHClA) have little influence over the activation energy of HCl elimination. The branched-chain can lower the energy barrier of HCl elimination at the carbon sites near the branch chain. The initiation of PVC decomposition might be due to the allylic chlorine atoms or branches connected with the carbon atom on the main-chain. A conjugated polyene free radical is more easily converted into aromatic compound through a series of isomerizations, cyclization and dehydrogenation, but it is difficult for formation of the radical because of the very high dissociation energy of CAC bond of the conjugated polyene. The presence of hydrogen can obviously reduce the energy barriers in formation paths of small molecule products. In the second stage of PVC pyrolysis, the conjugated polyene formed in the first stage could be decomposed in parallel reaction channels: one is the evolution of aromatics, another is the formation of small molecule products.
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Fig. 8. Optimized molecular structures in formation paths of small molecule hydrocarbons.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51266002, 11564005), the Innovation Group Major Research Project of Department of Education in Guizhou Province (No. KY[2016]028, KY[2017]035) and the Open Research Funds of Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle (Nanchang Hangkong University) (No. ES201880055). References Aboulkas, A., El harfi, K., El Bouadili, A., 2010. Thermal degradation behaviors of polyethylene and polypropylene. Part I: Pyrolysis kinetics and mechanisms. Energ. Convers. Manage. 51, 1363–1369. Aracil, I., Font, R., Conesa, J.A., 2005. Semivolatile and volatile compounds from the pyrolysis and combustion of polyvinyl chloride. J. Anal. Appl. Pyrol. 74, 465– 478. Bhaskar, T., Tanabe, M., Muto, A., Sakata, Y., Liu, C.F., Chen, M.D., Chao, C.C., 2005. Analysis of chlorine distribution in the pyrolysis products of poly(vinylidene chloride) mixed with polyethylene, polypropylene or polystyrene. Polym. Degrad. Stab. 89, 38–42. Cao, Q., Yuan, G., Yin, L., Chen, D., He, P., Wang, H., 2016. Morphological characteristics of polyvinyl chloride (PVC) dechlorination during pyrolysis process: influence of PVC content and heating rate. Waste Manage. 58, 241– 249. Castro, A., Soares, D., Vilarinho, C., Castro, F., 2012. Kinetics of thermal dechlorination of PVC under pyrolytic conditions. Waste Manage. 32, 847–851. Diaz Silvarrey, L.S., Phan, A.N., 2016. Kinetic study of municipal plastic waste. Int. J. Hydrogen Energ. 41, 16352–16364. Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Scalmani, G., Barone, V., Mennucci, B., Petersson, G.A., Nakatsuji, H., Caricato, M., Li, X., Hratchian, H.P., Izmaylov, A.F., Bloino, J., Zheng, G., Sonnenberg, J.L., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Vreven, T., Montgomery, J.A., Peralta, J.E., Ogliaro, F., Bearpark, M., Heyd, J.J., Brothers, E., Kudin, K.N., Staroverov, V.N., Keith, T., Kobayashi, R., Normand, J., Raghavachari, K., Rendell, A., Burant, J.C., Iyengar, S. S., Tomasi, J., Cossi, M., Rega, N., Millam, J.M., Klene, M., Knox, J.E., Cross, J.B.,
Bakken, V., Adamo, C., Jaramillo, J., Gomperts, R., Stratmann, R.E., Yazyev, O., Austin, A.J., Cammi, R., Pomelli, C., Ochterski, J.W., Martin, R.L., Morokuma, K., Zakrzewski, V.G., Voth, G.A., Salvador, P., Dannenberg, J.J., Dapprich, S., Daniels, A.D., Farkas, O., Foresman, J.B., Ortiz, J.V., Cioslowski, J., Fox, D.J., 2013. Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT. Gui, B., Qiao, Y., Wan, D., Liu, S., Han, Z., Yao, H., Xu, M., 2013. Nascent tar formation during polyvinylchloride (PVC) pyrolysis. P. Combust. Inst. 34, 2321–2329. Huang, J., Liu, C., Wu, D., Tong, H., Ren, L., 2014. Density functional theory studies on pyrolysis mechanism of b-O-4 type lignin dimer model compound. J. Anal. Appl. Pyrol. 109, 98–108. Huang, J., He, C., Wu, L., Tong, H., 2016. Thermal degradation reaction mechanism of xylose: A DFT study. Chem. Phys. Lett. 658, 114–124. Huang, J., He, C., Li, X., Pan, G., Tong, H., 2018. Theoretical studies on thermal degradation reaction mechanism of model compound of bisphenol A polycarbonate. Waste Manage. 71, 181–191. Jordan, K.J., Suib, S.L., Koberstein, J.T., 2001. Determination of the degradation mechanism from the kinetic parameters of dehydrochlorinated poly(vinyl chloride) decomposition. J. Phys. Chem. B 105, 3174–3181. Kim, S., 2001. Pyrolysis kinetics of waste PVC pipe. Waste Manage. 21, 609–616. Kim, S.S., Kim, S., 2004. Pyrolysis characteristics of polystyrene and polypropylene in a stirred batch reactor. Chem. Eng. J. 98, 53–60. Kumagai, S., Hasegawa, I., Grause, G., Kameda, T., Yoshioka, T., 2015. Thermal decomposition of individual and mixed plastics in the presence of CaO or Ca (OH)2. J. Anal. Appl. Pyrol. 113, 584–590. Parthasarathi, R., Romero, R.A., Redondo, A., Gnanakaran, S., 2011. Theoretical study of the remarkably diverse linkages in lignin. J. Phys. Chem. Lett. 2, 2660–2666. Shen, Y., Zhao, R., Wang, J., Chen, X., Ge, X., Chen, M., 2016. Waste-to-energy: dehalogenation of plastic-containing wastes. Waste Manage. 49, 287–303. Singh, R.K., Ruj, B., 2016. Time and temperature depended fuel gas generation from pyrolysis of real world municipal plastic waste. Fuel 174, 164–171. Starnes, W.H., 2002. Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride). Prog. Polym. Sci. 27, 2133–2170. Starnes, W.H., 2005. Structural defects in poly(vinyl chloride). J. Polym. Sci. Pol. Chem. 43, 2451–2467. Zhang, Y., Liu, C., Chen, X., 2015. Unveiling the initial pyrolytic mechanisms of cellulose by DFT study. J. Anal. Appl. Pyrol. 113, 621–629. Zhao, Y., Truhlar, D.G., 2008. Density functionals with broad applicability in chemistry. Acc. Chem. Res. 41, 157–167. Zheng, J., Cui, P., Tian, X., Zheng, K., 2007. Pyrolysis studies of polyethylene terephthalate/silica nanocomposites. J. Appl. Polym. Sci. 104, 9–14. Zhou, H., Wu, C., Onwudili, J.A., Meng, A., Zhang, Y., Williams, P.T., 2016. Influence of process conditions on the formation of 2–4 ring polycyclic aromatic
Please cite this article in press as: Huang, J., et al. Thermal decomposition mechanisms of poly(vinyl chloride): A computational study. Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.03.033
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hydrocarbons from the pyrolysis of polyvinyl chloride. Fuel Process. Technol. 144, 299–304. Younker, J.M., Beste, A., Buchanan III, A.C., 2012. Computational study of bond dissociation enthalpies for lignin model compounds: b-5 Arylcoumaran. Chem. Phys. Lett. 545, 100–106.
Yuan, G., Chen, D., Yin, L., Wang, Z., Zhao, L., Wang, J.Y., 2014. High efficiency chlorine removal from polyvinyl chloride (PVC) pyrolysis with a gas-liquid fluidized bed reactor. Waste Manage. 34, 1045–1050. Yu, J., Sun, L., Ma, C., Qiao, Y., Yao, H., 2016. Thermal degradation of PVC: A review. Waste Manage. 48, 300–314.
Please cite this article in press as: Huang, J., et al. Thermal decomposition mechanisms of poly(vinyl chloride): A computational study. Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.03.033
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Thermal decomposition mechanisms of poly(vinyl chloride): A computational study Jinbao Huang a, Xinsheng Li b,⇑, Guisheng Zeng c,⇑, Xiaocai Cheng a, Hong Tong a, Daiqiang Wang a a
School of Mechatronics Engineering, Guizhou Minzu University, Guiyang 550025, China School of Resources, Environment and Materials, Guangxi University, Nanning 530004, China c Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, China b
a r t i c l e
i n f o
Article history: Received 23 January 2018 Revised 9 March 2018 Accepted 19 March 2018 Available online xxxx Keywords: Poly(vinyl chloride) Thermal decomposition mechanisms Density functional theory
a b s t r a c t The studies on the pyrolysis mechanisms of waste PVC contribute to development and application of pyrolysis technology for mixed waste plastics. In the article, the thermal decomposition mechanisms of model compound of poly(vinyl chloride) (PVC) have been investigated by employing density functional theory methods at M06-2X/6–31++G(d,p) level in order to illuminate the elimination of HCl and the formation of hydrocarbons. Various possible pyrolysis paths for the formation of main products were proposed, and the thermodynamic and kinetic parameters in every path were calculated. The calculation results show that the HCl elimination can occur through the concerted reaction and the energy barrier of HCl elimination changes from 167.4 to 243.3 kJ/mol; allyl group can obviously reduce the activation energy of HCl elimination, and the branched-chain can lower the energy barrier of HCl elimination at the carbon sites near the branch chain; a free radical is more easily converted into aromatic compound through a series of isomerizations, cyclization and dehydrogenation; the conjugated polyene could be decomposed in parallel reaction channels: one is the evolution of aromatics, another is the formation of small molecule products. The above analysis is consistent with previous experimental results and analysis. Ó 2018 Published by Elsevier Ltd.
1. Introduction Plastic products are highly convenient for people’s use on account of their resistance to degradation, versatility, light weight and low price. The production of plastic increased on an average of 10% every year on a global basis since 1950 as continuous innovation and research for better product in the field occurred (Diaz Silvarrey and Phan, 2016). The short cycle life of plastics leads to the emergence of a large number of plastic wastes. The municipal plastic wastes (MPW) comprise a mixture of thermoplastics (such as polyethylene, PE; polypropylene, PP; polystyrene, PS; poly(vinyl chloride), PVC; and polyethylene terephthalate, PET). The rate of MPW generation has increased steadily at 5% per year while that of MPW recycling is only at 3% per year (Singh and Ruj, 2016). The remains are either incinerated or disposed in landfills. Pyrolysis, a thermochemical decomposition, provides an excellent alternative to transform MPW into energy fuels or valuable chemicals. ⇑ Corresponding authors. E-mail addresses: [email protected] (J. Huang), [email protected] (X. Li), [email protected] (G. Zeng).
There have been a number of researches on the pyrolysis of individual and mixed plastics with and without PVC (Aboulkas et al., 2010; Kim and Kim, 2004; Kumagai et al., 2015; Zheng et al., 2007). It is well known that the thermal decomposition of mixed plastics containing PVC generates HCl, which is corrosive, and some chlorinated hydrocarbons, which act as precursors of toxic emissions such as polychlorinated dibenzodioxins (PCDD), dibenzofurans (PCDF) and polychloro biphenyls (PCB) upon combustion afterwards (Bhaskar et al., 2005; Cao et al., 2016; Shen et al., 2016). Among all sorts of plastics, PVC is one of the most important but also potential environmentally harmful polymers for the content of chlorine. Therefore, studies on the thermal decomposition mechanism of PVC are an important consideration in developing pyrolysis technology for mixed waste plastics. Yuan et al. (2014) reported high efficiency chlorine removal from polyvinyl chloride (PVC) pyrolysis with a gas-liquid fluidized bed reactor, and the experimental results showed that dechlorination efficiency is mainly temperature dependent and 300 °C is a proper reaction temperature; under this temperature 99.5% of Cl removal efficiency can be obtained within reaction time around 1 min after melting is completed. Zhou et al. (2016) investigated the influence
https://doi.org/10.1016/j.wasman.2018.03.033 0956-053X/Ó 2018 Published by Elsevier Ltd.
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of process conditions on the formation of polycyclic aromatic hydrocarbons (PAH) during the pyrolysis of PVC; slow pyrolysis produced higher HCl yield, and lower gas and tar yield than fast pyrolysis; the PAH yield obtained from the slow pyrolysis was much lower compared to fast pyrolysis. The reaction kinetics of the thermal decomposition of PVC was studied by Castro et al. (2012), and a kinetic model was developed for the decomposition temperatures lower than 340 °C, in which almost all chlorine is removed from the pure PVC, with an activation energy of 133.8 kJ/ mol. Gui et al. (2013) investigated the effects of peak temperature, holding time, and heating rate on the formation of nascent tar during the polyvinylchloride pyrolysis, and proposed a new four-stage mechanism, including (1) dechlorination accompanied with inner cyclization, (2) aromatic chain scission, (3) release of quasi-3 rings or 3 rings group, and (4) release of 2 rings group. In general, PVC thermal degradation mainly consists of two stages: in the first stage, dehydrochlorination was the main reaction of PVC decomposition, leading to the release of HCl and the formation of conjugated polyene; in the second stage, aromatic hydrocarbons are formed from cyclization reactions of the conjugated polyene and also small molecule hydrocarbons are generated (Aracil et al., 2005; Jordan et al., 2001; Kim, 2001; Starnes, 2002; Yu et al., 2016). Quantum chemistry theoretical methods is increasingly being applied to predict possible reaction pathways and to investigate the detailed thermal decomposition mechanism (Huang et al., 2014, 2016, 2018; Younker et al., 2012). The pyrolysis mechanism of PVC is complex and experimental studies are hard to get insight into the detailed decomposition mechanism. In the present study, the thermal decomposition processes of model compound of PVC (shown in Fig. 1) are investigated by using density functional theory methods M062X with the 6-31++G(d,p) basis set in order to expound the elimination of HCl and the formation of aromatic hydrocarbons. Due to computational ‘‘cost”, we used a model compound of PVC (an oligomer containing 4 repeated units) instead of actual PVC polymers to investigate the pyrolysis mechanism of PVC. The model compound of PVC holds a chemical structure similar to the real PVC and is an intermediate appeared in the process of PVC pyrolysis, so the pyrolysis mechanism of model compound of PVC is similar to that of PVC and there is a similar product distribution during the pyrolysis of both PVC model and PVC. 2. Calculation methods The M06-2X hybrid exchange-correlation functional (Zhao and Truhlar, 2008), which is confirmed to be more accurate than the method B3LYP in previous studies (Parthasarathi et al., 2011; Zhang et al., 2015), has been employed in all calculations. All calculations have been performed with the version of Gaussian 09 (Frisch et al., 2013) program package. The calculation details are the same as those in the literature (Huang et al., 2018).
Fig. 1. Model compound of poly(vinyl chloride) (MPVC) and the BDE values marked in blue, calculated by using M06-2X/6-31++G(d,p) (kJ/mol).
3. Results and discussion 3.1. Bond dissociation energies The values of the bond dissociation energies (BDEs) in MPVC, calculated by using M06-2X/6-31++G(d,p), are shown in Fig. 1. On the basis of these calculations, the weakest bond in PVC is the single bond between the chlorine atom and carbon atom (352.9 kJ/mol), and the average BDE value of branched-chain CACl bond for MPVC is 355.6 kJ/mol. Compared with CACl bond, the BDE of main-chain CAC bond is obviously higher, and the average BDE value of main-chain CAC bond for MPVC is 373.8 kJ/mol. Therefore, the cleavage of the branched-chain CACl bond is more likely to occur in the thermal decomposition of PVC. Experimental results of the thermal decomposition of PVC (Castro et al., 2012; Gui et al., 2013; Yuan et al., 2014; Zhou et al., 2016) showed that the main products are HCl, aromatic hydrocarbons, small molecule hydrocarbons (for instance, methane, ethane, ethylene) and so on. Based on related experimental results, we propose various possible decomposition paths for the evolution of the main products, and the kinetic parameters in all paths were calculated.
3.2. Elimination mechanism of HCl The first decomposition step in the PVC pyrolysis is the elimination of HCl. Schemes 1 and 2 describe seven proposed thermal decomposition paths for elimination mechanism of HCl and the kinetic schematic diagram is shown in Fig. 2. The optimized molecular structures in these decomposition paths are given in Figs. 3 and 4. In path (1), the elimination of HCl starts from the left end of the model compound 1; in path (2), the elimination of HCl starts from the right end of the model compound 1; in paths (3) and (4), the elimination of HCl starts from the middle of the model compound 1; in path (5), the rupture of main chain CAC bond occurs, meanwhile there is a transfer of chlorine atom from one carbon atom to another carbon atom. Paths (6) and (7) describe the elimination of HCl for the model compound with branched-chain ACH2CH2Cl (or ACHClCH3). In reaction path (1), model compound 1 is decomposed into intermediate 2 and HCl via four-member ring transition state TS1 with an energy barrier of 214.9 kJ/mol through a concerted reaction, and the elimination reaction of HCl absorbs an energy of 92.4 kJ/mol; the intermediate 2 is decomposed into intermediate 3 and HCl via transition state TS2 with an energy barrier of 207.4 kJ/mol, and the elimination reaction of HCl absorbs an energy of 61.3 kJ/mol; the intermediate 3 is decomposed into intermediate 4 and HCl via transition state TS3 with an energy barrier of 203.5 kJ/mol, and the elimination reaction of HCl absorbs an energy of 61.7 kJ/mol; the intermediate 4 is decomposed into conjugated polyene 5 and HCl via transition state TS4 with an energy barrier of 225.4 kJ/mol, and the elimination reaction of HCl absorbs an energy of 54.5 kJ/mol. In reaction path (2), model compound 1 is decomposed into intermediate 6 and HCl via transition state TS5 with an energy barrier of 243.3 kJ/mol, and the elimination of HCl absorbs an energy of 81.6 kJ/mol; the intermediate 6 is decomposed into intermediate 7 and HCl via transition state TS6 with an energy barrier of 205.8 kJ/mol, and the elimination of HCl absorbs an energy of 57.2 kJ/mol; the intermediate 7 is decomposed into intermediate 8 and HCl via transition state TS7 with an energy barrier of 183.0 kJ/mol, and the elimination of HCl absorbs an energy of 60.9 kJ/mol; the intermediate 8 is decomposed into conjugated polyene 5 and HCl via transition state TS8 with an energy barrier of 177.8 kJ/mol, and the elimination of HCl absorbs an energy of 64.2 kJ/mol.
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Scheme 1. The proposed thermal decomposition paths for the elimination of HCl.
In reaction path (3), model compound 1 is decomposed into intermediate 9 and HCl via transition state TS9 with an energy barrier of 216.7 kJ/mol, and the elimination of HCl absorbs an energy
of 83.1 kJ/mol; the intermediate 9 is decomposed into intermediate 10 and HCl via transition state TS10 with an energy barrier of 187.8 kJ/mol, and the elimination of HCl absorbs an energy of 61.9 kJ/-
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Scheme 2. The proposed thermal decomposition paths for the elimination of HCl.
mol; the intermediate 10 is decomposed into intermediate 11 and HCl via transition state TS11 with an energy barrier of 176.3 kJ/mol, and the elimination of HCl absorbs an energy of 61.1 kJ/mol. In reaction path (4), model compound 1 is decomposed into intermediate 12 and HCl via transition state TS12 with an energy barrier of 213.2 kJ/mol, and the elimination of HCl absorbs an energy of 83.9 kJ/mol; the intermediate 12 is decomposed into intermediate 13 and HCl via transition state TS13 with an energy barrier of 194.2 kJ/mol, and the elimination of HCl absorbs an energy of 66.5 kJ/mol; the intermediate 13 is decomposed into intermediate 8 and HCl via transition state TS14 with an energy barrier of 226.3 kJ/mol, and the elimination of HCl absorbs an energy of 55.3 kJ/mol. In reaction path (5), model compound 2 is decomposed into chloroethylene 14 and intermediate 15 via transition state TS15 with a higher energy barrier of 361.3 kJ/mol through a rupture of main chain CAC bond, and the reaction absorbs an energy of 113.9 kJ/mol; the intermediate 15 is decomposed into intermediate 16 and HCl via transition state TS16 with an energy barrier of 203.3 kJ/mol, and the elimination of HCl absorbs an energy of 63.9 kJ/mol; the intermediate 16 is decomposed into intermediate 17 and HCl via transition state TS17 with an energy barrier of 227.3 kJ/mol, and the elimination of HCl absorbs an energy of 56.0 kJ/mol. Chloroethylene 14 can be decomposed into ethyne and HCl via transition state TS18 with an energy barrier of 302.2 kJ/mol, and the elimination of HCl absorbs an energy of 119.4 kJ/mol.
In reaction path (6), model compound 18 is decomposed into intermediate 19 and HCl via transition state TS19 with an energy barrier of 187.5 kJ/mol, and the elimination of HCl absorbs an energy of 76.7 kJ/mol; the intermediate 19 is decomposed into intermediate 20 and HCl via transition state TS20 with an energy barrier of 195.4 kJ/mol, and the elimination of HCl absorbs an energy of 60.4 kJ/mol; the intermediate 20 is decomposed into intermediate 21 and HCl via transition state TS21 with an energy barrier of 167.4 kJ/mol, and the elimination of HCl absorbs an energy of 64.9 kJ/mol; the intermediate 21 is decomposed into intermediate 22 and HCl via transition state TS22 with an energy barrier of 227.5 kJ/mol, and the elimination of HCl absorbs an energy of 55.0 kJ/mol. In reaction path (7), model compound 23 is decomposed into intermediate 24 and HCl via transition state TS23 with an energy barrier of 199.9 kJ/mol, and the elimination of HCl absorbs an energy of 65.8 kJ/mol; the intermediate 24 is decomposed into intermediate 25 and HCl via transition state TS24 with an energy barrier of 202.7 kJ/mol, and the elimination of HCl absorbs an energy of 70.4 kJ/mol; the intermediate 25 is decomposed into intermediate 26 and HCl via transition state TS25 with an energy barrier of 204.1 kJ/mol, and the elimination of HCl absorbs an energy of 56.9 kJ/mol. By comparing energy barriers of the HCl elimination in all reaction paths, we can find that the activation energy of concerted reaction of HCl elimination, which changes from 167.4 kJ/mol to 243.3 kJ/mol, is significantly lower than the average BDE value of
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Fig. 2. The kinetic schematic diagram in the elimination processes of HCl. Energy values are in kJ/mol.
CACl bond (355.6 kJ/mol), so the concerted reaction may be the major reaction channel and free radical reaction is less likely to occur with regard to the HCl elimination in the thermal decomposition of PVC. Allyl group (ACH@CHACHClA) generated after a HCl elimination can obviously reduce the activation energy of HCl elimination at allylic chlorine site, for example, in path (2), the energy barrier of first HCl elimination is 243.3 kJ/mol, but the energy barrier of second HCl elimination is 205.8 kJ/mol and the energy barrier of third HCl elimination is 183.0 kJ/mol and the energy barrier of fourth HCl elimination is 177.8 kJ/mol. However, butenyl group (ACH@CHACH2ACHClA) generated after a HCl elimination might have little influence over the activation energy of HCl elimination, for example, in path (1), the energy barrier of first HCl elimination is 214.9 kJ/mol, and the energy barrier of sec-
ond HCl elimination is 207.4 kJ/mol, and the energy barrier of third HCl elimination is 203.5 kJ/mol, and the energy barrier of fourth HCl elimination is 225.4 kJ/mol. In path (5), the energy barrier of the rupture of main chain CAC bond (361.3 kJ/mol) is significantly higher than that of HCl elimination, and the energy barrier of HCl elimination of chloroethylene is higher than that of HCl elimination of PVC, so the reaction path (5) is less likely to occur in the first stage of PVC thermal decomposition. In the process of synthesis of poly(vinyl chloride), some defect structures may occur, such as conformational isomer (Starnes, 2005). Paths (6) and (7) give the elimination reaction of HCl for the model compounds 18 and 23 of PVC isomers. It is inferred from kinetic analysis that the branched-chain (ACH2CH2Cl or ACHClCH3) has a significant effect on the activation energy of
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Fig. 3. Optimized molecular structures in the thermal decomposition paths for elimination of HCl. Some key interatomic distances are given in Å.
HCl elimination at the carbon sites near the branch chain, while the activation energy of HCl elimination at the carbon sites far away from the branch chain is rarely affected. The activation energy of HCl elimination at tertiary carbon site in path (6) is lower than that of HCl elimination in path (1), and the activation energy of HCl elimination at the carbon site connected with the branch chain in path (7) is also lower than that of HCl elimination in path (1).
Based on the above analyses, dehydrochlorination reactions, which result in the release of HCl and the formation of conjugated polyene, might occur through a concerted reaction with fourmember ring transition state in the first stage of PVC thermal decomposition; the initiation of PVC decomposition might be due to the random allylic chlorine atoms or branches connected with the carbon atom on the main-chain, which is consistent with previous reports (Starnes, 2002; Yu et al., 2016).
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Fig. 4. Optimized molecular structures in the thermal decomposition paths for elimination of HCl. Some key interatomic distances are given in Å.
3.3. Evolution mechanisms of aromatics In the second stage of PVC thermal decomposition, aromatic hydrocarbons can be evolved through cyclization reactions of the conjugated polyene. Scheme 3 describes the proposed evolution processes of aromatic hydrocarbons and the kinetic schematic diagram is shown in Fig. 5. The optimized molecular structures in evolution processes of aromatics are given in Fig. 6. In reaction path (8), model compound 5 can be transformed into isomer 27 via transition state TS26 with an energy barrier of 258.9 kJ/mol through the rotation of dihedral angle C(2)AC(3)AC(4)AC (5); the isomer 27 can be transformed into isomer 28 via transition state TS27 with a low energy barrier of 21.9 kJ/mol through the rotation of dihedral angle C(3)AC(4)AC(5)AC(6); the isomer 28 can be transformed into isomer 29 via transition state TS28 with a low energy barrier of 23.1 kJ/mol through the rotation of the dihedral angle C(1)AC(2)AC(3)AC(4). The isomer 29 can further be transformed into isomer 30 via transition state TS29 with an energy barrier of 97.4 kJ/mol through cyclization and the cyclization reaction releases an energy of 77.8 kJ/mol. The compound 30 can be transformed into compound 31 through a dehydrogenation reaction with a dissociation energy of 317.5 kJ/mol, or decomposed into radicals 32 and 34 through CAC homolytic cleavage with a dissociation energy of 315.7 kJ/mol. The compound 31 can be transformed into styrene 35 via transition state TS30 with an energy barrier of 103.2 kJ/mol through a dehydrogenation, or decomposed into benzene 33 and vinyl 34 via transition state TS31 with an energy barrier of 110.9 kJ/mol. Styrene 35 and ethylene can polymerize to form compound 36 via transition state TS32 with an
energy barrier of 120.3 kJ/mol through cycloaddition reaction. The compound 36 can be decomposed into compound 37 and H2 via transition state TS33 with an energy barrier of 165.2 kJ/mol. The compound 37 can be transformed into compound 38 through a dehydrogenation reaction with a dissociation energy of 330.4 kJ/mol, and the compound 38 can further be transformed into naphthalene 39 via transition state TS34 with an energy barrier of 139.5 kJ/mol; the compound 37 can also decomposed directly into naphthalene 39 and H2 via transition state TS35 with a high energy barrier of 425.7 kJ/mol. Styrene 35 and chloroethylene can polymerize to form bicyclic compound 40 via transition state TS36 with an energy barrier of 122.3 kJ/mol through cycloaddition reaction. The compound 40 can be decomposed into compound 41 and H2 via transition state TS37 with an energy barrier of 168.0 kJ/mol. The compound 41 can decomposed into naphthalene 39 and HCl via transition state TS38 with an energy barrier of 145.3 kJ/mol; the compound 41 can also transformed into free radical 38 through a dechlorination with a dissociation energy of 273.6 kJ/mol. In reaction path (9), model compound 5 can be decomposed into vinyl and free radical 42 through CAC homolysis with a high dissociation energy of 488.4 kJ/mol. Radical 42 can undergo isomerization to form isomer 43 via transition state TS39 with a low energy barrier of 25.6 kJ/mol; the isomer 43 can undergo isomerization to form isomer 44 via transition state TS40 with an energy barrier of 209.8 kJ/mol; the isomer 44 can undergo isomerization to form isomer 45 via transition state TS41 with a low energy barrier of 21.2 kJ/mol; the isomer 45 can undergo isomerization to form isomer 46 via transition state TS42 with a low energy barrier
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Scheme 3. The proposed evolution processes of aromatic hydrocarbons in thermal decomposition.
of 9.1 kJ/mol. The isomer 46 can further transformed into isomer 32 via transition state TS43 with a low energy barrier of 8.3 kJ/mol through cyclization and the cyclization reaction releases an energy of 236.7 kJ/mol. The radical 32 can be transformed into benzene via transition state TS44 with an energy barrier of 110.7 kJ/mol through a dehydrogenation. In reaction path (10), model compound 17 can undergo isomerization to form isomer 47 via transition state TS45 with a low
energy barrier of 25.3 kJ/mol; the isomer 47 can undergo isomerization to form isomer 48 via transition state TS46 with an energy barrier of 235.4 kJ/mol; the isomer 48 can undergo isomerization to form isomer 49 via transition state TS47 with a low energy barrier of 23.3 kJ/mol. The isomer 49 can further transformed into isomer 50 via transition state TS48 with an energy barrier of 88.2 kJ/mol through cyclization and the cyclization reaction releases an energy of 103.9 kJ/mol. The compound 50 can be transformed
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Fig. 5. The kinetic schematic diagram in evolution processes of aromatics. Energy values are in kJ/mol.
into compound 32 through a dehydrogenation with a dissociation energy of 314.6 kJ/mol. Based on a comparison of rate-determining steps in the evolution processes of aromatics, it can be found that a free radical
(such as 42) is more easily converted into aromatic compound through a series of isomerizations, cyclization and dehydrogenation, but it is difficult for formation of the radical. The energy barrier of HCl elimination of chlorinated aromatic hydrocarbon
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Fig. 6. Optimized molecular structures in evolution processes of aromatics.
(145.3 kJ/mol) is obviously lower than that of aliphatic hydrocarbon (167.4 243.3 kJ/mol), so there is the possibility of formation of chlorinated aromatics before all HCl eliminations in PVC thermal decomposition.
3.4. Formation processes of small molecule hydrocarbons Scheme 4 describes the proposed thermal decomposition paths for the formation of small molecule hydrocarbons and the
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Scheme 4. The proposed thermal decomposition paths for the formation of small molecule hydrocarbons.
kinetic schematic diagram is shown in Fig. 7. The optimized molecular structures in the formation processes of small molecule hydrocarbons are given in Fig. 8. In reaction path (11), model compound 5 undergoes homolytic thermal scission reaction to generate radicals 42 and 34 with a high dissociation energy of 488.4 kJ/mol. The vinyl 34 can be transformed into ethyne 54 via transition state TS49 with an energy barrier of 171.3 kJ/mol through a dehydrogenation, or into ethylene 51 through a hydrogenation reaction with a binding energy of 454.6 kJ/mol. Ethylene 51 can further be transformed into ethyl 52 via transition state TS50 with a low energy barrier of 15.3 kJ/mol through a hydrogenation and the hydrogenation reaction releases an energy of 154.8 kJ/mol; ethyl 52 can be transformed into ethane 53 through a hydrogenation reaction with a binding energy of 317.3 kJ/mol. In reaction path (12), the radical 42 can be decomposed into radical 55 and ethyne 54 via transition state TS51 with an energy barrier of 198.2 kJ/mol, and the radical 55 can further be transformed into butadiene 56 through a hydrogenation reaction with a binding energy of 462.1 kJ/mol. In reaction path (13), model compound 11 can be transformed into the radical 57 via transition state TS52 with a low energy barrier of 14.3 kJ/mol through a hydrogenation; the radical 57 can be decomposed into
compound 58 and methyl via transition state TS53 with an energy barrier of 184.8 kJ/mol, and the methyl can further be transformed into methane through a hydrogenation reaction with a binding energy of 433.0 kJ/mol. In reaction path (14), model compound 11 can be transformed into the radical 59 via transition state TS54 with a low energy barrier of 22.4 kJ/mol through a hydrogenation, and the radical 59 can be decomposed into radical 60 and propylene 61 via transition state TS55 with an energy barrier of 165.6 kJ/mol. Through comparison of energy barriers of rate-determining steps in all formation paths of small molecule hydrocarbons, it is easy to know that the dissociation energy of CAC bond of the conjugated polyene is very high, so it is difficult to form small molecule products CnHm; however, the presence of hydrogen can obviously reduce the energy barriers in formation paths of small molecule products. In the evolution processes of aromatics, some hydrogen radicals can be formed, which contribute to the formation of small molecule products. Therefore, in the second stage of PVC pyrolysis, the conjugated polyene formed in the first stage could be decomposed in parallel reaction channels: one is the evolution of aromatics, another is the formation of small molecule products.
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Fig. 7. The kinetic schematic diagram in formation of small molecule hydrocarbons. Energy values are in kJ/mol.
4. Conclusions Waste PVC is one of the potential environmentally harmful polymers for the content of chlorine, and then studies on the pyrolysis mechanisms of PVC are an important consideration in developing pyrolysis technology for mixed waste plastics. In the present study, the thermal decomposition mechanisms of model compound of PVC were investigated by using density functional theory methods M062X with the 6-31++G(d,p) basis set in order to clarify the elimination of HCl and the formation processes of aromatic hydrocarbons and small molecule hydrocarbons. The main conclusions are made as follows: (1) The HCl elimination can occur through the concerted reaction in the thermal decomposition of PVC, and the energy barrier of HCl elimination changes from 167.4 kJ/mol to 243.3 kJ/mol, which is significantly lower than the average BDE value of CACl bond (355.6 kJ/mol). (2) Allyl group (ACH@CHACHClA) generated after a HCl elimination can obviously reduce the activation energy of HCl
(3)
(4)
(5)
(6)
elimination, however, butenyl group (ACH@CHACH2ACHClA) have little influence over the activation energy of HCl elimination. The branched-chain can lower the energy barrier of HCl elimination at the carbon sites near the branch chain. The initiation of PVC decomposition might be due to the allylic chlorine atoms or branches connected with the carbon atom on the main-chain. A conjugated polyene free radical is more easily converted into aromatic compound through a series of isomerizations, cyclization and dehydrogenation, but it is difficult for formation of the radical because of the very high dissociation energy of CAC bond of the conjugated polyene. The presence of hydrogen can obviously reduce the energy barriers in formation paths of small molecule products. In the second stage of PVC pyrolysis, the conjugated polyene formed in the first stage could be decomposed in parallel reaction channels: one is the evolution of aromatics, another is the formation of small molecule products.
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Fig. 8. Optimized molecular structures in formation paths of small molecule hydrocarbons.
Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51266002, 11564005), the Innovation Group Major Research Project of Department of Education in Guizhou Province (No. KY[2016]028, KY[2017]035) and the Open Research Funds of Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle (Nanchang Hangkong University) (No. ES201880055). References Aboulkas, A., El harfi, K., El Bouadili, A., 2010. Thermal degradation behaviors of polyethylene and polypropylene. Part I: Pyrolysis kinetics and mechanisms. Energ. Convers. Manage. 51, 1363–1369. Aracil, I., Font, R., Conesa, J.A., 2005. Semivolatile and volatile compounds from the pyrolysis and combustion of polyvinyl chloride. J. Anal. Appl. Pyrol. 74, 465– 478. Bhaskar, T., Tanabe, M., Muto, A., Sakata, Y., Liu, C.F., Chen, M.D., Chao, C.C., 2005. Analysis of chlorine distribution in the pyrolysis products of poly(vinylidene chloride) mixed with polyethylene, polypropylene or polystyrene. Polym. Degrad. Stab. 89, 38–42. Cao, Q., Yuan, G., Yin, L., Chen, D., He, P., Wang, H., 2016. Morphological characteristics of polyvinyl chloride (PVC) dechlorination during pyrolysis process: influence of PVC content and heating rate. Waste Manage. 58, 241– 249. Castro, A., Soares, D., Vilarinho, C., Castro, F., 2012. Kinetics of thermal dechlorination of PVC under pyrolytic conditions. Waste Manage. 32, 847–851. Diaz Silvarrey, L.S., Phan, A.N., 2016. Kinetic study of municipal plastic waste. Int. J. Hydrogen Energ. 41, 16352–16364. Frisch, M.J., Trucks, G.W., Schlegel, H.B., Scuseria, G.E., Robb, M.A., Cheeseman, J.R., Scalmani, G., Barone, V., Mennucci, B., Petersson, G.A., Nakatsuji, H., Caricato, M., Li, X., Hratchian, H.P., Izmaylov, A.F., Bloino, J., Zheng, G., Sonnenberg, J.L., Hada, M., Ehara, M., Toyota, K., Fukuda, R., Hasegawa, J., Ishida, M., Nakajima, T., Honda, Y., Kitao, O., Nakai, H., Vreven, T., Montgomery, J.A., Peralta, J.E., Ogliaro, F., Bearpark, M., Heyd, J.J., Brothers, E., Kudin, K.N., Staroverov, V.N., Keith, T., Kobayashi, R., Normand, J., Raghavachari, K., Rendell, A., Burant, J.C., Iyengar, S. S., Tomasi, J., Cossi, M., Rega, N., Millam, J.M., Klene, M., Knox, J.E., Cross, J.B.,
Bakken, V., Adamo, C., Jaramillo, J., Gomperts, R., Stratmann, R.E., Yazyev, O., Austin, A.J., Cammi, R., Pomelli, C., Ochterski, J.W., Martin, R.L., Morokuma, K., Zakrzewski, V.G., Voth, G.A., Salvador, P., Dannenberg, J.J., Dapprich, S., Daniels, A.D., Farkas, O., Foresman, J.B., Ortiz, J.V., Cioslowski, J., Fox, D.J., 2013. Gaussian 09, Revision D.01, Gaussian, Inc., Wallingford CT. Gui, B., Qiao, Y., Wan, D., Liu, S., Han, Z., Yao, H., Xu, M., 2013. Nascent tar formation during polyvinylchloride (PVC) pyrolysis. P. Combust. Inst. 34, 2321–2329. Huang, J., Liu, C., Wu, D., Tong, H., Ren, L., 2014. Density functional theory studies on pyrolysis mechanism of b-O-4 type lignin dimer model compound. J. Anal. Appl. Pyrol. 109, 98–108. Huang, J., He, C., Wu, L., Tong, H., 2016. Thermal degradation reaction mechanism of xylose: A DFT study. Chem. Phys. Lett. 658, 114–124. Huang, J., He, C., Li, X., Pan, G., Tong, H., 2018. Theoretical studies on thermal degradation reaction mechanism of model compound of bisphenol A polycarbonate. Waste Manage. 71, 181–191. Jordan, K.J., Suib, S.L., Koberstein, J.T., 2001. Determination of the degradation mechanism from the kinetic parameters of dehydrochlorinated poly(vinyl chloride) decomposition. J. Phys. Chem. B 105, 3174–3181. Kim, S., 2001. Pyrolysis kinetics of waste PVC pipe. Waste Manage. 21, 609–616. Kim, S.S., Kim, S., 2004. Pyrolysis characteristics of polystyrene and polypropylene in a stirred batch reactor. Chem. Eng. J. 98, 53–60. Kumagai, S., Hasegawa, I., Grause, G., Kameda, T., Yoshioka, T., 2015. Thermal decomposition of individual and mixed plastics in the presence of CaO or Ca (OH)2. J. Anal. Appl. Pyrol. 113, 584–590. Parthasarathi, R., Romero, R.A., Redondo, A., Gnanakaran, S., 2011. Theoretical study of the remarkably diverse linkages in lignin. J. Phys. Chem. Lett. 2, 2660–2666. Shen, Y., Zhao, R., Wang, J., Chen, X., Ge, X., Chen, M., 2016. Waste-to-energy: dehalogenation of plastic-containing wastes. Waste Manage. 49, 287–303. Singh, R.K., Ruj, B., 2016. Time and temperature depended fuel gas generation from pyrolysis of real world municipal plastic waste. Fuel 174, 164–171. Starnes, W.H., 2002. Structural and mechanistic aspects of the thermal degradation of poly(vinyl chloride). Prog. Polym. Sci. 27, 2133–2170. Starnes, W.H., 2005. Structural defects in poly(vinyl chloride). J. Polym. Sci. Pol. Chem. 43, 2451–2467. Zhang, Y., Liu, C., Chen, X., 2015. Unveiling the initial pyrolytic mechanisms of cellulose by DFT study. J. Anal. Appl. Pyrol. 113, 621–629. Zhao, Y., Truhlar, D.G., 2008. Density functionals with broad applicability in chemistry. Acc. Chem. Res. 41, 157–167. Zheng, J., Cui, P., Tian, X., Zheng, K., 2007. Pyrolysis studies of polyethylene terephthalate/silica nanocomposites. J. Appl. Polym. Sci. 104, 9–14. Zhou, H., Wu, C., Onwudili, J.A., Meng, A., Zhang, Y., Williams, P.T., 2016. Influence of process conditions on the formation of 2–4 ring polycyclic aromatic
Please cite this article in press as: Huang, J., et al. Thermal decomposition mechanisms of poly(vinyl chloride): A computational study. Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.03.033
14
J. Huang et al. / Waste Management xxx (2018) xxx–xxx
hydrocarbons from the pyrolysis of polyvinyl chloride. Fuel Process. Technol. 144, 299–304. Younker, J.M., Beste, A., Buchanan III, A.C., 2012. Computational study of bond dissociation enthalpies for lignin model compounds: b-5 Arylcoumaran. Chem. Phys. Lett. 545, 100–106.
Yuan, G., Chen, D., Yin, L., Wang, Z., Zhao, L., Wang, J.Y., 2014. High efficiency chlorine removal from polyvinyl chloride (PVC) pyrolysis with a gas-liquid fluidized bed reactor. Waste Manage. 34, 1045–1050. Yu, J., Sun, L., Ma, C., Qiao, Y., Yao, H., 2016. Thermal degradation of PVC: A review. Waste Manage. 48, 300–314.
Please cite this article in press as: Huang, J., et al. Thermal decomposition mechanisms of poly(vinyl chloride): A computational study. Waste Management (2018), https://doi.org/10.1016/j.wasman.2018.03.033