Catalytic Dechlorination and Charring Reaction of Polyvinyl Chloride

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Catalytic Dechlorination and Charring Reaction of Polyvinyl Chloride (PVC) by CuAl-LDH Yang Chen, Shuhua Zhang, Xiuxiu Han, Xu Zhang, Mengting Yi, Siyuan Yang, Dayang Yu, and Weijun Liu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03271 • Publication Date (Web): 01 Jan 2018 Downloaded from http://pubs.acs.org on January 2, 2018

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Catalytic Dechlorination and Charring Reaction of Polyvinyl Chloride (PVC) by CuAl-LDH Yang Chen1, Shuhua Zhang1*, Xiuxiu Han1, Xu Zhang1, Mengting Yi1, Siyuan Yang1, Dayang Yu1 and Weijun Liu2* 1 College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China 2 College of Mechanical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China

ABSTRACT: The catalytic effect of CuAl-LDH on dechlorination and carbonization of polyvinyl chloride (PVC) during pyrolysis were investigated. The thermal degradation and combustion behaviors were researched via thermogravimetric analysis (TGA) and cone calorimetry (CONE). The released gases were evaluated in detail by TGA-FTIR. Subsequently, Elemental analysis (EA) and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS) were employed to fully characterize the solid residues. The results of TGA and CONE showed that the addition of CuAl-LDH brought about the PVC degradation earlier and the char residues increase significantly. Besides, the results of TGA-FTIR, Element analysis, SEM-EDS and Raman analysis revealed that the existence of CuAl-LDH accelerated the

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dehydrochlorination and promoted the char forming of PVC. CuAl-LDH maybe a potential catalyst applied to treat PVC waste into carbon materials to realize the material and chemical recycling, which possessed tremendous environmental and economic benefits.

KEYWORDS: PVC, CuAl-LDH, Pyrolysis, Dehydrochlorination, Carbonization

1. INTRODUCTION As consequence of the non-flammability, excellent corrosion resistances, electrical insulating properties, remarkable mechanical properties, ease of processing and low cost, PVC has attracted considerable attention over the past few decades, which has been commonly used as an important thermoplastic material in various of fields such as building, packaging, electric and electronic products, furniture etc.1, 2 At the same time, a high amount of PVC-containing wastes were generated consequently, which are increasingly considered as a major environmental issue.3 At present, the most widely used disposal method for PVC wastes are landfill and incineration. But the landfill is becoming more and more unreasonable due to the loss of calorific/chemical value and diminishing viable landfill capacity. The incineration is also highly contentious because of the emission of toxic pollutants although it can effectively cut down the quantity/ volume and produce recyclable heat energy.4 with the intention of solving the problem of energy recovery and the environmental problems caused by the degradation of PVC, a new technique has been developed and employed over the past years. Pyrolysis is considered as an optimal for the treatment of PVC waste compared with the landfill and incineration, which can convert the waste PVC to secondary valuable materials and reduce the emission of detrimental substances.5, 6 Thermal degradation of PVC is a very complicated process because of its high chlorine content. The initial step is dehydrochlorination, which will lead to the formation of conjugated


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polyene sequences.7, 8 Subsequently, these polyenes can undergo cyclization reactions that not only generate benzene and other aromatics but also accompany by a certain number of chlorinated aromatic hydrocarbon intermediates, which result in serious environmental problems.9 However, the polyene segments can also carry out the isomerization, rearrangement and Cross-linking reactions to form the network structure, which can effectively decrease the production of volatile aromatics increasing the mass of stable hydrocarbons.10, 11 Finally, the leftover products are pyrolyzed into liquid oil, gases and solid char at high temperatures.12 Therefore, it is vital that the prior removal of chlorine from PVC in the form of hydrogen chloride at a relatively lower temperature inhibiting the generation of hazardous dioxins, followed by the stabilization of the dechlorinated polyene fractions to convert into char with the temperature elevated. The most commonly used methods paid on the pyrolysis of PVC, including co-pyrolysis of PVC with biomass, coal and other plastics, catalytic dechlorination of raw PVC or Cl-containing oil and hydrothermal treatment using subcritical and supercritical water.13 Among the three treatment methods, because the catalytic pyrolysis can lower the reaction temperature and promote the release of hydrogen chloride to remove chloride effectively, which is accepted as a means with great promise for PVC recycling. Zhou et al. used La-MgO as catalyst, which exhibits high activity and stability for both dehydrochlorination and degradation process of PP/PVC mixtures leading to the improvement in degradation rate and oil quality.14 Zhou et al. synthesized an Al-Mg catalyst for catalytic pyrolysis of PP/PVC, LDEP/PVC, PS/PVC and LDPE/PS/PP/PVC mixtures. The chlorine was removed from the oil significantly.15 Lopez Urionabarrenechea et al. upgraded the chlorinated oils from the pyrolysis of PVC-containing


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plastics using Red Mud, and the catalysis of Red Mud resulted in light liquid fractions with very low chlorine content.16 Layered double hydroxide as a kind of alkali additive has been widely used in PVC resin. Based on the ultimate chemical composition and material properties, LDH can be classified into different fields of applications. It is mostly used as thermal stabilizers to restrain the thermal degradation of PVC during the processing and practical applications.17, 18 Besides, it has found that LDH might be the effective flame retardants and smoke suppressants for PVC.19, 20 Recently, the addition copper containing LDHs to PVC matrix to prepare PVC/CuAl-LDH composites has drawn much attention. Yang et al. has reported that the CuAl layered double hydroxide modified with dodecylbenzene sulfate (SDBS) was added to PVC resin, the obtained PVC/CuAl-LDH nanocomposites showed enhanced flame-retardant properties and thermal stability.21 Liu et al. has studied the influence of copper containing LDHs on the thermal stability and smoke suppression property of PVC, the results indicated that the combination of copper with LDHs can improve the thermal stability and smoke suppresion property of PVC composites effectively.22 However, the current studies are still focused on the impact of hydrotalcite on PVC stability, flame retardancy and smoke suppression. It is rarely reported on exploring the carbonization of plastic wastes or of utilizing copper containing LDHs as an effectively waste-toenergy catalyst. In this study, the effect of CuAl-LDH on PVC thermal degradation and combustion behavior were investigated. Whereafter, the charring residues produced in the pyrolysis process were extracted, separated and purified. The morphology and chemical composition of the charring residues were examined by SEM and EDS. This work attempted to provide a promising and


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environmental friendly catalyst with efficient char effect for PVC composites. More important was to open up a new avenue to prepare carbon materials from PVC.

2. MATERIALS AND METHODS 2.1. Materials.

The PVC resin was kindly provided by Shang hai Chlor-Alkali Chemical

Industry Stock Co., Ltd. Di-n-octylo-phthalate (DNOP) was supplied by Aladdin Chemical Co., Ltd. CuAl-LDH and MgAl-LDH were synthesized by hydrothermal and co-precipitation method, respectively. It has been described in the previous report.23

2.2. Preparation of PVC/LDH composites.

The test samples were prepared as

follows: 1.75g different type of LDH was dispersed into 35g PVC powder containing 14g DNOP, followed by vigorous grinding for 30 min at room temperature until the viscosity of mixture was significantly reduced so that it can flow. Then, the pasty compounds were molded at 100°C for 1 h to give square pieces with 105×105×2 mm3.

2.3. Characterization. Thermogravimetric analysis-infrared spectrometry (TG-IR) was performed using the SII TGA/DTA 6300 thermogrametric analyzer coupled with Nicolet 6700 FTIR spectrophotometer. The heating rate was 20 °C min-1 from 50 °C to 600 °C under nitrogen atmosphere. Total smoke release (TSR), Smoke production rate (SPR), and the burning residues were measured by a FTT cone calorimeter according to ISO 5660 at a heat flux 25 kW/ m2. The samples were molded and then cut to square pieces with a size of 105×105×2 mm3. The char residues element analysis was carried out with Vario EL Ⅲ element analyser. The morphology and chemical composition of the residual char collected after the cone calorimeter tests were observed by scanning electron microscopy (Hitachi S-3400N) and energy dispersive spectroscopy (Ametek), respectively. The structure of the char residues was characterized by


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using the Raman spectroscopy method (DXR532 model, made by Thermo Fisher Company, USA). The excitation source was a 532 nm wavelength laser with a power of 400 mW, and the detection range of the spectrometer was from 200 to 2000 cm−1 with a distinguishability superior of 5 cm−1.

RESULTS AND DISCUSSION 3.1 TG analysis. The pyrolysis process of PVC and its composites were investigated by TGA in the temperature range of 50–600 °C. The TG and DTG curves are presented in Fig. 1. It can be seen that the degradation of PVC as well as PVC/LDH composites present two distinct weight loss stages. The first region range from 50°C to 400°C and the second is in the range between 400 and 600°C. The first stage is attributed to the dehydrochlorination of PVC and volatilization of the plasticizer while the second one is owing to the further degradation of the molecular chains after dechlorination.24 From the DTG curves, as shown in the Fig. 1(a), At the first stage, the onset of thermal degradation temperature, the temperature at the maximum rate and the termination degradation temperature are virtually identical for PVC and PVC/MgAl-LDH, but the maximum mass loss rate of the pure PVC is higher than that of PVC composite with MgAl-LDH, suggesting that the introduction of MgAl-LDH remarkably enhance the thermal stability of PVC and defer its degradation. This can be explained by the fact that the hydrogen chloride gas formed by PVC dehydrochlorination is absorbed by means of interlayer anion exchange to inhibit the autocatalytic degradation process, in addition, the hydrogen chloride could react with MgAlLDH layers to form corresponding chloride salts.25 However, when the CuAl-LDH is added into the PVC resin, the degradation process of PVC presents completely different results. The


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temperature at the maximum weigh loss rate of PVC/CuAl-LDH composite appeared at 295°C is lower than that of PVC resins. Meanwhile, the degradation temperature at the weight loss of the first stage spans a narrower temperature range from 236°C to 330 °C. This indicates that the CuAl-LDH can effectively stimulate the dehydrochlorination of PVC and significantly accelerate the degradation of PVC during the first stage. This is because the released hydrogen chloride is absorbed by CuAl-LDH to form a number of Lewis acids sites, which would give rise to the further dehydrochlorination of PVC. From the TG curves, as shown in Fig. 1(b), at the beginning of second stage, we can find that there is no much distinction between PVC/MgAl-LDH and PVC/CuAl-LDH in terms of the mass lose rate through by the degradation of the first stage. But at the end of thermal degradation, the PVC/CuAl-LDH displays higher residue compared to PVC/MgAl-LDH. What is more, it can be obtained from DTG profiles that the maximum mass loss rate of the PVC/CuAl-LDH is inferior to that of neat PVC and PVC/MgAl-LDH at the second stage. Nevertheless the temperature at the maximum rate of the second stage appears at a higher site. These results suggest that the CuAl-LDH not only inhibit the molecular chain fracture, but also acts as an effective catalyst to facilitate the intermolecular cross-linking reaction with the formation a lot of more stable char.26, 27


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Figure 1.DTG and TG curves of PVC, PVC/MgAl-LDH and PVC/CuAl-LDH 3.2 TG-FTIR analysis. With the purpose of getting more information about the influence of CuAl-LDH on the volatile components during the pyrolysis of PVC and its composites, the evolved gaseous volatiles were examined by TG-IR, as presented in Fig. 2. The FTIR spectra showed the dynamic changes of the PVC and PVC/LDH composites at four different temperatures, which corresponds to the temperature that the hydrogen chloride stared releasing, the temperature at the maximum rate of the first stage, the termination releasing temperature of the hydrogen chloride, and the temperature at the maximum rate of the second stage, respectively. There are no FTIR absorption peaks generated at temperature of 200 °C for PVC and PVC/LDH, but with temperature increasing, the plasticizer first breaks down. The absorptions at 2934 cm-1, 2866 cm-1 and 1466 cm-1correspond to the methylene (−CH2−) group. Moreover, the absorption peaks at 910/1072/1123/1256 cm-1 and1806/1868 cm-1 can be ascribed to the C−O−C and C=O bonds, respectively.28 These results fully demonstrate that the gaseous products are derived from the DNOP. Then, PVC begins to decompose at a higher temperature. The appearance of the absorption band at 671 cm-1 represents the stretching vibration of C−Cl bond. The C−Cl bond in the PVC structure has lower bond energy compared to the C−C and C−H bonds. So, once the PVC is pyrolyzed, the C−Cl bonds will be given priority to break, which will lead to the release of hydrogen chloride.29, 30 A group of absorption peaks in 3100-2600cm-1 region are found which is ascribed to asymmetrical stretching of H−Cl.31 Moreover, when the temperature at the highest loss rate of the first stage, the intensity of the absorption peak reaches its maximum. Whereas, it is clear that the hydrogen chloride and other gaseous products intensity of IR spectra reduced


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apparently at the temperature around 400 °C, which indicates that the hydrogen chloride has been completely removed and char begins to form. Finally, as the increasing of temperature, only very weak peaks are found at the infrared spectra, this is because the dehydrochlorinated product continues with cracking at an elevated temperature, resulting in the release of low aliphatic hydrocarbon.32 Beyond that, Fig. 2 shows the FTIR spectra of PVC, PVC/MgAl-LDH and PVC/CuAl-LDH. It can be found that, for the PVC/CuAl-LDH composites, the temperature at which hydrogen chloride begins to release is obviously earlier than other materials. And, when all the hydrogen chloride is released the corresponding temperature was 326 °C, Compared with the PVC and PVC/MgAl-LDH , which is advanced by 61 °C and 50 °C, respectively. In addition, there is a legible change taken place at 671 cm-1, a new characteristic peak can be observed at the maximum mass loss rate temperature, which belongs to the stretching vibration of C−Cl bond. However, the absorption intensity of the C−Cl vibration of PVC/CuAl-LDH at this temperature is considerably lower than that of PVC and PVC/MgAl-LDH. These results show that the addition of CuAl-LDH greatly accelerates the dehydrochlorination leading to the release of hydrogen chloride in large quantities within a narrower temperature region. This could be explaining by the fact that the low-energy C−Cl is first broken under the action of CuAl-LDH so that more chlorine is released in the form of hydrogen chloride. Then the resulting hydrogen chloride reacts with CuAl-LDH to produce potent Lewis acids which heavily promotes the generation of hydrogen chloride.


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Figure 2.FTIR spectra of volatile products for (a) PVC, (b) PVC/MgAl-LDH and (c) PVC/CuAl-LDH at different temperatures 3.3 Cone calorimetry and Element analysis. To further investigate the effect of CuAl-LDH on promoting the char formation process of PVC compounds, the combustion tests were performed by cone calorimeter. The char residues of different PVC compounds are obtained from 600 °C and the digital photos are shown in Fig. 3. We can see that there are almost no residues left for the pure PVC after burning. Although both PVC/MgAl-LDH and PVC/CuAlLDH composites present a large amount of residue, there is an obvious dissimilarity in the morphology of the residue. Compared to the loose and dilapidated residue of PVC/MgAl-LDH composites, the residue of PVC/CuAl-LDH composite contraction forms a blocky porous structure with a thickness of about 15 mm. This may be attributed to that there is some char exist in the residue, which acts as a frame and plays a supporting role so that no collapse occurred. At the same time, the smoke density is also monitored. The total smoke production and smoke production rate curves of PVC, PVC/MgAl-LDH and PVC/CuAl-LDH are shown in Fig. 4. The total smoke release is 2624, 2223 and 1677 m2/m2 for PVC, PVC/MgAl-LDH and PVC/CuAlLDH, respectively. The release of black smoke dropped significantly, especially, the addition of CuAl-LDH reduced the yield of smoke by 36%. As we know, the black smoke is produced due to incomplete combustion of aromatic hydrocarbons generated by the degradation of PVC, and


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the cyclization of polyolefins is favorable for the generation of aromatic hydrocarbons.33 The decreasing in the amount of smoke released means the formation of a more stable polycyclic conjugate structure under the action of CuAl-LDH. After that, Element analysis is chosen to inspect the carbon content of the PVC/MgAl-LDH and PVC/CuAl-LDH residues. As listed in Table 2, the carbon content is largely differing, which the PVC/MgAl-LDH is only 1.22%, yet, the PVC/CuAl-LDH reaches 8.92%. The carbon content of acid-washed PVC/CuAl-LDH residues is nine times that of untreated samples reaching 80.56%. This indicates that the introduction of CuAl-LDH promotes the linear unsaturated structures to translate into the ring-shaped conjugated structures. After that more compact char layers are formed through the cross-linking reactions during combustion, which can increase diffusion control of the oxygen, suppress the smoke production leading to an increase in the yields of char and a decrease in the releasing amount of smoke.34

Figure 3.The photographs of residues after CONE test


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Table 1. Elemental analysis in solid residue Sample

C (%)

H (%)

N (%)

PVC/MgAl-LDH

1.22

1.47

≤0.05

PVC/CuAl-LDH

8.92

0.51

≤0.05

PVC/CuAl-LDH(acid-washed)

80.56

1.82

≤0.05

Char residues acid-washed process: the volume of aqua regia to char residues is 3: 1; Soak at 25 °C for 24 h.

Figure 4.Total smoke production and Smoke production rate curves for PVC, PVC/MgAl-LDH and PVC/CuAl-LDH samples 3.4 SEM-EDS analysis. In order to understand the relationships between the carbon chains structure and char formation, the structural and chemical changes for the residue char which were treated with aqua regia after cone calorimeter tests were studied in detail. The morphology and microstructure were investigated by SEM. The detailed images are given in Fig. 5. It could be seen that the char residues present smooth and compact surface. The char layer was improved by the cross-linking reactions, by which a large number of denser char layer was generated.35 and these closely packed chars layer exhibit excellent barrier property, decreasing the decomposition of the matrix. In addition, some cavities at the fractured surface


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were found, which were derived from the dissolution of the acid to the solid particles. These solid particles are dispersed uniformly and embedded in the char layer, which bond together matrix to form larger aggregates contributing not to the heat insulation improvement but also to the flame retardancy enhancement. The chemical constitutions of the charring residue were investigated by EDS. The EDS results were presented in Fig. 5b. The predominant elements in residue were C, Cu and Al. A small amount of Cl, mostly coming from the aqua regia was also detected. Due to the high solubility, most of the chlorides were removed by washing treatment. Among them, C element occupied an absolute superiority in number, which constructed the foundation of the decomposition residue. Thanks to the efficient carbonization reactions promoted by CuAl-LDH at the early stage of pyrolyzation. More carbon chains form polycyclic cross-linked structure, which imply denser morphology as well as higher carbon content.36

Figure 5.SEM image of the acid-washed char residue and the EDS spectrum result corresponding to the specific region marked in figure. 3.5 Raman analysis. Raman spectrum was used to characterize carbon structure of the acidwashed char residue, and the results are presented in Fig. 6. The D band (at 1339 cm−1) and G


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band (at 1591 cm−1) correspond to the polycyclic aromatic species, which reflect the sp3 hybridized amorphous carbon structure and sp2 hybridized C=C structure in char samples separately.37 The degree of graphitization is evaluated by the IG/ID ratio, which is defined as the intensity value ratio of G peak to D peak. The increase in the graphitization degree of char residues indicates that the presence of CuAl-LDH may promote the intermolecular cross-linking, cyclization and aromatization reaction leading to the formation of lager size polycyclic aromatic species with graphitized structure.

Figure 6.Raman image of the acid-washed char residue 3.6 Catalysis process and mechanism of CuAl-LDH. The catalytic mechanism of the dehydrochlorination and char formation of the CuAl-LDH were explicated by the fact that the HCl were firstly absorbed by CuAl-LDH to form CuCl2/AlCl3 with a lot of Lewis acidic sites which can combine with electrons and radicals to accelerate the dehydrochlorination of PVC. Then the carbenium ion and the conjugated unsaturated structure were formed by the further dehydrochlorination of the molecular. After that, the carbenium ion reacted with the unsaturated structure and olefin to form intermolecular cross-linking resulting in a network structure. At the


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same time, these conjugated olefin segments can also undergo isomerization, cyclization, aromatization and rearrangement reactions to form the ring structures. Finally, these cross-linked unsaturated structures and the ring structures form the polycyclic aromatic hydrocarbons structures which is the precursor of char. 21, 38

CH2 CH CH2 CH

CH Cl

Cl

CH2

CH Cl

Cl

CuAl-LDH R1 CH

CH CH

CH CH

CuCl2/AlCl3

R1 CH

CH

+

CH CH CH R4

CH

R2

R1 CH

CH CH

R1 + R2

CH

CH

R3 CH

CH

CH

CH R2

CH CH

CH R4 CH

CuCl3/AlCl4

H

+ R3 CH

CH

CH

CH R2 + CuCl2/AlCl3 + HCl

CH

CH CH

CH CuCl3 /AlCl4

Char Scheme 1. The charring mechanism of the PVC by CuAl-LDH


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CONCLUSIONS In this study, the CuAl-LDH exhibited extraordinary catalysis in the pyrolysis and combustion of PVC. TGA results showed that not only the release temperature was advanced but also the char residues were increased. According to the characterized of TGA-FTIR, FTIR, EDS, EA analysis, SEM for gaseous volatiles and solid char, the catalysis of CuAl-LDH was determined which enabled to accelerate the dehydrochlorination of PVC and promoted the conjugated olefin undergo isomerization, cyclization, aromatization and rearrangement reactions to produce polycyclic aromatic hydrocarbons structures with larger size. The addition of CuA-LDH decreases the release of aromatics and increases the yield of solid char significantly. The results of this work show that the CuAl-LDH may be applied to PVC thermal degradation processes to carry out material and chemical recycling.

AUTHOR INFORMATION Corresponding Author 1 *Address: College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Shanghai 201620, China. Tel: +86-21-67791214; Fax: +86-21-67791214; E-mail: [email protected]. Corresponding Author 2 *Address: College of Mechanical Engineering, Shanghai University of Engineering Science, Shanghai

201620,

China.

Tel:

+86-21-67791385;

Fax;

+86-21-67791385;

E-mail:

[email protected].

ACKNOWLEDGEMENT


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The authors gratefully acknowledge the supporting from the fund of UniversityStudents' Research and Innovation Projects ofChina (No. 201710856013).

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Catalytic Dechlorination and Charring Reaction of Polyvinyl Chloride

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