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The Thermal Transformation Mechanism of Chlorinated Paraffins: an Experimental and Density Functional Theory Study

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The Thermal Transformation Mechanism of Chlorinated Paraffins: an Experimental and Density Functional Theory Study JOURNAL OF ENVIRONMENTAL SCIENCES 75 (2019) 378– 387 Available online at www.sciencedirect.com ScienceDirect www.elsevier.com/locate/jes The thermal transformation mechanism of chlorinated paraffins: An experimental and density functional theory study Shanzhi Xin1,2, Wei Gao1,4, Dandan Cao1, Kun Lv1,6, Yaquan Liu5, Chunyan Zhao5,⁎, Yawei Wang1,3,4,⁎, Guibin Jiang1 1. State Key Laboratory of Environmental Chemistry and Ecotoxicology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China 2. Hubei Key Laboratory of Industrial Fume and Dust Pollution Control, Jianghan University, Wuhan 430056, China 3. Institute of Environment and Health, Jianghan University, Wuhan 430056, China 4. University of Chinese Academy of Sciences, Beijing 100049, China 5. School of Pharmacy, Lanzhou University, Lanzhou 730000, China 6. Environment Research Institute, Shandong University, Jinan 250100, China ARTICLE INFO ABSTRACT Article history: The increasing production and usage of chlorinated paraffins (CPs) correspondently increase the Received 12 April 2018 amount of CPs that experience thermal processes. Our previous study revealed that a significant Revised 24 May 2018 amount of medium-chain chlorinated paraffins (MCCPs), short-chain chlorinated paraffins Accepted 25 May 2018 (SCCPs) as well as aromatic and chlorinated polycyclic aromatic hydrocarbons (Cl-PAHs) were Available online 8 June 2018 formed synergistically during the thermal decomposition of CP-52 (a class of CP products). However, the transformation mechanisms of CP-52 to these compounds are still not very clear. Keywords: This article presents a mechanistic analysis on the decomposition of CP-52 experimentally and Decomposition mechanism theoretically. It was found that CP-52 initially undergoes dehydrochlorination and carbon chain Chlorinated paraffins cleavage and it transformed into chlorinated and unsaturated hydrocarbons. Cyclization and Chlorinated aromatic hydrocarbons aromatization were the most accessible pathways at low temperatures (200–400°C), both of Density functional theory (DFT) which produce mostly aromatic hydrocarbons. As the temperature exceeds 400°C, the hydrocarbons could decompose into small molecules, and the subsequent radical-induced reactions become the predominant pathways, leading to the formation of Cl-PAHs. The decomposition of CP-52 was investigated by using density functional theory and calculations demonstrating the feasibility and rationality of PCB and PCN formation from chlorobenzene. The results improve the understanding of the transformation processes from CP-52 to SCCPs and Cl-PAHs as well as provide data for reducing their emissions during thermal-related processes. © 2018 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V. Introduction (POPs) (Ma et al., 2014; Wang et al., 2017; Zeng et al., 2011). In May 2017, SCCPs were listed in Annex A of the Stockholm Short-chain chlorinated paraffins (SCCPs) are a type of Convention by the Persistent Organic Pollutants Review chlorinated paraffins (CPs) that have attracted attention due Committee (POPRC, 2017). However, when compared to to having similar properties to persistent organic pollutants studies regarding the environmental occurrence, distribution ⁎ Corresponding authors. E-mails: [email protected] (Chunyan Zhao), [email protected] (Yawei Wang). https://doi.org/10.1016/j.jes.2018.05.022 1001-0742 / © 2018 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V. JOURNAL OF ENVIRONMENTAL SCIENCES 75 (2019) 378– 387 379 and fate of SCCPs, data on the sources of SCCPs in the transformation of CP-52 and provide data for controlling the environment and pathways to humans are still scarce (Bayen emissions of SCCPs and Cl-PAHs synergistically during thermal et al., 2006). processes. Generally, SCCPs are mixed with other CPs in commercial products, and the majority of SCCPs will remain in the polymers that contain CPs during the entire life cycle of CPs 1. Materials and methods (Zhang et al., 2017). However, these polymers are typically used in products that are likely to be involved in high- 1.1. Materials, experiment and product analysis temperature processes, such as plastic/polymer processing or open burning, and the SCCPs along with other contaminants Commercial CP-52 (chlorination degree 52% ± 2%) was se- can be released into the environment unintentionally. Our lected as the feedstock from a chemical company in Ningbo previous study revealed that the commercial CP-52 released a City, China. It was in the form of transparent viscous oil and considerable amount of SCCPs and medium-chain chlorinated contained 10.9 wt% of SCCPs and 85.5 wt% of MCCPs. The paraffins (MCCPs) into the gas phase when temperatures reach dominant carbon and chlorine atom homologs of SCCPs were – 200 400°C (Xin et al., 2017). C13 and Cl7–8, while those in MCCPs were C14 and Cl8. The Currently, numerous studies have been conducted focusing experiments were performed in a bench-scale furnace at 200, on the thermal decomposition of polyvinyl chloride (PVC) (Gui et 300, 400, 600 and 800°C under oxidative and inert conditions. al., 2013; Yu et al., 2016). Nevertheless, the thermal decomposi- The volatile fraction (VF) was absorbed by downstream tion characteristics of PVC were much different with that of CPs absorption bottles with 50 mL dichloromethane and hexane since the former is a polymer with a high molecular weight (1:1, V/V) mixtures. After that, the VF was concentrated and while CPs are generally n-alkanes with shorter carbon chain analyzed by an Agilent 7200 GC-QTOF mass spectrometer with 13 length (Camino and Costa, 1980; Marongiu et al., 2003; Schinkel 10 ng of 1,5,5,6,6,10-hexachlorodecane ( C10) in a 100 mg/mL et al., 2017; Sosa, 1975). For instance, the pyrolysis of PVC is solution in cyclohexane (Andover, USA) as the internal characterized by two distinct stages. The first stage is the standard. A negative chemical ionization mode was used to dehydrochlorination of the polymer while the second stage analyze the concentration of the SCCP and MCCP congeners in corresponds to the further decomposition of dechlorinated PVC the VF, while an electron ionization source was used to detect (Yu et al., 2016). For CPs, studies indicated that they were the organic compounds (aliphatic hydrocarbons, PAHs and degraded with a single-stage process involving dehydrochlori- chlorinated aromatics). The organic compounds were then nation and the subsequent decomposition (Sosa, 1975; Xin et al., identified by the NIST library. The chlorinated alkanes with 2017). carbon chain lengths of nine (C9) were detected qualitatively So far, only a few studies have investigated the thermal by comparing the ratios of quantitative ion to the qualitative degradation mechanism of CPs or polychlorinated alkanes ion. Furthermore, a micro-pyrolyzer (CDS 5200) coupled (Sosa, 1975), and evaluated the thermal stability of chlorinated with gas chromatography–mass spectrometry (Py–GC–MS) paraffins with 18% to 38% chlorine by DSC. The dehydrochlo- was used to investigate the primary decomposition products rination of CPs with 70% chlorine generated conjugated double of CP-52 since it can suppress the secondary reaction largely bonds and inter-molecular addition occurred between the (Wang et al., 2012). In each experiment, 0.15 ± 0.01 mg of the double bonds, leading to a crosslinked chlorinated residue sample was loaded into a quartz tube and placed into the (Camino and Costa, 1980). Bergman et al. (1984) analyzed the pyroprobe. The pyrolysis temperature was set at 400°C, with a degradation products from polychlorinated alkanes with chlo- heating rate of 20°C/msec and pyrolysis time of 20 sec. A rine content ranging from 59% to 70% by GC–MS and proposed detailed summary of the experimental procedures and the some possible pathways. However, the mechanisms regarding product analysis and quantification methods can be refer- the formation of chlorinated and unchlorinated aromatic enced in Appendix A and our previous study (Xin et al., 2017, compounds from CPs are still not yet well understood. 2018). In our previous work, the distribution profiles of SCCPs, MCCPs and chlorinated polycyclic aromatic hydrocarbons 1.2. Calculation methods (Cl-PAHs) as a function of temperature were clarified. These data showed the synergistic emissions of SCCPs, MCCPs and In this computational study, all geometries and transition states Cl-PAHs. However, it lacks a mechanistic evaluation about the are optimized using density functional theory with the B3LYP chemical transformation pathways of SCCPs and the relation- functional and the 6-311+G(d,p) basis set. The zero-point ship between SCCPs and Cl-PAHs which adds to the challenges vibrational energy (ZPVE) corrections were also included in the in identifying effective technologies to eliminate emission. energy calculations. The transition state (TS) was determined Moreover, the source reduction techniques of SCCPs, MCCPs by four steps: first, we speculate an initial structure for the and Cl-PAHs lie in the need to fully understand their formation transition state; then, the Berny algorithm method was used to mechanisms. For this reason, in this study, the density optimize the initial structure for the transition state. In the third functional theory (DFT) methods were applied to elucidate the step, frequency analysis was performed to confirm the TS. All intermediate reactions
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