Electrophilic Chlorination of Naphthalene in Combustion Flue Gas

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Environmental Processes

Electrophilic Chlorination of Naphthalene in Combustion Flue Gas Dan Wang, Haijun Zhang, Yun Fan, Meihui Ren, Rong Cao, and Jiping Chen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.9b00350 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 6, 2019

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Electrophilic

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Combustion Flue Gas

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Dan Wang,†,‡ Haijun Zhang,*,† Yun Fan,† Meihui Ren,†,‡ Rong Cao,†,‡

4

Jiping Chen†

5 6 7 8

†

Chlorination

of

Naphthalene

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in

CAS Key Laboratory of Separation Sciences for Analytical Chemistry, Dalian Institute

of Chemical Physics, Chinese Academy of Sciences, Dalian, Liaoning, 116023, China ‡

University of Chinese Academy of Sciences, Beijing 100049, China

TOC/Abstract Art

9 10 11 12 13 14 15 16 17 18 19

*

20

Phone: +86-411-8437-9972, fax: +86-411-8437-9562; e-mail: [email protected]

Corresponding Authors


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Naphthalene chlorination is an important formation mechanism of polychlorinated

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naphthalenes (PCNs) in combustion flue gas. In this study, a total of 21 metal chlorides

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and oxides were screened for their activities in the electrophilic chlorination of

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naphthalene. Copper (Ⅱ) chloride exhibited the highest activity at 200–350 °C, followed

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by copper (Ⅰ) chloride. Copper (Ⅱ) chloride primarily acted as a strong chlorinating

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agent to facilitate chlorine substitution on naphthalene. Iron (Ⅱ and Ⅲ) chlorides were

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only highly active at 200–250 °C. At 250 °C, the average naphthalene chlorination

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efficiency over CuCl2·2H2O was 7.5-fold, 30.2-fold and 34.7-fold higher than those

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over CuCl, FeCl3·6H2O and FeCl2·4H2O, respectively. The other metal chlorides were

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less active. Under heated conditions, copper (Ⅱ) and iron (Ⅲ) chlorides were

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transformed to copper (Ⅰ) and iron (Ⅱ) chlorides via dechlorination, and then

32

transformed

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oxychlorination cycles of copper and iron species, respectively. The results obtained

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suggest that electrophilic chlorination of naphthalene in combustion flue gas is

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primarily driven by dechlorination-oxychlorination cycles of copper and iron species,

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and the reaction produces a selective chlorination pattern at 1 and 4 positions of

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naphthalene.

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INTRODUCTION

to

oxychlorides

and

oxides,

thereby

forming

dechlorination-

39

Polychlorinated naphthalenes (PCNs) are a group of common persistent organic

40

pollutants (POPs),1 and some PCN congeners have toxicity comparable to those of

41

polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs).2,

42

production of PCN formula has been prohibited since the 1980s.4, 5 Currently, PCNs in

43

environmental matrices, especially the atmosphere,6, 7 mainly originated from thermal

44

treatment applications, such as municipal solid waste incineration (MSWI),8, 9 iron ore

45

sintering (IOS),10 and metal smelting and coking.11, 12 The emission level of PCNs


3

The industrial


46

during most industrial thermal processes was reported to be the same order of

47

magnitude as that of PCDD/Fs.13 To control the emission of PCNs, it is necessary to

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understand the mechanisms of PCN formation within industrial thermal processes.

49

Several previous studies have proposed that PCNs in the post-combustion zone can

50

be formed, together with PCDD/Fs, through de novo synthesis from macromolecular

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carbon and the coupling of precursors such as chlorinated phenols.14, 15 In MSWI fly

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ash and stack gases, the levels of tri- to penta-CNs were found to be correlated with

53

those of tri- to hexa-CDDFs.9 Moreover, the chlorination of unsubstituted naphthalene

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also plays an important role in PCN formation during thermal processes. Naphthalene,

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as a typical of polycyclic aromatic hydrocarbons (PAHs), can be largely formed via a

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hydrogen-abstraction/acetylene-addition mechanism during the combustion of

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chlorine-containing substances.16, 17 The levels of naphthalene in MSWI stack gases

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were determined to be 1–50 ug/Nm3,18 which were 2–3 orders of magnitude higher than

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those of total PCNs.13 A strong correlation has been observed between the levels of less-

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chlorinated PCNs and naphthalene in MSWI flue gases.19

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The combustion of a chlorine-containing substance is usually accompanied by the

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generation and volatilization of high levels of HCl,20 which provides chlorine for the

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chlorination of naphthalene in the post combustion zone. In 1990, Hoffman et al.21, 22

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confirmed that the electrophilic substitution mechanism predominated over the

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aromatic chlorination reaction on the surface of MSWI fly ash in HCl-containing

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atmospheres. They proposed the possible mechanism: HCl first reacted with iron (Ⅲ)

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sites on the fly ash surface and then produce iron (Ⅲ) chloride species that act as

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chlorinating agents. The putative mechanism for the electrophilic chlorination of

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aromatic compounds is widely believed to be according to the following principle.

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Chloride cations (Cl+) are generated by the polarization and dissociation of Cl2 under


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Lewis acid catalysts, and Cl+ attacks the aromatic ring accompanied by the removal of

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a hydrogen ion (H+) (Scheme 1).23

73 74

Large quantities of chlorine and metal elements can be detected in the MSWI fly

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ash,24–27 which implies an abundance of classical Lewis acid catalysts containing

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chlorine, such as FeCl3, AlCl3, ZnCl2 and CuCl2.28 In the post-combustion zone, HCl

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can be converted to Cl2 via the Deacon process though the catalysis of metal chlorides

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and oxides present in MSWI fly ash.29–31 In addition, the dechlorination of variable-

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valence metal chlorides such as CuCl2 and FeCl3, the oxidation of metal chlorides,32

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and chlorine exchange between NaCl and metal oxides in the presence of SiO2 could

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also contribute a lot to the Cl2 in combustion flue gas.20, 33, 34 Alternatively, CuCl2 and

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FeCl3 were found to act as chlorinating reagents that directly react with aromatics via

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an electrophilic substitution mechanism.35–37

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Two recent laboratory studies have substantiated the claim of PCN production via

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the electrophilic chlorination of naphthalene during municipal solid waste

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combustion.23, 38 Nevertheless, the chemical features of the chlorination process are still

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largely unknown. In this study, we investigated the influence of Cu, Fe, Mn, Cr, Co, Ni,

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Al, Ti, Pb and Zn chlorides and oxides on the formation of PCNs by simulating the

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atmosphere and status of combustion flue gas. Furthermore, changes in phase and

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surface state of copper and iron chlorides before and after reaction were characterized

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by X-ray diffraction (XRD). With the aim of drawing a map of the PCN chlorination

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route and identifying the key control points, the congener-specific profiles of PCNs

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under our set condition were analyzed by a high-resolution gas chromatography



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coupled with high-resolution mass spectrometry (HRGC–HRMS). The results obtained

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provide important insights into the thermodynamic and kinetic characteristics of PCN

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formation in combustion flue gas.

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EXPERIMENTAL SECTION

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Chemicals. Parent naphthalene (purity: > 99.7%) was purchased from Macklin.

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The PCN calibration standard solutions (PCN-MXA and PCN-MXC) were obtained 13

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from the Wellington Laboratories Inc. (Guelph, Canada).

C12-labeled internal

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standard stock solution (ECN-5217, 13C10-CN-2; ECN-5520, 13C10-CN-6; ECN-5102,

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13

103

5260,

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Andover, MA, USA). The details of other chemicals and reagents are shown in

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Supporting Information (SI).

C10-CN-27, -42, -52, -67, -73, and -75) and recovery standard stock solution (ECN13

C10-CN-64) were purchased from Cambridge Isotope Laboratories (CIL,

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Experimental Details and Chemical Characterization. To explore the

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electrophilic chlorination mechanisms of naphthalene, we prepared a reaction medium

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that imitates the smoke dust in the flue gas of solid waste incineration. The reaction

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medium was a mixture of silicon dioxide (SiO2), catalytic metal species and sodium

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chloride (NaCl). A total of 21 catalytic metal species, including Cu, Fe, Mn, Cr, Co, Ni,

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Al, Ti, Pb and Zn chlorides and oxides, was adopted. In each reaction medium, the mass

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percentages of catalytic metal ions and chlorine were fixed at 0.5% and 10%,

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respectively. The remainder SiO2, catalytic metal species and NaCl were mixed and

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ground in a mortar for 15 min. A blank reaction medium without any catalytic metal

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species was prepared by mixing only SiO2 and NaCl.

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The chlorination experiment was conducted in a quartz tube flow reactor (length: 200

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mm, internal diameter: 4 mm), which was installed in the injection port of modified gas

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chromatograph (SI Figure S1). The quartz tube flow reactor was divided into two zones:


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the reaction zone and adsorption zone, which were packed with 0.2 g of reaction

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medium and 0.2 g of florisil adsorbent, respectively. Approximately 0.2 g of glass wool

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was first packed into the quartz tube flow reactor as a supporter, followed by florisil

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adsorbent. The reaction zone and adsorption zone were isolated using 0.2 g of glass

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wool. The height of the packed bed was approximately 24 mm. The reaction zone of

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the quartz tube flow reactor was inserted into the metal block heater of the gas

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chromatograph injector to achieve accurate control of the reaction temperature. The

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absorption zone of the quartz tube flow reactor was placed in the gas chromatograph

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oven, in which the temperature was kept below 30 °C by switching out the heating

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system. A capillary column was connected to the outlet of the quartz tube flow reactor

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to guide the exhaust gas into an absorbing bottle filled with n-hexane solution.

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An aliquot of naphthalene solution (solvent: n-hexane) was injected into the injection

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port of the modified gas chromatograph. Vaporized naphthalene was transported to the

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reaction bed by the purge gas with flow rate of 6 mL/min. The reaction atmosphere was

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controlled by adjusting the volume ratio of O2 (0, 5%, 10% and 21%) and N2, and the

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reaction temperature was controlled by changing the injector temperature. The radial

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variation in the reaction bed temperature was deemed to be negligible because of the

136

small tube diameter. After the set temperatures (100, 150, 200, 250, 300 and 350 °C)

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were reached, the reaction bed was continuously heated for 2 min, and subsequently,

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the naphthalene solution was injected to conduct a catalytic reaction for 20 s. By the

139

end of the reaction, the purge gas flow was first stopped, and then the reaction tube was

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immediately taken out from the metal block heater of the gas chromatographic injector

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and then quenched in a mixture of ice and water to stop the reaction. Due to the small

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volume of purge gas (2 mL), above 99.5% of formed PCNs still remained in the reaction

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zone. Only < 0.5% of formed PCNs was transported into adsorption zone (glass wool



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and florisil), and no PCNs was detected in the exhaust gas.

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To explore the naphthalene electrophilic chlorination mechanism, the crystal phase

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variations in Cu and Fe species before and after reaction were measured using XRD on

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a PANAlytical X'pert Pro diffractometer. Data were recorded in the range of 5-90° 2θ

148

with a step size of 0.033° and a counting time of 19.68 s/step.

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Sample Pretreatment and PCN Analysis. After the reaction, the reaction

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medium and florisil adsorbent were removed from the quartz tube flow reactor and put

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into a 100 mL conical flask. The internal wall of the quartz tube flow reactor was

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cleaned by rinsing with a mixture of dichloromethane (DCM)/n-hexane (3:1, v:v) 5

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times, and the rinse solution was also put into the 100 mL conical flask. The formed

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PCNs were extracted with the mixture of DCM/n-hexane (3:1, v:v) in an ultrasonic

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extractor for 1 h. Prior to extraction, 10 μL of a

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solution (a mixture of ECN-5217, ECN-5520 and ECN-5102) was spiked, and the

157

extract volume was adjusted to 50 mL. The extract was cleaned up by eluting through

158

an anhydrous sodium sulfate (5.0 g) column. The cleaned extract was concentrated to

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approximately 1 mL by a rotary evaporator and evaporated to approximately 20 μL in

160

a vial under a gentle stream of nitrogen. Finally, 10 μL of 13C10-labeled PCN recovery

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standard solution (ECN-5260) was added and then subjected to instrumental analysis.

13

C10-labeled PCN internal standard

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To evaluate the role of naphthalene chlorination process in the PCN formation in real

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combustion flue gases, the experimentally observed homologue and isomer distribution

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of PCNs were compared with real fly ash samples from MSW incinerators, IOS plants

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and steel smelting (SS) plants in China. The details on the sample collection and

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preparation are shown in SI. Elemental abundance in these fly ash samples was

167

analyzed by X-ray fluorescence (PANalytical Zetium XRF), and the result is shown in

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SI Table S2.


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The analysis of all PCN congeners was performed using an Autospec Ultima high

170

resolution mass spectrometer interfaced with a Hewlett-Packard 6890 Plus gas

171

chromatograph (HRGC–HRMS) equipped with a Rtx-5 MS capillary column with

172

length 60 m, i.d. 0.25 mm and film thickness 0.25 μm. Detailed information about the

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HRGC/HRMS method is shown in SI.

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Quality Assurance and Quality Control. Before each run, the quartz tube flow

175

reactor was cleaned and then baked in a furnace at 650 °C for 1 h. All laboratory

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glassware for sample preparation was rinsed with the mixture of dichloromethane

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(DCM)/n-hexane (3: 1, v: v) 3 times before use. PCNs were quantified using the isotope

178

dilution HRGC-HRMS method.39 The recoveries of the internal standards (13C10-

179

labeled PCN congeners) were in the range of 30–106%. Each treatment of naphthalene

180

chlorination was conducted in triplicate, and a blank experiment (without the reaction

181

substrate naphthalene) was performed for each treatment. The yields of PCNs in the

182

reaction samples were corrected by the recoveries of the internal standards and the

183

average PCN levels in blank experiment samples.

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RESULTS

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Activities of Different Metal Species. A total of 21 metal chlorides and oxides

186

were screened for naphthalene chlorination activity. The screening fulfilled the

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following two criteria: (1) possible existence in MWSI fly ash (SI Table S2) and (2)

188

possible participation in an electrophilic aromatic substitution reaction. The catalytic

189

reactions were conducted at 250 °C under an atmosphere of 10% O2 + 90% N2. The

190

catalytic activity was indicated by both PCN yield and chlorination efficiency, which

191

was defined as mmol of PCNs per mol of metal ion and mmol of Cl substituents per

192

mol of metal ion, respectively. As shown in Figure 1, copper and iron chlorides

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exhibited higher activities. CuCl2·2H2O was the most active catalyst, followed by CuCl,



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CuCl2·3Cu(OH)2, FeCl3·6H2O and FeCl2·4H2O. The average yield of PCNs over

195

CuCl2·2H2O was 0.7 mmol/mol metal ion, which was 3.6-fold, 3.7-fold, 18.9-fold and

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14.7-fold higher than those over CuCl, CuCl2·3Cu(OH)2, FeCl3·6H2O and FeCl2·4H2O,

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respectively. However, the other metal chlorides (MnCl2·4H2O, CrCl3·6H2O,

198

CoCl2·6H2O, NiCl2·6H2O, ZnCl2, PbCl2 and AlCl3·6H2O) were less active. Among the

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metal oxides, CuO and Fe2O3 showed minor catalytic activity, with a PCN yield of

Electrophilic Chlorination of Naphthalene in Combustion Flue Gas

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