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Catalysis and Kinetics

Low-temperature pyrolysis-catalysis coupled system for tetrachlorobenzene efficient removal: Condition optimization and decomposition mechanism Pingping Liu, Xiaosheng Yuan, Huarui Ren, Yanke Yu, Ning Xu, Jinglian Zhao, and Chi He Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00095 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 22, 2018

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Energy & Fuels

Low-temperature pyrolysis-catalysis coupled system 1

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2 5 6 7 8

3 1. Rotor flowmeter; 2. Gas inlet; 3. Ice bath; 4. Fly ash and TeCB 5. Quartz sand; 6. Catalyst bed; 7. Quartz wool; 8. Glass beads

Graphical abstract

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1

Low-temperature pyrolysis-catalysis coupled system for tetrachlorobenzene

2

efficient removal: Condition optimization and decomposition mechanism

3

Pingping Liu, Xiaosheng Yuan, Huarui Ren, Yanke Yu, Ning Xu, Jinglian Zhao, Chi He*

4

Department of Environmental Science and Engineering, State Key Laboratory of Multiphase

5

Flow in Power Engineering, School of Energy and Power Engineering, Xi'an Jiaotong University,

6

Xi'an 710049, Shaanxi, P.R. China

7 8

*

9

Tel./Fax: 86 29 82663857; E-mail: [email protected] (C. He)

To whom correspondence should be addressed:

1


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Energy & Fuels

ABSTRACT

11

The decomposition of tetrachlorobenzene (TeCB) emitted from simulated fly ash was studied

12

in a low-temperature pyrolysis-catalysis coupled system. The influences of catalyst support,

13

active component type, active phase/assistant ratio, vanadium loading, and catalyst calcination

14

temperature on TeCB conversion were comprehensively investigated. The optimal catalyst

15

composition and preparation condition were determined through single factor experiments.

16

Moreover, the effects of reaction temperature, space velocity, and pollutant initial concentration

17

on TeCB decomposition efficiency were explored by orthogonal experiments. The possible

18

mechanism for PeCB decomposition over prepared V2O5-WO3/TiO2 catalysts was proposed

19

based on reaction byproduct composition and distribution. It is found that the TeCB

20

decomposition efficiency is positively correlated with reaction temperature, while negatively

21

correlated with gas hourly space velocity (GHSV) and TeCB initial concentration. GHSV has the

22

most significant effect on TeCB decomposition, followed by reaction temperature and TeCB

23

concentration. Under optimum condition of space velocity of 600 h-1, reaction temperature of

24

350 °C, and TeCB initial concentration of 0.5 vol.%, the TeCB conversion can reach up to 94.1%.

25

The key substeps of TeCB decomposition are aromatic pollutant adsoprtion and nucleophilic

26

substitution and intermediates electrophilic substitution. The TeCB molecules were first adsorbed

27

on catalyst surface, and then decomposed into low chlorinated aromatics and aromatic and

28

aliphatic hydrocarbons as phenolates, benzoquinone, aldehydes, and carboxylic acids.

29

Keywords: Tetrachlorobenzene; Low-temperature pyrolysis; Catalytic destruction; Condition

30

optimization; Decomposition mechanism

2


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INTRODUCTION

32

The rapid economic development and improved living standards have led to great increase of

33

municipal solid waste (MSW).1 As one of waste-to-energy technologies, incineration is

34

considered as a strategic option for MSW reduction and disposal because it is possible to obtain

35

70% mass and 90% volume reduction in waste treatment.2,3 However, the emission of fly ash

36

from incineration is one of major and serious threats towards the environment and society.4

37

Municipal solid waste incineration (MSWI) produces fly ash, which accounts for approximately

38

3-5 wt.% of the original waste amount. The MSWI fly ash contains a large amount of toxic heavy

39

metals, such as mercury, plumbum, cadmium, and chromium, and highly toxic dioxins such as

40

polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). These

41

pollutants are extremely harmful to soil and fresh water ecosystems, especially when they are

42

bioaccumulated through earthy and aquatic food webs.5 In China, such PCDD/Fs emissions have

43

been restricted to 0.1 abd 0.5 ng I-TEQ Nm-3 for municipal incinerators.

44

Numerous methods for decomposing PCDD/Fs have been developed, such as activated carbon

45

adsorption, UV photolysis and photocatalytic oxidation, plasma discharge technology, and

46

selective catalytic reduction (SCR) technology.6-13 Only the activated carbon adsorption and SCR

47

technology has entered industrial applications while the other two technologies are still in

48

laboratory research stage or semi-industrial experiment stage. Activated carbon adsorption with

49

bag dedusting technology only entraps dioxins in the gas phase and has no substantial reduction

50

effect on dioxin; SCR technology will affect the stability of operation due to catalyst poisoning;

51

UV photolysis is less effective due to energy efficiency issues. Plasma technology is difficult to

52

bring to the market because of its high equipment costs and operating costs. In addition, low 3


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temperature pyrolysis technology was widely used to degrade dioxins in waste incineration fly

54

ash due to simple operation, low energy consumption, and high removal rate, however, there is

55

still possible to cause secondary pollution.14 Therefore, it is necessary to add a process to reduce

56

secondary pollution after the pyrolysis process. Catalytic oxidation shows several advantages in

57

complete and direct decomposition of chlorinated volatile organic compounds.15 It can be

58

anticipated that the low-temperature pyrolysis-catalysis combined technology can significantly

59

reduce the activation energy of dioxins and decomposition of dioxin at lower temperature, which

60

is a promising method with great potential to integrate energy saving and reduce the formation of

61

secondary pollutant.

62

During the past decades, a number of catalyst systems for the total oxidation of chlorinated

63

compounds were reported. Most of them were focused on three types of catalysts based on noble

64

metals (e.g., Pd, Pt, Rh, Ni, Au, and Ir),16,17 transition metals (e.g., V, W, Mn, Fe, and Cu)18-20 and

65

zeolites.21-23 Noble metal-based catalysts are catalytically more active at low temperature in

66

comparison to transition metal oxides and zeolites, however, they are expensive and susceptible to

67

fast deactivation by chlorine poisoning.21 The activity of zeolites was related to their acid

68

properties. However, the formation of polychlorinated compounds and deposition of coke on

69

these acidic catalysts are to be resolved. In general, transition metal oxides are less active than the

70

noble metals, but they can resist chlorine deactivation to a larger extent and the cost of transition

71

metal oxides is much lower.24 Amongst, the V2O5-WO3/TiO2 catalysts are successfully applied to

72

destroy chlorinated volatile organic compounds effectively.25 For instance, Xu et al. revealed that

73

the V2O5-WO3/TiO2 catalysts were effective in decomposition of pentachlorobenzene.26 Yang and

74

co-workers proposed that V2O5-WO3/TiO2 catalyst had superior activity for PCDD/F 4


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decomposition.27

76

Due to high toxicity of PCDD/PCDFs, some model compounds such as tetrachlorobenzene,

77

pentachlorobenzene, 1,2-dichlorobenzene, and 2-chlorophenol were employed to predict the

78

dioxin destruction behaviors in laboratory study.26,

79

feasibility of low-temperature pyrolysis-catalysis coupled technology for TeCB decomposition,

80

and to figure out the effects of catalyst type and composition and operating conditions such as

81

reaction temperature, space velocity, and pollutant initial concentration on TeCB conversion.

82

Moreover, the optimum reaction conditions and mechanism for TeCB decomposition were also

83

put forward.

84

2. MATERIALS AND METHODS

85

2.1. Chemicals

28-30

This paper aims to investigate the

86

Tetrachlorobenzene (TeCB) was purchased from Shanghai Shanpu Chemical Co., Ltd.

87

Ammonium tungstate and ammonium metavanadate were purchased from Tianjin Fu Chen

88

Chemical Reagent Plant. Activated alumina and coconut shell activated carbon were purchased

89

from Xiangyuan water purification materials plant, Henan Province. Titanium dioxide was

90

purchased from Xingtai chemical plant, Shanghai Jinshan. The reagents are of analytical grade

91

and other reagents are commercially available.

92

2.2. Catalyst preparation

93

All catalysts used in the experiments were prepared by an impregnation method. Typically, a

94

certain amount of oxalic acid was dissolved in deionized water at 60 °C. Ammonium tungstate

95

and ammonium metavanadate were then added in batches and kept stirring for 1 h. After that, the

96

TiO2 support (dried at 105 °C for 2 h in advance) was added and stirred at 60 °C for 4 h. The 5


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sample was then transferred to an oven and dried at 105 °C for 12 h, followed by calcined in

98

muffle furnace at 450 °C for 5 h to obtained the V2O5-WO3/TiO2 (VWTi) catalyst (contents of

99

V2O5 and WO3 are 5.0 and 10.0 wt. %, respectively). In addition, the VWTi catalysts with

100

different calcination temperatures, vanadium loadings, and V/W ratios (w/w) were also

101

synthesized. For comparison, CuO-WO3/TiO2 (CuWTi) and MnOx-WO3/TiO2 (MnWTi) catalysts

102

(contents of CuO/MnOx and WO3 are 5.0 and 10.0 wt. %, respectively) were further prepared by

103

the same method using copper nitrate and manganese acetate as metal precursors, respectively.

104

2.3. Catalyst characterizations

105

N2 sorption isotherms were measured at 77 K on a SSA-4000 apparatus (Builder, China). Prior

106

to the measurements, the samples were evacuated for 4 h under vacuum at 473 K. The total pore

107

volume was estimated from the amount of nitrogen adsorbed at a relative pressure (P/P0) of ca.

108

0.99. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method,

109

and the pore size distribution was derived from the desorption branch of the N2 isotherm using the

110

Barrett-Joyner-Halenda (BJH) method. X-ray diffraction (XRD) measurements were performed

111

using a powder diffractrometer (XRD-7000, Shimadzu Ltd., Japan) with Cu-Kα radiation. The

112

tube voltage was 40 kV, and the current was 40 mA. XRD diffraction patterns were obtained in 2θ

113

range of 10-80° (scanning rate of 4°/min). Fourier transform infrared (FT-IR) spectra were

114

recorded at room temperature on a Bruker Tensor 37 FT-IR spectrometer with 64 scans at an

115

effective resolution of 4 cm-1. The morphology of the adsorbents was observed by field emission

116

electron scanning microscopy (FE-SEM, JSM-6700F, JOEL, Japan).

117

2.4. Decomposition activity

118

Schematic diagram of the low-temperature pyrolysis-catalysis combined system is displayed in 6


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Scheme 1. The low-temperature pyrolysis-catalysis combined reactor is a three-section heating

120

tube furnace actually, and the tubes were charged with simulated TeCB-containing fly ash, quartz

121

sand, catalyst and underfill from top to bottom. The upper section is heated to control the

122

pyrolysis temperature, the middle section is heated to control the catalytic reaction temperature,

123

and the lower section is heated to ensure that the gas is kept in the vaporized state and absorbed

124

by the absorption liquid. During the experiment, aeration was performed for 10 min before

125

warming, and the residual gas in the reactor was discharged. First of all, the catalyst (40-60 mesh)

126

in the middle part of the reactor was heated to bring the catalyst into the activation temperature.

127

The specific temperature was set according to the experimental requirements. Then heating the

128

simulated fly ash in the upper part the reactor, and the pyrolysis temperature was set according to

129

the experiment. In this experiment, K type thermocouple is connected with the display instrument

130

and electronic regulator. We can set the temperature through the electronic regulator, and measure

131

the temperature by the K-type galvanometer. After the required temperature was reached, the air

132

pump was turned on and the flow rate was adjusted according to the experimental requirements

133

and the flow rate was controlled by rotor flow meter and stable-flow valve. The reaction

134

conditions of single factor experiments are total flow rate of 200 mL min-1, pyrolysis temperature

135

of 300 °C, TeCB concentration of 1.0 vol.%, reaction time of 60 min, and catalyst dosage of 10

136

mL. The chlorine-containing fly ash was extracted with Soxhlet for 24 hours after the reaction.

137

The concentration of TeCB in the absorption liquid was determined by gas chromatograph

138

(Beifen 3420A, China) with external standard method. A certain amount of TeCB was dissolved

139

in a toluene solution to prepare a standard solution of TeCB from 10 to 400 mg·L-1 to draw the

140

standard curve by using the gas chromatograph (Table S1 and Fig. S1). Agilent 6890-5973 7


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GC-MS system equipped with a six-port valve (0.25 mL), capillary column (DB-dioxin, 60 m ×

142

0.25 mm (i.d.) × 0.15 µm (film)), and

143

detection of the tail gas composition with an injection volume of 10 µL and the fragment size

144

ranges from 33 to 450 amu. Surface groups of the catalyst were characterized on a Bruker

145

tensor37 FT-IR with wavenumber ranges from 4000 to 400 cm-1.

146

63

Ni electroncapture detector (µ-ECD), was employed for

The TeCB conversion (η) is calculated as: m0 − m1 ×100% m1

147

η=

148

where m0 and m1 are the initial and residual masses of TeCB (mg), respectively.

149

3. RESULTS AND DISCUSSION

150

3.1. Optimization of catalyst

151

3.1.1. Effect of catalyst support

(1)

152

In this work, three typical supports, that is, activated carbon, TiO2, and γ-Al2O3, were selected

153

to study the effect of catalyst supports on TeCB conversion, as shown in Fig. 1. Only 54% of

154

TeCB can be converted with single pyrolysis process. The decomposition efficiency of TeCB

155

respectively increases by 8.1% and 7.6% when activated carbon or γ-Al2O3 is added, while the

156

decomposition efficiency of TeCB increases by 14.9% with the assistance of TiO2. Which

157

indicates that TiO2 is an efficient support in catalytic decomposition of TeCB. Bertinchamps et

158

al.31 believed that titania induces a spreading of the entire set of active phase as a well-dispersed

159

monolayer over its surface. Xiang et al.32 reported that TiO2 has a certain catalytic effect on the

160

decomposition of TeCB, while SiO2 prevents the homogeneous spreading of the active phase in

161

the form of a well-dispersed monolayer but promotes the formation of poorly dispersed

8


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crystallites of active phase. This observation was also proved by Bond et al.33 who compared VOx

163

monolayer supported on TiO2, Al2O3, and SiO2. They pointed out the preferential spreading of

164

VOx on TiO2 rather on Al2O3. Moreover, they demonstrated the formation of V2O5 crystallites on

165

SiO2, even below the theoretical monolayer coverage.

166

3.1.2. Effect of active component

167

V2O5-WO3 (VW), MnOx-WO3 (MnW), CuO-WO3 (CuWi) were used as the active phase and

168

introduced to TiO2 support by the impregnation method. The effect of different active components

169

on TeCB conversion is shown in Fig. 2. The activity difference of these catalysts is not obvious in

170

low temperature region (200-250 °C), while the effect of active component becomes significant

171

when further increase the reaction temperature. For instance, 98.6 % and 89% of TeCB can be

172

respectively converted over VWTi and CuWTi at 400 °C, much higher than that over MnWTi

173

catalyst (79.2%).

174

3.1.3. Effect of catalyst assistant

175

The effect of V2O5/WO3 ratio (1 : 1, 1 : 2, and 1 : 3, w/w) on decomposition efficiency of TeCB

176

was investigated, as shown in Fig. 3. It is shown that all V2O5/WO3-contained catalysts have

177

much higher catalytic activity than that of TiO2. TeCB decomposition efficiency increases firstly

178

and then decreases to some extend with the increasing of V2O5/WO3 ratio. Catalysts with

179

V2O5/WO3 ratio of 1 : 1 and 1 : 3 possess similar catalytic activity, while the activity of catalyst

180

with V2O5/WO3 ratio of 1 : 2 is obviously higher than the other two groups. Albonetti et al.34

181

proposed that the TiO2/WO3 catalyst has a higher number of Brønsted acid sites compared to that

182

of TiO2 sample, which play an important role in the oxidation of o-dichlorobenzene over

183

TiO2-based catalysts. Wang et al.35 found that WO3 has a positive effect on the dispersion of 9


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vanadium sites on the surface of V2O5/TiO2, and strengthen the interaction between vanadium

185

oxide and TiO2 support.

186

3.1.4. Effect of vanadium loading

187

As shown in Fig. 4, the activity of the catalyst increases firstly with the increasing of vanadium

188

loading, and the catalyst with vanadium loading of 5.0 wt.% possesses the highest activity with

189

more than 98% of TeCB converted at 400 °C. Compared with pure TiO2, all catalysts present

190

higher TeCB oxidation capacity in the presence of vanadium oxide. However, the catalytic

191

activity decreases to some extend when the loading of vanadium is 7.0 wt.% (about 94.9% TeCB

192

converted at 400 °C). Yu et al.36 found that excessive vanadium loading could block the pore

193

space on TiO2 surface and suppressed the adsorption capacity of catalyst.

194

3.1.5.Effect of calcination temperature

195

The effect of calcination temperature (400, 450, and 500 °C) of catalyst on decomposition

196

efficiency of TeCB is shown in Fig. 5. It can be found that the catalyst activity is enhanced

197

continuously with the increasing of calcination temperature, and the best sintering temperature is

198

500 °C. The activity of catalyst calcined at 400 °C is almost the same as that of TiO2 support as

199

the V2O5 active phase is derived from the decomposition of NH4VO3 precursor in air at high

200

temperature. Zhang et al.37 revealed that the final decomposition product of NH4VO3 under Ar

201

conditions is V2O5 in temperature range of 350-450 °C, while it is very difficult to decompose

202

NH4VO3 under air conditions at 400 °C. Chen et al.38 found that when the calcination temperature

203

is lower than 500 °C, the active vanadium component cannot completely converted into V2O5,

204

and some other vanadium oxides such as VO, VO2, and V2O3 are formed, which would decrease

205

the whole catalytic activity. 10


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3.2. Catalyst characterizations

207

The catalyst with optimal formula was analyzed by XRD and SEM, as shown in Figs. 6 and 7.

208

It can be observed that all studied catalysts exhibit characteristic diffraction peaks of anatase TiO2,

209

and rutile TiO2 cannot observed in TiO2 support. The characteristic peaks of V2O5 and WO3 can

210

be observed, indicating the presence of V2O5 and WO3 crystals on the surface of catalyst. Rutile

211

TiO2 is observed over the fresh and used V2O5-WO3/TiO2 catalysts, indicating that the anatase

212

TiO2 is partly transformed to rutile phase after loading vanadium oxide even the presence of WO3

213

plays a role of inhibiting the transfer of anatase TiO2 to rutile phase, which has a negative effect

214

on catalytic activity.39 The specific surface area and porosity of prepared catalysts are shown in

215

Table S2. The specific surface area of pure TiO2 is larger than that of V2O5-WO3/TiO2 catalyst,

216

while both the average pore size and pore volume of V2O5-WO3/TiO2 increase to some degree

217

compared with TiO2 support. The FE-SEM images of various catalysts are shown in Fig. 7. It can

218

be seen that all particles are evenly distributed over TiO2 support. Small aggregates can be

219

observed after loading of vanadium and tungsten, which results in the formation of some

220

interparticle large holes. No significant change can be found in morphology of the fresh and used

221

catalysts.

222

3.3. Optimization of reaction condition

223

According to the actual combustion condition of municipal solid waste incineration reactors, a

224

series of tests were carried out at different temperatures (250-400 °C), TeCB concentrations (0.5

225

vol.%-1.5 vol.%), and space velocities (300-1500 h-1), as shown in Fig. 8. Experimental settings

226

are shown in Table 1. The effect of reaction temperature on TeCB decomposition efficiency is

227

shown in Fig. 8A. TeCB conversion increases from 78.2% to 98.9% when reaction temperature 11


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increases from 200 to 400 °C. Gao et al.40 believed that the dechlorination of hexachlorobenzene

229

on solid supports is based on surface solid-gas reaction. According to the classic crystal chemistry

230

theory, metallic ions and O2- on the surfaces of crystal are unsaturated, which can form free

231

electrons in vacancies for chlorine abstraction. The free electrons increase with increasing of

232

temperature, resulted in much higher dechlorination efficiency. In catalytic decomposition of

233

PCDD/Fs, the polyaromatic compounds are strongly adsorbed on the Lewis acid sites of

234

V2O5-WO3/TiO2 catalysts by interacting with the p-orbitals of aromatic systems.41,42 VOx is

235

considered as the active phase of vanadium-tungsten supported catalysts. The active lattice

236

oxygen over V5+Ox firstly oxidize pollutants on catalyst surface with V5+Ox reduced to V4+Ox, and

237

then the reduced vanadium is reoxidized in the present of oxygen. Higher temperature enhances

238

catalyst activity, and the TeCB adsorption and decomposition cycle proceeds more quickly.

239

The effect of gas hourly space velocity (GHSV) on TeCB decomposition is shown in Fig. 8B.

240

In general, the increase of GHSV reduces TeCB decomposition efficiency due to the decrease of

241

contact time between pollutant molecules and catalyst. As show in Fig. 8B, the TeCB conversion

242

decreases from 95.0% to 80.7% with increasing of space velocity from 300 to 1800 h-1, similar

243

with the results reported by Wu et al.43 and Xu et al.26. Considering the pyrolysis process, the

244

combined process can achieve better synergistic effect with a relative low space velocity.

245

The influence of TeCB concentration on TeCB conversion is illustrated in Fig. 8C. TeCB

246

concentration has a significant negative influence on the decomposition of TeCB, which reduces

247

from 94% to 80% when the TeCB concentration increases from 0.5 vol.% to 1.5 vol.% due to the

248

limited pyrolysis energy and catalyst active site, in agreement with the results proposed by Ren et

249

al.44 As is known, the TeCB content in actual waste fly ash is much lower than 0.5 vol.%, which 12


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250

means that the reported technology could achieve higher decomposition rate toward

251

polychlorinated benzene substances in fly ash under actual treatment process.

252

According to the results of single factor experiment, the effects of reaction temperature, gas

253

hourly space velocity, and TeCB concentration were analyzed by orthogonal test, as shown in

254

Tables 2 and 3. It can be revealed that the TeCB decomposition efficiency decreases continuously

255

with the increasing of gas hourly space velocity from 300 to 1800 h-1 and TeCB inlet

256

concentration from 0.5 to 1.5 vol.%, while the TeCB decomposition efficiency increase with the

257

increasing of reaction temperature from 250 to 400 °C. The larger the range is, the more obvious

258

the effect is. The optimum conditions for TeCB decomposition are the reaction temperature of

259

350 °C, the space velocity of 600 h-1, and the concentration TeCB of 0.5 vol.%, under which

260

around 94.1% of TeCB can be removed.

261

3.4. Product distribution and proposed TeCB decomposition mechanism

262

The main products in tail gas absorption solution are illustrated in Fig. 9. Many dechlorination

263

products such as 1,3-dichlorobenzene (12.1 min), trichlorobenzene homologue (13.5/13.9 min),

264

and 1,2,3,5-tetrachlorobenzene (17.9 min) can be detected in the tail gas. Besides, other reaction

265

intermediates as cyclosiloxane hydrocarbon compounds (15.6/16.5 min), phenol (18.8 min),

266

benzoquinone (20.3 min), and phthalic acid (22.1 min) are also found (Table 4). Short-chain

267

alkanes and olefins can combine and react with SiO2 in fly ash due to the occurrence of

268

dechlorination/hydrogenation and partial cracking of TeCB in pyrolysis and catalysis process,

269

resulting in the formation of cyclosiloxane hydrocarbons. The presence of phenol, benzoquinone,

270

and phthalic acid indicates that chlorine in TeCB is completely removed as well as the occurrence

271

of catalytic oxidation reactions. FT-IR spectra of the fresh and used V2O5-WO3/TiO2 catalysts 13


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272

were collected and displayed in Fig. 10. The band at 500-800 cm-1 can be assigned to the

273

vibration of Ti-O bond of TiO2 and the small band near 3500 cm-1 corresponds to the -OH

274

group.45 The band at 2350 cm-1 represents the C=O bond,46 assigning to the adsorbed CO2 and CO

275

over catalyst surface. The bands at 1750 and 1700 cm-1 also represent the C=O, which can be

276

attributed to the presence of carbonyl intermediates as quinone and aldehydes.47 The bands

277

between 1475 and 1395 cm-1 represent the C-O and the C-H.48

278

Based on the results of GC-MS and FT-IR, a possible TeCB decomposition route over

279

synthesized catalyst was proposed, as shown in Fig. 11. The first step of TeCB decomposition is

280

believed to be a nucleophilic substitution with the formation of surface phenolates (A), in

281

agreement with the results reported by Lichtenberger and Amiridis49 and Xu et al.26. Since the

282

C-Cl bond in aromatic halide is weaker than the C-H bond, it is more prone to be attacked by

283

nucleophiles.50 Firstly, the aromatic compounds are adsorbed on catalyst via nucleophilic attacks

284

on chlorine positions of aromatic rings. The remaining aromatic rings are subsequently oxidized

285

or opened, resulting in the occurrence of some non-aromatic intermediates. TeCB offers four

286

positions for such a nucleophilic attack. Consequently, the oxidation of TeCB is their dissociative

287

adsorption on a vanadium oxide site via one or more Cl abstracted simultaneously or in sequence.

288

The presence of oxygen molecules in nucleophilic reagent plays an active role in the oxidation of

289

chlorinated compounds, and chlorine atoms in adsorbed TeCB can be gradually substituted by

290

surface oxygen. As such, the first step in the oxidation of TeCB on catalyst is the cleavage of C-Cl

291

bond, in consistent with the conclusions reported by Brink et al..50

292

The second step is the electrophilic substitution of the adsorbed partially dechlorinated species

293

(C-E). The proposed electrophilic attack on this species is expected to be faster than the 14


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Page 16 of 41

294

corresponding attack on the adsorbed tetrachlorobenzene derivatives, since halogen substituents

295

are deactivating toward electrophilic substitution reactions. Chlorine is known to be an ortho-para

296

director during electrophilic substitutions. One of chlorine atoms is abstracted and replaced by a

297

surface oxygen species during nucleophilic substitution, and the ortho-para-directing effect of the

298

Cl substituent is then enhanced by the surface oxygen. The electrophilic substitution could also

299

result in the bond breaking of the aromatic ring to give a nonaromatic intermediate (B), which

300

reacts rapidly to form surface maleates (F), acetates (G), and aldehydes (H). Some of the partial

301

oxidation products formed on the surface can undergo further reaction to form the final reaction

302

products (i.e., CO, CO2, HCl, and H2O).

303

The reaction mechanism of TeCB decomposition can be determined as a coexistence of

304

multiple reactions. Amongst, the catalysis of heavy metal on surface of fly ash, the generation of

305

oxygen on surface of the catalyst, and the cleavage of C-Cl bond of TeCB are three key processes

306

in the whole decomposition reaction. The catalyst provides a place for decomposition reaction,

307

and also enhances the generation of oxygen and reactive oxygen species, promoting the

308

decomposition efficiency of pollutants.

309

4. CONCLUSIONS

310

In summary, the feasibility of decomposition of tetrachlorobenzene by using a low-temperature

311

pyrolysis-catalysis combined technology was demonstrated. The V2O5-WO3/TiO2 has higher

312

catalytic activity than that of CuO-WO3/TiO2 and MnOx-WO3/TiO2 catalysts, and TeCB in the

313

simulated fly ash can be effectively decomposed by the V2O5-WO3/TiO2 catalyst. Amongst,

314

V2O5-WO3/TiO2 catalyst with V2O5 loading of 5.0 wt.% and V2O5/WO3 weight ratio of 1 : 2

315

possesses the best TeCB decomposition efficiency. TeCB conversion increases from 78.2% to 15


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316

98.9% when the reaction temperature increases from 200 to 400 °C, while the TeCB

317

decomposition efficiency shows an obvious reduction when increase the gas hourly space velocity

318

or TeCB inlet concentraion. Orthogonal test indicates that more than 94% of TeCB can be

319

removed under condition of reaction temperature of 350 °C, TeCB concentraion of 0.5 vol.%, and

320

gas hourly space velocity of 600 h-1. The main substeps of TeCB decomposition are aromatic

321

pollutant nucleophilic substitution and intermediates electrophilic substitution. It is shown that

322

TeCB can be finally converted to CO, CO2, H2O, and HCl, while several kinds of reaction

323

byproducts such as low chlorinated aromatics and aromatic and aliphatic hydrocarbons.

324

ASSOCIATED CONTENT

325

Supporting Information: GC peak areas of TeCB-toluene standard solutions, standard curve of

326

TeCB, and textural property of catalysts. This material is available free of charge via the Internet

327

at http://pubs.acs.org.

328

ACKNOWLEDGMENTS

329

This work was financially supported by the National Natural Science Foundation of China

330

(21477095,

21677114),

the

National

Key

Research

and

Development

Program

331

(2016YFC0204201), and the Fundamental Research Funds for the Central Universities

332

(xjj2017170). The valuable comments from the editor and anonymous reviewers are much

333

appreciated.

334

REFERENCES

335

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oxidation of PCDD/F on a V2O5-WO3/TiO2 catalysts: Effect of chlorinated benzenes and

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loaded sulfated zirconia and mordenite catalysts. Applied Catalysis A: General 2008, 335,

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Fe-Ca-Ox/TiO2 catalysts for low temperature 1,2-dichlorobenzene oxidation. Applied Catalysis A:

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Enhanced activity of zeolites by chemical dealumination for chlorinated VOC abatement. Applied

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Qi, T. Catalytic combustion of chlorobenzene on the Ln modified Co/HMS. Applied Catalysis B:

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pentachlorobenzene in simulated flue gas by a V2O5-WO3/TiO2 catalyst. Chemosphere 2012, 87,

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PCDD/F-containing gas stream generating system applied in catalytic decomposition of gaseous

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dioxins over V2O5-WO3/TiO2-based catalysts. Chemosphere 2008, 73, 890-895.

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dual redox-acid functionalities in VOx/TiO2/ZSM-5 catalyst for simultaneous abatement of

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PCDD/Fs and NOx from municipal solid waste incineration. Applied Catalysis B: Environmental

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23


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Energy & Fuels

Tables Table 1 Parameters for single-factor experiments. Experiment number

Reaction temperature/°C

GHSV/h-1

TeCB concentration/vol.%

1

250

600

1

2

275

600

1

3

300

600

1

4

325

600

1

5

350

600

1

6

375

600

1

7

400

600

1

8

300

300

1

9

300

600

1

10

300

900

1

11

300

1200

1

12

300

1500

1

13

300

1800

1

14

300

600

0.5

15

300

600

0.75

16

300

600

1.0

17

300

600

1.25

18

300

600

1.5 24


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 41

Table 2 Parameters for orthogonal experiments. Factor

GHSV/h-1

Temperature/°C

TeCB concentration/%

Level 1

300

300

0.5

Level 2

600

320

0.75

Level 3

1200

350

1

25


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Energy & Fuels

Table 3 Results of multi-factor orthogonal experiments. Experiment number

GHSV/h-1

Reaction temperature/°C

TeCB concentration/vol.%

Decomposition efficiency/%

1

300

300

1.0

90.2

2

300

325

0.5

92.4

3

300

350

0.75

93.5

4

600

300

0.75

89.7

5

600

325

1.0

90.6

6

600

350

0.5

94.1

7

1200

300

0.5

87.3

8

1200

325

0.75

89.2

9

1200

350

1.0

88.6

The average of indicator 1

92.1

89.1

89.8


91.5

90.8

90.8


88.4

92.1

91.3

Range

3.1

3.0

1.5 26


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 41

Table 4 Composition of tail gas absorption solution. Retention time/min

Substance

Molecular formula

12.1

1,3-dichlorobenzene

C6H4Cl2

13.5

1,2,4-trichlorobenzene

C6H3Cl3

13.9

1,3,5-trichlorobenzene

C6H3Cl3

15.6/16.5

Tetradecamethylcycloheptasiloxane

C14H42O7Si7

17.9

1,2,3,5-tetrachlorobenzene

C6H2Cl4

18.8

Phenol

C6H5OH

20.3

Benzoquinone

C6H4O2

22.1

Phthalic acid

C8H6O4

27


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Energy & Fuels

Figure captions Fig. 1 Effect of catalyst support on TeCB decomposition efficiency. Fig. 2 Effect of active phase composition on TeCB decomposition efficiency. Fig. 3 Effect of V2O5/WO3(w/w) on TeCB decomposition efficiency. Fig. 4 Effect of vanadium loading onTeCB decomposition efficiency. Fig. 5 Effect of catalyst calcination temperature on TeCB decomposition efficiency. Fig.6 XRD patterns of various samples. Fig. 7 FE-SEM images of (a) TiO2, (b) Fresh VWTi, and (c) Used VWTi. Fig. 8 TeCB decomposition under different reaction conditions. Fig. 9 Reaction product distribution of TeCB decomposition over VWTi catalyst at 350 °C. Fig. 10 FT-IR spectra of the fresh and used VWTi catalysts. Fig. 11 Proposed TeCB decomposition mechanism over VWTi catalyst. Scheme 1 Schematic diagram of the low-temperature pyrolysis-catalysis combined system.

28


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Page 30 of 41

Fig. 1

29


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Energy & Fuels

Fig. 2

30


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Page 32 of 41

Fig. 3

31


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Energy & Fuels

Fig. 4

32


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Page 34 of 41

Fig.5

33


Page 35 of 41

Anatase TiO2 Rutile TiO2 V2O5 WO3

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Used-VWTi

Fresh-VWTi

TiO2 support

10

20

30

40

50

60

70

80

2θ (°) Fig. 6

34


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

a

Page 36 of 41

b

100 nm

100 nm

c

100 nm

Fig. 7

35


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Energy & Fuels

Fig. 8 36


Energy & Fuels

Cl O Si O

30

Si O

Si

Si

Si O

Cl

O

O

Cl

Cl

Si O Si

25

Intensity/a.u. (105)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 38 of 41

Cl

OH

Cl

20

O

O

Cl Cl

OH

15

O HO Cl

10

Cl O

Cl

5 Cl

0 9

12

15

18

21

Retention time/min

Fig. 9

37


Page 39 of 41

Fresh VWTi

Transmittance/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

4000

3500 (-OH)

Used VWTi 2350 (C=O)

1750 (C=O)

1395 (C-H)

690 (Ti-O)

3500

3000

2500

2000

1500

1000

500

Wavenumber/cm-1 Fig. 10

38


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 40 of 41

Fig. 11

39


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Energy & Fuels

Scheme 1

40


Graphical abstract

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