Low-temperature hydrodechlorination of chlorobenzenes on platinum-supported alumina catalysts

Applied Catalysis A: General 250 (2003) 247–254

Low-temperature hydrodechlorination of chlorobenzenes on platinum-supported alumina catalysts Yoshihito Hashimoto, Akimi Ayame∗ Department of Applied Chemistry, Muroran Institute of Technology, 27-1 Mizumoto-cho, Muroran, Hokkaido 050-8585, Japan Received 4 February 2003; received in revised form 28 March 2003; accepted 28 March 2003

Abstract Hydrodechlorination of chlorobenzenes on platinum (Pt)-supported ␥-alumina and alumina Lewis superacid (AmLSA) catalysts was carried out at room temperature and ambient pressure using a fixed bed flow reactor and a semi-batch reactor. Both the catalysts indicated good activity for the hydrodechlorination, but the former was superior to the latter. The hydrodechlorinations of reactants C6 H6−x Clx (x = 1, 2, 3) proceeded step-wisely to benzene and then cyclohexane via C6 H6−y Cly (y = x−1). The reactions seem to be promoted by the contribution of spillover hydrogen formed on the Pt-supported catalysts. The catalysts deactivated with reaction time and the amount of chlorine that accumulated on the Pt-supported ␥-alumina catalyst in the hydrodechlorination of 1,4-dichlorobenzene for 3 h was near to that estimated from the converted reactant molecules. When the deactivated catalysts were treated in a stream of hydrogen above 503 K, the original activity was completely restored, but the deactivation phenomenon with reaction time was observed again. © 2003 Elsevier B.V. All rights reserved. Keywords: Chlorobenzene; Dichlorobenzene; Hydrodechlorination; Pt-supported ␥-alumina catalyst; Fixed bed flow reactor; Benzene; Cyclohexane

1. Introduction The decontamination processing of chloroorganic compounds is strongly required because of their high toxicity and persistency in the environment. Especially, dioxines and PCBs cause much abnormality and carcinogenocity, so establishing a processing method for them is very important. High-temperature incineration technology was developed for PCBs disposal [1], and in order to suppress dioxines generated from the burning process of Cl-containing chemical products and home-wastes, ∗ Corresponding author. Tel.: +81-143-46-5720; fax: +81-143-46-5736. E-mail address: ak [email protected] (A. Ayame).

new-model incinerators have been developed [2]. Some microbiological studies for dechlorination of such chloroorganic compounds have also been carried out [3–6]. For catalytic dechlorination of chloroorganic compounds, Coq et al. studied the dechlorination over the Pd catalysts supported on Al2 O3 , graphite, and AlF3 [7]. Juszczyk et al. reported the results obtained on Pt/Al2 O3 catalyst [8]. Rao et al. reported that Pt-bimetallic catalysts were effective [9]. However, these catalytic dechlorinations were carried out at temperatures higher than 413 K. On the other hand, Balko et al. had reported that Pd-catalyzed liquid-phase hydrodechlorinations of monochlorobenzene (MCB), dichlorobenzene (DCB), trichlorobenzene (TCB) and tetrachlorobenzene proceeded effectively at low

0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00319-3

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temperatures such as 303 K [10]. Except for this report, few publications about the low-temperature hydrodechlorination of chloroorganic compounds are available. One of the authors has succeeded in preparation of alumina Lewis superacid (AmLSA) [11–13] and the benzene alkylations with chloropropanes [20,21] and propene [14,15,22] at 273–293 K on the AmLSA catalysts. However, the AmLSA catalysts deactivated with reaction time due to the accumulation of basic poly-substituted benzene products. The deactivation was also effectively depressed by supporting Pt on the AmLSA and adding hydrogen into the feed gas [15]. The depression of the catalyst deactivation or accumulation of basic products was due to the contribution of spillover hydrogen formed on the Pt/AmLSA catalysts [15,19]. Experimental cases where such spillover hydrogen was formed on the bifunctional metal-metal oxide catalysts have been reported by Conner and Falconer [16], Ebitani et al. [17], and Zhang et al. [18]. If one could use the spillover hydrogen effectively, however, catalytic hydrodechlorinations of polychloro-substituted aromatic compounds such as chlorobenzenes, PCB, and dioxines would be permitted. This would make it possible to convert selectively polychloro-substituted aromatic compounds to pure aromatic compounds without destruction of the aromatic skeletal structures. In the present work, hydrodechlorinations of monochlorobenzene, 1,4-dichlorobenzene and 1,3,5-trichlorobenzene on the Pt catalysts supported on ␥-alumina and the alumina Lewis superacid [11] were carried out at 298 K under ambient pressure of hydrogen. The hydrodechlorinations were performed using a conventional fixed bed flow reactor for gasphase reaction and a fixed bed semi-batch reactor for liquid-phase reaction, which was newly developed in this work.

JRC-ALO-6 (180 m2 g−1 ; 1.6 mm spherical), which were the Reference Catalysts of the Catalyst Society of Japan. A 50 ml aliquot of hexachloroplatinic acid aqueous solution (including 1.40 × 10−4 mol as platinum metal) was dropped at 1 ml min−1 into the pure water suspending the alumina (5.451 g). The suspension was mixed well and then left to stand for 90 min. The impregnated aluminas were filtrated and dried, followed by calcination in a stream of 20 ml min−1 oxygen at the temperature which was elevated at 7.5 K min−1 from 298 to 723 K and kept at 723 K for 3 h. The oxidized materials were then reduced in a stream of 18 ml min−1 hydrogen at 3 K min−1 from 298 to 573 K and kept for 3 h at 573 K. The amount of Pt supported was 0.5 wt.%, referred to the carriers. The JRC-ALO-1 and ALO-6 were used in a liquid-phase hydrodechlorination and a gas-phase one, respectively. The catalytic performances of the Pt-supported catalysts (Pt/ALO-1 and Pt/ALO-6) using the two kinds of aluminas were almost comparable, as shown in Table 3. 2.1.2. Pt/AmLSA Pre-determined weights of the alumina carrier and of amorphous platinum were separately put into two calcination tubes. The amorphous platinum was previously prepared on coverglass using a high-frequency Ar+ sputtering instrument. The alumina was dehydrated at 1073 K, then exposed to dry chlorine, followed by outgassing at the same temperature. The chlorine-treated alumina has strong Lewis acid sites, so it is called “alumina solid Lewis superacid [11–13]”. After the chlorination, Pt was deposited on the AmLSA as PtCl2 by a CVD method using chlorine and finally reduced by hydrogen at 673 K. The detailed catalyst preparations were described in the previous papers [14,15,19]. The AmLSA samples prepared from ALO-1 and ALO-6 are denoted by AmLSA-1 and AmLSA-6, respectively. The surface area of the Pt/AmLSA-1 was 101.6 m2 g−1 .

2. Experimental 2.2. Chemicals 2.1. Catalyst 2.1.1. Pt/Al2 O3 Transient aluminas (mainly ␥-form) used were JRC-ALO-1 (160 m2 g−1 ; 2–4 mm spherical) and

Platinum (99.9%) and hexachloroplatinic acid (98.5%) were supplied by Fuchikawa Metal Kogyo Co. and Mitsuwa Pure Chem. Co. Ltd., respectively. Chlorine (Nissan Syoji Co. Ltd.; 99.5%)

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Fig. 1. Fixed bed flow reactor system for gas-phase reaction.

was distilled three times in a vacuum system and stored in a 3 l glass flask. Hydrogen (Air water Inc.; 99.9999%) was dehydrated by passing it through the molecular sieve 4A absorber cooled with liquid nitrogen. Nitrogen (Air water Inc.; 99.99%) was dehydrated by passing it through a diphosphorus pentaoxide-packed glass column. Monochlorobenzene (Kanto Chemical Co. Ltd.; 98%) was distilled three times in a vacuum system. 1,4-Dichlorobenzene (>99%) and 1,3,5-trichlorobenzene (99%) supplied by Tokyo Kasei Co. Ltd. were used without further purification.

2.3. Reaction systems The schematic figures of the fixed bed flow reactor system for gas-phase reaction and the fixed bed semi-batch reactor for the liquid-phase reaction are shown in Figs. 1 and 2, respectively. In the batch reactor, hydrogen was introduced below the catalyst bed and the reactants were automatically stirred by the rising hydrogen bubbles. The reaction temperature was controlled by keeping the reactor in a water bath. Reaction products were analyzed by a TCD gas chromatograph (Yanako G-1800) equipped with

Fig. 2. Liquid-phase semi-batch reactor (this reactor was displaced with the reactor port shown in Fig. 1).

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a 5% Bentone34 + DIDP/Uniport HP column (3 mm φ × 2 m) and a FID gas chromatograph (HITACHI G-3500) equipped with a CP-Sil13CB capillary column (0.32 mm φ × 25 m) for analysis of halocarbons.

3. Results and discussion 3.1. Reaction on AmLSA and γ-alumina The AmLSAs indicate very high or explosive activity to the benzene alkylation with 2-chloropropane at room temperature [20]. So, the benzene alkylation with chloropropanes have been examined at 273 K [21]. On the basis of the results, an attempt was made to test the activity of the AmLSA catalysts against the hydrodechlorination of MCB using the fixed bed flow reactor. However, the AmLSA catalysts did not result in any hydrogenated products. Of course, the ␥-aluminas used were also inactive to the hydrodechlorination at near room temperature. 3.2. Reaction on Pt/AmLSA The gas-phase hydrodechlorinations of MCB and DCB using the Pt/AmLSA-6 were carried out at 298 K for 3 h (Table 1). The catalyst was accurately active to the reactions, but the activities decreased gradually with time, as shown in Fig. 3. The products were benzene and cyclohexane; in the case of DCB, monochlorobenzene was also observed. The yield of cyclohexane as a final product decreased with deactivation of the catalyst. The low conversion of MCB was due to the small W/F. If the elimination rate was given by x/(W/F), the rates of MCB and DCB were

Fig. 3. Gas-phase hydrodechlorination of 1,4-dichlorobenzene on the (䊉) Pt/AmLSA-6 and (䊊) Pt/ALO-6 catalysts. Catalyst weight: 0.25 g, temperature: 298 K, 1,4-dichlorobenzene: 6.42 × 10−5 mol h−1 , and H2 : 4 ml min−1 .

(1.6–2.3) ×10−4 and (4.3–6.1) ×10−5 mol h−1 g−1 , respectively. Since the reaction temperature used was below the boiling points of benzene, cyclohexane, and chlorobenzenes as reactant, at initial stages some carbon-imbalance was observed between the inlet and outlet of the reactor. This is due to the adhesion or adsorption of the reactants and products on catalyst surfaces and on the inside wall of reactor part, as shown in Fig. 1. The carbon-imbalance, however, decreased with reaction time and after 30–40 min was hardly observed. 3.3. Gas-phase reaction on Pt/alumina catalysts Fig. 3 shows a typical reaction time dependence of the DCB conversion in hydrodechlorination of

Table 1 Gas-phase hydrodechlorination of chlorobenzenes on Pt/AmLSA-6 catalysts Reactant

W/F (g h mol−1 )

Reaction time (min)

Conversion (%)

C6 H5 Cl C6 H5 Cl C6 H4 Cl2

46.5 3900

60 300 60 300

1.1 0.72 23.6 16.9

relm (mol h−1 g−1 )

Composition (%) C6 H6

C6 H12

– –

47.6 43.8

52.4 56.2

2.37 × 10−4 1.55 × 10−4

0 15.5

0 0

100 84.5

6.05 × 10−5 4.33 × 10−5

The AmLSA-6 was prepared from JRC-ALO-6. Catalyst weight (W): 0.25 g, temperature: 298 K, and hydrogen flow rate: 4 ml min−1 (constant).

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Table 2 Gas-phase hydrodechlorination of chlorobenzenes on Pt/Al2 O3 catalysts Reactant

Alumina used

W/F (g h mol−1 )

Reaction time (min)

Conversion (%)

C6 H4 Cl2 C6 H5 Cl

ALO-6 ALO-6 ALO-1

46.5 3900 46.5

relm (mol h−1 g−1 )

Composition (%) C6 H5 Cl

C6 H5

C6 H12 55.4 76.4

60 300

12.3 4.5

– –

– –

44.6 23.6

60 300

97.6 98.4

– –

– –

0 0

60 300

13.1 5.2

– –

– –

44.6 24.5

0 0

0 0

2.2 5.5

7.7 3.5

C6 H4 Cl2

ALO-6

3900

60 300

95.9 54.5

– –

C6 H3 Cl3

ALO-6

3900

60 300

61.9 31.1

12.9 45.1

100 100 55.4 75.5 100 100 77.2 45.9

2.64 × 10−3 9.68 × 10−4 2.50 × 10−4 2.52 × 10−4 2.82 × 10−3 1.12 × 10−3 2.46 × 10−4 1.40 × 10−4 1.59 × 10−4 0.80 × 10−4

Catalyst weight (W): 0.25 g, temperature: 298 K, hydrogen flow rate: 4 ml min−1 (constant).

DCB on the Pt/ALO-6 catalyst at 3900 g h mol−1 , compared with that for the Pt/AmLSA-6 catalyst. The activity was larger than that of Pt/AmLSA-6, but the activity decrease with time was very large. The product in this case was only cyclohexane. Although the activity decreased to about a half of the initial one, no formation of benzene was observed, as shown in Table 2. The experimental results for the reactions of MCB and TCB are also shown in Table 2. The elimination rates of MCB at the initial stage and after 5 h on the Pt/ALO-6 were 11 and 6 times those on the Pt/AmLSA-6 catalyst, respectively, and the benzene formation was slightly smaller than the Pt/AmLSA-6. When the contact time (W/F) increased up to 3900 g h mol−1 , no activity decrease was apparently observed and the product became cyclohexane only. Furthermore, the activity and the product composition of Pt/ALO-1 were very similar to those of Pt/ALO-6. So, the direct comparison of the results for liquid-phase reaction on the Pt/ALO-1 catalyst, which are described later, with those for the gas-phase reaction shown in Table 2 would be admitted. The conversion of TCB was smaller than those for DCB and MCB, but the formation rate of hydrogen chloride, 4.3 × 10−4 mol g−1 h−1 , at the initial stage was close to 4.9 × 10−4 mol g−1 h−1 for DCB and was rather larger than 2.5 × 10−4 mol g−1 h−1 for MCB. The presence of DCB and MCB in the products on the TCB hydrodechlorination suggests that the amount

of active hydrogen (spillover hydrogen, [16–19]) required to complete the hydrodechlorination of TCB eliminated is insufficient. The variations of conversion and product composition with W/F in the gas-phase hydrodechlorination of DCB were examined by changing the catalyst weight (W) from 0.025 to 0.5 g. The results obtained at 5 h after starting the reaction are shown in Fig. 4. When the W/F increased the conversion and the content of cyclohexane in the product mixture increased linearly, while the MCB and benzene decreased rapidly to trace amounts. These phenomena indicate that the hydrodechlorination proceeds step-wisely via MCB and then benzene. Comparison of the data shown in Tables 1 and 2 and Fig. 3 shows that the Pt/alumina catalysts were more active than the Pt/AmLSA catalysts. Since the AmLSA has so strong Lewis acid sites with H0
− 14.5 that a very stable ␲-complex with λmax = 294 nm is formed between planar benzene molecule and the strong Lewis acid site, few of the coordinated benzene molecules would be desorbed [11–13]. Also, the surface area of the AmLSA is about half of the original transient aluminas [13]. Although it is not yet determined that either the strong Lewis acidity or the lowering in surface area is the principal factor, an ensemble effect of the characteristics mentioned above seems to be the origin for the low activity of the Pt/AmLSA catalysts.

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Fig. 4. Dependencies of conversion and product composition on contact time in the gas-phase hydrodechlorination of 1,4-dichlorobenzene on the Pt/ALO-6 catalyst. Reaction conditions are the same as those described in Fig. 3 and catalyst weight was variable. These plots were given by data obtained after 5 h from the starting of the reaction.

3.4. Liquid-phase reactions The liquid-phase hydrodechlorinations were carried out using the semi-batch reactor shown in Fig. 2. The reaction was started by switching from N2 -flow to H2 -flow using the four-way glass stopcock 2b. The flow rate of hydrogen was 4 ml min−1 constant. Chlorobenzenes were diluted to 0.8 mol l−1 using n-hexane as solvent and in each reaction 5 ml was used. Results obtained are shown in Table 3. The Pt/ALO-1 catalyst indicated moderate activities to

MCB, DCB, and TCB. In all the reaction, the catalytic activities decreased with time. With the deactivation, the catalyst lost the ability to supply hydrogen or spillover hydrogen. So, the formation of MCB in the hydrodechlorination of DCB and also DCB in the reaction of TCB increased with the deactivation. A similar phenomenon was observed in the gas-phase reaction of TCB on the Pt/ALO-6 catalyst (Table 2). In the liquid-phase reaction of DCB, the Pt/ AmLSA-1 catalyst indicated about the same activity as the Pt/ALO-1 but the product compositions were

Table 3 Liquid-phase hydrodechlorination of chlorobenzenes using the semi-batch reactor Reactant

Catalyst

Reaction time (min)

Conversion (%)

relm (mol h−1 g−1 )

Composition (%) C6 H4 Cl2

C6 H5 Cl

C6 H6

C6 H12

C6 H5 Cl

Pt/ALO-1

60 180

3.0 3.8

– –

– –

14.5 16.9

85.5 83.1

4.80 × 10−4 2.04 × 10−4

C6 H4 Cl2

Pt/ALO-1

60 180

1.4 2.4

– –

52.3 60.3

5.3 4.0

42.3 35.7

2.24 × 10−4 1.28 × 10−4

Pt/AmLSA-1

60 180

1.1 2.4

– –

40.8 17.4

0 0

59.2 82.6

1.76 × 10−4 1.28 × 10−4

Pt/ALO-1

60 180

0.08 0.1

62.0 68.2

0.5 0.3

36.6 32.7

1.28 × 10−5 0.53 × 10−5

C6 H3 Cl3

0.9 0.4

Catalyst weight: 0.25 g, temperature: 298 K, 0.8 mol l−1 n-hexane solution: 5 ml, and hydrogen flow rate: 4 ml min−1 (constant).

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different. The fact that more cyclohexane was formed on the Pt/AmLSA-1 catalyst than on the Pt/ALO-1 catalyst seems to be due to a strong interaction between benzene-like reaction intermediates and the strong Lewis acid sites on the AmLSA-1 surface. The results are also obviously different from those for the gas-phase reaction of DCB. This may be due to solvent effects of n-hexane. 3.5. Deactivation and reactivation The Pt-supported alumina and AmLSA catalysts were gradually deactivated in the course of the gasand liquid-phase hydrodechlorinations. An exception was the gas-phase reaction of MCB at W/F = 3900 g h mol−1 in Table 2. This implies that the contact time (W/F) was so large that hydrogen supply to MCB as it reacted was sufficient. In order to reveal causes of the deactivation, the amounts of chlorine remaining on the Pt/ALO-6 catalyst before and after the gas-phase DCB hydrodechlorination and on the catalyst treated (reactivated) in hydrogen above 503 K for 1 h were measured by the Volhard method [13] (Table 4). The chlorine on the fresh catalyst is due to H2 PtCl6 used as platinum source in the catalyst preparation. Some chlorine, 1.26 × 10−3 mol g−1 , was detected on the catalyst after the reaction for 3 h. So, the chlorine that accumulated during the reaction was 7.6 × 10−4 mol g−1 . These results suggest that the deactivation was produced by the adsorption of chlorine or chlorine ion produced in the course of hydrodechlorination. In addition, the amount of adsorbed chlorine was near to the value estimated from the total number Table 4 Amounts of chlorine on the Pt/Al2 O3 -6 catalyst before and after the gas-phase hydrodechlorination of DCB Catalyst

Cl− on the catalyst (mol g−1 cat )

Fresh Deactivateda Reactivatedb

5.01 × 10−4 1.26 × 10−3 5.24 × 10−4

a After the C H Cl hydrodechlorination for 3 h. Reaction 6 4 2 conditions: catalyst weight, 0.25 g; temperature, 298 K; C6 H4 Cl2 , 6.42 × 10−5 mol h−1 ; and H2 , 4 ml min−1 . b After reducing the deactivated catalyst in a stream of 20 ml min−1 H2 at 503 K for 1 h.

Fig. 5. Regeneration effects of the deactivated Pt/ALO-6 catalyst, which were observed in the 1,4-dichlorobenzene hydrodechlorination. Catalyst weight: 0.1 g, and other conditions are the same as those described in Fig. 3. The flow rate of H2 or N2 used in the regeneration of the deactivated catalyst was 20 ml min−1 .

of moles of DCB that reacted; the DCB that reacted for 3 h was 1.60 × 10−4 mol, so the number of the evolved chlorine atoms was 1.93 × 1020 . The number of chlorine atoms that remained on the catalysts was 1.14 × 1020 atoms/0.25 g. These results indicate strongly that the chlorine is present on both the alumina surface and Pt particles, since the number of Pt metal atoms on 0.25 g of the Pt/Al2 O3 catalyst is 3.86 × 1018 atoms even if Pt dispersion is assumed to be 100%. Fig. 5 shows the regeneration effects of the deactivated catalysts. The regeneration was carried out by treating the deactivated catalysts at 503–573 K in the stream of hydrogen or nitrogen of 20 ml min−1 for 1 h. After the hydrogen reduction above 503 K, the original catalytic activity was completely restored, but the deactivation phenomena with reaction time were again observed. The activities of the catalysts treated in the nitrogen stream rapidly decreased with time, although the initial activity was obtained. From the characteristics in the deactivation phenomena of the catalysts regenerated by hydrogen and nitrogen, one can explain that all chlorines on the Pt particles and the alumina carrier are desorbed or eliminated by hydrogen treatment at high-temperature, while, in the case of nitrogen treatment, chlorines on the Pt particles are scavenged and those on the alumina remain as they are.

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4. Conclusion The Pt/AmLSA and Pt/Al2 O3 catalysts showed prominent activity to the hydrodechlorination of chlorobenzenes to benzene and cyclohexane at 298 K and ambient pressure. Even in the liquid-phase reaction system they functioned effectively as catalyst. As the hydrodechlorination could not occur without Pt, it is speculated that hydrogen molecules are dissociatively adsorbed on platinum surface, the hydrogen atoms spillover on the alumina surface as proton and hydride, and the spilled-over hydrogens attack chlorobenzene molecules adsorbed on the alumina surface. Also, it was revealed that the catalytic hydrodechlorination proceeded step-wisely as 1,4-dichlorobenzene → monochlorobenzene → benzene → cyclohexane. References [1] N. Yoshida, So-da to Enso 548 (1995) 367. [2] H. Tsuboi, Nensho Kenkyu 114 (1998) 11. [3] S.A. Boyd, M.J. Zwiernik, J.F. Quensen, Environ. Sci. Technol. 32 (1998) 3360. [4] W.W. Mohn, B. Kuipers, W.R. Cullen, Environ. Sci. Technol. 33 (1999) 3579. [5] K.A. Deweerd, D.L. Bedard, Environ. Sci. Technol. 33 (1999) 2057.

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