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Catalytic dechlorination of aromatic chlorides with noble-metal catalysts under mild conditions: approach to practical use
Applied Catalysis B: Environmental 27 (2000) 97–104
Catalytic dechlorination of aromatic chlorides with noble-metal catalysts under mild conditions: approach to practical use Yuji Ukisu∗ , Satoshi Kameoka, Tatsuo Miyadera National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 16 November 1999; received in revised form 25 February 2000; accepted 25 February 2000
Abstract Catalytic dechlorination of aromatic chlorides was carried out in a solution of NaOH in 2-propanol with a carbon-supported Rh-based catalyst (Rh-Pt/C) at temperatures below 35◦ C. It was found that the dechlorination rate of aromatic chlorides (chlorobenzene, p-chlorotoluene, and 4-chlorobiphenyl) is strongly dependent upon the substituents. The dechlorination rate is hardly affected by the presence of water (ca. 10%), even under aerobic conditions, although the catalytic activity is suppressed significantly in the presence of acetone (ca. 5%). The catalyst life was evaluated both in a batch system and in a continuous-flow system. The catalytic activity gradually decreased, probably because of an accumulation of NaCl on the catalyst surface. The deactivated catalyst could be reactivated by washing it with water. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Aromatic chloride; Dechlorination; Noble-metal catalyst; Catalyst life
1. Introduction Halogenated organic compounds have been used on a large scale in the chemical, petrochemical, and electronic industries. The disposal of organic wastes containing halogen has become a major environmental and social problem, because most of them are toxic and thermally stable, accumulating in the surroundings for long time periods. Incinerating halogenated compounds is feasible, but often leads to the release of even more toxic compounds such as chlorinated dioxins and furans. The development of a highly efficient, safe alternative technology for detoxifying organic halides has been anticipated. ∗ Corresponding author. Tel.: +81-298-61-8219; fax: +81-298-61-8208. E-mail address: [email protected] (Y. Ukisu)
Catalytic hydrodehalogenation with homogeneous or heterogeneous catalysts is recognized as a facile and efficient procedure [1–3]. However, the practical application of catalysts to the dehalogenation of organic halides is always accompanied by the problem of the deactivation of the catalyst. In the reaction with homogeneous catalysts, structural changes in the metal complex sometimes take place and lead to deactivation of the catalyst [1]. In heterogeneous catalyst systems with solid catalysts, the activity change may be caused by adsorption of halogen [4], halogenation of catalyst [5,6], surface composition change of catalyst [7,8], and formation of oligomers and coke [9,10]. For practical use, the development of catalysts that maintain their catalytic activity for a prolonged time is an essential problem. Moreover, commercial catalysts must overcome interference by concomitants contained in organic wastes, because industrial wastes sometimes
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consist of mixtures of organic compounds, including compounds undesirable for the catalytic reaction. We reported that organic halides such as polychlorinated biphenyls (PCBs) and chlorobenzenes undergo complete dehalogenation in a solution of NaOH in 2-propanol in the presence of carbon-supported noble-metal catalysts (Rh/C, Pd/C, Ru/C, and Pt/C), among which Rh/C and Pd/C are the effective catalysts for the reaction [11,12]. The catalytic hydrodehalogenation reaction was performed under mild conditions (<82◦ C, 1 atm) with no use of molecular hydrogen as the source of hydrogen. We discussed the reaction mechanism and concluded that 2-propanol is the source of hydrogen, including hydrogen transfer from 2-propanol to organic halides [12]. CH3 CH(OH)CH3 → CH3 C(O)CH3 + 2H∗
(1)
Ar-Cl + 2H∗ → Ar-H + HCl
(2)
HCl + NaOH → NaCl + H2 O
(3)
Combining Eqs. (1), (2) and (3), we get Ar-Cl + CH3 CH(OH)CH3 + NaOH → Ar-H + CH3 C(O)CH3 + NaCl + H2 O
(4)
A characteristic of this catalytic system is that the substitution of halogen by hydrogen occurs selectively without hydrogenation of the aromatic ring. Furthermore, the dehalogenation of aliphatic halides was found to proceed catalytically, although the dehalogenation rate was much slower than that of aromatic halides [13]. A composite Rh-Pt catalyst was found to be effective for the dechlorination of aromatic chlorides even at ambient temperatures [14]. The hydrogen species arising from the dehydrogenation of 2-propanol on Pt particles is likely to control the reduced state of Rh and facilitate the generation of active Rh species [14]. In this study, we examined the catalytic properties of Rh-Pt/C and Pd/C, which are strong candidates for practical use, under various conditions to explore the practicality of this catalyst system. The dechlorination reaction was performed in a solution of aromatic chlorides with different substituents. The influence of water and acetone was examined because they are formed during the reaction (Eq. (4)) and are likely to become concomitants when the solvent is reused in a closed system. Moreover, the catalyst life was evaluated both
by repeated reaction in a batch system and by continuous reaction in a flow system.
2. Experimental 2.1. Materials and catalysts Carbon-supported noble-metal catalyst powder, Rh-Pt/C (Rh: 3 wt.%, Pt: 1 wt.%), was used for the dechlorination reaction. The Rh-based catalyst was prepared as described in a previous paper [14]. Pd/C (Pd: 5 wt.%), which was used as a reference, was purchased from N.E. Chemcat. The catalysts were dried at 200◦ C for 30 min in an N2 flow and kept in a dessicator. All chemicals and gases were of high-purity grade and used without further purification. 2.2. Catalytic reaction The catalytic dechlorination reaction of aromatic chlorides was carried out in a solution of NaOH in 2-propanol with supported noble-metal catalysts at temperatures below 35◦ C. The reaction in a batch system was performed in a test tube or a flat-bottomed flask according to the procedure outlined in a previous paper [12]. In the experiments evaluating concomitants (water or acetone), water was added to the solution before adding catalyst, whereas acetone was added to the solution containing the catalyst. In order to evaluate the catalyst life, the solution containing the catalyst was centrifuged after the dechlorination reaction, and the catalyst was re-used after removing the liquid phase. The reaction in a flow system was carried out using a Pyrex glass column (i.d.=12 mm, l=300 mm) with a dropping funnel on the top. The catalyst (0.10 g) was mechanically mixed with 20 g of glass beads (37–63 m). The mixture (volume: 13 cm3 ) was packed into the column. A solution of NaOH in 2-propanol (60 mmol l−1 , 20 ml) was passed through the catalyst bed. Then a 2-propanol solution containing p-chlorotoluene (20 mmol l−1 ) and NaOH (60 mmol l−1 ) was introduced to initiate the dechlorination reaction. The temperature of the solution was 26◦ C, and the flow rate was 18±2 ml h−1 . The solution emerging from the end of column was collected
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periodically and diluted with n-hexane for subsequent GC analysis. The concentrations of reactant and product were determined using a GC (HP 6890) equipped with an FID and a column of DB-Wax (30 m×0.32 mm, 0.5 m film-thickness). n-Dodecane was used as the internal standard. 2.3. Surface analysis The surface composition of fresh and used catalyst was analyzed by X-ray photoelectron spectroscopy (XPS) with Al K␣ radiation as the excitation source. Binding-energy values were referred to the C(1s) peak at 285.0 eV. The catalyst after the reaction in the batch system was separated from the solution, washed with hexane or water, and dried in air prior to analysis by XPS.
3. Results and discussion 3.1. Dechlorination of organic chlorides having different functional groups The catalytic dechlorination reaction of a mixture containing equimolar amounts of chlorobenzene, p-chlorotoluene, and 4-chlorobiphenyl was performed in a solution of NaOH in 2-propanol using Rh-Pt/C at 35◦ C. Fig. 1 shows the time dependence of the conversion of these organic chlorides. The result demonstrates that the catalytic system is capable of treating a mixture of aromatic chlorides, although the dechlorination rate depends on the substituents on the aromatic chlorides and decreases in the order: chlorobenzene > 4-chlorobiphenyl > p-chlorotoluene. The chlorine-free products (benzene, toluene and biphenyl) were obtained in high yield (>90%) without loss of material balance. The effect of the substituents has been recognized in the homogeneous dechlorination of aryl chlorides catalyzed by palladium complexes; the dechlorination reaction was found to be retarded by electron-donating substituents such as CH3 , CH3 O, and NH2 and enhanced by electron-withdrawing substituents such as CHO and CN [15]. Therefore, we conclude that the low dechlorination rates for 4-chlorobiphenyl and p-chlorotoluene
Fig. 1. Catalytic dechlorination of a mixture containing equimolar amounts of chlorobenzene, p-chlorotoluene, and 4-chlorobiphenyl. Reaction conditions: 2-propanol (7.5 ml); each reactant (16 mmol l−1 ); NaOH (100 mmol l−1 ); catalyst (Rh-Pt/C) (15 mg); temperature (35◦ C); atmosphere (N2 ).
arise from their electron-donating substituents (phenyl and methyl). In addition to the inductive effect of the substituents, the affinity of substrates for the catalyst should be considered in the heterogeneous system. Indeed, the difference in the adsorption constant was evaluated on Pd/C for various compounds (Ph-I, Ph-Br, Ph-Cl, Ph-F, etc.) [16]. When the aromatic chlorides were supplied individually to the reaction instead of the mixture, the dechlorination rate decreased in the order: chlorobenzene > p-chlorotoluene > 4-chlorobiphenyl. This is different from the earlier mentioned result of the mixture, implying that the affinity of aromatic chlorides to the catalyst surface depends on the substituents and affects the reaction rate. 3.2. Influence of concomitants and atmosphere The influence of acetone and water, which are formed as products during the reaction (Eq. (4)), on the dechlorination of p-chlorotoluene was examined. When the same solvent is used repeatedly in the reaction without purification, acetone and water will become possible inhibitors, because it is difficult to remove them completely from 2-propanol by distillation, unlike the chlorine-free organic products (toluene, biphenyl, etc.) having higher boiling points.
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Fig. 2. Catalytic dechlorination of p-chlorotoluene in the presence of H2 O. Reaction conditions: 2-propanol (15 ml); p-chlorotoluene (25 mmol l−1 ); NaOH (60 mmol l−1 ); Rh-Pt/C (30 mg); temperature (35◦ C); atmosphere (N2 ).
Fig. 3. Catalytic dechlorination of p-chlorotoluene in the presence of acetone. Reaction conditions: 2-propanol (15 ml); p-chlorotoluene (25 mmol l−1 ); NaOH (60 mmol l−1 ); Rh-Pt/C (30 mg); temperature (35◦ C); atmosphere (N2 ).
When the dechlorination reaction of p-chlorotoluene was carried out in the presence of water (5–10%), the dechlorination rate was not affected significantly compared with that in the absence of water, as shown in Fig. 2. The tolerance of the catalyst for water has practical advantages because industrial organic wastes sometimes contain water. Furthermore, the catalyst can be regenerated by washing it with water, as described further. In the presence of acetone (2–5%), on the other hand, the dechlorination reaction was retarded considerably, as shown in Fig. 3, indicating that acetone behaves as a strong inhibitor. It has been reported that the activity of the catalytic dehydrogenation of 2-propanol decreases in the presence of acetone, because hydrogenation of acetone occurs competitively on a noble-metal catalyst (Eq. (1)) [17]. This implies that acetone becomes a competitive hydrogen acceptor and interferes with the transfer of hydrogen to an organic halide. In addition, acetone is likely to affect the activation of the Rh species because of the shortage of active hydrogen required to reduce Rh; no dechlorination occurred when the catalyst was added to a solution containing acetone. Therefore, acetone was added to the solution last as described in Section 2. In our experiments, the catalytic dechlorination reaction has always been performed in an N2 atmosphere to ensure safety during each experiment
[11–14]. Fig. 4 shows the influence of the type of atmosphere (N2 or air) on the catalytic dechlorination of p-chlorotoluene with Rh-Pt/C at 35◦ C. Even in an atmosphere of air, the dechlorination reaction proceeded unimpeded to produce toluene in high yield (>90%). This finding is important because it is convenient not to have to replace air with N2 .
Fig. 4. Catalytic dechlorination of p-chlorotoluene in an atmosphere of air or N2 . Reaction conditions: 2-propanol (7.5 ml); p-chlorotoluene (25 mmol l−1 ); NaOH (60 mmol l−1 ); Rh-Pt/C (15 mg); temperature (35◦ C).
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3.3. Evaluation of the catalyst life in a batch system In order to evaluate the catalyst life, the dechlorination of p-chlorotoluene was repeated several times with the catalyst recovered after each reaction. Fig. 5A and B show the conversion change in the repeated reactions with Rh-Pt/C at 35◦ C and Pd/C at 50◦ C, respectively. The molar amount of p-chlorotoluene in each run was 1.9×10−4 mol, which was in large
101
excess of the molar amount of noble-metal on the catalyst: Rh (4.4×10−6 mol) and Pd (7.0×10−6 mol) in Rh-Pt/C and Pd/C, respectively. Each reaction was run for 90 min, which was the time necessary to complete the reaction in the first trial. In the case of Rh-Pt/C, the conversion decreased gradually and resulted in 58% in the fifth run. Before the next run, the liquid phase was removed by decantation, and the used catalyst was then stirred in a solution of NaOH in 2-propanol (60 mmol l−1 , 7.5 ml) at 35◦ C for 15 min. This treatment is believed to remove the adsorbed molecules soluble in 2-propanol and reduce the oxidized Rh species with the hydrogen arising from dehydrogenation of 2-propanol [14]. The conversion after this treatment increased to 77% (sixth run), probably because the catalyst surface became cleaner and/or reduced. When this treatment was performed after each run, the decrease in the activity was reduced (see the gray bar in Fig. 5A). The treatment at higher temperature was more effective (sixth run). On the other hand, the feature of the dechlorination reaction with Pd/C is different. The conversion of p-chlorotoluene dropped significantly in the second run, followed by little decrease in the conversion. After the used catalyst was washed with a solution of NaOH in 2-propanol (sixth run), the conversion increased in a fashion similar to that observed in the Rh-Pt/C system. 3.4. Application to a continuous-flow system
Fig. 5. Repeated dechlorination reaction of p-chlorotoluene in a batch system by using (A) Rh-Pt/C at 35◦ C and (B) Pd/C at 50◦ C. Reaction conditions: 2-propanol (7.5 ml); p-chlorotoluene (25 mmol l−1 ); NaOH (60 mmol l−1 ); catalyst (15 mg); reaction time (90 min); atmosphere (N2 ). The gray bars indicate the conversion after washing with a solution of NaOH in 2-propanol (60 mmol l−1 , 7.5 ml) at 35◦ C after every reaction, although the treatment was not done in the case of white bars. The 6th run was performed after washing with a solution of NaOH in 2-propanol at 35◦ C (white) or 82◦ C (gray).
In addition to the catalytic dechlorination of p-chlorotoluene in the batch system, the reaction was examined using a continuous-flow system. The reaction was performed by passing a 2-propanol solution containing p-chlorotoluene (20 mmol l−1 ) and NaOH (60 mmol l−1 ) through the catalyst bed, which consisted of Rh-Pt/C (0.10 g) and glass beads (20 g). The dilution of catalyst by glass beads was necessary to control the reaction rate and evaluate the catalyst life. Fig. 6 shows the change in the p-chlorotoluene conversion during the flow reaction. The dechlorination reaction proceeded successfully; toluene was detected at all times with no loss of the material balance against p-chlorotoluene. The dechlorination of p-chlorotoluene was completely achieved at the beginning of the reaction, but a gradual decrease in the conversion was observed; the conversion had
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ing the catalytic activity; washing of the catalyst with water restores the activity [8]. 3.5. Surface analysis of the catalyst before and after the reaction
Fig. 6. Catalytic dechlorination of p-chlorotoluene in a continuous-flow system. Reaction conditions: solvent (2-propanol); p-chlorotoluene (20 mmol l−1 ); NaOH (60 mmol l−1 ); catalyst bed, Rh-Pt/C (0.10 g)+glass beads (20 g); flow rate (18±2 ml h−1 ); temperature (26◦ C). Treatment I: 20 ml of 2-propanol solution of NaOH (60 mmol l−1 ) was flowed. Treatment II: 20 ml of H2 O was flowed, followed by 20 ml of 2-propanol solution of NaOH (60 mmol l−1 ).
dropped to 80% after an 8 h reaction. The total molar amount of consumed p-chlorotoluene for 8 h was estimated to be 2.6×10−3 mol, which is in large excess of the amount of Rh (2.9×10−5 mol) supported on the catalyst. After 20 ml of a solution of NaOH in 2-propanol was passed through the catalyst bed (Treatment I), the conversion increased to some extent, as observed in the batch system. However, the conversion gradually diminished again when the dechlorination reaction started. Another treatment was examined to recover the catalytic activity; water (20 ml) was passed through the catalyst bed, followed by a solution of NaOH in 2-propanol (Treatment II). After this treatment, the catalytic activity recovered completely, implying that water-soluble compounds accumulated on the catalyst surface during the reaction, leading to a retardation of the catalytic activity. It has been reported that in the catalytic hydrodehalogenation reaction, the addition of bases such as NaOH and NH3 is effective in suppressing the deactivation of catalysts, because the halogen ions responsible for the poisoning are removed from the catalyst surface [4,8]. Salts produced such as NH4 Cl are likely to deposit on the catalyst surface decreas-
The surface composition of the Rh-Pt/C catalyst before and after the dechlorination reaction of p-chlorotoluene was analyzed by XPS. As shown in Fig. 7A, an intense C(1s) peak due to the catalyst support (active carbon) was observed at 285.0 eV before the reaction. The Rh(3d3/2 , 3d5/2 ) peaks, which were detected at 314.0 and 309.0 eV, respectively, are ascribed to the oxidized Rh species rather than Rh metal [18,19]. The storage of the catalyst in air causes oxidation of the Rh species on the catalyst, although the hydrogen species arising from the dehydrogenation of 2-propanol on Pt particles is likely to make a reduced Rh species during the reaction [14]. The Pt(4f5/2 , 4f7/2 ) peaks, which were observed at 75.0 and 72.1 eV, respectively (not shown in the figure) are ascribed to metallic Pt. After the dechlorination reaction of p-chlorotoluene (0.56 mmol) was performed at 35◦ C with Rh-Pt/C
Fig. 7. XPS spectra of the Rh-Pt/C catalyst before and after the dechlorination reaction of p-chlorotoluene. (A) The catalyst before the reaction; (B) the used catalyst, which was washed with n-hexane; (C) after the used catalyst (B) was followed by washing with water.
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Scheme 1. Surface composition of Rh-Pt/C.
(15 mg) in the batch system (7.5 ml), the catalyst was separated and washed three times with n-hexane (total ca. 25 ml) to remove adsorbed organic compounds such as p-chlorotoluene and toluene, followed by drying in an N2 flow at 200◦ C. An intense Cl(2s) peak appeared at 272.3 eV together with a decrease in the intensity of the C and Rh peaks (Fig. 7B). The binding energy of Cl(2s) was in good agreement with that of NaCl supported on active carbon by the usual impregnation method. An intense peak due to Na(1s) was observed at 1072.3 eV (not shown in the figure), although it was difficult to distinguish between the peaks ascribed to NaOH and NaCl. These results strongly suggest that NaCl layers accumulate on the catalyst surface and cover the active site and/or carbon support thereby reducing the catalytic activity. The theoretical amount of NaCl produced was in excess of the catalyst’s weight and enough to cover the whole surface area of the catalyst. In the XPS spectra of the used catalyst after it had been washed several times with water (total ca. 35 ml), the Cl peaks completely disappeared (Fig. 7C). The change in surface composition of Rh-Pt/C during the dechlorination reaction is illustrated in Scheme 1. Freshly prepared catalyst consists of Rh and Pt particles dispersed on the support, although the Rh species is oxidized by exposure to air [14]. Catalyst support such as active carbon and TiO2 are effective to control the dispersion and the electronic state of noble metal [14]. The oxidized Rh species is reduced by hydrogen species arising from dehydrogenation of 2-propanol
on Pt particles [14]. When dechlorination of organic chlorides proceeds, produced NaCl covers the catalyst surface to reduce the catalytic activity. The accumulated NaCl on the surface is removed by washing with water.
4. Conclusion The catalytic dechlorination of aromatic chlorides was completed in a solution of NaOH in 2-propanol with a carbon-supported noble-metal catalyst (Rh-Pt/C) at temperatures below 35◦ C. The reaction was affected by the substituents of the aromatic chlorides; the dechlorination rate was retarded by electron-donating substituents such as methyl and phenyl. Acetone was found to be a possible inhibitor, because it behaves as a hydrogen acceptor thus disturbing hydrogen-transfer to an aromatic halide. The dechlorination reaction of p-chlorotoluene proceeded successfully in a continuous-flow system as well as in a batch system at ambient temperature. Although a gradual decrease in catalytic activity was observed, probably because of the accumulation of NaCl on the catalyst surface, the catalyst recovered completely after it was washed with water. References [1] R.A.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129.
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[2] V.V. Grushin, H. Alper, Chem. Rev. 94 (1994) 1047. [3] A.R. Pinder, Synthesis (1980) 425. [4] M.A. Aramendia, V. Borau, I.M. Garcia, C. Jimenez, J.M. Marias, F.J. Urbano, Appl. Catal. B 20 (1999) 101, and references therein. [5] J. Estelle, J. Ruz, Y. Cesteros, R. Fernandez, P. Salagre, F. Medina, J-E. Sueiras, J. Chem. Soc., Faraday Trans. 92 (1996) 2811. [6] B. Coq, G. Ferrat, F. Figueras, J. Catal. 101 (1986) 434. [7] P. Bodnariuk, B. Coq, G. Ferrato, F. Figueras, J. Catal. 116 (1989) 459. [8] P. Forni, L. Prati, M. Rossi, Appl. Catal. B 14 (1997) 49. [9] E.S. Creyghton, M.H.W. Burgers, J.C. Jansen, H. van Bekkum, Appl. Catal. A 128 (1995) 275. [10] H.C. Choi, S.H. Choi, O.B. Yang, J.S. Lee, K.H. Lee, Y.G. Kim, J. Catal. 161 (1996) 790.
[11] Y. Ukisu, S. Iimura, R. Uchida, Chemosphere 33 (1996) 1523. [12] Y. Ukisu, T. Miyadera, J. Mol. Catal. A 125 (1997) 135. [13] Y. Ukisu and T. Miyadera, Nippon Kagaku Kaishi (1998) 369 (in Japanese). [14] Y. Ukisu, S. Kameoka, T. Miyadera, Appl. Catal. B 18 (1998) 273. [15] Y. Ben-David, M. Gozin, M. Portnoy, D. Milstein, J. Mol. Catal. 73 (1992) 173. [16] H. Wiener, J. Blum, Y. Sasson, J. Org. Chem. 56 (1991) 6145. [17] Y. Saito, M. Yamashita, E. Ito, N. Meng, Int. J. Hydrogen Energy 19 (1994) 223. [18] J.S. Brinen, A. Melera, J. Phys. Chem. 76 (1972) 2525. [19] Y. Okamoto, N. Ishida, T. Imanaka, S. Teranishi, J. Catal. 58 (1979) 82.
Catalytic dechlorination of aromatic chlorides with noble-metal catalysts under mild conditions: approach to practical use Yuji Ukisu∗ , Satoshi Kameoka, Tatsuo Miyadera National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 16 November 1999; received in revised form 25 February 2000; accepted 25 February 2000
Abstract Catalytic dechlorination of aromatic chlorides was carried out in a solution of NaOH in 2-propanol with a carbon-supported Rh-based catalyst (Rh-Pt/C) at temperatures below 35◦ C. It was found that the dechlorination rate of aromatic chlorides (chlorobenzene, p-chlorotoluene, and 4-chlorobiphenyl) is strongly dependent upon the substituents. The dechlorination rate is hardly affected by the presence of water (ca. 10%), even under aerobic conditions, although the catalytic activity is suppressed significantly in the presence of acetone (ca. 5%). The catalyst life was evaluated both in a batch system and in a continuous-flow system. The catalytic activity gradually decreased, probably because of an accumulation of NaCl on the catalyst surface. The deactivated catalyst could be reactivated by washing it with water. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Aromatic chloride; Dechlorination; Noble-metal catalyst; Catalyst life
1. Introduction Halogenated organic compounds have been used on a large scale in the chemical, petrochemical, and electronic industries. The disposal of organic wastes containing halogen has become a major environmental and social problem, because most of them are toxic and thermally stable, accumulating in the surroundings for long time periods. Incinerating halogenated compounds is feasible, but often leads to the release of even more toxic compounds such as chlorinated dioxins and furans. The development of a highly efficient, safe alternative technology for detoxifying organic halides has been anticipated. ∗ Corresponding author. Tel.: +81-298-61-8219; fax: +81-298-61-8208. E-mail address: [email protected] (Y. Ukisu)
Catalytic hydrodehalogenation with homogeneous or heterogeneous catalysts is recognized as a facile and efficient procedure [1–3]. However, the practical application of catalysts to the dehalogenation of organic halides is always accompanied by the problem of the deactivation of the catalyst. In the reaction with homogeneous catalysts, structural changes in the metal complex sometimes take place and lead to deactivation of the catalyst [1]. In heterogeneous catalyst systems with solid catalysts, the activity change may be caused by adsorption of halogen [4], halogenation of catalyst [5,6], surface composition change of catalyst [7,8], and formation of oligomers and coke [9,10]. For practical use, the development of catalysts that maintain their catalytic activity for a prolonged time is an essential problem. Moreover, commercial catalysts must overcome interference by concomitants contained in organic wastes, because industrial wastes sometimes
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consist of mixtures of organic compounds, including compounds undesirable for the catalytic reaction. We reported that organic halides such as polychlorinated biphenyls (PCBs) and chlorobenzenes undergo complete dehalogenation in a solution of NaOH in 2-propanol in the presence of carbon-supported noble-metal catalysts (Rh/C, Pd/C, Ru/C, and Pt/C), among which Rh/C and Pd/C are the effective catalysts for the reaction [11,12]. The catalytic hydrodehalogenation reaction was performed under mild conditions (<82◦ C, 1 atm) with no use of molecular hydrogen as the source of hydrogen. We discussed the reaction mechanism and concluded that 2-propanol is the source of hydrogen, including hydrogen transfer from 2-propanol to organic halides [12]. CH3 CH(OH)CH3 → CH3 C(O)CH3 + 2H∗
(1)
Ar-Cl + 2H∗ → Ar-H + HCl
(2)
HCl + NaOH → NaCl + H2 O
(3)
Combining Eqs. (1), (2) and (3), we get Ar-Cl + CH3 CH(OH)CH3 + NaOH → Ar-H + CH3 C(O)CH3 + NaCl + H2 O
(4)
A characteristic of this catalytic system is that the substitution of halogen by hydrogen occurs selectively without hydrogenation of the aromatic ring. Furthermore, the dehalogenation of aliphatic halides was found to proceed catalytically, although the dehalogenation rate was much slower than that of aromatic halides [13]. A composite Rh-Pt catalyst was found to be effective for the dechlorination of aromatic chlorides even at ambient temperatures [14]. The hydrogen species arising from the dehydrogenation of 2-propanol on Pt particles is likely to control the reduced state of Rh and facilitate the generation of active Rh species [14]. In this study, we examined the catalytic properties of Rh-Pt/C and Pd/C, which are strong candidates for practical use, under various conditions to explore the practicality of this catalyst system. The dechlorination reaction was performed in a solution of aromatic chlorides with different substituents. The influence of water and acetone was examined because they are formed during the reaction (Eq. (4)) and are likely to become concomitants when the solvent is reused in a closed system. Moreover, the catalyst life was evaluated both
by repeated reaction in a batch system and by continuous reaction in a flow system.
2. Experimental 2.1. Materials and catalysts Carbon-supported noble-metal catalyst powder, Rh-Pt/C (Rh: 3 wt.%, Pt: 1 wt.%), was used for the dechlorination reaction. The Rh-based catalyst was prepared as described in a previous paper [14]. Pd/C (Pd: 5 wt.%), which was used as a reference, was purchased from N.E. Chemcat. The catalysts were dried at 200◦ C for 30 min in an N2 flow and kept in a dessicator. All chemicals and gases were of high-purity grade and used without further purification. 2.2. Catalytic reaction The catalytic dechlorination reaction of aromatic chlorides was carried out in a solution of NaOH in 2-propanol with supported noble-metal catalysts at temperatures below 35◦ C. The reaction in a batch system was performed in a test tube or a flat-bottomed flask according to the procedure outlined in a previous paper [12]. In the experiments evaluating concomitants (water or acetone), water was added to the solution before adding catalyst, whereas acetone was added to the solution containing the catalyst. In order to evaluate the catalyst life, the solution containing the catalyst was centrifuged after the dechlorination reaction, and the catalyst was re-used after removing the liquid phase. The reaction in a flow system was carried out using a Pyrex glass column (i.d.=12 mm, l=300 mm) with a dropping funnel on the top. The catalyst (0.10 g) was mechanically mixed with 20 g of glass beads (37–63 m). The mixture (volume: 13 cm3 ) was packed into the column. A solution of NaOH in 2-propanol (60 mmol l−1 , 20 ml) was passed through the catalyst bed. Then a 2-propanol solution containing p-chlorotoluene (20 mmol l−1 ) and NaOH (60 mmol l−1 ) was introduced to initiate the dechlorination reaction. The temperature of the solution was 26◦ C, and the flow rate was 18±2 ml h−1 . The solution emerging from the end of column was collected
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periodically and diluted with n-hexane for subsequent GC analysis. The concentrations of reactant and product were determined using a GC (HP 6890) equipped with an FID and a column of DB-Wax (30 m×0.32 mm, 0.5 m film-thickness). n-Dodecane was used as the internal standard. 2.3. Surface analysis The surface composition of fresh and used catalyst was analyzed by X-ray photoelectron spectroscopy (XPS) with Al K␣ radiation as the excitation source. Binding-energy values were referred to the C(1s) peak at 285.0 eV. The catalyst after the reaction in the batch system was separated from the solution, washed with hexane or water, and dried in air prior to analysis by XPS.
3. Results and discussion 3.1. Dechlorination of organic chlorides having different functional groups The catalytic dechlorination reaction of a mixture containing equimolar amounts of chlorobenzene, p-chlorotoluene, and 4-chlorobiphenyl was performed in a solution of NaOH in 2-propanol using Rh-Pt/C at 35◦ C. Fig. 1 shows the time dependence of the conversion of these organic chlorides. The result demonstrates that the catalytic system is capable of treating a mixture of aromatic chlorides, although the dechlorination rate depends on the substituents on the aromatic chlorides and decreases in the order: chlorobenzene > 4-chlorobiphenyl > p-chlorotoluene. The chlorine-free products (benzene, toluene and biphenyl) were obtained in high yield (>90%) without loss of material balance. The effect of the substituents has been recognized in the homogeneous dechlorination of aryl chlorides catalyzed by palladium complexes; the dechlorination reaction was found to be retarded by electron-donating substituents such as CH3 , CH3 O, and NH2 and enhanced by electron-withdrawing substituents such as CHO and CN [15]. Therefore, we conclude that the low dechlorination rates for 4-chlorobiphenyl and p-chlorotoluene
Fig. 1. Catalytic dechlorination of a mixture containing equimolar amounts of chlorobenzene, p-chlorotoluene, and 4-chlorobiphenyl. Reaction conditions: 2-propanol (7.5 ml); each reactant (16 mmol l−1 ); NaOH (100 mmol l−1 ); catalyst (Rh-Pt/C) (15 mg); temperature (35◦ C); atmosphere (N2 ).
arise from their electron-donating substituents (phenyl and methyl). In addition to the inductive effect of the substituents, the affinity of substrates for the catalyst should be considered in the heterogeneous system. Indeed, the difference in the adsorption constant was evaluated on Pd/C for various compounds (Ph-I, Ph-Br, Ph-Cl, Ph-F, etc.) [16]. When the aromatic chlorides were supplied individually to the reaction instead of the mixture, the dechlorination rate decreased in the order: chlorobenzene > p-chlorotoluene > 4-chlorobiphenyl. This is different from the earlier mentioned result of the mixture, implying that the affinity of aromatic chlorides to the catalyst surface depends on the substituents and affects the reaction rate. 3.2. Influence of concomitants and atmosphere The influence of acetone and water, which are formed as products during the reaction (Eq. (4)), on the dechlorination of p-chlorotoluene was examined. When the same solvent is used repeatedly in the reaction without purification, acetone and water will become possible inhibitors, because it is difficult to remove them completely from 2-propanol by distillation, unlike the chlorine-free organic products (toluene, biphenyl, etc.) having higher boiling points.
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Fig. 2. Catalytic dechlorination of p-chlorotoluene in the presence of H2 O. Reaction conditions: 2-propanol (15 ml); p-chlorotoluene (25 mmol l−1 ); NaOH (60 mmol l−1 ); Rh-Pt/C (30 mg); temperature (35◦ C); atmosphere (N2 ).
Fig. 3. Catalytic dechlorination of p-chlorotoluene in the presence of acetone. Reaction conditions: 2-propanol (15 ml); p-chlorotoluene (25 mmol l−1 ); NaOH (60 mmol l−1 ); Rh-Pt/C (30 mg); temperature (35◦ C); atmosphere (N2 ).
When the dechlorination reaction of p-chlorotoluene was carried out in the presence of water (5–10%), the dechlorination rate was not affected significantly compared with that in the absence of water, as shown in Fig. 2. The tolerance of the catalyst for water has practical advantages because industrial organic wastes sometimes contain water. Furthermore, the catalyst can be regenerated by washing it with water, as described further. In the presence of acetone (2–5%), on the other hand, the dechlorination reaction was retarded considerably, as shown in Fig. 3, indicating that acetone behaves as a strong inhibitor. It has been reported that the activity of the catalytic dehydrogenation of 2-propanol decreases in the presence of acetone, because hydrogenation of acetone occurs competitively on a noble-metal catalyst (Eq. (1)) [17]. This implies that acetone becomes a competitive hydrogen acceptor and interferes with the transfer of hydrogen to an organic halide. In addition, acetone is likely to affect the activation of the Rh species because of the shortage of active hydrogen required to reduce Rh; no dechlorination occurred when the catalyst was added to a solution containing acetone. Therefore, acetone was added to the solution last as described in Section 2. In our experiments, the catalytic dechlorination reaction has always been performed in an N2 atmosphere to ensure safety during each experiment
[11–14]. Fig. 4 shows the influence of the type of atmosphere (N2 or air) on the catalytic dechlorination of p-chlorotoluene with Rh-Pt/C at 35◦ C. Even in an atmosphere of air, the dechlorination reaction proceeded unimpeded to produce toluene in high yield (>90%). This finding is important because it is convenient not to have to replace air with N2 .
Fig. 4. Catalytic dechlorination of p-chlorotoluene in an atmosphere of air or N2 . Reaction conditions: 2-propanol (7.5 ml); p-chlorotoluene (25 mmol l−1 ); NaOH (60 mmol l−1 ); Rh-Pt/C (15 mg); temperature (35◦ C).
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3.3. Evaluation of the catalyst life in a batch system In order to evaluate the catalyst life, the dechlorination of p-chlorotoluene was repeated several times with the catalyst recovered after each reaction. Fig. 5A and B show the conversion change in the repeated reactions with Rh-Pt/C at 35◦ C and Pd/C at 50◦ C, respectively. The molar amount of p-chlorotoluene in each run was 1.9×10−4 mol, which was in large
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excess of the molar amount of noble-metal on the catalyst: Rh (4.4×10−6 mol) and Pd (7.0×10−6 mol) in Rh-Pt/C and Pd/C, respectively. Each reaction was run for 90 min, which was the time necessary to complete the reaction in the first trial. In the case of Rh-Pt/C, the conversion decreased gradually and resulted in 58% in the fifth run. Before the next run, the liquid phase was removed by decantation, and the used catalyst was then stirred in a solution of NaOH in 2-propanol (60 mmol l−1 , 7.5 ml) at 35◦ C for 15 min. This treatment is believed to remove the adsorbed molecules soluble in 2-propanol and reduce the oxidized Rh species with the hydrogen arising from dehydrogenation of 2-propanol [14]. The conversion after this treatment increased to 77% (sixth run), probably because the catalyst surface became cleaner and/or reduced. When this treatment was performed after each run, the decrease in the activity was reduced (see the gray bar in Fig. 5A). The treatment at higher temperature was more effective (sixth run). On the other hand, the feature of the dechlorination reaction with Pd/C is different. The conversion of p-chlorotoluene dropped significantly in the second run, followed by little decrease in the conversion. After the used catalyst was washed with a solution of NaOH in 2-propanol (sixth run), the conversion increased in a fashion similar to that observed in the Rh-Pt/C system. 3.4. Application to a continuous-flow system
Fig. 5. Repeated dechlorination reaction of p-chlorotoluene in a batch system by using (A) Rh-Pt/C at 35◦ C and (B) Pd/C at 50◦ C. Reaction conditions: 2-propanol (7.5 ml); p-chlorotoluene (25 mmol l−1 ); NaOH (60 mmol l−1 ); catalyst (15 mg); reaction time (90 min); atmosphere (N2 ). The gray bars indicate the conversion after washing with a solution of NaOH in 2-propanol (60 mmol l−1 , 7.5 ml) at 35◦ C after every reaction, although the treatment was not done in the case of white bars. The 6th run was performed after washing with a solution of NaOH in 2-propanol at 35◦ C (white) or 82◦ C (gray).
In addition to the catalytic dechlorination of p-chlorotoluene in the batch system, the reaction was examined using a continuous-flow system. The reaction was performed by passing a 2-propanol solution containing p-chlorotoluene (20 mmol l−1 ) and NaOH (60 mmol l−1 ) through the catalyst bed, which consisted of Rh-Pt/C (0.10 g) and glass beads (20 g). The dilution of catalyst by glass beads was necessary to control the reaction rate and evaluate the catalyst life. Fig. 6 shows the change in the p-chlorotoluene conversion during the flow reaction. The dechlorination reaction proceeded successfully; toluene was detected at all times with no loss of the material balance against p-chlorotoluene. The dechlorination of p-chlorotoluene was completely achieved at the beginning of the reaction, but a gradual decrease in the conversion was observed; the conversion had
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ing the catalytic activity; washing of the catalyst with water restores the activity [8]. 3.5. Surface analysis of the catalyst before and after the reaction
Fig. 6. Catalytic dechlorination of p-chlorotoluene in a continuous-flow system. Reaction conditions: solvent (2-propanol); p-chlorotoluene (20 mmol l−1 ); NaOH (60 mmol l−1 ); catalyst bed, Rh-Pt/C (0.10 g)+glass beads (20 g); flow rate (18±2 ml h−1 ); temperature (26◦ C). Treatment I: 20 ml of 2-propanol solution of NaOH (60 mmol l−1 ) was flowed. Treatment II: 20 ml of H2 O was flowed, followed by 20 ml of 2-propanol solution of NaOH (60 mmol l−1 ).
dropped to 80% after an 8 h reaction. The total molar amount of consumed p-chlorotoluene for 8 h was estimated to be 2.6×10−3 mol, which is in large excess of the amount of Rh (2.9×10−5 mol) supported on the catalyst. After 20 ml of a solution of NaOH in 2-propanol was passed through the catalyst bed (Treatment I), the conversion increased to some extent, as observed in the batch system. However, the conversion gradually diminished again when the dechlorination reaction started. Another treatment was examined to recover the catalytic activity; water (20 ml) was passed through the catalyst bed, followed by a solution of NaOH in 2-propanol (Treatment II). After this treatment, the catalytic activity recovered completely, implying that water-soluble compounds accumulated on the catalyst surface during the reaction, leading to a retardation of the catalytic activity. It has been reported that in the catalytic hydrodehalogenation reaction, the addition of bases such as NaOH and NH3 is effective in suppressing the deactivation of catalysts, because the halogen ions responsible for the poisoning are removed from the catalyst surface [4,8]. Salts produced such as NH4 Cl are likely to deposit on the catalyst surface decreas-
The surface composition of the Rh-Pt/C catalyst before and after the dechlorination reaction of p-chlorotoluene was analyzed by XPS. As shown in Fig. 7A, an intense C(1s) peak due to the catalyst support (active carbon) was observed at 285.0 eV before the reaction. The Rh(3d3/2 , 3d5/2 ) peaks, which were detected at 314.0 and 309.0 eV, respectively, are ascribed to the oxidized Rh species rather than Rh metal [18,19]. The storage of the catalyst in air causes oxidation of the Rh species on the catalyst, although the hydrogen species arising from the dehydrogenation of 2-propanol on Pt particles is likely to make a reduced Rh species during the reaction [14]. The Pt(4f5/2 , 4f7/2 ) peaks, which were observed at 75.0 and 72.1 eV, respectively (not shown in the figure) are ascribed to metallic Pt. After the dechlorination reaction of p-chlorotoluene (0.56 mmol) was performed at 35◦ C with Rh-Pt/C
Fig. 7. XPS spectra of the Rh-Pt/C catalyst before and after the dechlorination reaction of p-chlorotoluene. (A) The catalyst before the reaction; (B) the used catalyst, which was washed with n-hexane; (C) after the used catalyst (B) was followed by washing with water.
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Scheme 1. Surface composition of Rh-Pt/C.
(15 mg) in the batch system (7.5 ml), the catalyst was separated and washed three times with n-hexane (total ca. 25 ml) to remove adsorbed organic compounds such as p-chlorotoluene and toluene, followed by drying in an N2 flow at 200◦ C. An intense Cl(2s) peak appeared at 272.3 eV together with a decrease in the intensity of the C and Rh peaks (Fig. 7B). The binding energy of Cl(2s) was in good agreement with that of NaCl supported on active carbon by the usual impregnation method. An intense peak due to Na(1s) was observed at 1072.3 eV (not shown in the figure), although it was difficult to distinguish between the peaks ascribed to NaOH and NaCl. These results strongly suggest that NaCl layers accumulate on the catalyst surface and cover the active site and/or carbon support thereby reducing the catalytic activity. The theoretical amount of NaCl produced was in excess of the catalyst’s weight and enough to cover the whole surface area of the catalyst. In the XPS spectra of the used catalyst after it had been washed several times with water (total ca. 35 ml), the Cl peaks completely disappeared (Fig. 7C). The change in surface composition of Rh-Pt/C during the dechlorination reaction is illustrated in Scheme 1. Freshly prepared catalyst consists of Rh and Pt particles dispersed on the support, although the Rh species is oxidized by exposure to air [14]. Catalyst support such as active carbon and TiO2 are effective to control the dispersion and the electronic state of noble metal [14]. The oxidized Rh species is reduced by hydrogen species arising from dehydrogenation of 2-propanol
on Pt particles [14]. When dechlorination of organic chlorides proceeds, produced NaCl covers the catalyst surface to reduce the catalytic activity. The accumulated NaCl on the surface is removed by washing with water.
4. Conclusion The catalytic dechlorination of aromatic chlorides was completed in a solution of NaOH in 2-propanol with a carbon-supported noble-metal catalyst (Rh-Pt/C) at temperatures below 35◦ C. The reaction was affected by the substituents of the aromatic chlorides; the dechlorination rate was retarded by electron-donating substituents such as methyl and phenyl. Acetone was found to be a possible inhibitor, because it behaves as a hydrogen acceptor thus disturbing hydrogen-transfer to an aromatic halide. The dechlorination reaction of p-chlorotoluene proceeded successfully in a continuous-flow system as well as in a batch system at ambient temperature. Although a gradual decrease in catalytic activity was observed, probably because of the accumulation of NaCl on the catalyst surface, the catalyst recovered completely after it was washed with water. References [1] R.A.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129.
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