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Rh-based catalysts for catalytic dechlorination of aromatic chloride at ambient temperature
Applied Catalysis B: Environmental 18 (1998) 273±279
Rh-based catalysts for catalytic dechlorination of aromatic chloride at ambient temperature Yuji Ukisu*, Satoshi Kameoka, Tatsuo Miyadera National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 22 January 1998; received in revised form 8 May 1998; accepted 11 May 1998
Abstract Catalytic dechlorination of chlorotoluene to toluene was carried out using several supported Rh-based catalysts in a 2-propanol solution of NaOH at ambient temperature (278C). A carbon-supported Rh catalyst (Rh/C) showed high catalytic activity, although an induction period was involved in the reaction and the activity of the catalyst reduced during storage in air. The existence of Pt on the Rh catalyst was effective in overcoming the activity reduction by exposure to air and gave the reaction without any induction period. The composite Rh±Pt catalyst supported on TiO2 as well as on carbon was much more active for the reaction than the catalysts supported on SiO2, MgO and Al2O3. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Chlorotoluene; Dechlorination; Rh; Catalyst; 2-Propanol; Sodium hydroxide
1. Introduction The disposal of organic wastes containing halogen atoms is a problem of great urgency, since conventional incineration of these wastes has been recognized to be associated with the unexpected formation of more harmful compounds such as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). Removing halogen from organic halides by dehalogenation treatment is an effective procedure for diminishing the toxicity such that the dehalogenated organic wastes can be incinerated without formation of the dioxin-like compounds.
*Corresponding author. Tel.: +81 298 588219; fax: +81 298 588208; e-mail: [email protected]
A number of chemical degradation procedures of organic halides have been suggested including the catalytic dehalogenation process. Molecular hydrogen (pressure, ca. 1±50 atm) has been often used as a hydrogen source to complete hydrodehalogenation of aromatic halides in the liquid phase [1]. In recent years, studies of catalytic transfer hydrodehalogenation using a hydrogen-donor and a catalyst have been extensively done, in which several hydrogen-donors such as alcohols and formates have been used in combination with homogeneous and heterogeneous catalysts [2]. The hydrogen-transfer process generally proceeds under relatively mild conditions, having advantages not only in reducing energy cost but also suppressing the formation of undesirable byproducts. Moreover, the process without using molecular hydrogen is prominent in respect of safety of operation.
0926-3373/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0926-3373(98)00048-4
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Y. Ukisu et al. / Applied Catalysis B: Environmental 18 (1998) 273±279
We have reported that organic halides such as polychlorinated biphenyls (PCBs) and chlorobenzenes undergo complete dehalogenation in a 2-propanol solution of NaOH in the presence of carbon-supported noble-metal catalysts, in which Rh/C is the most effective catalyst for the reaction [3,4]. The catalytic dehalogenation reaction was performed under mild conditions (<828C, 1 atm), including hydrogen transfer from 2-propanol to organic halides. The proposed reaction scheme is as follows: CH3 CH OHCH3 ! CH3 C OCH3 2H
(1)
Ar-Cl 2H ! Ar-H HCl
(2)
HCl NaOH ! NaCl H2 O
(3)
(Eq. (1))(Eq. (2))(Eq. (3)) give ArÿCl CH3 CH OHCH3 NaOH ! ArÿH CH3 C OCH3 NaCl H2 O
(4)
where H* indicates a hydrogen-transfer species. It is worthwhile to make an effort to develop more active catalysts, since the catalysts used in the previous study were supported noble-metal catalysts, in which carbon was exclusively used as the catalyst support and a constant amount of noble metal (5 wt%) was contained. In this study, we used chlorotoluene as a reactant and examined a number of supported Rh-based catalysts to explore the possibility of active catalysts, which function even at ambient temperature. Five kinds of materials (active carbon, TiO2, SiO2, Al2O3 and MgO) were employed for the catalyst support and composite supported catalysts containing Rh±Pt or Rh±Pd were prepared. The effects of composition of noble metals and catalyst supports for the dechlorination reaction were examined. 2. Experimental 2.1. Materials and catalysts Catalyst supports used in this work were active carbon powder (Kanto Chemicals; BET surface area, 940 m2 gÿ1), TiO2 powder (Wako; anatase, 54 m2 gÿ1), SiO2 powder (Nippon Aerosil; 200 m2 gÿ1), gAl2O3 powder (Condea Chemie; Puralox, 200 m2 gÿ1) and MgO powder (Wako; 10 m2 gÿ1). The precursor
complexes used for catalyst preparation were [Rh(CH3COO)2]22H2O, H2PtCl66H2O and (NH4)2PdCl4. The supported catalysts were prepared by immersing a support into an aqueous solution containing an appropriate amount of each precursor complex or mixed ones to the expected content (percentage by weight) of noble metals and then reduced by adding an aqueous solution of NaBH4 dropwise. The reduced catalyst was ®ltered, washed by a large amount of water and dried in N2 ¯ow at 2008C for 30 min. 2.2. General procedure In a typical example, a 2-propanol solution (3.5 ml) containing p-chlorotoluene (20 mmol lÿ1) and NaOH (60 mmol lÿ1) was added to a test tube with a rubber cap. After the air in the test tube was completely replaced by nitrogen, an appropriate amount of supported noble-metal catalyst (10 mg) was added. The test tube was in water, controlled at 278C, and the reaction mixture was stirred vigorously with a magnetic stirrer. During the reaction, an aliquot of the reaction mixture was taken and diluted by a suitable solvent for analysis. The concentrations of p-chlorotoluene, toluene and acetone were determined using capillary GC (HP 6890) equipped with an FID and a column of DB-Wax (30 m0.32 mm, 0.5 mm ®lm-thickness). n-Dodecane was used as the internal standard. 3. Results and discussion 3.1. The features of catalytic dechlorination reaction Fig. 1 shows a typical time course of catalytic dechlorination of p-chlorotoluene in a 2-propanol solution of NaOH with a carbon-supported catalyst containing Rh and Pt (denoted as Rh±Pt/C) at 278C, in which the molar amount of NaOH exceeded that of p-chlorotoluene (NaOH/Cl3). The initial concentration of p-chlorotoluene was 22 mmol lÿ1, which corresponds to 3500 ppm. The concentration of p-chlorotoluene decreased with an increase in toluene concentration, ensuring the material balance between them. A high yield of toluene (>90%) was achieved not only in this case but also with all noble-metal
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Fig. 1. Typical example of catalytic dechlorination of p-chlorotoluene with an Rh-based catalyst. Reaction conditions: p-chlorotoluene, 22 mmol lÿ1; NaOH, 60 mmol lÿ1; 2-propanol, 3.5 ml; Rh± Pt/C (Rh/Pt1/1 wt%), 10 mg; temperature, 278C.
Fig. 2. Catalytic dechlorination of p-chlorotoluene with carbonsupported noble-metal (1 wt%) catalysts. Reaction conditions: pchlorotoluene, 20 mmol lÿ1; NaOH, 60 mmol lÿ1; 2-propanol, 3.5 ml; catalyst, 10 mg; temperature, 278C.
catalysts used in this work. When a mixture containing an equivalent molar amount of o-, m- and p-chlorotoluene was used as a reactant, no signi®cant difference in the dechlorination rate was observed among the isomers. This is probably because chlorotoluenes have less steric effect than polychlorinated compounds such as PCBs and trichlorobenzene, in which the halogen reactivity depended upon the substituted position on the aromatic ring [3,4]. In the following experiments, therefore, p-chlorotoluene was employed as a reactant and the ef®ciency of the catalysts was evaluated in terms of the conversion of p-chlorotoluene.
supported noble-metal catalysts, in which the concentration of noble metal was 5 wt% [4]. The amount of acetone produced after the 180-min reaction was determined to be 0.13 mmol for Rh/C, 0.18 mmol for Pd/C and 0.11 mmol for Pt/C, which were in excess of the initial amount of Cl in the substrate (0.070 mmol), indicating that enough hydrogen species to complete dechlorination was formed on the all catalysts (see the stoichiometry of Eq. (4)) and some of the hydrogen species were evolved as gaseous hydrogen. Therefore, it is reasonable to conclude that the difference in the dechlorination activity was mainly controlled by the ef®ciency of hydrogentransfer process (Eq. (2)) [4].
3.2. The dechlorination reaction with supported noble metal (Rh, Pd or Pt) catalysts Three carbon-supported noble-metal (1 wt%) catalysts (Rh/C, Pd/C and Pt/C) were examined for the dechlorination reaction of p-chlorotoluene at 278C. As shown in Fig. 2, Rh/C exhibited very high activity and the reaction was completed within 180 min, while Pd/ C was less active and no reaction occurred on Pt/C. This activity order is in good agreement with the author's previous results obtained in catalytic dechlorination of 1,2,4-trichlorotoluene at 428C with carbon-
3.3. The activity change of Rh catalyst after storage in air Although the freshly prepared Rh/C catalyst was remarkably effective for the dechlorination reaction even at room temperature, the catalyst kept in air for several days was found to exhibit no activity until reduced with H2. Fig. 3 shows the feature of decline in the catalytic activity of the Rh/C catalyst by exposure to air. To ensure the completely reduced state of the catalyst, the stored sample was pre-reduced in H2
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Fig. 3. Activity change in catalytic dechlorination of p-chlorotoluene with Rh(1 wt%)/C at 278C. (a) Catalyst stored in air for 19 h; (b) catalyst stored in air for 68 h; (c) catalyst reduced with H2 at 2508C; and (d) fresh catalyst reduced by NaBH4. Reaction conditions are the same as in Fig. 2.
¯owing at 2508C for 30 min before exposure to air. When a sample exposed to air for 19 h was used, an induction period (ca. 50 min) was observed before the dechlorination reaction was initiated (Fig. 3(a)). As shown in Fig. 3(b), prolonged exposure of the sample to air (for 68 h) resulted in no conversion of p-chlorotoluene even after 240 min. The deactivated catalyst recovered its activity when it was treated with H2 at 2508C for 30 min (Fig. 3(c)), but did not when treated with N2 at 2008C. These results suggest that a physisorbed molecule, such as H2O, is not associated with a decrease in the activity and the reduced state of Rh is an important factor to initiate the dechlorination reaction at room temperature. Wang et al. reported that oxidation of Rh on Rh/SiO2 occurred in air even at room temperature, while much higher temperatures (>5008C) were required for oxides (Rh2O3) to be detected by electron diffraction [7]. Furthermore, it was found that treatment of the Rh2O3 surface species in H2 at 1508C produced complete reduction to Rh metal [7]. Similar redox behavior of surface Rh species has been observed in Rh-based catalysts supported on TiO2 [10]. It can be seen that the fresh catalyst reduced by NaBH4 (Fig. 3(d)) completed the dechlorination reac-
tion slower than the H2-treated sample, apparently due to the exposure of the freshly reduced Rh metal to air during the preparation procedure. Thus, the catalytic dechlorination reaction at room temperature is sensitive to the reduced state of Rh on the surface. In reactions at higher temperature, however, the high catalytic activity was ensured without the pretreatment by H2 as reported in the author's previous papers [3,4]. It should be noted that the dechlorination rate following the induction period was almost the same as that of pre-reduced catalysts (see Fig. 3(a),(c) and (d)). Thus we may assume that reduction of Rh species is likely to proceed during the induction period, followed by the dechlorination reaction once the reduction of Rh is completed. In fact, the catalyst is always in a reducing atmosphere, since analysis of acetone by GC has revealed that dehydrogenation of 2-propanol occurs even during the induction period to produce active hydrogen species. 3.4. Composite noble-metal catalysts for the dechlorination reaction In order to develop an active and durable catalyst, several composite catalysts containing Rh and another noble metal (Pt or Pd) were examined. Fig. 4 shows
Fig. 4. Difference in catalytic dechlorination activity of pchlorotoluene at 27 with Rh±Pt/C (Rh/Pt2/1 wt%), Rh(2 wt%)/ C and Pt(1 wt%)/C. Reaction conditions are the same as in Fig. 2.
Y. Ukisu et al. / Applied Catalysis B: Environmental 18 (1998) 273±279
Fig. 5. Catalytic dechlorination of p-chlorotoluene with carbonsupported composite catalysts at 278C. (a) Rh/Pt1/1 wt%; (b) Rh/ Pt2/1 wt%; (c) Rh/Pt3/1 wt%; and (d) Rh/Pd1/1 wt%. The dotted line indicates the result with Rh(1 wt%)/C reduced by H2 at 2508C. Reaction conditions are the same as in Fig. 2.
the temporal course of conversion of p-chlorotoluene with a carbon-supported composite catalyst containing Rh and Pt (Rh±Pt/C; Rh, 2 wt%; Pt, 1 wt%), Rh(2 wt%)/C and Pt(1 wt%)/C. Although the reaction with Rh/C included an induction period and Pt/C exhibited no dechlorination activity, Rh±Pt/C gave a high reaction rate without any induction period. Moreover, it is noteworthy that the composite catalyst preserved its activity even after a long period of storage; the catalyst stored in air for over 60 days exhibited the same activity as a fresh one (not shown in ®gure). The activity of the composite Rh±Pt catalyst increased as the content of Rh (1±3 wt%) increased, as shown in Fig. 5(a), (b) and (c), indicating again that Rh is responsible for the catalytic dechlorination reaction. When Rh±Pt/C (Rh: 3 wt%, Pt: 1 wt%) was used, higher concentrations of p-chlorotoluene were entirely dechlorinated even at 278C: 44 mmol lÿ1 (7000 ppm) within 3 h and 60 mmol lÿ1 (9500 ppm) within 6 h. It should be noted that the reaction pro®le with Rh± Pt/C (Rh, 1 wt%; and Pt, 1 wt%) is in good agreement with that of Rh(1 wt%)/C reduced by H2 at 2508C (Fig. 5, dotted line). This implies that Pt metal plays an important role in either (i) keeping the reduced state
277
of Rh during storage, or (ii) restoring the Rh species immediately to a reduced one at the initial step of the reaction. The ®rst hypothesis is probable if a bimetallic particle or site consisting of Pt and Rh, which is tolerant to air forms on the support. Indeed, the existence of Rh±Pt alloy particles on supports such as SiO2 and Al2O3 has been suggested by spectroscopic techniques and chemisorption study, although their surface composition is strongly dependent upon their preparation procedure, gas-phase environment and temperature history [6±9]. For example, it has been reported on Rh±Pt/SiO2 that treatment in H2 at temperatures >5008C is required for the homogenization of alloy particles, while segregation and surface enrichment of Rh occur at temperatures <3008C [7]. The catalyst used in this work is rather like the latter case, because it underwent no high-temperature treatment. From analysis of GC, a large amount of acetone was detected during the dechlorination reaction with Rh±Pt/C, which corresponds to the sum of the acetone detected with Rh/C and Pt/C. This result leads us to assume that Rh and Pt particles preferably segregate and hydrogen species formed on Pt metal is likely to activate Rh metal. In fact, hydrogen spillover on supported noble-metal catalysts has been found to occur during dehydrogenation of 2-propanol because much more acetone in liquid phase was detected than H2 evolved [11]. Moreover, it has been reported that hydrogen spillover takes place from one metal to another in supported bimetallic particles [12]. Another composite catalyst, Rh±Pd/C, was less active than Rh±Pt/C with a short induction period as shown in Fig. 5(d). The amount of acetone with the Rh±Pd catalyst was much more than that with the Rh±Pt catalyst. These results imply that hydrogen species formed on Pd is less effective for the activation of Rh, probably because of the high ability of Pd to occlude hydrogen. In addition, a large amount of produced acetone may retard the hydrogen-transfer step (Eq. (2)), because the produced hydrogen species is consumed competitively by the hydrogenation of acetone [5]. 3.5. The effect of catalyst support for the catalytic activity Fig. 6 shows the features of catalytic dechlorination of p-chlorotoluene with Rh±Pt catalysts supported on
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Fig. 6. Catalytic dechlorination of p-chlorotoluene at 278C with a composite Rh±Pt catalyst (Rh/Pt1/1 wt%) supported on various supports. Reaction conditions are the same as in Fig. 2.
various supports (active carbon, TiO2, SiO2, Al2O3 and MgO). Both TiO2- and carbon-supported catalysts displayed signi®cant activities for the reaction, while the SiO2- and MgO-supported catalysts were less active and conversion with the Al2O3-supported catalyst leveled off within 60 min. The difference in the dechlorination rate between high- and low-activity catalysts was more than 20 times. Nevertheless, the amount of acetone produced during the dechlorination reaction over SiO2- and MgO-supported catalysts was ca. 30% of the active catalysts such as carbon- and TiO2-supported catalysts. These results indicate that the ef®ciency of hydrogen-transfer (Eq. (2)) is quite low on the less active catalysts. It should be emphasized, moreover, that the activity of Rh±Pt/TiO2 was kept after prolonged storage in air (not shown in ®gure) as observed in the case of the carbon-supported catalyst. On the other hand, the maximum conversion with the Al2O3-supported catalyst decreased after storage in air: ca. 40% of a fresh one after 7 days. It is interesting that TiO2 is effective as a catalyst support, although it has much smaller surface area than active carbon and leads to disadvantage in increasing the dispersion of supported metals. In recent years, various supported Rh catalysts have been studied in several reactions such as CO dissociation [13], CO hydrogenation [14,17] and dehydrogenation
of benzene and toluene [15], demonstrating that Rh/ TiO2 is more active than Rh/SiO2 and Rh/Al2O3. The high activity of Rh/TiO2 has been attributed to the interface sites consisting of Rh and TiOx suboxides [16] or the electronic properties of Rh controlled by the support [17]. Furthermore, it is well known that the morphology of metal species on TiO2 depends on the precursor complex used in catalyst preparation and the reduction temperature [18], including the strong metal±support interaction (SMSI) effect caused by high-temperature reduction [19,20]. Thus, the catalytic dechlorination activity of the Rhbased catalyst was found to strongly depend on the catalyst support. Rh-based catalysts supported on various supports will be characterized using spectroscopic techniques such as XPS, XRD and TPD in our laboratory. 4. Conclusions Composite Rh±Pt catalysts (Rh±Pt/C and Rh±Pt/ TiO2) displayed signi®cant activities for catalytic hydrogen-transfer hydrodechlorinartion of chlorotoluene in a 2-propanol solution of NaOH at ambient temperature (278C). 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 sites. The activated Rh metal is responsible for hydrogen transfer from 2-propanol to chlorotoluene at low temperatures.
References [1] A.R. Pinder, Synthesis (1980) 425. [2] R.A.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129. [3] Y. Ukisu, S. Iimura, R. Uchida, Chemospher 33 (1996) 1523. [4] Y. Ukisu, T. Miyadera, J. Mol. Catal. 125 (1997) 135. [5] Y. Saito, M. Yamashita, E. Ito, N. Meng, Int. J. Hydrogen Energy 19 (1994) 223. [6] R.E. Lakis, C.E. Lymana, H.G. Stenger Jr., , J. Catal. 154 (1995) 261. [7] T. Wang, L.D. Schmidt, J. Catal. 71 (1981) 411. [8] M. Chen, T. Wang, L.D. Schmidt, J. Catal. 60 (1979) 356. [9] Y. Zhu, L.D. Schmidt, Surf. Sci. 129 (1983) 107. [10] J.M. Herrmann, J. Disdier, P. Pichat, A. Fernandez, A. Gonzalez-Elipe, G. Munuera, C. Leclercq, J. Catal. 132 (1991) 490.
Y. Ukisu et al. / Applied Catalysis B: Environmental 18 (1998) 273±279 [11] T. Ogawa, Y. Saito, 72th Annual Meeting of Chem. Soc. Jpn., 4D135, 1997, in Japanese. [12] W.C. Conner, J.L. Falconer, Chem. Rev. 95 (1995) 759. [13] A. Erdohelyi, F. Solymosi, J. Catal. 84 (1983) 446. [14] T. Ioannides, X.E. Verykios, J. Catal. 140 (1993) 353. [15] T. Ioannides, X.E. Verykios, J. Catal. 143 (1993) 175. [16] M.E. Levin, M. Salmeron, A.T. Bell, G.A. Somorjai, J. Catal. 106 (1987) 401.
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[17] Y. Mori, T. Mori, A. Miyamoto, N. Takahashi, T. Hattori, Y. Murakami, J. Phys. Chem. 93 (1989) 2039. [18] E.J. Braunschweig, A.D. Logan, A.K. Datye, D.J. Smith, J. Catal. 118 (1989) 227. [19] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [20] S.J. Tauster, S.C. Fung, R.T.K. Baker, J.A. Horsley, Science 211 (1981) 1121.
Rh-based catalysts for catalytic dechlorination of aromatic chloride at ambient temperature Yuji Ukisu*, Satoshi Kameoka, Tatsuo Miyadera National Institute for Resources and Environment, 16-3 Onogawa, Tsukuba, Ibaraki 305-8569, Japan Received 22 January 1998; received in revised form 8 May 1998; accepted 11 May 1998
Abstract Catalytic dechlorination of chlorotoluene to toluene was carried out using several supported Rh-based catalysts in a 2-propanol solution of NaOH at ambient temperature (278C). A carbon-supported Rh catalyst (Rh/C) showed high catalytic activity, although an induction period was involved in the reaction and the activity of the catalyst reduced during storage in air. The existence of Pt on the Rh catalyst was effective in overcoming the activity reduction by exposure to air and gave the reaction without any induction period. The composite Rh±Pt catalyst supported on TiO2 as well as on carbon was much more active for the reaction than the catalysts supported on SiO2, MgO and Al2O3. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Chlorotoluene; Dechlorination; Rh; Catalyst; 2-Propanol; Sodium hydroxide
1. Introduction The disposal of organic wastes containing halogen atoms is a problem of great urgency, since conventional incineration of these wastes has been recognized to be associated with the unexpected formation of more harmful compounds such as polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs). Removing halogen from organic halides by dehalogenation treatment is an effective procedure for diminishing the toxicity such that the dehalogenated organic wastes can be incinerated without formation of the dioxin-like compounds.
*Corresponding author. Tel.: +81 298 588219; fax: +81 298 588208; e-mail: [email protected]
A number of chemical degradation procedures of organic halides have been suggested including the catalytic dehalogenation process. Molecular hydrogen (pressure, ca. 1±50 atm) has been often used as a hydrogen source to complete hydrodehalogenation of aromatic halides in the liquid phase [1]. In recent years, studies of catalytic transfer hydrodehalogenation using a hydrogen-donor and a catalyst have been extensively done, in which several hydrogen-donors such as alcohols and formates have been used in combination with homogeneous and heterogeneous catalysts [2]. The hydrogen-transfer process generally proceeds under relatively mild conditions, having advantages not only in reducing energy cost but also suppressing the formation of undesirable byproducts. Moreover, the process without using molecular hydrogen is prominent in respect of safety of operation.
0926-3373/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0926-3373(98)00048-4
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Y. Ukisu et al. / Applied Catalysis B: Environmental 18 (1998) 273±279
We have reported that organic halides such as polychlorinated biphenyls (PCBs) and chlorobenzenes undergo complete dehalogenation in a 2-propanol solution of NaOH in the presence of carbon-supported noble-metal catalysts, in which Rh/C is the most effective catalyst for the reaction [3,4]. The catalytic dehalogenation reaction was performed under mild conditions (<828C, 1 atm), including hydrogen transfer from 2-propanol to organic halides. The proposed reaction scheme is as follows: CH3 CH OHCH3 ! CH3 C OCH3 2H
(1)
Ar-Cl 2H ! Ar-H HCl
(2)
HCl NaOH ! NaCl H2 O
(3)
(Eq. (1))(Eq. (2))(Eq. (3)) give ArÿCl CH3 CH OHCH3 NaOH ! ArÿH CH3 C OCH3 NaCl H2 O
(4)
where H* indicates a hydrogen-transfer species. It is worthwhile to make an effort to develop more active catalysts, since the catalysts used in the previous study were supported noble-metal catalysts, in which carbon was exclusively used as the catalyst support and a constant amount of noble metal (5 wt%) was contained. In this study, we used chlorotoluene as a reactant and examined a number of supported Rh-based catalysts to explore the possibility of active catalysts, which function even at ambient temperature. Five kinds of materials (active carbon, TiO2, SiO2, Al2O3 and MgO) were employed for the catalyst support and composite supported catalysts containing Rh±Pt or Rh±Pd were prepared. The effects of composition of noble metals and catalyst supports for the dechlorination reaction were examined. 2. Experimental 2.1. Materials and catalysts Catalyst supports used in this work were active carbon powder (Kanto Chemicals; BET surface area, 940 m2 gÿ1), TiO2 powder (Wako; anatase, 54 m2 gÿ1), SiO2 powder (Nippon Aerosil; 200 m2 gÿ1), gAl2O3 powder (Condea Chemie; Puralox, 200 m2 gÿ1) and MgO powder (Wako; 10 m2 gÿ1). The precursor
complexes used for catalyst preparation were [Rh(CH3COO)2]22H2O, H2PtCl66H2O and (NH4)2PdCl4. The supported catalysts were prepared by immersing a support into an aqueous solution containing an appropriate amount of each precursor complex or mixed ones to the expected content (percentage by weight) of noble metals and then reduced by adding an aqueous solution of NaBH4 dropwise. The reduced catalyst was ®ltered, washed by a large amount of water and dried in N2 ¯ow at 2008C for 30 min. 2.2. General procedure In a typical example, a 2-propanol solution (3.5 ml) containing p-chlorotoluene (20 mmol lÿ1) and NaOH (60 mmol lÿ1) was added to a test tube with a rubber cap. After the air in the test tube was completely replaced by nitrogen, an appropriate amount of supported noble-metal catalyst (10 mg) was added. The test tube was in water, controlled at 278C, and the reaction mixture was stirred vigorously with a magnetic stirrer. During the reaction, an aliquot of the reaction mixture was taken and diluted by a suitable solvent for analysis. The concentrations of p-chlorotoluene, toluene and acetone were determined using capillary GC (HP 6890) equipped with an FID and a column of DB-Wax (30 m0.32 mm, 0.5 mm ®lm-thickness). n-Dodecane was used as the internal standard. 3. Results and discussion 3.1. The features of catalytic dechlorination reaction Fig. 1 shows a typical time course of catalytic dechlorination of p-chlorotoluene in a 2-propanol solution of NaOH with a carbon-supported catalyst containing Rh and Pt (denoted as Rh±Pt/C) at 278C, in which the molar amount of NaOH exceeded that of p-chlorotoluene (NaOH/Cl3). The initial concentration of p-chlorotoluene was 22 mmol lÿ1, which corresponds to 3500 ppm. The concentration of p-chlorotoluene decreased with an increase in toluene concentration, ensuring the material balance between them. A high yield of toluene (>90%) was achieved not only in this case but also with all noble-metal
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Fig. 1. Typical example of catalytic dechlorination of p-chlorotoluene with an Rh-based catalyst. Reaction conditions: p-chlorotoluene, 22 mmol lÿ1; NaOH, 60 mmol lÿ1; 2-propanol, 3.5 ml; Rh± Pt/C (Rh/Pt1/1 wt%), 10 mg; temperature, 278C.
Fig. 2. Catalytic dechlorination of p-chlorotoluene with carbonsupported noble-metal (1 wt%) catalysts. Reaction conditions: pchlorotoluene, 20 mmol lÿ1; NaOH, 60 mmol lÿ1; 2-propanol, 3.5 ml; catalyst, 10 mg; temperature, 278C.
catalysts used in this work. When a mixture containing an equivalent molar amount of o-, m- and p-chlorotoluene was used as a reactant, no signi®cant difference in the dechlorination rate was observed among the isomers. This is probably because chlorotoluenes have less steric effect than polychlorinated compounds such as PCBs and trichlorobenzene, in which the halogen reactivity depended upon the substituted position on the aromatic ring [3,4]. In the following experiments, therefore, p-chlorotoluene was employed as a reactant and the ef®ciency of the catalysts was evaluated in terms of the conversion of p-chlorotoluene.
supported noble-metal catalysts, in which the concentration of noble metal was 5 wt% [4]. The amount of acetone produced after the 180-min reaction was determined to be 0.13 mmol for Rh/C, 0.18 mmol for Pd/C and 0.11 mmol for Pt/C, which were in excess of the initial amount of Cl in the substrate (0.070 mmol), indicating that enough hydrogen species to complete dechlorination was formed on the all catalysts (see the stoichiometry of Eq. (4)) and some of the hydrogen species were evolved as gaseous hydrogen. Therefore, it is reasonable to conclude that the difference in the dechlorination activity was mainly controlled by the ef®ciency of hydrogentransfer process (Eq. (2)) [4].
3.2. The dechlorination reaction with supported noble metal (Rh, Pd or Pt) catalysts Three carbon-supported noble-metal (1 wt%) catalysts (Rh/C, Pd/C and Pt/C) were examined for the dechlorination reaction of p-chlorotoluene at 278C. As shown in Fig. 2, Rh/C exhibited very high activity and the reaction was completed within 180 min, while Pd/ C was less active and no reaction occurred on Pt/C. This activity order is in good agreement with the author's previous results obtained in catalytic dechlorination of 1,2,4-trichlorotoluene at 428C with carbon-
3.3. The activity change of Rh catalyst after storage in air Although the freshly prepared Rh/C catalyst was remarkably effective for the dechlorination reaction even at room temperature, the catalyst kept in air for several days was found to exhibit no activity until reduced with H2. Fig. 3 shows the feature of decline in the catalytic activity of the Rh/C catalyst by exposure to air. To ensure the completely reduced state of the catalyst, the stored sample was pre-reduced in H2
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Fig. 3. Activity change in catalytic dechlorination of p-chlorotoluene with Rh(1 wt%)/C at 278C. (a) Catalyst stored in air for 19 h; (b) catalyst stored in air for 68 h; (c) catalyst reduced with H2 at 2508C; and (d) fresh catalyst reduced by NaBH4. Reaction conditions are the same as in Fig. 2.
¯owing at 2508C for 30 min before exposure to air. When a sample exposed to air for 19 h was used, an induction period (ca. 50 min) was observed before the dechlorination reaction was initiated (Fig. 3(a)). As shown in Fig. 3(b), prolonged exposure of the sample to air (for 68 h) resulted in no conversion of p-chlorotoluene even after 240 min. The deactivated catalyst recovered its activity when it was treated with H2 at 2508C for 30 min (Fig. 3(c)), but did not when treated with N2 at 2008C. These results suggest that a physisorbed molecule, such as H2O, is not associated with a decrease in the activity and the reduced state of Rh is an important factor to initiate the dechlorination reaction at room temperature. Wang et al. reported that oxidation of Rh on Rh/SiO2 occurred in air even at room temperature, while much higher temperatures (>5008C) were required for oxides (Rh2O3) to be detected by electron diffraction [7]. Furthermore, it was found that treatment of the Rh2O3 surface species in H2 at 1508C produced complete reduction to Rh metal [7]. Similar redox behavior of surface Rh species has been observed in Rh-based catalysts supported on TiO2 [10]. It can be seen that the fresh catalyst reduced by NaBH4 (Fig. 3(d)) completed the dechlorination reac-
tion slower than the H2-treated sample, apparently due to the exposure of the freshly reduced Rh metal to air during the preparation procedure. Thus, the catalytic dechlorination reaction at room temperature is sensitive to the reduced state of Rh on the surface. In reactions at higher temperature, however, the high catalytic activity was ensured without the pretreatment by H2 as reported in the author's previous papers [3,4]. It should be noted that the dechlorination rate following the induction period was almost the same as that of pre-reduced catalysts (see Fig. 3(a),(c) and (d)). Thus we may assume that reduction of Rh species is likely to proceed during the induction period, followed by the dechlorination reaction once the reduction of Rh is completed. In fact, the catalyst is always in a reducing atmosphere, since analysis of acetone by GC has revealed that dehydrogenation of 2-propanol occurs even during the induction period to produce active hydrogen species. 3.4. Composite noble-metal catalysts for the dechlorination reaction In order to develop an active and durable catalyst, several composite catalysts containing Rh and another noble metal (Pt or Pd) were examined. Fig. 4 shows
Fig. 4. Difference in catalytic dechlorination activity of pchlorotoluene at 27 with Rh±Pt/C (Rh/Pt2/1 wt%), Rh(2 wt%)/ C and Pt(1 wt%)/C. Reaction conditions are the same as in Fig. 2.
Y. Ukisu et al. / Applied Catalysis B: Environmental 18 (1998) 273±279
Fig. 5. Catalytic dechlorination of p-chlorotoluene with carbonsupported composite catalysts at 278C. (a) Rh/Pt1/1 wt%; (b) Rh/ Pt2/1 wt%; (c) Rh/Pt3/1 wt%; and (d) Rh/Pd1/1 wt%. The dotted line indicates the result with Rh(1 wt%)/C reduced by H2 at 2508C. Reaction conditions are the same as in Fig. 2.
the temporal course of conversion of p-chlorotoluene with a carbon-supported composite catalyst containing Rh and Pt (Rh±Pt/C; Rh, 2 wt%; Pt, 1 wt%), Rh(2 wt%)/C and Pt(1 wt%)/C. Although the reaction with Rh/C included an induction period and Pt/C exhibited no dechlorination activity, Rh±Pt/C gave a high reaction rate without any induction period. Moreover, it is noteworthy that the composite catalyst preserved its activity even after a long period of storage; the catalyst stored in air for over 60 days exhibited the same activity as a fresh one (not shown in ®gure). The activity of the composite Rh±Pt catalyst increased as the content of Rh (1±3 wt%) increased, as shown in Fig. 5(a), (b) and (c), indicating again that Rh is responsible for the catalytic dechlorination reaction. When Rh±Pt/C (Rh: 3 wt%, Pt: 1 wt%) was used, higher concentrations of p-chlorotoluene were entirely dechlorinated even at 278C: 44 mmol lÿ1 (7000 ppm) within 3 h and 60 mmol lÿ1 (9500 ppm) within 6 h. It should be noted that the reaction pro®le with Rh± Pt/C (Rh, 1 wt%; and Pt, 1 wt%) is in good agreement with that of Rh(1 wt%)/C reduced by H2 at 2508C (Fig. 5, dotted line). This implies that Pt metal plays an important role in either (i) keeping the reduced state
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of Rh during storage, or (ii) restoring the Rh species immediately to a reduced one at the initial step of the reaction. The ®rst hypothesis is probable if a bimetallic particle or site consisting of Pt and Rh, which is tolerant to air forms on the support. Indeed, the existence of Rh±Pt alloy particles on supports such as SiO2 and Al2O3 has been suggested by spectroscopic techniques and chemisorption study, although their surface composition is strongly dependent upon their preparation procedure, gas-phase environment and temperature history [6±9]. For example, it has been reported on Rh±Pt/SiO2 that treatment in H2 at temperatures >5008C is required for the homogenization of alloy particles, while segregation and surface enrichment of Rh occur at temperatures <3008C [7]. The catalyst used in this work is rather like the latter case, because it underwent no high-temperature treatment. From analysis of GC, a large amount of acetone was detected during the dechlorination reaction with Rh±Pt/C, which corresponds to the sum of the acetone detected with Rh/C and Pt/C. This result leads us to assume that Rh and Pt particles preferably segregate and hydrogen species formed on Pt metal is likely to activate Rh metal. In fact, hydrogen spillover on supported noble-metal catalysts has been found to occur during dehydrogenation of 2-propanol because much more acetone in liquid phase was detected than H2 evolved [11]. Moreover, it has been reported that hydrogen spillover takes place from one metal to another in supported bimetallic particles [12]. Another composite catalyst, Rh±Pd/C, was less active than Rh±Pt/C with a short induction period as shown in Fig. 5(d). The amount of acetone with the Rh±Pd catalyst was much more than that with the Rh±Pt catalyst. These results imply that hydrogen species formed on Pd is less effective for the activation of Rh, probably because of the high ability of Pd to occlude hydrogen. In addition, a large amount of produced acetone may retard the hydrogen-transfer step (Eq. (2)), because the produced hydrogen species is consumed competitively by the hydrogenation of acetone [5]. 3.5. The effect of catalyst support for the catalytic activity Fig. 6 shows the features of catalytic dechlorination of p-chlorotoluene with Rh±Pt catalysts supported on
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Fig. 6. Catalytic dechlorination of p-chlorotoluene at 278C with a composite Rh±Pt catalyst (Rh/Pt1/1 wt%) supported on various supports. Reaction conditions are the same as in Fig. 2.
various supports (active carbon, TiO2, SiO2, Al2O3 and MgO). Both TiO2- and carbon-supported catalysts displayed signi®cant activities for the reaction, while the SiO2- and MgO-supported catalysts were less active and conversion with the Al2O3-supported catalyst leveled off within 60 min. The difference in the dechlorination rate between high- and low-activity catalysts was more than 20 times. Nevertheless, the amount of acetone produced during the dechlorination reaction over SiO2- and MgO-supported catalysts was ca. 30% of the active catalysts such as carbon- and TiO2-supported catalysts. These results indicate that the ef®ciency of hydrogen-transfer (Eq. (2)) is quite low on the less active catalysts. It should be emphasized, moreover, that the activity of Rh±Pt/TiO2 was kept after prolonged storage in air (not shown in ®gure) as observed in the case of the carbon-supported catalyst. On the other hand, the maximum conversion with the Al2O3-supported catalyst decreased after storage in air: ca. 40% of a fresh one after 7 days. It is interesting that TiO2 is effective as a catalyst support, although it has much smaller surface area than active carbon and leads to disadvantage in increasing the dispersion of supported metals. In recent years, various supported Rh catalysts have been studied in several reactions such as CO dissociation [13], CO hydrogenation [14,17] and dehydrogenation
of benzene and toluene [15], demonstrating that Rh/ TiO2 is more active than Rh/SiO2 and Rh/Al2O3. The high activity of Rh/TiO2 has been attributed to the interface sites consisting of Rh and TiOx suboxides [16] or the electronic properties of Rh controlled by the support [17]. Furthermore, it is well known that the morphology of metal species on TiO2 depends on the precursor complex used in catalyst preparation and the reduction temperature [18], including the strong metal±support interaction (SMSI) effect caused by high-temperature reduction [19,20]. Thus, the catalytic dechlorination activity of the Rhbased catalyst was found to strongly depend on the catalyst support. Rh-based catalysts supported on various supports will be characterized using spectroscopic techniques such as XPS, XRD and TPD in our laboratory. 4. Conclusions Composite Rh±Pt catalysts (Rh±Pt/C and Rh±Pt/ TiO2) displayed signi®cant activities for catalytic hydrogen-transfer hydrodechlorinartion of chlorotoluene in a 2-propanol solution of NaOH at ambient temperature (278C). 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 sites. The activated Rh metal is responsible for hydrogen transfer from 2-propanol to chlorotoluene at low temperatures.
References [1] A.R. Pinder, Synthesis (1980) 425. [2] R.A.W. Johnstone, A.H. Wilby, I.D. Entwistle, Chem. Rev. 85 (1985) 129. [3] Y. Ukisu, S. Iimura, R. Uchida, Chemospher 33 (1996) 1523. [4] Y. Ukisu, T. Miyadera, J. Mol. Catal. 125 (1997) 135. [5] Y. Saito, M. Yamashita, E. Ito, N. Meng, Int. J. Hydrogen Energy 19 (1994) 223. [6] R.E. Lakis, C.E. Lymana, H.G. Stenger Jr., , J. Catal. 154 (1995) 261. [7] T. Wang, L.D. Schmidt, J. Catal. 71 (1981) 411. [8] M. Chen, T. Wang, L.D. Schmidt, J. Catal. 60 (1979) 356. [9] Y. Zhu, L.D. Schmidt, Surf. Sci. 129 (1983) 107. [10] J.M. Herrmann, J. Disdier, P. Pichat, A. Fernandez, A. Gonzalez-Elipe, G. Munuera, C. Leclercq, J. Catal. 132 (1991) 490.
Y. Ukisu et al. / Applied Catalysis B: Environmental 18 (1998) 273±279 [11] T. Ogawa, Y. Saito, 72th Annual Meeting of Chem. Soc. Jpn., 4D135, 1997, in Japanese. [12] W.C. Conner, J.L. Falconer, Chem. Rev. 95 (1995) 759. [13] A. Erdohelyi, F. Solymosi, J. Catal. 84 (1983) 446. [14] T. Ioannides, X.E. Verykios, J. Catal. 140 (1993) 353. [15] T. Ioannides, X.E. Verykios, J. Catal. 143 (1993) 175. [16] M.E. Levin, M. Salmeron, A.T. Bell, G.A. Somorjai, J. Catal. 106 (1987) 401.
279
[17] Y. Mori, T. Mori, A. Miyamoto, N. Takahashi, T. Hattori, Y. Murakami, J. Phys. Chem. 93 (1989) 2039. [18] E.J. Braunschweig, A.D. Logan, A.K. Datye, D.J. Smith, J. Catal. 118 (1989) 227. [19] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [20] S.J. Tauster, S.C. Fung, R.T.K. Baker, J.A. Horsley, Science 211 (1981) 1121.