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Carbon nanotube supported Pd catalyst for liquid-phase hydrodehalogenation of bromobenzene
CARBON
4 6 ( 20 0 8 ) 2 1 3 7–21 4 3
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Letters to the Editor
Carbon nanotube supported Pd catalyst for liquid-phase hydrodehalogenation of bromobenzene Long Chena,b, Keli Yanga,b, Haitao Liua,b, Xiaolai Wanga,* a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui Road 18, Lanzhou 730000, PR China b Graduate University of the Chinese Academy of Sciences, Beijing 100039, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
A multi-walled carbon nanotube (MWCNT) supported Pd catalyst has been prepared by
Received 4 January 2008
conventional impregnation for the liquid-phase hydrodehalogenation of bromobenzene
Accepted 27 September 2008
under mild reaction conditions. Compared with conventional supports such as activated
Available online 1 October 2008
carbon, Al2O3, SiO2 and MgO, MWCNTs as a catalyst support offer not only better performance but also a substantial reduction in the amount of Pd required. Ó 2008 Elsevier Ltd. All rights reserved.
Aryl halides are one of the most widespread and persistent toxic pollutants. The disposal of these organic wastes is a vital environmental issue. Among the several methods proposed for their destruction, catalytic hydrodehalogenation (HDH) is of increasing interest because it excludes the formation of more toxic compounds such as dioxins and has a comparatively low reaction temperature. Activated carbon (AC) supported Pd catalysts have been proved to be effective in the liquid-phase HDH [1–4]. However, most of these catalysts have a high Pd loading (5–10 wt.%) which make the cost of catalysts high. Recently, Calvo et al. [3,4] studied the liquidphase HDH of 4-chlorophenol over AC-supported Pd catalysts with a Pd content as low as 0.5%. Although, the performance of these catalysts was frankly good in terms of both activity and selectivity, the amount of catalyst they used in their experiments was relatively large and the reactant concentration in aqueous phase was low. Thus, the development of low Pd content and active catalysts for HDH reaction requires further study. On the other hand, carbon nanotubes (CNTs) have attracted a lot of interest in the synthesis, characterization and applications due to their unique structural, mechanical and electronic properties. The use of CNTs as catalyst sup-
ports, especially in liquid-phase reactions, seems to be one of the most promising fields among all their applications. Some researchers have reported that CNT-supported catalytic nanoparticles exhibited much better activity and selectivity than conventional supports in some catalytic reactions [5]. Nevertheless, to the best of our knowledge, few report in the literature regarding the use of CNT-supported metal catalysts for liquid-phase HDH reaction. In this work, we report the excellent performance of CNT-supported Pd catalyst with a relatively low Pd content (2 wt.%) in liquid-phase HDH of bromobenzene. High quality multi-walled CNTs were synthesized by chemical vapor deposition using Co/La2O3 as catalyst and ethylene as carbon source in a tubular quartz reactor [6]. A sonochemical process was used to treat as-grown CNTs in mixed nitric and sulfuric acids to create surface functional groups for Pd nanoparticle deposition as described previously [7]. The catalysts were prepared as follows. About 200 mg of the treated CNTs was impregnated in PdCl2 aqueous solution with the loading of Pd fixed at 2 wt.% (w/w). After a sonication at room temperature for about 4 h, the excess water in the mixture was removed by a rotary evaporator at 80 °C. The
* Corresponding author: Fax: +86 931 8277787. E-mail address: [email protected] (X. Wang). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.09.049
2138
CARBON
4 6 ( 2 0 0 8 ) 2 1 3 7 –2 1 4 3
Table 1 – Pd loadings and TOF values of Pd catalysts supported on various supports Support Pd loading (wt.%)a CNTs AC SiO2 Al2O3 MgO
2.04 1.85 1.72 1.74 1.37
Conversion (%)b TOF (h 1)c 69 21 12 18 6
691 232 143 211 90
a Determined by inductively coupled plasma atomic emission spectrometry. b Reaction time: 30 min. c Mol bromobenzene converted per mol Pd per hour.
Fig. 1 – Conversion of bromobenzene as a function of reaction time over various catalysts.
resulting solid was calcined at 300 °C for 3 h and then reduced with hydrogen at 250 °C for 2 h. For comparison, Pd catalysts supported on conventional supports such as AC, Al2O3, SiO2
and MgO (supplied by Chemical Reagent Factory of Tianjin University, China) were prepared by the same method except that the supports were used as-received. Liquid-phase hydrodebromination reaction was conducted using a glass reactor equipped with a reflux-condenser at atmospheric pressure. In a typical reaction procedure, 4.8 mmol of bromobenzene was dissolved in 8 mL of metha-
Fig. 2 – SEM image (a), TEM image (b) and XRD pattern (c) of Pd/CNT catalyst.
CARBON
2139
4 6 ( 20 0 8 ) 2 1 3 7–21 4 3
Table 2 – Textural properties and particle size distribution of supported Pd catalysts Catalyst
SBET (m2/g)
Dpore (nm)a
Pd/AC Pd/CNTs
1040.0 201.6
4.2 20.3
Vpore (cm3/g)a
Pd particle size distribution (nm)
0.3 1.3
1–5 1–4
Pd mean particle size (nm)b 1.8 2.8
a BJH method. b Determined by TEM from more than 400 particles.
nol containing 2 mL of hydrazine hydrate (85 wt.%) as hydrogen donor. The reaction was started by vigorous stirring of the reaction mixture with 50 mg of catalyst at 60 °C for a period of time. The products were analyzed by gas chromatography and toluene was used as the internal standard. Blank tests carried out in the absence of Pd indicate that CNTs show no activity in bromobenzene HDH. Meanwhile, benzene was found to be the only product in the reaction over the catalysts used in this work. Fig. 1 presents the conversion of bromobenzene as a function of reaction time over various catalysts. A comparison of the activity data, obtained under the same reaction conditions, reveals that Pd/CNTs showed significantly higher activity than the catalysts supported on conventional supports. After 2 h of reaction, bromobenzene was completely converted over Pd/CNTs. In contrast, the conversions of bromobenzene were only 58%, 31%, 25% and 12% over AC, SiO2, Al2O3 and MgO-supported catalysts, respectively. The Pd loadings and turnover frequency (TOF) values during 30 min of reaction over various supports are summarized in Table 1. The TOF value was as high as 691 h 1 over Pd/CNTs, in comparison to 232, 143, 211 and 90 h 1 over AC, SiO2, Al2O3 and MgO supported catalysts, respectively. Thus, Pd/CNTs can be used as a potential catalyst with enhanced activity for the hydrodebromination of bromobenzene. Scanning electron microscope (SEM) image (Fig. 2a) shows that the Pd/CNT catalyst was composed of fibrous structures with diameter between 10 and 30 nm and length up to a dozen micrometers. Transmission electron microscope (TEM) image (Fig. 2b) confirms that metallic Pd nanoparticles with diameter between 1–4 nm distributed randomly on the external wall of CNTs. X-ray diffraction (XRD) pattern of Pd/CNTs (Fig. 2c) only shows (0 0 2), (1 0 1) and (0 0 4) reflections of graphite. No diffraction peaks corresponding to Pd particles were observed, indicating the high dispersion of Pd nanoparticles on the surface of CNTs, consistent with previous TEM results. Table 2 compares the textural properties and particle size distribution between Pd/AC and Pd/CNT catalysts. It can be seen that the Pd/CNTs comprises mainly mesopores and macropores, whereas the Pd/AC has a microporous structure with a much higher specific surface area. Although the Pd particle size distribution was similar on both catalysts, higher metal dispersion was observed on Pd/AC than that on Pd/CNTs (mean particle size: 1.8 nm for Pd/AC versus 2.8 nm for Pd/ CNTs), which might be attributed to the high surface area of AC (1223.2 m2/g) used in this study. The high activity observed on Pd/CNTs in liquid-phase bromobenzene HDH might be explained by the high external surface area and the complete absence of micropores in the
CNTs which prevent significant mass transfer limitation. In contrast, the presence of a large fraction of micropores inside AC was expected to significantly hinder the reactant and product diffusion which resulted in a lower activity despite the high specific surface area. Similar result has been observed by Vu et al. in the selective hydrogenation of cinnamaldehyde over CNT and AC-based catalysts (Pt, Ru and Pt– Ru) [5]. In addition, it has been reported that electron can transfer from the nanotube support to metal particles [5,8]. The increase in electronic density around Pd nanoparticles might promote the adsorption and activation of bromobenzene, thus improving catalyst activity. In conclusion, the use of CNTs as catalyst support could induce peculiar activity and selectivity in bromobenzene HDH. Further detailed investigations of the structure-activity relationship of Pd/CNTs and the HDH reaction of various aryl halides over this catalyst are under way.
R E F E R E N C E S
[1] Amorim C, Yuan G, Patterson PM, Keane MA. Catalytic hydrodechlorination over Pd supported on amorphous and structured carbon. J Catal 2005;234:268–81. [2] Arcadi A, Cerichelli G, Chiarini M, Vico R, Zorzan D. Pd/CCatalyzed transfer reduction of aryl chlorides with sodium formate in water. Eur J Org Chem 2004(16):3404–7. [3] Calvo L, Mohedano AF, Casas JA, Gilarranz MA, Rodrı´guez JJ. Treatment of chlorophenols-bearing wastewaters through hydrodechlorination using Pd/activated carbon catalysts. Carbon 2004;42:1377–81. [4] Calvo L, Gilarranz MA, Casas JA, Mohedano AF, Rodrı´guez JJ. Hydrodechlorination of 4-chlorophenol in aqueous phase using Pd/AC catalysts prepared with modified active carbon supports. Appl Catal B 2006;67:68–76. [5] Vu H, Gonc¸alves F, Philippe R, Lamouroux E, Corrias M, Kihn Y, et al. Bimetallic catalysis on carbon nanotubes for the selective hydrogenation of cinnamaldehyde. J Catal 2006;240:18–22. [6] Chen L, Liu H, Yang K, Wang J, Wang X. The effect of reaction temperature on the diameter distribution of carbon nanotubes grown from ethylene decomposition over a Co–La–O catalyst. Mater Chem Phys, doi:10.1016/j.matchemphys.2008.05.081. [7] Xing Y. Synthesis and electrochemical characterization of uniformly-dispersed high loading Pt nanoparticles on sonochemically-treated carbon nanotubes. J Phys Chem B 2004;108(50):19255–9. [8] Choi HC, Shim M, Bangsaruntip S, Dai H. Spontaneous Reduction of Metal Ions on the Sidewalls of Carbon Nanotubes. J Am Chem Soc 2002;124:9058–9.
4 6 ( 20 0 8 ) 2 1 3 7–21 4 3
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/carbon
Letters to the Editor
Carbon nanotube supported Pd catalyst for liquid-phase hydrodehalogenation of bromobenzene Long Chena,b, Keli Yanga,b, Haitao Liua,b, Xiaolai Wanga,* a
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Tianshui Road 18, Lanzhou 730000, PR China b Graduate University of the Chinese Academy of Sciences, Beijing 100039, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
A multi-walled carbon nanotube (MWCNT) supported Pd catalyst has been prepared by
Received 4 January 2008
conventional impregnation for the liquid-phase hydrodehalogenation of bromobenzene
Accepted 27 September 2008
under mild reaction conditions. Compared with conventional supports such as activated
Available online 1 October 2008
carbon, Al2O3, SiO2 and MgO, MWCNTs as a catalyst support offer not only better performance but also a substantial reduction in the amount of Pd required. Ó 2008 Elsevier Ltd. All rights reserved.
Aryl halides are one of the most widespread and persistent toxic pollutants. The disposal of these organic wastes is a vital environmental issue. Among the several methods proposed for their destruction, catalytic hydrodehalogenation (HDH) is of increasing interest because it excludes the formation of more toxic compounds such as dioxins and has a comparatively low reaction temperature. Activated carbon (AC) supported Pd catalysts have been proved to be effective in the liquid-phase HDH [1–4]. However, most of these catalysts have a high Pd loading (5–10 wt.%) which make the cost of catalysts high. Recently, Calvo et al. [3,4] studied the liquidphase HDH of 4-chlorophenol over AC-supported Pd catalysts with a Pd content as low as 0.5%. Although, the performance of these catalysts was frankly good in terms of both activity and selectivity, the amount of catalyst they used in their experiments was relatively large and the reactant concentration in aqueous phase was low. Thus, the development of low Pd content and active catalysts for HDH reaction requires further study. On the other hand, carbon nanotubes (CNTs) have attracted a lot of interest in the synthesis, characterization and applications due to their unique structural, mechanical and electronic properties. The use of CNTs as catalyst sup-
ports, especially in liquid-phase reactions, seems to be one of the most promising fields among all their applications. Some researchers have reported that CNT-supported catalytic nanoparticles exhibited much better activity and selectivity than conventional supports in some catalytic reactions [5]. Nevertheless, to the best of our knowledge, few report in the literature regarding the use of CNT-supported metal catalysts for liquid-phase HDH reaction. In this work, we report the excellent performance of CNT-supported Pd catalyst with a relatively low Pd content (2 wt.%) in liquid-phase HDH of bromobenzene. High quality multi-walled CNTs were synthesized by chemical vapor deposition using Co/La2O3 as catalyst and ethylene as carbon source in a tubular quartz reactor [6]. A sonochemical process was used to treat as-grown CNTs in mixed nitric and sulfuric acids to create surface functional groups for Pd nanoparticle deposition as described previously [7]. The catalysts were prepared as follows. About 200 mg of the treated CNTs was impregnated in PdCl2 aqueous solution with the loading of Pd fixed at 2 wt.% (w/w). After a sonication at room temperature for about 4 h, the excess water in the mixture was removed by a rotary evaporator at 80 °C. The
* Corresponding author: Fax: +86 931 8277787. E-mail address: [email protected] (X. Wang). 0008-6223/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.carbon.2008.09.049
2138
CARBON
4 6 ( 2 0 0 8 ) 2 1 3 7 –2 1 4 3
Table 1 – Pd loadings and TOF values of Pd catalysts supported on various supports Support Pd loading (wt.%)a CNTs AC SiO2 Al2O3 MgO
2.04 1.85 1.72 1.74 1.37
Conversion (%)b TOF (h 1)c 69 21 12 18 6
691 232 143 211 90
a Determined by inductively coupled plasma atomic emission spectrometry. b Reaction time: 30 min. c Mol bromobenzene converted per mol Pd per hour.
Fig. 1 – Conversion of bromobenzene as a function of reaction time over various catalysts.
resulting solid was calcined at 300 °C for 3 h and then reduced with hydrogen at 250 °C for 2 h. For comparison, Pd catalysts supported on conventional supports such as AC, Al2O3, SiO2
and MgO (supplied by Chemical Reagent Factory of Tianjin University, China) were prepared by the same method except that the supports were used as-received. Liquid-phase hydrodebromination reaction was conducted using a glass reactor equipped with a reflux-condenser at atmospheric pressure. In a typical reaction procedure, 4.8 mmol of bromobenzene was dissolved in 8 mL of metha-
Fig. 2 – SEM image (a), TEM image (b) and XRD pattern (c) of Pd/CNT catalyst.
CARBON
2139
4 6 ( 20 0 8 ) 2 1 3 7–21 4 3
Table 2 – Textural properties and particle size distribution of supported Pd catalysts Catalyst
SBET (m2/g)
Dpore (nm)a
Pd/AC Pd/CNTs
1040.0 201.6
4.2 20.3
Vpore (cm3/g)a
Pd particle size distribution (nm)
0.3 1.3
1–5 1–4
Pd mean particle size (nm)b 1.8 2.8
a BJH method. b Determined by TEM from more than 400 particles.
nol containing 2 mL of hydrazine hydrate (85 wt.%) as hydrogen donor. The reaction was started by vigorous stirring of the reaction mixture with 50 mg of catalyst at 60 °C for a period of time. The products were analyzed by gas chromatography and toluene was used as the internal standard. Blank tests carried out in the absence of Pd indicate that CNTs show no activity in bromobenzene HDH. Meanwhile, benzene was found to be the only product in the reaction over the catalysts used in this work. Fig. 1 presents the conversion of bromobenzene as a function of reaction time over various catalysts. A comparison of the activity data, obtained under the same reaction conditions, reveals that Pd/CNTs showed significantly higher activity than the catalysts supported on conventional supports. After 2 h of reaction, bromobenzene was completely converted over Pd/CNTs. In contrast, the conversions of bromobenzene were only 58%, 31%, 25% and 12% over AC, SiO2, Al2O3 and MgO-supported catalysts, respectively. The Pd loadings and turnover frequency (TOF) values during 30 min of reaction over various supports are summarized in Table 1. The TOF value was as high as 691 h 1 over Pd/CNTs, in comparison to 232, 143, 211 and 90 h 1 over AC, SiO2, Al2O3 and MgO supported catalysts, respectively. Thus, Pd/CNTs can be used as a potential catalyst with enhanced activity for the hydrodebromination of bromobenzene. Scanning electron microscope (SEM) image (Fig. 2a) shows that the Pd/CNT catalyst was composed of fibrous structures with diameter between 10 and 30 nm and length up to a dozen micrometers. Transmission electron microscope (TEM) image (Fig. 2b) confirms that metallic Pd nanoparticles with diameter between 1–4 nm distributed randomly on the external wall of CNTs. X-ray diffraction (XRD) pattern of Pd/CNTs (Fig. 2c) only shows (0 0 2), (1 0 1) and (0 0 4) reflections of graphite. No diffraction peaks corresponding to Pd particles were observed, indicating the high dispersion of Pd nanoparticles on the surface of CNTs, consistent with previous TEM results. Table 2 compares the textural properties and particle size distribution between Pd/AC and Pd/CNT catalysts. It can be seen that the Pd/CNTs comprises mainly mesopores and macropores, whereas the Pd/AC has a microporous structure with a much higher specific surface area. Although the Pd particle size distribution was similar on both catalysts, higher metal dispersion was observed on Pd/AC than that on Pd/CNTs (mean particle size: 1.8 nm for Pd/AC versus 2.8 nm for Pd/ CNTs), which might be attributed to the high surface area of AC (1223.2 m2/g) used in this study. The high activity observed on Pd/CNTs in liquid-phase bromobenzene HDH might be explained by the high external surface area and the complete absence of micropores in the
CNTs which prevent significant mass transfer limitation. In contrast, the presence of a large fraction of micropores inside AC was expected to significantly hinder the reactant and product diffusion which resulted in a lower activity despite the high specific surface area. Similar result has been observed by Vu et al. in the selective hydrogenation of cinnamaldehyde over CNT and AC-based catalysts (Pt, Ru and Pt– Ru) [5]. In addition, it has been reported that electron can transfer from the nanotube support to metal particles [5,8]. The increase in electronic density around Pd nanoparticles might promote the adsorption and activation of bromobenzene, thus improving catalyst activity. In conclusion, the use of CNTs as catalyst support could induce peculiar activity and selectivity in bromobenzene HDH. Further detailed investigations of the structure-activity relationship of Pd/CNTs and the HDH reaction of various aryl halides over this catalyst are under way.
R E F E R E N C E S
[1] Amorim C, Yuan G, Patterson PM, Keane MA. Catalytic hydrodechlorination over Pd supported on amorphous and structured carbon. J Catal 2005;234:268–81. [2] Arcadi A, Cerichelli G, Chiarini M, Vico R, Zorzan D. Pd/CCatalyzed transfer reduction of aryl chlorides with sodium formate in water. Eur J Org Chem 2004(16):3404–7. [3] Calvo L, Mohedano AF, Casas JA, Gilarranz MA, Rodrı´guez JJ. Treatment of chlorophenols-bearing wastewaters through hydrodechlorination using Pd/activated carbon catalysts. Carbon 2004;42:1377–81. [4] Calvo L, Gilarranz MA, Casas JA, Mohedano AF, Rodrı´guez JJ. Hydrodechlorination of 4-chlorophenol in aqueous phase using Pd/AC catalysts prepared with modified active carbon supports. Appl Catal B 2006;67:68–76. [5] Vu H, Gonc¸alves F, Philippe R, Lamouroux E, Corrias M, Kihn Y, et al. Bimetallic catalysis on carbon nanotubes for the selective hydrogenation of cinnamaldehyde. J Catal 2006;240:18–22. [6] Chen L, Liu H, Yang K, Wang J, Wang X. The effect of reaction temperature on the diameter distribution of carbon nanotubes grown from ethylene decomposition over a Co–La–O catalyst. Mater Chem Phys, doi:10.1016/j.matchemphys.2008.05.081. [7] Xing Y. Synthesis and electrochemical characterization of uniformly-dispersed high loading Pt nanoparticles on sonochemically-treated carbon nanotubes. J Phys Chem B 2004;108(50):19255–9. [8] Choi HC, Shim M, Bangsaruntip S, Dai H. Spontaneous Reduction of Metal Ions on the Sidewalls of Carbon Nanotubes. J Am Chem Soc 2002;124:9058–9.