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Dehydrogenation of ethane over vanadium, cobalt and nickel based catalysts
Zeolites and Related Materials: Trends, Targets and Challenges Proceedings of 4th International FEZA Conference A. Gédéon, P. Massiani and F. Babonneau (Editors) © 2008 Elsevier B.V. All rights reserved.
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Dehydrogenation of ethane over vanadium, cobalt and nickel based catalysts Libor apek,* Lukáš Vank, Jií Adam, Lucie Smoláková University of Pardubice, Faculty of Chemical Technology,Department of Physical Chemistry, Nám. s. Legií 565, CZ-532 10 Pardubice, Czech Republic,e-mail: [email protected]
Abstract The contribution deals with the catalytic performance of vanadium-, cobalt- and nickelbased catalysts in oxidative dehydrogenation of ethane. Vanadium, cobalt and nickel species were loaded to microporous (MFI) and mesoporous (HMS) materials, and supported on inorganic support (alumina). The activity of catalysts was compared at 600 °C (9 % ethane and 2.5 % oxygen in helium, and W/F 0.12 g.s.cm-3). The most effective catalytic system was Ni-Al2O3, which also was tested at varied oxygen concentration, reaction temperature, and W/F. The most favorable set up corresponded to 46 % in the ethane conversion, 30 % in the ethene yield 30 %, and 0.91 g(C2=).gcat-1.h-1 in the ethene productivity for Ni-Al2O3. The activity of Ni-Al2O3 was stable for 6 hours in time-onstream. Keywords: ODH, ethane, vanadium, nickel, cobalt
1. Introduction Oxidative dehydrogenation (ODH) of ethane to ethene offers an attractive alternative to traditional processes of ethene productions, i.e. catalytic dehydrogenation, fluid catalytic cracking and steam cracking. However, a low ethene yield and an insufficient selectivity to ethene still prevent industrial application of ODH of ethane. A large variety of different catalysts have been studied in this process [1]. Although, the most of catalytic system exhibits the ethene yield up to 20 % [1], there also have been reported some superior catalytic systems, such as Sr1.0La1.0Nd1.0OX, achieving the ethene yield approximately 40 % [2]. With this work we contribute to the systematic comparison of the activity of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. The individual metal species were loaded to microporous (MFI) and mesoporous materials (HMS), and supported on inorganic alumina.
2. Experimental MFI (Si/Al 12.5) and γ-Al2O3 were purchased from Zeolyst and Eurosupport, respectively. The hexagonal mesoporous silica (HMS, 835 m2/g, and average pore diameter 3.7 nm) was synthesized at ambient conditions according to the procedure reported by Tanev and Pinnavaia [3]. Co-MFI was prepared by Co(II) ion exchange using cobalt acetete in water at 50 °C. Others metal loaded HMS, alumina, and MFI catalysts were prepared by impregnation of nickel acetate tetrahydrate, cobalt acetate tetrahydrate and vanadyl acetoacetonate, respectively, in ethanol. The final form of the catalyst was received by its calcination at 600 °C in air. The catalysts were characterized by means of X-ray fluorescence (determination of metal content) and UVVis spectroscopy (determination of metal species distribution) [4]. The ODH of ethane
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was carried out in a quartz through-flow micro-reactor at 600 and 650 °C and atmospheric pressure, typically with 200 mg of the catalyst (0.25-0.50 mm) diluted with 1 cm3 of silicon carbide inert. The typical reaction mixture consisted of 9.0 vol. % C2H6, 2.5 vol. % O2 and a rest of He was kept at a total flow of 100 ml.min-1, i.e. W/F (weight of the catalyst / total flow) 0.12 g.s.cm-3.
3. Results and discussion Fig. 1 compares the activities of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. It is to be pointed out that metal oxide-like species was not present at any of the catalysts, as its presentation is generally the reason in the activity-selectivity decrease. The absence of metal oxide-like species was evidenced by the absence of its characteristic bands in the UV-Vis spectra of hydrated and dehydrated catalysts (not shown in the Figure). The activity of catalysts was compared (i) at 600 °C, (ii) using reaction mixture of 9.0 vol. % ethane and 2.5 vol. % oxygen in helium, and (iii) contact time W/F 0.12 gcat.s.ml-1. These reaction conditions represent the most effective reaction conditions for V-HMS catalysts [4] The ethane conversions, the ethene yields and the selectivity to ethene varied between 13-30 %, 5-16 %, and 37-78 %, respectively, depending on the type of metal species (Co, Ni, V) and support material (Al2O3, HMS, MFI).
80 60
X S Y
Co-
40 20
%
0 60
3.6
3.3
3.3
3.1
2.9
3.9
3.8
Ni-
40 20 0 60
2.7
V-
40 20 0
2.9
HMS
Al2O3
MFI
Figure 1 Ethane conversion (X), ethene yield (Y) and selectivity to ethene (S) in ODH of ethane over Co-, Ni-, and V- loaded -Al2O3, -HMS, and MFI catalysts (wt. % of Co, Ni, and V are given in an figure for individual catalysts). Reaction conditions: 9.0 vol. % ethane, 2.5 vol. % O2 and He, 200 mg catalyst, total flow 100 ml.min-1, W/F 0.12 g.s.cm-3, and 600 °C.
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Table 1 Activity of Ni-Al2O3 (3.1 wt. % Ni) in ODH of ethane. Reaction conditions: 9.0 % ethane, 2.5 or 5.0 % O2 and He, 200 (W/F 0.12 g.s.cm-3) or 400 (W/F 0.24 g.s.cm-3) mg catalyst, total flow 100 ml.min-1, and 600 or 650 °C. Catalyst Ni-Al2O3 Ni-Al2O3 Ni-Al2O3 Ni-Al2O3
T °C 600 650 650 650
O2 mcat % 2.5 2.5 2.5 5.0
mg 200 200 400 400
X (C2H6) % 19.8 30.4 35.4 45.6
X (O2) % 57.6 90.6 100 86.9
S S Y (C2H4) (CH4) (C2H4) % % % 78.8 1.2 15.6 76.1 2.9 23.1 74.1 4.5 26.2 64.7 3.7 29.5
Productivity g(C2=).gcat-1.h-1 0.48 0.71 0.81 0.91
The following differences were observed among the individual catalysts: (i) Stability. The catalytic activity (Fig.1) was analyzed in steady state conditions after 2 hours in time-on-stream. The activities of metal loaded HMS and Al2O3 catalysts were stable in time-on-stream for 6 hours. On the other hand, the metal (Co-, Ni-, V-) loaded MFI catalysts had significant initiative activity, but it slowly decreased in time-onstream. (ii) By-products. The main products in ODH of ethane were ethene and carbon oxides. No oxygenates were detected at all tested catalytic systems. Trace concentrations of methane were detected for V-based catalysts. Ni-based catalysts, and Co-HMS and CoAl2O3 had the methane yields up to 0.5 % (selectivity to ethene up to 1.5 %). On the other side, Co-MFI surprisingly supported formation of cracking product methane (the selectivity to methane 18 %). (iii) Selectivity to ethene. It is well known that the selectivity has to be comparing at the same degree of conversion for parallel-consecutive reaction, such as ODH of ethane. Thus, it is hard to compare the selectivity-conversion behavior of the all catalysts based on the data shown in Fig. 1. However, it was clearly evidenced that Ni-Al2O3 catalyst was highly selective to ethene even at high ethane conversion (20 %). Co-Al2O3 and VAl2O3 had the ethane conversion 25 % and the selectivity to ethene ca 64 %. In order to compare the selectivity to ethene for all metal-Al2O3 catalysts at iso-conversion conditions (ca 25%), Ni-Al2O3 was also tested at higher catalyst weight. At the ethane iso-conversion conditions (ca 25 %), the selectivity to ethene increased in order NiAl2O3 (77 %) > Co-Al2O3 (66 %) ≈ V-Al2O3 (63 %). (iv) Type of support. Co-, V- and Ni-Al2O3 catalysts were more active in comparison with the corresponding metal loaded -HMS and -MFI catalysts. Metal loaded MFI catalysts exhibited slow decrease in its activity (see above). (v) Type of metal species. The efficiency of alumina based catalysts increased in order Ni-Al2O3 > V-Al2O3 ≈ Co-Al2O3 (see also the selectivity to ethne). Although, V-HMS catalyst had higher ethane conversion in comparison with Ni-, and Co-HMS catalysts at the same reaction conditions (Fig.1), at the ethane iso-conversion comparison (ca. 13 %) the selectivity to ethene increased in order Ni-HMS (73 %, Fig.1) > V-HMS (61 % [4]) > Co-HMS (41 %). V-based catalysts represent one of the most active and studied catalytic systems in ODH reactions [1]. Co-based catalysts were mainly studied in its zeolite form for ODH of ethane and propane [5,6]. The activity of Ni-based catalysts was reported in ODH of ethane with inconsistent results. Nakamura et al. [7] reported high activity of NiO loaded MgO, while negligible one for Ni-Al2O3. Chang et al. [8] excluded Ni-MFI from being a good ODH catalyst due to extremely high activity to generate methane. On the
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other hand, Heracleous et al. [9] reported high activity of Ni-Al2O3 starting at 400 °C (W/F 0.54 g.s.cm-3, lower contact time in comparison with that used in this work). Ni-Al2O3 represented the most effective catalytic system here and it is in contrast to the results reported in literature [7]. It is well known that the oxygen and ethane concentrations, contact time, and reaction temperature represent important variables affecting the activity/selectivity of ODH catalysts. In order to achieve the ethene yield (the ethene productivity) as high as possible for Ni-Al2O3 catalyst, but keeping the selectivity to ethene above 55 %, the catalyst was tested at various reaction variables (oxygen concentration, reaction temperature, and W/F). The ethane conversion and the ethene yield increased with the increasing reaction temperature, the oxygen concentration, and the W/F (Table 1). The most favorable set up corresponded to 46 % in the ethane conversion, 30 % in the ethene yield 30 %, 65 % in the selectivity to ethene, and 0.91 g(C2=).gcat-1.h-1 in the ethene productivity for Ni-Al2O3.The selectivity to methane was only 3.7 %, which was significantly lower than that reported in literature [8]. The most of the catalytic systems reported in the recent review [1] exhibits the ethene productivity below 0.7 g(C2=).gcat-1.h-1, thus the productivity of NiAl2O3 (0.91g(C2=).gcat-1.h-1) represents the over average value.
4. Conclusion The contribution deals with the catalytic performance of V-, Co-, and Ni-based microporous (MFI), mesoporous (HMS) and alumina catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. Ni-, V- and CoAl2O3, and V- and Ni-HMS were effective catalysts in ODH of ethane. However, NiAl2O3 had the best selectivity-conversion behavior. The most favorable set up corresponded to 46 % in the ethane conversion, 30 % in the ethene yield 30 %, 65 % in the selectivity to ethene, and 0.91 g(C2=).gcat-1.h-1 in the ethene productivity for NiAl2O3. The activity was stable for 6 hours time-on-stream.
Acknowledgement The authors gratefully thank to the Grant Agency of Czech Republic for financial support (projects No. 104/07/P038 and No.203/08/H032) and Ministry of Education, Youth and Sports (MSM0021627501).
References [1] F. Cavani, N. Ballarini, A. Cericola, Catal. Today, 127 (2007) 113. [2] S. A. R. Mulla, O.V. Buyevskaya, M. Bearns, Appl. Catal. A, 226 (2002) 73. [3] P. T. Tanev, T. J. Pinnavaia, Science, 267 (1995) 865. [4] L. apek, J. Adam, T. Grygar, R. Bulánek, L. Vradman, G. Košová-Kuerová, P. imanec, P. Knotek, Appl. Catal. A, in press. [5] Y.-F. Chang, G.A. Somorjai, H. Heinemann, J. Catal., 154 (1995) 24. [6] K. Nowinska, A. Waclaw, A. Izbinska, Appl. Catal. A, 243 (2003) 225. [7] K. I. Nakamura, T. Miyake, T. Konishi, T. Suzuki, J. Mol. Catal. A, 260 (2006) 144. [8] Y. F. Chang, G. A. Somorjai, H. Heineann, Appl. Catal. A, 96 (1993) 305. [9] E. Heracleous, A. F. Lee, K. Wilson, A. A. Lemonidou, J. Catal., 213 (2005) 159.
1175
Dehydrogenation of ethane over vanadium, cobalt and nickel based catalysts Libor apek,* Lukáš Vank, Jií Adam, Lucie Smoláková University of Pardubice, Faculty of Chemical Technology,Department of Physical Chemistry, Nám. s. Legií 565, CZ-532 10 Pardubice, Czech Republic,e-mail: [email protected]
Abstract The contribution deals with the catalytic performance of vanadium-, cobalt- and nickelbased catalysts in oxidative dehydrogenation of ethane. Vanadium, cobalt and nickel species were loaded to microporous (MFI) and mesoporous (HMS) materials, and supported on inorganic support (alumina). The activity of catalysts was compared at 600 °C (9 % ethane and 2.5 % oxygen in helium, and W/F 0.12 g.s.cm-3). The most effective catalytic system was Ni-Al2O3, which also was tested at varied oxygen concentration, reaction temperature, and W/F. The most favorable set up corresponded to 46 % in the ethane conversion, 30 % in the ethene yield 30 %, and 0.91 g(C2=).gcat-1.h-1 in the ethene productivity for Ni-Al2O3. The activity of Ni-Al2O3 was stable for 6 hours in time-onstream. Keywords: ODH, ethane, vanadium, nickel, cobalt
1. Introduction Oxidative dehydrogenation (ODH) of ethane to ethene offers an attractive alternative to traditional processes of ethene productions, i.e. catalytic dehydrogenation, fluid catalytic cracking and steam cracking. However, a low ethene yield and an insufficient selectivity to ethene still prevent industrial application of ODH of ethane. A large variety of different catalysts have been studied in this process [1]. Although, the most of catalytic system exhibits the ethene yield up to 20 % [1], there also have been reported some superior catalytic systems, such as Sr1.0La1.0Nd1.0OX, achieving the ethene yield approximately 40 % [2]. With this work we contribute to the systematic comparison of the activity of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. The individual metal species were loaded to microporous (MFI) and mesoporous materials (HMS), and supported on inorganic alumina.
2. Experimental MFI (Si/Al 12.5) and γ-Al2O3 were purchased from Zeolyst and Eurosupport, respectively. The hexagonal mesoporous silica (HMS, 835 m2/g, and average pore diameter 3.7 nm) was synthesized at ambient conditions according to the procedure reported by Tanev and Pinnavaia [3]. Co-MFI was prepared by Co(II) ion exchange using cobalt acetete in water at 50 °C. Others metal loaded HMS, alumina, and MFI catalysts were prepared by impregnation of nickel acetate tetrahydrate, cobalt acetate tetrahydrate and vanadyl acetoacetonate, respectively, in ethanol. The final form of the catalyst was received by its calcination at 600 °C in air. The catalysts were characterized by means of X-ray fluorescence (determination of metal content) and UVVis spectroscopy (determination of metal species distribution) [4]. The ODH of ethane
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was carried out in a quartz through-flow micro-reactor at 600 and 650 °C and atmospheric pressure, typically with 200 mg of the catalyst (0.25-0.50 mm) diluted with 1 cm3 of silicon carbide inert. The typical reaction mixture consisted of 9.0 vol. % C2H6, 2.5 vol. % O2 and a rest of He was kept at a total flow of 100 ml.min-1, i.e. W/F (weight of the catalyst / total flow) 0.12 g.s.cm-3.
3. Results and discussion Fig. 1 compares the activities of vanadium-, cobalt- and nickel- based catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. It is to be pointed out that metal oxide-like species was not present at any of the catalysts, as its presentation is generally the reason in the activity-selectivity decrease. The absence of metal oxide-like species was evidenced by the absence of its characteristic bands in the UV-Vis spectra of hydrated and dehydrated catalysts (not shown in the Figure). The activity of catalysts was compared (i) at 600 °C, (ii) using reaction mixture of 9.0 vol. % ethane and 2.5 vol. % oxygen in helium, and (iii) contact time W/F 0.12 gcat.s.ml-1. These reaction conditions represent the most effective reaction conditions for V-HMS catalysts [4] The ethane conversions, the ethene yields and the selectivity to ethene varied between 13-30 %, 5-16 %, and 37-78 %, respectively, depending on the type of metal species (Co, Ni, V) and support material (Al2O3, HMS, MFI).
80 60
X S Y
Co-
40 20
%
0 60
3.6
3.3
3.3
3.1
2.9
3.9
3.8
Ni-
40 20 0 60
2.7
V-
40 20 0
2.9
HMS
Al2O3
MFI
Figure 1 Ethane conversion (X), ethene yield (Y) and selectivity to ethene (S) in ODH of ethane over Co-, Ni-, and V- loaded -Al2O3, -HMS, and MFI catalysts (wt. % of Co, Ni, and V are given in an figure for individual catalysts). Reaction conditions: 9.0 vol. % ethane, 2.5 vol. % O2 and He, 200 mg catalyst, total flow 100 ml.min-1, W/F 0.12 g.s.cm-3, and 600 °C.
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Table 1 Activity of Ni-Al2O3 (3.1 wt. % Ni) in ODH of ethane. Reaction conditions: 9.0 % ethane, 2.5 or 5.0 % O2 and He, 200 (W/F 0.12 g.s.cm-3) or 400 (W/F 0.24 g.s.cm-3) mg catalyst, total flow 100 ml.min-1, and 600 or 650 °C. Catalyst Ni-Al2O3 Ni-Al2O3 Ni-Al2O3 Ni-Al2O3
T °C 600 650 650 650
O2 mcat % 2.5 2.5 2.5 5.0
mg 200 200 400 400
X (C2H6) % 19.8 30.4 35.4 45.6
X (O2) % 57.6 90.6 100 86.9
S S Y (C2H4) (CH4) (C2H4) % % % 78.8 1.2 15.6 76.1 2.9 23.1 74.1 4.5 26.2 64.7 3.7 29.5
Productivity g(C2=).gcat-1.h-1 0.48 0.71 0.81 0.91
The following differences were observed among the individual catalysts: (i) Stability. The catalytic activity (Fig.1) was analyzed in steady state conditions after 2 hours in time-on-stream. The activities of metal loaded HMS and Al2O3 catalysts were stable in time-on-stream for 6 hours. On the other hand, the metal (Co-, Ni-, V-) loaded MFI catalysts had significant initiative activity, but it slowly decreased in time-onstream. (ii) By-products. The main products in ODH of ethane were ethene and carbon oxides. No oxygenates were detected at all tested catalytic systems. Trace concentrations of methane were detected for V-based catalysts. Ni-based catalysts, and Co-HMS and CoAl2O3 had the methane yields up to 0.5 % (selectivity to ethene up to 1.5 %). On the other side, Co-MFI surprisingly supported formation of cracking product methane (the selectivity to methane 18 %). (iii) Selectivity to ethene. It is well known that the selectivity has to be comparing at the same degree of conversion for parallel-consecutive reaction, such as ODH of ethane. Thus, it is hard to compare the selectivity-conversion behavior of the all catalysts based on the data shown in Fig. 1. However, it was clearly evidenced that Ni-Al2O3 catalyst was highly selective to ethene even at high ethane conversion (20 %). Co-Al2O3 and VAl2O3 had the ethane conversion 25 % and the selectivity to ethene ca 64 %. In order to compare the selectivity to ethene for all metal-Al2O3 catalysts at iso-conversion conditions (ca 25%), Ni-Al2O3 was also tested at higher catalyst weight. At the ethane iso-conversion conditions (ca 25 %), the selectivity to ethene increased in order NiAl2O3 (77 %) > Co-Al2O3 (66 %) ≈ V-Al2O3 (63 %). (iv) Type of support. Co-, V- and Ni-Al2O3 catalysts were more active in comparison with the corresponding metal loaded -HMS and -MFI catalysts. Metal loaded MFI catalysts exhibited slow decrease in its activity (see above). (v) Type of metal species. The efficiency of alumina based catalysts increased in order Ni-Al2O3 > V-Al2O3 ≈ Co-Al2O3 (see also the selectivity to ethne). Although, V-HMS catalyst had higher ethane conversion in comparison with Ni-, and Co-HMS catalysts at the same reaction conditions (Fig.1), at the ethane iso-conversion comparison (ca. 13 %) the selectivity to ethene increased in order Ni-HMS (73 %, Fig.1) > V-HMS (61 % [4]) > Co-HMS (41 %). V-based catalysts represent one of the most active and studied catalytic systems in ODH reactions [1]. Co-based catalysts were mainly studied in its zeolite form for ODH of ethane and propane [5,6]. The activity of Ni-based catalysts was reported in ODH of ethane with inconsistent results. Nakamura et al. [7] reported high activity of NiO loaded MgO, while negligible one for Ni-Al2O3. Chang et al. [8] excluded Ni-MFI from being a good ODH catalyst due to extremely high activity to generate methane. On the
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other hand, Heracleous et al. [9] reported high activity of Ni-Al2O3 starting at 400 °C (W/F 0.54 g.s.cm-3, lower contact time in comparison with that used in this work). Ni-Al2O3 represented the most effective catalytic system here and it is in contrast to the results reported in literature [7]. It is well known that the oxygen and ethane concentrations, contact time, and reaction temperature represent important variables affecting the activity/selectivity of ODH catalysts. In order to achieve the ethene yield (the ethene productivity) as high as possible for Ni-Al2O3 catalyst, but keeping the selectivity to ethene above 55 %, the catalyst was tested at various reaction variables (oxygen concentration, reaction temperature, and W/F). The ethane conversion and the ethene yield increased with the increasing reaction temperature, the oxygen concentration, and the W/F (Table 1). The most favorable set up corresponded to 46 % in the ethane conversion, 30 % in the ethene yield 30 %, 65 % in the selectivity to ethene, and 0.91 g(C2=).gcat-1.h-1 in the ethene productivity for Ni-Al2O3.The selectivity to methane was only 3.7 %, which was significantly lower than that reported in literature [8]. The most of the catalytic systems reported in the recent review [1] exhibits the ethene productivity below 0.7 g(C2=).gcat-1.h-1, thus the productivity of NiAl2O3 (0.91g(C2=).gcat-1.h-1) represents the over average value.
4. Conclusion The contribution deals with the catalytic performance of V-, Co-, and Ni-based microporous (MFI), mesoporous (HMS) and alumina catalysts in ODH of ethane. Representative catalysts contained between 2.9 and 3.9 wt.% of metal. Ni-, V- and CoAl2O3, and V- and Ni-HMS were effective catalysts in ODH of ethane. However, NiAl2O3 had the best selectivity-conversion behavior. The most favorable set up corresponded to 46 % in the ethane conversion, 30 % in the ethene yield 30 %, 65 % in the selectivity to ethene, and 0.91 g(C2=).gcat-1.h-1 in the ethene productivity for NiAl2O3. The activity was stable for 6 hours time-on-stream.
Acknowledgement The authors gratefully thank to the Grant Agency of Czech Republic for financial support (projects No. 104/07/P038 and No.203/08/H032) and Ministry of Education, Youth and Sports (MSM0021627501).
References [1] F. Cavani, N. Ballarini, A. Cericola, Catal. Today, 127 (2007) 113. [2] S. A. R. Mulla, O.V. Buyevskaya, M. Bearns, Appl. Catal. A, 226 (2002) 73. [3] P. T. Tanev, T. J. Pinnavaia, Science, 267 (1995) 865. [4] L. apek, J. Adam, T. Grygar, R. Bulánek, L. Vradman, G. Košová-Kuerová, P. imanec, P. Knotek, Appl. Catal. A, in press. [5] Y.-F. Chang, G.A. Somorjai, H. Heinemann, J. Catal., 154 (1995) 24. [6] K. Nowinska, A. Waclaw, A. Izbinska, Appl. Catal. A, 243 (2003) 225. [7] K. I. Nakamura, T. Miyake, T. Konishi, T. Suzuki, J. Mol. Catal. A, 260 (2006) 144. [8] Y. F. Chang, G. A. Somorjai, H. Heineann, Appl. Catal. A, 96 (1993) 305. [9] E. Heracleous, A. F. Lee, K. Wilson, A. A. Lemonidou, J. Catal., 213 (2005) 159.