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Comparison of carbon dioxide and carbon monoxide with respects to hydrogenation on Raney ruthenium catalysts under 1.1 and 2.1 MPa
Applied Catalysis A: General 168 (1998) 345±351
Comparison of carbon dioxide and carbon monoxide with respects to hydrogenation on Raney ruthenium catalysts under 1.1 and 2.1 MPa Kaoru Takeishia, Yuko Yamashitab, Ken-ich Aikab,* a
Department of Materials Science and Engineering, Faculty of Engineering, Shizuoka University, 5-1, Jouhoku 3-choume, Hamamatsu 432, Japan b Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received 23 June 1997; received in revised form 20 November 1997; accepted 4 December 1997
Abstract Hydrogenation of carbon monoxide and carbon dioxide (CO/H2CO2/H21/3) on Raney ruthenium catalysts was carried out under pressures of 1.1 and 2.1 MPa using an autoclave in the presence of water at 353 and 433 K. Hydrocarbons, whose distribution followed the Schulz±Flory pattern, were produced from CO±H2, together with a trace amount of methanol. The activities for their production have been much improved under high pressure, but the selectivity of methanol production decreased, probably due to the poisoning of active sites by water. The activity for methane production from CO2±H2 at 433 K under 1.1 MPa was much higher than that under atmospheric pressure. The rate of methane synthesis was 3.0 mmol gÿ1 hÿ1 and the selectivity for methane formation was 98% at 353 K, suggesting the practical use of this catalyst. The activity and selectivity of CO and CO2 hydrogenation are discussed from mechanistic standpoints. # 1998 Elsevier Science B.V. Keywords: Raney; Ruthenium; Hydrogenation; Methane; Methanol; Carbon dioxide; Carbon monoxide
1. Introduction A characteristic feature of Raney Ru in CO or CO2 hydrogenation is the high activity. Ru itself is an active element for CO hydrogenation and Raney structure provides high surface area (30±60 m2 gÿ1). Another feature we have found is that methanol can be produced from CO and H2 over water±vapor-treated Raney Ru [1±3]. The water±vapor-pretreatment oxi-
*Corresponding author. Fax: 81-45-924-5441 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00364-5
dized both aluminium and Ru atoms (probably adjacent to Al) on the surface of the catalysts. Existence of cationic Ru (Run) stabilized with the oxidized Al (Al2O3) has been discussed as the active center of methanol synthesis keeping the C±O bond, while metallic parts (Ru0) activating H atoms. In the previous report, CO and CO2 are compared with respect to hydrogenation over the same Raney Ru catalysts under 80 kPa. CO hydrogenation gave hydrocarbons including those with longer chain and methanol, while CO2 hydrogenation gave methane almost exclusively, but the activity of the latter was three
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orders of magnitude higher than that of the former. A mechanistic explanation was given and the competition of CO and CO2 in the hydrogenation was also studied. In this study, the authors tried to supply more practical data applicable to industry. The reaction was performed at high pressure (1.1±2.1 MPa) using the same Raney Ru catalysts suspended in water, which was preferable to remove the heat of reaction and stabilize the air sensitive catalysts. The interesting points of this study are two. (1) Is higher methanol production from CO±H2 expected under high pressure (kinetic and equilibrium standpoints)? (2) Is higher methane production from CO2± H2 expected under high pressure (activity and selectivity)? 2. Experimental 2.1. Catalyst and apparatus Raney Ru alloys (Al/Ru50/50 in w/w) were kindly manufactured by Dr. S. Miura at Tokyo Institute of Technology. They were prepared by melting Ru and Al metal at around 2100 K under Ar atmosphere using an arc furnace. The Ru powder (99.95%) used for the alloy preparation was supplied by The EndouPlatinum Corporation and the Al metal (99.999%) was purchased from The Nilaco Corporation. The alloy (usually 1.0 or 3.0 g) was leached with about 5 mol dmÿ3 potassium hydroxide aqueous solution at 373 K until H2 evolution ceased (about 2 h). After cooling, the sample was washed several times with decantation until the pH of the solution was decreased to 7. Further washings followed some times. The leached sample powder (Raney Ru catalyst) usually decreased the weight to half (0.5 or 1.5 g). The catalyst was transferred to the autoclave reactor together with a portion of the water (1.5 or 3.0 cm3). The autoclave was made of stainless steel and its volume was 200 cm3. Its maximum pressure and temperature for use were 30 MPa and 573 K, respectively. The reactor was connected to mass ¯ow controllers (Brooks 5850TR for H2, CO, CO2, and Ar) by a stainless steel tube (SUS-316) and Swagelok ®ttings, and gas samplers to gas chromatographs (GCs) (Shimadzu GC-8APT and GC-8APF, and Yanagimoto
G3800) for analysis. H2 (99.99999%, A Grade of Toyo Sanso) and Ar (99.9999%, S Grade of Nippon Sanso) were used without further puri®cation. CO (99.95%, R Grade of Takachiho Trading) and CO2 (99.99%, R Grade of Showa Tansangas) were puri®ed passing both through an Oxygen-Trap (Chromatography Research Supplies) and a Molecular Sieves 5A Filter (Shimadzu), respectively. For reduction of the catalyst, H2 was fed into the autoclave. The feeding of pressurized H2 and degassing was continued several times until no contamination such as air was detected by GCs. The autoclave was heated by a ribbon heater with a PID temperature controller (Shimaden SR-22). The gas, catalyst, and water were stirred vigorously by a magnetic stirrer (Advantec SR-550) and a Te¯on TFE stirring bar (Triangle type). The catalyst was reduced twice for 2 h at 473 K under 0.6 MPa pressure of H2. 2.2. CO and CO2 hydrogenation over Raney Ru After the reduction process, the autoclave was cooled and the reaction mixture (H2/CO/ArH2/ CO2/Ar6/2/1) was fed into the autoclave. The feeding of the pressurized gas and degassing were continued several times until the analyzed ratio of the reaction mixture became H2/CO/ArH2/CO2/Ar6/ 2/1. The reactant, the catalyst, and water were stirred vigorously by the magnetic stirrer and the stirring bar. The reaction was usually carried out at 353 or 433 K, and the reaction time was either 5 or 7 h. Before the reaction, the pressure of the reactant was adjusted to 1.1 or 2.1 MPa at room temperature. After heating, the pressure increased a little with the increase of the temperature. Fig. 1 is an example of the pressure changing. The reaction was stopped by rapid cooling using water±ice mixture, before the pressure decreased to about half its value. In this paper, the reaction pressure means the autoclave pressure at room temperature before heating and starting the reaction. The gas phase methanol and hydrocarbons with the carbon number of 1±8, recovered as products, were quantitatively analyzed by a ¯ame ionization detector (FID) of the GCs with two stainless columns ®lled with Porapak Q (80±100 mesh, 2 m long, 3 mm) and Porapak R (80±100 mesh, 1.5 m long, 3 mm) in series. Methane, ethane, CO, CO2, and Ar (internal
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Fig. 1. Example of the time course of the reaction pressure: (*) CO/H2 reaction on Raney Ru (1.5 g) at 353 K for 7 h (the conversion was 16%.); (~) CO2/H2 reaction on Raney Ru (0.5 g) at 433 K for 5 h (the conversion was 40%.).
standard for GC analysis) separated by activated charcoal (80±100 mesh, 3 m long, 3 mm) were detected by FID and the thermal conductivity detector (TCD). The solvent water (10 mm3) with products of liquid phase was injected to the GC, and methanol and hydrocarbons with the carbon number of 4±8 were detected. 3. Results 3.1. CO hydrogenation Logarithms of the reaction rates (produced amounts divided by the reaction time) are plotted against the carbon numbers of hydrocarbon and methanol in Fig. 2. The product distribution obeys the Schulz± Flory mechanism. The reaction rates were increased
with increasing temperature and pressure. However, the product distribution did not change remarkably. Conversion of CO hydrogenation on a Raney Ru catalyst (0.5 g) at 433 K under 1.1 MPa for 7 h was 11%, and that of the reaction on the catalyst (1.5 g) at 353 K under 1.1 MPa for 7 h was 16%, and that on Raney Ru (1.5 g) at 353 K under 2.1 MPa for 7 h was 40%. Selectivity of methanol formation was in a range of 0.08±0.1%, much less than the values observed under 80 kPa with the closed circulation system. 3.2. CO2 hydrogenation Logarithms of the reaction rates (produced amount divided by the reaction time) are plotted against the carbon numbers of hydrocarbon and methanol in Fig. 3. Conversion of CO2 hydrogenation on a Raney
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Fig. 2. Distribution of products obtained from CO hydrogenation by using an autoclave: (*) CO/H2 reaction on Raney Ru (1.5 g) at 353 K under 2.1 MPa (first pressure) for 7 h; (&) CO/H2 reaction on Raney Ru (0.5 g) at 433 K under 1.1 MPa (first pressure) for 7 h; (&) CO/H2 reaction on Raney Ru (1.5 g) at 353 K under 1.1 MPa (first pressure) for 7 h.
Ru catalyst (0.5 g) at 433 K under 1.1 MPa for 5 h was 40%, and that on the catalyst (1.5 g) at 353 K under 1.1 MPa for 5 h was 41%, and that on Raney Ru (0.5 g) at 353 K under 2.1 MPa for 7 h was 10%. The reactivity was much higher than that of the CO hydrogenation. The product distribution also obeyed the Schulz±Flory mechanism. However, the slope was very steep and the propagation factor was low. The main product was methane. The others were hydrocarbons of which carbon numbers were 2±5, and methanol. The selectivity of methanol formation was as low as 0.0±0.03%. Selectivity to methane was extremely high: 97±98%. Especially, the activity of methane formation became much higher than those under atmospheric pressure using the circulation system. In the previous report [3], we pointed out that Raney Ru was a very effective catalyst for methane synthesis from CO2 and H2 compared to the supported Ru catalysts. This is substantiated by the results of this work.
Fig. 3. Distribution of products obtained from CO2 hydrogenation by using an autoclave: (}) CO2/H2 reaction on Raney Ru (0.5 g) at 353 K under 2.1 MPa (first pressure) for 7 h; (~) CO2/H2 reaction on Raney Ru (0.5 g) at 433 K under 1.1 MPa (first pressure) for 5 h; (~) CO2/H2 reaction on Raney Ru (1.5 g) at 353 K under 1.1 MPa (first pressure) for 5 h.
3.3. Reaction temperature dependency of methane formation from CO2 and H2 The CO2 hydrogenation temperature was varied from 353 to 433 K every 20 K under 1.1 MPa on a Raney Ru catalyst (1.1 g). After each reaction, the catalyst was reduced twice for 2 h at 473 K under 0.6 MPa pressure of H2. The reaction time was changed in the range of 2.5±7 h depending on the reaction rate. The average conversion was 44%. The formation rate of methane was plotted against the reciprocal reaction temperature in Fig. 4. From the slope, the apparent activation energy for methane was estimated to be 33 kJ molÿ1. This was much less than the values reported previously: 81 kJ molÿ1 on Raney Ru under 80 kPa [3], 67±70 kJ molÿ1 on Ru/Al2O3 [4], and 82 kJ molÿ1 on Ru/SiO2 [5,6]. The activity of CO2 hydrogenation on Raney Ru at 493 K is estimated to be much higher than that of CO hydrogenation on ultra ®ne iron particles studied at 493 K under 3 MPa [7,8].
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Fig. 4. Arrhenius plots for the formation of methane from CO2 and H2 on the Raney Ru catalyst (1.1 g) under 1.1 MPa. (This pressure was before heating and the reaction. The details are in the text).
4. Discussion 4.1. Low selectivity to methanol from CO±H2 One of the initial purposes was to improve methanol selectivity under the high pressure. When Raney Ru was treated with H2O±He at 573 K, the selectivity to methanol was even 60±80% at the reaction temperature of 353 K [1±3]. Under the condition of this study, it was about 0.1%. This may be because of the lack of H2O±He pretreatment for the catalysts in this experiment, in contrast to the previous study using the circulation system [1±3], and hence, the absence of an appropriate amount of cationic Ru and Al, Run and Al3. With intent of forming Run and Al3 on the catalyst surface, we tried H2O±Ar pretreatment, heating and stirring with water at 373 K under 0.3 MPa pressure of Ar for 12 h. However, this pretreatment did not enhance the selectivity to methanol. Rather it made the catalyst somewhat less active. We have speculated that the presence of both of Run and Al3 on the surface of the catalyst is
indispensable for methanol formation [2]. However, these cationic active sites for methanol formation (without dissociation of C±O bonds) seem to be retarded by water present in the autoclave in this experiment. H2O molecules have higher polarity than CO molecules, and H2O is suggested to interact easily with those cationic active sites. On the other hand, synthesis of methane and hydrocarbons was not retarded by water, probably because the active sites, Ru metal (with dissociation of C±O bonds), had less interaction with H2O. The activity and selectivity of hydrocarbons' formation by using the autoclave were much higher compared with those formed by using the closed circulation system, because the autoclave was operated at higher pressures and temperatures. 4.2. Total pressure effect on the activity for CO±H2 and CO2±H2 reactions In the previous report, the tentative reaction mechanism and rate expression were proposed, and these explained well the competition between CO±H2 and CO2±H2 reactions. Two rate expressions are cited
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from the previous report (Eqs. 1±9 and 2±7) [3]. For CO hydrogenation the same expression (Eq. 1±9) is used here. rCO reaction rate k0 C H
k00 PCO P1:5 H2 =PH2 O 1 K1 PCO 2
(1)
However, for CO2 hydrogenation, the adsorption of CO (retardation) was assumed to be negligible to explain the high activity under the previous condition. Here in this case, we introduce the CO adsorption term as the main species adsorbed during CO2 hydrogenation. K5
CO2 H2 CO a H2 O CO main adsorbed species
(2)
K5 PCO2 PH2 =PH2 O 1 K5 PCO2 PH2 =PH2 O (3)
Eqs. 2±3, 2±6, and 2±7 in the previous paper are rewritten as follows: C active species
K4 PCO2 P2H2 =P2H2 O 1 K5 PCO2 PH2 =PH2 O
(4)
H active species
K3 P0:5 H2 1 K5 PCO2 PH2 =PH2 O
(5)
rCO2 reaction rate kC H
2 kK3 K4 PCO2 P2:5 H2 =PH2 O
1K1 PCO2 PH2 =PH2 O 2 (6)
Now Eqs. (1) and (6) tell us the total pressure effect, but these are not linear functions. Ignoring water pressure (as constant), two extreme cases are considered as follows: rCO / PCO P1:5 H2
for low coverage P2:5 total
1:5 Pÿ1 CO PH2
for high coverage P0:5 total
rCO2 / PCO P2:5 H2 0:5 Pÿ1 CO PH2
for low coverage P3:5 total ÿ0:5 for high coverage Ptotal
Thus, the total pressure dependency on CO hydrogenation and CO2 hydrogenation ranges from 0.5 to 2.5 order and from ÿ0.5 to 3.5 order depending on the coverages. If we compare methane production of code number 4 in Table 1 of the previous study at 353 K
(77 nmol gÿ1 hÿ1 and 41 mmol gÿ1 hÿ1 for CO and CO2 hydrogenation) under 80 kPa, the activities in this study (Figs. 2 and 3) under 1.1 MPa are 25 and 24 times higher respectively for the pressure increase of 14 times. Both total pressure dependencies are a little higher than 1st order. This situation may indicate high CO(a) coverage for CO hydrogenation and intermediate coverage for CO2 hydrogenation. 4.3. Selectivity in CO and CO2 hydrogenation Schulz±Flory plots of Figs. 2 and 3 tell us that the product distribution did not change much from the result obtained under low pressure [3]. The activity of CO2 hydrogenation was still 3 orders of magnitude higher than that of CO hydrogenation. The surface state discussed in the previous paper seems still to be appreciable to this condition. A higher concentration of adsorbed hydrogen than the surface carbide is assumed in the CO2±H2 system even under high pressure. Of course, the new situation may be the predominance of CO(a). However, the model (Eqs. (2)±(6)) suggests that CO(a) coverage can still increase under extremely high pressure of CO2, where the activity may saturate. The pressures studied here, 1.1±2.1 MPa, appear to be practical for hydrocarbon production of higher compound from CO±H2 and methane from CO2±H2 on Raney Ru. 5. Conclusion In CO2 hydrogenation using an autoclave, the activity and selectivity of methane formation are enhanced at higher pressures (1.1 and 2.1 MPa). However, selectivity of methanol formation from CO and H2 was not increased. The reason is suggested that the active center (Run±Al3) for methanol production is retarded by water. This research substantiated that the Raney Ru catalysts are more attractive than supported Ru catalysts for methane formation from CO2 and H2 and for hydrocarbon production from CO and H2 at elevated pressures. Acknowledgements The authors are grateful to former Professor Kazunori Tanaka of Shizuoka University for his helpful discussions and ®nancial support.
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A part of this work has been carried out as a research project of the Japan Petroleum Institute commissioned by the Petroleum Energy Center with a subsidy from the Ministry of International Trade and Industry. References [1] T. Yano, Y. Ogata, K. Aika, T. Onishi, Chem. Lett., (1986) 303.
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[2] K. Takeishi, K. Aika, J. Catal. 136 (1992) 252. [3] K. Takeishi, K. Aika, Appl. Catal. A: General 133 (1995) 31. [4] F. Solymosi, A. ErdoÈhelyi, M. Kocis, J. Chem. Soc. Faraday Trans. 1 77 (1981) 1003. [5] T. Inui, M. Funabiki, Chem. Lett., (1978), 251. [6] T. Inui, M. Funabiki, M. Suehiro, T. Sezume, T. Iwana, Nippon Kagaku Kaishi, (1978) 517. [7] E. Kikuchi, H. Itoh, M. Miyazaki, Y. Morita, Sekiyu Gakkaishi 29 (1986) 317. [8] H. Itoh, E. Kikuchi, Y. Morita, Sekiyu Gakkaishi 30 (1987) 324.
Comparison of carbon dioxide and carbon monoxide with respects to hydrogenation on Raney ruthenium catalysts under 1.1 and 2.1 MPa Kaoru Takeishia, Yuko Yamashitab, Ken-ich Aikab,* a
Department of Materials Science and Engineering, Faculty of Engineering, Shizuoka University, 5-1, Jouhoku 3-choume, Hamamatsu 432, Japan b Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226, Japan Received 23 June 1997; received in revised form 20 November 1997; accepted 4 December 1997
Abstract Hydrogenation of carbon monoxide and carbon dioxide (CO/H2CO2/H21/3) on Raney ruthenium catalysts was carried out under pressures of 1.1 and 2.1 MPa using an autoclave in the presence of water at 353 and 433 K. Hydrocarbons, whose distribution followed the Schulz±Flory pattern, were produced from CO±H2, together with a trace amount of methanol. The activities for their production have been much improved under high pressure, but the selectivity of methanol production decreased, probably due to the poisoning of active sites by water. The activity for methane production from CO2±H2 at 433 K under 1.1 MPa was much higher than that under atmospheric pressure. The rate of methane synthesis was 3.0 mmol gÿ1 hÿ1 and the selectivity for methane formation was 98% at 353 K, suggesting the practical use of this catalyst. The activity and selectivity of CO and CO2 hydrogenation are discussed from mechanistic standpoints. # 1998 Elsevier Science B.V. Keywords: Raney; Ruthenium; Hydrogenation; Methane; Methanol; Carbon dioxide; Carbon monoxide
1. Introduction A characteristic feature of Raney Ru in CO or CO2 hydrogenation is the high activity. Ru itself is an active element for CO hydrogenation and Raney structure provides high surface area (30±60 m2 gÿ1). Another feature we have found is that methanol can be produced from CO and H2 over water±vapor-treated Raney Ru [1±3]. The water±vapor-pretreatment oxi-
*Corresponding author. Fax: 81-45-924-5441 0926-860X/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0926-860X(97)00364-5
dized both aluminium and Ru atoms (probably adjacent to Al) on the surface of the catalysts. Existence of cationic Ru (Run) stabilized with the oxidized Al (Al2O3) has been discussed as the active center of methanol synthesis keeping the C±O bond, while metallic parts (Ru0) activating H atoms. In the previous report, CO and CO2 are compared with respect to hydrogenation over the same Raney Ru catalysts under 80 kPa. CO hydrogenation gave hydrocarbons including those with longer chain and methanol, while CO2 hydrogenation gave methane almost exclusively, but the activity of the latter was three
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orders of magnitude higher than that of the former. A mechanistic explanation was given and the competition of CO and CO2 in the hydrogenation was also studied. In this study, the authors tried to supply more practical data applicable to industry. The reaction was performed at high pressure (1.1±2.1 MPa) using the same Raney Ru catalysts suspended in water, which was preferable to remove the heat of reaction and stabilize the air sensitive catalysts. The interesting points of this study are two. (1) Is higher methanol production from CO±H2 expected under high pressure (kinetic and equilibrium standpoints)? (2) Is higher methane production from CO2± H2 expected under high pressure (activity and selectivity)? 2. Experimental 2.1. Catalyst and apparatus Raney Ru alloys (Al/Ru50/50 in w/w) were kindly manufactured by Dr. S. Miura at Tokyo Institute of Technology. They were prepared by melting Ru and Al metal at around 2100 K under Ar atmosphere using an arc furnace. The Ru powder (99.95%) used for the alloy preparation was supplied by The EndouPlatinum Corporation and the Al metal (99.999%) was purchased from The Nilaco Corporation. The alloy (usually 1.0 or 3.0 g) was leached with about 5 mol dmÿ3 potassium hydroxide aqueous solution at 373 K until H2 evolution ceased (about 2 h). After cooling, the sample was washed several times with decantation until the pH of the solution was decreased to 7. Further washings followed some times. The leached sample powder (Raney Ru catalyst) usually decreased the weight to half (0.5 or 1.5 g). The catalyst was transferred to the autoclave reactor together with a portion of the water (1.5 or 3.0 cm3). The autoclave was made of stainless steel and its volume was 200 cm3. Its maximum pressure and temperature for use were 30 MPa and 573 K, respectively. The reactor was connected to mass ¯ow controllers (Brooks 5850TR for H2, CO, CO2, and Ar) by a stainless steel tube (SUS-316) and Swagelok ®ttings, and gas samplers to gas chromatographs (GCs) (Shimadzu GC-8APT and GC-8APF, and Yanagimoto
G3800) for analysis. H2 (99.99999%, A Grade of Toyo Sanso) and Ar (99.9999%, S Grade of Nippon Sanso) were used without further puri®cation. CO (99.95%, R Grade of Takachiho Trading) and CO2 (99.99%, R Grade of Showa Tansangas) were puri®ed passing both through an Oxygen-Trap (Chromatography Research Supplies) and a Molecular Sieves 5A Filter (Shimadzu), respectively. For reduction of the catalyst, H2 was fed into the autoclave. The feeding of pressurized H2 and degassing was continued several times until no contamination such as air was detected by GCs. The autoclave was heated by a ribbon heater with a PID temperature controller (Shimaden SR-22). The gas, catalyst, and water were stirred vigorously by a magnetic stirrer (Advantec SR-550) and a Te¯on TFE stirring bar (Triangle type). The catalyst was reduced twice for 2 h at 473 K under 0.6 MPa pressure of H2. 2.2. CO and CO2 hydrogenation over Raney Ru After the reduction process, the autoclave was cooled and the reaction mixture (H2/CO/ArH2/ CO2/Ar6/2/1) was fed into the autoclave. The feeding of the pressurized gas and degassing were continued several times until the analyzed ratio of the reaction mixture became H2/CO/ArH2/CO2/Ar6/ 2/1. The reactant, the catalyst, and water were stirred vigorously by the magnetic stirrer and the stirring bar. The reaction was usually carried out at 353 or 433 K, and the reaction time was either 5 or 7 h. Before the reaction, the pressure of the reactant was adjusted to 1.1 or 2.1 MPa at room temperature. After heating, the pressure increased a little with the increase of the temperature. Fig. 1 is an example of the pressure changing. The reaction was stopped by rapid cooling using water±ice mixture, before the pressure decreased to about half its value. In this paper, the reaction pressure means the autoclave pressure at room temperature before heating and starting the reaction. The gas phase methanol and hydrocarbons with the carbon number of 1±8, recovered as products, were quantitatively analyzed by a ¯ame ionization detector (FID) of the GCs with two stainless columns ®lled with Porapak Q (80±100 mesh, 2 m long, 3 mm) and Porapak R (80±100 mesh, 1.5 m long, 3 mm) in series. Methane, ethane, CO, CO2, and Ar (internal
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Fig. 1. Example of the time course of the reaction pressure: (*) CO/H2 reaction on Raney Ru (1.5 g) at 353 K for 7 h (the conversion was 16%.); (~) CO2/H2 reaction on Raney Ru (0.5 g) at 433 K for 5 h (the conversion was 40%.).
standard for GC analysis) separated by activated charcoal (80±100 mesh, 3 m long, 3 mm) were detected by FID and the thermal conductivity detector (TCD). The solvent water (10 mm3) with products of liquid phase was injected to the GC, and methanol and hydrocarbons with the carbon number of 4±8 were detected. 3. Results 3.1. CO hydrogenation Logarithms of the reaction rates (produced amounts divided by the reaction time) are plotted against the carbon numbers of hydrocarbon and methanol in Fig. 2. The product distribution obeys the Schulz± Flory mechanism. The reaction rates were increased
with increasing temperature and pressure. However, the product distribution did not change remarkably. Conversion of CO hydrogenation on a Raney Ru catalyst (0.5 g) at 433 K under 1.1 MPa for 7 h was 11%, and that of the reaction on the catalyst (1.5 g) at 353 K under 1.1 MPa for 7 h was 16%, and that on Raney Ru (1.5 g) at 353 K under 2.1 MPa for 7 h was 40%. Selectivity of methanol formation was in a range of 0.08±0.1%, much less than the values observed under 80 kPa with the closed circulation system. 3.2. CO2 hydrogenation Logarithms of the reaction rates (produced amount divided by the reaction time) are plotted against the carbon numbers of hydrocarbon and methanol in Fig. 3. Conversion of CO2 hydrogenation on a Raney
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Fig. 2. Distribution of products obtained from CO hydrogenation by using an autoclave: (*) CO/H2 reaction on Raney Ru (1.5 g) at 353 K under 2.1 MPa (first pressure) for 7 h; (&) CO/H2 reaction on Raney Ru (0.5 g) at 433 K under 1.1 MPa (first pressure) for 7 h; (&) CO/H2 reaction on Raney Ru (1.5 g) at 353 K under 1.1 MPa (first pressure) for 7 h.
Ru catalyst (0.5 g) at 433 K under 1.1 MPa for 5 h was 40%, and that on the catalyst (1.5 g) at 353 K under 1.1 MPa for 5 h was 41%, and that on Raney Ru (0.5 g) at 353 K under 2.1 MPa for 7 h was 10%. The reactivity was much higher than that of the CO hydrogenation. The product distribution also obeyed the Schulz±Flory mechanism. However, the slope was very steep and the propagation factor was low. The main product was methane. The others were hydrocarbons of which carbon numbers were 2±5, and methanol. The selectivity of methanol formation was as low as 0.0±0.03%. Selectivity to methane was extremely high: 97±98%. Especially, the activity of methane formation became much higher than those under atmospheric pressure using the circulation system. In the previous report [3], we pointed out that Raney Ru was a very effective catalyst for methane synthesis from CO2 and H2 compared to the supported Ru catalysts. This is substantiated by the results of this work.
Fig. 3. Distribution of products obtained from CO2 hydrogenation by using an autoclave: (}) CO2/H2 reaction on Raney Ru (0.5 g) at 353 K under 2.1 MPa (first pressure) for 7 h; (~) CO2/H2 reaction on Raney Ru (0.5 g) at 433 K under 1.1 MPa (first pressure) for 5 h; (~) CO2/H2 reaction on Raney Ru (1.5 g) at 353 K under 1.1 MPa (first pressure) for 5 h.
3.3. Reaction temperature dependency of methane formation from CO2 and H2 The CO2 hydrogenation temperature was varied from 353 to 433 K every 20 K under 1.1 MPa on a Raney Ru catalyst (1.1 g). After each reaction, the catalyst was reduced twice for 2 h at 473 K under 0.6 MPa pressure of H2. The reaction time was changed in the range of 2.5±7 h depending on the reaction rate. The average conversion was 44%. The formation rate of methane was plotted against the reciprocal reaction temperature in Fig. 4. From the slope, the apparent activation energy for methane was estimated to be 33 kJ molÿ1. This was much less than the values reported previously: 81 kJ molÿ1 on Raney Ru under 80 kPa [3], 67±70 kJ molÿ1 on Ru/Al2O3 [4], and 82 kJ molÿ1 on Ru/SiO2 [5,6]. The activity of CO2 hydrogenation on Raney Ru at 493 K is estimated to be much higher than that of CO hydrogenation on ultra ®ne iron particles studied at 493 K under 3 MPa [7,8].
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Fig. 4. Arrhenius plots for the formation of methane from CO2 and H2 on the Raney Ru catalyst (1.1 g) under 1.1 MPa. (This pressure was before heating and the reaction. The details are in the text).
4. Discussion 4.1. Low selectivity to methanol from CO±H2 One of the initial purposes was to improve methanol selectivity under the high pressure. When Raney Ru was treated with H2O±He at 573 K, the selectivity to methanol was even 60±80% at the reaction temperature of 353 K [1±3]. Under the condition of this study, it was about 0.1%. This may be because of the lack of H2O±He pretreatment for the catalysts in this experiment, in contrast to the previous study using the circulation system [1±3], and hence, the absence of an appropriate amount of cationic Ru and Al, Run and Al3. With intent of forming Run and Al3 on the catalyst surface, we tried H2O±Ar pretreatment, heating and stirring with water at 373 K under 0.3 MPa pressure of Ar for 12 h. However, this pretreatment did not enhance the selectivity to methanol. Rather it made the catalyst somewhat less active. We have speculated that the presence of both of Run and Al3 on the surface of the catalyst is
indispensable for methanol formation [2]. However, these cationic active sites for methanol formation (without dissociation of C±O bonds) seem to be retarded by water present in the autoclave in this experiment. H2O molecules have higher polarity than CO molecules, and H2O is suggested to interact easily with those cationic active sites. On the other hand, synthesis of methane and hydrocarbons was not retarded by water, probably because the active sites, Ru metal (with dissociation of C±O bonds), had less interaction with H2O. The activity and selectivity of hydrocarbons' formation by using the autoclave were much higher compared with those formed by using the closed circulation system, because the autoclave was operated at higher pressures and temperatures. 4.2. Total pressure effect on the activity for CO±H2 and CO2±H2 reactions In the previous report, the tentative reaction mechanism and rate expression were proposed, and these explained well the competition between CO±H2 and CO2±H2 reactions. Two rate expressions are cited
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from the previous report (Eqs. 1±9 and 2±7) [3]. For CO hydrogenation the same expression (Eq. 1±9) is used here. rCO reaction rate k0 C H
k00 PCO P1:5 H2 =PH2 O 1 K1 PCO 2
(1)
However, for CO2 hydrogenation, the adsorption of CO (retardation) was assumed to be negligible to explain the high activity under the previous condition. Here in this case, we introduce the CO adsorption term as the main species adsorbed during CO2 hydrogenation. K5
CO2 H2 CO a H2 O CO main adsorbed species
(2)
K5 PCO2 PH2 =PH2 O 1 K5 PCO2 PH2 =PH2 O (3)
Eqs. 2±3, 2±6, and 2±7 in the previous paper are rewritten as follows: C active species
K4 PCO2 P2H2 =P2H2 O 1 K5 PCO2 PH2 =PH2 O
(4)
H active species
K3 P0:5 H2 1 K5 PCO2 PH2 =PH2 O
(5)
rCO2 reaction rate kC H
2 kK3 K4 PCO2 P2:5 H2 =PH2 O
1K1 PCO2 PH2 =PH2 O 2 (6)
Now Eqs. (1) and (6) tell us the total pressure effect, but these are not linear functions. Ignoring water pressure (as constant), two extreme cases are considered as follows: rCO / PCO P1:5 H2
for low coverage P2:5 total
1:5 Pÿ1 CO PH2
for high coverage P0:5 total
rCO2 / PCO P2:5 H2 0:5 Pÿ1 CO PH2
for low coverage P3:5 total ÿ0:5 for high coverage Ptotal
Thus, the total pressure dependency on CO hydrogenation and CO2 hydrogenation ranges from 0.5 to 2.5 order and from ÿ0.5 to 3.5 order depending on the coverages. If we compare methane production of code number 4 in Table 1 of the previous study at 353 K
(77 nmol gÿ1 hÿ1 and 41 mmol gÿ1 hÿ1 for CO and CO2 hydrogenation) under 80 kPa, the activities in this study (Figs. 2 and 3) under 1.1 MPa are 25 and 24 times higher respectively for the pressure increase of 14 times. Both total pressure dependencies are a little higher than 1st order. This situation may indicate high CO(a) coverage for CO hydrogenation and intermediate coverage for CO2 hydrogenation. 4.3. Selectivity in CO and CO2 hydrogenation Schulz±Flory plots of Figs. 2 and 3 tell us that the product distribution did not change much from the result obtained under low pressure [3]. The activity of CO2 hydrogenation was still 3 orders of magnitude higher than that of CO hydrogenation. The surface state discussed in the previous paper seems still to be appreciable to this condition. A higher concentration of adsorbed hydrogen than the surface carbide is assumed in the CO2±H2 system even under high pressure. Of course, the new situation may be the predominance of CO(a). However, the model (Eqs. (2)±(6)) suggests that CO(a) coverage can still increase under extremely high pressure of CO2, where the activity may saturate. The pressures studied here, 1.1±2.1 MPa, appear to be practical for hydrocarbon production of higher compound from CO±H2 and methane from CO2±H2 on Raney Ru. 5. Conclusion In CO2 hydrogenation using an autoclave, the activity and selectivity of methane formation are enhanced at higher pressures (1.1 and 2.1 MPa). However, selectivity of methanol formation from CO and H2 was not increased. The reason is suggested that the active center (Run±Al3) for methanol production is retarded by water. This research substantiated that the Raney Ru catalysts are more attractive than supported Ru catalysts for methane formation from CO2 and H2 and for hydrocarbon production from CO and H2 at elevated pressures. Acknowledgements The authors are grateful to former Professor Kazunori Tanaka of Shizuoka University for his helpful discussions and ®nancial support.
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A part of this work has been carried out as a research project of the Japan Petroleum Institute commissioned by the Petroleum Energy Center with a subsidy from the Ministry of International Trade and Industry. References [1] T. Yano, Y. Ogata, K. Aika, T. Onishi, Chem. Lett., (1986) 303.
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