Carbon Dioxide Reforming of Methane To Produce Synthesis Gas

896

Energy & Fuels 1996, 10, 896-904

Carbon Dioxide Reforming of Methane To Produce Synthesis Gas over Metal-Supported Catalysts: State of the Art Shaobin Wang and G. Q. (Max) Lu* Department of Chemical Engineering, The University of Queensland, St. Lucia, QLD 4072, Australia

Graeme J. Millar Department of Chemistry, The University of Queensland, St. Lucia, QLD 4072, Australia Received November 14, 1995X

Carbon dioxide reforming of methane produces synthesis gas with a low hydrogen to carbon monoxide ratio, which is desirable for many industrial synthesis processes. This reaction also has very important environmental implications since both methane and carbon dioxide contribute to the greenhouse effect. Converting these gases into a valuable feedstock may significantly reduce the atmospheric emissions of CO2 and CH4. In this paper, we present a comprehensive review on the thermodynamics, catalyst selection and activity, reaction mechanism, and kinetics of this important reaction. Recently, research has centered on the development of catalysts and the feasible applications of this reaction in industry. Group VIII metals supported on oxides are found to be effective for this reason. However, carbon deposition causing catalyst deactivation is the major problem inhibiting the industrial application of the CO2/CH4 reaction. Ni-based catalysts impregnated on certain supports show carbon-free operation and thus attract much attention. To develop an effective catalyst for CO2 reforming of CH4 and accelerate the commercial application of the reaction, the following are identified to be the most important areas for future work: (1) selection of metal and support and studying the effect of their interaction on catalyst activity; (2) the effect of different promoter on catalyst activity; (3) the reaction mechanism and kinetics; and (4) pilot reactor performance and scale-up operation.

Introduction In recent years, considerable attention has been paid to global warming due to the greenhouse effect. The reduction and utilization of greenhouse gases such as carbon dioxide and methane is therefore becoming more and more important. Catalytic reforming of methane with carbon dioxide to synthesis gas has been proposed as one of the most promising technologies for utilization of these two greenhouse gases as carbon-containing materials.1 The synthesis gas, produced by the reaction, has a high CO content which is effective for the synthesis of valuable oxygenated chemicals.2,3 Unfortunately, there is no established industrial technology for carbon dioxide reforming of methane, in spite of potentially attractive incentives with economical and environmental benefits. The principal reason for this is the carbon-forming reaction, which quickly deactivates conventional reforming catalysts if used without the presence of steam. No effective commercial catalyst to date exists which operates without carbon formation. In the past decade, efforts have focused on the development of catalysts which show high activity and * Author to whom all correspondence should be addressed. Phone: 61 7 33653708. Fax: 61 7 33654199. Email: [email protected]. X Abstract published in Advance ACS Abstracts, May 1, 1996. (1) Wang, S.; Lu, G. Q.; Tang, H. S. Proc. 23rd Austr. Chem. Eng. Conf. 1995, 2, 42-47. (2) Alyea, E. C.; He, D.; Wang, J. Appl. Catal. 1993, 104, 77-85. (3) Burch, R.; Petch, M. I. Appl. Catal. 1992, 88, 39-60.

S0887-0624(95)00227-1 CCC: $12.00

stability for methane partial oxidation4-11 and methane dry reforming with carbon dioxide12-21 to syngas. Nickelbased catalysts4,5,11-17 and noble metal-supported catalysts (Rh, Ru, Pd, Pt, Ir)6-11,18-21 were found to have (4) Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. J. Catal. 1991, 132, 117-160. (5) Choudhary, V. R.; Samsare, S. D.; Mammam, A. S. Appl. Catal. 1992, 90, L1-L5. (6) Ashcroft, A. T.; Cheethan, A. K.; Foord, J. S.; Green, M. L. H.; Grey, C. P.; Murrell, A. J.; Vernom, P. D. F. Nature 1990, 344, 319321. (7) Jones, R. H.; Ashcroft, A. T.; Waller, D.; Cheetham, A. K.; Thomas, J. M. Catal. Lett. 1991, 8, 169-171. (8) Hochmuth, J. K. Appl. Catal. B: Environ. 1992, 1, 89-100. (9) Vernon, P. D. F.; Green, M. L. H.; Cheetham, A. K.; Ashcroft, A. T. Catal. Lett. 1990, 6, 181-186. (10) Poirier, M. G.; Trudel, J.; Guay, D. Catal. Lett. 1993, 21, 99111. (11) Torniainen, P. M.; Chu, X.; Schmidt, L. D. J. Catal. 1994, 146, 1-10. (12) Gadalla, A. M.; Bower, B. Chem. Eng. Sci. 1988, 42, 30493062. (13) Gadalla, A. M.; Sommer, M. E. Chem. Eng. Sci. 1989, 44, 28152829. (14) Kim, G. J.; Cho, D. S.; Kim, K. H.; Kim, J. H. Catal. Lett. 1994, 28, 41-52. (15) Swaan, H. M.; Kroll, V. C. H.; Martin, G. A.; Mirodatos, C. Catal. Today 1994, 21, 571-578. (16) Chang, J. S.; Park, S. E.; Lee, K. W. Stud. Surf. Sci. Catal. 1994, 84, 1587-1594. (17) Zhang, Z.; Verykios, X. E. J. Chem. Soc., Chem. Commun. 1995, 71-72. (18) Ashcroft, A. T.; Cheethan, A. K.; Green, M. L. H.; Vernom, P. D. F. Science 1991, 352, 225-226. (19) Rostrup-Nielsen, J. R.; Hansen, J. H. B. J. Catal. 1993, 144, 38-49. (20) Qin, D.; Lapszewicz, J. Catal. Today 1994, 21, 551-560.

© 1996 American Chemical Society

Carbon Dioxide Reforming of Methane

promising catalytic performance in terms of conversion and selectivity for both reactions. The catalysts based on noble metals are reported to be less sensitive to coking compared to the nickel-based catalysts for CH4 + CO2 reaction.18-21 However, considering the high cost and limited availability of noble metals, it is more practical to develop improved Nisupported catalysts which exhibit stable operation for a long period of time. In this paper, we review the catalytic reaction of CO2 reforming of methane on metal-supported catalysts, covering the thermodynamics, catalyst activity, reaction mechanisms, and kinetics.

Energy & Fuels, Vol. 10, No. 4, 1996 897

Figure 1. Schematic diagram of the chemical energy transmission system (CETS).

Background and Prospects The major source of synthesis gas, which is mainly used as a feedstock for methanol synthesis and FischerTropsch conversions, is from the steam reforming reaction.

CH4 + H2O f CO + 3H2

∆H298 ) 206 kJ/mol (1)

However, this process suffers from severe limitations like high energy requirements, a high H2/CO product ratio, and poor selectivity for carbon monoxide. The partial oxidation reaction of methane though has several advantages over steam reforming, notably its greater selectivity to synthesis gas production, its exothermicity, and the more desirable CO/H2 ratio of the product. However, it must be operated at very high temperatures and pressures without catalysts. Recent efforts have concentrated on reducing the temperature by using group VIII metal-based catalysts.4-11 Although this reaction is mildly exothermic, a small decrease in CO selectivity results in a large increase in the reaction temperature. Besides, a high methane conversion coupled with high space velocity (reaction rate per unit volume) leads to the production of a large amount of heat in a small catalyst zone. It is difficult to remove the reaction heat from the reactor, particularly for a large operation, and therefore, the process becomes very hazardous and difficult to control. In addition, Ni catalyst deactivation caused by carbon deposition is another problem.

2CH4 + O2 f 2CO + 4 H2

∆H298 ) -71 kJ/mol (2)

A proposed alternative to the steam reforming process is reforming CH4 with CO2. This reaction was first proposed by Fischer and Tropsch in 1928.22 This reaction has several advantages. It produces a synthesis gas with a lower H2/CO ratio and has higher energy efficiency in conversion to hydrocarbons. Because of the high endothermicity of the reaction, it can be used in energy transfer from solar energy to chemical energy, energy storage in the form of CO and H2, and transporting nuclear energy. When a source of CO2 is available it is likely that CO2/CH4 reforming will become a promising industrial process. Carbon dioxide is a byproduct of many industrial processes and available for utilization. For instance, (21) Richardson, J. T.; Paripatyadar, S. A. Appl. Catal. 1990, 61, 293-309. (22) Fischer, V. F.; Tropsch, H. Brennst.-Chem. 1928, 3(9), 39-46.

Figure 2. Flow scheme of Eva-Adam process.

power plants emit a large amount of CO2 at relatively high temperature. Landfill gas commonly consists of 50% CH4 and 50% CO2. Natural gas contains a large CO2 content and digestion of industrial wastewater can produce methane and CO2. In these cases, CO2 reforming of methane may be the most effective way in utilizing these two greenhouse gases. Using a CO2-rich feedstock over an unspecified catalyst, the CO2 reforming of hydrocarbon for the production of synthesis gas has been commercialized as the Calcor process.23,24 So far there is no established commercially industrial process for CO2 reforming of CH4 due to the fatal problem of coking. Renewed interest in this conversion has been ascribed to its potential use for industry and energy storage as well as the associated environmental benefits. Fujimoto et al.25 reported that using two-stage reactor process composed of methane reforming by CO2 and FischerTropsch reaction, the synthesis of liquid hydrocarbons from CO2 and CH4 was achieved. Figure 1 shows the basic concepts in chemical energy transmission systems (CETS). CO2 reforming of CH4 can act as a preferred thermochemical reaction. This reversible endothermic reaction is driven to equilibrium by fossil, nuclear, or solar energy, so that products (synthesis gas) store the incident energy. These products are saved for later use or transported through pipelines to a location where the reverse reaction, exothermic, releases the energy. One of the applications, the Eva-Adam process (Figure 2) has been established in Germany, the United States, Israel, and the former Soviet Union.21 The process consists of steam reforming methane for the forward reaction and methanation for the reverse reaction. The products of CO/H2 are transported to remote site via a pipeline where the reverse reaction proceeds and then CH4 is sent back to the original site after removing the water from the product mixture. Since 1980, the CO2 reforming-methanation cycle has been studied as an alternative to the steam reforming-methanation cycle for the solar energy application.26-32 This process has several advantages com(23) Reitmeier, R. E.; Atwood, K.; Bennett, H. A., Jr.; Baugh, H. M. Ind. Eng. Chem. 1948, 40, 620-626. (24) Teuner, S. Hydrocarbon Process. 1985, 64(5), 106-107. (25) Fujimoto, K.; Omata, K.; Nozaki, T.; Yamazaki, O.; Han, Y. Energy Convers. Mgmt. 1992, 33, 529-539.

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Wang et al.

pared with the Eva-Adam process: e.g., the heat of reaction is higher than that of steam reforming of methane at the same conditions; all the reactants and products are in the vapor phase and no liquid products are formed, resulting in an easy operation and fewer side reactions. The disadvantage is the carbon deposition under certain conditions. McCrary et al.27 have reported a successful closedloop Solchem system including the CO2-CH4 reforming-methanation cycle. Nickel catalyst deterioration has been resolved by the use of Rh-on-stainless-steel catalyst. Using CO2 reforming of methane as the vehicle for storage and transport of solar energy, a solar chemical heat pipe was tested at the Weizmann Institute in Israel.32 High conversions were achieved for both the endothermic reforming reaction and the exothermic methanation reaction. The whole process was carried out in a closed loop and the performance was satisfactory. In the Catalytically Enhanced Solar Absorption Receiver (CAESAR) project, carbon dioxide reforming of methane in a solar volumetric receiver/ reactor was successfully demonstrated.31 The total solar power absorbed reached up to 97 kW and the maximum methane conversion was almost 70%. Receiver efficiencies ranged up to 85% and chemical efficiencies were at 54%. Apart from the solar chemical energy systems, nuclear energy transporting application was also reported.33 In addition, it was recommended for seasonal storage of energy in the form of CO and H2.34 Therefore, the reaction of methane reforming with CO2 is very promising both from industry and environmental protection points of view. If the catalyst deactivation can be successfully resolved, the process can be readily commercialized. Thermodynamics of CO2 Reforming of Methane The corresponding carbon dioxide reforming reaction is described as

CO2 + CH4 f 2CO + 2H2

∆H298 ) 247 kJ/mol (3)

∆G° ) 61770 - 67.32T This reaction is highly endothermic and is equally favored by low pressure but requires a higher temperature. A reverse water-gas shift reaction occurs as a side reaction.

CO2 + H2 f H2O + CO

∆H298 ) 41 kJ/mol

(4)

∆G° ) -8545 + 7.84T Under conditions of stoichiometric CO2 reforming, car(26) Chubb, T. A. Sol. Energy 1980, 24, 341-345. (27) McCrary, J. H.; McCrary, G. E.; Chubb, T. A.; Nemecek, J. J.; Simmons, D. E. Sol. Energy 1982, 29, 141-151. (28) Fraenkel, D.; Levitan, R.; Levy, M. Int. J. Hydrogen Energy 1986, 11, 267-277. (29) Fish, J. D.; Hawn, D. C. J. Sol. Energy Eng. 1987, 109, 215220. (30) Tokunaga, O.; Osada, Y.; Ogasawara, S. Fuel 1989, 68, 990994. (31) Buck, R.; Muir, J. F.; Hogan, R. E.; Skocypec, R. D. Solar Energy Mater. 1991, 24, 449-463. (32) Levy, M.; Levitan, R.; Rosin, H. Rubin, R. Solar Energy 1993, 50, 179-189,. (33) Hanneman, R. E.; Vakil, H.; Wentorf, R. H., Jr. Proc. Intersoc. Energy Convers. Eng. Conf. 1974, 9, 435-441. (34) Andujar-Peral, J. M. Proc. Intersoc. Energy Convers. Eng. Conf. 1986, 21(2), 695-701.

Figure 3. Equilibrium conversion for CH4 reforming with CO2 with the feed ratio of CO2/CH4 ) 1:1 at various pressures.35 Table 1. Limiting Temperatures for Reactions of the CO2/CH4 System reaction temp (°C) a

3a

4b

5b

6a

640

820

700

557

Lower limit. b Upper limit.

bon deposition occurs as in the Boudouard reaction

2CO f CO2 + C

∆H298 ) -172 kJ/mol

(5)

∆G° ) -39810 + 40.87T and methane cracking

CH4 f C + 2H2

∆H298 ) 75 kJ/mol

(6)

∆G° ) 21960 - 26.45T The standard free energy change was employed to calculate the miminum operating temperatures for CO2 reforming, CH4 cracking, and the upper limiting temperatures of the other side reactions (4) and (5). Assuming ∆G° ) 0, the upper or lower limiting temperatures for reactions 3-6 will be obtained (Table 1). CO2/ CH4 reaction can proceed above 640 °C accompanied by methane cracking reaction, while above 820 °C reverse water-gas shift and the Boudouard reactions could not occur. In the temperature range of 557-700 °C carbon will be formed from methane cracking or the Boudouard reaction. Figure 3 shows the equilibrium conversion vs temperature at total pressures of 0.01-0.1 atm with CO2/ CH4 ) 1. At a fixed temperature, conversions at lower total pressures are always higher than those at higher total pressures. At the pressure of 0.01 atm, conversion reaches 90% at 550 °C while at 0.1 atm the conversion does not reach 90% until 700 °C. Figure 4 shows the limiting temperature curve for carbon deposition for different values of the CO2/CH4 ratio and total pressures. For the same feed ratio, the temperature limit for carbon deposition increases as the pressure increases. Clearly, carbon deposition is thermodynamically possible for a CO2/CH4 reforming feed ratio of 1:1 at temperature up to 870 °C at 1 atm and 1030 °C at 10 atm. In addition, at a given pressure, the temperature limit increases as the CO2/CH4 feed ratio decreases. This mean that using excess CO2 in the feed may avoid carbon formation at lower temperatures. Figure 5 shows the equilibrium composition of the reforming reaction at the limiting temperatures and (35) Nakamura, J.; Uchijima, T. Shokubai 1993, 35, 478-484.


Figure 4. Effects of feed ratio of CO2/CH4 on limiting temperatures below which carbon deposits at various pressures.12


Figure 6. Effect of the feed ratio on the conversion of CH4 and the product distribution36

Figure 7. Band of temperature in which carbon deposition and carbide formation are suppressed at the operating pressure of 1 atm.12

Figure 5. Equilibrium composition of CO2/CH4 system at (a) 1 atm and (b) 10 atm.12

different CO2/CH4 feed ratios at pressures of 1 and 10 atm. It can be seen that water always forms in the reaction system. Its equilibrium concentration increases as the feed ratio increases. The formation of water is reflected by lower hydrogen concentration than CO due to the reverse water gas shift reactionsthe main water formation route. The amount of CO is always higher than that of hydrogen, which means that ratios of CO/H2 are above unity. This may imply that the carbon is formed in the CO2/CH4 system. Compared to 1 atm, the pressure of water at 10 atm is higher indicating the reverse of the water gas shift reaction is favorable at higher pressures. From Figure 5, it is also seen that CH4 conversion increases as the feed ratio increases and that at given feed ratio, CH4 conversion decreases as the pressure increases. For a given pressure, CO2 conversion increases as the feed ratio decreases. Takano et al.36 investigated the effect of the feed ratio of CO2/CH4 on carbon dioxide reforming of methane on (36) Takano, A.; Tagawa, T.; Goto, S. J. Chem. Eng. Jpn. 1994, 27, 723-731.

supported nickel catalysts as shown in Figure 6. The experimental data agreed with the thermodynamic calculations. At a CO2/CH4 ratio of 0.5, an equimolar amount of H2 and CO was produced. When the feed ratio was larger than 1.0, the yield of H2 became lower because of the reverse water gas shift reaction. To prevent carbon formation, operation temperatures must be higher than the limiting temperatures as indicated in Figure 4, which means that more energy will be required. At high temperatures, however, nickel carbide may form on the surface of nickel-based catalysts and accordingly an upper-temperature limit is needed to prevent its formation. Figure 7 shows the lower and upper temperature limits to prevent carbon deposition and carbide formation at pressure of 1 atm. At the feed ratio of CO2/CH4 ) 1:1, the optimum temperature is between 870 and 1040 °C. Sacco et al.37 studied the thermodynamics of carbon deposition on Fe, Co, and Ni in the gas mixtures of CH4-H2-H2O-CO-CO2 using the phase diagram. The carbon deposition order is Fe . Co . Ni. Figure 8 represents a typical phase diagram for the Ni/NixC/NiO/ C/gas equilibrium at 900 K and 1 bar pressure. Any gas mixture composed of CO, CO2, H2O, and CH4, and H2 can be plotted on this triangular plane. Nickel carbide and carbon are favored to form in the dry mixture of CH4 and CO2. When water is present in the mixture, Ni or NiO is favored to be the catalytic component. Thermodynamic calculations by Dibbern et al.38 on H2O-CO2-CH4 systems showed that carbon deposition would be prevented by introducing sufficient (37) Sacco, A., Jr.; Geurts, F. W. A. H.; Jablonski, G. A.; Lee, S.; Gately, R. A. J. Catal. 1989, 119, 322-341. (38) Dibbern, H. C.; Olesen, P.; Rostrup-Nielsen, J. R.; Tottrup, P. B.; Udengaard, N. R. Hydrocarbon Process. 1986, 65, 71-74. (39) Yamazaki, O.; Nozaki, T.; Omata, K.; Fujimoto, K. Chem. Lett. 1992, 1953-1954.

900


Wang et al. Table 2. Summary of Catalytic Reforming of CO2/CH4 in the Literature catalyst

Figure 8. Phase diagram of Ni-C-H-O at 900 K and 1 bar pressure.37

steam into the system. Various fixed values of the H2/ CO ratios in the product gas can be achieved by introducing identified H2O/CH4 and CO2/CH4 ratios. However, carbon-free operation with CO2-CH4 feed should be possible if the reaction is controlled kinetically by suitable catalysts such as Rh,18,19,21 Ru,18,19,21 Ir,18 and Ni supported on MgO,39 metal sulfide (MoS2, WS2),40 or after passivation with H2S.41 The SPARG process, namely sulfur passivated steam reformer, has been attempted to resolve the problem of carbon formation.19,38 Sulfur may block the nickel catalyst surface, which means that nickel cannot catalyze the formation of carbon because of the ensemble effect. However, the passivation process suffers from lower catalytic activity and high operating temperatures. Catalysts Methods of Catalyst Preparation. Different methods for catalyst preparation influence the catalyst activity for CO2 reforming reaction. There are few reports comparing the activities of catalysts prepared by different methods. The incipient wetness impregnation method is usually used to prepare metal supported catalysts. An improved vacuum wetness technology was employed by Ashcroft et al.18 who obtained highly dispersed noble metal catalysts which retained sufficient activity to reform the methane and carbon dioxide efficiently, while suppressing carbon deposition. Adding promoter to the catalyst may improve the catalytic activity and selectivity. In this case, the catalysts are often synthesized by the coprecipitation method. Using this technique, Gadalla and Sommer42 prepared three catalysts with different compositions of NiO, MgO, and Al2O3, which demonstrated high CH4 conversion in CO2/CH4 reaction. Fujimoto et al.25,39 prepared Ni/MgO-CaO catalyst by coprecipitating the hydroxides from aqueous solution of nickel, magnesium, and calcium. In this way, the catalyst exhibited a stable activity for more than 3 days without coke formation. Recently, Chang et al.16 reported the different activity of pentasil zeolite-supported nickel catalysts synthesized by two different methods, namely, (1) solid-state reaction and (2) incipient wetness method. KNiCa/ZSI catalyst prepared by incipient wetness method gave much less activity than the one prepared by solid-state (40) Osaki, T.; Horiuchi, T.; Suzuki, K.; Mori, T. Catal. Lett. 1995, 35, 39-43. (41) Rostrup-Nielsen, J. R. J. Catal. 1984, 85, 31-43. (42) Gadalla, A. M.; Sommer, M. E. J. Am. Ceram. Soc. 1989, 72, 683-687.

Ni/NaY Ni/Al2O3 Ni/SiO2 Pd/NaY Pt/NaY KNiCa/Al2O3 KNiCa/SiO2 KNiCa/ZSI Rh/TiO2 Rh/SiO2 Rh/Al2O3 Ni/Al2O3 Pd/Al2O3 Ru/Al2O3 Rh/Al2O3 Ir/Al2O3 Co,MgO/C Ni/CaO-MgO Rh/Al2O3 Ru/Al2O3 Ru/Eu2O3 Ir/Eu2O3 Ru/MgO Rh/MgO Pt/MgO Pd/MgO Ni/Al2O3 Ni/MgO-Al2O3 Ni/CaO-Al2O3 Ni/CaO-TiO2-Al2O3

CO2/CH4 conversion (%) temp (K) ref 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 1:1 2.38:1 2.38:1 2.01:1 2.01:1

84.0 36.3 14.9 29.2 156.3 17 21 78 88.2 5.1 85.1 80-90 70-75 60-70 85-90 85-90 65-75 80 85 83 75 88 90 88 85 84 100 86 88 100

873 873 873 873 873 923 923 923 893 893 893 1050 1050 1050 1050 1050 923 1123 1073 1073 923 1000 963 963 963 963 1213 1211 1211 1223

14 14 14 14 14 44 44 44 45 45 45 18 18 18 18 18 46 25 21 21 47 47 48 48 48 48 12 12 12 13

reaction technique. KNiCa/ZSI(1) catalyst showed over 90% conversion at 800 °C. It maintained its high performance for over 140 h at 800 °C without deactivation by coke formation. They proposed that it could be due to the melting effect of metallic precursor mixtures via solid-state reaction which was different from that of impregnation. Metal Activity. CO2-reforming of methane had already been proposed by Fischer and Tropsch as a substitute for the steam reforming process and most of the group VIII metals were identified as the preferred catalysts. Few other metals were reported to be effective for this reaction. Table 2 lists some results of CH4/CO2 reaction over group VIII metals supported catalysts. Most of the group VIII metals (Rh, Ru, Ni, Pt, Pd, Ir, Co, Fe) except Os are more or less effective for catalysis of the CH4/ CO2 reaction. Tokunaga et al.43 compared the activities of catalysts such as Ni, Fe, and Co supported on γ-Al2O3. Ni-Al2O3 was found to be the most active catalyst. According to thermodynamic studies, Ni-containing catalysts for CO2 reforming are prone to cause carbon (43) Tokunaga, O.; Ogasawara, S. React. Kinet. Catal. Lett. 1989, 39, 69-74. (44) Park, S. E.; Nam, S. S.; Choi, M. J.; Lee, K. W. Energy Convers. Mgmt. 1995, 36, 573-576. (45) Nakamura, J.; Aikawa, K.; Sato, K.; Uchijima, T. Catal. Lett. 1994, 25, 265-270. (46) Guerrero-Ruiz, A.; Rodriguez-Ramos, I.; Sepulveda-Escribano, A. J. Chem. Soc., Chem. Commun. 1993, 487-488. (47) Perera, J. S. H. Q.; Couves, J. W.; Sankar, G.; Thomas, J. M. Catal. Lett. 1991, 11, 219-225. (48) Takayasu, O.; Hirose, E.; Matsuda, N.; Matsuura, I. Chem. Express 1991, 6, 447-450. (49) Solymosi, F.; Kutsan, Gy.; Erdohelyi, A. Catal. Lett. 1991, 11, 149-156. (50) Sakai, Y.; Saito, T.; Sodesawa, T.; Nozaki, R. React. Kinet. Catal. Lett. 1984, 24, 253-257. (51) Xu, H.; Sun, X.; Fan, Y.; Xu, G.; Liu, J.; Yu, W.; Zhou, P. J. Petrochem. Eng. 1992, 21(3), 147-153.



Table 3. Catalytic Activities of Metals on Various Supports

metal activity 1. Al2O3 Rh > Pd > Ru > Pt > Ir Rh > Pd > Pt . Ru Ir > Rh > Pd > Ru Ni > Co . Fe Ni > Co . Fe Ru > Rh Ru > Ru 2. SiO2 Ru > Rh > Ni > Pt > Pd Ni > Ru > Rh > Pt > Pd . Co 3. MgO Rh > Ru > Ir > Pt > Pd Ru > Rh > Ni > Pd > Pt Ru > Rh ∼ Ni > Ir > Pt > Pd Ru > Rh > Pt > Pd 4. Eu2O3 Ru > Ir 5. NaY Ni > Pd > Pt

metal loading (wt %)

temp (K)

ref

1 0.5-1 1 9 10 0.5 0.5

823 823-973 1050 773-973 1023 873 923-1073

49 50 18 51 43 21 21

1 0.5

973 893

35 35

0.5 1 1 1

1073 973 823 913

20 35 19 48

1-5

873-973

47

2

873

14

formation resulting in deactivation of catalysts12 and it has proven impossible to avoid carbon formation under low CO2/CH4 ratios using nickel catalysts.18 To prevent carbon deposition, CO2/CH4 ratios above unity should be used. This constraint limits the application of Ni catalysts. Therefore, noble metals have been studied in recent years, and results showed that they were less sensitive to coking. Using catalysts based on platinum group metals, Ashcroft et al.18 examined the stoichiometric CO2-reforming reaction at 1050 K and atmospheric pressure and achieved 90% yield of synthesis gas without carbon deposition on Ru, Rh, and Ir (Table 2). Catalysts based on Ni, Ru, Rh, Pd, Ir, and Pt were also compared for CO2 reforming of methane. Ru and Rh showed high selectivity for carbon-free operation. The sequence of activity for CO2 + CH4 reaction or carbon-free formation was Ru, Rh > Ir > Ni, Pt, Pd.19 For Rh and Ru, the conversion and deactivation tests showed that Rh was much more stable.21 Table 3 illustrates the activity order for metals dispersed on various supports. It is seen that the combination of metal and support affects the resultant metal activity. Even for the same support there are conflicting conclusions deduced by various researchers. This may be due to the different operation conditions. Among the first row of group VIII metals (Fe, Co, and Ni), Ni shows the highest activity, which is justified by the thermodynamics presented in the previous section. Rh is a superior component in respect of the high activity and the coking-free operation if supported on Al2O3. Ru may be better than Rh if MgO and SiO2 are used as supports. Other noble metals such as Pd and Pt showed high activity if supported on Al2O3, while Ni catalysts with MgO or zeolite support demonstrated less coking ability. Apart from group VIII metals, other metal systems were recently used for dry methane reforming with CO2. A manganese-based catalyst gave high yield of synthesis gas without carbon deposition being detected at 1200 K. This catalyst, however, had significantly lower activity than the group VIII metals.52 Rhenium sup(52) Mirzabekova, S. R.; Mamedov, A. Kh.; Aliev, V. S.; Krylov, O. V. Kinet. Katal. 1992, 33, 591-596.

ported on alumina was also tested as a catalyst for dry reforming of methane. At temperatures above 973 K, Re was actually more active than Ir for the dry reforming reaction at stoichiometric CH4:CO2 ratios. However, the activity of Re catalyst decreased dramatically at lower temperatures. At about 873 K, methane conversion was less than 5%.53 Loadings of metals on supports also affect the activity of the catalysts. Low loadings (about 1-5 wt %) of the noble metals are usually sufficient because of their effective performances. Higher loadings are required for Ni and Co catalysts in which the metal-support interaction are stronger than in the cases of heavier metals. Perera et al.47 studied the effects of noble metal loadings (Figure 9). It is demonstrated that higher loading may increase the catalyst activity. Different amounts of Rh supported on La2O3 catalysts were also tested for CO2/CH4 reforming.54 At low ratio of CO2/ CH4 the CH4 and CO2 conversions on 0.2 and 1.5% Rh/ La2O3 were similar; as the feed ratio increased the 1.5% Rh/La2O3 catalyst showed the higher conversion for both reagents. Kim et al.14 researched the dependence of the amount of Ni loaded over NaY on the formation rate of H2 and CO (Figure 10). The activity strongly depended on the Ni amount over NaY, and the maximum conversion of CO2 and CH4 was obtained at 3.3 wt % Ni loading. The formation rate of H2 and CO decreased sharply when the amount of Ni exceeded more than 2 wt %. Ni catalysts supported on SiO2 with 40, 44, and 60% metal component were tested for CO2/CH4 reforming. The conversions of CH4 were similar for 40 and 60% Ni/SiO2, while 44% Ni/SiO2 showed lowest catalytic activity.36 As stated above, carbon deposition over catalysts is the fatal problem for CO2 reforming of methane. Although some noble metals show high activity and selectivity for carbon-free operation, high cost and limited availability of noble metals prevent the commercial use of this reaction. It is, therefore, more practival to develop an improved nickel-based catalyst which exhibits stable operation for a long period of time. Recently, a novel Ni/La2O3 catalyst was developed which exhibited high activity and excellent long-term stability for CO2 reforming of methane to produce synthesis gas. This catalyst is stable at temperatures as low as 823 K without losing its catalytic activity.17 The results reported by Takayasu et al.55 indicated that physically mixed SiO2 with Ni catalyst had a special feature which minimized deposited carbon. Catalysts of Ni-supported on or incorporated into zeolite promoted by alkali and alkaline-earth metals were also proposed. Ni/Y-zeolite catalyst exhibited high catalytic activity and conversion at about 850 K.16,44 Effect of Support on Catalyst Activity. The nature of the oxide support greatly affects the catalyst activity due to the varying active surface area and acidbase property. Carbon dioxide reforming involves the adsorption and dissociation of CO2 on catalysts. Since (53) Claridge, J. B.; Green, M. L. H.; Tsang, S. C. Catal. Today 1994, 21, 455-460. (54) Gronchi, P.; Mazzocchia, C.; Rosso, R. D. Energy Convers. Mgmt. 1995, 36, 605-608. (55) Takayasu, O.; Takegahara, Y.; Matsuura, I. Energy Convers. Mgmt. 1995, 36, 597-600.

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Figure 9. CH4 conversion as function of metal loading at various temperatures.47

Figure 11. Effect of metal oxides mixed with Rh/SiO2 on conversion of CH4 at 893 K.45 Figure 10. Effect of Ni loading over NaY on the catalytic activity in CO2/CH4 reaction.14

CO2 is well-known as an acid gas, adsorption and dissociation of CO2 may be improved with a basic catalyst. The role of catalyst support on the activity of Ni for reforming methane with CO2 was first studied by Gadalla and Bower.12 The catalytic activity, selectivity, and stability of the Ni catalysts varied greatly with different support of Al2O3, Al2O3-SiO2, Al2O3-MgO, and Al2O3-CaO. Ni on Al2O3, Al2O3-MgO, and Al2O3CaO resulted in high conversion and the catalysts with supports containing MgO and CaO were more stable. However, the Ni-Al2O3-SiO2 catalyst was deactivated rapidly and was shattered. Erdohelyi et al.56 investigated catalytic reaction of methane with CO2 over palladium supported on TiO2, Al2O3, SiO2, and MgO and concluded the order of activity as follows: Pd/TiO2, Pd/ Al2O3, Pd/SiO2, Pd/MgO. Takano et al.36 also worked on Ni catalysts with various supports. The catalytic activity varied depending on the type of support according to the following order, Al2O3 > SiO2 with MgO > SiO2. The addition of MgO to SiO2-supported catalyst increased the catalytic activity but decreased the stability. The role of support was also evaluated on Rh/SiO2 catalysts mixed physically with metal oxides such as Al2O3, TiO2, and MgO. It was clear that Al2O3, TiO2, and MgO promotes the catalytic activity of Rh/SiO2 remarkably, which indicates the synergetic effect (Figure 11). Ni on lanthanum-modified alumina was tested as catalyst for the CO2/CH4 reforming reaction.57 The results demonstrated that the modified catalysts were less active than the unmodified catalyst. However, the stabilities of modified catalysts were higher than that (56) Erdohelyi, A.; Cserenyi, J.; Papp, E.; Solymosi, F. Appl. Catal. 1994, 108, 205-219. (57) Blom, R.; Dahl, I. M.; Slagtern, A.; Spjelkavik, A.; Tangstad, E. Catal. Today 1994, 21, 535-543.

Table 4. Effect of Support on Catalyst Activity

activity order Ru Al2O3 > TiO2 > SiO2 TiO2 > Al2O3 > SiO2 Pd TiO2 > Al2O3 > NaY > SiO2 > MgO > Na-ZSM-5 TiO2 > Al2O3 > SiO2 > MgO Rh YSZ > Al2O3 > TiO2 > SiO2 . MgO Al2O3 > SiO2 > TiO2 > MgO Ni Al2O3 > SiO2 Al2O3 > SiO2 NaY > Al2O3 > SiO2 SiO2 > ZrO2 > La2O3 > MgO > TiO2

temp (K)

metal loading (wt %) ref

893 893

0.5 0.5

45 45

773

5

59

773

1

56

923 773

0.5 1

60 61

800-1000 873 873 823

40 10 2 4

36 30 14 15

of unmodified catalyst. The difference in activity and stability was due to the sintering degree of nickel on the support. Rh-supported catalysts (Rh/SiO2, Rh/ La2O3-SiO2, and Rh/La2O3) were investigated by Gronchi et al.54 The La2O3 showed a positive effect on the conversion and selectivity due to its basicity. Table 4 shows the effect of support on catalytic activity. Nakamura et al.45 found that the effect of support on catalytic activity was in the following order Al2O3 > TiO2 > SiO2 in view of CH4 and CO2 conversion, while by the turnover frequency for CH4 conversion, the order of activity was TiO2 > Al2O3 > SiO2. They believed that the significant effect of support may be due to a direct activation of CH4 or CO2 by metal oxides and the difference of particle size of the metal. From Table 4 it can be generally deduced that Al2O3 is much better as a support than SiO2 and MgO for the CO2/ CH4 reaction. Addition of alkali promoter to catalysts was effective in preventing coke formation from methane during steam reforming. Similarly, for the reaction of CO2 reforming of methane, adding basic promoter such as



alkali or alkaline-earth oxides may also change the nature of the support, because CO2 is adsorbed strongly on the surface of basic catalysts and covers a large part of the surface at lower CO2 partial pressure, which will prevent carbon deposition on catalysts. Yamazaki et al.39 developed a catalyst of Ni/MgO-CaO. It showed high basicity and lower coke forming ability attributed to the addition of CaO. Activity of the Co,MgO/C system was compared with that of Co/C catalyst and the results suggested that the role of MgO is to avoid occurrence of the Boudouard reaction.46 Ni/CaO-Al2O3 catalyst was also found to have higher reaction rate and stability than Ni/Al2O3. This is related to a relatively enhanced reactivity of the Cβ and Cγ deposited on the catalyst.58 Kim et al.14 studied the reaction of CO2 with CH4 over Ni metals introduced into different cation-exchanged Y-zeolite and concluded that Ni supported on Y-zeolite with alkali cations had much higher catalytic activity than Ni/HY, Ni/MgY, and Ni/Y. Carbon Formation and Activity. Carbon deposited on the catalyst comes from two routes. Experiments showed that the deposited carbon is primarily derived from the CO2 molecular route, while a very small amount of carbon present on the working catalyst surface is derived from the CH4 molecular route.56,60 Generally, the carbon deposits comprise various forms of carbon which are different in terms of reactivity. Three types of carbonaceous species, R-C, β-C, and γ-C species exist on the Ni catalysts. While the R-C species is suggested to be responsible for CO formation, the less active β-C and γ-C species are causing catalyst deactivation.58 It is known that among the deposited carbons, only a stable form, possibly arising from the CO disproportionation, would poison the Ni particles and that other less stable forms arising from methane activation are rapidly accumulated on the catalyst poisoning the catalysts to a lesser extent. The carbon species originally produced are believed to be atomic carbon. They are known to be very reactive and an important intermediate in the CO2 + CH4 reaction. In fact, it has been reported that CO2 dissociated into carbon and two oxygen atoms on metal and that it was possible that carbon and oxygen on surface recombined to form CO. There are reports that coke gasification by H2, steam, or CO2 is promoted by nickel. Therefore, it is expected that water produced in the system may react with reactive surface carbon to give H2, CO, and CO2 as follows:

C + H2O f CO + H2

(7)

C + 2H2O f CO2 + 2H2

(8)

C + CO2 f 2CO

(9)

Verykios17

Zhang and found that carbon originating from the CH4-CO2 mixture on Ni/La2O3 catalyst may be active, promoting the reaction via the participation of itself, or by its interaction with a component migrated from the La2O3 carrier. Solymosi et al.49 found that (58) Zhang, Z.; Verykios, X. E. Catal. Today 1994, 21, 589-595. (59) Masai, M.; Kado, H.; Miyake, A.; Nishiyama, S.; Tsuruya, S. Stud. Surf. Sci. Catal. 1988, 36, 67-71. (60) Tsipouriari, V. A.; Efstathiou, A. M.; Verykios, X. E. Catal. Today 1994, 21, 579-587. (61) Erdohelyi, A.; Cserenyi, J.; Papp, E.; Solymosi, F. J. Catal. 1993, 141, 287-299.

Table 5. Parameters of Power Law Equationa catalyst

m

n

Ea (kJ/mol)

ref

Ni/SiO2 Ni/Al2O3 Ni/Al2O3 Ni/SiO2 Rh/Al2O3 Ni/Al2O3 Pd/SiO2 Ni/SiO2 Ni/MgO Ru/MgO Rh/MgO Pd/MgO Ir/MgO Pt/MgO Rh/Al2O3

0.8 1.0 1.0 0.02-0.05 0.02-0.09 0.6 0.47 -0.30 1.0 1.0 1.0 1.0 1.0 1.0 0-1

0 0(1)b 0 0.5-0.6 0.4-0.5 0.3 0.36 0.16 0 0 0 0 0 0 0-1

40.1 43.7 76.0 63.0

36 36 36 50 50 43 56 40 19 19 19 19 19 19 21

92.4 141.2

a R ) KP m P n b CH4 CO2 . For pressure lower than 8.4 kPa, the reaction order is almost 1; for pressure in the range of 8.4-67.6 kPa the reaction order is 0.

Table 6. Comparison of Experimental Results with Model Calculations21 conversion (%) product (mol %) (dry) CH4 CO H2 CO2

exptl

model

equilibrium

70.5

68.3

72.7

8.7 45.4 37.8 8.1

9.6 44.3 36.6 9.5

8.0 46.2 38.6 7.2

carbon deposits formed on selected platinum group metals reacted toward hydrogenation. Their results indicated that the decomposition of methane leads to the formation of more reactive carbon on Rh than other Pt group metal surfaces. Sacco et al.37 reported that the primary source of surface carbon on Ni is CH4, whereas on Fe and Co it appears to be CO. The carbon from CH4 route is more active than that from CO, resulting in higher activity of Ni and less coking. Rudnitskii et al.62 studied the kinetics of the carbon formation in CO2:CH4 mixture containing 17-25% CH4 catalyzed by Ni/Al2O3. It exhibited that carbon deposition started at 720-770 K and increased as the temperature was raised. Above 920-970 K the carbon began to disappear. Kim et al.14 also studied the properties of carbons deposited on the catalysts and found that coke formation increased as the reaction time and the amount of Ni on the support increased. Mark and Maier63 studied the behavior of the surface carbon formed in CO2/CH4 reaction on Rh/Al2O3 catalyst and found that 90% of the surface carbon arising from the decomposition of methane on Rh catalyst is highly reactive and reacts extremely fast with CO2 to give CO. They proposed that the decisive factor for carbonizationfree CO2 reforming is a rapid reaction of the active carbon with CO2, which avoids a high surface coverage of carbon. Such condition can only be achieved when both reactants are added simultaneously. Reaction Mechanism and Kinetics The reaction of CO2 with CH4 is expected to proceed by the following steps: (1) dehydrogenation of methane to form surface carbon and hydrogen, (2) dissociative (62) Rudnitskii, L. A.; Solboleva, T. N.; Alekseev, A. M. Reac. Kinet. Catal. Lett. 1984, 26, 149-151. (63) Mark, M. F.; Maier, W. F. Angew. Chem., Int. Ed. Engl. 1993, 33, 1657-1660.

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Wang et al.

adsorption of CO2 and H2, and (3) reduction of CO2 to CO. It has been proposed that the possible mechanism for the reaction of CO2 with CH4 as follows:

CH4 f C(a) + 4H(a)

(10)

CO2(g) f CO(a) + O(a)

(11)

C(a) + O(a) f CO(a)

(12)

CO(a) f CO(g)

(13)

2H(a) f H2(g)

(14)

The mechanism of CO2 reforming may not differ significantly from steam reforming.19 A simplified reaction sequence for the CO2 reforming may involve the following two irreversible steps, namely, the activation of methane followed by the surface reaction with adsorbed oxygen atoms:

with temperature. Previous investigations of the reaction between CH4 and CO2 mainly dealt with the screening test of catalysts. Little research has focused on the reaction mechanism and the kinetics. Therefore, no general expression has so far been derived. Some researchers studied the reforming kinetics and obtained the reaction rate expressed approximately by a simple power-law equation (Table 5). Sakai et al.50 considered that the half-order and zero-order dependence of the reaction rates on the partial pressure of CO2 or CH4 suggested that CO2 participates in the reaction via the dissociative adsorption mechanism, being described as CO2 ) CO + O, whereas CH4 participates in the reaction as strongly adsorbed species dehydrogenated to CHx and (4 - x)H. Richardson and Paripatyadar13 studied the CO2/CH4 reforming kinetics at a pressure of 1 atm. Using linear regression analysis, a rate equation was obtained as

CH4 + * f CH3* + H*

(15)

R ) KrKCO2KCH4PCO2PCH4/(1 + KCO2PCO2 + KCH4PCH4)2

CH3* + * f CH2* + H*

(16)

CH2* + * f CH* + *

(17a)

Calculated and measured rates were correlated with a regression coefficient of 0.988. The data from a typical run are shown in Table 6. Agreement between experiment and model fitting is quite good.

CH* + * f C* + H* CHx* + O* f CO + (x/2)H2 + 2*

(17b)

CO2 + * f O* + CO

(19)

CH4 f CH3 + H

(20)

CO2 + H f CO + OH

(21)

CH4 + O f CH3 + OH

(22)

CH3 f CH2 + H

(23)

CH2 f CH + H

(24)

CHx f C + xH

(25)

CHx + O f CO + xH

(26)

CHx + CO2 f 2CO + xH

(27)

2H f H2

(28)

2OH f H2O

(29)

(18)

Experiments proved that methane promotes the dissociation of carbon dioxide on catalysts. The promotion of the dissociation of carbon dioxide is attributed to the effect of hydrogen in the decomposition of methane. It has been demonstrated that a small amount of hydrogen can greatly facilitate this process.56,61 Assuming the above effect, Erdohelyo et al. proposed the reaction mechanism of CO2 + CH4 on Rh and Pd catalysts as follows:

The kinetics for CO2 reforming depends on the type of catalyst and probably the mechanism is changing

Conclusion CO2 reforming of CH4 to produce syngas is a very important reaction with relevance to the chemical industry and environment. It has a great potential to be used as energy transformation and storage system due to its reversibility (methanation), thus offering effective vehicles for transporting and storing solar and nuclear energies. The thermodynamics of the reaction dictates carbon formation in the catalysts resulting in catalyst deactivation. This key problem, however, can be alleviated by using noble metals as catalysts. However, the cost and availability of noble metals limit the application of this reaction. Ni-based catalysts impregnated on certain supports also show high catalytic activity without coking and thus deserve more attention from the economic point of view. Based on research reported in the literature, to avoid catalytic coking, the operating temperature must be in an optimal range (for CH4/CO2 ratio of 1, 870-1040 °C). It is shown that alkali or alkaline oxides of high basicity can prevent the formation of carbon. To develop effective catalysts for CO2 reforming of CH4 with no coking and accelerate the commercial application of the reaction, the following areas deserve attention for future research: (1) selection of optimal metals and supports including alloys, and the effect of their interaction on catalyst activity; (2) the effect of different promoter such as alkaline oxides on catalyst activity; (3) reaction mechanism and kinetics; and (4) pilot reactor performance and scale-up operation. EF950227T

Carbon Dioxide Reforming of Methane To Produce Synthesis Gas

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