Conversion of Greenhouse Gases to Synthesis Gas and Higher

such as discharge power, wall temperature, flow rate, mixing ratio of methane to carbon dioxide, and temporal stability were studied. The conversion o...
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Energy & Fuels 2001, 15, 395-402

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Conversion of Greenhouse Gases to Synthesis Gas and Higher Hydrocarbons Kui Zhang,† Ulrich Kogelschatz,‡ and Baldur Eliasson*,‡ ABB Corporate Research Ltd. 5405 Baden-Dattwil, Switzerland Received July 18, 2000. Revised Manuscript Received December 4, 2000

The reaction of methane with carbon dioxide to produce synthesis gas (H2 + CO), gaseous hydrocarbons (C2-C4), and higher hydrocarbons was investigated over Quartz fleece, zeolite X, zeolite HY, and zeolite NaY catalysts promoted by dielectric-barrier discharges (DBDs) at low temperature and ambient pressure. Zeolite NaY is the most promising catalyst for producing synthesis gas (H2 + CO) and liquid hydrocarbons (C5+) with high methane and carbon dioxide conversions. The important variables affecting the activity and selectivity of zeolite NaY catalyst such as discharge power, wall temperature, flow rate, mixing ratio of methane to carbon dioxide, and temporal stability were studied. The conversion of CH4 was 67%, and that of CO2 was 40%. The yield of synthesis gas was 47%, and the selectivity to liquid hydrocarbons (C5+) was 34% when the reaction was performed at a wall temperature of 423 K, gas pressure of 1 bar, molar ratio of CH4 to CO2 of 1, feed gas flow rate of 200 mL/min, and input power of 500 W. The present study shows that zeolite NaY has potential application in the production of synthesis gas (H2 + CO) and liquid hydrocarbons (C5+) in a dielectric-barrier discharge reactor at low temperature and ambient pressure.

Introduction The consumption of all fossil fuels (i.e., coal, natural gas and oil) leads to CO2 formation. CO2 itself has until now little value and contributes approximately 75% to the man-made greenhouse effect.1 In addition to causing an increase in the global mean temperature, CO2 emission also contributes an extensive waste of natural carbon source. Emission control and utilization of carbon dioxide are challenges to mankind. Carbon dioxide fixation has attracted much attention, due to environmental considerations and its large scale availability.2 Baiker3 has reviewed recent progress on the catalytic conversion of carbon dioxide to valuable chemicals. Catalysis, either homogeneous or heterogeneous, is a promising approach to CO2 fixation. One of the ideas investigated presently is to remove CO2 from flue gas of power plants and use it in a catalytic process to synthesize useful chemicals.4,5 * Corresponding author. Email: [email protected]. † On leave from School of Chemical Engineering at Tianjin University, Tianjin 300072, P. R. China. ‡ ABB Corporate Research Ltd. (1) Eliasson, B.; Simon, F. G.; Egli, W. Hydrogenation of CO2 in a Silent Discharge. In Non-thermal Plasma Techniques for Pollution Control; Penetrante, B. M., Schultheis, S. E., Eds.; NATO ASI Series; Springer-Verlag: Berlin, 1993; Vol. G34, part B, pp 321-337. (2) Halmann, M. Chemical Fixation of Carbon Dioxide: Methods for Recycling CO2; CRC Press: Boca Raton, FL, 1993. (3) Baiker, A. Catalytic Conversion of Carbon Dioxide to Valuable Chemicals. In Creenhouse Control Technologies; Riemer, P., Eliasson, B., Wokaun, A., Eds.; Elsevier Science Ltd.: New York, 1999; pp 391396. (4) Eliasson, B. CO2 Chemistry: An Option for CO2 Emission Control. In Carbon Dioxide Chemistry: Environmental Issues; Paul, J., Pradier, C. M., Eds.; The Royal Society of Chemistry: Cambridge, 1994; pp 5-15. (5) Eliasson, B.; Egli, W.; Kogelschatz, U. Pure Appl. Chem. 1994, 66, 1275-1286.

Methane is the second most important gas responsible for the greenhouse effect. It contributes 23% of the total greenhouse forcing.6 Methane is the main constituent of natural gas. The conversion of natural gas and, in particular, its principal component, methane, to other useful products has been the subject of intense study over the past decade. Many authors have reviewed the progress on the direct conversion of methane to oxygenates and higher hydrocarbons, with particular attention on the production of methanol and ethene.7-12 At present, commercial processes for the conversion of methane to useful products are all indirect processes in which methane is first converted to synthesis gas (syngas) and a mixture of CO and H2. Syngas is converted to fuels by the Fischer-Tropsch process and to various chemicals, especially methanol and gasoline via the MTG process.13 Syngas is also the main source of hydrogen for refinery processes and ammonia synthesis. The principal routes for the conversion of methane to syngas include: steam reforming (H2/CO > 3), CO2 reforming (H2/CO < 1), and partial oxidation (H2/ CO < 2).1414(i) Steam reforming

CH4 + H2O ) CO + 3H2 - 229.7 kJ/mol

(1)

(6) Houghton, J. Global Warming-The Complete Briefing, 1st ed.; Lion Publishing: Oxford, 1994; pp 29. (7) Jones, C. A.; Leonard, J. J.; Sofranko, J. A. Energy Fuels 1987, 1, 12. (8) Parkyns, N. D. Chem. Ber. 1992, 841. (9) Fox, J. M. Catal. Rev.sSci. Eng. 1993, 35 (2), 169. (10) Lee, J. S.; Oyama, S. T. Catal. Rev. -Sci. Eng. 1988, 30(2), 249. (11) Pitchai, R.; Klier, K. Catal. Rev.sSci. Eng. 1986, 28, 13. (12) Parkyns, N. D.; Warburton, C. I.; Wilson, J. D. Catal. Today 1993, 18, 385. (13) Chang, C. D. Stud. Surf. Sci. Catal. 1988, 36, 127. (14) Pusakas, I. CHEMTECH 1995, 43-49.

10.1021/ef000161o CCC: $20.00 © 2001 American Chemical Society Published on Web 02/01/2001

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is a mature technology and the conventional process for the production of syngas from natural gas and other feedstocks. This is a highly endothermic reaction, resulting in a relatively costly and energy-intensive generation of syngas. Steam reactors generally run with larger than stoichiometric amounts of steam in order to prevent the deposition of carbon on the catalyst. This results in a H2/CO ratio in the syngas in the range of 3.4-5.0, which is much higher than the ratio required for the Fischer-Tropsch process or methanol synthesis. The ratio can be adjusted to the required value through the water gas shift reaction

CO + H2OdCO2 + H2

(2)

or the purging of hydrogen for use as a supplemental fuel, or a combination of the two. (ii) The reaction of methane with carbon dioxide

CH4 + CO2 ) 2CO + 2H2 - 247 kJ/mol

(3)

produces syngas with a H2/CO ratio close to 1.0, which has higher energy efficiency. Like steam reforming, this reaction is also strongly endothermic (247 kJ/mol). It can be utilized to transfer and store energy in the form of CO and H2. Although the conversion of CO2 and CH4 to syngas has potential applications in industry and also has environmental advantages, it has only occasionally been commercialized. A major problem in the reaction is the deactivation of the catalyst because of the deposition of carbon under reaction conditions. (iii) The partial oxidation of methane by oxygen

CH4 + 0.5O2 ) CO + 2H2 + 38 kJ/mol

(4)

generates syngas with a CO/H2 ratio close to 2.0, the ideal ratio for the Fischer-Tropsch process and methanol synthesis. It is a slightly exothermic reaction. Without catalyst, the reaction proceeds only at very high temperatures. The fast reaction of methane with oxygen results in the complete oxidation products of H2O and CO2, which is not desirable because expensive hydrogen is converted to water. So far, the partial oxidation of methane to syngas has been restricted to laboratory studies. The dielectric-barrier discharge (DBD), commonly used for producing a nonequilibrium plasma at atmospheric pressure, is an effective tool for generating energetic electrons. The temperature of the electrons is from 10 000 to 100 000 K, while the actual gas temperature remains near ambient temperature. Through electron impact ionization, dissociation, and excitation of the source gases, active radicals and ionic and excited atomic and molecular species are generated, which can initiate plasma chemical reactions. DBDs present a mature technology used to produce ozone on an industrial scale.15 The fundamental aspects of nonequilibrium plasma phenomena have been discussed in the literature.16-18 A great advantage of the silent discharge over (15) Kogelschatz, U.; Eliasson, B. Ozone Generation and Applications. In Handbook of Electrostatic Process; Chang, J. S., Kelly, A. J., Crowley, J. M., Eds.; Marcel Dekker: New York, 1995; pp 581-605. (16) Eliasson, B.; Egli, W.; Kogelschatz, U. Pure Appl. Chem. 1994, 66 (6), 1279-1286. (17) Eliasson, B.; Kogelschatz, U. IEEE Trans. Plasma Sci. 1991, 19 (2), 309-323.

other discharges is that the average energy of electrons can be influenced by changing either gas pressure (or gas density) or gap width. Thus, in a simple way, the chemical process under investigation can be optimized. In principle, nonisothermal cold plasmas offer a range of potential advantages compared to conventional thermal activation and synthesis methods and should be able to compete with conversional heterogeneous and homogeneous catalytic processes.19 Kogelschatz et al.20 recently reviewed the novel applications of DBDs. Investigations on the destruction of hazardous chemicals such as NOx and SOx21,22 from flue gases, H2S, and NH323 and treatment of volatile organic compounds24-26 have been conducted. Recently, several research works on methane conversion for producing carbon and hydrogen,27 higher hydrocarbons,28-30 oxygenates,31-34 and syngas have been performed.35,36 Eliasson et al.34 investigated the hydrogenation of CO2 to methanol in a dielectric-barrier discharge with and without catalyst in the DBD gap. Comparison of experiments shows that dielectric-barrier discharge can effectively lower the temperature range of optimum catalyst performance. The simultaneous presence of the discharge shifts the temperature range of maximum (18) Venugopalon, M.; Veprek, S. Kinetics and Catalysis in Plasma Chemistry. In Topic in Current Chemistry: Plasma Chemistry IV; Boschke, F. L., Ed.; Springer-Verlag: New York, 1983. (19) Badyal, J. P. S. Top. Catal. 1996, 3, 255-264. (20) Kogelschatz, U.; Eliasson, B.; Egli, W. J. Phys. IV 1997, 7, C447-C4-46. (21) Masuda, S.; Nakao, H. IEEE Trans. Ind. Appl. 1990, 26, 374. (22) Higashi, M.; Uchida, S.; Suzuki, N.; Fujii, K. IEEE Trans. Plasma Sci. 1992, 20, 1. (23) Chang, M. B.; Tseng, J. Environ. Eng. 1996, 122, 41. (24) Rosacha, L. A.; Anderson, G. K.; Bechtold, L. A.; Coogan, J. J.; Heck, H. G.; Kang, M.; McCulla, W. H.; Tennant, R. A.; Watnuck, P. J. Treatment of Hazardous Wastes Using Silent Discharge Plasmas. In Non-thermal Plasma Technique for Pollution Control; Penetrante, B. M., Schultheis, S. E., Eds.; NATO ASI Series; Springer-Verlag: Berlin, 1993; Vol. G34, Part B. (25) Yamamoto, T.; Lawless, P. A.; Ramanathan, K.; Ensor, D. S.; Ramsey, G. H.; Plaks, N. Application of Corona-Induced Plasma Reactors to Decomposition of Volatile Organic Compounds. In Proceedings from the Eighth Particle Control Conference; Electric power research institute report No. EPRI GS-7050; Sec. 10, 1-11. (26) Yamamoto, T.; Ramanathan, K.; Lawless, Phil A.; Ensor, David S.; Newsome, J. R.; Olaks, N.; Ramsey, G. H. IEEE Trans. Ind. Appl. 1992, 28 (3), 528-533. (27) Babariotskii, A. I.; Deminskii, M. A.; Demkin, A. I.; Zhivotov, V. K.; Potapkin, B. V.; Poteknin, S. V.; Rusanov, V. D.; Ryazantsev, E. I.; Etievan, C. Khim. Vysokikh Energii 1999, 33 (1), 49-56. (28) Liu, C.; Mallinson, R.; Lobban, L. J. Catal. 1998, 179, 326334. (29) Liu, C.; Mallinson, R.; Lobben, L. Appl. Catal., A 1997, 178 (1), 17-27. (30) Thanyachotpaiboon, K.; Chavadej, S.; Caldwell, T. A.; Lobban, L.; Mallinson, R. G. Kinet. Catal. 1998, 44 (10), 2252-2257. (31) Bhatnager, R.; Mallinson, R. G. The Partial Oxidation of Methane under the Influence of an AC Electric Discharge. In Methane and Alkane Conversion Chemistry; Bhasin, M. M., Slocum, D. N., Eds.; Plenum: New York, 1995; pp 249. (32) Larkin, D. W.; Caldwell, T. A.; Lobban, L.; Mallinson, R. G. Energy Fuels 1998, 12 (4), 740. (33) Kozlov, K. V.; Michel, P.; Wagner, H.-E. Synthesis of Organic Compounds from CH4-CO2 Mixtures in Barrier Discharges with Different Dielectric Materials. In 14th International Symposium on Plasma Chemistry; Proceedings of the 14th International Symposium on Plasma Chemistry, Prague, Czech Republic, Aug 2-6, 1999; Hrabovsky, M., Konrad, M., Kopecky, V., Eds.; Institute of Plasma Physics: Praque, Czech Republic, 1999; Vol. 4, pp 1849-1854. (34) Eliasson, B.; Kogelschatz, U.; Xue, B.; Zhou, L. Ind. Eng. Chem. Res. 1998, 37, 3350-3357. (35) Zhou, L.; Xue, B.; Kogelschatz, U.; Eliasson, B. Energy Fuels 1998, 12, 1191-1199 (36) Kogelschatz, U.; Zhou, L.; Xue, B.; Eliasson, B. Production of Synthesis Gas through Plasma Assisted Reforming of Greenhouse Gases. In Greenhouse Gas Control Technologies; Eliasson, B., Riemer, P., Wokaun, A., Eds.; Elsevier Science: New York, 1999; pp 385-390.

Conversion of Greenhouse Gases to Synthesis Gas

catalyst activity from 493 to 373 K, a much more desirable temperature range. On the other hand, the presence of the catalyst increases the methanol yield and selectivity by more than a factor of 10 in the discharge. Kogelschatz and Zhou35,36 found that DBDs are an efficient tool for converting the greenhouse gases CH4 and CO2 to synthesis gas (syngas H2/CO) at low temperature and ambient pressure. Syngas produced in this system can have an arbitary H2/CO ratio, mainly depending on the mixture ratio of CH4/CO2 in the feed gas. Specific electric energy, gas pressure, and temperature hardly influence syngas composition. The amount of syngas produced strongly depends on the electric energy input. They also found that high reaction temperatures lead to wax formation and carbon deposition. Low gas pressures favor the formation of syngas. Zeolites are crystalline compounds built from AlO4 and SiO4 tetrahedra which are interlinked through common oxygen atoms to give a three-dimensional network through which long channels run. The interior of the pore system, with its atomic-scale dimensions, is the catalytically active surface of zeolites. The inner pore structure depends on the composition, the zeolite type, and the cations. The accessibility of the pores for molecules is subject to definite geometric or steric restrictions. The intensity of the natural Coulombic electric field in zeolite structure reaches 1 V/Å, which can lead to a charge-based selectivity in zeolites.37 The electric properties of a zeolite can be affected by the structure, the SiO2 to Al2O3 ratio, ion exchange, and other modification methods. Zeolites as catalysts have been used in organic catalysis and many industrial processes38-40 because of their special catalytic properties. Zeolites have been found to be active for methane conversion to produce more valuable hydrocarbons.41-47 Greenhouse gas conversion promoted by plasma has not been studied in detail over zeolite catalysts, except for a few papers reporting on the plasma catalytic methane conversion to higher hydrocarbons and acetylene28,29 and the direct conversion of methane and carbon dioxide to higher hydrocarbons over zeolites.48 The objective of the present study is to investigate the effect of zeolite catalysts on the CO2 reforming of (37) Chen, N. Y.; Weisz, P. B. AIChE Symp. Ser. 1967, 73, 86. (38) Espeel, P.; Parton, R.; Toufar, H.; Martens, J.; Holderich, W.; Jacobs, P. Zeolite Effects on Organic Catalysis. In Catalysis and Zeolites-Fundamentals and Applications; Weitkamp, J., Puppe, L., Eds.; Springer: New York, 1999; pp 377-436. (39) Blauwhoff, P. M. M.; Gosselink, J. W.; Kieffer, E. P.; Sie, S. T.; Stork, W. H. J. Zeolites as Catalysts in Industrial Process. In Catalysis and Zeolites-Fundamentals and Applications; Weitkamp, J., Puppe, L., Eds.; Springer: New York, 1999; pp 437-538. (40) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape Selective Catalysis in Industrial Applications, 2nd ed.; Marcel Dekker: New York, 1996 (41) Shepelev, S. S.; Ione, K. G. React. Kinet. Catal. Lett. 1983, 23, 319. (42) Han, S.; Martenak, D. J.; Palermo, R. E.; Pearson, J. A.; Walsh, D. E. J. Catal. 1994, 148, 134. (43) Wang, L. S.; Xu, Y. D.; Wong, S. T.; Cui, W.; Guo, X. X. Appl. Catal., A 1997, 152, 173 (44) Chen, L. Y.; Liu, L. W.; Xu, Z. S.; Li, X. S.; Zhang, T. J. Catal. 1995, 157, 190. (45) Choudhary, V. R.; Kinage, A. K.; Choudhary, T. V. Science 1997, 275, 1286. (46) Guczi, L.; Sarma, K. V.; Borko, L. Catal. Lett. 1996, 39, 43. (47) Rigby, A. M.; Kramer, G. J.; van Santen, R. A. J. Catal. 1997, 1, 170. (48) Eliasson, B.; Liu, C.; Kogelschatz, U. Ind Eng. Chem. Res. 2000, 39, 1221-1227.

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CH4 for producing syngas and liquid hydrocarbons promoted by DBDs at ambient conditions and to demonstrate the feasibility of the reaction over zeolite NaY for producing synthesis gas and liquid hydrocarbons (C5+). The present study shows that zeolite NaY has potential application in the production of syngas and liquid hydrocarbons from CH4 and CO2 under DBD conditions at relatively lower temperatures and ambient pressure. Experimental Section 1. Material. Zeolite NaX (VEGOBOND, SiO2/Al2O3 ) 2.65 (mole ratio), surface area (m2/g) ) 680) was obtained from Condea Augusta SPA Research Center and zeolite NaY from Linde company (LZY-62, SiO2/Al2O3 ) 4.7 (mole ratio), surface area (m2/g) ) 635). The zeolites were calcined in air by heating at 16 K/min to 773 K for 4 h to remove the water adsorbed in the pores of zeolite before use. Zeolite HY was prepared from the sodium form by ammonium exchange with 1 M NH4C1 solution at 363 K for 4 h. The operation was repeated 4 times, and the solid was washed with distilled water until it became C1- free. The product was dried overnight at 423 K and then calcined at 823 K for 4 h. The degree of H+ exchange was 90%. The quartz fleece used to hold the catalyst powder was bought from Swiss Composite Shop and was untreated before use. 2. Catalytic Reactions. The experimental setup used in the experiment was a cylindrical dielectric-barrier discharge (DBD) reactor, the same as that described before.34-36 The outer steel cylinder served as the ground electrode. An alternating sinusoidal high voltage of up to 20 kV amplitude (peak to peak) and about 30 kHz frequency was applied to the center electrode, which was connected to a metal brush pressing a metal foil against the inner surface of the quartz tube. The power supply (Arcotec corona generator CG 20) could feed between 50 and 500 W into the discharge reactor by adjusting the amplitude (and frequency) of the applied voltage. The power dissipated in the discharge was measured by electronically integrating the product of voltage and current. In addition, a LeCroy Model LC 334A oscilloscope was used to record the voltage-charge Lissajous diagrams. The discharge was maintained in an annular discharge gap of 1 mm radial width and 310 mm length, formed by the outer steel cyclinder of 54 mm id and an inserted cyclindrical quartz tube of 52 mm od and 2.5 mm wall thickness, giving a discharge volume of of about 50 mL (without catalyst). To hold the catalyst in discharge gap, we distributed 4.0 g of zeolite powder equally on the surface of a quartz fleece first and then we wrapped the quartz fleeces around the outer surface of the quartz tube, which was put into the steel tube. Prior to the reaction, the catalyst was treated at 473 K in a carbon dioxide flow of 100 mL/min to clean the surface of zeolite catalyst. The feed gases, CH4, and CO2, were introduced into the reactor from high-pressure bottles via mass flow controllers (MFCs), admitting a total gas flow of 200 to 600 mL/min. A back pressure valve at the exit of the dielectric barrier reactor was used to regulate the pressure in the reactor between 1 to 5 bar. The preselected temperature of the reactor could be maintained by a closed loop of recirculating oil from a thermostat. A MTI (Microsensor Technology Inc.) dual-module micro gas chromatograph (MTI 200H) with a TCD (thermal conductivity detector) was used to analyze the gaseous products and methanol as well as H2O using a Poraplot Q column and a molecular sieve 5 Å plot column. It was connected to the outlet of the reactor by heated tubing (423 K) in order to avoid condensation. The liquid products were collected in a dry ice

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Table 1. Catalytic Activities and Product Selectivity of Zeolite Catalysts catalysta gas pressure (bar) wall temperature (K) CH4/CO2 (molar ratio) flow rate (ml/min) discharge power (W) conversion (mol %) CH4 CO2 yield (mol %) H2 CO C2-C4c-e H2O carbon number selectivity (mol %) CO CH3OH C2c C3d C4e C5+f

fleeceb

NaX

1 423 1 200 500

1 423 1 200 500

HY 1 423 1 200 500

NaY 1 423 1 200 500

68.4 39.7

60.9 38.0

62.9 36.9

66.6 39.9

18.1 24.9 4.8 5.3

18.9 20.6 4.7 5.1

21.6 20.9 6.7 4.6

25.7 22.8 4.5 7.0

46.0 0.2 9.2 5.2 10.4 29.7

41.9 0.1 10.2 5.3 10.5 32.1

41.8 0.2 10.7 5.5 24.4 17.5

42.7 0.1 9.1 4.2 9.9 34.1

a Zeolite amount: 4.0 g. b Only quartz fleece. c C H + C H + 2 6 2 4 C2H2. d C3H8 + C3H6. e C4H8 + C4H10. f Selectivity to C5+ and other oxygenates from carbon balance.

trap (after GC) for further analysis. All results are reported in moles percent. The mass balance of the reaction was obtained by adding a controlled flow of nitrogen as reference gas to the exit of the reactor in order to monitor the change of volume flow in reaction. The conversion of methane and carbon dioxide is defined as

conversion (reactant)i ) {[(reactant)i(in) - (reactant)i(out)]/ (reactant)i(in)} × 100% i ) CH4, CO2 The yield of products is defined as

yield (mol %)i ) product]i/[(CH4)in + (CO2)in] i ) H2, CO, C2-C4*, H2O C2-C4* ) C2H2 + C2H4 + C2H6 + C3H6 + C3H8 + C4H10 + C4H8 The selectivity of products is defined as

selectivity (product)i ) {[(number of carbon) × (product)i]/(carbon number converted)} × 100% i ) CO, C2H2, C2H4, C3H6, C3H8, C4H10, C4H8, CH3OH

Results and Discussions 1. Catalytic Activity. For a better understanding of the influence of zeolite catalysts on methane and carbon dioxide conversion under plasma conditions, we performed experiments at the reaction temperature of 423 K, gas pressure of 1 bar, molar ratio of methane to carbon dioxide of 1, total flow rate of 200 mL/min, and input power of 500 W. The results of CO2 reforming CH4, promoted by dielectric-barrier discharge using zeolite NaY, HY, NaX, and fleece as catalysts, are summarized in Table 1. Under plasma conditions, all catalysts we used exhibited high methane conversion (over 60%) and carbon dioxide conversion (over 37%). The highest methane

conversion reached 68% over the fleece. Experimental results showed that methane conversion decreases in the order fleece (without catalyst) > NaY > HY > NaX. The highest CO2 conversion reached 40% when using zeolite NaY as the catalyst. CO2 conversion decreases in the order NaY > fleece >NaX > HY. The products in the reaction include synthesis gas (H2 + CO), H2O, methanol, gaseous hydrocarbons (C2-C4), and higher hydrocarbons (C5+). Synthesis gas (H2 + CO), H2O, methanol, and gaseous hydrocarbons (C2C4) were measured with online gas chromatograph (MTI 200H). Higher hydrocarbons (C5+) were collected in a dry ice trap (after GC), weighed, and analyzed. The liquid products collected contain C5 to C11 hydrocarbons and some oxygenates. Compared to the results from fleece, only zeolite NaY is beneficial to enhancing the yield of synthesis gas (H2 + CO), while zeolite HY is helpful to increasing the yield of gaseous hydrocarbons (C2-C4) in the reaction. The yield of synthesis gas follows the order NaY > fleece > HY > NaX. According to the results of carbon number selectivity, the selectivity to CO was much higher than that of other products over all catalysts under plasma conditions, indicating that the carbon dioxide reforming of methane promoted by DBDs over zeolite catalyst can be used for producing synthesis gas. It is interesting to note that the C4 selectivity over zeolite HY reached 24%, indicating that zeolite HY is beneficial to enhancing the selectivity to C4 in the reaction. Experimental results listed in Table 1 show that carbon number selectivity to C5+ over zeolite NaY is the highest among all catalysts at high methane and carbon dioxide conversion. Higher hydrocarbon (C5+) selectivity over zeolite NaY reached 34%. Higher hydrocarbon (C5+) selectivity over different catalysts follows the order NaY > NaX > fleece > HY, indicating that zeolite NaY plays an important role in DBDs in obtaining high selectivity to synthesis gas and higher hydrocarbons (C5+). In the reaction, a film of solid materials was formed on the external surface of the quartz tube due to plasmainduced polymerization and the formation of wax when using fleece as the catalyst. DTA analysis result revealed that the formation of polymer materials and wax in the reaction was strongly suppressed by using zeolite NaY as the catalyst. Zeolite NaY is beneficial to the reaction under plasma conditions. Zeolite NaY has a higher SiO2 to Al2O3 molar ratio than zeolite NaX and contains a framework system of supercages, which is connected by a three-dimensional array of large diameter channels and which could provide a suitable electric field to affect the reactions under plasma conditions, thereby favoring high activity, higher hydrocarbon selectivity to C5+, and high synthesis gas yield. All the above experiments indicated that NaY was the most promising catalyst in producing synthesis gas and higher hydrocarbons (C5+) for the reaction under our experimental conditions, and more detailed studies were carried out on this catalyst, as described below. 2. Temporal Stability. Figure 1 shows the conversion of methane and carbon dioxide and the yield of CO, gaseous hydrocarbons (C2-C4), H2O, and H2 over zeolite the NaY catalyst as a function of time at 423 K, a flow

Conversion of Greenhouse Gases to Synthesis Gas

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Table 2. Temporal Stability of CO2 Reforming of CH4a selectivities (mol %) time (min)

CO

CH3OH

C2H2 + C2H4

C2H6

C3H6 + C3H8

C4H8

C4H10

i-C4H10

C5+*

50 100 200 300

42.7 41.3 41.1 40.1

0.09 0.08 0.08 0.08

0.3 0.4 0.4 0.4

8.8 8.9 8.7 8.8

4.2 4.9 4.8 4.9

1.2 1.2 1.3 1.3

1.3 1.5 1.5 1.6

7.5 9.1 9.2 9.5

34.1 34.1 34.5 34.8

a Pressure, 1 bar; flow rate, 200 ML/min; wall temperature, 423 K; CH /CO in the feed, 1:1; power, 500 W. Selectivity to C + and 4 2 5 other oxygenates from carbon balance.

Figure 1. Temporal stability of CO2 reforming CH4: (a) conversion and (b) yield. Pressure, 1 bar; flow rate, 200 mL/ min; wall temperature, 423 K; CH4/CO2 in the feed, 1:1; power, 500 W. *C2-C4 ) C2 + C3 + C4, C2 ) C2H6 + C2H4 + C2H2C3 ) C3H8 + C3H6, and C4 ) C4H8 + C4H10.

rate of 200 mL/min, a molar ratio of methane to carbon dioxide of 1:1, and an input power of 500 W. As Figure 1a shows, the plasma operates in a stable way. The product yields do not change significantly within the range of times tested (Figure 1b). From Table 2, one can see that the reaction time has no apparent influence on the carbon number selectivity. Results in Figure 1 and Table 2 indicate that the reaction under plasma conditions is stable and the formation of wax and polymer materials has no significant influence on the activity and selectivity of the reaction over the zeolite NaY catalyst under our experimental conditions. 3. Influence of the Discharge Power. In a mixture of CH4/CO2 ) 1 (molar ratio), a total flow rate of 200 mL/min, a reactor wall temperature of 423 K, and a pressure of 1 bar, we studied the effects of power variation on the reaction. Experimental results are depicted in Figure 2. It is clear from Figure 2a that increasing the power in the discharge leads to a substantial increase in the conversion of CH4 and CO2. The conversion of methane was always higher than that of carbon dioxide in the power range tested. The yield of syngas, water, and gaseous hydrocarbons (C2-C4) increases with increasing input power (Figure 2b). In the range tested, syngas is the main product and

Figure 2. Influence of the discharge power on CO2 reforming of CH4: (a) conversion and (b) yield. Pressure, 1 bar; wall temperature, 423 K; flow rate, 200 mL/min; CH4/CO2 in the feed, 1:1. *C2-C4 ) C2 + C3 + C4, C2 ) C2H6 + C2H4 + C2H2C3 ) C3H8 + C3H6, and C4 ) C4H8 + C4H10.

increases significantly with increasing power. For example, if the power was increased from 100 to 500 W, the yield of syngas increased from 12% to 42.6%. The H2/CO molar ratio of around 1 is hardly dependent on the power in the range from 100 to 500 W. This is in agreement with our previous work without using catalysts.36 At 300 W, the yield of water and gaseous hydrocarbons (C2-C4) reached 6% and 4%, respectively, and higher input power cannot increase these yields substantially. Table 3 shows that lower power is beneficial to obtaining higher selectivity of CO, methanol, and gaseous hydrocarbons. The carbon number selectivity to CO, CH3OH, C2, and C3 decreased with increasing discharge power, while the carbon selectivity to C5+ and C4 increased. When the discharge power was raised from 100 to 300 W, C5+ selectivity increased from 15% to 30%, and C4 selectivity increased from 1.1% to 8.2%, while C2 selectivity decreased from 20% to 14%, C3 selectivity decreased from 7.8% to 5.2%, and CH3OH selectivity decreased from 0.2% to 0.1%. High input power (over 300 W) is necessary for generating higher selectivity to higher hydrocarbons (C5+). This means that increased input power converts methanol and light

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Table 3. Influence of the Discharge Power on CO2 Reforming of CH4a selectivities (mol %) power (W)

CO

CH3OH

C2H2 + C2H4

C2H6

C3H6 + C3H8

C4H8

C4H10

i-C4H10

C5+*

100 200 300 400 500

56.7 47.3 42.5 41.8 42.6

0.0 0.2 0.1 0.1 0.09

1.6 0.7 0.5 0.3 0.3

18.1 20.5 13.2 10.4 8.8

7.8 6.5 5.2 4.5 4.2

0.0 0.0 0.0 1.0 1.2

1.1 1.6 1.3 1.3 1.3

0.00 6.7 6.9 7.0 7.5

14.7 16.5 30.2 33.9 34.1

a Pressure: 1 bar; wall temperature, 423 K; flow rate, 200 ML/min; CH /CO in the feed, 1:1. Selectivity to C + and other oxygenates 4 2 5 from carbon balance.

Figure 3. Influence of the mixing ratio on CO2 reforming of CH4: (a) conversion and (b) yield. Pressure, 1 bar; wall temperature, 423 K; flow rate, 200 mL/min; power, 500 W. *C2-C4 ) C2 + C3 + C4, C2 ) C2H6 + C2H4 + C2H2C3 ) C3H8 + C3H6, and C4 ) C4H8 + C4H10.

hydrocarbons such as C2 and C3 into higher hydrocarbons such as C4 and C5+. This also supports a mechanism of higher hydrocarbon chain buildup from methyl and other radicals. The above experiment reveals that the CO2 reforming of CH4 over zeolite NaY depends strongly on discharge power. In the reaction, there is a room for reducing the energy consumption by optimizing catalysts in dielectricbarrier discharge reactors. 4. Influence of the Mixing Ratio. For a better understanding of the reagent composition on the reaction, we performed experiments by varying the molar ratio of methane to carbon dioxide from 0.5 to 3 in the feed gas. The total flow rate of feed gas was 200 mL/ min all the time. A power of 500 W was applied to the feed gas. The gas pressure in the reactor was 1 bar, and the reactor wall temperature was 423 K. Figure 3a illustrates the effects of feed gas composition on the reaction under plasma conditions by using zeolite NaY as the catalyst. With an increase in the molar ratio of methane to carbon dioxide from 0.5 to 3.0, the conversion of methane decreases slightly. The conversion of carbon dioxide reached a maximum (40%)

when the ratio of CH4 to CO2 was 1. At ratios lower than 1, the conversion of methane decreased with increasing molar ratio, while the conversion of carbon dioxide increased. In contrast, at CH4/CO2 ratios above 1, the conversion of methane and carbon dioxide both decreased with increasing molar ratio. It is clear from Figure 3b that the yield of hydrogen and gaseous hydrocarbons (C2-C4) increased with increasing ratio. The ratio of H2/CO increased sharply when the ratio of CH4 to CO2 increased. For example, when the ratio increased from 0.5 to 3, the yield of hydrogen increased substantially from 18% to 40%, and the yield of gaseous hydrocarbons (C2-C4) increased from 2% to 10%. In contrast, increasing the molar ratio lowered the yield of CO and H2O. The molar ratio of H2/CO increased from 0.54 to 2.94 when the CH4/CO2 ratio increased from 0.5 to 3.0. As a consequence, it is possible to control the composition of synthesis gas by adjusting the molar ratio of feed gases. The carbon number selectivity versus molar ratio of CH4 to CO2 in the reaction is listed in Table 4. Increasing the molar ratio leads to an increase of selectivity to gaseous hydrocarbons (C2, C3, C4) and to a decrease of the selectivity to CO. The selectivity increase of gaseous hydrocarbons follows the order C4 > C3 > C2. The selectivity to C5+ reached a maximum (34%) when the molar ratio of methane to carbon dioxide was 1. At molar ratios lower than 1, the carbon number selectivity to higher hydrocarbons (C5+) increased strongly from 25% to 35%. In contrast, increasing the molar ratio further can reduce the selectivity to higher hydrocarbons (C5+). 5. Influence of the Flow Rate. Figure 4 shows the influence of the flow rate on the reaction using zeolite NaY as the catalyst under plasma conditions. Increasing the flow rate reduces the conversion of methane and carbon dioxide quickly in the range of 200 to 600 mL/ min (Figure 4a). The influence of flow rate on the product yields is dipicted in Figure 4b. It is apparent that increasing the flow rate results in a decrease of CO, H2O, and H2 yield quickly at the beginning. When the flow rate increased from 200 to 400 mL/min, the yield of CO decreased from 22.8% to 14.4%, the yield of H2 from 25.7% to 13%, and the yield of H2O from 7% to 4.3%. When the flow rate is above 400 mL/min, the yield of synthesis gas (H2 + CO), H2O and gaseous hydrocarbons (C2-C4) changes more slowly with increasing flow rate from 400 to 600 mL/min. At the same time, increasing the flow rate decreased the yield of gaseous hydrocarbon (C2-C4) from 4.5% to 3.8%. From Figure 4b, it is also possible to control the ratio of H2/CO (molar ratio) from 0.74 to 1.13 by changing the flow rate of feed gases. A lower flow rate is beneficial to producing synthesis gases (H2

Conversion of Greenhouse Gases to Synthesis Gas

Energy & Fuels, Vol. 15, No. 2, 2001 401

Table 4. Influence of the Mixing Ratio of CH4/CO2 on CO2 Reforming of CH4a selectivities (mol %) CH4/CO2 (molar ratio)

CO

CH3OH

C2H2 + C2H4

C2H6

C3H6 + C3H8

C4H8

C4H10

i-C4H10

C5+*

0.5 1 2 3

62.8 42.7 32.7 26.5

0.0 0.09 0.09 0.1

0.0 0.3 0.7 1.0

7.1 8.8 10.2 11.3

2.3 4.2 7.7 10.0

0.0 1.2 2.2 2.8

0.2 1.3 2.3 2.9

2.7 7.5 15.0 19.8

24.8 34.1 29.0 25.6

a Pressure, 1 bar; wall temperature, 423 K; flow rate, 200 mL/min; power, 500 W. Selectivity to C + and other oxygenates from carbon 5 balance.

Table 5. Influence of the Flow Rate on CO2 Reforming of CH4a selectivities (mol %) flow rate (mL/min)

CO

200 400 600

42.7 40.9 45.0

CH3OH

C2H2 + C2H4

C2H6

C3H6 + C3H8

C4H8

C4H10

i-C4H10

C5+*

0.09 0.2 0.2

0.3 0.5 0.9

8.8 12.8 17.4

4.2 4.2 7.5

1.2 0.7 0.0

1.3 1.8 2.1

7.5 10.1 10.8

34.1 28.8 16.1

a Pressure, 1 bar; wall temperature, 423 K; CH /CO in the feed, 1:1; power, 500 W. Selectivity to C + and other oxygenates from 4 2 5 carbon balance.

Figure 4. Influence of the flow rate on CO2 reforming of CH4 (a) conversion and (b) yield. Pressure, 1 bar; wall temperature, 423 K; CH4/CO2 in the feed, 1:1; power, 500 W. *C2-C4 ) C2 + C3 + C4, C2 ) C2H6 + C2H4 + C2H2C3 ) C3H8 + C3H6, and C4 ) C4H8 + C4H10.

+ CO) with higher H2/CO ratios. In contrast, higher flow rate reduces the yield of synthesis gas (H2 + CO) and is helpful to decreasing the molar ratio of H2/CO. Carbon number selectivities are summarized in Table 5. Increasing the flow rate results in a rise in the selectivity to CO (from 42.7% to 45%) and gaseous hydrocarbons (C2, C3, C4). On the other hand, selectivity to C5+ hydrocarbons decreased when the flow rate increased. When the flow rate was above 400 mL/min, the selectivity to C5+ decreased quickly from 28.8% to 16.1%, while the selectivity to C3 increased slowly (from 4.2% to 7.5%) and the selectivity to C4 increased a little (from 12.6% to 12.9%) in the range of 400 to 600 mL/ min. The selectivity to C3 increased almost linearly with

increasing flow rate. The selectivity to methanol increased a little when the flow rate increased. A low flow rate is beneficial to obtaining high selectivity to higher hydrocarbons (C5+). On the other hand, high flow rates are helpful to obtaining high selectivity to CO and gaseous hydrocarbons (C2, C3, C4). From Table 5, it is apparent that the selectivity to CO does not increase much with a significantly decreasing conversion of carbon dioxide when the flow rate from 200 to 600 mL/min increases. CO formation could be explained from the electron impact dissociation or dissociative attachment of CO2 in the plasma and the reverse water gas shift reaction. It can be expected that the newly formed CO is in an excited stage and will be more inclined to further react with other plasma species, e.g., H and C2H3, to produce additional hydrocarbons or oxygenates. In addition to CO, the dissociation reactions of CO2 will also generate some oxygen species. Some excited atomic species such as O(1D) are active species for the generation of methyl radicals from CH4.49 The hydrocarbon chain buildup presumably starts from the formation of methyl radicals. O(1D) is also active for methanol formation from CH4.50 6. Influence of the Wall Temperature. Temperature is one of the most important parameters affecting the speed of reaction in thermal chemical reactions. Equilibrium calculations demonstrate that normal chemical reactions between CH4 and CO2 cannot be expected at temperatures lower than 500 K. For endothermal reactions, normally high temperatures are required to add enthalpy. In a mixture of CH4/CO2 ) 1, a total flow rate of 200 mL/min, an input power of 500 W, and a pressure of 1 bar, we studied the effects of temperature variation on the reaction. Figure 5 shows the results. It is clear from Figure 5a that increasing the wall temperature from 323 to 423 K slightly decreased the conversion of methane from 67% to 66.6% and reduced the conversion of carbon dioxide from 41% to 40%, indicating that wall temper(49) Oumghar, A.; Legrand, J. C.; Diamy, A. M.; Turillon, N. Plasma Chem. Plasma Processing, 1995, 15, 87. (50) Parnis, J. M.; Hoover, L. E.; Pederson, D. B.; Patterson, D. D. J. Phys. Chem. 1995, 99, 13528.

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Table 6. Influence of the Wall Temperature on CO2 Reforming of CH4a selectivities (mol %) wall temperature (K)

CO

CH3OH

C2H2 + C2H4

C2H6

C3H6 + C3H8

C4H8

C4H10

i-C4H10

C5+*

323 373 423

40.0 40.5 42.7

0.1 0.1 0.09

0.2 0.3 0.3

9.1 8.8 8.8

5.4 4.6 4.2

0.0 0.0 1.2

0.9 1.5 1.3

5.8 7.1 7.5

38.4 37.1 34.1

a Pressure, 1 bar; flow rate, 200 ML/min; CH /CO in the feed, 1:1; power, 500 W. Selectivity to C + and other oxygenates from carbon 4 2 5 balance.

hydrocarbons, and methanol slightly. When the temperature increased from 323 to 423 K, the selectivity to C4 hydrocarbons increased from 6.7% to 9.9% and CO from 41.0% to 42.7%; at the same time, the selectivity to higher hydrocarbons (C5+) decreased from 38.4% to 34.1%. In summary, wall temperature does not significantly affect the reaction activity over zeolite NaY catalyst under dielectric-barrier discharge conditions at the temperature range tested. The action of zeolite NaY cannot be determined in the sense of a traditional catalyst in the reaction. 3. Conclusions

Figure 5. Influence of the wall temperature on CO2 reforming of CH4: (a) conversion and (b) uield. Pressure, 1 bar; flow rate, 200 mL/min; CH4/CO2 in the feed, 1:1; power, 500 W. *C2-C4 ) C2 + C3 + C4, C2 ) C2H6 + C2H4 + C2H2C3 ) C3H8 + C3H6, and C4 ) C4H8 + C4H10.

ature does not significantly affect the reaction under plasma conditions. Figure 5b shows the influence of the wall temperature on the yield of CO, H2, H2O, methanol, and gaseous hydrocarbons (C2-C4). Increasing temperature increased the yield of CO from 22.4% to 22.8%, gaseous hydrocarbons (C2-C4) from 4.3% to 4.5%, and H2O from 5% to 7%, while it decreased the yield of H2 from 27.8% to 25.7%. The yield of synthesis gas (H2 + CO), H2O, and gaseous hydrocarbons (C2-C4) does not change significantly within the range of wall temperature tested. The influence of wall temperature on carbon number selectivity is listed in Table 6. Increasing wall temperature leads to the decrease of selectivity to C2, C3,

The reaction of methane with carbon dioxide to produce synthesis gas (H2 + CO), gaseous hydrocarbons (C2-C4), and liquid hydrocarbons (C5+) over quartz fleece, zeolite NaX, zeolite HY, and zeolite NaY promoted by dielectric-barrier discharges at low temperature and ambient pressure was studied. Zeolite NaY is the most promising catalyst for producing synthesis gas (H2 + CO) and liquid hydrocarbons (C5+) with high methane and carbon dioxide conversions. The reaction depends strongly on the input power. Increasing the discharge power at optimum flow rate is beneficial to enhancing the conversion of feed gases, the yield of synthesis gas, and the selectivity to liquid hydrocarbons (C5+). Surprisingly, the composition of synthesis gas (H2/CO) is hardly dependent on power in the range of 100 to 500 W. For high CO2 and CH4 conversion and high selectivity to liquid hydrocarbons (C5+) to be obtained, the optimum molar ratio of CH4 to CO2 is 1:1. It is possible to control the composition of the synthesis gas (H2/CO) by adjusting the molar ratio of feed gases. Increasing the flow rate of feed gases reduces the conversion of CH4 and CO2, the yield of synthesis gas (H2 + CO), and the selectivity to liquid hydrocarbons (C5+). Good temporal stability was observed, and the wall temperature has no decisive influence on the reaction under plasma conditions when using zeolite NaY as the catalyst. Acknowledgment. The assistance of A. Bill, E. Killer, M. Kraus, and B. Xue was greatly appreciated. EF000161O