Thermochemical Conversion of CH4 to C2-Hydrocarbons and H2 over

methane was selectively converted to C2-hydrocarbons and hydrogen, while at a high ... conversion of natural gas to C2-hydrocarbon and hydrogen utiliz...
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Energy & Fuels 2001, 15, 463-469

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Thermochemical Conversion of CH4 to C2-Hydrocarbons and H2 over SnO2/Fe3O4/SiO2 in Methane-Water Co-Feed System T. Shimizu, Y. Kitayama, and T. Kodama* Department of Chemistry & Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan Received September 14, 2000

Highly endothermic conversion of methane to C2-hydrocarbons and hydrogen was catalytically demonstrated over the SnO2/Fe3O4/SiO2 at 1173 K under co-feeding of methane and steam. The main products were C2H4, C2H6, CO, CO2, and hydrogen. At a low W/F value (a high GHSV), methane was selectively converted to C2-hydrocarbons and hydrogen, while at a high W/F value (a low GHSV), steam reforming of methane to produce CO and hydrogen became dominant. At the GHSV ) 4000-12000 h-1, 87-94% of C2 selectivity with 4-6% of methane conversion was obtained at the C2-formation rate ) 4-8 mmol-C h-1 g-1-cat. This process offers the efficient conversion of natural gas to C2-hydrocarbon and hydrogen utilizing high-temperature heat such as concentrated solar radiation below 1173 K.

Introduction Efficient utilization of solar or nuclear high-temperature heat below 1273 K is a current subject for research.1-3 In this point of view, several high-temperature endothermic reactions have been studied, such as multistep water splitting reaction,4-6 methane reforming,7-9 and coal gasification10,11 for high-temperature thermochemical processes utilizing solar or nuclear heat. Optimal operating temperatures for converting concentrated solar radiation into chemical-free energy range from 800-1500 K for a blackbody solar cavityreceiver under peak solar flux intensities between 1000 and 12000 kW m-2.12,13 Our attention has been directed to conversion of methane to C2-hydrocarbons (C2-HCs) and hydrogen which is expressed by the following equation:

2CH4 f C2H8-2n + nH2

(1)

This reaction is highly endothermic and so-called dehydrogenative coupling of methane. If the desired C2* Author to whom correspondence should be addressed. Tel.: +8125-262-7335. Fax: +81-25-262-7010. E-mail: [email protected]. (1) Fletcher, E. A. J. Minn. Acad. Sci. 1983/84, 49, 30-34. (2) Sizmznn, R. Chimia 1989, 7-8, 202-206. (3) Tamaura, Y. Solar Thermal 2000, Proceedings of the 10th SolarPACES International Symposium on Solar Thermal Concentrating Technologies; Kreetz, H., Lovegrove, H. K., Meike, W., Eds.; Meetings Manager Pty Ltd.: Sydney, Australia, 2000; pp 189-192. (4) Nakamura, T. Solar Energy 1977, 19, 467-475. (5) Bilgen, E.; Joels, R. K. Int. J. Hydrogen Energy 1985, 10, 143155. (6) Lundgerg, M. Int. J. Hydrogen Energy 1993, 18, 369-376. (7) Steinfeld, A.; Kuhn, P.; Karni, J. Energy 1993, 18, 239-249. (8) Wo¨rner, A.; Tamme, R. Catal. Today 1998, 46, 165-174. (9) Steinfeld, A.; Brack, M.; Meier, A.; Weidenkaff, A.; Wuillemin, D. Energy 1998, 23, 803-814. (10) Gregg, D. W.; Taylor, R. W.; Campbell, J. H.; Taylor, J. R.; Cotton, A. Solar Energy 1980, 25, 353-364. (11) Flechsenhar, M.; Sasse, C. Energy 1995, 20, 803-810. (12) Fletcher, E. A.; Roger, L. M. Science 1977, 197, 1050-1056. (13) Steinfeld, A.; Schubnell, M. Solar Energy 1993, 50, 19-25.

HC is ethylene, which is an important raw material in the industrial production of commodity organic chemicals or synthetic fuels, the endothermic heat is 202 kJ mol-1-C2H4. Thermodynamic considerations show that the equilibrium conversion to C2-HCs via dehydrogenation exceeds 17% at T > 1173 K.14 In practice, however, it is difficult to approach equilibrium closely under dynamic conditions below 1273 K. To accelerate the oxidation of CH4 to C2-HCs, active surface oxygens of metal-oxide catalysts are necessary for the initial step, i.e., for H2 abstraction from methane. The reaction processes of methane and surface oxygen are represented by

2CH4 + nOS f C2H8-2n + nH2O

(2)

H2O f H2 + OS

(3)

where OS represents the surface oxygen of the original metal-oxide catalyst. In the catalytic reaction, the feeding of steam facilitates replenishment of the surface oxygens by the process of eq 3, especially under dynamic conditions. From this point of view, the conversion of methane to C2-HCs and hydrogen using water as an oxidant has been studied by some investigators in recent years.15-18 Generally oxidative coupling of methane (OCM) with oxygen has been widely studied over various catalysts.14,19-26 This reaction is, however, exothermic and not a calorifically upgrading process, in which hydrogen abstracted is removed as water:

2CH4 + n/2O2 f C2H8-2n + nH2O

(4)

Furthermore, it is frequently true that the selectivity to C2-HCs by this oxidative coupling of methane is not high enough because of the nonselective oxidation of (14) Keller, G. K.; Bhasin, M. M. J. Catal. 1982, 73, 9-19.

10.1021/ef000200w CCC: $20.00 © 2001 American Chemical Society Published on Web 02/27/2001

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methyl radical by oxygen molecules to produce CO2 and CO. Effects of steam on the oxidative coupling of methane with oxygen have been also investigated.25,26 The effect of water was different on each catalyst. In the presence of steam the active Mg-Li-O catalysts were able to facilitate the oxidative coupling of methane to give 18-22% conversion with selectivity to hydrocarbons around 30% at low temperatures 99%) by cycling redox mode in which methane and water were fed alternately (15) Li, X.-H.; Tomishige, K.; Fujimoto, K. Catal. Lett. 1995, 36, 2124. (16) Kodama, T.; Shimizu, T.; Miura, S.; Watanabe, Y.; Kitayama, Y. Energy 1997, 22, 859-866. (17) Omata, K.; Ehara, T.; Kawai, I.; Yamada, M. Catal. Lett. 1997, 45, 245-248. (18) Kodama, T.; Shimizu, T.; Aoki, A.; Kitayama, Y. Energy Fuels 1997, 11, 1257-1263. (19) Sofranko, J. A.; Leonard, J. J.; Jones, C. A. J. Catal. 1987, 103, 302-310. (20) Otuka, K.; Jinno, K.; Morikawa, A. J. Catal. 1986, 100, 353359. (21) Jones, C. A.; Leonard, J. J.; Sofranko, J. A. J. Catal. 1987, 103, 311-319. (22) Lunsford, J. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 970980. (23) Pak, S.; Lunsford, J. H. Appl. Catal. A 1998, 168, 131-137. (24) Murata, K.; Hayakawa, T.; Hamakawa, S.; Suzuki, K. Stud. Surf. Sci. Catal. 1998, 119, 331-336. (25) Chang, Y.-F.; Somorjai, G. A.; Heinemann, H. J. Catal. 1993, 141, 713-720. (26) Maitra, A. M.; Sacchetta, C.; Tyler, R. J. Stud. Surf. Sci. Catal. 1994, 81, 261-268.

over Sr2SnO4 oxides at 1023 K.17 In the previous paper, we also examined the two-step thermochemical process using a redox system of iron oxide by the alternate methane-water feeding mode with temperature swing.18 We reported that methane reacted with Fe3O4 supported on SiO2 and was selectively converted to C2-HCs and hydrogen at temperatures between 1123 and 1173 K. However, in comparison to the normal single-step continuous feeding mode of reactants, the two-step cyclic mode has some industrial penalties, such as the needs for alternate feeding of reactants and temperature swing. In the present work, selective conversion of methane to C2-HCs and hydrogen was catalytically demonstrated over the SnO2/Fe3O4/SiO2 catalyst under co-feeding of methane and steam at 1173 K. Experimental Section Synthesis of Materials. Three different metal-oxide catalysts, SnO2/SiO2, Fe3O4/SiO2, and SnO2/Fe3O4/SiO2 catalysts were prepared. The SnO2(10 wt %)/Fe3O4(10 wt %)/SiO2 (SnFeS) catalyst was prepared as follows. First, the SnO2(10 wt %)/SiO2 (SnS) was prepared and then it was plated with Fe3O4 (10 wt %) by the aerial oxide method of Fe(II) suspension.27-31 The powder of porous SiO2 (silica aerosil: the specific surface area ) 382 m2 g-1) was suspended in distilled water. The pH of the suspended solution was around 4. Then, SnCl4 was dissolved in the suspended solution and was titrated with 0.05 mol dm-3 NH3 solution for hydrolysis. The product was collected by centrifuging at 14000 rpm. After washing the product with distilled water and then with acetone, it was dried in vacuo at room temperature. The dried powder was calcined at 1173 K in air. The SnS thus prepared was then plated with Fe3O4 by the aerial oxidation method of aqueous suspensions of the Fe(II) hydroxide according to the procedure reported previously.27,28 Fe3O4 can be plated on a substrate surface of various substances such as organic compounds (PET, PMMA, and Teflon), a glass (slide glass), and a metal (stainless steel) directly in an aqueous solution of ferrous salt (the aerial Fe2+-oxidation wet method).29-31 Hydrolyzed ferrous ions FeOH+ in an aqueous solution of ferrous salt are adsorbed on a substrate surface which react into Fe3O4 associated with air oxidation of the FeOH+ ion.31 Thus, here we expected that Fe3O4 was deposited on the surface of the suspended SnS powder by this aerial Fe2+-oxidation wet method. The SnS powder was previously suspended in oxygen- and CO2-free distilled water. After passing N2 gas through for a few hours, FeCl2 was dissolved in the solution having SnS suspension. The solution was adjusted to pH ) 9 by adding 0.075 mol dm-3 NaOH solution to form Fe(OH)2. After heating to 338 K, air was passed through the suspension while keeping the pH at 9 by adding 0.075 mol dm-3 NaOH solution. The product was collected by centrifuging at 14000 rpm. After washing the product with distilled water and then (27) Kiyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 1646-1650. (28) Tamaura, Y.; Buduan, P. V.; Katsura, T. J. Chem. Soc., Dalton Trans. 1981, 1807-1811. (29) Abe, M.; Tamaura, Y.; Goto, Y.; Kitamura, N.; Gomi, M. J. Appl. Phys. 1987, 61, 3211-3213. (30) Tamaura, Y.; Abe, M.; Itoh, T. J. Chem. Soc. Jpn. 1987, 11, 1980-1987. (31) Abe, M.; Tamaura, Y. J. Appl. Phys. 1984, 55, 2614-2616.

Conversion of CH4 to C2-Hydrocarbons and H2

Energy & Fuels, Vol. 15, No. 2, 2001 465

tional, fixed-bed, continuous-flow reactor made of a quartz tube with an inner diameter of 7 mm and a length of 450 mm. A 0.5 g sample of the catalyst was placed at the center of the reactor. The reactor was heated to 1173 K in an electric furnace while passing N2 gas through the catalyst in the reactor. After reaching 1173K, a CH4/H2O or CH4/Ar/H2O mixture was fed to the reactor to carry out methane conversion to C2-HCs and hydrogen. The CH4/H2O or CH4/Ar/H2O mixture was prepared by passing CH4 or CH4/Ar mixture through the distilled water at 333 K. The partial pressure of steam in the CH4/H2O or CH4/Ar/ H2O mixture was estimated to be 20% from the H2O vapor pressure at 333 K and 1 atm. The steam in the effluent was condensed in a cooling trap connected to the outlet of the reactor. The dry effluent was analyzed by gas chromatography with TCD (Shimadzu, GC-4C) and FID (Shimadzu, GC-R1A) to determine the gas composition. The formation rates of the gaseous carbon products, such as C2-HCs, CO, and CO2, were determined on a carbon base as follows:

rCxOyHz ) x × FCxOyHz,out/[weight of the catalyst] (9) The CxOyHz indicates a gaseous carbon product of interest. FCxOyHz,out is the molar flow rate of the CxOyHz for the outlet of the reactor, which is determined by

FCxOyHz,out ) PCxOyHz × Fout

(10)

PCxOyHz is the partial pressure of the CxOyHz in the dry effluent and Fout is the molar flow rate of the dry effluent. The formation rate of hydrogen was determined by the following equation: Figure 1. XRD patterns of the solid phases of (a) the SnO2/ Fe3O4/SiO2, (b) the SnO2/SiO2, and (c) the Fe3O4/SiO2 before use of the reaction.

with acetone, it was dried in vacuo at room temperature. The powder was then calcined at 1173 K in an N2 atmosphere. The Fe3O4(10 wt %)/SiO2 (FeS) was also prepared by plating the porous SiO2 powder with Fe3O4 in the same procedure. The product was washed, dried, and calcined in N2 in the same way used for the SnFeS. The activity of the SiO2-support was also tested for comparison. It was calcined at 1173 K in an N2 atmosphere prior to the reaction. The catalysts were subjected to X-ray diffractometry (XRD) with Cu KR radiation (Rigaku, RAD-γA diffractometer). The XRD patterns of the SnFeS, SnS, and FeS were shown in Figure 1. For the SnFeS, the peaks due to SnO2 and the small and broadened peaks due to Fe3O4 were observed together with strong peaks of SiO2 (tridymite) in the XRD pattern (Figure 1a). In the XRD pattern of the SnS (Figure 1b), only the peaks due to SnO2 were observed along with the peaks of the SiO2support. For the FeS (Figure 1c), the small and broadened peaks due to Fe3O4 were observed with the strong SiO2 peaks. The BET surface areas of the catalysts were determined by nitrogen adsorption (Shimadzu, Micromeritics Flow Sorb II 2300), and are listed in Table 1. Mode of Reactor Operation. The reactions were carried out at atmospheric pressure using a conven-

rH2 ) PH2 × Fout/[weight of the catalyst]

(11)

PH2 is the partial pressure of hydrogen in the dry effluent. The yield of the gaseous carbon product was estimated on a carbon base using the following equation:

YCxOyHz ) x × FCxOyHz,out/FCH4,in

(12)

FCH4,in is the molar flow rate of CH4 for the inlet of the reactor. The methane conversion was estimated by

CCH4 ) 1 - FCH4,out/FCH4,in

(13)

Results and Discussion We first carried out the reaction of methane over the SnFeS catalyst without feeding H2O at 1173 K. Only methane (PCH4 ) 1.0 atm) was fed to the reactor containing 0.5 g of the SnFeS at the CH4 flow rate of 48 Ncm3 min-1(“N” refers to normal conditions, i.e., 273 K, 1 atm). Figure 2a shows the formation profiles for gaseous carbon products and hydrogen in the effluent. In the reaction, some amount of H2O may be evolved as the gaseous product by the reaction between methane and the oxide catalyst, but, in our experimental setup, H2O evolution could not be analyzed. As can be seen in Figure 2a, the main products were C2-HCs and hydrogen, and CO and CO2 were scarcely formed in the reaction. Seventy percent of C2 products was ethylene

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Table 1. C2 Yield and Selectivities of Various Products in the Methane-Water Co-Feed Reaction over Metal-Oxide Catalysts at 120 min of the Reactiona catalyst SiO2-support SnS (10 wt %-SnO2) FeS (10 wt %-Fe3O4) SnFeS (10 wt %-SnO2 10 wt %-Fe3O4)

BET surface area/m2 g-1

CCH4/ %

YC2/ %

carbon material balance

Rb

SC2H4

SC2H6

382.2 17.3 0.2 0.4

9.4 6.2 4.0 6.3

1.6 3.5 3.5 5.5

0.31 0.91 0.97 0.98

4.6 1.2 1.0 1.1

14.0 44.1 63.8 63.3

3.4 12.6 24.6 23.8

selectivity/% SC2 SCO 17.4 56.7 88.3 87.1

9.2 12.9 0 0

SCO2

SC

3.0 26.7 10.9 12.6

70.4 3.7 0.8 0.3

a A CH /CO mixture (P 3 -1 and 4 2 CH4 ) 0.8 atm and PH2O ) 0.2 atm) was fed to the catalyst (0.5 g) at the CH4 flow rate of 48 Ncm min at 1173 K. b The ratio of the observed H2 amount evolved to the calculated H2 amount evolved.

Figure 3. XRD patterns of the solid phases of the SnFeS after use of the reactions when (a) feeding only methane for 60 min or (b) feeding the CH4/H2O mixture for 300 min, over the SnFeS at 1173 K and at the CH4 flow rate of 48 Ncm3 min-1.

Figure 2. Time variations of the formation rates of gaseous carbon products and hydrogen when feeding (a) only methane (PCH4 ) 1.0 atm) or (b) the CH4/H2O mixture (PCH4 ) 0.8 atm and PH2O ) 0.2 atm) over the SnFeS (0.5 g) at the CH4 flow rate of 48 Ncm3 min-1 and at 1173 K.

and the rest was ethane. The formation rate of C2-HCs initially reached about 7 mmol-C h-1 g-1-cat, but it rapidly decreased to about 3 mmol-C h-1 g-1-cat in 60 min of the reaction, and then became constant. The formation rate of hydrogen showed a profile similar to that of the C2 product. In the XRD pattern of the SnFeS after 60 min of the reaction (Figure 3a), the peaks due to SnO2, which had been observed before the reaction, completely disappeared and the peaks due to β-Sn appeared, along with the strong peaks of the SiO2support. The small and broadened peaks due to Fe3O4 also disappeared after 60 min of the reaction, but any

other peaks due to iron compounds did not appear in the XRD pattern. In this study, we could not clarify the behavior of the iron compound phase during the reaction. The SnFeS after 120 min of the reaction gave an XRD pattern similar to that after 60 min of the reaction. These results indicate that the SnO2 phase in the SnFeS was completely reduced with methane in the initial stage of the reaction. The initial reaction (in 60 min) would be roughly written by the following equation:

CH4 + SnO2 f C2-HCs + H2 + H2O + β-Sn

(14)

The degradation of the activity of the SnFeS for the C2 and H2 formation will be due to the fact that the lattice oxygens of the SnO2 phase, which were active for the oxidative coupling of methane (eq 2), were consumed by the reduction of SnO2 phase with methane in the initial stage of reaction. Figure 2b shows the formation profiles when feeding a CH4/H2O mixture to the SnFeS under the similar reaction conditions. The CH4/H2O mixture (PCH4 ) 0.8 atm and PH2O ) 0.2 atm) was fed to the SnFeS (0.5 g) at the CH4 flow rate of 48 Ncm3 min-1 and at 1173 K. The main products were C2-HCs, hydrogen, and carbon dioxide. Seventy percent of C2 products was ethylene

Conversion of CH4 to C2-Hydrocarbons and H2

Energy & Fuels, Vol. 15, No. 2, 2001 467

and the rest was ethane, which was the same as the case of Figure 2a. The initial C2-formation rate was about 7 mmol-C h-1 g-1-cat, which corresponded with that when feeding only CH4 (Figure 2a). However, the initial C2-formation rate was retained almost constant for 300 min when feeding the CH4/H2O mixture (Figure 2b). The H2-formation rate was also kept constant in 300 min of the reaction. In the XRD pattern of the SnFeS after 300 min of the reaction (Figure 3b), the peaks due to SnO2 remained unchanged together with strong peaks of the SiO2-support. These results indicate that, when feeding H2O with methane over the SnFeS, the active surface-lattice oxygens of the SnO2 phase for the C2 and H2 formation, which are consumed via the oxidative coupling of methane (eq 2), are replenished by H2O splitting (eq 3). This will be the reason the degradation of the activity did not occurred when feeding the CH4/H2O mixture. The material balance for carbon was estimated by the following equation:

carbon material balance ) [MCO2 + MCO + 2 × MC2H4 + 2 × MC2H6 + MCH4]/[FCH4,in × t] (15) where t represents the reaction time. MCxOyHz represents the total molar amount of the gaseous product or reactant CH4 in the effluent during the reaction, which is determined by

MCxOyHz )

∫0t FC O H ,out dt x

y

z

(16)

The integral on the right-hand side of eq 16 was evaluated graphically as the area under FCxOyHz,out against t curve. 0.98 of the carbon material balance was obtained for the case of Figure 2b. This indicates that methane decomposition to bulk carbon,

CH4 f C + 2H2

(17)

was negligible in this reaction. Thus, the following reaction proceeded over the SnFeS when feeding the CH4/H2O mixture:

CH4 + H2O f C2-HCs + CO2 + H2

(18)

In the present work, we could not determine the hydrogen material balance because the evolved H2O could not be analyzed in our experimental setup. Thus, we checked the chemical stoichiometry between the evolved amounts of hydrogen and gaseous carbon products as follows. If the bulk carbon deposition from methane is negligible, hydrogen should be produced via the following reactions:

2CH4 f C2H4 + 2H2

(19)

2CH4 f C2H6 + H2

(20)

CH4 + 2H2O f CO2 + 4H2

(21)

CH4 + H2O f CO + 3H2

(22)

In this case, according to the chemical stoichiometry for the above equations, the amount of H2 evolved can be calculated from the evolved amounts of C2H4, C2H6, CO2,

and CO:

calculated H2 amount evolved ) 4 × MCO2 + 3 × MCO + 2 × MC2H4 + MC2H6 (23) The ratio of the observed H2 amount evolved to the calculated H2 amount evolved (R) should be one if the bulk carbon deposition is negligible:

R ) MH2/[calculated H2 amount evolved] (24) MH2 is the observed total amount of H2 evolved during the reaction. The R value close to one indicates that the hydrogen is produced by the reaction of eqs 19-22 and that the bulk carbon deposition from methane is negligible in the reaction. When the R value exceeds one, the significant occurrence of the carbon deposition is indicated. The R value for the reaction of Figure 2b was estimated to be 1.1, indicating that the carbon deposition of eq 17 was negligible here. Based on the measured amounts of C2H4, C2H6, CO2, and H2 at the steady state at 300 min of the reaction, the catalytic reaction corresponds to

15CH4 + 2H2O f 5C2H4 + 2C2H6 + CO2 + 16H2 (25) The activity and selectivity of the SnFeS were compared to those of the SiO2-support, SnS (10 wt %-SnO2), and FeS (10 wt %-Fe3O4) under the similar reaction conditions of feeding the CH4/H2O mixture. The results are listed in Table 1. For the SiO2-support, the C2 yield was 1.6%. The carbon material balance was only 0.3 and the R value largely exceeded one, suggesting that a significant carbon deposition occurred in the reaction with the SiO2-support. To determine the total molar amount of bulk carbon deposited during the reaction (MC), the catalyst after use of the reaction was subject to combustion of the catalyst in an O2 flow at 1173 K: the effluent was collected in a bottle to replace water and the total amount of evolved COx (mainly CO2) was determined by gas chromatography. Here we defined the selectivity for carbon products including deposited bulk carbon:

SCxOyHz ) x × MCxOyHz [MCO2 + MCO + 2 × MC2H4 + 2 × MC2H6 + MC]

(26)

The selectivities are listed in Table 1. For the SiO2support, the SC (the selectivity for the deposited bulk carbon) was 70%. For the SnS, the C2 yield was improved to 3.5%, and the SC was reduced to 3.7%. However, the SCOx (SCO2 + SCO) increased to 40% [SC2 (SC2H4 + SC2H6) ) 57%]. The FeS also showed 3.5% of the C2 yield. The SC and SCOx were reduced to 0.8 and 11%, respectively. With the SnFeS, the best C2 yield of 5.5% was obtained. The SC were best reduced to 0.3% and SCOx was only 13%, giving the high C2 selectivity over 87%. Figure 4 shows the yields of the gaseous carbon products at reaction temperatures between 1123 and 1223 K when feeding the CH4/H2O mixture (PCH4 ) 0.8 atm and PH2O ) 0.2 atm). The R values ranged from 0.9 to 1.1, indicating that the methane decomposition

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Figure 4. Temperature variations of yields of gaseous carbon products when feeding the CH4/H2O mixture (PCH4 ) 0.8 atm and PH2O ) 0.2 atm) over the SnFeS (0.5 g) at the CH4 flow rate of 48 Ncm3 min-1. The yields were calculated at 120 min of the reaction.

Figure 6. Effect of the W/F on (a) the C2 yield, the C2 formation rate, (b) the product distribution, and the R value. The CH4/H2O mixture (PCH4 ) 0.8 atm and PH2O ) 0.2 atm) was fed to the SnFeS (0.5 g) at 1173 K with various flow rates of CH4 feed. The yields were calculated at 120 min of the reaction.

Figure 5. Effect of PCH4 in the reactant on (a) the C2 yield, the C2 formation rate, (b) the product distribution, and the R value. The CH4/Ar/H2O mixture was fed to the SnFeS (0.5 g) at the total flow rate ) 72 Ncm3 min-1 and at 1173 K. PH2O in the reactant was set to 0.2 atm. The yields were calculated at 120 min of the reaction.

to bulk carbon was negligible. The C2 yield reached a maximum at 1173 K and decreased at a higher temperature. At 1223 K, a significant increase in CO yield was observed. It might be attributed to the formation

of CO by steam reforming of methane of eq 22 which becomes thermodynamically more favorable at higher temperature. The influences of PCH4 in the reactant on the C2 yield, C2 formation rate, and distribution of the gaseous carbon product were studied for the SnFeS at 1173 K. A CH4/Ar/H2O mixture, containing argon gas as diluent, was fed over the SnFeS. The total flow rate of the reactant gas was set to 72 Ncm3 min-1 and PH2O in the reactant was 0.2 atm. Figure 5a shows the variation of the methane conversion, C2 yield and C2-formation rate. The C2 yield slightly increased with increasing PCH4 while the C2-formation rate greatly and linearly increased. The distribution of gaseous carbon product is given together with the R value in Figure 5b. The R values of 0.9-1.1 indicated that the methane decomposition to bulk carbon was negligible. The C2 selectivity gradually increased and the CO2 selectivity decreased with an increase in PCH4. A higher PCH4 is favorable for obtaining higher C2-formation rate, C2 yield, and C2 selectivity. Experiments on a W/F dependence were carried out (Figure 6), where W refers the weight of the SnFeS and F represents the flow rate of CH4 feed. The PCH4 and PH2O in the reactant were 0.8 and 0.2 atm, respectively. The R values of 0.9 to 1.1 indicated the negligible methane decomposition to bulk carbon (Figure 6b). The

Conversion of CH4 to C2-Hydrocarbons and H2

methane conversion increased monotonically with increasing W/F (Figure 6a). The C2 yield rapidly increased at W/F < 3 g-cat h Ncm-3 (GHSV > 4000 h-1) but then became constant. The C2-formation rate rapidly decreased with increasing W/F. The C2 selectivity showed the maximal peak of 94% at 1.7 g-cat h Ncm-3 (GHSV ) 6000 h-1), and decreased at a higher W/F. On the contrary, the CO selectivity was negligibly small at W/F < 3 g-cat h Ncm-3 (GHSV > 4000 h-1), but significantly increased at a higher W/F. The CO2 selectivity was kept almost constant at around 10%. At a low W/F value (a high GHSV), methane was selectively converted to C2HCs and hydrogen at a high C2-formation rate, while at a high W/F value (a low GHSV), steam reforming of methane to produce CO and hydrogen became dominant. There have been several studies which reported a highly selective coupling of methane using H2O as a main oxidant over metal-oxide catalysts.15-18 Li et al. studied the oxidative coupling of methane under cofeeding of methane and water over perovskite oxide catalysts.15 They achieved ∼95% C2 selectivity at 0.8% methane conversion over a SrTi0.4Mg0.6O3-δ catalyst at 1123 K, in which the C2-formation rate was 1.3 mmol-C h-1 g-1-cat. Omata et al. demonstrated the two-step cyclic reaction in which methane and water were fed alternately to the metal-oxide catalyst at 1023 K.17 They reported that a Sr2Sn0.9Bi0.1O4 catalyst resulted in 1.01.9 mmol-C h-1 g-1-cat of the C2-formation rate with >99% of C2 selectivity. With a Sr2Sn0.9Ni0.1O4 catalyst,

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3.0 mmol-C h-1 g-1-cat of C2 formation rate with 99% of C2 selectivity was obtained. In the present paper, by the SnO2/Fe3O4/SiO2, we achieved the higher C2-formation rate of 4-8 mmol-C h-1 g-1-cat with 87-94% C2 selectivity at 4-6% methane conversion when the W/F ranged in 0.9-2.6 g-cat h Ncm-3 (GHSV ) 4000-12000 h-1) (Figure 6). Conclusion Methane was selectively converted to C2-hydrocarbons and hydrogen over the SnO2(10 wt %)/Fe3O4(10 wt %)/ SiO2 catalyst in the methane/steam co-feeding system at 1173 K. By feeding H2O with methane, the active surface-lattice oxygens of the SnO2 phase are replenished by H2O splitting, resulting in the high activity and stability. In comparison with earlier studies, our experimental data gave the higher C2-formation rate with a high C2 selectivity. This process offers the efficient endothermic reaction for converting natural gas to C2H4 and hydrogen with upgraded calorific values, utilizing solar or nuclear heat as the energy source of hightemperature process heat below 1173 K. Acknowledgment. This research was financially supported by a Grant-in-Aid for Science Research No. 10558072 from Ministry of Education, Science and Culture. EF000200W