High-Temperature Conversion of CH4 to C2-Hydrocarbons and H2

High-temperature solar chemistry for converting solar heat to chemical fuels. T Kodama. Progress in Energy and Combustion Science 2003 29, 567-597 ...
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Energy & Fuels 1997, 11, 1257-1263

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High-Temperature Conversion of CH4 to C2-Hydrocarbons and H2 Using a Redox System of Metal Oxide T. Kodama,* T. Shimizu, A. Aoki, and Y. Kitayama Department of Chemistry and Chemical Engineering, Faculty of Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-21, Japan Received May 12, 1997. Revised Manuscript Received August 7, 1997X

A high-temperature thermochemical process using a redox system of metal oxide is proposed for converting CH4 to C2-hydrocarbons (C2-HCs) and H2. Reactions were performed in a twostep redox cycle. In the first high-temperature and endothermic step, methane is reacted with metal oxide to produce C2-HCs and the reduced metal oxide. The reduced metal oxide is reoxidized with water to generate H2 at a low temperature in the second step. A thermodynamic analysis showed that redox systems of Fe3O4/FeO, SnO2/SnO, and WO3/WO2 are promising for the twostep process. The redox system of Fe3O4 was experimentally examined. Highly selective conversion could be repeated with SiO2-supported Fe3O4 (Fe3O4/SiO2) to produce C2-HCs (mainly C2H4) and H2 alternately in the different steps at temperatures from 1123 to 1173 K; evolution of COx and deposition of bulk carbon were scarcely observed. Experimental studies using unsupported Fe3O4 showed that the formation of C2-HCs in the first high-temperature step occurred favorably for the reduction from Fe3O4 to FeO in comparison to that from FeO to R-FeO. The two-step process using Fe3O4/SiO2 is superior to the production efficiencies of C2-HCs and H2 obtained by the direct single-step conversion of CH4, which offers the efficient conversion of natural gas utilizing high-temperature heat such as concentrated solar radiation.

Introduction Efficient utilization of high-temperature heat such as concentrated solar radiation is a current subject for research.1,2 Direct and efficient thermochemical conversion to fuels and chemicals is desired. The goal is an industrially important endothermic process that can be driven by high-temperature heat. Direct conversion of methane to C2-hydrocarbons (C2HCs) plus hydrogen is an attractive reaction for utilization of natural gas (NG):

nCH4 f CnH4n-2y + yH2

(1)

The reaction is highly endothermic. If the desired C2HC 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. However, the above direct conversion of CH4 has severe thermodynamic limitations below 1500 K.3,4 At the high temperatures in excess of 1500 K, system design becomes difficult because of material problems.5 Furthermore, radiation losses are much increased at the high temperatures. Optimal operating temperatures for converting concentrated solar radiation into the chemical-free energy range from 800 to 1500 K for a blackbody solar cavity-receiver.1,6 * Author for correspondence. Telephone: +81-25-262-7335. Fax: +81-25-263-3174. E-mail: [email protected]. X Abstract published in Advance ACS Abstracts, October 1, 1997. (1) Fletcher, E. A. J. Minn. Acad. Sci. 1983, 49, 30-34. (2) Sizmann, R. Chimia 1989, 7-8, 202-206. (3) Keller, G. K.; Bhasin, M. M. J. Catal. 1982, 73, 9-19. (4) Sofranko, J. A.; Leonard, J. J.; Jones, C. A. J. Catal. 1987, 103, 302-310. (5) Nakamura, T. Sol. Energy 1977, 19, 467-475.

S0887-0624(97)00069-8 CCC: $14.00

One of the other routes from methane to C2-HCs and hydrogen is via steam reforming of methane. A typical steam reformer operates at 15-30 atm and 10731173K with a Ni/Al2O3 catalyst.7 The synthesis gas produced can be converted to higher hydrocarbons by the Fischer-Tropsch (FT) reaction. However, in the FT reaction there is severe limitation of selectivity for a desired higher hydrocarbon owing to the Schulz-Flory (SF) product distribution; the SF distribution limits the selectivity for C2-HCs to a maximum of 30%.8 The most promising method for avoiding the SF limitations is via methanol conversion over shape selective zeolites.9 However, steam reforming and methanol synthesis are still required. An important challenge in a hightemperature process is the simple process for converting CH4 to C2-HCs and H2. Here we propose the simple thermochemical process using a redox system of metal oxide:

2CH4 + MxOy f C2H8-2y′ + MxOy-y′ + y′H2O (2) MxOy-y′ + y′H2O f MxOy + y′H2

(3)

It consists of two steps: (1) methane is oxidized using metal oxide as an oxidant to produce C2-HCs and the reduced metal oxide; (2) the reduced metal oxide is oxidized back with water to form hydrogen in the second step. A diagram of the thermochemical scheme is (6) Steinfeld, A.; Schubnell, M. Sol. Energy 1993, 50, 19-25. (7) Rostup-Nielsen, J. R. Catalysis Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer: Berlin, 1984; Vol. 5, p 1. (8) Dry, M. E. Catalysis - Science and Technology; Anderson, J. R., Boudart, M., Eds.; Springer-Verlag: Berlin, 1981; Vol. 1, p 159. (9) Potmeier, U.; Litterer, H.; Baltes, H.; Herzog, W.; Leupold, E. I.; Wunder, F. A. Chem.-Ing.-Tech. 1982, 54, 590-592.

© 1997 American Chemical Society

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H2O + 3FeO f Fe3O4 + H2 (second step)

(6)

This process offers the efficient net reaction for upgrading NG, utilizing solar heat as the energy source of hightemperature process heat. Thermodynamic Analysis Figure 1. Diagram of the thermochemical scheme of the twostep process.

depicted in Figure 1. The two-step process realizes reaction 1 as the net reaction. The first-step reaction 2 of the oxidative conversion of CH4 to C2-HCs appears to solve the thermodynamic barrier problem. The second-step reaction 3 of the water decomposition is slightly exothermic and can proceed at low temperatures. C2-HCs and H2 are derived in different steps, eliminating the need for high-temperature product separation, as opposed to the direct single-step conversion. It is believed that gas-phase oxidations and adsorbed oxygen depress the C2-HC selectivity in methane oxidation. Keller and Bhasin first reported that methane reacts with the lattice oxygens of various metal oxides to form C2-HCs when methane alone is fed to the metal oxides.3 There have been many studies published on the activity of the lattice oxygens in methane oxidation.3,4,10-16 Recently, Buyevskaya et al. reported that the surface-lattice oxygen of MgO is responsible for methyl radical formation resulting in C2-HCs, while adsorbed O2 takes part in the complete oxidation reaction.14 However, their purpose is to find the active catalysts for the following reaction

2CH4 + x/2O2 f C2H8-2x + xH2O

(4)

where water rather than hydrogen is coproduced. Therefore, although the lattice oxygens of several metal oxides are reported to be active for the methane oxidation to C2-HCs, there has been no attention to the fact that the reduced metal oxides have sufficient reactivity to decompose H2O to H2 at low temperatures. There have been few reports on the investigation of the reactivity of redox systems of metal oxides for the purpose of realizing the efficient conversion of CH4 to C2-HCs and H2. In the present paper, highly selective conversion of CH4 to C2-HCs and H2 is demonstrated by the following two-step process below 1173 K.

2CH4 + xFe3O4 f C2H8-2x + x(3FeO + H2O) (first step) (5) (10) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J. Am. Chem. Soc. 1985, 107, 5062-5068. (11) Otuka, K.; Jinno, K.; Morikawa, A. J. Catal. 1986, 100, 353359. (12) Jones, C. A.; Leonard, J. J.; Sofranko, J. A. J. Catal. 1987, 103, 311-319. (13) Moggridge, G. D.; Badyal, J. P. S.; Lambert, R. M. J. Catal. 1991, 132, 92-99. (14) Buyevskaya, O. V.; Rothaemel, M.; Zanthoff, H. W.; Baerns, M. J. Catal. 1994, 146, 346-357. (15) Yu, Z.; Yang, X.; Lunsford, J. H.; Rosynek, M. P. J. Catal. 1995, 154, 163-173. (16) Mallens, E. P.; Hoebink, J. H. B.; Marin, G. B. J. Catal. 1996, 160, 222-234.

The variations of ∆G° of reactions 2 and 3 with temperature, when CnH4n-2y′ ) C2H4, are shown in Figure 2 for several candidates of redox pairs of metal oxides.17,18 The direct conversion of CH4 to C2H4 and H2 requires temperatures above 1500 K, as shown in Figure 2a. H2O can be decomposed to H2 at low temperatures by the reactions listed in Figure 2b. However, the methane conversion to C2H4 by reaction (2) cannot proceed below 1500 K for the TiO2/Ti2O3, Nb2O5/NbO2, Cr2O3/Cr, Ta2O5/Ta, and FeO/Fe redox pairs (Figure 2a). The redox pairs of Fe3O4/FeO, SnO2/ SnO, and WO3/WO2 reduce the temperature required for reaction 2 below 1500 K. The iron oxide is the most abundant and least expensive material among them. The conversion of methane to C2H4 using Fe3O4 as an oxidant can proceed at temperatures around 1150 K. FeO can decompose water vapor to produce H2 and Fe3O4 at low temperatures below 650 K. In the present work, the redox system of Fe3O4 is experimentally studied. The two-step process using a redox system of Fe3O4 would appear to solve the thermodynamic barrier problem, but there is no indication that a highly selective reaction would result. The oxidation of methane to COx is expected to readily occur in the reaction of methane with the metal oxide. Furthermore, deposition of bulk carbon via methane decomposition, which is thermodynamically favorable at temperatures above 900 K, may significantly occur at high temperatures around 1150 K:

CH4 f C + 2H2

(7)

High selectivities for C2-HCs are essential properties for the working material of iron oxide. Experimental Section Preparation of Materials. Two different Fe3O4 samples were examined as a working material for the proposed reactions: one was normal or unsupported Fe3O4, and another was SiO2-supported Fe3O4. Normal or unsupported Fe3O4 powder was synthesized by oxidation of aqueous suspensions of the Fe(II) hydroxide according to the procedure reported previously.19,20 FeSO4 was dissolved in oxygen- and CO2-free distilled water prepared by passing N2 gas through the water for a few hours. The solution was adjusted to pH ) 9 by adding 3.0 mol dm-3 NaOH solution to form a hydroxide suspension. Air was passed through the alkaline suspension during oxidation for 5 h at 338 K while the pH of the solution was kept constant by adding 3.0 mol dm-3 of NaOH solution. The product was collected by decantation. After being washed (17) JANAF Thermochemical Tables, 3rd ed.; National Bureau of Standards: Washington, DC, 1985. (18) The NBS Tables of Chemical Thermodynamic Properties, Selected Values for Inorganic and C1 and C2 Organic Substances in Sl Units, International Bureau of Standards. J. Phys. Chem. Ref. Data, Suppl. 2 1982, 11. (19) Kiyama, M. Bull. Chem. Soc. Jpn. 1974, 47, 1646-1650. (20) Tamaura, Y.; Buduan, P. V.; Katsura, T. J. Chem. Soc., Dalton Trans. 1981, 1807-1811.

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Figure 2. Variations of ∆G° of the reactions (a) 2CH4 + MxOy f C2H4 + MxOy-2 + 2H2O and (b) MxOy-2 + 2H2O f MxOy + 2H2 with temperature for several candidates of redox pairs of metal oxides.

Figure 3. Schematic illustration of the apparatus used to study the two-step process. first with distilled water and then with acetone, it was dried in vacuo at 333 K for a day. The SiO2-supported Fe3O4 (Fe3O4/SiO2) was prepared by applying the above aerial oxidation method for preparation of Fe3O4. The powder of porous SiO2 (silica aerogel, specific surface area ) 226 m2 g-1) was previously suspended in oxygen- and CO2-free distilled water. After N2 was passed for a few hours, FeCl2 was dissolved in the solution having a SiO2 suspension. The solution was adjusted to pH ) 9 by adding 0.15 mol dm-3 NaOH solution to form Fe(OH)2. The content of iron in the Fe3O4/SiO2 was about 7% on a weight basis. After it was heated to 338 K, air was passed through the suspension while the pH was kept constant at 9 by adding 0.15 mol dm-3 NaOH solution. Glass plate and fine particles such as polymer tonner particles can be plated with Fe3O4 in an aqueous solution by this aerial oxidation method.21,22 Thus, here we expected that the suspended porous SiO2 was plated with Fe3O4 that was formed via the oxidation of Fe(OH)2. The product was collected by centrifuging at 14 000 rpm. After being washed with distilled water and then with acetone, it was dried in vacuo at 333 K for a day. The powder was then heated at 673 K for 2 h in an H2/H2O atmosphere to reduce byproducts of R-Fe2O3 and R-FeOOH components to Fe3O4. The Fe3O4 samples thus prepared were calcined at 1173 K (21) Abe, M.; Tamaura, Y.; Goto, Y.; Kitamura, N.; Gomi, M. J. Appl. Phys. 1987, 61, 3211-3213. (22) Tamaura, Y.; Abe, M.; Itoh, T. J. Chem. Soc. Jpn. 1987, 11, 1980-1987.

in an N2 atmosphere prior to the high-temperature reactions of eqs 5 and 6. Characterization of Materials. The BET surface areas of the unsupported and SiO2-supported Fe3O4 were determined by nitrogen adsorption (Shimadzu, Micromeritics Flow Sorb II 2300) to be 6.3 and 0.3 m2 g-1, respectively. The samples were subjected to X-ray diffractometry (XRD) with Cu KR radiation (Rigaku, RAD-rA diffractometer). Only strong peaks of spinel structure of magnetite appeared in the XRD pattern of the unsupported sample. For the Fe3O4/SiO2 sample, broad and small spinel peaks were observed together with strong peaks of SiO2 (tridymite) in the XRD pattern. Mode of Reactor Operation. The reactions were performed at atmospheric pressure using a conventional fixedbed continuous-flow reactor of a quartz tube with an inner diameter of 6 mm and a length of 330 mm. The apparatus is illustrated in Figure 3. The material was packed in the center of the reactor. The volume of the material used was 1.0-4.0 cm3. The reactivity and selectivity for the reactions of eqs 5 and 6 were examined. The first step in eq 5 is referred to as the CH4-coupling step and the second step in eq 6 as the H2Odecomposition step. The CH4-coupling step was first performed. The reactor was heated to 1173 K in an electric furnace while N2 gas passed through the material in the reactor. After reaching 1173 K, a CH4/He mixture was fed to the reactor at a flow rate of 12-36 cm3 min-1 to carry out methane conversion to C2-HCs. R-Al2O3 (80 m2 g-1) was examined as an inert material under similar conditions. The

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reactivity of support of SiO2 was also tested for the reaction. They were calcined at 1173 K in an N2 atmosphere prior to the reaction. For the Fe3O4/SiO2 sample, the H2O-decomposition step was carried out after the CH4-coupling step. The reactor was cooled to 573 K while N2 gas was passed. While raising the temperature to 1173 K, the reduced Fe3O4/SiO2 reacted with H2O by mixed H2O/N2 gas passing through the reactor at an N2 flow rate of 6 cm3 min-1. A mixture of H2O/N2 gas was produced by N2 gas passing through the distilled water at 353 K. For repetition of the two-step process with Fe3O4/SiO2, the CH4-coupling step was carried out for 60 min at 1173 K while only CH4 was fed to Fe3O4/SiO2 (1.0 cm3) at a flow rate of 36 cm3 min-1. The H2O-decomposition step was performed for 60 min at 1123 K when an H2O/N2 mixture passed through the reactor at an N2 flow rate of 6 cm3 min-1. The gaseous products in both steps were determined by gas chromatography (Shimadzu, GC-4C) with a TCD (Shimadzu, GC-4C) and an FID (Shimadzu, GC-4BM). The yields of gaseous carbon products were monitored during the CH4coupling step using the following equation:

YCxOyHz ) xFCxOyHz,out/FCH4,in

(8)

The CxOyHz indicates a carbon product of interest. Fj,in and Fj,out are the molar flow rates of species j for the inlet and outlet of the reactor, respectively. Total yields of the products after the CH4-coupling step were determined by

Figure 4. Time variations of yields of products of (a) hydrocarbons and (b) COx when a CH4/He mixture (the molar fraction of CH4 ) 30%) is fed to the unsupported Fe3O4 (1.0 cm3) at a flow rate of 12 cm3 min-1 and at 1173 K.

total YCxOyHz ) x[total molar amount of CxOyHz evolved]/ [FCH4,intHC] (9) where tHC represents the reaction time for the CH4-coupling step. The deposition of bulk carbon from methane was determined by combustion of the materials after use for the CH4-coupling step 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. The total yield of bulk carbon was determined by

total Ycarbon ) [total molar amount of evolved COx]/ [FCH4,intHC] (10)

Results and Discussion The reactivities of unsupported Fe3O4 and Fe3O4/SiO2 in the CH4-coupling step were studied first. Figure 4 shows time variations of yields of hydrocarbons, CO, and CO2 when a CH4/He mixture (the molar fraction of CH4 ) 30%) is fed to the unsupported Fe3O4 at 1173 K. Formation of C2-HCs (mainly C2H4) was observed during the first hour. The C2H4 yield increased in the first 35 min but then decreased to zero after 80 min, whereas the yields of CO2 and CO increased significantly after 35 min of the reaction. In the XRD pattern of the solid phase of the unsupported Fe3O4 after use for 35 min of the reaction, the spinel peaks of Fe3O4 disappeared and strong peaks of FeO were observed (Figure 5a).23,24 However, only peaks of R-Fe appeared in the XRD pattern after 80 min (Figure 5b).25 The lattice oxygens of magnetite were consumed via methane oxidation, and magnetite was reduced first to FeO and then to R-Fe. (23) Powder Diffraction File, International Centre for Diffraction Data, Newtowne Square, PA, 1969; Card No. 19-629. (24) Powder Diffraction File, International Centre for Diffraction Data, Newtowne Square, PA, 1956; Card No. 6-0615. (25) Powder Diffraction File, International Centre for Diffraction Data, Newtowne Square, PA, 1956; Card No. 6-0696.

Figure 5. XRD pattern of the solid phase of the unsupported Fe3O4 after use of the CH4-coupling step (shown in Figure 4) for (a) 35 min and (b) 80 min.

These results suggest that the formation of C2-HCs occurs favorably for the reduction from Fe3O4 to FeO in comparison to that from FeO to R-Fe. Significant formation of COx occurs in the course of the reduction of FeO to R-Fe. However, the yields of C2-HCs were much lower than that of COx through the CH4-coupling step using unsupported Fe3O4, which indicates that almost of the lattice oxygens of magnetite were consumed for methane oxidation to COx. Figure 6 shows the results of the product yields during the CH4-coupling step using Fe3O4/SiO2 at 1173 K. A much higher level evolution of C2-HCs was observed in comparison with unsupported Fe3O4. Main C2-HCs was also C2H4. Evolution of COx was also

Conversion of CH4

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Table 1. Total Yields and Selectivities of Products after Performing the CH4-Coupling Stepa at 1173 K

carbon

total conversion of CH4/%

C2H4

C2-HCs

CO

CO2

carbon

16.8 12.8 0.3 0.2

17.0 13.6 3.9 39.0

0.7 4.7 64.4 0.7

0.7 6.0 83.4 1.1

0.4 0 0.7 59.7

0 0 2.3 38.0

98.9 94.0 8.4 1.2

total yield of prouct/%

material

reaction period/min

C2H4

C2-HCs

CO

CO2

empty tube R-Al2O3 SiO2 Fe3O4/SiO2 Fe3O4 (unsupported)

300 300 300 300 100

0.20 0.11 0.64 2.51 0.29

0.27 0.11 0.82 3.25 0.39

0 0.07 0 0.03 23.5

0 0 0 0.09 15.0

selectivity of product/%

a A CH /He mixture [partial pressure of CH ) 30% (a molar fraction)] was fed to the reactor with/without material at a flow rate of 4 4 12 cm3 min-1. The volume the material used was 4.0 cm3 except that for unsupported Fe3O4 (1.0 cm3).

Figure 6. Time variations of yields of products of (a) hydrocarbons and (b) COx when a CH4/He mixture (the molar fraction of CH4 ) 30%) is fed to Fe3O4/SiO2 (4.0 cm3) at a flow rate of 12 cm3 min-1 and at 1173 K.

detected in the very initial stage of the reaction, but it did not occur after 30 min. C2-HCs were selectively produced for 800 min, and then the yields became zero. The yields of the higher HCs then rapidly decreased to zero at 3 h. The formation of COx was efficiently suppressed by SiO2-supporting for Fe3O4. The lattice oxygens of Fe3O4 were consumed selectively for methane oxidation to C2-HCs on SiO2 support. The reactions were also carried out for 300 min using Al2O3, SiO2, and an empty reactor tube. The total yields of the products after 300 min of the reaction are shown in Table 1. A direct single-step conversion of CH4 to C2-HCs and H2 was examined using the empty reactor tube. Table 1 shows the total yields and selectivities for the products after performing the CH4-coupling step using unsupported Fe3O4, Fe3O4/SiO2, SiO2, and R-Al2O3. A direct single-step conversion of CH4 to C2-HCs and H2 was examined using an empty reactor tube under similar reaction conditions. The yield of C2-HCs was only 0.27%. R-Al2O3 was also tested as an inert material in which the yield of C2-HCs was 0.11%. These results indicate that the yield of C2-HCs by the direct singlestep conversion was at most 0.3% by the 300 min reaction at 1173 K. For the SiO2 support, the yield of C2-HCs was only 0.82%. However, it was much improved to 3.3% with Fe3O4/SiO2 being 13 times as large as that by the single-step conversion. Significant

deposition of bulk carbon was observed for Al2O3 and SiO2 at ae total yield of 13-17%. The selectivities for carbon were more than 94% over these materials. The deposition of bulk carbon was efficiently suppressed over Fe3O4/SiO2 (total Ycarbon ) 0.3%). The carbon deposition also scarcely occurred over unsupported Fe3O4. These results indicate that the methane decomposition to bulk carbon was suppressed over Fe3O4. The total yields of CO and CO2 were less than 0.1% with Fe3O4/SiO2. The selectivity for C2-HCs was more than 80%. Highly selective conversion of CH4 to C2-HCs successfully occurred using Fe3O4/SiO2. Significant evolution of H2 was observed during the CH4-coupling step. The evolved H2 amounts were much larger than those expected from the reaction 2CH4 f C2H4 + 2H2, which were estimated from the C2H4 amounts evolved. The evolution of H2 will be mainly due to methane decomposition to bulk carbon. However, the evolved H2 amounts were still larger than those expected from CH4 f C + 2H2 if we used the total yields of carbon in Table 1 for the calculation of the expected H2 amount according to CH4 f C + 2H2. For example, when pure SiO2 or Al2O3 is used, it is 1.4-1.7 times larger than the expected values. The total yields of bulk carbon in Table 1 came from the carbon deposited only on the materials after use for the CH4-coupling step. The carbon deposition from methane, however, appeared to occur to some extent on the inside wall of the vacant quartz reactor tube; black soot on the inside wall of the vacant reactor tube was visible after the CH4coupling step. For example, when using Fe3O4/SiO2 for the CH4-coupling step, the soot deposition was not observed on the inside tube wall on which the Fe3O4/ SiO2 had been loaded, which would be suppressed by the presence of Fe3O4. But soot deposition could be seen on parts of the vacant tube of both sides of the Fe3O4/ SiO2. In the present work, we could not determine the amount of the carbon deposited on the quartz reactor tube. We just determined the carbon deposition on the materials. The extra amount of H2 evolved would be due to methane decomposition to bulk carbon on the quartz reactor tube. During the CH4-coupling step, it is possible that a surface catalytic process may also be involved. The small amounts of surface carbon that are formed during the CH4-coupling step over Fe3O4/SiO2 (Table 1) may, for example, be an Fe carbide phase. Transition metal carbides have been shown to be capable of activating CH4 for the formation of C2H4.26 To make sure that the proposed redox process involving the Fe3O4-FeO system is a major reaction and that the catalytic reaction is minor in the CH4-coupling step, we corroborated the (26) Wang, D.; Lunsford, J. H.; Posynek, M. P. Top. Catal. 1996, 3, 289-297.

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Figure 7. Variations of production rates for hydrocarbons at 100 min of the reaction time as a function of the partial pressure of CH4 (molar fraction) when a CH4/He mixture is fed to Fe3O4/SiO2 (4.0 cm3) at a flow rate of 12 cm3 min-1 and at 1173 K.

Figure 8. Relation between the production rate of total C2HCs and space velocity (SV) of the reactant of CH4 when only CH4 is fed to Fe3O4/SiO2 at 1173 K.

mass balance between the used Fe3O4 and produced C2H4 according to the proposed redox process. The mole amount of Fe3O4 in the used Fe3O4/SiO2 for the CH4coupling step of Figure 6 was 1.8 mmol. According to the proposed first-step reaction, 2CH4 + 2Fe3O4 f C2H4 + 6FeO + 2H2O, the amount of C2H4 produced should be 0.9 mmol. Thus, if the C2H4 evolution in the CH4coupling step is caused by the proposed reaction, it should be terminated, as can be seen at 850 min of the reaction time in Figure 6, after producing 0.9 mmol of C2H4. However, when pure SiO2 is used for the CH4coupling step under reaction conditions similar to those of Figure 6, 0.4 mmol of C2H4 is produced in 850 min. Since SiO2 is not reducible by CH4, the C2H4 evolution is due to thermal dissociation of methane to C2H4 and H2 over SiO2. This thermal dissociation would occur concurrently with the above proposed reaction between Fe3O4 and CH4 over Fe3O4/SiO2. Thus, the expected amount of C2H4 evolved in the CH4-coupling step of Figure 6 is the sum of 0.9 and 0.4 mmol. The total amount of evolved C2H4 in Figure 6 was experimentally 1.2 mmol in 850 min, which was very close to the expected amount (1.3 mmol). This calculation for the mass balance will support that a proposed reaction occurring between Fe3O4 and CH4 is the main reaction in the CH4-coupling step. The surface catalytic process over carbides would be a minor reaction in the CH4coupling step. To make this point more clear, the surface analysis for used Fe3O4/SiO2 will be required, which is now in progress. The CH4-coupling step with Fe3O4/SiO2 was also carried out at various partial pressures of CH4 in the CH4/He mixture under similar reaction conditions (at aflow rate of 12 cm3 min-1 and at 1173 K). The profiles of the product yields, which are similar to those of Figure 6, were observed for pCH4 ) 30-100%. Figure 7 shows the variations of production rates for hydrocarbons as a function of pCH4. The production rates per unit of surface area, observed at 100 min of the reaction time, were plotted against the pCH4. The C2H4 production rate greatly increased with increasing pCH4. A higher pCH4 is favorable for obtaining higher production efficiency of C2-HCs. The evolution of C2-HCs during the CH4-coupling step occurred over Fe3O4/SiO2 in a shorter time at a higher pCH4 . For example, the evolution of C2-HCs was

observed in the initial 120 min of the reaction when only CH4 was fed to Fe3O4/SiO2, while it continued for 800 min at pCH4 ) 30% as shown by Figure 6. This will come from the fact that the lattice oxygens of Fe3O4 were more rapidly consumed via methane oxidation to C2H4 with increasing the pCH4 because the production rate of C2H4 greatly became faster. From thermodynamic data, methane conversion to C2H4 can occur using the phase transition from Fe3O4 to FeO at 1173 K while it cannot proceed using that from FeO to R-Fe as noted before (Figure 2a). The formation of C2-HCs over Fe3O4/ SiO2 may also be mainly attributable to the reduction of Fe3O4 to FeO. Chemical analysis for the surface of the Fe3O4/SiO2 after use for the CH4-coupling step will be needed to clarify this point; in the XRD pattern of the Fe3O4/SiO2 used, any peaks due to FeO and R-Fe were not detected. Further work is in progress. Figure 8 shows the relation between the production rate of total C2-HCs and space velocity (SV) of the reactant of CH4 when only CH4 is fed to Fe3O4/SiO2 at 1173 K. The production rate rapidly increased at SV e 1000 h-1 but then gradually increased with increasing SV. The Fe3O4/SiO2 after the CH4-coupling step reacted with water vapor for the H2O-decomposition step. An H2O/N2 mixture was passed through the Fe3O4/SiO2 as the reactant gas while the temperature was raised from 573 to 1173 K. H2 evolution was observed at T > 723 K, and strong peaks of the evolution appeared at 1123 K. The evolution of COx and O2 was scarcely observed during the reaction, indicating that the oxygen of H2O was incorporated into the lattice of the solid phase. Thus, we determined the reaction temperature of the H2O-decomposition step at 1123 K. Figure 9 shows the results of the repetition of the twostep process using Fe3O4/SiO2 in the temperature range 1123-1173 K; the production rates of total C2-HCs and H2 were plotted against the reaction time. C2-HCs and H2 were produced alternately in repeating the steps. A high initial production rate of C2-HCs (4 carbon mmol h-1 m-2) in the CH4-coupling step was retained for repeating. More than 70% of the C2-HCs produced was C2H4, and CO and CO2 formation was scarcely observed in every CH4-coupling step. H2 continued to be evolved in the H2O-decomposition steps.

Conversion of CH4

Energy & Fuels, Vol. 11, No. 6, 1997 1263

Figure 9. Results of the repetition of the proposed two-step thermochemical cycle using Fe3O4/SiO2. The CH4-coupling step was carried out for 60 min at 1173 K while only CH4 is fed to Fe3O4/SiO2 (1.0 cm3) at a flow rate of 36 cm3 min-1 (SV ) 2450 h-1). The H2O-decomposition step was performed for 60 min at 1123 K when passing an H2O/N2 mixture at an N2 flow rate of 6 cm3 min-1.

Conclusions Methane was selectively converted to C2-HCs and H2 by a two-step thermochemical process using Fe3O4/SiO2. The selective conversion could be repeated at a temperature range from 1123 to 1173 K. It was found that the formation of C2-HCs in the CH4-coupling step occurred favorably for the reduction of Fe3O4 to FeO in comparison to that of FeO to R-Fe from the XRD results of the reaction using unsupported Fe3O4. It was found

that the high-temperature methane decomposition to bulk carbon was efficiently suppressed over Fe3O4. Our two-step process is superior to conversion of CH4 by a direct single-step reaction. This process offers the efficient endothermic net reaction for converting natural gas to C2H4 and H2 with upgraded calorific values, utilizing concentrated solar radiation as the energy source of high-temperature process heat below 1173 K. EF9700691