Optimization of C1-Oxygenates for the Selective Oxidation of Methane

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Energy & Fuels 2001, 15, 44-51

Optimization of C1-Oxygenates for the Selective Oxidation of Methane in a Gas-Phase Reaction of CH4-O2-NO at Atmospheric Pressure Tetsuya Takemoto,† Kenji Tabata,*,†,‡ Yonghong Teng,† Shuiliang Yao,† Akira Nakayama,† and Eiji Suzuki†,‡ Research Institute of Innovative Technology for the Earth (RITE), 9-2, Kizugawa-dai, Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan, and Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5, Takayama-cho, Ikoma, Nara 630-0101, Japan Received April 26, 2000. Revised Manuscript Received August 19, 2000

The optimization of C1-oxygenates for the selective oxidation of CH4 in the gas-phase reaction of CH4-O2-NO has been examined with kinetic models utilizing the software packages of CHEMKIN III. The variations of simulated selectivities of the products in the reactions reproduced well the experimentally obtained values except for those of CH3OH in a region of less than 10% CH4 conversion. The effects of several experimental conditions, i.e., SV, CH4/O2, NO concentration, NO/NOx (x ) 1,2) on the selectivities of C1-oxygenates were examined. The ratio of CH4/O2 ) 2 and 0.5% NO concentration for the feed gas were the best reaction conditions for getting the highest yield of C1-oxygenates through experimental conditions, and its yield was 57% at 10% CH4 conversion. All of the simulated values of the selectivities of C1-oxygenates could not exceed this value in the examined reaction conditions. If the decomposition reactions of CH2O and CH3OH with OH radicals in the course of the reactions were omitted, the predicted value of C1oxygenates would be 77% at 10% CH4 conversion.

Introduction The selective oxidation of CH4 with O2 to C1-oxygenates (CH3OH and CH2O) is an important process for the effective use of natural gas resources and also for the minimization of energy consumption. In recent years, many researchers have studied the selective oxidation of natural gas with various types of catalysts. However, the products mostly comprise CO, CO2, and H2O with only trace formation of CH3OH and CH2O.1-3 Accordingly, the metal oxide catalysts that have been examined cause successive oxidation. The gas-phase reaction without catalysts seems to have the advantage of yielding C1-oxygenates because the difficulty of the desorption stage from the surface of catalysts could be liberated but the control of a chain reaction could be difficult. Formation of CH3OH and/or CH2O with CH4 and O2 in a gaseous reaction has been reported.2,4-14 * Author to whom correspondence should be addressed. Tel.: +81774-75-2305. Fax: +81-774-75-2318. E-mail: [email protected]. † Research Institute of Innovative Technology for the Earth (RITE). ‡ Nara Institute of Science and Technology (NAIST). (1) Taylor, S. H.; Hargreaves, J. S.; Hutchings, G. J.; Joyner, R. W. Methane and Alkane Conversion Chemistry, Plenum: New York, 1995, pp 339-345. (2) Pitchai, R.; Klier, K. Catal. Rev.-Sci. Eng. 1986, 28, 13-88. (3) Pak, S.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. 1994, 98, 11786-11790. (4) Arutyunov, V. S.; Basevich, V. Y.; Vedeneev, V. I. Russ. Chem. Rev. 1996, 65, 197-224. (5) Mackie, J. C.; Catal. Rev. Sci. Eng.1991, 33, 169-240. (6) Foster, N. R. Appl. Catal. 1985, 19, 1-11. (7) Gesser, H. D.; Hunter, N. R.; Prakash, C. B. Chem. Rev. 1985, 85, 235-244.

The rate-determining step of the selective oxidation of CH4 is the first hydrogen abstraction from CH4. Therefore, initiators, or sensitizers, have been examined in order to reduce the activation energy of the first hydrogen abstraction from CH4.14-16 The promotion effect of nitrogen oxides for CH4 selective oxidation in a gaseous reaction has been reported.13,14,17-20 Recently, Ban˜ares et al. reported a high yield of CH3OH and CH2O over V2O5/SiO2 catalysts in the presence of NO.21 (8) Burch, R. G.; Squire, G. D.; Tsang, S. C. J. Chem. Soc., Faraday Trans. 1989, 85, 3561-3568. (9) Baldwin, R. R.; Hopkins, D. E.; Norris, A. C.; Walker, R. W. Combust. Flame 1970, 15, 33-46. (10) Thomas, D. J.; Willi, R.; Baiker, A. Ind. Eng. Chem. Res. 1992, 31, 2272-2278. (11) Krylov, O. V. Catal. Today 1993, 18, 209-302. (12) Feng, W.; Knopf, F. C.; Dooley, K. M. Energy Fuels 1994, 8, 815-822. (13) Amano, T.; Dryer, F. L. Twenty-Seventh Symposium (International) on Combustion, The Combustion Institute, 1998, Pittsburgh, pp 397-404. (14) Bromly, J. H.; Barnes, F. J.; Muris, S.; You, X.; Haynes, B. S. Combust. Sci. Technol.1996, 115, 259-296. (15) Wharren, B. K. Catal Today 1992, 13, 311-320. (16) Burch, R.; Squire, G. D.; Tsang, S.C. Appl. Catal. 1989, 46, 6987. (17) Smith, D. F.; Milner, R. T. Ind. Eng. Chem. 1931, 23, 357360. (18) Irusta, S.; Lombardo, E. A.; Miro, E. E. Catal. Lett. 1994, 29, 339-348. (19) Han, L. B.; Tsubota, S.; Haruta, M.; Chem. Lett. 1995, 931932. (20) Otsuka, K.; Takahashi, R.; Amakawa, K.; Yamanaka, I. Catal Today 1998, 45, 23-28.; Otsuka, K.; Takahashi, R.; Yamanaka, I. J. Catal. 1999, 185, 182-191. (21) Ban˜ares, M. A.; Cardoso, J. H.; Hutchings, G. J.; Bueno, J. M. C.; Fierro, J. L. G. Catal. Lett. 1998, 56, 149-153.

10.1021/ef000087+ CCC: $20.00 © 2001 American Chemical Society Published on Web 12/15/2000

Optimization of C1-Oxygenates for CH4 Oxidation

The highest yield of C1-oxygenates reached 7% at atmospheric pressure. Very recently, we reported a comparatively high yield of C1-oxygenates in the gas phase of CH4-O2 with a small amount of NOx (x ) 1 or 2) at atmospheric pressure.22,23 We also proposed a reaction scheme for the conversion of CH4 to CH3OH and CH2O in the gas phase of CH4-O2-NOx by means of theoretical calculations at the MP2 (frozen core) and CCSD(T) levels.24 It was found that in both NO and NO2, a nitrogen atom showed a higher activity for the cleavage of the C-H bond of CH4 than did an oxygen atom in NOx. Furthermore, the activation energies were calculated as 65.6 kcal/mol for a nitrogen atom of NO and 37.6 kcal/mol for that of NO2, indicating that NO2 had a higher activity for the hydrogen abstraction from CH4 than NO. This decrease of the transition barrier was experimentally verified by the linear enhancement of CH4 conversion with the concentration of NO2 in CH4-O2-NOx.23 The simulations of these gaseous reactions with kinetic and thermodynamic parameters have been discussed.5,14 Bromly et al. examined the kinetic models in CH4-O2-NOx.14 The predictions with their kinetic models for the oxidation reactions in an atmospheric pressure were reported to be in good agreement with their experimental data over the entire range of conditions, though the main product of the reaction was CO, and the formation of C1-oxygenates was not reported. However, a comparatively high yield of C1-oxygenates was obtained in the experiments for the gas-phase selective oxidation of CH4-O2-NOx at atmospheric pressure.23 In this study, we therefore examine the appropriateness of kinetic models utilizing the software packages of CHEMKIN III for the gas-phase selective oxidation of CH4-O2-NO, making a comparison between the experimentally observed variations of selectivities as functions of CH4 conversions and the variations obtained from the simulations. The optimized reaction conditions (SV, CH4/O2, NO concentration, NO/NOx) for getting the highest yield of C1-oxygenates were also examined. Experimental Section Activity Test. All of the experiments were carried out using a single-pass flow reactor made of a quartz tube with an inside diameter of 7.0 mm at atmospheric pressure. A heated length with an electric furnace was 100 mm. The reaction temperature was controlled from the outside of the quartz tube with a thermocouple at the center position of the heated zone. The total flow rate was 240 cm3 min-1 (STP) and space velocity (SV) was 3740 h-1 except for the experiments of effects of SV. SV was calculated by dividing the gas flow volume per 1 h at 298 K and 1 bar by the volume of the vacant heated zone of the reactor. The standard gas composition (CH4: 55.6 mol %; O2: 27.7 mol %; NO: 0.5 mol %; He: 16.2 mol %) was controlled with a mass flow controller. The ratio of CH4 to O2 was stabilized at 2.0 except for the experiments of effects of (22) Teng, Y.; Tabata, K.; Sakurai, H.; Suzuki, E. Appl. Catal. A 2000, 190, 283-289. (23) Tabata, K.; Teng, Y.; Yamaguchi, Y.; Sakurai, H.; Suzuki, E. J. Phys. Chem. A 2000,104, 2648-2654.; Teng, Y.; Tabata, K.; Ouyang, F.; Dai, L.; Karasuda, T.; Yamaguchi, Y.; Suzuki, E. Chem. Lett. 1999, 991-992. (24) Yamaguchi, Y.; Teng, Y.; Shimomura, S.; Tabata, K.; Suzuki, E. J. Phys. Chem. A 1999, 103, 8272-8278.

Energy & Fuels, Vol. 15, No. 1, 2001 45 the ratio. All experimental data were obtained by changing reaction temperature. The products were analyzed with two on-line gas chromatographs serially connected. A thermal conductivity detector (activated carbon) and a flame ion detector (Gaskuropack 54), using helium as a carrier gas, were used. The carbon balance before and after the reaction exceeded 95%. Measurements were carried out after 30 min of reacting at each experimental condition, and all experimental data were taken at least three times to ensure the reproducibility. The variations of selectivities of products as functions of CH4 conversions were calculated. Different CH4 conversions were accomplished by changing reaction temperature. Methods of Simulation. We used a one-dimensional, i.e., a simple plug-flow model as the reacting flows in this study. Additionally, we used an isothermal reactor model. The simulations were carried out with the CHEMKIN III program25,26 utilizing the thermodynamic database by Kee et al.27 and elementary reactions with Arrhenius parameters by Grimech 2.1.1.28 To apply for the selective oxidation of methane in CH4-O2-NOx, the original elementary reactions in Grimech 2.1.1 were modified. The elementary reactions which were related to N, NH, NH2, NH3, NNH, NCO, HCN, H2CN, HCNN, HCNO, HOCN, and HNCO were omitted from Grimech 2.1.1. Furthermore, the elementary reactions which were related to NOx, HONO, HONO2, CH3NO2, CH3ONO, CH2NO2, CH3NO, CH3OO, and CH3OOH were supplemented with the data reported by Arutyunov et al.,4 Mackie,5 Bromly et al.,14 and Tricot et al.29 As a total, 288 elementary reactions with Arrhenius parameters were utilized (Table 1). The SENKIN package was used as the application code of calculations.26 The variations of concentrations of CH4, CH3OH, CH2O, CO, CO2, C2H4, C2H6, and CH3NO2 at atmospheric pressure in the course of time were examined after the input of the experimental conditions (feed gas composition, reaction temperature, reaction pressure). From these data, the variations of selectivities of products as functions of CH4 conversions were simulated. All experimental data were compared with the simulated data at the same CH4 conversion.

Results and Discussion The variations of selectivities from the experimentally observed data of products are shown (Figure 1). Each experimental selectivity at different CH4 conversion was obtained by changing reaction temperature. The ratio of CH4 to O2 was 2, and the concentration of NO was 0.5%. The formation of C1-oxygenates (CH3OH and CH2O) were observed together with those of CO, CO2, and a small amount of both CH3NO2 and C2H6. All of the differences of obtained selectivities through several runs were less than 1%. The variations of these experimentally obtained selectivities of products were compared with the simulated data which were calculated from a one-dimensional analysis of the reacting flow (25) Kee, R. J.; Rupley, F. M.; Meek, E. CHEMKIN-III: A Fortran Chemical Kinetic Package for the Analysis of gas-Phase Chemical and Plasma Kinetics, 1996, Report SAND 96-8216 Sandia National Laboratories, Livermore, CA. (26) Lutz, A. E.; Kee, R. J.; Miller, J. A. SENKIN: A Fortran Program for Predicting Homogeneous Gas-Phase Chemical Kinetics with Sensitivity Analysis, 1988, Report SAND 87-8248, Sandia National Laboratories, Livermore, CA. (27) Kee, R. J.; Rupley, F. M.; Miller, J. A. The CHEMKIN Thermodynamic Data Base, 1990, Report SAND 87-8215B Sandia National laboratories, Livermore, CA. (28) Bowman, C. T.; Hanson, R. K.; Davidson, D. F.; Gardiner, W. C., Jr.; Lissianski, V.; Smith, G. P.; Golden, D. M.; Frenklach, M.; Goldenberg, M. Grimech 2.1.1, Web site: http://www.me.berkley.edu/ gri_mech/, Dec.,1997. (29) Tricot, J. C.; Perche, A.; Lucquin, M. Combust. Flame 1981, 40, 269-291.

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Energy & Fuels, Vol. 15, No. 1, 2001

Takemoto et al.

Table 1. The Mechanism of the Selective Oxidation of Methane and the Parameters of the Equation: k ) ATn exp(-E/RT) No

reaction

Ac

n

E

ref No

1.20E+17 -1 0 28 1 2O+M f O2 + H2/2.40/H2O/15.40/CH4/2.00/CO/1.75/CO2/3.60/C2H6/3.00/Ar/0.83 a 5.00E+17 -1 0 28 2 O + H + M f OH + M H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 1.88E+21 -2.8 0 14 3 O + O2 f O3 4 O + O3 f O2 + O2 4.80E+12 0 4090 14 5.00E+04 2.67 6290 28 5 O + H2 f H + OH 2.00E+13 0 0 28 6 O + HO2 f OH + O2 9.63E+06 2 4000 28 7 O + H2O2 f OH + HO2 8 O + CH f H + CO 5.70E+13 0 0 28 8.00E+13 0 0 28 9 O + CH2 f H + HCO 8.00E+13 0 0 14 10 O + CH2 f CO + H + H 3.00E+14 0 1920 14 11 O + CH2 f CH + OH 12 O + CH2(S) f H2 + CO 1.50E+13 0 0 28 1.50E+13 0 0 28 13 O + CH2(S) f H + HCO 3.00E+13 0 0 14 14 O + CH2(S) f CO + H + H 8.43E+13 0 0 28 15 O + CH3 f H + CH2O 1.02E+09 1.5 8600 28 16 O + CH4 f OH + CH3 6.02E+14 0 3000 28 17 O + CO + M f CO2 + Ma H2/2.00/O2/6.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/3.50/C2H6/3.00/Ar/0.50 18 O + HCO f OH + CO 3.00E+13 0 0 28 19 O + HCO f H + CO2 3.00E+13 0 0 28 3.90E+13 0 3540 28 20 O + CH2O f OH + HCO 1.00E+13 0 0 28 21 O + CH2OH f OH + CH2O 1.00E+13 0 0 28 22 O + CH3O f OH + CH2O 3.88E+05 2.5 3100 28 23 O + CH3OH f OH + CH2OH 1.30E+05 2.5 5000 28 24 O + CH3OH f OH + CH3O 5.00E+13 0 0 28 25 O + C2H f CH + CO 1.02E+07 2 1900 28 26 O + C2H2 f H + HCCO 27 O + C2H2 f OH + C2H 4.60E+19 -1.41 28950 28 1.02E+07 2 1900 28 28 O + C2H2 f CO + CH2 3.00E+13 0 0 28 29 O + C2H3 f H + CH2CO 1.92E+07 1.83 220 28 30 O + C2H4 f CH3 + HCO 3.50E+13 0 2832 14 31 O + C2H4 f CH3CO + H 8.02E+13 0 0 14 32 O + C2H5 f CH3HCO + H 1.32E+14 0 0 28 33 O + C2H5 f CH3 + CH2O 8.98E+07 1.92 5690 28 34 O + C2H6 f OH + C2H5 35 O + HCCO f H + 2CO 1.00E+14 0 0 28 1.00E+13 0 8000 28 36 O + CH2CO f OH + HCCO 1.75E+12 0 1350 28 37 O + CH2CO f CH2 + CO2 2.59E+13 0 0 14 38 O + CH3O2 f CH3O + O2 2.29E+12 0 1351 14 39 O + CH2CO f HCO + HCO 7.00E+12 0 1940 14 40 O + CH3HCO f OH + CH3CO 2.50E+12 0 47800 28 41 O2 + CO f O + CO2 1.00E+14 0 40000 28 42 O2 + CH2O f HO2 + HCO 43 O2 + C2H3 f CH2O + HCO 4.00E+12 0 -250 14 1.00E+12 0 0 14 44 O2 + C2H3 f C2H2 + HO2 2.00E+08 1.5 30100 14 45 O2 + C2H2 f HCCO + OH 1.70E+13 0 47780 14 46 O2 + H2 f 2OH 3.97E+13 0 56900 4 47 O2 + CH4 f CH3 + HO2 2.05E+13 0 44911 14 48 O2 + CH3OH f CH2OH + HO2 6.03E+13 0 51870 14 49 O2 + C2H6 f HO2 + C2H5 1.40E+09 1 0 14 50 O2 + HCCO f CO2 + CO + H 51 H + O3 f OH + O2 8.43E+13 0 950 14 2.80E+18 -0.86 0 28 52 H + O2 + M f HO2 + Ma O2/0.00/H2O/0.00/CO/0.75/CO2/1.50/C2H6/1.50 N2/0.00/Ar/0.50 3.00E+20 -1.72 0 28 53 H + 2O2 f HO2 + O2 9.38E+18 -0.76 0 28 54 H + O2 + H2O f HO2 + H2O 3.75E+20 -1.72 0 28 55 H + O2 + N2 f HO2 + N2 7.00E+17 -0.8 0 28 56 H + O2 + Ar f HO2 + Ar 8.30E+13 0 14413 28 57 H + O2 f O + OH 58 2H + M f H2 + Ma 1.00E+18 -1 0 28 H2/0.00/H2O/0.00/CH4/2.00/CO2/0.00/C2H6/3.00/Ar/0.63 9.00E+16 -0.6 0 28 59 2H + H2 f 2H2 6.00E+19 -1.25 0 28 60 2H + H2O f H2 + H2O 5.50E+20 -2 0 28 61 2H + CO2 f H2 + CO2 a 2.20E+22 -2 0 28 62 H + OH + M f H2O + M H2/0.73/H2O/3.65/CH4/2.00/C2H6/3.00/Ar/0.38 3.97E+12 0 671 28 63 H + HO2 f O + H2O 64 H + HO2 f O2 + H2 2.80E+13 0 1068 28 1.34E+14 0 635 28 65 H + HO2 f 2OH 1.21E+07 2 5200 28 66 H + H2O2 f HO2 + H2 1.00E+13 0 3600 28 67 H + H2O2 f OH + H2O 1.10E+14 0 0 28 68 H + CH f C + H2 b 2.50E+16 -0.8 0 28 69 H + CH2(+M) f CH3(+M) LOW/3.200E+27 -3.140 1230.00/ TROE/0.6800 78.00 1995.00/5590.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70/ Ma

reaction

Ac

n

E

ref

70 H + CH2(S) f CH + H2 3.00E+13 0 0 28 71 H + CH2(S) f CH2 + H 2.00E+14 0 0 14 b 72 H + CH3(+M) f CH4(+M) 1.27E+16 -0.63 383 28 LOW/2.477E+33 -4.760/2440.00/ TROE/0.7830 74.00 2941.00 6964.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70/ 73 H + CH4 f CH3 + H2 6.60E+08 1.62 10840 28 74 H + HCO(+M) f CH2O(+M)b 1.09E+12 0.48 -260 28 LOW/1.350E+24 -2.570 1425.00/ TROE/0.7824 271.00 2755.00 6570.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70/ 75 H + HCO f H2 + CO 7.34E+13 0 0 28 76 H + CH2O(+M) f CH2OH(+M)b 5.40E+11 0.454 3600 28 LOW/1.270E+32 -4.820 6530.00/ TROE/0.7187 103.00 1291.00 4160.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00 77 H + CH2O(+M) f CH3O(+M)b 5.40E+11 0.454 2600 28 LOW/2.200E+30 -4.800 5560.00/ TROE/0.7580 94.00 1555.00 4200.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00 2.30E+10 1.05 3275 28 78 H + CH2O f HCO + H2 79 H + CH2OH(+M) f CH3OH(+M)b 1.80E+13 0 0 28 LOW/3.000E+31 -4.800 3300.00/ TROE/0.7679 338.00 1812.00 5081.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00 80 H + CH2OH f H2 + CH2O 2.00E+13 0 0 28 81 H + CH2OH f OH + CH3 1.20E+13 0 0 28 82 H + CH2OH f CH2(S) + H2O 6.00E+12 0 0 28 83 H + CH3O(+M) f CH3OH(+M)b 5.00E+13 0 0 28 LOW/8.600E+28 -4.000 3025.00/ TROE/0.8902 144.00 2838.00 45569.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00 84 H + CH3O f H + CH2OH 3.40E+06 1.6 0 28 85 H + CH3O f H2 + CH2O 2.00E+13 0 0 28 86 H + CH3O f OH + CH3 3.20E+13 0 0 28 87 H + CH3O f CH2(S) + H2O 1.60E+13 0 0 28 88 H + CH3OH f CH2OH + H2 1.70E+07 2.1 4870 28 89 H + CH3OH f CH3O + H2 4.20E+06 2.1 4870 28 90 H + CH3OH f CH3 + H2O 2.00E+14 0 5310 14 91 H + CH3O2 f CH3O + OH 9.64E+13 0 0 14 92 H + C2H(+M) f C2H2(+M)b 1.00E+17 -1 0 28 LOW/3.750E+33 -4.800 1900.00/ TROE/0.6464/132.00 1315.00 5566.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 93 H + C2H2(+M) f C2H3(+M)b 5.60E+12 0 2400 28 LOW/3.800E+40 -7.270 7220.00/ TROE/0.7507 98.50 1302.00 4167.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 94 H + C2H3(+M) f C2H4(+M)b 6.08E+12 0.27 280 28 LOW/1.400E+30 -3.860 3320.00/ TROE/0.7820 207.50 2663.00 6095.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 95 H + C2H3 f H2 + C2H2 3.00E+13 0 0 28 96 H + C2H4(+M) f C2H5(+M)b 1.08E+12 0.454 1820 28 LOW/1.200E+42 -7.620 6970.00/ TROE/0.9753 210.00 984.00 4374.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 97 H + C2H4 f C2H3 + H2 1.33E+06 2.53 12240 28 98 H + C2H5(+M) f C2H6(+M)b 5.21E+17 -0.99 1580 28 LOW/1.990E+41 -7.080 6685.00/ TROE/0.8422 125.00 2219.00 6882.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 99 H + C2H5 f H2 + C2H4 2.00E+12 0 0 28 100 H + C2H6 f C2H5 + H2 1.15E+08 1.9 7530 28 101 H + HCCO f CH2(S) + CO 1.00E+14 0 0 28 102 H + CH2CO f HCCO + H2 5.00E+13 0 8000 28 103 H + CH2CO f CH3 + CO 1.13E+13 0 3428 28 104 H + HCCOH f H + CH2CO 1.00E+13 0 0 28 105 H2 + CO(+M) f CH2O(+M)b 4.30E+07 1.5 79600 28 LOW/5.070E+27 -3.420 84350.00/ TROE/0.9320 197.00 1540.00 10300.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 106 OH + H2 f H + H2O 2.16E+08 1.51 3430 28 107 2OH(+M) f H2O2(+M)b 7.40E+13 -0.37 0 28 LOW/2.300E+18 -0.900 -1700.00/ TROE/0.7346 94.00 1756.00 5182.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70

Optimization of C1-Oxygenates for CH4 Oxidation

Energy & Fuels, Vol. 15, No. 1, 2001 47

Table 1 (Continued) No

reaction

Ac

n

E

108 2OH f O + H2O 3.57E+04 2.4 -2110 109 OH + O3 f HO2 + O2 1.14E+12 0 2000 2.90E+13 0 -500 110 OH + HO2 f O2 + H2O 111 OH + H2O2 f HO2 + H2O 1.75E+12 0 320 DUPLICATE 5.80E+14 0 9560 112 OH + H2O2 f HO2 + H2O DUPLICATE 113 OH + C f H + CO 5.00E+13 0 0 114 OH + CH f H + HCO 3.00E+13 0 0 115 OH + CH2 f H + CH2O 2.00E+13 0 0 116 OH + CH2 f CH + H2O 1.13E+07 2 3000 3.00E+13 0 0 117 OH + CH2(S) f H + CH2O 118 OH + CH3(+M) f CH3OH(+M)b 6.30E+13 0 0 LOW/2.700E+38 -6.300 3100.00/ TROE/0.2105 83.50 5398.00 8370.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00 5.60E+07 1.6 5420 119 OH + CH3 f CH2 + H2O 120 OH + CH3 f CH2(S) + H2O 2.50E+13 0 0 121 OH + CH3 f CH2O + H2 1.02E+12 0 0 1.00E+08 1.6 3120 122 OH + CH4 f CH3 + H2O 123 OH + CO f H + CO2 4.76E+07 1.228 70 124 OH + HCO f H2O + CO 5.00E+13 0 0 3.43E+09 1.18 -447 125 OH + CH2O f HCO + H2O 126 OH + CH2OH f H2O + CH2O 5.00E+12 0 0 127 OH + CH3O f H2O + CH2O 5.00E+12 0 0 1.44E+06 2 -840 128 OH + CH3OH f CH2OH + H2O 129 OH + CH3OH f CH3O + H2O 6.30E+06 2 1500 130 OH + C2H f H + HCCO 2.00E+13 0 0 2.18E-04 4.5 -1000 131 OH + C2H2 f H + CH2CO 132 OH + C2H2 f H + HCCOH 5.04E+05 2.3 13500 133 OH + C2H2 f C2H + H2O 3.37E+07 2 14000 4.83E-04 4 -2000 134 OH + C2H2 f CH3 + CO 135 OH + C2H3 f H2O + C2H2 5.00E+12 0 0 136 OH + C2H4 f C2H3 + H2O 3.60E+06 2 2500 3.54E+06 2.12 870 137 OH + C2H6 f C2H5 + H2O 138 OH + CH2CO f HCCO + H2O 7.50E+12 0 2000 139 OH + CH2CO f HCO + CH2O 2.80E+13 0 0 -620 140 OH + CH3HCO f H2O + CH3CO 3.37E+12 0 141 2HO2 f O2 + H2O2 1.30E+11 0 -1630 DUPLICATED 4.20E+14 0 12000 142 2HO2 f O2 + H2O2 DUPLICATED 143 HO2 + O3 f OH + O2 + O2 8.43E+09 0 1200 2.00E+13 0 0 144 HO2 + CH2 f OH + CH2O 145 HO2 + CH3 f OH + CH3O 2.00E+13 0 760 146 HO2 + CH4 f CH3 + H2O2 9.04E+12 0 24640 1.50E+14 0 23600 147 HO2 + CO f OH + CO2 148 HO2 + CH2O f HCO + H2O2 1.00E+12 0 8000 149 HO2 + CH3OH f H2O2 + CH2OH 9.64E+10 0 12580 2.29E+11 0 1550 150 HO2 + CH3O2 f CH3O2H + O2 151 HO2 + CH2OH f CH2O + H2O2 1.21E+13 0 0 152 HO2 + C2H6 f C2H5 + H2O2 1.32E+13 0 20470 5.80E+13 0 576 153 C + O2 f O + CO 154 C + CH2 f H + C2H 5.00E+13 0 0 155 C + CH3 f H + C2H2 5.00E+13 0 0 3.30E+13 0 0 156 CH + O2 f O + HCO 157 CH + H2 f H + CH2 1.11E+08 1.79 1670 158 CH + H2O f H + CH2O 1.71E+13 0 -755 4.00E+13 0 0 159 CH + CH2 f H + C2H2 160 CH + CH3 f H + C2H3 3.00E+13 0 0 161 CH + CH4 f H + C2H4 6.00E+13 0 0 5.00E+13 0 0 162 CH + CO(+M) f HCCO(+M)b LOW/2.690E + 28 -3.740 1936.00/ TROE/0.5757 237.50 1652.00 5069.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00 163 CH + CO2 f HCO + CO 3.40E+12 0 690 164 CH + CH2O f H + CH2CO 9.46E+13 0 -515 5.00E+13 0 0 165 CH + HCCO f CO + C2H2 166 CH2 + O2 f OH + HCO 1.32E+13 0 1500 167 CH2 + H2 f H + CH3 5.00E+05 2 7230 3.20E+13 0 0 168 2CH2 f H2 + C2H2 169 2CH2 f C2H3 + H 2.00E+13 0 0 2.40E+14 0 9936 170 2CH2 f CH3 + CH

ref No

reaction

Ac

n

E

28 171 CH2 + CH3 f H + C2H4 4.00E+13 0 0 14 172 CH2 + CH4 f 2CH3 2.46E+06 2 8270 28 173 CH2 + CO(+M) f CH2CO(+M)b 8.10E+11 0.5 4510 28 LOW/2.690E + 33 -5.110 7095.00/ TROE/0.5907 275.00 1226.00 5185.00/ 28 H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 174 CH2 + C2H3 f C2H2 + CH3 3.00E+13 0 0 28 175 CH2 + O2 f CO2 + H2 4.38E+09 0 -5825 3.73E+10 0 -5825 28 176 CH2 + O2 f CO + H2O 28 177 CH2 + O2 f CH2O + O 5.47E+09 0 -5825 28 178 CH2 + HCCO f C2H3 + CO 3.00E+13 0 0 28 179 CH2(S) + N2 f CH2 + N2 1.50E+13 0 600 28 180 CH2(S) + Ar f CH2 + Ar 9.00E+12 0 600 181 CH2(S) + O2 f H + OH + CO 2.80E+13 0 0 1.20E+13 0 0 182 CH2(S) + O2 f CO + H2O 183 CH2(S) + H2 f CH3 + H 7.00E+13 0 0 28 184 CH2(S) + H2O(+M) f CH3OH(+M)b 2.00E+13 0 0 28 LOW/2.700E + 38 -6.300 3100.00/ 14 TROE/0.1507 134.00 2383.00 7265.00/ 28 H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00 28 185 CH2(S) + H2O f CH2 + H2O 3.00E+13 0 0 28 186 CH2(S) + CH3 f H + C2H4 1.20E+13 0 -570 28 187 CH2(S) + CH4 f 2CH3 1.60E+13 0 -570 28 188 CH2(S) + CO f CH2 + CO 9.00E+12 0 0 28 189 CH2(S) + CO2 f CH2 + CO2 7.00E+12 0 0 28 190 CH2(S) + CO2 f CO + CH2O 1.40E+13 0 0 28 191 CH2(S) + C2H2 f CH2 + C2H2 4.00E+13 0 0 28 192 CH2(S) + C2H6 f CH3 + C2H5 4.00E+13 0 -550 28 193 CH2(S) f CH2 1.00E+13 0 0 28 194 CH3 + O2 f O + CH3O 2.68E+13 0 28800 28 195 CH3 + O2 f OH + CH2O 3.60E+10 0 8940 28 196 CH3 + O2(+M) f CH3OO(+M) 7.80E+08 0.2 0 28 197 CH3 + O3 f CH3O + O2 1.57E+12 0 0 28 198 2CH3(+M) f C2H6(+M)b 2.12E+16 -0.97 620 28 LOW/1.770E + 50 -9.670 6220.00/ 28 TROE/0.5325 151.00 1038.00 4970.00/ 14 H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 14 199 2CH3 f H + C2H5 4.99E+12 0.1 10600 28 200 CH3 + HCO f CH4 + CO 2.65E+13 0 0 201 CH3 + CH2O f HCO + CH4 3.32E+03 2.81 5860 28 202 CH3 + CH3O2 f CH3O + CH3O 2.71E+13 0 0 203 CH3 + CH3OH f CH2OH + CH4 3.00E+07 1.5 9940 14 204 CH3 + C2H4 f C2H3 + CH4 2.27E+05 2 9200 28 205 CH3 + C2H6 f C2H5 + CH4 6.14E+06 1.74 10450 28 206 HCO + H2O f H + CO + H2O 2.24E+18 -1 17000 14 207 HCO + M f H + CO + Ma 1.87E+17 -1 17000 28 H2/2.00/H2O/0.00/CH4/2.00/CO/1.50/CO2/2.00 C2H6/3.00 28 208 HCO + O2 f HO2 + CO 7.60E+12 0 400 14 209 HCO + CH2OH f CH3OH + CO 1.21E+14 0 0 14 210 HCO + CH2OH f CH2O + CH2O 1.81E+14 0 0 14 211 HCO + HCO f CH2O + CO 3.01E+13 0 0 14 212 CH2OH + O2 f HO2 + CH2O 1.80E+13 0 900 28 213 CH3O + O2 f HO2 + CH2O 6.00E+10 0 2600 28 214 CH3O + CH4 f CH3OH + CH3 1.32E+14 0 15073 28 215 CH3O + CO f CH3 + CO2 1.57E+13 0 11800 28 216 CH3O + H2O2 f CH3OH + HO2 1.50E+12 0 4000 28 217 CH3O + CH3O f CH2O + CH3OH 1.80E+13 0 0 28 218 CH3O + CH2O f HCO + CH3OH 1.00E+11 0 3000 28 219 2CH3O2 f CH3O + CH3O + O2 1.02E+11 0 0 28 220 2CH3O2 f CH2O + CH3OH + O2 4.46E+16 0 0 28 221 CH3O2 + CH4 f CH3O2H + CH3 6.02E+11 0 21520 28 222 CH3O2 + CH2O f CH3O2H + HCO 2.83E+11 0 12010 223 CH3O2 + H2 f CH3O2H + H 2.17E+12 0 22320 224 CH3O2 + CH3O f CH3O2H + CH2O 9.03E+11 0 0 225 CH3O2 + CH3OH f CH3O2H + CH2OH 3.00E+11 0 14410 28 226 CH3O2 + H2O2 f CH3O2H + HO2 1.51E+11 0 13010 28 227 CH3O2H f CH3O + OH 4.00E+15 0 43040 28 228 C2H + O2 f HCO + CO 5.00E+13 0 1500 28 229 C2H + H2 f H + C2H2 4.07E+05 2.4 200 28 230 C2H4(+M) f H2 + C2H2(+M)b 8.00E+12 0.44 88770 28 LOW/7.000E + 50 -9.310 99860.00/ 14 TROE/0.7345 180.00 1035.00 5417.00/ 14 H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70

ref 28 28 28

14 14 14 14 28 28 28 28 28 28 28

28 28 28 28 28 28 14 28 14 28 28 14 14 28

28 28 28 14 28 28 28 28 28 28 14 14 14 28 28 14 4 4 4 5 4 4 4 4 4 4 4 4 4 28 28 28

48

Energy & Fuels, Vol. 15, No. 1, 2001

Takemoto et al.

Table 1 (Continued) No

reaction

Ac

n

E

ref No

231 C2H5 + O2 f HO2 + C2H4 8.40E+11 0 3875 28 232 HCCO + O2 f OH + 2CO 1.60E+12 854 28 233 2HCCO f 2CO + C2H2 1.00E+13 0 0 28 234 N2O + O f N2 + O2 1.40E+12 0 10810 28 235 N2O + O f 2NO 2.90E+13 0 23150 28 236 N2O + H f N2 + OH 4.40E+14 0 18880 28 237 N2O + OH f N2 + HO2 2.00E+12 0 21060 28 238 N2O(+M) f N2 + O(+M)b 1.30E+11 0 59620 28 LOW/6.200E + 14 0.000 56100.00/ H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 239 HO2 + NO f NO2 + OH 2.11E+12 0 -480 28 240 HO2 + NO f HONO2 2.23E+12 -3.5 2200 14 241 NO + NO + O2 f NO2 + NO2 1.20E+09 0 -1050 14 242 NO + O3 f NO2 + O2 1.08E+12 0 2720 14 243 NO + CH3O f CH2O + HNO 1.22E+12 0 -390 14 244 NO + CH3O2 f CH3O + NO2 2.53E+12 0 -358 14 245 NO + HCO f CO + HNO 7.24E+13 -0.4 0 14 246 NO + O + M f NO2 + Ma 1.06E+20 -1.41 0 28 H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 247 NO2 + O f NO + O2 3.90E+12 0 -240 28 248 NO2 + H f NO + OH 1.32E+14 0 360 28 249 NO2 + H f HONO 1.40E+18 -1.5 900 14 250 NO2 + H2 f H + HONO 1.20E+13 0 29000 14 251 NO2 + O3 f NO3 + O2 7.23E+10 0 4870 14 252 NO2 + CH4 f CH3 + HONO 1.20E+13 0 30000 14 253 NO2 + CH3 f CH3O + NO 1.39E+13 0 0 14 254 NO2 + CH3 f CH3NO2 5.00E+11 0 0 29 255 NO2 + CH3O f CH2O + HONO 6.02E+12 0 2285 14 256 NO2 + CH3OH f HONO + CH2OH 3.67E+11 0 21400 14 257 NO2 + CH2O f HONO + HCO 2.00E+11 0 16060 14 1.70E+13 0 0 14 258 NO2 + HCO f CO + HONO

reaction

Ac

n

E

ref

259 NO2 + CO f CO2 + NO 8.91E+13 0 33800 14 260 NO2 + CH3HCO f CH3CO + HONO 2.51E+10 0 12400 14 261 NO3 + OH f HO2 + NO2 1.38E+13 0 0 14 262 NO3 + O f O2 + NO2 1.02E+13 0 0 14 263 NO3 + H f NO2 + OH 6.00E+13 0 0 14 264 NO3 + HO2 f O2 + HONO2 5.60E+11 0 0 14 265 NO3 + HO2 f O2 + NO2 + OH 2.00E+12 0 0 14 266 NO3 + NO3 f NO2 + NO2 + O2 5.12E+11 0 4840 14 267 NO3 f O2 + NO 2.05E+08 1 12122 14 268 NO3 + NO2 f NO + NO2 + O2 3.25E+10 0 2960 14 269 NO3 + NO f NO2 + NO2 1.08E+13 0 -219 14 270 HNO + HNO f H2O + N2O 9.00E+08 0 3080 14 271 H + NO + M f HNO + Ma 8.95E+19 -1.32 740 28 H2/2.00/H2O/6.00/CH4/2.00/CO/1.50/CO2/2.00/C2H6/3.00/Ar/0.70 272 HNO + O f NO + OH 2.50E+13 0 0 28 273 HNO + H f H2 + NO 4.50E+11 0.72 660 28 274 HNO + OH f NO + H2O 1.30E+07 1.9 -950 28 275 HNO + O2 f HO2 + NO 1.00E+13 0 13000 28 276 HONO + OH f H2O + NO2 1.69E+12 0 -517 14 277 HONO + HONO f H2O + NO2 + NO 2.30E+12 0 8350 14 278 HONO + O f OH + NO2 1.00E+12 0 0 14 279 HONO2 + OH f H2O + NO3 1.03E+10 0 -1240 14 280 CH3NO2 + H f H2CNO2 + H2 7.50E+12 0 10000 14 281 CH3NO2 + O f H2CNO2 + OH 1.51E+13 0 5350 14 282 CH3NO2 + OH f H2CNO2 + H2O 1.49E+13 0 5740 14 283 CH3NO2 + H f HONO + CH3 3.27E+12 0 3730 14 284 CH3NO2 + H f CH3NO + OH 1.40E+12 0 3730 14 285 CH3NO2 + CH3 f CH4 + H2CNO2 7.08E+11 0 11140 14 286 H2CNO2 f CH2 + NO2 1.00E+13 0 36000 14 287 H2CNO2 + HONO f CH3NO2 + NO2 1.00E+12 0 0 14 288 CH3NO2 f CH3ONO 2.90E+14 0 67000 14

a +M; Enhanced/Reduced third body efficiencies.25 b (+M); Troe form for the pressure dependent reaction25 unit; mol, cal, cm, s. c The (E + N) part of the expression should be interpreted as ×10N.

Figure 1. The experimental values and the simulated results of the selectivities of the products as functions of CH4 conversions in the gas-phase selective oxidation of CH4-O2-NO at atmospheric pressure. The composition of the feed gas was CH4 (55.6 mol %), O2 (27.7 mol %), NO (0.5 mol %), and He (16.2 mol %). SV ) 3740 h-1. Experimental data: CH3OH (O), CH2O (0), CO (]), CO2 (4), C2H6 (3), CH3NO2 (×); simulated data: CH3OH (b), CH2O (9), CO ([), CO2 (2), C2H6 (1), CH3NO2 (+). Reaction temperatures were 540 °C (CH4 conversion ) 0.74%), 550 °C (1.71%), 560 °C (5.3%), 562 °C (10.0%), and 570 °C (40.6%), respectively.

with the CHEMKIN III. The experimentally obtained values of selectivities of CO and CH2O were raised and lowered, respectively, with the increase of CH4 conversions, and the simulated results of these species were in good agreement. The simulated results of selectivity of CH3OH also agreed with the experimentally obtained values except for the region which was less than 10% CH4 conversion. The experimentally obtained values of selectivity of CH3OH increased linearly until 10% CH4

conversion but the simulated results decreased gradually with CH4 conversions in all of the examined regions. The experimentally obtained selectivities of both CH3NO2 and C2H6 were small, and the simulated results of these selectivities agreed well. Concerning the differences between the experimental values and the simulated results of the selectivity of CH3OH in the region of less than 10% CH4 conversion, we examined the effects of the space velocity (SV) on the selectivities in order to assess the appropriateness of the simulations with CHEMKIN III. The experimental values and the simulated results of selectivities of all of the products at SV ) 470 h-1 are shown in Figure 2. All experimental data and simulated values were obtained with the same method in Figure 1. All of the products were the same as those observed at SV ) 3740 h-1. The experimental values of selectivities of CO and CH2O at SV ) 470 h-1 were raised and lowered, respectively, in comparison to those at SV ) 3740 h-1, and the simulated results of CH2O selectivity exceeded the experimental values at SV) 470 h-1. The experimental values of CH3OH selectivity at SV ) 470 h-1 in the region of less than 10% CH4 conversion decreased. This tendency was clearly different from that at SV ) 3740 h-1. Furthermore, these experimental values were smaller than those at SV ) 3740 h-1. Since the experimentally obtained selectivities of both CH2O and CH3OH were lowered and that of CO was raised at SV ) 470 h-1, the subsequential oxidation of the produced C1-oxygenates to CO progressed more at the lower SV value. The variations of experimental values of CH3OH selectivity at SV ) 470 h-1 and 3740 h-1 were different, therefore we examined the distribution of temperatures in the reactor because the temperature profile close to

Optimization of C1-Oxygenates for CH4 Oxidation

Energy & Fuels, Vol. 15, No. 1, 2001 49

CH3O + CH4 f CH3OH + CH3

Figure 2. The experimental values and the simulated results of the selectivities of the products as functions of CH4 conversions in the gas-phase selective oxidation of CH4-O2-NO at atmospheric pressure. The composition of the feed gas was CH4 (55.6 mol %), O2 (27.7 mol %), NO (0.5 mol %), and He (16.2 mol %). SV ) 470 h-1. Reaction temperatures were 475 °C (CH4 conversion ) 0.7%), 500 °C (3.2%), 525 °C (7.5%), 530 °C (10.0%), 550 °C (35.0%), and 575 °C (35.0%), respectively. The symbols in the figure are the same as those in Figure 1.

Figure 3. Temperature profiles from the inlet to the outlet at 475 and 575 °C in the reactor, respectively. SV ) 470 and 3740 h-1. CH4 conversions at 475 °C were 0.7% at 470 h-1 and 0.15% at 3740 h-1, and those at 575 °C were 35% at 470 h-1 and 41% at 3740 h-1, respectively.

the inlet of the feed gas could be different. Since the selective oxidation of methane is highly exothermic reaction, the existence of temperature distribution could be expected along the length of the reactor as reported by Foulds et al.30 We showed the temperature profiles at two different reaction temperatures (475 and 575 °C) from the inlet to the outlet of the reactor at both SV ) 470 and 3740 h-1 (Figure 3). The slopes of temperatures near the inlet of the feed gas at SV ) 3740 h-1 were larger than those at 470 h-1, especially at 475 °C, i.e., at a lower CH4 conversion. These different slopes of the temperature profiles were assumed to affect the selectivities, especially on the different variations of experimental CH3OH selectivities. We suggested the following equation as the main route for the formation of CH3OH in the previous paper:23

This equation therefore could be affected by the existence of temperature distribution close to the inlet of feed gas, and this temperature distribution should also affect the results of the simulated selectivities because we utilized the isothermal model for the simulations. The slopes of the simulations at SV ) 470 and 3740 h-1 in the region of less than 10% CH4 conversion were different as shown in Figures 1 and 2. We examined the optimized reaction conditions in order to get the highest yield of the C1-oxygenates. First, we examined the effects of the ratio of CH4/O2 in the feed gas (Figure 4). The experimental selectivities of both CH2O and CH3OH changed slightly in the examined regions, and the simulated results of these species were close to the experimental values. The total selectivities of C1-oxygenates, i.e., CH2O + CH3OH were seems to have the highest value at around CH4/O2 ) 3-5 in the simulated results, but the experimental values of the selectivity of CH2O + CH3OH had the highest value at CH4/O2 ) 2, although the difference of total selectivities of C1-oxygenates between the experiments and the simulation was small. Both of the selectivities of CH2O and CH3OH were lowered in the region over CH4/O2 ) 6 and that of CO increased. These decreases of selectivities of CH2O and CH3OH could be explained by the higher reaction temperature even at the same 10% CH4 conversion.23 Second, we examined the effects of NO concentration in the feed gas (Figure 5). The effects of NO on the simulated values of the selectivities of CH2O and CH3OH were different, i.e., the simulated selectivity of CH2O increased with the concentration of NO, then stabilized over 0.5% NO, but that of CH3OH decreased linearly in all of the examined regions. These variations of the simulated results of both CH2O and CH3OH selectivities as functions of NO concentrations could be explained as follows: The main reaction route of CH3OH formation was suggested as eq 1.23 However, three formation routes were suggested for the formation of CH2O:23

CH3O + O2 f CH2O + HO2

(2)

CH3O + NO f CH2O + HNO

(3)

CH3O + NO2 f CH2O + HNO2

(4)

Therefore, the simulated values of selectivity of CH2O could be raised by eqs 3 and 4 with the increase of NO. Concerning the reason of the stabilization of the simulated line of CH2O selectivity in the region over 0.5% NO, we speculated that OH radicals which were produced in the course of the reactions23 decomposed some of the produced CH2O as follows:

CH2O + OH f CHO + H2O

(5)

The simulated line of CH3OH decreased constantly. This decrease could be explained by the following equations: 23

CH3OH + NO2 f CH2OH + HNO2 (30) Foulds, G. A.; Gray, B. F. Fuel Processing Technol. 1995, 42, 129-150.

(1)

and

(6)

50

Energy & Fuels, Vol. 15, No. 1, 2001

Takemoto et al.

Figure 4. The experimental values and the simulated results of the selectivities of the products as functions of CH4/O2 in the feed gas of CH4-O2-NO at atmospheric pressure. SV ) 3740 h-1. Reaction temperatures were 538 °C (CH4/O2 ) 1), 562 °C (CH4/O2 ) 2), 568 °C (CH4/O2 ) 4), and 645 °C (CH4/O2 ) 9), respectively. The symbols in the figure are the same as those in Figure 1.

Figure 6. The experimental values and the simulated results of the selectivities of the products as functions of NO/NOx in the feed gas of CH4-O2-NO at atmospheric pressure. The ratio of CH4/O2 was 2. SV ) 3740 h-1. Reaction temperatures were 562 °C (NO/NOx ) 1), 540 °C (NO/NOx ) 0.5), and 550 °C (NO/NOx ) 0), respectively. The symbols in the figure are the same as those in Figure 1.

Figure 5. The experimental values and the simulated results of the selectivities of the products as functions of NO concentration in the feed gas of CH4-O2-NO at atmospheric pressure. The ratio of CH4/O2 was 2. SV ) 3740 h-1. Reaction temperatures were 588 °C (NO ) 0.13%), 562 °C (0.5%) and 525 °C (1.5%), respectively. The symbols in the figure are the same as those in Figure 1.

Figure 7. The experimental values and the simulated results of the selectivities of the products as functions of CH4 conversions in the gas-phase selective oxidation of CH4-O2-NO at atmospheric pressure. The composition of the feed gas, SV, and experimental values were the same as those in Figure 1. The elementary reactions of the decomposition reactions of CH2O and CH3OH with OH: CH2O + OH f HCO + H2O; CH3OH + OH f CH2OH + H2O; CH3OH + OH f CH3O + H2O were omitted from the simulations in Figure 1. The symbols in the figure are the same as those in Figure 1.

CH3OH + OH f CH2OH + H2O

(7)

The experimental values of CH3OH and CH2O selectivities at 0.13% NO were smaller and larger, respectively, than their simulated values at the same concentration of NO. These differences could be explained by the higher reaction temperature at 10% CH4 conversion with a smaller concentration of NO.23 From these simulated results and experimental values, the highest yield of the C1-oxygenates was expected at 0.5% NO. As mentioned above, the activation energies of hydrogen abstraction from CH4 with NO or NO2 were reported to be 65.6 and 37.6 kcal/mol, respectively.24 We therefore assumed that the ratio of NO to NOx might affect the selectivities of the products. However, both of the experimental values and the simulated results were hardly affected with the ratio of NO to total NOx (Figure 6). The total concentration of NOx was 0.5%. Therefore, the total yield of C1-oxygenates was assumed to be hardly affected by this ratio.

Last of all, we simulated the selectivities of the products on the assumption that the decomposition reactions of the produced CH2O and CH3OH with OH radicals in eqs 5, 7, and the following eq 8 were omitted from the course of the reactions (Figure 7):

CH3OH + OH f CH3O + H2O

(8)

The simulated selectivity of CH2O was raised in all of the examined regions but that of CH3OH was raised only in the region over 10% conversion of CH4. The simulated yield of C1-oxygenates at 10% conversion of CH4 was 77% with this modified simulation. Since the experimentally obtained C1-oxygenates at 10% conversion of CH4 was 57%, the OH radicals in the reaction gas were assumed to have strong effects on the decrease of the yield of C1-oxygenates in the products.

Optimization of C1-Oxygenates for CH4 Oxidation

Conclusions The kinetic simulations with the software package of CHEMKIN III well and appropriately reproduced the experimentally obtained selectivities of the products in the gas-phase selective oxidation of CH4-O2-NO except for that of CH3OH in the region of less than 10% conversion of CH4. We therefore utilized these simulations in order to examine the optimized conditions for getting the highest yield of C1-oxygenates. The effects of several experimental conditions, i.e., SV, CH4/O2, NO concentration, NO/NOx, were examined, but all of the simulated values of selectivity of C1-oxygenates did not

Energy & Fuels, Vol. 15, No. 1, 2001 51

exceed the experimentally obtained value. The highest yield of experimentally obtained C1-oxygenates at 10% conversion of CH4 was 57% but if the decomposition reactions of both CH2O and CH3OH with OH radicals in the reaction gas were omitted, the simulated selectivity of C1-oxygenates would attain to 77%. Acknowledgment. We acknowledge financial support of the New Energy and Industrial Technology Development Organization (NEDO). Y. Teng was supported by a Fellowship from the NEDO. EF000087+