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Energy & Fuels 1996, 10, 1278-1279
Low-Temperature Direct Conversion of Methane to Methanol with Filament Wire Initiation Method Yasushi Sekine*,† and Kaoru Fujimoto Department of Applied Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 113, Japan Received January 15, 1996 1. Introduction Many attempts at direct methane oxidation have been made with the aim of using natural gas, in either a catalyzed system or noncatalyzed system.1-3 The problem in the noncatalyzed system is that because of the high stability of methane, high temperature (∼673 K) is required for methane activation4 and thus high temperature generates many side reactions that make COx and invaluable products. So we tried to initiate the CH4-O2 reaction by using high-temperature NiCr filament wire to maintain gas phase temperature very low. In the present study, the location of methane activation was separated from that of the reaction. On and around the electrically heated small Ni-Cr filament wire (∼1450 K), a small amount of methane and oxygen was reacted to generate methyl radical, which diffuses into the gas phase maintained at 420-520 K. In the gas phase, the formed methyl radical may generate a series of chain reactions which include methanol formation.
Figure 1. Batchwise reactor made of SUS316, inner volume ) 350 cm3.
2. Experimental Section Experiments were conducted with a batchwise reaction system under high-pressure condition. The apparatus is shown in Figure 1. A Ni-Cr filament (0.4 mm diameter, 100 cm length) was set at the center of the reactor (made of SUS316, inner volume 350 cm3) and electrically heated up to 1450 K. From the top of the reactor, well-mixed methane and air was introduced into the reaction chamber. The gases reacted in the reactor, and products were taken out from the side hole through a pressure regulator. The main chamber was maintained at constant temperature (about 473 K) by an outer heater so as to prevent the condensation of products. Products were analyzed at 30 min intervals with on-line gas chromatography (Shimadzu GC14A and Hitachi GC164).
Figure 2. Sequence of CH4 oxidation.
chain reactions smoothly.4
CH4 f CH3• + H•
(1)
CH3• + O2 f CH3OO•
(2)
CH3OO• + CH3• f 2CH3O•
(3)
CH3O• + CH4 f CH3OH + CH3•
(4)
3. Results and Discussion 3.1. Reaction Model. On and around the electrically heated Ni-Cr wire at the center of the reactor, a small amount of methane and oxygen molecules is dissociated thermally to make methyl radical, and other free radicals are formed. The formed radicals diffuse into the reaction chamber and collide with oxygen molecules. The methyl radical seems to have very short life (about 10-2-10-3 s), but once the methyl peroxide radical (CH3OO•) is formed, it will promote the following †
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[email protected]. (1) Han, L.; Tsubota, S.; Kobayashi, T.; Haruta, M. J. Chem. Soc., Chem. Commun. 1995, 1, 93-94. (2) Feng, W.; Knopf, F.; Dooley, K. Energy Fuels 1994, 8, 815-822. (3) Olav, T.; Onsager, et al. Ind. Eng. Chem. Res. 1995, 34, 10441059. (4) Krylov, O. V. Catal. Today 1993, 18, 209-302.
Thus, we estimate a series of chain reactions which involves an initiation by methane activation on and around the high-temperature filament wire and its oxidation by oxygen to make methyl peroxide radical and methoxy radical formation (Figure 2). 3.2. Change in the Product Selectivity with Reaction Time. Figure 3 shows the changes in methane conversions and product selectivity with process time. Reaction conditions were 473 K and 40 kg/cm2.
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Energy & Fuels, Vol. 10, No. 6, 1996 1279
Figure 4. Pressure effect. Temperature 473 K, CH4/air ) 4/1. Figure 3. Product selectivity with reaction time. Pressure 40 kg/cm2, temperature 453 K, CH4/air ) 4/1.
After a certain induction period (about 100 min), methane conversion started. At an early stage of the reaction, methanol selectivity was about 90%. As the reaction time passed, conversions of methane and oxygen increased to reach about 4% and 100%, respectively, at 600 min. Also, as the time passed, a small amount of hydrogen was formed. As the methane conversion increased, the methanol selectivity decreased quickly in compensation for the increases in COx (mostly CO) and H2 formation. The final selectivity was methanol 22%, CO2 12%, CO 55%, and C2 hydrocarbons 10% carbon based. These phenomena suggest that methanol is formed selectively from methane and oxygen. But methanol is decomposed to carbon monoxide and hydrogen gas successively. 3.3. Effect of Reaction Pressure. It was tried to estimate the effect of reaction pressure by using a batchwise reactor changing the reaction pressure and the same gas mixture and wattage of filament. At the same methane conversion level, C2 hydrocarbons were predominant products at lower pressure (shown in Figure 4). And pressure increased, the selectivity of methanol formation also rose. Therefore, it is apparent that methanol formation requires high pressure. The reason is as follows: the elementary reaction (2) is dependent on pressure and under high pressurized condition, equilibrium of reaction 2 shifts to right side, and so the formed methyl radical will be smoothly converted to methyl peroxide radical and recombination of CH3 to C2 hydrocarbon is difficult. 3.4. Oxygen Conversion Level and MeOH Selectivity. It is clear that from the data in Figure 5 oxygen conversion and MeOH selectivity have an apparent relation, although when oxygen conversion was around 100%, MeOH selectivity was only 30% and its level was much higher at lower oxygen conversion level.
Figure 5. O2 conversion and MeOH selectivity. CH4/air ) 4/1, pressure 40 kg/cm2, temperature 473 K.
This phenomenon suggests that the remaining oxygen in the reactor chamber may decompose the produced methanol successively. So it is required that the formed methanol should be isolated from the nonreacted remaining oxygen and be taken out quickly from the reaction chamber so as to avoid the oxidation of methanol. 4. Conclusion From these results, we propose the mechanism of this reaction as follows: Methyl radicals which were generated on and around the high-temperature filament wire reacted smoothly with oxygen at the temperature even as low as 473 K and turned to methoxy radical by way of methyl peroxide radical, and finally to methanol. The methanol is not stable under the reaction conditions and thus is decomposed successively (by way of radical mechanism) to CO and H2. And also, it can be concluded that high-temperature filament wire can initiate the methane oxidation with oxygen to make methanol selectively at 473 K. EF960009F
