Direct synthesis of acetylene from methane by direct current pulse discharge

摘要

Direct synthesis of acetylene from methane by direct current pulse discharge Shigeru Kado,* Yasushi Sekine and Kaoru Fujimoto Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: tt97208@mail.ecc.u-tokyo.ac.jp Received (in Cambridge, UK) 25th September 1999, Accepted 1st November 1999 In non-catalytic direct conversion of methane to acetylene by using direct current pulse discharge under conditions of ambient temperature and atmospheric pressure, the selectivity of acetylene from methane was 95 at methane conversion ranging from 16 to 52; coexisting oxygen was very effective in removing deposited carbon and stabilized the state of discharge.Methane, which is a major constituent of natural gas, is so stable that high reaction temperatures such as 1273 K or higher are required for its pyrolysis to ethylene or acetylene.1 Although high reaction temperature is favorable for high conversion, higher temperature promotes the consecutive decomposition of the products to carbon.1,2 Recently, direct conversion of methane to higher hydrocarbons using plasma technology has been studied to improve the selectivity and yield of the desired products.3ndash;8 For example, in microwave plasma reaction, acetylene was produced with a selectivity of 90 from methane at as low a reaction pressure as 4.5 kPa.6 The plasma catalytic conversion of methane by using dc corona discharge was also found to produce acetylene with high selectivity and yield under atmospheric pressure in the temperature range 343ndash;773 K.The nature of the catalyst surface in contact with the plasma was very important for homogeneous activation of methane and NaY zeolite gave the highest yields of C2 hydrocarbons. The highest yield of C2 hydrocarbons (32) was obtained in a hydrogen-containing plasma at a flow rate of 10 cm3 min21.7 In this study, dc pulse discharge was applied to non-catalytic direct conversion of methane at ambient temperature and under atmospheric pressure to prepare C2 hydrocarbons selectively, with high methane conversion. A flow type reaction apparatus which was composed of a Pyrex glass tube of 4.0 mm internal diameter was used as the reactor.Reactant gas which was premixed at a given ratio was fed at a constant flow rate. Stainless steel electrodes of 2 mm diameter were inserted from each end of the reactor as shown in Fig. 1 and the distance between the electrodes was 1.5 mm. A dc pulse discharge was initiated by supplying a negative high voltage with a dc power generator. The wave signal was observed by a digital oscilloscope and the pulse duration was ca. 10 ms. All the reactions were conducted at atmospheric pressure and ambient temperature without any catalyst and all products were analyzed by gas chromatography. Product selectivity was defined as follows; selectivity () = yield of the product (Carbon mmol)/sum of all the products (Carbon mmol) 3 100.Upon dc pulse discharge, methane was activated readily to form acetylene with a selectivity of 90 in the absence of catalyst. Fig. 2 shows methane conversion, yield of C2 hydrocarbons and selectivity of acetylene, ethylene and ethane vs. supplied power. While methane conversion increased up to 52 by the increasing power supply, acetylene selectivity was very stable at ca. 95. Applying a discharge for methane activation thus led to the selective formation of acetylene with high C2 yield and stable selectivity, not observed in conventional homogenous gas phase reactions.Other hydrocarbon products such as prop-1-yne and buta-1,3-diyne were at 1. Some carbon deposition occurred on both electrodes and the reactor wall during the reaction and eventually resulted in unstable discharge and sometimes in cessation. In order to prevent carbon deposition during discharge, O2 and Ar were added to give a resultant feed gas composition CH4ndash;O2ndash;Ar = 5+1+4.Reaction results under a flow of 10 cm3 min21 at normal temperature and pressure (NTP) and 25 W power supply are given in Table 1. Carbon monoxide was produced as well as carbon dioxide (selectivity 1 to CO2), in addition to C2 compounds. Comparing results with those of the reaction with pure methane, there was little difference in C2 yield, which indicates that the increased methane conversion was essentially due to the carbon monoxide formed.Additionally, the composition of C2 compounds was not much affected. In the reaction using pure methane, the C2 yield was 40.6 and the amount of acetylene in the C2 products was 94.4, while the reaction in the presence of O2 gave 91 acetylene selectivity in terms of non-CO/CO2 carbon products with yields of C2 compounds and CO of 38 and 20, respectively. These results suggest that the precursor of the deposited carbon is converted to carbon monoxide by reaction with oxygen.Also the reaction in the presence of O2 led to stable discharge. The effect of total flow rate (from 3 to 225 cm3 min21 in the presence of O2) with a supplied power of ca. 25 W is also shown in Table 1. Methane and oxygen conversion was remarkably Fig. 1 Schematic diagram of the reactor. (a) 2.0 mm diameter stainless steel electrode, (b) 4.6 mm internal diameter quartz tube, (c) thermocouple, (d) negative high voltage, (e) ground, (f) direction of the flow gas, (g) direct current high voltage power supply, (h) digital oscilloscope.Fig. 2 Effect of supplied power on conversion and selectivity. Reaction conditions: pure methane, 10 cm3 min21 (NTP) flow rate, ambient temperature, 0.1 MPa, 1.5 mm electrode distance, (-) methane conv., (5) C2 yield, (2) acetylene selectivity, (frac12;) ethylene selectivity, (8) ethane selectivity. This journal is copy; The Royal Society of Chemistry 1999 Chem. Commun., 1999, 2485ndash;2486 2485affected by the total flow rate.At 3 cm3 min21 total flow rate, oxygen conversion reached 100 and methane conversion was 84. On the other hand, the selectivity towards acetylene was scarcely affected while the C2 yield increased to ca. 50. The selectivity to carbon monoxide increased slightly as the total flow rate was decreased. If carbon monoxide was formed by a chain reaction, its selectivity should increase more drastically with decreasing flow rate.Thus, the behavior of carbon monoxide selectivity to the total flow rate indicated that carbon monoxide formation was separate from C2 compound formation and derived from a coke precursor. We also attempted to stabilize the discharge by using hydrogen as the coexisting gas, since using oxygen caused the formation of carbon monoxide which decreased the selectivity to C2 compounds. The reaction results were much the same as for pure methane, and H2 did not stabilize discharge as well as O2.However, dilution of methane by hydrogen such that CH4+H2 = 1+4 was effective for stabilization of the discharge. Table 2 shows methane conversion, C2 yield and selectivities to each C2 compound at various H2 concentrations from 0 to 90. Up to 50 hydrogen concentration, a marked effect of H2 was not observed. However as the hydrogen concentration was increased, methane conversion and C2 yield increased from 44 (pure methane) to 57 (CH4+H2 = 1+2). At 80 hydrogen concentration, methane conversion and acetylene selectivity decreased slightly, and selectivity towards ethane and ethylene increased.At conditions of 80 hydrogen concentration, the excessive concentration of hydrogen interfered with electron attack on methane, leading to drastic reduction in methane conversion and to an increase in ethane selectivity. In conclusion, when a dc pulse discharge was applied for methane activation, acetylene was produced directly and selectively.Oxygen removed deposited carbon by oxidation to form carbon monoxide, and stabilized the state of discharge. Dilution of methane with hydrogen up to 80 also stabilized the state of discharge without changing the product selectivity. We assume that the formation of acetylene is via dimerisation of CH radicals which form by dc discharge, and that the high selectivity to acetylene is a characteristic feature of short range pulse discharge. Notes and references 1 F.G. Billaud, F. Baronnet and C. P. Gueret, Ind. Eng. Chem. Res., 1993, 32, 1549. 2 A. Holmen, O. Olsvik and O. A. Rokstad, Fuel Process. Technol., 1995, 42, 249. 3 M. S. Ioffe, S. D. Pollington and J. K. S. Wan, J. Catal., 1995, 151, 349. 4 C. Liu, A. Marafee, B. Hill, G. Xu, R. Mallinson and L. Lobban, Ind. Eng. Chem. Res., 1996, 35, 3295. 5 A. Marafee, C. Liu, G. Xu, R. Mallinson and L. Lobban, Ind. Eng. Chem. Res., 1997, 36, 632. 6 K. Onoe, A. Fujie, T. Yamaguchi and Y. Hatano, Fuel, 1997, 76, 281. 7 C. Liu, R. Mallinson and L. Lobban, J. Catal., 1998, 179, 326. 8 K. Thanyachotpaiboon, S. Chavadej, T. A. Caldwell, L. L. Lobban and R. G. Mallinson, AIChE J., 1998, 44, 2252. Communication 9/06914C Table 1 Effect of coexisting oxygen and total flow ratea Flow rate Conv. () Yield () Selectivity () (NTP)/ Feed gas cm3 min21 CH4 O2 C2 CO C2H2 CO CH4 10 40.6 mdash; 39.9 mdash; 94.4 mdash; CH4ndash;O2ndash;Ar (5+1+4) 3 83.9 100.0 49.6 33.8 56.2 40.3 CH4ndash;O2ndash;Ar (5+1+4) 10 58.9 69.9 38.1 20.2 58.9 33.8 CH4ndash;O2ndash;Ar (5+1+4) 225 6.8 9.7 4.9 1.9 62.3 27.9 a Reaction conditions: ambient temperature, 0.1 MPa, 1.5 mm electrode distance, 25 W supplied power. Table 2 Effect of hydrogen concentration on conversion and selectivitya H2 Selectivity () concentration CH4 conv. C2 yield () () () C2H6 C2H4 C2H2 0 44.3 44.2 0.6 2.8 94.6 50 46.3 46.2 1.1 4.4 94.2 67 57.4 57.1 1.2 5.7 92.7 80 47.0 46.6 2.9 8.1 88.2 90 11.8 11.4 17.7 15.2 63.7 a Reaction conditions: 10 cm3 min21 (NTP) flow rate, ambient temperature, 0.1 MPa, 1.5 mm electrode distance, 4 mA dc. 2486 Chem. Commun., 1999, 2485ndash;2486