Liu - Zeolite Plasma Methane Conversion

Jointly published by Akadémiai Kiadó, Budapest and Kluwer Academic Publishers, Dordrecht

React.Kinet.Catal.Lett. Vol. 74, No. 1, 71-77 (2001)

RKCL3917 ZEOLITE-ENHANCED PLASMA METHANE CONVERSION DIRECTLY TO HIGHER HYDROCARBONS USING DIELECTRICBARRIER DISCHARGES Changjun Liua ,b*,Baldur Eliassonb,c, Bingzhang Xueb,c, Yang Lia,b and Yaquan Wanga a

State Key Laboratory of C1 Chemical Technologies Tianjin University-ABB Plasma Greenhouse Gas Chemistry Laboratory Tianjin University, Tianjin 300072, P.R.China c ABB Corporate Research Ltd., Baden, CH-5405, Switzerland

b

Received May 9, 2001 Accepted June 21, 2001

Abstract A zeolite-enhanced plasma methane conversion with pure methane feed using dielectric-barrier discharges (DBDs) at atmospheric pressure has been conducted. This plasma methane conversion over NaX has led to a selective production of light hydrocarbons. Keywords: Methane conversion, dielectric-barrier discharge plasma, zeolite

INTRODUCTION The significant potential benefit of economically converting methane, the principal component of natural gas, to more valuable hydrocarbons has been well established. An intense investigation has been conducted to convert methane directly to higher hydrocarbons or oxygenated hydrocarbons catalytically. However, there is still no proven technology for such direct catalytic methane conversion. It is important to investigate continuously the catalytic methane conversion, while to exploit some alternative technologies simultaneously. The plasma is a promising alternative for methane conversion.

0133-1736/2001/US$ 12.00. © Akadémiai Kiadó, Budapest. All rights reserved.

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The methane conversion using plasmas can lead to a much higher methane conversion (close to 100% in some cases) [1-10]. There are usually two ways for plasma methane conversion, one is indirect plasma methane conversion via syngas step [1,2], and the other is direct plasma methane conversion to more valuable hydrocarbons [3-10]. The former one produces syngas first using plasma and then produces more valuable hydrocarbons from syngas catalytically. The pilot test of syngas production using plasmas has been reported recently [11,12]. The direct way produces more valuable hydrocarbons from methane using plasmas without through syngas step that attracts more attentions recently. However, co-reactant or oxidant or dilution gas is needed to sustain a stable operation of plasma methane conversion directly [3-10], which leads to a reduction in the selectivity or yield of objective products. Especially, there are few reports for the use of pure methane feed for plasma conversion (at atmospheric pressure) to clean liquid fuels. In this paper, we present a zeoliteenhanced methane conversion directly to higher hydrocarbon using dielectricbarrier discharge (DBD) plasmas over NaX zeolite with pure methane feed. EXPERIMENTAL The DBD reactor applied here, as shown in Fig. 1, is similar to that used in previous investigations on plasma methane conversion with carbon dioxide as co-reactant using DBDs [3]. The gas flow is subjected to the action of DBD in an annular gap (the radical width is 1 mm and its length is 300 mm) formed between an outer steel cylinder maintained at constant temperature and an inner quartz tube. A high voltage generator working at about 30 kHz applies the power. When the amplitude of applied ac electric field reaches a critical value, breakdown is initiated in the gas. Once breakdown is initiated at any location within the discharge gap, charge accumulates on the dielectric resulting in an opposite electric field. This opposite electric field locally reduces the external electric field in the gap and interrupts the current flow within a few nanoseconds. By this mechanism individual current filaments called microdischarges are formed. Their duration depends on the pressure, the properties of gas involved, the dielectric and zeolite used. A large number of such microdischarges will be generated when a stable DBD operation is established, esp. in the case that zeolite presents. The principal advantages of the DBD are that non-equilibrium conditions can be easily established at atmospheric pressure and that the entire electrode area is effectively used for discharge reactions. The feed gas, methane, was introduced into the system flowing through the reactor. Two MTI (Microsensor Technology Inc., M200H) dual-module micro gas chromatograph containing a Poraplot Q column, an Al2O3 " S" column and

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a molecular sieve 5Å. Plot column with TCD detectors were used to detect gaseous products. The liquid sample was analysed at the Laboratoire d'Analyse at the Shell Raffinerie de Cressier/Switzerland with gas chromatographs. It contains more than 200 branched higher hydrocarbon (HHC) components in the range of C5 to C11 that represents a better fuel production. A HP6890 GC with FID was also used to analyze the carbonaceous species adsorbed on the plasmaused zeolite. NaX zeolite was used here to inhibit the formation of carbon black and to increase the yield of gasoline and/or light hydrocarbons. The zeolite was obtained from Condea Augusta and applied as received. In each case about 9 g zeolite powder was introduced into the discharge gap. The loading of zeolite has also been described elsewhere [3]. The gas temperature was measured using two thermocouples: one has been located in the gap between the dielectric and the grounded electrode, while the other has been placed in the exit of active region of DBD plasmas. A TGA study was performed for Temperature Programmed Oxidation (TPO) characteristics of DBD plasma-used zeolite using a Shimadzu TGA-50 system under an air stream (flow rate: 20 mL/min). 2.319 mg of DBD plasma-used NaX zeolite sample was mounted in a platinum cell. The reaction temperature with the TPO was raised from room temperature with a ramping rate of 10°C/min.

Fig. 1. A schematically representative of DBD plasma reactor system

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RESULTS AND DISCUSSION The effective active region for DBD plasma methane conversion is within the gap between the dielectric (quartz) and the grounded electrode (the outer stainless steel cylinder), as shown in Fig. 1. The following reactions would occur at the initiation of DBD plasma CH4 conversion: CH4 → CH3 + H

(1) CH2 + H

(2) CH + H

(3)

carbon atom select. (wt%)

C+H

(4)

10 9 8 7 6 5 4 3 2 1 0 2

3

4

5

6

7

8

9

carbon number

Fig. 2. The carbon atom selectivity via carbon number from the experiment of pure methane feed with NaX (zeolite amount: 9.0 g, input power: 500 W, flow rate: 150 mL/min, pressure: 1 bar, gas temperature: 150°C, frequency: around 30 kHz, methane conversion: 26.8%)

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In addition to carbon black, the radical chain reactions will start from CH3, CH2 and CH to generate higher hydrocarbons. The experimental investigation, however, shows the presence of zeolite within DBD plasmas can stop the reaction at the step of reaction (3). The DBD plasma methane conversion has therefore become sustainable with pure methane feed. The liquid sample collected directly from the effluents of DBD reactor has a carbon number range from C5 to C11, upon the results of GC analysis. Figure 2 presents carbon atom selectivity with the increasing carbon number in the range from C2 to C11. From Fig. 2, the most selective product of DBD plasma methane conversion is C2 hydrocarbons. With increasing carbon number, the carbon atom selectivity decreases at the beginning. To our surprise, the curve shows a jump in the carbon atom selectivity at the carbon number from 5 to 8. The oily carbonaceous species were also produced on the zeolite. Table 1 shows a GC analysis of the composition of oily carbonaceous species from the DBD reactions using pure methane feed over NaX zeolite. Before this GC analysis, the plasma-used zeolite was put into a pure CCl4 solution. The carbonaceous species were thereby dissolved into the CCl4 solution to form a mixture. Then the GC analysis was conducted to analyze the component of this mixture. Table 1 also shows that a significant amount of branched hydrocarbons have been produced that are characteristics of DBD plasma reactions.

Table 1 Composition of carbonaceous species on DBD plasma-used NaX (pure methane feed; flow rate: 150 mL/min; input power: 500W; gas temperature: 150°C; pressure: 1 bar; methane conversion: 26.8%; catalyst weight: 9.0 g ) Component

≤ C15 C15 iC16+iC17 C17 iC18 C18 iC19 C19 iC20

Composition (mol%) 5.47 14.00 12.50 3.32 1.48 0.35 3.76 0.95 1.13

Component

C20 C21 iC21+C22+iC23 C23 iC24 C24 iC25 iC26 C26 >C26

Composition (mol%) 2.51 4.31 8.99 1.64 9.62 5.58 3.60 7.80 10.77 2.21

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TPO characteristics of DBD plasma-used zeolite Thermal analysis of DBD plasma-used zeolite shows a significant weight loss of ca. 280°C, which can be attributed to the oxidation of carbonaceous species (the components were shown in Table 1). A continuous weight loss is shown in the TGA spectrum (Fig. 3) that suggests a wide range of carbonaceous deposits. The TGA indicates an around 30% weight loss after the TPO measurement. It is clear that a large amount of carbonaceous species have been formed on the zeolite during the DBD plasma methane conversion. Further investigation is being conducted to understand the mechanism of interaction between zeolite and DBD plasmas.

Fig. 3. TGA spectrum of DBD plasma-used NaX zeolite from the experiment of pure methane feed with NaX (zeolite amount: 9.0 g, input power: 500 W, flow rate: 150 mL/min, frequency: around 30 kHz, pressure: 1 bar, gas temperature: 150°C, methane conversion: 26.8%)

CONCLUSION The present investigation confirmed the effectiveness of DBD plasma in converting pure methane directly into higher hydrocarbons in the presence of zeolite without using co-reactant or dilution gas. The liquid hydrocarbons produced are highly branched that represents a better fuel production.

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Acknowledgements. This investigation was supported in part by ABB Corporate Research Ltd., Switzerland, Key Fundamental Research Project of Ministry of Science and Technology of China (G1999022402) and National Natural Science Foundation of China (29806011). REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

C.-J. Liu, G.-H. Xu, T. Wang: Fuel Processing Tech., 58, 119 (1999). H. Lesueur, A. Czernichowski, J. Chapelle: Int. J. Hydrogen Energy, 19, 139 (1994). B. Eliasson, C.-J. Liu, U. Kogelschatz: Ind. Eng. Chem. Res., 39, 1221 (2000). C.-J. Liu, R. Mallinson, L. Lobban: J. Catal., 179, 326 (1998). S.L. Yao, A. Nakayama, E. Suzuki: AIChE J., 47, 419 (2000). S.L. Yao, F. Ouyang, A. Nakayama, E. Suzuki, M. Okumoto, A. Mizuno: Energy & Fuels, 14, 910 (2000). M. Okumoto, B.S. Rajanikanth, S. Katsura, A. Mizuno: IEEE Trans. on Ind. Appl., 34, 940 (1998). J.C. Legrand, A.-M. Diamy, R. Hrach, V. Hrachova: Vacuum, 52, 27 (1999). S. Kado, Y. Sekine, K. Fujimoto: Chem. Comm., 2485 (1999). M.A. Malik, X.Z. Jiang: Plasma Chem. Plasma Processing, 19, 505 (1999). Rentech, Thermal conversion begins testing plasma torch natural gas-to-syngas facility, Hydrocarbon Online, May 4, 1999. A. Czernichowski, personal communication.