Direct Conversion of Methane into Methanol and Formaldehyde in an RF Plasma Environment I: A Preliminary Study

The direct conversion of methane (CH4) into methanol (CH3OH) and HCHO in an Argon (Ar) 50W radio-frequency plasma system was applied and the effects of various feed compositions, CH4/O2 ratio, and plasma discharge areas were compared. Additionally, the effects of various methane to oxygen ratios and plasma discharge areas were studied. It was found that in an Ar stream, the CH3OH conversion ratio in the CH4/O2 plasma system was higher than that in CH4/CO, CO/H2 and CH4/H2/O2 plasma systems. The conversion of CH4 reached 19.1% at CH4/O2 = 40/60; the yield of CH3OH was 1.12% and 16.0% CO, the major product, was produced. A larger plasma discharge area, resulting in a longer residence time, corresponded to higher CH4 conversion, but a lower CH3OH conversion ratio, because of further decomposition into CO and CO2. Interestingly, no carbon deposition was observed in the RF plasma system, and the carbon balance was between 0.94 and 1.19.


INTRODUCTION
Methane (CH 4 ) is a major contributor to the human-caused greenhouse effect. Much attention has been paid to the conversion of methane into higher hydrocarbons and easily transportable liquids, such as methanol (CH 3 OH) and formaldehyde (HCHO). It is expected that methane will become increasingly important in the production of energy and chemicals during this century (Roth, 1994;Brown and Parkyns, 1991). Of all available direct conversion processes, the partial oxidation of methane into useful oxygenates has great potential and is considered to be one of the greatest challenges for catalysis (Gesser, et al. 1985;Crabtree, 1985;Krylov, 1993).
Direct conversion technology could significantly impact the realization of more efficient energy, conserving resources and protecting the atmosphere by reducing the concentration of exhausted greenhouse gas. The difficulty in the direct conversion of methane, both catalytically and thermally, concerns the high stability of the C-H bond in the methane molecule, which exceeds that in all other hydrocarbons (Lunsford, 2000). Catalytic methods of methane conversion must eliminate the need for high reaction temperatures and the poisoning of catalysts (Lodeng et al., 1995). Accordingly, the direct synthesis of methanol from methane can support a new synthetic process that consumes much less energy.
Nonthermal plasma has been used recently to excite small, stable molecules, using energetic electrons and no heating of gases. Energetic electrons are generated using either a corona, a pulse discharge, a microwave discharge, or a dielectric-barrier discharge (Suib et al. 1993;Liu et al. 1999;Huang et al. 2000). The products of methane conversion obtained using nonthermal plasma are mainly ethylene, acetylene, hydrogen, carbon monoxide, carbon dioxide, and some oxygenates. Some of results have led to the formation of carbon black and plasma polymerized carbon film. Very few studies have used a radio-frequency (RF) plasma system to examine the direct conversion of methane into a useful liquid fuel.
In this work, the direct conversion of methane into methanol in an RF plasma system was applied and the effects of various feed compositions were compared. Additionally, the effects of various methane to oxygen ratios and plasma discharge areas were studied. Figure 1 illustrates the RF plasma reactor used and schematically depicts the experiment' ssetup.

METHODS
Reactants CH 4 /CO/Ar, CO/H 2 /Ar, CH 4 /O 2 /Ar, and CH 4 /O 2 /H 2 /Ar were metered using Brooks Type 5850E mass flow controllers at a total flow of 100 cm 3 .min -1 . They were then separately introduced into the reactor (4.5 cm I.D. ×15 cm height). The inside of the reactor was separated into two sections by a FTIR quantification data were obtained through a carbon balance to evaluate the significance of both deposition and condensation in the sampling and analyzing apparatus.
CH 4 was partially oxidized at 13.3 mbar and 50W. The conversion of CH 4 (X CH4 ) and fraction of total input carbon converted into CH 3 OH (F CH3OH ), HCHO (F HCHO ) and C 2 H 2 + C 2 H 4 + C 2 H 6 (F C2 ) were calculated as, [CH 3 OH] p is the amount of CH 3 OH produced (%) [HCHO] p is the amount of HCHO produced (%) [C 2 H 2 + C 2 H 4 + C 2 H 6 ] p is the amount of C 2 H 2 + C 2 H 4 + C 2 H 6 produced (%).   Table 2 presents the effect of a CH 4 /O 2 feeding concentration on the conversion of CH 4 . Pure CH 4 has typically been regarded as rather chemically inactive, but easily decomposes in the RF plasma system.

METHANOL AND FORMALDEHYDE SYNTHESIS WITH VARIOUS CH 4 TO O 2 RATIOS
Unlike in other studies (Matsumoto, et al. 2001), X CH4 was less than 12% at a 2% CH 4 feeding concentration, and a 50 cm 3 /min -1 total flow rate, while X CH4 was about 4% with a 5.0 mL/min -1 CH 4 flow rate (Taylor et al. 2000). X CH4 with typical solid catalysts reported by Otsuka (2001) was 0.7% to 18.5%. In the RF system, applying 50W yielded 14.8 to 38.1% CH 4 conversion, while in the DBD system, 100W of input power yielded 5.97% CH 4 conversion (Jiang et al. 2002). Matsumoto (2001) stated that the intensities in Ar excitation drastically declined upon the introduction of a feeding concentration of CH 4 of only 2% into the Ar stream, and their system exhibited a large energy transfer from excited Ar species to reactant molecules. This suggestion was extended to the RF system. X CH4 was higher in the RF plasma system with Ar inflow (CH 4 /O 2 at 30/60 and 50/40) than in that without (CH 4 /O 2 = 40/60, 50/50).
No carbon deposition occurred in the Matsumoto et al. RF system, and the carbon balance was 0.97 to 1.19.  and F C2 were 1.12, 11.9 and 1.56% with CH 4 /O 2 ratios of 40/60, 8/8 and 8/8, respectively. The highest F CO and F CO2 were 24.9 and 23.1% with CH 4 /O 2 ratios of 8/8 and 50/50, respectively. Moreover, a larger plasma discharge area, resulting in a longer residence time, led to higher CH 4 conversion, but a lower CH 3 OH conversion ratio, owing to further decomposition into CO and CO 2 . Therefore, the CH 4 /O 2 /Ar plasma system of reactor A was selected as the basis for the further investigation of the effects of experimental parameters on X CH4 , F CH3OH and F HCHO .