Reactions Of Methane And Methanol In The Plasma Of Dielectric Barrier Discharge

The manufacture of chemicals with alternative feedstock such as methane and methanol is an important way to alleviate the problems of petroleum exhaustion and increasingly serious environmental pollution. This study was focused on the plasma reaction of methane and methanol in dielectric barrier discharge reactor, with the purpose of, on the one hand, gaining insight into the governing factor and the mechanism of the methane to higher hydrocarbon reaction and, on the other hand, investigating the possibility of one-step synthesizing ethylene glycol from methanol. The following results and conclusions were obtained:1. For the plasma reaction of methane in dielectric barrier discharge reactor, systematic investigations were given, first of all, to the effects of reaction conditions such as methane flowrate, input power and discharge frequency, and reactor structural parameters such as high voltage electrode diameter, reactor outer diameter and the length of discharge zone, on the methane conversion and product distribution. Results indicate that, the effects of the reaction conditions and higher hydrocarbon selectivity can mainly attributed to the effect of the electron density of plasma.2. By employing the in-situ optical emission spectrum, the spectral signals of excited state CH, C2 and H species were measured in the plasma of methane dielectric barrier discharge. According to the linear relationship of the spectral signal intensity of the excited state CH species with the conversion of CH4, we speculated that the activation of methane in dielectric barrier discharge reactor was involved by CH free radical. The reaction of CH free radical with methane generating C2H5 free radical should be the main reaction of methane activation.3. For the plasma reaction of methanol in dielectric barrier discharge reactor, considerable effort was given to the reactor design. Fortunately, ethylene glycol was directly obtained as high-value product by using a metal ground-electrode double dielectric barrier reactor and H2 carrier gas. Under the optimization condition of CH3OH flow rate 0.02 mL/min, H2 flow rate 80 mL/min, discharge frequency 12.0 kHz, input power 11 W, reaction temperature 300 ℃, and reaction pressure 0.1 MPa, the conversion of CH3OH and the selectivity of ethylene glycol reached 15.8% and 71.5%, respectively. The energy efficiency of ethylene glycol production at this experiment condition was estimated to be about 42.55 g/kWh. The reaction was carried out continuous over 100 h and all the performance indicators could be kept stable.4. According to the diagnostic results of the in-situ optical emission spectrum, we concluded that during the plasma reaction of methanol, H2 was decomposed into H atom in the discharge zone by cumulative excitation, the C-H bond of CH3OH was then selectively dissociated via the collision of H atom, producing CH2OH free radical (H + CH3OHâ†' CH2OH + H2). Finally, the coupling of CH2OH free radical producing the desired ethylene glycol. H2 pomoted the generation of CH2OH free radical, via the active form of H atom, by reducing the reaction activation energy. But it was not consumed during the process, when leaving the reactor the active H atoms recombined into H2 molecules. Based on the behavior of H2, as well as the fact that the presence of H2 can enhance both methanol conversion and ethylene glycol selectivity, we believe that hydrogen took the role of catalyst in the direct synthesis of ethylene glycol with methanol in the dielectric barrier discharge reactor.5. Systematical investigations show significant influences of reaction conditions such as input power, discharge frequency, methanol flow rate, reaction temperature and pressure on the direct synthesis of ethylene glycol with methanol. With the help of the in-situ optical emission spectrum, we knew that, when in the suitable range, the above-mentioned conditions favored the synthesis of ethylene glycol by enhancing the catalytic action of H2(favor the dissociation of H2 molecular into active H atom); However, when beyond the suitable range, the above-mentioned conditions hampered the synthesis of ethylene glycol by weakening the catalytic action of H2 and, meanwhile, triggered the non-catalytic decompositions of methanol.