Comparative investigations on plasma catalytic methane conversion to higher hydrocarbons over zeolites

Zeolites are an important class of industrial catalyst. In this investigation, the application of zeolites for plasma catalytic methane conversion (PCMC) to higher hydrocarbons at very low gas temperatures (room temperature to 200 degrees C) has been addressed. Zeolites NaY, HY, NaX, NaA, Linde type 5A and Na-ZSM-5 have been tested for the application in PCMC. The products contain Ct hydrocarbons (acetylene, ethane and ethylene), other carbon species including carbon deposits and trace C-3(+) hydrocarbons, and syngas (H-2+CO), depending upon co-reactant or dilution gases added to the feed. A streamer corona discharge, a cold plasma phenomenon, has been found to be the most effective and efficient at inducing plasma catalytic activity over zeolites. The order of the zeolites tested from good to poor for sustaining the desired streamer discharges is NaY, NaOH treated Y > KY > NaX > NaA > Linde Type 5A > Na-ZSM-5. Oxygen, carbon dioxide, hydrogen (with or without oxygen added in a small amount), steam and nitrogen have been tested as co-reactants or dilution gases for PCMC over zeolites. Experimental results showed that the selectivity to higher hydrocarbons decreases in the order H-2>H-2+O-2>H2O>N-2>N-2+O-2>CO2>O-2, while the methane conversion decreases in the order N-2+O-2>N-2>O-2>CO2>H-2+O-2>H2O>H-2. All the co-reactants tested here, except hydrogen, can induce high methane conversions during plasma catalytic reactions. Small amounts of oxygen added to hydrogen can improve significantly the plasma reactivity of hydrogen over zeolites. This has led to a very selective net production of hydrogen and higher hydrocarbons (especially acetylene). (C) 1999 Elsevier Science B.V. All rights reserved.