Secondary reactions on metal-zeolite catalysts used in synthesis gas conversion☆

The mechanism of synthesis gas conversion over cobalt-ZSM-5 catalysts to gasoline-range hydrocarbons has been examined. A correlation exists between the methane yield and the percentage aromatics in the liquid hydrocarbon product. This could be due to heat transfer effects or to the hydrogenolysis of propane and butane over cobalt, leading to the formation of additional methane. The formation of additional alkanes (propane and butane) is known to accompany the aromatization process. The percentage of methane made by the bifunctional catalyst 4.2% Co SiO 2 + ZSM-5 was 1.7 times that made by the Co SiO m2 catalyst with a feed of H 2 CO = 1 at 280 ° C , 21 atm, and WHSV = 0.77. An attempt was made to inhibit secondary hydrogenolysis reactions by the addition of copper to the cobalt catalyst. Under similar process conditions, this provided a very small help in keeping down the increase in percentage of methane upon the addition of ZSM-5. Addition of 5 and 10% propane to the synthesis gas showed no additional methane made by hydrogenolysis. If indeed some of the methane produced over cobalt-ZSM-5 catalysts is coming from hydrogenolysis of light alkanes, it is a small amount.