Conversion of synthesis gas to aromatics at medium temperature with a fischer tropsch and ZSM-5 dual catalyst bed

Abstract

The production of base chemicals such as olefins and aromatics from synthesis gas is of great interest from both an academic and an industrial point of view. We prepared a bulk iron Fischer Tropsch catalyst promoted with potassium capable of converting synthesis gas to olefins and combined it with an H-ZSM-5 zeolite to convert the formed olefins to aromatics on the zeolite acid sites. All experiments were performed in stacked bed mode to avoid migration of potassium promoters from the iron catalyst to the zeolite, which can deactivate acid sites in the zeolite. Operating at a pressure of 20 bar and temperatures between 250 °C and 300 °C instead of high temperatures of 300−350 °C led to a methane selectivity of only 7–14 %C. Olefins in the range of C4–C8 were converted to aromatics in the initial section of the zeolite bed, while further into the zeolite bed C2-C3 olefins oligomerized to longer olefins and alkylated light aromatics forming heavier aromatic hydrocarbons. Investigation of the influence of CO conversion on the aromatization of olefins showed a maximum in aromatic selectivity (excluding CO2) of 18 %C at medium CO conversion (XCO = 26–47 %). This work provides detailed insight into the chemical events that occur in bifunctional catalyst beds and can contribute to the development of an industrial process to convert synthesis gas to aromatics in a single reactor.