Influence of Olefin- and Aromatic- Cofeed on the Carbon Deposition during Fischer-Tropsch to Olefins reaction

Abstract

Currently aromatic compounds are synthesized by catalytic reforming of the naptha fraction and is dependent on the crude oil reserves. A breakthrough was done by developing a promoted iron catalyst (FTO-catalyst) that was able to form lower olefins with a high olefin/paraffin- ratio from synthesis gas. More work has been done to form aromatics from synthesis gas via lower olefins (FTA) using a bifunctional catalyst that consist of an FTO-catalyst mixed with a H-ZSM-5 zeolite. However during this reaction the formation of carbon was observed, which led to a pressure buildup in the reactor. This carbon formation is unwanted and therefore further investigated in this research project. In this study, iron-based Fischer-Tropsch catalysts promoted with sodium, sulfur and manganese were prepared. These were run under Fischer-Tropsch conditions cofeeding propene and toluene to mimic the reaction conditions during Fischer-Tropsch to Aromatics. It was found that cofeeding 1 ml/min Propene increased the carbon deposition significantly for all catalysts, with exception of the 6% Fe 0.010Na 0.010S 0.10Mn catalyst. For manganese promoted catalysts a cofeed of 1 ml/min toluene decreased the carbon deposition. From the TGA data it was found that the carbon deposition possessed 3 carbon species, which can be assigned to amorphous carbon, carbon filaments and graphitic carbon. Feeding CO without olefin or aromatic cofeed had carbon filaments as the main carbon deposit, 65.8%, followed by amorphous carbon, 20.3%, and lastly graphitic carbon with 13.9%. Cofeeding olefins slightly increased the amount of graphitic carbon, percentagewise. Cofeeding Toluene however increased the percentage of carbon filaments to 81.4% whilst decreasing both amorphous and graphitic carbon percentages. It was also found that a manganese promoter in combination with Toluene cofeed inhibited the production of graphitic carbon.