Selectivity loss in Fischer-Tropsch synthesis: The effect of carbon deposition

Abstract

Polymeric carbon was deposited over Siralox supported cobalt catalysts by an ethylene treatment at 230 °C and 260 °C. Cobalt catalysts with cobalt particle sizes of 10 and 14 nm were studied. Temperature programmed hydrogenation and Raman spectroscopy analyses of the deposited species were found to be similar to those formed during Fischer-Tropsch synthesis. The cobalt catalysts were tested in high-throughput 16 parallel reactors at 20 bars and 220 °C, using a H2/CO ratio of 2.12. Data were collected at different space times to compare activities and selectivities at iso-conversion levels. Ethylene treated catalysts showed lower activity that corresponded roughly to the loss of CO adsorption sites. Additionally, a decrease of the heavy products selectivity, an increase in the methane selectivity and a decrease of the olefin to paraffin ratio was observed after the ethylene treatments. Unlike the loss in activity, the deselectivation level was found to depend on the amount of deposited carbon. This phenomenon can be explained by a steric effect of the deposit, but an electronic effect cannot be excluded, but is challenging to prove.