Development of a chemical selective iron Fischer Tropsch catalyst

Abstract

In the chemical industry C2–C4 olefins are critical building blocks for high value commodity products. One of the key challenges in Fischer Tropsch Synthesis (FTS) technology development is to manipulate process conditions and/or catalytic systems in order to suppress methane selectivity while still producing mainly C2–C4 products with maximum olefin content. Sasol developed an iron catalyst promoted with sodium and sulphur that yielded a fairly high proportion of C2–C4 products with lower than expected methane selectivity. Due to the unexpected low methane selectivity in combination with high olefin selectivity necessitated the evaluation of other promoters similar to sulphur in combination with alkali promoters (i.e. sodium and potassium) in order to understand whether the combination of sulphur and sodium is unique or whether other alkali and non-metal promoters will yield similar results or even improve the selectivity further. By employing high throughput experimentation, a total of 306 catalysts were synthesised and tested in order to identify whether other possible non-metal FTS promoters could either match or improve the selectivities observed for sodium and sulphur and to also identify potential optimum promoter levels for maximum C2–C4 olefin selectivity and minimum methane formation. The results showed that from the series of catalysts evaluated sulphur and sodium yielded the highest olefin selectivity in combination with low methane selectivity and that a mole ratio of at least 2:1 Na:S is required for optimum C2–C4 selectivity. Apart from the statistical analysis of data and subsequent comparison of catalyst selectivity, the catalysts were also evaluated based on the expected Schulz Flory selectivity correlation. For unique catalyst selectivity a significant shift should be observed. In this paper the results of high throughput experimentation are presented and discussed using the direct relations of C2–C4 selectivity and methane selectivity in order to distinguish unique catalyst systems that are selective towards C2–C4 olefin production.