Novel Fischer-Tropsch catalyst

25 February 2012 - Catalysis

It is not every week that an inorganic chemist is making headlines in the Dutch newspapers or even the Dutch television news bulletins. The inorganic chemist is Krijn de Jong from Utrecht University and the headline-making news is a new catalyst for the Fischer-Tropsch (FT) process. The Science article was a joy to read (DOI) but it does have problems.
Problem number 1 concerns the title Supported Iron Nanoparticles as Catalysts for Sustainable Production of Lower Olefins. The intended feedstock for olefin production is biomass and hence the sustainability claim. But the new iron oxide catalyst works on syngas, the carbon monoxide hydrogen mixture that can be produced from any carbon source (biomass included) at an industrial scale. No biomass was anywhere near the Utrecht laboratory.
What the new catalyst does particularly well is converting CO/H2 to directly to the lower olefins (ethylene, propylene) instead to the higher ones. And these lower olefins are what plastics are made from. With the present state of technology these lower olefins are only accessible in a very inefficient way by cracking higher olefins.
Regular iron-based FT catalysts tend to degrade mechanically in typical FT process conditions (high temperature, high pressure) and therefore the new catalyst has iron oxide particles deposited on carbon nanofiber (CNF) as a solid support. Surface oxidized nanofiber (surface area 150 square meter per gram) was impregnated with a water solution of ammonium ferric citrate and then heated for 2 hours at 255°C. Silicon carbide particles were added and this ensamble was reduced using hydrogen although is it unclear what is being reduced and why. Note that although the article consistently mentions iron particles the true catalyst is an iron oxide.

Sure enough, compared to bulk iron or compared to several other supported iron oxides, at 20 bar and 340 °C, Fe2O3/CNF displays selectivity towards formation of the lower olefins at the expense of methane formation combined with a high carbon monoxide turnover. On the other hand the production of carbon dioxide (clearly a useless by-product) appears to be up. problem number two with this article is that this piece of information is presented in the supporting information and not in the article itself.
Problem number three concerns the requirement for sulfur and sodium as trace elements. An unsuspecting reader may get the idea that catalyst design involves testing a huge amount of element combinations before one of them actually works. As it happens an interview in the Dutch newspaper NRC Handelsblad with Krijn de Jong (title translates roughly as plastics from garbage incineration) reveals more. Apparently the discovery of the catalyst by one of his Ph.D students is a nice case of serendipity because reactions were found to work well with one particular bottle of ammonium ferric citrate where the sodium and sulfur were hiding but not at all with a second bottle. Should this piece of information not be in the article as well? Does the reaction require both sulfur and sodium or just one of them?