(119e) Fischer-Tropsch Synthesis Using H2/CO/CO2 Syngas Mixtures Over Iron and Cobalt Based Catalysts: Insights Into Reaction Pathways

With the depleting resource of crude oil all over the world, the Fischer-Tropsch Synthesis (FTS) process, in which synthesis gas is converted into a complex multi-component mixture consisting of linear and branched hydrocarbons and oxygenated products, becomes a promising route to meet the continuously increasing demand for liquid fuels and chemical feed stocks. Among the reported FTS catalysts, iron and cobalt are used commercially at temperatures between 473K and 573K, and at 10 to 60 bar pressure. The synthesis gas, a mixture of predominantly CO and H₂ with different H₂/CO ratios, can be produced from coal, natural gas or biomass. In some cases, CO₂ may be a significant component in the synthesis gas. It is therefore interesting to gain insight into the reaction pathways for FTS using H₂/CO/CO₂syngas mixtures over iron and cobalt based catalysts.

FTS experiments over iron and cobalt based catalysts have been carried out in a fixed-bed micro reactor. Two feed gases, one of composition H₂:CO:CO₂ = 3:0:1 and the other of H₂:CO:CO₂ = 2:1:0 were fed to the reactor and the reactions monitored.  Mixtures of various proportions of the two feed gases were fed to the reactor, thus varying the ratio of CO, CO₂ and H₂ stoichiometrically. The reaction conditions for the iron based catalyst were a total synthesis pressure of 20bar, flow rate of 60ml/min/g.cat and temperature of 523K; and for the cobalt based catalyst, total synthesis pressure of 20bar, flow rate of 60ml/min/g.cat and temperature of 473K.

The results show that CO₂ rich feeds produce products that are mainly short chain hydrocarbon products while CO rich feeds shift the product composition to a Fischer"CTropsch type (mainly long chain hydrocarbons) product on both iron and cobalt based catalysts. The products distributions of hydrocarbons, described by the Anderson-Schulz-Flory (ASF) distribution, were calculated for each runs for both iron and cobalt based catalysts. The single alpha product distribution is found with CO₂ rich feeds, while a two-alpha product distribution is found when using CO rich feeds. Masuku^[1] et. al. considered the effects of Vapour-Liquid Equilibrium (VLE) on the FT  product distribution, and showed that at high alphas, when a liquid layer is formed on the catalyst, a 2-alpha distribution will be measured experimentally.  For CO₂ rich feeds the alpha value of the product distribution is very low and all the products (short chain) are in the gas phase, and we therefore predict that the product distribution follows a single alpha distribution. However for CO rich feeds the alpha value of the product distribution is higher resulting in products in both the gas and liquid phases, so that the effect of the VLE results in a two-alpha product distribution for the total hydrocarbon products.

We find similar results to those found in the literature in that the olefin to paraffin ratio changes as a function of carbon number. Increasing the carbon number decreases the olefin to paraffin ratio (excluding O₂/P₂). However, we found that the ratio of (O-olefin, P-paraffin, n-carbon number) is equal to a constant value () for all experiments, irrespective of the proportions of CO, CO₂ and H₂ on the catalyst′s surface for both iron and cobalt based catalysts.  We therefore postulate that the product distribution may be determined by equilibrium considerations. The value of  and the FTS reaction mechanism will be discussed for both iron and cobalt based catalysts.

Keywords: Fischer-Tropsch Synthesis, CO Hydrogenation, CO₂ Hydrogenation, Product Distribution, ASF, Vapour-Liquid Equilibrium, Olefin to Paraffin Ratio, Thermodynamic Equilibrium.

Reference:

[1] Cornelius Mduduzi Masuku, Diane Hildebrandt and David Glasser, Thermodynamic Prediction of the Chain Propagation Probability (α) for the Fischer-Tropsch Reaction, 2007 AIChE Spring National Meeting.