(166a) On the Kinetics and Mechanism of Fischer-Tropsch Synthesis on a Highly Active Iron Catalyst Supported on Silica-Stabilized Alumina

Kinetic modeling is a useful tool for understanding the reactions that occur on the surface of heterogeneous catalysts. Historically, for Fischer-Tropsch Synthesis (FTS) catalysts of choice have been supported cobalt or unsupported iron¹. Attempts to make active supported iron catalyst had been unsuccessful until Keyvanloo et al. recently reported a highly active and stable iron catalyst supported on a silica stabilized alumina (AlSi)². Because industrially viable supported iron catalysts had not been created prior to this development, there exists essential no reports of kinetic modeling of supported Fe catalysts in the literature.  The goal of this study was to develop a rate model representative of the kinetic behavior of this highly active supported iron catalyst at 250°C.

Reaction kinetic experiments were carried out in a fixed-bed reactor operated under differential conditions (X_CO < 21%) for a 40 wt% Fe/ 3 wt% Cu 1.6 wt% K supported on AlSi Support. The reactor was operated at 250°C with P­_H2 and P­_CO ranging from 2.0-9.0 atm and 3.0-8.9 atm respectively. The rate data were fit to a variety of models, including models based on H assisted and unassisted CO dissociation pathways. To differentiate between models, some of which have a high degree of parity, a lack of fit test was performed on each of the models resulting in relatively sensitive statistical parameter (p-value).

Initially, the data were fit to a power law model. With respect to H₂ the reaction order was 0.877. The order with respect to CO was -0.221. The overall negative effect of CO partial pressure implies that CO readily adsorbs on the surface of the catalyst thus limiting the sites available for H₂ dissociation. The relatively high H₂ order suggests that the hydrogenation steps could be the rate limiting steps.

Eight derived two parameter rate expressions and one semi empirical model were fit to the data; however, none of these models fit the data with any statistical significance as evidenced by a significant trend in the residuals with respect to H₂ partial pressures and p-values less than 0.10. In an attempt to better fit the data, the H₂ dependence was adjusted for each of the derived models to maximize the p-value, resulting in a semi-empirical model that fit the data well. Three parameter models were explored that had similar partial pressure dependencies as the best semi-empirical model. The best fitting models suggest that supported Fe FT catalysts follow a direct CO dissociation pathway, that carbon is one of the most abundant species on the surface of the catalyst, and that the hydrogenation of either C* or CH* is the rate determining step for Fischer-Tropsch synthesis.

References

1. Botes, F. G.; Niemantsverdriet, J. W.; van de Loosdrecht, J., A comparison of cobalt and iron based slurry phase Fischer–Tropsch synthesis. Catalysis Today 2013, 215 (0), 112-120.

2. Keyvanloo, K.; Mardkhe, M. K.; Alam, T. M.; Bartholomew, C. H.; Woodfield, B. F.; Hecker, W. C., Supported Iron Fischer–Tropsch Catalyst: Superior Activity and Stability Using a Thermally Stable Silica-Doped Alumina Support. ACS Catalysis 2014, 4 (4), 1071-1077.