(563i) Influence of Long Time on Stream (TOS) in the Morphological and Structural Characteristics of Iron-Based Catalyst during FTS. A Closer Look at the Deactivation Routes

Iron catalysts is mostly used in the Fischer-Tropsch synthesis (FTS) because of its prize and ubiquitous in nature. Sintering, oxidation and carbon deposition during Fischer-Tropsch synthesis has long been postulated as some major deactivation mechanisms. However, to date there is no consistent picture. This paper presents deactivation modes of a commercial catalysts after running for long TOS by means of oxidation, sintering and graphitisation for iron-based catalyst. Furthermore, it presents results on the deactivation of an industrial Fe/SiO₂ catalyst as obtained by XRD, XPS, FTIR, HRTEM and TGA. These analyses were performed to study the phases of the iron catalyst and the nature of the carbon deposited on the catalyst of the spent industrial Fe/SiO₂ catalyst samples withdrawn from a fixed bed reactor operating under realistic FTS conditions, and it was concluded that sintering can be ruled out as a major deactivation mechanism if extended TOS are used. Finally, these data together with all relevant literature were used to create a common view on the sintering, oxidation behaviour of metallic Fe during FTS. The apparent discrepancies in literature on the sintering behaviour of Iron are most likely due to the lack of direct measurements of the iron clusters and due to the comparison of catalysts with varying crystallites sizes, compared at different reactor partial pressures of hydrogen and water (P_H2O/P_H2). As such sintering cannot be predicted because of time variations due to fluctuations in the amount of water, CH₄, butane and other hydrocarbons in the reactor.