(66b) Deactivation of Fe Based Fischer-Tropsch Catalyst-A Critical Problem

Fischer-Tropsch synthesis (FTS) is the most intensively studied system in heterogeneous catalysis. Fischer-Tropsch synthesis catalytically converts syn gas, a mixture of CO and H2 to hydrocarbons through a surface polymerization reaction. FTS is a strongly exothermic reaction. Often the catalysts and reaction conditions are tuned in such a way to obtain a wax consisting of long hydrocarbon chain products (C20+). Iron-based FTS catalyst precursors consist of Fe2O3 nanocrystals. Promoters are often added to these nanocrystals in order to improve catalyst performance. A typical catalyst contains promoters like copper to enhance catalyst reducibility, potassium to improve CO dissociation and silica or zinc oxide to improve the attrition resistance of the catalyst which in turn also improves the amount of iron atoms interacting with the gas phase. Iron based Fischer-Tropsch catalysts undergo two types of deactivation during use. One is due to the physical degradation of the catalyst because it destroys the integrity of the catalyst. This is considered as a serious problem because it forms fine particles that cannot be easily separated from the heavy wax products. It increases the viscosity of the slurry which may cause extreme pressure drops etc. The other deactivation pathway can be classified as being due to chemical or phase change. The iron based FT catalyst system is one of the oldest and perhaps most studied systems known in the field of heterogeneous catalysis. However, even after more than of 80 years of research many important questions remain unanswered. The exact structural composition of the active sites and the deactivation mechanism of this catalyst are still not clearly understood. Transformation of active phase to inactive phases, carbon deposition, pore blocking, sintering and deposition of other chemicals such as sulfur on the catalyst are considered as the main reasons of Fe based Fischer-Tropsch catalyst deactivation. Different deactivation mechanisms of Fe based FT catalyst will be discussed at the conference.