Title

CHAIN GROWTH IN THE FISCHER-TROPSCH SYNTHESIS

Date of Completion

January 1987

Keywords

Engineering, Chemical

Degree

Ph.D.

Abstract

Methanation and hydrocarbon synthesis from CO and H$\sb2$ was studied using 10 wt% Ni/Al$\sb2$O$\sb3$ and 10 wt% Fe/Al$\sb2$O$\sb3$ catalysts at 1 atm. Transient responses to the switches between various feed mixtures, especially isotopically labeled feeds, to a gradientless microreactor were used to obtain information on the amounts and reactivities of intermediates on and in the catalysts. On Ni/Al$\sb2$O$\sb3$, about 0.5 monolayer of adsorbed CO and smaller amounts of C are present during reaction. Infrared and CO/D$\sb2$ suggest that significant amounts of CH$\sb{\rm x}$ (x = 1-3) are not present. Chain growth experiments with $\sp{13}$CO/H$\sb2$ indicate that the hydrocarbons are formed primarily from C, but the results also indicate that the process is not fully understood. All steps in methanation and chain growth appear to be fast, except for the limiting steps CO$\sp\* \to$ C and C $\to$ CH. On Fe/Al$\sb2$O$\sb3$, methanation and chain growth occur via CH, with all steps except the initial conversion of CH apparently being fast. Bulk iron carbides and an inactive form of carbon, different from C on Ni/Al$\sb2$O$\sb3$, do not participate significantly. The CH deactivates slowly with time on stream; only a small portion produces most of the products after 2 h on stream.^ Other work concerned H$\sb2$ chemisorption on various supported metal catalysts. Using TPD, an unusual spike was found after low temperature adsorption on Ru/SiO$\sb2$. The activated adsorption of H$\sb2$ on Fe/SiO$\sb2$, Fe/Al$\sb2$O$\sb3$, Ni/AlO$\sb3$, and Rh/TiO$\sb2$ was compared. It was suggested that, by analogy to titania-supported metals, the origin of the activation barrier may lie in a decoration phenomenon. It was proposed that SiO$\sb{\rm x}$ and AlO$\sb{\rm x}$ species may have been derived from small amounts of support which dissolved in the impregnating solution. ^