Fischer-Tropsch synthesis: Using deuterium tracer coupled with kinetic approach to study the kinetic isotopic effects of iron, cobalt and ruthenium catalysts

Deuterium tracer studies coupled with kinetic approach were developed to accurately determine the kinetic isotopic effects (KIE) for the CO hydrogenation and the formations of CO₂, CH₄ and light hydrocarbons (C₂-C₄) over iron, cobalt and ruthenium catalysts. Pronounced inverse kinetic isotope effects (IKIE) (k_co,H/k_co,D = 0.64–0.66) over the Fe and Co catalysts and a moderate IKIE (k_co,H/k_co,D = ∼0.8) on the Ru catalysts for the CO hydrogenation were obtained under typical FTS conditions. The IKIE was also found in CH₄ formation, but the Co and Ru catalysts showed stronger IKIE than the Fe catalysts (k_CH4/k_CD4 = 0.6 vs 0.87). Regardless of catalyst type, only a minor normal KIE for the CO₂ formation was obtained (k_{CO2, H}/k_{CO2, D} ˜ 1.1). The IKIE was found on various light 1-olefins and n-paraffin hydrocarbons as well. All the IKIEs can be explained by thermodynamic and kinetic isotope effects. The results suggested that deuterium was involved in quasi-equilibrated and hydrogenation steps that determined KIE. However, the Fe, Co and Ru catalysts showed different deuterium isotopic effect for the hydrocarbon selectivity. Deuterium promoted chain growth, thus lowered CH₄ and light hydrocarbons selectivities and enhanced C₅₊ selectivity on both the Fe and Co catalysts; while it enhanced the hydrogenation rate to the low hydrocarbons and suppressed the formation of heavier hydrocarbons on the Ru catalysts. Deuterium isotopic effect on the formation of 1-olefins, n-paraffins and 2-olefins in C₁₀-C₁₈ range (in liquid phase) was also studied in detail. Deuterium was found to enhance 1-olefins formation, but suppressed or only slightly changed n-paraffins formation over the Fe and Co catalysts. However, deuterium led to greater amount of n-paraffins and less 1-olefins on the Ru catalysts. The results suggested that the IKIE originated from different hybridization changes of carbon in C–H (sp¹->sp² and/or sp²->sp³) on the Fe, Co and Ru catalysts, which implied different reaction pathways on the Fe, Co and Ru catalysts.