The Importance of Olefin Readsorption and H2/CO Reactant Ratio for Hydrocarbon Chain Growth on Ruthenium Catalysts

Abstract

Abstract The synthesis of high molecular weight hydrocarbons on Ru catalysts requires the readsorption of primary α-olefin products. Such a readsorption step initiates a surface chain by reversing the β-hydrogen abstraction reactions: the chain termination step that forms an α-olelin. On Ru catalysts. 60 to 90% of C21+ products require at least one readsorption event. These readsorption steps become increasingly imptortant as chain size increases. This occurs because the diffusive removal of α-olefins from catalyst particles slows down significantly with increasing molecular size leading to long intraparticle residence times that favor secondary readsorption reactions. Chain growth probability and paraffin content therefore increase with molecular size. As a result, carbon number distributions do not obey simple Flory kinetics, an observation previously attributed to multiple chain growth sites and to the higher solubility of larger hydrocarbons in Fischer-Tropsch liquids. At low H2/CO reactant ratios, where termination to olefins should be favored, large chains terminate only as paraffins. This again reflects the importance of diffusion-enhanced olefin readsorption which not only dominates bed residence time effects for large hydrocarbons but also weakens the effects of changes in reactant concentration. High H2/CO reactant ratios lead to higher concentrations of hydrogen adatoms on Ru surfaces and thus favor chain termination to paraffins by hydrogen addition steps. Consequently, the contribution of olefin readsorption to chain growth and product molecular weight decreases markedly with increasing H2/CO ratio.