The importance of olefin readsorption and H[sub 2]/CO reactant ratio for hydrocarbon chain growth on ruthenium catalysts

Journal Article · Mon Feb 01 04:00:00 EST 1993 · Journal of Catalysis; (United States)

DOI:https://doi.org/10.1006/jcat.1993.1051· OSTI ID:6236403

Madon, R J; Iglesia, E ^[1]

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1. Exxon Research and Engineering Co., Annandale, NJ (United States)

The synthesis of high molecular weight hydrocarbons on Ru catalysts requires the readsorption of primary [alpha]-olefin products. Such a readsorption step initiates a surface chain by reversing the [beta]-hydrogen abstraction reactions: the chain termination step that forms an [alpha]-olefin. On Ru catalysts, 60 to 90% of C[sub 21] + products requires at least one readsorption event. These readsorption steps become increasingly important as chain size increases. This occurs because the diffusive removal of [alpha]-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 H[sub 2]/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 H[sub 2]/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 H[sub 2]/CO ratio. 22 refs., 9 figs., 2 tabs.

• OSTI ID: 6236403
• Journal Information: Journal of Catalysis; (United States), Journal Name: Journal of Catalysis; (United States) Vol. 139:2; ISSN 0021-9517; ISSN JCTLA5
• Country of Publication: United States
• Language: English

Related Subjects

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CHEMISTRY
400201* -- Chemical & Physicochemical Properties
ALKANES
ALKENES
CARBON COMPOUNDS
CARBON MONOXIDE
CARBON OXIDES
CATALYSIS
CATALYSTS
CATALYTIC EFFECTS
CHALCOGENIDES
CHEMICAL PREPARATION
CHEMICAL REACTION KINETICS
CHEMICAL REACTIONS
DIFFUSION
ELEMENTS
HETEROGENEOUS CATALYSIS
HYDROCARBONS
HYDROGEN
KINETICS
MOLECULAR WEIGHT
NONMETALS
ORGANIC COMPOUNDS
ORGANIC POLYMERS
OTHER ORGANIC COMPOUNDS
OXIDES
OXYGEN COMPOUNDS
PARAFFIN
POLYMERIZATION
POLYMERS
REACTION KINETICS
REFRACTORY METAL COMPOUNDS
RUTHENIUM COMPOUNDS
SORPTIVE PROPERTIES
SURFACE PROPERTIES
SYNTHESIS
TRANSITION ELEMENT COMPOUNDS
WAXES