PRIMARY AND SECONDARY REACTION PATHWAYS IN RUTHENIUM-CATALYZED HYDROCARBON SYNTHESIS

Residence time studies show that n-paraffins, alpha-olefins, and cis-2-olefins are primary products during hydrocarbon synthesis on Ru catalysts. Their formation, as well as that of branched isomers, is consistent with previously proposed surface reactions of alkyl groups on metal surfaces. Secondary hydrogenation and hydrogenolysis of alpha-olefins are inhibited by the water product of the synthesis step. However, alpha-olefin readsorption and surface chain initiation and cis-to-trans isomerization take place as secondary reactions. The decrease in alpha-olefin selectivity with increasing CO conversion and molecular size reflects the greater extent of readsorption as bed and pore residence times of alpha-olefins increase. Readsorption of alpha-olefins and chain initiation increases with molecular size because the rate of removal of olefins from liquid-filled catalyst pores is decreased due to intraparticle diffusion limitations. This diffusion-enhanced olefin readsorption accounts for the observed deviations of carbon number distributions from those predicted by Flory polymerization kinetics. Chain growth probability increases with chain length until an asymptotic value is reached. Readsorption decreases the contribution of termination via hydrogen abstraction to chain growth kinetics and leads to a heavier, more paraffinic product. In effect, differences in selectivity between small and large hydrocarbons are due to the increasing influence of pore residence times and the decreasing influence of bed residence times on secondary reactions as olefin size increases.