Enhanced Fischer-Tropsch Synthesis Rates by the Combined Presence of Aqueous and Organic Media in Biphasic Systems

We report that when the Fischer-Tropsch synthesis (FTS) catalyzed by Ru particles supported on multiwall carbon nanotubes (Ru/CNT) is conducted in a biphasic decalin/water solvent mixture, the observed rate is significantly higher than in single-phase organic or aqueous medium. Multiwall carbon nanotubes of varying wettability and doped with Ru nanoparticles were tested as catalysts and stabilizers of biphasic emulsions in a batch reactor under FTS reaction conditions (H-2:CO = 4:1, 220 degrees C, 55 bar). First, in comparison to the runs conducted in pure organic solvent, both rate and selectivity increased with the addition of a second (aqueous) phase. Likewise, a large increase in reaction rate was found when a second (organic) phase was incorporated, relative to that measured in pure aqueous phase. Notably, this rate increase was more substantial when the nanotubes employed were more hydrophobic and exhibited higher oil wettability. At the same time, for a given (Ru/CNT) catalyst, the rate increased with the oil/water solvent ratio, suggesting that not only the promotion by water but also the interaction of the catalyst surface with the organic solvents at the oil/water interfaces plays a key role in enabling higher rates. Because the rate changes are not due to changes in rates of mass transfer, it is concluded that the intrinsic kinetics was enhanced. Among the possible explanations for the enhanced kinetics, we discuss here some concepts recently proposed in the literature, including the water-promotion of H-assisted CO dissociation and the disruption of dense CO surface layers by the organic solvent.