(532c) Investigating the Effect of Mass Transfer in Supercritical Phase Higher Alcohol Synthesis Over a Cu-Co Based Catalyst

There is sustained interest in the synthesis of higher alcohols from synthesis gas (syngas) based on the potential for producing high octane value fuels and fuel additives. Higher alcohols are also valuable in view of their applicability as either raw materials or solvents for producing high value-added chemicals. The abundance of diverse carbonaceous sources for producing syngas, including coal, natural gas and biomass, makes the catalytic synthesis of higher alcohols from syngas appealing as an alternative to petroleum resources. The Cu-Co catalyst is one of the most common catalyst types used in the synthesis of higher alcohols (HAS) from syngas. It is believed that the nature of the interaction between the Cu and Co species in these catalysts significantly impacts the CO conversion and the selectivity towards higher alcohols.

Since the HAS process is so highly exothermic in nature, a supercritical fluid (SCF) can be introduced as a reaction medium in order to better manage this exothermic process while also providing other benefits. SCFs are attractive media for various chemical reactions because of their unique properties, including enhanced heat transfer and mass transfer operations. Introducing a supercritical solvent into HAS can facilitate single phase operation while simultaneously providing gas-like viscosity and liquid-like diffusivity.  As such, introduction of a SCF in this heterogeneous catalytic process can enhance the in situ extraction of low volatility products from catalysts pores, diminish transport limitations, and integrate desorption and separation processes. In addition, higher alcohol synthesis from syngas involves a significant reduction in volume due to the stoichiometry of the reaction.  As such, higher selectivity towards C₂₊OH can be expected when applying a properly selected SCF medium into the process based on the application of elevated operating pressures while simultaneously managing the significant exotherm.

In this work, a Cu-Co based catalyst was investigated for the synthesis of higher alcohols from syngas under both gas phase and supercritical phase conditions. The catalyst was prepared by coprecipitation methods and tested in a fixed bed reactor using supercritical hexanes as the reaction medium. The influence of catalyst pellet size on the catalytic performance, the mass transfer of reactants, and the alcohol product distribution has been evaluated under both gas phase and supercritical hexanes phase conditions. The formation of CH₄ was significantly reduced under supercritical hexanes reaction conditions and the effect of various operating conditions on product yield and selectivities have been systematically determined. The results of these catalytic investigations, both with and without the supercritical hexanes medium, demonstrate that the supercritical medium has a significant effect on the selectivity and the productivity towards higher alcohols.