(113d) Effect of Supercritical Hexanes Reaction Medium and H2/CO Molar Ratio On the Synthesis of Higher Alcohols From Syngas Over a K-Promoted Cu-Co-Zn Catalyst

Optimizing the production of fuels and high-value chemicals from single-carbon compounds such as methane, carbon monoxide, and carbon dioxide has been an area of ongoing research for more than eighty years. Syngas, a mixture of carbon monoxide and hydrogen, can be generated by steam reforming natural gas or by gasifying coal or biomass and has been utilized industrially to produce a wide range of hydrocarbon products in a process known as Fischer-Tropsch Synthesis (FTS). FTS is typically carried out over either an iron or cobalt based catalyst and is a highly exothermic, surface-catalyzed polymerization reaction. By replacing the cobalt FT catalyst with a copper based catalyst, alcohols—as opposed to olefins and paraffins—can be primarily produced from syngas in a similarly unselective and exothermic alcohol synthesis reaction. Higher alcohols (e.g. ethanol and butanol) formed from the conversion of syngas are high-value feedstock chemicals that can also be used as both fuels and fuel additives due to their high octane ratings and low emissions compared to gasoline and diesel. However, traditional gas phase synthesis of higher alcohols from syngas has suffered from low productivity and selectivity.

In gas phase FTS, high rates of diffusion allow the reactants to readily access the active sites of the catalyst, but poor heat management in fixed-bed operation can result in deactivation of the catalyst and increased production of unwanted side products such as methane and carbon dioxide. On the other hand, in slurry phase FTS, the higher thermal conductivity of the liquid reaction medium allows for enhanced heat transfer; however, interphase mass transfer limitations reduce the accessibility of the reactants to the catalyst active sites. Supercritical fluids, which have physical properties intermediate between those of liquids and gases (e.g. vapor-like diffusivities and liquid-like thermal conductivities), have been shown to prevent catalyst deactivation, to facilitate heat removal, to reduce the formation of side products, and to promote carbon chain growth in FTS. Since supercritical phase operation has benefited FTS and since FTS and higher alcohol synthesis (HAS) are similar reactions, a supercritical reaction medium has the potential to improve the synthesis of higher alcohols compared to traditional gas phase operation. Additionally, incorporating cobalt into the copper based HAS catalyst has been shown in past studies to improve the selectivity towards higher alcohols.

In this study, the synthesis of higher alcohols from syngas was carried out in a fixed-bed reactor loaded with a Cu-Co based catalyst. The K-promoted Cu-Co-Zn catalyst was prepared using a co-precipitation and incipient wetness method, and hexanes were selected as the supercritical fluid reaction medium because the critical temperature of hexane falls just below the operating temperature for HAS. In the first part of this study, the effect of a supercritical environment on HAS was investigated by varying the hexane to syngas molar ratio while maintaining a constant H₂/CO ratio. Because a supercritical fluid medium will affect the relative rates of diffusion of the reactants to the catalyst active sites, the second part of this study involved varying the H₂/CO molar ratio in the feed in order to study the effect of syngas composition on catalytic performance under gas phase as well as supercritical phase conditions. Compared to gas phase operation at the same conditions, the use of supercritical hexanes resulted in a significant decrease in the selectivity towards methane and CO₂ and an increase in higher alcohol productivity.