(240a) Effect of Supercritical Solvents On the Synthesis of Higher Alcohols Over a Cu-Co Based Catalyst

There is sustained interest in the synthesis of higher alcohols from synthesis gas (syngas) due to the potential for the production of 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 various 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. However, due to the highly exothermic nature of the alcohol synthesis reactions and the underlying kinetics for these reactions, this procedure suffers from poor productivity and selectivity towards higher alcohols. These limitations have prevented the development of a large scale and economically feasible process.

Supercritical fluids (SCFs) have unique properties, especially density depended properties, which are intermediate between those of a liquid and a gas. The density of a SCF, while lower than that of a liquid, is considerably higher than that of a gas. The diffusivity, while lower than that of a gas, is significantly higher than that of a liquid. These characteristics of a SCF provide several incentives for considering SCF reaction media in fixed bed higher alcohol synthesis, such as enhanced heat transfer relative to gas phase operation, improved accessibility to catalytic active sites and increased thermal uniformity.

Higher alcohol synthesis from syngas involves an overall reduction in the number of moles (i.e. volume reduction) and is highly exothermic. As such, the high pressure, dense phase solvent conditions of a supercritical reaction medium affords the opportunity to enhance the reactions via Le Chatelier’s principle and to improve heat transfer relative to gas phase operation due to the improved heat transfer capacity of the SCF. Higher selectivity towards C₂₊OH can be expected when applying a properly selected SCF medium to the process.

In this work, 0.5 wt% K promoted CuCo/ZnO/Al₂O₃ was investigated as a catalyst for the synthesis of higher alcohols from syngas. The catalyst was prepared by co-precipitation methods and tested in a fixed bed reactor using a supercritical fluid as the reaction medium. Several types of supercritical solvents were tested, including polar and non-polar hydrocarbons. In addition, argon was utilized as an inert balancing gas for the comparison of gas phase and SCF phase operations. The results of experiments performed when using supercritical hexanes as the reaction media for this catalytic system illustrate that the presence of the SCF medium improves the heat transfer from the catalyst bed as evidenced by a significant reduction in the formation of CH₄. Furthermore, the results of these catalytic investigations demonstrate that the presence of the supercritical medium has a significant effect on the selectivity and the productivity towards higher alcohols.