Hydrogenation of Carbon Monoxide on Supported Ruthenium Catalysts

Abstract

The effects of catalyst support and dispersion of ruthenium on the activity and selectivity of CO hydrogenation have been studied. The correlation between the information on the beha vior of adsorbed species, obtained by physicochemical methods such as infrared spectroscopy (IR) and the reactive thermal desorption (RTD), and the catalytic features, determined by a steady-state reaction, have also been discussed.
Hydrogenation of CO at temperatures below 230°C and under pressurized conditions gave hydrocarbons with carbon atoms up to about 40. However, hydrocarbons of C₁₃ and higher did not come out of the catalyst bed during the reaction. They effused when the bed was treated at around 400°C in the flow of inert gas and gave product distributions which follow the Schulz-Flory law. The specific acti vity for CO conversion was dependent upon the metal dispersion, but the temperature and the pressure effect made a proper estimation difficult for the activity of supported ruthenium catalysts. For both SiO₂- and Al₂O₃- supported catalysts, the apparent activation energy of CO conversion was independent of the supports, but dependent on the dispersion (Table 4). It was around 20 kcal/mol and about 35 kcal/mol for the highly dispersed catalysts (H-catalyst) and for poorly dispersed catalysts (L-catalyst), respectively.
The specific activity for the H-catalysts was found to be higher than that for L-catalysts at atmospheric pressure, but an opposit result was found at 6 atm. Product distributions were characterized by the Schulz-Flory distribution law. The probability of chain growth (α) was independent of the supports, but dependent on the dispersion under a pressurized conditions. The a values for the H- and L-catalysts were 0.75 ± 0.02 and 0.87±0.01, respectively. The a values were smaller and the selectivity of methane was larger at 1 atm than at 6 atm (Table 5). The Ru/TiO₂ catalyst was found to produce high-molecular-weight hydrocarbons and less methane ev en at higher temperature and at low pressure.. The correlation of the chain growth hydr ocarbons and the metal dispersion obtained by both IR and the steady-state reaction coincided with each other, but the reactivity of adsorbed CO, obtained from RTD, did not exhibit a good agreement with those deduced from the steadystate reaction.