Investigation of the catalytic behavior and chemical nature of alkali promoted copper and copper-cobalt-chromium oxide catalysts for the conversion of synthesis gas to methanol and higher alcohols
The work completed for this dissertation has endeavored to identify the active phase(s) for the synthesis of normal alcohols by carbon monoxide hydrogenation with the alkali promoted copper-cobalt-chromium catalyst system patented by the Institut Francais du Petrole. Initial work investigated the effects of a variety of preparation parameters on catalytic activity and selectivity. It was found that special attention must be paid to catalyst calcination temperature. At low calcination temperatures, alcohol selectivities as great as 70 wt % with a production yield of 0.12 g alcohol/g catalyst/hr were obtained. This is the greatest selectivity and yield yet to be found for this catalyst system outside of the inventors' laboratories. Using X-ray diffraction and X-ray photoelectron spectroscopy, the chemical nature of the catalyst was evaluated as a function of calcination temperature. The activated catalyst was comprised of copper metal and a cobalt-chromium spinel. Oxygenate production correlated with the dispersion of copper metal in the catalyst. Poorer copper dispersion and a decrease in oxygenate formation was observed with increasing calcination temperature. The picture of the active catalyst which evolved was that copper metal was supported on a cobalt-chromium spinel. This picture contrasts sharply with previous speculation that cobalt metal and copper(I) chromite were the active phases;An additional avenue of research which resulted from work with the copper-cobalt-chromium system was the investigation of alkali promotion of unsupported copper metal. Although copper metal alone was inactive for carbon monoxide hydrogenation, alkali promoted unsupported copper catalysts were found to be active and selective methanol synthesis catalysts. The methanol synthesis rate increased by an order of magnitude from Li to Cs. It was determined that the role of the alkali is to stabilize the formation of Cu[superscript]+ species, active centers for methanol synthesis. The differences in activity for the alkali series were found to be a result of differences in the concentration of Cu[superscript]+ species at the surface and not electronic effects.