Reaction and Surface Characterization Study of Higher Alcohol Synthesis Catalysts

Abstract

An Engelhard, Zn/Cr spinel, methanol-synthesis catalyst (Zn-0312) has been promoted with Cs and tested for isobutanol and methanol synthesis using a 1:1 CO and H2syngas feed stream. Two temperatures (400 and 440°C) and two pressures (1000 and 1500 psig) were tested, and the highest isobutanol product rates are attained using the higher temperature and pressure. Increasing the Cs-promotor concentration to 3 wt% decreases the hydrocarbon byproduct rate and increases the total alcohol production rate, alcohol selectivity, and isobutanol production rate. Above 3 wt% Cs, however, these values decrease as does the rate of hydrocarbon formation. A product isobutanol production rate of 116 g/kg-h can be attained using the 3 wt% Cs-containing catalyst at 440°C and 1500 psig. This production is greater than that obtained using a K-promoted Zn/Cr spinel catalyst indicating that the Cs is a better promotor for isobutanol production, which is consistent with previous findings. Surface-characterization studies performed on the 3 wt%-Cs-containing catalyst indicate that the near-surface region contains primarily ZnO with very little Cr. According to X-ray photoelectron spectroscopy (XPS) data, Cs is present as Cs2Cr2O7and maybe Cs2O. These catalysts are reduced prior to tested in the reactor. Therefore, the catalyst was characterized after reduction in 1×10−7Torr of H2for 4 h at 250°C. Ion-scattering spectroscopy (ISS) data indicate that the outermost atomic layer of the as-prepared catalyst contains primarily C and O. Reduction results in enrichment of the outermost atomic layer with Cs, but smaller amounts of O, Na, Cl, Cr, and Zn also are present. While aging for 5 days in the reactor, the surface becomes O rich and the Zn-to-Cs atom ratio increases. These data suggest that Cs-promoted ZnO is the active catalyst phase and that neither Cr nor the spinel structure are required.