Reaction and surface characterization study of higher alcohol synthesis catalysts .2. Cs-promoted commercial Zn/Cr spinel

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 H-2 syngas feed stream, Two temperatures (400 and 440 degrees 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 degrees 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 Cs2Cr2O7 and maybe Cs2O., These catalysts are reduced prior to tested in the reactor, Therefore, the catalyst was characterized after reduction in 1 x 10(-7) Torr of H-2 for 4 h at 250 degrees 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, (C) 1997 Academic Press.