Reaction and Surface Characterization Study of Higher Alcohol Synthesis Catalysts: VII. Cs- and Pd-Promoted 1:1 Zn/Cr Spinel

Abstract

A 1:1 Zn/Cr spinel catalyst promoted with 5.9 wt% Pd and 1, 3, or 5 wt% Cs has been tested for methanol and isobutanol synthesis using a 1:1 H2:CO syngas feedstream. The reactor was operated at two different temperatures (400 and 440°C) and two different pressures (1000 and 1500 psig). The higher temperature and pressure settings result in the highest isobutanol production rates over most of the catalysts examined. As the amount of Cs promotor is increased at 440°C and 1500 psig, the total alcohol production rate increases and the methanol-to-isobutanol ratio decreases. A maximum isobutanol production rate of 142 g/kg-h and a methanol-to-isobutanol mole ratio of 1.0 are attained using the 5.0 wt% Cs/ 5.9 wt% Pd-promoted Zn/Cr catalyst. Furthermore, the addition of 3 and 5 wt% Cs results in an increase in the selectivity to total alcohols. The addition of Pd to these catalysts enhances both the total alcohol production rate and isobutanol production rate compared to Cs-promoted Zn/Cr catalysts which do not contain Pd. Surface characterization studies using X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) were performed on the 5 wt% Cs/5.9 wt% Pd-promoted catalyst. The XPS and ISS data reveal that the fresh catalyst surface consists primarily of ZnO, ZnCr2O4, Cs2O and a small amount of PdO2. Since the catalysts are typically reduced before testing in the reactor, the catalyst sample was again characterized after a reductive pretreatment in 1×10−7Torr of H2at 300°C for 4 h. This pretreatment results in removal of the carbon contamination, thereby exposing underlying ZnO, Cs, and Pd. Characterization studies were also performed on a 5 wt% Cs/5.9 wt% Pd-promoted catalyst which had been removed from the reactor after 5 days of testing. The XPS data indicate that the near-surface region remains enriched in Cs and Pd and that the PdO2present on the pretreated catalyst is converted to PdO during reaction. Apparently, the reaction process results in agglomeration of the ZnO film present on the reduced catalyst, which is the active catalytic phase in higher alcohol synthesis (HAS).