Reaction and surface characterization study of higher alcohol synthesis catalysts .1. K-promoted commercial Zn/Cr spinel

A commercial Zn/Cr spinel methanol synthesis catalyst obtained from Engelhard was promoted with potassium and tested for methanol and isobutanol synthesis using a syngas feedstream (1 : 1 H-2 and CO). An equal molar mixture of methanol and isobutanol would be an ideal feedstock for production of methyl tertiary butyl ether (MTBE) for use as an additive to gasoline to increase the octane number and reduce air pollution, Of the reaction conditions examined, the higher temperature, 440 degrees C, and higher pressure, 1500 psig, result in the highest isobutanol production rate over most of the catalysts examined, The effect of the K promotor loading also was examined. Increasing the amount of K from 0 to 1 wt% increases the hydrocarbon by-product rate, but the addition of 3 or 5 wt% K decreases the hydrocarbon by-product rate consequently increasing the selectivity to total alcohols. Increasing the K loading above 3 wt%, however, raises the methanol-to-isobutanol mole ratio. The use of the 3 wt% K-containing: catalyst results in the lowest methanol-to-isobutanol mole ratio of 1.5 which is near the desired value of 1.0 for MTBE synthesis. An isobutanol production rate of 103 g/kg-cat/hr is attained using a 1 wt% K-promoted Zn/Cr spinel catalyst. Surface-characterization studies were performed on the 3 wt% K-containing catalyst. Ion-scattering spectroscopy data reveal that the outermost surface layer of the fresh catalyst consists of C, O, Cr, and Zn, Since the catalysts are reduced prior to testing in the reactor, surface studies also were performed after reducing the catalyst in 1 x 10(-7) Torr of H-2 for 4 hr at 250 degrees C. The addition of the alkali is necessary for higher alcohol synthesis (HAS), and the pretreatment results in migration of the K promotor to the outermost surface layer where the catalytic reactions occur, X-ray photoelectron spectroscopy (XPS) data are consistent with these results and also reveal that the near-surface regions of the nontreated and H-2-treated samples consist primarily of ZnO. The K is present as K2Cr2O7 or K2CrO4. Although the outermost atomic layer of the reduced catalyst contains a large amount of K, relatively little is observed in the near-surface region using XPS, indicating that most of the K is located at the surface. The catalytically active surface for HAS consists of alkali species lying an top of a ZnO phase, The chemical state of the Cr species is altered during reaction. The K2Cr2O7 or K2CrO4 present on the fresh and reduced catalyst is converted to Cr2O3, Cr(OH)(x) and Cr-o with age, but no other phases of Zn other than ZnO are observable before or after aging. The Zn/Cr spinel is not important for activity toward isobutanol production and acts only as a high-surface-area support. (C) 1997 Academic Press.