Reaction and surface characterization study of higher-alcohol synthesis catalysts XII: K- and Pd-promoted Zn/Cr/Mn spinel

Abstract

The synthesis of higher alcohols from syngas is a potentially attractive route for the downstream production of methyl tertiary-butyl ether (MTBE), an oxygenated gasoline additive. One of the most effective types of higher alcohol synthesis (HAS) catalysts is a Zn/Cr spinel promoted with either Cs or K. Many studies have suggested that the Cr, required to form the high surface area spinel structure, is detrimental in HAS because it may enhance hydrocarbon formation. For this reason catalysts were examined in the present study in which Mn was substituted into the spinel thereby replacing some of the Cr. Two different K and Pd-promoted Zn/Cr/Mn spinel catalysts were tested for HAS using a 1:1 CO/H 2 syngas feedstream. These catalysts were promoted with 2.25 wt% K, 5.9 wt% Pd and 1.0 wt% K, 9.0 wt% Pd. The former performs better under all four reactor conditions examined (pressures of 1000 and 1500 psig and temperatures of 400°C and 440°C). At 440°C and 1500 psig the isobutanol production rate is 179 g/kg h, the total alcohol production rate is 304 g/kg h and no significant changes in the product stream composition occur over a five-day test period. The methanol-to-isobutanol mole ratio is 1.3, which is very close to the desired value of 1.0 for MTBE production. This is the most active HAS catalyst described in the literature. Surface characterization data, obtained using X-ray photoelectron spectroscopy and ion scattering spectroscopy, indicate that the Mn content in the near-surface region of the catalyst is small. The characterization data also demonstrate that the reductive pretreatments given to HAS catalysts prior to testing result in the elimination of C-containing contamination, adsorbed water and hydroxyl groups and either expose underlying K and Pd promotors or possibly induce the migration of the K and Pd to the outermost surface layers. The pretreated catalyst surface has a layered structure with an enriched K- and Pd-containing layer at the surface above the ZnO layer over the spinel support.