REACTION AND SURFACE CHARACTERIZATION STUDY OF HIGHER ALCOHOL SYNTHESIS CATALYSTS - IX - PD-PROMOTED AND ALKALI-PROMOTED ZN CR-BASED SPINELS CONTAINING EXCESS ZNO/

A Zn/Cr spinel support material was prepared which contains excess ZnO and then was promoted with 5.9 wt% Pd and varying amounts ranging fro m 0 to 7 wt% of either K or Cs. Each of these catalysts was tested at four different reactor operating conditions (T of 400 or 440 degrees C and P of 1000 or 1500 psig) for higher alcohol synthesis (HAS) using a syngas feedstream (1:1 CO:H-2) after reductive pretreatment. High is obutanol production rates in conjunction with low methanol-to-isobutan ol mole ratios (less than or equal to 1.0) and low hydrocarbon byprodu ct rates are desired. For the K-promoted catalysts the highest isobuta nol production rates are obtained at the higher pressure and temperatu re settings of 1500 psig and 440 degrees C, and methanol-to-isobutanol mole ratios below the ideal value of 1.0, which is required for downs tream methyl tertiary-butyl ether (MTBE) synthesis, are obtained. The Cs-promoted catalysts generally yield higher isobutanol production rat es than the K-promoted catalysts. The highest isobutanol production ra te of 170 g/kg-h is obtained using the 3 wt% Cs-promoted catalyst at 1 000 psig and 440 degrees C. The lower pressure is economically advanta geous with regard to process costs. Most importantly, this isobutanol production rate is quite high compared to others presented in the lite rature and demonstrates that Pd does enhance the synthesis of isobutan ol. The catalytic activity remained stable over a 5-day test period fo r each catalyst. X-ray photoelectron spectroscopy (XPS) and ion scatte ring spectroscopy (ISS) were used to characterize these catalysts. The results obtained from these techniques indicate that pretreating the catalysts in 1 x 10(-7) Torr of H-2 at 300 degrees C for 4 h causes an enrichment of the near-surface alkali-promotor concentration. A layer ed structure forms which consists of this enriched alkali layer and an enriched Pd and ZnO layer which reside above the Zn/Cr spinel support material. The reaction process decreases the thickness of the alkali and ZnO layers and seems to trap the Pd in a thin layer above the supp ort material but below the alkali promoters which reside at the surfac e. The surface areas of these catalysts are about 80 m(2)/g, and they are stable with aging. The primary role of the Zn/Cr spinel is to prov ide a high surface-area support for the promoted ZnO. (C) 1998 Academi c Press.