EFFECT OF THE K-MO INTERACTION IN K-MOO3 GAMMA-AL2O3 CATALYSTS ON THE PROPERTIES FOR ALCOHOL SYNTHESIS FROM SYNGAS

A series of K-MoO3/gamma-Al2O3 catalysts was prepared by varying the K/Mo atomic ratios between 0 and 1.5, maintaining molybdenum content as constant. The structures of the samples were characterized by several techniques (LRS, XRD, XPS, ESR, TPR, 02, CO and H-2 chemisorption, and TPD) and accounted for the catalytic properties for alcohol synthesis from synthesis gas. It is revealed that for the potassium-promoted samples an interaction between the potassium promoter and the supported molybdenum component occurs. The amount of the species resulting from this interaction gets saturated at a K/Mo ratio of 0.8. On the oxidic samples, K-Mo interaction species which may contain potassium cations and an Mo7O246- Unit are formed. This retards, to some extent, the sulfidation and reduction of Mo(VI) to Mo(IV). The sulfidation of the interaction species leads to the aggregation of MoS2 and the formation of surface ''K-Mo-S'' species. At K/Mo ratios above 0.8, KCl crystallites form; some amount of KCl, which is not aggregated, covers the exposed molybdenum surface, and decreases the amount of ''K-Mo-S'' species and Mo(CUS) sites. The activity toward alcohol formation over the sulfided samples is in parallel with the extent of the K-Mo interaction, with the maximum achieved at K/Mo ratio of 0.8, while alcohol selectivity monotonically increases with K/Mo ratio. It is tentatively proposed that the surface ''K-Mo-S'' species and Mo(CUS) sites are responsible for the alcohol and hydrocarbon formations, respectively. The large increase in alcohol selectivity at K/Mo ratios above 0.8 could be attributed to the effect of the unaggregated KCl which preferentially covers the Mo(CUS) sites. The high C2+OH/C1OH ratio in the alcohol product distribution is probably due to the substantial amount of the SH species present on the surface of the sulfided samples during reaction. (C) 1994 Academic Press. Inc.