Structure of Co–K–Mo/γ-Al 2O 3 catalysts and their catalytic activity for mixed alcohols synthesis

Abstract

A series of oxidized Co–K–Mo/γ-Al 2O 3 catalyst samples, prepared by impregnating oxidized K–Mo/γ-Al 2O 3 samples with an aqueous solution of calculated Co(NO 3) 2 and then calcining in air at temperatures of 350°C, 500°C, 650°C and 800°C, respectively, were sulfided and then investigated for the activity in the synthesis of mixed alcohols from CO hydrogenation under conditions of P=5.0 MPa, T=350°C and GHSV=4800 h −1. The results demonstrate that the addition of cobalt promoter is favorable to the formation of higher alcohols and the optimum calcination temperatures after impregnating with Co 2+ are about 500–650°C. The structure of Co and Mo species on the oxidized and sulfided samples was determined by X-ray diffraction (XRD), laser Raman spectrum (LRS) and extended X-ray absorption fine structure (EXAFS). For oxidized sample calcined at 350°C, cobalt exists as Co 3O 4 species and covers the surface of K–Mo–O species, the structure of K–Mo–O species is the same as that in K–Mo/Al 2O 3 sample. The interaction between cobalt and K–Mo–O species is relatively weak. After sulfidation, cobalt exists as sulfide crystallites with an octahedral coordinated structure, and molybdenum as MoS 2 crystallites. For oxidized samples calcined at 500–650°C, Co component interacts with the K–Mo–O species and destroys the long-range order of the K–Mo–O species gradually. After sulfidation, Co tends to exist as a sulfide with tetrahedral coordinated structure and Mo still as MoS 2 crystallites. For oxidized sample calcined at 800°C, Co component exists mainly as CoAl 2O 4 species, most of which cannot be sulfided during sulfidation.