Activated-carbon-supported K–Co–Mo catalysts for synthesis of higher alcohols from syngas

Abstract

A series of activated-carbon-supported K–Co–Mo catalysts (K–Co–Mo/AC) were prepared by using a sol–gel method combined with incipient wetness impregnation. The catalyst structure was characterized by X-ray diffraction (XRD), N₂ adsorption–desorption, X-ray absorption fine structure spectroscopy and X-ray photoelectron spectroscopy, and their catalytic performance toward synthesis of higher alcohols from syngas was investigated. The large surface area and pore volume of the support facilitated the distribution of metal particles and high dispersion of metals. At low Mo loading, the Mo atoms on the activated carbon surface were mainly tetrahedrally coordinated Mo⁶⁺ species. With an increase in the Mo loading, the coordination environment of the surface Mo atoms gradually transformed from tetrahedrally coordinated Mo⁶⁺ to octahedrally coordinated Mo⁴⁺, indicating that an increase in the Mo loading promoted the reduction of Mo⁶⁺ species to Mo⁴⁺ species. After reduction, it was observed that a type of a lower state Mo^δ+ (1 < δ < 4) species was present on the surface, which is suggested to be responsible for alcohol synthesis. Compared to the unsupported catalyst, K–Co–Mo/AC exhibited a significantly higher activity for alcohol formation. In particular, the C₂₊ alcohol content increased significantly. This can be attributed to the fact that the supported catalysts have a high active surface area, and the mesoporous structure is suggested to prolong the residence time of intermediates to form alcohols in the pores to some extent, thus promoting the formation of higher alcohols. Under the conditions of a Mo/AC weight ratio of 40% and a reduction temperature of 798 K, the K–Co–Mo/AC catalyst exhibited the highest activity for alcohol formation, which may be attributed to the high content of Mo^δ+ (1 < δ < 4) species on the surface.