Study Of Preparation, Structure And Performance Of K-Co-Mo/AC Catalysts For Synthesis Of Higher Alcohols From Syngas

The catalytic conversion of syngas to higher alcohols is one of the important topics in the field of C1chemistry. Higher alcohols is the excellent gasoline additive to enhance the octane number and has the potential to replace the severely polluting methyl tert-butyl ether (MTBE). Furthermore, higher alcohols can be used as a new generation of clean power fuel. Moreover, higher alcohols are the important chemical raw materials. Among reported catalyst systems for the synthesis of higher alcohols, Mo-based catalysts has attracted significant attention as they exhibit high resistance to sulfur poisoning and deactivation by coking. On the basis of our previous study, a series of activated carbon supported K-Co-Mo catalysts with different Mo loading were prepared by a modified sol-gel method combined with incipient wetness impregnation. The catalyst structure was characterized by X-ray diffraction (XRD), N2adsorption-desorption isotherm, X-ray absorption fine structure spectroscopy (XAFS) and X-ray photoelectron spectroscopy (XPS), and their catalytic performance toward synthesis of higher alcohols from syngas was investigated in detail. The main research conclusions obtained are listed below:(1) The effect of Mo loading on the catalyst structure were investigated. The results indicated that the active components had a high dispersion on the surface of activated carbon. The incorporation of active metals did not change the mesoporous structure of the activated carbon support. At low Mo loading, the Mo species were dispersed as a monolayer on the activated carbon surface, the surface Mo atoms were tetrahedrally or octahedrally coordinated Mo6+species. With an increase in the Mo loading, the surface tetrahedrally coordinated. M66+species were gradually transformed to octahedrally coordinated Mo4+, indicating that an increase in the Mo loading promoted the reduction of Mo6+species to Mo4+species. After reduction, a lower state Moδ+(1<δ<4) species was present on the catalyst surface, which is regarded as the active site for alcohol synthesis.(2) The effect of Mo loading on catalytic performance for higher alcohol synthesis were investigated under the conditions of reduction temperature at773K, reaction temperature at573K,5.0MPa,2400h-1. The catalyst with a Mo/AC weight ratio of40%exhibited the best activity for alcohol synthesis. The STY of total alcohol was126.2g·kg1·h-1, approximately6times as high as that of the unsupported sample, the selectivity for total alcohol also increased from11.5%to31.4%. Especially, methanol production was remarkably inhibited, MeOH/C2+OH decreased to0.54. Ethanol became the predominant product. Combining with the characterization results, it is suggested that the mesoporous structure of the catalyst can prolong the residence time of reaction intermediates in the pores to some extent, thus promoting the formation of C2+OH. Besides, the supported K-Co-Mo catalysts have a high active surface area, which is also conducive to the alcohol synthesis.(3) The effect of reduction temperatures on the structure and catalytic performance was investigated. Under the conditions of reduction temperature at798K, the K-Co-Mo/AC catalyst exhibited the highest activity for alcohol formation, which may be attributed to the high content of Moδ+(1<δ<4) species on the catalyst surface. Under the conditions of573K,5.0MPa and3600h"1, the stability of the K-Co-Mo/AC catalyst with a Mo/AC weight ratio of40%was tested continuously for100h. The CO conversion and selectivity for alcohol remained unchanged after100h reaction, indicating that the catalyst has a good stability.