Study Of Activation Mechanism Of A Mo-based Catalyst For The Synthesis Of Higher Alcohols

The conversion of syngas to higher alcohol (C2+ alcohols) is one of the important topics in the field of Cl chemistry. Higher alcohols are alternative additives for the improvement of the octane number in gasoline and can also be used as clean fuels and petrochemical feed stocks. Therefore, several catalytic systems have been developed for this reaction since the last decades. Among them, the alkali metal promoted Mo-based catalyst is regarded as one of the most promising candidates due to the excellent resistance to sulfur poisoning and carbon deposition. We have previously developed a specific K-Co-Mo catalyst with excellent performance for the higher alcohol synthesis. On this basis, the present work aimed at a systematic study on the activation of the K-Co-Mo catalyst. Different reducing gases were used to activate the catalyst. The catalyst structure and CO adsorption properties were characterized by X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and the catalytic performance for the higher alcohols synthesis was investigated. We discuss the relationship between the structure and catalytic performance. The main conclusions were summarized as below:(1) The activated carbon supported K-Co-Mo catalysts were prepared via a sol-gel method combined with pore volume impregnation and reduced under different activation atmospheres, including pure H2, syngas (H2/CO=2/1), and pure CO. A high-pressure fixed-bed flow reactor was designed and built to evaluate the catalytic performance of the catalyst for the higher alcohol synthesis from CO hydrogenation. For the different gases reduced catalysts, the optimal temperature of reduction was 623 K. For the CO-reduced catalyst, the CO conversion and space-time yield (STY) of total alcohols was the lowest. The H2-reduced catalyst had the highest CO conversion but the lowest alcohol selectivity. By contrast, the syngas-reduced catalyst showed the highest catalytic performance for alcohol synthesis.(2) The reduced catalysts had an amorphous structure. The different reducing atmospheres led to different distribution of active species, thus exerting a significant impact on the catalytic performance. The pure H2-reduced catalyst showed a high reduction degree. A large amount of metallic Co0 and low valence state Moφ+ (0<φ<2) species existed in the reduced catalyst, which favored a super activity for CO dissociation and hydrogenation unfavorable to the alcohol formation. In contrast, the reduction capacity of the pure CO treatment was rather weak as the main Mo and Co species in the catalyst were present under the form of Mo4+ and Co2+. Consequently, the CO dissociation and hydrogenation were inhibited. The syngas treatment had an appropriate reduction capacity and produced a large amount of Mo6+ species and multivalent state Co species on the surface of the catalyst. Their synergistic effects enhanced the cooperativity and equilibrium between the CO dissociation, hydrogenation and CO insertion and thus promoted the formation of higher alcohols.