Co-Mo-K Sulfide-Based Catalyst Promoted by Multiwalled Carbon Nanotubes for Higher Alcohol Synthesis from Syngas

Using homemade multiwalled carbon nanotubes (CNT) as the promoter, sulfurized Co-Mo-K catalysts (denoted CoiMojKk-x%CNT) were prepared by the coprecipitation method. Their catalytic performance for the synthesis of higher alcohols from syngas was evaluated and compared with that of the CNT-free counterpart (CoiMojKk). Appropriate incorporation of a minor amount of CNT into CoiMojKk led to a significant increase in CO conversion and the selectivity for the higher alcohols. Under the reaction conditions of 5.0 MPa, 623 K, V(H2):V(CO):V(N2) = 60:30:10, and GHSV = 3600 ml/(g·h), the observed space-time-yield of total (C1–4) alcohols reached 241.5 mg/(g·h) with 21.6% of CO conversion over the Co1Mo1K0.3-10%CNT catalyst, which was 1.84 times that over the Co1Mo1K0.3 catalyst. Ethanol became the dominant product of the CO hydrogenation under the conditions mentioned above. The water-gas-shift (WGS) side reaction was inhibited to a great extent over the CNT-promoted catalyst. The results of catalyst characterization indicated that the addition of a small amount of CNT into the Co1Mo1K0.3 catalyst did not cause an obvious change in the apparent activation energy for the conversion of CO but led to an increase in the molar percentage of the catalytically active Mo species (Mo4+) in the total amount of Mo at the surface of the working catalyst. On the basis of the temperature-programmed desorption results, it could be inferred that under the conditions of the higher alcohol synthesis, there existed a considerably larger amount of reversibly adsorbed H species on the CNT-promoted catalyst, which would generate a surface microenvironment with high steady-state concentration of the adsorbed H species on the catalyst and thus increase the rate of a series of surface hydrogenation reactions. In addition, high steady-state concentration of adsorbed H species on the surface of the catalyst would effectively inhibit the WGS side reaction. These factors considerably contribute to the increase in the yield of the main product.