Molybdenum-Based Catalysts
Molybdenum (Mo)-containing catalysts are another class of complex metal catalysts that can be used in the synthesis of alcohols from syngas. A number of researchers have observed that product distribution shifts from hydrocarbons to alcohols when alkali metals are added to Mo-based catalysts during the CO hydrogenation reactions [59-61]. The promoting effect of alkalis (on MoS2) for alcohol formation was found to increase in the order Li < Na < Cs < Rb < K, suggesting that moderate basic promotion is desired. During this work Tatsumi and coworkers suggested that the role of K on Mo/SiO2 is to preserve the surface MoO2 species which is active for alcohols by retarding the reduction of MoO2 species to metal.
In more recent work, Xiang and coworkers have investigated a K/^-Mo2C catalyst modified using Fischer-Tropsch metals Fe, Co and Ni for their performance in CO hydrogenation [62]. The results revealed that Ni — or Co-modified K/b-Mo2C catalysts were shown to be highly active and selective towards the synthesis of mixed alcohols, especially for the C2+OH production, but Fe caused a negative effect. Promotion effects of Co, Ni and Fe on Mo-based catalyst during CO hydrogenation are shown in Table 13.8. Alcohol yields, as well as the yields of other products, in a CO hydrogenation experiment using Mo-based catalyst and Co, Ni and Fe as promoters are shown in Table 13.8. [62]. Ni or Co led to a decrease in apparent activation energies for C1-C4 alcohols, whereas Fe caused a slight increase for methanol and the decrease for C2- C4 alcohols. As a result, they concluded [62] that the promotion effects of Fischer-Tropsch elements on Mo-based catalyst were different and followed the sequence: Ni > Co > Fe for the activity and Ni ~ Co > Fe for the C2+OH alcohol selectivity.
Molybdenum-based catalysts with cobalt and potassium sulfide as promoter is another system of interest [63,64]. These catalysts were obtained by modifying a Mo(MOVS)/SiO2 system prepared by metal oxide vapor synthesis (MOVS), and have also been used as a catalyst for higher alcohol production [65]. With the addition of nickel and potassium or cesium, these catalysts exhibited much higher activity and alcohol production, both by a factor of about two, compared to analogous non-MOVS catalytic systems. Alyea and coworkers claimed that excellent performance of Mo(MOVS) catalyst allows it to be used at a lower temperature to increase the selectivity for alcohols, or to be incorporated with more alkali, which again can enhance the production of alcohols, especially higher alcohols [65].
Bian et al. reported [66] the application of high-temperature calcined K-MoO3/y-Al2O3 catalysts for mixed alcohols synthesis from syngas. Their results indicated that with increased Mo-loading of MoO3/Al2O3 from 0.05 to 0.25, the total yields of mixed alcohols and hydrocarbons decreased, but the selectivity to mixed alcohols was enhanced sharply from 3% to 50%. With increased Mo-loading, MoO3/Al2O3 from 0.25 to 0.45, the CO conversion was enhanced,
but the selectivity to mixed alcohols leveled off. On these catalysts, condensation reaction of low alcohols to form branched C4 alcohols also occurred at the same time; furthermore, with increased Mo-loading, activity of the alcohols condensation became high. These activity experiments for mixed alcohol synthesis and the structural measurements of the catalysts indicated that the dispersion state of Mo species and the content of unreduced Mo species influenced the total CO conversion, and that the acidity of the catalyst controlled the selectivity in mixed alcohols.