Co to produce higher alcohols has b

Co to produce higher alcohols has been a source of controversy.Bailliard-Letournel et al.63 claimed, on the basis of spectroscopicevidence, that a CoCu surface alloy was responsible for theselectivity to higher alcohols. Baker et al.64 believed the role of Cuwas to moderate the hydrogenation activity of Co to suppresshydrocarbon formation, without necessarily participating in thereaction itself.When Mo alone was addedto a similar system (50% CuLa2Zr2O7),it deactivated the catalyst for methanol synthesis without increasing the formation of higher alcohols.62 In contrast, addition of Mo to a 5% Co/50% CuLa2Zr2O7 formulation increasedthe yield of higher alcohols, with both activity and selectivityattaining a maximum at 3% Mo. The increased yields fromMo addition were attributed to the hydrogen absorptioncapability of Mo oxides. In contrast to Mo, the other GroupVIB elements (Cr and W) did not increase the yield of higheralcohols (Figure 8).Mixtures of both alkali and nonalkali promoters can also bepromising.23 For example, Hofstadt et al.46 used nonalkalipromoters such as MnO, Cr2O3, and ThO2 with methanolsynthesis catalysts (CuO/ZnO/Al2O3/K) to increase the selectivity toward higher alcohols. MnO was particularly found toimprove the selectivity toward ethanol. Cr2O3- and ThO2-promoted catalysts favored the formation of propanol andbutanol, respectively. The promoters were assumed to influencethe Cuþ/Cu0 ratio and thereby change the balance of surfacespecies as described in Section 2.10.Inui et al.65,66 found that physically mixing Fe/Cu/Al/K andCu/Zn/Al/K catalysts increased the space time yield of ethanolfrom CO2 hydrogenation over that of the Fe-based catalyst alone.When alumina-supported Pd was combined with a mixture ofCu/Zn/Al/K and Ga-promoted Fe/Cu/Al/K, the ethanol spacetime yield increased by 48%. This catalyst mixture gave anethanol yield more than twice the value expected on the basisof a Schulz
Flory plot. Pd and Ga were added to optimize theoxidation state of the catalyst by hydrogen spillover and inversespillover, respectively.It can be inferred from the above findings that the role of apromoter is dependent on reaction conditions, catalyst composition, and support; nevertheless, promoters do play animportant role in enhancing ethanol and higher alcohol selectivity. An optimum amount of promoter is always necessary toachieve this goal.