Higher alcohol synthesis over Na-modified Co-Cu catalysts: Co2C formation and effect of surface composition

Higher alcohol synthesis (HAS) via hydrogenation of CO is a potential route for providing important basic chemicals, alternative fuels, and additives in gasoline blends. The Institute Francais du Petrolehas reported on selective alkali-doped Co-modified Cu/ZnO/Al2O3 methanol synthesis catalysts in HAS. These catalysts undergo several structural changes during activation and during the first hoursunder reaction conditions. An initial deactivation was observed for the sample with low Na loadings within the first 10 hr in HAS, while for samples with higher Na loading, the initial degree of conversion remained essentially constant. Also, the product distribution in terms of alcohol selectivity strongly depends on the alkali loading since the selectivity to higher oxygenates increased remarkably with increasing Na content. The initial deactivation observed for the Na-free catalyst was accompanied with a shift in product distribution presumably as a result of surface alloying of Cu and Co. Meanwhile, the structural support of ZnO and Al2O3 is largely collapsed for the Na-loaded samples. The presence of alkali compounds favors dissociative adsorption of CO, leading to enhanced C-C bond formation, but also results in carbidization of Co sites forming Co2C. The Co-Co2C interface is active in HAS and for the Na-rich samples a constant degree of conversion and a constant product distribution was obtained during HAS.