Role of Carbonaceous Supports and Potassium Promoter on Higher Alcohols Synthesis over Copper-Iron Catalysts

The identification of an effective copper-iron catalyst for the direct conversion of synthesis gas into higher alcohols is hindered by the low solubility limit of Cu in Fe and the limited understanding of structural and electronic descriptors in such multicomponent systems. Here, commercial carbonaceous carriers are shown to produce an efficient material only if they enable control of the size and location of metal species through confinement in adequately sized channels, with conical carbon nanofibers being more adequate than carbon nanotubes. Application of a sol-gel route was preferred to other deposition methods to avoid excessive Cu aggregation, associated with enhanced CO2 formation. A bulk Cu/Fe ratio of 2 permitted one to balance the different tendencies of Cu and Fe toward agglomeration, i.e., to form numerous Cu particles of moderate size and hinder the dispersion of the Fe phase and in turn the Fischer-Tropsch activity. Promotion by tiny amounts of potassium was instrumental to further increase the size of the Fe particles and enhance their proximity to Cu. These structural features were associated with a more facile Cu-mediated reduction of the Fe phase and a more pronounced hydrogen activation ability, based on thermal characterization under reaction conditions, and maximized the higher alcohols selectivity (47%) and the olefins fraction among hydrocarbons (50%). An in-depth kinetic analysis over the top performer provided guidelines to optimize temperature, pressure, H-2/CO ratio, and residence time, leading to a space time yield of 0.53 g(HA) g(cat)(-1) h(-1). This value is almost twice as high as that of the state-of-theart bimodal silica-supported CuFe system and could be maintained for 100 h on stream.