Tuning the Fischer–Tropsch reaction over CoxMnyLa/AC catalysts toward alcohols: Effects of La promotion
Selective synthesis of mixed alcohols from syngas (CO + H 2 ) via Fischer–Tropsch synthesis (FTS) is an alternative one-step one-pot method. Here we report a series of Co-based catalysts supported by activated carbon (AC) and promoted by Mn and La, namely, Co x Mn y La/AC, over which the selectivity to alcohols is usually ∼26.8% and up to 31.7% at elevated reaction pressure, accompanied by a large amount of olefin byproducts (∼30%) that can be further converted to alcohols via a hydroformylation process. Detailed characterizations are performed for the structural properties of catalysts, including physisorption, chemisorption, microscopy, crystal structure, and surface element compositions. The active site of Co x Mn y La/AC catalysts is the highly dispersed metallic Co on the surface of Co 2 C nanoparticles (Co@Co 2 C), similar to that of Co x Mn/AC catalysts reported previously, while that of Co y La/AC catalysts is a combination of metallic Co and Co 2 C phases (Co–Co 2 C). The result indicates that Mn promoter plays a determinative role in the formation of active sites over Co x Mn y La/AC catalysts. Nevertheless, the La promoter tends to aggregate on the surface of cobalt nanoparticles in the form of La 2 O 3 to promote the CO insertion step and enhance the H 2 chemisorption, so that it can both facilitate the formation of alcohols and make the FTS product class shift to light components. The result proves that the structure of active sites can be established by adding a strong promoter such as Mn, and a second promoter, such as La, can be used to tune the FTS reaction toward alcohols by further modifying its surface properties.
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