CO2 reactivity on Fe–Zn–Cu–K Fischer–Tropsch synthesis catalysts with different K-loadings

In the presence of low cost H2, the hydrogenation of pure CO2 streams (e.g., from carbon capture processes) or CO/CO2 mixtures (e.g., from biomass of coal gasification) to fuels is a potential solution to convert the primary greenhouse gas into a valuable building block for the synthesis of high-added value products. Iron based Fischer–Tropsch catalysts, known for their reverse water gas shift activity, are good candidates to run such process. However, their reactivity depends on the catalyst formulation which may vary significantly in the presence of supports or promoters. Also, the H2/COx inlet ratio may strongly influence the catalyst activity and selectivity. With the purpose of gaining more insights into the reactivity of carbon dioxide, the catalytic performances of K-promoted 100Fe/10Zn/1Cu samples have been comparatively studied in CO, CO2, and CO/CO2 hydrogenation processes with low H2/COx ratios varying from 0.5 to 1. We have found that traditional iron-based catalysts for the Fischer–Tropsch synthesis can be successfully used to obtain high added-value products from CO2 even in the presence of H2 deficient feed streams. In the case of pure CO2 hydrogenation, the most promising results are obtained with strong potassium promoted iron catalysts which grant interesting selectivities toward middle distillates by favoring CO2 adsorption. In the presence of strongly adsorbed CO, CO2 is instead hardly converted independently on the K-loading. The presence of CO2 does not affect the product distribution either. Nevertheless, CO2 has a key-role in preventing the CO shift to CO2, thus improving the overall economy of the conversion process and avoiding a net CO2 production. Interestingly, upon increasing the K-loading, the CO conversion rate is decreased, both in the presence and in the absence of CO2, possibly as a result of the very strong CO adsorption on the catalytic surface. Such result is however complicated by the presence of initial deactivation phenomena, whose rates may be also related to the K-loading of the catalyst. Accordingly, the best catalyst formulation appears to be significantly different depending on the presence of CO in the feed.