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Supported Iron Nanoparticles as Catalysts for Sustainable Production of Lower Olefins

The production of commodities for the petrochemical industry from sustainable sources is of major and ever-increasing interest. Among the many catalytic processes proposed, the Fischer–Tropsch (FT) synthesis, developed originally to produce fuels from coal or natural gas, and one of the few applied at industrial scale since the early 40s, is shaping up as one of the most interesting options. FT catalysts (typically ironor cobalt-based) convert syngas (CO+H2) into a mixture of CO2, CH4 and hydrocarbons, often referred to as “synfuel”, that contains paraffins, a-olefins and oxygenated compounds (alcohols). Owing to the complex mixture of reaction products, manufacturing of a specific commodity (e.g. , ethylene) from synfuel requires a downstream treatment and separation process, which involves increased production costs and revalorization of the undesired product. Therefore, improvement of the catalyst selectivity towards a desired group of products is of great interest ; this would simplify the FT downstream process, reducing its carbon footprint and improving its economic feasibility. Iron-based bulk catalysts are used typically for the production of olefins through the FT process. Active sites are iron carbide species formed during catalyst activation and the initial stages of the reaction. The role and contribution of each of this species in the FT synthesis is still a matter of controversy. One important limitation of this type of catalysts is its low mechanical strength. Supported iron catalysts can overcome this limitation but are less active than bulk catalysts because of the strong interaction of iron with the support. 5] The FT synthesis comprises a complex array of reactions: 1) reactant adsorption and (non-)dissociative activation, 2) formation of methylenic intermediate species ( [CH2]* ), 3) surface polymerization of these monomeric units to form growing hydrocarbon chains (adsorbed on the catalyst active sites) and 4) desorption of these hydrocarbon chains as paraffins, olefins or oxygenate compounds, depending on the termination mechanism. The polymeric nature of the hydrocarbon synthesis gives a well-known product distribution that, generally, fits the Anderson–Schulz–Flory (ASF) model. The occurrence of side reactions (olefin readsorption) and mass transfer phenomena, which influence the product selectivity, have also been reported. 8] Product selectivity depends on the relative rates of each of the steps mentioned above. Although key parameters that control the FT product selectivity have been investigated intensively, the characteristics of the active sites controlling the product selectivity remains unknown. K. P. de Jong et al. have made an important advance in the development of supported iron catalysts. The authors report the conversion of syngas to short olefins over two iron-based catalysts supported on a-Al2O3 and carbon nanofibers (CNF), respectively, with the highest product selectivity (61 wt % C) ever reported. In addition, both catalysts keep methane production at a low level and show good activity and stability. This has been achieved by the thoughtful design of the catalyst at the nanoscale by combining the use of a weakly interactive support with the homogenous distribution of iron (oxide) nanoparticles modified with promoters (Figure 1).