Conventional Fe-based low temperature Fischer Tropsch synthesis (FTS) catalysts are commonly synthesized using impregnation or co-precipitation method with iron salt and promoters, followed by a high temperature calcination step. It is known that the traditional impregnation preparative methods have certain drawbacks, including slow diffusion of the metal salt into the pores of the support during impregnation because of the high viscosity of water, poor dispersion of metals, and inhomogeneous distribution of the active compounds.
In this study, a new catalyst was prepared using pre-synthesized iron oxide nanoparticles that were controllably deposited onto a support using a gas-expanded liquid precipitation technique (GXL). The effectiveness of this novel catalyst system was tested for FTS performance in a fixed bed tubular reactor. A GXL is a mixed solvent composed of a compressible gas like CO2 or ethane dissolved in an organic solvent. Due to the expansion of the liquid-organic solvent upon gas dissolution, the viscosity, diffusivity and solubility are tunable through simple adjustments in the applied pressure. The lower viscosity, higher mass transfer coefficient, and diminished interfacial properties in the GXL enhance the diffusivity of the iron particles into the catalyst pores. Using the GXL technology, previously synthesized iron oxide nano-particles can be efficiently deposited onto the surface, and inside the pores, of a desired support material via CO2 pressurization of a mixture of a support material, organic solvent, and dispersed iron oxide particles. This method allows us to generate a supported iron catalyst with a narrow size distribution of iron oxide nano-particles for the Fischer Tropsch synthesis.
This work reports the preparation and optimization of these new nano iron oxide catalysts and their catalytic performance in terms of suitable activity and product selectivity. These nano iron oxide catalysts were compared to conventional catalysts prepared by the incipient wetness methods using the same iron oxide loading. In addition, the influence of the type of support material employed, the iron oxide loading amount, as well as the surface areas of the support materials on the FT catalytic performance have been evaluated.