(601f) Catalytic Performance of Supported Nano-Iron Catalysts Synthesized By a Gas-Expanded Liquid Deposition Technique in Fischer-Tropsch Synthesis

Supported iron catalysts for low temperature Fischer Tropsch Synthesis (FTS) as prepared by impregnation of iron salt precursors on support materials have met with limited success. While the support materials provide high surface areas, iron species tend to interact strongly with traditional support materials such as SiO₂ and Al₂O₃. As such, the activity and selectivity towards long chain hydrocarbons for these catalysts were found to be inferior to those prepared by a precipitation method. However, supported iron catalysts provide a few potential advantages over precipitated catalysts, such as 1) better mechanical strength in order to withstand degradation, 2) enhanced dispersion of the iron, and 3) better control over the size and distribution of the iron species on the surface, etc. Therefore, there is a need to develop new synthesis methodologies to create improved supported iron catalysts in order to further enhance catalyst activity and selectivity towards hydrocarbons.

In this study, a new catalyst was prepared using pre-synthesized iron oxide nanoparticles that were controllably deposited onto a SiO₂ support using a gas-expanded liquid precipitation technique (GXL). In general, gas expanded liquids are solvent systemsâ??typically a mixture of an organic solvent and a compressible gas, such as CO₂â??that enable controlled deposition of pre-synthesized nanoparticles, based on their size, onto a support surface. By controlling the applied CO₂ pressure, and hence the solvent strength, one can systematically deposit/precipitate nanoparticles of desired sizes onto the support materials. The iron oxide nanoparticle dispersion used in this study contains oleic acid stabilized iron oxide nanoparticles dispersed in an n-hexane solvent. We induced the precipitation of larger sized particles from the initial nanoparticle dispersion, thereby depositing these larger particles onto the surface of the support while enabling the removal of the smaller sized particles by removal of the residual hexane solution. The ability to exclude the smaller sized particles from those that are deposited onto the catalyst support is important since supported iron FT catalysts with nanoparticles of small size have lower activity due to their stronger interaction with the support surface. As an added benefit, the lower viscosity and higher mass transfer coefficient in the GXL can enhance the diffusivity of the iron particles into the catalyst pores during deposition, resulting in a more uniform distribution of the nanoparticles compared to traditional methods.

The objective of this work is to explore the benefits of using the GXL technique in preparing SiO₂ supported catalysts with moderate iron loadings for Fischer-Tropsch synthesis applications. The catalytic performance of GXL prepared Fe/SiO₂ catalysts with different iron loadings were evaluated in terms of CO conversion, CH₄ selectivity, C5+ selectivity and C5+ productivity. In addition to the effect of iron loading, we have also investigated the effect of reaction conditions, such as reaction temperature, on the catalytic performance of our GXL catalysts in order to understand the influence of operating conditions on the selectivity and productivity of desired hydrocarbon products.