Gas expanded liquids (GXL) are solvent systems which enable the controlled deposition of presynthesized iron oxide nanoparticles, based on their size, onto a surface through the use of compressible gas such as CO 2 . By controlling the applied CO 2 pressure, and hence the solvent strength, one can systematically deposit/precipitate nanoparticles of desired sizes. In this study, a technique using a GXL has been developed to controllably deposit iron oxide nanoparticles onto oxidic materials such as alumina and silica. The nanomaterials generated using this technique were then tested for their effectiveness as catalysts in Fischer−Tropsch (FT) synthesis in a fixed bed reactor system under standard low temperature FT conditions. The catalytic performance of these nanoscale iron FT catalysts was compared to control catalysts that were prepared by traditional incipient wetness methods using the same iron loading. Characterization of these GXL-synthesized materials demonstrated that the iron oxide nanoparticles were deposited in a size controlled manner onto the oxidic support. A CO conversion of 18% was observed using a silica catalyst generated using this GXL technique along with a 69% selectivity toward C 5+ hydrocarbons. The nanoscale iron catalysts prepared by the GXL technique employed in this study exhibit weaker interactions between the iron oxide nanoparticles and the support material, offering a new strategy to investigate the effect of metal−support interaction on the activity and selectivity of iron based FT synthesis catalysts.

No abstract

No abstract

Nanoparticles have highly size-dependent properties which need to be harnessed for their appropriate use in various applications. One method that has been used to obtain monodisperse nanoparticles involves the initial synthesis of a relatively polydisperse nanoparticle dispersion using conventional methods, followed by the size-selective separation of these nanoparticles using a variety of different processing techniques. Through prior investigations in our group, we have demonstrated that application scale quantities of ligand-stabilized metal nanoparticles can be precisely separated based upon their size via a sequence of pressure-induced precipitations from gas-expanded liquid (GXL) mixtures (typically a mixture of an organic solvent and compressed CO2 gas). However, this solvent-based process requires relatively high applied pressures of CO2 (e.g., ∼41.4 bar with n-hexane as the solvent and dodecanethiol as the ligand) to effectively precipitate these nanoparticles from the solvent + CO2 mixture. In this paper, through methodic selection of solvents and ligands of different steric structures, we have aimed to manipulate the nature of the solvent–ligand interaction so as to precipitate and size-selectively fractionate gold nanoparticles at lower pressures in these GXL systems. Constitutional isomers of n-hexane were chosen as the solvents in this study while dodecanethiol and its isomer, tert-dodecanethiol, were used as the stabilizing ligands. Ultimately, it was deduced that using a combination of branched solvent (2,2-dimethylbutane) and branched ligand (tert-dodecanethiol) caused the nanoparticles to precipitate from solution at only 6.9 bar. This almost 6-fold reduction in pressure can be attributed to the reduced solvent–ligand interactions in the system relative to straight-chain solvent and straight-chain ligand system. The impact that this altered solvent–ligand interaction has on effectiveness of the GXL size-selective fractionation process is also detailed in this paper. Hence, this study illustrates that one can tune the process parameters critical to the GXL size-selective fractionation process by simply changing the physical interactions between the nanoparticle ligand and the solvent.

No abstract