Cobalt catalyst supports for Fischer-Tropsch synthesis
Sammanfattning: In the Fischer-Tropsch (FT) synthesis, CO and H2 (synthesis gas) are converted into hydrocarbons that can be further upgraded to high-quality fuels and chemicals. Different carbon sources such as natural gas, coal and biomass can be used as feed-stocks for the synthesis gas. In commercial applications, supported cobalt catalysts are commonly used in the FT synthesis, especially when the synthesis gas emanates from natural gas and when the desired final product is diesel. The activity and selectivity of a cobalt catalyst is dependent on several parameters, one of them being the support.The present thesis is focused on the design, synthesis and characterization of alumina and silica materials (with and without Ce and Zr promoters) with non-conventional morphology, and evaluation of their feasibility as cobalt supports in the FT synthesis.Ce- and Zr-alumina nanoparticles were synthesized by co-precipitation in water-in-oil microemulsion. The obtained product is amorphous alumina with highly dispersed promoters, resulting in strong cobalt-support interactions and low cobalt reducibility. By increasing the calcination temperature of the Ce-promoted support, crystalline CeO2 is obtained which apparently increases the cobalt reducibility and thereby the catalytic activity (per gram catalyst). The small pore size of the materials may induce diffusion limitations on the reactants arrival and/or result in very small cobalt particles, which favour methane over long-chain hydrocarbons. Successful preparations of pore expanded mesoporous silicas with 1D, 2D and 3D pore structures via the atrane route, combined with the addition of swelling agents, have been demonstrated. The advantage of this method is that pore expansion can be achieved at mild conditions and there is no need for a post-synthesis process using an autoclave system. In larger silica support pores, larger cobalt particles will be formed and the weaker the cobalt-support interactions will be. This generally results in a higher cobalt reducibility for larger-pore supports and thereby a higher catalytic activity.
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