Studies On Ti, Zr Doped SBA-16as Supports Of Cobalt Catalysts For The Fischer-tropsch Synthesis

Fischer-Tropsch synthesis （FTS） is a promising way to convert coal, biomass and naturalgas to clean fuels and chemicals. It has been shown that the number of available surfacecobalt metal atoms determines the catalytic activity, which is significantly influenced by thetexture and surface chemical properties of the supports.SBA-16（cubic, Im3m）, with three-dimensional structures, which also allows fasttransport of reactants and products. Particularly, its cages accommodate metal nanoparticlesand small pore entrances to prevent metal leaching of the particles confined in themesoporous cages. Ti, Zr doped mesoporous silica composites may enhance its applicationas catalyst support. So, Ti, Zr was doped into the framework of SBA-16, to discuss theeffects of the titanium and zirconium contents on the structure of the supports and theactivity of the catalysts for FTS.Ti, Zr doped SBA-16with different Si/Ti or Si/Zr atomic ratios were synthesized.These mesoporous materials were loaded with15wt.%cobalt by incipient wetnessimpregnation and the catalysts were characterized by XRD, N₂adsorption-desorption, ICPelemental analysis,²⁹Si solid state MAS NMR, TEM, H₂-TPD, H₂-TPR, as well as O₂titration. The catalytic performance for FTS was tested in a fixed bed reactor. The mainresults as follows:1. The structure of Ti and Zr doped SBA-16are well retained after the introduction of Tiand Zr. Ti and Zr enter the framework of SBA-16. With increasing Ti or Zr doping contentin the samples, chemical shift of all peaks are not significantly shifted. However, theintensity of the three peaks is changed significantly, showing that the local environment ofSi has changed.The element composition of the ordered Ti and Zr doped SBA-16isdetermined by ICP, confirmed that there are no lose of Ti and Zr in the preparation process.2. TEM images of the catalysts before reduction by H₂show that the Co₃O₄nanoparticles are uniformly encapsuled inside the cages and most of them are regularlyarrange.The addition of Ti or Zr to the supports affects the reduction process of the catalysts.The reduction temperature for the peaks shift to higher temperature with increasing Tidoping content.With Ti and Zr doped into the framework of SBA-16, the CO conversionincrease and the methane and C₂-C₄light gases selectivities decrease, while reversedchanges were observed for the selectivity to liquid C₅₊products. The CO conversion for thetitania and zirconia impregnated catalysts are lower than that Ti, Zr doped catalysts.Textural properties of support and nanoparticle size of active metal are two of theimportant factors which influence the catalytic properties. In previous investigations, particle sizes of active metal were changed along with the support pore sizes, loading ofactive metal, preparation methods of catalysts and other factors, which may have maskedthe particle size effects. The effects of the particle sizes of Ru on the catalytic performancesin FTS were investigated using catalysts with different particle sizes of active metal,supported on the same support and with the same loading. These catalysts werecharacterized by XRD, N₂adsorption-desorption, H₂-TPR and TEM. Their catalyticperformances for FTS were tested in a fixed bed reactor. The main results as follows:The intensity of the major diffraction peak of Ru （2θ=43.8°） increases with increasingamounts of the citric acid used in the catalysts preparation, indicating that the Ru crystallitesize is larger at higher citric acid content.The dispersion and the amount of H₂consumedincrease with the decrease of the Ru particle size, indicating the increased number of activesites. The degree of reduction and the amount of O₂consumed show reverse trend.Decreasing Ru particle size leads to an enhanced interaction between Ru and SBA-16, andthe reducibility decreased.The selectivity to C₅₊products for FTS increased with increasingparticle sizes.The methane selectivity and the C₂-C₄light gases selectivity decrease withincreasing particle size. The Ru catalysts confined in the mesoporous SBA-16with a Ruparticles size of5.3nm shows the best performance.