Studies On The Structure And Catalytic Performance Of SBA-15 Supported Cobalt And Ruthenium Fischer-Tropsch Synthesis Catalysts

SBA-15 with different pore sizes have been synthesized using P123 (EO₂₀PO₇₀EO₂₀) as template agent. SBA-15 was used as a support to prepare cobalt and pore-confined ruthenium catalysts. All cobalt catalysts were prepared by incipient wetness impregnation. X-ray diffraction ( XRD ) , N₂ adsorption desorption, transmission electron microscopy, temperature programmed reduction ( TPR ) , H₂-temperature programmed desorption (H₂-TPD), X-ray photoelectron spectroscopy (XPS), diffusive reflectance infrared fourier spectroscopy and O₂ titration were used to characterize these catalysts. The activity of Fischer-Tropsch synthesis (FTS) was measured in a fixed bed reactor. Pore-confined ruthenium catalyst was prepared by surface modification. The effect of cobalt loading, promoter, pore size, surface modification and pore confinement on the structure and FTS performance of the catalyst have been investigated.1. For zirconium promoted Co/Al₂O₃ catalysts, the CoAl₂O₄ spinel phase content on the prepared catalysts decreased with the increase of zirconium loading, indicating that Zr-added could inhibit CoAl₂O₄ formation. The addition of zirconium to the Co/Al₂O₃ catalyst caused the increase of cobalt cluster size. Zr addition has been shown to improve the Fischer–Tropsch synthesis activity and C₅₊ selectivity of Co/Al₂O₃ catalyst. This could be explained by the increase of active metal cobalt sites and reducibility.2. High cobalt loaded Co/SBA-15 (30 wt.%) catalysts with different pore sizes were prepared by incipient wetness impregnation. The reduction of the catalysts took place in two stages with Co₃O₄ reduction to CoO and then to Co⁰. The first stage reduction was facile regardless of the catalyst pore size while the second stage reduction was much easier on the catalysts with larger pore. After reduction, cobalt particles were found to be distributed on both the exterior and interior surfaces of the support. Compared to the catalyst with smaller pores, the catalysts with larger pore had more CO adsorption sites with both the linear and bridge types. The catalysts with larger pore led to larger cobalt cluster size, lower dispersion and higher reducibility, which gave rise to more bridge-type adsorbed CO favoring for FTS. CO conversion increased and then decreased with the pore size in the range studied (3.4-15.7 nm). The catalysts with larger cobalt cluster size showed higher C₅₊ selectivity for FTS.3. 30 wt.%Co/SBA-15 catalysts with pore size of 6.4 nm and different ruthenium loading (0.05-0.5%) were prepared by incipient wetness impregnation. The addition of a small amount of Ru promoter to Co/SBA-15 shifted the reduction temperature of both steps (Co₃O₄→CoO and CoO→Co0) to lower temperatures and suppressed the formation of Co²⁺ and Co³⁺ species. After reduction, ruthenium atoms were encapsulated partially with cobalt clusters. There was no strong electronic interaction between metal cobalt and ruthenium, however, hydrogen spillover from ruthenium to cobalt oxide clusters occurred. With increasing ruthenium loading, catalyst reducibility increased and cobalt atoms became enriched at the surface of support. Moreover, the peak intensities for both the linear and bridge types CO adsorption increased with the increase of ruthenium loading, enhancing the catalytic activity.4. SBA-15 was treated with different TMCS (trimethylchlorosilane) content and the obtained materials were used to prepare cobalt catalysts. Compared to cobalt catalyst (Co/S) prepared from un-treated SBA-15, cobalt catalyst (Co/S-m) prepared from SBA-15 treated with TMCS exhibited higher CO conversion, lower CH4 selectivity and higher C₅₊ selectivity. The treatment of support led to the increase of the catalyst reducibility. For Co/S-m, CO conversion and C₅₊ selectivity increased with increasing TMCS content. The increase in CO conversion was ascribed to the increase of the catalyst reducibility. For cobalt catalysts, the increase in Co₃O₄ crystallite diameter was due to the surface modification of support, resulting in the increase C₅₊ selectivity.5. Ru nanoparticles confined in the channels of mesoporous SBA-15 with different pore sizes were prepared and investigated as catalysts for the FTS. The product distribution deviation from Anderson-Schulz-Flory rule was observed. By incorporating Ru nanoparticles into the pore of SBA-15 using surface modification, the catalyst activity was found to be significantly affected by pore size instead of particle size or other parameters.