Improved Cobalt Catalysts for Fischer-Tropsch Synthesis in the Gas-to-Liquids Process
Fischer-Tropsch synthesis (FTS) is at the core of the Gas-to-Liquids process that converts natural gas to hydrocarbons that are further upgraded to produce diesel, jet fuel, lubricants, and waxes. Currently, titania and alumina are the preferred supports for cobalt (Co) nanoparticles used in FTS. Despite having a higher surface area and excellent mesoporosity, silica is avoided as a support due to large Co particles that form following the conventional method of air calcination of the Co nitrate precursor and subsequent activation in H2 (hereafter referred to simply as "the conventional method"). The problem is due to the weak interaction between Co oxides and silica. The working hypothesis of this dissertation is that the application of alternative treatments, alternative Co precursors, and the incorporation of promoters can widely modify the strength of the interaction between Co species and the support. Our aim is to modify the Co metal active site density through alterations in both Co nanoparticle size and degree of Co reduction following H2 activation. Using this approach led to improvements in the performance of supported Co catalysts. Direct reduction of uncalcined supported Co catalysts prepared with Co nitrate and incorporating reduction promoters (Pt, Re, Ru, or Ag) outperformed catalysts prepared by the conventional method for CO conversion and C5+ selectivity over Co/SiO2, Co/TiO2, and Co/Al2O3. Direct reduction of Pt promoted 12%Co/silica, 12%Co/titania, and 25%Co/alumina catalysts exhibited CO conversions 3.7, 1.9, and 1.1 times that of the corresponding unpromoted catalysts prepared by the conventional method, and 63% and 25% higher than Pt-Co/SiO2 and Pt-Co/TiO2 prepared by the conventional method. No major change in CO conversion was found between Pt-Co/Al2O3 catalysts prepared using direct reduction of Co nitrate versus the conventional approach. Temperature programmed reduction in H2 of uncalcined catalysts combined with synchrotron methods (XANES and EXAFS) revealed that Co nitrate decomposes to intermediate Co oxides including a spinel (Co3O4) formed from oxidation of Co2+ species by NOx that was produced from Co nitrate decomposition. This spinel converted to CoO prior to the formation of Co0. Promoters (Pt, Re, Ru, or Ag) facilitate reduction of CoO to Co0. Chemisorption and EXAFS revealed smaller Co nanoparticles when direct H2-reduction of the nitrate was used relative to the conventional approach. With Co/titania and Co/alumina catalysts, improvements using direct reduction of Co nitrate were less profound. These supports presented strong interactions with Co oxides even with air calcination that stabilized small Co nanoparticles once activated in H¬2.Additional investigations to modify the interaction between silica and Co species focused on varying the Co precursor. In contrast to traditional Co/silica catalysts, both uncalcined and air calcined Co acetate catalysts exhibited strong interactions between Co species and silica stabilizing smaller Co particles after H2-activation. Pt-Co/silica catalysts prepared from Co acetate had higher activity compared to the conventional catalyst; however, residual carbide hampered selectivity, driving up light gas selectivity. Future work is needed to remove surface carbon and improve the C5+ selectivity. Finally, Co chloride was also investigated, but very weak interactions with silica resulted in large Co particles for all permutations of this catalyst. The active site density was further diminished by adsorbed chlorine, resulting in virtually no activity.