Studies On The Nano-alumina Supported Cobalt Catalyst For Fischer-Tropsch Synthesis

Catalytic synthesis of liquid fuels and valuable chemicals using natural gas and coal aswell as renewable biomass resources by Fischer-Tropsch synthesis （FTS） is a well establishedtechnology. Supported cobalt catalysts, which show good selectivity for long-chain paraffin,strong resistance to deactivation and low activity for water gas shift reaction, have beenwidely used as catalyst for the FTS. It was found that the performance of supported cobaltcatalysts was significantly influenced by the type and structure of the support. Alumina is oneof the most frequently used supports for cobalt catalyst due to its favorable mechanicalproperties, strong resistance to attrition and controllable surface properties.Many good works on the effect of alumina as support of cobalt catalyst for FTS havebeen reported, and it was found that surface area, porosity, crystallite phase, as well as surfacechemical properties of the alumina support influenced significantly the catalytic performancesof the catalysts. However, there are scarely any reports on the effect of morphology ofnanostructured alumina support on performance of cobalt FTS catalysts. In this work,nanostructured aluminas with controllable morphologies were synthesized and used assupport of cobalt catalysts for FTS. The supported cobalt catalysts were characterized bytransmission electron microscopy （TEM）, X-ray diffraction （XRD）, temperature programmedreduction （TPR）, N₂adsorption-desorption， hydrogen chemisorption and oxygen titrationmethod. The activity of the cobalt catalysts for FTS was evaluated on a fixed bed reactor. Theeffect of morphology of the nanostructured alumina support on performance of the catalysts,the preparation methods on the dispersion and the FTS performance have been investigated.It was found that the morphologies of nanostructured alumina significantiy influencedthe structure and reducibility, and the FTS catalytic performance of the supported cobaltcatalysts. The nanofiber alumina supported nano-cobalt catalysts prepared by ultrasonicimpregnation were shown to have good dispersion and reducibility, and the best FTSperformance. The main results are as follows:（1） The nanostructured alumina with controlled morphologies such as particles, rods, andfibers have been prepared using Al（NO₃）₃·9H₂O reagent as alumina source. The nanofiberalumina with various lengths was synthesized using aluminum isopropoxide as precursor.（2） The nanostructured alumina supports undergo reconstruction, including nanoparticleaggregated and pore shrinking, during the catalyst preparation by incipient wetnessimpregnation, resulting in higher dispersion of cobalt.（3） The pore structure of alumina support was decided by the stacking of primal particleof nanostructured alumina. Compared with nanoparticles and nanoroads, the nano-fiber alumina supported cobalt catalysts have stable, interconnected and bird nest-like porestructure exhibited the enhanced FTS catalytic performance.（4） Compared with short nanofiber, the long nanofiber alumina with larger pore size andpore volume show more stable pore structure during catalyst preparation. The presence of thestable larger pores in long nanofiber supported catalyst may facilitate the diffusion ofreactants and products, and inhibit the sintering of the metallic cobalt cluster during reaction.（5） The alumina nanofiber supported nano-cobalt catalysts were prepared by ultrasonicimpregnation method. The cobalt particles were distributed as independent particles on thiscatalyst instead of cobalt aggregate. The catalyst showed good FTS catalytic performance,even at the high temperature with high activity, still showed low CH₄selectivity and high C₅₊selectivity.（6） The nanofiber alumina support has undergone reconstruction during the catalystpreparation by ultrasonic impregnation. The random rearrangement of primal nanofiberalumina particle and cobalt oxide particle improve the stability of the cobalt oxide particle.Compared with catalyst prepared using incipient wetness impregnation, the catalyst preparedby ultrasonic impregnation have smaller contact area between alumina support and the cobaltspecies, resulting in weaker metal-support interaction, and good catalysts reducibility.