Preparation Of Cobalt-based Catalyst For Fisher-Tropsch Synthesis

Cobalt-based Fischer-Tropsch synthesis catalyst has strong CO hydrogenation activity and chain growth ability,and is suitable for reaction system with higher hydrogen-carbon ratio of raw material gas.Simultaneously,it has low activity for water gas shift reaction,and is difficult for the carbon accumulated on catalyst during reaction.In addition,the resulting product has a low selectivity for oxygen-containing compounds.Therefore,the cobalt-based catalysts are considered as the first choice for the preparation of long-chain hydrocarbons,and have good industrial application prospects.In this paper,two methods were used to modify the SiO₂ carrier,then the Co/SiO₂ catalyst was prepared by the equal volume impregnation method.The structure of the modified SiO₂ carrier catalyst were investigated.The Fischer-Tropsch synthesis performance of catalysts were tested by a fixed bed reactor.The process conditions for the preparation of pure phase hcp-Co by liquid phase reduction with hydrazine hydrate as reducing agent were investigated and the performance of Fischer-Tropsch synthesis reaction was tested.（1）The SiO₂ support was surface-modified with different concentrations of ammonium carbonate solution at 50℃,and Co/SiO₂ catalyst was prepared.It was found that the modification of ammonium carbonate solution can reduce the concentration of silanol on the surface of SiO₂,and the modification has a certain pore-expanding effect,so that the pore size of SiO₂ carrier increases and the specific surface area decreases.The evaluation results of the Fischer-Tropsch synthesis reaction showed that the SiO₂ carrier modified by 25 g/L ammonium carbonate solution possessed the highest activity and the highest C₅₊selectivity,under the reaction conditions of 220℃,2 MPa and space velocity 3600 ml/（h·g-cat.）.（2）Different amounts of cobalt aluminate were coated on the surface of SiO₂carrier,and Co/SiO₂ catalyst was prepared by using it as carrier.It was found that cobalt aluminate could be uniformly coated on the surface of SiO₂ carrier to reduce the hydrophilicity of SiO₂ carrier,as well as reduce the interaction between Co and the SiO₂ carrier.After the catalyst prepared by the cobalt aluminate modified SiO₂carrier is reduced by H₂,the produced Co⁰ is solitarily in hcp-Co,so that the Fischer-Tropsch synthesis activity of the catalyst is increased.The evaluation results of the Fischer-Tropsch synthesis reaction of the catalyst showed that the 10%content of cobalt aluminate modified SiO₂ carrier had the highest activity and the highest C₅₊selectivity,under the reaction conditions of 220℃,2 MPa and space velocity 3600ml/（h·g-cat.）.（3）The sequential Co（NO₃）₃·6H₂O,anhydrous ethanol,hydrazine hydrate and NaOH were applied as the cobalt source,solvent,reducing agent,and auxiliary agent,respectively.The ultrafine cobalt powder was prepared by the liquid phase reduction method by changing the manner of adding hydrazine hydrate and NaOH.The optimum preparation process was applied to prepare the pure phase hcp-Co.In addition,the Fischer-Tropsch synthesis performance of different crystalline ultrafine cobalt powders was further investigated.It was found that with the increase of hcp-Co ratio in ultrafine cobalt powder,the activity of the catalyst increased,and the hcp-Co activity of the pure phase was the highest.