Preparation Of Cobalt-based Zeolite Catalysts And Study On Their Performance In Fischer-Tropsch Synthesis Reaction

Syngas,a mixture of CO and H₂,as abundant source,large reserves and wide application,is usually derived from coal,natural gas and biomass and other carbon-contained materials.Syngas can be directly transformed into N-,S-and aromatics-free clean liquid fuel via Fischer-Tropsch synthesis?FTS?reaction,and this process is considered to be one of the effective ways to synthesize alternative fuel and high value-added chemicals in the post-petroleum era.However,FTS products follow the law of Anderson-Schlulz-Flory?ASF?distribution,which is difficult to selectively synthesize desired carbon-numbers hydrocarbons.At present,the modified catalysts and optimized processes are two efficient ways to improve the selectivity of intermediate hydrocarbons,which tune the product distribution within middle chain hydrocarbons(C₅-C₁₁)by accelerating the secondary reaction.Herein,we mainly developed novel cobalt-based zeolite catalysts for highly selective formation of gasoline(C₅-C₁₁)via FTS.Several characterization techniques like XRD,BET,SEM,TEM,TPR,TPD and Py-IR were employed to study the effect of the pore channel,morphology,acidity and metal site on the catalyst.?1?A series of zeolite supported cobalt catalysts were fabricated by wetness impregnation and solvent-free methods.We employed the modified NaCAN zeolite for FTS reaction,its acid strength and Na⁺content were regulated by ion-exchange technique.As the results demonstrated,Co/NaCAN catalyst without strong Br?nsted acid site,exhibited a gasoline selective of 78.3%with a relatively high CO₂selectivity.Co/HCAN catalyst showed a lower CO₂ selectivity,comparing to Co/NaCAN catalyst,owing to the suppression of water-gas shift?WGS?reaction rate by decreasing the Na⁺content.Compared with the conventional zeolites?ZSM-5,MOR,Beta and HY?supported cobalt catalysts,Co/NaCAN catalyst with specially one-dimensional pore structure and suitable acid strength was more favorable for the synthesis of gasoline hydrocarbons.?2?A series of Co/MFI catalysts and cobalt-imbedded Co@MFI catalysts are fabricated by a combination of impregnation and solvent-free techniques.Consequently,Na-type zeolite was successfully transformed into H-type zeolite with low Na⁺content through ion-exchange approach.We systemically investigated the effect of Co particle site,acid strength and acid site distribution on the performance of FTS reaction.As the results demonstrated,the gasoline selective on Co@MFI catalyst was usually higher than that on Co/MFI catalyst.The Co@S1 catalyst exhibited the highest selectivity to gasoline,which reached as high as 68.8%.The acidity of Co@MFI catalyst was analyzed by NH₃-TPD and Py-IR,respectively,and the acid strength of the catalysts follow the order of Co@HZSM-5>Co@NaZSM-5>Co@S1.Considering that too strong acid strength on Co@HZSM-5 will cause a higher carbon chain cracking reaction rate,and lead to high selectivity to low carbon-numbers hydrocarbons.However,Co@S1 catalyst,with the lowest acidity and opening structure,was more favorable for the synthesis of gasoline hydrocarbons,and meanwhile,suppress the CH₄ selectivity.?3?A series of zeolite supported cobalt-based catalysts and cobalt-imbedded Co@MOR/ZSM-5 composite zeolite catalysts were synthesized by a combination of impregnation and solvent-free method.We evaluated the different morphology and structure with varied crystallization time on the catalytic performance for FTS reaction.As SEM images demonstrated,Co@MOR/ZSM-5 with a crystallization time of 72h,illustrates the most perfect composite crystal morphology,and this is the key factor for its catalytic performance.As FTS reaction results present,Co@MOR/ZSM-5 exhibits the highest gasoline selectivity,reaching 72.3%.Therefore,the composite zeolites has a close relationship with the products performance from FTS reaction.