Synthesis Of ZIF-67-derived Cobalt/Carbon Hollow Materials For Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis is proposed as a promising way to solve the facing energy crisis considering that the raw materials used for Fischer-Tropsch synthesis may be obtained from a wide range of sources and the products can be of high value.However,the products of the Fischer-Tropsch synthesis are quite complex.Therefore,the focus of current research is how to improve the activity of Fischer-Tropsch synthesis and regulate the reaction selectivity to generate high value-added products such as C₅₊hydrocarbons.The unique structure of catalysts can improve the selectivity of specific products,and the different structures of exposed active sites can also affect the activity of the reaction.Therefore,it is an effective way to regulate the activity and selectivity of Fischer-Tropsch synthesis by changing the structure of the catalysts.However,it is still a challenge to prepare the catalysts with regular morphology and uniform size through structure control.On the other hand,it is also of significance to understand the structure-activity relationship of catalysts and the particular reaction mechanism of the Fischer-Tropsch synthesis.In this thesis,we used ZIF-67 as a template,and synthesized H-Co@NCNHP with hollow carbon shells and hollow cobalt nanoparticles by a developed reduction-oxidation-reduction strategy.Compared with the materials obtained by direct pyrolysis of ZIF-67,the H-Co@NCNHP possesses hollow structure and significantly improved surface density of cobalt via the removal of carbon film on the surface of cobalt nanoparticles,which will be beneficial to enhance the activity of Fischer-Tropsch synthesis.In addition,carbon nanotubes and lower coordination number are also present in the catalyst,which can improve the ability to dissociate CO.In the Fischer-Tropsch reaction,it was found that the existence of hollow carbon shell showed confinement effects on the secondary products,remarkably improving the selectivity of C₅₊products.On the other hand,in situ infrared spectra prove that hollow cobalt nanoparticles can produce more-CH₂-species in the reaction process,and the active sites on the hollow cobalt nanoparticles have stronger interaction with the secondary products,enabling the formation of more and longer secondary products.Besides,as the chains of these secondary products grow,their escaping probability from the carbon shell is decreased,and in turn increases the confinement of the carbon shell,which further improves the C₅₊selectivity of the catalyst in the Fischer-Tropsch synthesis.The catalyst exhibited excellent reactivity and extremely high C₅₊selectivity and the maximum values of CTY and STY_C5+respectively was1.85×10^-5mol g_cat^-1s^-1and 1.67×10^-5mol g_cat^-1s^-1at 230?,H₂/CO=2,2.0 MPa,28400 m L g_cat^-1h^-1,which outperformed most of the catalysts reported so far.Moreover,the catalyst also showed good stability.After continuous reaction for 120 h,the selectivity did not change significantly with only a slight decrease in the CO conversion.