A Theoretical Study On Carbon Chain Growing In Fischer-Tropsch Synthesis On Y-Zeolite Supported Co Catalyst

The process of preparing fuel oil and chemicals through the Fischer-Tropsch synthesis reaction using synthesis gas as a raw material has become an important component of the petrochemical route,and it is a great significance for the development of high value-added chemicals and for solving the contradiction between energy supply and demand in China.The difficulty in the development of Fischer-Tropsch synthesis technology is to control the selectivity of the target product.The zeolite-based bifunctional Fischer-Tropsch synthesis catalyst has high selectivity for distillate hydrocarbons in a specific range including gasoline,diesel and wax.In this study,the DFT calculation method is used to study the carbon chain growth process of the Y zeolite supported Co clusters catalyst from a micro perspective.It would clarify the catalytic mechanism of the zeolite-based Fischer-Tropsch bifunctional catalyst and provide a theoretical basis for the development of this type catalyst.In this paper,Co clusters of different sizes and structures are loaded on the 18T zeolite cluster model with Y zeolite supercage structure,and a Co/Y model with stable structure is optimized.After loading,some electrons on the metal cluster are transferred to Y zeolite,the positive charge on the surface of the Co cluster increases overall.Through the adsorption of key species on Co/Y during the process of CO activation and carbon chain growth,it is found that CO adsorbs on six different adsorption sites on Co/Y and Co₁₃.The main form is vacancy adsorption.The introduction of Y zeolite enhances the adsorption capacity of Co₁₃ for CO species and most intermediate reactive species.The activation and dissociation process of CO in Co/Y was investigated.The most likely dissociation path is CO?HCO?H₂CO?CH₂.The introduction of zeolite has little effect on the generation of HCO,but it has a significant inhibitory effect on the next two steps.There is no obvious difference between the continuous hydrogenation of CH on Co/Y and Co₁₃ to produce CH₄ as a by-product.The reaction energy barriers of CH+H and CH₂+H are close,and the presence of the carrier has little effect on the process.Different types of carbon chain growth reactions in the range of C₁?C₄ were calculated.The CH_x-CH_x coupling reaction between C₁ species mainly completes the carbon chain growth through CH₂+CH₂ coupling,and the reaction energy barrier is0.50 e V.CH+CH₃ is more likely to occur on Co/Y,and CH₂+CH₃ is easier to perform on Co₁₃.In the growth of C₂?C₄,CH₂ has a lower energy barrier for the steric hindrance reaction compared to CH,and the introduction of steric hindrance will not increase the difficulty of carbon chain growth.Different intermediates have different difficulty in the process of carbon chain growth.The introduction of Y zeolite has different effects on different reactions.It can promote the reaction of CH₂CH₂+CH₂,CH+CH₂CH₃ and CH₂CH₂CH₂+CH₂,and inhibit the reaction of CH₂CH₂+CH₂CH₂and CH+CH₂CH₂CH₃.