Synthesis And Catalytic Performance Of Nano-confined Cobalt-based Fischer-tropsch Catalyst

Since the industrial revolution,the use of fossil resources（coal,oil,and natural gas）to produce fuels and chemicals has caused severe damage to the carbon cycle,and there is an urgent need for the scientific community to promote the development of sustainable carbon source alternative technologies.Fischer-Tropsch（FT）synthesis is a classic route of synthesis gas（CO/H₂）chemistry,which can transform synthesis gas into clean energy and chemicals.Among them,cobalt-based Fischer-Tropsch catalysts have low reaction temperature,high long-chain hydrocarbon selectivity and low water gas shift reaction activity,etc.These advantages of cobalt-based Fischer-Tropsch catalysts are widely used in scientific research and industrial applications.A series of cobalt-based catalysts were mainly prepared by the coated method.To excluding the effects of SiO₂ shell,ZrO₂ shell and cobalt morphology on catalyst deactivation,product selectivity and Fischer-Tropsch performance.Catalyst prepared by impregnation and solid-phase grinding methods with ZrO₂ as the support was used to research the change of metal cobalt crystal phase after reduction.（1）To study the main cause of catalyst deactivation,a series of the model silica-encapsulated core-shell Co@SiO₂ catalysts with different thickness of the SiO₂ shell layer have been synthesized by coated method.The experimental results show that the thickness of porous silica layer and the reaction temperature play crucial roles in helping to find the origin of catalyst deactivation.At a high temperature（240°C）,the Co@SiO₂catalysts with high content of light hydrocarbons and which has strong fluidity to diffuse out of catalyst,but due to the high catalytic activity at high temperature,generate high water vapor concentration inside the catalyst particles.Thus,the oxidation of metal Co is the main reason for the deactivation of 20Co@SiO₂ and 30Co@SiO₂ with the thicker shell layer,while 40Co@SiO₂ with the thinner shell layer facilitates the water vapor diffuse out of the catalyst surface and the pore channels to alleviate catalyst deactivation.At a low temperature（220°C）,the Co@SiO₂ catalyst initially reseachs a relatively high catalytic activity,and then rapidly deactivates for all investigated catalysts,which is mainly attributed to the stronger ability of chain growth at low temperatures,then these products blocking of pore channels of silica layer and the covering of catalyst active sites leading to the syngas inefficient access onto the active sites.（2）ZrO₂ and SiO₂ were used as supports to study the change of the metal cobalt crystal phase after catalyst reduction and the effect of ZrO₂ shell on the Fischer-Tropsch performance of Co@Zr O₂catalyst.When the support is SiO₂,the metallic cobalt of the catalyst after reduction is all fcc-Co,and when the support is ZrO₂,the reduced metallic cobalt has hcp-Co and fcc-Co crystal phases.This phenomenon may be related to the force between support and metal.The interaction between the SiO₂ support and the cobalt species is stronger,while the interaction between the ZrO₂ support and the cobalt species is weaker,resulting in a different metal cobalt phase after reduction;The Co@ZrO₂ catalyst was prepared by the coated method.The study found that the selectivity of methane is significantly reduced and the selectivity of olefins such as C₂⁼-C₄⁼is improved,which is due to the zirconium materials promoted CO dissociation,increase the C*/H at the active site ratio and suppressed the termination of paraffins,thus the selectivity of C₅₊and unsaturated olefins is significantly enhenced.（3）Long rod-shaped Co₃O₄ was prepared by hydrothermal method,and Co@SiO₂catalyst was prepared with SiO₂ as the shell layer.The effect of Co₃O₄ morphology on Fischer-Tropsch product selectivity was investigated.The methane and C₅₊selectivity of the granular 20C@SiO₂ catalyst was 37%and 48.9%,respectively,while that of the long rod-shaped 20Co@SiO₂ catalyst was 20.5%and 69.9%,respectively.It shows that the long rod-shaped cobalt tends to undergo chain growth reaction,and the research found that the long rod-shaped cobalt catalyst liquid phase product is mainly composed of intermediate components.Mainly due to the increaseed with the thickness of shell layer,the chain growth factor（?）gradually decreased which reduced the carbon chain length of the Fischer-Tropsch product.Thereof the selectivity of C₅-C₁₁ can reach 59.7%.