Cobalt Particle Size Effects On The Selective Fischer-Tropsch Synthesis Of Middle Distillate From Syngas Over Cobalt-Based Catalysts

With the development of industry,the demand for liquid fuels is increasing.However,as the main source of clean liquid fuel,petroleum reserves continue to be consumed.The use of other non-petroleum carbon resources to generate clean liquid fuels has gradually attracted social attention.Fischer-Tropsch synthesis(FTS)reaction can convert syngas(obtained by processing natural gas,etc.)into hydrocarbons,and the products with a very low content of sulfur are more environmentally.Cobalt-based catalysts are one of the traditional FTS catalysts,which have the advantages of high activity as well as high selectivity for long-chain hydrocarbons.Generally,the product selectivity of conventional FTS follows the Anderson-Schulz-Flory(ASF)distribution,which is not selective toward the middle distillate products.Improving the selectivity of middle distillate to break the ASF distribution becomes a hot research topic nowadays,while it is still a great challenge.The chemical properties of metallic cobalt,as the active phase of the cobalt-based catalyst,can affect the performance of the FTS reaction to a certain extent,especially the particle size.Therefore,studying the effect of cobalt particle size in FTS provides a meaningful research basis for further enhancing the selectivity of middle distillate.This subject aims to study the effect of cobalt particle size in FTS,especially the selectivity of middle distillate products,and to improve the middle distillate selectivity which is higher than the maximum value of ASF distribution.This subject prepares a series of cobalt-based catalysts with different cobalt particle sizes and the specific research contents are as follows:1.The effect of cobalt particle size is discussed based on zeolite supported cobalt-based catalyst since the specific characteristic of zeolite in producing middle distillate hydrocarbons.Firstly,Z5 support with hierarchical pore structure and mild acidity was obtained after treating with sodium hydroxide and tetrapropylammonium hydroxide.Then Z5 supported Co-based catalysts with different cobalt particle size(4.5-16.0 nm)were prepared by an incipient wetness impregnation(IWI)method.The experimental results show that cobalt particle sizes of catalyst effect the cobalt reduction degree and dispersion significantly.The Co dispersion decreases with the increase of the cobalt particle size.With the increment of the particle size from 4.5 to9.9 nm,the reduction degree of cobalt species increases.Meanwhile,the reduction degree does not change much when the particle size is higher than 9.9 nm.The influence of the cobalt particle size in FTS is as follows.With the increment of cobalt particle size,the CO conversion and jet fuel production selectivity both increase first and then decrease gradually.Among them,the highest CO conversion(58.7%at 240 ^oC)is achieved at the cobalt particle size of 9.9 nm.Unlike other ZSM-5 supported cobalt-based catalysts exhibiting higher gasoline product selectivity,the Z5 supported cobalt-based catalyst shows higher jet fuel selectivity(43.5%)when the cobalt particle size is 14.4 nm.After reduction-oxidation(RO)treatment,the cobalt particle size becomes smaller.The catalysts after RO treatment also keep the similar influence of the cobalt particle size in FTS and the selectivity of jet fuel becomes higher.Among them,the catalyst with the cobalt particle size of 8.3nm shows higher jet fuel selectivity(47.0%).2.Core@shell structure is one of the strategies to stabilize metallic crystallites since the unique structure can resist agglomerating and sintering during FT reaction.Besides,the spatial confinement structure to some extent can tailor the product distribution toward middle distillate.Based on these,in order to prevent the increase of selectivity of low carbon number hydrocarbon products caused by the acidity of zeolite and cobalt reduction degree,a new method was used to prepare a series of Co@Si O₂ catalysts with different cobalt particle sizes ranged from 6.8 to 20.7 nm.Experimental results show that the statistical distribution range of cobalt particle size in the core@shell structured catalyst is narrower than that of the supported catalyst,indicating that the core@shell structure can better control the cobalt particle size.In addition,the size of the cobalt particles in the core@shell structure catalysts will affect the Co dispersion and reduction degree.When the particle size of the cobalt increases from 6.8 to 20.7 nm,the Co dispersion decreases and the reduction degree increases.The influence of the cobalt particle size in FTS is as follows.With the increment of cobalt particle size,the CO conversion and the selectivity of the diesel fraction hydrocarbons both increase first and then decrease.The highest CO conversion(83.7%at 230 ^oC)is achieved when the cobalt particle size reaches 10.2nm.The product selectivity of diesel fuel can reach 48.0%when the cobalt particle size reaches 8.3 nm,higher than the maximum value of the ASF distribution(40%).