Study On Effect Of Novel Supports And Co-catalysts On The Performance Of Cobalt-based Fischer-tropsch Synthesis Catalysts

Fischer-Tropsch synthesis technology can convert coal,natural gas and biomass into cleaner and more efficient carbon resources via syngas,thus effectively alleviating the global energy crisis and environmental pollution problems.Cobalt-based catalyst is considered to be one of the most promising Fischer-Tropsch synthesis catalysts due to its high chain growth capacity,low water-gas shift reaction,and less carbon deposition and poisoning.Traditional cobalt-based catalysts,however,still have some drawbacks,such as low conversion rate at low temperature and wide ranges of product distribution.In order to study the above problems,this thesis explores a novel cobalt-based catalyst system,which has achieved the following results:?1?By respectively using the novel supporter BN and Si₃N₄ to support Co to serve as catalyst,it was found that the active phase Co exists as the surface-centered cubic structure?Co-fcc?on both supporter surfaces.The CO conversion rate of both catalysts is only about 30%because the surface of the supporters was of fewer functional groups,which was not conducive to the adsorption of reactant molecules.In terms of selectivity,Co/BN and Co/Si₃N₄ catalysts facilitate the formation of C₅-C₁₂ because there are weak acidic sites on their carrier surfaces,which can inhibit hydrocracking and isomerisation of long-chain hydrocarbons during the reaction,and the catalysts contain micro-mesopore structures that are beneficial to the mass conveyance of reactants and products and also promote the growth of CH₂ monomer chains.Overall,the Co/Si₃N₄ catalyst produces more C₅₊because the Si₃N₄ support facilitated the uniform dispersion of the active phase Co and the Co particle size was less than 10nm.?2?ZnCo₂O₄ with spinel structure was prepared by co-precipitation method and hydrothermal method,which was used as a precursor to prepare Co/ZnO catalyst with high dispersion of Co on ZnO via in-situ hydrogenation reduction.The novel cobalt-based catalyst exhibited an extremely high CO conversion rate in Fischer-Tropsch synthesis reaction at a low temperature?220??,and the catalytic performance is also stable.In addition,ZnCo₂O₄ prepared by co-precipitation method presented better Fischer-Tropsch performance after hydrogenation reduction because it has higher specific surface area and Co content,and the suitable grain size of metalic Co for Fischer-Tropsch synthesis.?3?The spinel-type Zn-Co-O solid solution Zn_0.75Co₁O_x and Zn₁Co₁O_x with different ratios of Zn and Co were prepared by co-precipitation method.Similarly,they were used as catalyst precursors for Fischer-Tropsch reaction and their catalytic performance was evaluated after in-situ hydrogenation reduction.It was found that the activity of the catalyst was significantly reduced,mainly because the interaction between the atoms in the system changed,resulting in a change in the reduction temperature of the precursor.As a result,the reduction temperature of CoO and ZnO overlapped in the second step of reduction,and some of the ZnO was reduced in the hydrogenation reduction process.The produced metallic Zn covered the surface of the active phase,leading to a substantial reduction in active sites.?4?Pure phase Co₃Mo₃N and ZrO₂-Co₃Mo₃N with additives were prepared,and their catalytic performance for Fischer-Tropsch reaction at different reaction temperatures was further evaluated.It was found that the Co₃Mo₃N catalyst is active no matter in low-temperature or high-temperature,and the product is mainly short-chain hydrocarbons.This was because the Mo species present in the system facilitated the hydrogenation reaction of carbon species adsorbed on the catalyst surface,and was also beneficial to cracking and isomerization of long chain hydrocarbons.In addition,the Co₃Mo₃N catalyst generated a large amount of C₂-C₄oxides in the low temperature reaction,and the addition of the acidic ZrO₂ promoter inhibited the formation of oxides and regulated the product distribution.