MOF Derived Cobalt Nanocatalysts For Fischer-tropsch Synthesis

Fischer-Tropsch synthesis(FTS)technology can produce clean liquid fuels and high value-added products through the conversion of syngas(CO+H2),that can be from coal,natural gas,biomass and other carbon-containing energy sources.FTS can relieve the crisis of Chinese shortage of petroleum resources,the clean products can alleviate environmental issues arising from the use of coal resources as well.The product distribution of FTS depends both on the structures of catalysts and the reaction conditions.Cobalt(Co)-based FTS catalysts have high catalytic activity and stability,low water gas activity,and difficult to deactivate caused by carbon deposits.Most importantly,they are particularly effective in promoting chain growth and C5+ long chain hydrocarbons selectivity.The metal loading,particle size and dispersion are important factors affecting the performance of supported catalysts.The traditional Co-based catalysts are commonly use carriers as Si O2,Al2O3,Ti O2,etc,which will form multiple species that are difficult to reduce,decreasing the amount of active metals and the catalytic activity.Metal-organic framework(MOF)material has been developed rapidly in recent years.Its unique skeleton structure,metal uniformly distributed at the atomic level and adjustable pores make it promising precursors for the synthesis of nanomaterials.ZIF-67 derived catalysts are prepared in this study.Under different pyrolysis conditions,active metals or metal oxides are dispersed on graphitizated carbon.The core-shell structure catalysts Co3O4@C and Co@C perform well in FTS.Next,ZIF-8 was selected as template,and ZIF-67 was grown on ZIF-8 by epitaxial growth,followed by the pyrolysis in N2 atmosphere to prepare Co-embedded porous carbon nanocages(CoPCN)hollow nanoreactors.When ZIF-67 is directly used as the template to prepare Co3O4@C and Co@C catalysts,the Co@C catalyst generated under an inert atmosphere has a higher catalytic activity,but compared to Co3O4@C catalyst prepared in air condition,it has relatively higher CH4 selectivity.After the comparison of the wax phase and water phase products with traditional catalysts,the new prepared nanocatalysts broke the traditional product ASF law and narrowed distribution of carbon chains.At the same time,Co3O4@C nanoreactors with mesoporous structure have higher short carbon chain growth ability,but the alcoholic carbon chain in the water phase is shorter,because the Co@C microporous structure hinders the release of the primary product and cause the secondary reaction.CoPCN catalysts with around 30% Co loading that is self-reducing and well dispersed on the graphitized carbon support.The contradiction of active metal with high loading and high dispersion is well solved.Co PCN can maintain good stability after 120 h reaction and still has uniformly dispersed on the carbon support.Different calcination temperatures lead to changes in the size of Co particles,The corresponding high temperature pyrolysis catalysts have a higher CO conversion and higher C5+ selectivity,lower CH4 selectivity and higher short chain hydrocarbon(C2-C4)selectivity,because with increasing temperature the particle size of Co increases,which is suitable for the growth of carbon chains.At the same time,with the decrease of carbon content,more active sites are exposed to improve the catalytic activity of the catalyst.