Studies On Preparation Of Co₂C Catalyst And Its Selective Regulation In Syngas Conversion Reaction

Fischer-Tropsch synthesis has been studied for nearly a hundred years,but it is hardly to regulate the desired products selectivity.In particular,some high value-added products,such as lower olefins and some special alkanes,are key blocks in the chemical industry for the production of plastics and fine chemicals.Traditionally,these chemicals are primarily obtained via naphtha cracking or catalytic cracking petroleum.With the rapid depletion of petroleum resources,the direct conversion of syngas to lower olefins by Fischer-Tropsch synthesis without any intermediate steps has received extensive attention.Co₂C with exposed special crystal facet is a Fischer-Tropsch synthesis catalyst with higher lower olefins selectivity,but how to control the formation of this Co₂C and how to obtain high lower olefins selectivity under high activity conditions have remained unresolved.On the other hand,it is also important to further regulate the selectivity of products by coupling it with another catalyst.In this thesis,two different CoMn oxide catalysts,Co₂MnO_4.5 and Co₂MnO₄,were designed and synthesized by melting method.The phase transformations in the Fischer-Tropsch synthesis process were characterized.The key method to the formation of CoMn oxide with special crystal planes was studied.The olefin matathesis catalyst is then coupled with the CoMn oxide catalyst to further effectively control the selectivity of the single product.The above catalysts have been characterized by in-situ/ex-situ techniques,and the structure-activity relationship of these catalysts has been studied.The main discovers are as follows:Co₂MnO_4.5 and Co₂MnO₄ can be reduced to Co_xMn_1-xO through different reduction temperature treatments.Co_xMn_1-xO can form Co₂C exposed?101?and?020?special crystal planes under in-situ reaction conditions.The DFT calculations determined that x was ranged from 0.63 to 0.85,so we determined that Co_xMn_1-x-x O?x=0.63-0.85?was the precursor to form the exposed surface of Co₂C.This catalyst achieved a higher lower olefins selectivity?51%?at high carbon monoxide conversion?>90%?while also exhibited better stability.The Co₂MnO_4.5 catalyst was coupled with the olefin matathesis catalyst to make a huge change in the product distribution of the syngas reaction.The mechanical mixing of Co₂MnO_4.5 with W/ZSM-5 and W/USY caused a huge change in the product distribution of olefins,from the original propylene selectivity to butene and pentene.When we mixed Co2MnO4.5 with W/USY,the product distribution changed most sharply,and the selectivity of the iso-butane reached 51.5%.This coupled catalytic system is a secondary reaction process,and the acidity and pore structure of the molecular sieve will have great effects on the product distribution.