Synthesis Of Cobalt-based MFI Zeolite Catalyst And Its Application In Light Alkanes Dehydrogenation

Light olefins（ethylene,propylene and butene）,as the basic raw materials for the production of bulk chemicals,is an important indicator to measure the development level of petrochemical industry of a country.At present,light olefins are mainly obtained through traditional oil-based technologies,such as naphtha steam cracking and liquid catalytic cracking.However,with the increasing in the price of crude oil due to the over-exploitation of natural resources such as coal and oil,and the decreasing in the price of natural gas which is rich in light alkanes due to the shale gas revolution in the United States,the dehydrogenation of light alkanes to olefins has greatly aroused the interest of researchers.Among them,non-oxidative dehydrogenation of light alkanes is favored by researchers,because of its relatively acceptable alkanes conversion,high selectivity of olefins and catalyst stability.Pt-based and Cr-based catalysts are two kinds of paraffins dehydrogenation catalysts that have been commercialized and have been widely developed in the world.However,the rapid deactivation of Pt-based catalysts,reduction and sintering of active sites,and the price of metal Pt are expensive;The toxicity of Cr-based catalysts and the complexity of Cr species.These problems greatly limit the application of Pt-and Cr-based catalysts.As a result,a large number of other non-noble metal-based catalysts（Fe,Co,Ni,V and Zn,etc.）have been developed and used to replace Pt-and Cr-based catalysts.At present,active metals are often encapsulated into the framework or pore of various zeolite with the addition of coordination agents by hydrothermal synthesis method to form the irreducible-M^δ+-O^δ--structure,so as to achieve high selective dehydrogenation of alkanes to produce alkenes.However,the method has not been widely popularized because of the ununiversality of the coordination agent,the low loading of the encapsulated metal and the low performance of the catalyst.In the work,based on the previous research work and aimed at the active metal Co species,Co-based zeolite catalysts were prepared by hydrothermal synthesis method combined with MFI zeolite,in order to achieve high activity,high selectivity and high stability dehydrogenation of light alkanes.The specific research is as follows:（1）The metal Co species were encapsulated into the MFI type zeolite catalyst framework by hydrothermal synthesis method without the addition of any coordination agent.The obtained Co@MFI zeolite catalyst has the structure of-Co^δ+-O^δ--,which is atomically dispersed,and the loading of Co species in the catalyst can be controlled freely within the range of 0～10 wt%.Using XRD,XPS,XAS,SEM,STEM,H₂-TPR and DFT characterization techniques,it has been proved that Co species in Co@MFI zeolite catalyst exist in the form of tetra-coordination divalent Co（Ⅱ）,and can resist the reduction of H₂before 600℃.A series of ethane and propane evaluation experiments show that the irreducible-Co^δ+-O^δ--structure has very high alkane activity,and can selectively activate the C-H bond of the alkane,but not sensitive to the C-C bond,thus achieving very high olefin yield.Among them,the 6 wt%Co@MFI catalyst achieved thermodynamic equilibrium conversion under current conditions and close to 100%ethylene selectivity in the ethane dehydrogenation reaction.In the propane dehydrogenation reaction at 550℃,the propane conversion of 6 wt%Co@MFI reached 16%,the selectivity of propylene was more than 96%,and the catalyst stable within 150 h.TG,H₂-TPR and Raman characterization show that the deactivation of Co@MFI catalyst is mainly caused by carbon deposition when the reaction temperature is below 600℃.When the reaction temperature is higher than 600℃,the deactivation of catalyst is also related to the deanchoring,reduction and sintering of Co species.（2）Based on the high temperature deactivation of 6 wt%Co@MFI catalyst（>600℃）,it was used to modify the electron state of Co species in the catalyst by adding P additive in situ to improve the stability of the catalyst.XPS and H₂-TPR showed that the addition of P could transfer some electrons to Co species in the catalyst and form a strong interaction,which increased the initial temperature of H₂reduction of Co species by 50℃.The stability of the 6wt%Co P_x@MFI catalyst modified by P is greatly improved in the ethane dehydrogenation experiment at 650℃.At the reaction temperature of 600℃,the 6 wt%Co P₁@MFI catalyst showed very high stability of ethane dehydrogenation.The ethane conversion reached 11%,the ethylene selectivity was close to 99%,and there was no significant deactivation in the 50h reaction period.（3）Based on the long period instability of the 6 wt%Co@MFI catalyst,dilute nitric acid was used to pre-treat the catalyst to remove excess Co species in the catalyst.The characterization and dehydrogenation experiments showed that the removal of ineffective Co species on the catalyst surface could greatly improve the long-term stability of the catalyst.Under the reaction conditions of 30%C₂H₆/70%N₂,0.2g catalyst,2000 m L/g/h and 600℃,the conversion of ethane can reach 21%,the selectivity of ethylene is more than 99%,and no significant deactivation occurs over 80 h period.