Study On Conversion Of Ethanol To Light Olefins Over Heteroatom Doped ZSM-5 Catalyst

In recent years,with the increasing demand for basic chemical raw materials such as ethylene and propylene,and the requirements of the concept of low-carbon environmental protection and sustainable development,the production of light olefins from traditional petrochemical resources will be greatly limited in the future.The technology of producing light olefins（ETO）from bioethanol from biomass has attracted much attention.This technology promotes the deep utilization of biomass resources and makes it possible for people to get rid of the dependence on traditional petrochemical resources.At present,the core problem of ETO technology is the development of efficient catalysts.The catalysts with high selectivity,high stability and low cost become the focus of researchers.In this paper,ZSM-5 zeolite with the function of shape selection catalysis was selected as the research object.ZSM-5 zeolites were modified by doping Zr and P atoms.The effect of Zr and P doping on the catalytic performance of ZSM-5 zeolite for ETO reaction was studied by experiments and simulation.Finally,it provides theoretical guidance for the development of efficient ETO catalyst.Firstly,ZSM-5 zeolite doped with Zr was synthesized by impregnation method.The effects of calcination temperature,Zr/Al ratio and reaction temperature on the selectivity and yield of light olefins in ETO reaction and the catalytic stability of catalyst were investigated.The catalysts were characterized by XRD,N₂ adsorption desorption and NH₃-TPD.In the way of simulation,the cluster model of HZSM-5 and Zr/ZSM-5 zeolite was established.The electronic structure properties of HZSM-5 and Zr/ZSM-5 models,the adsorption energy of ethanol on the model and the energy barrier of ethylene protonation were calculated by density functional theory（DFT）.The results show that the introduction of Zr reduces the adsorption energy of ethylene and propylene,and improves the selectivity for light olefins;At the same time,the acidity of the catalyst surface was significantly changed（the amount of strong acid decreased and weak acid increased）,and the stability of ethanol conversion to propylene was improved.The optimum yield of propylene was 31.84%at 500℃with Zr/ZSM-5 catalyst（Zr/Al=0.4,calcination temperature at 550℃）.On Zr/ZSM-5 zeolites,the decrease of ethylene adsorption energy and the increase of ethylene protonation energy barrier indicate that the introduction of Zr can inhibit the strong adsorption and oligomerization of ethylene,thus improving the carbon deposition resistance and prolonging the service life of ZSM-5 zeolites.Then,ZSM-5 zeolites with different P/Al molar ratios were prepared by impregnation method,and characterized by NH₃-TPD and tested for ETO catalytic performance.In the aspect of simulation,the ZSM-5 cluster model with P atom doped in the framework and phosphoric acid doped out of the framework was built.Density functional theory（DFT）is used to calculate the electronic structure properties of P-doped ZSM-5 model,the adsorption energy of ethanol,and the energy barrier of ethylene protonation reaction.The results show that the weak acid content increases with the increase of the doping amount of P,while the strong acid content decreases with the increase of the doping amount of P.When the reaction temperature is 550℃,the yield of propylene increases first and then decreases with the increase of P/Al ratio.When P/Al=0.5,the yield of propylene reaches the maximum value of 30.5%.The yield of ethylene increased with the increase of P/Al ratio.When P/Al=1,the yield of ethylene reached90.4%.The doping of P atom changes the electronic structure properties of the ZSM-5 zeolite model,which makes the stability of the ZSM-5 zeolite improved after the doping of P atom.The introduction of P atom is beneficial to the stable adsorption of ethanol molecules.The adsorption energy of the ZSM-5 model doped with ethylene in the P atomic framework is reduced,which can prevent the excessive adsorption and deep polymerization of ethylene.The adsorption energy of ethylene and propylene on the ZSM-5 model doped with phosphoric acid skeleton is the minimum,and the light olefins is easier to be desorbed directly from the catalyst surface,which improves the selectivity of the light olefins.