Preparation And Modification Of ZrO₂ Catalyst And Its Catalytic Performance For CO Hydrogenation To Light Olefins

Light olefins(C2=-C4=)as important commodity chemicals are generally produced by steam cracking of naphtha.In recent years,non-petroleum route to produce light olefins has become one of the research hotspots.Two pathways have been developed for direct synthesis of olefins from syngas.One is based on the Fischer-Tropsch synthesis(FTS),and the other occurs via the formation of methanol,and then the conversion of methanol into olefins.Its production from FTS has attracted great research interest because it provides an effective way for production of light olefins from coal,natural gas,and biomass.However,in FTS,the product distribution generally follows the Anderson-Schultz-Flory(ASF)rule,and thus,the selectivity of C2=-C4=hydrocarbons are lower.As a catalyst or catalyst carrier,ZrO2 has excellent catalytic performance in the field of catalysis,especially in the isomerization reaction with high isobutylene selectivity.It is of great significance to explore the influence of different phases of ZrO2 and its modification on the product distribution of CO hydrogenation.At present,the research on ZrO2 catalyst is mainly focused on improving the selectivity of isobutene.However,there are few studies on the effect of phase structure of ZrO2 catalyst and additive modification on the selectivity of light olefins.The influence of modified ZrO2 catalyst on the product distribution needs further study.In our study,the preparation of ZrO2 catalysts with different phase structures and the influence of Zn modification on the product distribution of CO hydrogenation were investigated.The structure and surface properties of the catalysts were characterized by XRD,Raman spectroscopy,N2 adsorption-desorption,TEM,NH3-TPD,CO2-TPD,XPS,ESR and H2-TPD methods.The relationship between the phase structure and surface properties of the catalysts and the product selectivity was explored.The main research contents and results are as follows;(1)Tetragonal(t-ZrO2),composite(mt-ZrO2)and monoclinic(m-ZrO2)catalyst samples were prepared by surfactant assisted coprecipitation and hydrothermal methods,respectively.The results show that the product selectivity of CO hydrogenation is closely related to the crystal structure of ZrO2 catalyst.With the increase of monoclinic phase(m-ZrO2)content,the selectivity of CH4 decreased from 31.4%to 9.2%,and the selectivity of light olefins is increased from 39.9%to 66.9%with an O/P value of 6.7.The monoclinic ZrO2 has a large specific surface area and the most probable pore size.Its also has higher dispersion and alkali to acid ratio to produce more oxygen vacancies,which is conducive to the adsorption of CO and H2,and then more olefin products are produced during CO hydrogenation.(2)The study of reaction conditions shows that shows that it was appropriate at 400? and 2 MPa over m-ZrO2 catalyst during CO hydrogenation.A series of Zn modified ZnxZryOz catalysts were prepared by impregnation method.With the increase of Zn content,the selectivity of CH4 decreased from 11.7%to 8.8%,and the selectivity of CO2 decreased from 52.3%to 43.7%.The selectivity of light olefins over Zn1Zr100Oz catalysts was 66.3%with an O/P value of 7.4.Over Zn1Zr200O2 catalyst sample,the selectivity of light olefins can reach 65.8%with isobutylene content of 93.3%in C4 hydrocarbon fraction.Characterization results showed that the addition of Zn increased the oxygen vacancy,which then increased the catalytic activity and selectivity of isobutylene during CO hydrogenation.