Synthesis And Applications Of Hydrothermally Stable ZnAl₂O₄ Supported Catalysts

Nanomaterials are generally different from bulk materials in physical and chemical properties,which are related to surface effects,interfacial effects,size effects and the like.For example,nano-sized gamma alumina(y-Al2O3)has a specific surface area of 200 m2/g or more.Larger surfaces can carry more active metal sites,so high specific surface area γ-Al2O3 is widely used as a carrier in heterogeneous catalysis.Nanostructured materials has become one of the new ways to develop new functional materials.However,nano structures tend to agglomerate under harsh conditions causing the loss of nano-sized grains,thereby losing the advantages of nanostructures.It is therefore important to prepare highly stable nanostructured materials and apply them to industrial processes.In recent years,spinel mixed metal oxides(AB2O4)have attracted more research interest worldwide.These oxides are inexpensive and very stable with excellent physical and chemical properties.Among them,ZnAl2O4 is used as a catalyst carrier in various heterogeneous catalytic reactions.However,nanostructured and high purity spinel oxides are currently difficult to obtain commercially.This paper developed a very simple process for the preparation of spinel nanomaterials,resulting in a phase pure ZnAl2O4 nanomaterial with a particle size of about 10 nm.Preparation involves dissolving a mixture of zinc nitrate and aluminum nitrate in isopropanol followed by hydrothermal treatment.After calcination at a relatively low temperature(～600 ℃)for 5 hours,nano-sized ZnAl2O4 was obtained.The preparation method is low in cost,simple in process and environmentally friendly.The hydrothermal stability of the synthesized ZnAl2O4 nanoparticles was investigated.XRD,N2 low temperature adsorption and desorption,SEM and TEM characterization confirmed that the ZnAl2O4 prepared by this method has excellent hydrothermal stability without significant changes in structure and properties after hydrothermal treatment at 200 ℃ for two weeks.Combined with the theoretical calculation results,we attribute the high hydrothermal stability to the Zn-rich structure of ZnAl2O4 synthesized by this method.The PdZn/ZnAl2O4 catalyst was prepared by using its characteristic zinc-rich structure and applied in the aqueous phase hydrogenation of glycerol.The PdZn/ZnAl2O4 catalyst maintained high activity after ten cycles without significant changes in structure and properties.This hydrothermally stable nanocarrier material is further applied to other catalytic processes.A 5%Ni/ZnAl2O4 catalyst was prepared by incipient wetness impregnation method and studied for methanation reaction.With Al2O3 as a reference,the stability tests showed that both catalysts are active with high CH4 selectivity and low CO2 selectivity.Ni/Al2O3 catalyst deactivates at a faster rate than that of Ni/ZnAl2O4.The rapid deactivation of the Ni/Al2O3 catalyst is attributed to the sintering and growth of the Ni particles,while less sintering of Ni particles was observed for Ni/ZnAl2O4 catalyst.The deactivation of Ni/ZnAl2O4 catalyst was further studied.Ni loss was found to be the main reason for the deactivation of Ni/ZnAl2O4 catalyst.The Ni/ZnAl2O4 catalyst has excellent resistance to metal sintering compared with the Ni/Al2O3 catalyst in the syngas methanation reaction.When the conventional incipient wetness method is used to prepare a high metal loading catalyst,metal particles are dispersed on the carrier with relatively weak interaction between the active component and the carrier and thus the reduced stability.The lattice replacement method described herein attempts to abtain a highly stable metal catalyst.In this paper,a 12%Ni/ZnAl2O4@NiAl2O4 catalyst was prepared by lattice replacement and compared with 12%Ni/ZnAl2O4 and 12%Ni/Al2O3 catalysts prepared by impregnation method,and all three catalysts were compared in the steam reforming of methane.The initial conversion of the 12%Ni/Al2O3 catalyst was close to 50%.After 45 hours of reaction,the conversion decreased to about 25%.The initial conversions of the Ni/ZnAl2O4 and 12%Ni/ZnAl2O4@NiAl2O4 catalysts were about 3 5%and 40%,respectively.After the 45-hour life test s conversion and selectivity of the catalyst remained unchanged.The deactivation of the 12%Ni/Al2O3 catalyst was further investigated.It was found that carbon deposition during the reaction resulted in the blocking of active sites,resulting in decreased catalyst activity.Both 12%Ni/ZnAl2O4 and 12%Ni/ZnAl2O4@NiAl2O4 catalysts have good resistance to carbon deposition and thus good catalyst stability.