| With the development of nanotechnology and characterization techniques in recent years,people have gradually realized that the size effect of metal particles have a great impact on the activity and selectivity of catalytic reactions in heterogeneous catalysis.Reducing the size of active metal particles can effectively improve the utilization of metal atoms.On the other hand,the ethylene production of China ranks second in the world,and ethylene is mainly derived from steam-cracking of naphtha and heavy oil.The two-stage hydrogenation process is often employed to remove the highly unsaturated hydrocarbons(alkadienes and alkynes)in the team-cracking products to avoid the poison and deactivation of downstream catalysts in the industry.In order to enhance the catalytic performance of the selective hydrogenation catalysts and the utilization rate of precious metals,this research modified the support by adding magnesium oxide in an appropriate manner,and developed two new types of support for selective hydrogenation catalysts.The results are summarized as follows:1)A one-step hydrothermal method was used to synthesize MgAl2O4@MgO support with the unique layered flower-like architectures of magnesium oxide on the surface.The support had high specific surface area and large pore size,and the basicity of the support is enhanced,which is favorable to inhibit the formation of oligomer in selective hydrogenation.Pd/MgAl2O4@MgO catalyst was prepared by incipient wetness impregnation method,which was applied for selective hydrogenation of isoprene.For comparison,commercial MgAl2O4,γ-Al2O3 and MgO supports were also used to prepare Pd catalysts.The results showed that the Pd/MgAl2O4@MgO catalyst exhibited superior catalytic performance with high activity and high selectivity.The catalysts were studied by CO chemisorption,in situ CO-IR spectra,XPS,HAADF-STEM and a series of characterization techniques.Because the unique layered flower-like architectures of magnesium oxide on the surface of Pd/MgAl2O4@MgO catalyst,the support can effectively prevent the aggregation of active metals during high-temperature calcination,and then it is conducive to the improvement of the Pd dispersion and active sites for the hydrogen activation.At the same time,the addition of magnesium increases the electron density of metal Pd,this weakens the adsorption energy of semi-hydrogenation products.Pd/MgAl2O4@MgO catalyst exhibits excellent performance for selective hydrogenation of isoprene.2)Based on the above research,the commercial γ-Al2O3 was modified with magnesium oxide.Al2O3-xMgO with the unique layered flower-like architectures of magnesium oxide uniformly distributed on the surface were obtained,and the basicity of the support could be adjusted according to the amount of magnesium addition.Supported Pd catalysts were prepared by incipient wetness impregnation method with Al2O3-xMgO and commercial γ-Al2O3,which was applied for selective hydrogenation of isoprene.The results show that the magnesium oxide contents have significant effect on the hydrogenation activity.The Pd/Al2O3-0.12MgO catalyst showed the best catalytic performance with isoprene conversion and selectivity of semi-hydrogenation products was 93%and 99%,respectively.The results of SEM,CO chemisorption,in situ CO-IR spectra,XPS,HAADF-STEM characterization techniques showed that the enhance of the Pd dispersion and the utilization rate of precious metals resulted from the unique layered flower-like architectures of magnesium oxide on the surface of Pd/Al2O3-xMgO catalysts.At the same time,the electron transfer from Mg to Pd increases the Pd electron density.Therefore,Pd/Al2O3-xMgO catalyst showed the remarkable activity and selectivity for selective hydrogenation of isoprene. |
