| Promoters have been studied extensively since heterogeneous catalysts have been used industrially.Commercial catalysts tend to involve multiple promoters to enhance the activity,selectivity,lifetime and structural integrity. Generally,in acid catalysis and hydrogenation reactions,promoter effects can be dramatic and unexpected.In this paper,the promotion effect of ZrO2 in Ni/SiO2 aerogel catalyst is investigated systematically.ZrO2 has received considerable attention as a promoter due to chemical inertness,higher thermal stability and a special combination of surface properties,namely the preservation of both acidic and basic sites,on the one hand,and reducing and oxidizing properties,on the other.It is reported: introduction of a small amount of zirconium into the support leads to a significant increase of the dispersion of supported metal oxide species, especially,on silica.Silica aerogel is an excellent catalytic support due to its desirable characteristics,such as inertness,high specific surface area,pronounced mesoporosity,and good thermal stability.Among the silica-supported catalysts,Ni/SiO2 has been investigated widely.In this paper,ZrO2 promoted Ni/SiO2 catalysts were prepared via three different routes:(ⅰ) impregnation ZrO2-SiO2 composite aerogels with a aqueous solution of Ni(NO3)2,catalyst code Ni/Zr-Si;(ⅱ) impregnation SiO2 aerogels with a mixed aqueous solution of Ni(NO3)2 and ZrO(NO3)2·2H2O, catalyst code Ni-Zr/Si;(ⅲ) one-pot sol-gel procedure from precursors Ni(NO3)2/ZrO(NO3)2·2H2O/Si(OC2H5)4,catalyst code Ni-Zr-Si.For comparison,non-promoted catalysts Ni/Si and Ni-Si also prepared respectively by impregnation Ni(NO3)2 with silica aerogel and by one-pot sol-gel process of Ni(NO3)2/Si(OC2H5)4.The above prepared catalysts were characterized by X-ray diffraction (XRD),temperature programmed reduction(TPR),ammonia temperature-programmed desorption(NH3-TPD),N2 adsorption-desorption isotherms and Fourier transform infrared(FTIR).The Liquid-phase hydrogenation of maleic anhydride(MA) was evaluated over these catalysts. And the adsorptions of MA,SAH and GBL on these catalysts surfaces were also investigated by FT-IR.The results revealed that the different preparation routes result in a difference among the obtained catalysts,concerning the crystal structure and composition,surface acidity,mixed level of each component,texture,and catalytic selectivity.The main results and conclusions are summarized below:1.Comparing to 30Ni/Si the promoted catalyst 30Ni/Zr-Si has higher activity and selectivity to GBL,which is due to increased surface acidity and the decreased interaction between NiO and SiO2 and the Ni coverage that was induced by ZrO2 promoter.2.The 30Ni-Zr/Si catalyst has lower activity and selectivity than that of 30Ni/Zr-Si.The most possible reason is about the decreased interaction between NiO and SiO2 that was induced by ZrO2,and the poor structural suitability between the SAH and the catalyst.3.The catalyst 30Ni-Zr-Si also has lower activity and selectivity.The main reason is that the content of Ni is too low to increase the GBL selectivity.4.The main reason for the higher activity and selectivity of 30Ni-Si catalyst is the higher SBET and pore volume over the other four catalysts.
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