Influence on the reduction degree of Ni-B/SiO2 amorphous catalyst and its role in selective hydrogenation of acrylonitrile

The Ni-B/SiO2 amorphous catalyst was prepared by KBH4 reduction at room tem perature. The activities of the as-prepared catalysts with various Ni loadi ng amounts were measured during the selective hydrogenation of acrylonitril e to propionitrile and compared with those of the corresponding Ni/SiO2 cat alysts obtained by H-2 reduction at 773 K. For the Ni-B/SiO2 amorphous cata lyst, its activity per weight Ni (R-H) first increased and then decreased w ith the increase of Ni loading. The maximum activity was obtained at 6.4 wt .% Ni. However, the R-H of Ni/SiO2 remained at the highest value up to 5.0 wt.% Ni. Then, it decreased gradually with the further increase of Ni loadi ng. For the same kind of catalyst (Ni-B/SiO2 or Ni/SiO2), the TOF values re mained almost constant irrespective of Ni loading, showing that their activ ity was mainly dependent on the metal surface area (S-Ni) The S-Ni of Ni/Si O2 catalyst is simply affected by the surface dispersion (mean particle siz e) due to its perfect reduction degree. However, the S-Ni of Ni-B/SiO2 cata lyst was affected by both the surface dispersion and the reduction degree s ince the reducing ability of KBH4 is relatively poor. With the increase of Ni loading, the SNi first increased owing to the increase of reduction degr ee and then decreased due to the gathering of Ni particles at high Ni loadi ng, which could account for the change of the RH With Ni loading. If one co mpares between the two kinds of catalysts, the higher TOF value of the Ni-B /SiO2 indicates that it was more active than Ni/SiO2 this may be attributed to the modification of the nature of Ni active sites by the alloying B. Bo th the structural effect and electronic effect have been discussed based on various characterizations. (C) 2000 Elsevier Science B.V. All rights reser ved.