The role of different Ni sites in supported nickel catalysts for butene dimerization under industry-like conditions

Active sites for butene dimerization are single Ni n+ ( n = 1 and/or 2) moieties which are formed by oxidative addition of Brønsted sites to Ni 0 precursor species during conditioning and destroyed by reaggregation to Ni 0 clusters during deactivation. Octene is formed by subsequent insertion of butene into the Ni–H bond followed by β-H elimination. An industrial 20% NiO/SiO 2–Al 2O 3 catalyst and model systems prepared by impregnation of two differently acidic SiO 2–Al 2O 3 supports with Ni(Cp) 2 or Ni(COD) 2 have been analyzed by spectroscopic in situ studies and catalytic CSTR tests during butene oligomerization under industrially relevant conditions. Active and selective sites for linear C 8 target olefins are single Ni n+ species ( n = 1 and/or 2) which form during conditioning by oxidative addition of Brønsted sites to Ni 0 precursor species. This is connected with a switch from an acid-catalyzed ionic mechanism to a metal-catalyzed coordinative mechanism, which shifts the product composition from strongly branched olefins to more linear olefins. Deactivation at extended times on stream is due to reaggregation of the active Ni single sites to Ni 0 clusters. Operando EPR performed for the first time in flowing butenes at 20 bar clearly shows that such clusters are not active. Brønsted sites play a crucial role in stabilizing the Ni single sites.