Study of the isomerization of C-13 labelled methylpentanes on oxygen modified bulk tungsten carbides

The reactions of labelled 2- and 3-methylpentanes were carried out on three different oxygen modified bulk tungsten carbides varying by their oxygen t reatment temperature. In all cases, isomerization took place via bond shift mechanisms; no cyclic mechanism was involved in contrast to metals like Pt , Pd or Ir where such mechanism occurs. Alkoxy and sigma-alkyl intermediate s were put forward as adsorbed species responsible for isomerization for bu lk tungsten carbides treated by O-2 at moderate temperature (350 degrees C) over a short period of time (5 min) and at high temperature (700 degrees C ) for 4 h, respectively. These intermediate species are respectively correl ated to an acidic and a metallic behavior of the catalytic surfaces. The in crease of the O-2 treatment time at 350 degrees C results in the presence o f both kinds of these mechanisms for isomerization, a bifunctional behavior takes place where metallic and acidic characters are present. To explain t he presence of terminal-C-13 on acidic catalysts, a "protonated cyclopropan e alkoxy species'' intermediate responsible for isomerization was proposed. The driving force for the formation of such species being the presence of stable alkoxy species, general reaction pathways are discussed for the diff erent kinds of catalysts.