Selective Multiphase-Assisted Oxidation of Bio-Sourced Primary Alcohols over Ru- and Mo- Carbon Supported Catalysts

The oxidation of representative bio-based benzyl-type alcohols has been successfully carried out in a multiphase (MP) system comprised of three mutually immiscible liquid components as water, isooctane, and a hydrophobic ionic liquid as methyltrioctylammonium chloride ([CH3(CH2)6CH2]3N(Cl)CH3), a heterogeneous catalyst (either ad-hoc synthesized carbon-supported Mo or a commercial 5 % Ru/C), and air as an oxidant. The MP-reaction proceeded as an interfacial process with Mo/C or Ru/C perfectly segregated in the ionic liquid phase and the reactant(s)/products(s) dissolved in the aqueous solution. This environment proved excellent to convert quantitatively benzyl alcohols into the corresponding aldehydes with a selectivity up to 99 %, without overoxidation to carboxylic acids. The nature of the catalyst, however, affected the operating conditions with Ru/C active at a lower T and t (130 °C, 4–6 h) compared to Mo/C (150 °C, 24 h). The phase confinement was advantageous also to facilitate the products isolation and the recycle of the catalyst. Notably, in the Mo/C-catalyzed oxidation of benzyl alcohol, benzaldehyde was achieved with unaltered selectivity (>99 %) at complete conversion, for five subsequent reactions through a semicontinuous procedure in which the catalyst was reused in-situ, without ever removing it from the reactor or treating it in any way.