Highly selective conversion of phenol to cyclohexanol over Ru/Nb2O5-nC18PA catalysts with increased acidity in a biphasic system under mild conditions

Abstract

Cyclohexanol is an essential intermediate in the chemical industry for the synthesis of nylon and other materials, but the direct hydrogenation of phenol to produce cyclohexanol is problematic owing to excessive energy consumption associated with the harsh reaction conditions. In this regard, novel composite catalysts (Ru/Nb₂O₅-nC18PA) coated with octadecylphosphonic acid (C18PA) were prepared via an impregnation method to be applied for the efficient hydrogenation of phenol to produce cyclohexanol in a biphasic system consisting of water and decalin solvent. Under a hydrogen pressure of 12 bar and 80 °C, nearly 100% conversion of phenol and 93% yield of cyclohexanol were achieved within 4 hours. With the introduction of more Lewis acid sites the aromatic ring is activated, which promotes the HYD (hydrogenation) path of phenol, thereby giving improved selectivity to cyclohexanol. A mechanism-exploration experiment was carried out and showed that the strong binding effect between the surface of the Ru/Nb₂O₅-nC18PA catalysts and the benzene ring allows the aromatic ring in phenol to be hydrogenated, increasing the selectivity to cyclohexanol. This biphasic catalytic system enhances the potential value of lignin via the catalytic hydrogenation of lignin-derived phenols.