Selective functionalization of aromatic compounds. I. O-aminoalkylation of phenol and o-tert-butylphenol
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Hydrogenation of phenol is still an important process in the petrochemical industry, being used to obtain cyclohexanone, the essential feedstock for the synthesis of adipic acid and caprolactam [1,2,3]. Hydrogenation products of substituted phenols are used as feedstock for synthesis of lubricants, additives to oils and fuels, surfactants, and perfumes. The significant importance of alkyl-substituted phenols hydrogenation can be seen nowadays, in view of lignocellulose treatment and bio-fuels production [4,5].
Hydrogenation of phenols may be carried out in the gaseous or in the liquid phase, in the presence of VIII group metals (Fe, Co, Ni, Ru, Rh, Pd, Ir, Pt), deposited onto carbon, silica or alumina [6,7,8,9]. For the first time hydrogenation of phenol in the liquid phase was carried out in 1903, using NiO as a catalyst [10]. Cyclohexanol was obtained as the only reaction product after 55 h. Gas-phase hydrogenation of phenol was realized in the same year, with cyclohexanol and cyclohexanone obtained as the main reaction products and benzene, cyclohexane, and cyclohexene as minor reaction products [11]. In industry gas-phase hydrogenation of phenol is carried out at 120–170 °C and hydrogen pressure of 0.3–5 MPa in the presence of Pd/Al2O3 catalyst [3,6,7,12,13,14,15,16]. This is the main process to obtain cyclohexanone from phenol. Mixed metal oxides and sulfides can also be used as catalysts, resulting in aromatic or alicyclic compounds [6,10].
Reaction rate and product distribution in the liquid-phase hydrogenation of phenol depend on several factors, such as nature of the metal, support and solvent; temperature, hydrogen pressure, and the presence of acid or base additions [7]. Thus, the rate of phenol hydrogenation for carbon-supported catalysts was found to decrease in the order: Rh > Ru > Pd > Pt [9,17]