Catalytic performance of Pd-doped α-MnO2 for oxidation of benzyl alcohol under solvent-free conditions

Liquid-phase selective oxidation of benzyl alcohol to benzaldehyde is one of the most important processes in both laboratory and chemical industry processes due to the remarkable values of benzaldehyde in the production of flavours, fragrances, and biologically active compounds. In the traditional processes for selective oxidation of benzyl alcohol using a stoichiometric or excessive amount of toxic and expensive inorganic oxidants, such as ammonium permanganate in aqueous acidic medium, a large amount of toxic waste is produced. A few studies on the benzyl alcohol-to-benzaldehyde oxidation by environmentally clean oxidants (O₂ or H₂O₂) in the presence of organic solvents (e. g., toluene, p-xylene, and trifuorotoluene) have been reported; however, the usage of organic solvent is neither economical nor environmental friendly. Even though the solvent-free oxidation of benzyl alcohol to benzaldehyde using tert-butylhydroperoxide (TBHP) as oxidant has been reported, the co-product of tert-butanol from the consumption of TBHP will be left in the reaction solution, necessitating further separation. Therefore, various heterogeneous catalysts have been developed for solvent-free selective oxidation of benzyl alcohol using flowing air or oxygen; however, in most of these systems, the reaction temperature is still high (> 130 ℃) and/or conversion/selectivity is still low. Thus, the development of efficient heterogeneous catalysts for the solvent-free se-lective oxidation of benzyl alcohol with high selectivity and yield using molecular oxygen from air as the oxidant at low temperature is needed. In this study, Pd-doped α-MnO₂ nanorods were prepared from Pd-doped δ-MnO₂ precursors via a co-precipitation and in situ calcination transformation method. These catalysts were extensively characterized by various techniques, such as N₂ adsorption, X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS). The SEM and TEM results indicate that there are no obvious Pd nanoparticles on the surface of α-MnO₂ nanorods, signifying the possible doping of Pd into the lattice of α-MnO₂. The reduction temperature of pureα-MnO₂ is around 390 ℃, while the doped Pd could greatly promote α-MnO₂ reduction to lower temperatures at around 200 ℃. The applications of Pd-doped α-MnO₂ nanorods as catalysts for selective aerobic oxidation of benzyl alcohol to benzaldehyde under solventfree conditions with molecular oxygen were investigated. As compared with pure α-MnO₂, the Pd-doped α-MnO₂ nanorods show enhanced catalytic activity for selective oxidation of benzyl alcohol under solvent-free conditions with O₂, which can be attributed to the beneficial presence of oxidized palladium species and enhanced oxygen mobility resulting from the doping Pd species. In these Pddoped α-MnO₂ nanorods, when Pd (3%) -MnO₂ was used as catalyst, a 39% conversion of benzyl alcohol was achieved. It is much higher than the 18. 3% conversion when pure α-MnO₂ used as catalyst at 110 ℃ and reaction time of 4 h.