Synergistic catalysis of interlayer-ordered PtFe nanoalloy and Pd nanoparticles: high-efficiency oxidation of p-cresol to p-hydroxybenzaldehyde under base-free conditions

Abstract

The efficient oxidation of p-cresol to p-hydroxybenzaldehyde (p-HBA) under base-free conditions is a significant challenge in industry. We supposed that the sluggish kinetic problems due to the lack of hydroxide ions (OH⁻) may be addressed if reactive oxygen species (ROS, such as H₂O₂, O₂˙⁻, ¹O₂, HO· etc.) are introduced. Over optimized active sites in catalysts, molecular oxygen (O₂) can be adsorbed and activated into nucleophilic ROS and facilitate the oxidation of p-cresol to p-HBA under base-free conditions. Herein, an ordered intermetallic PtFe nano-catalyst supported over active carbon (PtFe/C) and a supported Pd catalyst over S doped carbon (Pd/S-C) with varied size from a single atom catalyst (SAC) to ca. 5.58 nm nanoparticles (NPs) were developed and employed in aerobic oxidation of p-cresol into p-HBA. The highest p-HBA yield up to >99% was obtained over PtFe/C and Pd/S-C combined catalysts, much higher than literature reported results. Radical quenching experiments and EPR spectroscopy confirmed that singlet oxygen (¹O₂), superoxide radicals (O₂˙⁻), and hydroxyl radicals (HO·) participated in the reaction path. Based on experimental data, a new catalytic mechanism for aerobic oxidation of p-cresol into p-HBA, involving ROS, was proposed. In this mechanism, Pd/S-C and PtFe/C catalysts played key roles in O₂ adsorption, activation, and ROS transformation and synergistically facilitated the oxidation of p-cresol to p-HBA.