Highly efficient benzyl alcohol valorisation via the in situ synthesis of H2O2 and associated reactive oxygen species

Abstract

The selective oxidation of chemical feedstocks via in situ production of reactive oxygen species (H₂O₂, ˙OOH, ˙OH, ˙O₂⁻), represents an attractive, environmentally friendly alternative to the use of stoichiometric oxidants. Within this contribution, we demonstrate the efficacy of the in situ approach to the selective oxidation of benzyl alcohol to the commodity chemical benzaldehyde, with the alloying of Au with Pd shown to be key in significantly promoting catalytic performance. The immobilisation of AuPd nanoalloys, particularly on to a γ-Al₂O₃ carrier, is demonstrated to result in high selective utilisation of H₂ (ca. 80%), overcoming a major hurdle that has often precluded the adoption of the in situ approach to chemical synthesis on a commercial scale, while also achieving yields of benzaldehyde in excess of 60%, over successive experiments, representing a significant step towards competitiveness with traditional oxidative processes reliant on stoichiometric oxidants. Evaluation of catalyst performance towards individual reaction pathways (i.e. H₂O₂ direct synthesis and benzyl alcohol oxidation in the presence of preformed H₂O₂), analysis by EPR spectroscopy and radical quenching experiments, indicates that reactive oxygen-based species (ROS), rather than H₂O₂, are primarily responsible for the observed catalysis. While the origin of these oxygen-based radicals is not fully understood, we consider that they are generated primarily as reaction intermediates formed during H₂O₂ synthesis over active metal surfaces.