Selective oxidation of alcohols to aldehydes via heterogeneous photocatalysts: a critical review

Photocatalytic oxidation of alcohols to aldehydes under mild conditions has attracted increasing attention as a more sustainable alternative to conventional thermal oxidation. This transformation is relevant to the synthesis of high-value intermediates for pharmaceuticals, fragrances, and fine chemicals and can potentially proceed using molecular oxygen under visible-light or solar irradiation. In this critical review, we summarize the state of the art in heterogeneous photocatalytic alcohol oxidation, with emphasis on (i) reaction pathways and reactive oxygen species, (ii) the influence of alcohol structure, including aromatic versus aliphatic and primary versus secondary alcohols, (iii) the role of the reaction medium, particularly water, and (iv) photocatalyst and cocatalyst design strategies for improving conversion and, more importantly, aldehyde and ketone selectivity while suppressing overoxidation. Finally, we highlight key challenges associated with meaningful comparisons among published studies and with process scale-up, and propose practical benchmarking and reporting recommendations for future research.