Photocatalytic coupled redox cycle for two organic transformations over Pd/carbon nitride composites
Heterogeneous photocatalysis offers a means for “green” organic transformations. Generally, photogenerated electrons and holes are not utilized simultaneously and effectively. Here we report a coupling approach to promote simultaneously two organic transformations of Ullmann C-C coupling reaction and value-added aromatic alcohol oxidation reaction in inert solvent and anaerobic condition using a carbon nitride based composite (Pd/CN-450) prepared by loading Pd nanoparticles (NPs) on CN-450 (N defects and O dopants co-modified g-C3N4 with high crystallinity). Upon photo-excitation of the photocatalyst, the activation of carbon-halogen bonds of the absorbed aryl halides can be promoted by the photogenerated electrons on Pd NPs and the aromatic alcohols undergo dissociation, dehydrogenation, and finally can be oxidized by the captured photogenerated holes from CN-450 in the presence of Brønsted base. N defects, O doping, and improved crystallinity, controlled by changing the post-annealing temperature in molten salts, enhance visible light absorption and improve charger carrier separation of the functionalized carbon nitride. The smaller size and high dispersity of Pd NPs give the higher surface area-to-volume ratio resulting in efficiency adsorption and activation of reactant molecules on Pd NPs and allow for effective interfacial interaction of CN-450 with Pd NPs for promoting electrons transfer from CN-450 to Pd NPs. As a result, Pd/CN-450 displays superior photocatalytic activity of the coupled reaction compared to Pd/g-C3N4 (Pd NPs supported pristine g-C3N4). Moreover, the coupled reaction system has general applicability for various substrates and shows reusability.