Fotocatálise mediada por nitretos de carbono cristalinos aplicada na valorização de nitroaromáticos

Abstract

Carbon nitrides are a class of inorganic polymeric compounds composed of carbon and nitrogen with a C3N4 ratio. Over the past decades, carbon nitrides have been extensively studied due to their significant photocatalytic potential for energy conversion reactions and organic synthesis. However, many of the materials reported in the literature exhibit low crystallinity and reproducibility. In this work, highly crystalline carbon nitrides were synthesized by the thermal condensation of melamine in the presence of NaCl, thus achieving greater control during the polymerization process. The obtained material has a poly(heptazine imide) (PHI) structural organization with negative charges stabilized by Na+ cations. The ionic nature of this metal allows for the substitution of alkali cations by transition metals through a cation exchange process in aqueous medium. The PHI structure enables strong coordination of transition metals to nitrogen atoms with high dispersion, which can be classified as single-atoms. Single-atom-based catalysts represent the latest frontier in maximizing surface area per mole of metal, combined with a coordination environment that simulates coordination compounds, thus acting as a bridge between homogeneous and heterogeneous catalysis. In addition to the possibility of metal coordination, the PHI structure is also susceptible to protonation in the presence of strong acids, leading to the formation of a new structure with acidic properties (H-PHI). Materials synthesized by these two approaches were applied in reactions of great chemical interest. Ni-PHI was used in hydrogen transfer reactions to reduce nitrobenzene to aniline using H2O under sustainable and renewable reaction conditions. Meanwhile, H-PHI was employed in the one-step photocatalytic synthesis of quinoline derivatives from nitrobenzene and ethanol under mild conditions.