Catalisadores baseados em nitretos de carbono grafíticos e metais de transição: síntese e aplicações

Abstract

Carbon nitrides are polymeric compounds with a C3N4 stoichiometric ratio and formed by sp2 nitrogen bonds. Most of the works in the literature uses polymeric carbon nitride as a catalyst for application in organic reactions, however, this substance has poor crystallinity, high structural disorder and low reproducibility. In this study, we synthesized crystalline carbon nitrides through a thermalcondensation process with NaCl, controlling the structural organization during the polymerization and with high reproducibility. These compounds, organized as poly(heptazine imide), or PHI, possess cations stabilized between its layers of carbon nitride, which can be exchanged for transition metals such as Fe, Ni, Co and Cu. These metals coordinate with the PHI structure covalently, bonding to N atoms and stabilizing as highly dispersed metal sites, which can be called single atoms. Single atoms catalysis allows a higher efficiency per mole of metal and an environment that resembles metallic complexes, all in stable and recyclable supports. Catalysts with single atoms stabilized in crystalline carbon nitride have been applied in different organic reactions with major industrial appealing. The Fe-PHI tested in this work showed a high activity for benzene photo-oxidation reactions, generating relatively high conversions and high selectivity for phenol (61% of conversion and 99% of selectivity). Detailed analyzes verified that the active sites are formed by iron-oxo species, the formation of these species is promoted by light. The Ni-PHI was efficient in the conversion of nitrobenzene into azo and azoxybenzene compounds in a coupling mechanism, where Ni sites are responsible for the abstraction of an oxygen atom, such reactions are facilitated in ethanol and in the presence of NaBH4. The Cu-PHI was the most effective catalyst in the photoconversion of methane to methanol (and other oxygenated products) in the presence of H2O2, the results indicated that Cu atoms react photocatalytically with peroxide, forming Cu-OH species on the surface of the catalyst, which react with methyl radicals present in the solution. The results obtained with Cu-PHI showed the highest values (2900 μmol.g-1) reported, so far, for methane oxidation reactions under ambient conditions.