Biocarvões de casca de laranja modificada com cloreto de cobre: avaliação da atividade catalítica na obtenção de um derivado hexahidroquinolínico

Abstract

In multicomponent organic reactions, such as the synthesis of hexahydroquinoline derivatives, the use of catalysts is often required to reduce reaction times and enable milder synthesis conditions, particularly in processes that demand high energy. Metal-based compounds, such as those derived from copper, have been employed as catalysts in organic reactions. However, issues such as high solubility hinder their reuse. Thus, the use of supports becomes a strategy to overcome this limitation. The valorization of orange peel (Citrus sinensis), a residue abundantly generated in Brazil, for the production of biochars emerges as a promising option for the development of new catalysts. This potential is mainly associated with the possibility of obtaining materials with high specific surface area and thermal stability, characteristics that make them suitable as supports for catalytic species. Therefore, this research aimed to prepare and characterize catalysts obtained from the pyrolysis of orange peel modified with copper chloride and to evaluate their effect on the yield of the multicomponent synthesis reaction of ethyl-2-methyl-5-oxo-4-phenyl-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate. The catalysts were produced through different routes of copper chloride incorporation into the orange peel biomass and characterized by techniques such as X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, scanning electron microscopy (SEM), zeta potential analysis, and thermogravimetry (TG). XRD results indicated the predominant formation of the mineral clinoatacamite in materials modified with copper chloride without pH adjustment, whereas metallic copper was formed mainly when the biomass pretreatment involved pH increase in the presence of the copper salt. These findings demonstrate that the pre-pyrolysis incorporation method is decisive for the copper species formed in the biochars. The presence of copper during pyrolysis showed a catalytic effect on the release of oxygenated groups and on the production of more graphitized biochars. However, the developed biochars did not improve the yield of the hexahydroquinoline derivative synthesis reaction. Nonetheless, the results show that biochar does not negatively affect the reaction yield, indicating its potential as a support for other catalytic species.