Investigação das propriedades ácido-base dos catalisadores de Cu/Zn/Nb utilizados na produção de metanol a partir da hidrogenação de CO2

Abstract

The use of CO₂ as an industrial input could become a sustainable alternative for the production of chemical commodities, such as methanol, while also contributing to reducing the concentration of this gas in the atmosphere and mitigating its negative effects. Copper-based catalysts have been intensively investigated for the hydrogenation of CO₂ to methanol. In recent studies, Cu/Zn catalysts promoted with Nb exhibited higher CO₂ conversion and greater selectivity to methanol compared to the non-promoted Cu/Zn catalysts and those promoted with other metals; however, this behavior has not been fully understood. Typically, in CO₂ hydrogenation catalytic reactions, an increase in surface basicity can favor the CO₂ adsorption step. Nevertheless, the addition of Nb generally leads to an increase in the acidity of the materials, which at first appears to be counterintuitive. In this context, the objective of the present study was to contribute to the understanding of the acid-base properties of Cu and Zn catalysts promoted with different Nb mass fractions. The materials were synthesized by co-precipitation and characterized by X-ray diffraction (XRD), N₂ physisorption, temperature-programmed desorption of CO₂ and NH₃ (TPD-CO₂ and TPD-NH₃), transmission electron microscopy (TEM), and X-ray fluorescence (XRF). From the characterizations performed, it was possible to verify that the materials exhibited crystallinity, morphology, and textural properties similar to those obtained in previous studies. Based on the results of TPD-CO₂, it was observe that the basicity of the materials with different Nb contents did not show a considerable variation. On the other hand, the TPD-NH₃ results showed significant variations in the acidity of the materials with increasing Nb content. The Cu50/Zn45/Nb5 material, which had previously shown the best catalytic performance for methanol production, also exhibited lower acidity than the other materials, suggesting that lower acidity may be the determining factor for maximizing methanol production in this material. Compared to the literature, the TPD-CO₂ and TPD-NH₃ results are consistent, indicating good reproducibility. The lower acidity may be related to the high calcination temperature (450 °C).