Estruturas metalorgânicas como catalisadores para hidrogenação do CO2

Abstract

The hydrogenation of CO2 to commercially relevant products such as alcohol, coupled with the use of renewable energy, can curb indiscriminate emissions of this gas, and reduce dependence on fossil resources. Due to their excellent performance, copper catalysts have been extensively investigated for CO2 hydrogenation. However, this reaction presents challenges regarding product selectivity, catalyst stability, and the use of non-fossil co-reactants. Thus, MOFs can be used as supports for Cu catalysts and have the potential to address some of these gaps, as they can generate interfaces that protect active sites, preventing deactivation and stabilizing intermediates in the reaction, thereby enhancing the selectivity of desired products. Moreover, using water as a co-reactant in the CO2 hydrogenation reaction allows for the use of a clean hydrogen source. Based on this, this work aims to synthesize Cu/UiO-67 for application in CO2 hydrogenation, using water steam and H2 as co-reactants. The impact of precursors and copper loading, the influence of the metal center, the addition of promoters, and the synthesis method of these materials were evaluated in reactions between CO2 and H2, always aiming for the best catalytic performance concerning ethanol production at atmospheric pressure. In a second stage, reactions between CO2 and water steam were conducted to optimize reaction parameters and identify the influence of oxidized species on alcohol formation. Through XRD, SEM, TEM, FTIR-ATR, TGA, N2 physisorption, TPD-CO2 and H2-TPR, it was possible to study, respectively, the crystalline structure, morphology, characteristic bands of these materials, thermal stability, textural properties, basicity, and reducibility profile of the materials. From the hydrogenation of CO2 using water vapor or H2, the formation of alcohols was observed in all investigated catalysts. Based on the studies conducted, it was possible to conclude that catalysts with a Zr metal center and potassium addition showed better performance, especially K-Cu/UiO-67, which achieved a productivity of 102 µmolEtOH.h-1.gCu- 1, a 32% increase compared to the unpromoted catalyst. This behavior may be associated with the increase in basic sites capable of improving CO2 stabilization on the catalyst surface. The use of water enabled an increase in methanol productivity at low temperatures, and with the variation of the activation temperature, there was a continuous increase in ethanol productivity reaching 18.2 µmolEtOH.h-1.gCu-1 at Tactivation=260°C, with Treaction=180°C and a H2O/CO2 molar ratio of 1, indicating a positive influence of oxidized species such as Cu+ in alcohol formation.