Estudo da reforma do biogás, utilizando-se catalisadores a base de Ni suportados em zeólitas: Efeito da hierarquização da estrutura e da síntese a partir de fontes alternativas de Si

Abstract

The increasing search for sustainable energy sources and the urgency to reduce the impacts of climate change have stimulated the use of biogas. In this context, dry reforming of methane (DRM) emerges as a promising approach to convert biogas, composed of methane (CH4) and carbon dioxide (CO2), into syngas, a versatile and high value-added product. However, DRM is an endothermic process that requires high temperatures, leading to the deactivation of nickel (Ni) catalysts due to sintering and carbon formation. In this work, catalysts of nickel (Ni) supported on hierarchical ZSM-5 and USY zeolites were prepared, using desilication and dealumination/desilication methods, respectively. The study investigated how the hierarchical porous structure of these supports affected the dispersion of Ni particles, the metal-support interaction and, consequently, the catalytic activity and stability during DRM. The use of sugarcane bagasse ash (SCBA), an agricultural residue, as a source of silicon for zeolite synthesis is also considered. For the characterization of the catalysts, a series of analyses were used, such as Nitrogen Adsorption, X-ray Fluorescence Spectroscopy (XRF), X-ray Diffraction (XRD), Temperature-Programmed Reduction (TPR), Temperature-Programmed Desorption of CO2 (TPD-CO2), Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS), and Thermogravimetric Analysis (TGA). The catalytic tests were performed at 800 °C, under atmospheric pressure, and initially with a CH4/CO2 ratio of 1. For the best-performing catalyst, additional tests were conducted using different CH4/CO2 ratios and with the addition of O2. The results revealed that the creation of mesopores in both zeolites increased the initial conversion of CH4 and CO2. It was observed that the catalyst supported on the hierarchical ZSM-5 zeolite demonstrated superior performance, exhibiting greater stability and less carbon formation and sintering. This result was attributed to a better dispersion of the NiO/Ni2+ species, the optimization of the metal-support interface and the strong basicity of the material, with an increase in the accessibility of the active Ni sites. The CH4/CO2 ratio and the addition of O2 influenced the conversion, syngas selectivity, and carbon formation for this catalyst. On the other hand, the catalyst supported on the hierarchical USY zeolite presented inferior performance, due to its high microporosity, which limited the reducibility and the accessibility of the reactants to the catalytic sites. The preparation of new catalysts, modifying only the sequence of Ni impregnation in relation to the treatment steps, confirmed this behavior. The effect was more pronounced when Ni impregnation was carried out before or between the zeolite modification stages. The catalyst prepared with the USY zeolite synthesized with SCBA showed an increase in catalytic stability, with the presence of silica in its composition. The creation of hierarchical zeolites from residues, such as SCBA, offers a promising path for the development of more efficient and sustainable catalysts for DRM.