Síntese e caracterização de zeólitas embrionárias e cristalinas do tipo ZSM-5 e sua aplicação na conversão de CO2 a CH4

Abstract

This work addressed the synthesis, characterization, and catalytic application of ZSM-5 type zeolites with different crystallinity stages (embryonic and crystalline), modified with the transition metals nickel (Ni) and ruthenium (Ru), aiming the conversion of carbon dioxide (CO₂) into methane (CH₄) via methanation reaction. The study sought to understand how the zeolite structure, the Si/Al ratio, and the nature of the active metal influence the activity and selectivity of the catalysts, as well as to propose possible reaction mechanisms for the tested catalysts. Two synthesis methodologies (A and B) were investigated, mainly varying the amount of structure-directing agent. X-ray diffractograms (XRD) confirmed the amorphous nature of the embryonic samples and the characteristic structure of ZSM-5 for the crystalline samples, highlighting the difference in the degree of structural ordering between the two. The zeolites synthesized by route A presented superior structural and textural properties and were selected for the subsequent stages of the work. The zeolites were used as support for Ni and Ru metallic particles in contents ranging from 2.5 to 10% w/w. Metallic impregnation significantly altered the structure of the materials, promoting a reduction in specific surface area and an increase in the average pore diameter. For catalytic CO₂ methanation tests, the 10NiZ50c catalyst (crystalline sample with a Si/Al ratio of 50 impregnated with 10% Ni) showed a conversion of ~84% and selectivity greater than 88%. The 5RuZ50c catalyst exhibited a conversion of 82.5% and a selectivity of 95.9%. In situ investigation of the reaction mechanism, conducted by diffuse reflection infrared Fourier Transform spectroscopy (DRIFTS), allowed the identification of intermediate species and distinct reaction pathways for Ni and Ru. The results demonstrated that the combination of hierarchically structured zeolites and appropriately dispersed transition metals allows for the adjustment of the acid-base, structural, and electronic properties of the catalysts, promoting high efficiency and selectivity in CO₂ methanation. These results reinforce the potential of modified ZSM-5 zeolites as versatile and efficient catalysts for CO₂ conversion, highlighting their relevance in the development of sustainable technologies aimed at carbon valorization and mitigation.