Esterificação de glicerol com ácido benzoico sobre zeólitas H-ZSM-5 hierárquicas incorporadas com Zr ou Nb
Macedo, Vinícius de
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Acid zeolites are microporous aluminosilicates, which exhibit negative charged AlO4- tetrahedrons in its lattice balanced by protons (H+), generating Brønsted acid sites (BAS). The Lewis acidity of zeolites is commonly accepted as caused by the presence of extraframework Al species (Alef) and also by heteroatoms, such as Ti4+, Sn4+, Zr4+ or Nb5+ tetra-coordinated in its framework. The acidic sites of these heteroatoms are considered to be water tolerant. It has been shown that by adjusting the textural and acidic properties of H-ZSM-5 and H-mordenite zeolites by partially removing Si, Al and or by incorporating the aforementioned atoms, the activity of these catalysts in esterification reactions has been improved. In this context, it is proposed to use acid zeolites, modified by post-synthesis treatments of desilication and Zr or Nb incorporation, to be used in the direct esterification between glycerol and benzoic acid without the use of solvents for the synthesis of α-glycerol monobenzoate (α-MBG). The α-MBG is an important molecule used as a platform in the development of drugs such as carvedilol and propranolol. However, its synthesis is poorly studied, its formation is slow and it can be degraded in acidic media, which stimulates studies to improve its synthesis. So, this thesis presents the incorporation of different levels of Zr or Nb in desilicated ZSM-5 zeolites using the solid state incorporation approach. The results obtained in the XRD diffractograms of the incorporated zeolites identified the formation of the tetragonal phase of zirconia at high Zr contents and the pseudohexagonal phase of niobium in all Nb contents. The chemical mapping images obtained by EDX showed that at high Zr contents there was a high agglomeration of this atom, mainly at the edges of the zeolite crystals, while the Nb agglomerated forming particles. The bands of the DRIFTS spectra referring to the hydroxyl vibrations (νOH) of silanol groups showed an increase in intensity after desilication, indicating an increase in the population of these species. After incorporating Zr or Nb, these intensities decreased, indicating the formation of Si-O-Zr or Si-O-Nb bonds and effective incorporation of these atoms into the structure of zeolites. The DRIFTS obtained after adsorption/desorption of pyridine identified that the relationship between Lewis and Brønsted acid sites (LAS/BAS) of the incorporated zeolites decreased. However, only with samples incorporated with Zr, as the increase in its content, the LAS/BAS also increased. It was observed that the selectivity of the reaction using the zeolites was 100% to α-MBG and the sample desilicated by a concentration of 0.2 mol L-1 of NaOH incorporated with the maximum content of Zr reached the highest absolute yield registered of 350 µmol under the conditions used in this thesis.
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