Síntese e caracterização de redes metalorgânicas e a busca pela atividade biomimética tipo peroxidase

Abstract

Metal-organic frameworks (MOF) are porous materials made up of metallic ions coordinated with organic ligands, forming two-dimensional and three-dimensional structures. Since their appearance, these structures have attracted attention for several applications considering their properties, such as high surface area, pore volume, and the possibility of structural adjustments. In this work, metalorganic frameworks constituted by the cobalt(II) ion and 2-methylimidazole as the organic ligand, denominated as ZIF-67, were synthesized at room temperature in three different morphologies. The ZIF-67 networks with microplate (ZIF-67(MP)), nanosheet (ZIF 67(NF)), and rhombic dodecahedron (ZIF-67(DR)) morphology were obtained by altering the nature of the solvent in the synthetic procedure. The morphological and elemental composition of the materials were characterized by MEV-FEG-EDS technique, revealing that, in addition to the desired morphology, the materials are made up of carbon, oxygen, nitrogen, and cobalt, without the presence of contaminants. The ATR - FTIR and X-Ray Diffraction techniques were used to check lattice formation and crystalline phase analysis. The BET method was used to determine porosity and surface area, and ZIF-67(DR) had a surface area of 1,223.86 m2 g-1 and a porosity of 0.72 cm3 g-1. For ZIF-67(NF) and ZIF-67(MP) the surface area was estimated as 104.49 m2 g-1 and 38.18 cm3 g-1, respectively, and the porosity was estimated as 0.47 cm3 g-1 for the ZIF-67(NF) and 0.048 cm3 g-1 for the ZIF-67(MP). The synthesized ZIF-67 networks were applied as catalysts for the oxidation of the phenolic substrate 2 methoxyphenyl (guaiacol) in the presence of H2O2, showing mimetic behavior of the peroxidase enzymes. The specific activity (SA) and steady-state kinetic constants of the ZIF-67 catalysts were determined by UVVis spectroscopy through monitoring the formation of the tetraguaiacol product, which absorption occurs at 470 nm. ZIF-67(MP) initially showed the highest specific activity (SA = 0.006 U mg-1), followed by ZIF-67(NF) (SA = 0.003 U mg-1) and finally the ZIF-67(DR) (SA= 0.002 U mg-1). The reaction parameters of temperature, pH, and catalyst concentration were optimized by changing one and keeping the others constant. Thus, it was determined that the optimal reaction conditions, among the parameters studied, are at a temperature of 50 ºC, pH 9, and a catalyst concentration of 0.284 mg mL-1. With the reaction parameters adjusted, the kinetic constants (Km and Vmáx ) for the three morphologies of ZIF-67 networks were determined by the Michaelis-Menten method and linearization of double reciprocal graphics. The three networks exhibited enzymatic kinetic behavior, reaching the active site saturation plateau. The ZIF-67(DR) network was the mimetic catalyst with the highest affinity for guaiacol and H2O2 substrate and promoted the highest conversion rate. [resumo fornecido pelo autor]