Ativação da ligação c-h no metano por complexos NbX 5 (X= F, Cl, Br e I) e espécies CoO m n+ (m = 1, 2; n = 0, 1, 2): mecanismo de reação, parâmetros estruturais e eletrônicos

Abstract

The current and future energy demands of the society aim for sustainable and durable resources that do not contribute to global climate change and energy instability. Therefore, researches have been carried out in order to promote the use of alternative sources for energy production that are more efficient from the environmental, economic, and social point of view. The methane activation in the gas phase by transition metals has attracted great attention from researchers because of its scientific and industrial importance. In this aspect, the present work is an effort in order to evaluate, from the computational standpoint, the reactivity of the Niobium complexes NbX5 X = (F, Cl, Br and I) and CoO m n+ oxides (m = 1, 2; n = 0, 1, 2) in the methane CH bond activation process, which corresponds to the limiting step of the methane conversion process to high economic value products. These species were chosen, in principle, because cobalt oxides are promising candidates for the methane to methanol conversion. The main motivation for the choice of niobium complexes is the great abundance of natural reserves of this mineral (98%) in Brazil, particularly in Minas Gerais. Initially, in this work, a detailed investigation was carried out by using different theoretical methodologies to analyze the structural, electronic and binding properties of the species in question. The aim of these analyzes was to evaluate and validate the methodologies employed in order to understand the ones that best describe these species and their chemical characteristics. From these results, the methane C-H bond activation process was investigated considering all possible known reaction mechanisms in the gas phase. It should be noticed at this point that there are currently no studies in the literature involving Niobium complexes in the C-H bond activation. As for cobalt oxides, no study was found in which all mechanisms considered here were investigated and compared to each other. In general, the results indicated that the electrostatic ΔEelast and orbital ΔE orb energies were responsible for the stability of the niobium complexes and cobalt oxides. In relation to the reactivity of the species studied, the overall results for the niobium complexes indicated better thermodynamic and kinetic conditions for the NbF 5 complex. Regarding the cobalt oxides, the low electron density of these species and the oxo ligands are important for the kinetics and thermodynamics of the reaction, and the OHM mechanism is thermodynamically more favorable than the DHA. Among the investigated cobalt oxides, CoO 2 ++ showed better kinetic and thermodynamic performance in the reaction with methane.