Avaliação da oxidação da Lignina do pseudocaule da bananeira catalisada por Co3O4: uma investigação experimental e teórica das propriedades da ligação BO4 com abordagem DFT, QTAIM e NCI

Abstract

Lignin is a natural macromolecule that, in general, is formed by the aromatic units syringuyl, guiacyl and hydroxyphenyl (S, G and H units, respectively). From its structure it is possible to obtain molecules with high added value, however, the processes come up against the need to break the bond that unites these aromatic units, the βO4 bond, which, in general, requires aggressive conditions at high temperatures and pressures. Focusing on the development of an efficient method for lignin oxidation, but under mild conditions, the oxidation of banana lignin was experimentally studied in the presence of atmospheric oxygen gas and Co3O4 as a heterogeneous catalyst under a temperature of 80°C and ambient pressure. The ideal concentration of the catalyst found was 1% (mol/mol), which after 54 hours of reaction was sufficient for the oxidation of the structure. In the evaluation by computational modeling via DFT (Density Functional Theory) using Kohn-Sham boundary orbitals, it was observed that oxidation abruptly changes the electronic topology, reducing the binding energy in βO4 for all models considered and makes them more reactive. Through QTAIM (Quantum Theory of Atoms in Molecules) and NCI (Non-Covalent Interactions), the main intramolecular interactions and their properties related to βO4 bond were investigated. It was observed that lignins formed by G and S units (with methoxyl linked to the second aromatic unit) have a smaller HOMO-LUMO gap and, therefore, are less stable compared to models that do not have methoxyl (H units). Thus, as they have more repulsive and destabilizing interactions that make the βO4 bond weaker, however, in some specific conformations, hydrogen bonds are formed that have the opposite effect. This allows us to conclude that hardwood-derived lignins are naturally more reactive and have lower βO4 bond dissociation energy, compared to softwood-derived lignins and herbaceous plant biomass. However, all types of lignin become more reactive and weaken the βO4 bond after oxidation. The results achieved in this work contributed to the construction of an understanding at a micro and macroscopic level about the properties of lignin.