Adsorção de moléculas de glicerol e água em MgO(001)

Abstract

Magnesium oxide is an inorganic material that has several applications, even as a constituent of refractory materials. These materials are able to maintain their physicochemical characteristics at high temperatures and can be used in converters, refining furnaces and steelworks ships, magnesia bricks are an example of refractory material. The industries in this branch report that the process of hydration of the materials represents a problem, since there is an increase in mass and volume that causes a decrease in the shelf life of the material. There are experimental and theoretical studies on the MgO hydration process, however there are few that seek surface protection, some investigated antihydration agents include citric acid and tartaric acid. In the Brazilian market there is an excess of glycerol due to the transesterification reactions of vegetable oils that produce biodiesel and glycerin. The accumulation of by-products is not desirable, which justifies the use of raw materials in new processes. In the present work it is verified whether the adsorption of the glycerol molecule on the MgO(001) surface is capable of inhibiting the hydration process, as observed for other molecules. The study was carried out by ab initio calculations based on the DFT. In the first part, the MgO(001) surface model was constructed and adsorption of glycerol and water molecules was carried out, for comparative purposes, and adsorption energy, charge density difference plots and Bader charges were calculated. These calculations were repeated after adding a new water molecule to the initial systems. Finally, thermodynamic and CI-NEB calculations were performed for the coadsorption systems. It was concluded that the adsorption of a glycerol molecule is favored over the adsorption of a water molecule. During the coadsorption process, proton migration from molecules to the surface was observed. In these systems, there is a predominance of ion-dipole interactions favorable to energy lowering, with adsorption energies lower than could be predicted. In the coadsorption of water and glycerol molecules, two systems were generated, in one there is the formation of the organic anion 2,3-dihydroxypropan-1-olate, MgO (001)−H2O−H + gli− and in the second, formation of hydroxide, MgO(001)−gli−H + OH− . The protonation of MgO(001) happens spontaneously up to 134 K, 240 K and 281 K for MgO (001)−H2 O−H + OH− , MgO(001)−H2 O−H + gli− and MgO (001)−gli−H + OH− , respectively. The kinetics showed that there is only considerable activation barrier to protonation in the system. In general, it is not possible to state that the presence of glycerol inhibits the formation of brucite, however it can be considered a decrease in the rate of hydroxide formation.