Estudo teórico da adsorção de siloxanos sobre superfícies da gama-alumina

Abstract

Alumina, Al2O3, is a material widely used in the industry. The phase is the most α stable product in the calcination of boehmite, ­ AlO(OH), bayerite ­Al(OH) γ α 3, and gibbsite ­Al(OH)γ 3, at temperatures above 1.000°C. At intermediate temperatures, different phases of alumina can be observed, which are termed transition aluminas ( , , , , e ). The phase η γ χ δ κ θ γ of this oxide is known as an extremely important material in a number of industrial processes acting as an adsorbent, a catalyst or a catalyst support. This transition alumina is extensively used in petroleum and petrochemical industries as a support for transition­metal sulfides Co(Ni)MoS in hydrotreatment (HDT) process. Polydimethylsiloxane (PDMS) is a polymer with chemical formula [(CH3)2SiO]n applied in the oil industry as a drilling fluid, but its application as an anti­foaming agent in oil refineries deserves attention, because of the problem associated with catalyst deactivation in the HDT process. Its degradation can occur at temperatures exceeding 400°C in non­catalytic thermal processes. So, some oil fractions which follows to the HDT process, as naphtha or diesel, may already be carrying to the reactor some oligomers like siloxanes, silanes, and silanols, in very low concentrations, but enough for catalyst deactivation due to the accumulation of silicon over the surfaces. In the present work, the Densidty Functional Theory (DFT) was used for the modeling of the (100) and (110) surfaces of ­alumina as well as of the active phase of MoS/ ­Al γ γ 2O3 catalyst. With the structural models it was possible to perform simulations of properties like the 27Al and 29Si Solid­State Nuclear Magnetic Resonance spectral parameters as well as the vibrational frequencies of the normal modes associated with the surfaces hydroxyl groups. With this set of simulated data, it was possible to reassess a number of experimental works related to the characterization of the oxide surfaces and the catalyst poisoning. From statistical thermodynamics, the presence of tri­coordinated Lewis acid sites AlIII in samples subjected to thermal treatment was discussed and a free energy analysis considering the first steps of the catlyst poisoning process suggested that the Brønsted acid sites of the support are consumed preferentially.