Avaliação catalítica do catalisador Co/SBA-15 promovido pelo óxido de cério na reação de Fischer-Tropsch.

Abstract

The purpose of this work is to evaluate the effect of cerium oxide-promoted Co/SBA-15 catalysts in obtaining liquid fuels from Fischer-Tropsch synthesis (FTS). For this purpose, CeO2/SBA-15 samples were prepared by different impregnation methods. Then, samples of CeO2/SBA-15 prepared by physical dispersion under different calcination conditions were evaluated. Subsequently, the cobalt catalysts promoted by the 20Co/xCeO2/SBA-15 cerium oxide (where x = 1, 2.5, 5, 10 and 20%), and a promoter-free cobalt catalyst 20Co/ SBA-15. The SBA-15 molecular sieve was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N2 physisorption. The catalysts were characterized by X-ray diffraction, N2 physisorption, programmed temperature reduction, inductively coupled plasma atomic emission spectroscopy (ICP-OES) and H2 chemorption. The FTS was performed in a fixed bed reactor and lasted 16 hours under the following conditions: P = 2MPa; T = 220 ° C and H2/CO = 2. The SBA-15 diffractograms confirmed the well-organized hexagonal structure of this sieve. MET micrographs exhibited the well-ordered hexagonal symmetry of the SBA-15A pores. The SBA-15A presented a BET area of 614 m2.g-1, pore volume of 2.5 cm3.g-1 and uniform pore distribution with predominance of pores around 11 nm. The SBA-15B had a BET area of 598 m2.g-1, a pore volume of 1.1 cm3.g-1 and a uniform pore distribution with a predominance of pores around 8 nm. The physical dispersion technique was defined as the most suitable to prepare the SBA-15 supported CeO2, since the 10CeO2/SBA-15_DF sample presented smaller crystallite size (5nm) and a percentage of CeO2 equal to 10%. Calcination at T = 350 ° C, 5 hours, 1 °C/min, under air flow, 1000 mL / min.gcat presented the best result for not showing traces of nitrate in the 20CeO2/SBA-15_C2 sample. The 20Co/SBA-15 catalyst exhibited cobalt oxide spinel (Co3O4) as the only crystalline phase, while cerium oxide (CeO2) promoted diffractograms of cobalt catalysts indicated the presence of spinel (Co3O4) and cerium oxides with fluorite centered face (CeO2) cubic structures in all catalysts. There was a decrease in BET area after incorporation of cerium and cobalt oxides in relation to SBA-15, this behavior being more pronounced for the 20Co/20CeO2/SBA-15 catalyst, where a decrease of 614 m2.g-1 occurred. to 300 m2.g-1. The superficial reduction of cerium oxide (CeO2 → CeO2 − x) promoted the displacement of the peaks related to the reduction of Co3O4 → CoO, to lower temperatures as the CeO2 content increased in the catalysts. A heterogeneous distribution was observed in the cobalt (Co0) metal phase support for the 20Co/SBA-15 catalyst and from the CeO2 and Co0 phases for the 20Co/5CeO2/SBA-15 catalyst. Well distributed cobalt metal particles (mean diameter 8.4 nm) were found on the SBA-15 support and some agglomerated regions in the 20Co/SBA-15 catalyst. The results obtained from the SFT showed that the 20Co/2,5CeO2/SBA-15 catalyst presented better CO conversion compared to the other catalysts (53%), while the 20Co/10CeO2/SBA-15 and 20Co/20CeO2/SBA-15 deactivated more quickly and had lower CO conversion (45 and 36%, respectively). TOF and CTY decreased as cerium oxide content increased in the catalysts. Cerium oxide as promoter of Co/SBA-15 catalysts has been found to result in new, more active and more selective catalysts for the diesel and gasoline fractions.