Síntese e análise de catalisadores de ferro suportados em carbono ativado para síntese de Fischer-Tropsch

Abstract

The aim of this work was to synthesize iron catalysts supported on polymer-based activated carbons, for the Fischer-Tropsch synthesis. The preparation of the catalysts was performed by incipient wetness impregnation method using an aqueous solution of iron nitrate nonahydrate to obtain samples with approximately 55 % of iron. Two samples were prepared (FeCP1 and FECP2) and characterized by energy dispersive X-ray fluorescence (EDXRF), X-ray diffraction (XRD), nitrogen adsorption measurements, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM-EDS) and temperature-programmed reduction (TPR). The samples were also submitted to catalytic tests using different conditions of temperature (513, 528 and 543 K), pressure (20, 25 and 30 atm), and H2:CO molar ratio of 1 and 0.5. EDXRF data showed considerable difference in content of impregnated metal for both catalysts. FeCP2 exhibited an iron load close to the value expected while FeCP1 presented an iron load significantly lower than expected. XRD patterns showed a higher degree of crystallinity of the sample FeCP2, whereas FeCP1 and both supports used (CP1 and CP2) were found to be semi-crystalline. FeCP2 catalyst presented two active phases, α-Fe2O3 and γ-Fe2O3, while FeCP1 showed only one phase, α-Fe2O3. The analysis of the textural characteristics revealed a decrease in the values of the specific area, pore volume and pore diameter after the introduction of the metal into the support, which was more noticeable with the FeCP2 catalyst. XPS patterns indicated oxygen functional groups on the support surface and the presence of Fe+3 as the predominant active phase on both catalysts. The spectrum of both catalysts also showed a satellite peak which shows the presence of another valence state similar to the iron carbide. Images obtained by SEM revealed irregular shape and surface, being the particles present in FeCP2 greater than those on FeCP1 due to the crystallinity of the former. EDS data showed that approximately half of the iron percentage present in the catalyst bulk is on the surface. The presence of iron carbide on the catalyst surface can be inferred by using this spectrum analysis too. TPR graphics demonstrated a higher stability of the FeCP2, due to the three-step reduction of iron oxide instead of two as shown for the FeCP1. According to the results of the catalytic tests FeCP2 exhibited a better efficiency for the production of hydrocarbons in the C5-C9 range, for the same conditions of temperature, pressure and molar ratio. However, the decrease in the molar ratio disfavors the production of heavy hydrocarbons.