Síntese de catalisadores para a desidrogenação oxidativa do propano a partir de precursores do tipo hidrotalcita no sistema Zn-Fe-Al

Abstract

In this work, catalysts based on hydrotalcite-like precursors were synthesized with nominal formula Zn1-x(FeyAl1-y)x(OH)2(CO3)x/2, in order to be evaluated in oxidative dehydrogenation of propane (ODHP). The precursors were prepared by two methods, coprecipitation at low supersaturation and titration, with different contents of metals. The catalysts were prepared by calcination of the precursors at 550 ˚C. The precursors were characterized by XRD, ICP- OES, TGA and FTIR. The catalysts were characterized by XRD, N2 adsorption and tested in the ODHP. The chemical analysis indicated that all metal used was introduced in the solid phase, except for the series trivalent molar fractions equal to 0.22 (series X22), which had a significant Zn fraction in the liquid phase. The XRD shows that the desired phase (hydrotalcite-like phase) was obtained in all synthesis containing Al, series X20 and X25 (trivalent molar fractions equal to 0.2 and 0.25 respectively), but ZnO and Zn(OH)2 have also been found. The size of the unit cell calculated with those results confirms the presence of carbonate as the compensation anion. Moreover, in the X20 series, the unit cell size a reached a maximum value that could be related to a possible incorporation limit of either Fe or Zn in the desired structure. The presence of Fe2+ can also be considered and associated to the elimination of Zn of the desired structure. The series X22 resulted in amorphous materials and the series X33 (trivalent molar fraction equal to 0.33), in crystalline ZnO, Zn(OH)2 and amorphous material. The infrared characterization corroborated the presence of carbonate in all precursors but, in some cases, nitrate was found in small quantities. The XRD showed that the catalysts are composed of ZnO and magnetite (Fe2.962O4). The propene yields in the ODHP were very similar among all catalysts with the maximum value being around 3 % which is probably related to low selectivity observed (< 10%). The selectivity increased with the temperature which is uncommon in this type of reactions and could mean that the direct dehydrogenation is taking place instead of the oxidative one. The catalysts with the highest surface area and with the highest iron content exhibited the best yields.