Estudio preliminar de la síntesis de ferritas de cobalto cubiertas por una capa mesoporosa de silicalita de titanio con potencial aplicación para reacciones de oxidación selectiva : Estudio preliminar de la influencia de la relación molar de titanio ti/(ti+si) en la síntesis de ferritas de cobalto cubiertas por una capa densa de sílice y una capa mesoporosa de silicalita de titanio.

Abstract

Titanium silicalites are catalysts commonly used in the selective oxidation of olefins. However, their recyclability is limited by their particle size, so the synthesis of catalysts with a magnetic core surrounded by a titanium silicalite shell is proposed as an alternative to this problem. In this work, cobalt ferrites coated with a dense silica layer and a mesoporous titanium silicalite layer were synthesized, varying the Ti/(Si+Ti) molar ratio from 2.5 to 3. The magnetic core was obtained from the hydrothermal synthesis of cobalt ferrite with iron nitrate and cobalt nitrate precursors at pH 10, using hexadecyltrimethylammonium bromide as nucleation agent. The dense silica layer was obtained from sol-gel synthesis using silicon tetraethoxide, distilled water, ammonium hydroxide and ethanol. The mesoporous titanium silicalite layer was synthesized by hydrothermally assisted sol-gel, employing silicon tetraethoxide, titanium butoxide, distilled water, tetrapropylammonium hydroxide, ethanol and hexadecyltrimethylammonium bromide. Two molar ratios of titanium butoxide were tested to obtain Ti/(Si+Ti) molar ratios of 2.5 and 3. The synthesized catalysts were characterized by X-ray diffraction (XRD), atomic absorption (AA), infrared spectroscopy (FTIR), Inductively Coupled Plasma Optical Emission spectroscopy ICP-OES, nitrogen physisorption and scanning electron microscopy (SEM). XRD results confirmed the formation of cobalt ferrite. SEM micrographs showed that the coating of the mesoporous layer was achieved. FTIR confirmed the formation of silica matrix and Si-O-Ti bonds related to the formation of Ti active sites. The specific surface area of both catalysts (FCR-Ti 2.5 and FCR-Ti 3) were 603 m2g-1. The pore size distribution (PSD) of FCR-Ti 2.5 catalyst was <3.5nm, while the PSD of FCR-Ti 3 was 3-4 nm. The total Ti content (ICP-OES) in FCR-Ti 2.5 was 0.78 mmol Ti g-1cat and in FCR-Ti 3 was 0.91 mmol Ti g-1cat. The conversions obtained with FCR-Ti 2.5 and FCR-Ti 3 catalysts were 20% and 18%, respectively. This catalytic activity was attributed to the fact that although the FCR-Ti 3 catalyst presented a higher Ti content, probably not all the Ti was in tetrahedral coordination.