Abstract
The chromium mesoporous materials have been used in many catalytic reactions. Some studies have been devoted to determining the nature of chromium species and to characterizing the chemical structure. The different chromium species depend on synthetic methods and the support used. In this work we prepared mesoporous SBA-15 supported chromium materials referred to in this document as xCrSBA-15, was prepared by the incipient wetness impregnation method. The materials were characterized by X-ray diffraction (XRD), nitrogen (N2) adsorption–desorption isotherms, hydrogen temperature-programmed reduction (H<sub>2</sub>-TPR), scanning electron microscopic (SEM), energy dispersive X-ray spectroscopy (EDS), diffuse reflectance ultraviolet-visible (UV–vis) and Fourier transform IR (FT-IR). Chromium in SBA-15 forms different types of chromates and Cr<sub>2</sub>O<sub>3</sub>, causing morphological and textural changes and generating different catalytic centers. The catalytic conversion of 2-propanol to xCrSBA-15 materials were carried out by in a quartz reactor with a loading of 30 mg of the material at 300°C. Before the catalytic test, the material was activated in two ways: with N<sub>2</sub> and reduced at a H<sub>2</sub>/Ar flow (MxCrSBA-15). The catalytic conversion of 2-propanol to xCrSBA-15 did not increase with reduction (MxCrSBA-15) but decreased deactivation. All the materials had an acidic behavior, which is related to the chromate species in the materials. These results suggest that the more acidic materials favor the formation of coke during the deactivation process, and coke reduces the Cr<sup>3+</sup> to Cr<sup>2+</sup> species, thus decreasing the acidity of the materials.
Highlights
Porous materials have been intensively studied with regard to technical applications as catalysts and catalyst supports
Tredja [27] reported a comparative study of the incorporation of chromium species via incipient wetness impregnation into mesoporous silica of MCM-41 type, SBA-3 and SBA-15, where they found that the formation of active centers on different silica supports strongly depends on the preparation conditions used in their synthesis
The 2CrSBA-15 coloring exhibits the presence of the Cr6+ species, due to the rapid oxidation of Cr3+ to Cr6+ in the nitrate salt solution before being impregnated [31], while the 5CrSBA-15 and 10CrSBA-15 colorings are green, which is a characteristic color indicating the presence of Cr3+ species, such as Cr2O3 [20]
Summary
Porous materials have been intensively studied with regard to technical applications as catalysts and catalyst supports. The most important advantage of these materials is their wall thickness, which is significantly thicker than those of the MCM-type silica [6] These mesoporous materials exhibit a neutral framework, which limits their application in catalytic reactions [7]. Cuesta Zapata et al [20] reported that, when 2-propanol is decomposed on MCM-41 chromium/silica materials with different hydrothermal treatments, these treatments improved the chromium/silica interaction thorough chromasiloxane ring formation These anchored Cr3+ species developed highly unsaturated Lewis acid sites responsible for the 2-propanol dehydration. Tredja [27] reported a comparative study of the incorporation of chromium species via incipient wetness impregnation into mesoporous silica of MCM-41 type, SBA-3 and SBA-15, where they found that the formation of active centers on different silica supports strongly depends on the preparation conditions used in their synthesis. The type and strength of catalytic centers on materials were determined to the conversion of 2-propanol in the non-reduced and reduced materials with H2 before the reaction
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