Texture and morphology-directed activity of magnesia-silica mixed oxide catalysts of ethanol-to-butadiene reaction

Abstract

Magnesia-silica mixed oxide catalysts of different texture and morphology were prepared for the ethanol-to-butadiene (ETB) reaction. Magnesia of low and high specific surface area (SSA) were made by thermal decomposition of magnesium nitrate. High-SSA MgO was prepared using hard-templating (HT) method. Mesoporous carbon, obtained by carbonizing a resorcinol-formaldehyde polymer was used as template. The carbon pores were saturated by Mg(NO3)2 solution and calcined then in order to decompose the nitrate and combust the carbon to get high-SSA MgO. The processes induced by latter calcination were followed by Thermogravimetry-Differential Scanning Calorimetry (TG-DSC) method. To obtain MgO-SiO2 catalysts both magnesia samples were wet-kneaded (WK) with a silica aerogel and a structured mesoporous SBA-15 silica material, having lower and higher SSA, respectively. Morphological and textural properties of these mixed oxide catalysts were characterized by means of N2 physisorption, X-ray powder diffraction (XRD), Transmission Electron Microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Energy Dispersive X-ray Spectroscopy (EDX). Ranking the catalysts was attempted according to their acidity and basicity, i. e., by the concentration and strength of their acidic and basic sites. Therefore, the samples were characterized by their adsorption interaction with molecules, having either basic or acidic character. The adsorption of CO2 and NH3 was studied by Temperature-Programmed Desorption (TPD) method, whereas that of the pyridine and CDCl3 by Fourier Transform Infrared Spectroscopy (FT-IR). The WK changed the structure of the parent oxides, generated Mg-O-Si bonds, acidity, and basicity. The XPS and EDX results showed that the surface Mg content of the mixed oxide samples made from the low-SSA MgO was higher than that of the bulk phase, while that of the samples made of the high-SSA MgO was lower. This demonstrates that MgO prepared by use of a carbon template proved to be more reactive in the WK process and generated more Mg-O-Si bonds. The mixed oxide catalysts containing high-SSA MgO showed always higher activity and butadiene (BD) selectivity than the corresponding catalyst containing low-SSA MgO. The higher BD selectivity of these catalysts is related to their subtle acidity-basicity balance. Nevertheless, the measured selectivities did not show correlation with any of the parameters characterizing the acid-base properties of the catalysts. The highest BD yield was obtained at 425 °C achieving 75% ethanol conversion level and 50% BD selectivity.