An analysis of the chemical, physical and reactivity features of MgO–SiO₂ catalysts for butadiene synthesis with the Lebedev process

Abstract

In MgO–SiO2 catalysts for the transformation of bio-ethanol into butadiene, the generation of more selective sites is achieved by the dispersion of isolated Si4+ atoms in MgO.

Ethanol to 1,3‐Butadiene Conversion by using ZrZn‐Containing MgO/SiO2 Systems Prepared by Co‐precipitation and Effect of Catalyst Acidity Modification

“…The decrease in the ethanol dehydration along with the increase in the ethanol dehydrogenation as the Mg:Si molar ratio was increased is in agreement with a reduction in the catalyst acidity, as expected when the Mg:Si increases. [10,18] On the other hand, interestingly, the effect of the Mg:Si molar ratio on 1,3-BD selectivity was different between MgO-SiO2 systems and MgO-SiO2 systems containing ZrO2 and ZnO. While a smooth rise in 1,3-BD selectivity was verified as the Mg:Si molar ratio increased for pure MgOSiO2 samples, Figure 1 (a), the same trend was not observed for catalysts containing ZrO 2 and ZnO, Figure 1 We have previously studied the effect of the Mg:Si molar ratio for Zn(II) and Zr(IV) containing MgO-SiO 2 systems, using the wet-kneading method in the MgO-SiO 2 preparation.…”

“…However, some reports have recently ruled out the aldol condensation as the main path, suggesting instead that crotyl alcohol is produced through the reaction between an activated form of ethanol and acetaldehyde. [18,19] Due to the specific features of the catalyst that are required for this cascade reaction, materials with multifunctional properties have been studied, especially MgO-SiO2 systems. [2,5] This is related to the fact that Mg-O pairs might act as Lewis acid-Brønsted basic sites and the silanol functionality as a Brønsted acid which are necessary for ethanol dehydrogenation, [20] acetaldehyde condensation, [21] crotonaldehyde reduction and the further crotyl alcohol dehydration to 1,3-BD.…”

“…[12,14,23,[28][29][30] Conversely, besides catalyst composition, the catalyst preparation method is of paramount importance for 1,3-BD formation, since different acid-basic features may be obtained depending on synthesis conditions. [18,22,23,31] Due to this, it has been reported different optimum Mg-to-Si molar ratios for 1,3-BD formation, depending on the synthesis procedure employed. [13,18,23] Among the catalyst preparation procedures, different methods have been investigated such as physical mixtures of MgO and SiO2, [11,13] wet-kneading, [10][11][12][13]22,23] sol-gel, [18,24] impregnation [13] and co-precipitation.…”

“…[18,22,23,31] Due to this, it has been reported different optimum Mg-to-Si molar ratios for 1,3-BD formation, depending on the synthesis procedure employed. [13,18,23] Among the catalyst preparation procedures, different methods have been investigated such as physical mixtures of MgO and SiO2, [11,13] wet-kneading, [10][11][12][13]22,23] sol-gel, [18,24] impregnation [13] and co-precipitation. [22,25,26] Whereas it has been proven that a physical mixture between precursor oxides is not suitable for 1,3-BD formation, since resultant catalysts show similar features to single MgO and SiO2 phases, wet-kneading methods have been the most widely discussed in the literature with less attention being dedicated to sol-gel and co-precipitation methods.…”

“…[22,27] These unique characteristics are described as the key factor for the conversion of ethanol into 1,3-BD, since an intrinsic basic-acid sites distribution is obtained on these materials. [22,27] Using a sol-gel method, Ochoa et al [18] observed that the Mg:Si molar ratio affected the number, strength and distribution of basic-acid sites, the surface area and the crystalline structure of the catalysts, impacting on product distribution. A comparison between wet-kneading and a co-precipitation method, at the same Mg:Si ratio, has pointed to the former as more suitable for ethanol conversion into 1,3-BD.…”