Abstract
The work presented in this PhD Thesis provides new insights into the underlying reasons that make SiO2-MgO materials excellent catalysts for the ethanol-to-butadiene Lebedev process. In particular, the preparation technique of choice affects the structural properties of the resulting SiO2-MgO materials considerably and therefore the distribution, i.e. nature and amount,
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of acidic-basic sites. Given the complex cascade of elementary steps required, striking a precise balance in the latter is key to obtaining a high butadiene yield and performance in general. Moreover, the adsorption of atmospheric CO2 on the active sites of these solid materials can also affect their acid-base properties, forming a number of carbonate species whose nature and thermal stability depend on the preparation method used for the SiO2-MgO samples.
During the wet-kneading preparation method used for the SiO2-MgO materials significant chemical changes occur in the primary components. Moreover, a number of preparation parameters (e.g., size of SiO2 and MgO and relative amounts of those) affect the extent of those structural changes. Indeed, the different parameters employed for catalyst preparation affect the amount of MgO and magnesium silicates in the resulting materials. The formation of such magnesium silicates has been previously reported, but their function in catalysis is still unclear. In this PhD work, the magnesium silicates are proposed to improve butadiene yield by facilitating the aldol condensation step.
It is well known that dehydrogenation promoters enhance the performance of SiO2-MgO catalyst materials for the ethanol-to-butadiene conversion. In this PhD work, Cu-promoted SiO2-MgO catalysts gave superior butadiene yield. Structurally such systems are very complex due to the presence of three distinct metal oxides and, additionally, of compounds resulting from the interaction of those. Nevertheless, the characterization techniques used showed that a large portion of copper present replaces magnesium in the crystal lattice sites of MgO; the solid solution formed between CuO and MgO then affects positively the performance of resulting materials by reducing the number of acidic/strong basic sites which favor ethanol dehydration, while slightly increasing basic sites of moderate strength. In addition to CuxMg1-xO solid solution, other Cu species are also present, albeit in lower amounts, and thought to influence catalysis. Specifically, CuO sub-nanometric clusters are, in this investigation, proposed to facilitate aldol condensation. Relatedly, catalyst materials that do not contain such species due to the preparation conditions employed (as a result of the preparation method chosen for SiO2-MgO materials or due to the order of addition of copper on the latter) give lower butadiene yield. This shows that the preparation of SiO2-MgO also influences the nature of the Cu present in the promoted samples. Finally, Cu2+ is extensively reduced during reaction to generate both Cu+ and Cu0, the latter being predominant. Metallic copper was then recognized to be the species responsible for the promotion of ethanol dehydrogenation.
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