Conversão de bioetanol em acetato de etila: efeitos das propriedades eletrônicas do metal e estruturais dos suportes nos catalisadores Cu-Ag/ZrO2 e Cu/ZrO2/Al2O3
Freitas, Isabel Cristina de
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In both industry and academia the interest in biomass conversions into manufactured products has grown rapidly in recent years. Various products can be obtained from alcohol chemistry such as acetaldehyde, ethyl acetate, hydrogen, and others. Although copper-based catalysts are commonly used in the dehydrogenation of ethanol to acetaldehyde, recently Cu/ZrO2 catalysts have also revealed themselves to be highly active in the onepot transformation of ethanol into ethyl acetate. In this work, we studied the one-pot synthesis of ethyl acetate from ethanol by using two separate series of catalysts, Cu-Ag/ZrO2 and Cu/ZrO2/Al2O3. The characterization of the supports and catalysts were performed by N2 physisorption, X-ray diffraction, thermogravimetry, temperature programmed reduction, Fourier transformed infrared spectroscopy of the adsorbed CO (DRIFTS-CO), X-ray photoelectrons spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS). In the first part of this work, the study of the series of Cu-Ag/ZrO2 catalysts suggested that the Ag-Cu surface alloying alter the electronic properties in the catalysts surface, as verified by DRIFTS-CO. XPS measurements showed that increasing the Ag content in Cu-Ag/ZrO2 catalysts lead to a higher ratio Cu+/Cu0 on the surface. The predominance of Cu+ species on the surface led to catalysts with higher selectivity to acetaldehyde over ethyl acetate. The second part of this work studied the influences of Cu active phase dispersion on the ZrO2/Al2O3 support. Different Cu+/Cu0 species ratio, as verified by XPS, were obtained by varying the Cu content on ZrO2/Al2O3. Furthermore, the EXAFS results revealed that for the reduced catalyst increasing the Cu loading led to an enlargement in the size of the Cu0 particle. DRIFTS-CO showed that CO adsorption into these catalysts may be occurring on the surface of the copper xvi particles deposited on Al2O3 and/or ZrO2, since ZrO2 may not to be covering the whole Al2O3. Catalytic tests show that the products distribution in the one-pot ethanol conversion is affected by Cu0/Cu+ species ratio on the surface of the catalyst as well as the acid/base properties of the support. The increase of the Cu loading up to 30 wt. % favored the selectivity to ethyl acetate (67 % at 250 oC), due to an optimized Cu0/Cu+ species ratio on the surface of ZrO2/Al2O3 support. Theoretically, by increasing the relative Cu0 concentration, the formation of acyl species increases, which condenses with ethoxy species formed on Zrδ+ and Cu+ Lewis acid sites for the formation of ethyl acetate. Additionally, the Cu content of 30 wt. % provides greater surface coverage of the support ZrO2/Al2O3 suppressing the formation of acetaldehyde condensation products on the ZrO2 surface, such as crotonaldehyde; and further methyl ethyl ketone and diethyl ether formed in the alumina acid sites.