Influence of hydrotalcite/rosasite precursors over Cu/Zn/Al mixed oxides on ethanol dehydrogenation

Cu/Zn/Al mixed oxides catalysts were obtained from their corresponding layered precursors synthesized by coprecipitation of components varying M(II)/M(III) molar ratios of 2, 3, and 4 while maintaining Cu2+/Zn2+ molar ratio at 1.0. Crystalline phases concentration in the precursors was determined by Rietveld refinement analysis. Hydrotalcite (HTLc) phase concentration decreased from 79 to 32 wt % as M(II)/M(III) molar ratio increased from 2 to 4, respectively. A constant value of M(II)/M(III) molar ratio of 1.3-1.4 was determined for HTLc formation independently of the M(II)/M(III) molar ratio used for the synthesis. The excess of M(II), Cu and Zn components, segregate as Rosasite phase. Therefore, mixed oxides with different surface properties were obtained depending on the precursor crystalline phase component. Precursor with mainly HTLc phase generates mostly a homogenous Cu/Zn/Al mixed oxide where Al atoms hinder Cu reduction. On that sample no Cu-0 was detected by XPS, then, hydrogen consumption during its reduction process was related to the presence of anionic vacancies, where H2 can be heterolytically dissociated and stored in its hydric form, generating highly active hydro/dehydrogenation sites, with highest Lewis acidity. When the mixed oxides come from a precursor with mainly Rosasite phase, mixed oxides decompose into Cu-0, ZnO, and Al2O3 during reduction, where metallic Cu-0 acts as dehydrogenating sites during ethanol dehydrogenation to acetaldehyde. In fact, higher Cu surface con-centration favors the dehydrogenation route, increasing the yield towards acetaldehyde, whereas a high Al surface concentration enhances the dehydration path increasing diethyl ether yield.

Automated summary

Cu/Zn/Al mixed oxide catalysts were synthesized from layered precursors, and the crystalline phase composition of the precursor determined the surface properties of the catalyst. Precursors mainly composed of HTLc phase resulted in a mixed oxide with high Lewis acidity and active hydro/dehydrogenation sites, while precursors mainly composed of Rosasite phase decomposed into Cu-0, ZnO, and Al2O3 during reduction, with Cu-0 acting as the dehydrogenation site.