Influence of active sites and the reaction conditions on the ethanol upgrading over Nb₂O₅/ZrO₂ based multifunctional catalysts

Nb₂O₅/ZrO₂, Cu–Nb₂O₅/ZrO₂ and Ag–Nb₂O₅/ZrO₂ were prepared by incipient wetness impregnation and evaluated in the ethanol conversion into higher value-added products. The catalysts were characterized by X-ray fluorescence, X-ray diffraction, N₂-adsorption–desorption, transmission electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, temperature-programmed reduction, temperature-programmed desorption of CO₂ and NH₃, and FTIR pyridine adsorption. The catalytic activity and selectivities were evaluated at different temperatures and space velocities. ZrO₂ exhibited strong acid sites and led to ethylene as the main product. The addition of niobium yielded a surface Nb₂O₅ overlayer and also crystalline nanoparticles, and also increased the acid site density of the catalyst. The impregnation procedure yielded Ag and CuO nanoparticles highly dispersed over the Nb₂O₅/ZrO₂ surface, promoting ethanol dehydrogenation and consequently increasing acetaldehyde selectivity. Besides, results revealed that a decrease in temperature favors ethanol dehydrogenation to acetaldehyde, while an increase in temperature favors ethanol dehydration to ethylene. Moreover, 1,3-butadiene and ethyl acetate selectivities were increased by higher contact times. Nevertheless, the higher acid sites density and the presumable deactivation of Ag and Cu dehydrogenation sites led to high ethylene formation, which reinforces the importance of a suitable balance of acid and basic sites. (author)