A study of ethanol dehydrogenation to acetaldehyde over copper/zinc aluminate catalysts

Catalysts composed of copper supported on ZnAl2O4 were prepared by conventional impregnation of a commercial zinc aluminate powder using copper nitrate water solutions. The fresh catalysts were characterized by XRD, skeletal IR and DR-UV–vis spectroscopies, FE-SEM microscopy, BET and pore volume measurements. The catalysts were tested in the conversion of ethanol (96% assay, 6.9% vol in nitrogen) at GHSV 10,000 h−1. The spent catalysts were characterized by FESEM and DR-UV–vis. These catalysts are very efficient for the dehydrogenation of ethanol to acetaldehyde, with selectivities in excess of 95% at low conversion, persisting also at total conversion, allowing yields up to 90%. The most active species appear to be on copper metal nanoparticles grown over Zn-poor substoichiometric spinel nanoparticles. The catalysts reduce themselves on stream. The high selectivity at low temperature is in part due to the ability of copper to kill the dehydration activity of the zinc aluminate support to diethyl ether. The selectivity to acetaldehyde decreases at very high temperature (> 673 K) due to overconversion of acetaldehyde to thermodynamically more stable products such as methane, acetone, propene and carbon oxides, as well as to increased competition with the more favored dehydration reaction. IR studies show the intermediate role of surface ethoxy-groups.