Steam Reforming of Ethanol to Acetaldehyde and Acetone Over Al-Doped ZnO Catalysts

Ethanol steam reforming with a water to ethanol molar ratio of 7:1 was investigated over pure ZnO and Al-doped ZnO catalysts with up to 10 mol% Al³⁺ synthesized via coprecipitation. This synthesis route yielded wurtzite ZnO, with Al being incorporated into the ZnO lattice at low doping levels. Al doping was found to alter the reaction pathway of ethanol steam reforming by suppressing the consecutive acetaldehyde conversion to acetic acid and further to acetone. Continuous kinetic experiments using a plug flow reactor resulted in almost full conversion and an acetone selectivity of 53% at 450°C over pure ZnO. Feeding acetaldehyde and acetic acid confirmed a consecutive multistep reaction network starting with ethanol dehydrogenation to acetaldehyde, followed by its conversion to acetic acid and a subsequent decarboxylative ketonization to acetone and CO₂. Upon Al doping, the specific surface area increased by about a factor of two, but conversion was hardly changed. Instead, the acetaldehyde selectivity increased, whereas acetone and CO₂ formation decreased, indicating that Al incorporation selectively suppresses ketonization. Overall, acetone formation via ethanol steam reforming was found to be a strongly structure-sensitive reaction over Al-doped ZnO, with its surface acid-base properties strongly depending on the Al content.