Identifying the Rate-Determining Step of the Oxidative Dehydrogenation of Ethanol on Iron Molybdate Catalysts through Isotope Exchange Experiments

This is the authors' abstract. We don't add key points for this paper.

The oxidative dehydrogenation (ODH) of ethanol (EtOH) to acetaldehyde (AcH) represents a promising route for the upgrading of bioethanol into value-added chemicals. Iron molybdate catalysts, widely used in the industrial oxidation of methanol to formaldehyde, have also shown high activity and selectivity for the ODH of EtOH. However, the reaction mechanism on these catalysts remains to be fully elucidated. In this study, isotope exchange experiments with selectively deuterated EtOH isotopologues (CH3CH2OD, CH3CD2OH, and CD3CH2OH) were used to identify the rate-determining step (RDS) of EtOH ODH on Fe2(MoO4)3. No kinetic isotope effect was observed when the hydroxyl hydrogen or the C2 hydrogen atoms were replaced by deuterium, indicating that O–H and C2–H bond cleavages are not rate-determining. In contrast, substitution of the C1 hydrogen atoms resulted in a pronounced primary kinetic isotope effect (kH/kD ≈ 2.5), demonstrating that C1–H bond cleavage constitutes the rate-determining step in AcH formation. Additional hydrogen–deuterium exchange reactions involving EtOH, AcH, and water were identified by mass spectrometry and confirmed by 2H NMR spectroscopy. These findings establish the mechanism of C–H activation and clarify the nature of the rate-determining step during the ODH of EtOH on iron molybdate catalysts.