Influence of Metal Oxide Support Acid Sites on Cu-Catalyzed Nonoxidative Dehydrogenation of Ethanol to Acetaldehyde

Selective ethanol dehydrogenation to acetaldehyde is an important reaction because acetaldehyde is a useful commodity chemical and can serve as a building block to produce other high-value products. Cu surfaces are known to selectively convert ethanol to acetaldehyde by driving sequential dehydrogenation steps. However, there remain questions regarding the rate-limiting step in the reaction and the role acidic supports play in promoting catalytic activity. In this work, both questions are addressed by performing kinetic measurements as a function of support acid characteristics, Cu dispersion, reaction temperature, ethanol pressure, and in situ pyridine poisoning. By coupling kinetic measurements with probe molecule infrared spectroscopy and microkinetic modeling, it is shown that at low ethanol partial pressure (∼1 mbar) the reaction rate is minimally dependent on the support, with a rate-limiting step of O–H cleavage occurring with similar turnover frequency on all Cu surface sites. In contrast, at high ethanol partial pressure (∼70 mbar), the Cu surface becomes poisoned with reactive intermediates, and supports with strong Lewis acidity (TiO2 and Al-doped ZrO2 supports) promote the rate of ethanol conversion to acetaldehyde by over an order of magnitude, compared to supports with weak acidity (ZrO2). It is further inferred that at high ethanol pressure for Cu catalysts on supports with strong Lewis acidity, the rate-limiting step switches to Cα-H cleavage occurring at interfacial Cuδ+ sites, while the initial O–H cleavage occurs on support acid sites. This demonstrates that the role of support acid sites and the rate-limiting step for ethanol dehydrogenation on Cu catalysts are dependent on reaction conditions and more generally provides an understanding of how active metals and acidic supports could act cooperatively to drive catalytic processes.