(664g) The Influence of Support Acid Sites on Non-Oxidative Dehydrogenation of Ethanol to Acetaldehyde over Supported Cu Catalysts

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 chemicals. Cu surfaces are known to selectively convert ethanol to acetaldehyde by driving sequential dehydrogenation steps with the rate limiting step being C_α-H bond cleavage in the conversion of ethoxy to an acetyl species^1–3. Ethanol is also known to react at Lewis acid sites on metal oxide supports through facile O-H cleavage, although difficulty of C_α-H bond activation leads primarily to the formation of diethyl ether and ethylene^2,4,5. In this work, we elucidate the impact of acid sites on oxide supports (TiO₂, ZrO₂, and Al doped ZrO₂) on ethanol conversion to acetaldehyde over supported Cu catalysts.

To develop insights into the role of acidic oxide supports on ethanol conversion over Cu nanoparticles, a series of catalysts were synthesized with variations in Cu particle size (by controlling weight loading and calcination temperature) and Lewis acid site density on the support (by using TiO₂ or ZrO₂ and adding acidity to ZrO₂ through Al(NO₃)₃*9H₂O). Cu particle size and dispersion were characterized with low temperature CO chemisorption, N₂O titration, and STEM imaging. Support acidity was characterized via pyridine probe molecule IR and NH₃ temperature programmed desorption. It was observed that the addition of Lewis acid sites on the support dramatically enhanced dehydrogenation activity while retaining complete acetaldehyde selectivity associated with pure Cu catalysts. Ethanol pressure dependent reactivity measurements over a few orders of magnitude of ethanol pressure showed that over non-acidic supports, reactivity quickly saturates as a function of pressure, while the addition of acid sites minimized ethanol inhibition until much higher ethanol pressures. In-situ pyridine poisoning of support acid sites demonstrated significantly decreased ethanol conversion rates and inhibited positive dependence of rate on ethanol pressure that was seen with the acidic support surfaces. Relying on the kinetic information obtained from these studies, in combination with Cu particle size dependent measurements on acidic and non-acidic supports, and microkinetic modeling, it was proposed that when Cu is supported on oxides that contain a significant number of strong Lewis acid sites, the rate limiting C_α-H bond cleavage occurred at the Cu/support interface, while the initial O-H cleavage likely occurred on the support acid sites. This study helps clarify the role of support acid sites on ethanol dehydrogenation to acetaldehyde over Cu catalysts and, in general, provides an understanding of how active metals and acidic sites on supports could act cooperatively to drive catalytic processes.

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