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. Ethanol is also known to react at Lewis acid sites on metal oxide supports through facile O-H cleavage, although difficulty of Cα-H bond activation leads primarily to the formation of diethyl ether and ethylene. 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 and Lewis acid site density on oxide supports. 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 the positive dependence of rate on ethanol pressure seen for the non pyridine poisoned case. Relying on the kinetic information obtained from these studies, in combination with Cu particle size dependent measurements on acidic and non-acidic support, and microkinetic modeling, it was proposed that when Cu is supported on oxides that contain a significant number of acid sites that the rate limiting Cα-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.

Influence of support acid sites on Cu-catalyzed non-oxidative dehydrogenation of ethanol to acetaldehyde

2019 Fall National Meeting

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