Application of thiolate self-assembled monolayers in selective alcohol oxidation for suppression of Pd catalyst deactivation

• Thiolate modification suppresses Pd catalyst deactivation in alcohol oxidation.
• Side reactions of the aldehyde product cause Pd poisoning and deactivation.
• Coverage of the thiolate modifier controls side reactions of the aldehyde product.
• Thiolate modifiers are stable during the oxidation reaction under mild conditions.

Rapid deactivation often reduces the effectiveness of Pt-group catalysts in the low-temperature aerobic oxidation of alcohols. Here, we report the use of thiolate self-assembled monolayers (SAMs) as activity promoters in the liquid-phase oxidation of trans-2-hexen-1-ol (HOL) under low air pressure; the oxidation rate was significantly improved, particularly at high conversion, in contrast to the frequently observed activity decrease induced by SAM modification in hydrogenation reactions. Reaction studies indicated that in the regime of low air pressure, catalyst deactivation was mainly caused by the carbonaceous poisons generated from the reaction product 2-hexenal (HAL). Infrared spectroscopy revealed that HAL produced surface residues on Pd that poisoned the catalyst, hypothetically through decarbonylation and dimerization. This poisoning can be suppressed by the SAM modifiers, in a manner that was controllable by varying thiolate surface coverage through use of either 1-adamantanethiol (AT) or 1-octadecanethiol (C18) SAMs.

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