(689d) Suppressing Pd Catalyst Deactivation in Alcohol Oxidation Using Thiolate Self-Assembled Monolayer Modification

The selective oxidation of alcohols is of great importance in the production of fine chemicals and pharmaceuticals. Heterogeneous Pt-group catalysts have been widely investigated for this process because of the ability to activate molecular oxygen and alcohols. However, these catalysts have often been reported to deactivate rapidly. One of the frequently reported factors responsible for deactivation is the strongly adsorbed species on the catalyst surface, which can be generated by decarbonylation and dimerization of the aldehyde product.

Recent studies have shown that thiolate self-assembled monolayers (SAMs) can be applied as Pt-group catalyst modifiers for hydrogenation reactions. One of the advantages of these modifiers is that their surface coverage can be tuned in a straightforward way: the coverage mainly depends on the bulkiness of the tail group. Though thiolate SAMs may block active sites to poison the catalysts, they have also been shown to limit deposition of carbonaceous species on the surface to increase the catalytic activity. However, this group of modifiers have not yet been applied for Pt-group catalysts in oxidation reactions. It is therefore desirable to use alcohol oxidation reactions as a probe to investigate the thiolate SAM modifiers for deactivation control.

Here, we show that the deactivation of a supported palladium catalyst can be successfully limited by thiolate SAMs modification in the liquid phase oxidation of trans-2-hexen-1-ol. While the native Pd catalyst suffered from rapid deactivation, the oxidation rate was improved on the modified catalysts, particularly at high conversion, without a significant change in selectivity to the desirable aldehyde product. This reactivity and selectivity on the modified catalyst was maintained during two additional recycle reactions. Kinetic studies combined with infrared spectroscopy indicated that the aldehyde product generated surface poisons through decarbonylation and dimerization, which can be suppressed by thiolate SAMs. The effect was controllable by varying the thiolate surface coverage through use of either 1-adamantanethiol (AT) or 1-octadecanethiol (C18) SAMs. This result suggests that thiolate SAMs can be a promising tool for enhancing the catalytic activity in low-temperature alcohol oxidation reactions.