Control of Pd catalyst selectivity with mixed thiolate monolayers

• Mixed monolayers of 1-adamantanethiol and 1-octadecanethiol on Pd were prepared through co-adsorption.
• By adjusting monolayer composition, selectivity and activity for benzyl alcohol hydrogenation were controlled.
• The deoxygenation selectivity reached 98% at nearly full conversion for octadecanethiol-modified catalyst.
• Mixed monolayers strongly suppressed decarbonylation while retaining significant deoxygenation activity.

Metal catalysts coated with self-assembled monolayers have been found to exhibit improved performance in a number of reactions, but previous work has focused on deposition of uniform layers from a single precursor. In this study, we investigate how the use of variable-composition monolayers formed from mixtures of 1-adamantanethiol (AT) and 1-octadecanethiol (C18) can be used to control catalyst performance and provide insights into both monolayer ordering and surface reaction mechanisms. Benzyl alcohol hydrodeoxygenation to toluene was used as a probe reaction for these studies. The mixed monolayer composition was controlled by varying the proportion of AT to C18 in an ethanol solution used for deposition. For AT-modified catalysts, toluene selectivity was higher than that for the uncoated catalyst. Increases in the C18 surface fraction (yC18) in the monolayers resulted in further increases in toluene selectivity, with selectivity approaching 98% near complete conversion at full C18 coverage. Infrared spectra collected after dosing CO or benzyl alcohol also indicated that increasing yC18 lowered the ratio of bridging and three-fold adsorption on high coordination sites compared to linear adsorption on low coordination sites. These results suggested that the mechanism for selectivity enhancement was at least partly due to selective poisoning of terrace sites and reduction of contiguous active sites. Whereas selectivity was observed to follow a continuous increasing trend with increasing thiolate coverage, trends in reaction rates were more complex. A sharp 60% decrease in the rate of the undesired decarbonylation (DC) reaction was observed when yC18 was increased from zero to 0.15. This sharp decrease at low yC18 was attributed to filling in defects in AT monolayers by C18. Control of mixed monolayer composition allowed for tuning of the catalyst performance such that the rate of toluene production could match that over the uncoated catalyst, while the decarbonylation rate was decreased by an order of magnitude, providing an active, highly selective catalyst. Beyond catalyst design, these investigations demonstrate that catalyst evaluation can serve as a valuable tool for understanding monolayer organization.

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