Stability of self-assembled monolayer coated Pt/Al2O3 catalysts for liquid phase hydrogenation

• Recycle studies were conducted on thiol self-assembled monolayer-coated catalysts.
• Selectivity was dependent on both surface and near-surface interactions.
• Selectivity from near-surface interactions was more sensitive to recycling.
• A recycle–regeneration procedure could maintain selectivity improvement.
• Aging in air decreases the efficacy of thiol SAM coating on selectivity improvement.

Thiolate self-assembled monolayers have recently been demonstrated to be effective catalyst modifiers for selectivity control, but these studies have not extensively explored the long term stability of these modifiers or the effects of specific reaction conditions. Here we investigate how the performance of thiolate-modified Pt/Al2O3 catalysts is affected by recycling and regeneration, using the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol as a probe reaction. Although modification of Pt catalysts with 3-phenyl-1-propanethiol results in high selectivity due to a ligand-specific interaction between modifier and reactant during the first use of a given catalyst, repeated recycling was shown to decrease the efficacy of this mechanism due to increasing disorder in the monolayer. However, selectivity and order could be stabilized using a thiol regeneration step or by feeding dilute concentrations of thiols in the reaction mixture. Similarly, aging in air was shown to decrease the order of the thiol and reduced the selectivity improvement of both a 3-phenyl-1-propanethiol and an octadecanethiol (C18) modified Pt/Al2O3 catalyst. These studies show that the ligand-specific interactions between the SAM and the reactant are particularly sensitive to conditions that can degrade the monolayer.

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