Catalyst Site Selection via Control over Noncovalent Interactions in Self-Assembled Monolayers

One strategy for controlling selectivity in surface-catalyzed reactions is to precisely control the types of surface sites available for reaction. Here, we show that such control can be achieved on Pd/Al₂O₃ catalysts modified by alkanethiol self-assembled monolayers (SAMs) by changing the length of the modifier's alkyl tail. Density functional theory (DFT) calculations show that thiolates with short alkyl chains preferentially bind to undercoordinated Pd step sites but that adsorption on (111) terrace sites is more favorable at higher chain lengths due to greater stabilization by van der Waals interactions. Linear alkanethiol SAMs with chain lengths ranging from six to 18 carbon atoms were deposited on Pd/Al₂O₃ catalysts to probe this predicted effect experimentally. Infrared spectroscopy measurements conducted after CO adsorption confirmed that increases in alkyl chain length resulted in increasing specificity in poisoning of terrace sites. The catalysts were also evaluated for furfuryl alcohol hydrogenation, a structure-sensitive probe reaction. Selectivity to the desired hydrodeoxygenation to methylfuran increased from <20% to >60% as chain length increased from six to 18 carbons, consistent with increasing efficiency for thiolate blocking of Pd terrace sites. DFT models demonstrate that the presence of thiolates strongly suppressed decarbonylation reactions and that step sites surrounded by thiolates could still be active for hydrodeoxygenation. This work demonstrates that site availability, and thus catalyst selectivity, can be tuned by changing the architecture of SAM precursors.