(25b) Controlling the Availability of Specific Active Sites On a Pd Catalyst Using Thiolate Self-Assembled Monolayers

Self-assembled monolayers (SAMs) have recently been utilized as modifiers to control reactivity in heterogeneous catalysis. One of the major advantages of SAM-based modifiers is their ability to form highly ordered, consistent structures on metal surfaces. This consistency is presumably attributable to the thermodynamically limited assembly process, which also facilitates connections between catalysis and surface science studies. In addition, both the attachment chemistry and tail groups can easily be tailored to create monolayers with desirable surface properties. These features potentially make SAMs useful modifiers for studying the effect of controlled surface modification on catalytic reactivity.

One key route for controlling reaction selectivity is to prepare catalysts that exhibit only specific types of sites required for desired product formation.  Altering the surface density of alkanethiolate SAM modifiers by altering the steric bulk of the tail function dramatically affects catalytic reactivity by controlling the availability of different active sites on supported Pd/Al₂O₃ catalysts. In this presentation, the relationship between modifier structure and the presence of specific active sites will be discussed.  Examples will be presented for reactions demonstrating dramatic reactivity changes by controlling site availability.  This technique simultaneously serves both to control reaction selectivity and provide insight into the reaction mechanism.