(734d) Tuning Catalyst Activity Using Self-Assembled Monolayers
One of the biggest problems in heterogeneous catalysis is that of poor selectivity. Often dopants are utilized to tune the properties of various catalysts to improve selectivity or activity. Over the last decade self-assembled monolayers (SAMs) have been developed as a means to control the selectivity of reactions on supported metal catalysts, but little focus has been made in the ability to tune activity. Here, we demonstrate how self-assembled monolayers can be modified in a rational design strategy to tune the surface electronic properties. In doing, so we are able to control the rate of some reactions. Similar strategies have been performed with metal-ligand complexes and semiconductors materials, but have not been applied to heterogeneous catalysts. Two such approaches will be presented 1) functionalizing metal oxide catalysts with phosphonate monolayers and 2) using palladium catalysts with thiolate monolayers. Firstly, when 1-propanol is reacted over titania three main pathways are possible: dehydration, dehydrogenation, and condensation. At low temperatures (~250°C), dehydrogenation is favored on the native catalyst, closely followed by condensation. However, upon functionalization of titania with phosphonic acids with various organic tail functionalities (methyl, octadecyl, benzyl, etc), dehydration becomes the dominant pathway. More importantly, depending on the substituents of the benzylic tail (e.g. 4-aminobenzylphosphonic vs 3-fluorobenzylphosphonic acid), the activity of dehydration can be tuned over a wide range, allowing far higher dehydration activities than on the uncoated catalysts. The applicability of this approach to other catalysts will be demonstrated with results from noble metals functionalized with thiolate monolayers.