(736b) Selective Hydrogenation Using Supported Metal Catalysts Modified With Alkanethiol Self-Assembled Monolayers
Of particular interest and difficulty in chemical processes is the chemoselective conversion of molecules containing more than one reactive functional group. To bias a catalyst to produce the less energetically favorable product, we deposit alkanethiol self-assembled monolayers (SAMs) on the surface of supported metal catalysts. These SAMs form a well-ordered structure on the metal catalyst surface, altering the surface chemistry through steric and electronic effects thereby influencing the selectivity of a reaction.
The development of novel alkanethiol SAM coated selective catalysts has wide and promising applications to the field of catalysis. Here we display the usefulness of selectively hydrogenating molecules with multiple functional groups such as epoxybutene, polyunsaturated fatty acids, and α,β-unsaturated aldehydes.
Our group has previously used SAMs to modify supported metal hydrogenation catalysts such as Pd/Al2O3 for the selective hydrogenation of 1-epoxy-3-butene (EpB) [1,2]. Reaction of the epoxide ring is more preferred on an uncoated catalyst but alkanethiol SAM coated catalysts were found to increase the selectivity to the desired 1-epoxybutane from around <20% to >90% under equivalent conditions in the gas phase and similarly in the liquid phase where solvent-SAM interactions were shown to influence the selectivity of the reaction.
Liquid phase studies are now being employed to study larger, less-volatile reactant molecules which have increased ability to interact with the tail groups of the alkanethiol SAMs. The selective hydrogenation of 18-carbon polyunsaturated fatty acids to monounsaturated products was studied over alkanethiol coated Pd/Al2O3 and showed an increased selectivity to monounsaturated fatty acids as compared to the uncoated Pd/Al2O3 catalyst reaction case. Additionally, thiol tail groups have been used to specifically direct the reaction pathway of cinnamaldehyde, an α,β-unsaturated aldehyde, over a Pt/Al2O3 catalyst. Selective hydrogenation of cinnamaldehyde was performed using phenylated SAMs where interaction between the phenyl ring of the reactant (the large terminal end of the cinnamaldehyde molecule) and the SAM was used to direct hydrogenation of either the double bond or the aldehyde over a Pt/Al2O3 catalyst. Control of the near surface environment of a catalyst system, as studied here, provides a unique, interesting, but not yet fully understood method for controlling the reaction environment.
1. Marshall, S., O’Brien, M., Oetter, B., & Corpuz, A. (2010). Controlled selectivity for palladium catalysts using self-assembled monolayers. Nature Materials, 9
2. Kahsar, K. R., Schwartz, D. K., & Medlin, J. W. (2012). Liquid- and vapor-phase hydrogenation of 1-epoxy-3-butene using self-assembled monolayer coated palladium and platinum catalysts. Applied Catalysis A: General 445-446