Ionic liquids improve performance of single-atom supported catalysts

Isolated metal atoms dispersed on the surface of a support material offer unique opportunities in heterogeneous catalysis. For example, this class of single-site catalysts uses precious metals with maximum efficiency. In addition, because of their relative simplicity compared with standard multiatom nanoparticle catalysts, single-atom supported catalysts allow researchers to deduce valuable mechanistic details far more easily. But the single-atom variety is tough to tailor. And although modifying catalyst synthesis methods sometimes improves performance, it’s not always clear why. A team led by Alper Uzun of Koç University and Bruce C. Gates of the University of California, Davis, reports that treating single-atom iridium complexes supported on γ-alumina with 1,3-dialkylimidazolium ionic liquids improves the complexes’ catalytic properties for reasons the team quantifies via high-resolution X-ray absorption spectroscopy (ACS Catal. 2017, DOI: 10.1021/acscatal.7b02429). Specifically, the team prepared atomically dispersed Ir(CO)₂complexes on alumina, coated them with various ionic liquids, and used them to catalyze partial hydrogenation of 1,3-butadiene, an important industrial process. The team found that the ionic liquids significantly increased selectivity for butenes—in some cases, from roughly 50% to 80%. They attribute the enhancement to electron donation from the ionic liquid to iridium, a process they examined in detail spectroscopically.