Ion exchanged zeolites are used in catalytic applications ranging from hydrocarbon upgrading to emissions abatement. Under some conditions, ions can be liberated from the zeolite framework and mobilized by solvent molecules, coupling attractive features of homogeneous and heterogeneous catalysis. Here, I discuss our recent results for metal-exchanged zeolite catalyzed Wacker oxidation and oxidative activation of ethane where we show that metal ions are solvated under reaction conditions, and solvation impacts the reaction mechanism.

The first example, Wacker oxidation, is a well-known redox process involving Cu and Pd ions where the formation of acetaldehyde is speculated to occur on Pd²⁺ ions. While the homogeneous counterpart proceeds by having Cl^- stabilize the metal ions, the anionic AlO₂^- sites of metal-exchanged zeolites can charge compensate Pd, eliminating the need for Cl^-. A major challenge with the heterogeneous catalyst is deactivation due to coking and sintering. We show that the Pd ions are solvated by H₂O and NH₃ under reaction conditions, and relative fractions of these two molecules compete with organic molecules, consequently suppressing oligomerization and coke formation.

Similarly, NH₃-solvated Co ions supported on MFI zeolites catalyze oxidative activation of ethane. Rates measured at different characteristic separations between cobalt ions demonstrate reactions among C₂H₆ and O₂ occur at isolated mononuclear cobalt sites, however, the addition of NH₃ forms oxygen-bridged binuclear cobalt-amine complexes. Computational results and UV-vis spectroscopy show that these binuclear species form by dimerization of mobile Co-amine complexes. Interestingly, confinement within zeolite frameworks stabilizes these complexes relative to their homogeneous analogues.