Mechanistic Insights into Adsorptive and Catalytic Reactions from Controllable Distributions of Metal Cations (Pd, Pt, Ni, Cr, Cu) as [M‐OH] ⁺¹ /1Al or M ⁺² /2Al in Zeolites

Abstract

Abstract Anchoring divalent metal ions in the same zeolite framework with similar Si/Al ratio selectively as zeolite‐bound M +2 or [M +2 ‐OH] +1 cationic species enables critical comparison of the species’ intrinsic reactivity for industrially and fundamentally relevant reactions. H‐BEA zeolites with similar Si/Al ratios but differing framework Al siting were used to anchored multiple divalent metal cations (Ni, Pd, Pt, Cr, Cu) in the zeolite micropores. State‐of‐the‐art infrared (IR) spectroscopy, electron paramagnetic resonance (EPR) measurements, including two‐dimensional pulsed HYSCORE EPR, extended X‐ray absorption fine structure (EXAFS), and density functional theory (DFT) calculations together provide unambiguous evidence for the selective formation of divalent metal cations as M +2 /2Al species (for H‐BEA prepared in the conventional hydroxide media), and [M +2 OH] +1 /1Al species for H‐BEA prepared in HF. Solid‐state proton‐decoupled triple‐quantum magic‐angle spinning (3Q MAS) NMR measurements confirmed contrasting Al distributions in the two H‐BEA zeolites, which led to a contrasting divalent cation speciation. The reactivities of the two cationic species were explored for catalytic and adsorptive applications in both organometallic homogeneous and heterogeneous catalysis. This work demonstrates their divergent reactivity in ethylene dimerization, ethylene oxidation (Wacker process), selective catalytic reduction (SCR) of NO, NO adsorption, and methane oxidation. Both M +2 /2Al and [M +2 OH] +1 /1Al cations are both active for ethylene dimerization, but [M +2 OH] +1 /1Al species show higher reaction rates for each Pd, Ni, Pt. [M +2 OH] +1 /1Al is active for acetaldehyde formation in Wacker ethylene oxidation. A new active site for ethylene oligomerization is proposed that possesses a terminal OH group (Cr‐OH) in Phillips catalysts evident by a nearly inactive isolated Cr +2 /2Al species that contrast an active Cr─OH motif.