Achieving controllable distribution of metal cations (Pd, Pt, Ni, Cr, Cu) in a zeolite either as [M(II)-OH]/1Al or M(II)/2Al provides novel mechanistic insights for adsorptive and catalytic reactions

Directing divalent metals to form either M(II) or M(II)-OH cationic species on the same support could provide an opportunity to compare their reactivity, if successful. Utilizing H-BEA zeolites with similar Si/Al ratios but with differing framework Al siting, we 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 2-dimensional pulsed HYSCORE EPR, EXAFS, and density functional theory (DFT) calculations provide unambiguous evidence for the selective formation of divalent metal cations as M+2/2Al species (for conventional zeolite prepared in the hydroxide media), and [M(II)-OH]+1/1Al species for H-BEA prepared in HF. This allowed us to explore their reactivities in the most important catalytic and adsorptive applications, which was not previously possible both in organometallic homogeneous and heterogeneous catalysis: we provide evidence for their divergent reactivity in ethylene dimerization, ethylene oxidation (Wacker process), selective catalytic reduction (SCR) of NO, NO adsorption, and methane oxidation. M+2/2Al and [M(II)-OH]+1/1Al species are both active for ethylene dimerization (including the first example of Pt/Zeolite reactivity for ethylene oligomerization), with [M(II)-OH]+1/1Al being more active in all cases (Pd, Ni, Pt). [M(II)-OH]+1/1Al is active for acetaldehyde formation in Wacker ethylene oxidation. We also propose the existence of a new active site with a terminal OH group (Cr-OH) in Phillips catalysts for ethylene oligomerization, as isolated Cr in BEA zeolite is virtually inactive, whereas Cr-OH shows activity in ethylene dimerization