Octahedral-based redox molecular sieve M-PKU-1: Isomorphous metal-substitution, catalytic oxidation of sec-alcohol and related catalytic mechanism

- Octahedral aluminoborate: alternative candidate to prepare redox molecular sieves.
- High substitution upper-limit and strong accommodation capacity for some transition metals.
- Sec-alcohols oxidized to ketones or other over-oxidative products.
- Cr3+ functioned as catalytic center to modulate the formation of free radicals.
- A mechanism proposed to demonstrate the probable reaction pathway.

Octahedral-based redox molecular sieves M-PKU-1 (MCr, Fe) were synthesized by isomorphous metal-substitution and used as catalysts for catalytic dehydrogenation of sec-alcohols using H2O2 as oxidant. Various characterizations, such as X-ray diffraction Rietveld refinement and XPS spectrum, confirmed that transition metals were embedded successfully inside the PKU-1 framework with a high level (about ∼50 atom %) and presented in the valence state of +3. Molecular probe analyzes suggested that Cr sites catalyzed the quick formation of active OH radicals, however strongly suppressed the generation of superoxide O2- ions. Under the performed reaction conditions, 10%Cr-PKU-1 exhibited excellent catalytic performances (>99 % selectivity) and kept favorable recyclable stability. A hypothetical mechanism was proposed, which involved a Cr3+-Cr2+-Cr3+ circle when the oxidative dehydrogenation reaction happened. Furthermore, qualitative and quantitative analyzes were performed to illustrate the stepwise by-products generated in the probable pathway due to the over-oxidization, but the selectivity to the two proposed pathways seemed to be in equal portions and didn't have any obvious preference. Obviously, our preliminary results still merit further exploration; we believe however, they would provide helpful information to better understand the structure-activity relationship and the key function of Cr-PKU-1 in the catalytic activation of H2O2.

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