NH 3 -Driven Benzene C-H Activation with O 2 that Opens a New Way for Selective Phenol Synthesis.

Catalytic benzene C-H activation toward selective phenol synthesis with O ₂ remains a stimulating challenge to be tackled. Phenol is currently produced industrially by the three-steps cumene process in liquid phase, which is energy-intensive and not environmentally friendly. Hence, there is a strong demand for an alternative gas-phase single-path reaction process. This account documents the pivotal confined single metal ion site platform with a sufficiently large coordination sphere in β zeolite pores, which promotes the unprecedented catalysis for the selective benzene hydroxylation with O ₂ under coexisting NH ₃ by the new inter-ligand concerted mechanism. Among alkali and alkaline-earth metal ions and transition and precious metal ions, single Cs ⁺ and Rb ⁺ sites with ion diameters >0.300 nm in the β pores exhibited good performances for the direct phenol synthesis in a gas-phase single-path reaction process. The single Cs ⁺ and Rb ⁺ sites that possess neither significant Lewis acidic-basic property nor redox property, cannot activate benzene, O ₂ , and NH ₃ , respectively, whereas when they coadsorbed together, the reaction of the inter-coadsorbates on the single alkali-metal ion site proceeds concertedly (the inter-ligand concerted mechanism), bringing about the benzene C-H activation toward phenol synthesis. The NH ₃ -driven benzene C-H activation with O ₂ was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH ₃ on Pt ₆ metallic cluster/β and Ni ₄ O ₄ oxide cluster/β. The NH ₃ -driven selective oxidation mechanism observed with the Cs ⁺ /β and Rb ⁺ /β differs from the traditional redox catalysis (Mars-van Krevelen) mechanism, simple Langmuir-Hinshelwood mechanism, and acid-base catalysis mechanism involving clearly defined interaction modes. The present catalysis concept opens a new way for catalytic selective oxidation processes involving direct phenol synthesis.
(© 2019 The Chemical Society of Japan & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)