NH3 -Driven Benzene C-H Activation with O2 that Opens a New Way for Selective Phenol Synthesis

Abstract

Catalytic benzene C-H activation toward selective phenol synthesis with O₂ 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 O₂ under coexisting NH₃ 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, O₂ , and NH₃ , 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 NH₃ -driven benzene C-H activation with O₂ was compared to the switchover of the reaction pathways from the deep oxidation to selective oxidation of benzene by coexisting NH₃ on Pt₆ metallic cluster/β and Ni₄ O₄ oxide cluster/β. The NH₃ -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.

Keywords: Benzene C−H activation toward selective phenol synthesis; Confined single-sites; Indispensable NH3 effect; Inter-ligand concerted mechanism; Single Cs+ and Rb+ site/β zeolite catalysis.