Direct hydroxylation of aromatic compounds by a palladium membrane reactor

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

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.

A Pd-TSH composite membrane reactor for one-step oxidation of benzene to phenol

Pd membranes with excellent stability and flux were prepared by modified electroless plating, and a loose TSH zeolite with intraparticle hollows was joined on the Pd membrane by a covalent bonding method. The prepared Pd-TSH composite membrane shows outstanding performance for phenol production in the one-step oxidation of benzene to phenol.

Direct Hydroxylation of Benzene to Phenol over TS-1 Catalysts

We synthesized a TS-1 catalyst to directly hydroxylate benzene to phenol with H2O2 as oxidant and water as solvent. The samples were characterized by FT-IR (Fourier Transform Infrared), DR UV-Vis (Diffused Reflectance Ultraviolet Visible), XRD (X-ray diffraction), SEM(scanning electron microscope), TEM (Transmission Electron Microscope), XPS (X-ray photoelectron spectroscopy), ICP (inductively coupled plasma spectrum), and N2 adsorption-desorption. A desirable phenol yield of 39% with 72% selectivity was obtained under optimized conditions: 0.15 g (0.34 to the mass of benzene) TS-1, 5.6 mmol C6H6, reaction time 45 min, 0.80 mL H2O2 (30%), 40.0 mL H2O, and reaction temperature 70 °C. The reuse of the TS-1 catalyst illustrated that the catalyst had a slight loss of activity resulting from slight Ti leaching from the first run and then kept stable. Almost all of the Ti species added in the preparation were successfully incorporated into the TS-1 framework, which were responsible for the good catalytic activity. Extraframework Ti species were not selective for hydroxylation.

Photocatalytic oxidation of benzene to phenol using dioxygen as an oxygen source and water as an electron source in the presence of a cobalt catalyst

The present study reports the first example of photocatalytic hydroxylation of benzene with O₂ and H₂O, both of which are the most green reagents, under visible light irradiation to afford a high turnover number.

Photocatalytic hydroxylation of benzene to phenol by dioxygen (O₂) occurs under visible light irradiation of an O₂-saturated acetonitrile solution containing [Ru^II(Me₂phen)₃]²⁺ as a photocatalyst, [Co^III(Cp*)(bpy)(H₂O)]²⁺ as an efficient catalyst for both the water oxidation and benzene hydroxylation reactions, and water as an electron source in the presence of Sc(NO₃)₃. The present study reports the first example of photocatalytic hydroxylation of benzene with O₂ and H₂O, both of which are the most green reagents, under visible light irradiation to afford a high turnover number (e.g., >500). Mechanistic studies revealed that the photocatalytic reduction of O₂ to H₂O₂ is the rate-determining step, followed by efficient catalytic hydroxylation of benzene to phenol with H₂O₂, paving a new way for the photocatalytic oxygenation of substrates by O₂ and water.

Structure of the Active Platinum Cluster and Reaction Pathway of the Selective Synthesis of Phenol from Benzene and Oxygen Regulated with Ammonia on a Platinum Cluster/β-Zeolite Catalyst Studied by DFT Calculations

DFT calculations were used to investigate the structure of the active Pt cluster and the catalytic reaction pathway for the selective synthesis of phenol from benzene and molecular oxygen regulated with ammonia on a Pt cluster/β-zeolite catalyst that was reported to be active for the selective hydroxylation of benzene only in the coexistence of ammonia. It was found that Pt5-Pt6 clusters were active for the direct synthesis of phenol, and they provided the reaction sites for bond rearrangements among ammonia, oxygen, and benzene; furthermore, the coexistence of ammonia was crucial for the selective oxidation of benzene to phenol, as it suppressed benzene combustion to CO2 and promoted the selective synthesis of phenol. It was further found that water coexisting in the system also played a significant role in desorbing phenol on the Pt cluster surface, which resulted in promotion of the overall selective synthesis of phenol. The energy diagram for the reaction sequences and the structures of the transition states were obtained, which indicated the origin of the Pt/β catalysis.