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 Ni4O4 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.

Unprecedented Catalysis of Cs+ Single Sites Confined in Y Zeolite Pores for Selective Csp3–H Bond Ammoxidation: Transformation of Inactive Cs+ Ions with a Noble Gas Electronic Structure to Active Cs+ Single Sites

“…Zeolite Y-supported alkali metal ion and alkaline earth metal ion catalysts were prepared by a reliable ion-exchange method, , followed by a controlled treatment at 573 K, using the corresponding metal nitrates as precursors. A typical Cs + (2 wt %)/Y sample was synthesized as follows.…”

“…Recently, we found a single alkali metal ion site platform in a β zeolite pore that selectively promoted benzene C sp2 −H activation toward phenol synthesis. 1,2 The catalytic performance of alkali metal ion sites incorporated in zeolite pores depended strongly on the kind of zeolites, and ZSM-5, mordenite, and Y zeolites were almost inactive as supports. No C sp3 −H activation catalysis of alkali metal ions and oxides has been achieved to date, although a variety of effects of alkali metal additives on metal oxide catalysis are known.…”

“…The Cs + /Y catalyst without a beneficial redox property showed a high benzonitrile yield (92.7% yield; 94.6% conversion and 98.0% selectivity at 623 K) and a high NH 3 utilization efficiency (almost no extra consumption) in toluene ammoxidation employed as a test reaction; to the best of our knowledge, this is the highest yield without NH 3 loss for the benzonitrile synthesis from toluene with O 2 + NH 3 in a single-step gas-phase reaction. There is always a competition between the ammoxidation to benzonitrile and undesirable NH 3 oxidation to N 2 and NO x on catalyst surfaces, [1][2][3]11,23 whereas the Cs + /Y was found to catalyze no NH 3 + O 2 reaction when toluene was not present (Table 1). The simple ion-exchange method for the chemically confined Cs + /Y catalyst fabrication in this study is also advantageous in industrial usage.…”

Nontraditional Aldol Condensation Performance of Highly Efficient and Reusable Cs+ Single Sites in β-Zeolite Channels

“…Alkali and alkaline-earth metal-ion (M n + ) single site catalysts in β-zeolite pores (M n + : Na + , Mg 2+ , K + , Ca 2+ , Rb + , Sr 2+ , Cs + , Ba 2+ ) were prepared by a reliable ion-exchange method using the corresponding metal nitrates as precursors in a similar way to that in our previous study. − In the present study, the obtained samples were activated and stabilized by calcination at 500 °C for 1 h. Typically, a Cs + (1 wt %)/β sample was synthesized as follows. CsNO 3 (0.146 g) was dissolved in 10 mL of deionized water in a 50 mL Erlenmeyer flask, to which solution 9.9 g of NH 4 + -β with SiO 2 /Al 2 O 3 = 25 (molar ratio) was added with stirring.…”

“…However, recently, we reported that a Cs + single site/β catalyst without a beneficial redox property showed catalytic benzene C sp 2 −H hydroxylation with N 2 O or O 2 + NH 3 toward phenol in gas phase. 42,43 We also reported that a Cs + single site/Y catalyst, which was inactive for the benzene hydroxylation, showed a high efficiency for the selective ammoxidation of toluene and its derivatives with O 2 + NH 3 in gas phase. 44 In the present study, we have found unprecedented efficient performances of Cs + single-ion sites confined in β-zeolite channels (1 wt % Cs + -loading) for selfand cross-aldol condensation reactions involving C−C bond formation in liquid phase for the first time.…”