Re Clusters in HZSM-5 Pores for Direct Phenol Synthesis

Re Clusters in HZSM-5 Pores For Direct Phenol Synthesis 

The three-step cumene process, including the liquid-phase reactions and using sulfuric acid, is energy-consuming, environmentally unfavorable and disadvantageous for practical operation the process also produces as an unnecessary byproduct acetone, stoichiometrically. Furthermore, the intermediate, cumene hydroperoxide, is explosive and cannot be concentrated in the final step, resulting in a low one-path phenol yield, ( 5%, based on the amount of benzene initially used). Thus, direct phenol synthesis from benzene in one-step reaction with high 

Previous studies have used many oxidants for direct phenol synthesis from benzene, such as O2 [74-80], H2O2 [81-89], N2O [90-99], Hj -1- Oj [100, 101], air/ CO [102] and O2/H2O [103], Among these oxidants, the selective oxidation of benzene with economically and environmentally favorable O2 has been nominated as one of the ten most difficult challenges for catalysis [104—106] and, indeed, there have been no reports on the direct phenol synthesis with greater than 5% conversion and 50% selectivity over the last 40 years. 

A novel HZSM-5-supported N-interstitial Reio cluster catalyst has now exhibited the highest phenol selectivity (93.9%) using O2 as the sole oxidant at a good benzene conversion of 9.9% [107]. Although rhenium with various valences possesses oxophilic and redox properties similar to molybdenum and vanadium, it has not been widely used as catalyst for selective oxidation because of the easy sublimation of Re207. However, rhenium acts as a good component of oxidation catalysts under reductive atmosphere, for example, coexisting with methanol [108-110] and ammonia [111-114]. 

In this section, we discuss the high performance of the Rejo cluster/HZSM-5 catalyst, its active structure and dynamic structural transformation during the selechve catalysis, and the reaction mechanism for direct phenol synthesis from benzene and O2 on this novel catalyst [73, 107]. Detailed characterization and determination of active Re species have been conducted by XRD, Al solid-state MAS NMR, conventional XAFS and in situ time-resolved energy dispersive XAFS, which revealed the origin and prospects of high phenol selectivity on the novel Re/HZSM-5 catalyst [73]. 

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