Active Re Clusters Entrapped in ZSM-5 Pores

4 Direct Phenol Synthesis from Benzene and Molecular Oxygen 67 

A phenol synthesis by one-path benzene conversion of 5.8% proceeded on the Reio cluster/HZSM-5 (19) catalyst in the steady-state reaction, but only the inactive 

The selectivity of phenol may depend on the existence of heterogeneous Re species. If there are some intermediate structures with low phenol selectivity, the total phenol selectivity would decrease. On this supported Re catalyst, the following three phenomena are key to achieving high phenol selectivity (i) there were no unfavorable intermediates during the decomposition of the active Reio cluster with O2, (ii) the Re monomer changed to a completely inactive species and (iii) selective interaction between benzene and O2 to produce phenol occurs on the Reio clusters in the pores [171]. 

Molecular oxygen was the active oxidant for the phenol synthesis and the maximum phenol selectivity (93.9%) was achieved on the N-interstitial Re cluster supported on HZSM-5. Molecular oxygen was activated in the space between the two Reg octahedral cores of the Rejo cluster and benzene concertedly reacted on the activated O2 with the very low activation energy of 24 kj mol . The pore size of HZSM-5 is 5.5 A, which is similar to the size of the Ree-octahedral cluster framework, 

The dynamic XAFS study of the catalytically active structures under the reaction conditions should enable us to further develop novel catalysts for direct phenol synthesis from benzene and the efficient activation of molecular oxygen. 

10 Tada, M. and Iwasawa, Y. (2005) Annual Review of Materials Research, 35, 397-426. 

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