Photon Factory Activity Report 2005 #23 Part B (2006)
Chemistry

12C, NW2A/2005G208

In-Situ XAFS Study on Re/Zeolite Catalysts for Direct Phenol Synthesis fromBenzene with Molecular Oxygen
Yohei UEMURA1, Mizuki TADA1, Rajaram BAL1, Yasuhiro INADA2, Masaharu NOMURA2, andYasuhiro IWASAWA*1
1
Department of Chemistry, Graduate School of Science, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan
2PF, KEK.
Introduction
Phenol is one of the most important chemicals andprepared by cumene processes with three-steps acid-basereactions in industry. The direct phenol synthesis frombenzene with molecular oxygen is one of the most
attractive subjects, but the selective oxidation is still
difficult. There were no reports on selective oxidation
catalysts for the direct phenol synthesis with high phenol
selectivity. We found that a H-ZSM-5-supported Re
catalyst exhibited high selectivity (88% for steady-state
reaction and 94% for pulse reaction) for the direct phenol
synthesis from benzene and O2 [1]. The structure ofactive species in zeolite and the structural changes in the
catalytic reaction were investigated by in-situ step-scan
XAFS and time-resolved energy-dispersive XAFS(DXAFS).
Experimental
HZSM-5 (SiO2/Al2O3 = 19) supported Re catalyst
was prepared by a CVD method of CH3ReO3 at 333 K.
The obtained catalyst was pretreated in a He flow at 673
K for 1 h and oxidized in an O2/He gas flow. The
reduction of the Re catalyst was carried out in a NH3 flowat 553 K for 2 h. In-situ XAFS measurements wereperformed with the gas O2 at 553 K after the treatment
with NH3 (selective oxidation), while the in-situ
formation of active Re cluster was performed under NH3(the flow rate of NH3 is around 20 ml/min) at 553 K after
the reaction of O2.
Re LIII-edge step-scan XAFS was measured in atransmission mode with a Si(111) double-crystal
monochromator at a BL-12C station. Time-resolved
energy-dispersive XAFS was measured at a NW2A
station of PF-AR and Si(111) bent crystal was used as apolychromator. A self-scanning photodiodo array wasused as a position sensitive detector. X-rays weredetected by position-sensitive PDA recording every 50 msor 100 ms. The time-resolved measurements started bythe injection of the reaction gases (O2, O2 and benzene,and NH3) to the evacuated system. Background of these
XAFS spectra were subtracted by AUTOBK program andthe obtained EXAFS oscillations were Fouriertransformed and fitted in R-range by FEFFIT program.
Results and Discussion
The active structure of the Re/HZSM-5 catalyst forthe selective benzene oxidation was found to be Re10
cluster with Re-Re bonds at 0.276 nm by Re LIII-edge
EXAFS. Figure 1 shows time-resolved energy-dispersive

XANES spectra of true active Re catalyst at Re LIII-edge
measured at 553 K by the exposure to O2 (reaction feed).
The peak top of the XANES region increased when theselective oxidation proceeded, indicating that the active7+Re cluster converted to Re monomers during theselective oxidation reaction in the presence of O2.
µttime [ms]energy [eV]

Figure 1. The series of in-situ energy-dispersive XANES
spectra at 553 K in the presence of molecular oxygen(13.9 kPa). These spectra were recorded every 150 ms
with the exposure time of 50 ms.
Figure 2 shows that changes in the intensity of0.40

0.39

0.38

µt0.370.36050100150x10200250

3time [ms]Figure 2. Changes in the intensity of Re LIII-edge XANES
at 553 K in the presence of molecular oxygen (16.0 kPa).
These spectra were recorded every 1000 ms with theexposure time of 100 ms. (blue:10612 eV, red:10595 eV)
XANES during the selective oxidation process. The
structural changes by benzene and molecular oxygen
were completed within 100 ms as shown in Figure 2. On
the other hand, the reduction to the active Re cluster muchslowly proceeded and the existence of intermediate
structures was suggested.
[1] R. Bal, M. Tada, T. Sasaki, and Y. Iwasawa, Angew.
Chem. Int. Ed. 45, 448-452 (2006) (Selected as Hot Paperof Angew. Chem. Int. Ed.).
* iwasawa@chem.s.u-tokyo.ac.jp

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