Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

10B, 12C/2003G092

XAFS Study on Re/Zeolite Catalysts for Direct Phenol Synthesis from Benzenewith Molecular Oxygen
Mizuki TADA1, Rajaram BAL1, Yohei Uemura1, and Yasuhiro IWASAWA*1
Department of Chemistry, Graduate School of Science, The University of Tokyo
Hongo, Bunkyo-ku, Tokyo, 113-0033, Japan

Experimental
HZSM-5 supported Re catalysts were prepared by aconventional impregnation method using NH4ReO4 anda CVD method with CH3ReO3 at 333 K. The bothcatalysts were pretreated in a He flow at 673 K for 1 h.
The selective catalytic reactions were conducted in a
fixed bed down flow reactor with the reaction gases ofHe: O2: NH3: benzene = 46.4: 12.0: 35.0: 6.6 (mol%),
analyzed by on-line GC.
XAFS spectra at Re LIII-edge were measured in atransmission mode at 15 K at the BL-10B and 12Cstations. The samples were measured without exposing toair. Background subtraction was conducted by theAUTOBK program, and Fourier-transformed spectra
were fitted in R space by FEFFIT program.
Results and Discussion
It was found that Re/HZSM-5 catalysts are active forthe selective oxidation of benzene with molecular oxygen
when ammonia was coexisted. The CVD catalyst wasalmost 18 times more active than the impregnated catalyst.
The selectivity was much higher on the CVD catalyst(88%) than on the impregnated catalyst (28%). The
catalytic performances depended on the kind of zeolites.
Figure 1 shows Re LIII-edge EXAFS Fouriertransforms and curve fitting spectra for the CVD
Re/ZSM-5 sample after NH3 treatment and the steady state oxidative reaction. It is obvious that the structure of
the Re species changes remarkably by NH3. The NH3treatment caused a drastic transformation to Re clusterwith Re-Re bonds at 0.276±0.001 nm (CN=5.1±0.3).
There were two types of Re-O bonding attributed to
Re=O at 0.173±0.001 nm (CN=0.5±0.2) and longer ReObridge and/or Re-Olattice at 0.202±0.001 nm(CN=2.4±0.3). The structure of the Re cluster wasmodelled by DFT as a N-interstitial Re9 clustercomposed of two octahedral Re6 frameworks with planeshare to each other. The treatment with NH3 at 553 K
reduces Re oxidation state to the lower valence proved by
ReLI XANES and XPS.

3FT [k χ(k)]20100-10-200123456456-1R /10 nm
203Introduction
Phenol is an important chemical in industry but has
been produced from benzene by the three-steps ofcumene process. Direct phenol synthesis from benzene is
one of the important subjects. However, no selective
catalyst with good performance in an economically andenvironmentally favourable benzene-O2 system has been
discovered to date.
We have found Re/HZSM-5
promoted the direct phenol synthesis from benzene with88% (steady-state) and 94% (pulse) selectivity using
molecular oxygen as an oxidant under co-existing NH3.

FT [k χ(k)]1100-10-200123-1

R /10 nm

Figure 1. Fourier-transformed Re LIII-edge EXAFS spectraof Re cluster after the NH3 treatment (top) and Re monomer
after the reaction (bottom).

The NH3 pretreated catalyst did not produce phenol
when only benzene was pulsed. This indicates that thelattice oxygen of the Re cluster is not involved for theselective oxidation. The pulse of benzene + O2 on theNH3 pretreated catalyst produced phenol with 94%
selectivity, which demonstrates that the Re cluster isactive species.
In the pulse experiment the phenol formation rate
became zero after 4 pulses. Thus the coexistence of NH3
is indispensable for the phenol formation, and the Re
cluster is not stable under O2 in the absence of NH3. The
active Re cluster was transformed to inactive Re
monomers with Re=O bonds at 0.173±0.001 nm(CN=3.7±0.2) and Re-O bond at 0.213±0.003 nm(CN=1.3±0.7) as proved by EXAFS.
In presence of ammonia about 3 - 4 % of the [ReO4]
monomers are estimated to be transformed to Re9 clusters,
which are responsible for the formation of phenol. A
mechanism for the phenol formation may involve oxygen
free radical formation on Re9 cluster. Alternatively,
molecular oxygen associatively adsorbed on the partiallyreduced Re9 cluster may be the active oxygen species forthe formation of phenol from benzene. On the latter casethe maximum selectivity would be 93.9%.
* iwasawa@chem.s.u-tokyo.ac.jp

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