Photocatalytic Hydroxylation
FULL PAPER
C
OH (see above), and benzene is more easily oxidized than
cies that introduces the O2-derived O atoms into the hy-
droxylated product. The findings in the present study also
reveal that, during the photocatalytic oxidation, hvb is not
BA (one-electron oxidation potential: EBA0 =2.6 V and
Ebenzene0 =2.1 V vs. NHE),[44] the direct oxidation pathway
should be more important for benzene than for BA. It
means that the 18O abundance of phenol (from H218O)
would be higher than that of BA-OH, which is completely
contradictory to our experimental observations shown in
Table 1. On the other hand, if the hydroxylation processes
of BA and benzene only took place away from the surface
of TiO2 (in the bulk solution), no difference in isotope abun-
dance between BA-OH and phenol would be measured
+
the only active species that initiates the oxidation. Even in
the absence of hvb+, the substrate can still be oxidized by
the OH formed from the reduction of O2 by ecbꢀ. The con-
C
tribution of this pathway cannot be neglected for the photo-
catalytic oxidation, and its proportion is comparable to that
of hvb+, at least for the hydroxylation of aromatic com-
pounds. Our experiments also show that the photocatalytic
oxidation products of the adsorbed substrates bear less
oxygen from oxidant O2 than those of the unadsorbed ones.
(i.e., 16O%
= O%
= O%·OH). It is not consistent
phenol
16
16
BAꢀOH
with our experimental results either. Therefore, to explain
the observed isotope abundance difference between BA-OH
and phenol, both the surface reaction and bulk solution re-
action should be considered at the same time.
Other supports for the different oxygen sources of the
photocatalytic hydroxylation between the surface reaction
Experimental Section
Materials: TiO2 (P25, ca. 80% anatase, 20% rutile; surface area, ca.
50 m2 gꢀ1) was kindly supplied by the Degussa Company (Clausthal-Zel-
lerfeld, Germany). H218O was purchased from Jiangsu Changshu Chemi-
cal Limited (Changshu, Jiangsu, P.R. China), the isotope abundance of
which was 98%. 18O2 (18O: 97%) was purchased from Cambridge Isotope
Laboratories, Inc (Andover, USA). The mass spectrometry analysis of
the post-reaction solvent/atmosphere showed that the oxygen isotope
abundance of the 18O-enriched reagent did not change significantly (<
2%) in all the reactions. Benzoic acid (BA), benzene, nitrobenzene, ben-
zonitrile, and their hydroxylated products were all of analytical grade,
and obtained from the Beijing Chemical Company (Beijing, P.R. China).
5,5-Dimethyl-1-pyrroline-N-oxide (DMPO), a spin trapping reagent for
ESR measurements, was supplied by Sigma–Aldrich Co. (Shanghai, P.R.
China). Horseradish peroxidase (POD), which was used in the measure-
ment of H2O2, was purchased from the Huamei Biologic Engineering Co.
(Luoyang, Henan, China), while N,N-dialkyl-p-phenylenediamine (DPD)
was from Merk (p.a.) (Whitehouse Station, NJ, USA). Chlorotrimethylsi-
lane (TMSCl) and 1,1,1,3,3,3,-hexamethyldisilazane (HMDS) were pur-
chased from Acros Organics (Beijing, P.R. China). All reagents were
used as received without further purification.
C
and bulk solution reaction came from the free OH quench-
ing experiments (Table S2 in the Supporting Information).
Both tert-butyl alcohol (tBuOH) and isopropanol (iPrOH)
[5a,45]
C
were reported to capture preferably free OH,
because
their low adsorption on TiO2 makes them unfavorable to
+
C
compete with BA for hvb or surface-bound OH (details of
the estimation and discussion about the proportions of the
C
OH captured by tBuOH or iPrOH on the TiO2 surface and
in the bulk solution are shown in Tables S3 and S4 in the
Supporting Information). In the spin-trapping ESR experi-
ment, the addition of iPrOH markedly weakened the
C
trapped OH signal with the appearance of the signal of
DMPO-C(OH)Me2 (Figure 2; DMPO=5,5-dimethyl-1-pyr-
roline-N-oxide, a spin-trapping reagent used in the ESR
measurements), indicating the quenching effect of iPrOH on
Photocatalytic reactions: A 100 W Hg Lamp (ToshibaIn SHL-100UVQ)
was used as the light source for the photocatalytic reaction. In a typical
H218O-isotope-labeled reaction, TiO2 (P25, 2 mg) was dispersed in H218O
(1 mL) with a given concentration of BA under aerated conditions (16O2).
Prior to irradiation, the suspension was magnetically stirred in the dark
for about 30 min to ensure the establishment of an adsorption/desorption
equilibrium. After irradiation for a definite time, the oxygen isotope
abundance of the hydroxylated product was analyzed by HPLC-ESI
method (Agilent LC 1200/Ion Trap 6310) with C-18 column (250 mmꢁ
2.1 mm). Each measurement was repeated at least three times to assure
the accuracy. The standard deviation of MS analysis is estimated to be ca.
0.6%. To minimize the disturbance caused by the change of substrate
concentration and the formation of intermediates, only the initial stage
(0–2 h, <7% of substrate conversion) of photocatalytic oxidation was
studied. Figure S1 in the Supporting Information gives the typical HPLC
chromatograms (acquired by UV and MS detectors) and ESI-MS spectra.
The measured isotope abundance of the product was corrected with the
oxygen isotope abundance of solvent H2O and the natural isotope abun-
dance of the product [Eqs. (10) and (11)] in which Cp, Cn, and Cw are the
18O percentages of the measured isotope abundance of the product, natu-
ral isotope abundance of the product, and measured isotope abundance
of solvent H2O, respectively.
C
OH. Accordingly, the isotope labeled experiments showed
that the presence of tBuOH and iPrOH decreased the pro-
portion of 16O in the hydroxyl group of BA-OH from 18.2%
to 13.9% and 11.8%, respectively, for the 1 h irradiated sys-
tems (Table S2, entries 2 and 3), indicating again the impor-
tant role of free OH, which mainly initiates the hydroxyl-
ation of unadsorbed substrates, in the oxygen incorporation
from O2.
C
Conclusion
We studied the photocatalytic hydroxylation of aromatics by
18O-labeling method. The experiments show that 18O-isotope
labeling is a powerful method to identify the oxygen incor-
poration pathway in the photocatalytic hydroxylation. Our
experimental results do not support the mechanism that the
addition of molecular O2 onto the cationic radical or a radi-
cal HO adduct of the substrate could incorporate the O
atom of O2 into the hydroxylated product. In contrast, we
show that the reduction of O2 by ecb is indispensable for
the O2 incorporation. The formed intermediate H2O2 plays
an essential role in the O2 incorporation process. The OH
generated from the reduction of H2O2 is the final active spe-
Cp ꢀ Cn
ð10Þ
ð11Þ
H2O% ¼
ꢂ 100
Cw ꢀ Cn
ꢀ
Cw ꢀ Cp
Cw ꢀ Cn
O2% ¼
ꢂ 100
C
In the experiments using 18O2 as oxidant, the photocatalytic oxidation of
Chem. Eur. J. 2012, 18, 2030 – 2039
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
2037