Highly efficient and selective sunlight-induced photocatalytic oxidation of cyclohexane on an eco-catalyst under a CO2 atmosphere

Article abstract of DOI:10.1039/c2gc16594e

Highly efficient and selective sunlight-induced photocatalytic oxidation of cyclohexane to cyclohexanone and cyclohexanol was achieved on TiO₂ (P25) modified with iron oxide under a CO₂ atmosphere.

Full text of DOI:10.1039/c2gc16594e

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COMMUNICATION
Highly efficient and selective sunlight-induced photocatalytic oxidation of
cyclohexane on an eco-catalyst under a CO atmosphere
2
Yusuke Ide,* Hideya Hattori, Shuhei Ogo, Masahiro Sadakane and Tsuneji Sano
Received 12th December 2011, Accepted 23rd February 2012
DOI: 10.1039/c2gc16594e
Highly efficient and selective sunlight-induced photocatalytic
oxidation of cyclohexane to cyclohexanone and cyclohexanol
was achieved on TiO (P25) modified with iron oxide under a
2
CO atmosphere.
2
Production of chemicals and fuels through heterogeneous photo-
catalysis driven by sunlight is one of the most compelling objec-
tives in modern chemistry. Titanium dioxide is a promising
material for the purpose due to availability, low toxicity, and
chemical stability; however, it responds only to UV light, occu-
pying 3–4% of sunlight, and tends to be nonselective for
1
synthetic reactions. Accordingly, after the discovery of the
2
,3
photocatalytic nature of TiO2, great endeavors are continu-
4
ously being done to modify TiO by doping with metal and
2
5
6
nonmetal elements or hybridization with organic dyes and
7
nanoparticles, as well as to design novel catalysts such as
8
–11
12–16
molecular-sieve-like TiO2
and non-TiO2 materials,
to
achieve efficient and selective photocatalysis under sunlight.
Besides catalyst design, there have been several studies on the
influence of reaction environments on heterogeneous photo-
Fig. 1 Diffused reflectance spectra of (—) P25 and ( ) FeO@P25,
(●) action spectrum in cyclohexanone formation on FeO@P25, and (
)
radiation spectrum of solar simulator used in this study.
10,17–23
catalysis.
Very recently, we have found that sunlight-
induced photocatalytic oxidation of aqueous benzene to phenol
An iron oxide-modified P25 (named as FeO@P25) was syn-
over TiO -supported gold nanoparticles was substantially
2
thesized using iron(III) acetylacetonate complex according to the
improved to give a higher yield and selectivity when the reaction
28
literature. In the XRD pattern and TEM image of FeO@P25,
2
3
was conducted under a CO atmosphere. This result motivated
2
diffraction peaks only due to TiO were detected and particles
2
us to investigate various sunlight-driven photocatalytic selective
oxidations on more economically favorable catalysts under CO2.
Selective oxidation of cyclohexane to cyclohexanone and
cyclohexanol is one of the most important synthetic reactions,
since the partially oxidized products are an important intermedi-
ate in ε-caprolactam synthesis, which is used in the manufacture
of nylon polymers. However, there have been few efficient
photocatalytic processes under sunlight or visible light
other than P25 were not observed, respectively. The adsorption
spectrum of FeO@P25 showed a shoulder centered at 500 nm
(
Fig. 1). These results imply the presence of molecular-level iron
28,29
oxide adjacent to the P25 surface.
An iron(III) acetylaceto-
nate complex has been reported to irreversibly react with surface
titanol groups to form Ti–O–Fe covalent bond via ligand
28
exchange reaction. The amount of the immobilized iron oxide
3+
−2
in FeO@P25 was estimated to be 0.81 Fe ions nm . Taking
24–27
irradiation.
efficient and selective sunlight-induced cyclohexane oxidation
In this communication, we report a high level of
−2
the amount (more than 4 groups nm ) of titanol on P25 into
30,31
account,
iron oxide on P25 is thought to exist as patchy
2
8,29
on TiO (P25) modified with iron oxide
by conducting the
2
molecular-level particles, rather than a contentious ultrathin
layer.
Photocatalytic conversions were carried out by photo-
irradiation with solar simulator (San-Ei Electric Co., Ltd) to a
reaction under CO . This photocatalyst is economically favor-
2
able, as well as environmentally benign, since iron is harmless
and abundant in nature.
mixture of catalyst (30 mg) and O -saturated acetonitrile
2
(18 mL) solution of cyclohexane (2 mL) in a stainless-made
closed container equipped with Pyrex glass (75 mL) under con-
Department of Applied Chemistry, Graduate School of Engineering,
Hiroshima University, 1-4-1 Kagamiyama, Higashi-Hiroshima 739-
trolled atmosphere at 42 °C, with shaking. The CO pressure was
2
tuned by changing the amount of the added dry ice in the
mixture and the irradiation was started after the sublimation of
8
7
527, Japan. E-mail: yusuke-ide@hiroshima-u.ac.jp; Fax: +81-82-424-
606; Tel: +81-82-424-7606
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a
Table 1 Results of cyclohexane oxidation on P25, FeO@P25 and FeO@SiO
Yield/μmol
2
under simulated sunlight
b
c
d
e
Entry
Catalyst
P25
Added CO
2
/μmol (kPa)
CHone
CHnol
CO
2
[CHone + CHnol] TON
[CHone + CHnol] Selectivity/%
f
1
2
3
4
5
6
7
8
9
0
39.7
24.6
47.1
30.5
29.6
30.3
3.6
2.5
6.3
5.4
4.8
18.8
78.8
4.3
6.2
9.2
13.6
8.0
15.1
3.3
126.0
183.0
252.4
821.2
373.3
503.5
Trace
n.d.
—
—
—
—
—
—
5.3
3.7
9.3
7.9
7.1
38
206
6.3
9.2
14
71.7
51.7
59.6
19.8
41.0
32.4
>99.9
>99.9
>99.9
30.0
909 (51)
1818 (102)
2273 (127)
2727 (153)
3409 (191)
13.7
10.0
Trace
Trace
Trace
Trace
Trace
7.0
60.7
Trace
Trace
Trace
n.d.
f
FeO@P25
0
455 (26)
909 (51)
n.d.
1
0
1818 (102)
2727 (153)
75.1
50.6
n.d.
n.d.
n.d.
n.d.
154.7
n.d.
1
1
36.2
g
1
1
1
1
1
1
2
3
4
5
6
7
909 (51)h
>99.9
>99.9
>99.9
>99.9
26.2
909 (51)
i
909 (51)j
909 (51)
k
0
0
f
FeO@SiO
2
n.d.
—
—
a
d
b
Solar simulator irradiation time, 6 h; catalyst, 30 mg; O
2
-saturated solution of cyclohexane (2 mL) in acetonitrile (18 mL). Cyclohexanone.
d
e
2
Cyclohexanol. Estimated on the basis of Fe amount. [formed CHone] + [formed CHnol]/[formed CHone] + [formed CHnol] + 1/6[formed CO ]
100. In air. Irradiation time, 12 h. Irradiation time, 24 h. Sunlight with a wavelength shorter than 420 nm was cut-off. After the reaction (entry
k
f
g
h
i
j
×
9
), the catalyst was recovered, washed with acetonitrile, and then used for reaction. Argon was purged.
cyclohexanone formation on FeO@P25 and P25 under typical
conditions. When the reaction was done on FeO@P25 under 51
kPa of a CO atmosphere for 24 h, the best result, with a TON
2
value of more than 200 and a selectivity of ca. 100%, was
attained (Table 1, entry 13). A notable finding is that even the
reaction on FeO@P25 in air gives a selectivity of ca. 100%, and
by conducting the reaction under 51 kPa of CO , even when the
2
irradiation time was prolonged, the yield is substantially
improved with a selectivity maintained at ca. 100% (Table 1,
entries 7, 9, 12 and 13, and Fig. 2, left (a)). To the best of our
knowledge, 100% selectivity is the highest among those that had
been reported for photocatalytic cyclohexanone production
24–27
under visible light.
Also, the yield (more than 200 TON)
was considerably higher than those obtained in other selective
24–27
photocatalytic cyclohexane oxidations under visible light.
For example, 68% selectivity and 2.5 TON for cyclohexanone
25
production were obtained on Cr–Si binary oxide and 99%
selectivity and 22 TON for cyclohexanone and cyclohexanol
production were attained on hydrophobically modified Cr–Si
binary oxide. In the present study, it is also worth mentioning
that the photocatalytic performance of pristine P25 is improved
to give a higher yield with a selectivity maintained to some
Fig. 2 (left) CO
2
pressure-dependence of photocatalytic oxidation
activity of cyclohexane to cyclohexanone on (a) FeO@P25 and (b) P25
under sunlight irradiation for 6 h. (right) Comparison between (●) CO
pressure-dependence of cyclohexanone formation on (a) FeO@P25 and
b) P25 (plots same to those in Fig. 1) and (○) that of cyclohexanone
2
27
(
adsorption from acetonitrile on (a) FeO@P25 and (b) P25.
extent by conducting the reaction under CO (Table 1, entries 1
2
the added dry ice. The container was placed by ca. 30 cm away
from the light source to irradiate 1 solar (1000 W m )-power
and 3, and Fig. 2, left (b)). All of the results described above
suggest that effective and selective sunlight-induced organic
synthesis is possible even on cheap and abundant photocatalysts
−2
light to the mixture. CO and organic compounds were quantitat-
2
ively analyzed by GC-TCD (the measurement accuracy was
within 1.0%) and GC-FID, respectively. Toluene was used as
internal standard for GC-FID. Pristine P25 and iron oxide-
modified SiO (named as FeO@SiO )† were also used for com-
just by conducting the reaction under CO atmosphere.
2
Interestingly, a high level of effective and selective cyclo-
hexane oxidation was attained only when FeO@P25 was irra-
diated by sunlight including UV light (320 nm < λ) (Table 1).
2
2
parison. For all photocatalytic reactions, only three products,
cyclohexanone, cyclohexanol, and carbon dioxide, were detected
by gas chromatographic analyses.
P25 produced much larger amount of CO when irradiated by
2
sunlight (excitation with UV light (λ < 420 nm), Fig. 1), result-
ing in much lower selectivity (Table 1, entries 1–6). It is well-
known that valence band holes possess strong oxidation power;
therefore, highly selective cyclohexane oxidation is difficult on
Table 1 summarizes all the results and Fig. 2 (left) shows the
CO pressure dependence of the yield and the selectivity for
2
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P25, even under CO . On the other hand, when FeO@P25 was
under an appropriate CO2 loading (ca. 50 and 100 kPa for
2
irradiated by sunlight, only with a wavelength longer than
FeO@P25 and P25, respectively) a balanced combination
4
1
20 nm (only molecular iron oxide was excited, Table 1, entry
4), smaller amount of cyclohexanone formed on FeO@P25 if
between the CO -modified surface properties of the catalysts and
2
the polarity of the liquid phase makes the formed cyclohexanone
and cyclohexanol promptly desorb from the catalysts surface,
which prevents the successive oxidation of the partially oxidized
products.
compared to that obtained by sunlight irradiation (320 nm < λ,
Table 1, entry 9), although similar selectivity (ca. 100%) were
attained. Also, no products were detected on FeO@SiO under
2
identical conditions (Table 1, entry 17), showing that the iron
oxide functioned as the photocatalysis of cyclohexane oxidation
As shown in Table 1 (entry 15), FeO@P25 was able to be
reused without significant loss of the activity. Moreover, it was
possible to synthesize the catalyst at larger scale by increasing
the amount of the added P25, iron source and solvent. These
facts show merits of the present system for practical applications.
In summary, we have reported a highly effective and selective
sunlight-induced photocatalytic oxidation of cyclohexane, in
only when it was adjacent to TiO . Moreover, no products were
2
detected on FeO@P25 without any irradiation, which revealed
that iron oxide did not work as a thermocatalyst. From these
observations, we considered a possible role of iron oxide in the
selective cyclohexane oxidation over FeO@P25 as follows: elec-
trons, which transfer from both the TiO valence band by UV
acetonitrile, to cyclohexanone and cyclohexanol on TiO (P25)
2
2
excitation, and iron oxide by visible light excitation (possibly
modified with iron oxide under a CO atmosphere. The present
2
2
8,29
due to the d–d transition of molecular iron oxide)
to the
success opens up new opportunities to synthesize a wide variety
of fine chemicals in an economically and environmentally favor-
able fashion.
TiO conduction band, effectively reduce adsorbed O to gener-
2
2
−
ate O . The obtained superoxide anion plays an important role
2
in the selective cyclohexane oxidation over FeO@P25. Lower
amount of cyclohexanone formed on FeO@P25 when only the
iron oxide was excited (Table 1, entries 9 and 14), which was
explained by that smaller amount of electrons, which were
Notes and references
2
8
†FeO@P25 was synthesized according to the literature: P25 (1.0 g,
Nippon Aerosil) was added to 6.5 × 10− mol L of a iron(III) acetyla-
cetonate in a mixed solvent (100 mL, ethanol/hexane = 3 : 17 v/v) and
the mixture was stirred at room temperature for 24 h. The product was
separated by centrifugation (3000 rpm, 20 min), washed repeatedly with
the same solvent, and calcined at 500 °C for 1 h. The reaction was
4
−1
−
necessary to generate O2 , were transferred to the conduction
band of TiO . The action spectrum (from 330 to 460 nm) in
2
cyclohexanone formation on FeO@P25 was in good agreement
with the UV-vis spectrum of P25 rather than that of FeO@P25
repeated three times. FeO@SiO
where SiO (Wako gel Q-63) was used instead of P25.
Adsorption tests were done in a similar way that conducted in photoca-
2
was synthesized in a similar way,
(Fig. 1), supporting the above hypothesis. The mineralization of
2
cyclohexane and the successive oxidation of cyclohexanone and
cyclohexanol hardly occurred to give only trace amounts of CO2
on FeO@P25 (especially under lower loading levels of CO2),
since iron oxide efficiently prevented the interactions between
bulky molecules, cyclohexane and the partially oxidized pro-
ducts, with the valence band holes on the P25 surface. The
‡
talytic conversions except that 10 mg of the catalyst and a solution of
cyclohexanone (2 mL) in acetonitrile (18 mL), which was not bubbled
with O , were mixed.
2
1
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2
3
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2
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