Photoactivation of molecular oxygen by an iron(III) porphyrin with a magnesium aluminum layered double hydroxide for the aerobic epoxidation of cyclohexene

Article abstract of DOI:10.1002/cctc.201402131

The photocatalytic aerobic epoxidation of cyclohexene proceeded under visible-light irradiation only if 5,10,15,20tetrakis(pentafluorophenyl)porphyrin iron(III) chloride ([Fe^III(TPFPP)Cl]) was combined with Mg-Al layered double hydroxide (Mg-Al LDH) as a solid base in chloroform. The OH groups in the Mg-Al LDH exchanged with the axial Cl ligands in [Fe ^III(TPFPP)Cl] to afford [Fe^III(TPFPP)OH], which functioned photocatalytically in the presence of Mg-Al LDH but was inactive in its absence. The band in the action spectrum agreed well with the Soret band in the UV/Vis absorption spectrum of [Fe^III(TPFPP)OH]. The spectral changes indicated the reduction of [Fe^III(TPFPP)OH] to [Fe ^II(TPFPP)] under visible-light irradiation, which in turn activates O₂ that could be used for the epoxidation of cyclohexene.

Full text of DOI:10.1002/cctc.201402131

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DOI: 10.1002/cctc.201402131
Photoactivation of Molecular Oxygen by an Iron(III)
Porphyrin with a Magnesium Aluminum Layered Double
Hydroxide for the Aerobic Epoxidation of Cyclohexene
Kentaro Teramura,*[a, b, c] Hideo Tsuneoka,[a] Kentaro Ogura,[a] Takashi Sugimoto,[a]
Tetsuya Shishido,[b, d] and Tsunehiro Tanaka*[a, b]
The photocatalytic aerobic epoxidation of cyclohexene pro-
ceeded under visible-light irradiation only if 5,10,15,20-
tetrakis(pentafluorophenyl)porphyrin iron(III) chloride ([FeIII-
(TPFPP)Cl]) was combined with Mg-Al layered double hydrox-
ide (Mg-Al LDH) as a solid base in chloroform. The OH groups
in the Mg-Al LDH exchanged with the axial Cl ligands in [FeIII-
(TPFPP)Cl] to afford [FeIII(TPFPP)OH], which functioned photo-
catalytically in the presence of Mg-Al LDH but was inactive in
its absence. The band in the action spectrum agreed well with
the Soret band in the UV/Vis absorption spectrum of [FeIII-
(TPFPP)OH]. The spectral changes indicated the reduction of
[FeIII(TPFPP)OH] to [FeII(TPFPP)] under visible-light irradiation,
which in turn activates O2 that could be used for the epoxida-
tion of cyclohexene.
Introduction
Molecular oxygen is an important oxidizing agent but because
it is very difficult to activate at ambient temperature and pres-
sure, studies on selective oxidation by O2 are reported rarely.
However, there are promising photocatalytic systems that can
be used for environmentally friendly and selective oxidation by
O2. Many research groups have focused on methods to acti-
vate O2, although the resulting photocatalytic activities remain
insufficient.[1] Recently, a number of reports have described ex-
tensive investigations of photocatalytic selective oxidation
using an organic complex (photosensitizer) immobilized on
a solid or organic–inorganic hybrid material. Yamashita et al.[2]
found that tris(2-2’-bipyridine)ruthenium(II) ([Ru(bpy)32+]) com-
plexes contained in a series of Y-type zeolite cages with vari-
ous extra-framework alkali metal cations exhibited photocata-
lytic activity for the selective oxidation of styrene and a-methyl
styrene. Y-type zeolites with light alkali metal cations acted as
more efficient hosts than those that contained heavy alkali
metal cations for the acceleration of photocatalytic epoxida-
tion. Shiragami et al.[3] reported that the silica-gel-supported
hydroxo(tetraphenylporphyrinato)antimony(V)
complex,
[SbTPP(OH)]+/SiO2, exhibited good activity for the photocata-
lytic oxidation of cyclohexene under visible-light irradiation.
Maldotti et al. reported photocatalytic aerobic oxidation using
porphyrin complexes[4] and/or a polyoxometalate[5] under UV-
light irradiation. In their study, an 5,10,15,20-tetrakis(penta-
fluorophenyl)porphyrin iron(III) chloride ([FeIII(TPFPP)Cl])/Nafion
system in ethanol or 2-propanol exhibited a good activity for
the selective photo-oxidation of cyclochexene under UV-light
irradiation (330–400 nm).[6] The main product was trans-cyclo-
hexane-1,2-diol monoethyl ether, and allylic oxidation products
(2-cyclohexen-1-ol and 2-cyclohexen-1-one) were formed in
very minor amounts. Hence, they suggested that the reaction
proceeds through the formation of 1,2-epoxycyclohexane,
which undergoes nucleophilic attack by the alcohol solvent
under acid conditions.
[a] Prof. Dr. K. Teramura, H. Tsuneoka, K. Ogura, T. Sugimoto,
Prof. Dr. T. Tanaka
Department of Molecular Engineering, Graduate School of Engineering
Kyoto University
Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2561
E-mail: teramura@moleng.kyoto-u.ac.jp
We have investigated an environmentally friendly CO photo-
oxidation[7] as well as the selective photocatalytic oxidation of
alkanes,[8] olefins,[9] and alcohols[9d] in the gas phase and vari-
ous hydrocarbons in the liquid phase[10] by using an orthovana-
date-like ((V=O)O3) species (i.e., a vanadium oxo complex) dis-
persed on various solid materials (e.g., SiO2 and Al2O3).[11] We
also determined that the selective photocatalytic oxidation of
alcohols[12] and amines[13] in the presence of O2 takes place
over niobium oxide (Nb2O5) under visible-light irradiation. The
photoactivation mechanism for the reaction of alcohol over
Nb2O5 is different from the typical electron transfer mechanism
on semiconductor photocatalysts: the alkoxide species ad-
sorbed on Nb2O5 is activated and transfers an electron to the
[b] Prof. Dr. K. Teramura, Prof. Dr. T. Shishido, Prof. Dr. T. Tanaka
Elements Strategy Initiative for Catalysts & Batteries (ESICB)
Kyoto University
1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245 (Japan)
[c] Prof. Dr. K. Teramura
Precursory Research for Embryonic Science and Technology (PRESTO)
(Japan) Science and Technology Agency (JST)
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
[d] Prof. Dr. T. Shishido
Department of Applied Chemistry, Graduate School of Urban Environmental
Sciences
Tokyo Metropolitan University
1-1 Minami-Osawa, Hachioji, Tokyo 192-0397 (Japan)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201402131.
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conduction band to reduce Nb5+ to Nb4+, which leaves a hole
in the alkoxide.[12c,13b]
1,2-epoxycyclohexane was obtained as the main product, and
2-cyclohexen-1-ol, 2-cyclohexen-1-one, cyclohexanol, and cy-
clohexanone were generated as byproducts. These products
were hardly formed in the absence of irradiation, [FeIII-
(TPFPP)Cl], or Mg-Al LDH, which indicates that the Fe-porphy-
rin complex works photocatalytically in cooperation with Mg-
Al LDH. The time course of the formation of 1,2-epoxycyclo-
hexane in the presence of a) O2 and b) Ar is shown in Figure 2.
In our previous communication, we reported the photocata-
lytic aerobic oxidation of cyclohexene in the presence of [FeIII-
(TPFPP)Cl] and various solid metal oxides and found that
a solid base was necessary to obtain epoxy compounds.[14]
Solid bases such as MgO, CaO, and ZnO enhanced the conver-
sion of cyclohexene to 1,2-epoxycyclohexane with high selec-
tivity, whereas amphoteric metal oxides such as Al2O3, ZrO2,
and TiO2 showed moderate additive effects. However, inert ma-
terials such as SiO2 and solid acids such as Ta2O5, Nb2O5, and
WO3 did not enhance the photocatalytic activity. If a solid base
catalyst such as Mg-Al layered double hydroxide (LDH) was
used, the maximum conversion of cyclohexene to 1,2-epoxycy-
clohexane could be achieved with high selectivity. The [FeIII-
(TPFPP)Cl]/Mg-Al LDH system functions as an effective photo-
catalyst for the aerobic epoxidation of alkenes under visible-
light irradiation, although [FeIII(TPFPP)Cl] is not immobilized on
the Mg-Al LDH. Our results reveal that solid bases, particularly
Mg-Al LDH, enhance the selectivity for 1,2-epoxycyclohexane
in the photocatalytic aerobic epoxidation of cyclohexene. The
[FeIII(TPFPP)Cl]/Mg-Al LDH system can be used for the effective
epoxidation of various alkenes under visible-light irradiation,
although “free-base porphyrins” can be used as photosensitiz-
ers in allylic oxidation. In this study, we attempted to identify
the active species in the photocatalytic epoxidation process
under visible-light irradiation using the [FeIII(TPFPP)Cl]/Mg-Al
LDH system and cyclohexene.
Figure 2. Time course of the formation of 1,2-epoxycyclohexane obtained in
the photocatalytic reaction of cyclohexene in the presence of a) O2 and
b) Ar after 8 h irradiation. Conditions: [FeIII(TPFPP)Cl]: 5.0ꢁ10À2 mm; cyclo-
hexene: 12.5 mm; Mg-Al LDH: 100 mg; solvent: chloroform; volume: 4 mL.
The photocatalytic epoxidation promoted by the [FeIII-
(TPFPP)Cl]/Mg-Al LDH system hardly proceeds in the absence
of O2, but the rate of 1,2-epoxycyclohexane formation is en-
hanced dramatically in its presence. The amount of 1,2-epoxy-
cyclohexane evolved in the presence of O2 (11.0 mmol) was ten
times that in the presence of Ar (1.14 mmol) after 8 h irradia-
tion. This indicates strongly that molecular O2, activated by the
[FeIII(TPFPP)Cl]/Mg-Al LDH system, plays a major role in the
photocatalytic aerobic epoxidation of cyclohexene. Hence, it
can be concluded that both [FeIII(TPFPP)Cl] as a homogeneous
catalyst and the solid base Mg-Al LDH as a promoter contrib-
ute to this photocatalytic process and activate molecular O2
through a visible-light response.
Results and Discussion
The yields of the oxidation products obtained in the photoca-
talytic aerobic oxidation of cyclohexene with and without [FeIII-
(TPFPP)Cl], Mg-Al LDH, and visible-light irradiation after 8 h (as
control experiments) are shown in Figure 1. In the presence of
[FeIII(TPFPP)Cl] and Mg-Al LDH under visible-light irradiation,
The time courses of the formation of the oxidation products
and the decrease in the cyclohexene substrate during the pho-
tocatalytic aerobic oxidations using a) [FeIII(TPFPP)Cl] and
b) H2TPFPP with Mg-Al LDH under the optimized conditions
are shown in Figure 3. In the presence of [FeIII(TPFPP)Cl], the
main 1,2-epoxycyclohexane product was formed at an initial
rate of 2.35 mmolhÀ1. In addition, minor amounts of 2-cyclo-
hexen-1-ol, 2-cyclohexene-1-one, cyclohexanol, and cyclohexa-
none were generated (Figure 3a). Clearly, the carbon balance
was maintained after 8 h of photoirradiation (Figure S1). In the
liquid phase, the amount of cyclohexene as a substrate was
detected as 31.4 mmol, and oxidation products were generated
in the following quantities: 1,2-epoxycyclohexane, 8.86 mmol;
2-cyclohexen-1-ol, 0.87 mmol; 2-cyclohexene-1-one, 1.39 mmol;
cyclohexanol, 1.21 mmol; and cyclohexanone, 1.50 mmol. Addi-
tionally, we observed 26.2 mmol CO2 in the gas phase; howev-
Figure 1. Yields of oxidation products obtained in the photocatalytic aerobic
oxidation of cyclohexene in the presence or absence of [FeIII(TPFPP)Cl] and
Mg-Al LDH under various conditions after 8 h irradiation. 1,2-Epoxycyclohex-
ane (&), 2-cyclohexen-1-ol (&), 2-cyclohexen-1-one (&), cyclohexanol (&),
and cyclohexanone (&). Conditions: [FeIII(TPFPP)Cl]: 5.0ꢁ10À2 mm; cyclohex-
ene: 12.5 mm; Mg-Al LDH: 100 mg; solvent: chloroform; volume: 4 mL.
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ties and cyclohexene consumption rate were virtually constant
after 20 h because of the competitive decomposition of [FeIII-
(TPFPP)Cl]. The porphyrin catalyst was not detected by UV/Vis
spectroscopy in the solution after 24 h reaction. The time
course for the consumption of [FeIII(TPFPP)Cl], estimated from
the reduction of the Soret band in the UV/Vis absorption spec-
tra, was consistent with that of the formation of 1,2-epoxycy-
clohexane, which indicates that the rate of epoxide formation
depends on the concentration of [FeIII(TPFPP)Cl] (Figure S2).
Even after the porphyrin complex was consumed completely,
oxidation could be resumed by the addition of fresh [FeIII-
(TPFPP)Cl] to the reaction mixture (Figure 4). In this case, the
Figure 3. Time courses of the formation of oxidation products and cyclohex-
ene consumption during photocatalytic aerobic oxidation a) in the [FeIII-
(TPFPP)Cl]/Mg-Al LDH/chloroform system and b) in the H2TPFPP/Mg-Al LDH/
chloroform system under the optimized conditions. Cyclohexene (*),
1,2-epoxycyclohexane (*), 2-cyclohexen-1-ol (*), 2-cyclohexen-1-one (*),
cyclohexanol (*), and cyclohexanone (*). Conditions: [FeIII(TPFPP)Cl] or
H2TPFPP: 5.0ꢁ10À2 mm; cyclohexene: 12.5 mm; Mg-Al LDH: 100 mg; sol-
vent: chloroform; volume: 4 mL.
Figure 4. Dependence of the addition of [FeIII(TPFPP)Cl] on the formation of
1,2-epoxycyclohexane in the photocatalytic aerobic oxidation of cyclohexene
in the [FeIII(TPFPP)Cl]/Mg-Al LDH/chloroform system. Conditions: [FeIII-
(TPFPP)Cl]: 1.0ꢁ10À2 mm; cyclohexene: 12.5 mm; Mg-Al LDH: 100 mg; sol-
vent: chloroform; volume: 4 mL.
er, the amount of cyclohexene consumed for the formation of
CO2 (4.37 mmol) is half as much as that for the formation of
1,2-epoxycyclohexane (8.86 mmol). The cyclohexene conversion
and selectivity for 1,2-epoxycyclohexane after 20 h photoirra-
diation were 52.4 and 54.5%, respectively. The turnover
number (TON) of cyclohexene, defined as the amount of prod-
ucts after 20 h (26.2 mmol) normalized to the addition of [FeIII-
(TPFPP)Cl] (0.2 mmol), reached 131. However, if the free-base
porphyrin (H2TPFPP) was used as the catalyst, only the allylic
oxidation products 2-cyclohexen-1-ol and 2-cyclohexen-1-one
were obtained (Figure 3b). H2TPFPP is generally known to act
as a photosensitizer under visible-light irradiation to generate
singlet oxygen species.[15] Singlet oxygen can abstract an allylic
hydrogen atom easily. As the epoxidation of cyclohexene pro-
ceeds preferentially in our photocatalytic [FeIII(TPFPP)Cl]/Mg-Al
LDH system, it is apparent that the main active O2 species in-
volved in the reaction is not singlet oxygen and that the mech-
anism of 1,2-epoxycyclohexane formation is different from that
of the allylic oxidation products.
concentration of [FeIII(TPFPP)Cl] was 1.0ꢁ10À2 mm. After 8 h,
the amount of 1,2-epoxycyclohexane remained virtually con-
stant and no [FeIII(TPFPP)Cl] was detected in the solution by
UV/Vis spectroscopy, which indicates its complete decomposi-
tion. The epoxide yields after the second and third additions of
catalyst were more or less the same as the yield obtained after
the first addition. This showed that the decomposition of [FeIII-
(TPFPP)Cl] competed with the formation of 1,2-epoxycyclo-
hexane.
The photocatalytic aerobic epoxidation of cyclohexene does
not proceed without Mg-Al LDH (Figure 1). The Fe-porphyrin
complex and the Mg-Al LDH function cooperatively in the oxi-
dative transformation under visible-light irradiation. The time
courses of the formation of 1,2-epoxycyclohexane and the con-
sumption of the substrate during the photocatalytic aerobic
oxidation with the [FeIII(TPFPP)Cl]/Mg-Al LDH system if a) Mg-Al
LDH was removed after 4 h and b) Mg-Al LDH was added after
4 h are shown in Figure 5. In both cases, the epoxide was gen-
erated only in the presence of Mg-Al LDH and none was pro-
duced in its absence.
The amounts of oxidation products increased with the pho-
toirradiation time and were correlated inversely with the cyclo-
hexene concentration shown in Figure 3a. The product quanti-
After the photocatalytic aerobic oxidation, the solid base
was examined by filtering the suspension to recover the Mg-Al
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Figure 6. UV/Vis absorption spectra of [FeIII(TPFPP)Cl] a) before (c) and
b) after (c) the addition of Mg-Al LDH and c) with an aqueous NaOH
solution (c).
[FeIII(TPFPP)OH] in acetonitrile in the presence of water.[16] Their
reported UV/Vis spectrum is similar to that shown in Figure 6b.
[FeIII(TPFPP)OH] was synthesized by the following method.
[FeIII(TPFPP)Cl] was mixed with aqueous NaOH at room temper-
ature for 1 h, and the resulting solution was extracted with
chloroform. The UV/Vis absorption spectrum of the solution is
consistent with that of [FeIII(TPFPP)OH] reported by Agarwala
and Bandyopadhyay[16] (Figure 6c). It was concluded that the
axial Cl ligands in [FeIII(TPFPP)Cl] are exchanged with the OH
groups in Mg-Al LDH.
Figure 5. Time courses of 1,2-epoxycyclohexane formation (*) and cyclo-
hexene consumption (*) in the photocatalytic aerobic oxidation of cyclo-
hexene in the [FeIII(TPFPP)Cl]/Mg-Al LDH/chloroform system in the case of
a) removal of Mg-Al LDH after 4 h and b) addition of Mg-Al LDH after 4 h.
Conditions: [FeIII(TPFPP)Cl]: 5.0ꢁ10À2 mm; cyclohexene: 12.5 mm; Mg-Al
LDH: 100 mg; solvent: chloroform; volume: 4 mL.
The apparent quantum efficiency of the photocatalytic aero-
bic oxidation of cyclohexene as a function of the wavelength
of incident light (action spectrum) is shown in Figure 7. The
band in the action spectrum is in good agreement with the
Soret band in the UV/Vis absorption spectrum of [FeIII-
(TPFPP)OH] (Figure 6b). Accordingly, the charge separation of
the Soret band contributes to the rate of photocatalytic aero-
bic epoxidation of cyclohexene under visible-light irradiation.
Although we performed cyclohexene oxidation using [FeIII-
(TPFPP)OH] alone, only a tiny amount of 1,2-epoxycyclohexane
LDH. The Mg-Al LDH was washed thoroughly with methanol,
followed by dissolution in aqueous H2SO4 solution (10%). The
resultant solution became cloudy upon titration with aqueous
AgNO3, which indicates the formation of AgCl (Figure S3).
Hence, it is likely that chloride ions are retained in the Mg-Al
LDH after the reaction. A suspension of Mg-Al LDH and chloro-
form without [FeIII(TPFPP)Cl] was irradiated, filtered, washed, re-
dissolved, and titrated. However, this sample did not become
cloudy upon titration with aqueous AgNO3. The photocatalytic
activity was dependent on the amount of Mg-Al LDH
introduced. The amount of 1,2-epoxycyclohexane
evolved increased gradually with the increasing
amount of Mg-Al LDH introduced up to 100 mg (Fig-
ure S4). Accordingly, the chloride ions must be de-
rived not from chloroform but from [FeIII(TPFPP)Cl].
We have confirmed that [FeIII(TPFPP)Cl] is not ad-
sorbed on Mg-Al LDH; therefore, the axial Cl ligand
in [FeIII(TPFPP)Cl] moves to the Mg-Al LDH surface.
The UV/Vis absorption spectra of [FeIII(TPFPP)Cl]
before and after the addition of Mg-Al LDH and
NaOH are shown in Figure 6. Initially, a typical Soret
band appears at l=410 nm, and two Q bands are
observed at l=567 and 629 nm (Figure 6a). After
the addition of Mg-Al LDH, the Soret band is blue-
shifted to l=406 nm, and only one Q band appears
Figure 7. UV/Vis absorption spectrum of [FeIII(TPFPP)Cl] after the addition of Mg-Al
LDH (c), and apparent quantum efficiency for the photocatalytic aerobic oxidation of
at l=564 nm (Figure 6b). Agarwala and Bandyopad-
hyay reported that [FeIII(TPFPP)Cl] is converted to cyclohexene as a function of the wavelength of incident light (*).
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was observed. It was not surprising that the epoxide was also
generated immediately upon the introduction of Mg-Al LDH
into the [FeIII(TPFPP)OH] solution. Therefore, we conclude that
solid bases such as Mg-Al LDH not only facilitate ligand ex-
change with [FeIII(TPFPP)OH] but also act as promoters in the
presence of [FeIII(TPFPP)OH] for the photocatalytic aerobic ep-
oxidation of cyclohexene.
As mentioned above, [FeIII(TPFPP)OH] rather than [FeIII-
(TPFPP)Cl] is activated under visible-light irradiation. The UV/
Vis absorption spectrum of [FeIII(TPFPP)Cl] with Mg-Al LDH
after degassing by the freeze–pump–thaw method followed
by visible-light irradiation is shown in Figure 8. The UV/Vis ab-
which is generated from [FeIII(TPFPP)Cl] with Mg-Al LDH in the
dark, is reduced to [FeII(TPFPP)] under visible-light irradiation.
The Soret band of [FeII(TPFPP)] was diminished in the presence
of O2 under visible-light irradiation, which indicates that O2 is
activated by [FeII(TPFPP)] not in the dark but under visible
light, and the active oxygen species decomposes [FeII(TPFPP)]
(autolysis) as shown in Figure 9a and b. However, a substrate
such as cyclohexene inhibits the decrease in the Soret band of
[FeII(TPFPP)] because epoxidation proceeds in preference to au-
tolysis (Figure 9c). It is recognized widely that an Fe-porphyrin
complex such as P-450 activates molecular O2 to result in the
formation of a high-valence Fe-oxoporphyrin complex that
could be used for the epoxidation of alkenes.[18] Currently, it is
not clear whether such a high-valence Fe-oxoporphyrin com-
plex is generated in our photocatalytic system. We conclude,
however, that [FeII(TPFPP)], which is converted from [FeIII-
(TPFPP)Cl] via [FeIII(TPFPP)OH] in the presence of Mg-Al LDH
under visible-light irradiation, acts as a photoactive species in
our system, and also plays a major role in the generation of
active oxygen species that trigger the epoxidation of cyclohex-
ene.
Conclusions
Solid base Mg-Al layered double hydroxide (LDH) enhances
the conversion of cyclohexene and the selectivity for 1,2-epox-
ycyclohexane in the photocatalytic aerobic epoxidation of cy-
clohexene in the presence of 5,10,15,20-tetrakis(pentafluoro-
phenyl)porphyrin iron(III) chloride ([FeIII(TPFPP)Cl]) under visi-
ble-light irradiation. The experimental results clarified that the
epoxidation proceeds only if both [FeIII(TPFPP)Cl] and Mg-Al
LDH coexist in chloroform in the presence of O2 under visible-
light irradiation. The OH group of Mg-Al LDH was exchanged
with the axial Cl ligand in [FeIII(TPFPP)Cl]. The resulting [FeIII-
(TPFPP)OH] could be used as a photocatalyst in the presence
of Mg-Al LDH. The redox reaction between FeIII and FeII takes
place under visible-light irradiation. O2 is activated by [FeII-
(TPFPP)] under irradiation with visible light, and active oxygen
species are generated that could be used for the epoxidation
of cyclohexene. The homogeneous–heterogeneous [FeIII-
Figure 8. UV/Vis absorption spectra of [FeIII(TPFPP)Cl] with Mg-Al LDH
a) after degassing by the freeze–pump–thaw method and b) after 10 min
visible-light irradiation. The dashed spectra show the changes in Figure 8b
in the dark between 10 and 270 min.
sorption spectrum after 10 min irradiation (Figure 8b) was dif-
ferent from that prior to irradiation (Figure 8a). The Soret band
at l=406 nm was shifted to l=413 nm, and the absorbance
increased in intensity after photoirradiation. The UV/Vis absorp-
tion spectrum after 10 min of visible-light irradiation was re-
stored gradually to the original UV/Vis absorption spectrum of
[FeIII(TPFPP)OH] in the dark with isosbestic points; however,
the absorbance of the restored spectrum was not perfectly
consistent with that of the origi-
nal spectrum because a small
amount of [FeIII(TPFPP)OH] de-
composed under irradiation. Bar-
tocci et al.[17] reported that the
reversible peak change is com-
parable to the redox reaction
between FeIII and FeII in the case
of iron(III) meso-tetrakis(2,2-
dichlorophenyl)porphyrin
(Fe-
(TDCPP)) under photoirradiation
(l>350 nm). The UV/Vis absorp-
tion spectrum of [FeIII(TPFPP)OH]
reduced by Na2S2O4 was almost
consistent with that after visible-
light irradiation (Figure S5). We
Figure 9. Decrease in the Soret band of [FeII(TPFPP)] in the presence of O2 a) in the dark, b) under visible-light irra-
concluded that [FeIII(TPFPP)OH], diation, and c) under visible-light irradiation together with cyclohexene.
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[2] K. Mori, M. Kawashima, K. Kagohara, H. Yamashita, J. Phys. Chem. C
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ble-light induced aerobic epoxidation of cyclohexene
effectively.
[4] a) A. Maldotti, C. Bartocci, G. Varani, A. Molinari, P. Battioni, D. Mansuy,
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Experimental Section
[FeIII(TPFPP)Cl] was purchased from Sigma–Aldrich Co. Mg-Al LDH
was fabricated by the following coprecipitation method. An aque-
ous solution that contained Mg(NO3)2 and Al(NO3)3 was added
slowly to aqueous NaOH at RT while the pH was kept at 10.0. The
resulting suspension was filtered after aging for 12 h at 333 K. The
filtrate was washed with distilled water and dried at 378 K for 12 h
in an oven. Prior to use, cyclohexene was distilled to remove water
and impurities and was subsequently passed through an alumina
column to remove any peroxides. Chloroform was used as re-
ceived. The photocatalytic reaction was performed in a closed
batch system using a Pyrex glass with a flat glass ceiling window
for illumination and a rubber balloon to maintain atmospheric O2
pressure. The synthesized Mg-Al LDH (100 mg) was dispersed in
a chloroform solution (4 mL) of [FeIII(TPFPP)Cl] (5.0ꢁ10À2 mm) and
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with O2 for 5 min and then irradiated with a 200 W Hg-Xe lamp
(San-Ei Electric Co., Ltd., UVF-204S Type C) equipped with fiber
optics, a collective lens, a mirror, and a L-42 cutoff filter (l>
400 nm). To determine the amount of CO2 evolved, the reaction
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was performed in
a conventional closed circulating system
equipped with an online GC with thermal conductivity detection
(TCD) using a GC-8A chromatograph (Shimadzu Corp.). For action
spectrum measurements, a monochromatic light device (JASCO)
equipped with a CT-10T monochromator (Bunko-Keiki Co., Ltd.)
and a SO-X500L 500 W Xe lamp (Ushio) was used. The organic
products were analyzed GC (GC-2014, Shimadzu Corp.) with flame-
ionization detection (FID) and a GC-mass spectrometer (Shimadzu,
GCMS-2010). UV/Vis absorption spectra were obtained by using
a JASCO Corp. V-670 spectrometer. Spectralonꢂ, supplied by Lab-
sphere Inc., was used as a standard reflection sample.
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This study was partially supported by the Program for Improve-
ment of Research Environment for Young Researchers from Spe-
cial Coordination Funds for Promoting Science and Technology
(SCF) commissioned by the Ministry of Education, Culture, Sports,
Science and Technology (MEXT), Japan; a Grant-in-Aid for Young
Scientists (B), 21760628, 2009, from the Japan Society for the Pro-
motion of Science (JSPS); and the Program for Elements Strategy
Initiative for Catalysts & Batteries (ESICB), commissioned by
MEXT, Japan.
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Keywords: epoxidation
·
iron
·
oxidation
·
oxygen
·
photochemistry
Received: March 13, 2014
Revised: April 17, 2014
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Published online on July 9, 2014
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