Hydrophobic Cr-Si mixed oxides as a catalyst for visible light-induced partial oxidation of cyclohexane

Article abstract of DOI:10.1039/c0nj00423e

Photocatalytic oxidation of cyclohexane (CHA) with molecular oxygen has been performed with hydrophobic Cr-Si mixed oxides as a catalyst under visible light irradiation (λ > 400 nm). The Cr-Si catalysts with highly dispersed chromate species were prepared by hydrolysis of tetraethyl orthosilicate (TEOS), organoalkoxysilane (1,2-bis(triethoxysilyl)ethane: BTESE), and Cr(NO₃)₃ followed by calcination. All of the Cr-Si catalysts promote partial oxidation of CHA to cyclohexanol (CHA-ol) and cyclohexanone (CHA-one) with high selectivity (>99%). Among the Cr-Si catalysts prepared with different TEOS and BTESE compositions, the catalyst prepared with 10% BTESE demonstrates the highest catalytic activity. The CHA vapor adsorption and electron spin resonance analysis reveal that the enhanced catalytic activity is due to the increased hydrophobicity of the catalyst surface, which enhances the access of CHA to the excited state chromate species. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/c0nj00423e

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www.rsc.org/njc | New Journal of Chemistry
Hydrophobic Cr–Si mixed oxides as a catalyst for visible light-induced
partial oxidation of cyclohexanew
Yasuhiro Shiraishi,* Hiroshi Ohara and Takayuki Hirai
Received (in Montpellier, France) 2nd June 2010, Accepted 26th July 2010
DOI: 10.1039/c0nj00423e
Photocatalytic oxidation of cyclohexane (CHA) with molecular oxygen has been performed with
hydrophobic Cr–Si mixed oxides as a catalyst under visible light irradiation (l 4 400 nm).
The Cr–Si catalysts with highly dispersed chromate species were prepared by hydrolysis of
tetraethyl orthosilicate (TEOS), organoalkoxysilane (1,2-bis(triethoxysilyl)ethane: BTESE), and
Cr(NO3)3 followed by calcination. All of the Cr–Si catalysts promote partial oxidation of CHA to
cyclohexanol (CHA-ol) and cyclohexanone (CHA-one) with high selectivity (499%). Among the
Cr–Si catalysts prepared with different TEOS and BTESE compositions, the catalyst prepared
with 10% BTESE demonstrates the highest catalytic activity. The CHA vapor adsorption and
electron spin resonance analysis reveal that the enhanced catalytic activity is due to the increased
hydrophobicity of the catalyst surface, which enhances the access of CHA to the excited state
chromate species.
1. Introduction
therefore be improved by an increase in the surface hydro-
phobicity of catalyst. The surface of silica materials can easily
be made hydrophobic by an introduction of hydrophobic
organic fragments. These organosilicas are synthesized
by two principal ways such as post-synthesis10 and direct
synthesis.11 The former consists of a reaction of solid silica
materials with an organochloro- or organoalkoxysilanes,
where the organic fragments are anchored on the material
surface. The latter consists of a hydrolysis of TEOS and
organosilanes such as 1,2-bis(triethoxysilyl)ethane (BTESE).
In the latter method, organic fragments are homogeneously
introduced into the silica framework, and a larger amount
of organic fragments are incorporated in the materials than
with the former method. In addition, the surface hydro-
phobicity is easily controlled by changing the composition
of TEOS and BTESE. Furthermore, the materials, when
calcined at high temperature, still show high surface hydro-
phobicity, even though the organic fragments are completely
decomposed.12
The partial oxidation of cyclohexane (CHA) to cyclohexanol
(CHA-ol) and cyclohexanone (CHA-one) has attracted much
attention because these products are the intermediates in
e-caprolactam synthesis.1 Of particular interest is the catalytic
CHA oxidation in heterogeneous systems with molecular
oxygen (O2) as an oxidant.2 Photocatalytic CHA oxidation
in liquid/solid heterogeneous systems with O2 has also been
studied extensively with various catalysts such as TiO2,3 Fe
porphyrin-modified TiO2,4 polyoxotungstate-modified SiO2,5
and V2O5-impregnated Al2O3.6 Some of these systems
promote partial oxidation of CHA with very high selectivity
(498%).3b,f,g,4a,5 All of these systems, however, require UV
light for catalyst activation.
Earlier, we reported a new photocatalytic CHA oxidation
process with Cr–Si binary oxide catalysts (Cr–Si catalysts),
prepared by hydrolysis of tetraethyl orthosilicate (TEOS) and
Cr(NO3)3 followed by calcination.7 The Cr–Si catalysts
containing highly dispersed chromate species promote partial
oxidation of CHA to CHA-ol and CHA-one with high
selectivity (499%) in a MeCN/catalyst heterogeneous system
under irradiation of visible light (l 4 400 nm).7
The rate of catalytic reactions in heterogeneous systems
depends on the surface hydrophobicity of catalysts because the
surface hydrophobicity strongly affects the affinity of reactant
with catalytic active site.8 Silica materials are basically hydro-
philic due to a large number of surface silanol groups,9 while
CHA is hydrophobic. The rate of CHA oxidation would
The purpose of the present work is to improve the rate of
CHA oxidation, while maintaining high oxidation selectivity.
We prepared hydrophobic Cr–Si catalysts by a hydrolysis of
TEOS, BTESE, and Cr(NO3)3 followed by calcination. We
hypothesized that the hydrophobic catalyst surface enhances
the access of hydrophobic CHA to the photoexcited chromate
species and improve the catalytic activity. We used four
kinds of Cr–Si(x) catalysts prepared with different BTESE
compositions [x (mol%) = BTESE/(BTESE + TEOS) ꢀ 100;
x = 5, 10, 20, and 50] and studied the effects of organic
modification on the catalytic activity and selectivity. We found
that Cr–Si(10) catalyst prepared with 10% BTESE shows the
activity twofold higher than that of Cr–Si(0) prepared without
BTESE,7 while maintaining high selectivity. The CHA vapor
adsorption and ESR analysis were carried out to clarify the
mechanism for oxidation enhancement on the Cr–Si(10)
catalyst.
Research Center for Solar Energy Chemistry, and Division of
Chemical Engineering, Graduate School of Engineering Science,
Osaka University, Toyonaka 560-8531, Japan.
E-mail: shiraish@cheng.es.osaka-u.ac.jp; Fax: +81 6-6850-6273;
Tel: +81 6-6850-6271
w Electronic supplementary information (ESI) available: N2 adsorption
and desorption isotherm of pg-Cr–Si(x) and Cr–Si(x) (Fig. S1). See DOI:
10.1039/c0nj00423e
c
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(at 400–530 nm; through the NaNO2 filter). After photo-
irradiation, the gas phase product was analyzed by GC-TCD
(Shimadzu; GC-8A). The resulting solution was recovered
by centrifugation and analyzed by GC-FID (Shimadzu;
GC-1700), where the product identifications were made by
GC-MS (EI) (Shimadzu; GCMS-QP5050A).
2. Experimental
2.1 Materials
All of the reagents were supplied from Wako, Tokyo Kasei,
and Sigma-Aldrich and used without further purification.
Water was purified by the Milli Q system. JRC-TIO-4 TiO2
particles (equivalent to Degussa P25) were supplied from the
Catalysis Society of Japan and used as a reference catalyst.
2.4 ESR measurement
The spectra were recorded at the X-band using a Bruker
EMX-10/12 spectrometer with a 100 kHz magnetic field
modulation at a microwave power level of 1.0 mW,15 where
microwave power saturation of the signals does not occur. The
magnetic field was calibrated using a 1,10-diphenyl-2-picryl-
hydrazyl (DPPH) as standard. Catalyst (50 mg) was placed
in a quartz ESR tube and treated with 100 Torr oxygen
(1 Torr = 133.3 Pa) at 673 K for 1 h. The tube was then
evacuated at 473 K for 2 h and cooled to room temperature.
Required quantity of O2 or CHA was introduced to the tube.
The tube was placed on an ESR sample cavity and photo-
irradiated at 77 K using a 500 W Xe lamp (USHIO Inc.). After
0.5 h of photoirradiation, measurement was started with
continued photoirradiation.
2.2 Preparation of Cr–Si(x) catalysts
Five kinds of Cr–Si(x) catalysts [x (mol%) = BTESE/(BTESE +
TEOS) ꢀ 100; x = 0, 5, 10, 20, and 50] were prepared by
calcination of the corresponding precursor gels, pg-Cr–Si(x),
according to the procedure described previously,7 as follows:
in a typical synthesis for x = 10, TEOS (45 mmol; 9.76 g),
BTESE (5 mmol; 1.84 g), and Cr(NO3)3ꢁ9H2O (0.05 mmol;
0.02 g) were dissolved in ethylene glycol (8.4 g) and stirred at
363 K for 3 h. Water (40 g) was added to the mixture and
stirred at 363 K for 5 h. The resultant was dried at 383 K for
12 h, affording a green-white powder of the precursor gel,
pg-Cr–Si(10). The gel was calcined at 773 K for 5 h under air
affording a yellow-white powder of Cr–Si(10). The properties of
pg-Cr–Si(x) and Cr–Si(x) are summarized in Tables 1 and 2,
respectively.
2.5 Other analysis
The Cr content of the catalyst was analyzed by an inductively-
coupled argon plasma atomic emission spectrophotometer
(Nippon Jarrell-Ash; ICAP575 Mark II) after washing with
HF.16 BET surface area and pore size distribution of the
catalysts were determined by N2 adsorption–desorption
analysis at 77 K using a BELSORP 18PLUS-SP analyzer
(BEL Japan, Inc.). The CHA monolayer adsorption capacity
per unit surface area of catalysts (qCHA) was determined by the
vapor adsorption measurement at 298 K using the BELSORP
system. Diffuse reflectance UV-vis spectra of the catalysts were
measured on a UV-vis spectrophotometer (Jasco Corp.; V-550
with Integrated Sphere Apparatus ISV-469) using BaSO4 as
reference. IR spectra were measured using an FT-IR-610
infrared spectrophotometer (Jasco Corp.). Particle size
distribution was determined using a Horiba LA-910 laser
light-scattering particle size analyzer.
2.3 Photoreaction
The respective catalysts (50 mg) were added to MeCN (9 mL)
containing CHA (1 mL) within a Pyrex glass tube (20 cm3;
f = 10 mm), and the tube was sealed using a rubber septum
cap. O2 gas was bubbled through the solution for 5 min at 273 K.
Each tube was photoirradiated with magnetic stirring by a Xe
lamp (2 kW; Ushio Inc.),13 filtered through an aqueous
NaNO2 (3 wt%) solution14 to give light wavelengths of
l 4 400 nm. The solution temperature during photoirradiation
was 313 K, and the light intensity was 16.0 mW mꢂ2
Table 1 Properties of the precursor gels, pg-Cr–Si(x)
Cr (mol%)a SBETb/m2 gꢂ1 Vpc/mm3 gꢂ1 Dpd/nm
pg-Cr–Si(0)
pg-Cr–Si(5)
0.076
0.077
789
764
748
734
675
684
629
583
514
477
3.5
3.3
3.1
2.8
2.8
pg-Cr–Si(10) 0.077
pg-Cr–Si(20) 0.078
pg-Cr–Si(50) 0.072
3. Results and discussion
3.1 Synthesis and properties of catalysts
a
b
=Cr/(Cr + Si) ꢀ 100. BET surface area determined by N2
The Cr–Si(x) catalysts [x (mol%) = BTESE/(BTESE +
TEOS) ꢀ 100; x = 0, 5, 10, 20, and 50] were synthesized by
calcination of the precursor gels, pg-Cr–Si(x), prepared by
c
adsorption–desorption data (Fig. S1, ESIw). Total pore volume.
Average pore diameter (=4Vp/SBET).
d
Table 2 Properties of Cr–Si(x) catalysts
Cr (mol%)a
SBETb/m2 gꢂ1
Vpc/mm3 gꢂ1
Dpd/nm
Particle sizee/mm
Vmf/mmol gꢂ1
qCHAg/molecules nmꢂ2
Cr–Si(0)
Cr–Si(5)
Cr–Si(10)
Cr–Si(20)
Cr–Si(50)
0.076
0.084
0.080
0.084
0.077
804
769
608
557
316
729
522
452
351
213
3.6
2.7
3.0
2.5
2.7
71.5
63.7
62.7
63.0
71.7
2.04
2.52
2.18
1.70
0.91
1.53
1.97
2.16
1.84
1.73
a
b
c
d
=Cr/(Cr + Si) ꢀ 100. BET surface area determined by N2 adsorption–desorption data (Fig. S1, ESIw). Total pore volume. Average pore
e
f
g
diameter (=4Vp/SBET). Determined by static laser scattering analysis. Monolayer adsorption capacity of CHA (p/p0 = 0.10–0.30). Adsorp-
tion capacity of CHA per unit surface area = (Vm/103) ꢀ (10ꢂ18/SBET) ꢀ N (N = Avogadro’s number).
c
2842 New J. Chem., 2010, 34, 2841–2846 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Fig. 2 IR spectra of Cr–Si(x).
observed for related materials.12a This is due to the condensation
of silanol (Si–OH) groups. During calcination, decomposition
of ethane fragments (Si–C cleavage) leads to a sequential reaction
between the resulting Si atom and the adjacent silanol group,
forming a siloxane (Si–O–Si) group.18 Fig. 2 shows IR spectra of
the catalysts. The Si–OH absorption observed at ca. 3500 cmꢂ1
decreases with an increase in the BTESE amount in the precursor
gel. This indicates that the removal of ethane fragments by
calcination leads to a silanol condensation and, hence, results
in an increase in surface hydrophobicity of the catalysts.
Fig. 1 UV-vis diffuse reflectance spectra of Cr–Si(x) and Cr2O3.
hydrolysis of TEOS, BTESE, and Cr(NO3)3. The properties of
the precursor gels and catalysts are summarized in Tables 1
and 2, respectively. As shown in Table 2, all of the Cr–Si(x)
catalysts contain 0.075–0.085 mol% Cr. Fig. 1 shows the
UV-vis diffuse reflectance spectra of the catalysts. These
catalysts show three distinctive absorption bands centered at
250, 350 and 450 nm, assigned to a ligand-to-metal charge
transfer (LMCT) band from O2ꢂ to Cr6+ transitions of the
chromate species, which are highly dispersed on silica matrices
and isolated from each other.17 In contrast, bulk Cr2O3 shows
red-shifted absorption band at 500–800 nm (dotted line),
assigned to a d–d transition of the octahedral Cr3+ species
in a Cr2O3 cluster.17 Such red-shifted absorption is not
observed for Cr–Si(x) catalysts, indicating that these Cr–Si(x)
catalysts contain highly dispersed chromate species.
3.2 Photocatalytic properties
Photocatalytic oxidation of CHA with O2 was carried out with
Cr–Si(x) catalysts. Table 3 summarizes the yields of CO2 and
partially-oxidized (PO) products (CHA-one and CHA-ol)
obtained by visible light irradiation (l 4400 nm) for 5 h by
a Xe lamp. As shown in runs 1–5, all Cr–Si catalysts promote
CHA oxidation under visible light irradiation. As shown in
run 6, a bulk Cr2O3 does not promote oxidation. This suggests
that, as described previously,7 the polymerized Cr3+ species
are inactive, and the highly dispersed chromate species are the
active species.
Table 2 (final column) summarizes the CHA monolayer
adsorption capacity per unit surface area of the catalysts
(qCHA) determined by the CHA vapor adsorption analysis.
The qCHA values of Cr–Si(x) (x = 5, 10, 20, 50) are 41.73,
which are higher than that of Cr–Si(0) (1.53). This indicates
that the surface hydrophobicity of catalysts prepared with
BTESE are indeed higher than that prepared without BTESE,
even after the ethane fragments are removed by calcination, as
As shown in runs 1–5, Cr–Si(x) prepared with BTESE
shows high PO product selectivity (499%), as does Cr–Si(0)
prepared without BTESE. This indicates that the increased
Table 3 Results of photocatalytic oxidation of cyclohexane (CHA) under visible light irradiationa
Yields/mmol
b
PO product selectivityc (%)
TONd
CHA-one
CHA-ol
CO2
Run
Catalyst
1
2
3
4
5
6
7
8
9
10
Cr–Si(0)
Cr–Si(5)
Cr–Si(10)
Cr–Si(20)
Cr–Si(50)
5.9
9.1
10.2
5.7
4.4
N.D.
3.7
10.0
9.8
3.0
4.0
4.2
2.8
2.1
N.D.
1.9
3.8
3.9
0.9
o0.2
o0.2
1.3
0.5
0.2
N.D.
11.0
1.0
1.1
0.3
499
499
99
99
99
14
19
22
12
10
e
Cr2O3
e
TiO2
50
99
99
97
Cr–Si(10)f
Cr–Si(10)g
Cr–Si(10)h
22
21
4
1.0
a
b
Reaction conditions: catalyst, 50 mg; MeCN, 9 mL; CHA, 1 mL; O2, 1 atm; photoirradiation time, 5 h; 4400 nm. Detection limit is
c
d
0.2 mmol. =[(CHA-one + CHA-ol)/(CHA-one + CHA-ol + (1/6)CO2)] ꢀ 100 (ref. 5). =[(CHA-one yields) + (CHA-ol yields)]/(Cr amount
e
on the catalyst). Catalyst, 10 mg. After the reaction (run 3), the catalyst was recovered by filtration and washed with MeCN, and used for
f
g
reaction. 2nd reuse. Without MeCN (10 mL CHA was used for reaction).
h
c
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Fig. 3 Pore size distribution of (open keys) pg-Cr–Si(x) and (closed keys) Cr–Si(x).
surface hydrophobicity of the catalysts scarcely affects the
oxidation selectivity. As shown in run 7, a common TiO2
photocatalyst (JRC-TIO-4; equivalent to Degussa P25) shows
a selectivity (50%) much lower than that of Cr–Si catalysts.
These data suggest that the Cr–Si(x) systems promote selective
oxidation of CHA under visible light irradiation. It must be
noted that the catalyst is reusable for further reaction. After
the reaction with Cr–Si(10) (run 3), the catalyst was recovered
by filtration. ICP analysis of the resulting solution does not
detect Cr leaching. In addition, as shown in runs 8 and 9, the
recovered catalyst, when reused for further CHA oxidation,
shows almost the same activity and PO product selectivity as
does the virgin catalyst (run 3). The data indicate that the
catalyst is reusable without loss of activity and selectivity.
Cr–Si(x) have a smaller surface area than the corresponding
pg-Cr–Si(x). In particular, Cr–Si(x) prepared with larger
BTESE amount shows a significant decrease in the surface
area. This indicates that the shrinkage of silica framework
actually occurs during the removal of ethane fragments by
calcination. Fig. 3 shows the pore size distribution of
pg-Cr–Si(x) and Cr–Si(x). The Cr–Si(x) prepared with larger
BTESE amount shows much broader pore distribution than
the corresponding pg-Cr–Si(x). These data clearly indicate
that the Cr–Si(x) prepared with larger amount of BTESE
has a constricted structure and contains plugged pores. In
such case, the CHA molecules cannot access the chromate
species confined within the plugged pores. This probably
results in very low catalytic activity on the Cr–Si(20) and
(50) catalysts. The pore plugging is supported by the CHA
adsorption capacity data. As shown in Table 2 (final column),
the qCHA values of Cr–Si(20) and (50) are much lower than
that of Cr–Si(10). This is because the plugged pores suppress
the CHA adsorption.
3.3 Photocatalytic activity
Table 3 (final column) summarizes the change in turnover
number (TON) for PO product formation [=(amount of PO
products formed)/(amount of Cr on the catalyst)] during 5 h
photoirradiation. The TON values increase with an increase in
the BTESE amount in the precursor gel; the value of Cr–Si(0)
is 14, whereas that of Cr–Si(5) and (10) catalysts are 19 and 22,
respectively. The increased catalytic activity is due to the
enhanced access of CHA to the photoexcited chromate species
on the catalyst surface. As shown in Table 2 (final column), the
CHA monolayer adsorption capacity per unit surface area
(qCHA) of Cr–Si(5) and (10) catalysts are higher than that of
Cr–Si(0). This indicates that a larger quantity of CHA is
adsorbed onto the hydrophobic surface of the catalysts; in
other words, the chromate species are surrounded by a larger
quantity of CHA. This therefore enhances the access of CHA
to photoexcited chromate species, resulting in an acceleration
of CHA oxidation on the Cr–Si(5) and (10) catalysts.
3.4 Properties of photoexcited chromate species
The properties of chromate species on the Cr–Si(x) catalysts
were studied by ESR analysis. The structure and catalytic
mechanism of Cr–Si(0) have been described in our previous
work,7 as shown in Scheme 1. Fig. 4.i (bold line) shows the
ESR spectra of Cr–Si(0), (10), and (50) catalysts measured
in vacuo at 77 K. The spectra consist of two major signals,
assigned to the reduced tetrahedral (Td5+; g> = 1.975,
gJ = 1.960) and square-pyramidal (Sp5+; g> = 1.987,
gJ = 1.900) species.19 The Sp species scarcely form excited
state and Td species act as the active site for oxidation.7 This is
confirmed by photoirradiation of the samples: as shown in
Fig. 4.i (normal line), photoirradiation leads to a decrease in
5+
5+
As shown in Table 3, the TON values of Cr–Si(20) and (50)
are much lower than that of Cr–Si(10), although these
catalysts are prepared with larger amount of BTESE. This is
because calcination of the precursor gel prepared with larger
amount of BTESE leads to a shrinkage of silica framework
and produces plugged pores, which suppresses the access of
CHA to the chromate species. As shown in Table 1 (second
column), the surface area of the precursor gels (pg-Cr–Si(x))
prepared with different BTESE amount are similar to each
other (670–780 m2 gꢂ1). As shown in Table 2 (second column),
the Td signal, while the Sp signal scarcely changes. The
5+
Td
signal decrease on these samples is due to the photo-
reduction of Cr5+ to Cr4+ (formation of Td4+* species). The
formation of Cr3+ and Cr2+ species can be ruled out because
distinctive Cr3+ signal does not appear in the absence and
presence of O2.19 This indicates that the Td4+* species acts as
the active site for these samples. The photoexcitation mechanism
of chromate species on Cr–Si(10) and (50) catalysts is therefore
explained with that of Cr–Si(0):7 as shown in Scheme 1a, the
catalyst prepared with a sol–gel method contains Cr species
c
2844 New J. Chem., 2010, 34, 2841–2846 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010

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Scheme 1 Photocatalysis mechanism of chromate species on Cr–Si(0) and (10) catalysts.
Fig. 4 ESR spectra (77 K) of (a) Cr–Si(0), (b) Cr–Si(10), and (c) Cr–Si(50) catalysts, when measured (i) in vacuo (bold) without photoirradiation
and (normal) with photoirradiation and measured (ii) with photoirradiation in the presence of 1 Torr CHA. The signal indicated by closed circle
symbol (g = 2.003) is assigned to the carbonaceous residues derived from BTESE.
arranged homogeneously with Si species. The reduction of
Td6+ to Td5+ in vacuo occurs due to a terminal oxygen (OT)–Si
higher than that of Cr–Si(0). This clearly suggests that the
enhanced access of CHA to the hydrophobic surface of
Cr–Si(10) promotes efficient CHA oxidation (Table 3).
bond formation associated with H2O removal from the adjacent
5+
species, photoinduced
Si species.20 On the stabilized Td
As shown in Table 3, Cr–Si(50) shows catalytic activity
much lower than Cr–Si(10). The surface area and qCHA data
(Table 2) suggest that the decreased activity of Cr–Si(50) is due
to the pore plugging by calcination, which suppresses the CHA
access to the Td4+* species. This is confirmed by ESR analysis
with Cr–Si(50). As shown in Fig. 4c.i, the spectra obtained
in vacuo without and with photoirradiation are similar to that
of Cr–Si(0) and (10), indicating that, on Cr–Si(50), Td4+* is
also produced by photoirradiation. As shown in Fig. 4a and
b.ii, photoirradiation of Cr–Si(0) and (10) with CHA produces
a [CHA–Td5+*] signal. However, as shown in Fig. 4c.ii,
Cr–Si(50) scarcely shows the [CHA–Td5+*] signal, where the
charge transfer between OT and Cr occurs easily, thus promoting
a formation of Td4+* species (Scheme 1c).
Fig. 4a and b.ii show the ESR spectra of Cr–Si(0) and (10)
obtained with CHA (1 Torr) under photoirradiation at 77 K.
A new Td5+* signal (g> = 1.978; gJ = 1.967) appears on both
samples. The OT formed by photoinduced electron transfer
has an electrophilic character and acts as a positive hole.21 The
new Td5+* signal is assigned to the [CHA–Td5+*] complex
4+
formed via a H attraction by the electrophilic OT on Td
*
(Scheme 1d), which leads to oxidation of Cr4+ to Cr5+.7 The
CHA oxidation on Cr–Si(10) therefore proceeds via a reaction
of this complex with O2, as is the case for Cr–Si(0)
(Scheme 1d). It must be noted that the intensity of
[CHA–Td5+*] signal on Cr–Si(10) is larger than that on
Cr–Si(0). This indicates that the Td4+* species on Cr–Si(10)
reacts efficiently with CHA and produces larger quantity of
[CHA–Td5+*] complex. This is because the access of CHA to
the Td4+* species is enhanced on the hydrophobic surface of
Cr–Si(10). The result agrees with the qCHA values of Cr–Si(10)
Td5+* signal still remains even after the CHA addition. This
4+
indicates that, on Cr–Si(50), the reaction of CHA with Td
*
is suppressed strongly. The results again suggest that the
plugged pores on Cr–Si(50) suppress the CHA access to the
Td species, resulting in low catalytic activity of Cr–Si(50).
It must be noted that, in the present process, the solvent
MeCN is important for catalytic activity. As shown in Table 3
(run 10), the product yields during reaction in pure CHA are
c
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much lower than that obtained with MeCN (run 3). One of the
possible reasons for this effect is the stability enhancement of
the Td4+* species in polar MeCN. This is supported by ESR
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5+
analysis. As shown in Fig. 4.i, the Td spectra for Cr–Si(0),
(10), and (50) catalysts obtained in vacuo are similar. Photo-
irradiation of the catalysts leads to an intensity decrease due
to the formation of Td4+* species. The intensity decrease
becomes smaller with an increase in the BTESE content of
the catalysts, indicating that the quantity of Td4+* decreases
4 (a) R. Amadelli, M. Bregola, E. Polo, V. Carassiti and A. Maldotti,
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with the BTESE content increase. The Td4+* species has a
charge-transferred electronic structure.21 The decreased Td
*
4+
quantity is probably because the Td4+* species is destabilized
by hydrophobic surface of the catalyst. Addition of polar
MeCN to the solution probably stabilizes the Td4+* species
as compared to the case with pure CHA. This may be the
reason for the activity improvement by the MeCN addition.
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4. Conclusion
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Hydrophobic Cr–Si(x) catalysts containing highly dispersed
chromate species were synthesized by a hydrolysis of TEOS,
BTESE, and Cr salts. These catalysts were used for CHA
oxidation with O2 under visible light irradiation. All Cr–Si(x)
catalysts promote partial CHA oxidation with high selectivity.
Among the catalysts, Cr–Si(10) prepared with 10% BTESE
shows the highest catalytic activity. The enhanced activity of
Cr–Si(10) than that of Cr–Si(0) prepared without BTESE is
due to the increase in surface hydrophobicity of the catalyst,
which accelerates the reaction of CHA with the excited state
chromate species (Td4+*). In contrast, Cr–Si(x) prepared with
420% BTESE shows the decreased activity. This is because
the calcination of precursor gel leads to a shrinkage of the
pores. This suppresses the access of CHA to the Td4+* species
confined within the plugged pores.
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This work was supported by the Grant-in-Aid for Scientific
Research (No. 19760536) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan (MEXT).
H.O. thanks the Japan Society of Promotion of Science (JSPS)
Research Fellowships for Young Scientist.
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2846 New J. Chem., 2010, 34, 2841–2846 This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010