Aerobic benzylic C-H oxidation catalyzed by a titania-based organic-inorganic nanohybrid

Article abstract of DOI:10.1039/c6ra10191g

The incorporation of Co(OAc)₂ into ascorbic acid-coated TiO₂ nanoparticles easily provided a new heterogeneous visible-light active titania-based photocatalyst (TiO₂/AA/Co) which was characterized by different techniques such as FT-IR, XPS, ICP-AES, TGA and TEM. A red-shift of the band-edge and a significant reduction of the band-gap (2.8 eV) were demonstrated by UV-DRS. The as-prepared TiO₂/AA/Co nanoparticles possess excellent photocatalytic properties under visible light for the aerobic benzylic C-H oxidation of a structurally diverse set of benzylic alcohols and benzylic hydrocarbons in ethyl acetate as a safe solvent. The spectral results and leaching experiments revealed that the structural integrity of the solid catalyst is well preserved after being reused many times.

Full text of DOI:10.1039/c6ra10191g

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Aerobic benzylic C–H oxidation catalyzed by
a titania-based organic–inorganic nanohybrid
Cite this: RSC Adv., 2016, 6, 54649
Maasoumeh Jafarpour,* Fahimeh Feizpour and Abdolreza Rezaeifard*
The incorporation of Co(OAc)
2
into ascorbic acid-coated TiO
2
nanoparticles easily provided a new
/AA/Co) which was characterized by
heterogeneous visible-light active titania-based photocatalyst (TiO
2
different techniques such as FT-IR, XPS, ICP-AES, TGA and TEM. A red-shift of the band-edge and
a significant reduction of the band-gap (2.8 eV) were demonstrated by UV-DRS. The as-prepared
Received 20th April 2016
Accepted 1st June 2016
2
TiO /AA/Co nanoparticles possess excellent photocatalytic properties under visible light for the aerobic
benzylic C–H oxidation of a structurally diverse set of benzylic alcohols and benzylic hydrocarbons in
ethyl acetate as a safe solvent. The spectral results and leaching experiments revealed that the structural
integrity of the solid catalyst is well preserved after being reused many times.
DOI: 10.1039/c6ra10191g
www.rsc.org/advances
The urgent need for more sustainable chemical processes
has prompted the development of mild and selective oxidation
1
. Introduction
In recent years, much attention has been devoted to different methods based on the use of green reagents and solvents. In
1
–3
kinds of photocatalytic reactions such as water splitting, CO
2
this context, the direct use of oxygen as an oxidizing reagent is
4
31–34
reduction to “solar fuels”, and organic syntheses that use a very desirable feature for modern synthetic methods.
photocatalytic and photochemical reactions for “valorization”
Transition metal-based systems have been successfully used
of petroleum feedstocks and biomass. In this regard, among for the aerobic C–H oxidation to the corresponding carbonyl
5
various photocatalytic materials, most studies have been compounds. In this regard, several challenges exist, as the need
2
focused on TiO , because of its strong oxidizing power, high of low pressures of O especially in ammable organic solvents,
2
chemical inertness, high catalytic activity, long-term stability, mild reaction conditions, low catalyst loadings, and avoidance
6
low solubility in water, low toxicity, and low cost. However, of costly or toxic additives. Another main issue is the functional
recombination of photogenerated electron/hole pairs and the group tolerance and the chemoselectivity for the desired
inability to respond to visible light, restrict its use only to the transformation when other groups susceptible to oxidation are
7
35
narrow range of UV light (<5% of total sunlight). To overcome present. As far as heterogeneous catalyst is concerned, only
this limitations, traditional easily realizable methods such as a few limited reports on Co-based C–H oxidation, mostly
8
–10
11–13
nanosizing,
loading,
metal-ion and anion doping,
noble metal involving Schiff base complexes, have been presented by
1
4–16
17,18
36,37 38,39
and dye sensitization,
have been developed. different research groups,
including us.
In this work, a novel heterogeneous titania-based photo-
and surfactant templating. The sensitization by catalyst has been prepared by simple incorporating of Co(OAc)
surface adsorbates (including biomolecules) that do not absorb to ascorbic acid-coated TiO nanoparticles (Scheme 1). Its
visible light by themselves, is a less investigated method for visible-light photocatalytic activity promoted signicantly
19,20
Further developments in this line are self-assembly,
bio-
21,22
23
templating
2
2
24,25
visible light activation of wide band gap semiconductors.
For example, titanium dioxide nanoparticles modied with folic acetate as a safe solvent through a synergistic effect of titania
benzylic C–H oxidation of alcohols and hydrocarbons in ethyl
26,27
acid
cancer cells as a result of binding by folate receptors, oen over spectra along with ICP analysis conrmed that TiO
may be selectively accumulated at the surface of human with Co ascorbic acid complex (Scheme 2). The UV-vis and FT-IR
/AA/Co
expressed at the surface of this type of cells. The photosensi- catalyst maintain its structural integrity under the photo-
tization and surface modication of TiO lms with
catalytic oxidation conditions.
+)-ascorbic acid (AA) shis the onset of the photoresponse of
2
2
8
2
L
(
29,30
2
TiO up to 600 nm.
2
. Experimental
2
.1. General remarks
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science,
University of Birjand, Birjand, 97179-414 Iran. E-mail: mjafarpour@birjand.ac.ir;
rrezaeifard@birjand.ac.ir; rrezaeifard@gmail.com; Fax: +98 5632202515; Tel: +98
All chemicals were purchased from Merck or Fluka. Powder
X-ray diffraction (XRD) experiments were performed on
a Bruker D8-advance X-ray diffractometer with Cu Ka (l ¼
5
632202516
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2
Scheme 1 Schematic representation of the procedure for the fabrication of TiO /AA/Co nanohybrid.
polymerization between citric acid, ethylene glycol and
complexes is developed and ultimately sol became more viscous
as a wet gel. In the nal step of sol–gel process, the wet gel was
ꢁ
fully dried by direct heating on the hot plate at 150 C for 6 h
leading to a black powder. Then the calcination was done at
ꢁ
ꢁ
ꢀ1
6
00 C for 3 h with heating rate 5 C min . Finally, TiO2
4
0
nanoparticles with a white color were obtained.
2.3. Preparation of TiO
2
/AA nanohybrid
To 1.0 g TiO
2
particles was gradually added 10.0 mL of 0.01 M
ascorbic acid in water over a period of 20 min at room
temperature under ultrasonic agitation. Then, the reaction
mixture was stirred at room temperature for 8 h. Aerwards, the
2
Scheme 2 Aerobic benzylic C–H oxidation catalyzed by TiO /AA/Co
nanohybrid.
˚
1
.54178 A) radiation. FTIR spectra were recorded on a NICOLET
spectrometer. Thermogravimetric analysis (TGA) of powders
was performed in air by Shimadzu 50. TEM images were ob-
tained with a 906 E instrument (Zeiss, Jena, Germany). Samples
for TEM experiment were prepared by dispersion in ethanol,
sonicating for 30 min, and evaporating one drop of the solution
onto a 200 mesh formbar-coated copper grid. XP spectra were
ꢀ
10
recorded using a BESTEC GMBH (10
mw) with Al anode.
Diffuse reectance UV-vis spectra were recorded using an
Avantes spectrometer (Avaspec-2048-TEC model). Progress of
each reactions was monitored by TLC using silica-gel SIL G/UV
254 plates and also by GC on a Shimadzu GC-16A instrument
using a 25 m CBP1-M25 (0.32 mm ID, 0.5 mm coating) capillary
column. NMR spectra were recorded on Bruker Avance DPX 250
and 400 MHz instruments.
2
.2. Fabrication of TiO nanoparticles
2
To a solution of TiCl (0.05 mol) in a mixture of double-distilled
3
water and absolute ethanol (1 : 1) was added citric acid
(
0.15 mol) and ethylene glycol (0.15 mol) subsequently. The
ꢁ
resulting mixture was dissolved at 45 C under ultrasound for
1
5 min to give a clear violet solution. The solution was reuxed
ꢁ
at 120 C for 8 h which turned into a metal–citrate homoge-
neous complex with a little color change from clear violet to
black-violet. Aer cooling down, in order to bring about the
required chemical reactions for the development of polymeri-
zation and evaporation of the solvent, the sol was further slowly
ꢁ
heated at 90 C for 6 h in an open bath until a beige wet gel was
2 2 2
Fig. 1 FT-IR spectra of (a) nanostructure TiO (b) TiO /AA (c) TiO /AA/
obtained. During continuous heating at this temperature, the Co nanohybrid.
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product was centrifuged and washed by distilled water. Finally, progress was monitored by GC, and the yields of the products
ꢁ
TiO /AA nanohybrid was obtained aer drying for 4 h at 100 C. were determined by GC and NMR analysis.
2
2
.6. General procedure for aerobic oxidation of benzylic
2
.4. Preparation of TiO
To 0.2 g of TiO /AA nanohybrid was gradually added 1.0 mmol
Co(OAc) dissolved in ethanol over a period of 20 min at room
2
/AA/Co nanohybrid
hydrocarbons
2
2
To a mixture of benzylic hydrocarbons (1 mmol) and TiO /AA/
Co nanocatalyst (0.12 mol%, 0.006 g) in ethyl acetate (1 mL)
was added NHPI (10 mol%, 0.016 g) and the reaction mixture
2
temperature under ultrasonic agitation. Then, the as-obtained
mixture was reuxed for 12 h. Aerwards, the product was
centrifuged and washed by ethanol. Finally, TiO /AA/Co nano-
hybrid was obtained aer drying for 12 h at 100 C.
ꢁ
was stirred at 70 C under air, ultrasonic agitation and visible
2
ꢁ
light for the required time. The reaction progress was moni-
tored by GC, and the yields of the products were determined by
GC and NMR analysis.
2.5. General procedure for aerobic oxidation of benzyl
alcohols
2.7. Reusability of catalyst
To a mixture of benzyl alcohols (1 mmol) and TiO /AA/Co To a mixture of 1-phenylethanol (1 mmol) and TiO /AA/Co
2
2
nanocatalyst (0.06 mol%, 0.003 g) in ethyl acetate (1 mL) was nanocatalyst (0.06 mol%, 0.003 g) in ethyl acetate (1 mL) was
added NHPI (N-hydroxyphthalimide) (15 mol%, 0.024 g) and the added NHPI (15 mol%, 0.024 g) and the reaction mixture was
ꢁ
ꢁ
reaction mixture was stirred at 70 C under air, ultrasonic stirred at 70 C under air, ultrasonic agitation and visible light
agitation and visible light for the required time. The reaction for 25 min. Aer completion of the reaction, TiO /AA/Co
2
2
Fig. 2 XPS spectra of TiO /AA/Co nanohybrid (a) wide scan, (b) Ti 2p, (c) Co 2p, and (d) O 1s.
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ꢁ
nanocatalyst was separated by centrifuging followed by stability up to higher 200 C. The organic parts decomposed
ꢁ
decantation (3 ꢂ 5 mL ethanol). The isolated solid phase completely at 655 C.
(
TiO /AA/Co nanocatalyst) was dried under reduced pressure
2
and reused for next runs. Catalyst recovery was also investigated
in the aerobic oxidation of benzylic hydrocarbons in ethyl
acetate according to the above mentioned procedure.
3
.2. Catalytic activity
Catalytic experiments were initiated with the oxidation of
-phenylethanol (1 mmol) under air in the presence of NHPI,
1
knowing that the catalyst-free condition has not ability to
trigger the reaction under any conditions. Different factors were
screened to nd the optimum reaction conditions, with respect
to the conversion and selectivity of the 1-phenylethanol
oxidation.
3
. Results and discussion
3
.1. Synthesis and characterization of catalyst
At the rst step of this investigation, TiO nanoparticles of
approximately 18–20 nm diameters were prepared by the PC
2
40
A systematic examination of the solvent nature was per-
formed in various solvents, such as acetonitrile, acetic acid,
dichloroethane (C H Cl ), ethyl acetate (EtOAc), ethanol, water,
sol–gel method. The particles were subsequently coated with
ascorbic acid layer followed by complexation with easily avail-
able Co(OAc)2 to yield the corresponding supported nano-
catalyst (Scheme 1) containing 1.3 wt% Co according to ICP-AES
analysis. Therefore, each gram of heterogeneous catalyst
contains 0.22 mmol Co or Co ascorbic acid complex.
2
4
2
water/EtOAc and EtOH/EtOAc using 0.06 mol% of TiO /AA/Co in
2
the presence of NHPI (15 mol%) at different temperatures
The XRD pattern of TiO
2
nanoparticles exhibited two
different phases of TiO
2
including anatase (tetragonal, a ¼ b ¼
˚
˚
˚
3
2
.782 A, c ¼ 9.502 A) and rutile (tetragonal, a ¼ b ¼ 4.584 A, c ¼
˚
41,42
.953 A).
Fig. 1a shows the FTIR spectrum of TiO nanoparticles. The
2
ꢀ
1
presence of major bands at 450–775 cm is attributed to the
stretching vibrations of Ti–O groups. Broad peaks at 3400 and
ꢀ
1
1
638 cm , correspond to the surface adsorbed water and
43,44
hydroxyl groups.
Fig. 1b conrms the successful fabrication
of TiO
2
/AA composite. Vicinal hydroxyl groups of furan ring of
AA act as bidentate ligand forming a chelate-type coordination
45
bond with Ti atoms. C–O stretching vibration of Ti–O–C is
ꢀ
1
46
appeared in the range of 927 and 1010 cm
. The strong peak
ꢀ
1
at 1320 cm assigned to the O–H enediol of free acid dis-
appeared in the spectra of the complex indicating the coordi-
47
nation of the OC3 and OC2 atoms to the Ti centers.
Fig. 1c demonstrates signicant spectral changes in the
FT-IR spectra of TiO /AA/Co compared with those of TiO and
TiO /AA (Fig. 1a and b). The appearance of a weak band at
65 cm rationalized to stretching vibrations of the Co–O bond
conrming the complexation of Co with AA coated TiO . Major
2
2
2
ꢀ
1
6
2
2
Fig. 3 TEM of TiO /AA/Co nanohybrid.
ꢀ1
bands at 500–750 cm are attributed to the stretching vibra-
tions of Ti–O groups (Fig. 1c).
XPS was applied to detect elements containing nano-
composite and their oxidation states. The binding energies of
the particles were calibrated by taking the C 1s peak as reference
(
285 eV). The respective peaks for Ti 2p and Co 2p and O 1s
conrms the presence of these elements in the nanocatalyst
Fig. 2a). The high resolution narrow-scan XPS spectra of Ti 2p
(
and Co 2p and O 1s peaks of the sample are shown in Fig. 2b–d,
respectively. Two signals positioned at 458 and 464 eV represent
2
2
2
the Ti P3/2 and Ti P1/2. Also the peak of Co P3/2 located at
2
ꢃ782.89 eV and that of Co P1/2 at ꢃ799 eV, conrms that Cobalt
48–50
is present mainly as Co(II).
The binding energy at 531 eV is
48–51
attributed to the contribution of oxygen (O 1s).
Transmission electron microscopy (TEM) observations
clearly revealed spherical nanoparticles of TiO /AA/Co
2
composite with size ranging between 10–15 nm (Fig. 3). The
TGA curve of TiO
2
2
/AA/Co nanoparticles (Fig. 4) demonstrates its Fig. 4 The TGA of TiO /AA/Co nanohybrid.
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Fig. 5 The screening of the (i) solvent nature (ii) temperature (iii) NHPI amount (iv) catalyst amount on the oxidation of 1-phenylethanol (1 mmol)
catalyzed by TiO /AA/Co after 25 min under air, ultrasonic agitation and visible light.
2
Fig. 7 The screening of reaction conditions on the oxidation of 1-
phenylethanol (1 mmol) in EtOAc (1 mL) using NHPI (15 mol%) cata-
lyzed by TiO
2
/AA/Co (0.06 mol% Co). (A) Open flask under air, ultra-
ꢁ
Fig. 6 The screening of various oxidant on the oxidation of 1-phe- sonic agitation and visible light at 70 C, (B) capped flask under air,
ꢁ
nylethanol (1 mmol) in EtOAc (1 mL) using NHPI (15 mol%) catalyzed by ultrasonic agitation and visible light at 70 C, (C) open flask under air,
ꢁ
magnetic stirrer and visible light at 70 ꢁC, (D) capped flask under air,
TiO /AA/Co nanohybrid (0.06 mol%) at 70 C after 25 min under air,
ultrasonic agitation and visible light.
2
ꢁ
magnetic stirrer and visible light at 70 C, (E) using a continuous stream
ꢀ1
ꢁ
of O (5–7 mL min ) at 70 C.
2
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a
2
Table 1 Oxidation of benzylic alcohols by TiO /AA/Co nanocomplex
b
c,d
Entry
Alcohol
Time (min)
Product
Yield % (isolated yield%)
1
15
15
85 (76)
2
78 (70)
3
4
10
40
89 (84)
85 (78)
5
25
92 (85)
6
7
8
9
30
20
60
80
83 (76)
90 (84)
55 (47)
38
1
1
1
0
1
2
25
60
40
100 (93)
78 (70)
95 (88)
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Table 1 (Contd.)
b
c,d
Entry
Alcohol
Time (min)
Product
Yield % (isolated yield%)
1
1
3
55
72 (65)
4
5
60
84 (77)
1
120
26
a
ꢁ
The molar ratio of substrate/NHPI/catalyst was 100 : 15 : 0.06. The reactions were run under air, ultrasonic agitation and visible light at 70 C in
b
1
c
d
EtOAc (1 mL). The products were identied by H NMR spectroscopy. GC yield. The numbers in parenthesis are yields of isolated products. The
selectivity of products were >99% based on GC analysis.
under air (Fig. 5) and visible light. The best efficiency was ob- alcohols to the related ester and carboxylic acids was observed,
ꢁ
tained in EtOAc at 70 C (Fig. 5ii). The reaction was further respectively.
optimized with respect to amount of NHPI and catalyst
Fig. 5iii).
The chemoselectivity of the procedure was prominent.
While, the primary benzylic alcohol containing sulde group,
(
Inspection of the results in Fig. 5iii revealed that the effi- oxidized to the corresponding aldehyde (84% yield at 60 min),
ciency of oxidation was dependent on the NHPI amount, so that sulde group remained completely intact (Table 1, entry 14). It
the reaction did not proceed in the absence of NHPI under any should be noted that a modest yield was observed in the
conditions. NHPI has been reported to act as a free radical oxidation of 2-adamantanol (entry 15, 26%), while, no reactivity
52,53
oxidation catalyst.
Therefore, a radical mechanism may be toward linear aliphatic and allylic alcohols was observed under
suggested for title oxidation system using air similar to previous different conditions.
36,54–59
reports in the presence of cobalt complexes.
This was
The promising results obtained in the oxidation of benzyl
further supported by retarded oxidation of benzyl alcohol in the alcohols prompted us to apply this catalytic system for oxida-
presence of radical scavengers such as 2,6-di-tert-butyl-4- tion of benzylic hydrocarbons. Initially, optimized conditions
methylphenol under the same conditions.
used for oxidation of 1-phenyl ethanol (Table 1, entry 10) was
The oxidizing potential of other common oxidants such as applied for the oxidation of ethyl benzene which led to 65%
O , H O , TBHP, UHP, Oxone® and tetra-n-butylammonium acetophenone aer 1 h. This result induced us to nd a new
2
2 2
Oxone® (TBAOX) was evaluated under optimized conditions optimization conditions for efficient oxidation of benzylic
Fig. 6). The same oxidation efficiency was observed by contin- hydrocarbons. Ethyl benzene was oxidized completely aer 1 h,
(
uous stream of oxygen instead of air, albeit, air is superior from when 0.12 mol% TiO
2
/AA/Co and 10 mol% NHPI was employed
ꢁ
economic stand point. It should be noted that, the high oxida- in ethyl acetate at 60 C under air, visible light and ultrasonic
tion performance was observed under ultrasonic agitation with agitation (Table 2, entry 1). The good/high yields (60–98%) and
an open ask (Fig. 7). Under the optimized conditions using excellent selectivity (>99%) of ketones were achieved in the
0
.06 mol% TiO
2
/AA/Co nanocatalyst and 15 mol% NHPI under oxidation of other benzylic hydrocarbons (Table 2). The reaction
ꢁ
air, visible light and ultrasonic agitation at 70 C, 1-phenyl- also proceeded smoothly with less reactive 4-nitroethylbenzene
ethanol converted completely to the acetophenone within (entry 3), and 66% of the corresponding ketone was formed as
25 min.
a sole product within 4 h. The method possesses novelty
To establish the general applicability of the method, various regarding the chemoselectivity. 2-Phenyl ethanol oxidized
benzylic alcohols were subjected to the catalytic oxidation selectively to 2-hydroxy acetophenone while, hydroxyl group was
protocol (Table 1). Structurally and electronically diverse tolerated in the reaction (Table 2, entry 8). Moreover, ada-
benzylic alcohols were generally excellent substrates for this mantane (entry 9) gave only 2-adamantanone with 39%
system. No overoxidation of secondary and primary benzylic conversion.
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a
2
Table 2 Oxidation of benzylic hydrocarbons by TiO /AA/Co nanohybrid
b
c,d
Entry
Alkyl benzene
Product
Time (h)
1
Yield % (isolated yield%)
1
97 (86)
2
3
4
0.75
95 (85)
66 (57)
71 (65)
4
2
5
6
7
8
1.3
2
98 (92)
75 (67)
61 (54)
57 (49)
3
4
9
5
39
a
ꢁ
The molar ratio of substrate/NHPI/catalyst was 100 : 10 : 0.12. The reactions were run under air, ultrasonic agitation and visible light at 60 C.
b
1
c
d
The products were identied by H NMR spectroscopy. GC yield. The numbers in parenthesis are yields of isolated products. The selectivity
of products were >99% based on GC analysis.
3
.3. Investigation of photocatalytic properties
(Table 3). Accelerated reactions under light radiation particu-
larly UV light demonstrated the photocatalytic effect of TiO
core on the oxidation efficiency of TiO /AA/Co nanohybrid
Table 3, entry 3).
To verify this important issue, band gap energy values of
as-prepared TiO , TiO /AA and TiO /AA/Co were assayed. The
UV-vis-DR spectra of different samples along with tangent
drawn for band gap calculation are presented in Fig. 9. The
absorption threshold of pure titania at ꢃ400 nm indicates the
major light absorption in the UV region. Nevertheless,
2
The photocatalytic activity of the TiO /AA/Co nanoparticles on
2
2
the oxidation performance of title system was explored when it
was replaced by another nanometal oxides (Fig. 8). Actually, no
oxidation activity was observed in the oxidation of 1-phenyl-
ethanol using MoO3, m-ZrO2, Fe O and TiO2 and their
nanocomposites such as TiO /AA, TiO /Co, AA/Co, Fe O /AA,
Fe O /AA/Co, MoO /AA, MoO /AA/Co and ZrO /AA/Co under the
same conditions.
To support this claim, oxidation of 1-phenylethanol under
UV light, visible light as well as in dark was investigated
(
40
60
2
2
2
3
4
2
2
3 4
3
4
3
3
2
2
a red-shi of the band-edge of TiO /AA/Co to wavelengths
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Fig. 10 Recycling of the catalytic system for oxidation of 1-phenyl-
2
Fig. 8 The comparison of catalytic activity of TiO /AA/Co with other
ethanol and ethylbenzene under air and ultrasonic agitation using
nanocomposites and nanooxometals on the oxidation of 1-phenyl-
ꢁ
NHPI at 70 C in EtOAc.
ꢁ
ethanol (1 mmol) in EtOAc (1 mL) using NHPI (15 mol%) at 70 C after 25
min under air, ultrasonic agitation and visible light.
61
formula and Tauc plot, revealed signicant reduction of the
band-gap of TiO /AA/Co nanoparticles (2.8 eV) compared with
those of TiO /AA (3.1 eV) and especially TiO nanoparticles
2
a
Table 3 The screening of photocatalytic activity of TiO
2
/AA/Co
2
2
(
3.2 eV). Accordingly, a synergistic effect of cobalt ascorbic acid
b
Entry
Catalyst
Condition
Yield %
Time (min)
complex and TiO nanoparticles can be considered for visible-
2
light photocatalytic activity modication of title nanocatalyst.
1
2
3
4
5
6
7
8
9
TiO
TiO
TiO
TiO
TiO
TiO
TiO
TiO
TiO
2
2
2
2
2
2
2
2
2
UV light
UV light
UV light
Visible light
Visible light
Visible light
Darkness
Darkness
Darkness
10
5
100
5
8
100
2
25
25
15
25
25
25
25
25
25
/AA
/AA/Co
3.4. The catalyst reuse and stability
Recovery of TiO /AA/Co nanocatalyst was easy, efficient and
2
/AA
/AA/Co
practical. The catalyst was recovered by centrifuging and
decantation of the reaction mixture. It was then washed with
ethanol as safe solvents, dried under vacuum, and used directly
for next round of reaction without further purication. The
/AA
/AA/Co
1
3
a
The molar ratio of substrate/NHPI/catalyst was 100 : 15 : 0.06. The catalyst proved to be reusable over at least ve times in the
ꢁ
reactions were run under air and ultrasonic agitation at 70 C in
EtOAc (1 mL). The yield of products were obtained by GC analysis.
oxidation of ethylbenzene and 1-phenylethanol without
noticeable loss of activity (Fig. 10). Co and Ti were not detected
b
in the reaction solutions according to ICP-AES analysis using
hot ltration method. When, the solid-free ltrates were then
higher than 400 nm demonstrates higher visible light absorp-
tion than those of TiO , and TiO /AA samples, making it more
favorable for photocatalytic applications under visible light
stirred continuously under standard reaction conditions for the
above mentioned substrates, the yields of acetophenone were
less than 5% aer 12 h. More evidences for stability of catalyst
were achieved by comparison of FT-IR and UV-vis spectra of
2
2
radiation. Diffuse reectance UV-vis using the Kubelka–Munk
2 2 2
Fig. 9 Diffuse reflectance UV-vis spectra of (a) bare TiO NPs, (b) TiO /AA and (c) TiO /AA/Co.
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2
Fig. 11 Comparative FTIR (left) and UV-vis spectra (right) of fresh TiO /AA/Co nanocatalyst (a) with used one (b) after 5 times reuses.
2
Table 4 The comparison of catalytic activity of TiO /AA/Co nanoparticles with other previously reported catalysts for oxidation of 1-phenyl
ethanol
Entry
Catalyst
Catalyst amount
Conditions
Time (h)
Conv. (%)
Ref.
ꢁ
a
1
2
3
4
5
TiO
[CoL
CoL
CoPc@cell
Bis[2-[{1-phenylethylimino}methyl
phenolato-N,O]-cobalt
Cobalt complex
2
/AA/Co
@SMNP]
@TiO
0.06 mol%
5 mol%
2.8 mol%
0.05 g
EtOAc/air (70 C)
0.4
14
3
5.5
5.5
100
100
93
91
94
b
ꢁ
2
CH
CH
3
CN/O
CN/O
2
(70 C)
(70 C)
r.t.
(80 C)
38
39
62
63
ꢁ
2
2
c
3
2
o-Xylene/O
2
ꢁ
5 mol%
CH
3
CN/O
2
ꢁ
6
7
8
9
0.25
CH
3
CN/O
2
(60 C)
22
13
1
2.8
13
1.5
6
100
53
95
80
99
40
34
79
64
65
66
67
65
68
69
70
ꢀ1
ꢁ
Co(II)/ZnO
1.1 mmol g
5 mol%
0.01 g
0.05 mmol
45 mg
PEG-600, CO
CH CN/NBS (80 C)
2 2
/O (70 C)
ꢁ
Co(II) acetylacetonate
3
d
e
ꢁ
Co–Pc
[Bmim]Br /O
PEG-600, CO
2
(70 C)
ꢁ
10
11
12
13
CoCl
Cds/TiO
TiO
Au (DP673)/P25
2
$6H
2
O
2
/O
2
(70 C)
ꢁ
2
, hn
CH
BTF /O
3
CN/O
2
(5 C)
f
2
25 mg
20 mg
2
r.t.
g
2
Toluene/O
2
r.t.
4
a
d
b
c
This work. Cobalt Schiff base complex on the starch coated magnetic nanoparticles. Cobalt(II) phthalocyanine immobilized on cellulose.
e
f
g
2
Cobalt(II) phthalocyanine. 1-Butyl-3-methyl-imidazolium bromide. Benzotriouride. Au (DPy)/P25 catalysts prepared at higher calcination
temperatures (>673 K), deposition–precipitation (DP) method.
used TiO
2
/AA/Co with fresh one. These results clearly showed mmol) acetophenone was secured in 89% yield in the oxidation
/AA/Co within
that the structure of the catalyst remained almost intact aer of 1-phenyl ethanol in the presence of TiO
2

ve times recovering (Fig. 11).
25 min.
Table 4 shows the merit of this operationally simple catalytic
protocol in comparison with those previously reported methods, in
terms of oxygen source, conversion rate, catalyst loading and 4. Conclusions
especially, conditions used in the oxidation of 1-phenylethanol as
In conclusion, the nanocrystalline TiO surface modied with
cobalt ascorbic acid complex under ultrasonic agitation
2
model substrate. For example full oxidation of 1-phenylethanol to
acetophenone took only 25 min, under air in EtOAc as a safe
solvent and using a low catalyst loading of 0.06 mol%.
Therefore, the title methodology is environmentally benign
because of using air as an oxygen source, visible light as energy
source, ethyl acetate as a safe reaction media, reusing of an active
catalyst with very low catalyst loading, easy isolation of organic
products, and nally, no need for toxic reagents, or solvents.
These advantages make the method amenable to scalability
readily. As an example, under a semi-scaled up procedure (20.0
produced a novel low band-gap photocatalyst (TiO
size ranging between 10 and 15 nm. The band gap energy of
TiO decreased signicantly (0.4 eV) aer coating with cobalt
2
/AA/Co) with
2
ascorbic acid complex, according to DRS analysis. Enhanced
visible light photocatalytic activity was observed towards
aerobic benzylic C–H oxidation of benzylic alcohols and
benzylic hydrocarbons. The efficiency, selectivity and oxidative
stability of catalyst well documented. A synergistic effect of
2
cobalt ascorbic acid complex and TiO nanoparticles on the
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visible-light photocatalytic activity well demonstrated. The use 19 M. Nieto-Suarez, G. Palmisano, M. L. Ferrer, M. C. Gutierrez,
of air as oxygen source, visible light as energy source, ethyl
acetate as a safe reaction media as well as reusing of catalyst
S. Yurdakal, V. Augugliaro, M. Pagliaro and F. del Monte, J.
Mater. Chem., 2009, 19, 2070–2075.
with very low catalyst loading providing the scalability of the 20 J. G. Yu, L. J. Zhang, B. Cheng and Y. R. Su, J. Phys. Chem. C,
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methods are cost effective which enable the industrially 21 H. Zhou, T. X. Fan and D. Zhang, ChemCatChem, 2011, 3,
important reactions to be carried out efficiently under aerobic
513–528.
and practically attainable conditions.
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E. M. Sabio, D. Zhang and Q. X. Guo, Adv. Mater., 2010, 22,
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Acknowledgements
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3 J. Sarkar, V. T. John, J. He, C. Brooks, D. Gandhi, A. Nunes,
G. Ramanath and A. Bose, Chem. Mater., 2008, 20, 5301–
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Support for this work by Research Council of University of Bir-
jand is highly appreciated.
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