Cu(ii) vitamin C tunes photocatalytic activity of TiO2 nanoparticles for visible light-driven aerobic oxidation of benzylic alcohols

Article abstract of DOI:10.1039/d0ra00075b

The incorporation of Cu(OAc)₂ into ascorbic acid coated TiO₂ nanoparticles easily provided a new heterogeneous visible-light active titania-based photocatalyst (TiO₂-AA-Cu(ii)) 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 reduction of the band-gap (2.8 eV vs. 3.08 for TiO₂) were demonstrated by UV-DRS and Tauc plots. The combination of the as-prepared TiO₂-AA-Cu(ii) nanoparticles with TEMPO and molecular oxygen (air) afforded an active catalytic system for the selective oxidation of diverse set of benzylic alcohols under solvent-free conditions. A photoassisted pathway was confirmed for oxidation reactions evidenced by good correlation between apparent quantum yield (AQY) and diffuse reflectance spectra (DRS) of the as-prepared nanohybrid. The spectral data and recycling experiments demonstrated the structural stability of the title copper photocatalyst during oxidation reactions.

Full text of DOI:10.1039/d0ra00075b

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PAPER
Cu(II) vitamin C tunes photocatalytic activity of TiO2
nanoparticles for visible light-driven aerobic
oxidation of benzylic alcohols†
Cite this: RSC Adv., 2020, 10, 12053
Narges Pourmorteza, Maasoumeh Jafarpour, * Fahimeh Feizpour
and Abdolreza Rezaeifard
*
The incorporation of Cu(OAc)
2
into ascorbic acid coated TiO
2
nanoparticles easily provided a new
-AA-Cu(II)) 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 reduction of the band-gap (2.8 eV vs. 3.08 for TiO ) were demonstrated by UV-DRS and Tauc plots.
The combination of the as-prepared TiO -AA-Cu(II) nanoparticles with TEMPO and molecular oxygen
air) afforded an active catalytic system for the selective oxidation of diverse set of benzylic alcohols
photoassisted pathway was confirmed for oxidation reactions
heterogeneous visible-light active titania-based photocatalyst (TiO
2
2
2
(
Received 3rd January 2020
Accepted 15th March 2020
under solvent-free conditions.
A
evidenced by good correlation between apparent quantum yield (AQY) and diffuse reflectance spectra
DRS) of the as-prepared nanohybrid. The spectral data and recycling experiments demonstrated the
structural stability of the title copper photocatalyst during oxidation reactions.
DOI: 10.1039/d0ra00075b
(
rsc.li/rsc-advances
2
+
reports regarding the combination of Cu and ascorbate
1
Introduction
1
1,12
(
vitamin C) halt tumor growth in mice.
As a part of our ongoing project, developing new visible light
Transition metal ions are playing an important role in bio-
1
,2
logical processes in the human body. They are found either driven photocatalytic systems by surface modication of TiO
2
13–15
at the active sites or as structural components of a good nanoparticles by vitamins–metal complexes,
in the present
3
,4
number of enzymes. Thus, study of the coordination chem- study, a novel heterogeneous nanostructured bio-photocatalyst
istry of biologically important metal ions with bio-relevant is prepared by incorporation of simple and easily available
ligands has been always an active research subject in bio- Cu(OAc)
inorganic chemistry. In this line of research, metal containing particles under ultrasonic agitation. Copper–vitamin C complex
vitamins are increasingly becoming important particularly in supported on TiO nanoparticles is explored as an active pho-
2 2
compound into ascorbic acid-coated TiO nano-
2
drug design and nutrition, with the hope of improving and tocatalyst for the selective oxidation of benzylic alcohols via
enriching the quality of existing vitamins, thereby serving as ideal green process, i.e. using molecular oxygen in air as oxidant
5
–7
a better substitute as chemotherapeutic agents.
In the and be free of any solvent under visible light irradiation
search for novel metal complexes, that combine high activity (Scheme 1). Spectral data, leaching experiment and recycling
with low toxicity, the study of metal complexes has continued test proved the stability of the as-prepared nanohybrid making
8
–10
to attract attention of some coordination chemists.
This it competent catalyst for practical goals.
has also provided useful outlets for basic research in coordi-
nation chemistry. Among metal containing vitamins, the Cu(II)
vitamin C complex attracted the most attention, because both
Cu and vitamin C are essential for the biosynthesis of the
pigment melanin and are preferentially absorbed by the
malignant melanoma tissues. There are also invaluable
2
. Experimental
2
+
2
Note: general remarks and step by step preparation of TiO -AA-
Cu(II) nanohybrid catalyst are given in ESI.†
2.1. General procedure for aerobic oxidation of benzylic
alcohols catalyzed by TiO -AA-Cu(II) nanohybrid
2
Catalysis Research Laboratory, Department of Chemistry, Faculty of Science,
University of Birjand, Birjand, 97179-414, Iran. E-mail: mjafarpour@birjand.ac.ir;
Jafarpouryas@gmail.com; rrezaeifard@birjand.ac.ir; Tel: +98 5632202515; +98
To a mixture of benzyl alcohols (0.125 mmol) and TiO -AA-Cu(II)
2
nanohybrid (3 mg) was added TEMPO (0.0192 mmol) and the
ꢀ
reaction mixture was stirred at 70 C under air, visible light
5
†
1
632202516
using a full spectrum CFL bulb (40 W) and solvent free condi-
tions for the required time. The reaction progress and the yields
Electronic supplementary information (ESI) available. See DOI:
0.1039/d0ra00075b
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Scheme 1 Aerobic oxidation of benzyl alcohols in the presence of TiO -AA-Cu(II) nanohybrid.
of the products were monitored by GC. The isolated products of Cu(II) with AA coated TiO
were obtained by plate chromatography eluted with n-hexane/ samples show a broad peak at around 3000 cm corresponding
2
. As depicted in Fig. 1, all of the
ꢁ
1
ꢁ
1
ethylacetate (10 : 3).
to the hydroxyl groups. Broad peak at 1625 cm corresponds to
the surface adsorbed water. The bands in the region of 500–
ꢁ
1
7
50 cm are attributed to the stretching vibrations of Ti–O
3
. Result and discussion
17
groups. The coating of AA on TiO (Fig. 1b) is substantiated by
2
3.1. Synthesis and characterization of TiO -AA-Cu(II)
2
C–O stretching vibration of Ti–O–C appeared in the range of 927
and 984 cm
nanohybrid
ꢁ1 18
ꢁ1
. The strong peaks at about 1411 and 1560 cm
First, TiO2 and TiO -AA nanoparticles were synthesized and are attributed to the acetate group. The appearance of a band at
2
ꢁ
1
16
puried according to our previous report (for experimental 563 cm in Fig. 1c is rationalized to the stretching vibration of
details, see the ESI†). For fabrication of TiO -AA-Cu(II) nano- Cu–O bond.
19
2
The survey XPS spectrum of TiO -AA-Cu(II) nanohybrid
hybrid, 0.2 g of TiO
2
-AA is dispersed in 5 mL deionized (DI)
2
reveals the presence of Ti 2p, Cu 2p, O 1s and C elements
water through sonication for 5 min. Then, 1.0 mmol Cu(OAc)
2
(
Fig. 2). Two broad peaks at 458 and 464 eV are attributed to the
dissolved in ethanol is dropped into the suspension under
ultrasonic agitation at room temperature for 20 min. The as-
obtained mixture is reuxed for 12 h (Scheme S1†). Aer-
20
Ti 2p3/2 and Ti 2p1/2, respectively. The peak at 531 eV is
assigned to the O 1s. Also, the signals at 935.32 and 955.28 eV
21
22–24
wards, the obtained product is centrifuged, washed with correspond to 2p3/2 and 2p1/2 of Cu(II), respectively.
ꢀ
ethanol and dried at 100 C for 12 h under vacuum. The Cu
TGA curve of catalyst (Fig. S2†) demonstrates its thermosta-
ꢀ
content of as-prepared catalyst was 1.23% based on ICP-AES bility up to 200 C and the organic parts decomposed
ꢀ
2
completely at 678 C.
analysis.
TEM images of nanohybrid exhibited
morphology with an average size of 10–15 nm (Fig. S1†).
Comparative FT-IR spectra of TiO , TiO -AA and TiO
Cu(II) nanohybrid depicted in Fig. 1, conrms the complexation
Tauc plot, (ahn) vs. (hn), was used to determine the optical
a
spherical
bandgap. As shown in Fig. 3, the band gap of TiO (3.08 eV)
reduced gradually during modication with AA (TiO -AA, 3.0 eV)
2
and complexation with Cu(II) (Cu-AA-Cu(II), 2.8 eV) making it
a visible-light sensitive photocatalyst.
2
2
2
2
-AA-
3
.2. Photo-induced aerobic oxidation of benzylic alcohols
Oxidation of 4-chlorobenzyl alcohol (0.125 mmol) in the pres-
ence of TiO -AA-Cu(II)/TEMPO photocatalytic system was
2
chosen as a model reaction. A series of experiments was con-
ducted to optimize the effect of solvent, amount of catalyst and
TEMPO, temperature, and nature of oxidants (Fig. S3 and S4†).
According to optimization experiments the reaction
proceeds more effective in the absence of any solvent (Fig S3a†).
A temperature variation study showed that the conversion
steadily increases from 50 to 90% as the temperature increases
ꢀ
from 25 to 70 C with maintaining the selectivity (Fig S3b†). A
survey of the results shown in Fig. S3c† revealed that a catalyst
loading of 3 mg (0.4 mol% based on Cu) leads to the desired
yield of product. Also, we found that TEMPO is essential to
trigger the reaction and access to the highest catalytic perfor-
mance (Fig S3d†). When air was replaced by pure O as well as
2
commercially available oxidants, less activity and selectivity
were observed (Fig. S3e†). The parent materials such as TiO2,
2 2
TiO -AA, AA, Cu(OAc) and also ZnO were inferior under opti-
2 2 2
Fig. 1 FT-IR spectra of (a) nanostructure TiO (b) TiO -AA (c) TiO -
AA-Cu(II) nanohybrid.
mized conditions (Fig. S4†). Thus, title catalytic aerobic
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Fig. 2 XPS spectra of TiO -AA-Cu(II) nanohybrid (a) wide scan, (b) Ti 2p, (c) Cu 2p, and (d) O 1s.
procedure exhibits the highest efficiency under visible light
Next, we subjected various benzylic alcohols to the reaction
irradiation, using a benzyl alcohol : TEMPO molar ratio of 6.5 conditions to assess the scope of the method. Our data summa-
(
0.125 : 0.0192) and 3 mg (0.4 mol% based on Cu) catalyst under rized in Table 1 demonstrated the high efficiency of the title
ꢀ
solvent free conditions at 70 C.
photocatalytic system for the visible light-induced aerobic oxida-
tion of structurally and electronically different benzyl alcohols.
2 2 2
Fig. 3 UV-Vis spectra and DRS of (a) bare TiO NPs, (b) TiO -AA and (c) TiO -AA-Cu(II) nanohybrid.
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a
2
Table 1 Oxidation of benzylic alcohols in the presence of TiO -AA-Cu(II)
b
c
Entry
Alcohol
Product
Isolated yield (%)
1
96
96
2
3
4
94
96
5
6
89
95
7
8
94
60 (3 h)
9
68 (3 h)
1
0
98
1
1
1
2
68 (5 h)
78 (5 h)
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Table 1 (Contd.)
b
c
Entry
Alcohol
Product
Isolated yield (%)
1
3
4
85
d
1
85 (6 h)
a
ꢀ
Reaction condition: 0.125 mmol alcohol, TEMPO (0.0192 mmol), cat. (0.003 g) and the reactions were run under solvent free conditions at 70 C for
b
2
h under air and visible light (CFL, 40 W). The products were identied by comparison with authentic sample retention times of GC analysis and
c
d
NMR spectra. The selectivity of products were >99% based on GC analysis. 15% of (4-methanesulnyl-phenyl)-methanol was obtained.
Fig. 4 Dependence of 4-chloro benzaldehyde yield on the irradiation wavelength (A), and the action spectrum of the photocatalytic reaction, in
which the light driven conversion is plotted against the irradiation wavelength (B).
However, the oxidation performance affected by electronic prop-
erties of substrates. Electron-withdrawing groups on the phenyl
rings of alcohols retarded the reaction (Table 1, entries 8 and 9).
Both primary and secondary benzylic alcohols delivered the cor-
responding carbonyl compounds in good to excellent yields. It
should be mentioned that no trace of ester or benzoic acid was
observed resulting from over oxidation of secondary and primary
alcohols, respectively. To demonstrate the chemoselectivity of the
method, 4-methylsulfanyl benzyl alcohol carrying oxidative
sensitive sulfur atom was subjected to the oxidation procedure.
The related carbonyl compound was formed as major product
(85%) accompanied with 15% of the pertinent sulfoxide (4-
methanesulnyl-phenyl)-methanol resulting from sulde group
oxidation (Table 1, entry 14). It should be noted that the photo-
catalytic system was unable to oxidize the aliphatic alcohols and
heterocyclic alcohols under different conditions.
Fig. 5 Photocatalytic action spectrum for synthesis of 4-chlor-
3.3. Photocatalytic activity
2
obenzaldehyde using TiO -AA-Cu(II) photocatalyst.
The photocatalytic activity of TiO
2
-AA-Cu(II) nanohybrid was
investigated by the oxidation of 4-chlorobenzyl alcohol under
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2
Table 2 Comparison of oxidative activity of TiO -AA-Cu(II) nanohybrid with other Cu based catalysts in the oxidation of benzyl alcohol
Entry
Catalyst
TiO -AA-Cu(II)
Catalyst (mol%)
Conditions
Time (h)
Yield (%)
Ref.
ꢀ
ꢀ
1
2
3
4
5
6
7
8
9
2
0.003 g (0.4 mol%)
1
2
5
70 C/s.f/TEMPO/vis. light/air
2
9
96
90
97
94
80
97
86
92
77.8
87
96.9
79
This work
a
b
SBA@AP -LPro -CuOAc
Salophen-copper(II)
CuCl
50 C/toluene/TEMPO/Cs
2
CO
3
/O
2
26
27
28
29
30
31
32
33
34
35
36
37
rt/MeCN/TBHP/NaOH/air
24
10
24
18
9
26
24
20
2.5
24
6
c
rt/DMAP /TEMPO/H
2
O/air
d
e
ꢀ
ꢀ
PS -PEG-TD
2
Cu(OAc)
2
20
80 C/heptane/TEMPO/air
allyl
f
[Cu(L
)
2
]
0.025 mmol
5
5
31 mg
1
10 wt%
10
5
25 C/toluene/TEMPO/air
g
h
CuBr/UiO-66-NH-PC
[MCM-41-bpy-CuI]
CuO-rectorite
rt/CH
3
CN/TEMPO/NMI /air
ꢀ
50 C/EtOH/TEMPO/NH
3
/air
ꢀ
50 C/H
2
O/K
2
CO
3
/TEMPO/O
O/NaCl/O
2
ꢀ
10
11
12
13
(NH
4
)
4
[CuMo
6
O
18(OH)
6
]
60 C/MeCN : H
2
2
ꢀ
Cu–Mn oxide/C
80 C/CH
2
Cl
O/TEMPO/O
OH/L-proline/Na
2
/TEMPO/O
2
ꢀ
CuFe
CuBr
2
O
4
100 C/H
2
2
rt/CH
3
2
CO
3
/TEMPO/air
92
a
e
b
c
d
(
3-Aminopropyl) triethoxysilane. (S)-1-(3,5-Dichloro-2-hydroxybenzyl) pyrrolidine-2-carboxylic acid. 4-Dimethylaminopyridine. Polystyrene.
f
g
h
Triazine-based polyethyleneamine dendrimer. N-Allyl-o-hydroxyacetophenoniminato. Picolinoyl chloride. N-Methylimidazole.
air as oxidant. The visible-light source for the photocatalytic photochemical method including a visible light-induced
reaction was a 40 W CFL bulb (400 to 800 nm) equipped with catalyst with low dosage and high recyclability, desired
different cut-off lters. A reaction subjected on the full spec- reaction time, high product yield and excellent selectivity and
trum irradiation (without any cut-off lters) gives 98% 4-chlor- particularly environmentally benign conditions used in this
obenzaldehyde (GC yield). Nevertheless, the product yield work (solvent-free condition under air and visible light irra-
decreased to 70, 62, and 49%, by employing 450–800, 550–800, diation) qualify all requirements of an efficient catalytic
and 600–800 nm lters, respectively. The contributions of the system for applied goals.
photoassisted pathway by deducting the thermal contribution
(
product formed at dark; 30%) are 41, 11, 19 and 27% for 400–
4
Conclusion
4
50, 450–550, 550–600 and 600–800 nm, respectively (Fig. 4).
These results are consistent with the UV-visible absorption In conclusion, an efficient and clean aerobic method for visible
spectrum and band gap value of 2.8 eV of TiO
-AA-Cu(II) nano- light-driven photooxidation of benzylic alcohols over TiO -AA-
hybrid and demonstrate the highest visible-light photocatalytic Cu(II) nanohybrid in combination with TEMPO under solvent-
performance of catalyst at 400–450 nm (Fig. 4).
free condition was developed. The catalyst showed desired
2
2
More evidence for the photocatalytic activity of as-prepared recyclability and no copper leaching was observed during
catalyst was obtained by apparent quantum yield for 4-chlor- reaction, which demonstrates its potential for industrial appli-
obenzaldehyde formation (AQY) calculated with the following cation. Good correlation between apparent quantum yield
equation: [AQY (%) ¼ (%) Yvis ꢁ Ydark ꢂ2/(photon number (AQY) and diffuse reectance spectrum (DRS) of the as-prepared
25
entered into the reaction vessel) ꢂ 100]. Fig. 5 clearly shows nanohybrid conrmed a photoassisted pathway for the title
a good correlation between AQY and diffuse reectance spec- oxidation reactions. The use of air as oxidant and visible light as
trum (DRS) of the TiO
-AA-Cu(II) nanohybrid conrming once a safe light source under solvent-free condition in the presence
2
again a photoassisted pathway for title oxidation reactions.
of a reusable and selective bio-relevant oxidation catalyst offer
an ideal green protocol, making it more appropriate for applied
goals.
3
.4. Catalyst recovery and reuse
The stability and reusability of TiO
assessed during 4 runs (Fig. S5†). At the end of each run, 2 mL
2
-AA-Cu(II) nanohybrid were
Conflicts of interest
ethanol as a green solvent was added, followed by centrifu- The authors declare no conict of interest.
gation to isolate the residual catalyst. The catalyst was dried
under vacuum and reused for next runs. The negligible loss of
activity was observed during 4 runs revealed the structural
Acknowledgements
stability of the catalyst. More evidences for this important Support for this work by Research Council of University of Bir-
issue were obtained by monitoring the FT-IR spectra of catalyst jand and ‘‘Iran national Science Foundation’’ (Grant No.
during 4 cycles (Fig. S6†) as well as lack of copper in ltrate 96013225) is highly appreciated.
based on leaching hot ltration test combined with ICP-AES
analysis.
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´
´
1
1
1
1
1
1
1
1
2
2
2
6
2
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