Cu2O/TiO2 as a sustainable and recyclable photocatalyst for gram-scale synthesis of phenols in water

Article abstract of DOI:10.1016/j.mcat.2021.111810

A green and straightforward protocol was developed for the synthesis of phenols from aryl boronic acid using an inexpensive and available Cu₂O/TiO₂ photocatalyst under visible light and sunlight. This approach proceeded in mild reaction conditions in water and the presence of air as a green oxidant, resulting in the corresponding phenols in good to excellent yields. Sunlight was also a sustainable source for this photochemical reaction. Heterogeneous nano photocatalyst was successfully recovered in 8 consecutive runs. It is noteworthy that, the photocatalyst exhibited high activity for the large-scale synthesis of phenols.

Full text of DOI:10.1016/j.mcat.2021.111810

Molecular Catalysis 514 (2021) 111810
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Cu O/TiO as a sustainable and recyclable photocatalyst for gram-scale
2
2
synthesis of phenols in water
*
Mina Tavakolian, Kimia Keshavarz, Mona Hosseini-Sarvari
Nano Photocatalysis Lab, Department of Chemistry, Shiraz University, Shiraz 7194684795, IR Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
A green and straightforward protocol was developed for the synthesis of phenols from aryl boronic acid using an
inexpensive and available Cu O/TiO photocatalyst under visible light and sunlight. This approach proceeded in
Green chemistry
Photoredox catalysis
Visible-light
Phenol
2
2
mild reaction conditions in water and the presence of air as a green oxidant, resulting in the corresponding
phenols in good to excellent yields. Sunlight was also a sustainable source for this photochemical reaction.
Heterogeneous nano photocatalyst was successfully recovered in 8 consecutive runs. It is noteworthy that, the
photocatalyst exhibited high activity for the large-scale synthesis of phenols.
Gram-scale
Introduction
non-environmentally solvents. Besides these catalysts, photocatalysts
have been also employed to change harsh reaction conditions to milder
The last decade has witnessed remarkable progress of photoredox
catalysts particularly in the organic chemistry field.[1] Indeed in many
cases, these catalysts could effectively proceed with the conventional or
non-proceeded organic transformations in milder and safer pathways
which well meet the principles of green chemistry. In this line, semi-
conductors as cost-effective and benign photocatalysts have attracted a
lot of attention. But most common and popular semiconductors such as
(Scheme 1).[11-17]
2 2
Recently, we synthesized and characterized Cu O/TiO and suc-
cessfully employed it as a photocatalyst for the synthesis of aryl phos-
phonates under visible light.[18] In this study by using this privileged
photocatalyst, we prepared various phenols via visible-light mediated
oxidative hydroxylation in water as a green solvent and gram- scale.
TiO
2
could not be applied for all photocatalytic targets in the
Experimental
visible-light region due to its wide bandgap and rapid recombination of
ꢀ
+
photogenerated electron/hole (e /h ) pairs. Thus, many efforts have
been devoted to the development of procedures based on an effective
visible-light-driven-photocatalyst. One of these promising approaches is
General information
Phenylboronic acid derivatives were commercially available. All
other reagents and all solvents were obtained from commercial sources
and purified using standard methods. Column chromatography was
2
hetero-junctions with other materials, particularly, p-type Cu O semi-
conductor due to the environmentally friendly, non-toxicity, and
,
1
13
cost-effectiveness of It. [2] [3]
accomplished on small columns of silica gel. H NMR (300 MHz) and
C
Phenols and their derivatives are privileged compounds often used in
the synthesis of valuable natural and pharmaceutical products.[4]
Among the various procedures to the synthesis of phenols the oxidative
hydroxylation of boronic acids is more considerable due to the easy
availability and low toxicity of phenylboronic acid derivatives. To this
end various catalytic systems have been introduced including AgNP
NMR (76 MHz) spectra were run on Bruker Avance DRX in pure
deuterated chloroform (CDCl3). Chemical shifts are given in the δ scale
in part per million (ppm) and J in Hz. Data for 1H NMR are reported as
follows: chemical shift (δ ppm), multiplicity (s= singlet, d= doublet, t=
triplet, q= quartet, m= multiplet, dd= doublet of doublets, ddd=
doublet of doublet of doublet, td= triplet of doublet), coupling constants
(Hz), and integration. Melting points were determined in open capillary
tubes in a Buchi-510 oil melting point apparatus. Power X-ray diffrac-
montK-10,[5] Ru@imine-nanoSiO
8] GO,[9] Au-PVP,[10] etc.[11] Although these catalysts demonstrated
high reactivity toward this transformation, they suffer from some
drawbacks such as using strong oxidants, bases, and
2 2 3
,[6] Pd/bentonite,[7] acidic Al O ,
[
tion patterns were recorded with Cu K
α (λ= 1.54178 Å) radiation.
Scanning electron microscopy (SEM) was done at TESCAN-Vega 3. The
*
Corresponding author at: Department of Chemistry, Shiraz University, Shiraz 7194684795, IR Iran.
E-mail address: hossaini@shirazu.ac.ir (M. Hosseini-Sarvari).
https://doi.org/10.1016/j.mcat.2021.111810
Received 11 June 2021; Received in revised form 30 July 2021; Accepted 31 July 2021
Available online 27 August 2021
2
468-8231/© 2021 Elsevier B.V. All rights reserved.

M. Tavakolian et al.
Molecular Catalysis 514 (2021) 111810
Table 1
Table 2
a,b
The effect of various reaction conditions on the hydroxylation of phenylboronic
Scope of phenylboronic acids.
a
acid.
Entry
PC (x
mg)
Solvent
Base
Light
Time
(h)
Yield
(%)
1
2
3
4
5
6
5
5
5
5
5
5
H
2
O
K
K
K
K
K
K
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
White
White
White
White
White
White
4
4
4
4
4
4
90
CH
3
CN
35
EtOH
40
Toluene
DMSO
trace
20
CH
3
CN/H
2
O
65
(
1:9)
7
8
9
5
5
5
5
5
5
5
5
5
5
5
5
3
10
-
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
H
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
2
O
K
K
2
CO
3
White
White
White
White
White
White
White
Blue
2
4
4
4
2
4
4
4
4
4
4
8
4
4
8
8
4
4
40
70
40
50
91
75
35
70
50
20
35
10
50
92
trace
8
3
PO
4
Na
2
CO
3
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
NaOH
Cs CO
2
3
NEt
-
3
K
K
K
K
K
K
K
K
K
K
K
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
2
CO
3
3
3
3
3
3
3
3
3
3
3
Green
Red
Violet
-
White
White
White
White
White
White
b
5
Cu
2
O (5)
(5)
45
30
a
TiO
2
Reaction conditions: aryl boronic acid (0.3 mmol), K CO3 (0.3 mmol), H O
2
2
a
(0.5 ml), under air atmosphere irradiated with a 15 W LED.
Reaction conditions: phenylboronic acid (0.3 mmol), Base (0.3 mmol), Sol-
b
Isolated Yield.
vent (0.5 ml), under air, room temperature, isolated Yield.
b
under argon atmosphere.
amount of nanoparticles (Cu and Ti) was measured by inductively
coupled plasma (ICP, Varian, Vista-pro). The lamps for irradiation were
compact fluorescent lamps (CFL) 15 W white, blue LED 12 W, green LED
1
2 W, red LED 12 W and violet LED 40 W.
Preparation of Cu
2 2
O/TiO photocatalyst[18]
Cu O/TiO nanostructure composite oxides were synthesized via the
2
2
modified subsequent method. In this way, 13.2 g (0.066 mol) of cupric
acetate monohydrate was dissolved in 600 mL of distilled water, and
then 3.2 mL (0.011 mol) of PEG-300 was added under vigorous stirring.
Subsequently, tetrabutyl titanate (0.002 mmol, 0.7 g dissolved in 3 mL
of ethanol) was added dropwise into the resulting suspension of cupric
acetate. After producing a white precipitate, NaOH (5mL, 5 M) and
hydrazine (15 mL, 5 M) were added dropwise under stirring at room
Fig. 1. Recovery of Cu O/TiO2 photocatalyst for the reaction of phenyl-
2
◦
temperature. The resulting mixture was kept at 12–14 C for 15 min.
boronic acid.
After that, nanoparticles were collected using a centrifuge and washed
with water (500 mL) and acetone (50 mL). Finally, the material was
reaction was completed and separation of the catalyst by centrifuge, the
reaction mixture was extracted with EtOAc and water. The organic layer
◦
dried in a vacuum oven for 3 h at 90 C. The resulting material contained
6
mol% Ti and 31 mol % Cu.
2 4
was dried over anhydrous Na SO and the solvent was then removed
under reduced pressure to get the crude product. Pure products were
obtained after column chromatography on silica using n-hexane and
ethyl acetate mixture as eluent.
General procedure for the ipso-hydroxylation of aryl boronic acids using
Cu O/TiO photocatalyst
2
2
Under air atmosphere, Cu
2
O/TiO
2
(0.5 mol %, 5 mg) was added to a
Results and discussion
mixture of aryl boronic acid (0.3 mmol), K
2
CO
3
(0.3 mmol, 41.46 mg),
◦
and H
2
O (0.5 mL) and then stirred at 25 C under irradiation of white
2 2
First, our study was devoted to the synthesis of Cu O/TiO photo-
LEDs (15 W) (note that the reaction temperature was maintained by
circulating cooling water), the reaction was monitored by TLC. After the
catalyst via our previous report.[18] To evaluate the potential of this
photocatalyst for the synthesis of phenols, phenylboronic acid was
2

M. Tavakolian et al.
Molecular Catalysis 514 (2021) 111810
2 2
Fig. 2. a) SEM image and b) XRD pattern of the Cu O/TiO nanoparticles after eight runs.
Scheme 1. Various conditions for oxidative hydroxylation of boronic acids.
absence of a base 35% yield was only obtained. In the next step, we
change the source of light, and the reaction was carried out in the
presence of blue, green, and purple lights, and a higher yield was ob-
tained using white LED. It is also worth mentioning that conducting the
reaction in the absence of light under the same reaction conditions led to
a much lower yield (10% within 8h), indicating the key role of light
(
Table 2, entry 18). The reaction conditions were next surveyed by
employing the different amounts of catalyst (Table 2, entries 19-21),
among them 5 mg was found to afford the best result under the other-
wise mentioned reaction condition (entry 1). The yield of the reaction
was significantly decreased in the presence of an argon atmosphere,
indicating the key role of air as an oxidizing agent. For comparison, the
Scheme 2. Gram-scale synthesis of phenol.
reaction was conducted using Cu
catalyst under obtained optimal reaction conditions (Table 1, entries 23,
4). As expected in these cases the desired product was obtained in a
lower yield than Cu O/TiO as photocatalyst.
2 2
O and TiO (P25) alone as photo-
2
2
2
Encouraged by this promising data, we then tested the scope and
generality of the present system for various phenylboronic acids
(
(
Table 2). Aryl boronic acids with donating substituent at para position
2b) delivered a higher yield than electron-withdrawing groups in the
Scheme 3. Synthesis of phenol from potassium phenyl trifluoroborate using
Cu O/TiO catalyst.
2
2
same position (2f-i). Interestingly, this photocatalytic system could
successfully be applied for ortho-substituted phenylboronic acid which
suffers from steric hindrance but in lower yields (2d, 2e). Furthermore,
chosen as a model substrate. To start optimizing the reaction condition,
first, the effect of solvent was tested in the presence of K CO under
2
3
1
-naphthalenylboronic acid, and 2-naphthalenylboronic acid also suc-
irradiation with white LED (15 W) using atmospheric air as an oxidant,
and water was found to be the best solvent. The model reaction then
took place in the presence of different organic and inorganic bases.
cessfully participated in the reaction, furnishing the corresponding
products in good yields (2j, 2k). It should be noted that in all cases no
by-products were not observed.
2 3 2 3
Although Cs CO gave the same result in a shorter reaction time, K CO
To further demonstrate the high potential and efficiency of this
was selected as the best from an economic point of view. As well, in the
3

M. Tavakolian et al.
Molecular Catalysis 514 (2021) 111810
Scheme 4. Synthesis of phenol under sunlight irradiation.
Scheme 6. Control experiments.
the recovered catalyst after 8 runs was analyzed with SEM, XRD, and
ICP. The SEM image of the recovered catalyst (Fig. 2a) showed that the
2 2
Cu O/TiO photocatalyst could successfully maintain its structure dur-
ing the recovery process. Furthermore, the XRD spectra of the recovered
catalyst well established no obvious structural changes (Fig. 2b). Based
on the ICP analysis the recovered catalyst contained 6 mol% Ti and 31
mol% Cu like the fresh catalyst. Together, these data indicated that the
nano photocatalyst is highly stable against aggregation or collapse even
after several at the present study.
e- e- e- e-
2
.17 ev
e- e-
Finally, we performed some control experiments[19-21] under
identical conditions to confirm that the reaction took place through a
radical pathway (Scheme 6). When the reaction was conducted in the
presence of TEMPO (1 equiv) as a radical scavenger only 10 % yield of
the phenol was obtained, as well as the reaction in the presence of
triethanolamine (TEA) as hole scavenger could not proceed even after
h+ h+
h+ h+
Cu O
3.2 ev
h+
TiO2
h+
2
1
2 h. Moreover, as described in Table 1, the reaction in the absence of
light could not proceed. These observations led us to propose a mech-
anism based on the radical pathway. As shown in Scheme 7 photoexci-
2 2
tation of TiO /Cu O nanoparticles generated electrons and holes
ꢀ
+
(
e /h ). First, electrons in the TiO
2
side of catalysts converted the O
2
Scheme 7. Proposed mechanism.
•ꢀ
into O
2
, which generated intermediate B via a single electron transfer
process. This intermediate coupled with hydroxyl radical formed via
holes of photocatalyst to deliver intermediate C. Subsequently aryl
group migration from boron to oxygen followed by losing OH produced
boronate ester D. consequently the corresponding phenol 2a was ob-
tained by hydrolysis process.
system, a scale-up reaction of phenylboronic acid (1.83 g, 15 mmol) was
investigated. As shown in Scheme 2, phenol product could be success-
fully obtained in 84% yield (1.51 g), albeit in a longer reaction time (12
h).
Further studies revealed that the present photocatalytic system could
successfully be employed for the synthesis of phenol from potassium
phenyl trifluoroborate in 88% yield (Scheme 3).
Conclusion
In the next step, the model reaction was conducted under sunlight as
In this study, we successfully employed Cu O/TiO as a sustainable
2
2
◦
shown in scheme 4. The maximum temperature of the day was 28 C and
and durable photocatalyst for the synthesis of phenol derivatives via a
radical pathway under visible light. Advantages of this protocol are the
use of water as a green solvent, and air as green and inexpensive oxidant,
broad substrate scope, and high functional group tolerance. This
promising protocol not only delivered phenol derivatives in good to
excellent yield but could also be applied for their gram-scale synthesis.
Remarkably, this heterogeneous photocatalyst could be reused 8 times
with maintaining reactivity. Further photochemical reactions using this
privileged photocatalyst under visible light are being pursued in our
laboratory.
◦
the minimum was 26 C. Interestingly, under these conditions the phenol
product was obtained in 90% within 3 h. an Increase in the reaction rate
is due to the higher intensity of sunlight compared to the LEDs.
Due to the importance of reusability of heterogeneous catalysts from
industry and economical point of view, in the next part of this study, we
2 2
checked the reusability of Cu O/TiO photocatalyst for the model re-
action. Fortunately, our photocatalyst could be reused 8 times without
loss of activity, demonstrating the high efficiency of our protocol to the
synthesis of phenol derivatives in the green pathway (Fig. 1).
To investigate the structural deviations and stability of the catalyst,
4

M. Tavakolian et al.
Molecular Catalysis 514 (2021) 111810
Declaration of Competing Interest
[7.] S. Bora, B. Chetia, Novel CuCl
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water at room temperature, J. Organomet. Chem. 851 (2017) 52–56.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[
8.] M. Khazaei, A. Khazaei, M. Nasrollahzadeh, M.R. Tahsili, Highly efficient reusable
Pd nanoparticles based on eggshell: green synthesis, characterization and their
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The manuscript was written through the contributions of all authors.
All authors have approved the final version of the manuscript.
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