Selective photocatalytic oxidation of aromatic alcohols to aldehydes with air by magnetic WO3ZnO/Fe3O4.: In situ photochemical synthesis of 2-substituted benzimidazoles

Article abstract of DOI:10.1039/d0ra08403d

Recently, visible light-driven organic photochemical synthesis has been a pioneering field of interest from academic and industrial associations due to its unique features of green and sustainable chemistry. Herein, WO3ZnO/Fe3O4 was synthesized, characterized, and used as an efficient magnetic photocatalyst in the preparation of a range of 2-substituted benzimidazoles via the condensation of benzyl alcohol and o-phenylenediamine in ethanol at room temperature for the first time. The key feature of this work is focused on the in situ photocatalytic oxidation of benzyl alcohols to benzaldehydes under atmospheric air and in the absence of any further oxidant. This new heterogeneous nanophotocatalyst was characterized via XRD, FT-IR, VSM and SEM. Short reaction time, cost-effectiveness, broad substrate scope, easy work-up by an external magnet, and excellent product yield are the major advantages of the present methodology. A number of effective experimental parameters were also fully investigated to clear broadness and generality of the protocol. This journal is

Full text of DOI:10.1039/d0ra08403d

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Selective photocatalytic oxidation of aromatic
alcohols to aldehydes with air by magnetic
WO3ZnO/Fe3O4. In situ photochemical synthesis of
2-substituted benzimidazoles†
Cite this: RSC Adv., 2020, 10, 40725
Bozhi Li,a Reza Tayebee,
b Effat Esmaeili,c Mina S. Namaghib and Behrooz Malekib
*
Recently, visible light-driven organic photochemical synthesis has been a pioneering field of interest from
academic and industrial associations due to its unique features of green and sustainable chemistry. Herein,
WO3ZnO/Fe3O4 was synthesized, characterized, and used as an efficient magnetic photocatalyst in the
preparation of a range of 2-substituted benzimidazoles via the condensation of benzyl alcohol and o-
phenylenediamine in ethanol at room temperature for the first time. The key feature of this work is
focused on the in situ photocatalytic oxidation of benzyl alcohols to benzaldehydes under atmospheric
air and in the absence of any further oxidant. This new heterogeneous nanophotocatalyst was
characterized via XRD, FT-IR, VSM and SEM. Short reaction time, cost-effectiveness, broad substrate
scope, easy work-up by an external magnet, and excellent product yield are the major advantages of the
present methodology. A number of effective experimental parameters were also fully investigated to
clear broadness and generality of the protocol.
Received 1st October 2020
Accepted 13th October 2020
DOI: 10.1039/d0ra08403d
rsc.li/rsc-advances
A photocatalyst is capable of absorbing light and producing
electron–hole pairs to force chemical reactions.18 Among the
1. Introduction
various photocatalysts, a great deal of attention has been paid to
semiconducting metal oxides because of their compatible band
gap in the visible region. Usually, the absorption of a photon by
a semiconductor with an energy equal to or greater than the
band gap energy can transfer an electron from the valence band
to the conduction band, leaving a hole in the conduction band
with a high oxidation capacity. The generated hole can absorb
electrons from the hydroxyl ions in water to generate highly
unstable and reactive $OH. Therefore, the excited electron can
also react with the oxygen gas to produce an O2À radical-anion.
The created electron–hole pair can follow different routes to
react with the organic materials adsorbed on the surface of the
photocatalyst. Notable features such as optimum band gap,
wide surface area, good stability and reusability and suitable
morphology are intended from an effective photocatalyst. In
addition, the performed photocatalyst should be fabricated
under environmentally friendly and cost-effective condi-
tions.18–20 Furthermore, the photocatalyst concentration, reac-
tant concentration, type of the chemical transformation,
temperature, kind and intensity of the light source can directly
affect the performance and effectiveness of a photochemical
reaction.21,22 Semiconducting metal oxides such as zinc, tita-
nium, tungsten, vanadium, and cerium oxides are the most
familiar photocatalysts, which have been used extensively in
electronic and chemical technology, hydrogen production,
storage, and environmental remediation because of their
Recently, green energy issues have attracted considerable
attention to develop newer effective sustainable energy
resources and environmentally benign routes to achieve
important synthetic pharmaceuticals broadly used in the
human life. The visible region of the solar energy (ꢀ43%) is the
most useful alternative to perform many green organic photo-
chemical reactions under catalytic conditions due to its
sustainable and abundant nature.1 Therefore, a great deal of
concern is devoted to the photocatalytic degradation of organic
wastes in water resources, efficient photocatalytic oxy-
functionalization of organic compounds and selective organic
transformations under ambient environmentally friendly
conditions, in particular with atmospheric air as a natural
oxidant, under solar light.2–6 Among various organic trans-
formations, the selective photochemical oxidation of alcohols
to carbonyl compounds is of prime importance due to its
signicant role in the synthesis of a variety of ne chemicals,
confectionaries, fragrances, and pharmaceutical industries.7–17
aDepartment of Food Science and Engineering, Jinzhou Medical University, Jinzhou,
China. E-mail: rtayebee@hsu.ac.ir
bDepartment of Chemistry, School of Sciences, Hakim Sabzevari University, Sabzevar,
96179-76487, Iran
cDepartment of Chemistry, Payame Noor University (PNU), Tehran, 19395-4697, Iran
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra08403d
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Scheme 1 General route for the preparation of different 2-substituted benzimidazoles.
suitable electronic structure, charge transfer characteristics, experiments. The light intensity of the UVC lamp was equal to
and light absorption properties.18,22–24 1020 mW cmÀ2 as measured using a Spectroline model DRC-
Among nitrogen-containing heteroaromatics, benzimid- 100X digital radiometer combined with a DIX-365 radiation
azoles are vital core materials to generate different and signi- sensor (Shokofan Tosee, Iran). 1H and 13C-NMR spectra were
cant drugs. They exhibit various biological activities such as recorded using a 300 MHz Bruker AVANCE III spectrometer in
antiviral,25 anti-inammatory,26 antibacterial,27 antiparasitic,28 DMSO-d6 using TMS as the internal reference. Melting points
analgesic,29 antihypertensive,30 antituberculosis,31 and anti- were determined in open capillary tubes using a Stuart BI
protozoal effects.32 Considering such a wide range of biological Branstead Electrothermal cat. no. IA9200 apparatus and
functionalities, extensive efforts have been made to the progress uncorrected. The morphology of the synthesized nanomaterials
of efficient strategies for the synthesis of various 2-substituted was studied using a Tescan 3Mira 3-XMU eld emission scan-
benzimidazoles. Condensation of 2-aminothiophenol, o-phe- ning electron microscope (FESEM). Fourier transform infrared
nylenediamine, and 2-aminophenol with aromatic aldehydes (FT-IR) spectroscopy was performed using a Shimadzu 8700
under acidic conditions is the well-known method to prepare Fourier transform spectrophotometer in the range of 400 to
these drugs33 by the mediation of various catalysts such as 4000 cmÀ1 with KBr pellets. UV-visible spectra were recorded
magnetic catalysts,34 porous catalysts,35 metal–organic frame- using a Photonix UV-visible array spectrophotometer. X-ray
work catalysts,36 ionic liquid catalysts,37 heavy metal catalysts,38 diffraction (XRD) patterns were acquired using an Xpert MPD
graphene oxides39 and so on. Moreover, quinoxalines are also diffractometer with Cu Ka radiation at 30 mA and 40 keV and
important heteroaromatic compounds that have drawn exten- a scanning rate of 3ꢁ minÀ1 in the 2q domain from 5ꢁ to 80ꢁ. The
sive attention for use in many pharmaceuticals,40,41 various room-temperature magnetic measurements were carried out
biofunctional molecules,42 semiconductor materials43 and using a Vibrating Sample Magnetometer model of Magnetic
organic synthons.44
Daghigh Kavir, MDKB.
To the best of our knowledge, WO3ZnO/Fe3O4 was prepared
and investigated for the rst time as an ideal magnetic nano-
photocatalyst in the synthesis of 2-substituted benzimidazoles
via the reaction of ortho-substituted anilines with various
substituted benzyl alcohols in the presence of air as a natural
oxidant under the irradiation of a high-pressure Hg lamp
(Scheme 1). The impacts of various experimental conditions
such as solvent type, reactant concentration, temperature,
contact time, and source of illumination on the efficacy of the
photocatalytic synthesis were also monitored. Furthermore, we
have used different benzyl alcohols with different electron-rich
and electron-decient groups in the condensation with
anilines.
2.2. Preparation of Fe3O4 nanoparticles by a chemical co-
precipitation method
Fe3O4 magnetic nanoparticles were prepared according to the
previously reported procedure.45 FeCl3$6H2O (10.3 g) and
FeCl2$4H2O (3.9 g) were dissolved in 100 mL deionized water,
purged with N2 for 15 min, and heated to 85 ꢁC. Thereaer,
15 mL of NH4OH solution (32%) was added drop-wise as well.
The precipitated solid material was separated using an external
magnet aer 15 min and washed thoroughly with a NaCl solu-
tion (0.1 M, 3 Â 10 mL). Finally, the obtained dark powder was
ꢁ
dried overnight in a vacuum oven at 80 C for 3 h.
2.3. Preparation of Fe3O4 nanoparticles via bioreduction of
S. cumini seed extract (Fe3O4 SMNT)
2. Experimental section
2.1. Materials and methods
Fe3O4 SMNT was prepared by an easy and environmentally
Zinc chloride, sodium hydroxide, zinc acetate, sodium tung- friendly method. First, 2.20 g of FeCl3$6H2O and 6.60 g of
state, benzyl alcohols, and other starting materials and solvents sodium acetate were dissolved in 50 mL of freshly prepared S.
were purchased from Merck and used without further treat- cumini seed extract. The resulting solution contained poly-
ment. A high-pressure mercury vapor lamp (NAVIFLUX, 400 W, saccharides and other biomolecules, and the mixture was
Berlin, NARVA) with a lamp operating current of 3.25 A and agitated at 67 ꢁC for 2 h. Aer 2 h, the resulting solution became
a nominal voltage of 230 V was used for the photocatalytic a black homogeneous dispersion, indicating generation of
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Fe3O4 SMNT. The colloidal solution was cooled to 25 ꢁC and the precipitate was collected, washed with distilled water, and air-
black precipitate was separated using an external magnetic dried at room temperature. To fabricate WO3ZnO/Fe3O4, the
eld, washed three times thoroughly with ethanol and dried preparation method for WO3ZnO was repeated as described,
ꢁ
overnight in vacuum at 90 C overnight.
but, in the presence of the as-synthesized Fe3O4 nanoparticles
(0.3 g).
2.4. Preparation of zinc oxide nanoparticles
2.6. Preparation of melamine/H3PW12O40 nanoparticles
In a typical route, 12.6 g of hydrated zinc acetate was dissolved
in 450 mL of double-distilled water under stirring until it was First, 0.1 g of melamine was added to 10 mL of chloroform and
completely dissolved. Thereaer, the solution was heated to le on the stirrer for 1 h to dissolve. During this process, pure
50 ꢁC, and 500 mL of absolute alcohol was slowly added under concentrated sulfuric acid was added drop-wise to the solution
stirring. Then, 6 mL of H2O2 (47%) was added drop-wise and to obtain an oily solution. Thereaer, 0.1 g of H3PW12O40 was
mixed using a magnetic stirrer until the solution becomes clear. dissolved in 5 mL of deionized water and slowly added to the
ꢁ
This solution was treated for 24 h and, then, dried at 80 C for above oily mixture. The nal mixture was stirred at room
several hours to obtain the white nanopowder of zinc oxide. The temperature to yield a white powder. Aer stirring for 2 h, the
attained zinc oxide was washed with double-distilled water to white solid powdered material was ltered, washed several
completely remove contaminants. Aer that, ZnO nanoparticles times with water and dried under an open-air autoclave.
were dried at 80 ꢁC to attain complete conversion and growth of
the crystals.46
2.7. General experimental procedure for the synthesis of 2-
substituted benzimidazoles
2.5. Preparation of WO3ZnO and WO3ZnO/Fe3O4
nanoparticles
A mixture of benzyl alcohol (1 mmol), o-phenylenediamine (1
mmol), and WO3ZnO/Fe3O4 nanophotocatalyst (0.01 g) was
First, 30 mL of NaOH (4 M) was added drop-wise into a 20 mL taken in 10 mL ethanol at room temperature. Then, the stirring
solution of zinc acetate (1 M) and Na2WO4$2H2O (2 mL, 0.5 M). reaction mixture was exposed to the HP mercury light irradia-
Then, the obtained suspension was maintained at room tion under mild air bubbling (1 mL minÀ1), and progress of the
temperature under mild continuous stirring for 3 h to form the reaction was monitored by TLC. Aer completion of the reac-
precursor. Aer that, the obtained suspension was subse- tion, the magnetic nanophotocatalyst was separated easily from
quently transferred into a 250 mL ground-glass-stopped conical the homogeneous reaction mixture using an external magnet.
ask and aged at 95 ꢁC for 10 h. Finally, the generated Finally, the yellow product appeared by adding ice-water to this
Fig. 1 FESEM images of WO3 (a), ZnO (b), and WO3ZnO/Fe3O4 (c) nanoparticles.
Fig. 2 TEM images of WO3 (a), ZnO (b), and WO3ZnO/Fe3O4 (c) nanocomposites.
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3.1.1. FESEM. Electron microscopy is eld emission elec-
tron microscopy that enables surface photography and
morphology of the nanoparticles with a magnication of about
500 000 times and with a resolution of about 1–20 nm. Field-
emission scanning electron microscopy was used to monitor
the morphology and particle distribution in WO3 (Fig. 1a), ZnO
(Fig. 1b), and WO3ZnO/Fe3O4 (Fig. 1c). From FE-SEM images, it
can be seen that the prepared nanocomposite had a uniform
surface morphology with a concomitant irregular distribution.
The FESEM image of WO3ZnO/Fe3O4 nanoparticles showed
random and irregular distribution of nanoparticles. The surface
of the material also showed layered and plate-like nano-
particles. In addition, the TEM images of WO3 (Fig. 2a), ZnO
(Fig. 2b), and WO3ZnO/Fe3O4 (Fig. 2c) are also provided. It can
be found that the surface of the WO3–ZnO nanocomposite
constructed revealed layered-shape particles with an average
thickness of about 27 nm and with effectively enhanced surface
area, providing superior photocatalytic performance.
Fig. 3 FT-IR spectra of Fe3O4, WO3ZnO, and WO3ZnO/Fe3O4.
3.1.2. FT-IR spectroscopy. Fig. 3 shows the FT-IR spectra of
Fe3O4, WO3ZnO, and WO3ZnO/Fe3O4 in the range of 400–
4000 cmÀ1. The peak absorbed at 587 cmÀ1 belonged to the Fe–
O vibration and the broad absorption peak at 3300–3500 cmÀ1
was due to the surface hydroxide ions.47 The absorption band at
428 and 587 cmÀ1 were due to the ZnO moiety, whereas a weak
and nearly wide band at 883 cmÀ1 would belong to WO3.48,49
3.1.3. XRD. The XRD method is the most common tech-
nique to identify the crystal structure and crystallinity of
a powdered material. Therefore, the relative amount of crystal
phases along with the average particle size of WO3ZnO/Fe3O4
can be easily determined by this technique. The phase analysis
of ZnO, WO3, and WO3ZnO (WZnO) was performed by XRD
(Fig. 4). The XRD patterns of ZnO (Fig. 4a) and WO3 (Fig. 4b)
solution. The precipitated product was ltered and re-
crystallized in ethanol to bring further puried products. For
the sake of completeness, the NMR characterization data for 2-
substituted benzimidazoles (3a–m) are provided in Table 3.
3. Results and discussion
3.1. Characterization and physicochemical properties of
WO3ZnO/Fe3O4 nanoparticles
The physicochemical and morphological properties of the
supported WO3ZnO/Fe3O4 nanophotocatalyst were explored
using the most informative techniques such as FESEM, FT-IR,
VSM, and XRD.
Fig. 4 Wide-angle XRD patterns of ZnO (a), WO3 (b), and WO3ZnO/Fe3O4 (c).
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Fig. 5 XPS survey spectrum of WZnO (a), W 4f (b), Zn 2p (c), and O 1s (d) in WO3ZnO.
were in good agreement with the literature. The peaks observed attributed to the vacancy oxygen atoms of ZnO and surface-
in the 2q range of 31.8, 34.4, 36.2, 47.5, 56.6, 62.9, 68.04 and chemisorbed hydroxyl groups, respectively.54,55 Moreover, the
69.03 were, respectively, assigned to the crystalline planes of binding energy values of 36.2 and 37.8 eV were due to the spin–
(100), (002), (101), (102), (110), (103), (112) and (201) due to the orbit components of 4f7/2 and 4f5/2 for W 4f peaks, respectively
presence of ZnO, which matched well with the JCPDS le no. 89- (Fig. 5b), conrming the W6+ form in the nanocomposite.
0510.50 The XRD pattern of WO3ZnO/Fe3O4 nanoparticles was
almost comparable to that of pure ZnO nanoparticles.51 The
intensity of all diffraction patterns was not altered, which
indicated that the ZnO crystallinity did not change aer doping
with WO3.52 Noteworthy, the weak diffraction peak with the 2q
value of 28.2 can be indexed to the hexagonal phase of WO3
(JCPDS no. 33-1387).53 The low intensity of WO3 diffraction
would be due to the low amount and amorphous phase of
tungsten oxide among ZnO nanoparticles.
3.1.4. X-ray photoelectron emission spectroscopy (XPS) of
WZnO. The supercial chemical conformation and valance
states of W, Zn, and O in WZnO were disclosed by XPS. The
spectra were calibrated according to the C 1s line with the
corresponding binding energy of 286.5 eV. The XPS spectra
survey in Fig. 5a conrmed no other elements rather than Zn,
O, W, and C. The C 1s peak originated from the carbon tape
used in the instrument. The high-resolution deconvoluted XPS
spectra of Zn 2p, W 4f, and O 1s of WZnO are shown in Fig. 5b–
d. The spin–orbit components of 2p3/2 and 2p1/2 due to Zn 2p
peaks were observed at 1021.9 and 1044.9 V, respectively, con-
rming that Zn exists in the form of Zn2+ ions. The O 1s spec-
trum shows two peaks at 532.6 and 533.7 eV, which are
Fig. 6 VSM magnetization curves of Fe3O4 (a) and WO3ZnO/Fe3O4 (b)
nanoparticles.
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Fig. 9 Effect of the catalyst amount on the photocatalytic efficacy of
WO3ZnO/Fe3O4. The reaction conditions are the same as described
for Fig. 7. Reactions were performed for 2.5 h. Yield% was increased to
18 and 31% in the absence of photocatalyst after 4 and 24 h,
respectively.
Fig. 7 Photocatalytic activity of some simple or mixed metal oxides in
the synthesis of 2-substituted benzimidazoles. o-Phenylenediamine (1
mmol), benzyl alcohol (1 mmol), and 2 mg of each photocatalyst were
mixed in 10 mL ethanol under the irradiation of a high-pressure Hg
lamp at 25 ꢁC for 3 h. Yield% refers to the isolated yield.
a high-pressure Hg lamp for 3 h. This study was planned to
achieve the best photocatalyst in the desired condensation
reaction (Fig. 7). According to the attained ndings, WO3ZnO/
Fe3O4, prepared by the co-precipitation method, behaved
signicantly much better that the bio-synthesized counterpart
wads found as the best photocatalyst to attain 92% yield aer
3 h. Aer that, the mixed metal oxide WO3Zn/MgO showed good
photocatalytic activity and provided 67% yield during the same
time. Noteworthy, doping of WO3ZnO with Mg (ꢀ10 wt%) was
constructive and increased the yield% from 50 to 67%, respec-
tively. Comparison of WO3ZnO with ZnO clearly conrmed that
3.1.5. VSM. The magnetic property of WO3ZnO/Fe3O was
compared with bare Fe3O4 by vibrating sample magnetometry
(VSM) at 25 ꢁC (Fig. 6). The saturation magnetization of the bare
magnetic and WO3ZnO/Fe3O4 nanoparticles was ꢀ61 and 9.0
emu gÀ1, respectively. These results conrmed the formation of
the well-dened crystalline structure for the magnetite nano-
particles. This study conrmed that a shell of WO3ZnO signi-
cantly decreased the saturation magnetization. However, the
saturation magnetization of WO3ZnO/Fe3O4 was high enough to
guarantee easy separation of it using an external strong magnet
from the reaction mixture.
3.2. Photocatalytic preparation of 2-substituted
benzimidazoles and studying different reaction parameters
3.2.1. Studying the photocatalytic activity of some simple
or mixed metal oxides, semi-conductors, and fused N-hetero-
cycles. In these experiments, a mixture of o-phenylenediamine
(1 mmol), benzyl alcohol (1 mmol), and 2 mg of each photo-
catalyst was prepared in 10 mL ethanol and illuminated with
Fig. 10 Effect of benzyl alcohol : o-phenylenediamine ratio on the
photocatalytic efficacy of WO3ZnO/Fe3O4. Here, 10 mg of WO3ZnO/
Fe3O4 was used in all cases. Reactions were performed for 2.5 h.
Table 1 Studying different irradiation sources on the photocatalytic
efficacy of WO3ZnO/Fe3O4
Catalyst
Light source
Yield%
WO3ZnO/Fe3O4 chemical
WO3ZnO/Fe3O4 chemical
WO3ZnO/Fe3O4 chemical
WO3ZnO/Fe3O4 chemical
WO3ZnO/Fe3O4 chemical
High pressure Hg lamp
Xenon lamp
White LED
No light
Sunlight
89
38
17
4
Fig. 8 Effect of the reaction time on the condensation reaction. The
reaction conditions are the same as described for Fig. 7; 20 mg of
WO3ZnO/Fe3O4 was used in all cases.
45 (8 h)
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Table 2 Synthesis of different 2-substituted benzimidazoles catalyzed by WO3ZnO/Fe3O4
Entry
1
Benzyl alcohols
Product
Yield (%)
92
Mp (ꢁC)
263–265
2
3
95
96
291–293
289–291
4
5
93
97
228–230
211–213
6
7
98
98
274–276
310–312
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Table 2 (Contd.)
Entry
Benzyl alcohols
Product
Yield (%)
Mp (ꢁC)
8
92
281–283
9
93
255–257
10
11
98
90
>300
291–293
12
90
>330
13
88
270–272
doping with WO3 is constructive and improves the photo- a little less reactive than ZnO in this reaction. Interestingly,
catalytic efficacy of the semi-conductor in the desired photo- Fe3O4 is an effective photocatalyst in the synthesis of 2-
chemical condensation reaction. It seems that TiO2 is just substituted benzimidazoles, and it attains 33% yield. This
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Table 3 NMR spectral data of the prepared 2-substituted benzimidazoles
Product
NMR spectral data of 2-substituted benzimidazoles (3a–m)
1H NMR (300 MHz, DMSO-d6); d 2.39 (s, 3H), 7.20–7.22 (m, 2H), 7.36–7.39 (d, 2H), 7.55–7.66 (m,
2H), 8.09–8.11 (d, 2H), 12.85 (s, 1H, NH) ppm; 13C NMR (75 MHz, DMSO-d6); d 21.44, 111.68, 119.18,
122.05, 122.77, 126.87, 127.95, 129.98, 135.45, 140.02, 144.20, 151.87 ppm
1H NMR (300 MHz, DMSO-d6); d 7.23–7.26 (m, 2H), 7.55–7.64 (m, 4H), 8.17–8.20 (m, 1H), 8.26–8.28
(m, 1H), 12.77 (brs, 1H, NH) ppm; 13C NMR (75 MHz, DMSO-d6); d 111.94, 119.45, 122.47, 123.14,
123.74, 128.85, 129.91, 132.43, 135.63, 144.25, 150.74 ppm
1H NMR (300 MHz, DMSO-d6); d 7.20–7.28 (m, 2H), 7.55–7.57 (d, 2H), 7.64–7.71 (m, 3H), 8.19–8.23
(d, 2H), 13.00 (s, 1H, NH) ppm; 13C NMR (75 MHz, DMSO-d6); d 111.89, 119.45, 122.31, 123.26,
128.61, 129.54, 129.55, 134.96, 135.50, 144.23, 150.63 ppm
1H NMR (300 MHz, DMSO-d6); d 7.34–7.36 (m, 2H), 7.42–7.45 (m, 2H), 7.51–7.54 (m, 1H), 7.72–7.74
(m, 2H), 8.42–8.45 (m, 1H), 10.45 (brs, 1H, NH) ppm
1H NMR (300 MHz, DMSO-d6); d 7.25–7.30 (m, 2H), 7.59–7.61 (m, 1H), 7.68–7.70 (m, 1H), 7.75–7.80
(m, 1H), 7.86–7.91 (t, 1H), 7.99–8.07 (m, 2H), 13.09 (s, 1H, NH) ppm; 13C NMR (75 MHz, DMSO-d6);
d 112.14, 122.37, 123.56, 124.73, 124.77, 131.36, 131.41, 131.44, 133.11, 135.10, 144.09, 147.79,
149.44 ppm
1H NMR (300 MHz, DMSO-d6); d 7.24–7.26 (m, 2H), 7.58–7.59 (m, 2H), 7.61–7.67 (m, 1H), 8.17–8.20
(m, 1H), 8.26–8.27 (m, 1H), 13.03 (brs, 1H, NH) ppm; 13C NMR (75 MHz, DMSO-d6); d 112.05, 119.77,
122.04, 123.24, 128.80, 128.82, 131.10, 132.85, 134.56, 135.54, 143.69, 143.76, 147.17 ppm
1H NMR (300 MHz, DMSO-d6); d 7.28–7.29 (d, 2H), 7.60–7.72 (d, 2H), 8.36–8.44 (m, 4H), 13.27 (s, 1H,
NH) ppm; 13C NMR (75 MHz, DMSO-d6); d 112.24, 119.92, 122.74, 124.01, 124.67, 127.80, 135.67,
136.49, 144.30, 148.20, 149.45 ppm
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Table 3 (Contd.)
Product
NMR spectral data of 2-substituted benzimidazoles (3a–m)
1H NMR (300 MHz, DMSO-d6); d 7.25–7.26 (m, 2H), 7.51–7.56 (m, 2H), 7.70–7.72 (m, 2H), 8.22–8.25
(d, 1H), 8.41 (s, 1H), 13.18 (brs, 1H, NH) ppm; 13C NMR (75 MHz, DMSO-d6); d 111.98, 119.53,
122.42, 122.72, 123.36, 123.70, 125.88, 129.39, 131.64, 132.89, 135.46, 144.10, 150.09 ppm
1H NMR (300 MHz, DMSO-d6); d 7.24–7.26 (m, 2H), 7.58–7.59 (m, 2H), 7.61–7.64 (m, 1H), 7.69–7.70
(m, 1H), 8.17–8.20 (m, 1H), 8.26–8.27 (m, 1H), 13.03 (brs, 1H, NH) ppm; 13C NMR (75 MHz, DMSO-
d6); d 112.00, 119.54, 122.40, 123.39, 125.51, 126.51, 130.00, 131.41, 132.69, 134.23, 135.43, 144.13,
150.20 ppm
1H NMR (300 MHz, DMSO-d6); d 7.24–7.29 (m, 2H), 7.64–7.66 (m, 2H), 8.12–8.15 (d, 2H), 8.31–8.34
(d, 2H), 13.18 (brs, 2H, NH, OH) ppm; 13C NMR (75 MHz, DMSO-d6); d 123.02, 126.95, 130.41,
132.05, 134.43, 150.65, 167.37 ppm
1H NMR (300 MHz, DMSO-d6); d 7.24–7.25 (m, 3H), 7.56–7.58 (m, 2H), 7.62–7.64 (m, 2H), 8.22–8.23
(d, 2H), 13.10 (brs, 1H, NH) ppm; 13C NMR (75 MHz, DMSO-d6); d 115.68, 122.68, 126.58, 126.98,
129.26, 129.43, 129.52, 130.41, 137.37, 143.03, 151.60 ppm
1H NMR (300 MHz, DMSO-d6); d 6.76–6.81 (m, 1H), 6.96–6.99 (m, 1H), 7.15–7.20 (m, 1H), 7.42–7.45
(m, 1H), 7.54–7.60 (m, 4H), 8.09–8.11 (m, 2H), 8.63 (s, 1H), 8.78–8.81 (m, 2H), 9.79 (s, 1H, NH) ppm;
13C NMR (75 MHz, DMSO-d6); d 115.46, 117.20, 117.99, 118.01, 121.23, 122.78, 125.39, 125.95,
127.64, 128.31, 128.49, 128.99, 129.39, 130.49, 131.21, 131.37, 135.36, 137.38, 144.35, 145.89, 156.58
ppm
1H NMR (300 MHz, DMSO-d6); d 7.24 (s, 2H), 7.62–7.70 (m, 4H), 8.22–7.25 (m, 2H), 13.02 (s, 1H,
NH) ppm; 13C NMR (75 MHz, DMSO-d6); d 111.89, 119.45, 122.31, 123.22, 128.45, 128.62, 129.26,
129.51, 131.24, 134.94, 135.54, 144.26, 150.67 ppm
nding promoted us to use magnetite as a core in the nal eld. Since fused nitrogen-containing materials such as
nanophotocatalyst WO3ZnO/Fe3O4 for the two reasons of graphitic carbon nitride and melem have been used as photo-
increasing the photocatalytic efficacy and simplifying the catalysts in the degradation of colored organic wastes, we tested
separation of the nanophotocatalyst using an external magnetic for the rst time commercial melamine as the trimer of
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cyanamide by acting as a photocatalyst and found that it can
produce 36% yield aer 3 h. Although this material did not
show high photocatalytic activity, this nding is important
because melamine is a major byproduct of most petrochemical
industries. Finally, to improve the photocatalytic activity of
melamine, phosphotungstic acid (H3PW12O40) was selected as
a strong superacid to activate the lone pair of nitrogen and form
a kind of ionic liquid. However, the prepared material showed
only a little progress in the photocatalytic activity and led to
40% yield aer 3 h.
3.2.2. Effect of the reaction time on the condensation
reaction. Initial attempts to optimize the reaction time in the
synthesis of 2-substituted benzimidazoles were made using
benzyl alcohol as the model substrate in the presence of
WO3ZnO/Fe3O4 under the reaction conditions mentioned in
Fig. 7 description. This experiment conrmed that 2.5 h dura-
tion is adequate to acquire 89% yield (Fig. 8).
3.2.3. Studying the impact of catalyst amount on the pho-
tocatalytic efficacy. In this case, we carried out some reactions
with different amounts of the photocatalyst in order to optimize
the exact quantity of WO3ZnO/Fe3O4. As shown in Fig. 9, 0.01 g
of the photocatalyst was appropriate for the present pathway
under the standard reaction conditions. However, a further
increment in the amount of catalyst affected neither the yield%
nor the reaction time.
3.2.4. Studying the effect of benzyl alcohol : o-phenyl-
enediamine molar ratio. The effect of changes in the ratio of raw
materials was investigated, and it was found that the product
yield increases with the increase in ratio of benzyl alcohol : o-
phenylenediamine (Fig. 10). Clearly, the yield% was enhanced
with the mentioned ratio. However, the 1 : 1 ratio was chosen as
the optimum, because no signicant improvement was attained
at higher ratios.
3.2.5. Studying different irradiation sources on the photo-
catalytic condensation reaction. In another important experi-
ment, the effect of different light sources was investigated, and
it was found that different lights have a profound effect on the
photocatalytic efficacy of the synthetic route. The attained
results showed that only 4% yield was gained under dark
conditions, whereas the product yield was improved to 17% by
means of a 50 W white LED. A commercial xenon lamp (1000 W)
yielded 38%; however, the best result (89%) was attained with
UV-Vis irradiation under similar reaction conditions, as
selected for the optimum (Table 1).
Fig. 11 Studying recyclability of WO3ZnO/Fe3O4 under the optimized
reaction conditions.
and gave the corresponding products in good to excellent yields
with no signicant effect on the efficiency of the trans-
formation, although benzyl alcohols with electron-withdrawing
substituents gave relatively better yields than those containing
electron-donating substituents. Therefore, substituted benzyl
alcohols with various functional groups such as NO2, Cl, and
CH3 reacted smoothly to deliver the desired products.56,57
A mixture of aromatic alcohol (1 mmol), o-phenylenediamine
(1 mmol), and WO3ZnO/Fe3O4 nanophotocatalyst (0.01 g) was
reacted in 10 mL ethanol at room temperature for 2.5 h. The
stirring reaction mixture was exposed to the light source under
mild air bubbling (1 mL minÀ1) and progress of the reaction was
monitored by TLC. Yield% refers to the isolated product.
3.2.7. Hot ltration test. A hot ltration test was arranged
to ensure that the photocatalytic activity originates from the
whole WO3ZnO/Fe3O4 and not from the species, which may be
released to the reaction mixture. For this purpose, 10 mg of
WO3ZnO/Fe3O4 was added to a mixture of benzyl alcohol and o-
phenylenediamine in ethanol under the standard reaction
conditions and the reaction was started as explained in the
Experimental section for 2.5 h. At this part, the product yield of
47% was obtained aer 45 min. Thereaer, the photocatalyst
was separated using an external magnet and the reaction was
continued for 150 min in the absence of photocatalysts. No
more than 2% was added to the last yield and, eventually, 49%
product was obtained in this experiment. This experiment
obviously conrmed that the nanophotocatalyst retained its
heterogeneous nature and none of the fragments had been
leached to ethanol as a strong polar solvent.
3.2.8. Recovery and reusability of WO3ZnO/Fe3O4. Recovery
and reusability of the WO3ZnO/Fe3O4 photocatalyst are valuable
properties from commercial and economic points of view,
which must be evaluated in this project. Therefore, the reus-
ability of this photocatalyst was investigated by repeatedly
separating the catalyst from the reaction mixture and, then,
reusing for over ve consecutive runs. Aer each run, the
catalyst was isolated from the reaction mixture using an
external magnet, washed with copious amounts of ethanol and
dried. Then, the recovered catalyst was applied for a subsequent
new batch of the reaction (Fig. 11). This study clearly showed
that the photocatalytic activity of WO3ZnO/Fe3O4 was preserved
and no signicant decrease was observed during reusing
A mixture of benzyl alcohol (1 mmol), o-phenylenediamine (1
mmol), and WO3ZnO/Fe3O4 nanophotocatalyst (0.01 g) was
taken in 10 mL ethanol at room temperature for 2.5 h. The
stirring reaction mixture was exposed to the light source under
mild air bubbling (1 mL minÀ1) and progress of the reaction was
monitored by TLC.
3.2.6. Synthesis of different 2-substituted benzimidazoles
catalyzed by WO3ZnO/Fe3O4. By consideration of the optimized
reaction conditions in hand, we investigated the scope and
generality of the process with respect to o-phenylenediamine
and benzyl alcohol using some substituted benzyl alcohols
(Table 2).56 Both electron-withdrawing and electron-donating
groups on the phenyl ring of alcohol were used as substrates
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can be catalytically oxidized with air in the presence of
WO3ZnO/Fe3O4 to form benzaldehyde (I). Then, phenylenedi-
amine reacts with the in situ generated aldehyde to produce an
imine intermediate (II), with concomitant release of a water
molecule. The imine intermediate is in equilibrium with the
corresponding dihydrobenzimidazole (III), and subsequently, it
can be catalytically dehydrogenated in the presence of air and
by the mediation of the prepared nanocatalyst to form the
desired nal benzimidazole product.58
4. Conclusions
We have developed a straightforward photochemical synthesis
strategy of 2-substituted benzimidazoles from the condensation
of benzyl alcohols with o-phenylenediamine in the presence of
WO3ZnO/Fe3O4 in ethanol. This new heterogeneous nano-
photocatalyst was characterized by XRD, FT-IR, VSM and SEM.
It is noteworthy that all produced benzimidazoles were ob-
tained in good to excellent yields. The used photocatalyst shows
excellent recovery and reusability without any signicant loss of
the photocatalytic activity. The key feature of this work is the in
situ photocatalytic oxidation of a range of benzyl alcohols to
benzaldehydes under atmospheric air and in the absence of any
further oxidant. In addition, the present protocol is also viable
to convert secondary aromatic alcohols to quinoxalines.
Fig. 12 XRD patterns of the fresh (a) and final reused photocatalyst (b).
experiments. Furthermore, the XRD pattern of the nal reused
catalyst was compared to the fresh one and no signicant
change was observed (Fig. 12). This study clearly conrmed
stability of the photocatalyst aer reusing experiments.
3.2.9. Proposed reaction pathway. A plausible reaction
pathway is disclosed for this green and practical methodology,
as described in Scheme 2. Initially, the benzyl alcohol substrate
Scheme 2 Plausible mechanism for the synthesis of different 2-substituted benzimidazoles catalyzed by WO3ZnO/Fe3O4.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
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