Enhanced Catalytic Properties of Molybdenum Promoted Mesoporous Cobalt Oxide: Structure-Surface-Dependent Activity for Selective Synthesis of 2-Substituted Benzimidazoles

Article abstract of DOI:10.1002/cctc.201801085

High-valent molybdenum ions were substituted into the cobalt oxide lattice through a one step, sol-gel method and investigated for selective synthesis of 2-substituted benzimidazoles. Catalyst synthesis involves surfactant assisted soft templating inverse micelle method, which forms mesopores by interconnected intraparticle voids. Substitutional doping of Mo⁶⁺ resulted in materials with modified structural, morphological, surface, and redox properties. The catalytic activity increased with Mo concentration until an optimum amount (3 % Mo incorporation). Modified material shows lattice expansion, increased surface oxygen vacancies, and high surface area, which are responsible for the higher catalytic activity in selective benzimidazole synthesis reaction. A strong correlation between surface properties of the catalyst and the product selectivity was observed and plausible mechanistic and kinetic data are proposed and collected, respectively.

Full text of DOI:10.1002/cctc.201801085

Accepted Article
Title: Enhanced Catalytic Properties of Molybdenum Promoted
Mesoporous Cobalt Oxide: Structure-Surface-Dependent Activity
for Selective Synthesis of 2-Substituted Benzimidazoles
Authors: Chandima Weerakkody, Dinithi Rathnayake, Junkai He He,
Biswanath Dutta, Peter Kerns, Laura Achola, and Steve L.
Suib
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10.1002/cctc.201801085
ChemCatChem
Enhanced Catalytic Properties of Molybdenum Promoted Mesoporous
Cobalt Oxide: Structure-Surface-Dependent Activity for Selective Synthesis
of 2-Substituted Benzimidazoles
Chandima Weerakkody, Dinithi Rathnayake, Junkai He, Biswanath Dutta, Peter Kerns,
Laura Achola, Steven L. Suib*
Department of Chemistry and Institute of Materials Science, University of Connecticut,
55 North Eagleville Road, U-3060, Storrs, CT 06269, United States
Abstract
High-valent molybdenum ions were substituted into the cobalt oxide lattice through a one
step, sol-gel method and investigated for selective synthesis of 2-substituted
benzimidazoles. Catalyst synthesis involves surfactant assisted soft templating inverse
micelle method, which forms mesopores by interconnected intraparticle voids.
Substitutional doping of Mo6+ resulted in materials with modified structural,
morphological, surface, and redox properties. The catalytic activity increased with Mo
concentration until an optimum amount (3ꢀ% Mo incorporation). Modified material
shows lattice expansion, increased surface oxygen vacancies, and high surface area,
which are responsible for the higher catalytic activity in selective benzimidazole
synthesis reaction. A strong correlation between surface properties of the catalyst and the
product selectivity was observed and plausible mechanistic and kinetic data are proposed
and collected, respectively.
1
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1. Introduction
Benzimidazole derivatives exhibit a broad spectrum of pharmacological and biological
applications in clinical medicines and bioactive compounds.1 Recently reported
pharmacological products containing benzimidazole rings have been used to treat
diseases such as ischemia-reperfusion injury, hypertension, and obesity. In addition,
benzimidazole derivatives have been reported as antivirus, antibacterial, and antiparasitic
agents as well as inhibitors of the hepatitis C virus, RNA polymerase, and angiotensin.2
Due to the growing interest of these compounds in the field of drug development and the
pharmaceutical industry, a number of synthetic protocols have been introduced recently.
They include palladium catalyzed tandem carbonylation cyclization reactions of o-
phenylenediamine3, palladium catalyzed tandem dehydration-coupling reactions of 2-
bromoaniline4, reductive cyclization reactions of o-nitroaniline with aldehydes5,
4
cyclization of o-phenylenediamines by CO2 in the presence of H2 , trimethylsilyl chloride
promoted reactions of o-phenylenediamine and aldehyde1, and condensation of o-
phenylenediamines and carboxylic acids6. Among these systems, the condensation
reaction of o-phenylenediamines and aldehydes under oxidative condition turned out to
be the most facile method to synthesize 2-substituted and 1,2-disubstituted benzimiazole
derivatives. This method is preferred in terms of applications as a wide range of 2-
substituted benzimidazole derivatives could be obtained due to the availability of a high
variety of aldehydes.7 However, these reactions require either stoichiometric amounts of
hydrogen acceptors or strong bases in order to achieve efficient and selective
benzimidazole yields, which has been an issue of high interest.8–10
2
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An alternative protocol is the one-pot tandem synthesis from amines in the presence of a
bifunctional catalyst, where amines are in situ oxidized to aldehydes followed by
cyclocondensation with amine compounds. Different oxide based heterogeneous catalytic
systems have been explored for the synthesis of benzimidazole derivatives including
alluminosilicates, iron oxide, cobalt oxide, zinc oxide, copper oxide, and molybdenum
oxide. However, lower yields of 2-benzimidazole have emerged as a major concern due
to the formation of 1,2-disubstituted derivatives as by-products. Zhang et al. reported
Lewis acidity of the catalyst plays a crucial role for the selective synthesis of of 2-
substituted benzimidazoles by evaluating catalytic activity of different acid catalysts
including ZrCl4, HfCl4, SnCl4, and InBr3.2 Various other metal salts such as Mg(ClO4)2,
NH4Ce(NO3)2, and LiBr showed either lower selectivity or no activity.2 Similarly, MoO3-
SiO2 has been used as bifunctional catalysts for selective synthesis of 2-benzimidazoles
in the presence of H2O2.11 MoO3 incorporated ZrO2 exhibits strong solid acidity and
excellent catalytic activity for use in organic transformation reactions.12,13
Considerable progress has been made employing complexes of base metals (Fe, Co, Mn,
Ni) in various dehydrogenation reactions.14 Cobalt catalysts have been reported in
hydrogenation reactions of olefins, ketones, imines, and CO2.15–18 Co3O4 has emerged as
an interesting catalytic material and has gained much attention due to superior oxygen
storage/release and redox properties due to the multivalent nature.19 Mesoporous
materials possess high surface areas and large pore volumes, which provide more
accessible active sites and facilitate diffusion of reactants and products, for efficient and
enhanced catalytic performance compared to their non porous counterparts.20,21
In the process of developing low loading, environmentally sound, affordable, stable and
3
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selective meal oxide catalysts for greener organic reactions, we report mesoporous cobalt
oxide as a mild oxidation catalyst which can be further modified to achieve bifunctional
catalytic effects for the synthesis of 2-benzimidazoles. Synthesis procedures for
mesoporous Co3O4 involve sol-gel processes where inverse surfactant micelles act as soft
templates for the mesopores.22 Moreover, the use of molybdenum as a promoter to adjust
the acidic strength of cobalt oxide was explored for the selective synthesis of 2-
substituted benzimidazole derivatives. Effects of the incorporation of molybdenum cation
into the Co3O4 unit cell were further explored by evaluating chemical and morphological
characteristics, and the catalytic activity of the material.
2. Experimental
2.1. Catalyst preparation
Mesoporous cobalt oxide was synthesized by dissolving 5 g (0.027 mol) of
Co(NO3)2·6H2O in a solution containing 34 g (0.66 mol) of 1-butanol, 2.4 g (0.038 mol)
of HNO3, and 2.5 g (4.31 x 10-4 mol) of P123. The resulted mixture was heated at 120 °C
for 5 hours and the formed solid was calcined at 150 °C for 12 hours. The obtained
powder was calcined at 300 °C for 4 hours to get the crystalline mesoporous Co3O4. To
prepare the Mo promoted Co3O4, certain amounts of (NH4)6Mo7O24.4H2O were dissolved
in a minimum amount of water and added to the initial mixture according to the metal
loading ratio and all the other steps are similar to the procedure described above. The
metal dopant amounts were 1, 3, and 5 mol % with respect to the total metal precursors.
The samples were labeled as X%Mo-Co3O4, where X is 1, 3, or 5 standing for the molar
percentage of molybdenum loading in Co3O4.
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2.2. Catalyst characterization
Powder X-ray diffraction (PXRD) data were collected (2θ = 5-75°) using a Rigaku
Ultima IV diffractometer with Cu Kα radiation (λ = 1.5406 Å) at an operating voltage of
40 kV and a beam current of 44 mA. N2 sorption analysis was performed on a
Quantachrome Autosorb-1-1C automated sorption system. The samples were degassed at
150 °C for 4 h prior to each measurement to remove physically adsorbed species. The
surface areas were obtained by the Brunauer-Emmett-Teller (BET) method, and the pore
size distributions were calculated by the Barrett-Joyner-Halenda (BJH) method using the
desorption data.
To measure the acid/base character of synthesized materials, CO2 sorption experiments
were performed on the Quantachrome Autosorb-1-1C automated adsorption system. All
samples were heated under vacuum at 150 ˚C for 6 h prior to experiments. The adsorption
studies of all materials were done at room temperature (25 ˚C). A water circulating bath
was used to control the temperature.
Morphological characterization was examined with a Zeiss DSM 982 Gemini field-
emission scanning electron microscope (FE-SEM) with a Schottky emitter at an
accelerating voltage of 2.0 kV. Elemental mapping of the materials was carried out on an
Oxford AZtec Energy microanalysis system equipped with an X-Max 80 silicon drift
detector. High-resolution transmission electron microscopy (HR-TEM) images were
collected by a JEOL 2010 FasTEM microscope operating at 200 kV with an energy
dispersive X-ray spectroscopy (EDS) detector. The samples were dispersed in ethanol
and mounted on a copper grid coated with carbon film. X-ray Photoelectron
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Spectroscopy (XPS) measurements were carried out on a PHI model 590 spectrometer
with multi probes (ΦPhysical Electronics Industries Inc.), using Al Kα radiation (λ =
1486.6 eV) as the radiation source. The binding energies reported in this study were
corrected for specimen charging by referencing them to the C 1s peak (set at 284.6 eV).
Raman measurements were performed on a Renishaw 2000 Raman microscope attached
to a charge-coupled device (CCD) camera with an Ar+ laser at 514.4 nm as the excitation
source and the instrument was calibrated using a silicon wafer as the reference.
Temperature-programmed mass spectrometry analyses were conducted in
a
programmable tube furnace equipped with a gas analyzer MKS coupled with a quadruple
mass selective detector. About 50 mg of materials were packed in a quartz tube reactor
mounted into the tube furnace. The loaded samples were pretreated in an inert gas flow
(Ar) at 150 °C for 1 h to clean the catalyst surface before each test. For H2-TPR
measurements, 5% H2/ Ar flow was passed through the catalyst bed at a flow rate of 50
sccm, while the temperature was ramped from room temperature (RT) to 800 °C with a
heating ramp rate of 10 °C/min.
2.3. Catalytic activity measurements
In a typical reaction, benzylamine (0.6 mmol), o-phenylenediamine (0.5 mmol), meso
Mo-Co3O4, and 1-butanol (1 mL) were placed in a 25 mL round-bottom flask (two-
necked flask for time- dependence studies, where the second neck was used as a sample
port). The flask containing the reaction mixture with a reflux condenser attached was
immersed in a silicone oil bath preheated to 100 °C. The reaction mixture was refluxed
with constant stirring (500 rpm) for the required time under atmospheric air. After the
6
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reaction, the mixture was cooled, the catalyst was removed by filtration, and the filtrate
was analyzed by gas chromatography-mass spectroscopy (GC-MS) methods using a
7820A GC system connected with a thermal conductivity detector of 5975 series MSD
from Agilent Technologies. A nonpolar cross-linked methyl siloxane column with
dimensions of 12 in × 0.200 mm × 0.33 m was used. For recycle experiments, the catalyst
was separated by filtration at the end of the reaction, washed several times with ethanol,
and heated in the oven at 250 °C for one hour.
3. Results
3.1. Physicochemical properties
3.1.1. PXRD and TEM
Figure 1(a) shows wide angle PXRD patterns for pure Co3O4 and Mo promoted Co3O4
with different Mo loading amounts. Up to 3% Mo loaded Co3O4 materials show similar
XRD patterns, which represent the typical Co3O4 spinel crystal structure (JCPDS #
090418). 3%Mo-Co calcined at 150 °C shows a mixture containing Co3O4 and
C4H6CoO4·4H2O indicating the incomplete transformation of the material into Co3O4,
which is consistent with the data reported in literature (Figure S1).23 In 5%Mo-Co3O4
material a small peak appears around 26°, which can be indexed to the cobalt molybdate
(CoMoO4) crystal phase. For comparison, pure CoMoO4 was synthesized and the PXRD
of CoMoO4 matches the peaks in 5%Mo- Co3O4 PXRD. The average crystallite size was
calculated using the Scherrer equation (τ = Kλ / B cos θ where τ = crystallite size, K =
shape factor, λ = x-ray wavelength, and B = line broadening at half maximum). A trend
of crystallite size decreasing with dopant amounts up to 3%Mo loadings was observed
7
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and the 5% material has a crystallite size close to pure Co3O4 (Figure 4). Crystallite size
of these materials are in the range of 11.2 to 14.7 nm and the largest size was observed
for bare Co3O4 material.
The change of nanoparticle sizes upon the Mo loading was also observed in TEM
analyses and all the materials exhibited crystalline walls with well defined lattice fringes
(Figure 3). All TEM images were taken with the same magnification for better
evaluation of the change of the particle sizes and each particle was composed of
nanoparticle aggregates. With the dopant amount increasing from 0% to 3%, a decrease
in particle size was observed and all the materials showed uniform pore structure. In HR-
TEM the d spacings were calculated as 0.25 nm and 0.48 nm for (311) and (111) planes,
respectively (Figure 3(e)).
3.1.2. N2 and CO2 sorption studies
Barrett-Joyner-Halenda (BJH) desorption pore diameters decreased with the introduction
of Mo into Co3O4 (Figure 1(c)). Pure cobalt oxide has a pore diameter of 20 nm and this
decreased to 15.5 nm in the 3%Mo-Co3O4 material. Again, 5%Mo has a pore diameter of
20.6 nm, which is close to that of pure Co3O4. As shown in Figure 1(b), all the materials
exhibit type IV isotherms followed by type I hysteresis loops indicating the mesoporosity
of the material and the surface area increased from 70 to 96 m2/g when going from pure
to 3%Mo-Co3O4. All materials showed uniform pore size distributions in BJH pore size
analyses except the 5%Mo-Co3O4 sample. In that way the 5%Mo-Co3O4 material does
not follow the same trend as other materials in consideration. Structural parameters of
each material are summarized in Table 1.
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We also performed CO2 adsorption studies using CO2-BET and the CO2 adsorption
curves for Co3O4, 3%Mo-Co3O4, and CoMoO4 materials as shown in Figure 1(d). Higher
numbers of basic sites provides highly active sites for CO2 adsorption, hence reducing the
capability for adsorption. Despite the lower surface area, undoped Co3O4 gave the highest
CO2 adsorption among all three materials. CO2 adsorption curves for the 3%Mo sample
deviated from that of undoped Co3O4 significantly to a lower adsorption capacity, which
can be ascribed to the fewer basic sites in the 3%Mo material. We checked the CO2
adsorption for CoMoO4 as a comparison and gave the lowest adsorption among all three
materials.
Table 1. Structural parameters of X%Mo-Co3O4 catalyst (X=0, 1, 3 and 5)
Mo loading %
BET surface
area (m2/g)
BJH Des. pore
diameter (nm)
Scherer
crystallite
size (nm)
14.74
d-spacing (nm)
0
1
3
5
70
96
86
91
20.0
18.5
15.5
20.6
0.242201
0.242529
0.242732
0.242612
11.34
11.20
13.29
9
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Figure 1. Characterization of as-prepared materials: (a) Wide angle XRD patterns, (b) N2
sorption isotherms, (c) BJH desorption pore size distributions, and (d) CO2 adsorption
curves for X%Mo-Co3O4 materials, X=0, 1, 3 and 5
Figure 2. SEM images of (a) Co3O4, (b) 1%Mo-Co3O4, (c) 3%Mo-Co3O4 and (d) 5%Mo-
Co3O4 materials
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Figure 3. (a-d) TEM images of (a) Co3O4, (b) 1%Mo-Co3O4, (c) 3%Mo-Co3O4 and (d)
5%Mo-Co3O4 (e) High magnification TEM of 3%Mo-Co3O4
3.1.3. Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy
(EDX)
Morphological differences in each material could be clearly seen in SEM images given in
Figure 2. Pure Co3O4 has spherical particles with rough surfaces and the morphology
gradually changed to flower like structures consisting of nanosize flakes. A homogeneous
distribution of Mo could be observed in EDX mapping for 1% and 3%Mo promoted
samples (Figure S2). However, 5%Mo-Co3O4 material showed two types of particles,
large round shape particles similar to pure Co3O4 along with minor impurity particles
deposited on large particles. These small particles contained Mo/Co in 1:1 ratio according
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to elemental analyses via EDX mapping. This observation is consistent with PXRD data
as we observed peaks for CoMoO4 in 5%Mo-Co3O4.
Changes in surface area, pore structure, and morphology of Co3O4 suggest successful
introduction of Mo into the crystal structure up to 3%Mo concentration. Higher Mo
loading did not create significant morphological or structural changes to the original
Co3O4 structure but formed an additional phase, identified as CoMoO4 mixed metal oxide
by PXRD.
Figure 4. Crystallite size and d-spacing values calculated using PXRD data for X%Mo-
Co3O4 materials, X=0, 1, 3 and 5
3.1.4. Raman spectroscopy
Figure S3 shows the Raman spectra of pure and Mo doped Co3O4 nanoparticles. Five
Raman bands were identified located at 195, 479, 522, 616, and 686 cm-1. They
correspond to A1g, Eg and F2g lattice vibrations of Co2+ and Co3+ cations situated at
tetrahedral and octahedral sites of the cubic Co3O4 structure.24,25 The A1g mode is related
to vibrations of octahedral sites and Eg and F2g modes are arising from combined
vibrations of tetrahedral sites and octahedral oxygen vibrations.26 Peak shifts towards
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higher wave number were observed in 1 and 3% Mo loaded materials. At the same time,
increased intensity was observed in Raman peaks with increasing Mo loadings. No peaks
were observed for CoMoO4 phase as compared to the Raman spectra of pure CoMoO4
(Figure S3).
3.1.5. X-ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy is a useful technique to study the surface properties and
metal valence states of metal oxide materials. Co 2p spectra for 0% to 5%Mo-Co3O4
samples are given in Figure 5(a), and Co 2p peaks at binding energies of ∼779 and 795
eV are characteristic of Co3+, while the peaks at ∼781 and 797 eV correspond to Co2+.27–
29
The Co2+/Co3+ ratio was calculated for each sample using areas under the peak in
deconvoluted spectra and the values are given in Table 2. Bare Co3O4 has the highest
Co3+ percentage of 59% and went down significantly upon Mo insertion where 1%Mo
and 3%Mo samples contained 46% and 40% of Co3+ respectively. However, 5%Mo
material has comparatively higher Co3+ amounts of 48%, which is a little lower than pure
Co3O4, but higher than both the 1% and 3% samples. Similar trends were observed for
the lattice oxygen where the Mo introduction decreased lattice oxygen density up to 3%
doping concentration. Deconvoluted O 1s spectra in Figure 5(b) have two major oxygen
contributions, with peaks centered at 529.5 and 531.5 binding energies, which can be
attributed to lattice oxygen and surface adsorbed oxygen species, respectively.23,30 The
increased asymmetry to the higher binding energy side of the O 1s peak indicates the
increase in adsorbed oxygen species compared to lattice oxygen in turn suggests the
formation of more oxygen vacancies.31 Density of oxygen vacancies increased
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significantly from 45% to 61% from bare Co3O4 to 3%Mo-Co3O4. However the 5%Mo
material did not fall in the trend with characteristics in between pure Co3O4 and 1%Mo
materials, which might be due to the incomplete insertion of Mo into the Co3O4 lattice.
Furthermore XPS data reveal that Mo present in the material is mostly Mo6+ and Mo5+
can be seen as a minor phase. Deconvoluted Mo 3d spectra for all samples give nearly
similar Mo6+/Mo5+ ratios, indicating the molybdenum loading amount does not have a
significant effect on the oxidation state of Mo in the material.
Table 2. Summary of area percentages of different elemental components obtained from
the deconvoluted XPS spectra
Sample
% Area
O 1s
Co 2p 3/2
Co3+
Mo 3d
Co2+
41.4
54.2
59.8
51.5
OL
OV
Mo6+
N/A
94.7
90.0
91.5
Mo5+
N/A
5.3
Co3O4
58.6
45.8
40.1
48.5
55.1
49.4
38.9
49.0
44.9
50.6
61.1
51.0
1%Mo-Co3O4
3%Mo-Co3O4
5%Mo-Co3O4
10.0
8.5
14
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Figure 5. X-ray photoelectron spectroscopy (XPS) analysis: (a) Co 2p and (b) O 1s
deconvoluted XPS spectra of Co3O4 and X%Mo-Co3O4 (X=1,3, and 5)
15
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Figure 6. H2-TPR profiles of mesoporous Co3O4 and 3%Mo-Co materials. The
measurements were conducted from room temperature to 800 °C (10 °C/min) under a
stream of 5% H2/Ar with a flow rate of 50 sccm.
3.2. Catalytic activity evaluation
Liquid phase batch mode reactions of benzylamine with o-phenylenediamine were used
as model reactions to investigate the structure activity relationships of Mo modified
Co3O4 catalysts. Reactions were carried out using 1-butanol as the solvent, refluxed under
air at 100 °C and with no additives. Conversion and selectivity of each catalyst were
investigated for the two possible products, which are 2-substituted benzimidazole (1a)
and 1,2-disubstituted benzimidazole (1b). Apparently, the control experiment with no
catalysts did not form any products for this reaction. Bare Co3O4 gave 45% conversion
after three hours and the product (1a) selectivity was 72%. Interestingly, within 3 hours
3%Mo-Co3O4 catalyst showed enhanced product selectivity of 96% and a conversion of
58%. When the reaction time increased to 5 hours, 100% conversion was achieved with
97% selectivity. However, the selectivity dropped by 17% when 5%Mo-Co3O4 was used
as the catalyst. The 5%Mo sample contains CoMoO4 as an impurity phase in the material.
16
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Thus, pure CoMoO4 was used as the catalyst under similar reaction conditions. CoMoO4
showed high selectivity (67%) for 1,2-disubtituted benzimidazole, which was not the
product of interest. 3%Mo-Co catalyst was recycled and reused, and the second reuse
cycle gave 90% conversion with 92 % selectivity.
1a
1b
Table 3. Conversion and selectivity of X%Mo-Co3O4 and CoMoO4 catalysts for
benzimidazole synthesis reaction
Catalyst
Time/h
Conversion %
Selectivity
1a
1b
NA
28
16
4
No catalyst
Co3O4
3
3
3
3
3
3
5
5
0
45
50
58
55
30
100
90
NA
72
84
96
79
33
97
92
1%Mo-Co3O4
3%Mo-Co3O4
5%Mo-Co3O4
CoMoO4
21
67
3
3%Mo-Co3O4
a
3%Mo-Co3O4
8
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Reaction conditions: Catalyst 10 mg, Butanol 1ml, 100 °C, o-phenylenediamine 0.5 mM,
benzylamine 0.6 mM, reflux under air, Conversions were determined by GC-MS based
on the concentration of o-phenylenediamine. a Recycled catalyst.
The 3%Mo-Co3O4 material was identified as the best catalyst for the selective synthesis
of 1-substituted product and was selected as the catalyst for further studies. To investigate
the versatility of the catalyst towards a variety of substrates, a series of different starting
materials were subjected to similar reaction conditions and the conversions and
selectivities were calculated for the 1-substituted product.
Table 4. Catalytic activity of 3%Mo-Co3O4 catalyst for the reaction of different
substituted benzylamine and diamine towards 1-substituted benzimidazole synthesis
Entry
1
Benzyl amine
Diamine
NH2
NH2
NH2
NH2
% Conversion % Selectivity
100
97
NH2
2
60
67
NH2
Cl
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3
100
90
NH2
NH
NH2
NH2
4
5
6
7
95
92
85
63
93
82
62
62
NH2
H3CO
H3C
F
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
NH2
OCH3
8
9
70
93
83
76
NH2
OCH3
NH2
NH2
NH2
F
F
F
NH2
Reaction conditions: Catalyst 10 mg, Butanol 1ml, 100 °C, diamine 0.5 mM, substituted
benzylamine 0.6 mM, reflux under air, 5h. Conversions were determined by GC-MS
based on the concentration of o-phenylenediamine
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3.3. Reaction mechanism and kinetics
The high catalytic activity and selectivity of Mo-Co3O4 towards the desired reaction
motivated us for the investigation of mechanistic details. Trace amounts of benzaldehye
were found to be present in the initial stage of reaction mixture according to the GC-MS
results. To further evaluate the fate of benzylamine in the reaction mixture when there is
no diamine present, a separate experiment was carried out with only benzylamine as the
starting reagent and Mo-Co3O4 as the catalyst. The major product obtained was the
corresponding imine with trace amounts of benzaldehyde. In this reaction pathway, the
first step is aldehyde formation, which then reacts with another molecule of amine to give
imine.32 No imine product was observed, indicating the reaction of benzaldehyde with
diamine happens at a faster rate compared to benzylamine, in the presence of diamine in
the reaction mixture. Scheme 1 gives the suggested reaction mechanism for the formation
of benzaldehyde from benzylamine where H+ abstraction is the rate determining step.32
The generation of the benzylamine radial intermediate caused the reduction of Co3+ to
Co2+ on the catalyst surface, which led to the facile release of labile lattice oxygen.33,34
Lattice oxygen combines with the protons (H+) and electrons released from metal ion
reduction to form water.
20
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ChemCatChem
Co3+
Co2+
- H+
NH2
NH2
NH2
RDS
Co3+
Co2+
- H+
NH2
NH
O
Scheme 1. Reaction pathway of oxidation of benzylamine to benzaldehyde
The surface acidity of the catalyst has a considerable effect on determining the rate of
formation of benzaldehyde as proton abstraction is the rate determining step in the given
reaction pathway (Scheme 1). Basic sites on the catalyst surface could enhance the rate
by promoting H+ abstraction, thereby generating the aldehyde at a higher rate. For
instance, Cs promoted MnOx catalyst has been employed for amine to imine conversion
and showed remarkable enhancement in catalytic activity compared to bare MnOx.32
Here, Cs incorporation was identified as the factor to increase the basic sites of the metal
oxide material, which in turn helps to improve the conversion efficiency of amine to
imine.
Mo doping in Co3O4 catalyst has an influential effect on the selectivity of the product
formation between 1-substituted and 1,2-substituted benzimidazole. CO2 adsorption data
show an enhancement of surface acidity when Mo was incorporated into the Co3O4
crystal lattice. Therefore, to determine the relationship between surface acidity of the
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10.1002/cctc.201801085
ChemCatChem
catalyst and rate of formation of benzaldehyde, two sets of kinetic data were collected
using bare Co3O4 and 3%Mo-Co3O4 as the catalyst (Figure 7).
Figure 7. Kinetic plots for the oxidation of benzylamine catalyzed by Co3O4 and 3%Mo-
Co3O4
A proposed reaction mechanism for the dehydrogenative coupling of aromatic diamine
and aldehyde is given in Scheme 2. Oxidation of amine produces aldehyde, which then
couples with the diamine, forming a Schiff base imine intermediate 2 by water
elimination. This will then be subjected to cyclization to produce 2-phenyl-2,3-dihydro-
1H-benzimidazole, which rapidly dehydrogenates to form the corresponding 1-
substituted benzimidazole product, 4.35 However, a higher rate of production of aldehyde
will lead to formation of diimine intermediate 5, which subsequently converts to 1,2-
disubstituted benzimidazole derivative.
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10.1002/cctc.201801085
ChemCatChem
H
N
Ph
- H2O
NH2
NH2
O
N
Ph
- H2
H
N
Ph
Ph
N
H
N
H
NH2
1
2
3
4
- H2O
Ph
O
H
N
H
- H2
N
N
N
Ph
Ph
Ph
Ph
N
N
Ph
Ph
7
6
5
Scheme 2. Plausible steps involved in benzimidazole synthesis from 1,2 diaminobenzene
and benzaldehyde, catalyzed by Mo-Co3O4 catalyst
Kinetic data reveal that bare Co3O4 produces benzaldehyde at a higher rate compared to
Mo-Co3O4, when used as the catalyst. This is in good agreement with the observed
product selectivity with two catalysts, where the catalyst with more acidic character,
shows high selectivity towards 2-disubstituted benzimidazoles.
4. Discussion
With regard to sustainable synthesis, an environmentally benign pathway for the
synthesis of benzimidazoles with high product selectivity would be highly desirable. Bi-
functional nature of the cobalt catalyst in terms of basicity and dehydrogenative
properties could be considered as major contributions for more proficient benzimidazole
synthesis.35 However, acid/base and redox properties of the catalyst are the key factors to
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10.1002/cctc.201801085
ChemCatChem
determine the product selectivity of benzimidazole synthesis reactions. In this work we
demonstrate molybdenum introduction into the Co3O4 lattice, which has a great influence
on creating acidic sites and oxygen vacancies in the material. Reduced CO2 adsorption by
Mo-Co3O4 material compared to bare Co3O4 (Figure 1(d)) indicates that the number of
acidic sites has been increased upon Mo addition. Successful incorporation of Mo into the
Co3O4 lattice could be observed up to 3%Mo loading concentration. When the Mo
addition increased to 5% molar concentration, formation of CoMoO4 mixed metal oxide
was identified as a separate phase. According to the XPS data, molybdenum was mainly
found in its 6+ oxidation state. The Co3+/Co2+ ratio decreased significantly with the
increased Mo concentration implying that Mo6+ substitutes for Co3+ in the Co3O4 crystal
lattice (Table 2). This would enhance the formation of oxygen vacancies on the catalyst
surface due to charge compensation, which in turn is responsible for the formation of
surface defects that can act as active sites for the adsorption and activation of reactant
molecules.36,37 XPS data shows significant increase in surface oxygen vacancy density of
the CO3O4 material upon molybdenum addition. Increased oxygen vacancy density
indicates the enhancement of the surface acidic character of the material.38 Reduced
basicity of the catalyst, control the rate of formation of benzaldehyde facilitating the
selective formation of 2-benzimidazole according to the reaction pathway shown in
Scheme 2.
Catalysts with higher activity toward oxidation reactions generally have higher
reducibility.39 H2-TPR measurements were performed to compare the redox properties of
molybdenum promoted and pure cobalt oxides (Figure 6). The reducibility of Co3O4 is
highly dependent on the dispersion state of particles, thus large particles are usually
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10.1002/cctc.201801085
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reduced to metallic cobalt in a single step while nanoparticles goes through a two step
reduction process.23 The reduction behavior of pure cobalt oxide starts with a sharp peak
around 350 °C, which can be attributed to the reduction of Co3+ to Co2+ (Co3O4 → CoO),
followed by a broad peak in the range of 400-500 °C corresponding the reduction of CoO
to metallic Co. In the H2-TPR profile of Mo-Co3O4, both reduction peaks shifted to
higher temperatures, indicating the lower reducibility of the material compared to pure
Co3O4.
Strain caused by the large density of oxygen defects creates a lattice expansion and
therefore, nanocrystallites preferentially grow in the direction of (311) planes.40,41 D-
spacings calculated using Bragg’s law indicate the variation of inter-planar distances
upon Mo incorporation (Figure 4). Scherrer crystallite sizes were found to be decreasing
with increasing d(311), implying a reciprocal relationship with crystallite size and d
spacing.42,43 Stronger and sharper Raman peaks were observed for Mo-Co3O4 and the
peaks slightly shifted to a higher wave number. This effect can be attributed to the
particle size enhancement in Mo-Co3O4, as the Raman peak intensity is dependent on the
grain size of the material.44 Doping molybdenum produced Raman peak shifts to a higher
energy compared to that of pure Co3O4, as a result of lattice deformation.45
Catalysts reported for benzimidazole synthesis have both acidic and basic sites. Thus,
acidic strength of the catalyst is an important factor in directing the product
selectivity.35,46,47 Therefore, careful adjustment of acid/base nature would promote the
synthesis of a new catalytic material with enhanced activity for selective organic
transformations. Molybdenum incorporation makes the material more acidic and the
amount of Mo6+ in the lattice determines the strength of acidity in each catalyst.
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ChemCatChem
Having analyzed the optimum reaction conditions for 2-phenylbenzimidazole, the
substrate scope and limitations were explored for different types of amine substrates.
Many substituents on the phenyl group of benzylamine, such as p-Me, p-OMe, m-Cl, p-
Cl, were tolerant to the oxidative cyclization (Table 4). Different 1,2-diaminobenzene
derivatives were also tested under similar reaction conditions. Electron donating or
electron withdrawing groups on the aromatic ring of 1,2-diaminobenzenes were well
tolerated under these cyclization reaction conditions, and the desired benzimidazoles
were obtained in good yields.
Enhanced surface characteristics of the modified catalyst, showed a remarkable effect on
the selective synthesis of 2-phenylbenzimidazole. This reaction is likely to follow the
pathway of coordination of Schiff bases (imine intermediate, 2) onto the acid sites of the
catalyst, followed by cyclization and dehydrogenation to give desired product as shown
in Scheme 2.47 Stabilization of intermediate 2 (Schiff base), by the acidic sites of the
catalyst, hinders the reaction of a second molecule of aldehyde with the imine
intermediate, thereby inhibiting the formation of disubstituted product. Combined effects
of acid-base strength, O2 vacancy density, and surface defects play a major role in
controlling the product selectivity in the above-mentioned reaction mechanism. Overall,
careful modification of mesoporous cobalt oxide catalyst by introducing molybdenum in
minor quantities has been recognized as a tool to develop a unique catalytic material for
selective synthesis of benzimidazoles.
5. Conclusion
In conclusion, a one-pot, highly selective, and efficient catalytic protocol has been
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developed for the synthesis of 2-substituted benzimidazoles. Catalytic activity of Co3O4
has been improved by introducing molybdenum as an impurity into the lattice structure,
where Mo6+ replaces the Co3+ in the crystal lattice, which alters the surface oxygen
vacancies, surface defects, and redox properties of the original material. The reaction is
applicable to a wide range of diamine and benzylamine substrates with excellent yields.
A plausible mechanism is proposed. This developed synthesis protocol provides an
attractive pathway for selective benzimidazole production, being environmentally benign,
and cost effective, using an earth-abundant metal, and forming H2O and H2 as the sole
byproducts.
Acknowledgement
This work was supported by the Chemical, Geochemical and Biosciences Division of the
Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy under
Grant DE-FGO2-86ER13622.A000. The HR-TEM studies were performed using the
facilities in the UConn/Thermo Fisher Scientific Center for Advanced Microscopy and
Materials Analysis (CAMMA). We thank the Bioscience Electron Microscopy
Laboratory of the University of Connecticut (UConn) for use of Nova NanoSEM and
Oxford EDS purchased through NSF grant #1126100.
Keywords: Lattice expansion, Mesoporous materials, Molybdenum substitution, Surface
oxygen vacancies, 2-Substituted benzimidazoles.
References
1
2
J. Wan, S. Gan, J. Wu and Y. Pan, Green Chem., 2009, 11, 1633–1637.
Z. H. Zhang, L. Yin and Y. M. Wang, Catal. Commun., 2007, 8, 1126–1131.
27
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801085
ChemCatChem
3
4
5
R. J. Perry and B. D. Wilson, J. Org. Chem., 1993, 58, 7016–7021.
C. T. Brain and S. A. Brunton, Tetrahedron Lett., 2002, 43, 1893–1895.
M. P. Surpur, P. R. Singh, S. B. Patil and S. D. Samant, Synth. Commun., 2007,
37, 1375–1379.
6
L. M. Dudd, E. Venardou, E. Garcia-Verdugo, P. Licence, A. J. Blake, C. Wilson
and M. Poliakoff, Green Chem., 2003, 5, 187–192.
7
8
9
10
F. Rajabi, S. De and R. Luque, Catal. Letters, 2015, 145, 1566–1570.
Z. Xu, D. Wang, X. Yu and Y. Yang, Adv. Synth. Catal., 2017, 359, 3332–3340.
T. Hille, T. Irrgang and R. Kempe, Chem. A Eurpean J., 2014, 20, 5569–5572.
R. Ramachandran, G. Prakash, S. Selvamurugan, P. Viswanathamurthi, J. G.
Malecki and V. Ramkumar, Dalt. Trans., 2014, 43, 7889–7902.
K. D. Parghi and R. V. Jayaram, Catal. Commun., 2010, 11, 1205–1210.
S. B. Rathod, M. K. Lande and B. R. Arbad, Bull. Korean Chem. Soc., 2010, 31,
2835–2840.
11
12
13
14
J. Dawody, M. Skoglundh and E. Fridell, J. Mater. Sci. Lett., 2004, 209, 215–225.
E. C. Tyo, C. Yin, M. Di Vece, Q. Qian, G. Kwon, S. Lee, B. Lee, J. E. Debartolo,
S. Seifert, R. E. Winans, R. Si, B. Ricks, S. Goergen, M. Rutter, B. Zugic, M.
Flytzani-Stephanopoulos, Z. W. Wang, R. E. Palmer, M. Neurock and S. Vajda,
ACS Catal., 2012, 2, 2409–2423.
15
I. Bauer, H. Kn, C. À. H. Bond, F. Cross and C. Arylalkylation, Chem. Rev., 2015,
115, 3170–3387.
16
17
P. J. Chirik, Acc. Chem. Res., 2015, 48, 1687–1695.
A. Mukherjee, A. Nerush, G. Leitus, L. J. W. Shimon, Y. Ben David, N. Angel, E.
Jalapa and D. Milstein, J. Am. Chem. Soc., 2016, 138, 4298–4301.
S. Chakraborty, P. Bhattacharya, H. Dai and H. Guan, Acc. Chem. Res., 2015, 48,
1995–2003.
W. Song, Z. Ren, S. Y. Chen, Y. Meng, S. Biswas, P. Nandi, H. A. Elsen, P. X.
Gao and S. L. Suib, ACS Appl. Mater. Interfaces, 2016, 8, 20802–20813.
N. D. Wasalathanthri, A. S. Poyraz, S. Biswas, Y. Meng, C. H. Kuo, D. A. Kriz
and S. L. Suib, J. Phys. Chem. C, 2015, 119, 1473–1482.
C. Weerakkody, S. Biswas, W. Song, J. He, N. Wasalathanthri, S. Dissanayake, D.
A. Kriz, B. Dutta and S. L. Suib, Appl. Catal. B Environ., 2018, 221, 681–690.
A. S. Poyraz, C.-H. Kuo, S. Biswas, C. K. King’ondu and S. L. Suib, Nat.
Commun., 2013, 4, 2952.
W. Song, A. S. Poyraz, Y. Meng, Z. Ren, S. Y. Chen and S. L. Suib, Chem.
Mater., 2014, 26, 4629–4639.
18
19
20
21
22
23
24
25
26
B. S. Yeo and A. T. Bell, J. Am. Chem. Soc., 2011, 133, 5587–5593.
B. Rivas-murias, J. Raman Spectrosc., 2017, 48, 837–841.
J. Yang, H. Liu, W. N. Martens and R. L. Frost, J. Phys. Chem. C, 2010, 114, 111–
119.
27
Z. Ren, Y. Guo, Z. Zhang, C. Liu and P.-X. Gao, J. Mater. Chem. A, 2013, 1,
9897.
28
29
Y. Ding, L. Zhu, A. Huang and X. Zhao, Catal. Sci. Technol., 2012, 2, 1977–1984.
M. C. Biesinger, B. P. Payne, A. P. Grosvenor, L. W. M. Lau, A. R. Gerson, R. St
and C. Smart, Appl. Surf. Sci., 2011, 257, 2717–2730.
30
P. Zhang, X. Li, Q. Zhao and S. Liu, Nanoscale Resech Lett., 2011, 6, 323–330.
28
This article is protected by copyright. All rights reserved.

10.1002/cctc.201801085
ChemCatChem
31
32
33
M. J. Jackman, A. G. Thomas and C. Muryn, J. Phys. Chem. C, 2015, 119, 13682–
13690.
S. Biswas, B. Dutta, K. Mullick, C.-H. Kuo, A. S. Poyraz and S. L. Suib, ACS
Catal., 2015, 5, 4394–4403.
V. D. Makwana, Y.-C. Son, A. R. Howell and S. L. Suib, J. Catal., 2002, 210, 46–
52.
34
35
36
37
38
A. Grirrane, A. Corma and H. Garcia, Science (80-. )., 2008, 322, 1661–1665.
P. Daw, Y. Ben-David and D. Milstein, ACS Catal., 2017, 7, 7456–7460.
X. Liu and B. Shan, Catal. Sci. Technol., 2017, 7, 4462–4469.
L. Wang, Y. Yu, H. He, Y. Zhang, X. Qin and B. Wang, Sci. Rep., 2017, 12845.
C. Mrabet, N. Mahdhi, A. Boukhachem, M. Amlouk and T. Manoubi, J. Alloys
Compd., 2016, 688, 122–132.
39
40
W. Shan, W. Shen and C. Li, Chem. Mater., 2003, 88, 4761–4767.
B. Santara, P. K. Giri, K. Imakita and M. Fujii, J. Phys. D. Appl. Phys., 2014, 47,
215302.
41
42
43
J. Sheng, U. Welzel, E. J. Mittemeijer, J. Sheng, U. Welzel and E. J. Mittemeijer,
Appl. Phys. Lett., 2010, 97, 153109.
C. R. Aita, M. C. Cisneros-morales and E. E. Hoppe, J. Phys. Chem. C, 2012, 116,
26679–26680.
R. Banerjee, E. A. Sperling, G. B. Thompson, H. L. Fraser, S. Bose, P. Ayyub, R.
Banerjee, E. A. Sperling, G. B. Thompson and H. L. Fraser, Appl. Phusics Lett.,
2003, 82, 4250–4252.
44
45
46
47
C. Stella, N. Soundararajan, K. Ramachandran, C. Stella, N. Soundararajan and K.
Ramachandran, AIP Adv., 2016, 87104, 87104–87114.
A. Jha, D. Jeong, Y. Lee, I. W. Nah and H. Roh, RSC Adv., 2015, 5, 103023–
103029.
A. Teimouri, A. Najafi, H. Salavati and L. Ghorbanian, J. Mol. Catal. A. Chem.,
2013, 373, 38–45.
A. Teimouri and A. N. Chermahini, J. Mol. Catal. A Chem., 2011, 346, 39–45.
29
This article is protected by copyright. All rights reserved.