Functional POM-catalyst for selective oxidative dehydrogenative couplings under aerobic conditions

Article abstract of DOI:10.1016/j.mcat.2021.111396

Development of selective and efficient reusable catalytic systems for sustainable chemical production under benign conditions is attractive and received much attention. Herein, we report a rod-shaped octadecyl trimethylammonium functionalized Keggin-type polyoxometalate [PMO₁₂O₄₀] hybrids (OTA-POM) as an efficient heterogeneous catalyst for selective oxidative dehydrogenative couplings under aerobic conditions without any additive or external base. The catalyst recovery and subsequent five successive recyclability studies of hybrid POM confirms the heterogeneous nature of present catalytic system.

Full text of DOI:10.1016/j.mcat.2021.111396

Molecular Catalysis 502 (2021) 111396
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Functional POM-catalyst for selective oxidative dehydrogenative couplings
under aerobic conditions
,
,
Elavarasan Samaraja, Ekambaram Balaramanb *, Sasidharan Manickama *
a Energy Storage, Conversion and Catalysis Laboratory, SRM Research Institute and Department of Chemistry, SRM Institute of Science & Technology, Kattankulathur,
Chennai, 603203, India
b Department of Chemistry, Indian Institute of Science Education and Research (IISER) Tirupati, Tirupati, 517507, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Development of selective and efficient reusable catalytic systems for sustainable chemical production under
benign conditions is attractive and received much attention. Herein, we report a rod-shaped octadecyl trime-
thylammonium functionalized Keggin-type polyoxometalate [PMO12O40] hybrids (OTA-POM) as an efficient
heterogeneous catalyst for selective oxidative dehydrogenative couplings under aerobic conditions without any
additive or external base. The catalyst recovery and subsequent five successive recyclability studies of hybrid
POM confirms the heterogeneous nature of present catalytic system.
Polyoxometalates
Heterogeneous catalyst
Oxidative dehydrogenation
Benzaldehydes
Imines
Phenyl benzimidazoles
1. Introduction
aldehydes and the use of stoichiometric amount of harmful reagent such
as acid chlorides [40,41]. Alternatively, past two decades, one-pot
Selective oxidation of alcohols to carbonyl compounds receives
paramount interest owing to their applications in pharmaceuticals, fine
chemicals, vitamins, and fragrances [1–5]. Conventionally, stoichio-
metric amounts of Mn salts, Cr salts, hypochlorite, hypervalent iodine,
O2/base-metal catalysts, and oxalyl chloride have been used as oxidants,
which generate huge undesirable wastes [6–16]. Moreover, alkali ad-
ditives such as potassium tert-butoxide, carbonate, and organic bases are
in common place to achieve high yield of product under mild conditions
[17–20].
tandem homo-coupling of amines or cross-coupling of amines and al-
cohols were also explored using precious metal catalysts [42–62].
However, harsh reaction conditions, use of base/ligand additives, and
expensive precious metals hamper their large-scale synthetic utility.
Similarly, 2-benzimidazole and its derivatives find wide-spread appli-
cations in pharmaceutical industry as antiulcer, antivirals, antifungals,
and antihypertensive agents [63–67]. Over the years, benzimidazoles
were prepared by condensation of carboxylic acids with o-aminoanilines
or by oxidative coupling with aldehydes [68,69]. However, the major
limitation of these methods is its competitive formation of 2-substituted
or 1,2-substituted benzimidazoles and use of toxic precious metals.
Recently, polyoxometalates (POMs), anionic metal-oxygen clusters,
received great attention in the field of oxidation catalysis [70–75].
However, unmodified POMs as catalysts show poor selectivity, reus-
ability, and difficulty in separation of the catalysts. To heterogenize the
POMs and overcome these issues, different strategies such as impreg-
nation into porous matrix, entrapment in sol-gel supports, and surface
grafting, have been studied [76–79]. In this context, Wei et al. investi-
gated POMs as an inorganic ligand to chelate different metals as well as
iodine for aerobic oxidation of alcohols and homo-coupling of amines to
imines [80]. Later, Vafaei-Nezhad et al. proposed ion-pair immobiliza-
tion with organic cations with Amberlite by electrostatic attraction to
heterogenize the catalyst [81–85]. On extension of this strategy, Ikegami
Furthermore, most powerful methodologies established with organ-
ometallic catalysts (Scheme 1) involve noble metals such as Ru, Rh, Pd,
Os, Ir, Pt, Au as well as base metals like V, Mn, Fe, Co, and Cu [21–27]. In
the organometallic catalysts, the ligands play a key role during the
course of reaction. In addition, the ligands as either additive or
metal-complexes during the reaction are sensitive to air/moisture and
pose challenges to durability and recyclability. Hence, development of
stable, recyclable, and efficient protocol in absence of base, additives,
and ligands under aerobic conditions would be more sustainable but
challenging.
Similarly, imines and their derivatives are important synthons in
organic chemistry since the C=N can readily react to give wide range of
heterocycles, natural products, pharmaceutically and biologically active
compounds [28–39]. Conventional synthesis of imines involving
* Corresponding authors.
E-mail addresses: eb.raman@iisertirupati.ac.in (E. Balaraman), sasidham@srmist.edu.in (S. Manickam).
https://doi.org/10.1016/j.mcat.2021.111396
Received 17 July 2020; Received in revised form 28 December 2020; Accepted 30 December 2020
Available online 20 January 2021
2468-8231/© 2021 Elsevier B.V. All rights reserved.

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
et al. reported an emulsion-based poly(N-alkylacrylamide) polymer and
15 mL of distilled water and the pH was adjusted to 1 using o-phosphoric
acid (solution A). Na2MoO4.2H2O (4 mmol) was dissolved in 35 mL of
distilled water (Solution B). Solution B was added to solution A slowly
and stirred for 24 h. The obtained solid was filtered off and washed with
hot water and ethanol three times. The final product was dried at 80 ◦C
for 12 h and used as a catalyst.
3-
phosphotungstic acid [Keggin-type polyoxometalates, [PW12O40
]
catalyst for oxidation of alcohols with H2O2, where the catalyst was
recovered upon addition of diethylether [86]. Hou et al. demonstrated
ionic liquid conjugate of POMs for olefin epoxidation [87]. Similarly,
amino-functionalized organic cation with [PW12O40
]
3- was also reported
by Wang et al. for efficient oxidation of alcohols with H2O2 [88,89]. Of
late, Keshavarz et al. reported an acidic ionic liquid
piperazinium-polyoxometalate hybrid catalyst for selective oxidation of
alcohols [90]. Although, the protocols for oxidation have now been
improved especially by using H2O2, the development of cheap catalyst
for direct activation of molecular oxygen (O2) would be highly sus-
tainable but more challenging.
2.3. Synthesis of benzaldehydes, imines, and phenyl benzimidazoles
Benzyl alcohol (1 mmol), OTA-POM (0.01 mmol) and toluene 1 mL
were taken in a 60 mL reaction tube. After adding the reactants, the tube
was filled with O2. Then the reaction mixture was stirred at 130 ◦C for 18
h. After completion of reaction, the content was cooled down to room
temperature and catalyst was filtered off through celite pad and washed
with ethyl acetate (3 × 10 mL). Solvents from the filtrate were removed
using rotary evaporator and the crude samples were analysed using TLC
and GC techniques. Column chromatography was performed to purify
the samples using hexane and ethyl acetate as eluents. The above gen-
eral procedure was followed for the synthesis of imines and benzimid-
azoles. In case of imine synthesis, anilines (1.1 mmol) were used and for
benzimidazoles synthesis, 1.2 mmol of o-phenylenediamines were used.
Herein, we report room temperature synthesis of octadecyl-
trimethylammonium cation (OTA) functionalized Keggin-type phos-
phomolybdate [PMo12O40
]
3- in one-step, where phosphoric acid used in
the synthesis acts as both phosphorous source as well as to maintain the
pH of reaction medium. The synthesised OTA-POM acts as an efficient
reusable catalyst for the synthesis of aldehydes, imines, and benzimid-
azoles by aerobic route under base-, ligand-, and additive- free condi-
tions using environment friendly O2 (Scheme 2).
2. Experimental
2.4. Leaching and recycling test
2.1. Materials and methods
To study leaching of any active POM into solution, hot filtration test
was performed considering benzyl alcohol oxidation as a test reaction.
As depicted in benzaldehyde synthesis, the same experiment was carried
out with benzyl alcohol (1 mmol), for initial period of 9 h by maintaining
all other reaction parameters identical. The OTA-POM was recovered
from the reaction medium and the filtrate was kept under the same re-
action conditions to continue further for 18 h. The course of reaction was
followed by monitoring conversion and product formation using gas
chromatography (GC). In the recyclability test, the OTA-POM was
separated by centrifugation of solid catalyst from the first reaction run,
washed carefully with ethanol, dried in a vacuum oven at 60 ◦C for 12 h
to remove any occluded organic chemicals on the catalyst surface. The
dried catalyst was reused for benzaldehyde synthesis for another five
successive cycles under the identical conditions.
Sodium molybdate dihydrate, octadecyltrimethylammonium chlo-
ride, and o-phosphoricacid were purchased from Sigma-Aldrich and
used without any further purification. Solvents were procured from
Fisher scientific and used as received. Powder X-ray diffraction (XRD)
pattern was recorded with a Bruker (D8 Advance, Davinci) with CuK
α
rays (λ =1.5418 Å). The morphology was observed using transmission
electron microscope (HRTEM) acquired with a JEOL (JEM-2100 Plus)
and scanning electron microscope using FEI, Quanta 200. Fourier
transform infrared spectra of the samples were recorded using Shimadzu
IR Tracer-100. CHN elemental analysis was performed using Vario EL III
Elementar instrument. Thermogravimetric (TG) analysis was carried out
using Netzsch-STA 2500 instrument (10 ◦C/minute). Organic reaction
progress was monitored by thin-layer chromatography and gas chro-
matography (Agilent 7890B with HP5 capillary column fitted with FID
detector). 1H and 13C NMR spectral data were acquired on Bruker
Avance-III 500 MHz instrument.
3. Results and discussion
3.1. Catalyst characterization
2.2. Synthesis of phosphomolybdate catalyst (OTA-POM)
Prior to catalysis, the OTA-POM hybrid catalyst was characterized by
XRD, FTIR, FE-SEM, HRTEM, elemental analysis by CHN, ICP, and XPS
techniques. Fig. 1 and inset show XRD reflections at 2.64◦, 3.12◦ and
Octadecyltrimethylammonium chloride (1 mmol) was dissolved in
Scheme 1. One step synthesis of benzaldehydes, imines, and benzimidazoles.
2

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
Scheme 2. Synthesis of OTA-POM catalyst via self-assembly.
9.23◦ 2θ corresponding to OTA cation [91]. The appearance of peak
broadening and peak shifting towards low-angle for OTA-POM (2.54◦)
signifies an increase in crystal lattice due to exchange of Na+ by [OTA]+
cations and the broad peak feature indeed confirms presence of organic
molecules in the OTA-POM. The intense peak at 2θ ~9◦ is attributed to
the Keggin-type ion [PMo12O40
]
3- [92].
To get further insight, FT-IR spectra of OTA-POM was recorded
(Fig. S1). The FT-IR spectra showed characteristic bands at 3000ꢀ 2800
cm-1 indicating the presence of OTA ions in the hybrid materials similar
to OTAC material. In addition, FT-IR spectra also shows the presence of
Keggin structure from the characteristic bands at 1100ꢀ 750 cm-1 cor-
-
–
responding to PO , Mo-O, and Mo-O-Mo vibrational frequencies of
phosphomolybdic acid (PMA). Thus, the FT-IR spectrum of OTA-POM
reveals the existence of both the PMA and OTA ions vibrational fre-
quencies clearly indicating the formation of (OTA)3PMo12O40 hybrid
catalyst. The FE-SEM images (Fig. 2A-G) of OTA-POM show primary
crystals, however, agglomerates in to larger crystals of size ~ 1ꢀ 2 μm.
The EDX spectrum of OTA-POM (Fig. 2B) further confirms the existence
of different elements like C, O, N, Mo, and P (elemental mapping). In
addition, elemental analysis of OTA-POM by CHN analysis found to
contain 28.95 %, 5.06 %, and 1.54 % of carbon, hydrogen, and nitrogen,
Fig. 1. XRD pattern of OTA-POM.
Fig. 2. (A) FE-SEM image; (B) EDX spectrum; (C-G) Elemental mapping of C, O, N, P, and Mo; (H-J) TEM images; and (K) electron diffraction (ED) pattern of
OTA-POM.
3

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
respectively, corroborating the formation of hybrid (OTA)3PMo12O40
materials. ICP-AES analysis was performed to determine the concen-
trations of Mo and P present in the sample, and were found to be 6.12 %
and 1.21 %, respectively.
Table 1
Optimization of aerobic oxidation of benzyl alcohol using OTA-POMa.
TEM images (Fig. 2H-J) also show the rod-like morphology with size
feature of ~1
μ
m in length. In addition, electron diffraction (ED) pattern
3-
of OTA-POM (Fig. 2K) shows diffused ring pattern for [PMo12O40
]
Entries
Catalyst mol %
T (oC)
Solvents
t (h)
Yield (%)b
suggesting the amorphous nature due to the presence of the octadecyl-
trimethylammonium cations (OTA). The X-ray photoelectron spectros-
copy (XPS) was recorded to further confirm the different elements
present in the OTA-POM. Fig. 3A exhibits the survey spectrum revealing
the presence of C, N, O, and Mo corroborating the EDX observation; the
C 1s spectrum (Fig. 3B) can be deconvoluted into two peaks, where the
1
1.0
1.0
1.0
1.0
1.0
1.0
1.0
1.0
0.66
0.33
1.0
1.0
1.0
1.0
NIL
130
130
130
130
130
130
130
130
130
130
120
110
130
130
130
H2O
48
48
48
48
48
24
18
12
18
18
18
18
18
18
18
Trace
83
2
ACN
3
DMF
32
4
Ethanol
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
41
5
>99
>99
>99
80
6
7
binding energy at 284.1 eV is attributed to sp3 hybridized C C bond
8
–
9
77
present in the alkyl chain and the peak at high binding energy of 285.6
10
11
12
13c
14d
15e
49
eV is attributed to the C-N bond present in OTA ions.
50
The O 1s spectrum (Fig. 3C) of OTA-POM showed a single broad peak
that can be deconvoluted into two peaks centred at 530.2 eV and 531.9
eV corresponding to O2- of polyoxometalates and Mo-O, respectively
[93]. The XPS signals (Fig. 3D) at 232.2 eV and 235.4 eV are ascribed to
the spin-orbit coupling of Mo6+ 3d5/2 and 3d3/2, respectively [94]. In
addition, the TGA result of OTA-POM (Fig S2) indicates the decompo-
sition temperature starts at 280 ◦C, showing high thermal stability of
OTA-POM which can be used for the organic transformation reactions
even at high temperature. Thus, all characterization results suggest the
formation of functional [PMo12O40] hybrid material.
33
72
14
Trace
a
Reaction conditions: Benzyl alcohol (1.0 mmol), Solvent (1 mL) under O2
atm.
b
c
GC analysis based on benzyl alcohol.
Reaction carried out under air atm.
Reaction performed under Ar atm.
No catalyst.
d
e
non-polar toluene as the best choice to achieve complete conversion of
benzyl alcohol and aldehyde selectivity. The optimal reaction time
(entries 6–8) was found to be 18 h, and shorter reaction duration of 12 h
reduced the conversion and product yield (Table 1, entry 8). Control
experiments with catalyst loading revealed that reducing catalyst
amount to 20 mg and 10 mg lowered the yields to 77 % and 49 %,
respectively (Table 1, entries 9–10).
3.2. Aerobic oxidation of benzyl alcohols to benzaldehydes
In preliminary investigation, the catalytic activity of OTA-POM was
tested for the oxidation of benzyl alcohol (1a) to benzaldehyde (2a)
under base-free conditions. Our initial catalytic run with 30 mg catalyst
at 130 ◦C for 48 h using water as solvent did not result in benzaldehyde
and subsequently, we proceed to scrutinize different solvents. Screening
of solvents with different polarities (Table 1, entries 1–5) indicated that
Similarly, reactions performed at lower temperature of 120 and 110
◦C, (Table 1, entries 11–12) furnished lower yield of 50 and 33 %,
Fig. 3. XPS spectra of OTA-POM: (A) Survey scan; (B) C1 s; (C) O1 s; and (D) Mo3d.
4

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
respectively. Likewise, the reaction progress sluggishly under air
ambiance (Table 1, entry 13) to give 72 % yield. More importantly, the
benzaldehyde yield decreased to 14 % in Ar atmosphere (Table 1, entry
14) indicating the importance of O2 for oxidative transformation.
Without catalyst the reaction did not occur (Table 1, entry 15). Thus,
conditions optimization with solvents, catalyst loading, and temperature
suggested that toluene as the best solvent and using 30 mg catalyst at
130 ◦C with 1 atm O2 as optimal reaction conditions.
3.3. Coupling of amines to imines using OTA-POMs
Encouraged with the aerobic oxidation of benzylic alcohols to
benzaldehydes over OTA-POMs, we envisioned that, if polyoxometalate-
catalyst is also a good catalyst for imine synthesis through aerobic
addition reactions of alcohols and amines to imines under base-free
conditions (Table S1). Screening of solvents with different polarity,
catalyst amount, and reaction temperature indicated that an optimum
reaction conditions with 30 mg catalyst at 130 ◦C in toluene solvent with
O2 (1 atm). Simple benzyl alcohol and aniline reacts to give a quanti-
tative yield of imine product (4aa). Under optimized conditions, a series
of primary alcohols were coupled with anilines and substituted anilines
(Table 3) to give imines in good to excellent yields. When coupled with
anilines, benzyl alcohols with the electron donating –CH3 and –OCH3
groups at o- and p-positions have little influence over the conversion and
selectivity (4ca, 4 da, & 4ea) than the p-substituted compound (4ba)
possibly due to higher nucleophilicity of anilines. Besides, benzyl
alcohol with electron-withdrawing groups also furnishes good to
excellent yield (80 %, 4fa and 97 %, 4ga).
We next examined the substrate scope of OTA-POM catalyst system
for alcohols with diverse functional groups (1a-m) and electronic
properties (Table 2). All substituted benzyl alcohols with both electron-
donating and electron withdrawing groups underwent the reaction
smoothly to give the corresponding aldehydes. The presence of electron
donating ꢀ CH3 at m- and p–positions affords 96 % (2c) and 92 % yields
(2b), respectively. Meanwhile, the reaction seems to be sensitive to
steric hindrance as revealed by lower yield obtained for ortho-
substituted benzyl alcohol than the meta- and para-substituted ana-
logues. For instance, o-CH3 (2d), o-OCH3 (2e) and o-Cl (2 g) substituted
compounds led to lower yields of 64 %, 70 %, and 68 %, respectively. To
our surprise, electron withdrawing NO2 group at ortho-position affords
an excellent yield of 97 % to the corresponding 2-nitrobenzaldehyde
(2f).
Regarding aniline substrate scope (4ab-4 ah) similar to benzyl al-
cohols, electron donating group at p- or o-positions (4ab and 4ad) af-
fords lower yields compared to m-OCH3 (4ac, 94 %). The presence of
halogen substituents like –Cl and –Br groups at p-position consistently
afford lower yields (4ae, 62 % and 4af, 60 %) due to presence of
deactivating halogens in the nucleophiles.
Furthermore, m,m-di-NO2-benzyl alcohol gave quantitative yield
(2k, >99 %) for the corresponding aldehyde regardless of steric and
electronic effect. Switching from chloro to fluoro substituent at the p-
position did not affect the alcohol conversion and selectivity (2 h, 91 %).
However, 3,5-di-Cl-Ph analogue consistently showed lower yield for the
corresponding aldehyde (2 L). Interestingly, heteroaromatic 2-pyridine-
methanol and 3-pyridinemethanol also afford excellent yields of 93 %
(2i) and >99 % (2 j), respectively to the corresponding products
demonstrating high functional group tolerance of present catalytic
protocol.
Furthermore, exploration of electron-poor heteroaromatic amines
substrates like pyridine-2-amine (3 g) and pyrazine-2-amine (3 h) af-
fords moderate yields 4 ag (67 %) and 4 ah (52 %), respectively.
Furthermore, the aliphatic amine also readily couples with benzyl
alcohol to the desired imine product (4ai, 78 %). Thus, the present
catalytic protocol demonstrates the suitability of OTA-POMs towards
imine synthesis under base- and ligand-free conditions.
However, the aliphatic alcohols fail to undergo aerobic oxidation
under identical conditions. More importantly, the POMs catalyst ex-
hibits high chemo selectivity towards selective oxidation of cinnamyl
3.4. Synthesis of benzimidazole derivatives using OTA-POMs
–
alcohol to the corresponding aldehyde without affecting the C C bond
–
Next, we examined the efficacy of OTA-POMs towards cascade re-
action between benzyl alcohol and o-phenylenediamine to obtain 2-
substituted benzimidazole. Table S2 shows the effect of solvents,
(2 m). All the substrates showed high turnover number (TON) ranging
from 64-100.
Table 2
Synthesis of aldehydes from benzyl alcohols using OTA-POMsa and the turnover numbers (TON).
a
Reaction conditions: Benzyl alcohols (1.0 mmol), OTA-POM (1.0 mol%) and toluene (1.0 mL), temperature 130 ◦C, O2 atmosphere, reaction time 18 h. bGC analysis
and yields based on benzyl alcohols. cValues in the parenthesis refers to turnover number (TON).
5

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
Table 3
Synthesis of imines from benzylalcohols and anilines using OTA-POM a.
a
Reaction conditions: Benzyl alcohols (1.0 mmol), aniline (1.2 mmol), OTA-POM (1.0 mol%), toluene (1.0 mL), temperature 130 ◦C, O2 atmosphere, reaction time
18 h. bGC analysis and yields based on benzyl alcohols. cValues in the parenthesis refers to turnover number (TON).
temperature, and catalyst loading for reaction involving o-phenyl-
enediamine and benzyl alcohols. The optimization condition for benz-
imidazole formation was found to be 130 ◦C in a toluene solvent for 18 h
under O2 ambience (1 atm). With the optimized reaction conditions in
hand for the cascade reaction of benzimidazole, we next examined the
substrate scope of various substituted benzyl alcohols as summarized in
Table 4.
The coupling of unsubstituted benzyl alcohol with o-
Table 4
Synthesis of Phenyl benzimidazoles from benzylalcohols and o-Phenylenediamaine using OTA-POMa.
a
Reaction conditions: Benzyl alcohols (1.0 mmol), o-phenylenediamines (1.2 mmol), OTA-POM (1.0 mol%), toluene (1.0 mL), temperature 130 ◦C, O2 atmosphere,
reaction time 18 h. bGC analysis and yields based on benzyl alcohols. cValues in the parenthesis refers to turnover number (TON).
6

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
phenylenediamine gives the corresponding 2-phenylbenzimidazole in
quantitative yield (6aa, >99 %). When R-group on aromatic ring of
alcohol was appended with electron-donating groups such as p-CH3
(6ba), m-CH3 (6ca) and o-CH3 (6 da), and p-OCH3 (6ea), it showed
facile conversion to give the corresponding benzimidazole compounds
with good yields (72–92 %, Table 4). On the other hand, when R-group
is appended with electron-withdrawing functionalities like –Cl (6fa) and
–F (6ga) exhibited higher yields (90–100 %) and nature of substituents
on the aromatic ring has less influence over benzimidazole formation.
Significantly, the hetero atom containing benzyl alcohols such as 2-Pyr-
idinemethanol (1 h) and 3-Pyridinemethanol (1i) afford excellent yields
>99 % and 98 %, respectively, despite of electron poor nature of starting
material. On the other hand, substituent effect on the aromatic ring of o-
phenylenediamine has no impact on the conversion and benzimidazole
derivatives as evidenced from 4-methyl-o-phenylenediamine (5a), 4-Cl-
o-phenylenediamine (5b) and 4-NO2-o-phenylenediamine (5c) which
affords the target products with 96 - >99 % yield.
Fig. 5. Recyclability test of OTA-POM.
3.5. Hot filtration and recyclability tests
To verify any homogeneous catalysis by leached metals present in
the homogeneous solution, we have performed leaching test for reaction
of aerobic oxidation of benzyl alcohol (1a) to benzaldehyde (2a)
(Fig. 4). In a typical leaching test reaction, the OTA-POM catalyst was
deliberatively removed from the reaction vessel after 9 h (~44 % con-
version) and the reaction was continued with filtrate under the same
reaction conditions for another 9 h. Absence of any noticeable progress
in the reaction confirms the absence of catalytic activity by homoge-
neous Mo-species.
Furthermore, the unique advantage of heterogeneous catalysts is
their recovery and recyclability. To investigate reusability of OTA-POM
catalyst, we performed oxidation of benzyl alcohol with benzaldehyde
under optimized reaction conditions. After the first reaction run, the
catalyst was separated by centrifugation, washed thoroughly with water
and ethyl acetate to remove adsorbed organics and finally vacuum dried
at 80 ◦C. The catalyst was reused for 5 successive catalytic runs under
identical conditions and the yield was decreased from 100 % to 88 %
after 5 successive reaction runs (Fig. 5).
Based on literature reports, a plausible reaction pathway for the
aerobic oxidation of benzyl alcohol is depicted in Scheme 3. Initially the
Mo(VI) oxidises the benzyl alcohol (1a) utilizing molecular O2 where Mo
(VI) is reduced to Mo(V) accompanied by removal of H2O to produce
benzaldehyde (2a) [95]. To prove the reversibility oxidation state, after
the completion of reaction, the recovered catalyst was analysed by XPS
(Fig S3). The survey spectra (A) and the binding energy values of Mo(VI)
(Mo3d) is evident from XPS spectrum (B) indicating robustness of pre-
sent catalytic protocol. In the catalytic cycle for imine synthesis (path
C), the addition of aniline forms an intermediate hemiaminal (11) [96]
Scheme 3. A plausible reaction mechanism of aerobic oxidation of
benzyl alcohols.
followed by dehydration to give imine (4aa). In pathway D, the initial
intermediate (12) undergoes cyclization (13) [97] followed by dehy-
drogenation utilizing O2 and OTA-POM to afford phenyl benzimidazole
(6aa).
4. Conclusion
In conclusion, we have developed a catalytically active octadecyl-
trimethylammonium functionalized Keggin-type polyoxometalate
[PMO12O40] hybrid as an efficient heterogeneous catalyst for oxidation
reactions under aerobic conditions. Characterization by XRD, SEM, and
TEM suggested hybrid architecture with rod-shaped morphological
features. The CHN, ICP and XPS analyses confirmed the presence of
various constituent elements and composition. The OTA-POM efficiently
catalyzed cascade reaction involving oxidative dehydrogenative
coupling of o-phenylenediamine with diverse benzyl alcohol to the
corresponding 2-substituted benzimidazole in quantitative yield (up to
Fig. 4. Leaching test of OTA-POM.
7

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
[11] D.B. Dess, J.C. Martin, Readily accessible 12-I-5 1 oxidant for the conversion of
primary and secondary alcohols to aldehydes and ketones, J. Org. Chem. 48 (1983)
4155–4156, https://doi.org/10.1021/jo00170a070.
>99 %) under base, ligand, and additive-free conditions. The OTA-
POM/O2 catalyst system also exhibited oxidative coupling between
benzyl alcohol and substituted anilines to the corresponding unsym-
metrical imines in good to excellent yields (60–99 %). Tested as oxida-
tion catalyst under aerobic conditions, the functional POM efficiently
catalyzed the benzylic alcohols to the corresponding aldehydes under
base-, additive-, and ligand-free conditions with wide tolerance for
functional groups. Verification of leaching of the active metals by a hot
filtration test as well as reusability of the retrieved OTA-POM catalysts
indicated its high stability in the aerobic oxidation of benzyl alcohols.
All experimental results demonstrated the robustness of OTA-POM cat-
alytic protocol in terms of versatility, recovery/reusability, and afford-
ability. We believe that the synthesis of OTA-POM catalyst paves the
way for a new family of robust heterogeneous catalysts for other
important and selective organic transformations.
[12] M.S. Sigman, D.R. Jensen, Ligand-modulated palladium-catalyzed aerobic alcohol
oxidations, Acc. Chem. Res. 39 (2006) 221–229, https://doi.org/10.1021/
ar040243m.
[13] R.A. Sheldon, I.W.C.E. Arends, G.J. Ten Brink, A. Dijksman, Green, catalytic
oxidations of alcohols, Acc. Chem. Res. 35 (2002) 774–781, https://doi.org/
10.1021/ar010075n.
´
¨
[14] G. Csjernyik, A.H. Ell, L. Fadini, B. Pugin, J.E. Backvall, Efficient ruthenium-
catalyzed aerobic oxidation of alcohols using a biomimetic coupled catalytic
system, J. Org. Chem. 67 (2002) 1657–1662, https://doi.org/10.1021/jo0163750.
¨
¨
¨
[15] B. Join, K. Moller, C. Ziebart, K. Schroder, D. Gordes, K. Thurow, A. Spannenberg,
K. Junge, M. Beller, Selective iron-catalyzed oxidation of benzylic and allylic
alcohols, Adv. Synth. Catal. 353 (2011) 3023–3030, https://doi.org/10.1002/
adsc.201100210.
[16] R. Noyori, M. Aoki, K. Sato, Green oxidation with aqueous hydrogen peroxide,
Chem. Commun. 3 (2003) 1977–1986, https://doi.org/10.1039/B303160H.
[17] J.M. Hoover, S.S. Stahl, Highly practical copper(I)/TEMPO catalyst system for
chemoselective aerobic oxidation of primary alcohols, J. Am. Chem. Soc. 133
(2011) 16901–16910, https://doi.org/10.1021/ja206230h.
[18] L. Peng, J. Zhang, Z. Xue, B. Han, X. Sang, C. Liu, G. Yang, Highly mesoporous
metal-organic framework assembled in a switchable solvent, Nat. Commun. 5
(2014) 4465, https://doi.org/10.1038/ncomms5465.
CRediT authorship contribution statement
Elavarasan Samaraj: Methodology, Data curation, Formal analysis,
Writing - original draft. Ekambaram Balaraman: Writing - review &
editing. Sasidharan Manickam: Supervision, Funding acquisition,
Writing - original draft.
[19] X. Kang, J. Zhang, W. Shang, T. Wu, P. Zhang, B. Han, Z. Wu, G. Mo, X. Xing, One-
step synthesis of highly efficient nanocatalysts on the supports with hierarchical
pores using porous ionic liquid-water gel, J. Am. Chem. Soc. 136 (2014)
3768–3771, https://doi.org/10.1021/ja5001517.
[20] J.M. Hoover, B.L. Ryland, S.S. Stahl, Copper/TEMPO-catalyzed aerobic alcohol
oxidation: mechanistic assessment of different catalyst systems, ACS Catal. 3
(2013) 2599–2605, https://doi.org/10.1021/cs400689a.
Declaration of Competing Interest
[21] C. Parmeggiani, F. Cardona, Transition metal-based catalysts in the aerobic
oxidation of alcohols, Green Chem. 14 (2012) 547–564, https://doi.org/10.1039/
C2GC16344F.
We, the authors declare that we have no known competing financial
interests and no personal relationships with anybody to influence the
work reported in this paper.
[22] C. Parmeggiani, C. Matassini, F. Cardona, A step forward towards sustainable
aerobic alcohol oxidation: new and revised catalysts based on transition metals on
solid supports, Green Chem. 19 (2017) 2030–2050, https://doi.org/10.1039/
C7GC00406K.
[23] S.K. Hanson, R. Tom Baker, J.C. Gordon, B.L. Scott, A.D. Sutton, D.L. Thorn,
Aerobic oxidation of pinacol by vanadium(V) dipicolinate complexes: evidence for
reduction to vanadium(III), J. Am. Chem. Soc. 131 (2009) 428–429, https://doi.
org/10.1021/ja807522n.
Acknowledgements
M. S. thanks Department of Chemistry, SRM SCIF, Nanotechnology
Research Centre, and Interdisciplinary Institute of Indian system of
medicine (IIISM) of SRMIST for providing research facilities. EB ac-
knowledges IISER-Tirupati and funding from SwarnaJayanti Fellowship
(DST/SJF/CSA-04/2019-2020 and SERB/F/5892/2020-2021).
¨
¨
¨
[24] M.T. Raisanen, A. Al-Hunaiti, E. Atosuo, M. Kemell, M. Leskela, T. Repo, Mn(II)
acetate: an efficient and versatile oxidation catalyst for alcohols, Catal. Sci.
Technol. 4 (2014) 2564–2573, https://doi.org/10.1039/C4CY00255E.
[25] H. Song, B. Kang, S.H. Hong, Fe-catalyzed acceptorless dehydrogenation of
secondary benzylic alcohols, ACS Catal. 4 (2014) 2889–2895, https://doi.org/
10.1021/cs5007316.
[26] G. Zhang, K.V. Vasudevan, B.L. Scott, S.K. Hanson, Understanding the mechanisms
of cobalt-catalyzed hydrogenation and dehydrogenation reactions, J. Am. Chem.
Soc. 135 (2013) 8668–8681, https://doi.org/10.1021/ja402679a.
[27] C. Chen, B. Liu, W. Chen, Copper/imidazolium/TEMPO-catalyzed aerobic
oxidation of benzylic alcohols in water, Synthesis (Stuttg). 45 (2013) 3387–3391,
https://doi.org/10.1055/s-0033-1339929.
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.mcat.2021.111396.
[28] J.A. Ma, Catalytic asymmetric synthesis of α- and β-amino phosphonic acid
derivatives, Chem. Soc. Rev. 35 (2006) 630–636, https://doi.org/10.1039/
References
B517100H.
¨
[29] A. Domling, Recent developments in isocyanide based multicomponent reactions in
[1] A. Rahimi, A. Azarpira, H. Kim, J. Ralph, S.S. Stahl, Chemoselective metal-free
aerobic alcohol oxidation in lignin, J. Am. Chem. Soc. 135 (2013) 6415–6418,
https://doi.org/10.1021/ja401793n.
applied chemistry, Chem. Rev. 106 (2006) 17–89, https://doi.org/10.1021/
cr0505728.
¨
[30] A. Erkkila, I. Majander, P.M. Pihko, Iminium catalysis, Chem. Rev. 107 (2007)
[2] R.W. Dugger, J.A. Ragan, D.H. Brown Ripin, Survey of GMP bulk reactions run in a
research facility between 1985 and 2002, Org. Process Res. Dev. (9) (2005)
253–258, https://doi.org/10.1021/op050021j.
5416–5470, https://doi.org/10.1021/cr068388p.
[31] J. Gawronski, N. Wascinska, J. Gajewy, Recent progress in Lewis base activation
and control of stereoselectivity in the additions of trimethylsilyl nucleophiles,
Chem. Rev. 108 (2008) 5227–5252, https://doi.org/10.1021/cr800421c.
[3] J.S. Carey, D. Laffan, C. Thomson, M.T. Williams, Analysis of the reactions used for
the preparation of drug candidate molecules, Org. Biomol. Chem. 4 (2006)
2337–2347, https://doi.org/10.1039/B602413K.
´˜
[32] M. Ordonez, H. Rojas-Cabrera, C. Cativiela, An overview of stereoselective
synthesis of α-aminophosphonic acids and derivatives, Tetrahedron 65 (2009)
[4] Y. Wang, J.L. DuBois, B. Hedman, K.O. Hodgson, T.D.P. Stack, Catalytic galactose
oxidase models: biomimetic Cu(II)-phenoxyl-radical reactivity, Science (80-.) 279
(1998) 537–540, https://doi.org/10.1126/science.279.5350.537.
17–49, https://doi.org/10.1016/j.tet.2008.09.083.
[33] S.F. Martin, Recent applications of imines as key intermediates in the synthesis of
alkaloids and novel nitrogen heterocycles, Pure Appl. Chem. 81 (2009) 195–204,
https://doi.org/10.1351/PAC-CON-08-07-03.
[5] G. Tojo, M. Fernandez, Oxidation of Alcohols to Aldehydes and Ketones: a Guide to
Current Common Practice (Basic Reactions in Organic Synthesis), 2006.
[6] R.V. Stevens, K.T. Chapman, H.N. Weller, Convenient and inexpensive procedure
for oxidation of secondary alcohols to ketones, J. Org. Chem. 45 (1980)
2030–2032, https://doi.org/10.1021/jo01298a066.
[34] S. Kobayashi, Y. Mori, J.S. Fossey, M.M. Salter, Catalytic enantioselective
formation of C-C bonds by addition to imines and hydrazones: a ten-year update,
Chem. Rev. 111 (2011) 2626–2704, https://doi.org/10.1021/cr100204f.
[35] J.H. Xie, S.F. Zhu, Q.L. Zhou, Transition metal-catalyzed enantioselective
hydrogenation of enamines and imines, Chem. Rev. 111 (2011) 1713–1760,
https://doi.org/10.1021/cr100218m.
[7] D.G. Lee, U.A. Spitzer, The aqueous dichromate oxidation of primary alcohols,
J. Org. Chem. 35 (1970) 3589–3590, https://doi.org/10.1021/jo00835a101.
[8] J.R. Holum, Study of the chromium (VI) oxide-pyridine complex, J. Org. Chem. 26
(1961) 4814–4816, https://doi.org/10.1021/jo01070a009.
[36] J. Adrio, J.C. Carretero, Novel dipolarophiles and dipoles in the metal-catalyzed
enantioselective 1,3-dipolar cycloaddition of azomethine ylides, Chem. Commun.
47 (2011) 6784–6794, https://doi.org/10.1039/C1CC10779H.
[9] R.J. Highet, W.C. Wildman, Solid manganese dioxide as an oxidizing agent, J. Am.
Chem. Soc. 77 (1955) 4399–4401, https://doi.org/10.1021/ja01621a062.
[10] K. Omura, D. Swern, Oxidation of alcohols by "activated" dimethyl sulfoxide a
preparative, steric and mechanistic study, Tetrahedron 34 (1978) 1651–1660,
https://doi.org/10.1016/0040-4020(78)80197-5.
[37] C.S. Marques, A.J. Burke, Advances in the catalytic asymmetric arylation of Imines
using organoboron reagents: an approach to chiral arylamines, ChemCatChem 3
(2011) 635–645, https://doi.org/10.1002/cctc.201000369.
[38] T.R. Ramadhar, R.A. Batey, Allylation of Imines and Their Derivatives with
Organoboron Reagents: Stereocontrolled Synthesis of Homoallylic Amines,
8

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
Synthesis (Stuttg), 2011, pp. 1321–1346, https://doi.org/10.1055/s-0030-
ribofuranosyl)benzimidazole, J. Med. Chem. 41 (1998) 1252–1262, https://doi.
org/10.1021/jm970559i.
1258434.
[39] M. Nielsen, D. Worgull, T. Zweifel, B. Gschwend, S. Bertelsen, K.A. Jørgensen,
Mechanisms in aminocatalysis, Chem. Commun. 47 (2011) 632–649, https://doi.
org/10.1039/C0CC02417A.
[64] W.A. Craigo, B.W. LeSueur, E.B. Skibo, Design of highly active analogues of the
pyrrolo[1,2-a]benzimidazole antitumor agents, J. Med. Chem. 42 (1999)
3324–3333, https://doi.org/10.1021/jm990029h.
[40] K.C. Nicolaou, C.J.N. Mathison, T. Montagnon, New reactions of IBX: oxidation of
nitrogen- and sulfur-containing substrates to afford useful synthetic intermediates,
Angew. Chemie - Int. Ed. 42 (2003) 4077–4082, https://doi.org/10.1002/
anie.200352076.
[65] W. Guba, L.G. Green, R.E. Martin, O. Roche, N. Kratochwil, H. Mauser, C. Bissantz,
A. Christ, M. Stahl, From astemizole to a novel hit series of small-molecule
somatostatin 5 receptor antagonists via GPCR affinity profiling, J. Med. Chem. 50
(2007) 6295–6298, https://doi.org/10.1021/jm701144e.
[41] T. Mukaiyama, A. Kawana, Y. Fukuda, J.I. Matsuo, Oxidation of various secondary
amines to imines with N-tert-butylphenylsulfinimidoyl chloride, Chem. Lett.
(2001) 390–391, https://doi.org/10.1246/cl.2001.390.
[66] L.J. Goossen, T. Knauber, Concise synthesis of telmisartan via decarboxylative
cross-coupling, J. Org. Chem. 73 (2008) 8631–8634, https://doi.org/10.1021/
jo801937h.
´
¨
[42] A.H. Ell, J.S.M. Samec, C. Brasse, J.E. Backvall, Dehydrogenation of aromatic
amines to imines via ruthenium-catalyzed hydrogen transfer, Chem. Commun. 10
(2002) 1144–1145, https://doi.org/10.1039/B202117J.
[67] S. Gangula, C.R. Elati, A. Neredla, S.R. Baddam, U.K. Neelam, R. Bandichhor,
A. Dongamanti, An improved process for the production of lansoprazole:
investigation of key parameters that influence the water content in final API, Org.
Process Res. Dev. 14 (2010) 229–233, https://doi.org/10.1021/op900258b.
[68] P.N. Preston, Synthesis, reactions, and spectroscopic properties of benzimidazoles,
Chem. Rev. 74 (1974) 279–314, https://doi.org/10.1021/cr60289a001.
[69] M. Banerjee, A. Chatterjee, V. Kumar, Z.T. Bhutia, D.G. Khandare, M.S. Majik, B.
G. Roy, A simple and efficient mechanochemical route for the synthesis of 2-aryl
benzothiazoles and substituted benzimidazoles, RSC Adv. 4 (2014) 39606–39611,
https://doi.org/10.1039/C4RA07058E.
[43] M. Largeron, A. Chiaroni, M.B. Fleury, Environmentally friendly chemoselective
oxidation of primary aliphatic amines by using a biomimetic electrocatalytic
system, Chem. - A Eur. J. 14 (2008) 996–1003, https://doi.org/10.1002/
chem.200700876.
[44] G. Jiang, J. Chen, J.S. Huang, C.M. Che, Highly efficient oxidation of amines to
imines by singlet oxygen and its application in ugi-type reactions, Org. Lett. 11
(2009) 4568–4571, https://doi.org/10.1021/ol9018166.
[45] K. Yamaguchi, N. Mizuno, Efficient heterogeneous aerobic oxidation of amines by a
supported ruthenium catalyst, Angew. Chemie - Int. Ed. 42 (2003) 1480–1483,
https://doi.org/10.1002/anie.200250779.
[70] A. Patel, S. Pathan, Solvent free selective oxidation of styrene and benzyl alcohol to
benzaldehyde over an eco-friendly and reusable catalyst,
undecamolybdophosphate supported onto neutral alumina, Ind. Eng. Chem. Res.
51 (2012) 732–740, https://doi.org/10.1021/ie2020608.
[46] T. Sonobe, K. Oisaki, M. Kanai, Catalytic aerobic production of imines en route to
mild, green, and concise derivatizations of amines, Chem. Sci. 3 (2012)
3249–3255, https://doi.org/10.1039/C2SC20699D.
[71] G. Peng, Y. Wang, C. Hu, E. Wang, S. Feng, Y. Zhou, H. Ding, Y. Liu,
Heteropolyoxometalates which are included in microporous silica, CsxH3-
xPMo12O40/SiO2 and CsyH5-yPMo10V2O40 /SiO2, as insoluble solid bifunctional
catalysts: synthesis and selective oxidation of benzyl alcohol in liquid-solid
systems, Appl. Catal. A Gen. 218 (2001) 91–99, https://doi.org/10.1016/S0926-
860X(01)00622-6.
[47] H. Yuan, W.J. Yoo, H. Miyamura, S. Kobayashi, Discovery of a metalloenzyme-like
cooperative catalytic system of metal nanoclusters and catechol derivatives for the
aerobic oxidation of amines, J. Am. Chem. Soc. 134 (2012) 13970–13973, https://
doi.org/10.1021/ja306934b.
[48] R.D. Patil, S. Adimurthy, Copper-catalyzed aerobic oxidation of amines to imines
under neat conditions with low catalyst loading, Adv. Synth. Catal. 353 (2011)
1695–1700, https://doi.org/10.1002/adsc.201100100.
[72] Z. Weng, G. Liao, J. Wang, X. Jian, Selective oxidation of benzyl alcohol with
hydrogen peroxide over reaction-controlled phase-transfer catalyst, Catal.
Commun. 8 (2007) 1493–1496, https://doi.org/10.1016/j.catcom.2006.11.041.
[73] R. Zhang, W. Ding, B. Tu, D. Zhao, Mesoporous silica: an efficient nanoreactor for
liquid-liquid biphase reactions, Chem. Mater. 19 (2007) 4379–4381, https://doi.
org/10.1021/cm071718h.
[49] R.R. Donthiri, R.D. Patil, S. Adimurthy, NaOH-catalyzed imine synthesis: aerobic
oxidative coupling of alcohols and amines, European J. Org. Chem. (2012)
4457–4460, https://doi.org/10.1002/ejoc.201200716.
¨
[50] F. Su, S.C. Mathew, L. Mohlmann, M. Antonietti, X. Wang, S. Blechert, Aerobic
[74] A.C. Dengel, W.P. Griffith, B.C. Parkin, Studies on polyoxo- and polyperoxo-
metalates. Part 1. Tetrameric hetero polyperoxotungstates and
oxidative coupling of amines by carbon nitride photocatalysis with visible light,
Angew. Chemie - Int. Ed. 50 (2011) 657–660, https://doi.org/10.1002/
anie.201004365.
heteropolyperoxomolybdates, J. Chem. Soc. Dalt. Trans. (1993) 2683–2688,
https://doi.org/10.1039/DT9930002683.
[51] B. Chen, L. Wang, W. Dai, S. Shang, Y. Lv, S. Gao, Metal-free and solvent-free
oxidative coupling of amines to imines with mesoporous carbon from macrocyclic
compounds, ACS Catal. 5 (2015) 2788–2794, https://doi.org/10.1021/
acscatal.5b00244.
[75] O. Bortolini, S. Campestrini, F. Di Furia, G. Modena, G. Valle, Metal catalysis in
oxidation by peroxides. 27. Anionic molybdenum-picolinate N-Oxido-Peroxo
complex: an effective oxidant of primary and secondary alcohols in nonpolar
solvents, J. Org. Chem. 52 (1987) 5467–5469, https://doi.org/10.1021/
jo00233a032.
[52] L. Blackburn, R.J.K. Taylor, In situ oxidation-imine formation-reduction routes
from alcohols to amines, Org. Lett. 3 (2001) 1637–1639, https://doi.org/10.1021/
ol015819b.
´
[76] S. Benadji, P. Eloy, A. Leonard, B.L. Su, K. Bachari, C. Rabia, E.M. Gaigneaux,
Preparation and characterization of HMS supported 11-molybdo-vanado-phos-
phoric acid for selective oxidation of propylene, Microporous Mesoporous Mater.
130 (2010) 103–114, https://doi.org/10.1016/j.micromeso.2009.10.019.
[77] E. Grinenval, X. Rozanska, A. Baudouin, E. Berrier, F. Delbecq, P. Sautet, J.
M. Basset, F. Lefebvre, Controlled interactions between anhydrous keggin-type
heteropolyacids and silica support: preparation and characterization of well-
defined silica-supported polyoxometalate species, J. Phys. Chem. C 114 (2010)
19024–19034, https://doi.org/10.1021/jp107317s.
[53] M. Tamura, K. Tomishige, Redox properties of CeO2 at low temperature: the direct
synthesis of imines from alcohol and amine, Angew. Chemie - Int. Ed. 54 (2015)
864–867, https://doi.org/10.1002/anie.201409601.
[54] F. Pohlki, S. Doye, The mechanism of the [Cp2TiMe2]-catalyzed intermolecular
hydroamination of alkynes, Angew. Chemie - Int. Ed. 40 (2001) 2305–2308,
https://doi.org/10.1002/1521-3773(20010618)40:12<2305::AID-
ANIE2305>3.0.CO;2-7.
´
´
¨ ¨
[55] J.S. Johnson, R.G. Bergman, Imidotitanium complexes as hydroamination
catalysts: substantially enhanced reactivity from an unexpected
cyclopentadienide/amide ligand exchange, J. Am. Chem. Soc. 123 (2001)
2923–2924, https://doi.org/10.1021/ja005685h.
[78] A. Molnar, C. Keresszegi, B. Torok, Heteropoly acids immobilized into a silica
matrix: characterization and catalytic applications, Appl. Catal. A Gen. 189 (1999)
217–224, https://doi.org/10.1016/S0926-860X(99)00278-1.
[79] Y. Hanada, M. Kamada, K. Umemoto, H. Kominami, Y. Kera, Dispersion of
phosphovanadates on silica gel chemically modified with silane coupling agents
having an amino group and their catalytic activities for methanol oxidation, Catal.
Letters 37 (1996) 229–233, https://doi.org/10.1007/BF00807759.
[80] H. Yu, Y. Zhai, G. Dai, S. Ru, S. Han, Y. Wei, Transition-metal-Controlled inorganic
ligand-supported non-precious metal catalysts for the aerobic oxidation of amines
to Imines, Chem. - A Eur. J. 23 (2017) 13883–13887.
[56] Y. Li, Y. Shi, A.L. Odom, Titanium Hydrazido and imido complexes: synthesis,
structure, reactivity, and relevance to alkyne hydroamination, J. Am. Chem. Soc.
126 (2004) 1794–1803, https://doi.org/10.1021/ja038320g.
[57] S.A. Ryken, L.L. Schafer, N,O-Chelating Four-Membered Metallacyclic Titanium
(IV) Complexes for Atom-Economic Catalytic Reactions, Acc. Chem. Res. 48 (2015)
2576–2586, https://doi.org/10.1021/acs.accounts.5b00224.
[58] D. Srimani, M. Feller, Y. Ben-David, D. Milstein, Catalytic coupling of nitriles with
amines to selectively form imines under mild hydrogen pressure, Chem. Commun.
48 (2012) 11853–11855, https://doi.org/10.1039/C2CC36639H.
[59] S. Chakraborty, H. Berke, Homogeneous hydrogenation of nitriles catalyzed by
molybdenum and tungsten amides, ACS Catal. 4 (2014) 2191–2194, https://doi.
org/10.1021/cs5004646.
[81] M. Keshavarz, M. Vafaei-Nezhad, Design and characterization of l-Prolinate-
Amberlite as a novel heterogeneous organocatalyst and its catalytic application in
the synthesis of pyrazol-derivates, Catal. Letters 146 (2016) 353–363, https://doi.
org/10.1002/chem.201703185.
[82] M. Keshavarz, Ion-pair immobilization of l-prolinate anion onto cationic polymer
support and a study of its catalytic activity for one-pot synthesis of spiroindolones,
J. Iran. Chem. Soc. 13 (2016) 553–561, https://doi.org/10.1007/s13738-015-
0765-y.
[60] K. Barta, P.C. Ford, Catalytic conversion of nonfood woody biomass solids to
organic liquids, Acc. Chem. Res. 47 (2014) 1503–1512, https://doi.org/10.1021/
ar4002894.
[83] I. Ahmed, R. Farha, M. Goldmann, L. Ruhlmann, A molecular photovoltaic system
based on Dawson type polyoxometalate and porphyrin formed by layer-by-layer
self-assembly, Chem. Commun. 49 (2013) 496–498, https://doi.org/10.1039/
C2CC37519B.
[61] B. Gnanaprakasam, J. Zhang, D. Milstein, Direct synthesis of imines from alcohols
and amines with liberation of H2, Angew. Chemie - Int. Ed. 49 (2010) 1468–1471,
https://doi.org/10.1002/anie.200907018.
[62] Y. Zhai, M. Zhang, H. Fang, S. Ru, H. Yu, W. Zhao, Y. Wei, An efficient protocol for
the preparation of aldehydes/ketones and imines by an inorganic-ligand supported
iron catalyst, Org. Chem. Front. 5 (2018) 3454–3459, https://doi.org/10.1039/
C8QO00833G.
[84] M. Kuzminska, T.V. Kovalchuk, R. Backov, E.M. Gaigneaux, Immobilizing
heteropolyacids on zirconia-modified silica as catalysts for oleochemistry
transesterification and esterification reactions, J. Catal. 320 (2014) 1–8, https://
doi.org/10.1016/j.jcat.2014.09.016.
[63] A.R. Porcari, R.V. Devivar, L.S. Kucera, J.C. Drach, L.B. Townsend, Design,
synthesis, and antiviral evaluations of 1-(substituted benzyl)-2-substituted-5,6-
dichloro benzimidazoles as nonnucleoside analogues of 2,5,6- trichloro-1-(β-D-
[85] D. Li, J. Song, P. Yin, S. Simotwo, A.J. Bassler, Y. Aung, J.E. Roberts, K.
I. Hardcastle, C.L. Hill, T. Liu, Inorganic-organic hybrid vesicles with counterion-
and pH-controlled fluorescent properties, J. Am. Chem. Soc. 133 (2011)
14010–14016, https://doi.org/10.1021/ja204034g.
9

E. Samaraj et al.
Molecular Catalysis 502 (2021) 111396
[86] H. Hamamoto, Y. Suzuki, Y.M.A. Yamada, H. Tabata, H. Takahashi, S. Ikegami,
A recyclable catalytic system based on a temperature-responsive catalyst, Angew.
Chem. Int. Ed. 44 (2005) 4536–4538, https://doi.org/10.1002/anie.200500574.
[87] Y. Qiao, Z. Hou, H. Li, Y. Hu, B. Feng, X. Wang, L. Hua, Q. Huang, Polyoxometalate-
based protic alkylimidazolium salts as reaction-induced phase-separation catalysts
for olefin epoxidation, Green Chem. 11 (2009) 1955–1960, https://doi.org/
10.1039/B916766H.
[92] V. Balaga, J. Pedada, H.B. Friedrich, S. Singh, Tuning surface composition of Cs
exchanged phosphomolybdic acid catalysts in C H bond activation of toluene to
benzaldehyde at room temperature, J. Mol. Catal. A Chem. 425 (2016) 116–123,
https://doi.org/10.1016/j.molcata.2016.10.007.
[93] H. Ji, H. Ju, R. Lan, P. Wu, J. Sun, Y. Chao, S. Xun, W. Zhu, H. Li, Phosphomolybdic
acid immobilized on ionic liquid-modified hexagonal boron nitride for oxidative
desulfurization of fuel, RSC Adv. 7 (2017) 54266–54276, https://doi.org/10.1039/
C7RA10697A.
[88] Y. Leng, J. Liu, P. Jiang, J. Wang, Heteropolyanion-based polymeric hybrids:
Highly efficient and recyclable catalysts for oxidation of alcohols with H2O2, RSC
Adv. 2 (2012) 11653–11656, https://doi.org/10.1039/C2RA22348A.
[89] Y. Leng, P. Zhao, M. Zhang, J. Wang, Amino functionalized bipyridine-
heteropolyacid ionic hybrid: a recyclable catalyst for solvent-free oxidation of
benzyl alcohol with H2O2, J. Mol. Catal. A Chem. 358 (2012) 67–72, https://doi.
org/10.1016/j.molcata.2012.02.012.
[94] L. Huder, C. Rinfray, D. Rouchon, A. Benayad, M. Baraket, G. Izzet, F. Lipp-
Bregolin, G. Lapertot, L. Dubois, A. Proust, L. Jansen, F. Duclairoir, Evidence for
charge transfer at the interface between hybrid phosphomolybdate and epitaxial
graphene, Langmuir 32 (2016) 4774–4783, https://doi.org/10.1021/acs.
langmuir.6b00870.
[95] A.M. Khenkin, L.J.W. Shimon, R. Neumann, Preparation and characterization of
4ꢀ
[90] M. Shahabi, M. Tabatabaee, M. Keshavarz, Novel piperaziniumpolyoxometalate-
based acidic ionic liquids as highly efficient catalysts for the selective oxidation of
alcohols, Catal. Commun. 131 (2019), 105748, https://doi.org/10.1016/j.
catcom.2019.105748.
new ruthenium and osmium containing polyoxometalates, [M(DMSO)3Mo7O24]
(M = Ru(II), Os(II)), and their use as catalysts for the aerobic oxidation of alcohols,
Inorg. Chem. 42 (2003) 3331–3339.
[96] S. Elavarasan, A. Bhaumik, M. Sasidharan, An efficient mesoporous Cu-organic
¨
[91] Y. Liu, Q. Zhong, T. Zhang, Preparation of cationic surfactant intercalated
graphene oxide and quantitative determination of the interlamellar spacing,
fullerenes nanotub, Carbon Nanostruct. 23 (2015) 196–202, https://doi.org/
10.1080/1536383X.2013.787598.
nanorod for friedlander synthesis of Quinoline and click reactions, ChemCatChem
11 (2019) 4340–4350, https://doi.org/10.1002/cctc.201900860.
[97] K. Bahrami, M. Mehdi Khodaei, F. Naali, Mild and highly efficient method for the
synthesis of 2-arylbenzimidazoles and 2-arylbenzothiazoles, J. Org. Chem. 73
(2008) 6835–6837, https://doi.org/10.1021/jo8010232.
10