Incorporation of Keggin-based H3PW7Mo5O40into bentonite: synthesis, characterization and catalytic applications

Article abstract of DOI:10.1039/d1ra01179k

The Keggin-based molybdo-substituted tungstophosphoric acid, H₃[PW₇Mo₅O₄₀]·12H₂O, were synthesized and incorporated with a bentonite clay by using a wetness impregnation method. The catalysts were cha

Full text of DOI:10.1039/d1ra01179k

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PAPER
Incorporation of Keggin-based H PW Mo O into
3
7
5 40
bentonite: synthesis, characterization and catalytic
Cite this: RSC Adv., 2021, 11, 11244
applications†
Dipak S. Aher,
Kiran R. Khillare and Sunil G. Shankarwar
*
The Keggin-based molybdo-substituted tungstophosphoric acid,
3 7 5 2
H [PW Mo O40]$12H O, were
synthesized and incorporated with a bentonite clay by using a wetness impregnation method. The
catalysts were characterized using several methods, such as inductively coupled plasma-atomic emission
spectroscopy (ICP-AES), Fourier transform infrared spectroscopy (FT-IR), X-ray diffractometry (XRD),
scanning electron microscopy energy-dispersive X-ray spectroscopy (SEM-EDS), transmission electron
microscopy (TEM), and thermogravimetric and differential thermal analysis (TG-DTA). This extremely
active catalytic system provides a green strategy for the synthesis of 1,8-dioxo-octahydroxanthene and
ꢀ
1
,8-dioxo-decahydroacridine derivatives under solvent free conditions at 80 C with a good reaction
Received 12th February 2021
Accepted 26th February 2021
mass efficiency, effective mass yield, and excellent atom economy. Both the surface acidity and catalytic
activity sharply increased after H
the PW Mo /bentonite catalyst can be conveniently recovered and reused numerous times without
demonstrating a significant loss in activity.
3 7 5 2
[PW Mo O40]$12H O was impregnated with bentonite clay. In addition,
DOI: 10.1039/d1ra01179k
7
5
rsc.li/rsc-advances
Bentonite, a clay mineral which is abundant on earth,
consists of layers of two tetrahedral silica sheets sandwiching
1
. Introduction
24,25
Polyoxometallates (POMs) or heteropoly acid (HPAs) are a large one octahedral alumina sheet.
It is well-documented that
family of distinct d-block transitional metal-oxide, anionic bentonite clays can be used as an efficient support owing to
cluster structures of metals, they have inspired several recent their common fascinating features, such as their higher surface
1–6
research activities in comprehensive elds of science. POMs acidity, excellent thermal stability, and effortlessly controlled
26
can, in some instances, be assembled with precise control structure and morphology.
regarding the placement of different heteroatoms such as W,
Multicomponent reactions (MCRs) provide a convenient
7–9
Mo, or V within their molecular frameworks. Predominantly, approach for the rapid synthesis of complex molecules from
the Keggin-type POMs are also characteristically templated by simple starting materials, without the isolation of intermedi-
2
7
a central tetrahedral template {XO
4
} (X ¼ P, Si, Nb, B, Ge, etc.) ates. The MCRs have achieved signicant importance in
28
that can have a prominent impact on the properties of the medicinal and organic chemistry, because these reactions
anion, these are mostly associated with a change in the charge minimize the consumption of catalysts and solvents, thus
10–15
and polarization of the metal-oxo shell.
more of the W atoms can be substituted by different hetero- compared to multistep synthesis.
atoms that comprise inner transition metals, such as Co, Ni, been classied as oxygen-containing molecular families, pos-
Furthermore, one or permitting the minimization of waste, time, effort and cost as
29,30
Xanthene derivatives have
31
Mo, V and so on to yield transition-metal substituted sessing a highly reactive inbuilt pyran ring. In addition, they
8,15–17
POMs.
tivity and effects processes such as the acidity and stability of compounds and dyes,
This compositional modication tunes the reac- have been used as synthetic precursors for several organic
32–34
such as uorescents and chiroptical
However, the major problems with POMs are molecular switches (examples include, rosamine, uorone,
18,19
the POMs.
their low thermal stability, low surface area, high solubility in uorescein, rhodamine etc.).
33,35
The core structure of the
20
polar solvents, and their recovery and reusability. For the xanthene derivatives shows a multitude of physicochemical and
effective access the catalyst needs to be supported on a suitable pharmacological properties, such as anti-inammatory, anti-
36
21–23
37
38
20
39
support material.
malarial, analgesic, anticancer, antiviral, antibacterial
38
properties, and so on. Additionally, acridine derivatives,
constituting a 1,4-dihydropyridine (DHP) ring skeleton, are very
important compounds owing to their pharmacological proper-
Department of Chemistry, Dr Babasaheb Ambedkar Marathwada University,
Aurangabad 431004, MS, India. E-mail: shankarwar_chem@yahoo.com
40
ties. Various members of this family are currently used for
cytostatic and antitumor activities (for example, nitracrine
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra01179k
1
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Fig. 1 Structures of some known xanthenes and acridines.
41
derivatives), platelet antiaggregatory activities, the treatment to perform, cost effective, and offers the reusability of the
of Alzheimer's disease, strong anti-cancer treatments (for catalyst, and perhaps offers a greener method, is highly
example, amsacrine derivatives), cardiovascular diseases attractive.
41,42
including hypertension and diabetes, and so on.
The struc-
In this regard, and in a continuation of our interest in the
tures of some xanthene and acridine derivatives are shown in development of modied novel Keggin-type POMs for different
60,61
Fig. 1.
As yet, several procedures have been employed for synthe- a series of molybdo-substituted tungstophosphoric acid H
sizing many of these organic derivatives using various catalysts, [PW Mo 40]$12H O catalysts that were prepared and impreg-
nated with bentonite clay (Scheme 1). Their effectiveness was
p-dode- explored for the preparation of 1,8-dioxo-octahydroxanthene
40 and and 1,8-dioxo-decahydroacridine derivatives in excellent yields
[Cmmim] and with a short reaction time.
acid catalysed organic transformations,
herein, we report
3
-
7
5
O
2
3
+
43
42
44
3
such as Fe montmorillonite, NiFeTi CLDH6, SmCl ,
4
5
46,47
48
HClO
4
–SiO
2
,
InCl
3
/ionic liquid,
2 4
FSG-Hf(NPf ) ,
49
cylbenzenesulfonic acid, Amberlyst-15, [MIMPS]
3
PW12O
4
7
2ꢁ
50
51
[
[
TEAPS]
3
PW12
O
40
53
,
SO
4
/ZrO
2
,
4
[Hbim]BF ,
52
54
55
56
BF4], SBSSA, [Hmim]TFA, CaCl2, SBNPSA, thiourea
57
58
dioxide,
(
SbCl /SiO ,
and 2,4,6-trichloro-1,3,5-triazine
3
2
2
. Results and discussion
59
TCT). However, these types of catalyst possesses some
disadvantages, such as an inability to recycle the catalyst, the 2.1 Catalytic characterization
use of expensive reagents, use of reux conditions, prolonged
2.1.1 FT-IR analysis. The Fourier transform infrared spec-
work up, hazardous organic solvents and low reaction yields.
Therefore, it has become important to nd an alternative route
for the synthesis of xanthene and acridine derivatives. Hence,
the development of a novel, contemporary reaction that is easy
troscopy (FT-IR) spectra of the bulk PW
7
Mo
Mo
5% are depicted in Fig. 2. For the bulk PW Mo , the charac-
5
, bentonite clay and
PW
7
Mo
5
/bentonite composites with PW
7
5
loading from 10–
2
7
5
teristic bands exhibited at 1070 (P–O in central tetrahedral),
a
Scheme 1 Schematic illustration showing the synthetic procedure for the fabrication of the PW
7
Mo
5
/bentonite catalyst.
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ꢁ
1
vibrations were detected at 993 cm and the bending vibration
ꢁ1
of Al–Al–OH was determined at 913 cm
for the parent
bentonite. Additionally, the other two bands at 790 and
ꢁ1
6
91 cm
vibrations, respectively. However, the characteristics peaks of
the 10%, 15%, 20% and 25% PW Mo supported bentonite
correspond to the Si–O–Al and Si–O–Si bending
25
7
5
clays have a similar pattern to that of the parent bentonite, as
only small shis in frequency appeared owing to the electro-
7 5
static interactions between the Keggin structure of PW Mo and
22,62
the parent bentonite.
.1.2 XRD analysis. The X-ray diffractometry (XRD) analysis
of the bulk PW Mo , plain bentonite and PW Mo /bentonite
2
7
5
7
5
composites (with PW
7
Mo
5
loading from 10, 15, 20 and 25%) in
ꢀ
ꢀ
3
< 2q < 80 are depicted in Fig. 3. The bulk PW
7
Mo
5
catalyst
ꢀ
exhibited major diffraction peaks, for example 2q, at 7–10 ,
ꢀ ꢀ ꢀ ꢀ
1
4.5 , 20.7 , 25.44 and 34.4 , demonstrating the characteristic
17
features of the Keggin structure and the crystalline state.
ꢀ
ꢀ
Sharp reections exhibited at 2q ¼ 7 and 20 are characteristic
of the amorphous material of plain bentonite, and the one at 2q
ꢀ
¼
27 corresponds to a-quartz. For the XRD patterns of the
different loading values (10, 15, 20 and 25%) of PW
bentonite, the peaks were observed to be less intense, demon-
strating that during the impregnation of PW Mo on bentonite,
the clay lost some of its crystallinity compared to the parent
bentonite. Most of the diffractograms of PW Mo /bentonite
indicate that no separate crystal phases were detected and the
q value of quartz was unaffected, even aer exchanging the
7 5
Mo on
Fig. 2 FT-IR analysis of bulk PW
7
5
Mo , bentonite and the PW
7
Mo
5
/
7
5
bentonite catalyst.
7
5
ꢁ
1
9
58 (terminal M]O
d
), 873 (M–O
b
–M), and 762 cm (M–O
c
–M)
2
1
7
coincide with the asymmetric vibrations in the Keggin unit.
26,63
bentonite clay with PW Mo .
This indicates that PW Mo5
7
7
5
The strong characteristic bands of the Si–O stretching
was well distributed over the surface of bentonite.
.1.3 SEM and TEM analysis. The surface morphology and
2
texture of the prepared samples were examined using scanning
electron microscopy (SEM) and transmission electron micros-
copy (TEM) analysis (Fig. 4a–d). Fig. 4a shows the agglomerated
irregular-shaped crystalline structure with smooth particles for
bulk PW Mo at 2 mm magnication. The SEM images of
7
5
bentonite and 20% PW Mo /bentonite are depicted in Fig. 4b
7
5
and c respectively. The result clearly reveals that the surface
morphology of the 20% PW Mo /bentonite catalyst is quite
dissimilar from that of the parent bentonite clay. Subsequently,
it was conrmed that PMo are homogeneously dispersed
over the surface of bentonite.
In addition, the morphology of the 20% PW Mo /bentonite
7
5
7
W
5
7
5
samples was investigated using TEM. The smaller cloudy-like
features and dark coloured spot in Fig. 4 indicate that the
functional components of PW
lized and aggregate with the bentonite clay support.
.1.4 EDX analysis and elemental mapping images. The
chemical compositions of the bulk PW Mo and 20% PW Mo
7 5
Mo were successfully immobi-
2
7
5
7
5
/
bentonite were conrmed using energy-dispersive X-ray spec-
troscopy (EDS) analysis as shown in Fig. S1.† The results ob-
tained for the fresh PW Mo catalyst clearly show the
7
5
occurrence of P, W and Mo (Fig. S1a†). Furthermore, the EDX
analysis of 20% PW Mo /bentonite (Fig. S1b†) showed the
presence of P, W, Mo, Na, Ca, Al and O elements, conrming the
formation of 20% PW Mo /bentonite.
7
5
7
5
Additionally, the elemental mapping images (EDS) of the
catalyst proved the uniform distribution of the elements such
7 5 7 5
Fig. 3 XRD analysis of bulk PW Mo , bentonite and the PW Mo /
bentonite catalyst.
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7 5 7 5 7 5
Fig. 4 FE-SEM images of bulk PW Mo (a), bentonite clay (b), 20% PW Mo /bentonite (c) and the TEM images of 20% PW Mo /bentonite (d).
as W, Mo, P, Na, Ca, Al and O in the prepared bulk PW
the 20% PW Mo /bentonite catalyst (Fig. S2a and b†).
7
Mo
5
and Table 1 Effect of PW
7
Mo
5
loading on the support bentonite for the
a
model reaction
7
5
2
.1.5 TG-DT analysis. The thermal stability of bulk PW Mo5
7
b
c
Entry
Catalyst
Pure bentonite
Time (min)
Yield (%)
and 20% PW Mo /bentonite were studied using thermogravi-
7
5
metric and differential thermal analysis (TG-DT) (Fig. 5). The
three types of crystallographic water molecule obtained in the
POMs are hydration water, protonized water and structural
1
2
3
4
5
6
85
45
35
15
5
65
70
70
85
92
92
Bulk H
3
7
PW Mo
5
O
2
40$12H O
10% PW
15% PW
20% PW
25% PW
7
7
7
7
Mo
5
5
5
5
/bentonite
/bentonite
/bentonite
/bentonite
Mo
Mo
Mo
1
7,60
water, as depicted in Fig. 7.
PW Mo
The TG curve for the bulk
ꢀ
7
5
shows the total weight loss of 8.78% below 520 C,
5
which demonstrates that 12-water molecules were lost. The rst
mass loss at 70 to 150 C, was about 7.38% because of the loss of
a
Reaction conditions: 1 : 2 ratio of benzaldehyde, 5,5-dimethyl-1,3-
ꢀ
ꢀ
b
cyclohexanedione and different catalysts (100 mg) at 80 C. Reaction
c
the absorbed water molecule. The second weight loss of 2.92% progress monitored by TLC. Isolated yields, bold values highlight the
ꢀ
best result.
appeared as an endothermic peak at 350 C for 2.18 molecules
of protonized water. The third exothermic peak appeared at
7 5 7 5
Fig. 5 TG-DT analysis of (a) PW Mo and (b) the 20% PW Mo /bentonite catalyst.
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ꢀ
5
20 C, owing to the weight loss of 1.2% for the structural water Table 2 Optimization of the reaction conditions for the model
a
ꢀ
reaction
molecule. The exothermic peak centred at 520 C is attributed to
the decay of the Keggin unit of PW Mo to the individual oxides,
suggesting that the POMs will persist up to 520 C (Fig. 5a).
The thermogravimetric analysis (TGA) of 20% PW Mo
bentonite shows a weight loss at 80 to 175 C, of about 9.8%
owing to the weight loss of the adsorbed water molecules. A
gradual mass loss of about 3% is observed up to 630 C, which is
due to the release of structural water. From this it can be seen
that the thermal stability of the bulk PW Mo materials has
noticeably improved, which may be due to the strong interac-
7
5
ꢀ
b
c
Entry Catalyst (mg) Temperature ( C) Time (min) Yield (%)
ꢀ
7
5
/
1
2
3
4
5
6
7
8
9
—
20
50
80
100
100
100
100
120
80
80
80
80
80
50
70
100
80
180
120
75
65
5
55
50
5
Trace
55
60
75
92
80
83
92
92
ꢀ
ꢀ
7
5
14
10
tion between PW Mo and bentonite clay (Fig. 5b).
7
5
a
Reaction conditions: 1 : 2 ratio of benzaldehyde, 5,5-dimethyl-1,3-
b
7 5
cyclohexanedione and 20% PW Mo /bentonite. Reaction progress
monitored by TLC. Isolated yields, bold values represent the best
result.
c
2
.2 Catalytic activity test
The catalytic activity of the newly prepared PW
7
5
Mo /bentonite
was explored for the synthesis of substituted xanthenes. More-
over, to accomplish the best reaction conditions for the
a
Table 3 Effect of different solvents on the model reaction
xanthene synthesis, the effect of PW
7 5
Mo /bentonite loading, the
ꢀ
b
c
Entry Solvent
Temperature ( C) Time (min) Yield (%)
amount of PW Mo /bentonite, the results in various solvents,
7
5
different temperatures and comparison of different catalysts
were obtained. A model reaction between benzaldehyde and 5,5-
dimethyl-1,3-cyclohexanedione was studied to optimize these
parameters.
For a demonstration of the effect of the PW Mo catalyst
7 5
loading on bentonite for the synthesis of 3a, different catalyst
1
2
3
4
5
6
7
MeOH
EtOH
EtOH–H O (1 : 1) Reux
Reux
Reux
25
20
10
35
5
55
65
68
75
45
35
92
2
PEG-400
CH CN
110
3
Reux
Reux
80
PhMe
Solvent free
25
5
loadings were employed, such as the parent bentonite, bulk
a
Reaction conditions: 1 : 2 ratio of benzaldehyde, 5,5-dimethyl-1,3-
PW
The parent bentonite gave a moderate yield of the product, but
it took longer for the reaction to reach completion (Table 1, highlight the best result.
entry 1). Bulk PW Mo gave a reasonable catalytic ability in
terms of the reaction time and the yield of the desired product
Table 1, entry 2). It was observed that both the yield of the
7 5
Mo , 10%, 15%, 20%, and 25% w/w, as shown in Table 1.
b
cyclohexanedione and 20% PW
progress was monitored by TLC. Isolated yields, bold values
7 5
Mo /bentonite (100 mg). Reaction
c
7
5
(
product and the reaction time improved upon increasing the detected aer extending the reaction time and increasing the
catalyst loading up to 20% w/w (Table 1, entries 3–5). Addi- temperature.
tionally, an increase in the amount of catalyst does not affect the
reaction time and the yield of the product as a result of the tions over various organic solvents, the model reaction was
leaching of the catalyst from the support (Table 1, entry 6).
performed in dissimilar solvents and the results are represented
Next, we performed the model reaction using various in Table 3. If MeOH, EtOH, EtOH–H O (1 : 1), and polyethylene
amounts of 20% PW Mo
/bentonite to assess the effect of the glycol (PEG-400) are used as the solvents, moderate yields of the
catalyst on the synthesis of xanthene derivatives at 80 C. It was products are obtained (Table 3, entries 1–4), whereas, if CH
To study the advantages of the solvent-free reaction condi-
2
7
5
ꢀ
3
CN
observed that in the absence of a catalyst, product 3a was only and PhMe are used, a poor yield of the product is obtained
afforded in trace amounts (Table 2, entry 1), even aer 3 h. By (Table 3, entries 5 and 6). However, when the reaction was
increasing the amount of the catalyst up to 100 mg, a notable carried out under thermal solvent-free conditions, a superior
alteration was observed in the percentage yield (Table 2, entries yield of the product was obtained within a short reaction time.
2
–4). There is no signicant improvement in the yield observed
upon increasing the amount of 20% PW Mo
/bentonite catalyst synthesis of 1,8-dioxo-octahydroacridines (Table 4) was carried
up to 120 mg, because the additional catalyst does not increase out under the same reaction conditions as used for the
the rate of the reaction (Table 2, entry 9).
synthesis of the corresponding 1,8-dioxo-octahydroxanthenes.
To further extend the reaction scope of the catalyst, the
7
5
Hereaer, the effects of the temperature and time were We carried out the reaction in a 1 : 2 : 1 ratio of aldehydes,
studied, which play an important role and affect the reaction 5,5-dimethyl-1,3-cyclohexanedione and ammonium acetate
ꢀ
ꢀ
OAc) with 20% PW Mo /bentonite at 80 C, which afforded
7 5
kinetics to a large extent. Various temperatures (50–100 C) were (NH
used to carry out the model reaction for different time periods the 1,8-dioxo-decahydroacridine derivatives (4a–4n) in excellent
4
(
5–180 min). Hence, the best result was obtained in the pres- yields (92–79%) within a short period of time (Table 5). In
ꢀ
ence of 100 mg of 20% PW
7
Mo
5
/bentonite at 80 C (Table 2, addition, the aromatic aldehydes containing both electron
entry 5). It was observed that no substantial change was withdrawing and electron donating groups afforded the
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Table 4 Synthesis of 1,8-dioxo-octahydroxanthenes using the 20% Table 5 Synthesis of 1,8-dioxo-decahydroacridines using the 20%
a,b,c
a,b,c
7
PW Mo
5
/bentonite catalyst
7 5
PW Mo /bentonite catalyst
a
Reaction conditions: 1 : 2 ratio of aldehyde, 5,5-dimethyl-1,3-
Mo /bentonite (100 mg) stirred at
0 C. Isolated yields. Melting points.
cyclohexanedione and 20% PW
8
7
5
ꢀ
b
c
31,42,43,52
a
Reaction conditions: 1 : 2 : 1 ratio of aldehydes, 5,5-dimethyl-1,3-
OAc and 20% PW Mo /bentonite (100 mg)
cyclohexanedione, NH
stirred at 80 C. Isolated yields. Melting points.
4
7
5
ꢀ
b
c
48,49,64–66
products with high yields. The work-up procedure was very
simple and included the addition of hot ethanol at the end of
the reaction, ltering the reaction mixture to separate off the
catalyst, and nally recrystallizing the products from ethanol.
Quantication of the effectiveness of a reaction with the aim
of waste reduction continues to drive the development of novel
green chemistry metrics, such as the E-factor, atom economy,
reaction mass efficiency, optimum efficiency, and practical
67,68
mass yield.
environmental performance of a chemical reaction.
The E-factor is the mass ratio of waste to desired product and
These metrics serve to quantify the efficiency or
69
70
the atom efficiency. The lower the value of the E-factor, the
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Table 7 Comparative study of 20% PW
7
Mo
5
/bentonite with previously
reported catalysts for the model reaction
Reaction
conditions
Yield
Time (%) Ref.
Entry Catalyst
3
+
ꢀ
1
2
3
Fe montmorillonite
EtOH, 100 C 6 h
94 (ref. 43)
95 (ref. 50)
85 (ref. 51)
2
ꢁ
ꢀ
SO
4
/ZrO
2
EtOH, 70 C
MeOH, rt
8 h
45
[Hbim]BF
4
min
4
[Cmmim][BF
4
]
Solvent free, 80 150
87 (ref. 52)
ꢀ
C
min
10 h 98 (ref. 53)
5
6
SBSSA
[Hmim]TFA
Reux, EtOH
Solvent free, 80 3 h
85 (ref. 54)
ꢀ
C
ꢀ
7
8
9
CaCl
SBNPSA
Thiourea dioxide
2
DMSO/85–90 C 4 h
85 (ref. 55)
93 (ref. 56)
96 (ref. 57)
Reux, EtOH
Reux
2 h
45
min
50
min
50
10
11
12
SbCl
3
/SiO
2
Solvent-free,
93 (ref. 58)
92 (ref. 59)
ꢀ
120 C
2,4,6-Trichloro-1,3,5-
triazine (TCT)
20% PW
Solvent-free,
ꢀ
120 C
min
7
Mo
5
/bentonite
Solvent free, 80 5 min 92 (This
ꢀ
C
work)
derivatives. The result shows that 20% PW
7
Mo
5
/bentonite is
superior to some of the previous reported catalysts in terms of
the excellent yield and short reaction time (Table 7, entry 12).
Fig. 6 Radar chart of the measured green metrics for the synthesis of
1
,8-dioxo-octahydroxanthene (3a–n) and the 1,8-dioxo-decahy-
droacridine derivatives (4a–n).
2.3 Plausible mechanism for the reaction
The 20% PW Mo /bentonite catalyses the synthesis of the 1,8-
7
5
dioxo-octahydroxanthene derivatives by stimulating the
carbonyl group of the aromatic aldehydes (1), making it more
liable to nucleophilic attack by dimedone (2), to form inter-
mediate (a), followed by Michael addition of another molecule
of dimedone to form the intermediate (b). Intramolecular
cyclization occurs aer the successive removal of water, which
results in the desired product (3a–3n) and regenerates the 20%
PW Mo /bentonite catalyst in the reaction mixture (Fig. 7).
more ecocompatible the reaction. An E-factor ranging from 0.1
to 0.6 highlights the greenness of the protocol, as shown in
Fig. 6.
We measured the green chemistry metrics for both the 1,8-
dioxo-octahydroxanthenes and 1,8-dioxo-octahydroacridines
derivatives to afford 3a and 4a under the optimized reaction
conditions and compared these with their ideal value, as
demonstrated in Table 6. The result shows that the values of the
green chemistry metrics such as the E-factor, atom economy
7
5
(
AE), reaction mass efficiency (RME), effective mass yield (EMY) 2.4 Recyclability of the 20% PW
7
Mo
5
/bentonite catalyst
and optimum efficiency (OE) are close to their ideal values, as
demonstrated here (see the ESI† for detailed calculations).
A comparison of the catalytic efficiency of the 20% PW Mo /
7 5
bentonite catalyst was performed with other reported catalysts
for the synthesis of the 1,8-dioxo-octahydroxanthene
To study the reusability of the catalyst (20% PW
bentonite), recycling studies were performed using a 1 : 2 ratio
of benzaldehyde, 5,5-dimethyl-1,3-cyclohexanedione and 20%
PW Mo /bentonite catalyst (100 mg), which were observed to be
7 5
the optimized reaction conditions for a specied time period.
Aer the reaction reached completion (monitored by thin layer
7
5
Mo /
Table 6 Quantitative evaluation of green chemistry metrics for 3a and chromatography (TLC)), the reaction mixture was diluted using
4
a
hot ethanol (10.0 mL) and ltered to separate the catalyst. The
crude product was obtained by solvent evaporation under
reduced pressure and recrystallized from ethanol. The recov-
ered catalyst was washed with ethanol (10 mL) and dried over-
night for further reuse. The results (Table 8) reveal that the
catalyst stability was maintained for up to six cycles. Aer the
sixth cycle, the recovered catalyst was characterized using FT-IR
and powder XRD analysis. The characteristic peaks at 1623,
Product
Ideal value Product 3a 4a
Entry Green chemistry metrics
1
2
3
4
5
E-Factor
Atom economy (AE)
0
0.19
90.67
83.39
83.39
91.97
0.44
75.38
69.34
69.34
91.98
100%
Reaction mass efficiency (RME) 100%
Effective mass yield (EMY)
Optimum efficiency (OE)
100%
100%
ꢁ
1
1005, 913, 787 and 687 cm for the FT-IR spectra (Fig. S3a†)
11250 | RSC Adv., 2021, 11, 11244–11254
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7 5
Fig. 7 Probable mechanism for the synthesis of 1,8-dioxo-octahydroxanthenes using the 20% PW Mo /bentonite catalyst.
a
Table 8 Recycling study of the catalyst for the model reaction
The elemental content was measured using inductively
coupled plasma-atomic emission spectroscopy (ICP-AES),
ARCOS, on a simultaneous ICP Spectrometer. The FT-IR
spectra were obtained using a Bruker ALPHA (Eco-ATR) spec-
trophotometer. The XRD pattern was obtained using a Bruker
AXS Company, D8 ADVANCE diffractometer (Germany). Scan-
ning electron microscopy (SEM) images and elemental analysis
of the catalyst were employed using a FEI Nova NanoSEM 450
combined with a Bruker XFlash 6I30 instrument for energy-
dispersive X-ray spectroscopy (EDX), with a scanning electron
electrode at 15 kV. TEM was performed on a HR-TEM: Jeol/JEM
b
c
No. of runs
Time (min)
Yield (%)
1
2
3
4
5
6
5
5
5
5
5
5
92
92
91
89
87
85
a
Reaction conditions: 1 : 2 ratio of benzaldehyde, 5,5-dimethyl-1,3-
b
cyclohexanedione and 20% PW
progress monitored by TLC. Isolated yield.
7
Mo
5
/bentonite (100 mg). Reaction
c
2100 at 200 kV voltage. TG and DTA were performed on a SHI-
MADZU, DTG-60H simultaneous DTA-TG apparatus. TLC was
used to monitor the reaction on Merck silica plates, and the
imaging was accomplished using iodine/ultraviolet light. The
ꢀ
ꢀ
ꢀ
ꢀ
and 2q ¼ 20.70 , 24.88 , 26.89 , and 31.63 for the XRD pattern
Fig. S3b†) remained the same.
(
melting points of all the synthesized compounds were deter-
1
mined in open capillary tubes and are uncorrected. The
H
3. Experimental
1
3
NMR and C NMR spectra were recorded on a Bruker Avance
DRX-400, in pure DMSO-d or CDCl solvents using TMS as an
internal reference standard.
3
.1 Materials and general characterization
Sodium molybdate (Na MoO $2H O), sodium tungstate (Na
WO $2H O), disodium phosphate (Na HPO ), and bentonite
6
3
2
4
2
2
-
4
2
2
4
clay powder were purchased from Molychem in India and used
without further purication. All the chemicals and solvents
3.2 Catalyst preparation
used in the organic synthesis were purchased from Alfa Aesar, Synthesis of the Keggin type POMs with the general formula
Merck and Sigma-Aldrich.
H
3
[PW
7
Mo
5
O
40]$12H
2
O was performed according to a previous
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2021 The Author(s). Published by the Royal Society of Chemistry
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8
report by Huixiong. Briey, disodium phosphate (0.58 g, (10.0 mL) and ltered for catalyst separation. The crude product
Na HPO ) and the desired amount of sodium tungstate (9.46 g, was obtained by solvent evaporation under reduced pressure
2
4
Na WO $2H O) were dissolved in deionised water (DI). The and recrystallized from ethanol. The recovered catalyst was
2
4
2
ꢀ
resulting mixture was stirred for 30 min at 90 C. Then, an washed with ethanol (10 mL) and dried overnight for further
aqueous solution of sodium molybdate (4.95 g, Na MoO $2H reuse. The recrystallized xanthene derivatives were character-
O) was added to the above heated solution, and the pH was ized using FT-IR, H and C NMR spectroscopy. The NMR
2
4
2
-
1
13
maintained close to 1.5–2 using sulfuric acid (H
mixture was heated at 90 C for 6 h. Finally, the cooled solution spectroscopic data are provided in the ESI.†
2
SO
4
). The spectral data for product 4a are given below; the remaining
ꢀ
was extracted with diethyl ether. The powder H
3
[PW
7
Mo
5
O
40]$
3.4.1 3,3,6,6-Tetramethyl-9-phenyl-3,4,6,7,9,10-hexahy-
ꢁ
1
1
2H O was obtained aer the concentrated etherate solution droacridine-1,8(2H,5H)-dione (4a). IR (ATR, n cm ): 661, 833,
2
1
was dried in a vacuum. The results of the ICP-AES elemental 1035, 1154, 1237, 1363, 1582, 1630, 2956, 3286. H NMR (400
analysis revealed that the atomic ratio of P/W/Mo is almost MHz, CDCl ) d (ppm) ¼ 1.09 (s, 6H, 2 ꢂ CH ); 1.24 (s, 6H, 2 ꢂ
3
H
3
maintained at 1.12/7.12/5.01 which corresponds to the formula CH
3
), 2.51–2.33 (m, 8H, 4 ꢂ CH
2
), 5.55 (s, 1H, CH), 7.11 (d, J ¼
H
3
[PW
In a typical synthesis of 10% PW
PW Mo was dissolved in dry methanol solution and added 27.5, 29.8, 31.5, 32.8, 47.1, 115.7, 125.9, 126.8, 128.1, 138.1,
7
Mo
5
O
40]$12H
2
O (PW
7
Mo
5
).
8.2 Hz, 2H, Ar-H), 7.19 (d, J ¼ 6.9 Hz, 2H, Ar-H), 7.26 (s, 1H, Ar-
13
7
Mo
5
/bentonite, 0.5 g of H), 7.27 (s, 1H, NH). C NMR (100 MHz, CDCl
3
) d
C
(ppm) ¼
7
5
dropwise into the ask containing 4.5 g of bentonite clay and 189.5, 190.6.
stirred for 3 h at room temperature. The resulting mixture was
ꢀ
heated to 80 C until complete evaporation of the liquid part
4. Conclusion
had occurred. The solid residue was then calcined in an oven at
2
2
7 5
00 K for 5 h. A series of PW Mo /bentonite loadings (10, 15, 20,
5 wt%) were prepared using a similar method.
In conclusion, we have successfully synthesized a novel Keggin-
type molybdo-substituted tungstophosphoric acid (H PW
Mo 40$12H O) catalyst, which has been impregnated with
acidied bentonite clay. The catalytic activity of PW Mo /
3
7
-
5
O
2
3.3 General procedure for synthesis of the 1,8-dioxo-
7
5
octahydroxanthene derivatives
bentonite was probed using the one-pot synthesis of the 1,8-
dioxo-octahydroxanthene and 1,8-dioxo-decahydroacridine
A
mixture of aromatic aldehyde (3.0 mmol), 5,5-
derivatives. The 20% PW
7 5
Mo /bentonite exhibited a higher
dimethylcyclohexane-1,3-dione (6.0 mmol), and 20% PW
7
Mo
5
/
ꢀ
catalytic activity than the bulk PW Mo catalyst, as well as 10%,
7
5
bentonite catalyst (100 mg) was heated at 80 C under solvent-
free conditions for a suitable time, as indicated by TLC. Aer
the reaction was complete, the reaction mixture was diluted
using hot ethanol (10.0 mL) and ltered for catalyst separation.
The crude product was obtained by solvent evaporation under
reduced pressure and recrystallized from ethanol. The recov-
ered catalyst was washed with ethanol (10 mL) and dried over-
night for further reuse. Recrystallized xanthene derivatives were
7 5
15% and 25% PW Mo /bentonite. The effects of various
parameters, such as the catalyst loading, amount of catalyst,
effect of the solvents, inuence of the temperature on the rate of
reaction, comparison of different catalysts and green metrics,
were discussed in detail. The PW Mo /bentonite at 20% was
7
5
found to be the most active, stable, and reusable catalyst. The
PW Mo /bentonite catalysts have great potential for application
7
5
1
13
as protable catalysts for encouraging acid-catalysed organic
transformations under environmentally friendly conditions and
processes.
characterized using FT-IR, H and C NMR spectroscopy. The
NMR spectral data for product 3a, synthesized using the model
reaction, are given below; the remaining spectroscopic data are
provided in the ESI.†
3
.3.1 3,3,6,6-Tetramethyl-9-phenyl-3,4,5,6,7,9-hexahydro-
Conflicts of interest
ꢁ1
1
1
H-xanthene-1,8(2H)-dione (3a). IR (ATR, n cm ): 685, 835,
1
035, 1242, 1364, 1581, 1664, 2958. H NMR (400 MHz, CDCl
(ppm) ¼ 1.09 (s, 6H, 2 ꢂ CH ), 1.23 (s, 6H, 2 ꢂ CH ), 2.37 (m,
H, 4 ꢂ CH ), 5.54 (s, 1H, CH), 7.09 (d, J ¼ 8.2 Hz, 2H, Ar-H),
.22 (t, 1H, Ar-H), 7.26 (t, J ¼ 8.4 Hz, 2H, Ar-H). C NMR (100
) d
(ppm) ¼ 27.5, 29.8, 31.5, 32.8, 46.5, 47.1, 115.7,
25.9, 126.8, 128.3, 138.1, 189.6, 190.6.
3
)
There are no conicts to declare.
d
8
7
H
3
3
2
1
3
Acknowledgements
MHz, CDCl
3
C
The author SGS is thankful for nancial assistance in the form
of a minor research project (STAT/VI/RG/DEPT/2019–20/337–38)
and UGC-DST SAP from Dr B. A. Marathwada University, Aur-
angabad (MS), India. The author DSA gratefully acknowledges
the University Grants Commission (UGC), New Delhi (India) for
a senior research fellowship (SRF).
1
3
.4 General procedure for synthesis of the 1,8-dioxo-
decahydroacridine derivatives
mixture of aromatic aldehyde (3.0 mmol), 5,5-
dimethylcyclohexane-1,3-dione (6.0 mmol), ammonium acetate
A
(
(
NH OAc, 3.0 mmol) and the 20% PW Mo /bentonite catalyst
4 7 5
100 mg) was heated at 80 C under solvent-free conditions for
References
ꢀ
a suitable time as indicated by TLC. Aer the reaction was
complete, the reaction mixture was diluted using hot ethanol
1 X. Liu, Y. Zhang, G. Zhao, J. Zhang, S. Ren and G. Fang, ACS
Omega, 2019, 4, 18487–18494.
11252 | RSC Adv., 2021, 11, 11244–11254
© 2021 The Author(s). Published by the Royal Society of Chemistry

View Article Online
Paper
RSC Advances
2
A. Enferadi-Kerenkan, T.-O. Do and S. Kaliaguine, Catal. Sci. 29 Y. Liu, X. H. Zhang, J. Ren and G. Y. Jin, Synth. Commun.,
Technol., 2018, 8, 2257–2284.
2004, 34, 151–157.
3
4
E. Raee and S. Eavani, RSC Adv., 2016, 6, 46433–46466.
J. Lan, J. Lin, Z. Chen and G. Yin, ACS Catal., 2015, 5, 2035–
30 S. Brauch, S. S. van Berkel and B. Westermann, Chem. Soc.
Rev., 2013, 42, 4948.
2
041.
31 Subodh, N. K. Mogha, K. Chaudhary, G. Kumar and
D. T. Masram, ACS Omega, 2018, 3, 16377–16385.
32 G. Shabir, A. Saeed and P. Ali Channar, Mini-Rev. Org. Chem.,
2018, 15, 166–197.
5
6
Y. Zhang, Z. Shen, J. Tang, Y. Zhang, L. Kong and Y. Zhang,
Org. Biomol. Chem., 2006, 4, 1478–1482.
D. Borkin, E. Morzhina, S. Datta, A. Rudnitskaya, A. Sood,
M. Torok and B. Torok, Org. Biomol. Chem., 2011, 9, 1394– 33 S. A. Hilderbrand and R. Weissleder, Tetrahedron Lett., 2007,
401. 48, 4383–4385.
H. Aghayan, A. R. Khanchi, T. Youse and H. Ghasemi, J. 34 B. B. Bhowmik and P. Ganguly, Spectrochim. Acta, Part A,
Nucl. Mater., 2017, 496, 207–214. 2005, 61, 1997–2003.
H. Wu, M. Zhou, Y. Qu, H. Li and H. Yin, Chin. J. Chem. Eng., 35 R. Singh and G. Panda, Org. Biomol. Chem., 2010, 8, 1097.
1
7
8
9
2
009, 17, 200–206.
36 S. Girault, P. Grellier, A. Berecibar, L. Maes, E. Mouray,
P. Lemi `e re, M.-A. Debreu, E. Davioud-Charvet and
C. Sergheraert, J. Med. Chem., 2000, 43, 2646–2654.
37 C. Teixeira, N. Vale, B. Perez, A. Gomes, J. R. B. Gomes and
P. Gomes, Chem. Rev., 2014, 114, 11164–11220.
A. O. Terent'Ev, I. A. Yaremenko, V. A. Vil', I. K. Moiseev,
S. A. Kon'Kov, V. M. Dembitsky, D. O. Levitsky and
G. I. Nikishin, Org. Biomol. Chem., 2013, 11, 2613–2623.
1
0 C. Tagusagawa, A. Takagaki, K. Takanabe, K. Ebitani,
S. Hayashi and K. Domen, J. Phys. Chem. C, 2009, 113, 38 A. Kumar, L. Rout, L. S. K. Achary, R. S. Dhaka and P. Dash,
7421–17427. Sci. Rep., 2017, 7, 42975.
1 J. Xu, R. W. Gable and C. Ritchie, Acta Crystallogr., Sect. C: 39 S. Hatakeyama, N. Ochi, H. Numata and S. Takano, J. Chem.
Struct. Chem., 2018, 74, 1384–1389. Soc., Chem. Commun., 1988, 64, 1202–1204.
2 E. Rezaei-Seresht, F. M. Zonoz, M. Estiri and R. Tayebee, Ind. 40 A. Amoozadeh, S. Rahmani, M. Bitaraf, F. B. Abadi and
Eng. Chem. Res., 2011, 50, 1837–1846. E. Tabrizian, New J. Chem., 2016, 40, 770–780.
3 L. R. V. da Conceicao, L. M. Carneiro, D. S. Giordani and 41 M. Gensicka-Kowalewska, G.
Cholewi n´ ski
H. F. de Castro, Renewable Energy, 2017, 113, 119–128. K. Dzierzbicka, RSC Adv., 2017, 7, 15776–15804.
4 L. D. Chavan and S. G. Shankarwar, Chin. J. Catal., 2015, 36, 42 G. Rathee, S. Kohli, N. Singh, A. Awasthi and R. Chandra,
054–1059. ACS Omega, 2020, 5, 15673–15680.
5 N. C. Coronel, M. J. da Silva, S. O. Ferreira, R. C. da Silva and 43 G. Song, B. Wang, H. Luo and L. Yang, Catal. Commun., 2007,
R. Natalino, ChemistrySelect, 2019, 4, 302–310. 8, 673–676.
6 J. E. Molinari, L. Nakka, T. Kim and I. E. Wachs, ACS Catal., 44 A. Ilangovan, S. Malayappasamy, S. Muralidharan and
011, 1, 1536–1548. S. Maruthamuthu, Chem. Cent. J., 2011, 5, 81.
7 H. Cai, X. Wu, Q. Wu and W. Yan, Dalton Trans., 2016, 45, 45 L. Q. Wu, Y. F. Wu, C. G. Yang, L. M. Yang and L. J. Yang, J.
4238–14242. Braz. Chem. Soc., 2010, 21, 941–945.
8 J. Li, X. Wang, W. Zhu and F. Cao, ChemSusChem, 2009, 2, 46 X. Fan, X. Hu, X. Zhang and J. Wang, Can. J. Chem., 2005, 83,
77–183. 16–20.
9 Z. Xie, H. Wu, Q. Wu and L. Ai, RSC Adv., 2018, 8, 13984– 47 S. M. Vahdat, S. Khaksar, M. Akbari and S. Baghery, Arabian
3988. J. Chem., 2019, 12, 1515–1521.
0 L. Rozic, B. Grbic, N. Radic, S. Petrovic, T. Novakovic, 48 M. Hong and G. Xiao, J. Fluorine Chem., 2012, 144, 7–9.
1
1
1
1
1
1
1
1
1
1
2
2
and
1
2
1
1
1
Z. Vukovic and Z. Nedic, Appl. Clay Sci., 2011, 53, 151–156.
1 V. V. Bokade and G. D. Yadav, Appl. Clay Sci., 2011, 53, 263–
49 B. Das, P. Thirupathi, I. Mahender, V. S. Reddy and Y. K. Rao,
J. Mol. Catal. A: Chem., 2006, 247, 233–239.
2
71.
50 S. S. Kahandal, A. S. Burange, S. R. Kale, P. Prinsen, R. Luque
and R. V. Jayaram, Catal. Commun., 2017, 97, 138–145.
2
2
2 G. Yadav, J. Catal., 2003, 217, 88–99.
3 L. Y. Zhang, S. Y. Cai, J. H. Mo, G. T. Wei, Z. M. Li, R. C. Ye 51 K. Venkatesan, S. S. Pujari, R. J. Lahoti and K. V. Srinivasan,
and X. M. Xie, Mater. Manuf. Processes, 2015, 30, 279–284. Ultrason. Sonochem., 2008, 15, 548–553.
4 A. d. N. de Oliveira, M. A. Barbosa de Lima, L. H. de Oliveira 52 A. N. Dadhania, V. K. Patel and D. K. Raval, J. Saudi Chem.
Pires, M. Rosas da Silva, P. T. Souza da Luz, R. S. Angelica, Soc., 2017, 21, S163–S169.
G. N. da Rocha Filho, C. E. F. da Costa, R. Luque and 53 K. Niknam, F. Panahi, D. Saberi and M. Mohagheghnejad, J.
L. A. Santos do Nascimento, Materials, 2019, 12, 1431. Heterocycl. Chem., 2010, 47, 292.
5 D. S. Moraes, R. S. Ang ´e lica, C. E. F. Costa, G. N. Rocha Filho 54 M. Dabiri, M. Baghbanzadeh and E. Arzroomchilar, Catal.
and J. R. Zamian, Appl. Clay Sci., 2010, 48, 475–480. Commun., 2008, 9, 939–942.
Ilangovan, S. Muralidharan,
2
2
2
2
2
6 R. Liu, X. Xia, X. Niu, G. Zhang, Y. Lu, R. Jiang and S. He, 55 A.
Appl. Clay Sci., 2015, 105–106, 71–77.
P.
Sakthivel,
S. Malayappasamy, S. Karuppusamy and M. P. Kaushik,
7 R. A. Valiulin, L. M. Halliburton and A. G. Kutateladze, Org.
Tetrahedron Lett., 2013, 54, 491–494.
Lett., 2007, 9, 4061–4063.
8 A. Domling, W. Wang and K. Wang, Chem. Rev., 2012, 112,
56 F. Rashedian, D. Saberi and K. Niknam, J. Chin. Chem. Soc.,
2010, 57, 998–1006.
3083–3135.
©
2021 The Author(s). Published by the Royal Society of Chemistry
RSC Adv., 2021, 11, 11244–11254 | 11253

View Article Online
RSC Advances
Paper
5
7 P. S. Bhale, S. B. Dongare and Y. B. Mule, Chem. Sci. Trans., 64 G. M. Ziarani, A. Badiei, M. Hassanzadeh and S. Mousavi,
015, 4, 246–250. Arabian J. Chem., 2014, 7, 335–339.
8 Z. H. Zhang and Y. H. Liu, Catal. Commun., 2008, 9, 1715– 65 M. Nasr-Esfahani, M. Montazerozohori and T. Abdizadeh, C.
2
5
1719.
R. Chim., 2015, 18, 547–553.
5
6
9 Z.-H. Zhang and X.-Y. Tao, Aust. J. Chem., 2008, 61, 77.
0 D. S. Aher, K. R. Khillare, L. D. Chavan and S. G. Shankarwar,
ChemistrySelect, 2020, 5, 7320–7331.
66 T.-S. Jin, J.-S. Zhang, T.-T. Guo, A.-Q. Wang and T.-S. Li,
Synthesis, 2004, 2004, 2001–2005.
67 C. R. McElroy, A. Constantinou, L. C. Jones, L. Summerton
and J. H. Clark, Green Chem., 2015, 17, 3111–3121.
68 R. A. Sheldon, ACS Sustainable Chem. Eng., 2018, 6, 32–48.
6
6
6
1 D. S. Aher, K. R. Khillare, L. D. Chavan and S. G. Shankarwar,
RSC Adv., 2021, 11, 2783–2792.
2 S. K. Bhorodwaj and D. K. Dutta, Appl. Clay Sci., 2011, 53, 69 M. Tobiszewski, M. Mar ´c , A. Gałuszka and J. Namie ´s nik,
47–352. Molecules, 2015, 20, 10928–10946.
3 L. V. Chopda and P. N. Dave, ChemistrySelect, 2020, 5, 2395– 70 R. A. Sheldon, Green Chem., 2007, 9, 1273.
400.
3
2
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