Effect of heteropoly acids on structure, electrochemical behavior, acidic properties and catalytic activity of zwitterionic-type ionic liquid

Article abstract of DOI:10.1016/j.ica.2019.119086

Zwitterionic-type ionic liquid (CDIPS) was prepared by the reaction of 1,1′-carbonyldiimidazole (CDI) with 1,3-propane sultone (PS). ¹H NMR, XRD, FT-IR and CHNS confirm that 1,3-propane sultone was attached to two N atoms. Inorganic–organic hyb

Full text of DOI:10.1016/j.ica.2019.119086

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Effect of heteropoly acids on structure, electrochemical behavior, acidic prop-
erties and catalytic activity of zwitterionic-type ionic liquid
Ezzat Rafiee, Shabnam Shahebrahimi
PII:
S0020-1693(19)30838-2
https://doi.org/10.1016/j.ica.2019.119086
ICA 119086
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Inorganica Chimica Acta
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Please cite this article as: E. Rafiee, S. Shahebrahimi, Effect of heteropoly acids on structure, electrochemical
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doi: https://doi.org/10.1016/j.ica.2019.119086
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Effect of heteropoly acids on structure, electrochemical behavior, acidic properties and
catalytic activity of zwitterionic-type ionic liquid
Ezzat Rafiee,a, b, * Shabnam Shahebrahimia
a Department of Inorganic Chemistry, Faculty of Chemistry, Razi University, Kermanshah,
67149, Iran.
b Institute of Nano Science and Nano Technology, Razi University, Kermanshah, 67149, Iran.
* Corresponding author: Tel./Fax: +98 833–4274559; E-mail: ezzat_rafiee@yahoo.com ,
e.rafiei@razi.ac.ir
'Declarations of interest: none
1

Abstract:
Zwitterionic-type ionic liquid (CDIPS) was prepared by the reaction of 1,1'-carbonyldiimidazole
(CDI) with 1,3-propane sultone (PS).1H NMR, XRD, FT-IR and CHNS confirm that 1,3-propane
sultone was attached to two N atoms. Inorganic–organic hybrids based on SO3H-functionalized
cations and Keggin-type heteropoly acids as anions (H3PW12O40, H3PMo12O40, and H4SiW12O40;
HPAs) were prepared and characterized. The electronic properties of the center-metal ions in
hybrid salts and their electrochemical behavior were studied through UV and cyclic voltammetry
(CV) respectively. Catalytic activities of the organic-inorganic HPA salts were studied in the
synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]-xanthene-11-one derivatives. Relationship
between catalytic activity and both of acidic strength and the number of acid sites of the catalysts
was discussed and proved by potentiometric titration. The catalyst was readily recovered and
reused for four runs. Moreover, the best catalyst was characterized more by FT-IR, XRD,
SEM/EDX, ICP-OES, BJH and CHNS.
Keywords: Organic-inorganic hybrid composites, Zwitterions, Heterogeneous catalysis,
Polyoxometalates.
1.
Introduction
Heteropoly acids (HPAs) are a category of polyoxometalates (POMs), which include one or more
heteroatoms, transition metal cations (addenda atoms) and oxide anions. HPAs are useful in green
technology because of strong Brönsted acidities. Also these compounds have redox potentials and
high thermal stabilities [1,2]. Separation and reusing of homogeneous HPA catalysts are extremely
difficult, therefore homogeneous HPA catalysts are replaced with heterogeneous analogs [3]. One
2

the strategies is heterogenization of HPAs which prepared insoluble organic-inorganic HPA-salts
(here after hybrid HPAs). They are produced through a combination of various organic cations
with heteropolyanions [4]. The synthesis and expansion of hybrid HPAs as ionic liquids (ILs) have
improved properties of HPAs. In fact, this combination not only makes heterogeneous HPAs but
also have the advantages of both organic cation and heteropolyanions [4, 5].
In organic-inorganic hybrids, organic cations can be propane sulfonate (PS) or butane sulfonate
(BS) functionalized organic cations that synthetic path of them is a reaction of the neutral
nucleophiles with sultone to provide zwitterions [6, 7]. The combination of these zwitterions with
Keggin type POMs are obtained SO3H-functionalized POM-based ionic liquids (POM-ILs) or
organic-POM salts. ILs are zwitterionic compounds that have low-melting point (below 100 °C)
[8]. Organic-POM salts are a family of a new POM salts and have higher melting points compare
with normal ILs (above 100 °C) [9].
Hybrid HPAs as organic-POM salts are satisfactory as acid catalysts for synthetic process owing
to their unique properties such as Brønsted acidic properties, low solubility in organic solvents,
high melting point, and high mass transfer resistance [10, 11].
Xanthene derivatives widely used as antibacterial, anti-proliferative, anti-inflammatory, and
antiviral compounds [12, 13]. Additionally, these compounds have been used as dyes, pH sensitive
fluorescent materials, and in laser technology [14, 15]. There are some methods and different kind
of catalysts which have been used for synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]-xanthene-
11-one [16-18].
We have synthesized and characterized new hybrid HPAs based on bi-SO3-functionalized 1,1'-
carbonyldiimidazole and different HPAs. Afterward, these salts are utilized as catalyst for the
3

synthesis of xanthene derivatives by multi-component condensation of 2-naphthol, aryl aldehydes
and dimedone.
2.
Experimental
2.1. Materials and methods
All used substances and solvents were bought from Fluka, Aldrich or Merck Company and used
without further purification. FT-IR spectra were recorded as KBr pellets using a Shimadzu 470
spectrophotometer in the range of 450–4000 cm−1. Scanning electron microscopy (SEM) and
energy dispersive X-ray spectroscopy (EDX) were used to know the morphology and composition
of the catalyst on a model TE-SCAN. A quantitative bulk elemental composition of the catalyst
was evaluated by inductively coupled plasma optical emission spectroscopy (ICP-OES) VARIAN
VISTA PRO. X-ray diffraction (XRD) patterns were collected with a diffractometer on Rigaku,
1
Japan, RINT 2500 V using Cu-Kα radiation. H NMR spectra data (for SO3H functionalized
31
cation) and P NMR spectra data (for hybrid HPAs) were recorded by UNITY Inova 500 MHz
Varian in D2O as the solvent and TMS and 85 wt.% H3PO4 respectively as standard. Elemental
analyses (CHNS) were performed using a EURO EA3000 Elemental Analyzer. The potential
variation was measured with a Metrohm 713 pH meter using a double junction electrode. The
absorption spectra of UV-vis were achieved with T80 double beam UV-vis spectrophotometer
(NORDANTEC-GmbH) diode-array spectrometer using quartz cells of 1 cm optical path and the
concentration of samples is 0.1 g/L in water. The electrochemical behavior of the catalyst was
studied in dimethyl sulfoxide (DMSO) (0.5 M NaClO4). A glassy carbon electrode, Ag/AgCl
electrode and Pt electrode were used as the working electrode, the reference electrode and the
auxiliary electrode respectively. Cyclic voltammograms (CV) were obtained at a scan rate 0.1 V/s
in ambient temperature (20 ± 5 °C) with autolab PalmSens 3. The porosity was measured with
4

sorptometer Kelvin 1042 using nitrogen adsorption at 77 K. Differential scanning calorimetry
(DSC) measurements were done on a SDT Q600 instrument with a scanning rate of 10 °C/min in
air. Thermogravimetric analysis was carried out using a SDT Q600 instrument at heating rate of
10 °C/min under nitrogen. The xanthene products were characterized by 1H NMR using a Bruker
Avance 400 MHz NMR spectrometer.
2.2. Preparation of the catalysts
2.2.1. Preparation of 1,1ʹcarbonyl-bis(3-sulfopropyl) diimidazole (CDIPS):
To 10 mL of solution of 1,1' carbonyldiimidazole (1 M in acetonitrile) 0.025 1,3-propane sultone
(PS) was added slowly within 5 min and stirred for 15 h at 50 °C. The reaction mixture was washed
with ether five times and dried at 100 °C for 3 h. The final product CDIPS was white powder with
a yield of 79.5% (Scheme 1).
O
O
O
O
S
O
O
N
O
O
N
N
S
N
N
O
+ 2
N
S
N
N
O
O
Scheme 1. Synthetic reaction for CDIPS production.
2.2.2. Preparation of hybrid HPAs:
For the synthesis of hybrid HPA catalysts, H3PW12O40 (PW), H3PMo12O40 (PMo) and H4SiW12O40
(SiW) were used as HPAs. The resulting CDIPS (4.0, 4.0 and 2.7 mmol) was added to an
aqueous solution of PW, PMo and SiW (10 mL of 0.27, 0.27 and 0.13 M) respectively. The
mixture was stirred at room temperature for 24 h. Water was removed in a vacuum to give the
final products (CDIPS)x(PW)y, (CDIPS)x(PMo)y and (CDIPS)x(SiW)y which were respectively
5

white, green and white powders. These compounds are stable in aqueous solution and they are
not hydrolyzed.
2.3. Acidity measurement
Acidity of hybrid HPAs was investigated by potentiometric titration. 0.05 g of hybrid HPA was
added to 90 mL of acetonitrile and stirred for 3 h. Titration of the mixture was done with 0.05 N
solution of n-butylamine in acetonitrile. The potential difference was analyzed with a Metrohm
713 pH meter using a double junction electrode.
2.4. Synthesis of 12-aryl-8,9,10,12-tetrahydrobenzo[a]-xanthene-11-ones
A blend of 2-naphthol (0.576 g, 4 mmol), dimedone (0.560 g, 4 mmol), aryl aldehyde (4 mmol)
and hybrid HPA (0.020 g) in a 40 mL round-bottomed flask connected to a reflux condenser, was
stirred in an oil-bath at 100 °C. The progress of the reaction was continuously monitored by TLC.
After finishing of the reaction, as indicated by TLC tests, the catalyst was recovered and washed
with ethyl acetate (the product and catalyst are soluble and insoluble in warm EtOAc respectively).
Then the catalyst was dried and reused in a new reaction. For extraction of the product, EtOAc
was removed. After removing of EtOAc by rotary evaporator, EtOH (90%) was added to residual,
due to the high concentration of the product, the product crystals precipitated from EtOH solvent
and the pure product was recrystallized.
3.
Results and Discussion
The synthesis of hybrid HPAs was performed at two steps reaction. In the first step, sulfonation,
the reaction of 1,1'-carbonyldiimidazole (CDI) with 1,3-propane sultone (PS) was prepared the
zwitterionic-type compound (CDIPS) (Scheme 1).
6

1
1
This product analyzed by H NMR (Fig. 1 and Fig. S1). Based on the results of H NMR, 1,3-
propane sultone was connected to each two N atoms in 1,1'-carbonyldiimidazole molecule
(product A) and one N atom (product B) in CDIPS synthesis procedure. H4-A and H4-B of
imidazole rings were characterized as singlet at δH 8.73 and 8.63 ppm respectively with the ratio
of the signals area 0.92:0.95 which was close to expected value 1:1 of A and B products. Signals
at 7.42-7.43 and 7.41-7.42 were related to H5-H6 of A and B respectively. Two triplet signals at δH
4.24 and 4.28 were related to H1-A and H1-B respectively (Fig. S1). The ratio of the signals area
was close to 2:1 according to expected stoichiometric values of H1 atoms in A and B (4 H1:2 H1).
Other signals at δH 2.77-2.84 and 2.16-2.23 were related to H3-A and H3-B, also signals at δH .16-
2.23 were related to H2-A and H2-B.
Table 1. Results of CHNS elemental analysis.
CDIPS
CDIPSPW
CHNS
Experimental Data Calculated Data Experimental Data
Calculated Data
4.747
C (wt.%)
H (wt.%)
N (wt.%)
S (wt.%)
37.140
5.019
38.230
4.902
5.027
1.120
1.634
1.864
0.600
13.454
15.080
13.720
15.680
1.700
1.947
In the second step, CDIPS with PW, PMo, and SiW were mixed and with the creation of ionic
interaction between them, hybrid HPAs were produced. 31P NMR spectrum of CDIPSPW showed
7

a sharp and characteristic signals at -15.57 and -0.49 related to phosphorous in the tetrahedral
position of the Keggin unit (Fig. 1b) and the structure of HPA has been preserved.
(a)
Fig. 1. (a) 1H NMR spectrum of CDIPS and (b) 31P NMR spectrum of CDIPSPW.
For more structural investigations, we were studied hybrid HPAs with different HPAs using FT-
IR. In the FT-IR spectra (Fig. 2), the structural and bonding changes were investigated in the hybrid
HPAs by characteristic bands of Keggin anions. The vibrational bands of Keggin type HPAs were
compared with hybrid HPAs in Table 2. In CDIPSPMo compound, according to the position bands
in the [PMo12O40]3- [19], the vibrational bands of P-O bond and Mo=O bond and Mo-Oe-Mo (edge
8

sharing) bond showed no change in band position (Table 2). The vibrational bands of Mo-Oc-Mo
(corner-sharing) band had red-shift from 871 to 866 cm−1. In the case of CDIPSSiW, the Keggin
structure of [SiW12O40]4- [20] showed a change in the band positions. The vibrational bands of
W=O terminal stretching shifts from 983 to 978 cm-1, and stretching of W-Oe-W intra bridges
between edge-sharing WO6 octahedral changes from 781 to 797 cm-1. The vibrational bands
stretching of Si-O and W-Oc-W inter bridges between corner-sharing WO6 octahedral showed no
apparent change in band positions and were in 1018 and 926 cm-1 respectively. Typical bands for
absorptions of PW [21] in the CDIPSPW compound are P-O stretching, W=O stretching, and W-
Oc-W which showed no changes in band position (Table 2). Also for W-Oe-W band showed change
from 814 to 810 cm-1. Besides, vibrations at 1033 and 1152 cm-1 were attributed to stretching mode
of sulfonic groups in the CDIPS cation, and N-C (sp3) quaternization stretching frequency at 1579
cm-1. The results exhibited that the amplification and weakening of the bonds of the HPAs were
due to the interaction between the SO3H-functionalized cations and the HPAs (Fig. 2).
Table 2. The vibrational bands of Keggin type HPAs and hybrid HPAs (M= W or Mo, X= P or Si).
Entry
Type of Cat.
PW
M=O
984
983
961
983
978
960
M-Oc-M
896
M-Oe-M
814
X-O
1080
1019
1064
1080
1018
1063
1
2
3
4
5
6
SiW
924
781
PMo
871
779
CDIPSPW
CDIPSSiW
CDIPSPMo
895
810
926
797
866
779
9

Fig. 2. FT-IR spectra of (a) PW, (b) CDIPS, (c) CDIPSPW, (d) reused CDIPSPW in left, and (a) PMo, (b)
CDIPSPMo, (c) CDIPS, (d) CDIPSSiW, (e) SiW in right.
For the study of the electronic properties of the center-metal ions, UV-vis is a very suitable method.
Most of HPAs have characteristic bands in the middle of the range UV-vis. The spectra of hybrid
HPAs are shown in Fig. 3. The HPAs in their non-reduced form have a unique band that is
depended on ligand to metal charge transfer (LMCT) band. This band is seen in the UV area below
400 nm [22]. The absorption bands at 202 nm (CDIPSPW) and 215 nm (CDIPSPMo) relate to the
O→P transition and the band at 203 nm (CDIPSSiW) is owing to the O→Si transition. The
medium broad energy band appears at 254 nm for CDIPSPW, 263 nm for CDIPSSiW, and 233
nm for CDIPSPMo are due to the LMCT of O2-→W6+ and O2-→Mo6+ in the Keggin unit of HPAs
[23-25]. The CDIPS cation has an intense and wide absorption band at approximately 208 nm,
related to the π-π* transition of the imidazole is unique [26]. Due to the presence of the index
10

LMCT band of pure PW and SiW, around 265 and PMo around 240 nm (Fig. 3), the results indicate
that in three compounds of hybrid HPAs, Keggin structure of HPAs is maintained in spite of
interaction with CDIPS cation.
Fig. 3. UV-Vis spectra of hybrid HPA with H2O as solvent.
CV is used to study electrochemical behavior of hybrid HPAs. Fig. 4 shows the CV of the organic
cation, HPA and hybrid HPAs for comparison. The shift in the potential of some characteristic
peak of CDIPSHPA, obviously exhibits the interaction between HPA and CDIPS. The amount of
the shifts in different peaks are reported in Table 3. Investigation of CVs of CDIPSPW and
CDIPSSiW shows that the produced hybrid HPAs have suitable electrochemical synergistic
behavior. In CV of CDIPSPMo, some peaks have shifted and some merged.
The redox peaks of W(VI)/W(V) and Mo(V)/Mo(IV) are effective in electron transport reaction
[27]. The comparison of these peaks in CV of HPAs and hybrid HPAs display that there is no
significant difference. Therefore, the difference in their performance cannot be seen in
electrocatalytic processes.
11

Table 3. Electrochemical data.
Potential Shift
Catalyst
PW
Epc (V)
Epa (V)
ΔΕpc (V)a
ΔΕpa (V)b
-0.198
-0.749
-1.250
-0.198
-0.749
-1.250
0.202
-0.098
-0.648
-1.099
-0.098
-0.648
-0.949
0.302
-
-
-
-
-
CDIPSPW
0.150
0.102
0.202
-0.148
-0.348
-0.698
-0.999
-1.249
0.202
PMo
-0.348
-0.799
-1.099
0.302
0.002
-
-
-0.148
-0.398
-0.698
-1.199
-0.698
-1.049
-
0.200
-0.100
-0.250
-0.200
CDIPSPMo
-0.448
-1.049
-1.299
-0.598
-0.949
-0.050
-
0.050
SiW
12

-1.549
-0.698
-1.049
-1.349
-1.299
-0.598
-0.949
-1.099
-
-
-
-
CDIPSSiW
0.200
0.200
a Epc (HPA) - Epc (CDIPSHPA)
b Epa (HPA) - Epa (CDIPSHPA)
Fig. 4. CV of organic cation, HPAs and organic-inorganic HPA salts (0.03 g) in DMSO (0.5 M NaClO4), scan rate
0.1 V/s, ambient temperature (20 ± 5 °C).
HPAs known as bi-functional compounds. These compounds and their derivatives have
electrotransfer ability and also are Brønsted acids. The acidity of hybrid HPAs is measured by
potentiometric titration with an organic base. Curves of Fig. 5 show titration curves of different
13

hybrid HPAs. In these titration curves, the initial electrode potential (Ei) exhibits the maximum
strength of the acid sites and value from which the plateau is reached (mmol amine/mmol catalyst)
indicates the total number of acid sites (na) (mmol/g) that are present in the titrated solid [28]. As
displayed in Fig. 5, all produced hybrid HPAs have very strong acid sites, Ei > 100 mV. It seems
that combination of HPA with organic cation decrease Ei of HPA. Although all of the salts are still
strong acids (Ei > 100 mV).
According to the titration curves, it is detected acidic strength of CDIPSPW is more than
CDIPSPMo and CDIPSSiW (Fig. 5b). Ei values for compounds of CDIPSPW, CDIPSPMo and
CDIPSSiW are 362, 327.5 and 274.6 mV respectively. Also, the number of acidic sites of
CDIPSPW is 12 mmol/g that is higher than other HPA salts.
Fig. 5. Potentiometric titration curves of different organic-inorganic HPA salts and different HPAs.
Catalytic activities of these salts are studied for the synthesis of 12-aryl-8,9,10,12-
tetrahydrobenzo[a]-xanthen-11-ones. The synthetic reaction is the one-pot three-component
condensation reaction of aryl aldehydes with 2-naphthol and dimedone (5,5-dimethylcyclohexane-
1,3-dione) under solvent-free conditions.
14

The catalytic activity of different salts of CDIPSHPA can be attributed to Ei and na of these salts.
There is a direct correlation between Ei and na with catalytic activity of different salts (Table 4 and
Fig. 6). So, CDIPSPW has a better catalytic activity due to its high acidic strength and higher
number of acidic sites than other salts.
Fig. 6. The correlation of acidic strength and the number of acid sites of CDIPSHPA with catalytic activity after
10 min.
Table 4. Effect of acidic strength and the number of acid sites on catalytic activity of HPAs and CDIPSHPA
after 10 min.
Entry
Type of catalyst
PW
Ei (mV)
650.00
484.30
397.70
362.00
327.50
274.60
na (mmol/g)
1.8
Yeild (%)
1
2
3
4
5
6
85
70
65
94
68
40
PMo
5.0
SiW
1.2
CDIPSPW
CDIPSPMo
CDIPSSiW
12.0
10.0
8.0
15

The catalytic activities of different hybrid HPAs are investigated and results show that CDIPSPW
is the best catalyst among other hybrids (Fig. 7a).
Fig. 7. (a) Investigation of activity of different catalysts in the model reaction, (b) Effect of the amount of
CDIPSPW on the model reaction.
The optimum amount of CDIPSPW is determined by examination of different amounts of the
catalyst (Fig. 7b). The results indicate that the optimum amount of CDIPSPW is 2 mol %.
The yield of the product in model reaction is 10% after 10 min at room temperature. But this
reaction efficiently gives the product in excellent yield and short reaction time at 100 °C (Table 5,
entries 1 and 2). To study the influence of the solvent, reaction is carried out in dichloromethane,
water, acetonitrile, ethyl acetate, and ethanol (Table 5, entry 3-7). The catalytic performance in
solvent system shows low yield or reaction dose not proceed.
16

Table 5. Optimization of conditions of model reaction in the presence of CDIPSPW a.
Entry
Temp. (℃)
r.t.
Solvent
Neat
Time (min)
Yield (%)
1
2
3
4
5
6
7
10
10
10 b
100
Neat
94
100
CH2Cl2
H2O
20 (10)
20
62 (35)
100
51
0
100
CH3CN
EtOAC
EtOH
20
100
20
0
100
20
0
a Benzaldehyde (1mmol), 2-naphthol (1 mmol), dimedone (1 mmol) and CDIPSPW (2 mol %)
b Yield of the product was 80% after 80 min.
According to above optimization, 2 mol % of the CDIPSPW in solvent free conditions at 100 °C
are chosen as the optimal reaction conditions for further investigations.
Supplemental identifications for CDIPSPW as an optimal catalyst for further investigation are
completed. The XRD patterns of CDIPS, PW, and fine crystalline powder of CDIPSPW, have
been shown in Fig. 8. XRD results show new pattern and new crystallization. The thermal stability
of CDIPSPW is investigated by means of TGA and DSC. The TGA curve of CDIPSPW (Fig. 8)
shows a weight loss of 2.0% below 200 °C is ascribed to water of hydration. The weight loss
observed between 200 and 400 °C is due to the decomposition temperature of the organic cation.
3-
The weight loss above 500 °C is mostly owing to the decomposition of PW12O40 to WO3 and
P2O5 oxides. Thermoanalytical technique of DSC is shown endothermic (heat absorption for the
17

loss of water molecules) and exothermic (heat evolution for the decomposition of the organic
cation) processes (Fig. 8).
Fig. 8. XRD patterns of (a) CDIPS, (b) CDIPSPW, and (c) PW in left, TGA and DSC curves of CDIPSPW in right.
The content of W and S are evaluated quantitatively by ICP-OES, which indicated 43.65% and
1.18% respectively. Based on these data, CDIPS to PW molar ratio is 1:1 depend to neutralization
of electrical charge with a molecular formula of [(CDIPS) (HPW12O40)] (Scheme 2). The
procedure of synthesis of CDIPSPW is shown in Scheme 2 as an example.
O
O
O
+ H3PW12O40
O
O
N
S
N
N
O
S
N
N
O
O
O
S
O
O
2-
O
HPW12O40
N
N
S
N
OH
OH
Scheme 2. Preparation of CDIPSPW.
18

The data of CHNS for CDIPSPW confirm that molar ratio is 1:1 (Table 1). Elemental analysis
using EDX (Fig. 9a) also showed the presence of N, P, W, O and S. Morphology of CDIPSPW is
investigated by SEM (Fig. 9b, 9c). According to the images of SEM, there is no specific shape for
the morphology of CDIPSPW.
The porous nature of the catalyst is studied using the N2 adsorption-desorption isotherm and BJH-
plot, those are shown in Fig. (9d, 9e). Based on the isotherm curve, CDIPSPW is macro-porous
with weak affinities. The macro-porosity of the cavities leads to the availability of more acidic
sites. Therefore, increasing the number of acidic sites is due to the size of the cavities which is
related directly to high catalytic activity of this catalyst (Fig. 6).
To show diversity in catalytic activity of this catalyst, a series of benzaldehyde derivatives with 2-
naphthol and dimedone are examined under the optimum reaction conditions and the results are
reported in Table 6. The recovery and reusability are an important factor for a catalyst. The
recycling of CDIPSPW is studied in several runs of model reaction. The yield of the product for
the first run reduce to 85% after fourth run (Fig. 10a).
The reused catalyst is studied by SEM/EDX (Fig. 10b, 10c). Morphology of the reused CDIPSPW
compare with fresh CDIPSPW has almost identical with less agglomeration that is probably owing
to abrasion in solvent free conditions. Data of EDX exhibit the presence of several elements
including C, N, P, W, O and S without important alteration in weight percent of the elements. Also,
there is no noteworthy alteration in the FT-IR spectrum (Fig. 2d in left). Moreover, the acidic
property of reused CDIPSPW is investigated by potentiometric titration (Fig. 5a). Strength of
acidic sites and number of acidic sites of reused CDIPSPW (after run 2) are decreased toward fresh
CDIPSPW (Ei and na values are 271.3, 362.0 mV and 5, 12 mmol/g respectively). The decline in
efficiency of reused CDIPSPW can be justified through reduce in acidic properties.
19

Fig. 9. (a) EDX pattern (b, c) SEM images (d) Nitrogen adsorption-desorption isotherm (e) BJH plot of CDIPSPW.
20

Table 6. The synthesis of xanthene derivatives using CDIPSPW as catalyst a.
R
O
O
O
OH
O
Catalyst
Solvent free, 100 °C
+
+
H
O
R
Entry
R
Time (min)
Yield (%) b
1
2
H
10
8
94
95
95
90
94
94
91
94
85
87
4-Cl
3-Cl
4-Br
3
10
13
10
8
4
5
4-NO2
3-NO2
3-OH
4-OH
4-CH3
6
7
17
7
8
9
25
22
10
4-OCH3
a Benzaldehyde (1mmol), 2-naphthol (1 mmol), dimedone (1 mmol) and CDIPSPW (2% mol)
in solvent free conditions at 100 °C.
b
1
Isolated yield; products were identified by comparing their H NMR with those in authentic
samples [18, 29].
21

Fig. 10. (a) Reusability of CDIPSPW in the model reaction (b) SEM image and (c) EDX pattern of reused
CDIPSPW.
4.
Conclusions
New hybrid HPAs have been synthesized based on bi SO3-functionalized 1,1'-carbonyldiimidazole
and different HPAs. These novel salts are fully characterized by several techniques including FT-
IR, UV-Vis, CV, XRD, TGA, DSC, CHNS, SEM-EDX, ICP-OES. Investigation of CV of the
hybrid HPAs shows that the electrochemical property of HPA is kept in CDIPSHPA. Based on
results of ICP-OES, molar ratio of CDIPS to PW is 1:1 in CDIPSPW depend to neutralization of
electrical charge with a molecular formula of [(CDIPS) (HPW12O40)]. The acidity measurement
shows a direct correlation between Ei and na with catalytic activity of different salts. The use of
22

these salts is investigated in the synthesis of xanthene derivatives from 2-naphthol with divers aryl
aldehydes and dimedone as an efficient and reusable catalyst. CDIPSPW has a better catalytic
activity due to its high acidic strength and higher number of acidic sites than other salts.
Acknowledgements
The authors thank the Razi University Research Council for support of this work.
Declarations of interest: none
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