Solvent responsive self-separation behaviour of Br?nsted acidic ionic liquid-polyoxometalate hybrid catalysts on H2O2 mediated oxidation of alcohols

Article abstract of DOI:10.1016/j.poly.2020.114993

Two solvent responsive self-separative ionic liquid-based POM hybrids [DEDSA]₃[PW₁₂O₄₀] and [DEDSA]₃[PMo₁₂O₄₀] were prepared by combination of diethyldisulphoammonium chloride [DEDSA]Cl with Keggin phosphotungstic acid and phosphomolybdic acid. Properties like water tolerance, high activity, high thermal stability and reusability in these POM hybrid compounds facilitate a new type of POM based catalysis used for oxidation of organic alcohols using hydrogen peroxide as oxidizing agent. Due to varied solubility of the hybrid with different solvents, the hybrid can effectively catalyze the oxidation reaction in homogenous condition and then can finally switch back to heterogenous system and convert as self-precipitating catalyst on addition of suitable solvent at the end of the reaction, which made the recovery and reuse of the hybrid very convenient. The catalysts are characterized via different analytical and spectroscopic tools like FT-IR, ¹H NMR, ¹³C NMR, ³¹P NMR, TGA, UV–Visible, Powder XRD techniques, Raman etc. It was found that the prepared material was highly acidic, thermally stable in nature and was recycled up to 7 times without much loss of catalytic activity.

Full text of DOI:10.1016/j.poly.2020.114993

Polyhedron 196 (2021) 114993
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Solvent responsive self-separation behaviour of Brønsted acidic ionic
liquid-polyoxometalate hybrid catalysts on H2O2 mediated oxidation of
alcohols
⇑
Niharika Kashyap, Sukanya Das, Ruli Borah
Department of Chemical Sciences, Tezpur University, Napaam, Tezpur 784028, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Two solvent responsive self-separative ionic liquid-based POM hybrids [DEDSA]3[PW12O40
] and
Received 9 October 2020
Accepted 18 December 2020
Available online 25 December 2020
[DEDSA]3[PMo12O40] were prepared by combination of diethyldisulphoammonium chloride [DEDSA]Cl
with Keggin phosphotungstic acid and phosphomolybdic acid. Properties like water tolerance, high activ-
ity, high thermal stability and reusability in these POM hybrid compounds facilitate a new type of POM
based catalysis used for oxidation of organic alcohols using hydrogen peroxide as oxidizing agent. Due to
varied solubility of the hybrid with different solvents, the hybrid can effectively catalyze the oxidation
reaction in homogenous condition and then can finally switch back to heterogenous system and convert
as self-precipitating catalyst on addition of suitable solvent at the end of the reaction, which made the
recovery and reuse of the hybrid very convenient. The catalysts are characterized via different analytical
and spectroscopic tools like FT-IR, 1H NMR, 13C NMR, 31P NMR, TGA, UV–Visible, Powder XRD techniques,
Raman etc. It was found that the prepared material was highly acidic, thermally stable in nature and was
recycled up to 7 times without much loss of catalytic activity.
Keywords:
Homogenous catalysis
Heterogenous catalysis
IL-POM
Organic-inorganic hybrid composites
Solvent-responsive
Self-separative
Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction
led to exploration of these materials in catalysis, medicine, materi-
als science, nanotechnology, molecular magnetism etc. [10–20].
Oxidation of primary and secondary alcohols to carbonyl com-
pounds are being recognized as fundamental essentials for labora-
tories, pharmaceuticals and other chemical manufacturing
industries due to their vital role in organic synthesis [1–3]. Many
conventional CrO3 and K2Cr2O7 based oxidant in acidic medium
produce over oxidation product of primary alcohol i.e. –COOH from
intermediate aldehyde which remains a problem in addition to sto-
ichiometric amount of oxidants, organic solvents, restriction for
acid sensitive substrates, release of toxic reagents and inorganic
side products to the environment [4–6]. Accompanied by the above
mentioned various environmental issues, efforts have been made
to produce reasonable oxidation protocols using environmentally
benign oxidants having high active oxygen content and producing
water as only byproduct at best [7,8]. Polyoxometalates (POMs) are
structurally diverse combination of stable metal–oxygen anionic
clusters of early transition metals (e.g. M = Mo, V, Ta, W, Nb) at
their higher oxidation states in presence of heteroatoms (e.g.
X = P, As, Si, Ge) [9]. The wide variation of composition, shapes,
sizes and charge densities provide numerous properties which
They show excellent multi-electron redox cycles without major
change of their structures in homogeneous or heterogeneous
phases [21–27]. Moreover, they are less toxic as compared to the
chromate-based oxidant. The solubility of POMs in polar solvent
increases their activity as redox catalysts for the oxidation of alco-
hol using H2O2 as oxidant in homogeneous phase [11,12,28]. The
recyclability limitation of homogeneous POMs was solved through
heterogenization of the POMs on inert support with high surface
area materials. But these supported catalysts also showed slow
reaction rates for aggregation and leaching from the support due
to weaker binding interaction between them [16]. Consequently,
many organic–inorganic hybrid POMs were designed to effectively
deal with the problems faced by the POM catalytic systems with
modification of their characteristic structural nature like high ther-
mal stability, Brønsted-acidic sites, rich redox properties have been
isolated [24–27].
Ionic liquids (ILs) have been used extensively for designing
task-specific catalysts, used as reaction media and also functional-
ized solvent because of their unique physical properties such as
wider range of melting points to be in liquid state, high thermal
stability, viscous nature, appropriate solvation behavior etc.[29].
The presence of organic cation into the POM framework increases
⇑
Corresponding author.
E-mail address: ruli@tezu.ernet.in (R. Borah).
https://doi.org/10.1016/j.poly.2020.114993
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
the porosity of the structure and also modify their polarity through
hydrophobic interaction which may work as controlling factor for
varied solvent responsive behaviour of the hybrid depending on
the nature of organic cation. These modifications of the POM struc-
tures may facilitate them as a new type of POM-based catalysts
having great potential in heterogeneous catalysis. The variation
of organic cations in to the POMs provides an efficient approach
to synthesize multifunctional materials based on the nature of
organic part in molecular level [30]. Task-specific (TSILs) ionic liq-
uids are an important group of ionic liquids with growing applica-
tions in synthesis, catalysis, electrochemistry etc. in which
functional groups are covalently tethered to the cation or anion
or both the ions [31]. Brønsted acidic ionic liquids were designed
by tethering acid groups to the ILs and this type of TSILs along with
providing acidic functionality to acid catalyzed reactions also helps
in avoiding strong mineral acids with volatile harmful acid vapors
[32]. Similarly basic TSILs are also designed and were investigated
for various applications [33,34]. Ion-pairing of organic cation with
the POM anion generates new type of hydrophobic ionic liquid-
based hybrid POMs (IL-POMs) as stable solid acidic material with
high melting points that can be utilized as heterogeneous catalyst
in suitable solvent [35,36].
Modification of the POMs by incorporation of organic moiety
has improved the catalytic activity as well as recovery and
reusability related issues. Till date several IL-POM hybrids have
been studied for base catalyzed reactions, acid catalyzed reactions,
oxidation reactions etc. with unique self- separation properties as
catalysts based on additive solvent, temperature variation and
other stimuli responsive [35–41]. Chen et al. in 2014 developed
mesoporous task specific hybrid catalyst of ionic liquid and polyox-
ometalate, [TMGHA]2.4H0.6PW by self-assembly of tetramethyl-
guanidinium functionalized with hydroxyl and amino groups
ionic liquid cation with Keggin phosphotungstic anion. Presence
of hydroxyl and amino groups in IL cations of the catalyst leads
to surface wettability property and acted as efficient triphasic cat-
alyst for water-mediated oxidation of alcohols [36]. Hou et al.
developed two aprotic N-methyl-alkylimidazolium polyoxometa-
lates [HDIm]2[W2O11] and [DMIm]2[W2O11] as reaction induced
self-separation catalysts for olefin epoxidation [39]. The study of
Brønsted acidic N-alkylsulfonic acid-based IL-POM hybrids was
first conducted by Keshavarz et al. as highly self-separation
catalyst for esterification reaction [40]. The use of SO3H-function-
alized imidazolium POM hybrid as alternative heterogeneous cata-
lyst for conc. H2SO4 in nitration of aromatics was reported by
Saikia et al. [41]. No reports are found for designing of direct N-
SO3H functionalized ammonium based Keggin POMs and their cat-
alytic studies in oxidation of alcohol as solvent-sensitive self-sep-
aration catalyst in the presence of sulfonic groups through
variation of interactions with different solvents. Therefore, herein
we aimed to develop direct N-SO3H functionalized diethyldisul-
foammonium salts of Keggin anions, [DEDSA]3[PM12O40] of phos-
photungstic acid and phosphomolybdic acid (Scheme 1) as
solvent-responsive recyclable catalysts for oxidation of alcohols
using 30% H2O2 as green oxidant at 65 °C after characterization
with various analytical techniques.
2. Experimental section
2.1. Materials and methods
The required chemicals were collected from Merck and Tokyo
Chemical Industry in pure form. Infrared spectra were recorded
on Perkin Elmer MIR/FIR-FTIR spectrophotometer. JEOL ECS
400 MHz spectrophotometer was used to take 1H ,13C and 31P
nuclear magnetic resonance (NMR) spectra (ppm scale) in DMSO d6
as solvent. Shimadzu UV 1800 spectrophotometer was utilized to
study the Hammett acidity plots of the POM hybrid materials.
Thermogravimetric analysis (TGA) was performed on Shimadzu
TGA-50. Scanning electron microscopy (SEM) images were
acquired from JEOL JSM-6390LVSEM besides energy-dispersive X-
ray (EDX) images. A RENISHAW BASIS SERIES 514 LASERS with
green Argon ion laser was utilized to obtain the Raman analyses.
Powder X-ray diffraction (PXRD) patterns were noted using a
Rigaku Multiflex instrument using
a nickel filtered Cu K
(0.15418 nm) radiation source and scintillation counter detector.
UV–Visible diffusion reflectance (DRS) spectra were collected using
Shimadzu 2450 spectrophotometer. Elemental analyses were car-
ried out on PerkinElmer 20 analyzer. Monitoring of oxidation reac-
tion was done by HPLC method using C18 reverse phase column
and UV detector at 254 nm. Equal mixture of water and acetonitrile
was employed as mobile phase at a flow rate of 1 mL/min. The
oxidation products were also analyzed using GC–MS having GC
Scheme 1. Synthesis of [DEDSA]3PM12O40
.
2

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
model (7890A), MS model (240 Ion trap) and column (Agilent
function (H0) was calculated using Eq. (1) by measuring the
absorption differences [I]/ [HI]+ where, pK(I)aq is the pKa value of
the basic indicator.
19091 J-413 HP-5 30 m ꢀ 320
l
m ꢀ 0.25
lm).
2.2. Preparation of diethyldisulfoammonium salts of Keggin anions
[DEDSA]3[PM12O40] where M = Mo (VI), W(VI)
H0 ¼ pKðIÞaq: þ log ½Iꢂ=½HIꢂþ
ð1Þ
The -SO3H functionalized POM hybrid salt [DEDSA]3[PM12O40
]
3. Results and discussion
where M = Mo (VI), W(VI) of diethyldisulfoammonium cation
3-
containing polyoxometalate Keggin anion [PMo12O40
]
or
The preparation of diethyldisulfoammonium salts of Keggin
anions [DEDSA]3[PM12O40] where M = Mo (VI), W(VI) were carried
out in two step method (Scheme 1) as mentioned in the experi-
mental section. The pure IL-POM hybrid salts were analyzed by dif-
ferent analytical techniques for structural confirmations, thermal
stability study and for the Brønsted-acidic natures. The redox prop-
erties of the IL-POMs were investigated as solvent responsive self-
separation catalyst for selective oxidation of benzylic alcohol to
aldehyde and secondary alcohol to keto compound.
[PW12O40
]
3- were prepared in two steps (Scheme 1). Firstly, the ini-
tial ionic liquid diethyl disulfoammonium chloride ([DEDSA]Cl) was
prepared by dropwise addition of chlorosulfonic acid (30 mmol)
using dropping funnel into a stirring solution of diethylamine
(15 mmol) in dry CH2Cl2 (15 mL) in a two neck 100 mL round bot-
tom flask at 0 °C for 10 min with continuous stirring [42]. Then it
was stirred for one hour at room temperature to get a separate layer
of the [DEDSA]Cl in dichloromethane solvent. Repeated washing of
the reaction mixture with dry CH2Cl2 (3 ꢀ 10 mL) and followed by
decantation remove most of the soluble impurities in dichloro-
methane. The crude [DEDSA]Cl was obtained as pure light red yel-
lowish liquid after drying in vacuum oven at 50 °C. The formation of
this ionic liquid was confirmed from 1H NMR and 13C NMR (Fig. S1).
3.1. FT-IR analysis
The existence of Keggin structures of heteopolyanions in the IL-
POM hybrids were confirmed by comparing their characteristic
peaks with the IR spectra of heteropolyacids in Fig. 1(a) & 1(b). It
In second step, 1 mmol of the heteropolyacid (HPA), H3PMo12
-
O40ꢁnH2O (PMA) or H3PW12O40ꢁnH2O (PTA) was dissolved in
20 mL of distilled water at room temperature in a 100 mL round
bottom flask by continuous stirring. To the stirred solution of
HPA, 3 mmol of [DEDSA]Cl was added dropwise within 5 min.
Immediately yellow and white precipitation were observed for
is well defined that
a-Keggin anion is a combination of a [XO4]
[DEDSA]3[PMo12O40
] and [DEDSA]3[PW12O40] respectively. The
reaction was continued to stir for another 12 h to complete the
precipitation of IL-POM hybrids in water. The product mixture
was centrifuged to obtain the IL-POMs i.e. [DEDSA]3[PW12O40
]
and [DEDSA]3[PMo12O40] as water insoluble solids which were
washed 3–4 times with distilled water to get analytically pure
product. Then the isolated solid products were dried in vacuum
oven at 80 °C for 12 h to get 97–99% yields.
2.3. Procedure for oxidation of alcohols
In a typical experiment, a homogeneous solution of 1 mmol of
1-phenylethanol and 3 mol% of [DEDSA]3[PW12O40] catalyst in
CH3CN (2 mL) was stirred at 65 °C using 30% H2O2 (3 mmol) as oxi-
dant for 2 h in a 100 mL two necked round bottom flask fitted with
an air condenser in an oil bath. After completion of the reaction as
monitored from thin layer chromatographic technique, the ace-
tonitrile solvent was evaporated under reduced pressure and CH2-
Cl2 was poured over the remaining product mixture. Catalyst self-
precipitates in CH2Cl2 and product being soluble in the CH2Cl2 was
separated by simple filtration. The catalyst was washed with more
amount of the CH2Cl2 solvent (2 ꢀ 2 mL) followed by distilled
water. The spent catalyst was reactivated after drying in vacuum
oven at 80 °C for 5 h to make ready for next cycle of the oxidation
reaction. The crude product mixture obtained from evaporation of
the organic solvent was analyzed by HPLC (Fig. S2) as well as GC–
MS to monitor the conversion of the substrates and detection of
desired products.
2.4. Procedure for Hammett acidity determination of ionic liquid
material
The Hammett acidity determination was done by mixing an
equimolar amount of basic indicator 4-nitroaniline (5 mg/L,
pKa = 0.99) and the ionic liquid material ([DEDSA]Cl / IL-POM
hybrids) in CH3CN solution [43]. The absorbance values of basic
indicator [I] in the acidic solution of ionic liquids reduced with
increasing acidity of the ionic liquid systems. The Hammett acidity
Fig. 1. FT-IR spectra of (a) PMA and [DEDSA]3[PMo12O40
[DEDSA]3[PW12O40] respectively.
] and (b) PTA and
3

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
tetrahedron of P or Si and [MO6] octahedra. The [MO6] octahedral
units are edge shared into four C3v [M3O13] groups and are con-
nected to the three-fold axes of the central [XO4] tetrahedron.
The metal atoms are situated at the centres of distorted Cs octahe-
dra with one terminal MꢃOt bond. The MꢃO4c bonds attributes
four-coordinate oxygen atoms that connects 3 units of [MO6] and
[XO4] unit. The MꢃO2c1 bonds and Mo–O2c2 bonds attributes
two-coordinate oxygen atoms in which the MꢃO2c1 bonds con-
nects the [MO6] octahedra into [M3O13] groups whereas the
MꢃO2c2 bonds connect the [M3O13] units together [44]. Herein,
both the POM hybrids displayed peaks correspond to PAO bending
(593 cmꢃ1), asymmetric stretch of MꢃO2c2ꢃM bonds involving
edge sharing MO6 octahedra (714–810 cmꢃ1), asymmetric stretch
of MꢃO2c2ꢃM bonds of corner MO6 octahedra (866–898 cmꢃ1),
asymmetric MꢃOt stretch (960–990 cmꢃ1), asymmetric PAO
stretch (1049–1097 cmꢃ1) and bending vibrations of water mole-
cules in the secondary structures of the Keggin species (1623–
1628 cmꢃ1). The tethering of -SO3H groups to the diethyldisul-
foammonium cation can be attributed from antisymmetric SAO
stretch (1147–1166 cmꢃ1), overlapping bands of symmetric SAO
stretch with the asymmetric PAO stretch and NAS stretch with
the asymmetric stretch of MꢃO2c2ꢃM bonds of corner MO6 octahe-
dra were observed. Very weak band of CAN stretch for aliphatic
amine at around (1020–1250 cmꢃ1) was observed. The two ethyl
groups directly attached to the ammonium cation also showed
CAH rocking (1350–1392 cmꢃ1), CAH bending (1450–1470 cmꢃ1
and CAH stretching vibrations (2844, 2943 & 3091–3109 cmꢃ1
in the respective IR spectra of the IL-POMs [40,44–47].
)
)
A weak intensity broad band at 3433 cmꢃ1 compared to the
heteropolyacids indicates minimum amount of strongly H-bonded
water molecules in rigid framework of phosphomolybdate anion
and phosphotungstate anion along with –OH bond of sulfonic
groups. The reduction in broadening of the OAH band above
3000 cmꢃ1 in the POM hybrids justifies the presence of smaller
hydration sphere around the heteropolyanion environment which
is also observed in case of deshielding in the resonance peaks in the
respective 31P NMR spectra of the hybrids [48].
3.2. NMR analysis
Proton NMR analysis of the [DEDSA]3[PMo12O40] and [DEDSA]3-
PW12O40 in DMSO d6 displayed two –CH3 groups as triplet at
1.12 ppm, two methylene groups as quartet at 2.87 ppm and one
broad singlet for two –SO3H protons at 8.14–8.15 ppm (Fig. 2).
Fig. 2. 1H NMR spectra of (a) [DEDSA]3[PMo12O40] and (b) [DEDSA]3[PW12O40
]
.
4

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
The 13C NMR of these POM hybrids showed two peaks at 41.83–
42.1 ppm and 11.6 ppm corresponding to ethyl groups of ammo-
nium cation (Fig. S3). The 31P NMR spectrum of [DEDSA]3[PMo12
-
O
40] hybrid salt expressed one peak at ꢃ3.65 ppm (Fig. 3(a))
which was observed in higher frequency region compared to
ꢃ3.51 ppm peak of the phosphomolybdic acid (Fig. S4(a)). In case
of [DEDSA]3PW12O40 there is similar kind of frequency shift of
the hybrid to ꢃ15.15 ppm is observed (Fig. 3(b)) whereas a single
peak appeared at ꢃ14.72 for the phosphotungstic acid (Fig. S4
(b)) indicating strong ionic interaction between the organic cation
and the Keggin anion. The shift in the resonance peaks in 31P indi-
cates dehydration in the IL-POM hybrids which ascribes the pres-
ence of smaller hydration sphere in the polyanion environment
noticed for dynamic dehydration equilibrium [49,50].
3.3. TGA analysis
Fig. 4. TGA curves of [DEDSA]3[PW12O40], [DEDSA]3[PMo12O40], [DEDSA]Cl, H3-
PMo12O40ꢁnH2O and H3PW12O40ꢁnH2O.
Thermogravimetric analysis (TGA) showed absence of physi-
sorbed water (Fig. 4) for the two IL-POM hybrid [DEDSA]3[PM12
-
degradation at 250 °C which can be expected for stepwise removal
of the two –SO3H group of the diethyldisulphoammonium moiety.
The TGA pattern of [DEDSA]3[PMo12O40] displayed its thermal
O40] samples up to 100 °C as compared to approximately 12%
loss of the physisorbed water in case of the precursor [DEDSA]Cl
ionic liquid. The chloride based ionic liquid expressed another
Fig. 3. 31P NMR spectra of (a) [DEDSA]3[PMo12O40] and (b) [DEDSA]3[PW12O40].
5

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
stability till 330 °C, whereas it was observed up to 440 °C for the
[DEDSA]3[PW12O40] with residue of the Keggin anions [51]. The
least amount of physisorbed water in the POM hybrids can also
be evidenced from the respective FT-IR spectra and 31P NMR.
Although the amount of cation can also be derived from the TGA
curves from single step decomposition between 430 °C and
600 °C of the hybrid samples, but in both cases the aproximate %
mass loss of three [DEDSA] cations are slightly 6–7% lower than
istic sharp intense peak at 6.78° of the phosphotungstic acid in
addition to small intensity peak at 2ɵ = 7.05° like the hybrid of
[DEDSA]3[PMo12O40]. However, intense peak at 8.55° and 18.46°
observed in the H3PW12O40ꢁnH2O reduced drastically in [DEDSA]3[-
PW12O40]. In this hybrid, new peaks are found with lower intensity
within 2ɵ values of 11°–17°. The disappearance of some sharp
peaks and appearance of new peaks in the IL-POM hybrids as com-
pared to their parent acids can be expected from reorganization of
crystal structures of the heteropolyacids in the IL-POM hybrids in
presence of large organic cation involving self assembly of the Keg-
gin anions and the organic cations via electrostatic force, hydrogen
bonding etc [48,52,53].
the actual stoichiometric amount i.e. 19% for the [DEDSA]3[PW12
-
O
40] and 27% for the [DEDSA]3[PMo12O40]. This can be attributed
for encapsulation of some fragments of the organic cationic part
with the Keggin anions through strong H-bonding interaction
under the degradation temperature.
3.5. Raman analysis
3.4. Powder XRD analysis
Raman spectra of the hybrids in Fig. 6(a) and (b) showed resem-
3-
blance of characteristic bands of the Keggin anions [PMo12O40
]
The powder XRD patterns of both the POM-hybrid samples con-
3-
3-
and [PW12O40
]
observed from Bridgeman’s assignments [44,54].
firmed the presence of Keggin structures of [PMo12O40
]
and
A very sharp-edged peak at 992 cmꢃ1 for the phosphomolybdate
hybrid is observed for symmetric stretch of Mo-Ot bond coupled
asymmetrically to PAO stretching motion along with merging of
asymmetric stretch of Mo-Ot bond around 970 cmꢃ1. Another peak
at 884 cmꢃ1 is assigned for asymmetric stretch of Mo-O2c2-Mo
3-
[PW12O40
]
in Fig. 5(a) and Fig. 5(b). Typical sharp diffraction
peaks of the H3PMo12O40ꢁnH2O are observed at 6.80° and 26.59°
[Fig. 5(a)]. In case of [DEDSA]3[PMo12O40], additional small inten-
sity peaks appeared at around 7–7.75° along with one sharp inte-
sity peak at 6.80, while the other prominent peak of
H3PMo12O40ꢁnH2O at 26.59° almost disappeared for the hybrid
bonds. Combined stretching and bending vibrations of Mo-O2c1
-
[Fig. 5(a)]. The existence of Keggin structures in [DEDSA]3[PW12
-
O
40] [Fig. 5(b)] can be attributed from the appearance of character-
Fig. 5. Powder XRD analysis patterns of (a) [DEDSA]3[PMo12O40] and H3PMo12O40
ꢁnH2O (b) [DEDSA]3 [PW12O40] and H3PW12O40ꢁnH2O.
-
Fig. 6. Raman spectra of (a) [DEDSA]3[PMo12O40] and (b) [DEDSA]3[PW12O40].
6

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
Mo, Mo-O2c2-Mo and OAPAO bonds could be attributed to med-
ium intensity broad shoulder at 605 cmꢃ1. The small intensity
sharp peak at 246 cmꢃ1 is accounted for overlapping symmetric
stretch of Mo-O4c and bending of the intra ligand Mo-O2c2-O bond.
Remaining few very weak Raman bands are related to complex
Raman modes of the polyoxometalate anion.
Similarly, in case of the [DEDSA]3[PW12O40], a very sharp band
at 1002 cmꢃ1 is assigned to the symmetric stretch of terminal W-
Ot bond. The medium intense band at 910 cmꢃ1 is observed for
asymmetric stretch of W-O2c2-W bond. A very weak intensity
peaks is observed at 664 cmꢃ1 which could be considered for com-
sity peaks of phosphotungstate Keggin anion at 259, 347 and
402 nm expressed oxygen to metal charge transfer transition
(LMCT) for non-reduced form of the HPA Keggin anion [55],
Fig. 7(b). Similarly, for the Keggin anion of the phosphomolybdic
acid displayed the weaker LMCT transitions at 218, 347 and
455 nm in Fig. 7(a) [56]. As compared to the HPAs, the Keggin
anions of the POM-hybrid samples shifted the position of LMCT
transitions to shorter wavelength which indirectly reflect the exis-
tence of strong ionic interaction between the constituent ion-pairs
[56] and also further observed in the Hammett acidity order. For
example, the highest absorption peak of the HPA of phospho-
tungstate anion shifted from 402 nm to 339 nm in the [DEDSA]3[-
PW12O40] hybrid. Likely, with the [DEDSA]3[PMo12O40], it was
shifted to 396 nm from 455 nm.
bined stretching and bending vibrations of W-O2c1-W and W-O2c2
-
W and OAPAO bonds. The weak band at 226 cmꢃ1 could be allot-
ted for W-O4c stretching and W-O2c2-O bending vibrations.
3.7. SEM analysis
3.6. UV–Vis diffuse reflectance spectra
The surface morphology of the hybrid salts [DEDSA]3[PMo12O40
]
The UV–Vis diffuse reflectance spectra of the IL-POM hybrids
and [DEDSA]3[PW12O40] were studied in SEM images at the same
magnification in Fig. 8. The SEM topography reveals micrometer
sized irregular agglomeration through extensive secondary inter-
actions of intermolecular H-bonds of the two –SO3H functionality
of the ammonium cation with the oxygen atoms of Keggin anions.
The fluffy like clustering of aggregates on the surface of [DEDSA]3[-
PMo12O40] will reduce the number of acidic sites on its surface as
compared to the smaller size distribution of aggregates in case of
and the HPAs are represented in Fig. 7(a) & (b). The retention of
3-
Keggin structures [PM12O40
]
(M = W or Mo) within the hybrid
compounds can be supported from their identical UV–Vis DRS
absorption patterns against the respective HPAs. The weak inten-
Fig. 7. UV –Vis DRS of (a) [DEDSA]3[PMo12O40] and (b) [DEDSA]3[PW12O40].
Fig. 8. SEM images of (a) [DEDSA]3[PMo12O40] and (b) [DEDSA]3[PW12O40].
7

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
the [DEDSA]3[PW12O40] which was further confirmed by the Ham-
mett acidity study (Fig. 10). The exact reason for differences in the
sizes of aggregates are unknown and it is beyond the scope of this
study.
3.8. EDX analysis
Energy Dispersive X-ray (EDX) spectrum of the [DEDSA]3[-
PMo12O40] in Fig. 9(a) confirmed C, N, O, P, Mo and S as the con-
stituent elements of the hybrid compound, whereas the
spectrum of [DEDSA]3[PW12O40] in Fig. 9(b) displayed all the peaks
of constituent elements except phosphorus. It can be accounted for
low abundance of this element in surface of the Keggin structured
anion as compared to high abundances of W and O elements. At the
same, the presence of P in the phosphotungstate Keggin anion can
be confirmed from the 31P NMR spectrum of [DEDSA]3[PW12O40],
Fig. 3(b).
3.9. Hammett acidity determination using UV–Visible spectrometry
Fig. 10. Hammett acidity plot of [DEDSA]3[PMo12O40] and [DEDSA]3[PW12O40] with
respect to parent ionic liquid [DEDSA]Cl.
The Brønsted acidities of [DEDSA]Cl and the IL-POM hybrid salts
i.e. [DEDSA]3[PMo12O40] and [DEDSA]3[PW12O40] were evaluated
as a function of the Hammett acidity (H0) values as per standard
method by UV–visible spectrophotometer using 4-nitroaniline as
basic indicator in acetonitrile (Table S1 & Fig. 10) [43]. Their acidi-
ties can be arranged in descending order depending on ionic
strength of the constituent ion-pairs as follows: [DEDSA]
[PW12O40] ˃ [DEDSA]3[PMo12O40] ˃ [DEDSA]Cl. The greater ionic
Fig. 9. EDX patterns of (a) [DEDSA]3[PMo12O40] and (b) [DEDSA]3[PW12O40].
8

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
strength between the ammonium cation with the Keggin anions
made both POM hybrids more Brønsted acidic than the [DEDSA]
Cl involving weaker ionic strength of the chloride anion. Between
the two POM hybrids, the presence of higher number of acidic sites
with the phosphotungstate hybrid can be described because of
minimum sizes of aggregates as evidenced from the SEM image,
Fig. 8(b). Similarly, for the phosphomolybdate hybrid, the bigger
size clusters exposed minimum number of the acidic sites in the
SEM image, Fig. 8(a).
entry 4). So, this amount of catalyst was taken as the standard
amount to study the effect of solvents for the model reaction in
Table 2. Using the same amount of [DEDSA]3[PMo12O40] catalyst,
the product yield was seen to decrease at 65 °C (Table 1, entry
9). As the amount of this catalyst was increased to 3.5 mol%, the
yield of acetophenone increased satisfactorily (Table 1, entry 10).
It was also observed that negligible conversion of alcohol occurred
in absence of catalyst using 30% H2O2 (3 mmol) at 65 °C in acetoni-
trile (Table 1, entry 11). The redox properties of both of these POM
hybrid catalysts were observed to be slightly different with respect
to the catalyst amount which was better for the [DEDSA]3[PW12
-
3.10. Catalytic activity
O40] catalyst (Table 1, entry 4). This can be attributed for varied
coordination abilities of Mo(VI) and W(VI) cations and also differ-
ent metal–oxygen bond order in their respective in situ generated
active species peroxophosphotungstate {PO4[WO(O2)2]4}3- and per-
oxophophomolybdate {PO4[MoO(O2)2]4}3- as proposed in Scheme 3
for catalytic oxidation of alcohol involving hydrogen peroxide as
oxidant [57]. During the oxidation process, the reaction intermedi-
ate of alcohol with {PO4[MoO(O2)2]4}3- will be more stable with
refilling of electrons in three vacant orbitals 4f, 4d, and 5 s of the
Mo species as compared to the intermediate of {PO4[WO(O2)2]4}3-
involving two vacant orbitals, 5d and 6 s of the W cation. When
the temperature was increased from 65 °C to 82 °C, the oxidation
of alcohol was completed within 1.5 h with lowering of percentage
yield of acetophenone through formation of byproduct that can be
attributed for formation of over oxidized product at 82 °C involving
H2O2 as oxidant (Table 1, entry 5).
3.10.1. Optimization of reaction condition
To study the redox properties of POM hybrid catalysts, initially
we optimized the amount of [DEDSA]3[PW12O40] by taking 0.5, 1, 2
and 3 mol% of the catalyst for oxidation (Scheme 2) of 1-pheny-
lethanol (1 mmol) in CH3CN (2 mL) using 30% H2O2 (3 mmol) at
different temperatures (0 °C, 25 °C, 45 °C, 65 °C, 82 °C) for the spec-
ified reaction time as included in Table 1 (entries 1–8). Excellent
catalytic activity was observed with 3 mol% of the [DEDSA]3[PW12
-
O
40] at 65 °C for 2 h to produce 98% yield of acetophenone (Table 1,
3.10.2. Effect of solvent study
Solvent study was conducted by performing the oxidation of 1-
phenylethanol in polar solvent including H2O, MeOH, CH3CN,
EtOAc at 65 °C and in non-polar solvent like CH2Cl2, hexane in
reflux temperature with the optimized amount of [DEDSA]3[PW12
40] catalyst for 2–8 h (Table 2). The results in Table 2 expressed
solvent dependence catalytic activity of the POM hybrid with var-
-
O
Scheme 2. Model reaction for oxidation of alcohol.
Table 1
Study of temperature effects and catalyst amount for the model reaction.
Entry
Reaction Temperature (°C)
Time(h)
Catalyst amount[a] (mol%)
Yield %[b]
1.
0
8
3
0
2.
3.
4.
5.
6.
7.
8.
9.
25
45
65
82
65
65
65
65
65
65
8
6
2
1.5
2
2
2
2.5
2.5
8
3
3
3
3
0.5
1
2
3
3.5
0
0
58
98
82
16
38
70
88
97.5
<1
10.
11.
[a]
Reaction conditions: (i) using [DEDSA]3[PW12O40] catalyst: 1-phenylethanol (1 mmol), 30% H2O2 (3 mmol), CH3CN (2 mL) (entries 1–8); (ii) using [DEDSA]3[PMo12O40
]
catalyst (entries 9,10); (iii) without catalyst (entry 11).
[b]
Yield % of acetophenone based on GC analysis.
Table 2
Study of solvent effects for the model reaction using [DEDSA]3[PW12O40] catalyst.
Entry
Solvent
Time (h)
Temperature[a] °C
Yield%[a],[b]
1.
2.
3.
4.
5.
6.
7.
H2O
CH3OH
CH3CN
CH2Cl2
n-Hexane
Solvent-free
Ethyl acetate
8
5
2
8
8
3
3
65
65
65
39
68
65
65
45
78
98
10
8
60
66
[a]
[b]
Reaction conditions: 1-phenylethanol (1 mmol), 30% H2O2 (3 mmol), [DEDSA]3PW12O40 (3 mol%) catalyst, CH3CN (2 mL).
Yield % based on starting substrates in GC analysis.
9

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
ied yields of oxidized product which were observed to proceed
through either homogeneous or heterogeneous catalytic phases
in reaction media. The catalyst was seen to be soluble in MeOH,
CH3CN as polar solvents (Table 2, entries 2, 3) except water making
the reaction as homogenous catalysis and the percentage yield of
products were raised to higher levels. At the same time, the insol-
ubility of catalyst in H2O, CH2Cl2 and n-hexane converts the reac-
tion as heterogenous catalysis and pulling down the percentage
yields (Table 2, entries 1, 4, 5). In case of ethyl acetate solvent,
the catalyst was found to be sparingly soluble in the reaction med-
ium making the reaction system non homogenous in nature that
may be accounted for moderate yield of the product (Table 2, entry
7). The self-precipitation of the [DEDSA]3[PW12O40] catalyst from
the reaction mixture was done by pouring CH2Cl2 solvent in the
crude product mixture of oxidation obtained Fig. 11(a) after evap-
oration of the homogeneous solution of reaction in acetonitrile
under reduced pressure. The catalyst was recovered and reused
after filtration of the dichloromethane solution of oxidation pro-
duct by keeping in vacuum oven for 5 h at 80 °C. The other polyox-
ometalate catalyst [DEDSA]3[PMo12O40] also displayed similar
solubility behavior with these solvents which was represented in
Fig. 11(b) for the model reaction in acetonitrile. The solvent depen-
dence behavior of the IL-POM hybrid towards polar and non-polar
solvent can be considered as a combined outcome of secondary
interactions including H-bonding, ion–dipole interactions etc. for
each ionic component of the POM with the solvent molecules.
The inability of water to solubilize the catalyst can be expected
for insufficient H-bonding interaction with the water molecules
in presence of two hydrophobic ethyl groups tethered to the
ammonium cation of hybrid salt. This factor may become a prefer-
ential condition for self-aggregation of the POM hybrid in water
through intermolecular H-bonding interactions involving the
POM anion and -SO3H groups of the ammonium cation of the
hybrid material. The non-polar solvents like dichloromethane, hex-
ane etc. are favorable for self-aggregation of the POM hybrid. In
MeOH and CH3CN, each of the ionic component of POM hybrid
can make sufficient intermolecular H-bonding as well as ion–
dipole interactions with the solvent molecules in presence of slight
amount of miscible water which may increase the solubility of the
catalyst.
alyst in case of p-methoxy benzyl alcohol and p-hydroxy benzyl
alcohol (Table 3, entries 4,8) because of strong mesomeric effect
as compared to p-chloro benzyl alcohol (Table 3, entry 6). The elec-
tron donating substituents in benzyl alcohol expressed excellent
yields (Table 3, entry 3) while the substrates with electron with-
drawing group showed relatively low reactivity (Table 3, entries
5, 6) as observed from GC analysis. The % yield of oxidised product
of p-chloro benzyl alcohol was increased to some extent after
increasing the catalyst amount up to 4 mol% (Table 3, entry 7)
within 6.5 h reaction at 65 °C in CH3CN. Surprisingly, the optimized
amount of catalyst did not work with cyclohexanol even after
increasing the catalyst amount upto 4 mol % (Table 3, entry 9) at
the temperature of 65 °C. Acyclic 2° alcohols like 1-phenylethanol
and p-chloro-1 phenylethanol proceded efficiently for 2–3 h reac-
tion with varied amount of the catalyst (Table 3, entries 1, 11). In
case of oxidation of 4-phenyl-1-buten-4-ol, a rearranged
a, b
unsaturated keto compound was formed after shifting of double
bond position without oxidation (Table 3, entry 10). No carbonyl
products were obtained under the optimized condition for oxida-
tion of primary alcohol other than benzylic alcohol (Table 3, entry
12).
3.10.4. Plausible reaction mechanism
The mechanism (Scheme 3) demonstrates that the Keggin
polyanion in the IL-POM hybrids gets degraded in presence of
H2O2 to an active metal peroxo intermediate {PO4[MO(O2)2]4},3-
which is found to be responsible for the oxidation reaction
[7,58,59]. This was further evidenced from respective IR spectrum
of peroxo intermediate isolated after treatment of the [DEDSA]3-
PW12O40 with H2O2 solution at room temperature (Fig. S5). The spec-
trum displayed peaks at 857 cmꢃ1 for (OAO) vibration, 555 cmꢃ1
t
for symmetric metal–oxygen (W–O2) stretching frequency and
677 cmꢃ1 for asymmetric (W–O2) stretching frequency which also
proved the involvement of peroxophosphometalate intermediate in
the oxidation reaction [60,61].
3.10.5. Recyclability of catalyst
The catalytic recyclability was studied by taking 3 mmol of 1-
phenylethanol as model substrate with the optimized amount of
[DEDSA]3PW12O40 catalyst for 2 h reaction in CH3CN at 65 °C. The
catalyst was precipitated out from the homogeneous solution of
acetonitrile through addition of dry CH2Cl2 which is insoluble in
CH2Cl2. The POM catalyst was then washed with hexane and then
reactivated in vacuum oven at 80 °C for 5 h for next catalytic run.
Fig. 12 displays reusability profile of the spent catalyst for seven
consecutive cycles. Similar catalytic activity with each cycle was
observed till third run with modest decrease in yield after the
3.10.3. Substrate scope study
The substrate scope study of oxidation reaction was investi-
gated in acetonitrile solution for different substituted benzyl alco-
hol or cyclic/acyclic secondary alcohol under the optimized
condition using [DEDSA]3[PW12O40] as catalyst (Table 3). The rate
of oxidation was found to be relatively high with 3 mol% of the cat-
Fig. 11. Photograph of (a) [DEDSA]3[PW12O40] and (b) [DEDSA]3[PMo12O40] switching from homogenous reaction medium in left testube to heterogenous one by self-
precipitating in testube kept in right after evaporation of acetonitrile solvent and pouring dry CH2Cl2 afterwards.
10

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
Table 3
Substrate scope for oxidation of alcohols using IL-POM hybrid.
Entry
1.[a]
Substrate
Product
Time (h)
2
Yield (%)[c]
98
1a
2a
1b
2.[a]
88.5(2b)9.5
(2c)
6
2
2b 2c
3.[a]
92.5
3a
3b
4.[a]
1
98.5
4a
4b
5.[a]
6
79.4
5a
6a
7a
5b
6b
7b
6.[a]
6.5
54.9
7.[b]
6.5
60
(continued on next page)
11

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
Table 3 (continued)
Entry
8.[a]
Substrate
Product
Time (h)
1
Yield (%)[c]
93.1
8a
9a
8b
9b
9.[b]
6
3
0
10.[a]
68.4
10a
10b
11.[b]
3
6
77.2
11a
12a
11b
12b
12.[a]
0
[a]
[b]
[c]
Reaction conditions: alcohol (1 mmol), H2O2 (3 mmol), [DEDSA]3PW12O40 (3 mol%), CH3CN (2 mL), 65 °C.
Reaction conditions: alcohol (1 mmol), H2O2 (3 mmol), [DEDSA]3PW12O40 (4 mol%), CH3CN (2 mL), 65 °C.
GC yields based on starting substrates.
Scheme 3. Mechanism of oxidation of alcohol using [DEDSA]3PM12O40 via peroxo
intermediate formation.
Fig. 12. Reusability diagram of the [DEDSA]3[PW12O40] for the model reaction.
12

N. Kashyap, S. Das and R. Borah
Polyhedron 196 (2021) 114993
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.poly.2020.114993.
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