New niobium heteropolyacid included in a silica/alumina matrix: Application in selective sulfoxidation

Article abstract of DOI:10.1016/j.cattod.2020.10.034

A new heteropolyacid, derived from phosphomolybdic acid containing niobium (PNbMo), was synthesized and included in silica, alumina, and silica/alumina matrixes by sol-gel technique, in order to use it as heterogeneous catalyst. The matrixes confer stability to the active phase, good textural and morphological properties. PNbMo and included materials were characterized by ³¹P-NMR, FT-IR, UV–vis, TGA, SEM, TEM, XRD, N₂ physisorption, and potentiometric titration, and tested as catalyst in the sulfoxidation of diphenyl sulfide under suitable conditions. The redox activity was compared with phospomolybdic acid and correlated with the edge energy obtained from UV–vis absorption spectra. The best results in diphenyl sulfide sulfoxidation were obtained with PNbMo-SiAl-4:1 (92% conversion and 95% selectivity at 3 h) and with PNbMo-Si (92% conversion and 94% selectivity at 4 h). The reuse of these catalysts was evaluated.

Full text of DOI:10.1016/j.cattod.2020.10.034

Catalysis Today xxx (xxxx) xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
New niobium heteropolyacid included in a silica/alumina matrix:
Application in selective sulfoxidation
a,
a,
a,b
María B. Colombo Migliorero *, Valeria Palermo *, Gustavo P. Romanelli , Patricia
G. V a´ zqueza
a
Centro de Investigaci o´ n y Desarrollo en Ciencias Aplicadas “Dr. Jorge J. Ronco” CINDECA, (CONICET-CIC-UNLP), Facultad de Ciencias Exactas, Universidad
o
Nacional de La Plata, Calle 47 N 257, B1900AJK, La Plata, Argentina
b
C a´ tedra de Química Org a´ nica, Facultad de Ciencias Agrarias y Forestales, Universidad Nacional de La Plata, Calles 60 y 119 s/n, B1904AAN, La Plata, Argentina
A R T I C L E I N F O
A B S T R A C T
Keywords:
A new heteropolyacid, derived from phosphomolybdic acid containing niobium (PNbMo), was synthesized and
included in silica, alumina, and silica/alumina matrixes by sol-gel technique, in order to use it as heterogeneous
Silica/Alumina
Niobium
catalyst. The matrixes confer stability to the active phase, good textural and morphological properties. PNbMo
Heteropolyacid
Sulfoxidation
31
and included materials were characterized by P-NMR, FT-IR, UV–vis, TGA, SEM, TEM, XRD, N
2
physisorption,
and potentiometric titration, and tested as catalyst in the sulfoxidation of diphenyl sulfide under suitable con-
ditions. The redox activity was compared with phospomolybdic acid and correlated with the edge energy ob-
tained from UV–vis absorption spectra. The best results in diphenyl sulfide sulfoxidation were obtained with
PNbMo-SiAl-4:1 (92% conversion and 95% selectivity at 3 h) and with PNbMo-Si (92% conversion and 94%
selectivity at 4 h). The reuse of these catalysts was evaluated.
1
. Introduction
4
Park et al. [16] prepared H PW11NbO40 by hydrothermal treatment
and evaluated its redox properties by electrochemical method and
UV–vis absorption. Whereas, Zalomaeva et al. [17] reported on the use
of Nb-monosubstituted Lindqvist and Keggin polyoxotungstates as cat-
alysts in the sulfide oxidation in acetonitrile, Maksmchuk et al. [18] used
Nb-monosubstituted Lindqvist polyoxotungstates in the epoxidation of
alkenes, and Ren et al. [19] described a mixed Nb/W Dawson
polyoxometalate.
Heteropolyacids (HPAs) are an important family of compounds
largely used in catalysis [1–3]. They are composed of metal oxide
clusters of early transition metals. The principal heteropolyanion is the
nꢀ
Keggin structure (XM12
XO
O
40 ), which consists of a central tetrahedron
5
+
5+
4+
(
4
) encircled by twelve MO
6
that form M
3
O
13 (X = P , As , Si
,
3
+
6+
6+
and B ; and M = W and Mo ) [4]. One of the main Keggin heter-
opolyacids used as acid and redox catalyst is phosphomolybdic acid
Other niobium species used in catalysis are W-Nb-O and W-V-Nb-O
mixed oxides, with hexagonal tungsten bronze structure, as catalysts in
the one-pot oxidehydration of glycerol into acrylic acid [20]. Also, some
3
(H PMo12O40, PMA), due to its strong acid strength and oxidation
power. The substitution of one or more Mo atoms by V, which enhances
the catalytic performance of HPA, has been widely reported in the
literature [5–8]. Our research group has studied the catalytic capacity of
PMA by substitution of one Mo atom by V, Bi, La, and Y [9–11]. The
results encouraged us to employ other elements, in order to enhance the
catalytic performance of PMA.
5 5
niobium catalysts (NbCl , Nb(OEt) , mesoporous niobium silicate, Nb
(Cp) Cl on silica) were used in sulfide [21] and alkene [22,23] oxida-
2
2
tions with hydrogen peroxide.
Recently, Yang et al. [24] prepared multilayer-like polyoxometalates
(HNbMoO
Nb , and MoO
was used in the etherification of 5-hydroxymethylfurfural.
6
) by calcination of a stoichiometric mixture of Li
2 3
CO ,
Niobium was chosen due its position in the periodic table: Nb be-
longs to the same period as Mo, and to V group. Besides, to our
knowledge, there are no reports on Nb doped PMA in the literature.
Nevertheless, some reports indicate the of use of niobium in HPAs: for
2
O
5
3
and subsequent acidification. This solid acid catalyst
On the other hand, HPAs could be dissolved in various reaction
media; for that reason many strategies to convert them to a heteroge-
neous catalyst have been developed by immobilization on a support
example, Nb
2
O
5
is used as support of HPA [12–15].
*
Corresponding authors.
E-mail addresses: mbmigliorero@quimica.unlp.edu.ar (M.B. Colombo Migliorero), vpalermo@quimica.unlp.edu.ar (V. Palermo).
https://doi.org/10.1016/j.cattod.2020.10.034
Received 26 March 2020; Received in revised form 12 August 2020; Accepted 21 October 2020
Available online 29 October 2020
0
920-5861/© 2020 Elsevier B.V. All rights reserved.
Please cite this article as: María B. Colombo Migliorero, Catalysis Today, https://doi.org/10.1016/j.cattod.2020.10.034

M.B. Colombo Migliorero et al.
Catalysis Today xxx (xxxx) xxx
[
25], or by formation of an insoluble ionic material [4]. One strategy is
to include the HPA in a solid matrix, such as silica, titania, and alumina
26,27], in order to turn it more stable and facilitate its isolation and
2.1.2. Heteropolyacid synthesis
PNbMo was synthesized by hydrothermal procedure [8] using a
stoichiometric mixture of MoO (110 mmol), Nb (5 mmol), H PO
[
3
2
O
5
3
4
recovery from the reaction media.
85% (w/v) (0.58 mL) suspended in distilled water (150 mL). The
◦
Sol-gel methodology allows the synthesis of micro and mesoporous
materials with specific properties [28], for example, when HPAs are
incorporated into a silica matrix, they provide stability to the Keggin
structure and hinder the solubility in the reaction medium [9,29], and
the alumina increases the catalytic activity [30].
mixture was magnetically stirred for 6 h at 75 C. Then, it was cooled to
room temperature, and the insoluble materials were removed by filtra-
tion. The solution was evaporated at room temperature, and finally a
dark green solid (PNbMo) was obtained.
Here, we describe the synthesis of phosphomolybdic acid doped with
Nb, a new HPA derived from PMA, and included in silica, alumina, and
two mixed silica/alumina frameworks, which confer stability to the
active phase and good textural and morphological properties. The solid
samples were characterized by several techniques (FT-IR, UV–vis,
potentiometric titration, physisorption, among others).
2.1.3. Matrixes synthesis
Silica, alumina, and mixed SiAl-1:1 and SiAl-4:1 matrixes, were
prepared according to reported methodology [37] (See Supplementary
Material).
2.1.4. Synthesis of HPAs included in silica
The new materials were examined as heterogeneous catalyst in a
sulfoxidation reaction. This reaction was selected because many sulf-
oxides are important in the synthesis of biologically active molecules
and natural products [31–33], asymmetric ligands [34], and oxygen
transfer agents [35]. For sulfide oxidation, HPA acts together with
aqueous hydrogen peroxide, instead of stoichiometric oxidants (nitric
acids, trifluoroacetic acid) [36]. Selective oxidation of diphenyl sulfide
PNbMo-Si (PNbMo included in silica) was prepared by mixing tet-
raethoxysilane (TEOS, 16 mmol) with absolute ethanol (80 mmol) under
nitrogen atmosphere at room temperature. Then, under atmospheric
conditions and stirring, a solution of PNbMo (0.75 g, 0.36 mmol) in
absolute ethanol (36 mmol) and water (1.07 mL) were added, and
continued stirring for 2 h to allow sol gelation. Finally, wet gel was aged
until dry at room temperature.
(
sulfoxidation) in an eco-friendly reaction condition, using aqueous
For comparative purposes, PMA included in silica (PMA-Si) was
synthesized in the same way as PNbMo-Si.
◦
hydrogen peroxide as oxidant and ethanol as solvent at 25 C, was
chosen as reaction test (Scheme 1).
2
.1.5. Synthesis of HPAs included in alumina
2
. Experimental
For PNbMo-Al (PNbMo included in alumina) synthesis, aluminum
tri-sec-butoxide (TSBAl, 16 mmol) was mixed with absolute ethanol
(174 mmol) under nitrogen atmosphere at room temperature. The
mixture was retired from the controlled atmosphere and stirred until
2
2
.1. Catalyst synthesis
◦
.1.1. General
homogeneity. Then, hot distilled water (29.24 mL at 98 C) was added
◦
Chemicals were acquired from Aldrich, Fluka and Merck, and used
and magnetically stirred at 90 C. After 0.5 h, 0.75 g (0.36 mmol) of
◦
without further treatment. The reaction products were identified by
comparative thin layer chromatography (TLC).
PNbMo was added and stirred 3 h at 90 C. Finally, the gel was aged at
◦
110 C for 12 h.
For comparative purposes, we synthesized PMA included in alumina
Scheme 1. Selective oxidation of diphenyl sulfide.
Table 1
HPA content, textural properties, and acid strength of samples.
2
3
Sample
HPA content (% w/w)
Specific surface area (SBET) (m /gr)
Pore volume (cm /gr)
Pore size
E
0
(
Å)
(mV)
PMA
100
100
0
19
0.02
0.01
0.35
0.30
0.26
0.20
0.32
0.32
0.76
0.70
0.45
0.70
0.50
0.90
4
7
822
768
126
377
358
195
86
PNbMo
7
SiO
2
760
702
498
280
339
347
299
267
303
302
289
262
19
20
PMA-Si
15
18
0
PNbMo-Si
21
Al
2
O
3
28
PMA-Al
14
16
0
37
PNbMo-Al
4
74
SiAl-1:1
101
107
59
295
269
185
354
377
379
PMA-SiAl-1:1
PNbMo-SiAl-1:1
SiAl-4:1
12
16
0
93
PMA-SiAl-4:1
PNbMo-SiAl-4:1
10
20
73
140
2

M.B. Colombo Migliorero et al.
Catalysis Today xxx (xxxx) xxx
3
1
Fig. 1. P-NMR spectra of PNbMo (left) and PMA (right).
(
PMA-Al) in the same way as PNbMo-Al.
2
.1.6. Synthesis of HPAs included in the mixed silica-alumina matrixes
PNbMo-SiAl-1:1 was prepared by mixing TEOS (8 mmol), TSBAl (8
mmol), and absolute ethanol (127 mmol) under nitrogen atmosphere at
room temperature. The mixture was retired from the controlled atmo-
sphere and stirred until homogeneity. Then, hot distilled water (15.15
◦
◦
mL at 98 C) was added and magnetically stirred at 90 C. After 0.5 h,
◦
0
.75 g (0.36 mmol) of PNbMo was added and stirred 3 h at 90 C.
◦
Finally, the gel was aged at 110 C for 12 h. For PNbMo-SiAl-4:1 syn-
thesis, TEOS (13 mmol), TSBAl (3.25 mmol), and absolute ethanol (99
mmol) were mixed under nitrogen atmosphere. The mixture was retired
from the controlled atmosphere and stirred until homogeneity. Then hot
◦
distilled water (6.70 mL at 98 C) was added and magnetically stirred at
◦
9
0 C. After 0.5 h, 0.75 g (0.36 mmol) of PNbMo was added and stirred
◦
for 3 h. Finally, the gel was aged at 110 C for 12 h.
For comparative purposes, we synthesized PMA included in PMA-
SiAl-1:1 and PMA-SiAl-4:1 mixed oxides.
Finally, all solids were washed with ethanol and the final amount of
HPA (Table 1) in the samples was calculated considering the final mass
of the catalysts, the HPA mass used in the synthesis and not-incorporated
HPA (remaining in the washing liquor).
Fig. 2. UV–vis spectra of ethanolic solution of PNbMo and PMA.
Scanning electron microscopy (SEM) with energy dispersive X-ray
spectroscopy (EDS) analysis was made with a Philips 505 scanning
electron microscope using an accelerating voltage of 25 eV. The solid
samples were metallized with Au. The chemical composition of the
materials was analyzed by X-ray scattering.
2
.2. Catalyst characterization
The 31P-NMR spectrum of PNbMo, in deuterated aqueous solution,
PNbMo was also analyzed by transmission electron microscopy
(TEM), in a JEOL 100 CX instrument, working at 100 kV. The sample
was ultrasonically dispersed in ethanol and placed on a Formar® grid.
The total catalyst acidity was obtained by potentiometric titration by
suspending the solid (0.025 g) in 45 mL of acetonitrile and titrating with
a solution of n-butylamine in acetonitrile (0.025 N). The electrode po-
tential (mV) variation was measured using the 794 Basic Titrino device
with a Solvotrode electrode.
was registered in Bruker Avance 300 equipment at a frequency of 121
MHz.
The UV–vis spectrum of solution of PNbMo in ethanol was measured
with a Perkin Elmer Lambda 35 UV–vis double beam spectrophotom-
eter, in the range 200ꢀ 1100 nm at room temperature.
Thermogravimetric analysis (TGA) of bulk PNbMo was carried out
with Shimadzu 50 equipment, in aluminum plate and air flow. The
◦
◦
heating rate was 5 C/min from 20 to 600 C.
The specific surface area (SBET), pore volume, and the mean pore
diameter of the included catalysts were determined by nitrogen
adsorption/desorption. The samples were previously degasified for 700
Fourier transform infrared spectra (FT-IR) were acquired using
Bruker Vertex 70v equipment. The solid samples were ground and mixed
with KBr, and pellets were formed. The measuring range was 400ꢀ 4000
◦
min at 100 C below 30 mm Hg, and the isotherms were obtained at -196
ꢀ
1
◦
cm
.
C using Micromeritics ASAP 2020 equipment.
X-ray diffraction (XRD) patterns were recorded by means of a PAN-
alyical X’Pert Pro 3373/00 device, using Cu K radiation (λ =1.5417 Å),
Ni filter, 20 mA and 40 kV in the high voltage source, scanning angle
α
2.3. Catalytic activity
◦
◦
◦
(
2θ) from 5 to 40 , scanning rate 2 (2θ)/min.
For the diphenyl sulfide oxidation test, catalysts (12 mg of bulk
3

M.B. Colombo Migliorero et al.
Catalysis Today xxx (xxxx) xxx
Fig. 3. FT-IR spectra of PNbMo and PMA.
Fig. 4. FT-IR spectra of matrixes and included PNbMo.
PNbMo or PMA; 266 mg of included HPA), diphenyl sulfide (1 mmol),
2 5
Fig. 5. XRD patterns of PNbMo, PMA, and Nb O .
Keggin structure [39,40]. From these spectra it is possible calculate the
absorption edge energy, which can be correlated with the redox prop-
erties of the HPA [41]. The edge energy for PNbMo is 3.22 eV, lower
than the value calculated for PMA (3.36 eV), due to stabilization of the
lowest unoccupied molecular orbital (LUMO) by the d-orbitals of
niobium. A similar effect was observed by the replacement of molyb-
denum by vanadium in PMA [10].
ethanol (8 mL), and aqueous H
2
O
2
, 35% (w/v) (1.5 mmol) were mixed
◦
in a glass reactor and stirred at 25 C.
The reaction advance was examined by gas chromatography: sam-
ples of the reaction mixture (0.1 mL) were withdrawn after each hour,
treated with dichloromethane (0.5 mL) and distilled water (0.5 mL). The
organic layer was dried on anhydrous Na
by gas chromatography in a Shimadzu 2014 with a Supelco column
SPB-1, 30 m x 0.32 mm x 1 m) and a flame ionization detector.
2 4
SO , filtered off, and analyzed
The curve obtained by TGA (Supplementary Material, Fig. S1) shows
(
μ
◦
◦
that PNbMo has a weight loss of 12.6% between 20 C and 200 C, which
corresponds to physisorbed water and allow calculating the hydration
3
. Results and discussion
.1. Characterization
The 31P-NMR spectrum of PNbMo (Fig. 1) shows a major chemical
◦
◦
number of HPA (13 for PNbMo). From 200 C to 450 C, there is a small
weight loss (1.45%) that corresponds to constitution water. The absence
3
◦
of major weight loss indicates that until 600 C, the structure of HPA is
thermally stable [42].
The FT-IR spectrum of PNbMo (Fig. 3) presents four bands at 1063
shift at -3.16 ppm that can be associated with the Keggin unit of phos-
phomolybdate (the characteristic signal of PMA is at -3.88 ppm). It can
be observed that the signal shifts to downfield by the presence of an
atom different to Mo. The peak at -1.59 ppm can be assigned to lacunar
species PMo11 [38].
ꢀ 1
ꢀ 1
ꢀ 1
ꢀ 1
cm , 962 cm , 871 cm , and 780 cm , which are in agreement with
ꢀ 1
the characteristic bands of Keggin structure PMA: 1064 cm (P-Oa),
ꢀ 1
ꢀ 1
ꢀ 1
9
62 cm (Mo = Od), 871 cm (Mo-Ob-Mo), 780 cm (Mo-Oc-Mo),
where Oa connects the central tetrahedral P with Mo atoms, Ob links
MoO octahedra by the corners, Oc shares the octahedra edges, and Od
6
The UV–vis spectrum of PNbMo solutions is shown in Fig. 2. For
comparative purposes, the spectrum of PMA is also included. The band
assigned to oxygen-metal transfers at 210ꢀ 260 nm can be seen in both
spectra, and the band at 240ꢀ 500 nm is assigned to octahedral Mo into
are terminal atoms, bonded to one Mo atom [43]. This confirms that the
synthesized PNbMo presents Keggin structure. A shoulder is observed
for the P-Oa band, which could be assumed as a parameter of Mo-Oa
interaction (a strong interaction corresponds to a weak stretching) [44].
4

M.B. Colombo Migliorero et al.
Catalysis Today xxx (xxxx) xxx
Fig. 6. SEM x1000 (left) and TEM (right) micrographs of PNbMo.
Fig. 7. SEM x1000 (left) and TEM (right) micrographs of PNbMo-Si.
Fig. 8. SEM x1000 (left) and TEM (right) micrographs of PNbMo-Al.
ꢀ
1
◦
Fig. 4 shows the 1500ꢀ 400 cm region of the FT-IR spectra of
matrixes and included PNbMo: PNbMo-Si, PNbMo-Al, PNbMo-SiAl-1:1,
and PNbMo-SiAl-4:1. The characteristic signals of pure matrixes can be
observed and the PNbMo bands (indicated by gray lines) are overlapped
The characteristic peaks of Keggin structure in the intervals 6.5ꢀ 10 ,
◦
◦
16.5ꢀ 23 , and 25ꢀ 36 2θ are identified [47]. For comparison, Nb
2 5
O
pattern is included in the figure.
In X-ray diffraction patterns of included catalysts (Supplementary
Material, Fig. S3) the PNbMo signals are masked by the matrixes pat-
terns, which indicates a high dispersion. PNbMo-Si diffraction pattern is
typical of an amorphous material, for PNbMo-Al the peaks appear at
with those of the matrix. Al
2
O
3
has the characteristic bands of the
-
1
-1
boehmite structure:
ν
O-H 3300ꢀ 3500 cm , δH
2
O 1635 cm , and δO-H
-
1
-1
1
070 cm . The band at 620 cm of γ-alumina [45,46] was not observed.
◦
◦
◦
◦
SiO
2
has the characteristic bands of the amorphous silica structure:
near 15 , 30 , 40 , and 50 2θ, corresponding to the boehmite structure
[30,48]. PNbMo-SiAl-1:1 and PNbMo-SiAl-4:1 present the same shape of
an amorphous material, because the broad bands of the silica masked
the alumina and PNbMo signals.
-
1
-1
-1
ν
O-Si-O 1100 cm , δO-Si-O 800 and 1196 cm ,
ν
Si-OH 950 cm , δSi-OH
-
1
-1
1
640 cm , and δO-H 3500 cm [27]. In the spectra of the SiAl mixtures,
the bands corresponding to both materials can be observed: 3400, 1635,
-
1
1
100, and 800 cm . (Full spectra are in Fig. S2 in Supplementary
Material)
X-ray diffraction patterns of PNbMo and PMA are presented in Fig. 5.
The SEM and TEM images of PNbMo morphology are shown in Fig. 6.
This HPA forms agglomerates of different sizes.
Figs. 7, 8, 9, 10 show the SEM and TEM micrographs of catalysts,
5

M.B. Colombo Migliorero et al.
Catalysis Today xxx (xxxx) xxx
Fig. 9. SEM x1000 (left) and TEM (right) micrographs of PNbMo-SiAl-1:1.
Fig. 10. SEM x1000 (left) and TEM (right) micrographs of PNbMo-SiAl-4:1.
Fig. 12. Potentiometric titration with n-butylamine of included PNbMo
Fig. 11. Potentiometric titration with n-butylamine of PNbMo and PMA.
and matrixes.
where laminar silica morphology and irregular morphology of alumina
can be observed for PNbMo-Si and PNbMo-Al respectively [49]. For
mixed silica-alumina materials, irregular morphology and size are
appreciated in both catalysts. However, an increase in size is observed
when the amount of Si is higher.
associated with the proton mobility of PNbMo within the framework.
The curves are similar to those of the pure matrixes, and the same
behavior was found for included PMA (Supplementary Material,
2
Fig. S4). The acid strength of SiO and SiAl-4:1 increases when the HPA
2 3
is included in them, but when the HPA is included in Al O or SiAl-1:1
The total acidity of PNbMo was obtained by potentiometric titration
with n-butylamine (Fig. 11). The maximum strength is indicated by the
matrix, the acid strength of the matrix decreases.
Nitrogen adsorption/desorption isotherms of mixed frameworks and
HPAs included in them are shown in Figs. 13, 14 . They present type II
curves, characteristic of solids with large pores or nonporous [50].
Meanwhile, silica, PNbMo-Si, and PMA-Si are type I isotherms (Sup-
plementary Material, Fig. S5), indicating a microporous material. A
initial potential (E
given by the value of meq amine/g solid in the plateau. PNbMo has very
strong acid sites, but a little lower than PMA (E
0
= 768 mV), and the total number of acid sites is
0
= 822 mV).
The acid strength and the number of acid sites of PNbMo decrease
when it is included in the matrixes (Table 1, Fig. 12). This could be
6

M.B. Colombo Migliorero et al.
Catalysis Today xxx (xxxx) xxx
Material, Fig. S6), characteristic of mesoporous materials. The specific
surface area (SBET) of matrixes and included PNbMo catalysts is listed in
Table 1.
3
.2. Catalytic activity
Diphenyl sulfide conversion (Scheme 1) was used to evaluate the
catalytic performance of the new HPA doped with Nb. In a blank test, no
conversion is observed when the reaction occurs in the absence of a
2 5
catalyst, and only 15% conversion is reached with Nb O after 24 h of
reaction (Fig. 15.a).
The incorporation of niobium in the Keggin structure results in a
higher diphenyl sulfide conversion with respect to PMA. This agrees
with the lower absorption edge energy calculated for PNbMo with
respect to PMA obtained from UV–vis spectra, a result similar to that
found when vanadium is incorporated in PMA structure [10].
The best results are obtained with PNbMo at 7 h (94% conversion
and 94% diphenyl sulfoxide selectivity); at longer reaction times the
selectivity to sulfoxide decreases (70% at 24 h), because it suffers the
oxidation to diphenyl sulfone. Meanwhile, PMA produces only 36% of
conversion at 7 h (100% diphenyl sulfoxide selectivity) but 98% of
conversion and 94% of selectivity after 24 h (Table 2 and Fig. 15.a and
b).
Fig. 13. Nitrogen adsorption/desorption isotherms of SiAl-1:1 and SiAl-1:1-
included catalysts.
Then, the catalytic activity of included HPA was tested. The results
show that the PNbMo catalysts produce higher values in diphenyl sulfide
conversion than PMA catalysts (Table 2, Fig. 16.a). The matrixes without
HPA inclusion were tested under the same reaction conditions. No
2 2 3
conversion was observed using SiO after 24 h, but when Al O was
used, 82% conversion and 65% selectivity were obtained. The mixed
matrixes produced lower conversion and high selectivity (up to 95%)
(
Supplementary Material, Figure S7).
With respect to niobium-containing catalysts, high diphenyl sulfide
conversion (near 100%) was reached after 5 h for all of them. However,
at this time, the selectivity toward diphenyl sulfoxide was moderate
(
Table 2, Fig. 16.b) due to the subsequent oxidation of diphenyl sulf-
oxide to diphenyl sulfone. Taking into account the conversion and
selectivity, the best results were obtained at 4 h with PNbMoSi (92%
conversion and 94% selectivity), and at 3 h with PNbMo-SiAl-4:1 (92%
conversion and 95% selectivity).
Fig. 14. Nitrogen adsorption/desorption isotherms of SiAl-4:1 and SiAl-4:1-
PNbMo-Si and PNbMo-SiAl-4:1 were selected to evaluate the reuse in
the same reaction conditions. They were isolated from the reaction
medium, washed with ethanol, dried under vacuum for the reuse. Tak-
ing into account the optimal reaction time for each catalyst, a slight
decrease in the catalytic activity was achieved in the reuse of PNbMo-
SiAl-4:1 at 3h: the conversion decreased from 92% to 82% in the
included catalysts.
lower SBET value than that of pure silica is probably due to pore blocking
for the PNbMo inside the silica structure. On the other hand, pure
alumina and HPA-Al present type IV isotherms (Supplementary
Fig. 15. Diphenyl sulfide conversion (a) and diphenyl sulfoxide selectivity (b) using bulk catalysts. Reaction conditions: catalyst (12 mg), diphenyl sulfide (1 mmol),
◦
ethanol (8 mL), and aqueous H
2
O
2
, 35% (w/v) (1.5 mmol), at 25 C.
7

M.B. Colombo Migliorero et al.
Catalysis Today xxx (xxxx) xxx
Table 2
a
Diphenyl sulfide conversion and diphenyl sulfoxide selectivity obtained with the catalysts .
Catalyst
PMA
Time (h)
Conv. (%)
Sel. (%)
Catalyst
PNbMo
Time (h)
Conv. (%)
Sel. (%)
1
2
3
4
5
6
7
2
100
100
100
100
100
100
100
94
1
2
4
100
100
100
99
4
26
26
71
83
87
94
97
8
3
13
29
31
36
98
4
5
99
6
98
7
94
2
4
24
70
PMA-Si
1
4
100
100
100
99
PNbMo-Si
1
2
27
56
81
92
96
98
98
99
100
99
96
94
88
83
76
42
2
3
4
5
6
7
18
29
41
53
62
72
99
3
4
99
5
99
6
98
7
2
4
85
24
PMA-Al
1
2
3
4
63
74
76
77
76
93
92
91
91
92
PNbMo-Al
1
2
76
90
91
87
86
81
59
3
91
4
96
2
4
24
100
PMA-SiAl-1:1
1
2
3
4
5
6
17
34
57
68
82
86
99
100
100
98
PNbMo-SiAl-1:1
1
2
40
72
95
90
86
81
76
71
–
3
84
96
4
93
95
5
96
93
6
99
2
4
84
24
100
PMA-SiAl-4:1
1
2
3
4
5
6
22
52
100
99
95
97
93
92
82
PNbMo-SiAl-4:1
1
2
31
74
99
97
95
68
–
76
3
92
87
4
100
100
100
100
98
5
99
6
–
2
4
100
24
–
a
◦
Reaction conditions: catalyst (bulk: 12 mg or included: 266 mg), diphenyl sulfide (1 mmol), ethanol (8 mL), aqueous H
O
2 2
, 35% (w/v) (1.5 mmol) at 25 C.
Fig. 16. Diphenyl sulfide conversion (a) and diphenyl sulfoxide selectivity (b) using included HPA. Reaction conditions: catalyst (266 mg), diphenyl sulfide (1
◦
mmol), ethanol (8 mL), and aqueous H
2
O
2
, 35% (w/v) (1.5 mmol), at 25 C.
reuse, and the selectivity toward diphenyl sulfoxide was 95% and 97%,
in the first and second run, respectively. While, a bigger decrease was
obtained with PNbMo-Si in the reuse after 4 h: the conversion dropped
from 92% to 71%, and the selectivity was 94% and 99% in the use and
reuse, respectively.
present better performance in the reuse.
4. Conclusions
A heteropolyacid (PNbMo) derived from PMA doped with niobium
was synthesized by hydrothermal method. The Keggin structure was
confirmed by FT-IR, XRD, and 31P-NMR characterization. The catalytic
activity was tested in the diphenyl sulfide sulfoxidation, compared with
These results show that both matrixes are efficient for the inclusion
PNbMo, allowing the use as recyclable catalyst in the selective sulfox-
idation of diphenyl sulfide. Moreover, mixed framework SiAl-4:1,
8

M.B. Colombo Migliorero et al.
Catalysis Today xxx (xxxx) xxx
PMA and correlated with the absorption edge energy calculated from
UV–vis spectra.
[9] V. Palermo, G.P. Romanelli, P.G. V a´ zquez, J. Mol. Catal. A Chem. 373 (2013)
1
42–150.
´
´
[
10] V. Palermo, A.G. Sathicq, P.G. Vazquez, H.J. Thomas, G.P. Romanelli, React. Kinet.
The heteropolyacids were included in silica, alumina, and mixed
silica/alumina matrixes by sol-gel technique, in order to convert PNbMo
to a heterogeneous catalyst. The solids were widely characterized, and
the catalytic performance was tested. Silica and mixed frameworks
conferred stability, hindered the solubility, and provided good textural
and morphological properties. The redox property of the heteropolyacid
was maintained when it was included in a matrix, and good results were
Mech. Catal. 104 (2011) 181–195.
´
[11] O. D’alessandro, A. Sathicq, V. Palermo, L. Sanchez, H. Thomas, P. Vazquez,
T. Constantieux, G. Romanelli, Curr. Org. Chem. 16 (2012) 2763–2769.
[
[
12] M. Ziolek, Catal. Today 78 (2003) 47–64.
13] E. Caliman, J.A. Dias, S.C.L. Dias, F.A.C. Garcia, J.L. De Macedo, L.S. Almeida,
Microporous Mesoporous Mater. 132 (2010) 103–111.
[
[
[
[
[
14] L.R.V. da Conceiçao, L.M. Carneiro, D.S. Giordani, H.F. de Castro, Renew. Energy
1
13 (2017) 119–128.
15] S. An, D. Song, Y. Sun, Y. Guo, Q. Shang, Microporous Mesoporous Mater. 226
(2016) 396–405.
2 2
achieved in the diphenyl sulfide sulfoxidation using aqueous H O ,
16] D.R. Park, S.H. Song, U.G. Hong, J.G. Seo, J.C. Jung, I.K. Song, Catal. Lett. 132
under mild reaction conditions. The best results were obtained with
PNbMo-SiAl-4:1 (92% conversion and 95% selectivity at 3 h) and with
PNbMo-Si (92% conversion and 94% selectivity at 4 h). The reuse of the
catalysts with best performance was evaluated, and only a slight
decrease in the conversion was detected. These catalysts are promising
for the sulfoxidation of more complex sulfides.
(
2009) 363–369.
17] O.V. Zalomaeva, N.V. Maksimchuk, G.M. Maksimov, O.A. Kholdeeva, Eur. J. Inorg.
Chem. 2 (2019) 66–68.
18] N.V. Maksimchuk, G.M. Maksimov, V.Y. Evtushok, I.D. Ivanchikova, Y.A. Chesalov,
R.I. Maksimovskaya, O.A. Kholdeeva, A. Sol ´e -Daura, J.M. Poblet, J.J. Carb o´ , ACS
Catal. 8 (2018) 9722–9737.
[19] Y. Ren, Y. Hu, Y. Shan, Z. Kong, M. Gu, B. Yue, H. He, Inorg. Chem. Commun. 40
(
2014) 108–111.
[
[
[
[
[
[
20] A. Chieregato, F. Basile, P. Concepci o´ n, S. Guidetti, G. Liosi, M. Dolores,
C. Trevisanut, F. Cavani, J.M. L o´ pez, Catal. Today 197 (2012) 58–65.
21] M. Kirihara, J. Yamamoto, T. Noguchi, A. Itou, S. Naito, Y. Hirai, Tetrahedron 65
(2009) 10477–10484.
CRediT authorship contribution statement
María B. Colombo Migliorero: Methodology, Investigation, Data
curation, Formal analysis, Writing - review & editing. Valeria Palermo:
Methodology, Supervision, Investigation, Formal analysis, Writing -
original draft, Writing - review & editing. Gustavo P. Romanelli:
Conceptualization, Methodology, Supervision, Formal analysis, Writing
22] I.D. Ivanchikova, N.V. Maksimchuk, I.Y. Skobelev, V.V. Kaichev, O.A. Kholdeeva,
J. Catal. 332 (2015) 138–148.
23] S. Dworakowska, C. Tiozzo, M. Niemczyk-Wrzeszcz, P. Michorczyk, N. Ravasio,
R. Psaro, D. Bogda, M. Guidotti, J. Clean. Prod. 166 (2017) 901–909.
24] F. Yang, J. Tang, R. Ou, Z. Guo, S. Gao, Y. Wang, X. Wang, Appl. Catal. B Environ.
2
56 (2019), 117786.
-
review & editing, Funding acquisition, Project administration. Patricia
25] E.F. Freitas, M.F. Paiva, S.C.L. Dias, J.A. Dias, Catal. Today 289 (2017) 70–77.
G. V a´ zquez: Conceptualization, Methodology, Supervision, Formal
analysis, Writing - review & editing, Funding acquisition.
[26] G. Pasquale, P. V ´a zquez, G. Romanelli, G. Baronetti, Catal. Commun. 18 (2012)
1
15–120.
´
´
[
27] V. Palermo, A. Sathicq, T. Constantieux, J. Rodríguez, P. Vazquez, G. Romanelli,
Catal. Letters 145 (2015) 1022–1032.
Declaration of Competing Interest
[28] S. Soltani, U. Rashid, S.I. Al-Resayes, I.A. Nehdi, Energy Convers. Manage. 141
(
2017) 183–205.
[
29] G. Romanelli, P. V a´ zquez, L. Pizzio, N. Quaranta, J. Autino, M. Blanco, C. C a´ ceres,
Appl. Catal. A Gen. 261 (2004) 163–170.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
[30] S. Thanasilp, J.W. Schwank, V. Meeyoo, J. Mol. Catal. A Chem. 380 (2013) 49–56.
[31] E.N. Prilezhaeva, Russ. Chem. Rev. 70 (2001) 897–920.
[
32] A. Kalir, H.H. Kalir, Biological activity of sulfoxides and sulfones, in: S. Patai,
Z. Rappoport (Eds.), The Chemistry of Sulphur-Containing Functional Groups,
Whiley, New York, 1993, pp. 957–973.
Acknowledgments
[
[
[
[
33] J. Legros, J.R. Dehli, C. Bolm, Adv. Synth. Catal. 347 (2005) 19–31.
34] I. Fern ´a ndez, N. Khiar, Chem. Rev. 103 (2003) 3651–3705.
35] V.Y. Kukushkin, Coord. Chem. Rev. 139 (1995) 375–407.
36] F. Liu, Z. Fu, Y. Liu, C. Lu, Y. Wu, F. Xie, Z. Ye, X. Zhou, D. Yin, Ind. Eng. Chem.
Res. 49 (2010) 2533–2536.
The authors thank L. Osiglio, J. Tara, M. Theiller, I.D. Lick, G. Kürten,
M.J. Ya n˜ ez, and A. Caneiro for the experimental measurements. We also
thank UNLP (X774), CONICET (PIP0321), and ANPCyT (PICT 2013-
[
[
[
[
37] M.B. Colombo Migliorero, V. Palermo, G.P. Romanelli, P.G. V a´ zquez, An. Asoc.
Quim. 106 (2019) 47–55.
0
409; PICT 2017-2174) for the financial support.
38] A.B. Vishnikin, T.Y. Svinarenko, H. Sklen a´ ˇr ov a´ , P. Solich, Y.R. Bazel, V. Andruch,
Talanta 80 (2010) 1838–1845.
Appendix A. Supplementary data
39] P. Moth ´e -Esteves, M. Maciel Pereira, J. Arichi, B. Louis, Cryst. Growth Des. 10
(
2010) 371–378.
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.cattod.2020.10.034.
40] R. de Paiva Floro Bonfim, L.C. de Moura, H. Pizzala, S. Caldarelli, S. Paul, J.G. Eon,
O. Mentr ´e , M. Capron, L. Delevoye, E. Payen, Inorg. Chem. 46 (2007) 7371–7377.
41] K.P. Barteau, J.E. Lyons, I.K. Song, M.A. Barteau, Top. Catal. 41 (2006) 55–62.
42] V. Sasca, M. S¸ tefǎnescu, A. Popa, J. Therm. Anal. Calorim. 56 (1999) 569–578.
43] L.R. Pizzio, P.G. V a´ zquez, C.V. C a´ ceres, M.N. Blanco, Appl. Catal. A Gen. 256
[
[
[
References
(
2003) 125–139.
[
[
1] L.M. Sanchez, H.J. Thomas, M.J. Climent, G.P. Romanelli, S. Iborra, Catal. Rev. 58
[44] C. Rocchiccioli-Deltcheff, A. Aouissi, M.M. Bettahar, S. Launay, M. Fournier,
J. Catal. 164 (1996) 16–27.
(
2016) 497–586.
2] M. Misono, Catalysis of heteropoly compounds (Polyoxometalates), Stud. Surf. Sci.
Catal. 176 (2013) 97–155.
[45] M. Jafar Tafreshi, Z. Masoomi Khanghah, Mater. Sci. 21 (2015) 28–31.
[
46] S. Music, D. Dragcevic, S. Popovic, Mater. Lett. 40 (1999) 269–274.
´
ˇ
´
´
[
[
3] S.-S. Wang, G.-Y. Yang, Chem. Rev. 115 (2015) 4893–4962.
4] N. Mizuno, K. Kamata, S. Uchida, K. Yamaguchi, Liquid-phase oxidations with
hydrogen peroxide and molecular oxygen catalyzed by polyoxometalate-based
compounds, in: N. Mizuno (Ed.), Modern Heterogeneous Oxidation Catalysis
Design, Reactions and Characterization, WILEY-VCH, Weinheim, 2009,
pp. 185–216.
[47] A. Popa, V. Sasca, E.E. Kiss, R. Marinkovic-Neducin, M.T. Bokorov, I. Holclajtner-
Antunovi ´c , Mater. Chem. Phys. 119 (2010) 465–470.
[48] D.N. Zambrano, M.O. Gosatti, L.M. Dufou, D.A. Serrano, M.M. Guraya, S. Perez-
Cat a´ n, Int. J. Min. Met. Mater. 10 (2016) 598–603.
[
49] M. Wannaborworn, P. Praserthdam, B. Jongsomjit, J. Nanomater. 2015 (2015),
519425.
[
[
[
[
5] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113–252.
6] I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171–198.
[
50] J.D. Wright, N.A.J.M. Sommerdijk, Sol-Gel Materials Chemistry and Applications,
Taylor & Francis Group, Boca Raton, 2001.
7] M.M. Heravi, S. Sadjadi, J. Iran. Chem. Soc 6 (2009) 1–54.
8] P. Villabrille, G. Romanelli, P. V ´a zquez, C. C ´a ceres, Appl. Catal. A Gen. 270 (2004)
1
01–111.
9