Vanadium-doped sodium phosphomolybdate salts as catalysts in the terpene alcohols oxidation with hydrogen peroxide

Article abstract of DOI:10.1039/d1ra04191f

In this work, we have explored the catalytic activity of Keggin-type heteropolyanions PMo_12?nV_nO₄₀^(3+n)?(n= 0, 1, 2, or 3) in the form of sodium salts in green oxidation routes of terpene alcohols with hydrogen peroxide. Nerol was the model molecule selected to assess the impacts of the main reaction parameters, such as temperature, catalyst load, and stoichiometry of reactants. The impacts of the presence of vanadium at different proportions (i.e., V₁, V₂, and V₃loads/per anion) in the structure of phosphomolybdate catalysts were assessed. All the catalysts were characterized by various techniques such as powder X-ray diffraction, attenuated diffuse reflectance infrared spectroscopy, ultraviolet-visible spectroscopy, thermogravimetric analysis, isotherms of adsorption-desorption of N₂measurements of surface area, scanning electronic microscopy, energy-dispersive X-ray spectroscopy, andn-butylamine potentiometric titration. Among the catalysts assessed, Na₄PMo₁₁VO₄₀was the most active and selective toward epoxides. The efficiency of this catalyst in the epoxidation of different terpene alcohols was investigated. Special attention was dedicated to correlating the composition and properties of the vanadium-doped phosphomolybdic catalysts with their catalytic activity.

Full text of DOI:10.1039/d1ra04191f

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Vanadium-doped sodium phosphomolybdate salts
as catalysts in the terpene alcohols oxidation with
hydrogen peroxide†
Cite this: RSC Adv., 2021, 11, 24072
a
a
a
Castelo Bandane Vilanculo, * Marcio Jose da Silva,
Alana Alves Rodrigues,
´
´
b
b
Sukarno Olavo Ferreira and Rene Chagas da Silva
(3+n)ꢀ
In this work, we have explored the catalytic activity of Keggin-type heteropolyanions PMo12ꢀn
V
n
O
40
(n
¼ 0, 1, 2, or 3) in the form of sodium salts in green oxidation routes of terpene alcohols with hydrogen
peroxide. Nerol was the model molecule selected to assess the impacts of the main reaction parameters,
such as temperature, catalyst load, and stoichiometry of reactants. The impacts of the presence of
vanadium at different proportions (i.e.,
1 2 3
V , V , and V loads/per anion) in the structure of
phosphomolybdate catalysts were assessed. All the catalysts were characterized by various techniques
such as powder X-ray diffraction, attenuated diffuse reflectance infrared spectroscopy, ultraviolet-visible
spectroscopy, thermogravimetric analysis, isotherms of adsorption–desorption of N
surface area, scanning electronic microscopy, energy-dispersive X-ray spectroscopy, and n-butylamine
potentiometric titration. Among the catalysts assessed, Na PMo11VO40 was the most active and selective
2
measurements of
Received 30th May 2021
Accepted 28th June 2021
4
toward epoxides. The efficiency of this catalyst in the epoxidation of different terpene alcohols was
investigated. Special attention was dedicated to correlating the composition and properties of the
vanadium-doped phosphomolybdic catalysts with their catalytic activity.
DOI: 10.1039/d1ra04191f
rsc.li/rsc-advances
To address this demand and make the oxidation reactions
more benign environmentally, the green and inexpensive
1
. Introduction
The development of catalysts that can achieve more selective oxidants molecular oxygen or hydrogen peroxide have been
and environmentally friendly oxidation routes of terpenic used in many catalytic processes to oxidize terpenic alcohols,
7
–9
compounds has received attention due to economic and envi- generating water as the only by-product. Nonetheless, it the
1
,2
ronmental reasons.
Terpenic alcohols are an abundant presence of a metal catalyst is always required to activate these
9
natural origin feedstock and occur in many plants; they are oxidants. Although molecular oxygen is the most abundant and
relevant platform molecules to produce key intermediates for cheapest oxidant, it is inammable, and difficult to handle if
10
the perfumery, avoring, ne chemicals, and pharmaceutical compared to hydrogen peroxide. In addition, in certain cases,
industries, being also used as ingredients for the formulation of high pressures of molecular oxygen are sometimes needed for
3
,4
10
cosmetics and household products. The oxidation of terpenic an efficient oxidation. On the contrary, hydrogen peroxide is
alcohol is a synthetic route of interest, leading to the formation a liquid, non-ammable, and an efficient oxidant at room
11
of valuable compounds such epoxides, through epoxidation of pressure.
the olenic double bond, or carbonylic compounds, aer the
Besides the green oxidant, in an oxidative process it is also
oxidation of hydroxyl groups. However, most of the oxidative desirable that the catalyst should be active, selective, and pref-
5,6
12
processes that are industrially used still consume hazardous erentially, easily recoverable. Different solid catalysts have
metal stoichiometric oxidants, which should be disposed of been widely developed to be used in oxidation reactions of
1
3,14
15
16
17
into the environment aer use.
terpenic alcohols.
Niobium, tungsten, titanium, and
1
8,19
various metal oxides
are only some examples of catalysts
used in epoxidation reactions of terpenic alcohols with
hydrogen peroxide.
a
Chemistry Department, Pedagogic University of Mozambique, FCNM, Campus of
Among the different catalysts, Keggin heteropolyacids
Lhanguene, Av. de Moçambique, Km 1, Maputo, Zipcode: 4040, Mozambique.
(
HPAs) belonging to the class of polyoxometalates (POMs) have
E-mail: castelovilanculo@gmail.com; Tel: +258 825573337
b
20,21
been widely used.
These POMs are well-dened metal–
Chemistry Department, Federal University of Viçosa, Minas Gerais State, 36590-000,
oxygen clusters, composed of oxygen atom bridges linking
transition metal atoms with high oxidation states, such as
Brazil
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra04191f
22,23
vanadium, molybdenum, or tungsten.
These versatile
1
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compounds have acidity and redox properties that make them evaluated their catalytic activity in the oxidation of terpene
24
efficient catalysts in a plethora of reactions. Although solid “in alcohols with hydrogen peroxide. All compounds were charac-
nature”, Keggin HPAs have a low surface area and are soluble in terized by powder X-ray diffraction, attenuated diffuse reec-
polar solvents, hampering their use as heterogeneous catalysts. tance infrared spectroscopy, thermogravimetric analysis,
Therefore, to circumvent this drawback they have been ultraviolet-visible spectroscopy, scanning electronic micros-
25–27
frequently used as solid-supported catalysts.
copy, energy-dispersive X-ray spectroscopy, and n-butylamine
An essential quality of Keggin HPAs consists of their easily potentiometric titration. The impacts of the main reaction
adjustable structure. A simple protons exchange by metal variables were investigated, using nerol as a model molecule. As
cations allows that their properties such as acidity strength, far as we know, vanadium-containing sodium phosphomo-
porosity, and surface area can be adequately tuned, making lybdates salts were not used yet as catalysts in the oxidation
28–32
them efficient catalysts in redox or acid-catalyzed reactions.
reaction of terpenic alcohols.
In addition, if the protons are exchanged by large ionic radius
cations such as cesium, potassium, or ammonium, they become
2
. Experimental section
33,34
insoluble catalysts in polar solvents.
This modication
keeps untouched the Keggin anion, which is the primary 2.1. Chemicals
structure of these catalysts.
All chemicals were purchased from commercial sources. Nerol,
There are still other alterations that allow improving the
redox potential of Keggin HPAs; the rst one consists of the
replacement of one MO unit (i.e., M ¼ Mo, W) generating
geraniol, and b-citronellol were all Sigma-Aldrich (99 wt%),
sodium carbonate (99.5 wt%) and diethyl ether (99.8 wt%) were
Proqu ´ı micos. Na HPO (99 wt%) was purchased from Riedel de
2
4
+
+
vacancies into the Keggin anion. Recently, Cs and Na cations
lacunar HPA salts were successfully used as catalysts in the
Hagen. Hydrated heteropolyacid H PMo O (99 wt%) was
3
12 40
acquired from Sigma-Aldrich. V2O5 (99.6 wt%), MoO3
35–37
oxidation of terpenic compounds with hydrogen peroxide.
The second modication is to ll the vacancy of lacunar HPA
(
(
99.5 wt%), H
$98 wt%), CH
3
PO
4 3 2 4
(85 wt%), NaVO (98 wt%), Na MoO
3
CN (99 wt%), were also purchased from Sigma-
38
salts with a transition metal cation. Metal doped phospho-
Aldrich. Aqueous hydrogen peroxide (35 wt%) was acquired
from Alphatec, and H SO
(95–98 wt%) was Din ˆa mica.
tungstic acid salts were effective catalysts in the oxidation
2
4
39–41
reactions of benzylic and terpenic alcohols.
Another modication that may improve the activity of Keg-
2
.2. Synthesis of the Na PMo VO
4 11 40
gin HPAs is to exchange one or more molybdenum atoms by
42
vanadium in the primary structure of catalysts. Such modi- The Na PMo11VO40 salt was synthesized according to the liter-
4
49
cation accelerates the steps of oxidation–reduction, which ature. Scheme 1 summarizes the main steps.
results in the improvement in activity and selectivity in oxida-
Typically, 15.8 g of MoO and 0.9 g of V were dissolved in
350 mL of deionized water and heated to point boiling. Then,
An example of improvement triggered by vanadium doping 1.2 g of H PO (85 wt%) was added and the resulting mixture
was reported during the isobutane oxidation. It was described was kept under reux for 6 h. A clear solution was obtained aer
that extra-Keggin O]VO species attach to the surface of the cooling to room temperature. The solid acid H PMo11VO40 was
–O–Mo single sites. obtained aer evaporating the solvent and recrystallization. To
Although V-free HPAs also catalyze the isobutane oxidation, synthesize the Na PMo11VO40 salt, aqueous solutions of Na CO
structures containing vanadium are generally more active. and H PMo11VO40 were mixed and heated at 333 K/3 h. Finally,
PMo11VO40 salt was obtained by solvent evaporation,
3
2 5
O
43–46
tion reactions.
3
4
47
4
4
4
+/5+
6+
heteropolyanions, thereby forming V
4
2
3
4
4
+/5+
6+
Thus, the aforementioned V
considered the active ones in H
Insoluble salts of vanadium-doped phosphomolybdic acid
–O–Mo
single sites are the Na
4
47,48
PVMo11
O
40
.
recrystallized from water, and subsequently drying at 373 K/5 h.
4
+
+
(
i.e., Cs and NH salts) demonstrated to be active in reactions
such as dehydrogenation of propane, oxidation of C –C
hydrocarbons, methacrolein oxidation.
denum HPAs have been used as solid-supported catalysts on
silica and molecular sieves (i.e., MCM-41, SBA-15), in oxidation
reactions of methacrolein and hydrocarbons.
4
10 2 40
2.3. Synthesis of the Na PMo V O
5
2
4
5
5
The catalyst was synthesized according to the original and
modied procedures (Scheme 2).
4
9–51
Vanadium–molyb-
57
Firstly, an aqueous solution (100 mL) containing Na HPO4
2
(
ca. 7.1 g) was added to a hot aqueous solution (100 mL) con-
taining 24.4 g of sodium metavanadate. Aer it has been cooled
to room temperature, 5 mL of concentrated H SO was slowly
52–54
Likewise,
sodium salts (i.e., Na PMo10 40) were supported on active
5
2
V O
55
2
4
carbon and used in aerobic oxidations. In a novel approach,
H PMo VO (shell) was supported on hydrothermally treated
4
11
40
Cs PMo VO (core) and successfully used to oxidize meth-
4
11
40
acrolein to methacrylic acid, achieving a higher efficiency than
+
4 3
bulky H PMo11VO40 or H PMo12O40, or than Cs salts of these
56
acids.
In this work, we synthesized sodium salts of phosphomo-
lybdic acid, containing Keggin-type heteropolyanions with
(
3+n)ꢀ
general formulae PMo12ꢀn
V
n
O
40
(n ¼ 0, 1, 2, or 3), and Scheme 1 Route of synthesis of Na
4
PMo11VO40.
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The porosimetry properties of the catalysts were studied by
N2 adsorption/desorption using NOVA 1200e High Speed,
Automated Surface Area, and Pore Size Analyzer Quantachrome
Instruments. Before the analyses, the samples were degassed by
1
h. The surface area of the solid catalysts was calculated by the
Brunauer–Emmett–Teller (BET) equation, which was applied to
the desorption/adsorption isotherms. To characterize the
surface of the salts, thin sections were selected and metalized
with carbon, for analysis with scanning electron microscopy
5 2 40
Scheme 2 Route of synthesis of Na PMo10V O .
added, and the solution developed a red color. Aerward, Na
2
-
(SEM) and energy dispersive X-ray spectroscopy (EDS), using
MoO $2H O (ca. 121 g) was dissolved in water (ca. 200 mL) and
4
2
a JEOL JSM 6010LA SEM.
added to the red solution under vigorous stirring. Concentrated
sulfuric acid (ca. 85 mL) was slowly added, and the hot solution
was cooled to room temperature. Finally, the solution was
extracted with ethylic ether, which was vapored under airow
Catalyst acidity strength was estimated by potentiometric
27
titration, as described by Pizzio et al. The electrode potential
variation was measured with a potentiometer (i.e., Bel, model
W3B). Typically, 50 mg of catalyst was dissolved in CH CN and
3
giving the solid acid H
dried at 343 K, and subsequently dry at 373 K/5 h.
To synthesize Na PMo10 40 salt, the aqueous solutions of
Na CO and H PMo V O were mixed and heated at 333 K/3
5 2
PMo10V O40. The resulting solid was rst
then titrated with n-butylamine solution in toluene (ca.
ꢀ1
0
.05 mol L ).
5
2
V O
2
3
5
10 2 40
2.6. Identication of main reaction products
hours. The Na PMo V O salt was obtained by solvent evap-
5
10 2 40
The main reaction products were identied in a Shimadzu GC-
010 gas chromatograph coupled with a MS-QP 2010 mass
oration and recrystallization from water, and dry at 373 K/5 h
Scheme 2).
2
(
spectrometer (i.e., electronic impact 70 eV, scanning range of m/
z 50–450). The purication, spectroscopic characterization, and
identication of all the products was previously published.
6 9 3 40
2.4. Synthesis of the Na PMo V O
31,35
A similar route to the described in Section 2.4. was used, except
by taking the required amount of sodium metavanadate and
sodium molybdate. Herein, the solution resultant developed
a cherry red color. Aer extraction with ethylic ether, it was
vapored and recrystallized in water. The Na PMo V O salt was
2
.7. Catalytic runs
Catalytic tests were carried out in a 25 mL three-necked glass
ask, tted with a reux condenser and sampling system, under
magnetic stirrer. Nerol was the model molecule. Typically, nerol

6
9 3 40
obtained from H PMo V O acid following the same proce-
6
10 3 40
(ca. 2.75 mmol) and H O
(ca. 34 wt%) were solved in CH CN
2(aq) 3
dure used for other sodium salts (Scheme 3).
2
(ca. 10 mL) and heated to 333 K. The addition of catalyst (ca.
0
.66 mol%) started the reaction.
The runs were monitored during 8 h, periodically collecting
2
.5. Characterization of catalysts
Infrared spectra were recorded on a Varian 660-IR spectrometer
aliquots and analyzing them in a GC equipment (Shimadzu
010, FID), tted with a Rtx®-Wax, capillary column (30 m
ꢀ1
at a wavenumber range of 400 to 1300 cm , which is the
ngerprint region of the main absorption bands of Keggin
heteropolyanions. UV-visible spectra were recorded on a AJX-
100 PC double bean Micronal spectrometer, tted with tung-
2

length, 0.25 mm i.d., 0.25 mm lm thickness). Gas chromato-
graphic conditions were as follows: 80 C (3 min); heating rate
ꢁ
6
ꢁ
ꢀ1
ꢁ
(10 C min ) until 240 C. Injector and detector temperatures
sten and deuterium lamps to provide visible and UV wave-
lengths, respectively. The spectra were obtained from CH CN
solutions 0.002 mol L , because it was the concentration used
in the most of catalytic runs.
ꢁ
ꢁ
were 250 C and 280 C, respectively.
3
ꢀ
1
3
. Results and discussion
Powder X-ray diffraction patterns of the salts with or without 3.1. Characterization of catalysts
vanadium were analyzed by X-ray diffraction (XRD) spectros-
copy using an X-ray Diffraction System Model D8-Discover
3
.1.1. FT-IR spectroscopy. The primary structure of the
Keggin anion of phosphomolybdate catalysts has phosphorus
as the central heteroatom, tetrahedrally coordinated to 4 oxygen
atoms and surrounded by 12 corner-sharing octahedra, each
one containing a Mo atom. There are 4 distinct types of oxygen
˚
Bruker using Ni ltered Cu-Ka radiation (l ¼ 1.5418 A),
working at 40 kV and 40 mA, with a counting time 1.0 s in the
ꢁ
diffraction angle (2q) ranging from 5 to 80 .
atoms; the rst linking the heteroatom to the Mo atom (O
other two types of oxygen atoms bridging two transition metal
atoms in adjoining octahedra (O and O ), and the last
a terminal oxygen atom (O ). The vibrations of these chemical
a
), the
b
c
d
ꢀ
1
bonds generate typical bands in the region 500 to 1200 cm of
the infrared spectrum, recognized as the ngerprint region. If
vanadium atoms replace molybdenum atoms in the Keggin
anion, may occur a decrease in the symmetry of the anion,
6 9 3 40
Scheme 3 Route of synthesis of Na PMo V O .
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Fig. 2 shows the infrared spectra of undoped and vanadium
doped phosphomolybdate acids and their respective sodium
salts. A slight shi toward lower frequencies of main absorption
bands was also noticed if compared to the infrared spectrum of
62
3
the Na PMo12O40 salt. Moreover, an increase in the replace-
ment level of Mo by V atoms triggered changes in the infrared
spectra, which was induced by a decrease in the symmetry of the
6
3,64
heteropolyanion.
a
Particularly, the splitting of P–O bond
ꢀ
1
stretching band (ca. 1080 cm ), as well as the presence of
shoulders were more noticeable in the vanadium-trisubstituted
65
phosphomolybdate anion.
In the infrared spectra of sodium vanadate–phosphomo-
lybdate salts (Fig. 1SM†), the band corresponding to Mo–O –Mo
stretching mode is slightly shied to lower wavenumber relative
to that of Na PMo12 40. This shi is due to vanadium incor-
poration into primary structure which forms Mo–O–V linkages
by replacing Mo from Mo–O–Mo bonds of Na PMo12
The shi toward lower frequencies suggests that the strength
Fig. 1 FT-IR/ATR spectra of undoped and doped-vanadium phos-
phomolybdate sodium salt and phosphomolybdic acid.
b
3
O
leading to the changes in the bands noticed in the infrared
58
spectrum of salts.
3
40
O .
To verify if during the synthesis some change occurred in the
structure primary (i.e., Keggin heteropolyanion), infrared
spectra of phosphomolybdic acid and their undoped and
vanadium-doped sodium salt were recorded (Fig. 1).
66
of the bond decreases when V(V) cations replaced Mo(VI) ions.
The shi is proportional to the number of vanadium atoms in
the heteropolyanion. Nevertheless, when it is not observed,
possibly may have occurred the elimination of part of vanadium
from the primary structure (Keggin) toward the secondary
ꢀ
1
The bands at 1073, 965, 885, and 775 cm wavenumbers
may be attributed to the asymmetric stretching of the phos-
phorus–oxygen bond (P–O ) of the central tetrahedron (PO ),
a
4
67
structure during a step of thermal treatment.
the asymmetric stretching of the bond between molybdenum
It was possible to assume that the Keggin structure of
sodium phosphomolybdate salt was retained because no rele-
vant change was noticed in infrared spectra before and aer the
vanadium doping. The impacts of vanadium load can be clearly
seen in the expanded infrared spectra (Fig. 3). Special attention
and different oxygen atoms; peripheral terminal oxygen atom
(
Mo–O
d
), stretching of the inter-and intra-octahedral bridges of
59
a Mo
3
O
13 group, Mo–O
b
–Mo and Mo–O
c
–Mo, respectively. As
a reference, dashed lines were inserted in infrared spectra
centered at the main absorption bands of phosphomolybdic
should be devoted to the absorption band assigned to the P–O
a
acids (Fig. 1 and 2). It is possible to note a shoulder in the P–O
a
ꢀ1
ꢀ1
bond stretching (ca. 1050 cm ) and for the absorption band
attributed to the vibration of the Mo–O bond placed around
bond stretching band (ca. 1080 cm ) in the infrared spectrum
of the Na PMo11VO40 salt (Fig. 1). This same effect was previ-
ously described in the literature.
d
4
ꢀ
1
60,61
950 cm .
No signicant changes
It can be veried that an increase in the number of vanadium
atoms led to a clear shi in the position of the bands (ca.
were observed in typical absorption bands of Keggin anion;
therefore, it can be concluded that their primary structure was
retained aer the synthesis of Na PMo11VO40 salt.
4
ꢀ
1
ꢀ1
1050 cm
and 950 cm ) toward lower wavenumbers. In
ꢀ1
addition, the band at 1050 cm in the FT-IR spectrum of
trisubstituted salt was broadened (Fig. 3). The IR spectra indi-
cate that the structure of the Keggin anion retains upon the
molybdenum substitution by vanadium.
3.1.2. UV-visible spectroscopy. The UV-visible spectroscopy
in solution is a simple tool to evaluate the potential activity of
HPAs as catalysts in the oxidation reactions carried out in the
Fig. 2 FT-IR spectra of unsubstituted (a) and substituted phospho- Fig. 3 Expanded FT-IR spectra of sodium phosphomolybdate salt and
molybdic acids (b–d), and their respective sodium salts. vanadium-substituted sodium phosphomolybdate salts.
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small bands at a wavelength lower than 205 nm (ca. 190–205
nm), which increased with a higher vanadium doping (Fig. 4).
The trisubstituted POMs were insoluble in CH CN and had
3
their UV-Vis spectra obtained in water (Fig. 2SM†). A similar
prole was observed for vanadium trisubstituted sodium salt.
3.1.3. Measurement of acidity strength of undoped and
vanadium-doped phosphomolybdate catalysts. The titration
curves of the pristine phosphomolybdic acid, sodium phos-
phomolybdate, and vanadium-(mono-, di-, or tri-) substituted
sodium phosphomolybdate salts are presented in Fig. 5. This
technique allows to quantify the total number of acid sites in
Fig. 4 UV-Vis spectra of undoped and doped phosphomolybdic the catalyst surface, calculated when the plateau of titration
catalysts.
curve is reached, and classify the acidity strength, in accordance
with the value of the initial potential electrode; E > 100 mV (very
liquid phase. The absorption edge in the UV-visible spectra of strong sites), 0 < E < 0 (weak
< 100 mV (strong sites), ꢀ100 < E
HPAs measures the energy necessary for “d–d”-type transitions, sites) and E
< ꢀ100 mV (very weak sites).
which involves the transfer of one electron from the Highest
According to Fig. 5, although have been observed a decrease
Occupied Molecular Orbital (HOMO) to the Lowest Unoccupied in the initial potential electrode (ca. E
¼ 680 mV, phospho-
Molecular Orbital (LUMO), or additionally, Ligand to Metal molybdic acid; to 400 and 370 mV, unsubstituted and
i
i
i
27
i
i
42
Charge Transfer (LMCT) transitions.
vanadium-monosubstituted sodium salts, respectively), the
Basically, the HOMO involves mostly the terminal oxygen protons exchange by sodium ions was not enough to diminish
atoms, consequently, its energy is not remarkably affected by the acid strength of the phosphomolybdate catalysts, conse-
changes in the HPA framework. Conversely, as the LUMO quently, both salts presented very strong acid sites (Fig. 5).
involves the bridging oxygen atoms and the d-orbitals of the Indeed, the presence of one vanadium atom had a minimum
metal framework, it can be affected by modications performed effect on the acidity strength of sodium salts, and Na
in the heteropolyanion. For instance, the substitution of one or and Na PMo11VO40 had titration curves almost similar with the
more Mo atoms by V modies the absorption edge and reects same initial E values. Nonetheless, an increase in vanadium
changes in the LUMO energy, as well as may also impact the load dramatically reduced the acidity strength of sodium
3 40
PMo12O
4
i
68
70,71
redox properties of cluster. To check how the vanadium phosphomolybdate salts (Fig. 5).
doping impacted these transitions, we recorded UV-Vis spectra
Villabrille et al. veried that equally to the observed herein,
of the phosphomolybdic catalysts before and aer the doping an increase in vanadium doping, lead to a diminution of the
72
with vanadium atoms (Fig. 4).
acidity strength of phosphomolybdic acids salts. They attrib-
The UV-Vis spectra were recorded in acetonitrile solutions. uted this effect to the increase in charge of heteropolyanion,
The most intense absorption band was observed at 219 nm with a consequent increase in the number of protons, reducing
wavelength for all the catalysts, which were assigned to the the acidity strength. Although this technique does not distin-
octahedrally coordinated Mo(VI) cations (Fig. 4). A second band guish the acid sites (i.e., Lewis or Brønsted), it can be useful to
5
1
was observed at 313 nm but with a weak intensity. The Mo(VI) compare the activity of these catalysts in oxidation reactions.
0
cations and V(V) have d congurations, therefore, these bands
were attributed to the Mo–O and V–O charge-transfer bands undoped and vanadium-doped phosphomolybdate catalysts.
i.e., LMCT transitions) involving octahedrally coordinated While infrared spectra of the HPAs offers data about the
3.1.4. Analyses of powder X-ray diffraction patterns of
(
69
Mo(VI) cations and oxygen atoms. The vanadium replacement primary
resulted in a shi to red of these absorption band edges to
a lower-energy region and the reinforced the absorbance. The
doping with vanadium(V) ions led to appearance of multiple
structure
of
Keggin
HPAs
(i.e.,
Keggin
Fig. 5 Potentiometric titration curves with n-butylamine of pristine
phosphomolybdic acid, sodium phosphomolybdate, and vanadium- Fig. 6 Hierarchical structure of vanadophosphomolybdate sodium
doped sodium phosphomolybdate salts.
salt (adapted from ref. 22).
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heteropolyanion), X-ray spectroscopy is important to assess the
secondary structure of these heteropolyanions (Fig. 3SM†). The
presence of metal cations and hydration water molecules may
affect the arrangement and symmetry of the unitary cell of
33,43
HPAs.
4
The hierarchical structure of Na PMo11VO40, which
comprises the primary, secondary, and tertiary substructures is
showed in Fig. 6.
Fig. 7 displays powder XRD patterns of phosphomolybdic
acid before and aer the protons exchange by sodium ions and
doping of the anion with vanadium. The X-ray diffraction peaks
ꢁ
ꢁ
was taken in the 2q ranges of 5.0 to 70.0 . The most signicant
peaks in the diffractogram of phosphomolybdic acid were
ꢁ
ꢁ
observed at 2q values of (5.0–10.0) , (18.0–23.0) , and (25.0–
ꢁ
30.0) counts.
The vanadium doping result in a higher crystallinity,
ꢁ
preserving the main diffraction peaks between 5 to 40 2q angle.
Nonetheless, there are new diffraction signals at 2q angles
Fig. 8 FT-IR spectra of unsubstituted (a) and substituted phospho-
molybdic acids (b–d), and their respective sodium salts.
ꢁ
ꢁ
greater than 40 (ca. 47, 50, and 63 2q angles), in the dif-
fractogram of the vanadophosphomolybdate sodium salt
(
Fig. 7).
hand, there works that report that
3
H PMo12O40 and
The literature describes that X-ray diffractograms of the
H
4
PVMo11 40 exhibited similar XRD patterns, consistent with
O
phosphomolybdic acid present well-dened peaks, which
suggests that the Keggin anion structure crystallizes in a body-
+
a triclinic lattice, but aer exchange protons by Cs cations, the
salts had different crystallinity patterns from the acid forms,
73
centered cubic system. Although this pattern depends on the
hydration level, the protons exchange is another aspect that also
affects the secondary structure. Nonetheless, comparing our
data with those reported in the literature we can conclude that
the secondary structure remained almost intact aer the
75,76
associated with a cubic crystallization lattice.
Although
+
+
+
smaller than Cs cations, Na ions are bigger than H ions and
maybe produce the same effect.
Comparing the crystallinity of vanadium doped salts and
their acid precursor, we can conclude that they are also highly
crystalline, mainly the monosubstituted Na
which showed a very intense peak, in the region of 5–10 2q
angle, besides two signicant peaks between 45–50 2q (Fig. 9).
The crystallinity of vanadium doped phosphomolybdic acids
and their sodium salts was compared in Fig. 8. Although the
sodium has an ionic radium greater than H ions, the prole of
diffractograms was relatively similar. It suggests that both
Keggin anion structure and the secondary structure was
retained. However, it is required a more deeply study to verify
how the vanadium load impacted these structures, mainly when
74
inclusion of one vanadium atom.
4
PMo11VO40 salt,
Fig. 8 shows the infrared spectra of undoped and vanadium
doped phosphomolybdate acids and their respective sodium
salts. In Fig. 8a, were highlighted the diffraction peaks of
phosphomolybdic acid which were observed at 2q ¼ 8.1, 8.9, 9.3,
ꢁ
ꢁ
ꢁ
1
1.1, 11.5, 17.9, 20.1, 26.1, 27.8, 28.3 and 28.9 are respectively
+
attributed to diffraction lines (001), (110), (10ꢀ1), (101), (011),
73
(012), (2ꢀ1ꢀ1), (34ꢀ2), (303), (203) and (132).
These values agree with the literature that suggests a cubic
7
2,73
crystalline structure for this heteropolyacid.
On the other
Fig. 7 Powder XRD patterns of phosphomolybdic acid before and Fig. 9 Powder XRD of vanadium (mono, di-, or tri)-substituted sodium
after the protons exchange by sodium and the vanadium doping. phosphomolybdate salts.
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three vanadium atoms are incorporated. It is possible that two
types of arrangement had been originated when this modica-
tion was performed, generating isomeric Keggin anions.
3.1.5. Thermal analyses of vanadium-containing sodium
phosphomolybdate salts. TG-DTG and DSC curves obtained
from vanadium-containing polyoxometalate salts were obtained
under nitrogen atmosphere revealing that two or three thermal
events have occurred (Fig. 10). The prole of the TG curve shows
a continuous decay of weight in the range of 298 to 973 K,
regardless of sodium salt. Nonetheless, it was faster and more
pronounced in the thermogram of the Na PMo V O .
Scheme 4 Sequence of thermal events proposed as the basis of
weight loss (adapted from ref. 47).
77
coordinated water molecules. According to the weight loss
percentage, it was possible to quantify the hydration water
molecules present in the sodium salts (Table 2SM†). It was
veried that as greater the number of Vanadium atoms, higher
was the number of water molecules (ca. 7, 10 and, 13 water
moles, in V1, V2, and V3 catalysts). Some of the changes
observed in XRD patterns of sodium salts (Fig. 8 and 9) can be
6
9 3 40
The rst weight loss occurred heating the sample from room
temperature up to 388 K, with a minimum in the DTG curve at
approximately 373 K. It was assigned to the release of physically
adsorbed water and some of the coordinate hydrating water
molecules. The second weight loss happened in the tempera-
ture range from 338 to 572 K and resulted in the formation of
anhydrous catalyst. The third event occurred over a wide
temperature range (ca. 653–803 K), and it was ascribed to the
decomposition of Keggin anion and total formation of oxides
mixture. This sequence of events was described by Jing et al.
aer they perform the thermal analysis of Keggin salts con-
78
ascribed to the different hydration levels.
3.1.6. Analyses of porosimetry of undoped and vanadium-
doped sodium phosphomolybdate salts. The N adsorption/
2
desorption isotherms provided information about the surface
area (BET), volume, distribution, and pores diameter of
molybdate sodium salt catalysts (Fig. 4SM†). According to
IUPAC rules, the isotherms were classied as being interme-
diate between type III and V. The same way, the slight hysteresis
loops in the isotherm plots were classied as H-3. The
vanadium-doped phosphomolybdate salts were classied as
a mesoporous material (ca. pores size between 5 to 50 nm), with
a higher range of pore size when the vanadium doping was
increased. It was ascribed to the capillarity condensation in
mesopores of the solid catalysts, a consequence of adsorption
on aggregates of platy particles.
3
ꢀ
49
taining hydrate PMo11VO40 anion (Scheme 4).
All the DTG curves had a decline at 473 K temperature,
however, while the curves obtained from the samples of Na
PMo O and Na PMo V O remained almost constant aer
3
-
1
2
40
4
10 1 40
this temperature (Fig. 10a and b), DTG curves of Na PMo V O
10 2 40
5
and Na PMo V O presented minimum at 723 and 573 K,
6
9 3 40
respectively (Fig. 10c and d). The literature describes that Keg-
gin heteropolyanions are decomposed to oxides at temperatures
equal or higher than 773 K, a resulted conrmed by the
2
ꢀ1
Keggin HPAs are solid with a low surface area (ca. < 5 m g ),
50,76
appearance of an endothermic peak in DSC curves.
peak was more visible in the DSC curves obtained from Na
PMo O and Na PMo V O , where it was observed at
This
which can be increased if the protons are exchanged by cations
3
-
˚
79
with an ionic radius higher than 1.3 A. Besides that, depend-
ing on the sort of cation, the salts can be insoluble in a polar
solvent. In this work, our intention was to assess the activity of
the vanadium salts dissolved in the solution, due to this reason,
small ions likewise sodium were selected to replace the protons.
Therefore, the increase in the surface area was not very
pronounced. However, an increase in vanadium load seems to
have also contributed to increasing the surface area of salts
1
2
40
6
9 3 4
temperatures of 773 and 743 K, respectively (Fig. 10a and d).
From Fig. 10, it was possible to calculate the number of water
molecules present in the salts, considering the percentage of
mass loss that occurred in the range of 298 to 523 K, which is
assigned to the loss of physisorbed, bounded, and structurally
(Table 1SM†).
3.1.7. SEM-EDS analyses of vanadium-containing heter-
opoly acid and its sodium salt. The vanadium-containing
sodium phosphomolybdate salts were submitted to SEM-EDS
analysis to characterize their surface. Fig. 5SM† shows SEM
images of the pure sodium phosphomolybdate salt and with
different loads of vanadium.
Comparing the SEM images of the vanadium-containing
heteropoly salt with the precursor heteropoly acid, it was
possible to note that there was a reduction in the particle sizes.
Specically, the particles of the Na
smaller than those of the undoped Na
4
PMo11VO40 are noticeably
PMo12 40 and, therefore,
3
O
apparently, the insertion of vanadium in the structure of anion
increased the surface area, as demonstrated by BET analysis.
The percentual elemental composition of sodium phospho-
Fig. 10 TG-DTG and DSC curves of unsubstituted (a) and vanadium- molybdate salts was conrmed by EDS analysis, which agrees
substituted sodium phosphomolybdate salts (b–d).
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Fig. 11 Effect of catalyst nature on kinetic curves (a), conversion and
products selectivity after 8 h (b) of nerol oxidation reactions by H
Reaction conditions: nerol (2.75 mmol), H (2.75 mmol), toluene
internal stander), catalyst (0.66 mol%), reaction time (8 h), tempera-
ture (333 K), CH CN (10 mL).
2 2
O .
2
O
2
(
3
with the theoretical values (Fig. 6SM†). No residual element was
found in the samples (i.e., the sulfuric acid component used in
the synthesis), a guarantee that they were adequately puried.
Fig. 12 Effect of the catalyst on the conversion and reaction selectivity
3.2. Catalytic tests
2 2
versus time of oxidation reactions of nerol with H O in presence of
H
3
PMo12 40 (a), Na PMo12 40 (b) and Na PMo12VO40 (c) catalysts.
O
3
O
4
3.2.1. Effect of the catalyst on nerol oxidation by hydrogen
peroxide. In the initial catalytic tests, we have studied the
activity of pristine phosphomolybdic acid, and their undoped
and vanadium-doped sodium salts in the oxidation reactions of
nerol, the selected model molecule (Fig. 11). The reaction
differently through the time; while in reaction with Na
VO40 the nerol epoxide was always the major product, in the
presence of H PMo O catalyst until 4 h reaction it happened
with another compound (i.e., nerol peroxide). Only from this
time, nerol epoxide became the main product.
4
PMo11-
35,36
conditions were chosen based on the literature.
The reaction conversions were Na PMo VO > H PMo O
12 40
3
12 40
4
11
40
3
>
Na PMo O , evidence that the vanadium doping had
3 12 40
Comparing the performance of the two most efficient cata-
a benecial effect on the performance of sodium phosphomo-
lybdate catalyst (Fig. 11a). Besides the highest initial rate and
conversion, this catalyst provided the greatest selectivity toward
oxidation products, being the nerol epoxide the major product
+
lysts, it can be concluded that both Brønsted (i.e., H
PMo12 40), and Lewis sites (i.e., vanadium sites, Na
VO40), seem to play a crucial role in the conversion of nerol to
the epoxide. However, Na PMo11VO40-catalyzed reaction con-
3
O ions,
H
3
O
4
PMo11-
(Fig. 11b). Nerol diepoxide and neraldehyde were the secondary
4
verted more quickly the reaction intermediates (i.e., nerol
peroxide) to the nal product (i.e., nerol epoxide) (Fig. 12a and
c) than acid-catalyzed reaction. Conversely, the undoped salt
products (Scheme 5).
The undoped catalyst achieved the lowest conversion (ca.
73%), it presented the lowest selectivity toward nerol epoxide.
3
(i.e., Na PMo12O40) was the less efficient catalyst, and its reac-
Moreover, it led to the highest formation of alkyl peroxides,
possible reaction intermediates, which are non-detected prod-
ucts by GC FID analysis but were calculated through the mass
balance of reactions (Fig. 11b).
In Fig. 12, it was described how the reaction selectivity and
the conversion varied during all the time. Fig. 12a and c show
that although the H PMo O or Na PMo VO -catalyzed
tion achieved the lowest conversion and nerol epoxide selec-
tivity, leaving consequently the highest amount of nerol
peroxide without being converted to the nal product (Fig. 12b).
3.2.2. Effect of vanadium load on nerol oxidation by
hydrogen peroxide. When one vanadium atom was introduced
in the Keggin phosphomolybdate anion, there was a signicant
enhancement in the conversion and selectivity toward epoxide
of oxidation reaction of nerol (Fig. 12). To verify if this effect is
proportional to the vanadium atoms load present in the cata-
lyst, tests were performed with the three vanadium salts
3
12 40
4
11
40
reactions had achieved the maximum conversion within the
rst hour of run, the selectivity of each process progressed

(Fig. 13).
When the content of vanadium was increased from V1 to V2,
only a slight decrease in the conversion and selectivity was
noticed (Fig. 13a and b). Nonetheless, when the sodium phos-
phomolybdate containing three vanadium atoms was the cata-
lyst, the reaction conversion was remarkably lower (ca. 58%),
although there was a signicant increase in the selectivity
toward neraldehyde (ca. 27%, Fig. 13b).
Scheme 5 Oxidation of nerol with H
2 2
O in the presence of phos-
phomolybdate catalysts in CH CN solutions.
3
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The selectivity of oxidation products reached in the vana-
dium salts-catalyzed reactions (i.e., mainly nerol epoxide and
neraldehyde) was as follows: Na PMo V O > Na PMo V O
4
11
1
40
5
10 2 40
>
Na PMo V O , the same trends observed in terms of
6 9 3 40
conversion.
When we focus on the selectivity of nerol peroxide, we can
realize that as higher was the reaction conversion, more quickly
the nerol peroxide was converted to oxidation products. It
means that in the rst hour of reaction, the highest selectivity of
nerol peroxide was found in the reaction with the less efficient
Fig. 13 Effect of vanadium doping on the activity of sodium phos- catalyst (i.e., Na PMo V O ). Once more, it can be attributed to
6
9 3 40
phomolybdate catalysts; kinetic curves (a), conversion, and products
selectivity after 8 h (b) of nerol oxidation with H . Reaction condi-
tions: nerol (2.75 mmol), H (2.75 mmol), toluene (internal stander),
catalyst (0.66 mol%), temperature (333 K), CH CN (10 mL).
the higher content of vanadium. The doping of phosphomo-
lybdate anion with three vanadium atoms drastically reduces
the acidity strength of the catalyst (Fig. 5), compromising its
performance. An aspect that distinguishes this reaction from
the others is the signicant formation of neraldehyde. None-
theless, since the epoxidation reactions oxidation of alcohol to
aldehyde involves different mechanisms, we will discuss them
in the next section.
2 2
O
2 2
O
3
A similar effect was noticed when TiO
doped cesium phosphomolybdate (i.e., Cs3+nPMo
2
-supported vanadium-
V O /
12ꢀn
n 40
TiO , n ¼ 0–3) catalysts were used in the benzyl alcohol oxida-
2
80
3.2.3. Mechanistic discussions. It is noteworthy that epox-
idation of allylic alcohols with hydrogen peroxide in the pres-
ence of metal catalysts such as tungsten, titanium,
molybdenum, or niobium are hydroxyl group assisted-reac-
tion to benzaldehyde. Despite the different oxidant (i.e.,
molecular oxygen) has been used, those authors veried that
4 2
while the Cs PMo11VO40/TiO -catalyzed reactions reached the
highest conversion, an increase in vanadium content led to
a decreasing in conversion. Moreover, likewise veried herein,
the aldehyde selectivity was also increased. Those authors
ascribed this effect to a change in redox properties (i.e., the
15–17,37,81–83
tions.
This reaction competes with the oxidation of
alcohol to aldehyde, and the chemoselectivity will depending on
the coordinate fashion of substrate to the metal catalyst.
Several authors have described that when Keggin HPAs are
the catalysts in oxidation with peroxide hydrogen of alcohols or
olens, peroxidized intermediates are the most probable active
4
+
5+
formation of V /V pairs), and the increase in protons mobility
80
triggered by the increase of the vanadium doping.
The decline in the performance of the catalyst observed
when a higher vanadium load was used can be justied by the
increase of energy barrier between HOMO and LUMO orbitals,
which difficult the reducibility of these di-or tri-substituted
15,17,37,84
species.
While in the epoxidation of alkenes with
hydrogen peroxide in the presence Ti-containing HPA catalysts
the hydroperoxide species (i.e., –OOH) are the most common
intermediates, in those with vanadium, molybdenum, or tung-
sten the oxo-intermediates (i.e. –O–O– species) are the most
80
heteropolyanions. The evolution of reaction selectivity along
the process was also monitored and the main data area shown
in Fig. 14.
85,86
favored.
Therefore, as the basis of the literature and our
experimental results we proposed that nerol epoxidation maybe
15,38
describes as in Scheme 6.
Herein, we suppose that the epoxidation is also a hydroxy
group assisted-reaction that involves a peroxidized vanadium-
doped phosphomolybdate intermediate (Scheme 6). Although
molybdenum atoms are also the active sites, our results showed
that the formation of nerol epoxide was more favorable when
Fig. 14 Effect of vanadium doping on the conversion and reaction Scheme 6 Propose of reaction pathway of vanadium-doped sodium
(3+n)ꢀ
selectivity versus time of Na3+nPMo11
V
n
O
40
(n ¼ 1–3) catalyzed phosphomolybdate salt-catalyzed nerol epoxidation reaction with
oxidation reactions of nerol with H
2
O
2
.
hydrogen peroxide (adapted from ref. 15, 17 and 38).
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one vanadium atom were present in the Keggin anion. Indeed,
Romanelli et al. described that the formation of the peroxide–
metal intermediate between the peroxide and the HPA catalyst
(
i.e., Mo or W addenda atoms) is more favorable in the presence
87,88
of vanadium atom.
This benecial effect was ascribed to the
reduction of the energy gap between HOMO and LUMO orbitals,
provoked by the presence of one vanadium atom, which favors
the reducibility of Keggin anions, as demonstrated in studies of
molecular orbital performed with vanadium-doped phospho-
89
molybdic acids.
It is important to highlight that even in the absence of
vanadium, the reaction efficiently proceeded; indeed, it was
more visible when a strong Brønsted acidity strength was
present, which is translated into better catalytic performance,
Fig. 15 Impacts of Na
4
PMo11VO40 catalyst load on the kinetic curves
(
a), conversion, and products selectivity after 8 h (b) of nerol oxidation
reactions with H . Reaction conditions: nerol (2.75 mmol), H
3
2.75 mmol), toluene (internal stander), temperature (333 K), CH CN
2
O
2
2 2
O
(
90
as in the case of H
strated the potentiometric titration curves, H
Na PMo11VO40 presented high values of initial electrode
potential, conrming that they have very strong acid sites undergone a fast interconversion of one electron (i.e., V /V ).
3
PMo12O
40-catalyzed reaction. As demon- (10 mL).
3
PMo12 40 and
O
4
4
+
5+ 92
27
(Fig. 5).
The literature suggests that a high load of vanadium can
89–92
The H PMo O catalyst in the solid-state present different promote this process.
Another important aspect is linked to
We the acidity of the catalyst. This catalyst was who presented the
3
12 40
+
+
+
90,91
3 5 2
protons in their structure (i.e., H , H O , and H O ).
believe that in solution, the less mobile protons linked to the lowest acidity strength (Fig. 5). Previously, it has been reported
Keggin anion can interact with hydrogen peroxide through that in Nb O -catalyzed nerol oxidation reactions the niobium
2
5
intermolecular hydrogen bonds. This effect favors the perox- oxide with the weakest acid sites proved the highest ner-
15
idation of the phosphomolybdate anion, which favor the aldehyde selectivity.
formation of an oxo-intermediate, likewise to the depicted in
3.2.4. Effect of the load of Na PMo VO catalyst on nerol
Scheme 6, however, having Mo as the atom present in oxo- oxidation by H O . The catalytic activity of Na PMo VO was
4
11
40
2
2
4
11
40
moiety. Consequently, this intermediate can transfer an evaluated using different concentrations, and the main results
oxygen atom to the nerol double bond. It is noteworthy that are displayed in Fig. 15. An increase in catalyst load enhanced
even with a low concentration of protons (i.e., Na PMo12O40), the initial rate of reactions, nonetheless, the kinetic curves had
3
which has only residual protons, the oxidation reaction was less the same prole, achieving the maximum conversion within the
efficient (Fig. 13). Conversely, with a typically protic catalyst (i.e., initial period, regardless of the catalyst load.
H
3
PMo12
O
40), the oxidation reaction reached a high conversion
However, the selectivity had a different behavior; within the
and selectivity toward nerol epoxide (Fig. 11).
rst hour of the reaction, the nerol was quickly converted to
On the other hand, the highest selectivity to neraldehyde was oxidation products, with a great quantity of alkyl peroxides,
achieved in the Na PMo V O -catalyzed reaction. In general,
6
9 3 40
such reactions can be explained as in Scheme 7, where the
hydroxyl group of alcohol is peroxidized, and further this
intermediate is decomposed by the catalyst to neraldehyde and
91
water.
Although the peroxidation of an alcohol can occurs without
a catalyst, we suppose that herein it participates either in the
transference step of an oxygen atom from oxidant to alcohol
(step 1, Scheme 4), as in the decomposition of the peroxide
intermediate to aldehyde (step 2, Scheme 7). To do it, it is
requiring that the catalyst should be easily oxidizable, it is,
Scheme 7 Propose of reaction pathway of Na
PMo V
6 9 3
O40-catalyzed Fig. 16 Variation of conversion and reaction selectivity versus time of
nerol oxidation reaction with hydrogen peroxide.
Na PMo11VO40-catalyzed nerol oxidation reaction with H
4
2 2
O .
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which were being gradually converted to nerol epoxide while the
reaction proceeds. Therefore, although the maximum conver-
sion has been quickly reached, it is required to carry the reac-
tion by longer times. Moreover, the decomposition of alkyl
peroxides to oxidation product (i.e., nerol epoxide) was more
efficient if a higher catalyst load was present in the solution.
This effect can be visualized in Fig. 16, which shows the
conversion and selectivity of reactions along the time.
Regardless of the catalyst load, all the runs achieved the
maximum conversion within 2 h of reaction, having always
nerol epoxide as the main product (Fig. 16).
Fig. 18 Influence of temperature on the kinetic curves (a), conversion
and products selectivity after 8 h of (b) Na
oxidation reactions of nerol with H . Reaction conditions: nerol
PMo11VO40 (0.083 mol%), CH CN
4
PMo11VO40-catalyzed
3
.2.5. Effect of oxidant load on the conversion and selec-
tivity Na PMo11VO40-catalyzed oxidation with H . As we re-
ported in our previous work, the amount of the oxidant
2 2
O
4
2 2
O
(
(
2.75 mmol), H O
2 2
10 mL).
(2.75 mmol), Na
4
3
15,17
could affect the substrate conversion and the product's selec-
tivity, mainly due to the presence of greater water amount when
a greater excess of oxidant is used. This effect was evaluated the
main results are in Fig. 17.
that it is converted), however, nerol epoxide and neraldehyde
were not detected.
The excess of peroxide had different impacts on reaction rate
and selectivity of products. Regardless of the oxidant load, the
maximum conversion was achieved within the rst hour of
reaction. Aerward, the conversion remained unaltered. None-
theless, keeping constant the catalyst load, a greater amount of
oxidant has no positive effect in converting the alkyl peroxides
to nerol epoxide. It suggests that a large excess of peroxide
compromises the formation of active species from the peroxi-
dized catalyst and the substrate. Possibly, the alcohol itself is
peroxidized, and the interaction of these species and the oxo-
catalyst its less favorable. Verifying the conversions in uncata-
lyzed reactions, we have found that using 1 : 1, 1 : 2, and 1 : 3
molar ratios, conversions of <5, 10, and 15% were reached,
however, no signicant amount of oxidation products was
detected. It assures that the substrate is peroxidized (i.e., once
3
.2.6. Assessment of the effect of temperature on the
PMo11VO40-catalyzed oxidation of nerol by H . To inves-
tigate the inuence of the temperature on the catalytic perfor-
mance of the Na PMo11VO40 salt, we have used a low catalyst
concentration (ca. 0.083 mol%), to do a more visible this effect
Fig. 18).
With a higher temperature the reactions became faster, due
Na
4
2 2
O
4
(
to the greater number of effective collisions, an effect that was
much more visible at 333 K (Fig. 18). Previously, we have seen
that the reactions carried out at 333 K achieved the conversion
maximum within the rst hour of reaction, regardless of the
reaction conditions (i.e., different stoichiometry, catalysts, etc.).
Nonetheless, it was happening because the reactions have been
performed with 0.66 mol% of catalyst load at 333 K. Herein,
Fig. 17 Effect of stoichiometry of the reactants on the conversion and Fig. 19 Effect of temperature on the conversion and reaction selec-
reaction selectivity versus time of Na PMo11VO40-catalyzed oxidation tivity versus time of Na PMo11VO40-catalyzed oxidation reactions of
reactions of nerol with H . Reaction conditions: nerol (2.75 mmol), nerol with H . Reaction conditions: Nerol (2.75 mmol), H (2.75
Na PMo11VO40 (0.66 mol%), temperature (333 K), CH CN (10 mL). mmol), Na PMo11VO409 (0.083 mol%), CH CN (10 mL).
4
4
2
O
2
2
O
2
2 2
O
4
3
4
3
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using the lowest catalyst load (ca. 0.083 mol%), only the reaction
When a tertiary allylic alcohol was tested (i.e., linalool,
carried out at 333 K had a similar behavior; those other reac- Fig. 20b), only a minimum formation of epoxide was noticed.
tions performed at temperatures lower than 333 K, had their The terminal double bond was less sensitivity to the epoxida-
conversions gradually increasing with the run time (Fig. 19).
tion, an evidence that this is a hydroxy group-assisted reaction,
When we analyzed the behavior of the reaction selectivity being thus less favorable herein. The major product was
through the time, we veried that at the temperature range of peroxide linalool. A cyclization product of linalool was also
2
98 to 323 K, an increase in temperature accelerated the reac- formed, similarly to the veried in a previous work, where the
35
tions as well as the conversion of nerol peroxide to nal lacunar sodium phosphotungstate was the catalyst.
oxidation products (i.e., nerol epoxide, aldehyde and sometimes
Finally, when primary alcohol without an allylic double bond
diepoxide) (Fig. 19). At the lowest temperature (ca. 333 K, was the substrate (b-citronellol, Fig. 20c), only a poor conversion
Fig. 18a), the decomposition of peroxide intermediate to nal was achieved, with a low selectivity toward aldehyde. Moreover,
products was compromised and aer 8 h of reaction the lowest the trisubstituted double bond remained almost intact, likewise
selectivity to nerol epoxide was achieved.
to the veried in the oxidation reactions of nerol or geraniol
PMo11VO40-catalyzed oxidation reactions with substrates.
: effect of the substrate. To verify the possible electronic
3
O
.2.7. Na
4
H
2
2
and steric effects on the reaction selectivity, we selected some
terpenic alcohols with different structures: geraniol (Fig. 20a),
a geometric isomer of nerol, linalool (Fig. 20b), a tertiary allylic Keggin-type heteropolyanions PMo1 VnO40
4
. Conclusions
(
3+n)ꢀ
2ꢀn
(n ¼ 0, 1, 2,
alcohol, and b-citronellol (Fig. 20c), a primary alcohol. The or 3) in the form of sodium salts were synthesized and evaluated
variation of conversion and reaction selectivity versus time are as catalysts in green oxidation routes of terpene alcohols with
shown in Fig. 20.
hydrogen peroxide. The effect of the main reaction parameters
Besides nerol, all three terpenic alcohols (i.e., geraniol, was assessed. Among the several sodium salts tested, the Na
4
-
linalool, b-citronellol) with double bonds that may be poten- PMo11VO40 was the most active and selective catalyst toward the
tially epoxidized were assessed. Geraniol and nerol are formation of nerol epoxide. We have come up with a sustainable
geometric isomers (i.e., E and Z isomers, respectively), and catalytic system for epoxidation reactions catalyzed by
showed a similar reactivity. It suggests that no stereochemistry vanadium-doped phosphomolybdate, using a very low loading
controlling occur on the oxidative process of these two alcohols. of catalyst (ca., 0.032 mol%), achieving good-to-excellent yields
Likewise veried in nerol oxidation, geraniol was quickly per- (ca. 85–95%) of nerol epoxide selectivity, using a stoichiometric
oxidized and then converted to epoxide as the main product. molar ratio of substrate oxidant (ca. 1 : 1). Temperature reac-
Nonetheless, different than oxidation of nerol, where no tion, catalyst load, as well as the content of vanadium on the
peroxide was detected at the end of the reaction, a signicant sodium phosphomolybdate were the key aspects that drive the
amount of the peroxidized substrate (ca. 20%) remained not efficiency of the process. The greatest activity of Na
4
PMo11VO40
converted aer 8 h of reaction. Moreover, only a minimum catalyst was ascribed to electronic and acidity properties, as
portion was converted to aldehyde.
demonstrated by the UV-Vis, FT-IR, n-butylamine potentio-
metric titrations analyses. This catalyst system provides an
efficient, convenient, and practical method for the synthesis of
epoxides from allylic alcohols from renewable origin feedstock
(i.e., terpene alcohols), and a friendly oxidant (i.e., hydrogen
peroxide).
Conflicts of interest
There are no conicts to declare.
Acknowledgements
The authors are grateful for the nancial support from CNPq
and FAPEMIG (Brasil). This study was nanced in part by the
Coordenaç ˜a o de Aperfeiçoamento de Pessoal de N ´ı vel Superior -
Brasil (CAPES) – Finance Code 001.
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RSC Adv., 2021, 11, 24072–24085 | 24085