Catalytic way of transforming 2,3-dimethylphenol to para-quinone with the use of vanadium-containing heteropoly acids

Article abstract of DOI:10.1016/j.apcata.2017.09.022

2,3-Dimethyl-p-benzoquinone (2,3-Me₂BQ) is a valuable chemical that is applied as a soft oxidizing and dehydrogenating agent and also as a synthon in preparing different complex products including pharmaceutical and biochemical substances. Keggin- and modified-type aqueous solutions of Mo-V-phosphoric heteropoly acids (Mo-V-P HPAs) with the gross compositions H_3+xPMo_12-xV_xO₄₀ (HPA-x) and H_aP_zMo_yV_x’O_b (HPA-x’), respectively, possessing high oxidation potential and simplicity of regeneration can serve as effective soft oxidants for obtaining such para-quinone from 2,3-dimethylphenol (2,3-Me₂P). The synthesized HPA catalysts with different vanadium content were characterized by a number of analysis techniques, such as ³¹P and ⁵¹V NMR spectroscopy, potentiometry, titrimetry, and pH measurement. It was found that the predominant formation of 2,3-Me₂BQ instead of corresponding diphenoquinone (DPQ) at one-electron oxidation is achieved by a consecutive optimization of reaction conditions, the most important among them being organic solvent and molar ratio of vanadium(V) to substrate. As was shown, the substitution of HPA-x by HPA-x’ allows one to increase the quinone selectivity and to decrease the optimal molar ratio of vanadium(V) to substrate. The highest yield of the desired quinone (97%) at total substrate conversion was obtained by using the biphasic water-benzene system at molar vanadium(V) to substrate ratio of 12. The temperature of 50 °C and an inert atmosphere were established to be the optimal reaction conditions. The aqueous HPA-10′ solution including the highest content of VO₂⁺ ions proved to be the most efficient catalyst among investigated HPAs. Carrying out catalyst regeneration at a separate stage provides the preservation of its activity and selectivity at the initial level for at least ten cycles.

Full text of DOI:10.1016/j.apcata.2017.09.022

AppliedCatalysisA,General549(2018)216–224
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Feature Article
Catalytic way of transforming 2,3-dimethylphenol to para-quinone with the
use of vanadium-containing heteropoly acids
Yulia A. Rodikova⁎, Elena G. Zhizhina, Zinaida P. Pai
Boreskov Institute of Catalysis SB RAS, prosp. Akad. Lavrentieva 5, 630090 Novosibirsk, Russia
A R T I C L E I N F O
A B S T R A C T
Keywords:
Benzoquinone
Heteropoly acid
Homogeneous catalysis
Selective oxidation
Biphasic system
2,3-Dimethyl-p-benzoquinone (2,3-Me2BQ) is a valuable chemical that is applied as a soft oxidizing and dehy-
drogenating agent and also as a synthon in preparing different complex products including pharmaceutical and
biochemical substances. Keggin- and modified-type aqueous solutions of Mo-V-phosphoric heteropoly acids (Mo-
V-P HPAs) with the gross compositions H3+xPMo12-xVxO40 (HPA-x) and HaPzMoyVx’Ob (HPA-x’), respectively,
possessing high oxidation potential and simplicity of regeneration can serve as effective soft oxidants for ob-
taining such para-quinone from 2,3-dimethylphenol (2,3-Me2P). The synthesized HPA catalysts with different
vanadium content were characterized by a number of analysis techniques, such as 31P and 51V NMR spectro-
scopy, potentiometry, titrimetry, and pH measurement. It was found that the predominant formation of 2,3-
Me2BQ instead of corresponding diphenoquinone (DPQ) at one-electron oxidation is achieved by a consecutive
optimization of reaction conditions, the most important among them being organic solvent and molar ratio of
vanadium(V) to substrate. As was shown, the substitution of HPA-x by HPA-x’ allows one to increase the quinone
selectivity and to decrease the optimal molar ratio of vanadium(V) to substrate. The highest yield of the desired
quinone (97%) at total substrate conversion was obtained by using the biphasic water-benzene system at molar
vanadium(V) to substrate ratio of 12. The temperature of 50 °C and an inert atmosphere were established to be
+
the optimal reaction conditions. The aqueous HPA-10′ solution including the highest content of VO2 ions
proved to be the most efficient catalyst among investigated HPAs. Carrying out catalyst regeneration at a se-
parate stage provides the preservation of its activity and selectivity at the initial level for at least ten cycles.
1. Introduction
reactive C]C and C]O sites in the structure, 2,3-Me2BQ may enter
into diverse reactions including amination, acylation, alkylation and
Developing the processes of industrial multistage low-tonnage pro-
duction of organic substances is an important area which intensifies
nowadays all over the world [1,2]. Being potential products and ‘plat-
form molecules’ in this area, quinones can serve as one- or two-electron
acceptors and also participate in a wide range of biological redox
processes, such as NAD- and FAD-dependent reactions, processes of
photosynthetic and respiratory energy transduction, and in reactions
catalyzed by a group of enzymes designated quinoproteins [3]. Due to
the ability of quinones to transfer both electrons and protons, they also
play an important biochemical role in understanding electron and
proton movement.
2,3-Dimethyl-p-benzoquinone (2,3-Me2BQ) is a valuable chemical
which is applied as a sensitizer in photorefractive systems and am-
perometric sensors and as a soft oxidizing and dehydrogenating agent
[4]. It is as well a very promising substrate for producing various
medical and physiologically active substances. Owing to the presence of
arylation as well as cycloaddition [5]. It allows one to apply this
compound as useful synthon for synthesis of substances with anti-
parasitic [6,7], anticancer [8], and antioxidant [9–11] activities
(Fig. 1). However, under conditions of redox processes, quinones and
hydroquinones have no sufficient stability. As a result, the task of se-
lective synthesis of benzoquinones has certain difficulties and requires
of developing new methods for their preparation including en-
vironmentally safe catalytic technologies.
Selective oxidation of phenols is one of the most valuable reactions
for preparing quinones with a simple structure, and a variety of dif-
ferent methods has emerged to date [4]. Previously, much attention has
been paid on developing effective oxidizing systems on the basis of
metal salts [12,13], transition metal complexes [14,15], oxide catalysts
[16–18], microporous and mesoporous heterogeneous catalysts
[19–24], with some of them showing excellent activity in oxidation of
phenols. However, the insufficient stability and low activity at reusing
⁎
Corresponding author.
E-mail addresses: rodikova@catalysis.ru, Julia.R.mailbox@gmail.com (Y.A. Rodikova).
http://dx.doi.org/10.1016/j.apcata.2017.09.022
Received 21 July 2017; Received in revised form 2 September 2017; Accepted 19 September 2017
Availableonline09October2017
0926-860X/©2017ElsevierB.V.Allrightsreserved.

Y.A. Rodikova et al.
AppliedCatalysisA,General549(2018)216–224
Fig. 1. Examples of valuable compounds obtained from 2,3-
Me2BQ.
along with hard regeneration conditions of these catalysts have limited
their commercialization. The out-of-date processes based on oxidizing
substituted anilines with MnO2/H2SO4 at 10 °C or Na2Cr2O7/H2SO4 at
20 °C are still used for preparing quinones [25]. As a consequence,
further improvement of developed processes or investigation of alter-
native catalysts is required in this area.
realize this reaction as the efficient catalytic one consists in applying
catalysts possessing both a high oxidative ability and a possibility to
rapid electron transfer. The shifting from acidic properties which are
typical for all HPA groups to redox dominance due to the introduction
into molybdenum-oxygen framework of vanadium(V) atoms (VV) al-
lows us to consider the aqueous Mo-V-P HPA solutions with Keggin
Polyoxometalates (POM) are an extensive class of isopoly and het-
eropoly compounds, the properties of which can change broadly de-
pending on elemental composition, the number of atoms in inorganic
framework, relative position of atoms and their binding method, and
also a counterion [26]. As a result, a large number of POMs currently
exists, but only some of them are used in practice [27].
H3+xPMo12-xVxO40 (HPA-x) and modified HaPzMoyVx’Ob (HPA-x’) gross
compositions as promising homogeneous oxidative catalysts that can be
used as soft oxidants for converting various organic compounds.
Previously, we showed that this kind of catalysts exhibits excellent ef-
ficiency in some transformations including oxidation of trimethyl-
phenol [42] and hydrolytic oxidation of cellulose [43]. At that, the
choice of oxidant determines the efficiency and the practicality of
systems, and molecular oxygen is regarded as a ‘greenest oxidant’. This
offers the significant opportunities for clean synthesis of fine and spe-
cial chemicals [44].
This work is aimed at studying the effect of the composition of
aqueous Mo-V-P HPA solutions and their oxidative properties as well
reaction conditions on reaction time and product distribution during
2,3-Me2BQ synthesis by oxidizing 2,3-Me2P in order to obtain the
maximal quinone selectivity and to ascertain the general patterns of this
reaction. The major problem limiting the utility of homogeneous cat-
alytic processes is the well-known difficulty in catalyst recovery and
recycling. In our work we used a biphasic water-organic system that
enables us to overcome the separation problem.
Today, the catalytic processes based on applying heteropoly (HP)
compounds are an actively developing field of investigations [28–31].
Among HP-compounds used in oxidative and/or acid catalysis, the
mixed-addenda Mo-V-phosphoric heteropoly acids (Mo-V-P HPAs,
Fig. 2) attract significant attention as perspective oxidative [32–37] and
bifunctional catalysts [38,39]. The interest of investigating such HP-
compounds in catalysis has considerably increased lately due to the
accumulation of data concerning their properties (especially owing to
information dissemination on their reversible oxidability and simplicity
of regeneration [40,41]). However, the solution both the general pro-
blems of this class of compounds (e.g. stability of catalysts) and the
particular issues of their application for improvement of specific cata-
lytic processes requires of extending investigations in this area.
In this work we investigated the synthesis of 2,3-Me2BQ by oxida-
tion of corresponding 2,3-dimethylphenol (2,3-Me2P). The key to
Fig. 2. Polyhedral (A) and component-bonding (B) models of
α-Keggin
[PM12O40
polyoxoanion
with
general
composition
]
3−, M – structure-forming metal (MoVI, VV).
217

Y.A. Rodikova et al.
AppliedCatalysisA,General549(2018)216–224
2. Experimental
To obtain the reduced HPAs without pollution with extraneous
compounds, prepared solutions in 35–40 mL volume were placed into
temperature-controlled reactor and quantitatively reduced with a 10 M
aqueous solution of hydrazine hydrate N2H4·H2O at 70 °C until nitrogen
evolution ceased. The necessary amount of hydrazine hydrate was
calculated from Eq. (5) subject to total vanadium(V) content. The vo-
lume of nitrogen evolved was determined by a burette. In all experi-
ments, the amount of nitrogen corresponded to the amount of N2H4
added with an accuracy of 1%.
2.1. Chemicals
For synthesis of HPA solutions, 85 wt.% H3PO4 (99.4%), V2O5
(99%), MoO3 (99%), and 30 wt.% H2O2 (special purity) all supplied by
Sibreakhim (Novosibirsk) were used. 2,3-Me2P (99%) was purchased
from Aldrich. 2,3,5,6-Tetramethyl-p-benzoquinone (DQ) was from
Merck-Schuchardt (99.9%) and was used as an internal standard in GC
analysis. All organic solvents (∼99%) were obtained from Sibreakhim
and used as received.
4VO+2 + N2H4 + 4H+ → 4VO2+ + 4H2 O+ N2
(5)
The concentration of vanadium(IV) (VIV) in fresh and regenerated
HPA solutions was determined by potentiometric titration with 0.1 N
KMnO4 in the presence of H3PO4 [47]. The average reduction degree of
the solutions (mred) was calculated according to Eq. (6), where ‘[HPA]’
is the initial concentration of HPA solution.
2.2. Catalyst synthesis
The modified-type aqueous HPA-x’ solutions of compositions
H13P3Mo15V6O74 (HPA-6′), H10P3Mo18V7O84 (HPA-7′), H11P3Mo18V8O87
(HPA-8′), and H17P3Mo16V10O89 (HPA-10′) were prepared by the method
of Odyakov et al. [45]. According to the method, vanadium(V) oxide,
V2O5, was dissolved in cold distilled water, and a cold diluted H2O2 so-
lution was added to the slurry at stirring. Upon gradual heating to room
temperature, the resulting dark-red mixture of peroxo-vanadium com-
pounds spontaneously decomposed with O2 evolving, forming a dark-or-
ange solution of decavanadic acid H6V10O28. The obtained H6V10O28 so-
lution was stabilized by adding diluted H3PO4 solution with forming
H9PV14O42, as detailed in Eqs. (1) and (2).
[VIV
x×[HPA]
]
[VIV
]
mred
=
=
[VIV] + [VV]
(6)
To estimate the thermostability of solutions prepared (the tem-
perature of solution decomposition, Td), the sample of solution (50 mL)
was heated in a thermostatic stainless steel autoclave to 130 °C under
vigorous stirring. After keeping at this temperature for 1 h, the solution
was cooled and filtered. In the absence of precipitation, heating was
repeated with increasing temperature by 5 °C.
In the next step, the prepared H9PV14O42 solution was gradually
introduced into boiling aqueous suspension of MoO3 and H3PO4 under
vigorous stirring (Eq. (3)). The resulting solution after the total
H9PV14O42 addition and complete MoO3 dissolution was evaporated to
required volume, giving homogeneous HPA-x’ solution with a specified
composition. The volume of prepared HPA solutions ranged within
0.25–+0H.3O2 L.
2.4. Catalyst testing
Oxidation of 2,3-Me2P was carried out in a jacketed glass reactor
under vigorous stirring (900 rpm) with a magnetic stir bar in a two-
phase system composed of an aqueous HPA solution and an organic
solvent with a dissolved substrate. In a typical experiment, the substrate
(0.033 ÷ 0.15 g) was dissolved in an organic solvent (OS), and the
resulting solution was introduced (entirely or dropwise) into reactor
with a certain amount of HPA solution. The experiments were per-
formed at different temperatures (25–50 °C) and molar ratio of vana-
dium(V) to substrate ([nVV]/[Su]) in the range of 4–21. The reactor was
equipped with a reflux condenser to avoid the loss of water and organic
solvent during the experiments. For studies under inert atmosphere, air
was substituted by N2 or CO2, and a slow stream of inert gas was kept at
the reactor inlet. In the course of reaction, a small volume of the re-
action solution was periodically sampled, and then the conversion of
2,3-Me2P and the yields (η) (or selectivities (S)) of products were de-
termined by GC analysis. After the reaction was completed, the organic
phase was separated from the catalyst solution in a separating funnel.
The product traces were additionally removed from the aqueous phase
by chloroform extraction (5 mL). Organic phase was washed twice with
water to ensure the total catalyst separation. In some cases the content
of catalyst components (Mo, V, P) in organic solution after catalyst
removing was determined by ICP AES method using a Perkin Elmer
Optima 4300 DV spectrometer. Their amount was within
10−8–10−10 mol L−1. The products were confirmed by comparison of
their GC retention times, GC–MS patterns, and/or infrared spectra with
those of the authentic samples.
2
2
V O5 ⎯⎯⎯⎯⎯⎯⎯→ VO(O2)+ + VO(O2)− ⎯⎯⎯→ H6V O28
2
10
2
(1)
(2)
−O
2
1.4H6V O28 + H3PO4 ⎯⎯→⎯ H9PV O42 + 1.2H2O
10
14
⟵
H3PO4 +
′
′
x
x
⎛
⎜
⎞
⎟
⎛
⎝
⎞
⎠
⎛
⎝
⎞
⎠
yMoO3 +
z−
H9PV O42 + cH2 O→ HaP MoyV Ob
14
z
x′
14
14
⎝
⎠
(3)
To compare the catalytic activity, the low-vanadium Keggin-type
aqueous solutions of H5PMo10V2O40 (HPA-2) and H7PMo8V4O40 (HPA-
4) were also synthesized by a similar technique according to Eq. (4) in
the volumes of 0.25 L [46].
(12−x)MoO3 + 0.5xV O5 + H3PO4 + 0.5xH2 O→ H3+xPMo12−xV O40
2
x
(4)
2.3. Catalyst characterization
The composition of freshly prepared and spent solutions was in-
vestigated by 31P and 51V NMR spectroscopy on a Bruker AVANCE 400
high-resolution NMR spectrometer at 162.0 and 105.24 MHz, respec-
tively, with 85% H3PO4 and VOCl3 as external standards.
Redox potentials (E) and pH values of the aqueous HPA solutions
were measured at room temperature using a pH-meter inoLab pH 730
(WTW, Germany). For pH registering, a combined pH-electrode ‘SenTix
41’ with an embedded temperature detector was used. The pH electrode
was calibrated in the usual manner using buffers with a pH of 1.09
(Hamilton) and 4.01 (WTW). To measure the E values, a combined
platinum electrode ‘SenTix ORP’ with a ceramic diaphragm was ap-
plied. The values of E are given in volts (V) relative to the Normal
Hydrogen Electrode (NHE). Potential constancy of HPA solutions was
2.5. Catalyst regeneration
After the experiments, the spent HPA solution was placed into a
thermostatic stainless steel autoclave equipped with a glass beaker-in-
sert. The Keggin-type HPA-4 solution was regenerated at the tempera-
ture of 140 °C and an oxygen pressure of 2–4 atm stirring for 35–40 min
[48]. The reduced modified-type HPA-x’ solutions and Keggin-type
HPA-2 solution were oxidized at 170 °C for 20 min at the same oxygen
pressure as for HPA-4. The regenerated HPA solutions were used for
repeated synthesis of 2,3-Me2BQ.
attained in 3–4 min with an accuracy of
achieved in 4–5 min accurate within
0.005 V; pH constancy was
0.01 pH units.
218

Y.A. Rodikova et al.
AppliedCatalysisA,General549(2018)216–224
2.6. Analytical methods
Conversion of 2,3-Me2P and product yields (selectivities) were de-
termined by GC analysis using a Hromos GH-1000 chromatograph with
a FID equipped with a St-WAX capillary column. DQ was used as an
internal standard.
GC–MS spectra were recorded on an Agilent Technologies 7000 GC/
MS Triple Quad instrument equipped with a GsBP1-MS capillary
column. In addition, the isolated powdered products were also analyzed
by infrared spectroscopy on a Vector 22 spectrometer using mixtures
with KBr.
3. Results and discussion
3.1. Composition of HPA-x and HPA-x’ solutions
Applying so-called ‘peroxide’ method to synthesize the described
above Mo-V-P HPA solutions has provided the complete dissolution of
the source oxides and the absence of extraneous substances (e.g., Na+
,
Cl−, SO4−) in comparison with other existing methods [49]. At that,
according to the gross-composition of prepared HPA solutions, the in-
creased amount of VV and H3PO4 and also the atomic P/(Mo+V) ratio
above 1/12 are the distinctive features of HPA-x’ as compared to HPA-x.
31P and 51V NMR spectra of freshly prepared Keggin-type HPA-2
and HPA-4 solutions and also of modified-type HPA-7′ and HPA-10′
solutions are shown in Figs. 3 and 4, correspondingly.
As can be seen from the spectra in Figs. 3 and 4, both the HPA-x and
HPA-x’ solutions are the close in composition complex mixtures of in-
dividual heteropoly anions (HP-anions) containing different number of
vanadium atoms.
Fig. 4. 31P and 51V NMR spectra of aqueous modified-type HPA-x’ solutions: (A) 0.25 M
HPA-7′ and (B) 0.25 M HPA-10′. The numbers 1–5 correspond to individual HP-anions
with x of 1–5.
According to the spectra in Fig. 4, the synthesized modified-type
HPA-x’ solutions have composition close to that of HPA-x ones. 31P
NMR spectrum of HPA-7′ solution consists of lines typical of Keggin-
type HP-anions with x of 1–5 and gives an average solution composition
+
close to H2PMo9V3O404− [45]. Besides, there is a broad signal of VO2
cation at 530–560 ppm overlapping with HPA lines in the 51V NMR
spectrum that complicates the precise determination of its concentra-
tion from such spectra. In the spectra of HPA-10′ solution there are the
lines relating to the isomeric HP-anions with x of 2–5 as well a broad
line from VO2+, with the abundance of high-vanadium HP-anions in-
creasing. It allows concluding that HPA-x’ solutions are actually a
mixture of H3PO4 with different Keggin-type acids where the excess of
vanadium(V) is present as VO2+ cations associated with HP-anions and
HzPO4(3−z)−. At that, for modified HPA solutions the 31P NMR lines due
to the individual HP-anions and the 51V NMR lines from these anions
shift downfield and upfield, respectively, relative to the same signals in
aqueous Keggin-type solutions. These shifts are explained by the more
protonation extent of existing in modified solutions HP-anions due to
higher acidity of HPA-x’. In all spectra for every individual HP-anion
except HPA-1 there is a ‘fine structure’ specified by the existence of
positional isomers [51–53].
The aqueous 0.40 M HPA-2 solution is an equilibrium mixture of
4−
individual Keggin-type anions Hx-1PMo12-xVxO40
with x of 1–3. The
0.20 M HPA-4 solution includes the indicated above Keggin-type anions
with x of 1–5. In addition, the 51V and 31P NMR spectra of this solution
contain broad lines at −544.8 and 0.53 ppm that can be attributed to
the weakly bound VVO2 and HzPO4(3−z)−/H3PO4 ions, respectively.
+
These signals are weak, and their labeling becomes complicated due to
overlapping the signals from individual Keggin-type anions. The
broadening of these signals can be associated with exchange reactions
progressing in solution between existing there different ions [50]. In the
+
spectrum of the HPA-2 solution the line from VO2 is very weak and
practically invisible, but its intensity increases with increasing x.
Thus, the prepared HPA-x and HPA-x’ solutions contain the Keggin-
4−
type heteropoly anions Hх-1PMo12-xVxO40
with different number of
vanadium atoms x, cations VO2 and H+, and also anions of phos-
+
phoric acid HzPO4(3−z)−, with the latter getting distinctly visible only
in the spectra of diluted solutions (with concentration of 0.05 mol L−1
)
[45]. Moreover, the strong broadening in the presented NMR spectra of
+
(3−z)−
lines from VO2 and HzPO4
ions allows us to assume the possi-
bility of exchanging between free and bonded states of these ions in
HPA solutions, and also of forming different products of their associa-
tion as outer-sphere complexes with HP-anions.
3.2. Physicochemical properties of HPA solutions
The ability of Mo-V-P HPA solutions to act as oxidants is based on
reversible transformations of vanadium atoms: VV ↔ VIV [54]. The
possibility of partial reducing VV to VIV during V2O5 activation with
H2O2 upon solution synthesis results in necessity to determine the exact
VV content. It was calculated as difference of total vanadium content
(CV) and concentration of VIV (CVIV) with the latter being estimated by
potentiometric titration. Amount of VIV was also used to calculate the
average reduction degree of solutions (mred) after synthesis. The
Fig. 3. 31P and 51V NMR spectra of aqueous Keggin-type HPA-x solutions: (A) 0.40 M
HPA-2 and (B) 0.20 M HPA-4. The numbers 1–5 correspond to individual HP-anions with
x of 1–5.
219

Y.A. Rodikova et al.
AppliedCatalysisA,General549(2018)216–224
Table 1
Physicochemical properties of freshly prepared HPA-x and HPA-x’ solutions.
HPA
CHPA, M
CV, Ma
CVIV, M
CVV, M
E, V
pH
mred·100%, %
Td, °Cb
H5PMo10V2O40
H7PMo8V4O40
0.40
0.20
0.25
0.25
0.25
0.25
0.80
0.80
1.50
1.75
2.00
2.50
0.03
0.03
0.07
0.09
0.11
0.12
0.77
0.77
1.43
1.66
1.89
2.38
1.074
1.033
1.084
1.097
1.104
1.112
−0.19
0.31
−0.32
−0.34
−0.32
−0.31
3.8
3.8
4.7
5.1
5.5
4.8
190
140
185
190
185
180
H13P3Mo15V6O74
H10P3Mo18V7O84
H11P3Mo18V8O87
H17P3Mo16V10O89
a
Total vanadium content that is calculated as (x × CHPA).
Temperature of solution decomposition.
b
obtained data along with some other properties of prepared HPA so-
lutions are presented in Table 1.
Table 2
Effect of solvents on product distribution upon 2,3-Me2P oxidation to 2,3-Me2BQ.a
According to Table 1, a somewhat lower vanadium(V) content
compared with calculated value is observed in prepared solutions. It is an
evidence for partial reduction of VV with H2O2 to VIV, with the VIV
content being less than 6% of the total one. All solutions both Keggin and
modified-type compositions are strong Brønsted acids with pH values
below 1. At that, the acidity of HPA-x’ solutions is higher as compared to
HPA-x that is explained by more content of H3PO4. The excess of H3PO4
seems to exist in modified solutions as partially dissociated anions
#
Solvent
Product selectivity, %
2,3-Me2BQ
DPQ
PPO
1b
2c
3d
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
without OS
Heptane
Cyclohexane
Benzene
Toluene
Trichloromethane
Trichloroethene
1-Pentanol
1-Hexanol
1-Octanol
2-Octanone
3-Hexanone
1-Etoxyoctane
1-Ethoxypentane
Ethyl acetate
Ethyl Benzoate
Hexanoic acid
Heptanoic acid
Octanoic acid
8
9.6
89.4
77.2
74.9
45.6
48.4
56.6
45.4
64.0
63.1
61.4
59.7
62.8
66.7
61.3
57.3
57.1
51.6
50.9
56.7
2.6
13.2
12.0
14.0
14.3
15.1
17.8
23.2
19.6
18.2
19.1
18.6
17.9
20.5
21.6
18.6
20.2
21.5
18.8
13.1
40.4
37.3
28.4
36.8
12.8
17.3
20.4
21.2
18.6
15.4
18.2
21.1
24.3
28.2
27.6
24.5
(3−z)−
+
HzPO4
connected with HP-anions and VO2 ions. It results in
+
stabilizing additional VO2 ions and growing their steadiness to
V2O5·nH2O deposition. As a consequence, such solutions acquire a higher
thermostability (see Td parameter in Table 1) that is of primary im-
portance to achieve minimal mred values after their regeneration [55].
The solutions synthesized also possess oxidation potential values
higher than 1 V. It is an important feature that allows using these HPA
solutions as oxidants. Fig. 5 demonstrates the dependence of E from
vanadium(IV) content for 0.25 M HPA-7′ and 0.20 M HPA-4 solutions.
It is evident from the figure that high vanadium content in HPA-x’ so-
lutions provides a less sharply decrease of E values upon their reduc-
tion. It allows us to consider HPA-x’ as promising oxidants for catalytic
reactions. To obtain the depicted in Fig. 5 curves, the HPA-7′ and HPA-4
solutions were partially reduced with N2H4, measuring the [VIV] con-
tent and oxidation potential after each reduction.
a
Reaction
conditions:
0.25 M
H13P3Mo15V6O74
(HPA-6′)
7.5 mL
(nVV = 1.07·10−2 mol), 2,3-Ме2P 0.066 g, air atmosphere, temperature 25 °С, organic
solvent volume 5.6 mL, molar [nVV]/[2,3-Ме2P] ratio 20, reaction time 65 min. Substrate
conversion is 100%.
b
Substrate conversion is 18%.
Substrate conversion is 93%.
Substrate conversion is 98%.
c
3.3. Effect of reaction conditions on the reaction time and product
distribution
d
Me2BQ. The influence of various reaction parameters, in particular,
organic solvent, temperature, atmosphere (inert or oxygen-containing
one), and molar [nVV]/[2,3-Me2P] ratio was examined in details.
Our initial experiments were focused on optimizing organic solvent
(OS). The investigation of this parameter was performed in the presence
of 0.25 M HPA-6′ solution (H13P3Mo15V6O74) at room temperature. The
results of solvent screening are summarized in Table 2.
The synthesized Mo-V-P HPA-x and HPA-x’ solutions have been
tested as catalysts for oxidation of 2,3-Me2P to corresponding 2,3-
The results presented in Table 2 indicate that organic solvent is an
active reaction member which significantly affects the selectivity of the
desired product. When using a solvent-free system, the reaction hardly
proceeded and led to quinone formation with yield of less than 2%.
Among the solvents examined, benzene and toluene gave the maximal
2,3-Me2BQ selectivity that was higher than 37%. The representatives of
carboxylic acids, esters and ketones showed moderate selectivities. In
contrast, nonpolar alkanes, such as heptane and cyclohexane, were not
suitable for the reaction. It is likely to be explained by their weak co-
ordination to the substrate molecules and the reaction intermediates.
Under the indicated in Table 2 reaction conditions, the preferred for-
mation of 2,2′,3,3′-tetramethyl-4,4′-diphenoquinone (DPQ) was ob-
served in all cases. Low-molecular aromatic polyethers with linear
structure (phenol ether resins, PPO) were also registered as by-products
(Fig. 6).
As shown in Table 2, the desired quinone was significantly obtained
when the reaction was performed using aromatic hydrocarbons and
Fig. 5. Oxidative potential (E) dependence on vanadium(IV) concentration for 0.25 M
HPA-7′ (■) and 0.20 M HPA-4 (▲) solutions.
220

Y.A. Rodikova et al.
AppliedCatalysisA,General549(2018)216–224
Table 3
Effect of reaction atmosphere on product distribution upon 2,3-Me2P oxidation to 2,3-
Me2BQ.a
Atmosphere
Product selectivity, %
2,3-Me2BQ
DPQ
N2
Air
CO2
58.8
54.3
58.7
37.3
39.3
38.2
a
Reaction conditions: 0.20 M H7PMo8V4O40 (HPA-4) 7 mL (nVV = 5.39·10−3 mol),
2,3-Ме2P 0.033 g, 50 °С, benzene 5 mL, molar [nVV]/[2,3-Ме2P] ratio 20, reaction time
40 min. Substrate conversion is 100%.
[56,57], the degree of such dissociation in HPA-1 solution is slight, but
its raising is observed with increasing x value and solution acidity and
decreasing solution concentration. It allows us to suppose that it is
VO2+ ions that take part in oxidation in the first place. The experiments
were carried out in the presence of HPA-6′ solution possessing high
Fig. 6. Products obtained upon 2,3-Me2P oxidation.
+
content of VO2 ions.
halogen-containing alkenes. It can be assumed that the selection of
optimal OS allows one to assure the effective solvation of substrate
molecules and intermediate compounds via donor-acceptor interac-
tions, destroying phenol self-associates and complicating radical re-
combination. At that, the initial 2,3-Me2P is an electron donor, whereas
benzene, toluene, and halogen-containing alkenes are electron accep-
tors. Another important property providing the efficient interaction of
substrate and the optimal OS is their high degree of polarizability. On
the basis of obtained 2,3-Me2BQ selectivity and higher acceptor prop-
erties compared to toluene, benzene was used by us for further ex-
periments as a model solvent. The selection of organic solvents was
based on their low water solubility, boiling temperature above 70 °C,
and stability into acid oxidizing media.
(13−x)H3+xPMo12−xVxVO40 + 12H+ ↔ (12−x)H2+xPMo13−xVxV−1O40
+ 12VVO+2 + H3PO4 + 12H2O
(7)
VVO+2
VIVO2+
+ H2 O↔
+ 2H+ + e
⇅
⇅
Hx+m+3PMo12−xVmIVVxV−mO40 ↔ Hx+m+2PMo12−xVmIV−1VxV−m+1O40 + H+ + e
(8)
Continuing our experiments on optimizing reaction conditions in
the presence of 0.20 M Keggin-type HPA-4 solution, we also ascertained
that application of inert atmosphere has no positive influence on the
reaction time, but it increases the 2,3-Me2BQ selectivity (Table 3). The
oxidation of 2,3-Me2P to 2,3-Me2BQ can be performed in air atmo-
sphere. However, oxygen absence results in raising the quinone se-
lectivity by 4% and also decreases the PPO formation. The observed
effect of inert atmosphere can be explained by the suppression of
forming unstable peroxide compounds arising from the interaction of
intermediate radicals with O2. The decomposition of such compounds
leads to receiving additional oxygen-containing intermediates that can
take part in PPO formation (see Fig. 6, 1). Based on product structures,
here we suppose that the 2,3-Me2P oxidation follows radical me-
chanism with forming several intermediate radicals.
At the next stage we investigated the influence of reaction tem-
perature. Raising the temperature from 25 to 50 °C enabled increasing
the selectivity of desired product from 40 to ∼70% and decreasing the
PPO formation (Fig. 7). It also favored the reduction in reaction time by
about 1.5 times (from 60 to 36 min). We believe that the observed rise
of 2,3-Me2BQ selectivity and the acceleration of the reaction can be
+
attributed to increasing VO2 ions content due to their formation via
Eq. (7) and also raising mobility of these ions. According to the data
Since the vanadium(V) atoms act as oxidation sites in HPA solu-
tions, the predominant formation of 2,3-Me2BQ can be achieved by
using optimal oxidant (VV) to substrate molar ratio. The molar [nVV]/
[2,3-Me2P] ratio was examined in a wide range (from 4 to 21) in the
presence of 0.20 M HPA-4 solution because VV can be a part of both
VVO2 ions and HP-anions (Table 4, Fig. 8).
+
As can be seen from Table 4, the change in indicated ratio affects
significantly both the reaction time and the product distribution. In-
creasing VV content promotes more rapid substrate conversion and fa-
vors higher 2,3-Me2BQ selectivity. When using low vanadium(V) to
substrate molar ratio, the reaction hardly proceeded and gave in-
complete substrate conversion. It allows us to conclude that if catalysts
do not possess the enough amounts of VVO2 ions, the intermediate
+
radicals mainly recombine thus resulting in predominant formation of
by-products. This testifies that the oxidation of 2,3-Me2P in the pre-
sence of HPA solutions passes through stepwise radical mechanism,
with the increase of vanadium content promoting the decrease of ra-
dical recombination [58]. At that, the rapid reduction of presented in
+
solution VVO2 ions to VIVO2+ is expected to result in changing lim-
Fig. 7. Product selectivity dependence on reaction temperature in the presence of
0.25 H13P3Mo15V6O74 (HPA-6′) solution: (■) – 2,3-Me2BQ, (●) – DPQ, (▲) – PPO.
Reaction conditions: catalyst 7.5 mL (nVV = 1.07·10−2 mol), 2,3-Ме2P 0.066 g, air at-
mosphere, benzene 5.5 mL, molar [nVV]/[2,3-Ме2P] ratio 20. Substrate conversion is
100%.
iting stage of reaction. The initial rapid process of electron transfer from
substrate to oxidant (VVO2+) is substituted by slow exchange processes
according to Eq. (8). As a result, incomplete substrate conversion and
predominant DPQ formation are observed in such cases (at molar [nVV]/
221

Y.A. Rodikova et al.
AppliedCatalysisA,General549(2018)216–224
Table 4
Effect of molar [nVV]/[Su] ratio on product distribution upon 2,3-Me2P oxidation to 2,3-Me2BQ.a
#
Vkat, mL
nVV, mol
[nVV]/[Su]
Conversion, %
t, min
Product selectivity, %b
2,3-Me2BQ
DPQ
PPc
1
2
3
4
5
6
7
8
4.2
8.3
0.00321
0.00642
0.00802
0.00963
0.01123
0.01203
0.01284
0.01364
0.01444
0.01524
0.01604
0.01685
4
8
13.4
28.4
39.1
54.3
83.2
91.6
97.3
100
100
100
100
100
75
75
70
70
55
50
50
45
42
42
40
38
8.7
63.2
59.3
54.0
52.7
51.3
46.9
40.8
38.6
38.2
37.7
37.1
36.6
5.1
5.2
4.8
2.1
0.7
0.4
–
–
–
–
–
16.8
24.3
31.6
38.3
45.3
53.2
56.2
57.5
58.1
58.9
59.6
10.4
12.5
14.6
15.6
16.7
17.7
18.8
19.8
20.8
21.9
10
12
14
15
16
17
18
19
20
21
9
10
11
12
–
a
Reaction conditions: 0.20 M H7PMo8V4O40 (HPA-4) solution (cVV = 0.77 mol L−1), 2,3-Ме2P 0.099 g, atm. N2, 50 °С, benzene, organic solvent volume 0.75·Vkat
The remaining content of products corresponds PPO.
.
b
c
2,3-dimethyl-4-(2′,3′-dimethylphenoxy)phenol.
3.4. Effect of HPA composition on the reaction time and product
distribution
As can be seen from Section 3.1, the HPA solutions are complex
equilibrium mixtures of different particles. Taking into account the
decisive influence of catalyst composition on selectivity of desired
quinone and reaction time, we investigated the 2,3-Me2P oxidation in
the presence of six synthesized HPA solutions with different vanadium
content under the obtained optimal conditions. As was noted above, the
vanadium(V) atoms in HPA solutions can exist as VO2+ ions and also be
a part of HP-anions, which affects the observable 2,3-Me2BQ selectivity.
So, the effect of HPA solution composition on the reaction was de-
termined in experiments where the HPA volume was calculated in such
a manner that it provided an equal content of VV atoms in the solutions.
The obtained results of 2,3-Me2BQ selectivity depending on the used
HPA solution are shown in Table 5.
According to Table 5, under identical reaction conditions the reac-
tion time and 2,3-Me2BQ selectivity change considerably by using dif-
ferent HPA solutions, in spite of the equal VV content in solutions and
the close values of their oxidation potentials. Both the selectivity of 2,3-
Me2BQ formation and the reaction rate rise with an increase in the
Fig. 8. Product distribution dependence on molar [nVV]/[Su] ratio in the presence of
0.20 M
H7PMo8V4O40
(HPA-4)
solution.
Reaction
conditions:
catalyst
number of vanadium atoms
2 > HPA-4 > HPA-6′ > HPA-7′ > HPA-8′ > HPA-10′.
x
in HPA gross-composition: HPA-
When
(cVV = 0.77 mol L−1), 2,3-Ме2P 0.099 g, atm. N2, temperature 50 °С, benzene, organic
solvent volume 0.75·Vkat. Products: ● – DPQ, ■ – 2,3-Me2BQ, ♦ – PPO, ▲ – PP.
using the Keggin-type HPA solutions the reaction slowly proceeded and
gave 2,3-Me2BQ with selectivity only about 50%. It can be explained by
the fact that the VV atoms are present in such solutions mainly as a part
of HP-anions (see Fig. 3). On the other hand, when oxidation of 2,3-
Me2P was performed in the presence of HPA-x’ solutions possessing
[2,3-Me2P] ratio ≤ 15, Fig. 8). It can be expected that application of
high-vanadium HPA-x’ solutions will allow one to decrease the optimal
vanadium(V) to 2,3-Me2P molar ratio since such solutions contain more
+
+
VO2 amount. Note that at using molar ratio lesser 16 the small
high content of VO2 ions stabilized by H3PO4, the predominant for-
amounts of 2,3-dimethyl-4-(2′,3′-dimethylphenoxy)phenol (PP) were
detected after experiments (Fig. 6).
mation of desired quinone was observed.
The results obtained let us suppose that during the reaction the
initial electron transfer from the substrate molecules occurs rapidly
Table 5
Effect of HPA composition on 2,3-Me2BQ selectivity and reaction time.a
HPA
CHPA, M
CVV, M
Vkat, mL
Е0, V
Е
fin, Vb
S2,3-Me2BQ, %
t, min
H5PMo10V2O40
H7PMo8V4O40
0.40
0.20
0.25
0.25
0.25
0.25
0.77
0.77
1.43
1.66
1.89
2.38
17.7
17.7
9.5
8.2
7.2
1.074
1.033
1.084
1.097
1.104
1.112
0.738
0.786
0.958
0.973
0.986
0.993
43.5c
56.4
72.2
78.4
83.1
92.1
50
45
36
35
32
32
H13P3Mo15V6O74
H10P3Mo18V7O84
H11P3Mo18V8O87
H17P3Mo16V10O89
5.7
a
b
c
Reaction conditions: HPA solution, nVV = 0.0136 mol, 2,3-Ме2P 0.099 g, atm. N2, 50 °С, benzene, organic solvent volume 0.75·Vkat. Substrate conversion is 100%.
Oxidation potential after reaction.
Substrate conversion is 92.3%.
222

Y.A. Rodikova et al.
AppliedCatalysisA,General549(2018)216–224
Table 6
Many-cycle testing of catalytic system HPA-10′/benzene upon 2,3-Me2P oxidation to 2,3-Me2BQ.a
b
#
[nVV]/[Su]
Vkat, mL
E0, V
рН0
E
fin, V
Method of Su addition
S2,3-Me2BQ, %
t, min
1
2
3
4
5
6
7
8
9
14
13
12
13
12
11
12
12
12
12
7.9
7.3
6.7
7.3
6.7
6.2
6.7
6.7
6.7
6.7
1.051
1.052
1.047
1.049
1.047
1.051
0.985d
0.933e
1.048
1.051
−0.24
−0.26
−0.23
−0.25
−0.23
−0.26
−0.15
−0.01
−0.24
−0.26
0.952
0.946
0.942
0.934
0.926
0.920
0.858
0.806
0.924
0.921
once
once
once
91.8
91.5
90.9
97.8
97.6
96.6
94.1
87.4
97.3
97.4
32
32
34
30
30
30
30
32
30
30
dropwisec
dropwise
dropwise
dropwise
dropwise
dropwise
dropwise
10
a
Reaction conditions: 0.25 M H17P3Mo16V10O89 (HPA-10′) solution, сVV = 2.16 mol L−1, 2,3-Ме2P 0.15 g, atm. N2, 50 °С, benzene, organic solvent volume 0.75·Vkat. Substrate
conversion is 100%.
b
Each cycle contains the oxidation of the substrate, the separation of the catalyst from the reaction products and the regeneration of the catalyst by oxygen.
The dropwise substrate addition was performed during 25 min.
The catalyst was reduced with a 10 M N2H4 solution to mred = 0.308.
The catalyst was reduced with a 10 M N2H4 solution to mred = 0.362.
c
d
e
mainly onto the readily accessible weakly-bound VO2+ ions. As their
concentration decreases, the complex stages of electron exchange
between the VO2+ ions formed during the reaction and VV atoms
present in HPA-ions get limiting. At that, the reaction rate decreases,
and the rate of interaction between the reaction intermediates begins
The 31P NMR spectrum of the regenerated HPA-10′ solution con-
tains groups of lines of the same Keggin-type anions as the freshly
prepared one. This spectrum is shifted by 2.3–3.0 ppm downfield after
reduction of the solution to mred ∼ 0.135 due to an increase of para-
magnetic ion VO2+ concentration and protonation of HP-anions. Under
the optimized reaction conditions, the highest 2,3-Me2BQ selectivity is
97%. The catalyst retains stability and selectivity at least 10 cycles.
Note that in absence of HP-anions the regeneration of VO2+/VO2+
oxidation system does not occur.
+
to exceed the rate of VO2 forming. The best selectivity of 2,3-
Me2BQ in 92% at total substrate conversion was obtained in the
presence of HPA-10′. In this case the formation of PPO was not ob-
served. Note that the final E values of HPA-x’ solutions after reaction
are higher than those of HPA-x. The high Efin values indicate the
possibility of decreasing the optimal [nVV]/Su molar ration below 17
for such solutions.
4. Conclusions
The efficient method of 2,3-Me2BQ synthesis via two-stage oxida-
tion of 2,3-Me2P in the presence of homogeneous Mo-V-P heteropoly
acid catalysts has been demonstrated. The consecutive optimization of
reaction parameters was shown to provide the transition from forming
diphenoquinone that is observed as the primary product to dominating
2,3-Me2BQ. It was found that applying appropriate organic solvent
(aromatic hydrocarbons, halogen-containing alkenes) allows one to
increase considerably the selectivity of the desired product likely due to
the effective solvation of the substrate and the intermediate compounds
via donor-acceptor interactions. Among the solvents examined, in this
paper we used benzene as a model solvent as it showed both high se-
lectivity and acceptor properties. The inert atmosphere was found to
exert no effect on the reaction time, however, it affects favorably the
selectivity of desired quinone.
3.5. Many-cycle catalyst testing
In the final series of experiments, the optimal HPA-10′/benzene
catalytic system was investigated in detail in terms of catalyst stability
and reusability. Some reaction parameters, such as oxidation potential
E, vanadium(V) to substrate molar ratio, and the method of substrate
introducing, were varied during many-cycle testing of the catalyst
(Table 6).
It is well known that the redox potential of HPA solutions E de-
creases upon the substrate oxidation and increases during the catalyst
regeneration. In contrast, the pH value in the course of the same re-
actions goes through the inverse alterations. As can be seen from
Table 6, the regenerated HPA-10′ solution has the decreased E0 and
increased pH values as compared to freshly prepared one (see Section
3.2, Table 1). It is explained by the fact that after the regeneration the
attained value of VIV concentration in catalyst solution (the average
reduction degree of solution mred) is higher compared to that obtained
after synthesis. In regenerated solution the mred value increases from
0.048 to 0.135 (from 4.8 to 13.5%) and remains at this level in the next
cycles. The observed rise in mred has no influence on the selectivity of
desired product since it is compensated by sufficient vanadium(V) to
substrate molar ratio.
The results obtained suggest that VV atoms are an indispensable
component for the phenol oxidation. As shown by the search for optimal
HPA composition with the use of Keggin-type H5PMo10V2O40 and
H7PMo8V4O40, and also modified-type high-vanadium H13P3Mo15V6O74
,
H10P3Mo18V7O84, H11P3Mo18V8O87, and H17P3Mo16V10O89, the highest
values of reaction rate and 2,3-Me2BQ selectivity are observed in the
presence of HPA-10′ solution possessing the largest content of VO2+ ions.
The substitution of HPA-4 solution by HPA-10′ one allows decreasing the
optimal [nVV]/Su molar ratio from 17 to 12. At the temperature of 50 °C
under optimized conditions, 2,3-Me2BQ was produced with 97% se-
lectivity at 100% 2,3-Me2P conversion. Virtually no polymerization of the
substrate occurs at that. The reduced HPA-10′ catalyst can be easily se-
parated, effectively regenerated by O2 and reused at least 10 cycles. The
acid function of HPA catalysts is also important for the total catalytic
process. It provides catalyst stability and allows us to perform rapid HPA-
10′ regeneration at temperatures up to 170 °C. It should be noted that in
our further investigation we will expand the experiments on solvent op-
timization towards composition, structure, ecological compatibility, and
physicochemical properties of different solvents and find the most ap-
propriate solvent taking into account the obtained data.
The results presented in Table 6 show that applying HPA-10′ solu-
tion having the highest content of VO2 ions among the synthesized
+
ones makes it possible to reduce the optimal molar ratio to 12 (or up to
1.5, if to consider it as [HPA]/[Su]). The dropwise substrate addition
provides the increase of 2,3-Me2BQ selectivity at least 6% compared to
the addition at once. The observed rise of product selectivity can be
explained by a significant decrease in instantaneous concentration of
substrate at such method of addition. The reduction of the catalyst
solution before reaction to mred ∼ 0.3 (30%) results in moderate de-
crease of 2,3-Me2BQ selectivity whereas the further reduction decreases
the catalyst activity considerably.
223

Y.A. Rodikova et al.
AppliedCatalysisA,General549(2018)216–224
Polyoxometalates vol. 2, John Wiley & Sons Ltd., Chichester, 2002.
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