Oxidation of 2,3,6-trimethylphenol to 2,3,5-trimethyl-1,4-benzoquinon over binary xerogels TiO2—SiO2 in the three-phase system

Article abstract of DOI:10.1007/s11172-017-1791-y

The technique of synthesis of 2,3,5-trimethyl-1,4-benzoquinon via catalytic oxidation of 2,3,6-trimethylphenol in H₂O₂—H₂O—PhCH₃ system with vibrational stirring is developed. Xerogels TiO₂ and TiO₂—SiO₂ were used as catalysts in the process. The effect of physicochemical properties of xerogels on the course and characteristics of oxidation is studied.

Full text of DOI:10.1007/s11172-017-1791-y

Russian Chemical Bulletin, International Edition, Vol. 66, No. 4, pp. 677—682, April, 2017
677
Oxidation of 2,3,6ꢀtrimethylphenol to 2,3,5ꢀtrimethylꢀ1,4ꢀbenzoquinon
over binary xerogels TiO2—SiO2 in the threeꢀphase system
Yu. V. Mikushina, A. B. Shishmakov, and L. A. Petrov
I. Ya. Postovsky Institute of Organic Synthesis, Ural Branch of the Russian Academy of Sciences,
22/20 ul. S. Kovalevskoy, 620041 Ekaterinburg, Russian Federation
Eꢀmail: Mikushina@ios.uran.ru
The technique of synthesis of 2,3,5ꢀtrimethylꢀ1,4ꢀbenzoquinon via catalytic oxidation of
2,3,6ꢀtrimethylphenol in H2O2—H2O—PhCH3 system with vibrational stirring is developed.
Xerogels TiO2 and TiO2—SiO2 were used as catalysts in the process. The effect of physicoꢀ
chemical properties of xerogels on the course and characteristics of oxidation is studied.
Key words: 2,3,5ꢀtrimethylꢀ1,4ꢀbenzoquinon, synthesis, titanium dioxide, silica dioxide.
The key semi product in vitamin E synthesis is 2,3,5ꢀ
trimethylꢀ1,4ꢀbenzoquinon (TMBQ), and one of the most
suitable method for its production is oxidation of 2,3,6ꢀ
trimethylphenol (TMP) which is an available feedstock.
Hydrogen peroxide is a commonly known and enviroꢀ
mentally friendly oxidizing agent of TMP. In recent years,
a considerable progress has been achieved in developing a
new process of catalytic oxidation of TMP by peroxide in
the presence of TiO2 and binary mesoporous oxides
TiO2—SiO2 of various structure.1—10 It was stated6 that
TMF oxidation proceeds through the Scheme 1.
As it follows from the above reaction scheme,
2,2´,3,3´,5,5´ꢀhexamethylꢀ4,4´ꢀbisphenol (BP), as well as
isomer epoxyderivatives and 2ꢀhydroxyꢀ3,5,6ꢀtrimethylꢀ
1,4ꢀbenzoquinon are registered as by products.
The activity of Tiꢀcontaining catalysts is mainly goverꢀ
ened by their structure, specific surface area, concentraꢀ
tion of oxygenꢀcontaining surface functional groups, and
phase composition.
solution of hydrogen peroxide and a substrate diluted in an
organic solvent miscible with water occur in a liquid phase.
Earlier,11 we elaborated a protocol for the synthesis of
TiO2—SiO2 binary xerogels by joint hydrolysis of aqueous
solution of sodium silicate and tetrabutoxytitanium (TBT).
This method allows us to produce xerogels by relatively
simple method with varying TiO2/SiO2 ratios. The relaꢀ
tionships between the number and the nature of surface
oxygenꢀcontaining groups and composition of the syntheꢀ
sized binary xerogels were estimated using wideꢀline
1H NMR spectroscopy. It was shown that the quantative
ratio of these surface groups and, consequently, acidꢀbase
properties may be altered significantly by varying the maꢀ
terial composition.11
In this work an attempt is made to synthesize TMBQ
using binary xerogels. Oxidation of TMP was conducted
in the system Н2О2—Н2О—PhCH3. In such system, the
target product (TMBQ, toluene layer) may be easily sepaꢀ
rated from the oxidizing agent and the catalyst (aqueous
layer) without extraction procedure. In this case, the reꢀ
action mixture is homogenized by vibrational mixing.
Oxidation of TMP is usually conducted in a liquid
phase heterogeneous reaction system in which aqueous
Scheme 1
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 0677—0682, April, 2017.
1066ꢀ5285/17/6604ꢀ0677 © 2017 Springer Science+Business Media, Inc.

678
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 4, April, 2017
Mikushina et al.
Xꢀray diffraction (XRD) analysis of the samples was carried
out on Rigaku DꢀMAXꢀ2200V diffractometer (Japan) equipped
with a vertical goniometer. The interval of the registrated angles
(2θ) was from 10 to 75° at a scanning rate of 1 deg min–1; CuꢀКα
radiation was used (40 kV, 30 mA, λ = 1.54184 Å). PDFꢀ2 data
base was used for the automatic data search.
Oxidation of TMP was conducted at 20 οС in a cylindrical
glass reactor (10 cm length, 3 cm diameter)equipped with hyꢀ
droseal without thermostating. Reactor was fixed on a rod of
MTA KUTESZ LE309 vibrator in a horizontal plane. A vibraꢀ
tional frequency was 50 Hz. A successful homogenization of the
reaction mixture was reached by moving the vibrator rod which
varied the amplitude of vibration.
A weighed sample of xerogel (0.025—0.327 g) was placed
into the reactor (mox). Then, 2 mL of a 0.005 mol L–1 solution of
TMP in toluene (special purity grade) and 1 mL of an aqueous
solution of hydrogen peroxide with concentration (C(H2O2))
varying from 0.26 to 3.73 mol L–1 were added. Immediately after
that the vibrational mixing was switched on. The reaction was
carried out for 60 min.
The contents of TMP and TMBQ were measures by gas
chromatography (GC) on Shimadzu GC 2010 (Shimadzu, Japan)
instrument equipped with the flame ionization detector
(GCꢀFID) and ZBꢀ5 capillar column (30 m length, 0.25 mm
i.d.). No internal standart was applied. The toluene layer of the
reaction mixture was analyzed. Nitrogen was used as carrier gas
(1.0 mL min–1). The introduced sample volume was 1.0 μL. The
chromatographic peak areas characterizing TMP and TMBQ
were calculated using specialized program. Based on the chroꢀ
matographic data, the calibration curves, which show the deꢀ
pendence of peak area on concentration of TMP and TMBQ,
were plotted.
To exclude movable contacts in the reactor, the entire
reactor was subjected to vibration. It is known that vibraꢀ
tional treatment intensifies heterogeneous processes in
chemical technology due to an increase in the contact
phase surface and a decrease in diffusional limitations
caused by distraction of the interlayer boarder and its turꢀ
bulization. Such activation and massꢀtransfer are espeꢀ
cially effective in the case of performance of multiphase
processes. Moreover, vibrational massꢀtransfer apparatus
are characterized by low metal and energy demands as
well as high productive capacity.12 We applied the indusꢀ
trial vibrational frequency of 50 Hz. A reactor incline of
45° slope of the reactor provides an intensive mixing of the
reaction mass. In future, the vibrational column type reꢀ
actor makes it possible to implement TMBQ synthesis in
a more efficient semiꢀflow regime.12
Thus, the purpose of the present work is to develop a
procedure for TMP oxidizing into TMBQ in the presence
of TiO2 and TiO2—SiO2 catalysts in the reaction system
H2O2—H2O—PhCH3 using vibrational massꢀtransfer. In
addition, it was of interest to investigate how physicoꢀ
chemical properties of xerogels affect the characteristics
of the oxidation.
Experimental
Titanium oxide (TiO2) xerogel was synthesized by hydrolyzꢀ
ing methanol solution of TBT (titanium(IV)butoxide, reagent
grade, 97%, Sigma—Aldrich) with threeꢀfold volumes of water
under vigorous stirring at 20 οС.
The conversion of TMP (Y (%)) was calculated from the
following equation:
To synthesize binary TiO2—SiO2 xerogels with TiO2 content
of 9 (1), 28 (2), 44 (3), and 66 (4) mol.%, methanol solutions of
TBT using 5, 20, 40, and 100 mL of TBT, respectively, were
prepared. The TBT; methanol volume ratio was 1 : 1. An equal
amount (25 mL) of aqueous solution of sodium silicate ("liquid
sodium glass", reagent grade, specification TU 6ꢀ15ꢀ433ꢀ92) and
75 mL of water were poured in every reaction glass. The reaction
mixtures were vigorously stirred to obtain uniform solution of
TBT and sodium silicate and after 2×75 mL of 1.2 M solution of
HCl was added and the stirring continued for 10 min.
To prepare SiO2, 75 mL of water was added to 25 mL of
sodium silicate and stirred to obtain a homogeneous mixture.
Hydrolysis was performed after addition of 75 mL of a 1.2 M
HCl solution under vigorous stirring at 20 οC.
The prepared individual and binary hydrogels were washed
by water and filtered under vacuum to delete NaCl and butanol.
The washed hydrogels were dried for 3 days at ∼20 °C and another
day at 100 °C. The powders were grinded and an aliquot of
<0.05 mm fraction was collected.
Y = (С°TMP – СTMP)•100/С°
,
TMP
where С°
is the starting concentration of TMP in toluene
TMP
(0.005 mol L–1), СTMP is concentration of TMP in toluene deꢀ
termined from chromatographic data after the prescribed reacꢀ
tion time (60 min).
The selectivity to TMBQ (S (%)) was determined using the
following equation
S = CTMBQ•100/Ccalc
,
TMBQ
where Ccalc
is a theoretical concentration of TMBQ in
TMBQ
toluene assuming that all the converted TMP is oxidized to
produce TMBQ, CTMBQ is concentration of TMBQ in toluene
determined from chromatographic data after 60 min of reacꢀ
tion time.
Results and Discussion
The specific surface area (SBET) of the samples was meaꢀ
sured by the multiple point BET method on a SoftSorbiꢀII ver.1.0
device. A special software (SoftSorbiꢀII) from a SORBIꢀМS deꢀ
vise was applied for calculation. Nitrogen gas (special purity
grade, RF state standart GOST 9293ꢀ74) was used as an adsorꢀ
bate. Prior to measurements the studied samples placed in caliꢀ
brated 1 mL ampules were cooled in liquid nitrogen up to 77.4 K.
The total surface was measured in the 4 to 12 m2 range with
a relative error not exceeding 5%.
Figure 1 shows the specific surface area of the oxides as
a function of titanium dioxide content in the binary xeroꢀ
gels. This dependence is described by a curve with a maxiꢀ
mum for the sample 1, which contains 9 mol.% of TiO2.
A similar character of dependence of SBET on xerogel comꢀ
position was observed for TiO2—SiO2 oxides which were
synthesized by joint hydrolysis of TBT and sodium silicate

Oxidation of 2,3,6ꢀtrimethylphenol
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 4, April, 2017
679
Ssp/m2 g–1
I(966)/I(1095)
0.8
400
0.7
0.6
0.5
0.4
0.3
0.2
0.1
350
300
250
200
20
40
60
80 [TiO2] (mol.%)
Fig. 1. Specific surface area (SBET) of the prepared oxides as
a function of titanium dioxide content in binary oxides.
20
40
60 [TiO2] (mol.%)
Fig. 2. Effect of TiO2 content in the binary oxides on the ratio of
the intensity of the absorption bands at 940—966 cm–1 (I(966))
and 995—1095 cm–1 (I(1095)) (based on the data from Ref. 7).
in aqueous or ammoniumꢀaqueous media followed by calꢀ
cination at 850 οС.13,14
In the IR spectra of SiO2 (see Ref. 11), the absorption
band at 1095 cm–1 corresponds to the asymmetric stretchꢀ
ing vibrations of Si—O—Si bond. The absorption band at
966 cm–1 is typical of the symmetric stretching vibrations
of linear Si—O bond. In the spectra of binary xerogels, the
peak at 966 cm–1 is overlapped with the absorption peak
in the region of 940—950 cm–1 which is characteristic of
Ti—O—Si bond. In the IR spectra of TiO2—SiO2 samples
with TiO2 content of 8, 25, 47, 73, and 88 mol.%, the
absorption band at 1095 cm–1 shifts to 1080, 1076, 1000,
995, and 950 cm–1, respectively.11 A decrease in the freꢀ
quency of the absorption band of the Si—O—Si asymmetric
stretching vibrations in the spectra of binary samples indiꢀ
cates distortion of the SiO4 tetrahedra caused by the presꢀ
ence of neighboringatom heaver than silicon. A character
of the frequency shifts of this band gives grounds to sugꢀ
gest that an increase in the TiO2/SiO2 ratio is accompaꢀ
nied by increasing content of Ti—O—Si groupings in the
binary xerogels. Further evidence is seen from the results of
calculations of the ratio of the intensities of the absorption
line at 940—966 cm–1 and the peaks typical of the asymꢀ
metric stretching vibrations of Si—O—Si bonds (Fig. 2).
Xꢀray diffraction analyses (Fig. 3) revealed the presꢀ
ence of finely dispersed phase of titanium dioxide in the
samples 1—4. The formation of Ti—O—Si groupings is
likely to stabilize the indicated TiO2 phase and prevents its
transformation to coarseꢀcrystalline anatase.
SBET for the binary xerogels approach SBET value of the
individual titanium dioxide. This finding is verified by the
results obtained for the calcinated (850 οС) TiO2—SiO2
xerogels with the molar ratio of the components from
0.07 : 0.93 to 0.73 : 0.07 (see Refs 13 and 14). The calciꢀ
nated TiO2—SiO2 xerogels (0.07 : 0.93 and 0.14 : 0.86)
contain the maximum concentration of Ti—O—Si groupꢀ
ings among the studied samples; at the same time,
TiO2—SiO2 (0.07 : 0.93) xerogel is notable for the maxiꢀ
mum value of SBET. An increasing concentration of titaniꢀ
I (arb. units)
14
12
SiO2
10
1
8
2
6
The peak observed in Fig. 1 may be explained by the
formation of silica globules with the distorted lattice reꢀ
sulted from the incorporation of titanium atoms into
the silica dioxide framework. At the same time an amount
of titanium atoms which can be incorporated into
the silica lattice is limited. Since the fraction of titanium
is low it can be uniformly distributed in the silica maꢀ
trix. Further increase in titanium content provokes the
formation of Ti—O—Si bonds on the surface of silica globules
which results in the increased fraction of Si—O(—Ti—
O—Ti—O)n grouping. As a consequence, the values of
3
4
4
2
A
A
TiO2
20
30
40 2θ/deg
Fig. 3. Fragments of the diffractogramms of the samples of SiO2,
TiO2, and binary oxides 1—4; A is anatase phase.

680
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 4, April, 2017
Mikushina et al.
Table 1. Dependence of TMP conversion on the composition of
xerogel at equal content of TiO2 in the reactor (based on the data
from Fig. 4)
um in the material reduces the values of SBET and the
fraction of Ti—O—Si groupings indicating the bond cleavꢀ
age in —Si—O(—Ti—O—Ti—O)n groupings located on
the surface of the globules. The analysis of the XRD reꢀ
sults obtained earlier15 for the xerogels calcinated at 850 °С
shows that calcination of the individual xerogel provides
the formation of the crystalline phase whereas the amorꢀ
phous phase persists in the xerogels with the TiO2—SiO2
molar ratio of 0.07 : 0.93. In the samples of calcinated
TiO2—SiO2 xerogels with the component ratio of above
0.14 : 0.86, the anatase phase was found. Moreover, in the
case of pure TiO2 xerogel, the anatase phase suffers transꢀ
formation into the rutil phase.
Xerogel
TMP conversion (%)
0.025 g TiO2 0.050 g TiO2 0.075 g TiO2 0.100 g TiO2
1
2
3
2
4
3
—
3
—
4
3
4
6
32
29
12
55
65
9
73
73
7
93
75
TiO2
A volcano plot describes the dependence of TMP conꢀ
version on the catalyst loading (mox = 0.025—0.327 g) at
С(H2O2) = 0.57 mol L–1 for the samples 3 and 4, which
contain 44 and 66 mol.% of TiO2, respectively (Fig. 4)..
Maximum TMP conversions of 75 and 12% were achieved
at mox = 0.1 g in the presence of individual TiO2 sample
and the sample containing 44% of TiO2, respectively. The
maximum conversion over the sample 4 containing 66%
of TiO2 was 93% at mox = 0.15 g. Further increase in the
catalyst loading reduces the rate of oxidation reaction,
apparently, because of insufficient mass transfer in the
reaction mixture caused by the limits of the vibrator caꢀ
pacity. Catalytic activities of the samples 1 and 2, with 9
and 28 mol.% of TiO2, respectively, are fairly low and
similar to the activity of SiO2 xerogel. This experimental
fact provides evidence of limited availability of the cataꢀ
lytically active titanium for TMP molecules. The reason
may be that the main portion of titanium in the samples
with low TiO2 content (samples 1 and 2) is concentrated
inside the silica globules.
xerogels of various composition (at С(H2O2) = 0.57 mol L–1).
The activities of pure TiO2 and sample 4 containing
66 mol.% of TiO2 are close. The observed difference may
be caused by the influence that exerts silica component in
the sample comprising 66 mol.% of TiO2 on distribution
of the catalyst in the reactor during the vibrational stirꢀ
ring. Another reason can be a difference in the phase comꢀ
position of titanium dioxide in the referred samples.
Under the applied experimental conditions, the optimal
value of mox, which allows us to properly compare the
activities of all the xerogels in the oxidative process, was 0.1 g.
Figures 5 and 6 exhibit TMP conversion and selectiviꢀ
ty towards TMBQ versus C(H2O2) at mox = 0.1 g.
The conversion of TMF is enhanced over pure TiO2
and sample 4 with 66 mol.% of TiO2 when С(H2O2) inꢀ
creases up to 1.9 mol L–1. With further rise in С(H2O2),
the reaction rate remains nearly constant. In the case of
the sample containing 44 mol.% of TiO2, the conversion
improves linearly with increasing С(H2O2) over the studꢀ
ied concentration range. It is worse noting that an inꢀ
crease in С(H2O2) does not accelerate oxidation of the
substrate over the samples with low TiO2 content.
Based on the data summarized in Table 1 it is possible
to compare the catalytic activities of titanium dioxide in
The effect of С(H2O2) on selectivity of SiO2 and the
samples with low TiO2 concentration (1 and 2) towards
Y (%)
100
Y (%)
4
4
100
80
TiO2
80
TiO2
60
40
20
60
3
40
20
2
SiO2
3
1
1
2
SiO2
C(H2O2)/mol L–1
0.05 0.10 0.15 0.20 0.25 0.30 mox/g
Fig. 4. Conversion of TMP (Y) vs. mox in the presence of SiO2,
TiO2, and samples 1—4; С(H2O2) = 0.57 mol L–1
1
2
3
Fig. 5. Conversion of TMP (Y) vs. C(H2O2) in the presence of
SiO2, TiO2, and samples 1—4; mox = 0.1 g.
.

Oxidation of 2,3,6ꢀtrimethylphenol
Russ. Chem. Bull., Int. Ed., Vol. 66, No. 4, April, 2017
N
681
S (%)
0.7
80
4
2
1
TiO2
70
60
50
40
30
20
10
0.6
0.5
0.4
3
1
2
0.3
SiO2
0.2
3
1
2
3
C(H2O2)/mol L–1
0.1
Fig. 6. Selectivity to TMBQ (S) vs. C(H2O2) in the presence of
SiO2, TiO2, and samples 1—4; mox = 0.1 g.
20
40
30
60 [TiO2] (mol.%)
Fig. 7. The correlation between TiO2content in the binary oxides
and the concentrations of H2O (1), H3O+ (2) and OH– groups in
the oxides (3) (N, per 1 mol of the sample).
TMBQ is described by curves with maxima at 30, 50, and
46%, respectively. In the presence of sample 3 (44 mol.%
of TiO2), a rise in С(H2O2) induces an increase in selecꢀ
tivity over the given range of concentrations. In the presꢀ
ence of highly active samples, the selectivity towards
TMBQ achieves its maximum at С(H2O2) = 0.57 mol L–1
and then experience a slight decline with increasing
С(H2O2).
Thus, the best result on TMBQ synthesis was obtained
when the reaction was conducted in the presence of 0.1 g
of sample 4 at С(H2O2) = 1.9 mol L–1. Under these conꢀ
ditions, conversion approaches 100 % for 60 min, with
71% selectivity towards TMBQ. We should note that there
are only peaks typical of toluene and TMBQ observed on
the chromatograms, since only toluene layer was analyzed.
The byꢀproducts of TMP oxidation could be concentrated
in the aqueous layer. Therefore, the toluene solution of
TMBQ as received may be directed to the following step of
the synthesis of vitamin E, i.e., hydrogenation of TMBQ
to trimethylhydroquinone.
Similar selectivities of TiO2 and sample 4, which conꢀ
tains 66 mol.% of TiO2, make it possible to conclude that
crystalline phase of titanium does not affect the selectiviꢀ
ty, although concentrations of water, hydroxonium ion,
and hydroxyl groups in these samples show considerable
differences (Fig. 7).
High oxidation rate over the sample 4 (see Fig. 5) is
probably caused by the highest fraction of titanium diꢀ
oxide among the synthesized xerogels and, correspondꢀ
ingly, by maximum concentrations of Н2О and Н3О+ on
the surface of this sample (see Fig. 7). Thus, the sample 4
containing 0.075 g of titanium dioxide exhibits higher catꢀ
alytic activity than the sample containing 0.1 g of pure
TiO2 (anatase), albeit the values of SBET of these samples
are equal.
Evidently, the highest activity can be expected from the
binary xerogel with the maximum content of TiO2 is conꢀ
centrated on the surface rather than in the bulk of silica
globules. At high value of SBET and low content of TiO2,
active sites of the catalyst are not available to the contact
with the oxidizing agent and the substrate in the studied
threeꢀphase system.
Based on the present results, the optimal composition
of the binary xerogel for TMP oxidation into TMBQ was
found. For that sample, the TiO2/SiO2 molar ratio was
0.66 : 0.34. A protocol for the synthesis of TMBQ via TMP
oxidation over TiO2—SiO2 xerogel (66 : 34, mol.%) in
Н2О2—Н2О—PhCH3 system was developed. Vibrational
stirring of the reaction mixture was employed in this methꢀ
od. At these conditions, 0.1 g of catalyst and Н2О2 conꢀ
centration of 1.9 mol L–1 provide full conversion of the
substrate for 60 min, and selectivity to TMBQ being 71%.
The produced TMBQ is characterized by the purity grade
of >98% and may be applied in further step of synthesis
of vitamin E without additional purification. It is postuꢀ
lated that an amorphous phase of titanium dioxide in
TiO2—SiO2 with the molar ratio of 0.66 : 0.34 shows higher
catalytic activity than anatase phase of individual TiO2
although specific surface area of the samples are nearly
the same.
This work was financially supported by the Ural Branch
of the Russian Academy of Sciences (Complex Program,
Project No. 15ꢀ21ꢀ3ꢀ6).
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in revised form October 10, 2016