Highly active and green mesostructured titanosilicate catalysts synthesized for selective synthesis of benzoquinones

Article abstract of DOI:10.1039/c4cy00313f

Two-dimensional hexagonal thick-walled mesoporous titanosilicate catalysts synthesized using various amounts of titanium were used for the synthesis of 2,3,5-trimethyl-1,4-benzoquinone (TMBO) by liquid-phase oxidation of 2,3,6-trimethylphenol (TMP-OH). These catalysts were also used for the oxidation of di-/tri-substituted phenols to produce 2,6-disubstituted p-benzoquinones (DSBQs). A promising chemical treatment method, for the preparation of green mesoporous TiSBA-15(6) or Washed TiSBA-15(6), was used for removal of non-framework TiO₂ nanoparticle species from the active surface, and the catalytic activity of the recovered mesoporous TiSBA-15(6) catalyst has been evaluated. To confirm the green aspects, recyclability and hot-catalytic filtration experiments were performed. Based on the experimental results, it was found that the green mesoporous TiSBA-15(6) is a highly active, recyclable, and promising heterogeneous catalyst for the selective synthesis of TMBO and DSBQs and produces >99% TMBO selectivity with 100% TMP-OH conversion at 353 K for 60 min and 90-100% DSBQs selectivity with 83-99% phenol conversion at 330 K for 1-5 h.

Full text of DOI:10.1039/c4cy00313f

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Highly active and green mesostructured titanosilicate
catalysts synthesized for selective synthesis
of benzoquinones†
Cite this: Catal. Sci. Technol., 2014,
4, 2674
M. Selvaraj
Two-dimensional hexagonal thick-walled mesoporous titanosilicate catalysts synthesized using various
amounts of titanium were used for the synthesis of 2,3,5-trimethyl-1,4-benzoquinone (TMBO) by
liquid-phase oxidation of 2,3,6-trimethylphenol (TMP–OH). These catalysts were also used for the oxidation
of di-/tri-substituted phenols to produce 2,6-disubstituted p-benzoquinones (DSBQs). A promising chemical
treatment method, for the preparation of green mesoporous TiSBA-15(6) or Washed TiSBA-15(6), was used
2
for removal of non-framework TiO nanoparticle species from the active surface, and the catalytic activity of
the recovered mesoporous TiSBA-15(6) catalyst has been evaluated. To confirm the green aspects, recy-
clability and hot-catalytic filtration experiments were performed. Based on the experimental results, it
was found that the green mesoporous TiSBA-15(6) is a highly active, recyclable, and promising hetero-
geneous catalyst for the selective synthesis of TMBO and DSBQs and produces >99% TMBO selectivity
with 100% TMP–OH conversion at 353 K for 60 min and 90–100% DSBQs selectivity with 83–99% phenol
conversion at 330 K for 1–5 h.
Received 12th March 2014,
Accepted 21st May 2014
DOI: 10.1039/c4cy00313f
www.rsc.org/catalysis
chlorine-containing by-products can occur and a significant
amount of copper-containing waste is produced when using
1
. Introduction
Quinone derivatives synthesized for the preparation of vita-
mins are of general interest in organic synthetic chemistry
and play an important role in biosystems. 2,3,5-Trimethyl-
high concentrations of CuCl . In this catalytic system, a
2
special apparatus containing corrosion-resistant coatings
might be required. Moreover, the environmentally unfriendly
stoichiometric oxidation of TMP–OH with manganese dioxide
(MnO ), nitric acid (HNO ), or other toxic reagents was
1
1
,4-benzoquinone (TMBO) is a key intermediate in the pro-
1
duction of vitamin E, which is the best fat-soluble biological
antioxidant. The demand for vitamin E is more than 30 000
2
3
1
previously the main industrial route for the production of
3
tons per year.
TMBO. Since 1983, the development of eco-friendly cata-
The industrial synthesis of vitamin E involves the produc-
tion of TMBO via the oxidation of 2,3,6-trimethylphenol
lytic methods for selective oxidation of organic compounds
has been a challenging goal. Many catalytic systems for the
4
(
TMP–OH) with molecular oxygen (or air) over the copper(II)
oxidation of TMP–OH to TMBO have been developed using
‘clean’ and inexpensive oxidants, such as molecular oxygen
and hydrogen peroxide, over homogenous catalysts such as
cobalt–Schiff base complexes, ruthenium salts, and hetero-
chloride (CuCl ) catalyst in various solvents, including two-
phase systems. This catalytic system affords a higher yield of
TMBO (>95%). However, after completion of the catalytic
reaction, this homogeneous catalyst system has well-known
drawbacks related to its separation, recycling and product
purification. Although the use of two-phase solvent systems
partially solves these problems, the product can still be
contaminated with traces of transition metals because of
metal complexation by organics and subsequent extraction
into the organic phase. In addition, the formation of
2
2
4
poly compounds. These catalytic systems still remain prob-
lematic for catalyst separation, and, therefore, the pure target
products could not be completely recovered from hazardous
transition metal compounds.
Since 2000, various titanium-containing silicate catalysts
such as Ti-MMM, Ti-MCM-41, and TiO –SiO have been used
2
2
for TMP–OH oxidation with aqueous hydrogen peroxide
; HP), as the oxidant, for the synthesis of TMBO with
2 2
(H O
5
yields of 80–98%. However, these catalysts (i.e. Ti-MMM,
Ti-MCM-41, and TiO –SiO aerogels) have low hydrolytic
School of Chemical and Biomolecular Engineering, Pusan National University,
Busan 609-735, Korea. E-mail: chems@pnu.edu; Fax: +82 51 512 8563;
Tel: +82 51 510 2397
2
2
stabilities because of their thin walls, and the activity of the
5
catalysts decreases dramatically after the first catalytic run.
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
c4cy00313f
In response to these problems, a few hydrothermally stable
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catalysts have been efficiently used to achieve a higher yield
of TMBO. Recently, mesoporous titanium–silicate catalysts
through the formation of 2,3,5-trimethylcyclohexa-2,5-
dienone radicals by a one-electron oxidation mechanism,
6
5
,6
synthesised by evaporation-induced self-assembly (EISA) pro-
duce a higher TMBO selectivity due to the increase of the
catalytic activity by the highly dispersed dimeric and/or oligo-
meric Ti species, and this catalytic activity is similar to that
of Ti/Ti-MMM-2, which is synthesised using the titanocene
which was first suggested by Kholdeeva's group.
In the
catalytic oxidation of TMP–OH, the TiSBA-15 catalysts pro-
duce TMBO, as a major product, as well as a C–C coupling
dimer, 2,2′,3,3′,5,5′-hexamethyl-4,4′-biphenol (HMBP–OH)
and a C–O coupling dimer, 2,3,6-trimethyl-4-(2,3,6-trimethyl-
phenoxy)phenol (TMPP–OH), as by-products. The order of the
catalytic activities of the catalysts with regards to TMBO selec-
tivity is as follows: TiSBA-15(6) > TiSBA-15(15) > TiSBA-15(20) >
TiMCM-41(40) > TiSBA-15(30) > TiSBA-15(60). TiSBA-15(6)
exhibits the best performance with 99% TMP–OH conver-
sion and 97% TMBO selectivity; its productivity is higher
than those of the other titanosilicate catalysts due to its
higher content of active titanium species on the inner pore
walls of SBA-15. Because the active titanium species on the
inner surfaces of the pore walls form the Lewis acid sites,
which can be confirmed by Fourier-transform infrared (FTIR)
6
a,b
dichloride precursor by the post-grafting method.
Hydrothermally stable and thick-walled mesoporous
titanosilicates (Ti-MMM-2 synthesised by the hydrothermal
method, and TiSBA-15 synthesized using the pH-adjusting
direct hydrothermal (pH-aDH) method) have been extensively
used in the synthesis of vitamin K via liquid-phase oxidation
3
7
,8
of 2-methyl-1-naphthol. However, to the best of our knowl-
edge, there have been no reports on the successful use of two-
dimensional hexagonal thick-walled mesostructured TiSBA-15
synthesized with rich Ti-species for the selective synthesis of
TMBO via liquid-phase oxidation of TMP–OH nor for the
selective synthesis of 2,6-disubstituted p-benzoquinones
6
e
spectroscopy with ammonia as a probe molecule, this results
5
–10
10a
(DSBQs) by oxidation of di-/tri-substituted phenols.
in many accessible active sites.
However, Trukhan et al.
During the 21st century, green catalytic technology for the
reported that the mesoporous TiSBA-15 catalyst synthesized
using sodium silicate solution as the silica source by a direct
acetic hydrothermal method produces only 77% TMBO
selectivity because sodium silicate solution is not favoured to
homogenously distribute the high amounts of active titanium
species on the surface of silica pore walls and does not sig-
nificantly improve the structural properties of TiSBA-15
because the sodium ions increase the basicity of the synthesis
gel. Moreover, the TiSBA-15 catalyst synthesised using TEOS
by the pH-aDH method has higher structural and textural
properties as compared to that synthesised using sodium
catalytic oxidation of phenols to their corresponding qui-
nones in halogen-free reaction systems has become a central
issue in both academia and industry. In our study, TMBO
has been synthesized by catalytic oxidation of TMP–OH over
mesoporous TiSBA-15 catalysts, and the selective synthesis of
TMBO has been conducted under a variety of reaction con-
ditions. The thick-walled mesostructured TiSBA-15 catalysts
have also been used for the synthesis of DSBQs by liquid-
phase oxidation of di-/tri-substituted phenols using HP as the
green oxidizing agent. The catalytic results for the selective
synthesis of TMBO and DSBQs using the mesostructured
TiSBA-15 catalysts synthesised with various amounts of tita-
nium have been correlated and compared.
8
,10a
silicate solution.
In our study, the results of inductively
coupled plasma-atomic emission spectroscopy (ICP-AES),
X-ray diffraction (XRD), N -sorption isotherm analysis, and
2
ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS)
strongly support that TiSBA-15(6) has higher catalytic activity
compared to the other mesoporous titanosilicate catalysts. The
ICP-AES studies show that the silica pore walls of TiSBA-15(6)
contain more titanium species compared to the other TiSBA-15
2
. Results and discussion
2
.1. Catalytic oxidation of TMP–OH with different
TiSBA-15 catalysts
8
29
Calcined mesoporous titanosilicate catalysts, i.e. TiSBA-15(6),
TiSBA-15(15), TiSBA-15(20), TiSBA-15(30), and TiSBA-15(60),
which have nSi/nTi values of 6, 15, 20, 30, and 60, respectively,
were synthesized using the pH-aDH method. Ti-MCM-41(40),
which has a n /n value of 40, was synthesized using a basic
catalysts. In addition, Si magic-angle spinning nuclear
magnetic resonance (MAS-NMR) spectroscopy (Fig. S1†) results
show that the relative signal intensity ratio (Q3/Q4 = 0.48) of
TiSBA-15(6) is much lower than that (Q3/Q4 = 0.8) of siliceous
8
7
SBA-15. This observation clearly supports the stabilization of
Si Ti
9
hydrothermal method. The synthesized mesoporous cata-
the active titanium species by the silanol groups (defect sites),
with huge amounts of Si–O–Ti formation. The UV-vis DRS spec-
troscopy results (Fig. S2†) prove that the calcined TiSBA-15(6)
catalyst contains more titanium hydroxyl moieties (Ti–OH)
on the inner surface of silica pore walls compared to the other
lysts were extensively used in the liquid-phase oxidation of
TMP–OH (Scheme 1). Initially, the catalytic oxidation of TMP–OH
was investigated, using the reaction conditions listed in Table 1,
6
c
TiSBA-15 catalysts. As Kholdeeva's group reported, UV-vis
DRS results further confirm that the high amounts of titanium
hydroxyl moieties can form 5-coordinated doubly bridged
hydroxo–Ti species (double site Ti active centre), as shown in
Scheme 2. Apparently, the large amounts of 5-coordinated Ti
species on the surface of SBA-15 generate its hydrophilic
nature, which is highly favourable to produce a higher
Scheme 1 Oxidation of TMP–OH.
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Table 1 Oxidations of TMP–OH over different TiSBA-15 catalystsa
n
Si/nTi ratio
f
TMP–OH
conversion (%)
TMBO
selectivity (%)
TOF
b
−1
Catalysts
Gel
Calcined
(min )
TiSBA-15(6)
6
18.9
33.4
42.0
59.4
88.6
42.0
21.4
21.4
88.6
—
99
71
60
49
35
97
68
57
45
33
52
97
100
32
3
3.0
3.8
4.0
4.7
4.9
3.7
3.4
3.4
4.9
—
TiSBA-15(15)
TiSBA-15(20)
TiSBA-15(30)
TiSBA-15(60)
TiMCM-41 (40)
TiSBA-15 (6)
TiSBA-15(6)
TiSBA-15 (60)
SiSBA-15
15
20
30
60
40
—
—
—
—
—
—
55
c
100
100
35
10
100
100
d
c
Washed TiSBA-15(6)e
21.4
21.4
>99
100
3.4
3.4
Washed TiSBA-15(6)
a
Reaction conditions: 50 mg of the catalyst; 1 : 3 ratio of TMP–OH to HP (10 mmol TMP–OH; 30 mmol of HP); 15 mL of MeCN; reaction
b
c
temperature, 353 K; reaction time, 60 min. The results of nSi/nTi ratios in the products are determined by ICP-AES. The results were
obtained after the 4th run. The results were obtained after the 7th run. TMP–OH was added to the reaction mixture by three portions
d
e
f
(3.33 mmol per 20 min). TOF = (moles of TMP–OH consumed)/(moles of Ti in the catalyst used × reaction time), determined from the initial
rates of TMP–OH consumption.
some related forms are responsible to achieve high TMBO
selectivity. Moreover, the presence of the “double site Ti active
centre” is required to ensure fast oxidation of key intermedi-
ates like phenoxyl radicals, thus preventing the formation of
dimeric by-products (HMBP–OH and TMPP–OH). As shown
in Table S2,† the similar catalytic activity for mesoporous
Scheme 2 Structure of titanium active sites bonded on the surface of
mesoporous silica.
6
b
Ti/Ti-MMM-2 was found by Kholdeeva's group. With increas-
ing TMBO selectivity, the density of Ti species increases but
the TOF decreases from TiSBA-15(60) to TiSBA-15(6). Hence,
the high Ti density does possibly improve the thick-walled pore
walls and is the key parameter to be carefully controlled
in order to obtain active and highly selective catalysts for
6
c
TMBO selectivity. In addition, the results of N -sorption
2
isotherm (Fig. S3†) and XRD (Fig. S4†) strongly indicate that
the physicochemical parameters (i.e. unit cell size, pore size,
surface area, pore volume, and wall thickness) of the TiSBA-
6
b
1
5(6) catalyst contribute to the improvement of its catalytic
TMP–OH oxidation. This reaction was also performed
using SiSBA-15 synthesized by the pH-aDH method. In this
case, only 10% TMP–OH conversion with very low TMBO
selectivity is observed (Table 1), indicating that the higher
catalytic activity is only obtained when the active titanium
species are incorporated into SBA-15. The catalytic activities
show that the hexagonal mesoporous TiSBA-15(6) catalyst with
thick pore walls plays an important role in the production of
8
activity in this liquid-phase catalytic oxidation. The elemental
compositions and the structural and textural properties of the
TiSBA-15 catalysts are listed in Table S1.† Compared to the cata-
lytic performance of various wall-natured mesoporous catalysts
with similar amounts of titanium, the TiSBA-15(20) catalyst has
higher catalytic activity compared to the Ti-MCM-41(40) cata-
9
,10b
lyst due to thicker pore walls.
is reported for mesoporous Ti-MMM-2.
The similar catalytic activity
6
b,c
6b
The above catalytic
TMBO with a higher TMBO selectivity.
results show that the thick pore walls of mesoporous
titanosilicate catalysts improve hydrolytic stabilities and cata-
lytic activities.
8
2.2. Performance of recyclable and Washed TiSBA-15(6)
The turnover frequency (TOF) of TiSBA-15 catalysts used in
the oxidation of TMP–OH was evaluated from the initial rates
of TMP–OH consumption. Although the TOF of TiSBA-15(6) is
much lower than that of other TiSBA-15 catalysts (Table 1),
the thick-walled TiSBA-15(6) produces higher TMP–OH con-
version (99%) and TMBO selectivity (97%) compared to
other TiSBA-15 catalysts (Table 1). Based on the experimental
results, it was found that TiSBA-15(6) has higher Ti(IV) dimers
compared to other TiSBA-15 catalysts, and the Ti(IV) dimers
are connected to the surface of silica pore walls through
Among the two-dimensional mesostructured titanosilicate
catalysts, TiSBA-15(6) and TiSBA-15(60) produce the highest
and lowest TMBO selectivity, respectively. Therefore, we have
investigated the reusability of TiSBA-15(6) and TiSBA-15(60)
under the reaction conditions listed in Table 1 to determine
their catalytic stabilities. Initially, the used TiSBA-15 catalysts
usually afford lower product selectivity because most of the
active sites are screened by unreacted organics, which create
the sintering effect on the catalyst surface; accordingly, the
catalysts have been regenerated by washing and calcination
before reuse. The used TiSBA-15 catalysts were washed four
times with acetone (ACO) and dried at 393 K overnight.
6
b,c
OH bridges, as confirmed by UV-DRS.
Therefore, the
5
-coordinated doubly bridged hydroxo–Ti species and
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Finally, the catalysts were calcined at 773 K for 6 h in air
to completely remove the organics and unreacted TMP–OH
molecules. The recovered recyclable and active catalysts, i.e.
TiSBA-15(6) and TiSBA-15(60), were reused for this catalytic
reaction (Table 1). The recyclability experiments were also
performed for each run before initiation of the catalytic reac-
tion. A decrease of TMBO selectivity from the first run to the
third run is observed for TiSBA-15(6) and after the 4th run, the
selectivity increases from 97 to 100%, and the rate of reaction
portions, as shown in Table 1. Furthermore, the XRD,
N -sorption isotherm, and UV-vis DRS results prove that
2
the structural and textural parameters, including the
“double site Ti active centre” coordinated to the surface of
Washed TiSBA-15(6), are maintained (Table S1).† As shown
in Table S2,† the catalytic activity of Washed TiSBA-15(6) is very
6
a
close to that of the mesoporous titanium–silicate catalyst.
The recycling results show that Washed TiSBA-15(6) is
unprecedented as a green catalyst among the calcined
mesoporous titanosilicate catalysts.
(
TOF) stabilizes. It is worth noting from the catalytic results
that the non-framework and inactive TiO nanoparticles, which
2
cause the screening effect on the active sites of the catalyst, are
completely removed. Moreover, the TMP–OH conversion and
TMBO selectivity remain constant from the 4th run to the
2
.3. Effect of reaction parameters
The Washed TiSBA-15(6) catalyst is the most active catalyst
among all of the calcined TiSBA-15 catalysts and used to
determine the optimal reaction parameters, i.e. temperature,
time, and reactant ratio. TMP–OH was oxidized at different
reaction temperatures and for different durations under the
reaction conditions listed in Fig. 1 and 2. When the reaction
temperatures and times are correspondingly increased from
303 to 353 K for 20 to 60 min, a high TMP–OH consumption
as well as high TMBO formation is observed with 100%
TMBO selectivity. With further increasing the temperatures
from 353 to 378 K, the TMP–OH conversion as well as
TMBO selectivity does not change, but the selectivities for
the by-products, HMBP–OH and TMPP–OH, increase with
decreasing temperature from 353 to 318 K because the
Lewis acid sites on the surface of the catalyst favour the
formation of HMBP–OH and TMPP–OH under these reaction
conditions. The catalytic results obtained using different tem-
peratures clearly show that the selectivity for the by-products
7
th run, as shown in Table 1, indicating that the leaching of
active titanium species from the mesoporous matrix is not
further observed; this is in good agreement with the ICP-AES
results for the filtrate solutions, in which the absence of active
titanium species is found. Our catalytic results clearly show
that the non-framework TiO nanoparticles could not be
2
further enhanced to achieve higher TMBO selectivity because
the surface area of TiSBA-15(6) is three times higher than that
5
,6,8
of TiO
2
nanoparticles.
Before using TiSBA-15(6) for the
oxidation of TMP–OH, the catalyst was washed more than four
times with acetone and dried at 393 K overnight. Finally
the treated catalyst was calcined at 773 K for 6 h in air to
2
completely remove the non-framework TiO nanoparticles
generating “Washed TiSBA-15(6)”, which was used in this
reaction as the green mesoporous TiSBA-15(6) catalyst to
determine its catalytic stability. The catalytic activity of
Washed TiSBA-15(6) is nearly the same as that of the recyclable
TiSBA-15(6) catalyst used in the 7th run, as shown in Table 1.
Moreover, the thick-walled silica mesopore improves stability
6
c
increases with decreasing temperature. The TOF increases
with increasing temperature from 303 K to 353 K. After that,
6
a–c
and prevents deactivation.
The UV-DRS spectra at the
2
50–270 nm region (Fig. S2†) show that both catalysts, i.e.
recyclable TiSBA-15(6) used in the 7th run and Washed
TiSBA-15(6), have the same amounts of active titanium
species, as confirmed by ICP-AES. These catalytic results
prove that the active titanium species could not further leach
from the surface of the SBA-15 matrix. Additionally, ICP-AES
analysis confirms the absence of titanium species. After
obtaining the results for the three runs, hot-catalyst filtration
experiments were performed twice at 353 K during the oxida-
tion of TMP–OH over calcined TiSBA-15(6). In this case,
the filtrate solutions contain a very small amount of the
conversion product (~7%), which may have formed by HP;
this confirms that the oxidation of TMP–OH occurs only on
the surface of the green TiSBA-15(6) catalyst. Therefore,
Washed TiSBA-15(6) has a higher catalytic activity and
6
,11
results in a green heterogeneous catalytic process.
Moreover, the ICP-AES results shown in Table S1† clearly
confirm that the few non-framework TiO nanoparticles were
2
completely removed by the washing treatments; similar
results were also observed for hot-catalyst filtrate solution of
TiSBA-15(6). Moreover, 100% completion of this catalytic
reaction is achieved when the TMP–OH is added in three
Fig. 1 Oxidations of TMP–OH at different temperatures over Washed
TiSBA-15(6). Reaction conditions: 50 mg of the catalyst; 1 : 3 ratio of
TMP–OH : HP (10 mmol TMP–OH, 30 mmol of HP); 15 mL of MeCN;
reaction time, 60 min.
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the TOF is still maintained (Fig. 1) when the temperature
increases from 353 K to 383 K. It is worth noting from the
results that the TMP–OH conversion as well as TMBO selec-
tivity is not affected even when the reaction is carried out
with a higher reaction temperature (383 K). When this liquid-
phase catalytic reaction is performed using a 1 : 3 ratio of
TMP–OH to HP under the reaction conditions listed in Fig. 3,
Washed TiSBA-15(6) gives a higher TMBO selectivity, and
−
1
the reaction rate (TOF = 3.4 min ) for the ratio of 1 : 3 is
higher as compared to other ratios because there is no deacti-
vation. When the amounts of HP (TMP–OH to HP ratios, 1 : 2,
1
: 5 and 1 : 7) are increased, the conversion as well as selectivity
is much lower than that of 1 : 3 ratio of TMP–OH to HP, due to
the formation of by-products HMBP–OH and TMPP–OH
(
Fig. 3). The diffusion rate of Washed TiSBA-15(6) as well as
TMP–H conversion decreases for ratios 1 : 1 and 2 : 5. The
TMBO selectivity does not also improve because the active
sites may be partially screened by unreacted organics deposited
on the inner pore walls. When this reaction is performed over
Washed TiSBA-15(6) using tert-butyl hydroperoxide (TBHP) as
the oxidizing agent instead of HP at the optimal temperatures,
times, and reaction ratios listed in Table 2, the TMBO selec-
tivity gradually decreases due to increasing selectivity for
HMBP–OH and TMPP–OH. However, as shown in Table 2, the
TMBO selectivity decreases almost threefold when molecular
oxygen (10 bar) is used as the oxidizing agent instead of HP
under the same reaction conditions because the active tita-
nium species may be inactive in molecular oxygen. When
pseudocumene is used instead of TMP–OH with urea hydrogen
peroxide (UHP) as the oxidizing agent and MeOH as the solvent
for longer reaction times (Table 2), the TMBO selectivity
decreases significantly but the selectivity for HMBP–OH
increases. In addition, a very small quantity of the 2,3,5-
Fig. 2 Oxidations of TMP–OH for different times over Washed TiSBA-15(6).
Reaction conditions: 50 mg of the catalyst; 1 : 3 ratio of TMP–OH : HP
(
10 mmol TMP–OH, 30 mmol of HP); 15 mL of MeCN; reaction
temperature, 353 K.
1
0c,12
trimethylhydroquinone by-product is formed.
From this
evidence, we found that the catalytic activity is not affected by
the water molecules of HP while the water molecules lead the
1
3
catalytic activity in this system. Comparing the catalytic
results obtained over Washed TiSBA-15(6) using different
reaction parameters, it is found that the excellent TMBO
selectivity is achieved via a reaction at 353 K for 60 min using
a 1 : 3 ratio of TMP–OH to HP.
Fig. 3 Various ratios of TMP–OH : HP over Washed TiSBA-15(6).
Reaction conditions: 50 mg of the catalyst; 10 mL of MeCN; reaction
temperature, 353 K; reaction time, 60 min.
Table 2 Oxidations of TMP–OH by TBHP over Washed TiSBA-15(6)a
f
−1
Temperature (K)
Time (min)
TMP–OH conversion (%)
TMBO selectivity (%)
TOF (min
)
298
333
353
353
353
333
333
30
30
30
60
60
35
44
46
95
96
25
32
39
42
81
78
31
61
2.4
3.0
3.1
3.2
3.3
0.8
0.6
b
c
60
180
d
e
55
a
b
Reaction conditions: 50 mg of Washed TiSBA-15(6); 1 : 3 ratio of TMP–OH to TBHP (10 mmol DTBP–OH; 30 mmol of TBHP); 15 mL of MeCN.
c
d
1
: 6 ratio of TBP–OH to TBHP (10 mmol DTBP–OH; 60 mmol of TBHP).
O
2
(10 bar) was used as the oxidizing agent. 1,2,4-
e
Trimethylbenzene was used instead of TMP–OH with urea hydrogen peroxide as the oxidizing agent and MeOH as the solvent. Conversion of
1
f
,2,4-trimethylbenzene. TOF = (moles of TMP–OH consumed)/(moles of Ti in the catalyst used × reaction time), determined from the initial
rates of TMP–OH consumption.
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2
.4. Effect of solvents
MeCN, as shown in Table 3. This is confirmed experimentally
when the same solvents were used at either higher or lower
The liquid-phase oxidation of TMP–OH was performed using a
variety of solvents, i.e. ACO, acetonitrile (MeCN), metha-
nol (MeOH), acetic acid (MeCOOH), ethanol, acetic anhydride
5
,6,13
reaction temperatures, as reported in the literature.
To
confirm the effect of the solvent on the catalytic activity, the
catalytic reaction is performed without a solvent at 353 K for
(
2
AC O), and nitromethane over Washed TiSBA-15(6), under
6
0 min, in which low TMP–OH conversion (21%) and low
the reaction conditions listed in Table 3 to achieve a higher
TMBO selectivity. MeCN is a common polar aprotic solvent
that can form complexes on the catalyst surface at ambient
temperature (353 K) and preferentially enables attack on the
active sites to achieve maximum TMP–OH conversion. When
this reaction is carried out with 5 mL of MeCN under similar
reaction conditions, the TMP–OH conversion (48%) and
TMBO selectivity (54%) decrease because the large amount
of TMP–OH may not completely dissolve to react with HP.
However, the conversion and selectivity were unaffected when
the reaction was carried out using 30 mL of MeCN under the
same conditions. When pseudocumene is used instead of
TMBO selectivity (28%) are obtained. A comparison of the
catalytic activities obtained with different solvents shows that
unsuitable solvents could not show high catalytic activities,
could not easily form complexes between the reactants and
the active surface of the catalyst, and may block most of the
8
,11,15,16
active sites on the catalyst surface.
MeCN is, there-
fore, found to be the best solvent under these reaction condi-
tions for the highly selective synthesis of TMBO.
2.5. Selective synthesis of DSBQs
To explore the scope and limitations of this liquid-phase
catalytic system, the mesoporous titanosilicate catalysts, i.e.
calcined TiSBA-15(6), recyclable TiSBA-15(6), and Washed
TiSBA-15(6), were used for the heterogeneous catalytic oxidation
of di-/tri-substituted phenols under the reaction conditions
listed in Table 4. When the calcined TiSBA-15(6) catalyst is
used to catalyse the oxidations of para-substituted 2,6-di-tert-
butylphenols (DTBP–OHs) such as 2,6-di-tert-butyl-4-methoxy-
TMP–OH with acetic anhydride (AC O) as the solvent, the for-
2
mation of TMBO is obtained with 78% selectivity. Almost a
similar result is confirmed when TMP–OH was catalyzed with
MeCOOH as the solvent at 353 K; however, the HMBP–OH
2
selectivity in MeCOOH at 330 K is higher than that in AC O.
Nitromethane is a slightly viscous and highly polar solvent
and yields 83% TMP–OH conversion with 90% TMBO selec-
tivity. But, under the same reaction conditions, the TMBO
selectivity is 70% in nitromethane and 89% in MeCN when
phenol (DTPB–OCH ) and 3,5-di-tert-butyl-4-hydroxybenzyl
3
acetate, the formation of di-tert-butyl-p-benzoquinone
2
,3,5-trimethlyphenol is used instead of TMP–OH; it is even-
(DTBBQO) with higher selectivity is observed after 5 h
when the elimination group is eliminated from the para posi-
tion (Table 4, entries 1 and 2). However, the catalyst is used
to catalyse the oxidation of DTBP–OH for 3 h (Table 4) and
gives the excellent catalytic activity with 100% DTBBQO
selectivity. 1,4-Diketones are easily generated on the aromatic
rings during these oxidations, and the elimination groups
are easily eliminated from the para positions of DTBP–OHs,
tually found that nitromethane has a much lower ability to
attack the active sites compared to MeCN. Compared to all of
the solvents tested, ethanol affords the lowest TMBO selec-
tivity. Although MeOH affords a high TMBO selectivity, it
is highly toxic and environmentally unsustainable. The order
of the dielectric constants of the solvents is nitromethane >
MeCN > MeOH > ethanol > ACO > AC
of the solvents, i.e. MeOH, MeCOOH, ACO, AC
nitromethane, afford lower TMBO selectivity compared to
1
4
2
O > MeCOOH. All
O, and
1
7
2
as experimentally confirmed elsewhere. The formation of
DSBQs depends on alkyl groups, which are electron-donating.
Table 3 Oxidations of TMP–OH in different solvents over Washed TiSBA-15(6)a
h
−1
Temperature (K)
Solvent (15 ml)
Dielectric constant
TMP–OH conversion (%)
TMBO selectivity (%)
TOF (min
)
353
333
330
330
353
348
353
353
333
353
353
353
353
MeCN
MeOH
ACO
37.5
33
21
6.2
6.2
24.3
37.5
37.5
21.0
—
39.4
37.5
39.4
100
91
81
43
75
84
48
100
70
21
61
90
83
>99
95
87
53
83
45
54
100
78
28
70
89
90
3.4
3.1
2.7
1.4
2.5
2.8
1.6
3.4
2.3
0.7
2.1
3.1
2.8
MeCOOH
MeCOOH
Ethanol
b
MeCNc
MeCN
d
e
2
Ac O
Solvent free
f
g
Nitromethane
f
g
MeCN
Nitromethane
a
b
f
Reaction conditions: 50 mg of Washed TiSBA-15(6); 1 : 3 ratio of TMP–OH to HP (10 mmol TMP–OH; 30 mmol of HP); reaction time, 60 min.
c
d
e
5
ml of the solvent. 30 ml of the solvent. 1,2,4-Trimetylbenzene was used instead of TMP–OH. Conversion of 1,2,4-trimethylbenzene.
g h
2,3,5-Trimethylphenol was used instead of TMP–OH. Conversion of 2,3,5-trimethylphenol. TOF = (moles of TMP–OH consumed)/(moles of
Ti in the catalyst used × reaction time), determined from the initial rates of TMP–OH consumption.
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Table 4 Oxidations of di/tri-substituted phenols over calcined, recyclable, and Washed TiSBA-15(6)a
g
−1
Entry
1
Time (h)
Substrate
Major product
Selectivity (%)
Conversion (%)
TOF (h
)
b
5
5
96
98
98
90
94
94
33
38
38
c
d
5
b
2
5
5
5
85
90
90
78
83
83
28
34
34
c
d
e
b
3
2
2
2
99
100
100
97
99
99
89
101
101
c
d
b
4
5
6
3
3
3
99
100
100
95
99
99
57
68
68
c
d
b
2
2
2
98
100
100
93
97
97
84
99
99
c
d
b
1
1
1
98
100
100
94
97
97
170
198
198
c
d
f
b
7
1
1
1
97
100
100
95
98
98
171
200
200
c
d
f
b
8
1
1
1
98
100
100
96
99
99
173
203
203
c
d
a
Reaction conditions: 50 mg of the catalyst; 10 mmol of the substrate; 30 mmol of HP; 10 mL of MeCN; reaction temperature, 330 K.
b
c
d
e
Calcined TiSBA-15(6). Washed TiSBA-15(6). The results were obtained after the 7th run, using recyclable TiSBA-15(6). 10 mL of hexane
f
g
was used as the solvent. A mixture of solvents (6 mL of dioxane : 4 mL of H
2
O). TOF = (moles of substrate consumed)/(moles of Ti in the
catalyst used × reaction time), determined from the initial rates of TMP–OH consumption.
At longer reaction times, a higher DTBBQO selectivity is
obtained; however, higher selectivities for 2,6-diisopropyl-p-
benzoquinone (DIPBQ) and 2,6-dimethyl-p-benzoquinone
(TMDPQ),
and
3,5,3′,5′-tetramethoxydiphenoquinone
(TMoDPQ), are formed with lower selectivities (1–16%).
Washed TiSBA-15(6) forms larger amounts of DSBQs com-
pared to calcined TiSBA-15(6), as shown in Table 4, because
(
DMBQ) are obtained at low reaction times (Table 4, entries
–6) because of steric effects, which decrease in the order
t-butyl > isopropyl > methyl; a similar trend was previously
4
2
the small number of TiO nanoparticles deposited on the cat-
alyst surface can be completely removed. Recyclable TiSBA-
15(6) has also the similar catalytic activity, as shown in
Table 4. The TOF values for recyclable TiSBA-15(6) and
Washed TiSBA-15(6) are higher than that for calcined TiSBA-
15(6). Hot-catalyst filtration experiments were also performed
twice at specific temperatures and times (not shown in
Table 4) during these oxidation reactions over calcined
TiSBA-15(6). The filtrate solutions contain very small
1
7
reported.
,6-disubstituted hydroquinones show the excellent selectiv-
ities for DSBQs (Table 4, entries 3, 7, and 8); however,
,6-disubstituted hydroquinones cost approximately four
At short reaction times, the oxidations of
2
2
times higher than 2,6-disubstituted phenols. Moreover,
the coupling products, i.e. TTBDPQ, 3,5,3′,5′-tetraisopropyl-
diphenoquinone (TIPDPQ), 3,5,3′,5′-tetramethyldiphenoquinone
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2
9
amounts of the conversion product (2–7%), which may be
formed via HP oxidation; this again confirms that the
Washed TiSBA-15(6) catalyst achieves, through a green
heterogeneous process, a higher catalytic activity for the
confirmed by UV-vis DRS and Si MAS NMR, interact with
H O to afford a titanium peroxide complex (1). Concurrently,
2
2
TMP–OH reacts with the titanium peroxide complex to form
the 2,3,5-trimethylcyclohexa-2,5-dienone radical (2 and 3),
which reacts with another titanium peroxide complex (4)
to give 2,3,5-trimethyl-4-hydroxycyclohexa-2,5-dienone with a
titanium–oxo radical complex (Ti–O radicals). 2,3,5-Trimethyl-
4-hydroxycyclohexa-2,5-dienone rearranges with titanium–oxo
radicals to produce the desired product, i.e. TMBO (Scheme 3),
with the regeneration of the “double site Ti active centre” (1a).
In addition, the reaction mechanism is significantly depen-
dent on the nature of the catalyst. Based on the previously
published literature and our results obtained from several
5
,6,11,16
selective synthesis of DSBQs.
Overall, the results show
that Washed TiSBA-15(6) is a very active, recyclable, and
unprecedented heterogeneous catalyst for the highly selec-
tive synthesis of DSBQs. A comparison of the results
obtained here with those reported in the literature shows
that the Washed TiSBA-15(6) catalyst has exceptional cata-
lytic activity for the oxidation of DSBQs compared to other
solid catalysts.
5
,6,10,16b,18–20
oxidation reactions in other studies,
it is evident
2
.6. Mechanism of TMP–OH oxidation
that a homolytic mechanism most likely operates in the
oxidation of alkane, alkyl aromatics, and di-/tri-substituted
phenols.
Generally, the oxidation of di-/tri-substituted phenols with a
suitable oxidizing agent and a perfect miscible solvent pro-
ceeds by a one-electron oxidation mechanism, which was first
5
,6,19
suggested by Kholdeeva's group.
Accordingly, we have
3
. Experimental
proposed a hypothetical reaction pathway for liquid-phase
3.1. Materials
oxidation of TMP–OH by H O in the presence of MeCN cata-
2
2
lyzed with Washed TiSBA-15(6) catalysts at 353 K for 60 min,
as shown in Scheme 3. Initially, the 5-coordinated doubly
bridged hydroxo–Ti species (double site Ti active centre), as
For the preparation of mesoporous titanosilicate catalysts, all
chemicals, i.e. triblock copolymer poly(ethylene glycol)-block-
poly(propylene glycol)-block-poly(ethylene glycol) (Pluronic
Scheme 3 Proposed reaction pathway of TMP–OH oxidation over Washed TiSBA-15(6).
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P123; molecular weight = 5800, EO PO EO ), tetraethyl-
using ICP-AES, XRD, N -sorption isotherm analysis, and
UV-vis DRS, based on published procedures.
2
0
70
20
2
9
orthosilicate (TEOS; 98%), hydrochloric acid (HCl; 37%), and
titanium isopropoxide (TIP; 99.9%), were purchased from
Aldrich Chemical Inc. and used as received without further
purification. Deionized water was used throughout the syn-
theses. For the selective synthesis of p-benzoquinones, all
chemicals, i.e. TMP–OH (≥97%), 2,3,5-trimethylphenol
3.5. Oxidation of TMP–OH
The liquid-phase oxidation of TMP–OH to TMBO was
performed under vigorous stirring in a thermostated quartz-
vessel reactor. In a typical experimental procedure, 50 mg of
TiSBA-15(6) was placed in the reactor with 10 mmol of
TMP–OH and 15 mL of MeCN as the solvent. The mixture
was stirred constantly while the temperature was slowly
raised to 353 K. After reaching 353 K, 30 mmol of HP was
carefully added to the reaction mixture through a septum
and the mixture was continuously refluxed for 60 min under
atmospheric pressure. After completion of the reaction, the
products were collected through the recovery of TiSBA-15(6).
To find the best catalyst, various mesostructured TiSBA-15
catalysts were used for the oxidation of TMP–OH under similar
reaction conditions. Various reaction parameters, i.e. the
time, temperature, and stoichiometric ratio of the reactants
(
(
2
99%), pseudocumene (98%), DTBP–OH (99%), DTBP–OCH
3
97%), 2,6-diisopropylphenol (97%), 2,6-dimethylphenol (99%),
,6-dimethylhydroquinone (>97%), 2,6-dimethoxyhydroquinone
>97%), 2,6-di-tert-butylhydroquinone, HP (30% in H O), TBHP
(
(
2
2
70% in H O), UHP (97%), MeCN (99.5%), ACO (99.5%,),
MeOH (99.8%), MeCOOH (99.7%), ethanol (≥99.5%), acetic
anhydride (≥98%), nitromethane (≥95%), hexane (99%), and
dioxane (>99%), were purchased from Sigma-Aldrich Inc.
(
USA), TCI (Japan), and ABI Chem (Germany) and used as
received without further purification.
3
.2. Synthesis of TiSBA-15 and Ti-MCM-41
The as-synthesized mesostructured TiSBA-15 catalysts with
Si/nTi ratios of 6, 15, 20, 30, and 60 were synthesized using a
gel with a TEOS/TIP/Pluronic P123/HCl/H O molar composi-
(
TMP–OH to HP ratios), were studied to determine the
n
optimal reaction conditions for Washed TiSBA-15(6). The
oxidation of TMP–OH was also performed with different
oxidants, i.e. HP, TBHP, UHP, and molecular oxygen (O ). In
2
2
tion of 1 : 0.0167–0.167 : 0.016 : 0.43 : 127 using the pH-aDH
8
method, based on a previously published procedure. The
the production of TMBO, two substrates, i.e. pseudo-
cumene and 2,3,5-trimethylphenol, were used instead of
TMP–OH. To identify the best solvent for TiSBA-15(6) reac-
tions, various solvents, i.e. MeCN, MeOH, ACO, MeCOOH,
ethanol, acetic anhydride, and nitromethane, were used for
the selective synthesis of TMBO.
The organic products collected after completion of the
reaction were extracted from the resulting mixture using
diethyl ether, cooled to room temperature, and compared
with the authentic samples by gas chromatography (GC;
Chromosorb WHP 80/100) using a flame ionization detector.
The products were further confirmed using a combined gas
chromatography-mass spectrometry system (GC-MS; Hewlett
G1800A).
samples were calcined at 813 K in air for 6 h for complete
removal of the template. The calcined mesoporous samples
with nSi/nTi ratios of 6, 15, 20, 30, and 60 were denoted as
TiSBA-15(6), TiSBA-15(15), TiSBA-15(20), TiSBA-15(30), and
TiSBA-15(60), respectively. The mesoporous Ti-MCM-41(40)
catalyst with a nSi/nTi ratio of 40 was hydrothermally synthe-
sized using cetyltrimethylammonium bromide (CTMABr) as
the structuring agent with a gel with a SiO /TIP/CTMABr/H O
2
2
molar ratio of 1 : 0.025 : 0.25 : 100, based on a previously pub-
9
lished procedure.
3
.3. Investigation of catalytic stability
The used mesoporous titanosilicate catalysts such as TiSBA-15(6)
and TiSBA-15(60) were regenerated by washing and calcination,
Furthermore, using various mesostructured titanosilicate
catalysts i.e. calcined TiSBA-15(6), recyclable TiSBA-15(6),
and Washed TiSBA-15(6), liquid-phase oxidations of di-/tri-
substituted phenols were performed in a vigorously stirred
thermostated glass-vessel reactor at a specific temperature
for a specific period of time in the presence of HP as the
green oxidizing agent. After completion of the reaction, the
organic products were extracted from the resulting mixture
using non-chlorinated solvents (diethyl ether/ethyl acetate/
hexane). All of the crude products were compared with
authentic samples by GC and confirmed via GC-MS. The C–C
coupling products in the reaction mixture were identified by
8
as reported previously, and the treated catalysts were denoted
as recyclable catalysts, i.e. TiSBA-15(6) and TiSBA-15(60).
TiSBA-15(6) was simply washed several times with acetone
for the removal of non-framework TiO nanoparticle species
2
deposited on the surface of silica pore walls and further treated,
8
based on the published procedure, to generate Washed TiSBA-15
or green mesoporous TiSBA-15(6), which was used for the cata-
lytic oxidation of TMP–OH at different optimal parameters.
3
.4. Characterization
1
The calcined, recyclable, and Washed TiSBA-15 mesoporous
H NMR spectroscopy after separating the used catalyst.
catalysts were characterized using ICP-AES, XRD, N -sorption
The recyclable mesoporous titanosilicate catalysts, i.e.
TiSBA-15(6) and TiSBA-15(60), were reused (50 mg of the
catalyst) for liquid-phase oxidations of di-/tri-substituted phe-
nols to determine their catalytic stabilities. The Washed
TiSBA-15(6) catalyst (50 g of the catalyst) was used in these
2
2
9
isotherm analysis, Si MAS NMR spectroscopy, UV-vis DRS,
field-emission scanning electron microscopy, and transmis-
8
sion electron microscopy, based on published procedures.
The mesoporous Ti-MCM-41(40) catalyst was characterized
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reactions to determine its catalytic activity. After completion
of each catalytic reaction, the catalyst was filtered and
analysed by ICP-AES to determine the percentage of titanium,
and the conversions of phenols and the selectivities for
DSBQs were calculated using standard formulas after GC and
GC-MS analyses. The spectral data for the C–C coupling prod-
3 (a) V. A. Bushmilyev, T. A. Kondratyeva, M. A. Lipkin and
A. V. Kondratyev, Khim.-Farm. Zh., 1991, 5, 65; (b)
M. Constantini, L. Krumenaker and F. Igersheim, French
Patent, 2552425, 1983.
4 (a) H. Chao-Yang and J. E. Lyons, European Patent, 93540,
1983; (b) D. L. Tomaja, L. H. Vogt and J. G. Wirth, J. Org.
Chem., 1970, 35, 2029; (c) S. Ito, K. Aihara and
M. Matsumoto, Tetrahedron Lett., 1983, 24, 5249; (d)
M. Shimizu, T. Hayakawa and K. Takehira, Tetrahedron Lett.,
1989, 30, 4711; (e) V. I. Korenskii, V. D. Skobeleva,
V. G. Kharchuk, I. P. Kolenko, V. L. Volkov, G. S. Zakharova
and V. N. Vinogradova, J. Gen. Chem. USSR, 1985, 55, 1750;
( f ) O. A. Kholdeeva, A. V. Golovin, R. I. Maksimovskaya and
I. V. Kozhevnikov, J. Mol. Catal., 1992, 75, 235.
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V. N. Parmon, RU Patent, 2164510, 2000; (b) N. N. Trukhan,
V. N. Romannikov, E. A. Paukshtis, A. N. Shmakov and
O. A. Kholdeeva, J. Catal., 2001, 202, 110; (c)
O. A. Kholdeeva, N. N. Trukhan, M. P. Vanina,
V. N. Romannikov, V. N. Parmon, J. Mrowiec-Bialon and
A. B. Jarzebski, Catal. Today, 2002, 75, 203; (d) J. Mrowiec-
Białoń, A. B. Jarzębski, O. A. Kholdeeva, N. N. Trukhan,
V. I. Zaikovskii, V. V. Kriventsov and Z. Olejniczak, Appl.
Catal., A, 2004, 273, 47; (e) O. A. Kholdeeva and
N. N. Trukhan, Russ. Chem. Rev., 2006, 75, 411.
6 (a) I. D. Ivanchikova, M. K. Kovalev, M. S. Mel'gunov,
A. N. Shmakov and O. A. Kholdeeva, Catal. Sci. Technol.,
2014, 4, 200–207; (b) O. A. Kholdeeva, I. D. Ivanchikova,
M. Guidotti, C. Pirovano, N. Ravasio, M. V. Barmatova and
Y. A. Chesalov, Adv. Synth. Catal., 2009, 351, 1877; (c)
O. A. Kholdeeva, I. D. Ivanchikova, M. Guidotti, N. Ravasio,
M. Sgobba and M. V. Barmatova, Catal. Today, 2009, 141,
330; (d) O. A. Kholdeeva, M. S. Melgunov, A. N. Shmakov,
N. N. Trukhan, V. V. Kriventsov, V. I. Zaikovskii,
M. E. Malyshev and V. N. Romannikov, Catal. Today,
2004, 91–92, 205; (e) A. Tuel and L. G. Hubert-Pfalzgraf,
J. Catal., 2003, 217, 343; ( f ) T. Maschmeyer, F. Rey,
G. Sankar and J. M. Thomas, Nature, 1995, 378, 159; (g)
M. Guidotti, L. Conti, A. Fusi, N. Ravasio and R. Psaro,
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ucts are as follows:
1
(
i) HMBP–OH. H NMR (400 MHz, CDCl ): δ 1.92 (s, 6H, CH ),
3
3
2
1
3
2
3
.20 (s, 6H, CH
3
), 2.21 (s, 6H, CH
3
), 6.71 (s, 2H, aromatic H
6
).
1
(
ii) TTBDPQ. H NMR (400 MHz, CDCl ): δ 7.72 (s, 4H),
3
.36 (s, 36H).
1
(
iii) TIPDPQ. H NMR (400 MHz, CDCl
3
): δ 7.54 (s, 4H),
.02 (septet, J = 7.2 Hz, 4H), 1.2 (d, J = 7.2 Hz, 24H).
1
(
iv) TMDPQ. H NMR (400 MHz, CDCl ): δ 7.73 (s, 4H),
3
.15 (s, 12H).
1
(
v) TMoDPQ. H NMR (400 MHz, d-DMSO): δ 8.3 (s, 4H),
.71 (s, 12H).
4
. Conclusions
Two-dimensional uniformly mesostructured TiSBA-15 cata-
lysts have been successfully used for the selective synthesis of
TMBO by liquid-phase oxidation of TMP–OH and further
used in the syntheses of DSBQs via liquid-phase oxidations of
di-/tri-substituted phenols. Washing studies show that the
green mesoporous TiSBA-15(6) catalyst has been completely
2
recovered with the removal of the non-framework TiO nano-
particle species and successfully used in these oxidations to
determine its catalytic activity. The catalytic stabilities of the
TiSBA-15 catalysts have been confirmed by the experimental
results of recyclability and hot-catalytic filtration experi-
ments. Recyclability studies of the oxidation reactions show
that many active titanium species are successfully incorpo-
rated into SBA-15 for improving the catalytic activity. The
results of the hot-catalyst filtrations clearly show that the
recyclable and Washed TiSBA-15(6) catalysts are excellent
heterogeneous catalysts in these oxidation reactions. Based
on the catalytic results, the green mesoporous TiSBA-15(6) is
found to be a highly active, recyclable, and environmentally
friendly solid catalyst with unprecedented catalytic activity
among the calcined mesoporous titanosilicate catalysts.
7
O. A. Kholdeeva, O. V. Zalomaeva, A. N. Shmakov,
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