User-friendly synthesis of highly selective and recyclable mesoporous titanium-silicate catalysts for the clean production of substituted p-benzoquinones

Article abstract of DOI:10.1039/c3cy00615h

Mesoporous titanium-silicates have been prepared following the evaporation-induced self-assembly (EISA) methodology and characterized by elemental analysis, XRD, N₂ adsorption, SEM, DRS UV-Vis and Raman techniques. The use of acetylacetone during synthesis allowed the formation of highly dispersed dimeric and/or small oligomeric Ti species, within the mesostructured silica network, to be realized. The materials catalyse oxidation of alkylsubstituted phenols to corresponding p-benzoquinones with 100% selectivity using the green oxidant-30% aqueous hydrogen peroxide. The titanium-silicates prepared by the convenient and versatile EISA-based procedure reveal the true heterogeneous nature of the catalysis and do not suffer from titanium leaching. They show advantages over other types of mesoporous Ti,Si-catalysts, such as TiO₂-SiO₂ mixed oxides and grafted Ti/SiO₂, in terms of the catalyst stability and reusability.

Full text of DOI:10.1039/c3cy00615h

Catalysis
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User-friendly synthesis of highly selective and
recyclable mesoporous titanium-silicate catalysts for
the clean production of substituted p-benzoquinones†
Cite this: Catal. Sci. Technol., 2014,
4, 200
Irina D. Ivanchikova, Mikhail K. Kovalev, Maxim S. Mel'gunov, Alexander N. Shmakov
and Oxana A. Kholdeeva*
Mesoporous titanium-silicates have been prepared following the evaporation-induced self-assembly (EISA)
2
methodology and characterized by elemental analysis, XRD, N adsorption, SEM, DRS UV–Vis and Raman
techniques. The use of acetylacetone during synthesis allowed the formation of highly dispersed dimeric
and/or small oligomeric Ti species, within the mesostructured silica network, to be realized. The materials
catalyse oxidation of alkylsubstituted phenols to corresponding p-benzoquinones with 100% selectivity
using the green oxidant – 30% aqueous hydrogen peroxide. The titanium-silicates prepared by the
convenient and versatile EISA-based procedure reveal the true heterogeneous nature of the catalysis and do
not suffer from titanium leaching. They show advantages over other types of mesoporous Ti,Si-catalysts,
Received 20th August 2013,
Accepted 5th October 2013
DOI: 10.1039/c3cy00615h
www.rsc.org/catalysis
such as TiO
2
–SiO
2
2
mixed oxides and grafted Ti/SiO , in terms of the catalyst stability and reusability.
6
by a sol–gel technique and catalysts obtained by grafting
Introduction
9
Ti(IV) precursors onto SBA-15 or commercial mesoporous
1
0
Oxidative transformations of substituted phenols offer
efficient access to quinones which are the building blocks
for a wide variety of bioactive compounds, e.g., vitamins E
silica. It was found that the key point to achieve high TMBQ
selectivity is the presence of Ti(IV) dimers or small oligomers
1
0c
on the surface of mesoporous silica.
Such species are
1
and K. Microporous titanium-silicates TS-1 and TS-2 are
manifested by the presence of a broad band centred at
250–290 nm in DR UV–Vis spectra. The presence of two
(several) adjacent Ti atoms is required to ensure fast oxidation
of the key intermediates, phenoxyl radicals, thus preventing
effective catalysts for the hydroxylation of phenol to hydro-
quinone and cathechol but they cannot be employed for
oxidation of bulky substituted phenols due to their pore
size limitation. On the contrary, H
2
1
0b,c
2
O
2
-based oxidations of
the formation of dimeric by-products.
The proper state of
substituted phenols readily occur over mesoporous titanium-
Ti centers in the grafted catalysts can be achieved either by
controlling the surface concentration of Ti (the optimum
3
–7
silicates. Pinnavaia and co-workers first reported oxidation
of 2,6-di-tert-butylphenol (DTBP) into a mixture of benzo-
quinone (BQ) and diphenoquinone (DPQ) using Ti-HMS and
−
2 10b,c
value lies within the range of 0.7–1.0 Ti nm )
or by using
9
,10c
di(poly)nuclear Ti(IV) precursors.
Although the grafted
3
Ti-MCM-41. Two groups independently reported the oxi-
catalysts produce TMBQ with excellent yields, they require the
use of concentrated (70%) H for a stable recycling
dation of 2,3,6-trimethylphenol (TMP) to 2,3,5-trimethyl-
2 2
O
10c
1
7
,4-benzoquinone (TMBQ, vitamin E precursor) with a
7–82% yield over Ti-MMM and Ti-MCM-41 catalysts pre-
behavior. Therefore, the development of a simple, user-friendly
procedure for the synthesis of mesoporous titanium-silicate
catalysts, which would combine excellent selectivity with
stable recycling performance for the oxidation of TMP and
other alkylphenols, with the environmentally friendly oxidant
4
5
pared by hydrothermal templated synthesis, under basic
conditions. The major by-product of this reaction was
2
,2′,3,3′,5,5′-hexamethyl-4,4′-biphenol (BP).
A hydrothermally stable mesostructured catalyst Ti-MMM-2
2 2
– 30% aqueous H O still remains a demanding task.
synthesized under weak acidic conditions showed a similar
TMBQ yield but revealed better resistance with respect to
8
deactivation with aqueous H
2
O
2
. An almost quantitative yield
of TMBQ was attained using TiO –SiO mixed oxides prepared
2
2
Boreskov Institute of Catalysis, Lavrentieva 5, Novosibirsk 630090, Russia.
E-mail: khold@catalysis.ru; Fax: +7 383 330 95 73; Tel: +7 383 326 94 33
†
Electronic supplementary information (ESI) available: SEM, N
2
adsorption
and Raman data for the reused catalyst. See DOI: 10.1039/c3cy00615h
200 | Catal. Sci. Technol., 2014, 4, 200–207
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Evaporation-induced self-assembly (EISA) is a simple
technique which enables rapid production of ordered thin
Synthesis of Ti,Si-catalysts. The batch composition was
typically: 1/TEOS : 25/C H OH : 6.7/H O : 0.0012/HCl : 0.15/CTAB :
2
5
2
1
1–18
silica films
and can be applied for the preparation of
0.018/Ti. In a standard procedure, 5.5 g of surfactant, CTAB,
was dissolved in 115 g of ethanol (95%) for 2 h at room
temperature. Then 20.8 g of TEOS, 12.0 g of 0.01 M HCl and
1
9
mesoporous materials in the form of fibres or powders.
The surfactant concentration progressively increases during
solvent evaporation and triggers the self-assembly of silica-
surfactant micelles and their further organization into
mesophases. The EISA method is widely used in the synthe-
1.52 g of the Ti(acac)
x
precursor solution were added
dropwise under vigorous stirring. The resulting clear sol was
left in an open Petri dish under ambient conditions until the
solvent had fully evaporated. The resulting solids were
calcined at 550 °C for 5 h in air with a temperature ramp of
2
0
sis of ordered mesoporous metal oxides. A few studies
demonstrated the utility of the EISA method for the synthesis
2
1–23
21
−1
of Ti-containing ordered silica films.
Ogawa et al.
1 °C min to remove the organic species. The freshly
reported the preparation of films with a Si/Ti ratio of 50 and
demonstrated their photocatalytic reactivity. Thin films with
Si/Ti = 30–80 containing mostly site-isolated Ti(IV) species
and with a DRS UV–Vis signal at 215 nm were prepared using
acetylacetone (acac) to retard the hydrolysis rate of Ti(IV)
calcined samples were characterized by elemental analysis, N
2
adsorption, XRD, DRS UV–Vis and Raman spectroscopy and
used for catalytic tests.
To estimate the hydrothermal stability and stability toward
aqueous H O , sample A was treated either with boiled water
2
2
2
2
23
isopropoxide. Hüsing et al. achieved titanium loading of
up to a Si/Ti ratio of 5 in silica films, using the EISA tech-
nique. Oligo(ethylene oxide) alkyl ether surfactant Brij56 was
used as both a template for mesostructure formation and a
complexing agent for Ti(IV). DRS UV–Vis studies of the calcined
films showed only a single intense band at 205 nm, charac-
for 6 h or with aqueous 30% H O for 1 h (15 mg of catalyst,
2 2
0.11 M H O , 3 mL MeCN, 25 °C), dried in air and calcined
2 2
at 550 °C before physicochemical measurements.
Catalytic oxidations
Catalytic oxidations were performed under vigorous stirring
2
4
teristic of isolated Ti(IV) atoms in tetrahedral coordination.
(
600 rpm) in thermostated glass vessels. Typical reaction
In the present work, we extended the EISA approach to the
preparation of mesostructured titanium-silicate catalysts. We
report a simple and versatile procedure for the synthesis of
highly efficient and reusable catalysts, with well-dispersed
di(oligo)nuclear active Ti centers, which allow for the oxida-
tion of alkylphenols to the target p-benzoquinones with 100%
conditions were as follows: phenolic substrate, 0.1 M; H
2
O
2
,
0
2
.35 M; catalyst 20 mg (Ti 0.006 mmol); MeCN, 1 mL, 80 °C,
0 min. The reaction was started by the addition of H
2
O
2
.
Samples of the reaction mixture were withdrawn periodically
during the reaction course by a syringe. The oxidation pro-
1
ducts were identified by GS-MS and H NMR spectroscopic
2 2
selectivity using 30% H O .
techniques. The yield of quinone and conversion of alkylphenol
were quantified by GC using an internal standard, biphenyl.
Turnover frequency (TOF) values were determined from the
initial rates of phenol consumption. Each experiment was
reproduced 3–5 times.
Catalyst reusability was examined in 4–6 time scaled
experiments (total reaction mixture volume 15–20 mL).
Following the reaction, the catalyst was filtered off and
washed with hot acetonitrile and acetone. Before use in the
next catalytic run, the catalyst was dried in air at room
temperature overnight and calcined in air at 250 °C for 2 h
and then at 550 °C for 4 h. The nature of the catalysis was
verified by hot filtration tests.
Experimental
Materials
Cetyltrimethylammonium bromide (CTAB, 99+%) and
tetraethylorthosilicate (TEOS, 98+%) were purchased from
Aldrich. Tetraethoxy titanium (IV) (TEOT, technical grade) and
acetylacetone (99%), 2,6-di-tert-butylphenol (DTBP, 99%),
2
,6-dimethylphenol (DMP, 99%) and 2,6-di-tert-butyl-1,4-
benzoquinone (DTBQ, 98%) were used as received from
Aldrich. 2,3,6-Trimethylphenol (TMP, 97+%) was obtained
from Fluka. Acetonitrile (Fluka) was dried and stored over
activated 4 A molecular sieves. Ethanol (95%) and other reac-
tants were obtained commercially and used without addi-
tional purification. The concentration of H O (ca. 30 wt% in
Instrumentation
2
2
water) was determined iodometrically prior to use. Deionized
water (EASY pure, RF, Barnsted) was used for the preparation
of catalysts.
GC analyses were performed using a gas chromatograph
Tsvet-500 equipped with a flame ionization detector and
a quartz capillary column (30 × 0.25) filled with Supelco
MDN-5S. GC-MS analyses were carried out using an Agilent
Synthesis of catalysts
7000B system with the triple-quadrupole mass-selective
Preparation of titanium precursor Ti(acac)
1.5 or 1.9 g) was mixed with acetylacetone (0.44 or 0.88 g)
and then water (0.16 g) and TEOT (1.0 g) were consecutively
added. The molar ratios of Ti : H O:acac were 1:2:1 (sample A)
or 1 : 2 : 2 (sample B). The resulting solutions were stirred for
1 h at room temperature before use in the next step.
x
. Ethanol (95%)
detector Agilent 7000 and GC Agilent 7890B (quartz capillary
column 30 m × 0.25 mm/HP-5ms). H NMR spectra were
recorded on a Bruker Avance-400 spectrometer (400.13 MHz).
XRD measurements were performed on a high precision
X-ray diffractometer mounted on a beamline No. 2 of VEPP-3
storage ring at the Siberian Synchrotron Radiation Center
1
(
2
~
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(
SSRC). The radiation wavelength was λ = 0.15393 nm. High
the synthesis, immediate precipitation of titanium dioxide
occurred. Furthermore, the use of acac allowed us not only to
avoid the TiO precipitation but also to generate a proper,
2
natural collimation of the synchrotron radiation beam, flat
perfect crystal analyzer and parallel Soller slit on the diffracted
beam, limited its azimuthal divergence and provided extremely
high instrumental resolution of the diffractometer in a small
angle region of 2θ = 0.5 ÷ 10° and higher.
di(oligo)meric state of Ti centers in the catalysts (vide infra).
The twenty-fold excess of the alcohol solvent (relative to
the concentration of CTAB at which the formation of micelles
normally starts) prevented the formation of any precipitate,
and the synthetic solution remained transparent after mixing
of all the reactants. Upon evaporation of the alcohol, the criti-
cal micelle concentration was achieved and self-assembly of a
silica–Ti-surfactant mesophase occurred, resulting in the for-
mation of transparent monoliths with a characteristic planar
size of ca. 1 cm and height of 3–5 mm without visible internal
defects (Fig. 1a). After calcination was used to remove the
organic template, the monoliths cracked to yield a solid with
particles of 0.1–2 mm (Fig. 1b). According to the EISA method-
ology, the composition of starting mixtures is directly reflec-
ted in the composition of the products, so that the introduction
of titanium into the final material was nearly quantitative,
as confirmed by the elemental analysis data (Table 1).
After calcination, the solids were ground to give a fine
powder. SEM images of ground sample A are shown in Fig. 2
(top). One can observe relatively sharp particles with a size of
100–500 μm. The particles have internal cracks and rows of
“bubbles”, which could be the reason for their further break-
ing, as confirmed by SEM images of sample A after the first
catalytic run (Fig. 2, bottom) and after 7 reuses in TMP oxi-
dation (Fig. S1 in ESI†). Indeed, the size of the particles
reduced to 50–200 μm after the catalytic runs, and the distri-
bution of particle sizes seems to be narrower relative to the
starting material.
Nitrogen adsorption measurements were carried out at
7
7 K using an ASAP-2400 instrument (Micromeritics) within
−4
the partial pressure range 10 –1.0. The catalysts were degassed
at 90 °C for 24 h before the measurements. Textural charac-
teristics were calculated using a comparative method reported
2
5
elsewhere. Pore size distributions were calculated from the
adsorption branches of the nitrogen isotherms by means of
the regularization procedure, using reference local isotherms
calculated in a cylindrical silica pore model in the framework
of the density functional theory (DFT) approach. Special soft-
ware provided by Quantachrome Corp. was used for this pur-
pose. Mean pore diameters were calculated as mathematical
expectation values from these distributions.
The state of titanium in the catalysts was probed by DR UV–Vis
spectroscopy under ambient conditions using a Shimadzu UV–VIS
2
−1
501PC spectrophotometer. FT-Raman spectra (3600–100 cm ,
−
1
3
00 scans, resolution 4 cm , 180° geometry) were recorded
using a RFS 100/S spectrometer (Bruker). Excitation of the
064 nm line was provided by a Nd-YAG laser (100 mW power
1
output). Scanning electron microscopy images were acquired
using a JEOL JSM-6460 LV microscope. Titanium content in the
filtrate was determined by ICP-AES using a Thermo Scientific
iCAP-6500 instrument.
Results and discussion
Catalyst synthesis and characterization
A sol–gel technique enables molecular-scale control, intimate
mixing of reactants and promotes Si–O–Ti bond formation
2
6
in titania–silica mixed oxides. However, the reactivities of
silicon and transition metal alkoxides in hydrolysis/condensation
reactions differ significantly and special measures are required
to avoid segregation of the active metal into a separate metal
oxide phase. Different strategies have been applied to adjust the
relative precursor reactivities, including pre-hydrolysis of TEOS,
the use of a more reactive silicon precursor, viz. tetramethyl
orthosilicate (TMOS) and/or modification of metal alkoxides
with complexing agents, such as peroxides, alkoxyalcohols,
aminoalcohols, β-diketones, β-ketoesters, carboxylic and phosphonic
Fig. 1 Sample A after evaporation under ambient conditions (a) and
after calcination (b).
2
6,27
acids.
Acetylacetone has been widely used as a hydrolysis-
Table 1 Physicochemical properties of Ti,Si-catalysts prepared by
EISA
retarding agent in the sol–gel synthesis of amorphous titania–
6
,26,27
silica
and the templated synthesis of ordered mesoporous
2
2,28
a
b
2
−1
c
3
−1
d
titanium-silicates.
to favour the formation of Ti dimers, the presence of which
is crucial for the selective oxidation of bulky phenols
On the other hand, acac is known
Sample Ti : acac
Ti, wt%
S
BET, m g
V, cm g
D, nm
2
7
A
B
A-1
A-7
1 : 1
1 : 2
1 : 1
1 : 1
1.43
1.36
1.43
1.43
1288
1105
1165
1000
0.70
0.70
0.64
0.48
3.1
3.4
3.0
2.7
e
f
10b,c
to p-benzoquinones.
Keeping all these facts in mind, we employed acac for the
stabilization of the Ti precursor (see Experimental for details).
The molar ratio of Ti/acac was chosen as 1 : 1 and 1 : 2
a
b
Molar ratio used in the synthesis. Elemental analysis data for
c
d
e
calcined samples. Mesopore volume. Mean pore diameter. After
1
f
run of TMP oxidation with H
2 2
O . After 7 runs of TMP oxidation;
(samples A and B, respectively). When no acac was used in
the reaction conditions as in Table 2 (entry 3).
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Fig. 4 Nitrogen adsorption–desorption isotherms for calcined samples
A and B (filled and empty symbols correspond to adsorption and
desorption, respectively).
Fig. 2 SEM images of calcined sample A: (top) before catalysis and
Variation in the amount of acac used during synthesis had
two consequences for the final material. First, the increase of
the acac/Ti molar ratio from 1 : 1 to 2 : 1 led to a slightly
increased mesopore diameter (3.4 nm versus 3.1 nm). Secondly,
the acac/Ti molar ratio affected the state of Ti centers.
(
bottom) after the 1st catalytic run (A-1).
The small angle XRD pattern of sample A is shown in
Fig. 3. The broad diffraction peak that can be indexed as (10),
in a two-dimensional hexagonal lattice with d10 ≈ 4.5 nm and
a corresponding lattice constant of a ≈ 5.2 nm, indicates a
long-range structural order in an array of regularly arranged
cylindrical mesopores with a uniform pore size. Treatments
It is well-known that the nature of Ti species in titanium-
silicate catalysts has a strong impact on their catalytic perfor-
2
,24
mance.
As was mentioned in the Introduction, well-dispersed
dimeric (small oligomeric) Ti centers are required for the
selective formation of p-benzoquinones in the oxidation of
9
,10c
with a MeCN solution of H
2
O
2
or boiling water produced no
alkylated phenols.
DR UV–Vis spectroscopy is a useful
changes in the lattice constant. At the same time, strong
broadening of the (10) reflection showed a high degree
of structural imperfection. Therefore, the material prepared
certainly represents a mesophase however it has a rather
poor organization.
The nitrogen adsorption–desorption isotherms for samples
A and B are presented in Fig. 4. Textural properties of the
materials (specific BET surface areas, mesopore volumes and
average mesopore diameters) acquired from the adsorption
data are given in Table 1. Importantly, the solids had no
micropores. A rough estimation of the silica wall thickness as
a difference between the lattice constant a and average
mesopore diameter D gives a value of ca. 2 nm.
technique for characterization of the local geometry and
coordination environment of the titanium ions in solids.
DR UV–Vis spectra of samples A and B are shown in Fig. 5.
Both samples reveal broad absorption bands centred at
235 (A) and 255 nm (B) although the Ti content was rather
low (the formal surface density of Ti calculated based on the
−
2
Ti content and surface area would be ca. 0.14 atom nm ).
Such broad absorptions centred at 235–260 nm are attributed
IV
29
to six-coordinated Ti dimers and small oligomers. The
long-wave shift observed for sample B relative to sample A
indicates a higher degree of Ti oligomerization in the former
sample. The absence of a characteristic absorption at 330 nm
Fig. 3 XRD patterns of calcined sample A: initial, after treatment with
Fig. 5 DR UV–Vis spectra of calcined samples A, B and sample A after
aqueous H
2
O
2
in MeCN (25 °C, 1 h) and after treatment with boiled
2 2
treatments with aqueous H O in MeCN (25 °C, 1 h) and with boiled
water for 6 h.
water for 6 h.
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typical of anatase microcrystallites is consistent with the
absence of the band at 145 cm in the Raman spectra of
the samples.
is 1 : 2). The efficiency of the oxidant utilization was, therefore,
53–54%. The conversion of TMP could be increased up to
100% by increasing the amount of hydrogen peroxide (entry 3).
In turn, the use of only a stoichiometric amount of H O led to
−
1
2
2
a significant decrease in both TMP conversion and TMBQ
selectivity (entry 6). Attempts to reduce the amount of catalyst
also resulted in decreasing substrate conversion, although the
product selectivity still remained high (entries 7 and 8).
Hydrothermal stability and stability toward H O
2
2
Hydrothermal stability is crucial for catalysts that are supposed
to be applied for oxidations with aqueous H O . Earlier, some
2
2
of us reported the synthesis of the hydrothermally stable
mesostructured titanium-silicate Ti-MMM-2 that demonstrated
The catalytic activity expressed by a TOF value was the
−
1
same for samples A and B (2.2 min ) and was a little higher
a fairly good resistance towards boiling water and dilute solu-
than the TOF values previously found for the grafted Ti/SiO
catalysts (1.4–1.8 min ). Taking into account that the EISA
samples had smaller or comparable mesopores (3.1–3.4 nm)
8
2
tions of H
the H
O
2 2
in MeCN. Ti-MMM-2 showed high activity in
−
1
2
O
2
-based oxidation of TMP, but the selectivity for TMBQ
never exceeded 82% as this catalyst contains mostly isolated Ti
atoms (DRS UV maximum at 208 nm). Interestingly, the use of
acac in the synthesis of Ti-MMM-2 did not allow us to obtain
samples with predominantly di(oligo)meric active Ti sites.
The hydrothermal stability of sample A was examined by
treatments with boiling water for 6 h and with 0.1 M solution
of H O in MeCN for 1 h at room temperature (the same tests
than the grafted Ti catalysts (e.g., 3.3 nm for Ti/SiO Nippon-
2
2
Kasei or 15.4 nm for Ti/SiO Davicat), we may suggest that no
internal diffusion limitation occurs in the TMP oxidation
over samples A and B.
To evaluate the scope of the catalytic material, two other
representative phenols, DTBP and DMP, were oxidized with
2 2
H O over sample A under standard reaction conditions. The
corresponding quinones, DTBQ and DMQ, formed with 100%
selectivity (Table 2, entries 4 and 5, respectively). The lower
conversion of DTBP as compared to TMP and DMP was most
likely due to its steric bulk.
2
2
8
were previously employed for Ti-MMM-2). Fig. 3 shows XRD
patterns of sample A after the treatments in comparison with
the initial catalyst. Both the position and width of the peaks
are very close, which confirms the good hydrothermal stability
of the solids synthesized by the EISA technique. DR UV–Vis
spectra of sample A after the treatments are given in Fig. 5.
One can see that the state of Ti centers changed insignifi-
Catalyst recyclability
cantly after the mild treatment with H
procedure caused a long-wave shift of the absorption, thus
indicating some further agglomeration of TiO oligomers.
2
2 2
O while the boiling
The recycling behaviour of sample A has been studied in
seven consecutive operation cycles of the TMP oxidation (con-
ditions of entry 3, Table 2; total TON = 120). As one can judge
from Fig. 6, the excellent (99–100%) selectivity towards TMBQ
remained unchanged while TMP conversion slightly decreased
(from 100 to 90%). A small decrease in reaction rate (TOF)
was observed, mostly in the first cycles, but starting from the
forth cycle, this parameter has been stabilized. In fact, the
decrease in TOF results in an increase in reaction time from
20 to 50 min, which is not crucial. More importantly, no drop
in TMBQ selectivity occurs after the catalyst recycling. This is
a clear advantage of the Ti,Si-catalysts prepared by the EISA
Catalytic study
The catalytic performance of the titanium-silicates prepared
by the EISA methodology has been explored in the oxidation
of TMP with 30% hydrogen peroxide in MeCN. The main
results are presented in Table 2. Under the reaction condi-
tions that had been previously determined as optimal for the
1
0b
production of TMBQ over Ti,Si-catalysts,
A and sample B revealed 100% selectivity toward the target
both sample
quinone (entries 1 and 2). The substrate conversion attained
method as compared to the grafted Ti/SiO catalysts, for which
2
9
5 and 93% for samples A and B, respectively, when a 1.75-fold
only the use of 70% H O allowed the selectivity to be kept
2
2
excess of the oxidant was employed (the reaction stoichi-
ometry for the oxidation of TMP with H O to produce TMBQ
2 2
quasi constant while with 35% H O the selectivity decreased
to 85% by the third reuse.
1
0c
2
2
2 2
Table 2 Oxidation of alkylphenols with H O
over Ti,Si-catalysts prepared by EISAa
b
−1
Entry
Substrate
[H O
2 2
], M
Sample
Substrate conversion, %
Product selectivity, %
TOF, min
1
2
3
4
5
6
7
8
TMP
TMP
TMP
DTBP
DMP
TMP
TMP
TMP
0.35
0.35
0.44
0.44
0.44
0.2
A
B
A
A
A
A
A
A
95
93
100
50
90
48
100
100
100
100
100
75
2.2
2.2
2.3
0.5
0.9
1.5
2.3
2.4
c
0.44
0.44
82
65
98
95
d
a
b
Reaction conditions: substrate, 0.1 M; catalyst, 0.006 mmol of Ti; CH
3
CN 1 mL; 80 °C; 20–40 min. TOF = (moles of TMP consumed)/(moles
c
d
of Ti × time), determined by GC from the initial rates of TMP consumption. 0.003 mmol of Ti. 0.0015 mmol of Ti.
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seven catalytic runs (Table 1; the corresponding N adsorption
2
isotherms and PSDs are given in Fig. S2 in ESI†). This decrease
is most likely due to shrinkage of the material subjected to
the turnover conditions and several calcinations during the
recycling. However, we should note that the reduction of
TOF values was more pronounced after the first use (Fig. 6);
therefore, it can hardly be attributed to the deterioration
of the textural properties. On the other hand, studies by
DR UV–Vis spectroscopy showed that the state of Ti in sample
A changed more drastically namely after the first catalyst use,
while very little difference was observed between the fifth and
seventh uses (Fig. 8). This agrees favourably with stabilization
of the catalytic activity after the fourth cycle (Fig. 6).
2 2
Fig. 6 Catalyst recycling (sample A) in TMP oxidation with H O .
Reaction conditions as in Table 2 (entry 3).
The observed long-wave shift in the DR UV–Vis spectra is a
manifestation of progressive oligomerization of titanium on
1
0c,29
Although there is no direct spectroscopic technique that
would allow us to give an unambiguous answer as to whether
the Ti di(oligo)meric species are truly incorporated within the
silica network or not, the higher stability of the EISA catalysts
the catalyst surface.
The growth of TiO oligomers initially
2
caused some decrease in TOFs however, more importantly, it
produced no negative effect on the reaction selectivity which
remained close to 100% during seven reuses (Fig. 6). The
Raman spectroscopic technique, which enables detection
of early stages in the emergence of anatase microcrystallites
2 2
toward deactivation with 30% H O might suggest that the
new procedure reported in this work favours insertion of the
Ti species into the silica mesostructure. Relatively thick silica
walls may be another factor responsible for the higher stability
to deactivation.
−
1 24,31
by the presence of an intense band at 140–145 cm ,
−1
identified just a very weak signal at ca. 150 cm (Fig. S3 in ESI†).
A rough estimation made from a comparison of the inten-
sity of this peak with the corresponding band of anatase
(
Fig. S3,† inset) showed that, even after the seventh cycle,
Catalyst stability under reaction conditions
the amount of anatase in the catalyst was below 1% relative
to the total Ti content. Earlier, it was demonstrated that the
presence of TiO
selective formation of p-benzoquinones. Therefore, the lack
of anatase and isolated Ti centres in the mesoporous titanium-
silicates, prepared by the EISA method, ensures the high
selectivity towards substituted p-benzoquinones.
The hot catalyst filtration test revealed neither TMP conver-
sion nor TMBQ formation in the filtrate after removal of the
catalyst (Fig. 7). This proves the true heterogeneous nature of
2
microcrystallites is detrimental for the
3
2
3
0
the catalysis in the catalytic system studied. In addition, the
amount of titanium in the filtrate determined by ICP-AES was
below 0.1 ppm. Accordingly, the content of titanium in the
catalysts remained the same after seven reuses (see Table 1).
Meanwhile, after the catalyst recycling, a progressive decrease
in the surface area and mesopore volume was detected (Table 1).
After the first run, the average mesopore diameter remained
practically unchanged but it reduced from 3.1 to 2.7 nm after
Conclusions
In summary, an original and convenient procedure for the
preparation of mesoporous titanium-silicate catalysts has
Fig. 7 Hot catalyst filtration test for TMP oxidation with H
2 2
O over
sample (filled and empty symbols correspond to substrate
A
Fig. 8 DR UV–Vis spectra of calcined sample A: initial (A) and after 1st
consumption and product yield, respectively). Reaction conditions as
in Table 2 (entry 1).
2 2
(A-1), 5th (A-5) and 7th (A-7) runs of TMP oxidation with H O .
Reaction conditions as in Table 2 (entry 3).
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been developed using the EISA approach. This procedure
does not require either rigorously controlled reaction condi-
tions or special handling, which makes it safe, cheap and
sustainable. Coupled with the use of acac as a Ti modificator,
the EISA methodology enables control of the Ti active center
state to favour the formation of Ti dimers or small oligomers
on the catalyst surface. The presence of such centers ensures
highly selective formation of p-benzoquinones during the
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