Synthesis of Cu(II)-containing TiO2-SiO2 binary xerogels by hydrolysis of a mixture of tetrabutoxytitanium, tetraethoxysilane, and copper(II) chloride in a water-ammonia atmosphere

Article abstract of DOI:10.1134/S1070427213030014

A Cu(II)-containing binary xerogel TiO₂-SiO₂ was synthesized by joint hydrolysis of tetrabutoxytitanium, teraethoxysilane and copper(II) chloride dissolved in their mixture. The synthesis was performed in a vapor of 10% aqueous ammonia under static conditions. EPR spectroscopy was used to examine the state of Cu(II) in the xerogel matrix. Data on specific features of the behavior of saccharose within xerogel pores under heating were obtained. The catalytic activity of the xerogel was tested by the kinetic method on model reactions of hydrogen peroxide decomposition and oxidative dehydrogenation of trimethylhydroquinone.

Full text of DOI:10.1134/S1070427213030014

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2013, Vol. 86, No. 3, pp. 297−303. © Pleiades Publishing, Ltd., 2013.
Original Russian Text © A.B. Shishmakov, M.S. Molochnikov, D.O. Antonov, O.V. Koryakova, A.S. Seleznev, L.A. Petrov, 2013, published in Zhurnal Prikladnoi
Khimii, 2013, Vol. 86, No. 3, pp. 321−327.
INORGANIC SYNTHESIS
AND INDUSTRIAL INORGANIC CHEMISTRY
Synthesis of Cu(II)-Containing TiO2–SiO2 Binary Xerogels
by Hydrolysis of a Mixture of Tetrabutoxytitanium,
Tetraethoxysilane, and Copper(II) Chloride
in a Water–Ammonia Atmosphere
A. B. Shishmakova, M. S. Molochnikovb, D. O. Antonovb, O. V. Koryakovaa,
A. S. Selezneva, and L. A. Petrova
aPostovskii Institute of Organic Synthesis, Ural Branch, Russian Academy of Sciences, Yekaterinburg, Russia
bUral State University of Forest Engineering, Yekaterinburg, Russia
e-mail: sas@ios.uran.ru
Received June 16, 2012
Abstract—ACu(II)-containing binary xerogelTiO2–SiO2 was synthesized by joint hydrolysis of tetrabutoxytitanium,
teraethoxysilane and copper(II) chloride dissolved in their mixture. The synthesis was performed in a vapor of
10% aqueous ammonia under static conditions. EPR spectroscopy was used to examine the state of Cu(II) in the
xerogel matrix. Data on specific features of the behavior of saccharose within xerogel pores under heating were
obtained. The catalytic activity of the xerogel was tested by the kinetic method on model reactions of hydrogen
peroxide decomposition and oxidative dehydrogenation of trimethylhydroquinone.
DOI: 10.1134/S1070427213030014
Previous studies have shown [1–4] that the polyfunc-
tional heterogeneous system having the form of a Cu(II)-
containing titanium dioxide gel (xerogel) is constituted
by self-organized copper structures of varied morphol-
ogy, which are catalytically active in hydroxyarene oxi-
dation reactions.
of these is hydrolyzed in an aqueous-alcoholicv medi-
um in the presence of catalysts, acids and alkalis. This
method does not require any intricate equipment and
provides a homogeneous distribution of the components
in the binary material. It has been suggested to perform
the reaction of hydrolysis of organometallic precursors
and copper(II) chloride simultaneously and without
an aqueous-alcoholic medium, which can simplify the
technology and yield a material with unusual sirface and
structural characteristics.
To proceed with research in this area, we suggest in
the present study to use as a support the TuO2–SiO2 bi-
nary system with low content of titanium. This system
is synthesized by the sol-gel method and has a homo-
geneous titanium dioxide distribution, with Ti–O–Si
bonds formed [5]. The conventional synthesis of Cu(II)-
containing xerogels of titanium and silicon consists
in that a xerogel (gel) of TiO2, SiO2, or TiO2–SiO2 is
formed by a particular method (sol-gel method, plasmo-
chemical synthesis, vapor-phase deposition, etc.) and
then impregnated with a copper(II) salt solution [6–11].
The most widely used method for synthesis of binary
oxides TiO2–SiO2 is the sol-gel technique employing ti-
tanium and asilicon alkoxides as precursors. A mixture
The goal of our study was to synthesize a Cu(II)-
containing binary TiO2–SiO2 xerogel by joint hydro-
lysis of tetrabutoxytitanium (TBT), tetraethoxysilane
(TEOS), an copper(II) chloride dissolved in a mixture of
these in a water–ammonia atmosphere and to examine
the physicochemical properties of the material obtained.
EXPERIMENTAL
To synthesize the binary xerogel TiO2–SiO2
297

298
SHISHMAKOV et al.
(7 mol % TiO2), a mixture of 9 mL of TEOS and 1 mL of
TBT was poured into a 30-mL round-bottom porcelain
cup with a diameter of 60 mm.
To study trhe decomposition of hydrogen peroxide,
the reactor was charged with 30 mL of an aqueous solu-
tion of H2O2 (1.05 M of H2O2) and a weighed portion
(0.5 g) of a calcined TiO2–SiO2–Cu(II) (TiO2–SiO2)
sample. The reaction was performed at 24°C with a ther-
mostating accuracy of ±0.2°C.
To synthesize individual oxides SiO2 and TiO2,
10 mL of TEOS or TBT was poured into the cup.
To obtain a Cu(II)-containing containing sample
TiO2–SiO2–Cu(II), a weighed portion (0.01 g) of
CuCl2·2H2O was preliminarily dissolved in TEOS/TBT.
The reaction of trimethylhydroquinone (TMHQ) ox-
idation was performed at 50°C. The reactor was charged
with 10 mL or an aqueous-ethanolic (water : ethanol =
1 : 1) solution of TMHQ with a concentration of 66 mM
and a weighed portion (0.9 g) of a calcined TiO2–SiO2–
Cu(II) (TiO2–SiO2) sample was introduced. The content
of Cu(II) in the reaction space was 0.024 mmol.
The cups with the precursors were placed in a des-
iccator (volume 3000 cm3) containing a vessel with
100 mL of 10% aqueous ammonia. The temperature in
the desiccator was 20°C.
The substances were kept in the desiccator for 93 h
and the contents of a cup were dried in air at 20°C for
48 h and in a drying box at 90°C for 24 h. Further, sam-
ples from the cup were charged into a quartz reactor and
calcined at 850°C)heating rate 10 deg min–1) in a flow of
air (feed rate 0.075 m3 h–1) for 1 h.
The kinetic measurements were made by terminating
the reaction and determining the content of the starting
substance. The thus obtained concentration functions
were approximated with polynomials. The initial reac-
tion rates were found by numerical differentiation and
interpolation. The determination error did not exceed
±10%.
The total porosity of the TiO2–SiO2 xerogel for water
was calculated from the difference between the mass of
a sample kept in 20 mL of water for 20 h and its before
the soaking (0.5 g) related to 1 g of the sample.
TMHQ was quantitatively determined by GLC on
a Chrom-4 chromatograph as described in [13].
The 0.01-g weighed portion of copper(II) chloride is
completely dissolved in TBT/TEOS without hydrolysis
of organometallic compounds.
IR spectra of solid powders were recorded with
a Perkin–Elmer Spectrum One IR Fourier spectrometer
with a diffuse reflection attachment (DRA) at frequen-
cies in the range 4000–370 cm–1. The band assignment
was made using the data of [12]. The band intensities
were processed and calculated using special applied
software of the spectrometer.
After the mixture is kept in a desiccator in a water–
ammonia atmosphere for 0.5 h, a light blue precipitate,
copper(II) hydroxide, is formed in the cup. In the course
of time, with increasing amount of ammonia dissolved
in TBT/TEOS, the precipitate is partly dissolved, with
the mixture colored dark blue as an ammonia complex
of copper(II), [Cu(NH3)4](OH)2, is formed. No hydroly-
sis of the organometallic precursors is observed in this
stage of the process. After the reaction mixture is kept
in the desiccator for 93 h, a solid transparent mass can
be found in the cup. This mass is divided by cracks into
3–8-mm fragments and its color is light blue in the main
bulk and bright blue at the up bottom. Apparently, the
ammonia complex of copper(II) is blocked within pores
of the xerogel formed at the liquid–gas phase boundary.
The specific surface area Ssp f the samples was de-
termined by the method of thermal desorption of nitro-
gen on a SoftSorbi-II ver. 1.0 instrument with an error
not exceeding ±5%.
Electron micrographs of TiO2–SiO2 were taken with
a GSM-5900LV electron microscope.
To study the behavior of saccharose within pores
of TiO2–SiO2–Cu(II) (TiO2–SiO2), a calcined xerogel
heated to 90°C was impregnated to saturation with a hot
(90°C) aqueous solution of saccharose (700-g L–1) and
then the grains were dried at 90°C for 2 days.
Drying at 90°C leads to an insignificant cracking of
particles of the material, with their transparence and col-
oration remaining unchanged.
Kinetic experiments were carried out in a thermo-
stated reactor equipped with a reflux. The reaction mix-
ture was agitated by bubbling of air at a flow rate of
3.1 L h–1. TiO2–SiO2–Cu(II) (TiO2–SiO2) grains to be
used in experiments were crushed to a <0.1 mm fraction
and wee kept in water for 15 min prior to addition of the
substrate.
BY the end of the annealing (850°C), the particles of
the material are green. On being kept in air, fine particles
again became light blue, with coarse articles remaining
green. It should be noted that the material with the mor-
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SYNTHESIS OF Cu(II)-CONTAINING TiO2–SiO2 BINARY XEROGELS
299
phological features described above is only obtained if
their hydrolysis and drying at 90°C.
the cup contents were not agitated in the course of hy-
drolysis in the desiccator. Otherwise, strongly cracked
nontransparent dirty blue particles are obtained immedi-
ately after the annealing.
The IR spectrum of the SiO2 xerogel contains
a strong absorption band of complex shape and peaks at
1155 and 1062 cm–1 and an absorption band peaked at
798 cm–1, associated with stretching vibrations of Si–O–
Si bonds. The complex-shape band is associated with
asymmetric stretching vibrations of SiO4 tetrahedra, and
the medium-intensity band at 798 cm–1, with their sym-
metric stretching vibrations [14]. The absorption band
peaked at 945 cm–1 characterizes symmetric stretching
vibrations of nonbridging Si–O– bands [15, 16]. The
high-intensity band at 435 cm–1 characterizes deforma-
tion vibrations of polyhedra. Absorption bands associ-
ated with stretching vibrations of hydroxyl groups and
water molecules appear in the range 3700–3000 cm–1.
A broad medium-intensity band at 1626 cm–1 corre-
sponds to deformation vibrations of water.
After the whole contents of the cup were ground,
the content of copper(II) in the binary matrix of the cal-
cined material was 0.027 mmol of Cu(II) per gram of
TiO2–SiO2.
On being calcined at 850°C, the TiO2–SiO2 binary
oxide matrix itself has the form of transparent col-
orless particles with grain sizes of 7–8 mm and Ssp
=
254.3 m2 g–1 (the grains are inhomogeneous, the po-
rosities of separate grains are within the range 0.28–
0.73 cm3 g–1).
The electron micrographs (Fig. 1) reveal the highly
porous globular structure of the oxide matrix.
The spectrum of the titanium dioxide xerogel
The values of Ssp for calcined SiO2 and TiO2 were
27.8 and 0.7 m2 g–1, respectively.
(a)
T
Figure 2a shows the IR spectra of the oxides upon
(a)
ν, cm–1
(b)
T
1 μm
(b)
ν, cm–1
0.1 μm
Fig. 2. IR spectra of (1) TiO2–SiO2, (2) SiO2, and (3) TiO2
after (a) drying at 90°C and (b) calcination. (T) Transmission
and (ν) wave number; the same for Fig. 4.
Fig. 1. Electron micrographs of the TiO2–SiO2 surface.
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300
SHISHMAKOV et al.
(Fig. 2a) contains a single broad high-intensity absorp-
tion band in the range 1000–400 cm–1, which is peaked
at 596 cm–1 and corresponds to stretching vibrations
of Ti–O bonds. The broad absorption band peaked at
3209 cm–1 is associated with the absorption by hydroxyl
groups and xerogel-bound water. The absorption band
peaked at 1633 cm–1 corresponds to deformation vibra-
tions of water. Three absorption bands peaked at 1126,
1095, and 1036 cm–1 are associated with deformation
vibrations of Ti–OH groups. Organic products formed
in hydrolysis of TBT, present in the xerogel, appear as
absorption bands at 3000–2800 cm–1 (stretching vibra-
tions of C–H) and bands at 1500–1350 cm–1 (deforma-
tion vibrations of –CH3– and –CH3.
1036 cm–1) disappear from the spectrum of titanium
dioxide.
The absorption bands of water (3700–3000,
1625 cm–1) are present in the TiO2–SiO2 spectrum,
which apparently results from sorption from the
atmosphere.
The absorption band of Si–O–Si, peaked at 1090 cm–1
in calcined SiO2, is shifted in the TiO2–SiO2 spectrum to
lower frequencies to 1071 cm–1.
The fact that the frequency of the absorption band
associated with stretching vibrations of Si–O bonds
becomes lower (to ~1100 cm–1) indicates that SiO4
tetrahedra are distorted by the presence of a heavier
neighboring atom bound via an oxygen atom to the
Si–O system under study.
The low content of titanium in the binary TiO2–
SiO2 sample gives now way of unambiguously stating
on the basis of the IR spectra that the Ti–O–Si bond is
formed; however, the observed shift of the peak of the
absorption band associated with symmetric stretching
vibrations of Si–O nonbridging bonds from 945 (SiO2)
to 955 cm–1 (TiO2–SiO2) can be regarded as resulting
from a superposition of the given band on the absorption
band of the Ti–O–Si group (940–960 cm–1) [5, 17].
Thus, the IR spectroscopic data indicate that Ti–O–
Si groups are present in the structure of the calcined
TiO2–SiO2 xerogel.
The IR spectrum of TiO2–SiO2–Cu(II) is fully
identical to that of TiO2–SiO2.
The EPR study of TiO2–SiO2–Cu(II) yielded an
anisotropic EPR spectrum with a well resolved hyperfine
structure in the range of the parallel orientation of the
magnetic moment of copper complexes with respect to
the external magnetic field, which indicates that isolated
copper complexes having the D4h axial symmetry are
formed. The complexes are situated rather far from each
other, which results in that there is no dipole-dipole
or exchange interaction between these complexes. It
follows from published data that, for this to be observed,
the distance between neighboring copper ions should be
no less than 2.5 nm.
An analysis of the IR spectrum of TiO2–SiO2–Cu(II)
demonstrated that it is fully identical to that of TiO2–
SiO2, which is apparently due to the low content of
Cu(II) in the binary matrix.
Figure 2b shows IR spectra of a material calcined
at 850°C. Calcination causes nearly full surface
dehydroxylation of a SiO2 sample, which is indicated
by the fact that the spectrum contains no absorption
bands at 3700–3000 cm–1 and those associated with
Si–O– nonbridging bonds. As a result of calcination,
the absorption bands of Ti–OH groups (1126, 1095, and
The parameters of the Cu(II) EPR spectra in the
samples under study and some published data are listed
in the table.
Parameters of the EPR spectra of Cu(II) in xerogels
Treatment
conditions
According to the parameters of the EPR spectrum of
TiO2–SiO2–Cu(II) dried at 20°C after being synthesized,
ammonia molecules are contained in copper complexes.
Each complex contains four ammonia molecules in the
equatorial plane of the elongated octahedron constituting
the nearest environment of copper ions. This conclusion
follows from the fact that the parameters measured
for the EPR spectra are close to those observed for
ammonia complexes of copper(II) in perfluorinated
sulfocation-exchange membranes [20] (see table) and in
ANKB-35 amphoteric ion exchanger (gII = 2.247, AII =
16.8 mT [21]. The remaining two coordination places
Sample
gII ± 0.005
AII ± 0.3, mT
TiO2–SiO2 Drying at 20°C
Drying at 90°C
2.241
2.276
17.7
16.2
Calcination at
300–850°C
2.354
12.5
TiO2 [18] Drying at 20°C
2.352
2.322
13.2
14.3
SiO2 [19]
The same
Cu(NH3)42+
[20]
2.239
18.5
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SYNTHESIS OF Cu(II)-CONTAINING TiO2–SiO2 BINARY XEROGELS
301
are occupied by oxygen atoms of dissociated hydroxy
groups on the TiO2–SiO2 surface, which form covalent
bonds with copper ions.
Upon a short-term (up to 3 h) drying of a sample
at 90°C, the parameters of the EPR spectrum and,
consequently, the composition of copper complexes
in the sample remained unchanged, characteristic of
copper tetraammoniates. The stability of complexes of
this kind against heating to up to 100°C has also been
noted for perfluorinated membranes [20, 22].Upon more
prolonged drying, the parameters of the EPR spectra
changed (see table). The changes in the parameters
of the EPR spectra reflect the decrease in the number
of nitrogen atoms in the first coordination sphere of
copper ions to two [23]. Presumably, the places freed
by ammonias molecules in the environment of copper
ions are occupied by oxygens of the binary matrix, with
bonds formed by the donor-acceptor mechanism.
Fig. 3. Scheme of Cu(II) ion complexes in the TiO2–SiO2
matrix.
T
After TiO2–SiO2–Cu(II) was calcined at various
temperatures, the same parameters of the EPR spectra
were observed, different from those for the uncalcined
sample. Comparison values measured in the spectra
with published data (see table) shows that complexes
with titanium dioxide functional groups are formed
in a calcined sample (Fig. 3), and this is just titanium
dioxide, rather than silicon dioxide for which smaller
values of gII and larger values of AII are characteristically
observed [19]. It turns out that, under the experimental
conditions, Ti–O fragments of the binary xerogel are
more active ligands with respect to copper(II) ions than
similar formations based on silicon dioxide.
ν, cm–1
Fig. 4. IR spectra of (1) TiO2–SiO2/saccharose upon drying
and (2) mechanical mixture of TiO2–SiO2 and saccharose.
Repeated EPR studies carried out a year after
samples were prepared demonstrated that tetraammonia
copper complexes in a sample dried at 20°C degraded,
with a part of ammonia molecules substituted by water
molecules from air or by oxygens from the binary matrix.
The parameters of the EPR spectra had the following
values: gII| = 2.295 and AII = 15.3 mT, which indicates
that only a single molecule of ammonia is present in
the first coordination sphere of copper ions [23]. At the
same time, the EPR parameters of samples calcined at
temperatures of 300–850°C remained unchanged.
τ, h
Fig. 5. Kinetic curves of hydrogen peroxide consumption.
(cH2O ) H2O2 concentration and (τ) time. (1) Without a catalyst
and i2n the presence of (2) TiO2–SiO2 and (3) TiO2–SiO2–
Cu(II); the same for Fig. 6.
Drying of a TiO2–SiO2 xerogel impregnated with a
saccharose solution results in that grains of the material
gradually change their color: become dark brown from
colorless, which indicates that saccharose undergoes
caramelization in pores of the binary oxide in the course
of dehydration. As additional evidence in favor serve IR
spectroscopic data, which demonstrate that the spectrum
of the thermally treated (90°C, 48 h) binary xerogel
with saccharose markedly differs from that of the TiO2–
SiO2/saccharose mechanical mixture (Fig. 4). A similar
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302
SHISHMAKOV et al.
behavior is observed for saccharose in pores of TiO2–
SiO2–Cu(II). It should be noted that no caramelization
is observed when saccharose is subjected to drying in an
individual TiO2 or SiO2.
action sphere has no effect on the course and parameters
of the TMHQ oxidation process (Fig. 6, curve 1); the
initial TMHQ oxidation rate was 40 mM h–1.
Presence of TiO2–SiO2–Cu(II) (copper(II) content
0.024 mmol) accelerates the reaction, with its initial
rate becoming 84 mM h–1. Thus, the initial reaction
rate in the presence of a Cu(II)-containing xerogel
is 2.1 times the initial rate of an uncatalyzed process.
Comparing the catalytic activity of copper(II) in TiO2–
SiO2–Cu(II) with the previously obtained data on the
activity of copper monocomplexes in TiO2 and SiO2
gels (with Ssp 320 m2 g–1) [1], we can conclude that
copper monocomplexes localized in the binary material
exhibit a substantially higher catalytic activity. The
initial oxidation rate of the substrate in the presence of
Cu(II)-containing gels of titanium and silicon dioxides
[copper(II) content in the reactor, 0.059 mmol] is
only 1.2–1.3 times the reaction rate in the absence of
copper and gels [1]. It should be noted that TiO2–SiO2
has no effect on a noncatalytic process because the
surface of the binary xerogel becomes dehydroxylated
as a result of calcination. Presence of hydroxyl groups
on the surface of TiO2 and SiO2 gels leads to TMHQ
sorption by the surface of the gels, which inhibits the
oxidation of the substrate in its noncatalytic oxidation
by atmospheric oxygen. The easy separation of TiO2–
SiO2–Cu(II) grains from the reaction product makes this
catalyst more technologically convenient as compared
with gel-type catalytic systems.
An analysis of the decomposition of hydrogen
peroxide on the samples synthesized in the study
demonstrated that TiO2–SiO2 exhibits a catalytic
activity in the model process (Fig. 5). Comparison of
the kinetic data suggests that, even though copper(II)
exhibits activity in the binary matrix, its influence on
the reaction is insignificant. It should be noted that the
interaction of H2O2 withTiO2–SiO2 leads to washing-out
of titanium from the binary oxide, as evidenced by the
appearance of a yellow solution coloration characteristic
of peroxy compounds of titanium. Keeping TiO2–SiO2
in an aqueous solution of hydrogen peroxide (H2O2
concentration 1.05 M), with periodic replacement of
the yellowed solution, demonstrated that the coloration
intensity of the solution decreases in the course of time,
with the solution becoming nearly colorless in 60 h.
The IR spectrum of TiO2–SiO2 at 850–1100 cm–1 and
the intensity ratio of the Ti–O–Si absorption band and
that associated with asymmetric stretching vibrations
of Ti–O–Si (947 cm–1) remained unchanged after the
sample was reacted with H2O2. This suggests that
titanium is washed out mostly due to the rupture of
Ti–O–TI bonds.
To find out whether the Cu(II)-containing binary
material can be used as a catalyst in organic syntheses, we
examined the effect of TiO2–SiO2–Cu(II) on the liquid-
phase reaction of TMHQ oxidation by atmospheric
oxygen into 23,6-trimethylbenzoquinone (TMBQ).
CONCLUSIONS
The kinetic data obtained are presented in Fig. 6. The
reaction of TMHQ oxidation occurs with a 98% selec-
tivity with respect to TMBQ, both without TiO2–SiO2–
Cu(II) (TiO2–SiO2) and in its presence. Introduction of
the binary oxide TiO2–SiO2 (0.9-g portion) into the re-
(1)Asingle-stage procedure for synthesis of a Cu(II)-
containing TiO2–SiO2 binary oxide was developed.
(2) It was shown that, in TiO2–SiO2–Cu(II) calcined
at 850°C, copper(II) is present in the form of complexes
and is coordinated with titanium dioxide functional
groups.
(3) It was demonstrated that impregnation of both
TiO2–SiO2 and TiO2–SiO2–Cu(II) with a saccharose
solution, followed by keeping at 90°C, results in the
caramelization of saccharose.
3
(4) It was found that Cu(II) contained in the
TiO2–SiO2 matrix exhibits catalytic activity in model
processes of hydrogen peroxide decomposition and
trimethylhydroquinone oxidation by atmospheric
oxygen.
τ, h
0.75
0.25
0.5
Fig. 6. Kinetic curves of TMHQ consumption. (cTMHQ) TMHQ
concentration and (τ) time.
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SYNTHESIS OF Cu(II)-CONTAINING TiO2–SiO2 BINARY XEROGELS
303
ACKNOWLEDGMENTS
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2011, vol. 12, no. 5, p. 357.
The study was financially supported by the Ural
Branch of the Russian Academy of Sciences (grant
no. 12-I-3-2058.
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Molecules, London: Methuen, 1958.
13. Kharchuk, V.G., Kolenko, I.P., Petrov, L.A., and
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