One-pot hydrothermal synthesis of mesoporous V-SBA-16 with a function of the pH of the initial gel and its improved catalytic performance for benzene hydroxylation

Article abstract of DOI:10.1007/s10562-012-0773-9

V-SBA-16 catalysts with uniform cubic mesoporous structure were prepared by direct hydrothermal method as a function of the pH of the initial gel and characterized by ICP, XRD, TEM, N₂ adsorption-desorption, DRUV-vis and Raman spectra. The pH of the initial gel in synthesis of V-SBA-16 show important effects on the maintenance of well ordered mesoporous structure, introduced vanadium content and the incorporation of vanadium into the network of SBA-16 type mesoporous material. The initial gel system with a pH value of 2.0 was found to be a suitable for incorporation of vanadium and retaining the mesostructure of SBA-16. The catalytic activities of V-SBA-16 catalysts were evaluated for the hydroxylation of benzene using molecular O₂ as the oxidant. The highest phenol yield of 30.4% with a selectivity of 90% and turnover number of 105 were obtained over the VS-2.0 (1.67) sample prepared at the initial gel system with pH value of 2.0, which is attributed to its high V content and uniform framework V species that highly dispersed on the well ordered SBA-16 type mesoporous materials.

Full text of DOI:10.1007/s10562-012-0773-9

Catal Lett (2012) 142:619–626
DOI 10.1007/s10562-012-0773-9
One-pot Hydrothermal Synthesis of Mesoporous V-SBA-16
with a Function of the pH of the Initial Gel and its Improved
Catalytic Performance for Benzene Hydroxylation
•
Lina Zhao Yongli Dong Xinlin Zhan
•
•
•
•
•
Yi Cheng Yujun Zhu Fulong Yuan
Honggang Fu
Received: 19 August 2011 / Accepted: 19 January 2012 / Published online: 29 March 2012
Ó Springer Science+Business Media, LLC 2012
Abstract V-SBA-16 catalysts with uniform cubic meso-
porous structure were prepared by direct hydrothermal
method as a function of the pH of the initial gel and char-
acterized by ICP, XRD, TEM, N2 adsorption–desorption,
DRUV—vis and Raman spectra. The pH of the initial gel
in synthesis of V-SBA-16 show important effects on the
maintenance of well ordered mesoporous structure, intro-
duced vanadium content and the incorporation of vanadium
into the network of SBA-16 type mesoporous material. The
initial gel system with a pH value of 2.0 was found to be a
suitable for incorporation of vanadium and retaining the
mesostructure of SBA-16. The catalytic activities of
V-SBA-16 catalysts were evaluated for the hydroxylation
of benzene using molecular O2 as the oxidant. The highest
phenol yield of 30.4% with a selectivity of 90% and
turnover number of 105 were obtained over the VS-2.0
(1.67) sample prepared at the initial gel system with pH
value of 2.0, which is attributed to its high V content and
uniform framework V species that highly dispersed on the
well ordered SBA-16 type mesoporous materials.
Keywords V-SBA-16 Á One-pot hydrothermal synthesis Á
pH Adjustment Á Benzene Á Hydroxylation
1 Introduction
In the family of mesoporous materials, SBA-n materials
exhibit large surface area, uniform pore sizes, thicker pore
walls and higher hydrothermal stability than M41S mate-
rials [1]. In particular, SBA-16 materials with a 3D cage-
like structure have attracted great attention in many fields
including catalysis, adsorption, separation and nanoscience
[2]. However, the absence of active sites in the framework
of these pure siliceous materials has limited their applica-
tions [3]. Many efforts have been devoted to the synthesis
of metal-containing SBA-15 and SBA-16 type mesoporous
materials by direct synthesis and impregnation methods
[4–11]. Nevertheless, it is very difficult to incorporate
metal species into the SBA-n silicious framework by direct
synthesis due to the difficulties in the formation of metal–
O–Si bonds under strongly acidic conditions [8]. Recently,
Gallo et al. attempted to incorporate aluminum into SBA-
16 framework using a pH-adjustment method through a
two-step route, obtaining a disordered SBA-16-related-
aluminosilicate material [9]. Park et al. synthesized the
Fe-SBA-16 by the adjustment of the molar ratio of a series
of reactants, such as the molar ratio of co-surfactant
n-butanol, TEOS, nH2O/nHCl ratio, Si/Fe ratio, and aging
conditions [10]. Shah et al. reported the hydrothermal syn-
thesis of Cu-SBA-16 by internal pH-modification method
using hexamethylenetetramine (HMTA) as a complexing
agent and internal pH-modifier [11]. The prepared products
possessed either an ordered structure with very low metal
content under strongly acidic conditions, or a poorly ordered
structure with higher amount of metallic species under weak
L. Zhao Á X. Zhan Á Y. Cheng Á Y. Zhu (&) Á F. Yuan (&) Á
H. Fu
Key Laboratory of Functional Inorganic Material Chemistry,
Ministry of Education, School of Chemistry and Materials,
Heilongjiang University, Harbin 150080,
People’s. Republic of. China
e-mail: yujunzhu@hlju.edu.cn
F. Yuan
e-mail: fulongyuan2000@yahoo.com
Y. Dong
Analysis and Testing Center, Heilongjiang Institute of Science
and Technology, Harbin 150027, People’s. Republic of. China
123

620
L. Zhao et al.
acidic condition. These studies showed that the synthesis
conditions, especially the pH value of initial gel, were an
important factor for the formation of well ordered structure
and the introduction of metal ions into SBA-16 framework in
the total hydrothermal synthesis process.
F-127 and P-123 in a solution of hydrochloric acid and
distilled water (solution A). TEOS and a certain amount of
ammonium metavanadate were mixed with deionized
water to get solution B. A set of samples were prepared by
changing the molar ratio of HCl to silicon. Briefly, the
starting molar composition was 0.0016 P-123:0.0037
F-127:x HCl:1.0 TEOS:117 H2O:0.06 NH4VO3, and the
amount of hydrochloric acid is fit for the final pH value
(pH = 0.5, 1.0, 1.5, 2.0 and 2.5). Solution B was stirred at
room temperature until the solution becomes clear, and
then it was added dropwise to solution A with magnetic
stirring at 313 K. The final mixture was stirred vigorously
for 24 h and then transferred to an autoclave and aged for
4 days at 373 K. The green solid precipitate was isolated
by filtration, washed and dried under vacuum at 323 K.
Finally, the precursor is calcinated in ambient air with a
heating rate of 1 K/min to 823 K and kept at this temper-
ature for 4 h. The prepared V-SBA-16 samples were
denoted to VS-n(x) (where n denotes the pH condition and
x represents V content determined by ICP).
Compounds of vanadium, due to their reactivity and
remarkable stability, have been utilized as efficient catalysts
for a variety of catalytic reactions [12–16], especially for the
catalytic selective oxidation of benzene to phenol [17–21].
Recently, various vanadium containing mesoporous molec-
ular sieves such as V-MCM-41 [22, 23], V-MCM-48 [24]
and V-SBA-15 [25, 26] have been prepared by hydrothermal
methods through different synthesis routes. Park and
co-workers [27] have firstly reported the synthesis of well
ordered V-SBA-16 with isolated framework V species and
examined the effects of reaction conditions including pH,
stirring temperature, aging and Si/V ratio. However, only a
narrow pH range was selected to investigate the synthesis of
V-SBA-16 materials, and most of their attention was paid to
the structure characterizations of V-SBA-16 samples with
very low initial V/Si ratio (1/200).
In previous work, we have reported the mesoporous VOx/
SBA-16 catalysts prepared by the impregnation method [28],
which exhibit efficient catalytic properties for the hydrox-
ylation of benzene. Nevertheless, the phenol yield (13.8%)
and turnover number (32.4) are still very low. In this paper,
uniform cubic mesoporous V-SBA-16 catalysts with highly
dispersed framework V species and relative high vanadium
content were prepared by a one-pot hydrothermal method as
a function of the pH of the initial gel. The influence of the pH
of the initial gel upon vanadium introduction, structure res-
ervation and V species dispersion were carefully character-
ized by ICP, XRD, TEM, nitrogen adsorption, DRUV—vis
and Raman spectra. The catalytic performances of V-SBA-
16 materials were first evaluated for the hydroxylation of
benzene using molecular oxygen as the oxidant. The rela-
tionship between vanadium species and catalytic activity
was also investigated.
2.2 Characterization
X-ray diffraction (XRD) measurements were carried out
on a Bruker D8 Advance X-ray diffractometer with Cu Ka
˚
(k = 1.5418 A) radiation (40 kV, 40 mA). Nitrogen
adsorption–desorption measurements were performed at
–196 °C on a Quantachrome Autosorb-1 automated gas
adsorption system. The samples were outgassed at 300 °C for
4 h before the measurement. The pore size distribution was
obtained from the analysis of the adsorption branch of the
isotherms using the BJH method. Vanadium content of the
samples was analyzed with inductively coupled plasma
atomic emission spectrometry allied analytical system
(Thermo iCAP 6000 ICP-OES) after an aliquot of the sample
was dissolved in a mixture of HF and HNO3. Transmission
electron microscopy (TEM) measurements were taken on a
JEM-2100 electron microscope operating at 200 kV. Raman
spectra were recorded with the laser Raman spectrometer (JY
HR800) equipped with an Ar? laser (458 nm). The laser was
operated at a power level of 20 mW measured at the sample
with a power meter (Coherent). The diffuse reflectance
UV—vis (DRUV—vis) spectra were measured with Shima-
dzu UV-2550, and BaSO4 was used as an internal standard.
All spectra were recorded in the wavelength range of
200–700 nm. TheKubelka–Munkfunction(F(R?))wasused
to convert diffuse reflectance data into absorption spectra.
2 Experimental
2.1 Synthesis of V-SBA-16
V-SBA-16 materials were synthesized under acidic con-
ditions, using tetraethyl orthosilicate (TEOS) and ammo-
nium metavanadate as silica and vanadium precursors,
respectively. Nonionic triblock copolymer surfactants
EO106PO70EO106 (F-127, BASF) and EO20PO70EO20
(P-123, BASF) were used as the structure-directing agents.
Typically, the samples were synthesized by a method based
on the direct hydrothermal synthesis procedure [29]. An
aqueous solution of copolymers was prepared by dissolving
2.3 Catalytic activity tests
Hydroxylation of benzene to phenol by O2 was typically
performed in stainless steel reactor using the mixture
(acetic acid:water, 2:1) as the reaction medium. 0.88 g
123

Synthesis of V-SBA-16 and its Catalytic Performance
621
benzene (11.3 mmol), 0.10 g catalysis and 1.50 g ascorbic
acid (8.5 mmol) were introduced into the reactor. The
reactor was charged with O2 (1.7 MPa) and the reaction
was held for 10 h at 80 °C.
The products were collected, filtered and analyzed by
gas chromatography (GC) fitted with an OV-1 capillary
column (30 m 9 0.25 mm 9 0.33 lm) connected to a FID
detector. Phenol, catechol and hydroquinone in the liquid
product were identified by GC-MS (Agilent 6890/5973 N).
The quantitative analysis of the mixture was determined by
the calibration curves, using toluene as the internal
standard.
incorporated into V-SBA-16 catalysts. The V content of the
samples prepared at pH of 1.0, 1.5, 2.0 and 2.5 are deter-
mined to be 0.38, 1.03, 1.67 and 1.60, respectively. This
trend is attributed to the change of V species from VO2? to
polymeric H2V10O284-, and these H2V10O284- species that
show interaction with the silicon alkoxide. The ICP results
imply that the pH at the initial gel in hydrothermal syn-
thesis plays an important role in the introduction of V
species. In summary, when the pH was adjusted to 2.0, a
relative higher V content of 1.67 was obtained.
3.1.2 XRD
The small-angle XRD (SAXRD) patterns of V-SBA-16
catalysts are shown in Fig. 1a. It is obvious that the pat-
terns of VS-n catalysts (n = 0.5, 1.0, 1.5 and 2.0) exhibit a
very strong reflection peak corresponding to (110) reflec-
tion peak, characteristics of the cubic (Im3 m) mesoporous
structure of SBA-16 [1, 4, 27, 31], indicating that ordered
mesoporous materials have been obtained at pH range from
0.5 to 2.0. In addition, two weak peaks attributed to
the(200) and (211) reflections in 1–1.5 degrees can also be
observed for the VS-1.5(1.03) and VS-2.0(1.67) sample,
show that well ordered V-SBA-16 mesoporous materials
have been synthesized within this pH range. However,
when the pH value is adjusted to 2.5, the pattern of
VS-2.5(1.60) displays a very weak peak at around 0.828
with the disappearance of higher order peaks, which
implies that the ordered long-range structure of mesopor-
ous SBA-16 is damaged in spite of a high amount of
vanadium that was introduced. The damage may be caused
by the absence of sufficiently strong electrostatic, hydrogen
bonding and van der Waals interactions between charge-
associated EO units and the cationic silica species at low
acidity, leading to the formation of disordered porous silica
[32, 33]. Moreover, as shown in Table 1, the d110 value and
unit-cell parameter (ao) of V containing SBA-16 catalysts
3 Results and discussion
3.1 Characteristics of V-SBA-16
The pH value is an important synthesis parameter for
effective vanadium incorporation and to obtain an ordered
mesoporous material [27, 30]. A serial of V-SBA-16 cat-
alysts at different pH range from 0.5 to 2.5 in the initial gel
were prepared and characterized by ICP, XRD, TEM,
nitrogen adsorption, DRUV—vis and Raman spectroscopy.
3.1.1 ICP-OES results
The vanadium content of V-SBA-16 samples prepared at
the initial gel system with different pH value was measured
by ICP–OES analysis (see Table 1). It is clear that the lack
of V present in the V-SBA-16 sample under the pH of 0.5
is due to the high acidity condition. The V species in the
solution exists as VO2? which could not co-condense with
Si(OC2H5)4, thus, it is difficult to introduce vanadium into
the structure of SBA-16 [26]. As a result, the VS-0.5(0) is
finally prepared as a pure siliceous SBA-16 like sample.
With the increase of pH, the V species was successfully
Table 1 Physicochemical parameters of VS-n(x) catalysts
Si/Va
d110
SBET
(m2g-1
VT
(cm3g-1
)
DBJH
(nm)
b
ao (nm)
c
d
e
Sample
pH
V content
(wt%)
)
VS-0.5(0)
VS-1.0(0.38)
VS-1.5(1.03)
VS-2.0(1.67)
VS-2.5(1.60)
a
0.5
1.0
1.5
2.0
2.5
0
?
220
81
10.64
12.09
12.27
11.31
–
15.04
17.09
17.35
15.99
–
933.5
997.6
946.3
943.8
685.9
0.87
0.99
1.05
1.13
1.10
5.46
7.83
6.64
6.58
0.38
1.03
1.67
1.60
50
52
4.35/6.57
The mol ratio of Si/V is calculated from the ICP results
The lattice constant was calculated by ao = 1.414 9 d(110)
BET specific surface area
b
c
d
e
Total pore volume
Pore diameter calculated by the adsorption branch of isotherm using the BJH method
123

622
L. Zhao et al.
110
(A)
(B)
VS-0.5(0)
VS-1.0(0.38)
VS-1.5(1.03)
VS-0.5(0)
VS-1.0(0.38)
200
211
200
VS-2.0(1.67)
VS-2.5(1.60)
VS-1.5(1.03)
211
VS-2.0(1.67)
VS-2.5(1.60)
1
2
3
4
10
20
30
40
50
60
2 Theta (degree)
2 Theta (degree)
Fig. 1 SAXRD patterns (a) and WAXRD patterns (b) of various VS-n(x) catalysts
(A)
(B)
VS-0.5(0)
VS-0.5(0)
VS-1.0(0.38)
VS-1.5(1.03)
VS-1.0(0.38)
VS-1.5(1.03)
VS-2.0(1.67)
VS-2.0(1.67)
VS-2.5(1.60)
VS-2.5(1.60)
25 30
5
10
15
20
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P/P0)
Pore diameter (nm)
Fig. 2 N2 adsorption–desorption isotherms (a) and pore size distributions (b) of VS-n(x) catalysts
(VS-1.0(0.38), VS-1.5(1.03), VS-2.0(1.67)) are larger than
that of the pure siliceous SBA-16 like sample (VS-0.5(0)),
which is attributed to the presence of vanadium in the
SBA-16 silicate framework [27, 29, 34]. This is consistent
with what would be expected since the radius of the V5?
cation is larger than that of the Si4? cation and the V–O
bond length is longer than the Si–O bond length [27].
The wide-angle XRD (WAXRD) patterns of V-SBA-16
catalysts are shown in Fig. 1b. All catalysts prepared in the
pH range of 0.5–2.5 show similar profiles as amorphous
SiO2 of the SBA-16 silica. No characteristic reflections of
the V2O5 phase were observed, which further confirms the
incorporation of V species into SBA-16 framework. These
highly dispersed V species can not be identified by XRD
methods [28].
3.1.3 N2 adsorption–desorption
The nitrogen adsorption–desorption isotherms of V-SBA-
16 catalysts are shown in Fig. 2. The VS-n samples
(n = 0.5, 1.0, 1.5 and 2.0) show type IV isotherms and H2
hysteresis loops, which are typical of the mesoporous SBA-
16 materials with 3D cage-like structures [1, 35, 36].
However, the VS-2.5(1.60) displays type IV isotherms with
123

Synthesis of V-SBA-16 and its Catalytic Performance
623
3.1.4 TEM
a typical H3 hysteresis loop, which indicates that the
ordered mesoporous structure of SBA-16 is destroyed.
These results are in good agreement with small-angle XRD
analysis. The physical structure parameters of V-SBA-16
samples are displayed in Table 1. The VS-n samples
(n = 0.5, 1.0, 1.5 and 2.0) exhibit large specific surface
area ([ 930 m2/g) with uniform pore size distribution
(5.46–7.83 nm) and pore volume ([0.87 m3/g), while the
VS-2.5(1.60) sample shows relatively poor structure
parameters due to its disordered mesoporous structure.
Furthermore, the pore diameter, BET surface area and total
pore volume of well ordered V containing SBA-16 cata-
lysts (VS-1.0(0.38), VS-1.5(1.03) and VS-2.0(1.67)) are
obviously larger than that of siliceous SBA-16 like mate-
rials VS-0.5(0), which confirm the incorporation of V
species into the SBA-16 framework due to the expansion of
the mesoporous structure [37]. It is in accordance with
the XRD results that the unit-cell parameter (ao) of
VS-n(x) catalysts is larger than that of the pure siliceous
SBA-16 like sample.
The TEM images of V-SBA-16 catalysts are shown in
Fig. 3. It can be clearly observed that the VS-0.5(0) sample
(Fig. 3a) has well ordered mesoporous cubic arrays along
the (111) and (100) directions and also exhibits a typical
3D cubic (Im3 m) pore structure of SBA-16 [1, 38, 39].
With the increase of the pH of the initial gel, the TEM
images of VS-1.5(1.03) (Fig. 3b) and VS-2.0(1.67)
(Fig. 3c) catalysts still show the ordered mesoporous
structure though the quality of pore ordering decreases
slightly. The VS-2.5(1.60) sample shows a disordered
mesoporous structure (see Fig. 3d), which is consistent
with the XRD and N2 adsorption–desorption results.
Moreover, the vanadium oxide (VOx) can also be clearly
observed on the surface of VS-2.5(1.60) catalyst.
From the XRD, TEM, ICP and N2 adsorption–desorp-
tion isotherms discussed above, it is clear that a function of
the pH of the initial gel plays an important role in the
synthesis of V-SBA-16 catalysts. The V species could not
Fig. 3 TEM images of V-SBA-16 catalysts: a VS-0.5(0), b VS-1.5(1.03), c VS-2.0(1.67) and d VS-2.5(1.60)
123

624
L. Zhao et al.
be introduced into SBA-16 material in the pH range lower
than 0.5, while the ordered mesoporous structure of SBA-
16 would be damaged in the pH range higher than 2.5.
Therefore, it is necessary to control the proper pH range of
1.0–2.0 at the initial gel system in order to obtain uniform
mesoporous V-SBA-16 catalysts with high V content.
the formation of extra-framework V2O5 species. These
microcrystalline V2O5 species can be easily observed by
Raman spectroscopy but can not be detected by XRD
measurement [28, 41, 43].
3.1.6 DRUV—vis spectra
3.1.5 Raman spectra
DRUV—vis spectra were also recorded in order to identify
the nature of observed coordination of vanadium species in
the V-SBA-16 catalysts. As shown in Fig. 5, there is no
obvious absorption band assignable to V species in the
range of 200–700 nm for the VS-0.5(0) sample which
agrees well with the ICP, XRD and Raman results. How-
ever, the spectra of VS-1.0(0.38), VS-1.5(1.03) and
VS-2.0(1.67) samples display an intense band centered at
about 262 nm with a shoulder at 303 nm, which can be
assigned to the charge-transfer transitions of the ligand O2-
to metal center V5? for the isolated tetrahedral VO4 species
(O3/2V = O) [40]. As far as the VS-2.5(1.60) is concerned,
the band assigned to VO4 species shifts to higher wavelength,
and one additional band is observed at about 481 nm which
could be associated with the presence of octahedral V5?
species, as above-mentioned V2O5 crystallites that have been
determined by Raman spectra [44, 45].
Raman spectroscopy is used to elucidate the nature of
vanadium species on the V-SBA-16 catalysts (see Fig. 4).
Figure 4a depicts the laser Raman spectra of various
V-SBA-16 samples. The spectra of VS-0.5(0) show no
typical Raman bands of V species, which is in consistent
with the ICP analysis that no vanadium is present in this
sample. With the increase of pH (1.0–2.0) in the initial gel
system, the spectra of V-SBA-16 samples exhibit an
intense band at around 1,040 cm-1 with a shoulder at
1,072 cm-1, which are assigned to terminal V = O stretch
vibration of monomeric tetrahedral VO4 species (Td-V5?
)
[40, 41]. Furthermore, the intensity of these bands increa-
ses with the increasing V content (see Table 1). However,
the VS-2.5(1.60) that was prepared at a pH 2.5 showed an
additional band at 998 cm-1 which is assigned to micro-
crystalline V2O5 species [41, 42]. These results reveal that
an aggregation of V species occurred on the surface of the
VS-2.5(1.60) catalyst at the pH of 2.5, which may lead to
The results of pore structure characterization and V
species analysis imply that the V species could not be
introduced into the SBA-16 structure for the VS-0.5(0)
262
303
1040
1072
998
VS-2.5(1.60)
481
VS-2.0(1.67)
VS-1.5(1.03)
VS-2.5(1.60)
VS-2.0(1.67)
VS-1.0(0.38)
VS-0.5(0)
VS-1.5(1.03)
VS-1.0(0.38)
VS-0.5(0)
600
800
900
1000
1100
1200
200
300
400
500
700
Raman shift (cm-1)
Wavelength (nm)
Fig. 4 Laser Raman spectrums of VS-n(x) catalysts
Fig. 5 DRUV—vis spectra of VS-n(x) catalysts
123

Synthesis of V-SBA-16 and its Catalytic Performance
625
sample due to the high acidity though it possesses a well
ordered mesoporous structure. When the pH at the initial
gel system is adjusted to 2.5, a relative high V content of
1.60 wt% was obtained for the VS-2.5(1.60) sample.
However, a low V dispersion is observed which is attrib-
uted to the change of partial V species from isolated VO4 to
agglomerated V2O5 crystallites along with the formation of
disordered mesoporous structure at the weakly acidic
condition. Thus, ordered mesoporous V-SBA-16 materials
containing monomeric tetrahedral framework V species
should be synthesized in the pH range of 1.0–2.0.
sample, which may be not only attributed to the decrease of
the V content, but also related to its low V dispersion with
the presence of crystalline V2O5 species and its disor-
dered mesoporous structure caused by low acidity during
preparation, in accordance with the characterizations of
pore structure and V species discussed above. From
VS-1.0(0.38) to VS-2.5(1.60) with the increasing the syn-
thesis pH value at the initial gel system, the TON of phenol
over V-SBA-16 shows great decline, which may be ascri-
bed to the increase of V content and incidental decrease of
V dispersion. Especially, the VS-2.5(1.60) sample with the
lowest V dispersion and disordered mesoporous structure
displays a relatively low TON of 95. These results indicate
that the isolated V species highly dispersed on uniform
mesoporous network play an important role in the
hydroxylation of benzene over V-SBA-16 catalysts.
The effect of the reaction time on catalytic benzene
hydroxylation has also been studied over VS-2.0(1.67)
sample shown in Table 2. At a the reaction time of 6 h, the
conversion of benzene and the yield of phenol are only
10.2 and 9.8%, respectively. At longer times, both the
conversion of benzene and the yield of phenol increase
quickly; and the yield of phenol around 30.4% could be
acceptable at the proper contact time for 10 h. However,
the selectivity to phenol decreases but the selectivity to
catechol and hydroquinone increases. This is due to the
phenol is the more active than benzene; when the reaction
time further increase, the rate of over oxidation occur
rapidly.
3.2 Catalytic evaluations
Figure 6 depicts the catalytic activities of benzene
hydroxylation over V-SBA-16 using O2 as the oxidant. It is
clear that the pure siliceous SBA-16 like of VS-0.5(0)
catalyst is inactive for oxidation of benzene to phenol.
With the substantial incorporation of V species, the
V-SBA-16 samples show the obvious catalytic activity of
benzene hydroxylation. Phenol is main product, benzo-
quinone, catechol and hydroquinone are the primary
byproducts and no benzene is oxidized to CO and CO2 for
the present research. The yield of phenol over V-SBA-16
catalysts increases quickly with increasing V content and
the corresponding increase of synthesis pH from 1.0 to 2.0
at the initial gel system. The highest phenol yield of 30.4%
with selectivity of 90% and turnover number of 105 were
obtained over VS-2.0(1.67) sample, which may be attrib-
uted to its high V content and the highly dispersed VO4
species on the well ordered SBA-16 type mesoporous
materials. However, an obvious decreases of yield and
selectivity for phenol was observed for VS-2.5(1.60)
4 Conclusion
In summary, the influences of the pH of the initial gel
system in the synthesis process of V-SBA-16 have been
300
100
Table 2 Effect of reaction time on catalytic activity over the
VS-2.0(1.67) sample
Sphenol
80
Time (h) Ca (%) Sb (%) Sc (%) Sd (%) Ye (%) TONf
200
TON
6
10.2
19.3
33.7
36.3
96.5
92.1
90.0
77.6
2.3
5.8
6.7
6.3
1.2
2.1
9.8
17.8
30.4
28.2
33.8
61.4
Yphenol
8
20
10
12
3.3
105
97.2
100
16.1
Reaction conditions: acetic acid:water = 2:1 (V:V), benzene
(11.3 mmol), catalyst (0.10 g), ascorbic acid (8.5 mmol), 1.7 MPa
O2, reaction temperature 80 °C
0
0
a
The conversion of benzene
VS-0.5(0) VS-1.0(0.38) VS-1.5(1.03) VS-2.0(1.67) VS-2.5(1.60)
b
The selectivity of phenol
Different catalysts
c
The selectivity of catechol
Fig. 6 Catalytic activity of benzene hydroxylation over different
V-SBA-16 catalysts (reaction condition:acetic acid and water (2:1) as
solvent, 11.3 mmol benzene, 8.5 mmol ascorbic acid, O2 pressure of
1.7 Mpa, 80 °C for 10 h, 0.10 g of catalyst)
d
The selectivity of hydroquinone
e
The yield of phenol
f
The moles of phenol formed per mol V amount of the VS-2.0(1.67)
123

626
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examined and shown a dependence on various factors, such
as vanadium introduction, reservation of mesoporous
structure and dispersion of V species which were carefully
studied by ICP, XRD, TEM, nitrogen adsorption–desorp-
tion, DRUV—vis and Raman spectroscopy. The results
imply the pH of the initial gel system plays an important
role in the synthesis of V-SBA-16, and show that well
ordered V containing SBA-16 type mesoporous materials
with highly dispersed isolated VO4 species can be syn-
thesized in the pH range of 1.0 to 2.0. The VS-2.0(1.67)
catalyst shows fascinating phenol yield of 30.4% with
phenol selectivity of 90.0% and high TON of 105 due to its
high V content and high dispersion of isolated vanadium
species in the framework of mesoporous SBA-16 type
materials. These isolated framework tetrahedral VO4 spe-
cies is a key factor to acquire the high TON in the benzene
hydroxylation reaction. The decrease of V dispersion and
appearance of the V2O5 crystallites are disadvantageous for
selective oxidation of benzene.
Acknowledgments This work is supported by the Natural Science
Foundation of China (20876034), Program for New Century Excellent
Talents in Heilongjiang Provincial University (1155-NCET-014),
Major Foundation of Educational Commission of Heilongjiang
Province of China (11531Z11) and Heilongjiang University Natural
Science Funds for Distinguished Young Scholar (JCL200802). The
authors also thank Dr. Alexander D. Gordon (Department of Chem-
istry, University of California, Riverside, USA) for his help in the use
of the English language.
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