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Microporous and Mesoporous Materials 110 (2008) 271–278

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Niobium rich SBA-15 materials – preparation, characterisation

and catalytic activity
M. Trejda a, A. Tuel b, J. Kujawa a, B. Kilos c, M. Ziolek a,*

a
Adam Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, PL-60-780 Poznan, Poland
b
Institut de Recherches sur la Catalyse, CNRS, 2 Av. A. Einstein, 69626 Villeurbanne, France
c
Chemical Sciences Division, Lawrence Berkeley National Laboratory, and Department of Chemical Engineering,
University of California, Berkeley, CA 94720, USA

Received 1 May 2007; received in revised form 7 June 2007; accepted 7 June 2007
Available online 14 June 2007

Abstract

Hexagonally ordered mesoporous niobiosilicates of SBA-15 type were synthesised by a new synthesis route in which the use of hydro-
chloric acid was avoided. This route allowed the incorporation of much higher amounts of niobium into the mesoporous network (Si/Nb
between 15 and 5) as compared to the conventional method of preparation. The location of niobium was determined by UV–Vis and
XRD techniques. The latter combined with N2 adsorption measurements allowed the estimation of texture/structure parameters. The
high niobium content in the NbSBA-15 had a significant impact on the sulphurisation of methanol leading to the formation of organic
sulphur compounds. It is well correlated with the acidity of the materials measured by pyridine adsorption followed by FTIR studies.
Such a relationship was not observed for oxidation of methanol and epoxidation of cyclohexene showing that the redox properties are
not linear related to the amount of niobium if Si/Nb is equal or below 15 (as applied in this work).
 2007 Elsevier Inc. All rights reserved.

Keywords: NbSBA-15; Niobium species; Methanol oxidation; Methanol sulphurisation; Cyclohexene oxidation; Pyridine adsorption

1. Introduction Texture/structure parameters, such as pore diameters,

and defect holes [1] were considered among others. More-
Niobium containing mesoporous molecular sieves of over, the isolation of Nb species in liquid phase oxidation
MCM-41 [1], SBA-3 [2], and SBA-15 [3] types have been with H2O2 [4], often related to the concentration of nio-
found to be attractive catalysts for the gas and liquid phase bium, were mentioned. To obtain a good isolation and high
oxidation of organic compounds [4]. The application of concentration of Nb, all niobium atoms must be included
solid catalysts in the liquid phase reactions gives rise to into the framework. This incorporation of the Nb protects
the heterogeneous system, which is advantageous from against excessively fast H2O2 decomposition, which other-
many points of view. According to the last trends, the wise occurs on dimers or oligomers of active metal species
use of clean oxidants, such as hydrogen peroxide, makes and decreases the activity. The use of traditional synthesis
the environmentally-friendly alternative possible compared methods did not allow the elimination of extra framework
to the traditional oxidation processes, which need an inor- niobium species in SBA-15 materials when the Si/Nb ratio
ganic oxidant in stoichiometric amounts. Several features was lower than 128 [3]. Therefore, in this work, we have
influence the catalytic activity of heterogeneous catalysts. applied a new synthesis route which allowed us to produce
niobium-rich SBA-15 materials in which Nb was located
exclusively in the framework. This synthesis method was
*
Corresponding author. Tel.: +48 61 8291243; fax: +48 61 8658008. environmentally friendly because it avoided the use of
E-mail address: ziolek@amu.edu.pl (M. Ziolek). hydrochloric acid. These materials were characterised in

1387-1811/$ - see front matter  2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.micromeso.2007.06.015
272 M. Trejda et al. / Microporous and Mesoporous Materials 110 (2008) 271–278

detail within this work and they were tested in gas and admitted at 373 K. After saturation with PY the samples
liquid phase oxidation reactions, i.e. the oxidation and were degassed at 373, 423, 473, 523 K in vacuum for
hydrosulphurisation of methanol and the oxidation of 30 min. Spectra were recorded at room temperature in
cyclohexene, respectively. the range from 4000 to 400 cm 1. The spectrum without
The aim of this study was to obtain better insight into any sample (‘‘background spectrum’’) was subtracted from
hexagonally ordered niobiosilicate materials containing a all recorded spectra. The IR spectra of the activated sam-
relatively high concentration of Nb species, their structure ples (after evacuation at 673 K) were subtracted from those
as well as catalytic activity. recorded after the adsorption of PY followed by various
treatments. The reported spectra are the results of this
2. Experimental subtraction.

2.1. Catalyst preparation 2.4. Methanol oxidation and sulphurisation

All mesoporous solids were synthesized using niobium Reactions were performed in a fixed-bed flow reactor.
pentachloride (NbCl5) and tetraethyl orthosilicate (TEOS) The pressed materials were granulated to 0.5 < B < 1 mm
as niobium and silica sources, respectively, without addi- size fraction. 0.02 g of the catalysts (calculated for the
tion of hydrochloric acid. In a typical preparation, NbCl5 dehydrated materials), were placed into the reactor. The
was dissolved in 126 mL H2O, followed by the addition samples were activated in helium flow (40 cm3 min 1) at
of 4 g of Pluronic P123 (Poly(ethylene glycol)-block- 723 K for 2 h. Next the temperature was decreased to
Poly(ethylene glycol)-block-Poly(ethylene glycol)-block) 573 K or 623 K for oxidation and sulphurisation, respec-
copolymer. After complete dissolution of the polymer, tively. The gas mixture of MeOH and O2, diluted by He,
9.1 mL TEOS were added and the mixture was vigorously was used for the oxidation process with a total flow rate
stirred at 313 K overnight. The gel was then placed in an of 40 cm3 min 1. The same total flow rate of MeOH/
oven and heated at 373 K for 24 h. After this period, the H2S/He mixture was used for the sulphurisation process
solid was filtered, washed with deionized water and air- (molar ratio of MeOH to H2S was 1:2). The reactor effluent
dried at room temperature. The last numbers in the catalyst was analysed using an online gas chromatograph (SRI
symbols denote the Si/Nb ratio in the solids. They corre- 8610 GAS) with FID and TCD or FPD (for sulphur) detec-
spond to the following Si/Nb ratios in the gels: 3 (5 – in tors. Hydrogen was used as a carrier gas. Substrates and
the solid), 5 (7.5), 9 (10), 15 (15). products were separated on a 30 m column filled with
GS-Q. The column was heated as follows: at 308 K for
2.2. Catalyst characterisation 40 min, then 10 K min 1 up to 508 K in the case of oxida-
tion process. For sulphurisation the heat program was as
XRD patterns were recorded at room temperature on a follows: at 318 K for 18 min, then 8 K min 1 up to 508 K.
Bruker AXS D8 Advance apparatus using Cu Ka radiation
(k = 0.154 nm), with a step size of 0.02o and 0.05o in the 2.5. Oxidation of cyclohexene with hydrogen peroxide
small-angle range and in the high-angle range, respectively.
N2 adsorption/desorption isotherms were obtained in a The reaction was performed at 318 K in the liquid-phase
Micromeritics ASAP equipment, model 2010. The samples using acetonitrile as a solvent. The catalytic reaction
(200 mg) were pre-treated in situ under vacuum at 573 K between cyclohexene and hydrogen peroxide was carried
for 3 h. The surface area was calculated using the BET out in a glass flask equipped with a magnetic stirrer, a ther-
method. The pore size distributions (PSD), the pore sizes mocouple, a reflux condenser and a membrane for sam-
(the maximum of the PSD), and the mesopore volumes pling. 0.04 g of a calcined catalyst was placed into the
were determined from the adsorption branch of isotherms. flask, and the solvent was added. The oxidation was con-
UV–Vis spectra were registered using a Varian-Cary 300 ducted by efficient stirring of a mixture of a solvent and
Scan UV–Visible Spectrophotometer. Catalyst powders a catalyst at 318 K. After stirring for 15 min at 318 K,
were placed into the cell equipped with a quartz window. cyclohexene (2 mmol) was added, followed by the drop
Spectra were registered in the range from 800 to 190 nm. wise addition of 35% hydrogen peroxide (2 mmol). Sam-
BaSO4 was used as a reference. ples were withdrawn at regular time intervals and analysed
by a gas chromatograph GC 8000 Top equipped with a
2.3. Acidity measurements capillary column DB-1 attached to a FID, operated with
a heating program: 340 K for 15 min, ramp 10 K min 1
Infrared spectra were recorded with a Bruker Vector 22 to 360 K (kept for 13 min). Some catalysts were examined
FTIR spectrometer using an in situ cell. Sample were in the second run, i.e. after 40 h of the reaction and separa-
pressed under low pressure into a thin wafer of ca. tion from the mixture by gyration. The catalyst was regen-
6 mg cm 2 and placed inside the cell. Catalysts were evac- erated before the second reaction by calcination at 673 K
uated at 673 K during 4 h and pyridine (PY) was then for 4 h.
 M. Trejda et al. / Microporous and Mesoporous Materials 110 (2008) 271–278 273

3. Results and discussion 1000

900
3.1. Synthesis

3 -1
800

Volume adsorbed, cm g
A series of compounds were prepared by varying the 700
amount of niobium chloride in the gel. Low Nb contents 600
did not lead to mesoporous compounds because of too
500
high pH values. Indeed, in the absence of HCl, the pH of
the solution was directly dependent on the amount of 400
e
NbCl5 present. Actually, relatively well-ordered mesopor- 300
ous compounds could be obtained with Si/Nb ratios rang- 200
ing between 3 and 15 in the gel. Four samples were selected,
100 c d 0.3
corresponding to Si/Nb ratios of 5, 7.5, 10 and 15 in the a b
calcined solids. 0
Relative pressure, p/p0

3.2. Texture/structure characterisation Fig. 1. N2 adsorption/desorption isoterms: (a) NbSBA-15-5; (b) NbSBA-
15-7.5; (c) NbSBA-15-10; (d) NbSBA-15-15; (e) NbSBA-15-32.

The characterisation of the prepared materials, with the

variety of Si/Nb ratios between 15 and 5 (the last number tribution became slightly wider with increasing niobium
in the catalyst symbol denotes Si/Nb ratio), is shown in content (Fig. 2).
Table 1. Texture parameters were calculated from nitrogen The location of niobium species in NbSBA-15 materials
adsorption measurements. The BET surface areas were can be deduced from XRD and UV–Vis measurements.
typical of SBA-15 materials and were in the range of The first technique was useful for the identification of crys-
710–840 m2 g 1 depending on the niobium content. The talline phases present in the matrix (so called extra frame-
smallest surface area was detected for the material with work species). Taking into account the high niobium
the highest Nb concentration (NbSBA-15-5; with the Si/ amount introduced during the synthesis (up to Si/
Nb ratio = 5) and the highest surface area was obtained Nb = 5) one could expect Bragg reflections corresponding
for the catalyst possessing Si/Nb ratio = 15, i.e. NbSBA- to the niobium species (e.g. from niobium oxide species
15-15. The clear relationship between Nb amount and the
texture parameters was only seen regarding the BET sur-
face areas, but it was not of linear dependence. The other
textural parameters did not change systematically in rela- 0.01
tion to Nb concentration. The N2 adsorption–desorption
Volume adsorbed, cm3g-1

isotherms (Fig. 1) characterised the difference between the

prepared catalysts. The isotherms were of type IV accord-
ing to the IUPAC classification [5] for all the samples
and exhibited the H1-type hysteresis loop, which is typical
NbSBA-15-15
of large-pore mesoporous solids. This hysteresis loop was
much narrow than that typically observed on NbSBA-15
NbSBA-15-10
materials prepared by the traditional route (Fig. 1e) [3].
That behaviour indicated the absence of plugs in ordered NbSBA-15-7.5
channels of the samples prepared within this work contrary
NbSBA-15-5
to that described in [3]. The positions of the inflection
points were clearly related to diameter in the mesoporous
10 20 30 40 50 60 70 80 90 100
range. The sharpness of the step well indicated for
Pore diameter, nm
NbSBA-15-15 and NbSBA-15-10 proved the uniformity
of the mesopore size distribution (PSD). The pore size dis- Fig. 2. Pore diameter distribution of prepared materials.

Table 1
Textural/structural parameters from low-temperature N2 adsorption/desorption
Catalysta BET, m2 g 1
Vtot, cm3 g 1
PD, nm (average) PSD max, nm Vmeso, % Vmicro, %
NbSBA-15-5 710 1.11 6.7 9.2 87 13
NbSBA-15-7.5 740 1.35 8.1 9.3 91 9
NbSBA-15-10 830 1.11 6.6 9.1 86 14
NbSBA-15-15 840 1.21 7.1 9.3 88 12
a
The last number stands for Si/Nb atomic ratio in the solid.
274 M. Trejda et al. / Microporous and Mesoporous Materials 110 (2008) 271–278

in the XRD patterns). Yet such peaks were not observed in fested by the infrared bands at 1550 cm 1 and two others
the high angle XRD range. These results suggested that the in the 1620–1640 cm 1 range. The number of BAS was esti-
niobium species introduced during the synthesis were mated using e1550 = 1.8 lmol 1 cm [7].
located in the walls of the materials or that crystalline nio- After pyridine adsorption and evacuation at 373 K for
bium species were very well dispersed on the walls. This 0.5 h, the infrared spectra showed characteristic bands at
result is in agreement with the N2 adsorption–desorption 1449, 1490, 1545, 1577, 1611, 1639 cm 1 (Fig. 4). One
isotherms, where the narrow hysteresis loop was observed. can notice that the intensity of these bands depended on
The complementary technique (UV–Vis) was applied to the amount of niobium species in the samples, i.e. Si/Nb
differentiate which case took place in the studied materials. ratio. The same occurrence is clearly showed in Table 2,
The UV–Vis spectra are shown in Fig. 3. All spectra con- where the number of LAS and BAS are exhibited. In
tained a broad band which covered two, poorly resolved Fig. 5 the number of LAS was plotted versus the amount
maxima at about 228 and 250 nm. The intensity of these of niobium species (Si/Nb ratio). It is important to stress
bands increases with the niobium content. These two bands the almost linear correlation between these two values.
corresponded to two different niobium species located in These results support earlier indications (from XRD and
the silica network. Different niobium species have been pre- UV–Vis) that the incorporation of niobium species into
viously reported for the niobiosilicate MCM-41 material SBA-15 material is efficient, i.e. it leads to the creation of
[6]. They were identified by FTIR and ESR study as isolated niobium species in the walls (not in the extra
Nbd+ and NbOd in mesoporous matrices. The octahedral framework locations).
niobium oxide species (in the extra framework positions), if
present, should generate a UV band at ca. 330 nm. Regard-
3.4. Methanol oxidation
ing the spectra presented in Fig. 3, one can notice the lack
of UV–Vis band at above 300 nm due to the extra frame-
The oxidation of methanol is an industrially important
work niobium species. This absence was a very important
process, which can be carried out towards various target
finding, which also indicated the location of niobium spe-
products, including formaldehyde among others. This reac-
cies inside (on) the walls of mesoporous materials (not in
tion has been also well adapted to the characterisation of
the extra framework positions).
the redox and acidic active species on the catalyst surface

3.3. Acidity measurements

0.05 1611
For the evaluation of the amount and type of acidic cen- 1639 1577 1449
tres, infrared spectroscopy combined with the adsorption 1545 1490
Si/Nb=15
of pyridine was applied. After the interaction of pyridine
Absorbance

with Lewis acid sites (LAS) the characteristic bands at Si/Nb=10

1450 and 1610 cm 1 appeared [7]. The intensity of the
first band was related to the number of LAS (with e1450 Si/Nb=7.5
= 1.5 lmol 1 cm [7]), whereas the position of the second
band characterised the strength of LAS [7]. Pyridine also Si/Nb=5
interacted with Brønsted acid sites (BAS) and was mani-

228
5 252
1800 1750 1700 1650 1600 1550 1500 1450 1400
-1
Wavenumber, cm

Fig. 4. FTIR spectra after adsorption of pyridine and evacuation at 373 K

for 0.5 h on NbSBA-15 materials with different Si/Nb ratios.
F(R)

NbSBA-15-5 Table 2
Number of BAS and LAS calculated per 1 g of the catalysts on the basis of
NbSBA-15-7.5
IR bands observed after desorption of pyridine at 373 K (e1450 =
NbSBA-15-10 1.5 lmol 1 cm, e1550 = 1.8 lmol 1 cm [7])
Sample Number of Number of Total number of
NbSBA-15-15 LAS · 1017 BAS · 1017 acid sites · 1017

200 300 400 500 600 700 800 NbSBA-15-5 670.8 127.1 797.9
NbSBA-15-7.5 563.2 89.5 652.7
Wavelength, nm NbSBA-15-10 508.8 90.4 599.2
NbSBA-15-15 372.0 64.7 436.7
Fig. 3. UV–Vis spectra of prepared materials.
 M. Trejda et al. / Microporous and Mesoporous Materials 110 (2008) 271–278 275

700 materials in methanol oxidation process, with the exception

of NbSBA-15-10, were similar, and reached ca. 30%. These
650
results suggested that the conversion of alcohol was inde-
pendent of the niobium content in the sample, and the dif-
600
ferent behaviour of NbSBA-15-10 material should be
LAS *10 /g

explained by some other factors. Indeed there were some

17

550
dissimilarity between the samples. The catalyst, which
500 showed the highest activity in methanol oxidation pro-
cesses, possessed the smallest average pore diameter, which
450
was only comparable to that in the richest niobium sample,
400
i.e. NbSBA-15-5. Moreover the pore size distribution in
NbSBA-15-10 was the most uniform, which can be an
350 advantage in methanol oxidation processes (see Fig. 2).
Before steady state is reached, the short initial period of
6 8 10 12 14 16
the catalyst activation was observed (Fig. 6). After that
Si/Nb ratio
period the conversion of alcohol stabilised on a certain
Fig. 5. Si/Nb ratio vs. LAS number for NbSBA-15 materials with level and did not change significantly during the reaction
different amount of Nb. time. Once again NbSBA-15-10 stood out from the other
samples. While there were some differences between the
[8–11]. The structural information concerning active sites conversion of methanol in the first step of the process
can be deduced from the selectivity and activity in metha- and the steady state for most catalysts, the activity of
nol oxidation processes. NbSBA-15-10 at the beginning of the reaction was quite
The average conversion of methanol (counted from all high.
experimental points during the whole reaction at 523 K The very similar conversion levels of methanol obtained
with the exception of the first one registered before steady for three catalysts were very convenient for the comparison
state was reached) and the selectivity to the different prod- of the selectivity (Table 3). Formaldehyde was the main
ucts are shown in Table 3. The conversion versus reaction product formed on all the samples. Its formation required
time is presented in Fig. 6. The activities of the prepared the presence of pairs of acid–base Lewis centres [10]. There
was no relationship between the selectivity to formaldehyde
and the amount of niobium species in the sample. A differ-
Table 3 ent phenomenon was previously observed. Wachs et al. [9]
Average conversion and selectivity during methanol oxidation at 523 K reported that lower amounts of Nb2O5 incorporated into
Catalyst Conversion of Selectivity, % silica resulted in the higher amounts of methyl formate pro-
CH3 OH, % HCHO HCOOCH3 (CH3)2O duced (i.e. the selectivity to formaldehyde decreases). Sim-
NbSBA-15-5 28 80 14 6
ilar behaviour was observed in our group in the case of iron
NbSBA-15-7.5 29 89 7 4 containing mesoporous molecular sieves [11]. The different
NbSBA-15-15 28 86 12 2 results obtained for niobium rich SBA-15 materials can be
NbSBA-15-10 51 87 12 1 explained by the partial location of Nb species inside the
mesoporous walls. In this position, Nb species were not
accessible for the reagents.
NbSBA-15-5 Besides the mentioned products of the reaction, i.e.
70 NbSBA-15-7.5
NbSBA-15-10 formaldehyde and methyl formate, a small amount of
60 NbSBA 15-15 dimethyl ether was also formed. The generation of the
Methanol conversion, %

later product did not need an oxidation step. The inter-

50 molecular dehydration process in the presence of acid
centres led to ether production. Therefore, the selectivity
40 to dimethyl ether points out the acidic character of cata-
lysts. In the case of materials showing the same conver-
30
sion level, one can notice the slight increase of dimethyl
20 ether selectivity from 2% (in the case of NbSBA-15-15
sample) to 6% (for NbSBA-15-5). These results suggested
10 that material richest in niobium were slightly more acidic
compared to those with lower Nb amounts, which is in
0
50 100 150 200 250 300 350 400 450
agreement with the pyridine adsorption study. This phe-
nomena could be important for the catalytic behaviour
Time, min
and will be shown in relation to the oxidation of cyclo-
Fig. 6. Activity in methanol oxidation process carried out at 573 K. hexene process.
276 M. Trejda et al. / Microporous and Mesoporous Materials 110 (2008) 271–278

3.5. Reaction between methanol and hydrogen sulphide

55

The sulphurisation of methanol has been studied using

Conversion of methanol, %
50
different kinds of catalysts based on metal oxides and sulp-
hides [12–16] as well as on zeolites [17–20]. Methanethiol 45
(CH3SH) and dimethyl sulphide (CH3SCH3) were the main
40
products of sulphurisation. In the presence of acid centres
dimethyl ether can also be formed as the result of the inter- 35
molecular dehydration process.
The average conversion of methanol and the selectivity 30
to different products are shown in Table 4 (counted from
all experimental points during the whole reaction at 25
523 K with the exception of the first one before the steady
20
state). As can be seen in Fig. 7, the methanol conversion 4 6 8 10 12 14 16
increased proportionally to the amount of niobium, which Si/Nb ratio
was not the case for the oxidation. The highest conversion,
which reached 53%, was obtained for NbSBA-15-5, Fig. 7. Si/Nb ratio vs. conversion of methanol in the sulphurisation
process for NbSBA-15 materials with different amount of Nb.
whereas 24% of methanol was converted on the NbSBA-
15-15. It is worthy of notice that a similar type of nio-
bium-containing material, i.e. mesoporous molecular sieve shown in Fig. 5, however, the reverse relationship was
of MCM-41 type with Si/Nb ratio of 32, showed 17% observed. Disregarding the difference in conversion level
methanol conversion [11], which also supported the role between the four samples, which obviously had a huge
of niobium loading in the catalyst on its activity in the impact on selectivity, this phenomenon could be easily
reaction. explained by the distance between the niobium species.
Although the selectivity to methyl ether was significant, To obtain a good selectivity towards sulphurisation prod-
the main product of the reaction was methanethiol. The ucts, the Lewis acid/base centres (Nbd+ and NbOd ) must
formation of this product required the presence of Lewis be close enough to interact with the same adsorbed mole-
acid/base centres. Acid centres were responsible for alcohol cule. When the distance between centres increased, the acid
adsorption on the catalyst surface, whilst basic sites were centres acted alone leading to intermolecular dehydration.
necessary for the abstraction of hydrogen from hydrogen
sulphide. As postulated previously [6,11], such centres have
been assigned to Nbd+ (Lewis acid centre) and NbOd 3.6. Cyclohexene oxidation
(Lewis base centre) species. This occurrence was supported
by the results obtained on siliceous MCM-41 and AlMCM- The oxidation of cyclohexene was applied to examine
41 [11]. In the case of a pure-silica material, no conversion the properties of the catalysts in liquid phase reactions.
was observed during the sulphurisation of methanol. By Besides the influence of the Nb content on the activity in
contrast, the AlMCM-41 showed 45% conversion, but liquid phase oxidation, this reaction also gave information
the main product was dimethyl ether (76% selectivity). on the isolation of niobium species. Moreover, the acidic
For both samples only the surface oxygen (e.g. O2 ) could character of the materials can be deduced from the selectiv-
act as a base centre, which was not sufficient to create more ity to diol [4]. The results of cyclohexene oxidation carried
sulphurisation products. Therefore, the high selectivity to out at 313 K for 40 h are shown in Table 5.
methanethiol observed on our samples can be explained The activities of the catalysts in cyclohexene oxidation
by the presence of the two kinds of above mentioned nio- process were higher than those of materials prepared by a
bium species. traditional route [3]. In the case of the latter, the conversion
Taking into account the selectivity, one could expect of cyclohexene was only 33% for Si/Nb = 32 and 16% for
dimethyl ether to increase with the number of acid centres Si/Nb = 128, which clearly demonstrated the influence of
in the sample, which was correlated with the Si/Nb ratio as niobium content on the catalyst activity. For the materials

Table 4
Average conversion and selectivity during methanol sulphurisation at 623 K
Catalyst Conversion of CH3 OH, % Selectivity, %
(CH3)2 O CH3SH (CH3)2 S (CH3)2 S2
NbSBA-15-5 53 13 76 10 1
NbSBA-15-7.5 44 15 76 8 1
NbSBA-15-10 32 23 68 9 <1
NbSBA-15-15 24 23 72 5 –
 M. Trejda et al. / Microporous and Mesoporous Materials 110 (2008) 271–278 277

Table 5
Conversion (after 10 and 40 h) and selectivity (after 40 h) in cyclohexene a cyclohexene
100 epoxide
oxidation diol

Conversion and selectivity, %

Catalyst Conversion, % Selectivity, % 90

Cyclohexene H2O2 Epoxide Diol Other 80

10 h 40 h 70

NbSBA-15-5 17 60 90 3 73 24 60
NbSBA15-5a (0.03 g) 27 66 89 1 71 28
50
NbSBA-15-7.5 24 43 89 7 64 29
NbSBA-15-10 32 49 89 14 42 44 40
NbSBA-15-15 25 72 92 8 59 33
30
NbSBA-15-15a (0.034 g) 30 64 91 9 64 27
NbSBA-15-32(ox)b 33 68 35 65 0 20
NbSBA-15-128(ox)b 16 46 32 68 0
10
NbSBA-15-32(Cl)b 65 97 50 50 0
a 0
Catalyst after regeneration.
b 0 300 600 900 1200 1500 1800 2100 2400
Examples from [3].
Time, min

cyclohexene
prepared by the new route, the conversion oscillated b epoxide
100
between 43% and 72%, depending on the samples. There diol

Conversion and selectivity, %

90
was no linear relationship between the cyclohexene conver-
sion and the niobium content in the samples. One thus 80

observes the following order of activity: NbSBA-15-15 > 70

NbSBA-15-5 >> NbSBA-15-10 > NbSBA-15-7.5. Taking 60
into account the role of niobium content in the samples
50
on the catalytic activity, one could expect the activity to
increase with the niobium content while keeping Nb species 40

isolated. It is well known that the isolation of niobium 30

species improves the selectivity to epoxide. In the case of 20
the prepared catalysts, the selectivity to epoxide was not
10
high and reached a maximum of 14% for NbSBA-15-10.
With the exception of the above-mentioned catalyst, the 0
0 300 600 900 1200 1500 1800 2100 2400
selectivity to epoxide increased with decreasing niobium
content. Time, min
The epoxide formation did not depend only on niobium Fig. 8. Conversion and selectivity in oxidation of cyclohexene for
isolation in the samples but also on the surface acidity. In NbSBA-15-15: (a) in the first run; (b) in the second run.
the presence of acid centres, epoxide can react further with
another molecule of hydrogen peroxide or water to form a
NbSBA-15-15, the selectivity after 40 h of reaction did
diol. Therefore, this factor is also important for consider-
not significantly change, implying the stability of the mate-
ing the catalyst selectivity. The methanol oxidation process
rial. Despite the same selectivity after 40 h reaction for the
showed that niobium-rich samples had a slightly more
first and second run (Fig. 8a and 8b, respectively), the for-
acidic character compared to the samples with the lower
mation of diol was faster after regeneration of the catalyst,
amount of niobium. These results were in good agreement
which was attributed to the modification of the catalyst
with the diol formation selectivity in cyclohexene oxidation
surface during the first run of the process. For NbSBA-
process. For the catalysts possessing the higher amount of
15-5 the selectivity to epoxide slightly decreased but one
niobium, the selectivity to diol increased (see Table 5).
cannot exclude that it was caused by some other factors,
A very important factor in processes carried out in
e.g. increase of the conversion.
liquid phase is the stability of the catalyst, i.e. their resis-
tance to leaching of the active species. Therefore, in the
case of materials possessing the highest and the lowest 4. Conclusions
amount of niobium, the oxidation of cyclohexene was also
carried out after catalyst regeneration. The results of these A new synthesis route was applied to prepare hexago-
experiments are shown in Table 5. While in the case of nally ordered mesoporous niobiosilicates. This route
NbSBA-15-15 the conversion decreased (or stayed on a allowed incorporation of high Nb concentrations in the
similar level if one included the loss of catalyst during silica walls without formation of extra framework niobium
regeneration) for NbSBA-15-5, the conversion increased species as previously observed in materials prepared using
in spite of a lower amount of catalyst. In the case of conventional methods [3]. Typically, solids with well-defined
278 M. Trejda et al. / Microporous and Mesoporous Materials 110 (2008) 271–278

mesopores could be obtained for Si/Nb ratios between 15 Acknowledgments

and 5.
The increase in Nb content did not systematically COST action D36, WG No D36/0006/06 and the Polish
change textural parameters; only BET surface areas Ministry of Science (Grant No. 118/COS/2007/03) are
decreased with increasing Nb concentration. UV–Vis and acknowledged for the financial support.
XRD results indicated the absence of Nb in extra frame-
work oxide particles (octahedrally coordinated). UV spec- References
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