Vanadium mesoporous silica catalyst prepared by direct synthesis as high performing catalyst in oxidative dehydrogenation of n-butane

Article abstract of DOI:10.1007/s10562-013-1158-4

The catalytic oxidative dehydrogenation (ODH) of light alkanes has great potential to be used for production of alkenes instead classically used dehydrogenation. We report successful direct synthesis of vanadium mesoporous silica material-n_Si/n_V = 10 which could be convenient catalyst for ODH of alkane. The extraordinary high productivity (1.92 kg _prod kg_cat^-1 h^-1) in n-butane ODH is 3-4 times higher in comparison to supported vanadium catalysts usually prepared by wet impregnation method. It ranks this catalyst among the best materials investigated so far in this reaction. Moreover, reported catalytic system exhibits catalytic activity stable for more than 8 h in the stream.

Full text of DOI:10.1007/s10562-013-1158-4

Catal Lett (2014) 144:50–55
DOI 10.1007/s10562-013-1158-4
Vanadium Mesoporous Silica Catalyst Prepared by Direct
Synthesis as High Performing Catalyst in Oxidative
Dehydrogenation of n-Butane
ˇ
Michal Setnicka Pavel Cicmanec
•
•
ˇ
Roman Bulanek Arnost Zukal Jakub Pastva
ˇ
•
•
´
ˇ
Received: 19 June 2013 / Accepted: 29 October 2013 / Published online: 23 November 2013
Ó Springer Science+Business Media New York 2013
Abstract The catalytic oxidative dehydrogenation (ODH)
of light alkanes has great potential to be used for produc-
tion of alkenes instead classically used dehydrogenation.
We report successful direct synthesis of vanadium meso-
porous silica material—nSi/nV = 10 which could be con-
regeneration [1]. However, the direct using of ODH reac-
tion over the present known and tested catalysts is still not
possible due to low alkenes selectivity and productivity,
insufficient for their applicability in industrial plants.
Therefore, it is important to develop suitable new catalytic
systems with high selectivity and productivity [2].
venient catalyst for ODH of alkane. The extraordinary high
-1
cat
productivity (1.92 kgprod kg h-1) in n-butane ODH is
A wide range of catalysts have been studied in ODH of
hydrocarbons as it was reported in many reviews before
[1–4] and most of them contained oxides of the transition
metals, mainly the vanadium oxides. Vanadium oxide
based catalysts are very often studied as catalysts in the
ODH reaction due to their lower temperature required for
the activation of C–H bonds and suitable geometric and
electron structure of VOX (tunable by used matrix) [1, 5].
Nevertheless, vanadium oxide catalysts can not be used in
its bulk form (leading to non selective reactions) but in the
form of dispersed vanadium oxide species embedded to the
matrix of practically inert material (silica, MgO, Al2O3,
ZrO2, TiO2 etc.). The support strongly affects the catalytic
performance of vanadium supported catalysts. The textural
properties and acid–base character of the support are the
most important factors, which influence dispersion of
vanadium and the retention period of reactants on the
surface. The MgO support was very often referred like the
most suitable support for vanadium based catalysts used in
ODH of n-butane, due its acid–base properties and good
selectivity. Nevertheless, MgO support has area only about
100–150 m2 g-1 what limits attainable vanadium loading
to achieve sufficient productivity.
3–4 times higher in comparison to supported vanadium
catalysts usually prepared by wet impregnation method. It
ranks this catalyst among the best materials investigated so
far in this reaction. Moreover, reported catalytic system
exhibits catalytic activity stable for more than 8 h in the
stream.
Keywords Vanadium oxide Á Mesoporous silica Á
Direct synthesis Á n-Butane Á Oxidative
dehydrogenation
1 Introduction
The oxidative dehydrogenation (ODH) of light alkanes has
great potential to be used in production of increasingly
demanded alkenes instead of classically used dehydroge-
nation (DH) which suffers from high energy consumption
and fast deactivation of catalysts requiring its frequent
ˇ
ˇ
ˇ
´
M. Setnicka (&) Á P. Cicmanec Á R. Bulanek
Department of Physical Chemistry, University of Pardubice,
Another very often used supports are silica based
materials with a large number of different support struc-
tures. Our recent study showed that the mesoporous silica-
based material which combines high surface area, proper
surface acidity and good thermal and hydrothermal stabil-
ity is suitable support for ODH of n-butane [6, 7]. The
´
Studentska 573, CZ532 10 Pardubice, Czech Republic
e-mail: michal.setnicka@upce.cz
A. Zukal Á J. Pastva
´
J. Heyrovsky Institute of Physical Chemistry, Academy of
Sciences of the Czech Republic, v.v.i., Dolejskova 2155/3,
ˇ
CZ182 23 Prague 8, Czech Republic
123

Mesoporous V-Silica in C4-ODH
51
method of the deposition of active vanadium species is
another very important parameter which can strongly
influence the degree of dispersion and coordination of VOX
units on the surface. The simple wet impregnation method
by vanadium salts (NH4VO3, vanadyl acetylacetonate) is
very often used for the deposition of VOX species. How-
ever, this method usually leads to materials with a broad
distribution of VOX species and also to formation non-
selective bulk-like vanadium oxide with octahedral coor-
dination [7, 8]. The isolated monomeric VOX species
(favourable in C4-ODH reaction) are formed only up to 4–5
wt% of V which is too low to achieve acceptable produc-
tivity (one of the most important parameters for industrial
applicability) [2]. Above this concentration, polymeric
VOX species with high polymerization degree and bulk like
V2O5 are generated and it means rapid loss in activity and
selectivity to required alkene. The direct hydrothermal
synthesis of mesoporous vanadosilicate is an alternative
method for incorporation of VOX species on the silica
surface or to the silica wall. This method usually leads to
catalysts with better dispersion and with higher perfor-
mance in ODH reactions [7, 9].
kinetic region (independently checked) and under steady
state conditions of reaction. The activity and selectivity of
catalysts were tested in temperature region 460–540 °C in
the dependence on contact time (W/F 0.015, 0.03, 0.06, 0.09
and 0.12 gcat s-1 cm-3) in order to obtain iso-conversion
data for comparison with previously published data. The
demanded weight of catalyst (grains 0.25–0.50 mm) was
mixed with 3 cm3 of inert SiC. The catalysts were pre-
treated in the oxygen flow at 450 °C for 2 h before each
reaction run. The input feed composition was n-C4H10/O2/
He = 5/5/90 vol%—with a total flow rate of 100 cm3
min-1 STP. The analysis of reaction mixture composition
was made by on-line gas chromatograph.
More details about characterization techniques (XRF,
N2 adsorption–desorption isotherm measurement, H2-TPR
and DR UV–Vis), analysis of reaction mixture and calcu-
lation of conversion, selectivity and productivity) which
were used for characterization of synthesized V-MSM can
be found in our previous papers [6, 7].
3 Results and Discussion
On the basis of our previous studies we choose the silica
materials with large volume of mesopores (V-SBA-15) as
the best support [6] and the direct synthesis as the most
suitable method for deposition of vanadium to the surface
[7, 9]. The high catalytic performance of the directly syn-
thesized V-SBA-15 was already reported for ODH of
propane [10]. In the present work we report simple one-pot
V-MSM synthesis based on Ref. [11] and its high catalytic
performance in ODH of n-butane.
3.1 Characterization
The concentration of vanadium determined from XRF was
6.50 0.15 wt% for calcined V-MSM catalysts and it
means that almost 90 % of vanadium from synthesis gel
was successfully incorporated into silica structure.
The Fig. 1 presents the N2 adsorption–desorption iso-
therms which provide macroscopic average information for
evaluating of structural properties of porous materials. This
isotherms resembles to the isotherms for directly synthe-
sized vanado-silicate materials published previously [11,
12] which are mark as SBA-15. Nevertheless the shape of
2 Experimental
V-MSM was synthesized by the following procedure. The
4.0 g Pluronic P123 (EO20PO70EO20) was dissolved
in 30.0 g of distilled water and stirred for 4 h. The
9.0 g tetraethyl orthosilicate (TEOS) and the appropriate
amount of ammonium metavanadate (NH4VO3) were
added directly to the homogeneous solution (nSi/nV = 10,
i.e. ca. 7.4 wt%). Then proper quantity of 0.30 M HCl was
added to adjust the pH value of reaction mixture to 3.0 (the
using pH 3 led to the material with the best distribution of
vanadium [11]). The gel was stirred for 24 h and then
maintained at 100 °C for another 48 h. The resultant dark-
green precipitate was collected, washed thoroughly with
distilled water and absolute ethanol for several times and
dried at 70 °C for 12 h. The as-prepared product was then
calcined in air stream at 550 °C for 6 h with a heating rate
of 1 °C min-1 to remove the template.
3.5
700
3.0
600
2.5
2.0
500
1.5
1.0
400
0.5
0.0
300
10
Pore Diameter, nm
100
200
100
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure, p/p0
The n-butane ODH reaction was carried out in a glass
Fig. 1 N2 adsorption/desorption isotherms of synthesized V-MSM
sample. The inset shows the corresponding pore size distributions
plug-flow fixed-bed reactor at atmospheric pressure in the
123

ˇ
M. Setnicka et al.
52
isotherm, BET surface area and especially hysteresis loop,
is not strictly the same neither as for SBA-15 silica or
V-SBA-15 catalysts prepared by impregnation [6] nor
material prepared by the same procedure in the literature
[11]. The total surface area (SBET) was 360 m2 g-1. This is
lower BET area compare with SBA-15 silica (which is
usually around 600–900 m2/g [6, 13]) or material prepared
by Gao et al. (580 m2/g [11]). It is probably due to lower
regularity of directly synthesized V-MSM caused by pre-
sence of vanadium in the structure. The isotherm was type
IV with smoother hysteresis loop exhibiting two poorly-
resolved condensation steps contrary to SBA-15 which
provides H1 type of loop with only one almost vertical
step. It indicates mesoporosity of the material with non-
uniformity of pore size. The catalyst contains negligible vol-
ume of micropores (obtained form t-plot employing Harkins–
Jura equation for calculation of thickness of adsorbed layer)
but large volume of mesopores (1.22 cm3 g-1) compared to
SBA-15 materials prepared by impregnation or by another
method [6, 7, 12]. The pore size distribution (PSD) was wider
in comparison with samples prepared by impregnation of
SBA-15 silica. The average diameter of mesopores (inset in
Fig. 1) determined by BJH algorithm (employing Harkins–
Jura equation for calculation of thickness of adsorbed layer)
from desorption branch of isotherm was 8 nm which is sig-
nificantly higher than in the case of pure SBA-15 silica (ca.
6 nm [6, 11, 13]). In addition, main peak in PSD curve is
accompanied by wide shoulder covering the pore diameter
range from 10 to 30 nm with small maximum at about 26 nm.
The XRD pattern of the V-MSM material together with
impregnated SBA-15 catalysts (3.6 V-SBA-15 from Ref. [6]
for comparison) are shown in Fig. 2. Impregnated sample
exhibits three well-resolved reflections at around 1.02, 1.73
and 1.98° corresponding to reflections (100), (110) and (200).
The XRD pattern of V-MSM catalyst shows only poorly
resolved reflections. This is caused by partial loss of regularity
order by broad pore size distribution. In addition, increase in
pore size leads to shift of diffraction lines to lower angles,
which can disappear in scattered light at very low angle (as in
the case interplanar distance for pores with diameter 26 nm
thediffractionangleshouldbelowerthan0.4°).Asaresult,we
do not see (100) diffraction line, whereas other two diffraction
[it means (110) and (200)] are visible as a broad feature in the
range of 2h from 1.2° to 1.5° due to shift to lower angle and
superposition of weak and broad lines corresponding to dif-
ferent pore size. As a conclusion, we can state, that our
material exhibits mesoporous character, with wider distribu-
tion of pore diameters in comparison to SBA-15 material with
BET area lower that typical mesoporous silicas but signifi-
cantly larger than common amorphous fumed silica. Despite
of mentioned irregularities catalysts with these parameters
still enable good dispersion of vanadium active species and
fast diffusion of reactant and products, respectively what it is
important for attainment high performance of catalyst.
The common problem of metal–silica materials catalysts
prepared by direct synthesis is the fact that some part of
metal can be buried in silica wall and can not participate as
active species in reaction. For the check of accessibility of
active vanadium species we carried out H2–TPR experi-
ment. The overall hydrogen consumption during H2-TPR
experiment corresponds to change of oxidation state during
the reduction process and it was 1.92 of electrons per
vanadium atom in this case (quantitative reduction of V?V
–
V?III). It indicates the fact that the most of vanadium oxo-
species are accessible for redox processes by hydrogen. We
reported previously [7] that only tetrahedrally coordinated
vanadium species in impregnated V-SBA-15 could be
reduced by n-butane. Therefore, we made independent H2–
TPR experiment with pre-reducted sample (reduction by n-
butane at 540 °C until water was produced). The result
obtained indicates that approximately 5 rel.% of vanadium
can be reduced only by H2 and hence this amount is
probably not accessible for C4-ODH reaction. This amount
agrees well with amount of octahedral bulk V2O5 oxide
(ca. 3 %) in this sample (see next paragraph). Therefore,
only the vanadium units with tetrahedral coordination were
taken into account for calculation of TOF value. .
3.6V-SBA-15
6.5V-MSM
Next very important parameter of vanadium-silica
samples is the local coordination of supported vanadium
oxide species. The DR UV–Vis spectroscopy is very often
used for characterization of materials from this point of
view. The Fig. 3 shows DR UV–Vis spectrum of dehy-
drated and fully oxidized V-MSM diluted by pure silica
[14]. The absorption under 3.2 eV which corresponds to
presence of the non-active vanadium bulk V2O5 oxide with
octahedral (Oh) coordination is very weak and the main
0
1
2
3
4
5
6
7
8
9
10
2 theta, °
Fig. 2 X-ray diffraction patterns for 3.6 V-SBA-15 [6] prepared by
impregnation of bare SBA-15 and 6.5 V-MSM prepared by direct
synthesis
123

Mesoporous V-Silica in C4-ODH
53
0.25
3.2 Catalytic Activity
The ODH of n-butane over investigated V-MSM catalyst
was studied at temperature 540 °C. Based on our previous
results and character of DR UV–Vis spectrum of our
V-MSM sample reported in the Fig. 3, high catalytic
activity and selectivity of this material can be expected.
The results of catalytic tests are presented in Table 1 and
the main reaction products identified in the reaction mix-
ture were: 1-butene (1-C4), cis- and trans-2-butene (c-C4
and t-C4), 1,3-butadiene (1,3-C4), cracking products, i.e.
methane, ethane, ethene, propane and propene (C1–C3) and
carbon oxides (COX). No oxygenate products were
observed and the carbon balance was 98 3 %. Therefore,
the products of the ODH reaction are the same as in the
case of impregnated VOX on porous silica catalysts
reported previously. The n-butane conversion over empty
reactor (reactor filled only by SiC) was lower than 0.5 %
under the same conditions (composition of feed and tem-
perature) therefore we can exclude participation of gas
phase reaction under this reaction conditions. The stability
of catalyst performance in time was almost the same as we
reported for our previously studied vanadium-silicate
materials prepared by wet impregnation method [6]. Data
which will be discussed in this paper were obtained after
2 h on-stream due to conformity with the catalytic data
obtained previously over impregnated samples [6]. V-MSM
catalysts and impregnated V-SBA-15 samples will be
compared under iso-conversion conditions at n-butane con-
version of 13 % (degree of oxygen conversion was ca. 30 %
at this n-butane conversion).
0.20
0.15
0.10
0.05
0.00
1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5
Energy, eV
Fig. 3 DR UV–Vis spectrum of V-MSM sample diluted by pure
silica for better resolution [14]
absorption bands are in the region 3.2–6.3 eV, which
belong to tetrahedrally (Td) coordinated VOX species [6,
14]. These units, especially the isolated VO4, play the role
of the most active catalytic sites in the ODH of n-butane
[7]. Relative amount of each type of VOX species on the
surface was determined from area of corresponding bands
of UV–Vis spectra [14]. The V-MSM sample contains
3 rel.% of Oh-coordinated species, 64 rel.% of oligomeric
Td-coordinated VOX species and 33 rel.% of most active
monomeric Td-coordinated VOX units. The high concen-
tration of tetrahedrally coordinated species above vana-
dium loading 4–5 wt%, which was previously reported [6,
7, 9, 14] as the limit for generation of Td species on the
silica surface, is the main benefit of directly synthesized
samples in comparison with samples prepared by others
method.
The maximum of catalytic activity of synthesized
V-MSM expressed by TOF (the criterion for the compar-
ison of catalytic activity) value was 47 h-1 (only vanadium
species with Td coordination were taken in account as an
Table 1 Results of catalytic tests over directly synthesized V-MSM and its comparison with the best catalysts published in literature for ODH of
n-butane
Catalytic
system
T
wV
X C4H10 Selectivity (%)
W/Fc Productivityd Ref.
to Deh.
(°C) (wt%) (%)
1-C4 c-C4 t-C4 1,3-C4 C1–Ca3 COx Deh.b
Mg3V2O8
Fe-Zn–O
V-MSM.
V–Ti–SiO2
V-ZrO2
550
450
540
500
330
3.5
50
19
13
8.5
43
13
13
31
0.030 4.65
0.200 2.23
0.018 1.92
0.017 1.65
0.074 1.02
0.057 0.57
0.052 0.37
[18]
64
11
14
1
11
11
0
5
1
35
36
49
65
59
49
72
59
34
[19]
6.4
2.8
6.6
3.6
9
23
14
14
11
This work
[21]
[22]
V-SBA-15_impr. 540
23
11
12
7
12
9
12
7
6
3
35
64
[6]
V-SBA-15_impr. 540
[6]
a
C1–C3 = sum of C1–C3 hydrocarbons
Deh. = sum of C4 alkenes
Contact time = gcat s cm-3
b
c
d
Productivity = kgC4-deh kg h-1
-1
cat
123

ˇ
M. Setnicka et al.
54
active species because bulk V2O5 with Oh structure is
almost inactive in ODH of n-butane [7, 15]). In our case,
TOF value is significantly higher then TOF value for the
vanadium impregnated SBA-15 catalysts (the best silica
based support for C4-ODH [6]), which reached TOF of 26
and 11 h-1 for sample with 3.6 and 9 wt% of V, respec-
tively [6] (investigated under the same reaction conditions).
Other authors published the best TOF value for vanadium-
silica catalysts in the nC4-ODH reaction ranging 22–35 h-1
[13, 16]. The apparent activation energy (EA) obtained
from data collected at different temperatures for V-MSM
material is 120 kJ mol-1. This value is similar to apparent
EA for ODH of n-butane on VOX–silica (110 kJ mol-1
[15]) or on VMgO catalysts (105 kJ mol-1 [17]). Because
these materials have different textural properties, it can be
expected that the rate-limiting step is most likely affected
only by the kinetic of ODH process. Therefore, the influ-
ence of other processes (e.g. diffusion) can be neglected.
The selectivity to desired products SC4-deh (in this case
1-C4, c-C4, t-C4 and 1,3-C4) is next very important
parameter of prepared catalysts. The Fig. 4 shows the
changes of the selectivity to C4-deh products with the
n-butane conversion. The results were obtained by chang-
ing the contact time (change of catalyst amount) at 540 °C.
The selectivity to C4-deh products dependence is almost the
same for impregnated (3.6 wt% of V) and synthesised
(6.4 wt% of V) samples. No only total C4-deh selectivity but
even the distribution of individual C4-deh products was the
same (cf. Table 1 and data from Ref. [6]) and it means that
both catalysts have comparable dispersion of VOX active
species which is the most important selectivity determining
parameter [6, 7].
The big advantage of directly synthesized samples is the
fact that the high selectivity value could be obtained even
over samples with the high vanadium concentration while
for samples prepared by impregnation remains high
selectivity to C4-ODH only up to vanadium concentration
around 4–5 wt% of V. The iso-conversion selectivity is
rapidly declining after reaching this concentration in the
case of impregnated samples (SC4-deh = 34 % for V-SBA-
15 with 9 wt% of V [6]) and the same behaviour was
previously published for two sets (impregnated and syn-
thesized) of V-HMS catalysts which were tested in ODH of
n-butane and propane [7, 9] where the decline of selectivity
was from 45 % for sample with 2 wt% of V to 15 or 2 %
for sample with 8 or 16 wt% of V, respectively). This drop
in selectivity of impregnated catalysts was previously
attributed to the increased population of oligomeric vana-
dium species and bulk vanadium oxide [7, 14] which are
generated during the impregnation procedure.
The comparison of different catalytic system tested in
one reaction but under different conditions (temperature,
feed composition, contact time etc.) is very difficult.
However, the value of productivity (kgprod. kg h-1) is
-1
cat.
generally accepted like the best criterion for this purpose
(in this paper we take the best published results and con-
ditions for individual systems reported in literature).
Moreover, the value of catalytic system productivity is very
important for its potential commercial applicability and the
lower limit (for this type of reaction) which is acceptable
-1
cat
for industrial using is 1 kgprod kg h-1 [2]. The maximum
productivity value obtained over our synthesized V-MSM
catalyst was 1.92 kgprod kg h-1. This value, according to
-1
cat
best of our knowledge, is one of the five highest C4-deh
productivity values which were published for ODH of
n-butane in literature (for summary see Table 1). The best
-1
cat
productivity (4.65 kgprod kg h-1 [18]) in n-butane ODH
70
65
60
55
50
45
40
was shown over vanadium containing hydrotalcite with
Mg3V2O8 as active phase. However, maximum of selec-
tivity to dehydrogenated products was only 31 % (under
published conditions) and it is too low for potential
industry application [2]. Second catalytic system with high
productivity is mixed Fe–Zn–oxide with published C4-deh
productivity around 2.2 kgprod kg h-1 [19] which is only
-1
cat
a bit higher than productivity obtained over our catalyst,
35
but it was obtained at lower temperature. Nevertheless,
further increase in productivity is impossible due to rela-
tively low thermal stability of this catalyst (above 500 °C
the phase transformation may occur [20]). Next two cata-
3.6V-SBA-15
6.5V-MSM
30
25
6
8
10 12 14 16 18 20 22 24 26 28 30
lysts, with C4-deh productivity higher than 1 kgprod
-1
cat
Conversion of n-butane, %
kg h-1, were based on the supported vanadium catalysts.
Fig. 4 Dependence of the nC4-ODH selectivity on the n-butane
conversion obtained at 540 °C over 3.6 V-SBA-15 [6] (W/F 0.03,
0.06, 0.09 and 0.12) prepared by impregnation (black circle) and
6.5 V-MSM (W/F 0.015, 0.03, 0.06 and 0.12) prepared by direct
synthesis (red diamond)
The C4-deh productivity achieved over vanadium oxide
supported on Ti–SiO2 matrix was 1.65 kgprod kg h-1
-1
cat
[21] and for catalyst with vanadium oxide supported on
ZrO2 1.02 kgprod kg h-1 [22]. However, Ti–SiO2 matrix
-1
cat
123

Mesoporous V-Silica in C4-ODH
55
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previously studied catalysts is very high productivity of butenes
-1
cat
which is about 1.9 kgprod kg h-1 and it is 3-times higher in
comparison to the best impregnated V–SBA-15 samples and
more than 10-times higher than impregnated samples with the
same concentration of vanadium. This catalyst productivity is
among the best reported (for ODH of n-butane) in open
literature.
22. Gazzoli D, De Rossi S, Ferraris G, Mattei G, Spinicci R, Valigi M
(2009) J Mol Catal A 310:17–23
Acknowledgments The authors thank to the project of the Czech
Science Foundation No. P106/10/0196 for financial support.
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