N-Butane oxidative dehydrogenation over VOX-HMS catalyst

Article abstract of DOI:10.1016/j.molcata.2011.05.004

The demand for olefins increases in recent years. Oxidative dehydrogenation (ODH) of n-butane is possible alternative to classical dehydrogenation, steam cracking and fluid catalytic cracking processes. The role of particular VO _X species supported on hexagonal mesoporous silica (HMS) in oxidative dehydrogenation (ODH) of n-butane was investigated on two sets of VO _X-HMS catalysts prepared by wet impregnation and direct synthesis differing in amount and distribution of VO_X species. The materials were characterized by XRF, N₂-BET isotherms, XRD, SEM, H ₂-TPR, O₂-TPO and DR UV-vis spectroscopy and tested in ODH of n-butane in the range of temperature from 460 to 540 °C. The highest activity and selectivity to olefins were reached on materials with high content of isolated monomeric VO_X units with tetrahedral coordination which are generated up to 4-5 wt.% of vanadium. The species with high degree of polymerization participate mainly on total oxidation reactions and those species are formed especially by wet impregnation.

Full text of DOI:10.1016/j.molcata.2011.05.004

Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
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Editor’s choice paper
n-Butane oxidative dehydrogenation over VOX-HMS catalyst
∗
ˇ
ˇ
M. Setnicˇka , R. Bulánek, L. Capek, P. Cicˇmanec
Department of Physical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska 573, 532 10 Pardubice, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
The demand for olefins increases in recent years. Oxidative dehydrogenation (ODH) of n-butane is possible
alternative to classical dehydrogenation, steam cracking and fluid catalytic cracking processes. The role
of particular VOX species supported on hexagonal mesoporous silica (HMS) in oxidative dehydrogenation
(ODH) of n-butane was investigated on two sets of VOX-HMS catalysts prepared by wet impregnation
and direct synthesis differing in amount and distribution of VOX species. The materials were character-
ized by XRF, N2-BET isotherms, XRD, SEM, H2-TPR, O2-TPO and DR UV–vis spectroscopy and tested in
ODH of n-butane in the range of temperature from 460 to 540 ◦C. The highest activity and selectivity to
olefins were reached on materials with high content of isolated monomeric VOX units with tetrahedral
coordination which are generated up to 4–5 wt.% of vanadium. The species with high degree of polymer-
ization participate mainly on total oxidation reactions and those species are formed especially by wet
impregnation.
Received 16 February 2011
Received in revised form 29 April 2011
Accepted 3 May 2011
Available online 11 May 2011
Keywords:
Vanadium oxide
Oxidative dehydrogenation
n-Butane
Hexagonal mesoporous silica
TPR
DR UV–vis
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
the decrease of selectivity to asked products with the increasing
conversion of butane. Thus the development of suitable catalyst is
Main task of today’s chemical industry is a production of a large
amount of organic compounds. Presently it is very important to
find alternative processes for production of these compounds from
more economically suitable raw materials and with smaller impact
to environment. For example we can use alkanes instead alkenes
because alkanes are by half cheaper compared to alkenes which
are easily available [1]. The global demand for C4-olefins increases
in recent years. In the year 1984 the world production of butenes
was 28.1 mil. tons [2]. In the year 2004 it was already 44 mil. tons
[3]. Similar situation is in the case of 1,3-butadiene whose produc-
tion was 1.3 mil. tons in the year 1983 in the USA [2] and nearly
2.1 mil. tons in year 2000 [4].
still important challenge.
Vanadium oxide based catalysts are very often used as cata-
lysts in ODH reaction. Their employment offers several advantages:
lower temperature for the activation of C–H bonds of reactants
(limitation of cracking and combustion reactions) and suitable geo-
metric and electron structure of VOX (tunable with used matrix
or temperature). The vanadium-oxide catalysts are hence suit-
able for insertion oxygen atom to molecule hydrocarbon. On the
other hand vanadium oxide catalysts cannot be used in its bulk
form (leads to non-selective reactions) but have to be used as
the well dispersed VOX species supported on the suitable support
[6–8].
Oxidative dehydrogenation (ODH) of n-butane is an alterna-
tive to classical dehydrogenation, steam cracking and fluid catalytic
cracking processes. The ODH reaction is thermodynamically favor-
able and can be proceed at temperature much lower compared to a
non-oxidative dehydrogenation. To a certain extent, the conditions
of ODH reaction reduce the problems of coke formation and catalyst
deactivation. But this reaction has still several unresolved problems
whose hinder its industrial use. The molecule of butane contains
four carbon atoms which enable a lot of consecutive reactions.
This makes possible the formation of lot of consecutive reaction
products as described in Ref. [5]. Moreover the formed alkenes are
approximately four times more reactive then butane [6]. It causes
The support strongly affects the catalytic performance. The tex-
tural properties and acid–base character of the support are the most
important factors [1]. The acid–base character has influence on (i)
the dispersion of the vanadium oxides [7,9] (ii) their structure [7,9]
and (iii) retention period of reactants and reaction intermediates
on the surface [6,7,10]. From this point of view the MgO support
(iso-electric point (IEP) ca. 12.5 [7]) seems as the best support for
ODH of alkanes because rising alkenes (more basic then alkanes)
are easy desorbed and it suppress consecutive reactions leading
to COX. Moreover acid character of V2O5 (IEP 1.4 [7]) facilitates
good dispersion of VOX species. On the other hand MgO supports
have area only about 100–150 m2 g−1 [11–13] what limits attain-
able vanadium loading. From this reason is required to find new
support materials with large surface.
The vanadium oxide (VOX) units supported on the surface of
amorphous SiO2 or ordered mesoporous silica (SBA-15) [14–16]
∗
Corresponding author. Tel.: +420 466 037 345; fax: +420 466 037 068.
E-mail address: Michal.Setnicka@student.upce.cz (M. Setnicˇka).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.05.004

2
M. Setnicˇka et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
Scheme 1. Three different type of VOX species exist on the HMS surface. Type I – isolated monomeric units with tetrahedral coordination, type II – one-dimensional oligomeric
units with distorted tetrahedral coordination and type III – two- and three-dimensional polymeric units in octahedral coordination.
were reported to be potentially suitable catalysts in the ODH of n-
butane in the past. The hexagonal mesoporous silica (HMS) is used
as support in this work. Advantages of the supported catalysts on
the HMS are: larger surface area (a good dispersion of active par-
ticles), thermal stability, good mechanical properties, weak acid
by ethanol and calcined in air at 450 ◦C for 20 h (with heat-
ing rate 1 ◦C/min) for the template removal. The various amount
(0.5–15.7 wt.% of V) of vanadium oxo-species was introduced onto
the silica support by the wet impregnation from EtOH solution of
vanadyl acetylacetonate for the first set of catalysts. Impregnated
samples were dried at 120 ◦C in air overnight and then calcined at
600 ◦C in air for 8 h (with heating rate 5 ◦C/min).
The set of synthesized VOX-HMS catalysts (1.2–14.0 wt.% of V)
was prepared by direct-synthesis method which was based on work
published by Reddy and Sayari [28]. The procedure was the same
as in the case of the HMS synthesis when the ethanol solution with
desired amount of VO(acac)2 was added to the reaction mixture
(DDA, H2O and ethanol) during the synthesis. Following procedure
was similar to HMS preparation. Finally, the solid was calcined in
flow of air at 450 ◦C for 20 h (with heating rate 1 ◦C/min).
˚
centers, system of one dimensional open channels with 35–40 A
pore diameter (easy mass transport of products) [17,18].VOX parti-
cle might be present at surface of catalysts in four different forms:
highly dispersed isolated monomeric units with tetrahedral coordi-
nation (sup-O)3V O (sup – atom of support) (type I – see Scheme 1),
one-dimensional oligomeric units connected by V–O–V bonds up
to distorted tetrahedral coordination (type II – see Scheme 1),
two-dimensional polymeric units in octahedral coordination called
oxide-like species (type III – Scheme 1) and bulk three-dimensional
V2O5 crystallites (type IV) [6,19].
The method of the deposition of active vanadium phase strongly
influences the nature and abundance of rising VOX species. The sim-
ple wet impregnation method of vanadium salt (NH4VO3, vanadyl
sulfate, vanadyl acetylacetonate) is very often used for the depo-
sition of VOX species [14,20–23]. However, impregnation methods
very often lead to materials with broad distribution of VOX species
including the VOX units with a low degree of dispersion or bulk
oxide. The direct hydrothermal synthesis of mesoporous vana-
dosilicate is an alternative method for incorporation of vanadium
species on the silica surface by the introduction of the required
amounts of metal salt to the synthesis gel. Despite of a great
progress in the development of advanced method for direct syn-
thesis of functionalized mesoporous silicas [24–26] but the exact
description and mastery this synthesis is still great challenge.
In the present paper, we report for the first time the comparison
of VOX-HMS prepared by wet impregnation and direct synthesis as
potential catalysts for ODH of n-butane and we want to contribute
to understanding of relationship between vanadium structure on
the HMS support and activity in ODH of n-butane. To analyze the
effect of vanadium concentration on the type of VOX structure, on
dispersion of VOX and their catalytic activity, the catalysts were
characterized by different techniques (XRF, N2-adsorption, XRD, DR
UV–vis and H2-TPR).
The investigated samples were denoted as I-VHMS-x and S-
VHMS-x for impregnated and synthesized samples, respectively,
where x is the vanadium content in weight percentage of V.
2.2. Elemental analysis
The vanadium content was determined by means of ED XRF by
ElvaX (Elvatech, Ukraine) equipped with Pd anode. Samples were
measured against the model samples (a mechanical mixture of pure
SiO2 and NaVO3) of granulated to the same size as catalysts.
2.3. Textural properties
The N2-adsorption isotherms were obtained at 77 K using the
through-flow chromatographic method. The relative pressure of
nitrogen was varied in the range of 0.01–0.30. The specific surface
area (SBET) was determined by the fitting of the experimental data
to the BET isotherm. The surface vanadium density (VOX surface
density, VOX nm−2) was calculated according to:
NAwV
Surface density =
· 10−18
(1)
MVSBET
where NA is Avogadro constant, wV mass fraction of vanadium in
V-HMS (wt.%), MV is molecular weight of vanadium (50.94 g mol−1
and SBET is the specific surface area of V-HMS catalyst (m2 g−1).
)
2. Experimental
The structure and crystallinity of catalysts was probed by X-
ray diffraction (D8-Advance diffractometer, Bruker AXE, Germany)
2.1. Preparation of catalysts
´
in the 2ꢀ range of 2–35◦ with Cu K␣ radiation (ꢁ = 1.5406 A) and
˚
scanning electron microscopy (SEM) using JSM-5500LV microscope
(JEOL, Japan).
Catalysts have been prepared in two ways (i) by impregnation
and (ii) by direct synthesis. The hexagonal mesoporous silica (HMS)
was used as support in both cases. The HMS was synthesized at
ambient conditions according to the procedure reported by Tanev
[27] by using dodecylamine (DDA, Aldrich) as a neutral structure
directing agent in the mixture of ethanol and re-distilled water.
After 20 min of homogenization was added tetraethylorthosili-
cate (TEOS, Aldrich) as a silica precursor. The reaction mixture
was stirred at RT for 18 h. The solid product was filtered, washed
2.4. DR-UV–vis spectroscopy
UV–vis diffuse reflectance spectra were measured using the
Cintra 303 spectrometer (GBC Scientific Equipment, Australia)
equipped with a SpectralonTM – coated integrating sphere using
a SpectralonTM discs as a standard. The spectra were recorded in

M. Setnicˇka et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
3
900
22
20
18
16
14
12
10
8
the range of the wavelength 200–850 nm. Before the spectra mea-
surement the samples were dehydrated and oxidized in the glass
apparatus under static oxygen atmosphere in two steps: 120 ◦C for
30 min and 450 ◦C for 60 min and subsequently cooled down to
250 ◦C and evacuated for 30 min. After the evacuation the samples
were transferred into the quartz optical cuvette 5 mm thick and
sealed under vacuum. For additional details you can see Ref. [22].
This procedure guaranteed complete dehydration and defined oxi-
dation state of vanadium for all catalysts. The obtained reflectance
spectra were transformed into the dependencies of Kubelka–Munk
800
700
600
500
400
300
200
100
0
polyvanadate
monolayer
function F(R ) on the absorption energy hꢂ using the equation:
6
∞
4
2
(1 − R
)
∞
∞
F(R ) =
(2)
∞
monovanadate
monolayer
2
2R
0
where R is the measured diffuse reflectance from a semi-infinite
∞
0
2
4
6
8
10
12
14
16
layer [29].
Concentration, wt.% of V
Fig. 1. Dependence of specific surface area (ring) and VOX surface density on vana-
dium concentration (square) for synthesized (full symbol) and impregnated (empty
symbol) V-HMS samples.
2.5. H2-TPR measurements
Hydrogen temperature programmed reduction (H2-TPR) was
used for study of redox behaviour and for distinguishing of
individual VOX species on the surface and the AutoChem 2920
(Micromeritics, USA) was used for the measuring. A 100 mg sam-
ple in a quartz U-tube micro reactor was oxidized in oxygen flow at
450 ◦C (2 h). The reduction was carried out from 30 ◦C to 900 ◦C with
a temperature gradient of 10 ◦C/min in flow of reducing gas (5 vol.%
H2 in Ar). The changes in hydrogen concentration were monitored
on TCD detector and simultaneously hydrogen consumption and
water formation was detected on quadruple mass spectrometer
OmniStarTM GDS 300 (Pfeiffer vacuum, Germany). The tempera-
ture programmed oxidation (TPO) experiments with the sample
S-VHMS-4.9 were performed by the same apparatus. The 100 mg
of sample was pre-reduced in the gas mixture (5 vol.% H2 in Ar) as
in the case of TPR. The oxidation was carried out using the gaseous
mixture (2 vol.% O2 in He) and various heating rates 8, 12 and
20 ◦C/min to obtain the apparent re-oxidation activation energy EA
using the Kissinger equation [30]:
The conversion was calculated using following equation:
ni0 − ni
Xi =
· 100
(4)
ni0
where ni0 is amount of key component in the input feed and ni
is the amount of key component in the output from reactor.
The selectivity to products was calculated as:
ꢂj · nj
ꢂi · (ni0 − ni)
Sj =
· 100
(5)
where vi, vj is number of carbon atoms of starting compound resp.
products, ni0 is molecular amount of key component in the input
feed, ni is the molecular amount of key component in the output and
nj is the molecular amount of product in the output from reactor.
Yield was calculated according to Sachtler [31]. The productivity
and turn-over-frequency (TOF) values per V atom was calculated
based on mass balance of the carbon. The TOF values were calcu-
lated as:
EA
2
ln Tmax − ln ˇ =
+ const
(3)
RTmax
n0n-C Xn-C MV
4
4
TOF =
(6)
where Tmax is the temperature of the maximum of oxidation peak
determined at particular heating rate ˇ.
mcatwV
where n0n-C4 is mol. flow n-butane (mol s−1), Xn-C4 conversion of n-
butane (%), MV is molar weight of vanadium (50.94 g mol−1), mcat is
weight of catalyst (g), wv is mass fraction of vanadium in catalysts.
2.6. Catalytic tests in ODH reaction
The n-butane ODH reaction was carried out in a glass plug-
flow fixed-bed reactor at atmospheric pressure in the kinetic
region (independently checked) and under steady state conditions
of reaction. Typically 400 mg of catalyst (grains 0.25–0.50 mm)
was diluted with 3 cm3 inert SiC to avoid the catalytic bed over-
heating. The catalysts were pre-treated in the oxygen flow at
450 ◦C for 2 h before each reaction run. The input feed composi-
tion was C4H10/O2/He = 10/10/80 vol.% – with a total flow rate of
100 cm3 min−1. The catalytic activity was compared in the range
460–540 ◦C at the steady state conditions. The composition of reac-
tion mixture analysis was made by on-line gas-chromatograph
CHROM-5 (Laboratorni pristroje Praha) equipped with a thermal
conductivity detector (TCD) for permanent gases and flame ioniza-
tion detector (FID) for combustible products of ODH reaction. The
n-butane and products of ODH reaction (butadiene, 1-butene, cis-
2-butene, trans-2-butene, propene and propane) were separated
using a packed column with n-octane on ResSilTM (Restek) at 20 ◦C.
The packed column Porapak Q (Supelco) was used for the analysis
of ethane, ethene, CO2 and acetaldehyde. The molecular sieve 13X
(Supelco) was used for the separation of permanent gases.
3. Results and discussion
3.1. Characterization of catalysts
3.1.1. Textural properties
The HMS materials provide the isotherm of type IV with hys-
teresis loop of type H4 which is typical for combination micro-
and mesoporous system of channels [20]. The average pore diam-
eter of pure HMS support was 4.2 nm with the mesopore volume
0.68 cm3 g−1 and specific surface BET 835 m2 g−1. Both sets of the
catalysts exhibit significant decrease of the SBET value with increas-
ing vanadium loading especially in the case of samples prepared by
the impregnation method where the sample containing 15.7 wt.%
of vanadium had a SBET value ca. only 90 m2 g−1. The decrease of
surface for the synthesized samples was only in the range from
800 to 490 m2 g−1. The dependence of surface decrease on con-
centration is shown in Fig. 1. This behaviour can be attributed
to partial destruction of the framework [21,22] or deposition of
vanadium oxo-species into mesopores which partially blocks these

4
M. Setnicˇka et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
Fig. 2. SEM images of V-HMS catalysts prepared by wet impregnation (sample I-VHMS-4.6, pictures A and C) and direct synthesis (S-VHMS-4.9 sample, pictures B and D).
pores [14,22]. On the other hand decrease in surface areas can indi-
cate that the VOX species are located inside the channel system on
the inner walls of mesoporous matrix.
The abundance of various vanadium oxide species on the surface
of silica based supports was previously related to the surface con-
centration of VOX species whose values are summarized in Table 1.
The value of 0.7 VOX per nm2 [32,33] is assumed to be a lim-
iting value for the preferential formation of isolated monomeric
units. The formation of various oligomeric and polymeric species is
expected after reaching this value.
This value of the surface density was reached for samples con-
taining more than 4 wt.% of vanadium for both sets of investigated
catalysts (see Fig. 1). The 3D bulk oxide species were formed after
reaching the surface VOX density higher than 10 VOX [8] or 7–8 VOX
units per nm2 [32]. Hence it can be seen that only impregnated
samples with highest concentration of vanadium contain higher
detectable concentration of these species. These results correspond
well with the information obtained from DR UV–vis spectroscopy.
The representative SEM images of impregnated (I-VHMS-4.6,
Fig. 2)A and C and synthesized V-HMS samples (S-VHMS-4.9, Fig. 2B
and D) are presented in Fig. 2. Both types of materials exhibit sim-
ilar morphology of particles. The size of the particles was analyzed
using Gwyddion software based on derivation of gray-scale signal
for determination of grains boundary and consecutively diameter
of circle with similar area. Any significant difference in particle size
distribution was not observed in individual samples. Size of par-
ticles exhibited Gaussian distribution from 0.2 to 0.8 ␮m. These
primary particles form large aggregates with dimensions about
20–150 ␮m. Typical orthorhombic V2O5 needles were not observed
in any sample. No V2O5 clusters were detected in SEM-EDX map-
ping of V content and the concentration of vanadium was spread
homogeneously in all parts of catalyst grains except I-VHMS-15.7.
than 35◦ contain no signal hence it is not showed here. Each
sample exhibits one low-angle diffraction peak with line at 2–3◦
attributable to a d100 diffraction (spacing about 4.4 nm) and the very
broad peak with low intensity among 15–35◦. In addition, a weak
broad shoulder around the 6◦ can be seen at synthesized samples
with high concentration of vanadium and we can attribute it to d210
diffraction. The peaks d100 and d210 are characteristics for the SiO2
hexagonal ordered structure and the broad low-intensity peak in
range 15–35◦ pertains to small amount of amorphous SiO2 which is
not organized to the HMS structure [21,34]. With rise in VOX load-
ing the d100 peak weakens in intensity and gradually disappears
which is results of the partial blocking of mesopores by VOX and the
decline in long-range order of hexagonal ordered structure [14]. All
the synthesized V-HMS catalysts do not contain crystallites V2O5
in whole range of concentrations. The impregnated V-HMS cata-
2250
background
HMS
I-VHMS-0.5
I-VHMS-2.1
I-VHMS-4.6
I-VHMS-8
I-VHMS-15.7
background
S-VHMS-1.2
S-VHMS-2.3
S-VHMS-4.9
S-VHMS-8.3
S-VHMS-12.3
S-VHMS-14
A
B
2000
1750
1500
1250
1000
750
500
250
0
5
10 15 20 25 30 35
5
10 15 20 25 30 35
2 θ, °
3.1.2. XRD analysis
Fig. 3 shows the small-angle XRD patterns, in the 2ꢀ range of
Fig. 3. X-ray diffraction patterns for V-HMS catalysts prepared by wet impregnation
(A) and by direct synthesis (B).
2–35◦, for pure HMS and V-HMS. The region of 2ꢀ value higher

M. Setnicˇka et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
5
Table 1
Chemical composition, results of physico-chemical characterization for both sets of investigated catalyst samples.
a
b
c
d
e
f
Sample name
cV (wt.%)
SBET (m2 g−1
)
VOX (nm−2
)
ε0 (eV)
Xmono
Tmax (◦C)
ꢃeg
S-VHMS-1.2
S-VHMS-2.3
S-VHMS-4.9
S-VHMS-8.3
S-VHMS-12.3
S-VHMS-14
I-VHMS-0.5
I-VHMS-2.1
I-VHMS-4.6
I-VHMS-8
1.2
2.3
4.9
8.3
12.3
14
0.5
2.1
4.6
8
803
0.18
0.43
0.95
1.76
2.95
3.35
0.07
0.39
0.99
2.39
20.62
3.75
3.74
3.70
3.57
3.42
3.38
3.78
3.73
3.68
3.50
2.45
0.88
0.87
0.81
0.62
0.41
0.37
0.97
0.85
0.78
0.52
557
575
578
592
594
591
573
563
565
1.70
1.76
1.90
1.74
1.74
1.68
1.98
2.14
1.83
1.97
1.87
636
611
558
493
493
803
648
546
394
90
577 (673)
595 (639)
I-VHMS-15.7
15.7
Not determined
a
Vanadium content determined by XRF method.
Specific surface area calculated following BET method.
VOX surface density (VOX nm−2).
Energy of absorption edge determined by Tauc [40].
Relative amount of monomeric species determined from ε0 = 3.148 + 0.681X.
Position of first (second) maxima of H2-TPR profile.
b
c
d
e
f
g
Average change of oxidation state during H2-TPR.
attributed to metal charge-transfer transitions of the O →V+V type
and d-d transitions of V+IV [36].
lysts up to loading of 8 wt.% of vanadium do not contain crystallites
V2O5 as well but over this concentration reflections of bulk-like
oxide VOX species can be observed. It is represented by three main
reflections at 15.4◦, 20.2◦ and 26.1◦ in the XRD pattern (see Fig. 3)
[14,20]. This observation is similar to results published by Karakou-
lia et al. [21] on V-HMS and V-MCM-41 who reported no presence
bulk V2O5 up to 8 wt.% or on the V-SBA-15 materials studied by
Liu [14]. Liu obtained the XRD signals of bulk oxides only when the
concentration of vanadium was over 13.4 wt.% of vanadium. It can
be said in conclusion that the presence or absence of V2O5 crystal-
lites in the silica based VOX catalysts depends on the preparation
method of samples. These results are in a good agreement with the
results from DR UV–vis spectroscopy as it will be discussed in next
paragraph.
The d–d absorption bands characteristic for the V+IV in the region
1.55 – 2.07 eV [35] were not observed in any spectra. The broad
obtained spectrum is superposition of several individual bands
with poorly defined maxima. The region 4–6 eV is usually attributed
to presence of isolated monomeric tetrahedrally (Td) coordinated
VOX species (type I denoted in introduction) [22,26,37]. The sec-
ond region 3–4 eV is attributed to presence of oligomeric distorted
tetrahedral VOX units (type II) [20,22]. The impregnated catalysts
with vanadium loading over 8 wt.% exhibit band bellow 3 eV and it
is attributed to 3D-octahedrally coordinated VOX species (type III)
[22,26]. The exact quantitative analysis or at least relative abun-
dance of individual species is not possible to obtain due to broad
and overlapping absorption bands and due to strongly nonlinear
dependence of Kubelka-Munk function on the concentration of
vanadium.
3.1.3. DR-UV–vis spectroscopy
Diffuse reflectance UV–vis spectroscopy provides information
about the nature and oxidation state of vanadium. The spectra for
both prepared sets of catalyst are presented in Fig. 4. The obtained
spectra are similar to spectra published previously [20,22,26,35,36]
for this catalytic system and contain several absorption bands
at region 190-850 nm (6.5–1.46 eV) which are conventionally
The method which can partially bypass this problem was devel-
oped for speciation of molybdenum oxide [38] and subsequently
applied for vanadium oxide species on surface [23]. This method is
based on the evaluation of absorption energy edge (␧0) from UV–vis
spectra using the expression introduced by Davis and Mott [39] or
by Tauc [40] in the form:
(F(R ) · hꢂ)2 ∝ (hꢂ − ε0)
(7)
∞
HMS
HMS
Based on the empirical linear correlation of the referent com-
pounds structure and the value of absorption energy edge Gao [23]
suggested the possibility to obtain mean value of covalent V–O–V
bonds (CVB) in the coordination sphere of central V(+V) cation and
estimate degree of the polymerization of VOX species.
A
B
S-VHMS-1.2
S-VHMS-2.3
S-VHMS-4.9
S-VHMS-8.3
S-VHMS-12.3
S-VHMS-14
I-VHMS-0.5
I-VHMS-2.1
I-VHMS-4.6
I-VHMS-8
I-VHMS-15.7
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Tian et al. [41] demonstrated that mechanic mixtures of sodium
ortho-vanadate Na3VO4 (3.83 eV, model compound for isolated
monomeric Td units) and sodium meta-vanadate NaVO3 (3.16 eV,
standard for linearly polymerized Td oligomeric units) exhibited
linear dependence of the ␧ value on the concentration of referent
0
compounds and suggested this method for the determination of the
relative amount of VOX monomeric units denoted as Xm value. The
values of ␧ and the Xm of our samples are in Table 1. Disadvan-
0
tage of this method is the fact that it cannot be used for materials
which include higher content of the octahedrally coordinated VOX
bulk-like oxide species (for us the sample I-VHMS-15.7).
The samples with the vanadium concentration less than approx-
imately 4-5 wt.% exhibit nearly the same linearly decreasing trend
of the ␧0 value on the VOX concentration and it depends less on the
method of their preparation. When the vanadium loading reaches
the value ca. 5 wt.% the samples prepared by the impregnation
2
3
4
5
6
2
3
4
5
6
Energy, eV
Fig. 4. Diffuse reflectance UV–vis spectra of dehydrated V-HMS samples prepared
by wet impregnation (A) and direct synthesis (B). DR-UV–vis spectrum of pure HMS
is displayed in both plots for comparison.

6
M. Setnicˇka et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
I-VHMS-0.5
I-VHMS-2.1
I-VHMS-4.6
I-VHMS-8
6
5
4
3
2
1
0
S-VHMS-1.2
S-VHMS-2.3
S-VHMS-4.9
S-VHMS-8.3
S-VHMS-12.3
S-VHMS-14
1200
1000
800
600
400
200
0
B
A
I-VHMS-15.7
200 300 400 500 600 700 800 900 300 400 500 600 700 800 900
0
2
4
6
8
10
12
14
Temperature, °C
Concentration, wt.% of V
Fig. 6. H2-TPR patterns of V-HMS samples prepared by impregnation method (A)
and by direct synthesis (B).
Fig. 5. The amount of monomeric species determined by Tian et al. [41] in depen-
dence on vanadium loading for impregnated samples (open red points and red line)
and for samples prepared by direct synthesis (full black points and black line). (For
interpretation of the references to color in this figure legend, the reader is referred
to the web version of the article.)
good agreement with the results obtained from our DR UV–vis
spectroscopy measurement. The high temperature peak most likely
belongs to the reduction of Oh coordinated 2D oligomers and bulk-
like 3D V2O5 crystallites [14,44]. The presence of V2O5 crystallites
was clearly proven by XRD results mentioned above.
method exhibit more rapid decrease of the ␧ value compared to
0
samples prepared by the direct synthesis. This effect indicates more
oligomeric units formation or a wider degree of theirs oligomer-
ization compared to samples prepared by the direct synthesis.
The values presented in Table 1 indicate that samples with low
VOX concentration contain at first mostly the isolated monomeric
Td units but the relative abundance of these species decreases
with the increasing of total vanadium loading. This effect can be
clearly seen from Fig. 5 where is displayed the dependence of
calculated amount of monomeric species on total vanadium con-
centration. The amount of monomeric units increases linearly and
these species represent significant part of the VOX species gener-
ated on the HMS surface but after reaching ca. 4–5 wt.% of vanadium
there is reached their maximal concentration and the amount of
these units starts to decrease. It is more important that both sets
of catalysts exhibit similar maximal concentration of monomeric
units and this concentration is reached at the relatively comparable
level of vanadium loading.
The overall hydrogen consumption was proportional to the con-
centration of vanadium and linearly increases with the vanadium
loading and corresponds to change of oxidation state of vanadium
from the VV to VIII. The changes of oxidation state of vanadium
cation are summarized in Table 1. The average ꢃe/V = 1.95 0.1 for
impregnated samples corresponds to change of oxidation state of
vanadium from the VV to VIII. The samples prepared by the direct
synthesis exhibit lower ꢃe/V value 1.75 0.1 and this effect can
be ascribed to non-complete extraction of vanadium oxide species
from the HMS lattice during the sample pretreatment.
The temperature programmed oxidation experiments were car-
ried out using the sample S-VHMS-4.9 and obtained TPO curves
consist of one oxidation peak (not shown here for sake of brevity).
The variation of the heating rate ˇ (8, 12 and 20 ◦C/min) in the
TPO causes the changes in position of maximum of this peak to the
temperatures 338, 356 and 395 ◦C. The apparent activation energy
of the catalyst reoxidation equal to 44 7 kJ mol−1 was calculated
using the Kissinger Eq. (3).
3.1.4. H2–TPR and TPO measurements
H2-TPR curves were measured to determine the scale of dis-
persion and type of the present active species. The obtained TPR
profiles are presented in Fig. 6 and results of their analysis are sum-
marized in Table 1. From the table is visible that samples prepared
by the direct synthesis exhibit only one symmetric reduction peak
at the temperature about 560–590 ◦C whereas the samples pre-
pared by the wet impregnation exhibit moreover the shoulder or
second reduction peak at the temperature about 640-680 ◦C. The
progressive shift of the major H2 consumption temperature peak
to higher values with increasing amount of vanadium are explained
by two ways (i) either this shift can be ascribed to the kinetic or the
thermodynamics of the process of VOX reduction and it is related
to the changes in H2 to surface vanadium ratio or to ratio of (gener-
ated H2O)/H2 during the reduction process with the increasing of
vanadium loading [30,42,43] or (ii) due to the gradual formation of
polymeric vanadium species [14].
3.2. Catalytic tests of n-butane ODH
Both synthesized and impregnated VOX-HMS catalysts were
active and selective in the ODH of n-butane with oxygen over the
V-HMS catalysts. The reaction products identified in the reaction
mixture were: methane (C1), ethane and ethene (C2), propane and
propene (C3), 1–butene (1-C4), cis- and trans-2-butene (c-C4 and t-
C4), 1,3-butadiene (1,3-C4), carbon oxides (CO and CO2) and traces
of acetaldehyde. The carbon balance was 98 3 % in all the cat-
alytic tests and no maleic-anhydride, other oxygenates or products
of significant catalyst coking were not found. The activity of cat-
alysts was stable at least for 10 hours time–on–stream (TOS) and
no loss of catalyst activity during the time reported elsewhere [45]
was observed. The activity of pure HMS was measured under the
same conditions to check reactivity of clear support. It was not
observed any activity up to 520 ◦C. The conversion of n-butane
only about 3% was detected at 540 ◦C (for more details see Table 2).
This conversion is remarkably lower in comparison to the conver-
sions reached over all tested catalysts. Nevertheless presence of
reactions in homogenous phase was proved after initialization by
The low temperature peak can be attributed on the basis of our
UV–vis spectra and according to literature either to reduction of
the solely monomeric highly dispersed VOX units (type I) [20] or
rather to reduction of Td both the monomeric and the oligomeric
3−
(VO4
) species (type II) [44]. The second interpretation is in the
n

M. Setnicˇka et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
7
32
30
28
26
24
22
20
18
16
14
12
10
8
B
A
6
4
2
0
0
2
4
6
8
10 12 14 16
2
4
6
8
10 12 14 16
Concentration, wt.% of V
Fig. 7. The activity of synthesized (A) and impregnated (B) V-HMS catalyst in ODH of
n-butane expressed by TOF factor at 460 ◦C (blue triangle and line), 500 ◦C (red circle
and line) and 540 ◦C (black square and line). (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of the article.)
matrix which is in the compliance with previously results published
[46].
3.2.1. Catalysts activity in C4-ODH reaction
The value of turn-over-frequencies (TOF) exhibits similar
dependence on the concentration of vanadium for both sets of cat-
alysts as shown in Fig. 7. The all low concentrated samples exhibit
approximately constant value of TOF factor (28 h−1 for 540 ◦C) until
the vanadium concentration ca. 4.5 wt.% is reached followed by a
rapid change of TOF to low value (5–7 h−1 for 540 ◦C). The very
similar dependences of TOF vs. V loading were obtained for the all
experiments carried out over both sets of catalysts in the temper-
ature range 460–540 ◦C (see Fig. 7) regardless that at the highest
temperature (540 ◦C) the conversion of oxygen was almost 100%
for samples with the high concentration of vanadium.
The vanadium concentration when the TOF starts to decrease
corresponds well with the maximal concentration of monomeric
VOX units as it is shown in Fig. 5 or reaching the level 0.7 VOX
per nm2 for vanadium surface density previously reported [32] as
limiting for obtaining the monolayer of monomeric units. Decrease
of TOF value clearly evidences that with subsequent increase of
vanadium content the significantly less active or non active species
in ODH of n–butane are generated and that is why the monomeric
VOX units should be taken as the most active species in the ODH of
n-butane.
3.2.2. Selectivity to all C4-ODH products
The selectivity to products of C4-ODH remained relatively con-
stant in whole temperature range as it can be seen from Fig. 8.
This character of selectivity vs. temperature dependence offers the
possibility to compare values of selectivity at the iso-conversion
conditions obtained at different temperatures of reaction, how-
ever this method is not as rigorous as comparison of iso-conversion
data obtained by variation of amount of catalyst used in reaction.
The values of selectivity to C4-olefins obtained for the conversion
7.5% are summarized in Table 2. Similarity of these values with the
C4-ODH selectivity values obtained at 540 ◦C confirms our assump-
tion of low dependence C4-ODH selectivity on temperature for our
materials under the given reaction conditions. Hence the both sets
values can be used for the comparison of selectivity to the C4-olefins
on studied materials.

8
M. Setnicˇka et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
55
50
45
40
35
30
25
20
15
10
5
olefins was published on V-Mg-O materials by Chaar [13] and high
selectivity to butadiene was ascribed to basic character of catalysts
compared to silica [11,13] as well as to presence of Mg3(VO4)2 with
isolated tetrahedral species which prohibits deep oxidation [1].
However our observation is in contrast to results published pre-
viously on SiO2 based catalysts. Owens [16] investigated vanadium
anchored on amorphous silica and he described only small selec-
tivity to butadiene. More similar results published Liu et al. [14]
who studied V-SBA-15 material which exhibited selectivity about
30% (from C4-olefins only) to butadiene.
The selectivity to dehydrogenation products as well as distribu-
tion of C4-olefins can be related to the acid-base character of the
catalysts as it was reported by Blasco et al. [12] on the base FTIR
of adsorbed pyridine or Liu et al. [14] on the basis of NH3-TPD and
widely discussed in many other works [1,6,7,10,48]. In our case
acid–base character of HMS support (IEP ca. 2) extend retention
period of reaction intermediates because the olefins (electron-
donating molecules with high electron densities at ꢄ bonds) are
stronger adsorbed on the surface of catalysts then paraffins [1,9,12].
This confirm values of adsorption heats on silanol groups (deter-
mined by FTIR spectroscopy) 27 kJ mol−1 [49] for n-butane and
30–36 kJ mol−1 [49,50] for butenes, respectively. The acidity fur-
ther increases with rising amount of vanadium and it implicates
that vanadium species act as an acidic site on silica and that is why
we need good distribution of monomeric VOX species [51].
It seems that more basic character of MgO support predestine
this support as the best support for the alkane ODH catalyst but
the situation is probably more complicated because according to
Albonetti et al. [10] the V/Al/O catalyst exhibit better performance
for the ethane ODH than V–Mg–O catalyst. The other effects can
control selectivity of ODH reaction and can prevail the influence of
acid-base characteristics of support.
B
A
0
460
480
500
520
540 460
480
500
520
540
Temperature, °C
Fig. 8. The conversion (black triangle and line), selectivity (red square and line) and
yield (blue circle and line) for representative sample prepared by direct synthesis (S-
VHMS-4.9, A) and by wet impregnation method (I-VHMS-4.6, B). (For interpretation
of the references to color in this figure legend, the reader is referred to the web
version of the article.)
It can be seen the drop of C4-olefins selectivity decreases with
increasing of the vanadium content from 63% for the sample
S-VHMS-1.2 to nearly non-selective catalyst I-VHMS-15.7 with
selectivity only about 10% (Table 2). The yield or productivity of
C4-ODH products increases with the amount of vanadium until
the concentration 4–5 wt.% is reached then these values start to
decrease. The maximal yield or productivity obtained was ca. 20%
or 0.74 g C4-olefins per gram of catalyst for one hour respectively
(S-VHMS-4.9). Similar behaviour was previously published for oxi-
dation of toluene over V-HMS prepared by similar procedure and
authors explain this behaviour on the base of different amount of
suitable and accessible VOX species but they didn’t propose more
detailed specification of these species [47].
This behaviour is probably due to increasing of abundance of
oligomeric species with Td and mainly Oh coordination because
these species contain the V–O–V bridging oxygen atoms. According
to mechanism introduced by Kung [6] the presence of this type
of oxygen allows the formation of alkoxide intermediate which is
furthermore oxidized to the products of total oxidation (CO and
CO2).
Although both sets of catalyst exhibit similar trends of conver-
sion on the vanadium loading the selectivity to C4–olefins is about
10% higher for the synthesized samples than for samples prepared
by impregnation method in the whole range of prepared concen-
trations even for samples with same value of VOX surface density.
Such difference of selectivity to C4-ODH products can be ascribed
to the VOX species with higher level of polymerization whose are
preferentially formed in the samples prepared by the wet impreg-
nation method. These species facilitate consecutive reactions and
formation of COX. These species cannot be detected on the basis
of analysis of VOX surface density value nor by the analysis of the
DR UV–vis spectra with the sufficient accuracy. We can presume
on presence of these species on the basis of TPR analysis as it was
mentioned above.
On the basis of this fact we can suppose that butadiene origi-
nate by direct two-step ODH reaction of the adsorbed hydrocarbon
complex over one or more active centers without desorption
intermediate to the gas phase. Similar mechanism was recently
proposed by Marcu et al. [52] on the basis of TAP measurements
over the tetravalent pyrophosphate catalysts. Moreover the large
surface area of catalysts facilitates good dispersion and isolation
of VOX species. This has resulted in limiting of deep oxidation of
forming intermediates to COX.
3.2.2.2. Selectivity to 1-butene and 2-butenes. The ratio of the selec-
tivity of 1-butene to sum of cis-/trans-2-butene is 1.5 for samples
with lowest concentration of vanadium. When we assume that the
first hydrogen abstraction occur on the secondary carbon atom this
value corresponds to statistical distribution of products (1-butene:
cis-2-butene: trans-2-butene) 3:1:1 which can be formed by the
abstraction of second hydrogen from adjacent methyl and methy-
lene groups. Based on this assumption Chaar et al. [13] attributes
this behaviour to fast radical dehydrogenation of adsorbed alkyl
intermediate. Similar effect was also observed on other catalytic
systems [7,11,45].
This statistical distribution ratio shifts to worth one at highest
concentration for both types of samples. This shift is due to unequal
participation of 1-butene in consecutive reactions and similar effect
was described by Lemonidou [45]. Blasco and Nieto [7] put this
shift of distribution of products to the context with acid character
of catalysts which probably influences the rate of the consecutive
reactions. It is known that 1-butene can isomerize to 2-butene if
acid site are present on the catalyst surface [7]. With increasing
VOX loading (IEP of V2O5 < SiO2) increases the acidity of material
and that is why the distribution of products shift from statistical
distribution of products to thermodynamic equilibrium. We can
finally say that distribution of products is predominantly controlled
by kinetic effects and not by thermodynamics of process because
3.2.2.1. Selectivity to 1,3-butadiene. When we make analysis of
selectivity of individual products of C4-olefins in detail it can be
found out that approximately 40-50% belongs to 1,3-butadiene.
The butadiene represents relatively high portion of C4-ODH prod-
ucts in the whole range of vanadium concentration, temperature
and conversion of n-butane and oxygen. Similar distribution of C4-

M. Setnicˇka et al. / Journal of Molecular Catalysis A: Chemical 344 (2011) 1–10
9
140
120
100
80
•
The isolated monomeric VOX species play the role of the most
active catalytic centre in the ODH of n-butane. The catalytic
behaviour of these units is similar for both sets of catalysts regard-
less of the method of their preparation.
The amount of isolated monomeric species is comparable for both
sets of materials up to total concentration 4–5 wt.% of vanadium
but the highest achievable amount of monomeric species was
slightly higher for catalysts prepared by direct synthesis.
The VOX species with higher degree of polymerization partici-
pate in undesired consecutive reactions with ODH products. The
direct synthesis method leads to lower extent of formation of
these species compared to wet impregnation.
The method of preparation influences the formation of oligomeric
species. The impregnated samples contain higher amount of octa-
hedrally coordinated species compared to samples prepared by
direct synthesis.
The most abundant selective product was 1,3-butadiene with
selectivity up to 30%. The total sum of selectivity to all C4-olefins
reached up to 65% over the catalysts prepared by direct synthe-
sis. The samples prepared by wet impregnation exhibit 10% lower
selectivity to C4-olefins compared to catalysts prepared by direct
synthesis in the whole range of vanadium concentrations. The
selectivity decrease with increasing vanadium content.
•
•
•
•
60
40
20
0
0
2
4
6
8
10
12
14
16
Concentration, wt.% of V
Fig. 9. The apparent activation energy in dependence on the vanadium loading for
impregnated (open red points and red line) and for samples prepared by direct
synthesis (full black points and black line). (For interpretation of the references to
color in this figure legend, the reader is referred to the web version of the article.)
the thermodynamic control should give the product distribution
(1:1:1.1) [7] and no remarkable isomerization of butenes occurs on
the surface of catalysts.
Acknowledgements
A financial support of the Grant Agency of the Czech Repub-
lic under the project no. P106/10/0196 and P104/07/0214 and
Ministry of Education of Czech Republic under project no. MSM
0021627501 is highly acknowledged.
3.2.3. Apparent activation energy
The activation energies corresponding to the transformation of
n-butane were determined in the temperature range from 460
to 540 ◦C using conversion data presented in Table 2 according
to Arrhenius relationship. The apparent activation energy (EA) in
dependence on the vanadium content is presented in Fig. 9. The
value of apparent EA is about 120 15 kJ mol−1 for samples with
low concentration of vanadium on the surface and it decreases to
EA = 40 8 kJ mol−1 for higher concentration. It is evident that the
change of rate-limiting step in mechanism of ODH n-butane occurs
as the vanadium loading increases. The rate-limiting step for low
concentration samples is supposed to be the activation of hydro-
carbon on the surface by abstraction of hydrogen from secondary
carbon [5]. The value 120 kJ mol−1 is similar to apparent EA for ODH
of n-butane on VOX–silica (110 kJ mol−1 [16]) or on VMgO catalysts
(105 kJ mol−1 [45]). Because these materials have different textural
properties, it can be expected that the apparent activation energy
values are most likely affected only by the kinetic of ODH process
for our materials as well. Therefore it can be neglected the influence
of other processes i.e. diffusion to this value.
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