Effect of water on oxidative scission of 1-butene to acetic acid over V2O5-TiO2 catalyst. Transient isotopic and kinetic study

Article abstract of DOI:10.1016/j.apcata.2010.03.043

The role of water in the oxidation of 1-butene to AcOH over VOx-TiO ₂ was investigated using spectroscopic and transient isotopic exchange methods. It was shown that the influence of water strongly depended on the temperature of reaction. In particular, DRIFTS and NH₃-TPD studies confirmed the temperature influence on the acidity and the amount of adsorbed water. XPS investigations suggested that not only oxygen from vanadia, but also from the lattice of titania was involved in the oxygen transfer during the reaction. Formation of oxidation products proceeded over two types of active vanadium oxide centers, i.e., VOH and VO. Hydrated vanadium species exhibited high selectivity towards AcOH formation. On the other hand, VO centers favored total oxidation. Kinetic model was developed for an unambiguous interpretation of the experimental results. Modelled reaction constants of the formation of AcOH over VOH centers were ca. 3.5 times higher than over VO centers. At the same time, the reaction rate constant of total oxidation in the presence of water was ca. 3.2 times lower than in dry flow. Estimated values suggested that in the presence of water the number of VOH centers was substantially lower than VO sites; however their contribution to the rate of AcOH formation was much higher.

Full text of DOI:10.1016/j.apcata.2010.03.043

Applied Catalysis A: General 391 (2011) 125–136
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Effect of water on oxidative scission of 1-butene to acetic acid over V2O5-TiO2
catalyst. Transient isotopic and kinetic study
Wladimir Supruna,∗, Ekaterina M. Sadovskayab, Christoph Rüdingerc, Hans-Jürgen Eberlec,
Michal Luteckia, Helmut Pappa
a Institut für Technische Chemie, Universität Leipzig, D-04103 Leipzig, Germany
b Boreskov Institute of Catalysis, SB RAS, 630090, Novosibirsk, Russia
c Wacker Chemie AG, Consortium für Elektrochemische Industrie GmbH, Zielstattstr. 20, D-81379, München, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The roleofwaterin the oxidation of1-butene toAcOHoverVOx-TiO2 was investigatedusing spectroscopic
and transient isotopic exchange methods. It was shown that the influence of water strongly depended
on the temperature of reaction. In particular, DRIFTS and NH3-TPD studies confirmed the temperature
influence on the acidity and the amount of adsorbed water. XPS investigations suggested that not only
oxygen from vanadia, but also from the lattice of titania was involved in the oxygen transfer during the
reaction. Formation of oxidation products proceeded over two types of active vanadium oxide centers,
i.e., VOH and VO. Hydrated vanadium species exhibited high selectivity towards AcOH formation. On
the other hand, VO centers favored total oxidation. Kinetic model was developed for an unambiguous
interpretation of the experimental results. Modelled reaction constants of the formation of AcOH over
VOH centers were ca. 3.5 times higher than over VO centers. At the same time, the reaction rate constant
of total oxidation in the presence of water was ca. 3.2 times lower than in dry flow. Estimated values
suggested that in the presence of water the number of VOH centers was substantially lower than VO
sites; however their contribution to the rate of AcOH formation was much higher.
Received 19 January 2010
Received in revised form 17 March 2010
Accepted 19 March 2010
Available online 27 March 2010
Keywords:
Vanadia–titania catalyst
Oxidation of 1-butene
Water
Acetic acid
O exchange
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
et al. [3] suggested a reaction mechanism without the participation
of water. Other authors [10,11] studied the influence of water vapor
Acetic acid (AcOH) is an important intermediate product for
the production of many value added chemicals like vinyl acetate.
Currently it is mainly produced by carbonylation of methanol
and oxidation of ethylene according to the Wacker process [1].
Some attempts were made in the last decades to develop a pro-
cess for the production of AcOH by catalytic gas phase oxidation
of light hydrocarbons on MeOx supported catalysts [2–7]. Vari-
ous redox catalysts were proposed for the selective oxidation of
C4-hydrocarbons mostly on the basis of vanadium oxide [3,4]. Espe-
cially VOx containing catalysts supported on TiO2 (anatase) have
shown promising results in the C4 oxidation [7]. Only a few older
articles [2–4] were devoted to the mechanism of the catalytic
oxidation of C4 hydrocarbons to AcOH. However, none of these
publications explained unambiguously the oxidative cleavage of
1-butene to AcOH and, hence, the mechanism is still a matter of
discussion [8,9]. Seiyama et al. [4] proposed an oxidative cleavage
mechanism, which involves water as one of the reactants and 2-
butanol as the primary intermediate product. In contrast, Kaneko
on the oxidative dehydrogenation of hydrocarbons (HC) on VOx cat-
alysts and observed different effects of water on the formation of
the reaction products. For example, in the oxidation of HCs at low
temperatures, the formation of carboxylic acid was promoted by
addition of water [8]. On the contrary, water was found to suppress
the rate of oxidative dehydrogenation reaction at high tempera-
tures [10,12]. D. Linke et al. studied catalytic partial oxidation of
ethane over VOx-containing catalyst to AcOH at 200–350 ◦C and
found that at higher temperatures above 300 ◦C higher water par-
tial pressures are necessary to suppress the formation of ethylene
and CO2 and thereby increase the selectivity to AcOH [13].
It was shown in the literature that water is essential additive
during catalytic gas phase oxidation and oxidative dehydrogena-
tion for higher conversion of hydrocarbon and higher selectivities
towards oxygenated products [13–25]. The water is generally
believed to influence the reaction in several ways, (i) it increases
the desorption rate of polar products and/or alters the adsorption –
desorption equilibria of educts, products and oxygen [14–18], (ii) it
depresses the coke formation by clearing the surface of the catalyst
[2,7,19], (iii) it modifies the catalyst surface, i.e., participates in the
reoxidation of catalytically active centers and facilitates the trans-
formation of surface-oxygenated complex to products [20–26], (iv)
∗
Corresponding author. Tel.: +49 341 9736300; fax: +49 341 9736349.
E-mail address: suprun@chemie.uni-leipzig.de (W. Suprun).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.03.043

126
W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
trie GmbH, München by mixing the aqueous slurry of TiO2, V2O5
Nomenclature
and vinyl acetate in the presence of air stream. Afterwards, the sam-
ple was first dried at 100 ◦C and then calcined at 400 ◦C for 5 hours in
air atmosphere. After calcination, catalyst was pressed and sieved
to 0.1 - 0.315. The catalyst was characterized by N2 adsorption
(ASAP 2000, micromeritics), NH3-TPD, H2-TPR, XPS and DRIFTS. The
BET-surface area of the catalyst was 79 m2/g and the average pore
diameter was 13 nm. A detailed description of the results of catalyst
characterization was reported elsewhere [27].
LV
the total number of surface sites (mol) per mole
the concentration (mol/mol)
␪
j
Cj
␶
the concentration (molar fraction)
the residence time (s)
ri
␣
␣
the rate of the elementary step i;
the matrix of stoichiometric coefficients.
relative concentrations of isotopic fractions in the
gaseous phase
ij
i
DRIFTS: Infrared spectra in diffuse reflection were recorded with
an infrared Fourier spectrometer Vector 22 (Bruker Optic GmbH;
software OPUS 3.1) combined with a diffuse reflection unit “The
Selector” (Specac Ltd., Kent, UK) and a stainless steel reaction cham-
ber (Specac Ltd., Kent, UK). Prior to the analysis, the catalyst was
saturated with water vapour in an air flow (50 ml/min) in tempera-
ture region 160–300 ◦C for 2 hours. After hydrothermal treatment,
the samples were placed in stainless steel cup inside the reaction
chamber and exposed to air for 1 hour at similar temperature to
the hydrothermal pretreatment. After activation, the system was
cooled down to 100 ◦C using dry nitrogen as purge gas. The spectra
were recorded with a resolution of 2 cm−1 in the range of 4000 -
␣
relative concentrations of adsorbed species
experimental concentration value in gaseous phase
or isotopic fraction
calculated concentration value
activation energy (kJ/mol)
j
xiexp
xicalc
Ea
j
number of experimental points
kinetic constant
k
K
equilibrium constant
n, m
reaction orders in kinetic equation concentration of
active sites
1100 cm−1
.
XPS: The measurements of V 2p, Ti 2p and C 1 s signals were
performed using the Mg anode (K␣ = 1253.6 eV) as X-ray source
(XR 50 from Specs GmbH). An accelerating voltage of 12.5 kV and a
pass energy of 30 eV were used for the measurements. The hemi-
sphere detector Phoibos 150 (Specs GmbH) was applied for the
detection of photoelectrons. The spectra analyses including satel-
lites and background subtraction (Shirley) were done with CasaXPS
software. Because of the electrostatic charging of the samples, the
binding energies were corrected. The binding energy value for C 1 s
BE = 284.5 eV was used as a calibration standard for the corrections.
TPD of ammonia: The measurements were performed using
a microreactor filled with catalyst (50 mg). The evolved gases
were analyzed with a quadrupole mass spectrometer (Pfeiffer GSD
301). Prior to the analysis, the catalyst was pretreated in He flow
(50 ml/min) at 160 ◦C K for 2 hours. After cooling down to 100 ◦C, the
adsorption experiments were carried out by consecutive pulses of
1 ml ammonia until complete saturation of the surface was reached.
After the removal of physisorbed ammonia by purging with helium,
the sample was heated from 100 up to 600 ◦C (heating ramp of
10 K/min) in a He flow and the desorption of NH3 was detected
by MS (m/e 16). Additionally, NH3-TPD measurements were car-
ried out after hydrothermal treatment (10 vol.-% of water vapor) of
catalyst at different temperatures.
it improves the heat exchange by dissipation of hot-spots [2,5,7].
All these effects contribute to the complexity of the investigated
catalytic process and therefore, the combination of several analyti-
cal techniques is required to fully explain, both surface and catalytic
properties of VOx-TiO2 in the oxidation of hydrocarbons.
Transient isotopic experiments are particularly suitable to elu-
cidate the role of water in the catalytic oxidation and oxidative
dehydration of hydrocarbons. Very recently, we investigated the
influence of water on the catalytic oxidation of 1-butene using tran-
sient isotopic-exchange of 18O2 and showed that 18O from H218O
was incorporated into AcOH [27]. Nevertheless, the transfer of oxy-
gen atoms from water to AcOH does not imply that water or OH
groups participate in the formation of AcOH. 18O can be introduced
in AcOH structure due to the exchange between AcOH and H218O
as well as between H218O and vanadium oxide centers. Sadovskaya
et al. [28] studied in the model system, the isotopic O-exchange
between water and oxygen from the monolayer of VOx-TiO2 in
temperature range 50 – 500 ◦C using SSITKA technique. It was found
that the oxygen exchange with the water proceeds over two types
of vanadium species, i.e., hydrated (V-OH) and dehydrated (V-O-V
and V=O), with different reactions rates and activation energies.
The aim of this communication is to clarify the role of water
in the mechanism of AcOH formation in the oxidation of 1-butene
over VOx-TiO2 catalysts. The present work is focused on the char-
acterization of surface properties, i.e., acidity, oxidation state of the
catalyst surface and water – catalyst surface interactions. Addi-
tionally the investigation of the reaction pathways of 1-butene
oxidation on these catalysts is carried out. For this purpose, the
dynamics of the reaction and the formation of oxidation products
in presence and absence of oxygen and/or water vapor were stud-
ied. The detailed steady-state and isotopic transient kinetic studies
were carried out to develop a consistent kinetic model for an unam-
biguous interpretation of the experimental results. In particular, the
investigations were aimed to explain the considerable differences
in the oxidation mechanism in presence and absence of water.
Adsorption experiments: The measurements were carried out in
the flow-through reactor with the catalyst packed in the basket
coupled with electronic balance (Rubotherm; resolution 0.01 mg).
Prior to the adsorption experiment the catalyst was pretreated in
dry flow of N2 at 300 ◦C or in the presence of 1-butene (1 vol.-
%) at 220 ◦C. After pretreatment, the water stream was introduced
into the flow (5 wt.-% water) and the change of catalyst weight was
recorded.
2.2. Isotopic-transient-experiments
The reaction was performed in a quartz reactor (i.D: 0.5 cm;
L: 10 cm) at atmospheric pressure in the temperature range of
160–280 ◦C using 100 – 200 mg of catalyst (fraction 0.1 – 0.3 mm)
diluted with 200 – 400 mg corundum of the same fraction (activa-
tion conditions: 280 ◦C in the mixture of 5 vol.-%O2 in He) and a
constant 1-butene to oxygen ratio of 1:4 (GHSV: 3600 h−1, const.).
The oxidation of 1-butene over the VOx-TiO2 catalyst was carried
out in presence and absence of both, H216O and H218O and the
oxygen isotopes 16O2 and 18O2. A reaction mixture contained 1.2
vol.-% 1-butene, 5 vol.-% O2 and balance He, whereas in case of the
2. Experimental section
2.1. Catalyst
The VOx-TiO2 catalyst (6 wt-% V2O5) was prepared by spray-
drying method of TiO2 (Millenium GmbH) and vanadium oxide
(Aldrich) in pilot plant of Consortium für Elektrochemische Indus-

W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
127
Fig. 1. Main reactions pathways for oxidative C-C-scission of 1-butene to C1-C3 carboxylic acids and total oxidation.
reaction in presence of water the mixture contained additionally
system of differential equations (2):
17 vol.-% water vapor.
ꢁ
ꢂ
N
i
ꢀ
For isotopic transient experiments cylinders containing 10 vol.-
∂Ci˛i
∂t
1
ꢀ
∂Ci˛i
∂ꢁ
∂Ci˛i
∂ꢁ
Ci
ꢂj
+
˛
+ Ci
= ˇ
rl˛l)
i
%
16O2 in He and 10 vol.-% 18O2 in He, respectively, were used
l=1
(purities: 18O2: 95.5 vol.-%, balance 16O2; H218O: 98.5 vol.-%, bal-
ance H216O). Before the isotopic experiments the catalyst was
activated with a mixture of 5 vol.-% 16O2 and He for 60 min and
cooled to the desired temperature. After steady state conditions
were achieved, 16O2 was replaced by 18O2. In case of experiments
with water H218O the catalytic reaction was carried out first in
absence of water and then in presence of H218O, which was dosed
by a Liquid-Flow-Evaporator (Bronkhorst Hi-Tech) to the feed. The
following isotopic transient experiments were performed: (i) in
presence of H216O and H218O, respectively, by replacement of 16O2
by 18O2, (ii) in absence of water by replacement of 16O2 by 18O2,
(iii) in absence and presence of water and 16O2.
(2)
N
j
ꢀ
∂˛j
=
rl˛l
∂t
l=1
with initial and boundary conditions:
t = 0 : ˛i = ˛0, ˛j = ˛0
i
j
ꢁ = 0 : ˛i = ˛input
i
where ˛i, ˛j – are the relative concentrations of isotopic frac-
tions in the gaseous phase and adsorbed species, respectively. This
model takes into account the change of isotope concentration in the
reagents and surface intermediates both with time and along the
catalyst bed. The form of the right parts of the equations is deter-
mined by the scheme of isotopic transfer from one substance to
another.
3. Calculation method
3.1. Model of transient regimes
3.3. The numerical analysis of experimental data
The kinetics model of transient regimes in a plug-flow reactor
can be written by the system of hyperbolic and ordinary differential
equations (1):
The numerical analysis of experimental data which corresponds
to the solution of the inverse problem of the system of equations (1)
or (2) was carried out through the minimization of next objective
R
functional:
ꢀ
dCj
dt
1 dCi
t
end
+
=
= LV
˛ijri j = 1, . . . , N1
ꢃ
N
ꢀ
ꢀ
dꢁ
F(ϑ) =
(xicalc − xiexp)2dt → min
(3)
¯
i=1
(1)
R
ꢀ
dꢂj
dt
i=1
o
˛ijri
j = N1 + 1, . . . , N2
i=1
where xiexp experimental concentration value in gaseous phase or
isotopic fraction, xi calc- calculated concentration value according
with initial and boundary conditions:
to the model (1) or (2).
0
t = 0 : Cj=j , ꢂj = ꢂ0
j
4. Results and discussion
4.1. 4.1.Catalytic activity and surface investigations of the
catalyst
ꢁ = 0 : Cj = Cinput
j
In present studies we investigated the effect of temperature on
the selectivity of the formation of AcOH from 1-butene in the pres-
ence of water. It needs to be mentioned that the reaction proceeds
via various intermediates such as 2-butanol, 2-butanone and 2,3-
butandione [3–6]. Ketone intermediate subsequently undergoes
C-C oxidative cleavage to C2-C3 aldehydes [29,30] which are then
selectively oxidized to corresponding carboxylic acids including
AcOH [31] (see Fig. 1). However, these intermediate steps proceed
very fast, thus, are not limiting. The limiting step is formation of
AcOH and desorption from the catalyst surface.
where Cj is the concentration (molar fraction) of the gaseous com-
ponent j; ␶ is the residence time (s); LV is the total number of surface
sites (mol) per mole of gas molecules present in the catalyst bed;
ꢂj is the concentration (mol/mol) of the surface species (including
the vacant sites); ri is the rate of the elementary step i; ˛ij is the
matrix of stoichiometric coefficients. The form of the right parts of
the equations is determined by the kinetics scheme of reaction.
3.2. Model of isotope transfer
The selectivity to AcOH in the oxidation of 1-butene in the pres-
ence of water depends on the reactivity of each intermediate, i.e.,
activation parameter, and temperature of the reaction. At the same
The model of isotope transfer carried out under steady-state
conditions in plug-flow reactor can be written as the following

128
W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
Table 1
addition of water results in the increase of the selectivity to AcOH,
but only at lower temperatures up till ca. 220 ◦C. At higher temper-
atures the difference is negligible indicating that influence of water
strongly depends on reaction temperature, i.e., prevalence of des-
orption of physisorbed water and dehydration of VOH and TiOH at
temperatures above 250 ◦C.
Conversion (X) and selectivities (S) to acetic acid, carbon dioxide and carbon monox-
ide during oxidation of 1-butene on VOx-TiO2 catalyst under dry gas flow condition
(Dry) and in presence of 17 vol-% water (Water).
Temperature (◦C)
Parameter
Conditions
160
200
220
250
280
At the same time the selectivity to COx was suppressed. The for-
mation of CO2 increased with the temperature from 5% to 35% under
dry flow and from 3% to 25% in the presence of water. CO was also
observed in the course of reaction, however, the selectivity was 3 –
4 times lower than for CO2. At low temperatures, 2-butanol is the
main product of the reaction with the selectivity above 20% (not
present in Table 1). However, due to its poor oxidative stability, its
amount decreases in the product composition at higher tempera-
tures to ca. 1.5%. The selectivity’s towards this intermediate were
slightly higher under wet conditions only at lower temperatures.
Increase of the reaction temperature suppressed the formation of
2-butanol due to its further oxidative C-C scission (not present in
Table 1). Additionally, the formation of furan and maleic anhy-
dride as byproducts was observed resulting from isomerization of
2-butene followed by dehydrogenation to butadiene (not show in
Table 1). However, this reaction pathway over VOx-TiO2 catalyst is
not favored as opposed to e.g., VPO. Thus, very low selectivities to
furan and maleic anhydride (below 1%) were observed.
Conversion (%)
Dry
Water
Dry
Water
Dry
Water
Dry
4
5
22
31
5
3
1.5
0.5
14
24
36
51
13
8
39
46
56
61
16
11
4.5
2.5
64
69
66
68
19
12
6
100
100
46
45
35
25
8
S CH3COOH (%)
S CO2 (%)
S CO (%)
3
1
Water
3
4.5
GHSV: 18000 h−1; 1-butene 1.4 vol-%; O2 4.0 vol-%.
The influence of water on the formation of products in partial
catalytic oxidation, oxidative dehydrogenation of C2-C3 hydro-
carbons [13–25] and selective oxidation of acrolein [32] is also
known from literature. Moreover, water influence on the selectivity
towards carboxylic acids during the oxidation of C2-C3 aldehydes
over VOx-TiO2 catalyst has been recently studied by our group [31].
The results showed that the selectivity increased in the presence
of water and the total oxidation pathway was depressed. In gen-
eral, the differences in activities (conversions) and selectivities in
the absence and presence of water at various temperatures can
be attributed to (i) different adsorption-desorption equilibriums
of reactant and product molecules and (ii) direct involvement of
water in the catalytic oxidation reaction. These results show that
the water and reaction temperature play an important role in the
formation of oxidation products. Obviously, this effect is associ-
ated with the modification of the surface of the catalyst and this
phenomenon strongly depends on the temperature. Thus, surface
sensitive techniques, i.e., DRIFT spectroscopy, XP spectroscopy and
TPD of ammonia were applied to study these effects in more details.
Additionally, the adsorption of water under anaerobic conditions
was carried out as a model experiment to elucidate the effect of
water on the properties of the catalyst.
Fig. 2. Correlation of 1-butene conversion with selectivity to acetic acid at different
temperatures in presence and absence of water. For reaction conditions, see Table 1.
time, the reaction temperature also has strong influence on the
adsorption-desorption equilibrium of the catalyst surface, water
and all the products of the reaction [24,34]. In the following section,
an experimental data about the influence of water and temperature
on the properties of the surface of the catalyst and their catalytic
performance will be presented.
Summarized results of 1-butene oxidation in the presence and
absence of water at different temperatures are shown in Table 1.
Under wet conditions, higher conversions of 1-butene and higher
selectivity to AcOH were observed in the temperature region of 160
– 250 ◦C.
Fig. 3a shows the DRIFT spectra of VOx-TiO2 catalyst after
pretreatment with water at different temperatures. The broad
Fig. 2 exhibits the comparison between conversion and selec-
tivity to acetic acid at different temperatures. It clearly shows that
Fig. 3. DRIF Spectra of VOx-TiO2 catalyst after pretreatment with water steam at different temperature. Hydrothermal treatment 15 vol% Water; balance Helium. Total Flow:
100 ml/min.

W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
129
Fig. 5. Formation of acetic acid in dry at wet flow during oxidation of 1- butene with
lattice oxygen from VOx-TiO2 catalyst in anaerobic condition.
Fig. 4. The NH3-TPD profiles for VOx-TiO2 catalyst after pretreatment with water
vapour at different temperature.
nal oxygen in vanadia species, bridging oxygen in titania matrix
(Ti-O-Ti) or bridging oxygen between vanadia species and titania
matrix (Ti-O-V), different products of partial and total oxidation are
formed such as COx and C1-C3 carboxylic acids and aldehydes. Due
to these reactions, vacancies in the matrix of the metal oxide are
formed serving as free water sorption centers. On the other hand,
the consumption of lattice oxygen inevitably leads to the reduc-
tion of vanadium oxide species and titanium oxide support. Thus,
XPS investigation was carried out to identify the oxidation states of
vanadium oxide surface species and titania matrix before and after
pretreatment with 1-butene.
absorption region between 3600 and 3000 cm−1 corresponds to the
physisorbed water. With the increase of the temperature from 160
to 350 ◦C the physisorbed water is removed from the surface of
the catalyst and the broad band disappears. Physisorbed molecu-
lar water also appears at the vibrational frequency of 1650 cm−1
and similar decrease of the band can be observed with increas-
ing temperature (see Fig. 3a). The sharp band at the frequency of
3650 cm−1 corresponds to the dissociatively adsorbed water or/and
OH groups from hydrated titania species. These findings stay in the
good agreement with the DFT calculations and experimental data
presented by Czekaj et al. [33]. The bands associated with V-OH
species could not be detected in the DRIFT spectra due to the low
content of vanadium in VOx-TiO2 catalyst, i.e., below 6 wt.-%.
The acidity of VOx-TiO2 catalyst is also substantially affected by
temperature. Fig. 4 shows the TPD of ammonia profiles of the cata-
lyst pretreated at three different temperatures for different periods
of time. In the case of catalyst pretreated at 160 ◦C the TPD-profile is
very broad with Lewis and Brønsted type of acidic sites not clearly
resolved. Nevertheless, there is a distinct shoulder in the profile at
higher temperature region likely corresponding to the strong acidic
centers of Brønsted type. Increase in the temperature of pretreat-
ment to 300 ◦C resulted in the decrease of the acidity of the catalyst.
In particular, the disappearance of Brønsted acidic centers can be
observed. Brønsted centers are mainly associated with OH groups
on the surface of the catalyst, either connected to vanadia species
or titania surface. Higher temperatures promote dehydration of the
surface resulting in the loss of Brønsted centers [24,34,35].
The signal of V 2p
of the freshly calcined catalyst is shown
3/2
in Fig. 7a. Binding energy of the signal corresponds to vanadium
species in the oxidation state +5. A small component at lower B.E.
can be attributed to vanadium species in lower oxidation state, i.e.,
V4+. After 6000 s TOS pretreatment of the catalyst with 1-butene
(see Fig. 5), XPS signal of V 2p 3/2 shifted to lower binding energies
indicating reduction of vanadium species, i.e., ca. 60% of vanadium
was found to be in the oxidation state +4 and +3 (see Fig. 7b).
More information could be derived from the Ti 2p
region of
3/2
XP spectrum. Fig. 7d and e shows the comparison of Ti 2p
3/2
signals of freshly calcined sample and after pretreatment with 1-
butene. Before the reaction, Ti 2p 3/2 signal mostly corresponds to
titania species on +4 oxidation state at 458.7 eV. Second compo-
nent found at lower binding energy at 457.2 eV is very small and
can be attributed to reduced titania surface species presumably on
The catalytic experiments in the presence and absence of water
under oxygen-free conditions were carried out to elucidate the role
of water in the mechanism of the formation of different oxidation
products (Fig. 5). After the input of 1-butene over freshly acti-
vated catalyst, the concentration of detected AcOH in the absence
of water reaches the maximum after 25 min TOS following by a
rapid decrease after 35 min TOS. In the presence of water, AcOH
was detected immediately after 1-butane input with higher con-
centration and over longer period of time. This suggests that the
oxygen from water is involved in the formation of AcOH. Addi-
tionally, faster detection of AcOH in the presence of water can be
explained by more agile removal and/or desorption of this polar
compound from the surface of catalyst.
Fig. 6 illustrates breakthrough curves for the adsorption of water
over pretreated and calcined catalysts. The results show that more
water is adsorbed on the catalyst pretreated with1-butene. In con-
trast, freshly calcined sample exhibited only very small uptake of
water. This can be explained by the reaction of 1-butene with lattice
oxygen. Depending on the origin of the oxygen, i.e., V = O termi-
Fig. 6. Breaktrhrough curves for the adsorption of water over freshly calcined cat-
alyst and after pretreatment with 1-buten at 220 ◦C for 120 min.

130
W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
Fig. 7. Experimental XPS spectra and curve-fitting of the V 2p 3/2 and Ti 2p 3/2 for freshly calcined (a, d) VOx-TiO2 catalyst and after treatment with 1-butene flow at 220 ◦
C
(b, e) and after pretreatment with 1-butene and water (c, f).
+3 oxidation state [36]. After the reaction, the whole signal of Ti
2p became much broader and a tailing at lower binding ener-
Fig. 9a shows concentration-time profile of AcOH in wet flow. The
AcOH was detected at the outlet of reactor immediately after the
input of 1-butene and the steady-state conditions were reached
after ca. 300 s. In contrast, in the absence of water, AcOH was
found only after ca. 200 s. from the addition of 1-butene to the flow
(Fig. 9b). In this case, the steady-state concentration of AcOH was 4
times lower than in the experiment in wet flow. After steady-state
conditions have been reached the continuous flow of water was
introduced into the gas feed. An impulse of increased concentration
of AcOH was detected. Afterwards, steady-state conditions have
been reached after ca. 1000 s. Additionally, such impulses were
detected also for other products of partial oxidation, i.e., 2-butanol
and MEK. The delay in the detection of AcOH in the beginning of the
experiment in dry flow can be explained by the saturation of the
catalyst surface with AcOH followed by its total oxidation. In the
beginning of the reaction, the total oxidation products are favored
due to the high content of vanadium oxide species on +5 oxida-
tion state. The reduction of these species in the curse of reaction
leads to the decrease of total oxidation reaction rate. Thus, AcOH
can be detected only after the rate of total oxidation reaction is
sufficiently small. It should also be mentioned that the reaction
order with respect to oxygen for total oxidation reaction is higher
than for selective oxidation of 1-butene to AcOH. No delay of the
AcOH detection in the wet flow experiment obviously indicates that
water effectively suppressed the total oxidation reaction.
The concentration of AcOH at the maximum of the desorp-
tion peak is much higher than starting concentration of 1-butene.
Thus, it can be assumed that this impulse is not connected with an
increase of the oxidation reaction rate. Instead, it indicates that the
addition of water increased the rate of desorption of the oxidation
products from the catalyst surface.
Amount of AcOH adsorbed on the catalyst surface in the dry
flow experiment can be estimated from the area of the desorp-
tion peak observed immediately after the addition of water. It
corresponds to ca. 0.5 × 1019 molecules/m2 which is close to the
monolayer coverage. Concentration of other intermediates, i.e., 2-
butanol and MEK was very low and can be neglected. Therefore, the
kinetic modeling was carried out taking into account simple model
3/2
gies appeared. Deconvolution of the signal showed much higher
contribution of reduced titania species at lower binding energies
suggesting strong influence of reactant and water on the surface
of VOx-TiO2 catalyst. This reductive effect can be explained by the
mechanism of reaction in which the reacting olefin consumes the
oxygen from vanadia species and lattice oxygen from titania sup-
port [37,38]. Resulting vacancies can later be hydrated with water
molecules; however the surface cannot be fully reoxidized. Fig. 7c
and f show the V 2p 3/2 and Ti 2p 3/2 XPS region for the pre-
treated catalyst with water and 1-butene. The spectra were taken
after 6000 s TOS (see Fig. 5). The results show that the surface con-
tent of reduced V and Ti species is not as prominent as in the case
of the catalyst pretreated with 1-butene only (see Fig. 7b and e).
Nevertheless, the surface is more reduced than in the case of fresh
calcined catalyst (see Fig. 7 a and d). The difference in the reduc-
tion degree of the surface for pretreated catalyst in the presence
and absence of water can be explained by much higher formation
of oxygenated products in the presence of water which can serve
as a potential oxygen source, thus, facilitating reoxidation [23].
Fig. 8 shows a proposed mechanism of the formation of lattice
vacancies and their hydration with water. The mechanism stays in
the agreement with the results of water adsorption experiment and
XPS studies over fresh and pretreated catalysts. In the real catalytic
oxidation process, the reaction proceeds in the presence of oxy-
gen via Mars van Krevelen mechanism. Thus, molecular oxygen is
adsorbed on the active center on the surface of the catalyst subse-
quently reacting with hydrocarbon. Formed vacancy, i.e., reduced
active center is then reoxidized by another oxygen molecule.
The second part of this work will be focused on the kinetic stud-
ies of the oxidation of 1-butene under aerobic conditions.
4.2. Transient kinetic studies
Prior to the transient studies, the catalyst was pretreated at
220 ◦C in oxygen atmosphere in the absence and presence of water
for 1 hour. After that time, 1-butene was introduced to the flow.

W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
131
Fig. 8. Mechanism of the oxidation with lattice oxygen from V- and Ti-Oxide during pretreament with 1-butene in anaerobic condition followed by adsoption of water.
Fig. 9. Time dependencies for the formation of acetic acid during oxidation of 1-butene on VOx-TiO2 catalyst at 220 ◦C in the presence of water (a) and in dry flow (b). Water
concentration 15 vol%. Open circles: experimental dates, dotted and solid curves: calculated. The arrows indicate the addition of 1-butene and water to the feed.
active species have been identified depending on the reaction con-
ditions, i.e., temperature, presence of water. This includes terminal
V=O, bridging V-O-V, and hydrated V-OH species. Hydrated vana-
dium oxide species have been proposed as the most selective in the
formation of AcOH [23]. Therefore, additional reaction equation,
i.e., hydration of oxidized vanadium species, has been introduced
Fig. 10. Simplified model of the transformation of 1-butene in partial and total
oxidation.
to the kinetic model.
with only one surface intermediate [X] - the adsorbed AcOH species
(Fig. 10).
It was assumed that the formation of AcOH and CO2 proceeds
+H
O
2
5) [ZO] −→ [ZOH]
As a consequence the following additional reactions involving
hydrated vanadium species need to be considered as well:
on one type of vanadium oxide active centers according to the
following reactions:
1a) 2[ZH] + O2 → [ZOH]
1) O2 + 2[Z] → 2[ZO]
2a) C4H8 + [ZOH]+(n−1)[−ZO→H]+m[ZO] [X1]
2) C4H8 + 4[ZO] → 2[X]
3a) [X1] → AcOH + m[Z] + n[ZH
]
3) [X] → AcOH + 2[Z]
4) AcOH + 4[ZO] → 2CO2 + 2H2O + 4[Z]
4a) C4H8 + 12[ZO] → 4CO2 + 4H2O + 12[Z]
Expanding the model with these terms allows more accurate
description of the experimental results (Fig. 9b, curve 2). Exact con-
centration of intermediate ꢂ[X] in dry flow calculated according to
the model equals to ca. 80% of monolayer coverage. In the presence
of water ꢂ[X] and ꢂ[X is not higher than few %. Calculated value of
Results of kinetic modeling following these assumptions are
presented in Fig. 9a and b. In the first model – curve 1a – it is
postulated that formation of CO2 mostly proceeds via parallel reac-
tions 4a (r4 = 0). In the second model – curve 1b – CO2 is formed
only by the oxidation of acidic acid (r4a = 0). The results show that
the second model describes the real experiment in dry flow more
adequately. Numerical analyses were carried out by varying des-
orption constant and total oxidation constant. The best results were
obtained in the case of high desorption constant and low total oxi-
dation constant in the presence of water. However, the model was
not consistent with the experimental results after desorption of
AcOH was completed. The model showed that the steady-state con-
ditions were reached immediately after desorption period, whereas
in experiment, a gradual increase in the concentration of AcOH was
observed over time. It is assumed that the increase is associated
with slow transformation of active sites. Different vanadium oxide
]
1
the hydration reaction constant k5 = 0.0005 s−1 is consistent with
the value determined experimentally in the model reaction [28].
Thus, it can be concluded that transformation of V=O to V-OH
species is responsible for a gradual increase in the concentration
of AcOH (Fig. 9b).
These results show that the addition of water has two main
effects: (i) an immediate effect, associated with the increase of
desorption rate of AcOH from the catalyst surface and suppres-
sion of total oxidation rate, (ii) a prolonged effect, associated
with the hydration of oxidized vanadium species which favor
the selective oxidation. Nevertheless, it cannot be unambiguously
concluded whether V-OH species are directly involved in the for-
mation of reaction products. It can also be assumed that they

132
W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
ox
VOH(VO)
only participate in the activation of 1-butene and oxygen transfer
proceeds over active lattice VO centers and/or adsorbed oxy-
gen.
and rate of reoxidation R
(Eq. (4)).
exch
VOH(VO)
18
2
18
O
2
W
˛
O + Rox
+ R
˛
H
VOH(VO)
18
VOH(VO)
˛
=
(4)
exch
VOH(VO)
ox
W
VOH(VO)
4.3. Isotopic kinetic studies
where: ˛1O8 = 0, ˛1H8 = 0.98.
O
Kinetic2 data of2 the isotope exchange with water obtained
with a similar sample of V2O5/TiO2 catalyst under temperature-
programmed (50–500 ◦C) and isothermal (200 ◦C) regimes have
recently been presented [28]. The exchange of oxygen atoms
between water and vanadium oxide was shown to be rather inten-
sive in this temperature range. The exchange involved both VOH
and VO sites. In comparison with water, the rate of dioxygen isotope
exchange with vanadium oxide is negligible. Rate constants of VOH
and VO oxygen exchange with adsorbed water (at 200 ◦C) and acti-
4.3.1. Analysis of the 16O, 18O isotope distribution in AcOH under
steady-state reaction conditions with H218O present in the feed
Three isotopomers of AcOH was formed, i.e., CH316O16OH,
CH316O18OH and CH318O18OH as a result of the reaction in pres-
ence of H218O. The total concentration of all three isotopomers was
the same as the concentration of formed AcOH in the presence of
H216O.
Under steady-state reaction conditions, at 220 ◦C the atomic iso-
18
AcOH
tope fraction of 18O in AcOH (˛
) amounted to:
exch
VOH
exch
VOH
exch
VO
vation energies were obtained: k
≈ 0.5 s−1, E
≈ 0, k
≈
2 ∗ CH3C18O18OH + CH3C16O18OH
18
AcOH
2 ∗ 10−3
s
−1, E
= 70 kJ/mol, as well as equilibrium constants of
˛
=
exch
2 ∗ (CH3C18O18OH + CH3C16O18OH + CH3C16O16OH)
VO
water adsorption KR ≈ 10 s–1, and adsorption heat Q = 45 kJ/mol.
= 0.63
It can be seen from Figure 11 that distribution of isotopic AcOH
After recalculation to our reaction conditions, the rates of iso-
−3
exch
VO
tope exchange equalled to: W
≈ 10 s−1 (per one site), and
exch
W
≈ 0.2 s−1
.
VOH
Rate of reoxidation of V-OH and V-O centers can be calculated
molecules observed in experiment strongly differs from calculated
18
AcOH
based on the rates of reactions proceeding on both centers. It is
assumed that the total oxidation takes place only over V-O center
and selective oxidation to AcOH over both V-O and V-OH cen-
ters. Formation of AcOH from 1-butene requires two oxygen atoms
transferred from active centers. Thus, three possible combinations
of active species participating in oxygen transfer can be proposed,
i.e., VOH+VOH, VOH+VO and VO+VO with corresponding pathway
contributions ı1, ı2 and ı3, where (ı1+ı2 + ı3 = 1). Reoxidation rates
for VO and VOH can be expressed as follow:
equilibrium distribution ˛
:
2
fA1c8O−H18(equilibrium) = (˛18
)
= 0.4
AcOH
fA1c6O−H18(equilibrium) = 2(˛A18cOH)(1 − ˛A18cOH) = 0.47
2
fA1c6O−H16(equilibrium) = (1 − ˛18
)
= 0.13
AcOH
Note that CH316O18OH fractions are much smaller than the equi-
librium values. As it will be shown below, this is a crucial point that
allows discriminating some variants of isotope label transfer.
During the oxidation of 1-butene in presence of H218O, two com-
petitive processes take place: isotope exchange with water, which
enriches active sites with 18O, and reoxidation of catalyst by 16O2
ox
VOH
R
= RAcOH(2ı1 + ı2)
ox
VO
R
= RAcOH(ı2 + 2ı3) + 3RCO
2
C
CO
C
= 0.003 s−1, RCO
=
= 0.0025 s−1 - rate of
AcOH
ꢀL
2
where RAcOH
=
2
ꢀL
v
v
(Fig. 12).
AcOH and CO2 formation calculated for one center (TOF).
18
Isotopic fraction of 18O in VOH (˛
) and VO (˛1V8O) depends on
VOH
Depending on the relation between rates of different reaction
exch
the ratio between the reaction rate of isotopic exchange W
ox
pathways ı1, ı2 and ı3, reoxidation rates R
can change in
VOH(VO)
VOH
the region between 0 (when ı1 = ı2 = 0) and 0.006 s−1 (when ı1 = 1).
In any case, this value is essentially lower than the rate of oxygen
exchange with water WVeOxcHh. Therefore, 18O isotope fraction in VOH
will be close to the isotope fraction in water vapor:
ꢄ
ꢄ
18
ꢄ
ꢄ
0.98
≥ ˛
≥ 0.97
VOH
ı
=ı =0
ı
=1
1
2
1
On the contrary, rate of VO sites reoxidation strongly exceeds
the rate of their isotope exchange with water, and at any ı1, ı2, ı3
the 18O fraction in VO will be close to isotope fraction in dioxygen:
ꢄ
ꢄ
ꢄ
ꢄ
18
0.075
≤ ˛VO ≤ 0.12
ı
=ı =0
ı
=1
1
2
1
With the known isotopic composition of the catalyst surface, we
can calculate the isotopic composition of AcOH that corresponds to
different ratios of the pathways proceeding on different sites.
A) Suppose that only VO sites take part in the formation of AcOH
(ı1 = ı2 = 0). In this case, the isotope fraction of 18O in AcOH at the
time of their formation will be equalled to ˛1V8O, and distribution
of isotopic AcOH molecules will be determined solely by proba-
Fig. 11. Comparison of experimental distribution of AcOH isotopomers under
steady state conditions in the presence of H218O and equilibrium distribution.
bility factor and will correspond to equilibrium distribution. If the
18
AcOH ↔ [H2
O
ADS
] exchange is ignored, the isotopic composition
16−18
18
of AcOH will be as follows: ˛
variant A1). If one of oxygen atoms in AcOH (most probably C-OН)
exchanges with [H2
= 0.12, f
= 0.21 (Fig. 13,
AcOH
AcOH
Fig. 12. Proposed scheme of 16O and 18O isotope transfer under steady state condi-
18
tions in the presence of H218O.
O
ADS
], this will provide an evident prevalence

W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
133
16−18
18
AcOH
Fig. 13. The values of f
and ˛
calculated by different models and compared to experimental values.
AcOH
18
of CH3C18O18OH fraction. The maximum value of ˛
may attain
AcOH
16−18
0.56, with f
= 0.88 (Fig. 13, variant A2). Alternatively, sup-
AcOH
posing both oxygen atoms of AcOH exchange equiprobably with
18
[H2
O
ADS
], the distribution of isotopic molecules will also corre-
18
spond to the equilibrium one. ˛
may attain the value observed
AcOH
16−18
in experiment, 0.63, with f
= 0.47 (Fig. 13, variant A3). Thus,
AcOH
in any case when the label appears in AcOH via isotope exchange,
16−18
the f
fraction will be greater or equal to the equilibrium one.
AcOH
B) Let us suppose that both VO and VOH sites take part in the
formationof AcOH. Thentheisotopefractions inAcOHwill bedeter-
mined as follows (ignoring the secondary exchange of AcOH with
water):
18
AcOH
˛
= (ı1 + 0.5ı2)˛18 + (0.5ı2 + ı3)˛18
VOH
VO
16−18
f
= ı12˛V18OH(1 − ˛18 ) + ı2(˛1V8OH(1 − ˛V18O) + ˛V18O(1 − ˛1V8OH))
VOH
AcOH
+ 2ı3˛18 (1 − ˛1V8O).
Fig. 14. Formation of different isotopomers of AcOH during the oxidation of 1-
butene on a VOx-TiO2 catalyst at 220 ◦C, in absence and presence of H218O. The
arrows indicate the addition of 1-butene and isotopic water to the feed.
VO
At a certain rate ratio of AcOH formation routes, namely at
state conditions, H218O was added to the feed resulting in a sharp
increase in the concentration of AcOH and then attaining a steady
state concentration level, which was 2–3 times higher than that of
AcOH formed in absence of water.
␦1 = 0.48, ␦2 = 0.24, ␦3 = 0.28, we obtain the calculated isotopic com-
position of AcOH that corresponds to that observed in experiment
(Fig. 13, variant B).
Analysis of isotopic distribution shows that V-OH center
directly participates in the formation of AcOH, although the cal-
culated values can be slightly overestimated due to exclusion
The model of transient regime upon addition of H218O is repre-
sented by a set of nonstationary kinetic equations (Eq. (1)) written
in accordance with the reaction steps 1–5 and 1a-3a. They were
supplemented with isotope kinetic equations (Eq. (5)) describing
changes in the concentration of isotopic AcOH molecules and the
18O fraction in VO (to simplify the calculation, 18O fraction in water
vapor and VOH was rounded off to 1):
of oxygen exchange between AcOH and adsorbed water, i.e.,
18
AcOH ↔ [H2
O
ADS
]. In the next part of the work the estimates were
refined by numerical modeling of the isotopic distribution of AcOH
molecules under transient conditions.
1818
ꢅ
ꢆ
dC
dC1818
1
ꢀ
AcOH
AcOH
18
+
+
= LV (r2a1 + r2a2
˛
+ r2(˛18
)
VO
2 − k3C1818
AcOHꢂZ2O) + LV 0.5k1C1816
VO
AcOH
dt
dꢁ
1618
AcOH
dC1618
ꢅ
ꢆ
dC
1
ꢀ
AcOH
= LV (r2a2(1 − ˛18 ) + 2r2˛18 (1 − ˛1V8O) − k3C1618
AcOHꢂZ2O) + LV k2C1616 − 0.5k1C1816
VO
VO
AcOH
AcOH
dt
dꢁ
(5)
1616
C
= CAcOH − C1818 − C1618
AcOH
AcOH
AcOH
18
VO
d˛
ꢂ
VO
= WexchꢂVO(1 − ˛18 ) − (r2a1 + 2r2 + 4r3)˛18
VO
VO
VO
dt
where, r2a1 = k2a2CBUT ꢂVOH and r2a2 = k2a2CBUT ꢂVOH - rates of reac-
4.3.2. Analysis of the isotope distribution in AcOH under transient
conditions after input of H218O
tions proceeding with the participation of VOH+VOH, VOH+VO
centers; r2a1 + r2a2 = r2a. Terms in parentheses correspond to iso-
Fig. 14 shows the formation of AcOH at 220 ◦C in presence of
H218O. The reaction was first studied in the absence of water, simi-
lar to the experiment described in chapter 4.2. After reaching steady
topic exchange between AcOH and [H2
O
]. If both oxygen AcOH
18
ADS
18
atoms are exchanged with [H2
O
ADS
] with the same probability,
then k1 = k2.

134
W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
Fig. 15. Modeling of transient regimes upon addition of H218O to the feed (points – experiment, lines – calculation).
If the rate constant of the formation of VOH centers remains
unchanged (k5 = 0.0005 s−1), then the calculated concentration of
isotopic AcOH molecules increases slower than in the experi-
ment (Fig. 15a). It can be proposed that during the desorption
of products, the probability of the transformation of unoccupied
active centers will be increased. Formally, it can be expressed as
dependency of rate constant of step 5 on the desorption rate, i.e.,
k5 = k50(1 + a(r2 + r3)), where k50 = 0.0005 s−1 – rate constant of the
transformation VO→VOH in the absence of reaction. Thus, after
addition of water to the reaction mixture, calculated rate for the
formation of VOH centers and consequently the amount of formed
isotopic AcOH molecules will increase (Fig. 15b and c). Calculated
variants showed in Fig. 15b and 15c differ in ki values. In the first
case (Fig. 15b) ki = 0, while in the other (Fig. 15c) ki > 0. The last
case (Fig. 15c) describes the dynamics of the AcOH formation the
at VOH + VO sites : ı2 = 0.28
at VO + VO sites : ı2 = 0.30
In this case, the calculated concentration of centers ꢂVOH and
ꢂVO equaled to ≈ 30% and ≈70%, of the total amount of vanadia
surface species, respectively. Thus, the turnover frequency (TOF =
R/ꢂ) over VOH centers is about three times higher than over VO
centers.
4.3.3. Analysis of 18O transfer from O2 to AcOH under
steady-state conditions (SSITKA)
Fig. 16a shows the change of 18O isotopic fraction in AcOH
18
AcOH
(˛
) after switching from 16O2 to 18O2 under steady-state
most adequately.
conditions in dry flow at 220 ◦C. Labelled 18O appears in AcOH
immediately after 16O2 → 18O2 switching and reaches the steady-
state conditions after ca. 3000 s.
18
Results of the modeling showed that exchange with [H2
O
ADS
]
strongly affects the isotopic distribution of AcOH molecules under
transient conditions. However, under steady-state conditions this
effect is negligible, and isotopic composition of AcOH is developed
mainly at the formation step. Contributions of the routes involving
different active sites to the total steady-state rate of AcOH forma-
tion were calculated in the view of subsequent isotope exchange of
AcOH with water:
Logarithmic dependency of the isotopic fraction on time, i.e.,
18
ln(1 − ˛
(t)) clearly exhibits convexity, typical for a reaction
AcOH
with buffer step [39,40]. The number of exchanged oxygen atoms
was calculated according to Eq. (6).
t
ꢃ
18
AcOH
NO
=
(1 − ˛
)dt × U × 2CAcOH/g/S ≈ 8 × 1019 at/m2,
(6)
at VOH + VOH sites : ı1 = 0.42
0
Fig. 16. Comparison of SSITKA experiments with 18O2 in dry (a) and wet flow (b).

W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
135
Fig. 18. Proposed scheme for oxygen transfer from gas phase to oxidation product
in the presence of water.
Fig. 17. Proposed scheme for oxygen transfer from gas phase to oxidation product
in the absence of water.
of isotope response. The curve has a pronounced sigmoid shape
and the 18O label in AcOH appears with a considerable delay, i.e.,
a few tens of seconds. The difference of the response curves in a
This number is one order of magnitude higher than the mono-
layer coverage. This indicates that oxygen from the bulk of
vanadium oxide serves as a buffer. Thus, the 18O transfer from O2
to AcOH in dry flow can be illustrated with the scheme showed in
dry flow is most clearly seen in logarithmic coordinates. In this
18
AcOH
case, the ln(1 − ˛
(t)) dependence is concave, which is typ-
ical for the consecutive mechanism of surface transformations.
Interestingly, the numerical modeling shows that concentration
of oxygen-containing intermediates appearing in the consecutive
mechanism strongly exceeds the monolayer coverage and is close
to oxygen concentration in the bulk of vanadium oxide. The scheme
of 18O transfer from O2 to AcOH describing most adequately the
results of experiment in presence of water vapor is shown in
Fig. 18.
Fig. 17.
18
AcOH
The scheme describes the isotope response ˛
quite accu-
rately (Fig. 16a, solid line). Estimated effective diffusion coefficient
of labeled oxygen atoms in the bulk of vanadium oxide equals to
D/L2 = 8 × 10–3 s–1
.
In presence of water vapor (H216O), the dynamics of isotope
response upon 18O2 substitution of 16O2 in the feed differs from
that in a dry flow. Firstly, the characteristic time of transient regime,
which is determined by a ratio of intermediates concentration to
conversion rate, strongly decreases due to a faster rate of the reac-
tion. Secondly, the isotope fraction of 18O in AcOH attains a level
that is much lower than the 18O fraction in O2, which is caused
by a concurrent process of isotope exchange of the catalyst oxy-
gen with H216O. Noteworthy is a profound change in the form
Taking into account that in a dry flow the reaction proceeds
mainly on VO sites while in the presence of water on VOH sites the
following conclusions can be drawn. VO centers are mainly formed
during direct interaction with gaseous oxygen. In turn, VOH centers
can be formed either from bulk oxygen or during interaction with
added or reaction water.
Fig. 19. Proposed mechanisms of selective oxidation (a) and total oxidation (b) of 1-butene.
Table 2
Elementary reactions and kinetic parameters used in the modelling of 1-butene oxidation with the formation of AcOH and CO2 following the mechanism proposed on Fig. 18.
Stages/Reaction
Rate expression
Constants (s−1
)
in absence of water
in presence of water
Catalytic reaction over VO centers
1) O2 + 2[Z] → 2[ZO]
r1 = k1CO ꢂ[Z]
>0.5
0.2
2
2) C4H8 + 4[ZO] → 2[X]
r2 = k2CC 8 ꢂ[ZO]
H
4
3) [X] → AcOH + 2[Z]
r3 = k3ꢂ[X]
0.001
2
0.024
0.6
4) AcOH + 4[ZO] → 2CO2 + 2H2O + 4[Z]
r4 = k4CAcOH ꢂ2
[ZO]
Catalytic reaction over VOH centers
1a) [ZH ] + [O]BULK → [ZOH]
1b) O2 → [O]BULK
BRUTTO
1a
r
= k1aCO ꢂ[Z
>0.5
H
]
2
2a) C4H8 + 4[ZOH] → 2[X1]
r2 = k2CC 8 ꢂ[ZOH]
0.7
>0.3
H
4
3a) [X1] → AcOH + 2[ZH
]
r3 = k3ꢂ[X
]
1
Transformation of centers
5) [ZO] ꢀ [ZOH]
r5 = k5(ꢂ[ZO] − KRꢂ
)
k50=0.0005
KR=2.5
k5 = k0(1 + 5(r3 +[rZ4O)H)]
5

136
W. Suprun et al. / Applied Catalysis A: General 391 (2011) 125–136
5. Summary
associated with two factors. Firstly, an increase in the desorption
rate of CxHyOz adsorbed products, notably carboxylic acids due to
addition of water. This effect appears immediately after the injec-
tion of water to the reaction feed. Secondly, gradual hydration of
the catalyst surface resulting in the formation of VOH species.
Kinetic modeling of transient isotopic exchange experiments
predicted the presence of two different types of active vanadium
oxide centers, i.e., VO and VOH. Hydrated centers exhibit high activ-
ity in partial oxidation of 1-butene in contrast to dehydrated VO
centers which favor total oxidation. Estimated turnover frequency
of 1-butene to AcOH over VOH centers is ca. 3 times higher than
over VO centers.
According to numerical modeling results of the oxidation of 1-
butene can be summarized by the scheme shown in Fig. 19.
The scheme illustrates that the selective oxidation of 1-butene
to AcOH proceeds in two different catalytic cycles, i.e., one involv-
ing VO and other VOH centers (Fig. 19a). Additionally, VO centers
participate in the total oxidation to CO2 and H2O (Fig. 19b). Rate
constants for the proposed mechanisms obtained from kinetic and
transient isotopic investigations are summarized in Table 2.
The obtained data shows that rate constants of steps 3, 4 and 5
(see Fig. 19) are influenced by the presence of water. The increase
of desorption rate constant of AcOH in the presence of water is
expected phenomenon and can be interpreted on the basis of
adsorption-desorption mechanism proposed by Tamaru [41]. In
their work, it was shown that desorption of reaction products dur-
ing catalytic conversions is accelerated by the presence of ambient
gas. In our case, water can be regarded as ambient medium influ-
encing desorption of reaction products.
Decrease in the AcOH total oxidation (step 4, Fig. 19b) in pres-
ence of water was explained in literature either by blocking the
active VO centers by adsorbed water [2–8] or transformation of
vanadia centers upon interaction with water [18,22,23]. Jehng et
al. used Raman spectroscopy to investigate the transformation of
vanadium oxide species at different temperatures [24]. It was found
that the ratio between VO and VOH centers strongly depended on
the pretreatment temperature. In our work, we showed that dur-
ing desorption of products in the course of reaction, the probability
of transformation of active centers increases. It is associated with
hydration of vacancies formed during the reaction of 1-butene with
lattice oxygen and desorption of oxygen containing products. It
should be emphasized that transformation of vanadium oxide cen-
ters strongly influences the total kinetic of reaction, i.e., conversion
and selectivities to different oxidation products. For example, the
estimated reaction constants of the formation of AcOH over VOH
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