FTIR study of β-picoline and pyridine-3-carbaldehyde transformation on V-Ti-O catalysts. the effect of sulfate content on β-picoline oxidation into nicotinic acid

Article abstract of DOI:10.1016/j.molcata.2013.09.028

Surface complexes of β-picoline, 3-pyridine-carbaldehyde and nicotinic acid on 20 wt.% V₂O₅/TiO₂ catalysts containing 0.07 and 6.3l wt.% SO₄^2- were studied by FTIR spectroscopy and temperature program

Full text of DOI:10.1016/j.molcata.2013.09.028

Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
FTIR study of ␤-picoline and pyridine-3-carbaldehyde transformation
on V–Ti–O catalysts. The effect of sulfate content on ␤-picoline
oxidation into nicotinic acid
∗
Yuriy A. Chesalov , Tamara V. Andrushkevich, Vladimir I. Sobolev, Galina B. Chernobay
Boreskov Institute of Catalysis, Russian Academy of Sciences, Prosp. Lavrentieva, 5, Novosibirsk 630090, Russian Federation
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 July 2013
Received in revised form
Surface complexes of ␤-picoline, 3-pyridine-carbaldehyde and nicotinic acid on 20 wt.% V O /TiO
2
2
5
2−
catalysts containing 0.07 and 6.3l wt.% SO4 were studied by FTIR spectroscopy and temperature pro-
grammed reaction technique. The water (both H2O and D2O), pyridine and ␤-picoline were used as
molecular probes for identification of surface acidity and the structure of sulfate species. Both samples
possess Lewis and Brönsted acidic sites. ␤-Picoline, 3-pyridine-carbaldehyde and nicotinic acid form
surface complexes of similar structure on both studied catalysts. But high sulfated catalyst contains
stronger Brönsted sites in greater amount. It leads to the formation of stronger bound surface complexes
of ␤-picoline, 3-pyridine-carbaldehyde and nicotinic acid. The strongly bound surface complexes are
transformed into by-products (COx, pyridine and 3-cyanopyridine). It resulted in substantial reduction
of nicotinic acid yield.
1
9 September 2013
Accepted 20 September 2013
Available online 1 October 2013
Keywords:
Vanadia–titania catalysts
Sulfates
␤
-Picoline oxidation
Nicotinic acid
FTIR spectroscopy
© 2013 Elsevier B.V. All rights reserved.
SO42 effect on the properties of the support makes it important to
study the effect of the sulfate content in supported vanadia–titania
catalysts on their acidity and adsorption properties as well as their
catalytic performance in the ␤-picoline oxidation.
The acid–base characteristics of the oxide material have three
major effects on the selective oxidation of hydrocarbons. They
affect the reactant molecule activation, the rates of competitive
transformation pathways and the rates of adsorption and desorp-
tion of the reactants and products [7].
−
1
. Introduction
Vapor-phase oxidation of ␤-picoline by oxygen from the air is
a promising “green chemistry” approach for developing a tech-
nology for the nicotinic acid production [1,2]. Nicotinic acid is
widely used in medicine, cosmetics, food industry, agriculture, and
as a monomer for various organic syntheses. Currently the global
annual nicotinic acid production exceeds 40 000 tons.
A novel method for the nicotinic acid production by single-
stage ␤-picoline oxidation with oxygen from the air in the presence
of water over a vanadia–titania oxide catalyst was developed at
Boreskov Institute of Catalysis SB RAS in the early 90s [3–5]. The
catalyst synthesis method uses the titanium oxide support that
is industrially produced by the sulfate technology. Such material
The effect of the acid-base properties is clearly revealed under
the catalyst promotion with basic (K, Na, Cs, Li) or acidic (P, W,
2−
SO4 ) agents. The alkali doping of V O /TiO catalysts for the
5
2
2
oxidative dehydrogenation of propane resulted in the decrease
of the acidity and increase of the basicity of the active centers.
The increase in the selectivity was ascribed to easier propene
desorption from the less acidic surface, preventing the consecutive
propene combustion [8–10].
contains up to 10 wt.% SO42
−
.
As it was demonstrated by us earlier [6], the FTIR spectra reveal
differences in the type, quantity and strength of acid sites as well as
in the composition and strength of surface complexes of ␤-picoline,
Modification of V O /TiO catalysts with sulfate results
5
2
2
3
-pyridine-carbaldehyde and nicotinic acid as a function of the
in growing acidity and promotes methanol oxidation to
dimethoxymethane (DMM) [11–14].
2
−
SO4 content in the anatase TiO . The surface aldehyde and nicoti-
2
nate complexes become dead-end forms on high sulfated TiO2.
Meanwhile, such complexes are weakly bound on low sulfated
TiO2 and are easily desorbed in the original form. The pronounced
For oxidation of C –C4 aldehydes to acids it was demonstrated
1
that the acid-base properties of the catalysts strongly affect the
direction of aldehydes transformation into the acids or deep oxida-
tion products. An increase of the V–Mo catalyst acidity by doping
with phosphorus increases the strength of the surface aldehyde
complexes, their conversion to deep oxidation products and low-
ers the selectivity to acids. Promotion with bases leads to higher
∗ Corresponding author. Tel.: +7 383 330 72 84; fax: +7 383 3308056.
E-mail address: chesalov@ngs.ru (Y.A. Chesalov).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.09.028

Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
119
SO42 to a certain content of SO4 . An aqueous solution of vanadyl
−
2−
basicity and decreases the selectivity due to the formation of
strongly adsorbed forms of the acids [15,16].
oxalate was added to the paste. Slurry was dried in lab spray-dryer
◦
(Buchi-290). Then the samples were calcined in air at 450 C.
Acid-base properties of catalysts are an important factor in
selective conversion of methyl aromatics and hetero aromatics.
Martin [17] demonstrated that the oxidation of these hydrocar-
bons to their aldehydes strongly depends on the nature, strength
and concentration of acid and base surface sites of the catalysts
and the acid–base properties of the reaction mixture. On promoted
vanadium catalysts the selectivity to substituted benzaldehydes
is closely related to the basic properties of the catalyst whereas
the activity strongly depends on the amount and strength of acid
sites. The selectivity of vanadium-containing catalysts to aldehydes
obtained from methyl- and hetero aromatic compounds (including
isomeric picolines) can be increased by adding alkaline cations or
organic bases blocking strong acid sites [17]. For toluene oxidation
the increase of the basicity of V–Ti catalysts doped with potas-
sium improves the selectivity to benzyl aldehyde and decreases the
selectivity to benzoic acid [18,19]. Kalevaru et al. [20] determined
2
.2. Samples characterization
Analysis of the sulfur content was performed by X-ray flu-
orescence spectrometry on an ARL-Advant’X analyzer with Rh
anode of the X-ray tube. Qualitative analysis of sulfate forms was
performed by FTIR spectroscopy. Infrared spectra of the samples
4000–200 cm , 30 scans, resolution 4 cm ) were obtained on
BOMEM MB-102 FTIR spectrometer by standard technique – press-
ing with CsI (2 mg and 500 mg CsI).
X-ray diffraction experiments were carried out on a Siemens
D-500 diffractometer with a monochromated CuK␣ radiation. FT-
Raman spectra (3600–100 cm , 300 scans, resolution 4 cm , 180
geometry) were recorded using a RFS 100/S spectrometer (Bruker).
Excitation of the 1064 nm line was provided by an Nd-YAG laser
100 mW power output).
− −1
1
(
−
1
−1
◦
that the highest possible acidity in a series of V O5/MgF2 catalysts
2
(
is required for selective oxidative ammonolysis of ␤-picoline to
nicotinonitrile.
Takehira et al. [21] studied the effect of the acidity of CrV1 PxO4
catalysts on their activity in ␤-picoline oxidation and selectivity to
−x
2.3. In situ FTIR experiments
nicotinic acid. NH -TPD and infrared study of adsorbed pyridine
showed an increase in the concentration of Lewis and Brönsted acid
3
In situ FTIR experiments were performed in a flow IR cell reac-
tor with a BOMEM MB-102 FTIR spectrometer. The sample powder
35–50 mg) was pressed into a self-supported 1 cm × 3 cm wafer.
The wafer was mounted into a quartz IR cell reactor with CaF
sites through replacing V with a small amount of P in CrVO . It was
4
(
found that the addition of P (x < 0.1) in monoclinic CrVO enhanced
4
2
the catalytic activity due to the cooperation between the acid sites
3
◦
windows and activated in an air flow (50 cm /min) at 300 C for
0 min.
To study the effect of water a mixture of 4%H O (or D O) in air
was introduced to the IR cell with VTi sample at 100–350 C.
-Picoline used for the adsorption was C5H NCH from Aldrich
and the redox properties of VO . An increase of the selectivity was
4
6
related by the authors to higher acidity. The higher acidity enhances
the selectivity, probably, due to better desorption of the formed acid
and prevention of its re-adsorption, thus, protecting the acid from
deep oxidation [21,22].
2
2
◦
␤
4
3
Chemical Company, Inc. (purity 99%). Pyridine-3-carbaldehyde
Modification of vanadia–titania catalysts with sulfates substan-
tially alters their acid and redox properties. Sulfated catalysts have
Lewis and Brönsted acid sites. The concentration and ratio of the
used for the adsorption was C5H NCHO from Aldrich Chemi-
4
cal Company, Inc. (purity 99%). The adsorption was carried out
◦
at 90–250 C by injecting ␤-picoline or pyridine-3-carbaldehyde
2−
Lewis and Brönsted sites depends on the SO4
content [23], the
(
1–2 ␮l) into air flowing through the IR cell reactor. 1–2 ␮g of crys-
amount, type and dispersion of vanadia [12,14,24–26], type of sul-
fate species [13], and temperature of the catalyst heat treatment
talline nicotinic acid C5H NCOOH (home-made sample obtained
4
◦
according [3], purity 99.8%) was introduced into air flow at 250 C
temperature of nicotinic acid desublimation is 237 C). All feed
pipelines were thermostated at 250 C.
[
23–25]. Lewis sites are transformed into Brönsted sites at higher
◦
(
2
−
of SO4 and water vapor contents, and decreasing calcination tem-
peratures.
◦
The background FTIR spectra of the samples were obtained at
each temperature for their further subtraction from the spectra of
species adsorbed on the samples.
Heinz et al. [27] studied vanadia–titania catalysts prepared
2−
using anatase TiO containing 0.5–1.5 wt.% SO4 . The authors con-
2
cluded that variation of the sulfate content in this range did not
have a significant effect on the acidity and catalytic properties in
␤
-picoline oxidation. However, in another paper of the same group
2.4. Catalysts test
the nicotinic acid yield was found to decrease from 84 to 74% when
the sulfate content was increased from 0.5 to 1.5 wt.% [28]. The
highest nicotinic acid yield (97%) was observed in this study over a
The temperature programmed gas-phase oxidation reactions
were performed in a quartz tube flow reactor with an internal diam-
eter of 6 mm, using shaped catalyst granules (d = 0.25–0.5 mm). 1 g
of VTi-1 and 150 mg VTi-2 diluted with crumb quartz were used.
catalyst supported on TiO2 with 0.5 wt.% SO42 [28].
−
The current study is devoted to the investigation of
vanadia–titania catalysts with substantially different sulfate
The reactive gas mixture (␤-picoline/O /He = 60 ml/min) was fed
2
content to elucidate clearly effect of SO42 content on the
−
into the reactor filled with the catalyst; the reactor was operated
under atmospheric pressure. The reaction temperature was mea-
mechanism of ␤-picoline oxidation.
◦
sured inside the reactor (± 1 C) using a thermocouple inserted into
the catalyst bed. The heating rate was 1 K/min. During the cat-
alytic runs, gas samples were analyzed periodically by integrated
online gas chromatography (GC) with a flame-ionization detector
(to determine the ␤-picoline conversion and selectivity to pyridine,
nicotinic acid, pyridine-3-carbaldehyde, 3-cyanopyridine) and a
thermal conductivity detector (to determine the selectivity to car-
bon oxides).
2
. Experimental
2.1. Samples preparation
TiO2 was prepared from a commercial (Euro Support Manufac-
turing Czechia) titanium hydroxide obtained by thermal hydrolysis
of a titanyl sulfate. The initial product contained 88% anatase mod-
ification of titania. A solid hydrogel was separated by decantation
from the initial aqueous suspension, washed with water to remove
Prior to the kinetic measurements, the catalysts were activated
◦
in the reactor at 400 C in a flow of O /He (1:3) for 1 h. The reactor
2
◦
was then cooled to 200 C and the feed was switched at this

1
20
Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
Table 1
Physicochemical characteristics of the samples.
2
2−
Sample
Content of V2O5, wt.%
Phase composition
BET, m /g
SO4 content, wt.%
VTi-1
VTi-2
20
20
TiO2 (anatase), V2O5
TiO2 (anatase), V2O5
13
140
0.07
6.31
−
1
temperature to the reactive gas mixture for 20–30 min in order to
reach steady-state initial conditions.
orthorhombic vanadia. Therefore, the band at 145 cm observed
in the spectra of VTi-1 and VTi-2 catalysts is a superposition of the
bands due to lattice vibrations of V O5 and TiO . Thus, according to
2
2
3
3
3
. Results and discussion
Raman spectroscopy, both samples consist of orthorhombic V2O5
and TiO2 of anatase modification. The bands due to the vibrations
of any sulfur-containing species are not detected in Raman spectra
of the samples.
.1. Samples characterization
.1.1. Bulk characterization
Physicochemical characteristics of the samples are listed in
FTIR spectra of VTi-1 and VTi-2 samples collected at room tem-
perature are shown in Fig. 2. A broad band with a maximum at
−
1
Table 1.
X-ray diffraction of VTi-1 and VTi-2 samples reveals TiO2 of the
anatase structure and orthorhombic vanadia phase. The VTi-2 sam-
ple with a higher content of SO42 has a greater surface area. It
appears that sulfate ions increase the surface area [29] and delay
the crystallization and sintering upon calcination [26].
Raman spectra of VTi-1 and VTi-2 samples collected at room
temperature are presented in Fig. 1. This figure also shows the
Raman spectra of anatase and orthorhombic vanadium oxide. The
spectra of VTi-1 and VTi-2 (Fig. 1, spectrum 3 and 4) are virtually
coincided. The bands at 995, 702, 642, 517, 485, 404, 305, 285, and
3460 cm is due to OH stretching vibrations of adsorbed water and
−
1
hydrogen-bonded hydroxyl groups. The band at 1630 cm char-
acterizes the bend vibrations of the adsorbed water. The bands at
−
−1
∼1020, 820, 380 and 298 cm are assigned to lattice vibrations of
orthorhombic V2O5 [30], whereas a broad band at 900–400 and a
−
1
band at 351 cm are due to lattice vibrations of anatase modifi-
cation of TiO2 [31]. In addition, the bands at 1200–1100, 970 and
−
1
670 cm due to asymmetrical (ꢀ3) and symmetrical (ꢀ1) stretching
and asymmetrical bending (ꢀ4) modes of sulfate groups, respec-
tively [32], are observed in the spectrum of VTi-2 catalyst. Three
−
1
bands at 1153, 1124, and 970 cm are observed in the region of
−
1
2−
1
45 cm
are observed in these spectra. The bands at 642, 517,
ꢀ(SO4 ) vibrations. The first two bands are due to triply degener-
−
1
4
04 cm are due to lattice vibrations of anatase TiO2 (see Fig. 1,
ate (for Td symmetry) ꢀ3 mode. The latter band is due to the totally
symmetric ꢀ1 mode (forbidden in the IR spectrum for the Td sym-
metry) [32]. The splitting of ꢀ3 vibration into two components and
the appearance of band due to ꢀ1 vibration in the infrared spectrum
indicates monodentate mode of coordination and C3v symmetry
−
1
spectrum 2), whereas the bands at 995, 702, 485, 305, and 285 cm
are assigned to lattice vibrations of orthorhombic V O5 (see Fig. 1,
spectrum 1). The bands with the maximum close to 145 cm are
observed both in the spectrum of anatase and in the spectrum of
2
−
1
for SO42 ion [32] in VTi-2 sample. The structures of mono- and
−
bidentate sulfate complexes are shown in Fig. 3. Previously, we
have shown in [6] that SO42 ions are bidentately coordinated to
−
titanium cations in the case of sulfated anatase TiO samples. Bridg-
2
ing coordination of sulfate ions is detected at a low sulfur content
2
2−
(
0.12 wt.%, S = 195 m /g), whereas chelate coordination of SO
is
4
2
observed at a large sulfur content (6.9 wt.%, S = 300 m /g). In the
case of VTi-2 sample (6.3 wt.%, S = 140 m /g) sulfate ions are mon-
2
odentately coordinated. The bands due to sulfate species are not
observed in the spectrum of VTi-1 sample because of low concen-
tration of SO4
4
2−
.
2
1
3
2
1
200 1000
2
1
1
000
800
600
400
200
1
-
1
4000
3000
1600
1200
800
400
Raman shift, cm
-1
Wavenumber, cm
Fig. 1. Raman spectra of orthorhombic V2O5 (1), TiO2 of anatase modification (2),
VTi-1 (3), and VTi-2 (4).
Fig. 2. FTIR spectra of VTi-1 (1) and VTi-2 (2) collected at ambient condition.

Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
121
O
O
O
O
O
O
O
O
2
1
S
S
S
O
O
O
O
M
M
M
M
bidentate chelate
sulfate complex
bidentate bridging
sulfate complex
monodentate
sulfate complex
Fig. 3. The structure of bidentate and monodentate sulfate complexes [32].
2
000
1800
1600
1400
1200
-1
3.1.2. Surface characterization
Wavenumber, cm
The state of the surface was studied by IR spectroscopy using
molecular probes (water and ␤-picoline). Water as a molecular
probe was used for the characterization of sulfate groups, whereas
◦
Fig. 4. FTIR spectra of VTi-1 (1) and VTi-2 (2) registered after 1-h activation at 350
in dry air.
C
␤
-picoline was used as basic molecular probe for the characteriza-
tion of Lewis and Brönsted acidity of the samples. It should be noted
that VTi-1 and VTi-2 catalysts in the form of self-supported wafer
the coordination mode of covalently bound sulfate because of the
impossibility collecting the full infrared spectrum due to the strong
absorption of the samples below 1200 cm caused by anatase and
vanadia lattice vibrations.
When the amount of adsorbed water is increased there are some
changes in the IR spectra (Fig. 5). In the case of VTi-1 sample (Fig. 5,
−
1
−1
are transparent only in the region 2200–1200 cm . The opacity of
the samples in the low-frequency region is caused by the strong
absorption due to lattice modes of titania and vanadia. The opacity
of the samples in the high-frequency region is due to the strong
scattering caused by optical heterogeneity related to two-phase
character of the samples.
spectrum 1) there is only an increase in the intensity of ı(H O) band
2
−
1
at 1640 cm . In the case of VTi-2 sample (Fig. 5, spectrum 2) there is
a change also in the spectrum of sulfate groups. The frequency of the
To study the effect of water a gas mixture of 4%H O in air was
2
◦
−1
−1
passed through a cell with the sample at 100–350 C. Fig. 1 shows
band at 1360 cm is reduced by ∼30 cm , and its intensity is also
−
−1
.
1
the spectra of vanadia–titania catalysts collected after activation
reduced. Simultaneously, the intensity of the band at 1285 cm
◦
at 350 C in dry air flow for 1 h. There is only a very weak band
is decreases, and its frequency is gradually reduced to 1230 cm
Perhaps this is due to fact that two processes run simultaneously:
−
1
at 2030 cm due to the first overtone of ꢀ(V O) vibration in the
spectrum of VTi-1 sample (Fig. 4, spectrum 1). The bands at 2030
symmetrization of SO tetrahedron leading to a decrease in ꢀ(S O)
4
−
1
(
ꢀ(V O)), 1620 (ı(H O)), 1360 and 1285 cm (ꢀ(S O) of surface
frequency and a change in the mode of sulfate group coordina-
tion resulting in a decrease in ꢀ(S O) band intensity. One can also
assume that the formation of monodentate sulfate groups occurs
with increasing amount of adsorbed water and hydroxyl groups on
the surface of the catalyst. As shown above (see Fig. 2) monoden-
tate sulfate, which is the main form in the case of the fully hydrated
2
sulfate groups [29,33,34]) are observed in the spectrum of VTi-2
sample (Fig. 4, spectrum 2). According to [35] the appearance of
−
1
bands in the frequency range 1400–1300 cm is typical for sul-
fated oxides such as ZrO , Al O and TiO2 and indicates covalently
2
2
3
bound sulfate species. In our case it is impossible to identify reliably
1
a
b
2
3
1
2
4
5
3
4
5
2200
2000
1800
1600
1400
1200
1800
1600
1400
-1
1200
-1
Wavenumber, cm
Wavenumber, cm
◦
◦
◦
◦
Fig. 5. FTIR spectra collected during passing a gas mixture 4%H2O in air through IR cell with VTi-1 (a) and VTi-2 (b) sample at 350 C (1), 300 C (2), 250 C (3), 200 C (4), and
◦
1
00 C (5).

1
22
Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
1
2
a
b
1
2
2
000
1800
1600
1400
1200
1800
1600
1400
1200
-
1
-1
Wavenumber, cm
Wavenumber, cm
◦
Fig. 6. FTIR spectra of VTi-1 (a) and VTi-2 (b) collected in dry air (1) and after 0.2 ␮l ␤-picoline adsorption (2) at 100 C.
bands at 1633, 1613, 1552 and 1471 cm 1 due to ꢀ(CC, CN) modes
of the 3-methylpyridinium ion [36–38] is observed in these spec-
tra. This surface complex is formed by the interaction of ␤-picoline
molecule with a strong Brönsted acid site of the catalyst. There are
no bands due ␤-picoline complexes with Lewis acid sites of cat-
alysts in the registered spectra. The intensity of the bands due to
3-methylpyridinium ion is ∼twofold in the spectrum 2 as compared
with the spectrum 1 (Fig. 7). The same volume of ␤-picoline (0.2 ␮l)
was injected into a cell with a catalyst in experiments shown in
Figs. 6 and 7. This amount was sufficient to completely cover the
surface of the catalyst VTi-1, but not enough to completely fill VTi-2
surface. The amounts of ␤-picoline enough to fill the surface of both
catalysts were evaluated under static conditions. A small volume of
␤-picoline (0.02 ␮l) was injected into the cell with VTi-1 or VTi-2
−
VTi-2 sample, is characterized by the bands at 1153, 1124, 970,
−
1
and 670 cm . In this case (the experiments described in Fig. 5b)
the sample were pressed into self-supported wafers and there is
−
1
almost complete absorption of infrared radiation in 1200–600 cm
region due to the lattice modes of anatase and vanadia. Thus, the
hydration of VTi-2 sample results in the change in symmetry of SO4
tetrahedron and in the mode of sulfate ions coordination.
We used ␤-picoline as a molecular probe to study the strong
Brönsted and Lewis acidity of vanadia–titania catalyst. The centers
capable to protonate the weak bases are considered as strong Brön-
sted acid sites. The adsorption of ␤-picoline was carried out at low
◦
temperature (100 C) to avoid its oxidation. It should be noted that
in our work ␤-picoline was also employed as a reactant.
Fig. 6 shows FTIR spectra of VTi-1 and VTi-2 collected in dry air
◦
◦
and after ␤-picoline adsorption at 100 C. Fig. 7 shows difference
samples at 100 C. This resulted in the appearance of the bands of
◦
FTIR spectra of ␤-picoline adsorbed on VTi-1 and VTi-2 at 100 C.
3-methylpyridinium ion in the spectra of both catalysts. The proce-
dure was repeated several times until a constant value of intensity
of the bands due to 3-methylpyridinium ion was achieved. The con-
centration of strong Brönsted acid cites was calculated taking into
account total amount of adsorbed ␤-picoline, catalyst weight and
a specific surface. The concentration of the centers is 35 ␮mol/g
2.7 ␮mol/m ) in VTi-1 catalyst and 190 ␮mol/g (1.4 ␮mol/m ) in
VTi-2. The surface concentration of these centers is less in the VTi-
catalyst probably due to unavailability of micropore volume for
The spectrum 1 in Fig. 7 is the difference between the spectra 2
and 1 presented in Fig. 6a. The spectrum 2 in Fig. 7 is the difference
between the spectra 2 and 1 demonstrated in Fig. 6b. Thus, the
spectra shown in Fig. 7 are due to surface species of ␤-picoline. The
2
2
(
2
adsorption of rather bulk molecule of ␤-picoline. The specific sur-
face of micropores is about 70% of the total surface of the sample.
In the case of VTi-2 sample 3-methylpyridinium ion formation is
accompanied by a low-frequency shift of ꢀ(S O) bands of surface
−
1
sulfate species for 15–20 cm (Fig. 6b) due to the symmetrization
2
of SO -group and the change in strength of Ti–O(S) bond. Thus,
4
at least part of strong Brönsted acid sites is associated with the
presence of sulfate ions on the surface of VTi-2.
As indicated above, self-supported wafers of vanadia–titania
catalysts used in this study are transparent only in the region
−
1
2
200–1200 cm
. The opacity of the samples in the high-
1
1400
frequencies region limits the opportunities for studying the
structure of the catalyst surface sites. To obtain the additional
information a model vanadia–titania catalyst was also examined.
This catalyst was synthesized by the same technique but calcined
at 400 C rather than at 450 C. The sulfur content in it is the same as
in the catalyst VTi-1. This sample has better optical characteristics
1
600
-
1
Wavenumber, cm
◦
◦
Fig. 7. Difference FTIR spectra of 0.2 ␮l ␤-picoline adsorbed on VTi-1 (1) and VTi-2
◦
(
2) at 100 C.

Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
123
1
2
3
1
2
3
4
000
3500
3000
2500
2000
1600
1200
-
1
-1
Wavenumber, cm
Wavenumber, cm
◦
◦
◦
Fig. 8. FTIR spectra collected during passing a gas mixture 4%H2O in air through IR cell with model VTi catalyst at 250 C (1), 150 C (2), and 100 C (3).
and is sufficiently transparent in the 4000–1200 cm 1 spectral
region. It allows measuring a spectrum of hydroxyl groups.
Fig. 8 demonstrates the spectrum of model VTi catalyst collected
−
of ꢀ(S O) vibrations (Fig. 8). Thus, hydroxyl group is not directly
bound to S atoms of sulfate groups, but is coordinated to vana-
dium ions. The first coordination sphere of these vanadium ions
also includes the oxygen atoms of sulfate groups. Hydroxyl group
also could be bound to the neighboring vanadium ions. The minor
shift of ꢀ(S O) bands of sulfate species caused by H-D exchange
could be due to the formation of hydrogen bonds between the pro-
tons of adsorbed water molecules (or the surface-OH groups) and
oxygen of sulfate groups.
◦
after 1-h activation of the sample in dry air flow at 250 C (spectrum
). The bands at 3650, 3440, 2030, 1620, 1365, and 1280 cm are
observed in the spectrum. The band at 3650 cm is due to the OH
stretching vibration of the surface hydroxyl group bound to two
surface V atom (type II) [39]. The broad band at 3700–2700 cm
(ꢀas(H O) and ꢀs(H O)) and the
band at 1620 cm (ı(H O)) characterize the adsorbed water. The
band at 2030 cm is assigned to the first overtone of ꢀ(V O) mode.
The bands at 1360 and 1285 cm characterize surface covalently
bound sulfate species (ꢀ(S O) mode) [29,33,34].
When the amount of adsorbed water is increased there are
significant changes in the IR spectra (Fig. 8, spectra 2 and 3). Increas-
ing the concentration of the adsorbed water (bands at 3440 and
620 cm ) is accompanied by an increase in the concentration
of surface hydroxyl groups (3650 cm ). There is also a change
in the spectrum of sulfate groups. A gradual low-frequency shift
−
1
1
−
1
−
1
−1
with a maximum at 3440 cm
2 2
−1
Adsorption of ␤-picoline on model VTi catalyst containing OH-
groups (Fig. 10, spectrum 1) results in a decrease in intensity of
2
−1
−
1
−1
the band at 3650 cm of surface OH-group (the band has nega-
tive intensity in difference spectrum). Simultaneously, the bands
at 3400–1800 (ꢀ(NH· · ·O), 3125, 3080 and 3047 (ꢀ(CH) of pyri-
dine ring), 2936 and 2885 (ꢀ(CH3)), 1633, 1613, 1552, and 1471
−
1
(ꢀ(CC, CN)), and 1390 (ı(CH3)) cm assigned to the vibrations of
−
1
1
3-methylpyridinium ion [36–38] appear in the spectrum (Fig. 10,
−1
−1
−1
(
∼20–30 cm ) of ꢀ(S O) bands at 1365 and 1280 cm is observed
and their intensity is also decreased. As mentioned above, this is
caused by two processes taking place simultaneously. The sym-
metrization of SO tetrahedron results in decrease of the frequency
4
of ꢀ(S O) vibrations, and the change of coordination mode of sul-
fate ion lead to decrease in the intensity of ꢀ(S O) bands. Both
of these processes are due to the increasing number of hydroxyl
groups and water molecules coordinated to surface vanadium
cations.
1
2
To further characterize the acid sites of the catalyst the proton-
containing groups were exchanged by a deuterium. ␤-Picoline was
used as a molecular probe to study the Brönsted and Lewis acidity
of vanadia–titania catalyst.
Passing a gas mixture 4% D O in air through IR cell with a model
2
◦
VTi catalyst at 200 C for 15 min results in the disappearance of the
bands at 3650 (ꢀ(OH)), 3440 (ꢀ(H O)) and 1620 (ı(H O)) cm and
−
1
2
2
3500
3000
2500
2000
1500
the simultaneous appearance of new bands at 2685 (ꢀ(OD)) and
2
-
1
−
540 (ꢀ(D O)) cm (Fig. 9). At the same time, the replacement of
2
1
Wavenumber, cm
protons by deuterium results in only minor shift of ꢀ(S O) bands
of sulfate species. However, there is a clear effect of the concen-
tration of hydroxyl groups and adsorbed water on the frequencies
◦
Fig. 9. FTIR spectra of model VTi catalyst at 200 C collected after 1-h activation in
dry air (1) and after passing a gas mixture 4%D2O in air through IR cell during 15 min
(2).

1
24
Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
groups are strong Brönsted acid sites of vanadia–titania catalysts.
Fig. 8 demonstrates that increasing the amount of adsorbed water
increases the number of hydroxyl groups. And, therefore, should
also increase the number of strong Brönsted acid sites. It is also
known that water significantly affects the rate of ␤-picoline oxida-
tion on VTi catalyst and selectivity to nicotinic acid [1]. We have
studied the effect of the amount of adsorbed water on the acidity
1
2
◦
of the model VTi catalyst. The research was carried out at 250 C.
This temperature is close to the optimum for the oxidation of ␤-
picoline into nicotinic acid on vanadia–titania catalysts. Due to fast
oxidation under such conditions ␤-picoline cannot be used as a
molecular probe. Therefore, we used a pyridine. Fig. 11a demon-
◦
strates a FTIR spectra of model VTi catalyst collected at 250 C after
◦
1
-h activation in dry air flow at 300 C (spectrum 1) and after H O
2
◦
−1
adsorption at 250 C (spectrum 2). The bands at 3650 cm due to
(OH) mode of surface hydroxyl groups and 1365 cm assigned
3500
3000
2500
2000
1500
−1
ꢀ
-
1
Wavenumber, cm
to ꢀ(S O) vibration of surface sulfate groups are observed in the
spectrum of model VTi catalyst before water adsorption. Adsorp-
tion of water results in ∼50% increase in intensity of ꢀ(OH) band,
low-frequency shift of ꢀ(S O) band and an appearance of the bands
◦
Fig. 10. Difference FTIR spectra of ␤-picoline adsorbed at 100 C on model VTi cat-
alyst containing OH- (1) and OD- (2) groups.
−
1
at 3440 and 1620 cm due to adsorbed water. Pyridine adsorption
results in the appearance of bands at 1635, 1606, 1536, 1486 and
spectrum 1). This surface complex is formed by the interaction of
-picoline molecule with a strong Brönsted acid site of the catalyst.
Adsorption of ␤-picoline on model VTi catalyst containing OD-
groups (Fig. 10, spectrum 2) results in a decrease in intensity of
of surface OD-groups. The bands at 3100,
079 and 3050 (ꢀ(CH) of pyridine ring), 2936 and 2885 (ꢀ(CH )),
−
1
1
1
447 cm
in the spectrum of dehydrated sample. The bands at
␤
−
1
635 and 1536 cm are assigned to vibrations of pyridinium ion
formed by interaction of pyridine with a strong Brönsted acid sites
−
1
−1
of the catalyst, the band at 1447 cm characterize surface com-
plex of pyridine with a Lewis acid sites of the catalyst, whereas the
the band 2685 cm
3
2
1
3
−
1
bands at 1606 and 1486 cm could be assigned to both surface
complexes [40]. Adsorption of pyridine after water pre-adsorption
results in ∼40% increase in the intensity of bands due to pyridinium
ion and ∼70% decrease in the intensity of band due to surface com-
plex of pyridine with Lewis acid sites. Thus, the adsorption of water
increases the amount of Brönsted acid sites and reduces the amount
of Lewis acid sites. At the same time, increasing number of Brönsted
acid sites (40%) correlates well with the increase in concentration
of surface hydroxyl groups (50%).
500–1800 (ꢀ(ND· · ·O), 1628, 1609, 1501, and 1457 (ꢀ(CC, CN)), and
−1
390 (ı(CH )) cm are observed in the spectrum of deuterated
3
analog of 3-methylpyridinium ion. The bands of ꢀ(NH· · ·O) (shift
−
1
−1
−1
∼
800 cm ) and ꢀ(CC, CN) at 1552 cm (shift ∼50 cm ) vibrations
are the most sensitive to H-D exchange in the measured spectral
region. It is clear that ꢀ(CC, CN) vibration is coupled. Deuteration
induced low-frequency shift of ꢀ(CC, CN) band is apparently due to
the contribution of ı(NH) vibration into this coupled mode. The
concentration of surface hydroxyl groups of model VTi catalyst
Thus, surface hydroxyl groups bound to vanadium ions are the
strong Brönsted acid sites of studied vanadia–titania catalysts. Sul-
phate ions are coordinated to the same vanadium ions as a hydroxyl
(
OH or OD) decreases after the formation of 3-methylpyridinium
ion (or it deuterated analog). Other ␤-picoline surface complexes
under these conditions are not formed. Thus, the surface OH (or OD)
a
b
2
1
2
1
4000 3000 1600 1400 1200 1700
1600
1500
1400
-
1
-1
Wavenumber, cm
Wavenumber, cm
◦
◦
Fig. 11. (a) FTIR spectrum of model VTi catalyst collected at 250 C after 1-h activation in dry air flow at 300 C (1) and FTIR spectrum of the same catalyst after H2O adsorption
◦
◦
◦
at 250 C (2). (b) Difference FTIR spectra of pyridine adsorbed at 250 C on model VTi catalyst after 1-h activation in dry air flow at 300 C (1) and after H2O adsorption at
◦
2
50 C (2).

Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
125
a
b
c
6
0'
2
0'
2
3
00'
0'
3
0'
5
'
2
0'
5
'
'
1
0'
1
1
'
1
'
1800 1700 1600 1500 1400 1800 1700 1600 1500 1400 1800 1700 1600 1500 1400
Wavenumber, cm-1
Wavenumber, cm-1
Wavenumber, cm-1
◦
◦
◦
Fig. 12. The difference FTIR spectra collected after introduction of ␤-picoline at 200 C (a) and 250 C (b) and pyridine-3-carbaldehyde at 160 C (c) in the cell with VTi-1
catalyst.
group or to the neighboring vanadium ions. As a result, sulfate ions
can modify the Brönsted acidity of vanadia–titania catalysts. Fur-
thermore, for the same reasons sulfate ions can also modify Lewis
acidity of the catalysts. The adsorption of water increases the num-
ber of Brönsted acid sites and reduces the amount of Lewis acid sites
due to the formation of additional hydroxyl groups. The structure
of sulfate complexes (the mode of coordination and the symmetry
of SO4 tetrahedron) is also determined by the number of surface
hydroxyl groups and adsorbed water. Thus, there is a mutual influ-
ence of sulfate groups and protic acid centers.
The transformation of 3-methylpyridinium ion proceeds signifi-
◦
◦
cantly faster at 250 C (Fig. 12b), than at 200 C (Fig. 12a). The bands
due to vibration of surface pyridine-3-carbaldehyde and nicotinate
are observed in the spectrum collected only 1 min after introduc-
tion of ␤-picoline. After 5 min, the bands due to ꢀ(C O) mode of
−
1
−1
molecularly adsorbed (1740 cm ) and gaseous (1770 cm ) nico-
tinic acid [6] are appeared in the spectrum.
The bands at 1721 (ꢀ(C O)), 1631, 1606, 1578, 1542, 1468
−
1
(ꢀ(CC, CN)), and 1411 (ꢀs(COO)) cm are observed in the spectrum
◦
registered at 160 C in 1 min after introduction of pyridine-3-
carbaldehyde (Fig. 12c). The value of the frequency due to ꢀ(C O)
vibration indicates the formation of hydrogen bonds between
pyridine-3-carbaldehyde molecule and Brönsted acid site of VTi-
1 catalyst through oxygen of a carbonyl group [41]. The bands at
3
.1.3. Transformation of ˇ-picoline and pyridine-3-carbaldehyde
on vanadia–titania catalysts
The study of the interaction of ␤-picoline with VTi-1 and VTi-
catalysts was carried out at two temperatures (200 and 250 C).
◦
−1
2
1631, 1606, 1542, and 1468 cm are assigned to substituted pyri-
−
1
The interaction of pyridine-3-carbaldehyde with the same catalysts
was studied at 160 C. Selecting a relatively low temperature for
the investigation of pyridine-3-carbaldehyde transformation was
caused by the high rate of it transformation on VTi catalysts at tem-
peratures above 200 C. It makes impossible studying the dynamics
dinium ions [36–38]. The presence of ꢀ(CC, CN) band at 1578 cm
◦
in the spectrum indicates the interaction of the molecule pyridine-
3-carbaldehyde with a Lewis acid centers via an unshared electron
pair of pyridine ring nitrogen [42–45]. Thus, we can assume that
two surface complexes of pyridine-3-carbaldehyde are formed
◦
−
1
of changes in the spectra.
(Table 3, SC2 and SC5). In addition, the band at 1411 cm due to
ꢀs(COO) mode of nicotinate surface complex (Table 3, SC3) is also
observed in the spectrum. With time the intensity of ꢀ(C O) band at
Fig. 12 demonstrates the difference FTIR spectra collected after
◦
◦
introduction of ␤-picoline at 200 C (a) and 250 C (b) and pyridine-
3
◦
−1
-carbaldehyde at 160 C (c) in the cell with VTi-1 catalyst.
1721 cm due to surface complexes of pyridine-3-carbaldehyde is
The bands at 1632, 1609, 1551, 1474 (ꢀ(CC, CN)) and 1390
reduced and the intensity of bands due to nicotinate surface com-
plex at 1569 and 1411 cm is increased. The band at 1740 cm
due to ꢀ(C O) mode of molecular adsorbed nicotinic acid (Table 3,
SC4) is also appeared in the spectrum.
−1
−1
−1
(
ı(CH )) cm
due to 3-methylpyridinium ion [36–38] (Table 2,
3
◦
SC1) are observed in the spectrum registered at 200 C in 1 min
after introduction of ␤-picoline (Fig. 12a). As indicated in Section
3
.1.2, this surface complex is formed by interaction of ␤-picoline
Fig. 13 demonstrates difference FTIR spectra collected after the
◦
◦
with strong Brönsted acid centers of vanadium–titanium oxide
catalysts. The bands at 1720 cm
pyridine-3-carbaldehyde surface complex [41] (Table 2, SC2) and
at 1412 cm assigned to ꢀs(COO) mode of nicotinate surface com-
plex [36] (Table 2, SC3) are appeared in the spectrum 10 min after
the introduction of ␤-picoline. With time the intensity of bands
at 1412 cm increases. The second band of niconitate complex at
1
1
introduction of ␤-picoline at 200 C (a) and 250 C (b) and pyridine-
−1
◦
due to ꢀ(C O) vibration of
3-carbaldehyde at 160 C (c) into the cell with VTi-2 catalyst. The
−
1
bands at 1633, 1612, 1553, and 1472 cm assigned to ꢀ(CC, CN)
−
1
vibrations of 3-methylpyridinium ion (Table 2, SC1) are observed
◦
in the spectrum registered at 200 C within 1 min after introduction
of ␤-picoline into the cell with VTi-2 catalyst (Fig. 13a). In 10 min
after ␤-picoline introduction, the spectrum reveals a new band at
−1
−
1
−1
564 cm (ꢀas(COO) is manifested in the spectra. And the band at
1418 cm due to ꢀs(COO) vibration of surface nicotinate complex
−
1
−1
740 cm due to ꢀ(C O) vibration of molecular adsorbed nicotinic
(Table 2, SC3). The ꢀ(C O) bands at 1740 and 1670 cm of two
acid (Table 2, SC4) [36] is also appeared in the spectra.
forms of molecular adsorbed nicotinic acid are also observed in

1
26
Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
c
a
b
6
0
3
0'
9
0'
2
0'
1
0'
3
0'
5
'
5
'
1
0'
1
'
1
'
1'
1
800
1700
1600
1500
1400
1800 1700 1600 1500 1400 1800
1700
1600
1500
1400
-1
-1
-1
Wavenumber, cm
Wavenumber, cm
Wavenumber, cm
◦
◦
◦
Fig. 13. Difference FTIR spectra collected after the introduction of ␤-picoline at 200 C (a) and 250 C (b) and pyridine-3-carbaldehyde at 160 C (c) into the cell with VTi-2
catalyst.
the spectrum (Table 2, SC4 and SC6, respectively). With time the
frequencies of bands assigned to ꢀas(COO) and ꢀs(COO) modes of
−
1
intensity of the bands at 1418, 1740 and 1670 cm is increased.
Two forms of molecular adsorbed nicotinic acid with the similar
frequencies in the spectra we have detected after nicotinic acid
adsorption on anatase TiO2 sample containing 6.9 wt.% of sulfate
surface nicotinates are also changed.
−
1
The absence of the band at 1770 cm of gaseous nicotinic acid
in the spectrum 2 could be associated with strong its readsorption
on strong Brönsted acid sites of VTi-2 catalyst. In the case of VTi-2
−
1
[
6].
The transformation of 3-methylpyridinium ion proceeds sig-
sample the intensity of ꢀ(C O) band 1740 cm due to molecular
adsorbed nicotinic acid complex SC4 is increased and molecu-
◦
◦
−1
nificantly faster at 250 C (Fig. 13b), than at 200 C (Fig. 13a). In
addition, the intensity of ꢀ(C O) band at 1670 cm
lar adsorbed nicotinic acid complex SC6 (ꢀ(C O) = 1670 cm ) is
−
1
of molec-
formed. SC6 formation is associated with increase in strength of
Brönsted acid sites in the sample VTi-2 due to the presence of
sulfate ions on the surface [6] (see also Section 3.1.1). The band
ular adsorbed nicotinic acid SC6 is significantly increased. The
−1
band at 1719 cm
assigned to ꢀ(C O) vibration of pyridine-3-
carbaldehyde surface complex SC2 is also observed in the spectrum
(Table 2).
The bands at 1720 (ꢀ(C O)), 1632, 1606, 1580, 1547, 1469
−
1
(
(ꢀ(CC, CN)), 1416 (ꢀs(COO)) cm
are observed in the FTIR
◦
spectrum registered at 160 C within 1 min after introduction of
pyridine-3-carbaldehyde (Fig. 13c). The bands at 1632, 1606, 1547
−
1
and 1469 cm
characterized substituted pyridinium ions. The
−1
presence of ꢀ(CC, CN) band at 1580 cm in the spectrum indicates
the interaction of pyridine-3-carbaldehyde with a Lewis acid cen-
ters via an unshared electron pair of nitrogen. Therefore, we can
assume that two surface complexes of pyridine-3-carbaldehyde
are formed (Table 3, SC2 and SC5). There is also ꢀs(COO) band
at 1416 cm due to nicotinate surface complex in this spectrum
Table 3, SC3). With time the intensity of bands due to pyridine-3-
carbaldehyde surface complexes is reduced (primarily band ꢀ(C O)
at 1720 cm ), whereas the intensity of bands due to nicotinate
complex (1565 and 1416 cm ) is increased. The spectra also
contain ꢀ(C O) bands at 1740 and 1670 cm
adsorbed nicotinic acid (Table 3, SC4 and SC6).
−
1
(
2
−
1
−1
−
1
due to molecular
Thus, the same series of surface complexes of similar structure
Tables 2 and 3) and the same sequence of it transformations are
(
1
observed for both catalysts. However, there are also noticeable dif-
ferences despite the similarity of the surface structures. Fig. 14
demonstrates the FTIR spectra collected 60 min after introduction
of ␤-picoline in the cell-reactor with VTi-1 (spectrum 1) and VTi-
◦
−1
2
(spectrum 2) catalysts at 250 C. The band at 1670 cm due to
1800
1700
1600
1500
1400
molecular adsorbed nicotinic acid is appeared in the spectrum of
-1
−1
Wavenumber, cm
VTi-2 catalyst. The band at 1770 cm due to gaseous nicotinic acid
is absent in this spectrum. The ratio of intensity of ꢀ(CC, CN) bands
at 1633 and ∼1610 cm is increased in the spectrum of VTi-2. The
Fig. 14. FTIR spectra collected 60 min after introduction of ␤-picoline in the cell-
reactor with VTi-1 (1) and VTi-2 (2) catalysts at 250 C.
−1
◦

Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
127
Table 2
Table 3
Surface complexes (SC) formed during ␤-picoline adsorption on VTi-1 and VTi-2.
Surface complexes (SC) formed during pyridine-3-carbaldehyde adsorption on VTi-1
and VTi-2.
SC structure
Wavenumbers (cm 1) and band assignment
−
SC structure
Wavenumbers (cm−1) and band assignment
VTi-1
VTi-2
VTi-1
VTi-2
CH3
H
N
H
C
O
1632 – ꢀ(CC, CN), 8a
1609 – ꢀ(CC, CN), 8b
1551 – ꢀ(CC, CN), 19a
1470 – ꢀ(CC, CN), 19b
1390 – ı(CH3)
1633 – ꢀ(CC, CN), 8a
1612 – ꢀ(CC, CN), 8b
1553 – ꢀ(CC, CN), 19a
1472 – ꢀ(CC, CN), 19b
N
H
H
1721 – ꢀ(C O)
1720 – ꢀ(C O)
O
O
O
1631 – ꢀ(CC, CN), 8a
1606 – ꢀ(CC, CN), 8b
1542 – ꢀ(CC, CN), 19a
1632 – ꢀ(CC, CN), 8a
1606 – ꢀ(CC, CN), 8b
1547 – ꢀ(CC, CN), 19a
1469 – ꢀ(CC, CN), 19b
SC1
SC2
1468 – ꢀ(CC, CN), 19b
H
H
C
C
O
O
N
.
.
H
1
721 – ꢀ(C O)
1720 – ꢀ(C O)
1580 – ꢀ(CC, CN), 8b
N
H
O
H
1720 – ꢀ(C O)
1719 – ꢀ(C O)
1578 – ꢀ(CC, CN), 8b
V
O
O
O
SC5
SC2
N
H
C
O
O
O
O
1633 – ꢀ(CC, CN), 8a
1609 – ꢀ(CC, CN), 8b
1569 – ꢀas(COO)
1636 – ꢀ(CC, CN), 8a
1614 – ꢀ(CC, CN), 8b
1565 – ꢀas(COO)
1466 – ꢀ(CC, CN), 19b
1416 – ꢀs(COO)
N
H
C
V
O
O
O
SC3
1466 – ꢀ(CC, CN), 19b
411 – ꢀs(COO)
O
V
1
SC3
1
1
1
1
1
633 – ꢀ(CC, CN), 8a
609 – ꢀ(CC, CN), 8b
564 – ꢀas(COO)
465 – ꢀ(CC, CN), 19b
412 – ꢀs(COO)
1633 – ꢀ(CC, CN), 8a
1614 – ꢀ(CC, CN), 8b
1558 – ꢀas(COO)
1466 – ꢀ(CC, CN), 19b
1418 – ꢀs(COO)
OH
C
O
N
H
O
1740 – ꢀ(C O)
1740 – ꢀ(C O)
OH
C
O
SC4
N
H
1740 – ꢀ(C O)
1740 – ꢀ(C O)
OH
C
O
N
H
O
SC4
H
–
1670 – ꢀ(C O)
OH
C
O
O
O
N
H
SC6
H
–
1670 – ꢀ(C O)
O
O
catalyst becomes possible as in the case of highly sulfated anatase
6].
The variation of the ratio of the intensities of ꢀ(CC, CN) bands
SC6
[
−
1
at 1633 and at ∼1610 cm
in the spectra of surface nicotinate
−1
ꢀ
(C O) = 1740 cm
picoline adsorbed on both samples.
Since the frequency of ꢀ(C O) band in the spectrum of SC4
1740 cm ) is close to those in the spectrum of gaseous nico-
tinic acid (1770 cm ), carboxyl group of SC4 weakly interacts
with acidic sites of the catalyst or do not interacts at all. Thus,
this surface complex interacts mainly with the surface via the
nitrogen atom of pyridine ring. Proton transfer from a strong
Brönsted acid center of the catalyst to the nitrogen atom of nico-
tinic acid results in the formation of substituted pyridinium ion
due to SC4 is observed in the spectra of ␤-
could be due to changing the bonding strength of these complexes
with the active centers of the catalyst. The spectrum of nicotinate
obtained by transformation of ␤-picoline on VTi-2 catalyst (Fig. 13,
spectrum 2) is similar to the spectrum of nicotinate SC7 formed
at pyridine-3-carbaldehyde oxidation on highly sulfated anatase
[6]. We have shown in [6] that strongly bound nicotinate SC7 is a
dead-end surface complex and it does not turn into nicotinic acid.
It is known that the difference between the frequencies of
ꢀas(COO) and ꢀs(COO) vibrations in the IR spectra of salts of
carboxylic acids is a structure-sensitive [32,46]. The difference
between these frequencies is determined by both the mode of
carboxylate ion coordination (mono- or bidentate) and by the
strength of the metal-carboxylate bond. Table 4 presents the
frequencies of ꢀas(COO) and ꢀs(COO), as well as the value of
ꢁ = ꢀas(COO) − ꢀs(COO) in the spectra of surface nicotinate formed
during the transformation of ␤-picoline on VTi-1 and VTi-2 cata-
lysts.
−
1
(
−1
(
complex structure is shown in Tables 2 and 3, SC4). Molecularly
adsorbed acid with the frequency of the band ꢀ(C O) = 1670 cm
−
1
interacts with the Brönsted acid sites of the catalyst both via
a nitrogen atom of pyridine ring and via an oxygen atom of
the carboxyl group (the structure of the complex is shown in
Tables 2 and 3, SC6). The formation of SC6 indicates not only
the increase of the strength of the proton centers but also the
increase of their surface density. Thus, two-centered interaction
between of molecule nicotinic acid and Brönsted centers of the
The frequencies of carboxyl group stretching vibration and the
value of ꢁ indicates bidentate coordination mode of nicotinate

1
28
Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
Table 4
1
00
The frequencies of ꢀas(COO) and ꢀs(COO) modes, and the value of
ꢁ
= ꢀas(COO) − ꢀs(COO) in the spectra of surface nicotinate formed during
о
3
␤
-picoline transformation on VTi-1 and VTi-2 catalysts at 250 C.
80
Catalyst
ꢀas(COO), cm 1
−
ꢀs(COO), cm
−1
−1
ꢁ, cm
VTi-1
VTi-2
1564
1558
1412
1418
152
140
60
1
4
0
carboxyl group on both samples. It is known [46] that in the case
of bidentate coordination mode, increasing the strength of the
metal-carboxylate bond reduces the value of ꢁ. Table 1 shows
2
20
−
1
that the value of ꢁ for VTi-2 sample is 12 cm
less than those
20
4
for VTi-1 sample. Therefore, it is expected that nicotinate stronger
bound to VTi-2 surface than to VTi-1 one. It should be noted that
the spectra of nicotinic acid adsorbed on VTi-1 and VTi-2 samples
15
◦
at 250 C (Fig. 15) virtually coincide with the spectra of products
10
of ␤-picoline and pyridine-3-carbaldehyde transformation on the
7
respective samples (Figs. 11 and 12).
5
5
0
6
3.1.4. Catalytic properties
The performance of VTi-1 and VTi-2 catalysts in ␤-picoline oxi-
dation was studied by the thermally programmed reaction method
in the temperature range of 240–315 C (Figs. 16 and 17). The con-
version range in this temperature interval was found to be 40–99%
for VTi-1 and 40–95% for VTi-2 catalysts. Nicotinic acid, pyridine-
240
260
280
300
o
◦
Temperature, C
Fig. 16. Temperature dependence of ␤-picoline conversion (1), nicotinic acid
yield (2) and selectivity to nicotinic acid (3), pyridine-3-carbaldehyde (4), 3-
cyanopyridine (5), pyridine (6) and COx (7) over VTi-1 catalyst.
3
-carbaldehyde, 3-cyanopyridine, pyridine and carbon oxides (CO
and CO ) were found to be the products of ␤-picoline oxidation on
2
both catalysts. Although nicotinic acid is the main product on both
catalysts, their selectivities are different. The maximum nicotinic
acid yield is 85% for VTi-1 and less than 50% for VTi-2.
pyridine-3-carbaldehyde. It almost completely transforms into
nicotinic acid on VTi-1. The contribution of its oxidation to
by-products – COx, pyridine, 3-cyanopyridine is less than 10%
Figs. 18 and 19 demonstrate the dependence of selectivity
to reaction products on ␤-picoline conversion. Nicotinic acid is
formed over vanadia–titania catalysts via two pathways. The par-
allel pathway involves direct ␤-picoline oxidation. The consecutive
mechanism includes the formation of pyridine-3-carbaldehyde
as an intermediate [47,48]. The dependence of the selectivities
on the ␤-picoline conversion (Figs. 18 and 19) demonstrates the
change in contributions of these pathways depending on the
sulfate content. The consecutive nicotinic acid formation pathway
predominates on the VTi-1 catalyst: the aldehyde selectivity
decreases with increasing ␤-picoline conversion and the nico-
tinic acid selectivity simultaneously grows. The contribution of
the parallel pathway becomes substantial on the VTi-2 catalyst
ꢀ
ꢀ
(
Sx −
S0). On the VTi-2 catalyst the aldehyde is converted only
to the side products. This conclusion is proved by the growth of the
selectivity to COx, pyridine and 3-cyanopyridine with the decrease
of the aldehyde selectivity, whereas the nicotinic acid selectivity
is not changed.
100
1
80
60
40
3
2−
with high SO4
content. It means that adsorption of ␤-picoline
(
weak base) is stronger on the catalysts containing more acid
2−
sites. The SO4
content has even more effect the destiny of
2
2
0
0
0
4
3
2
1
7
0
0
0
0
1
5
4
6
2
2
40
260
280
300
o
Temperature, C
1800
1600
1400
-
1
Wavenumber, cm
Fig. 17. Temperature dependence of ␤-picoline conversion (1), nicotinic acid
yield (2) and selectivity to nicotinic acid (3), pyridine-3-carbaldehyde (4), 3-
cyanopyridine (5), pyridine (6) and COx (7) over VTi-2 catalyst.
Fig. 15. FTIR spectra of nicotinic acid adsorbed on VTi-1 (1) and VTi-2 (2).

Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
129
1
00
nitrogenatom [43]. Pyridine-3-carbaldehydeis a strongerbase than
-picoline due to additional unshared electron pairs on the oxygen
atom of the carbonyl group. Both unshared electron pairs on the
nitrogen and the oxygen atoms form basic sites that may interact
with a pair of acid sites.
␤
9
0
1
8
0
The introduction of electronegative elements (S6+ in our case)
onto the surface should produce an inductive effect, making the
nearby metal cations less basic or more acidic. The introduction of
electropositive species such as alkali metal atoms should produce
a more basic or less acidic sites [43]. For instance, the introduction
of very electropositive P or Mo atoms to VOx/SiO2 decreases the
electron density on the vanadium atom making it more acidic. On
the contrary, the introduction of potassium results in an increase
of the electron density on the vanadium atom leading to stronger
basicity [10]. Correlations of the acid-base and catalytic properties
of vanadia–titania catalysts promoted with acids or bases have been
observed for selective oxidation of o-xylene, toluene, propylene,
benzene [8], methanol [49] and methylpyrazine [50,51].
The sulfate content in vanadia–titania catalysts affects the acid-
ity of the active sites, the strength of the surface complexes and
directions of their transformations into the reaction products.
Growing acidity of the adsorption sites leads to strengthening
the surface complexes formed by ␤-picoline and pyridine-3-
carbaldehyde. This effect is stronger for the latter reactant.
The ␤-picoline adsorption on the very acidic sample VTi-2
results in the formation of strongly bound surface complexes. These
complexes are not converted into the pyridine-3-carbaldehyde.
Instead, they are transformed following parallel pathways to
deeper oxidation products (nicotinic acid and COx) and to the
products of destruction (pyridine and 3-cyanopyridine). Strongly
2
0
2
10
0
3
15
1
0
5
0
5
4
40
60
80
100
Conversion, %
Fig. 18. Dependence of selectivity to nicotinic acid (1), COx (2), pyridine-3-
carbaldehyde, (3), 3-cyanopyridine, (4) and pyridine (5) upon ␤-picoline conversion
over VTi-1 catalyst.
The observed tendencies are the result of the effect of sulfates
on the catalyst acidity. The conception of the acid-base interac-
tions between gaseous reactants and surface acid sites of the solid
catalysts was developed by Stair [43]. ␤-Picoline and pyridine-3-
carbaldehyde are organic bases due to the presence of unshared
electron pair on the nitrogen atom. Adsorption of these reagents
required the presence of acid sites on the surface. The bases inter-
act with such sites by donation of the electron density from the
bound complexes of pyridine-3-carbaldehyde are converted on this
−
catalyst only into the side products. Nicotinates NC5H COO are
4
direct precursors of nicotinic acid and also have basic properties.
An increase of the catalyst acidity due to the higher SO42 con-
tent strengthens the nicotinates and decreases the selectivity to
nicotinic acid. The transformations of the surface complexes dur-
−
ing the temperature increase on the samples with different SO42
−
concentrations confirm described mechanism.
The SO42 content also affects further nicotinic oxidation.
Though, this effect is less pronounced than the ␤-picoline and
pyridine-3-carbaldehyde transformation to side products. The
amount of nicotinic subjected to deeper oxidation is higher on the
VTi-2 catalyst. The presence of a nitrogen atom with a unshared
electron pair in the pyridine ring determines the stronger reaction
of nicotinic acid to the higher amount of stronger acid sites in
VTi-2. It results in re-adsorption of the gaseous acid in a stronger
form (Fig. 15).
−
8
6
4
2
0
0
0
0
1
2
Takehira’s assumption [22] on increasing selectivity to nicotinic
acid due to its better desorption caused by increasing acidity is
not confirmed in the case of highly sulfated vanadium-titanium
0
5
sample. The negative effect of SO42 on the selectivity is likely
due to the greater strength of acid sites in the case of VTi cata-
lyst. Unfortunately, we cannot quantitatively compare the effect
of acidic centers in VCrP and VTi-SO4 catalysts since the different
methods for the determination of acidity were applied.
−
2
2
1
1
3
0
5
0
5
0
4
Thus, the modification of vanadia–titania catalysts with sulfates
increases the catalyst acidity expressed in the strength and concen-
tration of Brönsted acid sites, strengthens the surface complexes of
5
␤-picoline, pyridine-3-carbaldehyde and nicotinic acid, and, ulti-
mately, worsens the catalyst characteristics.
40
60
80
Conversion %
4. Conclusion
Fig. 19. Dependence of selectivity to nicotinic acid (1), COx (2), pyridine-3-
carbaldehyde, (3), 3-cyanopyridine, (4) and pyridine (5) upon ␤-picoline conversion
over VTi-2 catalyst.
Strong Brönsted and Lewis acid sites are shown to be present on
the surface of vanadia–titania catalysts. The strong Brönsted acid

1
30
Y.A. Chesalov et al. / Journal of Molecular Catalysis A: Chemical 380 (2013) 118–130
sites on these samples are hydroxyl groups coordinated to vana-
dium atoms rather than sulfur atoms of sulfate ions. The sulfate
ions are coordinated to the same vanadium ions as the hydroxyl
groups or to their neighbors. Their presence increases the strength
of the Brönsted acid sites. The concentration of strong Brönsted
acid sites substantially depends on the water concentration. Water
is adsorbed on coordinatively unsaturated vanadium ions (Lewis
acid sites) to form Brönsted acid sites.
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