In situ DRIFTS study of picoline oxidation over CrV0.95P0.05O4 catalyst

Article abstract of DOI:10.1039/b302571c

The catalytic behaviour of CrV_0.95P_0.05O₄ has been investigated in the selective oxidations of 2-, 3- and 4-picolines by in situ DRIFTS, and the model of picoline adsorption and the oxidation mechanism are proposed. Both Lewis and Bronsted acid sites were detected on the surface of CrV_0.95P_0.05O₄, and the number of the latter increased on the addition of steam in the reaction mixture, resulting in enhanced activity for selective oxidations. The enhanced activity due to water addition is interpreted by the fact that Bronsted acid sites are produced by the hydrolysis of V-O-Cr and activate picoline molecules by withdrawing the electrons of the pyridine ring, and at the same time, enable to accelerate the desorption of the acid products from the catalyst surface. Every 2-, 3- and 4-picoline was adsorbed on the catalyst surface via the N atom donating the electrons to the Bronsted acid sites, and the substituted methyl group was oxidized via hydrogen abstraction by surface oxide ion to form the radical intermediate, followed by oxygen insertion to produce the corresponding aldehyde and then acid. Even in the absence of gaseous oxygen, the oxygenated products were formed and observed over the catalyst surface by in situ DRIFTS. Thus, a Mars and van Krevelen mechanism was suggested for 2-, 3- and 4-picolines oxidations based on the spectral analysis. Both 2- and 4-picolines were more quickly oxidized than 3-picoline due to the inductive hyper-conjugative effect of nitrogen, resulting in an easy leaving of proton from the methyl group. 4-Picoline produced almost quantitatively isonocotinic acid, while 2-picoline afforded 2-picoline aldehyde as the main product due to the unstability of the acid product, i.e., the decarboxylation of picolinic acid took place to form pyridine.

Full text of DOI:10.1039/b302571c

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In situ DRIFTS study of picoline oxidation over CrV0.95P0.05O4
catalyst
T. Shishido, Z. Song, T. Matsushita, K. Takaki and K. Takehira*
Department of Chemsitry and Chemical Engineering, Graduate School of Engineering,
Hiroshima University, 1-4-1 Kagamiyama, Higashi-hiroshima 739-8527, Japan.
E-mail: takehira@hiroshima-u.ac.jp; Fax: +81 824 24 7744; Tel: +81 824 24 7744
Received 6th March 2003, Accepted 2nd May 2003
First published as an Advance Article on the web 22nd May 2003
The catalytic behaviour of CrV0.95
picolines by in situ DRIFTS, and the model of picoline adsorption and the oxidation mechanism are proposed.
Both Lewis and Brønsted acid sites were detected on the surface of CrV0.95 , and the number of the latter
0.05 4
P O has been investigated in the selective oxidations of 2-, 3- and 4-
P
0.05 4
O
increased on the addition of steam in the reaction mixture, resulting in enhanced activity for selective
oxidations. The enhanced activity due to water addition is interpreted by the fact that Brønsted acid sites are
produced by the hydrolysis of V–O–Cr and activate picoline molecules by withdrawing the electrons of the
pyridine ring, and at the same time, enable to accelerate the desorption of the acid products from the
catalyst surface. Every 2-, 3- and 4-picoline was adsorbed on the catalyst surface via the N atom donating the
electrons to the Brønsted acid sites, and the substituted methyl group was oxidized via hydrogen abstraction by
surface oxide ion to form the radical intermediate, followed by oxygen insertion to produce the corresponding
aldehyde and then acid. Even in the absence of gaseous oxygen, the oxygenated products were formed and
observed over the catalyst surface by in situ DRIFTS. Thus, a Mars and van Krevelen mechanism was
suggested for 2-, 3- and 4-picolines oxidations based on the spectral analysis. Both 2- and 4-picolines were more
quickly oxidized than 3-picoline due to the inductive hyper-conjugative effect of nitrogen, resulting in an easy
leaving of proton from the methyl group. 4-Picoline produced almost quantitatively isonocotinic acid, while
2-picoline afforded 2-picoline aldehyde as the main product due to the unstability of the acid product, i.e., the
decarboxylation of picolinic acid took place to form pyridine.
activation energy calculated from the 3-picoline consumption
was also the lowest over the most active catalyst. All the V spe-
Introduction
In our recent studies on selective oxidation of 3-picoline, a
4 4
cies are isolated as VO tetrahedra in the CrVO –I structure
series of chromium vanadates based on CrVO
a-MnMoO
catalyst family, including Cr1ꢀxAl
It was clearly shown that the CrVO
4
–I with
and considered to work as the active sites via reduction-oxida-
tion assisted by both chromium and phosphorus atoms. It is
concluded that 3-picloine is selectively oxidized on the V sites
by reduction–oxidation, further assisted by the Brønsted-
4
structure have been found to be a new promising
1
,2
3,4
x
VO
4
x 4
and CrV1ꢀxP O .
4
–I based structure was
important for the catalytic activity as well as the selectivity
to nicotinic acid. Moreover, both activity and selectivity were
significantly enhanced when CrVO –I structure was modified
either by replacing the Cr with Al or the V with P element,
and the latter replacement was more effective than the former.
The addition of water vapor into the reaction mixture substan-
tially promoted the selective oxidation of 3-picoline by erecting
Brønsted acid sites on the catalyst surface. It was found that
0.05 4
acid sites over the crystallized CrV0.95P O catalyst, which
showed the highest activity, i.e., 3-picoline conversion of
92.6% and nicotinic acid selectivity of 84.7% were obtained
4
ꢁ
3,4
at 360 C.
5
,6
Up to now, only a few papers have discussed the reaction
mechanism of 3-picoline oxidation, where no detailed datum
was shown concerning the reaction intermediates. Oxidation
mechanism of toluene has been frequently studied on vana-
7
–9
CrVO
amount of P resulted in a highly enhancement in the catalytic
4
–I was originally active and the addition of a small
dium based oxide catalyst.
Although both picoline and
toluene have a common six-membered cyclic structure, the nat-
ure of nitrogen atom in picoline is quite different from that of
the carbon atom in the toluene ring. Considering the electronic
configuration, the presence of nitrogen atom makes picoline a
much stronger base than toluene, and significantly affects the
adsorption behaviors and the reactivity. Furthermore, the
electron withdrawing ability of nitrogen in the pyridine ring
remarkably affects the reactivity of methyl group, i.e. the elec-
trons of methyl groups at the 2- and 4-positions are drawn to
pyridine ring due to the inductive and resonance effect of nitro-
gen, resulting in an easy leaving of proton from the methyl
group. On the other hand, the methyl hydrogen of 3-picoline
3
,4
3
activity. NH -TPD showed that an increase in the acid site
by replacing V with a small amount of P in CrVO –I. Favor-
4
able effect of water addition suggests that Brønsted acid assists
the selective oxidation, and this was confirmed by infrared
study of pyridine adsorption on the catalysts. The active cata-
lysts, CrV1 P O (0 < x < 0.1), alone revealed a weak reduc-
ꢀx
x
4
ꢁ
tion peak of V around 350 C in the TPR, and the peak
temperatures was the lowest over the most active catalyst,
4
i.e., CrV0.95
P
0.05
O
4
.
The reduction-oxidation property was
reversibly observed with the peak when the catalyst was
treated by H and O atmosphere alternately. Moreover, the
2
2
2
710
Phys. Chem. Chem. Phys., 2003, 5, 2710–2718
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DOI: 10.1039/b302571c

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is more stable than those of 2- and 4-picolines due to the
absence of the resonance effect.
Vapor-phase oxidation of 2-, 3- and 4-picolines over
CrV0.95P0.05O catalyst
1
0
4
In situ spectroscopic techniques have been widely used in
understanding the mechanistic details of chemical reaction at
the molecular level. In particular, IR spectroscopy is a well-
established method in the characterization of heterogeneous
reaction systems under working conditions. Diffuse reflectance
The catalytic tests were conducted in a fixed-bed reactor at
atmospheric pressure and at temperatures from 250 to
4
ꢁ
3,4
00 C, as reported previously. A Pyrex glass tube reactor
with 20 cm length by 8 mm id was used in an electrically heated
oven. The 0.2 g of the catalyst powder was made to pellet and
crushed into particles with the sizes of 26–42 mesh; these were
filled at the bottom of the tube diluted with quartz sand. The
catalyst was activated in an oxygen : nitrogen mixed gas flow
Fourier-transform infrared spectroscopy (DRIFTS) is
a
powerful tool for the investigation of adsorbed species and
active sites, because of its high sensitivity to surface species.
The in situ DRIFTS technique has been frequently employed
in investigating the active sites, the adsorbed species and the
intermediates on the catalyst surface under reaction condi-
tions. In the present study, we report the vapor phase oxida-
tion of 2-, 3- and 4-picolines over CrV0.95P0.05O4 by in situ
DRIFTS. We propose adsorption models and an oxidation
mechanism.
ꢀ1
ꢁ
(
to the reaction temperature under the O : N mixed gas flow.
18 : 33 ml min ) at 540 C for 1 h and then settled down
2
2
The liquid mixture of picoline and water was passed through a
ꢁ
vaporization zone at 200 C, mixed with oxygen and nitrogen
as the carrier gas and finally fed in to the reactor. The line from
the vaporizer to the reactor was heated to a temperature above
ꢁ
2
from 1 : 0 to 1 : 108 (molar ratio) under a constant gas hourly
50 C. The feed composition of picoline : H
2
O was varied
ꢀ1
ꢀ1
space velocity around 38 000 ml h (g-catalyst) . Water and
organic compounds were condensed after the reactor, and all
samples were analyzed by a FID gas chromatograph. The inor-
ganic components (CO, CO , O and N ) were analyzed by an
Experimental
2
2
2
Preparation of the catalysts
on-line TCD gas chromatograph.
The CrV0.95P0.05O4 catalyst was prepared following the
3
,4
method proposed by Touboul et al., as described previously.
Aqueous solutions of NH VO , Cr(NO ) ꢂ9H O and NH H PO
4
3
3 3
2
4
2
4
Results and discussion
were mixed, and the pH value of the solution was lowered to
0
3
.1–1.0 to form transparent and dark green solution by adding
N nitric acid aqueous solution. Then, the pH value was
The catalytic oxidations of 2-, 3-, and 4-picolines
ꢁ
The results of 2-, 3-, and 4-picoline oxidations over CrV0.95
raised and adjusted to 4.0 at 50 C by adding 3 N ammonia
aqueous solution, and stirred for 4 h, resulting in the forma-
tion of yellowish green precipitate. The precipitate was washed
P
0
0.05
O
4
catalyst in the absence of water (picoline : O
.032 : 0.73 : 4.56 mmol min ) are shown in Tables 1, 2 and
2
: N
2
¼
ꢀ1
ꢁ
3. In all cases, the conversions of 2-, 3-, and 4-picolines
increased with increasing the reaction temperature. Pyridine-
with de-ionized water, dried at 100 C overnight, and finally
ꢁ
calcined at 550–570 C in air for 6 h.
2
2
-carbaldehyde, CO and pyridine were produced as the main
products in 2-picoline oxidation, but picolinic acid was not
obtained (Table 1). This may be due to the fact that picolinic
acid is easily converted to pyridine by the decarboxylation
even at a low temperature. This is consistent with the result
Characterization of the catalysts
The CrV0.95
diffraction (XRD), Fourier transfer infrared spectroscopy
FT-IR), differential thermal analyses (DTA), BET specific
surface areas measurements, temperature-programmed deso-
rption of ammonia (NH -TPD), and temperature programmed
P
0.05
O
4
catalyst has been characterized by X-ray
11
reported by J a¨ r a˚ s et al. In the oxidation of 3-picoline
(
a
Table 1 2-Picoline oxidation over CrV0.95P0.05O4
3
4
Selectivity (%)
Yield (%)
reduction (TPR) as reported previously. In this work, diffuse
reflectance infrared Fourier-transform spectra (DRIFTS)
were recorded with a Nicolet MAGNA-IR 560 spectrometer
equipped with an in situ diffuse COLLECTOR diffuse reflec-
tance cell. Spectra were obtained from a narrow band MCT
Temperature/ Conversion
ꢁ
2-P.
Ald.
2-P.
Ald.
b
c
d
b
c
C
(%)
CO
x
P. A.
Pyr.
P. A.
250
11.5
13.0
18.6
29.8
34.3
10.9 89.1
14.6 83.5
18.1 78.3
36.7 57.6
30.9 56.5
0.0
0.0
0.0
0.0
0.0
0.0
1.6
10.2
10.9
14.5
17.1
19.4
0.0
0.0
0.0
0.0
0.0
ꢀ1
2
3
3
3
75
00
25
50
detector by 128 scans at 4 cm resolution. The catalyst sample
powder (ca. 40 mg) in an in situ diffuse reflectance cell with
3.0
4.8
ꢁ
NaCl windows was pretreated at 500 C for 1 h in a mixture
of dry N (30 ml min ) and O (4.5 ml min ), and cooled
10.5
ꢀ
1
ꢀ1
2
2
a
b
ꢀ1
ꢁ
ꢁ
2-Picoline : O : N ¼ 0.032 : 0.73 : 4.56 mmol min ¼ 1: 24 : 152 molar ratio.
to 30 C in N
2
flow. At 30 C, the spectra was recorded and
used as background. Picoline vapor was then introduced in a
2
2
c d
Pyridine-2-carbaldehyde. Picolinic acid. Pyridine.
ꢀ
1
(30 ml min ) flow for 10 min, and shut off. The chamber
was purged at 30 C for 30 min under N atmosphere to
2
N
2
ꢁ
a
remove the gas-phase species and the spectra were recorded
until the change was no longer observed in the spectra. Subse-
Table 2 3-Picoline oxidation over CrV0.95
P
0.05 4
O
ꢁ
ꢁ
ꢀ1
ꢀ1
Selectivity (%)
Yield (%)
quently, the sample was heated up to 150 C with 10 C min
under flowing N (30 ml min ) and O (4.5 ml min ), main-
ꢀ
1
2
2
Temperature/ Conversion
ꢁ
3-P.
COx Ald.
3-P.
Ald.
ꢁ
b
c
d
b
c
tained at this temperature for 1–2 s, and cooled to 30 C. At
ꢁ
C
(%)
N. A.
Pyr.
N. A.
3
0 C, the spectra were again recorded. Possible transforma-
3
3
3
3
3
00
25
50
60
75
6.1
8.9
32.6 67.4
42.4 53.4
57.1 27.5
45.2 22.0
43.9 18.5
0.0
0.0
0.0
4.2
3.1
2.9
5.0
4.1
4.8
6.0
6.9
7.0
0.0
0.0
tions of the adsorbed species were recorded in a similar way
at a series of increasing temperatures. All gases were delivered
to the sample at atmospheric pressure. The flow rate of each
gas was controlled by a STEC Model SEC400-MK3 mass flow
controller. Circulating water is used to cool the body of the
reaction chamber. The temperature was controlled by a ther-
mocouple in direct contact with the sample.
21.8
31.6
37.9
12.2
29.8
32.6
2.7
9.4
12.4
a
ꢀ1
3
ratio. Pyridine-3-carbaldehyde. Nicotinic acid. Pyridine.
-Picoline : O
b
2
: N
2
¼ 0.032 : 0.73 : 4.56 mmol min ¼ 1 : 24 : 152 molar
c
d
Phys. Chem. Chem. Phys., 2003, 5, 2710–2718
2711

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a
0.05 4
Table 3 4-Picoline oxidation over CrV0.95P O
Selectivity (%)
Yield (%)
Temperature/ Conversion
ꢁ
4-P.
4-P.
b
c
d
e
b
Ald I. A. Pyr. Others Ald. I. A.
c
C
(%)
CO
x
2
50
75
00
25
50
35.8
52.7
84.4
98.1
99.2
7.3 92.7
12.3 82.4
0.0
5.2
0.0
0.0
0.0
0.2
0.5
0.0
0.0
33.2
43.5
0.0
2.8
2
3
3
3
0.8 73.0 26.1
0.7 39.2 42.9
10.9 9.4 56.5
0.2
61.6 22.0
38.5 42.1
9.3 56.1
15.3
20.0
a
b
ꢀ1
4
-Picoline : O
2
: N
Pyridine-4-carbaldehyde. Isonicotinic acid. Pyridine. 4-Pyridinemetha-
2
¼ 0.032 : 0.73 : 4.56 mmol min ¼ 1 : 24 : 152 molar ratio
c d e
0
nol, 4-4 trimethylenedipyridine etc.
(
Table 2), the selectivity of pyridine-3-carbaldehyde decreased,
while the selectivity of nicotinic acid increased with increasing
ꢁ
the reaction temperature up to 375 C, suggesting that 3-pico-
line was oxidized to nicotinic acid via pyridine-3-carbaldehyde.
An appearance of pyridine above 325 C may be due to the
decarboxylation of nicotinic acid to form pyridine at the high
temperature. The main products from 4-picoline were pyri-
dine-4-carbaldehyde and isonicotinic acid, and pyridine, 4-pyr-
idinemethanol and 4-4 trimethylenedipyridine were produced
ꢁ
1
1
Fig. 2 Effect of water on 3-picoline oxidation over CrV0.95
catalyst. Reaction temperature, 360 C; N. A., Nicotinic acid; 3-P.
Ald., Pyridine-3-carbaldehyde; Pyr., Pyridine.
P
0.05
O
4
ꢁ
0
as by-products at the high temperature (Table 3). The selec-
tivity of both pyridine-4-carbaldehyde and isonicotinic acid
strongly depended on the reaction temperature. The selectivity
of isonicotinic acid gradually increased, while that of pyridine-
In the present study, the oxidations of 2- and 4-picolines
were carried out over CrV0.95P O catalyst in comparison
0.05 4
to 3-picoline oxidation. The results of 2-picoline oxidation
are shown in Fig. 1. Both conversion of 2-picoline and
yield of pyridine-2-carbaldehyde significantly increased with
increasing the amount of water, i.e., 18.6 and 14.5% in the
4
-carbaldehyde sharply decreased with increasing the reaction
temperature, also suggesting that 4-picoline was oxidized to
isonicotinic acid via pyridine-4-carbaldehyde. A total yield of
both pyridine-4-carbaldehyde and isonicotinic acid was 83.6%.
2
–4
absence of water, 34.4 and 30.3% at H
(
2
O : 2-picoline ¼ 10.8
In the previous papers, we reported that the presence of
excess amount of water enhanced the yield of nicotinic acid
in 3-picoline oxidation, where water converts Lewis acid sites
to Brønsted acid sites or creates Brønsted acid sites by the
hydrolysis of V–O–Cr bond on the catalyst surface. Moreover,
molar ratio), and 69.2 and 50.2% at H O : 2-picoline ¼ 108.
2
No picolinic acid was observed, even in the presence of a large
amount of water. The results of 3-picoline oxidation are shown
in Fig. 2 as a reference. Increasing the amount of water
resulted in an increasing yield of nicotinic acid production,
0.05 4
CrV0.95P O catalyst possesses reduction–oxidation sites
2
–4
as reported in the previous paper. The results of 4-picoline
oxidation are shown in Fig. 3. 4-Picoline was more quickly oxi-
consisting of V assisted by P, and the V reduction–oxidation
sites are effective for oxygen activation. Thus, it was concluded
that 3-picoline is selectively oxidized to nicotinic acid on the V
reduction-oxidation sites assisted by Brønsted acid sites on the
catalyst.
0.05 4
dized than 2-picoline over CrV0.95P O catalyst, and its
ꢁ
conversion was always above 80% at 325 C whether in the
presence of water or not. The addition of water remarkably
affected the product selectivity. The selectivity of pyridine-
4
-carbaldehyde decreased, while that of isonicotinic acid
Fig. 3 Effect of water on 4-picoline oxidation over CrV0.95
P
0.05
O
4
ꢁ
Fig. 1 Effect of water on 2-picoline oxidation over CrV0.95
catalyst. Reaction temperature, 300 C; 2-P. Ald., Pyridine-2-carbalde-
hyde; Pyr., Pyridine.
P
0.05
O
4
catalyst. Reaction temperature, 325 C; I. A., Isonocotinic acid; 4-P.
Ald., Pyridine-4-carbaldehyde; Pyr., Pyridine, 4-P.nol, 4-Pyridine-
methanol.
ꢁ
2
712
Phys. Chem. Chem. Phys., 2003, 5, 2710–2718

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ꢁ
CrV0.95
3
P
-picoline and pyridine adsorbed were recorded as a compari-
0.05
O
4
catalyst at 30 C. The spectra of both liquid
son. The observed IR absorption bands were assigned with
reference to the vibration spectra of neat picoline isomers
1
2–14
reported
of the absorption bands of 3-picoline adsorbed on
were achieved as below. Generally, the bands
and are collected in Table 4. The assignments
0.05 4
CrV0.95P O
ꢀ1
due to hydroxyl group appear in the 3000–4000 cm regions
and the bands between 3600–3700 cm
n(OH) of catalyst surface.
observed around 3620 and 3640 cm are assigned to –OH
groups on the surface of catalyst. This may be due to the inter-
action of –OH groups with the adsorbed 3-picoline resulted in
the decrease of the intensity that account for the negative
ꢀ
1
are assigned to
Thus, two weak negative bands
1
5,16
ꢀ
1
ꢀ
1
bands. The bands at 2884 and 2938 cm are due to n(C–H)
of CH groups, symmetric and asymmetric stretching vibra-
tions. The pyridine ring n(C–H) gives the bands at 3044 and
3
ꢀ
1
.
3
078 cm
Besides, another negative band was observed around 2050
Fig. 4 Apparent activation energy of the oxidation of 2-, 3-, and 4-
picolines over CrV0.95 catalyst. Molar ratio of H O : picoline,
0.8.
ꢀ1
ꢀ1
has been
cm . A similar negative band at 2048 cm
reported with toluene adsorbed on V O /TiO catalyst,
P
O
0.05 4
2
1
2
5
2
which was assigned to an overtone vibration (2n(V=O)) of
monomeric vanadyl species. It was reported that 3-picoline
1
6
increased with increasing the water amount. This indicates that
the presence of water was in favor of the oxidation of pyridine-
partially reduced vanadium ions via the coordination
through nitrogen atom. In fact, the negative band at
1
7
ꢀ1
4
nicotinic acid was 84.3% at 4-picoline : H
the presence of a large amount of water, no other by-products
were detected except for very small amount of pyridine, sug-
gesting that water was very efficient for the selective oxidation
of 4-picoline to isonicotinic acid.
-carbaldehyde to isonicotinic acid. The highest yield of iso-
2050 cm was also observed after the adsorption of pyri-
dine (Fig. 5c) or pyridine-3-carbaldehyde (not shown) in
our DRIFTS studies. Here, no V=O exist in the monoclinic
2
O ¼ 1 : 108. In
3
,10
crystal structure of CrV0 P0.05O4 ,
.95
negative band at 2050 cm
and therefore the
may be assigned the reduced
ꢀ
1
V–O groups due to the coordination through nitrogen of
3-picoline.
Apparent activation energies of the oxidation of 2- and 4-
picolines over CrV0.95P0.05O4 catalyst were calculated from
ꢀ
1
Absorption bands in the region of 1400–1600 cm
characteristic aromatic ring vibrations (8a,8b,19a,19b).
The bands observed at 1425, 1474, 1552, 1583, 1612 and
are
ꢁ
12,13
the rates of the picolines consumptions at 300–400 C in the pre-
sence of water (molar ratio of H O : picoline: 10.8) (Fig. 4). The
2
ꢀ
1
results obtained with 3-picoline is also shown as a reference.
The lower values were obtained with 2- and 4-picolines than
that with 3-picoline, among which 4-picoline showed the lowest
value. This may be due to the fact that the electron withdrawing
ability of nitrogen in the pyridine ring remarkably affects the
reactivity of methyl group, i.e. the electrons of the methyl group
at the 2- and 4-positions are drawn to pyridine ring due to both
inductive and resonance effect of nitrogen, resulting in an easy
leaving of proton from the methyl group. On the other hand,
the methyl hydrogen of 3-picoline is more stable than those of
1634 cm are assigned to skeletal stretching modes. These
bands appeared at somewhat higher frequencies than those
of liquid 3-picoline. This discrepancy can be interpreted by
the interaction of 3-picoline with acid sites on the surface
of CrV0.95P0.05O , i.e., a part of electron in the aromatic ring
4
is withdrawn through N atom and is donated to the acid
sites. Lewis (coordinatively unsaturated metal atoms) and
Brønsted (proton donating OH-groups) acid sites have been
observed in the previous study of probing acid sites on the
surface of the CrV0.95P0.05O catalyst upon pyridine adsorp-
4
1
0
3,10
the 2- and 4-picolines due to the lack of resonance effect.
small amount of P addition in CrVO catalyst was still effective
A
tion.
by the characteristic bands of pyridinium surface species
The presence of Brønsted acid sites was suggested
4
+
ꢀ1
3,10
for the oxidation of both 2- and 4-picolines.
(PyH ) at 1637 cm (n ).
Therefore, it is reasonable to
8
a
ꢀ
1
assign the band at 1634 cm as the 8a stretching vibrations
of 3-picoline adsorbed on Brønsted acid sites due to a proto-
nation of the N atom. The other bands of skeletal stretching
vibration of 3-picoline also appeared in the region of 1400–
DRIFTS of 3-picoline adsorbed
Fig. 5 shows the FTIR spectra of liquid 3-picoline (a), and
the DRIFTS of 3-picoline (b) and pyridine(c) adsorbed on
ꢀ1
1640 cm
.
ꢁ
Fig. 5 FTIR spectra of liquid 3-picoline (a), DRIFTS of 3-picoline (b) and pyridine (c) adsorbed on CrV0.95
P
O
0.05 4
. Temperature, 30 C.
Phys. Chem. Chem. Phys., 2003, 5, 2710–2718 2713

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0.05 4
Table 4 Assignment of IR bands of liquid 3-picoline, and both pyridine and 3-picoline adsorbed on CrV0.95P O catalyst
ꢀ1
Position/cm
Liquid 3-picoline
Adsorbed pyridine
Adsorbed 3-picoline
Assignment
Characteristic of
Pyridine ring
3030, 3084
2880, 2955, 2997
1414(19b), 1479(19a),
3089
—
3044, 3078
2884, 2933
n(C–H)
n(C–H)
n(C–C) or n(C–C,C–N)
3
–CH group
1448(19b), 1489(19a),
a
1539 (19b), 1575(8b),
1425, 1474, 1552,
1583, 1612, 1634
Pyridine ring
a
1577(8a), 1595(8b)
a
610(8a,8b), 1637 (8a)
1
1
383, 1454
226
–
1221
1388
1257
d(C–H)
n(C–N)
–CH
3
group
1
Pyridine ring
a
Suggested to originate from the Brønsted acid site.
Scheme 1
1
7
As to the 3-picoline adsorption, Andersson et al. proposed
two adsorption models that occur at different planes of V O .
3
3
atoms of CH group have been abstracted. n(C–H) bands of
1
ꢀ
pyridine ring at 3044 and 3078 cm decreased their intensities
with increasing temperature, which may be due to a transfor-
mation of adsorbent or a desorption of product. In addition,
2
5
-Picoline could either be adsorbed through nitrogen atom
to partially reduced vanadium ions, or through methyl carbon
atom to an oxygen species after an abstraction of hydrogen.
Based on the present IR results, i.e., the appearance of the
bands at somewhat higher frequencies, similar to the case of
pyridine adsorbed through N atom, it is considered that the
former model is more plausible than the latter. 3-Picoline is
adsorbed through N atom and its pyridine ring is oriented per-
pendicular to the surface. Scheme 1 shows the possible adsorp-
tion model of 3-picoline upon both Lewis (a) and Brønsted
acid sites (b).
ꢀ1
the broad band around 3400 cm due to n(OH), is interpreted
1
5,19
by a possible formation of water
6,20
or hydrogen bonding to
1
adsorbent.
It was reported that the infrared spectra of free carboxlic
acids are characterized by the strong absorptions at 1200–
1300 and 1700–1750 cm associated with the C–O and
1
C=O bonds of the carboxyl group, while the carboxylate
ꢀ1
2
ion has relatively strong symmetric (n ) and strong asymmetric
s
ꢀ
(nas) COO stretching vibrations at 1280–1400 and 1510–1650
ꢀ
1
21,22
cm , respectively.
Moreover, the absorptions at 1390 and
In situ DRIFTS during 3-picoline oxidation
In order to identify the intermediate species formed on the cat-
alyst surface at high temperature, the temperature dependent
IR spectra of 2-, 3- and 4-picolines and pyridine-3-carbalde-
hyde were studied on CrV0.95
between 150–400 C.
P
0.05
O
4
in the temperature range
ꢁ
The DRIFTS of 3-picoline adsorbed on CrV0.95
P
0.05
ꢁ
O
shown in Fig. 6, recorded at 30 C (a,a ), 150 C (b,b ),
2
4
4
are
0
ꢁ
0
ꢁ 0 ꢁ 0 ꢁ 0 ꢁ 0
00 C (c,c ), 250 C (d,d ), 300 C (e,e ), 350 C ( f,f ) and
00 C (g,g ). The observed absorption bands are listed in
ꢁ
0
Table 5 together with those of 2- and 4-picolines. A band at
718 cm observed in Fig. 6, is assigned to C=O vibration
ꢀ1
1
corresponding to the characteristic n (C=O) absorption of free
ꢀ
1 16,18
aldehyde around 1710 cm
carbaldehyde is produced at 150 C and physically adsorbed
.
This suggests that pyridine-3-
ꢁ
ꢀ
1
on the surface. Both bands at 2884 and 2938 cm in Fig. 6
are assigned to n(C–H) of CH
intensities when the temperature was raised from 30 to 300 C.
They are typical for the symmetric and asymmetric stretching
3
group and decreased gradually
ꢁ
0.05 4
Fig. 6 In situ DRIFTS of 3-picoline adsorbed on CrV0.95P O .
1
2,13
vibrations of CH group, respectively.
ꢁ
These bands are no
longer detected above 300 C, indicating that all hydrogen
ꢁ 0 ꢁ 0 ꢁ 0 ꢁ 0
Temperature, 30 C(a,a ), 150 C (b,b ), 200 C (c,c ), 250 C (d,d ),
3
ꢁ 0 ꢁ 0 ꢁ 0
300 C (e,e ), 350 C ( f,f ), and 400 C (g,g ).
2
714
Phys. Chem. Chem. Phys., 2003, 5, 2710–2718

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0.05 4
Table 5 Assignment of IR bands of 2-, 3- and 4-picolines adsorbed, followed by the reaction, on CrV0.95P O catalyst
ꢀ1
Position/cm
-Picoline
2
3-Picoline
4-Picoline
Assignment
Characteristic of
ꢃ3400
ꢃ3400
ꢃ3400
n(OH)
H
2
O or hydrogen bonding to adsorbent
Pyridine ring
group
3
2
1
1
1
065, 3086
856, 2932, 2968
3044, 3078
2884, 2933
3076
2868, 2929, 2963
n(C–H)
n(C–H)
n(C=O)
n(C=O)
ꢀCH
3
716
670
1718
—
1714
—
Physically adsorbed pyridine–carbaldehyde
Aldehydic adsorbate
Pyridine ring
455, 1473, 1489, 1542,
1425, 1474, 1552, 1583,
a
1612, 1634
1429, 1507, 1515,
1558, 1606, 1617, 1643
1584
n(C–C) or n(C–C,C–N)
a
a
1542, 1571, 1621, 1636
594
ꢀ
1
1588
n
n
as(COO )
Carboxylate
(picolinate, nicotinate, isonicotinate)
Carboxylate (picolinate, nicotinate, isonicotinate)
ꢀ
1
417
1412
1414
s
(COO )
a
Suggested to originate from the Brønsted acid site.
ꢀ
1
1
556 cm of aqueous nicotinate ion have been assigned to the
carboxylate stretching modes. An intensity of the band at
coordination modes: unidentate, bridging bidentate, and che-
2
3
21,25
lating bidentate.
spectra by their bands splitting of carboxylate absorption
These can be distinguished in infrared
ꢀ
1
412 cm shown in Fig. 6 (d–f), prominently increased with
1
increasing the temperature from 250 to 350 C. This band
ꢁ
ꢀ1
(Dn). The values of Dn are generally 350–500 cm for uniden-
tate binding, 150–180 cm for bridging, and 60–100 cm
ꢀ
ꢀ1
ꢀ1
can be assigned to the symmetrical vibration of the COO
while the asymmetrical vibration of the carboxylate
2
1–24
group,
for chelating. In general, there is little change in Dn between
carboxylate ion in solution and the bridging bidentate surface
species. As described above, the strong bands at 1412 and 1588
ꢀ
1
stretching appeared at 1588 cm . The assignments of the
bands due to carboxylate group were also confirmed by the
in situ DRIFTS study using pyridine-3-carbaldehyde, in which
the same spectra were observed for asymmetric and symmetric
ꢀ
1
cm (Figs. 6 and 7) are assigned to the carboxylate stretching
modes of nicotinate adsorbed on CrV0.95P0.05O . The carbox-
4
ꢀ
COO stretching vibration, with increasing the temperature
ꢁ
ylate band splitting of the adsorbed species (Dn ¼ 1588 ꢀ
ꢀ1
from 30 to 300 C (Fig. 7). These results suggest that nicotinate
1412 ¼ 176 cm ) is near to that of aqueous nicotinate ion
ꢀ
1 23
was formed and adsorbed on CrV0.95 O
picoline and pyridine-3-carbaldehyde. When the temperature
was increased above 350 C, the intensity of the band at 1412
P
0.05
4
in both cases of 3-
(1556 ꢀ 1390 ¼ 166 cm ), suggesting that a bridging biden-
tate nicotinate species is formed on the catalyst. However, any
shift of pyridine ring (C–C and C–N) skeletal stretching vibra-
tion was not observed when the nicotinate species formed, sug-
gesting that N atom of pyridine ring is still bound to the
catalyst surface. It was reported that salicylic and phthalic
acids were adsorbed on metal oxides via bidentate interactions,
involving coordination through both carboxylate and substitu-
ꢁ
ꢀ
1
cm turned to decrease. This may be due to the desorption
of nicotinate at higher temperature, which can also account
for the disapperance of pyridine ring n(C–H) bands at 3044
and 3078 cm at these temperatures. Another reason may
be the fact that COO groups are transformed to gaseous
CO , which contributes the band at 2345 cm (not shown).
Coordination of carboxylic acids usually takes place
through the carboxylate group, and there are three common
ꢀ
1
ꢀ
ꢀ1
26
2
ent groups. Therefore, a bridging bidentate surface nicoti-
nate species, coordinated through both N atom of pyridine
ring and a C–O bond of the carboxylate group, is the most
plausible structure of the intermediate. The Dn values for the
adsorbed species are too low to be normally considered as uni-
dentate carboxylate coordination, but expected to fit for brid-
ging bidentate coordination. Steric considerations are likely to
2
7
forbid tridentate coordination. Another FT-IR study also
reported the value Dn ( ¼ n ꢀ n ) with various carboxylates;
as
s
a unidentate structure is responsible when Dn is clearly larger
ꢀ1
than 140 cm , while a bidentate structure for the smaller Dn
than 140 cm . Recent study of the interaction between nico-
ꢀ
1
tinic acid and TiO surface immersed in water by ATR-FTIR
2
showed that hydrogen bonding between the protonated pyridi-
2
8
nic nitrogen and a vicinal OH surface group. This structure is
exactly consistent with the nicotinate adsorbed on the
Brønsted acid site as described above. Thus, the adsorption
model shown in Scheme 2 (M is the surface metal ion), is pro-
posed for nicotinate on the Lewis acid sites (a) and Brønsted
acid sites (zwitterionic surface species) (b) of metal oxide sur-
face.
In situ DRIFTS spectra of adsorption and oxidation of 2- and
4
-picolines
Fig. 8 shows the in situ DRIFTS spectra of 2-picoline adsorbed
on CrV0.95
marized in Table 5. The spectra (a,a ) are similar to those
0.05
P O
4
catalyst and the observed bands are sum-
0
reported recently on 2-picoline adsorbed on a hydrated silica
2
9
surface by Raman and FTIR spectroscopies. The authors
suggested that, on hydrated silica, the magnitude of the shift
observed for the ring breathing band of adsorbed 2-picoline
implies an interaction that is slightly stronger than simple
Fig. 7 In situ DRIFTS of pyridine-3-carbaldehyde adsorbed on
ꢁ
ꢁ
ꢁ
CrV0.95
P
00 C (d).
0.05
O
4
. Temperature, 30 C (a), 150 C (b), 200 C (c), and
ꢁ
3
Phys. Chem. Chem. Phys., 2003, 5, 2710–2718
2715

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Scheme 2
hydrogen bonding or an interaction with some Brønsted char-
acter; 2-picoline interaction with surface silanols through the
N atom is proposed. When 2-picoline was adsorbed on
0.05 4
CrV0.95P O , the ring breathing bands (1455, 1473, 1489,
ꢀ
1
542, 1571 and 1621 cm ) were observed at higher frequencies
1
than neat 2-picoline due to the interaction of 2-picoline with
ꢀ
1
the catalyst. The band at 1636 cm appeared suggesting a
protonation of the ring N atom by the interaction with
Brønsted acid sites. This may also lead to increase the shift
of the ring breathing bands. The presence of Brønsted acid
sites over the catalyst has been confirmed by pyridine adsorp-
0.05 4
Fig. 9 In situ DRIFTS of 4-picoline adsorbed on CrV0.95P O .
ꢁ
0
ꢁ
0
ꢁ
0
ꢁ
0
Temperature, 30 C (a,a ), 150 C (b,b ), 200 C (c,c ), 250 C (d,d ),
ꢁ
0
ꢁ
0
ꢁ
0
300 C (e,e ), 350 C ( f,f ) and 400 C (g,g ).
4
tion. Thus, referring to the vibration spectra of neat picoline
isomers
Fig. 9 shows the in situ DRIFTS of 4-picoline adsorbed on
1
2,13
and FTIR spectra of 2-picoline adsorbed on
CrV0.95
P
and the weak bands at 1429, 1515, 1558 and 1606
0.05
O
4
. The strong bands at 1507, 1617 and 1643
2
9
ꢀ1
silica, the model of 2-picoline adsorption over CrV0.95
is suggested to be similar to that of 3-picoline, i.e., adsorbed
P
0.05
O
4
cm
cm , shown in Fig. 9( a, a ), are mainly assigned to pyridine
ꢀ1
0
through the N atom.
The characteristic band at 1716 cm due to n(C=O) of pyr-
idine-2-carbaldehyde was observed above 150 C in Fig. 8(b–f )
ring (C–C and C–N) skeletal stretching modes. The band at
ꢀ1
ꢀ
1
1643 cm is ascribed to protonation of the ring N resulting
ꢁ
from the interaction with Brønsted acid sites on CrV0.95
in consideration of the same adsorption model as
described for 2 and 3-picoline adsorption. The temperature
-
ꢀ
1
together with the band at 1670 cm , and an intensity of the
0.05 4
P O
ꢀ1
band at 1716 cm distinctly increased with temperature. A
higher yield of aldehyde was observed in 2-picoline oxidation
than in 3-picoline oxidation (Tables 1 and 2). Increasing tem-
0
0
dependent spectral change is shown in Fig. 9(b,b –g,g ) . The
characteristic band of aldehyde expected at 1714 cm
ꢀ1
ꢁ
(C O) appeared at 150 C and disappeared above 250 C.
=
ꢁ
ꢁ
perature to 250 C caused the appearance of the bands at 1417
and 1594 cm assigned to carboxylate species accompanied
ꢀ
1
ꢀ1
Furthermore, the bands at 1414 and 1584 cm , which are
characteristic of the carboxylate group, grew prominently from
ꢀ
1
by a weakening of the bands at 2800–2900 cm from the
methyl group n(C–H). A contradiction arose, in that the inten-
sity of the former bands increased with temperature, indicating
that more carboxylate formed, but no picolinic acid was found
ꢁ
200 to 300 C, and then diminished at higher temperature. This
indicates that aldehyde was formed at the first step and then
transformed to isonicotinate. The desorption of isonicotinate
accounts for the intensities of both the 1414 and 1584 cm
ꢀ1
in the products, rather pyridine and CO
2
at higher tempera-
tures in the oxidation of 2-picoline (Table 1). It is likely that
the carboxylate is stable on the catalyst surface, but is unstable
and easily decarboxylated after it desorbs from the catalyst
surface. Based on the carboxylate species bands at 1417 and
bands reducing at higher temperature, coinciding with the rela-
tive decrease of the intensity of the pyridine ring C–H stretch-
ꢀ
1
ing vibration at 3076 cm
.
ꢀ1
ꢀ1
1
594 cm , Dn ¼ 1594 ꢀ 1417 ¼ 177 cm , thus, a bridging
Mechanism of 2-, 3-, and 4-picoline oxidations
bidentate structure is suggested, again, similar to that of nico-
tinate.
1
–4
Our previous works
suggested that the bridging oxygen in
the V–O–Cr bond is responsible for the catalytic activity dur-
ing the oxidation of 3-picoline. A partial replacement of V with
P (CrV0.95P O ) resulted in a significant increase in the activ-
0.05 4
ity. This may be due to an enhanced oxidizing ability of the V
reduction-oxidation sites as observed in an appearance of the
reduction peaks in TPR at low temperature. The enhance-
4
ment in catalytic activity of the V reduction-oxidation sites
on CrV0.95P0.05O may be assisted not only by the addition
4
4
of P but also by the Brønsted acidic properties.
The adsorption and oxidation of 3-picoline on
0.05 4
CrV0.95P O in the absence of gas-phase oxygen have been
studied by in situ DRIFTS (Fig. 10). The characteristic bands
of pyridine ring and methyl group, which are assigned to 3-
ꢁ
picoline on CrV0.95P0.05O , was observed at 100 C after the
4
2
purging in N flow (Fig. 10a). The characteristic bands of
ꢀ
1
the oxygen-containing species (1412, 1588 and 1718 cm
)
ꢁ
were also observed above 200 C (Fig. 10b and c) even under
the absence of gas-phase oxygen. This clearly indicates that lat-
tice oxygen of the catalyst involved in the oxidation process.
ꢀ1
The negative band at 2050 cm probably due to the partially
ꢁ
reduced V sites was disappeared after oxidation at 400 C for
20 min (Fig. 10d), whereas it was obtained even after the pur-
0.05 4
Fig. 8 In situ DRIFTS of 2-picoline adsorbed on CrV0.95P O .
ꢁ
0
ꢁ
0
ꢁ
0
ꢁ
0
Temperature, 30 C (a,a ), 150 C (b,b ), 200 C (c,c ), 250 C (d,d ),
ꢁ
0
ꢁ
0
ꢁ
3
00 C (e,e ), and 350 C ( f,f ).
ging in N
2
at 300 C. It is likely that the reduction-oxidation
2
716 Phys. Chem. Chem. Phys., 2003, 5, 2710–2718

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2
ꢀ
by a lattice oxide (O ) and interacts with the oxygen atom to
form a hydroxyl group. Another active lattice oxide attacks the
activated methyl group and forms an aldehyde-like intermedi-
ate. In order to compensate for the loss of the lattice oxide
ion, the oxygen adsorbed from the gas phase migrates to fill
the oxygen vacancy. Part of the reaction product, for example
pyridine–carbaldehyde, desorbs from the catalyst surface into
the gas phase. Part of pyridine–carbaldehyde still remains on
the catalyst surface and can be further oxidized to carboxylate
at higher temperature. The carboxylate desorbs to form car-
boxylic acid, and at the same time some pyridine and CO
x
form if decarboxylation is possible. If the temperature is very
high, then an appreciable amount of deep oxidation will take
place.
Water enhancement of the 2-, 3- and 4-picoline oxidations
can be explained by the formation of Brønsted acid site
(
ꢀOH groups). Some Lewis acid sites on the surface of
0.05 4
CrV0.95P O
catalyst could be transformed to Brønsted acid
4
sites in the presence of water. Moreover, some of the water
adsorbed on the catalyst surface can dissociatively hydrolyse
V–O–Cr bond and produce hydroxyl (–OH) groups as
Brønsted acid sites, again. The formation of the surface hydro-
xyls may also lead to a weakening of V–O bonds or a formation
of surface defect, which enables the rate of oxidation reaction
Fig. 10 In situ DRIFTS of 3-picoline adsorbed on CrV0.95
P
0.05
O
4
. In
for 20 min at
1
5,31–33
ꢁ
ꢁ
ꢁ
to increase.
Furthermore, it is well known that the resi-
N
2
at 100 C (a), 200 C (b), 300 C (c), and in O
ꢁ
2
/N
2
400 C (d).
dence time of an organic species depends on the desorption rate
of intermediate products formed on the surface. The ease of
desorption of an organic compound depends on acid-base
properties of the catalyst surface. A quick desorption of acidic
product requires high acidity and low basicity. Herein, the
addition of water results in an enhancement of the amount of
Brønsted acid sites on the catalyst surface, which accelerates
the hydrolysis and desorption of carboxylate from catalyst sur-
took place on the V sites. i.e., the original intensity of V-O
overtone band was almost recovered in the presence of gas-
phase oxygen. It is suggested that the oxidation of adsorbed
3-picoline is enhanced due to the replenishment of lattice oxy-
gen by gas-phase oxygen.
As described above, though there is a large difference in the
reactivity and the product selectivity between 2-, 3-, and 4-
x
face. Consequently, the total oxidation to CO can be blocked.
Finally, it is concluded that both V reduction–oxidation sites
and Brønsted acid sites are collaboratively responsible for the
high catalytic activity and the acid selectivity.
picolines oxidations over CrV0.95P0.05O , a good similarity
4
was observed in the adsorption and the oxidation behaviors
based on the analysis of in situ DRIFTS. On the basis of
5
experimental results and literature surveys, a possible Mars
,6
3
0
and van Krevelen mechanism of surface reactions over the
catalyst surface is proposed (Scheme 3). In the first step, a pro-
tonated picoline forms by the interaction with a Brønsted acid
site. Then, a hydrogen atom of the methyl group is abstracted
Conclusions
The vapor phase oxidations of 2-, 3- and 4-picolines on
CrV0.95P0.05O catalyst have been studied in connection with
4
Scheme 3
Phys. Chem. Chem. Phys., 2003, 5, 2710–2718
2717

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the results of in situ DRIFTS measurement of the adsorbed
species. Both 2- and 4-picolines were more active and showed
higher conversion than 3-picoline probably due to their iso-
meric molecular structure. Nicotinic acid and isonicotinic acid
were obtained from 3- and 4-picolines, respectively. Picolinic
acid was not obtained from 2-picoline, since picolinic acid
8
9
G. Busca, Catal. Today, 1996, 27, 457.
S. Besselmann, E. Loffler and M. Muhler, J. Mol. Catal., A, 2000,
62, 401.
R. M. Acheson, An Introduction to the Chemistry of Heterocyclic
Compounds, 1976.
1
1
1
0
1
S. J a¨ r a˚ s and S. T. Lundin, J. Appl. Chem. Biotechnol., 1977,
27, 499.
was easily decomposed to pyridine and CO
In situ DRIFTS study showed that both Lewis and Brønsted
acid sites were present on the surface of CrV0.95 , and
2
.
12 I. L. Tocon, M. S. Wooley, J. C. Otero and J. I. Marcos, J. Mol.
Struct., 1998, 470, 241.
1
3
J. F. Arenas, I. L. Tocon, J. C. Otero and J. I. Marcos, J. Mol.
Struct., 1999, 476, 139.
0.05 4
P O
the adsorptions of 2-, 3- and 4-picolines occurred through the
N atom to the acid sites on the catalyst surface. The bridging
bidentate adsorption of carboxylate through both N atom and
a C–O on the catalyst surface was suggested. Even though there
were observed some differences in the reactivity and the pro-
duct selectivity between 2-, 3- and 4-picoline oxidation over
CrV0.95P O , a good similarity was observed in the adsorp-
0.05 4
tion and the oxidation behaviors by in situ DRIFTS measure-
ment. Based on these results, a Mars and van Krevelen
1
4
G. A. H. Mekhemer, A. K. H. Nohman, N. E. Fouad and H. A.
Khalaf, Colloids and Surfaces A, 2000, 161, 439.
15 J. E. Sambeth, M. A. Centeno, A. Paul, L. E. Briand, H. J.
Thomas and J. A. Odriozola, J. Mol. Catal., A, 2000, 161, 89.
1
6
S. Besselmann, E. Loffler and M. Muhler, J. Mol. Catal., A, 2000,
62, 401.
A. Andersson, J. Catal., 1986, 100, 414.
1
1
1
7
8
M. Sanati and A. Andrsson, Ind. Eng. Chem. Res., 1991, 30,
12; 320.
3
19 M. Sanati and A. Andersson, J. Mol. Catal., 1993, 81, 51.
20 K. Kraub, A. Drochner, M. Fehlings, J. Kunert and H. Vogel,
J. Mol. Catal., 2000, 162, 413.
mechanism of surface reactions over CrV0.95P0.05O is proposed.
4
Addition of water greatly enhanced the activity and the
selectivity of the products in 2-, 3- and 4-picolines oxidations.
It is interpreted that a large amount of Brønsted acid sites were
formed on the catalyst surface in the presence of water, and
subsequently enabled the acceleration of the rate of oxidation
reaction.
2
1
R. C. Mehrotra and R. Bohra, Metal Carboxylates, Academic
Press, New York, 1983.
2
2
K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coor-
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23 F. Wang and K. A. Berglund, Ind. Eng. Chem. Res., 2000, 39,
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2
4
N. B. Colthup, L. H. Daly and S. E. Wiberley, Introduction to
Infrared and Raman Spectroscopy, 3rd edn., Academic press,
Inc., 1990.
2
2
5
6
N. W. Alcock, V. M. Tracy and T. C. Waddington, J. Chem. Soc.
Dalton, 1976, 2243.
K. D. Dobson and A. J. McQuillan, Spectrochim. Acta A, 2000,
56, 557.
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Phys. Chem. Chem. Phys., 2003, 5, 2710–2718