Oxidative ammonolysis of 3(4)-methyl- and 3,4-dimethylpyridines using vanadium oxide catalysts

Article abstract of DOI:10.1134/S1070363212120146

Oxidative ammonolysis of 3(4)-methyl- and 3,4-dimethylpyridines using vanadium oxide catalyst doped with Cr₂O₃, SnO₂, and ZrO₂ was studied. The yields of nitriles and conversion of the starting compounds were found to depend on the CH-acidity of the latter in the gas phase. The possible mechanisms of the formation of pyridine-3,4-dicarboxylic acid imide at the oxidative ammonolysis of 3,4-dimethylpyridine was discussed. The relation between the activity of the modified catalysts and the proton affinity of the vanadyl oxygen calculated by the extended Hueckel method was established.

Full text of DOI:10.1134/S1070363212120146

ISSN 1070-3632, Russian Journal of General Chemistry, 2012, Vol. 82, No. 12, pp. 1987–1993. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © P.B. Vorobyev, A.P. Serebryanskaya, 2012, published in Zhurnal Obshchei Khimii, 2012, Vol. 82, No. 12, pp. 2033–2039.
Oxidative Ammonolysis of 3(4)-Methyl-
and 3,4-Dimethylpyridines Using Vanadium Oxide Catalysts
P. B. Vorobyev and A. P. Serebryanskaya
Bekturov Institute of Chemical Sciences, ul. Ualikhanova 106, Almaty, 050010 Kazakhstan
e-mail: pavel.vr@mail.ru
Received November 10, 2011
Abstract—Oxidative ammonolysis of 3(4)-methyl- and 3,4-dimethylpyridines using vanadium oxide catalyst
doped with Cr2O3, SnO2, and ZrO2 was studied. The yields of nitriles and conversion of the starting compounds
were found to depend on the CH-acidity of the latter in the gas phase. The possible mechanisms of the
formation of pyridine-3,4-dicarboxylic acid imide at the oxidative ammonolysis of 3,4-dimethylpyridine was
discussed. The relation between the activity of the modified catalysts and the proton affinity of the vanadyl
oxygen calculated by the extended Hückel method was established.
DOI: 10.1134/S1070363212120146
Oxidative ammonolysis of mono- and dimethyl-
pyridine is the most rational method of obtaining the
practically important mono- and dicyanopyridines [1,
2]. The relationship between the structure and the
reactivity of monomethylpyridines in oxidative
ammonolysis was studied in [3]. Oxidative ammo-
nolysis of isomeric dimethylpyridines using the Cr2O3·
Al2O3 catalyst was studied at 360°C with the conver-
sion of ~20%. Under these conditions 3,4-di-
methylpyridine was converted to 4-cyano-3-methyl-
pyridine with the selectivity of 92% [4]. The different
order of entering the reaction of the substituents of
asymmetric dialkylpyridines was ascribed in [5] to the
difference in the deprotonation enthalpy values of
methyl substituents in the gas phase and to the surface
state associated with the acid center.
destructive oxidation reactions. Therefore the total
reactivity of the starting compounds was evaluated
from their conversion, and the reactivity of the methyl
groups, from the yields of 3-cyano- (IV), 4-cyano- (V),
and 3-methyl-4-cyanopyridines (VI).
The experimental data on the oxidative
ammonolysis of pyridines I–III using the V–Sn-oxide
catalyst under the standard conditions are shown in
Fig. 1. As can be seen, in the temperature range of
270–330°C the conversion of the starting material
increases in the sequence of I < III < II (Fig. 1a). The
reactivity of the methyl groups evaluated by the
monocyanides yields increases similarly: IV < VI < V
(Fig. 1b). The observed decrease in the yield of
compound VI as the temperature increases is due to its
conversion into pyridine-3,4-dicarboxylic acid imide
IX. The latter accumulates in the reaction products in a
significant amount.
The purpose of this work is the experimental and
quantum-chemical study of the relationship between
the reactivity of 3-methyl- (I), 4-methyl- (II) and 3,4-
dimethylpyridines (III) under the oxidative ammo-
nolysis conditions and some features of their structure,
of the possible formation mechanism of pyridine-3,4-
dicarboxylic acid imide IX from compound III, and
also of the effect of different oxide promoters on the
activity of vanadium oxide binary catalysts.
Similar pattern of the temperature effect on the
oxidative ammonolysis of compounds I–III was ob-
served when chromium(III) oxide and zirconium di-
oxide were used as modifiers of vanadium oxide catalyst.
Methylpyridines are weak CH-acids and can
eliminate a proton from a methyl group at the action of
a strong base to form a carbanion, for example, in the
liquid-phase deuterium exchange reaction [6]. This
fact should be taken into account when interpreting the
experimental data on the relative reactivity of the
The studied processes are characterized by a high
selectivity of transforming the methyl groups into the
nitrile groups and by the small contribution of the
1987

1988
VOROBYEV, SEREBRYANSKAYA
(a)
(b)
Temperature, °C
Temperature, °C
Fig. 1. The dependence on the temperature of the conversion of the starting materials (a) and the yields of the reaction products of
oxidative ammonolysis (b) on the catalyst with a molar ratio of V2O5: SnO2 = 1:2. (1) 3-methylpyridine and 3-cyanopyridine, (2) 4-
methylpyridine and 4-cyanopyridine, (3) 3,4-dimethylpyridine and 3-methyl-4-cyanopyridine, (4) pyridine-3,4-dicarboxylic acid
imide. The starting compound:oxygen:ammonia:water = 1:15:3:32. Here and hereinafter the feed rate of the reactants is 40 g l–1 of
the catalyst per hour.
starting materials. In the catalytic reaction
a
pyridine, in particular, in the low-temperature experi-
ments, is determined solely by the conversion of a
methyl group in the position 4, which has the lowest
ΔHd value. At higher temperature, the rate of the
compound VI accumulation is reduced as a result of its
consumption in the intramolecular cyclization stage to
form the reaction product IX (Fig. 1b).
nucleophilic oxygen of the surface can be used as a
proton acceptor. Obviously, the conversion of
compounds I–III depends on the value of
deprotonation enthalpy of methyl substituents
transformed into the cyano groups.
The experimental data on the CH-acidity in the gas
phase
[7–10]
are
known
only
for
The lone electron pair of the carbanion is located on
the highest occupied molecular orbital (HOMO). The
carbanion stability is determined by a degree of the
negative charge delocalization [11]. The more
delocalized is the lone electron pair, the lower is the
HOMO energy level and, therefore, the more stable is
the carbanion. In turn, the more stable is the carbanion,
the stronger the reaction (1) equilibrium is shifted to
the right and the faster is further transformation of the
carbanion into the nitrile. There is a direct relationship
between the ЕHOMO values of the carbanions resulting
from deprotonation of the methyl substituents of the
studied compounds, the values of conversion, and the
yields of the corresponding nitriles in the oxidative
ammonolysis.
monomethylpyridines. For dimethylpyridines these
features are not currently available. Therefore we
calculated the deprotonation enthalpies for the methyl
groups (ΔHd) based on the total energy values of the
initial molecules and the corresponding anions using
ab initio quantum chemical method (RHF/6-31G*).
PyCH3 ⇄ PyCH2− + H+ − ΔHd,
ΔHd = Еtot(PyCH2−) − Еtot(PyCH3).
(1)
A comparison of the experimental data (Fig. 1) and
the results of quantum chemical calculations (Table 1)
show that there is an inverse relationship between the
conversion of the starting compound, yield of the
corresponding nitrile, and the deprotonation enthalpy
of the methyl group of the starting compound (ΔHd):
the conversion of the reactants and yield of
corresponding nitrile increase in the sequence of I <
III < II as the deprotonation enthalpy decreases. It
should be noted that the reactivity of 3,4-dimethyl-
Thus, the conversion of the studied methylpyridine
derivatives into cyanopyridines (in the case of 3,4-
dimethylpyridine also the order of the substituents
entering into the reaction) is determined by the
deprotonation enthalpy values.
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OXIDATIVE AMMONOLYSIS OF 3(4)-METHYL-
1989
Table 1. The total energies of the molecules of pyridines I–III, the corresponding anions (–Etot), the deprotonation enthalpy
of the methyl groups in the gas phase (ΔHd), the energy of the higher occupied molecular orbitals of the anions (–EHOMO
calculated ab initio by the quantum-chemical method (RHF/6-31G )a
)
–Etot, au
ΔHd,
–ЕHOMO, (au)
Compound
kJ mol–1
kJ mol–1
molecules
anoins
285.6154165845
285.6168469440
324.6357600925
284.9670478655
284.9831732710
1702.3
1663.7
(0.0179) 47.0
(0.0371) 97.4
I
II
III
323.9848150532(3)
323.9982872763(4)
1709.0(3)
1673.7(4)
(0.0198) 52.0(3)
(0.0360) 94.5(4)
a ΔHd = [Еtot(anion) - Еtot(molecule)]·627.5·4.184.
3,4-Lutidine is similar to o-xylene in the ability to
the intramolecular cyclization. As known, one of the
primary products of the oxidative ammonolysis of o-
xylene is o-tolunitrile. Its nitrile group in the oxidation
reaction is able to react with the adjacent methyl
substituent to form phthalimide [12].
triple bond of the nitrile group resulting in phthalic
acid isoimide. The latter is easily converted into
phthalimide at a high temperature. For example, the
intermediate formation of the substituted benzoate ions
on the surface of the V2O5/Al2O3 catalyst in the xylene
oxidative ammonolysis was detected by the infrared
spectroscopy [13].
The mechanism of the imide ring formation in the
oxidative ammonolysis of o-xylene and 3,4-
dimethylpyridine is of great interest. Chukhno et al.
[12] suggested that the conversion of o-xylene to
phthalimide proceeded through the tolunitrile
formation, the oxidation of the methyl substituent
followed by the reaction of its oxidized form with the
Given the above and a similarity in the structure of
o-xylene and 3,4-dimethylpyridine, we suggest a
scheme for the imide IX formation in the pyridine III
oxidative ammonolysis, which includes the formation
of 4-cyanonicotinic acid VII and pyridine-3,4-di-
carboxylic acid isoimide VIII.
N−
N
HN
O
O
C
NH
C
C+
CH3
C
C
C
OH
O
CH3
C
CH3
O
O
N
N
N
N
N
VII
VI
VIII
IX
Etot –525.8233 au
III
Etot –525.7834 au
Probably, the conversion of VI into the imide IX
includes the surface oxidation of the methyl group at
the position 3 and the electrophilic attack of the proton
on the nitrogen atom of the nitrile group. In a scheme,
the proton belongs to the carboxy group, but the proton
source can be the hydrogen donors that are present in
the reaction mixture (ammonia, water vapor), which
are capable of the heterolytic rupture of the bonds
(H2N–H, HO–H) under the action of a catalyst. Such
mechanism of the VI → IX transformation is
supported by the results of our ab initio quantum
chemical calculations (RHF/6-31G*). A significant
polarization of the triple bond in the nitrile group is
characteristic of the isolated molecule of VI. As a
result, the carbon atom is positively charged (δC
+0.081), and the nitrogen atom is negatively charged
(δN −0.2365). This favors the electrophilic attack of the
proton on the nitrogen atom. The total energy level of
compound VIII (Etot = –525.7834274589 au) is higher
than that of imide IX (Etot = –525.8232800546 au),
which indicates the possibility of transforming the
thermodynamically less stable isoimide into the pyri-
dine-3,4-dicarboxylic acid imide.
We have observed that under the comparable
conditions of the oxidative ammonolysis of com-
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1990
VOROBYEV, SEREBRYANSKAYA
(a)
(b)
Temperature, °C
Temperature, °C
Fig. 2. The temperature dependence of the conversion of 3-methylpyridine (a) and the yield of 3-cyanopyridine (b) under the
conditions of oxidative ammonolysis using various catalysts. The starting compound:O2:NH3:H2O = 1:15.6:6:27.9. Here and
hereinafter: (1) V2O5–Cr2O3 (1:1), (2) V2O5–SnO2 (1:4), (3) V2O5–ZrO2 (1:4).
(a)
(b)
Temperature, °C
Temperature, °C
Fig. 3. The temperature dependence of the conversion of 4-methylpyridine (a) and the yield of 4-cyanopyridine (b) under the
conditions of oxidative ammonolysis using various catalysts. The starting compound:O2:NH3:H2O = 1:15.6:6:27.9.
pounds I–III the conversion of the starting compound
and the yields of mononitrile increases in the following
catalyst series: V2O5–Cr2O3 < V2O5–SnO2, < V2O5–
ZrO2 (Figs. 2–4). Generally, the activity and selectivity
of the vanadium oxide catalysts for the hydrocarbons
oxidation is caused by the presence of various forms of
the active oxygen on the catalyst surface. The double-
bonded oxygen atom (V=O) of the lattice plays an
important role [14]. One reason for the different
promoter activity of chromium(III), tin(IV), and
zirconium(IV) oxides on the catalytic activity of the
studied vanadium oxide contacts is the different
degrees of the influence of oxide promoters on the
reactivity of V=O bond involved into the proton
eliminating from the methyl group transforming into
the nitrile group. To test this hypothesis, we calculated
the proton affinity values of vanadyl oxygen for three
clusters simulating the active sites of the studied
catalysts containing the V2O5 fragments and various
oxide-promoters using the extended Hückel method
supplemented by the Anderson repulsive potential [15]
(Table 2).
It is presumed that the minimal surface clusters can
be used for such study because of the strong localized
character of the V=O and V–O bonds [16]. The
parameters for the V atoms are taken from [17]. The
calculations by the extended Hückel method were
performed by the optimization of the bond and
dihedral angles. The V=O (1.58 Å) and V–O bonds
lengths (1.83 Å), which were taken from the
experimental data on the crystal structure of V2O5 [16],
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OXIDATIVE AMMONOLYSIS OF 3(4)-METHYL-
(a)
1991
(b)
Temperature, °C
Temperature, °C
Fig. 4. The temperature dependence of the conversion of 3,4-dimethylpyridine (a) and the yield of 3-methyl-4-cyanocyanopyridine
(b) under the conditions of oxidative ammonolysis using various catalysts. The starting compound:O2:NH3:H2O = 1:15.6:6:32.1.
Table 2. The total energies of the vanadium-containing clusters and their protonated forms (Etot), the proton affinity of the
oxygen bonded with the vanadium ions (PA) calculated by the extended Hückel methoda
−Еtot, eV
PA,
Reaction
kJ mol–1
initial state
1330.8396
final state
1333.2727
O
O
O
O
O
O
O
O
234.9
Cr
Cr
Cr
Cr
+H+
O
O
O
O
V
V
V
V
O
O
O
O
H
q = 0, m = 1
q = 1, m = 1
O
O
1103.3136
1105.8436
244.2
Sn
Sn
+H+
O
O
O
O
O
O
O
V
V
V
V
O
O
O
O
O
H
q = 0, m = 1
q = 1, m = 1
O
O
1105.5254
1108.0626
244.9
Zr
Zr
O
+H+
O
O
O
O
O
V
V
V
V
O
O
O
O
O
O
H
q = 0, m = 1
a PA = (Еinitial – Еfinal)·23.07·4.184.
q = 1, m = 1
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VOROBYEV, SEREBRYANSKAYA
and the Cr–O (2.01 Å), Sn–O (2.06 Å) and Zr–O bond
lengths (2.04 Å) remained constant [18].
thesized by calcinating a mixture of oxides at 620°C
for 3 h.
The calculations show (Table 2) that the value of
the proton affinity of the vanadyl oxygen increases in
the catalyst series as follows: V2O5–Cr2O3, V2O5–
SnO2, V2O5–ZrO2, i.e., it changes in the same manner
as the activity of the studied catalysts under the
conditions of oxidative ammonolysis of 3(4)-methyl-
and 3,4-dimethylpyridines. This fact indicates that the
mechanism of the promoting action of chromium(III),
tin(IV), and zirconium(IV) oxides may be related to
their effect on the nucleophilic properties of the
vanadyl oxygen.
The reaction products were trapped in a scrubber of
airlift type irrigated with water. The conditions for the
chromatographic analysis of 3(4)-methylpyridine and
their transformation products are described in [21].
The conditions of the synthesis of 3-methyl-4-
cyanopyridine and pyridine-3,4-dicarboxylic acid
imide, analysis of the products of oxidative
ammonolysis of 3,4-lutidine are described in [22]. The
IR spectrum of 3-methyl-4-cyanopyridine contains the
band at 2232 cm–1 belonging to the nitrile group [23].
After recrystallization from ethanol, the imide melted
at 224°C (sublimation). IR spectrum, ν, cm–1: 1777.7,
1727.7 [νs,as(C=O)], 3015.4 (NH). Found, %: C 56.75;
H 2.20; N 18.40. C7H4N2O2. Calculated, %: C 56.76; H
2.70; N 18.90.
Thus, the study of the oxidative ammonolysis of
3(4)-methyl- and 3,4-dimethylpyridines using the
vanadium oxide catalysts doped with Cr2O3, SnO2, and
ZrO2 showed that there is a direct correlation between
the CH-acidity of the starting compounds in the gas
phase and the value of their conversion into the
corresponding nitrile of pyridinecarboxylic acid. This
favors the hypothesis of the mechanism that includes
the heterolytic rupture of the C–H bond of the methyl
group in the initial stages of the process. At the
oxidative ammonolysis of 3,4-dimethylpyridine on the
modified vanadium oxide catalysts the temperature
increase favors the intramolecular cyclization to form
pyridine-3,4-dicarboxylic acid imide. The activity of
the studied vanadium oxide catalysts in oxidative
ammonolysis of 3(4)-methyl- and 3,4-dimethyl-
pyridines changes in the same order as the proton
affinity values of the vanadyl oxygen calculated by the
extended Hückel method. This regularity suggests that
the promoters of different nature (Cr2O3, SnO2, ZrO2)
affect differently the nucleophilicity of the double-
bonded oxygen of the V2O5 lattice and catalytic
activity of the modified vanadium oxide catalysts.
The oxidation products were analyzed on a LKhM-
8MD chromatograph equipped with a thermal con-
ductivity detector. The stainless steel column
(l 3.5 m, d 3 mm) was used. An AG-5 charcoal
(0.25×0.50 mm) and Polysorb-1 (0.16–0.20 mm) were
used as an adsorbent for the determination of CO and
CO2, respectively. The columns were temperature-
controlled at 40°C. In all experiments, the balance on
the analytes was 95–100%.
The quantum chemical calculations were performed
by GAMESS software [24]. The calculations of the
clusters simulating the active sites of the modified
surface of catalysts were performed using a PMX
method by the program developed in the Laboratory of
Quantum Chemistry of Boreskov Institute of Catalysis
of Siberian Branch of the Russian Academy of
Sciences (Novosibirsk). The calculation procedure is
given in [25].
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