Influence of polyoxometalate structure in ammoxidation of 2-methylpyrazine

Article abstract of DOI:10.1016/j.catcom.2018.01.023

Keggin, Wells-Dawson and Preyssler structured polyoxometalates (POMs) were synthesized by different methodologies. The POMs were used as catalysts for ammoxidation of 2-methylpyrazine. The structural properties of POMs were determined by XRD, SEM, DRIFT, DRUV-vis, H₂-TPR, NH₃-TPD and N₂-physisorption. Preyssler POM sample showed highest 2-methylpyrazine conversion (69%) and 2-cyanopyrazine selectivity (98%) at 380 °C. Superior performance of Preyssler POM could be attributed to its redox behavior and relatively strong acid sites. Large number of ammonium ions in the secondary structure of Preyssler POM could also enhance the transformation of oxygenated 2-methylpyrazine species into 2-cyanopyrazine. A correlation between the amount of ammonium content of POMs and their catalytic ammoxidation activity was obtained.

Full text of DOI:10.1016/j.catcom.2018.01.023

Accepted Manuscript
Influence of polyoxometalate structure in ammoxidation of
2-methylpyrazine
Abdulmohsen Al-Shehri, Katabathini Narasimharao
PII:
DOI:
Reference:
S1566-7367(18)30029-3
doi:10.1016/j.catcom.2018.01.023
CATCOM 5302
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
4 December 2017
10 January 2018
18 January 2018
Please cite this article as: Abdulmohsen Al-Shehri, Katabathini Narasimharao , Influence
of polyoxometalate structure in ammoxidation of 2-methylpyrazine. The address for the
corresponding author was captured as affiliation for all authors. Please check if
appropriate. Catcom(2018), doi:10.1016/j.catcom.2018.01.023
This is a PDF file of an unedited manuscript that has been accepted for publication. As
a service to our customers we are providing this early version of the manuscript. The
manuscript will undergo copyediting, typesetting, and review of the resulting proof before
it is published in its final form. Please note that during the production process errors may
be discovered which could affect the content, and all legal disclaimers that apply to the
journal pertain.

ACCEPTED MANUSCRIPT
Influence of polyoxometalate structure in ammoxidation of 2-
methylpyrazine
Abdulmohsen Al-Shehri, Katabathini Narasimharao*
Department of Chemistry, Faculty of Science, King Abdulaziz University
P. O. Box 80203, Jeddah 21589, Kingdom of Saudi Arabia

ACCEPTED MANUSCRIPT
Abstract
Keggin, Wells-Dawson and Preyssler structured polyoxometalates (POMs) were
synthesized by different methodologies. The POMs were used as catalysts for
ammoxidation of 2-methylpyrazine. The structural properties of POMs were determined
by XRD, SEM, DRIFT, DRUV-vis, H2-TPR, NH3-TPD and N2-physisorption. Preyssler
POM sample showed highest 2-methylpyrazine conversion (69%) and 2-cyanopyrazine
o
selectivity (98%) at 380 C. Superior performance of Preyssler POM could be attributed
to its redox behavior and relatively strong acid sites. Large number of ammonium ions in
the secondary structure of Preyssler POM could also enhance the transformation of
oxygenated 2-methylpyrazine species into 2-cyanopyrazine. A correlation between the
amount of ammonium content of POMs and their catalytic ammoxidation activity was
obtained.
Keywords: Polyoxometalates; Keggin; Wells-Dawson; Preyssler; Ammoxidation; 2-
methylpyrazine
*Corresponding author, Fax: +966-26952292; Tel: +966-538638994; E-mail:
nkatabathini@kau.edu.sa

ACCEPTED MANUSCRIPT
1. Introduction
The polyoxometalates (POMs) possessed unique properties such as acid, base and
redox properties which render them as potential alternative catalysts to corrosive mineral
acids and other homogeneous reagents [1]. POMs with different structures are reported in
the literature; for instance, Keggin, Wells-Dawson, Anderson, Lindqvist and Preyssler
type POMs [2]. Among many POMs, Keggin compounds were widely utilized as
heterogeneous catalysts in different reactions [3]. Recently, other POMs majorly Wells-
Dawson [4] and Preyssler [5] POMs were also attracted considerable interest in catalysis.
In POMs, the three-dimensional arrangement of anions is usually called primary structure
and counter cations, water of crystallization and any additional molecules is known to be
the secondary structure [2]. In proton form, the acidic strength of POMs is higher than the
mineral acids; this is mainly due to large number of oxygen atoms in anions (40, 62 and
110 in Keggin, Wells-Dawson and Preyssler respectively) [3].
Catalytic ammoxidation of 2- methylpyrazine to 2-cyanopyrazine is a crucial step
in synthesis of 2-amidopyrazine, famously termed as pyrazinamide, an effective an anti-
Tuberculosis drug. The conventional multi-step synthesis process is unacceptable both
economically as well as environmentally due to multi-step synthesis and generates
number of byproducts [6]. It was previously observed that ammonium salt of Keggin
POM showed good performance in ammoxidation reaction [7, 8]. However, thermal and
hydrolytic stability of POMs is important and related to their primary and second
structures [9]. In addition, the nature and number of counter cations as well as the central
and poly atoms play an important role over the physico-chemical and catalytic properties
of POM materials. It was previously reported that the rate of ammoxidation is controlled

ACCEPTED MANUSCRIPT
by acidity and redox properties of catalyst [10]. Keggin, Wells-Dawson and Preyssler
structured ammonium tungstophosphates were synthesized and the materials were used as
catalysts for vapor phase ammoxidation of 2-methylpyrazine. A wide variety of
characterization techniques were used to study structural properties of the synthesized
samples and the performance of catalysts in ammoxidation of 2-methylpyrazine is
correlated with their structural properties. To best of our knowledge, this is the first
attempt to study the influence of structure of POM on the ammoxidation functionality.
2. Experimental
2.1. Synthesis of POMs
(i) Keggin POM: [(NH4)3PW12O40] was synthesized using stoichiometric amounts of
H3PW12O40 and NH4Cl by an ion exchange technique [11].
(ii) Wells-Dawson POM: [(NH4)6P2W18O62] was prepared as per the procedure reported
by Briand et al [12].
(iii) Preyssler POM: (NH4)13[NaP5W30O110] was produced by following the procedure
o
reported by Creaser et al [13]. The POM samples were calcined at 400 C for 4h under
the flow of air.
2.2. Characterization
Powder X-ray diffraction measurements of the samples were conducted using
Bruker diffractometer. DRIFT analysis of POM materials was performed on a Bruker
D70 spectrometer. Scanning Electron Microscopy (SEM) measurements were carried out
using a JEOL JSM840A instrument. The DRUV-vis spectra of the samples were
collected using a Thermo-Scientific evolution spectrophotometer. The textural properties
of samples were determined from N2-physisorption analysis using NOVA 3200e sorption

ACCEPTED MANUSCRIPT
system. H2-TPR, temperature programed decomposition and NH3-TPD analyses were
performed using Quantachrome CHEMBET-3000 instrument. The detailed preparation
procedures and characterization techniques are provided in the supplementary
information.
2.3. Catalytic ammoxidation of 2-methylpyrazine
The vapor phase ammoxidation of 2-methylpyrazine was performed in a fixed bed
catalytic reactor. Prior to the activity tests, the POMs were treated with ammonia for 2 h
o
at 300 C. The 2-methylpyrazine diluted with water, ammonia and air was used as the
feed (molar ratio of 2-methylpyrazine: H2O:NH3: air=1:13:7:38). The reaction was
carried out at a different temperatures.
The liquid products were collected by
condensation and the samples were analyzed by GC with SE-30 column and a flame
ionization detector.
3. Results and discussion
3.1. Powder X-ray diffraction (XRD)
Fig. 1 presents the XRD patterns of synthesized POM samples. The diffraction
peaks of Keggin sample is matched with the cubic structure of [NH4]3PW12O40. 4H2O salt
as reported in the literature [14] and absence of other diffraction peaks indicate the
presence of pure crystalline Keggin POM. The Wells-Dawson sample showed the major
XRD reflections of [NH4]6H6[P2O2(W2O7)9].7H2O phase as represented in JCPDS 04-
0443. Interestingly, this sample exhibited the reflections due to Keggin POM [JCPDS 50-
0305] and NH4Cl [JCPDS 01-0674] phases, which represent residue of the Wells-
Dawson POM synthesis. However, the intensity of the reflections due to Keggin POM
and NH4Cl are weak and the synthesized sample majorly possessed Wells-Dawson

ACCEPTED MANUSCRIPT
structured POM. A very similar results were previously reported by Arendt et al [15]. The
volume percentage of the Wells-Dawson phase (87%) of calcined sample was estimated
with the formula proposed by Toraya et al [16]. Powder XRD pattern of Preyssler
sample showed diffraction peaks at 2 = 6.6o, 8.8o, 12o, 13.6o, 17.6o, 25.5o and 33.7o. The
observed diffraction peaks for this sample are in accordance with reported for Preyssler
POM [17].
Preyssler
Wells-Dawson
Keggin
(NH ) NaP
W
O
5
4 13
30 110
(NH )
H P O (W O )9
6 2 2
2 7
4 6
(NH ) PW
O
4 3
12 40
NH Cl
4
(NH ) PW
4 3
O
12 40
5
10 15 20 25 30 35 40 45 50
2 (degrees)
Fig.1: Powder X-ray diffraction patterns of the samples
3.2. DRIFT analysis
The DRIFT spectrum (Fig. S1)
of the Keggin structured POM shows
-1
-1
-1
-1
characteristic bands at 1075 cm , 983 cm , 885 cm and 798 cm corresponding to (P-
Od), (W-Oa), (W-Ob-W) and (W-Oc-W) functional groups [18]. As shown in Fig. 2, the
DRIFT spectrum of [NH4]6P2W18O62 sample showed strong vibration bands at 1090 cm-1,
-1
-1
962 cm , 914 and 780 cm attributed to asymmetric stretching vibration of the PO4
tetrahedron, terminal W=O and W-O-W bonds, in agreement to the Wells-Dawson POM

ACCEPTED MANUSCRIPT
spectrum reported in the literature [19]. The Preyssler POM showed characteristic peaks
corresponding to P-O stretching at 1163, 1079 and 1022 cm-1, and two bands
-1
-1
corresponding to W-O-W bonding at 941 and 913 cm ; a band at 757 cm attributed to
W=O; and another band at 536 cm-1 related to P-O bending. The observed peaks are in
quite accordance with the literature report [20]. The DRIFT pattern of Preyssler POM is
different from Keggin and Wells-Dawson POMs, because its structural arrangement
contained of five [PW6O22] entities, which are resulted from [PW12O40]3- ions by
elimination of two sets of three corner-sharing WO6 octahedron [21]. In addition to the
characteristic bands of three POM structures, the DRIFT spectra of POM samples
-1
-1
exhibited two sharp IR bands at 1615 cm and 1410 cm due to N-H stretching and O-H
bending vibrations of NH4 and H2O moieties [7].
3.3. DRUV-vis analysis
DRUV-vis spectra of POMs are shown in Fig. S2. Appearance of multiple
absorption peaks in the UV-vis spectra of POMs is due to different structural oxygen
atoms presented in their structure [22]. Keggin POM showed DRUV-vis bands at 255 nm
and 320 nm. On other hand, the spectra of Wells-Dawson and Preyssler-POMs exhibited
absorption bands in the region of 220-550 nm. The peaks at lower wavelengths are
associated with ligand-metal charge transfer from O atoms to W(VI) species in the POM
structures. The position of the peak is at 320 nm for Keggin POM, which is
corresponding to W-O-W bonding and this peak is shifted towards higher wavelengths in
case of Wells-Dawson and Preyssler POMs [430 nm: Wells-Dawson, 460 nm: Preyssler].
The broadening of the peak could be related to increase in the number of the counter
cations in the POM structure [23].

ACCEPTED MANUSCRIPT
3.4. Textural properties
Fig. 2 represents the N2 adsorption-desorption isotherms for POM materials. The
three POM samples exhibited type-IV isotherms, which are characteristic of mesoporous
powders. However, an increase of adsorption at very low pressure was observed in all the
three samples, indicating that the samples also possessed micropores along with the
mesopores. Okamoto et al [24] and Inumaru [25] synthesized [NH4]3PW12O40 by
neutralizing the phosphotungstic acid with different ammonium sources and observed a
surface area between 90-130 m2 g-1. It was also previously reported that ammonium salt
of Keggin POM is microporous and/or mesoporous and the porosity of the material is
derived from aggregation of nanometer size crystallites of POMs [26].
114
Preyssler
95
76
57
Wells-Dawson
85
68
51
34
144
Keggin
108
72
36
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
P/Po
Fig. 2: N2 adsorption-desorption isotherms of POM samples
To investigate the particles aggregation in samples, FE-SEM analysis was
performed (Fig S3). It can be seen that Keggin POM exhibited aggregates of spherical
shape with size in the range of 1.0-0.5 μm. In contrast, Wells-Dawson and Preyssler

ACCEPTED MANUSCRIPT
POM samples form tightly packed aggregates of non-uniform particles having average
sizes about 0.5-0.25 μm. As expected, the three samples possessed different hysteresis
loops. Keggin and Wells-Dawson structured POMs showed H2 type hysteresis loop,
which reveals that the synthesized POM materials have bottle neck shaped pores [27].
Presence of H4 type hysteresis, which is associated with narrow pores is clearly observed
in Well-Dawson POM. The pore diameter, pore volume and BET surface area of the
POM samples are determined from the N2-physisorption measurements. The surface area
of Keggin POM (175 m2g-1) is significantly higher than those of both Preyssler (26 m2g-1)
and Wells-Dawson (14 m2g-1) structured POMs. The average pore diameter of Wells-
Dawson POM is the highest (35 Å) than other two POM samples [Keggin: 24 Å and
Preyssler: 20 Å]. The pore volume is the highest for Wells-Dawson structured POM,
0.113 cm3g-1, while Keggin and Preyssler structured POMs showed pore volumes 0.062
cm3g-1, 0.046 cm3g-1 respectively.
3.5. Catalytic ammoxidation of 2-methylpyrazine
Influence of reaction temperature on 2-methylpyrazine conversion and 2-
cynaopyrazine selectivity over three POM samples is presented in Fig. 3. All the POM
samples exhibited rise in 2-methylpyrazine conversion with increase of reaction
temperature from 360 to 400°C. Comparison of 2-methylpyrazine conversion and 2-
cyanopyrazine selectivity of the POMs samples reveals that Preyssler POM showed
superior performance than other two POMs under the identical reaction conditions. The
Preyssler POM offered a maximum conversion of 85% with 2-cyanopyrazine selectivity
of 96%, while Keggin structured POM is less active compared to Preyssler and Wells-
Dawson POM samples, demonstrated a maximum conversion of 66% with selectivity

ACCEPTED MANUSCRIPT
o
towards 2-cyanopyrazine of 82% at a reaction temperature of 400 C. The major
byproduct observed is the pyrazine and the formation of COx is negligible in all POM
catalysts. It is clear from the results that the selectivity to 2-cyanopyrazine decreased with
o
o
increase of reaction temperature from 360 C to 400 C in all the POM materials,
however the extent of decrease is small (from 98% to 96%) in Preyssler POM.
100
98
96
94
92
90
88
86
84
82
9800
Keggin
Wells-Dawson
80
Preyssler
70
60
50
40
360
370
380
390
400
Reaction temperature (oC)
Fig. 3: Influence of reaction temperature on 2-methylpyrazine conversion and 2-
cyanopyrazine selectivity
The variations in 2-methylpyrazine conversion and 2-cyanopyrazine selectivity
with reaction time also studied. The reaction was of the POM samples was investigated
under previously optimized feed composition (2-methylpyrazine, water, oxygen and
o
ammonia = 1:13:7:38) and at the reaction temperature of 380 C. The ammoxidation
activity of POM samples after 10 h is presented in Table S1. The results indicating that
Preyssler POM exhibited superior performance that it offered higher rate of reaction. The
reaction was continued for 24 h to perform the time on stream analysis and the results are

ACCEPTED MANUSCRIPT
presented in Fig. S4. The Preyssler POM has exhibited higher stability than other two
POM samples. The 2-methylpyrazine conversion and 2-cyanopyrazine selectivity have
not changed significantly over 24 h and the POMs are resistant to deactivation.
Keggin
Wells-Dawson
Preyssler
200
300
400
500
600
700
800
900
Temperature (oC)
Fig. 4: H2-TPR patterns for POM samples
It is well known that rate of ammoxidation is controlled by acidity and redox
properties of catalyst. The redox properties and strength of acid sites of the samples were
determined by H2-TPR and potentiometric titration of samples with n-butyl amine
respectively. The H2-TPR patterns of POM samples are shown in Fig. 4. From the H2-
TPR results, it can be understood that the synthesized three POM samples exhibited two
o
reduction peaks around 500-515 C and 790-800 °C. The first reduction peak for the three
samples can be attributed to the reduction of W6+ to a lower valence state W5+ and/or W4+
o
[28]. Since, the POM structures starts to decompose to WO3 and P2O5 (below 500 C) as
it observed in the temperature programed decomposition patterns. The reduction peak in
o
the 515 C represent not only the reduction of octahedrally coordinated W6+ in the POM
structure, but also the reduction of WO3 formed by the decomposition of POM in the

ACCEPTED MANUSCRIPT
reductive environment. The reduction of W4+ to lower oxidation states could have
happened at higher temperature (790-800 °C) [29].
3.0
Preyssler
2.5
2.0
1.5
1.0
0.5
0.0
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Wells-Dawson
0.8
Keggin
0.6
0.4
0.2
0.0
250 300 350 400 450 500 550 600 650
o
Temperature ( C)
Fig.5: NH3-TPD patterns of POM samples
Bondareva et al [32] proposed the ammoxidation mechanism that 2-
methylpyrazine transforms into oxygen containing intermediate complexes (aldehyde and
asymmetric carboxylate) by interacting with oxygen and Lewis acid sites of catalyst. It is
known that ammonia could partially reduce the catalyst and it also compete with the

ACCEPTED MANUSCRIPT
oxygen to adsorb on the surface of catalyst. Due to the competitive adsorption, ammonia
blocks some active sites and this is beneficial for the formation of nitrile, ammonia
arrests undesirable total oxidation of 2-methylpyrazine. Among the studied POMs,
Preyssler POM possessed more number of redox sites and ammonium cations in the
secondary structure. This is the main reason for the superior performance of Preyssler
+
POM. Centi et al [33] and Martin et al [34] reported that presence of NH4 ions
influences the catalyst acid-base properties and the reaction of oxygenated species with
+
NH4 ions proceeds fast to form nitrile and water [35]. The NH3-TPD profiles of the
POM samples are shown in Fig. 5. Along with NH3-TPD experiments, decomposition
behavior of three POM samples under neutral atmosphere also studied by the same
procedure except saturation of sample with ammonia at the beginning. The
decomposition patterns of POM samples are represented by red lines in Fig. 5. The
Keggin and Wells-Dawson samples showed main desorption peak between 350 and 530
o
oC, however the Preyssler POM showed peak in between 525 and 600 C. The NH3-TPD
samples clearly revealed that the Preyssler POM has the highest ammonium content
among the POM samples [Keggin: 3.84 mmolg-1, Wells-Dawson: 7.59 mmolg-1,
Preyssler: 18.16 mmolg-1]. The observed ammonium content include the ammonia
adsorbed at the acid sites of the POMs and evolved ammonia due to the decomposition of
the ammonium salts. It is known that POM samples generally possesses Lewis and
Brønsted acid sties [1]. It has been reported that desorption peak at lower temperature is
due to the ammonia adsorption over Lewis acid sites, while desorption peak at higher
temperature attributed to the ammonia adsorption on Brønsted acid sites [23].

ACCEPTED MANUSCRIPT
Pizzio et al [30] interpreted the potentiometric titrations results that initial
electrode potential (Ei) could be considered as the acid strength of surface sites and the
range, where the plateau is reached as the total number of acid sites. The acid strength of
sample can be considered as very strong when Ei > 100 mV, strong if 0 < Ei < 100 mV,
weak when -100 < Ei < 0 mV, and very weak site if Ei < -100 mV [31]. The acidic
strengths of Keggin, Wells-Dawson and Preyssler POM catalysts are found to be strong
(Fig. S5). The Preyssler POM showed high measured electrode potential implying the
presence of stronger acid sites than other two POMs.
A comparison among the ammonia content of POM, 2-methylpyrazine conversion
and 2-cyanopyrazine selectivity has been obtained (Fig. 6). The Preyssler POM possessed
much more ammonia content than other two POMs and offered highest 2-methylpyrazine
conversion and 2-cyanopyrazine selectivity. A good correlation among them indicates
that presence of more number of ammonium cations helps to obtain better 2-
methylpyrazine conversion and selectivity to 2-cyanopyrazine.
100
Sel. to 2-cyanopyrazine (%)
Con. of 2-methylpyrazine (%)
NH content (mmolg-1)
3
80
60
40
20
0
Keggin
Wells-Dawson
Preyssler
Fig. 6: Comparison of ammonia content of POM with ammoxidation activity; molar ratio
of 2-methylpyrazine: H2O:NH3: air=1:13:7:38; reaction temperature: 400 oC; reaction
time: 2h; catalyst weight: 1.0g

ACCEPTED MANUSCRIPT
Conclusions
Three different ammonium tungstophosphate POM samples with Keggin, Wells-
Dawson and Preyssler structures were successfully synthesized by different preparation
methods. The XRD, DRIFT, DRUV-vis, SEM, NH3-TPD and N2-physisorption
techniques were used to determine the physico-chemical properties of the samples. The
POM samples were used as catalysts for 2-methylpyrazine ammoxidation to produce 2-
o
cyanopyrazine in the range of 360-400 C. The catalytic activity results revealed that
Preyssler POM sample is efficient among the three samples, and it offered 69% of 2-
methylpyrazine conversion with 98% selectivity to 2-cyanopyrazine at 380 oC. The
superior catalytic performance of Preyssler POM is mainly due to the fact that this
sample possessed superior redox property, relatively strong acid sites and large number
of ammonium cations in its secondary structure, which enhanced the transformation of
oxygenated 2-methylpyrazine species into 2-cyanopyrazine. For the first time, a
correlation was deduced between the amount of ammonium content of the POM samples
and their catalytic ammoxidation activity.
Acknowledgements
This work was supported by the Deanship of Scientific Research (DSR), King
Abdulaziz University, Jeddah under grant No. (D-80-130-1438). The authors, therefore,
gratefully acknowledge the DSR technical and financial support.

ACCEPTED MANUSCRIPT
References
[1] I. Kozhevnikov, Catalysis by Heteropoly Acids and Multicomponent
Polyoxometalates in Liquid-Phase Reactions, Chem. Rev. 98 (1998) 171-198.
[2] M. Pope, Heteropoly and Isopoly Oxometalates, Springer, Berlin, 1983
[3] A. Predoeva, S. Damyanova, E.M. Gaigneaux, L. Petrov, Total oxidation of Cl-
containing VOCs over mixed heteropoly compounds derived catalysts, Catal. Today 128
(2007) 208-215.
[4] L. Briand, G. Baronetti, H. Thomas, The state of the art on Wells–Dawson
heteropoly-compounds: A review of their properties and applications, Appl. Catal. A:
Gen. 256 (2003) 37-50.
[5] S. Wu, J. Wang, W. Zhang, X. Ren, Preparation of Keggin and Preyssler
heteropolyacid catalysts on amine-modified SBA-15 and their catalytic performances in
esterification of n-butanol with acetic acid, Catal. Lett. 125 (2008) 308-314.
[6] A. Alshammari, V. N. Kalevaru, N. Dhachapally, A. Kockritz, A. Bagabas, A. Martin,
Nanosize gold promoted vanadium oxide catalysts for ammoxidation of 2-
methylpyrazine to 2-cyanopyrazine, Top Catal. 58 (2015) 1062-1068.
[7] K. Narasimharao, N. Lingaiah, I. Suryanarayana, P.S. Sai Prasad, A comparison of
structure and catalytic functionality of 12-molybdophosphoric acid and its ammonium
salt in the ammoxidation of 2-methylpyrazine to 2-cyanopyrazine, Catal. Lett. 90 (2003)
31-38.

ACCEPTED MANUSCRIPT
[8] K. Narasimharao, R. Gopinath and P. S. Sai Prasad, Highly selective molybdenum
phosphate catalyst for the ammoxidation of 2-methylpyrazine to 2-cyanopyrazine, Green
Chem. 3 (2001) 20-22.
[9] I. Kozhevnikov, Sustainable heterogeneous acid catalysis by heteropoly acids, J. Mol.
Catal. A: Chem. 262 (2007) 86-92.
[10] V. N. Kalevaru, N. Dhachapally, A. Martin, Catalytic performance of lanthanum
vanadate catalysts in ammoxidation of 2-methylpyrazine, Catalysts 6 (2016)10.
[11] D. Lapham, J. B. Moffat, Preparative effects on the surface area and pore structure
of microporous heteropoly oxometalates, Langmuir 7 (1991) 2273-2278.
[12] L.E. Briand, G.M. Valle, H.J. Thomas, Stability of the phospho-molybdic Dawson-
type ion P2Mo18O626− in aqueous media, J. Mater. Chem. 12 (2002) 299-304.
[13] I. Creaser, M. C. Heckel, R. J. Neitz, M. T. Pope, Rigid nonlabile polyoxometalate
(15-n)-
cryptates [ZP5W30O110
]
that exhibit unprecedented selectivity for certain lanthanide
and other multivalent cations, Inorg. Chem. 32 (1993) 1573-1578.
[14] N. Misono, M. Tateishi, M. Iwamoto. Direct oxidation of isobutane into methacrylic
acid and methacrolein over Cs2.5Ni0.08-substituted H3PMo12O40, J. Chem. Soc. Chem.
Commun. (1994) 1411-1412.
[15] E. Arendt, S. Ghislain, E.M. Gaigneaux, Operando investigation of the catalytic
behavior of Wells-Dawson heteropolycompounds in the oxidation of propene, Catal.
Today 155 (2010) 227-240.

ACCEPTED MANUSCRIPT
[16] H. Toraya, M. Yoshimura, S. Somiya, Calibration Curve for Quantitative Analysis
of the Monoclinic-Tetragonal ZrO2 System by X-Ray Diffraction, J. Am. Ceram. Soc. C
67 (1984) 119-121.
[17] H. Eshghi, A. Javid, A. Khojastehnezhad, F. Moeinpour, F. F. Bamoharram, M.
Bakavoli, M. Mirzaei, Preyssler heteropolyacid supported on silica coated NiFe2O4
nanoparticles for the catalytic synthesis of bis(dihydropyrimidinone)benzene and 3,4-
dihydropyrimidin-2(1H)-ones, Chinese J. Catal. 36 (2015) 299-307.
[18] C. Rcchioccioli-Delteheff, M. Fournier, R. Frank, Vibrational investigations of
polyoxometalates. 2. Evidence for anion-anion interactions in molybdenum (VI) and
tungsten (VI) compounds related to the Keggin structure, Inorg. Chem. 22 (1983) 207-
216.
[19] G.T. Baronetti, H. Thomas, C.A. Querini, Wells-Dawson heteropolyacid supported
on silica: isobutane alkylation with C4 olefins, Appl. Catal. A: Gen. 217 (2001) 131-141.
[20] M. Arabi, M. Amini, M. Abedini, A. Nemati, A. Alizadeh, Esterification of phthalic
anhydride with 1-butanol and 2-ethylhexanol catalyzed by heteropolyacids, J. Mol. Catal.
A: Chem. 200 (2003) 105-110.
[21] D. M. Ruiz, G. P. Romanelli, P. G. Vazquez, J. C. Autino, Preyssler catalyst: An
efficient catalyst for esterification of cinnamic acids with phenols and imidoalcohols,
Appl. Catal. A: Gen. 374 (2010) 110-119.
[22] P.E. Car, B. Spingler, S. Weyeneth, J. Patscheider, G.R. Patzke, All-inorganic 1D
chain-based architecture of a novel dimanganese-substituted Keggin polyoxotungstate,
Polyhedron 52 (2013) 151-158.

ACCEPTED MANUSCRIPT
[23] I. V. Kozhevnikov, Catalysis for Fine Chemical Synthesis, Catalysis by
Polyoxometalates, Wiley, New York, 2002, p. 2
[24] K. Okamoto, S. Uchida, T. Ito, N. Mizuno, Self-Organization of All-
Inorganic Dodecatungstophosphate Nanocrystallites, J. Am. Chem. Soc. 129 (2007)
7378-7384.
[25] K. Inumaru, '“Sponge Crystal”: a novel class of microporous single crystals formed
by self-assemby of polyoxometalate (NH4)3PW12O40 nanocrystallites, Catal. Surv. Asia
10 (2006) 151-160.
[26] M. Thommes, K. Kaneko, A. V. Neimark, J. P. Olivier, F. Rodriguez-Reinoso, J.
Rouquerol, K. S. W. Sing, Physisorption of gases, with special reference to the evaluation
of surface area and pore size distribution (IUPAC Technical Report), Pure Appl. Chem.
87 (2015)1051-1069.
[27] K.S.W. Sing, D. H. Everett, R.A.W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol,
T. Siemieniewska, Reporting physisorption data for gas/solid systems with special
reference to the determination of surface area and porosity (Recommendations 1984),
Pure Appl. Chem. 57 (1985) 603-619.
[28] K. M. Parida, S. Rana, S. Mallick, D. Rath, Cesium salts of heteropoly acid
immobilized mesoporous silica: An efficient catalyst for acylation of anisole, J. Colloid
Interface Sci. 350 (2010) 132-139.
[29] T.
Rajkumar, G. Ranga Rao, Porous hydrous zirconia supported 12-
tungstophosphoric acid catalysts for liquid-phase esterification of 2-ethyl-1-hexanol, J.
Mol. Catal. A: Chem. 295 (2008) 1-9.

ACCEPTED MANUSCRIPT
[30] L.R. Pizzio, P.G. Vazquez, C.V. Caceres, M.N. Blanco, Supported Keggin type
heteropolycompounds for ecofriendly reactions, Appl. Catal. A: Gen. 256 (2003) 125-
139.
[31] P. Villabrille, G. Romanelli, P. Vazquez, C. Caceres, Vanadium-substituted Keggin
heteropolycompounds as catalysts for ecofriendly liquid phase oxidation of 2,6-
dimethylphenol to 2,6-dimethyl-1,4-benzoquinone, Appl. Catal. A: Gen. 270 (2004) 101-
111.
[32] V.M. Bondareva, T.V. Andrushkevich, E.A. Paukshtis, N.A. Paukshtis, A.A.
Budneva, V.N. Parmon, Investigation of the reaction mechanism of methylpyrazine
ammoxidation on vanadia–titania catalyst by FTIR in situ, J. Mol. Catal. A: Chem. 269
(2007) 240-245.
[33] G. Centi, F. Marchi, S. Perathoner, Surface chemistry of V-Sb-oxide in relation to the
mechanism of acrylonitrile synthesis from propane. Part 2.-Reactivity towards ammonia and
relationship with catalytic behavior, J. Chem. Soc., Faraday. Trans., 92 (1996) 5151-5159.
[34] A. Martin, H. Berndt, B. Lücke, M. Meisel, Reaction pathway of benzonitrile
formation during toluene ammoxidation on vanadium phosphate catalysts, Topics in
Catal. 3 (1996) 377-386.
[35] Y. Goto, K. Shimizu, T. Murayama, W. Ueda, NH3-efficient ammoxidation of
toluene by hydrothermally synthesized layered tungsten-vanadium complex metal oxides,
Appl. Catal. A: Gen. 509 (2016) 118-122.

ACCEPTED MANUSCRIPT
Graphical Abstract
Wells-
Preyssler
Dawson
POM
Keggin
POM
POM
(NH4)3PW12O40
(NH4)6P2W18O62 (NH4)13NaP5W30O110
Vapor Phase Ammoxidation

ACCEPTED MANUSCRIPT
Highlights
 Keggin, Wells-Dawson & Preyssler structured tungstophosphates are synthesized.
 Synthesized POMs are tested for vapor phase ammoxidation of 2-methylpyrazine.
 Preyssler POM offered the best ammoxidation performance.
+
 Presence of more NH4 ions in Preyssler POM is responsible for high activity.
 First report on correlation of between POM structure and ammoxidation activity.