Effect of P/Fe ratio on the structure and ammoxidation
functionality of Fe-P-O catalysts
P. Nagaraju, Ch. Srilakshmi, Nayeem Pasha, N. Lingaiah,I. Suryanarayana, P.S. Sai Prasad *
Inorganic & Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad-500 007, India
Received 9 August 2006; received in revised form 12 April 2007; accepted 20 April 2007
Available online 25 April 2007
Abstract
Bulk FePO4 catalysts, with varying P/Fe atomic ratio in the range of 1–1.6, were prepared and characterized by XRD, FT-IR, TPR,
Potentiometric titration, Laser Raman, TEM, XPS and TG/DTA techniques in order to study the influence of P/Fe atomic ratio on the nature and
extent of the active phase formation. The data obtained from XRD and Laser Raman techniques suggested predominant formation of the quartz
type iron phosphate at close to stoichiometric P/Fe ratio, but as the ratio increased beyond 1.4 a progressive transformation of monomeric
phosphate into its polymeric form was observed. XPS spectra reflected the presence of iron in its 3+ state when P/Fe � 1.2 and exists as Fe2+ and
Fe3+ when P/Fe � 1.4. The catalytic properties of these iron phosphates were studied in the vapor phase ammoxidation, taking 2-methylpyrazine
(MP) to 2-cyanopyrazine (CP) as an example. The ammoxidation activity of the catalysts was found to be proportional to the extent of quartz phase
formed which in turn was proportional to the redox property, as observed by the oxidation functionality of the catalysts in benzyl alcohol
transformation. However, the selectivity to nitrile was found to be dependent on the acid strength of the catalysts.
# 2007 Published by Elsevier B.V.
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Applied Catalysis A: General 334 (2008) 10–19
Keywords: Iron phosphate; Ammoxidation; 2-Methyl pyrazine; 2-Cyano pyrazine
1. Introduction
Ammoxidation refers to the interaction of ammonia with a
reducible organic compound in the presence of oxygen [1,2].
For this reaction the catalyst should possess both acidic and
redox functionalities [3]. Vanadium based catalysts are
extensively studied in the ammoxidation reactions [4–6].
However, they operate at high reaction temperatures and
provide higher conversions but low product selectivity [7–10]
due to simultaneous decomposition of ammonia. It is reported
that the functionality of vanadium based catalysts can be
improved by converting them into their phosphates, such as
(VO)2P2O7 [11]. Selectivity plays a central role in developing
heterogeneous catalysts in industrial catalytic processes as
waste reduction and thereby environmental protection is the
order of the day [12]. Therefore, high selectivity at reasonable
* Corresponding author. Tel.: +91 40 27193163; fax: +91 40 27160921.
E-mail address: [email protected] (P.S.S. Prasad).
0926-860X/$ – see front matter # 2007 Published by Elsevier B.V.
doi:10.1016/j.apcata.2007.04.024
conversion has also been recognized as a better option in
ammoxidation catalysis [13].
In recent times much attention has been focused on metal
phosphates for various oxidation and oxidative dehydrogena-
tion reactions [14]. The existence of redox and acidic sites on
the surface of metal phosphates is found to be responsible for
the catalytic reaction [15]. In this regard iron phosphate, which
possesses both acidic and redox properties like molybdenum
and vanadium phosphates, has received less attention [16,17].
Recently, it has been found that iron phosphate shows a unique
property of offering very high selectivity in oxidative
dehydrogenation reactions as its oxygen insertion capacity is
low due to the absence of M O bond, unlike the phosphates of
vanadium and molybdenum [18–20]. A mixed valence iron
hydroxy phosphate compound is employed industrially in the
production of methacrylic acid from isobutyric acid [16,21].
Wang and Otsuka [22,23] have reported exceptional selectivity
for FePO4 in methane and ethane oxidation by O2, with high
yields to partially oxygenated products, observed in the
presence of H2 or N2O. FePO4 also exhibits high activity
and yield in the oxidation of methanol to formaldehyde [24].
Scheme 1. Generalized reaction scheme of ammoxidation of 2-methyl pyrazine.
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–19 11
As a part of our on-going study on ammoxidation of methyl
pyrazine (MP) to cyanopyrazine (CP) (Scheme 1), we have
recently reported [25] the activity and selectivity functionalities
of various bulk phosphate catalysts and revealed that FePO4
offers high selectivity (�98%) towards CP. However, very low
conversion (20%) has been obtained. It is felt that by optimizing
the reaction parameters and by identifying the proper P/Fe ratio
the conversion could be improved, retaining the high
selectivity. This has prompted us to take up a detailed study
on the FePO4 catalysts. In the present study various iron
phosphate catalysts were prepared with different P/Fe atomic
ratios ranging from 1 to 1.6 to study the effect of structural
changes on the catalytic activity and selectivity.
2. Experimental
2.1. Preparation of catalysts
Various FePO4 catalysts were synthesized adopting the
procedure followed by Wang et al. [26]. Required quantities of
aqueous iron nitrate (Fluka, Germany) and ammonium
dihydrogen phosphate (Loba Chemicals, India) were mixed
thoroughly such that the P/Fe ratio was maintained at a value in
the range of 1–1.6. The excess water was evaporated on a water
bath to yield a whitish yellow paste. The catalyst masses were
further dried at 393 K for 12 h in an air oven. Finally they were
calcined in air at 823 K for 4 h.
2.2. Characterization of catalysts
X-ray diffraction (XRD) patterns of the catalysts were
obtained on a Rigaku Miniflex diffractometer using Cu Ka
radiation. The extent of quartz phase formed, as represented by
the area of the peak at 2u = 25.8 A8 in the X-ray difractogram,
was calculated as a product of height of the peak and its full
width at half maximum (FWHM). BET surface area was
determined on a Micromeritics (Auto Chem-2910) instrument
with nitrogen physisorption at 77 K, taking 0.169 nm2 as the
cross sectional area of di-nitrogen. FT-IR spectra were recorded
on a DIGILAB (USA) spectrometer, with a resolution of
1 cm�1 using KBr disc method.
The Raman spectra of the samples were collected on a
UV–vis Raman spectrometer system (Horiba-Jobin Yvon
LabRam-HR) equipped with a confocal microscope, 2400/
900 grooves/mm gratings, and a notch filter. The UV laser
excitation at 325 nm was supplied by a Yag doubled diode
pumped laser (20 mW). The scattered photons were directed
and focused onto a single-stage monochromator and measured
with a UV-sensitive LN2-cooled CCD detector (Jobin Yvon
CCD-3000V). The catalyst samples in powder form (about
5–10 mg) were usually loosely spread onto a glass slide below
the confocal microscope for Raman measurements. Transmis-
sion electron microscope (TEM) photographs were obtained
using Tecnai-12 FEI instrument.
Temperature programmed reduction (TPR) was performed
in a flow system wherein a 5% H2/Ar mixture gas was
introduced into the reactor containing about 300 mg of the
sample at a flow rate of 30 ml/min with a temperature ramp of
10 K/min. Before the TPR analysis the catalyst was pretreated
in a flow of Ar at 523 K for 2 h. The consumption of hydrogen
was monitored using a thermal conductivity detector. XPS
measurements of the catalysts were conducted on a Kratos Axis
165 apparatus equipped with a dual anode (Mg and Al) using
the Mg Ka source. The C 1s binding energy of 284.6 eV was
used as a reference for determining the binding energies. A
charge neutralization of 2 eV was used to balance the charge up
of the sample. The binding energy values of the samples were
reproducible within �0.1 eV.
Diffuse reflectance spectra of the catalyst samples were
recorded in the UV–vis region (200–800 nm) with GBC Cintra
10e spectrometer at a slit width of 1.5 nm and a scan speed of
400 nm/min. Pellets were prepared by taking about 15 mg of
the catalyst sample in 185 mg of dried KBr powder, grinding
the samples thoroughly to ensure uniform mixing prior to
pelletization. The spectra were recorded at room temperature.
The TG/DT analysis was carried out on a Mettler-Toledo
apparatus. With a sample weight of ca. 30–40 mg, the tests were
performed under nitrogen flux in the temperature ranging from
298 to 1173 K and at a heating rate of 10 K/min.
The acidity of the solid samples was measured by the
potentiometer titration method. A known mass of solid,
suspended in acetonitrile, was stirred for 3 h and then the
suspension was titrated with a solution of 0.05 N n-butyl
amine in acetonitrile at a flow rate of 0.05 ml/min. The variation
in the electrode potential was measured with an instrument
(Automatic titrator, Schott GmbH, Germany) having a digital
pH meter, using a standard calomel electrode. The potentio-
metric titration was performed with a glass electrode. The
instrument was calibrated using standard buffer solutions. The
acidity of the catalysts measured by this technique enabled
determination of their strength [27].
2.3. Catalytic reaction
The ammoxidation reaction was carried out in a vertical
fixed bed, continuous down flow quartz micro-reactor under
atmospheric pressure. In a typical experiment, about 3 g of the
catalyst (sieved to 18/25 BSS mesh, to avoid mass transfer
limitations) diluted with an equal amount of quartz grains, was
packed between two layers of quartz wool in the reactor. The
upper portion of the reactor was filled with quartz beads that
served both as a pre-heater and a mixer for the reactants. An
Fig. 1. XRD patterns of (a) P/Fe 1.0; (b) P/Fe 1.1; (c) P/Fe 1.2; (d) P/Fe 1.4; (e)
P/Fe 1.6 catalysts; (~) corresponds to FePO4 (Q); (*) corresponds to
Fe3(P2O7)2; (*) Fe2P2O7.
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–1912
aqueous mixture of MP (MP:water = 1:2.5 v/v) was fed into the
reactor by means of a microprocessor controlled metering
pump (B. Braun, Germany). The molar ratio of the feed
was kept at MP:water:ammonia:air = 1:13:17:38, maintaining a
W/F liquid ratio = 2.5 g cm�3 h. The reaction temperature was
monitored by a thermocouple with its tip located in the catalyst
bed and connected by a PID-type temperature indicator-
controller. The reaction occurred in the flow integral mode. It
was assumed that there were no limitations to mass and heat
transfer. However, the present study is not devoted to the kinetic
aspects. The reaction was carried out at various temperatures
ranging from 633–693 K. The liquid product collected from a
cold trap kept at 263 K, after the catalyst had attained a steady
state, was analysed by gas chromatography. From the analysis
of non-condensable mixture, it was ensured that the quantity of
any organic species was negligible in the exit gas. The steady
state was attained after 1 h. The conversion was calculated
based on the disappearance of MP and the selectivity to CP was
calculated as follows:
% Selectivity ¼ moles of CP formed
moles of MP converted� 100
3. Results and discussion
3.1. Characterization of catalysts
3.1.1. BET surface area of the catalysts
The specific surface areas of the FePO4 catalysts are shown
in Table 1. The sample with the lowest phosphorus content
possessed a surface area of 7.4 m2/g. However, the surface area
decreased with an increase in phosphorus content. Ai et al. [28]
have also observed decrease in surface area with increase in
phosphorous content as well as calcination temperature. This
may be due to formation of poly-condensed phosphates that
filled the pores.
3.1.2. X-ray diffraction studies
Powder X-ray diffraction patterns of the catalysts calcined at
823 K are presented in Fig. 1. The FePO4 catalysts, with their P/
Fe ratio close to the stoichiometric value of 1.0, exhibited mainly
a quartz-like phase (2u = 25.88) with a small amount of the
tridymite phase represented by peaks appearing at 20.2, 20.958.However, the line at 2u = 20.28 could also be from the (1 0 0)
Table 1
The BET surface area and benzyl alcohol oxidation activitya data obtained for
the iron phosphate catalysts
(P/Fe)
ratio
Surface
area (m2/g)
Temperature (K)
523 543 563 583
1.0 7.4 25.8 28.2 33.5 41.2
1.1 4.5 30.0 34.5 37.8 46.5
1.2 3.8 20.8 23.6 28.0 36.8
1.4 1.6 16.2 21.0 24.6 31.3
1.6 1.2 12.3 19.2 21.4 27.6
a Percentage conversion with a (W/Fliquid ratio = 0.5 g cm�3 h, air flow =
60 ml/min).
plane of a quartz-like phase. Such a result has also been reported
by Wang et al. [26]. Beale and Sankar [29] have revealed that the
tridymite phase is normally formed at a temperature of 373 K
during heating of the precursor of iron phosphate prepared by
hydrothermal synthesis. The tridymite then gets transformed into
the quartz phase at temperature above 773 K. With increasing
P/Fe ratio from 1.2 to 1.6 there was a decrease in the intensity of
the XRD peaks corresponding to the quartz phase. The XRD
pattern of the catalyst with a P/Fe ratio 1.4 exhibited new peaks at
2u values of 14.48, 21.26, 24.84, 34.868. These peaks correspond
to the Fe3(P2O7)2 phase. The doublet at 2u: 29.8 and 31.08corresponds to the formation of Fe2P2O7 [16]. The intensity of
the lines related to Fe3(P2O7)2 and Fe2P2O7 were found to be
increasing with increase in P/Fe ratio of the catalysts.
3.1.3. Fourier transform infra red spectroscopy
The FT-IR spectra of the FePO4 catalysts are presented in
Fig. 2. The spectra of all the catalysts showed a very broad band
at 3400 cm�1 corresponding to surface hydroxyl groups. The
band at 1610 cm�1 was due to bending vibrational mode of
water molecules. It is well known that the peaks in the region
between 1200 and 500 cm�1 correspond to the symmetric and
asymmetric vibrations of the phosphate groups. The spectra of
all catalysts exhibited a broad band at 1063 cm�1 due to
asymmetric stretching mode of the PO4 group and a band at
500 cm�1 corresponding to the bending vibrational mode of
phosphate ion [30]. The spectra of catalysts with P/Fe ratio 1.4
and 1.6 showed shoulders at 744 and 548 cm�1 which could be
due to the symmetric stretching mode of (P–O–P) bond in
pyrophosphate and the band at 548 cm�1 to the bending mode
of (O–P–O) bond. Samuneva et al. [31] have also observed the
bands at 744 and 548 cm�1 for the pyrophosphate group. Thus,
Fig. 2. FT-IR spectra of (a) P/Fe 1.0; (b) P/Fe 1.1; (c) P/Fe 1.2; (d) P/Fe 1.4; (e)
P/Fe 1.6 catalysts.
Fig. 3. UV-DRS patterns of (a) P/Fe 1.0; (b) P/Fe 1.1; (c) P/Fe 1.2; (d) P/Fe 1.4;
(e) P/Fe 1.6 catalysts.
Fig. 4. Laser-Raman spectra of (a) P/Fe 1.0; (b) P/Fe 1.1; (c) P/Fe 1.2; (d) P/Fe
1.4; (e) P/Fe 1.6 catalysts.
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–19 13
the observation obtained from FT-IR spectroscopy also
confirms the formation of monophosphate at low P/Fe ratios
and pyrophosphate at higher P/Fe ratios.
3.1.4. UV-DRS studies
The UV-DRS spectra of the FePO4 catalysts are shown in
Fig. 3. The spectra showed a broad absorption band in the range
of 250–400 nm with multiplicity. The band at 230 nm could be
assigned to the P–O charge transfer transition and the bands at
320 and 380 nm to the Fe–O charge transfer transitions [32,25].
The spectra of catalysts, with P/Fe ratio 1.4 and 1.6, also
showed a broad band at around 500–800 nm corresponding to
the charge transition of Fe3+! Fe2+, due to formation of
pyrophosphate species. The results are in good agreement with
the XRD and FT-IR results.
3.1.5. Laser-Raman studies
Fig. 4 shows the UV Raman spectra of the FePO4 catalysts.
The catalysts exhibited two intense Raman bands at 1010 and
1053 cm�1 along with weak bands at 408, 440, 596, 665 and
1167 cm�1. It is known that stretching and bending vibrations
of phosphate groups occur at 1000–1200 and 400–700 cm�1,
respectively. Tetrahedrally coordinated iron gives the Raman
bands in the same region if the UV laser of 325 nm is used.
Raman bands at 1010 and 1053 cm�1 can be attributed to
alternatively connected tetrahedral FeO4 and PO4 groups,
respectively [33]. The intensity of the band at 1010 cm�1
decreased with increase in P/Fe ratio from 1 to1.4 and was
completely nullified in catalyst with P/Fe ratio 1.6. The
1053 cm�1 band broadened with increase in P/Fe atomic ratio
to 1.6. This broadening of the Raman band might be related to
the small particles of FePO4 as observed by Wang et al. [26].
Enrichment in phosphorous and formation of poly condensed
phosphates might be the reason for the decrease in size. The
bands at 1167 and 665 cm�1 can be ascribed to the presence
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–1914
of metaphosphate groups. It is clear from Raman analysis
that the structural changes are occurring with change in P/Fe
ratios. These results are in good agreement with XRD data.
3.1.6. TEM studies
Fig. 5 shows the results of TEM studies of the FePO4
catalysts. The catalyst materials consist of discrete spherical
particles ranging between 30 and 200 nm in size, although most
of them lie in between 30 and 100 nm in diameter. The
Fig. 5. TEM micrographs of (a) P/Fe 1.0; (b) P/Fe 1.1
morphological changes that took place with increase in P/Fe
ratio are interesting. Upon increasing the P/Fe ratio from 1.0 to
1.1, the particles became more spherical with increase in size.
Beale and Sankar [29] have also noticed increase in particle size
when tridymite changes to quartz phase in the FePO4 catalysts.
Finally as the P/Fe ratio increased to 1.6 the particles appeared
featureless with considerable reduction in size. These results
are in accordance with the Raman data where the peaks were
broadened due to smaller size of the particles.
; (c) P/Fe 1.2; (d) P/Fe 1.4; (e) P/Fe 1.6 catalysts.
Fig. 6. TPR profiles of (a) P/Fe 1.0; (b) P/Fe 1.1; (c) P/Fe 1.2; (d) P/Fe 1.4; (e)
P/Fe 1.6 catalysts.
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–19 15
3.1.7. Temperature programmed reduction studies
The TPR profiles for the FePO4 catalysts are presented in
Fig. 6. The catalysts with stoichiometric P/Fe ratio exhibited
one asymmetric reduction peak with a maximum at 976 K. This
Fig. 7. (A) Thermo gravimetric profiles of the catalysts: (a) P/Fe 1.0; (b) P/Fe 1.1; (c
various iron phosphate catalysts: (a) P/Fe 1.0; (b) P/Fe 1.1; (c) P/Fe 1.2; (d) P/Fe
peak could correspond to the reduction of quartz-phase FePO4
to Fe2P2O7 [16,26]. The TPR profiles of samples with
P/Fe > 1.1 showed three reduction peaks. The first reduction
peak appeared at 873 K. By reducing the catalysts under a
gaseous mixture of iso-butyric acid, water and nitrogen
Muniyama et al. [16] have identified the formation of
Fe3(P2O7)2 as an intermediate species for the formation of
Fe2P2O7 when the P/Fe ratio exceeds 1.0. The absence of the
low temperature peak in the catalyst with P/Fe = 1 and its
presence in all other high P-containing catalysts suggests that
this peak arises due to the formation of Fe3(P2O7) 2. The third
reduction peak at 1073 K might represent complete reduction
of the Fe species, probably to the metallic state. A progressive
decrease in hydrogen consumption and a corresponding
reduction in intensity of the first peak, due to the formation
of Fe3(P2O7)2 from FePO4 implies the formation of pyropho-
sphates at higher P/Fe ratios. These results are also in good
agreement with XRD, FT-IR, Laser Raman and TEM studies.
3.1.8. Thermal stability of the catalyst
The thermogravimetric profiles of FePO4 catalysts are
shown in Fig. 7a. The corresponding DTA curves are shown in
Fig. 7b. The sample with P/Fe = 1.0 appeared to have lost
weight in two steps. The first step was in the temperature range
323–573 K. The corresponding endothermic effect was
observed at 478 K in the DTA patterns (Fig. 7b) due to loss
of water molecules. The second step started at 598 K and ended
at 671 K, with the corresponding DTA peak at 632 K due to
phase transition of FePO4 into FePO4 (quartz) and FePO4
(tridymite). The catalysts with P/Fe > 1 displayed the weight
loss in three steps. The first step in the range 323–473 K due to
loss of physically adsorbed water and the second step from
) P/Fe 1.2; (d) P/Fe 1.4; (e) P/Fe 1.6. (B) Differential thermal analysis profiles of
1.4; (e) P/Fe 1.6.
Table 2
XPS results obtained for the iron phosphate catalysts
(P/Fe)
ratio
Binding
energy of
Fe 2p3/2 (eV)
Binding
energy of
P 2p (eV)
Binding
energy of
O 1s (eV)
Surface
P/Fe atomic
ratio
1.0 712.4 133.8 529.9 0.96
1.1 712.6 133.9 529.8 2.01
1.2 712.4 134.0 530.0 2.76
1.4 712.2 132.1 532.7 2.52
1.6 712.0 132.0 532.5 3.53
Fig. 8. Potentiometric titration profiles of the catalysts: (a) P/Fe 1.0; (b) P/Fe
1.1; (c) P/Fe 1.2; (d) P/Fe 1.4; (e) P/Fe 1.6.
Fig. 9. Variation of ammoxidation activity as a function of P/Fe atomic ratio at
different reaction temperatures.
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–1916
483–573 K corresponding to loss of crystalline water (in
NH4FeP2O7�1.5H2O) with DTA peak at 569 K. We expect
that NH4FeP2O7�1.5H2O was formed during the preparation
of the catalyst in the presence of ammonium ions, as
ammonium dihydrogen phosphate was used as the precursor.
The third high temperature peak particularly observed in the
cases of (d) and (e) patterns of Fig. 7b could be assigned to
the decomposition of NH4FeP2O7 into small quantities of
FePO4 (quartz/tridymite) and the remaining into polycon-
densed phosphate phases. Ai et al. [28] have also observed
an endothermic peak due to decomposition of NH4FeP2O7 in
the temperature of 723 and 823 K. Thus, the DT/TG analysis
also supports the idea of formation of FePO4 (quartz/
tridymite) at low P/Fe ratios and the polycondensed phases at
high ratios.
3.1.9. X-ray photo electron spectroscopy
The atomic ratios of P/Fe of each sample along with the
binding energies of Fe 2p3/2, Fe 2p1/2 are summarized in
Table 2. The binding energies of Fe 2p3/2 and Fe 2p1/2 for the
FePO4 catalysts were found to be 712 and 727 eV, respectively.
These values are in agreement with those reported for Fe+3
[26]. Even though no noticeable shift was observed with the
increase in P/Fe ratio, the peaks became broadened. This
indicates that Fe was present in its +3 oxidation state when
P/Fe � 1.2 and exists as Fe2+ and Fe3+ when P/Fe is 1.4 and 1.6
in the catalysts. The catalysts with P/Fe ratio � 1.1 showed the
binding energy of P 2p at 134.0 eV (Table 2). The binding
energy fits well with that reported in the literature (133.9 eV),
which is the characteristic of PO4 group [26]. With further
increase in P/Fe ratio to 1.4 and 1.6 the binding energy value
shifted to lower positions and reached a value of 132.0 eV. The
binding energy value of 132.0 eV agrees well with the
corresponding binding energy of P 2p in P2O7 group
(132.6 eV) reported in the literature [34]. Hence, XPS studies
also confirmed the formation of pyrophosphates at higher P/Fe
ratios.
All the catalysts with P/Fe ratio from 1 to 1.2 showed the
binding energy of O 1s around 530 eV whereas the catalysts
with P/Fe ratio of 1.4 and 1.6 showed slightly higher B.E. value
of 532.7 and 532.5 eV. This higher binding energy peak can be
attributed to the bridging oxygen in P–O–P of pyrophosphate
group [35]. The results obtained from XPS agreed well with
the results of XRD and FT-IR. Since XPS is a surface technique
the values of P/Fe ratio appear to be slightly higher than that of
the bulk.
3.1.10. Acidity measurements by potentiometric titration
The potentiometric titration curves obtained with n-
butylamine are shown in Fig. 8. The acidic strength of surface
sites have been assigned [27] according to the following ranges:
very strong site, E > 100 mV; strong site, 0 < E < 100 mV;
weak site�100 < E < 0 mVand very weak site E < �100 mV.
It can be seen from the figure that very strong acid sites were
present on the iron phosphate catalysts. The acid strength of the
catalysts increased initially with increase in P/Fe ratio reaching
a maximum at a value of 1.2 and decreased with further increase
in this ratio.
3.2. Catalytic functionality of FePO4 catalysts
3.2.1. The ammoxidation activity and its dependence on
the P/Fe ratio
Fig. 9 displays the variation in the ammoxidation activity
(expressed as percent conversion of MP) of the catalysts as a
function of P/Fe ratio at different reaction temperatures. The
activity patterns showed an initial increase up to a P/Fe ratio of
1.1. A maximum conversion of 69% was achieved on the
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–19 17
catalyst with P/Fe ratio 1.1 at 693 K. The conversion increased
with increase in reaction temperature. However, the activity
decreased with increase in P/Fe ratio beyond P/Fe = 1.1 and
reached a value of 32% at a ratio of 1.6. Ai et al. [28] have also
observed a maximum in the activity of iron phosphates with a P/
Fe ratio of unity or a little higher than unity in oxidative
dehydrogenation of iso-butyric acid. The decrease in activity at
higher P/Fe ratio could be due to the formation of iron
pyrophosphate species in these catalysts. It seems that the iron
pyrophosphate phase is inactive for the ammoxidation reaction.
Wang and Otsuka [22] have also found ferrous pyrophosphate
to be less active for methane oxidation than FePO4, suggesting
that methane activation is not dependent upon the presence of
Fe2+ in the coordination environment.
3.2.2. The influence of quartz phase
Attention has been focused on the identification of the
active species responsible for the activity. A clear indication
from the XRD patterns (Fig. 1) is that the peaks due to quartz
phase are predominant in these catalysts excepting the one with
P/Fe ratio 1.6. Fig. 10 shows a fairly good linear relationship
between the extent of quartz phase formed and the conversion
calculated at 673 K. Therefore, it can be presumed that the
quartz phase is the active phase whose formation depends on
the P/Fe ratio.
According to Beale and Sankar it is clear that Fe is
tetrahedrally co-ordinated in quartz phase of FePO4 [29].
Therefore, the freely available co-ordinatively unsaturated Fe
sites might be the active sites contributing to the activity.
The redox nature of iron phosphate catalysts contributing to
the conversion of MP, was further verified by carrying out
benzyl alcohol oxidation (to benzaldehyde), as a test reaction in
a fixed-bed reactor under vapor phase. The conversions
obtained are listed in Table 1. The oxidation activity increased
up to P/Fe ratio of 1.1 and then on decreased. Thus the
proportionality between conversion obtained in ammoxidation
reaction and the quartz phase formation implies that this
conversion is a function of redox property of the catalyst.
Fig. 10. Variation of ammoxidation activity at 673 K as a function of quartz
phase formation in the FePO4 catalysts: (a) P/Fe 1.0; (b) P/Fe 1.1; (c) P/Fe 1.2;
(d) P/Fe 1.4; (e) P/Fe 1.6.
3.2.3. Dependence of selectivity on acid strength
The variation in selectivity to CP with the P/Fe ratio,
obtained at different reaction temperatures, is shown in Fig. 11.
The selectivity to CP increased with increase in P/Fe ratio
reaching a maximum value of 98% on the catalyst with a P/Fe
ratio of 1.2 and then showed a decreasing trend. Even though
high selectivity is a special property of FePO4 catalyst, it
appears to be a function of P/Fe ratio.
In order to understand how the selectivity depends on acid
strength, the catalysts collected after the reaction were further
examined by FT-IR and the spectra are shown in Fig. 12. The
spectra of iron phosphate catalysts showed IR bands at 2370,
1625, 1060, 920, 980, 767 and 511 cm�1, along with broad
bands in the region 3200–3400 cm�1. The one at 3400 cm�1
was due to adsorbed OH groups. The bands at 1060, 980, 767,
511 cm�1 were due to stretching and bending vibrations of the
phosphate ion. An interesting observation was that the catalysts
with P/Fe ratio up to 1.2 exhibited a peak at 1402 cm�1
corresponding to the asymmetric stretching mode of ammo-
nium ion indicating the formation of an ammonium complex of
FePO4. This peak was conspicuously absent in catalysts with P/
Fe > 1.2. Thus, it should be noted that the formation of
ammonium complex was more facile in catalysts with low P/Fe
ratio. The high selectivity of the catalysts with P/Fe ratio of 1.0
to 1.2 could be due to the more easy formation of ammonium
complex [25] on the strongly acidic catalysts, (during the
course of the reaction), compared with the less acidic catalysts
containing polycondensed phosphates. The role of ammonium
complex in acting as a source of N atom for the formation of
cyanopyrazine, is in the same line with the results reported by
Marin et al. [36].
A comparison of the activity and selectivity of the present
catalysts is made with those reported in the literature.
Bondareva et al. [37] have reported an MP conversion of
70% and a combined (amidopyrazine and cyanopyrazine)
selectivity of about 90% on their titania supported vanadia
catalysts. The main byproduct was pyrazine, which formed as a
result of dealkylation of MP. However, the formation of amide
Fig. 11. Variation of selectivity to CP as a function of P/Fe atomic ratio at
different reaction temperatures.
Fig. 12. FT-IR spectra of various used iron phosphate catalysts: (a) P/Fe 1.0; (b)
P/Fe 1.1; (c) P/Fe 1.2; (d) P/Fe 1.4; (e) P/Fe 1.6.
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–1918
in the ammoxidation is found detrimental to meet the
pharmacopoeia standards. When compared with the data on
MP ammoxidation reported by us on FePO4 catalysts in a
previous communication [25] the performance of the present
catalyst reveals higher activity (45% against 20%) retaining the
same selectivity. After a thorough optimization study, it has
been found that the conditions for best activity are MP:water:-
ammonia:air = 1:13:17:38 and the W/F liquid = 2.5 g cm�3 h on
these catalysts. The difference in the performance of the FePO4
catalysts when compared to those given in the earlier report [25]
is due to the changes in the operating conditions as well as the
P/Fe ratio.
The main aim of the present work was to achieve near 100%
selectivity to cyanopyrazine alone, even with lower conver-
sions, so that the process could be made economically and
environmentally attractive by separation and recycle of the
unconverted MP. In this respect with 45 and 98% conversion
and selectivity values respectively are obtained on the catalyst
with P/Fe atomic ratio 1.2.
4. Conclusions
Iron phosphate predominantly exists in its quartz phase at
low P/Fe atomic ratios. Higher P/Fe ratio leads to the formation
of polyphosphates. The ammoxidation activity is proportional
to the quartz phase formation, which in turn is proportional to
the redox property of the catalysts.
Acidity increases with increase in phosphorous content
initially and beyond a P/Fe ratio of 1.2, the polyphosphate
formation decreases the acid strength. The higher the acid
strength the higher is the ammonium complex formation, which
again is proportional to the nitrile selectivity. In order to achieve
good CP yield, a judicious combination of the redox and acid
strength properties is required.
Acknowledgements
The authors thank the Director of Indian Institute of
Chemical Technology (IICT), Hyderabad, for giving permis-
sion to publish these results. Dr. B. Sridhar is acknowledged for
his help in the characterization of catalysts. Prof. I.E. Wachs of
Lehigh University, Bethlehem, USA is acknowledged for
providing UV-Raman analysis. The financial assistance of
CSIR in the form of a Task Force Project COR-0003 is highly
acknowledged.
References
[1] R.G. Rizayev, E.A. Mamedov, V.P. Visloveski, V.E. Sheinin, Appl. Catal.
A 83 (1992) 103.
[2] A. Martin, B. Lucke, Catal. Today 57 (2000) 61.
[3] S.K. Roy, P. Dutta, L.N. Nandi, S.N. Yadav, T.K. Mondal, S.C. Ray, S.
Mitra, P. Samuel, J. Mol. Catal. A 223 (2004) 211.
[4] V.M. Bonderava, T.V. Andruhkevich, R.I. Masksimovskaya, Kinet. Catal.
35 (1994) 129.
[5] Y. Zhang, A. Martin, H. Berndt, B. Lucke, M. Meisel, J. Mol. Catal. A 118
(1997) 205.
[6] V.M. Bonderava, T.V. Andruhkevich, O.B. Lapina, Catal. Today 61 (2000)
173.
[7] A. Andersson, J. Catal. 76 (1982) 144.
[8] A. Baiker, P. Zollinger, Appl. Catal. 10 (1984) 231.
[9] S.J. Kulkarni, R. Ramachandra Rao, M. Subramanyam, P. Kanta Rao, A.V.
Rama Rao, S. Farsinavis, Stud. Surf. Sci. Catal. 98 (1995) 161.
[10] K.V.R. Chary, G. Kishan, K. Ramesh, Ch. Praveen Kumar, G. Vidya Sagar,
Langmuir 19 (2003) 4548.
[11] M. Ai, J. Mol. Catal. A 114 (1996) 3.
[12] R.K. Grasselli, Catal. Today 99 (2005) 23.
[13] J.L. Callahan, R.K. Grasselli, AIChe J. 9 (1963) 755.
[14] A. Martin, K.V. Narayana, B. Lucke, Catal. Today 78 (2003) 311.
[15] J.B. Moffat, Catal. Rev. 18 (1978) 199.
[16] E. Muneyama, A. Kunishige, K. Ohdhan, M. Ai, J. Catal. 158 (1996) 378.
[17] M. Ai, K. Ohdhan, J. Mol. Catal. A 159 (2000) 19.
[18] M. Ai, K. Ohdhan, Appl. Catal. 150 (1997) 13.
[19] J.E. Miller, Appl. Catal. A 231 (2002) 281.
[20] M. Ai, Appl. Catal. A 234 (2002) 235.
[21] P. Bonnet, J.M.M. Millet, J. Catal. 161 (1996) 198.
[22] Y. Wang, K. Otsuka, J. Catal. 155 (1995) 256.
[23] Y. Wang, K. Otsuka, J. Catal. 171 (1997) 106.
[24] D. Klissurski, V. Rives, N. AbadzhJieva, Y. Pesheva, P. Pomonis, T.
Sdovkos, D. Petrakis, Chem. Commun. 1606 (1993).
[25] Ch. Srilaxmi, N. Lingaiah, P. Nagaraju, P.S. Sai Prasad, K.V. Narayana, A.
Martin, B. Lucke, Appl. Catal. A 309 (2006) 247.
[26] X. Wang, Y. Wang, Q. Tang, Q. Guo, Q. Zhang, H. Wan, J. Catal. 217
(2003) 457.
[27] P. Villabrille, P. Vazquez, M. Blanco, C. Caceres, J. Coll. Interf. 251
(2002) 151.
[28] M. Ai, E. Muneyama, A. Kunishige, K. Ohdan, J. Catal. 144 (1993) 632.
[29] A.M. Beale, G. Sankar, J. Mater. Chem. 12 (2002) 186.
P. Nagaraju et al. / Applied Catalysis A: General 334 (2008) 10–19 19
[30] P. Bonnet, J.M.M. Millet, C. Leclercq, J.C. Vedrine, J. Catal. 158 (1996)
128.
[31] B. Samuneva, P. Tzvetkova, I. Gugov, V. Dimitrov, J. Mater. Sci. Lett. 15
(1996) 2180.
[32] Q. Yuvan, Q. Zhang, Y. Wang, J. Catal. 233 (2005) 221.
[33] Y. Wang, X. Wang, Z. Su, Q. Guo, Q. Tang, Q. Zhang, H. Wan, Catal.
Today 93 (2004) 155.
[34] Y. Barbaux, M. Dekiouk, Appl. Catal. A 90 (1992) 51.
[35] J.M. Pemba-Mabiala, M. Lenzi, J. Lenzi, A. Lebugle, Surf. Interf. Anal. 15
(1990) 663.
[36] A. Martin, Y. Zhang, H.W. Zanthoff, M. Meisel, M. Baerns, Appl. Catal. A
139 (1996) L11.
[37] V.M. Bondareva, T.V. Andrushkevich, L.G. Detusheva, G.S. Litvak, Catal.
Lett. 42 (1996) 113.
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