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.

www.elsevier.com/locate/apcata

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: saiprasad@iict.res.in (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.