Antimony Substituted Ammonium 12-Molybdophosphoric Acid Catalysts for Gas Phase Chlorobenzene Oxidation

Article abstract of DOI:10.1007/s10562-020-03372-z

Abstract: Antimony (Sb) substituted Keggin structured ammonium salt of 12-molybdophsphoric acid (AMPA) materials were synthesized by acid precipitation method. The samples were obtained by increase of Sb atoms from 1 to 3 and corresponding decrease of Mo atoms from 11 to 9. The synthesized materials were thermal treated in the range of 300–500?°C. The catalytic performance of thermally treated Sb incorporated AMPA materials were examined for oxidation of chlorobenzene and the catalytic activity of Sb-AMPA samples was compared with activity of bulk AMPA. The materials were thoroughly analyzed by different techniques such as elemental analysis, TG, powder XRD, FTIR, SEM, N₂-physisorption, ³¹P MAS NMR, XPS, DR UV–Vis and H₂-TPR techniques. The characterization results clearly indicated that Sb-AMPA samples possessed high thermal stability, relatively large surface area and easy reducibility compared to bulk AMPA. The synthesized samples also possessed high content of surface adsorbed oxygen species, which are responsible for superior catalytic activity. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-020-03372-z

Catalysis Letters
https://doi.org/10.1007/s10562-020-03372-z
Antimony Substituted Ammonium 12‑Molybdophosphoric Acid
Catalysts for Gas Phase Chlorobenzene Oxidation
Abdulmohsen Alshehri1 · Katabathini Narasimharao1
Received: 22 May 2020 / Accepted: 28 August 2020
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
Antimony (Sb) substituted Keggin structured ammonium salt of 12-molybdophsphoric acid (AMPA) materials were syn-
thesized by acid precipitation method. The samples were obtained by increase of Sb atoms from 1 to 3 and corresponding
decrease of Mo atoms from 11 to 9. The synthesized materials were thermal treated in the range of 300–500 °C. The catalytic
performance of thermally treated Sb incorporated AMPA materials were examined for oxidation of chlorobenzene and the
catalytic activity of Sb-AMPA samples was compared with activity of bulk AMPA. The materials were thoroughly analyzed
by diferent techniques such as elemental analysis, TG, powder XRD, FTIR, SEM, N2-physisorption, 31P MAS NMR, XPS,
DR UV–Vis and H2-TPR techniques. The characterization results clearly indicated that Sb-AMPA samples possessed high
thermal stability, relatively large surface area and easy reducibility compared to bulk AMPA. The synthesized samples also
possessed high content of surface adsorbed oxygen species, which are responsible for superior catalytic activity.
Graphic Abstract
Keywords (NH4)3[PMo12O40] · Sb incorporation · Chlorobenzene · Gas phase oxidation
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1 3

A. Alshehri, K. Narasimharao
it is unclear whether the observed structural stability is due
to presence of doped Sb or K atoms which were already
presented in the secondary structure of AMPA. Recent
reports [14, 15] concluded that Sb in the primary struc-
ture of heteropolyacid could improve the thermal stability
(higher than 400 °C) and catalytic functionality.
1 Introduction
Degradation of chlorinated organic molecules to CO2,
HCl and H2O have gained tremendous importance due to
the fact that chlorinated organic compounds are toxic to
human beings even in low concentrations and also they
are responsible for the lessening of the ozone layer in the
stratosphere [1, 2]. The destructive removal of chlorinated
organic compounds by using catalytic oxidation is well
used process because it is not only efective but also eco-
nomical. It is a challenge to develop a catalyst that could
destruct a wide range of chlorinated organic compounds
with diferent concentrations and exhibit high selectivity
towards the formation of harmless products at relatively
low reaction temperatures [3].
In this work, Sb incorporated Keggin structured AMPA
samples were synthesized by a simple acid precipitation
method. The synthesized materials were used as heteroge-
neous catalysts for gas phase oxidation of chlorobenzene. A
thorough catalyst characterization was performed by using
TG, XRD, FT-IR, 31P MAS NMR, SEM, N2-physisorption,
XPS and H2-TPR techniques. The performance of the cata-
lysts in gas phase oxidation of chlorobenzene was correlated
with their chemical and structural properties.
Catalysts contained noble metals such as Pt, Pd [4]
and transition metal oxides such as V, Cu, Ni, Fe and Mn
oxides [5] were used for the decomposition of chlorin-
ated molecules. In fact, noble metal based catalysts are
highly active at low temperature, they sufer from disad-
vantages such as they are more expensive and deactivate
with in few hours of reaction. However, it was realized
that metal oxide catalysts are less expensive and are more
resistant to deactivation, but they sufer from less activity
[6]. Literature reports indicated that the mixed oxide based
catalysts such as V-Ti, V-Mo and V-W oxides showed bet-
ter catalytic activity and selectivity than the single oxide
catalysts [7, 8].
2 Experimental
2.1 Materials
The analytical grade chemicals utilized for the synthesis of
materials and gas phase oxidation reaction are obtained from
Aldrich, UK and the reagents were used as received without
adapting any purifcation procedures.
2.2 Synthesis of Sb Incorporated AMPA Catalysts
Keggin structured AMPA salt is thermally more stable
and efcient selective oxidation catalyst than its parent
acid; H3PMo12O40·xH2O [9]. It was previously reported
that expulsion of NH4+ ions from the secondary structure
and decomposition of primary [PMo12O40]3− (Keggin ion)
into parent Mo and P oxides beyond 380 °C is responsi-
ble for lower performance in gas phase catalytic reactions
[10]. Incorporation of transition metals such as V and Cu
metal ions into Keggin structure is a well reported method
to enhance the thermal stability of AMPA [10]. Heteropoly
acids based materials could be an efective catalysts for
decomposition of chlorinated organic molecules as these
materials were used in many acid-catalyzed and oxidation
reactions both in liquid and gas phase conditions [11]. It
was previously reported that the incorporation of transition
metals showed substantial infuence over the redox and
acid–base characteristics of heteropoly compounds [12].
Decades ago, Cavani et al. [13] observed that incorpora-
tion of Sb into AMPA enhanced the thermal stability of the
Keggin structure. The authors claimed that incorporation
of one Sb atom in the Keggin structure of AMPA shifted
the structure degradation temperature by 100 °C and it was
also observed that Sb atom was replaced some of the K+ or
NH4+ ions in the secondary structure of AMPA. However,
Acid precipitation method was used to synthesize Sb incor-
porated AMPA catalysts and the typical procedure is as fol-
lows. A calculated amounts of ammonium heptamolybdate
and diammonium hydrogen orthophosphate were dissolved
in minimum amount of distilled water by heating at the aque-
ous solution at 80 °C. Stoichiometric quantity of SbCl3 in dil.
HCl solution was added drop wise to the previously prepared
solution and the pH of the solution was maintained around
1. During the addition of dil. HCl, the color of the solution
progressively changed from green to light blue at the same
time, a precipitate was obtained. The total contents were
refuxed at 100 °C for 4 h, The precipitate was then fltered
and washed with distilled water for four times and dried at
120 °C for 12 h. The dried powder was thermally treated at
diferent temperatures in the range from 300 to 500 °C for
4 h. The atomic ratio of Mo:Sb has been varied from 11:1,
10:2 and 9:3 for AMPSb1, AMPSb2 and AMPSb3 series of
catalysts. For comparison study, bulk AMPA with out Sb
was prepared by acidifcation of stoichiometric amounts of
ammonium heptamolybdate, orthophosphoric acid, and the
excess amount of ammonium nitrate with nitric acid. The
resulting suspension was evaporated to dryness and the solid
thus obtained was thermally treated at the temperature range
of 300–500 °C for 4 h.
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Antimony Substituted Ammonium 12-Molybdophosphoric Acid Catalysts for Gas Phase Chlorobenzene…
benzene, chlorobenzene, O2, CO, CO2, N2 components and
2.3 Catalyst Characterization
to detect other hydrocarbons and chlorinated hydrocarbons.
The obtained data is accurate within a range of about 1% (in
relative) for the conversion of chlorobenzene.
The chemical composition (atomic ratios of Mo, Sb, and
P) of the samples was measured by using Perkin-Elmer
ICP-MS instrument. The thermogravimetric patterns of the
samples were obtained using Netsz thermal analyzer. The
XRD patterns of the samples were recorded using Bruker
D8 Advance X-ray difractometer using Ni fltered Cu Kα
radiation (λ=1.5405 Å). The FT-IR spectra of the samples
were recorded using Bruker D70 spectrometer. The nitrogen
physisorption measurements were carried out using Quan-
tachrome Autosorb ASiQ instrument. The SEM analysis
of the samples was carried out using JEOL Model JSM-
6390LV microscope. The XPS spectra of samples were
obtained using a PHI Quantera Scanning XPS Microprobe
operating in its large area mode. The 31P MAS NMR spec-
tra of the samples were recorded in a 400 MHz Bruker
spectrometer. The DR UV-Vis spectra of the synthesized
samples were obtained using Thermo-Scientifc evolution
spectrophotometer equipped with an integrating sphere in
the wavelength range of 200–800 nm. The H2-TPR measure-
ments of the samples were carried out using Quantachrome
CHEMBET-3000 instrument.
3 Results and Discussion
The thermal behavior of three dried Sb incorporated AMPA
materials was studied by thermogravimetric and diferen-
tial scanning calorimetry (TG/DSC) analyses. The TG pat-
terns of dried AMPSb1, AMPSb2 and AMPSb3 samples
are shown in Fig. 1. The TG results indicated that major
weight loss was occurred in two stages in all the samples.
The frst weight loss occurred between 140 and 360 °C could
be attributed to the removal of structural H2O molecules
and expulsion of NH4 ions from the Keggin structure. The
second weight loss between 360 and 540 °C is due to the
complete destruction of Keggin structure to form Mo and
P oxides.
It is well known that complete destruction of Keggin
structure in bulk AMPA generally occurs between 390 and
400 °C [16]. The TG results indicating that thermal decom-
position process is clearly delayed in Sb doped AMPA
samples; this is most probably due to the substitution of
Sb atoms in the secondary structure (in place of NH4 cati-
ons). This phenomenon is observed in three synthesized
Sb substituted AMPA materials, however it is high in case
of AMPSb3 material. The total weight loss observed for
AMPSb1, AMPSb2 and AMPSb3 samples is 20%, 18% and
14% respectively. The total weight loss in Sb substituted
AMPA materials is higher than bulk AMPA (around 11%).
This is probably due to high hydration levels of Sb [17] pre-
sented in the samples and increase of Sb atoms in AMPA
samples resulted the increase of H2O molecules. The DSC
curves of AMPSbx samples exhibited an endothermic peak
in the temperature range of 180–250 °C. The maximum tem-
perature of peak is at 210 °C indicating the loss of physically
adsorbed water in the pores of microcrystalline AMPA struc-
ture. Presence of a broad endothermic peak in the range of
400–540 °C could be attributed to destruction of secondary
2.4 Catalytic Oxidation of Chlorobenzene
Oxidation of chlorobenzene was carried in a continuous,
fxed bed micro-catalytic reactor (made of quartz, 8 mm
inner diameter, 30 cm long) and heated in an electric fur-
nace. The fow rates of air and nitrogen gases were meas-
ure using calibrated mass fow controllers. Catalyst sample
(250 mg diluted with 500 mg of quartz pellets) was placed
between two layers of the quartz wool in the reactor. Chlo-
robenzene was introduced into the reactor by N2 gas fow
through a saturator. The saturated stream was then mixed
with O2 and N2 gases to achieve the desired concentra-
tions, and preheated before entering into the reactor. The
concentration of chlorobenzene in the reactor feed is about
1300 ppm, unless otherwise stated. The total gas fow was
133 mL min−1, 85% N2 and 15% O2 along with 1300 ppm
chlorobenzene. An excess O2 was utilized to insure complete
oxidation of chlorobenzene. The gas hour space velocity was
maintained at 8000 h−1. The reaction was carried out at a
diferent temperature range from 150 to 350 °C. The reac-
tion temperature was maintained by means of PID tempera-
ture controller and a thermocouple placed co-axially with
the catalyst bed. The product stream was then collected for
every half an hour under steady state conditions obtained
after a few hours of operation. The formed HCl and Cl2
were both trapped at the exit of the reactor in a bubbler
containing 0.1 M aqueous KI solution. The product analy-
sis was carried out using GC equipped with four columns,
one methanizer, two FID and one TCD allowed to quantify
+
structure of AMPA by losing the crystalline H2O and NH4
cations. A short exothermic process presented in the range
of 600–630 °C could be due to the exothermic crystallization
of P, Mo and Sb oxides through destruction of Keggin struc-
ture. The area under the exothermic and endothermic peaks
assigned to the losses of mass corresponding to the H2O
and NH4+ species. These results could be related with both
number and nature of cations in the AMPA structure. Thus,
the complete decomposition of Sb incorporated AMPA salts
occurred 600 °C, 610 °C and 630 °C for AMPSb1, AMPSb2
and AMPSb3 samples respectively, which corresponds to the
broad exothermic efect with maxima for AMPSb3 sample.
1 3

A. Alshehri, K. Narasimharao
100
98
96
94
92
90
88
86
84
82
80
78
100
98
96
94
92
90
88
86
84
82
Exo
AMPSb1
7
6
5
4
3
2
1
0
Exo
AMPSb2
4
3
2
1
0
Endo
Endo
100
200
300
400
500
600
700
100
200
Exo
300
400
500
600
700
Temperature (oC)
Temperature (oC)
100
14
12
10
8
AMPSb3
98
96
94
92
90
88
86
Endo
6
4
2
0
100
200
300
400
500
600
700
Temperature (oC)
Fig. 1 TG/DSC patterns of as synthesized Sb incorporated AMPA samples
These observations indicating the presence of Sb enhanced
the thermal stability of AMPA structure.
XRD studies are revealing that the incorporation of Sb was
delayed the degradation of Keggin structure of AMPA.
These observations are in accordance with the results of
TG experiments. It was previously indicated that the incor-
porated foreign metal ions either could substitute Mo in
peripheral positions in Keggin anion (primary structure) or
act as cation by removing the NH4+ ions in the secondary
structure [19]. Marchal-Roch et al. [20] performed XRD
analysis of V incorporated AMPA [inside primary Keggin
ion, (NH4)4[PMo11VVO40] sample and the cubic phase is
stable up to 410 °C. However, the authors observed that
the cubic phase is stable only up to 390 °C, when the V
was substituted in the secondary structure of the Keggin
compound {(NH4)3–2x(VVIO)x[PMo12O40]}. The temperature
diference is not huge, however it was proposed that in case
of (NH4)4[PMo11VVO40], the V atoms were removed from
the Keggin anion after thermal treatment at 210 °C and the
expelled V cations occupied the counter-cation position.
FT-IR analysis is one of the important analytical tool
to analyze the structural features of synthesized Sb-
AMPA samples. The FT-IR spectra of AMPSb1, AMPSb2
and AMPSb3 samples thermally treated in the range of
300–500 °C are shown in Fig. S1 (electronic supplemen-
tary information). It was reported that the FT-IR spectrum
The powder XRD patterns of AMPSb1, AMPSb2
and AMPSb3 samples thermally treated in the range of
300–500 °C are shown in Fig. 2. The catalysts thermally
treated at 300 and 350 °C exhibited the refections due to
crystalline cubic structured AMPA [(NH4)3PMo12O40·4H2O,
JCPDS File No.9-412]. Presence of narrow intense refec-
tions due to highly crystalline α-MoO3 [molybdenite,
JCPDS 35-0609] phase was observed in case of AMPSb1
and AMPSb2 samples calcined at 400 °C along with minor
refections corresponding to AMPA phase. This is observa-
tion is indicating that decomposition of AMPA has started
in AMPSb1 and AMPSb2 samples at 400 °C. It is known
that the cubic structure of bulk AMPA is preserved until
380 °C and beyond this temperature the ammonium cations
evolves out from the Keggin structure [18]. The AMPSb3
sample thermally treated at 400 °C showed difraction peaks
corresponding to AMPA phase and did not showed any
refections corresponding to α-MoO3 phase. At 450 °C and
500 °C, AMPSb1 and AMPSb2 materials exhibited major
refections attributed to α-MoO3 due to the destruction of
Keggin structure. In contrast, AMPSb3 material shown
major refections due to AMPA at 450 °C. The results from
1 3

Antimony Substituted Ammonium 12-Molybdophosphoric Acid Catalysts for Gas Phase Chlorobenzene…
Fig. 2 Powder XRD patterns of a AMPSb1 b AMPSb2 and c AMPSb3 samples thermally treated at (a) 300 °C (b) 350 °C (c) 400 °C (d) 450 °C
and (e) 500 °C
of bulk AMPA calcined at 300 °C generally exhibits the
IR bands at 1060, 960, 890, 795 and 1425 cm−1 attributed
to asymmetric stretching vibrations for (P–Od), (Mo–Ot),
to understand the location of the incorporated Sb atoms. It
was previously reported that if any metal ion is substituted
in the peripheral position of Mo of the Keggin structure, it
would bring changes in the frequencies of IR vibrations of
Keggin structure due to decrease of the PO4 symmetry [22].
Therefore, if Sb atoms occupied in place of Mo, a shoulder
generally appears in (P–Od) band. However no shoulder was
observed in 1060 cm−1 band in the FT-IR spectra of Sb-
AMPA samples. From these results, it is clear that Sb atoms
were not replaced the Mo in the peripheral positions of Keg-
gin anion, but could have accommodated in the secondary
structure or segregated as isolated amorphous phase.
+
(Mo–Ob–Mo), (Mo–Oc–Mo) and NH4 functional groups
respectively [21]. The Sb substituted AMPA materials ther-
mally treated at 300, 350 and 400 °C exhibited the char-
acteristic bands due to Keggin ion along with a band at
1425 cm−1 corresponding to NH4+ ions (secondary struc-
ture). This observation indicating that the Keggin structure
is intact up to 400 °C in all the Sb substituted AMPA mate-
rials. At 450 °C, AMPSb1 sample did not show IR absorp-
tion band corresponding to NH4+ ion and bands attributed
to MoO3 at 982 cm−1, 863 cm−1 and 595 cm−1 are dominant,
however, AMPSb2 and AMPSb3 catalysts exhibited bands
related to Keggin and NH4+ ions. At 500 °C, AMPSb1 and
AMPSb2 samples showed only bands due to MoO3 phase
and AMPSb3 catalyst exhibited low intense IR bands cor-
responding to Keggin structure and the band corresponding
to NH4+ groups was vanished. These observations reveal-
ing that the Keggin ion is intact up to 450 °C for AMPSb1,
500 °C for AMPSb2 and above 500 °C for AMPSb3 mate-
rial. Therefore, the FT-IR spectral data also indicating that
incorporation of Sb into AMPA resulted increase of ther-
mal stability of Keggin structure, however it is important
To obtain precise information about the location of Sb
atoms, 31P MAS NMR analysis of the samples was per-
formed. Solid state 31P MAS NMR spectra of representa-
tive Sb-AMPA samples are shown in Fig. 3. The Sb-AMPA
samples thermally treated at 300 °C showed three resonance
peaks; a major sharp peak at − 4.5 ppm, and two broad
peaks at − 5.8 ppm and 2.1 ppm. All the samples exhib-
ited three P resonance peaks at low temperature treatment,
clearly indicating that the Sb-AMPA samples possessed
three diferent species. Black et al. [23] reported that pure
AMPA [(NH4)3PMo12O40·4H2O] phase exhibits a single
31P MAS NMR peak at − 4.45 ppm. The peak observed
1 3

A. Alshehri, K. Narasimharao
Fig. 3 31P MAS NMR spectra of representative Sb-AMPA samples
at − 4.5 ppm in the samples could be attributed to bulk
AMPA phase. The resonance peak appeared at − 5.8 ppm,
is due to AMPA with diferent degree of hydration, due to
high hydration levels of Sb [24] presented in the samples
the intensities of both peaks at 2.1 ppm and − 5.8 ppm are
increasing with increase of Sb atoms.
The AMPSb1 sample thermally treated at 450 °C exhib-
its a broad peak consists two resonances at 0.5 ppm and
0 ppm. It is well known that the phosphorous oxides such
and increase of Sb atoms. It was also reported that the 31
P
3−
MAS NMR peaks are sensitive to the number of crystalline
H2O molecules presented in Keggin structure, for instance,
chemical shift for H3PMo12O40·30H2O is − 3.9 ppm, while
it is shifted to − 2.9 ppm for H3PMo12O40 [25]. Loss of
H2O molecules in the crystal structure resulted NMR peak
shift towards positive ppm, however in case of Sb-AMPA
samples the peak shifted towards negative ppm, indicating
that the synthesized samples also contained Keggin struc-
ture compound with more number of H2O molecules. These
results are in quite accordance with TG and FT-IR results.
The peak at 2.1 ppm could be due to presence of Sb(PO4) as
as isolated PO4 and P2O5 species appears positive ppm
scale [26], the peaks appeared at 0.5 ppm and 0 ppm could
be assigned to oxides of P generated due to degradation of
Keggin unit. This sample also shows a small broad reso-
nance at − 8.3 ppm, which is due to the lacunary Keggin
ions (forms after expulsion of MoO6 octahedral species).
In contrast, AMPSb2 sample showed two resonance peaks,
frst one at − 4.5 ppm and another one at 0 ppm due to
presence of AMPA and phosphorous oxide. On other hand,
AMPSb3 sample showed very broad resonance peak cen-
tered at − 3.3 ppm due to Keggin species with diferent
1 3

Antimony Substituted Ammonium 12-Molybdophosphoric Acid Catalysts for Gas Phase Chlorobenzene…
stoichiometry. The obtained observations revealing that a
prominent degradation was happened in AMPSb1 material,
however increase of Sb atoms to 2 and 3 resulted existence
of Keggin structure even at 450 °C thus proves the stabiliza-
tion role of Sb atoms.
micro pores [28], but the Sb incorporated AMPA samples
treated at 300 °C exhibited sharp increase of adsorption at
low pressures. The Sb incorporated AMPA samples treated
at 400 °C samples exhibited the type H4 hystereses, which is
related to presence of narrow pores. On other hand, the sam-
ples treated at 500 °C exhibited type H2 hystereses, which
relates to presence of inkpot-shaped pores with narrow necks
and large spaces. These observations are in accordance with
results reported in the literature [29] that Keggin salts with
large cations such as Cs+, NH4+ are mesoporous in nature.
The BET surface area, pore volume and average pore
radius for the AMPSb1, AMPSb2 and AMPSb3 catalysts
thermally treated at diferent temperatures are tabulated in
Table 1. It was previously observed that the pure AMPA
possesses high surface area (around 150 m2 g−1) with micro
pore radius of 13 Å due to its microporous nature and has
The morphology of Sb-AMPA samples was studied using
SEM analysis. The SEM images of AMPSb1, AMPSb2 and
AMPSb3 samples thermally treated at 300, 400 and 500 °C
are shown in Fig. S2. The samples calcined at 300 °C exhib-
ited small, spherical particles ranging from 10 to 20 nm in
diameter. It was previously reported that cubic structured
AMPA nanoparticles exhibits spherical morphology due to
isotropic crystal growth [27]. Increase of treat temperature to
400 °C resulted in rise in particle size, as the three samples
clearly showed the presence of relatively large particles due
to agglomeration. However, it is interesting to observe that
the extent of agglomeration is less in case of AMPSb3 sam-
ple compared to AMPSb1 and AMPSb2 samples, indicating
the infuence of Sb stoichiometry on the size and morphol-
ogy of the AMPA. In contrast, the Sb-AMPA samples ther-
mally treated at 500 °C exhibited large irregular block-like
crystals with large size (around 100–150 nm), which are
mainly composed of decomposed Keggin structure AMPA,
to form Mo, P and Sb oxides. These results are in consistent
with the XRD and FT-IR results.
Table 1 The textural properties of Sb-AMPA samples treated at dif-
ferent temperatures
Catalyst
Surface
area
Pore
Average
pore
volume
(m2 g−1
)
(cm3 g−1
)
radius (Å)
AMPSb1-300
AMPSb1-400
AMPSb1-500
AMPSb2-300
AMPSb2-400
AMPSb2-500
AMPSb3-300
AMPSb3-400
AMPSb3-500
124
69
6
103
55
8
89
43
5
0.124
0.110
0.028
0.113
0.100
0.019
0.104
0.070
0.012
24
20
10
20
14
8
15
8
6
The N2-physisorption measurements were carried out to
study the textural properties of the synthesized samples. The
N2 adsorption–desorption isotherms of synthesized mate-
rials are presented in Fig. 4. The three Sb-AMPA materi-
als thermally treated at 300 °C showed type IV isotherms
(IUPAC standards), which is a representative of mesoporous
solids. It is well known that bulk AMPA possesses majorly
AMPSb1-500
AMPSb2-500
AMPSb3-500
AMPSb3-400
AMPSb3-300
AMPSb2-400
AMPSb2-300
AMPSb1-400
AMPSb1-300
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
o
o
o
P/P
P/P
P/P
Fig. 4 N2 adsorption–desorption patterns of the Sb-AMPA samples thermally treated at diferent temperatures
1 3

A. Alshehri, K. Narasimharao
average meso pore radius of 35 or 45 Å [30]. A decrease in
the surface area of the samples observed with Sb incorpo-
ration and also the thermal treatment from 300 to 500 °C
(85–10 m2 g−1), similar to that reported in the literature
Sb(V) atoms are occupying the vacant cationic position due
to release of NH3 and water from AMPA structure.
Liu et al. [37] observed that AMPA structure composed
of [PMo12O40]3− lattices with large cavities and channels
also exists in between the crystal lattices. Therefore, there
is clear possibility for free movement of metal cations in
between the crystal lattices and substitute NH4+ ions. Thus,
substitution of Sb(V) ions is possible all through the AMPA
crystal lattice and not limited to the surface. The XPS data
is also used to determine the elemental concentrations at
the surface of the Sb incorporated samples. The quantita-
tive data in the Table 2 shows the Sb concentration increas-
ing in concentration from samples AMPSb1, AMPSb2 and
AMPSb3 for all the samples calcined at 300, 400 and 500 °C.
The data also shows the Mo concentration is decreasing in
concentration when looking at samples 1 through 3. Due to
an overlap with O1s and the Sb3d5/2 peaks, curve ftting was
used to isolate the O1s peak area prior to making quantita-
tive calculations. The elemental concentrations of the cata-
lysts also measured using ICP-MS (Table 2) and compared
with the surface composition. The bulk and surface compo-
sition of the samples are almost equal; this could be due to
homogeneous nature of the materials.
+
[31]. It was previously reported that the expulsion of NH4
ions from the secondary structure of the Keggin after ther-
mal treatment at high temperature leads to decrease of spe-
cifc surface area [29]. A huge loss in surface area was also
noticed due to the complete degradation of Keggin unit that
occurs at very high temperatures. The loss of surface area
in case of Sb substituted AMPA samples could also due to
degradation of Keggin structure and pore blockage because
of Sb surface coverage.
High energy resolution X-ray photoelectron spectra
for AMPSb samples were collected for all the elements
to observe their chemical state as the quantity of Sb was
increased. As shown in the Fig. 5, substituting Sb for Mo
creates a second Mo chemical state. The Mo 3d5/2 peak for
AMPA and the primary peak for the other samples occurred
at approximately 233 eV. Confrmation of Mo6+ in the pure
AMPA (Fig. S3) can be obtained from sharpness of the sig-
nals of Mo 3d5/2 and Mo 3d3/2 levels in the XPS spectrum
[32]. As Sb is incorporated for Mo, the Mo concentration
decreases and a new Mo peak appears at approximately
232 eV. The intensity of this new peak increases as the
quantity of Sb increases. A similar efect is observed for
the samples treated at 400 and 500 °C. The new peak at
232 eV could be due to presence of reduced Mo(V) spe-
cies, as reduced Mo in heteropolyacid salts and oxides
generally show broad peaks at lower binding energy [33].
These results are quite corroborating with the thermolysis
pathway described in the literature [34]. The increase of
the intensity peak at 232 eV could be attributed to the rise
in the number of Mo(V) ions in the Sb substituted AMPA
materials as per electron exchange occurring as follows:
SbIII +2MoVI ↔SbV +2MoV.
The difusive refectance UV–Vis (DR UV–Vis) spectral
analysis was performed to distinguish the electronic prop-
erties of the Mo and Sb atoms in the Keggin compounds.
The DR UV–Vis spectra of the samples (Fig. S4) exhib-
ited ligand–metal charge transfer bands [O2− → Mo6+] in
the range of 215–225 nm, 280–300 nm and 380–400 nm
due to the diferent types of metal–oxygen ligands in the
AMPSbx compound structure. Presence of more than one
UV–Vis bands for metal incorporated Keggin structured
heteropoly compounds was previously reported other
researchers [38]. Cavani et al. [39] suggested that the size
and position of the lower charge transfer bands was infu-
enced by the composition of the cations of the Keggin
secondary structure. It is clear from the spectra that lower
charge transfer band is shifted in case of AMPSbx samples
indicating that composition of secondary structure is difer-
ent in these samples compared to bulk AMPA sample. In
addition, another large charge transfer band was observed
above 700 nm for AMPSbx samples, which could be
assigned to the d–d transition band of Mo5+ species in octa-
hedral coordination [40]. This observation indicating that
reduction of M6+ to Mo5+ by the Sb3+ in AMPSbx samples
[Sb3+ +Mo6+ →Sb5+ +Mo5+].
The ftting results of Sb3d and O1s peaks of AMPSbx
samples are also shown in Fig. 5. Due to the overlapping
of Sb3d5/2 with O1s peak, its binding energy could not be
determined accurately. All Sb3d3/2 XP peaks were correctly
ftted by Gaussian parameters with binding energy maxi-
mum of 540.8 eV and it could be attributed to Sb(V) species
[35]. The intensity of the peak at 540.8 eV increased with
increase of Sb atoms in the Keggin structure, revealing that
the number of Sb(V) species were increased with rise of
Sb(III) atoms in AMPA. According to Zeng et al. [36], Sb
3d3/2 peak corresponding to Sb(III) appear at 539.7 eV. The
absence of this peak suggesting that even though Sb(III) is
the starting material used for preparation of AMPSbx sam-
ples, it is readily oxidizing to Sb(V) during the preparation
of catalysts. It is clear that during the thermal treatment
The intensity of band at 700 nm is increasing with
increase of Sb content in the sample, which is indicating that
the extent of Mo reduction is a function of the Sb content;
more the Sb content, the higher the concentration of Mo5+
species [41]. The formation of Mo5+ species as a function
of the calcination temperature was also studied using DR
UV–Vis spectral analysis. The spectra of AMPSb3 sample
6+
below 450 °C, formation of [PMo55+Mo7 O40]8− and expul-
sion of Sb atoms from Keggin structure occurs. The expelled
1 3

Antimony Substituted Ammonium 12-Molybdophosphoric Acid Catalysts for Gas Phase Chlorobenzene…
AMPSb1-300
Mo 3d3/2
Mo3d
O1s
AMPSb1-300
P 2p1/2
P2p
AMPSb1-300
O 1s
Mo 3d5/2
P 2p3/2
Sb 3d5/2
Sb 3d3/2
144
222
136
128
124
140
132
242
238
226
545
540
535
530
234
230
525
Binding Energy (eV)
Binding Energy (eV)
Binding Energy (eV)
AMPSb3-300
O1s
AMPSb3-300
P2p
Mo3d
AMPSb3-300
Mo 3d3/2
O 1s
P 2p3/2
Sb 3d3/2
Mo 3d5/2
P 2p1/2
Sb 3d5/2
136
132
128
124
140
540
535
525
144
222
545
530
226
Binding Energy (eV)
242 238
234 230
Binding Energy
Binding Energy (eV)
AMPSb1-500
O1s
P2p
AMPSb1-500
Mo3d
AMPSb1-500
O 1s
P 2p3/2
Mo 3d5/2
Mo 3d3/2
P 2p1/2
Sb 3d3/2
Sb 3d5/2
545
540
Binding Energy (eV)
535
530
525
144
222
140
128
Binding Energy (eV)
124
136
132
230
226
238
234
242
Binding Energy (eV)
AMPSb3-500
AMPSb3-500
O1s
Mo3d
AMPSb3-500
P2p
O 1s
P 2p3/2
Mo 3d3/2
P 2p1/2
Mo 3d5/2
Sb 3d3/2
Sb 3d5/2
144 140 136
Binding Energy (eV)
128
132
124
226
Binding Energy (eV)
242
234
230
222
545
540
535
238
530
525
Binding Energy (eV)
Fig. 5 Deconvoluted X-ray photoelectron spectra of Sb-AMPA samples thermally treated at diferent temperatures
calcined at diferent temperatures shows that the extent of
Mo reduction increases with increase of calcination tempera-
ture; higher the calcination temperature the band at 700 nm
becomes dominant. These observations are supporting the
results observed from XRD, FT-IR and 31P MAS NMR
studies.
The H2-TPR profles for AMPA and Sb incorporated
AMPA samples calcined at 300 °C are shown in Fig. 6.
1 3

A. Alshehri, K. Narasimharao
compounds could be attributed to the reduction of Mo6+
to Mo4+. As shown in Fig. 6, with increasing Sb content,
the center of the major peak shifting to high temperatures
suggesting that the replacement of NH4+ ions by Sb, as it is
well known that the reducibilities of heteropoly compounds
were considerably infuenced by their counter cations. The
AMPASb3-300 catalyst is more stable than other samples
because its reduction temperature is higher, at which the
heteropoly compound was decomposed to Mo, P and Sb
oxides. The TG-DSC analysis of the synthesized samples
indicating that Keggin structure is decomposed completely
degraded to form crystalline oxides when temperature was
above 600 °C. Therefore, a small reduction peak appeared
around 530 °C could be assigned to the reduction of the
segregated Sb oxide. A similar observation was reported in
case of vanadium incorporated heteropolyacid catalysts, in
which the authors found a H2-TPR peak below 600 °C due
to reduction of vanadium oxide species [43]. The hydrogen
consumption values were obtained from the H2-TPR analysis
and the highest value was observed for the AMPSb3-300
sample (supporting information, Table S1).
Table 2 Bulk and surface elemental composition of Sb incorporated
AMPA catalysts calcined at diferent temperatures
Catalyst
Bulk composition
(ICP-MS)
Surface composition
(XPS)
P/Mo
ratio
P/Sb
ratio
P/Mo
ratio
P/Sb
ratio
AMPSb1-300
AMPSb1-400
AMPSb1-500
AMPSb2-300
AMPSb2-400
AMPSb2-500
AMPSb3-300
AMPSb3-400
AMPSb3-500
0.092
0.091
0.090
0.101
0.102
0.102
0.112
0.111
0.111
0.981
0.980
0.980
0.493
0.495
0.495
0.333
0.331
0.332
0.091
0.091
0.090
0.100
0.100
0.101
0.111
0.111
0.111
0.979
0.980
0.980
0.490
0.493
0.493
0.332
0.331
0.332
AMPASb3-300
The catalytic activity for the AMPA and Sb containing
AMPA samples treated at diferent temperatures was tested
for the oxidation of chlorobenzene reaction. The activity
tests were also performed for bulk AMPA samples for com-
parison purpose. A blank test was performed to investigate
the thermal decomposition of chlorobenzene using quartz
pellets and only 2% chlorobenzene conversion was observed.
Figure 7 shows the temperature dependency of the conver-
sion over the investigated catalysts. It is found that the cata-
lytic activity is greatly afected by the reaction temperature.
The conversion of chlorobenzene is gradually increased with
increasing the reaction temperature in all the catalysts.
AMPASb2-300
AMPASb1-300
AMPA-300
100
AMPA-300
AMPSb2-400
AMPSb3-400
AMPA-500
AMPSb1-300
AMPSb2-300
AMPSb3-300
AMPA-400
100
200
300
400
500
600
700
80
60
40
20
0
Temperature (oC)
AMPSb1-500
AMPSb2-500
AMPSb3-500
AMPSb1-400
Fig. 6 H2-TPR patterns for AMPA and Sb incorporated AMPA sam-
ples calcined at 300 °C
The bulk AMPA sample showed a major reduction peak at
650 °C. However the Sb contained AMPA samples exhibited
a small reduction peak around 530 °C with major reduction
peak in the range of 655–665 °C. As the heteropoly acids
generally decompose above 500 °C, the reduction peaks
could be attributed to the reduction of Mo and Sb oxides
formed by the degradation of the Keggin ion during the
H2-TPR experiments.
150
200
250
300
350
o
Reaction temperature ( C)
Sun et al. [42] reported that the reduction peak in the
range of 640–690 °C for metal substituted heteropoly
Fig. 7 Conversion of chlorobenzene of over AMPA and Sb incorpo-
rated AMPA samples thermally treated at diferent temperatures
1 3

Antimony Substituted Ammonium 12-Molybdophosphoric Acid Catalysts for Gas Phase Chlorobenzene…
As shown in Fig. 7, among the investigated catalysts,
AMPSb3-300 is found to possess the superior activity for
oxidation of chlorobenzene. The chlorobenzene conversion
reached to 90% at 350 °C. At this temperature, AMPA-300
sample ofered chlorobenzene conversion of 26%, and is
about 50% and 70% for AMPSb1-300 and AMPSb2-300 cat-
alysts respectively. For AMPSb3-300 catalyst, the complete
chlorobenzene conversion was achieved at 375 °C, however
AMPSb1-300 and AMPSb2-300 catalysts needs minimum
450 °C, which is 75 °C greater than that for AMPSb3-300
sample. All the investigated Keggin structured catalysts in
this study ofered around 99% selectivity to COx and no
other measurable by-products were obtained. The observed
trend in the Sb-AMPA catalysts is quite dissimilar from that
of Pt, Pd supported on alumina and alumina pillared clays
[44, 45], where large quantities of polychlorinated benzenes
were formed during oxidation of chlorobenzene. The change
in the catalytic activity of Sb incorporated AMPA catalysts
thermally treated in the range of 300–500 °C is due to the
fact that these samples possessed diferent redox properties
as a result of treatment temperature; a similar observation
was reported by Predoeva et al. [46] in case of vanadium
incorporated heteropolyacid samples.
more number of Sb atoms is the highly active. It is mainly
due to the fact that Keggin structured polyoxometalates are
multi-electron oxidants and Sb-containing heteropoly com-
pounds are the strongest oxidants [47]. Previously, Cavani
et al. [48] reported that substitution of one or more Sb atoms
in the Keggin compounds resulted in improvement in the
oxidizing capability of heteropoly compound because of Sb
redox potential. It can be concluded that the Sb containing
AMPA catalysts ofers dominant activity due to modifca-
tion of redox properties of heteropolyacid framework by the
Sb atoms.
The conversion of chlorobenzene at diferent chloroben-
zene concentrations over representative AMPSb3-300 cata-
lyst is shown in Fig. 8b. This study is essential majorly due
to the fact that the concentration of hazardous materials
usually changes in the industrial emissions. As shown in
fgure, the chlorobenzene conversion was decreased with
increase of concentration of chlorobenzene from 1300 to
5200 ppm. However, increase of reaction temperature from
150 °C to 350 °C resulted gradual increase of chloroben-
zene conversion. A similar trend was observed in case of
all the investigated catalysts. Further, the stability of the Sb
incorporated AMPA samples under the reaction conditions
was investigated.
Infuence of Sb atoms in the Keggin structured AMPA
catalysts over gas phase oxidation of chlorobenzene was
investigated. The catalytic activities AMPASb catalysts cal-
cined at diferent three diferent temperature are compared in
Fig. 8a. The highest activity was found in case of the cata-
lysts calcined at 300 °C. With the increase of Sb atoms in
AMPA structure, the catalytic activity is increased linearly.
When Sb atom reaches 3, the total conversion is reached
to 70% at 300 °C. The results clearly indicated increase
in catalytic activity of AMPA catalysts in chlorobenzene
oxidation after Sb atoms substitution and the catalyst with
The Fig. 9 shows the chlorobenzene conversion at 300 °C
over AMPSb1-400, AMPSb2-400 and AMPSb3-400 catalysts
for continuous 15 h. A minor decline of activity was observed
in the catalysts, however the extent of loss of activity is very
small in case of AMPSb3-400 catalyst compared to AMPSb1-
400 and AMPSb2-400 catalysts. The substituted Sb atoms are
enhancing the Keggin structure stability and also assisting for
the generation of the surface interactive Sb–O–Mo groups
(XPS results). In addition to the structural stability at high tem-
peratures, the redox ability of incorporated Sb–O–Mo groups
b 90
150oC
200oC
250oC
300oC
350oC
a
Reaction temperature: 300 oC
70
GHSV: 8000 h-1
calcined at 300 oC
calcined at 400 oC
75
60
45
30
15
0
60
50
40
30
20
10
calcined at 500 oC
0
1
2
3
1300
2600
3900
5200
Number of Sb atoms in AMPA
Chlorobenzene concentration (ppm)
Fig. 8 a Infuence of Sb atoms in AMPA catalysts over gas phase oxidation of chlorobenzene b Chlorobenzene conversion at diferent chlo-
robenzene concentrations over representative AMPSb3-300 catalyst
1 3

A. Alshehri, K. Narasimharao
75
70
65
60
55
50
45
40
35
30
catalysts were thermally treated at diferent temperatures in
the range of 300–500 °C. The synthesized catalysts were
analyzed by elemental analysis, thermogravimetric analysis,
X-ray difraction, Fourier transformed infrared spectroscopy,
scanning electron microscopy, N2-physisorption, 31P MAS
NMR, X-ray photoelectron spectroscopy, DR UV–Vis and
H2-temperature programed reduction techniques. The char-
acterization results indicated that AMPA samples main-
tained the Keggin structure after incorporation of Sb atoms
and the introduction of Sb infuenced the thermal stability
and redox properties of the AMPA catalysts. The catalytic
performance of the thermally treated Sb incorporated AMPA
catalysts were examined for the gas phase oxidation of chlo-
robenzene and the performance of the synthesized samples
was compared with activity of bulk AMPA samples. The
incorporation of three Sb atoms enhanced the chlorobenzene
conversion. Among the studied catalysts, AMPSb3-300 cata-
lyst ofered the best results; 90% conversion at 350 °C with
100% selectivity to COx. The improvement in catalytic activ-
ity is mainly due to change in the redox properties as the
XPS and H2-TPR results indicated that the electron transfer
between Sb3+ and Mo5+ caused increase in surface oxygen
species. The observed results revealed that the chloroben-
zene oxidation activity is infuenced by redox nature and
Keggin structure stability of Sb substituted AMPA catalysts.
AMPSb1-400
AMPSb2-400
AMPSb3-400
0
2
4
6
8
10
12
14
16
Time on stream (h)
Fig. 9 Time on stream analysis of the Sb incorporated AMPA sam-
ples
could expedite the direct involvement of Sb in activation of O2
and chlorobenzene molecules.
The stable chlorobenzene conversion levels in case of
AMPSb3-300 catalyst during time-on-stream analysis
may be because of presence of more number of Sb–O–Mo
species in the catalyst. The slight decrease of the activity
could be due to surface reduction of Sb-AMPA catalyst dur-
ing catalytic reaction as HCl and Cl2 are the products and
they usually make catalysts deactivated because of surface
decomposition. Paul et al. [49] reported that lattice oxygen
plays a crucial role in the selective oxidation reaction and
with increase of number of Mo5+ species could accelerate
the migration of the lattice oxygen, and lead to high selectiv-
ity to oxygenated products. It was reported in the literature
that the oxygen source for Keggin structured heteropoly
compounds catalyzing oxidation reactions is actually origi-
nating from the Keggin anion (bridging oxygen atoms of the
[PMo12O40]3− ion), which is eventually replenished by the
O2 in the reactants stream [50]. Therefore, the catalyst which
contained maximum surface redox sites would be the best
oxidation catalyst. From the characterization data it is clear
that the Sb-AMPA samples possessed high thermal stability,
considerably high surface area and easy reducibility. The
AMPSb3-300 sample possessed more number of Sb–O–Mo
interactive species on the surface of the catalyst, which are
responsible for superior chlorobenzene oxidation activity.
Acknowledgements This project was funded by the Deanship of Sci-
entifc Research (DSR) at King Abdulaziz University, Jeddah, under
Grant No. G-36-130-1440. The authors, therefore, acknowledge with
thanks DSR for technical and fnancial support.
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Afliations
Abdulmohsen Alshehri1 · Katabathini Narasimharao1
* Katabathini Narasimharao
Abdulaziz University, P. O. Box 80203, Jeddah 21589,
Kingdom of Saudi Arabia
nkatabathini@kau.edu.sa
1
Surface Chemistry and Catalytic Studies Group,
Department of Chemistry, Faculty of Science, King
1 3