Probing cobalt sites in CoAPO-11 via spectroscopic and activity studies

Article abstract of DOI:10.14233/ajchem.2017.20682

A systematic study undertaken for the oxidation state, coordination, stability of cobalt in CoAPO-11 structure by means of spectroscopic and catalytic activity, is presented. AlPO-11 containing Co(II) in the tetrahedral coordination was indicated by powder diffraction, electronic and EPR data. During calcination of Co(II) was partly converted to Co(III) and reverted back to Co(II) upon exposure to atmosphere. Apart from micropores, CoAPO-11 sample contains mesopores, which possibly arise from interparticle void space. The tetrahedral location of Co(II) was inferred from EPR data. The rectangular to spherical shapes of CoAPO-11 material was evident from electron micrographs. Activity studies revealed that o-cresol can selectively be converted to o-hydroxy benzoic acid. Moderate activity of CoAPO-11 along with spectroscopic analysis supports the fact that Co(II) is predominantly located in tetrahedral framework and do not convert to Co(III) effectively, as a consequence Co(II) are stable and do not display appreciable oxidative ability under mild reaction conditions.

Full text of DOI:10.14233/ajchem.2017.20682

Asian Journal of Chemistry; Vol. 29, No. 10 (2017), 2197-2202
ASIAN JOURNAL OF CHEMISTRY
https://doi.org/10.14233/ajchem.2017.20682
Probing Cobalt Sites in CoAPO-11 via Spectroscopic and Activity Studies
*
S. SAHOO and S.K. BADAMALI
Department of Chemistry, Utkal University, Vani Vihar, Bhubaneswar-751 004, India
*Corresponding author: Tel/Fax: +91 674 2567734; E-mail: skbuche@utkaluniversity.ac.in
Received: 31 March 2017;
Accepted: 30 May 2017;
Published online: 31 August 2017;
AJC-18518
A systematic study undertaken for the oxidation state, coordination, stability of cobalt in CoAPO-11 structure by means of spectroscopic
and catalytic activity, is presented.AlPO-11 containing Co(II) in the tetrahedral coordination was indicated by powder diffraction, electronic
and EPR data. During calcination of Co(II) was partly converted to Co(III) and reverted back to Co(II) upon exposure to atmosphere.
Apart from micropores, CoAPO-11 sample contains mesopores, which possibly arise from interparticle void space. The tetrahedral
location of Co(II) was inferred from EPR data. The rectangular to spherical shapes of CoAPO-11 material was evident from electron
micrographs.Activity studies revealed that o-cresol can selectively be converted to o-hydroxy benzoic acid. Moderate activity of CoAPO-
11 along with spectroscopic analysis supports the fact that Co(II) is predominantly located in tetrahedral framework and do not convert to
Co(III) effectively, as a consequence Co(II) are stable and do not display appreciable oxidative ability under mild reaction conditions.
Keywords: Aluminophosphate, CoAPO-11, o-Cresol.
metals aluminophosphate (MeAPO) emerged as one of the
INTRODUCTION
important catalytic materials for reaction in heterogenous
Aluminophosphate structures having corner sharing
building units of AlO4 and PO4 tetrahedron possessing crysta-
lline structure and well defined porous channel was first reported
by Wilson and Flanigen [1] at Union Carbide have been subse-
quently gained significant attention in the field of catalysis,
oil refining, petrochemicals and production of fine chemicals
[2-4]. Compared to microporous zeolitic analogues, AlPO
molecular sieves possess a larger variety of acid sites and
broader acid sites distribution.Among the aluminophosphates,
AlPO-11 having one dimensional pore system with a pore size
of 6.3 Å belongs to the AEL topology, contains 10 ring hexa-
gonal framework structure. Unlike the framework of a zeolite,
AlPO-11 is neutral and thereby do not display appreciable
catalytic functionality. However, a range of metals such as Fe,
Ni, V, Ti, Cu, Zn, Mg, Mn, Cr, Mo etc. partly substitutes the
framework aluminium in an AlPO which convert the substi-
tuted AlPO into an active oxidation and/or acid catalyst.
Therefore, over the period several attempts have been made
for isomorphous substitution of P(V) and/or Al(III) by transi-
tion metal ions to add catalytic functionalities [5-7].Alumino-
phosphates (AlPO) have unique properties that can be used as
hydrophilic adsorbents and their structure makes them
excellent catalysis themselves for isomerization and oxidation
of light paraffins [8]. Because of these valuable application
catalysis.
Among the metaloaluminophosphates (MeAPO),
CoAPO-11 represents one of the most proficient oxidation
catalytic systems. Several Co-substituted AlPOs have been
synthesized and characterized by employing a range of spectro-
scopic techniques [9]. Among the divalent metals studied,
cobalt containingAlPOs have attracted considerable attention
due to variable oxidation state of cobalt renders the solid a
redox catalyst [10].Active cobalt sites can be inserted into the
microporous AlPO-11 during synthesis, which facilitates the
successful incorporation of a hydrolyzable metal cation and
restricts the precipitation of their hydroxides along with
improving stability. Thomas and Raja have established the
substitution of metal ions particularly in AlPOs give rise to
obtain heterogeneous catalysts [11]. Further, the recent works
on aluminophosphate based mesoporous as well as micro-
mesoporous hybrid materials have broadened the scope of
future application of these materials [12-14]. It is largely
believed that isomorphous substitution of a Al(III) by Co(III)
results in a overall neutral and oxidation catalyst. On the other
hand, replacement of Al(III) by Co(II) leads to a Bronsted
acid sites [15]. The oxidative ability of CoAPO-11 materials
have been attributed to the interconversion of Co(II) and Co(III)
during the reaction.

2198 Sahoo et al.
Asian J. Chem.
Many researchers [16,17] have carried out the synthesis
in order to understand the oxidation state, location and role
and stability of cobalt in catalysis.
of cobalt containing AlPO-11 by adopting different methods
and evaluated the catalytic properties in various organic trans-
formations [17]. Apart from usual techniques such as XRD,
TG and FT-IR [18,19], a variety of spectroscopic methods
including DRUV-VIS [19] and EPR [20] were employed to
probe the location and oxidation of state of cobalt in the alumi-
nophosphate matrix. Beyond the spectroscopic investigation,
reactivity assessment is an important method to probe the cobalt
sites. Cobalt has the ability of activating molecular oxygen,
therefore can be used as a potential oxidation catalyst and some
important contributions are discussed below. Cobalt(II) substi-
tuted CoAPO-11 catalyst was studied with cyclodehydration
of butanol for the production of tetrahydrofuran in which acidic
behaviour was attributed for the catalytic performance [21].
Yumin et al. [22] investigated oxidation reaction of p-cresol
in presence of sodium hydroxide to protect the –OH group
and alkyl group undergoes oxidation reaction. They have
proposed a mechanism in which Co(III) plays a vital role in
initiating and propagating the reaction.At the same time, results
showed that Co(III) has better activity and selectivity than
Co(II). It has been reported that p-cresol can be oxidized to p-
hydroxy benzyl alcohol and eventually oxidized to p-hydroxy
benzaldehyde [16]. Among many synthetic methods, liquid
phase catalytic oxidation of p-cresol with molecular oxygen
appears most promising [23]. Varying cobalt in Co-saponite
showed the highest conversion of 92 % with > 90 % selectivity
to p-hydroxy benzaldehyde for liquid phase oxidation of p-
cresol under mild conditions [23]. Tian et al. [18] reported
that oxidation of cyclohexane over a series of metal containing
AlPOs in presence of molecular oxygen as oxidant and observed
that CoAPO-11 was the best catalyst yielding cyclohexanol
selectively. Oxidation of ortho/para-cresol was studied by She
et al. [24] to obtain corresponding hydroxy benzaldehyde by
using cobalt porphyrin complexes under homogeneous condi-
tion using NaOH and described that the electronic factor asso-
ciated with the ligand plays a decisive role in controlling the
selectivity of the product. Compounds like hydrotalcite have
been used as catalyst to get a high p-cresol conversion. The
autoxidation of p-cresol to p-hydroxy benzaldehyde by using
CoCl2, CoAPO-5 and CoAPO-11 catalysts in methanolic sodium
hydroxide solution were conducted [16] and observed activity
is solely attributed to the cobalt species present in homogenous
medium, i.e. cobalt is unstable inAlPO framework and released
into reaction medium during reaction.
EXPERIMENTAL
Synthesis of CoAPO-11: Microporous CoAPO-11 was
synthesized by hydrothermal method following the procedure
reported by Wilson and Flanigen [1] with the following gel
composition; 1.0Al2O3: 1.0 P2O5: 1.0 DPA: 0.1 CoO: 40 H2O.
Aluminium isopropoxide was used as the source of aluminium,
dipropylamine as structure directing agent and orthophosphoric
acid as a source of phosphorus. Aluminium isopropoxide was
kept in ageing overnight for the complete dissolution of the
mixture. An appropriate amount of orthophosphoric acid was
added with water and the resulting mixture was stirred for 2 h.
Then desired amount of cobalt nitrate was added and the
resulting mixture was allowed to stir for another 2 h. Dipropyl-
amine was added and the mixture was stirred for another 2 h.
The gel was transferred to a 175 mL autoclave lined with Teflon
and kept under aging at 448 K for 24 h. The product was
filtered, washed repeatedly with distilled water and dried at
363 K overnight. The synthesized sample was calcined in order
to remove the template by increasing the temperature from
room temperature to 823 K at a rate of 1 K min-1 under flow
oxygen. The sample was kept at 823 K for 6 h and then cooled
to room temperature under the flow of oxygen.
Characterization: Both as-synthesized and calcined
CoAPO-11 were characterized systematically by various spec-
troscopic and analytical techniques. Powder X-ray diffraction
(XRD) patterns were recorded using Brucker D8 diffractometer
with Cu Kα radiation. The diffraction patterns were recorded
in the 2θ range of 5-50°, with a scan speed and step size of
0.5° min–1 and 0.02°, respectively. Thermogravimetric analysis
(TGA) was performed under air with a Toledo-Mettler TG/
DTA/851 system. Fourier transform infrared (FT-IR) spectra
were recorded at room temperature on a Perkin Elmer 2000-
FT IR in the range 4000–400 cm–1 using KBr as a medium.
Diffuse reflectance ultraviolet and visible (DRUV-Vis) spectra
were recorded in the range 200-800 nm on Agilent Cary 100
UV-visible spectrophotometer using barium sulphate as the
reference. BET-surface area and pore size were determined
by analyzing the nitrogen adsorption-desorption isotherm at
77 K, usingASAP 2020, Micromeritics analytical system. The
crystal morphology of the materials was determined with
scanning electron microscopy (FESEM) by using Zeiss supra
40 vp instrument. Further, the morphology of the materials
was investigated with the aid of transmission electron micro-
scopy (TEM; Philips, CM200) operated at 20-200 kV. Electron
paramagnetic resonance (EPR) pattern was recorded in aVarian
machine at room temperature using tetracyanoethylene as
standard in the field range from 80-600 gauss.
Although considerable progress has been made to unravel
the oxidation state, coordination and location of cobalt in
aluminophosphate structure, still ambiguity remains regarding
role of cobalt species in catalysis. The present study relates to
the hydrothermal synthesis and systematic characterization of
cobalt containing aluminophosphate-11 (CoAPO-11) structure.
CoAPO-11 samples were characterized by various analytical
and spectroscopic techniques. Catalytic performance of the
CoAPO-11 was evaluated by performing liquid phase oxida-
tion of o-cresol. The probable structure property correlation
has been reported. While most of the earlier investigations
focused on spectroscopic studies, we have attempted to relate
both the spectroscopic data and catalytic activity performance
Catalytic reaction: In a typical catalytic reaction, o-cresol
(1 mmol, 0.108 g), acetonitrile (4 mL), catalyst (100 mg) and
35 % aqueous H2O2 or tert-butyl hydroperoxide (TBHP, 2
mmol) were placed in a double neck round bottomed flask
attached with condenser and allowed to react for 2-12 h in the
temperature range 343-353 K. The products were analyzed
using gas chromatography (Agilent 7890) connected to a HP-5
capillary column and flame ionization detector (FID). Substrate

Vol. 29, No. 10 (2017)
Probing Cobalt Sites in CoAPO-11 via Spectroscopic and Activity Studies 2199
100
conversion and product selectivity were determined using
calibration plots and standard reference samples.
In order to verify the stability of cobalt during reactions,
leaching tests was conducted as per the procedure described
below.Approximately 100 mg of sample was treated with 5 mL
of acetonitrile and 2 mL of tert-butyl hydroperoxde and stirred
for 333 K for 12 h. The sample was filtered, the filtrate was
collected and electronic spectra were recorded. Similarly,
electronic spectra were recorded for acetonitrile and tert-butyl
hydroperoxde containing cobalt nitrate and without cobalt
nitrate for comparison purpose.
98
96
94
92
90
88
RESULTS AND DISCUSSION
Catalyst colour: The colour of the as-synthesized CoAPO-
11 was royal blue in colour, which often considered the pre-
sence of Co(II) in the tetrahedral aluminophosphate framework
[20]. After calcination the colour of the sample was changed
to blue green which is usually assigned to presence of Co(III)
ions. However, gradually the colour of calcined sample reverted
back to blue maintaining Co(II) in tetrahedral coordination [25].
Powder X-ray diffraction: XRD patterns of the as-synthe-
sized and calcined CoAPO-11 are shown in Fig. 1 and the
patterns corresponded well to theAEL structure [1]. Formation
of CoAPO-11 structure is indicated by appearance of several
strong and weak reflections. Strong reflections at 2θ = 9.5,
20.4, 21.1, 22.1, 22.7 and 23.2 attributed to the planes (020),
(310), (002), (231), (112) and (240) respectively. Several weak
reflections at 2θ = 8.1, 13.2, 15.7, 24.6, 26.6, 28.6, 31.6, 32.9,
34.2, 37.8 assigned to the planes (110), (200), (121), (150),
(132), (042), (161), (152), (123), (233) were also observed.
CoAPO-11 is similar to the reflection pattern, which refers that
AEL structure is retained with cobalt insertion. Further incor-
poration of the cobalt into the AlPO-11 structure did not alter
the ordered AEL structure. Also calcination of CoAPO-11 did
notresultanycollapseofthestructure, whichissupportedbyabsence
of any signature of cobalt impurity phase in the X-ray analysis.
100
200
300
Temperature (°C)
Fig. 2. TG plot of as-synthesized CoAPO-11
400
500
600
CoAPO-11 in which 1.5 % of weight loss occurred in the initial
step corresponded to desorption of physically adsorbed water
molecule, 4.7 % weight loss occurred in the second step
corresponded to desorption of template desportion and in the
third step 3.9 % weight loss ascribed to the combustion of
organic species occluded in the channels of CoAPO-11 which
refers to the porous nature of material [18].
BET-surface area:The N2 adsorption–desorption isotherm
of CoAPO-11 molecular sieve is shown in Fig. 3. The adsor-
ption isotherm in pressure range p/po = 0.3 confirms the typical
microporous nature of CoAPO-11.A broad and flat hysteresis
observed between p/po = 0.4 to 1.0 is possibly due to meso/
macro porosity arising out of interparticle void space. Typical
BET surface area measurement resulted in a value of 128 m2
g-1 for CoAPO-11, which is an indicative of the microporous
nature of CoAPO-11 [26]. Pore volume was estimated to be
about 0.1 cm3 g-1. The pore size determined both by BJH and
DFT showed between 90-380 Å, which falls in the mesopore
region.
100
Desorption
80
(b)
Adsorption
60
40
20
0
(a)
10
20
30
(°)
Fig. 1. XRD pattern of CoAPO-11: (a) as-synthesized (b) calcined
40
50
2θ
0.0
0.2
0.4
Relative pressure (p/p0)
Fig. 3. N2 adsorption-desorption isotherm of calcined CoAPO-11
0.6
0.8
1.0
Thermogravimetric: Fig. 2 shows thermogravimetric
pattern of as-synthesized CoAPO-11. About 10 % of total
weight loss is observed in the thermogravimetry pattern of
FT-IR: Fig. 4 shows the FT-IR pattern of as-synthesized
and calcined CoAPO-11. Bands appearing at 3606 and 3445

2200 Sahoo et al.
Asian J. Chem.
(b)
(a)
630
580
535
(a)
(b)
(c)
400
500
600
Wavelength (nm)
700
800
4000
3500
3000
2500
Wavenumber (cm–1)
2000
1500
1000
500
Fig. 5. DRUV-Vis absorption spectra of CoAPO-11: (a) as-synthesized (b)
calcined and (c) calcined APO-11
Fig. 4. FT-IR spectra of CoAPO-11: (a) as-synthesized (b) calcined
cm-1 vibration can be assigned to ν(P-OH) and ν(CoP-OH)
bridged OH groups interacting with the oxygen atoms of the
framework [27]. Band at 1645 cm-1 ascribed to the bending
mode of –OH group. Similarly bands at 1227 and 1031 cm-1
are attributed to the asymmetrical stretching vibration of
exterior Al-O-P and Al-O-Co. The shoulder band appeared at
1227 cm-1 is considered as an evidence that Co(II) is incorpo-
rated into the framework of AlPO-11 [27]. Bands observed at
1127 and 731 cm-1 are assigned to interior asymmetrical and
symmetrical stretching vibration ofAlO4– and PO4+ tetrahedral
[27]. Two weak bands at 628 and 552 cm-1 assigned to the
distortion vibration of four membered rings and six-membered
rings structure of AlPO-11 is also observed. The band at 475
cm-1 is attributed to the bending vibration of the AlPO-11
double ring.
(a)
(b)
1.9
3.7
DR UV-visible spectra: Fig. 5 shows UV-visible spectra
of both as-synthesized and calcined CoAPO-11. Three absor-
ption peaks observed both in as-synthesized and calcined
at 535, 580 and 630 nm indicate the presence of cobalt in the
tetrahedral environment i.e. incorporated into the framework
of AlPO-11 [19]. The triplet bands are assigned to the 4A2(F)
→ 4T1(F), 4A2(F) → 4T1(P) and 4A2(F) → 4T2(F) d-d transition
of the divalent cobalt ions in tetrahedral coordination. From
the studies of UV-visible spectra Sponer et al. [19] have con-
cluded that cobalt was present as Co(II) in cobalt containing
aluminophosphate molecular sieves and did not change its
valence during calcination. Upon calcination, the colour of
CoAPO-11 was turned to blue green, however the electronic
absorption pattern remained the same. Such a trend is attributed
to distortion in tetrahedral Co(II) coordination. Appearance
of band in the region 250-500 nm is often considered as a signa-
ture of Co(III) [28], nevertheless in the present study such bands
are not noticed clearly, hence the presence of Co(III) was excluded.
Electron paramagnetic resonance spectra: The EPR
patterns of as-synthesized and calcined CoAPO-11 recorded
at room temperature are shown in Fig. 6. Both as-synthesized
and calcined CoAPO-11 showed signals geff at 1.9 and 3.7
assigned to Co(II) in a tetrahedral environment and in a
compressed tetrahedral environment, respectively. The broad
100
200
300 400
Field strength (Gauss)
500
600
Fig. 6. EPR of CoAPO-11: (a) as-synthesized (b) calcined
feature signal centered geff at 3.7 is due to high spin electronic
configuration [20]. Further both as-synthesized and calcined
CoAPO-11 showed signals having same line width of 47 gauss.
The relative intensity, i.e., intensities of geff 1.9 to 3.9 was esti-
mated and found to be equal, which indicates the concentration
of cobalt species remains same both in as-synthesized and
calcined sample.
Field emission scanning electron microscope: The
particle size and morphology of CoAPO-11 was probed by
FESEM and the micrographs are presented in Fig. 7. The
FESEM revealed uniform sized, spherical to distorted spherical
shapes having dimension between 10-15 µ [27]. At higher
resolution non-uniform and surface roughness appeared
showing rectangular shape rods having average size of about
5 µ. Fig. 8 shows the EDAX in which the sample contains Al,
P, Co and O. The elemental composition (in wt %) of CoAPO-
11 determined by EDAX analysis was Co (0.68), Al (19.36),
P (19.39), O (60.58). The cobalt content in the precursor gel
was 0.8 wt %, which is approximately 15 % less in the final
CoAPO-11. The shapes of crystals appeared to cuboid to

Vol. 29, No. 10 (2017)
Probing Cobalt Sites in CoAPO-11 via Spectroscopic and Activity Studies 2201
Fig. 7. FESEM of calcined CoAPO-11
Fig. 9. TEM of calcined CoAPO-11
CoAPO-11 Wt % Wt %
O
60.58 0.13
19.36 0.08
19.39 0.09
0.68 0.04
100.00
the electron diffraction patterns show different crystal planes
present according to probability. The observed electron diffrac-
tion pattern at 2.7 and 2.5 Å is attributed to the reflections (123)
and (233) respectively. The d-values determined by electron
Al
P
Co
Total
Al
O
CoAPO-11
30
20
10
0
P
diffraction patterns are in well agreement with the powder XRD
pattern. Further, aggregation of oxidic phases of cobalt was
not observed in electron micrographs.
Reactivity studies: The catalytic performance of CoAPO-
11 was evaluated by performing oxidation of o-cresol in liquid
phase using TBHP and H2O2 as the oxidant and results are
summarized in Table-1. o-Hydroxy benzaldehyde, o-hydroxy
benzoic acid and o-hydroxy benzyl alcohol were obtained as
oxidized product (Scheme-I). During the reactivity studies we
have achieved a maximum of 9.3 % substrate conversion and
o-hydroxy benzoic acid being the predominant product,
irrespective of variation in reaction conditions. For example
increase in temperature did not result remarkable increase in
substrate conversion. With the variation of the oxidant, H2O2
to TBHP the substrate conversion was decreased. Extending
the reaction studies in presence of TBHP in water or TBHP in
decane did not yield any noticeable enhancement in catalytic
performance (entry 4-7). The oxidative ability of cobalt in an
aluminophosphate matrix is largely attributed to the interplay
between Co(II) and Co(III) state [29] and lack of Co(III) sites
as evident from spectral studies, possibly results in moderate
performance of CoAPO-11.
Co
Co
0
5
10
15
keV
Fig. 8. EDAX of calcined CoAPO-11
spherical at higher resolution.As the magnification is increased
the surface appears to have made up of rectangular crystallites
having average dimension of about 2 µ seen in Fig. 7(c). Further
magnification showed the plenty of irregular surface roughness/
macroscopic voids [Fig. 7(d)].
Transmission electron microscope: TEM patterns are
shown in Fig. 9 reveals the needle shaped and uniform morpho-
logy of CoAPO-11 [29]. The electron diffraction pattern clearly
showed the crystalline nature of the CoAPO-11 material. The
crystal sizes were in the range of 4-10 nm. The bright spots in
The possibility of dissolution of CoAPO-11 framework
and leaching of cobalt into homogeneous medium to catalyze
TABLE-1
CATALYTIC OXIDATION OF o-CRESOL OVER CoAPO-11
Selectivity (%)
S-COOH
Temperature
(K)
Substrate
conversion (%)
Entry
Oxidant
Duration (h)
S-CHO
S-CH2OH
1
2
343
353
343
343
333
333
333
H2O2
H2O2
H2O2
TBHP
TBHP
TBHP
TBHP
7
7
7
12
7
10
10
9.3
7.2
1.7
–
–
–
11.5
8.9
–
–
–
59.7
58.7
100
–
–
–
28.6
33.0
3a
4b
5c
6d
7e
–
–
–
–
–
–
–
–
–
Reaction conditions: o-cresol (1.0 mmol): Oxidant (2.0 mmol): catalyst: 10 mg: solvent (acetonitrile): 4 mL
aH2O2 without CoAPO-11; bTBHP in water; cTBHP in water and CoAPO-11; dTBHP in decane; eTBHP in decane and CoAPO-11

2202 Sahoo et al.
Asian J. Chem.
COOH
CHO
CH2OH
CH3
Department of Chemistry, Delhi University, Delhi, India for
extending laboratory facility to carry out certain measurements.
OH
OH
OH
OH
CoAPO-11, oxidant
CH3CN, 6-12 h at 343-353 K
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Conclusion
In the present study, the successful synthesis of crystalline
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various spectroscopic investigation cobalt predominantly
remained as Co(II) in the tetrahedral framework of AlPO-11.
CoAPO-11 samples were observed to have uniform shape, never-
theless possessed considerable amount of surface roughness/void
space. The absence of Co(III) in CoAPO-11 do not favour an
interconversion between Co(II) and Co(III). As a consequence
catalytic performance of CoAPO-11 was moderate towards
oxidation of o-cresol under the chosen mild reaction conditions.
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ACKNOWLEDGEMENTS
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One of the authors, SS thanks University Grants Commission,
New Delhi, India for the award of Basic Scientific Research
fellowship. The authors are indebted to Prof. A. Sakthivel,