Magnetic Fe3O4@MgAl-LDH composite grafted with cobalt phthalocyanine as an efficient heterogeneous catalyst for the oxidation of mercaptans

Article abstract of DOI:10.1016/j.molcata.2015.03.001

Magnetically separable layered double hydroxide MgAl-LDH@Fe₃O₄ composite supported cobalt phthalocyanine catalyst was synthesized and used for the aerobic oxidation of mercaptans to corresponding disulfides under alkali free conditions. The catalyst exhibited excellent activity for the oxidation of mercaptans using molecular oxygen as an oxidant which can be effectively recovered by using an external magnetic field. In addition, the covalent immobilization of cobalt phthalocyanine to MgAl-LDH@Fe₃O₄ support prevents the leaching of the catalyst and improves its activity and stability.

Full text of DOI:10.1016/j.molcata.2015.03.001

Accepted Manuscript
Title: Magnetic Fe3O4@MgAl–LDH composite grafted with
cobalt phthalocyanine as an efficient heterogeneous catalyst
for the oxidation of mercaptans
Author: Pawan Kumar Kareena Gill Sunil Kumar Sudip K.
Ganguly Suman L. Jain
PII:
S1381-1169(15)00080-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.03.001
MOLCAA 9439
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
2-12-2014
12-2-2015
1-3-2015
Please cite this article as: Pawan Kumar, Kareena Gill, Sunil Kumar, Sudip
K.Ganguly, Suman L.Jain, Magnetic Fe3O4@MgAlndashLDH composite
grafted with cobalt phthalocyanine as an efficient heterogeneous catalyst for
the oxidation of mercaptans, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.03.001
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Magnetic Fe3O4@MgAl–LDH composite grafted with cobalt phthalocyanine as an efficient
heterogeneous catalyst for the oxidation of mercaptans
Pawan Kumara, Kareena Gillb, Sunil Kumar,b Sudip K. Ganguly,b*and Suman L. Jaina*
aChemical Sciences Division, CSIR-Indian Institute of Petroleum, Dehradun 248005, India
Email : sumanjain@iip.res.in
bRefining Technology Division, CSIR-Indian Institute of Petroleum, Dehradun India 248005;
Email: sganguly@iip.res.in
Highlights
►
► Magnetically separable layered double hydroxide composite
► Heterogeneous cobalt phthalocyanine catalyst for oxidation of
mercaptans under alkali free conditions ► Facile recovery of the
catalyst by using external magnet and efficient recycling without
change in catalytic activity ► Efficient conversion and higher
product yields ► Improved methodology for sweetening of
petroleum products
Graphical abstract

Abstract
Magnetically separable layered double hydroxide MgAl–LDH@Fe3O4 composite supported
cobalt phthalocyanine catalyst was synthesized and used for the aerobic oxidation of mercaptans
to corresponding disulfides under alkali free conditions. The catalyst exhibited excellent activity
for the oxidation of mercaptans using molecular oxygen as an oxidant which can be effectively
recovered by using an external magnetic field. In addition the covalent immobilization of cobalt
phthalocyanine to MgAl–LDH@Fe3O4 support prevents the leaching of the catalyst and
improves its activity and stability.
Keywords: Sweetening, Oxidation, Cobalt phthalocyanine, Immobilized catalys, Magnetically
separable catalyst
1. Introduction
Presence of mercaptans (RSH), in the petroleum products are undesirable due to their
foul odor and corrosive nature [1-4]. Therefore, removal of these mercaptans from refinery
streams is necessary before end use. The process of removing mercaptans or converting these
to less deleterious disulphides by catalytic oxidation is known as “sweetening”[5-6].
Sweetening involves the catalytic oxidation of the mercaptans to innocuous disulfides with
air by cobalt phthalocyanines in the presence of caustic soda as a co-catalyst [7-10].
However, the use of caustic is undesirable due to its hazardous nature and its disposal issues
due to the stringent environmental regulations[11]. Further, homogeneous nature of the
catalyst makes this process less attractive due to difficulty in recovery and re-cyclability of
the catalyst. One of the logical approaches to overcome both the limitations is the use of
solid bases as support to immobilize homogeneous cobalt phthalocyanine catalyst which not

only make the process caustic free but also make the catalyst easily recoverable and re-
cyclable [12-17]. In this regard, Liu et al reported bifunctional catalysts composed of
tetrasodium salt of cobalt(II)-tetrasulfophthalocyanine (Co(II)Pc(SO3Na)4, designated as
CoPcTs) supported on different Mg–Al mixed oxides for the alkali free oxidation of
mercaptans[18]. Recently, the nanosized support materials i.e. nanoparticles owing to their
large surface area have emerged as efficient alternatives for the immobilization of
homogeneous metal catalysts[19-22]. However, as the size of the particles is decreased,
separation of the catalyst via filtration or centrifugation becomes a difficult and time-
consuming procedure. The most appealing way to overcome this limitation is to catalyst with
magnetic properties, allowing easy separation of the catalyst by simply applying an external
magnetic field. Besides to the facile separation of the catalyst by external magnetic field, the
magnetic nanocomposite matrices act as the stabilizer of the nanoparticles and thus providing
a means to prevent aggregation[23-26]. Recently, magnetic composites have shown
tremendous interest and widely been used for various applications including as
photocatalysts for degradation of pollutants, water splitting for hydrogen production, and
selective organic transformations[27-32]. In a recent report Mi et al reported the novel
magnetic mixed metal Mg-Al layered double hydroxides (MgAl-LDH) supported gold
nanoparticles for the oxidation of selective oxidation of alcohols[33].
In continuation to our ongoing effort for developing improved process for sweetening
of etroleum products, herein we report novel magnetically separable MgAl–LDH@Fe3O4
(layered double hydroxide) composite covalently anchored with cobalt phthalocyanine for
the oxidation of mercaptans using molecular oxygen as an oxidant under alkali free
conditions. The catalyst showed excellent catalytic activity along with the facile recovery

using an external magnetic field. After completion of the reaction, the catalyst was readily
separated by using external magnet and reused for several runs without any significant loss
in catalytic activity.
2. Experimental
2.1.
Catalyst preparation
Iron(II) chloride (99.5%), iron(III) chloride (98%), mercaptans, cobalt phthalocyanine
98%, were purchased from Sigma-Aldrich. Cobalt phthalocyanine tetrasulfonic acid (CoPcS)
was synthesized by treating CoPc with chlorosulphonic acid as following the literature
procedure. All reagents were used without further purification. Deionized water was used in
all experiments.
2.1.1 Synthesis Synthesis of iron magnetic nanoparticles (MNP) [34]
Fine sized magnetic nanoparticles were synthesized by precipitation method using
ferrous chloride and ferric chloride in alkaline aqueous medium according to the literature
procedure. In briefly, 5.2 g FeCl3 (32 mmol) and 2.0g FeCl2 (15.74 mmol) were dissolved in
25ml deoxygenated water containing 0.85 mL of 12.1 N HCl solution. The obtained solution
was added drop-wise to 250 mL, 1.5 M NaOH with vigorous stirring to getting iron
nanoparticles. The resulting MNPs were separated by using the external magnet and
centrifugation washed with water and finally with 0.01 M HCl (500 mL).
2.1.2 Preparation of MgAl-LDH@MNP
In a 100 mL methanol/water (1/1) solution 0.104g MNPs were dispersed with the help
of ultrasonication for 15 min. To this suspension an alkaline solution containing 1.280 g
Na2CO3 and 1.600 g NaOH in 100 mL methanol/water (1/1) was added drop wise to get the
pH=10. Then, another solution containing 2.31 g Mg(NO3)2·6H2O and 1.12 g
Al(NO3)3·9H2O in 100 ml methanol/water(1/1) solution was added drop wise into the above

suspension under vigorous stirring at constant pH 10 maintained by simultaneous addition of
alkaline solution. The obtained gel was kept at 60 ᵒC for 24 h. final product was isolated by
decanting and washed with water five times and dried at 60 ᵒC for 24h.
2.1.3 Immobilization of tetrasulfonated cobalt phthalocyanine (CoPcS) to MgAl-LDH@MNP
In a typical synthesis, 0.25 g CoPcS and 2 g MgAl-LDH@MNP were added to a
round bottomed flask containing 100 mL ethanol/water (1/1) mixture and stirred for 24 hours
at 80 ᵒC. The synthesized heterogeneous material was thoroughly washed with ethanol, water
and then dried at 60 ᵒC for 24 h. Cobalt content as determined by ICP-AES was found to be
1.4 wt % (0.23mmol/g catalyst).
2.2 Catalyst characterization
Rough structural morphologies of materials were determined by using Scanning
electron microscopy on Jeol Model JSM-6340F. While fine structure of samples was
determine by High Resolution Transmission Electron Microscopy using FEI-TecnaiG2 Twin
TEM working at an acceleration voltage of 200 kV. For TEM analysis well dispersed
aqueous samples were deposited on carbon coated copper grid. Phase structure and
crystalline nature of samples were determined by XRD using a Bruker D8 Advance
diffractometer at 40 kV and 40 mA with Cu Kα radiation (λ= 1.5418 nm). FTIR spectra of
samples were recorded on Perkin–Elmer spectrum RX-1 IR spectrophotometer. UV-visible
light absorption pattern of materials were executed on Perkin Elmer lambda-19 UV-VIS-NIR
spectrophotometer by using BaSO4 as reference material. Thermal degradation pattern of
LDH@MNP and CoPcS@LDH@MNP was calculated by thermo gravimetric analysis with
the help of a thermal analyzer TA-SDT Q-600 in the temperature range 40 to 900 °C with
heating rate was 10 °C/min under nitrogen flow. Brauner Emmett Teller (BET) surface area,

pore volume, pore diameter and other surface properties of samples were obtained on
Micromeritics ASAP2010 working in liquid nitrogen at 77 K. Inductively coupled plasma
was used for the determination of cobalt content and was carried out by using inductively
coupled plasma atomic emission spectrometer (ICP-AES, DRE, PS-3000UV, Leeman Labs
Inc, USA). For ICP-AES samples were prepared by digesting 0.05 g catalyst with conc.
HNO3 and heated at 70 °C for 30 min and volume was making up to 10ml by adding de-
ionized water. The yield of product was determined by GC-MS. Mercaptan contents of
blended kerosene were determined by Mettler Toledo DL50 Rondolino potentiometer using a
platinum counter electrode.
2.3 General experimental procedure for oxidation of mercaptans
Water (10 mL) and heterogeneous catalyst (0.12 g) and mercaptan (10 mmol) were
charged in a 100 mL round bottomed flask. The obtained reaction mixture was heated at 70 ᵒC
in the presence of molecular oxygen under stirring. The reaction progress was monitored by
collecting the samples in half-an-hour interval, extracted with diethyl ether and analyzed by
GC-MS. In order to check the potential of developed catalytic system for sweetening of
petroleum products, the kerosene oil was blended with n-dodecane mercaptan (300 ppm). The
resulting sample was subjected for oxidation under optimized experimental conditions. The
samples were withdrawn at various time periods and unreacted mercaptan content was
determined by using a potentiostat having a platinum counter electrode. For comparison blank
reactions were also carried out by using MgAl-LDH@MNP without CoPcS.
3. Results and discussion
3.1 Synthesis and characterization of catalyst
At first Fe3O4 magnetic naoparticles were synthesized and subsequently coated with LDH

by a simple co-precipitation method as suggested in the literature. In a typical synthesis, solution
of Mg(NO3)2 and Al(NO3)3 in water/ethanol was precipitated in suspension of iron nanoparticles
with maintaining pH 10 by adding Na2CO3 and NaOH solution simultaneously. The obtained gel
was aged for 24h at 60 ᵒC to facilitate the growth of LDH platelets on Fe3O4 nanoparticles.
Finally CoPcS was immobilized to the synthesized MgAl-LDH@MNP (denoted as LDH@MNP)
by intercalation of CoPcS molecules between LDH interlayers (Scheme 1). The cobalt content as
determined by ICP-AES was found to be 1.4 wt % (0.23mmol/g catalyst).
Scheme 1: Synthesis of CoPcS@LDH@MNP catalyst
The rough microstructure of Fe3O4 magnetic nanoparticles, LDH@MNP and
CoPcS@LDH@MNP was explored with the help of scanning electron microscopy. SEM image
of MNP shows that very small sized particles around 15 nm to 20 nm were formed (Fig. 1). After
the coating of LDH on MNP the honey comb type morphology of composite revealed the
formation of efficient interlaced platelets. In case of CoPcS@LDH@MNP, the morphology
remained almost intact, suggesting that the attachment of complex molecules did not change the
surface morphology. As shown in EDX pattern, the weight percentage of iron in MNP was found
to be 52.00 wt %, which subsequently decreased to 13.59 % after the coating of LDH. The
remarkable decrease in the iron content of LDH@MNP confirmed the good coating of LDH on
MNP’s. The appearance of cobalt in EDX pattern of CoPcS@LDH@MNP revealed the presence

of cobalt phthalocyanine in composite.
Fig. 1: SEM image of a) MNP b) LDH@MNP and c) CoPcS@MNP, EDX pattern of d) MNP e) LDH@MNP and d)
CoPcS@LDH@MNP
TEM images clearly showed that particles of Fe3O4 were coated with LDH and were of in
the range of 15-20 nm. The ridge like structures in the TEM image of LDH@MNP was
confirming the LDH coating on MNPs. After the attachment of CoPcS to the LDH@MNP the
morphology remained almost similar, this was assumed due to the attachment of the CoPcS units
on the LDH support.

Fig. 2: TEM image of a) MNP b) LDH@MNP and c) CoPcS@LDH@MNP
The X-ray diffraction pattern of MNP, LDH@MNP and CoPcS@LDH@MNP is shown
in Fig. 3. Magnetic nanoparticles exhibited very diffuse peaks corresponding to Fe3O4 at 2θ =
35.4ᵒ, 43.6ᵒ, 57.4ᵒ and 63.2ᵒ due to 311, 400, 511, and 440 plane reflections (JCPDS Card No.
65-3107) [35]. The peak intensity was found to be low because of very small size of
nanoparticles. LDH@MNP gave sharp peaks at 2θ value 11.4ᵒ, 22.8ᵒ, 34.7ᵒ, 60.5ᵒ and 61.8ᵒ
respectively due to 003, 006, 009, 110, and 113 plane of LDH [36-38]. No peaks of MNP’s were
observable because of higher coating of LDH and small size of MNP’s. In analogy to the existing
literature reports [39-40], the XRD pattern of magnetic LDH containing tetrasulfonated cobalt
phthalocyanine was found to be identical with the precursor magnetic LDH (Fig. 3). Thus we
presume that the phthalocyanine complex was attached on the surface of magnetic LDH via
SO3- ions of CoPcS and not intercalated between the LDH layers by the ion-exchanged with
carbonate anions in the LDH.

Fig 3: XRD of a) Iron MNP b) LDH@MNP and c) CoPc(SO3H)4@LDH@MNP
BET surface area, pore diameter and pore volume of synthesized materials was measured
with the help of nitrogen adsorption desorption isotherm at 77 K. For LDH@MNP and
CoPcS@LDH@MNP the adsorption desorption isotherm was Type IV which according to
IUPAC recommendation suggested that the synthesized material was mesoporous in nature Fig.
S1a [41]. The specific BET surface area (SBET), mean pore diameter, and mean pore volume of
LDH@MNP was found 93.40 m2/g, 2.41 nm, and 0.28 cm3/g respectively Fig. S1b. Whether for
CoPcS@LDH@MNP these values were found to be 80.67 m2/g, 5.29 nm, and 0.20 cm3/g
respectively. The surface area of magnetic LDH containing CoPcS did not significantly
decreased after the attachment of CoPcS complex units to the magnetic LDH. This further
confirmed that the CoPcS complex units were attached to the surface of LDH and were not
intercalated between the LDH layers.
The UV-Vis absorption spectra of CoPcS exhibited two characteristic absorption band at

305 nm (Soret band) and 660 nm (Q band) respectively due to π→π* macrocycle ring transition
Fig. 4a [42]. LDH@MNP gave a strong absorption band at 310 nm [43] that after the
immobilization of CoPcS got slightly red shifted and Q band due to presence of CoPcS was also
observed at 665 nm. These changes in the UV-Vis spectra clearly indicated the attachment of
CoPcS complex to LDH@MNP.
Fig 4: UV-Visible spectra of a) CoPc(SO3H)4 b) LDH@MNP c) CoPcS@LDH@MNP
Vibrational spectra of CoPcS Fig 5a showed characteristics ring vibrations at 572 cm-1,
640 cm-1, 750 cm-1. The peaks at 923 cm-1, 1110 cm-1, and 1159 cm-1 were due to aromatic ring
vibrations, C-H bending vibrations, C=N vibration and pyrrole ring vibration of phthalocyanine.
The peaks at 1029 cm-1, 1321 cm-1, 1724 cm-1 were attributed due to the sulfonic group. Further
a peak at 3465 cm-1 was due to –OH stretch vibration of –SO3H group [44-45]. The FTIR spectra
of LDH@MNP revealed a strong peak at 1373 cm-1 due to asymmetric stretching vibration of

CO3-2 ions [46]. The peak at 659 cm-1 was correlated to the Fe-O stretch of MNPs. The peaks at
3471 cm-1 were observed due to –OH stretching of metal hydroxide and inter layer hydrogen
bonded water molecules. These interlayer water molecules further exhibited a band at 1637 cm-1
due to the bending vibration [47]. After immobilization of CoPcS to LDH@MNP no change in
vibration pattern were observed that may be because of low loading.
Fig. 5: FTIR of a) CoPcS; b) LDH@MNP; c) CoPcS@LDH@MNP
Thermal stability of LDH@MNP and CoPcS@LDH@MNP was determined by
thermogravimetric analysis. The thermogram of MNP’s showed a small weight loss at around
100 ᵒC due to loss of moisture and after that no sharp weight loss was observed (Fig. 6a).
Thermogram of LDH@MNP showed a small weight loss at 100 ᵒC to 150 ᵒC due to the loss of
entrapped water. A very small weight loss around 206 °C was due to loss of exposed –OH groups
presented on the outer surface of material. Another major weight loss around 396 ᵒC was
assumed due to dehydroxylation and decarbonation [48-50] Fig. 6b. The CoPcS@LDH@MNP

catalyst showed similar degradation pattern as LDH@MNP (Fig. 6c). The weight loss due to the
attached CoPcS units was not clearly identified which is probably due to the lower loading in
comparison to the amount of support.
Fig 6: TGA thermogram of a) MNP; b) LDH@MNP; c) CoPcS@LDH@MNP
3.2 Catalytic activity
The catalytic activity of the synthesized heterogeneous cobalt phthalocyanine catalyst
was tested for the oxidation of mercaptans in aqueous medium using molecular oxygen as
terminal oxidant (Scheme 2).
Scheme 2: CoPcS@LDH@MNP catalyzed aerobic oxidation of mercaptans to disulfides

A wide variety of mercaptans consisting of aliphatic, aromatic, and long chain aliphatic was
selectively oxidized to their corresponding disulfides in near quantitative yields without any
evidence for the formation of corresponding sulphonic acid. The results of these experiments are
summarized in Table 1. The conversion of the mercaptan to disulfide was determined by GC-MS
and the identity of the products was established by comparing their spectral data (1H NMR) with
those of authentic samples. In general aromatic mercaptans (Table 1, entries 6-9) were found to
be less reactive than the aliphatic ones (Table 1, entries 1-5), however in the case of aliphatic
mercaptans the reactivity decreases with the increase in chain length and accordingly the
mercaptans with longer chain length require more reaction time (Table 1, entries 4-5).
The effect of temperature was also evaluated by considering the oxidation of n-dodecyl
mercaptan as the representative reaction (Fig. 7). As shown the oxidation was found to be very
slow at room temperature and then the reaction was found to be increased with temperature.
Among the various temperatures studied 70 oC was found to be optimum for this transformation.
o
Further increase in temperature beyond 70 C affected the reaction adversely and provided
significantly lower yield of the desired disulfide. Further to check the effect of cobalt catalyst, we
performed the blank oxidation reaction of do-decane mercaptan using LDH@MNP as catalyst
under otherwise identical experimental conditions. The lower obtained yield of the
corresponding disulfide (36.4 %) confirmed that the CoPcS units on the LDH@MNP support are
the active catalytic sites which provided excellent catalytic activity for this reaction.

Fig. 7: Oxidation of n-dodecyl mercaptan using LDH@MNP and CoPcS@LDH@MNP at various temerpature
Next, in order to check the practical applicability of the developed catalytic system, we
performed the oxidation of kerosene oil sample blended with n-dodecane mercaptan (300 ppm).
As shown in Fig. 8, the oxidation of mercaptan in kerosene sample was efficiently occurred and
the concentration was found to be decreased from 300 ppm to 89 ppm, 67 ppm and 38.5 ppm at
60 oC, 65 oC and 70 ᵒC respectively.

Fig. 8: Oxidation of kerosene blended with n-dodecyl mercaptan using CoPcS@LDH@MNP at various temperature
Magnetic separation of the catalyst as shown in Fig. 9 provides an effective approach for
recovering the catalyst from the reaction mixture. At the end of the reaction, the catalyst was
recovered by placing a magnetic bar near the glass bottle. The catalyst particles were found to be
attracted to the magnet with time increasing and were almost totally attracted after 5 min, leaving
a clear solution (Fig. 9). Furthermore, we performed the recycling experiments to evaluate the
stability and recyclability of the developed heterogeneous CoPcS@LDH@MNP catalyst for
oxidation of kerosene oil sample blended with n-dodecyl mercaptan (300 ppm). The results are
summarized in Fig. 10. As shown the recovered catalyst showed almost similar catalytic activity
for four runs without showing any significant decrease in the catalytic activity. Moreover, the
resulted filtrate samples were analyzed by ICP-AES analysis to determine the leaching of the
active CoPcS catalyst during the reaction. The cobalt content in the recovered catalyst was found
to be almost same (1.34 wt %) as in the fresh one, indicating that the developed catalyst is quite
stable and the reaction is truly heterogeneous in nature.

Fig: 9 Magnetic recoverability of catalyst by external magnet
Fig: 10 Recycling experiment at 70 °C for the oxidation of kerosene blended with n-dodecyl mercaptan
.
4.
Conclusion
We have demonstrated an efficient magnetic nanoparticles coated with layered double
hydroxide (LDH@MNP) immobilized cobalt phthalocyanine for the oxidation of mercaptans to
disulfides using molecular oxygen as oxidant under alkaline free aqueous medium. The basic
sites presented on the LDH surface provided active sites for the covalent attachment of

tetrasulfonated cobalt phthalocyanine catalyst. Furthermore, the developed catalyst exhibited
excellent catalytic activity for the sweetening of kerosene oil samples which were blended with
do-decane mercaptan. After completion of the reaction, the catalyst was easily recovered from
reaction mixture by the use of external magnet and reused for several runs with the consistent
catalytic activity. Moreover, the presence of magnetic core provided facile recovery of the
catalyst from the reaction mixture by using external magnetic effect.
Acknowledgement
Authors are thankful to Director IIP for granting permission to publish these results. KG
kindly acknowledges Director, IIP and Director of Thapar University for providing opportunity
to work at CSIR-IIP. PK is thankful to CSIR New Delhi for providing fellowship under Emeritus
Scientist Scheme. Further, CSIR, New Delhi is kindly acknowledged for funding in CSC-0117
12th Five Year Projects. Dr. S. Bojja, CSIR-IICT is kindly acknowledged for providing TEM
analysis of the samples. Analytical department of the Institute is kindly acknowledged for
providing support in analysis of samples.
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Table 1: Oxidation of mercaptans using CoPcS@LDH@MNP[a]
Entry
1.
Mercaptan
Disulfide
T/h
4.0
Conv.
(%)[b]
Yield
(%)[c]
TOF/
h-1
97
96
24.0
SH
S
S
S
S
2.
3.
4.0
4.0
96
94
94
92
23.5
23.0
SH
S
S
SH
S
4.
5.
6.
4.5
4.5
5.0
94
90
88
93
87
86
20.7
19.3
17.2
SH
SH
S
4
6
4
6
4
6
S
S
S
SH
S
7.
8.
9.
5.0
5.0
5.0
87
84
81
86
81
80
17.2
16.2
16.0
S
S
SH
S
S
OMe
MeO
Cl
SH
MeO
S
S
Cl
SH
Cl
[a]Reaction conditions: mercaptan (10 mmol), catalyst (0.12 g), water (10 ml) under molecular oxygen at 70 ᵒC;
bdetermined by GC-MS; cIsolated yield