Palladium complex immobilized on graphene oxide-magnetic nanoparticle composites for ester synthesis by aerobic oxidative esterification of alcohols

Article abstract of DOI:10.1016/j.apcata.2014.10.004

A well dispersed magnetically separable palladium complex immobilized to magnetite-graphene oxide nanocomposite has been synthesized via solvothermal route. The successfully decorated graphene oxide sheets with the homogeneously dispersed iron nanoparticles and palladium(II) complex were proved by transmission electron microscopy. X-ray diffraction pattern of the synthesized catalyst revealed that palladium(II) complex was successfully immobilized to the support. Further, X-ray photoelectron spectroscopy confirmed that palladium is present in +2 oxidation state. The synthesized heterogeneous material was found to be an efficient catalyst for the oxidative esterification of alcohols with methanol by using molecular oxygen as oxidant. The developed catalyst was found to be more reactive than its homogeneous analog, i.e. PdCl₂(CH₃CN)₂. Similarly, no reaction occurred in the presence of Pd(0)/magnetite-graphene nanocomposite catalyst. These studies confirmed that the palladium(II) ions are the main catalyst and superior reactivity of heterogeneous palladium catalyst is attributed to the promotion role of graphene support in the adsorption of reactant alcohol and oxygen. After the reaction, the catalyst was easily separated by external magnetic effect and reused for several runs with consistent catalytic activity without any detectable leaching.

Full text of DOI:10.1016/j.apcata.2014.10.004

Applied Catalysis A: General 489 (2015) 17–23
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Palladium complex immobilized on graphene oxide–magnetic
nanoparticle composites for ester synthesis by aerobic oxidative
esterification of alcohols
Sanny Vermaa, Deepak Vermab, Anil K. Sinhab,∗, Suman L. Jaina,∗
a Chemical Sciences Division, CSIR – Indian Institute of Petroleum, Mohkampur, Dehradun 248005, India
b Catalytic Conversion Process Division, CSIR – Indian Institute of Petroleum, Mohkampur, Dehradun 248005, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A well dispersed magnetically separable palladium complex immobilized to magnetite–graphene oxide
nanocomposite has been synthesized via solvothermal route. The successfully decorated graphene oxide
sheets with the homogeneously dispersed iron nanoparticles and palladium(II) complex were proved
by transmission electron microscopy. X-ray diffraction pattern of the synthesized catalyst revealed
that palladium(II) complex was successfully immobilized to the support. Further, X-ray photoelectron
spectroscopy confirmed that palladium is present in +2 oxidation state. The synthesized heteroge-
neous material was found to be an efficient catalyst for the oxidative esterification of alcohols with
methanol by using molecular oxygen as oxidant. The developed catalyst was found to be more reac-
tive than its homogeneous analog, i.e. PdCl2(CH3CN)2. Similarly, no reaction occurred in the presence of
Pd(0)/magnetite–graphene nanocomposite catalyst. These studies confirmed that the palladium(II) ions
are the main catalyst and superior reactivity of heterogeneous palladium catalyst is attributed to the
promotion role of graphene support in the adsorption of reactant alcohol and oxygen. After the reaction,
the catalyst was easily separated by external magnetic effect and reused for several runs with consistent
catalytic activity without any detectable leaching.
Received 31 December 2013
Received in revised form 27 March 2014
Accepted 2 October 2014
Keywords:
Magnetically separable catalyst
Graphene oxide
Palladium
Esterification
Oxidation
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Recently, functionalized magnetic separable nanocomposite
materials have emerged as viable alternatives to conventional
Synthesis of esters particularly, methyl esters is one of the
important transformations in organic synthesis mainly due to their
wide spread applications in pharmaceutical chemistry, materials
science, and also as a protective group in organic synthesis [1]. The
conventional synthesis of methyl esters includes the reaction of
carboxylic acids or activated carboxylic acid derivatives such as
acid anhydrides or acid chlorides with methanol [2–4]. Alterna-
tive simple methodologies such as direct oxidative esterification of
aldehydes with alcohols by using stoichiometric amounts of strong
oxidants such as Na2Cr2O7, Ca(OCl)2, NaBrO3/NaHSO3, MnO2, and
N-iodosuccinimide have also been reported [5,6]. From the view-
point of green chemistry, direct oxidative esterification reactions
with alcohols, using molecular oxygen, catalyzed by reusable het-
erogeneous catalysts is an attractive and challenging area of current
research interest [7–13].
materials for organic transformations [14–16]. Besides 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. In the recent
years, magnetically separable palladium based nanocomposite
materials have been widely used for various organic transfor-
mations including oxidation, C C bond formation and coupling
reactions [17–20]. Very recently, few reports have been published
on the direct synthesis of esters via palladium catalyzed aerobic
oxidative esterification of alcohols by using molecular oxygen as
terminal oxidant [21–23]. However, in most of the cases difficult
recovery and non-recycling ability of the homogeneous palladium
catalysts make them of limited synthetic utility.
Herein we report the successful synthesis of magnetically sepa-
rable palladium(II) complex to graphene oxide, its characterization
and application in direct aerobic oxidative esterification of various
alcohols with methanol (Scheme 1). To the best of our knowledge,
this is the first report on heterogeneous palladium catalyst for the
direct oxidative esterification of alcohols using molecular oxygen
as oxidant.
∗
Corresponding authors. Fax: +91 135 2660202.
E-mail addresses: asinha@iip.res.in (A.K. Sinha), suman@iip.res.in (S.L. Jain).
http://dx.doi.org/10.1016/j.apcata.2014.10.004
0926-860X/© 2014 Elsevier B.V. All rights reserved.

18
S. Verma et al. / Applied Catalysis A: General 489 (2015) 17–23
Scheme 1. Synthesis of magnetically separable palladium(II) complex immobilized to graphene oxide 1.
2. Experimental
was allowed for continue stirring at room temperature for 24 h.
Subsequently, 80 ml water was slowly added and temperature of
reaction mixture was raised to 98 ◦C using an oil bath. After another
24 h, 200 ml of water was added, followed by another addition of
30% H2O2 (20 ml). Finally, oxidation product was filtered and puri-
fied by rinsing with 50 ml of 5% HCl solution. The filtrate cake was
repeatedly washed with copious amount of HPLC grade water until
the pH was about 6. This processed dark brown oxidized material
was dried in oven at 90 ◦C. The dried product was grounded with a
mortar and pestle to the fine powder.
2.1. Materials
Graphite powder and PdCl2 was purchased from Sigma–Aldrich
and used as received. KMnO4, H2SO4, NaNO3, HCl, NaOH, FeCl3,
ethylene glycol, and sodium acetate were obtained from Merck
(India). All the chemicals were of analytical grade and used without
further purification.
2.2. Techniques used
2.4. Synthesis of iron nanoparticles containing
graphene–magnetite nanocomposite
Fourier transform infrared spectroscopy (FT-IR) was conducted
by Perkin–Elmer spectrum RX-1 IR spectrophotometer. High res-
olution transmission electron microscopy (HR-TEM) and energy
dispersive X-ray spectroscopy (EDAX) of the nanocomposites was
executed using Phillips CM 200 operating at an acceleration volt-
age of 200 kV. Scanning electron microscopy (SEM) was performed
by Jeol Model JSM-6340F. For FE-SEM analysis aqueous dispersions
of GO and graphene–metal nanocomposites were deposited on the
glass slides, while very dilute aqueous suspensions were deposited
on carbon coated copper grids for HR-TEM analysis. The phase char-
acterization was carried out by X-ray diffractometer (XRD; model
No. PW1710). Sample for XRD was prepared by the deposition of
well dispersed graphene–metal nanocomposite on glass slide fol-
lowed by drying; the analysis was performed by using cobalt as
the target material. The conversions and selectivity of the products
were determined by high resolution GCMSD, EI, quadrapole mass
analyzer, EM detector. 1H NMR and 13C NMR spectra of the products
were performed at 500 MHz by using Bruker Avance-II 500 MHz
instrument. ICP-AES analysis was carried out by inductively cou-
pled plasma atomic emission spectrometer (ICP-AES, PS-3000UV)
by Leeman.
In the typical synthesis, graphene oxide (200 mg) was dispersed
in water (80 ml) through sonication for 30 min. The mixture of
ferrous sulphate (0.5 g, 1.7 mmol), PEG 4000 (1.0 g) and double dis-
tilled water (5 ml) was added slowly in to the GO solution with
vigorous stirring for 24 h. Total volume of solution was made up to
approx. 1 l by adding water. After 1 h, NaBH4 (1 g, 26 mmol) added
to this reaction mixture at 80 ◦C and kept it for next 2 h. The black
colored iron nanoparticles supported graphene oxide 2 was sepa-
rated by external magnet and washed with water and dried at 65 ◦C
under vacuum.
2.5. Synthesis of magnetically separable Pd(II) complex
immobilized to GO 1
A
solution of homogeneous palladium complex, i.e.
PdCl2(CH3CN)2 (0.25 g, 1 mmol) in dichloroethane (10 ml) was
mixed with magnetite–graphene nanocomposite (1 g) and the
resulting suspension was heated at 80 ◦C under stirring for 4 h. The
yellowish colored heterogeneous Pd+2/Fe/FeO/graphene nanocom-
posite was collected by magnetic separation and washed several
times with water, ethanol, and acetone, respectively. Finally the
sample was dried at 80 ◦C for 2 h under vacuum. The catalyst
was stored at room temperature without taking any precaution.
The weight percentage of palladium in the final catalyst was
found to be 10.23 wt% (0.96 mmol/g) as determined by induc-
tively coupled plasma-atomic emission spectroscopy (ICP-AES)
analysis.
2.3. Preparation of graphene oxide (GO)
Graphene oxide (GO) was prepared by harsh oxidation of
graphite powder. In a typical procedure, graphite powder (1.0 g),
NaNO3 (1.0 g) and H2SO4 (45 ml) were mixed in a reaction vessel
on an ice bath. This is followed by gradually addition of KMnO4
(6.0 g) under maintained temperature and then reaction mixture

S. Verma et al. / Applied Catalysis A: General 489 (2015) 17–23
19
Fig. 1. SEM image of: (a) magnetite–graphene nanocomposite; (b) EDAX of magnetite–graphene nanocomposite.
addition of NaBH4 to the above suspension at 80 ◦C resulted in the
formation of black colored graphene–magnetite nanocomposites.
High specific surface area of GO along with easily accessible ample
oxygen functionalities decorated on both side of GO nanosheets
provided a better support for anchoring of iron nanoparticles. The
SEM image of the synthesized magnetite–graphene nanocompos-
ite (Fig. 1), indicating that the iron nanoparticles with average
size of 30–40 nm were successfully combined with the graphene
sheets. Further, the EDAX analysis supported the deposition of the
iron nanoparticles onto the graphene surface with 8.0 wt.%. Further
the addition of PdCl2(CH3CN)2 complex into the Fe/FeO/graphene
oxide suspension in dichloroethane yielded magnetically separable
palladium(II) complex immobilized to graphene oxide as shown
in Scheme 1.
Catalyst characterization by X-ray diffraction (XRD) (Fig. 2) and
transmission electron microscopy (TEM) (Fig. 3) confirmed the suc-
cessful formation of palladium/Fe/FeO/graphene oxide 1. The X-ray
diffraction pattern (Fig. 2) indicated the crystallinity of the syn-
thesized heterogeneous material. In the XRD pattern of Fe/FeO/GO
nanocomposite (Fig. 2b), the broad peak observed at the 2ꢀ of 44.9◦
indicates the presence of zero-valent iron (␣-Fe) crystalline phases
Table 1
Oxidative esterification of benzyl alcohol with methanol under different reaction
conditions.a
Entry
Solvent
Base
Catalyst
Conv./select. (%)b
1
2
3
4
5
6
7
8
9
MeOH
MeOH
MeOH
MeOH
MeOH
DMSO
THF
MeOH
MeOH
MeOH
K2CO3
Na2CO3
Et3N
KOtBu
–
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
1
1
1
1
1
1
1
100/90
98/86
–
100/66c
–
Trace
–
Fe/FeO/GO
PdCl2(CH3CN)2
Pd(0)/Fe/FeO/GO
–
70/67d
–
10
a
Reaction condition: benzyl alcohol (1 mmol), catalyst (1 mol%), base (1.2 mmol),
solvent (3 ml) at 60 ◦C, reaction time 6 h.
b
Determined by GC–MS.
Benzaldehyde was obtained as a by-product.
Remaining was the unreacted benzyl alcohol.
c
d
2.6. General procedure for oxidative esterification of benzylic
alcohols
A 50 ml round bottomed flask was charged with heterogeneous
Pd catalyst 1 (1 mol%), K2CO3 (1.2 equiv.) and methanol (3 ml). The
base was soluble in the methanol and methanol acted as both
reactant and reaction media for the present transformation. The
resulting flask was sealed with septum, evacuated and back-filled
with oxygen. After that the benzyl alcohol (1.0 equiv.) was added to
the above suspension via syringe. The reaction mixture was stirred
at 60 ◦C for the time as given in Table 1 in the presence of oxy-
gen balloon. After completion, the mixture was cooled to room
temperature and catalyst was recovered by the influence of exter-
nal magnet. The crude product was dissolved in ethyl acetate and
washed successively with water to remove the base. The organic
layer was dried over anhydrous MgSO4 and concentrated under
reduced pressure. The residue so obtained was purified by column
chromatography using eluent ethyl acetate in hexane (20:80) to
give the pure desired product. The recovered catalyst was washed
with methanol, dried in vacuum and used for the subsequent runs.
3. Results and discussion
3.1. Synthesis and characterization of the catalyst 1
The synthetic process is indicated in Scheme 1. Graphene
oxide was prepared by oxidation of graphite powder under harsh
oxidizing conditions as following the literature procedure [25].
The synthesized graphene oxide was suspended into water and
treated with ferrous sulphate, PEG 4000 and stirred for 24 h. The
Fig. 2. XRD spectra of: (a) homogeneous PdCl2(CH3CN)2; (b) magnetite–graphene
nanocomposite; (c) magnetically separable palladium(II) complex immobilized to
graphene oxide.

20
S. Verma et al. / Applied Catalysis A: General 489 (2015) 17–23
Fig. 3. TEM of magnetically separable palladium(II) complex immobilized to graphene oxide.
while small peaks at 28.9◦ and 35.8◦ attributed to FeO crystalline
phase. In the XRD of Pd/Fe/FeO/GO, there is no C(0 0 2) diffrac-
tion peak, suggesting the disappearance of face-to-face stacking of
graphene nanosheets. This is probably due to the presence of pal-
ladium complex and iron nanoparticles acting as a spacer to keep
graphene nano-sheets isolated. Furthermore, the additional peaks
at 2ꢀ = 18.6, 28.3, 31.2o confirmed the presence of Pd2+ ions in the
synthesized heterogeneous material. The XRD pattern of homoge-
neous Pd(II) complex, i.e. [PdCl2(CH3CN)2] is shown in Fig. 2a. The
sharp peaks in the XRD pattern of 1 resemble well with the peaks
of homogeneous palladium complex, confirming that palladium
exists as Pd(II) complex in the synthesized material 1.
Thermal behavior of the prepared catalyst was determined by
thermogravimetric (TGA) analysis as shown in Fig. 5.
The weight percentage of palladium was found to be 10.23 wt.%
as determined by inductively coupled plasma-atomic emission
spectroscopy (ICP-AES) analysis.
The Pd oxidation states present in the synthesized catalyst 1
was determined by X-ray photoelectron spectroscopy (XPS) (Fig. 6).
Photoelectrons with binding energies of 337.7 and 342.9 eV are
observed, indicating that the Pd oxidation state in the synthesized
material is +2 (Fig. 6).
The morphology and structure of Pd(II) complex immobilized
to magnetite–graphene oxide nanocomposite was investigated in
detail by TEM analysis. The TEM image (Fig. 3a and b) shows clearly
that graphene oxide was decorated by palladium and iron nanopar-
ticles with the diameters in the range of 30–50 nm. The distribution
of magnetite as well as palladium complex on graphene sheets is
uniform, and no aggregated or free particles are detected. Probably
the electrostatic interaction between palladium ions and functional
groups of graphene oxide provided the homogeneous distribu-
tion of the palladium complex on the surface of graphene oxide.
The EDAX analysis (Fig. 3c), confirmed the presence of palladium
and iron nanoparticles homogeneously distributed throughout the
graphene surfaces.
FTIR spectrum [24–27] (Fig. 4) revealed the presence of various
functional groups like OH (3429 cm−1), COOH (1723 cm−1), C
O
(1074 cm−1) and C O (1633 cm−1) on the graphene oxide. The pres-
ence of these chemical functionalities makes the graphene oxide
as an ideal support for anchoring of metal ions. A clear band at
582 cm−1 related to Fe O suggested the successful grafting of iron
nanoparticles to the graphene nanosheets. Mild reducing ability of
polyethylene glycol might be responsible for the removal of some
oxygenated functional groups in graphene–magnetite nanocom-
posite (Fig. 4).
Fig. 4. FT-IR spectra of (a) graphene oxide; (b) magnetite–graphene nanocomposite;
(c) Pd(II) on magnetite–graphene nanocomposite 1.

S. Verma et al. / Applied Catalysis A: General 489 (2015) 17–23
21
Table 2
Oxidative esterification of different alcohols with methanol.a
Entry
Reactant
Product
Time (h)
6
Conv./select. (%)b
100/90
Yield (%)c
88
TOF (h−1
15
)
1
2
6
98/88
86
14.6
3
4
5
6
5
5
8
8
99/92
98/91
90/80
86/75
90
90
78
74
18.4
18.2
10
9.4
7
8
6
6
89/80
90/85
78
83
13.3
14.2
9
8
8
80/78
72/66
76
64
9.7
8.2
10
11
8
80/63
60
7.9
a
Reaction condition: substrate (1 mmol), catalyst 1 (1 mol%), K2CO3 (1.2 mmol), MeOH (3 ml) at 60 ◦C.
Determined by GCMS, remaining products were the corresponding aldehyde and unconverted alcohol.
Isolated yields.
b
c
3.2. Catalytic activity
experimental conditions. The reaction did not proceed and orig-
inal substrates were obtained at the end of the reaction (Table 1,
entry 8). Homogeneous palladium complex, i.e. PdCl2(CH3CN)2 was
found to be efficient but provided poor product yield as compared to
the heterogeneous catalyst (Table 1, entry 9). The superior reactiv-
ity of magnetically separable palladium(II) complex immobilized
to graphene oxide may be attributed to the promoting role of
graphene support in the adsorption of reactant alcohol and oxygen.
Characterization of the catalyst 1, suggested that the palladium(II)
complex was the real catalyst for the present transformation. To
verify this, we prepared magnetically separable graphene oxide
containing palladium (0) nanoparticles and studied the oxidative
esterification of benzyl alcohol under described experimental con-
ditions. The reaction did not proceed and original substrates were
recovered at the end (Table 1, entry 10).
The synthesized magnetically separable palladium(II) complex
immobilized on graphene oxide was tested for the oxidative ester-
ification of alcohols with methanol in the presence of base using
molecular oxygen as the oxidant (Scheme 2). Initially different
bases such as Na2CO3, K2CO3, NEt3 and KOtBu were tested for the
reaction of benzyl alcohol and methanol using molecular oxygen
as oxidant. Base screening showed that the organic base such as
triethylamine was found to be ineffective while a stronger base,
i.e. KOtBu gave the highest conversion albeit poor selectivity of the
desired ester (Table 1, entries 1–4). Potassium carbonate (K2CO3)
was found to be the best base from both conversion and selectivity
points of view. Likewise, no reaction was observed without addi-
tion of a base (Table 1, entry 5). Solvent effect was also investigated:
using MeOH as solvent as well as reactant afforded the best result.
Other solvents such as acetonitrile, DMSO, THF were found to be
ineffective for the reaction of benzyl alcohol with methanol under
similar experimental conditions (Table 1, entries 6–7). We also per-
formed the oxidative esterification of benzyl alcohol in the presence
of Fe/FeO/GO nanocomposite without palladium under identical
Further, we extended the reaction to a range of various aromatic
and aliphatic alcohols (Table 2) under described experimental
conditions. All the aromatic alcohols (Table 2, entries 1–8) with
electron releasing and withdrawing substituents underwent oxida-
tive methyl esterification in excellent yields. Benzylic alcohol
substituted with the strongly electron-withdrawing group such as

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S. Verma et al. / Applied Catalysis A: General 489 (2015) 17–23
Table 3
Recycling of the heterogeneous Pd(II) catalyst.a
Run
1
2
3
4
5
6
Yield (%)b
54
54
54.1
53.8
53.8
53.8
a
Reaction condition: benzyl alcohol (1 mmol), catalyst
1 (1 mol%), K2CO3
(1.2 mmol), MeOH (3 ml) at 60 ◦C, reaction time 3 h.
b
Reaction was stopped before the completion and the reaction mixture was ana-
lyzed by GC–MS.
in all cases the conversion of the benzyl alcohol to methyl ben-
zoate was found to be almost similar (54.1–53.8%) indicating that
the prepared catalyst was quite stable and efficiently recyclable
with consistent catalytic activity. Hot filtration test was performed
in the oxidative esterification of benzyl alcohol to confirm that no
leaching had occurred. The catalyst was refluxed in methanol for
3 h and then was separated by the filtration of hot reaction mix-
ture. The solvent was then charged with substrate and allowed the
oxidation under oxygen atmosphere for 10 h under identical reac-
tion conditions. GC analysis of the catalyst-free reaction indicated
that no oxidation of the substrate had occurred under these condi-
tions. The absence of palladium in the filtrate solution was further
confirmed by ICP-AES analysis, which indicated no detectable con-
tent of the Pd in the filtrate. Furthermore, the palladium content in
the recovered catalyst (10.21 wt%) was found to be same as in the
fresh one (10.23 wt%), indicating that the developed catalyst did
not show any leaching and the reaction was truly heterogeneous
in nature.
Fig. 5. TGA of (a) graphene oxide; (b) magnetite–graphene nanocomposite; (c) Pd(II)
on magnetite–graphene nanocomposite 1.
4. Conclusion
In conclusion, we described the first report on the use of mag-
netically separable palladium(II) complex immobilized to graphene
oxide as catalyst for the direct aerobic oxidative esterification of
alcohols with methanol. The developed method has been success-
fully applied to a range of substrates to give their corresponding
methyl esters in moderate to high yields. Furthermore, the use of
efficiently recyclable heterogeneous catalyst in conjunction with
molecular oxygen as the oxidant represents a step toward an envi-
ronmentally benign and sustainable process.
Acknowledgments
Fig. 6. XPS of Pd(II) complex immobilized to magnetite–graphene nanocomposite
1.
We are thankful to Director IIP, for his kind permission to publish
these results. SV and DV acknowledge CSIR, New Delhi and UGC,
New Delhi, respectively for their research fellowships.
1 ( 1mol%)
R
R
OH
OH
MeOH, 600C
K2CO3
O
Appendix A. Supplementary data
Scheme 2. Oxidative esterification of alcohols with methanol.
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.10.004.
p-NO2, was found to be equally reactive and afforded 78% yield of
the desired ester (entry 5). Finally, 2-pyridyl methanol (entry 8)
and cinnamyl alcohol (entry 9) were also transformed to the cor-
responding esters in high yields. However, aliphatic alcohols were
found to be less reactive and afforded poor yield of the desired
ester (Table 2, entries 10–13). All the products were confirmed by
comparing with their 1H, 13C NMR spectra with those of previously
known compounds.
Next we checked the stability and recycling of the heteroge-
neous catalyst by using the benzyl alcohol as the representative
example. The reaction was stopped after 3 h and catalyst was recov-
ered by the effect of external magnet and washed with methanol,
dried and reused for subsequent reaction. The recovered catalyst
was tested for six subsequent runs by stopping the reaction before
completion (Table 3). The reaction mixture was analyzed by GCMS,
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