Magnetic d-penicillamine-functionalized cellulose as a new heterogeneous support for cobalt(II) in green oxidation of ethylbenzene to acetophenone

Article abstract of DOI:10.1002/aoc.3774

A new efficient heterogeneous catalyst is introduced for the oxidation of ethylbenzene. The catalyst was obtained in three steps: functionalization of cellulose with d-penicillamine, deposition of Fe₃O₄ nanoparticles on cellulose–d-p

Full text of DOI:10.1002/aoc.3774

Received: 3 October 2016
DOI 10.1002/aoc.3774
Revised: 18 January 2017
Accepted: 26 January 2017
F U L L PA P E R
Magnetic D‐penicillamine‐functionalized cellulose as a new
heterogeneous support for cobalt(II) in green oxidation of
ethylbenzene to acetophenone
Sajjad Keshipour
| Kamran Adak
Department of Nanochemistry,
A new efficient heterogeneous catalyst is introduced for the oxidation of ethylben-
zene. The catalyst was obtained in three steps: functionalization of cellulose with
D‐penicillamine, deposition of Fe3O4 nanoparticles on cellulose–D‐penicillamine
and then anchoring of Co(II) to the magnetic cellulose–D‐penicillamine. High yield
and excellent selectivity were achieved for the oxidation of ethylbenzene to
acetophenone in ethanol under reflux conditions using H2O2 as a green oxidant.
Also, the recovered catalyst could be applied six times without a decrease in activity.
Nanotechnology Research Centre, Urmia
University, GCPO Box 165‐5715944931,
Urmia, Iran
Correspondence
Sajjad Keshipour, Department of
Nanochemistry, Nanotechnology Research
Centre, Urmia University, GCPO Box 165‐
5715944931, Urmia, Iran.
Email: s.keshipour@urmia.ac.ir
KEYWORDS
Funding information
Urmia University
acetophenone, cellulose, D‐penicillamine, ethylbenzene, heterogeneous catalysis, oxidation
1 | INTRODUCTION
synthesis of pharmaceuticals; (6) flavouring agent in non‐
alcoholic beverages, ice cream, candy, baked goods, gelatins
Cellulose is a carbohydrate polymer made up of repeating
β‐D‐glucopyranose units and consists of many hydroxyl
groups giving the cellulose molecule a high degree of func-
tionality. This most abundant natural biopolymer has charac-
teristic properties such as hydrophilicity, chirality,
biodegradability and high functionality. A great deal of atten-
tion has been directed in recent years at cellulose and its
derivatives focusing on their biological, chemical as well as
mechanical properties. Cellulose and its derivatives can be
used as supports since they are renewable, biodegradable
and non‐toxic.[1] There are some reports of cellulose
derivatives as supports of Cu(0) in aza–Michael addition,[2]
Pd(0) in Heck, Sonogashira, oxidation and reduction
reactions,[3–6] Cu(I)/Pd(0) in 1,3‐dipolar cycloaddition/direct
arylation sequence[7] and Co(II) in oxidation reaction.[8]
Acetophenone is a simple aromatic ketone with industrial
and natural sources[9] which has various applications includ-
ing: (1) in perfumery to impart an orange‐blossom‐like
odour; (2) catalyst for the polymerization of olefins; (3) pho-
tosensitizer for organic syntheses; (4) solvent for plastics and
resins; (5) chemical intermediate for the odorant ethyl methyl
phenylglycidate, the riot control agent 2‐chloroacetophenone,
2‐bromoacetophenone for dyes, 3‐nitroacetophenone and
and puddings, chewing gum and tobacco; (7) fragrance ingre-
dient in soaps, detergents, creams, lotions and perfumes; and
(8) solvent for synthesis of pharmaceuticals, rubber,
chemicals,
dyestuffs
and
corrosion
inhibitors.[10]
Acetophenone is mainly produced as a by‐product from
cumene oxidation to cumene hydroperoxide for the synthesis
of phenolacetone. Oxidation of ethylbenzene, acetylation of
benzene with acetyl chloride in the presence of aluminium
chloride, reaction of acetic and benzoic acids, reaction of
benzene and acetic anhydride, by‐product in the Hock phenol
synthesis and Wacker oxidation of styrene are other methods
for the manufacture of acetophenone.[11] Many green
approaches were introduced for the oxidation of ethylbenzene
to acetophenone such as aerobic oxidation catalysed by car-
bon nanotubes or carbon nanotubes filled with iron nano-
wires.[12,13] The selective oxidation of ethylbenzene using a
transition metal catalyst, such as Co(II), Mn(II) and Cu(II),
is an important approach for the production of
acetophenone.[14–17] In particular, cobalt‐based homoge-
neous and especially heterogeneous catalysts are mostly
investigated for the oxidation of ethylbenzene, because of
their plausible potential as commercial catalysts.[18–21]
Homogeneous catalysts have some drawbacks such as the
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KESHIPOUR AND ADAK
difficulty in product separation, irreversible deactivation,
self‐aggregation of active sites and low selectivity due to
secondary oxidation of acetophenone to the corresponding
carboxylic acid. However, currently the heterogenization of
homogeneous catalysts, or rather immobilization of metal
complexes on solid supports, has been used to address the
mentioned issues.
D‐Penicillamine (DPA) is used in severe and active rheu-
matoid arthritis. It is used mainly as a chelating agent in
heavy metal poisoning such as for Pb, Hg and Cu. DPA forms
stable complexes with Fe, Mn and Co cations.[22] Stable com-
plex of DPA with Co(II), active functional groups of DPA for
reaction with cellulose and catalytic oxidation ability of
Co(II) are three major reasons which directed us to the syn-
thesis of cellulose‐supported DPA–Co(II) as a new heteroge-
neous catalyst for the oxidation of ethylbenzene. In this
regard, for attaching Co(II) to cellulose for affording a bio-
compatible heterogeneous catalyst, DPA acts as a connector
with the ability to bond to both Co(II) and cellulose. Also,
Fe3O4 nanoparticles were added to the catalyst since a mag-
netic catalyst can be separated from a reaction mixture easily
with a magnet.
2 | EXPERIMENTAL
2.1 | Materials and methods
Cellulose was purchased from Merck with particle size ≤20%
less than 20 μm, ≤2% greater than 160 μm and ≥80% between
20 and 160 μm. DPAwas purchased from Sigma‐Aldrich with
≥99% purity and used without further purification. Anhy-
drous ethylbenzene was purchased from Sigma‐Aldrich with
99.8% purity and used without further purification. All other
reagents were purchased from Sigma‐Aldrich and used with-
out further purification.
X‐ray photoelectron spectroscopy (XPS) was performed
with a VG Multilab 2000 spectrometer (ThermoVGS) in an
ultrahigh vacuum. Transmission electron microscopy
(TEM) was performed with a LEO 912AB electron micro-
scope. Powder X‐ray diffraction (XRD) data were collected
with an XD‐3A diffractometer using Cu Kα radiation. Co
determination was carried out using flame atomic absorption
spectroscopy (FAAS; Shimadzu AA‐680 atomic absorption
spectrometer) with a hollow cathode lamp.
Recently, our research group introduced new cellulose‐
supported catalytic systems such as Pd nanoparticles
supported on N‐(2‐aminoethyl)acetamide‐functionalized
cellulose for the epoxidation of ethylbenzene,[4] Pd nanopar-
ticles deposited on ethylenediamine‐functionalized cellulose
for Heck and Sonogashira couplings,[5] Pd nanoparticles
supported on cellulose‐modified graphene quantum dots
for the reduction of nitro compounds,[6] Cu(I)/Pd(0)
on ethylenediamine‐functionalized cellulose for the 1,3‐
dipolar cycloaddition/direct arylation sequence[7] and
Co(II)–phthalocyanine‐modified cellulose in the oxidation
reaction of alkylarenes and benzyl alcohols.[8] The cellu-
lose‐supported catalytic systems have some advantages such
as heterogeneous recyclable catalysts, green biocompatible
support, high yield and selectivity, and in some cases green
solvent and mild reaction conditions. Herein, a new efficient
catalytic system is introduced for the oxidation of ethylben-
zene to acetophenone using DPA–Co(II)‐functionalized
magnetic cellulose (PACMC) as a heterogeneous magneti-
cally recoverable catalyst with H2O2 as a green oxidant in
ethanol (Scheme 1).
2.2 | Preparation of PACMC
In a typical procedure, a mixture of cellulose (2.00 g) and
NaOH (0.05 g) in 20 ml of water was stirred at 60 °C for
2 h to yield a yellow gel. Chloroacetic acid (0.5 g) was added
during 0.5 h to the mixture and stirring continued for 3.5 h at
60 °C to afford cellulose acetic acid. After filtration
and drying of the product in an oven at 70 °C, it was added to
a balloon containing 50 ml of a solution of N,N′‐
dicyclohexylcarbodiimide
(DCC;
0.5
g),
N,N′‐
dimethylaminopyridine (DMAP; 0.1 g) and DPA (0.8 g) in
dimethylsulfoxide. The mixture was stirred at 60 °C for
24 h. Then, 10 ml of acetone was added to the mixture to
increase the precipitate. Cellulose–DPA (5) was obtained as
a white solid after filtration, washing with acetone
(2 × 5 mL) and drying in an oven at 60 °C. Then, a mixture
containing 5 (2.00 g), FeCl2 4H2O (0.25 g), FeCl3 6H2O
(0.67 g) and water (30 mL), after 2 h stirring under nitrogen
atmosphere, was heated to 70 °C and treated with dropwise
addition of 30 ml of NH3 (30%) during 0.5 h under nitrogen
atmosphere. After addition of ammonia, the mixture was
stirred for 0.5 h, and then the grey solid was separated with
a magnet from the mixture and dried in 50 °C under nitrogen
atmosphere. The support containing Fe3O4 (2.00 g) was
added to a vessel containing CoCl2 6H2O (2 mmol) and
water (30 mL) under nitrogen atmosphere. After 24 h of
stirring, the catalyst was separated with a magnet as a grey
solid and dried in an oven at 50 °C under nitrogen
atmosphere.
SCHEME 1 Oxidation of ethylbenzene to acetophenone using
PACMC

KESHIPOUR AND ADAK
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2.3 | Determination of Co(II) on PACMC
using FAAS
PACMC (0.05 g) was added to 10 ml of a mixture of HCl and
HNO3 (3:1) and sonicated for 3 h. Then, the mixture was fil-
tered and the total volume of the filtrate made up to about
30 ml with distilled water. The final solution was aspirated
into the flame of an AAS instrument against a blank prepared
with cellulose. The Co(II) concentration was obtained using a
calibration curve prepared with Co solution standards.
2.4 | Typical procedure for oxidation of
ethylbenzene
In a typical procedure, ethylbenzene (0.10 g, 1.00 mmol) was
added to a round‐bottom flask containing the catalyst (0.05 g,
0.75 mmol), KOH (0.056 g, 1 mmol) and EtOH (5 mL). H2O2
(30%; 20 μL, 2.00 mmol) was added dropwise to the reaction
mixture and the mixture was stirred at 80 °C for 24 h. Upon
completion, the mixture was cooled to room temperature
and the catalyst separated with a magnet. Then, EtOH was
evaporated and the product purified with column chromatog-
raphy using n‐hexane–ethyl acetate (5:1) in 96% yield.
SCHEME 2 Synthesis of PACMC
information). The appearance of a new strong absorption band
at 1603 cm−1 for carboxylic acid group of compound 4 con-
firms the bonding of acetic acid group to cellulose. The addi-
tion of DPA to compound 4 is confirmed with appearance of a
new absorption band at 1757 cm−1 for carboxylic acid group
of compound 5. Complexation of Co(II) with DPA is observ-
able from the band shift of carboxylic group of compound 5
from 1757 to 1726 cm−1 in compound 7.
The detailed characterization of PACMC was performed
with energy‐dispersive X‐ray spectroscopy (EDS), XRD,
XPS, CHNS analysis, thermogravimetric analysis (TGA),
FAAS and TEM.
2.5 | Product characterization
1H NMR (300.13 MHz, CDCl3, δ, ppm): 2.57 (3H, s, CH3),
7.65–7.35 (3H, m, CH‐Ar), 7.96 (2H, d, 3JHH = 7.6, CH‐Ar).
13C NMR (75.47 MHz, CDCl3, δ, ppm): 26.3, 128.2, 128.7,
132.9, 137.3, 198.0.
EDS shows that Co and Fe are loaded on the support
(Figure 1). EDS also shows loading of S which indicates
modification of cellulose had been performed with DPA suc-
cessfully. The CHNS analysis shows that 3.43 wt% of the
3 | RESULTS AND DISCUSSION
3.1 | Catalyst synthesis
For the preparation of the catalyst, cellulose was converted to
cellulose acetic acid in two steps. The first was the reaction of
cellulose with NaOH solution to give activated cellulose as a
yellow gel. Then, chloroacetic acid was added to the reaction
mixture to afford cellulose acetic acid during 4 h.[4] The reac-
tion of cellulose acetic acid with DPA using activation with
DCC and DMAP led to the formation of 5. DCC/DMAP is
an activator for substitution of the hydroxyl group of carbox-
ylic acid with a nucleophile such as alcohol or amine.[23]
Fe3O4 nanoparticles were deposited on the support via in situ
oxidation of Fe(II) and Fe(III) with ammonia. Finally, Co(II)
was anchored to the DPA moiety of the support during 24 h
(Scheme 2).
3.2 | Catalyst characterization
All of these transformations were confirmed from Fourier
transform infrared (FT‐IR) spectra (see supporting
FIGURE 1 EDS analysis of PACMC

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KESHIPOUR AND ADAK
catalyst is DPA. The Co loading on the catalyst is determined
to be 0.089 g (1.5 mmol) per gram of the catalyst according
FAAS analysis.
The powder XRD pattern of PACMC shows characteris-
tic (101), (021), (002) and (040) diffraction peaks of cellulose
(Figure 2).[24] Fe3O4 nanoparticle diffraction peaks of (220),
(311), (400) and (440)[25] also indicate the loading of
magnetic nanoparticles on the catalyst, these peaks not being
observed in the XRD patterns of compounds 3 and 5 (see
supporting information).
acetate leads to a 15 °C decrease on the cellulose acetate
thermal stability. Fe3O4 deposition on the catalyst does
not change its thermal stability. But, complexation of Co(II)
with cellulose–DPA has a negative effect on thermal stabil-
ity with decomposition temperature above 262 °C. This
may be attributed to the catalytic effect of cobalt in cellu-
lose decomposition.
The TEM image of the catalyst indicates deposition of
Fe3O4 nanoparticles on the support with maximum average
particle size between 45 and 60 nm (Figure 4).
To confirm the catalyst elements, XPS analysis was per-
formed (Figure 3). The XPS analysis confirms the loading
of DPA on cellulose with S 2p doublet peak at 162.2 and
163.5 eV. The peaks related to Fe 2p at 710.9 and
724.9 eV, and Co 2p at 778.2 eV evidence the catalyst
preparation.
TGA was performed for compounds 3–7 to determine
the effect of various modifications according to Scheme 2
on cellulose thermal stability (see supporting information).
TGA of cellulose shows that it decomposes above
294 °C. Cellulose acetate with decomposition temperature
above 298 °C shows that the modification of cellulose with
acetate does not have any significant effect on cellulose
thermal stability. The reaction of DPA with cellulose
3.3 | Oxidation of ethylbenzene
The catalytic activity of PACMC was evaluated in the oxida-
tion of ethylbenzene to acetophenone. To this end, the oxida-
tion of ethylbenzene (1) with H2O2 was investigated for
optimization of the reaction conditions. It is found that
0.075 mmol of Co (Table 1, entries 1–3) and 1 mmol of
KOH with 2 mmol of H2O2 at EtOH reflux are the best reac-
tion conditions for the oxidation of ethylbenzene. Oxidation
of ethylbenzene proceeds to give acetophenone (2) as the sole
product with 100% conversion and 96% isolated yield in 24 h
without any by‐products. The product was characterized
1
using H NMR and 13C NMR spectroscopy. The reaction
yield is decreased in the absence of KOH (Table 1, entry
4). An amount of 2 mmol of H2O2 is necessary for obtaining
high yield which may be attributed to the decomposition of
some H2O2 at high temperatures. After screening a variety
of solvents, EtOH and MeCN are determined to be the best
solvents (Table 1, entries 6–10), EtOH being selected as the
solvent in accordance with green chemistry principles. Selec-
tivity in all of the runs summarized in Table 1 is 100% and
does not produce any by‐products.
FIGURE 2 XRD pattern of PACMC
FIGURE 3 XPS spectrum of PACMC
FIGURE 4 TEM image of PACMC

KESHIPOUR AND ADAK
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TABLE 1 Optimization of reaction conditions for oxidation of ethylbenzenea
Entry
Catalyst amount (Co, mmol%)
Solvent
EtOH
Conversion (%)
Yield (%)b
1
6.0
7.5
9.0
7.5
7.5
7.5
7.5
7.5
7.5
7.5
94
91
96
96
43
79
96
38
86
57
65
2
EtOH
100
100
49
3
EtOH
4c
5d
6
EtOH
EtOH
84
MeCN
H2O
100
47
7
8
MeOH
CH2Cl2
EtOH–H2O (1:1)
91
9
63
10
71
aReaction conditions: ethylbenzene (1 mmol), KOH (1 mmol), H2O2 (3 mmol), solvent (5 ml), reflux, 24 h.
bIsolated yield.
cWithout KOH.
dH2O2 (1.5 mmol).
Temperature is an important factor for this reaction. The
3.4 | Recyclability of catalyst
yield is decreased at low temperatures. The reaction temper-
ature curve for the oxidation of ethylbenzene shows that
80 °C give maximum yield of the reaction (Figure 5).
Potential Co and Fe leaching into the reaction mixture
was also analysed with FAAS. For this purpose, a sample
was taken through a syringe filter during the heterogeneous
oxidation reaction of ethylbenzene during 10 min, the solvent
was evaporated and the residue was dissolved in HNO3. The
analysis of these samples with FAAS shows that the Co and
Fe concentrations in the reaction solution are less than the
detection limit. The detection limits for Co and Fe are
obtained 14 and 9 μg l−1, respectively. This result indicates
that virtually no Co and Fe leach from the surface into the
solution. Also, the FT‐IR spectrum of the recovered catalyst
was obtained, with no significant difference being observed
compared to the spectrum of fresh catalyst (see supporting
information).
Recyclability of PACMC was examined in the oxidation reac-
tion of ethylbenzene. After carrying out the reaction, the cat-
alyst was separated with a magnet as a dark solid, washed
with acetone (2 × 5 ml) and reused. Only minor decreases
in the reaction yields are observed after six repeat cycles for
these reactions (Table 2). For the investigation of catalyst
TABLE 2 Successive trials using recoverable PACMC for the oxi-
dation of ethylbenzene
Trial
Catalyst amount (g)
Yield (%)b
1
2
3
4
5
6
0.05
0.04
0.04
0.04
0.04
0.04
96
95
95
95
95
95
aReaction conditions: ethylbenzene (1 mmol), KOH (1 mmol), H2O2 (2 mmol),
EtOH (5 ml), 24 h.
bIsolated yield.
FIGURE 5 Effect of temperature on reaction yield
SCHEME 3 Proposed mechanism

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KESHIPOUR AND ADAK
TABLE 3 Comparison of results obtained for oxidation of ethylbenzene to acetophenone
Entry
Catalyst
Solvent
Time (h)
Temp. (°C)
Yield (%)
Selectivity (%)
1
2
3
4
5
6
Fe3O4@SiO2‐NH2@CS‐Co(II)[26]
Acetic acid
MeCN
1
70
70
80
95
100
80
82.5 (conversion)
92 (isolated)
80.1
≥99
85
[27]
Co(II)@TiO2
6
Co‐MCM‐41[28]
Solvent free
Solvent free
Acetic acid
EtOH
24
0.2
8
26 (conversion)
24 (conversion)
83 (conversion)
96 (isolated)
[29]
CoTPP‐P(4VP‐co‐ST)/SiO2
98
SiO2/Al2O3‐APTMS‐BPK‐Co[30]
99
PACMC
24
100
stability, the amount of Co on the catalyst was determined
before and after six runs, 1.44 mmol per gram of catalyst
being obtained for both of them.
yield and excellent selectivity. The magnetically recoverable
catalyst is recyclable for six times without any significant
decrease in the catalyst activity. The comparison of the cata-
lyst with some recent reports showed that the introduced
method is a promising approach for the oxidation of ethyl-
benzene to acetophenone.
3.5 | Oxidation mechanism
Although the mechanism of this reaction has not been
established experimentally, the formation of species 8 and 9
is conceivable in the oxidation reaction of ethylbenzene in
the presence of H2O2 and Co(II) (Scheme 3). Compound 9
produces intermediate 10 with oxidation of Co(II) to Co(III).
The hydroxyl anion with taking H+ from intermediate 10
gives intermediate 11. Intermediate 11 under oxidation with
Co(III) gives product 2 and Co(II).
ACKNOWLEDGMENT
We gratefully acknowledge financial support from the
Research Council of Urmia University.
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How to cite this article: Keshipour S, Adak K.
Magnetic D‐penicillamine‐functionalized cellulose as a
new heterogeneous support for cobalt(II) in green
oxidation of ethylbenzene to acetophenone. Appl
Organometal Chem. 2017;e3774. https://doi.org/
10.1002/aoc.3774
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