Nitrogen-rich copolymeric microsheets supporting copper nanoparticles for catalyzing arylation of N-heterocycles

Article abstract of DOI:10.1016/j.apcata.2011.06.019

Nitrogen-rich copolymeric microsheets were synthesized through incomplete condensation of melamine and cyanuric chloride. The materials can be well-dispersed in polar solvent and perform excellently as supports or stabilizing agents for metal nanoparticles. The presence of nitrogen in the lattice improves their interaction with the copper nanoparticle precursor (Cu(II) ions). Monodispersed copper nanoparticles over these supports were obtained after reducing the absorbed precursors. The materials were characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS). The catalytic C-N coupling reaction was selected as the model reaction to evaluate the catalytic performance of the supported nanoparticles. The catalysts show above 90% yield of coupling products and retain activity after a five-run recycling test. Surface chemistry and morphology transforming during the catalytic process was further investigated. Nitrogen-rich copolymeric microsheets were proven to be excellent supports and stabilizing agents for metal nanoparticles.

Full text of DOI:10.1016/j.apcata.2011.06.019

Applied Catalysis A: General 403 (2011) 104–111
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Nitrogen-rich copolymeric microsheets supporting copper nanoparticles for
catalyzing arylation of N-heterocycles
Zhijun Huanga,b, Fengbo Lia,∗, Bingfeng Chena,b, Fei Xuea,b, Guochang Chena,b, Guoqing Yuana,∗
a Beijing National Laboratory of Molecular Science, Laboratory of New Materials, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China
b Graduate University of Chinese Academy of Sciences, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 May 2011
Received in revised form 15 June 2011
Accepted 20 June 2011
Available online 25 June 2011
Nitrogen-rich copolymeric microsheets were synthesized through incomplete condensation of melamine
and cyanuric chloride. The materials can be well-dispersed in polar solvent and perform excellently as
supports or stabilizing agents for metal nanoparticles. The presence of nitrogen in the lattice improves
their interaction with the copper nanoparticle precursor (Cu(II) ions). Monodispersed copper nanopar-
ticles over these supports were obtained after reducing the absorbed precursors. The materials were
characterized by powder X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray
photoelectron spectroscopy (XPS). The catalytic C–N coupling reaction was selected as the model reac-
tion to evaluate the catalytic performance of the supported nanoparticles. The catalysts show above 90%
yield of coupling products and retain activity after a five-run recycling test. Surface chemistry and mor-
phology transforming during the catalytic process was further investigated. Nitrogen-rich copolymeric
microsheets were proven to be excellent supports and stabilizing agents for metal nanoparticles.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Heterogeneous catalysis
Nanomaterials
Surface chemistry
Copper
Arylation
1. Introduction
involves the pyrolysis of nitrogen-rich precursors, which incor-
porating triazine rings or generating them through condensation
Graphite oxide (GO) [1,2] and chemically derived graphene
(CDG) [3–5] have been extensively explored as versatile build-
ing block for nanomaterials. Layered structures of GO are partially
oxidized graphitic sheets where epoxide and hydroxyl groups
are located on the basal planes. These functional groups facili-
tate the absorption and anchoring of ions, molecules and clusters
[6–8]. Such graphitic sheets are promising stabilizing agents for
catalytic metal nanoparticles [9–11]. Nitrogen-rich carbon nitride
(g-C3N4) is another type of graphene-like two-dimension (2D) lay-
ered structure [12–14]. Carbon nitride exhibits various possibilities
in building up lattice with exciting new properties, as predicted
by theoretical study predicts [15,16]. Most carbon nitride has a
very high thermal and chemical stability. Its electronic structure
is suitable for catalyzing many chemical reactions. Graphitic-
C3N4 (g-C3N4) and some incompletely condensed precursors have
shown promising applications in heterogeneous metal-free cat-
alysts [17–19], photocatalysts [18,20,21], and catalyst supports
[22,23]. However, poor porosity and lacking of mesostructure
for catalysis limit their applications. To enhance its performance,
mesostructured graphitic-C3N4 was synthesized through hard-
template methods [24–26]. The synthesis of g-C3N4 generally
and polymerization [26–29]. The thermal treatment results in poor
porosity in the bulk materials. However, incompletely condensed
fragment could be obtained under controlled synthesis conditions
[30]. In this work, nitrogen-rich copolymeric fragment sheets were
synthesized through controlled solution condensation of melamine
and cyanuric chloride. The resulting layered structures can be
well-dispersed in some polar solvents by sonication like graphite
oxide. Furthermore, the presence of nitrogen in the lattice improves
its electronic structure for the interaction with active catalytic
centers, such as ions, metal complexes, and nanoparticles. Well-
dispersed fragment sheets in water were used as stabilizing agents
for “bottom-up” synthesis of supporting metal nanoparticles and
the resulting nanocomposite was proven to be an active and recy-
clable catalyst for arylation of N-heterocycles.
In contrast to numerous efficient homogeneous Pd and Cu
catalyzed C–N coupling reactions [31,32], few really recyclable
catalysts for C–N coupling under heterogeneous conditions have
been developed. Many supports have been attempted specifi-
cally for the coupling of unsaturated N-heterocycles with aryl
halides. Majoral and co-workers reported an iminopyridine lig-
and grafted onto phosphorus dendrimers for supporting copper
(I), which catalyzed the coupling of aryl iodides or vinyl bromides
with pyrazole at 25–80 ◦C [33]. Ligands or copper complexes sup-
ported over silica, the apatite support, magnetic nanoparticles have
been designed for the coupling of N-heterocycles with aryl halides
∗
Corresponding authors. Tel.: +86 10 62634920; fax: +86 10 62559373.
E-mail addresses: lifb@iccas.ac.cn (F. Li), yuangq@iccas.ac.cn (G. Yuan).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.06.019

Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104–111
105
[34–36]. Cu or Cu2O nanoparticles are another type of active hetero-
geneous catalyst for the C–N coupling reaction [37,38]. In this study,
nitrogen-rich copolymeric sheets were synthesized and applied as
a support or stabilizing agent for copper nanoparticles. The catalytic
C–N coupling of N-heterocycles with aryl bromide was selected as
the model reaction to evaluate the catalytic performance of sup-
ported copper nanoparticles.
2. Experimental
2.1. Chemicals and materials
Cu(II)
acetate,
CuI,
sodium
carbonate,
potassium
carbonate, triethylamine, dimethylsulfoxidem (DMSO), N,N-
dimethylformamide (DMF) and pyridine were purchased from
Sinopharm Chemical Reagent Co. Ltd. All solvents were distilled
from powder CaH2 before using. Cyanuric chloride, melamine,
cesium carbonate, all the N-heterocycles and aryl bromine were
purchased from Aldrich. All the reagents were used as received.
2.2. Nitrogen-rich copolymeric microsheets supporting copper
nanoparticles
Melamine (12.61 g, 0.1 mol) was suspended in 100 mL dry DMF
and subsequently a pyridine solution (48.3 mL, 0.6 mol) was added,
then the mixture was cooled to 0 ◦C. A solution of cyanuric chlo-
ride (9.22 g, 0.05 mol) in 50 mL DMF was then added dropwise with
stirring. After 6 h at 0 ◦C, the reaction temperature was increased
to room temperature and stirred for an additional 24 h. The mix-
ture was cooled to 0 ◦C and cyanuric chloride (9.22 g, 0.05 mol) in
50 mL DMF was then added drop wise with stirring. The reaction
temperature was increased to room temperature and stirred for an
additional 12 h. Finally, the system was heated to 80 ◦C for 12 h, and
remained at 100 ◦C for 12 h. All above process was carried out under
nitrogen atmosphere. The flask was then cooled to room temper-
ature and the precipitates were collected by decanting out DMF
solution. The solid product was washed with methanol, acetone,
and dichloromethane subsequently and this process was repeated
twice. The solid was dried under vacuum at 100 ◦C.
50 mg of the support was dispersed in 50 mL deionized water by
sonication for 1 h. Copper acetate was used as the metal precursors
and the metal load was 5 wt%. After introducing Cu(II), the mixture
was treated under sonication for additional 1 h, and then hydrazine
hydrate was added. The resulting materials were collected by cen-
trifugation, washed with methanol, acetone, and dichloromethane.
The process was repeated twice. The solid was dried under vac-
uum at 100 ◦C and stored under nitrogen. The molar copper content
determined by XPS [Cu/(Cu + N + C + O)] is 7.66%.
2.3. Catalyst characterization
X-ray photoelectron spectroscopy (XPS) data were obtained
with an ESCALab220i-XL electron spectrometer from VG Scien-
tific using 300 W Al K␣ radiation. The base pressure was about
3 × 10−9 mbar. The binding energies were referenced to the C 1s
line at 284.6 eV from adventitious carbon. Eclipse V2.1 data analysis
software supplied by the VG ESCA-Lab200I-XL instrument manu-
facturer was applied in manipulation of the acquired spectra.
Transmission electron microscopy (TEM) was obtained by a JEOL
2010 TEM with an accelerating voltage of 200 kV. Powder X- ray
diffraction (XRD) data were recorded by using Rigaku D/max-2500.
The copper content in the catalyst was determined by inductively
coupled plasma atomic emission spectroscopy (ICP-OES) using
Perkin Elmer Optima 5300dv.
Fig. 1. (a and b) TEM images of the as-synthesized nitrogen-rich copolymeric
microsheets; (c) powder X-ray diffraction pattern of the sample.
nally to TMS (ı 0.0 ppm). Chemical shifts (ı) were reported as part
per million (ppm). 13C NMR spectra were referenced to CDCl3 (ı
77.16 ppm, the middle peak). Mass spectra were obtained with
positive electron spray ionization on Waters Micromass ZQ4000
instrument. Melting points were determined through a Buchi
instrument. Elemental analysis data were obtained with FLASH
EA1112.
1H NMR and 13C NMR spectra were recorded on a Varian-Unity
400 MHz NMR Spectrophotometer. Spectra were referenced inter-

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Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104–111
Scheme 1. Dispersing nitrogen-rich copolymeric microsheets in deionized water
by sonication and introducing of copper nanoparticles.
2.4. General procedure for catalytic C–N coupling of
N-heterocycles with aryl bromide
In an oven dried 10 mL round-bottom flask, catalyst (10 mol%
with respect to aryl bromine), aryl halide (1 mmol), N-heterocycles
(1 mmol), base (2 mmol), DMSO (2 mL) were stirred under argon
atmosphere at 120 ◦C. After the completion of the reaction, the
catalyst was precipitated with dichloromethane, filtered off and
washed with methanol followed by acetone and dried at room
temperature. The filtrate was diluted with dichloromethane and
washed with saturated NaCl solution. The organic layer was dried
over anhydrous MgSO4 and concentrated under reduced pressure
to get the crude product. Then the crude product was purified by
column chromatography to afford pure product. Detailed charac-
terization data of C–N coupling products are provided in Supporting
Information.
3. Results and discussion
3.1. Synthesis and characterization of nitrogen-rich copolymeric
microsheets supporting copper nanoparticles
Nitrogen-rich copolymeric sheets were characterized by pow-
der X-ray diffraction and transmission electron microscopy
(TEM). Fig. 1c is the powder XRD pattern and
a strong
reflection peak is clearly identified at 2ꢀ = 26.00–29.00◦ (d-
spacing = 0.330–0.312 nm), with which there appears an amor-
phous broad peak. The sharp peak is attributed to the interplaner-
spacing value for the (0 0 2) planes, which shows the similar layered
patterns as a graphite [39]. In addition, a peak at 2ꢀ = 19.66◦ (d-
spacing = 0.4512 nm) indicates the slight possibility of forming
oligomeric or polymeric crystals, which might be attributed to the
(1 0 0) peak of graphite-like C3N4 microcrystal [40–43]. Based on
TEM images (Fig. 1a and b), an array of layered structure can be
clearly discriminated. This confirms the layered structure described
by the powder XRD pattern.
Asymmetrical (0 0 2) line is divided into three compo-
nents related to polymorphism. They are identified as Peak-
1: 2ꢀ = 26.98◦ (d-spacing = 0.3302 nm), Peak-2: 2ꢀ = 27.58◦ (d-
spacing = 0.3231 nm), Peak-3: 2ꢀ = 28.54◦ (d-spacing = 0.3125 nm).
The presence of nitrogen in the lattice decreases d-spacing value of
(0 0 2) (d (0 0 2) for graphite is 0.340 nm) [44–47]. XPS was applied
to determine the chemical composition. The detailed data would
be discussed in the following sections. Elemental analysis indicates
that the nitrogen content is higher than 60 wt% and this proves the
substrate is really ‘nitrogen-rich’.
Fig. 2. (a) TEM image of nitrogen-rich copolymeric microsheets supporting copper
nanoparticles; (b) Cu 2p envelop e in XPS spectrum of supported copper nanoparti-
cles, binding energy of Cu 2p3/2 is labeled as 932.4 eV.
Nitrogen-rich copolymeric sheets could be well dispersed in
deionized water through sonication. Cu(II) ions are introduced
into the solution and the reducing agent is added after sonication.
Then light floccule is formed (Scheme 1). After post- treatment,
the resulting materials were characterized by TEM (Fig. 2a) and
XPS (Fig. 2b). TEM image shows that copper nanoparticles are kept
in a monodispersed state and particle size is under 10 nm. Most
copper nanoparticles have a filigree shape. Some nanoparticles
are located at the surface of the support and some are embed-
ded into gaps between layers. Chemical states of supported copper
nanoparticles were measured by X-ray photoelectron spectroscopy
(XPS). The binding energy of Cu 2p3/2 is 932.4 eV. This indicates
that supported copper species are kept in a zero-valent chemical

Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104–111
107
Fig. 3. (a) Coupling of benzene bromide and imidazole, the curve of imidazole con-
version with reaction time; (b) five-run test of recyclability for coupling of benzene
bromide and imidazole.
state. Herein, monodispersed copper nanoparticles over nitrogen-
enriched microsheets were obtained.
3.2. Catalytic results for arylation of N-heterocycles
Catalytic arylation of N-heterocycles was selected as a model
reaction to evaluate the catalytic activity of the as-synthesized
nanocomposite. Table 1 shows the results of screening experiment
of reaction conditions. DMSO is the best solvent. The best base
is Cs2CO3. The target product yield reaches 92% at reaction tem-
perature of 120 ◦C. Nitrogen-rich copolymeric microsheets have
chelating ability with copper species. They can act as the ligand for
CuI catalyzed coupling reaction [48] and the yield of this process is
75.7% (entry 8). This is an acceptable result for CuI-catalyzed C–N
coupling reaction [49,50]. However, leaching out of the CuI is very
serious after reaction and the recycling of the catalyst is not pos-
sible. Unsupported reduced copper specie (entry 10) shows very
low catalytic activity. Copper nanoparticles over graphite oxide
and activated carbon were tested and exhibited middle catalytic
activity.
Fig. 4. (a) TEM image of the used catalyst; (b) Cu 2p envelop e in XPS spectrum of
supported copper nanoparticles, binding energy of Cu 2p3/2 is labeled as 931.8 eV.
shows relatively little influence on the catalytic activity. The steric
effect plays a governing role on the activity of coupling reaction and
the electronic effect of substituting group is minute.
Catalytic stability and recyclability of supported metal nanopar-
ticles are further investigated through the model reaction of
benzene bromide with imidazole. The conversion of imidazole
increases gradually with reaction running (Fig. 3a). There is a five-
hour initiating stage. The curve reaches a plateau above 90% after
20 h running. The recyclability of the catalysts was studied in a
five-run recycling for coupling of benzene bromide with imida-
zole (Fig. 3b). After each run, the catalysts were isolated by adding
of dichloromethane, filtered off and washed with water followed
by acetone and dried at room temperature for the next run. The
activity slightly dropped run by run, and sharply decreased to less
than 50% (yield) in the fifth run. In our experiments, unsupported
Fifteen substrates, as shown in Table 2, were tested. The reaction
system has high activity for some simple substrates, but the activ-
ity decreases sharply when using benzoimidazole as N-heterocycle.
The activity of different aryl halide is ArI > ArBr > ArCl (entry 1 of
Table 2). The alteration of substituting groups of aryl bromide

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Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104–111
Table 1
Screening experiments of reaction conditions.
.
Entry
Catalyst
Reaction temperature (◦C)
Solvent
Base
Isolated yield (%)
1
2
3
4
5
6
7
8
9
Cu-NP
Cu-NP
Cu-NP
Cu-NP
Cu-NP
Cu-NP
Cu-NP
CuI
Cu(II) acetate
Cu(II) acetate/NH2NH2
Cu-GOb
Cu-ACc
120
120
120
120
120
110
90
120
120
120
120
120
DMSO
DMSO
DMSO
DMF
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
DMSO
NEt3
No product
37.2
64.8
76.1
92.0
63.5
29.7
75.7
15.1
K2CO3
K3PO4
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
a
10
11
12
32.2
56.3
48.6
Cu-NP: nitrogen-rich copolymeric microsheets supporting copper nanoparticles.
a
Cu(II) acetate was in situ reduced by NH2NH2.
Copper nanoparticles supported over graphite oxide (GO).
Copper nanoparticles supported over activated carbon (AC).
b
c
reduced copper species cannot be recycled. Copper nanoparticles
over graphite oxide and activated carbon show sharply activity
decrease in the second run.
after catalytic arylation of N-heterocycles. Further inspection of
Fig. 4a shows that copper nanoparticles transfer to the support
surface and aggregate to a certain extent. The main copper nanopar-
ticles have the size of around 10 nm. This indicates that copper
nanoparticles are catalytic sites and their involvement in the cat-
alytic process remodels their morphology. However, the copper
nanoparticles are kept in a well-dispersed state. The exact cop-
per load amount of the fresh catalyst and the used catalyst are
determined by ICP-OES and distinctly leaching of metal has not
been observed. Binding energy of Cu 3d3/2 of the used catalyst is
931.8 eV (Fig. 4b), which has −0.6 eV difference from Cu species of
the fresh catalyst (Fig. 2b). This illustrates that the copper nanopar-
ticles’ surfaces have adsorbed or chelated with some anions such as
Br−, CO32− in reaction mixture. Surface chemistry and morphology
transforming of the supported nanoparticles during catalytic aryla-
tion of N-heterocycles passivate their catalytic activity and directly
3.3. Surface chemistry and morphology transformation during
catalytic arylation of N-heterocycles
Surface chemistry and morphology of the supported catalysts
have subtle relation with several aspects of performance of the cat-
alysts. Greater insight into the catalytic behavior of the supported
nanoparticles can be gained through interpreting surface chem-
istry and morphology transformation during catalytic arylation of
N-heterocycles.
TEM image of the used catalyst (Fig. 4a) shows the dispersing
state of copper nanoparticles. Comparison with the fresh catalyst
(Fig. 2a) reveals that the particles grow bigger and lose their shape
Scheme 2. Defective graphitic structure from direct condensation of melamine and cyanuric chloride.

Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104–111
109
Table 2
Results of catalytic C–N coupling of various substrates.
Entry
Substrate
Het-NH
Product
Isolated yield (%)
Br
N
HN
1
95.1
67.7
N
N
HN
N
N
HN
>99
Br
N
HN
HN
N
N
N
2
3
92.0
98.2
Br
N
N
Br
N
HN
HN
N
N
4
5
95.0
97.0
N
N
N
Br
N
N
Br
Br
NPh2
HN
HN
N
NPh2
NPh2
6
7
77.3
74.6
N
N
N
N
NPh2
Br
N
N
N
N
N
HN
HN
8
9
65.2
85.7
O
O
O
N
Br
N
O
N
HN
HN
10
11
86.1
83.5
N
Br
Br
N
N
Br
12
13
37.3
61.0
N
HN
Reaction conditions: In 10 mL round-bottom flask, catalyst (10 mol% with respect to aryl bromine), aryl halide (1 mmol), N-heterocycles (1 mmol), base (2 mmol), DMSO
(2 mL) were stirred under argon atmosphere at 120 ◦C.
lead to the loss of activity in the recyclability test (Fig. 3b). This is
the main reason that sharp decrease in activity (below 50% yield)
in the 5th run, when compared to the 90% yield in the 1st run.
We drew a comparison about the main chemical components
of nitrogen-rich supports between the fresh catalyst and the used
catalyst. First of all, we should understand the way of building
up the lattice by basic units. Scheme 2 shows a graphite-like
ideal model structure from condensation between melamine and
cyanuric chloride. However, the actual materials contain various
defects distributed at random in the lattices, as labeled as 1, 2,
3, and 4. Such discontinuities affect the interaction of layers and
interplaner-spacing values. This directly causes the splitting of
(0 0 2) peak of powder XRD (Fig. 1c). The existing of defects in the
lattice leads to many terminal groups, as labeled as TG1, TG2, TG3,
and TG4. The latter two are hydrolysates of terminal groups com-
prising cyanuric chloride units. Fig. 5 shows the C 1s envelopes in
XPS spectra of the fresh catalyst and the used catalyst, fitting of the C
1s envelopes identifies two chemical states: carbon in the lattice (C
1s-1) and carbon of terminal groups (C 1s-2). According to the bind-
ing energy and corresponding bonding state, C 1s-1 is attributed to
carbon atoms around by nitrogen in the lattice and TG1/TG2. C 1s-2
is carbon atoms bonding with oxygen, such as TG3 and TG4. Binding
energy fluctuation of C 1s-1 and C 1s-2 is attributed to the error of
XPS determination. The atomic ratios of C 1s-1 and C 1s-2 have no
marked changes after reaction. This demonstrates that the lattice
keeps constant during the catalytic process. Fitting of O 1s envelope
in XPS spectrum of the fresh catalyst demonstrates two chemical
states: adsorbed H2O molecules (O 1s-1) and terminal groups of

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Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104–111
Fig. 5. C 1s envelopes in XPS spectra of the fresh catalyst and the used catalyst; peak
fitting is indicated.
TG3 and TG4 (O 1s-2) (Fig. 6). After reaction, the O 1s-2 core dis-
appears and only weak O 1s-1 peak is detected in XPS spectrum
of the used catalyst. This finding illustrates the disappearance of
active hydroxyls of TG3 and TG4 during the catalytic process. Fit-
Fig. 7. N 1s envelopes in XPS spectra of the fresh catalyst and the used catalyst;
peak fitting is indicated.
ting of the N 1s envelopes identifies two chemical states: nitrogen
atoms in triazine cycle (N 1s-1) and secondary amino bridge (N 1s-
2) (Fig. 7). Their binding energy and corresponding atom ratio show
negligible fluctuations. It is concluded that the main chemical com-
ponents of nitrogen-rich compolymeric sheets are kept constant
during the catalytic process and some active terminal groups are
transformed.
4. Conclusion
In summary, monodispersed copper nanoparticles over
nitrogen-rich copolymeric microsheets synthesized through
incomplete condensation of melamine and cyanuric chloride were
obtained. Copper nanoparticles are under 10 nm and have filigree
shape. The supported nanoparticles show high catalytic activity
for C–N coupling products of aryl bromide and N-heterocycles.
The catalytic reaction system is suitable for a broad spectrum
of aryl bromide substrates and retains activity after a five-run
recycling test. Surface chemistry and morphology of such catalytic
materials during the catalytic process were investigated through
XPS and TEM. The substrate was proven to be excellent supports
and stabilizing agents for catalytic metal nanoparticles.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.06.019.
Fig. 6. O 1s envelopes in XPS spectra of the fresh catalyst and the used catalyst; peak
fitting is indicated.

Z. Huang et al. / Applied Catalysis A: General 403 (2011) 104–111
111
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