Hydrothermal synthesis of imidazole functionalized carbon spheres and their application in catalysis

Article abstract of DOI:10.1016/j.cattod.2009.05.003

We describe the production and catalytic application of highly functionalized mesoporous carbonaceous materials, obtained via a one step green and cheap process relying on the hydrothermal carbonization of carbohydrates (e.g. glucose) in the presence of functional organic monomers (e.g. vinyl imidazole). The resulting materials could be further functionalized to feature imidazolium bromides on their surface and be used for various transesterification, Knoevenagel and Aldol reactions.

Full text of DOI:10.1016/j.cattod.2009.05.003

Catalysis Today 150 (2010) 115–118
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Hydrothermal synthesis of imidazole functionalized carbon spheres
and their application in catalysis
a
a
a
Rezan Demir-Cakan , Philippe Makowski , Markus Antonietti ,
b
a,
Frederic Goettmann , Maria-Magdalena Titirici *
a
Max-Planck Institute of Colloids and Interfaces, Colloid Chemistry Department, Scientific Campus Golm, 14424 Potsdam, Germany
b
Institut de Chimie S e´ parative de Marcoule, UMR 5257, ICSM Site de Marcoule, BP 17171, 30207 Bagnols sur Ceze Cedex, France
A R T I C L E I N F O
A B S T R A C T
Article history:
We describe the production and catalytic application of highly functionalized mesoporous carbonaceous
materials, obtained via a one step green and cheap process relying on the hydrothermal carbonization of
carbohydrates (e.g. glucose) in the presence of functional organic monomers (e.g. vinyl imidazole). The
resulting materials could be further functionalized to feature imidazolium bromides on their surface and
be used for various transesterification, Knoevenagel and Aldol reactions.
Available online 7 June 2009
Keywords:
Hydrothermal carbonization
Sustainable processes
Green chemistry functional carbon
materials
ß 2009 Elsevier B.V. All rights reserved.
Catalysis
1
. Introduction
does not involve organic solvents, catalysts or surfactants.
The resulting carbon is spherically shaped and its surface is
Functional porous carbon materials touch many aspects of
our daily lives being used as sorbents [1], filters [2], electrodes
3], catalysts [4] or sensors [5]. Carbon materials are particularly
attractive due to their remarkable properties such as light-
weight, high thermal resistance, and tuneable porosity as well as
exciting electronic properties. Among their many important
applications, catalysis has major importance. They are com-
monly used as a supportive material due to their stability at high
temperature and chemical inertness against solvents, reactants
or by-products.
decorated with oxygenated functional groups. A simplified
reaction mechanism for the formation of the carbon spheres
involves the dehydration of the carbohydrate into a furan-like
molecule (mainly 5-(hydroxymethyl)-2-furaldehyde) as the first
step and subsequent polymerization and carbonization as a
second step [7].
[
Moreover HTC is also a nice way to produce functionalized
carbons. Indeed, different water soluble vinylic monomers can be
added to the HTC mixture thus yielding materials that combine the
surface properties of the polymers with the structural, mechanical,
thermal, and electric properties of the carbon framework. Recently,
we reported HTC in the presence of acrylic acid as a functional
monomer leading to materials rich in carboxylate groups which are
The synthesis of nanostructured carbon materials generally
relies on very harsh conditions which are by far beyond the
sustainability values. In stark contrast to these high cost and
energy consuming technologies, hydrothermal carbonization
2
+
2+
successfully employed as adsorbents for Pb and Cd removal
from water [9].
(HTC) was described as a sustainable one-step methodology for
the synthesis of monodisperse hard carbonaceous particles [6–8].
This route involves the hydrothermal decomposition of various
aqueous carbohydrates solutions (herein glucose) in closed
autoclaves into carbonaceous material under mild hydrothermal
synthesis conditions (<200 8C, <20 h).
Here, we report on a straightforward methodology for the
production of carbonaceous materials containing imidazole groups
residing at their surface. This procedure is based on the hydro-
thermal carbonization of glucose in the presence of small amounts
of functional organic monomer (vinyl imidazole), inside a porous
network of a silica template, followed by subsequent removal of the
sacrificial template [10]. The resulting grafted imidazole moieties
were, in a second step, converted into the corresponding alkyl
imidazoliums and successfully employed as catalysts for various
reactions including transesterification, Diels–Alder or Knoevenagel
condensations.
This approach has the advantage of being very cheap and
mild following some important rules of green chemistry since it
*
Corresponding author. Tel.: +49 331 567 9508; fax: +49 331 567 9502.
E-mail addresses: magdalena.titirici@mpikg.mpg.de, titirici@mpikg.mpg.de
(M.-M. Titirici).
0920-5861/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2009.05.003

1
16
R. Demir-Cakan et al. / Catalysis Today 150 (2010) 115–118
Table 1
2
. Experimental
Elemental analysis of the imidazole functionalized carbon.
2.1. Incorporation of water soluble monomers
Sample
C %
H %
N %
O %
HC
66.70
68.22
68.84
69.70
69.58
4.51
4.81
5.26
5.83
6.24
–
29.79
24.56
22.49
18.66
15.96
In this strategy, the main precursor is a cheap water soluble
HC-1Im
HC-2Im
HC-5Im
HC-10Im
2.41
3.41
5.81
8.22
carbohydrate (glucose), while the organic monomer (vinyl
imidazole) is required in very small amounts in order to provide
the functionality. 1, 2, 5 and 10 wt.% of vinyl imidazole with
respect to the total glucose concentration (10 wt.%) have been
employed. In order to obtain porous functional carbonaceous
materials, a mesoporous silica template (Si-100, supplied by
Merck) was used as a sacrificial template. Thus, according to the
pore volume (1 mL/g), 1 g of silica was filled with 1 mL of the
glucose containing imidazole solution, and the filled particles
were hydrothermally treated at 190 8C for 16 h. After the
hydrothermal carbonization the silica template was removed
2.3.3. Transesterification
For this catalytic test, 25 mg of HC10ImBr and 2 mmol of
substrate were added to 5 mL of benzyl alcohol in a 50 mL Teflon
lined PAAR autoclave. The autoclave was heated in an oven at
150 8C for 72 h. The reaction products were analysed by GC–MS.
The conversion rates were determined on the basis of the initial
ester consumption.
4
using a 4 M aq. solution of NH HF2. The resulting samples were
named according to the monomer concentration added into the
reaction mixture, 1%, 2%, 5% and 10 wt.% vinyl imidazole; HC-1Im,
HC-2Im, HC-5Im and HC-10Im, respectively. The carbon material
obtained from 10% pure glucose solution without any monomer
was quoted HC.
3
. Results and discussion
3.1. Incorporation of water soluble monomers into hydrothermal
carbon
The successful incorporation of imidazole groups into the
2
.2. Formation of the imidazoliums
resulting materials was firstly confirmed by elemental analysis in
which the nitrogen contents increase with increasing the amount
of added monomer; that is 2.41% for HC-1Im and reached up to
In order to test the availability of these grafted functions for
other applications, we decided to test the catalytic activity of
the solids we obtained (as other functional hydrothermal
carbons have already proved to be good catalysts [11]).
Unfortunately, imidazoles themselves are neither strongly basic
nor strongly nucleophilic and thus have found only limited
catalytic applications [12]. On the opposite, imidazoliums
8
.22% in HC-10Im (Table 1). Furthermore, a clear difference
between the spectrum of the HC from pure glucose and the
imidazole containing samples are depicted according to FT-IR
spectrum (Fig. 1). Thus, besides the adsorption bands characteristic
to hydrothermal carbon (C–OH, C O, C C, C–O–C, etc.), two new
peaks are distinguished at 1458 cm and 3100 cm which are
attributed to C N and N–H vibrations, respectively. It can be
noticed that the intensity of the C N and N–H bands becomes
more pronounced the more monomer is added into the system.
À1
À1
(
especially imidazolium based ionic liquids) have found
numerous catalytic applications, on their own [13–16] or as
stable carbenes [17–19]. We thus alkylated HC-10Im by
refluxing it over night in toluene in the presence of one mass
equivalent of butyl bromide. The resulting powder was collected
by filtration, washed twice with toluene and will be labelled HC-
Also the peak of C–N is present in the imidazole containing
materials around 1000–1300 cm
À1
.
Zeta potential measurements also confirm the presence of the
imidazole ring anchored to our carbon materials (Fig. 2). Thus, the
HC has a negative zeta potential over all the pH range while the
imidazole containing materials show positive values at an acidic
pH due to the protonation of the nitrogen atom linked to the
carbon. The isoelectric point (IEP) increases from pH 2 in HC-1Im
up to 6 in HC-10Im clearly demonstrating the basic character of the
materials.
1
2
0Bu ImBr.
2.3. Catalytic tests
The substrates for the catalysis as well as the solvents were used
as purchased. All catalytic reactions were run in SCHOTT screw
capped glass tubes (160 mm length, about 10 mm inside diameter)
with stirring or in PAAR autoclaves without stirring. The solutions
were heated at the expected reaction temperature for 12–72 h.
GC–MS analyses were run on a Agilent Technologies, GC 6890N, MS
Fig. 3 shows SEM and TEM micrographs of the HC-10Im
functional materials. The morphology and the particle size of the
5
975 instrument.
2.3.1. Diels–Alder reaction
In order to test the usefulness of HC10ImBr in Diels–Alder
reaction, 25 mg of catalyst were weighted and added to a solution
of 2 mmol of substrates together with 2 mL of acetonitrile. The
solutions were then stirred and were heated at 90 8C for 48 h and
then analysed by GC–MS.
2.3.2. Knoevenagel condensations
In order to test the versatility of our material as
a
catalyst for Knoevenagel condensations, 2 mmol of aldehyde
or ketone and 2 mmol of nucleophile were mixed in 5 mL of
acetonitrile in the presence of 25 mg of HC10ImBr. The reaction
tubes were then heated to reaction temperature for 12–20 h.
The reaction products were analysed by GC–MS. The conver-
sion rates were determined on the basis of the nucleophile
consumption.
Fig. 1. FT-IR spectra of the pure and imidazole modified hydrothermal carbons.

R. Demir-Cakan et al. / Catalysis Today 150 (2010) 115–118
117
Fig. 2. Zeta potential of the pure and imidazole modified hydrothermal carbons.
Fig. 4. Nitrogen adsorption isotherms of the template (Si-100) and vinyl imidazole
initial silica template are successfully maintained through the
replication process. The TEM micrographs show that the mesopore
system of the silica template was also successfully reproduced and
functionalized hydrothermal carbon material (HC-10Im).
Knoevenagel and Aldol condensations [21] and (iii) transester-
ifications. The obtained results are summarised in Fig. 5.
As can be seen, when the Diels–Alder condensation of
naphthquinone and cyclohexadiene is run in the presence of
2
this is also confirmed by N adsorption measurements (Fig. 4). The
isotherms of Si-100 and HC-10Im show a sharp condensation in the
range of 0.7–0.8 relative pressure, corresponding to the existence
of mesopores. According to nitrogen adsorption, surface area of the
2
HC-10Bu ImBr (Fig. 5A), a nearly complete conversion of the
2
2
HC-10Im slightly decreased from 338 m /g to 170 m /g showing a
hysteresis loop with type IV isotherm.
reactants is obtained and anthraquinone (the re-aromatisation
product) represents around 25 mol% of the products. This confirms
that bromide ions are present in the catalyst and that they feature a
strong nucleophilicity. Similarly both the Knoevenagel condensa-
tion of benzaldehyde with malononitrile and the Aldol condensa-
tion of benzaldehyde with acetophenone are achieved by HC-
3.2. Imidazolium synthesis and catalytic tests
The successful alkylation of the surface imidazol moieties could
be evidenced by EDX showing the presence of bromine in our
solids (up to 1.5 at.%) (data not shown).
10Bu
Finally, HC-10Bu
2
ImBr in high yields under relatively mild conditions (Fig. 5B).
ImBr was deprotonated with a 2 mol% solution of
2
The catalytic activity of HC-10Bu
2
ImBr was tested on three
BenzONa in benzylalcohol and tested as N heterocyclic carbene in a
standard transesterification reaction, namely the reaction of
phenyl acetate with benzyl alcohol. Here, again the expected
benzyl acetate is obtained in a moderate yield (Fig. 5C).
reactions, which were recently reported to be promoted by
imidazolium halides: (i) the aromatisation of unsaturated six
rings (especially Diels–Alder condensation products) [20], (ii)
Fig. 3. Electron micrographs of the HC-10Im sample: (a and b) SEM; (c and d) TEM.

1
18
R. Demir-Cakan et al. / Catalysis Today 150 (2010) 115–118
Fig. 5. Some reactions catalyzed by HC-10Bu
2
ImBr: (A) the aromatisation of Diels–Alder condensates; (B) Knoevenagel and Aldol condensations; (C) transesterifications.
the range of applications of such materials can also be extended
towards adsorption and electrochemistry. Furthermore, this
method provides a versatile, green and cheap strategy towards
carbon materials with a high degree of functional groups, which
are determined by the choice of organic monomer.
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