Ionic liquid templated preparation of carbon aerogels based on resorcinol-formaldehyde: Properties and catalytic performance

Article abstract of DOI:10.1039/c2jm35258c

A series of carbon aerogels were prepared with or without using ionic liquids as templates. By varying the structures and contents of ionic liquids, carbon aerogels with different pore size distributions could be obtained. The effect of the ionic liquids on the properties of the final carbon aerogels was explored and the catalytic performance of the carbon aerogels in the selective oxidation of tetralin was studied.

Full text of DOI:10.1039/c2jm35258c

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 21852
www.rsc.org/materials
COMMUNICATION
Ionic liquid templated preparation of carbon aerogels based on
resorcinol–formaldehyde: properties and catalytic performance
ab
ab
a
a
Huimin Yang, Xinjiang Cui, Youquan Deng and Feng Shi*
Received 7th August 2012, Accepted 11th September 2012
DOI: 10.1039/c2jm35258c
A series of carbon aerogels were prepared with or without using ionic
liquids as templates. By varying the structures and contents of ionic
liquids, carbon aerogels with different pore size distributions could
be obtained. The effect of the ionic liquids on the properties of the
final carbon aerogels was explored and the catalytic performance of
the carbon aerogels in the selective oxidation of tetralin was studied.
weight phenol polymers, which offers a method for the targeted
synthesis of carbon aerogel.
Herein, we’d like to present our new results in carbon aerogel
preparation using ionic liquids as templates. In this process, one of
the key operations in the preparation of carbon aerogels, i.e. the
freezing or supercritical drying of the organic gels before pyroly-
5d,5e,10
sis,
is avoided. Moreover, carbon aerogels with a wide range of
pore sizes could be obtained by controlling the contents and struc-
tures of ionic liquids. These ionic liquid-oriented carbon aerogels can
be directly used as catalysts in the selective oxidation of tetralin under
mild conditions.
Introduction
Carbon aerogels have attracted extensive attention due to their
specific properties, such as low mass densities, high surface areas,
1
biocompatibility, energy storage performance, and so on.
Experimental
Commonly, carbon aerogels are prepared through the sol–gel poly-
merization of resorcinol with formaldehyde in aqueous solution,
drying of the organic gels and subsequent high temperature pyrolysis
Preparation of carbon aerogels
The carbon aerogels were prepared through a sol–gel polymerization
of resorcinol and formaldehyde with Na CO as a catalyst and ionic
2
in an inert atmosphere. Due to the specific physicochemical prop-
2
3
erties of carbon aerogels, they are expected to be used commercially
as gas adsorption devices, separators for heavy metals, electro-
chemical capacitors, catalysts or catalyst supports, and many other
liquids as templating agents. Typically, resorcinol (R, 2.20 g,
0 mmol), formaldehyde (F, 3.25 g, 40 mmol, 36.5% in water,
methanol stabilized) and 9 mL distilled water were added into a
00 mL polytetrafluoroethylene autoclave. Subsequently, 22.1 mg
Na CO (1 mol% to resorcinol) and a suitable amount of ionic liquid
2
3
promising applications. Thus, since the first work reported by Pekala
1
and co-workers through sol–gel polycondensation of resorcinol and
2a,4
2
3
formaldehyde, numerous articles have appeared concerned with
the synthesis of carbon aerogels and how these affect the final
were added into the mixture. After vigorous stirring for 1 h, the
ꢀ
autoclave was sealed and held at 80 C for one day. Then, the
5
structure of these materials. As is well accepted, the unusual prop-
autoclave was cooled to room temperature. The wet gels were put
ꢀ
erties of carbon aerogels are derived from its unique structure. Thus,
in order to develop carbon aerogels with potential applications, clean
and economic preparation of carbon aerogels with controllable
morphologies should be developed.
into a round-bottom flask and vacuum dried at 130 C for 3 h. After
ꢀ
ꢁ1
being pyrolyzed at 800 C for 5 h in nitrogen flow (20 mL min ),
about 3.2 g black solid was obtained. Several carbon aerogels were
prepared with or without ionic liquids as templates with the same
procedure and denoted as RF-no IL, RF-5%BMImCl (B ¼ butyl,
M ¼ methyl, Im ¼ imidazole), RF-10%BMImCl, RF-20%BMImCl,
The development of ionic liquid chemistry offers potential choice
6
in many fields. Typically, it was reported that ionic liquids act as a
7
versatile solvents in a number of polymerization reactions. More-
RF-2.5%BMImBF
4
, RF-10%BMImBF
(E ¼ ethyl), RF-10%OMImBF
and RF-10%BMImN(CN) . The
4 4
, RF-15%BMImBF , 20%
over, ionic liquids have been used widely as template agents or carbon
8
source in the fabrication of nano-structured organic materials.
9
Recently, del Monte et al. explored the utilization of ionic liquids as
BMImBF , RF-10%EMImBF
4
4
4
(
O ¼ octyl), RF-10%BMImPF
6
2
numbers in the name of carbon aerogel samples represent the mass
ratio of ionic liquids to resorcinol.
deep eutectic solvents (DESs) for the polycondensation of phenol and
aldehydes. In this process, ionic liquids acted as the solvent of starting
materials and as the promoter for the formation of high molecular
Characterization of the carbon materials
a
Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou, 730000, China. E-mail:
fshi@licp.cas.cn; Tel: +86-931-4968142
Nitrogen adsorption–desorption isotherms were recorded with an
ASAP2010 and ASAP2020 adsorption analyzer at 77 K. The powder
X-ray diffraction (XRD) patterns of the samples were recorded with
a Stoe STADI P automated transmission diffractometer equipped
b
Graduate School of the Chinese Academy of Sciences, Beijing, 100049,
China
2
1852 | J. Mater. Chem., 2012, 22, 21852–21856
This journal is ª The Royal Society of Chemistry 2012

View Online
with an incident beam curved germanium monochromator using Cu
Ka radiation. Scanning electron microscopy (SEM) investigations
1
were carried out with a JSM-6701F instrument. Transmission elec-
tron microscopy (TEM) measurements were performed with a JEM
2010 microscope. XPS measurements were conducted on a VG
ESCALAB 210 instrument provided with a dual Mg/Al anode
+
X-ray source, a hemispherical capacitor analyser and a 5 keV Ar
ion-gun. The spectra were recorded using non-monochromatic Mg
Ka (1253.6 eV) radiation.
4
Fig. 2 SEM pictures of RF-no IL (left) and RF-10%BMImBF (right).
Catalytic activity measurement
the formation of meso-porous carbon aerogel. The pore size of the
carbon aerogel is about 7 nm, Fig. 1(c). However, it turned to micro-
The catalytic activity was tested by the selective oxidation of tetralin.
Typically, 2.6 g (20 mmol) tetralin and 50 mg catalyst were put into a
4
porous carbon aerogel again if more BMImBF , i.e. 15 wt%, was
100 mL glass reactor equipped with magnetic stirrer. The reaction
ꢀ
added, Fig. 1(d). Subsequently, the structure of RF-10%BMImBF
4
was carried out at 80 C for 10 h under 1 atm oxygen. After reaction,
before carbonization was studied by removing the ionic liquid under
acetone refluxing. Clearly, it was composed of pores with diameters
of 3.6 and 6.9 nm, Fig. 1(e). The results suggested that the porous
structure was relatively stable after the formation of the gel. In other
words, the mesopores were maintained during carbonization but
reconstruction occurred.
the reaction mixture was cooled to room temperature and tested by
GC-FID (Agilent 7890A) using hexadecane as an internal standard.
Results and discussion
First, by applying BMImBF as template, the effect of the amount of
4
SEM characterization suggested that the addition of BMImBF
4
ionic liquid on the structure of the targeted carbon aerogels was
explored. Clearly, the variation of the amounts of ionic liquid affected
the structure of the carbon aerogels remarkably.
does not influence the morphology of the carbon aerogels signifi-
cantly. Two typical SEM pictures of RF-no IL and RF-10%
4
BMImBF were given in Fig. 2.
According to the nitrogen adsorption–desorption isotherms,
Fig. 1(a), there is no regular structure formation without the addition
of ionic liquid as template. Therefore, the curve of the pore size
distribution was not available by the BJH model. A micro-porous
Further on, the influence of the structures of ionic liquids on the
formation of carbon aerogels was explored. As we know from above,
the addition of 10 wt% BMImBF
4
as template produced meso-
porous carbon aerogels, so the same amount was chosen for the study
carbon aerogel was obtained if 2.5 wt% BMImBF was added,
4
Fig. 1(b). By raising the amount of BMImBF to 10 wt%, we found
4
Fig. 3
RF-10%EMImBF
and (e) RF-10%BMImCl; BJH pore distribution of (a) RF-10%
OMImBF ; DFT pore distribution of (e) RF-10%BMImCl and enlarged
N
2
adsorption–desorption isotherm of (a) RF-10%OMImBF
4
, (b)
Fig. 1
.5%BMImBF
RF-10%BMImBF
BJH pore distributions of (c) and (e).
N
2
adsorption–desorption isotherms of (a) RF-no IL, (b) RF-
, (c) RF-10%BMImBF , (d) RF-15%BMImBF and (e)
(before carbonization but removing ionic liquid) and
4
, (c) RF-10%BMImPF , (d) RF-10%BMImN(CN)
6
2
2
4
4
4
4
4
DFT pore distribution of (f) RF-10%BMImCl in the range of 0–2 nm.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21852–21856 | 21853

View Online
Table 1 Physicochemical properties of carbon aerogels
2
ꢁ1
3
Pore volume (cm g
ꢁ1
a
Density (g cm )
ꢁ3
Entry
Catalysts
S
BET (m g
)
Pore diam. (nm)
)
b
b
1
2
3
4
5
6
7
8
9
RF-2.5%BMImBF
4
386
559
490
394
391
570
481
422
511
590
—
7.0
—
—
—
0.44
0.19
0.5
RF-10%BMImBF
RF-15%BMImBF
RF-20%BMImBF
RF-10%EMImBF
4
b
b
4
—
—
0.75
0.74
0.67
0.67
0.77
0.36
0.36
0.15
b
b
4
4
b
b
—
7.5
—
0.53
RF-10%OMImBF
RF-10%BMImPF
4
b
b
6
—
—
—
—
b
b
RF-10%BMImN(CN)
RF-5%BMImCl
RF-10%BMImCl
2
b
b
—
0.6
—
—
c
b
10
c
2
—
—
0–80
b
b
1
1
1
2
RF-20%BMImCl
RF-no IL
20
371
—
—
0.16
0.77
b
b
a
b
The density was measured according to ISO 3953. The BJH model is no longer valid for micro- and macro-porous materials characterization, thus the
c
values are not given. Porosity distribution obtained by original density functional theory (DFT).
of other ionic liquids as templates, Fig. 3. The physicochemical
properties of different carbon aerogels were summarized in Table 1.
To our delight, meso-porous carbon aerogels were obtained with
further characterized by SEM and TEM. As expected, a carbon
aerogel with different morphology was formed. According to the
SEM measurement, this carbon aerogel was composed of a small
carbon particle of about 100 nm in size, Fig. 4(a). This structure was
confirmed by the TEM characterization, Fig. 4(b), and it further
suggested that the particle is composed of 20–30 nm particles with
micro-pores, Fig. 4(c) and (d).
ionic liquids OMImBF
4
as templates too, Fig. 3(a). The pore size of
was used as
this carbon aerogel is about 8.1 nm. If EMImBF
4
template, a micro-porous carbon aerogel was obtained, Fig. 3(b). The
changing of anions of ionic liquids influenced the structure of carbon
aerogels significantly, too. A micro-porous carbon aerogel was
One of the features of our experiments is that the ionic liquid
template is maintained in the wet gels without removing before
pyrolysis. It makes the operation simple but possibly contaminates
6
obtained if using BMImPF as template, Fig. 3(c). However, some
meso-porous structures should be contained in RF-10%EMImBF
4
,
and RF-10%BMImPF due to the formation of little hysteresis loops.
6
If BMImN(CN) was used to instead of BMImBF or BMImPF , the
2
4
6
meso-porous structure disappeared totally and a micro-porous carbon
aerogel formed, Fig. 3(d). Interesting results were obtained with
BMImCl as templating agent, which resulted in the formation of
remarkable amounts of micro-, meso- and macro-pores, Fig. 3(e) and
(f). This structure was only observed in the sample with BMImCl as
template. Therefore, based on the results given in Table 1 and Fig. 3, it
can be concluded that the structures of the anions and also the cations
affected the porous structure of the final carbon aerogels remarkably.
Due to the interesting observation of micro-, meso- and macro-
pore distributions of RF-10%BMImCl, Fig. 3(e) and (f), it was
Fig. 5 XPS spectra of (a) B1s of RF-10%BMImBF
BMImBF , (c) Cl2p of RF-10%BMImCl, (d) N1s of RF-10%BMImBF
top) and RF-10%BMImCl (bottom), (e) O1s of RF-10%BMImBF
4
, (b) F1s of RF-10%
4
4
(
4
(top),
RF-10%BMImCl (centre) and RF-no IL (bottom) and (f) C1s of RF-10%
Fig. 4 SEM (a) and TEM (b–d) pictures of RF-10%BMImCl.
BMImBF
4
(top), RF-10%BMImCl (centre) and RF-no IL (bottom).
2
1854 | J. Mater. Chem., 2012, 22, 21852–21856
This journal is ª The Royal Society of Chemistry 2012

View Online
we don’t remove the ionic liquids before pyrolysis, it doesn’t
contaminate the final carbon aerogels. Moreover, based on the
typical O1s and C1s spectra of carbon aerogels with or without the
4
addition of BMImBF and BMImCl, Fig. 5(e) and (f), similar O1s
and C1s spectra were observed. These results indicated that the
addition of ionic liquid only altered the porosity of the carbon aer-
ogels without changing the chemical valence of carbon in the final
materials. Moreover, the surface O/C ratios of RF-no IL, RF-10%
4
BMImBF , RF-5%BMImCl, RF-10%BMImCl and RF-20%
BMImCl were 0.26, 0.015, 0.06, 0.013 and 0.019, respectively. So it
could be imagined that the addition of ionic liquids as templates
reduced the quantity of surface oxygen.
In order to check the crystal structure of the obtained carbon
aerogels, the typical samples were characterized by XRD. As shown
in Fig. 6, typical diffraction patterns of amorphous carbon materials
were observed with or without the addition of ionic liquids as
templates. As a typical technique for meso-porous material charac-
terization, small-angle X-ray diffraction (SAXRD) was applied in the
Fig. 6 XRD diffraction patterns of (a) RF-no IL, (b) RF-10%
BMImBF , (c) RF-10%EMImBF , (d) RF-10%OMImBF , (e) RF-10%
BMImCl, (f) RF-10%BMImPF and (g) RF-10%BMImN(CN)
4
4
4
6
2
.
4
characterization of RF-10%BMImBF , Fig. 7, and there is no
observable diffraction pattern of typical crystallized meso-porous
material.
Carbon materials have been used extensively in catalysis as
supports or catalysts, and here, the selective oxidation of tetralin was
used as the model reaction to check the possibility of the carbon
aerogels to be used as catalysts directly, Table 2. The results suggested
that these carbon aerogels exhibited good activity in the selective
oxidation of tetralin with or without using ionic liquids as templates,
entries 1–9. Commonly, the catalytic activity could be improved
using ionic liquids as templates. By applying a typical carbon
aerogel, i.e. RF-10%BMImCl, as catalyst, the conversion of tetralin
to 1,2,3,4-tetrahydronaphthalen-1-ol and 3,4-dihydronaphthalen-1-
one reached 30% with 78% selectivity. According to the character-
ization results given in Fig. 3 and 4 and Table 1, the high activity of
RF-10%BMImCl might be derived from its specific structure, i.e. the
highly dispersed nano-particles, the presence of abundant micro-,
meso- and macro-pores and the low density.
4
Fig. 7 The SAXRD diffraction patterns of the RF-10%BMImBF .
the carbon aerogels. Therefore, the carbon aerogels were character-
ized by XPS to check the ionic liquid species. The extraneous species
possibly derived from the ionic liquids are B, F, Cl and N, and the
typical XPS spectra of these elements are given in Fig. 5. Clearly, they
were all unobservable on the surface of carbon aerogels even though
the ionic liquids were not removed before the pyrolysis. So, although
a
Table 2 Selective oxidation of tetralin catalyzed by typical carbon aerogels
b
Conversion /%
c
Selectivity /%
d
Yield /%
Entry
Catalysts
1 : 2
1
2
3
4
5
6
7
8
9
RF-no IL
RF-10%BMImBF
RF-10%EMImBF
RF-10%OMImBF
RF-10%BMImPF
RF-10%BMImN(CN)
RF-5%BMImCl
RF-10%BMImCl
RF-20%BMImCl
Activated carbon
Activated carbon
15.7
18.9
25
17.8
15.4
19.3
17.5
30
84
68
46
75.6
89
81.5
75
78
71
64
80
13.2
12.5
11.5
13.4
13.8
15.7
13
23.9
17
2.9
11.6
1 : 2.2
1 : 3.2
1 : 2.4
1 : 2.4
1 : 2.4
1 : 2.2
1 : 1.9
1 : 2.2
1 : 1.9
1 : 1.1
1 : 2.2
4
4
4
6
2
e
24
4.5
14.6
f
1
1
0
1
g
a
ꢀ
b
c
d
Reaction conditions: 2.6 g (20 mmol) tetralin, 50 mg catalyst, 80 C, 10 h, 1 atm O
S
2
.
S
Tetralin converted. The selectivities to 1 and 2. 1 and 2
2
e
produced. Average result of reactions repeated three times.
f
2
ꢁ1
g
ꢁ1
.
BET ¼ 1000 m g
.
BET ¼ 200 m g
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 21852–21856 | 21855

View Online
3
(a) C. Moreno-Castilla and F. J. Maldonado-Hodar, Carbon, 2005,
4
1
Adv. Mater., 2010, 22, 853–857; (d) C. Moreno-Castilla,
M. B. Dawidziuk, F. Carrasco-Marin and Z. Zapata-Benabithe,
Carbon, 2011, 49, 3808–3819; (e) B. F. Machado, H. T. Gomes,
P. Serp, P. Kalck, J. L. Figueiredo and J. L. Faria, Catal. Today,
Conclusion
3, 455–465; (b) A. C. Pierre and G. M. Pajonk, Chem. Rev., 2002,
02, 4243–4265; (c) G.-P. Hao, W.-C. Li, D. Qian and A.-H. Lu,
In summary, the influence of ionic liquid on the structure of carbon
aerogels was explored with resorcinol and formaldehyde as starting
agents. Carbon aerogels with micro- or meso-porous structures could
be controllably synthesized by varying the side chain of imidazolium
cation or the anions of ionic liquids. In this method, the ionic liquid
template remained inside the wet gel without removing before
pyrolysis. Catalytic activity measurement suggested these carbon
aerogels are potential catalysts for selective oxidation reactions. This
work offers a simple method for the controllable preparation of
carbon aerogels with tunable structures.
2
010, 149, 358–364; (f) J. Li, X. Y. Wang, Q. H. Huang,
S. Gamboa and P. J. Sebastian, J. Power Sources, 2006, 158, 784–788.
R. W. Pekala and F. M. Kong, Abstr. Pap. Am. Chem. Soc., 1989, 197,
113, POLY.
4
5
(a) A. Taguchi, J. H. Sm ꢀa tt and M. Lind ꢁe n, Adv. Mater., 2003, 15,
1
209–1211; (b) A. B. Fuertes, Chem. Mater., 2004, 16, 449–455; (c)
C. Liang and S. Dai, Chem. Mater., 2009, 21, 2115–2124; (d)
A. M. ElKhatat and S. A. Al-Muhtaseb, Adv. Mater., 2011, 23,
2
887–2903; (e) S. A. Al-Muhtaseb and J. A. Ritter, Adv. Mater.,
003, 15, 101–114; (f) H.-J. Liu, W.-J. Cui, L.-H. Jin, C.-X. Wang
and Y.-Y. Xia, J. Mater. Chem., 2009, 19, 3661–3667.
2
Acknowledgements
6
7
(a) M. Freemantle, Introduction to Ionic Liquids, 2010; (b)
Q. H. Zhang, S. G. Zhang and Y. Q. Deng, Green Chem., 2011, 13,
2619–2637; (c) A. C. Gujar and M. G. White, in Catalysis, ed. J. J.
Spivey and K. M. Dooley, 2009, pp. 154–190.
(a) P. Kubisa, Prog. Polym.Sci., 2009, 34, 1333–1347; (b)
K. A. Cavicchi, ACS Appl. Mater. Interfaces, 2012, 4, 518–526; (c)
G. Schmidt-Naake, I. Woecht and A. Schmalfuss, Macromol.
Symp., 2007, 259, 226–235; (d) K. H. Shaughnessy, S. J. P’Pool,
M. A. Klingshirn and R. D. Rogers, Abstr. Pap. Am. Chem. Soc,
2004, 227, U455; (e) J. M. Lu, F. Yan and J. Texter, Prog. Polym.
Sci., 2009, 34, 431–448; (f) S. Mallakpour and Z. Rafiee, Prog.
Polym. Sci., 2011, 36, 1754–1765.
We thank the National Natural Science Foundation of China
(21073208) and Chinese Academy of Sciences for supporting this
research.
Notes and references
1
2
(a) R. W. Pekala and D. W. Schaefer, Macromolecules, 1993, 26, 5487–
493; (b) L. W. Hrubesh, J. Non-Cryst. Solids, 1998, 225, 335–342; (c)
5
S.-R. Guo, J.-Y. Gong, P. Jiang, M. Wu, Y. Lu and S.-H. Yu, Adv.
Funct. Mater., 2008, 18, 872–879; (d) T. Baumann, M. Worsley and
J. Satcher, Abs. Pap. Am. Chem. Soc., 2011, 241, 418-POLY; (e)
N. Leventis, N. Chandrasekaran, A. G. Sadekar, C. Sotiriou-
Leventis and H. Lu, J. Am. Chem. Soc., 2009, 131, 4576–4577.
(a) R. W. Pekala, J. Mater. Sci., 1989, 24, 3221–3227; (b) K. Zhang,
B. T. Ang, L. L. Zhang, X. S. Zhao and J. Wu, J. Mater. Chem.,
8 (a) A. Z. Jia, J. Li, Y. Q. Zhang, Y. J. Song and S. X. Liu, Mater. Sci.
Eng., C, 2008, 28, 1217–1226; (b) N. Fechler, T. P. Fellinger and
M. Antonietti, Chem. Mater., 2012, 24, 713–719.
9 (a) D. Carriazo, M. C. Gutierrez, M. L. Ferrer and F. del Monte,
Chem. Mater., 2010, 22, 6146–6152; (b) M. C. Gutierrez, F. Rubio
and F. del Monte, Chem. Mater., 2010, 22, 2711–2719; (c)
M. C. Gutierrez, D. Carriazo, C. O. Ania, J. B. Parra, M. L. Ferrer
and F. del Monte, Energy Environ. Sci., 2011, 4, 3535–3544.
2
011, 21, 2663–2670; (c) S. Wang, W.-C. Li, G.-P. Hao, Y. Hao,
Q. Sun, X.-Q. Zhang and A.-H. Lu, J. Am. Chem. Soc., 2011, 133,
5304–15307; (d) J. Liu, S. Z. Qiao, H. Liu, J. Chen, A. Orpe,
D. Zhao and G. Q. Lu, Angew. Chem., Int. Ed., 2011, 50, 5947–5951.
1
10 O. A. Shlyakhtin and Y. J. Oh, J. Electroceram., 2009, 23, 452–461.
2
1856 | J. Mater. Chem., 2012, 22, 21852–21856
This journal is ª The Royal Society of Chemistry 2012