A synthetic route to ordered mesoporous carbon materials with graphitic pore walls

Article abstract of DOI:10.1002/anie.200352224

Carbonization of aromatic sources in an aluminosilicate mesoporous template yields mesoporous carbon materials with highly graphitic framework structures and excellent mechanical strength and thermal stability (see picture). TEM analysis shows discoid graphitic layers that are packed so that the most preferred orientation of the c axes is along the direction of the template channels.

Full text of DOI:10.1002/anie.200352224

Angewandte
Chemie
Mesoporous Carbon Materials
A Synthetic Route to Ordered Mesoporous
Carbon Materials with Graphitic Pore Walls**
Tae-Wan Kim, In-Soo Park, and Ryong Ryoo*
Recently, it has been discovered that a new class of
mesoporous carbons can be synthesized using ordered
mesoporous silica as a template.[1–5] The porous carbons are
characterized by regular arrangements of uniform mesopores
that exhibit Bragg X-ray diffraction (XRD) lines similar to
those of the mesoporous silica templates. The various pore
structures with tunable diameters (typically in the range of
2–10 nm) are attracting much attention for the develop-
ment of new adsorbents,[1i] catalysts,[1f] and electrode mate-
[*] Prof. R. Ryoo, T.-W. Kim, I.-S. Park
National Creative Research Initiative Center for Functional
Nanomaterials and
Department of Chemistry (School of Molecular Science—BK21)
Korea Advanced Institute of Science and Technology
Daejeon, 305-701 (Republic of Korea)
Fax: (+82)42-869-8130
E-mail: rryoo@kaist.ac.kr
[**] This work was supported in part by the Creative Research Initiative
Program of the Korean Ministry of Science and Technology, and by
the School of Molecular Science through the Brain Korea 21 project.
The authors also thank Iljin Nanotech for the donation of multi-
walled carbon nanotube samples, and LG Chem Research Park for
Raman spectroscopy measurements.
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DOI: 10.1002/anie.200352224
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rials.[1g–j,4a,5a] Similar to other carbon materials, it is expected
CMK-3G, respectively. As shown in Figure 1, the CMK-nG
carbons exhibit XRD peaks below 2q = 58, which is character-
istic of the highly ordered mesostructures, and similar to those
for CMK-n carbons synthesized with sucrose[1a,c,2a] or furfuryl
alcohol.[1b,4a] Furthermore, the CMK-nG carbons are charac-
terized by intense peaks at around 2q = 26, 45, 53, and 788,
which correspond to the (002), (101), (004), and (110)
diffractions of the graphitic frameworks, repsectively. The
diffraction intensities and peak widths are comparable to
those of multiwalled carbon nanotubes (MWCNT; Iljin
Nanotech, Seoul), and thus much more intense than those
of CMK-n carbons with amorphous frameworks, or activated
carbons. The powder XRD results are consistent with the
Raman spectra of different carbon species shown in Figure 2.
That is, the CMK-3G and MWCNT samples exhibit a very
narrow G-band as compared with the other carbons. The
transmission electron microscopy (TEM) images of the CMK-
3G mesoporous carbon in Figure 3 show hexagonal arrays of
carbon rods, 7 nm in diameter and 3 nm apart, similar to the
structure of CMK-3 reported in our previous work.[2] Most
interestingly, the TEM images indicate a stacking of the
discoid graphene sheets oriented perpendicular to the direc-
tion of the rods.
Figure 4 shows XRD patterns of the SBA-15 template
containing carbonaceous products after the carbonization of
acenaphthene at the given pyrolysis temperatures. The XRD
pattern after pyrolysis at 4008C shows a diffraction peak
centered at 2q = 268, which is very similar to the (002)
diffraction of the graphite structure. A mesophase pitch is
obtained by removing the SBA-15 template with HF. This
result indicates that a liquid-crystalline mesophase with a
d spacing of 0.36 nm is formed inside the SBA-15 channels
during pyrolysis at around 4008C. The mesophase pitch seems
to lead to graphitic frameworks, as the carbonization is
completed by further heating at 9008C. The discoid alignment
of the graphitic frameworks, as observed by TEM, is
consistent with the general tendency of the edge-on anchoring
of polycyclic aromatic hydrocarbons in mesophase pitch on
silica surfaces.[8]
that these mesoporous carbons should exhibit widely differ-
ent physicochemical properties depending on the detailed
structure of the frameworks. However, the synthesis of such
mesoporous carbons with pore regularity was hitherto limited
to carbon frameworks with an amorphous-carbon-like
nature.[1a,2a,4a]
Herein, we report a synthetic strategy to fabricate nano-
porous carbon materials with graphitic framework structures
through in situ conversion of aromatic compounds to meso-
phase pitch inside the silica templates. The carbon frame-
works are composed of discoid graphene sheets, which self-
align perpendicularly to the template walls during synthesis.
We have confirmed that this synthetic principle can be applied
with silica templates that have various pore structures, and
can provide materials exhibiting greater mechanical strength
under compression and thermal stability in air.
Various aromatic compounds such as acenaphthene,
acenaphthylene, indene, indan, and substituted naphtha-
lenes[6] are suitable as carbon sources for the present syn-
thesis. Aluminum is incorporated in silica templates following
the post-synthesis procedure.[7] First, the mesoporous tem-
plate and the carbon source are placed into an autoclave. The
aluminum sites on the silica wall act as catalysts to form in situ
mesophase pitch in the silica template pores at a low pyrolysis
temperature (4008C). Subsequently, the temperature of the
autoclave is increased to 7508C for carbonization of the
mesophase pitch in the template. After the autoclave was
cooled down, the product is further heated to 9008C under
vacuum in a fused-quartz reactor for the complete carbon-
ization of the carbon source. The carbon product is then
recovered by the removal of the silica template using an
aqueous solution of HF or NaOH.
We have confirmed that the synthesis method is appli-
cable to mesoporous silica templates with various structures,
such as MCM-48 (cubic Ia3d), SBA-1 (cubic Pm3n), and
SBA-15 (2-dimensional hexagonal p6mm) mesoporous silicas.
The resultant carbons using these silica templates with
acenaphthene are referred to as CMK-1G, CMK-2G, and
Figure 1. X-ray powder diffraction patterns for CMK-nG-type ordered mesoporous carbons in the low-angle region (below 2q=88), and other
carbon materials in the wide-angle region (108<2q<908).
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Figure 2. Raman spectra of CMK-3G ordered mesoporous carbon and
other carbon materials.
Figure 4. X-ray powder diffraction patterns for the SBA-15 template
and acenaphthene composites during pyrolysis. The asterisk highlights
the formation of a liquid-crystalline mesophase.
Figure 3. TEM image of CMK-3G (left) and its photomagnification
the carbons occurred in a narrow temperature range between
520 and 6808C for CMK-3G and MWCNTs, and between 400
and 6408C for CMK-3FA and CMK-3SC. Thus, the oxidation
temperature for CMK-3G is similar to that of nanotubes, and
significantly higher than that of mesoporous carbons with
furfuryl alcohol and sucrose as carbon sources.[1b,9] These
results provide additional confirmation of the graphitized
nature of the CMK-3G carbon frameworks, which is respon-
sible for the much improved thermal stability in air.
In conclusion, ordered mesoporous carbons with various
structures were synthesized through the in situ conversion of
aromatic compounds to a mesophase pitch using mesoporous
silicas MCM-48, SBA-1, and SBA-15 as templates. These
nanoporous carbons with highly graphitic frameworks exhibit
remarkably improved mechanical strength and thermal
stability, in comparison with mesoporous carbons with
amorphous frameworks. The underlying synthetic principle
presented in this report may be important for the design of
new mesostructured carbonaceous materials. Particularly, due
to the graphitic nature of the frameworks, the resultant
nanoporous carbons would attract much attention for the
development of new electrochemical applications, such as fuel
cells and lithium ion batteries. The edge-on graphitic structure
(right) and the corresponding electron diffraction pattern (inset).
In Figure 5, the mechanical strength of the CMK-3G
carbon is compared with those of other CMK-3 carbons
prepared using furfuryl alcohol (CMK-3FA) and sucrose
(CMK-3SC). The XRD intensity and peak resolution was
monitored after subjecting the carbon powders to a pressure
of 1.2 GPa for 1 hr in a mechanical press. This mechanical
stability test showed almost no change in XRD pattern due to
pressurizing in the case of CMK-3G. However, the XRD
intensity was reduced very substantially for other CMK-3
carbons with amorphous frameworks (approximately 70%
for CMK-3FA and 30% for CMK-3SC). This difference may
be attributed mostly to differences in the nature of the carbon
frameworks, either graphitic or amorphous.
Thermogravimetric weight changes were recorded in air
to evaluate the thermal stability of the graphitic mesoporous
carbon CMK-3G (Figure 6). The CMK-3G carbon and a
reference MWCNT exhibited almost the same weight-change
profiles. In contrast, CMK-3FA and CMK-3SC showed a
comparable weight-loss profile at a lower temperature.
Significant weight losses corresponding to the oxidation of
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exposed to pore surfaces may also give interesting properties
as a support for catalytic metal clusters.
Experimental Section
SBA-15 silica was prepared using EO20PO70EO20 (Pluronic P123,
BASF) and tetraethylorthosilicate (TEOS, Acros) according to the
procedure described elsewhere.[2a,10] In a typical synthesis batch, P123
(4.0 g) was dissolved in HCl solution (1.6m, 150 g) at 358C. TEOS
(8.50 g) was added to the solution while stirring at 358C, which was
continued for 15 min at the same temperature. The mixture was
placed under static conditions in an oven at 358C for 24 h, and then
heated to 1008C for 24 h. The SBA-15 product was filtered without
washing and dried in an oven at 1008C. The product was washed with
ethanol and calcined to remove the surfactant. Calcination was
performed in air by heating the sample to 4008C under static
conditions. Aluminum was incorporated within the calcined silica to
obtain AlSBA-15 with a Si:Al ratio of 20, following the impregnation
procedure using an aqueous solution of AlCl3 (98%, Junsei).[7] The
impregnation was carried out in a rotary evaporator at about 608C.
The aluminated sample was then immediately calcined in air at
5508C.
Other template samples such as MCM-48 and SBA-1 were
synthesized following the synthesis procedures reported in the
literature.[11,12] Aluminum was incorporated in the same way as for
SBA-15.
In a typical synthesis batch of CMK-3G, AlSBA-15 (3 g) and
acenaphthene (99%, Aldrich; 3.72 g) were combined in an autoclave
(7 cm3 capacity) that can withstand high temperatures and pressures
(made of INCONEL alloy 601). After being purged with Ar and
closed tightly, the autoclave was heated in a furnace while the
temperature was increased to 7508C at a rate of 2.58CminÀ1, and then
maintained for 2 h. The product was collected after the autoclave was
cooled to room temperature. (Care must be taken due to high-
pressure generation in the autoclave.) Subsequently, the product was
heated to 9008C under vacuum in a fused-quartz reactor. The carbon
product was released by the removal of the silica template with a HF
solution. The autoclave may be substituted with sealed quartz tubing
in cases where small amounts of sample are prepared.
Figure 5. X-ray powder diffraction patterns before (c) and after
(g) pressurization (100 mg carbon, 10 mm palletized diameter; 1 hr
at 1.2 GPa) for CMK-3G with acenaphthene as the carbon source,
CMK-3FA with furfuryl alcohol, and CMK-3SC with sucrose.
A sample of CMK-3FA was prepared with furfuryl alcohol (FA).
Typically, AlSBA-15 (1 g) was infiltrated with FA (0.9 mL; Aldrich)
by incipient wetness at room temperature. The FA/AlSBA-15
composite thus prepared was heated overnight in a closed vial at
908C to polymerize the FA. Then, the FA/AlSBA-15 composite was
heated under vacuum to 3508C at a rate of 1.88CminÀ1 in a fused-
quartz reactor. The sample was cooled to room temperature after
remaining at 3508C for 6 h. To this sample, FA (0.63 mL) was
subsequently added. The FA polymerization and heating to 3508C
was repeated once again. Carbonization of the carbon source was
completed in vacuum by heating to 9008C at a rate of 1.68CminÀ1
.
The carbon product was recovered by HF washing. A CMK-3SC
sample was synthesized with SBA-15 and sucrose, following the
procedure described elsewhere.[2a]
XRD patterns were recorded with a Rigaku MultiFlexdiffrac-
tometer operating with CuKa radiation at 2.0 kW. The 2q step width
was 0.018. Acquisition time was 1.2 s below 108, and 1.0 s elsewhere.
Raman spectra were recorded with a Bruker RFS 100/S spectrometer.
A DPY421 diode laser pumped Nd:YAG laser operating at a
wavelength of 1050–1060 nm and a power of 100 mW was used as
the radiation source. TEM images were obtained with Philips F20
Tecnai transmission electron microscope taken at 490000 ” magni-
fication at an operating voltage of 160 kV. The TEM images were
taken from thin edges of carbon particles mounted on a porous
carbon grid. The mechanical stability was tested with a 12 Ton E-Z
Press (ICL) with a 10 mm die set. Carbon samples (100 mg) were
pressurized to 1.2 GPa and maintained for 1 h. Thermal stability of
the carbon materials was investigated by recording weight changes on
Figure 6. Thermogravimetric curves for the CMK-3G (c), MWCNT
(d), CMK-3FA (g), and CMK-3SC (b) carbon materials in air.
The heating rate was 5.08CminÀ1
.
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a thermogravimetric analyzer (TA Instruments TGA 2050). Analysis
under air was carried out at a heating rate of 5.08CminÀ1
.
Received: June 25, 2003 [Z52224]
Keywords: carbon · graphitic framework · mechanical
.
properties · mesoporous materials · template synthesis
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