80% at pH 9. The CEMNPs-based sorbents are fully mobile and

have excellent sorption capacities, which substantially exceed

the capacities of carbon nanotubes and activated carbons.

0008-6223/$ - see front matter Crown Copyright � 2009 Published bydoi:10.1016/j.carbon.2009.01.015

* Corresponding author: Fax: +86 21 64252914.E-mail address: zhanliang@ecust.edu.cn (L. Zhan).

Fig. 5 – Metal ions adsorption isotherms on CEMNPs.

Fig. 4 – Effect of pH on adsorption of metal ions on CEMNPs.

1204 C A R B O N 4 7 ( 2 0 0 9 ) 1 1 8 9 – 1 2 0 6

Acknowledgements

This work was supported by the Ministry of Science and Educa-

tion through the Department of Chemistry, Warsaw University

under Grant No. N204 096 31/2160. M. Bystrzejewski thanks the

Foundation for Polish Science (FNP) for financial support.

R E F E R E N C E S

[1] Viard B, Pihan F, Promeyrat S, Pihan JC. Integratedassessment of heavy metal (Pb, Zn, Cd) highway pollution:bioaccumulation in soil, Graminaceae and land snails.Chemosphere 2004;55(10):1349–59.

[2] Chen JP, Yoon JT, Yiacoumi S. Effects of chemical and physicalproperties of influent on copper sorption onto activatedcarbon fixed-bed columns. Carbon 2003;41(8):1635–44.

[3] Pyrzynska K. Application of carbon sorbents for theconcentration and separation of metal ions. Anal Sci2007;23(6):631–7.

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[6] Tuzen M, Saygi KO, Soylak M. Solid phase extraction of heavymetal ions in environmental samples on multiwalled carbonnanotubes. J Hazard Mater 2008;152(2):632–9.

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[10] Stafiej A, Pyrzynska K. Adsorption of heavy metal ions withcarbon nanotubes. Sep Purif Technol 2007;58(1):49–52.

Carbon foams prepared by supercritical foaming method

Juan Li, Can Wang, Liang Zhan*, Wen-Ming Qiao, Xiao Yi Liang, Li-Cheng Ling

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, No. 130, MeiLong Road,

Shanghai 200237, PR China

A R T I C L E I N F O

Article history:

Received 7 October 2008

Accepted 9 January 2009

Available online 16 January 2009

A B S T R A C T

Multifunctional carbon foams with mean pore size smaller than 200 lm were prepared

by supercritical foaming, in which toluene and mesophase pitch were used as the super-

critical agent and carbonaceous precursor, respectively. The supercritical foaming behav-

iors and nucleation mechanisms were analyzed. According to the composition of

Elsevier Ltd. All rights reserved.

mesophase pitch and micrographs of resultant carbon foams, the interface between

light components and quinoline insoluble components is considered as the nucleation

site.

Crown Copyright � 2009 Published by Elsevier Ltd. All rights reserved.

Since the first development of carbon foam from thermo-

setting organic polymers in the late 1960s [1], it has attracted

much attention due to its special characteristics. Carbon

foams have an interconnected three-dimensional graphitic-

like microstructure arranged in a cellular fashion, which en-

dows them with attractive performances, such as low den-

sity, adjustable thermal and electrical conductivity, high

temperature tolerance and high mechanical strength [2,3].

Therefore, carbon foams show great promise for functional

elements or structure components in various applications

including heat exchangers, microwave absorbers, electrodes

for energy storage, catalyst supports and filters [4–7]. Gener-

ally, the pore sizes of carbon foams fabricated with tradi-

tional foaming processes [1–8] are around 200–600 lm.

However, carbon foams with mean pore size smaller than

200 lm can be expected to be applied as biological carbon

[9], catalyst supports, filters and foam-reinforced composites

[10–12].

The aim of this work is to utilize supercritical foaming

method to fabricate carbon foams with smaller pore size

and higher mechanical strength. The synthesis process can

be described briefly as following. Toluene (critical tempera-

ture (Tc), 591.7 K; critical pressure (Pc), 4.11 MPa) and naphtha-

lene based mesophase pitch (softening point, 548 K;

mesophase content, 100%; mean particle size, 200 lm) were

adopted as the supercritical agent and carbonaceous precur-

sor, respectively. About 20 g powder mesophase pitches and

210 ml toluene were placed in a cylindrical stainless steel

reactor (250 ml), and heated at 3 K/min up to the desired

supercritical temperature of 593 K and pressure of 12 MPa.

In all cases the depressurization was taking place quickly from

the foaming pressure to atmospheric pressure in about 20 s.

Subsequently, the resultant foam was oxidized at 2 K/min

from 300 to 553 K in an air flow rate of 0.7 l/min, and then

carbonized at 1133 K for 2 h under a nitrogen atmosphere.

Fig. 1a shows that the resultant carbon foam exhibits open

interconnected pores among most of the spherical cells with

a bulk density of about 0.4 g/cm3. The pore sizes around 20–

200 lm is obviously smaller than that of the foams derived

from the traditional foaming process. It should be attributed

to the supercritical foaming mechanisms. Generally, the

supercritical foaming process is based on the following steps:

(1) the mesophase pitch is saturated with supercritical tolu-

ene at the supercritical condition, forming a mesophase

pitch/toluene homogeneous phase; (2) the homogeneous

phase is quenched into a supersaturated state as a result of

thermodynamic instability caused by a rapid depressuriza-

tion process, and then phase separation takes place between

the melt and supercritical fluid, thus cell nuclei form; (3) cells

grow with the segregation and aggregation of toluene from

the pitch matrix, forming bubbles; and (4) bubbles are stabi-

lized with the viscosity of mesophase pitch rapidly increasing

because of the sudden pressure release. In addition, com-

pared with the traditional foaming process, the resultant

foam possesses smooth ligaments and cell walls as shown

in Fig. 1b. And the microcracks at the junctions, ligaments

and cell walls are significantly fewer and shorter, which make

the resultant foam have a higher mechanical strength.

To elucidate the nucleation mechanisms, carbon foam

(Fig. 1c) was derived from toluene-extracted mesophase pitch

under the same preparation conditions as stated previously,

in which 20 g mesophase pitches were pre-extracted with

210 ml toluene at 353 K for 1 h. It is of interest to note that

the resultant carbon foam has a lower pore density than that

of foams derived from the original mesophase pitch. Fig. 2

shows that the light components of the toluene-extracted

mesophase pitch including heptanes soluble (HS), heptanes

insoluble–toluene soluble (HI–TS), toluene insoluble–pyridine

soluble (TI–PS) and pyridine insoluble–quinoline soluble (PI–

QS) are lower than that of the original mesophase pitch.

The HS decreases from 11.3% to 0.46%, while quinoline insol-

uble (QI) increases by 32.2%. The light components have dif-

ferent physical or chemical performance with QI

components, so there have interface between them, more-

over, the amount of interface decreases with the decrease of

light component content. According to the results in Figs. 1

Fig. 1 – SEM micrographs of carbon foams: (a) carbon foam prepared with original mesophase pitch; (b) magnification of the

square section in (a); and (c) carbon foam prepared with toluene-extracted mesophase pitch.

C A R B O N 4 7 ( 2 0 0 9 ) 1 1 8 9 – 1 2 0 6 1205

and 2, the pore density of foams tends to be lower with the

decrease of light components, which indicates that the nucle-

ation site is directly related to the interface, especially the HI/

QI interface.

In conclusion, carbon foams with pore size distributed in

the range of 20–200 lm were prepared by supercritical foam-

ing. This typical foam presents smooth cell walls and has

few cracks at the junctions and ligaments. The composition

of mesophase pitch has a significant effect on the pore foam-

ing behaviors, and the light components/QI interface could

serve as the preferential nucleation site. Further study should

focus on the effects of supercritical forming conditions on the

microstructure of ligaments, junctions and microcracks.

Acknowledgments

This work was supported by the National Science Foundation

of China (Nos. 50730003, 50672025, 20806024) and the Research

Fund of China for the Doctoral Program of Higher Education

(No. 20070251008).

R E F E R E N C E S

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[2] Klett JW, McMillan AD, Gallego NG, Burchell TD, Walls CA.Effects of heat treatment conditions on the thermalproperties of mesophase pitch-derived graphitic foams.Carbon 2004;42(8–9):1849–52.

[3] Beechem T, Lafdi K, Elgafy A. Bubble growth mechanism incarbon foams. Carbon 2005;43(5):1055–64.

[4] Wang MX, Wang CY, Li TQ, Hu ZJ. Preparation of mesophase-pitch-based carbon foams at low pressures. Carbon2008;46(1):84–91.

[5] Liu MX, Gan LH, Zhao FQ, Fan XZ, Xu HX, Wu FR, et al.Carbon foams with high compressive strength derived frompolyarylacetylene resin. Carbon 2007;45(15):3055–7.

[6] Li SZ, Guo QG, Song Y, Liu ZJ, Shi JL, Liu L, et al. Carbon foamswith high compressive strength derived from mesophasepitch treated by toluene extraction. Carbon2007;45(14):2843–5.

[7] Chen Y, Chen BZ, Shi XC, Xu H, Hu YJ, Yuan Y, et al.Preparation of pitch-based carbon foam using polyurethanefoam template. Carbon 2007;45(10):2132–4.

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[9] Eiroa M, Vilar A, Kennes C, Veiga MC. Effect of phenol on thebiological treatment of wastewaters from a resin producingindustry. Bioresource Technology 2008;99(9):3507–12.

[10] Rios RVRA, Escandell MM, Sabio MM, Reinoso FR. Carbonfoam prepared by pyrolysis of olive stone under steam.Carbon 2006;44(7):1448–54.

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HS HI-TS TI-PS PI-QS QI0

20

40

60

80C

onte

nt /

%

Composition

original mesophase pitch toluene-extracted mesophase pitch

Fig. 2 – Component distribution of different mesophase

pitches.

1206 C A R B O N 4 7 ( 2 0 0 9 ) 1 1 8 9 – 1 2 0 6