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A porous carbon foam prepared from liquefied birch sawdust
Rui Wang • Wei Li • Shouxin Liu
Received: 3 August 2011 / Accepted: 23 September 2011 / Published online: 8 October 2011
� Springer Science+Business Media, LLC 2011
Abstract Carbon foam was prepared by submitting birch
sawdust to liquefaction, resinification, foaming, carbon-
ization, and activation steps. The foam was characterized
by TG and DTG, XRD, SEM, and nitrogen adsorption at
77 K. A mechanism for the formation of the porous carbon
foam was proposed. Solid non-graphitized lightweight
carbon foams with specific surface areas of 534–555 m2/g
and cell sizes of 100–200 lm were obtained, depending on
the carbonization or activation temperature used. The
intermediate liquefied birch-based resin foam exhibits
thermal stability superior to liquefied wood and inferior to
phenolic resin, and decomposes rapidly in two stages, at
285.7 and 412.9 �C, respectively. Further activation of the
carbon foam in a stream of nitrogen above 800 �C
improves the pore structure and homogeneity of the cell
size significantly. The matrix of the foams contains a large
number of micropores, and the microstructure becomes
more ordered as the activation temperature is increased.
Introduction
In recent years, the preparation of ultra-lightweight porous
carbon materials has attracted considerable attention
worldwide [1]. Carbon foam is a sponge-like material with
advantageous features such as low density (0.2–0.8 g/cm3),
large external surface area, and open cell structure [2–4].
Microcellular carbon foams with low density have been
developed for use as catalyst supports, adsorbents for liquid
or gas purification, porous electrodes and other battery
components [3–5].
For carbonaceous adsorbents, a large number of open
micropores are essential for fast adsorption kinetics. Acti-
vated carbon fibers often possess an extensive number of
open micropores without substantial formation of macro-
and mesopores because of the thin fibrous particles [3].
In general, carbon foam is characterized as an intercon-
nected open cell structure containing very few closed
pores. Typically, greater than 90% of the pores are open
[5].
To date, the precursors used to produce carbon foam
include coal, pitch, olive stones, polyimide, starch, and
thermosetting polymer [4–12]. However, the most com-
monly produced carbon foam is made by carbonization of
polymeric foams. These polymeric foams typically used
include phenol formaldehyde, resorcinol formaldehyde,
polyurethane, furfural resin, polyvinylidene chloride (PVC),
and polyacrylonitrile [8].
Considering the fact of huge amounts of sawdust waste
produced every year [13–15], which have result in exces-
sive consumption of resources and malignant pollutions of
environment, the applications of sawdust waste for the
preparation of advance materials have attracted great
concerns [16]. Carbohydrates can be a suitable carbon
precursor for preparing carbon foams. Carbohydrates are
considered hydrates of carbon and are represented by the
general formula Cx(H2O)y. They easily undergo dehydra-
tion to produce carbon [7]. Although, there are a few
publications related to the preparation of carbon foam from
cellulosic and lignocellulosic materials, the use of ligno-
cellulosic precursors is an interesting alternative because of
their low cost. Rios et al. [8] examined the experimental
conditions needed to pyrolyze olive stones to cause the
particles to swell and eventually form carbon foam. The
R. Wang � W. Li � S. Liu (&)
College of Material Science and Engineering, Northeast Forestry
University, Harbin 150040, China
e-mail: liushouxin@126.com
123
J Mater Sci (2012) 47:1977–1984
DOI 10.1007/s10853-011-5993-7
greatest expansion of the material occurred in the presence
of steam at a pressure of 1 MPa and the carbon foam
obtained exhibited a low density of 0.2–0.3 g/cm3 because
of the presence of meso- and macropores, mostly larger
than 1 lm in diameter.
The purpose of the present study was preparation of
carbon foam with a developed pore structure from low cost
and renewable materials. In the north part of China, large
amounts of birch sawdust are produced by the forest
industry annually and most of them were directly used as
burning or discarded, conversion of them to a novel
material may great significance and attracted considerable
interesting. To achieve this, birch sawdust was used as the
raw material, and was liquefied, converted to a resin,
foamed, carbonized, and activated. A porous carbon foam
with a highly developed pore structure and low density was
obtained. The prepared foam was characterized by thermo-
gravimetry (TG) and derivative thermogravimetry (DTG),
X-ray diffraction (XRD), scanning electron microscopy
(SEM), and nitrogen (N2) adsorption at 77 K.
Experimental
Materials
Sawdust of birch wood (30–80 mesh) was obtained from a
wood processing factory and used as lignocellulosic waste.
The polysorbate 80 used was chemically pure, and all other
chemicals were analytical reagents in accordance with the
China National Standards and used as received.
Liquefaction of birch sawdust
20 g of birch sawdust (80 mesh, dried at 105 �C for 8 h),
phenol (60 g), sulfuric acid (98%, 1 mL), and phosphoric
acid (85%, 1.7 mL) were added into a four-necked glass
reactor that was equipped with a mechanical stirrer, ther-
mometer, and condenser. The mixture was heated under
reflux at a temperature of 124–136 �C for 1 h. When the
reaction was finished, methanol (130 mL) was added to the
liquefied product. The mixture was filtered, the pH was adju-
sted to neutral using sodium carbonate, and then the mixture
was filtered again to remove the resulting precipitate. The
filtrate was concentrated by vacuum distillation at 50 �C to
remove the methanol, giving the liquefied birch wood.
Preparation of resin foam
Formaldehyde (37%, 100 mL), sodium hydroxide (1 g),
and distilled water (20 mL) were added to the above-
obtained liquefied birch wood and stirred until a homoge-
neous mixture was obtained. The mixture was reacted at
55–60 �C for 2 h, and then heated under reflux at 95–98 �C
for 1 h. The product was cooled to 60 �C and the pH
adjusted to neutral. Water was removed from the mixture
by vacuum distillation, leaving behind a resin with a suit-
able viscosity.
Polysorbate 80 (9 mL) and n-pentane (35 mL) were
immediately mixed with the resin and stirred strongly to
form a homogeneous mixture. Sulfuric acid (98%, 4 mL)
was added dropwise to the mixture, and then the mixture
was immediately poured into an open-top square plastic
box (6 9 6 9 6 cm). The mixture foamed and solidified
upon heating at 60 �C for 24 h, giving an liquefied birch-
based resin foam.
Preparation of carbon foam
The prepared resin foam was cut into small pieces
(6 9 2 9 0.5 cm), carbonized under a N2 atmosphere at
400 �C for 1 h, and then activated in a stream of N2 by
heating at 800 or 1000 �C for 1 h. A flowchart detailing the
preparation of the carbon foam is shown in Fig. 1a. Pho-
tographs of the liquefied birch wood-based resin foam,
carbonized foam and carbon foam activated at 800 �C are
shown in Fig. 1b–d.
Characterization
The BET surface area (SBET), pore volume, and pore size
distribution (PSD) of the prepared foams were determined
on an ASAP 2020 sorptometer (Micromeritics, USA) by
measuring their N2 adsorption at 77 K. Before measure-
ment, samples were out-gassed at 300 �C under a flow of
N2 for 6 h. The SBET values of the samples were calculated
by the Brunauer, Emmett, and Teller (BET) method. The
classical pore size model developed by Barrett, Joyner, and
Halenda (BJH) was used to calculate the PSD. The mor-
phology of the prepared foams was observed by SEM
(Quanta 200, FEI, Holland). Crystalline phases present in
the prepared foams were investigated by XRD using Cu Karadiation (Rigaku, Tokyo, Japan). The weight loss of the
prepared foams as a function of temperature was measured
using a thermogravimetric analyzer (Netzsch TG 209 F3,
Germany). The bulk density of the foam was determined by
weighing a block of known dimensions according to the
standard ASTM D1622-03.
Results and discussion
TG and DTG analysis
Figure 2 shows TG and DTG curves of the liquefied birch-
based resin foam, which was heated from 35 to 700 �C at a
1978 J Mater Sci (2012) 47:1977–1984
123
rate of 10 �C/min in a nitrogen atmosphere. The TG and
DTG curves show that the sample undergoes two distinc-
tively different stages during the heating process. The first
stage (35–330 �C) and the second stage (330–700 �C)
correspond to decreases in weight of 15.0 and 36.4%,
respectively. The weight loss in the first stage was caused
by the loss of residual moisture, and curing and foaming
agents. In the second stage, some small molecules are lost
through cleavage of intramolecular ether linkages and the
removal of terminal hydroxymethyl groups, and some
volatile substances are emitted because of the bond
cleavage at different positions of the phenolic resin main
chain [17–19]. According to the DTG result, two peaks
positioned at 285.7 and 412.9 �C indicate that maximum
decomposition rate of liquefied birch-based resin foam was
achieved at these temperatures. Also, it can be seen that the
thermal stability of the liquefied birch-based resin foam is
superior to that of liquefied wood [20–22] and inferior to
that of phenolic resin [23]. This indicates the chemical
modification of major components of liquefied wood dur-
ing polymerization of wood, phenol, and formaldehyde
increases the decomposition temperature of liquefied birch-
based resin foam over that of liquefied wood.
XRD analysis
Figure 3 shows XRD patterns of the liquefied birch-based
resin foam and carbon foams at different stages or activa-
tion temperatures. Similar to other carbon materials
[24–26], the carbon foams exhibit two broad diffraction
peaks at around 22 and 44�. These correspond to the (002)
and (100) diffraction signals, respectively, consistent with
disordered stacking of graphene layers. The interlayer
spaces between two adjacent carbon sheets (d002) and the
coherent domain size along the c-axis and a-axis (Lc and
La) were calculated using Bragg’s law and the Scherrer
formula, respectively (Table 1). The (002) diffraction peak
at 18.90� exists only in the noncarbonized precursors and
the (100) peak is not present in the pattern of the liquefied
birch-based resin foam indicating that the resin foam is
non-crystalline. The (002) diffraction peak centered at
17.16� in the XRD pattern of the foam carbonized at
400 �C arises from the amorphous component of the partly
carbonized polymer. Values of d002 are between 0.3903
Fig. 1 a Flowchart outlining
the preparation of a carbon foam
from birch wood; photographs
of b liquefied birch wood-based
resin foam, c carbon foam
carbonized at 400 �C, and
d carbon foam activated at
800 �C (scale bar, 20 mm)
Fig. 2 TG and DTG curves of liquefied birch-based resin foam
J Mater Sci (2012) 47:1977–1984 1979
123
and 0.5163 nm, which are higher than those of graphite
(0.3350 nm) [27] and coke treated at 1000–1350 �C
(0.3440 nm) [28], indicating a non-graphitized carbona-
ceous structure. It can be seen that the positions of the
(002) and (100) peaks shift slightly with temperature. The
(002) peak sharpens slightly and the intensity of the (100)
peak increases as the temperature increases. In addition, the
values of d002 decreased from 0.5163 to 0.3903 nm as the
temperature increased, suggesting that the stacking of
layers in the carbon foam prepared from liquefied wood
becomes more ordered at higher temperature, even though
it is a non-graphitized structure. Values of Lc and La
increase gradually with increasing temperature, illustrating
that the thickness of the stacked structure increases and the
graphitic structure of non-graphitized carbon foam devel-
ops along both the c-axis and a-axis. These properties are
different from those of other carbon materials such as
carbon fiber [26], and PVC and polyfurfuryl alcohol
carbons [29]. It is clear that the structure of the foams
has been completely changed during carbonization and
activation.
SEM analysis
SEM images of the resin foam and the prepared carbon
foams at different stages are shown in Fig. 4. The cell
structures of the prepared foams consist of an adjacent cell,
ligament and node, and the size of the cells ranges from
100 to 200 lm. The reason for the similarity of the cell
structure and size between the resin foam and carbon foams
is that the shape of the former is retained during solid-state
carbonization of the thermosetting resin [30]. Figure 4
shows that most of the cells are shaped as irregular pen-
tagons or hexagons, which is different from cells prepared
from other materials by pyrolysis, e.g., olive stones [8],
sucrose [7], coal tar pitch, and petroleum pitch [4]. The
carbon foams have a smaller mean cell size and more
uniform cell structure than the resin foam, caused by
shrinkage of the resin foam during carbonization. There are
many tiny bubbles with diameters of about 10 lm on the
ligaments and nodes of the cells that could have formed
from smaller cores of foaming agent. In addition, there is
one layer between each cell. As the temperature increases,
the surface of the material becomes smoother, the layer
between each cell becomes thinner and clearer, and layers
are broken, which leads to some closed cells opening.
Textural result
The textural characteristics of the activated carbon foam
samples are summarized in Table 2. The SBET of the liq-
uefied birch-based resin foam and foam carbonized at
400 �C are very small at only 0.09 and 0.29 m2/g,
respectively. The SBET of the sample activated at 800 �C
increased significantly to 555 m2/g. This observation,
which is similar to the result from carbonization of wood
tar pitch [31], can be understood by taking into account the
fact that large variations in the density of carbon occurs
during the condensation of aromatic rings to form two-
dimensional hexagonal networks between 400 and 800 �C.
Therefore, sparse regions are created, which give rise to
cracks in the foam matrix, and consequently form micro-
and mesopores. The surface area, and micropore area and
volume all decreased as the temperature increased, while
the external area and mesopore volume increased above
800 �C, which was probably caused by further decompo-
sition of carbon. It is interesting that the decrease in the
surface area of the micropores preceded the increase in the
surface area of the mesopores. This may indicate that for
foam activated at 1000 �C, the newly formed mesopores
are the result of micropore opening, which is similar to the
effect of temperature on modified activated carbon [32].
As shown in Table 2, the bulk density of the carbon
foam prepared from liquefied birch is about 0.02 g/cm3,
which is lower than that of similar foams [4, 8], indicating
Fig. 3 XRD patterns of liquefied birch-based resin foam and carbon
foams formed at different pyrolysis temperatures
Table 1 XRD parameters of carbonized and activated carbon foams
Temperature
(�C)
2h (002)
(�)
d002
(nm)
Lc
(nm)
2h (100)
(�)
La
(nm)
400 17.16 0.5163 0.4407 43.34 0.9085
800 21.85 0.4064 0.8660 43.94 1.2486
1,000 22.76 0.3903 1.2893 44.42 2.3391
1980 J Mater Sci (2012) 47:1977–1984
123
that the amount of char in the body of the foam is lower,
the foam is less dense and, consequently, more porous.
This result may be caused by the large number of micro-
and mesopores present in the ligaments, nodes, and layers
between cells in the structure of the foam.
The pore structure of the prepared foam can be deter-
mined from its adsorption isotherm and the corresponding
t-plot [4]. For a nonporous material, the t-plot is a straight
line passing through the origin. However, deviation from
this straight line indicates the presence of a porous struc-
ture, e.g., a downward deviation indicates the presence of
micropores, and an upward deviation, caused by capillary
condensation, indicates the presence of meso- or macrop-
ores. Carbon foam can be regarded as a carbon matrix
containing a certain number of cells in which there are
large number of micro- and mesopores. Figure 5a and b,
show N2 adsorption-desorption isotherms and the corre-
sponding t-plots, respectively, of foams activated at 800
and 1000 �C, and follow Type I adsorption. The curves
show a significant increase in the amount adsorbed at a
very low P/P0 of 0.15, which may correspond to the filling
of micropores. The corresponding t-plot shows a downward
deviation at low coverage, which indicates that foams
activated at 800 and 1000 �C are porous with a large
number of microstructures in the foam matrix. The
decrease in the quantity of nitrogen adsorbed as the acti-
vation temperature is increased shows that the pore struc-
ture in the matrix is related to the activation temperature.
Fig. 4 SEM images of a liquefied birch-based resin foam and carbon foams formed at b 400 �C, c 800 �C, and d 1000 �C (scale bar, 200 lm)
Table 2 Textural characteristics of the porous carbon foams pre-
pared at different activation temperatures
Activation temperature (�C)
800 1,000
SBET (m2/g) 555 534
Micropore area (m2/g) 497 462
Percentage of Micropore area (%) 89.5 86.5
External area (m2/g) 58 71
Percentage of external area (%) 10.5 13.3
Micropore volume (cm3/g) 0.23 0.21
Mesopore volume (cm3/g) 0.04 0.05
Total pore volume (cm3/g) 0.27 0.26
Average pore diameter (nm) 1.93 1.95
Bulk density (g/cm3) 0.0229 0.0210
J Mater Sci (2012) 47:1977–1984 1981
123
As shown in Fig. 5a, significant adsorption and desorption
at a high relative pressure between 0.95 and 1.0 indicates
that the matrix has mesoporous characteristics. It can also
be seen that the adsorption and desorption branches did not
close until lower pressure was reached. This phenomenon
is described as low-pressure hysteresis [33], and is related
to the unique cell pore structure of the carbon foam. The
layer between the cells is very brittle and easily broken,
and irreversible deformation occurred as the pressure
increased, which led to low-pressure hysteresis. Because
the distortion is not perfectly elastic, some molecules
became trapped and could only escape very slowly, or
possibly not at all, during the desorption run unless the
temperature is raised.
The PSD of the foams activated at 800 and 1000 �C are
shown in Fig. 5c. The results calculated from desorption
branches show that some of the pores in the foam matrix
possess diameters of about 4 nm, which implies that many
pores with diameters of below 2 nm must be present
because the average pore diameters for the samples are
1.93 and 1.95 nm at 800 and 1000 �C, respectively
(Table 2).
Mechanism analysis
According to the processing of liquefied birch-based car-
bon foam and the results from the corresponding TG, DTG,
XRD, SEM, and N2 adsorption analyses, a presumed
mechanism of formation of the carbon foams is proposed in
Fig. 6. In the first step, the foaming agent and liquefied
birch-based resin which is prepared from birch sawdust are
mixed well, so that there are a large number of cores of
foaming agent in the resin matrix. The cores grow and
connect constantly as the matrix is heated. When the resin
is cured, a large number of irregular pentagonal or hex-
agonal cells are formed. In the next step, the liquefied
birch-based resin foam is carbonized at 400 �C. This gives
a carbon foam which contains disordered stacked graphene
layers. At this point the surface area of the prepared foam is
very low. In the final step, a large number of micro- and
mesopores form at the node and ligament during activation
at high temperature (800 or 1000 �C) under inert atmo-
sphere, which increases the surface area of the foam. As the
activation temperature is increased, the carbon is ablated
further and some of the micropores become mesopores.
Conclusions
Liquefied birch sawdust can be used to prepare a light
weight porous carbon foam with a low bulk density of
0.0210–0.0229 g/cm2 by forming a resin, then a foam,
which is then exposed to carbonization and activation
steps. The liquefied birch-based resin foam decomposed in
two stages corresponding to the peak values of 285.7 and
412.9 �C, respectively. The product from pyrolysis of the
resin foam is composed of solid carbon foam with a non-
Fig. 5 a N2 adsorption–desorption isotherms, b corresponding t-plots, and c pore size distribution of carbon foams activated at 800
and 1000 �C
1982 J Mater Sci (2012) 47:1977–1984
123
graphitized carbonaceous structure. The prepared foams
consist of adjacent cells, ligaments, and nodes. The size of
the cell ranges from 100 to 200 lm and the shape is either
an irregular pentagon or hexagon. As the carbonization
temperature is increased, the stacking of layers of carbon
foam becomes more ordered, the cell becomes smaller and
more uniform, and the surface of the material becomes
smoother. The SBET of foams activated at 800 and 1000 �C
under N2 atmosphere is 555 and 534 m2/g, respectively.
Type I adsorption isotherms, low-pressure hysteresis, and a
downward deviation of the t-plot of the carbon foams
indicate that there are a large number of micropores in the
foam matrix.
Acknowledgement The present study was financially supported by
the Cultivation Project for Promoting Excellence in Research for
Ph.D. Degrees from the Northeast Forestry University (GRAP09),
Harbin, China.
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