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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