Characterization of the pore structure of nanoporous activated carbons produced from wood waste

Article (PDF Available)inHolzforschung 67(5) · July 2013with 94 Reads
DOI: 10.1515/hf-2012-0188
Abstract
Highly developed nanoporous carbon materials have been prepared by a two-stage thermocatalytic process. In the first step, alder (Alnus rhombifolia) and birchwood (Betula pendula) were carbonized with and without a dehydration catalyst (H3PO4); in the second step, the material was activated by means of NaOH. The dependence of the porous structure of activated carbons from process parameters was characterized by the novel limited evaporation technique. Specific surface areas, pore volumes, and radii were calculated according to the Derjaguin-Broekhoff-de Boer theory. The tests of activated carbons as electrodes in supercapacitors demonstrated their high potential for this application.
DOI 10.1515/hf-2012-0188Holzforschung 2013; 67(5): 587–594
Galina Dobele * , Darya Vervikishko , Aleksandrs Volperts , Nikolay Bogdanovich and
Evgeny Shkolnikov
Characterization of the pore structure of
nanoporous activated carbons produced from
wood waste
Abstract: Highly developed nanoporous carbon materi-
als have been prepared by a two-stage thermocatalytic
process. In the first step, alder ( Alnus rhombifolia ) and
birchwood ( Betula pendula ) were carbonized with and
without a dehydration catalyst (H
3 PO 4 ); in the second
step, the material was activated by means of NaOH. The
dependence of the porous structure of activated carbons
from process parameters was characterized by the novel
limited evaporation technique. Specific surface areas,
pore volumes, and radii were calculated according to the
Derjaguin-Broekhoff-de Boer theory. The tests of activated
carbons as electrodes in supercapacitors demonstrated
their high potential for this application.
Keywords: activated carbons, alder wood, benzene
desorption isotherms, birchwood, charcoal, limited
evapo ration technique (LET), sodium hydroxide activa-
tion, supercapacitor
*Corresponding author: Galina Dobele , Latvian State Institute of
Wood Chemistry, 27 Dzerbenes St., 1006 Riga, Latvia,
e-mail: gdobele@edi.lv
Darya Vervikishko and Evgeny Shkolnikov: Scientific Association
forHigh Temperatures , Russian Academy of Sciences,
13/2Izhorskaya Street, 125412 Moscow, Russia
Aleksandrs Volperts: Latvian State Institute of Wood Chemistry ,
27Dzerbenes Street, 1006 Riga, Latvia
Nikolay Bogdanovich: Northern Federal University , Naberezhnaja
Severnoy Dvini 17, 163002 Arkhangelsk, Russia
Introduction
The production of wood-based activated carbons (ACs) has a
rich history. Nowadays, ACs with a well-developed nanopo-
rous structure are used in ecology, medicine, atomic power
engineering, and modern devices for storage and transfer of
electric energy, just to mention a few. An example in the field
of wood technology is presented by G ü tsch and Sixta (2011) .
The production costs of ACs can be lowered by the use of
wastes from mechanical and chemical wood processing.
Highly porous carbons can be obtained by the ther-
mocatalytic activation of lignocellulosic materials as pre-
cursors (Torne -Fernandez etal. 2009 ). The conditions of
chemical activation such as the type and concentration
of chemical reagents, impregnation method, and param-
eters of carbonization and activation directly influence
the porosity, the pore size distribution (PSD), and the type
functional groups on the surface. Recently, microwave
pretreatment for charcoal production was also found to be
promising (Saito and Sato 2012 ).
From a compositional standpoint, the main ele-
ments of lignocelluloses (C, H, and O) volatilize during
a low-temperature stage (200 400 ° C). H and O volatilize
in disproportionally higher amounts than C, initially as
water and later on as hydrocarbons, tarry vapors, H
2 , CO,
and CO
2 . The proportion of C in the solid phase increases
from 40 – 50 % in the feedstock to 70 – 80 % after pyrolysis
between 250 ° C and 600 ° C (Antal and Gr ø nli 2003 ).
Each chemical activator has an optimal tempera-
ture range to interact with the feedstock, for example,
700 – 900 ° C for alkali metal hydroxides, 500 700 ° C
for zinc chloride, and 400 500 ° C for phosphoric acid
(Lozano -Castello etal. 2001 ; Idriss etal. 2007 ). As activa-
tors, the hydroxides of alkali metals have the most univer-
sal properties to improve the chemical reactions and are
able to react with the different types of organic feedstock,
including coal precursors and chars. The porous structure
of AC mainly depends on the type of feedstock, process
temperature, and concentration of the activating agent.
At temperatures of 200 400 ° C, heterocycles and C-O-
bonds degrade and alkali metal cations interact with the
oxygen-containing groups (Tamarkina etal. 2008 ). When
temperature rises, alkaline dehydration and dealkyla-
tion take place (Yoshizava etal. 2002 ). At 600 900 ° C, the
reduction of cation to metal and its intercalation into the
interlayer space of the crystallites structure are possible
(McKee 1983 ). Secondary carbon redox reactions occur in
the gas phase (Moulin and Kaptejn 1987 ).
In the initial process of alkaline activation, with
KOH or NaOH for example, the evolving CO
2 reacts with
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588G. Dobele et al.: Pore structure of activated carbons
an alkaline cation and carbonates are formed. This inac-
tivates alkali and leads to their overconsumption, which
can be diminished by a low-temperature precarbonization
of precursor to lower oxygen content, CO
2 development,
and the subsequent carbonate formation (Romanos etal.
2012 ). To this purpose, dehydration agents are required
(e.g., inorganic acids). The following thermocatalytic acti-
vation with alkali is more effective and more favorable for
the formation of the organic matrix of the final carbon
material (Kalinicheva etal. 2008a,b ).
For the time being, the most widespread methods
for the AC porosity assessment are mercury porosime-
try, nitrogen adsorption at 77 K, and, to a lesser extent,
benzene adsorption determined with a McBain balance
at 298 K. However, all of these approaches have flaws.
Mercury porosimetry cannot give information about
pores < 3 nm in diameter (and thus characterize micro-
porosity) and requires high pressures to force mercury
into the smallest of the mesopores, which can lead to
crushed or compressed samples; if the original mesopo-
rosity is destroyed, the results are meaningless (Geische
2006 ).
Nitrogen is used mostly for convenience without any
fundamental reason. Its adsorption isotherms sometimes
produce conflicting results (Herzog etal. 2006 ) due to the
fact that N
2 is not completely inactive at 77 K. N
2 isotherms
are used in the classic Brunauer-Emmett-Teller (BET)
approach to evaluate the surface areas of porous materi-
als. This assessment is recommended by the International
Union of Pure and Applied Chemistry and it is appropriate
for mesoporous materials (Herzog etal. 2006 ). However,
in the case of highly microporous ACs, which are ener-
getically and structurally inhomogeneous, the essen-
tial assumptions of the BET theory are not even fulfilled
approximately (Bansal and Goyal 2005 ) and lead to spe-
cific surface areas surpassing theoretical limits.
The adsorption of benzene on AC is most frequently
studied, and it was, for a long time, the model compound
to determine the porosity of ACs owing to its stability,
hydrophobicity, and symmetrical form and size of the mol-
ecule (Bottani and Tasc ó n 2008 ). For example, Dubinin
and Stoeckli (1980) and Loskutov (2000) developed their
theory of volume filling of micropores based on benzene
vapor adsorption. This theory provides a satisfactory
description of adsorption isotherms where the adsorp-
tion takes place largely in micropores. These isotherms
are usually obtained by the McBain method, which is very
time consuming.
This is the background of the present work, in which a
novel benzene sorption method called the limited evapo-
ration technique (LET) is proposed to assess the porosity
of microporous AC. More precisely, in the current work,
AC will be studied, which was prepared from wood by a
two-stage thermocatalytic method. First, wood samples
were carbonized with and without dehydration catalyst
(H
3 PO 4 ) and then the material was activated with the aid
of NaOH. The porous structure of AC (specific surface
area, volume, and PSD) will be evaluated by means of
the LET. In focus is the influence of wood species, acti-
vator content, and heating rate on the results. The per-
formance of the ACs will also be tested as electrodes in
supercapacitors.
Materials and methods
All percentages are presented based on weight.
Materials
Sawdust from mechanical processing from white alder ( Alnus rhombi-
folia ) and birchwood ( Betula pendula ; moisture content of 8 – 10 % )
was sieved and the fraction of 0.16 0.2 mm was carbonized. NaOH
(97 % ) was purchased from Sigma-Aldrich (Steinheim, Germany) and
H
3 PO 4 (87 % ; puriss.) was purchased Fluka (Lachner, Neratovice,
Czech Republic).
Synthesis of AC
ACs were synthesized in a two-stage process (Dobele etal. 2012a ); the
parameters are listed in Table 1 . In the  rst stage, the raw material was
pretreated at 400 ° C for 150 min. Heating rates: 1 ° Cmin -1 and 4 ° Cmin -1 .
In some pretreatment experiments, 200 g of the raw material were  rst
impregnated with H
3 PO 4 solution (2 % , 4 % , and 6 % based on the raw
material). In the second stage, the carbonized material was impregnat-
ed with a 40 % solution of NaOH. The ratio of activator mass to oven-
dried raw material was 50 % , 80 % , and 100 % . Then, the mixture was
pyrolyzed in steel crucible at 700 ° C for 90 min in Ar  ow (Nabertherm
oven L40/11, Lilienthal, Germany). Each AC was rinsed in deionized
water and 1 % acetic acid and again with deionized water until neutral
pH was obtained, and the material was oven dried at 105 ° C overnight.
The ash content in the obtained carbon materials was 1.8 2.4 % .
Limited evaporation technique
The porous structure of AC was tested by the LET method (Shkolnikov
etal. 1999 ). This method relies on the kinetics of adsorbate evapora-
tion from the cell under pseudo-equilibrium conditions. To reach this
condition, the adsorbate evaporation rate is limited at the cell outlet.
The measurement cell is situated on a balance and a constant dry air-
ow ensures vapor removal. Benzene was selected as the adsorbate.
The method is based on the theory that, under pseudo-equilib-
rium conditions, the rate of evaporation from the cell is exclusively
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G. Dobele et al.: Pore structure of activated carbons 589
Table 1 Synthesis conditions of AC (carbonization temperature 400 ° C, 150 min; pyrolysis temperature 700 ° C, 90 min) and characteristics
ofAC in terms of porosity.
Conditions of carbonization Characteristics of AC
Sample Heating rate
( ° C min - )
Content of
H
PO ( % )
Content of
NaOH ( % )
Pore volume
a
(cm
g - )
Total specific
pore area (m
g - )
Surface area
b
(m
g - )
AB--
c  .  
A--  .  
A--  .  
A--  .  
L--  .  
L--  .  
L--  .  
AP--   .  
AP--   .  
AP--   .  
a r p
< . nm.
b r p
< . nm (with pores).
c AB-- is from birchwood; the other samples are from alder wood.
the function of equilibrium vapor pressure inside the cell above the
sample. This leads to a desorption isotherm by measuring the mass
loss of adsorbate as a function of time without the evaluation of
vapor pressure, which can be determined by the speed of
evaporation from the cell (Shkolnikov and Volkov 2001 ).
Calculation of PSD
The pore radii were calculated from the obtained desorption iso-
therms based on the Broekho -de Boer equation for slit-like pore
desorption (Broekho and de Boer 1968 ). According to the theory
of Derjaguin-Broekho -de Boer for the slit-like pores, the equilib-
rium thickness of the adsorption  lm in micropores coincides with
the thickness of the  lm on the  at surface t . In this article, the tra-
ditional t -curve was used on the  at surface (Isirikyan and Kiselev
1962 ), which was observed by the adsorption of benzene on Stirling
graphitized carbon black.
The cumulative and di erential distributions of pore volumes V p
on their radii r p
are given by the equation:
-,
-
pp a
p
pp pp
Vr VtS
rrtt rr
⎛⎞⎛ ⎞
ΔΔΔ
⎜⎟⎜ ⎟
Δ+ΔΔΔ
⎝⎠⎝ ⎠
(1)
where S p
is the speci c surface area. This equation has been obtained
based on the Wheeler (1955) equations as applied to slit-like pores.
The speci c surface area was calculated from the cumulative PSD
taking into account the slit-like pore model:
max
.
i
V
p
p
p
V
dV
Sr
=
(2)
The calculation procedure and the possibility of application
of this approach are described by Shkolnikov etal. (2011) . The fact
that PSDs do not reach the abscissa axis is probably connected with
the speci c adsorption of benzene on pore surfaces (Vitkina etal.
2012 ).
Tests of electrochemical characteristics
ofAC
A single electrochemical cell modeling supercapacitor was applied.
It consists of two current collectors from thermo-expanded graphite,
two electrodes made of the investigated AC divided by porous separa-
tor (thickness of 10 μ m) lled with H
2 SO 4 aqueous solution as electro-
lyte. The electrodes of the supercapacitors (diameter of 23 mm and
thickness of 0.03 mm) were made of AC A-4-50 and A-4-100 (Table
1; compression of 0.1 MPa). The galvanostatic charging-discharging
tests (5 150 mA cm
-2 ) were made with potentiostat PS-10 (Elins LLC,
Russia). The cell voltage range was 0 1 V.
Results and discussion
Influence of the parameters on the porous
structure
The porous structure of the obtained carbon materials has
been investigated by the experimental and calculation
methods described in the experimental part. The follow-
ing parameters were in focus: the conditions of the car-
bonization in the first stage of the thermal treatment, the
quantity of the alkaline activator during pyrolysis in the
second stage, and the species of wood. The results are pre-
sented in Table 1 and Figures 1 – 3 .
Figure 1a shows that the elevated heating rates are
accompanied by the increase in the volume of pores
within the radius range of 0.5 1.5 nm. This is true for
both samples activated with 100 % and 50 % NaOH solu-
tions (samples A-4-100, L-1-100, A-4-50, and L-1-50, respec-
tively; Table 1). The pore structure of carbon considerably
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590G. Dobele et al.: Pore structure of activated carbons
changes depending on the quantity of the alkaline activa-
tor (Figure 1a and b). Elevated NaOH contents lead to an
increase of micropore and mesopore volumes.
Figure 2 shows the plots of differential PSD versus
the pore size radius r (nm) in two examples. The ranges
Figure 2 Plot of differential PSD (d V /d r ) vs. pore size radius r .
Variable: c.r. A-4-100 ( Δ ) 4 ° C min -1 and L-1-100 ( ) 1 ° C min -1 .
Figure 3 Plot of cumulative PSD vs. pore size r of alder and birch-
wood at a c.r. of 4 ° C min -1 (100 % NaOH input).
1.6
a
b
c
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
1.6
1.4
Vp (cm3 g-1)
1.2
1.0
0.8
0.6
0.4
0.2
0
1.6
1.8
2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0.1 1 10 100
Pore size r (nm)
Figure 1 Plots of pore volume ( V p
) vs. pore size radius ( r ) (cumula-
tive PSD).
(a) Variables: c.r. and amount of alkali. A-4-100 ( Δ ; 4 ° C min -1 c.r.,
100 % NaOH), L-1-100 ( ■ ; 1 ° C min -1 c.r., 100 % NaOH), A-4-50 ( ;
4 ° Cmin -1 c.r., 50 % NaOH), and L-1-50 ( ; 1 ° C min -1 c.r., 50 % NaOH).
(b) Variable: amount of alkali at a c.r. of 4 ° C min -1 . A-4-100 ( Δ ; 100 %
NaOH), A-4-80 ( ; 80 % NaOH), and A-4-50 ( ; 50 % NaOH). (c) Vari-
able: amount of phosphorous acid at a c.r. of 4 ° C min -1 and 100 %
NaOH. AP2-4-100 ( Δ ; 2 % H 3 PO 4 ), AP4-4-100 ( ◊ ; 4 % H 3 PO 4 ), and AP6-
4-100 ( □ ; 6 % H 3 PO 4 ).
of PSD for the samples obtained at different rates of car-
bonization (A-4-100 and L-1-100) coincide. In both cases,
the maximum of pore radii is ~ 0.77 nm. The volume and
surface area of micropores of sample A-4-100 exceed
those of sample L-1-100, which was heated at a slower rate
during carbonization (Table 1). The observed differences
seem to be the consequence of the different carbonization
rates (c.r.).
H 3 PO 4 intensified the c.r. of the raw material in the
course of thermal pretreatment and increased the yield
of the carbonizate. The interaction of dehydrating agents,
in the present case H
3 PO 4 , with the biomass components,
promotes the condensation and dehydration reactions
responsible for the carbonization of the material, which
increases the output of the carbonized product and
decreases the content of elemental oxygen (Jagtoyen and
Derbyshire 1998 ).
Figure 1c illustrates that the PSDs for the samples
obtained with the addition of 2 % , 4 % , and 6 % phos-
phoric acid exhibit a monotonous interrelation: the lesser
the content of H
3 PO 4 , the greater is the total pore volume
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G. Dobele et al.: Pore structure of activated carbons 591
(Table 1). Thus, the addition of small amounts of H
3 PO 4 in
the carbonization stage seems to be more prospective for
pore formation.
Figure 3 demonstrates the PSDs for the samples
obtained under the same conditions for birchwood and
alder and shows that the pore structures of the AC samples
are very similar. The volumes of micropores in the radius
range of 0.6 1 nm and their distributions are practically
the same. When the radius exceeds 1.2 nm, the curves
slightly diverge before taking equidistant positions. The
mesopore volume in AC alder is higher than that of AC
birchwood by 0.05 cm
3 g -1 .
The following interpretation of the porosity results
obtained by a two-stage thermocatalytic process is possi-
ble (Figure 4 ). In the first carbonization stage, the destruc-
tion and dehydration of polymeric wood components
take place (Strezov etal. 2007 ; Dobele etal. 2013 ). These
reactions depend on the presence of labile fragments and
oxygen-containing bonds in the precursor. The combina-
tion of destruction products influenced by the peculiari-
ties of the precursors structure leads to carbon- containing
microfragments, the formation of which are governed
only by statistical events, and which do not increase the
specific volume of micropores, because condensed liquid
products of pyrolysis fill the pores arisen during carboni-
zation (Inagaki etal. 2010 ).
Figure 4 Schematic representation of carbonization of wood precursors and their activation.
In the second stage of the thermocatalytic process,
the alkaline activator readily reacts with oxygen-
containing centers, and dehydrogenation, dehydra-
tion, and decarboxylation reactions occur (Marsh and
Rodriguez -Reinoso 2006 ). The intensity of these reac-
tions is highest beyond the melting point of NaOH
(i.e., > 318 ° C). Carbon transforms partially from the
sp3 -state to the aromatic sp2 -state (Inagaki 2010 ). A
part of the carbons is removed as volatiles and another
part forms flat (2D) multinucleus aromatic layers (gra-
phenes), which are left in the amorphous solid mate-
rial (Paris etal. 2005 ; Viswanathan etal. 2009 ; Volperts
etal. 2012 ; Dobele etal. 2012b ). Most of the remaining
non-C atoms are removed and graphene sheets continue
to grow laterally, eventually coalescing (Amonette and
Joseph 2009 ), but the dense packing of the graphenes
is hampered by sodium carbonate and bicarbonate,
originating from the decarboxylation process and the
subsequent reactions of CO
2 with alkali. The porous
carbon matrix is formed upon the subsequent removal
of inorganic compounds by washing. The presence of
graphenes in the amorphous AC with developed nano-
porous structure can be an attribute of unique sorp-
tion and electrochemical properties and makes them
desired candidates for electrodes in supercapacitors
(Brownson etal. 2011 ).
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592G. Dobele et al.: Pore structure of activated carbons
Application of AC in electrodes of
supercapacitors
Figure 5 a and b illustrates the results of electrochemi-
cal tests with the A-4-50 and A-4-100 samples as elec-
trode materials. The conditions of these experiments are
similar to those of a real device. The electrolyte in such
a system fills the macropores (which are the interspaces
between the particles of AC) and micropores (which are
located inside of the AC particles). The PSDs for A-4-50
and A-4-100 (Figure 1b) show that the micropore volume
( r < 1 nm) is 1.35 cm
3 g -1 for A-4-100 and 0.99 cm
3 g -1 for
A-4-50. The multiplication of the given pore volumes by
the density of sulfuric acid (1.28 g cm
-3 ) gives the mass of
acid in relation to the mass of dry AC (i.e., the amounts
of electrolyte in these pores are 1.73 and 1.27 g H
2 SO 4 g
AC
-1 , respectively). The real amounts of electrolyte that
were involved in experiments were 3.6 g H
2 SO 4 g AC
-1 for
A-4-100 and 2 g H
2 SO 4 g AC
-1 for A-4-50. It means that a
certain part of electrolyte resides in big macropores that
are formed by interspaces between AC particles. Macro-
pores should contain only a minimal volume of electro-
lyte to provide pore connectivity (i.e., not < 30 % of the
electrode volume). In our case, the amount of macropores
is excessive and is ballast in a supercapacitor system. The
main objective of the ongoing research is to optimize the
amount of macropores and to find the minimal amount
of micropores that would be sufficient for the electrical
double-layer formation.
The supercapacitors based on A-4-50 (Figure 5a)
show lower specific electrochemical characteristics based
on dry weight, but the amount of the electrolyte in this
system is lower than in the supercapacitors based on A-4-
100. This leads to the higher specific characteristics based
on the active weight (Figure 5b).
Conclusions
The novel benzene sorption method LET was applied for
the characterization of synthesized highly ACs. The defi-
nite specific surface areas, PSDs, and the electrochemical
properties of ACs can be obtained by the systematic varia-
tion of added H
3 PO 4 in the pretreatment (first stage) and by
the varying amounts of activators in the pyrolysis (second
stage). The tests of ACs as electrodes in supercapacitors
demonstrated promising results and a high potential for
this type of application.
Acknowledgements: The research leading to these results
has received funding also from the Latvian Budget (grant
1546), Latvian National Programme-5,2,2.4. The authors are
grateful to TEEMP, LLC, and personally to Dr. A.V. Dolgo-
laptev for the active help in the work organization. The work
was supported by the Ministry of Education and Science of
the Russian Federation (contract no. 16.526.12.6002).
Received October 31, 2012; accepted February 8, 2013; previously
published online March 14, 2013
1
2
3
4
5
6
7
8
9
0123456
Current (A g-1)
Energy density (Wh kg-1)
a) b.o. dry AC
b) b.o. dry AC + electolyte
Figure 5 Plots of energy densities of ACs vs. the electrical current.
ACs were produced at a c.r. of 4 ° C min -1 with two amounts of NaOH input: A-4-50 ( ; 50 % NaOH) and A-4-100 ( ; 100 % NaOH). Based on (a)
dry AC and (b) dry AC plus electrolyte.
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  • Article
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  • Chapter
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    The implementation of biorefinery concepts into existing pulp and paper mills is a key step for a sustainable utilization of the natural resource wood. Water prehydrolysis of wood is an interesting process for the recovery of xylo-oligosaccharides and derivatives thereof, while at the same time cellulose is preserved to a large extent for subsequent dissolving pulp production. The recovery of value-added products out of autohydrolyzates is frequently hindered by extensive lignin precipitation, especially at high temperatures. In this study, a new high-temperature adsorption process (HiTAC process) was developed, where lignin is removed directly after the autohydrolysis, which enables further processing of the autohydrolyzates. The suitability of activated charcoals as a selective adsorbent for lignin under process- relevant conditions (150 and 170 degrees C) has not been considered up to now, because former experiments showed decreasing efficiency of charcoal adsorption of lignin with increasing temperature in the range 20-80 degrees C. In contrast to these results, we demonstrated that the adsorption of lignin at 170 degrees C directly after autohydrolysis is even more efficient than after cooling the hydrolyzate to room temperature. The formation of lignin precipitation and incrustations can thus be efficiently prevented by the HiTAC process. The carbohydrates in the autohydrolysis liquor remain unaffected over a wide charcoal concentration range and can be further processed to yield valuable products.
  • Article
    The objective of this study was to investigate the carbonization induced by microwave treatment (MWT) of spruce (Picea sp.) wood, which was treated in a domestic microwave oven at 2.45 GHz. Structural changes were observed by elemental analysis, thermal analysis, and by IR and Raman spectroscopy. The degradation of the samples was completed within 3 min. The carbonization was comparable to that caused by heat treatment at ca. 420 degrees C. In the MWT samples, a graphitic layer was not detected, which behaves as a sensitizer for MW irradiation and sometime elevates the temperature drastically. This study shows that MWT leads to a carbonaceous material, and that the carbonization levels off with the loss of heat-inducing functional groups, such as O-H, CO-OH, C=O, upon MW irradiation. MWT is better suited as a kind of controlled and short pretreatment method before carbonization. Its suitability for porous absorbents should be investigated.
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    A recently developed dynamic desorption technique is used for obtaining vapor isotherms on porous materials. This gravimetric technique does not require any preliminary calibration and is based on analyzing the kinetics of liquid evaporation from a porous sample under quasi-steady state conditions. The crucial feature of the technique is concerned with the fact that no vapor pressure measurements are necessary. The technique is illustrated by desorption of benzene vapors from mesoporous silica MCM-41. To calculate the pore size distribution, the Derjaguin–Broekhoff–de Boer theory in its combination with the Wheeler model for capillary condensation is used. In the calculations, the reference data on benzene adsorption on a nonporous silica gel from two different sources (published by different authors) are applied. The mean mesopore sizes estimated from desorption isotherms are shown to be in a fair agreement with the calculations through the geometrical method based on the X-ray diffraction data. The dynamic desorption technique can serve as an additional tool for the characterization of a porous media.