Adsorption-stimulated deformation of microporous carbon adsorbent

Article abstract of DOI:10.1023/A:1023498530313

Deformation of the ACC microporous carbon adsorbent during adsorption of carbon dioxide, nitrogen, and argon in the temperature interval from 243 to 393 K and at pressures of 1-5·10⁶ Pa was studied. The effect of adsorbent contraction was found in the initial temperature interval at relatively low pressures. However, the negative value of relative linear deformation ΔL/L smoothly transforms into positive values with the pressure increase. Only the effect of adsorbent expansion is observed at high temperatures in the whole pressure interval. The dependence of the deformation effects for different systems on the adsorbent nature was revealed.

Full text of DOI:10.1023/A:1023498530313

354
Russian Chemical Bulletin, International Edition, Vol. 52, No. 2, pp. 354—358, February, 2003
Adsorptionꢀstimulated deformation of microporous carbon adsorbent
a
bꢀ
a
b
b
V. Yu. Yakovlev, A. A. Fomkin, A. V. Tvardovski, V. A. Sinitsyn, and A. L. Pulin
aTver State Technical University,
2
2 nab. Afanasiya Nikitina, 170026 Tver, Russian Federation.
Fax: +7 (082 2) 31 4190. Eꢀmail: atvard@tversu.ru
b
Institute of Physical Chemistry, Russian Academy of Sciences,
3
1 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 952 5681. Eꢀmail: fomkinaa@mail.ru
Deformation of the ACC microporous carbon adsorbent during adsorption of carbon
dioxide, nitrogen, and argon in the temperature interval from 243 to 393 K and at pressures of
6
1
—5•10 Pa was studied. The effect of adsorbent contraction was found in the initial temperaꢀ
ture interval at relatively low pressures. However, the negative value of relative linear deformaꢀ
tion ∆L/L smoothly transforms into positive values with the pressure increase. Only the effect
of adsorbent expansion is observed at high temperatures in the whole pressure interval. The
dependence of the deformation effects for different systems on the adsorbent nature was
revealed.
Key words: adsorption, active carbon, adsorption deformation, carbon dioxide, nitrogen,
argon, high pressures.
Studying adsorption of gases and vapors, researchers
It follows from Eq. (1) that noninertness of the adsorꢀ
1
usually imply that the solid is inert. This assumption is
valid for adsorbents with a relatively small specific surface
and is inappropriate for microporous adsorbents with
bent during adsorption can be mechanical, thermal, and
energetic. The mechanical character of noninertness apꢀ
pears in a change in the adsorbent sizes during adsorpꢀ
tion. To take into account the mechanical noninertness
2
nearly all atoms involved in the adsorption interaction.
Noninertness of an adsorbent is reflected primarily in
the behavior of thermodynamic functions of the adsorpꢀ
tion system, such as isosteric heat of adsorption, entropy,
and heat capacity. The studies in wide temperature and
pressure intervals show that for the correct calculation of
the thermodynamic characteristics of the system the adꢀ
sorption deformation of the adsorbent should be conꢀ
sidered.3
of the adsorbent, the terms (∂ν(a)/∂a) and v(a) reflectꢀ
T
ing a change in the adsorbent volume during adsorption
were introduced into the equation for the heat of adsorpꢀ
tion. The thermal noninertness is determined by a change
in the adsorbent volume with temperature under the
isosteric conditions (∂ν(a)/∂T ) . The energetic noninertꢀ
a
ness of the adsorbent is a consequence of a possible change
in the gradients and intensity of the sorption field in
microporous, related to local changes in the adsorption
field and induced, for example, by a highꢀenergy specific
interaction. This noninertness is accounted for by the
terms (∂lnP/(∂1/T )) and (∂P/∂a) .
For adsorption on microporous adsorbents, the isoꢀ
4
steric heat of adsorption q is determined by the equation
st
a
T
When real microporous adsorbents containing microꢀ,
mesoꢀ, and macropores are involved, adsorption in
micropores is accompanied by capillary condensation in
mesopores and layerꢀbyꢀlayer filling of the macropore
surface. Adsorption and deformation on the ACC microꢀ
porous carbon adsorbent are of special interest because
this adsorbent contains no mesopores and its macropore
surface is small. The correct calculation of the thermodyꢀ
namic characteristics of the adsorption system needs exꢀ
perimental data on adsorption deformation in the whole
region of temperatures and pressures, especially at high
pressures where the deformation makes the greatest conꢀ
,
(1)
where Z = Pv /(RT ) is the compressibility coefficient of
the gas phase at the pressure Р, specific volume v , and
temperature Т; v(a) = V /m is the specific reduced volꢀ
ume of the adsorbent—adsorbate adsorption system; V is
the volume of adsorbent—adsorbate system; m is the
weight of the degassed adsorbent; a is the adsorption value
defined as the full content; R is the universal gas constant.
g
g
1
0
1
0
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 338—342, February, 2003.
066ꢀ5285/03/5202ꢀ354 $25.00 © 2003 Plenum Publishing Corporation
1

Adsorptionꢀstimulated deformation
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
355
tribution to the thermodynamic characteristics of the
system.
8
7
6
5
4
9
The phenomenon of adsorption deformation was deꢀ
scribed in several works.5
,7—11
However, the region of
studied pressures and temperatures still remains insuffiꢀ
ciently wide for determination of general regularities of
this phenomenon. The purpose of this work is to obtain
experimental data on the adsorption deformation on the
ACC microporous carbon adsorbent in wide intervals of
pressures and temperatures. Adsorption was studied for
СО , N , and Ar, which possess different physicochemiꢀ
2
2
cal properties, in particular, different polarizabilities
Table 1), determining the energy of adsorption interꢀ
action.
(
4
3
Experimental
2
1
A model sample of the ACC microporous carbon sorbent
was prepared from silicon carbide at 900 K by removing silicon
Fig. 1. Scheme of dilatometer for measuring the adsorption deꢀ
formation of solids: 1, adsorbent; 2, quartz polished disks;
3, ampule; 4, rod; 5, core; 6, differential transformer; 7, tube;
with a chlorine flow followed by gaseous SiCl evolution.
4
A monolithic cylindrical sample with a length of 54.1 mm
and a diameter of 11.4 mm was used. Before measurements, the
adsorbent was evacuated for 6 h at 620 K to a residual pressure
of 0.8 Pa.
8
, vacuum valve; and 9, screw.
screw (9). A change in the sample length results in the displaceꢀ
ment of the core, which is detected by a V7ꢀ38 digital voltmeter
using a transformer. The dilatometer is connected to a highꢀ
pressure adsorption installation through the tube (7) and vacuum
To determine the characteristics of the microporous strucꢀ
ture, data on benzene adsorption at 293 K were processed on the
1
2
basis of the theory of volume micropore filling. The following
characteristics of the adsorbent sample were obtained: micropore
1
3
valve (8).
3
–1
During experiments, the temperature in the dilatometer secꢀ
tion containing the sample was maintained constant with an
error of ±0.2 K. The rest volume of the dilatometer along with
the induction converter and the highꢀpressure installation were
thermostatted in an air thermostat at 303 K.
volume W = 0.47 cm g , characteristic energy of adsorption
0
–
1
Е0 = 30 kJ mol , and characteristic micropore halfꢀwidth
х0 = 0.4 nm. An inductiveꢀtype dilatometer designed for meaꢀ
suring small deformations of solids during adsorption in pressure
7
and temperature intervals of 1—2•10 Pa and 77—570 K, reꢀ
The induction converter of displacements of the dilatometer
was calibrated at 303 K using a set of standard plates with a
thickness of 0.01–1.00 mm. The dilatometer was adjusted to
changes in temperature and gas pressure on a "model" of fused
quartz analogous to the adsorbent sample in shape and sizes.
spectively, was used. The scheme of the dilatometer is shown in
Fig. 1: the core (5) of the induction converter (6) of linear
displacements is connected with the rod (4) supported on the
quartz plate (2), which is placed on the sample (1). The latter is
placed inside the ampule (3) between the quartz disks (2). The
ampule is connected to the dilatometer body through the
Adsorption of СО was studied using a gravimetric vacuum
2
adsorption installation with the electronic compensation of the
weight change in three intervals with limits of 1, 10, and 100 mg.
The error of measurement did not exceed 1%. The gas pressure
was determined by two M10 and M1000 pressure bellows gauges
with measurement intervals of 1—1•103 and 10—1.5•10 Pa
and errors of ±0.1 and ±1.0 Pa, respectively.
Table 1. Relative deformation of the ACC adsorbent induced by
the adsorption of gases with different physicochemical paꢀ
rameters
5
30 d
Gases of special and high purity were used. The volume
Gas
|η|maxa
ηmaxb
Size of
molecule /Å
(α•10 )
c
/m3
fraction of CO , N , and Ar in these gases was at least 99.95,
2 2
%
9
9.9963, and 99.998%, respectively.
Results and Discussion
The plots of the relative linear deformation of the
СО2
N2
Ar
0.117 e
0.6 e
0.12
0.12 e
5.1/3.7
4.1/3.0
3.84
2.594
1.734
1.626
e
f
0.04
e
0.02
a
Maximum contraction.
Maximum expansion.
The nominator and denominator contain the length and width,
ACC carbon adsorbent vs. relative pressure of СО at
b
с
2
temperatures from 243 to 393 K are presented in Fig. 2.
As follows from the data in Fig. 2, in the region of low
temperatures (Т < 313 К) and initial region of equilibꢀ
rium pressures (Р < 0.25 MPa) the contraction of the
sample achieves the value of 0.12% at 243 К. At higher
respectively.
d
Polarizability.
At 243 К.
At 393 К.
e
f

356
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
Yakovlev et al.
3
(
∆L/L)•10 (rel. units)
pressures higher than 0.15 MPa was calculated using the
linearity of adsorption isosteres. The adsorption isotherms
are reversible and characterized by convexity toward the
pressure axis. With the pressure increase the adsorption
2
6
4
2
0
1
tends to the constant а , which is the limiting adsorption
3
4
0
value. It follows from similarity of the deformation (see
Fig. 2) and adsorption (see Fig. 3) curves that at high
pressures the adsorption in micropores makes the main
contribution to the deformation effect. Like the amount
adsorbed, the relative linear deformation of the microꢀ
porous carbon sorbent increases more slowly with the
pressure increase.
1
2
3
4
P/MPa
The results of studying the adsorption deformation of
the ACC adsorbent during nitrogen adsorption are preꢀ
sented in Fig. 4. They correspond in many aspects to the
Fig. 2. Relative linear deformation of the ACC microporous
carbon adsorbent as a function of the pressure during СО adꢀ
2
results for the ACC—СО system. In the region of relaꢀ
sorption at temperatures: 243 (1), 293 (2), 313 (3), and 393 K (4).
2
tively low pressures at 243 and 293 K, the contraction to
0
.046% is observed at 243 K, which then changes by the
pressures, the contraction is changed by expansion. At
higher temperatures of the studied interval (Т > 313 К),
the expansion was the only result of the pressure increase.
However, with the temperature increase the effect of relaꢀ
tive expansion decreases from 0.6% at 293 K to 0.25% at
expansion to 0.08% at a pressure of 4.5 MPa. The maxiꢀ
mum deformations upon contraction and expansion are
much smaller than those observed for the adsorption of
CO . When comparing the ACC—СО and ACC—N
2
2
2
systems, one should mention differences in the shape of
the curves along with the changes in the maximum values
of deformation. For nitrogen adsorption at pressures higher
than 1 MPa, the plots are almost linear, whereas for СО2
adsorption nonlinearity is observed in the entire pressure
interval. It is of interest that the inversion of adsorption
deformation for the ACC—N2 system (see Fig. 4) is
shifted toward high pressures. Its properties (intersecꢀ
tion of curves) can probably appear at pressures higher
than 5 MPa.
3
93 К. The plots of the relative linear deformation of
ACC vs. pressure at different temperatures show an inverꢀ
sion, that is, different sequence of curves at initial and
final pressures. As follows from the data in Fig. 2, for СО2
the inversion region lies approximately in an interval of
0
.3—0.5 MPa.
The character of the dependence of the relative linear
deformation under the isothermic conditions is caused, to
a great extent, by the behavior of the adsorption isoꢀ
therms. The isotherms of adsorption of CO on the ACC
2
microporous carbon adsorbent are presented in the a—P
coordinates in a pressure interval of 1—6•10 Pa and at
3
6
(∆L/L)•10 (rel. units)
4
temperatures from 243 to 393 K in Fig. 3. Adsorption at
3
0
0
0
0
0
.94
.74
.54
.34
.14
a/mmol g–1
1
2
1
1
1
9
7
5
3
2
3
4
–
–
–
0.06
0.26
0.46
1
2
3
4
P/MPa
1
0
1
2
3
4
5
P/MPa
Fig. 3. Isotherms of СО adsorption on the ACC microporous
Fig. 4. Relative linear deformation of the ACC microporous
2
carbon adsorbent at temperatures: 243 (1), 293 (2), 313 (3), and
carbon adsorbent as a function of the pressure during N adsorpꢀ
2
3
93 K (4).
tion at temperatures: 243 (1), 293 (2), 313 (3), and 393 K (4).

Adsorptionꢀstimulated deformation
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
357
3
scribed. The dimensions of molecules14 presented in
Table 1 are comparable with the adsorbent pore sizes.
Therefore, the initial contraction of the adsorbent is a
sequence of the interaction of adsorbed molecules with
the opposite micropore walls. This effect is observed for
relatively low temperatures. However, as the pressure inꢀ
creases the amount adsorbed also increases, the mean
distance between the adsorbed molecules decreases, and
repulsion forces increase. This results in an increase in the
internal pressure tending to stretch the adsorbent as obꢀ
served in the experiment.
The curves of adsorption deformation at high temꢀ
peratures behave in a different manner. In this region for
all systems under study, the adsorption deformation is
positive in the whole pressure interval. These differences
in the behavior of the adsorption deformation curves of
the ACC microporous adsorbent can be related to a change
in the adsorption mechanism: as the temperature inꢀ
creases, the transition from the partially localized adsorpꢀ
tion to the delocalized one occurs.
(
∆L/L)•10 (rel. units)
1
1
0
0
0
0
.2
.0
.8
.6
.4
.2
0
1
2
3
4
1
2
3
4
P/MPa
–
0.2
Fig. 5. Relative linear deformation of the ACC microporous
carbon adsorbent as a function of the pressure during Ar adsorpꢀ
tion at temperatures: 243 (1), 293 (2), 313 (3), and 393 K (4).
The adsorption deformation of the adsorbent exerts a
substantial effect on the calculation of the isosteric heat
of adsorption. According to our results, at pressures higher
than 4—5 MPa, corrections to the isosteric heat of adꢀ
sorption can reach 10—15% and more of the initial value.
This influence of the adsorption deformation is due to its
sharp increase at high pressures and, correspondingly, to
The smallest deformation effects of the ACC microꢀ
porous carbon adsorbent were found for Ar adsorption.
The plots of the adsorption deformation of ACC during
Ar adsorption at temperatures from 243 to 393 K and in
the pressure interval from 1•10 to 5•10 Pa are preꢀ
sented in Fig. 5. As follows from the data in Fig. 5, in the
lowꢀtemperature region (243—293 К) at pressures of
2
6
the approach of the (∂ν(a)/∂a) derivative to the specific
T
volume for the gas phase v . Taking into account the
g
0
.4—0.8 MPa, as for СО and N adsorption, the adsorꢀ
adsorption deformation of the adsorbent, corrections
to the work of the adsorbent contraction (expansion)
should be introduced in measurements of the differential
calorimetric heat of adsorption.
2
2
bent contraction is observed reaching 0.02% at 243 K and
going to the expansion region with the pressure increase.
The maximum expansion achieves the value of 0.12% at
2
43 К. Only adsorbent expansion is observed at high temꢀ
Thus, the deformation of the ACC microporous carꢀ
bon adsorbent was studied in temperature and pressure
intervals of 243—393 K and 1—5•10 Pa, respectively,
peratures (313—393 K) in the whole interval of measured
pressures. The region of inversion temperatures of the
deformation curves for this system is well pronounced
and lies at 1.5—2.0 MPa.
The limiting deformation effects of expansion and conꢀ
traction for the adsorption systems under study and some
physical characteristics of the gases are presented in
Table 1.
6
during adsorption of CO , N , and Ar. The differences
2
2
were revealed for the extent of deformation effects for
different systems depending on the adsorbate nature. In
the general case, the adsorption deformation is a sequence
of the summation of the forces of adsorbate—adsorbate
and adsorbate—adsorbent interactions. At small fillings in
the region of low temperatures, attraction forces of the
gas by the opposite walls of the adsorbent micropore are
predominantly manifested. They result in the contraction
of the sample in the initial region. The distance between
the molecules decreases with an increase in the micropore
filling, and attraction is replaced by repulsion to expand
the micropores and the sample on the whole.
It follows from the data presented in Table 1 that an
increase in the polarizability of gases increases the maxiꢀ
mum contraction of the sample at the lowest temperature
(
243 K). The maximum expansion depends to a less exꢀ
tent on the polarizability of gases. The maximum expanꢀ
sion values for the СО —ACC and Ar—ACC systems
were obtained at 243 K. The maximum ηmax value for the
2
N —ACC system was obtained at 393 K (at 243 K the
maximum expansion ηmax is 0.082%). For the first two
systems, the inversion of curves occurs, while for the
The greatest deformation effects are characteristic of
adsorption of gases with the highest polarizability. At high
temperatures (T > 313 К) for all systems, the adsorption
deformation is positive in the whole interval of pressures
studied.
2
N —ACC system the inversion will presumably occur at
2
higher pressures, which are beyond the experiment deꢀ

358
Russ.Chem.Bull., Int.Ed., Vol. 52, No. 2, February, 2003
Yakovlev et al.
The temperature inversion of the deformation curves,
which is specifically manifested for each of the systems
studied, was found.
Allowance for adsorption deformation needs introꢀ
duction of corrections in the isosteric heat of adsorption,
which are 10—15% in the region of high pressures.
7. D. J. C. Yates, Proc. Phys. Soc., 1952, 65, 80.
8
. O. K. Krasil´nikova, M. E. Sarylova, and L. A. Falko, Izv.
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Polyakova, V. V. Serpinski, and I. M. Guseva, J. Coll. Int.
Sci., 1994, 164, 114.
9
1
1
1
1
0. A. V. Tvardovski, A. A. Fomkin, Yu. I. Tarasevich, and A. I.
Zhukova, J. Coll. Int. Sci., 1999, 212, 426.
1. M. R. Warne, N. L. Allan, and T. Cosgrove, Phys. Chem.,
Chem. Phys., 2000, 2, 3663.
2. M. M. Dubinin, Adsorbtsiya i poristost´ [Adsorption and Poꢀ
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6
Received April 15, 2002;
in revised form September 13, 2002