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Micropore structure of coal
Micropore structure ofcoal Y. TODA, M. HATAMI*,S. TOYODA,Y. YosHrDAt and H. HONDAS
The adsorption of carbon dioxide at 298K, 273K and 253K by the vitrains of twelve Japanese coals and five foreign coals has been studied. The DubininPolanyi equation was used in order to interpret the isotherms. Equilibrium points at different temperatures fell satisfactorily on a single straight line of the Dubinin-Polanyi equation. It was shown that it is possible to estimate the differential heat of adsorption of these coals from an isotherm obtained at a single temperature if the isotherm follows the Dubinin-Polanyi equation. The heats of adsorption of carbon dioxide on coals amounted to 6-7.5 kcal/mol regardless of rank. The rate of adsorption of carbon dioxide on coals was measured at a constant pressure of 260 mmHg at 298K. The micropore volume, the gradient of the Dubinin plot and the rate of adsorption are discussed in relation to characteristics of their infra-red adsorption spectra. It is concluded that the micropore structure of coal is closely related to the number of hydrogen atoms bound directly to carbon atoms. A possible explanation of this is suggested.
MANYSTUDIESof the fine structure of coals by adsorption methods have been reported. Many years have been spent in the selection of suitable adsorbates and temperatures for experimental theories for interpretation of the isotherms, because the structure of coals is complex from both chemical and physical viewpoints. Dubininl and Spencer and Bond5 pointed out that it is questionable to use the BET equation to interpret isotherms for microporous materials, because m.icropore-filling by adsorbate occurs before monolayer formation and an adsorbate molecule is in a sense shared by several surfaces. Dubinin et cd-* have investigated extensively the isotherms for many adsorbents using various adsorbates, and developed the adsorption potential theory proposed by Polanyisy 7 to a rectilinear form, from which the micropore volume and a function of micropore size can be estimated. Marsh et al extensively studied the application of the Dubinin-Polanyi equation to the isotherms for the adsorption of carbon dioxide at room temperature on coalss, carbonized coals9 and carbonslo, and proved that the above application was satisfactory. In the present paper, we show that equilibrium points for carbon dioxide adsorption on coals at different temperatures (298K, 273K and 253K) fall on a single straight line of the Dubinin-Polanyi equation. The micropore volume or the apparent surface area and the function of micropore size for * Present address; Hokutan-Kasei
Co., Toda-Saitama, Japan t Present address; Government Industrial Development Laboratory, Sapporo-Hokkaido, Japan 9 Present address; Government Industrial Research Institute, Kyushu, Tosu, Kyushu, Japan
187
Y. TODA,
M. HATAMI,
S. TOYODA,
Y. YOSHIDA
AND H. HONDA
coals are deduced from the adsorption of carbon dioxide at 298K using the Dubinin-Polanyi equation. The rates of adsorption of carbon dioxide on coals were measured at a constant pressure of 260 mmHg at 298K. The changes of the estimated values are discussed in relation to the number of hydrogen atoms bound directly to carbon atoms, deduced from the infra-red spectrall.
THEORETICAL
The Dubinin-Polanyi
INTRODUCTION
equation for microporous materials takes the form log W = log W,J- B(RT log Po/P)~
(1)
When a plot of log W against (RT log Po/P)~ is rectilinear, the intercept at (RTlog Po/P)~ = 0 equals log WOand the gradient is B(= 2.303K//Ia), where R is the gas constant, W is the volume adsorbed in the absorbed state per gramme of coal at relative pressure P/PO, WO is the limiting micropore volume per gramme of coal, /3 is the affinity coefficient of adsorbate relative to nitrogen, and K is a constant characterizing the micropore size. The relation between the volume in the adsorbed state, W, and that in the gaseous state, V(cms/g STP), is w
=
VXM 22.4 x 103x pT
______-
where M is the molecular weight and pT (g/cma) is the density of adsorbate in adsorbed state at TK. Dubinina reported that, as a result of the high adsorption force field, the density of adsorbate in the adsorbed state is larger than that in the normal state over the range from boiling temperature to critical temperature, and that the density in the adsorbed state at TK can be obtained from the equations
and -- M b
per -
where b is the van der Waals constant, po and per are the densities in the bulk state at boiling temperature TbK and critical temperature T,rK respectively. Generally more than two isotherms are required to obtain the differential heat of adsorption, qrso, using the Clausius-Clapeyron equation qiso =
--2.mR
log Pl - log P2 (w_-l/Tz)v
If the plot of log W against (RT log Po/P)2 for the isotherms of different temperatures is rectilinear and the same regardless of temperature, it seems 188
MICROPORE STRUCTURE OF COAL
to be possible, however, to obtain the heat of adsorption from only a single isotherm by using equation (5) as follows. Let the values of W, P, PO, p at TI and Te be WI, PI, POI, pl and WZ,Ps,*Pss, ps respectively, then the following equations are obtained from equation (1) log WI = log Wo - B(RT1 log PoI/PI)~
(6)
log Wz = log Wo - B(RT2 log Po~/P~)~
(7)
From equation (2), with the condition V = VI = VZ, the equation log w2 = log Wl + log p1lp2 (8) is obtained. Then one can calculate the value of log PI from equation (6) and similarly the value of log P2 from equations (7) and (8). On the other hand, the value of Wl/Wo equals the filling ratio of micropores by adsorbate. Consequently the heat of adsorption at a desired filling ratio can be calculated from only a single isotherm by using equations (5), (6) (7) and (8), provided a suitable value of WI is chosen.
EXPERIMENTAL
Twelve Japanese coals and five foreign coals, ground to between 28 and 60 Tyler mesh sieve size and ranging in carbon content from 72.7 to 93.2 %, have been examined. The measurement was carried out for samples of vitrain-enriched fractions from each coal, prepared by a float-sink method. The conditions of sample preparation and the results of proximate and petrographic analysis have been reported elsewhere12. The compositions of the coals used are listed in Table 1. The adsorption of carbon dioxide was measured by using conventional volumetric apparatus. A sample of about 5 g was evacuated at about 373K until an ultimate pressure was reached of less than lO-5 mmHg. It took about 8-12 h to reach the ultimate pressure for all the samples, except Tempoku coal which required 30-40 h. The adsorption of carbon dioxide was measured at 298K. For some coals the experiment was also carried out at 273 and 253 K. At 298 K the rate of adsorption of carbon dioxide on coals was measured at a first adsorption point of a constant pressure about 260 mmHg. The micropore volume and the constant proportional to the function of micropore size were obtained from the intercept and the gradient respectively in the log-logs plot of the Dubinin-Polanyi equation. Table 2 shows the saturation vapour pressure of carbon dioxide13 and the densities of carbon dioxide in the adsorbed state14 calculated by using equations (3) and (4) with Tb = 216.4K, pb = 1.176 g/cm313 *, M = 44 g/mol and b = 42.8 cms/mo114. * The density of the liquid phase at the triple point was employed instead of that at the boiling point
189
g
Tempoku Chikubetsu Taiheiyo Kashima Takamatsu Bibai Akabira Miike Yitbari Southern Bell7 Hashima Mourat Michelt New River? Nishikawachi Itmannt Omine
72.7 16.8 71.8 78.1 79.8 81.1 83.4 84.5 86.2 86.6 87.2 87.6 88.4 89.0 89.8 90.7 93.2
(dCa5.j
* Kzorr and KaesO are the specific extinction t Foreign coal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 14’ 15 16
No. Sample 20.1 15.6 149 12.5 12.8 11.0 8.4 7.1 5.3 5.3 5.4 5.1 4.3 3.6 2.4 2.5 1.2 12.9 7.6 5.5 4.8 5.6 3.2 2.5 1.7 1.1 1.7 1.2 2.1 1.6 1.8 1.0 1.1 2.7
coefficients at 2920 cm-’ and 3030 cm-’ respectively
5.1 5.9 6.0 5.9 5.5 6.0 6,2 6.1 6.3 5.8 5.8 5.3 5.2 5.0 5.2 4.8 3.3
(%)
Moisture
39.8 45.0 48.2 48.0 40.6 44.1 43.2 42,7 40.2 36.1 344 28.1 25,9 243 22.9 18.2 6.7
V. M. (%)
0.204 0.372 0446 0.368 0.238 0.402 0.379 0422 0.489 0.344 0335 0.283 0.274 0,262 0244 0.210 0.070
-
0023 0.026 0.03 1 OG48 oG42 0.051 0.043 0.062 @OS9 0,069 0.073 0071
_ -
Kmo* (I/g cm)
K3030* U/s cm)
in the infra-red spectra, measured by Fujii el al”
4.7 3.3 5.0 4.1 2.4 3.4 1.2 3.6 2.4 1.4 2.2 2.3 1.8 1.5 2.7 2.4 4.1
Ash (%)
Table I Analytical data of coals
MICROPORE STRUCTURE OF COAL T&k 2 Characteristics Temperature
It took less samples, again Tempoku coal the adsorption required.
of carbon dioxide
(K)
Saturation vapour pressure tmmJ%)
Density in adsorbed state Wm3)
298 273 253
48 300 26 140 14700
1,038 1.080 1.113
than 24 h to reach the equilibrium state at 298 K for all the excepting Tempoku coal. For example it took about 96 h for to reach equilibrium state at 273 K and 260 mmHg. The more temperature decreased, the longer the time for equilibrium
-1.5
-2.0
-2.5
L
I
I
I
0
0.5
1.0
1.5
2.0
2.5
IRT log pa/p p.10~ Figure la Dubinin-Polanyi
plots for carbon dioxide adsorption coals 0 298K n 273K
191
on
Y. TODA, M. HATAMI, S. TOYODA, Y. YOSHIDA AND H. HONDA RESULTS
AND
DISCUSSION
As shown in Figures la and lb, the experimental points, which were calculated using equations (l), (2), (3) and (4), of the isotherms at different temperatures fell satisfactorily - perhaps surprisingly - on a single straight line for all the samples. This shows that the Dubinin-Polanyi equation may be satisfactorily applied to the adsorption of carbon dioxide on coals. Therefore, it should be possible to estimate the heat of adsorption by using equations (5)-W. Dubinin et all8 determined experimentally the carbon dioxide densities in the adsorbed state over the temperature range 186-233K using silica gels as adsorbents, which contain relatively large micropores and transitional pores in which capillary condensation occurs. But the use of carbon dioxide densities in the adsorbed state measured by Dubinin et al instead of those obtained from the equations made the deviation of points from the straight line larger than that shown in Figures la and lb. It is well known that coal contains smaller micropores and no transitional poreslg. Therefore, the carbon dioxide densities in the adsorbed state may differ from those in pores
-1.0
-1.5
3 cn 2 -20
-2.5
I,-0
0. 5
1.0 (RTLog
1.5
2.0
2.5
P~/~)~xIo-~
Figure Ib Dubinin-Polanyi
plots for carbon dioxide adsorption coals 0 298K n 273K n 253K
192
on
MICROPORE
STRUCTURE
OF COAL
of silica gel. For this reason the carbon dioxide densities were obtained by calculation using equations (3) and (4). In Figure 2, the heats of adsorption calculated from equations (5)-(g) and those obtained directly from two isotherms at 298K and 273K are plotted against the filling ratio, 8. In the former method, the same temperatures, 298K and 273K, were used for TI and Ts respectively.
A
A 750
l8 .
A 0
”
A
lA
’
0
A A
. A
0 a 0 0
I
600
I
I
I
0
I
I
0.5
Filling
ratio,
I@,
I 1.0
8
Figure 2 A comparison 00
Calculated
of heats of adsorption obtained by different methods from the equations, aA calculated from CO2 isotherms Oa Chikubetu .A Yfibari
As shown in Figure 2, the heats of adsorption from the former method agree with those from the latter method within an error of about 5 %. It is convenient, therefore, to obtain the heats of adsorption from the former method when very precise values are not required, because the former method reduces experimental effort. Figure 3 shows the relation between the 193
Y. TODA, M. HATAMI,
S. TOYODA,
Y. YOSHIDA AND H. HONDA
heat of adsorption (calculated from the former method) and the filling ratio for the other coals. As seen in Figures 2 and 3, the values decrease with increase in the value of 0. This is a normal tendency with adsorption phenomena. Judging from Figures 2 and 3 it may be said that the heats of adsorption of carbon dioxide on coals at the equal filling ratio are, approximately, almost equal regardless
0
A
A-L-L-L-
6000 O
0.5 Filling
ratio,
10 8
Figure 3 Variation of heats of adsorption with filling ratio 0 Tempoku, a Taiheiyb, n Bibai, l Nishikawachi, A Ornine
of the rank of coal : 6-7.5 kcal/mol* in the range of filling ratio 0.2-0.7. If the interaction forces or the affinity between carbon dioxide and coal surfaces of different rank differed significantly, whether owing to differences in surface nature or in pore size, this invariant relation between the heat of adsorption and the rank of coal would probably not be observed. The result mentioned above suggests that the use of carbon dioxide in the study of the fine structure * 1 kcal/mol = 4.187 kJ/mol
194
MICROPORE STRUCTURE OF COAL
0.10 -
I P
E 2 0.08 -’ 2”
I
0
I 0
I
o.ozL 70
O
QJ
\
I
0
I
I
80 Carbon
content
I 90
I
, ‘1.
Figure 4 Variation of micropore volume of coals with rank. Values obtained by Marsh et aI8 are included 0 Japanese coal, 0 Foreign coal, I Marsh et al
of coals, as Marshgal and Siemieniewskaa emphasized, is to be recommended because the interaction force between coal surface and carbon dioxide molecules does not depend significantly op the rank of coal. The micropore volume, We, is plotted against the rank of coal in Figure 4. Although the points scatter considerably, the micropore volume at first decreases with increase of rank and shows a minimum at about 85 % C. In order to compare with the surface areas obtained by Marsh and Siemieniewska from the carbon dioxide adsorption at 293 K using the Dubinin-Polanyi equation, the micropore volume is also expressed in terms of apparent surface area* in Figure 4. The values of surface area for Japanese coals are in general * The apparent surface area of coal can easily be obtained by multiplying the value of micropore volume, Wo, by 2626, provided that the sectional area of a carbon dioxide molecule is 185 AZ (1 A2 = 1O-2o ma) at 298K. The sectional area, 18.5 A2, was calculated from the density at 298K, 1.038 g/cm3, assuming that the molecules are spherical and packed most densely
195
Y. TODA,
M. HATAMI,
Tuble 3
S. TOYODA,
Y. YOSHIDA AND H. HONDA
Results for carbon dioxide adsorption
on coals -
NO.
7
8 9 10 11 12 13 14 14 15 16
Pore volume WO (ems/g)
Sample
0.0750 0.0579 0.0467 0.0482 0.0697 0.0475 0.0582 0.0362 0.0420 0.0433 0.0478 0.0527 0.0565 0.0533 0.0409 0.0579 0.0815
Tempoku Chikubetsu Taiheiyd Kashima Takamatsu Bibai Akabira Miike Yitbari Southern Bell Hashima Moura Michel New River Nishikawachi Itmann Omine
B value v30 X lo6 (cm3/g STP) 0.370 0.380 0.436 0.407 0.375 0.381 0.439 0.422 0.471 0.411 0.442 0.386 0.411 0.420 0.454 0.438 0.387
VW1
(cm3/g STP) 8.20 6.31 4.31 4.72 7.01 5.22 4.97 2.61 3.27 4.19 4.00 5.65 5.54 4.92 3.26 4.90 8.82
2.05 ;:z 2.43 2.70 2.27 2.88 1.54 2.17 1.30 240 2.83 3.30 2.19 1.11 1.98 3.70
0.8
0.6
0
I
I
I
70
I
80
Carbon
content,
“I.
Figure 5 Variation of VZO/Veqnt with rank T = 298K, P = 260 mmHg
196
I 90
v30
Vequt 0.250 0.475 0.465 0.515 0.385 0.435 0.580 0.590 0.665 0.310 0.600 0.500 0.595 0445 0.340 0.405 0.420
MICROPORE
STRUCTURE
OF COAL
smaller than those for foreign coals obtained by Marsh and Siemieniewska, as seen in Figure 4. The reason will be discussed later. Vso and VepUiin Table 3 are the volumes adsorbed from the gaseous state at 298K and 260 mmHg during the first 30 minutes, and at equilibrium, respectively. The value of Vso does not show any appreciable relation with the rank, as seen in Table 3. In Figure 5, the value of VXJV,,,~ is plotted against the rank. The value may be regarded as a function of micropore size, and the larger the value is, the larger is the micropore size. The value of Vso/VeclUi increases with increase of rank, shows a maximum at about 85 % C, and then decreases with the rank, although the experimental points scatter considerably. The micropore volume, WO, is plotted against the i&a-red adsorption value11 of K2920+ 2K3030 in Figure 6. As seen in Figure 6, the value of micropore volume decreases monotonically with increase of KZVZO + 2K30so. The symbols, KZV~O and Ksoso, are the specific extinction coefficients of the infra-red spectra at 2920 cm-1 and 3030 cm-l respectively; these are assigned
0.09
0.08
0.07
DO6 P i .z
0.05
P E” 004 1 9 al 003 b 0” b .s 0.02
0.01
I
0 0
O-l
I
I
I
I
0.2
0.3
0.4
0.5
K2g20+ 2K 3o3o( lg-* Figure
6 Variation
of micropore volume with the value of K2920
+
2Kaoro
197 Fuel-N
cm-’ )
0.6
Y. TODA, M. HATAMI, S. TOYODA, Y. YOSHIDA AND H. HONDA
to aliphatic and alicyclic CH hydrogen, and aromatic CH hydrogen, respectively. The VdUe Of &so $ 2&0so~~~ii is therefore considered to be proportional to the number of hydrogen atoms bound directly to carbon atoms. Figure 6 then suggests that the micropore volume in coal is strongly influenced by the concentration of hydrogen atoms bound directly to carbon atoms. Hydrogen atoms bound directly to carbon atoms probably exist on the periphery of a coal molecule because the hydrogen atom is univalent. Therefore, the pore walls of coal may consist mainly of hydrogen atoms bound directly to carbon atoms. As seen in Figure 4, in general the apparent surface area of Japanese coals is smaller than that of the foreign coal@ studied by Marsh and Siemieniewska, comparing coals of the same rank. It is well known that the hydrogen contents of Japanese coals are higher than those of foreign coalsi2~17.Therefore, the higher surface areas measured by Marsh and Siemieniewska may result from the fact that the numbers of hydrogen atoms in the form mentioned above in their coals were fewer than those in Japanese coals. The value of Vse/VepU(and the gradient of the Dubinin plot, B, are plotted against the value of &sse + 2&0s0 in Figures 7 and 8 respectively. The values of Vse/VevepU~ and B increase almost rectilinearly with the increase of &sso + =soao.
07 C
0.6
O 0”
8
0.5 0
0.4 >: ‘0
0
0
‘5
0 0
0
0 0
0.3
F 0.2
:i
O-l
I
0 0
0.1
I
0.2 K2g20+
0.3
2Kx3
(b-’
I
I
0.4 cm-’
0.5
0.6
)
Figure 7 Variation of V30/Veqclt with the value of Kzszo + z&030
198
MICROPORE STRUCTURE OF COAL
According to Dubinin’s theory, the larger the micropore size, the higher is the value of B. The value of Vse/Vequ(,as mentioned earlier, is a function of micropore size from the kinetic standpoint, because the larger the micropore size or entrance, the higher is the value of the ratio. From Figures 7 and 8 therefore, it may be said that the micropore size becomes larger as the number of hydrogen atoms bound directly to carbon atoms increases. An increase of the value of &gso + 2&0s0, that is to say, an increase of the occupation of pore walls by the relatively bulky hydrogen atoms, may result in the closing of ultrafine micropores and a narrowing only of the larger micropores. This effect may also explain the apparent increase of the values of V30/Vequi and B, which relate to average pore size. To sum up, from Figures 6, 7 and 8 it may be concluded that the pore structure of coal is strongly influenced by the existence of hydrogen atoms bound directly to carbon atoms; the micropore volume becomes smaller and the apparent micropore size becomes larger as the number of such hydrogen atoms increases. Plots of the micropore volume, the value of Vsc/VeqUiand the value of B against the total hydrogen content, H%, do not show any appreciable relation. It must be noted therefore, that the pore structure of coals measured
0.4
-
0.3
-
0.2
1
I
I
I
I
I
0
0.1
0.2
0.3
04
0.5
0.6
K2920+2K3030(lg-’ cm-’ ) Figure 8 Variation of the gradient of the Dubinin-Polanyi the value Of KZ~ZO+ 2K3030
199 Fuel-N’
plot with
Y. TODA, M. HATAMI, S. TOYODA, Y. YOSHIDA AND H. HONDA
by the carbon dioxide adsorption is apparently not affected by the existence of hydrogen atoms in the form of oxygen-containing functional groups. The reason for this may become clear from further studies. Resources Research Institute, KawaguchbSaitama, Japan
(Received 7 April 1970) (Revised 24 June 1970)
REFERENCES 1 Dubinin, M. M., ‘Chemistry and physics of carbon’, Vol. II, (Ed. P. L. Walker Jr.), Marcel Dekker, Inc., New York, 1966, p 51 2 Dubinin, M. M., ‘Industrial carbon and graphite’, Society of Chemical Industry, London, 1958, p 219 3 Dubinin, M. M. Chem. Rev. 1960, 60, 235 4 Dubinin, M. M., ‘Carbon, proceedings of the fifth conference’, Vol. I, Pergamon Press Inc., New York, 1962, p 81 5 Spencer, D. H. T. and Bond, R. L. Advan. Chem. Ser., Coal Sci. 1966,55, 724 6 Polanyi, M. Verhandl. deutsch. physk. Ges. 1914, 16, 1012 7 Polanyi, M. Trans. Faraday Sot. 1932, 28, 316 8 Marsh, H. and Siemieniewska, T. Fuel, Lond. 1965, 44, 355 9 Marsh, H. and Siemieniewska, T. Fuel, Lond. 1967, 46, 441 10 Lamond, T. G. and Marsh, H. Carbon 1964,1, 281 11 Fujii, S., Osawa, Y. and Sugimura, -H. Fuef, Lond. 1970, 49, 68 12 Sugimura, H., Osawa, Y., Hatami, M., Sato, H. and Honda, H. J. Fuel Sot., Japan 1966, 45, 199 13 Perry, J. H., ‘Chemical Engineers’ Handbook’, McGraw-Hill Inc., New York, 1950, p 254 14 Comings, E. W., ‘High pressure technology’, McGraw-Hill Inc., New York, 1956, p 483 15 Marsh, H. Fuel, Lond. 1965, 44, 253 16 Brown, J. K. J. Chem. Sot. 1955, p 744 17 Fuiii. S. and Tsuboi. H. Fuel. Lond. 1967. 46. 361 18 Dub&in, M. M., Bering, B. P., Serpinsky, V. V. and Vasil’ev, B. N., ‘Surface Phenomena in Chemistry and Biology’, (Ed. J. F. Danielli), Pergamon Press, London, 1958, p 172 19 Zwietering, P. and van Krevelen, D. W. Fuel, Lond. 1954, 33, 331
200
The adsorption of carbon dioxide at 298K, 273K and 253K by the vitrains of twelve Japanese coals and five foreign coals has been studied. The DubininPolanyi equation was used in order to interpret the isotherms. Equilibrium points at different temperatures fell satisfactorily on a single straight line of the Dubinin-Polanyi equation. It was shown that it is possible to estimate the differential heat of adsorption of these coals from an isotherm obtained at a single temperature if the isotherm follows the Dubinin-Polanyi equation. The heats of adsorption of carbon dioxide on coals amounted to 6-7.5 kcal/mol regardless of rank. The rate of adsorption of carbon dioxide on coals was measured at a constant pressure of 260 mmHg at 298K. The micropore volume, the gradient of the Dubinin plot and the rate of adsorption are discussed in relation to characteristics of their infra-red adsorption spectra. It is concluded that the micropore structure of coal is closely related to the number of hydrogen atoms bound directly to carbon atoms. A possible explanation of this is suggested.
MANYSTUDIESof the fine structure of coals by adsorption methods have been reported. Many years have been spent in the selection of suitable adsorbates and temperatures for experimental theories for interpretation of the isotherms, because the structure of coals is complex from both chemical and physical viewpoints. Dubininl and Spencer and Bond5 pointed out that it is questionable to use the BET equation to interpret isotherms for microporous materials, because m.icropore-filling by adsorbate occurs before monolayer formation and an adsorbate molecule is in a sense shared by several surfaces. Dubinin et cd-* have investigated extensively the isotherms for many adsorbents using various adsorbates, and developed the adsorption potential theory proposed by Polanyisy 7 to a rectilinear form, from which the micropore volume and a function of micropore size can be estimated. Marsh et al extensively studied the application of the Dubinin-Polanyi equation to the isotherms for the adsorption of carbon dioxide at room temperature on coalss, carbonized coals9 and carbonslo, and proved that the above application was satisfactory. In the present paper, we show that equilibrium points for carbon dioxide adsorption on coals at different temperatures (298K, 273K and 253K) fall on a single straight line of the Dubinin-Polanyi equation. The micropore volume or the apparent surface area and the function of micropore size for * Present address; Hokutan-Kasei
Co., Toda-Saitama, Japan t Present address; Government Industrial Development Laboratory, Sapporo-Hokkaido, Japan 9 Present address; Government Industrial Research Institute, Kyushu, Tosu, Kyushu, Japan
187
Y. TODA,
M. HATAMI,
S. TOYODA,
Y. YOSHIDA
AND H. HONDA
coals are deduced from the adsorption of carbon dioxide at 298K using the Dubinin-Polanyi equation. The rates of adsorption of carbon dioxide on coals were measured at a constant pressure of 260 mmHg at 298K. The changes of the estimated values are discussed in relation to the number of hydrogen atoms bound directly to carbon atoms, deduced from the infra-red spectrall.
THEORETICAL
The Dubinin-Polanyi
INTRODUCTION
equation for microporous materials takes the form log W = log W,J- B(RT log Po/P)~
(1)
When a plot of log W against (RT log Po/P)~ is rectilinear, the intercept at (RTlog Po/P)~ = 0 equals log WOand the gradient is B(= 2.303K//Ia), where R is the gas constant, W is the volume adsorbed in the absorbed state per gramme of coal at relative pressure P/PO, WO is the limiting micropore volume per gramme of coal, /3 is the affinity coefficient of adsorbate relative to nitrogen, and K is a constant characterizing the micropore size. The relation between the volume in the adsorbed state, W, and that in the gaseous state, V(cms/g STP), is w
=
VXM 22.4 x 103x pT
______-
where M is the molecular weight and pT (g/cma) is the density of adsorbate in adsorbed state at TK. Dubinina reported that, as a result of the high adsorption force field, the density of adsorbate in the adsorbed state is larger than that in the normal state over the range from boiling temperature to critical temperature, and that the density in the adsorbed state at TK can be obtained from the equations
and -- M b
per -
where b is the van der Waals constant, po and per are the densities in the bulk state at boiling temperature TbK and critical temperature T,rK respectively. Generally more than two isotherms are required to obtain the differential heat of adsorption, qrso, using the Clausius-Clapeyron equation qiso =
--2.mR
log Pl - log P2 (w_-l/Tz)v
If the plot of log W against (RT log Po/P)2 for the isotherms of different temperatures is rectilinear and the same regardless of temperature, it seems 188
MICROPORE STRUCTURE OF COAL
to be possible, however, to obtain the heat of adsorption from only a single isotherm by using equation (5) as follows. Let the values of W, P, PO, p at TI and Te be WI, PI, POI, pl and WZ,Ps,*Pss, ps respectively, then the following equations are obtained from equation (1) log WI = log Wo - B(RT1 log PoI/PI)~
(6)
log Wz = log Wo - B(RT2 log Po~/P~)~
(7)
From equation (2), with the condition V = VI = VZ, the equation log w2 = log Wl + log p1lp2 (8) is obtained. Then one can calculate the value of log PI from equation (6) and similarly the value of log P2 from equations (7) and (8). On the other hand, the value of Wl/Wo equals the filling ratio of micropores by adsorbate. Consequently the heat of adsorption at a desired filling ratio can be calculated from only a single isotherm by using equations (5), (6) (7) and (8), provided a suitable value of WI is chosen.
EXPERIMENTAL
Twelve Japanese coals and five foreign coals, ground to between 28 and 60 Tyler mesh sieve size and ranging in carbon content from 72.7 to 93.2 %, have been examined. The measurement was carried out for samples of vitrain-enriched fractions from each coal, prepared by a float-sink method. The conditions of sample preparation and the results of proximate and petrographic analysis have been reported elsewhere12. The compositions of the coals used are listed in Table 1. The adsorption of carbon dioxide was measured by using conventional volumetric apparatus. A sample of about 5 g was evacuated at about 373K until an ultimate pressure was reached of less than lO-5 mmHg. It took about 8-12 h to reach the ultimate pressure for all the samples, except Tempoku coal which required 30-40 h. The adsorption of carbon dioxide was measured at 298K. For some coals the experiment was also carried out at 273 and 253 K. At 298 K the rate of adsorption of carbon dioxide on coals was measured at a first adsorption point of a constant pressure about 260 mmHg. The micropore volume and the constant proportional to the function of micropore size were obtained from the intercept and the gradient respectively in the log-logs plot of the Dubinin-Polanyi equation. Table 2 shows the saturation vapour pressure of carbon dioxide13 and the densities of carbon dioxide in the adsorbed state14 calculated by using equations (3) and (4) with Tb = 216.4K, pb = 1.176 g/cm313 *, M = 44 g/mol and b = 42.8 cms/mo114. * The density of the liquid phase at the triple point was employed instead of that at the boiling point
189
g
Tempoku Chikubetsu Taiheiyo Kashima Takamatsu Bibai Akabira Miike Yitbari Southern Bell7 Hashima Mourat Michelt New River? Nishikawachi Itmannt Omine
72.7 16.8 71.8 78.1 79.8 81.1 83.4 84.5 86.2 86.6 87.2 87.6 88.4 89.0 89.8 90.7 93.2
(dCa5.j
* Kzorr and KaesO are the specific extinction t Foreign coal
1 2 3 4 5 6 7 8 9 10 11 12 13 14 14’ 15 16
No. Sample 20.1 15.6 149 12.5 12.8 11.0 8.4 7.1 5.3 5.3 5.4 5.1 4.3 3.6 2.4 2.5 1.2 12.9 7.6 5.5 4.8 5.6 3.2 2.5 1.7 1.1 1.7 1.2 2.1 1.6 1.8 1.0 1.1 2.7
coefficients at 2920 cm-’ and 3030 cm-’ respectively
5.1 5.9 6.0 5.9 5.5 6.0 6,2 6.1 6.3 5.8 5.8 5.3 5.2 5.0 5.2 4.8 3.3
(%)
Moisture
39.8 45.0 48.2 48.0 40.6 44.1 43.2 42,7 40.2 36.1 344 28.1 25,9 243 22.9 18.2 6.7
V. M. (%)
0.204 0.372 0446 0.368 0.238 0.402 0.379 0422 0.489 0.344 0335 0.283 0.274 0,262 0244 0.210 0.070
-
0023 0.026 0.03 1 OG48 oG42 0.051 0.043 0.062 @OS9 0,069 0.073 0071
_ -
Kmo* (I/g cm)
K3030* U/s cm)
in the infra-red spectra, measured by Fujii el al”
4.7 3.3 5.0 4.1 2.4 3.4 1.2 3.6 2.4 1.4 2.2 2.3 1.8 1.5 2.7 2.4 4.1
Ash (%)
Table I Analytical data of coals
MICROPORE STRUCTURE OF COAL T&k 2 Characteristics Temperature
It took less samples, again Tempoku coal the adsorption required.
of carbon dioxide
(K)
Saturation vapour pressure tmmJ%)
Density in adsorbed state Wm3)
298 273 253
48 300 26 140 14700
1,038 1.080 1.113
than 24 h to reach the equilibrium state at 298 K for all the excepting Tempoku coal. For example it took about 96 h for to reach equilibrium state at 273 K and 260 mmHg. The more temperature decreased, the longer the time for equilibrium
-1.5
-2.0
-2.5
L
I
I
I
0
0.5
1.0
1.5
2.0
2.5
IRT log pa/p p.10~ Figure la Dubinin-Polanyi
plots for carbon dioxide adsorption coals 0 298K n 273K
191
on
Y. TODA, M. HATAMI, S. TOYODA, Y. YOSHIDA AND H. HONDA RESULTS
AND
DISCUSSION
As shown in Figures la and lb, the experimental points, which were calculated using equations (l), (2), (3) and (4), of the isotherms at different temperatures fell satisfactorily - perhaps surprisingly - on a single straight line for all the samples. This shows that the Dubinin-Polanyi equation may be satisfactorily applied to the adsorption of carbon dioxide on coals. Therefore, it should be possible to estimate the heat of adsorption by using equations (5)-W. Dubinin et all8 determined experimentally the carbon dioxide densities in the adsorbed state over the temperature range 186-233K using silica gels as adsorbents, which contain relatively large micropores and transitional pores in which capillary condensation occurs. But the use of carbon dioxide densities in the adsorbed state measured by Dubinin et al instead of those obtained from the equations made the deviation of points from the straight line larger than that shown in Figures la and lb. It is well known that coal contains smaller micropores and no transitional poreslg. Therefore, the carbon dioxide densities in the adsorbed state may differ from those in pores
-1.0
-1.5
3 cn 2 -20
-2.5
I,-0
0. 5
1.0 (RTLog
1.5
2.0
2.5
P~/~)~xIo-~
Figure Ib Dubinin-Polanyi
plots for carbon dioxide adsorption coals 0 298K n 273K n 253K
192
on
MICROPORE
STRUCTURE
OF COAL
of silica gel. For this reason the carbon dioxide densities were obtained by calculation using equations (3) and (4). In Figure 2, the heats of adsorption calculated from equations (5)-(g) and those obtained directly from two isotherms at 298K and 273K are plotted against the filling ratio, 8. In the former method, the same temperatures, 298K and 273K, were used for TI and Ts respectively.
A
A 750
l8 .
A 0
”
A
lA
’
0
A A
. A
0 a 0 0
I
600
I
I
I
0
I
I
0.5
Filling
ratio,
I@,
I 1.0
8
Figure 2 A comparison 00
Calculated
of heats of adsorption obtained by different methods from the equations, aA calculated from CO2 isotherms Oa Chikubetu .A Yfibari
As shown in Figure 2, the heats of adsorption from the former method agree with those from the latter method within an error of about 5 %. It is convenient, therefore, to obtain the heats of adsorption from the former method when very precise values are not required, because the former method reduces experimental effort. Figure 3 shows the relation between the 193
Y. TODA, M. HATAMI,
S. TOYODA,
Y. YOSHIDA AND H. HONDA
heat of adsorption (calculated from the former method) and the filling ratio for the other coals. As seen in Figures 2 and 3, the values decrease with increase in the value of 0. This is a normal tendency with adsorption phenomena. Judging from Figures 2 and 3 it may be said that the heats of adsorption of carbon dioxide on coals at the equal filling ratio are, approximately, almost equal regardless
0
A
A-L-L-L-
6000 O
0.5 Filling
ratio,
10 8
Figure 3 Variation of heats of adsorption with filling ratio 0 Tempoku, a Taiheiyb, n Bibai, l Nishikawachi, A Ornine
of the rank of coal : 6-7.5 kcal/mol* in the range of filling ratio 0.2-0.7. If the interaction forces or the affinity between carbon dioxide and coal surfaces of different rank differed significantly, whether owing to differences in surface nature or in pore size, this invariant relation between the heat of adsorption and the rank of coal would probably not be observed. The result mentioned above suggests that the use of carbon dioxide in the study of the fine structure * 1 kcal/mol = 4.187 kJ/mol
194
MICROPORE STRUCTURE OF COAL
0.10 -
I P
E 2 0.08 -’ 2”
I
0
I 0
I
o.ozL 70
O
QJ
\
I
0
I
I
80 Carbon
content
I 90
I
, ‘1.
Figure 4 Variation of micropore volume of coals with rank. Values obtained by Marsh et aI8 are included 0 Japanese coal, 0 Foreign coal, I Marsh et al
of coals, as Marshgal and Siemieniewskaa emphasized, is to be recommended because the interaction force between coal surface and carbon dioxide molecules does not depend significantly op the rank of coal. The micropore volume, We, is plotted against the rank of coal in Figure 4. Although the points scatter considerably, the micropore volume at first decreases with increase of rank and shows a minimum at about 85 % C. In order to compare with the surface areas obtained by Marsh and Siemieniewska from the carbon dioxide adsorption at 293 K using the Dubinin-Polanyi equation, the micropore volume is also expressed in terms of apparent surface area* in Figure 4. The values of surface area for Japanese coals are in general * The apparent surface area of coal can easily be obtained by multiplying the value of micropore volume, Wo, by 2626, provided that the sectional area of a carbon dioxide molecule is 185 AZ (1 A2 = 1O-2o ma) at 298K. The sectional area, 18.5 A2, was calculated from the density at 298K, 1.038 g/cm3, assuming that the molecules are spherical and packed most densely
195
Y. TODA,
M. HATAMI,
Tuble 3
S. TOYODA,
Y. YOSHIDA AND H. HONDA
Results for carbon dioxide adsorption
on coals -
NO.
7
8 9 10 11 12 13 14 14 15 16
Pore volume WO (ems/g)
Sample
0.0750 0.0579 0.0467 0.0482 0.0697 0.0475 0.0582 0.0362 0.0420 0.0433 0.0478 0.0527 0.0565 0.0533 0.0409 0.0579 0.0815
Tempoku Chikubetsu Taiheiyd Kashima Takamatsu Bibai Akabira Miike Yitbari Southern Bell Hashima Moura Michel New River Nishikawachi Itmann Omine
B value v30 X lo6 (cm3/g STP) 0.370 0.380 0.436 0.407 0.375 0.381 0.439 0.422 0.471 0.411 0.442 0.386 0.411 0.420 0.454 0.438 0.387
VW1
(cm3/g STP) 8.20 6.31 4.31 4.72 7.01 5.22 4.97 2.61 3.27 4.19 4.00 5.65 5.54 4.92 3.26 4.90 8.82
2.05 ;:z 2.43 2.70 2.27 2.88 1.54 2.17 1.30 240 2.83 3.30 2.19 1.11 1.98 3.70
0.8
0.6
0
I
I
I
70
I
80
Carbon
content,
“I.
Figure 5 Variation of VZO/Veqnt with rank T = 298K, P = 260 mmHg
196
I 90
v30
Vequt 0.250 0.475 0.465 0.515 0.385 0.435 0.580 0.590 0.665 0.310 0.600 0.500 0.595 0445 0.340 0.405 0.420
MICROPORE
STRUCTURE
OF COAL
smaller than those for foreign coals obtained by Marsh and Siemieniewska, as seen in Figure 4. The reason will be discussed later. Vso and VepUiin Table 3 are the volumes adsorbed from the gaseous state at 298K and 260 mmHg during the first 30 minutes, and at equilibrium, respectively. The value of Vso does not show any appreciable relation with the rank, as seen in Table 3. In Figure 5, the value of VXJV,,,~ is plotted against the rank. The value may be regarded as a function of micropore size, and the larger the value is, the larger is the micropore size. The value of Vso/VeclUi increases with increase of rank, shows a maximum at about 85 % C, and then decreases with the rank, although the experimental points scatter considerably. The micropore volume, WO, is plotted against the i&a-red adsorption value11 of K2920+ 2K3030 in Figure 6. As seen in Figure 6, the value of micropore volume decreases monotonically with increase of KZVZO + 2K30so. The symbols, KZV~O and Ksoso, are the specific extinction coefficients of the infra-red spectra at 2920 cm-1 and 3030 cm-l respectively; these are assigned
0.09
0.08
0.07
DO6 P i .z
0.05
P E” 004 1 9 al 003 b 0” b .s 0.02
0.01
I
0 0
O-l
I
I
I
I
0.2
0.3
0.4
0.5
K2g20+ 2K 3o3o( lg-* Figure
6 Variation
of micropore volume with the value of K2920
+
2Kaoro
197 Fuel-N
cm-’ )
0.6
Y. TODA, M. HATAMI, S. TOYODA, Y. YOSHIDA AND H. HONDA
to aliphatic and alicyclic CH hydrogen, and aromatic CH hydrogen, respectively. The VdUe Of &so $ 2&0so~~~ii is therefore considered to be proportional to the number of hydrogen atoms bound directly to carbon atoms. Figure 6 then suggests that the micropore volume in coal is strongly influenced by the concentration of hydrogen atoms bound directly to carbon atoms. Hydrogen atoms bound directly to carbon atoms probably exist on the periphery of a coal molecule because the hydrogen atom is univalent. Therefore, the pore walls of coal may consist mainly of hydrogen atoms bound directly to carbon atoms. As seen in Figure 4, in general the apparent surface area of Japanese coals is smaller than that of the foreign coal@ studied by Marsh and Siemieniewska, comparing coals of the same rank. It is well known that the hydrogen contents of Japanese coals are higher than those of foreign coalsi2~17.Therefore, the higher surface areas measured by Marsh and Siemieniewska may result from the fact that the numbers of hydrogen atoms in the form mentioned above in their coals were fewer than those in Japanese coals. The value of Vse/VepU(and the gradient of the Dubinin plot, B, are plotted against the value of &sse + 2&0s0 in Figures 7 and 8 respectively. The values of Vse/VevepU~ and B increase almost rectilinearly with the increase of &sso + =soao.
07 C
0.6
O 0”
8
0.5 0
0.4 >: ‘0
0
0
‘5
0 0
0
0 0
0.3
F 0.2
:i
O-l
I
0 0
0.1
I
0.2 K2g20+
0.3
2Kx3
(b-’
I
I
0.4 cm-’
0.5
0.6
)
Figure 7 Variation of V30/Veqclt with the value of Kzszo + z&030
198
MICROPORE STRUCTURE OF COAL
According to Dubinin’s theory, the larger the micropore size, the higher is the value of B. The value of Vse/Vequ(,as mentioned earlier, is a function of micropore size from the kinetic standpoint, because the larger the micropore size or entrance, the higher is the value of the ratio. From Figures 7 and 8 therefore, it may be said that the micropore size becomes larger as the number of hydrogen atoms bound directly to carbon atoms increases. An increase of the value of &gso + 2&0s0, that is to say, an increase of the occupation of pore walls by the relatively bulky hydrogen atoms, may result in the closing of ultrafine micropores and a narrowing only of the larger micropores. This effect may also explain the apparent increase of the values of V30/Vequi and B, which relate to average pore size. To sum up, from Figures 6, 7 and 8 it may be concluded that the pore structure of coal is strongly influenced by the existence of hydrogen atoms bound directly to carbon atoms; the micropore volume becomes smaller and the apparent micropore size becomes larger as the number of such hydrogen atoms increases. Plots of the micropore volume, the value of Vsc/VeqUiand the value of B against the total hydrogen content, H%, do not show any appreciable relation. It must be noted therefore, that the pore structure of coals measured
0.4
-
0.3
-
0.2
1
I
I
I
I
I
0
0.1
0.2
0.3
04
0.5
0.6
K2920+2K3030(lg-’ cm-’ ) Figure 8 Variation of the gradient of the Dubinin-Polanyi the value Of KZ~ZO+ 2K3030
199 Fuel-N’
plot with
Y. TODA, M. HATAMI, S. TOYODA, Y. YOSHIDA AND H. HONDA
by the carbon dioxide adsorption is apparently not affected by the existence of hydrogen atoms in the form of oxygen-containing functional groups. The reason for this may become clear from further studies. Resources Research Institute, KawaguchbSaitama, Japan
(Received 7 April 1970) (Revised 24 June 1970)
REFERENCES 1 Dubinin, M. M., ‘Chemistry and physics of carbon’, Vol. II, (Ed. P. L. Walker Jr.), Marcel Dekker, Inc., New York, 1966, p 51 2 Dubinin, M. M., ‘Industrial carbon and graphite’, Society of Chemical Industry, London, 1958, p 219 3 Dubinin, M. M. Chem. Rev. 1960, 60, 235 4 Dubinin, M. M., ‘Carbon, proceedings of the fifth conference’, Vol. I, Pergamon Press Inc., New York, 1962, p 81 5 Spencer, D. H. T. and Bond, R. L. Advan. Chem. Ser., Coal Sci. 1966,55, 724 6 Polanyi, M. Verhandl. deutsch. physk. Ges. 1914, 16, 1012 7 Polanyi, M. Trans. Faraday Sot. 1932, 28, 316 8 Marsh, H. and Siemieniewska, T. Fuel, Lond. 1965, 44, 355 9 Marsh, H. and Siemieniewska, T. Fuel, Lond. 1967, 46, 441 10 Lamond, T. G. and Marsh, H. Carbon 1964,1, 281 11 Fujii, S., Osawa, Y. and Sugimura, -H. Fuef, Lond. 1970, 49, 68 12 Sugimura, H., Osawa, Y., Hatami, M., Sato, H. and Honda, H. J. Fuel Sot., Japan 1966, 45, 199 13 Perry, J. H., ‘Chemical Engineers’ Handbook’, McGraw-Hill Inc., New York, 1950, p 254 14 Comings, E. W., ‘High pressure technology’, McGraw-Hill Inc., New York, 1956, p 483 15 Marsh, H. Fuel, Lond. 1965, 44, 253 16 Brown, J. K. J. Chem. Sot. 1955, p 744 17 Fuiii. S. and Tsuboi. H. Fuel. Lond. 1967. 46. 361 18 Dub&in, M. M., Bering, B. P., Serpinsky, V. V. and Vasil’ev, B. N., ‘Surface Phenomena in Chemistry and Biology’, (Ed. J. F. Danielli), Pergamon Press, London, 1958, p 172 19 Zwietering, P. and van Krevelen, D. W. Fuel, Lond. 1954, 33, 331
200