Pore structure of low temperature chars

Pore structure of low temperature chars T. N. RoY and S. P. NANDI* Sorption of methanol, ethanol and n-butanol at P/Po=0.5 was determined on low temperatuje (500-700°C) chars produced from five samples of coal (carbon 80-847/oo). The chars show higher sorption for molecules of smaller size. A char from an 83YooC coal heat treated at 600°C shows sorption capacity for methanol and ethanol in the ratio 10:1. An activation energy of 9"6 kcal/mol was obtained for the diffusion of nitrogen (temperature 25-50°C) into the pore-system of the most selective char. Activation by air at low temperature (200-240°C) reduces the selective adsorption capacity of the char. Estimation of the micropore space of the activated chars were obtained from COo adsorption data by the application data by the application of the Dubinin equation. INTRODUCTION MOLECULAR SIEVE materials have been extensively used recently. The commercially available Linde molecular sieves are synthetic crystalline zeolites. In spite of their very high selectivity and high sorption capacity, the synthetic zeolites are unstable to acids and show a preference for polar molecules. A molecular sieve material based essentially on a carbon skeleton would be acid stable and non-polar. Coals of all rank show selective sorption as reported first by Anderson I et al. The sorption capacity of coals for normal butane is high compared to that for isobutane. As a result of the presence of molecular-size pores in coal, simple molecules like nitrogen and argon are taken into the micropore system by activated diffusion at room temperature. From a study 2 of the activated diffusion of argon on coals of different rank, it was found that the maximum activation energy was shown by a coal of about 84)~, carbon (d.m.f.). This implies that coals of this range of carbon content have the narrowest micropores. It was also shown 3 that the activation energy of diffusion increases on carbonization although there are exceptions to this 4. From these standpoints, it was desirable to take up a systematic study of the pore structure of some low-temperature chars. EXPERIMENTAL Five coals were selected for the study. The analyses of the coals are given in Table 1. The first part of the work was a comparative study of the selective adsorption capacity of the carbonized coals. Carbonization was carried out in a nitrogen atmosphere. The rate of heating was 5°C/min with a soak time of 2h at the desired temperature, and the carbonization temperatures employed were 500, 600 and 700°C. The particle size of the coal for carbonization was (--45 -}-65) mesh Tyler. Sorption of methanol, ethanol and n-butanol *Present Address: Department of Materials Science, Pennsylvania State University, University Park, Pennsylvania 346

PORE STRUCTURE OF LOW TEMPERATURE CHARS

Table 1 Analyses of coals

Coals

Carbon

Hydrogen

( % d.m.f)

( % d.m.f)

84.0 83.8 82.5 80-5 80.1

5'7 5'7 5.5 5.7 5.8

Chinakuri Dishergarh Poniati Assam (Low sulphur) Assam (High Sulphur)

was determined at 5 0 ~ saturation by a gravimetric method. A degassing temperature of 250°C was employed in all cases. The temperature range of sorption measurements was 42-48°C. For the purpose of comparison, sorption of these alcohols on two samples of active carbons was also determined. The sample of char having the best selectivity was subjected to detailed study. Low-temperature oxidation at 200, 215 and 240°C by air was carried out on the above mentioned sample with a view to achieving uniform activation so that only a very gradual increase in the micropore diameter results for all the pores in the particle. Surface area by the usual B.E.T. method (nitrogen sorption) was determined to get a comparative picture of the activation process. Adsorption of carbon dioxide at very low P/Po was carried out at 0°C and 25°C in a volumetric apparatus. Equilibration times of one hour were allowed for each point. The results were analysed by the Dubinin 5 adsorption equation. In addition, the rate of adsorption of nitrogen at constant pressure at 25 and 50°C was determined for the sample of char showing the best selectivity, and, from these data, the diffusion coefficient of nitrogen into the pore system, as well as the temperature coefficient of D, was determined.

E X P E R I M E N T A L R E S U L T S A N D DISCUSSION

(a) Comparative study of ihe selective sorption capacity of the chars Of the five samples of the coal three are from Ranigang field and the other two are of Assam coal field. The Ranigang coals were selected for reasons mentioned before, namely, that they have the narrowest micropores. Of the two Assam coals one was of low sulphur and the other of high sulphur content. The carbon content of the Assam coals were somewhat lower (80 ~'oC), but as the Assam coals show many interesting deviations in their properties (e.g. caking) it was thought worthwhile to study the pore structure of their low temperature chars. The particles of coal did not appear to undergo any appreciable size change as a result of the heat treatment. Sorption of methanol, ethanol and n-butanol was determined on the carbonized samples and the results are 347

T. N. ROY AND S. P. NANDI Table 2 Sorption of n-butanol, ethanol and methanol

Coal sample

Heat treatment temperature

Hit. loss percent

(°C) Chinakuri Dishergarh Assam coal (low sulphur) Poniati Assam coal (high sulphur) Active Carbon (S. Arcot lignite FRI made) Active carbon (commercial)

Percent weight increase at 50% of saturation on dry carbonized product n-Butanol

Ethanol

Methanol

500 600 700 500 600 700 500 600 700 500 600 700 500 600 700

7.4 23.6 27-0 25.5 29.2 30'5 30"9 33'i 36"9 28'4 29"1 31.3 26-9 31.7 33-7

1-0 0"4 0'5 0"2 0"1 0'2 0'2 0.3 -1. I 0.3 0"4 I-1 0"3 0"3

3"5 0.8 0"9 I "7 1"1 0.8 0.6 0-3 -2.0 0'8 0"6 2.0 0'8 0'3

8.1 7-2 5.6 6"1 6.7 4-4 6-8 1'2 -7.0 7"7 4-3 6-6 3-8 2.2

---

---

6-4 9'4

13-9 21"2

15.0 27.3

presented in Table 2. The s o r p t i o n c a p a c i t y o f two active c a r b o n s are also included in the table. H e a t t r e a l m e n t at lower t e m p e r a t u r e s was not used because c o n s i d e r a b l e quantities o f volatile m a t t e r would r e m a i n in the chars and this might r e n d e r it u n s t a b l e t o w a r d s solvents. It c a n be seen from Table 2 t h a t the socption c a p a c i t y for alcohols decreases with the increase in t e m p e r a t u r e o f carb o n i z a t i o n for all the chars studied. A c c o r d i n g to B a r r e r 6 the critical d i m e n sions o f m e t h a n o l , e t h a n o l and n - b u t a n o l are ~ - 4 A , ~ 5 A and m o r e that 5 . 5 A respectively. Therefore, the ratio o f the s o r p t i o n c a p a c i t y o f m e t h a n o l to e t h a n o l is a g o o d m e a s u r e o f selectivity (selective s o r p t i o n capacity) o f the sample. F o r the two A s s a m coals, the difference in s o r p t i o n c a p a c i t y for the c a r b o n i z e d samples p r e p a r e d at 500 and 600°C is high c o m p a r e d to the othe," three coal samples, and the c a p a c i t y o f the 700°C c h a r is very low indeed. T h e three R a n i g a n g coals fall in one g r o u p as their chars have similar s o r p t i o n properties. T h e 6O0°C heat treated chars show the best selectivity a n d at 700°C the s o r p t i o n c a p a c i t y falls p r o b a b l y due to n a r r o w i n g d o w n o f the m i c r o p o r e diameters. O f these chars, t h e 600°C heat treated P o n i a t i ( P o n i a t i 600) coal seems to be m a r g i n a l l y superior. T h e s o r p t i o n d a t a o f the active c a r b o n s show t h a t they are non-selective. There is h a r d l y any difference in the s o r p t i o n c a p a c i t y for m e t h a n o l and e t h a n o l a n d the ratio o f s o r p t i o n o f m e t h a n o l a n d n - b u t a n o l is a b o u t 3 as c o m p a r e d to a b o u t 27 for P o n i a t i 600. 348

PORE STRUCTURE OF LOW TEMPERATURE CHARS T h e r a t i o o f the s o r p t i o n c a p a c i t y o f m e t h a n o l to e t h a n o l is a b o u t 10 for P o n i a t i 600. M e t h a n o l a n d e t h a n o l are b o t h p o l a r molecules. Consequently, the preference shown to m e t h a n o l is p r i m a r i l y due to its smaller size. F r o m this it c a n be d e d u c e d t h a t the critical d i m e n s i o n s o f the m i c r o p o r e s are nearly 4 A . A s the chars are non-crystalline materials, the size o f their p o r e s c a n n o t be given with the s a m e degree o f confidence as in the case o f crystalline zeolite m o l e c u l a r sieves. A n o t h e r d r a w b a c k o f the chars is their c o m p a r a t i v e l y low s o r p t i o n capacity. T o increase capacity, a t t e m p t s were m a d e to activate P o n i a t i 600 which had the best selectivity.

(b) A l c o h o l sorption a n d B . E . T . a r e a s o f a c t i v a t e d P o n i a t i 600 T h e m a i n aim of the a c t i v a t i o n process was to increase the total c a p a c i t y w i t h o u t i m p a r i n g the selectivity. The a c t i v a t i o n t e m p e r a t u r e e m p l o y e d were 200, 215 a n d 240°C and air was the activating agent. A t low t e m p e r a t u r e s with a m i l d o x i d a n t it was h o p e d t h a t a c t i v a t i o n w o u l d be fairly uniform. A l t h o u g h the particle size ( - - 4 5 + 6 5 mesh) was p r o b a b l y t o o large for Table 3 Sorption of n-butanol, ethanol and methanol and surface area data

Temp. of oxidn (°C)

Time of oxidn (h)

o I 2 3 4

-200 200 200 200

0 340 370 510 950

+ ÷ -t-t-

5

6 7

215 215 215

168 403 615

8 9 10 11

240 240 240 240

18 29 37 46

No.

Wt. change (%) 0 5.3 3-7 3.0 2.2

B.E.T. smface Wt. increase at 50 ~ satn area on dry samples (%) (m2/g dry sample) n-Butanol Ethanol Methanol negligible negligible negligible 1.6 72-1

0.28 2.0

0.77 1.3 1.5 2.9 8.90

7-7 7-7 8.1 8.8 11.0

+ 4.3 + 5.9 -~- 2.3

0.3 7.3 49.6

1-9 2.7

2.1 6.0 7.7

8.7 9.9 10.5

- 1.0 - 4-9 -IO-0 -15.1

3.6 337.8 378.0 352.0

2-1 2-2 2.3 3.2

7.4 9.8 13.7 14.8

10-5 12.8 14-1 16.8

u n i f o r m activation, it was h o p e d that, if desired m a t e r i a l could be o b t a i n e d f r o m this particle size, it could be used w i t h o u t a n y further processing. The s o r p t i o n o f the alcohols was d e t e r m i n e d o n the activated samples. T h e l o w - t e m p e r a t u r e nitrogen B.E.T. surface areas were also d e t e r m i n e d . T h e results are presented in Table 3. Three sets o f d a t a were o b t a i n e d at o x i d a t i o n t e m p e r a t u r e s o f 200, 215 and 240°C. A t 200°C, o x i d a t i o n was very s l o w - e v e n after 950h there was a slight overall increase in weight. T h e 349

T. N. R O Y A N D S. P. N A N D I

increase in weight is of course due to the uptake of oxygen compensating for any loss of carbon or other elements by gasification. The 950h oxidized char did not give any colouration to IN alkali solution after standing overnight showing that compounds related to humic acids had not been produced. Again at 215°C the rate was slow, showing an over-all increase after 615h of oxidation. The oxidation at 240°C was faster showing 1 ~ weight decrease after 18h oxidation. It can be seen from the three sets of data that there was no measurable increase in total capacity (as given by uptake of methanol) before measurable B.E.T. areas were obtained. By that time, the selectivity as given by the ratio of methanol and ethanol had decreased considerably. From the data we deduce that, either uniform activation cannot be achieved for the particle size employed without altering the selectivity, or the pore structure of the chars are such that there are constrictions in some of the wider pores which are removed by oxidation. If the latter alternative is operating it is doubtful whether suitable selective adsorbent with high capacity could be obtained by any activation procedure.

(c) Sorption of carbon dioxide on actit~ated Poniati 600 In order to study the pore structure of the activated Poniati char in some detail, sorption of carbon dioxide was measured at 0°C and 25°C on some of 1.6l.t, 12

1'0 0"8 " 0"6 0'~,

0

I

I

I

I

I

-0.2-

2

t.

6

8

10

12

lt,~

~x 18[

I

-O.L,

Log2(Po/P) Figure ! Dubinin plot of CO2 sorption at 0°C on carbonized and subsequently oxidized (200°C) Poniati coal samples: (~.), Poniati 500; ~ , Poniati 600-OX-200-340; 13, Poniati 600-OX-200-990

350

PORE STRUCTURE OF LOW TEMPERATURE CHARS

the activated samples. The adsorption results have been plotted according to an equation derived by Dubinin 5 from Polanyi potential theory 7. The equation is: Log V ---- log V0 -- D' log 2 (Po/P)

(1)

where V ---- amount adsorbed at equilibrium pressure P Vo --=-micropore capacity Po = saturation vapour pressure of adsorbate at adsorption temperature D' = constant ---- the slope of the plot of V against log 2 (Po/P) The Dubinin plots are given in Figures 1-4. The samples have been designated e.g. Poniati 600-OX-200-340 meaning that 600°C heat treated Poniati char has been oxidized in air at 200°C for 340h. The Vo and D' values from the plots are given in Table 4. It can be seen that Vo values decreases on oxidation, that is, micropore capacity decreases. This is likely to happen if some of the micropores are enlarged to such an extent that the fields of the opposite walls no longer overlap. The D' values slightly decreases on oxidation. A decrease in the value of

1.6 1"/-, 12 >

o~ 1"0 o

08 0-6 0-4 02

0

2

h

6

8 10 12 Log z ( P o / p )

lh

16 \

\

Figure 2 Dubinin plots of CO~. sorption at 0°C on carbonized and subsequently oxidized (240°C) Poniati coal samples: O, Poniati 600; 0, Poniati 600-OX-240-18; A, Poniati 600-OX-240-29; r'l, Poniati 600-OX-240-37; m, Poniati 600-OX-240-46 351

T. N. ROY AND S. P. NANDI D ' points out to a n a r r o w i n g of pore diameter 8. L a m o n d and M a r s h 9 f o u n d a n increase of D' with activation which follows directly from the theory. Therefore, the s i m u l t a n e o u s decrease in V0 a n d D' with activation f o u n d in the present work c a n n o t be explained. The other interesting p o i n t is that the V0 values at 0°C are somewhat higher t h a n those o b t a i n e d at 25°C. This is in Table 4 Parameters of Dubinin equation from carbon dioxide sorption From 25°C isotherms

From O°C isotherms Sample

No.

D' × 102 1 2 3 4 5 6 7

Poniati 600 Poniati 600-OX-200-340 Poniati 600-OX-200-950 Poniati 600-OX-240-18 Poniati 600-OX-240-29 Poniati 600-OX-240-37 Poniati 600-OX-240-46

Vo 107-0 65-3 65"5 72-4 65"5 65"5 65'5

13'8 9.7 11'4 11' I 9.3 9'3 9.3

D' × 102

1/"o

13"4 10"8 10"5 11-1 10'0 10-0 9"8

91-2 63-1 52-5 66"1 56"3 56'3 52"0

2"0 1"8 1'6

1"2 >

1.0

o} o

0'B 0.6 0.z, 0.2 0 -0.2

2

1+

6

B

10

12

14

18

Log 2 (Po/P)

Figure 3 Dubinin plot of CO2 sorption at 25°C on carbonized and subsequently oxidized (200°C) Poniati coal samples: O, Poniati 600; /~, Poniati 600-OX-200-340; n , Poniati 600-OX-200-950

352

PORE S T R U C T U R E OF L O W T E M P E R A T U R E C H A R S

2'0 1'8 1"6 1'4

1.2 > _J

0'8

0.t, 0.2 01

0

I

i

[

2

4

5

I

I

t

I

~.

\

8 10 12 1~, 16 Lo92 (Po/P) Figure 4 Dubinin plot of COs sorption at 25°C on carbonized and subsequently oxidized (240°C) Poniati coal samples: O, Poniati 600-OX-240-18; A, Poniati 600-OX-240-29; r-I, Poniati 600-OX240-37; m, Poniati 600-OX-240-46 line with the findings of Walker and Shelef s who also reported V0 values o f 142 and 124 at 0°C and 25°C respectively for their sample A. This is probably due to the decreased packing density o f the sorbed layer at the higher temperature.

(d) The rate o f adsorption o f nitrogen on Poniati 600 To characterize the micropore structure o f Poniati 600 further the rates o f adsorption o f nitrogen at 25°C and 50°C at a constant pressure o f 2 8 c m H g were determined. Barrer and Brooks 10 had shown that for sorption occurring at constant gas pressure the following equation was applicable:

dt 1/2 Q oo -

O0. = T

.

(2)

where Qo, Q, and Qo~ refer to adsorption at time t = 0, t = t, and t = or, respectively; A is the surface area of the particles; v is the volume o f the particles; and D is the diffusion coefficient. The experimental results o f the rate study are given in Figure 5. The initial 353

T. N. R O Y A N D S. P. N A N D I

3"2~

I

2"B-

o o

2'4 ~2"0 I--Z

O

o

F

~1-6 u o

> 1.2 0"8

~

"

0.~ /~-'¢~°'S~" / 0"'T 0

4

I 8

I

12

I 16

I 20

i 24

l 28

i 32

ti#2 (Sl#a)

l 36

I 40

t 44

I 48

I 52

i 56

Figure 5 Rate of adsorption of nitrogen on Poniati 600 at 28 cmHg: O, 25°C; ~ , 50°C (weight of sample 2.5 g)

slopes of the curves have been used to calculate the diffusion coefficient. In order to obtain a numerical figure for D, the following approximations were made: (1) the micropore volume Vo obtained for the sample from carbon dioxide adsorption at 25°C has been converted to surface area by assuming a molecular cross-section of 17A ~ for CO~ and was used for A; (2) the reciprocal of methanol density was used for t,. Diffusion coefficients of 6.27 × 10-18cm2/s and 9.74 × 10-18cm2/s were obtained at 25 and 50°C respectively. The temperature effect on diffusion coefficient can be expressed by an Arrhenius-type expression: D =

Do e -e/nr

(3)

From the data an activation energy for diffusion of 9-6 kcal/mol is obtained. It must be added that the numerical value of D reported would change if the diffusion path length were estimated in a different way, but the temperature coefficient of D would be unaffected, because the diffusion path length is not temperature dependent. Barrer u showed in the case of K-mordenite that there is a pronounced effect of the size of the diffusing gas molecule on its activation energy (E) of diffusion. As the kinetic diameter of the gases increased from 2-9A (for H2) to 3.6A (Kr), E for diffusion in K-mordenite increased progressively from 2.5 to 10.0kcal/mol. The best estimate of the free diameter of the eightmember oxygen rings in mordenite (through which diffusion must take place) should be 3.9 A. as given by Barrer and Peterson r~. For the synthetic type A zeolite Reed and Breck t3 estimate that the opening to the structure is about 4.2A in diameter. With cation exchange by sodium and potassium the diameters are reduced to 3.5A and 3.2A, respectively. Activation energy of 354

PORE STRUCTURE OF LOW TEMPERATURE CHARS

diffusion of inert gases from helium to xenon from 3A zeolite was studied by Walker et al s. The diffusion of helium (kinetic diameter = 2.57A) was very rapid and showed no temperature dependence in the range of temperature used in the measurements. The diffusion of all the other gases was activated and E increased from 7.0kcal/mol for neon (2.79A) to 19.2kcal/mol for xenon (4.06A). The data on zeolites clearly show that when the size of the diffusing molecule approaches the size of the smallest opening to the structure, the value of E for diffusion becomes significant. In analogy to the diffusion data for zeolites, it can be deduced that the critical diameter of the pores in Poniati 600 should be similar to the size of a nitrogen molecule as an E value of 9.6 kcal/mol was found for the system. In sunanaary, the following points can be made: (1) Samples of coal of about 83~o carbon content on heat treatment at 600°C under inert atmosphere shows sorption capacity for methanol and ethanol in the ratio 10:1. (2) Activation of 600°C char by air in the temperature range 200-240°C reduces the selective sorption capacity. (3) The micropores in chars probably contain constrictions wider pores. Activation at low temperature removes the and reduces the selective adsorption capacity. If this model pore system is correct, an activation process would not obtaining high capacity selective adsorbents from chars.

in otherwise constrictions of the microbe helpful in

(4) Analysing the adsorption data on activated chars with the help of the Dubinin equation gives some insight into the pore structure, but the correlation of the parameters of this equation with the adsorption data is not straightforward. ACKNOWLEDGEMENT

The authors wish to thank Dr A. Lahiri, Director, Central Fuel Research Institute, for his constant encouragement and kind permission to publish this paper. Thanks are clue to Dr P. N. Mukherjee, Head, Division of Physical Chemistry Division, Central Fuel Research Institute, for his various suggestions during the progress of the work.

Central Fuel Research Institute, Jealgora, Dhanbad, hTdia

(Receit, ed 5 August 1969) (Ret,ised 10 March 1970)

REFERENCES I Anderson, R. B., Hall, W. K., Lecky, J. A., and Stein, K. C. J. Phys. Chem. 1956, 60, 1548

355

T. N. ROY AND S. P. NANDI 2 Nandi, S. P. and Walker, P. L., Jr., 'Coal Science', 1966, American Chemical Society, Washington, D.C., p 379 3 Walker, P. L., Jr., Austin, L. G., and Nandi, S. P., ~Physics and Chemistry of Carbon', Vol. 2, p 323, Marcel Dekker, New York, 1966 4 Marsh, H. and Siemieniewska, T. Fuel, Lond. 1967, 46, 441 5 Dubinin, M. M. Chem. Rev. 1960, 60, 235 6 Barrer, R. M., 'Non-Stoichiometric Compounds', 1964, Academic Press, New York, p 391 7 Young, D. M. and Crowell, A. D., 'Physical Adsorption of Gases', 1962, Butterworth, London, p 13 8 Walker, P. L., Jr. and Shelef, M. Carbon 1967, 5, 7 9 Lamond, T. G. and Marsh, H. Carbon 1963, 1,281 10 Barrer, R. M. and Brook, D. W. Trans. Faraday Soc. 1953, 49, 1049 11 Barrer, R. M. Trans. Faraday Soc. 1949, 45,358 12 Barrer, R. M. and Peterson, D. L. Proc. Roy. Soc. 1964, A280, 466 13 Reed, T. B. and Breck, D. W. J. Amer. Chem. Soc. 1956, 78, 5972

356