Jan., 1961
THESORPTION OF WATERVAPORBY CHARCOAL
37
THE SORPTION OF WATER VAPOR BY CHARCOAL AS INFLUENCED BY SURFACE OXYGEN COMPLEXES BYBALWANT RAIPURI,K. MURARIAND D. D. SINGH Department of Chemistry, Panjab University, Chandigarh, India Received February 3, 1960
A study of water sorption isotherms on charcoal coated with different amounts of variously disposed chemisorbed oxygen Show8 that it is the oxygen disposed as COz which enhances the sorption capacity of charcoal and not the rest of the combined oxygen. The area of the hysteresis loop also increases with increase in the amount of this oxygen. An appreciable amount of water, representing about one mole per mole of COP,is held so firmly by charcoal that it is not desorbed even on drying t o zero humidity. This enhances the hysteresis effect, which, however, persists though to a smaller extent, even on complete elimination of the combined oxygen. A few portions of the charcoals were also treated with The work of Lawson' and King and Lawson2 shows that the presence of chemisorbed oxygen in hydrogen at 400' for different intervals of time in a rotating tube of a/,' bore placed in a tube furnace. Hydrogen charcoal increases the low pressure sorption of water Pyrex waa led over the charcoal at the rate of 2 litera per hour. by charcoal and shifts the isotherms to lower pres- The samples, after the treatment, were allowed to cool in the sures than those corresponding to the same amount atmosphere of hydrogen and then transferred to stoppered of sorption in the absence of any such oxygen. bottles. The amount of oxygen retained ria well as its disposition in Similar results have been reported in more recent each sample was estimated by evacuating 2-g. portions at papers by Emmett,3 Dubinin and Z a ~ e r i n a , ~1200", by gradually raising the temperature in the same Healey, et aZ.,6 Anderson and Emmetts and Mc- manner as described above, collecting water in calcium tubes and analyzing the rest of the gases evolved in are of the view chloride Dermot and Amell.' Smith, et d.,* an Orsat-Lunge gas analysis apparatus in the usual way. A that the increase is due t o an extensive reaction of few of these samples were examined for total oxygen content water with oxygen complex producing a new surface by ultimate analysie as well. The values tallied very well complex. McDermot and Amell' believe that the with those obtained on degassing. Water Isotherms.-Water sorption isotherms were deteroxygen provides isolated active centers at which the mined a t 25".'0 sorption proceeds in the form of ' W u r n p ~ or ~~ "clusters"@ of water molecules which grow in size TABLE I by a mechanism of hydrogen bonding until they GASESEVOLVEDON EVACUATING AT 1200" THE VARIOUS merge, at a higher re!ative pressure, to fill the SAMPLES OF CHARCOAL smaller pores. Capillary condensation occurs whenc G a s e 8 evolved on evacuationTotal at 12000 oxygen ever a cluster reaches sufficient size to bridge the Description of COa CO H10 Ht evolved sample (mg./g.) (mg./g.) ( m d g . 1 (mg./g.) (mg./g.) walls of a pore. It appears that while the influence of chemisorbed Sugar charcoal oxygen-in altering water adsorption isotherms on Original 155.90 150.63 125.50 13.50 311.01 carbons has received a fair amount of attention, the Evacuated at: influence of the various forms in which it is dis300O 77.75 147.50 81.62 13.53 213.35 posed or removed on degassing (e.g., as CO, C02 500 47.35 137.88 76.05 13.25 180.78 and HzO) has not been investigated. The present 750 Nil 97.55 61.10 12.12 109.94 work, therefore, was undertaken. 1000 Nil 5.85 Nil 1.80 3.34 1200 Nil Nil Nil Nil Nil Experimental Materials .-Two samples of charcoal were prepared by the carbonization of cane sugar and coconut sheus in a limited suppl of air at 300" in a Pyrex glass vessel placed in a heat-laggedr nichrome roil for uniform electrical heating. The current waa controlled with a Variac transformer. The !hayoale were tramferred to wide trays and allowed to cool In air. Sugar charcoal as prepared was almost free of ash: the other sample was extracted with hydrofluoric acid to lower the ash content to about 0.14%. Several 10-g. portions of the charcoals were evacuated in a resistance tube furnace at 300,500,750,1000 and 1200". The temperature waa allowed to rise gradually and before it waa raised another 50" the complete elimination of the gases at the preceding temperature had been ensured. After degassing at each temperature the sample was allowed to remain and cool in vacuo and then transferred to a well stoppered bottle. (1) C. G. Lawaon, Trans. Faraday Soc., 83, 473 (1936). (2) A. King and C. G . Lawson, ibid., 80, 1094 (1934). (3) P. H. Emmett, Chcm. Reus., 48, 69 (1948). (4) M. M. Dubinin and E. D. Zaverina, J . Phya. Chem.. (U.S.S.R.). 21, 1373 (1947). (5) F. H. Healey, Y. F. Yu and J. J. Cheasick, THISJOURNAL, 69, 399 (1955). (6) R. B. Anderson and P. H. Emmett, ibid., 56, 756 (1952). (7) H. L. McDermot and J. C. Amell, ibid., 68, 492 (1954). (8) R. N. Smith, C. Pieroe and C. D. Joei, tbid., 68, 298 (1954). (9) C. Pieme and R. N. Smith, (bid., 64, 784 (1950).
Original Evacuated at: 300" 500 750 1000 1200
Coconut shell charcoal 77.18 130.15 80.50 11.15 202.06 55.27 116.51 25.32 109.33 Nil 73.06 Nil 4.43 Nil Nil
70.93 11.10 169.79 59.55 11.15 133.80 48.52 10.55 84.87 Nil 1.10 2.53 Nil Nil Nil
Sugar charcoal treated with hydrogen at 400" Treated for: 2 hr. 73.75 140.66 116.75 10.85 237.80 4 46.23 118.10 100.50 8.60 190.42 6 32.63 87.85 88.75 7.23 152.82 8 Nil 57.25 77.55 6.34 101.64 12 Nil 30.66 40.66 4.32 53.66
Discussion It is seen (Table I) that the original samples of sugar charcoal and coconut shell charcoal contain, respectively, about 31 and 20% of combined oxygen (10) B. R. Puri, 8. N. Khanna and Y. P. Myar, J . Sci. Ind. Rea., 18B, 67 (1969).
BALWANT RAI PURI,K, MURARIAND D. D. SINGH
Description of sample
Original Evacuated at 300" 500 750 1000 1200
Original Evacuated at 300 O 500 750
VOl. 65
TABLEI1 SORPTION-DESORPTION ISOTHERMS OF WATER VAPOR ON THE VARIOUS SAMPLES OF CHARCOAL Desorption values are given in parentheses. Amount sorbed ( /lo0 9.) at R.V.P. 0.00 2
0.09 3
0.18 4
0.00 (6.42)
2.61 (7.50)
3.82 (10.71)
0.00 (3.47) 0.00 (1.83) 0.00 (0.00) 0.00 (0.00) 0.00 (0.00)
1.06 (4.75) 0.86 (2.10) 0.78 (0.78) 0.80 (0.80) 0.81 (0.81)
2.64 (7.01) 1.28 (3.89) 1.22 (1.22) 1.22 (1.22) 1.21 (1.21)
0.00 (3.25)
0.20 (4.15)
4.01 (5.80)
0.00 (1.96) 0.00 (1.01) 0.00 (0.00)
0.75 (2.21) 0.62 (1.62) 0.54 (0.54) 0.55 (0.55) 0.55 (0.55)
2.02 (2.75) 1.20 (1.87) 1.00 (1.00) 1.01 (1.01) 1.00 (1.00)
1000
0.00
1200
(0.00) 0.00 (0.00)
0.28
0.48
0.68 8
Sugar charcoal 5.26 7.50 (14.15) (14.41)
9.72 (17.00)
13.21 (17.61)
4.01 (10.39) 2.11 (4.51) 1.81 (3.75) 1.79 (2.65) 1.79 (2.25)
6.22 (13.25) 5.61 (10.75) 5.52 (11.76) 5.78 (12.75) 5.96 (13.35)
9.94 (14.42) 9.76 (12.99) 11.00 (15.61) 11.12 (15.35) 11.52 (15.87)
6.40 (9.51) 6.52 (8.10) 6.25 (8.68) 7.15 (8.90) 7.20 (8.78) 7.49 (9.33)
5
8.38 6
4.53 (12.41) 3.40 (8.25) 3.19 (7.01) 2.78 (6.50) 2.90 (5.01)
Coconut shell charcoal 4.70 5.49 (7.49) (8.78) 3.21 (4.05) 2.01 (3.80) 1.49 (5.41) 1.50 (2.75) 1.52 (3.80)
4.89 (5.87) 5.11 (6.72) 4.72 (7.37) 4.81 (6.75) 4.78 (5.59)
7
0.89 9
0.996 10
18,70 (21.92)
37.55 (27.55)
12.49 11.74 (14.12) 13.78 (16.05) 13.68 (17.51) 13.70 (17.11)
19.52 (19.52) 16.01 (16.01) 18.50 (18.50) 18.97 (18.97) 19.01 (19.01)
8.51 (10.05)
9.49 (10.89)
12.95 (12.95)
7.74 (11.25) 8.24 (10.50) 9.00 (10.52) 9.02 (11.50) 9.60 (10.65)
10.29 (12.01) 9.50 (11.22) 10.51 (11.41) 10.00 ( 11.92) 10.61 (11.57)
12.50 (12.50) 11.72 (11.72) 12.34 (12.34) 12.10 (12.10) 12.19 (12.19)
7.31
11.50 (14.75) 10.39 (12.25) 9.00 (11.81) 8.94 (10.87) 8.75 (11.81)
17.11 (17.11) 14.23 (14.23) 13.85 (13.85) 12.51 (12.51) 12.45 (12.45)
( 15.79)
Sugar charcoal treated with hydrogen Treated for 2 hr. 4 6
8 12
0.00 (2.30) 0.00 (1.65) 0.00 (1.40) 0.00 (0.00) 0.00 (0.00)
1.41 (2.87) 1.25 (2.50) 0.75 (2.45) 0.53 (0.53) 0.51 (0.51)
3.25 (4.21) 2.75 (5.01) 2.20 (3.31) 1.51 (1.51) 1.46 (1.46)
4.49 (5.75) 4.15 (6.49) 3.51 (4.35) 2.85 (2.85) 2.76 (2.76)
which is desorbed at or below 1200" as CO, CO, and H20. When these samples are heated in vacuo at increasing temperatures or in hydrogen a t 400" for different intervals of time, the amount of the combined oxygen decreases progressively. It is also seen that the samples degassed at 750°, or heated in hydrogen at 400" for 8 hours, retain some oxygen disposed as CO and H 2 0 but none as CO, while those degassed at 1000" retain small amounts of oxygen disposed as CO only. A glance at the table shows that all the 17 samples included therein contain different amounts of the variously disposed oxygen. As regards water sorption isotherms (Table 11) it is seen that degassing at increasing temperatures or heating with hydrogen at 400" for different inter-
5.01 (7.75) 4.90 (7.75) 4.50 (5.85) 4.32 (4.85) 4.10 (4.60)
5.82 (9.35) 5.71 (8.65) 5.01 (6.87) 4.75 (6.40) 4.65 (5.75)
( 11.70)
7.10 (10 .OO) 6.11 (8.75) 6.00 (8.50) 5.51 (7.65)
vals of time results in lowering the water sorption capacity of charcoal, particularly at lower relative vapor pressures. These alterations cannot be attributed to surface area which, for a given charcoal, has been shown to remain almost the same irrespective of the deoxygenating treatment~.7~11J2 These observations when considered in the light of those recorded in Table I show marked influence of the combined oxygen in altering the isot,herms at lower relative pressures. Capillary effects appear to predominate, thereafter, in most cases. These results are in agreement with those reported by (11) P. H. Emmett and R. B. Anderson, J . Am. Chem. Sac., 67, 1492 (1945). (12) B. R. Puri, D. D. Singh and L. R. Shsrma, THIS JOURNAL, 62, 756 (1958).
Jan., 19G1
39
THESORPTION OF WATERVAPORBY CHARCOAL
But since the sorption values previous of the samples evacuated a t 750” do not materially differ from those of the samples of the same charcoal degassed at higher temperatures and since 750” is the temperature a t which the entire CO, complex is eliminated, il; appears that it is the oxygen disposed as CO, and not the total oxygen which, apart from capillary effects, influences the water sorption capacity of charcoal. It is significant to note that when degassed at 750” sugar charcoal retains about 11%and coconut shell charcoal about 8.5% oxygen disposed as CO and HzO yet little or no change in water sorption isotherms takes place when this oxygen is progressively eliminated on degassing a t 1000 and 1200”. Similarly, although the samples of sugar charcoal treated with hydrogen for 8 and 12 hours differ appreciably from one another in their oxygen content, disposed as CO and H,O, yet their water isotherms are almost identical. It appears, therefore, that the view of the previous ~ o r k e r s ~that - ~ the combined oxygen provides active sites for sorption of water vapor needs modification to the extent that it is the oxygen present as Cos-complex and not the rest of it which does so. Hysteresis.-The desorption isotherms of all the samples also were determined. The data are included in Table 11. It is seen that in the original as well as in the 300 and 500” degassed samples, all of which contained COz-complex, the two branches do not meet even at zero pressure. I n other words, the hysteresis loop persists throughout the entire range of vapor pressure. However, in the samples degassed at 750” and at higher temperatures which were completely free of oxygen disposed as COz, the two branches of the isotherms are seen to converge a t relative vapor pressures well above zero. Similar conclusion may be drawn from the data for the hydrogen treated samples. The two branches meet only when the C0,-complex is completely eliminated. The significance of these observations is that a certain amount of water sorbed by charcoal containing COS-complex is not desorbed even when the system is evacuated completely to constant weight a t 25”. This water may be regarded as “fixed” a t tJhech;trcoal surface containing C0,-cornplex by some mechanism probably involving hydrogen bonding. The C0,-content of each charcoal and the amount of water “fixed” by it are given in Table 111. It is evident that the greater t h i amount of C0,-complex in a charcoal the greater is the amount of water “fixed” by it. The weights of water “fixed” per mole of C 0 2 were also calculated. These values are recorded in column 4. It is interesting to note
that these values correspond to the “fixation” of one mole of water per mole of CO,. This shows that each site containing a “CO,-complex” can hold almost one mole of water in this intimate manner. TABLE I11 AMOUNTOF WATER“FIXED”BY CHARCOAL AND AREA OF HYSTERESIS LOOPIN RELATION TO THE C O r OF ~ CHARCOAL 4 m t . of water fixed Description of sample
Original Evacuated at: 300’ 500 750 1000 1200 Original Evacuated at: 300’ 500 750 1000 1200
COz-content (&/IO0 9.)
(g./IOO g.)
Sugar charcoal 15.59 6.42 7.78 4.74 Nil Nil Nil
3.47 1.83
.. ..
..
Amt. of water fixed per mole of CO? ( g . )
drea of hysteresis loop, squares
18.12
1049
19.59 17.08 ...
901 634 432 428 425
... ...
Coconut shell charcoal 7.72 3.25 18.37 5.53 2.53 Nil Nil Nil
1.96 1.01
.. ..
..
15.51 17.56
...
... ...
Sugar charcoal treated with hydrogen at 400’ Treated for: 2 hr. 7.38 2.30 13.80 4 4.62 1.65 15.71 6 3.26 1.40 18.89 8 Nil .. ... 12 Nil .. ...
745 498 443 360 358 349
750 596 486 335 328
It is also seen that the area of the hysteresis loop (col. 5) decreases considerably with decrease in the COS-complex and that when this complex is completely desorbed, either on evacuation at 750” or on treatment with hydrogen for 8 hours, the subsequent decrease in the area is quite insignificant. Thus the magnitude of the hysteresis loop also depends much more on oxygen disposed as CO, than on the rest of the chemisorbed oxygen. It may be pointed out that these results are not in accord with the observations of McDermot and knell7 who on the contrary found the hysteresis loop to increase on the elimination of the combined oxygen. It appears that the ‘Yixation” of water on act,ive sites presented by the C0,-complex makes a material contribution toward hysteresis in mater-carbon systems.
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