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Surface characteristics and surface behaviour of polymer carbons—V
Cur&nVol.18.pp.395-398 0 Pergamon Press Ltd.,1980. Printed in Great Britain
SURFACE
CHARACTERISTICS
BEHAVIOUR INTERACTION
OF POLYMER
OF POLYMER AQUEOUS
AND SURFACE CARBONS-V
CARBONS WITH SOLUTIONS
BROMINE
IN
R. C. BANSAL and T. L. DHAMI Department of Chemistry, Panjab University, Chandigarh 160014,India and
SAT PARKASH Fuel Sciences Division, Research Council of Alberta Edmonton, Alberta, Canada (Reeeiued 11 Decemfw 1979)
Abstract-Interaction of poIymer charcoais with bromine in aqueous sofutions results in the conversion of bromine into HBr and its fixation by the charcoals. The amount of bromine converted into HBr is related to the surface acidity of the char while the amount fixed depends upon the nature of the char and the history of its formation. PVDC and Saran charcoals adsorb appreciably larger amounts of bromine than PF and UF charcoals prepared at the same temperature. The adsorption decreases on outgassing the chars at 1OOOCand the decrease is only slight in case of PVDC and Saran chars. The adsorption of bromine involves two different mechanisms. In case of the highly porous charcoals such as those of PVDC and Saran prepared by the carbonisation of oxygen free polymers, the adsorption of bromine occurs largely in pores. In the case of less porous charcoals such as those of PF and UF prepared by the carbonisation of oxygen containing polymers, the adsorption of bromine takes place partly by addition at the unsaturated sites and partly by substitution in place of bonded hydrogen.
1. INTRODU~ON It has been shown in earlier publications
that the associated oxygen which comes off as CO, on evacuation influences the surface acidity and adsorption capacity of polymer carbons [l-3]. The role of this complex in influencing the interaction of sugar charcoals and commercial grade carbon blacks with bromine from aqueous solutions has also been reported [4-6]. For example, Puri et al.[4,5] observed that sugar charcoals and different carbon blacks chemisorb bromine when placed in contact with aqueous bromine solutions. The amount adsorbed increased progressively on increasing the evacuation temperature of the carbon upto 7OO’C, there being no significant increase on raising the temperature beyond 700°C. The adsorption of bromine was attributed to addition at the ethylenic double bond sites which are created by the elimination of that part of associated oxygen which is evolved as CDL on evacuation. Since the entire amount of this complex gets eliminated at temperatures lower than 800°C the adsorption of bromine remains more or less constant after this heat treatment temperature. Stearns and Johnson[6], while studying the interaction of channel blacks with aqueous bromine, observed appreciable adsorption of bromine at ordinary temperature. The amount adsorbed was found to increase with increase in the heat treatment temperature of the carbon. These workers attributed this adsorption to addition at the ethylenic type of double
bond sites as the heat of the reaction was about the same as the heat of bromination of olefines. Garten and Weiss[7], on the other hand, attribute this unsaturation in channel blacks before heat treatment to the presence of surface quinonic groups at sites where they cannot participate in resonance. The increase of unsaturation (i.e. bromine adsorption) on heat treatment was attributed to the generation of unsaturated side chains attached to the aromatic nucleus. Recently Puri et aI.[8,9] have further shown that the unsaturation in charcoals and carbon blacks which is produced by the elimination of the CO1complex and determined by adsorption of bromine is a definite quantity characteristic of a carbon at a given stage in the history of its formation and can be enhanced on surface oxidation of the carbon followed by evacuation. The interaction of carbons with bromine from non aqueous solutions and vapour phase has also been investigated by several workers [N-20]. Appreciable amounts of bromine are adsorbed but the mechanism of the adsorption is different in these cases. The adsorption of bromine from non aqueous solutions takes place largely by substitution for hydrogen and only small amounts may be fixed by addition at the unsaturated sites. The adsorption from vapour phase is reversible and involves physical forces. It appears from the above perusal of the literature that whatever be the mechanism of the creation of unsaturation in carbons, the adsorption of bromine
39.5
396
R. C. BANSAL. T.
L. DHAMI
from aqueous solution is a measure of this unsaturation. However, in none of these papers has the role of the porous nature of the carbons been taken as one of the factors which may also influence the adsorption of bromine. Since polymer charcoals are highly microporous, it was thought of interest to study the role of porosity in determining the adsorption of bromine from aqueous solutions. The present work was, therefore, undertaken. 2. EXPERIMENTAL Ten samples of polymer charcoals obtained by the carbonisation of different polymer percursors under varying experimental conditions were used in these investigations. The details of the procedures have been described elsewhere [20]. The amount of bromine fixed by each charcoal was determined by shaking mechanically in a thermostated shaker (30 f 1°C) 0.5 g charcoal with 50 ml of 0.1 N aqueous bromine solution in potassium bromide (2 moles per mole of bromine) in 100 ml stop pered Pyrex bottles (wrapped in thick black paper) for 24 hr necessary for the attainment of equilibrium [4,5]. The suspension was then filtered and the carbon sample repeatedly washed with distilled water. The filtrate and washings were made upto a known volume and an aliquot was analysed for free bromine and for hydrobromic acid by standard analytical procdures. A blank was run every time. There was no noticeable change in the composition of the solution in the absence of charcoal. The base adsorption capacity of the charcoal was determined by mixing 0.5 g sample of each charcoal with 0.2 N aqueous sodium hydroxide solution in the usual way [Z]. Desorption of bromine was carried out by heat treatment in nitrogen atmosphere. The bromine
and SAT PARKASH
evolved was collected in potassium and its amount determined as usual.
iodide solution
3. RESULTS AND DISCUSSION
The amounts of bromine fixed and converted into HBr on treatment of polymer charcoals with aqueous solutions of bromine are shown in Table 1. The presence of polymer charcoals results in the conversion of appreciable amounts of bromine into HBr and the extent of conversion is different in different cases. Puri and Bansal[S] showed that the extent of this conversion in the case of carbon blacks depended upon the surface acidity of the carbon black. The surface acidities of the various polymer charcoals, as determined by titration with 0.2 N sodium hydroxide solution are included in Table 1. The relationship between surface acidity and HBr formed for different polymer chars (Fig. 1) indicates a direct dependence of conversion on surface acidity. Each char is seen to fix appreciable amounts of bromine as well. The amount of bromine fixed depends upon the nature of the char and the history of its formation. For example PF and UF chars prepared at lower temperatures fix larger amounts of bromine and the amount fixed decreases as the temperature of preparation of the char is increased (see Table 1). This observation is not in agreement with the earlier work of Stearns and Johnson[6] who observed that the amount of bromine adsorbed increased with increase in the heat treatment temperature of the carbon during its preparation. It is also evident from the data in Table 1 that PVDC and Saran chars adsorb appreciably larger amounts of bromine than PF and UF chars prepared at about the same temperature. The results obtained on the adsorption of bromine and conversion to HBr on charcoals outgassed at
Table 1. HBr formed and bromine fixed on treatment of polymer charcoals with aqueous solutions of bromine mr formed (m.eq/g.)
sample Identification
_~~_~~_
Bromine fixed (m.eq /g)
Sodium droxide neutra $ ised (m.eq / g)
~~
FvDc-6cQ
18,s
x3.02
2.12
8aran-600
13.12
16.10
1.43
Saran-6OO(SteamAct.at PF-140 pF-400 m-600 PF-900
850°C)
3.10 15.a 8.42 2.06 0.23
18.24
0.50
21.70
1.83
18.72
0.97
10.04
0.42
5.23
0.16
17.01
0.76
UF-400
6.07
UF-650
4.12
9.22
0.78
UF-850
1.10
5.54
0.07
Surface characteristics and surface behaviour of polymer carbons-V
397
PVDC, Saran and steam activated Saran charcoals. This decrease in the adsoption of bromine on outgassing is surprising in view of the earlier work of Puri et aL[4,5] for sugar charcoals and carbon blacks showing an increase in the adsorption of bromine upon degassing. They attribute this increase in adsorption to the creation of unsaturated sites by the elimination of associated oxygen which is evolved as CO2 on evacuation. Thus it appears that the adsorption of bromine on polymer charcoals from aqueous solutions involves a different mechanism. PVDC and Saran chars are prepared from polymers which contain no oxygen as a part of their chemical structure. Therefore, there is little possibility of any oxygen becoming bonded to the carbon structure of the chars during their preparation. The associated oxygen which is given out as COz and CO on evacuation is picked up by the chars during their subsequent exposure to air [l-4]. These chars, therefore, have little or no unsaturation of the type suggested by I Puri et aI.[4,5]. A small amount of this unsaturation, 0.0 0.5 1.0 1.5 2.0 2. however, may be created on outgassing of the charcZCIDITY,m.e.,/, coals. On the other hand, these eharcoais are highIy Fig. 1. Surface acidity of polymer carbons in relation to porous and have large surface areas [BET (NJ surhydrobromic acid formed. face areas varying between 800 and 1200m2/g] and micropore volumes. Therefore, it is quite reasonable to believe that a large portion of the adsorbed bromine is present in the micro capillary pores. A 1ooo”C are presented in Table 2. There was little or no conversion of bromine into HBr in case of the small decrease in bromine uptake on outgassing at 1OCKYC (rather than an increase due to the creation of outgassed samples. This is due to the fact that the unsaturated sites on outgassing) may be attributed to outgassed samples are entirely free of any associated oxygen which is responsible for their surface a reorientation of the microporous structure on heat acidity [l]. Thus the conversion of bromine into IIBr treatment in such a way that some of the micropores in the presence of charcoals is a function of surface become inaccessible to bromine molecules. In case of PF and IJF chars, which are prepared by acidity which varies with the amount of CO1-complex the carbonisation of oxygen containing polymers, present on the surface of charcoals. appreciable amounts of oxygen become bonded to the The amount of bromine fixed (see Table 2) generally decreases on outgassing the charcoals in most carbon surface during their preparation. In addition, cases. The decrease, however, is smaller in the case of as shown by Fitzer et al.[22], these chars contain
I’
I
I
I
Table 2. HBr formed and bromine fixed on treatment of 1OOO”C-outgassed polymer charcoals with aqueous solutions of bromine Ii& formed (m eq /6)
Sample IdentFfication WDWOO sarall~00 SaranbCO(Staam
A&at
Bromine f lxed (m eq / 9)
0.29
16.75
0.28
15.25
85O*C) 0.21
13.w
PF-MO
0.15
9.02
PF400
0.18
9.50
PP-600
0.00
2.75
PF-900
0.00
4.90
UF-400
0.19
8.04
UF-tjSO
0.16
r,.?5
W&5G
0.00
5.25
R. C. BANSAL,T. L. DHAMIand SAT PARKASH
398
Table 3. Desorption of bromine on heat treatment at different temperatures Sample Menttiication
PvDc-600 Sarantn-GOO(Staam A&at
Bromine fixed (m eq / g)
3.8.02 85O*C) 18.24
PF-500
10.04
UF-550
9.22
appreciable amounts of bonded hydrogen as methylene groups. In these chars, therefore, the adsorption of bromine may take place partly by substitution of hydrogen at the methylene groups and partly by addition at the unsaturation sites. When these chars are outgassed at lOOo”C, a greater portion of the bonded hydrogen gets eliminated [22] causing a decrease in the adsorption of bromine on the outgassed samples. A small increase in adsorption of bromine due to the creation of unsaturated sites by the elimination of associated oxygen is offset by a big decrease in adsorption of bromine by substitution. The amounts of bromine adsorbed by PF-900 and UF-850 chars before and after outgassing at 1000°C support this view. These chars have been given heat treatment at 900 and 850°C respectively during their preparation. Therefore, these chars are devoid of any methylene hydrogen available for substitution by bromine. Since these chars do not have large surface areas and micropore volume. [BET (N2) surface areas being 1.3 m*/g for UF-850 char and 106 m*/g for PF-900 char] it appears that the adsorption of bromine on these samples occurs largely by addition at the unsaturated sites which remain more or less unchanged on outgassing. The desorption of bromine on heat treatment of the brominated samples (Table 3) further supports the two different mechanisms involved in the adsorption of bromine. In case of PVDC and Saran charcoals as much as SO”/, of the adsorbed bromine is desorbed (where large portion of bromine uptake, we believe, occurs in the microcapillary pores), compared to very little desorption from PF and UF brominated chars (where bromine uptake is both by substitution at the methylene groups and by addition at the unsaturated sites). ~e~~~w~e~ge~e~r-one of the authors (T.L.D.) is thankful to CSJR India for the award of a Research Fellowship.
Bromine
desorbed on heat at (at e g)
treatment
5o”c
loo0 c
150°c
2oo”c
5.41
6.10
7.01
7.52
5.04
6.91
7.42.
8.E12
0.22
0.58
0.24~
1.10
o.l.4
0.57
0.72
0.95
REFERENCES 1. R. C. Bansai and T. L. Dhami, Cur&on l&65 (1978). 2. R. C Bansal, T. L. Dhami and S. Parkash, Carbon 16, 389 (1978). 3. R. C. Bansal and T. L. Dhami, Carbon 1979 (in press). 4. B. R. Puri. N. K. Sandle and 0. P. Mahaian. _ J. Chem. Sot. 4880 (1963). 5. B. R. Puri and R. C. Bansal, Carbon 3, 533 (1966). 6. R. S. Stearns and B. L. Johnson, Ind. Engng Chem. 43, 146 (1951). 7. V. A. Garten and D. E. Weiss, Aust. 3. Chem. 3, 68 (1955). 8. B. R. Puri, 0. P. Mahajan and D. L. Gandhi, Indian J. Chem. 10,848 (1972). 9. B. R. Puri, 0. P. Mahajan and D. L. Gandhi, Chem.
Ind. 382 (1972). 10. J. J. Kipling, Adsorption from solutions of Nonelectrolytes, p. 66. Academic Press, London, New York (1965). 11. J. C. Macrae and R. Oxtoby, Fuel 44,409 (1965). 12. J. D. Brooks and T. McL. Spotswood, P&c. 5th Cczrbon Con& Vol. 1, p. 416. Pergamon Press, Oxford (1962). 13. J. W. Watson and D. Parkinson, Ind. Engng Gem. 47, 1053 (1955). 14. H. L. McDermot and J. C. Arnell, Can. .r. Chem. 34, 1114 (1955). 15. R. K. Sinha and P. L. Walker, Jr., Fuel 52, 153 (1973). 16. P. H. Given, M. E. Peover and W. F. Wyss, Fuel 44, 425 (1965). 17. B. R. Puri and K. C. Sehgal, Ind. J. Chem. 4, 206 (1966). 18. L. H. Reyerson and A. E. Cameron, J. Phys. Chem. 39, 181 (1935). 19. D. L. Gandhi, S. K. Sharma, A. Kumar and B. R. Puri, Ind. J. Chem. 13, 1317 (1975). 20. B. R. Puri and K. C. Sehgal, Ind. J. Chem. 5, 379 (1967). 21. J. Maire and J. Mering, chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 6, p. 125. Marcel Dekkec, New York (1971). 22. R. C. Bansal, T. L. Dhami and S. Parkash, Carbon 15, 157 (1977). 23. E. Fitter, K. Mueller and W. Schaefer, Chemistry und Physics of Curbon (Edited by P. L. Walker, Jr.), Vol. 7, pp. 237-383. Marcel Dekker, New York, (1971).
SURFACE
CHARACTERISTICS
BEHAVIOUR INTERACTION
OF POLYMER
OF POLYMER AQUEOUS
AND SURFACE CARBONS-V
CARBONS WITH SOLUTIONS
BROMINE
IN
R. C. BANSAL and T. L. DHAMI Department of Chemistry, Panjab University, Chandigarh 160014,India and
SAT PARKASH Fuel Sciences Division, Research Council of Alberta Edmonton, Alberta, Canada (Reeeiued 11 Decemfw 1979)
Abstract-Interaction of poIymer charcoais with bromine in aqueous sofutions results in the conversion of bromine into HBr and its fixation by the charcoals. The amount of bromine converted into HBr is related to the surface acidity of the char while the amount fixed depends upon the nature of the char and the history of its formation. PVDC and Saran charcoals adsorb appreciably larger amounts of bromine than PF and UF charcoals prepared at the same temperature. The adsorption decreases on outgassing the chars at 1OOOCand the decrease is only slight in case of PVDC and Saran chars. The adsorption of bromine involves two different mechanisms. In case of the highly porous charcoals such as those of PVDC and Saran prepared by the carbonisation of oxygen free polymers, the adsorption of bromine occurs largely in pores. In the case of less porous charcoals such as those of PF and UF prepared by the carbonisation of oxygen containing polymers, the adsorption of bromine takes place partly by addition at the unsaturated sites and partly by substitution in place of bonded hydrogen.
1. INTRODU~ON It has been shown in earlier publications
that the associated oxygen which comes off as CO, on evacuation influences the surface acidity and adsorption capacity of polymer carbons [l-3]. The role of this complex in influencing the interaction of sugar charcoals and commercial grade carbon blacks with bromine from aqueous solutions has also been reported [4-6]. For example, Puri et al.[4,5] observed that sugar charcoals and different carbon blacks chemisorb bromine when placed in contact with aqueous bromine solutions. The amount adsorbed increased progressively on increasing the evacuation temperature of the carbon upto 7OO’C, there being no significant increase on raising the temperature beyond 700°C. The adsorption of bromine was attributed to addition at the ethylenic double bond sites which are created by the elimination of that part of associated oxygen which is evolved as CDL on evacuation. Since the entire amount of this complex gets eliminated at temperatures lower than 800°C the adsorption of bromine remains more or less constant after this heat treatment temperature. Stearns and Johnson[6], while studying the interaction of channel blacks with aqueous bromine, observed appreciable adsorption of bromine at ordinary temperature. The amount adsorbed was found to increase with increase in the heat treatment temperature of the carbon. These workers attributed this adsorption to addition at the ethylenic type of double
bond sites as the heat of the reaction was about the same as the heat of bromination of olefines. Garten and Weiss[7], on the other hand, attribute this unsaturation in channel blacks before heat treatment to the presence of surface quinonic groups at sites where they cannot participate in resonance. The increase of unsaturation (i.e. bromine adsorption) on heat treatment was attributed to the generation of unsaturated side chains attached to the aromatic nucleus. Recently Puri et aI.[8,9] have further shown that the unsaturation in charcoals and carbon blacks which is produced by the elimination of the CO1complex and determined by adsorption of bromine is a definite quantity characteristic of a carbon at a given stage in the history of its formation and can be enhanced on surface oxidation of the carbon followed by evacuation. The interaction of carbons with bromine from non aqueous solutions and vapour phase has also been investigated by several workers [N-20]. Appreciable amounts of bromine are adsorbed but the mechanism of the adsorption is different in these cases. The adsorption of bromine from non aqueous solutions takes place largely by substitution for hydrogen and only small amounts may be fixed by addition at the unsaturated sites. The adsorption from vapour phase is reversible and involves physical forces. It appears from the above perusal of the literature that whatever be the mechanism of the creation of unsaturation in carbons, the adsorption of bromine
39.5
396
R. C. BANSAL. T.
L. DHAMI
from aqueous solution is a measure of this unsaturation. However, in none of these papers has the role of the porous nature of the carbons been taken as one of the factors which may also influence the adsorption of bromine. Since polymer charcoals are highly microporous, it was thought of interest to study the role of porosity in determining the adsorption of bromine from aqueous solutions. The present work was, therefore, undertaken. 2. EXPERIMENTAL Ten samples of polymer charcoals obtained by the carbonisation of different polymer percursors under varying experimental conditions were used in these investigations. The details of the procedures have been described elsewhere [20]. The amount of bromine fixed by each charcoal was determined by shaking mechanically in a thermostated shaker (30 f 1°C) 0.5 g charcoal with 50 ml of 0.1 N aqueous bromine solution in potassium bromide (2 moles per mole of bromine) in 100 ml stop pered Pyrex bottles (wrapped in thick black paper) for 24 hr necessary for the attainment of equilibrium [4,5]. The suspension was then filtered and the carbon sample repeatedly washed with distilled water. The filtrate and washings were made upto a known volume and an aliquot was analysed for free bromine and for hydrobromic acid by standard analytical procdures. A blank was run every time. There was no noticeable change in the composition of the solution in the absence of charcoal. The base adsorption capacity of the charcoal was determined by mixing 0.5 g sample of each charcoal with 0.2 N aqueous sodium hydroxide solution in the usual way [Z]. Desorption of bromine was carried out by heat treatment in nitrogen atmosphere. The bromine
and SAT PARKASH
evolved was collected in potassium and its amount determined as usual.
iodide solution
3. RESULTS AND DISCUSSION
The amounts of bromine fixed and converted into HBr on treatment of polymer charcoals with aqueous solutions of bromine are shown in Table 1. The presence of polymer charcoals results in the conversion of appreciable amounts of bromine into HBr and the extent of conversion is different in different cases. Puri and Bansal[S] showed that the extent of this conversion in the case of carbon blacks depended upon the surface acidity of the carbon black. The surface acidities of the various polymer charcoals, as determined by titration with 0.2 N sodium hydroxide solution are included in Table 1. The relationship between surface acidity and HBr formed for different polymer chars (Fig. 1) indicates a direct dependence of conversion on surface acidity. Each char is seen to fix appreciable amounts of bromine as well. The amount of bromine fixed depends upon the nature of the char and the history of its formation. For example PF and UF chars prepared at lower temperatures fix larger amounts of bromine and the amount fixed decreases as the temperature of preparation of the char is increased (see Table 1). This observation is not in agreement with the earlier work of Stearns and Johnson[6] who observed that the amount of bromine adsorbed increased with increase in the heat treatment temperature of the carbon during its preparation. It is also evident from the data in Table 1 that PVDC and Saran chars adsorb appreciably larger amounts of bromine than PF and UF chars prepared at about the same temperature. The results obtained on the adsorption of bromine and conversion to HBr on charcoals outgassed at
Table 1. HBr formed and bromine fixed on treatment of polymer charcoals with aqueous solutions of bromine mr formed (m.eq/g.)
sample Identification
_~~_~~_
Bromine fixed (m.eq /g)
Sodium droxide neutra $ ised (m.eq / g)
~~
FvDc-6cQ
18,s
x3.02
2.12
8aran-600
13.12
16.10
1.43
Saran-6OO(SteamAct.at PF-140 pF-400 m-600 PF-900
850°C)
3.10 15.a 8.42 2.06 0.23
18.24
0.50
21.70
1.83
18.72
0.97
10.04
0.42
5.23
0.16
17.01
0.76
UF-400
6.07
UF-650
4.12
9.22
0.78
UF-850
1.10
5.54
0.07
Surface characteristics and surface behaviour of polymer carbons-V
397
PVDC, Saran and steam activated Saran charcoals. This decrease in the adsoption of bromine on outgassing is surprising in view of the earlier work of Puri et aL[4,5] for sugar charcoals and carbon blacks showing an increase in the adsorption of bromine upon degassing. They attribute this increase in adsorption to the creation of unsaturated sites by the elimination of associated oxygen which is evolved as CO2 on evacuation. Thus it appears that the adsorption of bromine on polymer charcoals from aqueous solutions involves a different mechanism. PVDC and Saran chars are prepared from polymers which contain no oxygen as a part of their chemical structure. Therefore, there is little possibility of any oxygen becoming bonded to the carbon structure of the chars during their preparation. The associated oxygen which is given out as COz and CO on evacuation is picked up by the chars during their subsequent exposure to air [l-4]. These chars, therefore, have little or no unsaturation of the type suggested by I Puri et aI.[4,5]. A small amount of this unsaturation, 0.0 0.5 1.0 1.5 2.0 2. however, may be created on outgassing of the charcZCIDITY,m.e.,/, coals. On the other hand, these eharcoais are highIy Fig. 1. Surface acidity of polymer carbons in relation to porous and have large surface areas [BET (NJ surhydrobromic acid formed. face areas varying between 800 and 1200m2/g] and micropore volumes. Therefore, it is quite reasonable to believe that a large portion of the adsorbed bromine is present in the micro capillary pores. A 1ooo”C are presented in Table 2. There was little or no conversion of bromine into HBr in case of the small decrease in bromine uptake on outgassing at 1OCKYC (rather than an increase due to the creation of outgassed samples. This is due to the fact that the unsaturated sites on outgassing) may be attributed to outgassed samples are entirely free of any associated oxygen which is responsible for their surface a reorientation of the microporous structure on heat acidity [l]. Thus the conversion of bromine into IIBr treatment in such a way that some of the micropores in the presence of charcoals is a function of surface become inaccessible to bromine molecules. In case of PF and IJF chars, which are prepared by acidity which varies with the amount of CO1-complex the carbonisation of oxygen containing polymers, present on the surface of charcoals. appreciable amounts of oxygen become bonded to the The amount of bromine fixed (see Table 2) generally decreases on outgassing the charcoals in most carbon surface during their preparation. In addition, cases. The decrease, however, is smaller in the case of as shown by Fitzer et al.[22], these chars contain
I’
I
I
I
Table 2. HBr formed and bromine fixed on treatment of 1OOO”C-outgassed polymer charcoals with aqueous solutions of bromine Ii& formed (m eq /6)
Sample IdentFfication WDWOO sarall~00 SaranbCO(Staam
A&at
Bromine f lxed (m eq / 9)
0.29
16.75
0.28
15.25
85O*C) 0.21
13.w
PF-MO
0.15
9.02
PF400
0.18
9.50
PP-600
0.00
2.75
PF-900
0.00
4.90
UF-400
0.19
8.04
UF-tjSO
0.16
r,.?5
W&5G
0.00
5.25
R. C. BANSAL,T. L. DHAMIand SAT PARKASH
398
Table 3. Desorption of bromine on heat treatment at different temperatures Sample Menttiication
PvDc-600 Sarantn-GOO(Staam A&at
Bromine fixed (m eq / g)
3.8.02 85O*C) 18.24
PF-500
10.04
UF-550
9.22
appreciable amounts of bonded hydrogen as methylene groups. In these chars, therefore, the adsorption of bromine may take place partly by substitution of hydrogen at the methylene groups and partly by addition at the unsaturation sites. When these chars are outgassed at lOOo”C, a greater portion of the bonded hydrogen gets eliminated [22] causing a decrease in the adsorption of bromine on the outgassed samples. A small increase in adsorption of bromine due to the creation of unsaturated sites by the elimination of associated oxygen is offset by a big decrease in adsorption of bromine by substitution. The amounts of bromine adsorbed by PF-900 and UF-850 chars before and after outgassing at 1000°C support this view. These chars have been given heat treatment at 900 and 850°C respectively during their preparation. Therefore, these chars are devoid of any methylene hydrogen available for substitution by bromine. Since these chars do not have large surface areas and micropore volume. [BET (N2) surface areas being 1.3 m*/g for UF-850 char and 106 m*/g for PF-900 char] it appears that the adsorption of bromine on these samples occurs largely by addition at the unsaturated sites which remain more or less unchanged on outgassing. The desorption of bromine on heat treatment of the brominated samples (Table 3) further supports the two different mechanisms involved in the adsorption of bromine. In case of PVDC and Saran charcoals as much as SO”/, of the adsorbed bromine is desorbed (where large portion of bromine uptake, we believe, occurs in the microcapillary pores), compared to very little desorption from PF and UF brominated chars (where bromine uptake is both by substitution at the methylene groups and by addition at the unsaturated sites). ~e~~~w~e~ge~e~r-one of the authors (T.L.D.) is thankful to CSJR India for the award of a Research Fellowship.
Bromine
desorbed on heat at (at e g)
treatment
5o”c
loo0 c
150°c
2oo”c
5.41
6.10
7.01
7.52
5.04
6.91
7.42.
8.E12
0.22
0.58
0.24~
1.10
o.l.4
0.57
0.72
0.95
REFERENCES 1. R. C. Bansai and T. L. Dhami, Cur&on l&65 (1978). 2. R. C Bansal, T. L. Dhami and S. Parkash, Carbon 16, 389 (1978). 3. R. C. Bansal and T. L. Dhami, Carbon 1979 (in press). 4. B. R. Puri. N. K. Sandle and 0. P. Mahaian. _ J. Chem. Sot. 4880 (1963). 5. B. R. Puri and R. C. Bansal, Carbon 3, 533 (1966). 6. R. S. Stearns and B. L. Johnson, Ind. Engng Chem. 43, 146 (1951). 7. V. A. Garten and D. E. Weiss, Aust. 3. Chem. 3, 68 (1955). 8. B. R. Puri, 0. P. Mahajan and D. L. Gandhi, Indian J. Chem. 10,848 (1972). 9. B. R. Puri, 0. P. Mahajan and D. L. Gandhi, Chem.
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