Adsorption of bromine by carbons from solution in carbon tetrachloride

Adsorption of bromine by carbons from solution in carbon tetrachloride

Carbon, 1977. Vol. IS, pp. 173476. Pergaman Press. Printed in Great Britain ADSORPTION OF BROMINE BY CARBONS FROM SOLUTION IN CARBON TETRACHLORIDE ...

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Carbon, 1977. Vol. IS, pp. 173476.

Pergaman Press.

Printed in Great Britain

ADSORPTION OF BROMINE BY CARBONS FROM SOLUTION IN CARBON TETRACHLORIDE BALWANT RAI PURI, D. L. GANDHI and

0. P. MAHmNt

Department of Chemistry, Panjab University, Chandigarh, India (Received 1 October 1976)

Abstract-Adsorption of bromine from solution in carbon tetrachloride by a number of carbon blacks as well as sugar charcoal is shown to be partly reversible and partly irreversible. While reversible adsorption appears to be due to surface effects, irreversible adsorption is found to be only partly due to chemisorption in substitution for hydrogen and addition at unsaturated sites. A fairly large amount of it seems to be entrapped within some of the pores of the adsorbents or imbibed by the particles and requires energy of activation to diffuse out of the system.

1.

INTRODUCTION

Interactions

of charcoals[l] and carbon blacks[2] with bromine in aqueous solution at ambient temperature has been shown to result in chemisorption of bromine at unsaturated sites arising from the elimination of acidic CO+omplex. Similar interactions, using bromine dissolved in organic solvents, have also been reported. Weiler[3], for example, using bromine dissolved in carbon tetrachloride observed appreciable chemisorption of bromine by coking coals. Brooks and Spotswood[4] extended similar work to chars prepared from bituminous coals and reported fixation of 9 bromine atoms per 100 carbon atoms. Watson and Parkinson[5] as well as Shooter[6], using solutions in carbon tetrachloride and some other organic solvents, also found evidence for reversible as well as irreversible adsorption of bromine by carbon blacks. However, no suitable mechanism to account fully for each type of adsorption has been suggested. It was thought of interest, therefore, to examine the problem a little more in detail by working with a number of carbon blacks as well as sugar charcoal before and after outgassing at different temperatures. 2. ExPJmtMENTAL Fourteen samples of typical carbon blacks as well as a charcoal obtained by carbonizing recrystallized cane sugar on the addition of concentrated sulfuric acid[l] were used. Two portions of carbon (1 g each) were mixed separately with 100ml lots of bromine solution of 0.35 N concentration in stoppered Pyrex-glass bottles (125 ml capacity) wrapped in thick black papers. The suspensions were allowed to stand, with occasional shaking, in a thermostat maintained at 35“ (kO.05”) for 24 hr. An ahquot of the clear supernatant liquid in the case of one of the suspensions was then examined for drop in concentration of bromine by adding potassium iodide and a known excess of sodium thiosulfate and back titrating against standard iodine solution in the usual way. The solution was then also examined for HBr I’Material Sciences Department, Pennsylvania State University, University Park, PA 16802,U.S.A. CAR Vol. IS. No. 3X

formed, if any, by titrating against standard NaOH. To the second suspension as a whole, potassium iodide and a known excess of sodium thiosulfate were added and the mixture allowed to stand in the thermostat for 30 min, which in a number of preliminary experiments was found sufficient to permit “desorption” of the reversibly adsorbed bromine. An aliquot was then back titrated against standard iodine in the usual way. The drop in concentration of bromine in the first case gave the total amount of bromine adsorbed while that in the second case gave the amount of bromine adsorbed irreversibly. The difference between the two gave the amount of bromine adsorbed reversibly. Hydrogen contents were determined by ultimate (micro) analysis, specific surface areas by the conventional low temperature adsorption of nitrogen (BET) and surface unsaturations by estimating bromine values [ 1,2]. 2. RMJLTSANDDEC!UsSION The values for reversible and irreversible adsorption of bromine by the various carbon blacks as well as sugar charcoal together with specific areas, hydrogen contents and surface unsaturations of the carbons are given in Table 1. It is seen that the values for both types of adsorption, for a given carbon, are of the same order. Reversible adsorption appears to be due to surface effects. This was checked by plotting the amounts adsorbed reversibly against the corresponding specific surface areas, as shown in Fig. 1. It is seen that leaving aside the value for Carbolac, which is rather low for the exceptionally large specific surface area of this carbon, a straight line passing through most of the points can easily be drawn. However, the area occupied per bromine molecule comes out to be 48.9A2which is nearly twice of the theoretical value, viz. 24.1A2, as obtained from bond lengths and Van der Waafs radii[8]. An appreciable amount of hydrogen bromide is seen to be formed in every case. This, evidently, can not be due to hydrolytic adsorption of bromine, as has been reported in the case of similar reaction with aqueous bromine [l, 21. In the present study, carbons as well as carbon tetrachloride had been well dried before use. It appears highly probable that hydrogen bromide is formed

B. R. PURIet al.

174

Table 1. Reversible and irreversible adsorption of bromine by carbon blacks and sugar charcoal of different specific surface areas, surface unsaturations and hydrogen contents

Surface areat

Hydrogen contentS (3)

Carbon (1) Mogul Mogul-A Elf-O Spheron-C Spheron-4 Spheron-9 Philblack-A PhilblackPhilblack-E Vulcan-SC PhilblackKosmos-40 Carbolac Sugar charcoal

480

308 228 171 253 153 116 46 116 135 194 80 31 839 412

510 471 332 472 620 350 243 310 140 310 350 470 2984

Bromine adsorbed reversiblyt (4)

Bromine adsorbed irreversiblyS (5)

HBr formed+ (bromine fixed by substitution) (6)

Surface unsaturationS (bromine fixed by addition at unsaturated sites) (7)

171 132 105 144 86 65 26 68 72 111 38 11 226 210

132 131 124 148 117 118 32 57 61 122 45 24 183 393

23 18 25 19 21 23 11 16 13 11 12 8 32 140

16 14 13 70 41 42 11 21 22 64 18 9 % 46

Bromine unaccounted for* [5- (6 + 711 93 99 86 59 55 53 10 20 26 48 15 7 55 207

tAll values except that for sugar charcoal are as reported by Studebaker[‘l]. SExpressed as miUieq/lOO g. Table 2. Effect of outgassing carbon blacks and sugar charcoal on reversible and irreversible adsorption of bromine Surface unsaturationt (bromine fixed by addition at unsaturated sites) (6)

Bromine adsorbed reversiblyt (3)

Bromine adsorbed irreversiblyt (4)

HBr formed* (bromine fixed in substitution for hydrogen) (5)

335 328

171 183 173

132 267 221

23 13 nil

16 76 74

93 178 147

Carbolac Original Outgassed at 600°C Outgassed at looo”C

839t 815 760

226 232 201

183 291 272

32 20 11

% 168 170

55 103 91

Spheron-C Original Outgassed at 600°C Outgassed at 1000°C

253t

290 281

144 173 160

148 183 174

19 15 10

70 88 82

59 80 82

Vulcan-SC Original Outgassed at 600°C Outgassed at looo”C

194t 202 198

111 118 104

122 154 152

11 nil nil

64

47 74 76

Sugar charcoal Original Outgassed at 600°C Outaassed at IOOO”C

412 637 388

210 282 201

393 557 433

140 119 nil

Surface areat Carbon (1)

(;;I”’

Mogul Original Outgassed at 600°C Outgassed at looo”C

308t

80 76

46

380 375

Bromine unaccounted fort

[4-(5+6)1

207 58 58

Whese values are as reported by Studebaker[‘l]. SExpressed as millieq/lOOg. in substitution

of bromine for hydrogen present in the

carbon blacks and charcoal by the reaction C-H + Br, + /

C-Br f HBr /

as has been suggested by previous workers[5] as well. This indicates possibility of fixation of an equivalent

amount of bromine in substitution for hydrogen in the various carbons. Reference to hydrogen contents included in Table 1 shows that only a small fraction of it gets replaced by bromine. In this connection, it may be noted that Anderson and Emmett[9] as well as Puri and Bansal[lO] are of the view that considerable proportion of the combined hydrogen in carbons is dispersed in more than one layer in the interior of the particles.

Adsorption of bromine by carbons from solution in carbon tetrachioride

175

Surface area, m*/g Fig. 1. Reversibly adsorbed bromine against surface area. Table 3. Bromine retained on heating various brominated carbons at diierent temperatures Bromine retained (millieq/1OOg) MOglll

Temperature (“0

Spheron-C outgassed ‘at 6M)“C

outgassed at 600°C

35t 37 40 55 100 150 200 250 300 350 450

183 t77 173 146 126 116 101 95 85 71 63

267 255 249 238 213 193 142 126 98 63 57

Sugar

charcoal outgassed at 600°C 557 548 538 538 485 468 453 431 409 371 343

tThe temperature at which irreversible adsorption of bromine was observed initially. Surface unsaturation of each carbon was found to fall to zero after bromination. This indicates fixation of an equivalent amount of bromine by addition at the unsaturated sites. It is observed that after accounting for fixation of bromine by substitution and addition processes, a large amount of the irreversibly adsorbed bromine still remains unaccounted for (Table 1). In order to get some more insight, four of the carbon blacks as well as sugar charcoal were outgassed at 6OO’Cand at 1000°C before interaction with the bromine solution. The results are given in Table 2. The corresponding values for the original samples are also reproduced from Table 1 for ready reference. It is seen that the amount of HBr formed falls appreciably and even becomes zero presumably due to fall in hydrogen content on evacuation. The amounts of bromine adsorbed reversibly and irreversibly both increase on evacuation at 600°C and then fall slightly on further evacuation at 1000°C.The changes in the amounts of bromine adsorbed reversibly may be

at~buted to co~esponding alterations in the values of surface area. The increase in irreversible adsorption is, evidently, due to increase in surface unsaturation (co1 7) due to elimination of CO,-complex[l]. However, it is seen again, that after accounting for chemisorption of bromine by substitution and addition processes, a fairly large amount of the irreversibly adsorbed bromine still remains “unaccounted for”. Garten and Weiss[l l] have suggested that the presence of surface quinonic groups at sites where they cannot participate in resonance also creates unsaturation in carbons and this may account for considerable chemisorption of bromine. This view, however, does not appear to be tenable as the amount of bromine that remains “unaccounted for” is more in the case of carbon blacks outgassed at 1000°C (which are essentially free of oxygen) than in the case of the original carbon blacks. It appears quite likely that this bromine is entrapped within some of the pores of the adsorbents or imbibed by the particles themselves, as has been suggested by Sinha and Walker[l2] in their work on adsorption of bromine vapor by anthracite, and that it requires energy of activation to diffuse out of the system. In order to check this view, three of the brominated carbons containing irreversibly adsorbed bromine were heated gradually in a current of Nz. Bromine vapor was observed to come off as soon as the temperature was raised even sliihtly above 35”C,the temperature at which adsorption had been studied. This continued with increasing temperature as shown in Table 3. The amount retained at 450°C may be a part of the bromine chemisorbed by addition at the unsaturated sites.

1. B. R. Puri, N. K. Sandle and 0. P. Mahajan, I. Chem. Sot. 4880 (1963). 2. B. R. Puri and R. C. Bansal, Carbon 3, 533 (1965). 3. J. F. Weller, Fuel 14, 190 (1935). 4. J. D. Brooks and T. McL. Spotswood, Proc. Fiffh Carbon Conf., Vol. 1, p. 415. Pergamon Press, Oxford (1%2). 5. J. W. Watson and D. Parkinson, Id. Eng. &em. 47, 1053 (1955).

176

B. R. PURI et al.

6. P. V. Shooter, quoted by J. J. Kipling, Adsorption from Solutions and Nonelectrolytes, pp. 66. Academic Press, New York (1%5). 7. M. L. Studebaker, Rubber Chem. Technol. 30, 1400(1957). 8. L. Pauling, The Nature of the Chemical Bond, pp. 225, 260, 3rd edn. Cornell University Press, New York (l%O).

9. R. B. Anderson and P. H. Emmett, J. Phys. Chem. 56, 753

(1952). 10. B. R. Puri and R. C. Bansal, Chem. Indust. 574 (1%3). 11. V. A. Garten and D. E. Weiss, Austral. J. Chem. 3,68 (1955). 12. R. K. Sinha and P. L. Walker, Jr., Fuel 52, 153 (1973).