lDdjaD Journal of ChemistryVol. 19A, December 1980, pp.l146·1148

Molecular Sieve Properties of Polymer Carbons : Part I - Adsorp-tion of Nitrogen & Carbon Dioxide

R. C. BANSAL· & T. L. DHAMIDepartment of Chemistry, Panjab University, Chandigarh 160 014

and

SAT PARKASHFuel Sciences Division, Research Council of Alberta, Edmonton Alberta, Canada

Received 23 November 1979; revised and accepted 3 May, 1980

Adsorption isotherms of nitrogen at 77 K and of carbon dioxide at 195 K and 273 K on polymer charcoalsare generally of type-I of the BET classification. The adsorption is much larger in the case of polyvinylidenechloride (PVDC) and Saran charcoals than on polyfurfuryl (PF) and urea-formaldehyde (UF) charcoals. About90-95% of the total adsorption in the case of PVDC and Saran charcoals takes place at relative pressures lowerthan 0.2. This indicates that these charcoals are highly microporous. The surface areas of PVDC and Sarancharcoals obtained from adsorption of nitrogen at 77 K and from CO. adsorption at 273 K are comparable but thenitrogen surface areas of PF and UF charcoals are extremely small and several orders of magnitude lower thantheir CO. surface areas. These charcoals contain ultrafine micropores and only a part of this microporosity isaccessible to nitrogen at 77 K.

A number of workers+P studied the adsorptionisotherms of nitrogen at 77 K on Saran andPVDC charcoals before and in some cases

after activation at different temperatures. Theyfound that the adsorption of nitrogen was very fastand that appreciable amounts could be adsorbed atsaturation. The surface areas calculated from nitro-gen adsorption were comparable to those obtainedfrom CO2 adsorption at 273 K, indicating that thepores in these charcoals were large enough to ac-commodate both the nitrogen at 77 K and CO2at 273 K.

The internal porous structure of a carbon dependsupon the history of its formation and the startingmaterial from which it has been obtained. Since ourcharcoals were prepared from different polymer pre-cursors and since polymer carbons are consideredpotential carbon molecular sieves, it was thoughtworthwhile to study the adsorption isotherms ofnitrogen and carbon dioxide wi th a view to under-standing the molecular sieve behaviour of polymercarbons.

Materials and MethodsPolymer carbons were obtained by the carboni-

sation of four different polymers, viz., polyfurfurylalcohol (PF), polyvinylidene chloride (PVDC), urea-formaldehyde resin (UF) and Saran (a copolymer ofPVDC and PVC, 9:1) under different experimentalconditions.

Preparation of carbons - Polyfurfuryl char wasprepared following the method of Marsh and Wynne-Jones", In this method IN HCI (0.6 ml) was addedslowly and with vigorous shaking to furfuryl alcohol

1146

(100 ml). The acidified material was poured into asilica container and heated for 20 hr at 65°C and foranother 20 hr at 140°C. The resulting char wassubsequently carbonised in vacuo to the requiredtemperature (400-900°C).

The preparation of PVDC char involved poly-merisation of redistilled vinylidene chloride sealedin a pyrex bulb under fluorescent light for eightweeks. A hard polymerised vinylidene chlorideobtained was carboni sed under N:>.at 600°C (5°C!min) for 6 hr.

Saran charcoal was prepared by carbonising Saranpolymer tablets (I em diam.) in a resistance tubefurnace in vacuo. The polymer was heated gradually(5°C/min.) to a temperature of 180°C, and kept atthis temperature for 2 hr. The temperature wasthen raised to 600°C where it was maintained for12 hr. The sample was cooled to room tempera-ture in vacuo and washed with hot distilled watertill free from HCI.

For UF char, urea-formaldehyde resin was mouldedinto small tablets (1 em diam.) and carbonised ina resistance tube furnace under nitrogen atmosphere.The temperature of the furnace was raised gradually(5°C/min) to the required temperature and keptthere for another 12 hr. The sample was cooledunder nitrogen atmosphere.

All charcoals were powdered and passed througha 100 mesh sieve.

Adsorption measurements - Adsorption of nitrogenat 77 K and of carbon dioxide at 195 K and 273 Kwere studied volumetric ally using a self-fabricatedequipment after the well known BET apparatus.All charcoals were evacuated (10-4 torr) in situ

BANSAL et al. : MOLECULAR SIEVE PROPERTIES OF POLYMER CARBONS

at 11O°Cfor about 4 hr prior to adsorption measure-ments.

Results and DiscussionAdsorption isotherms - The adsorption isotherms

of nitrogen at 77 K on different polymer charcoalsare presented in Fig. 1. The adsorption occurs intwo steps; a slow but large initial adsorption followedby rapid but relatively smaller adsorption yieldinggenerally a parabolic isotherm with steep initialslopes. The isotherms for PF and UF charcoalsare not truely parabolic and show a rise in adsorptionat relative pressures near saturation . PVDC andSaran charcoals adsorb appreciably larger amountsof nitrogen than PF and UF charcoals although allthese charcoals have been prepared in the sametemperature range (600-650°C). The amount ofnitrogen adsorbed on PVDC charcoal is seventytimes larger than that adsorbed on UF charcoal andabout eight-times larger than that adsorbed on thePF charcoal. Furthermore, a larger proportion(90-95 %) of the total adsorption in the case ofPVDC and Saran charcoals takes place at relativepressures lower than 0.2. In the case of UF charcoal,however, appreciable amounts of nitrogen are notadsorbed until a relative pressure of 0.9 is reached.Since all these charcoals have been prepared at 600-650°C under identical conditions, it appears that theinternal porous structure of these charcoals dependsupon the starting material from which they have beenprepared. It may be mentioned that PVDC, sinceit contains stochiometric amounts of hydrogen forthe liberation of hydrochloric acid, on carbonisationproduces a charcoal with strong crosslinking betweenthe randomly oriented elementary crystallites result-ing in the development of a highly porous structure.

1-<>-<><:>- PVDC- CHARCOAL2 -6-6-6- SARAN- CHARCOAL3.•...•..•. SARAN-CHARCOAL STE •••• ACTMITED IIif 850·C~ PF-CHARCOAL5 -<HH>- UF - CHARCOAL

3

uuaw2IIIII:

£a.• 150I-z6~ 100

50

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200300 400 500 600 800£Q\jILIIIRIUM PR£5SUR£ (TORR)

Fig. 1- Adsorption isotherms of nitrogen on different polymercharcoals at 77 K

The nitrogen adsorption isotherm on Saran charcoalactivated in steam at 850°C is also included in Fig. 1.This isotherm is above the isotherms for Sarancharcoal or even PVDC charcoal at all relative pres-sures. This is consistent with the views presentedearlier by Kipling and Wilson", that activation ofcharcoal in steam results in opening up of some of thecapillary pores and increasing the length of theinternal capillaries by removing volatile matter.These pores could now accommodate larger amountsof nitrogen at the same temperature.

The adsorption isotherms of nitrogen on PF andUF charcoals prepared by carbonisation at diffe-rent temperatures (400-900°C) were also determined.The adsorption was found to be maximum in thecase of charcoals prepared at 600°. Lower adsorp-tion in the case of samples prepared at 400°C has beenattributed to improper carbonisationv-P whereasa slight decrease in adsorption in the case of thesample carbonised at 900°C is due to thermalshrinkage-s .

Adsorption isotherms of CO2 on polymer charcoalsat 273 K are presented in Fig. 2. It is seen that allthese charcoals adsorb appreciable amounts of CO2although the amounts adsorbed on PVDC, Saranand steam activated Saran charcoals are larger.The shape of the isotherms is essentially type-Iof the BET classification although the isothermsshow a slight increase in adsorption at higher relativepressures. The isotherms do not show a well-definedknee as in the case of nitrogen adsorption isotherms.

Surface area - The surface areas of different poly-mer charcoals were calculated using Langrnuir-s,BET15, Dubinin and Polanyi'" equations. Thesaturation vapour pressure and molecular area ofCO2 at 273 K were taken as 26, 140 torr (ref. 17)and 18.7 A 2 (ref. 18) respectively while that fornitrogen at 77 K were taken as 751 torr (ref. 19)and 16.2 A'2 (ref. 19) respectively. These surfaceareas are given in Table 1. It is seen that the surfacearea calculated from the three equations using eitherthe adsorption of nitrogen at 77 K or the adsorptionof CO2 at 273 K are different. This may be attri-

I •••.•••••• PVDC- CHARCOAL

2 _ SARAN-CHARCOAL

3 •......• SARAN-CHARCOAL STEAM ACTIVATED AT 850·C

4_ PF-4oo

5 ••.....•• PF- 600

Ii __ PF-900

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200 300 400 500 600, 700 100

EQUILIBRIUM PRESSURE (TORR)

Fig. 2 - Adsorption isotherms of carbon dioxide on polymer 1charcoals at 273 K

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INDIAN J. CHEM., VOL. 19A. DECEMBER 1980

TABLE1.- SURFACE AREAS OF POLYMER CHARCOALSBYNITROGENAND.CARBONDIOXlDRADSORPTION

Surface area (mo/g)

Sample N2 adsorption at 77 K CO. adsorption at 273 K

BET Dubi- Lang- BET Dubi- Lang-nin muir nin muir

PVDC-6oo 887 1076 1086 . 903 1017 1005Saran-6oo 787 1091 1097 798 1065 1049Saran-6oo· 1188 1298 1290 1138 1212 1250PF-600 105 151 138 288 359 297PF-900 106 133 141 163 229 296UF-400 1.0

(23.2)t 1.6 1.6 103 150 132UF-650 1.2

(27.2)t 2.9 3.2 198 246 232UF-850 1.3

(21.5)t 2.3 2.8 225 344 325

*Steam activated at 850°CtValues in parenthesis refer to N. adsorption at 195 K

buted to the varying assumptions made during thederivation of these equations.

The surface areas of PVDC, Saran and Steamactivated Saran charcoals calculated from' nitrogenadsorption at 77 K using anyone of these equationsare comparable with the surface areas obtained fromCO2 adsorption at 273 K using the same equation.However, in the case of PF and UF charcoals thenitrogen surface areas are extremely low as com-pared to their CO2-surface areas - the two valuesdiffering by a factor of several powers of ten in manycases. It is evident that UF and PF characoals,.in general, and UF charcoals in particular haveultrafine pores. The accessibility of nitrogen intothese pores is hindered because of the activated diffu-sion effects. Thus CO2, which is adsorbed at a muchhigher temperature, can have access to a larger pro-portion of the pores in these charcoals.

This receives further support from the fact thatwhen the surface areas ofUF charcoals was measuredby adsorption of nitrogen at 195 K, the BET values(shown in parentheses in Table 1) increased consi-derably. At higher temperature the rate of diffusion

1148

of nitrogen into ultrafine microcapillary pores in-creases so that the number of molecules entering thepores in a given time increases. This type of inacces-sibility to nitrogen adsorption at 77 K resulting in lowvalues of monolayer capacity has been observed byseveral workers in coals15,18,20 and several molecularsieve materials5,8,1O. Thus the polymer charcoalsprepared in the present study are promising aspotential carbon molecular sieves.

AcknowledgementOne of the authors (TLD) is thankful to the CSIR,

New Delhi for the award of a junior fellowship.

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