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Ad s o r p t i o n o f G a s e s a n d Va p o r s o n C a rb o n M o l e c u l a r

S i e v e s

I. P. Okoye, M. Benham , and K. M. Thomas*,

Northern Carbon Research Laboratories, Department of Chemistry, Bedson Building,University of Newcastle upon T yne, Newcastle upon T yne N E1 7R U, U.K., and

Hiden Analytical Ltd ., 420 Europa Boulevard, Warrington, WA5 5UN , U.K.

Received October 14, 1996. In Final Form: Jan uary 21, 1997X

The adsorption phenomena of oxygen and nitrogen on a carbon molecular sieve were studied above thecritical temperature ofthe adsorptives as a function ofpressure in order to understand further t he mechanismof air separ ation. The upta ke of both gases studied was virtually linear a t low equilibrium pr essures, inagreement with Henrys law, but deviation occurred at higher pressur es. The adsorption kinetics werestudied with different a mounts of preadsorbed gas for changes in pressur e of 11 kPa and pa rtial pressur ein helium of10 kPa. The gas adsorption kinetics obey a linear driving force mass tr ansfer model. Therat ios of the r ate constan ts (kO2/kN 2) for each pressur e increment were 35-43 for pure gases an d 21-27for gas/helium mixtures, an d th ese ratios clearly demonstr ate t he m olecular sieving char acteristics. Thepresence of water vapor is detr imental t o the operation of carbon m olecular sieves. The adsorption an ddesorption char acteristics of water vapor with different amounts of preadsorbed wat er were stu died forcomparison with oxygen and nitrogen adsorption over the pressure range 0-1.8 kPa for pressure stepsof 0.1 kPa. The resu lts ar e discussed in ter ms of th e mecha nism of gas separ at ion usin g car bon molecularsieves.

I n t r o d u c t i o n

The use of carbon molecular sieves in the separationand purification of mixtures of gases with very similarmolecular dimensions is of great interest in the chemicaland petrochemical indust ries. Awide ra nge ofcommer cialcarbon molecular sieves (CMS) have been manufacturedbyvaryingthetypeofprecursors,method and temperatureof carbonization, a ctivation procedure, pore blockingmethod, and passivation techn iques. These carbon mo-lecular sieves can be prepared from coal, petroleum ,biomass, a nd polymericpr ecursors 1,2 and a re used widelyfor gas separat ion 3 and storage4 applications. A typicalapplication is the indu str ial separa tion of air int o oxygen

and n itrogen by pressure swing adsorption (PSA). Thecapacities of this class of molecular sieves for oxygen andnitrogen adsorption are very sim ilar, but the rates of adsorption differ considerably. The P SA technique isbased on the difference between the kin etics of adsorpt ionofoxygen and n itrogen with oxygen a dsorption being muchfa s t e r t h a n n i t r og en a d s or p t ion . T h is d iffe r en ce i nadsorption kinetics is thought to be related to molecularsize. The kinetic diam eter of oxygen (0.346 nm ) is slightlysma ller th an th at of nit rogen (0.364 nm ). When a car bonsam ple with m olecular sieving cha ra cteristics comes intocontact with air, an oxygen-enriched adsorbed phase anda corresponding nitrogen-rich gas phase are producedinitially.

The pr esent study involved a n investigation of th e

kinetics of oxygen and nitrogen a dsorption on a carbonmolecular sieve with various amoun ts ofpr eadsorbed gasfor a series of pressu re steps . The effects of th e presenceof helium gas on th e adsorption kinetics and capacity ofthese gases were investigated. The presence of watervapor in the air is very detrimental to the performanceof carbon m olecular sieve m a terials. Therefore the

adsorption of water vapor was also investigated forcomparison and to understand the mechanism by whichwater vapor interferes with the separation process.

E x p e r i m e n t a l S e c t i o n

M a t e r i a l s U s e d . The commercial carbon molecular sieve(CMS) used in the present study was supplied by Air Productsand Chemicals Inc., U.S. The CMS was prepared by carbondeposition on a microporous substr at e. Helium, nitrogen, andoxygen (99.99% purity), supplied by BOC Ltd, were dried bypassage through drying tubes containing activated silica gels.

M e a s u r e m e n t o f A d s o r p t i o n K i n e t i c s . The kinetic mea-surements were carried out using the Intelligent Gravimetric

Analyser (IGA) supplied by Hiden Analytical Lt d. The IGAinstr umen t allows the adsorption-desorption isother ms and th ecorresponding kinetics of adsorption or desorption at eachpressure step t o be determined.5 The system consists of a fullycomputerized microbalance which automatically measures theweight of the carbon sample as a function of time with the gaspressure and sample temperature un der computer control. Thecarbon sample was outgassed to a const ant weight at 383 K and10-8 Pa prior to measurement of the isotherms. The pressurean d t em p erat u re were t h en s et t o t h e d es i red v al u e u n d ercomputer control, and the weight uptake was measured as afunction of time under isothermal conditions until equilibriumwas at tained. The approach to equilibrium was m onitored inreal time, an d a comput er algorithm was u sed to esta blish when99% gas uptake was achieved. These weight versus time datawere used to calculat e the adsorption kinetic param eters . After

equilibrium was achieved, the pressure was increased to thenext desired value and the weight versus time monitored. Theprocess was repeated un til sufficient a dsorption dat a points wereobtained for t he isotherm. In t his technique th e adsorptionkinetics for a given pressure step were measured for differentamounts of preadsorbed gases a dsorbed at equilibrium on thecarbon adsorbent. Asimilar pr ocedure was used for th e nitrogen/helium a nd oxygen/helium mixtures. In th is case the par tialpressure of the gas was increased in increments similar t o thepure gases. The total flow ra te used thr oughout the experimentswas 100 cm 3 m in-1. The adsorption and desorpt ion data for watervapor were obtained in a similar m ann er by first increasing an dt h en d ecreas in g t h e p res su re i n i n crem en t al s t ep s . Kin et i cmeasurements were obtained for oxygen, nitrogen, an d wat er

* Corresponding author. University of Newcastle upon Tyne. Hiden Analytical Ltd.X Abstract published in Advan ce ACS Abstracts, J une 15, 1997.(1) Metcalf, J. E.; Kawahata, M.; Walker, P. L. Fuel 1963, 42 , 233.(2) Moore, S . V.; Trimm, D. L. Carbon 1977, 15 , 177.(3) Sircar, S.; Golden, T. C.; Rao, M. B. Carbon 1996, 34 , 1.(4) Verma, S. K.; Walker, P. L. Carbon 1992 , 30 , 837. (5) Benh a m , M. J .; Ross, D. K. Z. Phys. Chem. 1989, 25 , 163.

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vapor a t 293K only. The isotherms are typically repeatable tob et t er t h a n (1%.

R e s u l t s

The carbon dioxide adsorption isotherm at 273 K forth e car bon molecular sieve was Type 1. The sur face ar eacalculated from the Langmuir isotherm gave a surfacearea of242 m 2 g-1 (based on an area of 1.7 10-19 m 2

per molecule) while the micropore volume obtained byextrapolation of th e Dubinin-Radush kevich equat ion was0.152 cm 3 g-1.

Adsorption isotherms of nitrogen and oxygen at 293 Kdeterm ined using the IGA are shown in F igures 1a and1b, respectively. There is litt le difference in th e isotherm sfor oxygen a nd n itrogen a nd t he gases in t he pr esence ofhelium. The upt akes ofboth gases as a function of pressure

are approximately linear at low pressures, but deviationsfrom linearit y are observed as pressur e increases. Theadsorption temperatu re used in this study was above thecritical temperatures of both nitrogen and oxygen, andtherefore, it is not possible to express the pressures int e r m s o f r e l a t i v e p r e s s u r e s i n c e t h e s a t u r a t e d v a p o rpressure (Po) does not exist under the aforem entionedconditions. The virial plots for nitrogen and oxygenadsorption are shown in Figures 2 and 3, respectively.These graphs ar e linear over a ma jor par t of the pr essureran ge but deviate a t low-pressure where Henr ys law isobeyed. In the low-pressure region of th e a dsorptionisother m, small errors pr oduce large errors in the virialplots. Good agreemen t was obta ined for the values ofA 0a n d A 1 for the adsorption of nitrogen gas and nitrogen in

h e li u m, a n d t h e v alu e s a r e g ive n i n T ab le 1 . T h eagreement for oxygen adsorption virial parameter wasless satisfactory (see Table 1). This is due to a higherdegree of scatter in the data points for oxygen/heliumm i xt u r e s . T h e p a r a m e t e r s o bt a i n ed fr om t h e v ir i a lequation graph for n itrogen a re similar to th ose obtainedpreviously.6 The isotherm s calculated from the virialcoefficients for nitrogen and nitrogen/helium mixtures areshown in Figure 1a while the isotherms calculated foroxygen and oxygen/helium mixtures are shown in Figure1b. It is appa ren t tha t in all cases th ere is good agreementbetween the isoth erms calculat ed from the virial equat ionparameters an d th e experimentally determined isotherms.Figure 4 shows the graph of nitr ogen an d oxygen u ptak eversus time. Compar ison of Figures 1 and 4 shows thatth e adsorption capacities for oxygen an d nitr ogen ar e verysimilar while the rates ofadsorption differ markedly. Thedifferences in the rate constants for the adsorption of pure gases an d corresponding gas/helium mixtures for agiven pr essure in crement are comparat ively small com-pared to the differences in the rat es ofoxygen and nitr ogen.Graphs of ln(1 - w t/w e) against t im e t where w t a n d w e

(6) Cole, J. H.; Everet t, D. H .; Marsha ll, C. T.; Paniego, A. R.; Powl,J. C.; Rodriguez-Reinoso, F . J. Ch em. Soc., Faraday Trans. 1 1974, 70 ,2154.

Figure 1. (a) Adsorpt ion isother ms for nitr ogen on th e car bonmolecular sieve; (9) N 2, (b) N 2 /He; determined at 293 K.Isotherm fitting using virial equation parameters in Table 1;s N 2, - - - N 2 /He. (b) Adsorption isotherms for oxygen on thecarbon molecular sieve; (9) O2, (b) O2 /He; determined at 293K. Isotherm fitting using virial equation parameters in Table1, s O2, - - - O2/He.

F i g u re 2 . Virial plots for nitrogen adsorption on carbonmolecular sieve at 293 K; (9) N 2, (b) N 2/He.

F i g u r e 3 . Virial plots for oxygen adsorption on the carbonmolecular sieve at 293 K; (9) O2, (b) O2/He.

T a b l e 1 . V i ri a l C o n s t a n t s f o r Ad s o r p t i o n o f O x y g e n a n d

N i t r o g e n o n t h e C M S

A 1/g mol-1 exp(A 0) 10 9/mol g-1 P a-1

N 2 -921 ( 25 4.109 ( 0.002N 2/He -928 ( 27 3.909 ( 0.002O2 -736 ( 7 3.756 ( 0.001O2/He -1015 ( 116 3.99 ( 0.01

Ad sorption of Gases an d V apors on CM S L an gm uir, V ol. 13, N o. 15, 1997 4055
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refer to the adsorbate weight uptake at t im e t a n d a tequilibrium, respectively, for the uptake of both gases at293 K are shown in Figur e 5. It is appar ent tha t the gasuptakes for both oxygen and nitrogen on this class of molecular sieve follow a linear driving force mass transferkinetic model already discussed elsewhere,7 where w t/w e) 1 - e-kt, with linear graphs of ln(1 - w t/w e) againsttim e. The rate of nitrogen and oxygen upta ke can becompa red in terms ofth e pseudo-first -order ra te consta nt ,k, which is determined from the gradient of the kineticplot as shown in Figure 5. The full kineticdat a presentedin Table 2 indicate t hat th e rat e of upt ake of nitr ogen ism uch slower and involves longer equilibrium tim escompared with th at of oxygen. Compar ison of th e rateconstants for gases and the gas m ixtures in helium iscomplicated by th e slightly different pressu re increment s.However, th e rat e consta nt s for both n itr ogen an d oxygen

adsorption for a given pressure increment increase slightlywith increasing initial pressure both for pure gases andfor mixtures with h elium. The kinetic selectivity for thecarbon molecula r sieves can be obtained from th e rat io ofthe rate constants for nitrogen and oxygen (see Table 2).It is apparent that the ratio decreases with increasingpressure.

Figure 6 sh ows th e adsorption-desorption isotherm ofwater determ ined at 293 K. I t is evident t hat the waterisotherm is Type V an d rema rk ably different from tha t ofnitr ogen or oxygen. This is due to th e different m echan ismofads orption which involves initial adsorpt ion on prima ry

centers followed by the growth of clusters of watermolecules ar ound these cent ers. The adsorption-desorp-tion hysteresis is small in these carbon molecular sieves,which is in contrast to adsorption on activated carbonswhich h ave a wider pore size distribution.8 In addition,the steep increase in water vapor uptake around p/p0 0.3-0.4 is lower than for active carbons (p/p0 0.45-0.65)which

have a wider pore size distr ibution. Figure 7 shows graph sof ln(1 - w t/w e) versus time for water vapor uptake forpressu re st eps of (a) 0-100 Pa , (b) 1617-1718 Pa , and (c)91 1-1012 Pa. It is appar ent that th e graphs are stra ightlines for >9 0% of t h e u p t a k e . T h is con fi r m s t h a t t h eadsorption kin etics follow a linear driving force masstra nsfer model for the pressur es steps used. The resultsalso show that the r ate consta nt s differ ma rkedly for thedifferent pr essure increments. Figure 8 shows a graphofra te constant versus water vapor pressure. This graphshows th ree distin ct regions corresponding to specificprocesses in the water adsorpt ion mecha nism. The initial

(7) Chagger, H. K.; Nda ji, F. E.; Sykes, M. L.; Thomas, K. M. Carbon1995, 33 , 1411.

(8) Foley, N.J .; For shaw,P . L.; Thom as,K.M.; St ant on,D.; Nor m an,P. R. La n g mu i r 1997, 13 , 2083.

F i g u r e 4 . Vari at ion of t h e g as u p t ak e w it h t i me for t h eadsorption of nitrogen and oxygen on the carbon molecularsieve at 293 K. Pressure ran ges: (a) pure gases, 55-66 kPa;(b) gas/helium mixtures, N 2 40.3-50.2 kPa, O2 41.2-48.9 kPa .

F i g u r e 5 . Variation of ln(1 - w t/w e) ag ai n s t t i me fo r t h eadsorption of N 2 an d O2 on the car bon m olecular sieve at 293K. Pressure range: 55-66 kPa.

T a b l e 2 . K i n e t i c D a t a fo r N i t r o g e n a n d O x y g e n G a sA d s o r p t i o n o n C a r b o n M o l e c u l a r S i e v e D e t e r m i n e d

a t 2 9 3 Ka

(a) Pure Gases

pressure(kPa)

pure N 2(k/s 10-4)

pure O2(k/s 10-4) kO2/kN 2

0-11 2.143 ( 0.011 83.5 ( 0.7 39.011-22 2.20 ( 0.01 88.9 ( 0.9 40.422-33 2.325 ( 0.009 99.5 ( 1.0 42.833-44 2.452 ( 0.014 99.6 ( 1.1 40.644-55 2.612 ( 0.011 104.1 ( 1.2 39.9

55-66 2.824 ( 0.013 105.1 ( 1.3 37.266-77 2.971 ( 0.014 109.3 ( 1.4 36.877-88 3.134 ( 0.016 111.3 ( 1.5 35.588-99 3.226 ( 0.017 113.9 ( 1.6 35.3

(b) Gas Mixtur es

p a r t ia lpressureN 2 (kPa)

N 2/He(k/s 10-4)

partialpressureO2 (kPa)

O2/He(k/s 10-4) kO2/kN 2

0.5-1 0.5 2 .6 62 ( 0 .0 22 0 .4-11.4 58.2 ( 0.5 21.910.5-2 0.4 2 .3 13 ( 0.011 11.4-20.2 62.7 ( 0.4 27.120.4-3 0.8 2 .4 44 ( 0.013 20.2-29.6 65.3 ( 0. 4 26.730.8-4 0.3 2 .5 72 ( 0.011 29.6-41.2 67.8 ( 0.4 26.440.3-5 0.2 2 .7 21 ( 0.019 41.2-48.9 69.9 ( 0.4 25.750.2-5 9.9 2 .8 48 ( 0.014 48.9-60.5 70.1 ( 0.5 24.659.9-6 9.7 2 .8 24 ( 0.016 60.5-68.8 72.1 ( 0.5 25.5

69.7-7 9.4 2 .9 07 ( 0.018 68.8-80.3 73.0 ( 0.5 25.179.4-8 9.1 3 .0 50 ( 0.023 80.3-89.4 74.7 ( 0.5 24.589.1-100.1 3.407 ( 0.019 89.4-1 01 .2 7 1. 9 ( 0.4 21.1

a Error bars obtained from the kinetic graphs.

F i g u r e 6 . Adsorption-desorption isotherms of water on thecarbon molecular sieve at 293 K;(b)adsorption,(O)desorption.

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stage indicates a fast rate constant of adsorption onprimary sites, while the slowest rate constant occurs atintermediat e pressure ran ge and this corresponds to thegrowth of clusters of water molecules around t he primar ycenters with in th e pressure r ange of 0.7-1.2 kPa leadingto bridging between pore walls and pore fill ing. Theincrease in r ate constant at high vapor pressures >1.2kPa corresponds t o the final stages of pore filling. Thereverse ap plies for th e desorption ofwa ter m olecules fromthe carbon molecular sieve. The adsorption and desorptionpar am eters for wat er vapor on th e car bon molecula r sieveare given in Table 3. It is apparent from these data tha t

the a dsorption-

desorption h ysteresis is small while therates constant s for adsorption and desorption are verys im i la r ov er a g iv en p r e ss u r e r a n g e. T h e s i m il a r it ybetween the adsorption and desorption rate const an ts wasobserved pr eviously for a ctive carbons where t he a dsorp-tion/desorption isoth erms sh owed significan t hyst eresis.8

The total water uptake at p/p 0 0.76 was 6.05 mmol g-1

which corr esponds t o a pore volum e of 0.109 cm 3 g-1. Theshape of the water adsorption isotherm indicates thathigher relative hum idities will not lead to greatly enha ncedadsorption of water. The presence of the hydrophobicsur faces and the ultr a-microporous str uctur e of th e carbonmolecular sieve produces an adsorbed phase which has alower density than water. This needs to be considered inth e calculation of pore volumes u sing water adsorption

data , and t he pore volume calculation needs to be tak enin this context.

D i s c u s s i o n

Generally, little att ention h as been paid t o the adsorp-tion of gases by solids at tem peratures well above th ecritical temperature ofthe adsorptives. Sofar most studiesin the l i terature h ave been carried out at tem peratureswellbelow th e gas critical tempera tu re where th e amoun tsadsorbed are much higher and th e pressures under whichadsorption takes place can be compared relative to thesaturat ed vapor pressures.9 Above t he critical temper-atu re th eoretical int erpreta tion of the r esults is possible

in terms ofn onideal gas theory but comparisons ofvar iousgases are difficult. However, it is well established tha tat sufficient ly low coverage, ad sorption occur s via adsor-bate-solid interactions with the exclusion of pairwiseintera ctions between the adsorbed molecules. The exclu-sion of this latter phenomenon ensur es tha t H enrys lawis obeyed reasonably. Thu s at low surface coverages th eadsorption obeys the equation n ) KHp, where n i s t h eam ount adsorbed per unit m ass of adsorbent at equi-librium pressure p a n d KH is Henrys law consta nt. TheLangmuir theory degenera tes to a mere empiricism, andthe calculation and m eaning of the saturation uptake

(9) Rodriguez-Reinoso, F. R.; Garrido, J.; Martin-Martinez, J. M.;Molina-Sabio, M.; Torregrosa, R. Carbon 1989, 27, 23.

T a b l e 3 . K i n e t i c D a t a f o r Wa t e r A d s o r p t i o n o n t h e C a r b o n M o l e c u l a r S i e v e a t 2 9 3 K

a dsor pt ion desor pt ion

pressurerange (Pa)

capacitya

(mmol g-1)rate constant(s-1 10-4)

pressurerange (Pa)

capacitya

(mmol g-1)rate constant(s-1 10-4)

0-100 0.158 59.98 ( 0.02 106-1 0.235 15.29 ( 0.01100-202 0.277 33.85 ( 0.01 207-106 0.363 35.42 ( 0.02202-304 0.405 19.78 ( 0.02 309-207 0.505 25.45 ( 0.0130 4-404 0.562 19.70 ( 0.01 410-309 0.679 18.55 ( 0.0140 4-506 0.766 19.30 ( 0.01 511-410 0.903 12.11 ( 0.0150 6-607 1.05 10.89 ( 0.02 613-511 1.20 10.12 ( 0.0360 7-709 1.49 4.86 ( 0.01 714-613 1.66 3.92 ( 0.03

70 9-809 2.06 4.41 ( 0.01 815-714 2.28 3.84 ( 0.0180 9-911 2.82 3.57 ( 0.01 916-815 3.03 4.20 ( 0.0191 1-1012 3.61 3.53 ( 0.01 1017-916 3.77 3.13 ( 0.03

1012-1114 4.26 3.93 ( 0.01 1116-1017 4.42 3.42 ( 0.041114-1212 4.78 3.25 ( 0.03 1217-1116 5.05 3.39 ( 0.011212-1314 5.27 3.88 ( 0.01 1319-1217 5.46 3.25 ( 0.011314-1414 5.52 6.73 ( 0.01 1420-1319 5.63 5.78 ( 0.041414-1515 5.67 8.76 ( 0.02 1521-1420 5.76 8.80 ( 0.031515-1617 5.79 10.96 ( 0.03 1622-1521 5.87 9.08 ( 0.011617-1718 5.90 10.08 ( 0.04 1723-1622 5.97 11.03 ( 0.011718-1820 6.05 10.59 ( 0.03 1820-1723 6.05 13.76 ( 0.01

a Refers to higher pressure in range.

F i g u r e 7 . Variation of water vapor uptake with time on thecarbon molecular sieve at 293 K. Pressure ranges: (a) 0-10 0

Pa; (b) 1617-1718 Pa; (c) 911-1012 Pa.

F i g u r e 8 . Variation of rate constant for water adsorption-desorpt ion on th e car bon molecula r sieve as a function of vaporpressure at 293 K; (9) adsorption, (b) desorption.

Ad sorption of Gases an d V apors on CM S L an gm uir, V ol. 13, N o. 15, 1997 4057
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(monolayer coverage) under this condition become com-pletely erroneous an d difficult t o inter pret. As a r esult,a pseudo or apparent monolayer instead of a true valuei s o b t a in e d fr om t h e l in e a r g r a p h of p/n versus p .Nicholson and Sing10 analyzed adsorption data using avirial equation and reported that at very low pressuresvirial expansions reduce t o Henrys law. The virialequation can be written in two forms:6

where n is the am ount adsorbed at pressure p a n d t h efirst coefficient of the t wo equations a re related by

T h e t e r m K0 is equal to t he Henrys law constant andtotallyd ependent on the intera ction between th e adsorbentsurfaces and the a dsorbed gas m olecules. Under t heconditions of the present study, low pressure a nd lowsur face coverage, it is meanin gless to extend t he an alysisbeyond the first and second term s in eq 2. Fu rth ermore,higher terms in eqs 1 and 2 are only important at muchhigher pressures. The values ofA 0 can be obtain ed fromthe gra phs of ln(n/p) versus n at a series of temperatur esT. Also from th e slope of th e plot of ln K0 against 1/T, thelimiting isosteric heat ofads orption can be estimat ed. Coleet al. investigated6 the adsorption of nitrogen, krypton,and xenon on various carbons over the temperature range27 3-398 K. The values of exp(A 0) a n d t h e A 1 values fornitrogen adsorption were in the range from 1.18 10-9

to 7.55 10-9 mol g-1 P a-1 and from 89 to 640 g mol-1,respectively. Compar ison of th ese values for exp(A 0) and

A 1 with th e values obtained in t his stud y at 293 K showstha t they are similar. Stud ies ofth e adsorption of oxygenand nitrogen on another sample from the same batch ofC M S o v e r t h e p r e s s u r e r a n g e 0-0.9 MPa gave virialequation graphs similar to th ose in Figures 3 and 4.11 Th egraphs h ad excellent linearity a nd A 1 values in th e range-67 4 ( 5 and -96 6 ( 5 g mol-1 for oxygen and nitrogen,respectively. The results also showed tha t th e kineticsfollowed a linear mass transfer kinetic model over thew h ol e p r e ss u r e r a n g e a n d t h a t t h e r a t e con s t a n t s foradsorption increase with increasing pressure over thepressure range 0-0.9 MPa for pr essur e steps of 0.1 MPa.

The carbon molecular sieve under study has a bimodalpore size distribut ion with a n ar row micropore size ran ge.Adsorption will occur at low relative pressures in t he ult ra-micropores, which are the more energetic sites.12-14 Th ekinetic results in Figures 4 a nd 5 and in Table 1 showclearly t he more r apid kinetics for oxygen a dsorptioncom pared with nitrogen, but both gases have sim ilar

adsorption capacities. Pr evious adsorption kineticst udiesshow that oxygen can penetrate the carbon pore structurem ore rapidly th an nitrogen.7,15,16 This is related to th emolecular size oft he oxygen which is slightly sma ller tha n

th e crit ical pore dimens ion responsible for th e selectivityof th e car bon molecular sieve. In cont ra st th e molecula rdimension ofnitrogen is slightly larger than oxygen, thusthere is a larger barrier for diffusion into the porousstructure.

The adsorption rat e constant s for N 2 a n d N 2 in He arevirtually ident ical within experimental error. However,the a dsorption r ate consta nt for O2 is significant ly highertha n tha t for O2 in helium. This suggests tha t, in the casewhere th e ra tes of adsorption a re fast, bulk diffusion inthe pores is a factor that influences the rate ofadsorption.

Ra o et al.17 have carried out mathematical modelingstu dies of th e diffusive potent ials within carbon molecularsieves. The model was based on that developed bySt eele18

for th e int era ction of gases with solid sur faces and involvedthe sum m ation of the a tom-atom interactions over theent ire solid. The intera ction was described by a Lenna rd-J ones function with pa ram eters derived from th e Lorent z-Berthelot com bining rules. The a pproach allowed th ediffusion of gases to be characterized along the porecenterline ofslit-shaped pores whose walls were composedofbasal planes. They concluded tha t twobarr iers existed,(1) ent ering t he pore an d (2) diffusion along th e pore, andtha t th e rate-limiting process was ent ry thr ough the poreaperture. The calculations pr edicted th at nitrogen an dcarbon dioxide diffused at similar rates in the pores. Theenergy barrier for pore entry for carbon dioxide was zerowhereas for nitrogen it was 24 kJ mol-1, indicating th atthe entry into the pore was the rate-lim iting process.Previous studies7 oft he adsorpt ion ofoxygen and nitr ogenat a tmospheric pressure on CMS similar to th e one usedin this study over the tempera tu re ran ge 275-333 K ha veshown differences in the pre-exponential factors andactivation energies. The activation for nit rogen adsorpt ionwas 34.6 kJ m ol-1, while the corresponding value foroxygen was 30.9 kJ mol-1. The pre-exponen tial factor fornit rogen was 650 s-1 wher eas th e correspondin g value foroxygen was 3500 s-1. T h e a c t iv a t ion e n er g y for t h ediffusion ofcarbon dioxide into th e car bon molecular sievewas mu ch lower,12 kJ mol-1. Theseresults showsimilartren ds to the theoretical studies. The carbon molecularsieve used in th is study was prepar ed bycarbon deposition.In this type of m aterial, the carbon deposition on thenonselective substr ate is heterogeneous a nd as a conse-quence reduction in the part icle size resu lts in the gra dua lreduction of the molecular sieving characteristics due tothe production of nonselective pat hways for gas adsorp-tion.7

Adsorption and desorption isotherms for water vaporon the CMS (see Figure 6)a re given in terms oft he amoun tadsorbed versus relative pr essure, p/po, h e r e p o i s t h esatu rat ed vapor pressur e ofwat er. The isoth erm for watervapor was t ype V.19 It is apparent from Figure 8 that t headsorption and desorption rat e constan ts determ ined fromwat er vapor adsorp tion on the car bon molecular sieve were

smaller than those ofoxygen and similar tonitrogen whilethe am ounts adsorbed were much larger even th ough thevapor pressur es were low compar ed with the gas pressures.The tr ansport ofwater molecules into th e carbon molecularsieve is different from oxygen and n itrogen, and t his ma ybe due to th e following reasons: (1) the plana r sh ape ofthe water molecule which will affect transport throughthe selective porosity an d (2) the a dsorption mechan ismwhich is different from that of both oxygen and nitrogen.The water adsorption isotherm and kinetics are similar

(10) Nicholson, D.;Sing,K. S. W. Colloid Science; Ever et t ,D.H.,Ed.;Chemical Society: London, 1979; Vol. 3, pp 1-62.

(11) I. P. OKoye and K. M. Thomas unpublished results.(12) Carrott, P. J. M.; Roberts, R. A.; Sing, K. S. W. Carbon 1987,

25 , 59.(13) Dubinin, M. M. In Characterisation of Porous Solids ; Gregg, S.

J., Sin g, K. S. W., Stoeckli, H. F., E ds.; Society of Chemical Indust ries:London, 1979; pp 1 -11 .

(14) Dubinin, M. M.; Astakhov, V. A. Adv. Chem. S er. 1971, 10 2, 69;J. Colloid Interface Sci. 1980, 75 , 34.

(15) Koresh, J.; Soffer, J. J. J. Chem . Soc., Faraday Trans. 1 1980,76, 2472.

(16) Nandi, S. P.; Walker, P. L. Fuel 1975, 54 , 169.

(17) Rao, M. B.; Jen kins, R. G.; Steele, W. A. Ext. A bstr. Programs Biennial Conf. Carbon 17th, 1985, 114.

(18) Steele, W. A. Surf. Sci. 1973, 36, 317.(19) Gregg, S. J.; Sing, K. S. W., Eds. Adsorption, Su rface Area, and

Porosity 2nd ed.; Academic Press: New York, 1982.

n/p ) K0 + K1p + K2p2+ ... (1)

ln (n/p) ) A 0 + A 1n + A 2n2+ ... (2)

K0 ) exp(A 0) ) H en r ys L aw con s t a nt (3 )

4058 L an gm u ir, V ol. 13, N o. 15, 1997 Ok oye et al.


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to those observed previously for an active carbon.8 Alsothe CMS has a relatively high uptake of water vapor.These observat ions suggest th at m olecular sieving effectsare of limited significance for water vapor adsorption inth is case. The adsorpt ion kinetics were found to occur inth ree stages over the pressure ra nge start ing with a veryfast uptake at the initial stage occurring on the primaryadsorption sites in th e carbon micropore stru ctu re. Ass h ow n i n F i gu r e 8 , i n t h e l ow -p r e ss u r e r e gi on , fa s tadsorption occurs a t low su rface coverage on pr imaryadsorp tion sites, for examp le, oxygen functionality in t he

carbon stru ctu re. Water molecules are thought to adsorbnot only on the available empty sites but also on adsorbedwater molecules forming groups of clusters around theprimar y sites. In th e int ermediate stage it was proposedthat these groups of clusters will eventually connect,bridging between pore walls which effectively approachpore filling at sufficiently high pressures. 20 At higherrelat ive pressu res, some cooperat ive effects are appa ren t.The adsorption of water vapor in significant quantitieswill lead to the f il l ing of available porosity and thisadsorbed wa ter will be difficult to desorb du e to th e slowkinetics for th e major part of the a dsorbed water upt ake.It h as n ot been possible to measur e oxygen a nd n itrogenadsorption in th e presence of water vapor due to the mu chlower quantities of oxygen and nitrogen adsorbed com-

pared with wat er vapor. In view of rates of adsorption-desorption being slowest in the pressure range wherechanges in adsorption capacity are greatest, it is likelytha t the detriment al effect of water vapor on molecularsieving is related to water vapor kinetics and blocking ofavailable porosity.

The amount of water vapor adsorbed at p/p o 0.76 wasequivalent to a pore volume of 0.109 cm 3 g-1, while themicropore volume obtained from t he Dubinin-Radush-kevich graph was 0.152 cm 3 g-1. The shape of the watervapor adsorption isotherm suggests that the capacity willn ot i n cr e a se g r ea t l y w it h fu r t h e r i n cr e a s e i n v a porpressure. The observation that the pore volum e deter-mined from water vapor adsorption was usually lowert h a n t h e t ot a l p or e v ol u m e ob t a in e d fr om n i t r og enadsorpt ion has been report ed previously.8,21-23 Values forthe r atio have ran ged from 0.22 to 0.9. In t he case of acarbon with a low extent of activation a value greaterthan 1 was obtained, and this was ascribed to activateddiffusion effects leading to an anomalouslylow pore volumecalculated from nitrogen adsorption at 77 K. In th e caseof adsorption of CMS m aterials t he total pore volum ecannot be obtained from nitrogen adsorption at 77 Kbecause of activated diffusion effects and the microporevolume determined from CO2 adsorption data is used forcom parison. The ratio of the water vapor uptake t o themicropore volum e was 0.72. Pr evious stu dies by Bra dleyet al.22 observed low values (0.22 and 0.28) for th e ra tiooft he pore volume estimat ed from water vap or adsorptiona t p/p

00.9 to total pore volume obtained from nitrogen

adsorp tion for two commer cially available active car bons.However, other studies8,21,23 have reported values in thera nge 0.7-0.8 for th e ratio which are similar to the resultsobtained for the carbon molecular sieve.

The rea son for th e lower pore volume det ermin ed fromwater vapor adsorption may be explained by differencesin th e stru ctur e of adsorbed water compar ed with water.T h e r e a s on for t h i s i s p o ss ib ly r e la t e d t o t h e w a t ermolecules associated with prima ry adsorpt ion center s andadjacent to hydrophobicsu rfaces have different st ru cturesto water. Carrot et al. have proposed that the low ratio

of water vapor uptake compared to nitrogen adsorptionfor Silicalite was due t o the inability of water to form athree-dim ensional hydrogen-bonded structure in thecylindrical micropores which have a diameter of