J.C.S. Faraday I, 1980,76,2507-2509Molecular Sieving Range of Pore Diameters of AdsorbentsBY JACOB KORESH AND ABRAHAM SOFFER*Atomic Energy Commission, Nuclear Research Center-Negev,P.O. Box 9001, Beer-Sheva, IsraelReceived 19th November, 1979The very sensitive molecular-dimension criterion of adsorption selectivity of molecular sieves changesrapidly to a molecular-mass criterion upon a slight enlargement of the adsorbent-pore dimension. Thisresulted in the inversion of the sequence of adsorbability of hydrogen and oxygen on carbons ofincreasingly wider pores. Considerable care must be taken in relating molecular-sieve effects on adsorp-tion to molecular dimensions of adsorbates.Adsorption on highly porous media may have molecular sieving effects. Accord-ingly, the adsorbability of molecules as well as some other adsorptive propertiesmay be different, and the question is how to discriminate molecular sieves (MS)from wide-pore adsorbents.Such a discrimination is of ultimate importance when-ever molecular dimensions are to be assessed from adsorbability on MS. MS areendowed with the unique property of discriminating very sharply between mol-ecules of similar width.’ Molecules which differ by merely 0.2-0.3 8, in width maybe adsorbed at rates which vary by several orders of magnitude.’ A difference of0.3 8, results in an apparently complete exclusion of the larger molecules; this isseen from adsorption isotherms as well as from studies of adsorption kinetics.l Thissensitivity is, however, rapidly lost once the pores are too open compared with themolecular width, but still in the ultramicroporous range ( < 7 8,).2-4 Under thesecircumstances, adsorbability as observed by both adsorption kinetics and isothermsmay be controlled by factors specific to open surfaces, in addition to moleculardimensions, and the assessment of the latter will consequently be impaired.The aimof this communication is to determine at what degree of pore opening does molecu-lar sieving change to “ordinary” adsorption on a microporous adsorbent.This work is essentially based on the ability to widen the ultramicroporousstructure to any desirable extent by mild activation steps. By such operations thecomplete closure of pores to a certain adsorbate may be gradually changed viamolecular sieving stages into the completely free, non-selective admission into widepores.EXPERIMENTALThe adsorption apparatus, methods of modification of pore dimensions and characteristics ofthe carbon starting materials have been described elsewhere.RESULTS AND DISCUSSIONSIn a previous study of the molecular dimensions of adsorbate molecules, hydro-gen molecules appeared to be wider than C02, acetylene and oxygen.’ This surpris-25025080.50.4iMI 03-E-ia 02-101-MOLECULAR SIEVE CARBONS------_ * A aa I 1 I I l I I l I I I I L I I I I I 1t/min1 2 3 4 5 6 7 8 9 1 0I I I 1 I I I I I IA A J AAAA ** A *A *aaaa*1A 0FIG.1.-Adsorption kinetics of oxygen on carbon C-200 and of oxygen and hydrogen on carbon C-300at 77K.Initial pressures 6&65Torr; sample weight 100mg. Tad = -196°C A, 02, 300"C,V, = 86.95 pmol min-'; *, Hz, 300"C, V, = 150 pmol min-l; 0, 02, 200"C, V, = 1.8 pmol min-'.ing result was in accordance with the average dimension calculated from the liquidmolar volume but not with the kinetic diameter calculated from gas-phase molecu-lar theory5 or from van der Walls radii.6 On the other hand, hydrogen has beencommonly considered smaller than the above higher-weight molecules, and studiesof the adsorption kinetics on mordenite MS presented by Barrer7 seem to supportthis view, since the sorption rate of hydrogen was far greater than that of oxygen,nitrogen and argon and close to that of helium.The following brief comparativestudy of the adsorption rates of hydrogen and oxygen on molecular sieve carbon(MSC) appears to settle this controversy. In fig. 1, the adsorption rate of oxygen ona MSC designated C-200 is plotted. On the same carbon and under the sameconditions, hydrogen is not adsorbed to any measurable amount (our experimentalresolution for adsorption was <0.1 ymol g-' for an adsorption rate <0.1 ymol g-'s-'). On the other hand, for the slightly wider pore, C-300 carbon, the rate ofhydrogen adsorption is at first faster than that of oxygen, and then becomes slowerthan it. The slowing down of H2 adsorption rate with elapsed time is a manifesta-tion of the much lower adsorption isotherm of H2 compared with that of 02.The high ratio between the adsorption rates for the C-200 carbon is undoubtedlytypical of the molecular sieving effect and shows repeatedly' that oxygen behaves asa thinner molecule than hydrogen. The inversed ratio of initial rates for the C-300carbon is not as great as that for the C-200 carbon.Recognizing furthermore thatthe C-300 carbon accomodates nitrogen,' which is a wider molecule than eitheroxygen or hydrogen,8 we are led to the conclusion that the pores of the C-300carbon are too wide to discriminate between oxygen and hydrogen. The MS selec-tivity of the C-200 carbon is, therefore, changed into a selectivity of wide pore solid,into which the light hydrogen molecule diffuses faster than oxygen. The largJ . KORESH AND A . SOFFER 2509decrease in the hydrogen adsorption rate with elapsed time is probably due to theproximity of the saturation of the adsorbent with hydrogen which should have amuch lower physisorption isotherm than that of oxygen.One should thereforeexamine the width of the C-300 carbon pores as compared with the dimensions ofoxygen and hydrogen, which are 3.28 and 3.44A, respectively.8 Xenon was com-pletely excluded from this carbon, indicating that its pores are narrower thanatomic diameter of xenon which is 3.94 A. Therefore an increase of only a fewtenths of Angstrom in pore dimension is sufficient to cancel the molecular sievingeffects of ultramicroporous solids. It is probably the large difference between themasses of hydrogen and oxygen which makes their adsorption selectivity rapidlyexceed the molecular sieving criteria. Nevertheless, great care must be taken incomparing the adsorbabilities of molecules of different dimensions on a single MSwhich can accomodate all of them. In case of MSC C-300 and C-200, it is theability to change the pore diameters very gradually which enable us to “meet” themolecular dimension very closely and observe the true molecular sieving sequenceof oxygen and hydrogen.J. Koresh and A. Soffer, J.C.S. Faraday 1, 1980,76,2457.*Y. Toda, N. Yuki and S. Toyoda, Carbon, 1972, 10, 13.3J. R. Dacey and D. G. Thomas, Trans. Faraday Soc., 1954, 50, 740.4S. S. Barton, M. J. B. Evans and B. H. Harrison, J . Colloid lnterface Sci., 1974, 49, 462.’G. L. Kington and A. C. Macleod, Trans. Faraday Soc., 1959, 55, 1799.’R. M. Barrer, Quart. Rev., 1949, 3, 293.* J. Koresh and A. Soffer, J.C.S. Faraday I, 1980, 76, 2472.L. Pauling, Nuture of the Chemical Bond (Cornell Univ. Press, Ithaca, N.Y., 3rd edn, 1960).(PAPER 9/1844