J.C.S. Farahy I, 1980,76,2457-2471Study of Molecular Sieve CarbonsPart 1.-Pore Structure, Gradual Pore Opening and Mechanism ofMolecular SievingBY JACOB KORESH AND ABRAHAM SOFFER"Atomic Energy Commission, Nuclear Research Center-Negev,P.O. Box 9001, Beer-Sheva, IsraelReceived 30th August, 1979The pore structure of a fibrous carbon molecular sieve has been studied by adsorption of COz,0 2 , CzHz, H2, N2, CO, Xe and SF6 as molecular probes. Apart from the negligible outer surfaceof the fibres, all adsorption sites possess molecular sieving properties. Mild activation steps enablethe graduated opening of critical pore dimensions in the range 3.1-5.681, which keeps adsorptionselectivity between molecules varying by merely 0.2 8, in cross-section 9 100 : 1 .The pore openingis effected by removing surface groups as COz and CO due to degassing at temperatures from 100to 700°C and by burning off skeletal carbon atoms in air at 400-450°C. Degassing at temperatures> 800°C leads to pore closure due to sintering. Removal of surface atoms must result in porewidening by steps as large as a few Angstroms, in contradistinction to the observed graduated poreopening.It is anticipated, therefore, that the fine discrimination between molecules of similar dimensionsis of kinetic-statistical origin, so that molecular sieving by pores, substantially greater than the mole-cules considered, is possible. The detailed model is based on the existence of a few rate-determiningconstrictions close to the outer surface of the fibres and of wider pores composing the major part ofthe pore volume.How-ever, constriction structure allows only a finite width of slits, still within a few Angstroms.High adsorption stereospecificity over a wide pore dimension range has enabled the studiedadsorbates to be ordered in a sequence of increasing critical molecular dimensions, which does notalways correspond with estimates based on gas-phase collision diameter or on bond length andVan der Waals radii.Adsorption behaviour of the flat benzene molecule suggests slit-like pores.Molecular sieve carbons compose a relatively new area in surface chemistry.Clear evidence of this property was observed by Franklin who found that fluidswere excluded from heat-treated coals in the order of their molecular size.found a reversal of the normal thermodynamic temperature dependence of physicaladsorption of small gaseous molecules on coals, a phenomenon related to activatedadsorption into pores a few Angstroms in dimension. laterpublished a thorough study of the molecular sieve properties of polyvinylidenechloride (PVDC) charcoal.Since then the subject has constantly been investigatedand revie~ed.~? As stated by Walker et aZ.,4 carbon molecular sieves may bepreferred to inorganic molecular sieves because of their stability at higher tempera-tures and in acidic media and also because of their lower affinity to water. Molecularsieve carbons are also attractive from the practical point of view. Molecular sievecarbons can be easily obtained by pyrolysis of many thermosetting polymers such aspoly(viny1idene chloride) (PVDC),3 poly(furfury1 alcohol),h cellulose, cellulosetriacetate, * saran copolymer, aswell as various coals such as coconutshell.12Pore structure can be opened by slight or extensive activation (oxidativeburnoff),13* l4 or closed by high temperature treatment in vacuum or under an inert2457MaggsDacey and Thomaspolyacryloni trile,l O and phenol formaldehyde2458 MOLECULAR SIEVE CARBONSgas atm0sphere.l The latter means of pore dimension design was best demonstratedby Lamond et a1.l5 who were able to reduce the pore dimension of a PVDC carbonin a few steps to 4.3 A by progressively increasing the high temperature treatment.Thus the gradual changes in pore structure which can be effected on mineralmolecular sieves by partial cation exchange l6.l 7 are also obtainable on carbons bymodifying the high temperature treatment. Since molecular sieve carbons are non-crystalline materials, their pore structure below 5A cannot be studied by X-raydiffraction analysis as is the case with most mineral molecular sieves. Transmissionelectron microscopy, on the other hand, is still insufficiently established for suchsmall pore structure dimensions. Analysis of the adsorption isotherms of molecularprobes of different cross-sections remains the most effective method for studies ofmolecular sieve carbons. By this method molecules of smaller cross-section arephysically adsorbed in preference to Iarger molecuIes.This is a highly sensitivemethod as demonstrated by numerous data where great differences in adsorbabilityare exhibited by molecules of very similar dimensions.l49 18* In this work a newapproach ,towards ultramicropore development in carbon is presented, by which,judging from molecular probe adsorption behaviour, critical pore dimensions of asingle starting material can be gradually increased for quite a large span, while thebandwidth of the pore distribution function remains very sharp. Some questionsconcerning pore structure and geometry are answered. Part of these deals with theouter surface roughness, the shape of pore cross-section and the change of cross-section along the pores.EXPERIMENTALAPPARATUSA conventional volumetric high vacuum system was used.Pressures were measuredwith a BLH type DHF transducer (0-20 p.s.i.). The gauge was calibrated by a ConsolidatedVacuum Corp. Mcleod gauge and a mercury differential manometer to an accuracy and aresolution of kO.1 Torr. The volumetric part had only glass and Teflon high vacuumstopcocks in order to allow work with organic gases. The analogue output of the pressuregauge was recorded so that fast adsorption kinetics, not easily followed by adsorptionbalances, could be monitored.MATERIALSCarbon cloth TCM 128 was supplied by Carbone-Lorraine, France. The density ofTCM 128 was 1.3 g cm3. Nitrogen and argon were Matheson " prepurified " products.Carbon monoxide was a research grade lecture bottle from Matheson. N2, Ar and COwere passed through a liquid nitrogen trap.High purity hydrogen and oxygen were pro-duced by thermal decomposition of uranium hydride and potassium permanganate, respec-tively. Carbon dioxide, acetylene and sulphur hexafluoride from Matheson were passedthrough a trap at -80°C and then cooled down to liquid nitrogen temperature and freedfrom permanent gases by evacuation. Benzene from Merck, A.R. grade, was dried bysoaking it in an excess of P205 within the vacuum line.Since several kinds of heat treatments were applied the following nomenclature wasadopted. In the case of only high temperature evacuation. the designation C-"C was used,for instance (2-200 for a carbon evacuated at 200°C. Unless specified otherwise, evacuationlasted 17 h. The designation for air oxidation treatment is Ox-time of oxidation inhours - temperature of oxidation in "C; the oxidation was performed in a moderate airoxygen stream over a carbon sampled in a tubular furnace, thermostatted to +2"C.Thegas flow rate, which was limited to avoid any temperature gradient along the sample,exceeded by far any reasonable uptake. Considerable care and accuracy were necessaryduring the oxidative activation in order to obtain reproducible adsorption specificityJ . KORESH AND A . SOFFER 2459Accurate oxidation times and temperatures were secured by first passing deoxygenated argonover the sample until temperature control was achieved, and then switching to oxygen or airflow, as necessary. The initial rates Vo tabulated in this paper were calculated from accurateoriginal expanded scale records of pressure against time plots and not from the figurespresented.Precautions were taken to use samples from the same batch of a few hundred g,while making the comparative study of adsorbability of different molecules. Differentbatches vary slightly in the activation conditions necessary to obtain the same adsorptiveproperties.RESULTS AND DISCUSSIONGRADUAL OPENING OF PORE STRUCTUREOPENING TO COz ADSORPTIONThe original carbon was found to be filled with water up to a volume of 0.12 cm3per g carbon. By pumping out the water at room temperature for 17 h, a productdesignated C-25 is obtained which, of the gases mentioned in the experimental section,adsorbs only carbon dioxide at the extremely slow rate shown in fig.1. Water canbe readsorbed into this carbon up to the same volume filling as that of the originalcarbon. Progressively increasing rates of CO, adsorption are achieved after furtherevacuation steps at increasing temperatures, as shown for 3 steps in fig. 1. Thecomplete series of initial rates are summarised in table 1. The overall lo4 foldincrease in kinetics is noteworthy.0.50.4UI M0 c3 8 0.3 z0.20.1-00- 0- A- aA0 AAA-0AAAa A0 8 a .AA AAtlminFIG. 1.-Adsorption kinetics of C 0 2 at 195 K on TCM 128 carbon fibres evacuated at varioustemperatures. Initial pressures in all experiments were 68-70 Torr. 0, 200°C, Vo = 21.93 pmolmin-I ; A, 100°C, V, = 8.23 pmol min-' ; ., 25"C, Vo = 0.8 pmol min-'.OXYGEN ADSORPTIONUptake of oxygen commences from C-200 and above on the high temperaturetreatment scale.Oxygen adsorption on C-200 is shown in fig. 2, together with acurve for C 0 2 adsorption reproduced from fig. 1. The pore structure is more ope2460 MOLECULAR SIEVE CARBONSto COz, and this is shown by the greater adsorption kinetics. Selectivity in favourof C02 is much greater in the more closed C-100. In this case, only a maximum of0.03 mmol per g oxygen can be adsorbed at an equilibrium pressure of 40 Torr,*while C 0 2 adsorption (fig. 1) amounts to 0.5 mmol g-1 at a pressure of 1.5 Torr.TABLE IN INITIAL RATES OF C02 ADSORPTION ON TCM 128 CARBON FIBRES AFTER VARIOUSHIGH TEMPERATURE TREATMENTSdesignation ofhigh temperature initial ratetreatment /pmol min-' g-l25°C-17 h 8 x lo-'100°C-17 h 8.237OO0C-17 h 2 .1 9 ~ 10'250°C-4 h 2.22x lo2750°C-21 h 7.81 x lo23OO0C-17 h 8.1 x 103..+ *0.1500Oao6* 0.035 10 20 30 LO 50 60 M 80t/minFIG. 2.-Adsorption kinetics of O2 on C-200 carbon at 195 K. Initial pressure 70 Torr. A, COz,Vo = 21.93 pmol min-' ; a, 02, Vo = 9 pmol min-', Tevac = 200°C.ACETYLENE ADSORPTIONThe degree of adsorption of acetylene at 195 K into the ultramicropores wasnext to that of oxygen. Thus the C-200 carbon, which adsorbs considerable amountsof oxygen, again exhibits only a minute outer surface adsorption of acetylene( z 0.024 mmol g-I at 51 Torr). Upon a slight increase in high-temperature treat-ment, acetylene penetrates into the pores of the resulting C-250 and C-300 carbonsas shown in fig.3. As with C02, the measurements with C2H2 were quite detailed,demonstrating again the very fine pore opening upon slight increase in high tempera-ture treatment.* Even this amount is attributed to the outer surface as discussed laterJ . KORESH A N D A . SOFFER-0 -A@In1 1 1 1 1 1 l 1 1 1 1 1 1 1 1 124610.05s- AA0 1 I I I I I 1 I- I M -0.3100 AA 0. A00AI 8010 50 100 150tlminFIG. 3.-Adsorption kinetics of acetylene at 195 K on C-250 and C-300 carbons.70 Torr, the C-300 refer to the upper time scale.Initial pressure0, 250°C, 21 h, Vo = 21.7 pmol min-' ; A,300"C, Vo = 360 pmol min-' ; W, 250"C, 5 h, V, = 2.16 pmol min-'.NITROGEN AND ARGON ADSORPTIONAccording to criteria of adsorbability identical to those of COz, O2 and C2H2,the series of adsorption experiments at 195 K was extended to carbons after furtherincreasing the evacuation temperatures.Thus argon and nitrogen adsorptions at195 K commence on the C-300 carbon and not on the C-250 carbon which adsorbsacetylene. Argon and nitrogen are adsorbed at about the same rate (fig. 4), i.e., theyhave similar critical cross-section diameters.00 A0 AA 0O At/min70 Torr. 0 , N2, Vo = 7 pmol min-' ; A, Ar, Vo = 4 pmol min-'. Tad = -80°C.FIG. 4.-Adsorption kinetics of argon and nitrogen at 195 K on C-300 carbon. Initial pressur2462 MOLECULAR SIEVE CARBONSA m *- Y 0' AW* aI I I I 1 I I 1 I 1CRITICAL DIMENSION OF HYDROGEN AND CARBON MONOXIDEIn order to eliminate thermal activation effects on adsorbability of variousmolecules, isotherms must be taken at the same temperature. Due to its low heatof adsorption,20 hydrogen cannot be practically adsorbed at 195 K, whereas manygases which differ greatly in volatility can. A comparative study of the adsorptionkinetics of 02, H2, N2 and CO was therefore performed at 77 K in order to placeH2 and also CO on the scale of critical molecular dimensions.Results are given infig. 5. Here again, stereospecificity is very high. Nitrogen adsorption on C-300carbon was at least two orders of magnitude lower than that on C-400 carbon at anytime. The tiny amount that adsorbs on C-300 is attributed to adsorption on theoutersurface of the fibres.Adsorption of oxygen commences at 77 K on the C-200carbon but that of hydrogen only on C-250. One can confidently deduce fromthese results that hydrogen is a larger molecule than oxygen. Also CO is slightlysmaller than N2 so that the sequence O2 < H2 < CO < N2 is obtained. The place-ment of H2 after O2 first suggested here is surprising.0-0 *O.lt-0 ******AmArntlminFIG. 5.-Adsorption kinetics of 02, H2, CO and N2 on carbon samples. O2 and H2 results aregiven for carbons at lowest high temperature treatment which enable their adsorption. Initialpressures in all samples were 65-70 Torr. 0, CO (4OO0C, 4 h), Vo = 32 pmol min-' ; *, N2(400"C, 4 h), Vo = 5 pmol min-' ; A, H2 (250°C), Vo = 5.5 pmol min-' ; 1, O2 (20O0C), VO =1.8 pmol min-'.Tad = 77 K.MECHANISM OF PORE OPENING BY HIGH TEMPERATURE EVACUATIONHigh temperature evacuation was found to be effective for pore opening up to400°C where adsorption kinetics increase considerably with evacuation time (table 2).From this temperature up to 800°C only slight changes in pore dimensions occurred.Thus xenon, the next larger molecule which we tried, can penetrate at a very slowrate into carbon C-700 but not into C-400. Quicker rates cannot be achieved byhigher temperature evacuation. Furthermore, pore closure by sintering takes placJ . KORESH AND A . SOFFER 2463above 800"C, as shown in table 2. Complete pore closure towards nitrogen adsorp-tion at 77 K is effected after evacuation at 1200°C.One can conclude that the mildpore opening by high temperature evacuation is due to the removal of surface oxygengroups as CO, and CO. The fact that it essentially terminates at = 400°C suggeststhat, at higher temperatures where surface groups can still be degassed as CO,,lsintering may already be taking place. The processing of organic fibres to producethe TCM 128 cloth includes high temperature treatment at temperatures as high as1200°C.22 This implies that initially the carbon cloth has no oxygen groups, sincethese are completely degassed above 1000"C.21 Exposure of the carbon to air isresponsible for the pore closure by oxygen chemisorption. We have observed thiseffect and so exposure of treated samples to air was avoided.TABLE 2.--INITIAL ADSORPTION RATES OF NITROGEN AT 77 K SHOWING OPENING AND CLOSUREOF ULTRAMICROPORES OF ?'CM 128 CARBON AFTER VARIOUS HIGH TEMPERATURE EVACUATIONStemp / "C 400 400 400 1000 1100 1200duration/h 1 4 17 2 3 3adsorption rate/ m o I g-l min-l 0.7 5 3240 200 13 0PORE OPENING BY MILD OXIDATIONA question of interest is now whether the well known oxidative activation can beused for a further extension of pore critical dimensions to achieve the same finegradation as that obtained by the high temperature evacuation.In order to meetthis goal, we chose air oxidation at z 400°C rather than steam or carbon dioxideactivation which can be performed only at considerably higher temperatures. Theresults of oxidative activation are given by the adsorption isotherms of xenon andsulphur hexafluoride presented in fig.6. Activation for 1 h (Ox-1-400 carbon) allowsthe adsorption of xenon but not of SF6, which requires 2 h activation at 420°C(Ox-2-420 carbon). The spherical molecules which have been used so far are Ar,Xe and SF6. The diameters of these molecules calculated from their liquiddensities 23 are 3.6, 3.94 and 5.02 A, respectively. The results show that evacuationat 400°C never opens the pores for xenon adsorption. Thus, diameters of3.6 < d/A< 3.94, 3.94 < d/A< 5.02 and 5.02 < d/A are suitable for the C-400,Ox-1-400 and Ox-2-420 carbons, respectively. The oxidative activation stepsdescribed above are therefore not as finely graduated as those of evacuation.Webelieve, however, that oxidation at lower temperatures and for longer times wouldalso enable fine and stepwise pore opening as for high temperature evacuation.ORDERING OF MOLECULES BY THEIR CRITICAL DIMENSIONSWe have demonstrated the possibility of almost continuous pore opening so thathigh stereospecificity in the adsorption of molecules of critical cross-section diametersranging from 3.1 to 5.6A is achieved. To our knowledge such continuous enlarge-ment has not yet been produced for any molecular sieve starting material. Thisfinding makes it possible to construct a cross-section diameter sequence for greatlydiffering molecules. Thus we deduce from the expderimental results obtained sofar that the critical molecular dimensions of the molecules studied change as follows :H20 < C02 < 0, < C2H, < H, < CO < N2 = Ar < Xe < SF62464 MOLECULAR SIEVE CARBONSA 0l I 1 I I I I 1 1 1 1 1 1 1 1 1 1 1 1 11 5 10 15 20p /TorrFIG.6.-Adsorption isotherms of xenon on carbon Ox-1-400 (0) and of SF6 on carbon Ox-2-420 (A).As far as we could determine, all available data for adsorbability on molecular sievematerials correlate with the above sequence. For instance, the preference of oxygenand carbon dioxide over nitrogen has been found for both ultramicroporous carbon l4and zeolites.24 Also, preference for carbon dioxide over acetylene has been reportedfor zeolites. This ordering can in principle be extended to any other adsorbatewhich can ultimately be accommodated by the pore structure.These results do notthoroughly correspond with collision diameter data,2 nor with bond lengths andvan der Waals radii.26 Discrepancies particularly arise with H, and C 0 2 . Thesewill be discussed in a following paper, in addition to an assessment of the numericalvalues of critical molecular dimensions based on geometrical considerations.PORE STRUCTUREAfter showing in the last section that apparently continuous widening of criticalpore dimensions can be effected by mild steps of evacuation, the elucidation of thepore structure which gives such a capability becomes of profound interest. In thefollowing section, activation procedures and adsorption isotherms will be carefullyanalysed in order to clarify the pore geometry.CONSTRICTIONS A N D PORE OPENINGAmong the first questions arising about pore structure is whether the critical poredimension is constant across the entire depth or whether there are a few constrictionswhich determine the sieving properties and which are followed by wider dimensionsinto which the adsorbate molecule is free to move.A better understanding of thepore opening by high temperature treatment would be of help in this respect. Hightemperature evacuation opens the pores by the removal of surface groups, resultingin an enlargement of pore volume. Degassing performed at temperatures not higherthan 400°C would remove mainly surface groups in the form of CO,. The porevolume increase which causes pore opening should therefore be :where AW, is the weight loss per g of carbon and dcoz is the solid density of COzJ .KORESH AND A . SOFFER 2465In cases of oxidation activation followed by 1 h evacuation at 400°C, the weight lossis the sum of the loss of carbon due to burnoff and that of CO, due to evacuation.The total experimental pore volume increase will therefore be :AW, AW,AVp= -+----&Oz dgraphitkTable 3 gives the experimental relative pore volume increments AVp/V,, due to hightemperature treatment. The pore volume Vp in the case of only high temperaturetreatment degassing was taken as that of total pore volume filling obtained from theC02 isotherm on C-300 carbon. In the case of oxidative activation, Vp was the totalpore volume filling by nitrogen. Weight losses were based on the dehydrated C-25carbon. If a constant pore width was prevailing for the pore system and enlarge-ment of pores by activation occurred evenly, i.e., by peeling off a layer from the pores,pore opening from a dimension which hardly enables CO, adsorption (C-100) to onewhich hardly enables N, adsorption (C-300) would require a relative pore dimensionincrease given by :for slit-shaped pores, and by :which is of higher values, for cylindrical pores.The magnitudes of 0 are the cor-responding critical molecular diameters. The values of (A Vp) /( V,), calculated accord-ing to eqn (3) are also given in table 3. The molecular diameters (a) which servedfor this calculation were estimated as 3.1 8, for C 0 2 , 3.28 A for 02, 3.44 8, for H2,3.59A for N2, 3.9411, for Xe and 5.0211, for SF,.23*Table 3 shows that the calculated values of (AVp)/(Vp), are z 3.5-6 times greaterthan the experimental ones, which is in contradiction to the model of homogeneouspore dimensions.This behaviour, on the other hand, can satisfactorily be explainedby assuming that, apart from few constrictions responsible for the molecular sievingeffect, the pore structure is relatively wide. Hence, a widening of the constrictionsonly (by high temperature treatment), and not of the whole pore depth, is necessaryto introduce increasingly larger molecules, so that experimental relative pore volumeincrements can be considerably below the values given in eqn (3) and (4). This isthe case encountered here. The portion of the total porosity occupied by thenarrower constriction may be at most equal to :f p = AVp -1- Q-Qcozv p Qcozprovided the whole relative volume increase is associated with it.Since in generalactivation is assumed to proceed over the whole pore walls, the above ratio, whichaccording to table 3 is z 17-28 %, is the ultimate value. The contribution of con-striction to the total porosity should therefore be considerably below that value.Once the constriction model is adopted, several unresolved peculiarities can beelucidated.* The diameters (T for the globular SF6 and Xe were calculated from molar densities of liquids.Shape correction factors were used for the other molecules2466 MOLECULAR SIEVE CARBONSTABLE 3 .-EFFECT OF HIGH TEMPERATURE TREATMENT ON RELATIVE PORE VOLUME EXPANSIONAND WEIGHT LOSS FROM TCM 128 CARBONtreatmentdegassing oxidation relative poreweight volume expansion largestmolecule/h /"C /h /"C (%) experimental calculated adsorbeddesignation time temp time temp loss ( %)c-100c-200C-250(2-250C-300c-400C-700OX-1 -400ox-2-420OX-1-450OX-2-4500x4450~ ~17 100 0.1 117 200 0.544 250 -17 250 0.7517 300 0.9717 400 1.192 700 1.31 400 2.52 420 5.51 450 11.32 450 18.64 450 29.40.351.742.423.133.834.17.812.715.221.426.6-co25.8 0 2C2H211.0 H215.8 N2N227 XeXe61.9 SF6SF6SF6SF6ABRUPT OR GRADUATED PORE OPENINGPore closure by sintering can basically be carried out to any extent.13 l 5 Unlikesintering, however, pore opening by degassing of surface groups or by burnoff ofsurface carbon atoms can be performed only uia abrupt steps, because of the atomocityof the material.Hence, removal of a surface group, or slight burnoff, must leavebehind a space of a few A units, similar to the dimension of the leaving group, whichresults in abrupt steps of local pore widening. This apparently contradicts theexperimental facts, where consecutive mild activation enables pore widening in stepsof at most 0.2A, as observed by our molecular probe adsorption experiments.Examples are the C-200 carbon which showed practically absolute selectivity for theadsorption of 0, in preference to N,, although both molecules differ by only 0.2Ain their van der Waals radii and by 0.3 A as calculated from both liquid density and 0values given above.Also C-400 carbon discriminates completely between N2 andXe while their diameters differ by only 0.1 or 0.33 A as calculated from liquid densityand Q values, respectively. The fact that molecular sieving carbons are quiteamorphous makes such high selectivity even more surprising since, unlike zeolite,their pore dimensions are not inherently determined by crystal structure. Further-more, and again because of abrupt abstraction of surface atoms, it is hard to imaginehow pore dimensions could be kept constant within +O.l A upon enlargement from3.1 A for CO, adsorption up to 5.02A for SF, adsorption. Presuming that thecritical pore dimensions' distribution function is indeed coarse, we anticipate that thehigh stereospecificity of molecular sieving carbons towards adsorbates, at least withinthe range studied (3.1-5.6 A), originates in kinetic-statistical effects rather than in thegreat homogeneity of pore dimensions.According to this approach there exists alayer composed of a series of a few critical passages or constrictions close to the outersurface of the adsorbent fibre. The contribution of this layer to the overall porosityis negligible, as is the amount adsorbed into it. The passage of a molecule througJ . KORESH AND A . SOFFER 2467this layer from the gaseous phase into the bulk of the porous adsorbent is the rate-determining step for adsorption. The probability for a molecule to pass throughsuch a layer is :NP(N) = Pi (6)i = 1where Pi is the probability of passing one constriction, i.For the sake of simplicityeqn (6) can be averaged to :P(N) E! pN. (7)In terms of the kinetic theorywhere Ea is the activation energy for passing one construction and p o is a frequencyfactor ; combining eqn (7) and (8) we obtainwhich shows that N constrictions in series produce the effect of multiplying N timesthe activation energy. Let us consider two adsorbate molecules A and B, which arevery close in their critical cross-section diameters. The larger one requires a slightlylarger activation energy for being " squeezed " through a constriction. However,although the difference AEAB can be quite small for molecules of similar dimensions,it is amplified N times due to the series of constrictions at the rate-determining layer.This relation is immediately obtained from eqn (9) written for each molecule.Hence,the ratio PA/PB between the adsorption rates of the two molecules assumes the form :P = PO exp -Ea/kT) (8)P(N) = p t exp (-NE,/kT) (9)Therefore, great sieving selectivity can be exhibited even if the molecular sieve has afairly wide pore distribution function.According to this model, criteria for adsorbability are kinetic in nature and do notoriginate from the true sieving effect. They should therefore be based on somepractical time of adsorption. Thus, a characteristic time of adsorption longer thansay 300 h determines a non-adsorbing molecule and a time shorter than 3 h determinesan adsorbing molecule.According to eqn (lo), N(EA-EB) = 1800 cal mol-1 at195K. For N = 10, EA-EB is merely 180calmol-l. Recognizing that E,originates in forcing a molecule to climb up the steep repulsive segment of its adsorp-tion potential, the value of 180 cal mol-1 is indeed a small difference, but neverthelessit suffices to provide great sieving selectivity owing to the multiplier N.Widening of the pores by a few A due to the abstraction of surface groups shouldby itself lead to an inhomogeneity of pore dimensions, i.e., to narrow and wide zones.However, only the constrictions in the layer close to the outer surface determine theadsorption kinetics. The molecules should be much more free to move deep intothe particle. Otherwise molecules of increasingly greater dimensions would still beadsorbed by passing fewer critical constrictions and the high stereospecificity wouldbe greatly impaired.The question of whether the characteristics of the outer layeroccur generally or are specific to the TCM 128 carbon are still under investigation.CONSTRICTION AS EVIDENCED BY THE ADSORPTION RATE OF co,It was shown above (fig. 1, tables 1 and 3) that following mild activations involvingonly 3.3 % of the relative pore volume expansion, the initial rate of adsorption of C 0 2could be increased by four orders of magnitude. Even more striking is the fact tha2468 MOLECULAR SIEVE CARBONSby evacuation for 4 h at 25"C, 12 wt % of adsorbed water are lost, whereas by evacuationfor a further 17 h, resulting in undetectable weight loss, C 0 2 adsorption is renderedpossible, as shown in fig.I. These observations can readily be explained on thebasis of the pore structure suggested above, namely a slight surface group removal issufficient to reduce N in eqn (9) to a value which allows a detectable adsorption rate.Homogeneous pore structure, on the other hand, would require much greater surfacegroup removal, which would be measurable, than could be obtained by prolonged0.40.3Lz-0.20 *I000*000I I I I I I I 1 1 I I l l t l l t l l l50 100 150 200t/minFIG. 7.-Adsorption rates of benzene on (2-300 and C-400 carbon at 25°C. Yo = 12 pmol min-'. n 1 TFIG. 8.-Dimensions of the benzene niolecule calculated from bond lengths and van der Waals radiiJ .KORESH AND A . SOFFER 2469evacuation at 25 or 100°C. A similar multifold is exhibited by acetylene (fig. 2) andby nitrogen (table 2). We believe that similar behaviour will be seen whenever anadsorbate is examined after several successive activation steps.157 10- M -- 5 - 8 k5 -GEOMETRY OF PORE CROSS-SECTION : ADSORPTION OF BENZENEA - - - a --a - A - AA0 -a0 - ---I 1 1 1 1 I I I I0.05 0.1 0.2 0.3 0.4The adsorption of the planar benzene molecule commences on the C-300 carbonat an extremely slow rate (fig. 7) and becomes faster and more significant on the C-400carbon. Benzene is considerably greater in width than the SF6 molecule (fig. 8)although its thickness is considerably smaller. Nevertheless, benzene is adsorbedon carbons from which sulphur hexafluoride is completely excluded.This behaviourindicates that the constrictions in the TCM 128 carbon are slit-shaped. Accordingto these results, the height of the slits in the C-300 carbon is < 5.02& which is thediameter of SF6 as calculated from its liquid density, and > 3.7& the thickness ofthe benzene molecule as obtained from Pauling’s van der Waals radii.AA0aFIG. 9.-Adsorption isotherms of nitrogen at the outer surface of the C-25 and C-200 carbons at77 K. a, Room temperature, S N ~ = 0.67 mZ g-1 ; A, 200°C, S N ~ = 0.86 m2 g-l.OUTER SURFACE POROSITYAn important characteristic of any molecular sieve adsorbent is the contributionof the more open porosity and of the outer, open surface, to the internal porosity.Such contributions reduce the overall stereospecificity of the adsorbent, and imposesome difficulties in analysing the behaviour of porous adsorbents. Selective adsorp-tion on such open surfaces can be performed using sufficiently large molecules to avoidpenetration into the ultramicropores.Thus, oxygen and larger molecules (in thesequence given above) could be selectively adsorbed on the outer surface of C-25carbon. A study of the adsorption on the outer surface of fibrous carbon would b2470 MOLECULAR SIEVE CARBONSadvantageous since the outer surface can be calculated from the fibre diameter providedthe diameter's distribution is not too large. The outer surface is given by :4-3 -rl bos , .CI P 2 -4 A = -Pd- 00- 0al ?-t l I 1 I I 1 1 1where p is the average porous solid density and d is the fibre diameter.For the carbonTCM 128 studied, A is 0.38 m2 g-l.Adsorption isotherms on large samples of TCM 128 carbon are given in fig. 8 fornitrogen on C-25 and C-200 carbons. Both are not penetrated by the adsorbate.Clear evidence of outer surface roughness and pore development is given by thegreater B.E.T. surfaces of 0.67 and 0.86 m2 g-l, respectively. This is confirmed byPIP01.2 m2 g-l.FIG. 11.-Adsorption isotherm at 77 K of O2 on the outer surface of a C-25 carbon. So2 =PIP0FIG. 12 -Adsorption isotherm of nitrogen on the C-400 carbon at 77 K. S N ~ = 1.2 m2 g-'FIG, 10.-S.E.M. micrograph of untreated TCM 128 carbon.[To face page 247J.KORESH AND A . SOFFER 2471the S.E.M. micrograph of the non-treated TCM 128 carbon shown in fig. 9. Anadsorption isotherm of O2 on the C-25 carbon is presented in fig. 10. Here thecorresponding B.E.T. surface, 1.2 m2 g-l, is almost twice that of N2. Thus molecularsieving effects already exist for the outer surface porosity. However, due to theshallow penetration, in our terms low N values, the adsorption rate is very fast andmolecular sieving selectivity appears to be poor. The ratio between moles of N2necessary for total pore volume filling shown in the isotherms of fig. 12 and the N2monolayer coverage of the outer surface is about three orders of magnitude, demon-strating the high degree of molecuiar sieving of the TCM 128 carbon.CONCLUDING REMARKSAdsorption experiments with molecular probes have been performed in order toobtain comprehensive information about pore structure and properties.A strikingresult of these experiments is that high stereospecificity between molecules of verysimilar dimensions can be achieved for the wide range 3.1-5.6 A developed by modify-ing the same starting material. To the advantages of carbon molecular sieves overzeolites given by Walker et aL4 one could add that, within a certain range, poredimensions may be tailored to any desirable value.We thank D. Rosen for technical assistance.R. E. Franklin, Trans. Faraday SOC., 1945, 45, 668.F. A. P. Maggs, Nature, 1952, 169, 793.J. R. Dacey and D. G. Thomas, Trans. 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