J. C. S. Faraday I, 76,180-1 95Zeolite SorbentsModification by Impregnation with SaltsBY RICHARD M. BARRER,* DAVID A. HARDING (IN PART) AND ARVIND SIKANDPhysical Chemistry Laboratories, Chemistry Department, Imperial College,London SW7 2AYReceived 1st March, 1979A study has been made of the effect of salt inclusion in certain zeolites upon the kinetics of sorp-tion of, and the molecule sieving behaviour shown towards, n-hexane, 2- and 3-methyl pentanes,2,2- and 2,3-dimethyl butanes, cyclohexane and benzene. The zeolites selected provided threetypical one dimensimal channel systems. They were zeolite L, offretite and H-mordenite. Thesalts used to impregnate the channels were KCl, KBr, KzS04 and K2Cr04, all comparatively stableto heating. The effects of salt concentration and salt type were investigated in each zeolite and alsoof washing and extraction of salt-impregnated sorbents.Among the above salts K2Cr04 gave thelargest effects. Access to the channels could be selectively controlled and changed by salt imbibitiongiving rise to various separation possibilities based on differences in size and shape of sorbatemolecules.Molecule sieving by a given zeolite sorbent can be altered in various ways. Oneof these is ion exchange : the ions may be of about the same size but have differentcharges (2Naf $ Ca2+); or they may have the same charge but different radii(Na + K+ ~t Rbf + Cs+). Exchange can modify zeolite sorbents very effectivelywhen the cations occupy window positions in three-dimensional channel networks orsit in one-dimensional (i.e.parallel non-intersecting) channel systems. 1-3 Secondly,one may change selectively the accessibility of intracrystalline pores to molecules ofdiffering dimensions by direct syntheses in which the A1 : Si ratio is altered and hencethe cation density in windows and cl~annels.~ Thirdly, one may pre-sorb into thepores strongly sorbed polar molecules which are immobile at the temperature atwhich separation of other mobile sorbates is to be effe~ted.~. Finally, one may bychemkorption attach groups to the anionic frameworks, as illustrated by reactions ofSiH4 with acidic silanols of H-zeolites and de-aluminated H-zeolites. Attachedgruups were found strongly to influence sorption and molecule sieving.has had success ininterpreting the accessibility of the intracrystalline pore space.* One-dimensionalchannel systems block more easily however and their behaviour has been consideredin terms of the effect of high energy barriers at intervals along each channel uponmolecule migration along it. ORecently changes in mobility of n-hexane and 2,2-dimethyl butane were investi-gatedl in the one-dimensional channels of offretite, after introducing, respectively, theexchange ions Lif, Naf, Kf, Csf, MeNHz, Me,NH;, Me3NH+ and Me4N+. Theinorganic ions were all too small greatly to influence sorption kinetics, but adequateamounts of Me,N+ blocked even n-hexane. However, organic exchange ions are notvery stable to heating and so a further method of selective channel blocking wasdesired.Zeolites soaked in salt solutions may imbibe the salts, according to aDonnan membrane equilibrium.12 Thermally stable salts can be selected and theIn three-dimensional channel networks percolation theory18R . M. BARRER, D . A. HARDING AND A . SIKAND 181anion dimensions varied systematically. It was thus of interest to establish whetherimbibed salts can function within one-dimensional channels as controllable intra-crystalline barriers selective for molecule sieving.EXPERIMENTALMATERIALSSynthetic offretite, zeolite L and mordenite (as H-Zeolon) were selected as sorbents. Thesorbates were the Cs hydrocarbons n-hexane, 2- and 3-methyl pentanes, 2,2- and 2,3-dimethylbutanes, cyclohexane and benzene. The salts used to impregnate the channels of thezeolites were KCl, KBr, KzS04 and K2Cr04.Crystallographic dimensions l3 of the ionswere taken as : K+, C1- and Br-, 2.66, 3.6 and 3.9A in diameter, respectively ; SO$- andCrOi-, 4.8 and 5.5 A for the respective tetrahedron heights and 5.5 and 6.3 A for the spheresjust circumscribing each tetrahedron.Some relevant properties of the Cs hydrocarbons (> 99 % pure) and of the zeolitesorbents are given in tables 1 and 2, respectively. Liquid densities, p, of hydrocarbons atTABLE 1 .-HYDROCARBONSrelative pressure at liquid densitymolecule 30°C and 63 Torr at 30°C dimensionsa/An-lrexane 0.34 0.649 4.0 ( t ) 4.9 (b) 10.3 (I)2-Me-pent ane 0.24 0.643 4.65 (h) 6-18 (6) 9.0 ( I )3-Me-pentane 0.27 10.643 b] 4.65 (h) 6.18 (6) 9.0 ( I )2,2-di-Me-butane 0.16 0.639 5.9 (h) 6.18(b) 7.77(1)2,3-di-Me-butane 0.22 0.652 4.65 (h) 6.18 (b) 7.77 ( I )cyclohexane 0.52 0.769 4.9 ( t ) 6.4 (b) 7.2 ( I )benzene 0.55 0.869 3.7 ( t ) 6.7 (b) 7.4 ( I )0 t = thickness ; b = breadth ; h = height ; I = length.The lengths refer to fully stretchedchains. * b Assumed density.TABLE 2.-zEOLITE SORBENTSwater and salt-free corresponding cell free dimensionszeolite cell compositions cell weights dimensionso of channels1A IA(K,TMA)-offretitea K J . O T ~ I * ~ [ A ~ ~ . ~ S ~ 14.203d 1290(K,TMA)-offretitea K~.~TMAI.I[AI~.~S~ 14.20 36] 1 1 262 hexag.(K,H)-offretitea K2.7H i.iW3.7Si 14.20361(outgassed 300°C)(K,H)-offretite(salt-impregnated :zeolite Lc JL~Na3[AWi270721 2456 hexag.zeolite L(outgassed 300°C)u Z 13.3 Z 6.4 1 ::: c 7.6K 3.8[AI 3& 14-20 3-61outgassed 360°C) JJa ," 18.4 ," 7.1(salt impregnated : Kg[A19Si270721 1 2504 c 7.5outgassed 360°C)outgassed 360°C)H-mordenite (H-Zeolona &*6[A16.61Si400961 1 2843 orthorhombica ", 18.1b ", 20.5 ", 6.7x7.0c ", 7.5 1 3093 J mordenite as above (salt K6*61[A16*6lSi4009dimpregnated : outgassed 360°C)a Approximate values because of treatments given or compositional differences from typeComposition of type material, ref.(19). materials, ref. (18). b Compositions from ref. (11).Compositions from ref. (17)182 ZEOLITE SORBENTS30°C were obtained from those at 20°C,'* assuming (ap/aTp) = 0.001 g ~ r n - ~ K-'. Forn-hexane, benzene and cyclohexane the values so found agreed with values at 30°C lS towithin < 0.25 %.The dimensions are based on those given for CH3, CH2 etc. by Pauliag.16The parent offretite of composition given in table 2 contained a small excess of base(eg., NMe40H), almost all of which was removed by outgassing at 360°C.'' The parentH-Zeolon (H-mordenite) was partly dealuminated l7 and the ideal composition when free ofmolecular water was H6.61[A16.61Si400,3.,2fOH)5.56]. It is assumed in the mordenitecompositions of table 2 that on outgassing at 360°C the -OH groups in " nests " resultingfrom de-alumination release all their water, although this is not exactly true.8 Also, aftertreatment with the potassium salts and outgassing at 360°C in presence of ex- solid salt,it is assumed that acidic H is replaced by K.The channels in offretite and mordenite are ofnearly uniform cross-section along their lengths. Those in offretite are cross-linked via 14-hedral gmelinite cages with 8-ring windows, so that if tetramethylammonium (TMA) ionsare removed from these cages it might be possible for n-hexane to enter and so migratebetween channels. None of the other Ca hydrocarbons could do so. The channels inmordenite are lined with side pockets, access to which is through an 8-ring window. Theends of n-hexane molecules might enter such pockets. In mordenite and in zeolite L thereare no lateral openings between channels large enough for passage of any c6 hydrocarbon.In zeolite L the free dimension of E 7.1 A refers to the narrowest parts along each channel,occurring at inter-vals of M 7.5 A.Between these parts the channel broadens considerably.L-PROCEDURES - .DI Parts of the parent (K,TMA)-offretite were calcined in air at 520°C for 16 h to convertthem to (K,H)-offretites. Samples of the parent (K,TMA)-form and the (K,H)-offretiteswere then soaked in the appropriate salt solutions for 16 h at 80°C. They were next filteredand similarly soaked in the solution twice more before being finally air-dried without re-moving adhering salts. (K,H)-offretites were outgassed at 360°C for at least 16 h prior touse as sorbents. Samples containing Th4A ions were, however, outgassed at 300°C becausehigher temperatures tended to decompose TMA. The TMA and water contents were foundby microanalysis and thermogravimetry.Several of the salt-impregnated sorbents werewashed on a Buchner funnel and associated residual salt determined. In these instances 0.5 gof sorbent were first washed for FZ 0.5 min with 10 cm3 of 0.1 mol dm-3 salt solution andsimilarly with 10 cm3 of distilled water, in order to remove as much external salt as possiblewith the least elution of intracrystalline salt. In other instances the salt-impregnated zeolitewas more severely extracted. = 0.5 g of zeolite was washed for E 1 min with 500 cm3 ofdistilled water. The sampIe was then filtered and stirred with 50 cm3 of distilled water for16 h at 80°C. This stage was repeated twice more. Various salt-impregnated mordeniteand zeolite L samples were made by the same impregnation procedures and some werewashed or extracted in the same way.Thus samples of each salt-bearing zeolite withadhering salt, in lightly washed or in extracted form, were available, as well as parent formsbefore any salt impregnation.Sorption measurements were made gravimetrically using silica spring balances, withsample weights in the range 190-280mg. The sample temperatures were 30°C and thepressure of sorbate vapour was 63 & 2 Torr.RESULTS AND DISCUSSIONSome of the experimental observations are summarised in tables 3 and 4. It isimmediately apparent that sorbent modification by salt impregnation can readily beeffected in ways that are selective as regards the shapes of the sorbate molecules.Kinetics of uptake (cm3 of sorbed liquid hydrocarbon against time, in a weight cor-responding with 1 g of outgassed salt-free zeolite) are compared for the various hydro-carbons in parent (K, TMA)- and (K, €3)-offretites in fig.l(a) and (b), respectively.The kinetics in the same two sorbents treated with 1 mol dm-3 K,CrO, are similarlR. M. BARRER, D. A . HARDiNG AND A . SIKAND 183expressed in fig. l(c) and (d) and may be compared inter se and with the correspond-ing behaviour in the parent zeolites. Only parts of the measured curves are shown ;runs were continued for + 1-4 days, according to the rate of uptake, before the finalsorptions in tables 3 and 4 were recorded, so that most of these refer to, or are veryclose to, equilibrium. Similarly fig. 2 and 3, respectively, show kinetics of hydro-carbon uptakes in parent zeolites L and H-mordenite and in these sorbents variouslytreated with KCl and K,CrO,.TABLE 3 .-SORPTIONS OF & HYDROCARBONS AT 30°C AND 63 2 TOIT IN MODIFIED OFFRETITES.SORPTIONS ARE GIVEN IN MOLECULES PER UNIT CELLhydrocarbon1no.sorbentn-hexane 2-Me- 3-Me- 2,2-diMe- 2,3-diW- cyclo-pentane pentane pentane pentane hexane benzene123456789101112131415(K,TMA)-OFF 1.03(K,TMA)-OFF in 3 mol dm-3 KCI 0.651(K,TMA)-OFF in 1 mol dm-3 K2CkO4 1.02(K,H)-OFF 1.156(K,H)-OFF in 3 mol dm-3 KC1 0.697(K,H)-OFF in 0.1 mol dm-3 KCl 1.136(K,H)-OFF in 3 rnol dm-3 KCl, washed 1.119(K,H)-OFF in 0.1 mol dm-3 K2Cr04 1.043(K,H)-OFF in 1 mol dm-3 K2CrO.q 1.012(K,H)-OFF in 1 mol dm-3 K2Cr04,(K,H)-OFF in 1 rnol dm-3 K2CrO4.washed 0.986extracted 1.23 1(K,H)-OFF in 0.1 mol dm-3 KBr 1.138(K,H)-OFF in 3 mol dm-3 KBr 0.376(K,H)-OFF in 0.1 mol dm-3 K2SO4(K,H)-OFF in 0.5 mol dm-3 K2SO41.1081.11 10.5710.0850.2220.7300.5100.0700.2780.1630.0550.6600.0820.22~0.2270.8620.1300.4390.1350.3310.1100.10~0.0980.1930.1000.0850.8390.38,0.083 -0.1120.0790.4010.1250.4880.3600.164 1.030.11~ 0.4510.126 0.4380.891 1.330.540 0.7410.325 0.5030.117 0.2g70.126 0.2060.080 0.1100.05~ 0.09~0.800 1.120.425 0.767- 0.2800.515 0.7440.358 0.543TABLE 4.-sORPTIONS OF cg HYDROCARBONS AT 30°C AND 63 & 2 Ton IN MODIFIED ZEOL- LAND MORDENITES.SORPTIONS ARE GIVEN IN MOLECULES PER UNIT CELLhydrocarbon1no.sorbent2-Me- 2,2-di-Me 2,3-di-Me cyclo-n-hexane pentane butane butane hexane benzene123456789101112131415zeolite Lzeolite L in 0.1 mol dm-3 KC1zeolite L in 3 mol dm-3 KClzeolite L in 0.1 mol dm-3 K2Ci-04zeolite L in 1 mol dm-3 K2Cr04zeolite L in 1 rnol dm-3 K2Cr04,zeolite L in 0.1 mol d m 3 KBrzeolite L in 0.1 mol dm-3 KzSO4extractedH-MORH-MUR in 0.1 mol dm-3 KClH-MOR in 0.1 mol dm-3 KCl, extractedH-MOR in 3 mol dm-3 KC1H-MOR in 0.1 rnol dm-3 &Cm4H-MOR in 0.1 mol dm-3 K~Cr04,H-MOR in 1 rnol dm-3 K2CrO4extracted2.22 1.87 1.171.82 1.40 0.980.535 0.255 0.2880.497 0.382 0.2920.062 0.081 0.03~2.165 1-76 1.3542.10 1.61 1.1741.64 1.10 0.9042.00 1.23 0.1611.97 1.76 1.361-18 0.85 0.830.73 0.324 0.2920.128 0.144 0.0921.15 0.90 0.780.175 0.144 0.1%1.62 ---0.1120.2501.76 -0.2721.131.160.2960.3160.1151.551.320.9440.4241.751.180.3910.1331.050.0092.291.530.4680.3870.1982.251.822.563.091.861.011.3440.2611.790.281840.12-0.10ZEOLITE SORBENTSap"""""" = - = - 0-0.040.0202/qh*FIG.l.-Sorption behaviour of C6 hydrocarbons, at 30°C and the relative pressures of table 1, in(a) (K,TMA)-offretite, (b) (K,H)-offretite, (c) (K,TMA)-offretite treated with 1 mol dm-3 K2Cr04and (4 (K,H)-offretite treated with 1 mol dm-3 K2Cr04. 0, n-hexane ; A, 2-Me-pentane ; V, 3-Me-pentane ; n, 2,2-di-Me-butane ; m, 2,3-di-Me-butane and X, benzene. Va is the volume sorbedexpressed as cm3 of liquid hydrocarbon per g of outgassed salt-free zeoliteR.M. BARRER, D . A. HARDlNG AND A . SIKAND0.120.100.080.06b 0.04-0.02185- - - L v - " n - - n Aw -- A : - A 3 # LL- *-U)(==-X-.---o---.- 0 fA- (dl -I . I I I I IFIG. 2.-Sorption behaviour of C6 hydrocarbons in zeolite L, at 30 "C and the relative pressures oftable 1 , in (a) parent L, (b)L treated with 0.1 mol dm-3 K2Cr04, (c) L treated with 3 rnol dm-3 KCL and(d) L treated with 1 mol dm-3 K2Cr04. The ordinates and symbols denoting the hydrocarbonsare as in fig. 1186 ZEOLITE SORBENTSAt 30°C and the relative pressures in table 1 equilibrium sorptions should ap-proach the saturation capacity of each C6 hydrocarbon because intracrystalline sorp-tion isotherms of these hydrocarbons in zeo1ites at this temperature are rectangular inform.At the same time the relative pressures are such that sorption on the smallexternal surfaces of the crystallites should be limited. Thus the uptakes, expressed intables 3 and 4 as molecules per unit cell, refer primarily to intracrystalline hydrocarbonuptakes except for the smallest of these, for which sorption on external surfaces couldbe a significant part.THE SALT-FREE ZEOLITESPassing through a unit cell of offretite there is one channel section of length E 7.6 A(table 2), whereas the full length of n-hexane is 10.3A (table 1). Despite this themaximum uptake of n-hexane in (K, TMA)-offretite was 1.03 molecules per unit cell.Accordingly, because the gmelinite cages are believed to be fully occupied by TMAions, the n-hcxane may either be coiIed as a helix against the channel wall, or bepuckered. In (K, H)-offretite, where the gmelinite cages me free of TMA, the maxi-mum uptake of n-hexane was 1.23 molecules per unit cell and some part of this couldrepresent insertion of a portion of the n-hexane into the cages, additionally to itspresence in the through channels.For benzene, of maximum length w7.4 a and,because of its dimensions (table l), no chance of entering gmelinite cages, the maxi-mum uptake also exceeds one molecule per unit cell. Thus at the closest packing ofbenzene the planes of the benzene rings may be tilted relative to the channel axes.For all branched chain C6 paraffins and for cyclohexane access to the salt-free(K, TMA)-offretite is restricted, often very greatly so (table 3, sorbent 1).Thisbehaviour is attributed to the presence of residual TMA ions in the through channelswhich are bulky enough to limit or hinder access of the equally bulky iso-, neo- andcyclo-paraffin molecules. However, in (K, H)-offretite where no TMA is presentthese bulky molecules were sorbed in amounts between 0.73 and 0.89 molecules perunit cell (table 3, sorbent 4). 2- and 3-Me-pentanes have fully extended lengths of9 A (table 1) and are less flexible within the channels than n-hexane, while 2,3- and2,2-di-Me-butanes have lengths of = 7.8 A ; within the channels their bulk allowslittle flexibility.Cyclohexane [dimensions 4.9 by 6.4 by 7.2 A (table l)] can also havelittle room for movement or flexibility in channels of 6.4A free diameter. When inaddition one considers the loose van der Waals contact between molecules, uptakesof less than one per unit cell (table 3, sorbent 4) are seen to be reasonable for iso-, neo-and cyclo-hexanes.In zeolite L and mordenite two wide channels traverse each unit cell so that up-takes in molecules per unit cell are about twice those in offretite (table 4, sorbents 1and 9). Allowing for this, for n-hexme and benzene the numbers of moleculessorbed are such that the arguments of the previous paragraph should also apply to thepacking of these hydrocarbons in each channel. In mordenite the 14-hedral gmelinite-type cages of offretite are replaced by side-pockets. In mordenite, however, a newfactor appears in that the uptakes of the most globular molecules, 2,2- and 2,3-di-Me-butane and cyclohexane, are drastically reduced (table 4, sorbent 9)’ even though thefree dimensions of the channels are greater than in offretite (table 1).It is suggestedthat, in this partially dealuminated H-mordenite, Al-bearing fragments derived fromthe framework by dealurnination are present in the wide channels in sufficient mountsto hinder or prevent entry of the most globular of the C6 hydrocarbons. Rates ofuptake of the various CB hydrocarbons in the parent H-mordenite are compared infig. 3(a). For n-hexane, 2-Me-pentane and benzene the sorption is rapid but the uptakeR .M. BARRER,TD. A. HARDING AND A . SIKAND 187differ considerably (table 4, sorb& 9). For 2,2- and 2,3-di-Me-butanes sorption isvery slow indeed, so that this sarbent has differentiated kinetically between the twogroups of molecules in a very clear manner. Zeolite L, having channels the freediameters of which vary from ~ 7 . 1 to x 13 A, with periodicity along the channel of0.080.060.04va 0.02A n n A - 1'002 c z0*02t- (Cl0 OB-- - L x-x- -- A+-b b - - 002- (d 10. I I 1 I I I I I I I I 102 0 4 06 0 8 10 12 14 16 18 2 0 2.2 2.4&lh+FIG. 3.-Sorption behaviour of Cg hydrocarbons, at 30°C and the relative presrmres of table 1, inH-mordenite partially dealuminated. (a) The parent zeolite ; (6) the same treated with a1 mol dm3KC1 ; (c) the same treated with 0.1 mol d ~ n - ~ KC1 and extracted and (a) the same treated with0.1 mol dm-3 Kt CrO4 and extracted.The ordinates and symbols are as in fig. 1I88 ZEOLITE SORBENTS257.5A between pairs of narrow or pairs of wide points, does not so differentiate(table 4, sorbent 1), although uptakes of 2,2-di-Me-butane and cyclohexane in par-ticular are reduced.THE SALT-BEARING ZEOLITESAfter impregnation with 3 mol dm-3 KC1 the (K, TMA)- and (K, H)-offretitesbecame similar (table 3, sorbents 2 and 6). Thus, with the possible exception ofbenzene, a given hydrocarbon was taken up to nearly the same extent in both sorbentswhile the sorption of each hydrocarbon was characteristic of that species and dependedstrongly on its shape.It is therefore likely that treatment with 3 mol dm-3 KCl dis-places TMA from the wide channels of offretite and that after outgassing [at 300°Cfor the salt-bearing (K, TMA)-form and at 360°C for the salt-bearing (K, H)-form]both forms contain only K+ and KCl in the wide channels and so behave much alike.(K, H)-offretite treated with 3 mol dm-3 KBr sorbed less n-hexane and benzene thandid this form treated with 3 mol dm-3 KCl, but 3-Me-pentane was sorbed nearlyequally in both sorbents.When (K, TMA)- and (K, H)-offretite were each treated with 1 mol dm-3 K2Cr04the effect on the final uptake of n-hexane was small (table 3, sorbents 1 and 3 andsorbents 4 and 9, respectively). In strong contrast, however, both these two salt-impregnated sorbents took up remarkably little cyclohexane, 2,2- or 2,3-di-Me-butane.The salt-bearing (K, H)-offretite also differentiated sharply between benzene md n-hexane (sorbeat 9, columns 3 and 9).The treatment of (K, TMA)- and (K, H)-offretites with 1 mol dm-3 K,Cr04resulted in marked changes in the rates of sorption of n-hexane [cf.fig. l(a) and (c)and fig. l(b) and (d)] and in very great changes for benzene and 2-Me-pentane in(K, TMA)-offretite and for all the hydrocarbons save n-hexane in (K, H)-offretite.On the other hand, in (K, H)-offretite treated with 0.1 mol d ~ n - - ~ KC1 sorption rateswere not sensibly diminished, although uptakes were altered for all the C6 hydro-carbons save n-hexane [cf. fig. l(b) and fig 4(a) and table 3, sorbents 4 and 51.In zeolite L treated with 3 mol dm-3 KCl uptakes were all much reduced withoutmarked differentiation between the hydrocarbons (table 4, sorbent 3).Whenimpregnated with 1 mol dm-3 K2Cr04 the uptakes, including that of n-hexane, werevery small, with benzene showing the best result (table 4, sorbent 5).The behaviour of H-mordenite after the treatment with 0.1 mol dm-3 KCI wasunexpected because after the treatment the sorptions of benzene, cyclohexane and theiso- and neo-parafEbs were much enhanced (table 4, sorbents 9 and 10) and therewere large increases in rates of uptake of 2,2- and 2,3-di-Me-butanes and cyclohexane[cf. fig. 3(a) and (b)]. Whatever had blocked the salt-free parent H-mordenite to thesemolecules appeared largely to have been removed by the treatment.On the otherhand, the H-moirdenite impregnated with 3 mol dm-3 KCl showed the expected reduc-tion in sorption capacity (table 4, sorbent 12). Although shape-selectivity is evidentthis selectivity is less marked than for the offretites 2 and 6 of table 3, but rather moreso than for zeolite L, sorbent 3 of table 4.EFFECTS OF WASHING AND OF EXTRACTIONWashing the (K, H)-offretite as described in the Experimental section, aftertreating it with 3 mol dm-3 KCl, considerably increased the uptakes of all the hydro-carbons and especially improved the rates of sorption of n-hexane [cJ: fig. 4(b) and (c)].There was at the same time some loss of selectivity as between n-hexane and the otherhydrocarbons (table 3, sorbents 6 and 7).In contrast, washing the (K, H)-offretit189after treating it with 1 mol dm-3 K2Cr04 if anything improved the selectivity for n-hexane (table 3, sorbents 9 and 10). As observed earlier, the washing was designed toremove externally adhering salt but as little as possible of that within the zeolite. Themore severe process of extraction (cf Experimental) of the (K, H)-offretite treated with1 rn~ljldrn-~ K,Cr04 restored much of the capacity of the parent salt-free (K, €3)-offretite to sorb 2-Me-pentane, cyclohexane and benzene (table 3, sorbents 9, 10 andR. M . BARRER, D. A . HARDING AND A . SIKAND0 a12n fi “ U U- o c - n A C h *U Y ” v0 I I I I I I I 1 I I I I ,0.640.02--0.06O * O 8 IFIG. 4.-Sorption behaviour of C6 hydrocarbons, at 30°C and the relative pressures of table 1 in(K,H)-offrdtites.(a) Treated with 0.1 mol d ~ n - ~ KCI ; (b) treated with 3 mol dm-3 KCl and (c)treated with 3 mol dm-3 KCl and washed. The ordinates and symbols are as in fig. 1190 ZEOLITE SORBENTS.\/@Fx;. 5.--Sorption behaviow of C6 hydrocarbons, at 30°C and the relative pressures of table I, in(K,H)-offretite treated with (a) 0.1 mol d m 3 K2Cr04 ; (b) 1 mol d w 3 K2CrO4 and washed and(c) 1 mol dm-3 K2Cr04 and extracted. The ordinates and symbols are as m fig. 1R . M. BARRER, D. A . HARDING AND A . SIKAND 19111). Again, while impregnation with 0.1 mol dm-3 K,CrO, caused a decrease inthe rate of sorption of n-hexane [fig. l(b) and fig. 5(a)] and treatment with 1 mol dm-3K2Cr04 diminished the rate still more [fig.5(a) and (b)], extraction produced a sorbentwhich once more sorbed n-hexane rapidly [fig. 5(b) and (c)]. The extraction, asdescribed in Experimental, was intended to remove both external and intra-crystallinesalt.When zeolite L previously impregnated with 1 mol d ~ n - ~ K,CrO, was extractedthe hydrocarbon uptakes became similar to those in the parent salt-free zeolite, asseen on comparing sorbents 1 and 6 of Table 4. Sorbent 6 can in turn be comparedwith the salt-treated, unextracted sorbent 5 in which the uptakes of the hydrocarbonsare greatly reduced. Zeolite L and mordenite treated with 0.1 mol dm-3 K2Cr04also gave reduced uptakes (sorbents 4 and 13 of table 4, respectively). Extraction ofthe mordenite sorbent 13 restored much of the ability to take up n-hexane, 2-Me-pentane and benzene and resulted in a higher uptake of 2,2-di-Me-butane andcyclohexane than in the parent salt-free H-mordenite (table 4, sorbents 14, 13 and 9).On the other hand, extraction of H-mordenite treated with 0.1 mol dm-3 KC1 causeda reduction in uptake with no marked improvement in selectivity (table 4, sorbents 11and 10).Evidently more than one factor operates in the extraction process.RELATIVE BLOCKING BY DIFFERENT SALTSThe relative ability of the four salts, K2Cr04, K2S04, KCl and KBr, to modify theuptakes of the c6 hydrocarbons is assessed in Table 5, at concentrations of 0.1 mold ~ n - ~ . Potassium chromate was consistently much the most effective in reducing theamounts sorbed, except for n-hexane in (K, H)-offretite where none of the 0.1 mol dm-3salts had any marked effect.Accordingly the (K, H)-offretite treated with 0.1 moldm-3 K2Cr04 can differentiate very well between n-hexane and the other c6 hydro-carbons (table 3, sorbent 8). In zeolite L a single sequence of blocking ability isobserved for all four salts towards each of the hydrocarbons. In offretite there aresome variations in the sequences but these variations do not represent large changes.TABLE 5.-RELATIVE EFFECT OF 0.1 m01 dm-3 SALTS IN BLOCKING HYDROCARBON UPTAKES INZEOLITES OF TABLES 3 AND 4zeolite and salt sequence in blockinghydrocarbon offretite zeolite L mordeniten-hexane KzCrO4 > KCI2-Me- pentane K2CrO4> KCI K2Cr04 > K2S04 > KCI > KBr K~Cr04 > KC13-Me-pentane2.2-di-Me-butane KzCrO4> KCI 2.3-di-Me-butane KzCr04 > KCI - -c yclo hexane K2Cr04 > KCIbenzene K2Cfi4 > KCIlittle blocking by 0.1 rnol dm-3 saltsKzCr04 > KBr > KCI > K2S04K2Cr04 > KBr > KCI > KzSO4KZCa4 > KBr > K2S04 > KCIK2Cfi4 > KCI, K2SO4 > KBrK2Cr04 > KzSO4 > KCl > KBrKzCrO4 > K2S01> KCI > KBrK2Cr04 > K2S04 > KCI > KBrK2Cfi4 > K2S04 =- KCI > KBr- -Tables 3 and 4 show that the zeolites all become increasingly blocked as the impreg-nating salt solutions are made more concentrated.The behaviour of H-mordenitetreated with 0.1 mol d ~ n - ~ KC1 as compared with parent salt-free H-mordenite isexceptional (table 4, sorbents 9 and lo), but going from 0.1 to 3 mol dm-3 KC1 reducesthe uptakes markedly (Table 4, sorbents 10 and 12).Further information on con-centration dependence was obtained in an additional study of offretite. Crystals of(K, TMA)-offretite were soaked in KCl solutions at room temperature for xl6 h192 ZEOLITE SORBENTSfiltered and dried and then part of each sample was calcined in air at 650°C to removeTMA. Crystals similarly soaked in K,Cr04 solutions were also prepared from(K, 33)-offretite which had been made from the (K, TMA)-offretite by calcining in air at520°C. The salt-bearing sorbents containing TMA were outgassed as before at 300°Cand those free of TMA at 360°C. Table 6 shows the effects of salt concentrationsupon the uptake of n-hexane, 3-Me-pentane and 2,2-di-Me-butane.For the(K, €3)-offretites the behaviour is regular : as salt concentration increases hydrocarbonuptake decreases. With K2Cr04 the amounts of the two branched chain hydro-carbons sorbed decreases at first very sharply and then more slowly with salt concen-tration.For the (K, TMA)-offretites the behaviour was different, due to the operation oftwo factors. The first of these was progressive elimination of blocking TMA ionsfrom the wide channels by ion exchange with potassium and the second was imbibitionTABLE 6.-SALT CONCENTRATION AND HYDROCARBON UPTAKE IN SALT-BEARING OFFRETITESmoles sorbed per unit celln-hexane 3-Me-pentane 2,Zdi-Me-pentanesolution andconcentration (K,TMA)-form (K,H)-form (K,H)-form (K,TMA)-form (K,H)-form00.5KCI 1 .o2.03.000.05K2Cr04 0.10.5w30.93 1.140.65 1.170.71 1.090.76 0.860.70 0.67- 1.14- 1.12- 1.11- 0.92- 0.570.860.370.280.120.090.15 0.890.11 0.820.19 0.620.07 0.390.22 0.31- 0.89- 0.24I 0.16 - 0.04 - 0.030.02va o.o\ 0.012.0 4.0dqh&FIG.6.-Effects of successive runs at 30 "C and the realtive pressure of table 1 on the sorption be-haviour in ofketite. Curves 1 and 2 refer to the second and eighth runs with parent (K,TMA)-offretite. The sorbate is 3-Me-pentane. Curves 3 and 4 are for benzene, the first and eighth runswith (K,TMA)-offretite treated with 1 mol dm-3 K2Cr04. The ordinate is as in fig. 1T 4t-1,1,IIIIII8 16 24 32 8t*/min+FIG.7.-Plots against l / t (t in min) of the fractional approach to equilibrium for the uptake of n-KCl. The lower curves refer to samples with TMA present and the upper curves to ignited samplesmolarities of the KCI solutions with which the crystals were treated. (b) (K,H)-offretites treated withcurves are the molaxities of the K2Cr04 solutions with which the crystal194 ZEOLITE SOWBENTSof the salt. These two opposed factors partially or largely offset one another as thetable shows.EFFECT OF SUCCESSIVE SORPTIONS ON A GIVEN SORBENTWith the (K, H)-offretites treated with a particular salt at a given concentrationthere was little change in the sorption capacity for a given C6 hydrocarbon before andafter a succession of sorption runs.With (K, T11MA)-offretite, salt-free or salt-bearing,there was a tendency, shown in fig. 6, for the hydrocarbon uptakes to increase withrepeated use. This behaviour may be associated with progressive elimination of TMAfrom the wide channels. With the salt-bearing forms of zeolite L and mordenitechanges in sorption with repeated use were small and no definite correlations wereobserved in that slight increases and decreases were noted.KINETICSFig.1-6 illustrate some kinetic runs and show that sorption rates can vary greatlyaccording to the sorbent, its treatment and the hydrocarbon. A more detailedexamination was made of sorption kinetics of n-hexane at 30°C in (K, TMA)- and(K, H)-offretites treated with KCl solutions at various concentrations [fig.7(a)] and(K, H)-offretiles similarly treated with K,CrO, solutions [fig. 7(b)], the proceduresbeing those described in the paragraph on relative blocking. The rates for n-hexanein (K, 33)-offretite decreased in a regular manner as the salt concentrations increased[fig. 7(a), top series of curves; and fig. 7(6)]. In the case of (K, TMA)-offretite thecompeting effects of TMA displacement and salt uptake are evident because the ratesequence is related to salt concentration in the order : 1 mol dm-3 > 2 mol >3 rnol dm-3 > 0.5 mol drr3.For n-hexane sorbing into (K, H)-offretite treated with 0.5 and 7z3 mol dm-3K2Cr04 the rates were sufficiently slow to obtain the first sections of the curves of(Q,- Q,)/(Q, - Q,) against dt, where Qt, Q, and Q, are uptakes at time t, whent = 0 and at equilibrium, respectively.These sections are linear and if the diffusivity,D, is constant the slope, S, of the plot iss = 4 0 -J-.I n :Thus D/Z2 = 0.196 S2.For the (K, H)-offretites treated with 0.5 and 7z3 mol dm-3 K2Cr04 the values ofD/Z2 at 30°C were, respectively, 7.4 x and 2.5 x min-'. If sorption rates arecontrolled by intra-crystalline diffusion I is the mean length of crystallites measured inthe c direction. If sorption is controlled by inter-crystallite diffusion I is the effectivemacroscopic depth of the bed. In the former instance taking I as 1 pm, which is inthe size range typical of synthetic zeolites, D in cm2 s-l would be 1.2 x 10-l2 and4.2 x for the two treated offretites.CONCLUDING REMARKSThe present study has established that access to the channels of typical zeolites inwhich the channel systems are one-dimensional can be selectively controlled andchanged by soaking the zeolites in solutions of different concentrations and of varioussalts. The sorbents were then often shape- and size-selective for the various C6hydrocarbon sorbates examined and this suggests possibilities for mixture separationR .M. BARRER, D. A. HARDING AND A. SIKAND 195provided interferences of the fast by the slow moving molecules are not dominant.This aspect has not yet been examined. Some possibilities axe :(1) Separation of straight from branched chain and cyclic paraffins. Sorbents 9 and10 of table 3. (2) Separation of straight chain paraffin and benzene from branchedchain paraffin and cycloparaffin.Sorbents 3, 6 and possibly 13 of table 3. (3)Separation of straight chain param, benzene and 2-Me-pentane from the two di-Me-butanes and cyclohexane. Sorbcnt 9 of table 4. (4) Separation of benzene fromcyclohexane. Sorbent 1 of table 3, and sorbent 9 of table 4.One may expect that salt-treated zeolites like offretite, zeolite L, mordenite andmazzite (zeolite Q) could be extended to give a variety of shape selective sorbentsadditional to those developed in this work. Separation possibilities like those indi-cated above need testing because of the possible interferences referred to abovebetween fast and slow moving diffusants, especially in one-dimensional channelsystems. Here much will depend on the relative affinities of slow and fast movingspecies for the intra-crystalline sorption sites and upon the mixture compositions.D. H. and A. S. acknowledge the support of the Wolfson Foundation and of AirR.M.B. acknowledges a recent award Products and Chemicals for part of this work.of a Leverhulme Emeritus Fellowship.R. M. Barrer, J. SOC. Chem. Ind., 1945,64,130, 132, and R. M. Barrer and L. Belchetz, J. SOC.Chem. Ind., 1945,64,131.R. M. Barrer, Trans. Faraday Soc., 1949, 45, 358.T. Takaishi, Y. Yatsurugi, A. Yusa and T. Kuratomi, J.C.S., Faraday I, 1974,70,97.R. M. Barrer and J. W. Baynham, J. Chem. SOC., 1956,2892.R. M. Barrer and L. V. C. Rees, Trans. Faraday SOC., 1954,50,852, 989.L. V. C. Rees and T. Berry in Molecular Sieves (SOC. Chem. Ind., London, 1968), p. 149.1977, Chicago.R. M. Barrer and J. C. Trombe, J.C.S. Faraday I, 1978,74,2798.J. M. Hammersley in Methods in Computational Physics (Academic Press, N.Y., 1963), vol. 1,p. 281.lo R. M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieves (Academic Press,London, 1978), p. 30 et seq.l1 R. M. Barrer and D. A. Harding, Separation Sci., 1974, 9, 195.l2 R. M. Barrer and A. J. Walker, Trans. Furaday Soc., 1964, 60, 171.Interatomic Distances (Spec. Pub. Chem. SOC., London, 1958), no. 11 ; L. Pauling, The Nature ofthe Chemical Bond (Cornell, Ithaca, 1940), p. 345.l4 J. A. Riddick and W. B. Bunger, Techniques of Chemistry, ed. A. Weissburger (Wiley, N.Y.,3rd edn, 1970), vol. 2.International Critical Tables (McGraw Hill, N.Y., 1926).R. M. Barrer and J. Klinowski, J.C.S. Faraday I, 1975,71, 690.London, 1978), p. 472.(Amer. Chem. SOC, 1971), no. 101, p. 155.' R. M. Barrer, R. G. Jenkins and G. Peeters in 4th Int. Con$ on Molecular Sieve Zeolites, Aprill6 L. Pauling, Nature of the Chemical Bond (Cornell, Ithaca, 1940), p. 187.lS R. M. Barrer, Zeolites and Clay Minerals as Sorbents and Molecular Sieues (Academic Press,l9 W. M. Meier and D. H. Olson, in Molecular Sieve Zeolites-.& Adv. Chem. Ser., 1971, 101(PAPER 9/339