Adsorption of Carbon Monoxide by Zeolite Y Exchanged withDifferent CationsB Y T. A. EGERTON~ AND F. s. STONE*$School of Chemistry, University of Bristol, Bristol BS8 ITSReceizied 5th July, 1972Adsorption of carbon monoxide has been studied on Zeolite Y in which Na+ ions have beenpartially exchanged for Zn2+, Mnz+, CoZ+, Ni2+, Cu2+, Ba2+, UO:' and Ce3+ ions. The divalentions all produced sites for specific CO adsorption, even at low degrees of exchange. Thus, unlikeCa2+ studied in earlier work, these ions do not have a total preference for internal sites in the prismsor sodalite units inaccessible to CO. At approximately 30 % exchange the affinity of divalent ionsfor internal sites decreases in the order Ca2+ > Ni2+, Mn2+, UOg+ > Cu2+ > Zn2+. However,even on the Zn-exchanged Y , only about 1 divalent ion in 10 acts as a site for specific adsorption ofCO.Very little CO was specifically adsorbed on Ce-exchanged Y, even at high degrees of exchange.This confirms that cerium ions favour internal sites.With ZnY, MnY, BaY and CeY, CO adsorption was rapid and reversible. Isosteric heats wereevaluated and, except for CeY, were substantially greater than on unexchanged Nay, confirmingthe presence of specific adsorption. The heats correlate with the electrostatic field strengths of thecations and with the shifts of the C-0 stretching frequency on adsorption. With NiY and CuY,adsorption was slow and is thought to reflect a gradual increase in the number of adsorption sitescaused by adsorbate-induced migration of divalent cations.A number of the systems studied in this work have not previously been investigated from thestandpoint of cation location.However, where comparisons are possible, the present results derivedfrom CO adsorption are shown to be in good agreement with those from other methods.The principal methods which have so far been used to study the location and co-ordination taken up by the charge-balancing cations in ion-exchanged syntheticfaujasites are the more obvious physical ones. Thus X-ray diffraction,l* electronre~onance,~ infra-red spectr~scopy,~ reflectance spectr~scopy,~~ Mossbauer spectro-scopy ' and magnetic susceptibility * are among the methods which have been appliedin individual cases. These methods are entirely sufficient in principle, and for themost part experimentally straightforward, but the interpretation of the results andthe establishment of firm conclusions is frequently difficult.The aim of the presentwork is to show that physico-chemical studies of specific adsorption can also be usedto give information on the siting of cations. The method is not as powerful as someof the physical ones, but it has the advantage of being applicable to a wide variety ofions. Moreover, it leads us closer to the chemical context of the problem, which isof growing importance now that many zeolites are being used as catalysts. Quiteapart from the matter of site occupation, there is much to be gained by studying thechemical interactions which occur between zeolite cations and the reactive gases ofinterest in catalysis.Carbon monoxide is an attractive choice for such work.The CO molecule ist present address : Department of Chemistry, Makerere University, P.O. Box 16020, Kampala,$ present address : School of Chemistry and Chemical Engineering, University of Bath, BathUganda.BA2 7AY.2T. A . EGERTON AND F. S . STONE 23small enough to enter the supercages of faujasite, but too large to enter the sodaliteunits or hexagonal prisms. The molecule has an asymmetric charge distribution andis easily polarized ; it is therefore sensitive to the strong electrostatic fields surroundingcations. It also interacts very specifically with transition metal ions, as a result ofits ability to act both as a weak donor and as a n-acceptor, and it is well known toform chemisorption bonds on transition metal oxides.A few years ago Angell and Schaffer showed by infra-red spectroscopy that COmolecules are indeed adsorbed close to multivalent cations in faujasite supercages, andat the same time Rabo, Angell and co-workers illustrated some of the potentialities ofanalyzing the cation-specific adsorption of CO on zeolites. We have since reporteda detailed study of calcium-exchanged Zeolite Y using CO as a probe molecule,''and in this paper we present results and conclusions on NaY exchanged with eightother ions, including several transition metal ions.Experiments on the influence ofoutgassing conditions have also formed part of the work, since there is increasingevidence to suggest that small amounts of residual water can exert a dominant effecton cation location.EXPERIMENTALMATERIALSAll samples used in the research were prepared from the same batch of synthetic sodiumfaujasite (Nay) which was kindly supplied by the Linde Division of the Union CarbideCorporation (SK-40, Lot No.51-31). The composition was in good agreement with theformula Na, 6(A102)5 6(Si02)13 6,264 H20.Aqueous solutions containing suspended Nay became alkaline on standing, so the solidwas washed with a sodium acetate+acetic acid buffer (pH 5) before being used in an ionexchange reaction. Small amounts of this buffer were also added to the first batch of asolution used in any ion exchange reaction. If these precautions are not observed, the highpH developed within the zeolite pores can lead to the incorporation of a hydroxylated formof some ions. With manganese, for instance, absence of buffer leads to the number ofequivalents of Mn2+ entering the zeolite exceeding the number of Na+ equivalents replaced,suggestive of Mn(OH)+ formation, and over a period of months samples change in colourfrom white to brown.Individual details of the ion exchange procedure employed in the preparation of samplesare summarized in table 1.A suspension of 5 g of pretreated NaY in 200 cm3 of theappropriate solution was stirred for 2 h. When necessary the sodium content was loweredfurther by repeated exchange. The zeolite was then filtered out, thoroughly washed anddried overnight at 70°C. Finally, it was stored over saturated ammonium chloride solutionfor at least three weeks before use.The water content of a fully hydrated ion-exchanged sample was determined within 0.2 %by dehydrating it at 400°C on a McBain balance.The extent of ion exchange was measuredwithin 1 % by flame photometric analysis for residual sodium. With MnY the sodiumanalysis was supplemented by a volumetric (EDTA) determination of manganese, and withNiY the nickel was analysed gravimetrically as the dimethylglyoximate. In both cases thenumber of equivalents of sodium lost equalled the number of equivalents of transition metaIion gained.In certain cases ion exchange can lead to collapse of a zeolite framework, and in additionsome exchanged zeolites which are stable in the hydrated form decompose when outgassed.Accordingly, selected samples were studied by X-ray diffraction both before and after use inadsorption measurements.Diffractograms were measured with a Philips PW 1051 diffracto-meter and Cu-K, radiation, using a sample rotator to improve reproducibility. No X-rayevidence of structural breakdown was observed. Low temperature (78 K) adsorptionisotherms of argon (c = 0.138 nm2) andlor nitrogen (a = 0.162 nm2) were also measure24on some samples. Surface areas (monolayer equivalent areas) were evaluated using bothpoint B l3 and Langmuir plot l4 methods, with the results shown in table 2. Only in thecase of MnY-75 * did the adsorption measurements suggest that substantial structuralbreakdown had occurred.co ADSORPTION ON ZEOLITE YTABLE 1 .-PREPARATION OF ION-EXCHANGED SAMPLEScationBa2fCe3+CO2+cu2+Mn2+Ni2+uo;+Zn2+ionicrlnm ion exchange solution t exchange/ "Cradius 12 temp.of0.134 0.05 M BaCI,0.107 0.005 M CeC130.072 0.1 M C0(N03)20.072 0.1 M C~(N03)20.080 0.05 M MnC120.069 0.005 M NiS042525251 0010010010010025252525252525250.05 M U02(CH3C00)2 100 u-0= 0.1710.074 0.02 M ZnC12 100100% degreeof exchange8417180803242872231407520465866333162t The concentrations are average values.TABLE 2.-sURFACE AREAS, AS DETERMINED BY ARGON AND/OR NITROGEN ADSORPTION AT 78 Ksurface area $argon adsorption NZ adsorptionoutgassing Langmuir Langmuirsample temp./"C point B plot point B plotNayCeY-40CeY-40Cey-71CeY-80CeY-80COY-80CUY-87MnY-75NiY-57ZnY-3 1ZnY-31ZnY-3 1ZnY-763503504803503 5080035035035035034057076035063564063060069070060559063562561061 5675 680 70065064061070569563 5 710 755605570 575648 680655645620635 625 640$ Areas are expressed as m2 g-' of hydrated zeolite.* The symbol preceding Y denotes the ion introduced by ion exchange, and the number denotesthe percentage of Na+ ions in the original Nay which have been replacedT. A .EGERTON AND F. S. STONE 25ADSORPTION MEASUREMENTSThe adsorption measurements were made volumetrically, as previously described. OGreat care was taken during the initial stages of dehydration, and only after many hourspumping was the outgassing temperature raised above 120°C.Except where otherwisestated, the samples were outgassed overnight at 350°C prior to the determination of eachadsorption isotherm.RESULTSThe results are presented in three sections. Section A contains the results ob-tained with ZnY, MnY and Bay, and includes determinations of heats of adsorption.These experiments complement the studies on NaY and CaY already reported. loIn section B we compare the behaviour of NiY, COY and CuY, and illustrate somenew features not found with the cations concerned in section A. In section C weplace CeY; in contrast to the ions in sections A and B, the new characteristic ofZeolite Y exchanged with cerium ions is that the solid shows little evidence of specificCO adsorption.A. CO ADSORPTION ON ZnY, MnY AND BaYFor these zeolites, the adsorption was rapid and equilibrium was always attainedwithin 30 min.The adsorption was reversible without hysteresis and isosteric heatswere calculated in the usual way from plots of (In P)e against 1/T.ZnYThe adsorption of CO on ZnY-31 and ZnY-62 was measured at five differenttemperatures after outgassing at 370°C. The set of adsorption isotherms for ZnY-62is shown in fig. 1, and heats of adsorption for both samples are given in table 3.At 0.6 em3 adsorbed per gram the heat of adsorption on ZnY-31 is 47 kJ mol-1 andthat on ZnY-62 is 52 kJ mol-1 ; both heats considerably exceed the value of 25kJ mol-I for CO adsorption on NaY at a coverage of 0.10 cm3 g-l.The similarityFIG. 1.-Adsorption of CO on ZnY-62 outgassedat 370°C: a, 0°C; e, 22.2"C; 9, 32.2"C;0, 40.9"C; c), 51.7"C.3I M-0 0.5 I .opressue/kN m-26 co ADSORPTION ON ZEOLITE Yin the heats for the two ZnY samples suggests that adsorption occurs on similar sites.Barrer and co-workers l4 have demonstrated that high heats of adsorption of polarmolecules on alkaline earth zeolites result from the strong electrostatic fields associ-ated with the divalent cations; clearly the high heats on ZnY have a similar cause.Previous work on CaY lo has shown that one adsorbed CO molecule is associatedwith each exposed divalent cation. If it is assumed that adsorption on ZnY proceedsin a similar manner it is possible to estimate the number of exposed zinc ions inZnY-31 and ZnY-62. To a first approximation the amount of specific adsorptionon the zinc cations may be calculated by subtracting the CO adsorption on NaY at1 kN m-2 from the corresponding adsorption on ZnY.Isotherms for CO adsorptionat 0°C on NaY and ZnY-31 are shown in fig. 2.TABLE 3.-HEATS OF ADSORPTION OF CO ON ZnY-3I AND ZnY-62heat of adsorption/kJ mol-1ZnY-3 1 ZnY-62coveragelcm3 g-1 8.7 cation1u.c. 17.4 cationlux.0.2 51 .O_+ 2.0 -0.4 48.0+ 2.0 -0.6 46.51 2.0 51.5k4.00.8 43.5k2.0 52.0+ 4.01 .o 39.0f 3.0 51.5+ 2.01.41.82.02.552.5+ 2.0I 5 1 .O_+ 2.0- 44.0+ 3.0- 36.5_+ 3.0-Ic.5 ! .C Ipressure/kN n r 2FIG. 2.-Comparison of 0°C isotherms for:a, NaY ; 0, Cay-36 ; 9, U02Y-33 ; 0 , MnY-31 ; 8, CuY-32; 0 , ZnY-31.For ZnY-31 the amount of specific adsorption calculated in this way is 1.04 cm3 g-l.This corresponds to 0.81 CO molecules per unit cell (u.c.) and hence implies thatthere are 0.81 exposed Zn ions per U.C.on ZnY-31. Altogether there are 8.65 Znions per U.C. of ZnY-31 and if they were all accessible they would be able to adsorb11.2 cm3 g-l of CO. A random distribution of Zn ions would correspond to aspecific adsorption of 4.6 cm3 g-l of CO.* It follows that the majority of the zinc* This is based on a distribution between 16 type I, 32 type I’ and 32 type IT sites, with only oneof a given trio of 11, IIo and 11’ sites occupied at a given timeT. A . EGERTON AND F. S .STONE 27ions are in sites inaccessible to CO. We conclude that they occupy the internal(I, 1’, or possibly 11’ lo) sites in the zeolite structure. On ZnY-62 the amount ofspecific adsorption was 2.73 cm3 g-l, which corresponds to 2.12 exposed Zn ions perU.C. compared with a total of 17.4 per U.C. in the zeolite. Again, the majority of thezinc ions are evidently inaccessible to CO.Fig. 2 affords a comparison of the 0°C isotherms for CO on ZnY-31 and Cay-36.In low-exchanged Cay, the preference of divalent ions for inaccessible sites is virtuallyabsolute.lo It is apparent that, although a high proportion of the Zn2+ ions inZnY-31 is in internal sites, the zeolite has considerably more accessible divalent ionsthan the corresponding calcium zeolite.A further difference between low-exchangedsamples of CaY and ZnY is their response to changes in outgassing temperatures.Increased outgassing temperature led to little change in the amount of CO adsorbedon low-exchanged CaY,l0 but with ZnY-31 outgassing at 570°C instead of 370°Cled to a significant increase in the amount of CO which could be subsequentlyadsorbed (fig. 3). When the isotherms in fig. 3 are corrected for the component ofFIG. 3.-The dependence of the 0°C adsorption onZnY-31 on the outgassing temperature. Out-gassing at : Q, 370°C ; 0 , 405°C ; 0, 505°C;@, 570°C ; 0, 750°C.5non-specific adsorption, it is found that the increased outgassing temperature hasled to an increase in the limiting value of the specific adsorption ; i.e.there are moreadsorption sites.Raising the outgassing temperature probably results in the removal of smallquantities of chemisorbed water from Zn ions in internal sites. There is no room forwater to be chernisorbed on Zn ions in the hexagonal prisms (site I), so we concludethat the inaccessible Zn ions in ZnY-31 are in the sodalite units. On removingchemisorbed water, which is believed to stabilize cations in the I’ and 11’ positions,the stability of ions within the sodalite units will be lowered and this could causethem to move to other sites, such as I1 or IIo, where they are accessible to CO. Itfollows that at least some of the ions in low-exchanged ZnY are in different sitesfrom those occupied by the calcium ions in low-exchanged Cay.On raising the outgassing temperature to 750°C the 0°C adsorption isotherm wasdepressed (fig.3), although separate surface area determinations (table 2) indicatedthat there was no marked loss of crystallinity. There may be an increased tendenc28 co ADSORPTION ON ZEOLITE Yfor the zinc ions to occupy the empty type I sites in the hexagonal prisms as the lasttraces of water are removed.MnYIsotherms were measured over a range of temperatures for MnY-22, MnY-31,MnY-40 and MnY-75. The 0°C isotherm for MnY-31 is compared with the 0°Cisotherms for Nay, Cay-36 and ZnY-31 in fig. 2. In fig. 4 is shown the set of adsorp-tion isotherms for MnY-22. Adsorption on all four samples was reversible, and forMnY-22, MnY-31 and MnY-40 the plots of (log P)o against 1 /T were linear.Themeasured heats are listed in table 4.FIG. 4.-CO adsorption on MnY-22 outgassed at350°C: 0 , 0°C; 0, 19.3"C; 0, 29.4"C; 0 ,42.2"C.c.5 1.0 1.5pressure/kN m-*TABLE 4.-HEATS OF ADSORPTION OF co ON MnYheat of adsorption */kJ mol-1coverage/cm' g-1 MnY-22 MnY-3 1 MllY-40 MnY-750.05 46.5 48.5 50.0 49.5 'f0.10 42.5 45.0 51.00.25 42.5 43.50.40 31.0 35.00.50 32.0* k 2.5 kJ moI-l. 7 This value was calculated from the best straight line drawn through thepoints between 103K/T = 2.36 and 3.18.Compared with Nay and CaY -36 (fig. 2) the appreciable curvature of the adsorp-tion isotherms and the increased heat of adsorption on low-exchanged MnY showthat Mn2+ is not like Ca2+ in having an absolute preference for sites inaccessible to CO.Mn2+ in Zeolite Y has a behaviour intermediate between Ca2+ and Zn2+.The specificadsorption on MnY-22, calculated in the same manner as that for ZnY-31, is 0.15cm3 g-l. This implies that 0.12 of the 6.1 Mn2+ ions per U.C. are accessible to CO :2.4 would be accessible if the manganous ions were randomly distributed. Thus thevast majority of Mn ions in MnY-22 are not available as adsorption sites for CO.Fig. 5 shows the CO adsorption on MnY as a function of Mn content. ThP r . A . EGERTON AND F . s. STONE 292.8FIG.as a- '*1 a5.-Adsorption of CO at 0°C and 100 N m-2 3function of thepegree of exchange : a, CeY ; 20 , MnY; 0, NiY; 0 , CuY. W9 I i-0 2 0 40 6 0 to% exchangeamount of adsorption increases rapidly beyond 50-60 % exchange, which stronglysuggests that a group of inaccessible sites becomes filled at this degree of exchangeand that further Mn ions which are introduced occupy sites accessible to CO.NowMnY-57 corresponds to a zeolite containing 16 Mn ions per U.C. Hence this resultprovides good evidence that the first Mn ions exchanged into the structure have astrong, but not absolute, affinity for a group of about 16 internal cation sites. Thesesites are probably those in the prisms (site I), of which there are 16 per unit cell.For MnY-75 the (log P)e against l / T plot showed appreciable curvature (fig. 6)103 KITand the value of the adsorption heat recorded in table 4 is only approximate. Thelow surface area of this sample (see table 2) indicated that some structural breakdownhad occurred.It i s possible that the curved heat plot is a consequence of the partialcollapse of the framework, although experiments in which the framework of MnY-430 co ADSORPTION ON ZEOLITE Ywas deliberately partially destroyed led neither to curved heat plots nor to a changein the heat of adsorpfion (fig. 6).BaY AND U02YBay-8 (2Ba2+ per unit cell) was outgassed at 350°C and CO adsorption wasmeasured at three different temperatures (fig. 7). The 0°C isotherm was thenFIG. 7.-Adsorption of CO on Bay-8 and U02Y-33 : e, 29.2"C on BaY outgassed at 350°C ; (>,17.9"C on BaY outgassed at 350°C ; 9, 0°C onBaY outgassed at 350°C; 8, 0°C on BaY out-gassed at 425°C ; 0, 0°C on U02Y outgassed at350°C.pressure/kN m-2redetermined after outgassing the zeolite at 450°C ; the higher outgassing temperaturecaused the adsorption to increase.The curved adsorption isotherms and the heatof adsorption of 36 kJ mol-1 (11 kJ mol-1 greater than on Nay) show that eventwo barium ions per unit cell affect the adsorption properties of Zeolite Y. How-ever, the specific adsorption is only 14 % of the amount which would be expected ifall the barium ions acted as adsorption sites. Thus barium behaves in a similarmanner to manganese and zinc.Adsorption on U02Y-33 was measured at 0°C only. The isotherm (fig. 7) showsthat the uranyl ion behaves like manganese and zinc also.B. CO ADSORPTION ON COY, NiY AND CuYThe isotherms for these three zeolites were markedly rectangular with a pro-nounced knee.For CuY and COY only 0°C isotherms were measured. For NiYthere was a gradual decrease in adsorption when the temperature was raised from0°C to 75"C, but as the isotherms were not reversible isosteric heats of adsorptioncould not be evaluated.COYAdsorption on COY-80 was measured at pressures between 0 and 150 N m-?The extent of specific adsorption was 1.5 cm3 g-l. If COY behaved as Cay, with allcations beyond the first 16 occupying accessible sites,1° there would be six exposedcobalt ions per unit cell and the specific adsorption would be - 8.2 cm3 g-l. Thelow adsorption shows that the cobalt ions have a preference for sites within the sodalit31cages, where they are hidden from CO molecules. The presence of cobalt ions intype 11' sites will help to compensate the negative charge on nearby oxygen windows.T .A. EGERTON AND F. S. STONENiY350°C, and on NiY-46 outgassed at 500°C.Adsorption was measured on NiY-20, NiY-46, NiY-57 and NiY-66 outgassed atRepresentative isotherms are shown in fig. 8. Clearly the initial steep sections ofFIG. &-Adsorption of CO on NiY: e, 0°Cadsorption on NiY-58; 0, -42°C on NiY-58;6,O"C on NiY-46 ; 9,O"C on NiY-46 outgassedat 500°C ; 0 , 0°C on NiY-20 ; (>, readsorptionat 0°C on NiY-20 after pumping overnight at 25°C.5the curves are associated with adsorption on the Ni2+ ions. Between 200 and 1500N the slopes of the isotherms are similar to the corresponding NaY plots. Thisindicates that tlie adsorption at high pressure is occurring on the aluininosilicateframework and on the residual sodium ions.The adsorption represented by boththe initial and final straight sections of the isotherms was rapid (equilibrium wasalways attained within 30 min) but that associated with the " knee'' was muchslower and in some cases equilibrium was established only after 36 h.The steep initial section of the isotherms suggests that CO adsorption on NiY israther strong. Since the isotherms were not reversible no heats could be evaluatedbut a nieasure of the adsorption strength was obtained by adsorbing known amountsof gas, pumping the system for a measured time, and then redetermining the adsorp-tion isotherm. When 0.6 cm3 g-l was adsorbed on NiY-57 only 0.12 cm3 8-l couldbe removed by pumping for 10 niin at room temperature.When 1.3 cm3 g-' wasadsorbed only 0.4 cm3 g-' was removed by pumping for 45 min at room temperature.All the adsorbed CO could be removed from NiY-20 by pumping overnight at roomtemperature.Fig. 5, in which the amount of CO adsorbed at 0°C and 100 N m-2 is plotted as afunction of the extent of nickel exchange, shows that the nickel ions do not have anabsolute preference for hidden sites, However, at approximately 50 % exchangethe amount of adsorption increases rapidly and this indicates that the first nickelions to be exchanged do have a marked preference for a group of 16 sites that are notaccessible to CO. Most probably these are type I sites32 co ADSORPTION ON ZEOLITE YIf one CO molecule could be adsorbed on every exposed nickel ion, and if everynickel ion in excess of the first 16 were exposed, 3.2 cm3 g-1 would be specificallyadsorbed on NiY-66.The measured specific adsorption was only 2.2 cm3 g-l.This implies that even when all the type I sites are filled not all the remaining nickelions are exposed. At high degrees of exchange some nickel ions must occupy typeI' and 11' sites.CUYCuY-87 rapidly adsorbed the first doses of CO, but a slow adsorption becameapparent at a coverage of - 3 cm3 g-'. Equilibrium then took up to 3 days to beattained. A second series of measurements was made in which each adsorption wasterminated after 30min, and a fresh dose of gas was then admitted. In this way" non-equilibrium isotherms " were determined for CuY-87 outgassed at 330°C,440°C and 640"C, and also for CuY-32 and CuY-50 outgassed at 350°C.Theisotherms are shown in fig. 9.FIG. 9.-Adsorption of CO on CuY: 0, "30min " 0°C isotherm on CuY-32 ; 9, " 30 min "0°C isotherm on CuY-50; 0, " 30 min" 0°Cisotherm on CuY-87 outgassed at 350°C; 6," 30 min " 0°C isotherm on CuY-87 outgassed at440°C ; Q, " 30 min " 0°C isotherm on CuY-87outgassed at 640°C ; (3, equilibrium measure-ments on CuY-87 outgassed at 350°C. Thedashed line is the 0°C isotherm onThe curved isotherm and increased adsorption, relative to Nay, show that inCuY-32 cupric ions are accessible to CO molecules. The specific adsorption is 15 %of that expected for a random distribution of cupric ions, or 6 % of that expected ifall the cupric ions were accessible. Thus the behaviour of Cu2+ is intermediatebetween that of Mn2+ and Zn2+. However, fig.5 shows that the change in adsorptionproperties at 50-60 % exchange is not as marked as for MnY. Therefore there is nostrong evidence that the inaccessible cupric ions in CuY-32 have a strong preferencefor type I sites. At higher degrees of exchange more Cu2+ ions are exposed to CO.The equilibrium isotherm for CuY-87 (outgassed at 350°C) implies that at equilibriumat least 10.4 Cu2+ ions per unit cell are accessible to CO. If all ions in excess of thefirst 16 were exposed, 8.4 Cu2+ ions would be accessible to CO. A more detailedinterpretation of the results in terms of site occupancy is complicated by the slowadsorption.C.CO ADSORPTION ON CeYCO adsorption was measured on CeY-40, CeY-71 and CeY-80. Representativeisotherms are shown in fig. 10 and 11. Adsorption on CeY-40 and CeY-71 waT. A . EGERTON AND F . S. STONE 33reversible and rapid. Heats of adsorption were determined for CeY-40 and CeY-71evacuated at 350°C and for CeY-40 evacuated at 460°C (table 5). No further changein the adsorption properties of CeY-40 occurred when the evacuation temperature wasincreased to 615°C. For both CeY-40 and CeY-71 the adsorption heat falls rapidlywith increasing coverage, and at pressures greater than 250 N r r 2 the adsorptionisotherm falls below the corresponding NaY plot. Both these facts indicate thatvery few cerium ions interact with adsorbed CO.The lowered adsorption relativeto NaY probably results from a depletion of the accessible sodium ions.0.40.3rlI M"EFIG. 10.-Adsorption of CO at 0°C on Cc '-40 2and CeY-71: 8, CeY-71 outgassed at 350°C ; 2 c.3(3, CeY-40 outgassed at 350°C; 0, CeY-40 out-gassed at 460°C ; 0, CeY-40 outgassed at 615°C. 4$3 p 0.1' 3 c.5 1.0pressure/kN m-2pressure/kN m-z1-25FIG. 11.-Adsorption of CO at 0°C on CeY-80:0, outgassed at 350°C for 16 h ; (>, outgassed at350°C for a further 32 h ; 0, outgassed at 460°Cfor 18 h ; 0, outgassed at 740°C for 14 h34 co ADSORPTXON ON ZEOLITE YCeY-80 outgassed for 16 h at 350°C adsorbed 3.5 cm3 g-l at 400 N in-’ and 0°C ;evacuation for a further 32 h led to an adsorption of 4.2 cm3 g-l. Neither isothermwas thermodynamically reversible but subsequent outgassing at 460°C led to areversible isotherm.The adsorption at 4-00 N m-’ was then 3.9 cm3 g-l. Thisdecreased when the outgassing temperature was raised to 740°C. The specificadsorption on CeY-80 is unexpected since Rabo and Lo-workers l5 found no cation-specific band in the spectrum of CO adsorbed on Lay. However, recent diffractionstudies have shown that there are differences between the cation distribution indehydrated cerium and lanthanum zeolites.’ Also the CeY-80, unlike the CeY-40and CeY-71, was prepared at 100°C and Sherry l6 has found that lanthanum exchangeat high temperatures is not fully reversible. The unusual adsorption behaviour ofCeY-80 may be a consequence of irreversibly exchanged ions.TABLE 5.-HEATS OF ADSORPTION OF co ON CeYheat of adsorption/kJ mol-1CeY-40 CeY-40 CeY-7 1outgassed outgassed outgassedcoverage/crnJ g-1 at 350°C at 460°C at 350°C0.01 37+ 1.0 42+ 2.0 40+ 2.00.05 29+ 1.0 27.5L4 2.0 42.5f 3.00.10 273- 1.0 28+ 2.0 29.5 *0.20 33k2.00.30 29f 2.00.40 28.5k2.0* This value is based on measurements made at only two temperatures.DISCUSSION1. SITE PREFERENCES OF DIFFERENT CATIONSAll divalent-ion-exchanged fornis of Zeolite Y examined show evidence of specificadsorption of CO. The extent of specific adsorption is variable and reflects thedistribution of cations between the different sites.We have indicated previously Othat these sites fall into three categories. Type I sites are inaccessible to all adsorbedmolecules.Cations in a second group are only accessible to molecules such as water(but not CO) which can enter the sodalite units. Cations in sites I1 and IIo interactwith all molecules which can enter the zeolite cages.Adsorption on low-exchanged zeolite is diagnostic for the distribution of multi-valent cations between accessible (supercage) and inaccessible (type I and sodalite)sites. An earlier studyio showed that Ca2+ ions at exchange levels below 50 %have an absolute preference for hidden sites. The present work shows that for otherdivalent ions the preference, although not absolute, is also strong and increases in theorderZn2+ < Cu2+ < Ni2+, Mn2+, UOZf < Ce3+, Ca2+.A more subtle problem concerns the distribution of the hidden cations betweenhexagonal prisms and the sodalite units.The most probable cause of a markedchange in adsorption properties at - 55 % exchange is preferential occupation of the16 hexagonal prisms per unit cell. Preferential occupation of each of the 8 sodaliteunits by only 2 divalent ions is unlikely since it is known that each sodalite unit canaccommodate at least 3 Ce3+ ionsa2 A compromise explanation that there are(16-x) divalent ions in the hexagonal prisms and x divalent ions in the sodalite unitsis inconsistent with the results of Dempsey and Olson.” These authors found thatin divalent X and Y zeolites there are two type I’ sites occupied for every empty site IT. A. EGERTON AND F. S . STONE 35We therefore conclude that the marked change in the adsorption characteristics ofMnY and NiY zeolites at 50-60 % exchange indicates that the cations mainly occupytype I positions at low degrees of exchange. The slow adsorption on CuY makesinterpretation of site preference difficult, but the adsorption results do not stronglysuggest that type I sites are occupied.Ions in the sodalite units are stabilized by small amounts of residual water, andraising the outgassing temperature removes this water.For ZnY and BaY theadsorption is more sensitive to the outgassing temperature than is the adsorption onNiY-46 and Cay-43. This indicates that for hidden zinc and barium ions thedistribution between the sodalite units and hexagonal prisms is more in favour of thesodalite sites than is the case for calcium and nickel ions.At 57 % exchange there are sufficient divalent cations to fill all 16 hexagonalprisms.If all type I sites are filled any further cations must occupy either sodalitesites or accessible sites. The adsorption results indicate that beyond 57 % exchangethe tendency to occupy the accessible sites decreases in the orderCa2+ > Ni2+ > Co2+.Of the ions investigated, only Ba2+ and U0$+ are sufficiently large to suggest thatthey might be excluded from the inaccessible sites for steric reasons. Barrer, Reesand Davies l8 found that about 16 sodium ions per unit cell of NaY cannot bereplaced by barium ions (Y = 0.134 nm). This suggests that at 25°C Ba2+ ionscannot enter the internal sites. However, since all the sodium can be replaced bypotassium (Y = 0.133 nm) we cannot conclude for certain that steric factors are thedominant ones, even for these three large ions.Since all the transition metal ionsare much smaller than barium it is clear that factors other than ionic radius areimportant in determining their site preferences.These factors include the following : (a) the difficulty of local charge compensationfor multivalent ions; (6) the need to attain a suitable coordination (in some casesby the addition of ligands from water); (c) the need to minimize the electrostaticenergy of the system (direct cation-cation repulsion must be avoided) ; (d) covalency,and directed bonds ; (e) crystal or ligand field stabilization.It would be naive to expect any single one of these causes to determine universallythe cation site preferences in Zeolite Y.Their relative importance will vary with thenature of the ion. Factors (a), (6) and (c) will all be influenced by the presence ofhydroxyl groups formed by the reactionM2+ + H20 -+ MOH+ + H+and the extent of this hydrolysis reaction will depend on the polarizing power of M2+.Since the formation of an MOH+ group stabilizes ions in the sodalite sites, the occupa-tion of sites I' and 11' will also depend both on the polarizing power of the charge-balancing cation and on the outgassing temperature. Crystal field stabilizationenergy will contribute to the stability of tetrahedral Co2+ in the sodalite sites, butcannot explain the affnity of Zn2+ ions for the sites.The distribution of the zincions must be a consequence of the tendency of Zn(I1) to form tetrahedrally directedcovalent bonds, a tendency which is reflected in the structure of many Zn(I1) com-pounds.2. COMPARISON OF THE SITE PREFERENCES WITH THOSE DEDUCED USINGOTHER TECHNIQUESIn this section the site preferences deduced from adsorption experiments are com-For the ions discussed in this paper X-ray diffraction results exist only for nickel,pared with the results of spectroscopic and X-ray diffraction experiments36 co ADSORPTION ON ZEOLITE Ycerium and copper. The nickel ions in a single crystal sample of nickel faujasite[Ni2,Ca,(A10z),,(SiOz),,,] occupy two-thirds of the type I sites and the remainingnickel ions are distributed between sites I1,II’ and I’., In NiY [Nil,Naz,H,(AI02),,(Si02)136] outgassed at 300”C, 10 nickel ions were found to occupy site I, and whenthe evacuation temperature was raised to 600°C the number of nickel ions in site Irose to 12.18 Both sets of results are in fair accord with the present conclusionsbased on CO adsorption.In dehydrated cerium faujasite the cerium ions occupy the I‘ positions.lg InCeX heated in Nz the ions mainly occupy site 1’, but site I and perhaps a site in thecentre of the ring of 12 oxygen atoms which joins adjacent supercages are alsooccupied.2 The adsorption results are consistent with the cerium ions in CeY-40and CeY-71 occupying site 1’.The results for CeY-80 suggest that a few cerium ionsare accessible to adsorbed CO.These accessible cerium ions, perhaps in the site be-tween the supercages, may correspond to the ions which Sherry found to be irreversiblyexchanged when LaY was prepared at high temperatures.The diffraction results for dehydrated Cu, 6Na24Y zeolite 2o indicate that copperions occupy both I and I’ sites. E.p.r. studies 21e 22 indicate that cupric ions occupytwo different environments in Zeolite Y, but no author has yet unambiguouslydefined these environments in terms of the known sites in the zeolite framework.The present CO measurements indicate that type I sites are not exclusively occupiedin either CuY-32 or CuY-50 and are consistent both with the X-ray diffraction andthe e.p.r. studies. The results also show that contrary to the assumption ofRichardson 23 not all the Cu2+ ions are in the supercages.Barry and Lay have measured e.p.r.spectra of Mn2+ probe ions in Zeolite Y.They deduced that in MnY-1 dehydrated at 300°C or above the Mn2+ ions are insites 11, I’ or 11’. Our results indicate that in MnY-32 there are occasional ions insite 11, but mostly the Mn ions are in internal sites, probably site I. The comparisonis not altogether valid because of the widely different concentrations of Mn2+ ionsin the two cases. In the e.p.r. experiments with dilute MnY-1, for example, the Mnions might be associated with a very few 6-membered rings rich in aluminium. Also,the curved heat plot for CO on MnY-75 indicates that the number of accessible Mnions may be temperature-dependent. As already pointed 0utY3 the various factorswhich determine site preference are very finely balanced in this case.In ZnY-67,Barry and Lay deduced from Mn2+ probe spectra that Zn2+ ions occupy sites 11,11‘ and 1’, and that the relative preference of the two ions for site I as compared withsite I1 or similar sites was Mn2+ > Zn2+. Our CO data are in full agreement withthis conclusion, although we doubt whether Zn2+ ions totaZZy shun site I.3 Amongstthe divalent ions we have investigated at - 30 % exchange, Zn2+ has the greatesttendency to show specific adsorption of CO and Ca2+ the least (fig. 2). At highdegrees of exchange, however, the amounts of CO adsorbed on ZnY and CaY aresimilar. For instance, the specific adsorption of 2.73 cm3 8-l on ZnY-62 at 0°Ccompares with 2.5 cm3 g-l on Cay-62, as interpolated from our data on Cay-54and CaY-64.1° This further emphasizes the point that comparison of behaviour atone degree of exchange is not necessarily a good index of relative site preference atsome other, widely different degree of exchange.measured the i.r. spectrum of CO adsorbed on Bay-80and concluded that the first 16 barium ions did not have an absolute preference forsite I, perhaps because of steric effects. However, the adsorption was less thanwould occur if all the Ba2+ were accessible to adsorbed molecules (16.4 instead of22.4 sites per u.c.).The present results confirm that barium is not totally excludedfrom inaccessible sites.Rabo and co-workerT. A . EGERTON AND F . S. STONE 37Rabo et aL9 found that the intensity of the infra-red band for CO on COY wasmuch less than that which would be expected if all Co2+ ions in excess of the first 16were to occupy sites accessible to CO.This result is confirmed in the present workand is attributed to cobalt having a high preference for the I' and 11' sites within thesodalite cages. Ions in these sites would be in quasi-tetrahedral coordination. Sucha coordination is indicated by reflectance spectra and by bulk magnetic suscepti-bility measurements * on dehydrated COY.3. THE NATURE OF THE co ADSORPTIONWe have previously shown that the presence of accessible calcium or zinc ionscauses a marked increase in the heat of adsorption of CO on Y Thepresent results for MnY and BaY confirm that divalent cations in general cause thisincreased adsorption heat.In fig. 12 the low coverage heats of CO adsorption onFIG. 12.-The relationship between the isostericheat of CO adsorption and (1) the i.r. stretchingfrequency of adsorbed CO (open circles) (2) theelectrostatic field of the charge-balancing cation(filled circles)u/cm-2180 2200 2220I I 12 0 I 1 I2 .o 3.0 4 .O10-lo fieldlV m-1Nay, Cay, ZnY and BaY are plotted as a function of the electrostatic field experiencedby the CO molecule. The field was calculated without taking shielding effects intoaccount and the distance of CO from the cation was taken as equal to the ionic radiusof the cation plus the van der Waals radius of CO (0.15 nm). Although the model iscrude, it is evident that the heat of adsorption is proportional to the field strengthexperienced by the CO molecule. All the points except that for CeY lie close to astraight line.It may be that the effective charge on the cerous ion is reduced by aprocess of the kindH20 + O:;mework + Ce3+ + Ce(OH)2+ + OH-.The fit in fig. 12 for CeY would be much more satisfactory if the charge is taken as+2 instead of +3. Since CO has negligible permanent dipole the increase of AHwith field, F, cannot be ascribed to a classical field dipole interaction pF, nor wouldthe classical polarization contribution, +aF2, to the energy of interaction lead to alinear increase in AH with F.Angel1 and Schaffer found that when CO was adsorbed on divalent-ion-ex-changed zeolites the CO stretching band occurred at higher than the gas phasefrequency.The shift, Av? was proportional to the electrostatic field acting on th38 co ADSORPTION ON ZEOLITE YCO niolecule. They suggested that under the influence of the eIectrostatic fieldthere was a transfer of the electrons in the lone pair orbit of the C atom towards thecation of the zeolite ; this process is analogous to the first stage in the formation of a~ - - n bond in a metal carbonyl. The good correlation between Av and AH (fig. 12)suggests that this mechanism also contributes to the high heat of adsorption onzeolites.Angell and Schaffer4 found an anomalous value for Av of CO on NiY andattributed it to the influence of the partially filled d orbitals of Ni2+. The anomalousstrength of the adsorption of CO on NiY which we observed may also be due to thesed orbitals, perhaps because they allow a small degree of back-bonding to occur.On both CuY and NiY samples a slow adsorption occurred.This probablycorresponds to an increase in the number of adsorption sites and is caused by a migra-tion of cations from inaccessible sites in the structure. Gallezot, Ben Taarit andImelik l8 report that NO and NH3 adsorption lowers the number of nickel ions insite I of NiY. Carbon monoxide, unlike ammonia and nitric oxide, is unable toenter the sodalite units of the zeolite structure.lO Therefore the migration of nickelions from inaccessible sites must be induced indirectly; perhaps the CO adsorptionperturbs the environment of the nickel ions by causing a distortion of the zeoliteframework. In NiY most of the inaccessible ions are in the hexagonal prisms, whilstfor CuY most of the inaccessible ions are in sodalite units. Gallezot, Ben Taaritand Imelik 2o have shown that for CuY molecules such as pyridine and butenereadily induce migration of Cu2+ ions from site I' into the supercages. Hence theinaccessible ions in NiY are more completely hidden and this explains why the slowmigration occurs to a much smaller extent in NiY than in CuY.We thank Dr. I. M. Rouse for many helpful discussions.D. H. Olson, J. Phys. Chem., 1969, 72,4366.F. D. Hunter and J. Scherzer, J. Catalysis, 1971,20,246.T. I. Barry and L. A. Lay, J. Phys. Chem. Solids, 1968,29,1395.C . L. Angell and P. C. Schaffer, J. Phys. Chem., 1966,70,1413.K. Klier and M. Ralek, J. Phys. Chem. Solids, 1968, 29, 95.T. A. Egerton, I. M. Rouse and F. S. Stone, to be published.W. N. Delgass, R. L. Garten and M. Boudart, J. Phys. Chem., 1969,73,2970,J. A. Rabo, C. L. Angell, P. H. Kasai and V. Schomaker, Disc. Faraday SOC., 1966, 41, 328.* T. A. Egerton, A. Hagan, F. S. Stone and J. C. Vickerman, J.C.S. Faraday I, 1972, 68,723.lo T. A. Egerton and F. S. Stone, Trans. Faraday Soc., 1970,66,2364.l1 E. Dempsey and D. H. Olson, J. Phys. Chem., 1970,74,305.l2 L. H. Ahrens, Geochim. Cosmochim. Acta, 1952,2, 155.l3 D. J. C. Yates, Canad. J. Chem., 1968,46, 1695.l4 R. M. Barrer and R. M. Gibbons, Trans. Faraday Soc., 1963,59,2569.l5 J . A. Rabo, C. L. Angell and V. Schomaker, Proc. 4th Int. Congress Catalysis, Moscow, 1968l6 H. S. Sherry, J. 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