J. Phys. Chem. B 1998, 102, 839-856
839
Cation Migration in Zeolites: An in Situ Powder Diffraction and MAS NMR Study of the Structure of Zeolite Cs(Na)-Y during Dehydration Poul Norby,† Faiza I. Poshni,† Alessandro F. Gualtieri,‡ Jonathan C. Hanson,§ and Clare P. Grey*,† Department of Chemistry, SUNY Stony Brook, Stony Brook, New York 11794-3400; Dipartimento Scienza delle Terra, UniVersity of Modena, Modena, Italy; and Chemistry Department, BrookhaVen National Laboratory, Upton, New York 11973 ReceiVed: September 16, 1997; In Final Form: NoVember 11, 1997
In situ synchrotron X-ray powder diffraction and 133Cs and 23Na MAS NMR have been used to investigate the cation migration and ordering in samples of cesium-exchanged zeolite NaY as a function of temperature and cesium cation-exchange level and during dehydration. Samples were prepared with cesium-exchange levels varying from 68 to 83% of the total cation content by carrying out repeated ion-exchange and calcination steps. Lower cesium content samples contain sodium cations in the sodalite cages and cesium cations in the supercages (SII and SIII) directly after the ion-exchange process. After dehydration at 350 °C and above, sodium cations are observed in the supercages (in SII) and in the double 6-rings (SI). A maximum of 8 cesium cations/unit cell (1/sodalite cage) migrate from the super to the sodalite cages, occupying SII′ and SI′ positions. The supercage sodium cations are then exchanged for cesium in the subsequent ion-exchange steps, increasing the cesium content. In situ X-ray data, collected during dehydration, showed that the sodium migration from SI′ to SI occurs initially (at >180 °C). This migration appears to be accompanied by, or drives, the migration of Cs+ from the supercages to the sodalite cages, which occurs at approximately the same temperature (>200 °C). It is not until >300 °C that the migration of SI′ sodium cations to SII is observed. Significant variations in the cation occupancies within the cages are seen at different temperatures in the dehydrated samples. For example, at 500 °C, there are an equal number of SII and SIII′ cesium cations in the lower cesium content sample. On cooling, the cesium cations order in the SII position, the SIII′ occupancy dropping from 12.5 to 7. An ordering scheme for the cations in the supercage is suggested to explain these observations. A number of resonances are seen in the 133Cs MAS NMR, which are assigned, making use of the occupancies obtained from the Rietveld refinements, to the various cesium positions. The lack of spinning sidebands associated with some of the resonances indicates that the some cations in the supercages are mobile, even at room temperature. When the temperature is raised, a number of SII and SIII resonances coalesce, as the cation mobility in the supercages increases.
Introduction Knowledge concerning the positions of the extraframework cations is critical to the understanding of the adsorption, separation, and catalytic properties of cation-exchanged zeolitic materials. Although the structure of a zeolite framework will define the size, shape, and interconnectivity of pores and cavities within the material, the nature and positions of the cations will control the electrostatic fields present within the zeolite pores, which can strongly influence the adsorption and reactivity of the sorbed molecules. The positions of the cations are often temperature-dependent, and it is, therefore, insufficient to know the structure at room temperature only: Determination of both the framework structure and the cation positions, under the actual working conditions of the material, becomes necessary in order to understand the physical properties of the material fully. Migrations of cations, between different cation positions, are often observed in zeolitic materials when adsorbing and * To whom correspondence should be addressed. † SUNY Stony Brook. ‡ University of Modena. § Brookhaven National Laboratory.
desorbing water. The cations in hydrated zeolites are typically coordinated to both the oxygen atoms of the framework and the water molecules, maximizing the coordination around the cations and stabilizing the cation positions. The coordination of the cation changes when the zeolite is dehydrated, and to obtain a more energetically favorable situation, either the framework can flex to accommodate the coordination requirements of the cation or the cation may move to a different position where it can obtain a higher coordination environment. Examples of significant framework distortions during dehydration are typically found in medium pore zeolites such as natrolite1 and zeolite Li-ABW.2 In these zeolites, the framework distortion causes the shape of the channels to change considerably, while preserving the topology and connectivity of the framework. The positions of the cations remain similar, but the change in the channel shape increases the coordination number of the cations. A combination of cation movement and framework distortions is, for example, observed in scolecite, mesolite, and laumontite.3,4 In larger pore zeolites, only minor changes to the framework are usually observed, but the changes in cation positions may be significant. This is particularly true for zeolites ion exchanged with bi- and trivalent cations where
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840 J. Phys. Chem. B, Vol. 102, No. 5, 1998 long-range migrations of the cations may occur to allow higher coordination sites to be occupied.5 There are now a number of studies where interactions between adsorbed molecules and extraframework cations have been demonstrated from crystallographic studies. For example, crystallographic studies have shown an interaction between the π electrons of benzene and the sodium cations on adsorbing benzene in faujasite-type zeolites.6 More recently, we have been studying the separations of hydrofluorocarbon mixtures such as HFC-134 (CF2HCF2H) and HFC-134a over basic molecular sieves and have shown that the interactions between HFC-134 (CF2HCF2H) and the sodium cations in zeolite Na-Y are strong enough to cause migration of cations from the sodalite cage into the supercage.7 Zeolite Cs-Y was shown to have the largest separation factor for the HFC-134/134a separation over a series of alkali-metal-exchanged Y zeolites.8 Subsequent calorimetry studies showed that Cs-Y also had the highest isosteric heat for HFC-134 adsorption.9 The Cs-Y used in these studies was prepared by ion-exchanging Na-Y and contained residual sodium cations. To rationalize the heats of adsorptions and separation factors obtained for this material, we required an accurate determination of the locations of the sodium and cesium cations in this structure, at the temperatures used for the adsorptions (20-90 °C)9 and separations (200 °C).8 We have, therefore, performed a joint 133Cs and 23Na MAS NMR spectroscopy and X-ray powder diffraction study of cesiumexchanged zeolite Y samples dehydrated at various temperatures under vacuum, and an in situ X-ray diffraction study of the dehydration process, to probe the structural changes that occur upon dehydration and to determine cation positions at elevated temperatures, as a function of the cesium-exchange level. A representation of the faujasite framework structure is given in Figure 1a, showing the tetrahedral framework sites. The faujasite structure can be viewed as being built from cubooctahedral dodecahedra (sodalite cages or β-cages) linked together at four of the eight 6-ring windows. This creates three different types of cages in the structure: the supercages (or R-cages), which are interconnected via 12-ring windows to four other supercages, the sodalite cages, which are connected via 6-ring openings to four adjoining supercages, and finally, the double 6-ring prisms, which link the sodalite cages together.10 An enlarged view of two sodalite cages connected via the double 6-ring prism is seen in Figure 1b, showing the positions of the SI, SI′, SII′, and SII cation sites. The SI site in the double 6-ring has six nearest oxygen neighbors at distances of approximately 2.5 Å. The double 6-rings are not very flexible, making it difficult to accommodate the large Cs cation in this position. The SI′ and SII′ positions may, however, be occupied by cesium cations. Both these positions are located on the 3-fold axes, with SI′ in front of the double 6-ring and SII′ in front of the 6-ring leading to the supercage. Both supercage sites (SII and SIII) can also be occupied by cesium cations: The SII position lies on the 3-fold axes in front of the 6-ring of the sodalite cage, while the SIII position faces one of the 4-rings in the supercage and lies on the intersection of two mirror planes. In some cases, cations have been located slightly off this high-symmetry SIII position, and these sites are collectively named SIII′. In anhydrous zeolite Na-Y, the sodium cations are mainly located on SII and SI′, with an additional lower occupancy of SI.6 Adjacent SI and SI′ positions cannot be simultaneously occupied, due to the very short distance between these two positions, and the same also hold true for the SII and SII′ sites. SIII is not significantly populated in zeolite Na-Y. Sodium cations are, however, found in several sites close to SIII in
Norby et al.
Figure 1. Representation of the faujasite structure showing only the tetrahedral atoms (a, top). The structure can be viewed as being built up from sodalite cages linked together to form double 6-rings (b, bottom). The positions of the cation sites SI, SI′, SII′, and SII are indicated.
zeolite Na-X, which also adopts the faujasite structure but has a lower Si/Al ratio and thus an increased sodium content.11 The geometrical constraints on simultaneous occupation of cation sites for mixed Cs/Na faujasites will be discussed later. A number of previous studies of the cesium exchange of synthetic faujasite zeolite Na-Y have been reported. In an early study, Sherry reported that the maximum level of cesium exchange is only 68%.12 Cesium cations (ionic radius 1.7 Å) are only able to exchange with the sodium cations in sites located in the supercage, since the entrance to the sodalite cage (2.2 Å) limits the passage of cesium cations into the sodalite cage during the ion-exchange process. Upon dehydration, the cesium cations were found to diffuse slowly into the sodalite cage, while the sodium cations migrated into the smaller hexagonal prisms and into the supercage.13,14 The sodium cations that have migrated to positions in the supercage can then be exchanged by repeating the ion-exchange process. Higher cesium exchange levels are, therefore, achieved by repeating the ion-exchange, calcination, and rehydration procedure a number of times.13 Recently, 100% exchange of Cs in zeolite Y was achieved by using a solid-state ion-exchange method involving the direct reaction of zeolite NH4-Y with CsOH.15 There are some differences in the cation positions obtained in the studies of the partially exchanged cesium faujasites and in the assignments of the 133Cs spectra: Koller et al.,13 in a joint MAS NMR and powder diffraction study of a 72% cesiumexchanged sample of dehydrated Cs(Na)-Y, found cesium
Cation Migration in Zeolites cations in SI′, SII, and SIII and sodium cation in SI and SII′. Cesium cations were also found in SI, which was surprising, given the size of this site. More recently, Malek et al., in a 133Cs MAS NMR study of bare and W(CO) -adsorbed 6 Cs(Na)-Y zeolite, proposed a different set of NMR assignments and a slightly different arrangements of cesium cations.14 No evidence for occupation of SI by cesium was seen. Rubidium cations were, however, detected in the SI position, in an X-ray single-crystal study of partially exchanged RbNa-X.16 Ultimately, a loss of crystallinity was observed, which was postulated to be due to the rubidium cations in these positions. Thus, the even larger cesium cations might also be expected to destroy the zeolite structure, if this site were occupied. One cesium cation was refined in the SI position, in CsNa-X, but no loss of crystallinity was observed, possibly due to the very low occupancy of this site.16 The combination of X-ray diffraction and 133Cs MAS NMR spectroscopy should lead to very detailed structural information concerning the cation environments in these materials. Cesium, with its larger cation radius can, in many cases, be easily distinguished from sodium in Rietveld refinements using powder diffraction data, due to the difference in cation-oxygen distances. The 100% abundant 133Cs(I ) 7/2) nucleus is very sensitive to its local environment, as is demonstrated by its large chemical shift range.17 The quadrupole coupling constants at 133Cs sites are typically very small, and hence the broadening of the 133Cs resonances, due to the second-order quadrupolar interaction, is insignificant. 133Cs is thus a favorable nucleus to use to study the short-range ordering and the dynamics of the extraframework cesium cations in hydrated and dehydrated zeolite samples.18 23Na(I ) 3/2) MAS NMR line shapes, in contrast, are dominated by second-order quadrupolar interactions, making it difficult to resolve the resonances from different chemical environments without the use of various solid-state NMR techniques or different field strengths.19 Experimental Section Sample Preparation. Zeolite Cs-Y was prepared by ionexchanging zeolite Na-Y (Strem Chemicals) with 0.1 M CsNO3 at a temperature of 85 °C over a period of 48 h. The sample was filtered and then washed with distilled water. This procedure was repeated three times. The samples were then dried at 100 °C in an oven for 48 h to remove excess water. Dehydration of the one-time-exchanged sample was carried out by ramping the temperature, under vacuum, to either 300, 350, 400, or 500 °C over a period of 12 h. The temperature was then held for a further 24 h. These samples are named 1xCsY300, 1xCs-Y350, 1xCs-Y400, and 1xCs-Y500, respectively. Further cesium exchange was achieved by successive dehydration and ion-exchange procedures. After the first ion exchange, the zeolite sample was first dehydrated at 100 °C for 24 h. The temperature was then ramped (under vacuum) to 450 °C over 24 h and was held there for an additional 48 h. Ion exchange was then carried out with a 0.1 M CsCl solution at 85 °C for 48 h. Three levels of cesium-exchanged zeolite were prepared: once, twice, and four times exchanged. These samples are labeled 1xCs-Y, 2xCs-Y, and 4xCs-Y, respectively. The samples were dehydrated at 500 °C for the MAS NMR experiments. The samples were placed in a glovebox under dry N2 and were then packed in either ZrO2 rotors for NMR experiments or capillaries, which were then flame-sealed, for powder diffraction. Elemental analysis obtained with ICP (Si, Al, and Na) and AAS (Cs) (Galbraith Laboratories) on the sample 1xCs-Y gave
J. Phys. Chem. B, Vol. 102, No. 5, 1998 841 a unit cell (u.c.) composition of Na18Cs38Si139Al53O384, corresponding to an exchange level of 68%, which is consistent with previous ion-exchange reports.12-14 29Si NMR gave a Si/Al ratio for this sample of 2.6, consistent with the elemental analysis. Thermal gravimetric analysis (TGA) (DuPont Corp.) determined a 16.7% (188 H2O molecules/u.c.) water loss when the sample was heated under air, at a ramping rate of 10 °C/min to 1000 °C. Dehydration is essentially complete by 450 °C, the two remaining water molecules (/u.c.) being lost by 600 °C. The TGA run was repeated at a slower ramping rate of 2 °C/min. Approximately 90% of the water is lost between 50 and 200 °C, leaving only approximately 23 water molecules/u.c. A more gradual loss of water then occurs between approximately 200 and 350 °C where approximately 21-22 molecules are lost. Dehydration was now almost completely achieved by 400 °C, the last water molecule (/u.c.) being removed by 550 °C. Chemical analysis on 4xCs-Y (University of New Mexico, Alberquerque) gave the composition: Cs43Na9Si134Al59O384‚ 108H2O, corresponding to an exchange level of 83%. MAS NMR. 133Cs MAS NMR experiments were performed with a 5.0 mm double-resonance Chemagnetics probe on a CMX-360 spectrometer at an operating frequency for 133Cs of 47.23 MHz. Small 133Cs flip angles of
