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Ion exchange and adsorption in layered phosphates
257
Materials Chemistry and Physics, 35 (1993) 257-263
Ion exchange and adsorption
in layered phosphates
A. Clearfield Department of Chemirby, Texas A&M Universi@ College Station, TX 77843 (USA)
Abstract The events leading up to the discovery of a brief description of their structures and phase the protons are present as hydronium a-zirconium phosphate and anhydrous salts chemical reactions. A brief description of ion-exchange resins is also presented.
crystalline (Y- and y-zirconium phosphates are described, along with aqueous ion-exchange behavior. The recent assertion that in the (Yions is refuted. A description of solid-solid ion exchange between is presented, along with the implications of such reactions for soft solid-solid exchange applied to zeolites and to strong acid organic
Introduction On the occasion of my 65th birthday I thought it worthwhile to review some of the history of the development of the chemistry of zirconium phosphates. In the late 1950s I was employed by the National Lead Company (now NL Industries) in their Titanium Alloy Division in Niagara Falls, New York. At that time, they were the largest supplier of zirconium chemicals in the world. My supervisor, Mr Warren Blumenthal, who was head of the chemistry division, asked me to search the literature for new potential uses of zirconium compounds. In doing so I became intrigued by papers that were just appearing in print [l-3] on the ionexchange properties of certain amorphous zirconium compounds, primarily the phosphate. I presented a report to the effect that these compounds might find important usage commercially and that we should initiate a project in that direction. This suggestion did not meet with approval, so I was left to find other means of studying these compounds. The opportunity presented itself as a result of my association with Niagara University. I had been teaching a crystallography course there in the evening and was asked to supervise the thesis research of Timothy Adams, an M.S. candidate. Tim prepared a dozen or so zirconium phosphate gels using ammonium phosphate as a precipitant. These products all exhibited strange compositions, which we later traced to the presence of NH3 in the gels and variable P:Zr ratios. Subsequent work with James Stynes, who was also an M.S. candidate, showed that the 2:l ratio could be achieved by use of excess phosphoric acid. The composition of the gel dried over P,O, was now always close to
0254-0584/93/$6.00
Zr(HPO,),-H,O. I happened to notice in Stynes’s Xray patterns that those samples which were heated in the excess acid gave an indication that a peak was forming at about 9 A. I told Jim to reflux the zirconium phosphate gel, and sure enough, peaks began to appear in the X-ray pattern. In this early work [4] the products were not fully crystalline, but we knew early on that the compound was layered from electron micrographs which showed plate-like hexagons [5, 61 and from the fact that the position of the first reflection in X-ray pattern depended upon which ion was exchanged. By now the National Lead Company was interested in the work, and eventually we obtained a patent [7] on the crystals. We first presented our results at the Gordon Research Conference on Ion Exchange in 1962. Kurt Kraus was sitting in the first row with Dave Schwartz from BioRad. My talk created quite a bit of excitement, as it was realized that now the exchange properties could be placed on a quantitative basis. For example, the formula could now be fixed at Zr(HPO,),.H,O, abbreviated cu-ZrP, with an ion-exchange capacity of 6.64 meq gg’ (or 6.62 meq gg’ considering the presence of 2% hafnium). Group 4 and 14 phosphates with the a-structure bear a resemblance to clay layers [S]. The general formula for this group of compounds is M”‘(HPO.&~H,O, and the layers consist of a central layer of octahedrally coordinated metal atoms sandwiched between tetrahedral phosphate layers [9]. The major difference from the clays is the fact that the phosphate groups are inverted (Fig. 1). Three oxygens of each phosphate group bridge across metal atoms and the fourth points into the interlamellar space. The ion-exchange capacity
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2.58 OH
OH
OH
OH
bH
OH
CiH
OH
bH OH
OH
that the pyramids were boron phosphate, BPO,. Apparently boron was removed from the Pyrex by the acid. Therefore, we next used 10-12 M H,PO, in quartz tubes and were successful in obtaining high-quality single crystals of the a-phase, which allowed us to carry out the crystal structure described above. Large single crystals can now be grown routinely by the HF method
1171.
l
P
0
0
Fig. 1. Schematic representation of the layered structure of (Yzirconium phosphate, Zr(HPO&.HrO. The water molecule is omitted.
of these solids depends upon the mass of the metal. For example, the exchange capacity for a-ZrP is 6.64 meq g-l and that for the isomorphous titanium phosphate is 7.76 meq g-‘, values that are 4-12 times those of clay minerals. All of this charge is concentrated in the oxygens not bonded to metal, which results in covalent bonding with the exchangeable protons. The water molecule resides in cavities between the layers but is not a hydronium ion [lo, 111. Exchange of Na’ for the protons occurs with some expansion of the layers, but in general no swelling occurs for the crystalline phases. Because of the lack of swelling of this class of compounds, they did not lend themselves to exchange of large species. However, it was demonstrated [12] that large charged species or complexes could be exchanged into zirconium phosphate if the layers were first spread apart by amine intercalation. In his dissertation [13] Jim Stynes also mentioned that a second phase was obtained when the gel zirconium phosphate was refluxed in 4 M HCl containing 2 M ammonium phosphate. Subsequently, Bob Blessing prepared that phase in more crystalline form as the sodium phase and in the acid form as y-zirconium phosphate [14]. At first we thought that y-ZrP was an expanded layer form of a-ZrP, but after examining its ion-exchange properties [15, 161 we knew that it had to be fundamentally different. Early on we recognized that structural information would be essential to understanding the ion-exchange behavior of these compounds. We therefore began a program to grow single crystals of the (Y-and y-phases. Our first attempts at growing single crystals of the (Yphase were carried out in sealed PyrexQDtubes. After heating the gel for 3 days in cont. H,PO, at 160-170 “C we obtained little pyramids. I couldn’t believe our good fortune, until we obtained X-ray data that showed
We were also able to grow single crystals of y-ZrP. These were obtained in sealed tube reactions run at = 180 “C in a mixture of 3 M HCl and 6 M NaH,PO,. After about 6 months very thin rectangular crystals 1 x 0.3 x 0.01 mm were obtained [18]. At the time these crystals were prepared, we only had film cameras to record X-ray data and the crystals were too thin to obtain the needed data. Subsequently, Robert Buckley grew y-Tip crystals in sealed tubes at 300 “C in cont. H,P04. The crystals were very thin squares. As we tried to mount them the layers kept sliding over one another, so that a single crystal would exfoliate into 3 or 4 thinner crystals. These also were too thin to provide useful data. However, a partial structure has now been provided from powder data [19] which shows that two different phosphate groups, POb3- and H,PO,‘-, are present (Fig. 2). Although the structure was not fully refined, it contains sufficient detail to provide a ready explanation for the high level of hydrolysis of phosphate groups when more than 50% of the exchange sites are occupied by cations [16]. The pK of the two protons on the same phosphate group must differ by about 5 units. Thus, up to 50% exchange, one site is preferentially occupied. At higher levels of exchange the second proton, which has a high pKvalue, must be displaced. Exchange generally occurs in basic
Fig. 2. Schematic representation of y-zirconium phosphate, Zr(PO,)(H,PO,).2HrO (from U. Costantino and R. Vivani, in M. Abe, T. Kataoka and T. Suzuki (eds.), New Developmentsin Zen Exchange, Elsevier, New York, 1991, used by permission).
259
product are the plateaux perfectly flat, as required by the phase rule for an invariant system (zero degrees of freedom). The uptake of ions into the bulk of the crystals results in the immediate formation of a new phase of the incoming cation. As more cation is taken up, the phase spreads out from the outside inward [21]. The gel, on the other hand, distributes the cations uniformly throughout its structure, forming a complete solid-solution. The titration curve is characterized by a continuous increase in pH with cation uptake. In between these two extremes, solid-solution phases of limited solubility are formed and the curves are altered in the direction of the plateaux with increasing crystallinity of the exchanger, until finally phases of definite composition are obtained. Thus each gel will exhibit somewhat different characteristics depending on its composition, temperature of precipitation, aging, etc. Another factor of importance in the gels is their ability to swell in water. We have been able to swell 0.5:48 to an interlayer spacing of 11.2 8, (7.6 A is the ideal value in the crystals). This swelling we attribute to the formation of H,O+ [5, 201. In the gel the phosphate groups are not in their equilibrium positions, and some of the protons may experience a high electrostatic repulsion from the poorly complexed or shielded Zr4’. Since the interlayer space has a high water content, some of the water molecules can readily interact with these protons to form hydronium ions. Uptake of a small amount of sodium ion results in even more swelling, to an interlayer spacing of 12.6 8, [20]. It is easy to see why the gels easily take up Cs’
solution, resulting in hydrolysis. Aiding the hydrolytic process may be the fact that the two cations approach each other closely, since both negative sites are on the same phosphate group, destabilizing this grouping. The H,PO,‘- would also have a weaker attachment to the metal atoms, since two - rather than three - oxygens are bonded to metal atoms.
Some aspects of the chemistry of gel zirconium phosphate Many workers continued to publish papers on the ion-exchange behavior of o+ZrP gels after the discovery of the crystalline compound. Agreement among these studies was generally poor, and one serious problem was the lack of proper characterization of the gel. If one thinks in terms of thermodynamics, each phase of a system must be precisely defined. This was not the case for the gels, so discrepancies continued to appear in the literature. In an important and comprehensive paper [20] we attempted to show that the exchange properties depended strongly upon the preparative procedure. Gels were refluxed in 12 M H,P04 for various lengths of time (from 10 to 400 h) and in various concentrations of H,PO, (from 0.5 to 12 M) for 48 h. In each case the composition was Zr(HPO&aH,O, but the ion-exchange titration curves differed markedly. These differences resulted from the differences in the degree of crystallinity of the preparations. Part of the data is reproduced in Fig. 3. Only in the fully crystalline
1
I 2
3
4 Meq
5
(OH)-/g
6
7
1
0 (b)
I
2
3
ME0
4
5
(i
7
8
OH- ADDED/g
Fig. 3. Potentiometric titration curves for n-ZrP of different crystallinities. (a) (0) O&48; (A) 2548; (0) 4548; (B) 12:48. (b) (A) 12:48; (0) 12:96; (0) 12:190; dashed line indicates fully crystalline compound. Numbers indicate concentration of H3P04 and time, respectively, for reflux of an amorphous gel.
260
whereas the crystals do not, since the large ions cannot gain access to the interlamellar space in the crystals. However, the surface of the crystals shows an affinity for the alkali cations in the same order as the gels: Cs’>Rb+>K+>Na+>Li+ [22]. An intere. ;ing feature of the gels is their large surface areas. Even a semicrystalline product, 2.5:48, had a surface area of 90 m2 g-l and therefore should provide an excellent medium for sorption and chromatographic purposes. One such commercial use is as a sorbent in a portable artificial kidney [23]. A zirconium phosphate gel that has been exchanged with Na+ to pH 7.4 is used. At this pH about two thirds of the exchange sites are occupied by Na + . The dialysate solution is treated with the enzyme urease to produce CO2 and ammonium ion, which is then removed by the zirconium phosphate. It is highly probable that the actual mechanism is the intercalation of NH, on the available proton sites. At pH = 7.4 more than half the NH,+ is converted to NH3. The concentration of Na’ in the dialysate is constant because it *s at equilibrium with the Na+-gel. Thus only the NH3 can access the unused exchange sites. Recently the formula for crystalline cu-ZrP has been questioned [24]. These authors claim that the crystals contain a larger amount of water than one mole per formula weight and that the water molecules are present as hydronium ion, H,O+ . The problem with this study is that the authors did not ensure that they obtained fully crystalline a-ZrP. It has been abundantly shown [20, 251 that the water content depends upon the crystallinity and the drying procedure. In Fig. 4 we
reproduce earlier data [20] upon which this statement is based. When fully crystalline and dried over P205 or CaSO, to constant weight, the total water loss is 11.95%, representing one mole of crystal water and one mole of water generated from the condensation of the monohydrogen phosphate groups. The assertion that the protons in a-ZrP are present as hydronium ions is also incorrect. Both an X-ray [lo] and a neutron diffraction study [ll] clearly showed that the protons were bonded to phosphate oxygens and that the water molecule possessed only 2 hydrogen atoms. The situation may be different at the surface. Here there are no steric constraints imposed by the cavity nature of cy_ZrP. Thus more than one water molecule per Zr atom may be present. In fact, Alberti et al. [26] have shown that the surface conductivity is much higher than the bulk conductivity. All of this information has been detailed in review articles [5, 27, 281.
Solid-solid
ion exchange
We have shown that solid a-layered compounds are able to undergo ion exchange with anhydrous solid salts [29]. For example, when NaCl is mixed with anhydrous cy-ZrP and heated, HCl is evolved and sodium-exchanged phases are obtained. The phases formed are Na-I, NaII, C, and F or G (Table l), successively, as the sodium ion content of the solid increases. The reactions are reversible and endothermic, since hydrogen ion is selectively favored by the exchanger. The rates of the reverse reactions have been measured [30]. The starting phase was Zr(NaPO,), (phase F) , and the sequence of reactions is as follows: Fz
CT
Na-11%
Na-1’4’-
kZrP
(1)
TABLE 1. Phases formed by the dehydration of partially and fully Na+-exchanged ru-zirconium phosphate Designation
Composition of phase
Interlayer’ spacing (A)
Fig. 4. Weight loss vs. temperature for zirconium phosphates of various crystallinities. 1248 refers to a product obtained by refluxing an amorphous gel zirconium phosphate in 12 M H,PO, for 48 h. It is the most crystalline sample and 0.8:48 the least crystalline sample of the group. All samples were dried at 45-50 “C except 0.8:48, which was equilibrated at a,=0.997.
A B C D E F G H Na-I Na-II l-ZrP n-ZrP
ZrNaH(PO& . 5H20 ZrNaH(PO&.HrO ZrNaH(PO& Zr(NaPO&
.3H,O
Zr(NaPO,), _Hz0 Zr(NaPO.& Zr(NaPO& Zr(NaPO& ZrNa0.2Ht.s(PO& ZrN%.aHi.z(POJz Zr(HPO,)z Zr(HPO&
11.8 7.9 7.3 9.8 8.4 8.4 7.6 7.6 6.86 7.2 7.41 7.15
261
The reactions were diffusion controlled and could be fit by an equation for a spherical particie in a wellstirred fluid of limited volume. Diffusion coefficients are of the order of IO-‘2 cm2 s-’ in the range 150-200 “C, with activation energies of 10.3, 12.7 and 15.9 kcal mol-’ for reactions (l), (2) and (3), respectively. Extrapolation of the diffusion coefficient values to 25 “C yields coefficients of the order of lo-l5 cm2 s-l. Thus the gas-solid reactions are considerably slower than ion-exchange reactions in solution, which typically have difIusion coefficients of the order of 10-l’ cm2 s-’ [31]. Equilibrium constants have been determined for the reactions shown in eqn. (1). By use of a suitable summation of the constants for this series of reaction steps it is possible to obtain the equilibrium constant for reactions of the type Zr(HPO,),
+ 2M + e
Zr(MPO,),
+ 2H’
(2)
These equilibrium constants represent selectivities of the various cations independent of solvation effects, which are normally present in solution ion exchange. Such values have been obtained [32] for Li” , Na’ and K’ exchange with H’. Pressure composition isotherms for Na’-H’ and Li’-H+ are shown in Figs. 5 and 6. The selectivities were found to be in the order H” > Li’ > Na’ > K’, corroborating the predictions of Eisenman’s theory [33] for the electrostatic effect. We also examined the exchange of solid first row transition metal chlorides with cz-ZrP. These cations are difficult to exchange in aqueous solution, because they are highly hydrated and too large to enter the cyZrP cavities. The usual procedure is to empioy acetates at elevated temperature, usually under reflux, to effect exchange [34]. The solid state reactions proceeded smoothly at temperatures above 250 “C. The phase obtained had an interlayer spacing of 7.87 A, which has been identified as one of the many high-temperature
Fig. 6. Pressure-composition isotherms for lithium-ion-exchanged zirconium phosphate +HCl(d. Temuerature of reaction t’“CI: (+) 80; (6) lob; (x) 120;(b) 1401 (A) 180. ’ ’
phases formed by aqueous exchange followed by dehydration [35]. The solid state chemistry of the layered zirconium phosphates exchanged partially or fully by cations is very complex. This is readily seen in the isotherms of Figs. 5 and 6, where each pressure increase denotes the formation of a new phase. Partial nonequilibrium phase diagrams have been published [28, 351 that show that partially exchanged rr-ZrP, on heating to different temperatures, yields new layered phases. At higher temperatures three-dimensional compounds are formed. In the case of the half-exchanged sodium ion phase, ZrNaH(PO,), .nH,O, pure the triphosphate, NaZr,(PO,),, is obtained by heating to 400 “C and washing out the soluble remainder 1361.In some respects the exchange of ions into rw-ZrP followed by heating is a form of soft chemistry. Not only can new phases be prepared this way that cannot be prepared directly by solid state reactions, but there is evidence that on removal of the cation with acid, new layered proton~ntaining phases are obtained that cannot be prepared directly [36].
boor--------Soft chemistry 400 t
300 g P -
Fig. 5, Pressure-composition isotherms for ~ium-ion-exchanged zirconium phosphates+H~(g). Temperature of reaction (“C): (0) 140; (0) 180; (A) 200; (0) 260; (*) 280; (A) 300.
There are a number of cases where the layered phosphates have been utilized in a soft chemistry approach to prepare new phases. The most direct example is the synthesis of NH,Zr,(PO,), from a mixture of (YZrP and ammonia or simple amines [37]. This phase is impossible to obtain by a high-temperature solid state reaction because of the volatility of NH,. We have already mentioned the preparation of sodium dizirconium triphosphate, from NaZr,(P%, NaHZr(PO,), by heating to 400 “C. The reaction is probably as shown in eqn. (3): 2NaHZr(PO,),
“-, NaZr,(PO,),
+ NaP4
+ Hz0
(3)
262 The
sodium metaphosphate is readily removed by washing with water [38]. The triphosphates are normally prepared by solid state reactions at 900-1000 “C [39]. However, we have shown that, in addition to reaction (3), triphosphates can be obtained hydrothermally [40] in the temperature range 200-300 “C. These hydrothermal reactions have not been fully exploited. Another aspect of soft chemistry involving layered phosphates is their use as solid acids or bases. From the gas-solid isotherms of reaction (4), Zr(NaPO,),
i- HCl(g) e
Zr(HPO&
f 2NaCl
(4)
it has been determined that the free energy is negative, as is the enthalpy. The sodium-containing phases spontaneously react with HCl both in the gas phase and in solvent media. Thus Zr(NaPO,), can be used as a getter for HCl in certain reactions. We illustrate with the preparation of alkoxides from anhydrous halides (eqn- (5)): M=+X,+yROH
=
M(ORXX_,
+yHCl
(5)
No~ally, gaseous a~onia is added to remove the HCl, but if the ammonia is not perfectly dry, condensation reactions may take place. Sodium zirconium phosphate may be pre-added to the reaction mix in an amount sufficient to remove all the HCl. The solvent is chosen so as to keep the alkoxide dissolved when hot, and the insoluble zirconium phosphate and NaCl are filtered off and the alkoxide recovered on cooling. On heating, the NaCl-cr-ZrP mix easily regenerates Zr(NaPO& by the &id-solid reaction [41]. Another potentially useful reaction is the preparation of anhydrous salts. We examined the synthesis of acetates of ~~alent cations such as copper and nickel acetates. Reflnxing the anhydrous transition metal phase of cu-ZrP in glacial acetic acid resulted in the formation of the acetates in about 50% yield. Such reactions may prove useful where the carboxylates are readily hy drolyzed in aqueous systems. Complexes such as acetylacetonates have also been prepared in nonaqueous solvents by a similar technique. In the preceding reactions salt forms were used as reactants in which the layered framework acted as a base. However, cu-ZrP s~ntaneously intercalates ammonia and amines and can be used as a solid acid to neutralize such bases by intercalation. Reactions of this type have not been widely exploited.
Solid-solid exchange involving zeolites and organic resins Shortly after the discovery of solid-solid exchange with zirconium phosphate, we determined that zeolites behave in a similar fashion [42]. Zeolites A, Y and X
were mixed with anhydrous first row transition metal chlorides and heated. Temperatures of 350-450 “C were required to attain high conversions in reasonable times. Solid-Solid exchange in zeolites has been found to be quite general, reactions having been carried out with a variety of framework types and different salts 1431. There are several advantages to using solid-solid ion exchange: (i) one can target precisely the amount of cation to be exchanged; (ii) ions that are difficult to incorporate from aqueous solutions because of their high degree of hydration are readily exchanged, even into narrow-pore zeolites; (iii) salts that are readily hydrolyzed in aqueous solution, such as MoC&, are difficult to exchange; however, by the solid-solid route large amounts of catalytically active species can be incorporated into the desired framework; and (iv) solid-solid exchange does not require the use of large quantities of salt solution. Strong acid organic cation exchangers exhibit a similar reaction. A partially dried Dowexa =50-8x resin was brought into contact with CuCl,, ‘FeCl,, AlCl, and ZrOCl, 1411.In each instance, rapid exchange occurred with evolution of HQ. The reactions were strongly exothermic and some additional water vapor evolved as the temperature increased. Apparently, concentrated solutions of the ions formed at the surface of the beads, followed by rapid exchange with HCl evolution. The nature of the species exchanged was not determined. However, it would appear that further consideration of such phenomena may yield interesting and unusual results.
I wish to extend my sincere thanks to the Chemistry Division of the National Science Foundation for continued support for my research efforts, starting in 1964. I am also grateful to the many wonderful students whose labors are here briefly recounted.
References K.A. Kraus and H.O. Phillips, J. Am. Chem. Sot., 78 (1956) 644. K.A. Kraus, H.O. Phillips, T.A. Carfson and J.S. Johnson, I’roc. 2nd UN Conf: Peacefir Uses ofAtomic Energy, Geneva, 1958, Vol. 28, p. 3. C.B. Amphlett, .prOc. 2nd UN Conf. Peaceful Uses of Atomic Energy Geneva, 1958, Vol. 28, p. 17. A. Clearfield and J.A. Stynes, J. Inotg. Nucl. Chem., 26 (1964) 117. A. Clearfield, G.H. Nancollas and R.H. Blessing, in J.A. Marinsky and Y. Marcus teds.), Ion Exchange and Solvent Ekfraction, Vol. 5, Marcel Dekker, New York, 1973, pp. l-119. A. Clearfield, Prrq Cystul Growth Charact., 21 (1990) 1.
263 7 J.A. Stynes and A. Clear-held, US Put. No. 3 416 884 (Dec. 17, 1968). 8 A. Clearheld, Comments Inorg. Chem., 10 (1990) 89. 9 A. Clearheld and G.D. Smith, Znorg Chem., 8 (1969) 431. 10 J.M. Troup and A. Clearfield, Znorg Chem., 16 (1977) 3311. 11 J. Albertsson, A. Oskarsson, R. Tellgren and J.O. Thomas, J. Phys. Chem., 81 (1977) 1574. 12 A. Clearfield and R.M. Tindwa, Inorg Nucl. Chem. Lett., 15 (1979) 251. 13 J.A. Stynes, M.S. Thesti, Niagara University, Apr. 17, 1961. 14 A. Clearfield, R.H. Blessing and J.A. Stynes, J. Inorg. Nucl. Chem., 30 (1968) 2249. 15 J.M. Garces, Ph.D. Dissertation, Ohio University, Aug. 26, 1972. 16 A. Clearfield and J.M. Garces, J. Inorg. Nucl. Chem., 41 (1979) 903. 17 G. Alberti and E. Torracca, J. Ino=. Nucl. Chem., 30 (1968) 317. 18 A. Cleartield and R.H. Blessing, unpublished work. 19 A.N. Christensen, E. Krogh-Andersen, I. Krogh-Andersen, G. Alberti, M. Nielsen and MS. Lehman, Acta Chem. Stand., 44 (1991) 865. 20 A. Clearfield, A. Oskarsson and C. Oskarsson, Zen l&h. Membr., I (1972) 91. 21 G. Alberti, Act. Chem. Res., 11 (1978) 163. 22 G. Alberti, M.G. Bemasconi, M. Casciola and U. Costantino, J. Chromutogr., 166 (1978) 109. 23 A. Gordon, A. Lewin, J. Rosenfeld, M. Roberts and M.H. Maxwell, Proc. 6th Int. Congr. Nephrology, Florence, 1975, pp. 612617. 24 Ph. Colomban and A. Novak, J. Mol. Struct., 198 (1989) 277. 25 A. Cleartield and S.P. Pack, J. Inorg. Nucl. Chem., 37 (1975) 1283. 26 G. Alberti, M. Casciola, U. Costantino, G. Levi and G. Ricciardi, J. Znorg. Nucl. Chem., 40 (1978) 533.
27 A. Clearfield, in A. Clearheld (ed.), Inorganic Ion Exchange Materials, CRC Press, Boca Raton, FL, 1982, p. 1. 28 A. Clearfield, Annu. Rev. Mater. Sci., (1984) 205. 29 A. Clearfield and J.M. Troup, J. Phys. Chem., 74 (1970) 2578. 30 A. Clearheld and P. Jirustithipong, in P. Vashista, J.N. Mundy and G.K. Shenay (eds.), Fast Zon Transport in Solids, Vol. 1, Elsevier North-Holland, New York, 1979, p. 153. 31 S.H. Harvie and G.H. Nancollas, J Inorg. Nucl. Chem., 30 (1968) 273. 32 A. Clearfield, G.A. Day and S.P. Pack, J. Phys. Chem., 87 (1983) 5003. 33 G. Eisenman, Biophys. J., SuppZ., 2 (1962) 1578. 34 A. Clearfield and J.M. Kalnins, J. Inorg Nucl. Chem., 38 (1976) 849. 35 A. Cleartield and S.P. Pack, 1. Inorg. Nucl. Chem., 42 (1980) 771; Mater. Res. Bull., 18 (1983) 1343. 36 A. Clearfield and AS. Medina, J. Inorg. Nucl. Chem., 32 (1970) 2775. 37 A. Clearlield, B.D. Roberts and M.A. Subramanian, Muter. Res. Bull., 19 (1984) 219. 38 A. Clear-held, W.L. Duax, A.S. Medina, G.D. Smith and J.R. Thomas, I. Phys. Chem., 73 (1969) 3424. 39 L. Hagman and P. Kierkegaard, Actu Chem. Stand., 22 (1968) 1822. 40 A. Cleartield, P. Jirustithipong, R.N. Cotman and S.P. Pack, Muter. Res. Bull., 15 (1980) 1603. 41 A. Clearfield and J.M. Troup, unpublished results. 42 A. Cleartield, C.H. Saldarriaga and R.C. Buckley, in J.B. Uytterhoeven (ed.), Proc. 3rd Znt. Conf. Molecular Sieves: Recent Progress Reports, Zurich, Switzerland, Sept. 3-7, 1973, University of Leuven Press, Leuven, Belgium, 1973, p. 241. 43 H.G. Karge and H.K. Beyer, in P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlovl (eds.), Zeolite Chemistry and Catalysis, Elsevier, Amsterdam, 1991, p. 43.
Materials Chemistry and Physics, 35 (1993) 257-263
Ion exchange and adsorption
in layered phosphates
A. Clearfield Department of Chemirby, Texas A&M Universi@ College Station, TX 77843 (USA)
Abstract The events leading up to the discovery of a brief description of their structures and phase the protons are present as hydronium a-zirconium phosphate and anhydrous salts chemical reactions. A brief description of ion-exchange resins is also presented.
crystalline (Y- and y-zirconium phosphates are described, along with aqueous ion-exchange behavior. The recent assertion that in the (Yions is refuted. A description of solid-solid ion exchange between is presented, along with the implications of such reactions for soft solid-solid exchange applied to zeolites and to strong acid organic
Introduction On the occasion of my 65th birthday I thought it worthwhile to review some of the history of the development of the chemistry of zirconium phosphates. In the late 1950s I was employed by the National Lead Company (now NL Industries) in their Titanium Alloy Division in Niagara Falls, New York. At that time, they were the largest supplier of zirconium chemicals in the world. My supervisor, Mr Warren Blumenthal, who was head of the chemistry division, asked me to search the literature for new potential uses of zirconium compounds. In doing so I became intrigued by papers that were just appearing in print [l-3] on the ionexchange properties of certain amorphous zirconium compounds, primarily the phosphate. I presented a report to the effect that these compounds might find important usage commercially and that we should initiate a project in that direction. This suggestion did not meet with approval, so I was left to find other means of studying these compounds. The opportunity presented itself as a result of my association with Niagara University. I had been teaching a crystallography course there in the evening and was asked to supervise the thesis research of Timothy Adams, an M.S. candidate. Tim prepared a dozen or so zirconium phosphate gels using ammonium phosphate as a precipitant. These products all exhibited strange compositions, which we later traced to the presence of NH3 in the gels and variable P:Zr ratios. Subsequent work with James Stynes, who was also an M.S. candidate, showed that the 2:l ratio could be achieved by use of excess phosphoric acid. The composition of the gel dried over P,O, was now always close to
0254-0584/93/$6.00
Zr(HPO,),-H,O. I happened to notice in Stynes’s Xray patterns that those samples which were heated in the excess acid gave an indication that a peak was forming at about 9 A. I told Jim to reflux the zirconium phosphate gel, and sure enough, peaks began to appear in the X-ray pattern. In this early work [4] the products were not fully crystalline, but we knew early on that the compound was layered from electron micrographs which showed plate-like hexagons [5, 61 and from the fact that the position of the first reflection in X-ray pattern depended upon which ion was exchanged. By now the National Lead Company was interested in the work, and eventually we obtained a patent [7] on the crystals. We first presented our results at the Gordon Research Conference on Ion Exchange in 1962. Kurt Kraus was sitting in the first row with Dave Schwartz from BioRad. My talk created quite a bit of excitement, as it was realized that now the exchange properties could be placed on a quantitative basis. For example, the formula could now be fixed at Zr(HPO,),.H,O, abbreviated cu-ZrP, with an ion-exchange capacity of 6.64 meq gg’ (or 6.62 meq gg’ considering the presence of 2% hafnium). Group 4 and 14 phosphates with the a-structure bear a resemblance to clay layers [S]. The general formula for this group of compounds is M”‘(HPO.&~H,O, and the layers consist of a central layer of octahedrally coordinated metal atoms sandwiched between tetrahedral phosphate layers [9]. The major difference from the clays is the fact that the phosphate groups are inverted (Fig. 1). Three oxygens of each phosphate group bridge across metal atoms and the fourth points into the interlamellar space. The ion-exchange capacity
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2.58 OH
OH
OH
OH
bH
OH
CiH
OH
bH OH
OH
that the pyramids were boron phosphate, BPO,. Apparently boron was removed from the Pyrex by the acid. Therefore, we next used 10-12 M H,PO, in quartz tubes and were successful in obtaining high-quality single crystals of the a-phase, which allowed us to carry out the crystal structure described above. Large single crystals can now be grown routinely by the HF method
1171.
l
P
0
0
Fig. 1. Schematic representation of the layered structure of (Yzirconium phosphate, Zr(HPO&.HrO. The water molecule is omitted.
of these solids depends upon the mass of the metal. For example, the exchange capacity for a-ZrP is 6.64 meq g-l and that for the isomorphous titanium phosphate is 7.76 meq g-‘, values that are 4-12 times those of clay minerals. All of this charge is concentrated in the oxygens not bonded to metal, which results in covalent bonding with the exchangeable protons. The water molecule resides in cavities between the layers but is not a hydronium ion [lo, 111. Exchange of Na’ for the protons occurs with some expansion of the layers, but in general no swelling occurs for the crystalline phases. Because of the lack of swelling of this class of compounds, they did not lend themselves to exchange of large species. However, it was demonstrated [12] that large charged species or complexes could be exchanged into zirconium phosphate if the layers were first spread apart by amine intercalation. In his dissertation [13] Jim Stynes also mentioned that a second phase was obtained when the gel zirconium phosphate was refluxed in 4 M HCl containing 2 M ammonium phosphate. Subsequently, Bob Blessing prepared that phase in more crystalline form as the sodium phase and in the acid form as y-zirconium phosphate [14]. At first we thought that y-ZrP was an expanded layer form of a-ZrP, but after examining its ion-exchange properties [15, 161 we knew that it had to be fundamentally different. Early on we recognized that structural information would be essential to understanding the ion-exchange behavior of these compounds. We therefore began a program to grow single crystals of the (Y-and y-phases. Our first attempts at growing single crystals of the (Yphase were carried out in sealed PyrexQDtubes. After heating the gel for 3 days in cont. H,PO, at 160-170 “C we obtained little pyramids. I couldn’t believe our good fortune, until we obtained X-ray data that showed
We were also able to grow single crystals of y-ZrP. These were obtained in sealed tube reactions run at = 180 “C in a mixture of 3 M HCl and 6 M NaH,PO,. After about 6 months very thin rectangular crystals 1 x 0.3 x 0.01 mm were obtained [18]. At the time these crystals were prepared, we only had film cameras to record X-ray data and the crystals were too thin to obtain the needed data. Subsequently, Robert Buckley grew y-Tip crystals in sealed tubes at 300 “C in cont. H,P04. The crystals were very thin squares. As we tried to mount them the layers kept sliding over one another, so that a single crystal would exfoliate into 3 or 4 thinner crystals. These also were too thin to provide useful data. However, a partial structure has now been provided from powder data [19] which shows that two different phosphate groups, POb3- and H,PO,‘-, are present (Fig. 2). Although the structure was not fully refined, it contains sufficient detail to provide a ready explanation for the high level of hydrolysis of phosphate groups when more than 50% of the exchange sites are occupied by cations [16]. The pK of the two protons on the same phosphate group must differ by about 5 units. Thus, up to 50% exchange, one site is preferentially occupied. At higher levels of exchange the second proton, which has a high pKvalue, must be displaced. Exchange generally occurs in basic
Fig. 2. Schematic representation of y-zirconium phosphate, Zr(PO,)(H,PO,).2HrO (from U. Costantino and R. Vivani, in M. Abe, T. Kataoka and T. Suzuki (eds.), New Developmentsin Zen Exchange, Elsevier, New York, 1991, used by permission).
259
product are the plateaux perfectly flat, as required by the phase rule for an invariant system (zero degrees of freedom). The uptake of ions into the bulk of the crystals results in the immediate formation of a new phase of the incoming cation. As more cation is taken up, the phase spreads out from the outside inward [21]. The gel, on the other hand, distributes the cations uniformly throughout its structure, forming a complete solid-solution. The titration curve is characterized by a continuous increase in pH with cation uptake. In between these two extremes, solid-solution phases of limited solubility are formed and the curves are altered in the direction of the plateaux with increasing crystallinity of the exchanger, until finally phases of definite composition are obtained. Thus each gel will exhibit somewhat different characteristics depending on its composition, temperature of precipitation, aging, etc. Another factor of importance in the gels is their ability to swell in water. We have been able to swell 0.5:48 to an interlayer spacing of 11.2 8, (7.6 A is the ideal value in the crystals). This swelling we attribute to the formation of H,O+ [5, 201. In the gel the phosphate groups are not in their equilibrium positions, and some of the protons may experience a high electrostatic repulsion from the poorly complexed or shielded Zr4’. Since the interlayer space has a high water content, some of the water molecules can readily interact with these protons to form hydronium ions. Uptake of a small amount of sodium ion results in even more swelling, to an interlayer spacing of 12.6 8, [20]. It is easy to see why the gels easily take up Cs’
solution, resulting in hydrolysis. Aiding the hydrolytic process may be the fact that the two cations approach each other closely, since both negative sites are on the same phosphate group, destabilizing this grouping. The H,PO,‘- would also have a weaker attachment to the metal atoms, since two - rather than three - oxygens are bonded to metal atoms.
Some aspects of the chemistry of gel zirconium phosphate Many workers continued to publish papers on the ion-exchange behavior of o+ZrP gels after the discovery of the crystalline compound. Agreement among these studies was generally poor, and one serious problem was the lack of proper characterization of the gel. If one thinks in terms of thermodynamics, each phase of a system must be precisely defined. This was not the case for the gels, so discrepancies continued to appear in the literature. In an important and comprehensive paper [20] we attempted to show that the exchange properties depended strongly upon the preparative procedure. Gels were refluxed in 12 M H,P04 for various lengths of time (from 10 to 400 h) and in various concentrations of H,PO, (from 0.5 to 12 M) for 48 h. In each case the composition was Zr(HPO&aH,O, but the ion-exchange titration curves differed markedly. These differences resulted from the differences in the degree of crystallinity of the preparations. Part of the data is reproduced in Fig. 3. Only in the fully crystalline
1
I 2
3
4 Meq
5
(OH)-/g
6
7
1
0 (b)
I
2
3
ME0
4
5
(i
7
8
OH- ADDED/g
Fig. 3. Potentiometric titration curves for n-ZrP of different crystallinities. (a) (0) O&48; (A) 2548; (0) 4548; (B) 12:48. (b) (A) 12:48; (0) 12:96; (0) 12:190; dashed line indicates fully crystalline compound. Numbers indicate concentration of H3P04 and time, respectively, for reflux of an amorphous gel.
260
whereas the crystals do not, since the large ions cannot gain access to the interlamellar space in the crystals. However, the surface of the crystals shows an affinity for the alkali cations in the same order as the gels: Cs’>Rb+>K+>Na+>Li+ [22]. An intere. ;ing feature of the gels is their large surface areas. Even a semicrystalline product, 2.5:48, had a surface area of 90 m2 g-l and therefore should provide an excellent medium for sorption and chromatographic purposes. One such commercial use is as a sorbent in a portable artificial kidney [23]. A zirconium phosphate gel that has been exchanged with Na+ to pH 7.4 is used. At this pH about two thirds of the exchange sites are occupied by Na + . The dialysate solution is treated with the enzyme urease to produce CO2 and ammonium ion, which is then removed by the zirconium phosphate. It is highly probable that the actual mechanism is the intercalation of NH, on the available proton sites. At pH = 7.4 more than half the NH,+ is converted to NH3. The concentration of Na’ in the dialysate is constant because it *s at equilibrium with the Na+-gel. Thus only the NH3 can access the unused exchange sites. Recently the formula for crystalline cu-ZrP has been questioned [24]. These authors claim that the crystals contain a larger amount of water than one mole per formula weight and that the water molecules are present as hydronium ion, H,O+ . The problem with this study is that the authors did not ensure that they obtained fully crystalline a-ZrP. It has been abundantly shown [20, 251 that the water content depends upon the crystallinity and the drying procedure. In Fig. 4 we
reproduce earlier data [20] upon which this statement is based. When fully crystalline and dried over P205 or CaSO, to constant weight, the total water loss is 11.95%, representing one mole of crystal water and one mole of water generated from the condensation of the monohydrogen phosphate groups. The assertion that the protons in a-ZrP are present as hydronium ions is also incorrect. Both an X-ray [lo] and a neutron diffraction study [ll] clearly showed that the protons were bonded to phosphate oxygens and that the water molecule possessed only 2 hydrogen atoms. The situation may be different at the surface. Here there are no steric constraints imposed by the cavity nature of cy_ZrP. Thus more than one water molecule per Zr atom may be present. In fact, Alberti et al. [26] have shown that the surface conductivity is much higher than the bulk conductivity. All of this information has been detailed in review articles [5, 27, 281.
Solid-solid
ion exchange
We have shown that solid a-layered compounds are able to undergo ion exchange with anhydrous solid salts [29]. For example, when NaCl is mixed with anhydrous cy-ZrP and heated, HCl is evolved and sodium-exchanged phases are obtained. The phases formed are Na-I, NaII, C, and F or G (Table l), successively, as the sodium ion content of the solid increases. The reactions are reversible and endothermic, since hydrogen ion is selectively favored by the exchanger. The rates of the reverse reactions have been measured [30]. The starting phase was Zr(NaPO,), (phase F) , and the sequence of reactions is as follows: Fz
CT
Na-11%
Na-1’4’-
kZrP
(1)
TABLE 1. Phases formed by the dehydration of partially and fully Na+-exchanged ru-zirconium phosphate Designation
Composition of phase
Interlayer’ spacing (A)
Fig. 4. Weight loss vs. temperature for zirconium phosphates of various crystallinities. 1248 refers to a product obtained by refluxing an amorphous gel zirconium phosphate in 12 M H,PO, for 48 h. It is the most crystalline sample and 0.8:48 the least crystalline sample of the group. All samples were dried at 45-50 “C except 0.8:48, which was equilibrated at a,=0.997.
A B C D E F G H Na-I Na-II l-ZrP n-ZrP
ZrNaH(PO& . 5H20 ZrNaH(PO&.HrO ZrNaH(PO& Zr(NaPO&
.3H,O
Zr(NaPO,), _Hz0 Zr(NaPO.& Zr(NaPO& Zr(NaPO& ZrNa0.2Ht.s(PO& ZrN%.aHi.z(POJz Zr(HPO,)z Zr(HPO&
11.8 7.9 7.3 9.8 8.4 8.4 7.6 7.6 6.86 7.2 7.41 7.15
261
The reactions were diffusion controlled and could be fit by an equation for a spherical particie in a wellstirred fluid of limited volume. Diffusion coefficients are of the order of IO-‘2 cm2 s-’ in the range 150-200 “C, with activation energies of 10.3, 12.7 and 15.9 kcal mol-’ for reactions (l), (2) and (3), respectively. Extrapolation of the diffusion coefficient values to 25 “C yields coefficients of the order of lo-l5 cm2 s-l. Thus the gas-solid reactions are considerably slower than ion-exchange reactions in solution, which typically have difIusion coefficients of the order of 10-l’ cm2 s-’ [31]. Equilibrium constants have been determined for the reactions shown in eqn. (1). By use of a suitable summation of the constants for this series of reaction steps it is possible to obtain the equilibrium constant for reactions of the type Zr(HPO,),
+ 2M + e
Zr(MPO,),
+ 2H’
(2)
These equilibrium constants represent selectivities of the various cations independent of solvation effects, which are normally present in solution ion exchange. Such values have been obtained [32] for Li” , Na’ and K’ exchange with H’. Pressure composition isotherms for Na’-H’ and Li’-H+ are shown in Figs. 5 and 6. The selectivities were found to be in the order H” > Li’ > Na’ > K’, corroborating the predictions of Eisenman’s theory [33] for the electrostatic effect. We also examined the exchange of solid first row transition metal chlorides with cz-ZrP. These cations are difficult to exchange in aqueous solution, because they are highly hydrated and too large to enter the cyZrP cavities. The usual procedure is to empioy acetates at elevated temperature, usually under reflux, to effect exchange [34]. The solid state reactions proceeded smoothly at temperatures above 250 “C. The phase obtained had an interlayer spacing of 7.87 A, which has been identified as one of the many high-temperature
Fig. 6. Pressure-composition isotherms for lithium-ion-exchanged zirconium phosphate +HCl(d. Temuerature of reaction t’“CI: (+) 80; (6) lob; (x) 120;(b) 1401 (A) 180. ’ ’
phases formed by aqueous exchange followed by dehydration [35]. The solid state chemistry of the layered zirconium phosphates exchanged partially or fully by cations is very complex. This is readily seen in the isotherms of Figs. 5 and 6, where each pressure increase denotes the formation of a new phase. Partial nonequilibrium phase diagrams have been published [28, 351 that show that partially exchanged rr-ZrP, on heating to different temperatures, yields new layered phases. At higher temperatures three-dimensional compounds are formed. In the case of the half-exchanged sodium ion phase, ZrNaH(PO,), .nH,O, pure the triphosphate, NaZr,(PO,),, is obtained by heating to 400 “C and washing out the soluble remainder 1361.In some respects the exchange of ions into rw-ZrP followed by heating is a form of soft chemistry. Not only can new phases be prepared this way that cannot be prepared directly by solid state reactions, but there is evidence that on removal of the cation with acid, new layered proton~ntaining phases are obtained that cannot be prepared directly [36].
boor--------Soft chemistry 400 t
300 g P -
Fig. 5, Pressure-composition isotherms for ~ium-ion-exchanged zirconium phosphates+H~(g). Temperature of reaction (“C): (0) 140; (0) 180; (A) 200; (0) 260; (*) 280; (A) 300.
There are a number of cases where the layered phosphates have been utilized in a soft chemistry approach to prepare new phases. The most direct example is the synthesis of NH,Zr,(PO,), from a mixture of (YZrP and ammonia or simple amines [37]. This phase is impossible to obtain by a high-temperature solid state reaction because of the volatility of NH,. We have already mentioned the preparation of sodium dizirconium triphosphate, from NaZr,(P%, NaHZr(PO,), by heating to 400 “C. The reaction is probably as shown in eqn. (3): 2NaHZr(PO,),
“-, NaZr,(PO,),
+ NaP4
+ Hz0
(3)
262 The
sodium metaphosphate is readily removed by washing with water [38]. The triphosphates are normally prepared by solid state reactions at 900-1000 “C [39]. However, we have shown that, in addition to reaction (3), triphosphates can be obtained hydrothermally [40] in the temperature range 200-300 “C. These hydrothermal reactions have not been fully exploited. Another aspect of soft chemistry involving layered phosphates is their use as solid acids or bases. From the gas-solid isotherms of reaction (4), Zr(NaPO,),
i- HCl(g) e
Zr(HPO&
f 2NaCl
(4)
it has been determined that the free energy is negative, as is the enthalpy. The sodium-containing phases spontaneously react with HCl both in the gas phase and in solvent media. Thus Zr(NaPO,), can be used as a getter for HCl in certain reactions. We illustrate with the preparation of alkoxides from anhydrous halides (eqn- (5)): M=+X,+yROH
=
M(ORXX_,
+yHCl
(5)
No~ally, gaseous a~onia is added to remove the HCl, but if the ammonia is not perfectly dry, condensation reactions may take place. Sodium zirconium phosphate may be pre-added to the reaction mix in an amount sufficient to remove all the HCl. The solvent is chosen so as to keep the alkoxide dissolved when hot, and the insoluble zirconium phosphate and NaCl are filtered off and the alkoxide recovered on cooling. On heating, the NaCl-cr-ZrP mix easily regenerates Zr(NaPO& by the &id-solid reaction [41]. Another potentially useful reaction is the preparation of anhydrous salts. We examined the synthesis of acetates of ~~alent cations such as copper and nickel acetates. Reflnxing the anhydrous transition metal phase of cu-ZrP in glacial acetic acid resulted in the formation of the acetates in about 50% yield. Such reactions may prove useful where the carboxylates are readily hy drolyzed in aqueous systems. Complexes such as acetylacetonates have also been prepared in nonaqueous solvents by a similar technique. In the preceding reactions salt forms were used as reactants in which the layered framework acted as a base. However, cu-ZrP s~ntaneously intercalates ammonia and amines and can be used as a solid acid to neutralize such bases by intercalation. Reactions of this type have not been widely exploited.
Solid-solid exchange involving zeolites and organic resins Shortly after the discovery of solid-solid exchange with zirconium phosphate, we determined that zeolites behave in a similar fashion [42]. Zeolites A, Y and X
were mixed with anhydrous first row transition metal chlorides and heated. Temperatures of 350-450 “C were required to attain high conversions in reasonable times. Solid-Solid exchange in zeolites has been found to be quite general, reactions having been carried out with a variety of framework types and different salts 1431. There are several advantages to using solid-solid ion exchange: (i) one can target precisely the amount of cation to be exchanged; (ii) ions that are difficult to incorporate from aqueous solutions because of their high degree of hydration are readily exchanged, even into narrow-pore zeolites; (iii) salts that are readily hydrolyzed in aqueous solution, such as MoC&, are difficult to exchange; however, by the solid-solid route large amounts of catalytically active species can be incorporated into the desired framework; and (iv) solid-solid exchange does not require the use of large quantities of salt solution. Strong acid organic cation exchangers exhibit a similar reaction. A partially dried Dowexa =50-8x resin was brought into contact with CuCl,, ‘FeCl,, AlCl, and ZrOCl, 1411.In each instance, rapid exchange occurred with evolution of HQ. The reactions were strongly exothermic and some additional water vapor evolved as the temperature increased. Apparently, concentrated solutions of the ions formed at the surface of the beads, followed by rapid exchange with HCl evolution. The nature of the species exchanged was not determined. However, it would appear that further consideration of such phenomena may yield interesting and unusual results.
I wish to extend my sincere thanks to the Chemistry Division of the National Science Foundation for continued support for my research efforts, starting in 1964. I am also grateful to the many wonderful students whose labors are here briefly recounted.
References K.A. Kraus and H.O. Phillips, J. Am. Chem. Sot., 78 (1956) 644. K.A. Kraus, H.O. Phillips, T.A. Carfson and J.S. Johnson, I’roc. 2nd UN Conf: Peacefir Uses ofAtomic Energy, Geneva, 1958, Vol. 28, p. 3. C.B. Amphlett, .prOc. 2nd UN Conf. Peaceful Uses of Atomic Energy Geneva, 1958, Vol. 28, p. 17. A. Clearfield and J.A. Stynes, J. Inotg. Nucl. Chem., 26 (1964) 117. A. Clearfield, G.H. Nancollas and R.H. Blessing, in J.A. Marinsky and Y. Marcus teds.), Ion Exchange and Solvent Ekfraction, Vol. 5, Marcel Dekker, New York, 1973, pp. l-119. A. Clearfield, Prrq Cystul Growth Charact., 21 (1990) 1.
263 7 J.A. Stynes and A. Clear-held, US Put. No. 3 416 884 (Dec. 17, 1968). 8 A. Clearheld, Comments Inorg. Chem., 10 (1990) 89. 9 A. Clearheld and G.D. Smith, Znorg Chem., 8 (1969) 431. 10 J.M. Troup and A. Clearfield, Znorg Chem., 16 (1977) 3311. 11 J. Albertsson, A. Oskarsson, R. Tellgren and J.O. Thomas, J. Phys. Chem., 81 (1977) 1574. 12 A. Clearfield and R.M. Tindwa, Inorg Nucl. Chem. Lett., 15 (1979) 251. 13 J.A. Stynes, M.S. Thesti, Niagara University, Apr. 17, 1961. 14 A. Clearfield, R.H. Blessing and J.A. Stynes, J. Inorg. Nucl. Chem., 30 (1968) 2249. 15 J.M. Garces, Ph.D. Dissertation, Ohio University, Aug. 26, 1972. 16 A. Clearfield and J.M. Garces, J. Inorg. Nucl. Chem., 41 (1979) 903. 17 G. Alberti and E. Torracca, J. Ino=. Nucl. Chem., 30 (1968) 317. 18 A. Cleartield and R.H. Blessing, unpublished work. 19 A.N. Christensen, E. Krogh-Andersen, I. Krogh-Andersen, G. Alberti, M. Nielsen and MS. Lehman, Acta Chem. Stand., 44 (1991) 865. 20 A. Clearfield, A. Oskarsson and C. Oskarsson, Zen l&h. Membr., I (1972) 91. 21 G. Alberti, Act. Chem. Res., 11 (1978) 163. 22 G. Alberti, M.G. Bemasconi, M. Casciola and U. Costantino, J. Chromutogr., 166 (1978) 109. 23 A. Gordon, A. Lewin, J. Rosenfeld, M. Roberts and M.H. Maxwell, Proc. 6th Int. Congr. Nephrology, Florence, 1975, pp. 612617. 24 Ph. Colomban and A. Novak, J. Mol. Struct., 198 (1989) 277. 25 A. Cleartield and S.P. Pack, J. Inorg. Nucl. Chem., 37 (1975) 1283. 26 G. Alberti, M. Casciola, U. Costantino, G. Levi and G. Ricciardi, J. Znorg. Nucl. Chem., 40 (1978) 533.
27 A. Clearfield, in A. Clearheld (ed.), Inorganic Ion Exchange Materials, CRC Press, Boca Raton, FL, 1982, p. 1. 28 A. Clearfield, Annu. Rev. Mater. Sci., (1984) 205. 29 A. Clearfield and J.M. Troup, J. Phys. Chem., 74 (1970) 2578. 30 A. Clearheld and P. Jirustithipong, in P. Vashista, J.N. Mundy and G.K. Shenay (eds.), Fast Zon Transport in Solids, Vol. 1, Elsevier North-Holland, New York, 1979, p. 153. 31 S.H. Harvie and G.H. Nancollas, J Inorg. Nucl. Chem., 30 (1968) 273. 32 A. Clearfield, G.A. Day and S.P. Pack, J. Phys. Chem., 87 (1983) 5003. 33 G. Eisenman, Biophys. J., SuppZ., 2 (1962) 1578. 34 A. Clearfield and J.M. Kalnins, J. Inorg Nucl. Chem., 38 (1976) 849. 35 A. Cleartield and S.P. Pack, 1. Inorg. Nucl. Chem., 42 (1980) 771; Mater. Res. Bull., 18 (1983) 1343. 36 A. Clearfield and AS. Medina, J. Inorg. Nucl. Chem., 32 (1970) 2775. 37 A. Clearlield, B.D. Roberts and M.A. Subramanian, Muter. Res. Bull., 19 (1984) 219. 38 A. Clear-held, W.L. Duax, A.S. Medina, G.D. Smith and J.R. Thomas, I. Phys. Chem., 73 (1969) 3424. 39 L. Hagman and P. Kierkegaard, Actu Chem. Stand., 22 (1968) 1822. 40 A. Cleartield, P. Jirustithipong, R.N. Cotman and S.P. Pack, Muter. Res. Bull., 15 (1980) 1603. 41 A. Clearfield and J.M. Troup, unpublished results. 42 A. Cleartield, C.H. Saldarriaga and R.C. Buckley, in J.B. Uytterhoeven (ed.), Proc. 3rd Znt. Conf. Molecular Sieves: Recent Progress Reports, Zurich, Switzerland, Sept. 3-7, 1973, University of Leuven Press, Leuven, Belgium, 1973, p. 241. 43 H.G. Karge and H.K. Beyer, in P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlovl (eds.), Zeolite Chemistry and Catalysis, Elsevier, Amsterdam, 1991, p. 43.