J. MATER. CHEM., 1994, 4( l), 9-12 Ion-exchange Properties of NASICON-type Phosphates with the Frameworks [TI,(PO,),I and [Ti, m7Alon3(P04)3] Naohiro Hiroset and Jun Kuwano" Department of Industrial Chemistry, Faculty of Engineering, Science University of Tokyo, 7 -3 Kagurazaka, Shinjuku-ku, Tokyo, 762 Japan Ion-exchange properties in aqueous A+ solution (A+=Li+, Na', K+, Ag', Rb+, NH:) are investigated for tne title materials at room temperature. In LiTi,(PO,), and Al-containing Li,.3Til.,AIo.,(P0,)3, Li ' is exchanged rapidly and selectively by Na' and Ag'. The exchange rate of Lil,3Til.7A10.3(P04)3 is faster than that of LiTi,(PO,),; this is probably related to a larger diffusion coefficient of Li' in the former due to its high Li' conductivity. Large ions such as K+ are not readily accepted by the anion frameworks.Accordingly, Lil,,Ti,,7Alo.3(P0,)3 has excellent Na+ selectivity with fast exchange rate to extract even a very small amount of Na' in reagent-grade KOH. The Na+-exchanged product of Lil,3Til.7A10.3(P04)3 can exchange Na' for Ag selectively. The reverse exchange also occurs easily. This material can + be used to recover Ag' in waste solutions. Some kinds of inorganic ion exchangers show excellent selec- tivity for a particular ion and high stability towards heat, chemical reagents and radioactivity compared with organic polymer resins. This stems from their rigid anion frameworks, which consist of covalently bonded 02-and multivalent cations; they behave like ion sieves. This 'ion-sieve effect' can be useful for collecting ions from sea water and waste solutions. The rate-determining step in the exchange is generally con- sidered to be the diffusion of the exchanging ions in the solid.',2 Fast-ion-conducting oxides, such as fi/fi"-A1203 or NASICONs, have rigid anion frameworks and exhibit high conductivities due to the large diffusion coefficients of the monovalent cations.This suggests that these materials should exhibit selective and fast-ion exchange even at room tempera- ture. Ono has reported, that the protonated NASICON-type compound, HZr,( PO,),, shows a selectivity for monovalent cations in aqueous solution in the order: Li', Na+>> Rb' >K+ >NH:; similar results were also described by Alberti.' However, so far most of the exchange properties of fast-ion conductors have been extensively studied only in molten salts6 in spite of the practical importance of ion exchange in aqueous solution.We have previously investigated the ion-exchange properties of Li+-conducting NASTCONs, LiTi2( PO,), [gbulk lo-, Scm-l (Aono et aL7+*)to Scm-' (Ando et al.9"0) at 25 "C] and its Al-containing form, Lil.3Til.7Alo.3(PO,), (Gbu1k% lop3 S cm-' at 25 0C)7-10 in aqueous solution, and have reported"*12 the rapid exchange rates of these materials and good selectivity of their frameworks for monovalent cations in the order: Ag' >Na' >Li+>> Kf , Rb+, NH;. Some of our previous have been recently confirmed by Mizuhara et al., who reported', exchange properties of LiTi,Zr2-x(P0,)3, and also by Hosono et a!., who reported', exchange properties of bulk, microporous LiTi,( PO,), pre-pared as a glass ceramic. This paper describes further ion-exchange properties of the title phosphates in aqueous solution at room temperature.The excellent selectivity of these materials towards a particular ion leads to possible applications. Experimental Samples of LiTi,( PO,), and Lil.3Til,7Alo,3( PO,), were pre- pared by conventional solid-state reactions. The starting mate- t Present address: Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB9 2UE, Scotland, UK. rials, Li2C0,, Ti02, A1203, and NH4H2P04 were mixed in hexane, dried and heated at 650 "C for 2 h to drive off' gases.The intermediates were ground thoroughly and calcined at 900 "Cfor 15 h. The products were reground and used in the following experiments. The average particle size was fcyund to be ca. 0.5-2 pm by SEM analysis. Ion-exchange treatment was carried out in 100ml of 0.25 moll-' aqueous solutions of alkali-metal chlorides and NH,Cl at 25 "C for 4days using 1 g amounts of sample. For the Ag' exchange, nitrate solution was used to avoid the precipitation of AgC1. The solutions were stirred continuously to disperse the exchanger particles and to enhance diffusion at the solid/liquid interface. Some experiments were carried out for different exchange times and with different initial c oncen-trations of the solutions. After the exchange treatment, the products were collected by filtration, washed with distilled water and dried under vacuum overnight at room temperature.The phases present in the samples before and after exchange attempts were identified by powder X-ray diffraction analysis (XRD) (Rigaku, RAD-IC; Kcr radiation with Ni filter). The Ag' concentration in some of the Ag+ exchange solutions was measured with an Ag' selective electrode (Toa Denpa; AG-125, 95% response time: 5-10 s); this allowed the exchange behaviour with exchange time to be studied Results and Discussion Exchange properties were also studied for the Na' and Ag' exchanged products of Lil.3Til,7Alo,3( PO,),. These results are summarized in Table 1 together with our previous results."*12 As reported ca.99% of Li + in LiTi:,( PO,), and Lil~3Til~7Alo,3( PO,), was exchanged by Na+ and Ag'. Fig. 1 and 2 show the variations of XRD patterns during the exchange. The end-member compound, LiTi2( PO,),, exhibited complete exchange in 28 days for Na' and in 4days for Ag'. The exchanged products were identified to be NaTi,( PO,), and AgTi,(PO,), using JCPDS cards (No. 33-1296, 35-737). In contrast, very rapid exchange was observed for Li1.3Ti1.7A10,3(PO,),. The exchange was almost completed in 30 min for Na' and in only 3 min for Ag'. The difference of exchange rate between Lil~,Ti,~7Al,~,(P04)3 and LiTiz(PO,), is probably due to the large diffusion coefficient of Li in the former, as expected from its high Li' conductivity 7-10 As evidently seen from these figures, the exchanged products form as separate phases, by a process of phase sepdration, J.MATER. CHEM., 1994, VOL. 4 Table 1 Extent of A+ exchange of MTi,(PO,),, Li,,3Til,7Alo,,(P04)3and the exchanged products of Li,~,Til~7Alo.,(P0,)3with 0.25 mol I-' A+ aqueous solution at 25 "Cfor 4 days" Li+ compound ionic radius/pm? 90 -LjTi2( PO4), NaTi,( PO,), N AgTi2( N KTiA PO413 N -Li1.3T1.7A10.3(Na +-exchanged product N Ag+-exchanged product N "C =complete exchange; P =partial exchange; N =no accompanied the exchange. A ? 0 10 20 30 Na+ Ag + K+ Rb' NH: 116 129 152 166 166 Ph Cb N N N -P' N N N -NN N N -NN N N Cb Cb N N N -C' N N N -NPC N N exchange.bPhase separation accompanied the exchange. 'Solid-solution formation B 0 A T A 40 10 20 30 40 2Oldegrees Fig. 1 X-Ray diffraction patterns of the exchanged products (triangles) during Na+ exchange of (A) LiTi,( PO,), and (€3) Li,,3Til,7Alo.3(PO,), (circles).A: (a) before exchange, (b)4, (c) 8, (d)28 days; B: (a) before exchange, (b)10, (c) 30 min rather than forming a range of partially exchanged solid solution. For Li1.,Til .7A10.3 (PO,),, excellent selectivity was observed in addition to the very rapid exchange. Fig. 3 shows the XRD patterns before and after treatment with 300ml of 3 mol 1-' KOH for 3 h. Surprisingly, the pattern after the treatment corresponded exactly to that of the Na+-exchanged product phase. It is clear that this compound had exchanged Li' for Na', which is contained in commercial KOH reagents as a small impurity (min.1 wt.%). From a thermodynamic view- point Kf exchange is expected to take place, but no K+-exchanged product was seen in the XRD pattern (Fig. 3). We also confirmed the absence of K+ in this product by atomic absorption spectrometry. This suggests that the K+ exchange is not preferred for kinetic reasons. As mentioned earlier, the rate-determining step in an exchange reaction can safely be assumed to be the diffusion of the exchanging ions in the solid. The diffusion coefficient is expressed by the Arrhenius equation using a pre-exponential factor, Do, and the Boltzmann constant, k,: D =Do exp(-AE/k, T) where AE is the activation energy of diffusion.Since large ions such as K+ require large activation energies to expand the relatively small framework of [Til.7Alo,3( PO,),], their diffusion coefficients in the framework should be very small. Thus, the framework acts as a sieve to K'. In NaTi,(P04)3, Na' was partially and selectively exchanged by Ag' leading to the formation of solid-solution Ag,Na, -xTi2( PO4), after a 4 day exchange treatment (Table 1).12 As expected, the Na+-exchanged product of Lil,3Til.7Alo.3(PO,), also exchanged Na' for Ag' and resulted in a fully exchanged product in 4 days. Fig. 4 shows the extent of Ag' exchange with time in Ag' exchange solutions for Lil~3Ti~~7Alo~3( and the Na +-exchanged PO,), product. The Na+-exchanged product exhibited a fast Ag+- exchange rate although not comparable to the very rapid Ag 'exchange of Lil.3Til,7Alo~3( PO,),.The reverse exchange, in which Ag+ in the Ag+-exchanged product of Li1.3Ti1.7A10.3( PO,), was partially exchanged by Na+ is shown in Table 1. When the Agf-exchanged product was treated with an NaNO, solution with a higher concen- tration of 1.5 moll-' the reverse exchange was almost com- pleted (Fig. 5). J. MATER. CHEM., 1994, VOL. 4 0 I 0 D I I 0 0 0 10 I 1 219/degrees Fig. 2 X-Ray diffraction patterns of the exchanged products (squares) during Ag+ exchange of (A) LiTi2(P04), and (B) Li1.3Ti1.7A103(P04)3 (circles).A: (a) before exchange, (b)1, (c)4 days; B: (a) before exchange, (b) 3, (c) 10 min 0 0 0 A A 10 20 30 40 2Bldegrees Fig.3 X-Ray diffraction patterns of (a) Li~,~Ti~,,Al~.,(PO~)~,(b)the Na+-exchanged product (triangles) after treatment with 3 mol I-' KOH containing a minimum of 1 wt.% Na+ and (c) KTi2(P04), (for comparison) A solid solution, Nal.,Til,7Alo.3( PO,),, which is probably the same composition as the Na+-exchanged product of Li1.3Ti1.7A10.3(PO,),, has been easily prepared by solid-state reaction at 1100"C using inexpensive Na salts. Moreover, the two compounds showed almost the same exchange behav- 3 80-v -awC 2 60-0X a 2 40-).0 c c2 20-01 tlmin Fig.4 The extent of Ag+ exchange uersus time in 0.034 mol 1-' AgNO, solution for Lil.3Til.7Alo,3( PO,), (circles) and the Na+- exchanged product (triangles).For each sample, 0.6 g was used. iour." This inorganic exchanger based on the [Til.7Alo.,(PO,),] framework is stable towards heat and chemical reagents such as strong acids (apart from phosphoric acid and acids with H20212315)and bases (see Fig. 3) and has a relatively large ion-exchange capacity (ca. 3.2 mequiv. g- '). Thus, this selective and rapid exchange (Table 1 and Fig. 4) for Ag' can be used to recover Ag' in waste solutitrns, e.g. those produced by plating industries and photographic processes. Conclusions (i) Very rapid and selective exchange of Na' and Ag+ for Li' takes place in the solid solution, Lil.,Til,,AlO., (PO,),; the exchange rate is faster than that of the end member, 10 20 30 40 2Bldegrees Fig.5 X-Ray diffraction patterns of the exchanged products of Lil,3Til,7Alo,3(PO& with sequential treatments: (a) Na+-exchanged product+) Agt-exchanged product+(c) treatment of (b) with 1.5 moll-' NaNO, LiTi,(PO,),.This is probably due to the higher Li+ conduc- tivity in the former. (ii) Large ions such as K+ have large activation energies for the replacement of Li+ in the small framework of these materials. The K+ is therefore sieved kinetically by the frameworks. This sieve effect is clearly seen by the preferential exchange of Na+ in the treatment of Lil.3Til.7Alo.3( PO,), with aqueous solution of a commercial KOH reagent which contains a small amount of Na' as an impurity.(iii) The solid solution, Lil.3Til.7Alo.3( PO,)3, may be exchanged sequentially by Na' and then Ag+. The Na/Ag exchange is reversible, but the Li/Na(Ag) exchange is not, under the conditions studied here. The Na+-exchanged prod- J. MATER. CHEM., 1994, VOL. 4 PO,),, i.e. Nal.3Til.7Alo.3(uct of Lil.3Til,7Alo,3( PO,),, can be used to recover Ag' in waste solutions. The research was supported in part by a grant from the Japan Private School Promotion Foundation. We are grateful to Professor A.R. West for useful suggestions. References 1 A. Clearfield, Chem. Rev., 1988,88, 125. 2 W. A. England, J. B. Goodenough and P. J. Wiseman, J. Solid State Chem., 1983,49, 289. 3 J. B. Goodenough, H. Y-P. Hong and J. A. Kafalas, Muter. Rex Bull., 1976,11,203; K.D. Kreuer, H. Kohier and J. Maier, in High Conductivity Solid Ionic Conductors, ed. T. Takahashi., World Scientific, Singapore, 1989, p. 242; G. Collin and J. P. Boilot, in Super Ionic Solids and Solid Electrolytes, ed. A. L. Laskar and S. Chandra, Academic Press, London, 989, p. 227. 4 A. Ono, J. Muter. Sci., 1984, 19,2691. 5 M. A. 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