Synthetic inorganic ion-exchange materials—XXIX Ion-exchange equilibria of crystalline antimonic(V) acid with organic cations

J. inorg, nacl. C~m. Vol.42, pp. 1753-1757 PergamonPress Ltd., I~0. Printedin GreatBritain

SYNTHETIC INORGANIC ION-EXCHANGE MATERIALS--XXIX I O N - E X C H A N G E E Q U I L I B R I A OF C R Y S T A L L I N E ANTIMONIC(V) ACID WITH ORGANIC CATIONS MITSUO ABE, KENJIRO YOSHIGASAKIand TAKASHI SUGIURA Department of Chemistry, Faculty of Science, Tokyo Institute of Technology,2-12-1,Ookayama,Meguro-ku, Tokyo 152,Japan (First received20 September 1979; acceptedfor publication 14 January 1980)

Abstract--A study of distribution coefficients as a function of concentration of hydrochloric acid indicates an "ideal*' 1:1 exchange reaction for the NH4+/H+ and CH3NH3+/H+ systems on crystalline antimonic(V) acid(CSbA). The ion-exchange isotherms have been measured for the NH4+, CH3NH3+, C2HsNH3+, (CH3hNH2+. isoC3HTNH3+, (CH3)3NH+, (CH3hN+ and (C2Hs)o~I÷/H+ systems on C-SbA at 25, 45 and 60°C. The selectivity coefficients vary with the equivalentfraction of the exchangingorganic cations. The selectivitysequence decreases in the order, NH4 + > CH3NH3 + > C2HsNH3 + > (CH3)2NH2+

which is parallel to the increasing order of van der Waals dimensions of organic cations. The ion-sieve effect is observed for (CH3)tN+ and (CzHs)4N+/H+ systems. The dimension of the window on C-SbA lattice can be evaluated to be 5-6 A. Overall and hypothetical (zero loading) thermodynamicdata have been derived for these ion-exchange reactions.

INTRODUCTION Theoretical aspect During the last two decades much attention has been The ion-exchange process may be represented by the paid to the development of synthetic inorganic ion- following equation for hydrogen ion/univalent organic exchange materials with respect to higher selectivities cations systems, and resistance at higher temperatures and in radiation fields than those of commercial ion-exchange resins [1-6]. FI+ + M+~I~I+H + (1) Among these ion exchangers, crystalline antimonic(V) acid(C-SbA) shows excellent ion-exchange properties where bar refers to C-SbA and M+ is an organic cation. with high selectivities for certain metal ions, such as The selectivity cuefficient is defined here by including the Na+[7,8], Sr2+[9], Cd2+[10], Hg2+ and Ag~[ll]. terms of molal activity coefficients (y) of organic cations Effective separation can be achieved for various metal in the equilibrating solution. ions or groups of metal ions by using a relatively small column of C-SbA[7-10]. _ Xr~" XH. y~ YH (2) K"MXM-g. Similar compounds to the C-SbA have been reported by different author, e.g. polyantimonic acid by Lefebvre et aL[12] and Baetsle et al.[13] and hydrated antimony where )(H and )fM are the equivalent fractions of the pentoxide by Girardi et al. [14]. C-SbA can be regarded as a zeolitic ion exchanger exchanging hydrogen ions and organic cations, and XH and X~ those in the solution. with a rigid structure from the studies of ion-exchange From the Kielland table[19], the activity coefficients equilibria for alkali metal inns/H + and alkaline earth metal ions/H + systems, and from X-ray diffraction for NIL +, CILNH3 +, C2HsNH3+ and (CH3hNH2 + are 0.80, 0.81, 0.815 and 0.81 at ionic strength 0.1, respecanalysis [15, 16]. The ion-exchange selectivities vary, depending on the tively, values which are nearly equal to the mean activity loading of the exchanging ions, which points to a steric coefficient of HC! at the same ionic strength. In dilute solution the ratio, YH./TM, will not deviate effect. If there is no available space in the window of the greatly from the ionic strength principle at 25.0°-60.0°C. lattice for an ingoing large cation, an ion-sieve effect may occur. There have been a few investigation of exchange Neglecting temperature dependence of the ratio ~ / y u involving organic cations in the crystalline zeolites, such will not give serious deviation in the values of K.M, especially for uni-univalent ion-exchange reaction[20]. as near-faujasite[17] and clinoptilolite[18]. The thermodynamic equilibrium constant is defined by This investigation has been extended to the ionexchange of H + in C-SbA by organic cations over various exchange compositions, from which the apK = kH M" f ~ (3) propriate thermodynamic data may be derived. f. 1753

M. ABE et aL

1754

where fH and fM are the activity coefficients of hydrogen ion and organic cations in the C-SbA. This can be evaluated by using the simplified treatment of GainesThomas equation[21,22], In K =

In K , M dXM

(4)

M=O

assuming that the change in the activity coefficients of water in exchangerand solution phases is negligible. The In KHMvS XM plots are generally linear for univalent ion-exchange reactions on a rigid inorganic ion exchanger, and thus In KHM is given by In KH M =4.606 C)fM + (In KHM)~M-,O

(5)

where term C is the Kielland coefficient. The thermodynamic equilibrium constant (natural logarithm scale) may be rewritten as In K -- In K , M+ 2.303 C(I - 2.~M).

(6)

The numerical value of K is equal to that of the KHM at XH = )(M = 0.5. The thermodynamic data, AG °, AH ° and AS°, were calculated in the usual way. EXPERIMENTAL

Preparation of C-SbA. The C-SbA was prepared as described previously[23]; 75 ml of antimony(V) pentachloride was preliminarily hydrolyzed by 75 ml of cold water," and was then hydrolyzed by 51 of water. The precipitate obtained was kept in the mother solution at 40°C for over 20 days, and then washed with cold demineralizedwater until free from chloride ions using a centrifuge (about 10,000rpm). After drying the precipitate, the product was ground and sieved in the appropriate fraction of 100-200 mesh size. The collected sample was rewashed with cold demineralizedwater in order to remove any adherent fine particle and to obtain a clear supernatant solution in the subsequent experiments. Distribution coefficients. The determination of the distribution coefficients (Kd) of the organic ions was carried out as follows: 0.250g of the C-SbA in hydrogen ion form was immersed in 25.0m] of a solution containing 1.0xl0-3M ammonium and methylammonium chloride in hydrochloric acid solution at different concentration at 25 and 45°C. Cation exchange isotherms. The cation exchange isotherms were measured by batch experiment for NH4*, CH3NH3÷, C2H~NH3+ and (CH3)2NH2+. 0.250g of the C-SbA in hydrogen ion form was immersed in 25.0ml of a mixed solution of varying ratio of alkylammonium chloride/hydrochloric acid in a sealed glass tube with intermittent shaking. The ionic strength in the mixed solution was adjusted to 0.1. The concentrations of the organic cations and the hydrogen ion in the solid phase and in the solution phase were deduced from the relative change between the initial and final concentrations in aqueous solution. Preliminary studies revealed that equilibrium was attained within 4 and 40 days for the ammonium and the alkylammoniumions at 25°C, respectively. The concentration of the alkylammoniumions in aqueous solution was determined by the spectrophotometric method using thymol[24,25], after decomposing alkylammoniumions by the Kjeldahl method. The emf titration method was employedfor determiningthe hydrogen ion concentrations with 0.02M of standard sodium hydroxide solution. The dissolution of C-SbA can be negligible for the determination of liberated H+ for the systems of NH4+/H+ and CH3NH3*/H+ exchange. With increasing number of carbon atoms in the organic cation for other systems studied, the solubility of C-SbA increases in a range of 10-5-10-3 tool of antimony per litter of supernatant solution. The concentration of the hydrogen ions liberated from solid phase was obtained by subtracting the equivalent amount of antimony in the solution phase,

by assuming that one mole of the monomer of HSb(OH)6 can be neutralized with one equivalent of sodium hydroxide. The concentration of antimony was determined by using a Varian-Techtron Atomic Absorption Spectrometer Model 1100. Maximum uptake of organic ions on C-$bA. The maximum uptake on C-SbA was measured by batch experiments for NH4+ and organic cations at 45±0.10C. 0.250g of the C-SbA in hydrogen ion form was immersed in 25.0ml of the alkylammonium chloride solution at the concentration of 0.1 M.

Thermal analysis and X-ray powder diHraction studies. The thermal analysis and the X-ray analysis were carried out for the C-SbA ion-exchangedwith different organic cations, after washing with water. The results of DTA and TGA showed that the adsorbed organic cations were decomposed completely to amines, and the amines removed from the C-SbA were burned out at 300-400°C. The water content of the ion-exchangedC-SbA was determined by assuming that the ion-exchanged C-SbA samples gave an empirical formula of Sb6Om3by heating at 80ff'C in the TGA[26]. TGA-DTA Thermoflex Analyser (Rigaku Denki Ltd. Japan) was employed for the TGA and DTA. X-ray powder diffraction data for the ion-exchanged C-SbA samples were obtained using a Model JDX-7E diffractometer (Japan Electric Optics Laboratory, Ltd., Japan), with Ni-filtered CuKa radiation. RESULTS AND DISCUSSION

Chemical and physical properties of C-SbA The results of TGA, DTA and X-ray powder diffraction analysis for C-SbA in I-I+ form showed good agreement with our earlier works[23,26]. The composition of the C-SbA can be written as Sb205 • 4H20.

Distribution coe~cients The Kd values for NH~+/H+ and CH3NH3+/H + systems were calculated in the usual manner (Fig. 1). The plots of log Kd vs Iog[H +] showed a linear relationship with a slope of -1, as expected for "ideal"l:l ionexchange reaction for both systems.

Maximum uptake of organic cations on C-SbA The maximum uptake was found to be 1.35, 0.54, 0.31, 0.18, 0.20, 0.10, 0.04 and 0.04 meq/g for NI-I4+, CH3NH3 +, C2H~NH3+, (CH3)2NH2+, iso-C3H7NH3 +, (CH3)3NH +, (CH3)4N+ and (C2Hs)4N+, respectively (Table 1). The uptake of the organic cations decreased with increasing van der Waals dimensions of the organic ions, except for iso-C3H7NI-I3+ (Table 1). Similar sequences have been found for the maximum uptake of organic cations on clinoptilolite[18].

o21 I0|

~ i0 "3

i I0 "2 [ H~moI/I

I0 "~

I

Fig. 1. Distribution coefficients for organic cations on C-SbA at different concentration of hydrochloric acid. C-SbA: O.~Og, organic cations: 10-~M, total volume: 25.01mi, temperature: 25°C.

Synthetic inorganic ion-exchangematerials--XXIX

1755

Table I. Analysisof the C-SbA exchanged with organic cations Cation

x direction (length)

y direction (width)

z direction (height)

uptake lattice constant (meq/g) (i)

+ H

H20/ Sb205

10.380

4.0

NH4+

2.86

2.86

2.86

1.35

10.339

3.2

+ CH3NH 3

4.91

4.00

4.00

0.54

10.408

3.7

C2H5NH3 +

5.90

4.88

4.00

0.31

10.395

3.8

(CH3)2NH2 +

6.42

4.28

4.00

0.18

10.389

3.8

iSO-C3H7NH3 +

6.52

5.56

4.77

0.20

10.388

3.8

(CH3)3NH +

6.42

6.10

4.17

0.I0

10.383

3.8

(CH3)4N+

6.42

6.10

6.22

0.04

10.378

4.0

0.04

10.379

3.9

(C2H5)4N+

DTA shows a sharp exothermic peak for C--SbA at 300-400°C, when organic cations are exchanged in bulk. The detection limit was 0.02 meq/g, and proportional to the amount exchanged. No exotherm is seen for (CHa)4N + and (C2HshN +, but the exchange results suggest uptake of 0.04 meq/g (which is higher than the experimental error of the exchange determination). Therefore, (CH3)4N+ and (C2HshN + can only be adsorbed in a surface process, which does not yield an exotherm in the DTA. This shows (Table 1) that the window dimension is < 6 It. When the cation is smaller than 6 A in two directions, it can pass the window of the structure without any resistance. For the exchange with (CH3hNH +, having only less than 6 A in one direction, the cation may pass its window with some distortion during lattice vibration. X-Ray powder diffraction analysis indicated that the lattice constants of the C-SbA exchanged with NIL + decreased continuously with the degree of the exchange; no formation of two phases was observed over the entire range of the ammonium ion studied. Slightly increased lattice constants were observed in the sample exchanged with organic cation, without change in the space group (Fd3m). Thus, a solid solution forms for the ion-exchange systems between organic cations and hydrogen ion in C-SbA.

Ion-exchange isotherms The ion-exchange isotherms for the NIL+/H +, CH3NH3+/H +, C2HsNH3+/H + and (CH3hNH:+/H + systems at 25°C, are shown in Fig. 2. Similar profiles were also obtained at 45°C and 60°C. The ion-exchange isotherms were normalized with theoretical capacity as discussed previously[15]. The theoretical capacity of 5.057 meq/g was calculated from the empirical formula of Sb205 • 4HH20by assuming that one antimony gave one hydrogen ion available for ionexchange[27]. For the NIL+/H + system, the slope of the isotherm rises steeply for low loading and then decreases as a function of equivalent fraction of NIL + in the solution phase, indicating that the exchange does not go to completion although the entering cation is initially preferred.

The slope of the isotherms decreases with the increasing size of organic cation. Similar isotherms were also observed for the systems of alkali metal ions/H + and transition metal ions/H + on C-SbA [15, 28].

Kieiland plot The plot of In KHMvS XM, which was referred to as a KieUand plot, gave fairly good linearity for the systems studied at the different temperatures, except for the NIL+/H + system. The linear KieUand plot was also observed for the alkylammonium ions/Na ÷ systems on clinoptilolite[18] and for the alkali metal ions/H + systems on the C-SbA[15]. As pointed out by Barfer et al.[18,29], if there was no available space for the ingoing large cations within the intracrystalline volume, the ion-exchange would become increasingly difficult after some but not all the ions in the

03

0.2

L~ OI

CH~ NH~

0

0.5

C. H. N~+

1.0

x. Fig. 2. Ion-exchangeisotherms of organic cationslH+ exchange systems on C-SbA at 25°C.

1756

M. ABE et al. Table 2. Overall thermodynamicdata on the ion-exchangereaction Temp.

Kie~land coefficient (c)

in K

NH4 +

CH3NH3 +

25

-8.05

-8.35

C2H5NH3 +

(CH3)2NH2+

-16. 4

-22

45

-8.04

-7.91

-12. 5

-17

60

-8.03

-6.10

-i0

-14

25

-ii.5

-21

-40

-53

45

-11.7

-17.

-31

-41

60

-11.9

-15.

-26

-37

24

31

AG ° (Kcal/ eq)

6.81

"0

5 4

12.4

1 cal = 4.18 J

exchanger were replaced. Incomplete exchange can also arise with large cations from lack of interstitial space even when no ion-sieve effect occurs. No steric effect was found in a range of equivalent fraction of 0-0.04 for the NH4+/H+ exchange. The absolute values of the Kielland coefficient increase with the increasing difference in the ionic radii between two exchanging cations, and the cations having large ionic radii may undergo larger steric effect than those of the smaller radii. Numerical values of Kielland coefficient were calculated by the least mean square method (Table 2). The absolute values increased in the order; NH4+ < CHaNH3+ < C2HsNH3+ < (CH3)2NH2+, which is parallel to the increasing order of the van der Waals dimensions. A similar sequence was found for the organic cations/H + systems on clinoptilolite[18]. Thus, the difference in maximum uptake for organic cations can he explained in terms of steric effect on C-SbA. The thermodynamic equilibrium constant, K, was evaluated from the eqn (4) by assuming that the relation between In KHM vS XM remains unity over the entire range from 0 to I of the J(M. The calculated AS° and AH ° can be regarded as unity because the plots of In K vs l i t showed fairly good linearity over the entire range of temperature studied. The overall thermodynamic selectivity series was found to be (Table 2); NH4+ > CH3NH3+ > C2HsNH3+ > (CH3)2NH2+, which is similar to the increasing order of the van der Waals dimensions.

Interpretation of the selectivity in the infinitesimal concentration of the organic cations The hypothetical thermodynamic data in infinitesimal concentration were calculated for, interpretation of the selectivity of the microquantities of organic cations on C-SbA. The values of (ln KMU)~w.o were obtained by extrapolating to "zero loading" of organic cations on Fig. 3. From these values, the hypothetical thermodynamic data were calculated by a similar treatment to that for the overall equilibrium constants. The accuracy of these values may be possibly higher than those of the latter. The values of ( A G ° ) ~ are positive for all systems, except the NH4+/H+ system (Table 3). This indicates

that ammonium ions cannot be exchanged preferably to the exchange site of the C-SbA. The hypothetical thermodynamic selectivity series was foimd to be the same order as the overall series. The ion-exchange reaction (eqn (I)) can be separated into the following two reactions; M+(aq.) + H+(gas)#M+(gas) + H+(aq.)

-A Y ° h y d r

M+(gas) + I~+~M + + H+(gas)

A Y°ex

where Y represents thermodynamic functions such as G, H and S. The numerical values of A Y° contribute to the difference between the thermodynamic function of hydration (A Y°hydr) of the ions in aqueous solution and that (AY°ex) of the exchanging cations. The values of AY°hydr were calculated from Arnett's table/30] in accordance with the values standardized to Rosseinsky's

4

•

0

-2

-6

~

-8 O

2HsNH~ (CH3)aNH+ I 0.I

I 0.2

I 03

Fig. 3. Selectivitycoefficientsof organiccations/H4- exchangeon

C-SbA at 25°C.

1757

Synthetic inorganic ion-exchange materials--XXIX Table 3. Hypothetical thermodynamic data on "zero loading" of the ion-exchange reaction

Temp.

NH4 +

25 45 60

M (in KH)~M+0

CH3NH3 +

C2H5NH3 +

(CH3)2NH2 +

7.2 7.0 6.8

-1.73 -1.59 -1.36

-2.5 -2.6 -2. 8

-3.8 -4.3 -4. 6

1.04

1.5

2.3

194.7

199.5

(AG°)~M+0

(kcal/eq)

-4.3

(AG°ex)~M+0

(kcal/eq)

179.1

191.7

(AH°ex)~M+0

(kcal/eq)

183. 0

193. 9

190

192

(AS °

(cal/deg-eq)

15

7

-16

-25

)=

+^

ex xM u

AG°hydr

(kcal/eq)

183.4

190.7

193.2

197.2

AH°hydr

(kcal/eq)

185.9

191.9

19~.0

197.1

AS°hydr

(cal/deg-eq)

8. 5

4. 2

-4. 2

-0. 2

table[31]. The calculated (AY°e~)~u-,o values are summarized in Table 3. The values of (AS°cx)~M_,o decrease in the order; NH4 ÷ > CH3NH3 ÷ > C2HsNH3 + > (CH3)2NH2 +, which parallels the values of AS°hydr. Frank and Evans[32] pointed out that highly ordered water structure, labelled as an "ice-berg", was formed around the solute when non polar solutes were dissolved in water. Frank and Wen[33] extended this hypothesis to the alkylammonium ions, and indicated that the highly ordered water structure caused the negative hydration entropy. For this reason, the hydration entropy of the alkylammonium ions may decrease considerably with the increasing order of the van der Waals dimensions. The magnitudes of (AS°~x)~M-,o can be evaluated to be almost the same as AS°hydr of organic cation. Furthermore, the C-SbA has a rigid structure and does not undergo appreciable dimensional change (0.2-0.5% in the lattice constant) during the ion-exchange because of the three-dimensional framework, like zeolite to the first approximation (Table 1). When less hydrated organic cations go to the hydrated sites, the degree of freedom may decrease in the system. If some of the water molecules on exchange go to the solution phase, a large decrease in the (AS°cx)~u-.o may also occur, because the net transfer of water molecules from the solid phase to the solution phase is an entropy producing process. Table 1 indicates that the number of the water molecules removed by the exchange of one molecule of organic cation increases with the increasing number of carbon atoms in the organic cation, although experimental error is involved in the determination of the water contents. Thus, decrease in the. (AS°cx)~M-.o can be explained in the terms of the water transfer from solid phase to solution phase. REFERENCES 1. C. B. Amphlett, Inorganic Ion Exchangers. Elsevier, Amsterdam (1964). 2. V. Vesel~' and V. Pek~irek, Talanta 19, 219(1972). 3. V. Pekfirek and V. Vesel~,, Talanta 19, 1245 (1972).

I[NC Vol. 42, No. 12--F

4. M. Abe, Bunseki Kagaku 23, 1254, 1561 (1974). 5. M. Qureshi, S. Z. Qureshi, J. P. Gupta and H. S, Rathore, Sep. Sci. Technol. 7,615 (1972). 6. A. K. De and A. K. Sen, Sep. Sci. Technol. 13, 519 (1978). 7. M. Abe and T. Ito, Bull. Chem. Soc. Japan 40, 1013 (1%7). 8. M. Abe, Bull. Chem. Soc. Japan 42, 2683 (1%9). 9. M. Abe and K. Uno, Sep. Sci. Technol. 14, 355 (1979). 10. M. Abe, Chem. Lett. 561 (1979). 11. M. Abe and M. Akimoto, Bull. Chem. Soc. Japan 53, 121 (1980). 12. J. Lefebvre and F. Gaymard, Compt. Rend. 266, 6911 (1965). 13. L. H. Baetsle and D. Huys, J. lnorg. Nucl. Chem. 30, 639 (1%8). 14. E. Girardi, R. Pietra and E. Sabhioni, J. Radional. Chem. 5, 141 (1970). 15. M. Abe, J. Inorg. Nucl. Chem. 41, 85 (1979). 16. M. Abe, Bull. Chem. Soc. Japan 52, 1386 (1979). 17. R. M. Barrer, W. Barrer and F, Grutter, Heir. Chim. Acta 39, 518 (1956). 18. R. M. Barrer, R. Papadopoulos and V. L. C. Rees, J. lnorg. Nucl. Chem. 29, 2047 (1%7). 19. R. Parsons. Handbook o/ Electrochemical Constants, pp. 26-27, Butterworths, London (1957). 20. O. D. Bonnet and L. L. Smith, J. Phys. Chem. 61, 1614 (1957). 21. G. L. Gaines and H. C. Thomas, J. Chem. Phys. 21, 714 (1953). 22. D. W. Breck, Zeolite Molecular Sieves, Structure Chemistry and Use, p.533. Wiley, New York (1974). 23. M. Abe and T. Ito, Bull. Chem. Soc. Japan 41,333 (1%8). 24. R. T. Rosam and D. de Langen, Anal. Chim. Acta 30, 56 (1964). 25. H. Hashitani and H. Yoshida, Bunseki Kagaku 17, 1136 0%8). 26. M. Abe, K. Kogyo Kagaku Zasshi 70, 2226 (1%7). 27. M. Abe and T. Ito, Bull. Chem. Soc. Japan 41, 2366 (1%8). 28. M. Abe and K. Sudoh, J. lnorg. Nucl. Chem. 42, 1051 (1980). 29. R. M. Barter and W. M. Meier, Trans. Faraday Soc. S4, 1074 (1958). 30. E. M. Arnett, F. M. Jones llI, M. Taagepera, W. G. Henderson, J. U Beauchamp. D. Holtz and R. W. Taft, J. Am. Chem. Soc. 94, 4724 (1972). 31. D. R. Rosseinsky, Chem. Bey. 65, 467 (1%5). 32. H. S. Frank and M. W. Evans, J. Chem. Phys. 13, 507 (1945). 33. H. S. Frank and Wen-Yang Wen, Disc. Faraday Soc. 29, 133 (1957).