Studies on crystalline zirconium phosphate—I

J. inorg, nucl. Chem,,1971,Vol. 33,pp. 3141to 3151.

STUDIES

ON

Pergamon Press.

Printed in Great Britain

CRYSTALLINE PHOSPHATE-

ZIRCONIUM I

I O N - E X C H A N G E D F O R M S OF a - Z I R C O N I U M P H O S P H A T E A. DYER, D. L E I G H * and F. T. OCON~" Department of Chemistry and Applied Chemistry, University of Saiford, Salford M5 4WT, Lancs., England

(Received 30 July 1970)

Abstract-Crystalline zirconium phosphate (H2ZrP) has been prepared in its a-modification and then converted to various ion-exchanged forms (M2~ZrP, M~ZrP). X-ray and thermoanalytical measurements have been made where M I = H +, Li +, Na +, K +, Rb +, Cs +, NH4 + and M u = Ca 2+, Sr z+, Ba2+. Conclusions are drawn as to the instability and interlayer forms of these ion-exchanged derivatives.

INTRODUCTION

Zn~coNIUM phosphate has been the subject of a series of investigations with a view to its use as an ion-exchange material[l]. The early work was concerned with amorphous gels, but in 1964 Clearfield and Stynes[2] reported the preparation of a crystalline form. This has now been identified as part of a series of layer structures which Clearfield and Smith [3] have subjected to X-ray single crystal analysis. They have designated the most stable of the layer structures as azirconium phosphate and this paper examines the instability and forms of ionexchanged modifications of the a-phase. The materials investigated are of the types M~IZrP, M~HZrP and MnZrp, where M I = Li +, Na +, K +, Rb +, Cs +, N H 4 + and M u = Ca 2+, Sr ~+, Ba2+. EXPERIMENTAL Crystalline a H~ZrP was prepared by the method of Clearfield and Stynes [2].

Preparation ofM21ZrP andM"ZrP ( M = Na +, K +, Rb ÷, Cs +, M n = Ca 2÷, Sr 2+, Ba2+) 0'01 kg of a-H~ ZrP was dispersed by slow additions of small amounts to 0.11 of I M radioactivelylabelled salt solution. This slurry was agitated continuously and maintained at pH 7, by small additions of M(OH) or M(OH)~, for several hours. The crystals were filtered, washed with several small volumes of deionized water, and dried for several days at 60°C in an air oven. In the preparation of Rb~ZrP about 90 per cent exchange was achieved by an additional percolation of the crystals by rubidium nitrate solution at pH 8. Percolation was also used for Ba and Cs, and maximum exchange seemed to be reached after 48 hr at pH 9. Elevated temperatures were sometimes used and a summary of experimental conditions is given in Table I. * Present address: Laporte Industries Ltd., Luton, Beds., England. tPresent address: Philippine Atomic Energy Commission, Manila, Philippines. I. C. B. Amphlett, Inorganic ion-exchangers. Elsevier, Amsterdam (1964). 2. A. Clearfield and J. A. Stynes, J. inorg, nucL Chem. 26, 117 (1964). 3. A. Clearfield and G. D. Smith,J. Colloid Interface Sci. 28,325 (1968). 3141

3142

A. DYER, D. LEIGH and F. T. OCON Table 1. Experimental conditions for the preparation of ionexchanged forms of a-zirconium phosphate

M

T (°C)

Exchange time (hr)

Na K Rb Cs Ca Sr Ba

22 22 60 102 22 60 102

7 9 18 48 18 18 18

Form of neutral salt

Isotope

NaCI KCI RbNOa CsCI CaClz SrC12 BaCIz

2~Na 42K S6Rb 1:~7Cs 4~Ca s'~Sr 1'~:~Ba

pH 7 7 8 9 7 7 9

Preparation ofM2~ZrP (M ~= Li +, NH4 +) andMtHZrP (M t = Li +, Na +, K +, NH4 +) Certain exchanges were carried out by a potentiometric titration method[3] in which small controlled additions of M O H solutions were made, over long time periods, to stirred dispersions of aH2ZrP in water.

Chemical analysis Materials were analyzed for zirconium and phosphate Contents as described by Alberti, Conte and Torracca[4]. Cation contents were obtained either by radiochemical analysis (M = Na ÷, K ÷, Rb ÷, Cs +. Ca 2+. Sr 2+, Ba~+) potentiometry (M = Li +, Na +, NH4 +) or in some cases by difference (M = Li +, NH4+).

Thermal analysis Thermal analyses were carried out on a Du Pont Modular Thermal Analysis System. Differential thermal analysis (DTA) was by the 900 module and thermogravimetric analysis (TGA) by the 950 attachment to the modular system. DTA was carried out at a heating rate of 20°C/min in an atmosphere of nitrogen.

X-ray analysis Powder photographs were usually obtained with a Philips camera, with an Ievins-Straumaris mounting, but occasionally a Guinier camera ~vas used. RESULTS

Chemical analyses are shown in Tables 2 and 3. The a-HzZrP batches prepared had compositions identical to those observed by other workers[2, 3] Table 2. Chemical constitution of batches of a H2ZrP prepared

Batch

ZrO2 (mole/kg)

P205 (mole/kg)

1 II !11 IV V VI

3"33 3.27 3.28 3.27 3.32 3.37

3" 18 3"29 3.30 3.30 3.31 3'24

H20(Total) (mole/kg) 6.75 6-69 6.63 6.67 6.47 6.61

Ratio ZrO2 : P205 : H~O 1.0 : 0.96: 2.03 1.0:1.01:2.05 1-0: 1.01:2.02 1.0: 1-01 : 2.04 1.0: 1.0:1.95 1.0:0.96:1.96

4. G. Alberti, A. Conte and E. Torracca, J. inorg, nttcl. Chem. 28,225 ~1966).

Studies on crystalline zirconium phosphate- I

3143

Table 3. Ionic composition and water losses of ion-exchanged forms of c~-zirconium phosphate

Cavity water M

Water loss Condensation water

M

present

Approx. temp.

(--- I % )

Mole

H Li

100 100

1 1.1

~ ~

Li

50

1 1.01 0.43

~ 350 ~ 200 200 ~ 300

Na Na K K Rb

100 50 100 50 90

1.38 1 1.05 0.31 0.56

~ --9 ~ ~ ~

300 170 200 300 300

Cs

61

0-17

~

400

Ca

100

2-4 0-96 0.69 4.47 0.75

--* 200 ~ 500 500 --> ~ 700 ~ 300

Sr Ba

100 66

r a n g e (°C) 170 170

Mole

Approx. temp. (range °C)

Total water loss(mole)

1

500 ~

2 2'10

0.57

350 ~

2.02

0.45

500 --~

0.40 0'16

550 ~ 600 ~

1-38 1.45 1.05 0'83 0.72

0.44

550 "-~

0"61 4"05

0-30

500 ---,

4.47 1.05

within the limits of experimental error. The results obtained for X-ray, potentiometric and thermal analyses also were in good agreement with published data and confirmed that the a-phase ofa-H2ZrP had been prepared. Tables 4 and 5 record the interlayer spacings (above 3A) computed from powder photographs for the various forms of zirconium phosphate prepared. Thermal analysis curves are shown in Fig. la, b (DTA) and 2a, b (TGA). A summary of weight losses observed by T G A is given in Table 3. DISCUSSION

ZrP containing Li Clearfield and Troup[5] have recently identified several phases of Li2ZrP. The material prepared in this work corresponds to Li2ZrP" 2.1H20 which is close to the phase G (Li2ZrP" 2"3H20) described by Clearfield and Troup as stable at room temperature. Phase G has an interlayer spacing of 8.80 A. They also identified a phase H (Li2ZrP'H20), formed in the region 80-100°C, by a spacing of 7.87 ,~. We have recorded spacings at 8.50 and 7.80 ,~ for a material which loses 1.1 H.,O when heated to about 170°C. This can be interpreted as a mixture of two phases (G, H) converting to a single phase H. A further mole of water is lost up to 350°C and features on the D T A curve at 320 and 350°C represent the formation of the anhydrous I and J phases envisaged by Clearfield and Troup. These workers also identify a series of phases with composition (1.33 Li 0.66) HZrP.xHzO where x = 0, 0.67, 1.33, 3.3. A single phase A (x = 3.3) was formed initially, which yields a mixture of phases A, B, C, D on dehydration in air. These 5. A. C l e a r f i e l d a n d J. T r o u p , J. phys. Chem. 7 4 , 3 1 4 ( 1 9 7 0 ) .

m.w

v.s m.s

v.w w

v.w v.w

s

7.56

4.~

3-57 3"53

3.29 3.21

3-18 3-02

H + (Ref.[3]) d(A) !

Table

w w

v.w

3-10

v.s

s

m.s

3.28 3-22

3"54

4"~

7-54

3.19

m.w

m.s

m.s

3.60

3-50

m

4.04

m

7.80

m

v.s

8-50

4.35

!

Li +

patterns

d(A)

diffraction

H + (This work) d(A) I

4. X-Ray

3-00

3.15

3-48 3.41 3.35 3-25

y.w

m.w

w m w

m

m

w

3.70 3.60

m

4-00

m

m w

4.55 4-40

4-22

m

m.w

m

m

7-20

7-70

8-40

9-90

50% Li + d(A) !

3.10

3.25

3-25 v.w

3.10v.w

3.45

3-75

3-90

4.10

4-30

s

m.w

m

w

w

s

s

w

v.w m

5-20 4-75

4.40

v.w v.w

v.s

6"70 6-50

7.65

3.45 m.w

3.54 m.s

3.62 s

3.80 s

4.17 m.w

4-55 m 4.40 m.s

7.20 v.w 6-70 v.w

8.51 v.s

3.02 m

3.17 v.s

3.53 m.w

3-65 v.w

4-0 v.s 3-89 s 3-79 v.w

4-35 m.s

4-60 m 4-50 m

8.90 v.s

10-70 w

3.02

3-45

3-75

4-20

4.60

5.30

7.50

8-90

m

m.s

v.s

m

s

w

s

w

50% K+ d(A) !

phosphate

K+ d(A) I

of a-zirconium

50% Na + d(A) I

K* forms

Na* d(A) I

o f H +, L i +, N a + a n d

0 Z

o

F"

F

.<

>

Studies on crystalline zirconium phosphate- 1

3145

<3

200

0

400 °C

600

o

Ld

I,,.

NH 4

Sr

0

200

400

600

"C

Fig. 1. (a, b) Differential thermal analysis curves of ion-exchangedforms of a-zirconium phosphate. phases have characteristic interlayer spacings o f 10.0, 8.55, 7.30 and 7.84 ,~ with x = 3.33, 1.33, 0-67 and 0, respectively. The material prepared in this work corresponds to LiHZrP.2.02H~O and has four high d values of reasonable intensity (9.90, 8.40, 7.70, 7.20,~). This may represent a mixed-phase system. T h e D T A shows several features up to 210°C tending to confirm the existence of mixed phases which are converted, with the

Jinc Vol. 33 No. 9 0

3146

A. DYER, D. L E I G H and F. T. OCON

16

I2

~

~

_

li Li

_

.~l~i," J

/ , ~ . X - - - - - Li/2

1.1-

Rb

o¢

0

2~)0

400

6 0

8 0

*C Fig. 2. (a, b) Thermogravimetric analysis curves of ion-exchanged forms of a-zirconium phosphate.

loss of 1.01 H~O (see Table 5), to a single phase LiHZrP. 1-01H20. This loses water (0.43 moles) at 250°C to give LiHZrP and finally condensation of phosphate groups occurs at above 500°C accounting for the final weight loss equivalent to 0.57 H20. These latter stages may include the formation of a glassy phase which subsequently recrystallizes, accounting for the observed exotherm at 560°C. ZrP containing Na Extensive examination of Na-exchanged forms of ZrP by Clearfield et al.[6] has shown that a multiplicity of phases can exist depending upon the amount of water incorporated into the structure. Our material (Na2ZrP.1.38H20) corresponds to the phase E observed by Clearfield (Na2ZrP'H20). It probably contains a small amount of phase D (Na2ZrP.3H20), as spacings of 4.17 and 3.10 A have been observed. Complete conversion to E occurs at 100°C according 6. A. Clearfield, W. L. Duax, A. S. Medina, G. D. Smith and J. R. Thomas, J. phys. Chem. 73. 3424 (1969).

Studies on crystalline zirconium phosphate-

I

3147

T a b l e 5. X - r a y d i f f r a c t i o n p a t t e r n s o f R b +. C s +, N H 4 +, C a 2+, S r ~÷ a n d B a 2+ f o r m s o f a zirconium phosphate Rb +

Cs +

d(A)

/

d(A) 12'64

!

N H4 + d(,~) i

C a 2+ d(A) 1

Sr 2+ d(A) !

a(A)

B a z+ I

v.w 10.12ms

9.58

v.s

7.54

m

9-02

6-56

7.54

9.82

v.s

7.54

m.w

v.s

m

4-44

m

4.17

m

4.00

m.s

4.44

m.w

4.44

v.s

4.28

w

4-31

m.s

4.06

m 4"00

w

3.88

m

3"71

v.w

3'58

w

3'00

s

4-53

v.s

s 4-42 4.04

3.97

v.w

3.92

3"79 3'75

m s

s 3.83

3.87

v.w

3.74

w

v.w

v.w

m.s

S

3.51

v.s

3.31

m.s

3.06

4-53

w

m.s 4.59

3.13

m.s

4.80

w

m

4"60

3.69

m.w.

v.w

w 4.86

4.67

7.52 7.23

3-52 3.46

v.s v.w

3.19

m

3-09

m.w

s

3.67

m.s

3-52 3.49 3'42

m.w w w

3.51

m.w

3-22

m

3.05

v.w

3.61

3.33

m.s m.w

3.62

3.10 3.05 3.02

s w m.w

w

m

to Ref. [6] and this is indicated by DTA. Phase E is said to lose the last mole of water at 165°C to give F Na2ZrP, which is converted to another anhydrous phase G on further heating. The D T A curve (Fig. 1) for Na2ZrP. 1.3 H20 is in general agreement with this concept, although it would seem that conversion of F to G under the conditions of DTA can take place at a temperature lower than the 400°C found by Cleartield et al. The N a H Z r P samples prepared correspond to phase B6 (NaHZrP-H20) which

3148

A. DYER. D. LEIGH and F. T. OCON

is converted to the anhydrous phase C[NaHZrP] at about 170°C. A further change takes place at 520°C due to phosphate group condensation to form a glassy state which crystallizes at 560°C. The thermal analyses confirm this series of changes and can be summarized in general agreement with the results of Clearfield et al. as B (NaHZrP.H20) ~ C (NaHZrP) 52o%jGlass ~6°°~ [NaZr2(PO4)z -too ~-- (-0.45 H20) --~ ~+ NaPO3 ZrP containing K The fully-exchanged form has almost identical d values from powder photographs to that observed by Torracca[7] for an anhydrous sample obtained by drying at 110°C. However, our sample retains 1.05 mole of water to 200°C and corresponds to a stable form of constitution K2ZrP" 1.05H20. The D T A confirms this, showing an endotherm corresponding to the water loss and a feature above 600°C. The weak interlayer spacings observed at 10.70, 3.79 and 3.65/~, are an indication that a small quantity of the phase K2ZrP.2.7 H~O identified by Torracca is present. The half-exchanged form loses 0.31 H20 up to 200°C; it has no interlayer spacing at 10.74 ,~, but shows the 8.90 ,~ spacing observed for the fully-exchanged form as well as a line at 7.50 A close to the interlayer spacing of the hydrogen form. Differential thermal analysis shows only a shallow endotherm about 200 C and a sharper feature above 600°C. This latter endotherm coincides with a weight loss of 0.44 H 2 0 and can be identified with phosphate group condensation. These observations are consistent with the view that the first stage of H + replacement by K ÷ occurs by exchange of HzO + species in interlayer positions, followed by lattice expansion with the reincorporation of water into the structure. The material prepared thus corresponds to K H Z r P with a small amount of KsZrP. H20. The presence of the d value of 3.75 A is consistent with potassium occupying sites between the ZrP layers. ZrP containing Rb The accommodation of 90% Rb into the zirconium phosphate structure causes expansion of the layer spacing. Thermal analysis indicates that 0.56 H20 are lost on heating to about 300°C. Comparison of X-ray and D T A evidence suggests some unchanged H~ZrP and further replacement of H ÷ by Rb ÷ might be possible. A weight loss equivalent to 0.16 H 2 0 is observed above 600°C representative of phosphate group condensation. ZrP containing Cs A very weak line equivalent to an interlayer spacing of 12.64 ,~ was observed and may indicate that the preparation of a fully-exchanged CssZrP is a possibility. The major product obtained contains about 60% exchanged Cs, which can be presumed to occupy sites intermediate to the principal layers (d = 3.74 A). This siting excludes virtually all water from the structure and thermal analyses indicate only changes associated with phosphate condensation. 7. E. Torracca, J. inorg, nucl. Chem. 31, I 189 (1969).

Studies on crystallinezirconiumphosphate- I

3149

ZrP c o n t a i n i n g NH4 Data from ammonium forms of ZrP are included for comparison purposes. The material corresponds to (NH4)~ ZrP.2 H20 and a more detailed study of this material will be reported in a future paper. ZrP c o n t a i n i n g the alkaline earth m e t a l s Ca and Sr can be fully exchanged into the phosphate structure and cause interlayer distances to expand. The compounds are CaZrP.4-05 H20 and SrZrP. 4.47 H~O, and X-ray measurements show that the SrZrP is very similar to that observed by Clearfield and Smith [8]. Thermal analysis data show that both forms lose about 2.5 H20 below 200°C. The Ba form is (0.66 Ba 0.66 H) ZrP. 1.05 H~O in which the interlayer spacing has not increased and Ba is sited between the major layers. High-temperature weight loss equivalent to 0.3 H~O is observed and a further 0.75 H20 is lost up to 300°C, presumably from sites between the layers. This behaviour is similar to that observed for Rb and Cs, and provides another instance of the tendency of barium to behave as a monovalent ion. This is well known in relation to the hydration characteristics of the barium ion and has been observed from self-diffusion and electrochemical phenomena [9, 10]. CONCLUSION The exchanges performed with Li and Na are generally in accord with those of other workers [5, 6], except that we have observed a range of compounds of formulation LiHZrP.xH~O. Multiple phases seem to be absent when Rb and Cs are introduced into zirconium phosphate, presumably because of the inability of these ion-exchanged forms to accommodate more than 1 mole of water per cation. However, there may be some phases of high water content and low thermal stability formed in the case of potassium [7]. In general, the water content and the increases in principal interlayer spacings are a function of exchanging ion-size (Fig. 3a, b) for the monovalent ions. There is also a simple relationship between ion-size and the water content of the partially-exchanged materials containing Li, Na, K (Fig. 3b). These simple comparisons suggest that the partially-exchanged materials obtained are ones in which H + has been replaced by a cation incorporated in an unhydrated condition into existing cavities in the crystal[11]. Entry to the cavities is through a restricting dimension of about 2.63A[11] which normally excludes the Rb and Cs ions; however, exchange of these ions has been shown to take place at elevated temperatures in an analogous manner to the replacement of Na by Rb and Cs in the zeolite analcite where these large ions progress through restricting dimensions of 2"3 ,~[12]. These exchanges are effected at pH > 7 which causes some hydrolysis but a large part of the crystallinity is apparently preserved, as the X-ray powder photographs show little evidence of amorphous material. 8. 9. 10. 1I.

A. Clearfieldand G. D. Smith,J. inorg, nucl. Chem. 30, 327 (1968). A. Dyer and R. B. Gettins,J. inorg, nucl. Chem. 32, 2401 (1970). E. Glueckauf,Trans. Faraday Soc. 61, 914 (1965). A. Clearfield,Ion-Exchange in the Process Industries. p. 311. Soc. Chem. Ind. Conf., London (1969). 12. R. M. Barrerand L. V. C. Rees, Trans Faraday Soc. 56, 709 (1960).

3150

A. DYER, D. LEIGH and F. T. OCON

Ca

'

o

",i,"

I

7

L

8

9

L

I

I0

II

12

Fig. 3. (a) Relationship between "cavity" water content and interlayer spacing (d) in ion exchanged forms of a-zirconium phosphate. [-I-M ~,M n - [M2~,M']ZrP (this work); ~)-LiHZrP (this work-for two lines); A-NaHZrP (this work); T-KHZrP (this work); ©-Li2ZrP'xH20 ( x = l , 2-3, 4) Ref.[510-I.33Li.0.66HZrP'xH20 (x=0.67, 1.33, 3.33, 4) Ref.[5]; A-Na2ZrP.xH~O (x = 1, 3) Ref.[6]; A-NaHZrP.xH20 (x = l, 5) Ref.[6].

I

"r

(~.6 Li

06

Na

,.0

1.2

K

1.4

Rb

t.6

Cs

Zonic radius,

Fig. 3. (b) Relationship between "cavity" water content and ion-size. O-maximum exchanged form; O - half-exchanged form. W h e n more than half the exchange capacity is taken up, further exchange requires displacement of hydrogens involved in interlayer hydrogen bonding1; as a c o n s e q u e n c e cations replacing hydrogen in these sites cause interlayer expansion (Fig. 3a, line Ill). C a and Sr ions do not s e e m to b e h a v e in this way (Fig. 3a, line I, Refs.[5, 8] and s e e m to o c c u p y sites b e t w e e n the layers (rather than in cavities); this m a y be a manifestation of the greater t e n d e n c y of these ions to exist as hydrated ions in aqueous solution in c o m p a r i s o n to the m o n o v a l e n t ions, Thus the balance between hydration energies, lattice energies and entropy changes involved in the ion exchange is such as to favour the r e p l a c e m e n t of hydrogen-bonded protons rather than the replacement of protons in the cavities, entry to which is restricted to unhydrated ions. T h e b e h a v i o u r of Ba, with its k n o w n anomalous hydration properties, would tend to confirm this view. Lithium m a y be small enough to enter the cavities as a hydrated species in the initial stages of exchange as implied by Fig. 3b; but a b o v e 50 per cent exchange p r e s u m a b l y it can occupy sites b e t w e e n

Studies on crystalline zirconium phosphate- I

3151

20 - HK ~. ~

I.Cf oCs

E

Bo

I 0

0.5

Condensation water,

.0

moles

Fig. 4. Relationship between unexchanged H + content and "condensation" water content for a-zirconium phosphate.

the layers as a hydrated ion (Fig. 3a, line 1). Leigh [ 13] has noted that dehydration of a sample of LizZrP.2 H20 with an interlayer spacing of 8-84 A, (compare to Ref.[5]) produces a material which on rehydration has a constitution of Li2ZrP. H~O and a spacing of 8.02 ,~. Finally, at low temperatures and high water vapour pressures the small monovalent forms of a zirconium phosphate can exist as unstable, highly-hydrated structures in which the observed interlayer spacing is a function of water content (Fig. 3a, line 11). In the foregoing discussion it has been assumed that certain weight losses can be assigned to condensation of phosphate groups in certain heteroionic forms of zirconium phosphates. Figure 4 shows a plot of unexchanged proton content against these weight losses which justifies this assumption. Acknowledgement-One of us !F.T.O.) wishes to thank The International Atomic Energy Agency and the Philippine Atomic Energy Commission for the provision of a fellowship under which this work was carried out. Provision of a research grant by the University of Salford to D.L. is acknowledged gratefully. We also wish to thank Mr. D. Brown and Mr. R. Olorunfemi for assistance with thermal and X-ray analysis. 13. D. Leigh. Ph.D. Thesis, University of Salford (1968).