Studies on crystalline zirconium phosphate-II

J. inorg,nucl.Chem.,197I, Vol.33, pp. 3153to 3163. PergamonPress. Printedin Great Britain

STUDIES

ON

CRYSTALLINE PHOSPHATE-

ZIRCONIUM II

S E L F - D I F F U S I O N OF C A T I O N S A. DYER and F. T. OCON* Department of Chemistry and Applied Chemistry, University of Salford, Salford M5 4WT, Lancs, England

(Received 30July 1970) Abstract-Cation self-diffusion studies have been carried out by radiochemical techniques for the ions Na +, K ÷, Rb +, Cs +, Ca 2+, Sr 2+ and Ba 2+ in a-zirconium phosphate and for Ca 2+ in y-zirconium phosphate. Thermodynamic parameters have been estimated for the diffusion processes and conclusions drawn as to the cation sitings and mobilities in these ion-exchange materials. INTRODUCTION

ALTHOUGH the kinetics of ion exchange in crystalline zirconium phosphate (ZrP) have received attention during the last few years[I-3], no information on cation self-diffusion kinetics is available. The interpretation of parameters measured from diffusion kinetics has proved of value in providing structural information in the inorganic ion-exchange materials, i.e. the zeolite minerals [4]. Recent work [5, 6] by Clearfield et al. has provided a detailed background of structural information on the various forms of crystalline ZrP. This work makes use of this background to interpret cation self-diffusion kinetics in terms of ionic mobility and siting for the ions Na ÷, K ÷, Rb ÷, Cs ÷, Ca 2÷, Sr2+, Ba '-'+ in a-ZrP and Ca 2÷ in yZrP. The diffusion processes have been followed by radioisotope techniques. EXPERIMENTAL The ~- and y-forms of H2ZrP were prepared as described by Clearfield, Blessing and Stynes[6]. The crystalline products were typified by potentiometric titration, X-ray powder diffraction, thermogravimetry and chemical analysis as described elsewhere[7]. The crystals were converted to the appropriate cationic forms, sedimented to ensure uniformity of crystallite size, and particle size estimations attempted.

S e l f diffusion experiments Experiments at a series of constant temperatures below 100°C were carried out as follows: 10 -3 kg of isotopically-labelled cationic forms of ZrP (MzI/MxIZrP) were placed in a thermostat at the reaction temperature for 20 min. 0.051 of a 10-~ M solution of M as chloride were equilibrated in the * Present address: Philippine Atomic Energy Commission, Manila, Philippines. I. G. H. Nancollas and V. Pekarek,J. inorg, nucl. Chem. 27, 1409 (1965). 2. J. Albertsson,Acta chem. scand. 20, 1689 (1966). 3. G. H. Nancollas and B. V. K. S. R. A. Tilak, J. inorg, nucl. Chem. 31, 3643 (1969). 4. A. Dyer and J. M. Fawcett, J. inorg, nucl. Chem. 28, 615 (1966). 5. A. Clearfield and G. D. Smith,J. Colloid interface Sci. 28,325 (1968). 6. A. Clearfield, R. H. Blessing and J. A. Stynes, J. inorg, nucl. Chem. 30, 2249 (1968). 7. A. Dyer, D. Leigh and F. T. Ocon,J. inorg, nucl. Chem. 33, 3141 (1971). 3153

3154

A. D Y E R and F. T. OCON

same way and then the solid and solution mixed at zero time. The slurry so formed was continuously agitated by a magnetic stirrer and maintained at the reaction temperature. Some experiments were made at temperatures below 0°C in a low-temperature thermostat bath, where additions of up to 50% ethyl alcohol were made to prevent the solution from freezing. Aliquots of slurry were taken at different time intervals, cooled, centrifuged, and samples of the clear supernatant withdrawn for radioisotope determinations. At temperatures above 100°C batch diffusion experiments in sealed quartz tubes were performed.

Radiochemical procedures The isotopes used are listed in the previous paper [7] and were as supplied by R.C.C., Amersham. All isotopes except l~aBa were determined by liquid scintillation counting, whereby 10 -a I. of aqueous radioactive salt solution was rendered miscible with a P P O / P O P O P toluene-based scintillator[8] (4 x 10-3 I.) by adding 7 x 10-a 1. ethanol, laaBa was measured by incorporating 5 x 10-~ I. of aqueous salt solution into 10 -2 I. of N.E 221 gel scintillator (Nuclear Enterprises Ltd.) Dispersion of solution in gel was assured by the use of an ultrasonic vibrator. The time interval between the addition of sample to the gel and the isotope measurement was standardized. Liquid scintillation measurements were made using a Nuclear Chicago Ambient counting system. X-ray powder photographs were obtained of samples after self-diffusion experiments had been carried out.

Thermal analyses Differential thermal analyses (DTA) and thermogravimetric analyses (TGA) of M2'ZrP and M " Z r P samples were carried out before and after the self-diffusion experiments. These were performed on a Du Pont Modular Thermal Analysis System; D T A was at a heating rate of 20°C/rain in an atmosphere of nitrogen. M2~ZrP and M n Z r P samples were also subjected to isothermal D T A in which they were slurried in a sample tube with the appropriate cation solutions. A chromel :alumel thermocouple was placed in the slurry and sealed into the tube with "Araldite". The tube was heated isothermally and heat content changes recorded by the Du Pont instrument. Alumina was used as a standard. RESULTS

Triplicate chemical analyses gave the formula of the sample of y-H2ZrP as

ZrP2"P2Os'3.3H20. Potentiometric data (Fig. 1), thermal analysis (Fig. 2), and IO-

//

I ~

/Tr

8-

4-

2

0

I

2

[

4

I

6

m-equll OH-/~) Fig. 1. Potentimetric titration curve of crystalline zirconium phosphates. (a-phase full lines, ~,-phase pecked lines-lines 1 and 111 this work, line II Ref. [3] in Part I of this series line IV Ref. [6]). 8. A. Dyer, J. M. Fawcett and D. U. Ports, Int. J. applied Radiation Isotopes 15,377 (1964).

Studies on crystalline zirconium p h o s p h a t e - I !

3155

<3

W

18

f /-

14

f .W:)

at

"i

2 0

I

200

1

400

I

600

I

800

Temperature, "C Fig. 2. Thermal analyses of a- and y-phases of ~irconium phosphate.

X-ray powder photography (Table 1) confirmed that the H2ZrP prepared was almost identical to that obtained by Clearfield, Blessing and Stynes. The exchanged forms of a-ZrP were as in Part I of this series [7]. Particle size was not measured with any satisfaction; inspection of electron microphotographs and visual microscopy showed the particles to be of uniform but irregular shape. For the purposes of solving the diffusion equation the particles were assumed to be spherical with an approximate diameter of 10 -6 m. Self-diffusion plots of the attainment of equilibrium (WJW®) with time are ir,o~Zigs. 3-7. When the migrating ion was Na, K, Cs or Ba in a-ZrP and Ca in y-ZrP the plots were analysed by a computer procedure[9] developed for a solution to the Carman-Haul equation[10]. This gave values for D, the cationic self-diffusion coefficient. For Rb, Ca and Sr migration in a-ZrP more than one process seemed to occur. ~or Rb, separation of the stages was experimentally difficult and n o / ) values were obtained. With Ca 2÷ and Sr 2+ the first process reached completion after about 9 min. This wa~ followed by a slow process, the extent of which was temperature dependent, and which was a precursor to a further stage by which complete s,elf-exchr.nge was reached. This final stag~,, 9. A. Dyer, R. B. Gettins and R. P. Townsend, J. inorg, nucl. Chem. 32, 2395 (1970). 10. P. C. Carman and R. Haul, Proc. R. Soc., Lond., 222A, 109 (195~,).

s

vs

m

m

m

s m

12.20

5-81

4.62

4.35

3.74

3.31 3-20

* R e f . [6].

I

m

m

vw

ms

in s m mw

4.62

4-38

4.07

3.74

3-52 3-33 3-21 3" 11

"~This w o r k .

vs

vw

6.45

5"81

v.s

12-22

!

T-H ZrPt

d(A)

T-H2ZrP*

d(A)

m vw

ms mw

s vw

m

3.05

3.63 3.44 3-29 m

3-33 3.23

3.61

4.06 3"87 3.74

4.28

4.53

5"00

7"23

10.12

d(A)t

vw m

w

m vw w

w

ms

w

vw

ms

t

3.61 3.46 3.33 3-24 3-08

3-87 3.71

4.28 4-09

4.49

5.89

w w mw mw vw

m vw

w vw

vs

vw

vw

vw

8.05 7.38

m

1

9.06

d(A)§

a-SrZrP

§After self-diffusion.

m m ms

m

vs

4-11 3"83

m

vs

4.42

4-59

m

vw

8.05

4-99

vs

/

9.82

d(A)§

~tBefore s e l f - d i f f u s i o n .

3-22 3.05

3.67 3.51

4-04 3.83

4.42

vs

w

mw

7.54

4.86 4.59

vs

1

a-CaZrP

9.82

d(A)t

T a b l e 1. X - r a y d i f f r a c t i o n d a t a f o r T- a n d t ~ - z i r c o n i u m p h o s p h a t e s

3-46 3"32 3" 19

3"71

4-06

4-42

4.71

5-69

6.37

m vs m

mw

m

w

m

vs

vw

mw

v.s

12"22

8.32

!

3"44 3.32 3-20 3.09

m vs m mw

m

m 4"06

w

4-27

m m

s

vw

vw

vs

t

4-44

4.71 4.61

5"81

6.32

8-12

12"22

d(A)§

T-CaZrP d(A)t

0 Z

O

g

.< m

.>

Studies on crystalline zirconium phosphate - I I t0

/~

2 2 5 C ." 44"C

3157

44"C

0.8

37"C o

10"5%

06

0-4

0.2

///

N°

0.8

0.6

I07"C

,,o

76oC

94 *C

0-4

02

Cs

I

I

I

5

I0

150

~'~

I

5

1

io

Fig. 3. The rates of diffusion of monovalent ions from a-zirconium phosphate.

although quite rapid, was capable of kinetic analysis if a time zero was estimated by straight line extrapolation from the intermediate and final stages. From this the final process was replotted (Fig. 8) and computer-analysed for D values as before. A similar analysis of the initial stages was impracticable due to its rapidity. Energies of activation (Ea) for cationic self-diffusion were calculated for the temperature ranges studied from the straight lines obtained when log10 D was plotted vs. 1/T°K assuming the Arrhenius equation D = Do exp (--EJRT) to hold (Fig. 9). Entropies of activation (AS*) were estimated[11] from the preexponent Do assuming the inter-ionic jump distance (d) to be 5 × 10-1° m. Free energies of activation (AG*) could then be calculated. The parameters obtained are listed in Table 2. As a consequence of the unusual kinetics obtained in some systems, thermal and X-ray analyses were 1 I. R. M . B a r t e r , Proc. phys. Soc., Lond. 52, 5 8 ( 1 9 4 0 ) .

3 i 58

A. D Y E R a n d F. T. O C O N

I

f

0.4

/f//~'~ ~ q C

23% --

25oc

-

Oil

Sr

o

|

I

I

2

I

4

I r

6

1

o

~'/',

I

2

I

4

6

rain

Fig. 4. T h e rates o f diffusion of C a 2+ a n d Sr 2+ from a-zirconium p h o s p h a t e at low t values.

lOf lgC

1.0 80"C

Z7" ~ ° C / 5 9 "C

0.9

/

0.8 ~80"6

~1

0.4

Ca

0.2

I 15

0

4~,

0.6

r

o.5

I 2o

2o

rnin

~('/-, min

Fig. 5. T h e rates o f diffusion o f C a 2+ a n d Sr 2+ from a-zirconium p h o s p h a t e at high t values.

Table 2. T h e r m o d y n a m i c p a r a m e t e r s m e a s u r e d for cation selfdiffusion in zirconium p h o s p h a t e s

Cation

Ea kJ/mole

AS* J/mole. degree

AG* kJ/mole

ZrP phase

Na K Cs Ca Sr Ba Ca

63.6 69.7 82.6 74.4 84.7 61.6 74.4

+41.4 + 35.1 + 11.8 - 1"4 -8-4 -51.7 - 1.4

+50 + 57.8 + 77.1 + 76.0 +84.7 +73.4 + 76.0

a t~ a a ct a ~,

Studies on crystalline zirconium p h o s p h a t e - 11

3159

1.0 1330C 119°C o.8

-

0.6

0.4

0.2

I

5 ,~¢'/,

I

I

15

Io

min

Fig. 6. T h e rates of diffusion of Ba z÷ from c~-zirconium phosphate.

1.0 08

-

75.5"C

-

0.6 0

O4i 0.2 0

I 5

1 10 V"1,

I 15

rain

Fig. 7. T h e rates of diffusion of Ca z+ from ~/-zirconium phosphate.

carried out on samples after self-diffusion. All samples showed slight reductions in water content as determined by TGA. X-ray and D T A results were identical to those obtained before self-diffusion except in the cases of t~-CaZrP and a-SrZrP (see Fig. 10 and Table 1), i.e. those systems in which anomalous diffusion kinetics occurred. Results from isothermal D T A showed features indicative of phase changes for a-CaZrP and a-SrZrP (Fig. 11).

3160

A. D Y E R and F. T. OCON

I l0 [

128.5%

~ 8 00C

~11 0.6

52*C

0.4

0.2

Sr

IF 0

I

I

J

1

I

I

5

I0

15

5

I0

15

rain

~',

¢0-, min

Fig. 8. The rates of diffusion of Ca 2+ and Sr ~+ from a-zirconium phosphate-final process replotted. 16.0

o o7 17,0

o I

2.4

I

I

l

I

I

I

2 "6

2"8

3"0

3-2

3-4

3'6

I/7",

°K

Fig. 9. Plot of logloD vs. I/T°K for cation self-diffusion in zirconium phosphates. ( ( 3 - a-ZrP; • - "y-ZrP). DISCUSSION

Monovalent ion migration in a-ZrP The possibility arises that the Na and K forms of c~-ZrP re-form as higher hydrates under the self-diffusion experimental conditions. However, Torraca [ 12], and Clearfield and Medina[13] have stated that dehydrated phases of ionexchanged forms of c~-ZrP rehydrate very slowly, or not at all. The isothermal 12. E. Torraca, J. inorg, nucl. Chem. 31, 1189 (1969). 13. A. Cleartield and A. S. Medina, J. inorg, nucl. Chem. 32, 2775 (1970).

Studies on crystalline zirconium phosphate - i I

3161

o W

k.

<~

i

o

~

c uJ

e

o

o t.iJ

200

0

400

600

800

oC

Fig. 10. Differential thermal analysis curves of cx-CaZrP and ~-SrZrP before (b) and after (a) self-diffusion studies.

o

,,i Ca

k

s-

UJ

Na K __Rb Cs

I 50

1 I00

1150

Fig. II. Isothermal differential thermal analysis curves of a-zirconium phosphates (Na, K, Rb, Cs, Ba at 45°C: Ca, Sr at 60°C).

DTA results (Fig. 11) show no signs of heat content changes for a-Na~2ZrP and c~-K2ZrP under self-diffusion conditions. X-ray and DTA data of samples after self-diffusion experiments are identical to those of starting materials. Also, Guinier X-ray powder patterns taken for slurries of these ion-exchanged forms with 0-1 M solutions of the appropriate cations* showed no changes in d spacing when compared to those of original materials. It seems reasonable to conclude from the comments above that the selfdiffusion parameters observed for Na + and K ÷ are for ions moving in expanded layer structures, of composition a-Na2ZrP.1.38 H~O and o~-K2ZrP.1.05 H20, respectively, which are thermally stable in aqueous salt solutions in the ranges of temperature studied. According to Ciearfield [14] each ion is coordinated to 4 *Suggestion from a Referee. 14. A. Clearfield, Ion-exchange in the Process Industries p. 311. Soc. Chem. ind. Conf., London (1969).

3162

A. D Y E R and F. T. OCON

phosphate oxygens and 2 water oxygens in hydrated phases of a-Na2ZrP. Thus the results in Table 2, and the appropriate figures, indicate both N a and K occupying homogeneous sites and the slightly larger value of Ea observed for K in comparison to Na migration is consistent with the ions participating in a rate-controlling step which necessitates the migration of an ion from a position in which it is coordinated in some measure to the phosphate oxygens. The fact that AS* is smaller for K than for Na is a reflection of the smaller amount of water which needs to be disturbed by the cation in moving between equivalent sites. The rubidium form apparently contains metal ions in two environments, in one or more phases. The self-diffusion results can be interpreted as about half of the rubidium occupying a position which is exchangeable at temperatures below 76°C and a remainder which requires a higher temperature to migrate. Due to the uncertainty in the nature of the material no further elucidation was attempted. The self-diffusion of Cs takes place in an unexpanded structure, which seems to be one phase, in almost anhydrous conditions. The parameters observed are for the progression of Cs from positions in cavities which are bounded by apertures of free diameter 2.63/~ [ 14]. Barrer and Rees [ 15] have measured energy barriers for the movement of Cs through a 2.3 A aperture in the zeolite analcite, again in near anhydrous conditions, and conclude that the E~ for this process is about 109 kJ/mole. The value observed in this work is less than this value in accordance with the larger aperture. The small positive AS* is a function of the low water content of the caesium zirconium phosphate. Divalent ion migration in a-ZrP Calcium and strontium. The primary rapid diffusion stages can be equated to the movement of hydrated ions relatively unhindered by the ct-ZrP structure. For Ca, about ~ of the ions are free to move in this way, where ½of the Sr ions have similar mobility. The remaining ions are "fixed" and, by the evidence presented in Part I [7], are present as hydrated ions linked through water molecules to phosphate oxygen. Hydrated ions present in these positions would mean an increase in aperture which may be such as to allow the almost unhindered passage of other hydrated ions between cavities, i.e. the primary stage of self-diffusion. The ions in "fixed" positions can only migrate when sufficient thermal energy is present to remove water from their hydration sphere. When this happens the cations are resited to positions in which they are coordinated to phosphate oxygens (and some water oxygens). They then migrate between these positions in a structure which has a reduced interlayer spacing, and Ea is dependent upon ion size in an analogous way to that observed for Na and K. The negative AS* values arise when the divalent ion moves from a position coordinated to phosphate oxygen and acquires a hydration shell which orders the water present. This is a well-known phenomenon for divalent ions in water, in contrast to monovalent ions which tend to disrupt a water environment[16]. The resiting of Ca and Sr appears to cause a phase change, as can be seen from the small, but distinct, differences in thermal and X-ray analyses carded out on samples before and after diffusion experiments. Also, the isothermal dta (Fig. 11) show that Ca 15. R. M. Barter and L. V. C. Rees, Trans. Faraday Soc. 58, 709 (1960). 16. E. Glueckauf, Trans. Faraday Soc. 61,914 (1965).

Studies on crystalline zirconium phosphate - 11

3163

and Sr forms of a - Z r P exhibit small exotherms after heating at temperatures close to those used in the diffusion runs. Barium. Barium is anomalous in that it does not cause layer expansion in a sample in which 66 per cent o f the cation sites are occupied by Ba. Presumably this means that Ba is sited in cavity positions rather than in interlayer positions and the reluctance o f the barium ion to form hydrated species precludes it from positions suggested for hydrated Ca and Sr species. T h e migration of Ba observed is between cavities via a 2.63 .~ aperture, so that E~ can best be compared to the large monovalent Cs ion. This again is consistent with the known properties of Ba in solution [7]. T h e high negative AS* observed can be explained in terms of the relatively large amounts of water present and also by the increase in number of pathways for migration available to a divalent ion in comparison to a monovalent ion moving in the same structure.

Calcium ion migration in y-Z~P It can be seen from Table 2 that the parameters for Ca ion movement in T-ZrP are identical to those observed for the Second-stage kinetics for Ca moving in a-ZrP. This may mean that the resiting of the ions suggested earlier creates a y- (or/3-) structure in which adjacent layers are aligned opposite one another, which does not seem unreasonable [6].

.4cknowledgements-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.