On the mechanism of ion exchange in crystalline zirconium phosphates—III

J. inorg,nucl.Chem., 1970,Vol.32. pp. 2775to 2780. PergamonPress. PrintedinGreat Britain

ON THE MECHANISM OF ION EXCHANGE IN CRYSTALLINE ZIRCONIUM PHOSPHATES-III THE DEHYDRATION BEHAVIOR OF SODIUM ION E X C H A N G E D PHASES OF a-ZIRCONIUM PHOSPHATE A. C L E A R F I E L D and A U G U S T O S. MEDINAI" Department of Chemistry, Clippinger Graduate Research Laboratories, Ohio University, Athens, Ohio 45701

(Received 5 January 1970) A l ~ t r a e t - T h e dehydration characteristics of two sodium ion exchanged phases of crystalline azirconium phosphate have been determined. The half-exchanged phase, Zr(NaPO~)(HPO4) • 5H20, was found to form two lower hydrates and an anhydrous phase. At a temperature of 390 ° or higher sodium dizirconium triphosphate, NaZr2(PO4)a, was obtained. The fully exchanged phase, Zr(NaPO4)2 " 3H~O, forms one lower hydrate and three anhydrous phases. The sodium ion may be eluted from all of these phases to recover the original a-zirconium phosphate crystals except for the fully exchanged phase heated to 800°C which yields a new form of zirconium phosphate. INTRODUCTION

RECENT ion exchange studies on zirconium bis(monohydrogen orthophosphate) monohydrate, Zr(HPO4)2-H20, hereafter called a-ZrP, have shown that two exchanged phases are formed during the uptake of sodium ions[l]. The two phases have the formulas Zr(NaPO4) (HPO4)" 5H20 and Zr(NaPO4)2.3H20 and will be referred to as phase A (Na + • 5H20) and phase D (2Na + • 3H20), respectively.* These phases lose water to form a number of partially hydrated and anhydrous phases[l]. It has now been found that a knowledge of the composition of these phases and the temperature range over which they are stable is necessary for a full thermodynamic description of the ion exchange process [2]. This paper will present the requisite phase data. EXPERIMENTAL a-ZrP crystals were prepared by refluxing a zirconium phosphate gel in 12M phosphoric acid as described previously [3]. These crystals as well as the exchanged phases were analyzed by the methods described in Ref. [3]. Preparation of phase D (2Na +- 3H20). A weighed quaatity of a-ZrP crystals was added to a 500 ml polyethylene bottle and a solution of 0.2M sodium chloride added in the ratio of 100 ml of solution per gram of crystals. Then a standardized solution of 0.200N sodium hydroxide was added in small increments with agitation until 8.85 meq. of hydroxide per gram of a-ZrP had been added. This amount represents a one-third excess over that required for complete replacement of the ex*See Ref. [1] for an explanation of this nomenclature. tThis paper is one of a series based upon the Ph.D. Thesis of A. S, Medina, to be presented to the Department of Chemistry, Ohio University in 1970. 1. A. Clearfield, W. L. Duax, A. S. Medina, G. D. Smith and J. R. Thomas, J. phys. Chem. 73, 3424 (1969). 2. A. Clearfield and A. S. Medina, In preparation. 3, A. Clearfield and J. A. Stynes, J. inorg, nucl. Chem. 26, 117 (1964). 2775

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changeable hydrogen ions. The mixture was then shaken for 2 days at 25 ___0.1°. A small portion of the solid was filtered through a disk of millipore MF paper and the wet disk taped to a microscope slide cover glass. This was inserted into a Norelco wide-angle diffractometer and the powder pattern of the wet solid recorded with CuK~ (nickel filtered, ,X= 1.5418 A) radiation. The only phase found to be present was phase D (2Na +. 3H20). The remainder of the solid was filtered off and blotted between filter papers to remove excess solution. A portion of the wet solid was packed into a plastic deep-well sample holder suitable for insertion into the Norelco X-ray diffractometer. The remainder of the solid was placed into a tared platinum crucible and weighed. Both the sample in the plastic holder and that in the platinum crucible were placed inside a dessicator over an aqueous solution of sulfuric acid to maintain a predetermined water activity. The platinum crucible was then weighed at various time intervals. Each time the sample was weighed the sulfuric acid solution was replaced so that the final vapor pressure at equilibrium was known. A typical weight loss vs. time curve is shown in Fig. 1. X-ray patterns showed that no phase change occurred during this drying period if the relative humidity was 51 per cent or higher at room temperature. Analysis of the dried solid gave: 32.24% ZrO2, 37-13 P20~, 16.22% NatO, loss on Ignition 14-18%. Required for Zr(NaPO4)2 • 3H20*: 32-51% ZrO2, 37.14% P~Os, 16.22% Na~O and 14-14% loss on ignition. I

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Fig. 1. Drying of phase D (2Na + • 5H20) without phase change at a relative humidity of 51 per cent.

Preparation of phase A (Na + • 5H20). A weighed quantity of a-ZrP crystals was added to a S00 ml polyethylene bottle and a solution of 0.IN sodium chloride added in the ratio of 100 mi of solution per gram of crystals. Then standardized 0.100N sodium hydroxide solution was added in the amount of 3.32 meq. per gram of a-ZrP. The mixture was shaken in a constant temperature bath at 25__0.1 ° for two days. An X-ray powder pattern taken of the wet solid showed only the presence of phase A (Na +. SHeO). The remaining solid was filtered off and treated in the same manner as for phase D (2Na +- 3HzO). A relative humidity of 85 per cent was satisfactory to effect drying without phase change as shown in Fig. 2. Found: 5.82% Na, 25" 10% loss on Ignition. Required for Zr(NaPO4) (HPO4) • 5H20: 5.80% Na, 25.24% loss on Ignition. • Corrected for the presence of 2 per cent Hf (formula wt. = 382.22).

Ion exchange in crystalline zirconium phosphates 3(3

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Fig. 2. Drying of phase A (Na + • 5H~O) without phase change at a relative humidity of 85 per cent.

Dehydration procedure. Dehydration studies were carded out both kineticaily and by static methods. Weight losses to 800"C were determined kinetically using a Tem-Pres Model TG-2A thermogravimetric unit programmed for a heating rate of 100" per hr. Equilibrium weight losses up to 350* were determined in a constant temperature oven which could be controlled to +--2 per cent. The phases were identified from their X-ray diffraction patterns [ 1]. RESULTS

The weight loss vs. temperature curves for phase A (Na +- 5H20) and phase D (2Na +. 3H~O) are given in Figs. 3 and 4, respectively. Since phase changes also occur at elevated temperatures without weight loss, a complete listing of the phases is given in Tables 1 and 2. I

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Fig. 3. Weight loss at different temperatures for PhaseA (Na +- 5H20 ).

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Fig. 4. Weight loss at different temperatures for Phase D (2Na +" 3H20).

Table 1. Phases formed by the dehydration and subsequent heating of phaseA (Na + • 5H~O) Phase

' Composition

Conditions of formation

PhaseAB (Na ÷" 4H20) Phase B (Na ÷. H20)

Zr(NaPO4)(H PO~). 4H20 Zr(NaPO4)(H PO4) • H20

Phase C (Na +) Sodium dizirconium triphosphate

Zr(NaPO4)(H PO,) NaZr2(PO4) 3

250-35 °, rel. hum. 40-50% 40°-72° in air or 25° at very low rel. hum. 900-350 ° above 390°

Table 2. Phases formed by the dehydration and subsequent heating of phase D (2Na +. 3H~O) Phase

Composition

Conditions of formation

Phase E (2Na +. HuO) Phase F (2Na +) Phase G (2Na +) Phase H (2Na +)

Zr(NaPO4) 2 • H20 Zr(NaPO4) 2 Zr(NaPO4) 2 Zr(NaPO4) 2

500-90° 140°-165 ° 200° 800 °

Phase A (Na +. 5H20) loses four moles of water on exhaustive drying at room temperature over phosphorus pentoxide or at 400-72 ° in air forming phase B (Na + • H~O). However, if phase A (Na ÷ • 5H20) is kept at a relative humidity of 40-50 per cent a mixture of phase A and a phase whose composition is believed to be Zr(NaPO4) (HPO4)" 4H~O [hereinafter called phase AB(Na ÷. 4H20)] is obtained. This phase was not observed previously [1]. It is difficult to obtain in pure form since at lower humidities or slightly higher temperatures it occurs along with phase B (Na ÷" H20). Its composition was estimated from its relative abundance as estimated from X-ray powder patterns and weight loss measure-

Ion exchange in crystalline zirconium phosphates

2779

ments. For example, when the solid mixture contained 4.92 moles of water approximately 10 per cent was phase A B and 90 per cent phase A (Na ÷ • 5H~O). At a water content of 3.5 moles the solid was estimated to be 20 per cent of the monohydrate and 80 per cent of phase AB. The X-ray pattern of phase A B (Na ÷. 4H20) is given in Table 3. Further heating of phase B (Na ÷- H20) to above 90 ° results in the loss of the last mole of water and formation of phase C (Na+). The steeply rising portions of the weight loss curves represent two phase regions whereas the plateaux indicate the presence of a single phase. For example, in the temperature range 720-90 °, mixtures of phase B (Na ÷. H20) and phase C (Na ÷) in varying proportions depending upon the amount of water lost, are obtained. Heating phase C above 350°C results in the condensation of the monohydrogen phosphate Table 3. X-ray patterns of new sodium exchanged a-zirconium phosphate phases Phase A B (Na +- 4H20)

Phase H (2Na +)

d

Ill o

d

1/1o

9.88 4.94 4.60 4.31 4.02 3.60 3-31 3.22 3.13 2.86 2.81 2.67 2"63

100 10 15 15 25 20 20 20 20 10 10 10 10

7"63 6.32 4-55 4.40 4-35 4.30 4.19 3.90 3.86 3.79 3-66 3-49 3"39 3.36 3.31 3"16 3.06 2"89 2"87 2'82 2"8O 2.64 2.58 2.50

100 5 15 25 40 40 20 40 50 50 20 35 30 30 30 12 12 15 35 20 15 60 30 12

group with formation of sodium dizirconium triphosphate. A somewhat lower temperature had been reported earlier[l]. It should be noted that the X-ray powder pattern attributed to zirconium orthophosphate,[4] Zr3(PO4)4, is almost identical to that of sodium dizirconium phosphate [5]. Since the orthophosphate was prepared by calcination of a gel zirconium phosphate prepared in alkaline 4. A. Burdese and M. L. Bolera, Ricerca Scient. 29, 2337 (1959). 5. L. Hagman and P. Kierkegaard, A c t a chem. scand. 22, 1822 (1968).

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solution, it is quite likely that s.uflicient sodium ion was exchanged to form NaZr2(PO4)a on heating. Phase D was found to dehydrate very slowly over phosphorus pentoxide at room temperature. However, at 50*-100 ° it was converted completely to phase E (2Na ÷. H20). Complet6 dehydration was attained by heating above 140°C. Usually the product was a mixture of two ~mhydrous phases, phase F (2Na ÷) and phase G (2Na÷). However, if the mixture was allowed to stand in air or in a dessicator for several days phase G reverted to F. When the dehydration was carded out at temperatures in excess of 2000C, then only phase G was obtained and it did not revert to phase F. In the temperature range 100-140 ° mixtures consisting of phase E (2Na ÷. H20), phase F (2Na +) and phase G (2Na ÷) were obtained. Although a plateau indicative of a phase of composition Zr(NaPO4) • 3/5H20 was observed in this temperature range, it was not detected in the X-ray patterns. Finally, heating phase G at 800 ° converted it to yet a third anhydrous phase, phase H(2Na+). This latter phase has not been reported before and therefore its X-ray powder pattern is given in Table 3. The phases listed in Tables 1 and 2 retain the ability to behave as ion exchangers. However, some of them rehydrate slowly or not at all and thus exhibit markedly altered selectivity coefficients. This has already been demonstrated in the case of a partially dehydrated lithium ion exchanged phase [6]. However, when the sodium ions are eluted from the dehydrated phases with acid, all of them with the exception of phase H revert to ot-ZrP crystals (after proper drying). In the case of phase H a new crystalline zirconium phosphate phase was obtained. This procedure of strongly heating a fully exchanged zirconium phosphate and eluting may be a general method of preparing new ion exchange phases. We are investigating this point. Acknowledgement-Acknowledgement is made to the National Science Foundation for financial support of this work under Grant No. GP- 10 i 50. 6. E. Torracca, J. inorg, nucl. Chem. 31, 1189 (1969).