Crystalline insoluble salts of polybasic metals - III

J. inorg,nucl.Chem, 1968,Vol.30, pp. 295 to 304.PergamonPressLtd.Printedin Great Britain

C R Y S T A L L I N E IN S O L UBLE SALTS OF POLYBASIC M E T A L S - I l l * P R E P A R A T I O N A N D ION E X C H A N G E PROPERTIES OF CERIUM(IV) P H O S P H A T E OF V A R I O U S C R Y S T A L L I N I T I E S G. ALBERTi, U. C O S T A N T I N O , F. D1 G R E G O R I O , P. GALLI and E. T O R R A C C A Laboratorio di Chimica delle Radiazioni e Chimica Nucleare del C.N.E.N. and lstituto di Chimica Generale ed lnorganica della Universit~t, Roma, Italy

(Received 25 May 1967) Abstract-The effects of temperature, digestion time, PO4/Ce ratio in solution and order of mixing the reactants on the composition and degree of crystallinity of cerium phosphate are reported. Four different cerium phosphate materials have been obtained; (1) amorphous CeP with P--64/C-e ~- 1.7, (2) microcrystalline CeP with PO4/Ce= 1.15, (3) microcrystalline CeP with PO4/Ce = 1-55, and (4) fibrous crystalline CeP with PO4/Ce - 2. Preparative procedures and some ion exchange properties of these materials are reported and discussed. Fibrous CeP has been employed for the preparation of inorganic ion exchange papers or membranes containing no binder.

IT IS well known that many phosphates of polybasic metals, precipitated in acid solutions, exhibit interesting ion exchange properties[l]. However, most of the work to date has been restricted to zirconium phosphate, and very little experimental work has been reported on titanium, cerium or thorium phosphates. These latter compounds have in fact been obtained as amorphous materials which show lower ion exchange" capacities and a greater tendency towards hydrolysis than zirconium phosphate[ 1]. Since the stability towards hydrolysis of these exchangers should be improved by increasing the degree of crystallinity, systematic investigations were undertaken in this laboratory on the preparation of crystalline or semicrystalline titanium, cerium and thorium phosphates or arsenates. Previous papers have described the synthesis and ion exchange properties of crystalline titanium phosphate[2, 3], and the present paper reports investigations on the preparation and ion exchange properties of some cerium phosphate materials of various crystallinities. Cerium phosphate (CeP) possessing some interesting ion exchange properties has been already prepared by Vissers[4], Cilley[5] and Rocco e t a/.[6]. However, these authors prepared only amorphous materials which were unstable towards hydrolysis, and nothing is known about the formula and structure of this exchanger. While this work was in progress, a paper by K6nig and Meyn has appeared which reports the preparation of two microcrystalline CeP materials[7]. However, no mention was made of the ion exchange properties of these materials. *This work has been supported by the "Consiglio Nazionale delle Ricerche". C. B. Amphlett, Inorganic Ion Exchangers. Elsevier, Amsterdam (1964). G. AIberti, P. Cardini-Galli, U. Costantino and E. Torracca, J. inorg, nucl. Chem. 29, 571 (1967). G. Alberti, G. Giammari and G. Grassini, J. Chromatogr. 28, 118 (1967). D. Vissers, P h . D . Thesis, University of Wisconsin, Madison (1959). W. A. Cilley, P h . D . Thesis, University of Wisconsin, Madison (1963). G. G. Rocco, J. R. Weiner and J. P. Cali, Physical Sciences Research, paper No. 73, A F C R L 64-1018, December (1964). 7. K. H. K6nig and E. Meyn,J. inorg, nucl. Chem. 29, 1153 (1967). 295 1. 2. 3. 4. 5. 6.

296

G. A L B E R T I et al. EXPERIMENTAL

Reagents. Erba RP products were used without further purification. Ce(SO4)2'4H20 was Merck pro analysi product. Analyticalprocedures. The P O J C e ratio in the solids was determined by the following procedure. About 200 mg of the sample were dissolved in 22 ml of hot 9 M H.~SO4. To this solution 0.1 N oxalic acid was added to completely reduce Ce(IV) to Ce(IlI). The solution was then diluted to 100 ml with distilled water and orthophosphate was determined as described previously[8]. Twenty-five miUilitres of solution were diluted again to 100 ml wi[h distilled water and Ce(llI) was determined spectrophotometrically at 253.5 mp~ according to the method of Greenhaus et al.[9]. Alkali metal ions were determined by flame photometry. Weight losses were determined by heating the sample to constant weight in a furnace regulated to +5°C. Titration curves were obtained by equilibrating several CeP samples (0.250g) with 50 ml of X-ray powder photographs were taken using CuK~ radiation. 0.1 N (NaCI + N a O H ) solutions. After shaking for 4 days at 25°C the supernatant liquids were analyzed for alkali metal and phosphate contents and their pH was measured. RESULTS

AND

DISCUSSION

When a solution of phosphoric acid is mixed with a solution containing Ce(IV) a precipitate of CeP is formed, the composition, degree of crystallinity and structure of which is strongly dependent on experimental conditions such as temperature, digestion time, PO4/Ce ratio in solution, rate and order of mixing, stirring etc. The effects of some of these factors are reported and discussed below.

Effect of temperature and PO4/Ce ratio in solution Several CeP samples were precipitated by slowly adding 1 vol of Ce(IV) solution in 0.5 M H2SO4 to 1 vol of phosphoric acid solution while stirring. Various concentrations of these solutions were employed in order to obtain different PO4/Ce ratios in solution. For each PO4/Ce ratio, the precipitation was carried out at different temperatures (20 °, 60 °, 95 ° and boiling temperature) with a fixed digestion time (4 hr) at the corresponding temperature. The phosphate/cerium(IV) ratio in the solids (PO4/Ce), degree of crystallinity and physical aspect of the various materials obtained under different experimental conditions are shown in Table 1. From these data the following conclusions can be drawn. (1) Only amorphous materials are obtained at room temperature; PO4/Ce is slightly increased by increasing PO4/Ce in solution. (2) By increasing the temperature and PO4/Ce ratio in solution, first fibrous semicrystalline, then fibrous crystalline materials are obtained with high PO4/Ce ratio. At 95°C, when PO4/Ce is higher than 100, POJC--e becomes nearly 2. (3) At boiling temperatures a microcrystalline CeP is obtained with PO4[Ce - 1.5. The same product is also obtained by refluxing any type of CeP material in H3PO4(1-6 M).

Effect of digestion time The variation of PO4/Ce and degree of crystallinity with the digestion time at 60 ° and 95°C is summarized in Table 2, which shows that the degree of crystal8. G. Alberti, A. Conte and E. Torracca, J. inorg, nucl. Chem. 28, 225 (1966). 9. H. C. Greenhouse, A. M. Feibush and L. Gordon, A nalyt. Chem. 29, 153 i (1957).

Crystalline insoluble salts of polybasic m e t a l s - 111

297

Table 1. P O 4 / C e ratio, crystallinity and physical aspect of various samples of cerium(IV) phosphate prepared at various temperatures and at various PO4/Ce ratios in solution; digestion time = 4hr

P O 4 / C e , crystallinity and physical aspect

Ce(IV)

Ft3P04

PO4/Ce

Molarity

Molarity

in solution

20 °

60 °

80 °

0.06 0.20 0.20 0.05 0.05 0.05 0.05 0-25

0.6 1 4 1 4 6 8 6

3 5 20 20 80 120 160 240

1.44 A 1-65 A

1.70 S . F .

1.70 S . F .

A M C.F. S.F.

= = = =

1.66 A

1.70 1-75 1.80 1.88

S.F. S.F. C.F. C.F.

95 °

1-87 C . F .

reflux

1.50 M 1.80 1-85 1.87 1.98 2-00 1-98

C.F. C.F. C.F. C.F. C.F. C.F.

1.60 M

1.55 M

Amorphous. Microcrystalline. Crystalline fibrous. S e m i c r y s t a l l i n e fil~rous,

linity is increased by increasing the digestion time. However, the different effects of the digestion time on P-O4/C'~ ratios must be outlined. At 60 °, PO4/Ce increases on increasing the digestion time only for low values of PO4/Ce. At high values of PO4/Ce, PO4/Ce first increases and then decreases on increasing the digestion time. At 95 ° PO4/Ce is lowered by increasing the digestion time and a microcrystalline product is finally obtained similar to that prepared by refluxing. Table 2. Effect of digestion time at 60 ° and 95°C on PO4/Ce ratio, crystallinity and physical aspect. (Abbreviations as in Table 1)

Time (hr) 2 4 8 10 20 24 40

PO4/Ce in solution = 5 Ce(IV)=0-20M;H~PO4= 1M 60° 95 ° 1-70 A

1.95 S.F.

P O J C e in solution = 20 Ce(IV)=0-05M:H~PO~ IM 60 ° 95 °

PO4/Ce in solution = 1211 Ce(IV)=0-05M;H:~PO~=6M 60 ° 95 °

1.80 S.F.

1.90 S.F.

1.70 S.[:.

1.85 C.F.

1-70 S.F. 1.75 C.F.

1.71 C.F.

1-80 C.F. 1.88 C.F. 2.00 C.F. 1.95 C.F. 1.83 C.F.

1-98 C.F. 1.86 C.F 1-74 C.F. 1-60 M 1.50 M

Effect of order of mixing By reversing the order of mixing a microcrystalline CeP with POJCe --- 1.15 is obtained when PO4/Ce in solution is low (Table 3). However, it can be seen from the same table that at very high PO4/Ce values (e.g. PO4/Ce = 120) a microcrystalline CeP with higher PO4/Ce ratio is obtained.

Preparative procedures and ion exchange properties of the various CeP materials Tables 1, 2 and 3 show that under different experimental conditions four different CeP materials can be obtained: (1) amorphous CeP, (2) microcrystalline CeP with P--O4/C-~ - 1.15, (3) microcrystalline CeP with PO4/Ce - 1.5, and (4) fibrous crystalline CeP with P--O4/C'7-e- 2. Detailed preparative procedures and some ion exchange properties of these CeP materials are reported below.

298

G. A L B E R T I et al. Table 3. PO4/Ce ratio of CeP materials obtained by adding 1 M H3PO~ solution to 0.25 M Ce(IV) solution in 0.5 M H2SO4. (Abbreviations as in Table 1) PO4/Ce(IV) in solution

Temperature (°C)

Digestion time (hr)

POJCe (M)

10 10 10 10 120 120

60 60 95 95 95 95

2 10 2 10 2 10

1"19 1"19 1"15 1"13 1"30 1'32

(1) Amorphous CeP One volume of 0.2 M Ce(SO4)24H20 solution in 0.5 M H2SO4 is added dropwise to 1 vol of a well-stirred 1 M H3PO4 solution. The precipitate was digested for 4 hr at room temperature and then filtered and washed free of sulfate ion with 0.1 M H3PO4. The precipitate was then washed with distilled water to pH4,

11¸

10 pH c

9

2[_ 0

I b

1

2

3

/a

4

5

6

Fig. 1. Ion exchange properties of amorphous CeP (PO4/Ce = 1.7) titrated with 0-I N (NaCI + NaOH) Abscissae: curve (a)m-equiv. O H - / g o f e x c h a n g e r driedover P205 curve (b) Na+-uptake (m-equiv./g) curve (c) phosphate released to the external solution (mmole/g).

Crystalline insoluble salts of polybasic metals -

1l I

299

giving a product with PO4/Ce = 1-7. This material is very readily hydrolysed and the POJCe ratio is strongly dependent on the extent of washing. The titration curve of amorphous CeP, shown in Fig. 1, is very similar to that found by Rocco et al.[6]. (2) Microcrystalline CeP with PO4/Ce = 1-15 One hundred millilitres of I M H3PO4 in 0-5 M H2SO4 were added dropwise to 40 ml of 0.25 M Ce(SO4)2.4H20 in 0-5 M H2SO4. The temperature was kept constant at 60°± 5°C. The precipitate was then stirred for about 4 hr at this temperature, washed with distilled water to pH 5.5 and dried over P205 to constant weight. An appreciable amount of sulphate remains in this material (=7%).

Table

4. d - v a l u e s f r o m X - r a y

powder patterns of crystalline CeP

Fibrous crystalline CeP

Microcrystalline CeP

( P O 4 = 1"98~

(P -1.55)

Microcrystalline CeP

\(.~10-95 v v s 6-26 v v w

10.46 s 6.26 s

10.34 v v w 7.97 v w

5.55 v w 5.35 w

5.26 v s 4.68 w

7.52 s 6-76 v w

5-08 v w 4-78 v v w

3.47 v s 3.31 w

5.13 v v w 4.94 s

4.14 s

3.12 w

4.67 v v w

3.87 w

2-95 v s

4.54 v v w

3.70 s 3.50 s

2.85 s 2-72 v w

4.31 v v w 4.11 v v w

3.31 v v w 2.98w

2.58 w 2.51 v w

3.98 w 3.30w

2.85 w 2.69 w

2.39 v v w 2.27 v v w

3.13 s 3.05 v w

2.42 v v w

2.17 w

2.94 v w

2.28 v v w 2.12 v v w

2-09 s 2.01 w

2-79 v w 2.70 v v w

1-80 v v w

1.94 w

2.64 v v w

1.87 w

2-60 s

1-83 v v w

2.56 v v w

1-79 w 1.76 s

2.49 v v w 2.44 w

1.74 v v w 1.68 v v w

2.17 v w 2.14 v w

1-64 v v w

2.03 v w 1-93 w 1-92 v v w 1.89 v v w 1.84 v v w

1.77 w 1-75 v w v v s = v e r y v e r y strong, vs = v e r y strong, s = strong, w = weak, v w = v e r y weak, vvw = very very weak.

300

G. A L B E R T I e t al.

The d-values of this crystalline material* are reported in Table 4, and the %wt. loss as function of temperature is reported in Fig. 4 (curve c). The titration curve (Fig. 2) shows that this material exhibits low ion exchange capacity in acid media, while at pH = 9 the capacity is - 3 m-equiv./g. This material is very stable towards hydrolysis. Figure 1 shows that only 0.25 mmole of phosphate per g of material are lost at pH 9. 10 $

B I

7

6

5

4

3, 2~

I ra

1J0

I

2'0

t

10

i

,'o

I

I

s0

I

10

I

1o

Fig. 2. Ion exchange properties of microcrystalline CeP (PO4/C-e = 1.15) titrated with 0" 1 N (NaCI + NaOH) Abscissae: curve (a) m-equiv. OH-/g of exchanger dried over P205. curve (b) Na+-uptake (m-equiv./g) curve (c) phosphate released to the external solution (mmole/g).

(3) Microcrystalline CeP with l~04/C-e = 1.55 This product can be obtained by refluxing any type of CeP material in concentrated HaPO4 (See Table 2). After refluxing, the precipitate is washed with distilled water to pH 5.5 and dried over P20~ to constant weight. The d-values of this material* are reported in Table 4. Figure 3 shows the titration curves with Na + ion, which show that this crystalline material does not exhibit appreciable exchange; similar results were obtained with Li ÷ ion. The % wt. loss as function of temperature is reported in Fig. 4 (curve a). *Recently a similar material has been obtained by K6nig and Meyn[7].

Crystalline insoluble salts of polybasic metals - 111

12

'b

pH

~

301

a

11

10, 9.

8.

0

1.0

2.0

3.0

4.0

5.0

Fig. 3. Ion exchange properties of microcrystalline CeP (PO4/Ce = 1.55) titrated with 0-1N (NaCI + NaOH) Abscissae: curve (a) m-equiv. O H - / g of exchanger dried over P20~ curve (b) Na+-uptake (m-equiv./g).

(4) Fibrous crystalline CeP with POjC-e = 1.98 (a) Preparation. One volume of 0-05 M Ce(SO4)2. 4H20 solution in 0.5 M H~SO4 was added dropwise ( - 3 ml/min) to 1 vol of a well-stirred solution of 6 M H3PO4. The temperature was maintained at 94 +__4°C. The slurry obtained under these conditions was stirred for 4 hr at this temperature, then washed free of sulphate ion with distilled water (pH ~- 4) and air dried. (b) Some stoichiometric considerations. The wt. loss curve (Fig. 4, curve b) shows that fibrous CeP dried over P205 loses 12.4% in weight at 800°C; chemical analysis showed that at 800°C Ce(IV) is completely reduced to Ce(III) and oxygen is evolved, reduction starting at -200°C. Taking into account the % wt. loss and the P O J C e ratio it can be assumed that the empirical formula representing fibrous CeP is CeO2" P20~'2H20, with a molecular weight of 350-1. From this formula 2 mole of water must be lost per mole of CeP on heating CeP at elevated temperatures. Furthermore, 0.25 mole of oxygen must also be lost per mole of CeP. Therefore, the calculated wt. loss is [(2 × 18+8)/350.1] × 100 = 12.57%, in good agreement with the experimental value. Assuming 350.1 as the molecular weight of fibrous CeP, it can be seen from Fig. 4 that 1 mole of water is lost between 110 and 160 °. Since pyrophosphate and Ce(IlI) are not formed in this temperature range, wt. loss due to condensation

302

G. ALBERTI et al.

2st 20

15 ¸ o

cn o;

10

10b"

"

200

300

400

500

600

700

800

900

temperature *C Fig. 4. % wt. loss of CeP as function of drying temperature

Abscissae: curve (a) microcrystalline CeP (POJCe = 1.55) curve (b) fibrous crystalline CeP (PO4/C-e= 1-98) curve (c) microcrystalline CeP (POJCe = 1.15). of acid p h o s p h a t e groups or to production of o x y g e n m u s t be excluded. T h u s there are two possibilities for weight loss in this range of temperature, i.e. hydration w a t e r or condensation water, corresponding to the following processes: (I) C e ( H P O 4 ) ~ ' H 2 0

,,~ Ce(HPO4)2 + H 2 0

(II) CeO(H~PO4)z

303

Crystalline insoluble salts of polybasic m e t a l s - III

An analogous uncertainty was found in determining the empirical formula of amorphous zirconium phosphate[10]. However, the latter is almost completely converted to Zr(HPO4h at 110°C; the formula Zr(HPO4)2"nH20 was thus chosen, since it seems very unlikely that the decomposition ZrO(H2PO4)~ ~ Zr(HPO4)2 + H20 will occur below l l0°C. On the other hand, fibrous CeP, as previously discussed, is only dehydrated to Ce(HPO4h at 160 °, and therefore the decomposition I I cannot be completely excluded. More data are thus necessary to establish the formula of fibrous CeP. At present we are working out the crystal structure of this material. The d-values are reported in Table 4. (c) Ion exchange properties. A sheet of CeP paper (about 1 mm thick) prepared as described below, was cut into small pieces (-0.1 cm 2) and dried over P205 to constant weight. Figure 5 shows the titration curve (curve a), Na uptake (curve b) and phosphate loss (curve c) for these pieces of CeP paper. The figure shows for comparison the titration curve (curve a') and Na uptake (curve b') obtained by Rocco for amorphous CeP[6] (dashed curves).

;

10 9

"

/ b~

?

/ / i

•

,'&

¢'/ / 7

tI / ;//

5' 4'

/

J

3, 23i

o

L

~

4

i

i

I

a

i

,

~

i

;

J

h

, -g-

Fig. 5. Ion exchange properties of fibrous crystalline CeP (]~4/C-'e = 1.98) (solid curves) titrated with 0-1 N (NaCI + NaOH). The dashed curves refer to amorphous CeP[6]. Abscissae: curves (a) and (a') m-equiv. OH-/g of exchanger dried over P20~ curves (b) and (b') Na+-uptake (m-equiv./g) curve (c) phosphate released to the external solution (mmole/g).

It can be seen that fibrous CeP exhibits a total higher ion exchange capacity than amorphous CeP, but the ion exchange capacity of amorphous CeP at low pH (<3.3) is higher than that of fibrous CeP. This phenomenon is also seen in zirconium phosphate[11] and titanium phosphate[2]. Crystalline materials are less readily hydrolysed than the corresponding amorphous materials, and therefore the higher total capacity of crystalline exchangers must be due to a larger 10. G. Alberti, E. Torracca and A. Conte, J. inorg, nucl. Chem. 28, 607 (1966). 11. A. Clearfield and J. A. Stynes, J. inorg, nucl. Chem. 26, 117 (1964).

304

G. ALBERTI et al.

P-OJlg~ ratio; at very low pH values, the higher ion exchange capacity of the amorphous materials may be due to the fact that in them the exchangeable hydrogen ions display a larger range of acidities than in the crystalline materials, so that some exchange may occur at lower pH values than in the crystalline ones. From the sodium uptake curve of fibrous CeP (Fig. 5, curve b) an experimental ion exchange capacity of 4.6 m-equiv./g is obtained at pH 10. Since at this pH - 0 - 6 mmole of phosphate are released, a total ion exchange capacity of 4.6 + 0.6 --- 5.2 m-equiv./g can be derived. Assuming a molecular weight of 350.1 for fibrous CeP, the hydrogen exchanged at pH 10 corresponds to (350-1 × 5.2.10-3)/2 = 0.91 equiv, for each phosphate group. This value is in good agreement with the formula Ce(HPO4)2.H~O. If, on the other hand, we represent fibrous CeP by the formula CeO(H2PO4)~, only one hydrogen of the H2PO4 group would be exchanged at pH 10. (d) Preparation of ion exchange paper and membranes from fibrous CeP. A very interesting property of fibrous CeP is the tendency to give flexible sheets similar to cellulose paper[12]. To prepare CeP paper, the fibrous CeP is suspended in water, filtered under suction on a Buchner funnel, and air dried. After removing from the filter a sheet of inorganic ion exchange paper is obtained. The mechanical properties of these sheets improve when the P O J C e ratio tends to a value of about two. This inorganic ion exchange paper can absorb water by capillarity and has been employed in this laboratory for chromatographic and electrophoretic studies, with results which will be published elsewhere. Thin inorganic ion exchange membranes containing no binder can also be obtained by decreasing the porosity of thick layers of material by pressure[ 12]. The mechanical and electrical properties of these membranes are at present under investigation. 12. G. Alberti and U. Costantino, Italian patent p e n d e n t n 36739A/67.