Characterization of the compounds formed from the interaction of divalent cations with inorganic molecular metal—oxygen cluster compounds

Catalysis, 80 (1993) 59-73 Elsevier Science Publishers B.V., Amsterdam

Journal of Molecular

59

MO20

Characterization of the compounds formed from the interaction of divalent cations with inorganic molecular metal-oxygen cluster compounds G.B. McGarveyl, N.J. Taylor and J.B. Moffat* Department of Chemistry and Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario N2L 3Gl (Canada); tel. (+ l-519)8851211, fax. (+ 1 519)7460435, e-mail; [email protected]

(Received February 26,1992; accepted September 28,1992)

Abstract Infrared spectroscopy, powder X-ray diffraction, differential thermal analysis and nitrogen adsorption-desorption measurements were carried out on a series of solids prepared from l2molybdophosphoric and 12-tungstophosphoric acids and the alkaline earth hydroxides. A single crystal investigation and data analysis were done for the Bax+/PM0,~0i; system. It is concluded that the reaction of simple divalent salts with 12-molybdophosphoric acid produces a mixture of the parent acid and a divalent compound rather than a divalent heteropoly salt. Key words: cluster compounds; divalent cations; heteropoly oxometalates; 12-molybdophosphoric acid; nitrogen; 12-tungstophosphoric acid

Introduction Inorganic molecular metal-oxygen clusters are of interest because of their remarkable structures and unusual properties [ 11.The areas of application, both actual and potential, of the materials range from medicinal, clinical and analytical chemistry through to solid state chemistry and heterogeneous and homogenous catalysis. The unique structure of heteropoly anions with well defined arrangements of anions makes them particularly attractive for model studies whereby the substitution of atoms can often be readily achieved. Of current interest in many disciplines which exploit the solid state structure of these clusters are the 12-heteropoly oxometalates which possess the socalled Keggin anion. Briefly, these anions are based on a central X0, tetrahedron (X=P5+, Si4+, As5+ ) which is surrounded by twelve octahedra of oxygen atoms with peripheral atoms near their centres (MO, units; M = W, MO, V). The anions possess overall Td symmetry, and by altering the choice of X and M it is possible to prepare several isostructural anions [ 1,2].Charge bal*Corresponding author. ‘Present address: Department of Chemistry, University of Calgary, Calgary, Alberta, Canada. T2N lN4. 0304-5102/93/$06.00

0 1993 - Elsevier Science Publishers B.V. All rights reserved.

60

G.B. McGarvey et al./J. Mol Catal. 80 (1993) 59-73

ante is achieved using a variety of cations including the proton, alkali cations, ammonium and alkylammonium cations. The choice of constituent atoms of the anion, as well as the choice of cation are important in determining the solid state properties of the 12-heteropoly oxometalates. The acid, H3PW12040, is known to exist in several hydrated states and has been shown to exhibit good protonic conductivity properties [ 31. The 12-tungstosilicate anion has been found to modify effectively glassy carbon electrodes which displayed improved exchange currents and hydrogen evolution from acidic solutions [ 41. In addition to the chemical changes described above, the structural and catalytic characteristics of 12-heteropoly oxometalates are also significantly influenced by the choice of cations and anions. Work in this laboratory has been concerned with the characterization of the porous structure and solid state structure of a series of 12-heteropoly salts prepared with a variety of monovalent cations [5-71. These studies demonstrated an approximately inverse relationship between the micropore volume and the intensity ratio (IJ &,) of two powder X-ray diffraction peaks when each is considered as a fimction of the cation diameter. That is to say, the micropore volume was a maximum for the salt with the lowest intensity ratio. The explanation for this phenomenon is based on the orientation and translation of the terminal oxygen atoms in the lattice which undergo varying degrees of distortion as the size of the cation changes thus altering the X-ray intensity in the [ 1101 plane. Concomitantly, the distortion or reorientation permits adsorbate molecules such as nitrogen to enter the interstitial voids between the primitive cubic arrangement of the anions. More recently, the ion exchange, structural, porous and catalytic properties of a number of salts of H,PW,,O,, and H,PMo,,O,, prepared with monovalent cations were studied [ 8-101. The investigations revealed the existence of single discrete phases of mixed cation composition and surface area which varied systematically with the cation composition. While the acid form and salts of monovalent cations have received the most attention, there are reports in the literature of studies describing the catalytic properties of 12-heteropoly salts prepared with divalent cations [ ll141. Unfortunately, the characterization of the surface and bulk properties of these materials has not been carried out in detail. To characterize these materials more fully, a study of the alkaline earth salts of 12-tungstophosphoric acid ( H3PW12040) and 12-molybdophosphoric acid (H,PMo,,O,,) has been made. Such an investigation allows for the examination of the effect of cation charge as well as the effect of changing the number of cations associated with the anion on the surface and structural properties. The materials have been characterized by measuring nitrogen adsorptiondesorption isotherms, differential thermal analysis curves, powder X-ray diffraction patterns, infrared spectra and a crystallographic study of a prepared single crystal to investigate the location of the cation in the lattice.

G.B. McGarvey

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61

Experimental Sample preparation The parent 12heteropoly acids were obtained from BDH (H,PW,,O,, and H,PMo~,O,~, both AnalaR grade) and were further purified by recrystallization and ether extraction. The barium, strontium, calcium and magnesium salts of each of the 12-heteropoly acids were prepared using double decomposition methods with the appropriate hydroxides as had been reported in the literature for nitrate and carbonate [ 14,151. Each catalyst was prepared with enough of the alkaline earth hydroxide to yield a salt of nominal stoichiometry A3(PM12040)2. The solution or suspension of the alkaline earth hydroxide was added gradually to a stirred, aqueous solution of the 12-heteropoly acid at 333343 K. The resulting homogeneous solutions were taken to dryness over a hot water bath in order to isolate the solids. Physical and structural determination procedures Infrared spectra were recorded in the fingerprint region for 12-heteropoly anions (2000-300 cm-’ ) on a Perkin-Elmer Model 983 spectrometer for samples which were pressed in a KBr matrix. X-Ray powder diffraction patterns were recorded on a Phillips Model loll/ 60 diffractometer using CuK, radiation filtered through nickel at 30 mA and 40 kV. Unless otherwise stated, samples were heated to 473 K for 2 h in air prior to analysis. Differential thermal analysis curves (DTA) were recorded on a DuPont Model 910 thermal analyzer equipped with an intermediate range DTA cell. All measurements were made in quartz sample holders in flowing nitrogen with alumina serving as the reference material at a heating rate of 10 K min-l to a maximum temperature of 923 K. Nitrogen adsorption-desorption isotherms were recorded at 77 K using a standard volumetric system. The samples were outgassed at 473 K and lop5 Torr for 2 h prior to measurement of the isotherm. Crystallographic study In order to reproduce the conditions used to prepare the polycrystalline samples the same method was used to introduce the Ba( OH), to the solution of H3PMo1204w However, instead of reducing the volume of the solution at an elevated temperature, the crystals were permitted to form over a period of several days. Due to the instability to desolvation and the acidic nature of the material, the only successful method to form a crystal of acceptable dimensions was to place a portion of the mother liquor in a Lindemann tube and allow the crystal to grow. A cylindrical crystal of diameter 0.3 x 0.35 mm was mounted on a Syntex P2, diffractometer. Accurate unit cell dimensions were derived from 15 reflec-

G.B. McGarvey et al./J. Mol Catal. 80 (1993) 59-73

62

tions, well distributed in reciprocal space. Data was collected by the 219-8scan technique using a scan width of 0.9” below K,, to 0.9” above K,,. Background measurements were made at the beginning and end of each scan for a total time equal to half the scan time. Two standard reflections (888; 999) were monitored every 100 measurements. These exhibited an 8% decay and were used to scale the data to a common level. All h k t I reflections were measured with 28152”. A total of 3518 measured reflections were merged to give 787 unique data, of which 367 were considered observed (12 30(I) ), and were used in the solution and refinement. The structure was solved by Patterson and Fourier techniques and refined by full matrix, least squares methods to R and R, values of 0.056 and 0.069. Results

Infrared spectroscopy is a convenient and powerful tool for investigating the primary (anion) structure of 12-heteropoly oxometalates [ 161. The highly symmetric anions give rise to a series of bands in the 300-1100 cm-l range which are sensitive to anion composition and can be used to verify the existence of the anion. Four bands are particularly characteristic of the Keggin anion and their frequencies are listed in Table 1. The triply degenerate asymmetric stretch of the central atoms-oxygen bond (v,, (P-O) ), the asymmetric stretch of peripheral atom-terminal oxygen atom bond (v,, (M-O,) ) and the M-O-M stretches of the inter- andintraoctohedral bridges (Ye ( M-0-M)inter) and (v,, ( M-0-M)intra) have been tabulated previously [ 161 and were compared with the results in the present study. The agreement between the present results and those reported in the literature confirms the existence of the 12-heteropoly anion structure for all of TABLE 1 Vibrational assignments for solids prepared from alkaline earth hydroxides and 12-heteropoly acids Alkaline earth/ heteropoly anion

v,w-0)

v,(M-0,)

v,(M-O-M)i,ti,

v,(M-O-M),,,

M$+/PW,,Ok Ca2+/PW,* Ok SS’/PW,,O~, Ba’+/PW,,O& KPWI&

1080 1079 1077 1080 1080

981 979 976 983 980

899 897 900 890 888

810 809 811 806 805

M$+/PMo,~O~ Ca2+/PMo120& S?+/PMo,,O& Ba2+/PMo120& H,PMo,&

1062 1061 1061 1063 1063

963 960 960 961 962

883 881 884 872 875

803 805 804 792 797

G.B. McGarvey

et al./J. Mol Catal. 80 (1993) 59-73

63

the materials studied. In addition to the bands associated with the Keggin anion, a sharp band at 1600 cm-l, attributed to the bending vibration of lattice water [ 171 was observed for each sample as were broad bands in the 3200-3400 cm-’ range assigned to water or the oxonium ion [ 171. Powder XRD patterns recorded in the present study indicate significant differences between the secondary structure of the parent acids and monovalent salts and those prepared in the present study. The patterns for the alkaline earth/l2-molybdophosphate systems are largely featureless, the only diffraction peak of significance is the one positioned at 28=8.75” (d= 10 A). The featureless patterns make assignment of a space group impossible for these materials and it is certainly not apparent from the present results that these materials belong to the Fd3m space group [ 181. Hence, only the position of the peak maxima and the interplanar d-spacings have been tabulated (Table 2 ). Similarly for the solids prepared from 12-tungstophosphoric acid, poorly resolved powder XRD patterns were obtained and the 26 and d-spacing values are listed in Table 3. It was evident from the XRD patterns that the incorporation of divalent cations into the heteropoly oxometalate resulted in a significant reorientation or reorganization of the secondary structure of the materials. Since infrared spectroscopy confirmed the presence of the Keggin anion, TABLE 2 X-Ray diffraction data for alkaline earth/PMo120& Alkaline earth/ heteropoly anion

28(“)

d (A)

Intensity (% )

Mi$+/PMo,,O:;

8.88 18.00

10.0 4.9

100

20.25

4.4

15.7 17.6

22.63 27.25

3.9 3.3

7.8 15.7

8.88 15.00 18.00 20.25

10.0 5.9 4.9 4.4

26.88

3.3

Ca’+/PMo,O&

Ba2+/PMo,,0~

100 19.6 21.7 23.9 19.6

a.75

10.1

10.25 25.50

8.6 4.3

80.0 70.0 100

26.50 29.50

3.4 3.0

90.0 65.0

8.43

10.5

12.18 17.55 19.80

7.3 5.1 4.5

100 8.7 14.1 41.3

28.30

3.2

21.7

G.B. McGarvey

64 TABLE X-Ray

et al./J. Mol Catal. 80 (1993) 59-73

3 diffraction

Alkaline earth/ heteropoly

data for alkaline earth/PW,zO& 20 (“1

d (8,

Intensity

8.75 17.75 20.13 27.00

10.1 5.0 4.4 3.3

100 36.8 41.2 39.7

28.50 36.38 37.63

3.1 2.5 2.4

35.3 10.3 14.7

46.25

2.0

7.3

9.00 18.25 20.38 24.75

9.8 4.9 4.4 3.6

100 21.1 28.2 28.2

( %)

anion

&?+/PW,2@,

Ca2+/PW,,0&

!V+/pw,,o:;

Ba2+/PW,zO:<

28.75

3.1

25.4

37.88

2.4

22.5

8.75 18.25 20.00 24.25

10.1 4.9 4.4 3.7

100 28.3 45.7 26.1

25.50

3.5

37.0

28.25 37.25

3.2 2.4

52.2 19.6

9.00

9.8

18.00 20.25 23.75 25.75

4.9 4.4 3.7 3.5

27.1 29.2 35.4 52.1

29.25

3.1

31.3

100

the loss of crystallinity must be attributed to the presence of the divalent cations and not the destruction of the anion. To investigate the temperature dependence of the reorientation of the secondary structure, Ba/PW was pretreated at various temperatures and the powder X-ray diffraction patterns were recorded. Figure 1 shows the diffraction pattern for HPW and Ba/PW pretreated at various temperatures. The difference between the patterns of the parent acid and the solid containing the divalent cations are immediately evident. In addition, the changes brought about by raising the pretreatment temperature are also evident. As the temperature exceeds 473 K the strong peak at 20~ 8.3 o disappears while a pair of peaks at 2& 11’ appears and persists through the higher pretreatment conditions. In the range 30” < 28~ 15O,the appearance of new diffraction peaks is accompanied by the decrease in intensity and the dis-

G.B. McGarvey

65

et al./J. Mol Catal. 80 (1993) 59-73

25°C

x:-

40

30

28

20

Fig. 1. The effect of pretreatment Ba2+/PW,,0&.

10

temperature

on the powder

X-ray

diffraction

pattern

of

appearance of some of the peaks observed at lower temperatures. The observed changes may be related to the role played by molecular water in determining the space group into which the material crystallizes. The principal features of the DTA thermograms for the alkaline earth/ heteropoly anion systems are summarized in Table 4 together with those for the two parent acids, for comparison purposes. The DTA thermograms for HPW and HPMo each display one endothermic and one exothermic peak as has been reported previously [ 19,201. With the parent acids the endotherm is attributed to loss of molecular water held on and in the solids. The high temperature exothermic peaks have been attributed to the decomposition of the anions. Introduction of the divalent cations into the 12-tungstophosphate series resulted in changes in the thermal behaviour of the materials. The decomposition temperatures for the PW,,Oi; anion, indicated by the temperature of the exothermic transition, remain relatively unchanged following the incorporation of the divalent cations. As was observed for the acids, the low temperature endothermic peak could be removed by pumping the sample at room temperature, indicating the the peak was due to water held in molecular form. The behaviour of the materials in the intermediate temperature range was significantly different from that of the parent acid. For the materials contain-

G.B. McGaruey et al./J. Mol Catal. 80 (1993) 59-73

66 TABLE 4

Thermal transitions (DTA) for alkaline earth/l2_heteropoly anions Alkaline earth/ heteropoly anion

Endotherm (K)

Exotherm (K)

H,PW,,O,, Mg2+/PWlzO$ Ca’+/PW,,O$c s?+/Pw,,ok Ba’+/PW,,O&

548 s 583 w, 608 m 518 m 563 m, 613 m 528 m, 558 s

863 m 868 s 833 s 828 m 833 w, 873 m

H,PMo,,O,, M%+PMo,,O:, Ca2+/PMoIz 0:; Sfl+/PMo,,O& Ba2+/PMolzO&

433 s, 463 w 403 m, 423 w, 598 w, 628 w, 648 m 473 w, 598 s, 638 w 413 s, 438 w, 543 m,br 458 w,br, 536 m

723 w 703 w,br 693 w, 718 m, 838 w,br 698 m, 803 w 723 w,br

473

673 Temp

a73

W)

Fig. 2. DTA thermograms from alkaline earth/PW,,O&

.

ing Ba2+ , SP, and MS+, two broadened and temperature shifted peaks were observed (Fig. 2). The additional peaks are believed to arise from the interaction of the divalent cation and residual protons with the heteropoly anion in processes whereby oxygen atoms are stripped from the Keggin anion. In order to investigate this phenomenon more fully, experiments were carried out in

67

G.B. McGarvey et al/J. Mol Catal. 80 (1993) 59-73

which the DTA experiment proceeded to a temperature which was higher than the endothermic peaks but lower than the decomposition temperature. Following cooling, the experiment was repeated and it was observed that the endothermic peaks were absent. The sample was cooled once again and exposed to water vapour for 12 h. Following this exposure, the DTA thermogram was recorded and the endothermic peaks were again in evidence indicating that the removal of oxygen atoms from the Keggin anion was reversible, at least in cases where the anion was not completely destroyed. 8.0-

PIP, Fig. 3. Nitrogen adsorption-desorption isotherm for S?+/PW,,O& TABLE 5 Nitrogen adsorption data for alkaline earth/l2-heteropoly anions Alkaline earth/ heteropoly anion

SBET(m2 g-‘)

GET

Mg+/PW120& CaZ+/PWIzOY& Sr”/PW,,O3,Ba’+/PW,,Oi;

18.5 17.9 20.5 1.8

6 5 5 90

Mg2+/PMo,,Os Ca’+/PMo,,Og S?+/PMo120k Ba’+/PMo,,O&

11.5 19.4 19.3 3.1

11 5 4 66

.

68

G.B. McGarvey et al/J. Mol Catal. 80 (1993) 59-73

The thermograms for the 12-molybdophosphate materials were also significantly different from those observed for the parent acid. Decomposition temperatures of the anions were not increased by the incorporation of the divalent cations (Table 4). Indeed the thermal behaviour of the 12-molybdophosphate materials is more complex than that for the parent acid with several endothermic peaks in evidence in the intermediate temperature range. The more complex thermal behaviour of the 12-molybdophosphate materials is in agreement with the observations that these catalysts exhibit more facile decomposition [ 19-211 and reduction [ 21,221 than the corresponding tungstophosphate catalysts. Nitrogen adsorption-desorption isotherm measurements The measurement of the nitrogen adsorption-desorption isotherms of the divalent materials revealed several interesting differences from previous investigations of monovalent salts. The isotherms were all consistent with those expected for a non-porous material. Figure 3 is a representative isotherm for Sr/PW. Note the small initial uptake of the adsorbate and the small quantity adsorbed even at high relative pressure. Table 5 lists the surface areas and CsET parameters for the materials studied in the present investigation. The small surface areas suggest a non-porous structure and the small C,,, values can be TABLE

6

Crystal data for Ba2+/PMoIzO$ mol.wt.= 1969.18 g mol-’ V=12597(2) As

Z=8 T=295 K

*8H,0a Structure = cubic space group = Fd3m pc= 2.076 g crne3

a=23.268(3) (A) (no. 227-O:) F(OOO) = 7376

/l=o.71073 A

p(MoK,)

“All values listed here are based on water molecules probably not located. TABLE

(fractional,

x 104) for Ba2+/PMo120&

Atom

x

Y

2

MO(~) P(l)

171.5(6) 1250

171.5 1250

1293.9(5)

O(l)

1634(g) 1740(5) 612(5) 2833(5) 2500

1634 1740 612 2833

O(2) O(3) O(4) O(5Y 0(6Y’

3757(14)

2500 1250

*nH,O

1250 1634 -15(3) 2687(3) 1164(4) 5000 1250

“The only water molecules found, but presumably

cm-’

located, though a significant

7

Atomic coordinates

=21.92

there are others.

number were

69

G.B. McGarvey et al./J. Mol Catal. 80 (1993) 59-73 TABLE

8

Bond lengths and bond angles for Ba*+/PMo,,Ok Bondlengths

(A)

Mo(l)..*Mo(l)’ MO(~)-O(1) MO(~)-O(2) MO(~)-O(3) MO(~)-O(4) P(l)-O(1)

3.40(2) 2.42(2) 1.90(l) 1.91(l) 1.69(l)

(X2) (X2)

1.55(l)

(X4)

Bond angles ( ’ ) O(l)-MO(~)-O(2) O(l)-MO(~)-O(3) O(l)-MO(~)-O(4) O(2)-MO(~)-O(2)’

83.4(6) 72.4(6) 171.2(6) 84.4(4)

(X2) (X2)

O(2)-MO(~)-O(3) O(2)-MO(~)-O(3)’ O(2)-MO(~)-O(4)

155.5 (4) 89.3 (4) 103.0(5)

(X2)

O(3)-MO(~)-O(3)’ O(3)-MO(~)-O(4) O(l)-P(l)-O(1)’

86.8(4) 101.4(5) 109.5(11)

(X2) (X2) (X2) (X6)

Fig. 4. View down the 100 axis of Ba2+/PMo120~~.

-nH,O

70

G.B. McGarvey et al./J. Mol Catal. 80 (1993) 59- 73

associated with unrestricted formation of the adsorbate monolayer, a phenomenon indicative of a non-porous surface. Crystallographic s tzdy The crystallographic data are listed in Table 6 and the fractional atomic coordinates and bond lengths and bond angles are listed in Tables 7 and 8, respectively. The structure of the anion is consistent with that of the PMo,, 0:~ anion as has been determined previously [ 24,251. As in previous studies, the location of all of the water molecules could not be determined. Interestingly, no evidence for the presence of Ba2+ cations was found in the difference maps at any point in the refinement. Figure 4 shows the arrangement of the anions in the unit cell viewed along the 100 axis. The cubic arrangement of anions is quite evident, as is the interstitial void space that is present between the anions.

Discussion

Infrared spectroscopic measurements have confirmed that the Keggin anion has been retained following the preparative procedures. The infrared bands for both of the anions studied here have been shown in a previous study to be extremely sensitive to pH modifications [26]. The formation of lacunary PMo,, O& and PW,, 0;; anions were found to result in significantly modified infrared spectra from those of the corresponding 12-heteropoly anions. In the present study there was no evidence for the presence of decomposition species of the 12-heteropoly anions. The intermediate temperature endothermic peaks from the DTA experiments resemble those observed for the parent acids but in most cases two or more peaks were observed indicating that more complex interactions take place between the divalent cations and the heteropoly anions. Based on the experimental evidence it is possible to envision the system behaving in such a manner that divalent salts are not formed, but instead the heteropoly acid remains in its original state and the divalent cations are present as some additional compound. If the divalent material is amorphous or poorly crystalline, the changes in the powder XRD patterns can be explained as resulting from a masking of the diffraction pattern of the acid. The study of the single crystal grown from the solution used in the preparation of the divalent materials supports this second interpretation of the interaction between the divalent cations and the heteropoly anions. The powder X-ray diffraction studies and the single crystal study suggest that the alkaline earth cations are not incorporated into the lattice and hence do not form salts with the trivalent 12-heteropoly anions. As expected, the features of the anion structure are similar to those that have been reported previously for the PMo,, 0:; anion [ 24,251. Of greater interest in the presem

G.B. McGarvey et al./J. Mol Catal. 80 (1993) 59-73

71

study were the efforts to locate the Ba2+ cations in the lattice. Previous studies of the 12-heteropoly salts prepared with monovalent salts have provided indirect and direct evidence for the location of the cations. For the primitive cubic H,PW,,O,,-6H,O system, the hydrated proton was determined to be on a 42m site and it was inferred from powder diffraction data that monovalent cations (i.e., Cs+ ) occupy the same crystallographic site [ 271 in heteropoly salts. Direct evidence for the location of potassium and ammonium cations was obtained from a study of the K3PMo1204,, and ( NH4)3PMo12040 salts and salts containing mixtures of these two cations [ 281. Once again the monovalent cations were found to occupy the 42m site in the primitive cubic lattice (Pn3m). Unexpectedly, there was no evidence from the refinement of the crystal data for the presence of the barium cation. There is precedent in the literature for the disorder of cations and the subsequent difficulties in locating them in the lattice [ 29,301. In each of these studies however, a certain number of the cations were located, whereas in the present case only water molecules were located. Based on these previous reports it was expected that at least a portion of the cations would be found. Thus, the crystallographic study confirmed the anion structure but yielded no evidence for the location or even the existence of the Ba2+ cation. This was despite the advantage of some knowledge of where to expect the cations to be located in the Fd3m crystal system. Previously published data have been compiled by Wyckoff for a series of alkaline earth salts of H,PW,,O,, and H,PMo,,O,, [ 181 as well as for several rare earth salts. In each case, for the 12-molybdophosphate salts, the space group was stated to be Fd3m with quoted lattice parameters (a,) of 23.1 A. Given the range in size of the divalent and trivalent cations that were considered, this appears to be somewhat fortuitous. Indeed, for several series of 12heteropoly salts with monovalent cations, the cubic lattice parameter was found to increase as the cation radius was increased [ 5-7 1. In view of the observation that the measured lattice parameters are almost identical to those of H,PMo,,O,,*29H,O it is apparent that the cations have not been incorporated into the lattice. Instead, for the polycrystalline samples, the acid coexists in a two phase system with the cation present as another salt. This explanation would account for several of the apparent contradictions in the experimental results. For example, the existence of the Keggin anion cannot be ruled out by the powder X-ray diffraction results. It is however, evident that a mixture of the heteropoly acid with a second poorly crystalline material could lead to the observed patterns. The infrared spectra also provide strong evidence for the presence of the Keggin anion, and the exothermic decomposition characteristics are sufficiently similar to support the idea that the Keggin anion is retained for all of the materials that were studied. The conventional methodology in which homogeneous solutions are evaporated to dryness is frequently employed for the preparation of a wide variety of inorganic salts. A number of crystallographic and ion exchange investigations have proven this to be a viable and successful method for the preparation

G.B. h4cGarvey et al/J. Mel Catal. 80 (1993) 59-73

72

of salts of metal-oxygen cluster compounds with monovalent cations. In contrast, evidence presented here clearly shows that the application of this method with divalent cations and the metal-oxygen cluster anions does not produce the divalent salts of these materials, either in their pure state or in the form of a mixture with some other substance. Although the present work does not provide any definitive explanations of the apparent inability of these salts to form under these circumstances it is clear that the retention of the crystallographic structure of the parent acid with the introduction of divalent cations is difficult, if not impossible on electrical neutrality and geometrical grounds. Indeed, although not proven here, it is believed that the formation of divalent salts of the metal-oxygen cluster compounds with Keggin structure is not possible under any preparative conditions, assuming the retention of the structure of the parent acid, regardless of the relative amounts of the anions and divalent cations employed in the preparation.

Acknowledgement

The financial support of the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.

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R.G.W. Wyckoff, Crystal Structures, Vol. 3, 2nd edn. Interscience, S.F. West and L.F. Audrieth, J. Phys. Chem., 59 (1955) 1069.

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of Inorganic

and Coordination

245, and references Compounds,

Wiley,

New York, 1960, p. 887.

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13

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