A novel open framework zincophosphate: Synthesis and characterization

F~UTTERWORTH I=iEINEMANN

A novel open framework zincophosphate: Synthesis and characterization N e v e n k a Raji~,* Nata/ia Z a b u k o v e c Logar,* a n d Vengzeslav Kau~zi/z*'t

*National Institute of Chemistry, and the "?Department of Chemistry and Chemical Technology, University of Ljubljana, Slovenia The crystallization of a reactive zincophosphate gel (which may contain Co(ll) component) in the presence of sodium cations at ambient temperature and pressure yielded products (abbreviated ZnPO-HEX or CoZnPO-HEX) with a novel anionic open framework topology. The Co(ll) ions isomorphously replace zinc(ll) in this zincophosphate lattice up to 30%. The hopeite phase appears as a precursor in the crystallization of these new products. The single crystal structure determination of CoZnPO-HEX reveals that the structure belongs to the hexagonal P61 space group with unit cell parameters a = 10.447(3) and c = 15.049(5) A. The structure consists of a three-dimensional tetrahedral framework containing one-dimensional six-ring channels. An infinite helix built up of sodium cations and water molecules has been found inside the channel. The helix plays an important structural role because a complete dehydration of the materials is the cause for the transformation of the hexagonal structure to an unknown crystalline phase. Kevword$: Zinc phospate; molecular sieves; open framework; zincophosphates; zincophospate characterization

INTRODUCTION In the last few years a great effort has been devoted to the investigation of open framework oxide materials a p a r t f r o m a l u m i n o s i l i c a t e s or a l u m i n o p h o s phates.l-3 The study of the zinc-phosphorus-oxygen 4-7 c system by Stucky and co-workers resulted in th discovery of a number of new open framework zincophosphates (ZnPO). As in the case of aluminophosphates, some structures are analogues to those of zeolites (e.g., ZnPO-sodalite 5 or ZnPO-faujasite6), whereas others represent new structural types, s-l° It is well known that the aluminophosphate framework, in contrast to the aluminosilicate one, offers a possibility for the incorporation of various heteroatoms.11 In view of the variety of applications of these materials, this feature is very important since it endows aluminophosphates with unique physicochemical properties. In this regard, we explored the possibility of incorporating Co(II) into a zincophosphate lattice with the aim of preparing CoZnPO materials. Following a slightly modified procedure for the synthesis of the zincophosphate with sodalite structure 5 (ZnPO-SOD), we succeeded in preparing not only Cosubstituted ZnPO-SOD (CoZnPO-SOD) but also a novel CoZnPO phase. Although the latter structure can also be obtained without the cobalt component, 12 this paper describes mainly the synthesis and charac-

Address reprint requests to Dr. Raji6 at the National Institute of Chemistry, Hajdrihova 19, POB 30, 61000 Ljubljana, Slovenia. Received 12 April 1995; accepted 21 June 1995 Zeolites 15:672-678, 1995 © Elsevier Science Inc. 1995 655 Avenue of the Americas, New York, NY 10010

terization of the novel hexagonal Co(II)-substituted zincophosphate material (acronym: CoZnPO-HEX). EXPERIMENTAL The new zincophosphate was prepared by combining the solutions of zinc acetate, and cobalt acetate, H3PO 4 (85%), and a solution of sodium hydroxide to give a gel of following composition: 1.75 Na~O : x CoO : (1 - x) ZnO : 0.625 P205 : 160 H20 (x varied from 0.0 to 0.5). The solutions of zinc/cobah acetate and sodium hydroxide were prepared by dissolving the salts in about one half of the required (by the gel formula) amount of water, and sodium hydroxide in the rest of water. The resulting gel was blended thoroughly to homogeneity (for about 30 min) and then left standing overnight at room temperature. If the reaction mixture containing the cobalt component is heated at 50°C, CoZnPO-HEX is obtained in approximately 1 h. The obtained solids were collected by filtration and dried at room temperature. Depending on the cobalt content in the reaction mixture (the value of x), the color of as-synthesized products varied from pale to dark royal blue. X-ray powder diffractograms (XRD) confirmed the presence of a same crystalline phase for x between 0.0 and 0.3, and could be indexed as a hexagonal unit cell (for an example, unit cell parameters for ZnPO-HEX are a = b = 10.448(2) A and c = 15.034(7) A). However, the solids obtained from systems with x greater than 0.3 contained mostly amorphous products. Suitable crystals for the single crystal X-ray analysis 0144-2449/95/$10.00 SSDI 0144-2449(95)00083-1

A novel open framework zincophosphate: IV. Rajid et el.

were obtained f r o m the p r o d u c t synthesized f r o m the reaction gel with x = 0.2. For that reason, the results o f f u r t h e r characterization are p r e s e n t e d mainly for the C o Z n P O - H E X p r o d u c t with x = 0.2.

085

coz~o-r~x,x=02

0.68 u~

Instrumentation

7~ 0 . 5 1

Elemental analyses and XRD m e a s u r e m e n t s were p e r f o r m e d o n all samples to check the chemical compositions and crystallinity. Chemical analysis was perf o r m e d by the inductively c o u p l e d plasma emission spectroscopy (Na, Zn, P, Co) and t h e r m o g r a v i m e t r i cally (H20). XRD analysis was carried out o n a Phillips 1 7 1 0 d i f f r a c t o m e t e r using CuK~ radiation (h = 1.5418 /~,). T h e p o w d e r diffraction data were anal y z e d by a t r i a l - e n d - e r r o r indexing program TREOR.13 Diffuse r e f l e c t a n c e spectra w e r e o b t a i n e d o n a Varian MS 80 u.v.-vis s p e c t r o p h o t o m e t e r in the 8 0 0 300 n m range, with MgO as a standard, alp MAS n.m.r, spectra were r e c o r d e d on a Varian VXR-300 spectrometer. T h e spectra were taken at a f r e q u e n c y o f 121,415 M H z a n d a spinning rate o f 4 - 5 kHz. C h e m i c a l shifts are r e p o r t e d relative to 85 wt% H a P O 4. T h e r m o g r a v i m e t r i c analyses were carried out on a Mettler T A 2 0 0 0 instrument. T h e m o r p h o l ogy o f the o b t a i n e d p r o d u c t s was studied using a scanning electron microscope (Jeol JSM-T220).

Single crystal structure analysis A 0.12 × 0.06 × 0.04 m m 3 crystal was used to collect r o o m t e m p e r a t u r e intensity data with M o K , radiation o n an E n r a f - N o n i u s CAD4 d i f f r a c t o m e t e r

Table 1 Crystallographic parameters Empirical formula

Zn24C°ePa°Na3°O16sHg°

Formula weight Crystal color, habit

418.17 Royal blue, prismatic

Crystal system

Hexagonal

a (~) c (~) V (A 3) Z

10.477(3) 15.049(5) 1430.6(10) 6

Space group T (°C)

20(1 )

P61

h(MoK~ (~)

0.71069

I~ (MoK~ (mm -~1 Absorption correction hkldata limits

5.30 None - 1 5 ~< h ~< 15, - 1 5 ~ < k~< 15, -19 ~< I ~< 19

Total data Observed data (/t> 2.5
10,566

527 527 68 0.070 0.061 0.80 + 0.30tanO 2.40 + 0.90tan0 60 (0 - 3 2), (1 - 2 4), (2 2 O)

~ 0.34 oA7 ~=.=_ 0

~ 350

300

, 400

, 450

, 500

q 550

, 600

, 650

, 700

, 750

800

850

WAVELENGTH,n m Figure 1 Diffuse reflectance spectra of dried CoZnPO reaction gel (a), and CoZnPO-HEX material (b).

(Table 1). T h e hexagonal unit cell p a r a m e t e r s were obtained o n the basis o f 25 reflections in the r a n g e o f 6.00-13.88 °, using the least squares m e t h o d . 10,566 reflection intensities were m e a s u r e d , yielding 1,205 u n i q u e intensities, a m o n g which 527 were considered to be observed (I /> 2.5cr(I)). T h e total s p h e r e o f reflection with i n d e x r a n g e - 15 ~< h ~< 15, - 1 5 ~< k ~< 15, - 1 9 ~< 1 ~< 19 was m e a s u r e d , and t h r e e s t a n d a r d reflections w e r e m o n i t o r e d e v e r y 20,000 s with an intensity c h a n g e o f 0.70%. N o absorption c o r r e c t i o n was applied (I~(MoK,~) = 5.30 m m - 1 ) because o f the small size o f the crystal. T h e structure was solved by direct m e t h o d s , followed by Fourier calculations. 17 n o n h y d r o g e n atoms were f o u n d and isotropically r e f i n e d (on Fo) by a full matrix least squares m e t h o d . T h e positions o f h y d r o gen atoms were not obtained. T h e empirical weighting function was used. In the final r e f i n e m e n t cycle t h e r e were 527 contributing reflections and 68 variables. T h e obvious lack o f intensity data caused some difficulties d u r i n g the structure d e t e r m i n a t i o n and also resulted in high final R values. S t r u c t u r e r e f i n e m e n t led to a straightforward framework structure. Large t e m p e r a t u r e factors o n n o n f r a m e w o r k atoms m i g h t indicate a

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The sources of atomic scattering and dispersion factors for neutral atoms Zn, P, and O were Cromer and Mann TM and Cromer and Liberman. 2°

Figure 2 XRD patterns of CoZnPO batches at different stages of crystallization.

Zeolites 1 5 : 6 7 2 - 6 7 8 , 1995

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A nove/open framework zincophosphate: N. Rajid et al.

t~ =

~

occupancy factors for the two Zn sites less than l and suggested the isomorphous substitution of Zn by an atom of lower atomic number (Co) predominantly at the Zn(l) site. The final R = 0.070 and R w = 0.061. The M U L T A N 8814 program and the Xtal 3.215 system were used for calculations and interpretation. All calculations were done at the University Computer Centre, Ljubljana. Detailed crystallographic parameters are available from the authors.

~

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t=30 min

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t=O min

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RESULTS AND DISCUSSION Reaction gels prepared in an otherwise identical way, but in which the molar fraction of the Co(II) component varied (or was omitted), yielded the products belonging to a same crystalline hexagonal phase. Elemental analyses show that the products correspond to following general formula

I,~?\,,fA"

Na6[C°xZnl-xPO4]6"yH20

,/" "%, ......";;'"'"~'""'";~'F;g'"'"'~'"""";'"'"-'d'"'=;'"'"~'"'~';;=:~

where x can reach a maximum value of 0.3. With a higher cobalt concentration, the reaction gel yielded mainly an amorphous product. This observation indicates that Co(II) may affect the gel chemistry and nucleation of this zincophosphate phase. T h e water amount varied with varying for x = 0.2, y reached the value of 9. To get an insight into the coordination geometry

x;

CoZnPO-HEX

/~ CoZ.,,,FO-HEX (x=0 21

CoZnPO-HEX

b

::

ZnPO-HEX

. . . . . . . . . . . 20

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~0

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so

10o 120

~/

140 160 l~O 200 2Zo Z~0 .'~0

".x0 30O 320 340 360 3S0

TEMPERATURE, C 101

L

C o Z a - I-IF--X ( x = 0 . 2 )

Figure 3 31p MAS n.m.r, spectra of a, CoZnPO batches at different stages of crystallization: the dried reaction gel (0 min), the batch after 30 rain, and the batch after 60 min, b, ZnPO-HEX and CoZnPO-HEX (x = 0.05 and x = 0.2) material.

lower symmetry of the group. However, the attempts to refine the structure in a lower symmetry resulted in no significant improvement, also because of the increased number of variables. The maximum and minimum heights on the difference Fourier map were located in the vicinity of heavy atoms. The trial refinement of the population parameters led to the

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Zeolites 15:672-678, 1995

~< ~ . . . . . . . . . . . . . . . .TEMPERATURE, ... C

Figure4 a, d.t.g, profiles of ZnPO(l) and CoZnPO(2)product, b, t.g. curve of the CoZnP0 (x = 0.2) material.

A novel open framework zincophosphete: N. Rajid et aL m

CoZn-HEX (200~)

Z IT]

L =

,

. . . .

Figure 5 XRD patterns of CoZnPO-HEX. Partially dehydrated (a), and after complete dehydration (b).

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25

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2 THETA

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and the position of Co(II) in the obtained products, diffuse reflectance spectra of the following systems were recorded: (1) the dried reaction gel (the value of x was 0.2); (2) the product obtained by crystallization of the above gel. Both spectra (Figure 1) exhibit three absorption maxima. For the dried reaction gel the absorption maxima are of lower intensity and are considerably broader than those for the obtained novel product. The spectrum of the latter shows a strong absorption with three-well distinct maxima (at 540, 580, and 625 nm), which is characteristic of the d-d electron transitions of tetrahedrally coordinated Co with a d 7 configuration. 16 The spectrum of the dried gel is broader, and the absorption is much less intense and may be said to arise from octahedral geometry of Co(II) ions. T h e powder pattern of this material (dried reaction gel), however, entirely corresponds to the hopeite zinc phosphate phase. These results have at least two consequences: 1. Since the structure of the hopeite consists of ZnO2(H20)4 octahedra and ZnO 4 tetrahedra (these polyhedra share corners and edges with PO 4 terahedra), 17 it could be concluded that during gel formation the Co(II) replaces the octahedrally coordinated fraction of zinc in the zincophosphate lattice. 2. The hopeite phase is a precursor in the crystallization of a novel hexagonal zincophosphate phase. This fact is rather intriguing since some recently reported 9 porous zincophosphates in contact with water underwent a complete transformation into the more stable hopeite. The progress of crystallization of the CoZnPOHEX product (x = 0.2) with time was followed by monitoring the change in the powder diffractograms of the batches at 50°C. This is demonstrated in Figure 2, which shows X-ray patterns of CoZnPO at different stages of crystallization. It can been seen that the transformation of the hopeite phase into the novel hexagonal one is essentially completed in the course of about 60 min.

The 3]p MAS n.m.r, measurements lead to the same conclusions as the diffuse reflectance spectroscopy and XRD results. Namely, the spectra o f CoZnPO solids (Figure 3a) exhibit strong side bands as ~___=,,,=m=R

-'~"

1

•

I

SEM micrographs of CoZnPO-HEX (x = 0.2). Lower panel shows the CoZnPO crystal at higher magnification. Figure 6

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A nove/open framework zincophosphato: N. Rajid et al.

a consequence of dipolar interactions between unpaired electrons on Co 2+ and the alp nucleus. This indicates that some 3~p nuclei are located very close to Co 2+, thereby supporting the isomorphous replacement of Zn(II) by paramagnetic Co(II). The position of the central peak of the dried gel is at 4.3 ppm and is in good agreement with the value previously reported 9 for the zinc phosphate with hopeite structure. The central peak of the CoZnPO-HEX product (Figure 3b) is at 9.07 ppm, that is, close to the value for ZnPO-HEX (see below). The bottom spectrum (Figure 3b) corresponds to ZnPO-HEX. A single sharp peak located at 8.1 ppm indicates that phosphorus is only in one environment in the hexagonal lattice. This is in accordance with a regular tetrahedral coordination with no P - O - P links. 6 As expected, the relative intensity of the central band increases with decreasing Co 2+ content, as also shown in Figure 3b for CoZnPO products with x = 0.05 and x = 0.2. D.t.g. curves of the ZnPO-HEX and CoZnPO-HEX (x = 0.2) products are given in Figure 4a. It is evident that the samples exhibit different thermal behavior, The maxima for the product containing the Co(II) component are at lower temperature than those of ZnPO. This indicates that water evolution from the hexagonal network depends on cobalt incorporation into the zincophosphate framework. A t.g.a, profile of CoZnPO sample is given in Figure 4b. It has four major weight losses corresponding to a gradual departure of water molecules. The first loss (50-80°C) can be attributed to evolution of one water molecule, In the 80-160°C range two water molecules leave the framework, the next four molecules are lost between 160 and 260°C, and the last two molecules are gone when the temperature has reached approximately 300°C. It should be mentioned that the SOD phase (with practically the same empirical formula) exhibits quite different thermal behavior. Namely, the sodalite framework loses all its lattice water upon dehydration at approximately 140°C. Since the arrangement of the water molecules and sodium cations in a sodalite cage is in the form o f a cubane-like cluster, it

Table 3 Bond distances (A)

Table 2 Fractional coordinates and isotropic temperature factors U (A2)

Zn(1) Zn(2) P(1) P(2) O(1 ) 0(2) 0(3) 0(4) 0(5) 0(6) 0(7) 0(8) Na(1) Na(2) OW(9) OW(10) OW(11)

676

x/a

v/b

z/c

U

0.4931(8) -0.1574(7) 0.169(1) 0.591 (1) 0.047(3) 0,310(3) 0.580(3) - 0.248(4) 0.619(3) 0.197(3) 0.504(3) -0.199(3) 0.690(3) 0.813(5) 0.419(8) 0.092(7) 0.418(8)

0.5082(8) 0.1584(7) 0.411(1) 0.832(1) 0.245(3) 0.428(3) 0,696(3) - 0.051 (4) 0.493(3) 0.462(3) 0.381(3) 0.260(3) 0.352(3) 0.794(5) 0.288(9) 0.020(8) 0.114(8)

0.5493(8) 0.5487(9) 0.573(1) 0.526(1) 0.581 (2) 0.617(2) 0.485(2) 0,525(2) 0.641(2) 0.479(2) 0.461(2) 0.450(2) 0.545(3) 0.438(3) 0.789(5) 0.662(4) 0.68(1)

0.0145(9) 0.0107(8) 0.010(2) 0.010(2) 0.012(5) 0.021(6) 0.017(6) 0.039(8) 0.020(7) 0.017(6) 0.019(7) 0.015(6) 0.076(6) 0.19(1) 0.17(2) 0.15(2) 0.17(2)

Zeolites 15:672-678, 1995

Zn(1)-O(2) Zn(1)-0(3) Zn(1)-O(5) Zn(1)-O(7) Zn(2)-O(1) Zn(2) - 0(4) Zn(2)-O(8) Zn(2)-o(6) Na(1)-0(5) Na(1)-0(7) Na(1)-0(8) Na(1)-O(6) Na(1)-0(3) Na(1)-0W(9)

1.95(3) 1.96(3) 1.97(4) 1.92(4) 1.92(3) 1.94(4) 1.99(4) 2.01(5)

2.75(5)

P(1)-O(1) P(1)-0(2) P(1)-0(6) P(1)-0(7) P(2)-0(3) P(2) - 0(4) p(2)-0(5) P(2)-0(8) Na(2)- 0(3) Na(2)- 0(4) Na(2)- OW(10) Na(2)- OW(11) Na(2)-OW(10)

2.57(9)

Na(2)-0W(9)

2.43(5) 2.46(5) 2.34(5)

2.44(6)

1.56(2)

1.54(4) 1.49(3) 1.53(6) 1.50(4) 1.52(3) 1.52(5) 1.45(4) 2.24(6)

2.41(7) 2.10(8) 2.4(1) 2.00(9) 2.59(9)

could be concluded that in the hexagonal phase the water molecules are bound in a different manner. However, the powder diffraction pattern of a completely dehydrated sample (Figure 5) shows that thermal treatment causes the phase transformation of the hexagonal product to an unknown crystalline phase, the same one that appears upon a total water removal from ZnPO-SOD. i s No explanation can be offered presently for this intriguing behavior. SEMs of CoZnPO product are shown in Figure 6. CoZnPO has prism-like shaped crystals with a length of about 0.05-0.07 mm. The lower panel in Figure 6 shows a CoZnPO crystal at higher magnification. It is worth noticing that the reaction gel containing the cobalt component yields larger crystals than in the absence of Co(II). The suitable transparent single crystal was selected from the CoZnPO sample with value of x = 0.2 for the single crystal structure determination. The single crystal X-ray analysis conTable 4 Bond angles O(2)-Zn(1)-0(3) O(2)-Zn(1)-O(5) O(2)-Zn(1)-O(7)

0(1)-P(1)-0(2)

107(2)

O(1)- P(1)-O(6) O(1)-P(1)-O(7) O(2)- P(1)-O(6)

112(1) 109(2) 111(2)

0(3)-Zn(1)-0(7) O(5)-Zn(1)- O(7)

(°) 123(2) 99(1) 117(1) 116(1) 101(1) 99(2)

0(2)- P(1)-0(7) O(6)- P(1)-O(7)

107(2)

O(1)-Zn(2)-O(4) O(1)-Zn(2)-O(8) O(1)-Zn(2)-O(6) O(4)- Zn(2)- O(8) O(4)- Zn(2)- O(6) O(8)- Zn(2)- O(6)

112(1) 115(1) 109(2) 110(1) 111(1) 98(2)

O(3)-P(2)-O(4) O(3)-P(2)-O(5) O(3)- P(2)- O(8) O(4)- P(2)- O(5) O(4)- P(2)- O(8) O(5)- P(2)- O(8)

107(2) 109(2) 109(2) 114(2) 109(2) 109(3)

O(5)0(5) 0(5) O(7)O(7)0(8) O(5)O(5)O(7)O(7)0(8) O(8)O(9)O(9)O(3)-

74(2) 169(1) 105(2) 109(2) 146(2) 78(2) 93(2) 56(2) 82(3) 119(2) 77(2) 114(2) 67(3) 130(3) 85(2)

O(3)- Na(2)- O(10) 0(3) - Na(2) - O(11) 0(3) - Na(2) - 0(4) O(3)- Na(2)- O(10) O(10)- Na(2)- O(11) O(10) - Na(2) - 0(4) O(10)- Na(2)- O(10) O(11)- Na(2)- O(4) O(11)- Na(2)- O(10) O(4)- Na(2)- O(10) 0(3) - Na(2) - 0(9) O(4)- Na(2)- O(9) O(10)- Na(2)- O(9) O(9)- Na(2)- O(10) O(9)- Na(2)- O(11)

134(3) 80(4) 63(2) 136(6) 138(6) 82(4) 80(4) 140(5) 87(6) 109(5) 104(3) 141(4) 86(5) 105(6) 108(7)

O(3)-Zn(1)-O(5)

Na(1)-O(7) Na(1) - 0(8) Na(1 ) - 0(6) Na(1)- O(8) Na(1)-O(6) Na(1 ) - 0(6) Na(1)-O(9) Na(1)-O(3) Na(1)- O(9) Na(1)- O(3) Na(1)- 0(9) Na(1)-O(3) Na(1)-O(3) Na(1)-O(6) Na(1)-O(6)

110(2)

A novel open framework zincophosphate: N. Rajid et aL

firms the previous findings: CoZnPO-HEX possesses the composition presented by following formula: Na6[Co0.2Zn0.sPO4]6 • 9H20 and contains the Co(II) ions as a constitutive part of the zincophosphate lattice. Further, CoZnPO-HEX is an o p e n f r a m e w o r k s t r u c t u r e f e a t u r i n g onedimensional six-ring channels occupied by the sodium cations and water molecules. Final atomic positional and thermal parameters are presented in Table 2, with selected bond distances in Table 3 and bond angles in Table 4. The structure of this novel hexagonal phase is built up of ZnO4, COO4, and PO 4 tetrahedra, forming a three-dimensional network in which all vertices are shared. For the framework there are two zinc atoms, two phosphorus atoms, and eight oxygen atoms in the asymmetric unit, as shown in Figure 7. The isomorphous substitution of Co(II) mostly into one of two different Zn- sites has been established, since some trial refinement of Zn occupancy factors and a calculated difference electron density map revealed a small excess of electrons at that site in a ZnPO model structure. The (zinc/cobalt)/phosphorus ratio is unity, and the calculated Z n - O and P - O distances are consistent with alternation of the (Co0.2Zn0.8) and P atoms at tetrahedral sites. T h e average Z n - O and P - O bond lengths are 1.96(3) and 1.51(3) A, respectively, and are in good agreement with the structural data for other zincophosphates. 7 This mode of connectivity g e n e r a t e s an a n i o n i c f r a m e w o r k s t r u c t u r e [Co O2Zn08PO4] - where charge compensation is provided by the extraframework sodium cations. It deserves mentioning that this tetrahedral framework (electronically equivalent to the aluminosilicate one) is in contrast to some recently obtained zincophosphates, where a variety of groups (ZnO 4, ZnO3OH 2, PO4, POaOH) form some different zincophosphate structures.6,s Within the framework, the linkage of tetrahedra generates four-and six-member rings whose connection forms six-member ring channels spreading in the [001] direction. The sodium cations and water molecules are trapped in small cavities that are connected by six-member pore openings. Six of eight framework oxygen atoms form links with one of the two

05

Figure 8 The position of helix found in CoZnPO-HEX openings. Na atoms (large white circles) are bonded to water molecules (small white circles).

sodium cations, leading to the average bond angles 111(24) ° for Z n ( C o ) - O - N a and of 115(15) ° for P - O - Na. The coordination of Na(1) and Na(2) can be described as a distorted octahedron. T h e Na(1) is bonded to four framework oxygen atoms with an average distance of 2.42(5) A, one additional framework oxygen at distance of 2.75(5)/~, and one water molecule at 2.57(9) A. The Na(2) is surrounded by two framework oxygens and three water molecules at an average distance of 2.2(2) A, and an additional water molecule is at a slightly greater distance o f 2.59(9) A. The most interesting feature of this new structure is the presence of an infinite helix that is located in the six-ring channel, as illustrated in Figure 8. The helix is built up of sodium cations and water molecules. In particular, the Na(2) atoms and water molecules make a chain consisting of the NaO 4 units, where the Na atom of each unit is bonded additionally to two framework oxygen atoms (forming above mentioned octahedral coordination of Na(2)). At the same time, each Na(1) octahedron is a constituent of the helix through one of its vertices, performing an extra stabilization of the helix in the structure. In view of these structural results, the phase transformation of the hexagonal framework upon dehydration becomes more understandable. The presence of a helix, which means an arrangement of water molecules, indicates that the water molecules play an important structural role in this novel zincophosphate structure. At the same time, the formation of a helix could be a probable reason for the transformation of initially f o r m e d hopeite into the new hexagonal phase.

REFERENCES Figure 7 Framework asymmetric unit of the CoZnPO-HEX structure, showing the atom labeling scheme.

1 Chert, J., Jones, R.H., Natarajan, S., Hursthouse, M.B. and Thomas, J.M. Angew. Chem. Int. Ed. Engl. 1994, 33, 639 2 Shen, J.-F., Suib, S.L. and Young, C.-L. J. Am. Chem. Soc. 1994, 116, 11020

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