Synthesis and ab initio structure determination of AlPO4·H2OH4 from powder diffraction data

Microporous

Materials,

245

2 (1994) 245-250

Elsevier Science B.V., Amsterdam

Synthesis and ab initio structure determination of ilPOb.H20-H4 from powder diffraction data Damodara M. Poojarya, Kenneth J. Balkus b**, S. J. Rileyb, Bruce E. GnadeC and A. Clearfield”,* ‘Department of Chemistry, Texas A & M University, College Station, TX 77843-3255, USA “Department of Chemistry, University of Texas at Dallas, Richardson, TX 75083-0688, USA ‘Texas Instrumc?s, Inc., Central Research, Dallas, TX 75265, USA

(Received 7 August 1993; accepted 26 October 1993)

Abstract

The crystal and molecular structure of aluminophosphate, AlPO,-H4, was determined solely from X-ray powder diffraction data and refined by Rietveld methods. The compound crystallizes in the monoclinic space group CZ/c with a= 7.1374(2)A, b=7.0945(2) A, c= 14.7377(5)A, and j?=99.102(1)“. The final agreement factors are Rp=0.095, Rwp=0.135, and R,=0.03. The Rietveld refined formula is AlP04*H,0. There are two independent Al atoms. One is coordinated by two water molecules and four phosphate oxygens. The other is tetrahedrally coordinated by phosphate oxygens. All the phosphate oxygens are involved in bridging the Al atoms. In the synthesis procedure it was necessary to include an amine template to preclude impurities even though H4 is a condensed phase and not a molecular sieve. Keywords:

molecular sieves; AlPO,-H4;

crystal structure; X-ray powder data

Introduction The discovery of aluminum phosphate molecular sieves [l] generated a flurry of activity in molecular sieve synthesis. As new structural types began to emerge there was renewed interest in a family of aluminum phosphates referred to as the H series discovered by D’Yvoire two decades earlier [2]. This was in part due to the potential relationship between the large-pore molecular sieve VPI-5 and AlP04-Hl [3,4]. Structural characterization of the aluminum phosphates Hl through H4, which were generally all thought to be molecular sieves [5], is incomplete. The structure of AlP04-H3 was determined [6] followed by the structure of A1P04-H2 [7]. Here we complete the series with a structural characterization of AlPO,-H4. As will be seen H4 is not a molecular sieve but rather an interesting condensed aluminum phosphate phase having half * Corresponding authors. 0927-6513/94/$7.00 0 1994 - Elsevier Science B.V. All rights reserved. SSDIO927-6513(93)EOO59-P

of the aluminum atoms four coordinate other half six coordinate.

and the

Experimental AlPOd-H4 was prepared as follows. Fresh aluminum hydroxide was prepared by adding 150 ml of deionized water to 8.2 g of melted aluminum isopropoxide (Aldrich). The hydrolysate was separated and washed with water by centrifugation. The aluminum hydroxide was suspended in 50 ml of deionized water, and 13 ml of 4.4 M orthophosphoric acid were added. The mixture was stirred for 2 h, and 2.4 ml of tripropylamine (Kodak) were added, followed by another 45 min of stirring. An additional 75 ml of water were added for a total molar ratio A1203/PZ05/TPA/Hz0 of 1: 1.1: 0.7 : 55. The gel was heated in a Parr reactor at 140°C under static conditions for 26 h. The white crystals were isolated by filtration, washed

246

with deionized water and dried at 90°C overnight. An electron micrograph of the crystals is shown in Fig. 1. Step scanned X-ray powder data for the finely ground sample (packed into a flat aluminum sample holder) were collected by means of a Rigaku computer-automated diffractometer. The X-ray source was a rotating anode operating at 50 kV and 180 mA with a copper target and graphite monochromated radiation. Data were collected between 2 and 80” in 28 with a step of 0.02” and a count time of 15 s per step. Data were mathematically stripped of the Ka2 contribution and peak picking was conducted by a modification of the double-derivative method [8]. The powder pattern was indexed by Ito methods [9] on the basis of the first twenty observed lines. The best solution which indexed all the lines (figure of merit = 52) indicated a monoclinic unit cell with lattice parameters a=7.146 A, b=7.103 A, c= 14.754 A, and /I = 99.1”. Systematic absences indicated the space group to be either Cc or C2/c.

D.M. Poojary et al. / Microporous Mater. 2 (1994) 245-250

Structure solution and refinement

Thermogravimetric analysis showed that the sample loses 14.74 wt.% water between 50 and 150°C. 31P Nuclear magnetic resonance (NMR) [lo] indicated a single phosphorus site while “Al NMR showed the presence of both tetrahedral and octahedral sites. Hence we assumed that the composition of the compound is AlP04*Hz0. Calculations based on this composition indicated that the unit cell should contain eight molecules of AlP04*Hz0 and therefore we chose the centric space group, C2/c. Integrated intensities were extracted from the profile over the range 11.5 < 28 < 65.5 by decomposition (maximum likelihood estimation, MLE) methods as described earlier [ 111. This procedure produced 77 reflections of which 64 were nonoverlapping. Among the thirteen overlapping intensities twelve had two contributors while the other had three components. The intensities of these overlapping peaks were divided by the

Fig. I. Scanning electron micrograph of AlPOd-H4 crystals at 4000 x magnification.

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number of contributors and included in the data set for structure analysis in the TEXSAN [ 121 series of single-crystal programs. Direct methods (MITHRIL) [13] solution yielded the position of the P atom, one Al atom on the two-fold axis and three 0 atoms of the phosphate group. The positions of the other Al atom and the remaining oxygen atoms were obtained by difference Fourier maps computed after constrained refinement of the above positions against the same data set. This model was used for Rietveld refinement in the generalized structure analysis system (GSAS) [14]. Using a utility program GRAPH [15], the raw data were transferred to the GSAS program package for full-pattern refinement. The pairs of peaks arising from the a, and CQdoublet were treated as separate reflections in the fixed intensity ratio of 2: 1. In the early stages of refinement, the atomic positions were refined with soft constraints consisting of both Al-O and P-O bond distances and O-O non-bonded distances. All atoms were refined isotropically. In the final cycles of refinement the shifts in all the parameters were less than their estimated standard deviations. Neutral atomic scattering factors were used for all atoms. No corrections were made for anomalous dispersion, absorption or preferred orientation.

Results and discussion

Crystallographic and experimental parameters are given in Table 1, final positional and thermal parameters in Table 2, bond lengths and angles in Table 3, and the final Rietveld refinement difference plot is in Fig. 2. The bridging nature of the phosphate oxygens and the coordination surrounding Al atoms is presented in Fig. 3. Figs. 4 and 5 show the packing of the groups in the lattice along band a-axes, respectively. The structure consists of a tetrahedral phosphate group bridging octahedral and tetrahedral aluminum atoms (Fig. 3). Aluminum atom All lies on a two-fold axis and is tetrahedrally coordinated by pairs of phosphate oxygen atoms 01 and 02. Pairs of oxygen atoms 03, 04 and that of the water molecule complete an octahedral coordination about A12. The bridging of phosphate groups

TABLE 1 Crystallographic data for AlPO,-H4 Parameter

Value

Pattern range (20) (“) Step scan increment (20) (“) Step scan time (s) Radiation source Wavelength (A) Space group a (A) b (A) c (A) B (“) Number of contributing reflections Number of geometric observations P-O distances and tolerance (A) AlO distances and tolerance (A) AlO distances and tolerance (A) O-O distances for PO4 (A) O-O distances for AlO, (A) O-O distances for AIOs (A) Number of structural parameters Number of profile parameters Statistically expected Rwp*

11.5-80 0.02 15 Rotating anode 1.5406, 1.5444 cz/c 7.1374(2) 7.0945(2) 14.7377(5) 99.102(l) 448 23 1.53(l) 1.73(l) 1.85(l) 2.55(l) 2.86 2.66(l) 27 11 0.02 0.135 0.095 0.030

Rwp’

BP” RF.

“Rwp =(Z&, - I,)2/Z[wI, ’ 1) “’ Rp =(ZlI,-- IJ~I,); expected Rwp = Rwp/(~~)“~; R,= x2 = WI, - Q2/(N,b, - Nv.,);

Wol-IFd>/WoI>.

TABLE 2 Positional and thermal parameters for AlPO,-H4

All Al2 Pl 01 02 03 04 O(W)

x

Y

z

UiaoP (AZ)

0.5 0 0.6876(4) 0.8045(4) 0.5365(6) 0.8134(7) 0.5835(7) 0.1771(l)

0.8720(2) 0

0.75 0.5 0.6219(2) 0.7050(3) 0.6588(3) 0.5769(3) 0.5536(3) 0.5886(3)

0.020(2) 0.025(2) 0.019(l) 0.024(2) 0.013(2) 0.016(2) 0.013(2) 0.020(2)

0.1387(4) 0.2397(7) 0.0174(7) 0.0002(7) 0.2789(7) 0.1480(7)

wise = B,,,/87?.

thus leads to a three-dimensional network of the structure (Figs. 4 and 5). The structure consists of chains of aluminum atoms linked by phosphate groups. These chains run along the diagonal directions and the adjacent chains are linked by P-O-Al bonds. The metal atoms and the phos-

D.M. Poojary et al. / Microporous Mater. 2 (1994) 245-250

248 TABLE 3

[ lo,1 61 yielded phase impurities or low crystallinity samples. However, in the presence of tripropylamine we obtained single-phase H4 with well formed crystals (Fig. 1). The morphology of these crystals is completely different from the “rice-like” crystals obtained from the template-free synthesis [lo]. It is interesting to note that good crystalline powders of AlPOd-Hl were also obtained using an organic template [3,4]. This study shows once again that crystal structures of many inorganic compounds may be solved from diffractometer X-ray powder data by ab initio methods [11,17,18]. The general technique employed by us is to use only reflections which have a single index in a direct method or a Patterson technique to obtain the initial model. This ensures that only correctly indexed reflections with accurate intensities are used to obtain the model. Structures have been solved with as few as 30-60 reflections [11,17-191. Once the initial structure fragment has been obtained the model is completed and refined using all the data (fullpattern Rietveld methods). While synchrotron radiation yields more highly resolved and more symmetrical peaks, the final results are not much different from those obtained with a highresolution rotating anode data set as shown by our results with VPI-5 [20].

Bond lengths (A) and bond angles (“) for AlPO,-H4 All-01 All-02 A12-03 A12-04 A12-O(W)

1.725(4) 1.746(4) 1.881(5) 1.814(5) 1.971(5)

Ol-All-01 Ol-All-02 Ol-All-02 02-All-02 03-A12-03 03-Al2-04 03-Al2-04 03-A12-O(W) 03-A12-O(W) 04-Al2-04 04-A1220(W) 04-Al2-O(W) O(W)-A12-O(W)

114.1(5) 103.6(2) 114.1(2) 107.6(4) 180 92.7(2) 87.3(2) 87.6(2) 92.4(2) 180 92.3(2) 87.7(2) 180

2x 2x 2x 2x 2x

2x 2x

2x 2x 2x 2x

Pl-01 Pl-02 Pl-03 Pl-04

Ol-Pl-02 Ol-Pl-03 Ol-Pl-04 02-PI-03 02-PI -04 03-PI-04

1.544(5) 1.545(5) 1.549(5) 1.523(5)

107.4(3) 110.9(3) 111.5(4) 106.4(3) 107.q3) 112.8(3)

2x 2x

phate group display a regular geometry with expected values for the bond parameters. It is curious that the structure of AlP04-H4 has eluded researchers until now. This in part may be due to the quality of crystals obtained by nontemplate procedures. Our attempts to prepare AlPOd-H4 by reported template-free methods I

B

I

2.0 2-THETA,

DEG

I

I

I

I

I

I

3.0

4.0

5.0

I

I 6

I

I

I

0

7

I 0 XlOE

8. 1

Fig. 2. Observed (+) and cakulated (-) profiles (X-ray intensity versus 26) for the Rietveld refinement of AlPO,-H4. curve is the difference plot on the same intensity scale.

The bottom

D.M. Poojary et al. 1 Microporous

Fig. 3. Plot of the AlPO,-H4

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Mater. 2 (1994) 245-250

structure showing the numbering scheme and the bridging nature of the phosphate groups.

Fig. 4. Packing of the structure in the unit cell down the b-axis. Chains of Al and P groups along the diagonal direction are linked by oxygen atoms.

Conclusions

Acknowledgements

We have determined the structure of AlPO,-H4, completing the structural characterization of the AlPO,-H(l-4) series. AlPOh-H4 is not a molecular sieve which supports previous adsorption and NMR data [lo].

We thank the Robert A. Welch Foundation (A.C. and K.J.B.) and the donors of the Petroleum Research Fund administered by the American Chemical Society (K.J.B.).

D.M. Poojary et al. / Microporous Mater. 2 (1994) 245-250

6 J.J. Pluth and J.V. Smith, Nature, 318 (1985) 165. 7 H.X. Li, M.E. Davis, J.B. Higgins and R.M. Dessau, Chem. Commun., 4 (1993) 403. 8 J.W. Visser, J. Appl. Crystallogr., 2 (1969) 89. 9 C.L. Mellory and R.L. Snyder, Ado. X-Ray Anal., 23 (1979) 121. 10 B. Duncan, M. Stocker, D. Gwinup, R. Szostak and K. Vinje, Bull. Sot. Chim. Fr., 129 (1992) 98. 11 P.R. Rudolf and A. Clearfield, Inorg. Chem., 28 (1989) 1706. 12 TEXSAN, Structure Analysis Package, Molecular Structure Corp., The Woodlands, TX, revised edition, 1987. 13 G.J. Gilmore, MITHRIL, A Computer Program for the Automatic Solution of Crystal Structures from X-Ray Data,

Fig. 5. Stereo view of the structure down the u-axis.

References 1 ST. Wilson, B.M. Lok and E.M. Flanigen, U.S. Pat., 4310440 (1982). 2 F. D’Yvoire, Bull. Sot. Chim. Fr., (1961) 1762. 3 A. Cleafield and J.O. Perez, in M.L. Occelli and H.E. Robson (Eds.), Molecular Sieues, Van Nostrand Reinhold, New York, NY, 1992, p. 266. 4 J.O. Perez, N.K. McGuire and A. Clearfield, Catal. Lett., 8 (1991) 145. 5 See, for example, R. Szostak, Handbook of Molecular Sieves, Van Nostrand Reinhold, New York, NY, 1992.

University of Glasgow, Glasgow, 1983. 14 A. Larson and R.B. von Dreele, GSAS, Generalized Structure Analysis System, LANSCE, Los Alamos National Laboratory, Los Alamos, CA, copyright 1985-1988 by the Regents of the University of California. 15 P.R. Rudolf and A. Clearfield, Acta Crystallogr., B41 (1985) 418. 16 R. Szostak, B. Duncan, R. Auello, A. Nastro and K. Vinje, in M.L. Occelli and H.E. Robson (Eds.), Molecular Sieves,

Van Nostrand Reinhold, New York, NY, 1992, p. 240. 17 P.R. Rudolf, C. Saldarriaga-Molina and A. Clearfield, J. Phys. Chem., 90 (1986) 6122. 18 D.M. Poojary, J.O. Perez and A. Clearfield, J. Phys. Chem., 96 (1992) 7709. 19 D.M. Poojary, H.-L. Hu, F.L. Campbell, III and A. Clearfield, Acta Crystallogr. Sect. B, in press. 20 D.M. Poojary and A. Clearfield, Zeolites, 13 (1993) 542.