7709
J. Phys. Chem. 1992, 96,7709-7714
Crystal Structures of Dehydrated VPI-5 and H1 Alumlnum Phosphates from X-ray Powder Data Damodara M. Poojary, Jaime 0. Perez: and Abraham Clearfield* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: December 13, 1991; I n Final Form: May 21, 1992) The crystal structures of the dehydrated phases of the 18-ring aluminum phosphates VPI-5 and H1 have been determined from X-ray powder data. Both phases crystallize in the hexagonal space group P63cmwith a = 18.6005 (6) A, c 8.3664 (4) A for VPI-5 and a = 18.6576 (7) A, c = 8.3284 (5) A for H1. In the latter phase, the atoms in the six-membered ring (6MR) were found to be disordered, requiring two partially occupied structures to define the atomic positions. One structure is the regular VPI-5 framework with all six-membered rings parallel to the c-axis direction. The other requires a rotation of the 6MR phosphate group to produce rows of 4MR, 6MR, and lOMR replacing the rows of all 6MR. The thermal instability of H1 is attributed to this disorder, which in turn is thought to arise from the low-phosphate content of HI.
Introduction The 18-ring aluminum phosphate VPI-513zis an important member of the aluminum phosphate family of porous compounds3 because of the large size of the micropore. In recent publications,4J we have presented evidence to show that there are actually two types of 18-ring aluminum phosphates. One phase, which has a ratio of A1 to P of very close to 1 and is thermally stable, is hereafter referred to as VPI-5. The second phase always exhibits a ratio of Al/P > 1 and is thermally unstable, converting to A1p04-8at temperatures in the range of 100-300 OC. This phase has been designated as H1 in conformity with the aluminum phosphate prepared earlier by d'Yoirea6 D'Yoire's synthesis resulted in a mixture of H1 with other aluminum phosphate phases. On heating, this mixture converted to the dense phase aluminum phosphate tridymite. However, Duncan et al.' prepared pure H1 from an amine-free, clear solution and showed that it converts to AlP04-8 on heating. In this paper, we report the results of an X-ray diffraction study on the anhydrous forms of VPI-5 and H1. This study was carried out in the hope of discovering a structural basis for the difference in properties of the two preparations. The framework structure of VPI-5 was originally deduced from a combination of X-ray powder data and structural modelingn2V8The proposed model is consistent with the theoretical net 81(1) as proposed by Smith and D y t r y ~ h .Richardson ~ et a1.I0 have refined the structure of the dehydrated form of VPI-5,which they termed AlP04-54,using timwf-flight neutron data. Although, as we shall see,the correct framework symmetry of VPI-5 is P63cm, these authors chose the higher symmetry space group fi3/mcm, and therefore, they could not distinguish between A1 and P in the structure. The structure of hydrated VPI-5 was refined separately by Rudolf and Crowder" and more recently by McCusker et a1.12 Rudolf and Crowder extensively explored the network of hydrogen-bonded water molecules in the pore, but their model did not include octahedral A1 atoms, although NMR studies have shown the existence of such species.I3 Because of this incomplete geometrical model, they could not achieve satisfactory refinement. Our own earlier study on hydrated H1, based on synchrotron powder data, on the other hand, revealed that the aluminum atoms entirely on the four-membered ring are six-coordinated. Despite this finding, we could not obtain a complete refinement for reasons which will become clear after the presentation of our new results reported here. The most complete structure solution and refinement to date was carried out on hydrated VPI-5 in space group P63 using synchrotron X-ray powder data.lz The relationship of our study to this structure will be deferred to the Discussion section. ExperimentalSection
Synthesis. The synthetic procedures for the preparation of VPI-5 and H1 have been given in detail previo~sly.~*~ Briefly
'Present address:
Universidad de Antioqua, Medellin,
Colombia.
TABLE I: Crystallographic Data for 18-Ring Aluminum Phosphates" VPI-5 H1 pattern range (2rP), deg 14-80 14-80 step scan increment (29), deg 0.01 0.01 1.o 1.o step scan time, s P63cm P6,cm space group 0,
c,
A A
no. of contributing reflns no. of geometric obsns P-O distances and tolerance, A A 1 4 distances and tolerance, A 0-0 distances to define P coord, A 0-0 distances to define AI coord, A no. of structural parameters no. of profile parameters
18.6005 (6)
8.3664 (4) 640 34
18.6576 (7) 8.3284 ( 5 ) 634 55
1.53 (2)
1.53 (2)
1.73 (2)
1.73 (2) 2.55 (2) 2.86 (2) 58 11
2.55 (2) 2.86 (2) 39 11
statistically expected R,, RF
0.043 0.11
0.04
RWP
0.119 0.09 1
0.117
RP
0.115 0.086
Rp = (Ello- &l/E&), RF = 'RWP ( c W ( l o - ~c)2/E[w~~])'/27 ((IFobl- lF~d))/(lFo~l), statistically expected R , = R , , / ( x ~ ) ~ x2 / * ,=
Ew(l0- U 2 / ( N o b- NVar).
stated, VPI-5was prepared from an aluminum phosphate gel and dipentylaminetemplate at 150 OC in 18-22.5 h. In contrast, the H1 product was prepared at 130 OC with dipropylamine or a combination of tributylamine and dipentylamine as the templating agents. Analytical data and X-ray powder patterns of these products have been published? In the case of the current samples, the analytical data were as follows. For VPI-5 All 18.8%; P, 21.11%; Al/P = 1.02, DPTA, 13.86% (TGA); H20, 4.52% (TGA). Calculated for A1P04-0.13DPTA-0.37H20. Al, 18.12%, P, 20.80%; DPTA, 13.64% H20, 4.47%. For DPA-prepared H1 Al, 17.85%; P, 17.66%; Al/P = 1.16; HzO, 24.6% (TGA). Calculated for A l I , 1 ~ 4 ( O H ) o . 1 6 ~ 2 . 3 4Al, H 218.30% 0 P 18.11% H20, 26.1%. The VPI-5 sample was dehydrated and deaminated by heating at 650 OC for 15 h. It was then allowed to cool in a desiccator and transferred to an air-tight sample holder in a drybox. The H1 sample could not be treated similarly because of its ready conversion to AlP04-8. Instead, it was kept at 50 OC under a continuously pumped vacuum (p = lo4 Torr) for 24 h. This procedure was showns to remove all but traces of water without conversion to AlP04-8. Data Collection. Step-scanned X-ray powder data were collected on the finely ground dehydrated samples of VPI-5 and H1 by means of a Rigaku RU 200 computer-automated powder diffractometer. The X-ray source was a rotating anode operating at 50 kV and 180 mA with a copper target and graphite-monochromated radiation (A = 1.541 84 A). Data were collected between 4 and 80 OC in 28 with a step size of 0.01 and a count time of 1 s/step. The data were transferred to a DEC MicroVAX-I1 computer, and the pattern was decomposed as described
0022-3654/92/2096-7109%03.00/0 0 1992 American Chemical Society
7710 The Journal of Physical Chemistry, Vol. 96, No. 19, 1992
earlier.14 Indexing was carried out using trial-and-error methods and refined by least-squares procedures. Details are given in Table 1
I.
Structure Solution and Refinement VPI-5. After unit-cell least-squares refinement, peab that could not be decomposed into their individual reflections were rejected, and the remaining starting set was used to generate the normal F,(hkl) and a(F,) values for the structure determination. Structure solution was attempted using 41 unambiguously indexed peaks in the angular range 10-61' in 28. The noncentric space group P6$m, which allows for the alteration of Al and P, was chosen in conformity with the observed systematic absences. Direct method solutions in the T E X S A N ~series ~ of programs revealed the positions of all the P and A1 atoms along with two oxygen atoms. Normal Fourier methods were applied to locate the remaining oxygen atoms. A limited refinement was conducted on these atoms with heavy damping and selective refinement of a few parameters per cycle. These refined positions produced a framework structure which agreed with the one proposed by Crowder et a1.8 with reasonable bond parameters about P and Al. The same structure was also determined independently by model-based Patterson methods using the same data set.8 Calculated atomic positions for VPI-S8were used for rotation and translation search in the present unit cell using the program PATSEE.16 Except for two oxygen atoms, the positions of all other atoms were very close to those determined by the above-described methods. Using a utility program GRAPH," the raw data were transferred to the GSAS'* program package for the Rietveld method of full-profile refinement. The pairs of peaks arising from Kal and Kaz doublets were treated as separate reflections in the f l e d intensity ratio of 2:l. The refinement was started with the profile coefficients obtained from the refinement of lanthanum boride powder data which was collected on the same instrument. Lanthanum boride is often chosen as a standard pattern for this purpose. While these profile coefficients duplicated the bulk of the X-ray profile very well, they were not satisfactory to describe the shape of the intense 100 and 200 reflections. GSAS allows the use of only one set of coefficients to describe the entire pattern. Therefore, these two low-angle reflections were not included in the refinement. However, their integrated intensities were determined separately so that values of IF,I - lFcl could be obtained for these reflections as a measure of correctness of the structure. Initial refinement of the scale, background, unit cell parameters and the zero-point error led to good agreement between the o b served and calculated patterns, indicating that the starting structural model was correct. Careful refinement of the profile function (number of coefficients = 11) and increasing the background coefficients to 12 showed a considerable improvement in the calculated pattern (R,,,, = 0.15). Atomic positions were then refined with soft constraints consisting of both A1-0 and P-O bond distances and 0-0 nonbonded distances. A1-0 and P-O distances were constrained to 1.73 and 1.53 A, respectively, with allowable refinement bounds of 0.2 A. The tetrahedral geometry around A1 and P was obtained by the appropriate distances between the bonded oxygen atoms. A total of 34 geometric constraints were included, and initially their weight was kept high to maintain a reasonable framework geometry. As the refinement progressed, the soft constraints were assigned less and less weight, but they could not be removed completely without reducing the stability of the refinement. This is the usual procedure for large structures with small data sets. The refinement of the atomic positions converged with R,, = 12.0. In the last stages of refinement, the atomic isotopic thermal parameters were refined. In the final refinement, the shifts in all the parameters were less than their estimated standard deviations. A difference Fourier computed at this stage was featureless. Neutral atomic scattering factors were used for all atoms. No corrections were made for anomalous dispersion, absorption, or preferred orientation. H-1. The raw data set was transferred to the GSAS program package for Rietveld full-pattern refinement. The refinement was started with the atomic positions and profile coeficients obtained
Poajary et al. from the refinement of the dehydrated VPI-5 structure. As in the case of VPI-5, the refinement was carried out using the data in the 28 range 14-80' for the same reasons given for VPI-5. The peak shape for H1 (fwhm = 0.15) was very similar to that of VPI-5. The powder patterns of these two samples exhibit significant differences in reflection intensities, with the largest differences being observable in the 21-23O 219 region. Initial refinement of the scale, background, and unit cell parameters showed an overall agreement between the observed and the calculated patterns, and as expected, the difference is large in the 20 range mentioned above. Refinement of the atomic positions with the soft constraints resulted in a distorted geometry for the group in all the six-membered rings. Additionally, the thermal parameters of the associated atoms were unacceptably high. Difference Fourier maps were therefore computed by removing one atom of these group at a time. These maps have shown that both the PO4and the A104groups in the six-membered rings are disordered. There were three electron density peaks for the oxygen atoms which link the P and A1 atoms of the six-membered ring, while the oxygens connecting the P atom to the four-membered ring showed only two separate positions. Interestingly,there were two peaks at a separation of about 2 A between the unbridged P(2) and Al(2) atoms along the c axis; one of them was at a bonding distance from P(2), while the other may be considered as an alternative position of the six-membered ring Al(2) atom. Among these disordered positions, one set of peaks corresponds to the positions of the respective atoms in the VPI-5 structure. Although there are other possibilities, the alternative model described below seems to be chemically feasible and explains most of the density peaks in the Fourier map. This proposed structure is derived from the regular VPI-5 structure by breaking two of the Al-0-P bonds at the A M connection (one in the plane viz. A1(2)-0(7)-P(2), and the other connecting the layers along the c axis viz. A1(2)-0(5)-P(2)) followed by a rotation of the phosphate groups. The rotation is such that the oxygen atom which was originally bonded to the Al atom in the adjacent layer now forms a new bond with the A1 atom in the plane of the six-membered ring. At the same time, the oxygen atom in the plane of the ring is moved to a new position where it can form a weak link with the disordered A1 atom in the adjacent layers in the opposite direction. The disordered positions of the other two oxygen atoms along with the two described above complete an alternative tetrahedral geometry around the P atom. The second position of the Al atom is also in a tetrahedral environment but with slightly longer bond distances. Based on the peak heights, the former positions were assigned with an occupancy of 0.65 and the latter with 0.35. This structural model was refined with a total of 55 geometric constraints. As in the case of the VPI-5 structure, the Al-0 and P-O distances were constrained to 1.73 and 1.53 A, respectively, with allowable refinement bounds of 0.2 A. The refinement converges with R,, = 0.117. Difference Fourier m a p computed at this stage showed some minor peaks in the region of the disordered atoms, but they could not be refined. However, a small peak near Al( 1) in the four-membered ring did refine to A 1 4 z 2.2 A. This oxygen undoubtedly belongs to a partial water molecule arising from the incomplete dehydration of H1 at 50 OC. Neutral atomic scattering factors were used for all atoms, and no corrections were made for anomalous dispersion, absorption, or preferred orientation. Results VPI-5. Crystallographic and experimental parameters are given in Table I, final positional and thermal parameters in Table 11, and bond lengths and angles in Table IV. The final Rietveld refinement difference plot (Figure 1) shows that all the essential features of the powder pattern have been reproduced. Since the data were collected on a calcined and completely dehydrated sample, the structure consists of only the framework atoms as shown in Figure 2. The formula in the asymmetric unit is therefore (AlP04)1.5, while that of the unit cell is (AlP04)18.The unit cell parameters obtained from the Rietveld refinement of the full pattern show that the a and b dimensions of the unit cell are
The Journal of Physical Chemistry, Vol. 96, No. 19, 1992 7711
Dehydrated VPI-5 and H1 Aluminum Phosphates
TABLE I V Average Bod Length (A) and Angles (deg)
TABLE Ik Pmitional and Thcrmrl Parameters for VPI-5 x ~~~~
z
Y
uh! A2
~
0.132 (9) 0.4177 (9) 0.190" Al(1) 0 Al(2) 0.8262 (6) 0.4891 (6) 0.175 (2) 0.008 (4) 0.5734 (6) 0.314 (2) 0.012 (4) P(l) 0 P(2) 0.8324 ( 5 ) 0.3332 ( 5 ) 0.296 (2) 0.017 (3) 0 0.4959 (9) 0.287 (3) 0.016 (5) O(1) 0.5781 (8) 0.493 (2) 0.134 (8) O(2) 0 O(3) 0.9075 ( 5 ) 0.3255 ( 5 ) 0.240 (3) 0.024 (7) O(4) 0.9242 (6) 0.5738 (7) 0.233 (3) 0.048 (9) O(5) 0.8250 (8) 0.3364 (7) 0.470 (3) 0.083 (4) O(6) 0.8156 (8) 0.3933 (8) 0.213 (3) 0.096 (8) 0.7565 ( 5 ) 0.2480 ( 5 ) 0.257 (3) 0.015 ( 5 ) O(7) " Held fixed in least-squarcs refinement. U, Bim/'8112.
P-0 A1-0 GP-0 GA1-0 PUAl P( 11-0 P(2)-0 Al( 1)-0 AK1)-0(W) A1(2)-0 A1(2A)-0 o-P(1)-0 O-P(2)-0 0-Al( 1)-0
TABLE IIk Pmitionrl and Thermal Parameters for H1 x Y z u,, A1 0.0526 (6) 0 0.4108 (10) 0.157" Al(1)
Al(2) P(1) P(2) O(1) O(2) O(3) O(4)
0.8276 (8) 0
0.8291 (8) 0 0
0.8971 (7) 0.9233 (3) 0.8325 (10) 0.801 (2) 0.7510 ( 5 )
O(5)
0.4894 (8) 0.134 (3) 0.014 (4) 0.5677 (8) 0.289 (2) 0.018 (4) 0.3340 (7) 0.258 (3) 0.027 (4) 0.4883 (10) 0.259 (4) 0.047 (9) 0.5894 (8) 0.460 (2) 0.027 (7) 0.3353 (7) 0.153 (3) 0.021 (6) 0.5534 (9) 0.191 (3) 0.093 (7) 0.3259 (9) 0.432 (3) 0.009 (4) 0.3889 (13) 0.166 ( 5 ) 0.10 (1) 0.2425 (6) 0.242 (4) 0.008 (8) 0.376 (2) 0.409 (3) 0.09 (3) 0.4917 (15) 0.346 (3) 0.14 (2) 0.348 (3) 0.336 (4) 0.02 (2) 0.3835 (12) 0.368 (4) 0.02 (1) 0.349 (2) 0.059 (3) 0.12 (4)
O-A1(2)-0 GA1(2A)-0 P-O-A1
av value VPI-5 1.52 1.72 109 110 154 H1 1.52 1.56 1.69 2.2 1.71 1.84 109 109 95 156 109 110 141
min/max 1.46 (2)-1.57 (1) 1.65 (2)-1.78 (1) 101 (1)-118 (1) 101 (1)-117 (1) 118 (1)-179 (1) 1.48 (1)-1.55 (1) 1.46 (1)-1-68 (1) 1.65 (1)-1.72 (1) 1.65(1)-1.76 (1) 1.78 (2)-1.95 (2) 102 (1)-115 (1) 92 (1)-127 (2) 77 (1)-120 (20) 150 (2)-162 (2) 102 (1)-116 (1) 85 (2)-127 (2) 101 (1)-167 (1)
O(6) O(7) O(W 0 A1(2A)* 0.8349 (14) 0(3A)b 0.9066 (14) 0(6A)b 0.8007 (14) 0(5A)b 0.843 (2) " Held fixed at least-squares refinement. A represents the alternative position of the distorted atom. I
;:I _ -
: : i
z
I
I
I
TI
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