J. Phys. Chem. 1992, 96, 3092-3096
3092
'"A
P(t) = -W.P(t)
(AI)
In this equation P = ( P I , P Z...,,PN)is the N-dimensional vector of the adsorbate occupation probability P,(t), P is the vector of the time derivative of Pn(t),and W is the trznsition matrix whose elements have the following form: wnm
=
(cwn-k + W n l 6 n m - wm-n k
('42)
Formally we can write the solution of equation (A2) as P(t) = e-wfP(0)
(A3)
which can be transformed into a solution of the following form: TIME (lO-'sec Figure 7. Total adsorbate population as a function of time for several
intensity prefactors. the system reaches the steady state at approximately the same time (of the order of 10 ps), no matter what initial state was used in our calculations; in other words, the system loses memory of the initial state on a picosecond time scale. Our results also show that a first-order kinetic behavior (mean first passage time approximation) can be assumed safely as long as the surface temperature is kept below 300 K; for higher temperatures more complicated kinetics must be used (multiple exponent behavior). Finally, we have presented a dynamical analysis of the desorption process induced by a phonon exitation source dramatically different from the thermal one. We have used a time-independent nonequilibrium phonon population to simulate an acoustically prepared perturbation. Our calculations have revealed an increase in the desorption rate constant due to the perturbation in the phonon population. Acknowledgment. We are grateful to Horia Metiu for very valuable discussions and advices. R.A. would like to thank the CDCHT (Grant C-479-90) of La Universidad de Los Andes for its financial support. We thank the reviewers for theirs useful comments.
Appendix A In what follows, we present how we obtained the expression for the time evolution of the population of a state n, P,(t), used in section 3.3 (eq 3.2). We start by rewriting the master equation (2.7) in matrix form as
P(t) = Te-*TtP(0)
(A4)
here we have used the matrix T that diagonalizes W. The diagonal matrix A of eigenvalues of W is represented as
A = TtWT (A51 The probability of finding the oscillator in the state n is thus given by PAt) = CCTnke-X%pm(0) k m
('46)
which can be rewritten as P,(t) = CPnke-*tf k
('47)
Appendix B The desorption rate constants presented in this article have been obtained by computing the mean first time passage, 7,of the adsorbed particles from the bound to the continuum Morse states. This time is given by6
Substitution of eq A3 for P,(t) and integration by parts yields
k;' = CCWnm-'Pm(0) m n
(B2)
which is the expression used in the computation of the rate constants. Registry No. Ar, 7440-37-1; W, 7440-33-7.
Modeling and MASNMR Spectroscopic Studies on Molecular Sieves. 1. The Nature of Organic Template Molecules in As-Synthesized AIP04-11 Host Latticet S.Prasad and R. Vetrivel* National Chemical Laboratory, Pune 41 1 008, India (Received: June 26, 1991)
We predict the location and conformation of the secondary amine template molecules inside the pores of A1P04-I1 molecular sieve. The results obtained by molecular modeling and the semiempirical quantum chemical calculations are used as tools for this purpose. I3C magic angle spin (MAS) NMR studies on as-synthesized and oven-dried AIP04-I 1 samples are also presented to illustrate the nature of the occluded species. The geometric and electronic requirements of the amines to be a successful structure-directing species for the synthesis of AIP04-11 are reported.
Introduction The studies on the nature of occluded template molecules in daoporous material improve our understanding of the role played NCL communication No. 5103.
by the former in the synthesis of the latter. Aluminophosphates with micropores in the range of 3-8 8, were reported by Wilson et There are protons attached to the bridging oxygen atoms, ( 1 ) Wilson, S . T.; Lok, B. M.; Messina, C. A.; Cannan, T.R.; Flanigen, E. M . J . Am. Chem. SOC.1982, 104, 1146.
0022-365419212096-3092%03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3093
Modeling and MASNMR Studies on Molecular Sieves which are present to compensate for the negative charge of the framework due to substitution of P or A1 by other elements, and there are also hydroxyl groups present due to nonstoichiometry and defects. Thus, the aluminophosphates are a new generation of acidic molecular sieve materials. AlP04-11, a member of the aluminophosphate series, is found to have a novel structure with a one-dimensional elliptical pore ~ y s t e m . ~The synthesis of AlPO.,-ll has been achieved using different secondary amines as the organic template molecules.s In this paper, we present the results of our molecular modeling and quantum chemical calculations to study the nature of the occluded template molecules and to interpret the relation between the pore architecture of AlP04-11 and the structural as well as electronic properties of template molecules. The results of I3C MASNMR studies are presented which provide information regarding the dynamic nature of the amines inside the pores of AlP04-11.
Methods and Experimental Section Standard force fields are used in the energy minimization calculations.6 All geometrical degrees of freedom such as bond length, bond angle, and dihedral angles are varied. In this process all atoms are moved and the molecule reaches a final geometry, where the molecular potential energy is minimum. From these calculations we obtain the dimensions of the amines at their minimum energy conformations. We also calculate the strain energy involved in the conformational changes from their equilibrium geometry. Semiempirical quantum chemical calculations using the MNDO (modified neglect of differential overlap) technique’ was used to study the electronic structure of the amines, cluster models of AlP04-11 framework, and the ammonia adsorption complexes over the oxygen sites in AlP04-11. The values of the electronic and total energies are used to decide the favorable adsorption sites; the analysis of molecular orbital energy as well as the contributions of various atomic orbitals to molecular orbitals and the electron distribution and partial charges calculated are also useful to determine the nature of interaction between amines and AlPO,-1 1 framework. The chemicals used in the synthesis and the synthesis procedure of AlP04-11 materials are described elsewhere.8 13Ccross polarization (CP) MASNMR spectra were recorded with a Bruker MSL 300 spectrometer (7 T magnetic field, 13Cfrequency of 75.47 MHz). A contact time of 1 ms, a pulse length of 5 ps, and a spinning speed of 3 kHz were employed. The chemical shifts were measured relative to the I3C signal of -CHgroup in adamantane. Results and Discussion Geometrical Properties of Amines. The equilibrium geometry of the amine molecules were obtained by the molecular force field method described by Gelin and Karplus9 using an integrated molecular modeling package.6 The dimensions calculated for various secondary amines in their equilibrium conformation are given in Table I. It was found by different authorsl*JOthat mostly secondary amines act as template molecules in the synthesis of AlP04-l 1. Tapp et a1.I0 predicted that only the secondary amines with lengths comparable to the unit cell dimension along the c axis (Le. 8.44 A) lead to successful synthesis of AlP04-11. However, our molecular modeling studies and other experimental studies8 show that even longer amine molecules could be a suc~
~
~
(2) Wilson, S. T.; Lok, B. M.; Flanigen, E. M. US.Patent 4310440, 1982.
(3) Wilson, S.T.; Lok, E. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. A C S S y m p . Ser. 1982, 218, 79. (4) Bennet, J. M.; Richardson, J. W., Jr.; Pluth, J. J.; Smith, J. V. Zeolites 1987, 7, 160. ( 5 ) Lok, E. M.; Cannan, T. R.; Messina, C. A. Zeolites 1983, 3, 282. (6) Molecular Modeling PackageQuanta and charmM-Release 2.1;
Polygen Corporation: Waltham, MA, 1990. (7) Dewar, M. J. S.;Thiel, W. J . A m . Chem. SOC.1977, 99, 4899. (8) Balakrishnan, I.; Prasad, S. Appl. Caral. 1990, 62, L7. (9) Gelin, B. R.; Karplus, M. Proc. Natl. Acad. Sci. U.S.A.1975,72,2002. (10) Tapp, N. J.; Milestone, N. B.; Bibby, D. M. Zeolites 1988, 8, 183.
TABLE 1: Dimensionso of the Pores in AIP04-ll as Well as of Certain Tvoical Amine Molecules and Their Electronic Properties charge charge density density on N on H, channel/molecule dimension. A lormembered channel 6.40 X 3.90 6.40 X 4.90 6-membered channel 3.40 X 2.15 4-membered channel 1.40 X 0.90 -0.24 0.08 NH, 4.25 X 2.75 X 2.25 -0.29 0.15 Me2NH -0.29 0.15 Et2NH 6.15 X 2.75 X 2.25 n-Pr2NH 9.25 X 2.75 X 2.25 -0.29 0.15 -0.29 0.11 i-Pr2NH 6.75 X 4.25 X 3.25 0.15 wBu~NH 11.75 X 2.75 X 2.25 -0.30 -0.29 0.11 i-Bu2NH 9.25 X 4.25 X 3.25 0.15 n-Pe2NH 14.25 X 2.75 X 2.25 -0.29 0.1 1 i-Pe2NH 11.75 X 4.25 X 3.25 -0.29 -0.31 0.11 i-Bu, n-PrNH 9.25 X 4.25 X 3.25 i-Pe, n-PrNH 10.50 X 4.25 X 3.25 -0.31 0.11 -0.30 0.10 n-HexNH2 8.25 X 4.75 X 4.00 -0.36 Et,N 6.00 X 6.00 X 6.00 ~~
For the amine molecules length, width and height are given on the basis of their van der Waals radii.
cessful template due to their conformational flexibility. From the molecular modeling methods, we are able to derive a general relation (1) to determine the dimension of amine molecules based on their van der Waals radii. For a typical amine molecule, RNHR’, where R # R’, the length LRNHRt could be derived as LRNHR~ = L R N H R /+~ L R W H R , / ~
(1)
where LRNHR and LRtNHR, are reported in Table I for different alkyl groups varying from methyl to pentyl. In a secondary amine molecule of the type RNHR’, where R and R’ are alkyl groups, the substitution of an n-alkyl group by an isoalkyl group reduces the length of the molecule by 1.25 A. The width and height of both normal and isoalkyl groups are well within the limits of the pore dimensions of the 10-member channel in AlP04-11 (Table I). There is a rotational degree of freedom for the amine molecules along the N-C bonds. We carried out conformational analysis on various secondary amine molecules given in Table I. For the equilibrium conformation, rotation along the N-C bond was tried. For a rotation of f30° along the N - C bond, the overall increase in energy did not exceed 5.0 kcal/mol for any of these secondary amine molecules. Within this flexible range of i3Oo, the decrease in the length of the amine molecules is ~ 1 . A. 5 It has been reported” that there is approximately one template molecule present in each channel unit (8.44 A along the straight channel) in the as-synthesized molecular sieve. Our simulation studies have shown that even secondary amine molecules which are longer than 8.44 A can fit into a channel unit of AlP04-I 1 due to conformational flexibility. Electronic Properties of Amine Molecules. Semiempirical MNDO calculations were carried out on different amine molecules in order to study the electronic factors involved in the specificity of secondary amines as structure-directing species. The net electron charge density on nitrogen as well as hydrogen attached to nitrogen (HN)of the amine molecules are given in Table I. The electron charge density on nitrogen increases in the order primary E secondary < tertiary amines. We notice that the absence of hydrogen attached to nitrogen in the case of a tertiary amine and almost zero charge on the hydrogens of methylene and methyl groups of tertiary and primary amines are partial reasons for their failure to act as templating agents for the synthesis of the AlP0411 framework. The net charge on nitrogen in secondary amines is found to be ==-0.30,independent of the dimension and nature of the alkyl (1 1) Tapp, N. J.; Milestone, N. B. Stud. SurJ Sci. Coral. 1988, 36, 639.
Prasad and Vetrivel
3094 The Journal of Physical Chemistry, Vol. 96, No, 7, 1992
TABLE 11: Results of MNDO Calculations on the Cluster Models of the AIP04-11 Lattice and the Ammonia Adsorption Complexes tot. charge tot. energy T-0-T energy density on for ammonia adsorption charge energy of density on the bridging adsorption angles, of the cluster ammonia, eV ammonia oxygen complex, eV dea cluster. eV no. model -2767.16 -0.63 2.29 -0.04 -2519.21 1 P1-01-AI1 167 -2768.69 -0.02 -0.50 1.41 -2519.86 131 2 P1-04-All -2768.44 -0.05 -0.61 1.65 -2519.85 3 140 P1-05-AI2 -2766.24 -0.06 1.75 -0.63 -2517.75 4 152 P 1-06-Al2 -2768.3 6 0.00 -0.66 2.95 -252 1.07 174 5 P2-02-AI2 -0.02 -2772.22 -0.62 2.17 -2524.15 6 P2-07'-A13 168 -0.02 -2761.78 -0.62 1.60 7 -2519.14 P2-05'-All 147 -2768.18 -0.64 -0.04 1.65 -2519.59 8 P2-06'-A11 140 -0.66 -0.05 -2766.03 2.99 -2518.78 9 P3-03-AI3 172 -0.68 -0.05 -2765.21 2.32 -2517.29 10 P3-07-AI2 166 -0.69 -2765.24 -0.04 3.38 -25 18.38 11 P3-08-Al3 175
H
H
(b)
Figure 2. (a) A typical representative dimeric cluster model showing the bridging oxygen site, 0 4 . (b) The geometry of the dimeric cluster-ammonia adsorption complex.
Figure 1. A repeating unit cell of the ordered AIP04-l 1 structure. The inset is the schematic representation of the AIP04-11 lattice which is formed by several such repeating units and their mirror images.
groups. However, the n-alkyl groups are more efficient than the isoalkyl groups in withdrawing electrons from the hydrogen attached to nitrogen. Among all the atoms, the hydrogen attached to the nitrogen is the atom with maximum positive charge and hence it is expected to have strong bonding interactions with the oxygen atoms of the A1P04-11 framework. The electron distributions among the methylene and methyl groups are almost constant for all the secondary amines. Overall, these amines are less polar with a typical dipole moment value of = 1.25 D and hence are expected to be weakly bound to the framework. Tapp and Milestone" reported that the template molecules desorbed at -200 OC, which is in agreement with the weak binding predicted by the above calculations. Calculations were carried for ammonia also, since ammonia was used as the model compound to study the interactions between amine molecules and the bridging oxygen in the AlP04-11 lattice. Cluster Models of the AIPO4-ll Lattice. Altogether, there are 1 1 possible bridging oxygen sites which are crystallographically distinct in the ordered AlP04-11 structure.I2 The geometry of the repeating unit cell containing six unique T sites and the 11 (12) Richardson, J. W., Jr.; Pluth, J. J.; Smith, J. V . Acta Crystallogr. 1988, 844, 376.
oxygens attached to them is shown in Figure 1. The inset shows the lattice of AlP04-11 formed by the presence of such repeating units. All 11 unique oxygen sites in AlP04-11 are simulated by suitable cluster models. Dimeric cluster models [ (OH)3Tl-OT2(OH),] where T1 = P and T2 = A1 were selected to model all the bridging oxygen sites in the AlP04-11 crystal structure reported by Richardson et a1.I2 The hydrogen atoms needed to maintain neutrality of the clusters are located at the nearest-neighbor T sites. A typical cluster model is shown in Figure 2a. Electronic structures of these clusters are calculated by the MNDO method. Cluster models, their total energies, and the electron densities on the bridging oxygens are reported in Table 11. The net electron density on the bridging oxygen atom is found to increase with increasing T-0-T angle and decreasing T-0 distance. The calculated electron density on the bridging oxygen atom indicates how strongly the oxygen atom can interact with the hydrogen atom of ammonia. Cluster Models of the AIP04-11 Lattice and Ammonia Adsorption Complexes. The electronic structure of amines indicates that the major interaction of the amine with the framework is expected to occur through the hydrogen attached to nitrogen and its nearest neighbors. Hence, ammonia adsorption over different oxygen sites in AlP04-11 will have corollary results as far as electronic interactions are concerned. A neutral ammonia molecule adsorbed through one of its hydrogen atoms onto the bridging oxygen site of the AlP04-11 cluster is considered. Ammonia is positioned in such a way that the O-HNHz bond lies at the center of the T-0-T angle on the obtuse side. The typical cluster model representing the ammonia adsorption complex is shown in Figure 2b. The binding energy values of ammonia to the 11 different
Modeling and MASNMR Studies on Molecular Sieves
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3095
oxygen sites in the AlP04-11 lattice are calculated and given in Table 11. The binding energy values of ammonia to the oxygen sites are calculated as follows: B E N H= ~ TEcomp~cx- (TEdimcriccluster + TENH,) where BE and T E are the binding energy and total energy, respectively. The binding energy of ammonia is a positive value, which is an artifact of the small cluster model. However, the binding energy gives the trend in the strength of adsorption of ammonia at various oxygen sites, which is expected to be the same for the amine molecules also. The analyses of molecular orbitals of the cluster models show that the highest occupied molecular orbitals (HOMO) are contributed to by the 2p orbitals of oxygen and aluminum, while the lowest unoccupied molecular orbitals (LUMO) are contributed to by the 1s orbital of hydrogen and 2p orbitals of oxygen and phosphorus. The 1s orbital of hydrogen in ammonia is contributing to the frontier HOMO in the adsorption complex. Since PO4 groups have unoccupied orbitals, electron donation from amines to the PO4 group is expected to occur. The favorable adsorption sites for ammonia are found to be 0 4 , 0 5 , 05', and 06'. Incidentally, the values of the T-0-T angles for these oxygen sites are minimums (Table 11). The ammonia prefers to adsorb on oxygen sites where the steric hindrance for ammonia to approach the oxygen site is minimum, and this argument is applicable to the secondary amines also. The amine molecule may undergo multiple site adsorption with the hydrogen atoms of the alkyl groups having interactions with other oxygen atoms of the framework. The topographic analysis of the framework indicates that the favorable oxygen adsorption sites predicted by electronic structure calculations lie a t different positions of the framework. 06' is on the wall common to 4member and 6-member channels, and hence the template molecule cannot physically approach this site. 0 4 is on the wall common to 6-member and 10-member channels, while 05 and 05' are on the walls common to 4-member and 10-member channels as could be made out from the Figure 1. It is encouraging to note that our findings are in correspondence with the studies13on the crystal structure analysis of the as-synthesized MnAPO-11 material, which show that the diisopropylamine is located inside the pore in such a way that nitrogen lies closer to 0 4 and 0 5 of the framework. I3C MASNMR Spectroscopic Studies. We also carried out N M R studies to obtain insight into the nature of template molecules in AlP04-11. The I3C C P MASNMR spectroscopic studies on as-synthesized AlP04-1 1 with di-n-propylamine, din-butylamine, and di-n-pentylamine template molecules indicate that they are chemically intact without undergoing any dissociation as shown in Figure 3. The asymmetric doublet pattern (-55 ppm) arising in I3C N M R spectra of carbons bonded to nitrogen can be attributed to be due to I4N-I3C quadrupolar effects as explained by Naito et al.I4 Otherwise unsplit signals for the methylene and methyl carbons indicate that both the alkyl groups in dialkylamines are in a uniform environment inside the AlP0,- 11 framework and both alkyl groups are in the same conformation. When the as-synthesized samples are dried at higher temperatures, the N M R spectrum shows a split in the methyl group signal, and a typical spectrum recorded for AIP04-11 with di-nbutylamine dried at 373 K is shown in Figure 4. Such a solid-state splitting could be due to environmental changes or due to conformational changes occurring in the occluded species. In the earlier s t u d i e ~ , ' ~it. 'was ~ reported that the templating molecules (13) Pluth, J. J.; Smith, J. V.; Richardson, J. W., Jr. J . Phys. Chem. 1988, 92, 2734. (14) Naito, A.; Ganapathy, S.;McDowell, C. A. J . Chem. P h p . 1981, 74, 5393. (15) Nagy, J. B.; Gabelica, Z.; Derouane, E.G. Zeolires 1983, 3, 43. (16) Boxhoorn, G.; van Santen, R. A.; van Erp, W. A,; Hays, G. R.; Alma,
N. C. M.; Huis, R.; Claque, A. D. H. Proceedings of the 6th International Zeolite Conference, Reno, NV; Olson, D., Bisio, A., Eds.;Butterworth: Boston, 1984; p 694.
,
60
a
1
50
I
I
I
40
I
30
'
I
1
20
IO
'
PPM
Figure 3. I3C C P MASNMR spectra recorded for as-synthesized AIP04-11 material with di-n-propylamine (a), di-n-butylamine (b), and din-pentylamine (c) as the template molecules.
I
PPM
Figure 4. I3CC P MASNMR spectrum recorded for AIPO,-ll material dried at 373 K showing the split in the methyl group signal. This material was prepared using di-n-butylamine as the template molecule.
existing in a uniform environment inside ZSM-5 and ZSM-11 frameworks give rise to unsplit signals. Boxhoorn et a1.I6 have provided evidence from 13C MASNMR studies that, in the case of the TPA ion inside ZSM-5 framework, the species undergoes conformational changes on heat treatment. Conformational changes between the two alkyl groups are expected to cause splitting in the methyl as well as the methylene
3096
J . Phys. Chem. 1992, 96, 3096-3100 amines may have more than one favorable adsorption site, and the values of the adsorption energies are expected to be comparable, thus facilitating its migration from one to another favorable site. This observation is in correlation with the findings from our electronic structure calculation that there are three oxygen sites for adsorption of basic molecules with comparable adsorption energies (Table 11).
I. I
lbl
Figure 5. (a) The geometry of the side pocket which is a double 6member ring. The access to this side pocket from the 10-member ring is shown by shading. (b) The schematic view of the channels along the c axis (box) and the perpendicular a axis.
group signals. However, since the splitting occurs only in the signal of the methyl group (Figure 4), the fact that the terminal methyl group alone is present in a different environment is brought (Jut. In this context, it is relevant to note a unique feature of the structure of AlPO,-1 1, namely the presence of interconnected double 6-member side pockets. There is access to these side pockets from the 10-member channel. The geometry of the side pocket and the schematic view of channels of AlP04-11 from two perpendicular directions are shown in Figure 5a and 5b, respectively. The nonbonded distance between A13 and P3 is 5.30 A. When 0.86 A for the ionic radii of A13+ (0.51 A) and Ps+ (0.35 A) is left out, the free dimension across the 6-member ring aperture is 4.44 A. This is an unusually large aperture for a 6-member ring compared to those in other molecular sieves, and this provides a hint to explain the cause of the split in the signal of the methyl group. On heating, the weakly bound amine molecule may shift its interaction site, resulting in the movement of the -CH fragment of the terminal methyl group into the 6member side pocket, and thus exists in a different environment. These results indicate that the template molecules are still chemically intact and they possess the freedom that allows the dynamic behavior inside the neutral AlP04-l 1 framework. The
Conclusions The following features are the outcome of the present study: 1. The favorable oxygen sites for the adsorption of amine molecules are 0 4 , 0 5 , and 05'as predicted by the electronic structure calculations. It is heartening to observe that the results are consistent with the findings from single crystal powder diffraction studies of as-synthesized MnAPO-11, that the diisopropylamine is hydrogen bonded to 0 4 and 05. 2. The occluded amine molecules in as-synthesized AlPO,-ll samples are chemically intact. At high temperatures, they have a dynamic freedom inside the pores as indicated by NMR studies. 3. In deciding the role of these amine molecules as template molecules, the geometric factor plays a relatively insignificant role compared to the electronic factor because of the conformational flexibility and the possibility of a methyl group fragment projecting into side pockets. A recent paper1' reporting the synthesis of AlP04-11 using 1-methylimidazole as the template also supports the fact that the geometry is not a significant factor to decide the templating ability of molecules. 4. As far as the electronic factor, there is a small but subtle difference in the electron density distribution among the primary, secondary, and tertiary amines. It appears that the amines may also play a role in modifying the pH of the precursor gel thereby facilitating the synthesis to different extents. It is difficult to estimate the pH modifier role played by template molecules from the present study. Hence, in the formation of AlP04-11, the electronic influence of amine molecules seems to be greater than the geometric factor. Acknowledgment. We thank Dr. P. R. Rajamohanan for the MASNMR experiments. This work was partly funded by the UNDP. (17) Czarnetski, B. K.; Jongkind, H.; Dogterom, R. J.; Stork, W. H. J. Appl. Catal. 1991, 75, L9.
Modeling and MASNMR Spectroscopic Studies on Molecular Sleves. 2. The Nature of Water Molecules in Hydrated AIP04-11 Host Latticet S. Prasad, I. Balakrishnan, and R . Vetrivel* National Chemical Laboratory, Pune 41 1 008, India (Received: July 23, 1991)
We report here the results of cluster calculations and MASNMR spectroscopic studies on the behavior of water molecules inside the pores of molecular sieve AlP04-1 1 . We have carried out MNDO calculations to understand the nature of adsorbed water, its interaction with AIP04-l 1 framework, and preferred sites of adsorption. The results indicate that 'the T3 sites (where T = AI or P) are the hydrophilic centers. Consistent with this, MASNMR studies indicate a shift in 27Aland ,IP signals corresponding to these sites only.
Introduction The calcined AlPO4-11 sample is reported' to have an ordered (alternating A104- and PO4+tetrahedra) structure with orthorhombic symmetry in the space group of Icm2. The asymmetric repeating unit consists of six crystallographically distinct tetraNCL Communication No. 5215.
hedral sites (3A1 + 3P). T1 and T2 (where T = AI or P) are located a t the junction of lo-, 6-, and 4-membered rings, while the T3 site is located at the junction of lo-, 6-, and 6-membered rings as &own in Figure 1 Calcined AlP04-11 is capable of (1) Richardson, Jr., J. W.; Pluth, J. J.; Smith, J. V. Acra Crysrallogr.1988, 8 4 4 , 361.
0022-365419212096-3096%03.00/0 0 1992 American Chemical Society
