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
Water Molecules in Hydrated AlP04-1 1
The Journal of Physical Chemistry, Vol. 96,No. 7, 1992 3097
0 Oxygen
Hydrogen
Figure 2. Typical cluster models showing different modes of adsorption of water over a TO, group. The geometry of each cluster model is explained in Table 11, and the TO4 cluster represents the A13 site.
Figure 1. Framework structure of AIP0,- 1 1. The edges of the unit cell are shown, and the view is along the c axis.
adsorbing water, and the results of many experimental techniq~es~?~ such as IR, NMR, and TGA-DTA have shown that AlP04-l 1 undergoes structural change reversibly on adsorption of water including overall reduction in the unit cell volume and pore volumea2 In the present study, the changes occurring in the XRD pattern and MASNMR profiles due to hydration are analyzed. Electronic structure calculations are undertaken to locate preferred sites of adsorption for water inside the AlP04-11 framework. 2 0 Further, it is revealed that quantitative correlations exist between Figure 3. Observed XRD profile for AIP04-1 1: (a) as-synthesized (AS) the additional N M R signals due to water adsorption and change sample, (b) calcined and dehydrated (CD) sample, and (c) fully hydrated in the environment of T sites. (FH) sample.
Experimental Section An AlP04-1 1 sample was synthesized using n-dibutylamine as the templating molecule, and the procedure is described in detail el~ewhere.~The as-synthesized (AS) sample of AlP04-11 was calcined in air at 723 K for 12 h to remove the occluded organic amine. The calcined sample was divided into two portions: One portion was dehydrated at 473 K, cooled in a desiccator over P2OS, and packed under a nitrogen atmosphere to give a calcined dehydrated (CD) sample. Another portion was kept for hydration in a desiccator containing saturated ammonium nitrate solution for 72 h to give a fully hydrated (FH) sample. The samples were analyzed by XRD and MASNMR techniques. The sample FH contained 0.13 g of water per gram of the catalyst. The X-ray powder diffraction profiles of the samples were generated using a Rigaku Giegerflux diffractometer with Cu K a source, and MASNMR studies were carried out with a Bruker MSL 300 spectrometer operating at a field of 7 T. 27Alspectra were recorded at a frequency of 78.2 kHz, with a pulse length of 2 ps and a spinning speed of 3-5 kHz. 31Pspectra were run at a frequency of 121.4 kHz with a pulse length of 4 p s and a (2) Tapp, N. J.; Milestone, N. B.; Bowden, M. E.; Meinhold, R. H. Zeolites 1990, 10, 105. (3) Khouzami, R.; Coudurier, G.; Lefebvre, F.; Vedrine, J. C.; Mentzen, B . F. Zeolites 1990, 10. 183. (4) Balakrishnan, I.; Prasad, S . Appl. C u d 1990, 62, L7.
spinning speed of 2-3 kHz. The chemical shifts were measured relative to aqueous AlC& solution and 85% H3P04 solutions, respectively. Model and the Calculation Methodology. The geometry and structure of each tetrahedral unit (either A104 or PO4) are obtained from the crystal structure reported' for AlP04-11. The unsaturated valency of the oxygen atoms in the tetrahedra is balanced by bonding a hydrogen atom to them, and the positions of these hydrogen atoms are the adjacent T atoms in the AlP04-1 1 structure. A typical cluster model representing the water adsorbed on a TO4unit is shown in Figure 2a. For each of the tetrahedra, the different possibilities of water approaching the T site through its oxygen end is considered. Molecular orbital calculations using the MNDO technique5 were adopted to calculate the electronic structure of each cluster model. The calculations were carried out for the water molecule, the bare tetrahedral cluster (Td), and tetrahedral clusters with water molecules adsorbed (adsorption complex). The adsorption energy of water (mads) a t different sites is calculated as mentioned below:
In all the adsorption complexes, the distance between the tetrahedral atom and the oxygen of a water molecule ( T atom ( 5 ) Dewar, M. J. S.; Thiel, W. J. A m . Chem. Sac. 1977, 99, 4899.
3098 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 TABLE I: Electronic Properties of H4T04(Where T Clusters Representing Different Sites in the AIP04-11 total T site oxygen sites energy AI 1 01, 0 3 , 05’, 0 6 ’ -1380.32 A12 02, 05, 06, 0 7 -1380.73 A13 0 3 , 07’, Ol’, 0 8 -1383.31 P1 01, 04, 05, 0 6 -1476.03 P2 0 2 , OS‘, 06‘, 0 7 ’ -1477.97 P3 03, 07, 07, 0 8 -1473.84 free H 2 0 -351.41
= AI or P) Framework net charge on T site 0.9 1 1.01 0.91 1.47 1.65
1.81
of H4T04 and oxygen of H 2 0 ) is kept at 1.5 A uniformly. The values of the net charge density on various atoms of the clusters are calculated using the Mulliken population analysis method.6 Results and Discussion X-ray diffraction profiles recorded for AlP04-11 under different conditions-as-synthesized (AS), calcined dehydrated (CD), and fully hydrated (FH)-are shown in Figure 3. The XRD profiles for AS and CD samples correspond to the orthorhombic Zcm2 space group as reported ear1ier.I~~However, the XRD pattern of the FH sample showed a different profile. The peaks at 28 = 9.5, 13.0, and 15.75 were shifted either to higher or lower 28 values of 9.80, 12.75, and 16.10. There were new peaks at 28 = 13.65, 14.6, and 17.5, and the peaks in the range of 28 = 20-30 showed splittings. Similar results have been reported by Tapp et aL2 for AlP04-l 1 and Khouzami et aL3 for SAPO-1 1, and the changes in the XRD profile pattern are attributed to a change in the crystal structure with lower symmetry, namely the Pna2, space group. The reduction in the intensity of the peaks at 28 = 8-10 may be an indication of the interaction of water molecules with the framework T sites. A similar reduction in peak heights in the XRD pattern at 28 = 8-10 with simultaneous appearance of peaks due to Al(V1) in 27AlMASNMR have been observed in related materials such as SAPO-3 I s and A1P04-89upon hydration. As mentioned earlier, the amount of water in the FH sample is 13 wt 5%. This amounts to the presence of approximately 20 water molecules in a unit cell (A120P200so)of AIPO,-11. Since both T atoms, namely A1 and P, are known to form stable compounds in octahedral coordination, it is likely that they transform from tetrahedral to octahedral coordination on interaction with water. Hence, assuming that each T site is adsorbing two water molecules, -25% of the T sites will be interacting with water molecules. Khouzami et ala3have reported the coordinates of atoms in hydrated SAPO-1 1, calculated by the DLS 76 program, which show the presence of only four water molecules per unit cell of AlPO,-l 1. The rest of the water molecules may be mobile in the pores of the AlP04-l 1 framework. IR studies2 also have shown the presence of free water molecules. Hence, the XRD studies alone are not capable of explaining the nature and location of all the adsorbed water. We have resolved the problem of understanding the behavior of water in AlP04-11 into two components, namely the nature of water interaction with the framework and the environmental changes occurring at T sites due to its adsorption. We adopted semiempirical quantum chemical cluster calculations and MASNMR spectroscopic studies, respectively, to gain information on the above two components. Nature of Interaction of HzO Molecules with AIP04-11 Framework. The total energy calculated for the six crystallographically distinct T sites (3A1 and 3P) modeled by the H,TO, cluster is given in Table I. Calcined and dehydrated AlPO,11(CD) shows a tendency for rapid uptake of water molecules. Since the AlP04-11 framework is a neutral one, there are no (6) Mulliken, R. S . J . Chem. Phys. 1955, 23, 1833. (7) Pluth, J. J.; Smith, J. V.; Richardson, Jr., J. W. J . Phys. Chem. 1988, 92, 2734. (8) Zubowa, H. L.;Alsdorf, E.; Fricke, R.; Neissendorfer, F.; Menden, J. R.; Scheirer, E.; Zeigan, D.; Zibrowius, B. J . Chem. Soc., Faraday Trans. 1 1990, 86, 2307. (9) Prasad, S.; Balakrishnan, 1. Inorg. Chem. 1990, 29, 4830.
Prasad et al. TABLE 11: Different Modes of Interaction of Water with the TO, Group in the AIP04-Il Framework As Shown in Figure 2, with the Value of Calculated Adsorption Energy for the A13 Site Given as a Typical Example interaction interaction of hydrogen of oxygen atom of atoms of adsorptlon water with water with T site of energy, oxygen of mode of interaction of water eV framework framework H 2 0 in the plane of 0-T-0 +8.80 strong strong with oxygen atom toward T site (Figure 2a) H20in the plane of 0-T-0 +27.09 strong weak with oxygen atom away from T site (Figure 2b) H 2 0 in a plane perpendicular +20.48 strong medium to 0-T-0 plane with hydrogen atoms toward oxygen of TO, group (Figure 2c) H 2 0 in a plane perpendicular +12.74 weak medium to 0-T-0 plane with hydrogen atoms away from oxygen of TO4 group (Figure 2d) TABLE 111: Electronic Properties of Water Adsorbed over the T3 (Where T = AI or P) Site ( T W x y g e n of H20Is 1.5 A) net charges on three atoms of the plane in adsorption oxygens which the water energy, of the oxygen molecule is present eV framework T site of H,O 03-A13-07’ (X2) 10.12 -0.72, -0.65 +1.11 -0.07 8.80 -0.74, -0.72 +1.14 -0.07 03-A13-08 10.16 -0.65, -0.63 +1.11 07’-A13-07’ -0.04 10.31 -0.66, -0.67 +1.14 07’-A13-08 (X2) -0.03 -17.46 -0.84, -0.33 +1.62 -0.06 03-P3-07 (X2) -0.87, -0.46 +1.74 -0.02 -17.78 03-P3-08 -0.63, -0.84 +1.88 -0.16 -13.66 07-P3-07 -0.80, -0.91 +1.94 -0.18 -12.33 07-P3-08 (X2) free H 2 0 -0.32
compensating cations present to interact with water. Hence, water could be assumed to interact with the framework oxygens through hydrogens and with framework T sites through oxygen. We have considered four different modes of interaction of a water molecule with the TO4group as shown in Figure 2a-d. The results of the calculations on these model clusters are summarized in Table 11. The interactions between the atoms of a water molecule and the AlP04-11 framework are qualitatively estimated from the values of net charge density on the atoms and bond orders calculated. It is evident from the data in Table I1 that the mode of interaction shown in Figure 2a is the energetically favorable mode. We have analyzed in detail this mode of interaction with all the six TO4groups given in Table I and each individual site. Hereafter, all the water adsorption complexes will be represented by the model shown in Figure 2a. There are six ways the mobile water molecules can approach the T site. The adsorption energy of water to a particular T site is assumed to be an average of the six values of adsorption energy calculated for the six ways of adsorption of water around a T site. For example, the different possibilities wherein the water molecule can lie in the plane of 0-T-0 for the A13 tetrahedron are illustrated below. A13 is bonded to four oxygens, namely 0 3 , 0 8 , and two 07’. The six possible planes through which oxygen can approach the A13 site are those planes containing three atoms as shown: 03A1308, 03Al307’, 07’4308, and 07’4307’ (03Al307’ and 07’Al308 are repeated). The results of the calculation on the above cluster models as well as for the cluster models representing water adsorption around the P3 site are given in Table 111. The absolute values of the calculated adsorption energy reflect the inherent negative charge on AlO, and positive charge on PO, tetrahedra. In reality, when A10, and PO4 are lying adjacent, the charges
The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 3099
Water Molecules in Hydrated AlP04-11
TABLE I V Geometric and Electronic Properties of Water Adsorbed over All Six T Sites net charges on (average) av adsorption oxygen av T-0-T energy of of the oxygen hydrogen site angle, den water, eV framework T site of H20 of H 2 0 +0.22 -0.56 +0.93 -0.12 All 146.0 11.93 A12 158.0 11.35 -0.61 +1.00 -0.12 +0.20 9.97 -0.68 +1.13 -0.05 +0.17 A13 171.8 -0.70 +1.64 -0.27 +0.27 PI 147.5 -13.21 P2 157.3 -13.88 -0.66 +1.69 -0.15 +0.26 P3 171.3 -15.17 -0.71 +1.85 -0.06 +0.27 will be uniformly distributed over all atoms leading to a comparable adsorption energy. Hence, only the qualitative trend in strength of adsorption over A1 and P sites could be derived from the results of the present calculations (Table 111). The average adsorption energy for adsorption of a water molecule over the A13 site is calculated from the values in Table 111 as 9.97 eV. Their magnitudes of adsorption energy are dependent on the distance of the water molecule from the T site and the size of the cluster model. We have chosen a small cluster model since we are interested in studying the interaction of the water molecule with a single T site, and the T-O(H20) distance is uniformly kept at 1.5 A for purposes of comparison. Thus, the complex having the smallest positive value of AHadswill be the energetically favorable mode of adsorption. As mentioned earlier, the magntiude of the adsorption energy given here is for qualitative ordering of adsorption sites only. The values of adsorption energy are found to depend on the polarization of the 0-A1-0 bonds. When the net negative charge on the framework oxygen atoms and the net positive charge on the aluminum atom are maximum, say as in the case of the 03A1308 plane shown in Table 111, the adsorption is favored. Oxygen of the water molecule gains the maximum negative charge in this case. The adsorption of water is found to take place by the transfer of electrons from the TO4 group. The net charges on various atoms themselves are found to depend on the geometry of TO4, which will be discussed in the following section. The procedure mentioned above is followed, and the adsorption energies for water at the rest of the five T sites were also calculated. The average adsorption energy of water obtained from six values of the adsorption energy are given in Table IV. They indicate that A13 and P3 are the preferred sites among the aluminum and phosphorus sites, respectively, for the adsorption of water. The water molecules are expected to be mobile around the T3 site; hence, the adsorption energies calculated for the water molecule in more than one plane are comparable in magnitude. The geometric reasons for the preference for these T sites is obvious since these sites have the largest T-0-T angles, thus posing minimum steric hindrance to the approaching water molecules. In addition to the steric factor, the electronic factor is also favorable where the T-0-T angles are maximum and the T-0 distancq are minimum. We have shown in our earlier worklo that the charge separation between T and 0 sites in the AlPO,-ll framework is maximum at sites where T-0 distances are short and T-0-T angles are large. The polarity and hence the electrostatic field play a significant role in the adsorption process. Although charge separation occurs inside water, still it remains intact and there is no indication of its dissociation from the calculated bond order values. Topologically, A13 and P3 sites occur at the junction of 10membered and two 6-membered rings as mentioned earlier. Statistically, T3 sites represent 20%of the T sites (where T is either A1 or P) since there are two T1 and T2 sites each for every single T3 site. The implication of these results will be described later in light of MASNMR results. Further studies on adsorption of water in a site between two TO4groups, and thus the possibility of a single water molecule changing the environment of two adjacent T sites, are being undertaken. ~~
(10) Prasad, S.; Vetrivel, R. J . Phys. Chem., in press.
(dl
x
l
I50
00
50 PPM
0
-50
100
!
50
h
0
C
-50
J
l
-100-15c
PPM
Figure 4. *'AI MASNMR spectrum for (a) calcined and dehydrated (CD) sample and (b) fully hydrated (FH) sample. ,'P MASNMR spectrum for (c) calcined and dehydrated (CD) sample and (d) fully hydrated (FH) sample.
27AIand 3'PMASNMR Spectroscopic Studies. A single resonance line at 34 ppm was observed in the 27Al MASNMR spectrum of the AS sample, indicating tetrahedrally coordinated AI in the ample.^ Parts a and b of Figure 4 are the 27Alspectra for CD and FH samples, respectively. The signal at 34 ppm is present in both. However, the additional signal in sample CD at 32.3 ppm appears only as a shoulder in the sample FH which also shows an additional peak at 23.7 ppm. These signals and the additional broadening observed in FH most probably arise from the quadrupole interaction with the nonzero electric field gradient on the aluminum nuclei with 5 / 2 spin. In a 27AlMASNMR study of zeolites ZSM-5 and Y, Kentgens et a1.I' have observed that quadrupole broadening is most promiment in the latter case which has a high A1 content. This broadening was found to depend on the nature of the chargecompensating cation, the chemical shift distribution, and the intensity of the magnetic field applied. In the case of AlPO,-1 I, which has a high A1 content but no charge-compensating cation, a similar broadening can be expected. A redistribution of electronic charges of aluminum in the presence of water molecules has been indicated by MNDO calculations. This must lead to an increased electric field gradient at the A1 nucleus and, hence, an increase in peak broadening as observed in the FH sample. In addition to the peaks in the tetrahedral region, a broad peak appeared in the high-field region of -15 to -25 ppm in the case of the FH sample. A peak in the region is characteristic of a 6-coordinated A1 species in the framework and has been reported in the case of related materials such as A1PO4-5,l2AlPO4-17,I3 VPI-5,14 A1PO4-H3,I5and AlP04-11 as well as SAPO-34.I6 It is attributed to interaction of water molecules with the AIO, tetrahedron17 and is recognized as evidence for the presence of chemisorbed water in the hydrated sample. (11) Kentgens, A. P. M.; Scholle, K. F. M. G . ;Veeman, W. S. J . Phys. Chem. 1983,87,4357. (12) Meinhold, R.; Tapp, N. J. J . Chem. Soc., Chem. Commun. 1990,219. (13) Blackwell, C.S . ; Patton, R. L. J . Phys. Chem. 1984,88, 6134. (14) Grobet, P. J.; Martens, J. A.; Balakrishnan, I.; Mertens, M.; Jacobs, P. A. Appl. Carol. 1989,56, L21. (15) Martens, J. A.; Verlinden, B.; Mertens, M.; Grobet, P. J.; Jacobs, P. A. ACS Symp. Ser. 1989,No. 398, 305. (16) Goepper, M.; Guth, F.; Delmotte, L.;Guth, J. L.;Kessler, H. Stud. Surf.Sei. Catal. 1989,49A, 857. (17) Muller, D.; Grunze, I.; Hallas, E.; Ladwig, G.Z . Anorg. Allg. Chem. 1983,500, 80.
3100
J . Phys. Chem. 1992,96, 3100-3109
In an earlier related study, Khouzami et al.3 had indicated the presence of an octahedral A1 species from X-ray diffraction measurements, but it was not confirmed by the analysis of a high-field *'A1 MASNMR spectrum. Tapp et al.,2 on the other hand, had not extended the spectral studies to include the high-field region and hence failed to notice the presence of this broad peak. This led to the conclusion that all the water molecules in the hydrated samples are only physisorbed. However, Goepper et a1.I6 have assigned the broad signal to the presence of octahedral Al. Granting that the broad peak in our case is due to a 6-coordinated A1 species, it should be possible to estimate the fraction of the total Al in the framework that is present in this state. This can be done by subtracting the area under the spinning sideband (6 to -70 ppm) from the area under the broad signal and relating it to the total area under the 27Alsignals. Following this procedure, it is found that nearly 20% of the total A1 may be assigned an octahedral coordination. Significantly, the fraction corresponds to the fraction of A13 sites (Figure 1) which are the favored sites for the location of water molecules on the basis of MNDO energy calculations. Further evidence for the existence and essential accuracy of the estimate of octahedral A1 species comes from the 31Pstudies. In the MASNMR spectra, a single peak at -30 ppm was observed for the AS sample., This peak was unaffected by calcination and dehydration in sample CD (Figure 4c). In sample FH, an additional peak at -23.4 ppm attributable to interaction of framework P with water molecules2J8was noticed. Such an interaction can cause deshielding of P, which causes an increase in the observed chemical shift. The area of this peak is, again, like as in the case of the octahedral A1 species, computed to be up to 20% of the total area under the 31Psignals. Naturally, therefore, it is the P3 sites which are involved in interaction with water, and the number of these sites corresponds to 20% of the total P sites (Figure 1). Thus, both 27Al and 31PMASNMR (18) Jahn, E.; Muller, D.; Becker, K . Zeolites 1990, 10, 151.
spectroscopic studies furnish data supporting the predictions of adsorption energy calculations. Conclusions The structural changes occurring in the AlPO,-1 1 framework due to hydration are evident from the XRD studies. The MNDO technique has been found to be useful for studying different modes of interaction of water with TO, groups in the AlP04-11 framework by cluster model calculations. 27Aland MASNMR spectra were recorded to study the environmental changes occurring at T sites due to water adsorption. Some of the notable findings of the present study are given below: 1. Water prefers to undergo three-site adsorption over the TO, group as shown in Figure 2a. 2. Among the different T sites, T3 (where T = A1 or P) shows favorable adsorption energy for water due its geometry (wide T-O-T angles). 3. The weakening of the 0-H bonds in water molecules is insignificant, showing that the latter do not dissociate. Hence, T sites are in a pseudooctahedral environment which can revert back to a tetrahedral environment on dehydration. 4. For the hydrated sample FH, 27Al and 31PMASNMR spectra show additional signals a t around -20 and -23.4 ppm, respectively, indicating interaction of the T sites with water molecules. 5 . In a quantitative analysis, ratios of intensities of the two signals in the N M R spectra of the hydrated sample have been calculated. These ratios of the intensities of the additional peaks (arising from water adsorption) to the corresponding tetrahedral peaks for Al and P are -2080, consistent with interaction between water molecules and the T3 sites causing distortion of the framework.
Acknowledgment. We thank Dr. P. R. Rajamohanan for MASNMR experiments. This work was partly funded by the UNDP. Registry No. H20, 7732-18-5.
Electron Spln Echo Envelope Modulation Studies of Cu2+ Interactions with Framework AI in Dehydrated X, Y, and A Zeolites Kbalid Matar and Daniella Goldfarb* Department of Chemical Physics, The Weizmann Institute of Science, 76 100 Rehovot, Israel (Received: August 5, 1991) The interactions of Cu2+cations with framework A1 in zeolites NaX, KX, Nay, and NaA in various stages of dehydration and after methanol adsorption were investigated by electron spin echo envelope modulation (ESEEM) spectroscopy. The FT-ESEEM spectra of all species studied showed contributions from two types of 27Alnuclei. The first type, referred to as first shell, consists of 27Alnuclei bonded to the oxygens to which the Cu2+is coordinated. They exhibit relativdp large isotropic hyperfine constants which manifest the firm binding of the Cu2+to the framework. The second type consists of A1 nuclei which are coupled to the Cu2+by weak dipolar interactions and are termed distant Al. Orientation selectiveESEEM experiments were carried out as well to obtain additional structural information. The modulation amplitude corresponding to distant A1 did not show any dependence on the irradiation position within the EPR powder pattern as the arrangement of those nuclei around the cation is approximately isotropic. The modulation amplitudes of the first shell AI in some cases showed strong orientation dependence which was interpreted in terms of the site geometry. The amount of adsorbed methanol in Cu-NaX has a considerable effect on the relative intensities of the peaks of the first shell AI and distant AI. This dependence is explained in terms of the changes in the 27Alquadrupole coupling constants of the two types of Al. Introduction Zeolites are microporous materials which have many applications as molecular sieves, ion exchangers, and catalysts.' The exchangeable cations play an important role in many of these
applications. Accordingly, considerable efforts were made through the years to obtain their location2and understand the factors which govern their distribution among the various sites* In hydrated zeolites the cations are mobile and undergo rapid exchange between the various sites. Upon dehydration they be-
( I ) Breck, D. W. Zeolite Molecular Sieues; John Wiley and Sons: New
(2) Mortier, W. J. Compilation of extraframework sites in zeolites; Butterworths: Guildford, 1982.
York, 1974.
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