J. Phys. Chem. 1994,98, 1579-1583
1579
Interaction of Water Molecules with the VPI-5 Lattice? S. Prasad Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei, Taiwan 107, Republic of China
R. Vetrivel' National Chemical Laboratory, Pune-411 008, India Received: July 27, 1993"
The application of cluster model calculations to understand the interaction of water molecules with the VPI-5 lattice is presented. The energetically favorable location of water molecules and their influence in altering the local symmetry of the VPI-5 framework are reported. The results indicate the presence of a water molecule in the vicinity of All and P2 sites which are correlating with M A S N M R and X-ray powder diffraction studies. W e also carried out thermal analysis and quantified seven different types of water molecules in the hydrated VPI-5. The dehydration to different extents will lead to rupture of the water chain leaving more void space inside VPI-5, and this has been demonstrated by xenon adsorption studies.
Introduction The synthesis and structure of a very large pore molecular sieve YPI-5 was first reported by Davis et al.l-z They confirmed a framework structure with a one-dimensional pore defined by 18 T sites (where T = A1 or P). The pore diameter in VPI-5 is 1.2 nm in comparison to that of 0.74 nm in 12-T ring zeolites such as zeolite Y. The structure is reported to be an ordered (regularly alternating A104- and PO4+tetrahedra) structure with hexagonal symmetry in the space group of P63cm containing two crystallographically distinct T sites. The large one-dimensional channel with a diameter of 1.2 nm gives it potential to be used in the separation of bulkier organic molecules and in the catalytic cracking of large hydrocarbons. The incorporation of large metal complexes resulting in enzyme-like active sites3and, more recently, the self-assembly of molecules such as c 6 0 on a quantum scale4v5 have been reported. However, the use of this molecular sieve primarily depends on the removal of the helical arrangement of water molecules without the breakdown of the framework structure. The work by Rudolf and Crowder6revealed thelocation of water molecules. All the water molecules were weakly bound and were located in the 18-T pore in the form of layers held together by H-bonding. The MAS N M R spectroscopic studies by Grobet et al.7 predicted three different coordinations for phosphorus atoms and the presence of A1 in octahedral coordination. Thus the structural predictions by N M R studies do not coincide with the earlier structure proposed by X-ray powder diffractometry.6 Recently in a high resolution synchrotron powder diffraction experiment, McCusker et a1.8 have refined the structure of VPI-5 with a lower symmetry space group, namely, P63. The refinement again shows an ordered structure which distinguishes three crystallographically distinct T sites, in contrast to two reported earlier, and reveals the presence of two water molecules near the All site. The All site is between two four-membered rings, and the adsorbed water molecules lead to octahedral coordination at the All site. Grobet et ai.: based on their high resolution DOR N M R studies at different field strengths, were also able to confirm the presence of two water molecules closer to the All site, thus forming an octahedral coordination for All. van Braam Houckgeest et a1.IO have shown that at ambient temperature, there are three signals of equal intensity in the 3lP MAS N M R spectrum. At temperatures above 60 OC, when the water molecules are t NCL communication No. 5657. Abstract published in Advance ACS Abstracrs, January 15, 1994.
0022-3654/94/2098- 1579%04.50/0
desorbed, there is a transition to higher framework symmetry and there are only two signals with their intensity in the ratio of 2:1. Similar results from N M R studies have also been reported by various other research groups."-13 Thus, the hydrogen-bonded water is the cause of the reduction of symmetry in the framework leading to three rather than two nonequivalent T sites, and this transformation is reversible. Rocha et al.11 have recently reported a reversible low temperature (-83 to -53 "C) structural transformation attributed to the freezing of water molecules inside the channels. The effect of calcining a t higher temperatures leading to irreversible transformation from VPI-5 to AlP04-8 is reported by Prasad and Ba1akri~hnan.l~However, Perez et a1.12 and Martens et al.15argue that VPI-5 is thermally stable in the absence of moisture. Amen et a l l 6 have shown that there is variation in the dimensions of the unit cell during dehydration and rehydration. Perez et propose that extensive exposure to water may lead to the interaction of water molecules with the P2 site and loss of phosphate groups. Consequently, the solubility of phosphate may ease the conversion of VPI-5 to AlP04-8. In a subsequent paper, Poojary et aI.17 compared the crystal structures of dehydrated forms of 18-T AlPOs-an ordered stable VPI-5 and a disordered unstable VPI-5, referred as H1. They have observed that A1-0-P angles of dehydrated VPI-5 are quite different from those of the dehydrated disordered VPI-5. They concluded that the angle factors are not responsible for the changes observed in N M R spectra. Thus there is no consensual agreement in the extent of affinity of water molecules to different TO4groups (where T = P or AI) and the mechanism of consequent phase transitions occurring in VPI-5. In this background, we report our studies on the cluster model calculations describing the interaction of water with the different TO4 sites in the VPI-5 framework and related experimental studies. The interaction energy values and the net electron density on various atoms obtained from the present calculations lead to a rationalization of the nature and location of water molecules in the hydrated form of VPI-5.
Theoretical and Experimental Section The method of generation of cluster models to study the adsorption of water molecule over each of the TO4 group is same as described in our earlier reports.ISJ9 It has been shown's that the nature and site of interaction of water molecules with a molecular sieve host lattice predicted from cluster calculations are in good correlation with the MAS N M R results. Thegeometry 0 1994 American Chemical Society
1580 The Journal of Physical Chemistry, Vol. 98, No. 6, 1994
Prasad and Vetrivel
TABLE 1: Geometry and Electronic Properties of K T 0 4 (where T = AI or P) Clusters Representing Different Sites in the VPI-5 Framework average T-0 distance (A) average 0-T-O angle (deg) average T-O-T (deg) total energy (eV) net charge on T site T site oxygen sites H4AIO4 Clusters All 01,02,03,04 1.83 105.80 150.51 -1379.54 0.96 A12 05,07,08,09 A13 06,010,011,012
1.73 1.76
109.44 109.41
P1
1.52 1.51 1.52
109.42 109.46 109.46
0-H = 0.96
H-O-H = 104.5
151.82 143.84
-1381.98 -1382.63
1 .oo 0.93
154.62 146.09 145.53
-1475.47 -1476.29 -1475.55
1.58 1.55 1.51
-351.41
go-0.32
H4P04 Clusters P2
P3
01,02,05,06 03,07,08,010 04,09,011,012
Water
TABLE 2 Electronic Properties of Water Adsorbed in the Interacting Planes over the A1 Sites and the Experimental Distances between Oxygen of Water and A1 Sites three atoms of the net charges on MAS NMR interacting plane average average average chemical shift oxygens of the framework where the water interaction interaction oxygen charge Al-OHZ of hydrated AI site molecule is present energy (eV) energy (eV) AI site of HzO on AI site distane (A) samples (ppm) AI I O3-All-02 21.02 -0.33 -0.36 0.97 -0.3 1 04-All42 21.09 15.00 -0.35 -0.34 0.88 -0.25 0.92 4.47 -10.4bJ 04-All43
2.88
Os-kd2-07 16.63 Os-Alfl9 16.17 07-Alfl9 16.37 Ah 06-AI3-0 I o 16.36 06-Ah412 15.33 01o-Al3-O 12 17.53 a From ref 5. From ref 6. From ref 20. Ah
16.39
16.41
-0.60 -0.46 -0.45 -0.56 -0.46 -0.51 -0.48
and structure of each tetrahedral unit (either A104 or PO4) is obtained from thecrystal structure reported8 for VPI-5. Crowder et al.2 did not locate the water molecules in their structural study. Grobet et al.’ and Wu et al.*O reported that half of the A1 sites in the 6-T ring are hexa coordinated. Later studies by McCusker et a1.* and Grobet et al.9 were able to shed some light on the location of water molecules. The unsaturated valency of the oxygen atoms in the tetrahedral cluster is balanced by bonding a hydrogen atom to them, and the position of these hydrogen atoms is the adjacent T atoms in the VPI-5 structure. Among the different models of interaction of a water molecule with the TO4group, the water molecule in the plane 0-T-0 with the oxygen atom pointing toward the T site is found to be the energetically favorable mode.18 Molecular orbital calculations using the MNDO technique21 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 interaction energy of water ( m a d s ) at 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 of H4T04 and oxygen of HzO) is kept at 0.15 nm uniformly. The values of the net electron density on various atoms of the clusters are calculated using the Mulliken population analysis method.22 VPI-5 was synthesized according to the procedure reported earlier,l4 using n-dibutylamine as the organic additive. The X-ray powder diffraction pattern showed the absence of impurity phases in the synthesized material. Thermal analysis was carried out in a ULVAC Sinku-Riko Differential Thermogravimetric Analyzer TGD 7000. Typically, 10 mg of the sample was used each time under the following conditions: reference, alumina; atmosphere, nitrogen; temperature range, 30-600 OC; scan rate range, 0.5-10 OCmmin-1. Partially dehydrated VPI-5 samples were prepared by vacuum treating about 0.5 g of samples at room
-0.63 -0.52 -0.56 -0.50 -0.50 -0.47 -0.48
0.91 1.01 1.00 0.96 0.96 0.99 0.96
-0.10 -0.21 -0.24 -0.18 -0.27 -0.28 -0.23
0.99
5.24
+43.6;b +41.6c
0.97
6.17
+41,6;b +43.6c
temperature. By this method 2.2%, 4.5%, and 9.6% of weight losses could be obtained and they are represented as VPI-5(2.2), VPI-5(4.5), and VPI-5(9.6), respectively. A VPI-5(23.2) fully dehydrated sample, corresponding to a weight loss of 23.2% was prepared by slow programmed heating of a sample up to 300 OC after evacuating a t 25 OC for 24 h. The xenon adsorption experiments were performed in a vacuum manifold.
Results and Discussion Interaction Energy of Water at DifferentSites. The total energy calculated for the three crystallographically distinct A1 and P sites modeled by the H4T04 clusters (where T = A1 or P) are given in Table 1. The geometry of different T sites, the 0 sites which are attached to each of the T sites, and the net electron density on T sites are included. The results of the calculation on the water molecule is also given in Table 1. Vogt and Louwen23 have correlated the charges calculated from the electronegativity equalization principle to N M R chemical shift. Higher charges on T sites are expected to lead to a shift toward higher field, but the charges on T sites given in Table 1 show that the variations are too small to distinguish the crystallographically distinct T sites. In the energetically favorable mode of adsorption, the oxygen of the water is expected to interact with the T site and the hydrogen of water with the oxygen of the framework. There are six planes through which a water can approach the T site. However, since it is evident from XRD studies638 that all the water molecules are located in the 18-T pores of VPI-5, only the three 0-T-0 planes of each T site which are approachable by water molecules in the 18-Tpore are considered as “interacting planes”. The other three 0-T-0 planes of each T site which are approachable from 4-T or 6-T pores are considered as “noninteracting planes” for the purpose of water adsorption. The electronic properties of water adsorbed on different interacting planes of H4A104 clusters representing thethreeAlsitesaregiveninTable2. Theinteraction energy of water to a particular T site can be determined as an average of all the three values, and the water molecule is assumed
Interaction of Water with VPI-5
The Journal of Physical Chemistry, Vol. 98, No. 6,1994 1581
TABLE 3 Electronic Properties of Water Adsorbed in the Interacting Planes over the P Sites and the Experimental Distances between Oxygen of Water and P Sites Three atoms of the net charges on average MAS NMR interacting plane average average P-OH2 chemical shift Oxygens Of the oxygen charge distance' of hydrated where the water interaction interaction P site molecule is present energy (ev) energy (ev) framework P site of HzO on P site (A) samples (ppm) PI P2 Pa
05Pl-06 O2-Pl-06 02-P1-06 O3-P2-O7 03-Pflio 07-pr010 04-Ps-09 04-Ps-012 oPp3-012
21.48 20.23 17.83 11.67 13.48 11.61 19.41 18.73 22.49
19.85 12.25 20.21
-0.35 -0.30 -0.74 -0.62 -0.81 -0.75 -0.36 -0.49 -0.30
-0.70 -0.67 -0.35 -0.74 -0.63 -0.69 -0.73 -0.71 -0.61
1.55 1.52 1.56 1.66 1.75 1.69 1.54 1.66 1.47
-0.20 -0.21 -0.16 -0.20 -0.39 -0.25 -0.18 -0.24 -0.26
1.54
5.23
-33.2b9C
1.70
4.80
-23.7;b -27.5'
1.56
4.90
-27.5;b -23.7'
From ref 5. From ref 9. e From ref 20. n
M
"Q W
Figure 1. Energeticallyfavorablemode of adsorptionof a water molecule over the All site leading to octahedral coordinationas reported in ref 8.
to be mobile around a specific T site. The average interaction energy of water with each site is also given in Table 2. The interaction of the water molecule with different T sites is dependent on the 0-T-0 angle (shown in Table l), and the net electron density values are an indication of the strength of this interaction. The net electron density on various atoms is also included in Table 2. The average interaction energy of water is more favorable for the All site rather than the A12 and A13 sites, although all the values are positive. The observed positive values for adsorption energy may partly be due to an artifact of the small cluster size considered to represent the tetrahedral sites and the representative T-water distance of 0.15 nm uniformly used here. However, we are comparing the same parameter calculated for different sites on a relative basis. The incoming water can approach the All site through three planes, namely, 413, 412, and 312. Out of these three modes, the approach of water along the41 3 plane, where the 0-A1-0 angle is a maximum of 169O,is the most favorable mode of adsorption. The hydrogen atom of the water molecule lies parallel to the 03-All-04 bonds. The experimental position of the water molecules shows that there are simultaneously two water molecules present leading to octahedral coordination at All as shown in Figure 1. As far as the A12 and A13 sites are concerned, the average 0-A1-0 angles are larger than in the All site and there is no significant variation in the 0-T-O angles of different planes in these sites as in the case of the All site. Thus, the results of our calculation confirm
that the water molecule is located near the All site and it approaches the All site specifically through the 41 3 plane. The average distances of all the seven water molecules from the three A1 sites are also given in Table 2, which also indicate that water molecules are closer to the All site. Particularly, two of the water molecules lie very close to the All site. Similar calculations were also carried out to study the interaction between the water molecules and the three crystallographically distinct P sites. The results of these calculations are presented in Table 3. The average interaction energy of water is more favorable for the P2 site than the P1 and P3 sites. Unlike in the All site, none of the 0-P-0 angles are large enough to lead to octahedral coordination. The average distance of water from the P sites is also given in Table 3. Correlations to the XRD and MAS NMR Studies. From the results presented in Table 2, it can be observed that the average adsorption energy of water in the interacting planes of the All site is favorable. Similarly the results presented in Table 3 indicate that the average adsorption energy of water in the interacting planes of the P2 site is favorable. The average distance of the water molecules from the different T sites reported froman X-ray diffraction study are also given in Tables 2 and 3. It is seen that the water molecules are closer to the All and P2 sites. The 31P MAS N M R spectrum of VPI-5 shows three signals approximately in the ratio of 1:l:l; the most shielded signal at -33 ppm has been attributed to P in the 4-T ring (P1 site). The other two signals at -27 and -23 ppm are attributed to P in the 6-T ring (P2 and P3 sites).7JO-*3 Davis et ~1.13have speculated that the origin of one of the above two signals is the interaction of "immobile" water molecules. H e et al.24 have given supporting evidence to this. Perez et al.12have shown by 31PMAS N M R studies that different P sites of the VPI-5 framework are differently affected by the water environment to yield separate resonances. In dehydrated VPI-5, the two signals coalesce to a single signal a t -27 Hence it is reasonable to assume an interaction of water molecules with the P site in the 6-T ring. Based on our modeling studies, we have observed that the interaction energy of water molecules with the P2 site is favorable and the average distance to the water molecules is the least (Table 3). P2 is specifically closer to two water molecules, which are also close to All. Hence, we assign the peaks at -23 and -27 ppm to P2 and P3 sites, respectively. However, based on heteronuclear correlation spectroscopy, Engelhardt and Veemanzs have assigned the -23.7 ppm peak to the P3 site and the -27.5 ppm peak to the P2 site (Le., reversal). Our water adsorption studies contradict this new assignment. The formation of octahedrally coordinated aluminum by the interaction of two water molecules with the A1 1 site has been proved beyond doubt by 27AlMAS N M R ~ t u d i e s . ~ From the average net charges on the T sites given in Tables 2 and 3, it is possible toqualitatively correlate the MAS N M R chemical shift to the polarizability. The net charges on oxygen atoms of the framework, which lie close to the water molecules, have less charge and hence cause more polarization around TO4.
Prasad and Vetrivel
1582 The Journal of Physical Chemistry. Vol. 98, No. 6, I994
I
+11.76b
I
I
\
t 23.35 30.0
30.0
36.0
54.0
42.0
78.0
54.0
48.0
102.0
60.0
150.0
126.0
TEMPERATURE ("C) Figure 2. TGA curves of hydrated VPI-5 at different heating rates: (a) 10 OCemin-l, (b) 5 OC-min-l, (c) 1 "C-min-', and (d) 0.5 OC.min-l. The inset
shows the expanded form of curve d. Thermal Analysis. The TGA traces for VPI-5 a t different heating rates are shown in Figure 2. Earlier we had found26 that there is no considerable weight loss above 150 OC, though the experiment was continued up to 600 O C . Hence, the TGA curves are shown up to 150 OC for the different heating rates. The total weight loss corresponds to 23.15% which compares with an earlier report27 of 23.40% weight loss in the temperature range 25-190 "C. The expanded trace is given for the heating rate of 0.5 OCSmin-1 in the range 3 0 6 0 OC as an inset in Figure 2. It can be observed that significant weight loss is complete in the range 30-60 OC and there are seven distinct steps of weight loss. Each step of weight loss corresponds approximately to 3.3%. We attribute these weight losses to the seven different water molecules present in the structure of VPI-5.8 Janchen et al.28have reported a two-step isotherm for the adsorption of water from calorimetric measurements. They obtained a distinct step after 12 water molecules/unit cell (UC), corresponding to a dihydrate species. Rocha et al.29 and Goldfarb et al.30 have also reported only two types of discrete water molecules, namely, framework water molecules (12/UC) and nonframework water molecules (30/ UC). The peaks attributed to HzO(1) and H20(II) in Figure 2 almost merge, and this is due to the fact that the water molecules I and I1 are strongly bound to All. These two water molecules are closer to the P2 site than the other water molecules. All the other five water molecules are hydrogen bonded to HzO(1) and H2O(II), and they lie almost a t the center of the 18-T pore of VPI-5. Of these five water molecules, the last one which is most weakly bound has to be at the center of the channel linking the helices to one another. Figure 3 shows the water molecule network inside the large pore of VPI-5. In fully hydrated VPI-5, where there are six sets of seven water molecules per unit cell, there is practically no void space. Because of the hexagonal symmetry, the top layer containing three sets of seven water molecules is just repeated in the second layer but with 60' rotation along the axis passing through the center of the 18-T pore. The dimension of void space
AL
@
P
0
FRAMEWORK 0
0
WATER 0
(Top L o y e r )
0
WATER 0
(Bottom Layer)
Figure 3. Reduction in the dimension of the 18-Tpore due to the presence of seven types of water molecules in the form of a triple helix. Theactual diameter of the void formed due to the removal of various types of water is given in Table 4.
in VPI-5 with different levels of water are shown in Table 4. Actually, there is no uptake of xenon by the fully hydrated VPI-5 (Figure 4), but after the I step of water desorption, the xenon uptake is as high as 50%. From the values given in Table 4, it is evident that after removal of the most weakly bound water, the water chain is ruptured and a void space of 0.41 nm is generated,
Interaction of Water with VPI-5
TABLE 4 Content'
The Journal of Physical Chemistry, Vol. 98, No. 6, 1994 1583
Void Space Variation in VPI-5 with Water
temperature
water content in VPI-5
("C)
P3Al301y7H20 (fully hydrated) P3A1301y6H20 P3Al301y5H20 P3Al301y4H20 P3Al3012.3H20 PaA1301y2HzO P3A1301ylHzO P3Al3012 (fully dehydrated)
35.0 39.0 42.0 45.0 51.0 54.0 60.0
diameter of void water being cumulative formed removed as weight loss (A) in Figure 3 (%) 0.3 none 4.1 5.8 7.4
0
2.7 6.2 8.5 11.2 14.7 19.3 23.2
111, VI
8.0 10.0 12.0
I, Diameter of 02-is considered as 2.7
W
VI1 V IV I1
I
A.
VPI-5 (23.2)
200
400
600
800
1000
PRESSURE (Torr)
Figure 4. Adsorption of xenon over VPI-5 samples at different levels of hydration. The numbers in the parentheses indicate the percent weight loss during dehydration.
thus accommodating the xenon molecules. This is expected since the void space available is too small (Figure 3 and Table 4) in the fully hydrated VPI-5. The TGA weight loss corresponding to step 1 is 2.7%. The bonds connecting the helices are ruptured creating a void space. The void formed for VPI-5(2.2) hence corresponds to 0.41 nm (Figure 3 and Table 4), and thus a xenon atom is able to diffuse into the channel. On progressive dehydration, the xenon uptake increases, but not markedly even for a fully dehydrated VPI-5(23.2). Owing to the neutrality of the framework and hence the absence of an electric field, VPI-5 has a low adsorption potential for xenon unlike zeolite molecular sieves. Hence, it is possible that the effective pore diameter of VPI-5 could be altered by carefully adjusting its hydration level.
Conclusions The exact location of water molecules in the hydrated form of VPI-5 and the nature of their interaction with the framework are identified. The proximity of water to All and Pz sites observed by XRD and MAS N M R studies are rationalized by the water interaction energy calculations. The interaction energy calculations are emerging as a reliable and an efficient method for the interpretation of the observed N M R signals. The net charge density calculated over the T sites is useful to assign the N M R
signals to corresponding crystallographically distinct sites. Careful thermal analysis experiments have shown the stepwise release of seven different kinds of water molecules inside the VPI-5 lattice. The removal of the most loosely bound HzO(VI1) creates a void space of 0.41 nm, thus accommodating xenon into the VPI-5 channel.
Acknowledgment. This work was partly funded by UNDP. One of us (S.P.) acknowledges the National Science Council of Taiwan for financial support. We are grateful to Dr. P. Ratnasamy, NCL, and Dr. S.-B. Liu, IAMS, for the encouragement to pursue this work. References and Notes (1) Davis, M. E.; Saldarriaga, C.; Montes, C.; Garces, J. M.; Crowder, C. E. Nature 1988, 331, 698. (2) Crowder, C. E.; Garces, J. M.; Davis, M. E. Adu. X-ray Anal. 1988, 32, 507. (3) Parton, R. F.; Uytterhoeven, L.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1991, 59, 395. (4) Gugel, A.; Mullen, K.; Reichert, H.; Schmidt, W.;Schon,G.; Schuth, F.; Spickermann, J.; Titman, J.; Unger, K. Angew. Chem., Int. Ed. Engl. 1993, 32, 556. (5) Anderson, M. W.; Shi, J.; Leigh, D. A.; Moody, A. E.; Wade, F. A.; Hamiton, B.; Carr, S.W. J. Chem. Soc., Chem. Commun. 1993, 533. (6) Rudolf, P. R.; Crowder, C. E. Zeolites 1990, 10, 163. (7) Grobet, P. J.; Martens, J. A.; Balakrishnan, I.; Mertens, M.; Jacobs, P. A. Appl. Catal. 1989, 56, L21. (8) McCusker, L. B.; Baerlocher,Ch.; Jahn, E.; Bulow, M. Zeolites 1991, 11, 308. (9) Grobet, P. J.; Samoson, A.; Geerts, H.; Martens, J. A,; Jacobs, P. A. J . Phys. Chem. 1991, 95, 9620. (10) van Braam Houckgeest, J. P.; Kraushaar-Czarnetski, B.; Dogterom, R. J.; de Groot, A. J. Chem. SOC.,Chem. Commun. 1991, 666. (1 1) Rocha, J.; Kolodziejski,W.; He, H.; Klinowski, J. J . Am. Chem. Soc. 1992, 114, 4884. (12) Perez, J. 0.;Chu, P. J.; Clearfield, A. J . Phys. Chem. 1991,95,9994. (13) Davis, M. E.; Montes, C.; Hathaway, P. E.; Arhancet, J. P.; Hasha, D. L.; Garces, J. M. J. Am. Chem. SOC.1989, 111, 3919. (14) Prasad, S.; Balakrishnan, I. Inorg. Chem. 1990, 29, 4830. (15) Martens, J. A,; Feijen, E.; Lievens, J. L.; Grobet, P. J.; Jacobs, P. A. J . Phys. Chem. 1991, 95, 10025. (16) Annen, M.; Young, D.; Davis, M. E.; Cavin, 0. C.; Hubbard, C. R. J . Phys. Chem. 1991,95, 1380. (17) Poojary, D. M.; Perez, J. 0.;Clearfield, A. J. Phys. Chem. 1992,96, 7709. (18) Prasad, S.;Balakrishnan, I.; Vetrivel, R. J . Phys. Chem. 1992, 96, 3096. (19) Prasad, S.; Vetrivel, R. J. Phys. Chem. 1992, 96, 3092. (20) Wu, Y.;Chmelka,B. F.;Pines,A.;Davis, M. E.;Grobet, P. J.; Jacobs, P. A. Nature 1990, 346, 550. (21) Dewar, M. J. S.;Thiel, W. J . Am. Chem. SOC.1977, 99, 4899. (22) Mulliken, R. S.J . Chem. Phys. 1955, 23, 1833. (23) Vogt, E. T. C.; Louwen, J. P. In Extended abstracts andprogram of Ninth International Zeolite Conference; Montreal, Canada, July 5-10, 1992; Higgins, J. B., et al., Eds.; Buttenvorth-Heinemann: Stoneham, 1992; RP108. (24) He, H.; Kolodziejski, W.; Klinowski, J. Chem. Phys. Lett. 1992,200, 83. (25) Engelhardt, G.; Veeman, W. J . Chem. SOC.,Chem. Commun. 1993, 622. (26) Prasad, S.;Gunjikar, V. G.; Balakrishnan, I. Thermochim.Acta 1991, 105, 265. (27) Ojo, A. F.; McCusker, L. B. Zeolites 1991, 11, 460. (28) Janchen, J.; Stach, H.; Grobet, P. J.; Martens, J. A.; Jacobs, P. A. In Extended abstractsandprogram of Ninth InternclrioMlZeolite Conference; Montreal, Canada, July 5-10, 1992; Higgins, J . B., et a l . , Eds.; Butterworth-Heinemann: Stoneham, 1992; A28. (29) Rocha, J.;Kolodziejski, W.;Gameson, I.; Klinowski, J. Angew. Chem., Int. Ed. Engl. 1992, 31, 610. (30) Goldfarb, D.; Li, H. X.;Davis, M. E. J . Am. Chem. Soc. 1992,114, 3690.
