Periodic DFT Studies of AlPO-11: The Role of Hydration on Structural

Jun 13, 2007 - Instituto Tecnológico Superior de Irapuato, Carretera Irapuato-Silao Km. 12.5, Irapuato, Guanajuato, 36821 México, Departamento de FÃ...
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9664

J. Phys. Chem. C 2007, 111, 9664-9670

Periodic DFT Studies of AlPO-11: The Role of Hydration on Structural Properties G. Herrera-Pe´ rez,† C. M. Zicovich-Wilson,‡ and A. Ramı´rez-Solı´s*,§,| Instituto Tecnolo´ gico Superior de Irapuato, Carretera Irapuato-Silao Km. 12.5, Irapuato, Guanajuato, 36821 Me´ xico, Departamento de Fı´sica, Facultad de Ciencias, UniVersidad Auto´ noma del Estado de Morelos, AV. UniVersidad 1001, CuernaVaca, 62209 Me´ xico, and Department of Chemistry & Biochemistry, UniVersity of California at Santa Barbara. Santa Barbara, California 93106-9510 ReceiVed: March 12, 2007; In Final Form: May 1, 2007

Periodic density functional theory B3LYP calculations are performed to study the structure of AlPO-11 and the possible effects of hydration on three different crystallographic T-sites. Previously tested atomic basis sets were used, and full geometry optimizations were done within the Ibm2 space group for anhydrous and for T1-, T2-, T3-hydrated structures. A detailed comparison of the relevant geometric parameters is done for the B3LYP optimized structures with and without hydration. The most stable ideal hydration coordinates water molecules to the T1-site and induces a significant change in the b cell parameter with respect to the anhydrous case, thus leading to better agreement with the experimental b value. However, improving the atomic basis sets and correcting for basis set superposition error (BSSE) yields energy differences between the various hydration situations on the order of 1 kcal/mol. This suggests that thermal mobility of water molecules inside the large pore of AlPO-11 will play a significant role in determining the average structure.

I. Introduction The aluminophosphates of general AlPO4‚nH2O composition were first synthesized by Wilson.1,2 These compounds are crystalline materials with features resembling those of zeolites, thus forming an important family among the molecular sieves. The microcrystalline structure of aluminophosphates is formed by an alternating series of TO4 units, where T is a tetrahedral aluminum (AlO4)-1 or phosphorus (PO4)+1 atom. These tetrahedral sites are linked by oxygen bridges (Al-O-P), and ionic alternance provides thus a neutral crystalline network. Aluminophosphates (AlPO’s) have special properties that make them good to be used as hydrophilic adsorbents and as supports for catalysts, and their primary structure makes them excellent catalysts themselves for isomerization and oxidation of light paraffins.3 These features arise from their well-defined pore size and a high degree of crystallinity. They differ from zeolites in that AlPO’s do not have extraframework ions to compensate the superficial ionic charge but, nevertheless, they also show superficial acidity. The spatial structure of AlPO-11 corresponds to the AEL ideal crystalline topology, which has 10 tetrahedral units (10T). Pore geometry is elliptic with major and minor axes of around 6.5 and 4.0 Å, respectively. Channels are formed along the [001] direction (growth in c) with an orthorhombic unit cell of Imma symmetry. The formation of the crystalline structure (PBU) is defined by repeating the 4T units along the axis, thus giving rise to the 10T pore with five 6T units4 (see Figure 1). The study of the crystallographic structure of aluminophosphates leads to the conclusion that some important factors can * Corresponding author. E-mail:[email protected]. † Instituto Tecnolo ´ gico Superior de Irapuato. ‡ Universidad Auto ´ noma del Estado de Morelos. § University of California at Santa Barbara. | On sabbatical leave from Depto. de Fı´sica, Facultad de Ciencias, Universidad Auto´noma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, Cuernavaca, Mor., CP 62209, Me´xico.

make the unequivocal identification of a structure a very difficult task. In particular, the degree of hydration has been a key issue that plays a crucial role in the definition of the microstructure, since it has been possible to identify changes in the geometry of the unit cell, changes in the symmetry space group,5 or, in some cases, even changes in the coordination number of Al.6 The crystalline structure of AlPO-11 (calcined as well as rehydrated) has been identified by the refinement of data obtained with pulsed-neutron diffraction experiments. These analyses reveal three types of crystallographic sites for Al sitting, T1/T2/T3 in 2:2:1 proportion, respectively.7 It has been shown,8 using several experimental techniques such as IR, NMR, and TGA-DTA, that water adsorption in AlPO-11 is a reversible process. When water is present, a reduction of the b unit cell parameter is observed.9 Meinhold et al.6 have shown (using XRD) that hydrated AlPO-11 has an orthorhombic bodycentered space group Ima2 with a ) 17.619, b ) 13.325, and c ) 8.406 Å, differing from its supposedly anhydrous values of a ) 17.940, b ) 13.833, and c ) 8.079 Å.9 As a consequence of the diminished symmetry, Peeters et al.9 have shown, using 27Al and 31P NMR, that five different T-sites (with 1:1:1:1:1 proportion) are generated in hydrated AlPO-11.5 Also we note that, from the crystallographic point of view, some striking inconsistencies have been found such as too short P-O distances (1.31 Å) and too large T-O-T angles (175°).7 Previous theoretical calculations have been performed on AlPO’s. We note in particular the periodic DFT studies of Cora` et al.,10 where the structure and acidity of AlPO-34 were addressed for divalent metal-ion-substituted networks. However, no theoretical study has, to the best of our knowledge, ever addressed the structure of AlPO-11. From a general perspective, it is important to determine, in a precise manner, the reference structure of the basic AlPO-11 (unsubstituted), since many experimental studies have addressed the Al intranetwork substitution for a range of metal ions such as Mg, Ca, Fe, Co, and Zn, among others.11

10.1021/jp072004x CCC: $37.00 © 2007 American Chemical Society Published on Web 06/13/2007

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Figure 1. Crystal chemical data [Al20P20O80]-AEL, orthorhombic crystalline system, channels along [0 0 1] and 10T ring viewed.

Since hydration seems to be a key factor in the determination of the structure of AlPO-11, we shall, as a first step, concentrate here on providing a truly reliable theoretical structure for anhydrous AlPO-11 and compare this with the three possible fully hydrated ideal structures. However, we recall that since our methodology relies on periodic (thus perfectly crystalline) structures, the resulting structures are relevant for describing only complete symmetric hydration on each of the studied T-sites, namely on T1-, T2-, and T3-sites, all coordinating water molecules to Al atoms. Since our periodic (static) study is done at 0 K, this means that no statistical factors are considered and the possible medium- or long-range fluctuations that are present in the real hydration patterns cannot be taken into account. Although the relative hydration energies have been corrected for the basis set superposition error (BSSE), they do not take into account the smaller zero-point energy (ZPE) corrections, which might also be important in the real system. In spite of the fact that several approximations are made in the physical models used to study hydration (most of them concerning the ideal static perfect hydration), this study provides fundamental insights on the role of water in the modification of the basic anhydrous structure of AlPO-11. II. Method and Computational Details Periodic density functional theory B3LYP calculations with localized atomic Gaussian basis sets were done for AlPO-11. This semi-empiric exchange-correlation hybrid functional has been chosen because it has proven to yield results in excellent agreement with experimental data for periodic DFT studies on other AlPO’s.12 The geometry optimizations started from the XRD refined structure of AlPO-117,13 and were done without any restrictions imposed except for those of the corresponding space group (Ibm2). We chose to use the z-axis as the pore axis; the original XRD structures were rotated according to this choice. The parallel version of CRYSTAL0614 code was used with the optimized Gaussian-type basis sets for Al, O, and P atoms taken from Cora´ et al.15 (later referred to as a B1 basis set), which have been successfully used for AlPO-34. The initial hydrated structures were built by placing the correct number of water molecules coordinated to the various T(1, 2, or 3)-sites of the anhydrous optimized structure. We recall here that the number of water molecules in each case is determined by the symmetry requirements of the Ibm2 space group. It is worth noting that CRYSTAL06 fully exploits the space symmetry in

the periodic calculations; therefore, the consideration of symmetric systems allows both significant savings in computational resources and ease in the interpretation of the results. All hydrated structures were fully optimized also within the same space group, as well as the anhydrous material. The primitive anhydrous unit cell contains 60 atoms and, even with the smallest basis set used here (B1), a 31G(p) for H and those reported in ref 15 for Al, P, and O, this yields a total of 920 Gaussian basis functions and 600 electrons per primitive unit cell. Concerning the numerical parameters of the CRYSTAL calculations, 21 k-points were used for sampling the Brillouin Zone (shrinking factor 4),16 the standard default tolerances for the integral evaluations and optimization thresholds are the default ones as given in the CRYSTAL manual.14 The exchangecorrelation functional is integrated numerically over a pruned grid of points. The integration is performed using 75 radial and 434 angular points, generated through the Gauss-Legendre and Lebedev schemes, respectively. Geometry optimizations were carried out using analytical gradient methods as implemented in CRYSTAL06, where the atomic positions as well as the cell parameters are simultaneously optimized. Richer basis sets (named B2 basis from here onward) with additional diffuse s and p orbitals on Al (88-31G (d))17 and O (8-411G(d))18 with the same exponents as B1 for H and P were used to make better estimates of the hydration energies by means of single-point calculations with the geometries obtained at the B3LYP/B1 level. In order to improve the accuracy in the energy estimation, new tolerances (6 6 6 7 14) for the integral evaluation in terms of AO and a denser grid (75 974 points) for the exchange-correlation integration have been considered. Given the rather small values of the hydration energies, these have also been corrected for basis set superposition error (BSSE) using both basis sets. III. Results and Discussion A. General. It should be stressed that these calculations have been made using the space group reported in the literature for AlPO-11; therefore, note that the optimized minimum energy geometries are symmetry-constrained and that other less symmetric structures might actually exist that belong to a subgroup of Ibm2. However this remains to be proved through more refined vibrational analysis (work in progress) that might lead to further full symmetry-unconstrained optimizations. Figure 2

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Figure 2. Optimized structures of AlPO-11: (A) anhydrous, (B) hydrated in T1-, (C) T2-, and (D) T3-sites. Projections along [0 0 1].

shows a schematic representation of the optimized anhydrous structure along the c axis. Three different hydrated structures have been built by placing four, four, and two water molecules coordinated to the T1-, T2-, and T3-sites, respectively. Figure 3 shows a pictorial representation of the three optimized hydration situations. Fully hydrated structures are obtained and the number of water molecules in each case simply reflects the stoichiometric relations of the T1/ T2/T3-sites inside the large pore of AlPO-11. The optimized geometries (unit cell or asymmetric units) of the anhydrous and hydrated structures are available upon request from the authors. We shall, from here onward, concentrate the discussion on the most important structural changes induced by the hydration on the different T-sites with respect to the anhydrous case. Table 1 shows the unit cell parameters (and volume) along with the overall energies for the anhydrous and the hydrated optimized structures. Table 2 shows the B3LYP-optimized T-O distances and Al-O-P angles compared with the available experimental data for AlPO-11.

B. Geometries: Analysis of Anhydrous vs Hydrated Structures. Let us first compare the ideal anhydrous structure with the experimental data of the calcined AlPO-11. From Table 1 it is clear that the optimized a and c unit cell parameters are in very good agreement with the experimental values, but the theoretical b value seems slightly larger (2.2%) than the experimental one. However, at this stage it is not possible to assert whether the presence of water has an overall lengthening or shortening effect in the b parameter with respect to the anhydrous case. Other possible reason for the discrepancy between the theoretical and the experimentally determined b parameters could come from the intrinsic inaccuracy of the exchange-correlation functional used here. However, two arguments can be given against such a possibility: (i) there is no a priori reason why such a defective exchange-correlation functional yields both a and c cell parameters in almost perfect agreement with experiment while leading to an overestimated b value, and (ii) it has been argued in ref 15 that only density functional calculations using the local density approximation

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Figure 3. Interaction of water molecules coordinated to (B) T1-, (C) T2-, and (D) T3-sites. Periodic optimized structure of AlPO-11 projections along [0-1 0].

yield a noticeable structural difference in this type of materials, with the usual underestimation (1-2%) of the lattice parameters due to an overestimation of the nonbonded (van der Waals) interactions. The latter interactions, however, modify the AlO-P angles but not the equilibrium bond distances. It is essential to recall that, using exactly the same basis sets as presently done, the more sophisticated hybrid exchangecorrelation B3LYP functional leads to optimized unit cell parameters that are within 0.04% of the experimental ones for berlinite (QUA-structured AlPO);15 this is a particularly important argument that supports the relevance of the present study. We naturally expect that, since we are dealing with the same type of bondings in AlPO-11, the B3LYP functional (and using the same atomic basis sets) this study provides the same kind of accuracy on this system. Now we focus on the analysis of intracell parameters. A quick look at Table 2 shows that the most important differences between the experimental vs theoretical results lie in the P-O and Al-O distances. In particular, we highlight the fact that the experimental P-O distances range from 1.31 to 1.66 Å, while the B3LYP values only range from 1.52 to 1.54 Å. Clearly, the 1.31 Å experimental value for the P(3)-O(3) pair is completely wrong, since even the simplest quantum-chemical cluster (3T) model for the local structure surrounding this bond predicts a much too high repulsion energy between the oxygen and phosphorus atoms at such a short internuclear distance. Next we focus on the Al-O distances, for which the experimental values range from 1.54 to 1.84 Å, while the B3LYP values range from 1.72 to 1.75 Å. Again we find a much too large variation of the experimental Al-O distances. Concerning the P-O-Al angles, it is immediately evident that the largest difference between the experimental and the theoretical anhydrous values is found for the P(1)-O(4)-Al(1) angle. The experimental value (131°) seems to be way too small, but note that it is not far (within 6°) from the value obtained with any of the three hydrated optimized structures.

In order to explain why there is such a large variation of Al-O and P-O distances in the experimental data, we note that the experimental intracell geometric parameters are actually obtained through a very indirect manner. They are secondary quantities derived from a set of interplanar distances, which are the primary truly reliable experimental data, averaged over long distances and recollecting signal intensity from hundreds or thousands of planes. The definition of interplanar distances involves several Al-O, P-O distances, Al-O-P, O-Al-O, and O-P-O angles at the same time, so that the Rietveld refinement provides a set of fitted interatomic distances and angles from averaged structures which are consistent with the whole set of interplanar distances obtained from the XRD data. We stress that contrary to what happens for these intracell parameters, the a, b, and c unit cell parameters are experimentally very well determined and, therefore, are much more reliable than the interatomic distances and angles. Concerning the hydrated structures, note that the presence of water molecules coordinated to any T-site in the pore does have an impact on the unit cell parameters. It is possible to distinguish three situations with respect to the anhydrous case: (i) one in which a and c increase while b decreasesshydration on T1-sites; (ii) one in which a and c decrease while b increasesshydration on T2-sites; and (iii) one where b and c increase while a remains practically unchangedshydration on T3-sites. Note that case (iii) implies the addition of only two water molecules per unit cell, half of what is considered for the other two coordination sites, simply due to symmetry requirements. Note that for cases (i) and (ii) the relative changes on the cell parameters are much larger, as compared with the anhydrous one, than for hydration on T3-sites. Once this has been pointed out, it seems that the slightly shorter b experimental value could actually be due to partially hydrated AlPO-11, preferentially over T1-sites. Recall that in the experimental case a small hydration is possible and that no absolute guarantee of perfect dehydration, even after calcination,

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Figure 4. Local geometries for the adsorbed water molecules on the different T-sites. Lines 1, 2, and 3 correspond to the T1-, T2-, and T3-sites. Columns A, B, and C correspond to the original anhydrous local geometry, deformed geometry with water adsorption, and closeup of the first neighbors of the hydrated T-sites, respectively.

TABLE 1: Comparison of Periodic DFT-B3LYP/B1 Optimized Unit Cell Parameters (Å) of Anhydrous vs Hydrated Structures (Differences with respect to anhydrous are in parentheses.) expta a ) 13.534 b ) 18.482 c ) 8.370 volume ) 2093.65 Å3 total energy (a.u.)b

anhydrous 13.516 18.892 8.364 2135.58 -8848.292276

T1-hydrated

T2-hydrated

T3-hydrated

13.692 (+0.176) 18.558 (-0.334) 8.475 (+0.111)

13.230 (-0.216) 19.264 (+0.372) 8.430 (-0.056)

13.488 (-0.028) 18.954 (+0.062) 8.425 (+0.061)

2153.43 -9153.877489

2148.43 -9153.855181

2154.00 -9001.086265

a Calcined from ref 7. b Total energy for the formula units [Al20P20O80], [Al20P20O80]‚4H2O, [Al20P20O80]‚4H2O, and [Al20P20O80]‚2H2O orthorhombic systems in the anhydrous, T1-hydrated, T2-hydrated, and T3-hydrated cases, respectively.

is possible given the extraordinary hygroscopic character of this material. In order to check if this is possible, we turn our attention now to the water adsorption energies of ideally hydrated structures. C. Local Geometries of the Adsorption Sites. Table 3 shows the parameters of the local geometry around the adsorption sites for the three cases. First of all, it should be noted that the concerned Al sites become 5-fold coordinated in all three cases. As shown on the values marked with an asterisk (/) in Table 3, for the three hydration sites, the Al-O bond located on the opposite side of the trigonal bipyramid becomes slightly longer

than the other three nearly in-plane Al-O bonds. Note that the Al(T)-OH2 distance is somewhat larger for the T1-case and is smallest for the T3-case. These local geometries are extracted from the fully unconstrained B3LYP/B1 optimized structures, but we stress again that the water H atoms have to comply with the space group determined for the anhydrous AlPO-11 material; therefore, this choice imposes some constraints on the geometry of the water admolecules. One might naturally ask if, for instance, the H-O-H angle may have unreasonable values imposed by the symmetry constraints. However, the values in Table 3 show that these constraints are soft enough to ensure

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TABLE 2: Most Relevant Intracell Geometric Parameters Obtained with Fully Unconstrained B3LYP/B1 Optimizations within the Experimentally Derived Space Group expta anhydrous

T1

T2

T3

P(1)-O(1) P(1)-O(4) P(1)-O(5) P(1)-O(6) P(2)-O(2) P(2)-O(5) P(2)-O(6) P(2)-O(7) P(3)-O(3) P(3)-O(7) P(3)-O(7) P(3)-O(8)

1.57 1.66 1.64 1.39 1.50 1.48 1.56 1.52 1.31 1.57 1.57 1.35

Distance (Å) 1.5377 1.5313 1.5363 1.5471 1.5395 1.5410 1.5413 1.5254 1.5245 1.5307 1.5307 1.5303

1.5360 1.5425 1.5431 1.5367 1.5477 1.5416 1.5303 1.5375 1.5224 1.5210 1.5210 1.5286

1.5386 1.5704 1.5361 1.5276 1.5357 1.5457 1.5385 1.5263 1.5344 1.5387 1.5287 1.5366

1.5388 1.5398 1.5388 1.5404 1.5381 1.5607 1.5400 1.5172 1.5227 1.5403 1.5403 1.5080

Al(1)-O(1) Al(1)-O(4) Al(1)-O(5) Al(1)-O(6) Al(2)-O(2) Al(2)-O(5) Al(2)-O(6) Al(2)-O(7) Al(3)-O(3) Al(3)-O(7) Al(3)-O(7) Al(3)-O(8)

1.64 1.84 1.58 1.60 1.68 1.74 1.54 1.67 1.80 1.69 1.69 1.63

1.7426 1.7325 1.7448 1.7525 1.7438 1.7431 1.7552 1.7239 1.7222 1.7308 1.7308 1.7280

1.7745 1.7833 1.7722 1.8182 1.7266 1.7459 1.7520 1.7174 1.7264 1.7359 1.7359 1.7416

1.7382 1.7616 1.7466 1.7456 1.7833 1.7822 1.8296 1.7434 1.7280 1.7450 1.7540 1.7274

1.7406 1.7426 1.7670 1.7355 1.7464 1.7529 1.7565 1.7060 1.7432 1.7700 1.7700 1.7911

P(1)-O(1)-Al(1) P(2)-O(2)-Al(2) P(3)-O(3)-Al(3) P(1)-O(4)-Al(1) P(1)-O(5)-Al(2) P(2)-O(5’)-Al(1) P(1)-O(6)-Al(2) P(2)-O(6’)-Al(1) P(3)-O(7)-Al(2) P(2)-O(7’)-Al(3) P(3)-O(8)-Al(3) Al(x)-OH2 O(4)-O(4) O(5)-O(5) O(7)-O(7) P(3)-Al(3) a

167 174 172 131 140 147 152 140 166 168 175

Angles (degrees) 144.174 145.996 142.254 146.291 175.957 176.106 148.154 137.088 145.253 151.844 142.626 138.765 132.058 133.694 135.920 134.021 156.130 167.372 159.103 159.964 175.957 170.431

139.615 147.745 170.461 125.966 136.437 143.837 133.694 134.021 167.372 159.964 170.431

TABLE 3: Local Geometric Parameters of the Water Adsorption T-Sites (The values corresponding to the Al-O and O-Al-O angles directly linked to the adsorption sites are marked with /.)

144.592 141.143 171.579 135.553 139.328 129.270 142.009 148.581 168.906 154.394 168.985

Hydration Parameters 2.1891 2.1415 2.1003 6.73 7.0373 7.0343 7.5584 7.0655 7.81 5.2689 5.4682 5.2555 5.2717 9.55 8.8441 9.1352 8.5942 9.2370 10.4464 10.7042 10.1811 10.3930

Calcined from ref 7.

that no unreasonable geometric parameters are actually forced either on the intramolecular geometry of water molecules, nor on the adsorption-induced O-Al-O angles. The largest changes (15°) of the O-Al-O angles around the adsorption sites with respect to the anhydrous reference structure are found for the T2- and T3-hydrations. Finally, in good accordance with the interaction energies (discussed in the next section), the O-H distances and the H-O-H angles of the adsorbed water molecules show very small changes (4°) with respect to the free molecule. D. Comparison of Hydration Energies on the Various T-Sites. In this section we discuss the hydration energies of the three T-sites and the effect of the basis set quality and the inclusion/exclusion of the BSSE. Table 4 shows the hydration energies obtained with the B1 and extended B2 basis sets, with and without the BSSE correction. In order to make a coherent comparison, Table 4 shows adsorption energies per water molecule on each of the T-sites. Therefore, the hydration energies are calculated as the difference between the optimized hydrated structure minus the sum of the optimized anhydrous

expt* anhydrous Al(1)-O(1) Al(1)-O(4) Al(1)-O(5) Al(1)-O(6) Al(2)-O(2) Al(2)-O(5) Al(2)-O(6) Al(2)-O(7) Al(3)-O(3) Al(3)-O(7) Al(3)-O(7) Al(3)-O(8) O(4) - O(4) O (1)-O(2)

1.64 1.84 1.58 1.60 1.68 1.74 1.54 1.67 1.80 1.69 1.69 1.63

Distance (Å) 1.742 1.774* 1.732 1.783* 1.744 1.772* 1.752 1.818* 1.743 1.726 1.743 1.745 1.755 1.752 1.723 1.717 1.722 1.726 1.730 1.735 1.730 1.735 1.728 1.741 4.18 4.24 6.36 5.37 Angles (degrees) 142.54 112.41 144.17 135.66 111.33 94.94 109.49 101.34 107.90 119.67 107.75 98.27 110.37 119.29 109.40 115.12 110.79 109.44 110.14 111.41 109.24 101.39 107.63 109.34 110.68 112.28 108.32 110.52 111.68 109.54 111.68 109.54 105.41 113.38 107.94 103.82 110.73 110.37 110.73 109.38

P(2)-O(2)-Al (2) P(1)-O (1)-Al (1) O(4)-Al(1)-O(6) O(5')-Al(1)-O(6) O(5')-Al(1)-O(4) O(1)-Al(1)-O(6) O(1)-Al(1)-O(4) O(1)-Al(1)-O(5) O(7)-Al(2)-O(6) O(5)-Al(2)-O(6) O(5)-Al(2)-O(7) O(2)-Al(2)-O(6) O(2)-Al(2)-O(5) O(2)-Al(2)-O(7) O(7)-Al(3)-O(8) O(7)-Al(3)-O(8) O(7)-Al(3)-O(7) O(3)-Al(3)-O(8) O(3)-Al(3)-O(7) O(3)-Al(3)-O(7) T(1,2,3)-OH2 H-O*H *H-OH

T1

T2

T3

1.738 1.761 1.746 1.745 1.783* 1.782* 1.829* 1.743* 1.728 1.745 1.754 1.727 4.23 4.86

1.740 1.742 1.767 1.735 1.746 1.753 1.756 1.706 1.743* 1.770* 1.770* 1.791* 4.21 5.00

147.35 139.65 108.57 111.30 107.14 106.81 114.50 110.34 97.98 92.58 124.20 99.88 117.10 114.06 110.34 110.34 102.52 114.70 110.54 107.41

Water-Related Distances (Å) 2.1891 0.9892 0.9706 0.9892 0.9753

2.1415 0.9677 0.9773

Water-Related Angles (degrees) H(1)-O(w)-H(2) 105.60 105.16 O(x) T(x) O(w)H(1) -35.55 -11.45 O(x) T(x) O(w)H(2) 79.40 98.35

141.14 144.59 108.54 107.54 110.40 109.39 110.06 110.06 113.85 107.44 107.38 109.95 109.12 109.12 95.24 95.24 121.23 94.57 118.25 118.25 2.1003 0.9703 0.9703 110.40 -65.24 65.24

a H* denotes the hydrogen atom participating in the hydrogen bond with the closest network oxygen. b Ox-Ty-OH2 angles, where Ox denotes the interlaminar O(1), O(2), and O(3) atoms, nearly perpendicular to the water-T hydration coordination axis.

TABLE 4: Hydration Energies (kcal/mol) per Water Molecule with Different Basis Sets, with and without BSSE Corrections method/hydration site

T1

T2

T3

B3LYP/B1 B3LYP/B1 with BSSE B3LYP/B2 B3LYP/B2 with BSSE

-10.10 -1.20 -4.40 -1.10

-7.50 1.80 -0.60 2.70

-11.43 -1.37 -3.10 -0.20

structure plus n times the energy of a single isolated water molecule (n ) 4 for T1- and T2- adsorption; n ) 2 for T3adsorption); this difference is finally divided by the corresponding value of n. The basis set superposition error corrected energies are calculated as

EadsBSSE ) {E(AlPO11+ nH2O) - [E(AlPO11)* E(AlPO11) + nE(H2O)* - nE(H2O)]}/n

9670 J. Phys. Chem. C, Vol. 111, No. 27, 2007 where E(AlPO11)* stands for the energy of AlPO-11 optimized hydrated geometry with the ghost basis set of the n adsorbed water molecules, and E(H2O)* stands for the energy of one of the space symmetry-equivalent water molecules (at their optimized adsorbed geometry) with the ghost basis set of all atoms that in the periodic structure are closer than 5.0 Å to the O atom. First of all, note that the B3LYP hydration energies are strongly dependent on the quality of the atomic basis sets used. For instance, with the B1 basis set, the adsorption preference yields T3 > T1 > T2; this trend remains even after inclusion of the BSSE correction with this basis set. However, when considering the larger B2 basis set, not only are the absolute values significantly reduced, but also, more importantly, the adsorption predominance trend is modified, now slightly favoring the T1-sites over the T3-sites, by only a 1.3 kcal/mol difference per water molecule. Again by applying the BSSE correction on the energies obtained with the larger basis set B2, the trend is conserved, but the adsorption energy difference between the T1- and T3-sites is further reduced to 0.9 kcal/ mol. Note that the hydration energies for the T2-case become positive after inclusion of the BSSE correction with both basis sets; physically this simply means that the repulsion between the intervening water molecules under such conditions would be greater than the overall energy gain related to their coordination on the T2-sites. Overall, our analysis reveals that, although the BSSE is essential to yield better quantitative hydration energies, basis set quality is more important to define the spatial adsorption trends, irrespective of whether the BSSE correction has been applied or not. At this point, we recall that adsorption of water molecules on T1-sites seems to have a shrinking effect on the b cell parameter with respect to the completely anhydrous AlPO-11. IV. Conclusions We have presented the results of periodic DFT calculations using a proven successful hybrid exchange-correlation functional to study the structure of AlPO-11 and the possible effects of hydration on three different crystallographic T-sites. Previously optimized atomic basis sets are used and full geometry optimizations were done within the Ibm2 space group for anhydrous and ideally T1-, T2-, T3-hydrated structures. A detailed comparison of the relevant geometric parameters is done for the B3LYP optimized structures with and without hydration. The theoretical optimized Al-O and P-O sets of interatomic distances show a very limited range of variation with and without hydration, while for the experimental ones, both ranges seem too large and physically unrealistic, especially the too short P-O (1.31 Å) and Al-O (1.54 Å) distances. The periodic B3LYP a and c cell parameters for the anhydrous case are in excellent agreement with experiment, while the experimental b value seems too short. Our explanation for this theoretical-experimental mismatch is that perhaps the calcined AlPO-11 was not completely anhydrous, allowing for the presence of some water molecules adsorbed, preferentially over T1-sites, inside the large pore. The most stable ideal hydration situation adsorbs water molecules on the T1-site and induces a significant change in

Herrera-Pe´rez et al. the b cell parameter with respect to the anhydrous case, making it 2% smaller than in the anhydrous case, in much better agreement with the widely used experimental data. Given the very hygroscopic nature of this material, we think this might actually be due to a not completely water-deprived sample. We are aware that a definitive theoretical proof can only come from refined finite-temperature ab initio molecular dynamics studies of hydrated AlPO-11 relaxing symmetry and exploring different stoichiometries. Since this would require long simulation times and involve even supercells of several dimensions to allow a statistical sampling with varying degrees of hydration, the unit cell being rather large (at least 60 atoms without any water), it is clear that such a study is presently out of reach even with the largest supercomputers available today. However, this careful and systematic study suggests, given the excellent theoretical-experimental agreement obtained previously for another member of the AlPO family with the present exchange-correlation functional/basis set combination, that the structural data widely used as reference for most AlPO11 studies actually corresponds to a partially hydrated structure. Acknowledgment. We thank the “FOMES2000 Co´mputo Cientı´fico” SESIC-SEP project for generous allocations of CPU time on the IBM-p690 supercomputer at UAEM. C.Z.W. and A.R.S. are thankful for support from CONACYT Grants No. 46983 and 45986-E, respectively. A.R.S. also acknowledges support from the UCMEXUS/CONACYT Program for a sabbatical year at UCSB. G.H.P. is thankful for support from Direccio´n de Investigacio´n y Postgrado de la Universidad de Guanajuato DINPO-UG through Project UG-CIQ-09-04. References and Notes (1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (2) Wilson, S. T.; Oak, S.; Lok, B. M.; Flanigen, E. M. U.S. Patent 4,310,440, 1982. (3) Hartmann, M.; Kevan, L. Chem. ReV. 1999, 99, 635. (4) Baerlocher, Ch.; Meier, W. M.; Olson, D. H. Atlas of Zeolite Framework Types; Elsevier: Amsterdam, 2001. (5) Barrie, P. J.; Smith, M. E.; Klinowski, J. Chem. Phys. Lett. 1991, 180, 6. (6) Meinhold, R. H.; Tapp, N. J. J. Chem. Soc., Chem. Commun. 1990, 219. (7) Richardson, J. W., Jr.; Pluth, J. J.; Smith, J. V. Acta Crystallogr., Sect. B 1988, 44, 367. (8) Prasad, S.; Vetrivel, I. R. J. Phys. Chem. 1992, 96, 3096. (9) Peeters, M. P. J.; de Haan, J. W.; van de Ven, L. J. M.; van Hooff, J. H. C. J. Phys. Chem. 1993, 97, 5363. (10) Saadoune, I.; Cora`, F.; Catlow, C. R. A. J. Phys. Chem. B 2003, 107, 3003. (11) Saadoune, I.; Cora`, F.; Alfredsson, M.; Catlow, C. R. A. J. Phys. Chem. B 2003, 107, 3012. (12) Cora`, F.; Catlow, C. R. A.; D’Ercole, A. J. Mol. Catal. A, Chem. 2001, 166, 87. (13) Pluth, J. J.; Smith, J. V.; Richardson, J. W., Jr. J. Phys. Chem. 1988, 92, 2734. (14) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; D’Arco P.; Llunell, M. CRYSTAL06 User’s Manual; Universita` di Torino, Italy, 2006. http://www.crystal.unito.it. (15) Cora`, F.; Catlow, R. J. Phys. Chem. B 2001, 105, 10278. (16) Monkhorst, H. J.; Pack, J. D. Phys. ReV. 1976, 13, 5188. (17) Montanari, B.; Civalleri, B.; Zicovich-Wilson, C. M.; Dovesi, R. Int. J. Quantum Chem. 2006, 106, 1703. (18) Civalleri, B.; Ugliengo, P. J. Phys. Chem. 2000, 104, 9419.