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Computational Study for Water Sorption in AlPO4-5 and AlPO4-11 Molecular Sieves Renjith S. Pillai and Raksh V. Jasra*,† Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research, Bhavnagar-364 002, India. †Present Address: Reliance Technology Group, Reliance Industries Limited, Vadodara Manufacturing Division, Vadodara-391346, India. Received May 2, 2009. Revised Manuscript Received November 29, 2009 The unusual water adsorption behavior in aluminophosphate molecular sieves AlPO4-5 and AlPO4-11 was studied using canonical Monte Carlo and periodic density functional theory (DFT) calculation. The number of adsorbed water molecules per cavity ranging from 1 to 12 were located inside the molecular sieves by canonical Monte Carlo simulation methods using a “compass” forcefield. The DFT calculations were done for optimizing each structure with and without adsorbed water molecules employing generalized gradient approximation with the Perdew-Burke-Ernzerhof exchange-correction functional. Both classical and quantum mechanical calculations have exhibited hydrogen bonding between adsorbed water molecules inside the main 12-membered ring. The Al-O-P angles were observed to decrease after adsorbing water molecules in geometry optimized AlPO4-5 and AlPO4-11 molecular sieves. DFT calculations illustrate that the initial loading of water in the large cavity is due to the mild acidity in the framework but the isobaric increase in loading is due to the abundant hydrogen bonding between adsorbed water at higher water loading.

1. Introduction AlPO4-5 and AlPO4-11 members of aluminophosphate molecular sieve family have alternating tetrahedra of Al and P linked together to form a microporous structure with one-dimensional ring channels.1-3 Unlike zeolites, AlPO4-5 and AlPO4-11 have a neutral framework with mild hydrophilicity. Aluminophosphates are interesting materials having potential as catalysts, catalyst supports, and molecular sieves in the chemical industry.4-8 Interest in the water-aluminophosphate interaction arises from the fact that water sorption shows an unusual isotherm in these materials. Water sorption isotherms in microporous solids like zeolites are generally of type-I as per BDDT classification. However, in AlPO4-5 and AlPO4-11, the water isotherm is reported9 to be very low at the initial relative pressure followed by a near isobaric rise in the relative pressure range of 0.25 to 0.30. This unusual sorption behavior of water was explained9 by two step water sorption, i.e., initially water sorption occurring in 6-membered ring channels, followed by capillary condensation in large 12-membered ring channels. Similar sorption isotherms for water have been reported for other aluminophosphate molecular *To whom correspondence should be addressed. E-mail: rvjasra@gmail. com; [email protected]. Tel.: þ91 265 6693935. Fax: þ91 265 6693934.

(1) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146–1147. (2) Wilson, S. T.; Oak, S.; Lok, B. M.; Flanigen, E. M. U.S. Patent 4,310,440, 1982. (3) Lee, S.; Raja, R.; Harris, K. D. M.; Thomas, J. M.; Johnson, B. F. G.; Sankar, G. Angew. Chem., Int. Ed. 2003, 42, 1520–1523. (4) Lischke, G.; Parlitz, B.; Lohse, U.; Schreier, E.; Fricke, R. Appl. Catal., A 1998, 166, 351–361. (5) Weckhuysen, B. M.; Rao, R.; Martens, J. A.; Schoonheydt, R. A. Eur. J. Inorg. Chem. 1999, 565–577. (6) Cora, F.; Alfredsson, M.; Barker, C. M.; Bell, R. G.; Foster, M. D.; Saadoune, I.; Simperler, A.; Catlow, C. R. A J. Solid State Chem. 2003, 176, 496–529. (7) Pastore, H. O.; Coluccia, S.; Marchese, L. Annu. Rev. Mater. Res. 2005, 35, 351–360. (8) Hartmann, M.; Kevan, L. Chem. Rev. 1999, 99, 635–664. (9) Newalkar, B. L.; Jasra, R. V.; Kamath, V.; Bhat, S. G. T. Microporous Mesoporous Mater. 1998, 20, 129–137.

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sieves, viz., AlPO4-11, -17, -20, and -31. Wilson et al.10 have attributed such a behavior to the mild hydrophilicity, whereas Thamm et al.11 have proposed a capillary condensation phenomenon in secondary pores of the AlPO4-5 structure to explain the observed behavior. Goldfarb et al.12 have investigated the water sorption using solid state NMR and have shown a reversible conversion of tetrahedral to octahedral Al coordination upon hydration. Furthermore, they distinguished two types of water molecules, namely the sorbed water molecules as those coordinated to framework Al and physisorbed molecules isotropically reoriented within the channel of AlPO4-5. Lachet et al.13 have explained the step adsorption isotherm of CH4 in AlPO4 at 77 K in terms of local molecular rearrangement away from the idea of condensation of gas into a liquid. Fuchs and coauthors14,15 have recently illustrated the step adsorption isotherm of water in hydrophobic zeolites such as silicalites-1 using molecular simulation. The unusual step in water adsorption isotherms at about p(H2O) = 8 Torr reported by Olson et al.16 for HZSM-5 samples having higher silica/alumina ratio in predominantly hydrophobic samples was attributed to the adsorption at framework silanol groups. It was further observed that this adsorption step reflects weak interaction as it occur at a higher relative pressure and also disappears for isotherms measured at temperatures higher than 40 °C. (10) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannan, T. R.; Flanigen, E. M. In Intra Zeolite Chemistry; Stucky, G. D., Dwyer, F. G., Eds.; ACS Syp. Ser. 218; American Chemical Society: Washington, DC, 1983; p 79. (11) Thamm, H.; Stach, H.; Jahn, E.; Fahlke, B. Adsorp. Sci. Technol. 1986, 3, 217–224. (12) Goldfarb, D.; Li, H. X.; Davis, M. E. J. Am. Chem. Soc. 1992, 114, 3690– 3697. (13) Lachet, V.; Boutin, A.; Pellenq, R. J. -M.; Nicholson, D.; Fuchs, A. H. J. Phys. Chem. 1996, 100, 9006–9013. (14) Cailliez, F.; Stirnemann, G.; Boutin, A.; Demachy, I.; Fuchs, A. H. J. Phys. Chem. C 2008, 112, 10435–10445. (15) Trzpit, M.; Soulard, M.; Patarin, J.; Desbeins, N.; Cailliez, F.; Boutin, A.; Demarchy, I.; Fuchs, A. H. Langmuir 2007, 23, 10131–10139. (16) Olson, D. H.; Hagg, W. O.; Borghard, W. S. Microporous Mesoporous Mater. 2000, 35-36, 435–446.

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DOI: 10.1021/la902629g

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Cailliez et al.14 reported an interesting molecular simulation study on water sorption in nanoporous silicalite having heterogeneous surface hydrophilicity. Nanoporous hydrophobic silicalite expectedly shows a type-V water adsorption isotherm. However, with the introduction of silanol defects on the surface, hydrophilic paches are formed in the structure resulting in heterogeneous surface configuration consisting of hydrophilic patches in a hydrophobic surface. It is concluded from simulation study of hydration in heterogeneous silicalite nanopores that such a configuration is thermodynamically stable in a wide range of reduced pressures. It is also concluded that such materials display water adsorption isotherms which is a combination of type-I indicating strong adsorption of a limited amount of water at a very low pressure and type-V isotherm that displays a plateau in the intermediate pressure range followed by a rather sudden filling of the nanopore volume. Floquet et al.17 using incoherent quasi-elastic neutron scattering (IQENS) explained that during water sorption in AlPO4-5, the growth of two helices ice-like structures of water forms from the initial sorbed water and is driven by the structural commensurability with the AlPO4-5 channel structure. Despite these efforts, the unique shape of the water sorption isotherm in AlPO4’s is far from clearly understood. Canonical Monte Carlo (CMC) simulation followed by the use of highly accurate quantum mechanical ab initio methods such as density functional theory (DFT)18,19 is a promising method to have an insight on the sorption behavior in aluminophosphate. The macroscopic properties such as isosteric heat, energetically favorable sorbate location inside the adsorbent cavities can be determined using canonical ensemble Monte Carlo simulation.20,21 DFT is widely used method for electronic structure calculation in chemistry and solid state physics because the approximations used in this theory were refined to better mode of exchange and correlation interactions. In many cases the results of DFT calculations of solid state systems agree quite satisfactorily with the experimental data. Furthermore, the computational costs are low as compared to traditional methods which are based on using the many electron wave functions like the Hatree-Fock theory.22,23 Theoretical calculations have so far been performed on AlPO4-5 and AlPO4-11 mainly to predict their acidic nature and structure of water inside the cavities.24,25 However, few theoretical studies are reported to understand the sorption phenomena in aluminophosphates.18,19,24-27 For example Fois et al.27 have examined the conformation of the triple helix formed by adsorbed water inside the VPI-5 aluminophosphate using Car-Parrinello ab initio molecular dynamic simulation. They have shown that rather flexible helices are formed in proximity to the VPI-5 channel walls with flexibility arising due to high thermal motion of water molecules. In the present paper, an attempt is made to understand the water adsorption mechanism (17) Floquet, N.; Coulomb, J. P.; Dufau, N.; Andre, G. J. Phys. Chem. B 2004, 108, 13107–13115. (18) Elanany, M.; Vercauteren, D. P.; Kubob, M.; Miyamoto, A. J. Mol. Catal. A: Chem. 2006, 248, 181–184. (19) Saadoune, I.; Cora, F.; Catlow, C. R. A. J. Phys. Chem. B 2003, 107, 3003– 3011. (20) Ramsahye, N. A.; Bell, R. G. J. Phys. Chem. B 2005, 109, 4738. (21) Pillai, R. S.; Peter, S. A.; Jasra, R. V. Langmuir 2007, 23, 8899–8908. (22) Koch, W.; Holthausen, M. C. A Chemist’s Introduction to Density Functional Theory; Wiley-VCH Verlag GmbH: Weinheim, Germany, 2001. (23) Accelrys, Materials Studio Release Notes, Release 4.1; Accelrys Software, Inc.: San Diego, CA, 2006. (24) Cora, F.; Catlow, C. R. A.; D’Ercole, A. J. Mol. Catal. A: Chem. 2001, 166, 87–99. (25) Saadoune, I.; Cora, F.; Alfredsson, M.; Catlow, C. R. A. J. Phys. Chem. B 2003, 107, 3012–3018. (26) Elanany, M.; Koyama, M.; Kubo, M.; Selvam, P.; Miyamoto, A. Microporous Mesoporous Mater. 2004, 71, 51–56. (27) Fois, E.; Gamba, A.; Tilocca, A. J. Phys. Chem. B 2002, 106, 4806–4812.

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in AlPO4-5 and AlPO4-11 molecular sieves using canonical Monte Carlo simulation and DFT calculations.

2. Computational Methods The Materials Studio software (Accelrys Software Inc., USA) with crystal builder in MS vizaulizer was used to build the crystal structure of AlPO4-5 and AlPO4-11 molecular sieves. The Materials Studio with Sorption module,28 utilizing canonical ensemble (fixed loading) Monte Carlo (MC) simulation methods was used for computation of energetically most favorable adsorbate position in the adsorbent structure. The simulations were performed on an IBM Mpro workstation running on windows platform. DFT was employed for optimization of water adsorbed AlPO4-5 and AlPO4-11 using MS CASTEP in IBM x3950 server running on Red Hat Linux Advanced Enterprise platform. 2.1. Monte Carlo Simulation Study. The crystallographic data of Qui et al.29 and Richardson et al.30 were used to construct the crystal structure of AlPO4-5 and AlPO4-11, respectively. Connolly surface was calculated using a water molecule as a probe in order to estimate the accessible micropore volume.31 Connolly surface with vdW scale factor 1.4 A˚ is overlaid on adsorbent structure, so that it is possible to visualize the accessible volume in different channels. The COMPASS forcefield was used to find the most energetically favorable adsorption site for water molecules in aluminophosphates cavities. Electrostatic interactions were modeled by Coulombic forces. van der Waals, London, and hydrogen-bonding interactions were modeled by a 6-9 Lennard-Jones potential.31-33 The functional forms of this forcefield are of the consistent forcefield (CFF) type. Charges and bonded terms were derived from Hatree Fock/6-31G* calculations, while the nonbonded parameters were initially transferred from the polymer consistent forcefield (pcff) and optimized using molecular dynamic (MD) simulations of condensed-phase properties. (The details of CFF and pcff forcefield are included in the Supporting Information.) The COMPASS force field was validated by various authors using molecular mechanical calculation and molecular dynamic simulation on a number of isolated molecules, liquids, and crystals.23 The calculated molecular structure, vibration frequencies, conformational properties for isolated molecules, crystal cell parameters and density, liquid density, and heat of evaporation agreed favorably with most experimental data.28,33,34 The canonical Monte Carlo (CMC) simulations were carried out for both AlPO4-5 and AlPO4-11 with 1, 3, and 12 water molecules in one unit cell at 303 K. As reported from experimental data after 3 molecule/unit cell, an isobaric increase of loading to 12 molecule/unit cell was observed. All these simulations were performed with fixed loading method in MS Sorption28 at 303 K using one unit cell of each model with a typical number of Monte Carlo (MC) steps ranging from 4 to 5  106. The evolution of the total energy over the MC steps was plotted in order to monitor the equilibration conditions. (The energy distribution profiles are included in the Supporting Information.) The Ewald summation was used for calculating electrostatic interactions, and the shortrange interactions were calculated with a cutoff distance of 15 A˚. The aluminophosphate structures were assumed to be rigid during the sorption process and were maintained fixed in their initial crystallographic data positions. Ideally simulation with flexible framework, especially at higher water loading, is desirable. However, we could not do that, as the MS sorption software package with us does not support such simulations. However, quantum mechanical methods like DFT are useful for identifying the inter (28) (29) (30) (31) (32) (33) (34)

Sorption. Materials Studio v4.2 Accelrys, Inc. San Diego, CA, 2006. Qiu, S.; Pang, W.; Kessler, H.; Guth, J. L. Zeolites 1989, 9, 440–444. Richardson, J. W.; Pluth, J. J.; Smith, J. V. Acta Cryst. B 1988, 44, 367–373. Fleys, M.; Thompson, R. W. J. Chem. Theory. Comput. 2005, 1, 453–458. Sun, H.; Ren, P.; Fried, J. R. Comput. Theor. Polym. Sci. 1998, 8, 229–246. Sun, H. J. Phys. Chem. B 1998, 102, 7338–7364. Rigby, D.; Sun, H.; Eichinger, B. E. Polym. Int. 1998, 44, 311–330.

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Figure 1. Schematic diagram of AlPO4-5 along the [100] plane. The atoms are colored as follows: pink for aluminum, violet for phosphorus, red for oxygen, and white for hydrogen.

atomic and intermolecular distances effectively and have been applied in this study. Studies on structural features of the framework upon adsorption using ab initio DFT have earlier been reported.22-24 It may be noted that to understand the macroscopic adsorption properties canonical and grand canonical Monte Carlo Simulation are widely used.13-15,20,21,31 However, the structural features of the adsorbent framework upon adsorption cannot be thoroughly understood in classical simulations followed by us as these simulations are mainly based on empirical parameters. It may further be noted that the Monte Carlo algorithm in MS Sorption28 used in this study treats the adsorbent framework as a fixed simulation box and does not give an accurate determination of the transition state.28 2.2. DFT Calculations. CASTEP a quantum mechanics based program that employs the density functional theory (DFT) plane-wave pseudopotential method to perform firstprinciple quantum mechanics calculations and explore the properties of crystals and surfaces in materials was used for the present quantum mechanical calculations.28,35,36 Ultrasoft pseudopotentials were used to describe the interactions of ionic core and valence electrons.37 Generalized gradient approximation (GGA) within the Perdew-Burke-Ernzerhof (PBE) scheme was employed to evaluate exchange-correlation energy according to the method described by White and Bird.38 To reduce computational cost, a kinetic energy cutoff of 300 eV was used for plane wave expansions in reciprocal space. The BFGS optimization method was used to find the ground state of primitive cells optimization.39 Total energy change was finally reduced to less than 0.2  10-5 eV/atom, and Hellman-Feynman forces acting on atoms were converged to less than 0.05 eV/A˚.

3. Results and Discussions 3.1. Canonical Monte Carlo Simulation of Water Sorption in AlPO4-5. The adsorption sites for water in AlPO4-5 were simulated with 1, 3, and 12 number of water molecules per unit cell in view of earlier experiment studies9 of water sorption in AlPO4-5, wherein an isobaric increase in loading ranging from 3 to 12 molecules/unit cell at 303 K was reported. In the AlPO4-5 unit cell, there are 24 equivalent tetrahedral sites (T) equally shared by P and Al. Besides, there are three types of oxygen: O1 and O3 connecting Al and P to form 12-membered rings (12 MRs) and O2 connecting the 12-membered rings with the 4-membered rings (4-MRs) and 6-membered rings (6-MRs) sheets along the c (35) Marzari, N.; Vanderbilt, D.; Payne, M. C. Phys. Rev. Lett. 1997, 79, 1337– 1340. (36) Poulet, G.; Sautet, P.; Tuel, A. J. Phys. Chem. B 2002, 106, 8599–8608. (37) Vanderbilt, D. Phys. Rev. B 1990, 41, 7892–7895. (38) White, J. A.; Bird, D. M. Phys. Rev. B 1994, 50, 4954–4957. (39) Pfrommer, B. G.; Cote, M.; Louie, S. G.; Cohen, M. L. J. Comput. Phys. 1997, 131, 133–140.

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direction. The schematic diagram of AlPO4-5 is presented in Figure 1. Canonical Monte Carlo simulation studies show that the water molecules are located only in the 12-membered ring channels. Figure 2 shows the configuration of adsorbed water molecules in the super cell [2  2  2] structure of AlPO4-5 along the c direction. The simulated structures show that with one water molecule loaded in the AlPO4-5 unit cell, the water molecule is hydrogen bonded with framework oxygen atoms. However, in the case of three water molecules adsorbed per unit cell, one of the water molecules in the large 12-membered cavity is hydrogen bonded with framework oxygen atom and the other two are hydrogen bonded between themselves. It is clear from the classically simulated structure of adsorbed water in AlPO4-5 that the framework oxygen atoms due to its mild hydrophilic nature provide an interaction with water at lower water adsorption; however, as loading increases to three water molecules, the interaction sites changes to the electronegative oxygens of sorbed water molecules. Therefore, as the relative pressure increases, the water molecule interacts strongly with initially adsorbed water molecules.9,27 The pores in AlPO4-5 at initial partial pressure are filled by water molecules interacting with framework oxygen atoms followed by an isobaric increase of loading which is due to extended hydrogen bonding to fulfill the chain of hydrogen bonded network with the initially adsorbed water. At lower loading water molecules are located close to the walls of the framework of the main AlPO4-5 channels as represented in Figure 2a, which has also been demonstrated by the neutron diffraction studies.17 Figure 2c shows a unidimensional chain of water molecules in large cavity due to the abundant hydrogen bonded network. Figure 3 shows the super cell [2  2  2] of AlPO4-5 with water loaded in one-dimensional channel along c direction controlled by 12 MRs. 3.2. DFT Studies of Water Sorption in AlPO4-5. The unit cell of AlPO4-5 with and without water molecules was geometry optimized using periodic DFT calculations. The AlPO4-5 structure for optimization without a water molecule was taken from the literature,29 and water loaded structures were simulated structure of canonical Monte Carlo study. Figure 4 shows a thin section of accessible solvent surfaces on super cell [2  2  2] structure of optimized AlPO4-5 unit cell at a probe radius of 1.4 A˚. This also shows that water molecules can be observed in main 12membered ring channels only. The occupied volume, free volume and surface area calculated using the probe radius of water in energy minimized and experimental crystallographic structures are shown in Table 1. Because of the small size of water molecules (2.8 A˚), the accessible volumes were obtained in large 12-membered ring straight pores. Figure 5 shows the energy minimized [2  2  2] super cell structures of AlPO4-5 with 0, 1, 3, and 12 number of adsorbed water molecules/unit cell. These energy minimized structures show that as loading in main cavity increases the distance between framework oxygen and hydrogen of water as well as hydrogen bonding between water molecules is observed to decrease. The optimized cell parameters are reported in Table 2, which show that cell length is increased slightly as 0.5, 0.57, and 0.26 A˚ for a, b and c, respectively, due to the water sorption. The shortest distance between hydrogen and framework oxygen is 3.368 A˚ in AlPO4-5 with 1 water molecule/unit cell. In quantum mechanically energy minimized structures of water loaded AlPO4-5, the hydrogen bond was observed to disappear even though canonical Monte Carlo simulation showed an existence of the hydrogen bonding between framework and water. Both classical and quantum mechanical calculation have exhibited hydrogen bonding between adsorbed water molecules inside the DOI: 10.1021/la902629g

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Figure 2. [2  2  2] super cell structure of AlPO4-5 unit cell with a different number of adsorbed water molecules (a) 1, (b) 3, and (c) 12 molecule/u.c. at 303 K.

Figure 3. Simulated super cell [2  2  2] structure of AlPO4-5 showing water loaded one-dimensional large cavity.

Figure 4. Accessible solvent surface of super cell [2  2  2] structure of energy minimized AlPO4-5 unit cell.

12-membered ring. Furthermore, two types of hydrogen bonds with angles ∼160° and ∼170° are obtained (not shown in figure) inside 12-membered ring, and these hydrogen bond angles are retained both at the lower and higher water loading in AlPO45 unit cell. The Al-O-P angle in water sorbed AlPO4-5 was observed to decrease by ∼2.2° compared to that without water inside the channel of the energy minimized unit cell structures. Therefore, the observed decrease in Al-O-P angle on water adsorption means that there is a decrease in acidity in AlPO4-5 on water adsorption. Nur and Handman40 had reported that a larger Al-O-P angle leads to a stronger acidity in AlPO’s. This was also reported by Elanany et al.18 from their study on tetravalent metal doped AlPO4-5. For a single water molecule loaded unit cell of AlPO4-5 shows a decrease of 2.2° in the water molecule facing the Al-O-P angle with respect to that Al-O-P angle in an energy minimized dehydrated AlPO4-5 unit cell structure. This results in framework oxygen atoms having the shortest distance from the hydrogen atom of the sorbed water molecule and a

Table 1. Occupied Volume, Free Volume, and Surface Area of AlPO45 Unit Cell Computed Using Water As a Probe

(40) Nur, H.; Hamdan, H. Mater. Res. Bull. 2001, 36, 315–322.

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unit cell of AlPO4-5 experimental theoretical

occupied volume (A˚3)

free volume (A˚3)

surface area (A˚2)

1230.22 1337.36

154.05 191.56

143.08 164.77

Table 2. Optimized Cell Parameters of AlPO4-5 and Its Water Loaded Structure cell length (A˚)

AlPO4-5 (literature) AlPO4-5 AlPO4-5(1H2O) AlPO4-5(3H2O) AlPO4-5(12H2O)

cell angle

a

b

c

R

β

γ

13.726 14.242 14.239 14.243 14.269

13.726 14.242 14.244 14.245 14.296

8.484 8.723 8.729 8.735 8.748

90 90.086 90.031 90.036 90.193

90 90.015 89.966 89.970 89.955

120 120.227 120.243 120.211 120.357

decrease in acidity of the framework. Energy minimized AlPO4-5 structures having 3 and 12 adsorbed water molecules per unit cell have Al-O-P angle values similar to the one observed with 1 Langmuir 2010, 26(3), 1755–1764

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Figure 5. Selected shortest distance of hydrogen with framework oxygens and the selected Al-O-P angle in the [2  2  2] super cell structure of energy minimized unit cell of (a) AlPO4-5, (b) AlPO4-5(1H2O), (c) AlPO4-5 (3H2O), and (d) AlPO4-5 (12H2O).

Figure 6. Fermi level orbital at isovalue 2  10-4 electrons/A˚ (a) AlPO4-5 and (b) AlPO4-5 (1H2O).

water molecule/unit cell. This also shows that as the water adsorption amount increases, the water oxygen atom coordinates with initially adsorbed water only. DFT calculations illustrate that the initial loading of water in the large cavity is due to the mild hydrophilicity in the framework but the isobaric increase in loading is due to the abundant hydrogen bonding between adsorbed water molecules. Moreover, the radial distribution results obtained from molecular dynamic (MD) simulation in water adsorbed one-dimensional VPI-5 also reported27 that bound water forms strong hydrogen bonds with free water and interacts only weakly with framework oxygens, and also forms an extended network of strong hydrogen bonds between free water molecules. The Fermi level orbital of the AlPO4-5 unit cell with and without adsorbed water (Figure 6) at iso value of 2  10-4 electron/A˚ also shows the homogeneity of the orbital is distorted during water adsorption. The most energetic orbital (Fermi level orbital) of the water sorbed AlPO4-5 shifted from framework oxygen to more electronegative oxygen atoms in the initially adsorbed water molecules. The partial density of states (PDOS)23 of the s-orbital shows a slight change in energy after water sorption in AlPO4-5 as shown in Figure 7. Furthermore, the Langmuir 2010, 26(3), 1755–1764

Figure 7. s-orbital partial density of states of AlPO4-5 and AlPO45(12H2O).

PDOS of s-orbital (Figure 7) shows an additional peak in water adsorbed AlPO4-5 at an energy of -6.3 eV which could be due to the hydration of aluminophosphate molecular sieves. The PDOS DOI: 10.1021/la902629g

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Pillai and Jasra Table 3. Occupied Volume, Free Volume, and Surface Area of AlPO411 Unit Cell Computed Using Water As a Probe

Figure 8. Partial density of states for hydrogen atom in AlPO45(1H2O), AlPO4-5(3H2O), and AlPO4-5(12H2O).

of the hydrogen atom at different water loading is shown in Figure 8. An additional peak is obtained at -4.0 eV in the case of 3 and 12 adsorbed water molecule/u.c., which might be extra s character developed due to the hydrogen bonding between water molecules. The Fermi level orbital and PDOS data show the strong interaction of the water molecule at lower loading with aluminophosphate framework. 3.3. Canonical Monte Carlo Simulation of Water Sorption in AlPO4-11. The AlPO4-11 structure has 10 tetrahedral units, and channels are formed along the [001] direction with an orthorhombic unit cell. AlPO4-11 aluminophosphate is formed by an alternating series of TO4 units, where T is the tetrahedral aluminum or phosphorus atom. These tetrahedral sites are linked by oxygen bridges. Canonical Monte Carlo (CMC) simulation was done to predict the water adsorption sites inside the AlPO4-11 molecular sieve. The fixed loading calculations were carried out with adsorption of 1, 3, and 12 water molecule/unit cell at 303 K. CMC studies of water sorption in AlPO4-11 show that water molecules at lower loading (1 water molecule/unit cell) consist of hydrogen bonds with framework oxygen inside the large cavity [Figure 9a]. At higher loading an extra hydrogen bonding network is formed between the water molecules. As loading increases the hydrogen bond length slightly decreases [Figure 9, panels b and c]. Figure 10 shows the presence of abundant network of hydrogen bonded water molecules in unidimensional large cavity of AlPO4-11 super cell [2  2  2] structure. 3.4. DFT Studies of Water Sorption in AlPO4-11. The periodic DFT calculations of the AlPO4-11 unit cell and its hydrates are reported elsewhere.26,41,42 In the presence of water reduction of b, the unit cell parameter is observed.41 To understand the configuration of the adsorbed water molecule inside the cavity, we have carried out the periodic DFT calculations using its primitive cell of AlPO4-11 [Al12P12O64] in order to reduce the computational expense. Table 3 shows occupied volume, free volume, and surface area of energy minimized primitive cell of AlPO4-11 using water as a probe. Figure 11 indicates the water (41) Peeters, M. P. J.; de Haan, J. W.; van de Ven, L. J. M.; van Hooff, J. H. C. J. Phys. Chem. 1993, 97, 5363–5369. (42) Herrera-Pe0 rez, G.; Zicovich-Wilson, C. M.; Ramırez-Solıs, A. J. Phys. Chem. C 2007, 111, 9664–9670.

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AlPO4-11 (primitive cell)

occupied volume (A˚3)

free volume (A˚3)

surface area (A˚2)

experimental theoretical

1014.27 1105.99

50.13 71.20

94.76 113.35

accessible pores of AlPO4-11. It shows that only the 12-membered ring cavity can accommodate the water molecules. The CMC simulation for water molecules in the primitive cell of AlPO4-11 was carried out based on the experimental observation that there is an isobaric increase in water loading from 3 to 12 molecules/unit cell. Figure 9b shows that the water molecules at lower loading are dispersed only in one cage. As the primitive cell of AlPO4-11 has one large cavity, it would be appropriate to calculate the water molecules in one large cavity to reduce the computational expense. The periodic DFT calculations were carried out for 1, 3, and 6 number of adsorbed water molecules per primitive cell and the optimized structures are shown in Figure 12. All optimized structures have shown hydrogen bonds between framework oxygen and between water molecules. The hydrogen bond lengths are changed as loading increased inside the large cavity. The framework oxygen containing the hydrogen bonded Al-O-P angle is decreased by ∼1.92° with one adsorbed water molecule per primitive cell. As a result, acidity of aluminophosphate decreases40 as observed in the case of water sorption in AlPO4-5.41The decrease in Al-O-P angle is maintained at higher water adsorption loading as obtained in 1 molecule/primitive cell as shown in Figure 12b. The acidity is reduced due to the initial loading indicating that the initial adsorption of water molecules in AlPO4-11 is because of a mild hydrophilicity of framework. An isobaric increase observed at water adsorption of 3-12 molecules/unit cell could be due to the extended hydrogen bonding between the water molecules inside the cavity. The hydrogen bond angle is 173.845° at water adsorption of molecules/primitive cell; however, at water adsorption of 3 molecules, it decreases to 168.767°. This type of hydrogen bonding network during water sorption in VPI-5 aluminophosphates was also reported by Fois et al.27 from molecular dynamic simulation studies. They have reported the formation of two well-defined layers of water molecules located at 1-2 and 4-5 A˚ from the center of the cavity of VPI-5. The water layer near the surface of VPI-5 corresponds to the flexible water helices. The Fermi level orbital with and without water sorption is given in Figure 13 at iso value of 2  10-4 electron/A˚ show homogeneity of AlPO4-11 orbital, which gets distorted post water sorption. The most energetic orbital (Fermi orbital) of the water sorbed AlPO4-11 changed from oxygen atoms of framework to oxygen atoms of water molecules upon water sorption. Figure 14 shows the partial density of states s-orbital of all atoms in AlPO411 and AlPO4-11(6H2O). The PDOS of hydrogen atom at different loading have minor changes after adsorption of water (Figure 15), which would be due to the hydrogen bonding. An extra peak is formed in the hydrogen PDOS of AlPO4-11 at higher loading and three molecules per primitive cell except in the case of one water molecule loaded structure. 3.5. Water Structure Inside the Channels of AlPO4-5 and AlPO4-11. In our earlier study,9 the water sorption at lower adsorption loading in both AlPO4-5 and AlPO4-11 was assumed to occur in the 6-membered hexagonal nanopores as schematically shown in Figure 16. Floquet et al.17 have studied neutron diffraction of water molecules located inside the AlPO4-5 and shown that the unusual behavior of water is due to the structural Langmuir 2010, 26(3), 1755–1764

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Figure 9. Simulated adsorption sites of water loaded in unit cell of AlPO4-11 with hydrogen bonding (a) 1, (b) 3, and (c) 12 molecule/u.c.

Figure 10. Simulated super cell structure of AlPO4-11 with water showing one-dimensional large cavity.

Figure 11. Accessible solvent surface of energy minimized AlPO411 primitive cell.

characteristics of AlPO4-5 framework porosity. Therefore, the structural characteristics of both AlPO4-5 and AlPO4-11 could be the main reason for the isobaric increase of water sorption in these materials. The trend in water sorption behavior in the initial region of the isotherm is the consequences of the presence of mild hydrophlilicity in the AlPO structure. However, a sharp rise in water uptake beyond this stage indicates the incidence of hydrophilic character on the same surface leading to facile water sorption.9,10 Similar observations of unusual isobaric water adsorption step in nanopores of high silica ZSM-5 and silicalite have been discussed in the literature.14,16 For example, an unusual Langmuir 2010, 26(3), 1755–1764

step in water adsorption isotherms reported by Olson et al.16 for HZSM-5 samples having higher silica/alumina ratio was attributed to the presence of structural defects (probably silanol groups) in predominantly hydrophobic samples. Cailliez et al.14 reported more insightful molecular simulation studies on water sorption in nanoporous silicalite wherein they have shown that on introducing silanol defects on the surface of hydrophobic silicalite hydrophilic patches are formed in the structure resulting in a heterogeneous surface configuration. They have further confirmed from a simulation study of hydration in heterogeneous silicalite nanopores that such a configuration is thermodynamically stable in a wide range of reduced pressure. It is well established that hydrophobic silicalite will show a type-V water adsorption isotherm, i.e., a plateau in the intermediate pressure range followed by a rather sudden filling of the nanopore volume. On the other hand, strongly hydrophilic solids such as zeolites show a type-I water adsorption isotherm indicating strong adsorption of limited amount of water at a very low pressure. Cilliez et al.14 from simulation studies have concluded that predominantly hydrophobic heterogeneous surfaces having patches of hydrophilicity display water adsorption isotherms which is a combination of type-I and type-V isotherms. Peeters et al.43 have observed a transition of the crystal phase in the case of AlPO4-11 upon hydration. However, the possibility of such a transition of the crystal phase in AlPO4-5 on hydration was not observed from the X-ray diffraction pattern of its hydrated state.43 The unit cell of AlPO4-5 contains 12 alternating tetrahedral oxide units each of Al and P. The structure consists of unidimensional 12-, 6-, and 4-membered ring channels parallel to the c axis, and the internal space of the 12-membered ring channel is created by the network of six 6-membered windows throughout the channel length. The 12- and 6- membered ring channels have pore diameters of 7.3 and 3.0 A˚, respectively. However, AlPO4-11 crystallizes in the orthorhombic system, and it has a 10membered ring channel, which is surrounded by 4-membered ring channels. The geometry optimization of the AlPO4-5 unit cell with adsorption of one water molecule in the 6-membered hexagonal nanopore cavity was carried out using DFT calculations, and the energy minimized structure is represented in Figure 17. This shows that higher binding energy is required to stabilize the water molecule in 6-membered rings than in the main 12-membered ring (43) Peeters, M. P. J.; de Haan, J. W.; van de Ven, L. J. M.; van Hooff, J. H. C. J. Phys. Chem. 1993, 97, 8254–8260.

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Figure 12. Selected Al-O-P angle and shortest distance between framework oxygen and hydrogen of water in (a) AlPO4-11, (b) AlPO411(1H2O), (c) AlPO4-11(3H2O), and (d) AlPO4-11(6H2O).

Figure 13. Fermi level orbital at isovalue 2  10-4 electrons/A˚ in (a) AlPO4-11 and (b) AlPO4-11(1H2O).

Figure 14. s-orbital partial density of states of AlPO4-11 and AlPO4-11(6H2O).

channel. The adsorption energy, ΔE (ΔE = EAlPO4-5(1H2O) EAlPO4-5 - EH2O) obtained from the periodic DFT calculations of adsorption of water molecule in the 6-membered ring and 12membered ring are 0.98 and 13.08 kJ/mol, respectively. The 1762 DOI: 10.1021/la902629g

Figure 15. Partial density of states of hydrogen atom in AlPO4-11 (1H2O), AlPO4-11(3H2O), and AlPO4-11(6H2O).

hydrogen bond length and hydrogen bond angles are 2.235 A˚ and 135°, respectively, which is shorter than the experimentally determined17 hydrogen bond length of water molecules in Langmuir 2010, 26(3), 1755–1764

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AlPO4-5. A water molecule inside the 6-membered cavity is more strained than a molecule in the 12-membered cavity. Therefore, the possibility of a water molecule getting sorbed inside the 6-membered ring channel has to be ruled out. From the recent

Figure 16. Schematic representation of the water location in the 6-membered ring channel of AlPO4-5.

Figure 17. [2  2  2] super structure of energy minimized AlPO45 unit cell with one water molecule adsorbed in narrow cavity.

Figure 18. Partial density of states of the hydrogen atom in AlPO4-5(1H2O).

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study,17 the calculated neutron diffractogram of the D2O confinement in the 6-membered ring secondary hexagonal nanopore had not predicted the experimental diffraction data at 4 D2O molecules in AlPO4-5. The partial density of states of hydrogen in one adsorbed water molecule with AlPO4-5 at main 12-membered and 6-membered ring channels is shown in the Figure 18, which shows a slight difference in s-orbital character of hydrogen in water. Therefore, the water molecules in the 12-membered ring channel and the 6-membered ring channel are having different types of coordination with framework oxygen. The water sorption can occur in the main 12membered ring hexagonal channel of AlPO4-5 molecular sieves. At initial stages of water adsorption in aluminophosphate molecular sieves, the water molecules are sorbed at the position close to the walls of the main 12-membered rings, but on increasing adsorption capacity, water extends to the center of the 12-membered ring. The simulated water density at lower and higher water loading in AlPO4-5 is observed only in main 12-membered ring channels as represented in Figure 19. It is clear from the figure that water molecules are positioned from the walls of 12-membered rings in lower loading and further the water molecules are accumulated at the center of the channel. Figure 20a shows a canonical Monte Carlo simulated chain of water molecules arranged in the hexagonal ice structure in the main 12-membered ring channel. This structure shows two chains of 12 water molecules hydrogen bonded with the inner surface oxygen by involving small angular deformation ((10). A two helix arrangement of water molecules inside the AlPO4-5 channels is comparable to the triple helix of water inside the VPI molecular sieve shown by Fois et al.27 They have explained the growth of the helicoidal water structure in the pores of VPI5 by both a static and dynamic point of view, which indicates the hydrogen bond network leads to the ice structure in the layer of molecules near the surface and close to the channel center. The strong octahedral coordination of water molecules is the basic factor, which drives the formation of water chains inside the VPI-5. As compared to the other microporous structures like zeolite, the VPI-5 aluminophosphate pores are too small to form a liquid phase at the center of the pore by hydrogen bonding. This specifies that properties like pore width, hydrophilicity, host-guest dipolar interactions, etc. have a crucial role in the formation of an ice structure in the aluminophosphate molecular sieves.27,44 The quantum mechanically optimized structure shows a helical water structure in AlPO4-5; the ice structure is formed due to the hydrogen bonding of water molecule as represented in Figure 20b. However, the optimized structure of water loaded AlPO4-11 shows a hexagonal ice structure inside the main 10-membered ring channels, making hydrogen bonding with framework oxygen as well as with the water molecules as represented in

Figure 19. Simulated water density field in [2  2  2] super cell structure of AlPO4-5 at (a) lower and (b) higher regime. Langmuir 2010, 26(3), 1755–1764

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Figure 20. Hexagonal ice-like structure inside the 12MRs of AlPO4-5: (a) simulated and (b) optimized.

Figure 21. Hexagaonal ice-like structure inside the 10MRs of AlPO4-11.

Figure 21. The “capillary condensation” phenomenon corresponds to the growth of a dense ice, the structure of which is a double helix commensurate with the AlPO4-11 channel structure.

4. Conclusions Canonical Monte Carlo and periodic DFT calculation were used to explore water adsorption inside the cavities of aluminophosphate molecular sieves AlPO4-5 and AlPO4-11. The DFT calculation used a gradient corrected and PBE exchange correlation function for periodic optimization of water sorbed in AlPO4(44) Fois, E.; Gamba, A.; Tabacchi, G.; Quartieri, S.; Vezzalini, G. J. Phys. Chem. B 2001, 105, 3012–3016.

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5 and AlPO4-11 molecular sieves. The simulated adsorption configurations of water molecules in both the aluminophosphate and its geometry optimized structures clearly show the existence of water in large 12-memebred cavities. The geometry optimized structure of a water molecule inside the 6-membered ring of AlPO4-5 is more strained than that in the 12-membered ring. The Al-O-P angle is decreased on water adsorption, which leads to a reduction in the acidity of AlPO4-5 and AlPO4-11 molecular sieves at initial loading of water inside the cavity of aluminophosphate. Therefore, the initial water sorption could be driven by the affinity of the aluminophosphate framework. The water molecules are hydrogen bonded with framework oxygen at lower loading. However, at higher water loading, water molecules inside the large cavities are made of hydrogen bonded chain. Therefore, the steep rise in water sorption in both AlPO4-5 and AlPO4-11 aluminophosphate molecular sieves is due to the extending hydrogen bonds started from the initially adsorbed water molecules inside the large cavity. Acknowledgment. We appreciate the financial support from Council of Scientific and Industrial Research (CSIR), New Delhi. R.S.P. thanks CSIR for financial support as a Senior Research Fellow. We also thank Dr. Casten Menke and Dr. Naseem A. Ramshye Accelrys, E.U. for their fruitful discussion on DFT calculations. Supporting Information Available: Short outline for CFF and pcff forcefield. Fractional coordinates of geometry optimized molecular sieves. Energy distribution profile obtained from CMC. The PDOS plots of hydrated and dehydrated aluminophosphate molecular sieves. This material is available free of charge via the Internet at http://pubs. acs.org.

Langmuir 2010, 26(3), 1755–1764