The Structural Directing Role of Water and Hydroxyl Groups in the

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The Structural Directing Role of Water and Hydroxyl Groups in the Synthesis of Beta Zeolite Polymorphs Yuriy G. Bushuev,* German Sastre, and J. Vicente de Juliań-Ortiz Instituto de Tecnologia Quimica, UPV - CSIC, AVda. de los Naranjos s/n, 46022 Valencia, Spain ReceiVed: August 10, 2009; ReVised Manuscript ReceiVed: October 6, 2009

The investigation of stability of as-made Beta zeolite polymorphs A, B, and C (BEA, BEB, and BEC) was carried out on the basis of lattice energy minimization and molecular dynamic atomistic simulations. The force field employed provided an excellent agreement between calculated and experimental enthalpies of formation of zeolites with respect to quartz. BEA and BEB are obtained at low water and KOH concentration in the initial mixture, while BEC forms at higher concentrations of both water and KOH. The four-component systems of unit cell composition (C12NH20+)6/zeolite/F-6/(H2O)l for BEA and BEB polymorphs and fivecomponent systems (C12NH20+)3/BEC/F-2/OH-/(H2O)l, (C18N2H302+)2/BEC/F-2/(OH-)2/(H2O)l were simulated for different water content, l. It was shown that the presence of water molecules stabilized the systems but to a different extent, in a way such that BEC is favored at higher water contents. Water amounts are rationalized in terms of the free volume available in the micropore, not totally filled by the organic. Defects have been found to form in BEC through interaction of hydroxyl anions with Si-O-Si linkages. The number and location of defects (silanol and siloxy groups) are in agreement with 29Si MAS NMR signals due to Q3 (Si) atoms. 1. Introduction A large number of new zeolites have been synthesized using organic molecules. Their role during the synthesis was initially explained by a templating effect,1-7 whose mechanism is under debate. A large templating effect was noticed when the shape of the molecule corresponded to the shape of the internal zeolite void surface. The van der Waals interaction energy between the organic molecule and the zeolite is used as a value for prediction of the templating activity. However, it was observed that some organic molecules can produce several zeolites depending on the synthesis conditions and, meanwhile, the zeolite may be synthesized with different organic molecules. Owing to the lack of a true templating effect, these molecules were rather called structure directing agents (SDAs).8 Zeolite Beta is a highly faulted intergrowth of two polymorphs, A (BEA) and B (BEB), which are normally found at a 60:40 ratio, with the initial gel concentration H2O/Si ) 7.5,9,10 and at a ratio of 15:85 with H2O/Si ) 15.11 The structure of the polymorph C (BEC) is closely related to those of polymorphs BEA and BEB and could be generated from polymorph BEA simply by the recurrent application to the building layers of a shear operation along both a and b axes. In this way, the space group of BEA (P4122; a ) b ) 12.66139 Å, c ) 26.40612 Å, 64 Si atoms in unit cell) is transformed into the more symmetrical polymorph BEC (P42/mmc; a ) b ) 12.6241 Å, c ) 13.12559 Å, 32 Si atoms in unit cell). BEB crystallized only with BEA and its crystallographic parameters were determined with low accuracy (C2/C; a ) b ) 17.70 Å, c ) 14.33 Å, β ) 114.89°). The three polymorphs have slightly different densities: 15.2 Si/(1000 Å3) (BEA); 15.7 Si/(1000 Å3) (BEB); 15.3 Si/(1000 Å3) (BEC). * To whom correspondence should be addressed. E-mail: yuriyb2005@ gmail.com. Permanent address: Ivanovo State University of Chemistry and Technology, Engelsa, 7, Ivanovo, Russia.

Only by changing the synthesis conditions by adding KOH and water in the initial mixture it is possible to synthesize BEC zeolite. It was noted that water concentration drastically changes the synthesis result.12,13 If reactions were carried out at low H2O/ SiO2 ratios, very open framework, all-silica molecular sieves could be produced. The direct dependence of the synthesis results on water content has stimulated us to suggest and prove the idea of the structural directing role of water molecules. All components of the initial mixtures are in the hydrated state. Dissolved ions have solvation shells whose composition differs from the composition of the mixture due to the effect of preferential solvation. This effect is enhanced during the silica polymerization and nucleation owing to the hydrophobic properties of the forming zeolite framework.14 Ions, occluded in cavities and channels during crystallization, may keep part of the hydration shells. The hydrophobicity of the SDA cations depends on their size and atomic charge distribution. As a rule, the smaller the charge density on the molecular surface, the larger the hydrophobicity. Studies of the different partitioning behavior of quaternary ammonium molecules between chloroform and water indicate that molecules with C/N < 10 strongly prefer water, while molecules with C/N > 16 strongly prefer chloroform.8 The all-silica zeolite frameworks show hydrophobic properties.15 Hydroxyl groups have strong hydrophilic properties, and it is reasonable to expect that most of them are hydrated at all of the stages during the hydrothermal zeolite synthesis. Several mechanisms of structural stabilization of zeolite frameworks by water molecules may be proposed. Water molecules in the framework fill the empty volume and mechanically prevent collapse of the structure. It was experimentally established that in some cases attempts of zeolite dehydration lead to the collapse of the crystal structure.16 The free energy of the zeolite is increased with the increase of the internal zeolite surface area. We may expect to reach the limit of stability of a

10.1021/jp907694g  2010 American Chemical Society Published on Web 12/15/2009

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tetrahedral framework with the decreasing surface curvature, as it was established for the aqueous solutions of hydrophobic particles.17 According to the Lum-Chandler-Weeks (LCW) theory18-22 there are two regimes of hydrophobic hydration. For small hard spheres immersed in water, the excess chemical potential is proportional to the particle volume and the structure of water close to tetrahedral is remaining in the hydration shells. For large particles with small curvature of the surface, it is impossible to keep the tetrahedral structure and part of the H-bonds are broken, and the free energy is increased proportionally to the surface area of a hard sphere. The crossover of the two regimes of hydration is reached at a radius of ca. 10 Å under ambient conditions for a hard sphere particle immersed in water. In the case of soft spheres immersed in water, the structural changes of hydration shells are more complicated and effects of H-bond breaking are visible for the spheres of radius larger than 4 Å. Dewetting of the molecular surface of the large particles is observed.23 The valuable result for understanding of the synthesis processes in pure silica as-made zeolites is the dewetting-induced collapse of hydrophobic particles. It was shown recently24 that when two hydrophobic nanoparticles are brought together closer than a characteristic critical distance, spontaneous drying takes place. The critical distance depends on the curvature of surfaces and sizes of the particles, and also of their strength of interaction with water. The structure of water near hydrophobic and hydrophilic surfaces was investigated.22,25-27 A dewetting effect of the hydrophobic silica surfaces was shown, as well as a decreasing hydrophobicity with temperature decrease. The hydrophilic surfaces induce wetting in a confined space. Experiments of water confined between a hydrophobic and a hydrophilic surface show that the hydrophobic surface encourages water to dewet, while the hydrophilic surface constrains water to absorb. It is reasonable to suppose that the processes of wetting-dewetting play an important role during the hydrothermal zeolite synthesis. The curvature of the surfaces of SDA cations and zeolite channels and cavities, the presence of hydrophilic hydroxyl groups, and the temperature and pressure during the synthesis are all expected to influence the water behavior with respect to the zeolite. This theory is applicable to any tetrahedral system, but in the case of the zeolites, the internal stress produced by large SDA particles may be sufficiently reduced by the formation of nontetrahedral fragments of framework, such as three- or fourmembered rings, and structural defects. The hydroxyl groups interact with silicon atoms on the cavities and the channel surfaces forming silanol groups. This process is accompanied with the breaking of one Si-O bond of the attacked atom, and a syloxy group is formed. Schematically, it may be represented as follows:

(sO3tSi-O-SitO3s) + OH- T (sO3tSisOH) + (O-sSitO3s) (1) The chemical shift of the attacked silicon atoms (Q3) differs from the chemical shift of tetrahedrally coordinated silicon atoms of the zeolite framework (Q4), and hence, these structural defects can be detected by 29Si MAS NMR. Defects locally break connectivity and may reduce the structural stress produced by tightly fitting SDA and water molecules. Apart from the generation of defects, the other mechanism of structural stabilization has mainly hydrophilic character. The energy of the system is decreased due to strong attractive interactions of water molecules with the components of the

Bushuev et al.

Figure 1. Chemical structures of organic cations used as structural directing agents in the zeolite syntheses: SDA1 is 4,4-dimethyl-4-azoniatricyclo[5.2.2.02,6]undec-8-ene (C12NH20+); SDA2 is diazoniapentacyclo[7.5.2.02,8.03,7.010,14]hexadec-15-enyl (C18N2H302+).

system. In the case of real zeolites, it is reasonable to expect both types of stabilization. The objective of this study is rationalizing the experimental results obtained for different gel compositions in terms of the relative stability of the as-made zeolites. Using molecular mechanic (MM) and molecular dynamic (MD) methods, we want to study the mechanism of zeolite framework stabilization due to the presence of water molecules. Also, the role of hydroxyl groups in the stabilization and defect formation in the framework will be investigated. 2. Methodology Section The classical methods of molecular mechanics (MM) and molecular dynamics (MD) were selected for the simulations of zeolite systems. Pure silica zeolites are crystals built from SiO2 units with preferably tetrahedral ordered positions of Si atoms. Several force fields have been proposed for the simulations of the silica-based materials. In the present work, we used the force field proposed in our previous study28 concerning the as-made SSZ-74 zeolite. Our force field for silica zeolite simulations is based on the CLAYFF potential,29 which was used for simulations of a wide class of materials: clays, nanotubes,30,31 and layered double hydroxides.32 Flexible modification of the single point charge (SPC) model29 was used for water. For the organic cations SDA1 (C12NH20+) and SDA2 (C18N2H302+), which are presented in Figure 1, we have used the force field by Oie et al.33 for the intramolecular SDA interactions, and the Kiselev force field34 for the intermolecular SDA-zeolite interactions. All of the components of the simulated materials were flexible. The total energy in the simulations is determined from the evaluation of the appropriate energy term for every atom-atom interaction in the system. We have chosen a force field with nonbonding contributions coming from electrostatic and van der Waals interactions, employing the Ewald method for summation of the long-range Coulombic interactions, and direct summation of the short-range interactions with a cutoff distance of 12 Å. The potentials and interaction parameters used for zeolites, fluoride, water, hydroxyl, and silanol groups are presented in a previous publication28 and can also be obtained as Supporting Information, which contains also GULP input files with the used keywords and estimations on the accuracy of calculations. The simulations were carried out with the General Utility Lattice Program (GULP).35 The simulation cells were converted to the P1 triclinic space group symmetry, allowing all crystallographic cell parameters and atomic positions to vary during the molecular mechanic (MM) optimization and molecular dynamic (MD) simulations. MD calculations were performed as the NPT canonical ensemble at P ) 0.1 MPa, using the leapfrog algorithm for the integration of the equation of motion. A time step of 1 fs was used in all of the MD runs.

Synthesis of Beta Zeolite Polymorphs

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Figure 2. Experimental and calculated enthalpies of transition (symbols with 95% confidence interval) vs their calculated counterparts. The solid line in each figure is the line on which all points would lie if both methods agreed perfectly. The dashed line is the linear fit of the experimental data. Molecular mechanic (MM) and molecular dynamic (MD) at T ) 298 K simulations with our potential.

The present paper is devoted to the investigation of stability of the Beta zeolite polymorphs, which have similar topological and thermodynamic properties. This task requires special demands from the force fields to be used. It was shown that calculated zeolite enthalpies, with respect to quartz, are very sensitive to the force field or DFT method employed.36 For a preliminary test of our force field, we selected quartz and 11 pure silica zeolites whose IZA37 codes are AST, BEA, CFI, CHA, IFR, ISV, ITE, MEL, MFI, MWW, and STT. The measured enthalpies38 of zeolite formation with respect to quartz at T ) 298 K (∆H298) with estimated uncertainties and a comparison between calculated and experimental values are shown in Figure 2. An excellent correlation between experimental values and computational results has been found. The agreement between MM and MD results means that the differences between the position of the local minimum on the potential energy surfaces of zeolites and quartz and the average total energy of the zeolites and quartz at T ) 298 K are small for this proposed force field, which means that it can be used successfully in both MM and MD simulations for the prediction of the zeolite stability. We made additional tests of our force field.28 Here, we present the results of the simulations of the fully hydroxylated (100) surface of R-cristobalite. The FFSiOH force field was proposed recently,39 based on the results of DFT calculations, for the simulation of silica materials containing silanol groups. The main features of the FFSiOH force field are core-shell model for oxygen atoms, Buckingham and Morse potentials, and a special function applied to describe hydrogen bonding. We used the CLAYFF force field29 for silanol groups and our force field28 for Si and O atoms. This force field contains widely used Coulombic, Lennard-Jones, harmonic, and three-body terms of interactions. An input file for GULP is included as Supporting Information.39 The calculated structures are presented in Figure 3. We use an overlay representation where the balls show the atom positions for the FFSiOH; meanwhile, the sticks and the tubes connect atom positions calculated with our force field. Both presented structures are close to each other. The silicon positions are extremely close, but oxygen positions are slightly different. There are some deviations of the surface structures calculated with FFSiOH and our force field (atoms on the surface are shown by tubes): Si-O-H angles are 120.2 and 119.4°, Si-O bond lengths in silanol groups are 1.64 and 1.73 Å, and O · · · O distances between H-bonded OH groups are 2.91 and 2.82 Å,

Figure 3. (a) Overlay of the hydroxylated R-cristobalite structures calculated with the FFSiOH force field (balls) and the force field used in the present paper (sticks). The atoms at the (100) face are shown by tubes, while those in the bulk are shown by sticks. Hydrogen O · · · H bonds between hydroxyl groups at the surface are shown by dotted lines. (b) The same structure from another point of view. Lines are used for the structure calculated with our force field, while circles point to the positions of atoms calculated with the FFSiOH force field, showing the close agreement between the geometrics obtained with the two force fields.

respectively. The frequencies of OH vibrations calculated with CLAYFF (3669 and 3715 cm-1) are ∼100 cm-1 higher than those calculated with the FFSiOH force field (3570 and 3617 cm-1). The former result is more close to the experimental data of frequencies of OH vibrations in the silica materials,14,15,40 and the latter result is similar to DFT data.39 Taking into account the previous28 results of the simulations, we may expect that our simple force field gives reliable information about the stability and structure of the complex silica materials. For as-made zeolite simulations, we have used a special force field28 including the OH- group and the water molecule. Hydroxyl groups are immersed in the materials and interact with their components. This approach opens a way for the simulation of a chemical reaction (see eq 1) between hydroxyl groups and silica atoms in the zeolite frameworks. We used a special procedure for preparing the starting configuration for the GULP program. Initially, the SDA cations were placed in the void space of the zeolite framework and we did a set of MD simulations at high temperature using universal Lennard-Jones potentials for intermolecular and intramolecular interactions,41 followed by energy minimization. In this special case, the positions of the Si atoms in the silica framework were fixed, and the configuration with minimum energy was selected.

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TABLE 1: Composition of the Final Product of the Hydrothermal Zeolite Synthesis9a

fraction of the reference element in the as-made material. We have suggested that the stoichiometric chemical composition of the materials may be presented by the formulas

KOH/Si H2O/Si

0.1

0.25

3.0 7.25

BEA + BEB BEA + BEB + BEC

BEA + BEB + BEC BEC

The crystallization conditions were 175 °C and 14 days. The initial gel composition was F/Si ) SDA1OH/Si ) 0.5. a

The positions of fluoride anions were then selected according to experimental 19F MAS NMR spectra data.9,10 The hydroxyl groups and water molecules were placed in the void space. The coordinates of all atoms of the system were then used as the starting point for GULP energy minimization with our force field. After that, we made an additional 50 ps MD simulation of the system at T ) 1000 K. Several intermediate high temperature structures were selected as the starting points for energy minimizations. The configuration with minimum energy was selected for analyses. The Materials Studio 4.0 molecular simulation program (Accelrys Software Inc., www.accelrys.com) was used for the calculation of the Connolly surface and volume of the void space in the unit cells. A Connolly surface42 is at the boundary between the Connolly probe (spherical particle) and the atoms as represented by their scaled van der Waals radii. Varying the Connolly probe radius of a Connolly surface tends to change the curvature at cusp and crest points on the van der Waals surface, but away from such areas, the Connolly surface follows the van der Waals surface. The scale factor and the probe radius are adjustable parameters. We used a probe radius of 1 Å and a scale factor of 1. 3. Results and Discussion 3.1. Analysis of the Experimental Data. The result of the hydrothermal synthesis of pure silica zeolites is very dependent on the composition of the initial mixture. According to the experimental data9 presented in Table 1, the change of water content and pH gives three different zeolites, which belong to the Beta family: polymorphs A (BEA), B (BEB), and C (BEC). In all of the cases, the template is the same, SDA1 (Figure 1). The mixture of BEA and BEB, in a proportion of 60:40, is formed at low concentration of water and KOH in the initial mixture. However, the BEC starts to form as water or KOH concentrations increase. And finally, BEC is dominant when the nucleation and crystallization occurs at a high concentration of both water and OH-. 3.1.1. Analysis of Chemical Compositions of As-Made BEC Materials. The experimental data of chemical composition were obtained only for as-made BEC materials, because only in that case is it possible to synthesize pure crystals using SDA1 and SDA2. The crucial difference of framework topologies is the presence of two double four-membered ring, D4R ([46]), small cavities in the BEC framework, which may accommodate only two fluoride ions (one at each cage). The 19F MAS NMR spectrum of as-made BEC has only one band at -38.3 ppm,10 which corresponds to the fluoride ions occluded in the D4R cavities. Its position and amount in the BEC unit cell are well established. The total molecular mass of the unit cell (including zeolite, SDA, fluoride and hydroxyl anions, and water) can be estimated according to the equation

M ) (nmref)/Wref

(2)

where n is number of atoms taken as the reference element for calculation, mref is the atomic mass, and Wref is the weight

(SDA1+)k(SiO2)32(F-)n(OH-)k-n(H2O)l 2+

-

-

(SDA2 )k(SiO2)32(F )n(OH )2k-n(H2O)l

(3) (4)

where k, n, and l are integers. The experimental values of Wref are presented in Table 2. The cases of k ) 3, n ) 2 for SDA1 and k ) 2, n ) 2 for SDA2 give the best correspondence between calculated weight fractions of elements in as-made BEC materials and experimental data. No combination of the k, n parameters is compatible with the choice l ) 0. Calling ∆M the mass attributed to water, the following two equations can be established:

∆M ) M - k · mSDA - mSiO2 - n · mF - p · mOH

(5) l ) INT(∆M/mH2O)

(6)

where mSDA, mSiO2, mF, mOH, and mH2O are the masses of the corresponding components of the materials. Estimations of l can be found in Table 2. The following condition between p, k, and n is obtained from the electric charge balance: p ) k - n (for SDA1) and p ) 2k - n (for SDA2). Finally, l is the closest integer number obtained from eq 6. The results depend on the reference element chosen for the calculation and are sensitive to the accuracy of the experimental data. Owing to the possibility of partial filling of the zeolite voids by SDA cations, which is very difficult to estimate, we consider that the calculation based on the experimental fluoride content data seems more reliable. The measured values of WF are less than 0.015, and the variation of WF on (0.001 gives a variation of l ( 10. It is clear that this numerical analysis does not allow one to find the exact water content, but the estimations allow one to establish clearly that water is present in as-made BEC. Since the presence of water is clear, we now use computational chemistry methods in order to investigate more accurately the water content. 3.1.2. Analysis of As-Made BEC 29Si MAS NMR Spectra. Experimental spectra for SDA1/BEC and SDA2/BEC as-made materials with their deconvolution are presented in Figure 4. The three well resolved bands at -110, -112, and -116 ppm were obtained for the calcined BEC zeolite.9 Using the empirical relation43 between the chemical shift (δ) of the 29Si MAS NMR spectrum and Si-O-Si angle (d)

δ)a+b·d

(7)

(where a ) -25.44 ppm and b ) -0.5793 ppm/deg), it was established that the three peaks correspond to three crystallographic types of Si atoms (noted as Si1, Si2, and Si3) in the BEC framework (Figure 5). The unit cell of BEC contains 16 Si1 atoms in the unit cell that form D4R cavities, 8 Si3 atoms forming two 4MR rings belonging to mtw cavities (425462), and the remaining 8 atoms belonging to the type Si2. The structural changes of as-made BEC materials produced by non-zeolite components may be estimated by means of the calculation of the normalized integral intensity (ki ) Ii/ΣIi) of the peaks from the following equation:

Ni ) N · ki

(8)

where N ) 32 is the total number of silicon atoms in the unit cell. The results of spectra deconvolutions and the peak assignations are presented in Table 3.



TABLE 2: Weight Fraction of the Chemical Elements (W, %), Residual Mass of the Unit Cell (∆M, au), and Number of Water Molecules (l) in the Unit Cell of As-Made BEC Zeolite Materials: (SDA1+)3(SiO2)32(F-)2(OH-)(H2O)l and (SDA22+)2(SiO2)32(F-)2(OH-)2(H2O)l SDA1 (C12NH20+)

SDA2 (C18N2H302+) Wcalc

Wcalc reference element F N C b

F b

1.4 1.26 1.3

N

C

∆M

la

reference element

F

N

C

∆M

l

1.54 1.39 1.44

15.9 14.3 14.77

202 509 413

11 28 23

F N C

1.3 1.43 1.49

1.91 2.11 2.19

14.8 16.28 16.90

380 111 13

21 6 1

a Different estimations of the water content come from the different experimental weight fractions taken as a reference for the calculation. Experimental data is taken for the element chosen as reference (bold). The other elements are calculated according to eqs 2, 5, and 6.

Figure 4.

29

Si MAS NMR spectra and deconvolutions of as-made with SDA1 (a) and SDA2 (b) BEC zeolite.

Figure 5. Three crystallographic positions of Si atoms in BEC zeolite.

From the spectra (Figure 4, Table 3), taking into account the material balance, the number of Si1 atoms in the defectless zeolite, and the bands overlapping, it is estimated that the relative population of Q4 Si1, Si2, and Si3 atoms is, in SDA1/BEC, NSi1 ) 16, NSi2 ) 7.4, and NSi3 ) 7.2 and, in SDA2/BEC, NSi1 ) 16, NSi2 ) 7.5, and NSi3 ) 5.1. It was established28,44,45 that the Q3 Si atoms of siloxy and silanol groups give the band at -100 ppm. The attachment of one hydroxyl group to the zeolite framework creates two Q3 Si atoms according to the reaction corresponding to eq 1. The band intensity calculation (eq 8) shows that there are approximately 1.4 Q3 Si atoms in the unit cell of as-made SDA1/BEC and 3.4 Q3 Si atoms in SDA2/BEC materials. This means that part of the OH- groups (the number of groups in the unit cells is 1 for SDA1/BEC and 2 for SDA2/BEC) in the statistical ensemble

is not attached to the framework as silanol groups and is located in channels or cavities. As for the location of the silanol groups, according to this data, hydroxyl groups are not attached to Si1 atoms, and this can be explained by the unlikely high repulsion that results between OH- and F-, the later ions occupying the D4R cavities, which are made of Si1 atoms. Therefore, silanol groups are expected to be of the type Si2-OH and/or Si3-OH. Another feature we would like to explain is the different width of the Si2 and Si3 peaks in BEC/SDA1 and SDA2/BEC (Figure 4). The corresponding values, wi, are listed in Table 3, and the differences can be rationalized as being due to two factors: (i) the closer interatomic SDA2-framework distances due to the better filling of voids (SDA2 is larger than SDA1) produces a stronger change in the chemical shift of the Si atoms; (ii) hydrogen bonds between water molecules and oxygen of -Si-O-Si- groups may make peaks broader, and therefore wider peaks may be associated with larger water content. 3.2. Results of Simulations. 3.2.1. Anhydrous As-Made BEC Materials. Three SDA1 and two SDA2 cations can be occluded in the unit cell of BEC zeolite containing 32 SiO2 units. This result corresponds to the chemical analysis data. SDA1 cations are located in the three channels parallel to a, b, and c axes of the unit cell, as presented in Figure 6a. The two SDA2 cations are in the channels parallel to the a and b axes (Figure 6b). Hydroxyl groups were put in the channels and the attached Si atoms, and their final positions were calculated during energy minimization. Although it was mentioned in the methodology section, we recall here that the type of force field used allows one to cleave the siloxane bond to give the silanol and siloxy groups, as described in eq 1. This important feature

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TABLE 3:

Bushuev et al.

29

Si MAS NMR Spectra Deconvolutions of As-Made BEC Zeolite (See Figure 4) SDA1 r2 ) 0.9958a

i

-δi, ppm

1 2 3 4 5 6 7

100.1 106.6 108.3 110.6 112.7 116.9 119.9

b

wi, ppm

SDA2 r2 ) 0.9982a

Ni

c

type of Si

3.51 1.4 Si-O, Si-OH 1.20 14.8 14.8 Si1 1.24 1.7 1.2 Si1, 0.5 Si2 1.47 6.2 6.2 Si2 1.42 0.7 0.7 Si2 2.18 6.5 6.5 Si3 1.50 0.7 0.7 Si3 compositiond: 1.4 Q3 (0.6 Si2 + 0.8 Si3); 30.6 Q4 (16 Si1 + 7.4 Si2 + 7.2 Si3)

-δi, ppm

wi, ppm

Ni

type of Si

99.6 104.9 106.9 108.3 110.7 117.2

3.24 2.40 1.56 1.48 3.74 3.97

2.8 2.1 12.9 1.6 7.5 5.1

2.8 Si-O, Si-OH 0.6 Si-OH, 1.5 Si1 12.9 Si1 1.6 Si1 7.5 Si2 5.1 Si3

composition: 3.4 Q3 (0.6 Si2 +2.8 Si3); 28.6 Q4 (16 Si1 + 7.5 Si2 + 5.1 Si3)

a 2

r is the coefficient of determination (COD). b δi is the ith peak position; wi is the width of the peak. c Ni is the average number of Si atoms in the unit cell corresponding to the peak; see eq 8. d The estimation was done according to the material balance, the number of Si1 atoms in defectless zeolite, 16, and the bands overlapping; see Figure 4.

Figure 6. Anhydrous as-made SDA1/BEC (a) and SDA2/BEC (b) zeolite structures. Only three SDA1 and two SDA2 cations are shown (sticks). The silanol groups (balls) are shown with their periodic images.

Figure 7. Silanol and siloxy groups (balls) are formed after reaction of hydroxyl groups Si3 (a) and Si3, Si2 (b) atoms of the BEC zeolite framework (sticks).

makes our force field very suitable for the calculation of the defects in a zeolite structure. During the simulations, hydroxyl groups become close to the Si2 atom in the case of SDA1/BEC, attach to the Si2 atom in the [100] channel (Figure 6a), and create a pair (according to eq 1) of a silanol and a siloxy group as it is shown in Figure 7a, while the Si atom of the siloxy group belongs to the Si3 crystallographic type. According to this, equal amounts of Q3 Si2 and Q3 Si3 atoms would be expected. These results are in good agreement with our 29Si MAS NMR analysis, according to which approximately 0.6 Si2 atoms and 0.8 Si3 atoms (see Table 3) give the Q3 bend of the SDA1/BEC spectrum.

In the unit cell of SDA2/BEC material, the Si2 atom in the [010] channel and the Si3 atom in the [100] channel were attacked by two OH- groups (Figures 6b and 7b). According to the simulation results, these two OH- groups create four Q3 sites: two siloxy groups with Si3 atoms, one silanol group with a Si3 atom, and one silanol group with a Si2 atom (Figure 7b). This suggests a 1:3 ratio between Q3 Si2 and Q3 Si3 atoms. From the SDA2/BEC spectrum analysis (see Table 3), it follows that approximately 0.6 Q3 Si2 atoms and 2.8 Si3 atoms give the Q3 signal. This agrees with the simulation results which indicate Si3O- as the main siloxy group. There is a dynamical equilibrium between the processes of attachment and detachment of OH- groups in the zeolite materials.46 The thermal fluctuations and the presence of SDA and water molecules may shift the reaction (eq 1) to the left side, and it is reasonable to expect that part of the OH- groups in the thermodynamic ensemble may be in the channels and the framework defects may disappear. This suggestion is supported by the experimental result of 18O exchange studies.47 It was shown that Si-O-Si bridges in zeolites are cleaved at measurable rates even under mild conditions at temperatures as low as 95 °C. Water molecules can form not only strong hydrogen bonds with bridging oxygens of the zeolite framework but demonstrate a stronger interaction, which may lead to Si-O-Si cleavage. Hydroxyl groups show higher chemical activity than water and may participate together with water in the oxygen exchange reaction. 3.2.2. Water in the Synthesis of Beta Zeolite Polymorphs. Void Fractions and Energy. For an investigation of the stability of Beta polymorphs and an explanation of results of zeolite syntheses, a set of MD and MM calculations were done for the systems of composition. BEA and BEB: [(SDA1+)6(SiO2)64(F-)6(H2O)l], and BEC: [(SDA1+)3(SiO2)32(F-)2(OH-)(H2O)l], [(SDA22+)2(SiO2)32(F-)2(OH-)2(H2O)l] at different l. The energy hypersurface of close packed multicomponent systems has many local minima, which may be separated by large barriers, and therefore, we made MD simulations at T ) 1000 K for every system composition with several quenches. The free volume of the systems was calculated using the Connolly method42,48 with a probe spherical particle radius of 1 Å. The Connolly surface divides the volume occupied by atoms and the free volume in the unit cell. The void fraction is the free volume per total volume of the unit cell. The results of the void fraction calculations are presented in Figure 8. The anhydrous SDA1/BEC material has approximately twice larger void fraction in comparison with the corresponding SDA1/BEA and SDA1/BEB or SDA2/BEC materials. It is very



Figure 8. Void volume fractions in as-made zeolites vs water content: (a) SDA1/BEA, SDA1/BEB, SDA1/BEC; (b) SDA1/BEC, SDA2/BEC materials. l is the number of water molecules, and NSi is the number of Si atoms in unit cells of BEA, BEB (NSi ) 64), and of BEC (NSi ) 32) zeolite.

unusual to have such a large void volume, as it contradicts the assumed molecular mechanism of the templating effect, and hence, it is reasonable to think that water is present in the material. The SDA1 cation directs to a zeolite with large void fraction but with large water content and high basicity of the initial mixture (Table 1). Water molecules were inserted in the materials with the aim of finding the stability reached by the systems as the water content increases. In the case of the SDA1 cations (Figure 8a), the rates of the free volume reduction are fast at small water contents in zeolites, but the void fraction remains approximately constant at high water concentration. In the first stage, water molecules occupy the free volume in the materials. In a second stage, it is possible to insert water molecules in the unit cell after the filling of free volume due to the flexibility of the zeolite frameworks and the SDA1 cations, without further decrease of free volume. There is no large room for water in SDA2/BEC material (Figure 8b), but the rate of free volume decrease is slower with respect to the SDA1/BEC system due to the large flexibility of the SDA2 cation. It is reasonable to expect that the number of water molecules in the as-made zeolite is related to the void space in the structures. The results of simulations indicate two regimes of zeolite filling (Figure 8). The saturation of the free volume by water is reached at 0.1-0.15 molecules per Si atom for BEA and BEC zeolites and at larger concentration for BEB. Taking into account possible fluctuations of the average number of the water molecules in a statistical ensemble, the following average water concentrations are expected at normal pressure: 3-7 water molecules per unit cell of BEC zeolite (32 Si atoms), 5-9 molecules for BEA (64 Si atoms), and 8-12 in the case of BEB (64 Si atoms). It is not possible to simulate the real process of nucleation and crystallization of zeolite systems, sometimes lasting several weeks, which is governed not only by thermodynamic but also by kinetic factors. Free energy calculations should be performed in order to solve the problem of the thermodynamic stability of these systems. It is a very complicated task especially if we have close packed multicomponent systems, and this cannot be solved at the present time using direct simulation techniques. For the investigation of the relative stability of the systems, we have compared the energies of their local minima obtained with the energy minimization method. The SDA1 + Beta-polymorph

systems have similar structures with not very large differences of framework topology. In all cases, the same SDA1 cation was used. For the calcined zeolite frameworks, the measured entropic contributions are found to be an order of magnitude smaller than those of the corresponding enthalpic terms. The entropies of transition from quartz at 298.15 K are BEA, 3.4 J · K-1 · mol-1; FAU, 3.2 J · K-1 · mol-1; MFI, 3.6 J · K-1 · mol-1; MTT, 4.2 J · K-1 · mol-1,49 and they span a very narrow range above quartz. It is reasonable to propose that the entropies of the BEA, BEB, and BEC are close to each other, and the statistical average free energies of the thermodynamic ensembles directly correspond to the minima of energies in the case of condensed systems. The last point is supported by the results of the MM and MD simulations of pure silica zeolites presented in Figure 2, which shows very good correlations between the experimental38 and calculated transition enthalpies of zeolites with respect to quartz. Energies of the systems calculated for different water contents are presented in Figure 9. The systems with the same SDA may be compared directly if the energy is calculated per Si atom, because the concentration of SDA is the same for BEA, BEB (6 SDA1 ions per 64 Si atoms in the unit cells), and BEC (3/32 ions per Si atom) as-made materials (Figure 9a). The anhydrous SDA1/BEC has the highest energy. The consecutive additions of water molecules stabilize the systems differently. The average stabilization energies for the SDA1/BEC and SDA2/BEC systems are equal to 62 and 54 kJ (mol H2O)-1, but for SDA1/ BEA and SDA1/BEB, the stabilization is significantly lower: 40 and 23 kJ (mol H2O)-1, respectively. The SDA1/BEB material is the most hydrophobic, and it is difficult to expect significant water content in BEB even if there is enough room for water molecules. The crossover point, l/NSi ) 0.0625 (Figure 9a), corresponds to a water content of two molecules per unit cell of BEC and four molecules per unit cell of BEA and BEB zeolites. At small water content, the energies of the as-made BEA and BEB zeolites are very close and they are the most stable polymorphs. This correlates with the experimental result (Table 1, H2O/Si ) 3.0, low KOH content), where BEA and BEB polymorphs crystallize together in concentrated aqueous solution. However, at higher water content, as-made BEC zeolite becomes more stable; meanwhile, BEA is getting slightly more stable than BEB. This, again, corresponds to the experimental

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Figure 9. Total energies per Si atom in as-made zeolites vs water content: (a) SDA1/BEA, SDA1/BEB, SDA1/BEC; (b) SDA1/BEC, SDA2/BEC materials.

observation of BEA and BEC materials (Table 1, H2O/Si ) 7.25), considering that the larger water content in the material correlates with the larger initial concentration of water in the experiments. It is difficult to estimate the thermodynamic properties of the system for l/NSi > 0.2 due to the increase of entropy of the system, which depends on the number of possible microstates with the same energy. The other source of complexity is that the energy of water clusters is very sensitive to the number of the molecules in it and to the topology of the cluster.50 There are several stable conformations for each water cluster, and the external electric field (due to ions) can change the population of the conformations. We made independent MD simulations for each system and minimized the energy starting from different molecular conformations in order to minimize the influence of these factors. According to the void fraction analysis (Figure 8), it is difficult to expect (at pressures close to normal) that the water content in any of the systems exceeds 0.2, because at such loading the insertion of water molecules is only possible at the expense of highly stressed framework and SDA ions. The fitting curves in Figure 9a at l/NSi > 0.2 show low energetic gain, which can only be achieved through high activation energies corresponding to stressed frameworks and cations, and therefore, l/NSi > 0.2 seems unlikely. Hydroxyl groups are an essential component during the synthesis of BEC materials because they are needed for charge compensation of cations, and an increase of their concentration shifts the reaction of zeolite synthesis toward BEC (Table 1, KOH/Si ) 0.25), which is the only polymorph containing OHin the formula units reported. BEC zeolite can be obtained with both SDA1 and SDA2 cations. However, the cations interact with the components of the system differently. The calculated total energy of the SDA1/ BEC or SDA2/BEC system contains all terms of interactions including the intermolecular. This is the reason of the difference between the energies of the SDA1 and SDA2 systems in Figure 9b. There are two regimes of structural stabilization for both SDA1 and SDA2 systems, with the crossover points (l/NSi ) 0.125 and 0.156, respectively) at four (SDA1) and five (SDA2) water molecules per unit cell of BEC. In the first stage (l ) 1-2) for the system containing the SDA2 cation, the energy decreases slowly. Beyond the crossover points, the stabilization

of the SDA1/BEC and SDA2/BEC systems per one added water molecule is approximately the same, and nearly no further energetic stabilization is reached. The presence of water does not make a qualitative difference with respect to the anhydrous systems, and at any water content, SDA1 seems a more stabilizing SDA. 3.2.3. Structure of As-Made Zeolites. Water plays a crucial role on the outcome of zeolite synthesis, stabilizing different zeolites at different rates with increasing water content, and this should have a structural explanation. Previously, we have estimated a water content between three and seven water molecules per unit cell of SDA1/BEC zeolite (32 SiO2 atoms). Now, we study two water loadings, one within such a range (l ) 5) and the other above such a range (l ) 8). Fragments of as-made SDA1/BEC zeolite structures are presented in Figure 10. Water molecules form compact clusters with typical features of pure water structure. Each water molecule has two or more hydrogen bonds with neighbors, and the molecules form rings and chains of H-bonds. The hydroxyl groups are in the hydrated state and behave as the center of the water clusterization. SDA1 cations additionally stabilize the water structure due to hydrophobic hydration. The system with eight water molecules shows an elongated, less compact cluster (Figure 10b) compared to the system with five molecules (Figure 10a). The cluster of eight water molecules is surrounded by five SDA1 ions; meanwhile, the cluster of five molecules has only four nearest SDA1 ions. The system with eight H2O molecules is less favorable owing to the absence of void space, and SDA1 ions change their conformations, forming more packed structures. For SDA2/BEC systems, we did several calculations with different OH- group distributions, which were sampled according to high temperature MD simulations. A fragment of the resulting structure for the system containing six water molecules per unit cell is presented in Figure 11. This is different from the structure shown in Figure 6b. One OH- group is attached to the Si3 atom at the channel wall, forming the Si3OH silanol group; meanwhile, the other OH- group is in the middle of the channel, not attached to the Si2 atom. Water molecules form two clusters, and the OH- groups are the centers of water clusterization. In the case of BEA and BEB zeolite systems at l ) 8, the branched or chain-like structure of water clusters is obtained



Figure 10. Five (a) and eight (b) water molecule clusters in as-made BEC zeolite. Only nearest to cluster SDA1 ions (sticks) are shown for clarity. Water molecules and silanol groups are displayed separately down to corresponding structures.

Figure 11. Six water molecules (sticks) inside the as-made SDA2/ BEC unit cell. The silanol (Si3OH) group, Si2 atom, and OH- group are shown with balls. SDA2 cations are not shown for clarity (see Figure 6b for comparison).

as a stable conformation (Figure 12), which is in agreement with our previous estimations of water content in BEA (l ) 5-9) and BEB (l ) 8-12). There are two reasons for this type of clusterization. On the one hand, there is not enough room for compact cluster formation, and no strong interaction between water and hydrophilic groups occurs. The second reason is the curvature of the Connolly surface of organic SDA ions hydrated as hydrophobic particles. The chain-like and branched clusters are typical water structures near the surfaces with small curvature, as is the case of SDA1. BEA and BEB, by not containing silanol groups or OH- groups, are more hydrophobic systems than BEC. On the other hand, in the case of BEC, the large number of H-bonds and direct strong hydrophilic interactions with hydroxyl (silanol) groups explains the fast rate of energy decrease with increasing water content at low water loading (Figure 9). 3.2.4. Influence of the Hydroxyl Groups on the Zeolite Framework Stability. Tetrahedral networks create Connolly surfaces with large curvature. It is not possible to generate a flat surface without breaking the bonds in a tetrahedral structure. This is a reason why the ice surface is usually covered by liquid water layers at a temperature less than that of the ice melting

point.51-53 This is not a unique property of water, and the quasiliquid layers on surfaces were found for noble gases, complex organic molecules, and metals below the melting points of these substances. Theoretical explanations have been proposed for this phenomenon.54,55 In the case of silica materials, we can expect disordered, amorphous layers on the surface of the crystals at a temperature far from the melting point. Recent studies have confirmed the existence of four-, three-, and two-membered rings (4MR, 3MR, 2MR) on the surfaces or interior of amorphous and crystalline silica, as well as vitreous silica.56,57 The 3MR are found in some zeolite frameworks such as MEI, OSO, RWY, and ITQ-33.36,58 According to the LCW theory of hydrophobic hydration, a tetrahedral network of water has the limit of stability. Zeolite frameworks contain tetrahedral and nontetrahedral motifs, with the latter being built, among others, from 3 MR, 4MR, and connectivity defects. Different curvatures may be associated with these motifs, and curvature regulates the sizes of cavities and the diameters of channels. Framework densities (number of T-atoms per nm3) correlate with size of the smallest rings in the network,59 and the smaller the ring size, the smaller framework density can be reached. It was shown17 that the energetic destabilization of zeolites by small rings depends on their concentration. Zeolites were described as a space-filling packing of face-sharing polyhedra. The topology of polyhedra determines the energy and the structure of the zeolite frameworks. It was proposed that extra-large-pore zeolites may be formed if the framework contains a sufficient number of small rings.60 The nontetrahedral motifs of BEC zeolite structure are 4MRs of silicon atoms. The T-T-T angles in 4MRs are equal to 90°, which differs significantly from the tetrahedral angle of 109.47°. Fluoride ions located in D4R cubic cavities or Ge atoms substituting Si in D4R stabilize the nontetrahedral motifs of BEC and other similar frameworks.61 Other examples are ITQ-3358 and ITQ-37 zeolites.62 There are 18-membered rings in the framework of ITQ-33, which contains 3MR and 4MR as nontetrahedral motifs of the zeolite framework. The framework of the chiral ITQ-37 silicogermanate zeolite has extra large 30membered rings and a gyroidal channel system, and the

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Figure 12. Eight water molecules inside as-made BEA (a) and BEB (b) zeolite. Only nearest to water clusters, SDA1 ions (sticks) are shown for clarity. Water clusters are displayed separately down to corresponding structures.

framework has defects of connectivity, where it is possible to see the fragments of opened and shifted 12-membered rings. The curvature of the surface is locally close to the curvature of BEC zeolite. The nontetrahedral motifs of the ITQ-37 framework are D4R cavities and defects. The hydroxyl groups attached to Ge atoms in the vertexes of D4R cavities prevent the 12-membered rings from closing. An ordered pore system of intermediate size with unusual chemical properties may be created by the presence of hydroxyl groups, as was done recently for SSZ-74 zeolite.45 An investigation of the regularities of tetrahedral and nontetrahedral motifs as well as their combination and distribution in the frameworks may help in the design of new materials. There are several theoretical methods for the investigation of the properties and topology of tetrahedral networks,17,36,60,63-65 for example, based on ring counting.66-69 According to other methods, the dihedral angles T-T-T-T or O-T-O-T (T is a tetrahedral atom, O is oxygen) are considered as the basic structural parameters, called “structons”. Four types of “structons” were used to analyze structural transformations in crystalline and amorphous silica.70 The R and β “structons” correspond to the different dihedral angles in the Si2O7 unit. It was shown that a T-T-T-T dihedral angle of 38° gives hierarchical modular self-similar fractal structures.71,72 The T3tTsTtT3 bond between T sites in T8 is called the centrally symmetric (c.s.) bond if the dihedral angle (T-T-T-T) is equal to 60, 180, or 300°, and it is called the mirror symmetric (m.s.) bond if the angle is equal to 0, 120, or 240°.73 T8 fragments of a T-network with c.s. and m.s. bonds are shown in Figure 13a.

These bonds are distorted in real crystals, and tetrahedral angles differ from these exact values. The proportion of c.s. and m.s. bonds defines the crystalline structure. All T-T-T-T bonds in β-cristobalites, cubic ice (Ic), and metal-Si are c.s. bonds. There are 3/4 of c.s. and 1/4 m.s. bonds in β-tridymite or hexagonal ice (Ih). Flat surfaces may be created on silica crystals when the surfaces are terminated by hydroxyl groups.46,74 Recently, the hypothetical structure of the fully hydroxylated (100) surface of R-cristobalite, a high temperature polymorph of quartz, was investigated.39 The Si atoms are tetrahedrally coordinated and form a T-network, which is built from “bonds” connecting Si atoms. It is not possible to form a surface with small curvature without breaking the chemical bond between Si and O atoms. The energetic cost is very large, but it is easy to break bonds and obtain a relatively stable phase by using OH- groups. The simulated (100) surface of R-cristobalite is presented in Figure 13b. Broken -Si-O-Si- bonds cover the surface and are terminated by hydroxyl groups, which results in chains of hydrogen bonds. All of the (H)O-Si-Si-Si- units have a central bond of the c.s. type. A fragment of BEC zeolite structure with a hydroxyl group attached to the Si3 atom on the wall of the zeolite channel is presented in Figure 13c. In this case, we have three c.s. bonds in the (H)O-Si3-Si-Si- units. All Si1-Si1 and Si3-Si3 bonds in 4MRs of the BEC framework are m.s. bonds. One Si3-Si3 m.s. bond transforms to a c.s. bond as a result of the OH- attachment previously described. The change of the bond conformation creates an energetic barrier between the defectless structure and the structure formed after



Figure 13. (a) Centrally symmetric (c.s.) and mirror symmetric (m.s.) T-T bond in the eight-membered fragments of the tetrahedral network; (b) c.s. bond in highlighted fragment (sticks) on the (100) cristobalite hydroxylated surface; (c) c.s. bonds formed in the BEC zeolite channel after the OH- group attachment. It can be seen that all three dihedrals of the type (H)O-Si-Si-Si have a central bond of c.s. type.

the OH- group attachment, where the two tetrahedral conformations are connected through an inversion path similar to the umbrella-related ammonia stereoisomers. Comparing the hydroxylated surface of R-cristobalite with the defects in BEC zeolite, it is possible to propose that the combination of the large curvature surfaces with locally flat patterns will give materials with new properties. Zeolites with ordered defects, such as SSZ-74, obtained through the use of OH- groups may be stable and show high catalytic activity. 4. Conclusions An investigation of stability of as-made Beta zeolite polymorphs was made on the basis of atomistic force field methods of computational chemistry, MM and MD. The employed force field has shown an excellent performance for the calculation of enthalpy of zeolites with respect to quartz. The B polymorph grows up only with A polymorph during the synthesis and there is no experimental data obtained for pure crystals. The results of the simulations were compared with the result of the chemical elemental analysis study of the SDA1/BEC and SDA2/BEC asmade materials and their 29Si MAS NMR spectra. The spectra were deconvoluted and explained on the basis of the assumption that, in the as-made materials, hydroxyl groups form silanol-siloxy defects. The amount of defects was estimated, and the crystallographic positions of defects in the BEC zeolite framework were proposed. Water plays a crucial role in zeolite synthesis. Changing the concentration of the initial gel, it is possible to steer the zeolite synthesis, and in this sense water and hydroxyl groups play a role of structural directing agents. Water appears in the form of monomers, dimers, or chain-like clusters in the A and B polymorphs. These structural elements are usual for water in hydrophobic media for which the dewetting effect was predicted and proved by computer simulations. The structural state of water in C polymorph is different from the case of A and B polymorphs and depends on the SDA cation. The large void space in the as-made SDA1/BEC zeolite and the presence of hydroxyl groups needed for charge compensation of organic cations create the conditions for water clusterization. The hydrophilic hydroxyl groups play the role of the centers of clusterization. The hydrophilic and hydrophobic effects act

together and provide a larger energy stabilization of the SDA1/ BEC material with respect to SDA1/BEA and SDA1/BEB. The synthesis with the bulky dipositively charged SDA2 organic cation gives BEC zeolite, which is the only polymorph containing hydroxyl groups. Two hydroxyl groups per unit cell create favorable conditions for water clusterization, but the smaller void space available prevents the formation of compact clusters. There is a dynamical equilibrium for attachment and detachment of hydroxyl groups with framework silicon atoms, and the calculations find they are mostly in the hydrated state, which make sense in terms of their hydrophilic nature. A molecular mechanism for the formation of framework defects was proposed. This is based on an interesting feature of our newly developed force field, which allows the dynamical treatment of the hydroxyl negatively charged group, both as a free entity and forming a silanol (tSiOH) group. According to our simulations, the hydroxyl group can attach to a silicon atom in the zeolite framework and cleave one SisOsSi bond. A siloxy-silanol pair is formed as a result of the hydroxyl group attachment. The local structural change of the framework produced by this pair formation may be described in terms of conformational transition of a structural unit defined as an eightmembered tetrahedral fragment. This means that an activation energy barrier separates the bonded state and the state with a broken SisO bond. The pair formation changes locally the topology of the framework, and the mirror symmetric (H)OsSi3s Si3sSi dihedral transforms into the central symmetric conformation, mimicking the behavior of hydroxylated surfaces of other silica polymorphs such as R-cristobalite. The different local hydrophilicity and hydrophobicity combined with the respective curvature of the surfaces are important parameters which might be tuned to give materials with new physicochemical properties. Acknowledgment. G.S. acknowledges Ministerio de Ciencia e Innovacion for funding through project MAT2007-64682. Y.G.B. acknowledges ITQ for a postdoctoral contract. Supporting Information Available: GULP input files with the employed potential and a movie file with visualization of the structure. This material is available free of charge via the Internet at http://pubs.acs.org.

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