Atomistic Simulations of Structural Defects and Water Occluded in SSZ

J. Phys. Chem. C 2009, 113, 10877–10886

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Atomistic Simulations of Structural Defects and Water Occluded in SSZ-74 Zeolite Yuriy G. Bushuev*,† and German Sastre Instituto de Tecnologia Quimica, UPV - CSIC, AVda. de los Naranjos s/n, 46022 Valencia, Spain ReceiVed: February 13, 2009; ReVised Manuscript ReceiVed: May 4, 2009

The amount and location of water trapped during the synthesis of pure silica SSZ-74 has been predicted by atomistic simulations, and the results are in agreement with XRD as well as 1H and 29Si MAS NMR previously published data of the material. With SSZ-74 being the first zeolite in which structural defects have been fully characterized by XRD and MAS NMR, we have completed a new force field parametrization which allows the possibility of simulating silanol and siloxy groups with improved accuracy, as well as water and the organic structure directing agents. The results of the simulations show that water is present in the as-made material, a fact not fully analyzed in the original characterization of the SSZ-74. Further, we find the amount and location of water as well as the structure of the defects and characterize the interaction of water and the defects. The computational results show a reasonable agreement with the experimentally reported data. 1. Introduction Pure silica zeolites are crystalline materials with tetrahedral frameworks. Being metastable states at the conditions not very far from normal and having smaller densities with respect to quartz, the zeolite structures have cavities and channels. Due to the use of structural directing agents (SDA), usually large organic cations, it is possible to synthesize different types of structures. A new type of zeolite called SSZ-74 was synthesized recently,1 whose structure contains ordered defects. The defects are fully characterized, and each defect can be described as one SiO2 vacancy. Siloxy (O3Si-O) and silanol (O3Si-O-H) groups compensate for the positive charge of the SDA cations used in the synthesis. Systems with ordered chemically active centers may have interesting properties and show catalytic activity. Hydrophilic siloxy and silicon groups locally compensate the mostly hydrophobic properties of the pure silica zeolite framework, which creates the possibility to absorb amphiphilic molecules. Interesting effects may be achieved in the case of selectivity between the structure of the absorbed molecule and the structure of framework defects. Water plays a significant role during the zeolite synthesis as the liquid solvent and as a directing agent in some cases.2 It is well established3 that changing the concentration of the initial solution drastically affects the result of the zeolite synthesis. Water molecules stabilize frameworks, and in some cases, attempts of zeolite dehydration lead to structure collapse.4,5 Usually, calcined pure silica zeolites show hydrophobic behavior. Ordered ionic systems of as-made zeolites are more hydrophilic. Due to strong electrostatic interactions, water molecules may hydrate ions if there is enough room for molecules in cavities and channels of the framework. According to the previous results of as-made SSZ-74 zeolite, defects contain not only ionic but also strong hydrophobic groups. Siloxy groups may be involved in hydrogen bonds (H-bonds) with water molecules acting as * To whom correspondence should be addressed. E-mail: yuriyb2005@ gmail.com. † Permanent address: Ivanovo State University of Chemistry and Technology, Engelsa, 7, Ivanovo, Russia.

proton acceptors, while silanol groups may participate in bonding as donors and acceptors of protons. The original publication on SSZ-741 provides a characterization through XRD as well as 1H and 29Si NMR MAS spectra of both the as-made and the calcined structure but does not analyze the water included in the samples. We have tried to investigate the presence of water in SSZ74 zeolite, and this is the main aim of this study. To achieve this goal, we have used computer simulations based on atomistic calculations within the molecular mechanics (MM) and molecular dynamics (MD) methodologies. In order to ensure the quality of our simulations, we have used a particularly well-suited force field, which we have improved in the present study by including parameters specially designed to simulate defects. We have compared the results of the computer simulations with the experimental data1 in order to validate our model. 2. Methodology Section Molecular mechanics and dynamics simulations were carried out with the General Utility Lattice Program6,7 (GULP). 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. We used Coulombic and Lennard-Jones (12-6) functions to calculate these terms as follows

EijCoulombic )

qiqj 4πε0Rij

EijLJ ) D0,ij[(R0,ij /Rij)12 - 2(R0,ij /Rij)6]

(1)

(2)

The parameters qi (partial charges), D0,ij, and R0,ij are obtained from a fitting of the force field, and ε0 is the dielectric permittivity of vacuum. The interaction parameters between unlike atoms are calculated according to the arithmetic mean rule for R0,ij (distance parameter) and the geometric mean rule for D0,ij (energy parameter) as follows

10.1021/jp9013306 CCC: $40.75  2009 American Chemical Society Published on Web 05/28/2009

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Bushuev and Sastre

1 R0,ij ) (R0,ii + R0,jj) 2

(3)

D0,ij ) √D0,iiD0,jj

(4)

Energies of intramolecular interactions were calculated using harmonic, three body, and torsion potentials according to

1 Eijharmonic ) kijh (rij - r0,ij)2 2

(5)

1 tb three body Eijk ) kijk (θijk - θ0,ijk)2 2

(6)

torsion t Eijkl ) kijkl [1 + cos(3φijkl)]

(7)

where the coefficients k are force constants, r0,ij represents the equilibrium bond length between i and j atoms, θijk is the bond angle, θ0,ijk is the equilibrium bond angle between i, j, and k atoms, and φijkl is the torsion angle between i, j, k, and l atoms. The parameters of the intermolecular interactions are presented in Table 1. We used the CLAYFF force field8 for the charges and nonbonding parameters of the Si and O atoms of zeolites, but the O-Si-O and Si-O-Si bend terms (eq 6) were included to reproduce quartz and zeolite structures. Flexible modification of the single point charge (SPC) water model of Berendsen et al.9 with stretch (eq 5) and bend (eq 6) terms of the CLAYFF force field were used. For the organic SDA component, 1,6-bis(N-methylpyrrolidinium)hexane [(CH3)N(C4H8)C6H12(CH3)N(C4H8)]2+, we have used the same approach as that in previous publications,10-13 which is to utilize the force field by Oie et al.14 for the intramolecular SDA interactions and the Kiselev force field15 for the intermolecular SDA-zeolite interactions. In the as-made material, defects contain two silanol and two siloxyl groups, in which a total charge of -1|e| compensates for the positive charge of the SDA cation. In the calcined material, each defect contains four silanol groups with 0 total charge. Electroneutrality requires that OH groups have different charges in both cases. We took the main part of the parameters for the OH group from the CLAYFF force field based on transferability reasons. The internal bond between the atoms of the OH group was considered through a spring term (Table 1), where the intramolecular electrostatic term is subtracted. This was implemented by the “molmec” option and the 0 covalent radius for Si in the GULP input file. In this way, OH anions, water molecules, and SDA cations are considered as a separate entity from the zeolite framework. The conditions imposed to the fitting procedure were a total charge of -1|e| to the hydroxyl group and the experimental 1.61 Å Si-O distance in the SiOH group of as-made SSZ-74. With these restraints, we obtained the charges of Oh and Ho, as well as the R0,OSi shown in Table 1. In the calcined material, we used the charge distribution from the CLAYFF force field, as well as the R0,OSi value (parameters for Os and Hs in Table 1). With the fitted parameters, two tests were performed regarding Si-O-H angles and OH frequencies. The optimal Si-O-H angles of silica glass surfaces have been calculated at 118° from electronic structure calculations,16 while a wide distribution (100-140°) was obtained from MD simulations.17 Our calculated Si-O-H angle distribution in as-made SSZ-74 is 129-140°. The discrepancy may be due to the fact that silanols

are occluded in the zeolite pores and inside of an electric field. Therefore, their properties may differ from the properties of silanol groups on the silica glass surface. The phonon frequencies calculated with our potential for OHgroups for as-made SSZ-74 (eight water molecules per unit cell) are in the range of 3597-3723 cm-1, which corresponds to the results of an IR spectroscopy investigation18,19 of silicalite samples (MFI topology), characterized by a high and low concentration of defects (internal SiOH nets generated by Si vacancies). The IR spectra have peaks at 3600-3800 cm-1. The main peak, centered at 3747 cm-1, was assigned to the vibration of isolated silanols on external surfaces. The shoulder at about 3720 cm-1 has been associated with isolated silanols located at internal positions. The three components at 3711, 3655, and 3540 cm-1 are associated with silanol groups in terminal positions or involved in H-bonded chains. The phonon spectrum contains the 3641-3752 cm-1 band. The blue shift of the band and the structural interpretation of the experimental IR spectra18,19 correspond to the results of our calculations. 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 dynamics (MD) simulations. The Ewald summation was used to evaluate the long-range Coulombic energy. MD calculations were performed as a NPT canonical ensemble at P ) 0.1 MPa and T ) 300 K using the leapfrog algorithm for the integration of the equations of motion. A time step of 1 fs was used in all of the MD runs. Each system was allowed to relax and equilibrate for 100 ps of MD simulation. The convergence of total energy and its components as well as the temperature and density were monitored during the equilibration period. GULP input files are given in the Supporting Information. 3. Results and Discussion 3.1. Initial Considerations of Defects in SSZ-74. The zeolite SSZ-74 synthesized by Baerlocher et al.1 has a very particular structure as it contains four silicon defects per unit cell. A defectless structure contains 96 SiO2 units, while the reported SSZ-74 contains 92 SiO2 units. The structure of a silicon defect is schematically presented in Figure 1. During nucleation and crystallization of the zeolite in aqueous media, some SiO2 units cannot accommodate in the perfect positions of the zeolite framework (Figure 1a), and then, cavities with structural defects are formed (Figure 1b). The OH- groups, which compensate for the SDA charge (4 SDA2+ molecules per unit cell) attach to the terminal -O3Si+ sites and form eight silanol groups (-O3Si-O-H) per unit cell (Figure 1c). During the calcination, organic SDA ions and water molecules are removed from the zeolite framework (Figure 1d). The large distances between terminal -O3Si sites and silanol groups prevent structural recombination. When calcination occurs, there are 16 silanol groups per unit cell, 8 of them formed due to a chemical reaction during calcination (see Figure 1b and c). 3.2. Anhydrous As-Made SSZ-74. In the reported structure, no information about water was presented. We have then performed a force field optimization of the as-made SSZ-74 zeolite, with chemical formula [(SSZ-74)(40)(4SDA2+)(8OH-)] (Figure 2). The unit cell contains four defects (40). Defects have structural differences; one of them is shown in Figure 2b. There are six possible distributions of two OH- groups among four defect sites; if we name O1-O4 as the candidate oxygens to host two protons (becoming silanol groups), the possibilities are O1|O2, O1|O3, O1|O4, O2|O3, O2|O4, and O3|O4. This

Structural Defects and Water Occluded in SSZ-74 Zeolite

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10879

TABLE 1: Parameters for the Force Field to Simulate Zeolite/Water/OH-/SDA Systems nonbond electrostatic and Lennard-Jones (LJ), eqs 1 and 2 species, interaction a

water hydrogen, electrostatic water oxygena zeolite silicona zeolite oxigena hydroxyl hydrogen hydroxyl oxygen silanol hydrogena silanol oxygena hydroxyl oxygen-silicon, LJb silanol oxygen-silicon, LJc water oxygen-silicon, LJ zeolite oxygen-silicon, LJ carbon in organic template-silicon, LJd nitrogen in organic template-silicon, LJd hydrogen in organic template-silicon, LJd carbon in organic template-any oxygen, LJd nitrogen in organic template-any oxygen, LJd hydrogen in organic template-any oxygen, LJd

symbol

qi (e)

Hw Ow-Ow Si-Si O-O Ho Oh-Oh Hs-Hs Os-Os Oh-Si Os-Si Ow-Si O-Si C-Si N-Si H-Si C-O N-O H-O

0.41 -0.82 2.10 -1.05 0.13 -1.13 0.425 -0.95

three body, eq 6

D0,ij (eV)

R0,ij (Å)

0.67388 × 10-2 0.79812 × 10-7 0.67388 × 10-2

3.5532 3.7064 3.5532

0.67388 × 10-2

3.5532

0.67388 × 10-2 0.231913 × 10-4 0.231913 × 10-4 0.231913 × 10-4 0.231913 × 10-4 0.18213 × 10-4 0.2991 × 10-4 0.17321 × 10-4 0.52922 × 10-2 0.8692 × 10-2 0.5033 × 10-2

3.5532 3.9 3.6298 3.6298 3.6298 3.5766 3.4032 3.2032 3.5766 3.3266 3.1266

harmonic, eq 5

i

j

k

tb (eV Å-2) kijk

θ0,ijk (deg)

i

j

kijh (eV Å-2)

r0 (Å)

O Si Hw

Si O Ow

O Si Hw

1.2614 1.4 3.9695

109.47 142.0 109.47

Hw Ho

Ow Oh

48.0595 48.0595

1.0 1.0

SDA three body,e eq 6 i

j

C2 H C C2 H H H H

C2 C2 N C2 C2 C1 C2 C1

tb kijk

k C2 C2 C N N N H H

SDA harmonic,e eq 5 -2

(eV Å )

θ0,ijk (deg)

3.56 2.50 6.87 3.56 2.50 2.50 2.06 2.06

110.4 109.0 109.0 105.2 109.0 109.5 109.1 109.2

i C C C

f

j

kijh (eV Å-2)

r0 (Å)

C N H

27.46 28.75 28.71

1.52 1.495 1.095

SDA torsion,e eq 7 i

j

k

l

t (eV) kijkl

C2 C2 C C C2 H

C2 C2 N N C2 C2

C2 C2 C C2 C2 C2

C2 H H C2 N H

0.0032 0.0050 0.0175 0.0035 0.0035 0.0053

a From ref 8. Terminal oxygen atoms in siloxy groups are of this type. b See Figure 1c. c See Figure 1d. d From ref 15. e From ref 14. f C means C1 or C2.

creates a combinatorial structural disorder. We have selected only one distribution. According to our MD calculations, there are several different conformations of the silanol groups. We have also computed a defectless SSZ-74 (96 SiO2), and we compare the results with the as-made material in Table 2, which also contains the experimental data. According to the experimental XRD data refinement, the positions of the oxygen atoms at defect sites for the as-made material differ considerably from the calculated as-made corresponding oxygen atoms (see columns 1 and 6 in Table 2). The calculated distances between corresponding atoms for the defectless and calcined structures (columns 4 and 5 in Table 2) are similar to the reported asmade distances, despite being totally different systems. The total charge of -2|e| in the defect sites and the same opposite sign charge of SDA ions surrounding the cavity create a large electric field inside of the cavity in the as-made zeolite. Silanol and siloxy groups change their conformational states due to the

flexibility of the bonds. The experimentally reported structure1 considers a close to symmetrical configuration of the four -O3Si sites in the defect (regardless if they are -O3Si-O or -O3Si-O-H), while this is not the case for the calculated structures, and this is the reason why our calculations of the as-made material in Table 2 do not agree with the experiment. Apart from the symmetrical average, we suggest that the asmade structure should contain something else not taken into account during the XRD data refinement; water was not included in such refinement. However, the 1H MAS NMR spectra, as we will see below, suggest the presence water. Therefore, in the following, we make a study of water included in the asmade SSZ-74 structure. 3.3. Hydrated As-Made SSZ-74. Regarding the amount of water to be considered, our choice ranges from one to two water molecules per defect. It is clear that the structure cannot accept three water molecules per defect, as can be clearly seen from

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Figure 1. Scheme of the structural transformation of the SSZ-74 zeolite structure; perfect zeolite (a), structure of vacancy (b), and structure of defect in as-made zeolite (c). A SDA2+ molecule (not shown) compensates for the charge; structure of the defect in calcined zeolite (d).

TABLE 2: Geometrical Parameters of the MM Calculated SSZ-74 Defectless (SiO2)96, Calcined Si92O176(OH)16, and As-Made (SiO2)92[(SDA2+(OH-)2)]4 · nH2O Zeolite Structures in the Regions Indicated in Figure 2 dij (Å) sites,ai-j

n)0 calc.

n)4 calc.

n)8 calc.

defectless calc.

calcined calc.

as-made expt.b

O1-O2 O1-O3 O1-O4 O2-O3 O2-O4 O3-O4 Si1-Si3 Si2-Si4

4.22 3.62 4.05 3.10 6.07 3.65 5.60 7.48

4.32 3.63 4.08 3.26 5.84 3.20 5.47 7.56

4.33 3.76 4.22 3.44 5.90 3.14 5.53 7.54

2.53 2.65 2.56 2.52 2.81 2.53 5.69 5.76

2.70 2.83 2.70 2.60 4.24 3.29 5.77 6.68

2.69 2.48 2.51 2.73 4.20 2.42 5.61 6.80

a Notation of atoms is from Figure 2b; (Sij is attached to Oj (j ) 1-4)). b From ref 1.

the lack of room, and therefore, zero, one, or two water molecules per defect will cover the reasonable possibilities. We

may expect different types of cavities filling in the real as-made material, and the population of the water molecules in defect sites may depend on the conditions of the synthesis. It is impossible to simulate a thermodynamic ensemble using the MM method. Even the MD or MC simulations of the final materials are very limited in our case because the unit cell contains as the maximum 16 water molecules, and for good statistics, we would need to simulate a much larger system. Moreover, water is trapped during the synthesis, and the final amount of water in the zeolite depends on kinetic not only thermodynamic factors. These are the reasons to study the limit of the number of the water molecules in the unit cell, n ) 0 and 8, and that for the intermediate case of n ) 4. First, we analyze the results corresponding to the unit cell characterization and compare them to the experimental as-made sample in Table 3. If we look at the a, b, and c cell parameters, it can be seen that the calculated hydrated systems give results closer to the experiment than the calculated anhydrous systems. This effect is even more marked in the unit cell volumes, where any of the calculated hydrated systems give volumes (ranging between 5318 and 5417 Å3) reasonably close to the experimental value of 5382 Å3, while the calculated anhydrous value is considerably lower (5285 Å3) (see Table 3). This indicates that a hydrated as-made sample looks like a better model than the initially considered anhydrous system. This force field was tested in zeolite structures, and it gave accurate unit cell volumes. The hydrated model is clearly more in agreement with the experimental data than the anhydrous model, but beyond that, in order to validate our two models regarding the water content (n ) 4 or 8 water molecules per unit cell), a different set of experimental data is required. Therefore, with the proposal that the hydrated as-made SSZ-74 is a better model than the anhydrous as-made SSZ-74, we now undertake a further analysis, based on NMR spectra of the as-made SSZ-74, to discriminate between the models containing four and eight water molecules per unit cell. 3.4. 1H MAS NMR Spectra of As-Made SSZ-74. We have used the data obtained from the as-made SSZ-741 (also included in the Supporting Information) and made a deconvolution of the peaks using Lorentzian functions. The results of our deconvolution of the 1H MAS NMR spectrum are presented in Figure 3a and Table 4. The spectrum of the as-made zeolite has three bands which indicate, at least, three types of hydrogens in the system. If we propose that the first peak corresponds to the hydrogen atoms of the SDA and take into account that the unit cell

Figure 2. The SSZ-74 as-made anhydrous zeolite structure; 2 × 2 × 2 unit cells (a) with only two SDA ions (balls) shown for clarity; the large cavity with the defects (b) with oxygen and hydrogen atoms of the silanol and siloxy groups presented by balls.



TABLE 3: Calculated and Experimental Geometrical Cell Parameters and Energy of Hydration for (SiO2)92[(SDA2+(OH-)2)]m · nH2Oa n/m/methodb

a (Å)

b (Å)

c (Å)

R (deg)

β (deg)

γ (deg)

V (Å3)

0/0/MM 0/0/MD 0/4/MM 0/4/MD 4/4/MM 8/4/MM 8/4/MD calcined, MM as-made, expt.

20.438 20.227 20.351 20.436 20.424 20.497 20.576 20.344 20.476

13.236 13.156 13.329 13.356 13.331 13.352 13.380 13.220 13.384

19.606 19.151 19.865 20.030 19.894 19.940 20.137 19.889 20.086

90.4 90.0 90.5 90.0 90.3 90.4 90.0 90.7 90.0

103.0 102.5 101.2 101.9 101.0 101.5 102.2 102.2 102.1

89.5 90.0 90.1 89.8 90.0 89.9 90.1 89.2 90.0

5168 4975 5285 5349 5318 5348 5417 5227 5382

∆Ec (kJ/mol)

-114 -73 -77

a The force field in Table 1 has been used for the calculations. b The number of H2O molecules/number of SDA2+(OH-)2 molecules in a unit cell containing 92 SiO2 units/method of calculation. c ∆E ) (En - E0)/n, En is energy of the system with n water molecules in the unit cell.

Figure 3.

1

H MAS NMR spectra and deconvolutions of as-made (a) and calcined (b) SSZ-74 zeolite.

TABLE 4: 1H MAS NMR Spectra Deconvolutions of As-Made and Calcined SSZ-74 Zeolite as-made zeolite r 2 ) 0.9978a i δi, ppm 1 2 3

2.60 5.70 10.47

b

c

wi, ppm 4.97 1.44 1.16

d

ki

calcined zeolite r 2 ) 0.9945 e

Ni

δi, ppm wi, ppm

0.853 136.0 0.072 11.4 0.075 12.1

3.04 5.13

0.79 3.87

ki

Ni

0.46 7.4 0.54 8.6

a 2 r is the coefficient of determination (COD). b δi is the ith peak position. c wi is the width of the peak. d Relative integral intensity of the peak. e Ni is the average number of H atoms in the unit cell corresponding to the peak.

contains four SDA ([C16H34N2]2+) molecules, this means 136 hydrogen atoms (from SDA molecules). The corresponding normalized integral intensity (ki ) Ii/ΣIi) of this peak is 0.853 (Table 4). Then, we take into account the following equation

Ni ) N · ki

(8)

where N is the total number of hydrogen atoms in the unit cell, and N1 ) 136. From eq 8, it is possible to estimate N ∼ 159.5 as the total number of hydrogen atoms. With N known, it is possible to use eq 8 for the other two peaks at 5.70 and 10.47 ppm, whose corresponding intensities, ki, are (see Table 4) 0.072 and 0.075, respectively. Therefore, the corresponding numbers of hydrogens (Ni) are 11.4 and 12.1. It is clear that the system contains a larger number of hydrogen atoms than an anhydrous system, and this is in agreement with our proposal of hydrated

as-made SSZ-74. An anhydrous system is not compatible with the spectrum in Figure 3a. Once the peak at 2.60 ppm (signal 1 in Table 4) is assigned to the SDA and the number of hydrogens of the other two peaks (signals 2 and 3 at 5.70 and 10.47 ppm in Table 4) has been calculated, it remains to identify what kinds of hydrogen atoms are responsible for these two peaks. According to the reported data,1 there are 8 OH groups and 136 SDA hydrogen atoms in the unit cell. With N ∼ 159.5 and N1 ) 136, the eight OH groups are unable by themselves to account for the remaining (N2 + N3 ) 23.5) hydrogen atoms in the system. An additional band due to hydrogen from water may well explain the spectra in which one of the two bands at 5.70 and 10.47 ppm will be due to hydroxyl groups and the other will be due to water. With 8 hydroxyl groups and the number of hydrogen atoms as 23.5, we propose the presence of approximately 8 H2O molecules per the unit cell as the average value per ensemble (8 OH + 8 H2O gives 24 hydrogens). The two peaks at 5.70 and 10.47 ppm may correspond to the two types of H-bonds in the system. A relation between the chemical shift and the H-bond strength can be estimated according to the following empirical linear equation20

δ ) a + b·d

(9)

where a ) 79.5 ppm, b ) -25.5 ppm/Å, δ (ppm) is the 1H chemical shift, and d (Å) is the distance between H-bonded

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TABLE 5: 29Si MAS NMR Spectra Deconvolutions of As-Made and Calcined SSZ-74 Zeolite as-made zeolite r2 ) 0.9981a i

-δi,b ppm

wi, ppm

Nic

Figure 6

1 2 3 4 5 6 7

100.4 103.3 104.9 106.5 108.7 112.9 117.2

2.76 1.38 1.24 1.81 2.60 4.37 1.34

7.3 3.2 4.1 5.8 8.4 60.4 2.8

b c,d,e e f f

calcined zeolite r2 ) 0.9985 -δi, ppm

wi, ppm

Ni

Figure 6

98.7 102.1 105.0 106.5 111.6 114.7

3.05 2.98 1.55 0.80 6.02 4.42

8.3 7.1 2.0 0.6 55.5 18.5

a e e f

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.

oxygen atoms. If we apply this equation to the peaks δ2 ) 5.70 and δ3 ) 10.47 ppm, this gives O · · · O distances of d2 ) 2.9 and d3 ) 2.7 Å, respectively. This empirical correlation has significant limitations and takes into account only a finite set of the systems. In the specific cases, other factors, for example, the distance between oxygen and hydrogen atoms, the electric field and the chemical properties of functional groups, or which atoms are involved in H-bond, affect the chemical shift. It can be said that the hydrogens of peak-2 will be involved in relatively weaker H-bonds than those of the peak-3, which will be stronger bonds. Our previous estimation of the number of hydrogens (N2 ) 11.4; N3 ) 12.1) completes the data available from which an assignment may be suggested. An indication of the presence of water in 1H MAS NMR spectra is the high spinning side band which arises mainly from the strong homonuclear dipole-dipole interaction. This feature is most marked when water molecules remain in fixed positions and are close to each other, but this is not the case in the present material. Dipolar interactions are significantly reduced20,21 in this case due to two-fold rotation (“flips” ) around the bisector axis of the water molecules. These water rotations were observed in our MD simulation. A movie showing this is presented in the Supporting Information. If the free volume in the zeolite cavities is enough to allow for the mobility of the water molecules, it is possible to consider such a situation as a special case of ionic aqueous solution. The mobility of the water molecules and OH groups may eliminate the high spinning side band on the 1H MAS NMR spectra, and in that case, it is difficult to distinguish between hydrogens coming from OH and water, especially if the OH groups strongly interact with Si atoms from the zeolite network. Therefore, as the assignment of the peaks belonging to OH groups and water is not easy from this data, we try to extract this information for the spectrum of the calcined sample. 3.5. 1H MAS NMR Spectrum of Calcined SSZ-74. The 1 H MAS NMR spectrum of calcined SSZ-74 zeolite is presented in Figure 3b. There are two bands in the spectrum with the centers at 3.04 and 5.13 ppm (see Table 5). The integral intensities of the peaks are close to each other. According to our assumption, calcination duplicates the number of internal silanol groups in the system due to the fact that the positive charge of the SDA must be given to the framework as protons. Two protons per SDA correspond to two protons per defect (see the scheme in Figure 1c and d), and overall, the number of silanol groups in the unit cell increases from 8 (before the calcination) to 16 (after the calcination). A comparison between the spectra of the as-made and calcined samples (see Table 4) shows a peak at 5.70 ppm (as-made SSZ-74) and a peak at 5.13

ppm (calcined SSZ-74), which may be assigned to silanol groups interacting. The fact that the peaks appear at similar chemical shifts in the as-made (5.70 ppm) and calcined (5.13 ppm) samples indicates that silanol-silanol interactions in the calcined material are of a similar weak nature as the silanol-water and water-water interactions in the as-made material. The sharp peak at 10.47 ppm disappeared after calcination, and this can be explained by the absence of water-water and water-silanol interactions in the calcined sample. The width of the peak at 5.13 ppm (calcined SSZ-74) is about twice larger than that corresponding to the peak at 5.70 ppm of the as-made zeolite spectrum (widths are 3.87 and 1.44 ppm, respectively, from Table 4), which indicates a wider distribution of H-bond strengths due to hindered rotation and vibration of silanol groups. Sterical obstacles created by water molecules make the peak sharper in the as-made material. A MM simulation of the calcined zeolite showed that four silanol groups in each cavity form three member cycles or chains. One hydrogen atom (out of a total of four) is unbonded. The interatomic distances are presented in Table 2. They are shorter than the corresponding distances in the simulated asmade material. A MD simulation showed the changing character of bonding with time. Due to vibration and rotation of groups, H-bonds break and form in the system. A movie, presented as Supporting Information, shows this effect. With only 16 silanol groups in the unit cell simulation, the statistics is poor. It is difficult to define the average number of H-bonds in the system, and this task is far beyond the purpose of this investigation, but nevertheless, the results of MM and MD simulations can explain the results of the 1H MAS NMR spectrum presented in Table 4. As previously commented, the analysis of the 1H MAS NMR spectrum of the as-made SSZ-74 suggested the hypothesis of the presence of, at least, eight water molecules per unit cell in the as-made zeolite. This water is expected to locate close to the defects at the large cavities, but we may also expect to find “loosely held” water molecules trapped in the small cavities of the zeolite network. 3.6. Further Analysis of the As-Made SSZ-74. The calculations above on the hydrated as-made SSZ-74 suggested the presence of one or two water molecules per large cavity. Now, we analyze, from the calculated data, the energetic aspects of both water contents. The calculations (see Table 3) show that in both models (four and eight water molecules per unit cell), the water stabilizes the system. The values (Table 3) show -114 kJ/(mol H2O) for the case of four water molecules per unit cell and from -73 to -77 kJ/(mol H2O) in the case of eight water molecules per unit cell. Both cases (four and eight water molecules per unit cell) show reasonable values which justify water held in the as-made zeolite. The energetic criteria can be used to establish whether each water content is feasible, but the values cannot be compared to decide which water content is most probable. From this energetic data, it can be said that both water contents are feasible, but a detailed analysis of the geometries of the optimized structures can give us an idea about the number of hydrogen bonds in each case, and this information can be compared with the data from the 1H MAS NMR spectra in order to find the water content of the as-made SSZ-74. 3.6.1. Analysis of As-Made SSZ-74 (Eight Water Molecules Per Unit Cell). The system containing eight H2O molecules in the four defects at the large cavities was simulated by the MM method. As illustration, configurations corresponding to two cavities are presented in Figure 4. Two water molecules in Figure 4a formed H-bonds with two siloxy groups (-O3Si-O)



Figure 4. Two molecular configurations illustrating water positions in large cavities of the SSZ-74 zeolite framework. Water molecules, silanol, siloxyl groups, and bridging oxygen atoms are highlighted by balls, and the zeolite framework is highlighted by tubes. Corresponding schemes are shown below for the sake of clarity.

with distances of 2.57 and 2.62 Å and with two bridging oxygens (-O3Si-O-SiO3-) with distances of 2.67 and 2.72 Å. Additionally, one weak H-bond was formed between silanol groups (-O3Si-OH) with a distance of 3.04 Å. A significantly different structure is presented by the other defect in Figure 4b. The first water molecule connects with a siloxy group and a bridging oxygen (O · · · O distances are 2.68 and 2.71 Å). The second water molecule forms three H-bonds. It forms two H-bonds with siloxy oxygens (O · · · O distances are 2.63 and 2.65 Å) and one H-bond with a silanol group (3.04 Å) as the proton acceptor. There are no H-bonds between silanol groups in this configuration. This is clearly a heterogeneous distribution as the two cavities shown contain different numbers and types of H-bonds. The other two defects in the simulated unit cell show significant differences from these two cavities. Further, and what is more important, this is the picture given by a MM calculation, but the MD simulations performed show water molecules in different configurations as the time evolves, where the mobility of the water molecules reflects changes of the H-bonding states (see Supporting Information). A proper statistical analysis of all of these configurations has been carried out. We have analyzed molecular configurations from the MD runs, and an average estimation shows that in the unit cell, the eight water molecules form six strong (2.55-2.64 Å) and one weaker H-bonds (2.89 Å) with siloxy groups, eight strong (2.55-2.67 Å) H-bonds with silanol groups, and one weaker bond (2.73 Å) with bridging oxygens. Additionally, seven relatively weak (2.89-3.04 Å) H-bonds are formed between silanol groups in the unit cell. As an overall averaged result,

we obtain 9 weak and 14 strong H-bonds. This count should not be considered on a high-quality quantitative basis but rather as an estimation. However, this simple analysis agrees with the experimental 1H MAS NMR spectrum of the as-made zeolite, as we illustrate below. The integral peak intensities, ki, analyzed above from the data in Table 4 and with the help of eq 8, give the estimation of the number of H-bonds. The values, from our previous estimations, of the number of hydrogens associated to the peaks at δ2 ) 5.70 and δ3 ) 10.47 ppm were N2 ) 11.4 and N3 ) 12.1. Now, we can propose that the peak at 10.47 ppm corresponds to the protons involving O3Si-OH · · · H2O and O3Si-O- · · · H2O interactions (strong interactions) and that the peak at 5.70 ppm mostly corresponds to the protons of silanol groups interacting with each other (weak interactions). This proposal is consistent with two other previously mentioned data; (i) the results of the computer simulations gave an estimation of a ratio between strong/weak H-bond interactions of about 14/9, which is not too different from the ratio observed from the NMR data of N3/N2 ) 12.1/11.4. Certainly, a difference between the two estimations appears, but as mentioned above, the accuracy of the value (14/9) should be taken as a qualitative estimation, and therefore, the agreement looks reasonable; (ii) the assignment of the peak at δ2 ) 5.70 ppm to the silanol groups is consistent with the fact that the groups are relatively weakly interacting. 3.7. 29Si MAS NMR Spectrum of As-Made SSZ-74. We now may attempt to explain 29Si MAS NMR spectra, which are presented in Figure 5a and Table 5. From the experimental data, we have made (Table 5) a different deconvolution (seven

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Figure 5.

29

Bushuev and Sastre

Si MAS NMR spectra and deconvolutions of as-made (a) and calcined (b) SSZ-74 zeolite.

peaks) than that proposed in the original paper.1 The estimated number of Si atoms corresponding to the peaks was calculated according to eq 8. A key point to interpret the results of the deconvolution of the 29Si MAS NMR spectra is the assignment of the Q3 and Q4 bands corresponding to silicons in terminal (-O3Si and -O3SiOH) groups and framework silicons, respectively, but in this case, the bands are much overlapped, which makes it difficult to distinguish the state of silicon atoms. Ours (Table 5) and the original deconvolution give a large total number of Si atoms with δSi > -110 ppm. If we assume that only the first three peaks (at -100.4, -103.3, and -104.9 ppm) correspond to Q3 atoms, we obtain, on average, 14.6 Si atoms of such a type per unit cell. Approximately 14.2 Si atoms give the shoulder deconvoluted by the peaks at -106.5 and -108.7 ppm (peaks 4 and 5 in Table 5). Therefore, peaks at δSi > -110 ppm give a total of 28.8 Si atoms, and this is an estimation of the total number of atoms in the unit cell under the influence of interactions changing the chemical shift of a tetrahedral framework Si environment and are associated to the defects in the as-made SSZ-74. According to the XRD data refinement already mentioned, the as-made material contains 8 siloxy and 8 silanol groups, which gives 16 Q3 Si atoms in the unit cell instead of 28.8. We believe this difference is due to the effect of the interaction between water molecules and some Q4 Si atoms. In order to check this hypothesis, we analyze the spectrum of the calcined material. 3.8. 29Si MAS NMR Spectrum of Calcined SSZ-74. This spectrum gives additional information because the number of Q3 Si atoms may be estimated from the deconvolution with higher accuracy due to less overlapping than in the previous case. The deconvolution (Table 5) gives 17.4 Q3 Si atoms in the unit cell. This is in agreement with our above hypothesis, that it is water (in the as-made material) that influences the chemical shifts of some Q4 Si atoms toward values typical of Q3 Si atoms; when water is removed by calcination, this influence disappears, and the peak corresponding to Q3 Si atoms corresponds entirely to the silanol groups. From our previous computer simulations, including MM and MD, we extract the SSZ-74 models of H-bonding displayed in Figure 6. Figure 6a shows an unperturbed silanol group, Figure 6e shows two interacting silanol groups, and Figure 6c and d shows interactions between the silanol group and water. The

Figure 6. Scheme of the molecule configurations extracted from the computer simulations. Silanol group (a); hydrated siloxy group (b); silanol group as the proton acceptor (c); silanol group as the proton donor (d); interaction of two silanol groups (e); H-bond between the water molecule and bridging oxygen (f).

interactions between water and a siloxy group (Figure 6b) and bridging oxygen (Figure 6f) are also shown. All of these types of H-bonding correspond to configurations observed from the analysis of the MD computer simulations in the as-made SSZ74, and we use these models to refer to the bands in the spectra. 3.9. Structural Interpretation of the NMR Spectra. The H-bonds formed in the system have different strengths and geometrical features. We have attempted to give structural interpretation of the 29Si MAS NMR spectra based on the differences between unperturbed (far from the defect) Q4 Si and perturbed (close to the defect) Q4 Si. We rely on the assumption that the larger the perturbation, the larger the effect on the chemical shifts relative to the main Q4 band (∼-112 ppm). This corresponds to the empirical relations between the chemical shift of the 29Si MAS NMR spectrum and the Si-O-Si angle22 (eq 9, where a ) -25.44 ppm, b ) -0.5793 ppm/deg, and d is the Si-O-Si angle). The calcined zeolite has only silanol groups, and according to our previous 1H MAS NMR spectrum analysis, more than half of the silanol groups form H-bonds. We have attributed


Figure 7. “Loosely held” water molecule occluded in the [5661] cavity of the zeolite framework.

the band at δSi ) -98.7 ppm to noninteracting or weak interacting silanol groups (Figure 6a). This corresponds to the most perturbed state of silicon atoms. The bands at -102.1 and -105 ppm are attributed to interacting silanol groups (Figure 6e), and the wide distribution of H-bond strengths is responsible for the overlap between these two peaks (Figure 5b). The state of silicon atoms in silanol groups depends on the type of hydrogen bonding. Each silanol group may participate in bonding as the proton donor and/or proton acceptor. The interpretation of the 29Si MAS NMR spectrum of the as-made zeolite is more complicated than that of the calcined. The spectrum shows the presence of four or five states of the silicon atoms in the ensemble corresponding to their crystallographic positions. We propose that the band at -100.4 ppm, which is farthest from the main, corresponds to Si of siloxy groups interacting with water (Figure 6b). The next two peaks at -103.3 and -104.9 ppm may be attributed to Si in silanol groups acting as the proton acceptor (Figure 6c) and donor (Figure 6d) with water molecules and forming H-bonds to each other (Figure 6e). This gives a large number of possible states which should increase the width of the band, as is, in fact, observed (Table 5). We suggest that the shoulder on the spectrum at -108 ppm is due to Si atoms under the influence of H-bonded bridging oxygens and SDA cations. A weak distortion of electron density on two Si atoms, close to the bridging oxygen, due to the hydrogen bonding with the water molecule (Figure 6f) may only slightly change the shape of the spectrum in the region close to the bands corresponding to Q4 Si atoms. 3.10. On the Existence of Trapped Water after Calcination of SSZ-74. Our results, in agreement with the experiments, considered approximately an average of eight water molecules in the unit cell, with the water molecules located at the defects. We have also considered the possibility that a certain amount of additional water molecules may be located not at defects but rather inside of small cavities. We have done one MM calculation for the system with nine H2O molecules in the unit cell. Eight water molecules were located as previously, and one additional water molecule was inserted in a small [5661] cavity of the zeolite framework. The calculated stabilization energy was -16.1 kJ/(mol H2O). The water molecule formed two H-bonds with bridging oxygens with bond lengths of 2.53 and 2.62 Å. A fragment of the structure is presented in Figure 7. Water molecules may be trapped in the zeolite framework cavities at nucleation and crystallization stages.23-25 On the basis of our calculations, we may say that such processes are not energetically favorable, but we should take into account that the process

J. Phys. Chem. C, Vol. 113, No. 25, 2009 10885 is governed mostly by kinetic factors. If trapped water molecules, which are loosely held, are present in the [5661] cavities of the zeolite framework, they should form H-bonds with oxygens (-Si-O-Si-) in the cavity, and this will affect the chemical shift of the silicon atoms connecting to the bridging oxygens. According to our analysis, these water molecules influence the 29Si MAS NMR spectrum of as-made material at δSi ∼ -107 ppm. It is important to note that this trapped water molecule in the small cavity should remain occluded even after the calcination.25 In the 29Si MAS NMR spectrum of the calcined zeolite, we can see a small shoulder at this region. It is difficult to estimate the amount of such molecules simulating the final zeolite structure with the standard computational methods. However, if we take into account that each water molecule may influence about four close Si atoms and that the estimated number of such Si atoms in calcined zeolite is 0.6 (Table 5), we can conclude that there are, on average, 3 trapped water molecules per 20 zeolite unit cells (1840 SiO2 units). This means that entrapped water molecules play a minor role in the zeolite synthesis, but it is, nevertheless, a non-negligible contribution. 4. Conclusions A new force field is proposed for simulation of zeolites containing silanol and siloxy groups. The force field was used for MM and MD simulations of the new type of as-made and calcined pure silica zeolites with a high ordered system of defects. The results of the simulations were compared with the result of XRD refinement and 1H and 29Si MAS NMR spectra. The spectra were deconvoluted and explained based on the assumption that in the as-made SSZ-74, water molecules are present in the defect regions. We estimate that the water content is about eight water molecules per unit cell. The location of the water molecules has been characterized, and we find that two water molecules per defect are about the expected content of the as-made SSZ-74. The water positions have been analyzed and rationalized in terms of H-bonds, and the following interactions have been found to be responsible of most of the features observed: water-silanol, water-siloxy, silanol-silanol, and water-bridging oxygen. All of these interactions have been justified throughout the results of the computer simulations and the analysis of the 1H and 29Si MAS NMR spectra of the asmade SSZ-74, with a reasonable agreement between computational and experimental results having been observed. This model is also compatible with the 1H and 29Si MAS NMR of the calcined material. Finally, the possibility of some additional water remaining occluded (and loosely bound) in the small [5661] cavities of the material has been investigated. This water is the only which remains occluded not only in the as-made material but also in the calcined material. 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: This includes GULP input files with the used potential, cif files, our final structures, and a movie file with visualization of MD calculations. Also a cif file with the hypothetical defectless SSZ-74 structure is included. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Baerlocher, C.; Xie, D.; Mccusker, L. B.; Hwang, S.-J.; Chan, I. Y.; Ong, K.; Burton, A. W.; Zones, S. I. Nat. Mater. 2008, 7, 631.

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(2) Bougeard, D.; Smirnov, K. S. Phys. Chem. Chem. Phys. 2007, 9, 226. (3) Zones, S. I.; Burton, A. W.; Lee, G. S.; Olmstead, M. M. J. Am. Chem. Soc. 2007, 129, 9066. (4) Sacerdoti, M. Microporous Mesoporous Mater. 2007, 102, 299. (5) Cruciani, G. J. Phys. Chem. Solids. 2006, 67, 1973. (6) Gale, J. D. J. Chem. Soc., Faraday Trans. 1997, 93, 629. (7) Gale, J. D.; Rohl, A. L. Mol. Simul. 2003, 29, 291. (8) Cygan, R. T.; Liang, J.-J.; Kalinichev, A. G. J. Phys. Chem. B 2004, 108, 1255. (9) Berendsen, H. J. C.; Postma, J. P. M.; van Gunsteren, W. F.; Hermans, J. Interaction models for water in relation to protein hydration. In Intermolecular Forces; Pullman, B. , Ed.; D. Reidel: Amsterdam, The Netherlands, 1981; p 331. (10) Sastre, G.; Fornes, V.; Corma, A. J. Phys. Chem. B 2002, 106, 701. (11) Sastre, G.; Lewis, D. W.; Catlow, C. R. A. J. Phys. Chem. 1996, 100, 6722. (12) Sastre, G.; Vidal-Moya, J. A.; Blasco, T.; Rius, J.; Jorda, J. L.; Navarro, M. T.; Rey, F.; Corma, A. Angew. Chem., Int. Ed. 2002, 41, 4722. (13) Sastre, G.; Leiva, S.; Sabater, M. J.; Gimenez, I.; Rey, F.; Valencia, S.; Corma, A. J. Phys. Chem. B 2003, 107, 5432. (14) Oie, T.; Maggiora, T. M.; Christoffersen, R. E.; Duchamp, D. J. Int. J. Quantum Chem., Quantum Biol. Symp. 1981, 8, 1.

Bushuev and Sastre (15) Kiselev, A. V.; Lopatkin, A. A.; Shulga, A. A. Zeolites 1985, 5, 261. (16) Baram, P. S.; Parker, S. C. Philos. Mag. B 1996, 73, 49. (17) Du, J.; Cormack, A. N. J. Am. Ceram. Soc. 2005, 88, 2532. (18) Bordiga, S.; Ugliengo, P.; Damina, A.; Lamberti, C.; Spoto, G.; Zecchina, A.; Spano`, G.; Buzzoni, R.; Dalloro, L.; Rivetti, F. Top. Catal. 2001, 15, 43. (19) Bordiga, S.; Roggero, I.; Ugliengo, P.; Zecchina, A.; Bolis, V.; Artioli, G.; Buzzoni, R.; Marra, G.; Rivetti, F.; Spano`, G.; Lamberti, C. J. Chem. Soc., Dalton Trans. 2000, 3921. (20) Eckert, H.; Yesinowski, J. P.; Silver, L. A.; Stolper, E. M. J. Phys. Chem. 1988, 92, 2055. (21) Eckert, H.; Yesinowski, J. P.; Stolper, E. M.; Stanton, T. R.; Holloway, J. R. J. Non-Cryst. Solids 1987, 93, 93. (22) Thomas, J. M.; Klinowski, J.; Ramdas, S.; Hunter, B. K.; Tennakoon, D. T. B. Chem. Phys. Lett. 1983, 102, 158. (23) Breger, I. A.; Chandler, J. C.; Zubovic, P. Am. Mineral. 1970, 55, 825. (24) Van Reeuwijk, L. P. The thermal dehydration of natural zeolites. Meded. Landbouwhogesch. Wageningen 1974, 74, 1–88. (25) Piccione, P. M.; Laberty, C.; Yang, S. Y.; Camblor, M. A.; Navrotsky, A.; Davis, M. E. J. Phys. Chem. B 2000, 104, 10001.

JP9013306

Atomistic Simulations of Structural Defects and Water Occluded in SSZ

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