J. Phys. Chem. C 2010, 114, 2989–2995
2989
Mechanism of F- Elimination from Zeolitic D4R Units: A Periodic B3LYP Study on the Octadecasil Zeolite C. M. Zicovich-Wilson,*,† M. L. San Roma´n,‡ and A. Ramı´rez-Solı´s† Depto. de Fı´sica, Facultad de Ciencias, and Centro InVestigaciones Quı´micas, UniVersidad Auto´noma del Estado de Morelos, AV. UniVersidad 1001, Col. Chamilpa, 62209 CuernaVaca (Morelos), Mexico ReceiVed: September 12, 2009; ReVised Manuscript ReceiVed: January 19, 2010
The mechanism of elimination of F- trapped inside the double four-membered ring (D4R) cages of octadecasil has been determined by means of periodic calculations at the B3LYP/VDZP//VTZP level of theory with the CRYSTAL06 program. Finite temperature and entropic effects are taken into account to study the efficiency of the elimination reaction, which is exothermic, requires acid media, and is catalyzed by cationic polar molecules with accessible protons, from which a hydronium ion has been considered in the present study. The mechanism consists of three steps that schematically correspond to hydrolysis of one SiOSi bridge in the D4R cage, extraction of the F- from the unit, and recondensation of the bridge with formation of slightly adsorbed HF · H2O in the large cavity that can be easily eliminated by diffusion. The limiting step is the first one and involves a quite low Helmholtz free energy barrier even at 100 °C (51.6 kJ mol-1) that slowly decreases with increasing temperature. 1. Introduction An interesting alternative to the traditional hydrothermal route of synthesis of porous silicates is the so-called fluoride route, which displays comparative appealing features.1,2 At variance with the former that requires basic media, the latter is performed in neutral solutions (pH 5-9), and the presence of F- ions as mineralizing agents allows dealing with reaction mixtures with very low or even null Al content. Moreover, the products obtained in this manner exhibit a quite large crystal size and low occurrence of structural defects, which permits synthesizing significantly hydrophobic materials suitable for interesting technological applications.2 A remarkable feature of the fluoride route is that it makes the synthesis of pure silica frameworks with [46] cages feasible, more commonly denoted as D4R units, which can not be obtained at all through the hydrothermal route. This fact has suggested that the fluoride anion could play the role of structuredirecting agent (SDA) toward the formation of such cages. This is partially evidenced by 19F MAS NMR and XRD characterization techniques that indicate the anion always appears occluded within the D4R cages, close to the center, in the as-synthesized materials.2,3 In addition, even when no D4R cages are formed, the F- still appears frequently associated to four- [five-]ring units in coordination with one of its constituent Si atoms in a trigonal bypyramid [SiO4/2F]- conformation.1,2,4–6 The presence and stability of F- inside the cages has been discussed in a previous computational DFT study in which the cluster model was adopted to represent the zeolitic moiety.7 On the basis of the Mulliken atomic populations, the authors suggested that the structure-directing effect toward the formation of D4R cages might involve a weak covalent bond between the central F and the Si atoms at the angles of the D4R unit. However, owing to the significant spatial confinement of the F * To whom correspondence should be addressed. E-mail: claudio@ servm.fc.uaem.mx. † Depto. de Fı´sica, Facultad de Ciencias. ‡ Centro Investigaciones Quı´micas.
atom within the cage,1 it is expected that the Mulliken atomic partition is not able to reliably assign the electrons to atoms due to basis set superposition effects. On the other hand, this kind of interaction is not actually evidenced by very sensitive characterization experiments, like 29Si NMR, because even with an apparent presence of F- within the cage, the observed chemical shifts of 4-fold coordinated Si remain similar to those of the clean zeolite, without any trace of Si-F interaction.1 Nonetheless, in a more recent work from some of us devoted to the structural and vibrational analysis of as-made TMI-F/SiITW,8 more detailed clues have been obtained on the nature of the interaction between the occluded anion and the silica framework. By analyzing the calculated dynamic Born atomic charges, validated by comparison with experimental IR intensities, it arises that the presence of F- inside the D4R cages enhances the ionicity of the Si-O bonds in it and, moreover, brings about a dynamic charge transfer between the anion and the framework. This global effect with time, causes an increase in the structural flexibility, directing the synthesis toward the formation of building units like D4R that are quite strained in the pure silica form without F-. A similar argument has been previously considered to explain the preferential location of framework cations more electropositive than Si, for instance, Ge and Ga, in fairly strained cages like [46] and [44].9,10 Experiments also provide proof that the anion is fully eliminated together with the cationic template after calcination without appreciable generation of structural defects.1,2 The way the occluded fluoride anion leaves the cage to finally be released out of the material has been a matter of controversy in the literature. In a computational study performed under the classical Born model on Octadecasil (AST code),11 the energy barrier for migration of the anion across the quite small four-member window of the cage has been estimated to be more than 3 eV, a fact that made the authors to conclude that either the F- does remain trapped inside the D4R cage, even upon calcination, or it is not actually trapped in the D4R units in the as-synthesized material. On the other hand, a detailed experimental study12 provides strong evidence of the absence of fluoride in the zeolite
10.1021/jp9088244 2010 American Chemical Society Published on Web 02/04/2010
2990
J. Phys. Chem. C, Vol. 114, No. 7, 2010
Zicovich-Wilson et al.
Figure 1. Si-AST framework, (100) view (left) and primitive unit cell (right).
after calcination and of its presence inside the D4R cages in the as-synthesized material, in contradiction with the previous statement. In that work, the authors propose that the fluoride anion migrates out from the D4R unit in a process that involves the opening of the cage by hydrolysis of one of the Si-O-Si bridges, allowing diffusion of the F- toward the larger cavity and through the channels up to elimination. A further recondensation of the same bridge restores the starting framework connectivity that, at the end of the process, appears without defects. This hypothesis is supported by a more recent paper in which full elimination of the SDA, and supposedly of the fluoride anion occluded in the D4R cage, in acid medium at quite low temperatures is reported for as-made beta zeolite.13 Although the previous mechanism of elimination appears to be the only one that explains the experimental results, it still leaves some unsolved issues: (1) it is generally assumed that the formation of defects in a crystal is entropically favored and the restoration to the perfect crystallinity should be a rather slow process, while experiments on fluoride elimination witness a fast recovery of the crystallinity upon fluoride remotion; and (2) it is also assumed that pure silica zeolites are thermally resistant structures even at temperatures higher than that employed in the calcination, therefore, it seems unlikely at first glance that the elimination of extraframework ions could involve the spontaneous breaking of the framework structure. In the present work a detailed computational study of the mechanism of fluoride elimination from the D4R units of asmade Octadecasil is performed through the periodic B3LYP approach, as implemented in the CRYSTAL06 program.14 Octadecasil has been chosen because it has been the object of previously mentioned works,11,12 and very accurate experimental results on the elimination of fluoride from their cages are available. Nonetheless, it is worth stressing that the present results can be transferred to every silica-zeolite that exhibits D4R cages. And, last but not least, the not too large number of atoms per unit cell (30) makes the electronic structure calculations feasible at a rather high level of accuracy. Accordingly, the present investigation includes the optimization of all transition states and stable species, its characterization as critical points of the energy hypersurface by means of the vibrational analysis, and the consideration of zero-point and thermal energy contributions. This permits the achievement of a truly reliable and realistic enough picture of the whole solid state process that takes place during the elimination reaction.
The main purpose of this work is to check the feasibility of a reaction path that involves hydrolysis-condensation of the siliceous structure so as to explain the removal of the F- from the D4R cages in zeolites. The present computational investigation has led to a mechanism similar to that proposed in ref 12, but with some additional features that are crucial to understand the whole process. One of the key points is that the mechanism, being similar in several aspects to typical heterogeneous catalysis reactions in zeolites as, for instance, in the occurrence of concerted steps, remarkably differs in that the transformation takes place in the solid phase and the catalyst is actually a diffusing extraframework molecular species that must be a small protonated polar molecule. In our calculations the hydronium cation has been considered to play this role since it is present in both elimination processes: the low temperature/acid medium and the high (calcination) temperature reactions, appearing in the latter as a consequence of the SDA thermal decomposition. 2. Computational and Modeling Details The host material Si-AST (Octadecasil) is here represented by means of a periodic model, which is depicted in Figure 1. At this point we must stress here that the overall charge of the system we model here is zero; because the formal charge of the fluoride anion occluded in the D4R cage is -1, to have a totally neutral system, as in the real situation, a positive counterion must be included, here represented by the hydronium ion arising from the intervening water molecule. A heterogeneous phase reaction like the one concerned in the present work is not, in general, expected to evolve simultaneously in all unit cells of the crystal, but in a process where at a given time a sparse distribution of sites are involved in the reaction, while most of the crystal remains practically unchanged. Therefore, each of these reacting sites behaves in practice as a kind of isolated defect surrounded by a region of the framework whose flexibility modes only allow a partial relaxation of the active part. The periodic model is obviously not fully suitable to represent this situation as it forces all the cells to evolve in phase along the reaction path neglecting the structural constraints imposed by the environment upon the nearly isolated reacting cell. On the other hand, a cluster model in which the border atoms were fixed at their positions in the perfect crystal is also inadequate because the active part will be extremely constrained by the environment along with the simultaneous lack for a good description of the very many long-range interactions present in
F- Elimination from Zeolitic D4R Units the real system. To overcome those shortcomings, a large enough cluster with tens of unit cells should be considered, which would be prohibitive in terms of computational requirements at a relatively high level of theory. In this work a kind of compromise between these two extreme approaches has been chosen: the periodic model is adopted owing to its advantages concerning the quite natural incorporation of long-range effects, a lower number of degrees of freedom compared to large enough finite models and lack of spurious border effects. However, to impose constraints similar to that provided by the whole lattice on an isolated reacting site, the cell parameters has been kept fixed into their experimental values for the tetragonal clean material (crystallographic cell: a ) b ) 9.196 Å, c ) 13.40 Å; primitive cell: a ) b ) c ) 9.337 Å, R ) β ) 121.00°; γ ) 88.29°).15 We also note that, given the large cell parameters, the interactions between neighboring reaction sites that involve small partial charges are expected to play a minor role in the overall shape and detailed topology of the potential energy surface. Calculations have been performed at the hybrid Kohn-Sham level of theory according to the B3LYP approach proposed by Becke16 as it is implemented in the LCAO code CRYSTAL06.17 Previous studies of zeolitic systems have successfully addressed energetic and structural properties carried out using this hybrid functional.8,10,18–21 The reliability of B3LYP in the estimation of reaction barriers has been recently tested for the first time with CRYTAL in periodic zeolitic systems,22 but the method has been widely employed to study catalytic reactions in zeolites with cluster models exhibiting good reliability in reproducing experimental data. Two different accuracy levels for the estimation of the electronic energy have been adopted in this work, namely, L1 and L2. In the former, a basis set that consists of a 6-31G(d,p) set for H, F, and O and 6-21G(d) for Si atoms is considered. The sets have been modified from their standard form by reoptimizing the last sp exponents for O and Si to 0.2798 and 0.13 bohr-1, respectively, leading to an optimal basis set for silica environments. The exponents of the d polarized shells are 0.5, 0.3, and 0.5 bohr-1 for O, F, and Si, respectively. This basis set has been employed in previous works involving silica with very good performance in geometry optimizations and frequency calculations, which indicates a quite good representation of the potential energy hypersurface for the nuclei.8 The accuracy in the calculation of the mono- and bielectronic integrals is tuned by the standard tolerances recommended in the code manual, while for the numerical integration in the DFT part, a large pruned grid with 75 points in the radial and 434 points in the angular parts (see LGRID keyword in the manual) around each nucleus has been adopted. The highest accuracy level, L2, involves a standard 6-311G(d,p) (for Si, 88-31G(d)23) basis set and the following set of tolerances for the integrals estimation (8, 8, 8, 8, and 16). Moreover, the integration grid for the electronic density is here much denser than in the previous case, albeit still being pruned, also displays 75 points in the radial part but includes up to 974 angular points (XLGRID keyword in the code manual14). In all cases, a Pack-Monkhorst24 shrinking factor 2 has been considered that leads to 8 k-points for sampling the Brillouin Zone. All structures have been fully optimized except for the cell parameters that have been kept fixed, as explained above. Transition state optimizations have been performed, adopting the procedure proposed by Nichols et al.25 A preliminary exploration of the potential energy hypersurface (PEH) has been
J. Phys. Chem. C, Vol. 114, No. 7, 2010 2991 performed by scanning along some selected valence internal coordinates. The definition of internal coordinates is performed using the valence redundant scheme originally proposed by Pulay et al.26 All these tools which facilitate the localization of transition states in complex PEH, including the saddle point optimization, have been implemented in a development version of the CRYSTAL06 code. Details of this implementation are provided elsewhere.22 The vibrational analysis has been performed only at the Γ-point. To estimate the thermal and zero-point contributions to the Helmholtz free energy, the harmonic approximation has been adopted. This means that only vibrational contributions to the entropy are here actually considered. In this approach the influence of phonon dispersion in the vibrational corrections to the free energy is neglected under the supposition that the long-range terms are practically the same in all species considered as they are just partially ionic and the system exhibits a quite large unit cell. This is expected to minimize the differential effects connected to phonon dispersion. Geometry optimizations and frequency calculations have been performed at the L1 level of accuracy which also includes the zero-point and thermal corrections to the free energy. At this level all TS structures exhibit just one negative eigenvalue of the Hessian warranting they are true saddle points of the PEH. As concerns the electronic contributions have been computed at the L2 level with single point calculations at the optimized geometries. Basis set superposition error (BSSE) corrections have also been performed at this level for the desorption of H2O and HF, adopting the Counterpoise scheme.27 3. Results and Discussion 3.1. Structural Features. The F-exclusion mechanism takes place in three steps, namely, (1) cage opening, (2) exiting of F- from the cage, and (3) cage closing. In the last step, the clean octadecasil structure is reached upon HF and H2O formation and further elimination from the zeolite. The notation adopted for the four stable species considered consists of two characters that indicate whether the cage is open (O) or closed (C) and two more to identify if the F- is inside (i) or outside (o) the cage. Transition states are labeled denoting the features that change in going from reactants to products. They appear separated by colons, for instance, C:O denotes the transition state when going from a closed to an open cage. Accordingly, the label Oi refers to the species in which the cage is open while the fluoride anion is still inside it. For transition states, the label O(i:o) indicates the one exhibiting the cage opened with the fluoride evolving from inside to outside. The structures of all stationary species along the reaction path are represented in Figures 2 and 3. To facilitate monitoring the structural evolution along the reaction path, some selected geometric parameters are provided in Table 1. In the table, Ow, Oz1, and Oz2 refer to the O atoms belonging to the water molecule and the Si-O-Si bridges protonated and interacting with water in Ci (see Figure 2), respectively. On the other hand, H+, Hw2, and Hw1 label the protons, originally belonging to the hydronium cation, interacting with Oz1, Oz2, and noninteracting, respectively, always in Ci. The reaction starts with the F- inside the cage and a hydronium, H3O+, placed in the large cavity. The hydronium may appear as a result of either the decomposition of the SDA at calcination temperatures or the acidic medium as in the previously mentioned case of low temperature elmination in zeolite β.13 In fact, the structure with hydronium is not a stationary point of the PEH. In the first stable structure
2992
J. Phys. Chem. C, Vol. 114, No. 7, 2010
Zicovich-Wilson et al.
Figure 2. Graphical representation and some selected structural parameters of the stable species considered in the reaction path. Distances are given in Å.
considered, the starting H3O+ evolves along the optimization process and changes its nature by donating a proton to one SiOSi bridge of the D4R unit, remaining a distorted water molecule with a strong H-bond with the very acidic Si(OH)Si bridge formed and a weaker interaction with the O atom of another Si-O-Si bridge belonging to the same cage in a neighboring cell. This is shown as Ci in Figure 2. The former interaction is to be ascribed to the partial transfer of the proton from the H3O+ to the O-bridging with HO distances of 1.08 and 1.39 Å, respectively. The second interaction is a characteristic H-bond with a HO distance of 1.83 Å. Concerning the F- anion, it moves out from the center of the cage, where it is located in the asmade samples, according to both experimental1,4 and theoretical7,8,28 evidence. The displacement of the fluoride anion, which in the original cage has barely enough room to reside inside the unit experimenting slight Pauli repulsion with the O electrons,7,8,28 is now possible because the interaction of the O-bridging with the proton gives rise to a weakening of the SiO bond, in accordance with the Gutmann rules,29 involving the lengthening of the bond and, therefore, the enlargement of the cage close to the Si angle. Then the Si atom changes its coordination state to 5-fold, displaying a bond with the F- (1.87 Å) and an elongation of the Si-O bond, that belongs to the protonated bridge, reaching 1.77 Å. The geometry of the SiO4F unit matches the corners of a trigonal bipyramid, similar to that appearing in zeolites synthesized by the F-route that do not feature D4R
cages.30 The Si-O bond that, together with the Si-F line, determines the principal axis, is 1.68 Å long, and the F-Si-O angle is 169°. The remaining Si-O bond lengths are 1.65 Å. The stable species Ci evolves to the Oi structure through the TS (C:O)i. In going from the reactant to the transition state, the structure exhibits global changes, revealing that the process is not localized. The Si-O distance increases to 2.77 Å, indicating a bond breaking, with a simultaneous recovering of the tetrahedrality of the remaining SiO3F unit (F-Si distance decreases to 1.68 Å and the F-Si-O angle of the atoms originally on the axis is now 134°). In a concerted way, the proton shared by the O-bridging and the water molecule is definitely transferred to the silica O atom, giving rise to a Silanol group with a hydrogen-bonded water molecule (OH distances: 0.99 and 1.71 Å, respectively). It turns out that in this TS the originally free water H atom approaches the fluoride owing to a slight electrostatic interaction. The geometry of the first intermediate, Oi, is very similar to (C:O)i, except for the geometry of the neighbors of the Si atom, which is evolving from 5- to 4-fold coordination. The distance between the Si to the O-bridging increases to 3.6 Å, while the F-Si-O angle corresponding to the atoms that previously lied on the original axis of the trigonal bipyramid now becomes 123°. Accordingly, the F atom approaches the limit between the newly opened cage and the cavity where the water molecule resides.
F- Elimination from Zeolitic D4R Units
J. Phys. Chem. C, Vol. 114, No. 7, 2010 2993
Figure 3. Graphical representation and some selected structural parameters of the active part of the transition states between the stable species. Distances are given in Å.
TABLE 1: Selected Distances (in Å) in All Stationary Points Along the Reaction Path distance +
Ow-H Ow-Hw1 Ow-Hw2 Si-F Si-Oz1 F-Hw1 Oz1-H+ Oz2-Hw2
Ci 1.39 0.98 0.98 1.87 1.77 4.84 1.08 1.83
(C:O)i 1.71 0.97 0.97 1.68 2.77 3.70 0.99 2.19
Oi 1.72 0.97 0.97 1.65 3.14 3.63 0.99 2.17
O(i:o)
Oo
(O:C)o
Co
1.88 0.97 0.97 1.61 3.59 2.90 0.98 2.25
1.99 0.97 0.97 1.62 3.25 1.90 0.99 2.27
1.18 0.99 0.98 1.80 1.83 1.61 1.31 2.0
1.56 0.97 0.97 3.09 1.62 0.97 2.54 2.73
In the next step, the F atom goes outside the cage and forms a hydrogen bond with the water molecule. In the TS, namely, O(i: o), the SiO3F unit acquires a quasi regular tetrahedral configuration and the silanol group is located 3.59 Å away from the Si atom. The electrostatic interaction between the water H and the F atoms appears to be enhanced as their distance is shortened to 2.90 Å. The Si-F bond turns out to be more covalent, in accordance with the increase of the sp3 character of the Si orbitals, featuring a bond length of 1.61 Å. This process leads to the intermediate called Oo, in which the F atom is fully outside the cage, a situation accompanied with an important modification of the geometry of the cage and the neighboring atoms. The distance between the silanol OH and the Si atom bonded to F slightly decreases to 3.25 Å, while the F atom forms a H-bond with the water H atom. The structure is additionally stabilized by two other H-bonds between
the remaining water H and one O atom located on the cavity surface and between the water O and the H atom of the silanol group. At this point the structure is quite tightly bound and this feature allows the easy restoring of the conectivity by recondensation of the SiOSi bridge of the cage, as it is explained in what follows. The process takes place in a concerted way; the TS structure is given in Figure 3, (O:C)o. The cyclic intermediate has six centers and involves the F anion, its neighboring Si atom, the silanol group, and the water molecule. Practically all distances in the cycle are far from their equilibrium values, as shown in Figure 3, because all atoms involved are going to break or form bonds. The Si atom takes once more a 5-fold coordination in a process that finally gives rise to the breaking of the bond with fluoride while restoring the Si-O-Si bridge. The F atom is forming a new bond with one water hydrogen while it is dissociating from the molecule, and the silanol H atom is being transferred to the water molecule. This structure is additionally stabilized by a H-bond between the remaining water H and an O atom of the cavity surface. The reaction gives as products an HF molecule, electrostatically interacting with two Si atoms of the cage and hydrogen-bonded to the water molecule, which is simultaneously interacting with the cavity surface through a H-bond. The silica structure recovers the original structure with practically the same symmetry of the pure material, while the geometry of the HF and the water molecule closely resembles that of the free molecules.
2994
J. Phys. Chem. C, Vol. 114, No. 7, 2010
Zicovich-Wilson et al.
Figure 4. Helmholtz free energy profile of the whole reaction path.
TABLE 2: Energy Differences (Electronic ∆E, Internal ∆U, and Free Helmholtz ∆F, at 373 and 773 K, in kJ mol-1) of the Stationary Species with Respect to the Starting Reactant Species, Ci ∆U (C:O)i Oi O(i:o) Oo (O:C)o Co SPrdsa a
∆F
∆E
373 K
773 K
373 K
773 K
55.9 52.8 57.1 6.2 40.1 -59.8 -28.2
56.1 56.6 58.3 11.4 33.7 -54.7 -44.6
54.3 58.1 57.2 13.0 30.1 -53.7 -57.7
51.6 42.4 50.5 -2.6 30.8 -71.7 -157.7
47.3 26.3 42.3 -18.5 29.2 -90.6 -283.3
Separated products: Si-AST + H2O(g) + HF(g).
It is expected that these molecules diffuse across the structure at calcination temperature, catalyzing the fluoride elimination in other still occupied cages up to exit from the crystallite, or diffuse through the solvent medium in the experiment performed at low temperature in acid medium. 3.2. Energetics of the Reaction Path. The electronic, internal, and free energies of the stationary points with reference to the initial reactants are documented in Table 2, while a graphical representation of the reaction profile in Helmholtz free energies is given in Figure 4. Two temperatures have been considered, namely, 373 and 773 K, which allows to compare the situations at low temperature/acid medium and calcination conditions, respectively. The reaction turns to be exothermic considering the situations where the water and HF molecules are either still trapped inside the material (see Co in Figure 2) or in the gas phase as the final product of the calcination reaction. Obviously, in the latter, the entropic effects extremely favor the formation of products as it arises from the free energies reckoned in Table 2. However, a less pronounced effect also takes place when the molecules remain occluded in the cavity and, quite remarkably, the products are more stable than the reactants even considering just the electronic energy term (∆E in Table 2). The preference of the F atom to reside outside
the cage in the form of HF probably obeys the tendency to decrease the electrostatic energy by reducing the charge separation of the systems. It is likely that a slight additional instability of the reactants may reside on the repulsion between the F- and the cage O atoms, owing to the close contact between them. Accordingly, the reaction is expected to be spontaneous from the thermodynamic point of view. As concerns the kinetics, it turns out that the activation energy of the limiting step, Ci f Oi, is quite low, as it ranges from 47.3 to 51.6 kJ mol-1 at 773 and 373 K, respectively. This explains why, even at quite low temperatures, the reaction takes place, requiring only the presence of a protic molecule capable of protonating one SiOSi bridge of the D4R cage and initiating the reaction. The hydrolysis process is favored by the fact that the Fis able to easily react with the SiO4 unit, probably owing to its quite small atomic radius, and substitute one of the O-bridging, giving rise to the opening of the cage. The stability of the first intermediate, Oi, is much more favored by the temperature than the TS, (C:O)i. This is because the former is less bounded than the latter and, therefore, the entropic effects, here computed considering only the vibrational contributions, are more significant. This agrees with the different energy trends given in Table 2, while the effect of adding the thermal energy and zero-point corrections to the electronic term to obtain the internal energies leads in principle to a slight increase of ∆U attributable to the kinetic energy permitted by the less strained situation. However, the significant stabilization displayed when adding the entropic term evidences the additional disorder associated to a less connected structure. The second reaction step, Oi f Oo, is quite fast owing to its small activation energy and is remarkably exothermic. The stability of Oo is to be attributed to the occurrence of several H-bonds connecting the water molecule with the silanol group, the F atom and one O atom at the cavity wall. In addition, the system seems to be flexible enough so as to allow a quite disordered statistical state that gives rise to a remarkable entropic stabilization favored by temperature in the case of Oi. The last reaction step that concerns the simultaneous elimination of F- and closing of the cage by condensation of the SiOSi
F- Elimination from Zeolitic D4R Units bridge, Oo f Co, is also quite fast in general, though it is expected to still be faster at low temperatures. The key feature that favors this passage is the formation of a TS, (O:C)o, that exhibits a cyclic structure of six centers formed in a concerted way. The structure is quite tightly bound as it is revealed by the quite low dependence on the temperature and its relative stability (even neglecting dynamic effects) makes fast and feasible the reconstruction of the original conectivity of the framework. 4. Conclusions The fluoride elimination reaction from the D4R cages has been a matter of much debate and carries a long story of research efforts. Although a previous cluster-based investigation suggested that this reaction could not occur, this very refined and exhaustive periodic study shows that it is actually possible due to a conjuction of effects arising only when the extended solidstate structure is considered and an acid medium acting as catalyst is involved. The present results are able to explain why the fluoride elimination takes place despite it seems to be counterintuitive, as discussed in the Introduction. The fact that the reaction starts with the breaking of one SiOSi bridge, even though it is energetically and kinetically unfavored, is here explained by the particular affinity of the F- to coordinate with the Si atoms. A concerted mechanism undergoes the substitution of the O-bridging by the fluoride anion in the 4-fold coordinated Si unit, allowing the opening of the cage with a quite low activation energy, although this is the limiting step of the reaction. For this reaction to occur, the presence of an occluded species able to protonate the O-bridging is mandatory, and the lack of this agent in previous studies is one the reasons for their failure to explain the experimentally observed F- elimination from the D4R cages. From the strictly entropic point of view, the elimination of structural defects in zeolitic systems is, in general, unfavorable; however, this study shows that the formation of a quite tightly bound six-center TS gives rise to a low energy barrier for the reconstruction of the framework connectivity, thus, overcoming the general trend to increase the structural disorder. We think this might also explain the remarkable lack of defects exhibited by Si-zeolites synthesized through the F-route. A very important conclusion one can draw from the present results is that the reaction is as kinetically and thermodynamically favorable that it does not require high (calcination) temperatures to take place. This strongly supports previous indirect experimental evidence that the anion is fully eliminated from D4R cages at 150 °C in acid media together with the SDA in as-made β zeolite.13 Obviously, much more experimental effort is required to characterize each of the involved species and validate that F- can be easily extracted from the D4R cages in acid medium even at quite low temperature. The reaction studied in this work is an example of solid state chemistry that exhibits very interesting similarities with heterogeneous catalysis. Actually, the process occurs through the interaction between a fluid and a solid phase and involve concerted mechanisms similar to those that frequently appear in catalytic reactions on zeolites. However, at variance with the usual catalytic reactions, here the chemical change takes place in the solid phase, while the catalyst is an extraframework species: the protic molecule here represented by a water molecule. The role of the concerted mechanisms is crucial for the feasibility of the reaction from both the thermodynamic and the kinetic points of view. The present study strongly suggests that the occurrence of concerted mecha-
J. Phys. Chem. C, Vol. 114, No. 7, 2010 2995 nisms involving several centers in zeolitic reactions do not exclusively take place in catalysis but also in a more general set of processes that includes the synthesis of the material itself. This opens the possibility to study several types of reactions whose mechanisms involve chemical changes occurring in both the solid and the fluid phases, mixing traditional catalysis with structural processes of the zeolite. Acknowledgment. We thank Miguel Camblor for fruitful discussions and suggestions. Unlimited CPU time on the IBMp690 supercomputer at UAEM and financial support through Projects “FOMES2000 Co´mputo Cientı´fico” and SEP-CONACYT No. 46983, respectively, are gratefully acknowledged. References and Notes (1) Villaescusa, L. A.; Camblor, M. A. Recent Res. DeV. Chem. 2003, 1, 93–141. (2) Caullet, P.; Paillaud, J. L.; Simon-Masseron, A.; Soulard, M.; Patarin, J. C. R. Chim. 2005, 8, 245–266. (3) Caullet, P.; Guth, J. L.; Hazm, J.; Lamblin, J. M. Eur. J. Solid State Inorg. Chem. 1991, 28, 345–361. (4) Villaescusa, L. A.; Wheatley, P. S.; Bull, I.; Lightfoot, P.; Morris, R. E. J. Am. Chem. Soc. 2001, 123, 8797–8805. (5) Matijasic, A. M.; Paillaud, J. L.; Patarin, J. J. Mater. Chem. 2000, 10, 1345. (6) Attfield, M. P.; Catlow, C. R. A.; Sokol, A. A. Chem. Mater. 2001, 13, 4708–4713. (7) George, A. R.; Catlow, C. R. A. Chem. Phys. Lett. 1995, 247, 408–417. (8) Zicovich-Wilson, C. M.; San-Roma´n, M. L.; Camblor, M. A.; Pascal, F.; Durand-Niconoff, S. J. Am. Chem. Soc. 2007, 129, 11512–11523. (9) Blasco, T.; Corma, A.; Dı´az-Caban˜as, M. J.; Rey, F.; Vidal-Moya, J. A.; Zicovich-Wilson, C. M. J. Phys. Chem. B 2002, 106, 2634–2642. (10) Hong, S. B.; Lee, S.-H.; Shin, C.-H.; Woo, A. J.; Alvarez, L. J.; Zicovich-Wilson, C. M.; Camblor, M. A. J. Am. Chem. Soc. 2004, 126, 13742–13751. (11) George, A. R.; Catlow, C. R. A. Zeolites 1997, 18, 67–70. (12) Villaescusa, L. A.; Barrett, P. A.; Camblor, M. A. Chem. Mater. 1998, 10, 3966–3973. (13) Jones, C. W.; Tsuji, K.; Takewaki, T.; Beck, L. W.; Davis, M. E. Microporous Mesoporous Mater. 2001, 48, 57–64. (14) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; Arco, P. D.; Llunell, M. CRYSTAL06 Users Manual; University of Turin: Turin, 2006; see http://www.crystal.unito.it. (15) Caullet, P.; Guth, J. L.; Hazm, J.; Lamblin, J. M.; Gies, H. Eur. J. Solid State Inorg. Chem. 1991, 28, 345. (16) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652. (17) Dovesi, R.; Saunders, V. R.; Roetti, C.; Orlando, R.; ZicovichWilson, C. M.; Pascale, F.; Civalleri, B.; Doll, K.; Harrison, N. M.; Bush, I. J.; Arco, P. D.; Llunell, M. CRYSTAL06 User’s Manual; Universita` di Torino: Turin, 2006, see http://www.crystal.unito.it. (18) Rimola, A.; Corno, M.; Zicovich-Wilson, C. M.; Ugliengo, P. J. Am. Chem. Soc. 2008, 130, 16181–16183. (19) Ugliengo, P.; Busco, C.; Civalleri, B.; Zicovich-Wilson, C. Mol. Phys. 2005, 18, 2559–2571. (20) Solans-Monfort, X.; Branchadell, V.; Sodupe, M.; Zicovich-Wilson, C. M.; Gribov, E.; Spoto, G.; Busco, C.; Ugliengo, P. J. Phys. Chem. B 2004, 108, 8278–8286. (21) Ugliengo, P.; Civalleri, B.; Dovesi, R.; Zicovich-Wilson, C. M. Phys. Chem. Chem. Phys. 1999, 1, 545–553. (22) Rimola, A.; Zicovich-Wilson, C. M.; Dovesi, R.; Ugliengo, P. J. Chem. Theory Comput., submitted for publication. (23) Nada, R.; Nicholas, J. B.; McCarthy, M. I.; Hess, A. C. Int. J. Quantum Chem. 1996, 60, 809–820. (24) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (25) Nichols, J.; Taylor, H.; Schmidt, P.; Simons, J. J. Chem. Phys. 1990, 92, 340–346. (26) Pulay, P.; Fogarasi, G. J. Chem. Phys. 1992, 96, 2856–2860. (27) Boys, S.; Bernardi, F. Mol. Phys. 1970, 19, 553. (28) Villaescusa, L.; Ma´rquez, F.; Zicovich-Wilson, C. M.; Camblor, M. A. J. Phys. Chem. B 2002, 106, 2796–2800. (29) Gutmann, V. The Donor-Acceptor Approach to Molecular Interactions; Plenum Press: New York, 1978. (30) Koller, H.; Wo¨lker, A.; Villaescusa, L.; Dı´az-Caban˜as, M. J.; Valencia, S.; Camblor, M. A. J. Am. Chem. Soc. 1999, 121, 3368–3376.
JP9088244
