Ab Initio Periodic Study on H-Clinoptilolite - ACS Publications

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On the Full Mechanism of Zeolite Dealumination in Aqueous Strong Acid Medium. Ab Initio Periodic Study on H-Clinoptilolite Karell Valdiviés Cruz, Anabel Lam, and Claudio Marcelo Zicovich-Wilson J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b09794 • Publication Date (Web): 10 Jan 2017 Downloaded from http://pubs.acs.org on January 15, 2017

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The Journal of Physical Chemistry

On the Full Mechanism of Zeolite Dealumination in Aqueous Strong Acid Medium. Ab initio Periodic Study on H-Clinoptilolite Karell Valdiviés-Cruz,† Anabel Lam,∗,† and Claudio M. Zicovich-Wilson∗,‡ Laboratorio de Ingenier´ıa de Zeolitas, Instituto de Ciencia y Tecnolog´ıa de Materiales (IMRE), Universidad de La Habana, 10400, La Habana, Cuba, and Centro de Investigaci´ on en Ciencias-IICBA, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62209, Cuernavaca (MOR), Mexico. E-mail: [email protected]; [email protected]

∗

To whom correspondence should be addressed Laboratorio de Ingenier´ıa de Zeolitas, Instituto de Ciencia y Tecnolog´ıa de Materiales (IMRE), Universidad de La Habana, 10400, La Habana, Cuba ‡ Centro de Investigaci´ on en Ciencias-IICBA, Universidad Autónoma del Estado de Morelos, Av. Universidad 1001, Col. Chamilpa, 62209, Cuernavaca (MOR), Mexico. †

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Abstract Periodic quantum-chemistry methods as implemented in the Crystal14 code were considered to study the mechanism of the dealumination of H-Clinoptilolite in aqueous acid medium. A series of models consisting of the primitive cell of a monoaluminated structure of HEU framework type together with water molecules and HCl as catalyzer were considered. Stable and transition state structures together with their relative Gibbs free energy values were computed at the hybrid DFT level of theory with dispersion correction (PBE0-D2) employing atomic basis sets. Four different steps for the mechanism have been considered. Each of them corresponds to the hydrolysis of the bonds that connect the Al atom with the framework providing as final product Al(OH)3 adsorbed on a silanol nest. Results show the free reaction energy for the whole acid dealumination is about -49 kJ mol−1 . The crucial way the HCl intervents in the reaction is studied in detail and the polar nature of most intermediates strongly suggests they are significantly stabilized by a solvation-like effect of the aqueous medium.

Introduction Zeolites are structurally ordered aluminosilicates exhibiting microporous structure with channels and cavities of molecular size. 1 Such a kind of materials are frequently employed as catalysts, effective adsorbents or molecular sieves 2 and may be obtained either from natural quarries or by chemical synthesis. The former way is economically competitive for technological purposes as natural zeolites are present on earth in huge quantities and easy to extract by mining. Against such advantages, the small pore diameter of such zeolites often makes difficult to use them in most required applications as, for instance, support for medicines or effective molecular sieves in the selective removal of organic contaminants. As a way to overcome the before-mentioned drawbacks it has been proposed to develop new materials from natural zeolites by enlarging their pore size through chemical modification of their structures. 3,4 2


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and exhibit 8- and 10-membered rings (MR), respectively. Channel C also displays 8-MRs but goes parallel to the conventional a axis. About twenty water molecules per unit cell may be occluded inside the crystalline material. Several types of compensation cations like Na+ , K+ and Ca2+ also appear. 1,7 Dealumination of acid Clinoptilolite in aqueous acid media takes place in approximately one hour at room temperature (RT) in an exothermic reaction. Accordingly, it is not expected to exhibit a high activation barrier. On the other hand, the reverse process, i.e., the redisolution of the EFAL and their incorporation to the framework hardly occurs. Clinoptilolite can be dealuminated up to 46% of the original Al content without perceptible crystallinity loss, yielding a thermally stable material upon lixiviation. 8 An analysis of the

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Al NMR

spectra allows detecting an increase in the amount of octahedral Al upon dealumination together with a simultaneous decrease of such a cation in tetrahedral coordination. 8,9 These results strongly suggests that removal of Al from framework has occurred upon treatment. It has been shown that such modified materials can be employed for instance in the industrial separation of large molecules as, for instance, n-paraffins. 10 Though pioneering works on dealumination of zeolites by Barrer 5,6 were published about 50 years ago, there is not a well established mechanism for the reaction when it is performed in aqueous strong acid medium. This is partially because the use of experimental techniques to such a scope is a very complex and costly task. On the other hand, the consideration of computational techniques to shed some light on that kind of problems appears to be an appealing alternative. Accordingly, in the last years several theoretical studies have been devoted to provide insights into another kind of dealumination process that occurs when zeolites are treated in calcination conditions. 11–14 Such a process is similar to the above mentioned dealumination under aqueous strong acid conditions in that the final products, namely EFAL and Silanol nests, are the same. However, the corresponding mechanisms and intermediate species are expected to exhibit significant differences. This is because in calcination conditions the reaction is mainly induced by temperature with low water

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content, while in the case here considered the high water and acid concentrations do play a non negligible role. The latter is strongly suggested by the experimental fact that reaction rather fastly takes place at room temperature (RT). On the other hand, H-Clinoptilolite has been subject of previous theoretical works. 9,15,16 In a preliminary study from our working group, 15 we have investigated the best models and methodologies to be employed for reproducing known structural and energetic properties of zeolites of HEU topology with different composition and water content. It was found that a methodology suitable for the study of such a family of compounds is the PBE0 functional together with a model consisting of 18 T sites. Such a model appeared to be enough to closely represent a H-Clinoptilolite with experimental Si/Al ratio and hydration degree. In a further work, 16 we have studied the energetics of the different possible pathways of dealumination of H-Clinoptilolite. This allowed to determine the most labile sites in dependence on the Si/Al ratio and the influence of the adsorbed water molecules in the stability of the species involved. Here we focus on the different steps of the mechanism for the dealumination under aqueous conditions and strong acid medium. To such a scope we investigate the reaction pathway by accurately determining the geometry of the corresponding Transition States (TS), stable intermediates and final product. This is accompanied by the calculation of the full energy profile of the reaction including the energy barriers of the steps with thermal contributions to the Gibbs free energy through vibrational analysis.

Methodology Electronic structure calculations were performed with the Crystal14 program for periodic systems 17,18 at the hybrid-DFT level of theory through PBE0 approach. 19 It has been shown that this strategy provides in general much more accurate reaction energy barriers than other DFT schemes 20 and, particularly for HEU zeolitic topologies, a rather reasonable

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description of their structural and energetic features as compared to known experimental data. 15 In addition, Grimme D2 correction 21 (PBE0-D2) was considered in order to include the dispersion forces contribution to the total energy of all the stationary states. Atomic charges have been computed by means of the Hirshfeld-I (HI) partition 22 recently adapted to LCAO periodic calculations. 23 Two basis set levels have been considered, namely valence double- and triple-ζ with polarization functions, from now on labeled as VDZP and VTZP, respectively. The former was considered in previously mentioned works, 15,16 and it was optimized for the study of zeolites and related materials. It is here employed for computation of properties connected with the shape of the PEH, namely geometry optimizations and frequency calculations. The second set is a standard one proposed by Schafer et al. 24 that provides a good calibration between computational cost and high quality for periodic calculations of this kind of systems. 25,26 The latter level has been considered for the calculation of properties that require accurate evaluation of the electronic structure such as total energy and atomic charges. The corresponding Hamiltonian matrix has been diagonalized in a set of k-points in reciprocal space generated according to the Pack-Monkhorst prescription 27 with shrinking factor 2 for sampling the first Brillouin zone (BZ). Different accuracy criteria has been considered for the integrations required in this kind of periodic calculations. In what concerns the computation of mono- and bi-electronic integrals employed in the coulomb and exchange series, were the {6, 6, 6, 6, 12} and {8, 7, 7, 7, 25} set of tolerances were used with the VDZP and VTZP basis sets, respectively. 17 The exchange-correlation contributions to the Kohn-Sham energy and forces have been numerically integrated also considering two accuracy levels. The exchange-correlation contributions to the Kohn-Sham energy and forces have been numerically integrated also considering two accuracy levels, namely (75,974) and (99,1454) pruned grids, are those that define the points in the radial and angular parts around the nuclei (see XLGRID and XXLGRID keywords in the code manual 17 ) were considered with the VDZP and VTZP levels were employed. The condition for the convergence of the SCF part was that

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the energy difference between two subsequent cycles must be less than 10−8 Hartree. Geometry optimizations were performed through gradient methods. Analytic gradients for both atomic positions and lattice parameters, and a pseudo-Newton algorithm (BFGS) for Hessian matrix update were considered to compute the minima of the Potential Energy Hypersurface (PEH). As concerns TS optimizations, the method proposed by Simmons and Nichols 28,29 has been considered. The Distinguished Reaction Coordinate (DRC) 30 strategy was also employed in order to explore the PEH for finding the starting structures of the TS optimizations by scanning along some selected valence internal coordinates. The internal coordinates are here defined using the Redundant Valence Coordinates scheme for defining the paths to follow along the search originally proposed by Pulay. 31 All these tools facilitate the localization of TS in complex PEH, including the saddle point optimization. The starting Hessian matrices for TS optimizations were computed by means numerical second derivatives. 30 All TS structures exhibit just one negative eigenvalue of the Hessian, which guarantees that they are true saddle points of the PEH. To ensure consistency between the optimized geometries and the previously mentioned approximations for integrals evaluation, the FINALRUN code option 17 with value 4 has been also considered. The Gibbs free energy (G) at 298 K and 1 atm for all stationary points of the PEH has been computed by considering vibrational thermal and zero-point contributions through phonon calculations at Γ in the BZ. Given the large size of the unit cell it is expected the considered sampling to be enough for a reasonable description of the thermodynamic state function differences here taken into account. These calculations require much higher accuracy than geometry optimizations and, consequently, the threshold considered for the SCF convergence was 10−10 Hartree. The different steps for dealumination under aqueous acid conditions have been studied by considering a model for H-Clinoptilolite consisting of a HEU zeolitic code primitive cell with a single Brønsted acid site. The Al atom is located at the T2 framework position, the one with largest Al content in experimental samples. 7,9 In addition, previous theoretical

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studies indicate that Al in such a position is the most favorable one for removal in the first dealumination of H-Clinoptilolite. 9,16 The acidic proton is placed at O4 position which is the most stable one for the OH-bridging in such a model. 16 Additionally, one HCl molecule is located at the neighborhood of the site. This system will be considered as the initial product (IP). Water molecules are sequentially included so as to simulate the different Al-O hydrolysis steps. The number of water molecules strictly required by stoichiometry, namely three, has been considered. This means that in the previously mentioned sequential strategy, the last Al-O bond cleavage did not require any water addition. The full dealumination reaction of H-Clinoptilolite in aqueous acid medium is here split into four steps, each one corresponding to the cleavage of a single Al-O bond from the four exhibited by the original Brønsted site. Each step consists of reactant, TS and product. The scheme considered to estimate the energetics of the inclusion of a water molecule so as to initiate the step was as follows. Starting from IP or the intermediate product of the foregoing step, a water molecule is included in the model. It is adsorbed on a SiOSi bridge in the widest channel, quite far away from the Al atom. Next, the model is fully optimized and a control is done to check if the obtained geometry around the Al atom keeps the geometry of the original reaction intermediate. The hydration energy for the next step is consequently computed by considering such a species as the reference instead of the intermediate product of the preceding step. Hereafter the O atoms around the Al site will be indicated according to the original labeling from Alberti. 7

Results and discussion Structural and electronic analyses of the reaction steps In the analysis of the chemical changes experimented by the atoms and ions along the dealumination path, the HI atomic charges (see Methodology) computed at PBE0-D2/VTZP computational level are considered. They are documented in Table 1. 8


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Figure 2 shows the local geometric features around the Brønsted acid site for the species involved in the first dealumination step of H-Clinoptilolite in HCl/H2 O medium. A view that includes all atoms of the whole primitive cell is also shown in Figs. S2 (a), (b) and (c) (see Supporting Information -SI- file). Table 1: Hirshfeld-I atomic charges (in |e|) for atoms in the nearest neighbors of the Al one. Atomic labels are provided in Figs. S1 and S2 in the Supplementary Information file.

H1 O1 O2 O4 O10 Al Ow1 Hw1 Hw1’ Ow2 Hw2 Hw2’ Ow3 Hw3 Hw3’ Cl H2

R1 TS1 0.61 0.59 -1.79 -1.79 -1.78 -1.80 -1.45 -1.32 -1.63 -1.62 3.02 3.02 -1.14 -1.32 0.53 0.61 0.48 0.52

-0.58 0.51

I1 R2 TS2 I2 R3 TS3 I3/R4 0.60 0.58 0.61 0.61 0.60 0.61 0.62 -1.78 -1.77 -1.80 -1.79 -1.76 -1.75 -1.74 -1.78 -1.77 -1.75 -1.76 -1.75 -1.61 -1.60 -1.34 -1.30 -1.33 -1.32 -1.32 -1.36 -1.36 -1.62 -1.58 -1.30 -1.29 -1.34 -1.36 -1.31 3.01 3.01 3.01 3.01 2.97 2.94 2.99 -1.39 -1.14 -1.35 -1.36 -1.20 -1.21 -1.26 0.64 0.55 0.61 0.62 0.58 0.60 0.60 0.53 0.49 0.50 0.50 0.49 0.51 0.51 -1.25 -1.44 -1.47 -1.29 -1.34 -1.52 0.59 0.62 0.64 0.60 0.64 0.66 0.59 0.66 0.66 0.61 0.56 0.63 -1.33 -1.32 -1.36 0.63 0.63 0.64 0.52 0.52 0.53 -0.54 -0.54 -0.60 -0.62 -0.62 -0.55 -0.56 -0.55 0.50 0.50 0.50 0.50 0.50 0.52 0.53 0.51

TS4 FP 0.57 0.48 -1.52 -1.60 -1.60 -1.53 -1.28 -1.26 -1.30 -1.51 2.88 3.01 -1.39 -1.51 0.59 0.67 0.50 0.60 -1.48 -1.53 0.63 0.62 0.62 0.61 -1.29 -1.28 0.60 0.61 0.53 0.51 -0.57 -0.54 0.48 0.54

R1 corresponds to the hydrated acid site in which a single water molecule coordinates with the Al atom. This is accommodated so as to exhibit a close to straight angle with the acidic OH group in O4-bridging position and the Al atom. The resulting AlO5 arrangement acquires a trigonal bi-pyramid conformation. This structure is similar to that reported in case of the thermally induced dealumination. 11,13,14 In the present case an additional HCl molecule is also involved. The molecule is partially dissociated in the complex, as the proton forms an OH-bridging together with the O10 atom. In the following analysis, it is assumed that an H atom and other one playing the role of acceptor, A, are H-bonded if the H· · ·A distance is shorter than 2.1 ˚ A. In accordance to 9




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As regards the structural features of the model that initiates this part of the reaction, R2, it turns out that, as in the previous step, the included water molecule also coordinates with the Al atom bringing about a trigonal bi-pyramid arrangement around it. Water molecules are accommodated along its principal axis. The Al-O10 bond length exhibits the largest value among the Al-O interactions lying on the equatorial plane. This suggests a preparation for further Al-O10 bond breaking. Such a weakening is likely due to the influence of the proton to which O10 is strongly bonded and the Cl− H-bonded to it. In fact, a certain degree of electron transfer does occur from O10 to the anion as exhibited by the HI charges. Accordingly, it arises from Table 1 that the atomic charge on O10 decreases from -1.62 |e| to -1.58 |e| while at the same time that of Cl− increases from -0.54 |e| to -0.60 |e| in going from I1 to R2. As in the previous step, transition state (TS2) and second intermediate (I2) structures are rather similar. In particular, the arrangement around the Al atom also exhibits a tetrahedral coordination shape where now two water molecules act as ligands. Comparing both geometries to R2, it arises that the Al-O bond lengths do not practically change along the reaction path apart from the one corresponding to the broken Al-O10 one. Bond lengths reported in Fig. 3 show that the latter increases from 1.88 ˚ A to 3.30 ˚ A along the path. A significant charge redistribution takes place around the Al atom during the process, as it is evidenced from atomic charges documented in Table 1. The formation of the silanol group on O10 makes it to decrease its absolute charge value in almost 0.3 |e| reflecting an ionicity loss. This change is accompanied by an increase of the anionic character of the O atoms belonging to the water molecules coordinated to the Al atom. Each of them (Ow1 and Ow2 in Table 1) features an electron gain of about 0.22 |e|. The increase in the charge of the water O atoms may be connected to the fact that the part of the framework containing the Al and O atoms bonded to it exhibits a local electron deficiency as a result of the breaking of the Al-O10 bond. This is to be at least partially compensated by the increase in the anionic character of the water O atoms so as to ensure electro-neutrality. A local charge polarization

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is therefore promoted accordingly. The charge separation also includes the Al and the Cl atoms with similar absolute charge values of ca 0.60 |e|. Remarkable differences between the dealumination reaction performed in HCl medium at RT and in calcination conditions 11,13,14 turn out from the present partial results. While in the latter the first and second steps of the reaction path involve water dissociation as the necessary way to promote the Al-O bond breaking, in the present case this occurs through the intervention of the partially dissociated H· · ·Cl molecule that acts in a role intermediate between Brønsted and Lewis acid. This is because the proton is partially transferred to O10 in a Brønsted-like process, but at the same time it remains quite strongly attached to the Cl atom with significant electron transfer between both atoms. Accordingly, the couple Hδ+ · · ·Clδ− acts as an electron attractor in the Lewis sense. As a result of this overall effect, in both steps dative Al-OH bonds are broken and the water molecules allow the Al to remain in four-fold coordination. It is important to note that other pathways were explored in these two steps and some of them included the possibility of dissociation of the water molecules that are coordinated to the Al atom. Nevertheless, the water dissociation mechanism is the energetically most favorable one. The three stationary points of the PEH involved in the third reaction step, namely R3, TS3 and I3, are schematized in Fig. 4 together with their most relevant atomic distances. A wider representation of the structures as they look within the primitive cell is also provided in the SI file [Figs. S2 (g), (h) and (i)]. At the starting of the step a water molecule adsorbs on I2 so as to give rise to R3. Figure 4 shows that, upon adsorption, the Al atom acquires five-fold coordination with the surrounding Oxygens once more displaying a trigonal bi-pyramidal arrangement. Structure R3 therefore exhibits three water molecules around the Aluminum, two of them being placed along the principal axis while the remaining one lying on the equatorial plane together with framework atoms O1 and O2. Water molecules are also connected to the framework and the Cl− anion through H-bonds (see Table S2 in the SI file). This provides a strong stabilization

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Concerning the structural features of the step it turns out from Fig. 5 that one of the adsorbed water molecules, namely that originally included in the first step, dissociates into a proton and an OH− anion. The proton is subsequently transferred to O1 so as to bring about a new Brønsted site, while the OH− remains coordinated to the Al atom. Accordingly, the latter keeps the original coordination folding and shape. As a result of such a process, some negative charge transfer befalls from the protonated atom O1 to the O atom of the formed hydroxyl, Ow1, as they evolve from -1.74 |e| and -1.26 |e| to -1.52 |e| and -1.39 |e|, respectively, in going to TS4, as it arises from Table 1. In the final product (FP) the OH group coordinated to the Al atom moves so as to establish a connection with the Si atom attached to O1 giving rise to a new double OHbridging. Atomic charges on O1 and Ow1 (see Table 1) become -1.60 |e| and -1.51 |e|, respectively, evidencing their anionic character. As the consequence of the previous process the arrangement between the Al and its neighboring O atoms becomes octahedral. It is worth noting that in all the steps of the reaction pathway the Al atom keeps a charge close to the formal one as it turns out from Table 1. This suggests it is mainly attached to the surrounding O and Cl atoms through electrostatic interactions. The coordination of Cl− with the Al center is more likely at the early stages of the H-CLI dealumination reaction, where the Al is more accessible. At the last stages, the presence of three water molecules and the increasing number of H-bonds make the migration of the Cl− more difficult. Consequently, we explored the possibility of coordinating the Cl to the Al center, but it was not possible in all the attempts we made. Therefore, it could be stated that there is a preference of the Al atom for the oxygens from the water molecules, instead of the Cl. If we now compare the HI charges on all the water oxygen atoms (Ow1, Ow2 and Ow3) with the Cl (see Table 1) along the pathway, it could be noticed that the Oxygens have a greater basic character than the Cl, so that they should preferentially coordinate with the Al center. The product of the dealumination reaction in strong acid medium is an Al(OH)3 molecule

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trapped into a silanol nest. As it turns out from the present study such a chemisorption involves the formation of the two double OH-bridges as shown in Fig. 5. As mentioned in the Introduction, this kind of EFAL is experimentally eliminated through a series of lixiviation treatments with dilute acid at RT. 8,9 The mechanism of the dealumination reaction performed in HCl medium at RT here considered and the one previously studied considering calcination conditions 11,13,14 display some remarkable differences not only as concerns the previously mentioned initial steps but also in the last stages of the reaction path. While, in studies of the process at high temperature the fourth step takes place upon a water adsorption followed by an Al-O bond hydrolysis, in the present mechanism there is no additional hydration and one adsorbed water dissociates bringing about a particular Al-O bond hydrolysis featured by the formation of a double OH-bridging. Additionally, instead of the separation of the the EFAL from the framework, 11,13,14 it here remains in an octahedral Al coordination and trapped into the formed silanol nest through two double OH-bridgings.

Energetics of the H-Clinoptilolite dealumination reaction Figure 6 shows the Gibbs free energy profile at RT for the reaction performed considering the PBE0-D2/VTZP approach. The optimized geometry and the thermal/zero-th point energy contributions were computed at the PBE0/VDZP computational level. In the SI file the profiles computed at PBE0/VDZP and PBE0/VTZP levels are shown (see Graphic S3). The explicit values of the different steps and contributions to the free energy are detailed in the SI file (Table S4). The dealumination reaction is dominated by electrostatic interactions. Nevertheless, in some events such as the water adsorption processes and other ones involving the formation of several H-bondings the dispersion forces have a non-negligible role. In fact, a comparison of all the profiles allows to remark the importance of taking into account the long range dispersion forces in the present pathway. 17




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aqueous medium. Because in the literature there is not reports of any mechanism for the H-CLI dealumination in neutral medium at calcination temperature, here we compare the energetic differences in the reaction mechanism of the H-CLI dealumination in strong acid medium at RT with the only theoretical reports available for zeolite dealumination. 11–14 Concerning the water adsorption that enables the formation of R2, it turns out that the corresponding reaction is endothermic. Besides, in R2 a reorganization of the Al site takes place as a result of an increase in its coordination. Both processes are determined by the stiffness of the Brønsted acid site. This means that a large number of water molecules must be present in the cavities so that such a hydration to be probabilistically favored. In experiments, this situation is fulfilled as the medium itself is aqueous and so a significant water excess is ensured to displace the equilibrium towards R2. It was shown in the previous subsection that the second reaction step involves the breaking of an Al-O bond, bringing about an Al ion of charge close to the formal one, +3.0 |e|, which is not compensated by the charge of the neighboring O atoms as it happens in the previous step. This is because in I2 two of the Oxygens in the first coordination sphere of the Al atom belong to water molecules that does exhibit smaller negative charges than those connected to Si atoms. The electro-neutrality of the whole arrangement is in a great part based on the charge on the Cl atom that, as previously mentioned, decreases 0.08 |e| in going from I1 to I2 (see Table 1). As a result of such an electronic rearrangement around the Al atom, a charge polarization is induced, which could be the main responsible for the high energy of TS2 and I2. It is likely that in this case the effect of the polarity of the aqueous medium would be crucial in the experiment so as to stabilize these stationary species. This features are once more radically different to the case of the dealumination reaction at calcination temperature. 13,14 According to the previously discussed stabilizing effect of water molecules around the positively charged Al atom, the addition of the third H2 O so as to form R3 entail a substantial stabilization of 136 kJ mol−1 as it is shown Fig. 6 (see also Table S4 in SI file). The water

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molecule mainly remains attached to the Al atom through electrostatic-like interactions but accompanied with a non-negligible electron dative character. At the same time, one of its H atoms form rather strong H-bonding with the Cl− . Both effects might be the main responsible for the remarkable stabilization of R3 that does provoke a displacement of the equilibrium toward reaction products. This effect highly favors the reaction to occur at RT in contrast with dealumination in neutral anhydrous conditions that require calcination temperatures to take place. 13,14 The energetic barrier of the third step is substantially large (79 kJ mol−1 see Fig. 6 and Table S4 in SI file) but, as in the previous step, it is likely that the presence of a significant number of water molecules in the medium largely contributes to stabilize the charge polarization and subsequent energetic penalty connected to the formation of the OH− ion upon dissociation of the water molecule. As a result of the rather high reactivity of the hydroxyl anion, it forms in I3 a double OH-bridging that stabilize the system leading it to the most stable of the intermediates. It is likely that before the lixiviation treatment the product of the reaction in strong acid medium could be the formation of such a very stable species. The last reaction step would take place during lixiviation that, in excess of water molecules, could not only stabilize TS4 but also displace the equilibrium towards products by the effect of the elimination of the Al(OH)3 from the reaction medium and subsequent formation of the silanol nest. The fourth step exhibits a calculated energetic barrier which is larger than that of the third one in ca 34 kJ mol−1 (see Fig. 6 and Table S4 in SI file). This might be ascribed to the differences in the way the components of the systems interact to each other in TS3 and TS4. While in both structures there is the same number of water molecules TS3 exhibits eight different H-bondings and, particularly, the Cl− ion is strongly stabilized at the center of the channel as a result of three interactions with neighboring H atoms (see Table S2 in the SI file). On the contrary, TS4 just exhibits four hydrogen bonds and the Cl− ion is linked to two H atoms.

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It is important to remark the role of the HCl is crucial at the early stages of the dealumination reaction of H-CLI at RT. The HCl molecule is dissociated after the first water molecule addition, forming a new Brønsted acid site and contribute to the weakening of the Al-O bonds and their successive hydrolyzations. Therefore, the presence of the HCl avoids that the water molecules be dissociated, changing the trends exhibited in the first stages of the dealumination reaction of other zeolites in neutral media at high temperature. On the other hand, the energetic barriers of the latest steps of the dealumination reaction are affected by the insufficient number of water molecules considered in our mechanism. And also because HCl is more dilute in the system, and its role decreases, while other factors become more important (hydrogen bonds, charge separation, framework constraints and the formation of double OH-bridgings). It is expected that if the appropriate number of water and acid molecules are considered, the barriers will be lower. This scenario contrasts with the solvation effects of dealumination in calcination conditions where there are only few water molecules in the zeolites channels and these high barriers could be overcome by the effect of temperature which increases the probability of thermally excited states.

Conclusions The dealumination reaction of H-Clinoptilolite in aqueous strong acid medium at RT has been studied in detail by considering high level theoretical techniques. Periodic models together with a hybrid functional and accurate atomic basis sets have been considered to estimate the full mechanism of the reaction and its energetic profile. The crucial influence of the acid reagent, the HCl molecule, in several instances of the reaction is highlighted. As a matter of fact, such a molecule just partially dissociates and keeps a strong H-bonding between its original atoms during the whole process. This allows a double action: the couple Hδ+ · · ·Clδ− acts as an electron-withdrawing agent through the proton that interacts with O atoms and as an H-bonding acceptor through the Cl moiety

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that displays a partial anionic character. In the two initial reaction steps, the linkage between the Al atom and the framework is broken through the HO-Al bond cleavage of the OH-bridgings. The water molecules play the role of stabilizing agents remaining coordinated to the Al atom. In the third and fourth steps, such molecules participate more actively undergoing dissociation in a partial hydrolysis of the O-bridgings. Such processes give rise to double OH-bridging that are weaker than the original Al-O-Si ones and able to allow separation of the EFAL in cleaning conditions. As compared to the mechanisms of dealumination at calcination conditions previously reported for several zeolitic frameworks 13,14 it turns out that the relevant differences between both kind of processes could make them to be considered as different reactions. This is based on the crucial role that the acid agent plays together with the aqueous medium in favoring the dealumination at least as concerns the case of H-Clinoptilolite. From the point of view of the material science it is expected that the elucidation of the acid dealumination mechanism of the H-Clinoptilolite could allow to gain more knowledge about a process with high impact in pore size, crystallinity and Si/Al ratio that allows new potential applications of this economically competitive mineral.

Acknowledgement K. Valdiviés-Cruz and A. Lam would like to honor and validate Professor Zicovish-Wilson’s invaluable contribution to this paper. Sadly, Dr. Zicovish-Wilson passed away during the revision process of the manuscript. The authors thank Mexican CONACyT for funding through project CB-178853. They are also indebted to Germán Rojas for his invaluable support in dealing with graphic formats and applications.

Supporting Information Available The following file is available free of charge.

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• SI.pdf: – Supplementary figures of: (i) the anhydrous acid site geometry alone and within the primitive cell; (ii) the different stationary species of the reaction mechanisms as appearing inside the primitive cell: (iii) the free energy profile of the reaction at RT computed at PBE0/VTZP and PBE0/VDZP levels.

– Supplementary Tables that provide: (i) Selected distances around the Al atom for each stationary species along the reaction path; (ii) detailed H-bond lengths and Mulliken atomic charges for the atoms involved in H-bonds; (iii) Detailed reaction energetics computed at PBE0-D2/VTZP, PBE0/VTZP and PBE0/VDZP levels. This material is available free of charge via the Internet at http://pubs.acs.org/.

References (1) Baerlocher, C.; Meier, W. M.; Olson, D. H. Atlas of Zeolite Framework Types; Elsevier: Amsterdam, 2001; URL: http://www.iza-structure.org/databases/. (2) Breck, D. W. Zeolite Molecular Sieves; Wiley and Sons: New York, 1973. (3) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pérez-Ram´ırez, J. On the Introduction of Intracrystalline Mesoporosity in Zeolites Upon Desilication in Alkaline Medium. Micropor. Mesopor. Mat. 2004, 69, 29–34. (4) Groen, J. C.; Peffer, L. A. A.; Moulijn, J. A.; Pérez-Ram´ırez, J. Mesoporosity Development in ZSM-5 Zeolite Upon Optimized Desilication Conditions in Alkaline Medium. Colloid. Surf. A 2004, 241, 53–58. (5) McDaniel, C. V.; Mäher, P. K. In Molecular Sieves; Barrer, R. M., Ed.; Soc. Chem. Ind.: London, 1968; p 186.

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Ab Initio Periodic Study on H-Clinoptilolite - ACS Publications

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