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Spontaneous proton transfer in Na zeolites
10 November 2000
Chemical Physics Letters 330 (2000) 457±462
www.elsevier.nl/locate/cplett
Spontaneous proton transfer in Na zeolites L. Benco a,*, T. Demuth a, J. Hafner a, F. Hutschka b a
Institut fur Materialphysik and Center for Computational Materials Science, Universit at Wien, Sensengasse 8, A-1090 Wien, Austria b Total Ranage Distribution, Centre Europ een de Recherche et Technique, B.P. 27, F-76700 Har¯eur, France Received 28 May 2000; in ®nal form 4 September 2000
Abstract First-principles room-temperature molecular dynamics (MD) simulations are conducted to investigate proton transfer (PT) in Na zeolites. The MD are performed on the unit cell containing two Al-sites, one of them saturated with H (acid site) and the second one with Na coordinated with three water molecules. The creation of the charged H3 O cations is suppressed by the Na cation. Spontaneous barrierless PT between the O-sites in the zeolite, however, is possible. The lifetime of the hydronium cation is extremely short (65 fs). The presence of Na cations leads to a modi®ed mechanism but does not suppress the proton transfer around the Al-site in zeolites. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Zeolites are acid catalysts with a large internal surface that are used in many technologically important chemical processes [1]. The detailed scenario of the reactions within zeolites is not known. The adsorption of molecules to the surface, preferably to acid sites, is considered an initial stage of a reaction. Interaction of the acid proton with an electronegative atom of the sorbent molecule (preferably oxygen atom) yields hydrogen-bonded surface structures. It is believed that the second step in any chemical reaction is proton transfer (PT) from the surface to the adsorbed molecule. The proton exchange between the zeolite and the sorbent molecule is investigated in numerous
* Corresponding author. Permanent address: Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dubravska cesta 9, SK-84236 Bratislava, Slovakia. Fax: +43-1-4277-9514. E-mail address: [email protected] (L. Benco).
ab initio simulations, predominantly for small sorbent molecules, such as H2 O and CH3 OH. The importance of the water molecule stems from its constant presence in natural zeolite structures and from the easy introduction/removal of water during both the pretreatment of solid samples and chemical processes. On the other hand, the methanol molecule represents the simplest reactant: two methanol molecules react in zeolites to give a diethylether molecule in the technologically important methanol-to-gasoline (MTG) process. Pioneering steps in the simulation of the MTG process in several zeolite frameworks have been recently taken by Stich et al. [2,3]. The proton exchange between the zeolite and the extra-framework particle depends on the basicity of the adsorbed molecule. While the more basic methanol molecule can form stable methoxonium cations, the attractive power of a single water molecule adsorbed in a zeolite is not large enough to produce a stable charged extra-framework particle (hydronium cation ± H3 O ). The
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 1 1 8 - 0
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desirable increase of basicity is achieved e.g. by the clustering of molecules at higher loadings [4,5]. Recent molecular dynamics (MD) simulations show, however, that the basicity of a single water molecule can be considerably increased for a period of time through the creation of secondary hydrogen bonds (HB's). These bonds are established between the H atoms of the adsorbed water molecule and the framework O-sites which are ®rst or second neighbors to the acid O-site. During the existence of the relatively strong secondary HB, i.e. when the H O distance is considerably shorter the attractive power of the O(water) than 2.0 A, atom is increased and the acid proton jumps from the zeolite framework to the water molecule, thus producing an extra-framework hydronium cation. In contrast to the simple proton exchange between the zeolite and the adsorbed molecule, the proton transfer to neighboring O-sites is a more complex process. It requires such a rearrangement of atoms within the extra-framework particle that the secondary HB becomes stronger than the primary HB to the acid site, and the proton returns to another framework O-site. The probability of a PT therefore increases with the increased lifetime of H3 O . Because the time evolution of the position of O-sites within the framework plays a role as well, PT between the O-sites becomes highly probable when the lifetime of H3 O is comparable with the period of the framework O±O vibrations. Details of spontaneous barrierless PT in Na-free zeolite, including the scenario and the time evolution of the geo-
Fig. 1. Orientation of the water molecule within the zeolite. (a) Adsorption to the acid site; (b) coordination to the Na atom. Because the presence of the Na atom in the zeolite evokes the reorientation of the water molecule it appears to counteract the formation of H3 O and proton transfer.
metrical parameters, are given in our recent paper [6]. The dynamics of H2 O in Na zeolites, however, considerably diers from the behavior in Na-free compounds. Both the acid site (AS) and the Na atom attract the adsorbed molecules and thus compete for the intrazeolite water. Adsorption to either of the centers requires reorientation of the water molecule as schematically displayed in Fig. 1. Because the creation of H3 O is not possible without water being adsorbed to the AS, the presence of Na appears to counteract the formation of H3 O and the process of the PT in zeolites. In this Letter, we simulate the time evolution of a zeolite containing two Al-sites, one of them saturated with a hydrogen atom and another one with an Na atom. The Al-sites are placed in second-neighbor positions within the eight-membered ring (8MR). The short distance between the Alsites allows the competition between the two centers for H2 O molecules to be studied. The hydration of the zeolite with three water molecules represents the interesting case when all three water molecules are connected to Na; one of them, however, is in contact with the acid site. The time evolution of interatomic interactions is compared with that of the Na-free zeolite [6]. 2. Results and discussion 2.1. MD simulations Periodic ®rst-principles density-functional calculations are performed with the Vienna ab initio simulation package VASP [7,8] using generalized gradient approximation (GGA) [9] and projector augmented wave (PAW) techniques [10,11]. The simulation temperature during the MD within the canonical ensemble varies around 300 K. More details on the computational method are given in our previous Letter [6]. The adsorption of water to the acid site of Na zeolite is simulated on gmelinite, a zeolite with a medium-sized unit cell. The hexagonal structure (P 63 mmc [12]) is composed of one type of the tetrahedral atom and four irreducible O atoms. The largest apertures of the channel system are
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twelve- and eight-membered rings (12MR and 8MR, respectively) [12]. The secondary building units, which are hexagonal prisms, also constitute elements of other important zeolites such as faujasites. The interaction between the zeolite acid site and the Na atom is explored for the structure with two Al-sites localized within the 8MR in the second-neighbor positions (cf. Fig. 2). The mechanism of adsorption of water molecules to both the AS and to the Na atom is tested for several concentrations of H2 O and for several initial locations of the adsorbed molecules within the zeolite framework [13]. While the AS can adsorb only one water molecule, the coordination sphere of Na comprises up to three molecules. At low concentration, e.g., one H2 O per cell, the at-
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tractive powers of the AS and the Na atom are similar. The water molecule only slightly prefers adsorption to Na [13]. Increase of the concentration of water leads ®rst to the saturation of the acid site, then completes the coordination sphere of Na. When it is completed any additional water is adsorbed within the channels of the zeolite [14]. As a response to the adsorption of water considerable local deformations of the zeolite structure can occur. Because the capacity of the framework to accommodate a proton depends on the local geometry [15], the deformation of the structure drives the `local chemistry' within the zeolite framework. Through the local deformation, the Osites of the zeolite can be both favored or disfavored for protonation. The concentration and the location of water can thus control proton transfer between the O-sites in zeolites. 2.2. Mechanism of proton transfer
Fig. 2. Mechanism of proton transfer within the 8MR. (a) The acid proton located in position O(4); (b) the acid proton transferred to position O(1).
Recent MD simulations of one acid site [6,13] have shown that the acid proton can migrate between O-sites neighboring the Al-site as soon as one water molecule is adsorbed to the acid site. This high mobility of acid protons in zeolites was not observed in previous MD simulations [4,5,16]. The reason could be that the dynamical interaction of water with a zeolite framework was investigated only for small unit cells, thus imposing constraints to such a deformation of the framework that supports proton transfer [6]. Reproduction of proton transfer through the simulation at the temperature of liquid nitrogen [13] implies a conclusion that proton transfer around the Al-site is a general phenomenon occurring in acid zeolites. In zeolites with a high concentration of Al-sites residual Na cations can interact with acid protons thus disturbing proton transfer around the Al-site. We have observed such an interaction for the structure displayed in Fig. 2a, where the water molecule, playing an active role in the PT, connects both to the AS and to the Na atom. Two Alsites are next-nearest-neighbors in the 8MR of the gmelinite framework. One Al-site is compensated by a proton connected to the O(4) position (Fig. 2a, left) and another one by the Na atom localized within the 12MR (not displayed) making
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contact to the O(3) position (Fig. 2a, right). Two water molecules are connected directly to the Na atom from the side of the 12MR (bottom of Fig. 2a). The third water molecule is placed between the two Al centers. It is adsorbed to the AS and at the same time it connects to Na. This double linkage to the two positively charged centers leads to an orientation of the molecule such that the O±H bonds are oriented approximately perpendicular to the line connecting Na and the AS (cf. the angle indicated in Fig. 2a). In this con®guration one of the hydrogen atoms points to the O(1) atom and establishes a secondary hydrogen bond to the framework. The water molecule is thus doubly connected to the zeolite and constitutes a six-membered ring composed of the Al atom, two framework O-sites, the acid proton and the O±H bond of the water molecule, respectively. The orientation of the water molecule as displayed in Fig. 2a is rather short lived. A quick reorientation is accomplished through a PT in which one proton of the adsorbed water molecule jumps to the framework O(1) position, and at the same time the acid proton jumps from the framework O(4)-site to the water molecule. The relaxation of all atomic positions of the two structures (Fig. 2a and b) favors structure (b) by 7.7 kJ/mol. The preference for the protonation of the O(1)-site is indicated by the Al±O±Si angles as well. In structure (a) the angle Al±O(1)±Si has an orientation favorable for the protonation of the O(1)-site. The value of this angle (146.1°) is higher than that of the angle Al±O(4)±Si (141.5°) in structure (b). This agrees well with the recently observed correlation between the stability of the protonated structure and the value of the angle of the unprotonated framework [15]. 2.3. Time evolution of geometry parameters A snapshot of the structure fragment of the Na zeolite with the adsorbed water molecule shortly before the PT is displayed in Fig. 3a together with an equivalent fragment of the Na-free structure shown in Fig. 3b. The similarity of both structures demonstrates that the same mechanism of PT applies. In both the Na zeolite and in the Na-free structure the
Fig. 3. The geometry of the 6MR established by the adsorption of the water molecule to the acid site and the creation of a secondary hydrogen bond to the framework O-site. (a) The Na zeolite with the acid site in O(4) position; (b) the Na-free zeolite with the acid site in O(1) position. In Na zeolite (a) the H2 O molecule connects to both the acid site and to the Na atom.
water molecule is locked in the 6MR around the Al-site and PT takes place within this 6MR. The time evolution of the selected geometry parameters is displayed in Fig. 4. The distance of the two protons from the framework O-sites (Fig. 4a) shows that PT in the Na zeolite is very fast. In contrast to the Na-free structure, there is no plateau indicating the existence of an H3 O ion. The crossing of the two O±H lines denoted by the vertical line (tPT ) indicates the PT. The O±O curves showing the distance of the adsorbed water molecule from the two framework O-sites (O(4), O(1)) are displayed in Fig. 4b. Due to the dynamics of the structure comprising both the deformation of the zeolite skeleton and the migration of the adsorbed water molecule, the position of H2 O is shifted from the O(4)-site toward the O(1)-site. The vertical line (tPT ) shows that the proton transfer is delayed by several femtoseconds after the moment when the water molecule approaches the O(1)-site. Fig. 4c displays the variation of the O±O±O angle, stressing the fact that PT is realized through tight bonding to both framework O-sites. Similar to the PT in the Na-free zeolite [6], the maximum value of the O±O±O angle occurs at the time of the PT as detected by the O±H distances in Fig. 4a. The strength of the hydrogen bond depends on the O±H O angle and reaches the largest value for an angle of 180°. For the purpose of assessing the
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Fig. 5. The time evolution of the O±H±O angles (cf. Fig. 3) (a) in the Na zeolite and (b) in the Na-free structure.
Fig. 4. Time evolution of the geometrical parameters within the 6MR (cf. Fig. 3). (a) The O±H distances; (b) the O±O distances and (c) the O±O±O angle.
variation of the strength of the hydrogen bonds, the time evolution of the O±H O angles for the two hydrogen bonds within the 6MR is displayed in Fig. 5. The angles for the Na zeolite (Fig. 5a) are compared with those for the Na-free zeolite (Fig. 5b). For the Na zeolite the variation of the values of both angles is synchronized and repeating with a period of 20 fs. At the moment of PT both angles reach their largest values. The PT is thus accomplished through the most favorable geometry with the O±H O angles at their maximum values. An interesting feature is that the transfer of both protons is practically simultaneous. This is in con-
trast to the Na-free structure for which the corresponding angles are displayed in Fig. 5b. Here, the ®rst proton transfer leads to the creation of a hydronium cation as indicated by the ®rst arrow. The lifetime of the H3 O cation has been previously estimated, based on the time evolution of the O±H distances, to be 50 fs. According to the evaluation in Fig. 5 the decay of the H3 O occurs shortly after the maximum value of the O(4)±H2±O(w) angle (cf. the second arrow), and the lifetime of the hydronium cation is 70 fs. In contrast to the longlived H3 O in Na-free zeolite, no hydronium cations are produced in the Na zeolite. Here the creation of H3 O is prohibited by the direct contact of the water molecule with the Na cation. 3. Conclusions First-principles MD simulations show that, in zeolites, the close proximity of the Na counterion
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to the adsorbed water molecules suppresses the creation of the long-lived H3 O cations. A spontaneous barrierless PT, however, is possible without the existence of such a cation and occurs through concerted double-proton transfer. The presence of Na cations thus leads to a modi®ed mechanism but does not suppress proton transfer around the Al-site in zeolites. Acknowledgements The work has been performed within the Groupement de Recherche Europeen `Dynamique Moleculaire Quantique Appliquee a la Catalyse', founded by the Council National de la Recherche Scienti®que, France, the Institut Francßais du Petrole (IFP), TOTAL Recherche et Development, and the Universitat Wien. Computing facilities at IDRIS, France, are kindly acknowledged. References [1] J.W. Ward, in: J. Rabo (Ed.), Zeolite Chemistry and Catalysis, ACS monograph no. 171, American Chemical Society, Washington, DC, 1976.
[2] I. Stich, J.D. Gale, K. Terakura, M.C. Payne, Chem. Phys. Lett. 283 (1998) 402. [3] I. Stich, J.D. Gale, K. Terakura, M.C. Payne, J. Am. Chem. Soc. 121 (1999) 3292. [4] Y. Jeanvoine, J.G. Angy an, G. Kresse, J. Hafner, J. Phys. Chem. 102 (1998) 7307. [5] V. Termath, F. Haase, J. Sauer, J. H utter, M. Parinello, J. Am. Chem. Soc. 120 (1998) 8512. [6] L. Benco, T. Demuth, J. Hafner, F. Hutschka, Chem. Phys. Lett. 324 (2000) 373. [7] G. Kresse, J. Furthm uller, Comput. Mater. Sci. 6 (1996) 15. [8] G. Kresse, J. Furthm uller, Phys. Rev. B 54 (1996) 11169. [9] J.P. Perdew, Y. Wang, Phys. Rev. B 45 (1992) 13244. [10] P.E. Bl ochl, Phys. Rev. B 50 (1994) 17953. [11] G. Kresse, D. Joubert, Phys. Rev. B 59 (1999) 1758. [12] E. Galli, E. Passaglia, P.F. Zanazzi, Neues Jahrb. Mineral. Monatsh. 1982 (1982) 145. [13] L. Benco, T. Demuth, J. Hafner, F. Hutschka, unpublished. [14] L. Benco, T. Demuth, J. Hafner, F. Hutschka, Microporous Mater., submitted. [15] L. Benco, T. Demuth, J. Hafner, F. Hutschka, J. Chem. Phys. 111 (1999) 7537. [16] E. Nusterer, P.E. Bl ochl, K. Schwarz, Chem. Phys. Lett. 253 (1996) 448.
Chemical Physics Letters 330 (2000) 457±462
www.elsevier.nl/locate/cplett
Spontaneous proton transfer in Na zeolites L. Benco a,*, T. Demuth a, J. Hafner a, F. Hutschka b a
Institut fur Materialphysik and Center for Computational Materials Science, Universit at Wien, Sensengasse 8, A-1090 Wien, Austria b Total Ranage Distribution, Centre Europ een de Recherche et Technique, B.P. 27, F-76700 Har¯eur, France Received 28 May 2000; in ®nal form 4 September 2000
Abstract First-principles room-temperature molecular dynamics (MD) simulations are conducted to investigate proton transfer (PT) in Na zeolites. The MD are performed on the unit cell containing two Al-sites, one of them saturated with H (acid site) and the second one with Na coordinated with three water molecules. The creation of the charged H3 O cations is suppressed by the Na cation. Spontaneous barrierless PT between the O-sites in the zeolite, however, is possible. The lifetime of the hydronium cation is extremely short (65 fs). The presence of Na cations leads to a modi®ed mechanism but does not suppress the proton transfer around the Al-site in zeolites. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Zeolites are acid catalysts with a large internal surface that are used in many technologically important chemical processes [1]. The detailed scenario of the reactions within zeolites is not known. The adsorption of molecules to the surface, preferably to acid sites, is considered an initial stage of a reaction. Interaction of the acid proton with an electronegative atom of the sorbent molecule (preferably oxygen atom) yields hydrogen-bonded surface structures. It is believed that the second step in any chemical reaction is proton transfer (PT) from the surface to the adsorbed molecule. The proton exchange between the zeolite and the sorbent molecule is investigated in numerous
* Corresponding author. Permanent address: Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dubravska cesta 9, SK-84236 Bratislava, Slovakia. Fax: +43-1-4277-9514. E-mail address: [email protected] (L. Benco).
ab initio simulations, predominantly for small sorbent molecules, such as H2 O and CH3 OH. The importance of the water molecule stems from its constant presence in natural zeolite structures and from the easy introduction/removal of water during both the pretreatment of solid samples and chemical processes. On the other hand, the methanol molecule represents the simplest reactant: two methanol molecules react in zeolites to give a diethylether molecule in the technologically important methanol-to-gasoline (MTG) process. Pioneering steps in the simulation of the MTG process in several zeolite frameworks have been recently taken by Stich et al. [2,3]. The proton exchange between the zeolite and the extra-framework particle depends on the basicity of the adsorbed molecule. While the more basic methanol molecule can form stable methoxonium cations, the attractive power of a single water molecule adsorbed in a zeolite is not large enough to produce a stable charged extra-framework particle (hydronium cation ± H3 O ). The
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 1 1 8 - 0
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L. Benco et al. / Chemical Physics Letters 330 (2000) 457±462
desirable increase of basicity is achieved e.g. by the clustering of molecules at higher loadings [4,5]. Recent molecular dynamics (MD) simulations show, however, that the basicity of a single water molecule can be considerably increased for a period of time through the creation of secondary hydrogen bonds (HB's). These bonds are established between the H atoms of the adsorbed water molecule and the framework O-sites which are ®rst or second neighbors to the acid O-site. During the existence of the relatively strong secondary HB, i.e. when the H O distance is considerably shorter the attractive power of the O(water) than 2.0 A, atom is increased and the acid proton jumps from the zeolite framework to the water molecule, thus producing an extra-framework hydronium cation. In contrast to the simple proton exchange between the zeolite and the adsorbed molecule, the proton transfer to neighboring O-sites is a more complex process. It requires such a rearrangement of atoms within the extra-framework particle that the secondary HB becomes stronger than the primary HB to the acid site, and the proton returns to another framework O-site. The probability of a PT therefore increases with the increased lifetime of H3 O . Because the time evolution of the position of O-sites within the framework plays a role as well, PT between the O-sites becomes highly probable when the lifetime of H3 O is comparable with the period of the framework O±O vibrations. Details of spontaneous barrierless PT in Na-free zeolite, including the scenario and the time evolution of the geo-
Fig. 1. Orientation of the water molecule within the zeolite. (a) Adsorption to the acid site; (b) coordination to the Na atom. Because the presence of the Na atom in the zeolite evokes the reorientation of the water molecule it appears to counteract the formation of H3 O and proton transfer.
metrical parameters, are given in our recent paper [6]. The dynamics of H2 O in Na zeolites, however, considerably diers from the behavior in Na-free compounds. Both the acid site (AS) and the Na atom attract the adsorbed molecules and thus compete for the intrazeolite water. Adsorption to either of the centers requires reorientation of the water molecule as schematically displayed in Fig. 1. Because the creation of H3 O is not possible without water being adsorbed to the AS, the presence of Na appears to counteract the formation of H3 O and the process of the PT in zeolites. In this Letter, we simulate the time evolution of a zeolite containing two Al-sites, one of them saturated with a hydrogen atom and another one with an Na atom. The Al-sites are placed in second-neighbor positions within the eight-membered ring (8MR). The short distance between the Alsites allows the competition between the two centers for H2 O molecules to be studied. The hydration of the zeolite with three water molecules represents the interesting case when all three water molecules are connected to Na; one of them, however, is in contact with the acid site. The time evolution of interatomic interactions is compared with that of the Na-free zeolite [6]. 2. Results and discussion 2.1. MD simulations Periodic ®rst-principles density-functional calculations are performed with the Vienna ab initio simulation package VASP [7,8] using generalized gradient approximation (GGA) [9] and projector augmented wave (PAW) techniques [10,11]. The simulation temperature during the MD within the canonical ensemble varies around 300 K. More details on the computational method are given in our previous Letter [6]. The adsorption of water to the acid site of Na zeolite is simulated on gmelinite, a zeolite with a medium-sized unit cell. The hexagonal structure (P 63 mmc [12]) is composed of one type of the tetrahedral atom and four irreducible O atoms. The largest apertures of the channel system are
L. Benco et al. / Chemical Physics Letters 330 (2000) 457±462
twelve- and eight-membered rings (12MR and 8MR, respectively) [12]. The secondary building units, which are hexagonal prisms, also constitute elements of other important zeolites such as faujasites. The interaction between the zeolite acid site and the Na atom is explored for the structure with two Al-sites localized within the 8MR in the second-neighbor positions (cf. Fig. 2). The mechanism of adsorption of water molecules to both the AS and to the Na atom is tested for several concentrations of H2 O and for several initial locations of the adsorbed molecules within the zeolite framework [13]. While the AS can adsorb only one water molecule, the coordination sphere of Na comprises up to three molecules. At low concentration, e.g., one H2 O per cell, the at-
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tractive powers of the AS and the Na atom are similar. The water molecule only slightly prefers adsorption to Na [13]. Increase of the concentration of water leads ®rst to the saturation of the acid site, then completes the coordination sphere of Na. When it is completed any additional water is adsorbed within the channels of the zeolite [14]. As a response to the adsorption of water considerable local deformations of the zeolite structure can occur. Because the capacity of the framework to accommodate a proton depends on the local geometry [15], the deformation of the structure drives the `local chemistry' within the zeolite framework. Through the local deformation, the Osites of the zeolite can be both favored or disfavored for protonation. The concentration and the location of water can thus control proton transfer between the O-sites in zeolites. 2.2. Mechanism of proton transfer
Fig. 2. Mechanism of proton transfer within the 8MR. (a) The acid proton located in position O(4); (b) the acid proton transferred to position O(1).
Recent MD simulations of one acid site [6,13] have shown that the acid proton can migrate between O-sites neighboring the Al-site as soon as one water molecule is adsorbed to the acid site. This high mobility of acid protons in zeolites was not observed in previous MD simulations [4,5,16]. The reason could be that the dynamical interaction of water with a zeolite framework was investigated only for small unit cells, thus imposing constraints to such a deformation of the framework that supports proton transfer [6]. Reproduction of proton transfer through the simulation at the temperature of liquid nitrogen [13] implies a conclusion that proton transfer around the Al-site is a general phenomenon occurring in acid zeolites. In zeolites with a high concentration of Al-sites residual Na cations can interact with acid protons thus disturbing proton transfer around the Al-site. We have observed such an interaction for the structure displayed in Fig. 2a, where the water molecule, playing an active role in the PT, connects both to the AS and to the Na atom. Two Alsites are next-nearest-neighbors in the 8MR of the gmelinite framework. One Al-site is compensated by a proton connected to the O(4) position (Fig. 2a, left) and another one by the Na atom localized within the 12MR (not displayed) making
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contact to the O(3) position (Fig. 2a, right). Two water molecules are connected directly to the Na atom from the side of the 12MR (bottom of Fig. 2a). The third water molecule is placed between the two Al centers. It is adsorbed to the AS and at the same time it connects to Na. This double linkage to the two positively charged centers leads to an orientation of the molecule such that the O±H bonds are oriented approximately perpendicular to the line connecting Na and the AS (cf. the angle indicated in Fig. 2a). In this con®guration one of the hydrogen atoms points to the O(1) atom and establishes a secondary hydrogen bond to the framework. The water molecule is thus doubly connected to the zeolite and constitutes a six-membered ring composed of the Al atom, two framework O-sites, the acid proton and the O±H bond of the water molecule, respectively. The orientation of the water molecule as displayed in Fig. 2a is rather short lived. A quick reorientation is accomplished through a PT in which one proton of the adsorbed water molecule jumps to the framework O(1) position, and at the same time the acid proton jumps from the framework O(4)-site to the water molecule. The relaxation of all atomic positions of the two structures (Fig. 2a and b) favors structure (b) by 7.7 kJ/mol. The preference for the protonation of the O(1)-site is indicated by the Al±O±Si angles as well. In structure (a) the angle Al±O(1)±Si has an orientation favorable for the protonation of the O(1)-site. The value of this angle (146.1°) is higher than that of the angle Al±O(4)±Si (141.5°) in structure (b). This agrees well with the recently observed correlation between the stability of the protonated structure and the value of the angle of the unprotonated framework [15]. 2.3. Time evolution of geometry parameters A snapshot of the structure fragment of the Na zeolite with the adsorbed water molecule shortly before the PT is displayed in Fig. 3a together with an equivalent fragment of the Na-free structure shown in Fig. 3b. The similarity of both structures demonstrates that the same mechanism of PT applies. In both the Na zeolite and in the Na-free structure the
Fig. 3. The geometry of the 6MR established by the adsorption of the water molecule to the acid site and the creation of a secondary hydrogen bond to the framework O-site. (a) The Na zeolite with the acid site in O(4) position; (b) the Na-free zeolite with the acid site in O(1) position. In Na zeolite (a) the H2 O molecule connects to both the acid site and to the Na atom.
water molecule is locked in the 6MR around the Al-site and PT takes place within this 6MR. The time evolution of the selected geometry parameters is displayed in Fig. 4. The distance of the two protons from the framework O-sites (Fig. 4a) shows that PT in the Na zeolite is very fast. In contrast to the Na-free structure, there is no plateau indicating the existence of an H3 O ion. The crossing of the two O±H lines denoted by the vertical line (tPT ) indicates the PT. The O±O curves showing the distance of the adsorbed water molecule from the two framework O-sites (O(4), O(1)) are displayed in Fig. 4b. Due to the dynamics of the structure comprising both the deformation of the zeolite skeleton and the migration of the adsorbed water molecule, the position of H2 O is shifted from the O(4)-site toward the O(1)-site. The vertical line (tPT ) shows that the proton transfer is delayed by several femtoseconds after the moment when the water molecule approaches the O(1)-site. Fig. 4c displays the variation of the O±O±O angle, stressing the fact that PT is realized through tight bonding to both framework O-sites. Similar to the PT in the Na-free zeolite [6], the maximum value of the O±O±O angle occurs at the time of the PT as detected by the O±H distances in Fig. 4a. The strength of the hydrogen bond depends on the O±H O angle and reaches the largest value for an angle of 180°. For the purpose of assessing the
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Fig. 5. The time evolution of the O±H±O angles (cf. Fig. 3) (a) in the Na zeolite and (b) in the Na-free structure.
Fig. 4. Time evolution of the geometrical parameters within the 6MR (cf. Fig. 3). (a) The O±H distances; (b) the O±O distances and (c) the O±O±O angle.
variation of the strength of the hydrogen bonds, the time evolution of the O±H O angles for the two hydrogen bonds within the 6MR is displayed in Fig. 5. The angles for the Na zeolite (Fig. 5a) are compared with those for the Na-free zeolite (Fig. 5b). For the Na zeolite the variation of the values of both angles is synchronized and repeating with a period of 20 fs. At the moment of PT both angles reach their largest values. The PT is thus accomplished through the most favorable geometry with the O±H O angles at their maximum values. An interesting feature is that the transfer of both protons is practically simultaneous. This is in con-
trast to the Na-free structure for which the corresponding angles are displayed in Fig. 5b. Here, the ®rst proton transfer leads to the creation of a hydronium cation as indicated by the ®rst arrow. The lifetime of the H3 O cation has been previously estimated, based on the time evolution of the O±H distances, to be 50 fs. According to the evaluation in Fig. 5 the decay of the H3 O occurs shortly after the maximum value of the O(4)±H2±O(w) angle (cf. the second arrow), and the lifetime of the hydronium cation is 70 fs. In contrast to the longlived H3 O in Na-free zeolite, no hydronium cations are produced in the Na zeolite. Here the creation of H3 O is prohibited by the direct contact of the water molecule with the Na cation. 3. Conclusions First-principles MD simulations show that, in zeolites, the close proximity of the Na counterion
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to the adsorbed water molecules suppresses the creation of the long-lived H3 O cations. A spontaneous barrierless PT, however, is possible without the existence of such a cation and occurs through concerted double-proton transfer. The presence of Na cations thus leads to a modi®ed mechanism but does not suppress proton transfer around the Al-site in zeolites. Acknowledgements The work has been performed within the Groupement de Recherche Europeen `Dynamique Moleculaire Quantique Appliquee a la Catalyse', founded by the Council National de la Recherche Scienti®que, France, the Institut Francßais du Petrole (IFP), TOTAL Recherche et Development, and the Universitat Wien. Computing facilities at IDRIS, France, are kindly acknowledged. References [1] J.W. Ward, in: J. Rabo (Ed.), Zeolite Chemistry and Catalysis, ACS monograph no. 171, American Chemical Society, Washington, DC, 1976.
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