A Periodic DFT Study of N2O4 Disproportionation on Alkali

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J. Phys. Chem. C 2008, 112, 5510-5519

A Periodic DFT Study of N2O4 Disproportionation on Alkali-Exchanged Zeolites X Evgeny A. Pidko,*,† Pierre Mignon,‡ Paul Geerlings,§ Robert A. Schoonheydt,‡ and Rutger A. van Santen† Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, NL-5600 MB EindhoVen, The Netherlands, Centrum Voor OpperVlaktechemie en Katalyse, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg, 23 B-3001 HeVerlee, Belgium, and Dienst Algemene Chemie, Vrije UniVersiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium ReceiVed: September 3, 2007; In Final Form: January 7, 2008

Adsorption and disproportionation of dinitrogen tetraoxide on sodium-, potassium-, and rubidium-exchanged zeolites X with Si/Al ratio of 1.18 were studied using density functional theory calculations with periodic boundary conditions. It is found that the stabilization and activation of most of the N2O4 isomers confined in the zeolitic cage does not follow Lewis acidity difference of the extraframework cations. This is also observed for the energetics of the N2O4 disproportionation reaction resulting in formation of a space-separated NO+‚‚‚NO3- ion pair. The reaction energy increases in the row NaX < RbX < KX. The strength of perturbations and, therefore, the low-frequency shift of the N-O stretching frequency of the adsorbed NO+ cations correlate well with the basicity of the zeolite (RbX > KX > NaX). However, this factor is not the relevant reactivity parameter for the N2O4 disproportionation in the cationic forms of zeolites. The higher activity for the disproportionation as well as the stronger molecular adsorption of N2O4 on RbX and KX zeolites as compared to that on NaX is ascribed to the features analogous to the molecular recognition characteristics of supramolecular systems. The steric properties of the zeolite cage and the mobility of the extraframework cations induced by adsorption are essential to shape the optimum configuration of the active site for N2O4 disproportionation.

1. Introduction Zeolites and related microporous materials are widely applied in different technological fields. They act as efficient heterogeneous catalysts, as adsorbents, and as molecular sieves in gas separation and purification. Such a broad spectrum of applications results from the possibility to rather easily tune the chemical and physical properties of the material by choosing the particular zeolite structure with required type and size of cavities and channels and by the introduction of extraframework active species into the zeolite matrix. Isomorphous substitution of tetrahedrally coordinated framework silicon with aluminum generates a negative charge on the oxygens of the framework. The compensation of the thus formed charge requires the presence of a countercation. Introduction of the extraframework cations to the zeolite leads to formation of conjugate acid-base pairs, where the cation is the Lewis acid and the framework oxygen atoms are the basic sites. In addition, there is a significant electrostatic field in the cage of the zeolites because of the exchangeable cations. Both the acidic and basic sites as well as the field within the zeolite are crucial for the reactivity of the material. Contrary to the well-understood mechanisms of chemical reactions catalyzed by Brønsted acid sites in the hydrogen forms of zeolites, the nature of chemical reactivity in zeolites modified with metal ions remains the subject of intense debate. Typically, zeolites with alkali and alkaline-earth cations compensating for * To whom correspondence should be addresesd. E-mail: [email protected]; tel: +31 40 247 2189; fax: +31 40 245 5054. † Eindhoven University of Technology. ‡ Katholieke Universiteit Leuven. § Vrije Universiteit Brussel.

the negative charge on the framework exhibit well-pronounced basic properties.1-4 The presence of such rather inert species enhances the electron density of the framework oxygens, which can act as basic sites. The extraframework cations, on the other hand, produce a rather strong electrostatic field in the zeolite cage and can in principle polarize molecules confined in the microporous matrix resulting in their activation. Usually, these two effects, that is, the presence of rather strong basic sites and the electrostatic field due to the exchangeable cations, are considered to account for the chemical reactivity of alkali and alkaline-earth exchanged zeolites.1-6 This paper aims to contribute to an understanding of the role of these factors in chemical reactions promoted by cation-exchanged low-silica zeolites. Recently, it has been found that the alkali and alkaline-earth exchanged low-silica faujasite-type zeolites promote selective oxidation of hydrocarbons with O2.5-11 The reaction takes place via either photochemical or thermal activation. The respective zeolitic cations do not show any redox properties and cannot dissociate either hydrocarbon or oxygen molecules. Thus, the activation of the reagents in these zeolites is different from that found in conventional processes.6,12,13 It has been suggested that the photoactivated oxidation of hydrocarbons (HC) by dioxygen molecules loaded into a zeolite cage proceeds via formation of a [HC+‚‚‚O2-] charge-transfer state.5,8 The high electrostatic field associated with the chargebalancing cations lowers the energy required to excite electron transfer from the hydrocarbon to O2 providing a route to selective oxidation at relatively low temperatures. A similar mechanism has been also assumed for the low-temperature

10.1021/jp077063p CCC: $40.75 © 2008 American Chemical Society Published on Web 03/18/2008

N2O4 Disproportionation on Cationic Zeolites X SCHEME 1: Relative Energies (B3LYP/6-31G(d)) and Geometric Parameters (Å) of the Selected N2O4 Isomers in Gas Phase from Ref 21a

J. Phys. Chem. C, Vol. 112, No. 14, 2008 5511 separated. Herein, we report a density functional theory study of adsorption and disproportionation of N2O4 on sodium-, potassium-, and rubidium-exchanged low-silica zeolites X in an attempt to separate the effects of the zeolitic cations and the framework oxygens. 2. Computational Details

a The geometric parameters of the symmetric D 2h N2O4 and of the O2NONO isomer calculated at PW91/PAW level of theory with periodic boundary conditions are presented in brackets for comparison.

selective oxidation of propane to acetone over alkaline-earth exchanged faujasites in the absence of irradiation.6,11 Lahr et al.14 reported recently a detailed investigation of H2 and CO oxidation over various ion-exchanged X and Y zeolites. Surprisingly, no correlation between the reactivity and the electrostatic field within the zeolite cages was observed. In agreement with these experimental findings, our recent quantumchemical calculations15,16 show that the electrostatic field is, indeed, not a key feature of such systems. Instead, molecular recognition features such as the relative orientation and the distance between the reagents confined in the zeolitic pore have been proposed to be the most important factors. Another interesting chemical property of the alkali-exchanged zeolites is the promotion of the disproportionation of nitrogen dioxide at low temperatures.17-20 Indeed, the gas-phase reaction is unfavorable (Scheme 1). Formation of a separated NO3-‚‚‚NO+ ion pair requires energies as high as 205 kJ/mol.21 Such endothermic reactions can be substantially stabilized in a polar media, for example, in solution via solvation. Adsorption measurements, theoretical modeling, and extensive reaction chemistry using cationic faujasites indicate that the intracrystalline void space shows high polarity.22 Different endothermic reactions are strongly facilitated in zeolite pores. The interaction between the occluded molecules and the zeolite framework displays the characteristics of a strong electrolyte or solvent.23,24 In particular, in the case of low-silica zeolites, the high concentration of framework aluminum and extraframework cations results in very strong “solvent effects”. This is similar to the effect of polar solvents in solutions with specific features because of the local environment and confined space in the zeolite pores.22 The disproportionation of nitrogen dioxide observed on alkaliexchanged zeolites X and Y has been recently used to characterize the basic properties of microporous materials.20,25 NO+ adsorbed on the negatively charged framework oxygens has been proposed as a new probe molecule for zeolite basicity associated to the negative charge around oxygen atoms.1 The stretching frequency of the adsorbed NO+ has been related to the oxygen basicity that increases along with the size of the exchangeable cation. On the other hand, the high reactivity of cation-exchanged zeolites toward N2O4 disproportionation has been attributed by Li et al.19 to the existence of rather mobile Lewis acid sites that are extraframework alkali and alkaline-earth cations. The reactivity of the alkali and alkaline-earth exchanged zeolites is determined by a combination of Lewis acidity of the exchangeable cations and the basicity of the framework oxygens. In addition, the geometrical properties of the zeolite cage as well as the high density of the exchangeable cations in it can be important for chemical activation of the molecules confined in the zeolite matrix.15,16 These factors cannot be easily

2.1. Methodology. Periodic density functional theory (DFT) as implemented in Vienna Ab Initio Simulation Package (VASP)26,27 was used to identify equilibrium structures and their energetics as well as to calculate vibrational frequencies of N2O4 adsorption complexes within alkali-exchanged zeolite X. All calculations were performed using the Perdew-Wang (PW91) form of the generalized gradient approximation for the exchange and correlation energies.28 The projected augmented waves (PAW) method29,30 was used to describe electron-ion interactions, and for valence electrons, a plane wave basis set was employed. The energy cutoff was set to 400 eV. The Brillouin zone sampling was restricted to the Γ-point.31 Cell parameters were initially optimized for each of the zeolites. The parameters obtained were then used in all calculations. Full geometry optimizations were performed for each structure with the fixed cell parameters using a conjugated gradient algorithm. Convergence was assumed to be reached when the forces on each atom were below 0.05 eV/Å. Vibrational frequencies of the adsorbed N2O4 species were calculated using the finite difference method as implemented in VASP. Small displacements (0.02 Å) of atoms from the N2O4 species and of zeolitic ions involved in direct interaction with N2O4 were used for the estimation of numerical Hessian matrix. The rest of the zeolitic atoms were kept fixed to their equilibrium positions. The stabilities of the adsorption complexes discussed below were evaluated on the basis of the relative energies (∆E) of the respective structures. These energies were calculated as

∆E ) E(N2O4‚zeolite) - E(zeolite) - E(N2O4) where E(N2O4‚zeolite) is the calculated total electronic energy of the adsorption complex, E(zeolite) is the total energy of the optimized zeolite structure, and E(N2O4) is the total energy of the isolated N2O4 molecule. For the adsorption complexes of N2O4 isomers on cationexchanged low-silica zeolties, in most cases pure electrostatic and covalent interactions, which are well described within GGA method, dominate the adsorption energy. The accuracy of the chosen method for such interaction within the adsorption complexes on zeolitic cations was verified by computing the adsorption energies of N2O4 on Na+ stabilized in a 4T cluster model using the current plane-wave DFT method with the PW91 functional as well as the hybrid DFT with the B3LYP functional and the MP2 method (see Supporting information). Both latter methods were utilized in combination with atom-centered basis sets of different quality. The results obtained using the current PW91/PAW method agree well with those computed by means of the higher level B3LYP and MP2 methods. Nevertheless, DFT methods are known to fail in description of dispersion interactions.32-36 This could in principle lead to some underestimation of the thus computed adsorption energies. However, the van der Waals contribution in our case is expected only for the intermolecular O‚‚‚O contacts, which are very weak ( K+ > Rb+ because of the decrease of the charge-to-radius ratio. As a result, the N2O4 adsorption energies should decrease along with the increase of the ionic radius of the exchangeable cations. In contrast to such

TABLE 1: Optimized Structural Properties of Fully Siliceous Faujasite and Alkaline-Exchanged Zeolites X Si-FAU

NaX

a ) b ) c ) 17.243 Å R ) β ) γ ) 60.00°

a ) b ) c ) 17.775 Å R ) β ) γ ) 60.00°

KX

RbX

a ) b ) c ) 18.033 Å R ) β ) γ ) 60.00°

a ) b ) c ) 18.186 Å R ) β ) γ ) 60.00°

Unit Cell Dimensions

Selected Interatomic Distances M(SI′)-O M(SII)-O M(SIII)-O

2.17-2.41 Å 2.23-2.46 Å 2.39-2.63 Å

2.56-2.78 Å 2.70-2.88 Å 2.76-3.26 Å

2.70-2.97 Å 2.84-3.04 Å 2.94-3.32 Å

N2O4 Disproportionation on Cationic Zeolites X

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Figure 2. Optimized structures and selected geometric parameters (Å) of the adsorption complexes of N2O4 on (a) SII and (b) SIII sites of NaX; (c) SII and (d) SIII sites of KX; and (e) SII and (f) SIII sites of RbX.

TABLE 2: Adsorption Energies (∆E, kJ/mol) and N-O Stretching Frequencies (cm-1) of the Adsorption Complexes of N2O4 in Cation-Exchanged X Zeolite NaX

KX

RbX

SII

SIII

SII

SIII

SII

SIII

∆E

-10

-11

-10

-15

-21

-26

b2u b3g ag b1u

1778 1754 1404 1264

1796 1732 1396 1272

ν(N-O) 1732 1707 1264 1234

1731 1695 1261 1234

1696 1670 1237 1216

1695 1665 1230 1216

expectations, the opposite trend is observed. The highest adsorption energies are calculated for the case of RbX, while the N2O4 adsorption on NaX is the weakest (Table 2). One of the reasons for this observation is the following. In addition to the primary interactions between the adsorbed molecule and the cations at sites SII or SIII, some weaker

interatomic contacts with the neighboring cations occur (Figure 2). With the increase of the ionic radius of the exchangeable cation, the number of these secondary interactions increases in parallel. The respective distances decrease. Indeed, in the case of the sodium form of zeolite X, the cations neighboring the adsorption site are located at a rather long distance from the adsorbed molecule and, therefore, cannot interact efficiently with it. These Na+ ions can only weakly polarize the adsorbed N2O4. On the other hand, the larger potassium ions in positions neighboring the adsorption site form contacts (3.390-3.848 Å), which are comparable with those (3.024-3.150 Å) involved in the primary N2O4‚‚‚cation interaction (Figure 2c and d). In the case of RbX, the primary and the secondary interactions are already characterized by very similar Rb‚‚‚O2NNO2 distances. Moreover, in this case, adsorbed N2O4 interacts not only with the cations nearest to the initial adsorption site but with almost all Rb+ in the supercage of zeolite X (Figure 2e and f). Thus,

5514 J. Phys. Chem. C, Vol. 112, No. 14, 2008 in spite of the weaker individual interatomic contacts between the adsorbed molecule and the extraframework cations, the number of the contacts increases with the increase of the ionic radius. This results in the unexpected enhancement of the N2O4 adsorption energy in the row NaX < KX < RbX. Recently, it has been shown that shielding of the exchangeable cations by the framework oxygens can lead to unexpected behavior of the adsorbed molecules.33,34 Obviously, the cations stabilized at SII sites of faujasite are better shielded as compared to those at SIII sites. This leads to the observed stronger adsorption of N2O4 on SIII cations. For the small sodium cation, this effect is not very well pronounced because of significant coordination saturation of Na+ both at the SII and the SIII sites. However, in the case of K+, the difference in adsorption energies becomes more significant. Indeed, a larger cation can be still rather well shielded by the oxygen atoms of the zeolitic sixmembered ring, while K+ at the SIII site is unsaturated. Further increase of the ionic radius, as in the case of RbX, results in equalization of the properties of the exchangeable cations at SII and SIII sites. 3.2. Adsorption of O2NONO Isomer. In the first step of the N2O4 disproportion event, the N-N bond breaks and the NO2 moiety distant to the adsorption site rotates. This results in formation of a close NO3δ-NOδ+ ion pair. It is a O2NONO (Cs) conformer bound with two oxygen atoms of the NO3 fragment to the alkaline cation and with a nitrogen atom from the NO moiety coordinated to the framework oxygens. This structure has been proposed earlier25 to be formed upon adsorption of N2O4 on cation-exchanged faujasites. Although the NO3 and NO moieties are still covalently bound in the O2NONO species in the gas phase, it has been shown by Wang et al.45 that this isomer already displays a large degree of ionic bonding character relative to the symmetric N2O4 (D2h) isomer. It has been also suggested that the thermal disproportionation of dinitrogen tetraoxide proceeds via formation of this isomer. Therefore, one expects that interaction of the partially negatively charged NO3δ- moiety with the extraframework cation and simultaneous coordination of the NOδ+ to the basic framework oxygens would facilitate the decomposition of the close ion pair and, as a result, would lead to the disproportionation products. Similar to the above case, the adsorbed O2NONO species can coordinate through the NO3δ- moiety to the exchangeable cation located at either SII or SIII site and through the NOδ+ to the neighboring framework oxygens. The initial guess structures for these adsorption complexes were constructed as follows. The starting geometry of the O2NONO molecule corresponds to that of the Cs gas-phase isomer. The negatively charged part was located at the same cation‚‚‚ON distances as calculated for the case of adsorption of the symmetric N2O4 (D2h) isomer. The distance between the nitrogen atom from the NOδ+ moiety and the basic zeoltic oxygens was kept at 3 Å, which is the sum of van der Waals radii of N and O atoms.44 Figure 3 shows optimized structures of the thus constructed adsorption complexes of O2NONO in NaX, KX, and RbX zeolite. The structure of the O2NONO isomer remains similar to the proposed one only in the case of adsorption on the SII site of NaX zeolite (Figure 3a). This is a rather unstable configuration (∆E ) +18 kJ/mol). In this structure, the NO bonds coordinated to Na+ are elongated by about 0.03 Å, while the one directed away from Na+ is shortened by more than 0.15 Å as compared to the geometry parameters of the gas-phase isomer (PW91/PAW, Scheme 1). The NOδ+ moiety is 1.993 Å distant from the nitro moiety, which is 0.321 Å larger than the respective interatomic distances in the gas-phase isomer. These

Pidko et al. strong perturbations suggest an increase of the ionic bonding character in the O2NONO isomer adsorbed on the SII site of NaX. Adsorption of O2NONO on the SIII site of NaX zeolite (Figure 3b) results in formation of a rather stable structure (∆E ) -45 kJ/mol) and, at the same time, in a more pronounced separation of the nitro and nitroso moieties (r(ON‚‚‚ONO2) ) 2.190 Å). The higher stability of this structure is most likely because of a more effective interaction of the nitro group with Na+ at site SIII as well as because of a more favorable coordination of NO+ to the framework oxygens at the SIII′ site that prevents repulsion with the neighboring SIII Na+. In the cases of potassium- and rubidium-exchanged X zeolites, adsorption complexes involving a well distinguishable O2NONO moiety are not found. Geometry optimization resulted in disproportionation of the adsorbed molecule and formation of a space-separated NO+‚‚‚NO3- ion pair (Figure 3c-f). All of these adsorption complexes are rather stable (Table 3). Independent of the initial geometry and of the type of the main adsorption site (SII or SIII), geometry optimization results in formation of a NO3- species coordinated to three exchangeable cations and of a NO+ bound to the SIII′ site. Two different types of coordination of the nitrosonium cation to the basic framework oxygens are found, that is, mono- (Figure 3c and e) and bidentate (Figure 3d and f). In the former case, the shortest distance between the NO3- and the NO+ moieties is realized (r(ON‚‚‚ONO2) ) 2.890 and 3.370 Å, respectively, for KX and RbX). This is, however, too long to be considered as an interatomic contact of significant energy. The adsorption energies do not strongly depend on the type of NO+ coordination to the zeolite framework (Table 3). Thus, the rather high stability of the discussed adsorption complexes on KX and RbX zeolites is due to an effective interaction of NO3- with three extraframework cations. The selected results of vibrational analysis of the adsorption complexes are presented in Table 3. The red shift of the NO stretching frequency (ν(N-O)NOδ+) of the adsorbed nitrosonium cation increases in the row NaX < KX < RbX. This reflects the enhancement of the interaction of the NO+ with the zeolite framework. This conclusion is also supported by the fact that the interatomic distances ON‚‚‚O between the NO+ species and the zeolitic O atoms decrease in this row. Simultaneously, the corresponding N-O bond elongates reflecting the stronger perturbation of the stronger interacting NO+ ion. Calculated frequencies of the N-O stretching vibrations of the NO3 moiety (ν(NO3), Table 3) coordinated to the extraframework cations show that, similar to the above discussed adsorption of symmetric D2h N2O4, the strength of perturbation does not correlate with the expected Lewis acidity strength of the exchangeable cations. The weakest red shift of ν(NO3) and the smallest geometry perturbations of the adsorbed nitro anion are observed for NaX zeolite, where the NO3 species coordinates only to one sodium ion. Therefore, the perturbation of the adsorbed NO3- ions is mainly controlled by the number of the intermolecular contacts formed upon adsorption and not by the strength of the individual contact interaction. 3.3. Adsorption of the NO+‚‚‚NO3- Ion Pair. We consider three different conformations of the NO+‚‚‚NO3- ion pair confined in the cation-exchanged zeolites X. In all cases, the nitro group is coordinated to the largest possible number of neighboring cations. The NO+ cation is initially located either at the SIII site of faujasite (structure I), that is, in the center of the four-membered ring next to the SII site, or at the SIII′ site nearest (II) or next nearest (III) to the six-membered ring of

N2O4 Disproportionation on Cationic Zeolites X

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Figure 3. Optimized structures and selected geometric parameters (Å) of the adsorption complexes of the initially O2NONO isomer on (a) SII and (b) SIII sites of NaX; (c) SII and (d) SIII sites of KX; and (e) SII and (f) SIII sites of RbX.

TABLE 3: Adsorption Energies (∆E, kJ/mol) and Selected Vibrational Frequencies (cm-1) of the Adsorption Complexes Formed via Confinement of O2NONO in Cation-Exchanged X Zeolite NaX ∆E ν(N-O)NOδ+ ν(NO3)assym′ ν(NO3)assym′′ ν(NO3)sym

KX

RbX

SII

SIII

SII

SIII

SII

SIII

+18

-45

-65

-65

-78

-80

2010 1487 1254 985

Frequencies 2017 1977 1436 1354 1259 1332 1018 1047

1982 1369 1322 1042

1942 1355 1324 1035

1975 1367 1315 1039

faujasite. The optimized structures of these adsorption complexes on NaX, KX, and RbX are shown in Figures 4, 5, and 6, respectively. Relative energies and selected vibrational frequencies of the respective adsorption complexes are summarized in Table 4. Structure I is found to be the least stable ion pair, whatever the cation. The nitrosonium cation in this structure is coordinated

to two framework oxygens of the four-membered ring (r(ON‚ ‚‚Oz) ) 2.135 and 2.215 Å for NaX, 2.078 and 2.134 Å for KX, and 2.073 and 2.096 Å for RbX) with significantly longer distances to the other zeolitic oxygens (2.847, 2.667, and 2.641 Å, respectively, for structure I on NaX, KX, and RbX zeolite). NO+ at the SIII site is significantly perturbed. The N-O bond is the longest found among the systems considered. The strong perturbations are also evident from the calculated ν(N-O)NOδ+ frequencies, which are very strongly red-shifted (Table 4). The shortening of the ON‚‚‚Oz interatomic distances together with the enhancement of the perturbations of the adsorbed NO+ species observed along with the increase of the ionic radius of the exchangeable alkaline ions is due to the increase of the basicity of the framework oxygen ions.1 Thus, one can conclude that location of the nitrosonium cation at the SIII site of zeolite X provides an effective binding of NO+ to the zeolitic oxygens. On the other hand, such a localization of NO+ results in a strong displacement of the original alkaline cation located at this site toward the distant SII cation. Because of such geometry

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Pidko et al.

Figure 4. Optimized structures and selected geometric parameters (Å) of the adsorption complexes of the NO+‚‚‚NO3- product of N2O4 disproportionation on NaX.

Figure 5. Optimized structures and selected geometric parameters (Å) of the adsorption complexes of the NO+‚‚‚NO3- product of N2O4 disproportionation on KX.

changes, the NO3- ion is coordinated to only two alkaline cations, although as it has been shown above, the most stable structures are formed when a 3-fold coordination is realized. Thus, despite the very effective interaction of the nitrosonium cation with the basic lattice oxygen ions, the overall energetics of structure I are unfavorable because of the unfavorable

distribution of the extraframework cations and the resulting lower efficiency of the stabilization of NO3-. Structures II and III are the most stable among the N2O4 isomers confined in the zeolites (Table 4). The nitro group in these complexes coordinates to three extraframework alkaline cations. Two cations (the SII and the initial SIII neighboring to

N2O4 Disproportionation on Cationic Zeolites X

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Figure 6. Optimized structures and selected geometric parameters (Å) of the adsorption complexes of the NO+‚‚‚NO3- product of N2O4 disproportionation on RbX.

TABLE 4: Adsorption Energies (∆E, kJ/mol) and Selected Vibrational Frequencies (cm-1) of the NO+‚‚‚NO3- Product of N2O4 Disproportionation Confined in Cation-Exchanged X Zeolite NaX ∆E ν(N-O)NOδ+ ν(NO3)assym′ ν(NO3)assym′′ ν(NO3)sym

KX

RbX

INa

IINa

IIINa

IK

IIK

IIIK

IRb

IIRb

IIIRb

+18

-69

-65

+9

-91

-90

-9

-82

-78

1991 1370 1336 1039

1991 1358 1341 1038

1863 1363 1322 1038

1972 1359 1329 1039

1991 1353 1329 1036

1955 1402 1314 1040

2017 1391 1351 1044

2023 1386 1351 1047

Frequencies 1881 1368 1328 1039

the NO+ ion) coordinate each to two oxygens atoms, while the SIII cation distant to the nitrosonium ion is bound to only one oxygen atom of NO3-. NO+ is stabilized at the SIII′ sites next (II, Figures 4-6) or next-nearest (III, Figures 4-6) to the sixmembered ring of the faujasite supercage. To minimize repulsion, the original SIII alkaline cation migrates from the initial cation site to the SIII′ position opposite to the one occupied by the NO+. This, in turn, allows coordination of this cation to the nitro group providing additional substantial stabilization of the system. The most stable structures are formed on KX zeolite (IIK and IIIK, Figure 5). The corresponding adsorption complexes on RbX zeolite (IIRb and IIIRb, Figure 6) are only slightly (by ∼10 kJ/mol) less stable (Table 4). This is in agreement with the lower Lewis acidity of the larger cations. However, in the case of NaX, the respective structures show lower stability. This is, most likely, because stabilization of the NO3- moiety with three Na+ requires much larger displacements of the sodium cations from their equilibrium positions as compared to the larger potassium or rubidium ions. Moreover, such displacements in the case of NaX zeolite require more energy because of the stronger binding of sodium ions to the zeolite framework. Both these effects remarkably destabilize the system. Structures IIRb and IIIRb (Figure 6) show very similar relative energies (-82 and -78 kJ/mol, respectively (Table 4)) to those

of the structures discussed above (Figure 3e and f) resulting from O2NONO adsorption on SII and SIII sites of RbX (-78 and -80 kJ/mol, respectively (Table 3)). Indeed, in spite of the different coordination of NO+ to zeolitic oxygens and some slight differences in interatomic distances, these systems show one general feature: bidentate coordination of the NO3- ion to Rb+ at SII and SIII sites along with additional monodentate coordination to another SIII ion. In the case of KX zeolite, such an environment of the nitro anion is realized only for structures IIK and IIIK (Figure 5), whereas the complexes resulting from adsorption of the O2NONO isomer (Figure 3c and d) involve less interatomic contacts between the exchangeable alkaline cations and the NO3- moiety. This results in less effective stabilization of the nitro group and, hence, in a lower (by ∼15 kJ/mol, Tables 3 and 4) stability of the latter structures. The calculated N-O stretching frequencies for complexes II and III (Table 4) show trends similar to those discussed in section 3.2. The increasing basicity of the zeolite framework (NaX < KX < RbX) is accompanied with the increase of the low-frequency shift of the ν(N-O)NOδ+ stretch. The same tendency is observed for the calculated N-O stretching bands of the NO3- anion. Although because of the deficiencies of the method used the calculated frequencies are somewhat different from those observed experimentally,20 the general trend is reproduced well (Figure 7).

5518 J. Phys. Chem. C, Vol. 112, No. 14, 2008

Figure 7. Comparison of the experimental20 and calculated ν(NO)NOδ+ (a) and ν(NO3)assym (b) stretching frequencies for the stable adsorption complexes on alkali-exchanged X zeolites. Arrows in b correspond to the frequency range where the asymmetric N-O stretching vibrations of NO3- adsorbed on Na, K, and Rb exchanged X zeolites were observed.20

4. Discussion Gas-phase N2O4 disproportionation is an unfavorable process. The O2NONO isomer and the NO+‚‚‚NO3- ion pair are less stable by 45 and by 205 kJ/mol, respectively, than the D2h symmetric isomer.21 The calculations presented here show that formation of even the least stable structures within zeolite matrix is significantly more favorable than the gas-phase isomerization. Indeed, N2O4 isomerization results in formation of rather polar structures, which are stabilized in the cage of cation-exchanged zeolite. The microporous matrix shows characteristics of polar solvents facilitating charge separation. Molecular adsorption of a nonpolar N2O4 on the alkaliexchanged X zeolites is relatively weak. Bonding within the adsorption complexes can be well described as induced polarization of the adsorbed molecules in the field of the exchangeable cations. Such interactions depend on the size of the alkali ion. The smaller the cation is, the stronger its polarizing ability and Lewis acidity are. As a result, one expects a decrease of the N2O4 adsorption energies simultaneously with the increase of the ionic radius of the zeolitic cations. However, the calculations presented show the opposite trend (Table 2). Upon adsorption, a high density of alkali cations in the zeolite results in interaction of N2O4 with several exchangeable ions. In the case of NaX zeolite, the smaller sodium ions are located too far to form significant interatomic contacts with the adsorbed molecule and can only slightly polarize it. On the other hand, when the ionic radius of the zeolitic cations increases, numerous addition contacts are formed between the N2O4 molecule confined in the supercage of X zeolite and the accessible

Pidko et al. extraframework cations (Figure 2). Despite the weaker individual contacts, the overall interaction energy increases in the row NaX < KX < RbX. These are the features analogous to the molecular recognition characteristics formulated for supramolecular systems.46 At the first stages of the disproportionation reaction, the nonpolar symmetric molecule rearranges to form an O2NONO isomer that involves a rather ionic bond between the O2NO and the NO moieties. Already in this structure, the former fragment bears an excessive negative charge, while the latter fragment is charged positively.45 Therefore, interaction of each fragment with the corresponding zeolitic counterion facilitates cleavage of this contact ion pair. The supercage of the cation-exchanged X zeolite provides numerous positively charged sites, that is, alkali cations, which can accommodate the O2NO moiety, while the electron-deficient NO fragment can be well stabilized via interaction with the basic framework oxygen atoms. The stronger such interactions are, the easier the cleavage of the contact ion pair is. In the case of NaX, the framework oxygen atoms have the lowest basicity1 and, hence, bind NOδ+ weaker. At the same time because of the smaller size, the supercage sodium cations cannot provide the appropriate configuration to stabilize the negatively charged fragment. On the other hand, the larger potassium and rubidium cations shape the optimum configuration of the faujasite supercage providing a more efficient stabilization of the NO3δ- group. This along with the enhanced basicity of the framework zeolite lattice facilitates the N2O4 disproportionation. As a result, a rather unstable adsorption complex involving a well-distinguished O2NONO isomer is found only in the case of NaX zeolite, while the contact ion pair confined within KX and RbX is completely cleaved resulting in a more stable isomer (Figure 3). For the structures that result from adsorption of the O2NONO isomer, a similar trend in the adsorption energies is found. The interaction energy increases along with the increase of the cation size. The lower adsorption energies calculated for the case of NaX zeolite are due to the factors described above. However, the weaker adsorption on KX as compared to that on RbX is due to a less favorable coordination of the nitro group to the exchangeable cations (Table 3, Figure 3). Indeed, in spite of lower framework basicity of KX, further structural rearrangement leads to formation of more stable (Table 4) structures IIK or IIIK involving distant NO3- and NO+ ions (Figure 5). In this case because of the rather close ionic radii, the extraframework rubidium and potassium ions provide similar configurations stabilizing the nitrate ion. Thus, the strength of the individual intermolecular contact becomes important. This shows the dominant effect of the exchangeable cations on the reactivity of the zeolite toward N2O4 decomposition, while the role of the framework basicity is only indirect. The red shift of the stretching frequencies of NO+ cation bound to the zeolite framework correlates well with the strength of the corresponding interaction and, hence, with the basicity of the zeolite. This effect has been experimentally reported by Marie et al.20 The perturbations of the N-O stretching frequencies of the adsorbed NO3- ions reflect the strength of interaction of the nitro anion with the exchangeable cations. There is a deviation between the values of ν(N-O)NOδ+ reported in ref 20 and those presented above. The systematic error due to deficiencies of the method and anharmonicity effects can be suppressed by using a multiplication coefficient. The calculated ν(N-O)NOδ+ and ν(NO3)assym reasonably agree with those observed experimentally (Figure 7a and b, respectively). How-

N2O4 Disproportionation on Cationic Zeolites X ever, the calculated values do not correspond precisely to those reported in ref 20. It illustrates that the theoretical model of a completely ion-exchanged zeolite X probably does not perfectly match with the experimentally used partially exchanged samples. Also, the experimental IR spectra of N2O4 adsorbed on cationic X zeolites show rather broad bands corresponding to the N-O stretching vibrations. This coheres well with the current theoretical finding. The strongest perturbations, that is, the strongest elongation of the N-O bond and the strongest red shift of ν(N-O)NOδ+ band, are observed in the cases of the least stable structures INa, IK, and IRb. The low stability (Table 4) of those is due to an unfavorable coordination of the NO3- ion to the exchangeable cations (Figures 4-6). Moreover, as can be seen from the data presented in Tables 3-4 and Figures 3-6, although different coordination of the nitrosonium cation to the zeolitic framework can affect its vibrational properties, the variation of the relative energy depending on this factor is very low. The stability of the adsorption complexes is mainly determined by the interactions of the nitro group with the exchangeable cations. 5. Conclusions Molecular adsorption and disproportionation of dinitrogen teraoxide on completely exchanged NaX, KX, and RbX zeolites were investigated using the periodic plane-wave DFT method. The molecular N2O4 adsorption is a precursor for the subsequent isomerization and dissociation leading to formation of NO3-‚‚‚NO+ ion pair. The calculated N2O4 adsorption energies increase along with the increase of the ionic radii of the extraframework zeolitic cations (NaX < KX < RbX). The increase of the size of the exchangeable cations reduces the space in the faujasite supercage and makes possible formation of multiple interatomic contacts between the adsorbed molecule and the extraframework cations located in the supercage of zeolite X. This results in an increase of the overall adsorbate-adsorbent interaction in spite of weaker individual contacts. A similar effect is also observed for adsorption of other N2O4 isomers. N2O4 disproportionation results in formation of a NO3- ion bound to extraframework cations and a NO+ cation attached to Lewis basic framework oxygen atoms. The calculations presented show that the stability of such structures is mainly controlled by the interactions between the negatively charged nitro group and the extraframework alkali cations, while the role of the interaction between the nitrosonium cation and basic sites of the zeolites is only minor. We conclude that the N2O4 disproportionation over alkali-exchanged low-silica faujasites is mainly because of the cooperative effect of the extraframework cations as well as because of the confinement effect of the zeolite matrix. This can be considered as the analogue of molecular recognition phenomena in organic supramolecular systems. Acknowledgment. This work was sponsored by the National Computing Facilities Foundation, which provided supercomputer facilities, with financial support from The Netherlands Organization for Scientific Research (NWO). P. M. acknowledges a fellowship of the K.U. Leuven. Financial support came also from the Concerted Research Action (GOA/2005/13) and the Center of Excellence (EF/05/009), both at the K.U. Leuven. Supporting Information Available: Details and results of the test DFT and MP2 calculations of N2O4 adsorption on Na+ cation stabilized in a 4T zeolite cluster. This material is available free of charge via the Internet at http://pubs.acs.org.

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