J. Phys. Chem. B 2005, 109, 11119-11125
11119
Interaction of Molecular Nitrogen and Oxygen with Extraframework Cations in Zeolites with Double Six-Membered Rings of Oxygen-Bridged Silicon and Aluminum Atoms: A DFT Study Hans Mikosch,*,† Ellie L. Uzunova,‡ and Georgi St. Nikolov*,‡ Institute of Chemical Technologies and Analytics, Vienna UniVersity of Technology, Getreidemarkt 9/E164/EC, Vienna A-1060, Austria, and Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria ReceiVed: October 21, 2004; In Final Form: April 15, 2005
The interaction of N2 and O2 with extraframework cations of zeolite frameworks was studied by DFT, using the B3LYP method. The extraframework cation sites located in the vicinity of the double six-member rings (D6R) of FAU zeolites (SI, SI′, SIII′) were considered and clusters with composition (Mn+)2/nH12Si10Al2O18, M ) Li+, Na+, K+, Ca2+, were selected to represent the adsorption centers. The cation sites SII in the center of single six-membered rings (S6R) were modeled by [MIH12Si4Al2O6]- and MIIH12Si4Al2O6 clusters. The adsorption energy of N2 and O2 is the highest for Li+ cations at the SIII′ cation sites, while for the SI′ and SII sites the adsorption energies decrease in the order Ca2+ > Na+ > Li+. The calculated small N2 adsorption energy for Li+ cations at SII sites suggests that these sites do not take part in the sorption process in agreement with results of NMR studies and Monte Carlo simulations. The N2 adsorption complexes with the extraframework cations are linear, while those of O2 are bent regardless of the extraframework cation location. The SIII′ cation sites are the most favorable ones with respect to N2 adsorption capacity and N2/O2 selectivity; the SII sites are less selective and the SI sites are not accessible.
Introduction Zeolite materials have low framework density. Their void volume reaches 50% of the crystal volume. The 3D network of channels and cavities with dimensions comparable to those of molecules provides permeability to different reactants and allows the active sites inside the crystal to become accessible.1,2 Zeolites possess remarkable catalytic properties and adsorption capacities, which are related to their molecular sieving properties. The dehydrated forms of zeolites find commercial application in separation processes of many organic substances and also air.3 The molecular sieving properties depend on the pore dimensions and also on the type and locations of the extraframework cations.1b The extraframework cations, which balance the framework charge resulting from Si,Al substitution, are easily exchangeable by other cations from solutions. Knowledge of the cation positions and their occupancy in such materials is important for understanding their properties. Li-exchanged zeolites (FAU, CHA) and in particular Li-LSX (Low-Silica X) find industrial application in air separation by pressure-swing adsorption.3 These zeolites contain double sixmember rings (D6R). The FAU unit cell consists of eight sodalite cages (truncated cubooctahedra), interconnected via D6R’s; they enclose larger supercages with a free diameter of 12.7 Å. The supercages are accessible via 12-membered-ring windows with a diameter of 7.4 Å.4 The adsorption sites at the different extraframework sites are shown in Figure 1. The site SI lies in the center of the D6R; SI′ sites are located at opposite sides of T6O6 faces of a D6R; site SII is at the center of a single six-membered ring (S6R) or displaced from this point into the supercage; site SIII is in the supercage on a 2-fold axis opposite † ‡
Vienna University of Technology. Bulgarian Academy of Sciences.
Figure 1. Schematic representation of the FAU zeolite framework with extraframework cation sites denoted. The framework oxygen atoms are not shown; they lie near the center of line segments, connecting T-atoms. The adsorption sites AS1 (at site SI′), AS2 (at site SII), and AS3 (at site SIII′) are denoted.
a single four-membered ring (S4R) between two 12-member rings; SIII′ sites differ in position from the SIII sites and are found near the edges of the 12-ring windows.5 The SIII and the SIII′ sites have low fractional occupancy. With the exclusion of SI sites, all extraframework sites are not described by unique crystallographic coordinates. In zeolite frameworks, the extraframework cations tend to approach the regions of high electron
10.1021/jp0451795 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/13/2005
11120 J. Phys. Chem. B, Vol. 109, No. 22, 2005 density.6 The Na+ cations avoid occupation of the SI sites in X zeolites,5a,b while K+ cations are able to occupy both SI′ and SI sites in one D6R.5d,e A number of closely spaced SIII, SIII′, and SIII′′ sites have been resolved by single-crystal X-ray diffraction studies of dehydrated Na-X.5a,c The location of Li+ cations at SIII′ sites has been the subject of some controversy: according to structural studies of Li-LSX by Ple´vert et al. both SIII and SIII′ were equally occupied.7 Feuerstein and Lobo obtained different results: Li+ cations were found only at SIII sites in the supercages of Li-LSX.8 Taking into consideration the temperature dependence of the SIII and SIII′ site occupancy in Li-X with a Si/Al ratio of 1.25, the authors suggested that the SIII site found in Li-LSX represents an average of dynamically exchanging cations manifested by jumps of the Li+ cations between SIII′ sites at opposite corners of S4R.8 The Ca2+ cations also migrate between SI′ and SI sites in zeolite X upon dehydration.9 The crystal structures of hydrocarbon (ethylene, acetylene, cyclopropane, benzene, mesitylene), methylamine, and ammonia adsorption complexes in Ca-X have been resolved and the displacements of Ca2+ cations from their equilibrium positions in the zeolite upon sorption have been determined.10 The theoretical cluster studies of zeolites rely on the possibility to segregate the large zeolite unit cell into smaller fragments which correspond to the secondary building units (SBU) of zeolite frameworks. The cage-shaped structures and the D6R’s (hexagonal prisms) are stable and individual compounds have been isolated;11 S6R’s are less stable and to a large extent flexible.12 The permeability of silicate and aluminosilicate frameworks by N2 and O2 and the accessibility of the different extraframework cation positions have been a subject of theoretical and experimental studies.7b,8b,13-18 In the 7Li MAS NMR spectra of Li-LSX a chemical shift upon N2 and O2 adsorption was observed only for Li cations at SIII sites. The lack of access to cations inside the sodalite cages and D6R was suggested, based on comparison of van der Waals radii of N2, O2 and the six-member-ring windows. The Li cations at SII sites in the supercages also did not exhibit chemical shift, which could hardly be explained by steric factors. The penetration of N2 and O2 into uncharged polyhedral silsesquioxanes (HSiO3/2)n has been examined by the Hartree-Fock method.13 These theoretical studies clearly show that both N2 and O2 are able to penetrate the zeolite framework via six-member rings, five-member rings, and even four-member rings and find equilibrium positions inside D6R, D5R, and D4R. The calculated energy barriers to N2 and O2 insertion in various cages were found to decrease in the order T8 > T10 > T12, where Tn is the number of tetrahedral Si atoms in the cage-shaped structures. The same trend holds for the selectivity with respect to N2 and O2. The dissociation energies of the N2 and O2 adsorption complexes with Li+ and Na+ cations in zeolites were calculated by DFT in the localdensity approximation and linear structures MX2 were obtained for both X2 ) N2 and O2; the size of the zeolite clusters was limited to S6R.14 The Monte Carlo (MC) simulations of the sorption equilibrium for N2 and O2 in Li-LSX indicated that the interaction energies of the adsorbate molecules with the cations at SIII and SIII′ sites are rather similar.15 Lithiumcontaining faujasites and chabazites have very good nitrogen sorption capacities, but other cations are still of interest, because of the low thermal stability of the fully lithiated forms. The differential enthalpies of N2 adsorption on monovalent (Li+, Na+, K+) and divalent (Ca2+, Ba2+, Sr2+, Mn2+) cationsubstituted X zeolites, measured by microcalorimetry,18 are in very good agreement with the results obtained by MC simulations and with the experimental isosteric sorption heats of N2
Mikosch et al.
Figure 2. Schematic representations of the D6R cluster models. Extraframework positions in the proximity of the D6R are denoted. Atoms in decreasing size are O, Mn+ (extraframework cation), and tetrahedral atoms (Si or Al). Oxygen atoms are large light gray circles; Si are black circles; and Mn+ are gray circles.
adsorption on Li-LSX.15 The selective cation positioning among the available extraframework sites remains a challenge as well.17 The aim of the present study is to discriminate quantitatively the different cation adsorption centers in FAU zeolites on the basis of their ion-exchange energies, the electrostatic potential created in the nearest zeolite environment, and the electronic structure and binding energies of the zeolite-cation-adsorbate complexes. For the purpose of comparison, the interaction of N2 and O2 with the “bare” cations, Li+, Na+, K+, and Ca2+, was considered. Methods Geometry optimizations were performed in Cartesian coordinates by the B3LYP method, which includes local and nonlocal terms as implemented in the Gaussian 98 package.19-22 D6R’s of composition (Mn+)2/nH12Si10Al2O18 with M ) Li+, Na+, K+, Ca2+ were selected to represent SI, SI′, and SIII′ sites, see Figure 2. The negative charges of the D6R clusters with Si,Al substitutions were compensated by an appropriate number of extraframework cations, which are included in the optimizations. The advantage of the neutral cluster models over those bearing a negative charge is important: unbalanced negative charges require diffuse functions and larger basis sets, which in turn may produce a significant basis-set superposition error. Significant shifts of the vibrational frequencies may be observed.23 The introduction of balancing point charges may prevent correct geometry optimization. Different Si,Al distributions in D6R’s were examined in order to compare the gasphase ion-exchange energies: clusters with C2h symmetry have Al-O-(Si-O)3-Al linkages; clusters with C2V symmetry have Al-O-(Si-O)2-Al linkages, the two Al atoms being in one T6O6 ring; and clusters with Cs symmetry have an Al-O-SiO-Al linkage in one T6O6 ring. S6R of compositions [MIH12Si4Al2O6]- and MIIH12Si4Al2O6 with maximum separation of the Al atoms within the ring were used to examine the SII sitesin the case of monovalent cations, the clusters were negatively charged. The standard 6-31G(d) basis set was employed for all atoms involved, except Ca2+ and K+, for which the basis was extended to 6-311G(d). Hydrogen atoms were used to terminate the clusters through Si-H and Al-H bonds. For the OH-containing fragments, polarization functions at the hydrogen atoms were included via the 6-31G(d,p) basis. Full structure optimization of the clusters representing the various sorption complexes was performed without geometry restrictions or fixed coordinates. The points that correspond to minima (global or local) on the potential energy surfaces of (Mn+)2/nH12Si10Al2O18, [MIH12Si4Al2O6]-, and MIIH12Si4Al2O6 clusters were identified by the absence of negative eigenvalues in the diagonalized Hessian matrix, which give rise to imaginary
N2 and O2 Adsorption to Zeolite Extraframework Cations
Figure 3. (A) HOMO contour plots for (a) N2 and (b) O2. Atomic nuclei are represented by open circles: the N2 atoms are light gray and O2 atoms are dark gray. The positive parts of the orbitals are depicted by filled squares and the negative part by open squares. Orbital contours correspond to 90% electron density. (B) Molecular electrostatic potential maps (au) of (a) N2 and (b) O2.
TABLE 1: Bond Lengths (R), Dissociation Energies (D0), and Quadrupole Moments (QM) of N2 and O2, Calculated by B3LYP
R, Å R(exptl),a Å Dzpe,b eV D0(exptl),a eV QM⊥,c D Å QM|,d D Å
N2 1Σ + g
O2 3Σ g
1.106 1.098 9.512 9.759 0.519 -1.039
1.214 1.208 5.307 5.116 0.153 -0.306
a Experimental values from ref 28. b Zero-point correction included. Perpendicular with respect to the X-X bond, X)N, O. d Parallel with respect to the X-X bond, X)N, O.
c
vibrational modes. For clusters containing extraframework cations at SIII′ sites, the imaginary frequencies found at e40 cm-1 attributed to extraframework cation displacements were ignored. The charge distribution was examined by natural population analysis (NPA), which yields results that are rather insensitive to basis set enlargement.24 This method permits us to discern both covalent and noncovalent effects in molecules. The electrostatic potential (EP) of the clusters was calculated from the B3LYP density. Extraframework Cation Distribution: Energetics and Electrostatic Potentials Electrostatic Potential Maps. The B3LYP calculated bond lengths, dissociation energies, and quadrupole moments of the two diatomics, N2 and O2, are in agreement with data reported in the literature, see Table 1. The binding schemes of N2 and O2 are rather different. In N2, the two atoms are connected by three bonding doubly occupied MOs; two lone pairs (LP) lie along the bond axis. The HOMO is a lone-pair MO, Figure 3. The O2 molecule has a triplet ground state: two bonding and four lone-pair π-MOs. The N2 molecule has a significant quadrupole moment and it is able to interact with the electric field gradient created by cations.25 The molecular electrostatic
J. Phys. Chem. B, Vol. 109, No. 22, 2005 11121 potential (MEP) maps of N2 and O2 also present marked differences, Figure 3. Two nucleophilic regions, related to the LP, are directed along the σ-bond of the N2 molecule with the lowest MEP values located at 4 Å from the σ-bond center. The EP of N2 is more contracted in space, as compared to O2. In the plane of the O2 molecule, four symmetric regions of negative MEP are created by the 4 LP orbitals. The nucleophilicity of O2 is much lower, compared to that of N2. Upon penetration of the zeolite framework, the adsorbate molecules are subjected to the electrostatic field created by the extraframework cations inside the zeolite cavities. The extraframework cations strongly influence the electrostatic potential in the nearest zeolite environment.26 Cations at SI′, SII, and SIII′ sites in FAU zeolites are all accessible for small-size adsorbates. The SI′ sites are accessible via the S6R windows of the sodalite cages, as their free aperture is sufficiently large to allow penetration of N2 and O2 molecules.27 The SII and SIII′ sites are positioned in the supercages and are easily accessible. Since the SIII′ cation sites provide a less favorable oxygen atom coordination in comparison with the other sites, cations at the SIII′ sites are expected to have the highest adsorption capacity, but these sites also have the lowest fractional occupancy.5a,b The calculated Si-O and Al-O bond lengths, as well as the ∠TOT bond angles and the distances between extraframework cations (Li+, Na+) and the nearest oxygen atoms, agree with experimental data for FAU zeolites, see Tables 2 and 3. The K+(SI′)-O(3) and Ca2+(SII)-O(2) internuclear distances are overestimated by the B3LYP calculations, while the Ca2+(SI′)-O(3) distance is underestimated, as compared to the experimental data. The small-size Li+ cations cause the largest bond angle deformations. They are strongly attracted by the negative framework charges and previous DFT studies revealed that Li+ cations are unlikely to occupy either SI or SI′ sites, when Al-O-(Si-O)n-Al linkages (n ) 1, 2) are present in the D6R.6 The EP inside the D6R is positive, when cations take the SI or SI′ sites. With K+ and Ca2+ cations at these sites, both the positive and negative EP decay in space faster than with Na+ cations. The presence of two K+ cations at closely spaced SI and SI′ sites has a smaller combined effect as compared to two Na+ cations at SI′ sites, see Figure 4. In the presence of Ca2+ cations, the negative EP regions are delocalized in an area away from the framework Al atoms and in the case when a Ca2+ cation occupies the SI′ site above the Si4Al2O6 plane, a negative EP region emerges in proximity of the Si6O6 plane of the D6R. With extraframework cations at the SIII′ sites or bridging hydroxyl groups, both the positive and negative EP’s decay in space in the following order: H+ (OH-) > Li+ > Na+ > K+. Two separate regions of negative EP emerge in these cases inside the D6R. A stronger negative EP is generated at the ring with Si,Al substitutions. In Na-X and K-X, the SIII′ sites have low occupancy factors, but in Li-LSX the SIII′ site occupancy is higher.5,7,8 The specific feature of cations at SIII′ sites to cause the appearance of a broad region of negative EP inside the D6R and in the vicinity of T6O6 rings can provide an explanation for the lack of interaction between adsorbate molecules and cations inside the sodalite cages and D6R, evidenced by NMR studies of Li-LSX.7b,8b Cation Site Preferences. In D6R’s with D3d symmetry the cations at SI′ sites have three oxygen atoms from a T6O6 ring as nearest neighbors and three more distant oxygen atoms from the same T6O6 ring as next nearest neighbors. Upon Si,Al substitution, the symmetry of the D6R is lowered to either C2V or Cs and the longer Al-O bonds cause slight deformation. This
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TABLE 2: Average Bond Lengths and Angles in S6R Fragments with Composition [Mn+H12Si4Al2O6](2-n)- and D6R Fragments with Composition Mn+2/nH12Si10Al2O18 (M ) Li+, Na+, K+, Ca2+) as Extraframework Cations, Calculated by B3LYP cluster model/ TOT linkages
RSi-O (Å)
-
RAl-O (Å)
∠Al-O-Si (deg)
∠Si-O-Si (deg)
[LiH12Si4Al2O6] / Al-O-(Si-O)2-Al [NaH12Si4Al2O6]-/ Al-O-(Si-O)2-Al
1.635
1.78
138.0
133.3
1.635
1.814
145.4
144.0
[KH12Si4Al2O6]-/ Al-O-(Si-O)2-Al
1.664
1.842
140.1
141.76
[CaH12Si4Al2O6]/ Al-O-(Si-O)2-Al
1.630
1.79
146.2
142.0
M2H12Si10Al2O18/ Al-O-(Si-O)3-Al M2H12Si10Al2O18/ Al-O-(Si-O)2-Al M2H12Si10Al2O18/ Al-O-Si-O-Al
1.635
1.763
145.9
149.7
1.630
1.755
148.1
149.4
1.636
1.766
145.9
150.8
1.649-1.651 1.631-1.651 1.596-1.649 1.595-1.655
1.695-1.755 1.701-1.714 1.664-1.785 1.716 -1.760
126.6-144.5
Li-LSX(exp)8 Na-X(exp)5a Ca-LSX(exp)29 Ca-X(exp)9a
130.4-141.4 125.9-140.8
TABLE 3: Internuclear Distances between Extraframework Cations and Nearest Framework Oxygen Atoms (Oz) before Adsorption (Req) and with the Adsorbate N2 or O2 (Rads.N2 and Rads.O2) and Displacements (∆R) of the Extraframework Cations from Their Equilibrium Positions upon Adsorption, As Calculated by B3LYP cation sites SII sites Li+ at SII Na+ at SII Ca2+ at SII SI′ sites Li+ at SI′ Na+ at SI′ K+ at SI′ Ca+ at SI′ SIII′ sites Li+ at SIII′ Na+ at SIII′ a
Req(M-Oz) (Å)
Req(M-Oz) exptl (Å)
Rads.N2(M-Oz) (Å)
Rads.O2(M-Oz) (Å)
2.050 2.302 2.402
2.000a 2.355b 2.276c
2.062 2.317 2.415
2.052 2.308 2.410
1.985 2.353 2.794 2.289
1.902a 2.267b 2.509d 2.544e
2.001 2.370 2.799 2.299
1.989 2.357 2.799 2.296
1.801 2.166
2.439b
1.811 2.173
1.808 2.171
∆Rads.N2f (Å)
∆Rads.O2f (Å)
0.064 0.088 0.001 0.030
0.007 0.065 0.001 0.001
Reference 8. b Reference 5a. c Reference 9a. d References 5d,e. e Reference 29. f ∆R is referred to the plane of the six T atoms in D6R’s.
in turn may shorten the M-O distance with the Al-bonded oxygen atoms and change the number of nearest oxygen atom neighbors, see Figure 5. This is valid also for the SII sites in S6R. A comparison of the gas-phase ion-exchange energies reveals that SII sites provide favorable oxygen-atom coordination to Na+ cations when a Si,Al distribution with maximum separation of negative framework charges within the S6R is attained, see Table 4. The Li+ cations do not form stable clusters with closely spaced Al atoms (Al-O-Si-O-Al linkages) and Ca2+ cations do not form stable D6R clusters with C2h or Cs symmetry at a Si/Al ratio of 5.6 The K+/Na+ ion exchange energies are high in the presence of closely spaced Al atoms within the S6R, but lower for the D6R, irrespective of the Si,Al ordering. The most favorable location of Ca2+ cations is at SI sites inside the D6R, with two Al in a T6O6 ring, separated by an Al-O(Si-O)2-Al linkage. The Na+ and K+ cations are readily exchanged by Li+ at all sites, though at a Si/Al ratio of 5, Li+ strongly prefer the SI′ sites in D6R with C2h symmetry and maximum separation of the framework negative charges. The SII sites are preferred next and then follow the SIII′ sites. The SIII′ sites provide a cation site in close proximity to a negative framework charge center; they are not sensitive to the type of linkages inside the D6R. In a dehydrated zeolite, the SIII′ cations can be regarded as nearly “bare”: they should be easily accessible and have high adsorption capacity.
Interaction of N2 and O2 with Cations. The molecules of N2 and O2 approach the extraframework zeolite cations end-on to form stable adsorption complexes with either C∞V (linear) or Cs (bent) symmetry, Figure 5. The side-on structures with C2V symmetry are not stable: for the “bare” cations they represent saddle points on the potential energy surface. The interaction of N2 or O2 with the “bare” cations Li+, Na+, K+, and Ca2+ is considerably stronger compared to the interaction of the same molecules with cations embedded in a zeolite environment, see Table 5. The M-X internuclear distances for the “bare” cation adsorption complexes MX2 (X2 ) N2, O2) are shorter, the adsorption energies considerably higher, and the adsorbate molecule polarization is very strong. Molecular nitrogen forms linear adsorption complexes with all cations, while molecular oxygen forms bent M-O-O adsorption complexes. While the B3LYP calculations indicated a linear Ca2+-OO adsorption complex for the “bare” Ca2+, geometry optimizations at the MP3 level led to bent M-OO configurations for all cations; this was confirmed by CCSD(T) calculations as well. According to the B3LYP calculations the energy needed to bend the angle ∠Ca2+-O-O from 180.0° to 160.0° is very small, 0.7 kJ mol-1. The angle ∠M-O-O is nearly constant for the monovalent “bare” cations and the energy barriers to form linear adsorption complexes M-OO are less than 0.15 kJ mol-1seven lower than for Ca2+. The linear
N2 and O2 Adsorption to Zeolite Extraframework Cations
J. Phys. Chem. B, Vol. 109, No. 22, 2005 11123
Figure 5. Schematic representation of the interaction of N2 and O2 with cations at SII sites in S6R and SIII′ sites in D6R. Atoms in decreasing size are O, Mn+ (Li+, Na+, Ca2+), tetrahedral atoms (Si or Al), and adsorbate molecule (N2, O2). Oxygen atoms are large light gray circles; Si atoms are black circles; Al atoms are white circles; Mn+ cations are gray circles; nitrogen atoms are gray; and oxygen atoms from O2 are dark gray.
Figure 4. Molecular electrostatic (Mn+)2/nH12Si10Al2O18 clusters.
potential
maps
(au)
of
adsorption complexes with N2 are more stable: 5 kJ mol-1 is needed to bend the angle ∠Li-N-N from 180.0° to 160.0°. The comparison of the N2 adsorption energies clearly shows that the SIII′ site is more favorable than the SI′ and SII sites, see Table 5. The Li+ cations at SIII′ sites have a much higher N2 adsorption energy (35.4 kJ mol-1), as compared to all other cations under study, and the N2/O2 selectivity is higher than the one deduced from the adsorption energies of the “bare” cations. Among the “bare” cations and for cations at the SI′ and SII extraframework sites, the highest N2 and O2 adsorption energies were obtained for Ca2+. Though the cation-adsorbate distance (M-XX) is the shortest for Li+ cations, the N2 and O2 adsorption energies to cations at SI′ and SII sites decrease in the order Ca2+ > Na+ > Li+, and particularly for Li+ at SII sites, the N2 adsorption energy is quite low. This result agrees
with previous Monte Carlo studies, according to which, Li+ cations at SII sites are not involved in the adsorption process.15 Adsorption energies were calculated for Li-X, Na-X, and Ca-X, based on the DFT-calculated data for the different cation sites, and they are in good agreement with experimental data, Table 6. Though Ca2+ cations at SI′ sites possess both a remarkable adsorption capacity and N2/O2 selectivity, they have very low occupancy in Ca-LSX,29 so the resulting selectivity is lower than for Li-X. The extraframework cations undergo certain displacements from their equilibrium positions upon interaction with the adsorbate molecules, see Table 3. The cation displacements at SI′ sites are less than 0.10 Å and the K+ cation positions practically remain unaltered. Those cations at SI′ sites, which are not involved in the adsorption process, are not displaced. Lengthening of the internuclear distances between the cations and the zeolite framework oxygen atoms (M-Oz) upon N2 adsorption was observed; for all cations it was within the range 0.007-0.017 Å. The cation displacements upon O2 adsorption are relatively small. Zeolite framework relaxation was also observed, more significantly in the S6R cluster models; this, however, indicates that the cluster size is not sufficient to correctly represent the adsorption center at SII sites. The changes in Si-O and Al-O bond lengths were minor. Larger cation shifts were obtained in the experimental structure studies of nitrogen-containing adsorption complexes (methylamine and ammonia) in Ca-X and alternating displacements of cations at the SII sites off the S6R plane were observed.10 Structural models larger than S6R’s are needed for the correct SII site representation; at the present stage the charge balance within the sodalite cage remains beyond our consideration. The M-X distances for N2 and O2 binding to Ca2+ at SII and SI′ sites are similar; Li-O(ads) and Na-O(ads) distances for the SII sites are similar too, and so are their heats of adsorption. No changes of the O-O bond length are observed for O2 upon adsorption, except for the “bare” cations, where the O-O bond length decreases; with Li+ the O-O bond length change is 0.004 Å as compared to the free O2 molecule. The angle ∠M-O-O is within the 127-132° range for both the SI′ and SIII′ sites and for all cations except K+, for which the angle is nearly the same as for the free cation. The K+ cations interact weakly with both N2 and O2 and there is practically no
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TABLE 4: B3LYP Calculated Cation Exchange Reaction Enthalpiesa for the Different Cation Sites prism (ring) configuration/ linkages/symmetry S6R; [MIH12Si4Al2O6]-/ Al-O-(Si-O)2-Al/C2h
K+(SII) + Na+ ) Na+(SII) + K+ Na+(SII) + Li+ ) Li+(SII) + Na+
-158.6 -117.9
S6R; [MIH12Si4Al2O6]-/ Al-O-Si-O-Al/C2 D6R; M2H12Si10Al2O18/ Al-O-(Si-O)3-Al/C2h
K+(SII) + Na+ ) Na+(SII) + K+ Na+(SII) + Li+ ) Li+(SII) + Na+ K+(SI′) + Na+ ) Na+(SI′) + K+ Na+(SI′) + Li+ ) Li+(SI′) + Na+ K+(SIII′) + Na+ ) Na+(SIII′) + K+ Na+(SIII′) + Li+ ) Li+(SIII′) + Na+ K+(SI′) + Na+ ) Na+(SI′) + K+ 2Na+(SI′) + Ca2+ ) Ca2+(SI′) + 2Na+ 2Na+(SI′) + Ca2+ ) Ca2+(SI) + 2Na+ K+(SIII′) + Na+ ) Na+(SIII′) + K+ Na+(SIII′) + Li+ ) Li+(SIII′) + Na+ K+(SI′) + Na+ ) Na+(SI′) + K+
-164.7 -118.5 -130.3 -120.1 -127.3 -90.7 -148.0 -509.8 -516.9 -126.3 -87.9 -141.4
D6R; M2H12Si10Al2O18/ Al-O-(Si-O)2-Al/C2V
D6R; M2H12Si10Al2O18/ Al-O-Si-O-Al/Cs a
∆H°, kJ mol-1
exchange reaction
At T ) 298.15 K and p ) 1 atm.
TABLE 5: Internuclear Distances (R) between the Metal Cation M+ ) Li+, Na+, Ca2+ at Extraframework Cation Sites and the Adsorbed Molecule X2 ) N2, O2; Net Natural Charges (Q); and Dissociation Energies for the Adsorption Complexes (∆E°)a R(M-X), Å
R(X-X), Å
∠M-X1-X2, deg
qM
interaction of N2 and O2 with “bare” cations 1.104 180.0 +0.99 1.104 180.0 +0.99 1.106 180.0 +1.97 1.104 180.0 +0.99 1.210 158.5 +0.99 1.211 158.0 +0.99 1.212 180.0 +1.96 1.212 159.6 +0.99
qX1
qX2
∆E°, kJ mol-1
-0.20 -0.16 -0.30 -0.12 -0.18 -0.14 -0.28 -0.11
+0.21 +0.17 +0.32 +0.13 +0.20 +0.15 +0.32 +0.12
62.2 42.4 106.0 18.8 47.1 30.5 91.4 10.4
N2 + Li+ N2 + Na+ N2 + Ca2+ N2 + K+ O2 + Li+ O2 + Na+ O2 + Ca2+ O2 + K+
2.089 2.448 2.460 2.901 1.981 2.358 2.293 2.777
N2 + LiZsN2 + NaZsN2 + CaZs O2 + LiZsO2 + NaZsO2 + CaZs
2.607 2.686 2.742 2.836 2.864 2.765
interaction of N2 and O2 with cations located at SII sites in S6R 1.104 180.0 +0.87 +0.01 1.104 180.0 +0.89 0.00 1.103 180.0 +1.84 -0.08 1.214 119.7 +0.88 0.00 1.214 122.9 +0.91 0.00 1.213 134.0 +1.86 -0.06
+0.01 +0.03 +0.11 +0.02 +0.03 +0.09
11.5 15.1 28.7 10.8 11.8 21.3
N2 + LiZ, Li+ at SIII′ N2 + NaZ, Na+ at SIII′ N2 + LiZ, Li+ at SI′ N2 + NaZ, Na+ at SI′ N2 + CaZ, Ca2+ at SI′ N2 + KZ, K+ at SI′ O2 + LiZ, Li+ at SIII′ O2 + NaZ, Na+ at SIII′ O2 + LiZ, Li+ at SI′ O2 + NaZ, Na+ at SI′ O2 + CaZ, Ca2+ at SI′ O2 + KZ, K+ at SI′
interaction of N2 and O2 with cations located at SI′ and SIII′ sites in D6R 2.172 1.103 180.0 +0.88 -0.08 2.543 1.104 180.0 +0.91 -0.07 2.352 1.104 180.0 +0.89 -0.07 2.586 1.104 180.0 +0.89 -0.04 2.684 1.103 180.0 +1.81 -0.08 3.034 1.104 180.0 +0.95 -0.04 2.145 1.213 127.2 +0.89 -0.08 2.513 1.214 128.1 +0.92 -0.08 2.504 1.212 128.2 +0.88 -0.07 2.597 1.214 127.4 +0.88 -0.06 2.642 1.213 131.5 +1.82 -0.09 2.913 1.213 160.6 +0.94 -0.04
+0.13 +0.10 +0.11 +0.08 +0.11 +0.03 +0.11 +0.10 +0.90 +0.09 +0.12 +0.02
35.4 26.1 18.8 22.4 27.2 8.5 24.7 18.1 12.7 17.1 18.3 4.5
a
Zs) H12Si4Al2O6; Z) H12Si10Al2O18; M ) Li+, Na+, Ca2+.
energy barrier along the ∠K+-O-O reaction coordinate. For the cations at the SI′ site (Li+, Na+, K+, Ca2+), the energy difference in favor of the bent configuration over the linear one is 1.9-2.2 kJ mol-1. All cation sites allow free rotation of the O2 molecule with an energy barrier lower than 1.0 kJ mol-1. The N-N bond length varies by (0.001 Å in the adsorption complexes; only the interaction with “bare” Ca2+ leads to an increase of the N-N bond length by 0.003 Å. The M-N internuclear distances decrease in the order Ca2+ > Na+ > Li+ for all cation sites and also for the “bare” cations. The Ca2+ cations preferably occupy SI sites in both FAU and CHA frameworks;9,29,30 the SI site in the D6R center is, however, not accessible for adsorption. The Ca2+(SI)-X2 (X2 ) N2, O2) equilibrium internuclear distance refers to a position of the adsorbate molecule at the top of the energy barrier on its way
to penetrate the T6O6 ring. Though this barrier was calculated to be sufficiently low for siliceous D6R,13 the adsorption complex could not be localized inside the D6R. Even for entirely siliceous structures, the adsorbate molecules undergo polarization when entering the zeolite cavities.13 In general, the atom that interacts directly with the extraframework cation acquires a partial negative charge or remains neutral, while the more distant atom acquires a positive charge, see Table 5. The “bare” cations induce very high polarization in the adsorbate molecule, particularly Ca2+. The cations at SIII′ sites have the highest potential to induce polarization; the same holds for Ca2+ at SI′ sites. The differential electron density maps show a redistribution of the electron density in the adsorbate molecule, as shown in Figure 6. The framework oxygen atom in position near the Li+ cation is also polarized, less during N2 adsorption,
N2 and O2 Adsorption to Zeolite Extraframework Cations TABLE 6: Dissociation Energies of the Adsorption Complexes, Calculated for X Zeolites,a Compared to Experimental Data ∆E°, kJ mol-1
∆E(exptl), kJ mol-1
23.5 20.7 28.5 15.1 15.4 20.9
24.80 ( 0.15b 19.00 ( 0.11b 27.00 ( 0.16b 13.8c
N2/Li-X N2/Na-X N2/Ca-X O2/Li-X O2/Na-X O2/Ca-X
a Dissociation energies for the specific cation sites from Table 4; cation distributions from ref 8 (Li-X), ref 5a (Na-X), and ref 29 (Ca-LSX). b Experimental data from microcalorimetric measurements, ref 18. c Experimental data from isosteric heats of adsorption, ref 15.
Figure 6. Differential electron density maps in the C2V plane of a D6R with composition Li2H12Si10Al2O18, describing the N2 and O2 adsorption to Li+ at SIII′ sites. Atomic nuclei are represented by open circles: the framework oxygen atoms are large circles, light gray; Si atoms are small black circles; Al atoms are white small circles; Li+ cations are gray circles; N2 atoms are light gray; and O2 atoms are dark gray. Negative parts (unfilled squares) and positive parts (filled squares) represent the regions of surplus and lack of electronic density in e Å-3.
more during the O2 adsorption. It is worthwhile noting that neither Si nor Al atoms undergo electronic density changes throughout the adsorption process. Conclusions DFT studies of N2 adsorption at SII, SI′, and SIII′ sites of FAU zeolites reveal a linear M-N-N unit. Conversely, O2 forms a bent structure, the M-O-O angle being smaller for cations at the SII sites than at the SI′ or SIII′ sites. Li+ cations at SII sites perform poorly with respect to both N2 adsorption and N2/O2 selectivity; the SII site is also less selective between N2 and O2 for the other cations (Na+, Ca2+) as compared to the SI′ and SIII′ sites. The Li+ cations at SIII′ sites have the highest adsorption energy among the examined cations and also a high N2/O2 selectivity. Adsorbate molecules undergo significant polarization during the adsorption process. A surplus electron density emerges at the oxygen atom interacting directly with the cation. The cations at SIII′ sites induce negative EP inside the D6R and at the T6O6 ring faces of the D6R. In cases where the SIII and SIII′ site occupancy is high, such as in Li-LSX, this effect would prohibit the adsorption of N2 and O2 on cations at SI and SI′ sites. Acknowledgment. The authors gratefully acknowledge CPU time at the Computer Center, Technical University Vienna, where most of the Gaussian 98 calculations were performed. References and Notes (1) (a) Newsam, J. M. In Solid State Chemistry: Compounds, Cheetham, A. K., Day, P., Eds.; Oxford University Press: Oxford, UK, 1992; Vol. 2, p 234. (b) Smith, J. V. Chem. ReV. 1988, 88, 149. (2) Rabo, J. A. In Zeolite Chemistry and Catalysis; ACS Monograph Series 171; Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976; p 332.
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