19728
J. Phys. Chem. 1996, 100, 19728-19734
Charge Sensitivity Analysis of Intrinsic Basicity of Faujasite-Type Zeolites Using the Electronegativity Equalization Method (EEM) R. Heidler,* G. O. A. Janssens, W. J. Mortier, and R. A. Schoonheydt Centrum Voor OpperVlaktechemie en Katalyse, Departement Interfasechemie, Katholieke UniVersiteit LeuVen, Kardinaal Mercierlaan 92, B-3001 HeVerlee, Belgium ReceiVed: May 29, 1996; In Final Form: July 25, 1996X
The Electronegativity Equalization Method (EEM) is used to study the dependence of the basicity of faujasitetype zeolites on the Si:Al ratio, the cation type, and the type of oxygen. The model structures were obtained by distance least-squares optimization within the F222 space group, and cations were located with the Monte Carlo technique, starting from experimental XRD positions. The main effect determining basicity, as indicated by the average negative charge on the oxygens, is the chemical composition of the framework. The average negative oxygen charge increases with decreasing Si:Al ratio, in agreement with the experimentally found increase of basicity. The secondary structure effect identifies the supercage oxygens O1 and O4 as the most basic. The details of the cation distribution further controls the basicity of these oxygens, which increases when Na+ is replaced by Cs+ on sites II and, especially, site III, also in agreement with experiment. Besides charges on oxygen, Fukui indices and regional sensitivities are also good indicators of basicity.
Brønsted acid sites in zeolites, the so-called bridging hydroxyls tSisOHsAlt, have been extensively studied, both experimentally and theoretically.1-8 Its acidity is chemical composition and structure dependent, the former being the most important and increases with increasing Si:Al ratio. As soon as the Si:Al ratio has reached the level that, statistically, no next nearest neighboring Al atoms (NNN) are present, the maximum acidity is reached.4-7 This was theoretically addressed recently using the electronegativity equalization method (EEM).8 Following a classical acid-base pair concept the surface oxygen is the conjugated base of the acidic bridging hydroxyl. The basicity of these oxygens is called the structural or intrinsic framework basicity.9-11 As for the bridging hydroxyl we expect the properties of the basic sites to depend on the chemical composition (Si:Al ratio and the exchangeable cations) as well as on the structure type. Up to now, the intrinsic basicity of zeolites has been studied experimentally, either directly, by looking at the properties of the framework atoms,9-12 or indirectly, by adsorption of probe molecules.13-15 The O(1s) binding energy, deduced from XPS spectra, reflects the electron density of the oxygen atom in the framework. It decreases with increasing Al content, but at a given Si:Al ratio it increases with increasing electronegativity of the cation.16,17 A decrease of the O(1s) binding energy is equivalent to an increase of the electron density around the O nucleus, or an increase of the negative charge on the oxygen, thus reflecting an increased basicity. XPS, however, gives only general trends and no distinction can be made between structurally different atoms. Similar results are obtained from calculations of averaged charges on framework oxygens with Sanderson’s model of electronegativity:18 the average negative charge on structural oxygens increases with the Al content of the framework and with decreasing electronegativity of the exchangeable cations.10,19,20 Pyrrole is proven to be an excellent probe for basicity of zeolites. Upon adsorption it interacts both with the exchangeX
Abstract published in AdVance ACS Abstracts, October 1, 1996.
S0022-3654(96)01561-4 CCC: $12.00
able cations and with the framework oxygens.10,11,13,14 Both the N(1s) binding energy envelope and the N-H stretching vibrations reveal several sites, reminiscent of the presence of basic sites with different strengths in the sense that different cations create different basic sites. The basicity increases in the sequence Li < Na < K < Rb < Cs.13,14 At low pyrrole loading the resolution is enhanced and the different N-H stretching vibrations were attributed to pyrrole interacting with different types of oxygens in EMT and FAU,15 O2 coordinated to cations at position II being most basic. The framework oxygens are potential basic sites, and the chemical composition of the framework, the crystallographic type of the oxygen, and the exchangeable cations (which coordinate with these oxygens) will determine their basic character. The electronegativity equalization method (EEM) is used to quantify the effect of those three parameters on the basic character of the oxygens. One can expect that the oxygen charges correlate with experimental data such as the O(1s) binding energies and the N-H stretching frequency of adsorbed pyrrole. Description of the Model The EEM method21-25 is a semi-empirical density functional theory26 method. It assumes an atom-in-molecule resolution, where each atom carries an effective charge qR. The energy of the ground state of each bonded atom in the molecule can be written as21 2 ER ) E* R + χ* RqR + η* RqR + k
∑
β*R
qβqR RRβ
(1)
χ denotes the electronegativity and η the chemical hardness. The asterisk * denotes the respective value for the neutral atom within the molecule. ER* is the energy of the neutral atom R within the molecule, i.e., without charge relaxation. The last term in eq 1 expresses the electrostatic Coulomb interaction at position R due to all surrounding atoms with charges qβ, and k is a conversion factor depending on the used dimensions for the charges and the distance. The indices R and β run over all atoms within the molecule (solid). The values of the parameters © 1996 American Chemical Society
EEM Analysis of Basicity in Faujasite-Type Zeolites
J. Phys. Chem., Vol. 100, No. 50, 1996 19729
χR* and ηR* have been calibrated in order to reproduce ab initio (STO-3G) charges.21 The electronegativity of each atom R is then defined within the framework of EEM as27
χR )
( ) ∂ER ∂qR
qβ
) χ* R + 2η* RqR + k
∑R
β*R
(2)
Rβ
For all atoms of the molecule in the ground state this value must be equal. This gives, together with the constraint that the charge of the molecule is constant (zero), N+1 equations for the N+1 unknowns (the N unknown charges and the averaged electronegativity). The native form
( )( ) ( ) k RRβ
2η* R
k RβR · · · k RnR 1
k RRn
· · ·
· · ·
k Rβn · · · 2η*n
· · ·
1
2η* β
· · ·
· · · k Rnβ
··
1
1
·
1
qR
-χ* R
qβ · · · qn
-χ* β · ) · · -χ* n
· · · 1 -χ
(3)
0
0
can easily be solved in order to get the charges and the averaged electronegativity. Furthermore it is possible to calculate the Fukui functions defined as
( ) dqR dN
fR ) -
(4)
V
( )( ) ( )
where N is the total number of electrons of the whole molecule. The Fukui functions can be calculated by solving the following set of equations22
2η* R
k RβR · · · k RnR 1
k RRβ
2η* β · · · k Rnβ 1
· · ·
k RRn 1
fR
k Rβn · ·· · · · · · · 2η* n
0 0 1 · ) · · · · · 0 1 -η 1
· · ·
0
· · ·
1
fβ · · · fn
(5)
For systems containing more than one interacting molecule, this set of equations needs to be expanded. The vector on the lefthand side contains then, besides the Fukui functions for one molecule, the so-called regional sensitivities of the atoms of the other molecule with respect to the molecule in question. These regional sensitivities characterize the response of the atoms of one molecule to a change of the total charge (or total number of electrons) in another interacting molecule28,29
( )
rRB ) -
∂qR ∂NB
(6)
where qR denotes the charge of atom R (of molecule A) and NB denotes the total number of electrons of molecule B interacting with molecule A. In our case molecule A will be the zeolite framework and molecule B the exchangeable cation. For our calculations the extra framework cations were modeled as point charges with their formal charge and with a charge of +0.75. The point charge model is reasonable taking into account the expected high ionicity of the cation-oxygen
bond. Therefore, in our calculations no charge transfer is permitted between the cations and the zeolite framework. Zeolite Models The symmetry of the pure Si form of faujasite (Figure 1; space group Fd3hm, one T-atom with a multiplicity of 192 and four O’s with a multiplicity of 96)30 was lowered to F222 [12 T-atoms (multiplicity of 16) and 24 O’s (multiplicity of 16)] in order to be able to model different Al distributions. The calculations were performed with Si:Al ratios of 1, 2, and 3, i.e., six, four, and three T-atoms were replaced by Al, respectively. The Al positions were chosen so as to obey the Loewenstein rule31 and to stay as close as possible to the binomial distribution (see Table 1) which is also in agreement with 29Si-NMR measurements.32 The models were DLS-optimized (DLS ) distance least squares)33 in order to adjust the bond distances and bond angles for the chosen Al distribution as close as possible to the prescribed values: d(Si-O) ) 1.61 Å, d(Al-O) ) 1.74 Å, ∠(T-O-T) ) 109.47°, ∠(O-T-O) ) 145°. In order to find the correct cation positions, a Monte Carlo analysis29 was performed with the starting positions for the cations taken from experimental XRD data.34,35 A solid state EEM calculation was performed with the cations as point charges at their starting positions in order to determine the potential for the cation distribution. This potential was then used in the Monte Carlo optimization. Note that the potential varies during the optimization and, consequently, must be updated. In order to avoid a too close approach between atoms only those steps were allowed which kept the distance between two atoms larger than the sum of the ionic radii. Results Monte Carlo Simulations for the Positions of the Cations. The results of our Monte Carlo simulations are summarized in Table 2. For cations at position I there are two possible locations, shifted out of the center of the hexagonal prism along the (111) axes. These positions are symmetric with respect to the center of the hexagonal prism, at which position I was determined by XRD. This imposes that cation I is coordinated to three oxygen atoms of type O3 and not to six oxygen atoms of type O1. Cation position III is a special position within Fd3hm (48f, symmetry 2.mm) which means that x ) y ) 0. Furthermore there is a symmetry relation between the two given positions in the Cs case (z′ ) 1.25 - z). These symmetry conditions, which are not constraints in the chosen space group F222, are preserved during the Monte Carlo run within the error limits. In Figure 2a and b the electrostatic potentials of the zeolite framework along the (111) and the (100) axes, respectively, are shown. The vertical lines indicate the cation positions as they were determined by XRD. Inspecting Figure 2, one finds that the Monte Carlo determined positions of the cations are as close as possible to the minima of the potential under the constraint that the distance between cation and nearest oxygen does not become lower than the respective sum of the ionic radii, i.e., 2.4 Å for Na+ and 2.7 Å for Cs+ (see also Figures 3-5). Average Oxygen Charges. Following the definition of intrinsic basicity given in the introduction the charge of the oxygen averaged over all framework oxygens was calculated (eq 3). Table 3 shows the results for the Si:Al ratios of 1, 2, and 3 for the pure Na form and for a (partially) Cs-exchanged form (positions II and III). The cations were positioned according to our Monte Carlo simulation (see above). Comparing columns 2 and 3 of Table 3 one finds that the averaged
19730 J. Phys. Chem., Vol. 100, No. 50, 1996
Heidler et al.
Figure 1. Faujasite structure (only T-atoms). The positions of the oxygens and of the cations are indicated.
TABLE 1: Binomial Distribution of the Number of Al Neighbors around Si Atomsa Si:Al ) 1 Si:Al ) 2 Si:Al ) 3 no. of Al probability (p ) 1) model (p ) 0.5) model (p ) 0.3h) model 0 1 2 3 4
1(1 - p)4 4(1 - p)3p 6(1 - p)2p2 4(1 - p)p3 1p4
0.0 0.0 0.0 0.0 1.0
0.0 0.0 0.0 0.0 1.0
0.0625 0.25 0.375 0.25 0.0625
0.25 0.125 0.25 0.125 0.25
0.198 0.395 0.296 0.099 0.012
0.111 11 0.555 55 0.222 22 0.111 11 0.000 00
ap
denotes the probability for a neighbor of Si to be an Al, i.e., the inverse of the Si:Al ratio. The numbers in the columns “model” describe the appropriate probabilities achieved with our zeolite models.
TABLE 2: Cation Coordinates Determined by Monte Carlo and by XRD position I
II III
I (Na)
oxygen charge depends very sensitively on the assumed charge of the cations. The higher the cation charge, the more negative the oxygens become. The negative oxygen charge increases significantly with increasing Al content. This is in agreement with experimentally observed increased basicity. The influence of the cation type on the oxygen charge is much weaker and shows only for Si:Al ) 1 the expected trend, i.e., an increased negative charge for the Cs case in comparison with the Na case. For the Si:Al ratios of 2 and 3 the obtained trend is weaker and seems to be opposite. In order to investigate the influence of the size of the cations on the different cation positions we calculated the dependence of the average oxygen charge for a Si:Al ratio of 1 on the
II (Cs) III (Cs)
standard deviation
coordinate
XRD-value
Na (Positions I, II, and III) x ) y ) z ) 0.125 x ) y ) z ) 0.1032 σ ) 5.3 × 10-4 x ) y ) z ) 0.125 x ) y ) z ) 0.1467 σ ) 2.0 × 10-4 mean value: 0.12498 deviation to 0.125: 1.5 × 10-5 x ) y ) z ) 0.3579 x ) y ) 6.897 × z ) 0.2535
10-3
σ ) 1.0 × 10-4
x ) y ) z ) 0.376
σ ) 1.4 × 10-2 σ ) 3.9 × 10-3
x)y)0 z ) 0.3721
Na (Position I) and Cs (Positions II and III) x ) y ) z ) 0.125 x ) y ) z ) 0.1032 σ ) 2.1 × 10-4 x ) y ) z ) 0.125 x ) y ) z ) 0.1467 σ ) 4.0 × 10-4 mean value: 0.12492 deviation to 0.125: 7.7 × 10-5 x ) y ) z ) 0.374 10-3
x ) y ) -1.506 z ) 0.2722 x ) y ) 0.2530 z′ ) 0.9780 1.25 - z′ ) 0.2720
σ ) 1.17 × 10-4 σ ) 1.06 × 10-2 σ ) 1.40 × 10-3 σ ) 1.10 × 10-2 σ ) 2.24 × 10-3
position of cation II (Figure 6) and III (Figure 7). For cations at position II larger cations cause the framework oxygens to be less negatively charged than smaller cations. For cations at position III the opposite trend was found, i.e., larger cations at position III cause the framework oxygens to be more negatively
EEM Analysis of Basicity in Faujasite-Type Zeolites
J. Phys. Chem., Vol. 100, No. 50, 1996 19731
Figure 4. Distance between cation II and its nearest oxygen neighbors in relation to the position along (111).
Figure 2. Electrostatic potential of the faujasite framework for a Si: Al ratio of 1, calculated with the EEM charges for the framework atoms under the influence of the cations at their Monte Carlo-optimized position, (a, top) along the (111) axis and (b, bottom) along the (100) axis. The vertical lines indicate the cation positions. Figure 5. Distance between cation III and its nearest oxygen neighbors in relation to the position along (100).
Figure 3. Distance between cation I and its nearest oxygen neighbors in relation to the position along (111).
charged than smaller cations at position III. The latter effect is stronger than the effect of cations at position II. Therefore, we obtain in the case of an Si:Al ratio of 1 the trend that the oxygen in the Cs-exchanged zeolite is more negatively charged than in the Na-exchanged form (see Table 3). For the Si:Al ratios 2 and 3 there are no cations at position III. Therefore the oxygen in the Cs-exchanged models is slightly less negatively charged than in the Na-exchanged case. Charge on Crystallographically Different Oxygen Types. EEM is able to calculate also local properties, i.e., to distinguish between the crystallographically different oxygen positions. As
Figure 6. Average oxygen charge for the Si:Al ratio of 1 in relation to the location of cations at position II.
a measure of basicity we propose to use, beside the charge of the oxygens, also their Fukui functions and their local sensitivities with respect to the cation charges. Figures 8-10 show the dependence of the charges of the different oxygen types on the location of the cations at positions I, II, and III, respectively. The vertical lines indicate the positions of Na+ and Cs+ according to our Monte Carlo simulation. The general trend is that O3 carries the smallest negative charge, while the negative charge of O1, O2, and O4 is dependent on the cation distance and its siting (II or III).
19732 J. Phys. Chem., Vol. 100, No. 50, 1996
Heidler et al.
Figure 7. Average oxygen charge for the Si:Al ratio of 1 in relation to the location of cations at position III.
Figure 8. Oxygen charges in relation to the position of the cation I along (111).
TABLE 3: Averaged Oxygen Charges for the Different Zeolite Models zeolite model
cation charge: +1 cation charge: +0.75 oxygen charge (e) oxygen charge (e)
Si:Al ) 1; Na(I) + Cs(II, III) Si:Al ) 1; Na(I, II, III)
-0.869 86 -0.867 31
-0.834 69 -0.832 78
Si:Al ) 2; Na(I′) + Cs(II) Si:Al ) 2; Na(I′, II)
-0.854 31 -0.855 11
-0.830 47 -0.831 07
Si:Al ) 3; Na(I) + Cs(II) Si:Al ) 3; Na(I, II)
-0.842 89 -0.843 20
-0.826 34 -0.826 58
The cations mainly affect the charges of the oxygens in direct coordination. Thus, the negative charge of O3 significantly increases with increasing distance of the point charge Na+ (Figure 8). The size of the cation at position II influences mainly the oxygens O1 and O4 and also O2. For larger cations at position II (i.e., higher values for the fractional coordinate x ) y ) z) the oxygens O1 and O4 become more negatively charged than for smaller cations at position II. The charge of oxygen O2 shows the opposite trend. The negative charges of O2 and O3 increase with increasing size of the cations at position III whereas O3 and O4 become less negatively charged. The effect on O1 and O2 is larger than the effect on O3 and O4, which is in agreement with the obtained dependence of the averaged oxygen charge on the size of the cations at position III. Because we do not allow any charge transfer between the cations and the framework, the Fukui functions of the oxygens do not depend on the cation distribution, i.e., the Fukui functions are completely independent of the charges of the oxygens (which of course depend on the cations, see above) and of the cation type. Therefore, the Fukui functions reflect only the intrinsic properties of the framework. As one can see from Table 4, the Fukui functions for the oxygens O1 and O4 are remarkably higher than the Fukui functions for the oxygens O2 and O3, meaning that O1 and O4 respond more easily to charge transfer. These are the oxygen types pointing into the supercages. Figure 11 illustrates a correlation between the topological density [calculated as (∑1/R)]36,37 and the Fukui functions. Oxygen atoms in less dense regions (like the surface oxygens) show a higher Fukui function and vice versa. For the T-atoms the opposite trend is obtained. In Table 5 the regional sensitivities of the different oxygen types with respect to the different cation positions for an Si:Al ratio of one are given, i.e., the negative derivatives of the charge of an atom R with respect to the number of electrons of the corresponding cation B [-(dqR/dNB)]. The values show that cations at position I induce a more negative charge on O3,
Figure 9. Oxygen charges in relation to the position of the cation II along (111).
Figure 10. Oxygen charges in relation to the position of the cation III along (100).
TABLE 4: Fukui Functions of the Different Atoms for Si:Al )1 atom type
Fukui function
atom type
Fukui function
O1 O2 O3 O4
0.013 31 0.009 23 0.004 13 0.014 52
Si Al
0.023 26 0.018 88
cations at position II do so for O2 and O4 (the oxygens of the hexagonal window in front of which position II is situated), and cations at position III induce a more negative charge on O1 and O4. The general rule which emerges is that the oxygens
EEM Analysis of Basicity in Faujasite-Type Zeolites
J. Phys. Chem., Vol. 100, No. 50, 1996 19733 TABLE 5: Regional Sensitivities (-(dqr/dNB)) of the Framework Oxygen with Respect to the Different Cation Positions regional sensitivity
Figure 11. Correlation between the Fukui functions of the framework oxygens and the topological densities, calculated as (∑1/R)36,37 for the Si:Al ratios of 1 and 3 for oxygen (a) and for the T-atoms (b). The solid lines are guides for the eyes.
in the coordination sphere acquire more negative charge while the charge of the other oxygens becomes less negative. This is in line with the expected framework polarization. Discussion Cation Siting. Powder X-ray diffraction analyses of cation positions in dehydrated zeolites give “average” sites. The space group is that of a purely siliceous faujasite (Fd3hm), and the cation positions are averaged values of all the cation positions that are possible in the real space group. This is borne out nicely by the Monte Carlo simulations. In addition, the cations are held strongly at their sites and cannot jump from site to site during the Monte Carlo procedure. Secondly, the strongest cation lattice interaction is at the shortest cation-oxygen distance () sum of ionic radii). As a consequence, two equivalent sites I are obtained: a cation at site I moves toward either site of the hexagonal prism to coordinate to three O3 oxygens at the preset minimum distance of approach, which is the sum of the ionic radii. The average position of these two sites I is the central site I position determined by XRD. In F222 the Monte Carlo procedure also preserves the symmetry relation between the two different sites III. Indicators of Structural Basicity. The trends in the average charges of the oxygen atoms, as calculated with EEM, confirm the basicity trends of zeolites, measured experimentally. Thus, the basicity of faujasites increases with increasing Al content of the lattice.10,16,17 With EEM the average charges of the oxygens decrease, i.e., the oxygens become more negative as the Al content of the lattice increases. Trends in average oxygen
oxygen
cation position I
cation position II
cation position III
O1 O2 O3 O4
0.009 57 0.000 94 -0.030 04 0.022 89
0.003 78 -0.015 02 0.006 77 -0.005 77
-0.008 84 0.000 78 0.007 30 -0.008 28
charges can therefore be used as indicators of framework or structural basicity of zeolites. Aluminium being harder than Si not only increases the global hardness of the faujasite framework, but also softens the oxygens.25 This softening is expressed here by the increase of the averaged negative charge and thus of the basicity. This result can of course be generalized for all structure types. With the Fukui functions it is possible to refine this picture. Indeed, the Fukui function measures the change of the charge of, e.g., the oxygens upon changing the total amount of charge of the zeolite. It describes therefore the capability of the oxygen atom to respond to a global charge change and can be regarded as a reactivity index of the atom considered (in our case oxygen). The oxygen atoms O1 and O4 are characterized by the largest Fukui indices (Table 4) and are therefore most reactive. Large Fukui indices also mean soft atoms and therefore the most basic ones. As O1 and O4 are forming the supercages, it follows that supercage oxygens are the most basic ones. In our earlier work25 this was formulated as O1 and O4, being in a hard environment (vacuum of the supercage), are the softest atoms. One could also state that oxygens in the less dense regions of the faujasite structure (in term of number of atoms per unit volume) are the most basic. This should also be true for other structure types. The larger the cavities and cages are, the more basic the surface oxygens will become. It is then an interesting research topic to study the relation between the curvature of the surface and the basicity of the oxygens. If O1 and O4 are the most basic oxygens, one could wonder whether they are also the preferential proton cariers in H-FAUtype zeolites. According to a recent neutron diffraction study,38 this is, however, not the case, as O1 and O3 were found to form OH groups. Proton distribution over the available oxygens is a matter of energy minimization. In this paper we studysfor a given energy-minimized cation distributionsthe sensitivity of the oxygens to several factors such as structure and cations. It is impossible to deduce proton distributions from our data. Effect of Exchangeable Cations. In this paper the cations are represented as point charges and differences between monovalent cations such as Na+ and Cs+ are only in the distance to the oxygens, the minimum distance taken as the sum of the ionic radii. It is remarkable that this simple approach leads to data in full agreement with the chemistry of zeolites and at the same time gives insight into why Cs+ zeolites are more basic than Na+ zeolites. This is already evidenced by the average oxygen charges of Table 3. It is shown there that only for Cs+ on site III the average oxygen charge becomes more negative. When there is no Cs+ at site III (such as for Si:Al ratios of 2 and 3) Cs+ has no effect on the average oxygen charge. At site III the exchangeable cations are coordinated to O4, and these become less negatively charged when changing from Na+ to Cs+. This decrease of negative charge on O4 (Figure 10) is more than compensated by its increase on O1, and therefore the overall basicity increases from Na+ to Cs+. The same reasoning applies for cations at site II. The negative charge on O2 and O3
19734 J. Phys. Chem., Vol. 100, No. 50, 1996 decreases when replacing Na+ with Cs+, but that of O1 and O4 increases. The general conclusion is that when Na+ is replaced by Cs+ at sites II and III, the basicity of the supercage oxygens, especially O1, increases, which is in full agreement with our reasoning based on the Fukui indices. The regional sensitivities of the different oxygen types with respect to the different cation positions describe the responses of the oxygens on a change in the charge of the cation. Therefore they show that high loadings of a certain cation position (higher averaged charge at this position) increases the negative oxygen charge of those oxygen types which are directly coordinated with the cation at this site (compare Table 5 and Figures 3, 4, and 5). High loadings of the cation position III will therefore increase the basicity of the oxygen types O1 and O4. The regional sensitivities give also measures for the influence of the cation charge on the different oxygen charges. Assuming other charges than +1.0 for the cations will rescale the obtained oxygen charges with a weight given by the respective regional sensitivity factors. Conclusions With the EEM method it is possible to calculate changes in charges on oxygens, which match the known trends in basicity of faujasite-type zeolites. Thus the basicity increases with decreasing Si:Al ratio, and this is reflected in an increase of the average negative charge on the oxygens. This is by far the most important parameter controlling the basicity. The influence of the structure type, as modeled here by the Fukui functions, shows that the O1 and O4 oxygens, i.e., those of the supercages or in the most open parts of the structure, are the most basic ones. The influence of the cations on these oxygens is modeled via the regional sensitivity factor. This secondary effect depends on the size of the cavities and cages (topological density) and calls for structures with large cavities if we wish to maximize basicity. The exchangeable cations attract negative charges to the oxygens in their coordination sphere, and this is more pronounced for Na+ than for Cs+. As a consequence O1 and O4 become more negatively charged, and therefore more basic, when Na+ is replaced by Cs+ in sites II and III. Thus, not only the charge but also charge changes on the oxygens determine the basicity trends of faujasite-type zeolites. In conclusion, O1 and O4 are the oxygen types which are most negatively charged; they are also the softest oxygens and most susceptible to charge changes upon replacing a small cation by a larger one. For these two reasons they are the most basic oxygens. As adsorption of molecules and catalytic reactions occur almost exclusively in the supercages, the basicity of faujasite-type zeolites is determined by these oxygens. Acknowledgment. R.H. is grateful for a grant of the European Community within the Human Capital and Mobility program. G.O.A.J. thanks the Flemish Institute for the Support of Scientific-Technological Research in Industry (IWT). The authors acknowledge financial support from the Flemish Government in the form of a Concerted Research Action (G.O.A.).
Heidler et al. References and Notes (1) Jacobs, P. A.; Uytterhoeven, J. B. J. Chem. Soc., Faraday Trans. 1 1979, 69, 359. (2) Freude, D. In Recent AdVances in Zeolite Science; Klinowski, J.; Barrie; P. J., Eds.; Studies in Surface Science and Catalysis 52, Elsevier: Amsterdam, 1989; p 169. (3) Jacobs, P. A. Catal. ReV.sSci. Eng. 1982, 24, 415. (4) Dempsey, E. J. Catal. 1975, 39, 155. (5) Pine, L. A.; Maher, P. J.; Wachter, W. A. J. Catal. 1984, 85, 466. (6) Barthomeuf, D. Mater. Chem. Phys. 1989, 93, 3677. (7) Stach, H.; Ja¨nchen, J.; Jerschkewitz, H.; Lohse, U.; Parlitz B.; Hunger, M. J. Phys. Chem. 1992, 96, 8480. (8) Janssens, G. O. A.; Toufar, H.; Baekelandt, B. G.; Mortier, W. J.; Schoonheydt, R. A. J. Phys. Chem. 1996, 100, 14443. (9) Barthomeuf, D. Stud. Surf. Sci. Catal. 1991, 65, 157. (10) Barthomeuf, D. Catal. ReV. (submitted for publication). (11) Huang, M.; Kaliaguine, S.; Auroux, A. Lewis Basic and Lewis Acidic Sites in Zeolites. In Zeolites: A Refined Tool for Designing Catalytic Sites; Bonneviot, L., Kaliaguine, S., Eds.; Elsevier: Amsterdam, 1995; p 311. (12) Barthomeuf, D. J. Phys. Chem. 1984, 88, 42. (13) Huang, M.; Adnot, A.; Kaliaguine, S. J. Catal. 1992, 137, 322. (14) Huang, M.; Kaliaguine, S. J. Chem. Soc., Faraday Trans. 1992, 88, 751. (15) Murphy, D.; Massiani, P.; Franck, R.; Barthomeuf, D. J. Phys. Chem. 1996, 100, 6731. (16) Okamoto, Y.; Ogawa, M.; Maezawa, A.; Imanaka, T. J. Catal. 1988, 112, 427. (17) Stoch, J.; Lercher, J.; Ceckiewicz, S. Zeolites 1992, 12, 81. (18) Sanderson, R. T. Chemical Bonds and Bond Energy, 2nd ed.; Academic Press: New York, 1976. (19) Barthomeuf, D.; de Mallmann, A. Stud. Surf. Sci. Catal. 1988, 37, 365. (20) Mortier, W. J. J. Catal. 1978, 55, 138. (21) Mortier, W. J.; Ghosh, S. K.; Shankar, S. J. Chem. Phys. 1986, 108, 4315. (22) Mortier, W. J. Struct. Bonding 1987, 66, 125. (23) Baekelandt, B. G.; Mortier, W. J.; Schoonheydt, R. A. Struct. Bonding 1993, 80, 187. (24) Janssens, G. O. A.; Baekelandt, B. G.; Toufar, H.; Mortier, W. J.; Schoonheydt, R. A. J. Phys. Chem. 1995, 99, 3251. (25) Baekelandt, B. G.; Janssens, G. O. A.; Toufar, H.; Mortier, W. J.; Schoonheydt, R. A. Acidity and Basicity of Solids; Fraissard, J., Petrakis, L., Eds.; NATO ASI Series C444; Kluwer Academic Publishers: Dordrecht, 1994; p 95. (26) Parr, R. G.; Yang, W. Density-Functional Theory of Atoms and Molecules; The International Series of Monographs on Chemistry 15; Oxford University Press: New York, 1989. (27) Parr, R. G.; Pearson, R. G. J. Am. Chem. Soc. 1983, 105, 7512. (28) Korchowiec, J.; Bredow, T.; Jug, K. Chem. Phys. Lett. 1994, 222, 58. (29) Toufar, H.; Baekelandt, B. G.; Janssens, G. O. A.; Mortier, W. J.; Schoonheydt, R. A. J. Phys. Chem. 1995, 99, 13876. (30) Meier, W. W.; Olson, D. H. Atlas of Zeolite Structure Types; Butterworths-Heinemann: Stoneham, MA, 1987. (31) Loewenstein, D. H. Am. Mineral. 1954, 39, 92. (32) Klinowski, J.; Ramdas, S.; Thomas, J. M.; Fyfe, C. A.; Hartman, J. S. J. Chem. Soc., Faraday Trans. 2, 1982, 78, 1025. (33) Baerlocher, C.; Hepp, A.; Meier, W. M. DLS-76: A Program for the Simulation of Crystal Structures; ETH: Zu¨rich, Switzerland, 1978. (34) Mortier, W. J.; Bosmans, H. J.; Uytterhoeven, J. B., J. Phys. Chem. 1972, 76, 650. (35) Mortier, W. J.; Costenoble, M. L.; Uytterhoeven, J. B. J. Phys. Chem. 1973, 77, 2880. (36) Bertaut, F. J. Phys. Radium 1952, 13, 499. (37) Fischer, R.; Ludwickzek, H. Monatsh. Chemie 1975, 106, 223. (38) Czjzek, M.; Jobic, H.; Fitch, A. N.; Vogt, T. J. Phys. Chem. 1992, 96, 1535.
JP9615619
