Oxygen Basicity in Alkaline Cation-Exchanged ... - ACS Publications

B-3001 HeVerlee, Belgium, and Eenheid Algemene Chemie (ALGC) Vrije UniVersiteit Brussel, Pleinlaan 2,. 1050 Brussels, Belgium. ReceiVed: April 3, 2007...
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12376

J. Phys. Chem. C 2007, 111, 12376-12382

Oxygen Basicity in Alkaline Cation-Exchanged Zeolite and the Effect of Isomorphous Substitution. Use of Hard Descriptors Pierre Mignon,*,† Paul Geerlings,‡ and Robert Schoonheydt† Centrum Voor OpperVlaktechemie en Katalyse, Katholieke UniVersiteit LeuVen, Kasteelpark Arenberg 23, B-3001 HeVerlee, Belgium, and Eenheid Algemene Chemie (ALGC) Vrije UniVersiteit Brussel, Pleinlaan 2, 1050 Brussels, Belgium ReceiVed: April 3, 2007; In Final Form: June 5, 2007

Density function theory calculations were carried out on 14T clusters wherein one of the 14 Si atoms was substituted by either B, Al, or Ga. The clusters were neutralized by one alkaline cation. Two accessible oxygen atoms, O1 and O4, bonded to the substituted atom are considered for their basic properties. The computed proton affinities and NO+ binding energies show the same trends as experimentally observed. These data point out that the O4 oxygen is the most basic for the B substitution and O1 is the most basic for the Al and Ga substitutions. In view of the hard character of the oxygen atom, hard descriptors are computed. Among them, the charge neither reproduces correctly the effect of the alkaline cation nor the effect of the isomorphous substitution. The local hardness reproduces well the isomorphous substitution but fails to predict the most basic site or the effect of the alkaline cation. The molecular electrostatic potential appears to be the best hard descriptor. It predicts well the effect of the monovalent cation as well as that of the isomorphous substitution. It is also able to predict which site is the most basic for a given substitution: O4 for B substitution and O1 for Al and Ga substitutions. In comparison with previous studies using soft indices to describe the oxygen basicity in zeolites, the molecular electrostatic potential is found to be a reliable and rigorous descriptor that can be used to predict both Lewis basicity and Brønsted-Lowry basicity.

Introduction Cation-exchanged zeolites are frequently used in catalysis and in separation by adsorption.1-8 The extraframework cation compensates the negative charge of the zeolite framework. A conjugate acid-base pair is then formed. The alkaline cation plays the role of the Lewis acid, and the framework oxygen is the Lewis base. Basicity is associated with the negative charge density on the framework oxygen, which depends on the structure type and chemical composition. The oxygen basicity has been shown to increase with increasing Al content of the framework.9-13 Numerous experimental measurements of the stretching frequency of the N-H group of adsorbed pyrrole have shown14-16 that basicity increases with the size of the exchangeable cation. Similar experiments measuring the stretching frequency of the adsorbed NO+ on zeolites Y and X, show that NO+ can be used to assess zeolite basicity.17,18 Basicity is also modified by isomorphous substitution. Trivalent atoms such as Al-, Ga-, and B-substituting Si have been studied experimentally in zeolite β. The basicity of the samples was investigated by IR spectroscopy of adsorbed pyrrole. The observed sequence for basicity was [B]-zeolite > [Ga]-zeolite > [Al]-zeolite.19 Various reactivity descriptors have been tested to describe the Si-O-Al-bridging oxygen reactivity in basic as well as acidic zeolites. In basic zeolites, methods such as standard population analysis failed to reproduce the effect of the alkaline cation on the oxygen basicity.20 Alternatively, Heidler et al. used successfully the electronegativity equalization method developed * To whom correspondence [email protected]. † Katholieke Universiteit Leuve. ‡ Vrije Universiteit Brussel.

should

be

addressed.

E-mail:

by Mortier21-24 to compute the charge of the bridging oxygen. Also in the DFT framework, Deka used soft descriptors to evaluate the effect of monovalent and bivalent cations on the basic oxygen.20 There have been no attempts to describe theoretically the effect of isomorphous substitution on oxygen basicity. In acid zeolites, the reactivity of Brønsted sites has been investigated using different approaches. It was studied by using a hard descriptor such as the net atomic charge on H.25 Soft descriptors were used as well to evaluate the effect of isomorphous substitution and alkaline cation on the acidity of the zeolite.26-29 In these studies, the sequence in Brønsted acidity was found to be B < Ga < Al. Calculations on small clusters also showed that the computed proton affinity reproduces the experimentally found sequence of acidity.30,31 Although both hard and soft descriptors have been used to describe the acidity of zeolite, reliable indices of basicity related to the properties of the hard bridging oxygen atoms have still not been tested. In the present study we will test hard reactivity indices to describe the basicity of the bridging oxygen. Indeed the main drawback of previous studies using the density functional theory (DFT)-based reactivity descriptors is the use of soft indices. Because of the hard character of the oxygen atom the use of the local softness, Fukui function, and derivatives are much less appropriate to describe its intrinsic properties. We propose instead that hard descriptors may be computed to reproduce and to predict the reactivity of the basic oxygen with respect to the nature of the alkaline cation and to the trivalent atom used to substitute Si. We will first evaluate the Brønsted and Lewis basicity by computing, respectively, proton affinities and adsorption energies of NO+ on the two accessible oxygen atoms, O1 and O4,

10.1021/jp072609t CCC: $37.00 © 2007 American Chemical Society Published on Web 08/02/2007

Oxygen Basicity in Alkaline Cation-Exchanged Zeolite

J. Phys. Chem. C, Vol. 111, No. 33, 2007 12377

[14T - O-] M+ + H+ T [14T - OH] M+

(1)

Also adsorption geometries and adsorption energies of the NO+ molecule have been calculated and corrected for the basis set superposition error. The binding energy was computed as the difference between the energy of NO+ bound to the 14T cluster and the energies of the isolated NO+ and zeolite cluster with each of them optimized separately, ∆E ) E([14T - O- NO+] M+) - E([14T - O-] M+) - E(NO+), according to the binding process

[14T - O-] M+ + NO+ T [14T - O- NO+] M+ (2) +

Figure 1. 14T cluster model with Al and Na . The oxygen atoms O4 and O1 bonded to the Al atom are the atoms for which the descriptors are tested to describe the oxygen basicity.

bonded to the substituted atom. From these results, we will establish a scale of the reactivity of the two considered oxygen atoms as a function of the alkaline atom and isomorphous substitution. We will then show the computed hard DFT-based reactivity descriptors:32 the net atomic charge, the molecular electrostatic potential (MEP), and the local hardness computed for the oxygen atoms O1 and O4. By comparing these reactivity descriptors with the proton affinity values and adsorption energies of NO+, we will estimate their ability to reproduce the effect of isomorphous substitution on the zeolite basicity. Computational Details The 14T cluster is taken from the sites II and III of the X-ray crystal structure of Faujasite33 (see Figure 1). The alkaline cation is placed in the center of the six-membered ring. The outer silicon atoms of the cluster were fixed during the optimization procedure. All optimizations have been performed at the B3LYP/6-31G* level, and then single point calculations were carried out at the B3LYP/6-311G* level of theory to compute the reactivity descriptors. Calculations were performed using the Gaussian 03 suite of programs.34 The Stuttgart/Dresden basis sets were used for the alkaline metal atoms.35 Small clusters have been shown to provide an adequate, qualitative picture of chemical reactions occurring at active sites in zeolites.36-38 For example, Rozanska et al.39 studied the isomerization and transalkylation of toluene and xylenes and found that the relative order of activation energies is conserved when comparing results obtained by using a small T4 cluster and periodic calculations. Also, the constraints imposed on the outer Si atoms of the 14T cluster allow us to keep a rigorous geometry of Faujasite structure. The cluster model we chose allows the isomorphously substituted T-atom to relax. The oxygen atoms considered for their basic properties are bound to at least one unconstrained Si atom, and they are free to move while the overall structure is maintained. From experimental X-ray data, a very low cation population of SIII site in Y zeolites was observed.40 Most cations are coordinated to SII, SI, and SI′ sites. However, SI or SI′ sites are not accessible by Rb+. Thus, we chose to place the cations over the six-membered ring (SII). The proton affinity was computed for the O4 and O1 oxygen atoms in all clusters as the difference between the energies of the unprotonated and protonated states both optimized separately for each alkaline metal and trivalent atom, ∆E ) E([14T O-] M+) - E([14T - OH] M+), according to the protonation process

The starting position of the NO+ molecule in the optimization procedure was close to the O4 atom. To describe the oxygen basicity, we choose hard reactivity descriptors. Net atomic charges may be used as an approximation to the local hardness.41 We thus computed charges obtained from a natural population analysis (NPA).42 The MEP gives a more detailed picture of the interaction of the charge density around a given atom with a hard partner (a proton). The MEP at position R is given by

V(R) )

∑A |R

ZA

A

- R|

F(r)

-

∫ |r - R| dr

(3)

where the first summation is over all the nuclei A with charge ZA at position RA within the molecule. It corresponds to the nuclear part of the MEP. The second term on the right side represents the electronic contribution (Vel(r)) and involves the electron density function (F(r)). We considered the minimum of the MEP around the basic oxygen atoms (O1 and O4) as a measure of the basicity that was already successfully applied in previous research.43-47 In the context of DFT descriptors, we chose to test the local hardness, η(r), for its ability to estimate the accumulation of negative charge at a well-defined point independently of the number of electrons of the system. It has been successfully used in the study of electrophilic aromatic substitution48 and in estimating the negative charge above substituted benzenes in the study of the stacking interaction between aromatic molecules.43,49 The local hardness is computed according to48

η(r) ) -

Vel(r) 2N

(4)

N is the number of electrons of the system, and Vel(r) is the electronic part of the electrostatic potential (eq 3). Vel(r) is the minimum around the considered oxygen. Results Geometry of the 14T Cluster. Table 1 shows selected geometrical parameters of the optimized clusters. Tr atom stands for the trivalent atom B, Ga, or Al, and T is the general name for the tetrahedral framework atoms. The B-O4 distances are quite large as compared to the B-O bond length observed for B-tetrahedra in crystals of hydroborate: 1.472-1.482 Å.50 It suggests no bonding between boron and the bridging oxygen O4. This is ascribed to the small size of B3+, which prefers tri- rather than tetracoodination. This has already been observed in previous theoretical studies on acidic zeolites30,31,51 where the bridging hydroxyl group resembles a terminal hydroxyl group.

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TABLE 1: Bond Lengths between the Substituted Atom (Tr) and O1 and O4 Oxygen Atoms in Å Li+ Na+ K+ Rb+ Cs+

Tr

Tr-O4

Tr-O1

B Al Ga B Al Ga B Al Ga B Al Ga B Al Ga

1.667 1.748 1.832 1.642 1.755 1.835 1.637 1.758 1.837 1.633 1.757 1.837 1.630 1.758 1.836

1.436 1.716 1.809 1.451 1.731 1.813 1.455 1.739 1.822 1.473 1.746 1.824 1.479 1.742 1.828

TABLE 2: Proton Affinities, PA (kcal/mol), Computed for the O4 and O1 Oxygen Atoms Li+ Na+ K+ Rb+ Cs+

Tr

PA (O4)

PA (O1)

B Al Ga B Al Ga B Al Ga B Al Ga B Al Ga

242.1 216.3 220.8 248.2 216.7 220.9 251.7 218.9 223.0 252.4 220.1 224.5 253.9 221.9 226.0

236.7 220.5 225.3 241.1 225.2 227.2 244.4 227.4 229.7 247.0 228.3 231.0 248.8 229.5 232.1

For Li+, Na+, and K+, the B-O1 distances are slightly smaller than the B-O bond length observed in crystals. Because of their size, these alkaline cations induce a relatively important stress on the 14T structure as compared to Rb+ and Cs+, which are further away from the framework oxygen atoms. Also, while the B-O4 distances decrease with increasing size of the charge-neutralizing alkaline cation, the B-O1 distances increase. Because of decreasing hardness of M+, the interaction M+‚‚‚O4 decreases with increasing size of M+. This leads to a slight strengthening of B-O4 and weakening of B-O1. In contrast, in the clusters with Al and Ga substitutions, both Tr-O1 and Tr-O4 distances increase slightly with increasing size of the alkaline cation. This shows the importance of the M+‚‚‚O4 interaction in the B-substituted clusters, stabilizing the negative charge of the basic terminal oxygen. Proton Affinities. Table 2 shows the proton affinities calculated for O4 and O1. These values are about 100 kcal/mol smaller than those computed at the same level of theory for a 8T cluster of the MCM-22 zeolite model.30 This difference may be due to the fact that in the latter model, no alkaline cation is present, or it may be due to the size of the model and to the constraints imposed in the optimization procedure. In the absence of alkaline cations, the proton affinity is computed for a negatively charged cluster and is thus larger than for a cluster neutralized with a monovalent cation. Also, the constraints imposed in our optimization procedure lead to less structural differences between the protonated and unprotonated states. Thus, the energy difference between the two states is smaller. Nevertheless, the effect of the isomorphous substitution is similar to the experimental basicity sequence.

The proton affinity increases with increasing size of the exchangeable cation irrespective of the Tr atom substitution. For each exchangeable cation, the sequence in proton affinity is the same as the sequence of experimental basicity: B . Ga > Al. One can see that between B and Ga the difference in the proton affinity values (28.7 kcal/mol for O4 with K+) is far larger than the difference between the proton affinity values of Ga and Al substitution (4.1 kcal/mol). Also, O4 shows a larger proton affinity for B substitution, while for the Ga and Al substitutions, O1 is the most Brønsted basic site. This is attributed to the absence of a bond between O4 and B, as discussed in the previous section, and thus to the higher reactivity of O4 for the B substitution. Figure 2 shows the geometries of the unprotonated system and the geometries of the systems protonated on O1 and O4 with K+ as the charge-neutralizing cation of the B-substituted cluster. In all cases, B is tricoordinated. For the unprotonated form, the K+ atom interacts with the terminal oxygen: Si-O-‚‚‚K+. In the case of protonated clusters, it is the bonding of the oxygen with the proton that leads to the B-O bond breaking and to the formation of the hydroxyl groups, Si-O4H and Si-O1H. Thus the B-O bond breaking is due to the small size of B and to the interaction/bonding of O1 or O4 with a cation/proton. Adsorption of NO+. The calculated adsorption energies of NO+ and selected geometrical parameters are given in Table 3. The adsorption energies of NO+ are negative and decrease with increasing size of the alkaline metal for B, Al, and Ga substitution. For a given alkaline cation, the order in adsorption energy is always B < Ga < Al. The more negative the adsorption energy of NO+, the larger the Lewis basicity. These sequences reflect the experimentally observed trends of basicity. Thus, for O1 and O4 the basicity increases when the size of the exchangeable cation increases. The effect of isomorphous substitution on the basicity agrees with the experimental data of B . Ga > Al. A large gap between the values of the B substitution and those of the Ga and Al substitutions is observed as was also seen from the proton affinity values. Table 3 shows that N-O4 distances are significantly smaller than N-O1 distances in the case of B substitution. The reverse holds true for Al and Ga substitution. Table 3 shows that in the case of B substitution, NO+ interacts with the O4 oxygen, while for the Ga and Al substitutions NO+ is adsorbed on the O1 atom, whatever the exchangeable cation (see Figure 3). In agreement with the proton affinity values, O4 is more basic in the case of the B substitution while O1 is the most basic site for Al and Ga substitutions. The starting position of NO+ in the optimization procedure was close to O4. Thus, NO+ finds its energetically most favorable position close to the oxygen atom with the highest Lewis basicity: O4 in the case of B substitution and O1 in the cases of Al and Ga substitutions. Charges, MEP, and Local Hardness. NPA charges, MEP, and local hardness values for O1 and O4 are given in Table 4. The variation of the oxygen charges with the nature of the monovalent cation is different for the B-substituted cluster than for Al- and Ga-substituted clusters. In the former case, the charge of O4 remains constant when the size of the alkaline cation increases, except for Li+. In the case of Al and Ga substitutions, the O4 charge decreases from Li+ to K+ and remains constant up to Cs+. In the B substitution, the O1 charge decreases with increasing size of the alkaline cation. In the Al and Ga substitution, the O1 charge increases from Li+ to Na+ then decreases slightly in the series K+, Rb+, and Cs+. The

Oxygen Basicity in Alkaline Cation-Exchanged Zeolite

J. Phys. Chem. C, Vol. 111, No. 33, 2007 12379

Figure 2. Geometries of the 14T cluster complexed with K+ and substituted with B. Left, no protonation; middle, O4-H; right, O1-H.

TABLE 3: Adsorption Energy of NO+ on the 14T Clusters (in kcal/mol)a Li+ Na+ K+ Rb+ Cs+

a

Tr

∆E

N-O4

N-O1

B Al Ga B Al Ga B Al Ga B Al Ga B Al Ga

-74.3 -60.5 -63.0 -80.3 -62.3 -63.8 -83.1 -63.4 -64.7 -84.4 -64.1 -65.9 -85.8 -64.8 -66.2

1.62 2.27 2.36 1.59 2.41 2.39 1.55 2.46 2.48 1.54 1.95 2.54 1.52 2.50 2.54

3.14 2.08 2.01 3.08 1.99 2.00 2.99 1.96 1.94 2.99 2.48 1.92 2.99 1.94 1.92

Distances from the N of NO+ and O4 and O1 oxygen atoms (in Å).

basicity is proportional to the negative charge density, according to Barthomeuf,2 on the O1 and O4 oxygen atoms, and the most basic oxygen (O1 or O4) is observed for the Al substitution. For every exchangeable cation, the basicity sequence is Al > Ga > B. Finally, O4 is more basic than O1 whatever the isomorphous substitution and whatever the monovalent cation, except for Li+. These trends do not correspond with those deduced from the NO+ adsorption energies and proton affinities. One may also notice that the charge variations are very small with respect to the alkaline cations. The values of the MEP (Table 4) become more negative when the size of the exchangeable cation increases, except on O4 for Al and Ga substitutions with Li+; and for O1 with Cs+ in the B substitution and with Rb+ in the Al substitution. More negative values of the MEP are associated with a more dense electron density. In the Lewis acid-base concept, a base is an electron pair donor, and a higher electron density or more negative MEP values are then indicative for a more basic atom. Thus, the MEP predicts that the basicity of the oxygen atom increases with increasing size of the alkaline cations. Comparing these values with those of the all Si cluster, -0.0486 and -0.0470 au for O4 and O1, respectively, we see that the isomorphous substitution and the presence of an alkaline metal increase clearly the oxygen basicity. For O4, the sequence in basicity deduced from the MEP values is B > Ga > Al for each exchangeable cation except Li+. For O1, the sequence is Ga > Al > B. For the B substitution, the MEP values are more negative for O4 than that of O1. For Al and Ga substitutions, the MEP values for O1 are more negative. It indicates that O4 is more basic for the B substitution, and O1

is the most basic for the Al and Ga substitutions. This agrees with the adsorption energies of NO+ and with the proton affinity values. Thus, the MEP can be used to predict the most basic oxygen for a given substitution, and it reproduces well the effect of the isomorphous substitution (the B substitution effect is only observable for O4 because only the B-O4 bond is broken). The local hardness (Table 4) of O1 is always larger than that of O4. For O1, it increases very slightly with the alkaline cation size. For O4, a small increase is observed in the Al- and Gasubstituted clusters except for Li+. An increase in the local hardness is expected to be associated with a larger basicity. The variation of η(r) with the alkaline metal is so small that it cannot be used with confidence to predict the effect of the exchangeable cation on the oxygen basicity. For the all Si clusters, the values are 0.0599 and 0.0584 au for O4 and O1, respectively. It shows that the isomorphous substitution leads to a net increase on the oxygen basicity for O1. For O4, only the B substitution leads to an increase in the local hardness. The effect of the isomorphous substitution on η(r) is similar to the effect on the MEP. For O4, the observed sequence corresponds to the adsorption of NO+ and the proton affinity values of B > Ga > Al (except for Li+). For O1, the observed sequence is Ga > B > Al. The effect of B substitution is small for O1, because the B-O1 bond is not broken as compared to O4. Discussion Acid-Base Concepts. Acidity and basicity are fundamental chemical concepts. In the frame of the Brønsted-Lowry theory, the acid is a proton donor and the base is a proton acceptor according to acid T base + H+. In accordance to the Lewis theory, an acid is an electron pair acceptor and a base is an electron pair donor where acid + base T acid-base. In this paper, both viewpoints have been implicitly used in our attempt to get insight into zeolite basicity. The study of the proton affinity refers to the Brønsted-Lowry acid-base concept applied to zeolites (eq 1). The proton affinities measure both basicity and acidity; the stronger the basicity of the oxygen, the weaker the conjugate acid. Thus Si-O4‚‚‚B is the strongest base for all alkaline cations of Table 2, and Si-O4H‚‚‚B is the weakest conjugate acid. This is clearly due to the lack of O4-B bond. O4 acts as a terminal negatively charged oxygen neutralized by the monovalent cation. This is not the case for Al and Ga substitutions. Here, Si-O1-Ga and Si-O1-Al are more basic than Si-O4-Ga and Si-O4-Al, respectively. The basicity sequence is thus B . Ga > Al. The sequence in acidity is opposite (Al > Ga > B), in agreement with experimental and theoretical works.30,31,52,53

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Figure 3. Geometries of the adsorbed NO+ for B-, Ga-, and Al-substituted clusters complexed with K+.

TABLE 4: NPA Charges, MEP Minimum, and Local Hardness Computed for the O4 and O1 Atoms (all Values in Atomic Units) NPA Li+ Na+ K+ Rb+ Cs+

η(r)

MEP

Tr

O4

O1

O4

O1

O4

O1

B Al Ga B Al Ga B Al Ga B Al Ga B Al Ga

-1.195 -1.300 -1.248 -1.176 -1.314 -1.276 -1.175 -1.319 -1.277 -1.179 -1.319 -1.278 -1.178 -1.319 -1.277

-1.127 -1.316 -1.240 -1.130 -1.298 -1.231 -1.137 -1.299 -1.236 -1.138 -1.303 -1.238 -1.141 -1.303 -1.241

-0.0754 -0.0732 -0.0776 -0.0824 -0.0713 -0.0772 -0.0826 -0.0728 -0.0785 -0.0848 -0.0747 -0.0807 -0.0853 -0.0757 -0.0817

-0.0728 -0.0765 -0.0890 -0.0808 -0.0879 -0.0911 -0.0815 -0.0909 -0.0930 -0.0844 -0.0907 -0.0946 -0.0839 -0.0920 -0.0948

0.0606 0.0593 0.0588 0.0608 0.0577 0.0586 0.0611 0.0577 0.0589 0.0610 0.0580 0.0590 0.0611 0.0580 0.0592

0.0625 0.0623 0.0633 0.0630 0.0625 0.0638 0.0632 0.0625 0.0640 0.0634 0.0628 0.0641 0.0636 0.0628 0.0642

The adsorption of NO+ refers to the Lewis acid-base concept. NO+ is the acid or electron acceptor and O1 or O4 act as the electron donor or base. Thus, the stronger the interaction, NO+‚‚‚ O1/O4, the more basic the oxygen is. O4 is found to be the most basic in the case of B substitution, and O1 is in the case of Al and Ga substitutions. The basicity sequence is B . Ga > Al, in agreement with the computed proton affinities. Therefore, both from the viewpoint of the Brønsted-Lowry concept and the Lewis concept of basicity (and acidity) the basicity sequence is B . Ga > Al; O4 is the basic site for the B substitution and O1 for the Ga and Al substitutions. The reason for this switch from O1 to O4 is known: O4 is a terminal oxygen in the case of the B substitution. Its negative charge is stabilized by the alkaline cation. DFT-Based Descriptors. We tested several hard descriptors for their ability to reproduce the effect of the alkaline cation and isomorphous substitution on the oxygen basicity. These descriptors should reflect the trends observed from the computed proton affinities and adsorption energies of NO+. The NPA charges do not reproduce the effect of the alkaline cation. The variations are very small and do not give a clear trend with respect to the cation size. Net atomic charges are already known to incorrectly reproduce the effect of the alkaline cation on the basicity of bridging oxygens.20 Our data confirm this conclusion. The effect of the isomorphous substitution on atomic charges is more prononced but opposite to the results obtained from the calculated proton affinities and NO+-binding energies. In contrast, atomic charges on H have been found to reproduce well the effect of the isomorphous substitution on zeolite acidity.28,48 Atomic charges are thus unable to reproduce the reactivity of a basic oxygen in cation-exchanged zeolites.

The MEP is the only descriptor that reproduces the effect of the exchangeable cation on the basicity. The basicity of the two oxygen atoms increases with the size of the cation whatever the isomorphous substitution. In the B substitution, O4 has the most negative MEP value and is the most basic; for Ga and Al substitution the MEP of O1 is the most negative, and thus O1 is the most basic oxygen. This agrees with the proton affinity values and adsorption energies of NO+. The MEP reproduces well the experimentally found sequence B > Ga > Al for O4. The more basic character of O1 for Ga substitution as compared to Al substitution is also well reproduced. For O1, the small effect of the B substitution is because the B-O1 bond is not broken. The local hardness reflects well the effect of isomorphous substitution on the oxygen basicity for O4. The sequence in basicity is B . Ga > Al. For O1, a higher basicity is found for Ga substitution as compared to Al substitution, and the small effect of the B substitution is also observed, as was observed from the MEP values. However, it fails to predict the basic site for a given substitution and the effect of the alkaline cation. In previous studies, soft descriptors were computed to assess the effect of alkaline cations.20 The local softness and the relative nucleophilicity reproduce well the effect of the cation. However, negative values of the local softness were obtained when the Mulliken population analysis was used. On acidic zeolites, the effect of isomorphous substitution was investigated by computing the same soft descriptors on the proton.28 The relative nucleophilicity, computed from the local softness, reproduces well the acidity sequence. However, the use of soft indices to describe the reactivity of an intrinsically hard atom such as O or H is counterintuitive and lacks reality. All descriptors used in this study correspond to the definition of Barthomeuf of the zeolite basicity as being the “negative charge around framework oxygen”.2 However among all hard descriptors used in the present study, the MEP is the more reliable descriptor of basicity as compared to charge and local hardness. The first reason is that the MEP is intrinsically related to the electron density and directly computed from it with its unique expression (eq 3). In contrast, atomic charge values depend largely on the population analysis used. The local hardness expression we used (eq 4) is an approximation using Vel for the electron/electron repulsion term.25 The second reason is that by definition the MEP is the interaction between a positive charge and the electron density, computed here around the considered basic oxygen. It is thus intuitively related to Lewis basicity. As seen in the calculation of the proton affinities and the NO+ adsorption energies, Lewis and Brønsted-Lowry basic properties of the oxygen are strongly related. Thus, the MEP also reproduces well the oxygen Brønsted-Lowry basicity. Although this hard descriptor is basis set dependent, the same

Oxygen Basicity in Alkaline Cation-Exchanged Zeolite trend can be expected for different basis sets, as was observed in a previous study.49 These findings are interesting from a conceptual point of view and the descriptors may be used at a larger scale to determine the most reactive sites of zeolites. For example, in Faujasite supercage one may compute a MEP map and look for the minima on the surface and thus determine the most basic sites and this for various Si/Al ratios and Al distributions. Also, other definitions of the local hardness may be tested54 for zeolites in the future. In the η(r) values, the proton affinity values, and the adsorption energies of NO+, we observe a larger gap between the effects of B and Ga substitutions as compared to the difference between Ga and Al substitutions. This gap is related to the B-O4 bond breaking that leads to the more important O4 basicity as compared to Ga and Al substitutions. Therefore, the terminal oxygen is more negatively charged and thus more basic than the bridging oxygen atoms. This can also be linked to the hardness of the trivalent atom. Indeed, using Sanderson’s electronegativity equalization principle, one can predict that the charge transfer between two atoms in a diatomic molecule is inversely proportional to the sum of the atomic hardnesses. By extending this to the present study, the harder the trivalent atom, the less charge transfer with the bridging oxygen is expected, and the more negatively charged55,56 the oxygen atom remains. From the hardness values of B, Ga, and Al (4.03, 2.90, and 2.77),57 one can directly see that B is substantially harder than Ga and Al. Thus the B substitution leads to a larger negative charge density on the oxygen, and larger oxygen basicity than Ga and Al substitutions. Conclusion Various descriptors were computed to assess the effect of the isomorphous substitution on the basicity of two oxygen atoms: O1 and O4 in Faujasite type zeolites. First, the computed proton affinities and NO+-binding energies show the same trends as experimentally obtained. In addition, they point out that the most reactive basic site differs for the B substitution and the Ga /Al substitutions. The O4 oxygen is the most basic for the B substitution, and O1 is the most basic for the Al and Ga substitutions. Among the computed hard descriptors, the charge and the local hardness do not reproduce correctly the effect of the alkaline cation. The local hardness reproduces well the effect of the isomorphous substitution. The MEP appears to be the best hard descriptor. It predicts well the effect of the monovalent cation as well as the isomorphous substitution. It is also able to predict which site is the most basic for a given substitution: O4 for B substitution and O1 for Al and Ga substitutions. In comparison with previous studies using soft indices to describe the oxygen basicity in zeolites, we have shown here that in line with the intrinsic hard character of the oxygen atom, the molecular electrostatic potential is a reliable and rigorous descriptor that can be used to predict both Lewis and BrønstedLowry basicity. Acknowledgment. P.M. is financially supported with the postdoctoral fellowship of the K.U. Leuven. This work was made possible by the Concerted Research Action (G.O.A.), IAPVI, and the Scientific Research Communities of the Fund of Scientific Research-Flanders. References and Notes (1) Barthomeuf, D. Stud. Surf. Sci. Catal. 1991, 65, 157. (2) Barthomeuf, D. Catal. ReV.sSci. Eng. 1996, 38 (4), 521-612.

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