Theoretical Study of 4, 4′-Bipyridine Adsorption on the Brønsted Acid

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Theoretical Study of 4,4′-Bipyridine Adsorption on the Brønsted Acid Sites of H-ZSM-5 Zeolite Emile Kassab,*,† Martine Castella`-Ventura,‡ and Yamina Akacem§ Laboratoire de Chimie The´orique, CNRS-UMR 7616, UniVersite´ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France, Laboratoire de Dynamique, Interactions et Re´actiVite´, CNRS-UMR 7075, UniVersite´ Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France, and Institut de Chimie, UniVersite´ des Sciences et de Technologie, LTMM-USTHB, BP32 Dar El Beida, Alger, Alge´rie ReceiVed: July 2, 2009; ReVised Manuscript ReceiVed: October 8, 2009

The adsorption of 4,4′-bipyridine (44BPY) on the Brønsted acid sites of H-ZSM-5 zeolite is investigated by density functional calculations, at the B3LYP level, using two basis sets. The H-ZSM-5 straight channel is simulated by two 10-membered ring clusters (18Si, 2Al), saturated by hydrogen atoms (2-10T). Two different aluminum distributions were employed to study this adsorption. Because of the size of the systems considered, this study represents a significant challenge to quantum chemical methods if enough accuracy is required. The energetic and vibrational results for the species formed upon concerted interaction of bidentate 44BPY ligand via its both nitrogen lone pairs with the Brønsted acid sites in a straight channel of H-ZSM-5 zeolite are presented. The potential energy surface for double proton transfer from the zeolite (Z) to 44BPY shows 2only one minimum corresponding to the formation of a bidentate ion-pair complex (44BPYH2+ 2 /Z ). The other bidentate complexes having either neutral (44BPY/Z) or monoprotonated (44BPYH+/Z-) hydrogenbonded structures are not stable, in agreement with a recent Raman spectrometry study reported for 44BPY occluded in acidic H-ZSM-5 zeolite in which the diprotonated dication 44BPYH2+ 2 appears as the unique species observed. The calculated frequencies and frequency shifts of the adsorbed 44BPY are in good agreement with the available experimental data. The interactions responsible for the vibrational properties are essentially local. The symmetric ring stretching mode 8a of 44BPY in the 1400-1600 cm-1 region is the most affected one upon adsorption as observed in Raman spectra. I. Introduction During the past decades, a number of experimental and theoretical studies on vibrational spectra of organic molecules adsorbed on Brønsted acid sites of several types of zeolites have been widely investigated.1-23 Despite the abundance of experimental investigations on vibrational spectra of pyridine adsorbed on zeolites,2-11 there are only few theoretical studies dealing with the vibrational properties of pyridine10,23 and none was devoted to other aromatic species. Experimental1-16,24-26 and theoretical17-23,26-49 methods for studying zeolite acid sites have begun to mature in recent years, and the complementarity of these approaches is a realistic goal. In our recent paper23 dealing with density functional calculations (DFT) of vibrational frequencies of adsorption complexes implicated in proton transfer between pyridine and zeolite, we found good agreement between experimental and calculated frequency shifts. Many previous ab initio and DFT studies have been reported in the past on open small clusters as models of zeolites. Such clusters do not correspond to any specific zeolite but rather to a generic tetrahedral subunit containing the Brønsted acid site in an unconstrained environment. These small clusters can give a good description of the adsorption of small molecules.18,19,26-36 However, for larger molecules such as aromatic species, with * To whom correspondence should be addressed. E-mail: emile.kassab@ lct.jussieu.fr. † Laboratoire de Chimie The´orique, Universite´ Pierre et Marie Curie. ‡ Laboratoire de Dynamique, Interactions et Re´activite´, Universite´ Pierre et Marie Curie. § Institut de Chimie, Universite´ des Sciences et de Technologie.

dimensions close to that of the 10-membered ring channels of H-ZSM-5, there can be multiple electrostatic interactions of the hydrogen atoms of the adsorbed molecules and the oxygen atoms of the zeolite framework.11,23,39-44 In addition, during the geometry optimization, these open clusters with OH dangling bonds introduce unrealistic structures due to intramolecular hydrogen bond formation. The extension of the cluster model toward more realistic representations of the zeolite structure is obviously desirable. The present theoretical work has been prompted by a recent Raman spectrometry study reported for 4,4′-bipyridine (44BPY) occluded in acidic H-ZSM-5 zeolites in which the diprotonated dication 44BPYH22+ appears as the unique species observed.6 In this paper, we present DFT results for energetic and vibrational properties of the species formed upon interaction of 44BPY with the Brønsted acid sites in straight channels of H-ZSM-5 zeolite simulated by two 10membered ring clusters with two different aluminum distributions. These large clusters represent a part of the wall of the H-ZSM-5 channel. Because of the size of the systems considered, this study represents a significant challenge to quantum chemical methods if enough accuracy is required. II. Models and Methods The straight channel of H-ZSM-57,50,51 is simulated by two 10-membered ring clusters (2-10T) arranged in parallel fashion as indicated in Figure 1. Each ring is composed of 9 silicon atoms and 1 aluminum atom. Starting from the symmetrical distribution of the two Al atoms, in which the two parallel 10membered rings can be perfectly superimposed (Cs symmetry) denoted as (Al1, Al1′) distribution (Figure 2), five other different

10.1021/jp9062302 CCC: $40.75  2009 American Chemical Society Published on Web 10/30/2009

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Figure 1. Schematic representation of the structure of H-ZSM-5 zeolite (from Database of Zeolite Structures50) and of the 2-10T cluster model: (A) front view; (B) top view. The Brønsted-active regions (2-3T) are represented in balls and sticks.

Al distributions can be generated by letting the first aluminum atom of the first 10-membered ring stay at its position (Al1), whereas the second aluminum atom is moved around the second 10-membered ring from left to right: one symmetrical distribution (Ci symmetry) in which the two Al atoms are placed in opposite position, denoted as (Al1, Al6′), and four unsymmetrical distributions denoted as (Al1, Al2′), (Al1, Al3′) (Al1, Al4′), and (Al1, Al5′) (Figure 2). The unsymmetrical distribution (Al1, Al5′) is simply obtained by exchanging the Si and Al atoms of the bridging active region SiOHAl in the (Al1, Al6′) symmetrical distribution (Figure 3). These two distributions, (Al1, Al6′) and (Al1, Al5′), are potential candidates for the study of the concerted double proton transfer reaction, as discussed in Energetics section. The dangling bonds of the two 10membered ring clusters were terminated with hydrogen atoms.

Because of the size of the system under study, it is yet hard to carry out calculations using large basis sets on all atoms of the system, since the geometry optimization and frequency calculations would require too much CPU time. Thus, as in the ONIOM approach, the system is divided into two parts, and each one is treated at a different level of calculation by using a mixed basis set: for each 10-membered ring, the Brønsted active regions, constituted by 3 tetrahedra (2Si, Al) (shown in Figure 1), are treated with the large standard 6-31+G* basis set, designated as SB, at the B3LYP level, while the rest of the clusters (14Si) are treated less accurately with 6-31G basis set at the same B3LYP level. This mixed basis set 6-31+G*(6T):6-31G(14T) is designated by MB. The 44BPY molecule is always treated as the cluster active regions, at the B3LYP/6-31+G* level. For the symmetrical (Al1, Al6′) distribution, the structure of the

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Figure 2. Schematic representation of the distributions of the two aluminum atoms, each located in a 10T-membered ring. The (Al1, Al1′), (Al, Al2′), (Al1, Al3′), (Al, Al4′), (Al1, Al5′), and (Al, Al6′) distributions are indicated by dotted lines.

system studied is completely optimized under symmetry constraint, whereas for the unsymmetrical (Al1, Al5′) distribution, the geometrical parameters of the two cluster rings (2-10T) and the two pyridyl rings of 44BPY are decoupled (the number of geometrical parameters to be optimized for the (Al1, Al5′) distribution is twice of the (Al1, Al6′) distribution). Because of the size of the system considered, we test the reliability of this basis set on the geometry optimization only for the symmetrical distribution (Al1, Al6′) in order to limit the number of geometrical parameters to be optimized, which significantly reduces the computation time. Calculated adsorption energies (Eads) were corrected for the basis set superposition error (BSSE) using the counterpoise corrected method.52 The vibrational zero-point energy (ZPE) corrections to the adsorption energies were calculated within harmonic approximation only with the mixed basis set. Normalmode analysis was performed for the adsorption complex to confirm its stability and to compare its vibrational frequencies with the Raman spectra.6 The frequencies computed for all species studied here were scaled by our scaling factors (see below) determined to give the best agreement between the experimental anharmonic and the calculated frequencies for 44BPY and its complex. To designate the normal modes of 44BPY, we used the convention adopted by Varsanyi for monosubstituted benzenes53 adapted from the Wilson notation for benzene.54 All calculations were done using the Gaussian03 program.55 III. Results and Discussion Energetics. The next-nearest-neighbor (NNN) rule suggests the strong Brønsted acidity occurs only on acid sites, of which the aluminum atom has no aluminum atoms at its next nearest neighbor.56-59 In addition, it has been reported that the nextnext-nearest-neighbor (NNNN) distribution is more acidic than the NNN one when the two Al atoms are located in the same ring.60 However, as the Al...Al distance is large enough in our two 10-membered ring clusters, the Al distribution has little effect on the acidity of the Brønsted sites. But, since we are interested in the double proton transfer from the cluster to the adsorbed bidentate molecule, the two negative charges on the two 10-membered ring clusters will be highly localized in the regions around the framework oxygen atoms next to the Al atoms. It is clear that the larger the distance between the two

Kassab et al. Al tetrahedra is, the weaker the repulsion energy between them. Thus, as shown in Figure 2, the Al · · · Al distance for the different distributions increasing in the following order: (Al1, Al1′) < (Al1, Al2′) < (Al1, Al3′) < (Al1, Al4′) < (Al1, Al5′) < (Al1, Al6′), the acidity increases in the same order, in terms of the repulsion energy. Thus, the two distributions (Al1, Al5′) and (Al1, Al6′) are best suited for studying the concerted double proton transfer. The 44BPY molecule can be placed between the two 10membered rings so that its two nitrogen atoms are directed toward the two bridging hydroxyl groups SiOHAl. The large distance between the two acidic sites situated in opposite position in these distributions strongly reduces the repulsion and spatial hindrance between pyridyl rings and the framework wall atoms. As resulting the two N atoms of 44BIPY can interact with Brønsted acid sites synchronously. The other distributions are not suitable for studying the concerted double proton transfer, since in these distributions, when a nitrogen atom of 44BPY can be in front of the bridge proton SiOHAl, the second nitrogen atom is in front of the O atom of the bridge SiOSi; thus, the second bridging SiOHAl proton is located at a distance farther away from the second N atom to be transferred. In addition, owing to the molecular dimension of 44BPY (about 7 Å) and the intermolecular hydrogen bond length (about 1.55 Å), the spatial effect is important. Because of the fact that the distance between the two acidic sites in these distributions is not large enough, strong repulsion and spatial hindrance take place between the pyridyl rings and the framework wall atoms. As resulting the two N atoms of 44BPY can not interact with Brønsted acid sites simultaneously. In the present work, we just focus on the two (Al1, Al5′) and (Al1, Al6′) distributions suitable for studying the concerted double proton transfer. The total energies of the optimized species involved in the adsorption process are calculated at the B3LYP/MB level for the two (Al1, Al5′) and (Al1, Al6′) distributions and also at the B3LYP/SB level only for the symmetrical (Al1, Al6′) distribution (Table 1). The diprotonation of 44BPY is considered to proceed via a concerted mechanism. The calculated adsorption energies for the concerted interaction of bidentate 44BPY ligand via its both nitrogen lone pairs with the Brønsted acid sites of the 2-10T clusters for these distributions are also summarized in Table 1. For both distributions, the potential energy surface for double proton transfer from the Brønsted acid sites of zeolite to 44BPY shows only one minimum corresponding to the 2formation of a bidentate ion-pair complex (44BPYH2+ 2 /Z ). The other bidentate complexes having either neutral (44BPY/Z) or monoprotonated (44BPYH+/Z-) hydrogen-bonded structures are not stable. Their optimized structures were obtained by fixing the OH distances in the zeolite bridging hydroxyl SiOHAl to 1.05 Å. When allowing the OH distances to relax, the direct transfer of one or two protons from the Brønsted acid sites of zeolite to 44BPY occurs, leading to the formation of the (44BPYH22+/Z2-) complex. We note that the monoprotonated species 44BPYH+ which could be formed on the surface Brønsted acid sites of H-ZSM-5 via one nitrogen lone pair of 44BPY has not previously been observed by Raman spectra.6 It is probably due to the small Si/Al ratio of H-ZSM-5 used in this work. The two (Al1, Al5′) and (Al1, Al6′) distributions have nearly the same calculated adsorption energy values (52.1 and 51.3 kcal/mol, respectively). This is not surprising since they have the same acidic site distances. To analyze the effect of further enlarging of the basis set, we have also performed only the geometry optimization of the bidentate ion-pair complex with the extended 6-31+G* basis

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Figure 3. Bidentate ion-pair complex of 44BPY with 2-10T cluster model: (A) front view and (B) top view for (Al1, Al6′) distribution; (C) top view for (Al1, Al5′) distribution.

set for the symmetrical (Al1, Al6′) distribution. Whatever the basis set used, the potential energy surface for double proton transfer shows only one minimum corresponding to the formation of a bidentate ion-pair complex (44BPYH22+/Z2-). The computed adsorption energies are 51.3 and 43.5 kcal/mol with MB and SB basis sets, respectively (Table 1). While geometry parameters change only slightly upon increasing the basis set (see below), the adsorption energy decreases by ∼ 7.0 kcal/ mol. However, the BSSE corrections decrease with increasing of the size of the basis set. When the adsorption energy is corrected for BSSE, the obtained results are very similar (40.0 and 37.4 kcal/mol with MB and SB basis sets, respectively). This indicates that, although the small mixed basis set MB leads

to higher interaction energies, these energies are reasonably corrected when the counterpoise correction is added. These results validate the use of the mixed basis set MB to model various effects such as adsorbate-cavity wall and long-range electrostatic interactions, using more realistic zeolite clusters, including more oxygen atoms of the zeolite framework. The ZPE corrections do not depend on the basis set; they are estimated to be less than 2.9 kcal/mol. Thus, the corrected adsorption energies including ZPE corrections are 37.1 and 34.8 kcal/mol with MB and SB basis sets, respectively. No experimental data on 44BPY adsorption are available for comparison. For adsorption of a large molecule such as pyridine and 44BPY in H-ZSM-5 zeolite, the interactions of the pyridyl

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TABLE 1: Optimized Geometrical Parameters a of the Species Involved upon 44BPY Adsorption on 2-10T Zeolite Cluster for the Symmetrical (Al1, Al6′) Distribution, Total Energies Etot (au), and Adsorption Energies Eads (kcal/mol) Calculated at the B3LYP Level with the two Basis Sets 6-31+G*(6T):6-31G(14T) (MB) and 6-31+G*(20T) (SB) H-ZSM-5 44BPY N 1C 2 C 2C 3 C 3C 4 C4C4′ C3C4C4′C5′ NH OaH AlOa SiOa AlOb SiOb SiO2 SiO1 SiO diag. OO AlOaSi OaAlOb SiObAl OSiO SiOSi Etot

44BPYH2+ 2

exptlb

SB

SB

1.47 37.2

1.340 1.396 1.404 1.485 38.0

1.352 1.387 1.408 1.493 44.3 1.021

-496.01662

244BPYH2+ 2 /20T

20T exptlc

1.59 ( 0.01 8.3 ( 0.2

-495.38764

44BPY/H-ZSM-5

109 ( 3 149 ( 3

Eads

MB

0.980 1.906 1.705 1.803 1.648 1.638 1.635 ∼1.69 ∼9.10 129.9 93.3 141.9 ∼111 ∼149 -7226.08439

SB

0.979 1.897 1.691 1.807 1.638 1.653 1.649 ∼1.65 ∼8.67 129.4 92.0 140.9 ∼109 ∼145 -7227.23032

MB

SB

1.344 1.384 1.404 1.480 35.9 1.080 1.537 1.811 1.665 1.814 1.642 1.653 1.660 ∼1.69 ∼9.05 135.9 97.8 139.0 ∼111 ∼145 -7721.55378 -7721.55511d -51.3 (-37.1)e -52.1 (-37.9)d,e

1.344 1.386 1.404 1.483 39.3 1.085 1.540 1.816 1.662 1.811 1.636 1.664 1.666 ∼1.65 ∼8.69 134.5 96.7 138.1 ∼109 ∼142 -7722.68712 -43.5 (-34.8)e

a Bond lengths in Å, and angles in deg. For atom numbering, see Figure 3. b Experimental value in ref 61. c Average experimental values in ref 50. d Etot and Eads for the (Al1, Al5′) distribution. e Eads corrected for ZPE and BSSE in parentheses.

hydrogen atoms with the oxygen atoms of the zeolite wall have more of a contribution to the adsorption energy than in H-Y zeolite,23,43 because the pyridyl ring has the same dimension of the 10-membered ring channels of H-ZSM-5, and this contribution increases with decreasing of the size of the zeolite pore wall. Figure 3 shows that the two ortho hydrogen atoms of each pyridyl ring of 44BPY have attractive interactions with the nearest bridging oxygen atoms of each 10-membered ring cluster (CH · · · O distances of 2.3-2.8 Å); the para hydrogen atoms have a negligible interaction with the oxygen atoms of the zeolite wall (CH · · · O distances more than 4.0 Å). The contribution of the active region (2-3T), around the Brønsted acid site (Figure 1), to the adsorption energy was estimated at the B3LYP/MB level. This contribution varies from the neutral structure (44BPY/2-10T) to the bidentate ion-pair complex (44BPYH2+ 2 / 2-10T2-). It was found that the values of the interaction energy between 44BPY and the Brønsted active region (2-3T), cut from the optimized adsorption complexes, represent ∼77% of the interaction energy between the neutral fragments of (44BPY/ 2-10T), and ∼86% of the interaction energy between the ionic fragments of (44BPYH22+/2-10T2-). Structures. For both Al distributions (Al1, Al5′) or (Al1, Al6′), the geometrical characteristics of the adsorption complexes with 44BPY are quite identical. Thus, only the calculated results obtained with (Al1, Al6′) distribution are discussed. The main optimized geometrical parameters determined at the B3LYP level with MB and SB basis sets for 44BPY and for 44BPYH22+ free and within the bidentate ion-pair complex formed with this 2-10T cluster model (Figure 3) are presented and compared with available experimental data in Table 1. Selected structural parameters of the cluster bare and within the complex are also given in Table 1. Previous experimental61 and recent theoretical studies62-66 have allowed to conclude that free 44BPY may be considered

as two aromatic pyridyl rings linked by a single bond in a nonplanar conformation (D2 symmetry). The molecular structure of dication 44BPYH22+ has only been investigated by X-ray crystal analysis;67 44BPYH22+ has scarcely been studied by quantum chemistry methods.63 As yet reported in our previous work,63 the present study shows that the dication structure is much closed to the neutral molecule one (Table 1). However, on the one hand, 44BPYH22+ is more strongly twisted than 44BPY (dihedral angle C3C4C4′C5′ larger by 6.3° in the ion), and its inter-ring bond C4C4′ is longer by 0.008 Å. On the other hand, on going from 44BPY to 44BPYH22+, geometrical modifications principally localized in the nitrogen region (lengthening of the N1C2 bond by 0.012 Å, increasing of the C2N1C6 angle by 6.2°) result from quarternization of the rings upon protonation of the nitrogen atoms. Finally, dication 44BPYH22+ may be described by two pyridinium rings linked by a single bond in a twisted conformation. The framework structure of the H-ZSM-5 zeolite has been studied by single crystal X-ray analysis.7,50,51 The average SiO distances, OSiO and SiOSi angles, as well as the range of the diagonal OO distances defining the size of the straight channel pore, are summarized in Table 1. In 2-10T cluster, the calculated lengths of the SiO bonds, not involved in the Brønsted acid site, are larger by less than 0.1 Å with respect to the experimental value. On the other hand, the diagonal OO distances are calculated to be larger by ∼0.6 Å. The larger diameter of the 10-membered ring obtained by calculation is due to the relaxation allowed during the optimization procedure. In comparison to experimental values, the OSiO and SiOSi angles are quite well calculated. Upon adsorption, the nitrogen atoms of 44BPY are directly protonated via a concerted double proton transfer from the zeolite Brønsted acid site to both nitrogen atoms, leading to a bidentate ion-pair complex (44BPYH22+/2-10T2-). The struc-

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TABLE 2: Scaled Frequencies (cm-1) of the In-Plane Modes in the 600-1600-cm-1 Region of 44BPY Free, 44BPYH22+ Free, and within its Bidentate Ion-Pair Complex (44BPYH22+/20T2-) Calculated at the B3LYP Level 44BPYH2+ 2

44BPY

44BPY/H-ZSM-5

approximate descriptiona,b,c

expd

SBe,f

exph,i

SBe,f,h

exph,i

244BPYH2+ 2 /20T MBe,h,k

(6b-B3) (1-A) (12-A) (18a-A) (9a-A) (14-B3) (Ω-A) (19a-A) (8a-A)

660 756 1000 1082 1219 1242 1297 1511 1607

663 750 995 1080 1232 1271 1292 1517 1615

644 j (-16) 757 (+1) 1016 (+16) 1075 (-7) 1216 (-3) 1250 j (+8) 1288 (-9) 1528 (+17) 1644 (+37)

639 (-24) 760 (+10) 1010 (+15) 1065 (-15) 1224 (-7) 1273 (+2) 1283 (-8) 1528 (+11) 1659 (+44)

645 (-15) 759 (+3) 1013 (+13) 1077 (-5) 1218 (-1) 1260 (+18) 1293 (-4) 1528 (+17) 1649 (+42)

648 (-15) 750 (0) 1007 (+13) 1079 (-1) 1222 (-10) 1298 (+27) 1296 (+5) 1533 (+16) 1660 (+45)

δr νCC δr βCH βCH νCC νir νCC νCC

a ν, stretching; β, bending; δ, deformation. b Assignments with Wilson notation in parentheses. c D2 symmetry. d ref 63. e Scaling factor of ∼0.981. f SB is the standard basis 6-31+G*. g Raman frequencies in the dibromomide salt in ref 69. h Frequency shift with respect to free 44BPY in parentheses. i Raman frequencies in ref 6. j Raman frequency in acid solution (pH e 1.2) in refs 70 and 6 for 6b and 14 modes, respectively. k MB is the mixed basis set 6-31+G*(6T):6-31G(14T).

tures of free and adsorbed 44BPYH22+ are somewhat different. The adsorption within the zeolite affects principally the C2C3 bond, which decreases significantly by ∼0.01 Å. All the other geometrical parameters of adsorbed 44BPYH2+ 2 are less modified (less than 0.005 Å) than the free dication ones. Contrary to isolated dication, adsorbed 44BPYH22+ is identically twisted as 44BPY, and its inter-ring bond C4C4′ distance decreases by ∼0.003 Å. The NH bond is longer in the adsorbed 44BPYH22+ than in the isolated dication; it is comparable to the length calculated in the ion-pair complexes formed between pyridine and protonic zeolite clusters.23 In the bidentate ion-pair complex, the NHOa angle is nearly linear (∼175.6°), and the bridging OaH distance is of ∼1.54 Å. The NH · · · Oa distance of ∼2.59 Å is significantly less than the sum of the van der Waals radii of the nitrogen atom (1.5 Å) and oxygen atom (1.4 Å). That suggests the occurrence of a hydrogen bond NH · · · Oa rather strong in the complexes, as it was yet shown for the ion-pair complexes with pyridine.23 Intermolecular hydrogen bonds formed between the ortho hydrogen atoms of 44BPYH22+ and the oxygen atoms of the clusters are weak (distances of 2.3-2.8 Å). The meta hydrogen atoms have a negligible interaction with oxygen atoms of the clusters (distances longer than 4.0 Å). Upon adsorption, the geometrical structure of the cluster is principally modified in the Brønsted active region. The AlOa bond and to a lesser extend the SiOa bond decrease (by ∼0.09 and ∼0.04 Å, respectively). The other bonds remain approximately unchanged (shifts less than 0.03 Å). In the same way, the bond angles OaAlOb and AlOaSi are the most modified (increase up to 6°). All other angles generally vary by less than 3°. On the other hand, the diagonal OaO distance in the bidentate ion-pair complex is significantly increased by ∼0.13 Å. In this complex, the computed average distance between the two ring 10T clusters (∼8.6 Å) is in good agreement with experimental data.7 Vibrational Properties. For the free 4,4′-bipyridine molecule (D2 twisted symmetry), the 54 vibrational modes are shared among 14A + 12B1 + 14B2 + 14B3 symmetry classes. For the diprotonated dication 44BPYH22+ (also D2 twisted symmetry), there are six additional modes related to the NH motions, and the 60 modes are transformed in 15A + 13B1 + 16B2 + 16B3. Adsorption of 44BPY on the Brønsted acid sites of 2-10T cluster leading to the bidentate ion-pair complex does not affect the molecular symmetry of 44BPYH22+. All vibrational modes of 44BPY and 44BPYH22+ are Raman active, whereas only the B1, B2, and B3 modes of vibration are IR active. 44BPY vibrational spectra have been studied principally by Raman and IR spectrometry in the solid phase and in

solution63,64,68 and also by some scarce ab initio and DFT 69 methods.63-65 44BPYH2+ 2 in the solid state as the dibromide salt and in H2O/HCl solution (pH e 1.8) has been investigated by Raman spectrometry.6,68,70 In an earlier exhaustive study based on ab initio and DFT calculations, we presented a detailed analysis of the vibrational properties of the protonated forms of 44BPY (44BPYH+ and 44BPYH22+).63 To our knowledge, only one Raman study has concerned the adsorption of dry pure 44BPY within dehydrated H-ZSM-5 zeolite.6 That is why we indicate in Table 2 the experimental values concerning the solid state of 44BPY63 and 44BPYH22+.69 Anyway, the observed frequencies of spectra in solution or in the solid phase are very similar (less than 6 cm-1). The Raman spectra of 44BPY and 44BPYH22+ are largely dominated by the totally symmetric modes, which are assigned without any ambiguity. Two in-plane B3 modes are also distinguishable in the Raman spectra: the CN and CC ring stretching mode 14 about 1250 cm-1, and the ring deformation mode 6b about 650 cm-1. Vibrational harmonic frequencies calculated with B3LYP method are usually systematically larger than the corresponding experimental values. They can be corrected by an empirical scaling factor, calculated, for each species, from the mean deviation of the computed frequencies from the observed values of the seven totally symmetric (A) in-plane modes in the 1700-600-cm-1 range, of which the assignment is certain. Whatever the species considered, a mean scaling factor of 0.980 has been determined. The scaled computed frequencies and frequency shifts for the A in-phase modes, the B1 corresponding out-of-phase modes below 1700 cm-1, and the two B3 modes 14 and 6b of 44BPY, and of 44BPYH22+ free or within the bidentate ion-pair complex formed with the 2-10T cluster model, only for the symmetrical (Al1, Al6′) distribution, are shown in Table 2 together with the experimental data. As can be seen from Table 2, a very good agreement is obtained between the scaled calculated frequencies and the experimental values for all species studied (mean absolute error of 8 ( 7 cm-1). The frequency shifts on going from 44BPY to 44BPYH2+ 2 either in the dibromide salt or occluded in H-ZSM-5 are also very well computed (mean deviation with respect to experiment of 5 ( 3 cm-1). The spectra of adsorbed species 44BPYH22+ exhibit features which are similar to those found for the bipyridinium dication in the dibromide salt. The vibrational frequencies of free and occluded 44BPYH22+ are quite identical (experimental shift of 3 ( 3 cm-1,69 and theoretical shift of 7 ( 5 cm-1), except for mode 14 (see below). This confirms the formation of the

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diprotonated species upon adsorption of 44BPY into the straight channel of H-ZSM-5, as observed by Moissette.6 All totally symmetric modes present nearly the same potential 63 energy distributions in 44BPYH2+ 2 as in 44BPY. The frequencies of these modes, except of mode 8a (CC ring stretching) and to a lesser extend of mode 19a (CN and CC ring stretching, and CH bending), are only slightly shifted on going from 44BPY to 44BPYH2+ 2 , in agreement with the experimental results (Table 2). The significant positive shift upon adsorption calculated for mode 8a is in very good agreement with experiment (∆νexp ) 42 cm-1, ∆νscal ) 45 cm-1). This shift is due to an electronic effect, and it may be compared to those already observed upon quaternization of the pyridyl rings.23,66 It reflects also the increase of the force constant related to the C2C3 stretching coordinate, principally involved for this mode. The less blue shift calculated for mode 19a, in perfect accordance with experiment, may be explained by the weak variations of the force constants relative to the CH bending coordinates predominantly implicated in this mode. The B1 corresponding out-of-phase modes present the same salient features, and the same remarks may be done. The inter-ring stretching mode Ω, which contains a large contribution of the C4C4′ bond stretching, is not significantly shifted upon adsorption. This indicates that quaternization has no meaningful influence on the inter-ring bond order. For B3 symmetry modes 6b and 14, the potential energy distributions are the same in the neutral molecule 44BPY and in the dication 44BPYH22+. Mode 6b, which is a pure in-plane deformation ring mode, shows a significant negative shift upon formation of the dication. This shift cannot be explained by the negligible variation of the force constant relative to the ring deformation coordinate. Mode 14, which is exclusively a CN and CC ring stretching mode, is only little shifted in free 44BPYH22+, in good agreement with experiment. On the other hand, it presents a significant shift in adsorbed 44BPY, a little overestimated by our calculation, but nevertheless in accordance with Raman results (∆νexp ) +18 cm-1, ∆νexp ≈ +28 cm-1). This difference between frequency shifts in free 44BPYH2+ 2 and in adsorbed 44BPYH22+ is related to the variations of CN and CC bond lengths (see above). Concerning the other in-plane vibrational modes of B3 and B2 symmetry, the lack of experimental results prevents any comparison. Nevertheless, we may expect a significant increase of the frequencies of 8b (ring stretching) and 19b (ring stretching, and NH and CH bending) modes, as found in the ion-pair complexes between pyridine and zeolitic clusters.23 To our knowledge, up to now, no experimental data is available concerning the monoprotonated species 44BPYH+. It should be noted that our previous theoretical study of the protonated forms of 44BPY63 allows to exclude the implication of 44BPYH+ in the Raman spectra observed by Moissette in H-ZSM-5 with small Si/Al ratios.6 IV. Conclusion The adsorption of 44BPY in the straight channels of H-ZSM-5 was investigated by B3LYP density functional calculations using two 10T-membered ring cluster to model the Brønsted acid sites of zeolite in order to provide insight into the nature of the species formed upon adsorption. Because of the size of the system studied, all adsorption characteristics, including vibrational frequencies, were calculated using a mixed basis set (631G(14T):6-31+G*(6T)) as in the ONIOM approach, while only the structural and energetic properties were obtained with the larger basis set 6-31+G*(20T). We considered the adsorption process proceeds through a concerted mechanism in which both

Kassab et al. nitrogen atoms of the bidentate molecule 44BPY bind simultaneously to the Brønsted acid sites of zeolite. Two aluminum distributions, (Al1, Al5′) and (Al1, Al6′), were chosen to study this concerted double proton transfer. For both Al distributions, our results showed that the double proton transfer from the Brønsted acid sites of zeolite to the nitrogen atoms of 44BPY immediately occurs upon adsorption. On the one hand, the potential energy surface calculated for this proton transfer reaction presents only one minimum corresponding to the formation of a bidentate ion-pair complex (44BPYH22+/Z2-). Thus, our calculations support the experimental observations that a specific diprotonated dication 44BPYH22+ is formed in H-ZSM-5 and exclude the presence of the neutral bidentate hydrogen bonded complex (44BPY/Z). However, we predict that the monoprotonated cation 44BPYH+ could be detected in zeolites of weak aluminum contents. The similar results obtained with the two basis sets MB and SB for structural and energetic properties allows us to explore more realistic clusters. On the other hand, all frequency values of adsorbed 44BPYH22+ are computed to be quite similar to those of the free diprotonated dication. Most of these frequencies are slightly shifted with respect to neutral molecule, except of 8a ring stretching mode which is strongly up-shifted by 44 cm-1, in good agreement with the available Raman data. References and Notes (1) Busca, G. Catal. Today 1998, 41, 191. (2) Parker, L. M.; Bibby, D. M.; Burns, G. R. J. Chem. Soc,. Faraday Trans. 1991, 87, 3319. (3) Buzzoni, R.; Bordiga, S.; Ricchiardi, G.; Lamberti, C.; Zecchina, A.; Bellussi, G. Langmuir 1996, 12, 930. (4) Barzetti, T.; Selli, E.; Moscotti, D.; Forni, L. J. Chem. Soc., Faraday Trans. 1996, 92, 1401. (5) Daniell, W.; Topsøe, N.-Y.; Kno¨zinger, H. Langmuir 2001, 17, 6233. (6) Moissette, A.; Batonneau, Y.; Bre´mard, C. J. Am. Chem. Soc. 2001, 123, 12325. (7) Nishi, K.; Kamiya, N.; Yokomori, Y. Microporous Mesoporous Mater. 2007, 101, 83. (8) Suzuki, K.; Aoyagi, Y.; Katada, N.; Choi, M.; Ryoo, R.; Niwa, M. Catal. Today 2008, 132, 38. (9) Floria´n, J.; Kubelkova´, L. J. Phys. Chem. 1994, 98, 8734. (10) Floria´n, J.; Kubelkova´, L.; Kotrla, J. J. Mol. Struct. 1995, 349, 435. (11) Kubelkova´, L.; Kotrla, J.; Floria´n, J. J. Phys. Chem. 1995, 99, 10285. (12) Lee, C.; Parrillo, D. J.; Gorte, R. J.; Farneth, W. E. J. Am. Chem. Soc. 1996, 118, 3262. (13) Bore´ave, A.; Auroux, A.; Guimon, C. Micropor. Mater. 1997, 11, 275. (14) Barthos, R.; Lo´nyi, F.; Onyestya´k, G.; Valyon, J. J. Phys. Chem. B 2000, 104, 7311. (15) Armaroli, T.; Bevilacqua, M.; Trombetta, M.; Alejandre, A. G.; Ramirez, J.; Busca, G. Appl. Catal., A 2001, 220, 181. (16) Busch, O. M.; Brijoux, W.; Thomson, S.; Schu¨th, F. J. Catal. 2004, 222, 174. (17) Buc˘ko, T.; Hafner, J.; Benco, L. J. Phys. Chem. 2004, 120, 10263. (18) Jiang, N.; Yuan, S.; Wang, J.; Jiao, H.; Qin, Z.; Li, Y.-W. J. Mol. Catal. A 2004, 220, 221. (19) Injan, N.; Pannorad, N.; Probst, M.; Limtrakul, J. Int. J. Quantum Chem. 2005, 105, 898. (20) Soscu´n, H.; Castellano, O.; Hernandez, J.; Arrieta, F.; Bermu´dez, Y.; Hinchliffe, A.; Brussin, M. R.; Sanchez, M.; Sierraalta, A.; Ruette, F. J. Mol. Catal. A 2007, 278, 165. (21) Nieminen, V.; Sierka, M.; Murzin, D. Y.; Sauer, J. J. Catal. 2005, 231, 393. (22) Tuma, C.; Sauer, J. Angew. Chem., Int. Ed. 2005, 44, 4769. (23) Castella`-Ventura, M.; Akacem, Y.; Kassab, E. J. Phys. Chem. C 2008, 112, 19045. (24) Joly, J. P.; Perrard, A. Langmuir 2001, 17, 1538. (25) Zheng, A.; Chen, L.; Yang, J.; Zhang, M.; Su, Y.; Yue, Y.; Ye, C.; Deng, F. J. Phys. Chem. B 2005, 109, 24273. (26) Ehresmann, J. O.; Wang, W.; Herreros, B.; Luigi, D.-P.; Venkatraman, T. N.; Song, W.; Nicholas, J. B.; Haw, J. F. J. Am. Chem. Soc. 2002, 124, 10868.

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