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2006, 110, 2963-2967 Published on Web 01/27/2006
Confined Space-Controlled Olefin-Oxygen Charge Transfer in Zeolites Evgeny A. Pidko* and Rutger A. van Santen Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, NL-5600 MB EindhoVen, The Netherlands ReceiVed: August 23, 2005; In Final Form: January 13, 2006
Density functional theory (DFT) calculations on the initial charge-transfer step in the photo-oxidation of alkenes in cationic zeolites are presented. The model system used represents a part of the Y-zeolite supercage containing two II sites with alkali-earth cations and coadsorbed 2,3-dimethylbutene-2 (DMB) and O2 on them. It is found that the electrostatic field of the zeolite cavity plays only a minor role for the stabilization of a charge-transfer state, whereas the relative orientation and the distance between the DMB and O2 molecules are the most important factors. On the basis of these results, the photo-oxidation considered is due to a confinement effect in which the reagents are oriented in a suitable “pre-transition state” configuration.
Introduction Recent photochemical studies of alkene-oxygen gas mixtures loaded in alkaline- and alkali-earth-exchanged zeolites revealed that selective partial oxidation can be induced by visible light.1-9 The UV-visible spectroscopic studies6,10,11 have shown that a weak continuous absorption tail in the visible extending into the red spectral region appears only when both olefin and O2 are present in the zeolite. This absorption is responsible for the light-induced oxidation of olefin. The corresponding alkene‚O2 contact charge-transfer bands in the liquid phase12 and in a solid oxygen matix13,14 are well established and lie in the UV spectral range. For instance, the continuous absorption band attributed to the olefin‚O2 charge transfer in the case of 2,3-dimethylbutene-2 trapped in solid O2 was detected at about 380 nm,13,14 whereas the same molecules loaded in NaY zeolite exhibit the charge-transfer band at 750 nm.10 It has been proposed that the interaction of the alkene‚O2 contact pair with the strong electrostatic field of the cation-exchanged zeolite upon coadsorption results in a very strong stabilization of the charge-transfer state. Such stabilization is thought to cause the large red shift of alkene‚O2 contact charge-transfer transitions from the UV range into the visible range. The excitation of the alkene‚O2 contact pair results in the formation of an [alkene+‚O2-] charge-transfer state. However, when the hydrocarbon molecule is coordinated to a positively charged cation, this excited state will be strongly destabilized. On the other hand, one expects a very strong stabilization of the charge-transfer state when the O2 molecule is adsorbed by the cation, while the hydrocarbon is located elsewhere far from the positively charged cationic adsorption sites and, at the same time, in the vicinity of oxygen. This picture is not easily reconciled with the experimental fact that the adsorption of alkenes by the cation-exchanged zeolites is much stronger * Corresponding author. E-mail:
[email protected]. Phone: +31 40 247 2189. Fax: +31 40 245 5054.
10.1021/jp0547562 CCC: $33.50
compared to O2 adsorption and, hence, the alkene will replace the adsorbed oxygen. To clarify this, a density functional theory (DFT) study of a model system containing 2,3-dimethylbutene-2 and O2 adsorbed in the Ca-, Mg-, and Sr-exchanged supercage of faujasite was performed. The calcium form of zeolite was chosen as a main object for our investigation, since the ionic radius of Ca2+ is similar to that of the Na+ ion, which was used in the experimental studies. On the other hand, the cluster that models the FAU zeolite exchanged with bivalent cations can be chosen to be smaller, which reduces computational requirements. Computational Details The M2Al4Si12O20H24 clusters, where M is Mg, Ca, and Sr, shown in parts a-c of Figure 1, respectively, were chosen for our model DFT calculations. In the following, these structures will be designated as MZ (M ) Mg, Ca, or Sr). The cluster represents a part of the wall of the faujasite supercage, containing two 6T rings (II sites) with a corresponding alkalineearth cation in each 6T ring, connected via three adjacent 4T silicon rings. To stabilize exchanged alkaline-earth cations, each 6T ring contains two aluminum atoms. The starting geometry of the clusters corresponds to the lattice of FAU zeolite according to X-ray diffraction (XRD) data.15 Dangling bonds are terminated by H atoms located 1.4 Å from each terminal Si atom and 1.5 Å from each terminal Al atom oriented in the direction of the next T (tetrahedral) site. Full geometry optimization was performed either for cluster models with adsorbed 2,3-dimethylbutene-2 (DMB) and O2 molecules or for clusters themselves, while the positions of boundary H atoms were fixed according to the initial coordinates. The quantum chemical calculations were carried out within the density functional theory (DFT) using the Gaussian 0316 program at the B3LYP/LanL2DZ level.17 The computed adsorption energies were corrected for basis set superposition error (BSSE) (EBSSE) using the counterpoise method.18 The spin state of O2 was assumed to be triplet in all of the computations presented below. © 2006 American Chemical Society
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Figure 1. Optimized structures and selected bond lengths (Å) of (a) MgZ, (b) CaZ, and (c) SrZ cluster models.
The energy of the electron transfer was estimated at the same computational level as the geometry optimization using the timedependent DFT method that is implemented in the Gaussian 03 program package. Partial optimization of the DMB‚O2 complex with a fixed distance between one of the O atoms and a carbon from the CdC bond was performed in order to compare computational results with those obtained experimentally14 for the complex stabilized in a solid O2 matrix. Results Figure 2b shows the optimized structure ([DMB‚O2]/CaZ) of 2,3-dimethylbutene-2 and dioxygen embedded in the CaZ cluster. The most important interatomic distances are also displayed in Figure 2. Coadsorption of these molecules to a single exchanged cation is energetically unfavorable due to the stronger adsorption of DMB (60 kJ/mol), which suppresses that of the O2 (14 kJ/mol). The energies of either individual adsorption or coadsorption of DMB and O2 are listed in Table 1. It is found that the DMB molecule is coordinated with the C1-C2 double bond to the Ca2+ cation (Ca2), whereas the O2 molecule is end-on adsorbed to another cation (Ca1). The changes, which occur in the geometry within the adsorbed molecules due to interaction with the alkaline-earth cations, are insignificant. This is consistent with the mainly electrostatic nature of the interaction of either DMB or O2 molecules with the alkaline-earth cations exchanged in the zeolitic cavity. For an adsorption complex in which the DMB is coordinated to one cation and the O2 molecule to the other, the computed energy of intermolecular DMB‚O2 charge transfer is equal to 1.67 eV (745 nm). The oscillator strength of it is f ) 0.078 (Table 2). The shapes of molecular orbitals involved in this electron excitation process are shown in Figure 3. One can see the interaction of the highest occupied molecular orbital (HOMO) of the DMB with the lowest unoccupied molecular orbital
(LUMO) of the dioxygen molecule. The orbitals involved in the charge-transfer process are, respectively, the bonding and antibonding molecular orbitals of a DMB‚O2 molecular complex formed in the zeolitic cage. They represent a linear combination of the occupied πβ-orbital of the DMB molecule and the unoccupied π*β-orbital of the O2 molecule. The finite value of the oscillator strength is due to their small but significant overlap. To investigate the effect of the electrostatic field in the zeolite cage on the stabilization of the charge-transfer state, the singlepoint time-dependent DFT calculation was performed for a free DMB‚O2 complex with exactly the same geometry as that in the [DMB‚O2]/CaZ system. In the absence of the zeolite framework, the computed charge-transfer energy (Table 2) slightly increases and becomes equal to 1.90 eV (653 nm), whereas the oscillator strength is only slightly lowered (f ) 0.057). Hence, we conclude that the low energy intermolecular charge transfer is due to the specific orientation between the DMB and O2 molecules in the ground state due to adsorption by the exchanged cations. To support this hypothesis, also coadsorption by Mg2+ and Sr2+ stabilized in the same cluster model was studied. The optimized structures and the most important interatomic distances for [DMB‚O2]/MgZ and [DMB‚O2]/SrZ are presented in parts a and c of Figure 2, respectively. The energies of individual adsorption and coadsorption of DMB and O2 by these clusters are listed in Table 1. Similar to the abovediscussed case of the CaZ model, coadsorption of 2,3-dimethylbutene-2 and dioxygen molecules by an isolated cation is improbable, because the former interacts much stronger with the cation (Table 1) due to the higher basicity of the alkene molecule compared with that of the O2. Similar trends in the coordination of the DMB and O2 molecule to the cations are detected. The alkene is adsorbed to the cation with the CdC bond, and the oxygen is adsorbed in an end-on fashion. However, in the case of the MgZ model, both the adsorbed
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J. Phys. Chem. B, Vol. 110, No. 7, 2006 2965
Figure 2. Coadsorption of 2,3-dimethylbuthene-2 and O2 on (a) MgZ, (b) CaZ, and (c) SrZ. All of the interatomic distances presented are in angstroms.
TABLE 1: Correcteda (∆EBSSE) and Uncorrected (∆E) for the BSSE Energies (kJ/mol) of Individual Adsorptionb and Coadsorptionc of O2 and DMB on MgZ, CaZ, and SrZ Clusters MgZ DMB/MZ O2/MZ [DMB‚O2]/MZ
CaZ
SrZ
∆E
∆EBSSE
∆E
∆EBSSE
∆E
∆EBSSE
63 31 91
44 15 55
81 31 110
60 14 72
86 28 108
65 9 70
a ∆E b BSSE ) ∆E - EBSSE. ∆E ) -{E([DMB‚O2]/MZ) - (E(DMB/ MZ) + E(O2/MZ))}. c ∆E ) -{E(molecule/MZ) - E(molecule/MZ)}.
TABLE 2: Optimized C‚‚‚O Distances (r) and the Estimated Energies (E) and Oscillator Strengths (f) of the Charge Transfer in the DMB·O2 Complex [DMB‚O2]/MgZ [DMB‚O2] from MgZa [DMB‚O2]/CaZ [DMB‚O2] from CaZa [DMB‚O2]/SrZ [DMB‚O2] from SrZa DMB‚O2 gas phase; partially optimizedb
rC‚‚‚O, Å
E, eV
f
3.957/3.807 3.957/3.807 2.877/2.860 2.877/2.860 3.782/3.754 3.782/3.754 2.150/2.655
0.80 1.25 1.67 1.90 1.21 1.41 2.69 4.22
0.007 0.005 0.078 0.057 0.008 0.007 0.099 0.070
a Data for the free DMB‚O complex with the geometry obtained 2 from the optimization of [DMB‚O2]/MZ (M ) Mg, Ca, or Sr). b Partially optimized gas phase DMB‚O complex with a constrained 2 C-O distance of 2.15 Å.
DMB and O2 molecules are located closer to the cations in the cluster and the distance between them is significantly larger. On the other hand, in the case of the [DMB‚O2]/SrZ structure, the DMB molecule is coordinated to both strontium ions (Sr1 and Sr2), while dioxygen is forced out from the cluster model
Figure 3. Shape of the calculated orbitals involved in the DMB‚O2 charge-transfer bonding (a) and antibonding (b) molecular orbitals of the DMB‚O2 complex.
and again the distance between the adsorbed molecules is significantly larger than in the case of the CaZ model. These effects are due to different ionic radii of the considered alkalineearth cations (Sr > Ca > Mg) and, hence, to the different space between the adsorbed molecules. Besides this, a strongly different relative orientation between the alkene and O2 is detected in the case of the magnesium containing cluster model. Instead of the end-on coordination of the dioxygen to the CdC bond of the DMB molecule adsorbed by CaZ and SrZ, the contacts between the O2′ atom from the O2 and protons from the methyl groups of the DMB molecule are found. The computed properties of the intermolecular DMB‚O2 charge transfer for [DMB‚O2]/MgZ and [DMB‚O2]/SrZ are listed in Table 2. One can see that the different relative configuration of the O2 molecule to the alkene strongly influences both the energy of the charge transfer and the oscillator strength. The increase of the distance between the
2966 J. Phys. Chem. B, Vol. 110, No. 7, 2006
Figure 4. Partially optimized structure of the free DMB‚O2 complex with a fixed C1-O1 distance at 2.150 Å.
DMB and O2 molecules results in a substantial decrease of the oscillator strength and, hence, of the probability of the intermolecular charge transfer. Surprisingly, increased separation of the adsorbed molecules from each other also results in a significant decrease of the energy of the corresponding electron excitation. Most likely, it is connected to the fact that with a larger intermolecular separation no DMB‚O2 complex is formed and, therefore, the ground state lies higher in energy. Also, one should note that we could not detect pure charge transfer between the DMB and O2 adsorbed on the MgZ cluster, since the lone pairs of the basic oxygens of the cluster (O5 and O6) are also involved. In the absence of the zeolite framework, in all considered cases, the corresponding charge-transfer band is blue-shifted and the oscillator strength slightly decreases (Table 2). The large difference between the charge-transfer energy in the free DMB‚O2 complex and that embedded in the MgZ cluster is most likely the result of the effect described above. Hashimoto and Akimoto14 estimated a distance of 4.1 Å between the positive (DMB) and negative (O2) ends of the dipole moment of the DMB‚O2 charge-transfer complex in a solid O2 matrix. One notes that this distance is realized when the shortest distance between one of the carbons (C1) of the CdC bond of DMB and the interacting O1 atom is equal to 2.15 Å (Figure 4). In this case, two charge-transfer absorption bands with rather high values of the oscillator strength were detected: 2.69 eV (462 nm, f ) 0.099) and 4.22 eV (294 nm, f ) 0.070). The latter value very well agrees with that reported in ref 14 (4.32 eV). On the other hand, the experimentally observed chargetransfer absorption spectrum14 exhibits a very broad absorption band overlapping both values. It is also noticeable that the model used does not take into account additional interactions of the complex with the other surrounding dioxygen molecules of the O2 matrix. Discussion A very strong decrease of the energy for intermolecular charge transfer between branched alkenes and oxygen embedded in alkaline-earth zeolites compared to the free state has been detected.6,10,11 Indeed, UV-visible spectroscopy of alkali- and alkaline-earth zeolite Y loaded with alkenes and O2 revealed a visible adsorption tail to be attributed to the hydrocarbon‚O2 contact charge-transfer transition.10 On the other hand, UV charge-transfer bands for similar contact complexes are wellknown in solid O2 matrixes.14 It has been suggested that the interaction of the high electrostatic field in cation-exchanged zeolites with the large dipole generated upon excitation of the hydrocarbon‚O2 to the charge-transfer state leads to the stabilization of the state by 1.5-3 eV. This results in a very strong red shift of the absorption from the UV region into the visible region.
Letters Indeed, the interaction of an O2 molecule with a cation strongly increases its electron affinity, but on the other hand, an adsorption of the hydrocarbon molecule to the exchanged cations would lead to a simultaneous increase of its ionization potential. Both of these effects cancel each other. It is known that alkenes are adsorbed on zeolites significantly stronger than oxygen. Thus, if we suppose that only the hydrocarbon molecules are adsorbed on the cation sites of zeolite and the O2 molecules are located elsewhere, the electrostatic field will be directed opposite to the direction of the charge transfer. Therefore, the observed absorption band should be blue-shifted compared to that for the gas phase. On the other hand, the results of our DFT model calculations presented above show that the estimated energies and the probabilities of the charge transfer between 2,3-dimethylbutene-2 and oxygen molecules with the same geometries are very close both in the presence and in the absence of the zeolite cluster. It is evident that the major factor in the shift of the experimental absorption band is the relative geometry of these molecules. The specific relative orientation, which is due to adsorption to the closely located cation sites, results in the formation of a molecular complex with the excitation energy of visible light. Absorption bands in the absence of the zeolite matrix were experimentally detected in a matrix of solid oxygen.13,14 In this case, the average distance between the hydrocarbon and oxygen molecules should be rather small and, in accordance with our theoretical results, this leads to a strong blue shift of the absorption band to the UV range and to a significant increase of the probability of the electron transition. The model DMB-O2 complex (Figure 4) has a distance of about 4.1 Å between the positive and negative ends of the dipole that are located at the opposite from the O2 part of the alkene and at the noninteracting oxygen atom (O2), respectively. It exhibits a charge-transfer band of 4.22 eV. This value agrees very well with that obtained experimentally (4.32 eV, ref 14). The specific orientation of the CdC double bond of the hydrocarbon to the O2 molecule results in the formation of a molecular complex with an overlap of the πβ-orbital of the DMB molecule and of the π*β-orbital of the O2 molecule. The optimum configuration was found in the case of coadsorption of these molecules on calcium-exchanged faujasite. The adsorption of the DMB and O2 on the nearest cation sites in the zeolite supercage results in their confinement in specific orientation that is suitable for a rather effective overlap of corresponding HOMOs and LUMOs and, hence, for the intermolecular charge transfer. One notes that the CsO distances (2.877 and 2.860 Å) in [DMB‚O2]/CaZ are significantly lower compared to the sum of corresponding van der Waals radii (3.1 Å19). On the other hand, when due to steric factors such a suitable configuration between the adsorbed molecules cannot be realized (as is found for [DMB‚O2]/MgZ and [DMB‚O2]/SrZ), the molecular complex is not formed and the effective charge transfer cannot be observed. Thus, one can expect a much lower activity of MgY and SrY zeolites in the photo-oxidation of 2,3-dimethylbuthene-2 in comparison with that of CaY. Conclusions The role of the zeolite in the photo-oxidation of alkenes with molecular oxygen is the complexation of the hydrocarbon and O2 to the exchanged cations, resulting in confinement of these molecules with a specific relative orientation. This leads to the formation of a π-π intermolecular complex. The interaction between alkene and oxygen in this complex occurs with a finite overlap of the involved orbitals. The formation of such a
Letters complex results in a significant transition moment of the intermolecular charge transfer. The role of the electrostatic field of the zeolite is only indirect. It stabilizes the reagents in a suitable “pre-transition state” configuration. Acknowledgment. R.A.v.S. acknowledges the University of California, Berkeley, for a Miller visiting professorship 2004 and for important discussions with Prof. H. J. Frei. References and Notes (1) Blatter, F.; Frei, H. J. Am. Chem. Soc. 1994, 116, 1812. (2) Blatter, F.; Sun, H.; Vasenkov, S.; Frei, H. Catal. Today 1998, 41, 297. (3) Sun, H.; Blatter, F.; Frei, H. J. Catal. Lett. 1997, 44, 247. (4) Sun, H.; Blatter, F.; Frei, H. J. Am. Chem. Soc. 1996, 118, 6873. (5) Sun, H.; Blatter, F.; Frei, H. Chem.sEur. J. 1996, 118, 6873. (6) Tang, S. L. Y.; McGarvey, D. J.; Zholobenko, V. L. Phys. Chem. Chem. Phys. 2003, 5, 2699. (7) Myli, K. B.; Larsen, S. C.; Grassian, V. H. Catal. Lett. 1997, 48, 199. (8) Larsen, R. G.; Saladino, A. C.; Hunt, T. A.; Mann, J. E.; Xu, M.; Grassian, V. H.; Larsen, S. C. J. Catal. 2001, 204, 440. (9) Xiang, Y.; Larsen, S. C.; Grassian, V. H. J. Am. Chem. Soc. 1999, 121, 5063. (10) Blatter, F.; Moreau, F.; Frei, H. J. Phys. Chem. 1994, 98, 13403.
J. Phys. Chem. B, Vol. 110, No. 7, 2006 2967 (11) Vasenkov, S.; Frei, H. J. Phys. Chem. B 1997, 101, 4539. (12) Coomber, J. W.; Hebert, D. M.; Kummer, W. A.; Marsh, D. G.; Pitts, J. N., Jr. EnViron. Sci. Technol. 1971, 4, 1141. (13) Hashimoto, S.; Akimoto, H. J. Phys. Chem. 1986, 90, 529. (14) Hashimoto, S.; Akimoto, H. J. Phys. Chem. 1987, 91, 1347. (15) Olson, D. J. J. Phys. Chem. 1970, 74, 2758. (16) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003. (17) Becke, A. D. Phys. ReV. 1988, A38, 3098. Becke, A. D. J. Chem. Phys. 1993, 98, 1372. Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (18) Simon, S.; Duran, M.; Dannenberg, J. J. J. Chem. Phys. 1996, 105, 11024. (19) Bondi, A. J. Phys. Chem. 1964, 68, 441.
