Origin of the Enhanced Interaction of Molecular Hydrogen with

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J. Phys. Chem. C 2010, 114, 13926–13934

Origin of the Enhanced Interaction of Molecular Hydrogen with Extraframework Cu+ and FeO+ Cations in Zeolite Hosts. A Periodic DFT Study Xavier Solans-Monfort,† Mariona Sodupe,*,† and Juergen Eckert*,‡ Departament de Quı´mica, UniVersitat Auto`noma de Barcelona, Bellaterra 08193, Spain, and Materials Research Laboratory, UniVersity of California Santa Barbara, Santa Barbara, California 93106 ReceiVed: May 7, 2010; ReVised Manuscript ReceiVed: June 23, 2010

The interactions of hydrogen molecules with Cu+ and FeO+ extraframework cations in chabazite were studied using periodic DFT (B3LYP) calculations. Dispersion forces were accounted for by adding Grimme’s correction to the DFT energy at the B3LYP-optimized geometries. Two potential binding sites are found for FeO+ cations inside chabazite as for Cu+. The preferred site I is located in the middle of the six-membered ring, where FeO+ coordinates to three oxygen atoms of the zeolite framework, while in the less stable site IV FeO+ coordinates to two oxygen atoms of the eight-membered ring (∆E ) 17.3 kJ mol-1). The interaction of H2 with FeO+ (Eads ) -17.5 kJ mol-1) is weaker than that with Cu+ (Eads ) -85 kJ mol-1), so that binding is only favorable at the more open site IV, where no framework reorganization is needed. For both CuCHA and FeOCHA, the geometry of the adsorbed H2-M+ complex is governed by M d f H2 σ* backdonation, which also contributes to a large extent to the final adsorption energy and controls the barrier to H2 rotation. The M-H distance, the polarization of the metal d orbital involved in the back-donation toward the H2 molecule, as well as the H2 charge density show that the M d f H2 σ* back-donation is larger in H2-Cu+ than in H2-FeO+. Both the binding energy and the barrier to rotation are therefore much greater for H2-CuCHA than for H2-FeOCHA (15.1 vs 7.4 kJ mol-1), in agreement with the available experimental data. A comparison between site I and site IV along with results on the possible adsorption of a second H2 molecule per cation site suggests that the different adsorption sites observed by experiment are more likely due to the presence of different metal cation environments rather than binding of more than one H2 per cation site. Introduction It has become widely recognized that the potential utilization of porous materials as a medium for hydrogen storage in mobile applications requires a substantial increase in binding energies relative to those available by simple physisorption, and this is likely to be achievable only by the better use of metals incorporated in such porous, hybrid materials.1-5 A considerable number of coordination polymers have therefore been reported to possess so-called open binding sites at metal ions that are part of the framework.1,6-11 These unsaturated metal binding sites are generated by the thermal removal of a ligand (typically water) from the in-framework metal. Some of these compounds have indeed been determined to have isosteric heats of adsorption that exceed 10 kJ/mol at low hydrogen loadings,6,7 but these values are still far short of what is required for ambient temperature operation of a storage system (>20 kJ/mol),5 and of what is in fact available from coordination of H2 to a metal center in organometallic compounds1,12-15 or even naked metal cations.16-19 The reason for this is that the hydrogen molecule does not approach the open in-framework metal sites sufficiently closely to form such a metal-dihydrogen complex, the cause of which may be of both steric and electronic nature. This has been demonstrated in several powder neutron diffraction studies,6,7,20-23 where, for example, a H2 to metal distance of 2.39 Å was found for the Cu site in HKUST-1,23 or 2.6 Å for * Corresponding authors. E-mail: [email protected] (M.S.); [email protected] (J.E.). † Universitat Auto`noma de Barcelona. ‡ University of California Santa Barbara.

Zn in MOF-74.22 Formation of a metal-dihydrogen complex, on the other hand, requires the H2 molecule to approach the metal center to within 1.6-1.7 Å.13,24 Zeolites may be viewed as a unique and highly suitable platform for systematic investigations of the effect on hydrogen binding of a variety of charge-compensating extraframework cations or other types of metals sites that can easily be introduced into some zeolites such as ZSM-5.25 Some of us have, in fact, previously reported the first example26 of nondissociative, molecular chemisorption of hydrogen in a porous material, socalled “overexchanged” Fe-ZSM-5. This conclusion was based on a comparison of the inelastic neutron scattering spectra (INS) of the rotational tunneling transitions, which closely resemble those of some well-characterized metal dihydrogen complexes of Fe.27,28 The existence of such molecular dihydrogen complexes in coordination compounds was first demonstrated by Kubas who prepared the tungsten dihydrogen complex W(η2H2)(CO)3(PCy3)2 in 1984.29,30 A large number of both neutral and cationic complexes of many transition metals have since been characterized and are described in an extensive review by Kubas.24 We note, however, that stable molecular dihydrogen complexes with first row transition metals are rare apart from the notable exception of Fe,27,28,31 while those with Cu or Ni have never been isolated. Despite the large number of studies on FeZSM5,32-36 the nature of the extraframework iron species in overexchanged FeZSM5 is unfortunately still not well-known, which in turn makes it difficult to rationalize the nature of H2 chemisorption. In fact, it has been suggested that many species can coexist and that

10.1021/jp104175n  2010 American Chemical Society Published on Web 07/22/2010

Interaction of Hydrogen with Cu+ and FeO+ Cations their relative abundance may be dependent on the way the FeZSM5 is prepared. FeO+, Fe(OH)2+, and other di-iron cations with bridged oxo or hydroxo ligands are the most frequently proposed species. More recently, we have confirmed37,38 with the use of inelastic neutron scattering (INS) that H2 binds molecularly in Cuexchanged ZSM-5 and forms an η2 complex with monovalent Cu-ions at two well-defined adsorption sites, which are associated with H2 rotational tunneling frequencies of (0.80 and (1.37 cm-1. These findings are in accord with prior IR and computational studies,39,40 which also provided evidence for the existence of such molecular chemisorption of hydrogen at Cu sites in ZSM-5. The interaction of H2 with Fe-ZSM-5 apparently is considerably weaker than that in Cu-ZSM-5, as the rotational tunneling lines occur at higher frequency (4.2 and 8.3 cm-1), that is, with lower rotational barrier, than in Cu-ZSM-5.26,37 Isosteric heats of adsorption for H2 in Cu-ZSM-5 were determined from room temperature adsorption isotherms and were found to be as high as 74 kJ/mol.38 This value is in qualitative agreement with our earlier calculations40 and more than 5 times higher than those in MOFs with open metal sites.1,20 Some of the underlying reasons for such differences were recently discussed on the basis of extensive INS studies of a wide range of ion-exchanged zeolites.25 These include not only the type of metal cation, and its oxidation state, but perhaps more important are the location, coordination, and hence the accessibility of the metal binding site to the relatively hydrogen molecule. This study also reported the first INS spectra for H2 adsorbed at Cu in the six-ring window site (similar to site I in chabazite) in partially ion-exchanged zeolite CuNaA with a tunneling transition observed at 2.3 meV. With the aim of understanding the nature of the interaction of H2 in Cu-ZSM5 and Fe-ZSM5, and to rationalize the observed differences, we have carried out a theoretical study on the adsorption of one and two H2 molecules at the binding sites of Cu+ and FeO+ cations in chabazite. The use of computational chemistry for providing insight into the interactions of H2 with porous materials (such as zeolites or MOFs) is now quite common,40-47 but most systems studied do not include open-shell transition metal cations. We have taken the FeO+ cation as the most active extraframework species in Fe-ZMS-5 based on previous studies of Bell et al., which have shown that this species binds N2O molecules more strongly than any others that were considered.48-50 We have also chosen chabazite as the model for the zeolite ZSM-5 because it has a much smaller unit cell than ZSM-5, which makes it possible to perform full ab initio periodic calculations at reasonable computational cost. This strategy has been successfully used in several previous computational studies of metal exchanged zeolites.40,43,44,51,52 We have also computed the energies of the barrier to rotation and compared the results to those derived from experimental measurements of the hindered rotor transitions of the bound H2 molecules by inelastic neutron scattering (INS). Finally, we have evaluated the possibility that a second H2 molecule can be bound at each cation site. Computational Details The chabazite framework was modeled using periodic boundary ab initio calculations on chabazite with Si/Al ) 11/1 (MCHA) (Figure 1). The periodic model is constructed by substituting one of the 12 equivalent Si atoms of the all-silica chabazite with an Al atom. The resulting negative charge is compensated by one Cu+ or one FeO+ cation per unit cell. We would like to point out that the ZSM-5 samples whose INS

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Figure 1. Zeolite models and metal cation siting sites inside chabazite framework.

spectra and isosteric heats of adsorption we use for comparison have a Si/Al ratio between 17 and 26,26,37 so that a Si/Al ratio of 11 is the closest we can obtain with the chabazite model. Both the atomic positions and the lattice parameters were fully optimized in the periodic calculations, in contrast to our previous study on H2-CuCHA40 in which the cell parameters were kept fixed. The Brillouin zone was sampled by 8 reciprocal k points. All calculations were performed with the hybrid B3LYP density functional53,54 with a double-ζ plus polarization quality basis sets for H, O, Si, and Al. A modified all-electron Ahlrich’s TZV(P) basis set was used for Cu,55 and a modified all-electron Ahlrich’s VTZ(P) basis set was used for iron.56 This basis set is equivalent to that used in our previous studies on adsorption processes in chabazite40,52,57 and, hereafter, will be referred to BS1. To analyze the effect of using even larger basis sets, we also performed calculations on 5T and 13T clusters (Figure 1b,c). A basis set called BS2 was used in these cluster calculations, which consists of the Dunning’s aug-cc-pVDZ basis set58 for H, O, Si, and Al and the all-electron WachtersHay basis, supplemented by diffuse and polarization functions for Cu and Fe.59 Because standard DFT methods such as B3LYP do not properly account for dispersion forces, the interaction energies obtained were corrected with Grimme’s empirical term (D),60,61 which has been shown to properly describe systems where dispersion forces are relevant,62-66 including adsorption energies in silica-based materials52,67,68 and other related systems.69,70 The C6ij parameters associated with the Cu+ and formally Fe3+ cations were set to zero because the reported values correspond to neutral atoms, which possess much larger polarizabilities than those of the cations. The s6 term was fixed at a value of 1.05 as suggested for B3LYP.60 The final adsorption energies are based on potential energies without including zero-point energies and are composed of two terms, the interaction energy (EINT) related to the energy difference between the adsorbed complex and the separated species as obtained from B3LYP and the dispersion term computed with the Grimme’s correction (D). The EINT term is also corrected for basis set superposition errors using the counterpoise procedure (CP) on the optimized geometries.71 Barriers to rotation for H2 molecule interacting with MCHA at site IV (vide infra for site definition) (M ) Cu+ or FeO+)

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Solans-Monfort et al.

SCHEME 1

were evaluated as follows. First, we located the rotational transition state using the 5T cluster model (Figure 1b). Because differences in the geometries between the cluster complex and its corresponding rotational transition state are mainly centered on the M-H2 fragment, we computed the potential energies at different values of the two torsion angles defined by H1X3M4O5 and H2X3M4O5 in Scheme 1, from 0° to 180° for H1X3M4O5 and from 180° to 0° for H2X3M4O5, in steps of 10°. In the course of this rotation, only the M-H2 fragment was allowed to relax (M ) Cu+ or FeO+); that is, the only parameters optimized upon rotation were the distances r and R shown in Scheme 1. The variation of these geometrical parameters (r and R) was then transferred from the cluster to the periodic model to obtain the barrier to rotation, which includes the effect of the framework.72 In the case of H2 interacting with Cu+ cations at site I, an equivalent strategy was adopted, but only the transition state geometry was taken into account because of the greater complexity of the system and the higher computational cost. The 13T cluster shown in Figure 1 was used in this case as a finite model for localizing the rotational transition state structure. The variations on the parameters r, R, and R (Scheme 1) between the adsorption complex and its rotational transition state in the cluster model were subsequently transferred to the periodic model for evaluating the rotational barrier, which includes the effect of the framework. Results and Discussion Our results are presented in the following manner. First, we will briefly discuss the siting of the cations inside the chabazite framework (MCHA where M ) Cu+ or FeO+). Second, we will describe the adsorption of one H2 molecule at each of these cation sites in the zeolite framework and evaluate the barrier associated with the H2 rotation. These results are then compared to those previously derived from the rotational tunneling INS spectra.26,37 Finally, the possibility of adsorption of a second H2 molecule at these sites will be discussed. We would like to point out that Cu+ is a closed-shell metal cation with a 3d10 (1S) electronic ground state while FeO+ is an open-shell system with a 6Σ+ ground state.73-75 Calculations were therefore carried out considering a singlet spin multiplicity for Cu+-CHA systems and a sextet high spin state for FeO+-CHA. Quartet states were not considered for the latter system because previous studies have shown the electronic ground state49 to be one of high spin multiplicity. Moreover, our preliminary 5T cluster calculations also confirm the high spin state to be the ground state for FeO+5T. Cation Siting in MCHA (M ) Cu+ and FeO+). Previous optimization with a constrained unit cell40 as well as with an unconstrained cell52 has shown that Cu+ can be found in two different extraframework sites in chabazite (see Figure 1): (i) site I in which Cu is located in the middle of the six-membered ring, coordinated to three different oxygen atoms with Cu · · · O distances ranging from 2.03 to 2.23 Å, CuCHA(I), and (ii) site IV, in which Cu+ is bound to two oxygen atoms with Cu · · · O

Figure 2. Cation sitings in MCHA (M ) Cu+ and FeO+). B3LYPoptimized distances (in angstroms) and relative energies (in kJ mol-1).

distances of 2.07 and 2.26 Å in the channel formed by the eightmembered ring CuCHA(IV). The energy difference between the two sites is 40.5 kJ mol-1 with CuCHA(I) being the preferred site. The same two sites were considered for siting of the FeO+ cation. The corresponding (FeOCHA(N), N ) I or IV) optimized geometries are shown in Figure 2. In site I, the Fe atom is positioned in the center of the six-membered ring coordinated to three oxygen atoms of the zeolite framework, as in CuCHA(I) but with somewhat larger M · · · O distances (2.12, 2.29, and 2.30 Å). The O atom of FeO+ is oriented toward the cavity of chabazite to reduce repulsion with the oxygen atoms of the sixmembered ring. The FeO+ distance (1.67 Å) is slightly larger than that found for an isolated FeO+ cation (1.64 Å).73,76 The FeO+ cation in site IV is only coordinated to two oxygen atoms of the zeolite framework with Fe · · · O distances of 2.09 and 2.10 Å as in the case of Cu+.40 Again, the O atom of FeO+ cation is located as far as possible away from the zeolite framework pointing toward the chabazite cavity, which indicates that the interaction between the O atom and the zeolite framework is repulsive. The Fe-O distance of 1.66 Å is very similar to that computed for FeOCHA(I) and is in agreement with previous cluster models where FeO interacts with two oxygen atoms of the zeolite.50,77-79 The higher coordination of FeO+ in site I makes this location energetically the most favorable as in the case of Cu+. However, the preference for site I is considerably less pronounced in FeOCHA as the energy difference between the two sites is only 17.3 kJ mol-1 (Figure 2). Binding of H2 in CuCHA and FeOCHA. Adsorption of one H2 molecule per copper site has previously been studied using cluster37,80 or ab initio periodic calculations.40,81 H2 sorption in CuCHA was studied by some of us40 by performing optimizations with a constrained unit cell. We will therefore only summarize the effect of full optimizations of the atomic positions and cell parameters, and the inclusion of dispersion forces to the total adsorption energy. Relaxation of the cell parameters

Interaction of Hydrogen with Cu+ and FeO+ Cations

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Figure 3. B3LYP-optimized structures for H2-MCHA complexes. Distances are in angstroms.

TABLE 1: First and Second H2 Adsorption Energies (in kJ mol-1) in MCHA(I) and MCHA(IV) (M ) Cu+ or FeO+)a H2-CuCHA(I) H2-CuCHA(IV) H2-FeOCHA(I) H2-FeOCHA(IV) (H2)-(H2)CuCHA(I) (H2)-(H2)CuCHA(IV)sp (H2)-(H2)CuCHA(IV)Td (H2)2-(H2)FeOCHA(IV)

Eads(1)b

EINT

D

-15.4 -70.5 (-85.0)c -9.0 -12.0 (-17.5)c -7.8 -5.5 (-8.0)c +2.6 -5.4 (-10.5)c

-7.2 -58.2 -0.7 -2.3 +3.4 +2.4 +11.4 +1.8

-8.2 -12.3 -8.3 -9.7 -11.2 -7.9 -8.8 -7.2

a The total adsorption energy (Eads) has been decomposed into two terms: the interaction energy (Eint), which includes counterpoise correction to BSSE (ref 71) and the dispersion correction (D). b Eads ) (EH2-CuCHA - EH2 - ECuCHA) - CP + D. c Estimated value considering basis set effect on the small complex.

Figure 4. Metal d orbitals involved in H2-M+ back-donation (M+ ) Cu+ (a,b) or FeO+ (c,d)).

was found not to change the main features of the adsorption processes. The optimized structures are displayed in Figure 3, and the associated adsorption energies are summarized in Table 1. Binding of H2 occurs both at sites I and IV by means of η2-side-on coordination of H2 in the O-Cu-O plane. The H-H bond is in fact activated by the formation of this dihydrogen complex as shown by the fact that it lengthens by about 0.06 Å. This is because of the back-donation from the dπ doubly occupied orbital of Cu (Figure 4a) to the σ* empty orbital of H2, as described in ref 40. The adsorption process at site I, however, requires an important geometry reorganization; that is, the Cu+ cation must move out of the plane of the six-membered ring to allow H2 to interact with Cu in a plane, which is defined by Cu+ and the two oxygen atoms of the zeolite

that are coordinated to it. Comparison with the previous results using a constrained unit cell40 shows that optimization of the cell produces greater stabilization of Cu+ at site I, which disfavors the adsorption of H2 at this site by about 5 kJ mol-1. This suggests that relaxation of the cell parameters favors the placement of Cu+ cation in the center of the six-membered ring. Interaction of H2 at site IV, on the other hand, requires a much smaller reorganization of the zeolite framework structure, and the adsorption energy at this site (-70.5 kJ mol-1) was found to be considerably higher than that at site I (-15.4 kJ mol-1). This difference in energy arises mainly from the EINT term (-58.2 vs -7.2 kJ mol-1). Furthermore, while the larger basis set for the 5T cluster model does not produce significant changes in geometry (see Figure S1 of the Supporting Information for further details), the adsorption energy shows a remarkable dependence on the basis set. Overall, our best estimate of the adsorption energy at site IV after including dispersion corrections and the effect of the larger basis set is 85 kJ mol-1, 11 kJ mol-1 greater than the experimental value.38 The interaction of H2 with FeO+ is considerably weaker than that with Cu+. This fact along with the greater hindrance of the zeolite framework at site I prevents the coordination of H2 to the iron cation of FeOCHA. In fact, the only minimum of the potential energy surface we have been able to locate corresponds to a H2 molecule weakly interacting with the O atom of the oxo group by an η1-end-on bent structure (Figure 3). Similar structures have been described for the interaction between H2 molecules and electron-rich atoms or anions.45,82-84 The FeO · · · H2 distance is 2.35 Å, and the geometrical parameters of both FeOCHA(I) and H2 fragments are not significantly affected by their interaction in agreement with a rather low adsorption energy of -9.0 kJ mol-1, which arises mainly from dispersion forces (-8.3 kJ mol-1). Binding of H2 at site IV occurs by coordination in the form of a distorted trigonal pyramid around the Fe3+ cation with an η2-side-on H2 in the apical position, coplanar with the FeO+ cation and relatively far from the metal center. The Fe · · · H2 distances are 2.22 and 2.29 Å (Figure 3). The H2 and FeOCHA(IV) fragments of the sorption complex therefore have an optimized structure almost identical to that before adsorption. The interaction between FeO+ and H2 in FeOCHA(IV) is much weaker (-12.0 kJ mol-1) than that observed in CuCHA(IV). The improved basis set produces an important shortening of the Fe · · · H2 distances (see Figure S2 of the Supporting Information for further details) and a non negligible increase of the interaction energy by 5.5 kJ mol-1, which in turn leads to our best estimate for H2 adsorption at FeOCHA(IV) of -17.5 kJ mol-1. We have previously described how H2 adsorption in CuCHA40 involves a significant charge transfer from copper to H2, which is responsible for the increase in the H-H distance and the large bathochromic shift of the H-H stretching frequency. This interaction is significantly different and much weaker in the case of H2-FeOCHA. H2 interacts at site I with the O atom of FeO by an η1-end-on bent structure, which maximizes the electrostatic interaction between the negatively charged O atom and the positive region of H2 quadrupole moment. Binding at site IV, on the other hand, takes place with the iron center in a η2side on orientation, which favors the interaction between the positively charged Fe center and the negative region of the H2 quadrupole moment. At this site, the coplanar orientation of H2 with respect to FeO+ is governed by back-donation between the mono-occupied dπ orbital of the FeO+ fragment and the empty σ* of H2 (Figure 4c). This back-donation is more efficient

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than the one that takes place in an orientation perpendicular to the dδ orbital (Figure 4d). This provides a barrier to rotation for H2, which may then be related to results from the inelastic neutron scattering measurements. Barrier to H2 Rotation and INS Spectra. An important aspect of our theoretical results for hydrogen binding at Cu and FeO+ sites in chabazite is that they can be related in a fairly direct way to experimental measurements of the hindered rotor transitions of the bound H2 molecules, which can readily be observed by inelastic neutron scattering (INS). This can be accomplished by deriving a barrier to H2 rotation from the theoretical results and comparing it to the one obtained from the INS measurements. In the latter case, the lowest observed transition (rotational tunneling transition) is related to the eigenvalues of the rotational Schro¨dinger equation for planar rotation of a dumbbell molecule in a double-minimum potential if the molecule forms a dihydrogen complex with the metal. In the case where the H2 molecules are merely physisorbed, or very weakly bound, the transitions are calculated by a phenomenological model for H2 in a double-minimum potential with two angular degrees of freedom instead. The calculated energy profile for the rotation of H2 in CuCHA at site IV is shown in Figure 5. The point of highest energy in the course of the rotation represents H2 interacting with Cu+ through a η2-side-on structure, which is symmetrically rotated by 90° with respect to the dihydrogen complex so that the H2 molecule is perpendicular to the (OZ)2Cu plane (Figure 5). This rotation produces an appreciable shortening of the H-H bond by 0.02 Å, which is associated with an elongation of the Cu · · · H distance by 0.04 Å. These changes result from the fact that in this orientation the only metal orbital able to participate in backdonation (Figure 4b) to the σ* H2 orbital is considerably less polarized by the zeolite framework than the one (dπ) (Figure 4a) involved in back-donation in the minimum energy structure. At 90°, therefore, back-donation is smaller, and, consequently, the Cu · · · H distance is longer and the H-H bond is shorter. The reduction in back-donation in the transition state structure is reflected by a relatively high barrier of 15.1 kJ mol-1. This value is in agreement with the previously published computed energy barriers obtained with cluster models (∆Eq ) 13.0 kJ mol-1) and reasonably close to the barrier derived from experiment (∆Eq ) 9 kJ mol-1).37 Dihydrogen rotation in H2-FeOCHA(IV) also induces a lengthening of the Fe · · · H distances ranging from 0.09 to 0.16 Å (see Figures 3 and 5), while the H-H bond length remains almost unchanged. The reason for this is the fact that backdonation is rather small for this Fe-dihydrogen complex (Figure 4c) so that rotation (Figure 4d) of the H2 does not induce significant changes in the H-H bond length. Because the H2 interaction with the metal cation is weaker and less sensitive to their relative orientation in H2-FeOCHA(IV) than in H2CuCHA(IV), the corresponding barrier to rotation is considerably smaller in the former. Our results indeed give a rotational barrier of 7.4 kJ mol-1 in H2-FeOCHA, which is almost 8 kJ mol-1 lower than that obtained for H2-CuCHA. This is in agreement with the experimental evidence26 for FeZSM5, which can be interpreted in terms of a model for planar rotation to give a barrier of 7.5 kJ mol-1. The analysis in the previous work26 derived rotation barriers under the assumption of H2 rotation with two degrees of freedom, whereas our calculated barriers are for planar rotation. We note that the experimentally derived barriers above refer only to the stronger of the two pairs of rotational tunneling peaks observed in the INS spectra for both the Fe-26 and the

Solans-Monfort et al.

Figure 5. B3LYP rotational barrier and transition state structure for (a) H2-CuCHA(IV) and (b) H2-FeOCHA(IV). Distances are in angstroms.

Cu-ZSM-537 systems. These data also contain a second set of peaks at higher frequency (lower rotation barrier) as well as a broad distribution of frequencies, which appears to be centered at zero energy transfer. The former suggests the presence of a well-defined second binding site for H2,37 while the latter points to the existence of an additional, broad distribution of adsorption sites. These observations may also be related to the isosteric heat of adsorption (Qst) for hydrogen in Cu-ZSM-5 obtained from the adsorption isotherms. The average value of Qst was found to decrease from a maximum value of 74 kJ/mol at the

Interaction of Hydrogen with Cu+ and FeO+ Cations lowest H2 loadings to below 40 kJ/mol as H2 is added and (apparently) other types of sites with lower binding strengths are occupied. The identities of H2 binding sites with smaller isosteric heat of adsorption and lower barriers to rotation are difficult to determine in the absence of more detailed structural information on Cu-ZSM-5, but some of these could be associated with less accessible extraframework cations. We have therefore evaluated the rotational potential energy surfaces for site I in H2-CuCHA. We also note that Nachtigallova et al.85 have previously shown that Cu+ cations in the MFI framework may be located at several different sites whose relative energies are very similar. Nonetheless, they also pointed out that the most typical Cu+ environment contains 2, 3, or 4 oxygen atoms of the zeolite framework at distances within 2.5 Å. Therefore, site IV in CuCHA is expected to be reasonably good for model Cu+ cations in the channel intersections of ZSM-5 bound to two oxygen atoms, while site I could be related to the less accessible, partially blocked Cu+ cations in ZSM-5. The reader should, however, be aware that the analogy between site IV and the channel intersection site of ZSM-5 has to be made with care despite the fact that the number of coordinating oxygen atoms of the framework is the same, because the size of the channels into which the cation is protruding is significantly smaller in the chabazite framework. This can lead to peculiar structures, as found for CaCHA-H2, in which H atoms are weakly interacting with oxygens of the zeolite framework.86 Nevertheless, this is not the case for the present system because here adsorption is dominated by the Cu+-H2 interaction, as shown by the much shorter M+-H2 distance (1.65 Å for Cu+-H2 vs 2.5 Å for Ca2+-H2). The transition state structure found with the 13T cluster model (see Figure S3 of the Supporting Information for further details) for site I reveals a rather more complex structural reorganization than that observed for site IV, in addition to elongation of the Cu-H distance and shortening of H-H bond. In fact, we find that the H2 molecule moves out the OZ-Cu-OZ plane and that this is accompanied by the displacement of the Cu+ cation to approach a third oxygen atom of the zeolite six-membered ring. Consequently, the barrier to rotation in this case arises not only from the loss of back-donation but also needs to be adjusted by the stabilization achieved by displacement of Cu+. This greater complexity of this rotation precluded us from performing a step by step scan so that we could only transfer the transition state r, R, and R (Scheme 1) and the variation from reactants to the periodic model. The resulting barrier to rotation was found to be 10.7 kJ mol-1, which is 4.4 kJ mol-1 lower than that observed for H2 interacting with the less hindered Cu+ cations of site IV and 3.3 kJ mol-1 higher than that observed for the FeO+ cation with less back-donation. This value may be compared to that obtained from an INS study of H2 adsorbed in partially exchange CuNaA, where Cu+ is also located in a six-ring window but may have a somewhat different geometry from the present case. The rotational tunneling transition of 2.3 meV in this case may be interpreted to give a barrier to rotation of 7.6 kJ/mol if the rotation has two degrees of freedom.25 Adsorption of a Second H2 Molecule to H2-CuCHA and H2-FeOCHA. Because adsorption of a second H2 molecule to either Cu+ and FeO+ cations might also explain the experimental data in terms of both the variety of binding sites (INS spectra) as well as the decrease in Qst at higher loadings, we have also investigated this possibility. Figure 6 shows optimized geometries of (H2)2-CuCHA(I), (H2)2-CuCHA(IV), and (H2)2-FeOCHA(IV), and the corresponding binding energies of the second H2 molecule are listed in Table 1. Only site

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Figure 6. B3LYP-optimized structures for (H2)2-MCHA complexes. Distances are in angstroms.

IV was considered in the case of iron exchanged chabazite because the binding at site I does not occur by way of the metal cation. Adsorption of a second H2 molecule interacting with Cu+ cation at H2-CuCHA(I) is not favorable, and the only minimum we have been able to locate finds the second H2 molecule interacting with one oxygen atom of the zeolite framework through a η1-end-on structure. The geometries of the zeolite fragment and the second H2 molecule are very similar to those of free H2 and H2-CuCHA(I). The O · · · H2 distance is quite large 2.67 Å, which suggests a weak interaction. In fact, the EINT term is destabilizing by +3.4 kJ mol-1, and it is only the dispersion forces that have the effect of making the adsorption energy of the second H2 molecule favorable by -7.8 kJ mol-1. As in the case of H2-FeOCHA(I), the η1-end-on orientation is governed by the electrostatic interaction between the negatively charged oxygen and the positive region of the H2 quadrupole moment. Inclusion of a second H2 molecule at site IV results in two minima for H2 interacting with the metal center (Figure 6). The most stable one, (H2)2-CuCHA(IV)SP, is similar to the H2-CuCHA(IV) fragment for one H2 molecule per Cu+ with the second H2 molecule in the apical position, far from the metal center (Cu · · · H around 3 Å). The H2 coordinates to Cu+ in η2side-on fashion, and it is perpendicular to the basal H2 molecule. This orientation is equal to that of the first H2 molecule in H2-FeOCHA(IV). The second and higher minimum energy structure has two H2 molecules close to the metal with Cu-H distances in the 1.8-1.9 Å range. If we consider the two H2 molecules as two dihydrogen ligands, the coordination around the metal center can be considered as pseudotetrahedral. The formation of this structure from H2-CuCHA(IV) + H2 requires a very substantial geometrical and electronic reorganization. Moreover, the H2 bond distances in the (H2)2-CuCHA(IV)Td complex are shorter than that observed for H2-CuCHA(IV) (0.77 instead of 0.80 Å), which indicates that the charge transfer from CuCHA to the two H2 molecules is much smaller than in the complex with a single H2 molecule. This conclusion is confirmed by the population analysis, which indicates that the two H2 molecules have negligible negative charges in (H2)2-CuCHA(IV)Td. This result can be explained by the fact

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that this geometrical reorganization avoids efficient backdonation from the polarized Cu dπ to the empty σ* H2 orbital40 and that back-donation is shared by the two H2 molecules (Figure 4b). The energy of (H2)2-CuCHA(IV)Td is 8.1 kJ mol-1 above the most stable (H2)2-CuCHA(IV)SP complex. The gain in stability, therefore, does not overcome the cost of deformation, although the interaction of the second H2 seems to be considerably stronger in the tetrahedral structure and shows that the trigonal pyramid coordination about copper is preferred. On the other hand, the (H2)2-CuCHA(IV)SP structure is -8.0 kJ mol-1 below H2-CuCHA(IV) + H2 asymptote when dispersion forces and basis set corrections are included even though the EINT term is not favorable for either of the two (H2)2-CuCHA(IV) complexes. This suggests that the adsorption of more than one H2 molecule per copper site in CuCHA(IV) is mainly driven by dispersive forces. The second H2 molecule binds to H2-FeOCHA(IV) in a trans configuration with respect to the first H2 molecule. This is not surprising in view of the fact that this position is the only remaining vacant site. The resulting geometry around the metal center is thus one of a distorted trigonal bipyramid with the two H2 molecules in the apical positions and far from the metal center. In fact, the second H2 molecule is even further than the first H2 molecule and the Fe · · · H distances are larger than 3.0 Å. The resulting interaction energy is rather weak, and its origin is mainly dispersive as for (H2)2-CuCHA(IV): the adsorption energy is -10.5 kJ mol-1, of which -7.2 kJ mol-1 arises from the dispersion term. We note that the second H2 molecule is coplanar to the first one, and all of the attempts to find a minimum energy structure with a perpendicular orientations spontaneously evolved to the structure presented here (Figure 6). This can be explained again by assuming that the relative orientation of H2 is determined by a very weak orbital interaction between the FeO+ dπ and the H2 σ* molecular orbitals. We conclude that binding of a second H2 molecule to either Cu+ or FeO+ sites of Si/Al ) 11 chabazite is not significantly favorable and find that the resulting adsorption energies for the second H2 molecule are of the same order of magnitude of that of an H2 molecule simply interacting with the zeolite framework. The second set of rotational tunneling peaks in the INS spectra (at lower barriers to rotation) as well as the decrease of the isosteric heat of adsorption upon increasing the H2 loading are more likely to be associated with less accessible binding sites than with adsorption of a second molecule to an already active site. Conclusions DFT(B3LYP) periodic calculations were performed to rationalize the origin of the enhanced interaction of H2 with extraframework cations (FeO+ and Cu+) inside chabazite. Adsorption energies were corrected by adding Grimme’s empirical parametrization for dispersion at the optimized DFT geometries. We considered binding one and two H2 molecules per cation site for all potential cations and obtained barriers to H2 rotation for the most relevant cases to compare to barriers derived from available INS rotational tunneling spectra. FeO+ can be located at two different sites as previously described for Cu+ cations:40,52 (i) the preferred site I where FeO+ is located in the middle of the six-membered ring and coordinated to three oxygen atoms of the zeolite framework and (ii) the less obstructed site IV where FeO+ points toward the eight-membered ring and it is only coordinated to two

Solans-Monfort et al. oxygen atoms of the zeolite framework. The energy difference between the two sites is 17.3 kJ mol-1, which demonstrates that the preference for site I is smaller than that for Cu+ cations (∆E ) 40.5 kJ mol-1).52 Adsorption of one H2 molecule at FeOCHA with FeO+ cation located in site I does not lead to an Fe-H2 complex because the H2-M interaction energy is not high enough to overcome the cost of FeOCHA distortion. H2 instead interacts with the oxygen atom of the FeO+ cation by an η1-H2 angular mode with an adsorption energy of -9.0 kJ mol-1. In contrast, H2 interacts with FeO+ cation located in site IV through a η2-H2 side-on mode with an Eads equal to -12.0 kJ mol-1 because virtually no structural reorganization is required. The H2-FeO geometry of the adsorbed complex is driven by the metal d f H2 σ* back-donation as in the case of Cu+. The Fe-H distances are, however, larger than those of Cu-H, and the metal d orbital involved in back-donation is less polarized toward H2, so that the overall back-donation is significantly smaller in the H2-FeO system. This observation explains the lower adsorption energy and results in a significantly lower barrier to rotation (∆Erotq ) 7.4 kJ mol-1 for H2-FeOCHA in site IV and 15.1 kJ mol-1 for H2-CuCHA in site IV). In both CuCHA and FeOCHA, therefore, H2 rotation on site IV implies a reduction in metal d f H2 σ* back-donation, so that rotation is more energetically demanding for H2-CuCHA, because of stronger backdonation in the latter. Adsorption of a second H2 molecule per cation site as well as the evaluation of the rotational barrier for the more hindered site I were considered in an attempt to understand the origin of the reduction in the overall isosteric heat of adsorption (Qst) experiments with increases in H2 loading and the evidence of additional binding sites in the inelastic neutron scattering rotational tunneling spectra. Adsorption of a second H2 molecule per metal site is only marginally favorable and driven by dispersive forces. In contrast, the H2-CuCHA rotation energy barrier of the more hindered site I correlates reasonably well with the experimental data of the weaker sites, the computed value being 3.6 kJ mol-1 higher in energy than the experimental one. More importantly, the computed rotation energy barriers for H2 in CuCHA (site IV and site I) and for H2-FeOCHA at site IV follow the experimentally observed trends: stronger CuZSM5 site > weaker Cu-ZSM5 site > stronger overexchanged FeZSM5 site. At this point, it is worth mentioning that Nachtigallova et al.85 have shown that Cu+ locates preferentially at sites where Cu+ interacts with 2, 3, and 4 oxygen atoms of the MFI framework. While the precise geometries of the sites in our chabazite models and those described for CuZSM5 sites are not identical, the values reported herein and, more significantly, the trends can be transferred to ZSM5. Overall, two key factors may be distinguished to favor the enhancing of H2-M interactions inside zeolite frameworks: (i) the accessibility of the metal cation to allow H2 coordination and (ii) the occupancy and polarization toward the incoming ligand of the d orbitals with the appropriate symmetry to interact with H2. Among the two metal cations considered here, both factors favor CuCHA because the presence of the oxygen atom of the FeO+ cation precludes the existence of a vacant site in the H2 OZ-Fe-OZ coordination plane, which is the orientation that maximizes the metal to H2 back-donation. Acknowledgment. We would like to thank P. A. Georgiev for helpful discussions. The financial support from MICINN through the CTQ2008-06381/BQU project and the use of the Barcelona Supercomputing Center (BSC) are gratefully ac-

Interaction of Hydrogen with Cu+ and FeO+ Cations knowledged. X.S.-M. thanks the Spanish government for a Ramo´n y Cajal contract. Work at UCSB was supported by the Office of Energy Efficiency and Renewable Energy, U.S. Department of Energy, under contract number DE-FC3605GO15004. Supporting Information Available: Figures S1-S3 showing the optimized geometries using 5T and 13T cluster models. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Dinca, M.; Long, J. R. Angew. Chem., Int. Ed. 2008, 47, 6766. (2) Ma, S. Q.; Eckert, J.; Forster, P. M.; Yoon, J. W.; Hwang, Y. K.; Chang, J. S.; Collier, C. D.; Parise, J. B.; Zhou, H. C. J. Am. Chem. Soc. 2008, 130, 15896. (3) Nouar, F.; Eckert, J.; Eubank, J. F.; Forster, P.; Eddaoudi, M. J. Am. Chem. Soc. 2009, 131, 2864. (4) Hulvey, Z.; Falcao, E. H. L.; Eckert, J.; Cheetham, A. K. J. Mater. Chem. 2009, 19, 4307. (5) Bhatia, S. K.; Myers, A. L. Langmuir 2006, 22, 1688. (6) Dinca, M.; Dailly, A.; Liu, Y.; Brown, C. M.; Neumann, D. A.; Long, J. R. J. Am. Chem. Soc. 2006, 128, 16876. (7) Dinca, M.; Han, W. S.; Liu, Y.; Dailly, A.; Brown, C. M.; Long, J. R. Angew. Chem., Int. Ed. 2007, 46, 1419. (8) Forster, P. M.; Eckert, J.; Heiken, B. D.; Parise, J. B.; Yoon, J. W.; Jhung, S. H.; Chang, J. S.; Cheetham, A. K. J. Am. Chem. Soc. 2006, 128, 16846. (9) Wang, X. S.; Ma, S. Q.; Forster, P. M.; Yuan, D. Q.; Eckert, J.; Lopez, J. J.; Murphy, B. J.; Parise, J. B.; Zhou, H. C. Angew. Chem., Int. Ed. 2008, 47, 7263. (10) Sava, D. F.; Kravtsov, V. C.; Eckert, J.; Eubank, J. F.; Nouar, F.; Eddaoudi, M. J. Am. Chem. Soc. 2009, 131, 10394. (11) Chen, B. L.; Ockwig, N. W.; Millward, A. R.; Contreras, D. S.; Yaghi, O. M. Angew. Chem., Int. Ed. 2005, 44, 4745. (12) Bakhmutov, V. I.; Bertran, J.; Esteruelas, M. A.; Lledos, A.; Maseras, F.; Modrego, J.; Oro, L. A.; Sola, E. Chem.-Eur. J. 1996, 2, 815. (13) Maseras, F.; Lledos, A.; Clot, E.; Eisenstein, O. Chem. ReV. 2000, 100, 601. (14) Millar, J. M.; Kastrup, R. V.; Melchior, M. T.; Horvath, I. T.; Hoff, C. D.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 9643. (15) Howdle, S. M.; Healy, M. A.; Poliakoff, M. J. Am. Chem. Soc. 1990, 112, 4804. (16) Kemper, P. R.; Bushnell, J.; Vankoppen, P.; Bowers, M. T. J. Phys. Chem. 1993, 97, 1810. (17) Kemper, P. R.; Bushnell, J.; Vonhelden, G.; Bowers, M. T. J. Phys. Chem. 1993, 97, 52. (18) Weis, P.; Kemper, P. R.; Bowers, M. T. J. Phys. Chem. A 1997, 101, 2809. (19) Kemper, P. R.; Weis, P.; Bowers, M. T.; Maitre, P. J. Am. Chem. Soc. 1998, 120, 13494. (20) Murray, L. J.; Dinca, M.; Long, J. R. Chem. Soc. ReV. 2009, 38, 1294. (21) Brown, C. M.; Liu, Y.; Neumann, D. A. Pramana 2008, 71, 755. (22) Liu, Y.; Kabbour, H.; Brown, C. M.; Neumann, D. A.; Ahn, C. C. Langmuir 2008, 24, 4772. (23) Peterson, V. K.; Liu, Y.; Brown, C. M.; Kepert, C. J. J. Am. Chem. Soc. 2006, 128, 15578. (24) Kubas, G. J. Chem. ReV. 2007, 107, 4152. (25) Eckert, J.; Trouw, F. R.; Mojet, B.; Forster, P.; Lobo, R. J. Nanosci. Nanotechnol. 2010, 10, 49. (26) Mojet, B. L.; Eckert, J.; van Santen, R. A.; Albinati, A.; Lechner, R. E. J. Am. Chem. Soc. 2001, 123, 8147. (27) Vandersluys, L. S.; Eckert, J.; Eisenstein, O.; Hall, J. H.; Huffman, J. C.; Jackson, S. A.; Koetzle, T. F.; Kubas, G. J.; Vergamini, P. J.; Caulton, K. G. J. Am. Chem. Soc. 1990, 112, 4831. (28) Eckert, J.; Blank, H.; Bautista, M. T.; Morris, R. H. Inorg. Chem. 1990, 29, 747. (29) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman, H. J. J. Am. Chem. Soc. 1984, 106, 451. (30) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120. (31) Eckert, J.; Albinati, A.; White, R. P.; Bianchini, C.; Peruzzini, M. Inorg. Chem. 1992, 31, 4241. (32) Bordiga, S.; Buzzoni, R.; Geobaldo, F.; Lamberti, C.; Giamello, E.; Zecchina, A.; Leofanti, G.; Petrini, G.; Tozzola, G.; Vlaic, G. J. Catal. 1996, 158, 486. (33) Lobree, L. J.; Hwang, I. C.; Reimer, J. A.; Bell, A. T. J. Catal. 1999, 186, 242.

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