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J. Phys. Chem. B 2004, 108, 8278-8286
Can Cu+-Exchanged Zeolites Store Molecular Hydrogen? An Ab-Initio Periodic Study Compared with Low-Temperature FTIR Xavier Solans-Monfort,† Vicenc¸ Branchadell,† Mariona Sodupe,† Claudio M. Zicovich-Wilson,‡ Evgueny Gribov,§ Giuseppe Spoto,§ Claudia Busco,§ and Piero Ugliengo*,§ Departament de Quı´mica, UniVersitat Auto` noma de Barcelona, Bellaterra 08193, Spain, Facultad de Ciencias, UniVersidad Autonoma del Estado de Morelos, AV. UniVersidad 1001, Col. Chamilpa, 62210 CuernaVaca (Morelos), Mexico, and Dipartimento di Chimica IFM, UniVersita` di Torino, Via P. Giuria, 7, 10125 Torino, Italy ReceiVed: March 25, 2004
Cu+-exchanged Si/Al 11:1 chabazite has been studied ab initio using the periodic CRYSTAL03 computer code with Hartree-Fock and the hybrid B3LYP Hamiltonians to characterize the structures and energetics of the Cu+ ion sitting preference and its interaction with H2. Two sites (I and IV) have been found to be stable for Cu+ ion: site I, the most stable one, envisaging coordination in a six-membered zeolite ring and site IV in which the Cu+ ion sits in the largest eight-membered ring. Interaction of H2 gives adsorption energies at B3LYP of -13 and -56 kJ/mol for sites I and IV, respectively. The B3LYP bathochromic harmonic H2 frequency shifts are 847 and 957 cm-1 for adsorption at sites I and IV, respectively, in good agreement with the shifts measured (1030 and 1081 cm-1) in the Cu-ZSM-5 system in which Cu+ ion is, respectively, three and bi-coordinated by the oxygen atoms of the zeolite framework. Analysis of the components of the adsorption energy, carried out within the cluster approach, revealed that charge transfer from the Cu(3dπ) orbital through the antibonding H2(σu) and orbital polarization play a significant role in the H2 adsorption energy, and cause the large bathochromic H2 frequency shift.
Introduction Zeolites have traditionally being used in many industrial processes, mainly as catalysts, ion exchangers, molecular sieves, and carriers.1 Since the critical reexamination of the real sorption properties of carbon nanotubes, filaments, and vapor-grown fibers2 with respect to H2, which showed that no more than 1 wt % of H2 is really trapped, metal-exchanged zeolites have been suggested as possible candidates for H2 storage, the reversible molecular hydrogen storage being critical for largescale application of hydrogen as fuel, in particular for mobile applications.3 The shape of the channels, the high surface area, as well as the interaction between the zeolite framework and H2 are determinant factors to control these properties. Thus, the study of H2 adsorption on metal-exchanged zeolites is of great interest. Since the pioneering work of Kubas et al.,4 the coordination of dihydrogen to transition metal complexes has been extensively studied.5 Moreover, the interaction of naked transition metal cations M+ with H2 has been considered by several authors both from experimental and theoretical points of view.6-11 The theoretical studies have shown that several factors, such as the cost of hybridization, the metal-ligand repulsion, the extent of charge transfer, and finally the charge-induced dipole and charge quadrupole electrostatic interactions influence the M+-H2 binding energy.5,10 Thus, the metal cation-H2 interaction is highly sensitive to the electronic structure of the metal cation. * Corresponding author. E-mail:
[email protected]. † Universitat Auto ` noma de Barcelona. ‡ Universidad Autonoma del Estado de Morelos. § Universita ` di Torino.
In zeolites, in which a Si atom has been substituted by an Al one, metal cations act as charge balancing ions of the negatively charged framework by coordinating to the lattice oxygen atoms so that their electronic properties can experience changes that ultimately could affect their interactions with adsorbed molecules such as H2.12-16 These changes vary, depending on the coordination environment of the metal. Therefore, the metal ability to bind H2 can strongly depend on the metal cation sitting site. To analyze the potentiality of a particular zeolite as a H2 storage material it is important to know the coordination, accessibility, and binding properties of the metal cation at the different positions where it can be located. For that, theoretical studies can provide important information and insights. However, although many studies have considered the interaction of naked transition metal cations with H2, to our knowledge no theoretical work has been performed on the adsorption of molecular hydrogen on metal-exchanged zeolites. In the present work, a theoretical study on the sitting sites of Cu+ ions on chabazite and the subsequent adsorption of the H2 molecule using a fully periodic ab initio simulation is addressed. Chabazite has been selected because its small unit cell allows fully ab initio periodic calculations with good basis sets to be performed on commodity personal computers. The interest in Cu+ as a metal compensating ion in zeolite is due to its high H2 affinity, as shown by gas-phase calculations on H2-Cu+ sporting a binding energy (66.9 kJ/mol), significantly larger than that for other cations such as Li+ (6.1 kJ/mol) or Na+ (10.3 kJ/mol).6,10,11 It is worth noting that recent experiments of the thermodynamics of H2 adsorption on Li-ZSM-5 zeolite17 gave a modest value of -6.5 kJ/mol for the adsorption enthalpy, which implies that Li-exchanged zeolites may be used for H2 storage in cryogenic conditions only. Because Cu+ chabazite
10.1021/jp0486651 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/25/2004
Can Cu+-Exchanged Zeolites Store Molecular Hydrogen? free from the co-presence of other counterions has unfortunately never been prepared, the periodic calculation results have been contrasted with new infrared measurements of the H2 stretching frequency when sorbed in a Cu+-exchanged ZSM-5 prepared in our laboratory. For the sake of comparison and of a deeper understanding of the Cu+-H2 interaction, ab initio calculations on selected clusters have also been performed. It is worth noting that most of the theoretical studies performed up to now on transition metal-exchanged zeolites have considered only the cluster approach. To our knowledge, only Sauer et al.18-20 and Limtrakul et al.21 have introduced periodic conditions to Cu+exchanged zeolites using embedded cluster approaches with different hybrid (QM/MM) schemes. Thus, the present work reports for the first time a fully periodic ab initio simulation on a transition metal-exchanged zeolite. Computational and Experimental Details Cu+-ZSM-5 was prepared by reaction of H-ZSM-5 (Si/Al ) 90) with gaseous CuCl,22,23 a procedure leading to the 1:1 substitution of the acidic protons with isolated extraframework Cu+ ions.13 Before H2 dosage, the zeolite sample (in the form of self-supporting pellets suitable for transmission IR measurements) was outgassed under high vacuum (
