Can Cu+-exchanged zeolites store molecular hydrogen?: An ab-initio periodic study compared with low-temperature FTIR

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 H-2. Two sites (I and IV) have been found to be stable for Cu+ ion: site 1, 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 H-2 gives adsorption energies at B3LYP of -13 and -56 kJ/mol for sites I and IV, respectively. The B3LYP bathochromic harmonic H-2 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(3d(pi)) orbital through the antibonding H-2(sigma(u)) and orbital polarization play a significant role in the H-2 adsorption energy, and cause the large bathochromic H-2 frequency shift.