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 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 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 π ) orbital through the antibonding H 2 (σ u ) and orbital polarization play a significant role in the H 2 adsorption energy, and cause the large bathochromic H 2 frequency shift.