Can Cu⁺-Exchanged Zeolites Store Molecular Hydrogen? An Ab-Initio Periodic Study Compared with Low-Temperature FTIR

Abstract

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.

New frontier in transmission IR spectroscopy of molecules adsorbed on high surface area solids: Experiments below liquid nitrogen temperature

“…This is mainly due to the structure of the Cu + ions, which are highly coordinatively unsaturated and thus easily accessible to molecules moving inside the zeolite channels [34,39,42]. Very recently, it has been reported that intrazeolitic Cu + species can also bind molecular H 2 at room temperature [30,93,94], a rather surprising finding when considering that the only Cu + (H 2 ) complex previously known was the [Cu(h 2 -H 2 )Cl] adduct isolated by cocondensation of CuCl and H 2 in an Ar matrix [95]. The interest of a microporous system able to reversibly adsorb H 2 at room temperature is self-evident, because it satisfies one of the pre-requisites for hydrogen storage [71].…”

“…It is widespread opinion that a large shift (of the order of À1000 cm À1 ) of theñ(HH) frequency in Cu + (H 2 ) molecular complexes in side-on configuration is the symptom of a strong interaction involving weak electrostatic (charge-induced dipole and charge quadrupole) and strong chemical (donation from the H 2 s orbital and back-donation from the 3dp Cu + orbitals) contributions [30,93,95,97]. The s donation in the 4s orbital and back-donation from the dp orbitals of Cu + are synergistic and lead to the formation of three center Cu + (H 2 ) covalent bonds, where back-donation weakens the H-H bond to a very large extent.…”

“…The s donation in the 4s orbital and back-donation from the dp orbitals of Cu + are synergistic and lead to the formation of three center Cu + (H 2 ) covalent bonds, where back-donation weakens the H-H bond to a very large extent. This is reflected in the relatively large interaction energy (60-80 kJ mol À1 ) [30,93,95]. The covalent character of the Cu + -H bonds in the side-on Cu + (H 2 ) species also accounts for the low intensity of theñ(HH) mode because the H-H stretching is not accompanied by strong dipole oscillations along the covalent Cu + (H 2 ) and H-H bonds.…”

Hydrogen adsorption in a copper ZSM5 zeoliteAn inelastic neutron scattering study

“…Following our work on the CoALPOs, we now report a preliminary INS study of molecular hydrogen interacting with a copper ZSM5. It has been claimed that the interaction of H 2 with Cu ZSM5 [13][14][15][16][17][18] and Fe ZSM5 [19] is strong.…”

Dihydrogen in zeolite CaX—An inelastic neutron scattering study

“…In protonated zeolites, binding of H 2 molecules increased with the Brønsted acidity of the zeolite [8]. From theoretical studies the H 2 interaction with alkali metal cations is dominated by the polarisation contribution [14]. That polarisation of the H 2 molecule [9] is significant in the binding we now show experimentally by inelastic neutron scattering (INS) [15].…”