Origin of the enhanced interaction of molecular hydrogen with extraframework Cu⁺ and FeO⁺ cations in zeolite hosts. A periodic DFT study

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 (ΔE 17.3 kJ mol-1). The interaction of H₂ with FeO⁺ (E_ads -17.5 kJ mol-1) is weaker than that with Cu⁺ (E_ads=-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 H ₂ 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 H ₂ charge density show that the M d f H₂ σ* back-donation is larger in H₂-Cu⁺ than in H ₂-FeO⁺. Both the binding energy and the barrier to rotation are therefore much greater for H₂-CuCHA than for H ₂-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.