Effect of the Al Siting on the Structure of Co(II) and Cu(II) Cationic Sites in Ferrierite. A Periodic DFT Molecular Dynamics and FTIR Study

Periodic DFT molecular dynamics and FTIR spectroscopy were used to investigate the cationic sites of ferrierite exchanged with Co(II) and Cu(II) and their complexes with NO. Particular attention was paid to the effect of the Al siting in six-membered rings forming the cationic sites on the structure of these sites and the corresponding binding energies of Me(II) (Me = Co and Cu). Our calculations show that both the cations upon binding to cationic sites induce a rearrangement of the local structure of the zeolite framework. The rearrangement is significant for the α and β-2 sites while it is minor for the β-1 site. Comparison of the observed and theoretical NO stretching frequencies of ferrierite Co(II) and Cu(II) complexes with a NO molecule permitted the assignment of IR bands to the individual types of cationic sites. For NO-Co-ferrierite, the IR bands found at 1956, 1941, and 1935 cm^–1 can be assigned to NO-Co complexes with Co(II) located in the α, β-1, and β-2 sites, respectively. Similarly for NO-Cu-ferrierite, the frequencies of 1864, 1912, 1904, and 1892 cm^–1 belong to NO-Cu complexes having Cu(II) accommodated in the α, β-1, β-2 (conformer 1), and β-2 (conformer 2) sites, respectively. The calculated adsorption energies are systematically higher for Co(II) than for Cu(II) for all the three sites and are in the order α > β-2 > β-1 for both the cations. Our computational results further reveal that upon binding Me(II) both the local structure of the zeolite framework as well as the binding energies of Me(II) strongly depend on the Al siting in the rings forming the cationic sites. The calculated relative binding energies of Me(II) are in the order β-1 > β-2 > α for both the cations. The general tendency of Me(II) accommodated in a cationic site to react is inversely proportional to the corresponding binding energies. We also showed that FTIR spectroscopy of complexes of NO and Me(II)-exchanged ferrierite can serve to identify the Al siting in the six-membered rings forming cationic sites.