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.