Theoretical studies of the coordination and stability of divalent cations in ZSM-5

The coordination of divalent metal cations to ZSM-5 has been investigated using gradient-corrected density functional theory (DFT). Coordination at both isolated charge-exchange sites and pairs of charge-exchange sites was considered for Co2+, Cu2+, Fe2+, Ni2+, Pd2+, Pt2+, Ru2+, Rh2+, and Zn2+. Thermodynamic calculations of the stability of M2+ to reduction to MO and demetalation to form MO, particles were also carried out. The results indicate that Cu2+, Co2+, Fe2+, and Ni2+ are coordinated preferentially to five-membered rings containing two Al atoms, which are located on the walls of the sinusoidal channels, whereas Pd2+, Pt-2+, Ru2+, Rh2+, and Zn2+ are coordinated preferentially to six-membered rings located on the walls of the sinusoidal channels. Examination of the stability of dimer cations of the form [M-O-M](2+) shows that such structures are not generally stable to hydrolysis, with the possible exception of [Cu-O-Cu](2+). The findings of these calculations are in good general agreement with experimental results.