Theoretical Studies of the Coordination and Stability of Divalent Cations in ZSM-5

Abstract

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 Co 2+, Cu 2+, Fe 2+, Ni 2+, Pd 2+, Pt 2+, Ru 2+, Rh 2+, and Zn 2+ . Thermodynamic calculations of the stability of M 2+ to reduction to M 0 and demetalation to form MO x particles were also carried out. The results indicate that Cu 2+, Co 2+, Fe 2+, and Ni 2+ are coordinated preferentially to five-membered rings containing two Al atoms, which are located on the walls of the sinusoidal channels, whereas Pd 2+, Pt 2+, Ru 2+, Rh 2+, and Zn 2+ 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.