Site Availability and Competitive Siting of Divalent Metal Cations in ZSM-5

Accommodation of divalent cations (M2+) and dimer cations ([MOM]2+) in zeolites requires the presence of two charge-exchange sites in close proximity to each other. Since each charge-exchange site is associated with a framework Al atom, this constraint requires that 2 Al atoms be located in next-nearest-neighbor T sites. The maximum number of divalent cations that can be accommodated by ZSM-5 as either Z-M2+Z− or Z−[MOM]2+Z− (where Z− represents a cation-exchange site in the zeolite) has been calculated for a given value of Si/Al, and the results are reported as the maximum value of M/Al. Two cases for the distribution of framework Al atoms are considered: random placement in T sites and placement governed by equilibrium thermodynamics. For both M2+ and [MOM]2+ the maximum value of M/Al is significantly less than 0.5 and 1.0, respectively, because of the required constraints on the Al–Al pairs, and decreases as the Si/Al ratio increases. For M2+ cations the distribution of specific cations (Co2+, Cu2+, Fe2+, Ni2+, Pd2+, Pt2+, and Zn2+) was determined using energies predicted from electronic structure calculations. M2+ cations are preferentially situated in rings containing either five or six T atoms. The manner in which the Al atoms are distributed among T sites affects the distribution of M2+ cations among different types of rings.