Site availability and competitive siting of divalent metal cations in ZSM-5

ZSM-5 zeolites containing charge-exchanged, divalent cations are active catalysts for several reactions: Cu-ZSM-5 for NO decomposition, Co-ZSM-5 for NO reduction by hydrocarbons, Zn-ZSM-5 for dehydrocyclization of C₆ and C₇ alkanes to aromatics, Pd-ZSM-5 for methane combustion, and Pt-ZSM-5 for acrylonitrile and acetonitrile production from butane, isobutane, propane or propene. The 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. Each charge-exchange site is associated with a framework Al atom, which requires that 2 Al atoms be located in the 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^- was calculated for a given value of Si/Al, and reported as the maximum value of M/Al. Two cases for the distribution framework of Al atoms are considered: random placement in T sites and placement governed by equilibrium thermodynamics. For M²⁺ and [MOM]²⁺, the maximum value of M/Al was significantly less than 0.5 and 1.0, respectively, due to required constraints on the Al-Al pairs, and decreased with increasing Si/Al ratio. For M²⁺ cations, the distribution of cations (Co²⁺, Cu²⁺, Fe²⁺, Ni²⁺, Pd²⁺, Pt²⁺ and Zn²⁺) were determined using energies predicted from electronic structure calculations. M²⁺ cations were preferentially situated in rings containing 5 or 6 T atoms. The manner in which Al atoms are distributed among T sites affected the M²⁺ cation distribution among different types of rings.