Structure and stability of zeolite offretite under Si⁴⁺/(Al³⁺, M⁺) substitution (M = Na, K): A first principles molecular dynamics study

A local density functional study of the structure and energetics of offretite, with a Si⁴⁺ ion substituted by (Al³⁺, M⁺) (M = Na, K), is presented. The calculations have been performed within the framework of the first principles molecular dynamics method using a periodically repeated unit cell with 55 atoms. It is found that the preferred site for K⁺ cations lies inside the cancrinite cage, in agreement with experiment. This site is related to Al substituting a Si atom at a T₁ site, which is one of the two inequivalent tetrahedral sites of offretite, and belongs to the hexagonal prism. Site selectivity is quite pronounced in the case of K⁺, as the large size of this cation gives rise to important steric effects in the offretite framework. For Na⁺, instead, the lowest energy site is located in the channel, near a window of the cancrinite cage, and corresponds to an Al in T₂. For this cation the spread in the relative substitution energies of the different binding sites is very small, the four sites of lower energies being in a range of ∼2.5 kcal/mol.