A local density functional study of the structure and energetics of offretite, with a Si4+ ion substituted by (A13+, 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 A1 substituting a Si atom at a TI 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 A1 in Tz. 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

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We have used density functional theory, both within the local density (LDA) and generalized gradient (GGA) approximations, to study the structure, energetics, and vibrational properties of zeolite offretite in the presence of different monovalent cations (H+, Na+, K+, and Cu+). We find that the spatial locations of the most favorable cation-binding sites are similar for the different cations, being related to the minima of the electrostatic potential. However, the relative stability of the sites does depend on the nature of the counterion, as well as on the Al/Si ratio and on the mutual interactions between cations. At low Al/Si ratios, the preferred site for H+ is in the channel, where it is accessible for reaction with incoming molecules. For both Na+ and Cu+, the most stable site is within the 6-fold ring of the gmelinite cage, but for Na+, two other sites are present within a few tenths of a kilocalorie/mole from the lowest site (small site selectivity). For K+, two sites, one inside the cancrinite cage and the other near the 8-fold ring of the gmelinite cage, are very close in energy, consistent with the X-ray experiments on natural hydrated and dehydrated offretites. Dynamical simulations have been carried out for H− and Na−offretite. The vibrational spectrum of the framework agrees well with the available experiment. OH stretching frequencies calculated for a number of different H+ locations show that more “open” positions, e.g., in the channel, have higher frequencies, in agreement with experiment.