Proton transfer in zeolites: a comparison between cluster and crystal calculations

We have calculated the adsorption energy of NH3 and NH4+ in a zeolite crystal and on four different clusters cut out from the crystal. The adsorption energies were calculated for two different geometries of the chabazite: a geometry determined with x-ray diffraction and a shell model optimized geometry. The effect of the geometry optimization of the crystal is large. In the crystal for which the geometry has been optimized the adsorbates are less stable. For NH3 the difference between the two geometries is 29 kJ mol-1 (adsorption energies -101 and -72 kJ mol-1), and for NH4+ the difference between the two geometries is 137 kJ mol-1 (-15 and 122 kJ mol-1). The adsorption energy of NH3 is a local process. The effect of the Madelung potential is relatively small: for the geometries we found -6 and +5 kJ mol-1. The adsorption of NH4+ is more affected by the Madelung potential: -53 and +23 kJ mol-1 for the two geometries. Clusters that have bonds saturated with hydrogen atoms close to the adsorbate do not give a proper description of the adsorption process: this type of boundary error can be as large as 60 kJ mol-1.