AB-INITIO EMBEDDED-CLUSTER STUDY OF THE ADSORPTION OF NH3 AND NH4+ INCHABAZITE

The adsorption of NH3 in acidic zeolites has been studied extensively experimentally. Therefore, it can be used very well to verify a model used in a quantum chemical calculation. Here, we present a calculation that, from a quantum chemical point of view, should give a reliable d escription of the adsorption process. We studied the adsorption of NH3 and NH4+ in chabazite with the embedded cluster method using a reason able basis set, applying the counterpoise correction and including ele ctron correlation. The geometry was partially optimized. With this cal culation we verified the reliability of our model and obtained informa tion that cannot be obtained experimentally. The adsorption energies o f hydrogen-bonding NH3 and of NH4+ were -70 +/- 10 kJ/mol and -120 +/- 15 kJ/mol, respectively. The latter value compares very well with the experimental heat of adsorption. NH4+ has a high coordination with th e zeolite wall; this is confirmed experimentally. A good geometry is o btained if the boundary of the embedded cluster is kept fixed to that of the zeolite crystal.