| Abstract
| - The adsorption of NH3 and H2O in acidic chabazite has been studied with the B3LYP method within thecluster approach (5T, 48T clusters) and the periodic approach adopting a Si/Al = 11/1 chabazite and a basisset of polarized double-ζ quality. The 5T cluster has been treated fully ab initio at the B3LYP level whereasthe 48T cluster has been treated with the ONIOM2 scheme using B3LYP as the high level of theory and theMNDO, AM1, and HF/3-21G methods as low levels of theory. Periodic calculations show that the adsorptionof NH3 in acidic chabazite takes place through an ion pair (NH4+−CHA-) structure, the computed adsorptionenergy being −32 kcal/mol. The adsorption of H2O leads to a hydrogen bonded (H2O−HCHA) complexwith the computed adsorption energy of −20 kcal/mol. All ONIOM combinations provide similar structuresto those obtained with periodic calculations. Adsorption energies, however, are sensitive to the low levelused, especially for NH3. The ONIOM B3LYP:HF/3-21G method is the one that provides more satisfactoryresults. Present results show that, for larger zeolites, the ONIOM scheme can be successfully applied whiledrastically reducing the cost of a fully ab initio treatment.
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