Combining ab initio techniques with analytical potential functions. A study of zeolite-adsorbate interactions for NH3 on H-faujasite

A hybrid computational scheme which proved successful in the description of crystalline silica polymorphs is extended to the calculation of zeolite-adsorbate interactions. As a demonstrative application ammonia adsorption on H-faujasite at site mi was investigated. The Hartree-Fock method (QM) is combined with shell model potentials (MM) based on ab initio data for the interaction of NH3 and NH4+ with zeolites. This 'mechanical' embedding scheme accounts for the constraints imposed by the lattice on the structure relaxation at the reaction site. The long range contribution of the electrostatic potential of the crystal to the reaction energies is obtained from the MM energies of the host (periodic lattice) and of the embedded zeolite model. Although it converges slowly with increasing size of the embedded cluster, the total QM/MM energies are remarkably stable. The embedding scheme allows to use basis sets sufficiently large and flexible to describe both the neutral complex and the ion pair structures well. Ammonium ions in zeolites are therefore correctly described as more stable than ammonia. After including corrections for electron correlation, basis set superposition error, zero point energy and thermal contributions, the predicted estimate for the energy of formation of NH4+ from NH3 and the acidic zeolite of 112 kJ/mol is in close agreement with the range of heats of adsorption inferred from microcalorimetry and temperature-programmed desorption of ammonia on H-faujasites.