Comparison of a combined quantum mechanics/interatomic potential function approach with its periodic quantum-mechanical limit: Proton siting and ammonia adsorption in zeolite chabazite

Comparison is made between a combined quantum mechanics/interatomic potenti al function approach (QM-Pot) and its fully quantum-mechanical limit, ab in itio calculation applying periodic boundary conditions. The Hartree-Fock (H F) method is combined with ab initio-parametrized ion pair shell model pote ntial functions. The CRYSTAL code is employed for the periodic Hartree-Fock calculations. The same double-/valence triple-zeta polarization basis sets are used in both the approaches. The proton siting and ammonia adsorption in a high-silica acidic zeolite catalyst, H-chabazite (Si/Al=11, space grou p P1, unit cell H-AlO2[SiO2](11)) are examined. The combined QM-Pot relativ e stabilities and reaction energies deviate from the periodic full QM resul ts by 4-9 kJ/mol only, which demonstrates the power of our combined approac h. This conclusion is also supported by comparison of the electrostatic pot ential inside the zeolite pore, calculated from the periodic wave function and by the QM-Pot approach. Framework oxygen O1 is found to be the preferre d proton site and on interaction with NH3 the proton is predicted to move t o NH3 yielding NH4+. The NH4+ surface species is coordinated to two framewo rk oxygen atoms, It is by 30-35 kJ/mol more stable than the neutral adsorpt ion complex of NH3. Evidence is produced that the failure of previous perio dic HF calculations to predict a stable NH4+ ion is due to the limitations of the minimum basis set used. (C) 1998 American Institute of Physics. [S00 21-9606(98)70847-5].