Theoretical studies on the properties of acid site in isomorphously substituted ZSM-5

The structure and electronic properties of the Bronsted acid site in B, Al or Ga isomorphously substituted ZSM-5 zeolites were studied by ab initio HF or DFT methods. The dependence of the calculation results on the employed methods, cluster size and basis sets was examined in details. Several measures for determining of the acidity including the proton affinity, the charge on proton, the hydroxyl group vibrational frequencies, gamma(OH), as well as the adsorption energy of NH3 showed that the acidity of the substituted ZSM-5 increases in the sequence: B-ZSM-5 < Ga-ZSM-5 < Al-ZSM-5, which is in good agreement with experimental results. Studies on the interaction of NH3 with the Bronsted acid site indicated that NH3 becomes protonated in contact with the zeolite cluster and the configurations in which the protonated NH3 interacts with two lattice oxygen atoms are favored energetically. In addition, the calculated adsorption energy of NH3 on Al-ZSM-5 is comparable with the experimental data. (C) 2002 Elsevier Science B.V. All rights reserved.