Abstract: The siting of B, Al or Ga and the bridging hydroxyl group in mordenite framework were studied by using ab initio HartreeFock methods. The influence of the presence of B or Ga in the framework on the Bronsted acidity of mordenite was investigated. It was shown that energetically, heteroatoms including B,Al or Ga prefer the same Tsites, T3 and T4, when replacing Si in mordenite framework. For the preferred substitution site T4, the chargecompensating proton prefers to attach to O10 in case of Al and Ga substitution while the proton prefers to bond with O2 or O10 in case of B substitution. The proton affinity calculations showed that the acidity of isomorphously substituted mordenite increases in the order:BMORGaMOR< AlMOR.

Key words: B, Al or Ga, Isomorphous substitution, Mordenite, Acidity, Ab initio