Periodic density functional investigation of Brønsted acidity in isomorphously substituted chabazite and AlPO-34 molecular sieves

The relative strength of Brønsted acid sites in isomorphously substituted chabazite (M-CHA; Me=Al³⁺, Ga³⁺, B³⁺, Sc³⁺ and Fe³⁺) and aluminophosphate-34 (MAlPO-34; Me=Ti⁴⁺, Si⁴⁺, Cr²⁺, Mn²⁺, Fe²⁺, Co²⁺ and Ni²⁺) have been investigated using periodic density functional theory. Adsorption energy of NH₃ and deprotonation energy are considered for measuring the acidity. It has been found that Al³⁺-CHA and Mn²⁺-AlPO-34 possess the strongest acidic character. Moreover, a correlation between deprotonation energy and adsorption energy of NH₃ has been observed.