Brønsted sites and structural stabilization effect of acidic low-silica zeolite A prepared by partial ammonium exchange

Abstract

It is demonstrated for low-silica zeolite 25NH4,Na-A (nSi/nAl = 1.0) with an ammonium exchange degree of 25% that the formation of acidic bridging OH groups (Si(OH)Al) after thermal deammoniation is accompanied by a weak dealumination of the framework. As the extra-framework aluminum species mainly exist predominantly in the form of Al3+ cations, they partially neutralize negative framework charges. The remaining framework charges are compensated by Na+ cations and hydroxyl protons of Si(OH)Al groups. Therefore, a lower number of Si(OH)Al groups is formed than expected by the stoichiometry of the ammonium exchange. For zeolite 25NH4,Na-A, only 17% of the introduced ammonium ions form Si(OH)Al groups after thermal deammoniation. A benefit of the cationic extra-framework aluminum species (Al3+) is their stabilizing effect exerted on the remaining framework aluminum atoms and the whole structure of the obtained zeolite 25H,Na-A. 1H MAS NMR signals of acidic OH groups were observed at 3.6 and 4.8 ppm and assigned to Si(OH)Al groups in the α- and β-cages, respectively. A low-field signal at 13.3 ppm is probably due to Si(OH)Al groups, however, involved in H-bondings (Si(OH⋯O)Al) with hydroxyl protons directed to very near oxygen atoms, such as expected in double 4-rings of the LTA structure.