Double interactions between ammonia and a series of alkali-exchanged faujasite zeolites evidenced by FT-IR and TPD-MS techniques

The acid–base properties of a series of cationic zeolites have been investigated by means of FT-IR and TPD-MS techniques using ammonia as probe molecule. The present work clearly demonstrates that the adsorption strength of ammonia on these cationic zeolites is strongly dependent on the Lewis acidity of counter-ions, indicating that the interaction between ammonia and cationic zeolites is dominated by an electrostatic interaction between counter-ions and the lone-electron pair of the nitrogen atoms. However, the variation of the bathochrome shift content of stretching vibration bands of ammonia adsorbed on these cationic zeolites revealed by FT-IR does not follow the strength of this electrostatic interaction. The second interaction between the negative charged oxygen atoms of the framework with the hydrogen atoms of the ammonia exists. The double interaction of ammonia with zeolites is for the first time experimentally evidenced. This experimental observation could be of great importance in characterization and in theoretical calculation of acid–base properties of cationic zeolites. A slight decomposition of ammonia even at room temperature is also observed and could be of great interest in the development of new catalysts for methylation of ammonia for methylamine production and in the conception of fuel cell using ammonia as hydrogen source.