Pressure-swing absorption and desorption behaviours of ammonia in bis(trifluoromethylsulfonyl)amide salts†

The absorption and desorption behaviours of NH₃ in bis(trifluoromethylsulfonyl)amide (TFSA) salts were investigated using the pressure-swing method. The effects of cation species and temperature on the NH₃ absorption behaviour of four TFSA salts, namely, Na[TFSA], K[TFSA], Mg[TFSA]₂, and Ca[TFSA]₂, were evaluated. NH₃ was absorbed by these solid TFSA salts, and high NH₃ desorption was observed for Na[TFSA] at 473 K and K[TFSA] at 300 K. The NH₃ absorption behaviour varied with the cation of the TFSA salt. Crystallographic refinement showed that the crystal lattice of Na[TFSA] expanded and contracted along the c-axis upon NH₃ absorption and desorption, respectively, indicating the coordination of NH₃ molecules with cation sites between the lattice layers. For the alkaline-earth metal TFSA salts, NH₄[TFSA] and amide compounds (Mg(NH₂)₂ or Ca(NH₂)₂) were formed after NH₃ absorption. Therefore, two absorption processes—coordination and dissociation of NH₃—occurred in the TFSA salts.