Zeolitic water content and adsorptive capacity for ammonia of microporous sepiolite

Abstract

Sepiolite from Amboseli, Tanzania, possesses a high adsorptive capacity for ammonia at 273 K and 298 K after outgassing below 378 K; the B.E.T. (N₂) area is ca. 360 m² g^–1. The adsorption of NH₃ and N₂ is greatly decreased by prior exposure of outgassed sepiolite to NH₃ followed by outgassing at 373 K, or by outgassing fresh sepiolite above 383 K. Desorption of NH₃ from the solid is accompanied by the loss of H₂O previously strongly held. Exchange of NH₃ with up to half the co-ordinately bound H₂O in sepiolite is postulated. A classification of ‘acidic sites’ on the surface on the basis of NH₃ retention is thus not possible. The adsorptive capacities for NH₃ and N₂ of sepiolite exposed to NH₃ can be restored to the original values by exposure to H₂O vapour for 24 h. Only partial regeneration is possible for sepiolite outgassed above 393 K.

Thermogravimetric studies in a vacuum and in air yield results which are compared with other data and with theoretical models of the sepiolite structure. The above effects are discussed in terms of reversible and irreversible bonding of adsorbed NH₃, of the exchange of NH₃ and structural H₂O, and of the ‘tilting’ of lattice channels of the sepiolite structure when it is heated in vacuo above 390 K.