Zeolitic water content and adsorptive capacity for ammonia of microporous sepiolite

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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.

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