Evolution thermique de zeolithes y chromees

Two series of ammonium—chromium exchanged zeolites, Cr_zNH_4yNa_xY obtained by ionic exchange and Cr_z′NH_4y′HY/T for which an intermediate form NH_4yNa_xY has been steamed, are investigated by use of TGA, DTA and X-ray. Introduction of chromium effectively stabilizes the structure. Measurements of hydration and hydroxylation levels show that dehydroxylation of the bulk occurs early due to the fixation of chromium ions and that thermostable hydroxyls are present. The stability of chromium exchanged zeolites occurs through the occupation of cavities by oxocations. Generally self-steaming takes place and zeolites change into a mixture of both wet- and dried-air treated forms.