STRUCTURAL EVOLUTION OF CALCIUM OR LANTHANUM-EXCHANGED (NH_4)_2SiF_6-DEALUMINATED Y ZEOLITE AFTER VARIOUS CHEMICAL TREATMENTS

摘要

The structural evolution of Y zeolite (Si/Al = 2.17) weakly dealuminated by hexafluorosilicate (Si/Al = 3.13), denoted Y_D, and exchanged with calcium (CaY_D)or lanthanum (LaY_D), has been studied after acid-base treatments at 80℃ close to the cation exchange conditions. The stability of the samples is followed by X-ray diffraction and solid-state NMR of ~(29)Si and ~(27)A1; Y_D zeolite is completely destroyed by treatment with acid at pH 2.5 and suffers serious degradation on treatment with alkali at pH 11.8. The introduction of calcium or lanthanum improves the stability of the zeolite in acid and base. In acid it is not destroyed until pH 1. At pH 2, silicon and aluminium are extracted and an amorphous phase is formed in the case of the calcium-exchanged zeolite. Base treatment at pH 13 does not affect CaY_D and leads to silicon extraction in the case of LaY_D. The introduction of lanthanum into YD leads to the formation of a new phase which is eliminated by acid-base treatment.