Repeatedly performed solid-state substitution of silicon for framework aluminium in Y zeolite using crystalline (NH4)2[SiF6] as dealumination agent

Abstract

Mixtures of NH₄Na-Y zeolite and crystalline (NH₄)₂[SiF₆] (applied in molar amounts corresponding to 0.38% of the framework aluminium) were subjected in covered but not hermetically closed dishes to heat treatment at temperatures below the decomposition point of the complex salt and subsequently washed with water or buffer solution. This process was twice repeated without any further intermediate treatment. The products were characterized by XRD, IR spectroscopy, without and with pyridine as probe molecule, ²⁹Si MAS NMR spectroscopy, ammonia TPE (thermal deammoniation) and wet chemical analysis. They proved to be highly crystalline with generally low concentrations of lattice defects (vacancies) due to desilication and/or non-substitutive dealumination. The products obtained after two subsequent dealumination steps exhibited X-ray crystallinities of 88–93% at dealumination degrees up to 70%. After the third successive dealumination step the X-ray crystallinity was found to amount to 80–88% and the dealumination degree to about 80%. With increasing dealumination degree the thermal and hydrothermal stability of the structure as well as the resistance of HF and LF bridging hydroxyls to thermal dehydroxylation gradually increased and the LF hydroxyls became accessible to pyridine.