Siliceous extra-framework species in dealuminated Y zeolites generated by steaming

The dealumination of NH4Y zeolite by steaming at 873 K and 1 bar water pressure is accompanied by a partial decomposition of the framework. X-ray amorphous aluminosilicate and silica gel are formed as extra-framework siliceous species in addition to the known polymeric aluminium ions. Source of the aluminosilicate and silica gel are Al-rich and Si-rich parts of the non-homogeneously crystallized FAU framework which are unstable during steaming. The Al-rich part consists of a Si/Al gradient as characterized by means of Molybdate experiments. The extra-framework siliceous species are identified by 29Si MAS NMR spectroscopic measurements as extra Si(4Al) and Si(0Al) signals which appear separated from the corresponding signal distribution of the zeolite framework. A certain portion of this silica gel superimposes, however, the Si(0Al) and Si(1Al) signal distribution of the zeolite framework. This “non-visible” part is quantified by the calculation of Si/Al ratio obtained by 29Si MAS NMR spectroscopy with respect to results obtained using the characteristic lattice parameter values.