Dealumination of Y zeolite with ammonium hexafluorosilicate: A SIMS-XPS study of the aluminum distribution

Abstract

A series of samples with different chemical Si Al ratios (3–7.2) and crystallinities above 80% were prepared by dealumination of a NH 4NaY zeolite with different amounts of ammonium hexafluorosilicate (from 0.25 to 1.15 the stoichiometric hexafluorosilicate/zeolite ratio). With all the samples, the Si Al ratio on the outer surface of the crystallites (measured by XPS) was much greater than was the Si Al ratio determined by chemical analysis. The chemical Si Al ratio was slightly greater than was the framework Si Al ratio estimated by X-ray diffraction or measured by 29Si n.m.r. For the most dealuminated sample, the concentration profile of Si and Al was determined through a combined XPS-SIMS study down to about 40 nm. It can be concluded that the gradient in the Si Al ratio in the crystallites was due both to a gradient of dealumination (dealumination is diffusion controlled) and to a selective deposit of silica on the outer surface.