Stability of zeolites under electron irradiation and imaging of heavy cations in silicates

Abstract

The aluminosilicate framework of synthetic ‘zeolites’ of type A (Na⁺-form) and ZSM-5, as well as the Na⁺-forms of faujasite (Fig. 1) in their so-called X- and Y-variants (fuller structural descriptions are given in ref s 1 and 2) have recently been imaged at near-atomic scale by high-resolution electron microscopy (HREM)^3–5. However, the as-prepared synthetic and naturally occurring zeolites, all of which contain alkali metal and alkaline-earth exhangeable cations and much constitutional, intracrys-tallite water, are converted within minutes into an amorphous state on examination in an electron beam of the intensity required to yield high-resolution images. The specimen has to withstand 10⁶ electrons Å⁻² bombardment within a few minutes. To extract further information from HREM about the local atomic structure, and, in particular, to identify the siting of the various cations that may be accomodated in these catalytically important and shape-selective materials, methods have to be developed which retain the structural integrity of the zeolite for a reasonable time under electron irradiation in vacuo. It has been shown³ that dehydration of the zeolite before observation significantly increases its lifetime in an electron beam. Here we show that the commercially important⁴ zeolite-Y may be chemically stabilized against electron irradiation damage by the simple expedient of incorporating uranyl (UO²⁺₂) ions into the structure beforehand. Beam sensitivities are decreased by factors of 10–100 in this way, but appropriate heat-treatment of the UO₂-exchanged zeolite is required to achieve this stability and hence to locate these heavy cations.