Making New Materials - Zeolites
Zeolites are important nanoporous materials with many applications, including use in laundry detergents, as industrial catalysts and for cleaning up nuclear waste. Zeolites have a cage-like structure which enables them to trap charged particles called cations. Scientists from the University of Birmingham have been using Diamond to investigate new ways of locating cations and nanoparticles within different types of zeolites, this is important in order to understand and improve their use.
Zeolite A is perhaps the prototypical and most studied synthetic zeolite. There are four commonly accepted sites for cations (see below), but frequently the cations are spread out over severally partially occupied sites and therefore it is difficult to find them. A group led by Dr Joseph Hriljac at the University of Birmingham is exploring the possibility of using the Pair Distribution Function (PDF) to locate the exact positions of cations within the pores of zeolite A.
A view of the zeolite A framework showing the accepted cation positions in yellowHigh energy (60 keV) X-ray scattering data was collected at beam line I15 for a series of zeolites with Na, Zn, Ca and Ag cations, in both hydrated and dehydrated states. Data was also collected for ITQ-29, the pure silicon form with no cations.
The plots above show the PDF’s of four systems for comparison, where each peak in a PDF corresponds to an interatomic distance in the solid. This data was collected at 60 keV in a matter of minutes, highlighting the excellent quality that can be obtained at I15. It is readily clear that the peaks associated with the aluminosilicate framework, such as that at ~1.7 Å due to the (Si,Al)-O bonds, are essentially the same for all patterns but others change dramatically as a function of cation. This is more readily seen below, comparing the PDF’s of the hydrogen form of zeolite A and a specially prepared sample of the Zn form of A. Work is now underway to build a model for the Zn system, a problem that has not yet been tractable using traditional crystallographic analysis with powder diffraction data.