Model-Free Search for Extra-Framework Cations in Zeolites Using Powder Diffraction

Abstract

The crystal framework of zeolites can usually be determined using prior chemical knowledge and traditional crystallographic techniques using direct methods or Patterson syntheses. However, a more difficult problem often occurs in the structure refinement stage, namely the location of extra-framework cations, a key-feature of any zeolite catalyst. To find these, the methods currently in use rely heavily on educated guesses, constrained Rietveld refinements and Fourier syntheses. Here, we demonstrate a more objective way of locating these cations from powder diffraction data using a maximum-entropy approach. We have successfully applied this procedure to synchrotron x-ray powder data pertaining to Na-A and Y aluminosilicate zeolites.