Pair distribution function as a probe for disorder in molecular sieves

The objective of my research was to adapt the technique called Pair-Distribution Function (PDF) to the study of local structure in molecular sieves in order to fill the gap in information that currently exists for the understanding of their structure-property relationship.; We have explicitly studied three different types of structural disorder in zeolites using this technique: static (permanent structural defects, e.g. stacking faults), dynamic (local atomic deviation from the average structure) and adsorbed phase disorder. In addition we have studied the reliability of the method in studying the amorphous structure of carbon molecular sieves.; We have used the PDF method to understand the faulted structure of zeolite beta, a zeolite that is used industrially as a catalyst for alkylation reactions. Zeolite beta's structure is described as an intergrowth of two hypothetical polytypes A and B. We showed that the current structural models of the two polytypes do not agree with experiment but optimization of the structures using force fields improved the agreement greatly. Using the PDF method we obtained better structural models for the two end polytypes of zeolite beta. These results show that the PDF method is a viable technique to analyze the local structure of zeolites with stacking faults where the end polytypes have the same local structure. However, given the complexity of some zeolite frameworks, for which many atomic positions need to be refined, the use of high-resolution patterns from both X-ray and neutron radiation is essential to obtain reliable refinements.; Next we used the PDF method to gain insight into the mechanism of contraction of zeolite chabazite, which is one of the most thermally contracting materials known. Using the PDF method we have shown that, contrary to what is commonly accepted, rigid unit modes are not causing the contraction of the zeolite with temperature.; In a third investigation we showed that the PDF method can be used to study the structure of industrially relevant supported metal clusters. Due to their cost, metal catalysts are usually supported in a matrix in a ∼1% wt. concentration. We synthesized a sample of 1.1% wt Pt supported in zeolite beta and characterized it using XRD, X-ray photoelectron spectroscopy, X-ray absorption, small angle scattering (SAXS), dark-field scanning transmission electron microscopy, and PDF. We were able to determine that the platinum clusters were in a reduced state, monodisperse and located mainly in the pores. (Abstract shortened by UMI.)...