2011 Spring Symposium
Lynne B. McCusker and Christian Baerlocher
Laboratory of Crystallography
Abstract — Structural information is essential to the understanding of zeolite chemistry. Whether it is the synthesis process, the adsorption properties, the ion-exchange selectivity, or the catalytic activity that is of interest, the key lies in the structure. However, zeolitic materials tend to be polycrystalline, so standard single-crystal methods of structure analysis cannot be applied. Fortunately, powder diffraction methods have developed enormously in the last 20 years, and as a result, structure analysis using powder diffraction data has become almost routine. Nevertheless, zeolite structures often pose a challenge to these methods.
For structure solution, i.e. when the zeolite framework structure is not known, the problems to be overcome are twofold: (1) the basic phase problem, which is central to all crystallographic structure analyses (including single crystals), and (2) the reflection overlap problem, which is specific to powder diffraction data. Over the years, our group has developed several different approaches to these problems, and a few of these (the use of chemical information to supplement the powder diffraction data, the adaptation of the charge-flipping algorithm to accommodate powder diffraction data, and the use of electron microscopy data to complement the powder diffraction data) will be described.
Once the basic framework structure is known, investigation of the details of the structure can begin. Where is the structure directing agent? Where are the isomorphously substituted atoms in the framework? What happens upon calcination? Where are the ions before and after ion exchange? Where are the sorbed molecules? Under favorable circumstances, these questions can be addressed by generating difference electron density maps using the measured powder diffraction pattern and the framework structure model. However, not all problems are amenable to this technique and even when they are, the interpretation of the maps still requires considerable patience and perseverance. The possibilities and limitations will be discussed.
Speaker’s Biography — Lynne McCusker has been studying zeolite crystal structures since she started her doctoral research with Karl Seff at the University of Hawaii in 1976. After spending three years as a postdoc in the group of Walter Meier at the ETH in Zurich, Switzerland learning powder diffraction techniques from Christian Baerlocher, she moved on to Texas A&M University to put these newly acquired skills into practice. In 1985, she moved back across the Atlantic for another postdoc, this time at Oxford University in Mike Glazer’s group. There she learned the value of synchrotron radiation and solved her first novel zeolite framework structure (Sigma 2). For this work, she received the Barrer Award from the British Zeolite Association in 1987 and the Physical Crystallography Award from the British Crystallographic Association in 1989. In 1988, she returned to the ETH and for the last 20 years has headed a group, together with Christian Baerlocher, devoted to the development of methodology for solving zeolite framework structures from powder diffraction data. In 2007, they received the Breck Award of the IZA (together with Osamu Terasaki) for their zeolite structural work. In 2010, she received the IZA award for the research she has done with Christian and for her service to the IZA community.