Pinpointing and Quantifying the Aluminum Distribution in Zeolite Catalysts Using Anomalous Scattering at the Al Absorption Edge
Ana B. Pinar, Przemysław Rzepka, Amy J. Knorpp, Lynne B. McCusker, Christian Baerlocher, Thomas Huthwelker, Jeroen A. van Bokhoven
Abstract
The location of aluminum in a zeolite framework structure defines the accessibility and geometry of the catalytically active sites, but determining this location crystallographically is fraught with difficulties. Typical zeolite catalysts contain only a small amount of aluminum, and the X-ray scattering factors for silicon and aluminum are very similar. To address this problem, we have exploited the properties of resonant X-ray powder diffraction across the Al K edge, where the aluminum scattering factor changes dramatically. By combining conventional synchrotron powder diffraction data with those collected at energies near the X-ray absorption edge, aluminum is highlighted. In this way, the different distributions of aluminum in two FER-type zeolites with identical chemical compositions but different catalytic properties could be determined unambiguously. The results are consistent with previous studies, but quantitative. This approach constitutes a major advance in our fundamental understanding of the relationship between zeolite structure and catalytic activity.