Deciphering Ordered But Noncrystalline Structures

Zeolites and layered silicate clay materials have a variety of applications, including catalysis and chemical separations. Although the materials are molecularly ordered, pinning down their exact structures can be difficult because they may not form crystals large enough for single-crystal X-ray diffraction. A new approach to determining structures of such solids combines powder X-ray diffraction, ²⁹Si nuclear magnetic resonance spectroscopy, and computer modeling (J. Am. Chem. Soc., DOI: 10.1021/ja311649m). A research team led by Bradley F. Chmelka of the University of California, Santa Barbara, and Sylvian Cadars of France’s National Center for Scientific Research demonstrated the approach on a nanocomposite clay made of silicate layers separated by an alkylammonium surfactant. The researchers obtained data on repeating structural units from the X-ray analysis and on silicon bonding from NMR. They used those data to narrow the number of possible framework structures. With density functional theory, they were able to identify two structures that best represent the nanocomposite.