Structural elucidation of microporous and mesoporous catalysts and molecular sieves by high-resolution electron microscopy.
Twenty years ago, one of us embarked (Bursill, L. A.; Lodge, E. A.; Thomas, J. M. Zeolitic structures as revealed by high-resolution electron microscopy. Nature 1980, 286, 111-113) on the study of zeolites (renowned for their electron-beam sensitivity) by high-resolution transmission electron microscopy (HRTEM). In the ensuing years, high-resolution imaging aided by optical diffractometry has yielded details of the open framework structures of a number of new aluminosilicate and aluminophosphate molecular sieves and catalysts. The nature of intergrowth and recurrently twinned structures, as well as new types of structural imperfection in hitherto uncharacterized materials, has also been elucidated. With continued improvements in instrumental development, encompassing higher accelerating voltages, better objective lenses and vacua, computational advances, and the arrival of slow-scan CCD detectors, electron crystallographic methods and HRTEM imaging now enable the ab initio three-dimensional structures of micro- and mesoporous solids, with their occluded structure-directing organic species, to be determined. High-resolution scanning transmission electron microscopy using subnanometric probes provides supplementary structural and ultramicro analytical information and electron spectroscopic imaging (at the attogram level). In its high-angle annular dark-field mode, it is capable of locating and determining the composition of individual nanoparticle catalysts (consisting of just a few atoms) supported on porous hosts.