Credit: © 2010 Wiley

Mesoporous crystals are important materials for catalysis and separation. As with so much of chemistry, controlling their properties relies on understanding and then tailoring their structures. This is somewhat problematic, however, because their crystallinity arises from the ordered distribution of pores, rather than of atoms as in normal crystalline materials. So instead of X-ray crystallography, electron crystallography is the technique of choice, but resolution has so far been limited to 1.5 nm. The mesopore walls are often formed using surfactant templates that can remain in the as-synthesized samples, further complicating their structural elucidation.

Now, Keiichi Miyasaka and Osamu Terasaki from the Korea Advanced Institute of Science and Technology and Stockholm University have developed1 self-consistent structural solutions for as-synthesized and calcined mesoporous crystals using electrostatic potential maps obtained from electron crystallography. Miyasaka and Terasaki rely on assessing the curvature of the pores to find an equipotential surface and thus a realistic representative unit cell. To confirm the accuracy of their results, they simulated what transmission electron micrographs would look like, based on the pore-wall structure they had determined. These agreed well with experimental micrographs.

The calcined sample had larger volumes because the silica wall contracts during calcination. Miyasaka and Terasaki also found that the walls in the calcined crystals were smoother than in the as-synthesized ones. Comparing mesoporous crystals with different structures (MCM-48 and AMS-10) showed that AMS-10 has lower surface complexity than MCM-48 and thus a lower surface area.