Surprises in the structural chemistry of zeolites

High-resolution electron microscopy, apart from strikingly confirming the correctness of the X-ray-based models for the skeletal structure of the aluminosilicate frameworks of zeolites, points to the existence of new families of ordered, crystalline microporous solids (e.g., with composition A_xB_xC_m?xO_2m · nH₂O, where A is an exchangeable monovalent cation, B is Al or Ga, C is Si or Ge, and x, m, n are integers.) It also reveals crystalline imperfections and unexpected superlattice structures in A-type and faujasitic zeolites, and the nature of the intergrowths in, for example, $\text{ZSM-5}\text{ZSM-11}$ materials. The short-range order of Si and Al within the aluminosilicate framework may be directly explored by magic-angle-spinning NMR (MASNMR) employing ²⁹Si and ²⁷Al nuclei. This technique probes the site symmetry and environment of these atoms. Al in tetrahedral as well as in octahedral sites may be readily identified and so may the populations of groups such as Si(OAl)₄, Si(OAl)₃, (OSi), etc., so that new information is obtained pertaining to Si, Al ordering in a variety of zeolitic solids.