Toward Understanding Extra-Large-Pore Zeolite Energetics and Topology: A Polyhedral Approach

Porous crystalline materials (e.g., zeolites, aluminophosphates) are widely used as catalysts, absorbents, and membranes. Two factors that largely determine the utility of any such material are its maximum pore\channel size and its energetic stability. Presently, the maximum pore size for materials stable enough to be of commercial use is approximately 8 A. Extending the applicability of porous crystalline materials to encompass the control of molecules significantly larger than simple gases, and thus be of potential interest in the areas of fine-chemicals, pharmaceuticals, and nanotechnology, requires stable materials with larger pores. Unfortunately, no systematic, rational method exists to guide the development of such materials. Here, we show how for a large class of porous crystalline materials certain considerations of the topology and energetics of the constituent cages, using topological descriptors and quantum mechanical calculations, respectively, can lead to definite predictions regarding the stability of large pore\channel materials. The analysis formally demonstrates why smaller rings in such structures naturally compensate and help to stabilize large pores. These new insights allow us not only to predict the relative thermodynamic stability of a range of desirable (but as yet unmade) porous materials, but also to give practical advice to the experimentalist to guide their actual synthesis