Delta Chem: A New Geometric Approach of Porosity for Symmetric Porous Materials

Porous materials, such as zeolites and metal-organic frameworks (MOFs), and zeolitic-organic frameworks (ZIFs), are frequently considered for shape-selective separations, molecular storage, and catalysis applications, mainly due to their hollow structures. The amount and chemical nature of sorbate molecules that may (or may not) be fitted inside their cavities, and hence the bulk of their applications, depend on their internal structure, that is, on their surface areas, available volumes, and shapes of their porosities. However, experimentally, the access to such strucutral information is somewhat limited and computationally can be expensive to calculate for structures of more than 100 atoms. Moreover, the large number of known and hypothetical structures reported makes computational geometry-based techniques particularly attractive to identify the most suitable structures for a desired application. In this context, Delta Chem is both a method and a program designed to quickly analyze porous structures, relying solely on their Cartesian coordinates, and characterize the shapes of their cages using regular convex polyhedra. The program also provides a systematic approach to determine the positions of the centers of porosity and the atoms that contribute to form the internal surfaces of these materials, as well as other geometric features of the porosities such as volumes and surface areas. It also includes a routine to compute the irreducible volumes of the cages, i.e., minimal regions of the cavities that can be used to represent the hole porosity shape via symmetry operations. The capabilities of the program are tested on well-studied porous systems, namely, Buckminsterfullerene, MOF-5, HKUST-1, UiO-66, and ZIF-8. As highlighted through the fullerene, it can equally be used to characterize the cavities of hollow molecules. Our approach is compared against other widely used polyhedra-based approaches for porous materials. Our results show that Delta Chem is a novel and systematic way of characterizing routinely porous materials and hollow molecules. Besides potential applications to systematically simplify computational studies of shape-dependent properties, like shape-selective catalysis and adsorption, Delta Chem can be used in many studies to generate basic geometrical models.