Metal–Organic Polyhedra and Metal–Organic Frameworks: Understanding How Discrete Versus Extended Structure Impacts Surface Areas and Pore Size Distributions

Certain discrete metal–organic polyhedra (MOPs) contain pores that also appear in extended metal–organic frameworks (MOFs) and MOP-based supramolecular frameworks. Here, we have selected a library of three MOPs (ZrMOP, CuMOP, and PdMOP) and their corresponding MOFs with analogous pores (UiO-66, HKUST-1, and the Co Sponge) to understand how parameters related to their sorption behavior differ. Specifically, we report the BET areas and pore size distributions for all six species. The BET areas for the MOPs follow the same trend as those for their MOF counterparts, with the highest shown by the ZrMOP (379 m2/g) and UiO-66 (1110 m2/g) pair. A discussion of the Zr-based materials includes comparisons to a known ZrMOP-based supramolecular framework that maintains extrinsic porosity via noncovalent interactions. The lowest areas were measured for the PdMOP (33 m2/g) and Co Sponge (80 m2/g) pair, a ramification of a phase transition that occurs during activation. Additionally, pore size distribution measurements suggest that the lower BET areas likely result from inaccessible internal cavities. Although the ZrMOP has the highest area per pore, its BET area is not as high as that of UiO-66 because the pores in an MOF share building blocks, greatly reducing the mass required to support a given number of pores, and the pores pack more tightly in space. MOFs are also more resilient to structural collapse upon activation, though we highlight some interesting examples for a loss of crystallinity that can increase the BET area of certain MOPs. Supramolecular chemistry can further enhance the properties of MOPs, where careful ligand or cage design can promote noncovalent interactions to enforce additional extrinsic porosity. Although MOFs remain top candidates for bulk storage and uptake due to their gravimetric and volumetric areas, for applications in thin films and membranes for separations chemistry, highly dispersed pores are desirable and the higher area per pore of MOPs is advantageous.