Separating Solids: Purification of Metal-Organic Framework Materials

摘要

A tremendous development in the area of functional, nanostruc-tured materials is the emergence of large numbers of structurally well defined, permanently microporous metal-organic framework materials (MOFs). Consisting of metal-ion or-cluster nodes and multitopic organic struts, such materials are often characterized by very large internal surface areas, low densities, and uniformly sized channels and pores. Among the many applications that may capitalize on these extraordinary properties are gas storage, chemical separations, and selective catalysis, MOFs are generally synthesized via one-pot solvothermal methods. Since purification of the resulting network solids is not feasible via the methods usually employed by chemists (distillation, recrystallization, chromatography, sublimation, etc.), a premium is placed on discovering conditions that yield pure products. Typically, discovery entails systematically evaluating scores of reaction conditions that differ only slightly from initial or refined conditions (e.g., temperature, solvent composition, reactant concentrations, reaction time, and even reaction vessel size). Alternatively, if sufficiently large crystals of distinct morphology or color are obtained, they can be manually separated from undesired byproducts -albeit, often in painstaking fashion. Nevertheless, isolation of pure materials is essential; closely structurally related porous materials can often differ enormously in terms of properties and functional behavior.