Postsynthetic Tuning of Metal–Organic Frameworks for Targeted Applications

Abstract

Metal-organic frameworks (MOFs) are periodic, hybrid, atomically well-defined porous materials that typically form by self-assembly and consist of inorganic nodes (metal ions or clusters) and multitopic organic linkers. MOFs as a whole offer many intriguing properties, including ultrahigh porosity, tunable chemical functionality, and low density. These properties point to numerous potential applications, including gas storage, chemical separations, catalysis, light harvesting, and chemical sensing, to name a few. Reticular chemistry, or the linking of molecular building blocks into predetermined network structures, has been employed to synthesize thousands of MOFs. Given the vast library of candidate nodes and linkers, the number of potentially synthetically accessible MOFs is enormous. Nevertheless, a powerful complementary approach to obtain specific structures with desired chemical functionality is to modify known MOFs after synthesis. This approach is particularly useful when incorporation of particular chemical functionalities via direct synthesis is challenging or impossible. The challenges may stem from limited stability or solubility of precursors, unwanted secondary reactivity of precursors, or incompatibility of functional groups with the conditions needed for direct synthesis. MOFs can be postsynthetically modified by replacing the metal nodes and/or organic linkers or via functionalization of the metal nodes and/or organic linkers. Here we describe some of our efforts toward the development and application of postsynthetic strategies for imparting desired chemical functionalities in MOFs of known topology. The techniques include methods for functionalizing MOF nodes, i.e., solvent-assisted ligand incorporation (SALI) and atomic layer deposition in MOFs (AIM) as well as a method to replace structural linkers, termed solvent-assisted linker exchange (SALE), also known as postsynthethic exchange (PSE). For each functionalization strategy, we first describe its chemical basis along with the requirements for its successful implementation. We then present a small number of examples, with an emphasis on those that (a) convey the underlying concepts and/or (b) lead to functional structures (e.g., catalysts) that would be difficult or impossible to access via direct routes. The examples, however, are only illustrative, and a significant body of work exists from both our lab and others, especially for the SALE/PSE strategy. We refer readers to the papers cited and to the references therein. More exciting, in our view, will be new examples and new applications of the functionalization strategies-especially applications made possible by creatively combining the strategies. Underexplored (again, in our view) are implementations that impart electrical conductivity, enable increasingly selective chemical sensing, or facilitate cascade catalysis. It will be interesting to see where these strategies and others take this compelling field over the next few years.

Supramolecular Organization in Confined Nanospaces

“…For example, strategies for the controlled incorporation of capped nanoparticles of various shape, composition and size into a ZIF can produce composites with good catalytic, magnetic and optical properties derived from the encapsulated nanoparticles . Another emerging strategy is the use of atomic layer deposition approaches ,. Also, self‐assembly methods were successfully exploited to confine nanowires with electron donor character into the periodic pores of electron‐poor MOFs acting as acceptors.…”

Evaluation of the BET Theory for the Characterization of Meso and Microporous MOFs

“…They are often compared to zeolites; however certain characteristics are often not present in zeolites including big pore sizes, high sorption capacities, and complex sorption behavior . New MOF structures with diverse properties are still emerging with ever increasing specific surface areas, that have to be determined in the course of MOF characterization . In this regard, correct measurements of the surface areas are valuable as this is a very important characteristic of microporous materials.…”

Incorporating the Thiazolo[5,4-d]thiazole Unit into a Coordination Polymer with Interdigitated Structure

“…Due to the immense quantity of possible inorganic and organic building units several applications are discussed (e.g., gas storage and separation, catalysis, sensing and heat transformation) [1][2][3][4][5][6][7][8][9][10][11][12][13]. Many different organic functionalities have been introduced into the frameworks either by a priori ligand functionalization or by post synthetic modification [14][15][16]. 4,4 -Biypridine based ligands have been used to construct a diverse set of different topologies (e.g., one-dimensional chains, ladders, two-dimensional and three-dimensional networks) [17].…”