Syntheses, crystal structures, and water adsorption behaviors of jungle-gym-type porous coordination polymers containing nitro moieties

A novel fabricated material with divergent chemical handles based on UiO-66 and used for targeted photodynamic therapy

A unique zirconium-based framework, used as a fabricated material to achieve dual functional chemical modification of UiO-66, was synthesized and demonstrated.

Flexibility in metal–organic frameworks derived from positional and electronic effects of functional groups

The position of functional groups and the subsequent e-density of the benzene rings in a bi-/tri-functionalized zinc-based metal–organic framework (MOF) have been controlled to reveal structural differences.

Compositional control of pore geometry in multivariate metal–organic frameworks: an experimental and computational study

Changing the ratio of the dicarboxylates, L : L′, in MOFs of the general formula [Zn₂(L)_2–x(L′)_x(dabco)] affords control of the pore geometry, through non-covalent interactions between the ligands.

Modulating adsorption and stability properties in pillared metal-organic frameworks: a model system for understanding ligand effects

Metal-organic frameworks (MOFs) are nanoporous materials with highly tunable properties that make them ideal for a wide array of adsorption applications. Through careful choice of metal and ligand precursors, one can target the specific functionality and pore characteristics desired for the application of interest. However, among the wide array of MOFs reported in the literature, there are varying trends in the effects that ligand identity has on the adsorption, chemical stability, and intrinsic framework dynamics of the material. This is largely due to ligand effects being strongly coupled with structural properties arising from the differing topologies among frameworks. Given the important role such properties play in dictating adsorbent performance, understanding these effects will be critical for the design of next generation functional materials. Pillared MOFs are ideal platforms for understanding how ligand properties can affect the adsorption, stability, and framework dynamics in MOFs. In this Account, we highlight our recent work demonstrating how experiment and simulation can be used to understand the important role ligand identity plays in governing the properties of isostructural MOFs containing interconnected layers pillared by bridging ligands. Changing the identity of the linear, ditopic ligand in either the 2-D layer or the pillaring third dimension allows targeted modulation of the chemical functionality, porosity, and interpenetration of the framework. We will discuss how these characteristics can have important consequences on the adsorption, chemical stability, and dynamic properties of pillared MOFs. The structures discussed in this Account comprise the greatest diversity of isostructural MOFs whose stability properties have been studied, allowing valuable insight into how ligand properties dictate the chemical stability of isostructural frameworks. We also discuss how functional groups can affect adsorbate energetics at their most favorable adsorption sites to elucidate how functional groups can affect the adsorptive performance of these materials in ways that are unexpected based on the isolated ligand's properties. We then highlight a variety of simulation tools that not only can be used to understand the differing molecular-level behavior of the adsorbate and framework dynamics within these isostructural MOFs, but also can shed light on possible mechanisms that govern the differing chemical stability properties among these materials. Lastly, we provide perspective on the challenges and opportunities for utilizing the structure-property relationships arising from the ligand effects described in this Account for the design of further MOFs with enhanced chemical stability and adsorption properties.

Adjusting the stability of metal-organic frameworks under humid conditions by ligand functionalization

The practical use of metal-organic frameworks (MOFs) in applications ranging from adsorption separations to controlled storage and release hinges on their stability in humid or aqueous environments. The sensitivity of certain MOFs under humid conditions is well-known, but systematic studies of water adsorption properties of MOFs are lacking. This information is critical for developing design criteria for directing future synthesis efforts. The goal of this work is to understand the influence of the extent of Zn-O bond shielding on the relative stabilities of MOFs belonging to same family of isostructural, noncatenated pillared MOFs [Zn(L)(DABCO)(0.5)], where L is the functionalized BDC (1,4-benzenedicarboxylic acid) linker. The different extent of Zn-O bond shielding is provided by incorporating a broad range of functional groups on the BDC ligand. The resulting MOFs have varying surface areas, pore sizes, and pore volumes. Stability is assessed through water vapor adsorption isotherms combined with powder X-ray diffraction (PXRD) experiments and surface area analyses. Our study demonstrates that integration of polar functional groups (e.g., nitro, bromo, chloro, hydroxy, etc.) on the dicarboxylate linker renders these MOFs water unstable compared to the parent MOF as these polar functional groups have a negative shielding effect; i.e., they facilitate hydrolysis of the Zn-O bond. On the other hand, placing nonpolar groups (e.g., methyl) on the BDC ligand results in structurally robust MOFs because the Zn-O bond is effectively shielded from attack by water molecules. Therefore, the anthracene- and tetramethyl-BDC MOFs do not lose crystallinity or surface area after water exposure, in spite of the large amount of water adsorption due to capillary condensation at ∼20% relative humidity (RH). This has been observed rarely in the MOF literature. The results of this work show that by ligand functionalization it is possible to adjust the water stability of a pillared MOF in both the positive and negative directions and, thus, provide an important step toward understanding the water adsorption behavior of MOFs.

Tuning the adsorption properties of UiO-66 via ligand functionalization

UiO-66 is one of the few known water-stable MOFs that are readily amenable to direct ligand substitution. In this work, UiO-66 has been synthesized with amino-, nitro-, methoxy-, and naphthyl-substituted ligands to impart polar, basic, and hydrophobic characteristics. Pure-component CO(2), CH(4), N(2), and water vapor adsorption isotherms were measured in the materials to study the effect of the functional group on the adsorption behavior. Heats of adsorption were calculated for each pure gas on each material. The results indicate that the amino-functionalized material possesses the best adsorption properties for each pure gas due to a combination of polarity and small functional group size. The naphthyl-functionalized material exhibits a good combination of inhibited water vapor adsorption and high selectivity for CO(2) over CH(4) and N(2).