This article examines the water stability and adsorption in metal organic frameworks. Topics discussed include ways to quantify water stability and existing MOFs and their stability.
Metal-organic frameworks (MOFs) have extremely high surface areas and pore volumes, tunable pore sizes and chemical functionalities and are an interesting addition to the field of porous materials. ...One of the key challenges that needs to be overcome to elevate MOFs to the applied level is their sensitivity to humid environments. The work reported here seeks to address this critical issue by examining two important contributors to MOF stability or instability: basicity (pKa value) of the pillar ligand and catenation of the framework. Catenation is the interpenetration or interweaving of two or more identical and independent frameworks. We demonstrate that, using catenation in combination with a pillaring strategy, it is possible to obtain water stable MOFs even when the pillar ligand has lower basicity (pKa value). This study shows that after 90% relative humidity (RH) exposure, comparing Zn-BDC-DABCO (DMOF) and Zn-BDC-BPY (MOF-508), MOF-508 is stable due to its two-fold interpenetration that prevents significant water adsorption. In contrast, comparing non-catenated isostructural pillared MOFs Zn-TMBDC-DABCO (DMOF-TM) and Zn-TMBDC-BPY (MOF-508-TM), MOF-508-TM is unstable since BPY is less basic (higher pKa) and less rigid than DABCO.
The surface area is one of the most important quantities for characterizing novel porous materials. The BET analysis is the standard method for determining surface areas from nitrogen adsorption ...isotherms and was originally derived for multilayer gas adsorption onto flat surfaces. Metal−organic frameworks (MOFs) are a relatively new class of crystalline, porous materials that have been shown to exhibit very large BET surface areas. These materials are microporous and possess surfaces that are far from flat. In some MOFs, adsorption occurs through a pore-filling mechanism rather than by layer formation. Thus, it is unclear whether BET surface area numbers reported for these materials are truly meaningful. Given the standard practice of reporting BET surface areas for novel porous materials, a critical test of the BET method is much needed. In this work, grand canonical Monte Carlo simulations were used to predict adsorption isotherms for nitrogen in a series of MOFs. The predicted isotherms were used as pseudoexperimental data to test the applicability of the BET theory for obtaining surface areas of microporous MOFs. BET surface areas calculated from the simulated isotherms agree very well with the accessible surface areas calculated directly from the crystal structures in a geometric fashion. In addition, the surface areas agree well with experimental reports in the literature. These results provide a strong validation that the BET theory can be used to obtain surface areas of MOFs.
The development of new adsorbents or new adsorption separation processes has the potential to bring enormous energy and cost savings to the chemical process industry. Achieving success in this area ...requires a detailed understanding of how a proposed adsorbent performs under realistic conditions, but most research studies focus on idealized process streams. This critical lack of data on adsorption of complex mixtures represents a major barrier to growth in the development of new separation systems. This commentary discusses the persistent nature of the mixture-adsorption knowledge gap, while providing some historical framework, and provides several strategies that researchers might adopt to move toward true multicomponent adsorption studies.
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 CO2, CH4, N2, 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 CO2 over CH4 and N2.
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.
Thermal stability and heat capacity of several metal–organic frameworks and their corresponding organic ligands have been investigated systematically using TGA-DSC technique. A simple notation system ...was created to present the local coordination environment around metal atoms in a secondary building unit (SBU). The heat capacity contributions of organic functional groups and SBUs were examined using the group-contribution method. Our results suggest that the thermal stability of MOFs is determined by the coordination number and local coordination environment instead of framework topology. Specific heat capacities (Cp ) of all examined MOFs exhibit comparable values as other solids including carbon nanotubes, zeolites, and minerals. The molar heat capacity contributions of SBUs in MOFs indicate similar abnormal thermal behavior as negative thermal expansion of MOFs.
UiO-66 is a Zr-based MOF that is being highly investigated for a wide variety of small molecule gas separations since it possess unprecedented thermal, chemical, and mechanical stability. In this ...work, we have investigated the performance of various functionalized variations of UiO-66 (UiO-66-OH, UiO-66-(OH)2, UiO-66-NO2, UiO-66-NH2, UiO-66-SO3H, and UiO-66-(COOH)2) towards ammonia removal from air. Functionalized UiO-66 analogs have been synthesized solvothermally and characterized using ammonia breakthrough measurements under dry and humid (80% RH) air conditions along with powder X-ray diffraction (PXRD) patterns and results from BET modeling of N2 adsorption isotherms. Counter to chemical intuition, our study demonstrates that the ammonia capacities of UiO-66-SO3H and UiO-66-(COOH)2 are lower than UiO-66-OH and UiO-66-NH2. This is due to significant reduction in the framework porosity (surface area and pore volume) upon functionalization with bulky functional groups such as –COOH and –SO3H. The –OH group is the least bulky functional group considered in the work and interacts favorably with ammonia. UiO-66-OH has a capacity of ~5.7mmol/g for ammonia under dry conditions which is very close to the ammonia removal goal of 0.1g/g MOF (or ~6mmol/g). However, we observed a decrease in the ammonia capacities of functionalized UiO-66 variations under humid conditions due to competition between water and ammonia molecules for adsorption on the active sites. Overall, balancing the water adsorption behavior and high selectivity and high capacity for ammonia is crucial to developing new adsorbents for ammonia removal from air.
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•UiO-66 and 6 variants were synthesized and studied for adsorption of ammonia from air.•Functional groups were chosen for high interaction with ammonia.•Hydroxyl groups interact the most effectively with ammonia at low concentration.•Breakthrough curves were measured on all materials using a microbreakthrough system.•Large-pore MOFs (>10Å) are necessary for efficient use of complex 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.
