Single-Component Adsorption Equilibria of CO2, CH4, Water, and Acetone on Tapered Porous Carbon Molecular Sieves.

Engineered carbon molecular sieves (CMSs) with tapered pores, high surface area, and high total pore volume were investigated for their CO2, CH4, water, and acetone adsorption properties at 288.15, 298.15, 308.15 K, and pressures of <1 bar. The results were compared with BPL carbon. The samples exhibited higher adsorption capacity for CO2 compared to BPL carbon, with Carboxen 1005 being the highest due to the presence of ultramicropores (pores smaller than 0.8 nm). Similar observations were made for CH4 except at 288.15 K. Although the CMSs exhibited higher hydrophobicity than BPL carbon, the latter had the highest acetone uptake for all investigated temperatures due to its higher oxygen content, which facilitates stronger interactions with polar VOC molecules. Heats of adsorption were calculated using the Clausius-Clapeyron equation after fitting the isotherms with the dual-site Langmuir-Freundlich model, and results largely corroborated the order of adsorption capacities of CO2, CH4, and water on the carbon materials.

Interrogating Encapsulated Protein Structure within Metal-Organic Frameworks at Elevated Temperature.

Encapsulating biomacromolecules within metal-organic frameworks (MOFs) can confer thermostability to entrapped guests. It has been hypothesized that the confinement of guest molecules within a rigid MOF scaffold results in heightened stability of the guests, but no direct evidence of this mechanism has been shown. Here, we present a novel analytical method using small-angle X-ray scattering (SAXS) to solve the structure of bovine serum albumin (BSA) while encapsulated within two zeolitic imidazolate frameworks (ZIF-67 and ZIF-8). Our approach comprises subtracting the scaled SAXS spectrum of the ZIF from that of the biocomposite BSA@ZIF to determine the radius of gyration of encapsulated BSA through Guinier, Kratky, and pair distance distribution function analyses. While native BSA exposed to 70 °C became denatured, in situ SAXS analysis showed that encapsulated BSA retained its size and folded state at 70 °C when encapsulated within a ZIF scaffold, suggesting that entrapment within MOF cavities inhibited protein unfolding and thus denaturation. This method of SAXS analysis not only provides insight into biomolecular stabilization in MOFs but may also offer a new approach to study the structure of other conformationally labile molecules in rigid matrices.

Adaptive Pore Opening to Form Tailored Adsorption Sites in a Cooperatively Flexible Framework Enables Record Inverse Propane/Propylene Separation.

A proposed low-energy alternative to the separation of alkanes from alkenes by energy-intensive cryogenic distillation is separation by porous adsorbents. Unfortunately, most adsorbents preferentially take up the desired, high-value major component alkene, requiring frequent regeneration. Adsorbents with inverse selectivity for the minor component alkane would enable the direct production of purified, reagent-grade alkene, greatly reducing global energy consumption. However, such materials are exceedingly rare, especially for propane/propylene separation. Here, we report that through adaptive and spontaneous pore size and shape adaptation to optimize an ensemble of weak noncovalent interactions, the structurally responsive metal-organic framework CdIF-13 (sod-Cd(benzimidazolate)2) exhibits inverse selectivity for propane over propylene with record-setting separation performance under industrially relevant temperature, pressure, and mixture conditions. Powder synchrotron X-ray diffraction measurements combined with first-principles calculations yield atomic-scale insight and reveal the induced fit mechanism of adsorbate-specific pore adaptation and ensemble interactions between ligands and adsorbates. Dynamic column breakthrough measurements confirm that CdIF-13 displays selectivity under mixed-component conditions of varying ratios, with a record measured selectivity factor of α ≈ 3 at 95:5 propylene:propane at 298 K and 1 bar. When sequenced with a low-cost rigid adsorbent, we demonstrated the direct purification of propylene under ambient conditions. This combined atomic-level structural characterization and performance testing firmly establishes how cooperatively flexible materials can be capable of unprecedented separation factors.

Surprising Use of the Business Innovation Bass Diffusion Model To Accurately Describe Adsorption Isotherm Types I, III, and V.

Using adsorption isotherm data to determine heats of adsorption or predict mixture adsorption using the ideal adsorbed solution theory (IAST) relies on accurate fits of the data with continuous, mathematical models. Here, we derive an empirical two-parameter model to fit isotherm data of IUPAC types I, III, and V in a descriptive way based on the Bass model for innovation diffusion. We report 31 isotherm fits to existing literature data covering all six types of isotherms, various adsorbents, such as carbons, zeolites, and metal-organic frameworks (MOFs), as well as different adsorbing gases (water, carbon dioxide, methane, and nitrogen). We find several cases, especially for flexible MOFs, where previously reported isotherm models reached their limits and either failed to fit the data or could not sufficiently be fitted due to stepped type V isotherms. Moreover, in two instances, models specifically developed for distinct systems are fitted with a higher R2 value compared to the models in the original reports. Using these fits, it is demonstrated how the new Bingel-Walton isotherm can be used to qualitatively assess the hydrophilic or hydrophobic behavior of porous materials from the relative magnitude of the two fitting parameters. The model can also be employed to find matching heats of adsorption values for systems with isotherm steps using one, continuous fit instead of partial, stepwise fits or interpolation. Additionally, using our single, continuous fit to model stepped isotherms in IAST mixture adsorption predictions leads to good agreement with the results from the osmotic framework adsorbed solution theory that was specifically developed for these systems using a stepwise, approximate fitting, which is yet far more complex. Our new isotherm equation accomplishes all of these tasks with only two fitted parameters, providing a simple, accurate method for modeling a variety of adsorption behavior.

Room-Temperature Synthesis of Metal-Organic Framework Isomers in the Tetragonal and Kagome Crystal Structure.

Two metal-organic framework (MOF) isomers with the chemical formula Zn2(X)2(DABCO) [X = terephthalic acid (BDC), dimethyl terephthalic acid (DM), 2-aminoterephthalic acid (NH2), 2,3,5,6-tetramethyl terephthalic acid (TM), and anthracene dicarboxylic acid (ADC); DABCO = 1,4-diazabicyclo[2.2.2]octane] have been synthesized via a fast, room-temperature synthesis procedure. The synthesis solvent was found to play a vital role in directing the formation of the Kagome lattice (ZnBD) versus tetragonal topology (DMOF-1). When N, N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO) was used as the synthesis solvent, the reaction resulted in the formation of ZnBD, whereas methanol, ethanol, acetone, N, N-diethylformamide (DEF), and acetonitrile each produced DMOF-1. Water adsorption isotherms of ZnBD and DMOF-1 were collected, and the materials were found to have similar adsorption characteristics and stabilities. Both MOFs degraded upon exposure to water at a relative pressure ( P/ Po) of 0.5 at 25 °C, but both are hydrophobic below a P/ Po of 0.4, displaying very little water adsorption. Additionally, CO2 adsorption isotherms of ZnBD were collected and compared to those previously reported for DMOF-1. ZnBD adsorbs less CO2 at low pressure compared to DMOF-1 but reaches a similar capacity at 20 bar. This adsorption behavior can be explained by the structural features of the materials, where ZnBD possesses large hexagonal pores (15 Å) compared to the smaller pore opening (7.5 Å) in DMOF-1. The heat of adsorption of CO2 on ZnBD was calculated to be ∼22 kJ/mol at zero coverage. Attempts to functionalize the Kagome lattice proved to be unsuccessful but instead resulted in a new method for producing functionalized DMOF-1 at room temperature. This was hypothesized to be a result of the steric effects imposed by the functional groups that prevent the formation of the Kagome lattice.

Structured Growth of Metal-Organic Framework MIL-53(Al) from Solid Aluminum Carbide Precursor.

The conventional synthesis of metal-organic frameworks (MOFs) through soluble metal-salt precursors provides little control over the growth of MOF crystals. The use of alternative metal precursors would provide a more flexible and cost-effective strategy for direction- and shape-controlled MOF synthesis. Here, we demonstrate for the first time the use of insoluble metal-carbon matrices to foster directed growth of MOFs. Aluminum carbide was implemented as both the metal precursor and growth-directing agent for the generation of MIL-53(Al). A unique needle-like morphology of the MOF was grown parallel to the bulk surface in a layer-by-layer manner. Importantly, the synthesis scheme was found to be transferrable to the production of different linker analogues of the MOF and other topologies. Given the variety of metal carbides available, these findings can be used as a blueprint for controlled, efficient, and economical MOF syntheses and set a new milestone toward the industrial use of MOFs at large-scale.

Probing Metal-Organic Framework Design for Adsorptive Natural Gas Purification.

Parent and amine-functionalized analogues of metal-organic frameworks (MOFs), UiO-66(Zr), MIL-125(Ti), and MIL-101(Cr), were evaluated for their hydrogen sulfide (H2S) adsorption efficacy and post-exposure acid gas stability. Adsorption experiments were conducted through fixed-bed breakthrough studies utilizing multicomponent 1% H2S/99% CH4 and 1% H2S/10% CO2/89% CH4 natural gas simulant mixtures. Instability of MIL-101(Cr) materials after H2S exposure was discovered through powder X-ray diffraction and porosity measurements following adsorbent pelletization, whereas other materials retained their characteristic properties. Linker-based amine functionalities increased H2S breakthrough times and saturation capacities from their parent MOF analogues. Competitive CO2 adsorption effects were mitigated in mesoporous MIL-101(Cr) and MIL-101-NH2(Cr), in comparison to microporous UiO-66(Zr) and MIL-125(Ti) frameworks. This result suggests that the installation of H2S binding sites in large-pore MOFs could potentially enhance H2S selectivity. In situ Fourier transform infrared measurements in 10% CO2 and 5000 ppm H2S environments suggest that framework hydroxyl and amine moieties serve as H2S physisorption sites. Results from this study elucidate design strategies and stability considerations for engineering MOFs in sour gas purification applications.

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.

Predicting Multicomponent Adsorption Isotherms in Open-Metal Site Materials Using Force Field Calculations Based on Energy Decomposed Density Functional Theory.

For the design of adsorptive-separation units, knowledge is required of the multicomponent adsorption behavior. Ideal adsorbed solution theory (IAST) breaks down for olefin adsorption in open-metal site (OMS) materials due to non-ideal donor-acceptor interactions. Using a density-function-theory-based energy decomposition scheme, we develop a physically justifiable classical force field that incorporates the missing orbital interactions using an appropriate functional form. Our first-principles derived force field shows greatly improved quantitative agreement with the inflection points, initial uptake, saturation capacity, and enthalpies of adsorption obtained from our in-house adsorption experiments. While IAST fails to make accurate predictions, our improved force field model is able to correctly predict the multicomponent behavior. Our approach is also transferable to other OMS structures, allowing the accurate study of their separation performances for olefins/paraffins and further mixtures involving complex donor-acceptor interactions.

Synthesis of cobalt-, nickel-, copper-, and zinc-based, water-stable, pillared metal-organic frameworks.

The performance of metal-organic frameworks (MOFs) in humid or aqueous environments is a topic of great significance for a variety of applications ranging from adsorption separations to gas storage. While a number of water-stable MOFs have emerged recently in the literature, the majority of MOFs are known to have poor water stability compared to zeolites and activated carbons, and there is therefore a critical need to perform systematic water-stability studies and characterize MOFs comprehensively after water exposure. Using these studies we can isolate the specific factors governing the structural stability of MOFs and direct the future synthesis efforts toward the construction of new, water-stable MOFs. In this work, we have extended our previous work on the systematic water-stability studies of MOFs and synthesized new, cobalt-, nickel-, copper-, and zinc-based, water-stable, pillared MOFs by incorporating structural factors such as ligand sterics and catenation into the framework. Stability is assessed by using water vapor adsorption isotherms along with powder X-ray diffraction patterns and results from BET modeling of N2 adsorption isotherms before and after water exposure. As expected, our study demonstrates that unlike the parent DMOF structures (based on Co, Ni, Cu, and Zn metals), which all collapse under 60% relative humidity (RH), their corresponding tetramethyl-functionalized variations (DMOF-TM) are remarkably stable, even when adsorbing more than 20 mmol of H2O/g of MOF at 80% RH. This behavior is due to steric factors provided by the methyl groups grafted on the BDC (benzenedicarboxylic acid) ligand, as shown previously for the Zn-based DMOF-TM. Moreover, 4,4',4″,4‴-benzene-1,2,4,5-tetrayltetrabenzoic acid based, pillared MOFs (based on Co and Zn metals) are also found to be stable after 90% RH exposure, even when the basicity of the bipyridyl-based pillar ligand is low. This is due to the presence of catenation in their frameworks, similar to MOF-508 (Zn-BDC-BPY), which has also been reported to be stable after exposure to 90% RH.

Determining Surface Areas and Pore Volumes of Metal-Organic Frameworks.

The surface area and pore volume of a metal-organic framework (MOF) can provide insight into its structure and potential applications. Both parameters are commonly determined using the data from nitrogen sorption experiments; commercial instruments to perform these measurements are also widely available. These instruments will calculate structural parameters, but it is essential to understand how to select input data and when calculation methods apply to the sample MOF. This article outlines the use of the Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method for the calculation of surface area and pore volume, respectively. Example calculations are performed on the representative MOF UiO-66. Although widely applicable to MOFs, sample materials and adsorption data must meet certain criteria for the calculated results to be considered accurate, in addition to proper sample preparation. The assumptions and limitations of these methods are also discussed, along with alternative and complementary techniques for the MOF pore space characterization.

Molecular-level insight into unusual low pressure CO2 affinity in pillared metal-organic frameworks.

Fundamental insight into how low pressure adsorption properties are affected by chemical functionalization is critical to the development of next-generation porous materials for postcombustion CO2 capture. In this work, we present a systematic approach to understanding low pressure CO2 affinity in isostructural metal-organic frameworks (MOFs) using molecular simulations and apply it to obtain quantitative, molecular-level insight into interesting experimental low pressure adsorption trends in a series of pillared MOFs. Our experimental results show that increasing the number of nonpolar functional groups on the benzene dicarboxylate (BDC) linker in the pillared DMOF-1 [Zn2(BDC)2(DABCO)] structure is an effective way to tune the CO2 Henry's coefficient in this isostructural series. These findings are contrary to the common scenario where polar functional groups induce the greatest increase in low pressure affinity through polarization of the CO2 molecule. Instead, MOFs in this isostructural series containing nitro, hydroxyl, fluorine, chlorine, and bromine functional groups result in little increase to the low pressure CO2 affinity. Strong agreement between simulated and experimental Henry's coefficient values is obtained from simulations on representative structures, and a powerful yet simple approach involving the analysis of the simulated heats of adsorption, adsorbate density distributions, and minimum energy 0 K binding sites is presented to elucidate the intermolecular interactions governing these interesting trends. Through a combined experimental and simulation approach, we demonstrate how subtle, structure-specific differences in CO2 affinity induced by functionalization can be understood at the molecular-level through classical simulations. This work also illustrates how structure-property relationships resulting from chemical functionalization can be very specific to the topology and electrostatic environment in the structure of interest. Given the excellent agreement between experiments and simulation, predicted CO2 selectivities over N2, CH4, and CO are also investigated to demonstrate that methyl groups also provide the greatest increase in CO2 selectivity relative to the other functional groups. These results indicate that methyl ligand functionalization may be a promising approach for creating both water stable and CO2 selective variations of other MOFs for various industrial applications.

Kinetic water stability of an isostructural family of zinc-based pillared metal-organic frameworks.

The rational design of metal-organic frameworks (MOFs) with structural stability in the presence of humid conditions is critical to the commercialization of this class of materials. However, the systematic water stability studies required to develop design criteria for the construction of water-stable MOFs are still scarce. In this work, we show that by varying the functional groups on the 1,4-benzenedicarboxylic acid (BDC) linker of DMOF [Zn(BDC)(DABCO)(0.5)], we can systematically tune the kinetic water stability of this isostructural, pillared family of MOFs. To illustrate this concept, we have performed water adsorption studies on four novel, methyl-functionalized DMOF variations along with a number of already reported functionalized analogues containing polar (fluorine) and nonpolar (methyl) functional groups on the BDC ligand. These results are distinctly different from previous reports where the apparent water stability is improved through the inclusion of functional groups such as -CH(3), -C(2)H(5), and -CF(3) which only serve to prevent significant amounts of water from adsorbing into the pores. In this study, we present the first demonstration of tuning the inherent kinetic stability of MOF structures in the presence of large amounts of adsorbed water. Notably, we demonstrate that while the parent DMOF structure is unstable, the DMOF variation containing the tetramethyl BDC ligand remains fully stable after adsorbing large amounts of water vapor during cyclic water adsorption cycles. These trends cannot be rationalized in terms of hydrophobicity alone; experimental water isotherms show that MOFs containing the same number of methyl groups per unit cell will have different kinetic stabilities and that the precise placements of the methyl groups on the BDC ligand are therefore critically important in determining their stability in the presence of water. We present the water adsorption isotherms, PXRD (powder X-ray diffraction) patterns, and BET surface areas before and after water exposure to illustrate these trends. Furthermore, we shed light on the important distinction between kinetic and thermodynamic stability in MOFs. Molecular simulations are also used to provide insight into the structural characteristics governing these trends in kinetic water stability.

Does Mixed Linker-Induced Surface Heterogeneity Impact the Accuracy of IAST Predictions in UiO-66-NH2?

To move toward more energy-efficient adsorption-based processes, there is a need for accurate multicomponent data under realistic conditions. While the Ideal Adsorbed Solution Theory (IAST) has been established as the preferred prediction method due to its simplicity, limitations and inaccuracies for less ideal adsorption systems have been reported. Here, we use amine-functionalized derivatives of the UiO-66 structure to change the extent of homogeneity of the internal surface toward the adsorption of the two probe molecules carbon dioxide and ethylene. Although it might seem plausible that more functional groups lead to more heterogeneity and, thus, less accurate predictions by IAST, we find a mixed-linker system with increased heterogeneity in terms of added adsorption sites where IAST predictions and experimental loadings agree exceptionally well. We show that incorporating uncertainty analysis into predictions with IAST is important for assessing the accuracy of these predictions. Energetic investigations combined with Grand Canonical Monte Carlo simulations reveal almost homogeneous carbon dioxide but heterogeneous ethylene adsorption in the mixed-linker material, resulting in local, almost pure phases of the individual components.

Structure and mobility of metal clusters in MOFs: Au, Pd, and AuPd clusters in MOF-74.

Understanding the adsorption and mobility of metal-organic framework (MOF)-supported metal nanoclusters is critical to the development of these catalytic materials. We present the first theoretical investigation of Au-, Pd-, and AuPd-supported clusters in a MOF, namely MOF-74. We combine density functional theory (DFT) calculations with a genetic algorithm (GA) to reliably predict the structure of the adsorbed clusters. This approach allows comparison of hundreds of adsorbed configurations for each cluster. From the investigation of Au(8), Pd(8), and Au(4)Pd(4) we find that the organic part of the MOF is just as important for nanocluster adsorption as open Zn or Mg metal sites. Using the large number of clusters generated by the GA, we developed a systematic method for predicting the mobility of adsorbed clusters. Through the investigation of diffusion paths a relationship between the cluster's adsorption energy and diffusion barrier is established, confirming that Au clusters are highly mobile in the MOF-74 framework and Pd clusters are less mobile.

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).

Effect of Loading on the Water Stability of the Metal-Organic Framework DMOF-1 [Zn(bdc)(dabco)0.5].

In this work, the degradation of the metal-organic framework (MOF) DMOF-1 as a function of water adsorption was investigated. As the quantity of water vapor adsorbed by DMOF-1 increases, degradation of the MOF from hydrolysis accelerates. Degradation was attributed to clustering of water molecules in the void space of DMOF-1, as seen in NVT Monte Carlo simulations. Our molecular simulations strongly suggest that degradation of DMOF-1 by water is driven by water adsorption at defect sites in the MOF. Interestingly, it was observed that DMOF-1 can remain stable if it adsorbs less water than the 1 mmol/g necessary to initiate degradation within the framework. Even though the rate of hydrolysis increases at higher temperatures, the degradation threshold for DMOF-1 remains 1 mmol/g regardless of temperature. This suggests that at sufficiently elevated temperatures (above ∼50 °C) DMOF-1 is stable toward water vapor at all relative humidities.

Unblocking a rigid purine MOF for kinetic separation of xylenes.

The separation of xylene isomers still remains an industrially challenging task. Here, porous purine-based metal-organic frameworks (MOFs) have been synthesized and studied for their potential in xylene separations. In particular, Zn(purine)I showed excellent para-xylene/ortho-xylene separation capability with a diffusion selectivity of 6 and high equilibrium adsorption selectivity as indicated by coadsorption experiments. This high selectivity is attributed to the shape and size of the channel aperture within the rigid framework of Zn(purine)I.