Understanding the Adsorption of Noble Gases in Metal–Organic Frameworks Using Diffuse Reflectance Infrared Fourier Transform Spectroscopy

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to study noble gas (i.e., He, Kr, and Xe) adsorption in metal-oxide frameworks (MOFs). To study the adsorption of infrared (IR)-inactive noble gases, we monitored the intensity of MOF stretching vibrations as a function of gas pressure using the DRIFTS spectra. Specific and nonspecific adsorption sites were identified in Ni/DOBDC (DOBDC = 1,4-dioxido-2,5-benzenedicarboxylate) and HKUST-1 (HKUST = Hong Kong University of Science and Technology) MOFs after noble gas adsorption, and the results were consistent with those obtained from single-crystal synchrotron X-ray diffraction (XRD) experiments and results published in the literature. Our results demonstrate that the DRIFTS technique can be an alternative approach for locating gas adsorption sites in MOFs. The adsorption selectivity of Xe/Kr for the two MOFs had qualitatively related to the peak saturation pressure in the DRIFTS spectrum, and the results are consistent with those obtained from isotherm data. In addition to the Xe adsorption process, we investigated the Xe desorption process by using the DRIFTS technique for HKUST-1 to validate the analysis of DRIFTS spectral data. We also studied competitive adsorption between Xe and trace amounts of water in the Ni/DOBDC

1:2 and 1:1 Ag(I)-Isonicotinamide Coordination Compounds: Five-Fold Interpenetrated CdSO₄ Network and the First Example of (Pyridine)N−Ag−O(Amide) Bonds

On the basis of acid−pyridine and acid−amide hydrogen bond synthons in dicarboxylic acid−isonicotinamide complexes and proton/silver mimicry in carboxylic acids, isonicotinamide was reacted with AgBF4 to afford different coordination compounds. The crystal structure of [Ag(C5H4NCONH2)2][BF4] 1 has linear tapes of Npyridyl−Ag−Npyridyl (180.0°) connected via N−H···O and N−H···F hydrogen bonds in square nets with CdSO4 topology. The 16 × 17 Å voids eschew 5-fold interpenetration of identical nets. [Ag(C5H4NCONH2)][BF4] 2 provides the first occurrence of near linear Npyridyl−Ag−Oamide bonds (165.4°). The similarity in motifs of carboxylic acid and Ag(I) is extended to heterosynthons for crystal engineering

1:2 and 1:1 Ag(I)-Isonicotinamide Coordination Compounds: Five-Fold Interpenetrated CdSO₄ Network and the First Example of (Pyridine)N−Ag−O(Amide) Bonds

On the basis of acid−pyridine and acid−amide hydrogen bond synthons in dicarboxylic acid−isonicotinamide complexes and proton/silver mimicry in carboxylic acids, isonicotinamide was reacted with AgBF4 to afford different coordination compounds. The crystal structure of [Ag(C5H4NCONH2)2][BF4] 1 has linear tapes of Npyridyl−Ag−Npyridyl (180.0°) connected via N−H···O and N−H···F hydrogen bonds in square nets with CdSO4 topology. The 16 × 17 Å voids eschew 5-fold interpenetration of identical nets. [Ag(C5H4NCONH2)][BF4] 2 provides the first occurrence of near linear Npyridyl−Ag−Oamide bonds (165.4°). The similarity in motifs of carboxylic acid and Ag(I) is extended to heterosynthons for crystal engineering

Generation of 2D and 3D (PtS, Adamantanoid) Nets with a Flexible Tetrahedral Building Block

The self-assembly of the flexible tetrahedral linker tetrakis[4-(carboxyphenyl)oxamethyl]methane acid with various transition metals (Cu, Co, and Mg) results in a 2D layered structure and 3D frameworks with PtS and adamantanoid topology. The PtS net exhibits permanent porosity, as confirmed by BET and gas adsorption experiments

Metal–Organic Frameworks for Removal of Xe and Kr from Nuclear Fuel Reprocessing Plants

Removal of xenon (Xe) and krypton (Kr) from process off-gases containing 400 ppm Xe, 40 ppm Kr, 78% N₂, 21% O₂, 0.9% Ar, 0.03% CO₂, and so forth using adsorption was demonstrated for the first time. Two well-known metal–organic frameworks (MOFs), HKUST-1 and Ni/DOBDC, which both have unsaturated metal centers but different pore morphologies, were selected as novel adsorbents. Results of an activated carbon were also included for comparison. The Ni/DOBDC has higher Xe/Kr selectivities than those of the activated carbon and the HKUST-1. In addition, results show that the Ni/DOBDC and HKUST-1 can adsorb substantial amounts of Xe and Kr even when they are mixed in air. Moreover, the Ni/DOBDC can successfully separate 400 ppm Xe from 40 ppm Kr and air containing O₂, N₂, and CO₂ with a Xe/Ke selectivity of 7.3 as indicated by our breakthrough results. This shows a promising future for MOFs in radioactive nuclide separations from spent fuels

Frustrated Organic Solids Display Unexpected Gas Sorption

Between nonsorptive solvate and desolvate forms of p-tert-butylcalix[5]arene lies a frustrated crystalline region in which there is rapid sorption of gases. From sorption studies, we conclude that the frustrated form is porous and that the intermediate form represents a new type of material that was previously unrecognized by traditional gas sorption trends or ideals for other active systems

Frustrated Organic Solids Display Unexpected Gas Sorption

Between nonsorptive solvate and desolvate forms of p-tert-butylcalix[5]arene lies a frustrated crystalline region in which there is rapid sorption of gases. From sorption studies, we conclude that the frustrated form is porous and that the intermediate form represents a new type of material that was previously unrecognized by traditional gas sorption trends or ideals for other active systems

Five New Pseudopolymorphs of <i>s</i><i>ym</i>-Trinitrobenzene

1,3,5-Trinitrobenzene (TNB) was recrystallized from about 30 solvents and five mixed solvents in around 50 experiments to yield five new solvates. All the solvates lose their solvent readily, and special precautions were required to determine their crystal structures. In all cases, the solvents that gave solvates are electron-rich aromatics leading to the conjecture that π···π donor−acceptor stacking interactions are important. This was confirmed from the crystal structures. Additionally, these structures are composed of hexagonal sheets in which TNB and solvent molecules are linked with weak C−H···O hydrogen bonds. The benzene solvate contains an unusual finite TNB motif not hitherto seen. The toluene and chlorobenzene solvates are not isostructural, and C−H···Cl interactions in the latter are unusually short and linear, accounting thereby for the failure of chloro-methyl structural exchange. The m-xylene and mesitylene solvates are similar to the toluene solvate. This study indicates that the use of terms such as solvate, pseudopolymorph, donor−acceptor complex, and molecular complex is a subjective matter, and also that a better definition for the term pseudopolymorph may be needed, especially because it occurs frequently in the pharmaceutical literature

Free Transport of Water and CO₂ in Nonporous Hydrophobic Clarithromycin Form II Crystals

Clarithromycin is a well-known antibiotic that exists in various polymorphic forms. This molecule can be sublimed to afford a guest-free form that displays the unexpected transport of molecules of water or carbon dioxide to voids within the channel-free crystal lattice

Metal−Organic Framework Isomers with Diamondoid Networks Constructed of a Semirigid Tetrahedral Linker

Solvothermal assembly of a semirigid tetrahedral carboxylate ligand tetrakis[4-(carboxyphenyl)oxamethyl]methane acid (H4X) with Cd(II) ion in different solvent systems yields three novel metal−organic framework isomers (1−3) based on different secondary building units (SBUs). Although all three frameworks have the same dia (diamondoid) topology, complexes 1 and 3 are noncentrosymmetric and complex 2 is centrosymmetric. One of the networks (1) shows cross-linked 3-fold interpenetration of the single dia net and exhibits permanent porosities, as confirmed by Brunauer−Emmett−Teller (BET) and selective CO2 adsorption