Anna M. Plonka

Metal–organic framework with optimally selective xenon adsorption and separation

Nuclear energy is among the most viable alternatives to our current fossil fuel-based energy economy. The mass deployment of nuclear energy as a low-emissions source requires the reprocessing of used nuclear fuel to recover fissile materials and mitigate radioactive waste. A major concern with reprocessing used nuclear fuel is the release of volatile radionuclides such as xenon and krypton that evolve into reprocessing facility off-gas in parts per million concentrations. The existing technology to remove these radioactive noble gases is a costly cryogenic distillation; alternatively, porous materials such as metal–organic frameworks have demonstrated the ability to selectively adsorb xenon and krypton at ambient conditions. Here we carry out a high-throughput computational screening of large databases of metal–organic frameworks and identify SBMOF-1 as the most selective for xenon. We affirm this prediction and report that SBMOF-1 exhibits by far the highest reported xenon adsorption capacity and a remarkable Xe/Kr selectivity under conditions pertinent to nuclear fuel reprocessing.

Mechanism of Carbon Dioxide Adsorption in a Highly Selective Coordination Network Supported by Direct Structural Evidence

Understanding the interactions between adsorbed gas molecules and a pore surface at molecular level is vital to exploration and attempts at rational development of gasselective nanoporous solids. Much current work focuses on the design of functionalized metal–organic frameworks (MOFs) or coordination networks (CNs) that selectively adsorb CO2. [1–9] While interactions between CO2 molecules and the p clouds of aromatic linkers in MOFs under ambient conditions have been explored theoretically, no direct structure evidence of such interactions are reported to date. Here we provide the first structural insight of such interactions in a porous calcium based CN using single-crystal X-ray diffraction methods, supported by powder diffraction coupled with differential scanning calorimetry (DSC-XRD), in situ IR/Raman spectroscopy, and molecular simulation data. We further postulate that such interactions are responsible for the high CO2/N2 adsorption selectivity, even in the case of a high relative humidity (RH). Our data suggest that the key interaction responsible for such selectivity, the room-temperature stability and the relative insensitivity to the RH of the CO2-CN adduct, is between two phenyl rings of the linker in the CN and the molecular quadrupole of CO2. The specific geometry of the linker molecule results in a “pocket” where carbon from the CO2 molecule is placed between two centroids of the aromatic ring. Our experimental confirmation of this variation on theoretically postulated interactions between CO2 and a phenyl ring will promote the search for other CNs containing phenyl ring pockets. Selective adsorption and sequestration of CO2 from sources of anthropogenic emissions, such as untreated waste from flue gas and products of the water gas shift reaction, is important to mitigate the growing level of atmospheric CO2. [10] Current separation methods use absorption in alkanolamine solutions, which are toxic, corrosive, and require significant energy for their regeneration. Hence microporous solid-state adsorbents, such as zeolites, hybrid zeolite–polymer systems, porous organic materials, and MOFs are proposed as alternatives, especially in combination with pressure swing processes. Rather than relying solely on tuning the pore diameters of microporous materials to select between gases based on size (the kinetic diameters of CO2, CH4 and N2 are 3.30, 3.76 3.64 , respectively ) selective separation relies on differences in electronic properties, such as the quadrupole moment and polarizability. Attempts to produce MOFs or CNs with adsorption properties competitive with those of commercially established aluminosilicate zeolites, relies on strategies that include pore surface modification with strongly polarizing functional groups, such as amines 7, 9,15] and desolvating metals centers 8, 16] to produce low-coordinated sites suitable for CO2 adsorption. The amine-functionalized materials offer a high selectivity toward CO2 adsorption, but a low effective surface area and thus, a low total uptake capacity. Strong interactions with polarizing functional groups, as well as with open metal sites presents other drawbacks including an increase in the costs for material regeneration. Furthermore, water effectively competes with CO2 at low-coordinated cation sites, impeding the performance of frameworks in commercial flue gas. We recently described a porous framework, CaSDB (SDB: sulfonyldibenzoate, compound 1) with a high CO2/N2 selectivity. At 0.15 bar of CO2 and 0.85 bar of N2, a typical composition of flue gas mixture from power plants, the selectivity is in the range of 48 to 85 at 298 K. CaSDB shows a reversible uptake of CO2 of 5.75 wt% at 273 K and 1 bar pressure and 4.37 wt% at room temperature, with heats of adsorption for CO2 and N2 of 31 and 19 kJmol , respectively. The as-synthesized compound contains not coordinated water molecules and is easily activated for gas adsorption by heating to 563 K in vacuum; remarkably the activated framework does not readsorb water, even if exposed to a RH greater than [*] A. M. Plonka, W. R. Woerner, Prof. Dr. J. B. Parise Department of Geosciences, Stony Brook University Stony Brook, NY 11794-2100 (USA) E-mail: john.parise@stonybrook.edu

Direct Observation of Xe and Kr Adsorption in a Xe-Selective Microporous Metal-Organic Framework

The cryogenic separation of noble gases is energy-intensive and expensive, especially when low concentrations are involved. Metal-organic frameworks (MOFs) containing polarizing groups within their pore spaces are predicted to be efficient Xe/Kr solid-state adsorbents, but no experimental insights into the nature of the Xe-network interaction are available to date. Here we report a new microporous MOF (designated SBMOF-2) that is selective toward Xe over Kr under ambient conditions, with a Xe/Kr selectivity of about 10 and a Xe capacity of 27.07 wt % at 298 K. Single-crystal diffraction results show that the Xe selectivity may be attributed to the specific geometry of the pores, forming cages built with phenyl rings and enriched with polar -OH groups, both of which serve as strong adsorption sites for polarizable Xe gas. The Xe/Kr separation in SBMOF-2 was investigated with experimental and computational breakthrough methods. These experiments showed that Kr broke through the column first, followed by Xe, which confirmed that SBMOF-2 has a real practical potential for separating Xe from Kr. Calculations showed that the capacity and adsorption selectivity of SBMOF-2 are comparable to those of the best-performing unmodified MOFs such as NiMOF-74 or Co formate.

Effect of ligand geometry on selective gas-adsorption: the case of a microporous cadmium metal organic framework with a V-shaped linker.

A microporous cadmium metal organic framework is synthesized and structurally characterized. The material possesses a 3-D framework with a 1-D sinusoidal chain and shows high selectivity for CO2 over N2. The selectivity is attributed to CO2 interacting with two phenyl rings of a V-shaped linker as estimated by the in situ XRD-DSC study.

In Situ Probes of Capture and Decomposition of Chemical Warfare Agent Simulants by Zr-Based Metal Organic Frameworks

Zr-based metal organic frameworks (MOFs) have been recently shown to be among the fastest catalysts of nerve-agent hydrolysis in solution. We report a detailed study of the adsorption and decomposition of a nerve-agent simulant, dimethyl methylphosphonate (DMMP), on UiO-66, UiO-67, MOF-808, and NU-1000 using synchrotron-based X-ray powder diffraction, X-ray absorption, and infrared spectroscopy, which reveals key aspects of the reaction mechanism. The diffraction measurements indicate that all four MOFs adsorb DMMP (introduced at atmospheric pressures through a flow of helium or air) within the pore space. In addition, the combination of X-ray absorption and infrared spectra suggests direct coordination of DMMP to the Zr6 cores of all MOFs, which ultimately leads to decomposition to phosphonate products. These experimental probes into the mechanism of adsorption and decomposition of chemical warfare agent simulants on Zr-based MOFs open new opportunities in rational design of new and superior decontamination materials.

Pressure induced topochemical polymerization of diiodobutadiyne: a single-crystal-to-single-crystal transformation

Diiodobutadiyne forms cocrystals with bis(pyridyl)oxalamides, based on halogen bonds between the pyridine groups of the host and the iodoalkynes of the guest. These interactions align the diyne for topochemical polymerization to form poly(diiododiacetylene) or PIDA. To induce polymerization, the crystals are subjected to pressures of 3.5 GPa or above. Previously, we reported spectroscopic evidence of this pressure-induced polymerization, but attempts to recover single crystals after pressure treatment were unsuccessful. Here we present direct structural evidence of clean single-crystal to single-crystal polymerization in these cocrystals. The structure of the polymer cocrystal was solved from single-crystal diffraction data and is supported by high pressure in situ Raman spectroscopy. Careful analysis of the structural changes suggests that increasing pressure changes the packing of host molecules, and that the flexibility of the pyridine ring orientation enables the polymerization. The new sigma bonds of the polymer form at the expense of the halogen bonds in the starting cocrystal; after polymerization, the iodine atoms are no longer ideally located for strong halogen bonding with the host.

β‐diopside, a new ultrahigh‐pressure polymorph of CaMgSi2O6with six‐coordinated silicon

Minerals containing silicon in four-fold coordination (IVSi4+) are common in crustal rocks, while those involving six-coordinated silicon (VISi4+) dominate the Earths lower mantle and determine its properties. Here we show a new type of phase transition determined by single-crystal high pressure X-ray diffraction experiments in a diamond anvil cell (DAC) using natural diopside (CaMgSi2O6), the archetypic member of clinopyroxene family, and one of the most abundant minerals of the Earths upper mantle. Above 50 GPa at ambient temperature diopside transforms to a previously unknown post-clinopyroxene phase,β-diopside, with half of the tetrahedralIVSi4+ layers converted to octahedral VISi4+coordination. This phase is most probably a metastable state that is kinetically accessible at room temperature and the transformation is fully reversible on decompression. This new type of phase transition provides important clues to the exact mechanisms of breakdown of clinopyroxene in the Earths mantle and may be expected to take place in other pyroxenes at pressures higher than previously explored.

Direct Structural Identification of Gas Induced Gate-Opening Coupled with Commensurate Adsorption in a Microporous Metal–Organic Framework

Gate-opening is a unique and interesting phenomenon commonly observed in flexible porous frameworks, where the pore characteristics and/or crystal structures change in response to external stimuli such as adding or removing guest molecules. For gate-opening that is induced by gas adsorption, the pore-opening pressure often varies for different adsorbate molecules and, thus, can be applied to selectively separate a gas mixture. The detailed understanding of this phenomenon is of fundamental importance to the design of industrially applicable gas-selective sorbents, which remains under investigated due to the lack of direct structural evidence for such systems. We report a mechanistic study of gas-induced gate-opening process of a microporous metal-organic framework, [Mn(ina)2 ] (ina=isonicotinate) associated with commensurate adsorption, by a combination of several analytical techniques including single crystal X-ray diffraction, in situ powder X-ray diffraction coupled with differential scanning calorimetry (XRD-DSC), and gas adsorption-desorption methods. Our study reveals that the pronounced and reversible gate opening/closing phenomena observed in [Mn(ina)2 ] are coupled with a structural transition that involves rotation of the organic linker molecules as a result of interaction of the framework with adsorbed gas molecules including carbon dioxide and propane. The onset pressure to open the gate correlates with the extent of such interaction.

Poly[(μ4‐adamantane‐1,3‐dicarboxylato‐κ5O1:O1′:O3,O3′:O3′)(μ3‐adamantane‐1,3‐dicarboxylato‐κ5O1,O1′:O3,O3′:O3′)dimagnesium]: a layered coordination polymer

The title compound, [Mg(2)(C(12)H(14)O(4))(2)](n), is the first example of an s-block metal adamantanedicarboxylate coordination polymer. The asymmetric unit comprises two crystallographically unique Mg(II) centers and two adamantane-1,3-dicarboxylate ligands. The compound is constructed from a combination of chains of corner-sharing magnesium-centered polyhedra, parallel to the a axis, connected by organic linkers to form a layered polymer. The two Mg(II) centers are present in distorted tetrahedral and octahedral coordination environments derived from carboxylate O atoms. Tetrahedrally coordinated Mg(II) centers have been reported in organometallic compounds, but this is the first time that such coordination has been observed in a magnesium-based coordination polymer. The bond valance sums of the two Mg(II) centers are 2.05 and 2.11 valence units, matching well with the expected value of 2.

Iodine Adsorption in Metal Organic Frameworks in the Presence of Humidity

Used nuclear fuel reprocessing represents a unique challenge when dealing with radionuclides such as isotopes of 85Kr and 129I2 due to their volatility and long half-life. Efficient capture of 129I2 ( t1/2 = 15.7 × 106 years) from the nuclear waste stream can help reduce the risk of releasing I2 radionuclide into the environment and/or potential incorporation into the human thyroid. Metal organic frameworks have the reported potential to be I2 adsorbents but the effect of water vapor, generally present in the reprocessing off-gas stream, is rarely taken into account. Moisture-stable porous metal organic frameworks that can selectively adsorb I2 in the presence of water vapor are thus of great interest. Herein, we report on the I2 adsorption capacity of two microporous metal organic frameworks at both dry and humid conditions. Single-crystal X-ray diffraction and Raman spectroscopy reveal distinct sorption sites of molecular I2 within the pores in proximity to the phenyl- and phenol-based linkers stabilized by the I···π and I···O interactions, which allow selective uptake of iodine.