Hoi Ri Moon

Air-stable magnesium nanocomposites provide rapid and high-capacity hydrogen storage without using heavy-metal catalysts

Hydrogen is a promising alternative energy carrier that can potentially facilitate the transition from fossil fuels to sources of clean energy because of its prominent advantages such as high energy density (142 MJ kg(-1); ref. 1), great variety of potential sources (for example water, biomass, organic matter), light weight, and low environmental impact (water is the sole combustion product). However, there remains a challenge to produce a material capable of simultaneously optimizing two conflicting criteria--absorbing hydrogen strongly enough to form a stable thermodynamic state, but weakly enough to release it on-demand with a small temperature rise. Many materials under development, including metal-organic frameworks, nanoporous polymers, and other carbon-based materials, physisorb only a small amount of hydrogen (typically 1-2 wt%) at room temperature. Metal hydrides were traditionally thought to be unsuitable materials because of their high bond formation enthalpies (for example MgH(2) has a ΔHf~75 kJ mol(-1)), thus requiring unacceptably high release temperatures resulting in low energy efficiency. However, recent theoretical calculations and metal-catalysed thin-film studies have shown that microstructuring of these materials can enhance the kinetics by decreasing diffusion path lengths for hydrogen and decreasing the required thickness of the poorly permeable hydride layer that forms during absorption. Here, we report the synthesis of an air-stable composite material that consists of metallic Mg nanocrystals (NCs) in a gas-barrier polymer matrix that enables both the storage of a high density of hydrogen (up to 6 wt% of Mg, 4 wt% for the composite) and rapid kinetics (loading in <30 min at 200 °C). Moreover, nanostructuring of the Mg provides rapid storage kinetics without using expensive heavy-metal catalysts.

A Comparison of the H2 Sorption Capacities of Isostructural Metal–Organic Frameworks With and Without Accessible Metal Sites: [{Zn2(abtc)(dmf)2}3] and [{Cu2(abtc)(dmf)2}3] versus [{Cu2(abtc)}3]

Various metal–organic frameworks (MOFs) have been prepared to obtain materials that show specific or multifunctional properties. Porous MOFs that contain free space where guest molecules can be accommodated are of particular interest because they can be applied in gas storage and separation, selective adsorption and separation of organic molecules, ion exchange, catalysis, sensor technology, and for the fabrication of metal nanoparticles. Secondary building units (SBUs) with a specific geometry have often been employed for the modular construction of porous MOFs as they make the design and prediction of molecular architectures simple and easy. In particular, {M2(CO2)4}-type paddlewheel clusters that can be formed from the solvothermal reaction of M ions and the appropriate carboxylic acid are widely used for the construction of porous frameworks. Three-dimensional porous frameworks with various topologies (Pt3O4, boracites, NbO, and PtS nets) can be built from paddlewheel-type metal cluster SBUs and trior tetracarboxylates, whereas pillared square-grid networks can be constructed from paddlewheel cluster SBUs and dicarboxylates in the presence of diamine ligands. Porous MOFs with accessible metal sites (AMSs) should have a higher hydrogen storage capacity than those without AMSs, although there are not yet enough experimental data to support this assumption. To determine the effect of AMSs in a MOF on H2 adsorption, the H2 uptakes should be compared for the same framework in the absence and presence of AMSs, or for two independent isostructural MOFs with and without AMSs. H2 uptake has previously been measured under several different outgassing conditions. Unfortunately, these experiments could not clearly demonstrate the effect of AMSs as the exact formula and structure at each stage were not known. Furthermore, even when coordinating solvent molecules are successfully removed with retention of the porous framework structure, the metal ion sometimes transforms its coordination geometry to the thermodynamically most stable form instead of keeping the AMSs. Herein we report two porous MOFs with the same NbOtype net topology, namely [{Zn2(abtc)(dmf)2}3]·4H2O·10dmf (1) and [{Cu2(abtc)(H2O)2}3]·10dmf·6 (1,4-dioxane) (2 ; H4abtc= 1,1’-azobenzene-3,3’,5,5’-tetracarboxylic acid ), and compare the gas adsorption data for the MOFs with and without AMSs. Heating crystals of 1 and 2 under precisely controlled conditions allowed us to prepare [{Zn2(abtc)(dmf)2}3] (1a ; SNU-4) and [{Cu2(abtc)(dmf)2}3] (2a ; SNU-5’), which have no AMSs, as well as [{Cu2(abtc)}3] (2b ; SNU-5), which has AMSs. The framework structure of 1a is the same as that of 1 and those of 2a and 2b are the same as that of 2, as evidenced by the PXRD patterns. Solid 1a, 2a, and 2b exhibit higher adsorption capabilities for N2, CO2, CH4, and H2 than other previously reported MOFs. In particular, 2b adsorbs 2.87 wt% of H2 gas at 77 K and 1 atm, which is the highest value for H2 sorption under these conditions amongst a variety of other MOFs. The N2, CO2, and CH4 adsorption capacities per unit sample volume for 2b, which has AMSs, are 140–160% higher than those for 1a and 2a, which have no AMSs. The H2 adsorption capacity of 2b is also higher than those of 1a and 2a [at 77 K and 1 atm, 2.87 wt% for 2b vs. 2.07 wt% for 1a and 1.83 wt% for 2a ; excess adsorbed H2 at 77 K and 50 bar: 5.22 wt% (total 6.76 wt%) for 2b vs. 3.70 wt% (total 4.49 wt%) for 1a], although this is mainly due to the lower molecular weight effect of 2b. The H2 sorption capacity ratios 2b/1a and 2b/2a per unit sample volume at 77 K and 1 atm are 105% and 120%, respectively, and the ratio 2b/1a at 77 K and 50 bar is 106%. Our measurements of the isosteric heat of H2 adsorption (zero-coverage isosteric heats are 7.24, 6.53, and 11.60 kJmol for 1a, 2a, and 2b, respectively) suggest that the enhanced H2 adsorption in 2b can be attributed to the stronger interaction of H2 molecules with the AMSs of the MOF. Yellowish block-shaped crystals of [{Zn2(abtc)(dmf)2}3]·4H2O·10dmf (1) were prepared by heating a dmf solution of Zn(NO3)2·6H2O and H4abtc at 100 8C for 12 h. Greenish block-shaped crystals of [{Cu2(abtc)(H2O)2}3]·10dmf·6 (1,4-dioxane) (2) were prepared by heating Cu(NO3)2·xH2O and H4abtc in a dmf/1,4-dioxane/H2O (4:3:1 v/v) mixture at 80 8C for 24 h. Solid 1 is insoluble in common organic solvents but is slightly soluble in water, where it dissociates into its building blocks. Solid 2 is insoluble in all common organic solvents and water. The temperaturedependent PXRD patterns show that the framework struc[*] Y.-G. Lee, H. R. Moon, Y. E. Cheon, Prof. M. P. Suh Department of Chemistry, Seoul National University Seoul 151-747 (Republic of Korea) Fax: (+82)2-886-8516 E-mail: mpsuh@snu.ac.kr

Nanoporous Metal Oxides with Tunable and Nanocrystalline Frameworks via Conversion of Metal–Organic Frameworks

Nanoporous metal oxide materials are ubiquitous in the material sciences because of their numerous potential applications in various areas, including adsorption, catalysis, energy conversion and storage, optoelectronics, and drug delivery. While synthetic strategies for the preparation of siliceous nanoporous materials are well-established, nonsiliceous metal oxide-based nanoporous materials still present challenges. Herein, we report a novel synthetic strategy that exploits a metal-organic framework (MOF)-driven, self-templated route toward nanoporous metal oxides via thermolysis under inert atmosphere. In this approach, an aliphatic ligand-based MOF is thermally converted to nanoporous metal oxides with highly nanocrystalline frameworks, in which aliphatic ligands act as the self-templates that are afterward evaporated to generate nanopores. We demonstrate this concept with hierarchically nanoporous magnesia (MgO) and ceria (CeO2), which have potential applicability for adsorption, catalysis, and energy storage. The pore size of these nanoporous metal oxides can be readily tuned by simple control of experimental parameters. Significantly, nanoporous MgO exhibits exceptional CO2 adsorption capacity (9.2 wt %) under conditions mimicking flue gas. This MOF-driven strategy can be expanded to other nanoporous monometallic and multimetallic oxides with a multitude of potential applications.

Luminescent Li-Based Metal–Organic Framework Tailored for the Selective Detection of Explosive Nitroaromatic Compounds: Direct Observation of Interaction Sites

A luminescent lithium metal-organic framework (MOF) is constructed from the solvothermal reaction of Li(+) and a well-designed organic ligand, bis(4-carboxyphenyl)-N-methylamine (H(2)CPMA). A Li-based MOF can detect an explosive aromatic compound containing nitro groups as an explosophore, by showing a dramatic color change with concurrent luminescence quenching in the solid state. The detection sites are proven directly through single-crystal-to-single-crystal transformations, which show strong interactions between the aromatic rings of the electron-rich CPMA(2-) molecules and the electron-deficient nitrobenzene.

Size-Controlled Synthesis and Optical Properties of Monodisperse Colloidal Magnesium Oxide Nanocrystals†

Author(s): Milliron, Delia J.; Urban, Jeffrey J.; Moon, Hoi Ri | Abstract: colloids ? luminescence ? metal oxides ? nanocrystals ? synthesis design

4,4′-Biphenyldicarboxylate sodium coordination compounds as anodes for Na-ion batteries

Novel 4,4′-biphenyldicarboxylate (bpdc) sodium salts with different compositions were evaluated for the first time as anodes for Na-ion batteries, and their crystal structures and corresponding electrochemical performances were analyzed. The structure of the bpdc-sodium salts was modified using precipitation and solvothermal methods to afford three different crystal structures with different degrees of deprotonation of the carboxylic acid (COOH) groups and different coordination of the water molecule, as determined by single crystal X-ray diffraction. The extent of deprotonation in bpdc-sodium salts not only affects their electrochemical performance, but also affects the corresponding reaction mechanisms. The fully deprotonated bpdc-disodium salt exhibits a promising electrochemical performance with a reversible capacity of about 200 mA h g−1 at ca. 0.5 V vs. Na/Na+, stable cycle performance over 150 cycles, and an excellent rate performance of 100 mA h g−1 even at a 20 C rate, which are better than those of the partially deprotonated bpdc-sodium salt. The sodiation–desodiation of bpdc-sodium salts proceeds in a two-phase reaction, regardless of the degree of deprotonation. However, unlike the fully deprotonated bpdc-disodium salt, which shows a reversible phase transition during sodiation and desodiation, the partially deprotonated bpdc-sodium salt exhibits an irreversible phase transition during cycling.

A transformative route to nanoporous manganese oxides of controlled oxidation states with identical textural properties

Nanoporous nanocrystalline metal oxides with tunable oxidation states are crucial for controlling their catalytic, electronic, and optical properties. However, previous approaches to modulate oxidation states in nanoporous metal oxides commonly lead to the breakdown of the nanoporous structure as well as involve concomitant changes in their morphology, pore size, surface area, and nanocrystalline size. Herein, we present a transformative route to nanoporous metal oxides with various oxidation states using manganese oxides as model systems. Thermal conversion of Mn-based metal–organic frameworks (Mn-MOFs) at controlled temperature and atmosphere yielded a series of nanoporous manganese oxides with continuously tuned oxidation states: MnO, Mn3O4, Mn5O8, and Mn2O3. This transformation enabled the preparation of low-oxidation phase MnO and metastable intermediate phase Mn5O8 with nanoporous architectures, which were previously rarely accessible. Significantly, nanoporous MnO, Mn3O4, and Mn5O8 had a very similar morphology, surface area, and crystalline size. We investigated the electrocatalytic activity of nanoporous manganese oxides for oxygen reduction reaction (ORR) to identify the role of oxidation states, and observed oxidation state-dependent activity and kinetics for the ORR.

Preparation of Co3O4 electrode materials with different microstructures via pseudomorphic conversion of Co-based metal–organic frameworks

To develop high-performance nanostructured metal oxide electrodes, it is important to understand their structural effects on electrochemical performances. Thus, the preparation of metal oxide materials that have well-tailored nanostructures is crucial for studies. However, while synthetic strategies to control the size of metal oxide nanoparticles are well-developed, the control of the higher level structures, namely microstructure, is not very well established. Herein, we present the synthesis of the two kinds of Co3O4 nanomaterials through pseudomorphic conversion so that the macroscopic morphologies of parent MOFs, such as plate-like and rod-like shape, are well-maintained. Both Co3O4 nanomaterials are composed of almost identical 10 nm-sized primary nanocrystals but with different nanoporous secondary structures and macroscopic morphologies such as plate and rod shapes. These Co3O4 nanomaterials were utilized as an electrode in lithium ion batteries (LIBs), and their electrochemical properties were comparatively investigated. It was revealed that the different cyclability and rate capability are attributed to their different microstructures. The pseudo-monolithic integration of primary and secondary structures at higher level was the governing factor, which determined the electrochemical performances of the Co3O4 electrode.

Coordination Polymer Open Frameworks Constructed of Macrocyclic Complexes

Publisher Summary This chapter describes assembly, functions, and single crystal-to-single-crystal transformations of coordination polymer open frameworks that incorporate Ni(II) or Cu(II) macrocyclic complexes. Coordination polymer open frameworks, having voids and channels of various aperture sizes and shapes, can be assembled using metal azamacrocyclic complexes and organic carboxylates as molecular building blocks. In the assembly, the azamacrocyclic complexes in square–planar geometries act as linear linkers for the carboxylate ligands that have linear, triangular, and tetrahedral shapes to provide primary 1D, 2D, and 3D networks. In the self-assembly of porous coordination polymers, free metal ions are commonly used as metal building blocks. In this process, macrocyclic complexes are also employed. The utilization of macrocyclic complexes offers several advantages over free metal ions in the assembly of multidimensional coordination polymer networks. Macrocyclic complexes simplify the coordination mode of a ligand because of the bulkiness of the macrocycle. Macrocyclic complexes are expected to prevent the interpenetration of the network that can be considered a major impediment in the construction of open structures.

Exploration of Gate-Opening and Breathing Phenomena in a Tailored Flexible Metal–Organic Framework

Flexible metal-organic frameworks (MOFs) show the structural transition phenomena, gate opening and breathing, upon the input of external stimuli. These phenomena have significant implications in their adsorptive applications. In this work, we demonstrate the direct capture of these gate-opening and breathing phenomena, triggered by CO2 molecules, in a well-designed flexible MOF composed of rotational sites and molecular gates. Combining X-ray single crystallographic data of a flexible MOF during gate opening/closing and breathing with in situ X-ray powder diffraction results uncovered the origin of this flexibility. Furthermore, computational studies revealed the specific sites required to open these gates by interaction with CO2 molecules.