Tuning the Selectivity of Nitrate Reduction via Fine Composition Control of RuPdNP Catalysts.

Herein, aqueous nitrate (NO3 - ) reduction is used to explore composition-selectivity relationships of randomly alloyed ruthenium-palladium nanoparticle catalysts to provide insights into the factors affecting selectivity during this and other industrially relevant catalytic reactions. NO3 - reduction proceeds through nitrite (NO2 - ) and then nitric oxide (NO), before diverging to form either dinitrogen (N2 ) or ammonium (NH4 + ) as final products, with N2 preferred in potable water treatment but NH4 + preferred for nitrogen recovery. It is shown that the NO3 - and NO starting feedstocks favor NH4 + formation using Ru-rich catalysts, while Pd-rich catalysts favor N2 formation. Conversely, a NO2 - starting feedstock favors NH4 + at ≈50 atomic-% Ru and selectivity decreases with higher Ru content. Mechanistic differences have been probed using density functional theory (DFT). Results show that, for NO3 - and NO feedstocks, the thermodynamics of the competing pathways for N-H and N-N formation lead to preferential NH4 +  or N2 production, respectively, while Ru-rich surfaces are susceptible to poisoning by NO2 - feedstock, which displaces H atoms. This leads to a decrease in overall reduction activity and an increase in selectivity toward N2 production. Together, these results demonstrate the importance of tailoring both the reaction pathway thermodynamics and initial reactant binding energies to control overall reaction selectivity.

3D-Printed Porous Supramolecular Sorbents for Cobalt Recycling.

Electronic waste recycling is a recognized global challenge that requires new strategies to bind and release critical materials selectively, such as cobalt present in lithium-ion batteries. To address this challenge, hierarchical 3D-printed porous polymer scaffolds bearing supramolecular receptors were prepared using vat photopolymerization and their cobalt binding profiles were examined as a function of matrix polarity. By combining high-resolution digital light processing (DLP) with polymerization-induced phase separation (PIPS), functional acrylic copolymer networks with micrometer-level precision of geometry and nanometer-level pores were generated. Covalent integration of a methacrylate-functionalized bisdicyclohexyl acetamide (BDCA-MA) receptor enabled binding and release of cobalt(II) chloride (CoCl2) via a solvent polarity switch mechanism involving a change in solvent from ethanol to water. The present structures proved reusable as shown by sustained high binding efficiency over five bind and release cycles. This platform represents a "green" and energy conscious method for future electronic waste recycling.

Low-Valent Metals in Metal-Organic Frameworks Via Post-Synthetic Modification.

Metal-organic frameworks (MOFs) provide uniquely tunable, periodic platforms for site-isolation of reactive low-valent metal complexes of relevance in modern catalysis, adsorptive applications, and fundamental structural studies. Strategies for integrating such species in MOFs include post-synthetic metalation, encapsulation and direct synthesis using low-valent organometallic complexes as building blocks. These approaches have each proven effective in enhancing catalytic activity, modulating product distributions (i.e., by improving catalytic selectivity), and providing valuable mechanistic insights. In this minireview, we explore these different strategies, as applied to isolate low-valent species within MOFs, with a particular focus on examples that leverage the unique crystallinity, permanent porosity and chemical mutability of MOFs to achieve deep structural insights that lead to new paradigms in the field of hybrid catalysis.

Mn-CUK-1: A Flexible MOF for SO2, H2O, and H2S Capture.

The environmentally benign metal-organic framework (MOF) CUK-1 based on 2,4-pyridine dicarboxylate has been prepared for the first time using Mn(II) as the inorganic node and water as the only solvent. Mn-CUK-1 shows reversible and efficient capture of H2O, SO2, and H2S. Compared to previously studied Co(II) and Mg(II) versions of the same MOF, Mn-CUK-1 also exhibited unique temperature-induced structural flexibility due to organic linker torsion, as detailed by variable-temperature single-crystal X-ray diffraction studies. Owing to this inherent solid-state flexibility, Mn-CUK-1 showed stepwise adsorption for polar gases, which induce structural deformations upon adsorption, while the nonpolar guest adsorbates were reversibly sorbed in a more classical manner. Notably, Mn-CUK-1 demonstrates the highest reported H2S capacity-to-surface area ratio among MOFs that are chemically stable toward this reactive acidic molecule. Moreover, Mn-CUK-1 displays exceptional structural stability in the presence of high relative humidity and corrosive gases and shows soft crystalline behavior triggered by changes in both the adsorption temperature and guest molecule identity.

Investigating H2 Adsorption in Isostructural Metal-Organic Frameworks M-CUK-1 (M = Co and Mg) through Experimental and Theoretical Studies.

A combined experimental and theoretical study of H2 adsorption was carried out in Co-CUK-1 and Mg-CUK-1, two isostructural metal-organic frameworks (MOFs) that consist of M2+ ions (M = Co and Mg) coordinated to pyridine-2,4-dicarboxylate (pdc2-) and OH- ligands. These MOFs possess saturated metal centers in distorted octahedral environments and narrow pore sizes and display high chemical and thermal stability. Previous experimental studies revealed that Co-CUK-1 exhibits a H2 uptake of 183 cm3 g-1 at 77 K/1.0 atm [ Angew. Chem., Int. Ed. 2007, 46, 272-275, DOI: 10.1002/anie.200601627], while that for Mg-CUK-1 under the same conditions is 240 cm3 g-1 on the basis of the experimental measurements carried out herein. The theoretical H2 adsorption isotherms are in close agreement with the corresponding experimental measurements for simulations using electrostatic and polarizable potentials of the adsorbate. Through simulated annealing calculations, it was found that the primary binding site for H2 in both isostructural analogues is localized proximal to the center of the aromatic rings belonging to the pdc2- linkers. Inelastic neutron scattering (INS) spectroscopic studies of H2 adsorbed in both MOFs revealed a rotational tunnelling transition occurring at around 8 meV in the corresponding spectra; this peak represents H2 adsorbed at the primary binding site. Two-dimensional quantum rotation calculations for H2 localized at the primary and secondary binding sites in both MOFs yielded rotational energy levels that are in agreement with the transitions observed in the INS spectra. Even though both M-CUK-1 analogues possess different metal ions, they exhibit similar electrostatic environments, modeled structures at H2 saturation, and rotational potentials for H2 adsorbed at the most favorable adsorption site. Overall, this study demonstrates how important molecular-level details of the H2 adsorption mechanism inside MOF micropores can be derived from a combination of experimental measurements and theoretical calculations using two stable and isostructural MOFs with saturated metal centers and small pore windows as model systems.

Low-Valent Metal Ions as MOF Pillars: A New Route Toward Stable and Multifunctional MOFs.

PCM-102 is a new organophosphine metal-organic framework (MOF) featuring diphosphine pockets that consist of pairs of offset trans-oriented P(III) donors. Postsynthetic addition of M(I) salts (M = Cu, Ag, Au) to PCM-102 induces single-crystal to single-crystal transformations and the formation of trans-[P2M]+ solid-state complexes (where P = framework-based triarylphosphines). While the unmetalated PCM-102 has low porosity, the addition of secondary Lewis acids to install rigid P-M-P pillars is shown to dramatically increase both stability and selective gas uptake properties, with N2 Brunauer-Emmett-Teller surface areas >1500 m2 g-1. The Ag(I) analogue can also be obtained via a simple, one-pot peri-synthetic route and is an ideal sacrificial precursor for materials with mixed bimetallic MA/MB pillars via postsynthetic, solvent-assisted metal exchange. Notably, the M-PCM-102 family of MOFs contain periodic trans-[P2M]+ sites that are free of counter anions, unlike traditional analogous molecular complexes, since the precursor PCM-102 MOF is monoanionic, enabling access to charge-neutral metal-pillared materials. Four M-PCM-102 materials were evaluated for the separation of C2 hydrocarbons. The separation performance was found to be tunable based on the metal(s) incorporated, and density functional theory was employed to elucidate the nature of the unusual observed sorption preference, C2H2 > C2H6 > C2H4.

Magnetism and Luminescence of a MOF with Linear Mn3 Nodes Derived from an Emissive Terthiophene-Based Imidazole Linker.

A new terthiophene-based imidazole luminophore 5,5'-(1H-thieno[3,4-d]imidazole-4,6-diyl)bis(thiophene-2-carboxylic acid) (TIBTCH2, 5) was synthesized in one step from previously reported 4,6-di(thiophen-2-yl)-1H-thieno[3,4-d]imidazole (DTTI, 4), and their photophysical properties were studied and compared accordingly. Under solvothermal conditions, reacting 5 with Mn(OAc)2 yielded a new three-dimensional metal-organic framework (MOF, 6) which was structurally defined by single-crystal X-ray diffraction. In 6, all Mn(II) ions octahedrally bind to carboxylate-O atoms to form a linear Mn3 secondary building unit (SBU) that contains three distinct coordination modes. Importantly, 6 exhibits dual functional properties of ligand-based emission and metal-based magnetic behaviors.

Hydro-thermal synthesis and crystal structure of poly[bis-(µ3-3,4-di-amino-benzoato)manganese], a layered coordination polymer.

The hydro-thermal synthesis and crystal structure of the title two-dimensional coordination polymer, poly[bis-(µ3-3,4-di-amino-benzoato-κ3 N 3,O,O')manganese(II)], [Mn(C7H7N2O2)2] n , are described. The Mn2+ cation (site symmetry ) adopts a tetra-gonally elongated trans-MnN2O4 octa-hedral coordination geometry and the µ3-N,O,O' ligand (bonding from both carboxyl-ate O atoms and the meta-N atom) links the metal ions into infinite (10) layers. The packing is consolidated by intra-layer N-H⋯O and inter-layer N-H⋯N hydrogen bonds. The structure of the title compound is compared with other complexes containing the C7H7N2O2 - anion and those of the related M(C8H8NO2)2 (M = Mn, Co, Ni, Zn) family, where C8H8NO2 - is the 3-amino-4-methyl-benzoate anion.

Reversible Solid-State Isomerism of Azobenzene-Loaded Large-Pore Isoreticular Mg-CUK-1.

A large-pore version of Mg-CUK-1, a water-stable metal-organic framework (MOF) with 1-D channels, was synthesized in basic water. Mg-CUK-1L has a BET surface area of 2896 m2 g-1 and shows stark selectivity for CO2 sorption over N2, O2, H2, and CH4. It displays reversible, multistep gated sorption of CO2 below 0.33 atm. The dehydrated single-crystal structure of Mg-CUK-1L confirms retention of the open-channel structure. The MOF can be loaded with organic molecules by immersion in hot melts, providing single crystals suitable for X-ray diffraction. trans-Azobenzene fills the channels in a 2 × 2 arrangement. Solid-state UV-vis spectroscopy reveals that azobenzene molecules undergo reversible trans-cis isomerization, despite being close-packed; this surprising result is confirmed by DFT-simulated UV-vis spectra.

In situ formation and solid-state oxidation of a triselenane NSeN-pincer MOF.

Controlled partial decomposition of 2-selenonicotinic acid in the presence of Co2+ or Ni2+ resulted in the in situ formation of an unusual MOF based on triselenane ligands (RSeSeSeR) coordinated to M2+ centers as NSeN-pincers. Post-synthetic oxidation by treatment with aqueous H2O2 facilitates its solid-state conversion into a RSeO2- molecular coordination complex, which was tracked via powder X-ray diffraction studies and by single-crystal structural resolution of the final product.

Rational Design of Rhodium-Iridium Alloy Nanoparticles as Highly Active Catalysts for Acidic Oxygen Evolution.

The oxygen evolution reaction (OER) is pivotal for renewable energy conversion and storage devices, such as water electrolyzers and rechargeable metal-air batteries. However, the rational design of electrocatalysts with suitably high efficiencies and stabilities in strongly acidic electrolytes remains a significant challenge. Here, we show the demonstration of sub-10 nm, composition-tunable Rh-Ir alloy nanoparticles (NPs) prepared using a scalable microwave-assisted method as superior acidic OER catalysts. The OER activities showed a volcano-shaped dependence on Ir composition, with Ir-rich NPs (Ir ≥ 51%) achieving better OER performance than pure Ir NPs, as reflected by lower overpotentials and higher mass activities. Most significantly, Rh22Ir78 NPs achieved a maximum mass activity of 1.17 A mg-1Ir at a 300 mV overpotential in 0.5 M H2SO4, which corresponds to a 3-fold enhancement relative to pure Ir NPs, making it one of the most active reported OER catalysts under acidic conditions. Density functional theory calculations reveal that owing to the synergy of ensemble and electronic effects by alloying a small amount of Rh with Ir, the binding energy difference of the O and OOH intermediates is reduced, leading to faster kinetics and enhanced OER activity. Furthermore, Rh-Ir alloy NPs demonstrated excellent durability in strongly acidic electrolyte. This work not only provides fundamental understandings relating to composition-electrochemical performance relationships but also represents the rational design of highly efficient OER electrocatalysts for applications in acidic media.

Porous Metal-Organic Framework CUK-1 for Adsorption Heat Allocation toward Green Applications of Natural Refrigerant Water.

The development of new water adsorbents that are hydrothermally stable and can operate more efficiently than existing materials is essential for the advancement of water adsorption-driven chillers. Most of the existing benchmark materials and related systems in this field suffer from clear limitations that must be overcome to meet global requirements for sustainable and green energy production and utilization. Here, we report the energy-efficient water sorption properties of three isostructural metal-organic frameworks (MOFs) based on the simple ligand pyridine-2,4-dicarboxylate, named M-CUK-1 [M3(µ3-OH)2(2,4-pdc)2] (where M = Co2+, Ni2+, or Mg2+). The highly hydrothermally stable CUK-1 series feature step-like water adsorption isotherms, relatively high H2O sorption capacities between P/P0 = 0.10-0.25, stable cycling, facile regeneration, and, most importantly, benchmark coefficient of performance values for cooling and heating at a low driving temperature. Furthermore, these MOFs are prepared under green hydrothermal conditions in aqueous solutions. Our joint experimental-computational approach revealed that M-CUK-1 integrates several optimal features, resulting in promising materials as advanced water adsorbents for adsorption-driven cooling and heating applications.

Shape-persistent pyrrole-based covalent organic cages: synthesis, structure and selective gas adsorption properties.

Two cryptand-like, shape persistent [2+3] imine cages (1 and 2) derived from oligopyrrolic precursors (diformyl dipyrrylpyridine 3 and diformyl bipyrrole 4) were prepared. These cages contain open cavities as inferred from solid state structural analyses and act as selective CO2 gas adsorbing materials in the solid state.

Structural characterization of heterogeneous RhAu nanoparticles from a microwave-assisted synthesis.

A microwave assisted method was used to synthesize RhAu nanoparticles (NPs). Characterization, based upon transmission electron microscopy (TEM), energy dispersive spectroscopy, and powder X-ray diffraction, provided the evidence of monomodal alloy NPs with a mean size distribution between 3 and 5 nm, depending upon the composition. Extended X-ray adsorption fine-structure spectroscopy (EXAFS) also showed evidence of alloying, but the coordination numbers of Rh and Au indicated significant segregation between the metals. More problematic were the low coordination numbers for Rh; values of ca. 9 indicate NPs smaller than 2 nm, significantly smaller than those observed with TEM. Additionally, no single-particle structural models were able to reproduce the experimental EXAFS data. Resolution of this discrepancy was achieved with high resolution aberration corrected scanning TEM imaging which showed the presence of ultra-small (<2 nm) pure Rh clusters and larger (∼3-5 nm) segregated particles with Au-rich cores and Rh-decorated shells. A heterogeneous model with a mixture of ultrasmall pure Rh clusters and larger segregated Rh/Au NPs was able to explain the experimental measurements of the NPs over the range of compositions measured. The combination of density functional theory, EXAFS, and TEM allowed us to quantify the heterogeneity in the RhAu NPs. It was only through this combination of theoretical and experimental techniques that resulted in a bimodal distribution of particle sizes that was able to explain all of the experimental characterization data.

Humidity-induced CO2 capture enhancement in Mg-CUK-1.

Kinetic CO2 adsorption measurements in the water-stable and permanently microporous Metal-organic framework material, Mg-CUK-1, reveal a 1.8-fold increase in CO2 capture from 4.6 wt% to 8.5 wt% in the presence of 18% relative humidity. Thermodynamic CO2 uptake experiments corroborate this enhancement effect, while grand canonical Monte Carlo simulations also support the phenomenon of a humidity-induced increase in the CO2 sorption capacity in Mg-CUK-1. Molecular simulations were implemented to gain insight into the microscopic adsorption mechanism responsible for the observed CO2 sorption enhancement. These simulations indicate that the cause of increasing CO2 adsorption enthalpy in the presence of H2O is due to favorable intermolecular interactions between the co-adsorbates confined within the micropores of Mg-CUK-1.

Highly selective room temperature acetylene sorption by an unusual triacetylenic phosphine MOF.

The new ligand tris(p-carboxyphenylethynyl)phosphine (P{C[triple bond, length as m-dash]CC6H4-4-CO2H}3) was used to synthesize a permanently porous Mn(ii)-based acetylenic phosphine coordination material, PCM-48. This triply-interpenetrated MOF contains 1-D microchannels that are decorated with electron-rich and adsorbate-accessible acetylenic moieties and phosphine lone pairs. PCM-48 has a moderate room-temperature C2H2 adsorption capacity (25.54 cm3 g-1) and displays high separation selectivities for C2H2 over CH4 (C2H2/CH4 = 23.3), CO2 (C2H2/CO2 = 4.3), and N2 (C2H2/N2 = 76.9) at 296 K.

Organoarsine Metal-Organic Framework with cis-Diarsine Pockets for the Installation of Uniquely Confined Metal Complexes.

ACM-1 is the first example of an organoarsine metal-organic framework (MOF), prepared using a new pyridyl-functionalized triarylarsine ligand coordinated to Ni(II) nodes. ACM-1 has micropores that are decorated with cis-diarsine coordination pockets. Postsynthetic metalation of ACM-1 with AuCl under facile conditions studied by single-crystal X-ray diffraction reveals the installation of dimeric Au2Cl2 complexes via the formation of As-Au bonds. The Au(I) dimers display exceptionally short aurophilic bonds (2.76 Å) induced by the rigidity of the MOF, which acts as a unique solid-state ligand.