Observation of an Intermediate to H2 Binding in a Metal-Organic Framework.

Coordinatively unsaturated metal sites within certain zeolites and metal-organic frameworks can strongly adsorb a wide array of substrates. While many classical examples involve electron-poor metal cations that interact with adsorbates largely through physical interactions, unsaturated electron-rich metal centers housed within porous frameworks can often chemisorb guests amenable to redox activity or covalent bond formation. Despite the promise that materials bearing such sites hold in addressing myriad challenges in gas separations and storage, very few studies have directly interrogated mechanisms of chemisorption at open metal sites within porous frameworks. Here, we show that nondissociative chemisorption of H2 at the trigonal pyramidal Cu+ sites in the metal-organic framework CuI-MFU-4l occurs via the intermediacy of a metastable physisorbed precursor species. In situ powder neutron diffraction experiments enable crystallographic characterization of this intermediate, the first time that this has been accomplished for any material. Evidence for a precursor intermediate is also afforded from temperature-programmed desorption and density functional theory calculations. The activation barrier separating the precursor species from the chemisorbed state is shown to correlate with a change in the Cu+ coordination environment that enhances π-backbonding with H2. Ultimately, these findings demonstrate that adsorption at framework metal sites does not always follow a concerted pathway and underscore the importance of probing kinetics in the design of next-generation adsorbents.

Understanding How Ligand Functionalization Influences CO2 and N2 Adsorption in a Sodalite Metal-Organic Framework.

In this work, a detailed study is conducted to understand how ligand substitution influences the CO2 and N2 adsorption properties of two highly crystalline sodalite metal-organic frameworks (MOFs) known as Cu-BTT (BTT-3 = 1,3,5-benzenetristetrazolate) and Cu-BTTri (BTTri-3 = 1,3,5-benzenetristriazolate). The enthalpy of adsorption and observed adsorption capacities at a given pressure are significantly lower for Cu-BTTri compared to its tetrazole counterpart, Cu-BTT. In situ X-ray and neutron diffraction, which allow visualization of the CO2 and N2 binding sites on the internal surface of Cu-BTTri, provide insights into understanding the subtle differences. As expected, slightly elongated distances between the open Cu2+ sites and surface-bound CO2 in Cu-BTTri can be explained by the fact that the triazolate ligand is a better electron donor than the tetrazolate. The more pronounced Jahn-Teller effect in Cu-BTTri leads to weaker guest binding. The results of the aforementioned structural analysis were complemented by the prediction of the binding energies at each CO2 and N2 adsorption site by density functional theory calculations. In addition, variable temperature in situ diffraction measurements shed light on the fine structural changes of the framework and CO2 occupancies at different adsorption sites as a function of temperature. Finally, simulated breakthrough curves obtained for both sodalite MOFs demonstrate the materials' potential performance in dry postcombustion CO2 capture. The simulation, which considers both framework uptake capacity and selectivity, predicts better separation performance for Cu-BTT. The information obtained in this work highlights how ligand substitution can influence adsorption properties and hence provides further insights into the material optimization for important separations.

An In-Situ Neutron Diffraction and DFT Study of Hydrogen Adsorption in a Sodalite-Type Metal-Organic Framework, Cu-BTTri.

Herein we present a detailed study of the hydrogen adsorption properties of Cu-BTTri, a robust crystalline metal-organic framework containing open metal-coordination sites. Diffraction techniques, carried out on the activated framework, reveal a structure that is different from what was previously reported. Further, combining standard hydrogen adsorption measurements with in-situ neutron diffraction techniques provides molecular level insight into the hydrogen adsorption process. The diffraction experiments unveil the location of four D2 adsorption sites in Cu-BTTri and shed light on the structural features that promote hydrogen adsorption in this material. Density functional theory (DFT), used to predict the location and strength of binding sites, corroborate the experimental findings. By decomposing binding energies in different sites in various energetic contributions, we show that van der Waals interactions play a crucial role, suggesting a possible route to enhancing the binding energy around open metal coordination sites.

Near-Perfect CO2/CH4 Selectivity Achieved through Reversible Guest Templating in the Flexible Metal-Organic Framework Co(bdp).

Metal-organic frameworks are among the most promising materials for industrial gas separations, including the removal of carbon dioxide from natural gas, although substantial improvements in adsorption selectivity are still sought. Herein, we use equilibrium adsorption experiments to demonstrate that the flexible metal-organic framework Co(bdp) (bdp2- = 1,4-benzenedipyrazolate) exhibits a large CO2 adsorption capacity and approaches complete exclusion of CH4 under 50:50 mixtures of the two gases, leading to outstanding CO2/CH4 selectivity under these conditions. In situ powder X-ray diffraction data indicate that this selectivity arises from reversible guest templating, in which the framework expands to form a CO2 clathrate and then collapses to the nontemplated phase upon desorption. Under an atmosphere dominated by CH4, Co(bdp) adsorbs minor amounts of CH4 along with CO2, highlighting the importance of studying all relevant pressure and composition ranges via multicomponent measurements when examining mixed-gas selectivity in structurally flexible materials. Altogether, these results show that Co(bdp) may be a promising CO2/CH4 separation material and provide insights for the further study of flexible adsorbents for gas separations.

Record High Hydrogen Storage Capacity in the Metal-Organic Framework Ni2(m-dobdc) at Near-Ambient Temperatures.

Hydrogen holds promise as a clean alternative automobile fuel, but its on-board storage presents significant challenges due to the low temperatures and/or high pressures required to achieve a sufficient energy density. The opportunity to significantly reduce the required pressure for high density H2 storage persists for metal-organic frameworks due to their modular structures and large internal surface areas. The measurement of H2 adsorption in such materials under conditions most relevant to on-board storage is crucial to understanding how these materials would perform in actual applications, although such data have to date been lacking. In the present work, the metal-organic frameworks M2(m-dobdc) (M = Co, Ni; m-dobdc4- = 4,6-dioxido-1,3-benzenedicarboxylate) and the isomeric frameworks M2(dobdc) (M = Co, Ni; dobdc4- = 1,4-dioxido-1,3-benzenedicarboxylate), which are known to have open metal cation sites that strongly interact with H2, were evaluated for their usable volumetric H2 storage capacities over a range of near-ambient temperatures relevant to on-board storage. Based upon adsorption isotherm data, Ni2(m-dobdc) was found to be the top-performing physisorptive storage material with a usable volumetric capacity between 100 and 5 bar of 11.0 g/L at 25 °C and 23.0 g/L with a temperature swing between -75 and 25 °C. Additional neutron diffraction and infrared spectroscopy experiments performed with in situ dosing of D2 or H2 were used to probe the hydrogen storage properties of these materials under the relevant conditions. The results provide benchmark characteristics for comparison with future attempts to achieve improved adsorbents for mobile hydrogen storage applications.

Reversible Capture and Release of Cl2 and Br2 with a Redox-Active Metal-Organic Framework.

Extreme toxicity, corrosiveness, and volatility pose serious challenges for the safe storage and transportation of elemental chlorine and bromine, which play critical roles in the chemical industry. Solid materials capable of forming stable nonvolatile compounds upon reaction with elemental halogens may partially mitigate these challenges by allowing safe halogen release on demand. Here we demonstrate that elemental halogens quantitatively oxidize coordinatively unsaturated Co(II) ions in a robust azolate metal-organic framework (MOF) to produce stable and safe-to-handle Co(III) materials featuring terminal Co(III)-halogen bonds. Thermal treatment of the oxidized MOF causes homolytic cleavage of the Co(III)-halogen bonds, reduction to Co(II), and concomitant release of elemental halogens. The reversible chemical storage and thermal release of elemental halogens occur with no significant losses of structural integrity, as the parent cobaltous MOF retains its crystallinity and porosity even after three oxidation/reduction cycles. These results highlight a material operating via redox mechanism that may find utility in the storage and capture of other noxious and corrosive gases.

Adsorption of two gas molecules at a single metal site in a metal-organic framework.

One strategy to markedly increase the gas storage capacity of metal-organic frameworks is to introduce coordinatively-unsaturated metal centers capable of binding multiple gas molecules. Herein, we provide an initial demonstration that a single metal site within a framework can support the terminal coordination of two gas molecules-specifically hydrogen, methane, or carbon dioxide.

Directly patternable, highly conducting polymers for broad applications in organic electronics.

Postdeposition solvent annealing of water-dispersible conducting polymers induces dramatic structural rearrangement and improves electrical conductivities by more than two orders of magnitude. We attain electrical conductivities in excess of 50 S/cm when polyaniline films are exposed to dichloroacetic acid. Subjecting commercially available poly(ethylene dioxythiophene) to the same treatment yields a conductivity as high as 250 S/cm. This process has enabled the wide incorporation of conducting polymers in organic electronics; conducting polymers that are not typically processable can now be deposited from solution and their conductivities subsequently enhanced to practical levels via a simple and straightforward solvent annealing process. The treated conducting polymers are thus promising alternatives for metals as source and drain electrodes in organic thin-film transistors as well as for transparent metal oxide conductors as anodes in organic solar cells and light-emitting diodes.