Adam J. Rieth

Water Oxidation and Oxygen Monitoring by Cobalt-Modified Fluorine-Doped Tin Oxide Electrodes

Electrocatalytic water oxidation occurs at fluoride-doped tin oxide (FTO) electrodes that have been surface-modified by addition of Co(II). On the basis of X-ray photoelectron spectroscopy and transmission electron microscopy measurements, the active surface site appears to be a single site or small-molecule assembly bound as Co(II), with no evidence for cobalt oxide film or cluster formation. On the basis of cyclic voltammetry measurements, surface-bound Co(II) undergoes a pH-dependent 1e(-)/1H(+) oxidation to Co(III), which is followed by pH-dependent catalytic water oxidation. O2 reduction at FTO occurs at -0.33 V vs NHE, allowing for in situ detection of oxygen as it is formed by water oxidation on the surface. Controlled-potential electrolysis at 1.61 V vs NHE at pH 7.2 resulted in sustained water oxidation catalysis at a current density of 0.16 mA/cm(2) with 29,000 turnovers per site over an electrolysis period of 2 h. The turnover frequency for oxygen production per Co site was 4 s(-1) at an overpotential of 800 mV at pH 7.2. Initial experiments with Co(II) on a mesoporous, high-surface-area nanoFTO electrode increased the current density by a factor of ~5.

High and Reversible Ammonia Uptake in Mesoporous Azolate Metal–Organic Frameworks with Open Mn, Co, and Ni Sites

A series of new mesoporous metal-organic frameworks (MOFs) made from extended bisbenzenetriazolate linkers exhibit coordinatively unsaturated metal sites that are responsible for high and reversible uptake of ammonia. Isostructural Mn, Co, and Ni materials adsorb 15.47, 12.00, and 12.02 mmol of NH3/g, respectively, at STP. Importantly, these near-record capacities are reversible for at least three cycles. These results demonstrate that azolate MOFs are sufficiently thermally and chemically stable to find uses in recyclable sorption, storage, and potentially separation of chemically challenging and/or corrosive gases, especially when designed to exhibit a high density of open metal sites.

Grand Challenges and Future Opportunities for Metal–Organic Frameworks

Metal–organic frameworks (MOFs) allow compositional and structural diversity beyond conventional solid-state materials. Continued interest in the field is justified by potential applications of exceptional breadth, ranging from gas storage and separation, which takes advantage of the inherent pores and their volume, to electronic applications, which requires precise control of electronic structure. In this Outlook we present some of the pertinent challenges that MOFs face in their conventional implementations, as well as opportunities in less traditional areas. Here the aim is to discuss select design concepts and future research goals that emphasize nuances relevant to this class of materials as a whole. Particular emphasis is placed on synthetic aspects, as they influence the potential for MOFs in gas separation, electrical conductivity, and catalytic applications.

Record Atmospheric Fresh Water Capture and Heat Transfer with a Material Operating at the Water Uptake Reversibility Limit

The capture of water vapor at low relative humidity is desirable for producing potable water in desert regions and for heat transfer and storage. Here, we report a mesoporous metal–organic framework that captures 82% water by weight below 30% relative humidity. Under simulated desert conditions, the sorbent would deliver 0.82 gH2O gMOF–1, nearly double the quantity of fresh water compared to the previous best material. The material further demonstrates a cooling capacity of 400 kWh m–3 per cycle, also a record value for a sorbent capable of creating a 20 °C difference between ambient and output temperature. The water uptake in this sorbent is optimized: the pore diameter of our material is above the critical diameter for water capillary action, enabling water uptake at the limit of reversibility.

Controlled Gas Uptake in Metal–Organic Frameworks with Record Ammonia Sorption

Ammonia is a vital commodity in our food supply chain, but its toxicity and corrosiveness require advanced protection and mitigation. These needs are not met efficiently by current materials, which suffer from either low capacity or low affinity for NH3. Here, we report that a series of microporous triazolate metal-organic frameworks containing open metal sites exhibit record static and dynamic ammonia capacities. Under equilibrium conditions at 1 bar, the materials adsorb up to 19.79 mmol NH3 g-1, more than twice the capacity of activated carbon, the industry standard. Under conditions relevant to personal protection equipment, capacities reach 8.56 mmol g-1, 27% greater than the previous best material. Structure-function relationships and kinetic analyses of NH3 uptake in isostructural micro- and mesoporous materials made from Co, Ni, and Cu reveal stability trends that are in line with the water substitution rates in simple metal-aquo complexes. Altogether, these results provide clear, intuitive descriptors that govern the static and dynamic uptake, kinetics, and stability of MOF sorbents for strongly interacting gases.

Highly Stereoselective Heterogeneous Diene Polymerization by Co-MFU-4l: A Single-Site Catalyst Prepared by Cation Exchange

Molecular catalysts offer tremendous advantages for stereoselective polymerization because their activity and selectivity can be optimized and understood mechanistically using the familiar tools of organometallic chemistry. Yet, this exquisite control over selectivity comes at an operational price that is generally not justifiable for the large-scale manufacture of polyfolefins. In this report, we identify Co-MFU-4l, prepared by cation exchange in a metal-organic framework, as a solid catalyst for the polymerization of 1,3-butadiene with high stereoselectivity (>99% 1,4-cis). To our knowledge, this is the highest stereoselectivity achieved with a heterogeneous catalyst for this transformation. The polymers low polydispersity (PDI ≈ 2) and the catalysts ready recovery and low leaching indicate that our material is a structurally resilient single-site heterogeneous catalyst. Further characterization of Co-MFU-4l by X-ray absorption spectroscopy provided evidence for discrete, tris-pyrazolylborate-like coordination of Co(II). With this information, we identify a soluble cobalt complex that mimics the structure and reactivity of Co-MFU-4l, thus providing a well-defined platform for studying the catalytic mechanism in the solution phase. This work underscores the capacity for small molecule-like tunability and mechanistic tractability available to transition metal catalysis in metal-organic frameworks.

Selective Vapor Pressure Dependent Proton Transport in a Metal–Organic Framework with Two Distinct Hydrophilic Pores

The mechanism of proton conductivity in porous solids (i.e., Grotthuss or vehicular) is related to the structure and chemical environment of the pores. Direct observation of structure-function relationships is difficult because state-of-the-art solid proton conductors are often amorphous. Here, we present a systematic elucidation of two distinct proton transport pathways within MIT-25, a mesoporous metal-organic framework that exhibits parallel channels of ∼27 Å and ∼4.5 Å width. We characterize transport through these pores using temperature- and humidity-dependent proton conductivity measurements and density functional theory. Through control of vapor pressure we are able to sequentially fill the small and large pores, promoting proton conductivity with distinct activation energies at low and high relative humidity, respectively.

Programming Framework Materials for Ammonia Capture

The surface of pores in covalent organic networks was chemically modified for better ammonia capture.

CCDC 1579914: Experimental Crystal Structure Determination

Related Article: Romain J.-C. Dubey, Robert J. Comito, Zhenwei Wu, Guanghui Zhang, Adam J. Rieth, Christopher H. Hendon, Jeffrey T. Miller, and Mircea Dincă|2017|J.Am.Chem.Soc.|139|12664|doi:10.1021/jacs.7b06841