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This Perspective summarizes progress in the development of O2-selective metal–organic frameworks for adsorptive air separations and identifies key metrics and design considerations toward optimizing material performance for practical applications.

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.

Oxygen is a critical gas in numerous industries and is produced globally on a gigatonne scale, primarily through energy-intensive cryogenic distillation of air. The realization of large-scale adsorption-based air separations could enable a significant reduction in associated worldwide energy consumption and would constitute an important component of broader efforts to combat climate change. Certain small-scale air separations are carried out using N

Mitigation of anthropogenic climate change is expected to require large-scale deployment of carbon dioxide removal strategies. Prominent among these strategies is direct air capture with sequestration (DACS), which encompasses the removal and long-term storage of atmospheric CO

Diamine-appended variants of the metal–organic framework M2(dobpdc) (M = Mg, Mn, Fe, Co, Zn; dobpdc4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) exhibit exceptional CO2 capture properties owing to ...

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 H

We present in this paper the results of the field testing at a California Landfill of a UV photodecomposition reactor (PhoR) used for the removal of siloxane impurities from landfill gas (LFG). Prior to its field testing, the PhoR technology was tested in the laboratory with simulated LFG and was shown to be capable of completely removing the trace siloxane compounds and to convert them into silica particulates. The key objective of the field test was to validate the ability of the PhoR system to treat real LFG in a practical setting. The field-scale PhoR again proved quite efficient in attaining complete siloxane removal at different concentrations in the real landfill environment. These promising findings have led us to propose a scaled-up, commercial size PhoR system, competitive to conventional adsorption systems that can be practically applied in existing landfill plants to obtain 99+% siloxanes removal rates without associated secondary emissions.

The large-scale deployment of carbon capture technologies is expected to play a crucial role in efforts to meet stringent climate targets set forth by the Paris Agreement, but current models rely heavily upon carbon dioxide removal (CDR) strategies for which viability at the gigatonne scale is uncertain. While most 1.5 and 2 °C scenarios project rapid decarbonization of the energy sector facilitated by carbon capture and sequestration (CCS), they generally assume that CCS units can only capture ∼90% of the CO2 in coal and natural gas combustion flues because this was previously considered the optimal condition for aqueous amine scrubbers. In this Perspective, we discuss a small but growing body of literature that examines the prospect of moving significantly beyond 90% capture-a concept we term deep CCS-in light of recent developments in materials and process design. The low incremental costs associated with performing varying degrees of deep CCS suggest that this approach is not only feasible but may also alleviate burdens placed upon CDR techniques facing significant barriers to large-scale deployment. We estimate that rapid deployment of deep CCS in deep decarbonization pathways could avoid more than 1 gigatonne of CO2 globally each year. The principles of deep CCS could also be applied directly to the CDR strategy of employing bioenergy with CCS, which could lead to a significant alleviation of the land and freshwater burden associated with this technology.

Coordinatively-unsaturated metal sites within certain zeolites and metal–organic frameworks can strongly adsorb various molecules. While many classical examples involve electron-poor metal cations that interact with adsorbates largely through electrostatic 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 H2chemisorption 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 situpowder neutron diffraction experiments enable crystallographic characterization of this intermediate, the first time that this has been accomplished for any material. Support 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.

Diamine-appended variants of the metal-organic framework M

The goal of this study is to assess the technical feasibility of remediating siloxane contaminants in biogas via a photochemical process. Specifically, we studied in the laboratory a process that involves the use of an ultraviolet (UV) photodecomposition reactor (PhoR) to convert siloxane trace impurities, commonly found in biogas produced in water treatment plants and landfills, into silica particulates. These can then be effectively removed from the reactor effluent with the use of a downstream filter. High siloxane conversions were obtained, which demonstrates the effectiveness of the technique. The proposed technology is presently being field-tested in a California landfill.

Carbon dioxide (CO2) has been shown to contribute to human health consequences indoors, such as shortness of breath, nasal and optic irritation, dizziness, and nausea. In this work, we explore the potential of metal–organic frameworks (MOFs) as highly-porous, crystalline sorbents for sensitive colorimetric CO2 detection. In particular, the zeolitic imidazolate framework (ZIF-8) is chosen as the sorptive material due to its chemical stability and tunable CO2 affinity. The colorimetric gas sensor is developed in methanol with three components: (i) MOF ZIF-8 as a high surface area adsorbent; (ii) ethylenediamine (ED) as the CO2-affinitive basic function; and (iii) phenolsulfonpthalein (PSP) as the pH indicator. Colorimetric assays and ratiometric analysis confirm a colorimetric response to variable CO2 concentrations of relevance to indoor air quality. The color response is attributed to a zwitterion mechanism whereby ED reacts with CO2 to form a zwitterionic intermediate. This intermediate is then deprotonated by the pH indicator, shifting the pH and inducing a color change. Given its simple fabrication, rapid and obvious response, and stability in ambient environment, the ZIF-8-based colorimetric sensor provides a promising route for an improved indoor air quality monitoring.