Selective Adsorption of Oxygen from Humid Air in a Metal–Organic Framework with Trigonal Pyramidal Copper(I) Sites

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Porous solids can accommodate and release molecular hydrogen readily, making them attractive for minimizing the energy requirements for hydrogen storage relative to physical storage systems. However, H 2 adsorption enthalpies in such materials are generally weak (−3 to −7 kJ/mol), lowering capacities at ambient temperature. Metal−organic frameworks with well-defined structures and synthetic modularity could allow for tuning adsorbent−H 2 interactions for ambient-temperature storage. Recently, Cu 2.2 Zn 2.8 Cl 1.8 (btdd [1,4]dioxin; Cu I -MFU-4l) was reported to show a large H 2 adsorption enthalpy of −32 kJ/mol owing to π-backbonding from Cu I to H 2 , exceeding the optimal binding strength for ambient-temperature storage (−15 to −25 kJ/ mol). Toward realizing optimal H 2 binding, we sought to modulate the π-backbonding interactions by tuning the pyramidal geometry of the trigonal Cu I sites. A series of isostructural frameworks, Cu 2.7 M 2.3 X 1.3 (btdd) 3 (M = Mn, Cd; X = Cl, I; Cu I M-MFU-4l), was synthesized through postsynthetic modification of the corresponding materials M 5 X 4 (btdd) 3 (M = Mn, Cd; X = CH 3 CO 2 , I). This strategy adjusts the H 2 adsorption enthalpy at the Cu I sites according to the ionic radius of the central metal ion of the pentanuclear cluster node, leading to −33 kJ/mol for M = Zn II (0.74 Å), −27 kJ/mol for M = Mn II (0.83 Å), and −23 kJ/mol for M = Cd II (0.95 Å). Thus, Cu I Cd-MFU-4l provides a second, more stable example of optimal H 2 binding energy for ambienttemperature storage among reported metal−organic frameworks. Structural, computational, and spectroscopic studies indicate that a larger central metal planarizes trigonal Cu I sites, weakening the π-backbonding to H 2 .

Section: Discussionmentioning

confidence: 92%

Section: Resultsmentioning

confidence: 99%

Geometric Tuning of Coordinatively Unsaturated Copper(I) Sites in Metal–Organic Frameworks for Ambient-Temperature Hydrogen Storage

Yabuuchi,

Furukawa,

Carsch

et al. 2024

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“…10 ppm . Chemical adsorption is a promising approach for CO separation, but the difficulty of desorption lowers the separation efficiency. ,− Moreover, the open metal sites (OMSs) of chemical adsorbents generally show poor air/O 2 /H 2 O stability and can chemically adsorb many undesired guests. ,− …”

Section: Introductionmentioning

confidence: 99%

Isomeric Porous Cu(I) Triazolate Frameworks Showing Periodic and Aperiodic Flexibility for Efficient CO Separation

Wang,

Zhang,

Chen

et al. 2024

Guest-induced (crystal-to-crystal) transformation, i.e., periodic flexibility, is a typical feature of molecule-based crystalline porous materials, but its role for adsorptive separation is controversial. On the other hand, aperiodic flexibility is rarely studied. This work reports a pair of isomeric Cu(I) triazolate frameworks, namely, α-[Cu(fetz)] (MAF-2Fa) and β-[Cu(fetz)] (MAF-2Fb), which show typical periodic and aperiodic flexibility for CO chemical adsorption, respectively. Quantitative mixture breakthrough experiments show that, while MAF-2Fa exhibits high adsorption capacity at high pressures but negligible adsorption below the threshold pressure and with leakage concentrations of 3–8%, MAF-2Fb exhibits relatively low adsorption capacity at high pressures but no leakage (residual CO concentration <1 ppb). Tandem connection of MAF-2Fa and MAF-2Fb can combine their advantages of high CO adsorption capacities at high and low pressures, respectively. MAF-2Fa and MAF-2Fb can both keep the separation performances unchanged at high relative humidities, but only MAF-2Fb shows a unique coadsorption behavior at a relative humidity of 82%, which can be used to improve purification performances.

“…However, various applications of MOFs demand water stability. , For example, any MOF intended for use that entails exposure to ambient air or industrial gas streams should be able to tolerate humidity. Such applications include water harvesting from air, ,− direct air capture of CO 2 and O 2 , − CO 2 capture from flue gas, and pollutant sensing in air . In addition, for some applications, a MOF must be stable even in liquid water.…”

Section: Introductionmentioning

confidence: 99%

Metal–Organic Framework Stability in Water and Harsh Environments from Data-Driven Models Trained on the Diverse WS24 Data Set

Terrones,

Huang,

Rivera

et al. 2024

Metal−organic frameworks (MOFs) are porous materials with applications in gas separations and catalysis, but a lack of water stability often limits their practical use given the ubiquity of water. Consequently, it is useful to predict whether a MOF is water-stable before investing time and resources into synthesis. Existing heuristics for designing water-stable MOFs lack generality and limit the diversity of explored chemistry due to narrowly defined criteria. Machine learning (ML) models offer the promise to improve the generality of predictions but require data. In an improvement on previous efforts, we enlarge the available training data for MOF water stability prediction by over 400%, adding 911 MOFs with water stability labels assigned through semiautomated manuscript analysis to curate the new data set WS24. The additional data are shown to improve ML model performance (test ROC-AUC > 0.8) over diverse chemistry for the prediction of both water stability and stability in harsher acidic conditions. We illustrate how the expanded data set and models can be used with a previously developed activation stability model in combination with genetic algorithms to quickly screen ∼10,000 MOFs from a space of hundreds of thousands for candidates with multivariate stability (upon activation, in water, and in acid). We uncover metal-and geometry-specific design rules for robust MOFs. The data set and ML models developed in this work, which we disseminate through an easy-to-use web interface, are expected to contribute toward the accelerated discovery of novel, water-stable MOFs for applications such as direct air gas capture and water treatment.