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Water is the most abundant and cleanest natural resource on earth, and it is the driving force of all nature. It not only affects food security, human health, and ecosystem integrity and maintenance, but is also an important driver of energy in industrial production and life. Importantly, water adsorption applications are considered to be highly energy-efficient and environmentally friendly technologies,1 including atmospheric water harvesting,2-4 desiccation of clean gases,5 indoor humidity control,6,7 and adsorptive heat transformation.8,9 However, current water adsorption-related applications are still constrained by properties of adsorbents, such as their low water uptake capacities, poor cyclic stabilities, limited feasibilities over a range of humidity conditions, and minimal commercial availabilities. Conventional nanoporous materials (e.g., silica gels, zeolites, and clays) were the first adsorbents used in water capture applications due to their low cost, commercial availability, and favorable water adsorption kinetics. However, these materials generally suffer from either low water uptake capacities or high regeneration temperature, limiting their use in practical water absorption applications.1,10 Metal-organic frameworks (MOFs), a class of crystalline porous materials, are assembled from inorganic nodes and organic linkers through coordination bonds.11,12 Benefiting from their exceptional porosity and surface area, tunable pore size and geometry, and highly tailorable and designable structures and functionalities, MOFs show considerable potential for gas storage and separation, heterogeneous catalysis, and other energy and environmental sustainability applications.13-17 In recent years, MOFs have also shown great potential for water vapor adsorption because of a growing understanding of the relationship between MOFs and water, as well as an increasing number of reports detailing MOFs that exhibit high water stability.1,4,9 Moreover, judicious design of the MOF structures enables control over their water adsorption properties and the water uptake capacities, which make MOFs ideal candidates for water adsorption-related applications. This review aims to provide an overview of recent advances in the development of MOFs for water adsorption, as well as to offer proposed guidelines to develop even better water adsorption materials. First, we briefly introduce the fundamentals of water adsorption, including how to ascertain key insights based on the shapes of water adsorption isotherms, descriptions of various water adsorption mechanisms, and a discussion on the stability of MOFs in water systems. Next, we discuss several recent reports have detailed how to improve water uptake capacity through the design and synthesis of MOFs. In particular, we highlight the importance of reticular chemistry in the designed synthesis of MOF-based water adsorbent materials. We then shift our focus to discussing the enormous potential of MOFs for use in selective water vapor adsorption applications with both theoretical and practical considerations considered. Finally, we offer our thoughts on the future development of this field in three aspects: chemistry and materials design, process engineering, and commercialization of MOFs for water adsorption. We hope that this review will provide fundamental insights for chemists and inspire them to synthesize MOFs with better water adsorption performance; and provide assistance to engineers researching MOF-based water adsorption devices and working towards the development of highly energy-efficient and environmentally friendly technologies with reduced carbon footprints.
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Insights into dual-functional modification for water stability enhancement of mesoporous zirconium metal–organic frameworks
The stability of metal–organic frameworks (MOFs) in water affects their ability to function as chemical catalysts, their capacity as adsorbents for separations in water vapor presence, and their usefulness as recyclable water harvesters. Here, we have examined water stability of four node-modified variants of the mesoporous MOF, NU-1000, namely formate-, Acac-, TFacac-, and Facac-NU-1000, comparing these with node-accessible NU-1000. These NU-1000 variants present ligands grafted to NU-1000's hexa-Zr( iv )-oxy nodes by displacing terminal aqua and hydroxo ligands. Facac-NU-1000, containing the most hydrophobic ligands, showed the greatest water stability, being able to undergo at least 20 water adsorption/desorption cycles without loss of water uptake capacity. Computational studies revealed dual salutary functions of installed Facac ligands: (1) enhancement of framework mechanical stability due to electrostatic interactions; and (2) transformation and shielding of the otherwise highly hydrophilic nodes from H-bonding interactions with free water, presumably leading to weaker channel-stressing capillary forces during water evacuation – consistent with trends in free energies of dehydration across the NU-1000 variants. Water harvesting and hydrolysis of chemical warfare agent simulants were examined to gauge the functional consequences of modification and mechanical stabilization of NU-1000 by Facac ligands. The studies revealed a harvesting capacity of ∼1.1 L of water vapor per gram of Facac-NU-1000 per sorption cycle. They also revealed retention of catalytic MOF activity following 20 water uptake and release cycles. This study provides insights into the basis for node-ligand-engendered stabilization of wide-channel MOFs against collapse during water removal.
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- PAR ID:
- 10399140
- Date Published:
- Journal Name:
- Journal of Materials Chemistry A
- Volume:
- 10
- Issue:
- 33
- ISSN:
- 2050-7488
- Page Range / eLocation ID:
- 17307 to 17316
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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The new material [RuGa]@NU-1000 incorporates Ru and Ga in 1.2 and 1.8 wt% respectively (molar ratio 1 : 2). It stems from the grafting of the heterobimetallic ruthenium gallate complex, [MeRu(η 6 -C 6 H 6 )(PPh 3 ) 2 ][GaMe 2 Cl 2 ] into the MOF material NU-1000. [RuGa]@NU-1000 shows enhanced adsorption of SO 2 , specially at low pressures (10 −3 bar) even when compared with other materials employing more expensive precious metals. Additionally, [RuGa]@NU-1000 samples need not be exposed to such harsh conditions for reactivation as they retain their adsorption properties after several cycles and preserve their porosity and structure. Thus, [RuGa]@NU-1000 is an excellent, selective material suitable for detection and precise quantification of SO 2 , with a lower cost compared to other MOFs incorporating precious metals.more » « less
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CALF-20, a Zn-triazolate-based metal-organic framework (MOF), is one of the most promising adsorbent materials for CO2 capture. However, competitive adsorption of water severely limits its performance when the relative humidity (RH) exceeds 40%, limiting the potential implementation of CALF-20 in practical settings where CO2 is saturated with moisture, such as post-combustion flue gas. In this work, three newly designed MOFs related to CALF-20, denoted as NU-220, CALF-20M-w, and CALF-20M-e that feature hydrophobic methyl-triazolate linkers are presented. Inclusion of methyl groups in the linker is proposed as a strategy to improve CO2 uptake in the presence of water. Notably, both CALF-20M-w and CALF-20M-e retain over 20% of their initial CO2 capture efficiency at 70% RH – a threshold at which CALF-20 shows negligible CO2 uptake. Grand canonical Monte Carlo (GCMC) simulations reveal that the methyl group hinders water network formation in the pores of CALF-20M-w and CALF-20M-e and enhances their CO2 selectivity over N2 in the presence of high moisture content. Moreover, calculated radial distribution functions indicate that introducing the methyl group into the triazolate linker increases the distance between water molecules and Zn coordination bonds, offering insights into the origin of the enhanced moisture stability observed for CALF-20M-w and CALF-20M-e relative to CALF-20. Overall, this straightforward design strategy has afforded more robust sorbents that can potentially meet the challenge of effectively capturing CO2 in practical industrial applications.more » « less
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null (Ed.)The mechanism of ethene hydrogenation to ethane on six dicationic 3d transition metal catalysts is investigated. Specifically, a combination of density functional theory (DFT), microkinetic modeling, and high throughput reactor experiments is used to interrogate the active sites and mechanisms for Mn@NU-1000, Fe@NU-1000, Co@NU-1000, Ni@NU-1000, Cu@NU-1000, and Zn@NU-1000 catalysts, where NU-1000 is a metal–organic framework (MOF) capable of supporting metal cation catalysts. The combination of experiments and simulations suggests that the reaction mechanism is influenced by the electron configuration and spin state of the metal cations as well as the amount of hydrogen that is adsorbed. Specifically, Ni@NU-1000, Cu@NU-1000, and Zn@NU-1000, which have more electrons in their d shells and operate in lower spin states, utilize a metal hydride active site and follow a mechanism where the metal cation binds with one or more species at all steps, whereas Mn@NU-1000, Fe@NU-1000, and Co@NU-1000, which have fewer electrons in their d shells and operate in higher spin states, utilize a bare metal cation active site and follow a mechanism where the number of species that bind to the metal cation is minimized. Instead of binding with the metal cation, catalytic species bind with oxo ligands from the NU-1000 support, as this enables more facile H 2 adsorption. The results reveal opportunities for tuning activity and selectivity for hydrogenation on metal cation catalysts by tuning the properties that influence hydrogen content and spin, including the metal cations themselves, the ligands, the binding environments and supports, and/or the gas phase partial pressures.more » « less
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Farha, Omar (Ed.)Metal-Organic Frameworks (MOFs) are advanced platforms for enzyme immobilization. Enzymes can be entrapped via either diffusion (into pre-formed MOFs) or co-crystallization. Enzyme co-crystallization with specific metals/ligands in the aqueous phase, also known as biomineralization, minimizes the enzyme loss as compared to organic phase co-crystallization, removes the size limitation on enzymes and substrates, and can potentially broaden the application of enzyme@MOF composites. However, not all enzymes are stable/functional in the presence of excess metal ions and/or ligands currently available for co-crystallization. Furthermore, most current biomineralization-based MOFs have limited (acid-) pH stability, making it necessary to explore other metal-ligand combinations that can also immobilize enzymes. Here, we report our discovery on the combination of five metal ions and two ligands that can form biocomposites with two model enzymes differing in size and hydrophobicity in the aqueous phase under ambient conditions. Surprisingly, most of the formed composites are single- or multi- phase crystals even though the reaction phase is aqueous, with the rest as amorphous powders. All 20 enzyme@MOF composites showed good to excellent reusability, and were stable under weakly acidic pHs. The stability under weakly basic conditions depended on the selection of enzyme and metal-ligand combinations, yet for both enzymes, 3-4 MOFs offered decent stability under basic conditions. This work initiates the expansion of the current “library” of metal-ligand selection for encapsulating/biomineralizing large enzymes/enzyme clusters, leading to customized encapsulation of enzymes according to enzymes stability, functionality, and optimal pH.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D2TA03851J
