Crystalline hydrogen bonding of water molecules confined in a metal-organic framework

Advanced Solid Lubrication with COK-47: Mechanistic Insights on the Role of Water and Performance Evaluation

Metal-organic framework (MOF) nanoparticles have attracted widespread attention as lubrication additives due to their tunable structures and surface effects. However, their solid lubrication properties have been rarely explored. This work introduces the positive role of moisture in solid lubrication in the case of a newly described Ti-based MOF (COK-47) powder. COK-47 achieves an 8.5-fold friction reduction compared to AISI 304 steel-on-steel sliding under room air. In addition, COK-47 maintains a similarly low coefficient of friction (0.1-0.2) on various counterbodies, including Al2O3, ZrO2, SiC, and Si3N4. Notably, compared to other widely studied MOFs (ZIF-8, ZIF-67) and 2D materials powder (MXene, TMD, rGO), COK-47 exhibits the lowest friction (≈0.1) under the same experimental settings. Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy, scanning electron microscope, and transmission electron microscopy indicate that the tribofilm is an amorphous film obtained by hydrolysis of COK-47 in the air with moisture. Density functional theory further confirms that water catalyzes the decomposition of COK-47, a crucial step in forming the tribofilm. This study demonstrates the idea of utilizing MOF and water-assisted lubrication mechanisms. It provides new insights into MOF applications in tribology and highlights interdisciplinary contributions of mechanical engineering and chemistry.

Gas-flow activation of MOFs: unlocking efficient catalysis through dynamic bonding

Metal-organic frameworks (MOFs), characterized by dynamic metal-ligand coordination bonding, have pivotal roles in catalysis, gas storage, and separation processes, owing to their open metal sites (OMSs). These sites, however, are frequently occupied by Lewis-base solvent molecules, necessitating activation to expose the OMSs for practical applications. Traditional thermal activation methods involve harsh conditions, risking structural integrity. This study presents a novel 'gas-flow activation' technique using inert gases like nitrogen and argon to eliminate these coordinating solvent molecules at low temperatures, thereby maintaining the structural integrity of MOFs. We specifically explored this method with HKUST-1, demonstrating that gas-flow activation at mild temperatures is not only feasible but also superior in efficiency compared to the conventional thermal methods. This approach highlights the potential for safer, more efficient activation processes in MOF applications, making it a valuable addition to the repertoire of MOF activation techniques. This activation function of inert gas flow allows HKUST-1 as a catalyst for the hydrogenation of acetophenone even at room temperature. In addition, it is demonstrated that this 'gas-flow activation' is broadly applicable in other MOFs such as MOF-14 and UTSA-76. Furthermore, the findings reveal that dynamic coordination bonding, the repeating transient dissociation-association of solvent molecules at OMSs, are key mechanisms in facilitating this activation, pointing towards new directions for designing activation strategies that prevent structural damage.

Time-efficient atmospheric water harvesting using Fluorophenyl oligomer incorporated MOFs

Adsorption-based atmospheric water harvesting (AWH) has the potential to address water scarcity in arid regions. However, developing adsorbents that effectively capture water at a low relative humidity (RH < 30%) and release it with minimal energy consumption remains a challenge. Herein, we report a fluorophenyl oligomer (FO)-incorporated metal-organic framework (MOF), HKUST-1 (FO@HK), which exhibits fast adsorption kinetics at low RH levels and facile desorption by sunlight. The incorporated fluorophenyl undergoes vapor-phase polymerization at the metal center to generate fluorophenyl oligomers that enhance the hydrolytic stability of FO@HK while preserving its characteristic water sorption behavior. The FO@HK exhibited vapor sorption rates of 8.04 and 11.76 L kg-1MOF h-1 at 20 and 30% RH, respectively, which are better than the state-of-the-art AWH sorbents. Outdoor tests using a solar-driven large-scale AWH device demonstrate that the sorbent can harvest 264.8 mL of water at a rate of 2.62 L kg-1MOF per day. This study provides a ubiquitous strategy for transforming water-sensitive MOFs into AWH sorbents.

A homochiral tartrate-bridged dinuclear chromium(III) complex anion with a resonance-assisted hydrogen bond for proton conduction

The synthesis of a homochiral building block based on L-tartrate-chromium(III) complex anions is reported. The dinuclear complex anion, which contains two bridging L-tartrate ligands and one aromatic N-donor ligand coordinated to chromium(III) ions, exhibits a boat conformation in which intramolecular resonance-assisted hydrogen bonding is present. The sodium L-tartrate-chromium(III) compound with the formula Na[Cr2(bpy)2(L-tart)2H]·9H2O (1) crystallizes from a methanol-water solution as a high water content material in the monoclinic space group P2. The as-synthesized compound is only stable at high relative humidity and undergoes structural transformations during drying, which are accompanied by water loss. However, these transformations are reversible and upon wetting, the material returns to its high water content structure. Based on a combination of experimental techniques (PXRD, in situ ATR-FTIR and EPR spectroscopy), the structure of the complex anions appears to be insensitive to the humidity variable processes (wetting and drying). Due to the presence of several hydrogen acceptor and donor groups in the L-tartrate-chromium(III) complex anion, we investigated the proton transport properties of a sodium L-tartrate-chromium(III) compound by impedance spectroscopy under dry and wet conditions at different temperatures. Since the relative humidity affects the structural transformations in this system, it also has a large influence on the proton conductivity, which varies by up to four orders of magnitude depending on the degree of hydration. These results confirm that the proton conductivity can be tuned in flexible structures in which non-covalent interactions determine the crystal packing.

Four Lanthanide(III) Metal-Organic Frameworks Fabricated by Bithiophene Dicarboxylate for High Inherent Proton Conduction

Currently, it is still a challenge to directly achieve highly stable metal-organic frameworks (MOFs) with superior proton conductivity solely through the exquisite design of ligands and the attentive selection of metal nodes. Inspired by this, we are intrigued by a multifunctional dicarboxylate ligand including dithiophene groups, 3,4-dimethylthieno[2,3-b]thiophene-2,5-dicarboxylic acid (H2DTD), and lanthanide ions with distinct coordination topologies. Successfully, four isostructural three-dimensional lanthanide(III)-based MOFs, [Ln2(DTD)3(DEF)4]·DEF·6H2O [LnIII = TbIII (Tb-MOF), EuIII (Eu-MOF), SmIII (Sm-MOF), and DyIII (Dy-MOF)], were solvothermally prepared, in which the effective proton transport will be provided by the coordinated or free solvent molecules, the crystalline water molecules, and the framework components, as well as a large number of highly electronegative S and O atoms. As expected, the four Ln-MOFs demonstrated the highest proton conductivities (σ) being 0.54 × 10-3, 3.75 × 10-3, 1.28 × 10-3, and 1.92 × 10-3 S·cm-1 for the four MOFs, respectively, at 100 °C/98% relative humidity (RH). Excitingly, Dy-MOF demonstrated an extraordinary ultrahigh σ of 1 × 10-3 S·cm-1 at 30 °C/98% RH. Additionally, the plausible proton transport mechanisms were emphasized.

Energy-efficient extraction of linear alkanes from various isomers using structured metal-organic framework membrane

Extraction of low concentration linear alkanes (C5-C7) from various isomers is critical for the petrochemical industry. At present, the separation of alkane isomers is mainly accomplished by distillation, which results in substantial energy expenditure. Metal-organic frameworks (MOFs) with well-tailored nanopores have been demonstrated to be capable of realizing molecule-level separation. In this study, oriented HKUST-1 membranes are formulated according to the morphology-biased principle and finally realized with a low dose synthesis method for terminating undesired crystal nucleation and growth. The fully exposed triangular sieving pore array of the membrane induces configuration entropic diffusion to split linear alkanes from mono-branched and di-branched isomers as well as their cyclical counterparts. Typically, the current separation technique consumes 91% less energy than vacuum distillation. Furthermore, our membranes can realize one-step extraction of normal-pentane, normal-hexane and normal-heptane from a ten-component alkane isomer solution that mimics light naphtha.

Synthesis and Characterization of a New Cu(II) Paddle-Wheel-like Complex with 4-Vinylbenzoate as an Inorganic Node for Metal-Organic Framework Material Design

A new Cu(II) paddle-wheel-like complex with 4-vinylbenzoate was synthesized using acetonitrile as the solvent. The complex was characterized by X-ray crystal diffraction, FT-IR, diffuse reflectance spectroscopy, thermogravimetric, differential scanning calorimetric, magnetic susceptibility, and electronic paramagnetic resonance analyses. The X-ray crystal diffraction analysis indicated that each copper ion was bound at an equatorial position to four oxygen atoms from the carboxylate groups of the 4-vinylbenzoate ligand in a square-based pyramidal geometry. The distance between the copper ions was 2.640(9) Å. The acetonitrile molecules were coordinated at the axial position to the copper ions. Exposure of the Cu(II) complex to humid air promoted the gradual replacement of the coordinated acetonitrile by water molecules, but the complex structure integrity remained. The EPR spectra exhibited signals attributed to the presence of a mixture of the monomeric (S = ½) and dimeric (S = 1) copper species in a possible 3:1 ratio. The magnetic studies revealed a peak at 50-100 K, which could be associated with the oxygen absorption capacity of the Cu(II)-vba complex.

Ultrafast Water H-Bond Rearrangement in a Metal-Organic Framework Probed by Femtosecond Time-Resolved Infrared Spectroscopy

We investigated the water H-bond network and its dynamics in Ni₂Cl₂BTDD, a prototypical MOF for atmospheric water harvesting, using linear and ultrafast IR spectroscopy. Utilizing isotopic labeling and infrared spectroscopy, we found that water forms an extensive H-bonding network in Ni₂Cl₂BTDD. Further investigation with ultrafast spectroscopy revealed that water can reorient in a confined cone up to ∼50° within 1.3 ps. This large angle reorientation indicates H-bond rearrangement, similar to bulk water. Thus, although the water H-bond network is confined in Ni₂Cl₂BTDD, different from other confined systems, H-bond rearrangement is not hindered. The picosecond H-bond rearrangement in Ni₂Cl₂BTDD corroborates its reversibility with minimal hysteresis in water sorption.

Structure and thermodynamics of water adsorption in NU-1500-Cr

Metal-organic frameworks (MOFs) are a class of materials with diverse chemical and structural properties, and have been shown to effectively adsorb various types of guest molecules. The mechanism of water adsorption in NU-1500-Cr, a high-performance atmospheric water harvesting MOF, is investigated using a combination of molecular dynamics simulations and infrared spectroscopy. Calculations of thermodynamic and dynamical properties of water as a function of relative humidity allow for following the adsorption process from the initial hydration stage to complete filling of the MOF pores. Initial hydration begins at the water molecules that saturate the open Cr³⁺ sites of the framework, which is then followed by the formation of water chains that extend along the channels connecting the hexagonal pores of the framework. Water present in these channels gradually coalesces and fills the hexagonal pores sequentially after the channels are completely hydrated. The development of hydrogen-bond networks inside the MOF pores as a function of relative humidity is characterized at the molecular level using experimental and computational infrared spectroscopy. A detailed analysis of the OH-stretch vibrational band indicates that the low-frequency tail stems from strongly polarized hydrogen-bonded water molecules, suggesting the presence of some structural disorder in the experimental samples. Strategies for designing efficient water harvesting MOFs are also proposed based on the mechanism of water adsorption in NU-1500-Cr.

Prospects for Simultaneously Capturing Carbon Dioxide and Harvesting Water from Air

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 CO₂  by purely engineered means. Because it does not require arable land or copious amounts of freshwater, DACS is already attractive in the context of sustainable development, but opportunities to improve its sustainability still exist. Leveraging differences in the chemistry of CO₂  and water adsorption within porous solids, here, the prospect of simultaneously removing water alongside CO₂  in direct air capture operations is investigated. In many cases, the co-adsorbed water can be desorbed separately from chemisorbed CO₂  molecules, enabling efficient harvesting of water from air. Depending upon the material employed and process conditions, the desorbed water can be of sufficiently high purity for industrial, agricultural, or potable use and can thus improve regional water security. Additionally, the recovered water can offset a portion of the costs associated with DACS. In this Perspective, molecular- and process-level insights are combined to identify routes toward realizing this nascent yet enticing concept.