Mesoporous Metal-Organic Frameworks with Exceptionally High Working Capacities for Adsorption Heat Transformation

Engineered Nanoporous Frameworks for Adsorption Cooling Applications

The energy demand for traditional vapor-compressed technology for space cooling continues to soar year after year due to global warming and the increasing human population's need to improve living and working conditions. Thus, there is a growing demand for eco-friendly technologies that use sustainable or waste energy resources. This review discusses the properties of various refrigerants used for adsorption cooling applications followed by a brief discussion on the thermodynamic cycle. Next, sorbents traditionally used for cooling are reviewed to emphasize the need for advanced capture materials with superior properties to improve refrigerant sorption. The remainder of the review focus on studies using engineered nanoporous frameworks (ENFs) with various refrigerants for adsorption cooling applications. The effects of the various factors that play a role in ENF-refrigerant pair selection, including pore structure/dimension/shape, morphology, open-metal sites, pore chemistry and possible presence of defects, are reviewed. Next, in-depth insights into the sorbent-refrigerant interaction, and pore filling mechanism gained through a combination of characterization techniques and computational modeling are discussed. Finally, we outline the challenges and opportunities related to using ENFs for adsorption cooling applications and provide our views on the future of this technology.

Fabrication of Two-Dimensional Homo-Bimetallic Porphyrin Framework Thin Films for Optimizing Nonlinear Optical Limiting

Developing efficient metal-organic framework (MOF) optical devices with tunable third-order nonlinear optical (NLO) properties is an important challenge for scientific research and practical application. Herein, 2D monometallic and hetero/homo-bimetallic porphyrin MOF thin films (ZnTCPP(M) M = H2, Fe, Zn) were fabricated using the liquid-phase epitaxial (LPE) layer-by-layer (LBL) method to investigate the metal substitution dependent third-order NLO behavior. The prepared homo-bimetallic ZnTCPP(Zn) thin film exhibited enhanced third-order NLO performance with a higher third-order nonlinear susceptibility of ∼4.21 × 10-7 esu compared to monometallic and hetero-bimetallic counterparts. Additionally, theoretical calculations were performed to complement the experimental findings and revealed that the enhanced NLO effect of the ZnTCPP(Zn) thin film is mainly attributed to the enhanced local excitation. These findings not only provide a comprehensive understanding of the relationship between metal types and the NLO behavior of porphyrin MOF thin films but also offer valuable insights into the design and optimization of NLO devices.

Improvement of water adsorption performance of UiO-66 by post-synthetic modification

Post-synthetic modification can be used for structural replacement or functional modification of materials after they have been formed or assembled. It can effectively combine various modification methods for metal-organic frameworks (MOFs) such as defect control, replacement of metal sites, or functionalization of ligands. In this work, organic ligands that incorporate N-functionalities or amino groups were introduced into defective UiO-66 through post-synthetic ligand exchange (PSE) to improve its water adsorption performance. Parameters such as water adsorption capacity, half adsorption value (α), and Henry constant KH were used to characterize the water adsorption performance. After PSE, new ligands in different molar ratios entered the skeleton of UiO-66. The N sites or amino groups on the ligands provided new sites for the adsorption of water molecules. The water adsorption capacity and hydrophilicity of all samples were significantly superior to those of LD-UiO-66, which had almost no defects. H-UiO-66-PyDC samples exhibited the highest ligand replacement ratio and a significant enhancement of water adsorption performance. Compared to the unchanged H-UiO-66, the water uptake of H-UiO-66-PyDC increased from 0.08 g g-1 to 0.23 g g-1 at P/P0 = 0.30 and α decreased from 0.36 to 0.28. After 20 water adsorption/desorption tests, the water uptake of all samples did not decrease, showing excellent cycling stability. These results suggest that the combination of defect modulation and PSE is a potential tool to make UiO-66 more appropriate for applications based on reversible adsorption.

Metal-Organic Frameworks for Greenhouse Gas Applications

Using petrol to supply energy for a car or burning coal to heat a building generates plenty of greenhouse gas (GHG) emissions, including carbon dioxide (CO₂ ), water vapor (H₂ O), methane (CH₄ ), nitrous oxide (N₂ O), ozone (O₃ ), fluorinated gases. These up-and-coming metal-organic frameworks (MOFs) are structurally endowed with rigid inorganic nodes and versatile organic linkers, which have been extensively used in the GHG-related applications to improve the lives and protect the environment. Porous MOF materials and their derivatives have been demonstrated to be competitive and promising candidates for GHG separation, storage and conversions as they shows facile preparation, large porosity, adjustable nanostructure, abundant topology, and tunable physicochemical property. Enormous progress has been made in GHG storage and separation intrinsically stemmed from the different interaction between guest molecule and host framework from MOF itself in the recent five years. Meanwhile, the use of porous MOF materials to transform GHG and the influence of external conditions on the adsorption performance of MOFs for GHG are also enclosed. In this review, it is also highlighted that the existing challenges and future directions are discussed and envisioned in the rational design, facile synthesis and comprehensive utilization of MOFs and their derivatives for practical applications.

Oriented Assembly of 2D Metal-Pyridylporphyrinic Framework Films for Giant Nonlinear Optical Limiting

The development of metal-organic frameworks (MOFs) with nonlinear optical (NLO) properties is of pronounced significance for optical devices. Herein, a series of 2D MOFs ZnTPyP(M) (TPyP = 5,10,15,20-tetrakis(4-pyridyl)porphyrin, M = Cu, Ni, Mn, H₂) films with [010]-orientation growth composed of ultrathin nanosheets from a pyridylporphyrinic ligand are first obtained by using a liquid-phase epitaxial (LPE) layer-by-layer (lbl) growth approach. ZnTPyP(M) films show a giant nonlinear optical limiting (OL) response and can be modulated by tuning the type of metalloporphyrinic ligands. As a result, ZnTPyP(Cu) film exhibits the highest nonlinear absorption coefficient of 5.7 × 10^-6 m/W compared to other reported NLO materials. Density functional theory calculations were consistent with the experimental results, revealing that the tunable π-π* local excitation and the increased delocalization of the metalloporphyrinic group regulate the NLO performance of ZnTPyP(M) films. These findings provide new insight into the effect of 2D porphyrinic MOFs toward the NLO response and offer new film candidates for nonlinear OL application.

Interpenetrated Metal-Porphyrinic Framework for Enhanced Nonlinear Optical Limiting

Structural interpenetration in metal-organic frameworks (MOFs) significantly impacts on their properties and functionalities. However, understanding the interpenetration on third-order nonlinear optics (NLO) of MOFs have not been reported to date. Herein, we report two 3D porphyrinic MOFs, a 2-fold interpenetrated [Zn₂(TPyP)(AC)₂] (ZnTPyP-1) and a noninterpenetrated [Zn₃(TPyP)(H₂O)₂(C₂O₄)₂] (ZnTPyP-2), constructed from 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP(H₂)) and Zn(NO₃)₂ (AC = acetate, C₂O₄ = oxalate). ZnTPyP-1 achieves excellent optical limiting (OL) performance with a giant nonlinear absorption coefficient (3.61 × 10⁶ cm/GW) and large third-order susceptibility (7.73 × 10^-7 esu), which is much better than ZnTPyP-2 and other reported OL materials. The corresponding MOFs nanosheets are dispersed into a polydimethylsiloxane (PDMS) matrix to form highly transparent and flexible MOFs/PDMS glasses for practical OL application. In addition, the OL response optimized by adjusting the MOFs concentration in the PDMS matrix and the type of metalloporphyrin are discussed in the ZnTPyP-1 system. The theoretical calculation confirmed that the abundant π-π interaction from porphyrinic groups in the interpenetrated framework increased the electron delocalization/transfer and boosted the OL performance. This study opens a new avenue to enhance OL performance by the construction of interpenetrated structures and provides a new approach for the preparation of transparent and flexible MOF composites in nonlinear optical applications.

Retention of a Four-Fold Interpenetrating Cadmium-Organic Framework through a Three-Step Single Crystal Transformation

Controlled hydration leads to four derivatives of a metal-organic framework consisting of cadmium ions, N¹,N¹,N⁴,N⁴-tetrakis(4-(pyridin-4-yl)phenyl)benzene-1,4-diamine, and coordinated and free nitrates. The balance of water coordination and the multitude of bonding of the weakly coordinated nitrate lead to a progressive change in the coordination number of the Cd²⁺ ions from eight to seven to six without great perturbation to the 4-fold interpenetration three-dimensional framework.

Porous Covalent Organic Polymers for Efficient Fluorocarbon-Based Adsorption Cooling

Adsorption-based cooling is an energy-efficient renewable-energy technology that can be driven using low-grade industrial waste heat and/or solar heat. Here, we report the first exploration of fluorocarbon adsorption using porous covalent organic polymers (COPs) for this cooling application. High fluorocarbon R134a equilibrium capacities and unique overall linear-shaped isotherms are revealed for the materials, namely COP-2 and COP-3. The key role of mesoporous defects on this unusual adsorption behavior was demonstrated by molecular simulations based on atomistic defect-containing models built for both porous COPs. Analysis of simulated R134a adsorption isotherms for various defect-containing atomistic models of the COPs shows a direct correlation between higher fluorocarbon adsorption capacities and increasing pore volumes induced by defects. Combined with their high porosities, excellent reversibility, fast kinetics, and large operating window, these defect-containing porous COPs are promising for adsorption-based cooling applications.

Encapsulation of Sulfur into N-Doped Porous Carbon Cages by a Facile, Template-Free Method for Stable Lithium-Sulfur Cathode

Lithium-sulfur (Li-S) batteries with a high energy density and long lifespan are considered as promising candidates for next-generation electrochemical energy-storage devices. However, the sluggish redox kinetics of electrochemistry and high solubility of polysulfide during cycling render insufficient sulfur utilization and poor cycling stability. Herein, a facile, template-free procedure based on controlled pyrolysis of polydopamine vesicles is described to prepare N-doped porous carbon cages (NHSC) as a new sulfur host, which significantly improves both the sulfur utilization and cycling stability. As NHSC shows a high pore volume, continuous electron and ion transport paths, and good catalytic activity, encapsulation of S nanoparticles into NHSC endows the resulting S@NHSC electrode with a good energy storage capacity and exceptionally high electrochemical stability. Consequently, a Li-S cell with the S@NHSC as the cathode achieves a high initial capacity of 1280.7 mAh g^-1 , and cycling stability over 500 cycles with the capacity decay as low as 0.0373% per cycle.

Water and Metal-Organic Frameworks: From Interaction toward Utilization

The steep stepwise uptake of water vapor and easy release at low relative pressures and moderate temperatures together with high working capacities make metal-organic frameworks (MOFs) attractive, promising materials for energy efficient applications in adsorption devices for humidity control (evaporation and condensation processes) and heat reallocation (heating and cooling) by utilizing water as benign sorptive and low-grade renewable or waste heat. Emerging MOF-based process applications covered are desiccation, heat pumps/chillers, water harvesting, air conditioning, and desalination. Governing parameters of the intrinsic sorption properties and stability under humid conditions and cyclic operation are identified. Transport of mass and heat in MOF structures, at least as important, is still an underexposed topic. Essential engineering elements of operation and implementation are presented. An update on stability of MOFs in water vapor and liquid systems is provided, and a suite of 18 MOFs are identified for selective use in heat pumps and chillers, while several can be used for air conditioning, water harvesting, and desalination. Most applications with MOFs are still in an exploratory state. An outlook is given for further R&D to realize these applications, providing essential kinetic parameters, performing smart engineering in the design of systems, and conceptual process designs to benchmark them against existing technologies. A concerted effort bridging chemistry, materials science, and engineering is required.

The Progress and Prospect of Zeolitic Imidazolate Frameworks in Cancer Therapy, Antibacterial Activity, and Biomineralization

The progressive development of zeolitic imidazolate frameworks (ZIFs), as a subfamily of metal-organic frameworks (MOFs), and their unique features, including tunable pore size, large surface area, high thermal stability, and biodegradability/biocompatibility, have made them attractive in the field of biomedicine, especially for drug delivery and biomineralization applications. The high porosity of ZIFs gives them the opportunity for encapsulating a high amount of therapeutic drugs, proteins, imaging cargos, or a combination of them to construct advanced multifunctional drug delivery systems (DDSs) with combined therapeutic and imaging capabilities. This review summarizes recent strategies on the design and fabrication of ZIF-based nansystems and their exploration in the biomedical field. First, recent developments for the adjustment of particle size, functionality, and morphology of ZIFs are discussed, which are important for achieving optimized therapeutic/theranostic nanosystems. Second, recent trends on the application of ZIF nanocarriers for the loading of diverse cargos, including anticancer medicines, antibiotic drugs, enzymes, proteins, photosensitizers, as well as imaging and photothermal agents, are investigated in order to understand how multifunctional DDSs can be designed based on the ZIF nanoparticles to treat different diseases, such as cancer and infection. Finally, prospects on the future research direction and applications of ZIF-based nanomedicines are discussed.

Molecular Insight into Fluorocarbon Adsorption in Pore Expanded Metal-Organic Framework Analogs

The rapid growth in the global energy demand for space cooling requires the development of more efficient environmental chillers for which adsorption-based cooling systems can be utilized. Here, in this contribution, we explore sorbents for chiller use via a pore-engineering concept to construct analogs of the 1-dimensional pore metal-organic framework MOF-74 by using elongated organic linkers and stereochemistry control. The prepared pore-engineered MOFs show remarkable equilibrium adsorption of the selected fluorocarbon refrigerant that is translated to a modeled adsorption-based refrigeration cycle. To probe molecular level interactions at the origin of these unique adsorption properties for this series of Ni-MOFs, we combined in situ synchrotron X-ray powder diffraction, neutron powder diffraction, X-ray absorption spectroscopy, calorimetry, Fourier transform infrared techniques, and molecular simulations. Our results reveal the coordination of fluorine (of CH2F in R134a) to the nickel(II) open metal centers at low pressures for each Ni-MOF analog and provide insight into the pore filling mechanism for the full range of the adsorption isotherms. The newly designed Ni-TPM demonstrates exceptional R134a adsorption uptake compared to its parent microporous Ni-MOF-74 due to larger engineered pore size/volume. The application of this adsorption performance toward established chiller conditions yields a working capacity increase for Ni-TPM of about 400% from that of Ni-MOF-74, which combined with kinetics directly correlates to both a higher coefficient of performance and a higher average cooling capacity generated in a modeled chiller.

A Hydrolytically Stable Vanadium(IV) Metal-Organic Framework with Photocatalytic Bacteriostatic Activity for Autonomous Indoor Humidity Control

Metal-organic frameworks (MOFs) with long-term stability and reversible high water uptake properties can be ideal candidates for water harvesting and indoor humidity control. Now, a mesoporous and highly stable MOF, BIT-66 is presented that has indoor humidity control capability and a photocatalytic bacteriostatic effect. BIT-66 (V₃ (O)₃ (H₂ O)(BTB)₂ ), possesses prominent moisture tunability in the range of 45-60 % RH and a water uptake and working capacity of 71 and 55 wt %, respectively, showing good recyclability and excellent performance in water adsorption-desorption cycles. Importantly, this MOF demonstrates a unique photocatalytic bacteriostatic behavior under visible light, which can effectively ameliorate the bacteria and/or mold breeding problem in water adsorbing materials.

Facile Synthesis of Magnetic Nitrogen-Doped Porous Carbon from Bimetallic Metal⁻Organic Frameworks for Efficient Norfloxacin Removal

Magnetic nitrogen-doped porous carbon (MNPC) has been prepared via self-catalytic pyrolysis of bimetallic metal-organic frameworks (MOFs). The as-obtained MNPC showed favorable features for antibiotics adsorption such as high specific surface area (871 m² g-1), high pore volume (0.75 cm³ g-1), porous structure, good graphitization degree, and rich N-doping. Moreover, the MNPC has magnetic properties due to the Co species, which is embedded with a high dispersion, so the absorbent can be easily separated. Based on the above excellent characteristics, the MNPC was used as the absorbent for norfloxacin (NOR) removal. The experimental maximum NOR adsorption capacity of MNPC was 55.12 mg g-1 at 298.15 K and a pH of 6.0 with an initial NOR concentration of 50 mg L-1. The data analysis of the kinetics revealed that the experimental data of NOR uptakes versus time agreed with the pseudo-second order model. The isotherm data analysis revealed the favorable application of the Freundlich model. Based on the adsorption results over a wide range of conditions, the dominant adsorption mechanisms were found to be pore-filling, electrostatic interaction, and the H-bond.