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.

When Polymorphism in Metal-Organic Frameworks Enables Water Sorption Profile Tunability for Enhancing Heat Allocation and Water Harvesting Performance

The development of thermally driven water-sorption-based technologies relies on high-performing water vapor adsorbents. Here, polymorphism in Al-metal-organic frameworks is disclosed as a new strategy to tune the hydrophilicity of MOFs. This involves the formation of MOFs built from chains of either trans- or cis- µ-OH-connected corner-sharing AlO4(OH)2 octahedra. Specifically, [Al(OH)(muc)] or MIP-211, is made of trans, trans-muconate linkers, and cis-µ-OH-connected corner-sharing AlO4(OH)2 octahedra giving a 3D network with sinusoidal channels. The polymorph MIL-53-muc has a tiny change in the chain structure that results in a shift of the step position of the water isotherm from P/P0 ≈ 0.5 in MIL-53-muc, to P/P0 ≈ 0.3 in MIP-211. Solid-state NMR and Grand Canonical Monte Carlo reveal that the adsorption occurs initially between two hydroxyl groups of the chains, favored by the cis-positioning in MIP-211, resulting in a more hydrophilic behavior. Finally, theoretical evaluations show that MIP-211 would allow achieving a coefficient of performance for cooling (COPc) of 0.63 with an ultralow driving temperature of 60 °C, outperforming benchmark sorbents for small temperature lifts. Combined with its high stability, easy regeneration, huge water uptake capacity, green synthesis, MIP-211 is among the best adsorbents for adsorption-driven air conditioning and water harvesting from the air.

Confined Water Cluster Formation in Water Harvesting by Metal-Organic Frameworks: CAU-10-H versus CAU-10-CH3

Several metal-organic frameworks (MOFs) excel in harvesting water from the air or as heat pumps as they show a steep increase in water uptake at 10-30 % relative humidity (RH%). A precise understanding of which structural characteristics govern such behavior is lacking. Herein, CAU-10-H and CAU-10-CH3 are studied with H, CH3 corresponding to the functions grafted to the organic linker. CAU-10-H shows a steep water uptake ≈18 RH% of interest for water harvesting, yet the subtle replacement of H by CH3 in the organic linker drastically changes the water adsorption behavior to less steep water uptake at much higher humidity values. The materials' structural deformation and water ordering during adsorption with in situ sum-frequency generation, in situ X-ray diffraction, and molecular simulations are unraveled. In CAU-10-H, an energetically favorable water cluster is formed in the hydrophobic pore, tethered via H-bonds to the framework μOH groups, while for CAU-10-CH3, such a favorable cluster cannot form. By relating the findings to the features of water adsorption isotherms of a series of MOFs, it is concluded that favorable water adsorption occurs when sites of intermediate hydrophilicity are present in a hydrophobic structure, and the formation of energetically favorable water clusters is possible.

Sorption-Based Atmospheric Water Harvesting: Materials, Components, Systems, and Applications

Freshwater scarcity is a global challenge posing threats to the lives and daily activities of humankind such that two-thirds of the global population currently experience water shortages. Atmospheric water, irrespective of geographical location, is considered as an alternative water source. Sorption-based atmospheric water harvesting (SAWH) has recently emerged as an efficient strategy for decentralized water production. SAWH thus opens up a self-sustaining source of freshwater that can potentially support the global population for various applications. In this review, the state-of-the-art of SAWH, considering its operation principle, thermodynamic analysis, energy assessment, materials, components, different designs, productivity improvement, scale-up, and application for drinking water, is first extensively explored. Thereafter, the practical integration and potential application of SAWH, beyond drinking water, for wide range of utilities in agriculture, fuel/electricity production, thermal management in building services, electronic devices, and textile are comprehensively discussed. The various strategies to reduce human reliance on natural water resources by integrating SAWH into existing technologies, particularly in underdeveloped countries, in order to satisfy the interconnected needs for food, energy, and water are also examined. This study further highlights the urgent need and future research directions to intensify the design and development of hybrid-SAWH systems for sustainability and diverse applications.

Monolithic Zirconium-Based Metal-Organic Frameworks for Energy-Efficient Water Adsorption Applications

Space cooling and heating, ventilation, and air conditioning (HVAC) accounts for roughly 10% of global electricity use and are responsible for ca. 1.13 gigatonnes of CO₂ emissions annually. Adsorbent-based HVAC technologies have long been touted as an energy-efficient alternative to traditional refrigeration systems. However, thus far, no suitable adsorbents have been developed which overcome the drawbacks associated with traditional sorbent materials such as silica gels and zeolites. Metal-organic frameworks (MOFs) offer order-of-magnitude improvements in water adsorption and regeneration energy requirements. However, the deployment of MOFs in HVAC applications has been hampered by issues related to MOF powder processing. Herein, three high-density, shaped, monolithic MOFs (UiO-66, UiO-66-NH₂ , and Zr-fumarate) with exceptional volumetric gas/vapor uptake are developed-solving previous issues in MOF-HVAC deployment. The monolithic structures across the mesoporous range are visualized using small-angle X-ray scattering and lattice-gas models, giving accurate predictions of adsorption characteristics of the monolithic materials. It is also demonstrated that a fragile MOF such as Zr-fumarate can be synthesized in monolithic form with a bulk density of 0.76 gcm^-3 without losing any adsorption performance, having a coefficient of performance (COP) of 0.71 with a low regeneration temperature (≤ 100 °C).

Coordination Properties of Hydroxyisophthalic Acids: Topological Correlations, Synthesis, Structural Analysis, and Properties of New Complexes

Hydroxyisophthalic acids are valuable polytopic ligands for the design of functional materials based on coordination polymers due to the variety of charges and coordination modes they possess. Herein, we describe the synthesis, thermal stability, nonlinear optical (NLO) and spectroscopic properties of five novel coordination compounds, [K₂ L(H₂ O)₂ ], [MgL(H₂ O)₂ ] ⋅ 3H₂ O, [CaL(H₂ O)₃ ], [SrL(H₂ O)₃ ] ⋅ H₂ O, [BaL(H₂ O)(H₂ O)₅ ], and one salt, (NH₄ )₂ L ⋅ 2H₂ O, with 4,5,6-trihydroxyisophthalic acid (H₂ L), which has not been tested in assembling crystalline coordination networks before. The peculiarities of the structural organization of the compounds were analyzed and compared with those for other hydroxyisophthalates. The coordination properties of hydroxyisophthalic acids were studied from the topological point of view, and a comparative topological analysis of coordination and H-bonded networks was performed. Structural correlations revealed in this study could be useful for the design of hydroxyisophthalate-based coordination networks, including porous metal-organic frameworks, proton conductors, and NLO materials.

Water Vapor Adsorption on CAU-10-X: Effect of Functional Groups on Adsorption Equilibrium and Mechanisms

Metal-organic frameworks (MOFs) possess unique flexibility of structure and properties, which drives them toward applications as water adsorbents in many emerging technologies, such as adsorptive heat transformation, water harvesting from the air, dehumidification, and desalination. A deep understanding of the surface phenomena is a prerequisite for the target-oriented design of MOFs with the required adsorption properties. In this work, we comprehensively study the effect of functional groups on water adsorption on a series CAU-10-X substituted with both hydrophilic (X = NH2) and hydrophobic (X = NO2) groups in the linker. The adsorption equilibrium is measured at P = 7.6-42 mbar and T = 5-100 °C. The study of water adsorption by a set of mutually complementary physicochemical methods (TG, XRD in situ, FTIR, and 1H NMR relaxometry) elucidates the nature of primary adsorption sites and water adsorption mechanisms.

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.

Robust synthesis routes and porosity of the Al-based metal–organic frameworks Al-fumarate, CAU-10-H and MIL-160

We confirm that the investigated Al-MOFs are robust with respect to reproducible synthesis and concomitant porosity as a prerequisite for applications.

Tunable Metal⁻Organic Frameworks for Heat Transformation Applications

Metal–Organic Frameworks (MOFs) are a subclass of porous materials that have unique properties, such as varieties of structures from different metals and organic linkers and tunable porosity from a structure or framework design. Moreover, modification/functionalization of the material structure could optimize the material properties and demonstrate high potential for a selected application. MOF materials exhibit exceptional properties that make these materials widely applicable in energy storage and heat transformation applications. This review aims to give a broad overview of MOFs and their development as adsorbent materials with potential for heat transformation applications. We have briefly overviewed current explorations, developments, and the potential of metal–organic frameworks (MOFs), especially the tuning of the porosity and the hydrophobic/hydrophilic design required for this specific application. These materials applied as adsorbents are promising in thermal-driven adsorption for heat transformation using water as a working fluid and related applications.

Scalable Green Synthesis and Full-Scale Test of the Metal-Organic Framework CAU-10-H for Use in Adsorption-Driven Chillers

The demand for cooling devices has increased during the last years and this trend will continue. Adsorption-driven chillers (ADCs) using water as the working fluid and low temperature waste energy for regeneration are an environmentally friendly alternative to currently employed cooling devices and can concurrently help to dramatically decrease energy consumption. Due to the ideal water sorption behavior and proven lifetime stability of [Al(OH)(m-BDC)] ∙ x H₂ O (m-BDC^2- = 1,3-benzenedicarboxylate), also denoted CAU-10-H, a green very robust synthesis process under reflux, with high yields up to 95% is developed and scaled up to 12 kg-scale. Shaping of the adsorbent is demonstrated, which is important for an application. Thus monoliths and coatings of CAU-10-H are produced using a water-based binder. The composites are thoroughly characterized toward their mechanical stability and water sorption behavior. Finally a full-scale heat exchanger is coated and tested under ADC working conditions. Fast adsorption dynamic leads to a high power output and a good power density. A low regeneration temperature of only 70 °C is demonstrated, allowing the use of low temperature sources like waste heat and solar thermal collectors.

Metal-Organic Frameworks in Adsorption-Driven Heat Pumps: The Potential of Alcohols as Working Fluids

A large fraction of global energy is consumed for heating and cooling. Adsorption-driven heat pumps and chillers could be employed to reduce this consumption. MOFs are often considered to be ideal adsorbents for heat pumps and chillers. While most published works to date on this topic have focused on the use of water as a working fluid, the instability of many MOFs to water and the fact that water cannot be used at subzero temperatures pose certain drawbacks. The potential of using alcohol-MOF pairs in adsorption-driven heat pumps and chillers is investigated. To this end, 18 different selected MOF structures in combination with either methanol or ethanol as a working fluid are considered, and their potential is assessed on the basis of adsorption measurements and thermodynamic efficiencies. If alcohols are used instead of water, then (1) adsorption occurs at lower relative pressures for methanol and even lower pressure for ethanol, (2) larger pores can be utilized efficiently, as hysteresis is absent for pores smaller than 3.4 nm (2 nm for water), (3) larger pore sizes need to be employed to ensure the desired stepwise adsorption, (4) the effect of (polar/apolar) functional groups in the MOF is far less pronounced, (5) the energy released or taken up per cycle is lower, but heat and mass transfer may be enhanced, (6) stability of MOFs seems to be less of an issue, and (7) cryogenic applications (e.g., ice making) become feasible. From a thermodynamic perspective, UiO-67, CAU-3, and ZIF-8 seem to be the most promising MOFs for both methanol and ethanol as working fluids. Although UiO-67 might not be completely stable, both CAU-3 and ZIF-8 have the potential to be applied, especially in subzero-temperature adsorption chillers (AC).