Water and Metal–Organic Frameworks: From Interaction toward Utilization

Abstract

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.

Screening metal-organic frameworks for adsorption-driven osmotic heat engines via grand canonical Monte Carlo simulations and machine learning

“…Among these adsorbents, MOFs attracted considerable attentions because of their outstanding adsorption performance due to the high volumetric surface area, structural diversity, and structural tunability ( Li et al., 2016 , 2019b ; Altintas et al., 2018 ; Kirchon et al., 2018 ). Screening potential MOFs from a vast number of MOF databases for adsorption-driven heat pumps and chillers has been extensively investigated in recent decades ( Liu et al., 2020 ; Shi et al., 2020 ). A high-throughput computational screening of MOFs for alcohol-based adsorption-driven heat pumps based on grand canonical Monte Carlo has been conducted in our previous study ( Li et al., 2019a ), from which the correlation between MOF structure property and their coefficient of performance (COP), as well as the top performers with the highest COP, were identified.…”

Metal‐Organic Frameworks as Sorption Materials for Heat Transformation Processes

“…[89] Such soft acid-soft base interactions are orbital controlled and have a higher covalency than charge-controlled (more ionic) hard acid-hard base interactions between metal ions and ligand donor atoms of low polarizability. [76,89,90] Besides or instead of thermodynamics, kinetics can be used to stabilize a material. In thermodynamics the Gibbs free energy (ΔG) determines stability/instability.…”

“…This includes steric shielding of the labile metal-donor atom bond, the increase of the material′s hydrophobicity, and the use of rigid, inflexible linkers and metal-cluster secondary building units, SBUs. [89,76,90,92] Rigid linkers and SBUs significantly reduce linker exchange, while steric shielding makes the labile metal-linker bonds difficult to access for water molecules. The approach of hydrophobicity increase distinguishes between internal and external hydrophobicity.…”

“…External hydrophobicity, on the other hand, prevents water molecules from penetrating into the pores of the material. [89,76] The difference between the stability towards liquid water and towards water vapor in ad/desorption cycles can be explained by the water phase change enthalpy, the head of adsorption which is released at the adsorption site, i.e., in the MOF. Such adsorption enthalpies have been calculated to lie in the range of ligand displacement energies.…”

“…A great effort has been spent finding materials that match the different boundary conditions and has lately been exhaustingly reviewed. [76] Amongst the above mentioned metal-organic frameworks, MIL-160(Al) has been evaluated as a promising material for heat transformation and storage. [105,108] The work of Schlüsener and co-workers demonstrated the possibilities to tune the adsorption characteristics by a mixed linker approach varying the linker from furandicarboxylic acid to isophthalic acid yielding a mixture of MIL-160 and CAU-10-H structure.…”