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.

Basic adsorption heat exchanger theory for performance prediction of adsorption heat pumps.

Adsorption modules are the core components of thermally driven adsorption heat pumps and chillers. Due to the transient nature of the adsorption and desorption processes, usually complicated numerical models are used for prediction of efficiency and heat flow rates. In this research article, we suggest a radically simplified calculation based on splitting up the ad- and desorption half cycle into a transient, strongly non-isothermal switching phase and a quasi-isothermal phase. In the quasi-isothermal phase, the heat flow rates can be calculated with relationship between temperature effectiveness (ϵ) and number of transfer units (NTU). Effective thermal resistances account for the heat and mass transfer processes. The prediction quality of our simple calculation in terms of heat flow rates is within ±20% compared with experimental data of two different sorption modules. The suggested method and its experimental validation lay the foundation of a basic adsorption heat exchanger theory.