Tunable Metal-Organic Frameworks Enable High Efficiency Cascaded Adsorption Heat Pumps

Rising global standards of living coupled with the recent agreement to eliminate<br>hydrofluorocarbon refrigerants are creating intense pressure to develop more sustainable climate<br>control systems. In this vein, the use of water as the refrigerant in adsorption heat pumps is highly<br>attractive, but such adsorption systems are constrained to large size and poor efficiency by the<br>characteristics of currently employed water sorbents. Here we demonstrate control of the relative<br>humidity of water uptake by modulating the pore size in a family of isoreticular triazolate metalorganic<br>frameworks. Using this method, we identify a pair of materials with stepped, nonoverlapping<br>water isotherms that can function in tandem to provide continuous cooling with a<br>record ideal coefficient of performance of 1.63. Additionally, when used in a single-stage heat<br>pump, the microporous Ni2Cl2BBTA has the largest working capacity of any material capable of<br>generating a 25 °C difference between ambient and chiller output.