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

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