J. Am. Chem. Soc. https://doi.org/10.1021/jacs.8b09655 (2018)

Climate control systems consume lots of energy — more than 44% of total energy used in US homes, for instance — so the development of sustainable adsorption heat pumps (AHPs) is urgent. Water is considered to be an ideal refrigerant to replace hydrofluorocarbons, but its development is hindered by the low adsorption efficiency and large size of current AHPs. Recent interest has focused on using metal–organic frameworks (MOFs) for AHPs because they offer accurate control over water uptake over a range of relative humidity. Now Dincă et al. from MIT report cascaded AHPs based on tunable MOFs with improved efficiency at lower source temperature. Moreover, water is used as the only refrigerant.

They choose Ni2Cl2BBTA (BBTA is 1H,5H-benzo(1,2-d),(4,5-d′)-bistriazole) to construct the AHPs and find that Ni2Cl2BBTA has a high working capacity, able to produce a lift of 25 °C in a single-stage AHP. Usually, multiple-stage cascaded cycles work better because the input thermal energy for regeneration can be used twice. They use this in tandem with another thermodynamically tunable MOF adsorbent, Co2Cl2BTDD. The small-pore Ni2Cl2BBTA and large-pore Co2Cl2BTDD have non-overlapping water isotherms, enabling continuous cooling with much higher efficiency. A coefficient of performance of 1.63 is achieved with a low driving temperature of 127 °C.