Ammonia Adsorption Performance of Zeolitic Imidazolate Frameworks for Cooling

Promoting the cooling performance of adsorption chillers (ACs) greatly relies on the exploration of high-performance adsorbent/refrigerant working pairs. Ammonia is not only an environmentally friendly refrigerant but also favorable for heat and mass transfer in ACs owing to its large vapor pressure and enthalpy of evaporation. Zeolite imidazolate frameworks (ZIFs) with excellent ammonia stability are identified as a class of potential adsorbents for practical ammonia-based ACs. However, high-performing ZIF/ammonia working pairs with excellent AC performance are still to be developed. In this work, the cooling performance including the coefficient of performance for cooling (COP_C) and the specific cooling effects (SCEs) of 26 ZIFs with the same composites but different topologies was evaluated by combining molecular simulation and mathematical modeling. Five high-performing ZIFs with COP_C > 0.45 and SCE > 250 kJ/kg were identified, among which -ZIF with the highest COP_C of 0.51 and -ZIF with the highest SCE of 354 kJ/kg both are promising to be synthesized and applied further. Besides, the quantitative structure-performance relationship (QSPR) was extracted that can help quickly identify and design high-performing ZIFs according to their ammonia adsorption isotherms and structural characteristics. Moreover, "S"-shaped adsorption isotherms with high saturation adsorption capacity (>0.2 g/g), suitable step position (0.2-0.4), and relatively low Henry's constant (<1 × 10^-5 mol/(kg·Pa)) are more favorable for excellent COP_C and SCE. From the perspective of structure characteristics, ZIFs possessing low crystal density (<0.9 g/cm³), high accessible surface area (>2000 m²/g), balanced largest cavity diameter (∼15 Å), and accessible pore volume (∼0.65 cm³/g) are beneficial for high-efficient cooling performance.