Ammonia Adsorption\nPerformance of Zeolitic Imidazolate\nFrameworks for Cooling

Abstract

Promoting the cooling performance of adsorption chillers\n(ACs)\ngreatly relies on the exploration of high-performance adsorbent/refrigerant\nworking pairs. Ammonia is not only an environmentally friendly refrigerant\nbut also favorable for heat and mass transfer in ACs owing to its\nlarge vapor pressure and enthalpy of evaporation. Zeolite imidazolate\nframeworks (ZIFs) with excellent ammonia stability are identified\nas a class of potential adsorbents for practical ammonia-based ACs.\nHowever, high-performing ZIF/ammonia working pairs with excellent\nAC performance are still to be developed. In this work, the cooling\nperformance including the coefficient of performance for cooling (COP_C) and the specific cooling effects (SCEs) of 26 ZIFs with\nthe same composites but different topologies was evaluated by combining\nmolecular simulation and mathematical modeling. Five high-performing\nZIFs with COP_C > 0.45 and SCE > 250 kJ/kg were identified,\namong which <i><b>gis</b></i>-ZIF with the highest\nCOP_C of 0.51 and <i><b>lta</b></i>-ZIF\nwith the highest SCE of 354 kJ/kg both are promising to be synthesized\nand applied further. Besides, the quantitative structure–performance\nrelationship (QSPR) was extracted that can help quickly identify and\ndesign high-performing ZIFs according to their ammonia adsorption\nisotherms and structural characteristics. Moreover, “S”-shaped\nadsorption isotherms with high saturation adsorption capacity (>0.2\ng/g), suitable step position (0.2–0.4), and relatively low\nHenry’s constant (<1 × 10^–5 mol/(kg·Pa))\nare more favorable for excellent COP_C and SCE. From the\nperspective of structure characteristics, ZIFs possessing low crystal\ndensity (<0.9 g/cm³), high accessible surface area (>2000\nm²/g), balanced largest cavity diameter (∼15 Å),\nand accessible pore volume (∼0.65 cm³/g) are beneficial\nfor high-efficient cooling performance.