Targeted tuning of water adsorption at P/P 0 = 0.2 in multivariate metal-organic frameworks for boosting water-sorption-driven refrigeration †

Abstract

Water-based adsorption refrigeration has emerged as a promising solution to reduce global energy consumption and environmental pollution caused by heating and cooling demand. While numerous metal-organic frameworks (MOFs) have been developed as water adsorbents, their cooling performance is often limited by insufficient water uptake at P/P 0 = 0.20.Herein, we report a strategy of optimizing the content and distribution of hydrophilic nitrogen sites in multivariate MOFs (MTV-MOFs) for targeted increase of water adsorption at P/P 0 = 0.20, thereby enhancing the cooling efficiency in refrigeration. Through the incorporation of pyrazine-2,6-dicarboxylic acid with two nitrogen sites into CAU-10H, a series of MTV-MOFs were designed and synthesized (CAU-10H/PDC-x%, x = 5/10/15). Amongst these MTV-MOFs, the optimal CAU-10H/PDC-10% exhibits an S-shaped water adsorption isotherm with significantly improved water uptake of 0.34 g g -1 at P/P 0 = 0.20, while has a negligible effect on initial water uptake below P/P 0 = 0.10. This water uptake at P/P 0 = 0.20 is notably higher than the pristine CAU-10H (0.28 g g -1 ), and even comparable to some benchmark materials such as MOF-303 (0.37 g g -1 ) and MIP-200 (0.36 g g -1 ). Theoretical calculations reveal that the optimized content and distribution of nitrogen sites render them unable to participate in the initial adsorption sites, and solely serve as secondary adsorption sites to bind with water molecules and thus promote the formation of larger water clusters at P/P 0 = 0.20. This precise improvement in water capacity at P/P 0 = 0.20 can optimize and maximize the working capacity (0.29 g g -1 ) and COP value (0.79) of CAU-10H/PDC-10% under a low driving temperature of 65 °C, which outperform those of some benchmark MOFs such as MIP-200 (0.11 g g -1 , 0.74) and KMF-2 (0.16 g g -1 , 0.62) for water adsorption-based refrigeration.