Boosting the water absorption of CAU-10: The role of hydrochloric acid in regulating defect concentration

Abstract

CAU-10-H is a promising water adsorbent for adsorption heat transformation systems due to its excellent hydrothermal stability and favorable "S" type isothermal adsorption characteristic. However, its water uptake capacity is relatively low. In this work, the hydrochloric acid modulation defect structure of CAU-10 is rationally developed to get the CAU-10-HCl-2 sample, which possesses high water absorption capacity and outstanding cycle stability. The concentrations of metal atom defects and ligand defects in CAU-10-HCl-2 are 5.8 % and 3.0 %, respectively, which are 29 times and 0.375 times those of the CAU-10 sample without hydrochloric acid addition. This indicates that hydrochloric acid modulation facilitates the production of metal atom defects while suppressing the emergence of ligand defects. The specific surface area, micropore volume, and total pore volume of CAU-10-HCl-2 are 526.6 m²/g, 0.20 ml/g, and 0.23 ml/g, respectively, up 17 %, 25 %, and 10 % from CAU-10. At a relative pressure P/P₀ of 0.2, the water uptake of CAU-10-HCl-2 reaches up to 0.29 g/g, which is a 14 % increase compared to CAU-10. Compared to the initial hydration, the water uptake of CAU-10 decreases by 16 % by the 30th cycle, while that of CAU-10-HCl-2 lowers by only 2 %. These results demonstrate that hydrochloric acid modulation is an efficient strategy to improve the water absorption capacity and cyclic stability of CAU-10. This study provides quantitative analysis and synthesis instructions for designing defect structures in CAU-10 via hydrochloric acid modulation.