(42e) Hierarchical Silica-LiCl Composites with Tailorable Composition and Adsorption Properties of Interest to Atmospheric Water Harvesting

Potable water scarcity is a major humanitarian challenge due to population growth, climate shifts and socio-economic changes. Atmospheric water harvesting, especially in remote, arid or underdeveloped areas, is a promising solution as a renewable source of water that can meet the current and future water consumption demands. Hierarchical silica sorbents with multimodal mesopores and macropores recently received significant interest as a promising technology for water harvesting. The water-stability and pore size distributions of these silica compounds result in improved water uptake across a wide relative humidity (RH) range, compared to strictly microporous sorbents and to pure hygroscopic salts. Incorporating hygroscopic salts into hierarchical mesoporous silicas is shown to enhance low RH water adsorption while maintaining its absorbent properties at high RH. In our previous work, the water sorption performance of LiCl-impregnated silica was focused on supports having primary mesopores of approximately 4nm in size, in addition to larger textural mesopores and macropores. In this work, precise and independent control of the mesopores and of the macropores of these hierarchal silicas is demonstrated. A matrix of silicas, synthesized with different templates and reaction conditions, showed independent variation of the macropore (50 to 3000nm) and primary mesopore sizes (2 to 6 nm). The synthesis, characterization, and water adsorption results of these silica variants, impregnated with hygroscopic salt LiCl using the incipient wetness impregnation method, are presented. As in previous studies, the water uptake scales with the salt loading in these composites. The effect of the silica mesopore size and pore volume on the water uptake performance of the silica-salt composites was evaluated in preventing the deliquescence of the sorbents. While small mesopores are responsible for the strong capillary forces that lead to stable sorbents at high salt loadings, a high mesopore volume is also required for achieving salt loadings of 50wt.% or greater.