Investigating the Effect of Defect Engineering via Soft Templating on the Water Adsorption Behavior of Metal–Organic Frameworks

With a rising global population and worsening climate change, adsorption-based atmospheric water harvesting provides one potential method to mitigate the water crisis by generating water from atmospheric air. However, before porous sorbents can be integrated into water generating systems, the structure-property relationship must be investigated to better understand the water adsorption behavior in these sorbents. Defects in MOF materials have been known to play an important role in tuning the adsorption behavior, stability, and separation efficiency through the tuning of the crystal structure. In this work, we investigated the role of defects, generated through soft templating, on the water adsorption behavior of both MOF-808 and MOF-801. MOF-808 and MOF-801 are both water stable, robust, zirconium MOFs, but their smaller pore volumes and pore sizes limit their applications for AWH. Through defect engineering, defective structures can be generated that can tune the adsorption behavior of MOF materials. Through soft templating, structure directed agents were added into the MOF synthesis, with the aim of growing crystals around the templates with larger pore sizes and pore volumes, and potentially developing more hydrophilic pores. Through soft templating, defective crystals were generated with smaller crystal sizes and larger macropores and interparticle voids. There was no distinct correlation between the introduction of defects and improved hydrophilicity or saturation capacity, most likely due to difficulties of MOF crystals successfully growing around the large template molecules. However, the decrease in crystal size and the increase in interparticle void spacing improved the mass transfer of water vapor into the sorbent materials, enhancing the dynamic water adsorption behavior. These results suggest that defect engineering of MOF-808 and MOF-801 via soft templating primarily alter the crystal size and morphology, and thus the mass transfer effect, of MOFs as opposed to the hydrophilicity and water adsorption capacity.