Construction And Application Of Two-dimensional Covalent Organic Frameworks With Atmospheric Water Harvesting Properties

The utilization of porous materials for water harvesting from the air and its application in the fields of water production in arid regions,heat pumps,desalination,proton conduction and humidity sensing has attracted a great deal of attention from researchers.Designing and synthesizing new atmospheric water harvesting materials with well-defined structures and good stability is the key for application in specific scenarios.Covalent organic frameworks（COFs）is an emerging class of crystalline porous materials with pre-defined structure,adjustable chemical composition,permanent porosity and excellent chemical stability,which is one of the ideal materials for capturing water from the air.This project aimed at the functionalization of COFs as an entry point and designed and synthesized a series of air-harvesting COFs containing different types of hydrophilic and hydrophobic pores through the precise selection of hydrophilic and hydrophobic sites in the building blocks,and explored applications in organic solvent trace water detection,humidity sensing,low-humidity air-harvesting and proton conduction1.Synthesis of two-dimensional covalent organic framework（COF-pe-1）and its application in trace water detection and humidity sensing:For water detection and humidity sensing,abundant hydrophilic groups and luminescent properties are crucial.Diformyl-2,6-dihydroxynaphthalene（2,6-DHNA）containing hydroxyl hydrophilic sites was reacted with 1,3,6,8-tetrakis（4-aminophenyl）pyrene（Py TTA）with fluorescent properties to synthesize COF-pe-1 with both atmospheric water collection and fluorescence response.Single-crystal diffraction analysis,infrared spectroscopy and solid-state NMR of the model compound confirmed the para-enolimine structure of COF-pe-1.COF-pe-1 has a high specific surface area(794 m²g^-1)and a homogeneous pore size（1.5 nm）.Powder X-ray diffraction and simulation analysis further demonstrate that COF-pe-1 is a layered framework structure with two-dimensional AA stacking.COF-pe-1 exhibits a typical S-shaped water vapor adsorption isotherm with rapid adsorption up to 410 cm³g^-1at RH=40%-50%and a maximum adsorption of 611 cm³g^-1.The combination of the electron-rich pyrene unit and the hydrophilic functionalized hydroxynaphthalene unit enables the framework to exhibit solvent-chromogenic and step water adsorption properties,which contribute to the sensitive colorimetric humidity sensing properties of COF-pe-1 when exposed to air.Meanwhile,the preferential binding of water molecules to hydrophilic hydroxyl sites also enhances the intramolecular charge transfer from pyrene unit to hydroxynaphthalene unit,which makes COF-pe-1 have significant fluorescence enhancement response to trace water in organic solvents,and the detection limits in methanol,acetonitrile and other solutions are below0.20%,which can be used as a trace water detection.The work provides a simple and versatile method for the detection and humidity sensing of trace water in organic solvents and provides an in-depth understanding of the sensing mechanism at the molecular level,which is essential for expanding the practical application of COFs.2.Synthesis of two-dimensional covalent organic framework（COF-ok-1）and its application in low humidity atmospheric water harvesting:To achieve low-humidity atmospheric water harvesting（AWH）driven by natural sunlight in arid regions,not only high-density hydrophilic sites are required to enhance the low-humidity water harvesting capacity,but also suitable hydrophobic sites are needed to improve its low-temperature regeneration capacity.COF-ok-1 was constructed by the reaction of diformyl-2,3-dihydroxynaphthalene（2,3-DHNA）containing adjacent hydroxyl groups with1,3,6,8-tetra（4-aminophenyl）pyrene（Py TTA）.Single-crystal diffraction analysis,infrared spectroscopy and solid-state NMR confirmed COF-ok-1 is an ortho-ketoenamine structure.COF-ok-1 was stable in extreme conditions such as 12M HCl,12M KOH solution and boiling water.COF-ok-1 has a high specific surface area(1194 m²g^-1)and a homogeneous pore size（1.3 nm）.Powder X-ray diffraction combined with theoretical simulations show that COF-ok-1 has a pore structure with hydrophilic ketone groups"face to face"and hydrophobic naphthalene rings"back to back".Due to the balanced hydrophilic and hydrophobic patterns in the pores,COF-ok-1 exhibits excellent low humidity water harvesting ability,reaching 414cm³g^-1at RH=34%,and also exhibits extremely low desorption temperature（45°C）.Outdoor AWH experiments have shown that a water harvesting system using COF-ok-1 as a sorbent can achieve AWH efficiency of 161 g kg^-1under natural solar irradiation,and the collected water fully satisfies the drinkable standard.This work provides an interesting idea for the development of efficient COF-based atmospheric water harvesting sorbents to solve water scarcity problems in arid regions.3.Synthesis of ultra-high water sorption capacity two-dimensional covalent organic frameworks（COFs）and proton conduction applications:proton exchange in fuel cells often takes place as hydrated protons under near-saturated humidity conditions,which requires proton-conducting materials that can provide a sufficient number of water molecules.COFs with both ultra-high porosity and hydrophilic groups can provide more adsorption sites and storage capacity for water molecules,thus achieving high performance proton conduction.Based on the[C₃+C₂]construction strategy,the large pore size COF-ok-2 and COF-pe-2 were synthesized by reacting 2,4,6-tris（4-aminophenyl）-1,3,5-triazine（TAPT）with 2,3-DHNA and2,6-DHNA,respectively.Infrared spectroscopy and solid-state NMR confirmed the structure of COF-pe-2 as para-enolimine and COF-ok-2 as ortho-ketoenamine.COFs have high specific surface areas,1424 m²g^-1for COF-ok-2 and 1742 m²g^-1for COF-pe-2,and large pore sizes,2.37 nm for COF-ok-2 and 2.82 nm for COF-pe-2.Powder X-ray diffraction combined with theoretical simulations indicate that COF-ok-2 and COF-pe-2 are two-dimensional framework structures with hexagonal pores.COF-ok-2 and COF-pe-2 have extremely high total water vapor adsorption amounts of 815.4 cm³g^-1and 1071.9 cm³g^-1,respectively.The proton conductivity of COF-pe-2 and COF-ok-2 were 1.27×10^-3S cm^-1and6.83×10^-5S cm^-1at 80°C and RH=98%.And activation energies of 0.37 e V and 0.39 e V,respectively,showing that the proton transfer mechanisms of both are jumping mechanisms.The higher conductivity of COF-pe-2 can be attributed to the denser hydrogen network formed by the high content of water molecules in its pore channel,which facilitates the more rapid water-mediated proton conduction.