Covalent Organic Frameworks (COFs) Atmospheric Water Remediation
Covalent organic frameworks (COFs) are constructed from organic building units to make frameworks with backbone structures that are entirely composed of light elements (e.g., B, C, N, O) —excellent for applications where metal leaching pose health risks. X-rays and electron diffraction show the architectural stability of gases moving in and out of the pores without deforming the COF, though capillary forces acting on the pore walls could cause hydrolysis. The gold standard to test this is measurement of the N2(g) or Ar(g) adsorption isotherm at 77 K.
Covalent Organic Frameworks for Atmospheric Water Harvesting
COFs are designed by linking pre-synthesized organic units through covalent bonds to create frameworks with properties suitable for atmospheric water harvesting. Their reticular design aids the integration of chemical functionalities for the fabrication of pore shapes. In the typical water isotherm of a highly crystalline microporous COF, at first water will strongly interact with the adsorptive sites and coalescence to form clusters. Then start filling the micropores (water–water interaction is dominant), and finally entirely fill the pores of the COF. The mechanism relies on the formation of water–imine interactions inside the framework.
For COFs to be atmospheric water harvesters their water isotherm should have a steep and step-shaped uptake at low relative humidity (RH): 15 to 40% RH; with the reflection point close to zero, large total volumetric water capacity, and low regeneration temperature. This method requires adsorbents with ample working capacity, rapid kinetics, low energy costs, and long-term stability under operating conditions. Best pore sizes range from 8 to 20 Å, especially 8 A.
The COF’s surface area is optimal when it crystallizes in the highest crystalline form with minimal defects. Perfect crystallites avoid the defects that cause large hysteresis loops in the water uptake isotherm, resulting in energy inefficiency. Sometimes a high level of interpenetration and/or different stacking modes of the structure leads to high crystallization.
High porosity does not mean the COF can take up water at low RH. A mesoporous COF structure may not work well if the active sites are not strong enough to interact with water molecules.
A COF with a shallow isotherm requires a higher release temperature, but the temperature swing
for regeneration in a high‐crystalline COF is smaller than for a low one, or amorphous/semicrystalline polymer.
Adsorptive sites on the framework determine the RH where the cutoff of the water isotherm occurs. A high degree of amorphous in the COF causes an inclined water isotherm resulting in low working capacity and inefficient energy usage.
The design of hydrophilic-hydrophobic COFs is possible due to the variety of organic building units. The hydrophilic pore environment is prone to shift the water sorption isotherm to a low RH. But adding hydrophilicity to the framework increases the isosteric heat of adsorption, requiring a higher regeneration temperature to release water. A good balance of hydrophilic & hydrophobic entities might reduce the regeneration temperature required.
Conclusion
COF chemistry has existed for only 20 years. In addition to synthesizing more COFs, methods for synthesizing COFs at industrial-scale production are essential. COFs should be grown on top of the commercial substrates commonly used in water harvesting devices. COFs must be stable under many cycles of water adsorption/ desorption—at least 10,000 cycles.
The energy required to cycle a COF must be calculated and optimization of water-harvesting devices perfected. Closer collaboration between chemists and engineers is necessary. At present, the cost of making COFs is higher than other options, but this can be resolved by large-scale production.
Covalent Organic Frameworks for Water Remediation
COFs have been explored for water remediation for a wide range of pollutants, including persistent organic pollutants, toxic metals, radioactive waste, etc. The development of COFs into macrophysical forms such as foams and film membranes have advanced water treatment. Granular adsorption of molecular pollutants is being replaced by rapid and efficient foam-based adsorption and chemo-selective membrane-based separation.
In low energy remediation, pollutants are removed from water by adsorption, ion exchange, molecular sieving, and chemical reaction-based detoxification:
- In adsorption, molecules are adsorbed on a porous platform through weak interactions. For example, COFs that are designed as acidic easily interact with basic molecular pollutants through acid–base interactions. A COF decorated with thioether units selectively interacts with mercury metal ions in water through Hg–S weak chemical bonding. The selection of COFs for target-specific micropollutants is possible by correlating their physical and chemical properties to the pollutant molecules.
- Ion exchange purifies water from ionic pollutants. Ionic porous platforms exchange cations and anions.
- Molecular sieving is a separation strategy.
- Chemical reactions subject organic contaminants in water to COF-based oxidation or catalytic processes, which degrade the toxic molecules into non-toxic fragments.
Adsorptive or membrane platforms provide a chemical environment for capturing molecules through weak inter-actions such as hydrogen bonding, pi-pi, and dispersion interactions. COF
ordered porosity enhances the access-ibility of decorated functional moieties towards pollutants. The chemical moieties are connected to the framework through the original design and/or post-synthetic modifi-cation.
It is sometimes difficult to form a framework with the desired functionalities through direct synthesis. Post-synthetic modification creates opportunities to graft a large set of functional moieties on pore walls and the surface of the framework. However, these post-synthetic modifica-tions result in partial functionalization of the network and/or decrease in porosity of the resultant material.
An unsubstituted olefin-linked 2D-COF was connected to 2,4,6-trimethyl-1,3,5-triazine (TMT) and 4,4′-biphenyldi-carbalde-hyde (BPDA). The chemical inertness of the olefin linkage in a range of pH values could be an important factor for the development of next-generation water purification materials.
Field Experiences with Atmospheric Water Harvesting COFs
One mesoporous COF has excellent water sorption performance, showing a cutoff at 35% RH. The b-keto-enamine linkages at the corners and bipyridine at the edges of the hexagon units showed high water-interaction, facilitating the formation of clusters in the pores. Despite having large pores, it exhibited a steep uptake of water isotherm at 35% RH with a total volumetric water capacity of 900 cm3 g–1 (0.72 gwater gCOF–1).
Another COF crystallized in a tetragonal structure. Its powder X-ray pattern showed very sharp diffraction lines. Its high crystallinity resulted in a perfect S-shaped water isotherm with no hysteresis. Imine linkages are likely water adsorptive sites that optimize the proximity of water to the imine, especially in this staggered structure. It can take up a total water capacity of 0.3 gwater gCOF–1; its water working capacity from 30 to 40% RH is 0.23 gwater gCOF–1. The low-temperature swing for its regeneration indicates that this COF is an energy-efficiency material for water harvesting. Another case used 2 hydrophilic COFs. One based on hydrazine linkage, the other on b-ketoenamine linkage. Both have the same honeycomb structure type and pore size of 13 Å. However, the second has an additional pore size distribution at 6 Å,
indicating that two types of crystalline domains and stacking modes were formed. The result was a lower surface area of 520 m2 g–1 compared to 1125 m2 g–1 and lower total water uptake capacity 0.3 gwater gCOF–1 at 15 °C, 90% RH compared to 0.4 gwater gCOF–1, The first COF showed a steep and step-shaped isotherm with a reflection point at 22% RH, making it a candidate for atmospheric water harvesting.
The amide version of imine/hydrazine, hydrazide, is a hydrophilic linkage that has potential for COF chemistry. However, synthesis of hydrazide-linked COFs via the de novo method is challenging due to the irreversible formation of its linkage. Imine can be successfully oxidized to amide, so a more stable hydrazine-linked COF was oxidized to form a hydrazide-COF, but the hydrazine was not fully converted to hydrazide. The maximum conversion was 26% in a square hydrazine-to-hydrazide COF with pore size ranging from 16 to 19 Å. Despite this, the hydrazide-COF exhibited good total water uptake capacity of 0.79 gwater gCOF–1, but with a left-shifted isotherm. The reflection point shifted to the lower RH of 45% RH.
In a COF using naphthalene-2,3-dione units in a o-ketoenamine-linked COF where dione and naphthalene functionalities act as hydrophilic and hydrophobic entities, respectively, the COF exhibited an S-shaped isotherm with the reflection point at 27% RH and a total volumetric uptake of 789 cm3 g–1 (0.63 gwater gCOF–1). The working capacity for the range of 27–34% RH was 0.33
gwater gCOF–1. The COF was heated to a temperature as high as 45 °C. The water sorption mechanism was studied by the force-field-based grand-canonical Monte Carlo method. Water mole-cules quickly interacted with dione hydrophilic groups, accelerating the formation of water clusters. The naphthalene hydrophobic groups narrowed the center of the pores and promoted the interconnection of opposite water clusters at the center until the pores were completely filled.
Small cage units were connected using 2,5-dihydro-xyterephthalaldehyde to generate a 3D-Cage COF with acs topology. This 3D COF adopted a 2-fold interpenetrated structure with –OH functional groups pointing into the pores, making the pore environment polar and allowing for water uptake at low RH. The water isotherm exhibited an S-shaped isotherm with steep uptake at 21 to 34% RH. At 40% RH, the water working capacity was 0.22 gwater gCOF–1.
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