Water cluster in hydrophobic crystalline porous covalent organic frameworks

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Water is the most conventional yet mysterious molecules embodying the earth ecosystems. Nature has designed them with a negative volume expansion at 4 °C for deep water fluidity at extreme weathers. Their large heat capacity is of paramount in minimising temperature fluctuation between days and nights. One exclusive feature in bulk water is the extensive hydrogen bonding network. This network is often soft and flexible, yet difficult to be broken. Disintegrating bulk water network into nanoclusters could alter its intrinsic properties, leading to exciting land of discovery. For instance, quick water flow with minimal resistance could be realised with small water cluster. Here, we propose organic microporous material can facilely break the extended hydrogen bonding networks of water molecules into nanoclusters. The nanospace in the material can host water molecules by actively disconnecting the hydrogen bonding network and rearranging into small clusters. These water nanoclusters can align along the single-file pseudohydrophilicity strips within the material for quick entry and exit through the nanospace without large energy barrier. The material is highly durable for multiple entry and exit of water molecules, retaining its crystalline porous structure upon cycling.

We design hydrophobic trigonal supermicroporous aromatic framework with polar C=N nucleation sites. Each wall in the trigonal channels is decorated with a C=N site. At the induction stage, water molecules selectively nucleate at the pseudohydrophilic C=N sites to form small clusters. Once the cluster size fits the channel size and shape, it penetrates the nanochannels. As a result, the entry point is found at pressure much lower than that of saturation pressure. Notably, as each channel is independent of each other in the framework, the adsorbed water cluster does not recombine into bulk water upon confinement. The water cluster size remains the same and can easily exit the material at similar pressure. This translates into easy adsorption of water vapour even at low pressure and precludes harsh heating for desorption of water. The small space in the channel allows high heat of adsorption when the water resides in the channel. The close distance of C=N strips with water in a small space allows strong confinement to occur.

The uptake of water can be switched off by imposing steric obstruction to the C=N strips. We found that simple decoration with methyl group next to the C=N site completely changes the uptake pressure to high pressure and strengthens the hydrophobicity of the entire framework by shielding the accessibility of C=N sites. In these cases, the C=N sites are sterically hindered by the methyl group. This leads to inefficient nucleation and clustering of water molecules. Without breaking into small cluster, bulk water could not easily penetrate the channels. Such control in material is not usual but we can easily achieve this by delicate design of the porous organic material.

 

Tetragonal and hexagonal topologies are compared to trigonal frameworks. With less strained angle and space, hexagonal and tetragonal microporous frameworks can achieve similar uptake pattern at a slightly larger pore size. Cycling stability of all these frameworks are excellent by retaining high accessibility to water clusters.

Our material combines high durability, porosity with diverse designability for water cluster adsorption. We foresee that various application such as water uptake, transport and separation as well as heat-pump energy conversion can be realised in porous organic frameworks.

For further information, please read our paper “Water cluster in hydrophobic crystalline porous covalent organic frameworks” published in Nature Communications from here.

 

Tan Ke Tian

Ph.D. student, National University of Singapore