Water-Ethanol and Methanol-Ethanol Separations Using in Situ Confined Polymer Chains in a Metal-Organic Framework

This study presents a straightforward approach for the in situ polymerization of poly(N-isopropylacrylamide) (PNIPAM) chains within the one-dimensional (1D) pores of the five-coordinated zinc-based metal-organic framework DMOF in order to obtain new MOF-based composites. The loading amount of PNIPAM within DMOF ⊃ PNIPAM composites can be tuned by changing the initial weight ratio between NIPAM, which is the monomer of PNIPAM, and DMOF. The guest PNIPAM chains in the composites block partially the 1D pores of DMOF, thus leading to a narrowed nanospace. The water adsorption studies reveal that the water uptake increased by increasing the loading of PNIPAM in the final DMOF ⊃ PNIPAM composites, indicating that the exposed amide groups of PNIPAM gradually alter the hydrophobicity of pristine DMOF and lead to hydrophilic DMOF ⊃ PNIPAM composites. The composite with the highest loading of PNIPAM displays a selective adsorption for water and methanol over ethanol when using equimolar mixtures of methanol-ethanol and water-ethanol. This is confirmed by the single-component adsorption measurements as well as ideal adsorbed solution theory molecular simulations. Additionally, the water stability of pristine DMOF has been greatly improved after the incorporation of PNIPAM in its pores. PNIPAM can undergo a phase transition between hydrophobic and hydrophilic phases in response to a low temperature change. This property is used in order to control the desorption of water and methanol molecules, thus enabling an efficient and cost-effective regeneration process.