An eco-friendly approach using a nonfluorous self-cleaning metal–organic framework composite and membrane for oil–water separation

Abstract

Here, we report a first-ever superhydrophobic metal–organic framework (MOF) material prepared from a presynthesized long-chain hydrocarbon-based linker. The nonfluorous MOF was synthesized using a palmitamdoterephthalic acid (H₂BDC-NH-R, R: –NH–CO–(CH₂)₁₄–CH₃) linker and zirconium salt. The material was systematically characterized by PXRD, FT-IR, EDX and FE-SEM experiments to confirm its crystallinity, functionality, elemental purity and particle nature. Furthermore, the water contact angle (WCA) measurement proved the material's superhydrophobicity. The MOF material was further utilized to design a superhydrophobic melamine sponge composite and silk membrane. Both the composite and membrane were applied for the selective separation of oil from the oil–water mixture. The separation efficiency in absorption and filtration-based methods was higher than 99%. The absorption capacity of the composite ranged between 43.8 and 97.2 g g⁻¹. The flux of oil–water separation was 58 263–47 416 L m⁻² h⁻¹, which is higher than for most reported materials. Both the membrane and composite displayed remarkable self-cleaning properties. The long-chain hydrocarbon-based linker is the reason behind the superhydrophobic nature of the MOF. The tiny nanostructure of the MOF on the surface of the composite and membrane causes the formation of the Cassie–Baxter state when they come in contact with water. The nonpolar–nonpolar interactions between MOFs and different oils are the reasons behind the selective oil–water separation. Since water is polar, the superhydrophobic composite and membrane inhibit the permeation of water through them. The mechanism of nonpolar–nonpolar interaction was supported by the ESP diagram of the linker, water and all the organic molecules in crude oil.