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. 2024 Mar 13;16(10):12672-12685.
doi: 10.1021/acsami.3c19536. Epub 2024 Feb 29.

Just Soaping Them: The Simplest Method for Converting Metal Organic Frameworks into Superhydrophobic Materials

Dimitrios A Evangelou  1 , Anastasia D Pournara  1 , Vasiliki I Karagianni  1 , Christos Dimitriou  2 , Evangelos K Andreou  3 , Yiannis Deligiannakis  2 , Gerasimos S Armatas  3 , Manolis J Manos  1
Affiliations

Just Soaping Them: The Simplest Method for Converting Metal Organic Frameworks into Superhydrophobic Materials

Dimitrios A Evangelou et al. ACS Appl Mater Interfaces. .

Erratum in

Abstract

The incorporation of superhydrophobic properties into metal organic framework (MOF) materials is highly desirable to enhance their hydrolytic stability, gas capture selectivity in the presence of humidity and efficiency in oil-water separations, among others. The existing strategies for inducing superhydrophobicity into MOFs have several weaknesses, such as increased cost, utilization of toxic reagents and solvents, applicability for limited MOFs, etc. Here, we report the simplest, most eco-friendly, and cost-effective process to impart superhydrophobicity to MOFs, involving a rapid (90 min) treatment of MOF materials with solutions of sodium oleate, a main component of soap. The method can be applied to both hydrolytically stable and unstable MOFs, with the porosity of modified MOFs approaching, in most cases, that of the pristine materials. Interestingly, this approach was used to isolate superhydrophobic magnetic MOF composites, and one of these materials formed stable liquid marbles, whose motion could be easily guided using an external magnetic field. We also successfully fabricated superhydrophobic MOF-coated cotton fabric and fiber composites. These composites exhibited exceptional oil sorption properties achieving rapid removal of floating crude oil from water, as well as efficient purification of oil-in-water emulsions. They are also regenerable and reusable for multiple sorption processes. Overall, the results described here pave the way for an unprecedented expansion of the family of MOF-based superhydrophobic materials, as virtually any MOF could be converted into a superhydrophobic compound by applying the new synthetic approach.

Keywords: MOFs; oil-in-water separation; porous materials; post-synthetic modification; superhydrophobic materials.

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Conflict of interest statement

The authors declare no competing financial interest.

Figures

Figure 1
Figure 1
Digital images of water droplets on thin films of (a) UiO-66, (b) UiO-66-green soap, and (c) UiO-66-Oleate-5 along with the determined WCA values. (d) Digital image of UiO-66 and UiO-66-Oleate-5 dispersing in water/chloroform mixtures. (e) Comparative PXRD data for UiO-66 and UiO-66-Oleate-5. (f) High resolution Zr 3d core-level photoelectron spectra of UiO-66 and UiO-66-Oleate-5. The peaks assigned to Zr 3d3/2 and 3d5/2 appear at 182.7/185.1 and 182.4/184.8 eV, respectively. (g) Suggested mechanism for the interaction of oleate anions with the UiO-66 framework.
Figure 2
Figure 2
Digital images of water droplets on thin films of (a) HKUST-1-Oleate-2 and (b) HKUST-1, along with the determined WCA values. (c) Nitrogen physisorption isotherms at 77 K for HKUST-1 and HKUST-1-Oleate-2. (d) Comparative PXRD data for HKUST-1 and HKUST-1-Oleate-2. (e) FE-SEM image of HKUST-1. (f) FE-SEM image of HKUST-1-Oleate-2. (g) Suggested mechanism for the interaction of oleate ions with the HKUST-1 framework.
Figure 3
Figure 3
(a) Digital image of a water droplet on a thin film of UiO-66-Oleate-1-Fe3O4 with the indication of the determined WCA value. (b) Digital stereoscopic image of a liquid marble based on the UiO-66-Oleate-1-Fe3O4 material. (c) A water droplet rolling over UiO-66-Oleate-1-Fe3O4 particles. (d) Controlling the motion of the liquid marble using an external magnet.
Figure 4
Figure 4
(a) Two-step in situ coating route for the isolation of UiO-66-Oleate@Cotton fabric, (b) Digital image of UiO-66-Oleate@Cotton fabric floating on the water surface in comparison to UiO-66@Cotton fabric. (c) Digital image of a water droplet on UiO-66-Oleate@Cotton fabric, along with the determined WCA value. (d) Comparative PXRD data of UiO-66, UiO-66-Oleate@Cotton fabric, and cotton fabric. (e) FE-SEM image of UiO-66-Oleate@Cotton fabric.
Figure 5
Figure 5
(a) Postsynthetic immobilization of UiO-66-Oleate-PMMA mixture onto the cotton fabric/fiber. (b) Digital image of UiO-66-Oleate-5-PMMA@Cotton fabric floating on the water surface in comparison to UiO-66-PMMA@Cotton fabric. (c) Digital image of a water droplet on UiO-66-Oleate-5-PMMA@Cotton fabric, along with the determined WCA value. (d) Comparative PXRD data of UiO-66, UiO-66-Oleate-5-PMMA@Cotton fabric, and PMMA@Cotton fabric. (e) FE-SEM image of UiO-66-Oleate-5-PMMA@Cotton fabric. (f) Digital image of UiO-66-Oleate-3-PMMA@Cotton floating on the water surface in comparison to cotton substrate. (g) Digital image of liquid droplets (dye solutions and water) on UiO-66-Oleate-3-PMMA@Cotton, along with the determined WCA value.
Figure 6
Figure 6
(a) Digital image of a water droplet on HKUST-1-Oleate-2-PMMA@Cotton fabric, along with the determined WCA value. (b) Comparative PXRD data of HKUST-1, HKUST-1-Oleate-2-PMMA@Cotton fabric, and PMMA@Cotton fabric. (c) FE-SEM image of HKUST-1-Oleate-2-PMMA@Cotton fabric. (d) Comparative PXRD data of HKUST-1-Oleate-2-PMMA@ Cotton fabric before and after water treatment.
Figure 7
Figure 7
Digital images demonstrating the oleophilicity and hydrophobicity of (a) UiO-66-Oleate@Cotton fabric, (b) UiO-66-Oleate-5-PMMA@Cotton fabric, and (c) HKUST-1-Oleate-2-PMMA@Cotton fabric. (d–g) Crude oil sorption from the water surface by HKUST-1-Oleate-2-PMMA@Cotton fabric. (h) Desorbed oil from the oil-laden fabric after its treatment with n-hexane and the regenerated HKUST-1-Oleate-2-PMMA@Cotton fabric. (i) Oil-free water solution and the HKUST-1-Oleate-2-PMMA@Cotton fabric after the 10th cycle of sorption/regeneration.
Figure 8
Figure 8
Vacuum pump oil-in-water emulsions with initial oil concentrations (a) C ≈ 540 ppm, (b) C ≈ 780 ppm, and (c) C ≈ 1040 ppm before and after a 20 h treatment with UiO-66-Oleate-5-PMMA@Cotton fabric. (d) Crude oil-in-water emulsion with initial oil concentration C ≈ 1005 ppm before and after a 20 h treatment with UiO-66-Oleate-5-PMMA@Cotton fabric. (e) Comparative PXRD data of UiO-66-Oleate-5-PMMA@Cotton fabric before and after demulsification of vacuum pump oil-in-water emulsion (with initial oil concentration of ≈1040 ppm) as well as after regeneration with n-hexane. (f) Comparative FTIR spectra of a vacuum pump oil sample, UiO-66-Oleate-5-PMMA@Cotton fabric before and after demulsification of vacuum pump oil-in-water emulsion (with initial oil concentration of ≈1040 ppm) as well as after regeneration with n-hexane. (g) Demulsification performance, evaluated on vacuum pump oil-in-water emulsion with initial oil concentration of ≈1040 ppm, of UiO-66-Oleate-5-PMMA@Cotton fabric during four cycles of sorption/regeneration.
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