Synthesis of superhydrophobic and high stable Zr-MOFs for oil-water separation

Highlights

• A post-modification approach is developed to synthesize superhydrophobic Zr-MOFs.

• The UiO-66-NH-C18 has better stability under alkaline conditions than UiO-66-NH_2.

• The oil-water separation efficiency can reach up to 99.9 %.

• The hydrophobic Zr-MOFs@sponge via dip-coating has high oil absorption ability.

Abstract

A universal strategy is developed to construct a superhydrophobic and superoleophilic metal-organic framework (MOF) by an amidation reaction between amino group and acid chloride of octadecanoyl chloride. The introduction of long alkyl chain reduced the surface free energy, yielding a superhydrophobic Zr-MOF (UiO-66-NH-C18) with a high water contact angle (WCA) of 151.7°. The adsorbed amount of water on the UiO-66-NH-C18 was only 5.59 mmol/g, which was just 28 % of that on the UiO-66-NH₂. More importantly, the hydrophobic modification improved the stability of Zr-MOFs under strong alkaline conditions (e.g., pH = 13). Furthermore, the UiO-66-NH-C18 can selectively absorb organic solvent from water and achieve continuous oil-water separation. The oil-water separation efficiency could reach up to 99.9 %. In addition, the UiO-66-NH-C18@sponge was also prepared through dip-coating of UiO-66-NH-C18 on the melamine sponge in the presence of PDMS solution. The resulting composite shows high adsorption capacities for various oils and organic solvents ranging from 32.3–66.1 g/g. The superhydrophobic UiO-66-NH-C18@sponge has potential as a promising absorbent for treatment of oily wastewater.

Introduction

Oil spills and industrial organic contaminants have caused serious water pollution, which lead to a long-term threat to public health and the ecological environment [1,2]. To date, various superhydrophobic materials have been developed for absorption and separation of oils from water [[3], [4], [5]]. Generally, superhydrophobicity is achieved by roughening the surface of hydrophobic materials or by modifying the rough surface with low surface energy chemicals [[6], [7], [8]].

Metal-organic frameworks (MOFs), constructed by metal-containing units with organic ligands, have received considerable interest due to their high porosities, rich structure and chemical functionalization, and showed potential applications in adsorption or separation [[9], [10], [11], [12], [13]]. Therefore, MOFs are promising candidates to prepare superhydrophobic materials. Currently, there are three main strategies for synthesizing hydrophobic MOFs. The first approach is using the ligands containing aromatic hydrocarbon moieties or fluorinated groups. For example, Zhang et al. introduced a facile method to fabricate hydrophobic UPC-21 with a water contact angle (WCA) of 145° by introducing multi-aromatic hydrocarbon units in the organic ligand. The UPC-21 exhibited good performance in the oil/water separation with an efficiency of 99 % [14]. Liu et al. prepared a superhydrophobic F-ZIF-90 through a one-step synthesis strategy using fluorine-functionalized imidazole-2-formaldehyde as an organic linker. The F-ZIF-90 exhibited high hydrophobicity with a WCA of 159.1° [15]. The second approach is constructing nano- to micrometer surface with high roughness. [16] For example, Rao demonstrated a new approach to synthesize a porous coordination polymers (PESD) with superhydrophobic surfaces as a result of nanoscaled corrugation of a predominantly organic surface at the crystal interface. The resulting materials had a WCA of above 150° [17]. Moreover, Post-modification with alkyl or fluoroalkyl groups is an effective common approach for the synthesis of hydrophobic MOFs. [18,19] For example, Liu et al. prepared a superhydrophobic zeolitic imidazolate framework (ZIF-90) via an amine condensation reaction between the aldehyde groups of ZIF-90 and amine groups of pentafluorobenzylamine. The resulting MOFs had a WCA of 152.4° and can be usd to remove 98 % bio-alcohol (ethanol, iso-propanol and butanol) from the bio-alcohol/water mixture [20]. Sun et al. reported a facile method to modify the external surface of MOFs with superhydrophobicity through incorporation of n-octadecylphosphonic acid. The modified MOFs had a WCA of >150° [21]. Generally, besides the superhydrophobic property, the MOFs should exhibit high water stability for the use in oil-water separation [22].

The Zr‐containing metal–organic frameworks (Zr-MOFs) formed by 2-aminoterephthalate ligands (UiO-66-NH₂) have attracted great attention due to its unique structural features [23,24]. In particular, the presence of free amino groups in the UiO-66-NH₂ framework allows the functionalization of UiO-66-NH₂ with various groups [25,26]. However, they can adsorbed water (vapor) in the water bearing media (e.g., water, air) due to its good hydrophilicity. For example, Zhu et al. [27] modified UiO-66-NH₂ with CO₂-preabsorbed polyethyleneimine (PEI) via Schiff base reaction. The resulting PEI-modified MOFs show higher CO₂ adsorption capacity and superior moisture endurance compared to pristine UiO-66-NH₂. In addition, UiO-66-NH₂ shows poor water stability when it is exposed to strong alkaline conditions. A promising strategy to overcome the intrinsic shortcoming of MOFs is post synthetic modification with functional groups [28]. In this study, we attempt to prepare superhydrophobic UiO-66-NH-C18 via the post-modification of UiO-66-NH₂ octadecanoyl chloride. Furthermore, the superhydrophobic Zr-MOFs was directly used or dip-coated on the surface of sponge for oil-water separation.

Herein, we report a facile method to prepare superhydrophobic UiO-66-NH-C18 via an amidation reaction of amino groups in UiO-66-NH₂ and acid chloride groups in octadecanoyl chloride. Furthermore, we prepared a superhydrophobic UiO-66-NH-C18@sponge via polydimethylsiloxane (PDMS)-assisted surface coating of UiO-66-NH-C18 on a melamine sponge (Scheme 1). The morphology and structure of UiO-66-NH-C18 and its composite was characterized by scanning electron microscopy (SEM), transmission electronic microcopy (TEM) and x-ray diffraction (XRD). The wetting behavior, water vapor adsorption, chemical and thermal stability of UiO-66-NH-C18 was then tested. Finally, the oil-water separation and oil absorption performance of UiO-66-NH-C18 and UiO-66-NH-C18@sponge were evaluated.