Water-stable MOFs-based core-shell nanostructures for advanced oxidation towards environmental remediation

Abstract

Metal organic frameworks (MOFs) find many potential applications because of their versatile physicochemical properties. Advanced oxidation is an important way for wastewater remediation to realize sustainable supply of clean water. However, due to the lack of water-stable MOFs with sufficient catalytic activity, the application of MOFs in advanced oxidation processes (AOP) for wastewater treatment is greatly hindered. In this study, by taking advantage of the rich pores of water stable MOFs, we develop a MOFs-based core (water stable MOFs)-shell (NiP) structure as an efficient catalyst for peroxymonosulfate (PMS) activation in AOP. Here, water stable MIL-96 as the MOFs is synthesized by a hydrothermal method, and the core-shell structured MIL-96@NiP is facilely synthesized through electroless coating of the NiP layer. The as-prepared core-shell structure demonstrates superior performance in catalytic degradation of rhodamine B (RhB), over performing the individual MOFs and NiP parts, suggesting the appearance of synergistic effect between MOFs and NiP in the core-shell structure. Furthermore, the catalyst demonstrates four consecutive runs without losing significant catalytic activity. Temperature has a significant role in faster degradation of RhB. A plausible degradation mechanism is proposed through classical quenching tests study, and oxygen singlet is found to play imperative part in removal of RhB.

Introduction

Metal-organic frameworks (MOFs) are crystallized compounds with one-, two- or three-dimensional structures, created through the self-assembly of metal ions or clusters and organic ligands with multiple binding sites in a periodic way, which show versatile physicochemical properties, such as crystalline nature, easily modifiable structure, topology, high specific surface area, uniform pore size, high porosity, and tunable band gaps. The unique features of MOFs make them suitable for a broad range of applications, such as gas storage, transportation or separation, catalyst, or catalyst support [[1], [2], [3], [4], [5]]. During the past, MOFs have been extensively exploited for gas adsorption and other gas storage and separation applications [[6], [7], [8], [9]]. More recently, their potential application in wastewater treatment was also recognized, for example, the removal of heavy metal ions from water by ion adsorption, the removal of organic pollutant by adsorption, and photocatalytic degradation of organic pollutants [7,[10], [11], [12], [13], [14]]. MOFs are also very attractive candidates for the synthesis of highly functional carbon composite materials. The MOFs derived composites find extensive applications in various fields [[15], [16], [17], [18], [19], [20]]. Interestingly, some MOFs derived composites can be synthesized from waste polyethyelene terephthate (PET) waste bottles [21].

Advanced oxidation process (AOP) is a promising technique for the quick degradation of organic pollutants in wastewater [1,[22], [23], [24], [25]], which is of particular attractiveness for the removal of industrial waste that is difficult to be degraded by conventional biodegradation method. During AOP, a catalyst is usually applied for the quick activation of some high-energy substance (such as H₂O₂ and PMS), and the generated free radicals with high oxidizing potential quickly decompose the organic through redox reaction. Considering the high surface area and tuneable properties, MOFs may also be applicable as catalysts in many chemical processes, including AOP for wastewater remediation. However, a big challenge of MOFs for use in aqueous media is the poor phase stability due to the easy attack of metal-organic bond by the polar water [26]. Recently, some water-stable MOFs have been successfully developed, for example, MIL-96 (MIL stands for Materials Institute of Lovousir), HKUST-1, and UiO-66 [6,[27], [28], [29]]. In particular, MIL-96 was successfully used for the adsorptive removal of organic and inorganic waste materials from wastewater [6,7]. Unfortunately, such water-stable MOFs showed low intrinsic catalytic activity in AOP. It is actually a big challenge to develop new MOFs with simultaneous high stability under water environment and high catalytic activity because of the limited selection of water stable MOFs.

Instead of trying to develop MOFs with simultaneous high activity for AOP and water stability, a more promising way is to form a composite catalyst by modifying the water-stable MOFs with some highly active AOP catalyst. The high surface area of MOFs may allow the well distribution of the second phase (the active materials), thus providing large active sites. A synergy may further be created between the MOF and the catalyst, resulting in enhanced catalytic performance. Actually, grafting of adenine into structure of MIL-100-Cr has been reported in adsorption removal of indole and quinoline from model fuel, showing promising results [30]. In that case, adenine provided electron-rich nitrogen species that helped in the formation of hydrogen bonds with adsorbate molecules. Incorporation of NH₂ groups also showed positive effects on performance of MIL-101-Cr for adsorptive removal of methyl orange [31]. Apart from conventional immobilisation of additional organic molecules and/or second metals into existing MOFs [32], core-shell structured MOF-based materials were also exploited for application in various fields, ranging from adsorption, catalytic degradation of water contaminates and biomedical applications [33,34]. For example, An et al. reported the building of core-shell structured bioMOF-11 to bioMOF-14, which showed superior performance for CO₂ adsorption as compared to the pristine MOFs without coating. Moreover, core-shell structures improved water stability of bioMOFs [35]. Similarly, ZIF-8-based core-shell structures with additional functionalities were also reported [36]. For example, by encapsulation of Pt and Fe₃O₄ [33] nanoparticles (NPs) with the help of surfactants into ZIF-8, improved magnetic and catalytic properties were demonstrated. Moreover, promising results were reported in hydrogenation and in molecular sieving by forming the core-shell structured MOFs [37].

In this study, we propose a MIL-96 core-NiP shell structured MOFs for AOP-based wastewater treatment. MIL-96 was specifically selected as a candidate MOF owing to its exceptional stability in aqueous environment under harsh conditions i.e. elevated temperature and strong acidic/basic conditions. A facile electroless deposition technique was used for the deposition of NiP shell onto MOF surface. The high surface area and rich porosity of MIL-96 allowed the well dispersion of NiP phase on its surface. The interaction between MIL-96 and NiP increased the catalytic activity of MIL-96 MOFs on the one hand, and reduced the loss of NiP phase during operation on the other hand. The NiP shell experienced oxidation with the formation of NiOOH, acting as efficient catalyst for the activation of PMS for the generation of free radicals. The interaction between MOF and the shell further increased the activity of NiP for PMS activation. The as-synthesized core-shell structure showed high performance for the removal of RhB through AOP, which is much superior to the pristine MIL-96 MOF and the NiP, confirming the synergistic effect.