ZIF-8 membrane supported on alumina hollow fiber with enhanced salt removal by forward osmosis

Highlights

• Zinc oxide was synthesised using electroless deposition for the support of ZIF-8.

• The salt rejection is based on size-exclusion molecular sieving mechanism.

• ZIF-8 was stable for 30 days in 40,000 ppm saline solutions.

Abstract

This work describes the development of ZIF-8 membranes supported on alumina hollow fiber via electroless deposition (ELD) of ZnO followed by solvothermal synthesis for water desalination. The relatively low operating temperature of ELD of ZnO provided an alternative method to fabricate pure-phase ZIF-8 membrane. As-prepared ZnO and ZIF-8 samples were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), water contact angle, and Fourier transform infrared spectroscopy (FTIR). The performance of ZIF-8 membrane was evaluated using forward osmosis (FO) using active layer facing feed solution. The well-intergrown membrane provided high water flux up to 12.25 L/m²h with reverse solute flux as low as 0.029 kg/m²h when using 100,000 ppm NaCl solution and water as the draw and feed solutions, respectively. Furthermore, the membrane showed high KCl (87.8%) and NaCl (88%) rejection and excellent CaCl₂ (95%) and MgCl₂ (98%) salt rejection in FO using 1.0 M dextrose solution as the draw solution. Interestingly, the rejection of AlCl₃ salt was only as high as 46% due to the instability of ZIF-8 in the AlCl₃ solution causing the loss of its crystallinity. The ZIF-8 material showed no degradation in various saline solutions (e.g., KCl, NaCl, CaCl₂, and MgCl₂) except for AlCl₃ solution even at a high concentration of 40,000 ppm for 720 h. The findings suggest that the prepared ZIF-8 membrane is a potential membrane for desalination application due to its excellent separation performance toward certain salts.

Introduction

Metal–organic frameworks (MOFs) are emerging novel porous materials, built by interconnecting metal building unit with organic linkers. The versatility of their molecular structure and functionality by changing metal or organic linkers enables these materials to be tailored for various applications, such as separations, gas storage, sensing, catalyst, and adsorption [1]. Recently, studies have been focused on the preparation of supported MOFs layer as a molecular sieving membrane for high-performance separation application. The MOFs, especially zeolitic imidazolate frameworks (ZIFs), have attracted interest as potential membrane materials. The ZIFs which consisted of zinc or cobalt metal and imidazolate-based linkers, possess high thermal and chemical stabilities, zeolite-like topology, and uniform pore sizes [2]. ZIF-8, a part of ZIFs family, is composed of zinc metal and 2-methylimidazole (Hmim) forming a sodalite zeolite-like topology. ZIF-8 has been a widely studied MOF membrane due to high thermal stability (up to 550 °C), high chemical resistance in various solvents, and relatively low synthesis temperature than any other MOFs [3]. In particular, for desalination application, ZIF-8 has shown to be stable in seawater while having small pore aperture (3.4 Å) in between kinetic diameter of most salt ions and water molecule for potential high salt removal capabilities. Hu et al. showed that, theoretically by simulation, ZIF-8 was capable of removing 100% of NaCl salt due to its small pore aperture [4]. Thus, ZIF-8 should be assembled in the form of a layer to act as a membrane for desalination applications. However, more effort is required to prepare high-quality MOF membrane than that to prepare MOF crystals.

Typically, pure-phase ZIF-8 membranes are mainly prepared via two methods, namely in situ or secondary growth. The in situ growth method is a direct synthesis of the ZIF-8 layer without any ZIF-8 nanocrystals seeded on the surface of the support in advanced. This method combines nucleation, growth, and intergrowth process of the crystals in one single step by solvothermal or hydrothermal synthesis [5,6]. However, the preparation of the membrane through in situ growth method is much difficult due to uneven crystals nucleation due to poor interaction and compatibility between ZIF-8 and material of the support, usually a ceramic-based material such as alumina. Therefore, the surface of the support had to be modified, deposited, or functionalized with a silane coupling agent, metal oxide with the same metal element of the framework or covalent linker such as polydopamine [[7], [8], [9], [10], [11], [12], [13], [14], [15], [16]]. These modifications are deployed to promote the heterogeneous nucleation of ZIF-8 crystals which lead to the well-intergrown membrane and improve the attachment of the membrane on the support.

The deposition of ZnO on support's surface for the preparation of the ZIF-8 membrane gains burgeoning attention due to different deposition methods that can be deployed, which can affect membrane performance. To date, various deposition methods have been introduced for the preparation of ZIF-8 layer including manual rubbing, sol-gel, hydrothermal, electrodeposition, atomic layer deposition, electrophoretic deposition, spray coating, and magnetron sputtering [15,[17], [18], [19], [20], [21], [22], [23], [24], [25]]. For instance, Zhang et al. successfully used the sol–gel method followed by the calcination process at 400 °C to deposit ZnO on alumina support for scalable ZIF-8 membrane preparation [26]. Moreover, Kong et al. utilized the calcination process as high as 700 °C to immobilize ZnO on the support surface by manual rubbing [17]. These methods, however, involve high temperature process or electrical power or combination of both. Alternatively, the electroless deposition (ELD) method is an autocatalytic coating process with the advantage of lower operating temperature without additional electrical energy [27]. This method has been used on a large scale for the plating of nickel. In 2016, Fu et al. prepared multiple morphologies of ZnO deposited on poly(ethylene terephthalate) support by ELD for optoelectronic application [28]. Thus, it is interesting how the different morphologies of ZnO deposited by ELD on different types of support could influence the morphologies and performance of the ZIF-8 membranes for desalination.

Herein, a novel synthesis strategy is proposed to prepare a pure-phase ZIF-8 membrane on alumina hollow fiber. The method focused on the ELD method to depositing the ZnO layer on the surface of alumina hollow fiber before membrane synthesis. To the best of our knowledge, no studies have been reported on depositing ZnO on alumina hollow fiber by ELD for the preparation of the ZIF-8 membrane for various applications, especially desalination. The relatively low-temperature process of ELD combined with low synthesis temperature of the ZIF-8 membrane give an alternative for energy-efficient technique and can be suitable for large scale production. The desalination performance of the membrane will be evaluated using forward osmosis (FO).