Aramid nanofiber and modified ZIF-8 constructed porous nanocomposite membrane for organic solvent nanofiltration
Graphical abstract
Introduction
Organic solvent nanofiltration (OSN) is a pressure-driven process that can effectively separate molecules in the 200–1000 g mol−1 range from organic liquids [1]. As a versatile, green technology, OSN is used in various applications in the pharmaceutical industry, in petrochemistry and in the food industry [1,2]. OSN membranes are often integrally skinned asymmetric membranes; such membranes have been commercialized due to their ease of preparation and controllable post-treatment [1]. However, the crosslinked dense structure of the conventional integrally skinned asymmetric polymeric membrane limits the permeability of organic solvents [2,3]. Designing a porous membrane structure to enhance solvent transport may bring new opportunities for the development of OSN membranes.
One-dimensional nanomaterials with diameter on nanometer scale emerged for the membrane fabrication in recent years. Examples are carbonaceous nanofibers [4], metal hydroxide nanofibers [5], nanowires [6], nanostrands [7], electrospun nanofibers [8] and aramid nanofibers [2,3,9,10]. Many studies focused on one-dimensional nanofibers as membrane building blocks via strategies of vacuum filtration [5,6] or electrospinning [8]. The high porosity, low tortuosity, high surface area, low weight, interconnected pores and thin film thickness are expected to bring great advantages compared to conventional polymer-based membranes [2]. However, membranes constructed with nanofibers could so far only be used in the ultrafiltration or microfiltration range for water treatment, due to their large pore size [[4], [5], [6], [7]]. It is a challenge to obtain nanofibrous films to be used in nanofiltration, capable of selecting molecules small than 1000 g mol−1. Moreover, the poor solvent resistance of nanofibers hardly meets the requirement of the membrane to be used in organic solvent nanofiltration [3]. In our previews work, Kevlar aramid nanofibers (ANFs) were found promising in OSN membrane fabrication due to its strong solvent resistance and robust mechanical strength [2,3,9]. The nanofibrous hydrogel membrane was transferred into a chemically stable OSN membrane through a facile thermal post-treatment. The regenerated hydrogen bonds among ANFs enable the membrane could withstand a variety of solvents [3,9]. However, the high density of the layered fibrous structure limits solvent transport [3]. Intercalating porous materials among nanofibers to produce a porous structure has potential to enhance the solvent permeability [3].
Metal organic frameworks (MOFs) are inorganic-organic crystalline compounds with a high surface area, controlled porosity, affinity for small molecules and a tunable chemical composition [11,12]. MOFs have been widely studied as fillers in thin film nanocomposite (TFN) membrane fabrication by introducing the nanofiller in the polyamide selective layer [[11], [12], [13], [14]]. The incorporation of porous MOFs leads to an exceptional increase of permeance in OSN and water desalination. Hybrid mixed-matrix membranes (MMMs) consisting of MOFs and polymers were also investigated [[15], [16], [17]]. However, an architecture with one-dimensional nanofibers and porous nanofillers for molecular separation in organic liquids has not been reported yet. Agglomeration of nanoparticles is one of the most cited problems associated with the physical blending method. It was reported that the coordinative bonding of polymers on the MOFs surface could effectively reduce the interface interactions among nanoparticles, thus enabling an exceptional colloidal stability [18,19]. Furthermore, it was found that branched polyethylene imine (BPEI) has strong non-covalent interactions with the ANFs, giving rise to an intensified solvent resistance and a higher solvent permeance [3]. Based on these results, the commercial zeolitic imidazolate framework-8 (ZIF-8) was selected in this work and coordinatively modified by polymers of BPEI to reduce the aggregation of nanoparticles [18]. Zeolitic imidazolate framework-8 is a typical MOF, and is extensively studied because of its high chemical and thermal stability [20,21]. ZIF-8 crystals are constructed from Zn2+ metal ions and 2-methylimidazole as linker, have the sodalite topology and a pore aperture and diameter of 3.4 Å and 11.4 Å respectively [21,22]. Intercalation of ZIF-8 nanoparticles into the dense laminar layer of ANF membrane interfering the hydrogen bond regeneration among aramid nanofibers is expected to produce a porous OSN membrane.
Herein, a novel nanocomposite membrane was constructed by one-dimensional Kevlar nanofibers and modified commercial ZIF-8 nanoparticles. Coordinative bonding of BPEI to the ZIF-8 was applied to address the aggregation of particles in the membrane matrix. The ANF nanocomposite membrane was facilely prepared by non-solvent induced phase inversion and thermal treatment. A porous nanocomposite OSN membrane with three-dimensional interconnected network was developed. The chemical stability and OSN performance of the resultant membrane for molecular separation were evaluated. This work intends to be a reference for designing and constructing nanocomposite OSN membrane with porous structure.
Section snippets
Chemicals and materials
Kevlar aramid fiber was purchased from Zhangjiagang Free Trade Zone Fengduo International Trade Co., Ltd., China. Branched polyethyleneimine (PEI) (average MW ~25000 Da) was obtained from Sigma-Aldrich BVBA. Potassium hydroxide (KOH, 85%) in the form of pellets was obtained from Acros Organics NV. Basolite® Z1200 (ZIF-8) (BASF, Antwerpen, Belgium) was purchased from Sigma-Aldrich. Dimethyl sulfoxide (DMSO, 99.5%), n-heptane (95%) from VWR International BVBA, ethanol (99%), methanol (99.9%),
Membrane preparation
Fig. 1a illustrates the preparation process of the ANF nanocomposite membranes. The destruction of hydrogen bonds among macromolecular chain leads to the dissociation of fibers on nanoscale [2]. As shown in Fig. 1 (b), a good distribution of ANFs was observed using transmission electron microscopy (TEM). The nanofibers were found to have a diameter of 13 ± 3 nm, which was measured by Image J software based on a higher resolution TEM image (Fig. S1). The commercial ZIF-8 with particles size of
Conclusion
In summary, a novel porous nanocomposite membrane was constructed by one-dimensional aramid nanofibers and modified ZIF-8. BPEI was coordinatively bonded with the ZIF-8 surface and reduced the aggregation of particles in the casting solution. XRD, EDS and SEM characterization demonstrated that ZIF-8 had an intact crystalline structure and a homogeneous distribution in the membrane matrix. The embedded ZIF-8 produced a porous intermediate layer of the ANF membrane by steric hinderance. The
CRediT authorship contribution statement
Yi Li: Conceptualization, Methodology, Validation, Investigation, Writing - original draft, Writing - review & editing, Visualization. Jian Li: Resources, Conceptualization, Validation, Formal analysis, Data curation. Raul Bahamonde Soria: Resources. Alexander Volodine: Formal analysis, Data curation. Bart Van der Bruggen: Validation, Supervision, Writing - review & editing, Project administration, Funding acquisition.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgments
Yi Li acknowledges the support provided by the China Scholarship Council of the Ministry of Education. The authors thanks Nancy Weyns from the department of earth and environmental sciences, KU Leuven for the XRD memsurment. The authors also thanks for the kind help from Indah Prihatiningtyas for the tensile strength measurement, Steff Van Loy for the BET anlynasis and Christine Wouters for the ICP-OES measurement from the department of chemical engineering, KU Leuven.
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