Amine-functionalized ZIF-8 nanoparticles as interlayer for the improvement of the separation performance of organic solvent nanofiltration (OSN) membrane

Highlights

• The OSN membrane was fabricated with covalent bonding between layers.

• The mZIF-8 interlayer helped to form an ultra-smooth and ultra-thin skin layer.

• OSN membrane permeance was effectively increased without sacrificing rejection.

• The OSN membrane kept super stability in DMF at 80 °C for 120 h.

Abstract

We successfully fabricated a kind of novel sandwich-like thin-film nanocomposite (TFN) polyamide (PA) membranes via interfacial polymerization (IP) method on polyimide (PI) support surface which was modified by a layer of nanosized amine-modified zeolitic imidazolate framework - 8 (mZIF-8) nanoparticles for organic solvent nanofiltration (OSN). Uniform distribution of the nanoparticles was obtained via directly immersing the PI support membrane in aqueous mZIF-8 suspension prior to the IP. A post-IP cross-linking procedure was conducted to enhance further the solvent resistance of the fabricated TFN OSN membranes. Ultra-smooth (average surface roughness of 13.7 nm) and ultra-thin (average thickness of about 33 nm) skin layer was achieved, together with high Rhodamine B (RDB, 479 Da) rejection (99.1%) and reasonable ethanol permeance (~28 L m⁻² h⁻¹ MPa⁻¹), as well as nearly unchanged RDB rejection after being submerged in DMF at high temperature (80 °C) for 120 h, indicating their very good resistance in very harsh strong polar solvent circumstance and the promising aspect of their industrial application.

Introduction

A green technology emerging recent years for the separation and purification of organic solvent system is organic solvent nanofiltration (OSN) [1], which has attracted great attention for vast potential to replace traditional separation processes such as distillation, extraction, and crystallization. OSN membranes are expected to maintain both good separation performance and good solvent resistance for industrial applications. To achieve this goal, membrane materials selection, membrane preparation methods, as well as the interactions between OSN membranes and solvents are of vital importance [[2], [3], [4]].

Currently, polyimide (PI), polyetherimide (PEI), polyacrylonitrile (PAN), polyphenylsulfone (PPSU) and polybenzimidazole (PBI) have been used as OSN membranes polymers [5]. Most of these polymers are used to fabricate asymmetric membranes through phase separation technique [[6], [7], [8]]. However, asymmetric membranes usually have smaller solvent permeance than thin-film composite (TFC) membranes [9] which are fabricated via interfacial polymerization (IP). It has been reported that polyamide (PA) TFC OSN membranes fabricated via IP on chemically stable support membrane surface and further activated by N, N−dimethylformamide (DMF) could achieve a dramatically increased solvent flux without sacrificing solute rejection [9,10].

TFC membranes can encounter compaction or physical aging, and the trade-off effect between permeability and selectivity [11]. Recently, thin-film nanocomposite (TFN) membranes manufactured by incorporating nanoparticles in the skin layer have attracted much attention due to their improved solvent permeance and solute rejection. The incorporated nanoparticles could create additional pathways within the PA layer to increase molecular transport through the intrinsic nanopores as well as the interfacial voids [12]. Till now, nano TiO₂ [13], carboxyl-functionalized multiwalled carbon nanotubes (MWCNT) [[14], [15], [16]], graphene oxide (GO) nanosheets [[17], [18], [19], [20]], graphene quantum dots (GQDs) [[21], [22], [23], [24]], metal–organic frameworks (MOFs) [25], as well as covalent–organic frameworks (COFs) [26] have been incorporated. For instance, Li et al. [27] incorporated amino-functionalized GQDs in the skin layer of the TFN membrane that they fabricated and achieved a great improvement in its solvent resistance and permeance. However, there are still some technical challenges to fabricate defect-free membranes using nanoparticles, since nanoparticles could easily aggregate and thus could result in increasing defects in the selective layer.

MOFs, as crystalline inorganic–organic hybrid materials, have good affinity to the polymeric chains, and the MOF−polymer interfacial interactions can be easily controlled, thus avoiding nonselective voids between the phases [25]. The early work of Livingston's group [25] indicated that TFN OSN membranes with the incorporation of nanosized MOFs nanomaterials increased the methanol permeance remarkably from 15 to 39 L m⁻² h⁻¹ MPa⁻¹. From then on, the research works of using MOFs for fabricating OSN membranes have become a hotspot [[28], [29], [30], [31], [32], [33], [34], [35], [36], [37]]. Some of them are not on the incorporation of MOFs for TFN membranes, but on using MOFs layer/film as the selective layer [36]. Some other fabrication of the MOFs layers can be seen from a latest comprehensive review by Xu's group [38].

Recently, Livingston's group [39] covered a nano-strand layer on a polyimide support membrane to control accurately the IP process and fabricated a type of sub-10 nm nanofilms with high separation performance. From then on, ideas of interlayer have gained much attention on the fabrication of membranes orienting aqueous systems to reduce the skin layer thickness and to increase the permeation flux [32,[40], [41], [42], [43], [44]]. For instance, Wang et al. [44] used eolitic imidazolate framework-8 (ZIF-8), a kind of MOFs materials, as a sacrificial template layer on single-walled carbon nanotubes modified PSF support membrane and fabricated TFN NF membranes which achieved 535 L m⁻² h⁻¹ MPa⁻¹ for water permeance and at least 95% for Na₂SO₄ rejection. However, most of the interlayered membranes are focused on aqueous nanofiltration (NF) membranes, few research works were carried out on OSN membranes [32]. Latterly, Sarango et al. [32] prepared a type of TFN OSN membrane via IP to form a PA layer on polyimide substrate surface which was modified by a layer of ZIF-8 nanoparticles (with sizes of 70 ± 10 nm) and achieved a rejection of 90% for Sunset Yellow (452 Da) and a methanol permeance of 87 L m⁻² h⁻¹ MPa⁻¹. Although the dye rejection is relatively low, this investigation gives us an indication that ZIF-8 can be used as interlayer for OSN membrane fabrication, since ZIF-8 has good chemical and thermal stability, they are stable in boiling water, benzene and sodium hydroxide solutions for more than 7 days [45,46].

For OSN membrane, not only high solvent permeance but excellent solvent resistant is preferred. Usually, hydrophobic, hydrogen bonding and π-π stacking interactions are considered between MOFs and the surrounding [47]. Owing to the chemistry nature, most MOFs could not chemically participate in the IP reaction. Covalent bonding of MOFs in the IP reaction could surely result in TFN OSN membrane with exceptional chemical stability. Nevertheless, little research work was carried out on this subject.

In this work, we conducted covalent bonding interaction between MOFs and the membrane polymer, and fabricated a novel TFN OSN membrane using IP reaction on PI support membrane surface which was modified by an ultra-thin interlayer of nanosized hydrophilic amino-modified ZIF-8 (mZIF-8) nanoparticles. We extensively investigated the factors influencing the surface morphology and separation performance of the prepared OSN membrane to study the effect of the interlayer on the regulation of the IP process. We also investigated their resistance to strong polar solvents.