Solvent Resistant Nanofiltration Research Articles

Polyethyleneimine crosslinkers: Towards greener crosslinking and simultaneous conditioning of solvent-resistant nanofiltration polyimide membranes?

Crosslinked polyimide is a state-of-the-art membrane material for solvent-resistant nanofiltration (SRNF) and gas separation aiming for applications that involve harsh solvents or plasticizing components in the gas stream, respectively. Preparation of these membranes usually requires a multistep procedure involving hazardous diamine monomers. In this study, two hyperbranched polyethyleneimines (PEIs) with different molecular weights (800 and 25 000 g mol−1) were explored as crosslinking agents in the simultaneous (SIM) crosslinking -procedure where crosslinking takes place during coagulation in the non-solvent bath. Because of their lower volatility compared to traditional diamine crosslinkers, the manipulation of these polymeric forms of crosslinkers is intrinsically less hazardous. It is hypothesized that the conditioning step, a required post-processing step in membrane upscaling to allow subsequent potting, handling, and storage, can be omitted when crosslinking with such polymeric crosslinkers. Results showed how presence of PEI in the non-solvent bath influenced the course of phase inversion, resulting in a range of different membrane morphologies and performances. Drying unfortunately resulted in brittle membranes which lost most of their permeance. Even when residual PEI was allowed to remain in the membrane as a conditioning agent, low dried membrane permeances were obtained. By using PEI or other state-of-the-art crosslinkers prior to drying, light was shed on the structure-performance relationships governing permeance decline when drying SRNF membranes.

Integrated membrane process for pretreating and desolventizing hexane-soybean oil miscella – A pilot plant study

The study aimed to develop a membrane desolventizing technology for solvent recovery in vegetable oil extraction. Pretreatment of crude miscella is a prerequisite for an organic solvent nanofiltration (OSN) based desolventizing process. Solvent-resistant ultrafiltration (SRUF) membrane employed in micelle-enhanced ultrafiltration (MEUF) of hexane-crude soybean oil miscella displayed high phospholipids (PL) rejection (98.5 %) and productivity (∼28 L/m2/h), accompanied with a moderate oil rejection (∼22 %). The PL-rich retentate fraction was alkali-treated to recover the oil in the stream (8.8 %). This complementary pretreatment approach met the quality requirements and maximized the yield of treated oil (>98 %) while reducing the chemicals in the process. A subsequent two-stage desolventizing of MEUF-treated miscella with solvent-resistant nanofiltration (SRNF) membrane reduced the oil-in-permeate to 1.37 %, achieving the set recycling criteria (<2 %). The results obtained from the pilot scale study (batch size 40 kg of crude miscella) revealed that the MEUF met the pretreatment requirements, and the subsequent OSN process recovered ∼63 % of hexane through a non-thermal route for recycling in the extraction process. The pilot-scale evaluation demonstrated the technical feasibility of the proposed membrane desolventizing process employing indigenous spiral-wound SRUF and SRNF membrane modules.

Strong impact of exposure to water/solvent mixtures on permeance of nanofiltration membranes

Water/solvent mixtures released from industrial processes, especially in pharma and fine chemical synthesis or in advanced oil production, are an environmental concern, necessitating the development of efficient remediation and recycling techniques. Solvent-tolerant nanofiltration (STNF), situated between aqueous nanofiltration (NF) and solvent-resistant nanofiltration (SRNF), offers a promising solution. This study investigates the potential of using aqueous NF- and SRNF-membranes to treat water/solvent mixtures. Commercial membranes (NF270, DuraMem® 300, DuraMem® 200) and a lab-synthesized polyvinylidene fluoride (XL-PVDF NF) membrane were used to treat dimethyl sulfoxide (DMSO), acetonitrile (ACN), Isopropyl alcohol (IPA), or dimethylformamide (DMF) mixtures with water. The addition of 10% DMSO, ACN, IPA, or DMF to water significantly reduced the permeance of these NF-membranes, and an increase in temperature failed to restore the initial permeance. Changes in both viscosity and surface tension of the water/solvent solutions strongly influence the permeance of the NF-membranes. Flory-Huggins and Zimm-Lundberg theories offered an explanation for the phenomenon where solvent clusters form within the dense polymer matrix of NF-membranes upon exposure to water/solvent mixtures, resulting in decreased water permeabilities. NMR analysis identified of the presence of small amounts of solvents in the STNF-membranes, even still after prolonged exposure to pure water permeation. The results emphasize the current shortcomings of aqueous and solvent-resistant membrane chemistries in handling water/solvent mixtures. Minor solvent additions significantly modify NF membrane permeance, emphasizing the need for innovative material design, especially given the variable solvent content in industrial applications.

Stereochemistry as a Parameter to Tune the Molecular Weight Cutoff in Solvent-Resistant Nanofiltration Membranes

Stereochemistry as a Parameter to Tune the Molecular Weight Cutoff in Solvent-Resistant Nanofiltration Membranes

Stepwise UV crosslinking in solvent: A novel method to enhance the solvent resistance of poly(aryl ether ketone) membranes

In this contribution, the viability of using UV to give separation membranes a high crosslinking degree and broad-spectrum solvent resistance was investigated. A series of poly(aryl ether ketone) polymers suitable for UV crosslinking were first designed. And a unique photocrosslinking method, stepwise UV crosslinking in solvent, was developed by understanding photochemical reactions and molecular dynamics. This method does not require the introduction of additional photoinitiators and crosslinking agents, and the membrane preparation is a green and efficient process with a theoretical 100% atomic utilization rate. Combining this method with the special polymer structure design, a solvent-resistant nanofiltration membrane that can withstand strong polar aprotic solvents can be obtained. The molecular weight cut off of the membrane in N,N-dimethylformamide was 300 g/mol, and the permeance was 1.154 L m−2 h−1 bar−1. This separation effect even exceeded that of most composite membranes. After testing, the membranes prepared using this method had the ability to maintain functional stability at 80 °C in N,N-dimethylformamide for one month. The excellence of membrane performance and the flexibility of regulatory methods both reveal the enormous potential of this method.

Preparation of a Solvent-Resistant Nanofiltration Membrane of Liquefied Walnut Shell Modified by Ethylenediamine.

N,N-dimethylformamide (DMF) has excellent chemical stability and is widely used as an aprotic polar solvent. In order to reduce production costs and reduce pollution to the surrounding environment, it is necessary to recycle and reuse DMF. Previous research has found that the thin film composite nanofiltration membrane prepared from liquefied walnut shells exhibited a high rejection rate in DMF, but relatively low permeance and mechanical strength. In order to increase permeance without compromising the separation performance, ethylenediamine (EDA) is used as a modifier to graft onto the structure of liquefied walnut shell through the Mannich reaction. Then, modified liquefied walnut shell as an aqueous monomer reacts with trimesoyl chloride (TMC) via the interfacial polymerization method on the EDA-crosslinked polyetherimide (PEI) membrane. The results show that the permeance of the prepared membrane is significantly improved by an order of magnitude, demonstrating a rejection rate of 98% for crystal violet (CV), and a permeance of 3.53 L m-2 h-1 bar-1 in DMF. In conclusion, this study reveals the potential of utilizing liquefied walnut shells as raw materials for preparing high-performance separation membranes and demonstrates that surface modification is a feasible approach to enhance permeance of membranes without sacrificing the rejection rate.

Molecular weight resolution of solvent resistant nanofiltration (SRNF) membranes

Engineering membranes for molecular separations in organic media is still a big challenge, especially when two closely resembling solutes have to be separated by allowing one to pass with the solvent and the other to be rejected. The so-called “resolution” of the membrane then becomes critical. Resolution reflects the difference in rejection values of the two (or more) solutes for that solvent-membrane system. Recently developed crosslinked poly(vinylidene difluoride) (XL-PVDF) and well-known crosslinked polyimide (XL-PI) membranes were prepared and tested for solvent resistant nanofiltration (SRNF). Rejections of 7 dyes (i.e. Rose Bengal (RB), Acid fuchsin (AF), Rhodamin B (RhB), Sudan Black (SB), Zinc tetraphenylporphyrin (ZTPP), and 55,10,15,20-Tetrakis(3,5-di-tert-butyl phenyl)porphyrin (TBPP) in a broad range of solvents, i.e. a set of polar solvents (water, ethanol, isopropanol), and a set of non-polar ones (butyl acetate, chloroform, xylene and toluene) were studied. As anticipated, the molecular weight cut-offs (MWCOs) of both membrane-types varied with the applied solvent type. Butyl acetate and chloroform filtrations respectively yielded the sharpest MWCO for XL-PVDF and XL-PI. The varying MWCO-curve per solvent is explained by the changing affinity of solvents and solutes for the membrane, as indicated by swelling and sorption experiments respectively.

Fluorinated solvent resistant nanofiltration membrane prepared by alkane / ionic liquid interfacial polymerization with excellent solvent resistance

The preparation of solvent resistant nanofiltration (SRNF) membranes with high permeance and retention is required for accurate chemical separation, yet the solvent resistance of the membranes still needs to be improved. Fluoropolymers with strong weather resistance and chemical stability are expected to improve the solvent resistance of solvent resistant nanofiltration membranes. However, the application of fluorinated monomers in interfacial polymerization has been limited because they are water-insoluble. In this paper, ionic liquids were added to the aqueous phase to improve the solubility of hexafluorobisphenol A (BPAF). Trimesoyl chloride (TMC) was used as organic phase monomer. Fluorinated polyarylester thin film composite (TFC) membranes were synthesized on polyacrylonitrile (PAN) base membranes utilizing interfacial polymerization. The results confirmed that the surface morphology and roughness of TFC membranes could be regulated by adding fluorinated monomer. The membrane sample PAN-F-0.5 exhibits a high permeance for methanol up to 124.05 L m−2 h−1 bar−1. The resulting membrane possesses a molecular weight cut off (MWCO) of∼450 g mol−1. PAN-F-0.5 still showed excellent performance after continuous immersion in DMF for 90 days (room temperature) and 7 days (80 °C), respectively. The results show that the TFC membrane has excellent filtration performance and solvent resistance.

Poly(aryl ether ketone) hollow fibers preparation with acid resistant spinnerets

Poly(ether ketone ketone) (PEKK) and poly(ether ether ketone) (PEEK) are high-performing thermoplastics applicable for solvent-resistant nanofiltration due to their outstanding resistance to harsh conditions. However, the polymers require strong acids for solubilization and use as dope solution for membrane preparation. This is the major limitation when targeting hollow fibers (HF) since the metallic spinning line and spinnerets could be prone to corrosion in long term. In this work, we propose the fabrication of an acid-resistant spinneret by 3D printing technology through stereolithography using acrylate and methacrylate-based resins. The hollow fibers are produced by interplaying spinneret designs and spinning conditions with an acid-resistant spinning line. The fabricated hollow fibers exhibited N,N-dimethylformamide (DMF) permeance of 1.4–2.7 L m−2 h−2 bar−1, with a molecular weight cut-off around 246 g mol−1 confirmed with more than 90% rejection of 1,3,5-tri-tert-butyl benzene (TTBB). Furthermore, the hollow fiber modules were tested at high-temperature filtration in DMF with confirmed membrane flux or structural dimension stability.

Performance investigation of a novel interfacially polymerized poly(urea-co-urethane) thin film nanocomposite on PAN membranes for solvent-resistant nanofiltration membrane application

Performance investigation of a novel interfacially polymerized poly(urea-co-urethane) thin film nanocomposite on PAN membranes for solvent-resistant nanofiltration membrane application

Calcium-alginate/HKUST-1 interlayer-assisted interfacial polymerization reaction enhances performance of solvent-resistant nanofiltration membranes

This study aimed to further enhance the solvent stability and flux of organic solvent-resistant nanofiltration (OSN) membranes. Based on hexane diamine cross-linked polyetherimide (PEI) ultrafiltration membranes, in-situ calcium alginate (CaAlg) hydrogel doped with HKUST-1 was constructed as an interlayer, and then the OSN membrane with an integral covalent bond structure was constructed from interfacial polymerization reaction and NN dimethylformamide (DMF) activation. The membrane showed ultra-thinness (polyamide (PA) layer thickness reduced from 97 to 11 nm), large surface area (surface roughness increased from 75.6 to 113 nm), high hydrophilicity (water contact angle reduced from 60.1° to 54.45°), and negative charge. The CaAlg-HKUST-1 interlayer limited the piperazine diffusion rate by hydrogen bonding and covalent bonding, etc., reduced the thickness of the PA layer, and linked the base membrane and the PA layer by its internal integral covalent bonding, which effectively enhanced the structural stability of the OSN membrane. The PA/CaAlg-HKUST-16/PEI membrane achieved a flux of 29.4 L·m−2·h−1 in ethanol and a rejection of 94.7 % in 0.1 g·L-1 methyl blue. Different dye rejection tests, solvent flux tests, DMF and acetone immersion tests, fouling resistance tests, and continuous operation tests show PA/CaAlg-HKUST-16/PEI membrane has better solvent resistance, dye separation performance, and stable membrane flux. Hence, it has great application prospects in the fields of organic solvents and other types of wastewater treatment.

Review on Liquid-Liquid Separation by Membrane Filtration.

Liquid-liquid separation is crucial in the present circumstances. Substitution of the conventional types of separation like distillation and pervaporation is mandatory due to the high energy requirement of the two. The separation of organic mixtures has a huge potential in industries such as pharmaceutical, fine chemicals, fuels, textile, papers, and fertilizers. Membrane-affiliated separations are one of the prime techniques for liquid-liquid separations. Organic solvent nanofiltration, solvent-resistant nanofiltration, and ultrafiltration are a few methods through which organic liquid-liquid separation can be attained. Implementation of such a technology in chemical industries reduces the time consumption and is cost efficient. Even though a lot of research has been done, attention is needed in the field of organic-liquid separation aided by membranes. In this review, various membranes used for organic mixture separations such as polar-nonpolar, polar-polar, and nonpolar-nonpolar are discussed with a focus on membrane materials, additives, separation theory, separation type, experimental setup, fouling mitigation, surface modification, and major challenges. The review also offers insights and probable solutions for existing problems and also discusses the scope of research to be undertaken in the future.

Two dimensional laminar MoS2 modified PTMSP membranes with improved organic solvent nanofiltration performance

Solvent resistant nanofiltration membranes with both improved permeability and long-term stability are in desperate demand for organic solvent recovery. In this study, a high-performance molybdenum sulfide (MoS2)-poly-trimethylsilylpropyne (PTMSP) nanocomposite membrane was prepared on polysulfone (PSf) through a facile solution casting method, which was used for efficient organic solvent recovery. Two-dimensional (2D) MoS2 was employed to construct interlayer channels that facilitated the passage of solvents and blocked small molecule solutes. Compared with the pristine PTMSP membrane, the methanol permeance of modified membrane increased by 47% (5.89 L m−2 h−1 bar−1) while remaining the retention rate of RB of 97.14%. The obtained membrane exhibited superior mechanical strength as well as excellent stability. Moreover, the resultant membrane also showed more precise molecular sieving capability than most reported polymer membranes. This work demonstrated that incorporating 2D MoS2 nanosheets into PTMSP polymer is a promising method to obtain organic solvent nanofiltration (OSN) membranes with excellent separation performance and favorable long-term stability.

Integration of thin film composite graphene oxide membranes for solvent resistant nanofiltration

Solvent resistance of graphene oxide (GO) makes it a promising material to construct membranes for precise separations with solvent-involved conditions. A thin GO laminate layer supported by a porous substrate displaying a thin film composite (TFC) membrane configuration is commonly employed, and integration of the composite structure is necessary to acquire desirable performance. While the substrate effects on solvent transport in supported laminates were rarely involved, here we prepared integrated GO laminates using diamine crosslinking as a precondition and then investigated the effects of supporting substrates with different porous structure properties on TFC membrane performance. It was found that even if the laminates were integrated with similarly stable interlayer spacing in solvents, the substrate played a key role in forming GO laminates with desirable molecular rejection properties in solvents. The dye molecule rejection performance of the TFC GO membrane was significantly changed from 3% to 92% by optimizing the supporting substrate with limited surface porosity and pore size. Additionally, the altered stacking and channel formation of the GO nanosheets by the substrate exhibited solvent permeation following different transport models. This study may provide considerations in designing functional GO membranes for solvent resistant filtrations with an emphasis on substrate optimization.

Fabrication of sustainable organic solvent nanofiltration membranes using cellulose–chitosan biopolymer blends

Membrane technologies have emerged as a promising alternative to energy-intensive separation processes in various industrial sectors. To address the sustainability challenge associated with the fabrication of separation membranes, a paradigm shift from the use of petrochemical-based raw materials to greener biobased sources is highly desired. In this study, blends of cellulose — as a plant-based material — and chitosan — obtained from shrimp farming waste and used as a biomass material — were investigated for the fabrication of oil and solvent-resistant nanofiltration membranes. The structural, thermal, mechanical, and morphological properties of the prepared membranes were characterized. Molecular simulations were performed to study the fractional free volume and interaction energy among membrane constituents. Adjusting the cellulose–chitosan ratio allowed fine-tuning the molecular sieving properties of the membranes, which exhibited outstanding separation performance and chemical stability even in harsh solvents, such as polar aprotic solvents, at the maximum temperature of 100 °C. Cellulose membranes containing 25 wt% chitosan achieved the lowest molecular weight cutoff value of 413 g mol−1 and a permeance of 24 L m−2 h−1 bar−1 in acetonitrile. The membranes showed stable separation performance over 7 days of continuous cross-flow nanofiltration. Moreover, the cellulose membranes blended with 10–25 wt% chitosan showed decreased water permeance from 52 to 38 L m−2 h−1 bar−1 and increased oil-removal efficiency from 73.8% to 98.6%. Furthermore, the membranes successfully underwent biodegradation, confirming their potential to close the loop of the sustainable lifecycle of membranes from cradle to grave.

Carbon molecular sieve membranes with integrally skinned asymmetric structure for organic solvent nanofiltration (OSN) and organic solvent reverse osmosis (OSRO)

Organic solvent nanofiltration is an energy-efficient separation process for solutes and solvents. Robust membranes that show stability in harsh environments are highly desirable. In this study, we developed free-standing integrally skinned asymmetric carbon molecular sieve membranes that synergize the advantages of stable carbon materials and porous polymer membranes. The membranes were prepared using a polyimide of intrinsic microporosity (PIM), known as 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)-3,3′-dimethylnaphthidine (DMN), via a phase inversion technique. Carbonization preserved the surface porosity and finger-like porous morphology of the membranes after structural rearrangement. The effects of pyrolysis temperature, membrane thickness, dope solution concentration, and polymer porosity on separation performance were investigated. The molecular sieving performance of the membranes was investigated using five solvents with different polarities. The membranes showed no swelling and high stability in strong acids, bases, and organic solvents, as well as an excellent rejection profile and reasonable permeance. The membrane pore size, molecular-weight cutoff, and performance were fine-tuned by controlling the pyrolysis temperature, dope solution concentration, and polymer porosity. The polymer porosity and the asymmetric structure of the membrane strongly affected the molecular sieving performance. Carbonizing porous 6FDA-DMN afforded a 10-fold higher permeance than that obtained by carbonizing nonporous 6FDA-m-phenylenediamine (mPDA). To the best of our knowledge, the developed membrane fabrication platform yielded one of the tightest, most robust, and highly solvent-resistant nanofiltration membranes reported thus far.

The significant role of enantiomeric excess of the monomer in binaphthalene-based polymer membranes

Numerous studies have been carried out to improve the membrane performance and stability of solvent-resistant nanofiltration membranes. This research aims at studying the influence of molecular structure on the membrane performance in a step-wise fashion, making it possible to specifically fine-tune membranes towards their application. For this purpose, a tunable binaphthalene di(acid chloride) is combined with several amine monomers. This choice of monomers allows to simultaneously study (1) the influence of the enantiomeric excess, since binaphthalenes show atropisomerism and (2) the influence of the number and position of the amine functionalities. By varying the amount and position of the amine functionalities relatively small changes in the filtration performance are observed. However, in case optically pure acyl chloride is used, the membrane performance excels independent of the amine used. The water and acetonitrile permeance of the optically pure membranes even doubles in comparison to the optically impure membranes, independent of the degree of e.e. (0, 25, 50 or 75 %e.e.), while maintaining a similar rejection. Hence, this study adds a polymer's enantiomeric excess to the toolbox of parameters that can be used to tailor membrane properties.

Rapid fabrication of fluorinated covalent organic polymer membranes for organic solvent nanofiltration

Covalent organic polymers (COPs) are prepared via the non-uniform covalent assembly of organic building blocks, endowing their structures with permanent pores, thereby rendering them suitable for diverse applications. The use of COPs in the fabrication of composite membranes can enhance their permeability, selectivity, and chemical stability. However, the existing COP synthesis processes are typically tedious, thus necessitating the development of rapid and simpler routes. We demonstrate an easily performed synthesis route for the rapid (less than 10 s) fabrication of COP-based composite membranes via interfacial polymerization. The membranes were directly prepared on a polyacrylonitrile substrate without transferring the COP layer onto a porous support. The hydrophobicity of the membranes was achieved by the integration of fluorine-rich groups along the polymer backbone. The obtained solvent-resistant composite membranes exhibited a toluene permeance of 11 L m −2 h −1 bar −1 and congo red (687 g mol −1 ) rejection levels of more than 95%. The remarkable performance, crosslinked polymer structure, and manufacturing scalability of the fabricated thin films make them attractive as solvent-resistant nanofiltration membranes. Ultrafast fabrication of covalent organic polymer composite membranes for high-permeance nanofiltration in toluene. • Rapid synthesis of covalent organic polymer (COP) membrane via interfacial polymerization • Hydrophobicity of COP by integrating fluorine atoms • Hydrophobic pathways for highly selective organic solvent permeance • COP membranes with excellent toluene permeance and high dye rejections • Solvent-stable COP thin films with long-term stability during organic solvent nanofiltration

Braid-reinforced polybenzimidazole (PBI) hollow fiber membranes for organic solvent nanofiltration (OSN)

A braid-reinforced polybenzimidazole (PBI) hollow fiber membrane has been reported, which was made by our in-house developed spinning devices. The braid elements not only remarkably increase the mechanical strength of the PBI hollow fiber membrane to more than 100 Mpa but also maintains its low swelling ratio of less than 2% even in harsh organic solvents compared with the self-support PBI hollow fiber. The effects of PBI dope concentration, flow rate and wetting agents for braid co-extrusion on the structure and separation performance of braid-reinforced hollow fibers have been investigated. The spinneret design and the braid handling device are also discussed. The recently developed braid-reinforced PBI hollow fiber membrane has a rejection of Rose Bengal (RB) over 90% in methanol (MeOH) with a permeance up to 3.6 Lm-2h-1bar-1 compared with the self-support hollow fiber under high pressure. In addition, the braid-reinforced membrane has a slightly higher rejection and a stable separation performance over 40 h than the self-support one because the former has a lower swelling ratio than the latter one. Due to the favorable interfacial bonding between the PBI coating layer and the reinforced elements, the braid-reinforced PBI hollow fiber membrane can withstand an operation pressure of exceeding 10 bar, which exhibits its great prospects for solvent resistant nanofiltration (SRNF) applications.

Effective interfacially polymerized polyarylester solvent resistant nanofiltration membrane from liquefied walnut shell

Effective interfacially polymerized polyarylester solvent resistant nanofiltration membrane from liquefied walnut shell