Organic Solvent Nanofiltration Performance Research Articles

Solvent-resistant polyimide aerogel film as ultrapermeable support for thin-film composite and covalent organic framework nanofiltration membranes

Organic solvent nanofiltration (OSN) membranes require robust selective layers and support materials. In this study, polyamide and polyester thin-film composites (TFCs) and covalent organic frameworks (COFs) were fabricated as selective layers on a polyimide aerogel support. The ultrapermeable support was stable in various organic solvents including harsh polar aprotic solvents such as N,N-dimethylformamide (DMF), dimethylsulfoxide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone for more than six months. The support enabled fabricating both amorphous and crystalline selective layers at both room and higher temperatures. Moreover, the polyimide aerogel support had 85% porosity, facilitating high solvent flux. The compatibility of the selective layers and support was excellent, and as low molecular weight cutoff values (MWCO) as 356 and as high as 591 g mol−1 were obtained in methanol at 10 bar for the COFs and polyester TFC membranes, respectively. Both experimental MWCO and permeance values of the COF membrane agreed with those predicted using the pore flow model. The effect of the operating temperature on the OSN performance was examined in DMF at 20 and 100 °C. Both membranes demonstrated constant permeance and rejection for more than nine days of continuous filtration.

Thin-Film Composite Membranes with a Carbon Nanotube Interlayer for Organic Solvent Nanofiltration.

Compared to the traditional chemical-crosslinking-based polymer, the porous polytetrafluoroethylene (PTFE) substrate is considered to be an excellent support for the fabrication of thin-film composite (TFC) organic solvent nanofiltration (OSN) membranes. However, the low surface energy and chemical inertness of PTFE membranes presented major challenges for fabricating a polyamide active layer on its surface via interfacial polymerization (IP). In this study, a triple-layered TFC OSN membrane was fabricated via IP, which consisted of a PA top layer on a carbon nanotube (CNT) interlayer covering the macroporous PTFE substrate. The defect-free formation and cross-linking degree of the PA layer can be improved by controlling the CNT deposition amount to achieve a good OSN performance. This new TFC OSN membrane exhibited a high dye rejection (the rejection of Bright blue B > 97%) and a moderate and stable methanol permeated flux of approximately 8.0 L m−2 h−1 bar−1. Moreover, this TFC OSN membrane also exhibited an excellent solvent resistance to various organic solvents and long-term stability during a continuous OSN process.

Polyoxometalate-modified halloysite nanotubes-based thin-film nanocomposite membrane for efficient organic solvent nanofiltration

Polyoxometalate-modified halloysite nanotubes (POM@MHNTs) were synthesized and doped into the polyamide (PA) matrix by interfacial polymerization to prepare doped POM@MHNTs thin-film nanocomposite (TFN) membranes for organic solvent nanofiltration (OSN). Likewise, thin-film composite (TFC) and TFN membranes doped with HNTs were fabricated. It is worth noting that the methanol flux of the TFN can be efficiently enhanced by tuning the contents of POM@MHNTs. The as-synthesized optimal sample (TFN-0.10) exhibits superior methanol flux of TFN-0.10 membrane of 14.80 L m−2 h−1 bar−1 (1.76 times of that of undoped TFC membrane and 1.12 times of that of doped HNTs membrane (TFN-HNTs-0.10). These are mainly due to the tubular structure of POM@MHNTs providing additional solvent transfer channels and the ridge-valley morphology of the membrane surface increasing the contact area between the membrane and the solvent. Meanwhile, the POM@MHNTs nanohybrids are similarly electrostatically attracted by the amide groups on the polyamide chains produced by interfacial polymerization, improving the compatibility of POM@MHNTs with polyamide membranes. TFN-0.10 OSN membrane showed stable chemical properties in medium polar organic (methanol, ethanol, THF), acid polar organic (acetonitrile), and strong polar organic (DMF) solvents. Prepared TFN OSN membranes exhibited strong long-term operation capability and organic solvent resistance after 80 °C DMF immersion for 7 days with inconspicuous separation performance changes. This work offers the prospect of using organic-inorganic hybrid modified nanomaterials to improve OSN performance.

Leaf-veins-inspired nickel phosphate nanotubes-reduced graphene oxide composite membranes for ultrafast organic solvent nanofiltration

Organic solvent nanofiltration (OSN) membranes provide extraordinary opportunities for environment friendly and cost-competitive solvent recovery in petrochemical and pharmaceutical purifications. Currently, membranes that based on two-dimensional (2D) materials are extremely attractive in ultrafast molecular separations owing to the atomic thickness and remarkable physicochemical properties of the 2D nanosheets. Inspired by the structures and functions of vessel element that transport water in leaf veins, nickel phosphate nanotubes-reduced graphene oxide (NPTs-rGO) ultrathin laminar membrane is successfully prepared for ultrafast organic solvent nanofiltration. The in-situ synthetic NPTs can not only contribute to the size-dependent ultrafast molecular separation of the rGO membrane by providing additional molecular transport pathways as well as enlarging the interlayer space of the rGO layer, but also enhance the stability of the resultant membrane. The optimum membrane exhibits a methanol permeance of 670 L m −2 h −1 bar −1 and a high rejection toward molecules greater than 2 nm (MWCO: 650 Da). More importantly, the NPTs-rGO membrane shows promising anti-swelling property under OSN operating conditions. Inspired by leaf veins, this strategy provides insight into designing high-performance lamellar membranes with tubular-channel network, which may promote the fabrication and application of lamellar membranes in OSN. • Inspired by leaf veins, nickel phosphate nanotubes-reduced graphene oxide (NPTs-rGO) composite was applied to develop organic solvent nanofiltration (OSN) membrane. • The OSN performance of the resultant membrane was markedly improved. • The NPTs in the membrane could create tubular transport pathways for solvent. • The resultant NPTs-rGO membrane has superior anti-swelling stability and mechanical strength.

Novel organic solvent nanofiltration membrane based on inkjet printing-assisted layer-by-layer assembly

Novel layer-by-layer (LBL) organic solvent nanofiltration (OSN) membrane was developed via inkjet printing of polyethyleneimine (PEI) and single walled carbon nanotube (SWCNT) on a polyketone (PK) membrane surface, followed by post-treatment using three different cross-linking agents: glutaraldehyde (GA), (±)-epichlorohydrin (ECH) and trimesoyl chloride (TMC). The effects of PEI and SWCNT concentrations, bilayer numbers, and cross-linking conditions in the formation of the selective layers were evaluated in terms of membrane OSN performances. PEI concentration of 10.0 g/L and SWCNT concentration of 1.0 g/L with eight cycles of printing bilayers were chosen as optimal conditions. GA cross-linking was found to give the best membrane performance, and thus GA was considered as the best cross-linking agent for inkjet-printed LBL membrane modification among the three kinds of cross-linkers. The (PEI/SWCNT)8-GA exhibited Rose Bengal (RB) rejection over 99% with high organic solvent permeances. Compared to the cross-linking time, cross-linking agent concentration was found to have a greater effect on the membrane modification in terms of rejection performance. Moreover, the inkjet-printed LBL membrane showed negligible changes in membrane weight and OSN performance after immersion in different organic solvents over a period of three weeks, indicating its high mechanical and chemical stability. Finally, the possible applications of our printed LBL membranes in the pharmaceutical and hemp industries were evaluated. Overall, our work could further develop inkjet printing method for LBL OSN membrane fabrications.

Fabrication of organic solvent nanofiltration membrane using commercial PVDF substrate via interfacial polymerization on top of metal-organic frameworks interlayer

Interfacial polymerization (IP) is a rapid and convenient approach to fabricating thin-film composite (TFC) membranes with high permeance and satisfactory rejection for organic solvent nanofiltration (OSN). However, it is normally difficult to directly conduct the IP approach on the commercial membrane substrate (particularly, in the microfiltration range) because the monomers on the substrate are unevenly distributed during the IP reaction. In the current work, Cu-TCPP nanosheets were in-situ grown on commercial PVDF microfiltration substrate (pore size of 0.45 μm) by using preinstalled Cu2O nanoparticles as a copper ion source. The Cu-TCPP nanosheets covered the large pores and penetrated the surface layer of the PVDF substrate. The obtained PVDF/i-Cu-TCPP was then used as a substrate to conduct the IP and to fabricate the PVDF/i-Cu-TCPP/PA membrane. It was found that i-Cu-TCPP which serves as an interlayer could store the aqueous solution of the amine and was critical to the formation of a thin and defect-free polyamide separation layer. In comparison, loading of the Cu-TCPP on the PVDF substrate by filtration led to the formation of PVDF/e-Cu-TCPP/PA, which however exhibited inferior separation performance as compared to the PVDF/i-Cu-TCPP/PA membrane. More importantly, the PVDF/i-Cu-TCPP/PA membrane exhibited higher tensile strength (6.0 vs. 2.8 MPa) compared to the PVDF/e-Cu-TCPP/PA membrane, due to the enhanced interaction (adhesion) between the PVDF substrate and the i-Cu-TCPP interlayer. Under optimal synthetic conditions, the permeance of the PVDF/i-Cu-TCPP/PA membrane was almost 4 times that of the PVDF/PA membrane (1.93 vs. 0.49 L m-2 h-1 bar-1) without compromising rejection (>96%). The optimal TFC membrane also showed excellent OSN performance towards various organic solvents and good long-term stability to VB12/ethanol mixtures (50 h). Furthermore, the concentration test of erythromycin in methanol solution indicated that the PVDF/i-Cu-TCPP/PA membrane has great potential in the pharmaceutical industry for drug concentration and organic solvent recovery.

Layer-by-layer aided β-cyclodextrin nanofilm for precise organic solvent nanofiltration

Synthetic interlayers with multiple functions and structural stability have a high potential to construct novel thin film composite (TFC) membranes with demanding permselective property for precise separations. Herein, a functional interlayer derived from the polyelectrolyte-based layer-by-layer (LBL) self-assembly is reported to prepare interlayer engineered organic solvent nanofiltration (OSN) membranes using β-cyclodextrin (β-CD) as a monomer via interfacial polymerization (IP) process. Compared to the interlayer-free OSN membrane, a smoother selective layer with higher crosslinking degree is obtained for our developed membranes, enabling an improved organic dye rejection (e.g., 91.9% for methyl orange and 98.8% for brilliant blue R). Importantly, the LBL-aided β-CD nanofilm exhibits competitive permeances towards both polar (e.g., methanol, 5.8 LMH/bar) and nonpolar solvents (e.g., hexane, 7.0 LMH/bar). These performances can be attributed to the multiple roles of LBL-based interlayer as preventing the intrusion of selective layer and creating dual linkages (i.e., amide and ester), along with tailoring the physicochemical properties of caped β-CD nanofilm. Besides, the newly developed OSN membrane can discriminate between dye molecules with similar molecular weights and same charge but different molecular configurations. Meanwhile, its good structural integrity and long-term stability are also demonstrated by the 7-day filtration test using tetracycline in methanol and β-carotene in hexane, respectively, as evidence of constant solute rejections (≥90%) during the entire filtration process. This work unravels the functions of LBL-based interlayer in enhancing the OSN performance, offering new insights into shaping the development of interlayer engineered OSN membranes for real-life applications.

Phosphonium-defoliated GO nanosheets as micropore booster of carbon membrane to improve separation of tetracycline from isopropanol

This study developed a supported carbon membrane (CM) to conduct organic solvent nanofiltration (OSN). The OSN performance was tuned up by dispersing phosphonium-defoliated graphene oxide (PGO) as filler (0.3 wt%) in the precursor (bitumen) coating. PGO was synthesized through anchoring phosphonium (Ph3PBu+) ions to the oxygenated groups dangled on individual GO sheets that take up 49 wt% of PGO. As PGO experiences elimination of the pendant organic functional groups to form a graphene-like sheet through the course of pyrolysis and carbonization that converts the precursor coating to CM, the graphene-like sheets thus act to induce in-situ face-to-face stacking of the planar polyaromatic hydrocarbons (PAHs) generated from the pyrolysis of bitumen in nanoscale. PGO achieved a superior efficacy of induction than GO because of the defoliating role of phosphonium ion that assures a higher dispersity of graphene-like sheets in the precursor coating. This mechanism in the end casts a higher pore volume in membrane CMPGO than in CMGO (600 vs. 334 μl/g) primarily associating with the micro and meso pores inside the carbonaceous grains (~15–20 nm) that constitute the membrane. These tiny pores connect with the grain boundaries (3–5 nm) to form a porous network in the membrane. Moreover, the resultant membrane operates through a dual interaction model of adsorption and size screening. Membrane CMPGO manifests 99.2~94.2% retention of tetracycline (TC) from the permeate (isopropanol) with an average permeance of 32 L/m2·h·bar over 30 h, whereas the control, CMGO, shows progressive declining from 82% to 30% in 10 h. CMPGO was also evaluated by using the oppositely-charged dyes of different sizes to examine the effect of solute identity on percolation.

Integration of P84 and porphyrin–based 2D MOFs (M−TCPP, M = Zn, Cu, Co, Ni) for mixed matrix membranes towards enhanced performance in organic solvent nanofiltration

• Four porphyrin-based MOFs were synthesized as nanofillers for membrane fabrication. • 2D porphyrin–based MOFs have high compatibility with the P84 polymeric matrix. • The optimal MMMs exhibited 3-folds enhancement in permeance without rejection loss. • Ultrasonic treatment of Cu-TCPP further increased dispersion and OSN performance. • Stable OSN performance (36 h) was observed for separating VB12/ethanol solutions. Mixed matrix membranes (MMMs) are promising candidates for organic solvent nanofiltration (OSN), but the thick separation layer and poor compatibility between polymer and fillers are two critical challenges needed to be solved. Herein, a series of tetrakis(4-carboxyphenyl)porphyrin (abbreviation: TCPP) based metal–organic frameworks (MOFs, M−TCPP (M=Zn, Cu, Co, Ni)) have been synthesized in the form of two–dimensional nanosheets, which were then used as thin fillers in P84 matrix to fabricate M−TCPP/P84 MMMs for the OSN application. The effects of the types of M−TCPP and loading contents in the resultant MMMs on the OSN performance were systematically investigated. All the obtained MMMs exhibited higher permeance and slightly declined rejection of brilliant blue R (BBR) as compared with the pristine P84 membranes, among which Zn-TCPP/P84 MMM showed the greatest permeance improvement due to its flower-like structure assembled by wrinkled nanosheets as building blocks. Moreover, the permeance of MMMs showed volcanic type curves as increasing the MOFs loading content, while the rejection did not sacrifice too much. The optimum loading contents for Cu-TCPP/P84 and Zn-TCPP/P84 MMMs were 1 wt% and 1.5 wt%, respectively. In addition, the water-stable Cu-TCPP MOF can be further disassembled by an ultrasonic probe to achieve monodispersed Cu-TCPP nanosheets. It is interesting to find that the permeance of the MMM incorporating 1.5 wt% of the monodispersed Cu-TCPP nanosheets was enhanced nearly 2-fold as compared with the MMM with the untreated Cu-TCPP MOFs and the rejection to BBR only slightly decreased (94.7 vs. 95.7%). Besides, the general applications of Cu-TCPP/P84 membranes for other organic solvents (methanol, isopropanol, acetone, acetonitrile, hexane, etc. ), diverse organic dye/ethanol separation, vitamins B12/ethanol long-term separation test (36 h) indicated that the M−TCPP−based MMMs have great potentials in the field of organic solvent-related separation.

An ultrapermeable thin film composite membrane supported by “green” nanofibrous polyimide substrate for polar aprotic organic solvent recovery

The preparation of polymeric-based thin film composite (TFC) membrane with high permeance and superior solvent resistance in organic solvent nanofiltration (OSN) applications always involves the consumption of toxic chemicals or high energy, which does not meet the requirements of green chemistry. In this work, intrinsic insoluble polyimide (PI) nanofiber substrates were obtained by a two-step method including electrospinning of poly (amic acid) synthesized by polycondensation reaction between various monomers and subsequent thermal imidization, and the optimized one with low thermal imidization temperature was employed as substrate for TFC membrane in strong polar aprotic solvents recovery, therefore achieving the green process. The effects of different dianhydride and diamine monomers and different thermal imidization temperatures on the solvent resistance and microscopic morphology of nanofiber substrates are also studied. The optimized monomers in this work contribute to a low-temperature imidization of PI nanofiber substrate. The solvent permeance of corresponding TFC OSN membrane with the insoluble PI nanofiber substrate reaches up to 11.2 L m −2 h −1 bar −1 with 99.7% rejection in rose bengal/DMF system, and good long-term stability. This work provides potential guidelines for developing next-generation OSN membranes under a green process. • Insoluble polyimide (PI) nanofibrous substrate was prepared facilely with low-temperature thermal imidization. • A green modification without toxic chemical solvent involved is applied. • PI nanofibrous substrate can withstand strong polar aprotic solvents for more than three months. • The high porosity and interconnected pores endow substrate superior solvent permeance. • TFC membrane exhibits outstanding OSN performance and long-term stability.

Organic solvent nanofiltration (OSN) membrane with polyamantadinamide active layer for reducing separation performance inconformity

Organic solvent nanofiltration (OSN) membranes suffer from separation performance inconformity (SPI) after the OSN application process in industry. In this work, a membrane and an indicator are proposed to solve this problem. Briefly, an adamantane (AD) structure is introduced into the active layer via 3-amino-1-adamantanol (AAMO) interfacial polymerization with acyl chloride to build a semi adamantane-amide active layer (SAAAL) on the aramid substrate. The substrate is also enhanced by the introduction of a certain amount of AAMO. The TFC membrane with SAAAL exhibits an “onion ring” surface morphology, lowered water contact angle, increased surface energy (γS) and acceptable OSN performance, in which the pure methanol (MeOH) permeance reaches 29.5 L·m−2·h−1·bar−1 and the removal ability toward fast green FCF (FCF) approaches 84.4%. In addition, a novel concept named the SPI index is proposed here to estimate the level of the SPI phenomena. According to the evaluation results based on five dyes in three solvents, the as-developed SAAAL-covered membranes show lower SPI indices compared to conventional aromatic polyamide layer (APAL)-covered membranes in the nanofiltration (NF) range. Long-term stability operation results also identify the potential of the AD structure in fabricated membranes showing both excellent OSN performance and mitigation of SPI phenomena.

Waltzing around the stereochemistry of membrane crosslinkers for precise molecular sieving in organic solvents

Crosslinking of polymeric membranes induces changes in both membrane stability and separation performance. Numerous membrane-crosslinking methods have been developed with the objective of obtaining improved membranes. However, none of these methods systemically investigated the stereochemical effects of the crosslinker in the pursuit of better stability and performance. Herein, we address this knowledge gap by presenting a systematic investigation of the stereochemistry of crosslinkers. The intrinsically microporous poly(ether-ether-ketone)-containing Tröger's base (iPEEK-TB) was synthesized and employed in the fabrication of organic solvent nanofiltration (OSN) membranes. Crosslinkers were carefully selected based on the stereochemical position of the two benzyl bromide functional groups, separated by distances of 4.3, 8.2, 8.5, and 12.4 Å and significant effects arising from crosslinking on membrane physical properties, morphology, and OSN performance were investigated. Crosslinked membranes showed excellent solvent resistance, mechanical flexibility, and thermal stability. As a function of crosslinking distance, the molecular weight cutoff (MWCO) values of the membranes varied in the range of 575–750 g mol−1. The para isomer of the crosslinkers resulted in higher permeance relative to membranes crosslinked with their counterpart ortho isomers, and vice versa, the ortho substitution resulted in higher solute rejection values compared with para isomers. An increase of 50% and 12% in acetonitrile permeance relative to the annealed benchmark membrane was observed upon the treatment using iPEEK-TB with 4,4′-bis(bromomethyl)biphenyl (p-BBMBP) and 2,2′-bis(bromomethyl)-1,1′-biphenyl (o-BBMBP), respectively, whereas a permeance decrease of approximately 23% and 32% was noted upon treatment with α,α′-Dibromo-p-xylene (p-DBX) and α,α′-Dibromo-o-xylene (o-DBX), respectively. The corresponding MWCO changes were found to decrease for all crosslinked membranes within the range of 12%–40%. The crosslinked membranes demonstrated stable performance in polar aprotic solvents such as N,N-dimethylformamide and N-methyl-2-pyrrolidone. The Molecular dynamic (MD) simulations supported the obtained performance results through the variations in the fractional free volume (FFV). This work demonstrates the importance of crosslinker selection for OSN membrane performance and solvent resistivity and opens new avenues for fine-tuning membrane stability and OSN performance.

Impact of crosslinking on organic solvent nanofiltration performance in polydimethylsiloxane composite membrane: Probed by in-situ low-field nuclear magnetic resonance spectroscopy

Polydimethylsiloxane (PDMS) has been widely used in gas separation, pervaporation, and organic solvent nanofiltration. The crosslinking and penetration behavior of PDMS has been of great importance for fabricating composite membranes. In this study, an in-situ low-field nuclear magnetic resonance (LF-NMR) spectroscopy was employed to probe the crosslink density of PDMS during the crosslinking process. The relationship between crosslink density and nanostructure of PDMS selective layer was systematically investigated. It was found when the PDMS network had low crosslink density (υe < 1.32 × 10-3 mol cm-3), the un-crosslinked PDMS chains could easily infiltrate into the pores of the substrate, leading to a defective active layer. Therefore, the rejection for Evans blue from organic solvents was below 3.9%. With the increasing crosslink density to 1.57 × 10-3 mol cm-3, the integrity of the PDMS selective layer was remarkably reinforced with a rejection of 99.0%. Moreover, the as-prepared membrane exhibited good performance for concentrating trace vitamin B12 from different solvents. The swelling property was responsive monitored with T2 relaxation time using LF-NMR.

Realizing ultrathin silica membranes with straight-through channels for high-performance organic solvent nanofiltration (OSN)

In this study, a thin membrane with vertically aligned mesoporous silica nanochannels was modified with long silane group molecules (E-MSTF-LTA) supported on an anodic aluminum oxide (AAO) for organic solvent nanofiltration (OSN). The ultrathin thickness, low tortuosity, ordered vertical channels and super-organophilic surface of E-MSTF-LTA endow it with an ultrahigh ethanol permeance (110 LMH/bar) over current state-of-the-art OSN membranes and a high acetone permeance of 360 LMH/bar. Furthermore, a 99% rejection of Evans blue (EB) was demonstrated, and a cut-off of approximately 660 Da was achieved. Besides, the modified Hagen-Poiseuille (HP) equation was applied to evaluate the theoretical permeance, which is consistent with the experimental results. To explore in depth the underlying principle behind the OSN performance of the vertically aligned E-MSTF-LTA membrane, the parameters, including the solvent viscosity, total Hansen solubility, solvent diameter, and solvent polarity were introduced to evaluate their influence on the permeance, while for solute transport behavior, the impact of the size exclusion effect on the solute rejection rate was examined. The results show that the viscosity is the critical factor for determining the solvent permeation, while the size exclusion effect dominates solute rejection. These findings open up a way to design next-generation vertically aligned OSN membranes and shed light on the OSN performance, both empirically and theoretically, using this system.

Strategy to Chemically Decorate Nanopores of a Carbon Membrane for Filtrating Polyphenolics from Ethanol

This study invents a post-pyrolysis modification approach to render the resulting carbon membrane (CM) competent for organic solvent nanofiltration (OSN). A bitumen coating on a porous stainless-steel disk (PSD) serves as the precursor for the intended carbon membrane (CM), which is attained through pyrolysis in Ar. The bitumen coating casts dual-pore networks in the CM because of the dominant asphaltene constituent in bitumen. The subsequent chemical decoration of CM was pursued through the following protocol: dopamine (DA) was deployed in the nanopores of CM via pressurized infiltration and followed by Tris buffer passes through to trigger in situ conversion of DA to polydopamine (PDA), which was affixed over the pore walls to furnish chemical affinity (termed as CMPDA). Additionally, the catechol moiety of PDA was arranged to chelate with the Zn2+ ion, aiming to trim the -OH anchor (termed as CMPDA-Zn) to probe the effect of chelate on separation. The three membranes (CM, CMPDA, and CMPDA-Zn) were thereafter assessed by the separation of ethanol or isopropanol from phenolics [tannic acid (TA)/tetracycline (TC)]. A significantly improved OSN performance [rejection (%) ↔ permeance (L/(m2·h·bar))] of CM vs CMPDA was observed: (i) for TA feed, 13% ↔ 85 L/(m2·h·bar) vs 83% ↔ 12 L/(m2·h·bar); and (ii) for TC feed, 20% ↔ 78 L/(m2·h·bar) vs 78% ↔ 12 L/(m2·h·bar). Compared to CMPDA, CMPDA-Zn further advances the rejection performance (∼89% for TA and ∼80% for TC) over 50 h separation. They are benchmarked by the latest literature results. The performance enhancements can be attributed to the spreading of PDA or PDA-Zn sites in the dual-pore networks, so that they are able to offer H-bonding and steric blocking roles, a chemicomechanical effect, to seize solute molecules over pore walls. It is this interfacial drag effect that sustains the solute rejection.

Surface engineering of intrinsically microporous poly(ether-ether-ketone) membranes: From flat to honeycomb structures

Surface engineering of polymeric membranes can induce subtle changes in membrane properties and enhance their performance. Numerous membrane surface modification methods have been developed to improve the material performance. However, these methods can be complex, thus limiting their practical applications. Herein, we present a simple method for fabricating membranes with honeycomb surfaces by controlling the polymer molecular weight (Mw). Spirobisindane-based intrinsically microporous poly(ether-ether-ketone) (iPEEK-SBI) homopolymers with low and high Mws were synthesized and used to prepare organic solvent nanofiltration (OSN) membranes. The significant effects of polymer Mw on its physical properties, membrane morphology, and OSN performance were systematically investigated. iPEEK showed excellent solution processability, high Brunauer–Emmett–Teller surface area, and remarkable thermal stability. Three mechanically flexible OSN membranes exhibiting honeycomb surfaces with different honeycomb cell sizes were prepared using iPEEK-SBI homopolymers with low Mws at concentrations of 27–39 wt% in N-methyl-2-pyrrolidone. By contrast, the use of iPEEK-SBI homopolymers with high Mws yielded membranes with flat surfaces. The Mw cutoffs of the membranes were fine-tuned in the range of 408–772 g mol−1 by adjusting the dope solution concentration. Although the Mw cutoffs were unaffected by polymer Mw, the membranes derived from the polymer with low Mw exhibited substantially higher solvent permeance (18%–26%) than that of the high Mw membrane prepared at the same dope solution concentration. Stable performance was demonstrated over seven days of continuous cross-flow filtration and a six-month aging of the membranes. This work shows the importance of surface engineering for OSN membranes by adjusting polymer Mw. These findings open a new avenue for fine-tuning the properties of OSN membranes.

GWF-NH2 enhanced OSN membrane with trifluoromethyl groups in polyamide layer for rapid methanol recycling

Textile dyeing industry (TDI) produced huge amount of methanol effluent. An organic solvent nanofiltration (OSN) membrane owns ultra-high methanol permeance was developed in this study with a highly reactive and chemically stable symmetrical dianiline structure into its active layer. The so-called “BHTTM” molecule owns double trifluoromethyl (CF3) which offers polarity and negative charge while the matrix was doped with positively charged processed glass wool fiber (GWF-NH2) to form a specific active layer/area. FT-IR and EDX results demonstrated the successful introduction of BHTTM and their specific morphology were captured by FE-SEM. The membrane offered molecular weight cut-off value of 441.9 in water but smaller in organic solvents. It displayed outstanding OSN performance in which the methanol permeance reached 117.2 ± 9.8 L m−2 h−1 MPa−1 with 99.5 ± 0.1% rejection of Chlorazol black and methanol effluent with Rose bengal was also totally decolorized (99.5 ± 0.2%).

Effects of functionalization on the nanofiltration performance of PIM-1: Molecular simulation investigation

As a unique class of microporous polymers, polymers of intrinsic microporosity (PIMs) have attracted considerable interest for organic solvent nanofiltration (OSN). Here, we report a molecular simulation study to investigate OSN through six PIMs (pristine PIM-1 and five functional PIMs). The permeabilities of four organic solvents (methanol, ethanol, acetonitrile and acetone) through PIM-1 are found to correlate well with a combination of membrane and solvent properties. In the presence of a solute (mometasone furoate), the predicted solvent permeabilities through PIM-1 agree well with available experimental data. The solute is 100% rejected in all the solvents; however, it has different effects on solvent permeation. For protic solvents (methanol and ethanol), the permeabilities are marginally reduced by the solute; this is attributed to the favourable interaction between the solute and protic solvent, thus the solute is less accumulated at the membrane interface. Upon substituting the cyano groups in PIM-1 by various functional groups (hydroxyl, amine, amidoxime, tetrazole and carboxyl), the functional PIMs generally exhibit larger swelling degrees in methanol and higher methanol permeability, surpassing many existing OSN membranes. This simulation study provides microscopic insights into the effects of functionalization on OSN and reveals the complex interplay of solute-solvent-membrane interactions in governing OSN performance.

Novel designed TFC membrane based on host-guest interaction for organic solvent nanofiltration (OSN)

Active layers with outstanding organic solvent nanofiltration (OSN) performances were designed novelly by adopting host-guest (HG) interaction to realize adamantane structure immobilization. During the fabrication process, β-cyclodextrin (β-CD) served as the “host” while adamantylamine/adamantanemethylamine (AMA/AMmA) worked as the “guest”. Stechiometrical experiment results demonstrated that one half molar ratio of the “guest” to the “host” molecular (1.0%, w/v) led to the best OSN performance. The successful immobilization of adamantane structure was verified by XPS, FT-IR and EDX results, and the adamantane immobilized membrane showed weakened “nodule-like” pattern and reduced surface roughness values compared with the control membrane. Acetone, for the first time, was proposed as the activation solvent for enhancement of OSN performance. After acetone activation, the morphology and topography of the adamantane immobilized membranes seldom changed but their pure methanol (MeOH) flux were quintupled (29.0 ± 2.6 L m−2 h−1) and the rose bengal (RB) removed from MeOH reached to 98.2 ± 0.1%. Those HG complexes also resulted in the reduction of molecular weight cut-off (MWCO) values from 466.6 Da to 436.9 and 428.4 Da of PCA101/2+ and PCAm101/2+, respectively. Besides, the PCAm101/2+ demonstrated excellent OSN performances in the long-term operation of active pharmaceutical ingredients (APIs) concentration of clarithromycin (CLA)/acetone.

Organic Dye Solution Nanofiltration by 2D Zn-TCPP(Fe) Membrane – leverage of chemical and fluid dynamic effects

This study investigates organic solvent nanofiltration (OSN) using the membranes composed of 2D Zn-TCPP(Fe) nanosheets. A group of organic dyes with different molecular weights, hydrophilic properties and functional groups was selected as probe solutes in ethanol, isopropanol and ethyl acetate, respectively, to examine their OSN characteristics through the Zn-TCPP(Fe) membrane. The intermolecular interactions of organic solvent, dye and Zn-TCPP(Fe) nanosheets profoundly influence the mass transfer in the nano channels of the lamellar membrane. Representatively, the isopropanol solution of Brilliant Blue G dye turns out to have a favorable OSN performance (>90 % rejection with an average permenance of 175 L∙m-2∙h-1∙bar-1) over 4h, whereas replacement of isopropanol by ethanol results in incipient loss of rejection. However, for this ethanol solution feed, incorporating an optimal dose of ZIF-8nanograins into the nanosheets via co-deposition, a particle-over-lamellae structure formed unveils a significant improvement on OSN. Such boost is attributed to a combinative effect, i.e., alleviating dye-nanosheet complexation so as to reduce disturbing the lamellar structure as well as to assist desolvation of the dye over the feed side. To comprehend how solvent-slolute-nanosheet interactions dominate OSN, besides the experimental characterizations, both visco-molar volume and Stokes-Einstein diameter that describe fluid dynamic properties of solute molecule in a solvent are employed to assist the analysis. Possessing hydrophilic characteristic, Zn-TCPP(Fe) nanosheet engages strong hydrophilic association with hydrophilic dye and solvent, leading to weak OSN performance. Fundamentally, there are mutual interactions among solvent, solute and nanosheets in this membrane system, in which the prevailed pair of them impacts OSN performance.