Ladder polymers of intrinsic microporosity from superacid-catalyzed Friedel-Crafts polymerization for membrane gas separation

A twisted imidazole-tethered aromatic polymer for high-performance membranes in vanadium-based redox flow batteries

We report on a twisted aromatic polymer, the first xanthene-based example bearing protonated imidazole groups. Its special architecture enables superior ion selectivity, conductivity, and chemical stability, providing excellent performance in both all-vanadium and iron-vanadium redox flow batteries.

High-performance anion exchange membranes based on poly(oxindole benzofuran dibenzo-18-crown-6)s functionalized with hydroxyl and quaternary ammonium groups for alkaline water electrolysis

Anion exchange membranes (AEMs) play a vital role in AEM water electrolysis (AEMWE), a promising technology for hydrogen production. However, developing AEMs that simultaneously achieve high OH- conductivity, robust alkaline stability and sustained operational performance in AEMWE remains challenging. In the present study, we report on a class of high-performance, readily synthesized AEMs based on poly(oxindole benzofuran dibenzo-18-crown-6)s functionalized with hydroxyl and quaternary ammonium groups. A super-acid-catalyzed Friedel-Crafts reaction was used to copolymerize dibenzofuran (DBF), dibenzo-18-crown-6 (DBC) and isatin to yield a series of poly(oxindole benzofuran dibenzo-18-crown-6)s (P(O-Fx-Cy)s). The hydrophilic and π-conjugated DBF units provide the membranes with additional free volume, while the bulky DBC units promote a favorable microphase morphology and impart resistance to hydroxide attack. Quaternization is achieved via a ring-opening reaction with glycidyl trimethyl ammonium chloride (GTA) without requiring a catalyst. The resulting side chains, featuring alkyltrimethylammonium cations with hydroxyl groups in the β-position, introduce hydrogen-bonding networks and enhance OH- conductivity. The optimized P(O-F50%-C50%)-GTA membrane exhibits a well-developed microphase structure, achieving a Cl- conductivity of up to 103 mS cm-2 at 80 °C. Moreover, the presence of DBC groups mitigates the degradation of the hydroxyl-containing side chains, enabling the membrane to retain 81 % of its original conductivity after 600 h of alkaline stability testing in 1 M KOH at 80 °C. In a PGM-free AEMWE, the P(O-F50%-C50%)-GTA membrane attains a current density of 3.8 A cm-2 at 2 V and 80 °C. These findings shows the potential of incorporating DBF and DBC moieties in the polymer main chain, along with the GTA-functionalized side chains, offering a promising pathway for advancing AEMs in AEMWE applications.

Recent Advances in Direct Synthesis of Functional Polymers of Intrinsic Microporosity Based on (Super)Acid Catalysis

Polymers of intrinsic microporosity (PIMs) are an emerging class of amorphous organic porous materials with solution processability, which are widely used in a multitude of fields such as gas separation, ion conduction, nanofiltration, etc. PIMs have adjustable pore structure and functional pore wall, so it can achieve selective sieving for specific substances. In order to meet the functional requirements of PIMs, two principal methods are used to synthesize functional PIMs, namely, post-modification of PIMs precursors and functionalization of monomers. A number of post-modification routes have been reported, however, the direct synthesis of functional PIMs with diverse groups still remains a challenge. The synthesis of PIMs through the acid-catalyzed polyhydroxyalkylation has been demonstrated to be an effective solution, exhibiting the advantages of wider substrates range, milder reaction conditions, and higher molecular weight. Recently, a series of functional substrates for direct synthesis of PIMs have been proposed. This article presents a review and summary of recent advances in synthesizing PIMs via acid-catalyzed polyhydroxyalkylation, and the synthesis route and structure-activity relationship are emphasized, which provides a versatile platform for the direct synthesis of functional PIMs.

Diffusion-Regulated Interfacial Polymerization of Hierarchically Microporous Polyamide Membranes for Permselective Gas Separations

Interfacial polymerization has emerged as a robust method for fabricating task-specific polyamide (PA) membranes. However, the limited microporosity of highly cross-linked PA membranes constrains their effectiveness in gas separation applications. Herein, we introduce an ionic liquid (IL)-regulated interfacial polymerization process to fabricate polyamide nanofilms incorporating kinked tetrakis (4-aminophenyl) methane monomers. In situ ultraviolet-visible spectroscopy demonstrates that the diffusion of 1,3,5-benzenetricarbonyl trichloride (TMC) toward the interface increases with the IL/H2O ratio, leading to the formation of a more compact membrane with a higher cross-linking degree. The PA-TAM7/3-60 min membrane exhibits a CO2 permeance of 29.8 GPU and a CO2/CH4 selectivity of 109, exceeding the 2008 Robeson upper bond. Additionally, the highly cross-linked structure imparts the membranes with notable plasticization resistance. Mixed-gas tests (CO2/CH4 = 50/50, v/v) reveal that the PA-TAM7/3-60 min membrane experiences only a 2% reduction in CO2 permeance and a 10% decrease in CO2/CH4 selectivity at a CO2 partial pressure of 300 PSIG, compared to its performance at 30 PSIG. The ease of tuning membrane structure and gas separation performance, along with its excellent plasticization resistance, underscores the potential of these PA membranes for task-specific gas separations.

Supported Ionic Liquid Membrane with Highly-permeable Polyamide Armor by In Situ Interfacial Polymerization for Durable CO2 Separation

Supported ionic liquid membranes (SILMs), owing to their capacities in harnessing physicochemical properties of ionic liquid for exceptional CO2 solubility, have emerged as a promising platform for CO2 extraction. Despite great achievements, existing SILMs suffer from poor structural and performance stability under high-pressure or long-term operations, significantly limiting their applications. Herein, a one-step and in situ interfacial polymerization strategy is proposed to elaborate a thin, mechanically-robust, and highly-permeable polyamide armor on the SILMs to effectively protect ionic liquid within porous supports, allowing for intensifying the overall stability of SILMs without compromising CO2 separation performance. The armored SILMs have a profound increase of breakthrough pressure by 105% compared to conventional counterparts without armor, and display high and stable operating pressure exceeding that of most SILMs previously reported. It is further demonstrated that the armored SILMs exhibit ultrahigh ideal CO2/N2 selectivity of about 200 and excellent CO2 permeation of 78 barrers upon over 150 h operation, as opposed to the full failure of CO2 separation performance within 36 h using conventional SILMs. The design concept of armor provides a flexible and additional dimension in developing high-performance and durable SILMs, pushing the practical application of ionic liquids in separation processes.

Enrichment of Nutmeg Essential Oil from Oil-in-Water Emulsions with PAN-Based Membranes

This study used polyacrylonitrile (PAN) and heat-treated polyacrylonitrile (H-PAN) membranes to enrich nutmeg essential oils, which have more complex compositions compared with common oils. The oil rejection rate of the H-PAN membrane was higher than that of the PAN membrane for different oil concentrations of nutmeg essential oil-in-water emulsions. After heat treatment, the H-PAN membrane showed a smaller pore size, narrower pore size distribution, a rougher surface, higher hydrophilicity, and higher oleophobicity. According to the GC-MS results, the similarities of the essential oils enriched by the PAN and H-PAN membranes to those obtained by steam distillation (SD) were 0.988 and 0.990, respectively. In addition, these two membranes also exhibited higher essential oil rejection for Bupleuri Radix, Magnolia Officinalis Cortex, Caryophylli Flos, and Cinnamomi Cortex essential oil-in-water emulsions. This work could provide a reference for membrane technology for the non-destructive separation of oil with complex components from oil-in-water emulsions.

Penetrant-induced plasticization in microporous polymer membranes

Penetrant-induced plasticization has prevented the industrial deployment of many polymers for membrane-based gas separations. With the advent of microporous polymers, new structural design features and unprecedented property sets are now accessible under controlled laboratory conditions, but property sets can often deteriorate due to plasticization. Therefore, a critical understanding of the origins of plasticization in microporous polymers and the development of strategies to mitigate this effect are needed to advance this area of research. Herein, an integrative discussion is provided on seminal plasticization theory and gas transport models, and these theories and models are compared to an exhaustive database of plasticization characteristics of microporous polymers. Correlations between specific polymer properties and plasticization behavior are presented, including analyses of plasticization pressures from pure-gas permeation tests and mixed-gas permeation tests for pure polymers and composite films. Finally, an evaluation of common and current state-of-the-art strategies to mitigate plasticization is provided along with suggestions for future directions of fundamental and applied research on the topic.

Alkali-Stable Anion Exchange Membranes Based on Poly(xanthene)

Poly(xanthene)s (PXs) carrying trimethylammonium, methylpiperidinium, and quinuclidinium cations were synthesized and studied as a new class of anion exchange membranes (AEMs). The polymers were prepared in a superacid-mediated polyhydroxyalkylation involving 4,4'-biphenol and 1-bromo-3-(trifluoroacetylphenyl)-propane, followed by quaternization reactions with the corresponding amines. The architecture with a rigid PX backbone decorated with cations via flexible alkyl spacer chains resulted in AEMs with high ionic conductivity, thermal stability and alkali-resistance. For example, hydroxide conductivities up to 129 mS cm^-1 were reached at 80 °C, and all the AEMs showed excellent alkaline stability with less than 4% ionic loss after treatment in 2 M aq. NaOH at 90 °C during 720 h. Critically, the diaryl ether links of the PX backbone remained intact after the harsh alkaline treatment, as evidenced by both ¹H NMR spectroscopy and thermogravimetry. Our combined findings suggest that PX AEMs are viable materials for application in alkaline fuel cells and electrolyzers.

9-Trifluoromethylxanthenediols: Synthesis and Supramolecular Motifs

The synthesis of four derivatives and the single-crystal X-ray structures of six 9-trifluoromethylxanthenediols (TFXdiols) I-VI are analyzed in this work. These compounds were obtained through superacid-catalyzed condensation of dihydroxybenzenes with 1,1,1-trifluoroacetone or 2,2,2-trifluoroacetophenone. The title molecules have a convex molecular structure due to their three fused rings of the xanthene moiety. We have found that, similar to resorcinol, the configuration of the hydroxyl groups is of great relevance for the crystal packing favoring either interactions above and below their molecular plane or lateral interactions that create layers. Considering that reports of TFXdiols are very scarce, our findings contribute to a better understanding of the molecular conformation and intermolecular interactions in their crystal structures. A similar analysis was extended to a fortuitous cocrystal obtained between 9-trifluoromethyl-9-(4'-fluorophenyl)-xanthenediol and 1,4-dihydroxybenzene, showing that these structures might be used to obtain cocrystals in the future.