Effective increase in permeability and free volume of PIM copolymers containing ethanoanthracene unit and comparison between the alternating and random copolymers

Polymer of Intrinsic Microporosity (PIM-1) Membranes Treated with Supercritical CO₂

Polymers of intrinsic microporosity (PIMs) are a promising membrane material for gas separation, because of their high free volume and micro-cavity size distribution. This is countered by PIMs-based membranes being highly susceptible to physical aging, which dramatically reduces their permselectivity over extended periods of time. Supercritical carbon dioxide is known to plasticize and partially solubilise polymers, altering the underlying membrane morphology, and hence impacting the gas separation properties. This investigation reports on the change in PIM-1 membranes after being exposed to supercritical CO₂ for two- and eight-hour intervals, followed by two depressurization protocols, a rapid depressurization and a slow depressurization. The exposure times enables the impact contact time with supercritical CO₂ has on the membrane morphology to be investigated, as well as the subsequent depressurization event. The density of the post supercritical CO₂ exposed membranes, irrespective of exposure time and depressurization, were greater than the untreated membrane. This indicated that supercritical CO₂ had solubilised the polymer chain, enabling PIM-1 to rearrange and contract the free volume micro-cavities present. As a consequence, the permeabilities of He, CH₄, O₂ and CO₂ were all reduced for the supercritical CO₂-treated membranes compared to the original membrane, while N₂ permeability remained unchanged. Importantly, the physical aging properties of the supercritical CO₂-treated membranes altered, with only minor reductions in N₂, CH₄ and O₂ permeabilities observed over extended periods of time. In contrast, He and CO₂ permeabilities experienced similar physical aging in the supercritical treated membranes to that of the original membrane. This was interpreted as the supercritical CO₂ treatment enabling micro-cavity contraction to favour the smaller CO₂ molecule, due to size exclusion of the larger N₂, CH₄ and O₂ molecules. Therefore, physical aging of the treated membranes only had minor impact on N₂, CH₄ and O₂ permeability; while the smaller He and CO₂ gases experience greater permeability loss. This result implies that supercritical CO₂ exposure has potential to limit physical aging performance loss in PIM-1 based membranes for O₂/N₂ separation.

Porous liquid zeolites: hydrogen bonding-stabilized H-ZSM-5 in branched ionic liquids

Porous liquids, as a newly emerging type of porous material, have great potential in gas separation and storage. However, the examples and synthetic strategies reported so far likely represent only the tip of the iceberg due to the great difficulty and challenge in engineering permanent porosity in liquid matrices. Here, by taking advantage of the hydrogen bonding interaction between the alkane chains of branched ionic liquids and the Brønsted sites in H-form zeolites, as well as the mechanical bond of the long alkyl chain of the cation penetrated into the zeolite channel at the interface, the H-form zeolites can be uniformly stabilized in branched ionic liquids to form porous liquid zeolites, which not only significantly improve their gas sorption performance, but also change the gas sorption-desorption behavior because of the preserved permanent porosity. Furthermore, such a facile synthetic strategy can be extended to fabricate other types of H-form zeolite-based porous liquids by taking advantage of the tunability of the counter-anion (e.g., NTf2-, BF4-, EtSO4-, etc.) in branched ionic liquids, thus opening up new opportunities for porous liquids for specific applications in energy and environment.

Effective Conversion of Amide to Carboxylic Acid on Polymers of Intrinsic Microporosity (PIM-1) with Nitrous Acid

Carboxylate-functionalised polymers of intrinsic microporosity (C-PIMs) are highly desirable materials for membrane separation applications. The recently reported method to afford C-PIMs was via an extensive base hydrolysis process requiring 360 h. Herein, a novel and effective method to convert PIM-CONH₂ to C-PIM using nitrous acid was studied. The chemical structure of C-PIM was characterised by ¹H NMR, ¹³C NMR, FTIR, elemental analysis, UV-Vis, TGA and TGA-MS. Complete conversion from amide to carboxylic acid groups was confirmed. Decarboxylation of C-PIM was also successfully studied by TGA-MS for the first time, with a loss of m/z 44 amu (CO₂) observed at the first degradation stage. TGA also revealed decreased thermal stability of C-PIM relative to PIM-CONH₂ under both N₂ and air atmosphere_. Gel permeation chromatography (GPC) analysis showed continuous molecular weight degradation of C-PIM with extended reaction time. Aromatic nitration was also observed as a side reaction in some cases.