Preparation and gas permeation of crown ether-containing co-polyimide with enhanced CO2 selectivity

Synthesis and enhanced CO2/CH4 selectivity of hyperbranched copolyimide membranes

A series of hyperbranched copolyimides (co-HBPIs), as the completely symmetrical tetraamine monomer successfully incorporated into the linear polyimide 6FDA-ODA, were synthesized under the chemical imidization after the gel formation was avoided, as to more effectively separate CO2/CH4. CO2/CH4 selectivity (74%) was observed to be improved significantly under 7‰ tetraamine contents at 2 bar and 35°C; thus, the acceptable CO2 permeability was maintained compared with the pure membrane, exceeding the 1991 upper bound. Moreover, the as-prepared membrane with 6FDA-ODA-7‰ was reported to exhibit higher CO2/CH4 selectivity (109.14) at 10 bar and 35°C. Accordingly, the hyperbranched copolyimides will be promising candidates as membrane materials for the further separation of CO2/CH4 gas pair.

Development of CO2-Selective Polyimide-Based Gas Separation Membranes Using Crown Ether and Polydimethylsiloxane

A series of CO2-selective polyimides (CE-PDMS-PI-x) was synthesized by copolymerizing crown ether diamine (trans-diamino-DB18C6) and PDMS-diamine with 4,4'-(hexafluoroisopropylidene) di-phthalic anhydride (6FDA) through the polycondensation reaction. The structural characteristics of the copolymers and corresponding membranes were characterized by nuclear magnetic resonance (NMR), infrared spectroscopy (ATR-FTIR), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray diffraction (XRD), and gel permeation chromatography (GPC). The effect of PDMS loading on the CE-PDMS-PI-x copolymers was further analyzed and a very good structure-property relationship was found. A well-distributed soft PDMS unit played a key role in the membrane's morphology, in which improved CO2-separation performance was observed at a low PDMS content (5 wt %). In contrast, the fine-grained phase separation adversely affected the separation behavior at a certain level of PDMS loading, and the PDMS was found to provide a flexible gas-diffusion path, affecting only the permeability without changing the selective gas-separation performance for the copolymers with a PDMS content of 20% or above.

Introducing hydrophilic ultra-thin ZIF-L into mixed matrix membranes for CO2/CH4 separation

Mixed matrix membranes (MMMs) were developed by mixing hydrophilically modified two-dimensional (2D) imidazole framework (named as hZIF-L) flakes into a Pebax MH 1657 (Pebax) matrix, and designed to separate carbon dioxide/methane (CO₂/CH₄) mixtures. The hZIF-L flakes were important for increasing the effectiveness of the MMMs. First, the tannic acid (TA) etched hZIF-L flakes have a large number of microporous (1.8 nm) and two-dimensional anisotropic transport channels, which offered convenient gas transport channels and improved the permeability of CO₂. Second, the TA molecules provide the surface of the ZIF-L flakes with more hydrophilic functional groups such as carbonyl groups (C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O) and hydroxyl groups (–OH), which could effectively prevent non-selective interfacial voids and filler agglomeration in the Pebax matrix, and also presented strong binding ability to water and CO₂ molecules. The satisfactory interface compatibility and affinity with the CO₂ molecule promoted its permeability, solubility, and selectivity. As a result, the MMMs exhibited the highest performance of gas separation with the hZIF-L flake weight content of 5%, at which the CO₂ permeability and CO₂/CH₄ selectivity were 502.44 barrer and 33.82 at 0.2 MPa and 25 °C, respectively.

Schematic diagram of CO₂ transfer in Pebax/hZIF-L mixed matrix membranes.