Blending PPO-based molecules with Pebax MH 1657 in membranes for gas separation

Truly combining the advantages of polymeric and zeolite membranes for gas separations

Mixed-matrix membranes (MMMs) have been investigated to render energy-intensive separations more efficiently by combining the selectivity and permeability performance, robustness, and nonaging properties of the filler with the easy processing, handling, and scaling up of the polymer. However, truly combining all in one single material has proven very challenging. In this work, we filled a commercial polyimide with ultrahigh loadings of a high-aspect ratio, CO₂-philic Na-SSZ-39 zeolite with a three-dimensional channel system that precisely separates gas molecules. By carefully designing both zeolite and MMM synthesis, we created a gas-percolation highway across a flexible and aging-resistant (more than 1 year) membrane. The combination of a CO₂-CH₄ mixed-gas selectivity of ~423 and a CO₂ permeability of ~8300 Barrer outperformed all existing polymer-based membranes and even most zeolite-only membranes.

The Effect of Solution Casting Temperature and Ultrasound Treatment on PEBAX MH-1657/ZIF-8 Mixed Matrix Membranes Morphology and Performance

Approximately two-thirds of anthropogenic emissions causing global warming are from carbon dioxide. Carbon capture is essential, with membranes proving to be a low cost and energy-efficient solution to alternative technologies. In particular, mixed matrix membranes (MMMs) can have higher permeability and selectivity than pure polymer membranes. The fabrication conditions affect the formation of defects within the membranes. In this work, MMMs were created using a PEBAX MH-1657 polymer and a ZIF-8 filler. The effect of casting plate temperature, varying from −5 °C to 50 °C, and the effect of ultrasound treatment time (80–400 min) and method (filler solution only, filler and polymer combined solution only and filler solution followed by combined solution) were investigated, aiming to reduce defect formations hence improving the performance of the MMMs. SEM images and permeation tests using pure CO2 and N2 gas, replicating flue gas for carbon capture, were used to investigate and compare the membranes morphology and performance. The results indicated that the MMMs maintained their permeabilities and selectivities at all tested casting temperatures. However, the neat PEBAX membranes demonstrated increased phase separation of the polyamide and polyether oxide phases at higher temperatures, causing a reduction in permeability due to the higher crystallinity degree, confirmed by DSC experiment. The MMMs fabricated at low ultrasound times displayed a large amount of aggregation with large particle size causing channeling. At high ultrasound times, a well-dispersed filler with small filler diameters was observed, providing a high membrane performance. Overall, defect-free membranes were successfully fabricated, leading to improved performance, with the best membrane resulting from the longest ultrasound time reaching the Robeson bound upper limits.

MOF/Polymer Mixed-Matrix Membranes Preparation: Effect of Main Synthesis Parameters on CO2/CH4 Separation Performance

Design and preparation of mixed-matrix membranes (MMMs) with minimum defects and high performance for desired gas separations is still challenging as it depends on a variety of MMM synthesis parameters. In this study, 6FDA-DAM:DABA based MMMs using MOF-808 as filler were prepared to examine the impact of multiple variables on the preparation process of MMMs, including variation in polymer concentration, filler loading, volume of solution cast per membrane area, solvent type used and solvent evaporation rate, and to identify their impact on the CO₂/CH₄ separation performance of these membranes. Solvent evaporation rate proved to be the most critical synthesis parameter, directly influencing the performance and visual appearance of the membranes. Although less dominantly influencing the MMM performance, polymer concentration and solution volume also had an important role via control over the casting solution viscosity, particle agglomeration, and particle settling rate. Among all solvents studied, MMMs prepared with chloroform led to the best performance for this polymer-filler system. Chloroform-based MMMs containing 10 and 30 wt.% MOF-808 showed 73% and 62% increase in CO₂ permeability, respectively, without a decrease in separation factor compared to unfilled membranes. The results indicate that enhanced gas separation performance of MMMs strongly depends on the cumulative effect of various synthesis parameters rather than individual impact, thus requiring a system-specific design and optimization.

Modulator-Mediated Functionalization of MOF-808 as a Platform Tool to Create High-Performance Mixed-Matrix Membranes

Modulator-mediated functionalization (MoFu) is introduced as a new and versatile platform tool to improve the separation performance of metal-organic framework (MOF)-based membranes, exemplified here by the creation of mixed-matrix membranes (MMMs) with enhanced CO₂ separation efficiency. The unique structure of MOF-808 allows incorporation of CO₂-philic modulators in the MOF framework during a one-pot synthesis procedure in water, thus creating a straightforward way to functionalize both MOF and corresponding MMM. As a proof of concept, a series of fluorinated carboxylic acids [trifluoroacetic acid (TFA), pentafluoropropionic acid (PFPA), and heptafluorobutyric acid (HFBA)] and nonfluorinated alkyl carboxylic acids (acetic acid (AA), propionic acid (PA), and butyric acid (BA)) were used as a modulator during MOF-808 synthesis. Two of the best MMMs prepared with 30 wt % MOF-TFA (100% increase in CO₂/CH₄ separation factor, 350% increase in CO₂ permeability) and 10 wt % MOF-PFPA (140% increase in CO₂/CH₄ separation factor, 100% increase in CO₂ permeability) scored very close to or even crossed the 2008 and 2018 upper bound limits for CO₂/CH₄. Because of its facile functionalization (and its subsequent excellent performance), MOF-808 is proposed as an alternative for widely used UiO-66, which is, from a functionalization point-of-view and despite its widespread use, a rather limited MOF.