A roadmap to enhance gas permselectivity in metal-organic framework-based mixed-matrix membranes

Performing gas separation at high efficiency with minimum energy input and reduced carbon footprint is a major challenge. While several separation methods exist at various technology readiness levels, porous membrane-based separation is considered as a disruptive technology. To attain sustainability and required efficiency, different approaches of membrane design have been explored. However, the selectivity-permeation trade-off and membrane aging have restricted further advancement. In this regard, a new generation composite made of organic polymers and metal-organic framework (MOF) fillers shows substantial promise. Organic polymer matrix allows easy processibility, but it has poor permselectivity for gas molecules. Metal-organic frameworks are excellent sieving materials; however, they suffer from poor processibility issues. A combination of these two components makes an ideal sieving membrane, which can potentially outnumber the existing energy intensive distillation strategies. In this perspective, we have discussed key indices that regulate gas permselectivity by a careful selection of the existing literature. While the target gas flux and selectivity values have been a part of many previous reviews and articles, we have presented a concise discussion on the interface design of the MOF-polymer membrane, morphology, and orientation control of MOF fillers in the matrix. Following this, a future roadmap to overcome challenges related to MOF-polymer interfacial defects is outlined.

Composite amine mixed matrix membranes for high-pressure CO2-CH4 separation: synthesis, characterization and performance evaluation

The key challenge in the synthesis of composite mixed matrix membrane (MMMs) is the incompatible membrane fabrication using porous support in the dry-wet phase inversion technique. The key objective of this research is to synthesize thin composite ternary (amine) mixed matrix membranes on microporous support by incorporating 10 wt% of carbon molecular sieve (CMS) and 5-15 wt% of diethanolamine (DEA) in polyethersulfone (PES) dope solution for the separation of carbon dioxide (CO₂) from methane (CH₄) at high-pressure applications. The developed membranes were evaluated for their morphological structure, thermal and mechanical stabilities, functional groups, as well as for CO₂-CH₄ separation performance at high pressure (10-30 bar). The results showed that the developed membranes have asymmetric structure, and they are mechanically strong at 30 bar. This new class of PES/CMS/DEA composite MMMs exhibited improved gas permeance compared to pure PES composite polymeric membrane. CO₂-CH₄ perm-selectivity enhanced from 8.15 to 16.04 at 15 wt% of DEA at 30 bar pressure. The performance of amine composite MMMs is theoretically predicted using a modified Maxwell model. The predictions were in good agreement with experimental data after applying the optimized values with AARE % = ∼less than 2% and R ² = 0.99.

Superior performance of surface-treated NaX@Pebax-1657 membranes for O2/N2 separation

In this study, the performances of mixed matrix composite membranes (MMCMs) containing surface-treated NaX nanocrystals (ST-NaX-NCs) were experimentally and theoretically investigated for O₂/N₂ separation. For this purpose, the MMCMs were fabricated by the casting solution method and characterized by various analyses. The results reveal that there is a robust interaction between the polymer chains and the ST-NaX-NCs, and that the ST-NaX fillers are uniformly dispersed in the polymer matrix. The incorporation of ST-NaX-NCs alters the PEBAX polymer chain packing arrangement resulting in decreased membrane transport behavior for both O₂ and N₂ gases. The MMCM containing 16.7% wt ST-NaX-NCs has drastically enhanced air separation properties, with a selectivity that is increased to 204% of that of the neat membrane. Moreover, the Lewis-Nielsen model was modified by considering non-ideal effects in mixed matrix membranes, like the clogging of filler pores and polymer chain hardening around the nanocrystals, to predict the gas permeation behavior through the MMCMs. The comparison of the experimental and model results reveals that the modified model can accurately predict the gas permeability and selectivity through the MMCMs.

Predicting CO2 Permeation through an Enhanced Ionic Liquid Mixed Matrix Membrane (IL3M)

Ionic liquid mixed matrix membranes (IL3Ms) were synthesized using polyethersulfone (PES) as the base polymer and silica-aluminophosphate (SAPO-34) as the dispersed particles, and their CO2 permeation was investigated. Three of the most widely used models for gas separation—the Maxwell, Lewis–Nielson, and Maxwell–Wagner–Sillar (MWS) models—were then applied to the membranes. Large deviations were found between the model predictions and experimental data. FESEM images suggested that local agglomeration and disorientation of the SAPO-34 particles within the membrane afforded substantial changes in the morphology. The MWS model, which considers the shape factor, was modified to incorporate the volume fraction of the wetted dispersed phase and the ideal shape factor. A direct relationship was found between the filler concentration and the shape factor. The modified model was shown to produce absolute and relative errors of less than 3%. When validated against data from the literature, the deviation remained within 5%. The modified model can be used to estimate the gas permeance of an IL3M.

Modeling the Effects of Interfacial Characteristics on Gas Permeation Behavior of Nanotube-Mixed Matrix Membranes

Theoretical approaches that accurately predict the gas permeation behavior of nanotube-containing mixed matrix membranes (nanotube-MMMs) are scarce. This is mainly due to ignoring the effects of nanotube/matrix interfacial characteristics in the existing theories. In this paper, based on the analogy of thermal conduction in polymer composites containing nanotubes, we develop a model to describe gas permeation through nanotube-MMMs. Two new parameters, "interfacial thickness" (a_int) and "interfacial permeation resistance" (R_int), are introduced to account for the role of nanotube/matrix interfacial interactions in the proposed model. The obtained values of a_int, independent of the nature of the permeate gas, increased by increasing both the nanotubes aspect ratio and polymer-nanotube interfacial strength. An excellent correlation between the values of a_int and polymer-nanotube interaction parameters, χ, helped to accurately reproduce the existing experimental data from the literature without the need to resort to any adjustable parameter. The data includes 10 sets of CO₂/CH₄ permeation, 12 sets of CO₂/N₂ permeation, 3 sets of CO₂/O₂ permeation, and 2 sets of CO₂/H₂ permeation through different nanotube-MMMs. Moreover, the average absolute relative errors between the experimental data and the predicted values of the proposed model are very small (less than 5%) in comparison with those of the existing models in the literature. To the best of our knowledge, this is the first study where such a systematic comparison between model predictions and such extensive experimental data is presented. Finally, the new way of assessing gas permeation data presented in the current work would be a simple alternative to complex approaches that are usually utilized to estimate interfacial thickness in polymer composites.

Prediction of concrete corrosion in sewers with hybrid Gaussian processes regression model

A hybrid Gaussian Processes Regression (GPR) model is to approach the evolution of the corrosion rate and corrosion initiation time, thereby supporting the calculation of service life of sewers.