Metal–organic frameworks (MOFs) represent the largest known class of porous crystalline materials ever synthesized. Their narrow pore windows and nearly unlimited structural and chemical features have made these materials of significant interest for membrane-based gas separations. In this comprehensive review, we discuss opportunities and challenges related to the formation of pure MOF films and mixed-matrix membranes (MMMs). Common and emerging separation applications are identified, and membrane transport theory for MOFs is described and contextualized relative to the governing principles that describe transport in polymers. Additionally, cross-cutting research opportunities using advanced metrologies and computational techniques are reviewed. To quantify membrane performance, we introduce a simple membrane performance score that has been tabulated for all of the literature data compiled in this review. These data are reported on upper bound plots, revealing classes of MOF materials that consistently demonstrate promising separation performance. Recommendations are provided with the intent of identifying the most promising materials and directions for the field in terms of fundamental science and eventual deployment of MOF materials for commercial membrane-based gas separations.
Equilibrium partitioning of ions between a membrane and a contiguous external solution strongly influences transport properties of polymeric membranes used for water purification and energy generation applications. This study presents a theoretical framework to quantitatively predict ion sorption from aqueous electrolytes (e.g., NaCl, MgCl2) into charged (i.e., ion exchange) polymers. The model was compared with experimental NaCl, MgCl2, and CaCl2 sorption data in commercial cation and anion exchange membranes. Ion sorption in charged polymers was modeled using a thermodynamic approach based on Donnan theory coupled with Manning's counter-ion condensation theory to describe non-ideal behavior of ions in the membrane. Ion activity coefficients in solution were calculated using the Pitzer model. The resulting model, with no adjustable parameters, provides remarkably good agreement with experimental values of membrane mobile salt concentration. The generality of the model was further demonstrated using literature data for ion sorption of various electrolytes in charged polymers, including HCl sorption in Nafion.
The ability of Ce-containing bioactive glasses to inhibit oxidative stress in terms of reduction of hydrogen peroxide, by mimicking the catalase enzyme activity is demonstrated here for the first time. The antioxidant properties of three bioactive glasses containing an increasing amount of CeO2 have been evaluated by following the degradation of hydrogen peroxide with time after immersion in H2O2 aqueous solutions with different concentration. XPS and UV-vis measurements allowed us to determine the Ce(3+)/Ce(4+) ratio in the bulk and on the glass surface, and to correlate it with the ability of the samples to show catalase mimetic activity. Interestingly, we have found that the bioactive glass with composition 23.2Na2O-25.7CaO-43.4SiO2-2.4P2O5-5.3CeO2 immersed in 0.1 M H2O2 aqueous solution is able to degrade 90% of it in 1 week. The reduction in bioactivity of the glasses with increasing CeO2 content is here rationalized in terms of a lower amount of phosphate groups available for the hydroxyapatite layer formation, after binding with cerium ions. In fact, classical molecular dynamics simulations revealed that the addition of CeO2 leads to the formation of cerium phosphate rich regions. The formation of an insoluble CePO4 crystalline phase is also observed by XRD analysis after thermal treatment of the glass samples.
Polymer membranes with ultrahigh CO 2 permeabilities and high selectivities are needed to address some of the critical separation challenges related to energy and the environment, especially in natural gas purification and post-combustion carbon capture. However, very few solution-processable, linear polymers are known today that access these types of characteristics, and all of the known structures achieve their separation performance through the design of rigid backbone chemistries that concomitantly increase chain stiffness and interchain spacing, thereby resulting in ultramicroporosity in solid-state chain-entangled films. Herein we report the separation performance of a porous polymer obtained via Ring-Opening Metathesis Polymerization (ROMP), which possesses a flexible backbone with rigid, fluorinated side chains. This polymer exhibits ultrahigh CO 2 permeability (> 21000 Barrer) and exceptional plasticization resistance (CO 2 plasticization pressure > 51 bar). Compared to traditional polymers of intrinsic microporosity (PIMs), the rate of physical aging is slower, especially for gases with small effective diameters (i.e., He, H 2 , and O 2 ). This structural design strategy, coupled with studies on fluorination, demonstrates a generalizable approach to create new polymers with flexible backbones and pore-forming side chains that have unexplored promise for small molecule separations.Membranes are a promising platform technology for energy-efficient chemical separations.Unlike other separation processes, membranes do not require thermal regeneration, phase changes, or moving parts. [1] Increasing the permeability of polymer membranes used for gas separations is essential for enhancing productivity and reducing membrane areas required for large-scale gas and vapor separations. [2] Specific membrane applications include natural gas purification, hydrogen separations, air separation, and CO 2 capture from flue gas. [3,4] Over the past decade, polymers of intrinsic microporosity (PIMs) have defined the state-ofthe-art for gas separations. [5,6] Their rigid and contorted backbone structures lead to excellent separation performance for a variety of challenging binary separations (e.g., CO 2 /N 2 , CO 2 /CH 4 , O 2 /N 2 ,
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