BACKGROUND Synthetic membranes are used for desalination, dialysis, sterile filtration, food processing, dehydration of air and other industrial, medical, and environmental applications due to low energy requirements, compact design, and mechanical simplicity. New applications are emerging from the water-energy nexus, shale gas extraction, and environmental needs such as carbon capture. All membranes exhibit a trade-off between permeability—i.e., how fast molecules pass through a membrane material—and selectivity—i.e., to what extent the desired molecules are separated from the rest. However, biological membranes such as aquaporins and ion channels are both highly permeable and highly selective. Separation based on size difference is common, but there are other ways to either block one component or enhance transport of another through a membrane. Based on increasing molecular understanding of both biological and synthetic membranes, key design criteria for new membranes have emerged: (i) properly sized free-volume elements (or pores), (ii) narrow free-volume element (or pore size) distribution, (iii) a thin active layer, and (iv) highly tuned interactions between permeants of interest and the membrane. Here, we discuss the permeability/selectivity trade-off, highlight similarities and differences between synthetic and biological membranes, describe challenges for existing membranes, and identify fruitful areas of future research. ADVANCES Many organic, inorganic, and hybrid materials have emerged as potential membranes. In addition to polymers, used for most membranes today, materials such as carbon molecular sieves, ceramics, zeolites, various nanomaterials (e.g., graphene, graphene oxide, and metal organic frameworks), and their mixtures with polymers have been explored. Simultaneously, global challenges such as climate change and rapid population growth stimulate the search for efficient water purification and energy-generation technologies, many of which are membrane-based. Additional driving forces include wastewater reuse from shale gas extraction and improvement of chemical and petrochemical separation processes by increasing the use of light hydrocarbons for chemicals manufacturing. OUTLOOK Opportunities for advancing membranes include (i) more mechanically, chemically, and thermally robust materials; (ii) judiciously higher permeability and selectivity for applications where such improvements matter; and (iii) more emphasis on fundamental structure/property/processing relations. There is a pressing need for membranes with improved selectivity, rather than membranes with improved permeability, especially for water purification. Modeling at all length scales is needed to develop a coherent molecular understanding of membrane properties, provide insight for future materials design, and clarify the fundamental basis for trade-off behavior. Basic molecular-level understanding of thermodynamic and diffusion properties of water and ions in charged membranes for desalination and energy applications such as fuel cells is largely incomplete. Fundamental understanding of membrane structure optimization to control transport of minor species (e.g., trace-organic contaminants in desalination membranes, neutral compounds in charged membranes, and heavy hydrocarbons in membranes for natural gas separation) is needed. Laboratory evaluation of membranes is often conducted with highly idealized mixtures, so separation performance in real applications with complex mixtures is poorly understood. Lack of systematic understanding of methodologies to scale promising membranes from the few square centimeters needed for laboratory studies to the thousands of square meters needed for large applications stymies membrane deployment. Nevertheless, opportunities for membranes in both existing and emerging applications, together with an expanding set of membrane materials, hold great promise for membranes to effectively address separations needs.

http://science.sciencemag.org/content/sci/356/6343/eaab0530.full.pdf

Maximizing the right stuff: The trade-off between membrane permeability and selectivity

Membranes with unprecedented solvent permeance and high retention of dissolved solutes are needed to reduce the energy consumed by separations in organic liquids We used controlled interfacial polymerization to form free-standing polyamide nanofilms less than 10 nanometers in thickness, and incorporated them as separating layers in composite membranes Manipulation of nanofilm morphology by control of interfacial reaction conditions enabled the creation of smooth or crumpled textures; the nanofilms were sufficiently rigid that the crumpled textures could withstand pressurized filtration, resulting in increased permeable area Composite membranes comprising crumpled nanofilms on alumina supports provided high retention of solutes, with acetonitrile permeances up to 112 liters per square meter per hour per bar This is more than two orders of magnitude higher than permeances of commercially available membranes with equivalent solute retention

http://science.sciencemag.org/content/sci/348/6241/1347.full.pdf

Sub–10 nm polyamide nanofilms with ultrafast solvent transport for molecular separation

Molecular separation with organic solvent nanofiltration: a critical review.

Materials research is key to enable synthetic membranes for large-scale, energy-efficient molecular separations. Materials with rigid, engineered pore structures add an additional degree of freedom to create advanced membranes by providing entropically moderated selectivities. Scalability - the capability to efficiently and economically pack membranes into practical modules - is a critical yet often neglected factor to take into account for membrane materials screening. In this Progress Article, we highlight continuing developments and identify future opportunities in scalable membrane materials based on these rigid features, for both gas and liquid phase applications. These advanced materials open the door to a new generation of membrane processes beyond existing materials and approaches.

Materials for next-generation molecularly selective synthetic membranes

Thin-film nanocomposite membranes containing a range of 50-150 nm metal-organic framework (MOF) nanoparticles [ZIF-8, MIL-53(Al), NH2-MIL-53(Al) and MIL-101(Cr)] in a polyamide (PA) thin film layer were synthesized via in situ interfacial polymerization on top of cross-linked polyimide porous supports. MOF nanoparticles were homogeneously dispersed in the organic phase containing trimesoyl chloride prior to the interfacial reaction, and their subsequent presence in the PA layer formed was inferred by a combination of contact angle measurements, FT-IR spectroscopy, SEM, EDX, XPS, and TEM. Membrane performance in organic solvent nanofiltration was evaluated on the basis of methanol (MeOH) and tetrahydrofuran (THF) permeances and rejection of styrene oligomers (PS). The effect of different post-treatments and MOF loadings on the membrane performance was also investigated. MeOH and THF permeance increased when MOFs were embedded into the PA layer, whereas the rejection remained higher than 90% (molecular weight cutoff of less than 232 and 295 g·mol(-1) for MeOH and THF, respectively) in all membranes. Moreover, permeance enhancement increased with increasing pore size and porosity of the MOF used as filler. The incorporation of nanosized MIL-101(Cr), with the largest pore size of 3.4 nm, led to an exceptional increase in permeance, from 1.5 to 3.9 and from 1.7 to 11.1 L·m(-2)·h(-1)·bar(-1) for MeOH/PS and THF/PS, respectively.

High flux thin film nanocomposite membranes based on metal-organic frameworks for organic solvent nanofiltration.

High flux membranes for organic solvent nanofiltration (OSN)—Interfacial polymerization with solvent activation

High flux hydrophobic membranes for organic solvent nanofiltration (OSN)—Interfacial polymerization, surface modification and solvent activation

La presente invention concerne une membrane composite de separation de gaz et/ou de nanofiltration d'une solution de flux d'alimentation comprenant un solvant et des solutes dissous et presentant un rejet preferentiel des solutes. La membrane composite comprend une couche de separation dotee d'une microporosite intrinseque. La couche de separation est formee de maniere appropriee par polymerisation interfaciale sur une membrane de support. De maniere appropriee, au moins un des monomeres utilises lors de la reaction de polymerisation interfaciale devrait posseder une certaine concavite, resultant en un polymere reticule dote de nanopores interconnectes et d'une membrane presentant une permeabilite amelioree. La membrane de support peut eventuellement etre impregnee d'un agent de conditionnement et peut eventuellement etre stable dans des solvants organiques, en particulier dans des solvants polaires aprotiques. La couche superieure de la membrane composite est eventuellement couverte de groupes fonctionnels pour modifier la chimie de surface. La membrane composite peut etre durcie dans le four pour ameliorer le rejet. Enfin, la membrane composite peut etre traitee avec un solvant d'activation avant la nanofiltration.

Membranes de séparation

La presente invention concerne une membrane composite pour la nanofiltration d'une solution de flux d'alimentation comprenant un solvant et des solutes dissous et montrant un rejet preferentiel des solutes. La membrane composite comprend un mince film polymere forme par polymerisation interfaciale sur une membrane support. La membrane support est de plus impregnee avec un agent de conditionnement et est stable dans les solvants polaires aprotiques. La membrane composite est facultativement traitee dans un milieu de desactivation, ou la reaction de polymerisation interfaciale peut etre desactivee et, dans certaines formes de realisation, la chimie de la membrane peut etre modifiee. Finalement, la membrane composite est traitee avec un solvant d'activation avant la nanofiltration.

Maria Fernanda Jimenez Solomon

Papers

Molecular separation with organic solvent nanofiltration: a critical review.

High flux membranes for organic solvent nanofiltration (OSN)—Interfacial polymerization with solvent activation

High flux hydrophobic membranes for organic solvent nanofiltration (OSN)—Interfacial polymerization, surface modification and solvent activation

Membranes de séparation

Membranes de nanofiltration de polyamide résistantes aux solvants