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

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Science  16 Jun 2017:
Vol. 356, Issue 6343, eaab0530
DOI: 10.1126/science.aab0530


  • From intrinsic permeability/selectivity trade-off to practical performance in membranes.

    Polymer membranes for liquid and gas separation applications obey a permeability/selectivity trade-off—highly permeable membranes have low selectivity and vice versa—largely due to broad distributions of free-volume elements (or pores in porous membranes) and nonspecific interactions between small solutes and polymers. We highlight materials approaches to overcome this trade-off, including the development of inorganic, isoporous, mixed matrix, and aquaporin membranes. Further, materials must be processed into thin, typically supported membranes, fashioned into high surface/volume ratio modules, and used in optimized processes. Thus, factors that govern the practical feasibility of membranes such as mechanical strength, module design, and operating conditions are also discussed.

  • Fig. 1 Upper-bound relations in polymer membranes.

    (A) O2/N2 separation (23); (B) water/salt separation (39); (C) protein/water separation in porous ultrafiltration membranes [1/Sa is the separation factor of bovine serum albumin (BSA) from water and other small solutes (e.g., salts and sugars)], and hydraulic permeability is the rate of transport through the membrane of water and any solutes not retained by the membrane (43); and (D) polymer electrolyte membranes, where ionic conductivity for membranes at a given level of water uptake (i.e., water sorption) is apparently limited by an upper-bound type relationship (44).

  • Fig. 2 Evolution of cavity (or pore) size distributions in membranes.

    Cavity (or pore) size distributions in separation membranes, ranging from (A) broad cavity-size distributions in dense polymers, such as those used in gas separations (113) and (B) water purification (e.g., porous ultrafiltration membranes) (114) to narrow pore size distributions in (C) isoporous UF membranes formed via self-assembly of block copolymers (~34-nm pore diameter) (52), (D) graphene nanomesh having ~30-nm pores prepared via block copolymer lithography (115), (E) potassium ion channel (116), (F) aquaporin (93), and (G) the effect of hourglass pore design parameters (pore opening angle, α, and aspect ratio, L/a) on water permeability (117).

  • Fig. 3 In designing mixed-matrix membranes to overcome the upper bound, compatibility between filler and polymer, filler particle size and shape, and homogeneous filler distribution are important factors.

    (Case 1) Molecular sieving fillers (e.g., CMS and zeolites) often lead to selectivity increases and permeability decreases (35). (Case 2) Molecular sieving fillers with nano size or nanosheet shapes (e.g., MOFs nanocrystals or 2D nanosheets) can improve both permeability and selectivity (55). (Case 3) Fillers with interfacial voids, even when homogeneously dispersed, can result in increased permeability and decreased selectivity (55).

  • Fig. 4 Morphology of state-of-the-art membranes.

    Examples of (A) hollow fiber gas separation membrane (118) and (B) flat sheet RO membrane (91).

  • Fig. 5 Effect of membrane support and operating conditions on separation characteristics of gas separation membranes.

    (A) Membrane support. (B) operating conditions. (A) shows the 2008 upper-bound data for CO2/N2 separation translated into the selectivity and permeance (i.e., pressure-normalized flux) that could be achieved by placing a thin (100 nm) membrane onto a slow (103 GPU), medium (104 GPU), or fast (105 GPU) porous support membrane. For brevity, only 20% of the 2008 upper-bound data have been shown on the selectivity-permeance plot. The dashed lines indicate the expected performance of materials lying on the upper bound. The star [left side of (A)] denotes permeability/selectivity of a hypothetical material with separation properties above the upper bound. The stars [right side of (A)] denote permeance and selectivity of thin-film composite membranes of this material using different supports, showing that a high flux support is needed to reach desirable performance (blue shaded region) for postcombustion CO2 capture (15). The procedure for generating the upper-bound lines in the selectivity-permeance plot is described here (119). (B) shows the effect of varying membrane selectivity on permeate vapor concentration for a vapor separation membrane operating at a feed/permeate pressure ratio of 20 and 1% vapor in the feed.

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