Membrane-Based Olefin/Paraffin Separations

Abstract

Efficient olefin/paraffin separation is a grand challenge because of their similar molecular sizes and physical properties, and is also a priority in the modern chemical industry. Membrane separation technology has been demonstrated as a promising technology owing to its low energy consumption, mild operation conditions, tunability of membrane materials, as well as the integration of physical and chemical mechanisms. In this work, inspired by the physical mechanism of mass transport in channel proteins and the chemical mechanism of mass transport in carrier proteins, recent progress in channel-based and carrier-based membranes toward olefin/paraffin separations is summarized. Further, channel-based membranes are categorized into membranes with network structures and with framework structures according to the morphology of channels. The separation mechanisms, separation performance, and membrane stability in channel-based and carrier-based membranes are elaborated. Future perspectives toward membrane-based olefin/paraffin separation are proposed.

Keywords: carrier‐based membranes; channel‐based membranes; framework structures; network structures; olefin/paraffin separations; structure–performance relationships.

Figure 1

The roadmap of membrane materials development for olefin/paraffin separations. The image of glassy polymer membrane was reproduced with permission.^{[ 14 ]} Copyright 2002, Elsevier. The image of PIM membrane was reproduced with permission.^{[ 15 ]} Copyright 2017, Elsevier. The image of ZSM‐2 membrane was reproduced with permission.^{[ 16 ]} Copyright 2001, Elsevier. The image of ZIF‐8 membrane was reproduced with permission.^{[ 17 ]} Copyright 2010, Elsevier. The image of polymer electrolyte membrane was reproduced with permission.^{[ 18 ]} Copyright 2001, Elsevier. The image of IL‐based membrane was reproduced with permission.^{[ 19 ]} Copyright 2018, Elsevier. The image of DES‐based membrane was reproduced with permission.^{[ 20 ]} Copyright 2017, American Chemical Society. The image of BN/IL membrane was reproduced with permission.^{[ 21 ]} Copyright 2019, John Wiley and Sons. The image of ZIF‐8 MMM was reproduced with permission.^{[ 22 ]} Copyright 2011, Elsevier. The image of Mg₂(dobdc) MMM was reproduced with permission.^{[ 23 ]} Copyright 2016, Springer Nature. The image of carbon membrane was reproduced with permission.^{[ 24 ]} Copyright 1999, American Chemical Society.

Figure 2

Schematic of channel‐based and carrier‐based membranes toward olefin/paraffin separations.

Figure 3

Chemical structures of rubbery and glassy polymers, as well as the diamine and dianhydride moieties of polyimides.

Figure 4

a) Space‐filling molecular model of a fragment of PIM‐1, showing its rigid, randomly contorted structure. Reproduced with permission.^{[ 37 ]} Copyright 2004, Royal Society of Chemistry. b) 3D visual of a triptycene building block, presenting its structurally fixed free volume elements and dimensions provided based on molecular dynamic simulation. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 38 ]} Copyright 2019, the Authors, Published by MDPI, Basel, Switzerland. c) Comparison of torsional rigidity of PIM‐1 and PIM‐EA‐TB. Reproduced with permission.^{[ 39 ]} Copyright 2013, American Association for the Advancement of Science. d) Molecular simulations showing the accessible free volume for H₂, N₂, and CH₄ in Pebax2533, PEEK‐WC, HyflonAD60x, and PIM‐MP‐TB. Reproduced with permission.^{[ 40 ]} Copyright 2019, Royal Society of Chemistry.

Figure 5

a) Primary building blocks of [SiO₄]⁴⁻ tetrahedron, b) secondary building blocks formed by the combination of primary building blocks with a specific spatial arrangement, and c) the framework structure of ZSM‐5 produced by the linkage of secondary building blocks with a unique system of channels and cages. The red and purple in the framework represent oxygen and silicon atoms, respectively, and the aluminium elements are not shown.

Figure 6

In ZIF‐8, the zinc ions are connected by MeIM ligands with coordinated bonding. a,b) The primary units of zinc–nitrogen tetrahedron expand to building blocks with a specific topology by sharing the same MeIM ligand between two adjacent zinc ions. c) The building blocks expand further to the framework structure with a unique system of channels and cages. All hydrogen atoms have been omitted for clarity. The cyan, blue, and grey in the structure represent the atoms of zinc, nitrogen, and carbon, respectively.

Figure 7

Preparation methods for ZIF‐8 membranes. Secondary growth method: a) the synthesis of ZIF‐8 hollow fiber membranes by combining the modification of polymer supports to optimize the seeding layer structure and microfluidic secondary growth. Reproduced with permission.^{[ 62 ]} Copyright 2019, American Chemical Society. b) The synthesis of ZIF‐8 membranes by an electrophoretic process to assemble ZIF‐8 seed crystals on the support and followed by secondary growth in the synthesis sol for a short period of time. Reproduced with permission.^{[ 63 ]} Copyright 2018, John Wiley and Sons. c) Hetero‐epitaxial growth of the ZIF membrane with two different layers (ZIF‐8 and ZIF‐67) on ZIF‐8 seed crystals. Reproduced with permission.^{[ 64 ]} Copyright 2015, American Chemical Society. d) The synthesis of ZIF‐8 membrane by the counter‐diffusion method. The support saturated with a metal precursor solution is placed in a ligand solution containing sodium formate. The diffusion of metal ions and ligand molecules cause the formation of a “reaction zone” at the interface. Rapid heterogeneous nucleation/crystal growth in the vicinity at the interface leads to continuous well‐intergrown ZIF‐8 membranes. Reproduced with permission.^{[ 65 ]} Copyright 2013, American Chemical Society. e) ZIF‐8 membrane prepared by dopamine‐mediated counter‐diffusion method. Dopamine was added into the metal ion solution, which functions as a competitive ligand to MeIM, chelating Zn²⁺ ions in the solution. Also, dopamine polymerized on the support surface to anchor Zn²⁺ ions and MeIM promotes the counter‐diffusion growth of the ZIF‐8 layer on the support. Reproduced with permission.^{[ 66 ]} Copyright 2019, Royal Society of Chemistry. f) A scheme depicting the IMMP approach for ZIF‐8 membranes in hollow fibers. The Zn²⁺ ions are supplied in 1‐octanol solution (light red) flowing through the bore of the fiber, whereas the MeIM linkers are supplied on the outer shell side of the fiber in an aqueous solution (light blue). The two precursors react and form a polycrystalline ZIF‐8 layer (dark blue) on the inner surface of the fiber. Reproduced with permission.^{[ 67 ]} Copyright 2014, American Association for the Advancement of Science.

Figure 8

Emerging fabrication methods for ZIF‐8 membranes. a) An electrochemical cell for ZIF‐8 membrane growth by electrochemistry deposition, whereby the substrate serves as a cathode. With the local in situ electric field that forms around the support by the current, inborn lattice distortion occurs and a ZIF‐8 membrane with stiffer framework structures is formed. Reproduced with permission.^{[ 71 ]} Copyright 2018, American Association for the Advancement of Science. b) Schematic of ZIF‐8 membrane formation process by a gel–vapor deposition method. Zn‐based sol was coated on ammoniated poly(vinylidene fluoride) hollow fibers and transformed to Zn‐based gel by heat treatment. The MOF membrane was formed directly through ligand vapor deposition, whereby the produced vapor reacted with the sensitive Zn‐based gel and substituted organic chains to form more stable ZIF‐8. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 73 ]} Copyright 2017, Springer Nature. c) Schematic of an all‐vapor‐phase ligand‐induced permselectivation membrane fabrication process. The pores of the support are first blocked with ZnO through atomic layer deposition. The impermeable ZnO deposits are converted to permselective ZIF‐8 membrane by means of ligand–vapor treatment. Reproduced with permission.^{[ 74 ]} Copyright 2018, American Association for the Advancement of Science. d) Schematic of ZIF‐8 membrane synthesis by a two‐step sol–gel transformation, including gel coating for precursor dispersion and thermal treatment for crystallization. Reproduced with permission.^{[ 75 ]} Copyright 2018, Royal Society of Chemistry.

Figure 9

a) A representation of facilitated transport in mobile carrier membranes and b) corresponding application in a supported liquid membrane composed of CuCl as olefin carriers and deep eutectic solvents. Reproduced with permission.^{[ 20 ]} Copyright 2017, American Chemical Society. c) A representation of facilitated transport in fixed carrier membranes and d) corresponding application in an Ag‐exchanged Na‐X membrane for propylene/propane separation. Reproduced with permission.^{[ 47c ]} Copyright 2019, American Chemical Society.

Figure 10

a–c) Tapping mode AFM surface images of boron nitride (BN)/nylon membrane, confirming the self‐assembly of BN nanosheets at horizontal and inclined directions. d1–d4) Snapshots of molecular simulations of reactive ionic liquids, which is composed of 1‐ethylimidazolium nitrate and silver nitrate, nanoconfined within BN nanochannels, showing the nanoaggregation of alkyl groups in ionic liquids and ordered arrangement of silver ions along the BN nanosheets; C: gray, H: white, O: red, N: blue, Ag: cyan: d1) all atoms, d2) NO₃ ⁻ anion of ionic liquids and silver salt, d3) silver cation, and d4) cation of ionic liquids. Reproduced with permission.^{[ 21 ]} Copyright 2019, John Wiley and Sons.

Figure 11

Representation of the solution–diffusion model of gas transport in dense polymer membrane, where the gradient in chemical potential is produced by the difference in partial pressure of component i across the membrane. Reproduced with permission from Zachary P. Smith.^{[ 105 ]}

Figure 12

a) A schematic of molecular sieving separation of molecules of two different sizes in membrane pores. Larger‐diameter molecules are excluded by molecular sieving. b) Diffusion coefficients of eight gases (CO₂, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈, n‐C₄H₁₀, and i‐C₄H₁₀) in Zr‐fum‐fcu‐MOF and 6FDA‐DAM at 35 °C and 106 kPa. The diffusivities of C₃H₈, n‐C₄H₁₀, and i‐C₄H₁₀ were also estimated from their sorption kinetics of Zr‐fum‐fcu‐MOF crystals; c) Typical conformers of a C₃H₈ molecule in the space‐filling (CPK) model: staggered (S), eclipsed‐S (ES), and eclipsed‐D (ED) representing conformers with one end in eclipsed conformation and both ends in eclipsed conformation, respectively. The carbons in the rotated ‒CH₃ are shown in different colors (black and green) for easier observation. d) Hypothesized triangular aperture that can distinguish “eclipsed” conformer (up) from “staggered” conformer (down) with the solid line triangle in the front plane. e) Schematic representation of the pore‐aperture size reduction of fum‐fcu‐MOF induced by reduced metal cluster size from Y‐fum‐fcu‐MOF (left) to Zr‐fum‐fcu‐MOF (right). Reproduced with permission.^{[ 81 ]} Copyright 2019, John Wiley and Sons.

Figure 13

Optimized structures of a) the PEO/AgBF₄ complex, b) the PEO/AgBF₄ complex coordinated by one ethylene, and c) the PEO/AgBF₄ complex coordinated by two ethylene molecules (transition state). Reproduced with permission.^{[ 115 ]} Copyright 2001, American Chemical Society. d) Schematic representation of Ag⁺‐propylene complexation. Reproduced under the terms of the Creative Commons Attribution 4.0 International License.^{[ 116 ]} Copyright 2006, the Authors, Published by Brazilian Society of Chemical Engineering.

Figure 14

Ethylene/ethane separation performance of channel‐based and carrier‐based membranes. The polymer upper bound is represented as a bold solid line according to ref. [123]. The grey and light blue oval areas highlight the separation performance of membranes with network structures and other kinds of channel‐based membranes, respectively. Membranes with framework structures are sub‐divided into zeolite and MOF membranes, represented by different shapes. Other kinds of channel‐based membranes are further sub‐divided into carbon membranes and MMMs, represented by different shapes. Here, membranes with network structures specifically refer to polymeric membranes. Carrier‐based membranes specifically refer to facilitated transport membranes. These two classes of membranes are represented by squares, but distinguished by different colors.

Figure 15

Propylene/propane separation performance of channel‐based and carrier‐based membranes. The polymer upper bound is represented as a bold solid line according to ref. [130]. The grey, light pink, and light blue oval areas highlight the separation performance of membranes with network structures, membranes with framework structures, and other kinds of channel‐based membranes, respectively. Membranes with framework structures are sub‐divided into zeolite and MOF membranes, represented by different shapes. Other kinds of channel‐based membranes are further sub‐divided into carbon membranes and MMMs, represented by different shapes. Here, membranes with network structures specifically refer to polymeric membranes, while carrier‐based membranes specifically refer to facilitated transport membranes. These two classes of membranes are represented by squares, but distinguished by different colors.

Figure 16

Strategies to inhibit plasticization in membranes. a) Cross‐linking strengthens interchain interaction and mitigates the “swelling” of polymer chains under higher feed pressure. Reproduced with permission.^{[ 86d ]} Copyright 2017, Royal Society of Chemistry. b) Introducing fillers could immobilize the polymer chains under high feed pressure and contribute to higher plasticization pressure. Reproduced with permission.^{[ 23 ]} Copyright 2016, Springer Nature. c) PDMS coating penetrates into the intercrystalline regions and hinders the framework flexibility of ZIF‐8, helping the membrane maintain high selectivity under high pressure difference across the membrane. Reproduced with permission.^{[ 76 ]} Copyright 2017, Royal Society of Chemistry.